Chemical Dust Suppression Technology and Its Applications in Mines (Open-pit Mines) 981169379X, 9789811693793

Table of contents :
Preface
Contents
1 Introduction
1.1 Dust Source in Open-Pit Mine
1.1.1 Dust Generated by Drilling Operation
1.1.2 Dust Generated by Drilling Operation
1.1.3 Dust Generated by Shoveling and Loading Operation
1.1.4 Dust Generated by Transportation Operation
1.1.5 Dust Generated by Dumping Operation
1.2 Calculation of the Intensity of Dust Source in the Mine
1.2.1 Source Intensity of the Mine
1.2.2 Calculation of the Source Intensity of the Mine
1.3 Development Status of Chemical Depression of Dust
1.3.1 Research Status of Dust Suppressants Abroad
1.3.2 Research Status of Domestic Dust Suppressants
1.3.3 Development Trend of Chemical Dust Suppressants
References
2 Mechanism of Production and Transport of Blasting Dust and Smoke of Open-Pit Mine and Study on Pollution Model
2.1 Mechanism of Production and Transport of Blasting Dust and Smoke of Open-Pit Mine [1]
2.2 Study on Pollution Model of Blasting Dust and Toxic Gases
2.2.1 Diffusion and Settling of Blasting Dust
2.2.2 Mathematical Model of Gaseous Substance Motion of the Dust and Smoke
2.2.3 Evaluation of Blasting Dust and Toxic Gases Pollution
2.2.4 The Emission Intensity of the Blasting Dust and Toxic Gases
2.3 Computer Numerical Simulation Study on Mine Blasting Dust and Toxic Gases
2.3.1 Computer Numerical Simulation of the Initial Motion of Explosive Gas
2.3.2 Numerical Simulation of Movement Process of Blasting Dust Particles
References
3 Chemical Suppression Technology of Open-Pit Mine Blasting Dust and Smoke
3.1 Control Measures on Open-Pit Mine Blasting Dust
3.2 The Development of Water Enriched Gelatin Stemming
3.2.1 The Development of Water Enriched Gelatin Stemming
3.2.2 Dust and Toxic Gases Reduction Effect of Water Enriched Gelatin Stemming
3.2.3 Dust Reduction Effect of Water Enriched Gelatin Stemming
3.2.4 Dust and Toxic Gases Reduction Mechanism of Water Enriched Gelatin Stemming
3.3 Development of Blasting Dust and Smoke Inhibitors of Open-Pit Mine
3.3.1 Dust Reduction Mechanism of Blasting Dust and Smoke Inhibitors
3.3.2 Component of Dust and Smoke Inhibitors
3.3.3 Monomer Experiment
3.3.4 Compounding Experiments of Surfactants and Hygroscopic Agents
3.3.5 Final Selection of Surfactant and Hygroscopic Agent
3.3.6 The Optimal Formula of the Dust and Smoke Inhibitor
3.4 Research on Blasting Dust Reduction with Foam
3.4.1 Formula Requirement of Foaming Agent
3.4.2 Development of Foaming Agent Formula
3.4.3 Experimental Study on Foaming Generator Performance
3.4.4 Experimental Results
3.4.5 Measurement of Foam Dust Removal Efficiency
References
4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard
4.1 Mechanics of Dust Production of Open Ore Stacking Yard [1–6]
4.2 Measures of Dust Prevention of Open Ore Stacking Yard
4.3 Development of Dust Suppressants of Open Ore Stacking Yard
4.3.1 The Dust-Settling Mechanism of Dust Suppressants of Open Ore Stacking Yard [21, 22]
4.3.2 Single-Factor Experiment
4.3.3 Orthogonal Experiment
4.4 Performance Study on Dust Suppressants of Open Ore Stacking Yard
4.4.1 The Basic Physicochemical Property of Dust Suppressants
4.4.2 Surface Curing Effect
4.4.3 Compressive Strength
4.4.4 Wind Erosion Resistance
4.4.5 Rain Resistance
4.4.6 Freeze–Thaw Resistance Property
References
5 Chemical Dust Suppression Technology of Road Surface of Strip Mine
5.1 Mechanisms of Dust-Raising on Road Surface of Transportation Roads of a Strip Mine [1, 2]
5.1.1 Dust Anchoring Load
5.1.2 Dust-Raising Generated by the Shear Friction of Automobile Tires on the Road Surface
5.1.3 Dust-Raising Generated by the Mechanical Wind Load of the Vehicle
5.1.4 Dust Raised by Natural Wind Current on the Road Surface
5.2 Influencing Factors and Control Measures of Dust-Raising on Transportation Roads in Strip Mines
5.2.1 Influencing Factors of Dust-Raising on the Road Surface of Strip Mines
5.2.2 Control Measures for Dust-Raising on Road Surface
5.3 Experimental Research on the Law of Dust-Raising Diffusion on Road Surface
5.4 Development of Compound Dust Suppressant Formula on the Road Surface
5.4.1 Performance Requirements of Dust Suppressants
5.4.2 Dust Suppression Mechanism of Dust Suppressant
5.4.3 Components of Dust Suppressant
5.4.4 Monomer Experiment
5.4.5 Orthogonal Experiment
5.4.6 Formula Optimization Experiment
5.4.7 Performance Characteristics of Dust Suppressant
5.5 Hygroscopic Dust Suppressant for Road Surface
5.5.1 Formula of Hygroscopic Dust Suppressant for Road Surface
5.5.2 Performance Experiment of Hygroscopic Dust Suppressant for Road Surface
5.6 Moist Dust Suppressant for Road Surface
5.6.1 Formula of Moist Dust Suppressant for Road Surface
5.6.2 Performance experiment of moist dust suppressant for road surface
References
6 Field Application of Mine Dust Suppressant
6.1 Application of Blasting Dust Suppressant in Open-Pit Mine
6.1.1 Application of Water-Enriched Gelatin Stemming
6.1.2 Application of Foam in Blasting
6.1.3 Application of Surfactant Solution in Blasting
6.2 Application of Dust Suppressant in Open-Pit Mine Yard
6.2.1 Industrial Test of Hygroscopic Dust Suppressant in Suppressing Dust of Stockpile
6.2.2 Dust Floating Test of Adhesive Dust Suppressant in Tailing Pond
6.2.3 Dust Suppression Test of Binding Dust Suppressant in Open Coal Pile
6.3 Application of Dust Suppressant on Pavement in Open-Pit Mine
6.3.1 Spring Road Dust Suppressant Industrial Experiment-Shougang Shuichang Iron Mine
6.3.2 Industrial Experiment of Road Dust Suppressant in Summer-Wulongquan Mine in Wuhan
6.3.3 Industrial Experiment of Pavement Dust Suppressant in Autumn-Sijiaying Mine, Hebei
6.3.4 Industrial Experiment of Road Dust Suppressant in Winter-Jianlin Mountain Iron Mine

Citation preview

Yuan Wang · Cuifeng Du · Jiuzhu Wang · Huaiyu Li

Chemical Dust Suppression Technology and Its Applications in Mines (Open-pit Mines)

Chemical Dust Suppression Technology and Its Applications in Mines (Open-pit Mines)

Yuan Wang · Cuifeng Du · Jiuzhu Wang · Huaiyu Li

Chemical Dust Suppression Technology and Its Applications in Mines (Open-pit Mines)

Yuan Wang School of Civil and Resource Engineering University of Science and Technology Beijing Beijing, China

Cuifeng Du School of Civil and Resources Engineering University of Science and Technology Beijing Beijing, China

Jiuzhu Wang School of Civil and Resources Engineering University of Science and Technology Beijing Beijing, China

Huaiyu Li School of Civil and Resources Engineering University of Science and Technology Beijing Beijing, China

ISBN 978-981-16-9379-3 ISBN 978-981-16-9380-9 (eBook) https://doi.org/10.1007/978-981-16-9380-9 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

This book is focused on the study of chemical dust suppression in mine dust pollution control by means of theories, experiments, computer simulation and case application, aiming at providing chemical dust suppression solutions for mining worksites. With the incentive of accelerating the application of chemical dust suppressants in mine dust population control, this book would hopefully be of guiding significance for efficient control of mine dust pollution. The book is composed of six chapters, including the following contents: (1) The book analyzed the dust source intensity of worksites and the mechanisms of dust generation and summarized the dust control measures for different mining worksites. (2) According to the mechanisms of dust generation in different mining worksites, targeted dust suppressants were developed. Through optimization by monomer experiment and orthogonal experiment, the optimum formula of different types of dust suppressants was obtained, and its properties were characterized. (3) The dust suppressant field application process was introduced and the economic benefits were analyzed. This book is expected to provide application references for researchers and engineering technicians engaged in environmental engineering, safety engineering, occupational health and mining metallurgical engineering, and it can also serve as the elective textbook for teachers and graduate students in the above disciplines. Beijing, China

Yuan Wang Cuifeng Du Jiuzhu Wang Huaiyu Li

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Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Dust Source in Open-Pit Mine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 Dust Generated by Drilling Operation . . . . . . . . . . . . . . . . . . . 1.1.2 Dust Generated by Drilling Operation . . . . . . . . . . . . . . . . . . . 1.1.3 Dust Generated by Shoveling and Loading Operation . . . . . 1.1.4 Dust Generated by Transportation Operation . . . . . . . . . . . . . 1.1.5 Dust Generated by Dumping Operation . . . . . . . . . . . . . . . . . 1.2 Calculation of the Intensity of Dust Source in the Mine . . . . . . . . . . 1.2.1 Source Intensity of the Mine . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Calculation of the Source Intensity of the Mine . . . . . . . . . . . 1.3 Development Status of Chemical Depression of Dust . . . . . . . . . . . . 1.3.1 Research Status of Dust Suppressants Abroad . . . . . . . . . . . . 1.3.2 Research Status of Domestic Dust Suppressants . . . . . . . . . . 1.3.3 Development Trend of Chemical Dust Suppressants . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Mechanism of Production and Transport of Blasting Dust and Smoke of Open-Pit Mine and Study on Pollution Model . . . . . . . . 2.1 Mechanism of Production and Transport of Blasting Dust and Smoke of Open-Pit Mine [1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Study on Pollution Model of Blasting Dust and Toxic Gases . . . . . . 2.2.1 Diffusion and Settling of Blasting Dust . . . . . . . . . . . . . . . . . . 2.2.2 Mathematical Model of Gaseous Substance Motion of the Dust and Smoke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Evaluation of Blasting Dust and Toxic Gases Pollution . . . . 2.2.4 The Emission Intensity of the Blasting Dust and Toxic Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Computer Numerical Simulation Study on Mine Blasting Dust and Toxic Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Computer Numerical Simulation of the Initial Motion of Explosive Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 1 2 2 2 3 3 3 3 10 11 13 15 16 19 19 20 21 23 27 30 34 34

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2.3.2 Numerical Simulation of Movement Process of Blasting Dust Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Chemical Suppression Technology of Open-Pit Mine Blasting Dust and Smoke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Control Measures on Open-Pit Mine Blasting Dust . . . . . . . . . . . . . . 3.2 The Development of Water Enriched Gelatin Stemming . . . . . . . . . . 3.2.1 The Development of Water Enriched Gelatin Stemming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Dust and Toxic Gases Reduction Effect of Water Enriched Gelatin Stemming . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Dust Reduction Effect of Water Enriched Gelatin Stemming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4 Dust and Toxic Gases Reduction Mechanism of Water Enriched Gelatin Stemming . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Development of Blasting Dust and Smoke Inhibitors of Open-Pit Mine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Dust Reduction Mechanism of Blasting Dust and Smoke Inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Component of Dust and Smoke Inhibitors . . . . . . . . . . . . . . . 3.3.3 Monomer Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4 Compounding Experiments of Surfactants and Hygroscopic Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5 Final Selection of Surfactant and Hygroscopic Agent . . . . . . 3.3.6 The Optimal Formula of the Dust and Smoke Inhibitor . . . . 3.4 Research on Blasting Dust Reduction with Foam . . . . . . . . . . . . . . . . 3.4.1 Formula Requirement of Foaming Agent . . . . . . . . . . . . . . . . 3.4.2 Development of Foaming Agent Formula . . . . . . . . . . . . . . . . 3.4.3 Experimental Study on Foaming Generator Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.5 Measurement of Foam Dust Removal Efficiency . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Mechanics of Dust Production of Open Ore Stacking Yard [1–6] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Measures of Dust Prevention of Open Ore Stacking Yard . . . . . . . . . 4.3 Development of Dust Suppressants of Open Ore Stacking Yard . . . 4.3.1 The Dust-Settling Mechanism of Dust Suppressants of Open Ore Stacking Yard [21, 22] . . . . . . . . . . . . . . . . . . . . . 4.3.2 Single-Factor Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Orthogonal Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

36 38 41 41 47 47 53 61 63 70 70 72 77 96 98 101 106 106 111 113 116 119 120 123 123 124 126 126 129 148

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4.4 Performance Study on Dust Suppressants of Open Ore Stacking Yard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 The Basic Physicochemical Property of Dust Suppressants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Surface Curing Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3 Compressive Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4 Wind Erosion Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5 Rain Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.6 Freeze–Thaw Resistance Property . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Chemical Dust Suppression Technology of Road Surface of Strip Mine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Mechanisms of Dust-Raising on Road Surface of Transportation Roads of a Strip Mine [1, 2] . . . . . . . . . . . . . . . . . . 5.1.1 Dust Anchoring Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2 Dust-Raising Generated by the Shear Friction of Automobile Tires on the Road Surface . . . . . . . . . . . . . . . . 5.1.3 Dust-Raising Generated by the Mechanical Wind Load of the Vehicle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4 Dust Raised by Natural Wind Current on the Road Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Influencing Factors and Control Measures of Dust-Raising on Transportation Roads in Strip Mines . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Influencing Factors of Dust-Raising on the Road Surface of Strip Mines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Control Measures for Dust-Raising on Road Surface . . . . . . 5.3 Experimental Research on the Law of Dust-Raising Diffusion on Road Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Development of Compound Dust Suppressant Formula on the Road Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Performance Requirements of Dust Suppressants . . . . . . . . . 5.4.2 Dust Suppression Mechanism of Dust Suppressant . . . . . . . . 5.4.3 Components of Dust Suppressant . . . . . . . . . . . . . . . . . . . . . . . 5.4.4 Monomer Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.5 Orthogonal Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.6 Formula Optimization Experiment . . . . . . . . . . . . . . . . . . . . . . 5.4.7 Performance Characteristics of Dust Suppressant . . . . . . . . . 5.5 Hygroscopic Dust Suppressant for Road Surface . . . . . . . . . . . . . . . . 5.5.1 Formula of Hygroscopic Dust Suppressant for Road Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.2 Performance Experiment of Hygroscopic Dust Suppressant for Road Surface . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Moist Dust Suppressant for Road Surface . . . . . . . . . . . . . . . . . . . . . . 5.6.1 Formula of Moist Dust Suppressant for Road Surface . . . . .

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155 156 157 159 160 162 164 165 167 167 168 168 170 171 174 174 176 178 185 185 186 190 192 213 223 224 230 230 232 233 233

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5.6.2 Performance experiment of moist dust suppressant for road surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 6 Field Application of Mine Dust Suppressant . . . . . . . . . . . . . . . . . . . . . . 6.1 Application of Blasting Dust Suppressant in Open-Pit Mine . . . . . . 6.1.1 Application of Water-Enriched Gelatin Stemming . . . . . . . . 6.1.2 Application of Foam in Blasting . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Application of Surfactant Solution in Blasting . . . . . . . . . . . . 6.2 Application of Dust Suppressant in Open-Pit Mine Yard . . . . . . . . . 6.2.1 Industrial Test of Hygroscopic Dust Suppressant in Suppressing Dust of Stockpile . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Dust Floating Test of Adhesive Dust Suppressant in Tailing Pond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Dust Suppression Test of Binding Dust Suppressant in Open Coal Pile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Application of Dust Suppressant on Pavement in Open-Pit Mine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Spring Road Dust Suppressant Industrial Experiment-Shougang Shuichang Iron Mine . . . . . . . . . . . . . 6.3.2 Industrial Experiment of Road Dust Suppressant in Summer-Wulongquan Mine in Wuhan . . . . . . . . . . . . . . . . 6.3.3 Industrial Experiment of Pavement Dust Suppressant in Autumn-Sijiaying Mine, Hebei . . . . . . . . . . . . . . . . . . . . . . 6.3.4 Industrial Experiment of Road Dust Suppressant in Winter-Jianlin Mountain Iron Mine . . . . . . . . . . . . . . . . . . .

243 243 243 252 254 267 267 269 274 278 278 288 299 321

Chapter 1

Introduction

Abstract This chapter mainly analyzes the main sources of mine dust, and takes an open-pit iron mine as an example to calculate the source strength of each process. In addition, this chapter summarizes the research status of chemical dust suppressant and discusses its development trend.

1.1 Dust Source in Open-Pit Mine Dust pollution in open-pit mines is mainly caused by the drilling, blasting, shoveling and loading, fragmentation, transportation, dumping and other operations during the mining process [1]. The dust content in the stope of an open-pit mine is correlated with the meteorological conditions and geological characteristics of the mine, mining technology, equipment type, mining intensity and so on.

1.1.1 Dust Generated by Drilling Operation Drilling rig is one of the main equipment for open-pit mine production, and the intensity of its dust generation is second only to loading and transportation equipment, ranking third among production equipment in terms of dust generation amount [2]. When the drilling speed of a rotary drill is 0.025 m/s, it will generate dust of 2– 3.5 kg/s with a diameter of 10–15 mm. If no preventive measures are taken, under the action of airflow, large operation areas of open-pit mine will be polluted. The dust content in the air, even distant from the drilling rig, far exceeds the national health standards.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Wang et al., Chemical Dust Suppression Technology and Its Applications in Mines (Open-pit Mines), https://doi.org/10.1007/978-981-16-9380-9_1

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1 Introduction

1.1.2 Dust Generated by Drilling Operation The dust in blasting operations is mainly generated from explosive-induced rock breakage, most of which are respirable dust. At the same time, the blasting will lift up the dust on the surface, increasing the dust concentration. The amount of blastinggenerated dust is relatively large, but the high dust concentration arising from this is maintained in the air for a short time. During blasting, the explosion products are mainly gases, such as CO2 , H2 O, CO, NO2 , O2 , N2 , SO2 , H2 S, etc., customarily known as blasting smoke, among which CO, nitrides (NO, NO2 ) and H2 S are hazardous gases. Whether hazardous gases are produced or not and the amount of production are related to the oxygen balance of explosives, the complete degree of explosive reaction and other factors.

1.1.3 Dust Generated by Shoveling and Loading Operation The dust generated by shoveling and loading operation is the most important source of dust in open-pit mines. The dust intensity of the electric shovels used in mines ranges between 400 mg/s and 2000 mg/s. The dust is generated by the shoveling and loading operation for the following reasons: the electric shovel breaks the rock when excavating the ore and rock, part of the dust sink down on the surface of the ore and rock, and the other part caused by friction and collision under vibration become the re-entrain dust. When the electric shovel is unloading materials to transportation or reloading equipment, a large amount of dust will be generated due to the drop. When using bulldozer or some other mining equipment to clean the floating coal on the muck piles and steps, a certain amount of dust will also be generated.

1.1.4 Dust Generated by Transportation Operation During vehicle transportation, the dust deposited on the transportation road affected by the compression, vibration and airflow when the vehicle passes will move irregularly, causing the re-entrain dust. During the transportation via a belt conveyor, due to the large cross-sectional area exposed to the air per unit time, a certain amount of dust will be generated under the action of airflow, belt vibration and other factors. Especially when no protective measure is taken, coal dust will be generated continuously, resulting in a waste of coal resources simultaneously.

1.1 Dust Source in Open-Pit Mine

3

1.1.5 Dust Generated by Dumping Operation When the transportation equipment discharges materials in the refuse dump, dust is generated due to the collision and friction of the rocks. Moreover, the large re-entrain dust generated due to the large drop of the refuse dump and the large amount of waste has a serious impact on the surrounding environment.

1.2 Calculation of the Intensity of Dust Source in the Mine 1.2.1 Source Intensity of the Mine According to the mining equipment, the main source intensity in the open-pit mine is shown in Table 1.1.

1.2.2 Calculation of the Source Intensity of the Mine Taking an iron mine as an experimental mine, the source intensity of mine drilling, blasting, shoveling and loading, fragmentation, transportation, dumping and other operation processes are calculated. (1)

Dust from drilling operation

Rotary drills as a commonly used drilling equipment in large open-pit mines mostly adopt wet dedusting method, making the dust moist and condensed, thus reducing the emission of dust. Through the actual monitoring of some mines, for the rotary drill with general dedusting efficiency, the emission of a single equipment is 1.05 kg/h, while for the equipment with good dedusting efficiency, the emission can be reduced to 0.22 kg/h. Dust generated by rotary drills can basically be regarded as a continuous point source. The fixed point with a long operating time and a small variation range generates relatively stable amount of dust. The layout of the measuring points is shown in Table 1.2. According to the diffusion model of the continuous point source on the ground, the concentration formula of the axis of the continuous point source on the ground is obtained [3]:   y2 z2 Q exp −( 2 ) exp(− 2 ) C(x, y, z, 0) = π uσ y σz 2σ y 2σz C(x, 0, 0, 0) =

Q π uσ y σz

(1.1)

(1.2)

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1 Introduction

Table 1.1 Main source intensity in the mine Mining equipment (dust source)

Source intensity, mg/s Coal mine

Metal mine

Building materials mine

Electric shovel

1000–1200

300–400

–

Dry-type

500–600

60–100

–

Wet-type

400–500

100–500

300–700

Dry-type

150–250

30–150

80–120

Wet-type

APF-1250 bucket shovel

11,000–12,000

–

–

Dry-type

1600–2000

–

–

Wet-type

3111–4/40 dragline

10,000–11,000

–

–

Dry-type

1400–1800

–

–

Wet-type

27 Belas truck

3000–4000

6000–12,000

12,000–15,000

No covering on the pavement

1000–2000

200–300

150–300

Pavement covered

–

80–150

–

Special covering on the pavement

Rotary drill, down-the-hole drill, rotary rig

600–1200

300–43,000

–

No dust extractor

50–80

20–250

–

With dust extractor or wet-type

Bench drill, trepanning machine

100–120

60–220

–

–

2–8

–

Bulldozer

1600–2000

200–250

1500–2500

Dry-type

–

–

200–400

Wet-type

Belt conveyor (per meter length)

35–50

–

–

Dry-type

15–25

–

–

Wet-type

Transfer point

80–100

–

–

Dry-type

45–60

–

–

Wet-type

Hourly capacity: 318t/h

–

80

–

Dry-type

Hourly capacity: 1340t/h

–

300

–

Dry-type

CM-518, CM-580A

–

140–1200

–

Dry-type

–

15–60

–

Wet-type

AKP-8 AKF-4

Truck

Drilling rig and bench drill

Bulldozer Belt conveyor and transfer point

Crusher

Dressed stone saw

Remarks

1.2 Calculation of the Intensity of Dust Source in the Mine

5

Table 1.2 Dust concentration at different distances in the downwind direction of rotary drill No

Different distances in the downwind direction m

Average concentration mg/m3

1m

2m

5m

10 m

Natural Concentration

Average wind velocity m/s

37.8000

37.2500

14.7500

7.5000

5.407

1.6

The following can be obtained from the above formula: Q = π uσ y σz C(x, 0, 0, 0)

(1.3)

The test conditions are as follows: the wind speed is 2.8 m/s. The on-site atmospheric stability level is determined based on the surface wind speed and the solar radiation level, which can be combined to obtain the atmospheric stability grade of B to C. According to the data table of the power function expression of the diffusion parameter, σ y = 1.2276 and σz = 0.6 at 5 m are obtained. Substitute into the above formula Q 1 = 186.18 mg/s. Same as above, at 10 m from the dust source, the diffusion parameter σ y = 2.31409, σz = 1.17189. speed 1.5 m/s). Substitute into the above formula Q 2 = 223.52 mg/s (wind n 1 Qi According to the formula of average source intensity Q = n , it can be obtained that Q = 196.35 mg/s. (2)

Dust from shoveling and loading operation

The dust pollution source of shoveling and loading is a kind of fixed continuous point pollution source, mainly caused by electric shovel. There is no outlet channel for dust generated by this pollution source, so that the dust is directly discharged into the atmosphere. Therefore, it is one of the main dust pollution sources. When measuring the emission intensity of the dust of the electric shovel, one or two sampling points are set at 15 m on the upwind side of the operating point of the electric shovel, and 2 or 3 sampling points are set up at 15–20 m on the downwind side of the operating point of the electric shovel, and the results are shown in Table 1.3. The concentration distribution curve is drawn according to the dust concentration distribution at different distances from the dust source, as shown in Fig. 1.1. The functional distribution is obtained by the concentration distribution curve fitting, and the concentration near the shoveling and loading point is calculated as 31.32 mg/m3 . The source intensity is calculated using the estimation method of flux production [4].   Q = (C − C0 ) · L y − L z v

(1.4)

Q—the amount of dust generated by unorganized emissions per unit time, mg/s;

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1 Introduction

Table 1.3 Dust concentration at different distances in the wind direction Distance group

5m

9m

11 m

13 m

Background concentration

1

34.3413

19.4103

10.4517

10.4517

5.633

2

29.862

14.931

7.4655

8.9586

6.436

3

18.51

16.4241

11.9448

7.4655

4.354

4

14.931

13.4379

13.4379

7.4655

5.874

5

19.33

14.931

11.9448

8.9586

4.739

Average mg/m3

23.394

15.826

11.048

8.659

5.407

Fig. 1.1 Concentration distribution curve of dust generated by electric shovel

24

Dust concentration/mg/m3

22 20 18 16 14 12 10 8 6 4

4

6

8

10

12

14

16

Distance from dust source/m

C—dust concentration at the test point on the leeward side, mg/m3 ; C0 —dust concentration on the upwind side of the operation point, mg/m3 ; v—the average velocity at the cross-section test point, m/s; L y —the width of the measuring section, m; L z —the height of the measuring section, m; L y = 4.3σ y , L z = 4.3σz ; σ y and σz are the horizontal diffusion parameter and vertical diffusion parameter at this distance, respectively. By using the method of determining the atmospheric stability, it is found that the on-site atmospheric stability is level B, so the horizontal and vertical diffusion parameters are σ y = 1.2276, σz = 0.6. And then, it can be calculated that Q 1 = 563.66 mg/s. The effective duration of electric shovel loading once is 50 s, so the amount of dust generated W = Q 1 · t = 28,183 mg. The loading capacity is 21.6 t. The dust Intensity of shoveling and loading operation Q c = W/Loading Capacity = 1304.76 mg/t.

1.2 Calculation of the Intensity of Dust Source in the Mine

(3)

7

Dust from transportation

Vehicle dust is a continuous line source. Therefore, when there is no wind or a wind direction without effect on the dust source diffusion in the direction of the vehicle (the same or opposite to the driving direction of the vehicle), the dust raised behind the vehicle will form a continuous dust layer with different concentrations in the vertical direction due to different dust particle sizes. C1 , C2 · · · Cn are the dust concentrations of different heights measured in real time within a short time. Then, the average concentration and the space volume of dust diffusion are calculated and converted into the amount of dust. Dividing it by the driving time or distance and load of the car, the dust intensity can be obtained (the time is relatively small, and it is continuous dust generation, and dust deposition is not considered). The selection of test points: the layout of test points is divided into three sections of different heights, different dips in different stopes are arranged in the test points, which are relatively representative road sections less influenced by wind action. Through synthesis, the concentration is obtained. The layout of specific measuring points is shown in Fig. 1.2. The experimental data is shown in Table 1.4. Figure 1.3 shows the concentration distribution curve and fitting figure at different heights. The formula of the fitting curve is shown in formula 1.5 C(h) = −17.8 + 409.7h − 591.9h 2 + 334.1h 3 − 67.3h 4

(1.5)

where C (h)—concentration value, mg/m3 ; Fig. 1.2 Layout drawing of measuring points for dust concentration at different heights

Table 1.4 Dust concentration at different heights Height/m

0.15 m

0.5 m

1.0 m

1.5 m

1.7 m

2.0 m

Average mg/m3

32.0188

75.834

64.0376

55.6116

43.8152

31.1762

Note The concentration value of background concentration point 6 is 7 mg/m3

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1 Introduction

Experimental value Fitting value

80

Dust concentrationmg/m3

Fig. 1.3 Fitting curve of concentration distribution at different heights

70

60

50

40

30 0.0

0.5

1.0

1.5

2.0

Distance from groundm

h—height from the road, m. According to the regression curve equation, the dust production of the vehicle driving this distance is calculated by using the formula (1.5) [5]: h 0 W =

C(h) · Sdh

(1.6)

0

where h0 —the diffusion height of dust at the rear of the vehicle, m; C (h)—the dust concentration at different heights from the road, mg/m3 ; S—The cross-sectional area of the road the vehicle has driven on, m2 . Set C (h) = 7 and the original concentration, through the solution, the upper limit of the integral, h0 = 2.26 m, and when the height of the rear part of the vehicle is 2.26 m from the road, the concentration after diffusion reaches the original concentration. The driving distance of the vehicle is 23.4 m, the width of the vehicle is 5.7 m, and the spreading width is 1 m. From the above formula, the dust production is: W = 18,344.07840 mg. The dust production intensity per unit time of the vehicle is: G = W/t = 3057.3464 mg/s. As calculated by formula (1.5), the amount of dust produced by a single vehicle driving for 6 s: W = 18,344.07840 mg. The intensity of dust production of a single vehicle: Q = Wt = 3057.3464 mg/s. (4)

Dust from dumping of ore and rock

On the downwind side of the operating point of fragmentation and dumping, 2 to 3 sampling points are set up, and the average value is obtained by measuring different groups of data. The calculation method is the same as that of the above shoveling and loading operation. The measured data are shown in Table 1.5.

1.2 Calculation of the Intensity of Dust Source in the Mine

9

Table 1.5 Dust concentration at different distances from the downwind side of the dumping site m

Downwind distance

Average concentration mg/m3

10 m

15 m

20 m

25 m

Measuring point of background concentration

43.2999

22.6453

20.4057

16.6107

6.5735

Note Average wind velocity at the test point: 2.5 m/s

Through the calculation of the flux generated, it is obtained that Q = 3059.06 mg/s. The average effective duration of dumping dust once is 60 s. Therefore, the amount of dust produced is W = G * t = 91,771.8 mg, with a loading capacity of 130 T. The intensity of dust production Q = W/Loading Capacity = 1412.12 mg/t. (5)

Dust from blasting

The amount of dust produced from blasting is related to factors such as the blasting method, the explosive quantity, and the physicochemical properties of the ore and rock, and the dust diffuses with the air flow produced by blasting and the natural airflow. The dust production is calculated using the concentration reverse method, that is, using the mode of pollutant transportation and diffusion in the atmosphere, and inversely calculating the pollutant production or emissions from the concentration values measured in the field, with the blasting pile as the center, X axis horizontally towards the wind direction, Y axis perpendicular to the wind direction, and Z axis vertically upward. A rectangular coordinate system is established, where the equation of pollution diffusion of instantaneous pollution point source is as follows.  1 (X − Vx t)2 2Q   exp − 2 σx2 (2π )3/2 σx σ y σz 

(Z − Vx t − V p t)2 Y2 + 2 + σy σz2

C(X, Y, Z , t) =

(1.7)

where C—the instantaneous dust concentration at the measuring point, kg/m3 ; Q—the dust emissions from blasting, kg; Vx , Vy , V p are the average wind velocity and dust sedimentation velocity in the X and Z directions, respectively, m/s; σx , σ y , σz are the diffusion coefficients in three directions, m. Thus, the dust emission represented by the instantaneous concentration is obtained:    (2π )3/2 σx σ y σz 1 (X − Vx t)2 C(X, Y, Z , t)/ exp − Q= 2 2 σx2

10

1 Introduction

Table 1.6 Dust concentration at the measuring point of blasting operation Coordinates of the measuring point

The first time

The second time

The third time

The fourth time

(30, 0, 20)

266.54 mg/m3

211.39 mg/m3

157.18 mg/m3

302.52 mg/m3

Note The concentration values which referring to the maximum concentration appear at 33 s, 29 s, 20 s, 38 s at the measuring point, respectively, with the average wind velocity of 1.1 m/s, 1.4 m/s, 1.5 m/s, 0.9 m/s

(Z − Vz t − V p t)2 Y2 + 2 + σy σz2



(1.8)

By testing several many blasting operations at fixed points, the data in the following table are obtained (Table 1.6). The diffusion parameters are obtained by checking the power function expression table of the diffusion parameters, which are σx = σ y = 4.11 and σz = 2.82, respectively, and the rest of the data are measured in the field. According to formula (1.1–1.8), the average dust emission of one blasting operation is calculated: Q = 222.837 kg. The average explosive quantity used per blasting is 26 (the final amount of explosives is calculated according to the unit explosive consumption). Therefore, the emission intensity of dust during blasting is 8.57 kg/t.

1.3 Development Status of Chemical Depression of Dust The dust suppression technology has been studied and applied for a long time. Based on the results of continuous research and practice, the achievements of dust suppression in the field of air purification are now among the top. However, dust elimination technology is a rather complicated technology with a lot of limitations. For example, many problems in the theory and methods of dust suppression still await further exploration; there still exist some shortcomings in the structure of various dust collectors; although the existing dust suppression technology can meet the required requirements in some places, it is not available economically because of high cost. These problems require continuous hard work to find a better solution. Since the twentieth century, especially in the last 50 years, chemical dust suppressant has been developed and applied, which became one of the relatively new and effective dust suppression methods. This method has been innovated and developed continuously since its emergence. Chemical dust suppression generally reduces the surface tension of the solution and enhance the wetting effect by means of surfactants [6, 7].

1.3 Development Status of Chemical Depression of Dust

11

1.3.1 Research Status of Dust Suppressants Abroad In the 1920s, some British researchers conducted laboratory study and measurement of the properties of some wetting agents and applied them to mines. At that time, one derivative of sulfonated hydrocarbons was found with better wetting properties than other compounds. The main research purpose of British scholars was to invent an effective new method to wet the dust on the floor of roadway, thus preventing dust from being generated from the walking of people and animals; on the other hand, they hoped to use wetting agents to suppress coal dust from drilling, blasting, loading and other operations [5]. By the 1960s, through an early study on the use of wetting agents to suppress the dust production of continuous mining machines by the Mining Administration of the United States, it was found that the dust reduction effect of aqueous solution containing wetting agents was about 30% higher than that without wetting agents [8]. After the 1980s, with the continuous development of environmental protection technology, developed countries have invested strong scientific research forces in developing chemical dust suppressants, which promoted its constant and steady development. In the development of application field, chemical dust suppressants have been expanded from the prevention and control of coal dust hazards to all kinds of non-coal mine dust pollution; dust binders and coagulants have also begun to functioned as curing agents for soil dams, subgrade, foundation in-situ and open-air materials, as well as anti-leakage agents for surface and underground geotechnical structures. In 1998, sulfonated melamine aldehyde resin and polyvinyl alcohol were initially used by Lahalih et al. as the main components to form a chemical dust suppressant, which was proved to have a good fixation effect on sand and dust, as well as good water corrosion resistance and pressure resistance [9]. In recent years, many countries have done a lot of work on chemical dust suppressants and published a large number of special reports. Some countries and regions have established a good market supply and demand relationship and achieved excellent economic and environmental benefits. In 2011, through the polymerization of natural glycerol, a by-product from biodiesel, Medeiros M A et al. obtained a polymer, which was an efficient dust suppressant. Through thermogravimetric analysis, the results showed that the type and concentration of initiators are of great importance to obtain a good polymerization rate. The dust suppressant derived from the natural glycerin product owns more efficient performance in dust suppression than that of the glycerin product oligomer [10]. In 2015, Gotosa J. et al. used molasses and alcohol wastewater as the raw material to prepare dust suppressants, which was economical and environmentally friendly. In addition, it had better actual dust suppression effect than pure water, and reduced the dust deposition rate of gravel sandy roads by 77–83% [11]. In 2016, Dou Goulan et al. studied the aqueous solutions of surfactants such as alkyl polyglycosides and sodium dodecyl benzene sulfonate, and discussed the interaction between sodium carboxymethyl cellulose and a super absorbent polymer.

12

1 Introduction

In terms of surface tension of the solution and emulsification, it is found that the super absorbent polymer has a better wetting effect on coal dust than sodium carboxymethyl cellulose [12]. In 2016, Gilmour et al. developed an environment-friendly dust suppressant with excellent dust suppression effect, which is also a microbial dust suppressant with bioremediation function. The application of the dust suppressant could not only minimize the health and safety risks of workers, but would also improve the productivity of mining operations and save costs. The microbial composition in the dust suppressant formula could provide long-term environmentally friendly conditions for bioremediation, soil regeneration and vegetation regeneration so as to further stabilize the road [13]. In 2019, Alvaro Gonzalez et al. found that the usage of magnesium chloride hexahydrate (6H2 O·MgCl2 ) could reduce dust on mining roads and improve transportation safety and mining efficiency. Studies also revealed that the application of brine of 6H2 O·MgCl2 on transportation roads can effectively reduce dust suppression water consumption, and even save about 99% of water, indicating that the byproduct of 6H2 O·MgCl2 can be used as a very promising material to address road water problem in open-pit mines [14]. Wang et al. [15] prepared five different types of wetting agents, measured the surface tension of different types of wetting agents by capillary rise method, and determined a wetting agent with better overall effect. In order to control pulverized coal, Xi et al. [16] proposed a compound dust suppressant blended with polyethylene oxide (g-PEO) and sodium dodecyl sulfate (SDS). Zhou et al. [17, 18] studied the performance of the developed cementing agents using infrared spectrum (FTIR), x-ray diffraction analysis (XRD), simultaneous thermal analyzer (TGA–DSC), scanning electron microscope (SEM) and other methods. According to the hydrophobicity of lignite, Fan et al. [19] designed a compound dust suppressant, which could reach a dust collection efficiency of 97.1% after industrial test. Shi et al. [20] chose an efficient dust suppressant compound scheme based on parameters such as surface tension, sedimentation time and contact angle, and studied the influence of surfactants on the surface wettability of coal samples. Despite an ideal dust suppression effect of dust suppression agent, it has the disadvantages of poor environmental protection and short validity period. Therefore, some scholars have also made improvements accordingly. Fan et al. [19] prepared a compound dust suppressant for the agglomeration of coal dust. Studies have shown that this dust suppressant can effectively restrain the diffusion of coal dust. Some scholars [21, 22] also proposed the surfactants with magnetized water by utilizing the synergistic effect of magnetization and surfactants. Huang et al. [23] developed an environmentally-friendly, efficient and composite moisturizing dust suppressant for rock dust, which is applicable to dust suppression on the copper ore transportation road, and the experimental research results proved the dust suppressant’s good dust suppression effect.

1.3 Development Status of Chemical Depression of Dust

13

1.3.2 Research Status of Domestic Dust Suppressants The research on chemical dust suppressants in China started late but developed rapidly. The soil stabilizer was the first research object. From 1960s to the end of 1970s, the relevant domestic units began to introduce foreign technology of sand stabilizer. On the basis of drawing overseas experience, relevant research of soil stabilizer was initiated according to the properties of sandy soil in China. The 1980s witnessed the rapid development of chemical dust suppressants in China, with remarkable progress in the research of chemical dust suppressants and application of a lot of patents. In 1990, Xiang et al. developed a new SDLY-type of chemical dust suppressant, and selected highly hydrophobic pulverized coal with a particle size of less than 70 µm as the test dust. When the temperature is 25 °C, the static dust-fall test was performed on 75 kinds of monomers using the static infiltration and sedimentation test method. 16 kinds of monomers were selected for formulation design, thus developing SDLY dust collector [24]. Since the 1990s, with the continuous emergence of chemical dust suppressant research results, and deep exploration of fields involved, the types of additives for inorganic and organic chemical dust suppressants have also been constantly expanded. Taking advantage of the binding effect of inorganic compound materials, inorganic chemical dust suppressants, such as cement, lime, NCS curing agent, compound extravasation agent, etc., can achieve the purpose of sand fixation and dust suppression. Organic chemical dust suppressants generally composed of crude oil, slag, etc., need to be emulsified by adding some emulsifiers. In 1997, Wu Chao et al. did a lot of work in this area and studied the influence of various types of additives on the performance of dust suppression [25]. In the twenty-first century, polymer chemical dust suppressants have developed rapidly in China. Polymer chemical dust suppressants are roughly classified into petroleum products, synthetic polymers, and biomass resources. In addition, due to its excellent water absorption and retention properties, super absorbent resin products are often used in dust suppressants for dust suppression, moisture absorption and moisturizing. In 2003, Yang Ming et al. developed the polymer dust suppressant with the main component of a compound sulfonated urea-melamine–formaldehyde resin. The experimental results showed that when the amount of formaldehyde was about 3 times the amount of urea and 4 times the amount of melamine, the ratio of the amount of sulfonating agent to the sum of urea and melamine not less than 1:1, a good watersoluble and stable dust suppressant could be obtained [26]. In 2008, taking watersoluble polymers such as polyvinyl alcohol, acrylic acid, ethylene glycol and starch as raw materials, Bai Li conducted the research on the film formation mechanism of polyvinyl alcohol solution, the graft copolymerization mechanism of polyvinyl alcohol and acrylic acid, the graft polymerization mechanism of starch and sodium acrylate and the catalytic oxidation modification of starch. She also investigated the solidification and dust suppression characteristics, solution viscosity behavior, solution permeability to coal and temperature resistance of protective curing layer of all kinds of solidified dust suppressants. The curing and dust suppression effects of

14

1 Introduction

the curing dust suppressant are tested through laboratory and on-site industrial-scale spraying experiments [27]. Although the lagging research on chemical dust suppressants in China compared with foreign countries, it has been increasingly applied with rapid development in recent years. In terms of the expansion of dust suppressant type and relevant application fields, researches have yielded great achievements. In 2012, Chai Qiang et al. synthesized the coal dust suppressant with excellent water solubility, adhesion and stability with oxidized starch, polyvinyl alcohol and acrylic acid. It can be sprayed on the surface of coal to form a layer with good viscosity, compressive strength, weather resistance and water retention, thus effectively suppressing dust, reducing production costs and preventing spontaneous combustion of coal during transportation [28]. In 2013, Li Yansong et al. prepared a dust suppressant with good effect in the process of blasting dust reduction. In this process, the dust collection speed is rapid, accelerating the rapid conversion of carbon monoxide and nitrogen oxides [29]. In 2015, Du Cuifeng et al. developed an antifreeze dust suppressant based on the characteristics and mechanism of dust in the low-temperature environment of open pit roads. This kind of dust suppressant with good moisture absorption, moisture release and adhesion can resist the wind erosion of 30 m/s, without freezing at the temperature of −33.4 °C. It is economical with low cost, technically ensuring the road dust control in open-pit mine in winter [30]. In the same year, Pan Haijun et al. prepared a dust suppressant with 0.05% rapid penetrant, 0.01% BS12, and 1% Na2 SiO3 as the synergists, which reduced the surface tension of pulverized coal by 61.73%, the initial contact angle by 53.66%, with a dust reduction rate of over 80%. It is more efficient on respirable dust suppression [31]. In 2018, Hu Hui et al. formulates a dust suppression with high water retention rate and dust suppression efficiency with calcium chloride solution, sodium polyacrylate, AEO-9, potassium formate, sodium lauryl sulfate, and glycerin as the main raw materials. The application performance indicators for the diluent of this kind of dust suppressant with different concentrations can better meet the application requirements of customers in different regions and different needs [32]. Based on the technology of microbial induced precipitation of carbonate rock, Jiang Yaodong et al. developed a green and environmentally friendly microbial dust suppressant and urease dust suppressant with no toxicity, degradability, strong adhesion and good ecological compatibility [33]. In 2019, Li Shufang et al. prepared a wetting dust suppressant with papermaking waste, which became a research focus of waste reuse in recent years. By extracting lignosulfonate from papermaking waste and grafting onto the branched chain of polyacrylamide with graft copolymerization, adding a small amount of surfactant to enhance the wetting effect, a wetting dust suppressant for spraying and dust suppression was prepared [34]. In the same year, Wang Zhenyu et al. used inorganic mineral bentonite to compound a variety of functional additives, combined the microwave polymerization method with the traditional solution polymerization method, used a four-factor three-level orthogonal experiment, and used the water loss rate, moisture absorption rate and pH to characterize the objects, and conducted intuitive analysis and variance analysis of the experimental results, finally, the best composition of dust suppressant was determined. At the same time, physical performance testing,

1.3 Development Status of Chemical Depression of Dust

15

comprehensive thermal analysis experiments, wind resistance, moisture retention and other performance verification experiments were performed, and a compound dust suppressant with strong outdoor adaptability was developed [35].

1.3.3 Development Trend of Chemical Dust Suppressants From the development and research status of chemical dust suppressants at home and abroad, it can be seen that with the development of dust suppressants in recent years, the dust suppression effect of traditional dust suppressants (such as wetting dust suppressants) can no longer meet people’s requirements, and they have been gradually eliminated by the times and science and technology. With the continuous development of chemical products and the wide application of various surfactants, super absorbents and other polymer materials, the dust suppression efficiency of chemical dust suppressants has been continuously improved, and new chemical dust suppressants have emerged in endlessly. Multi-component, multi-functional, costeffective and environment-friendly dust suppressants have become the development trend [36]. (1)

In terms of component development

Most surfactants in chemical dust suppressants have transitioned from anionic to nonionic; the inorganic components of chemical dust suppressants have expanded from ordinary halides to alkaline oxides, coal ash, clay, gypsum, kaolin, acid and so on. Organic components have expanded from general crude oil, coal residue oil, asphalt to edible oil residue, biological oil residue, lignin derivatives, resins, paraffin oil, cellulose filter media, polymers, polymer functional polymers and so on [37]. In particular, chemical dust suppressants with the formulation composed of natural and synthetic high polymers are becoming increasingly common. (2)

In terms of molecular composition

The chemical dust suppressants has expanded from unitary composition to multiple compositions, and some dust suppressants are even composed of more than 5 chemical materials; the production process is becoming more complicated. Despite a higher dust suppression efficiency and wider versatility, the cost is relatively high. (3)

In terms of function development

In order to improve the efficiency of chemical dust suppressants, in recent years, researchers have continuously developed developing special chemical dust suppressants, such as dust suppressants for tailing pond, dust suppressants for road dust control, dust suppressants for fine particle material piles and pulverized coal yards, dust suppressant for blasting operation, dust suppressant for high silica minerals, cement dust suppressant, dust suppressant for coal bunker, dust suppressant for house demolition, dust suppressant for lime plant, dust suppressant for sprayed concrete, etc. [38]

16

1 Introduction

With the development of economy and the advancement of science and technology, people s awareness of environmental protection has also gradually strengthened, and the traditional chemical dust suppressants with imperfect performance and hidden dangers of environmental pollution are on the way out of the stage of dust prevention and control. At the same time, from the perspective of the performance and economic point of various dust suppression products in the market, there still exist some common problems, such as high cost, inconvenience in use, single performance, poor durability and environmental pollution to some extent, which limit the large-scale popularization and application in dust suppression and sand stabilization. In recent years, under the influence of the development model of low-carbon economy, dust suppressants are developing towards the direction of environmentfriendly, energy-saving and cheap, simple process, long-term dust suppression and multi-functions. It is urgent to research and develop low-cost, multi-purpose, longacting, easy-to-use and environment-friendly polymer dust suppression products, and popularize and use them on a national scale.

References 1. Bai RC, Bai Y, Wang ZP, et al. Preliminary discussion on dust prevention and control in surface mine. Opencast Mining Technol. 2013;(04):76–77+82. 2. Bi SG. dust pollution control of open-pit mine. China Molybdenum Ind. 2000;24(5):33–5. 3. Jiang ZA, Chen SJ, Wen HF. Aerosol mechanics and applications. Beijing: Metallurgical Industry Press; 2018. p. 118–21. 4. Shen HG, Su SJ, Zhong Q. Principles and processes of air pollution control engineering. Beijing: Tsinghua University Press; 2009. p. 15–6. 5. Yang J, Shen HS. Measurement and calculation of dust strength produced by underground rock drill. Nonferrous Metals (Mining Section). 1997;(05):44–47. 6. Du C, Cai S, Jiang Z. Experimental study on controlling road dust raising in the open mine with the YCH dust suppressant. J Univ Sci Technol Beijing. 2007;S2:45–8. 7. Li J, Zhou F, Liu H. The selection and application of a compound wetting agent to the coal seam water infusion for dust control. Coal Preparat. 2016;36(4):192–206. 8. Anderson FG, Beatty RL. Dust control in mining tunneling and quarrying in the United States. Inf Circ. 1969;8407:1–10. 9. Lahalih SM. Development and evaluation of new multipurpose soil additives. Ind Eng Chem Res. 1998;37:420–6. 10. Medeiros MA, Leite CMM, Lago RM. Use of glycerol by-product of biodiesel to produce an efficient dust suppressant. Chem Eng J. 2011;180. 11. Gotosa J, Nyamadzawo G, Mtetwa T, et al. Comparative road dust suppression capacity of molasses stillage and water on gravel road in Zimbabwe. Adv Res. 2015;3(2):198–208. 12. Dou G, Xu C. Comparison of effects of sodium carboxymethylcellulose and superabsorbentpolymer on coal dust wettability by surfactants. J Dispersion Sci Technol. 2016;38(11):1542–6. 13. Gilmour, Weamout, Quebec. Dustsuppressant:US, US20160130489A1. 2016-05-12. 14. Gonzalez A, Aitken D, Heitzer C, Lopez C, Gonzalez M. Reducing mine water use in arid areas through the use of a byproduct road dust suppressant. J Clean Product. 2019;230. 15. Wang K, Ma X, Jiang S, et al. Application study on complex wetting agent for dust-proof after gas drainage by outburst seams in coal mines. Int J Min Sci Technol. 2016;26(4):669–75. 16. Xi Z, Feng Z, Li A. Synergistic coal dust control using aqueous solutions of thermoplastic powder and anionic surfactant. Colloids Surf, A. 2017;520:864–71.

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17. Zhou G, Fan T, Ma Y. Preparation and chemical characterization of an environmentally-friendly coal dust cementing agent. J Chem Technol Biotechnol. 2017;2699–2708. 18. Zhou L, Yang S, Hu B, et al. Evaluating of the performance of a composite wetting dust suppressant on lignite dust. Powder Technol. 2018;339:882–93. 19. Fan T, Zhou G, Wang J. Preparation and characterization of a wetting-agglomeration-based hybrid coal dust suppressant. Process Saf Environ Prot. 2018;113:282–91. 20. Shi G, Han C, Wang Y, et al. Experimental study on synergistic wetting of a coal dust with dust suppressant compounded with noncationic surfactants and its mechanism analysis. Powder Technol;2019. 21. Cui D, Baisheng N, Hua Y, et al. Experimental research on optimization and coal dust suppression performance of magnetized surfactant solution. Procedia Eng. 2011;26:1314–1321. 22. Zhou Q, Qin B, Wang J, et al. Effects of preparation parameters on the wetting features of surfactant-magnetized water for dust control in Luwa mine, China. Powder Technol. 2018;326:7–15. 23. Huang Z, Zhang L, Yang Z, et al. Preparation and properties of a rock dust suppressant for a copper mine. Atmos Poll Res. 2019. 24. Xiang XD, Deng CZ, Ding CY. Development of Sdly dedusting agent. Indus Saf Environ Protect. 1990;(5):13–16+49. 25. Wang PL, Wu C. Dust inhibition properties of sodium polyacrylate sol. J Central South Univ (Science and Technology).1997;(4):319–321. 26. Yang M, Zhang LD, Guo HY, et al. Synthesis of sand-fixation agent and its water-solubility. J Beijing Univ Chem Technol. (Natural Science Edition). 2003;(4):81–84. 27. Bai L. Study on the dust suppression solidifying agent being used in the transport and the piling of the bulk minerals. Taiyuan University of Technology;2008. 28. Lai SL, Chai Q, Wang B, et al. Preparation and application of dust suppressant in coal transportation. Speciality Petrochem. 2012;29(1):56–9. 29. Li YS, Jiang B, Du CF. Industrial experiment and study on reducing blasting dust and gas in roadway excavation. Nonferrous Metals (Mining Section). 2013;65(1):90–93. 30. Du CF, Wang Y, Ren JY. Development and characteristics of the formula of one anti- freezing road dust-depressor. Nonferrous Metals (Mining Section). 2015;67(1):1–6. 31. Pan XJ. Research and Application of a new efficient dust-depressor. Safety in Coal Mines. 2015;46(4):134–6. 32. Hu H, Wang YL, Wu LY, et al. Preparation and application of compound antifreeze dust inhibitor. Shandong Chem Indus. 2018;47(20):22–4. 33. Jiang YD. Research on new dust-depressor of calcite precipitation induced by microbe and enzyme. Southeast University;2018. 34. Li SF, Tian J, Xie H, et al. Study on preparation of wetting dust suppressant from papermaking waste and its properties. Safety Coal Mines. 2019;50(07):14–16+20. 35. Wang ZY. Study on the mechanism and properties of compound dust suppressant under microwave polymerization. Taiyuan University of Technology;2019. 36. Zhang TT, Wang Z. Application research process of chemical dust suppressant. Journal of Longdong University. 2016;27(5):55–8. 37. Wang SW. The research and development in anti-dust and anti-rain film of hypaethral coaldeposit. Xi’an University of Architecture and Technology;2008. 38. Yang P. Study on applied technology of dust suppression of surfactants in caving face. Shandong University of Science and Technology;2009

Chapter 2

Mechanism of Production and Transport of Blasting Dust and Smoke of Open-Pit Mine and Study on Pollution Model

Abstract According to the experiment and site investigation, this chapter divides the smoke dust after blasting in stope into three stages, analyzes the mechanism of its production and transportation, establishes the diffusion equation of blasting dust and the mathematical model of gaseous substance movement and puts forward the mathematical model and method for evaluating the pollution status of blasting dust and poison. Based on the mathematical model of blasting dust diffusion and movement, it conducts numerical simulation on the movement and diffusion of blasting dust by programming.

2.1 Mechanism of Production and Transport of Blasting Dust and Smoke of Open-Pit Mine [1] Pit blasting is the process that explosive gas carrying huge energy expands rapidly to do work at the moment of explosion, as shown in Fig. 2.1. Explosive action first forces rock fragmentation, and there are large amount of dust particles instantly produced at the moment of rock fragmentation. The dust rushes into the open pit space to accelerate the outward movement with the blasting impact kinetic energy, with the increase in the diffusion radius, the diffusion rate gradually decreases [2]. From the experiments and field surveys, it is known that the movement of dust and smoke generated after blasting can be divided into 3 stages: impact motion stage, mushroom cloud formation stage and diffusion stage. For single dust particle, it is affected by gravity, buoyancy and resistance in the three stages. Assuming the particle mass is constant, the direction of gravity is vertically downward and the direction of buoyancy is vertically upward, both of which do not change in magnitude with velocity. The resistance is in the opposite direction to the velocity of particle motion and the magnitude is proportional to the velocity [3]. In the impact motion stage, the velocity of dust particle is greater than that of the air mass, so the dust particles move along the outside of the pollution clouds in the air medium, and the dust particles decelerate and rise by gravity, air buoyancy and air resistance [4, 5]. In the mushroom cloud formation stage, due to the decrease of vertical static pressure and autogenous turbulence, explosive gas keeps sucking fresh air, resulting in rising air © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Wang et al., Chemical Dust Suppression Technology and Its Applications in Mines (Open-pit Mines), https://doi.org/10.1007/978-981-16-9380-9_2

19

20

6s

2 Mechanism of Production and Transport of Blasting Dust …

14s

18s

Fig. 2.1 Morphological evolution of blasting dust masses

mass increasing, volume expanding, the velocity of rise decreasing. After a period of time, the gas expansion rate exceeds the velocity of dust particle movement, and gradually becomes the material along the outer edge of the polluted air mass, at which point a mushroom-shaped cloud is formed. In the late expansion of the explosive gas movement, due to entrainment of a large amount of fresh air, after a certain period of time, it is approximately considered that the nature of smoke is similar to that of surrounding air. At this time, the relative impact of dust particles and explosive gas is small, which can be seen as their own independent diffusion movements. The dust particles in the air decelerate the rise, accelerating descent becoming uniform settlement.

2.2 Study on Pollution Model of Blasting Dust and Toxic Gases Blasting dust and smoke consists of gaseous substance and dust. The superposition of trajectories of gaseous substance and particulate matter motion form the motion trails and cloud shape of blasting dust and smoke clouds according to the motion characteristics of gaseous substance and dust. The gaseous substance of the blasting dust and smoke a multi-component mixture of gases and tiny aerosols with the same behavioral characteristics as gases. Blasting dust is a collection of finite particles with various shapes. Due to the short time for the impact motion stage and mushroom cloud motion stage of dust and smoke, generally less than 3 s, the volume of its pollution cloud is smaller compared to that of the pollution cloud in the diffusion motion phase. Therefore, when evaluating the status of smoke pollution, the volume of pollution clouds in the impact motion stage and mushroom cloud motion stage are ignored, and the blasting smoke pollution process is only related to the diffusion motion process. The blasting dust and smoke dispersion equation and the mathematical model of gaseous substance motion will be established, and the evaluation mathematical model and evaluation methods of blasting dust and toxic gases pollution status will be proposed in this section.

2.2 Study on Pollution Model of Blasting Dust and Toxic Gases

21

2.2.1 Diffusion and Settling of Blasting Dust (1)

Settling Motion

The movement of blasting dust out of the ground surface in the atmosphere goes through three stages: decelerated rise, accelerated degradation, and gravitational free settlement (migration with wind flow). The deceleration stage of dust movement is very short and has little effect on the settling process, which can be neglected in the evaluation of pollution status. Therefore, the settling motion of blasting dust and smoke can be treated as free settlement in the wind flow field. The force analysis of blasting dust is carried out and its equation of motion is given as: mp

 2 1 dv pz 1 = πC D kr ρd 2p vz + v pz + π d 3p (ρka − ρ p k g )g dt 8 6

(2.1)

where m p —dust mass, kg; vpt —vertical dust velocity, m/s; t—time, s; C D —dust movement resistance coefficient; kr —form coefficient of dust movement resistance; ρp —dust density, kg/m3 ; Vz —vertical wind velocity, m/s; ρ—density of gaseous substance of dust and smoke, kg/m3 ; ka —dust buoyancy form coefficient; kg —dust gravity form coefficient; dp —dust particle size, m; g—gravitational acceleration, g = 9.81 m/s2 . dv The dust in free settling motion dtpt = 0, then the following formula holds:  2 1 1 πC D kr ρd 2p vz + v pz + π d 3p (ρka − ρ p k g )g = 0 8 6

(2.2)

The equation for the final settling velocity of dust is solved from the above equation: v pzl

   4 ρ p k g − ρka = −vz + 3C D ρkr

(2.3)

where v pzl —final settling velocity of particles, m/s. In the airflow field with wind velocity of Vz , the dust settles uniformly. When it migrates with the wind flow, the velocity of motion in the vertical direction is the final settling velocity, and the velocity of motion in the horizontal direction is equal to the wind velocity. Then the motion velocity of uniformly settling dust is:

22

2 Mechanism of Production and Transport of Blasting Dust …

v pz = −vz + v px = vx



4(ρ p k g −ρka ) 3C D ρkr

(2.4)

where v px —horizontal dust velocity, m/s; vx —horizontal wind velocity, m/s. (2)

Diffusion Motion of the Dust

After going through the impact motion stage and mushroom cloud motion stage, the blasting dust particles enter the diffusion motion stage. In the diffusion motion stage, the particles all reach the uniform settling state and migrate with the wind flow. The time to complete the impact motion stage and mushroom cloud motion stage of blasting dust and smoke is very short, which can be considered that the particles of blasting dust and smoke are released into the atmosphere in an instant. Therefore, ignoring the volume of smoke pollution clouds before entering the diffusion movement, the diffusion of particles of blasting dust and smoke can be treated as instantaneous point source diffusion. In a coordinate system where the coordinate origin moves with the wind flow, the equation for the pollution concentration of instantaneous point source diffusion is given as follows:    1 x2 2Q P y2 z2 + 2+ 2 C p (x, y, z, t) =  exp − 3 2 σx2 σy σz (2π ) 2 σx σ y σz

(2.5)

where Cp —dust concentration, kg/m3 ; Q p —blasting dust emission, kg; x—horizontal coordinate of the downwind side of the muck pile, m; y—horizontal coordinate perpendicular to the wind direction, m; z—vertical coordinate, m; σ —mean square error, m. When blasting dust moves in the wind flow field, it moves with the wind flow in the horizontal direction, and free settlement occurs moving with the wind flow in the vertical direction. Therefore, for the circular definite coordinate system, the blasting dust concentration calculation equation shall be corrected by the local wind velocity and settling velocity. After diffusion for t time, the displacement of the center of the dust pollution cloud in the horizontal direction is: x 0 = vx t

(2.6)

2.2 Study on Pollution Model of Blasting Dust and Toxic Gases

23

where x0 —the displacement of the center of the dust pollution cloud in the horizontal direction, m. In the vertical direction, the pollution clouds move with the wind flow, and have free settlement by gravity, which appears the relative displacement between the air. The total displacement is the algebraic sum of the relative displacement between air and cloud under the action of wind displacement and free settlement, that is   z 0 = vx + v pzl t

(2.7)

where z 0 —the displacement of the center of the dust pollution cloud in the vertical direction, m. For the fixed coordinate system, x − x0 and z − z 0 separately take place of x and z in Eq. (2.5) and the calculation formula of pollution concentration distribution of blasting dust diffusion is obtained, that is, dust diffusion equation: C p (x, y, z, t) =

   2  z − vx t − v pzl t 1 (x − vx )2 2Q P y2 · exp − + + 3  2 σx2 σ y2 σz2 (2π ) 2 σx σ y σz

(2.8)

2.2.2 Mathematical Model of Gaseous Substance Motion of the Dust and Smoke Due to the low blasting dust concentration in the atmosphere, the effect of blasting dust motion on the motion of gaseous substance is negligible. The mathematical model of gaseous substance motion of the diffusion motion stage includes diffusion equation and mathematical model of cloud buoyancy movement. (1)

Diffusion Equation

The time for completing the impact motion stage and the mushroom cloud motion stage by blasting gaseous substance of the dust and smoke and particles together is very short, and the contamination scope before entering the diffusion motion is small relative to the contamination range after diffusion. Ignoring the volume of the dust and smoke cloud before entering the diffusion motion stage, the diffusion of the dust and smoke gaseous substance can be a point source diffusion. A certain period of time after blasting, it continuously spills large amounts of the dust and smoke gaseous substance from the muck pile. To simplify the problem, it is assumed that the overflow rate of the dust and smoke gaseous pollutants is stable and constant during the tiny time interval t, then the dispersion of blasting the dust and smoke gaseous substance can be treated as a stable point source diffusion during that time interval. The pollution concentration of continuous point source diffusion is calculated as:

24

2 Mechanism of Production and Transport of Blasting Dust …

   Q 1 y2 z2 C(x, y, z, t) = · exp − + 2 π v x σ y σz 2 σ y2 σz

(2.9)

where C—concentration of gaseous substance of the dust and smoke, kg/m3 ; Q—emission of gaseous substance of the dust and smoke per unit time, kg/s. The displacement of the center of the dust and smoke cloud gaseous substance in the horizontal direction at moment t after the start of diffusion motion is given by Eq. (2.6). In the vertical direction, the cloud of gaseous substance moves with the wind flow and is subjected to the air buoyancy, which produces a relative displacement between the air. Its total displacement is the algebraic sum of the displacement of the wind flow movement and the relative displacement of the air and the cloud under the action of buoyancy, i.e.:   z 0 = vz + v f t

(2.10)

where v f —motion velocity of clouds relative to air, m/s. For the fixed coordinate system, z − z 0 takes place of z in Eq. (2.9) and the calculation formula of pollution concentration distribution of blasting dust and smoke gaseous substance is obtained, that is, the diffusion equation: Q · exp C(x, y, z, t) = π v x σ y σz

(2)



  2   z − vz t − v f t 1 y2 − + 2 σ y2 σz2

(2.11)

The Upward Movement Velocity of the Cloud of the Dust and Smoke Gaseous Substance

In solving the upward movement velocity of the cloud of the dust and smoke gaseous substance, the temperature field of the atmosphere in the open-pit mine and the heat released into the air by blasting are required. To simplify the problem, it is assumed that the process of state change in the motion of the cloud of gaseous substance is adiabatic and the resistance in the uplifting process of the cloud is small, which can be negligible. A cloud of gaseous substance of volume Vw , after receiving heat q from the outside world, the formula of the increase in the working material temperature is: T =

q c p0 vw ρ

where Vw —the volume of the working material after heating, m3 ; ρ—the density of the working material after heating, kg/m3 ; c p0 —the constant-pressure specific heat of the working material, J/kg m3 .

(2.12)

2.2 Study on Pollution Model of Blasting Dust and Toxic Gases

25

The above formula can be transformed as:   q = T · C p0 Vw ρ

(2.13)

Since blasting dust and smoke enters the air with a small range of temperature change, the change process satisfies the ideal gas state equation, i.e.: RT =

p ρ

(2.14)

where R—gas constant, J/kg K, air R = 287.1 J/kg K; P—pressure, N; T —the working material temperature after heating, K. The air temperature which is not contaminated by blasting dust and smoke gaseous substance is T0 , and the temperature increases T after being polluted by blasting dust and smoke gaseous substance, T = T0 + T , the following formula holds: p Rρ

(2.15)

p − T0 Rρ

(2.16)

T0 + T = The above equation can be written as: T =

Substituting Eq. (2.16) into Eq. (2.12) to obtain: q = C p vw ρ

p − T0 Rρ

(2.17)

In the open thermal system, the working material (blasting dust and smoke gaseous substance) moves from position 1 to position 2, as shown in Fig. 2.2, and can be given by Eq. (2.17). Fig. 2.2 The changing process of blasting dust and smoke gaseous substance clouds

26

2 Mechanism of Production and Transport of Blasting Dust …

C p Vw1 ρ1

p1 − T01 Rρ1

= C pV w2 ρ2

p2 − T02 Rρ2

(2.18)

where the subscripts 1 and 2 indicate the state of the process. It can be solved from the above equation: ρ2 =

vw1 ρ1 T1 + R1 (vw2 p2 − vw1 p1 ) vw2 T02

(2.19)

In an open-pit mine, the air temperature at different heights can be measured, giving the law of the air temperature variations in the mine, i.e. the temperature distribution in the vertical direction is a known quantity. Therefore, the gas density of the blasting dust and smoke cloud at different heights can be calculated with Eq. (2.19), and the average parameters during the change of the blasting dust and smoke gaseous substance moving from position 1 to position 2 are given by the logarithmic average method. The average volume and average density are: Vw =

vw2 − vw1   ln vvw2 w1

(2.20)

ρ2 − ρ1   ln ρρ21

(2.21)

ρw =

mass of the blasting dust and smoke cloud is: m = Vw ρ =

vw2 − vw1 ρ2 − ρ1     ρ2 ln vvw2 ln ρ1 w1

(2.22)

the gravity of the blasting dust and smoke cloud is: FE = mg =

vw2 − vw1 ρ2 − ρ1     ·g ρ2 ln vvw2 ln ρ1 w1

(2.23)

and the buoyancy of the blasting dust and smoke cloud is: FF = Vw ρ0 g =

vw2 − vw1 ρ20 − ρ10     ·g ρ20 ln vvw2 ln ρ10 w1

(2.24)

Based on Newton’s Second Law of Motion, the equation of motion for the buoyant action of the blasting dust and smoke cloud is established. m

dv = FE − FF dt

(2.25)

2.2 Study on Pollution Model of Blasting Dust and Toxic Gases

27

Substituting Eqs. (2.22), (2.23) and (2.24) into Eq. (2.25), the equation of motion for buoyancy action during the diffusive motion of blasting dust and smoke gaseous substance is organized as follows: ρ20 − ρ10 ln(ρ2 − ρ1 ) dv =g− ·g · dt ρ2 − ρ1 ln(ρ20 − ρ10 )

(2.26)

The above gives the mathematical model of blasting dust and smoke gaseous substance in the process of diffusion motion. The model consists of diffusion equation and cloud buoyancy movement mathematical model, solving them simultaneously, and the pollutant concentration and movement process of blasting dust and smoke gaseous substance in the stage of diffusion motion can be calculated. By solving them simultaneously with the mathematical model of pollution state evaluation, the emission of blasting dust and smoke gaseous substance can be solved and the smoke pollution can be evaluated quantitatively.

2.2.3 Evaluation of Blasting Dust and Toxic Gases Pollution At the blasting moment, blasting dust is released into the atmosphere, and the toxic gases spill from the muck pile to the atmosphere for a period time after blasting, with the concentration of dust and toxic pollutants in the atmosphere constantly changing. When measuring the concentration of pollutants at the blasting site, the data available are dust settlement amount of fixed locations, the dust concentration of the muck pile downwind side at different distances and toxic gas concentration. How to quantitatively calculate the emission intensity of blasting dust and toxic pollutants based on the measured data is the problem to be solved by the pollution status evaluation. Blasting dust dispersion is instantaneous point source diffusion, and the blasting site measurement data is the settlement amount at a fixed point and the pollutant concentration at a given point on the downwind side. In the tiny time interval, the diffusion of blasting toxic gas can be as continuous point diffusion, and the blasting site measurement data is the pollutant concentration at the given point. The mathematical model of blasting dust and toxic gases emission intensity is established according to the type of pollution source and the characteristics of measured data. In pollution evaluation, the 1/10 of the concentration surface of the blasting cloud center (or central axis) concentration is generally used as the pollution boundary. To simplify the problem, the following assumptions are made: when calculating the pollution range in the direction of a coordinate axis, it is assumed that the pollutants are all concentrated in that coordinate, so the pollution range in the direction of that coordinate axis is independent of the other axes, and is only related to the wind speed and cloud diffusion time. (1)

The Scope of the Blasting Dust Pollution

28

2 Mechanism of Production and Transport of Blasting Dust …

The calculation formula for the pollutant concentration of blasting dust diffusion is given by Eq. (2.8). In the direction of x axis, the pollutant concentration at x = 0 is:

Cp (0, x, y, z, t) =

2Q p 1 exp − 3  2 (2π ) 2 σx σ y σz



(−vx t)2 σx2

+

y2 σ y2

 +

z − vz − v pzl t σz2

2 

(2.27)

The equation for calculating the pollution boundary along the x axis direction can be given based on the hypothesis: Cp (x, y, z, t)|x = Jx =

1 C p (0, x, y, z) 10

(2.28)

where Jx —pollution boundary along the x axis, m. Substituting Eqs. (2.8), (2.27) into Eq. (2.28), solve the pollution scope in the direction of x axis, that is: J x1,2 = vx ±



2σx2 (ln 10 + vx t)2

(2.29)

T time after diffusion, the center of the blasting dust cloud is marked at point O  (xt , yt , z t ), and the coordinate xt of point O  is given by the following equation: xt =

1 (Jx1 + Jx2 ) 2

(2.30)

Substitute Eq. (2.29) into the above equation to obtain xt . x t = vx t

(2.31)

T time after diffusion, the length of the pollutant L x in the direction of x axis is: L x = Jx2 − J X 1

(2.32)

Substitute Eq. (2.29) into the above equation to obtain L x .  L x = 2 2σx2 · ln 10 + (vx t)2

(2.33)

Similarly, solve the equation: Pollution boundary, range, center coordinates, and pollution length along the y 1 C p (x, 0, z, t) and axis and z axis can be calculated with C p (x, y, z, t)|x=Jy = 10 1 C p (x, y, z, t)|x=Jx = 10 C p (x, y, 0, t). The final solution results are given directly below. y axis:

2.2 Study on Pollution Model of Blasting Dust and Toxic Gases

 Jz1,2 = ± 2σ y2 ln 10   − 2σ y2 ln 10 ≤ y ≤ 2σ y2 ln 10 yt = 0  L y = 2 2σ y2 · ln 10

29

(2.34)

z axis:   2   Jz1,2 = vz + v pzl ± 2σz2 ln 10 + vz + v pzl t 2    2   2    2 vz + v pzl t − 2σz2 ln 10 + vz + v pzl t 2 ≤ z ≤ vz + v pzl t+ 2σz ln 10 + vz + v pzl t 2  2  L z = 2 2σz2 ln 10 + vz + v pzl t 2

(2.35)

The pollution length L p in any direction can be calculated with the following equation: Lp =



L 2x + L 2y + L 2z

(2.36)

Total pollution scope, i.e. the total volume of pollution clouds V pw : Vpw =

1 π L 3p 6

(2.37)

According to the calculation formula of pollution cloud volume, the shape of pollution cloud is a sphere, but the actual calculation result is not the same. In the actual calculation, as the wind speed and dust settling velocity are changing, so it is difficult to give the analytical solution directly. The method of solving the calculation is to solve with the equation for the stabilization process in the time T with the given time step T . The motion process of the dust and smoke clouds is the superposition of the solution results of finite T time, that is, replacing unsteady motion process with the stabilization process with finite T time. And the calculated shape of the dust and smoke clouds are tend to be irregular. (2)

The Scope of the Blasting Toxic Gas Pollution

Blasting toxic gas diffusion can be treated as continuous point source diffusion for a certain time interval, and the diffusion equation is given by Eq. (2.11). The diffusion scope of toxic gases along the x axis is up to the wind speed and the diffusion time. The frontier boundary of the toxic gases Jx2 is: Jx2 = vx t

(2.38)

The pollution scope along the x axis: 0 ≤ x ≤ vx t

(2.39)

30

2 Mechanism of Production and Transport of Blasting Dust …

T time after diffusion, the length of the pollutant in the direction of x axis is: L x = Jx2 = vx t

(2.40)

Pollution boundary, range, and pollution length along the y axis and z axis can be 1 1 C(x, 0, z, t) and C(x, y, Jz , t) = 10 C(x, y, 0, t). calculated with C(x, Jy , z, t) = 10 The final results are given directly below. y axis:  Jy1,2 = ± 2σ y2 ln 10   − 2σ y2 ln 10 ≤ y ≤ 2σ y2 ln 10  L y = 2 2σ y3 · ln 10

(2.41)

z axis:   2   Jz1,2 = vz + v pzl ± 2σz2 ln 10 + vz + v pzl t 2   2        2 2 vz + v pzl t − 2σz2 ln 10 + vz + v pzl t 2 ≤ z ≤ vz + v pzl t+ 2σz ln 10 + vz + v pzl t 2  2  L z = 2 2σz2 ln 10 + vz + v pzl t 2

(2.42)

The volume of pollutant at time t after continuous point source diffusion can be approximated by the following equation: Jx Vw =

π 2 L y + L 2z · d x 4

(2.43)

0

Having a slight transformation of the above equation, it can be written as: π Vw = 4

t

 vx L 2y + L 2z · dτ

(2.44)

0

2.2.4 The Emission Intensity of the Blasting Dust and Toxic Gases When measuring the concentrations of blasting dust and toxic gas, the location of the measuring point should be made within the scope of dust and toxic gases pollution. If it is outside the scope of pollution, the measured data is equal to zero or not the blasting dust and toxic gas concentration values. Based on the measured data, the blasting dust and toxic gases emissions and emission intensity are calculated.

2.2 Study on Pollution Model of Blasting Dust and Toxic Gases

(1)

31

Calculate the Dust Emission from the Dust Instantaneous Concentration at a Point at Different Times

The dust emission Q p expressed in terms of dust concentration from Eq. (2.8) is: Qp =

  2  n  z 0 − vz τ − v pzl t y02   1 (x0 − vx τ )2 1 C p (x, y, z, t) σx σ y σz · exp + + (2π )3/ 2 2 2 σx2 σ y2 σz2 i=1

(2.45)

Measurements for n times are carried out at the fixed point D(x0 , y0 , z 0 ), dust emissions are calculated as the arithmetic mean of emissions of n times, i.e. Qp =

n 1 Q pt n i=1

(2.46)

Substituting Eq. (2.45) into Eq. (2.46) to obtain the formula for calculating the blasting dust emissions using the measured dust concentration at a point at different times: ⎧ ⎡  2 ⎤⎫ n ⎨ 1 (x − v τ )2 ⎬  z 0 − vz τ − v pzl y02   1 x 0 3 2 / ⎦ ⎣ Qp = C p (x0 , y0 , z 0 , t) σxi σ yi σzi · exp + 2 + (2π ) 2 2 ⎩2 ⎭ 2n σ σ σ xi yi zi i=1

(2.47) where the subscript i indicates the result of the measurements for ith measuring. In practical calculations, the mean square error σ is a function of the meteorological parameters and the position of the dust cloud center. After the meteorological parameters and the position of the cloud center are determined, a lot of research has been done on the mean square error of the diffusion motion of dust and smoke. Sutton, Turner and others have proposed the calculation methods of mean square error of smoke and dust diffusion motion. When the smoke and dust diffusion distance is short, the difference of mean square error within the diffusion scope can be calculated by the formula given in Tables 2.1 and 2.2. (2)

Calculate the Dust Emission from the Dust Settlement Amount Per Unit Area at a Given Point

Table 2.1 Mean square error of turner instantaneous point source Downwind distance

X = 100 m

Square error

Mean

X = 400 m

σ y (m)

σz (m)

σ y (m)

σz (m)

Unstable

10

15

300

220

Medium

2

3.8

120

50

Stable

1.3

0.75

35

7

32

2 Mechanism of Production and Transport of Blasting Dust …

Table 2.2 Mean square error of dust and smoke diffusion proposed by Turner Stability classification of Distance to the downwind Horizontal Vertical Pasupuill side of the explosion pile x σ y (m) σz (m) (m) A

100 < x < 300

0.493x0.88

0.087x1.10

A

300 < x < 3000

0.493x0.88

exp[−1.67 + 0.9021lgx + 0.181(lgx)2 ]

B

100 < x < 500

0.337x0.88

0.135x0.96

B

100 < x < 2 ×

0.337x0.88

exp[−1.25 + 1.09lgx + 0.0018(lgx)2 ]

C

100 < x < 105

0.195x0.90

0.112x0.91

D

100 < x < 500

0.128x0.90

0.093x0.85

D

500 < x
t0 , the upper part of explosive air mass is in the buoyancy plume and the lower part z < z(tc ) is in the momentum jet region. The two segments are divided into 100 units equally in the z-direction along the axis direction and n units in the horizontal direction along the y and x radii directions, respectively, as shown in Fig. 2.3a, b. In the explosive air mass which has been divided, respective studies are made on the laws and physical properties of each unit air mass. The discussion of air mass boundary conditions are carried out from the explosive air mass as a whole, which Fig. 2.3 Explosive air mass unit division diagram

2.3 Computer Numerical Simulation Study …

35

is also a very important issue. The most important among the boundary conditions are boundary air mass coordinates (x[i][ j], y[i][n], z[i][n]) which respond to the entire explosive air mass contour, velocity, axis temperature T [i] and concentration c[i][n]. According to the above algorithm and the physical quantities to be solved, the program structure compiles a computer program, and the calculation steps are as follows: (1) (2)

(3) (4)

(2)

Comparison of the magnitude of the calculated time with the critical time t versus tc . Calculate critical height and calculate the physical properties of the axial air masses in the momentum jet region and buoyancy plume region z[i][ j], r [i], T [i], ω[i][ j] and c[i][0]; Calculate the physical nature and coordinates of each unit air mass in x − y and y − z, z[i][ j], ω[i][ j], c[i][ j]. Calculate the entire air mass boundary x[i][n], y[i][n], z[i][n] and z[1][ j], x[1][ j], y[1][ j], ω[1][ j], the structural block diagram of the calculation program can be easily drawn from the calculation steps of the program as shown in Fig. 2.4. Program Simulation Results and Analysis

From the block diagram of the program shown in Fig. 2.4 to prepare the program, the motion of the explosive gas mass process can be simulated with the given initial Fig. 2.4 Simulation block diagram of the initial motion of explosive gas

36

2 Mechanism of Production and Transport of Blasting Dust …

Fig. 2.5 Simulation of the initial motion of explosive gas

conditions, and take the following initial conditions according to field observations and previous results: Entrainment coefficient: α = 0.56 β = 0.085; Air density ρα = 0.79 kg/m3 (Standard air pressure, room temperature 20 °C). Density of explosive gas substance ρs = 1.22 kg/m3 Blasthole radius R = 0.04 m. Initial temperature of explosive gas substance T0 = 2 K still wind field u < 1 m/s. Given the above basic data, the simulation is solved to obtain a simulation of the single-hole blasting dust and smoke cloud motion process, as shown in Fig. 2.5.

2.3.2 Numerical Simulation of Movement Process of Blasting Dust Particles (1)

Programming Algorithm

Blasting dust and smoke consists of gas phase and solid phase and the initial phase is impact motion with high initial velocity. Because the particles are denser than the gas phase, so the inertia is large, which is in the frontier motion. With the increase of time, the momentum of dust and smoke decreases continuously by air resistance, the velocity of the particles decreases, and the substances in the frontier motion of the blasting dust and smoke cloud are gradually transformed into gaseous substances. They are dominated by diffusive motion and form a mushroom-shaped dust and smoke cloud in a certain time interval in the transformation from inertial motion to diffusive motion [6]. In this study, it is assumed that the mass of the particles is constant during the motion, so the inertial force due to mass change is 0. The dust particles are mainly affected by gravity, buoyancy and resistance, so the motion of dust particles is described using the particle method in this study, and the difference equation and trajectory equation for the velocity component of a single dust particle at each stage are obtained with difference scheme.

2.3 Computer Numerical Simulation Study …

37

In this algorithm, the motion parameters of single dust particles are calculated to describe the trajectory and compare the expansion velocity and expansion volume of dust particles along the outer edge of the motion and the outer edge of the explosive air mass to determine the profile and motion of the entire blasting smoke and dust cloud. (2)

Programming Structure

A computer program is developed to simulate the entire post-blasting motion, dispersion distance and the dust and smoke cloud profile combining the algorithm of the initial motion simulation of the blasting dust and smoke cloud with the problem to be solved in this study. It is very necessary to select an appropriate model to further study the dispersion pattern of toxic dust and smoke of the mine and to predict the distribution of pollutants and pollution levels in the mine. The calculation steps are as follows: (1)

(2)

(3)

(4)

For cast blasting, the dust source is mainly the dust deposited on the surface and generated during crushing. Representative   position   and particle size are selected as the calculating objects within −π 4, π 4 section; in loosening blasting, the dust is mainly the filler particles when the quality of the blasthole filling is not good. In the case of known explosive initial velocity, calculate the initial velocity component along each projection angle; Apply the above analysis and the results in 3.2 to simulate the initial motion of these mass points respectively and determine mass point of the outer edge of the motion at any moment by comparing the velocity and displacement between the mass points max{(x, y, z)}; Compare the mass point and the radius and height of explosive gas mass to determine the phase state of the outer edge of motion and the motion medium of the particle to determine the stress of the mass point; Compare the size between the mass point of outer edge of motion and the radius of the explosive gas mass at this angle to determine the shape of the blasting smoke and dust cloud.

By decomposition of above procedure calculation steps and analysis of the physical quantities to be solved, it is easy to simulate the program block diagram of the initial motion process of the blasting dust and smoke cloud, and the corresponding computer program will be compiled to perform the simulation under known initial conditions, the computer program block diagram is shown in Fig. 2.6. (3)

Stimulation Results

According to the above block diagram, the program is compiled and the motion process of the blasting dust and smoke cloud can be simulated under certain known initial conditions, and the following initial values are taken based on field observations and literature [7–9]:     The angle of thrown-off funnel −π 4, π 4 initial velocity of ejection v p0 = 40–80 m/s.

38

2 Mechanism of Production and Transport of Blasting Dust …

Fig. 2.6 Simulation program block diagram of initial movement of blasting dust and smoke cloud

Wind speed u = 3 m/s particles diameter d p = 20 − 1000 µm; Density of particles ρ p = 3000 kg/m3 density of gaseous substance ρs = 1.22 kg/m3 .  When Rep < 1 in the laminar region, C D = 24 Rep ; When 1 < Rep < 500 in the transition region, C D = √24 ; Rep

When 500 < Rep < 2 · 10 in the recirculation zone, C D = 0.44. 5

References 1. Zou CF, Deng YF. Study on dust production law and dust reduction technology in blasting stope. Min Res Dev. 2019;39(08):34–7. 2. Hu CH, Xia YM, Bu YY, et al. Numerical simulation of blasting dust movement rule in blind tunnel. Comput Simulat. 2015;32(08):235–238+264. 3. Li Y, Liu LS. Study on intelligent control of dust and harmful gas produced by blasting in metal mine. China High-Tech Enterprises. 2015;24:159–60. 4. Li YC, Liu TQ, Li Z, et al. Research on unsteady migration law of dust in blasting tunnelling space. J Saf Sci Technol. 2014;10(06):33–8. 5. Wang YT, Liu B, Ji WD. Study on dust control in blasting face. Shandong Coal Sci Technol. 2012;04:206–7.

References

39

6. Li HY, et al. Simulation of the initial movement process of blasting dust in metal mines. 1995;2:19–21. 7. Wang J, Shi H. Prediction of explosive toxic smoke. Min Technol. 1992;23:18–21. 8. Li HY, Zhang XK. Simulation of the initial movement process of blasting dust. Metal Mine. 1995;02:19–21. 9. Zhou SL, Wang R. The deep hole charge of open pit mine faces to detonation and dust fall. Min Technol. 1993;05:13–4.

Chapter 3

Chemical Suppression Technology of Open-Pit Mine Blasting Dust and Smoke

Abstract In this chapter, the mechanism of production and transportation of blasting dust is analyzed, and two dust suppressants, water-rich gelatin stemming and foam, are developed, and the dust suppression mechanism and effect of the two dust suppressants are analyzed.

3.1 Control Measures on Open-Pit Mine Blasting Dust Blasting dust and smoke belong to instantaneous and unorganized dust source. For the instantaneous unorganized emission source which has no condition for unified collection and purification, it can only take measures on the source of dust and smoke to reduce the amount of dust and smoke. The following is a brief description of the current status of research on the control technology for each source of smoke and dust, starting from the analysis of the factors affecting the amount of smoke and dust generated by each source. Explosives are composed of C, H, O, N and other elements. Explosion of explosives is a quick complicated chemical reaction process with high temperature and high pressure, instantly generating a large amount of heat, explosive gases, dust and shock waves. Blasting dust consists of three parts: fine particles crushed by rock that is in direct contact with the charge and under the impact and compression of ultra-high pressure shock wave and detritus used for blasthole filling. Dust of these two parts and the explosive gases generated from blasting rush out from the blasthole together and enter into the blasting clouds. In addition, the ground deposited dry dust in the blasting area enters into the explosive gases by the blasting shock wave vibration. The amount of CO in the explosive gas has something to do with the composition of the explosives and explosives dosage. The amount of blasting dust generated is connected with the unit consumption of explosives, the water content of blasting area rock (ore) and the amount of dust deposited on the ground and fillers. The higher the unit consumption of explosives, the greater the degree of rock fragmentation, the more dust generated; and the greater the water content of the blasting area rock, the less dust generated; the less dry dust deposited on the ground of the blasting area, the less dust generated. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Wang et al., Chemical Dust Suppression Technology and Its Applications in Mines (Open-pit Mines), https://doi.org/10.1007/978-981-16-9380-9_3

41

42

3 Chemical Suppression Technology of Open-Pit Mine …

Research on blasting dust and smoke control at home and abroad is mainly in two aspects, namely, ventilation and purification on the generated dust and smoke and reduction of the generation of blasting dust and smoke by changing the blasting process, changing the blasthole filling material and increasing the water content of the blasted ore. Diluted ventilation is usually used for the high concentration smoke and dust generated by blasting. Open-pit mines mainly rely on natural ventilation to dilute and remove the blast smoke and dust. Underground mines mainly use mechanical ventilation, generally using local fans to exhaust blasting dust and smoke into the mining roadway first, and then use the main fan to dilute and remove these blasting dust and smoke from the mine. For deep concave open-pit mines with mining depth over 200 m, the air flow below the closed circle is getting smaller and smaller, and it is difficult for fresh air outside the pit to flow into the pit, while the blasting dust and smoke inside the pit cannot be replaced by fresh air outside the pit in time, which can make the dust and smoke stay in the pit for a long time. In underground mines, if the local ventilation arrangement is not reasonable or the air duct leakage is serious, it will make the blasting operation surface accumulate high concentration of blasting fumes for a long time. The high concentration of blasting dust and smoke accumulated in the working area can cause blasting fumes poisoning. To avoid accidents, enterprises often increase the ventilation time, thus affecting the normal operation of mining production. Therefore, whether it is an open-pit mine or an underground mine, relying only on ventilation is not a good solution to the problem of blasting smoke pollution. On the one hand, spraying water on blasting dust and smoke can make water droplets combine with dust, increase the weight of dust, accelerate its settlement, which facilitates the elimination of dust in situ. On the other hand, when water droplets come into contact with soluble gases in blasting soot, such as NO2 , these gases dissolve into the droplets, which is conducive to reducing the concentration of toxic gases in blasting dust and smoke. Although spraying water on blasting dust and smoke has some effect, few metal mines adhere to this methods because of the high surface tension of water, low capture rate of respiratory dust generated by blasting and relatively complex process and high water consumption. Improving the blasting process is mainly the use of blasting methods or process that are conducive to reducing the generation of blasting dust and toxic gases, which are summarized as follows [1–3]: (1)

Ensure the length and quality of the blasthole filling

Improving the quality of the blasthole filling can make the explosion reaction adequate, not only reducing the amount of toxic gas and dust generation, but also reducing the unreaction and inadequate reaction of explosive particles from the charge surface thrown out of the blasting reaction area.

3.1 Control Measures on Open-Pit Mine Blasting Dust

(2)

43

Use the hole-bottom priming device

The hole-bottom priming is helpful to the sufficient explosion of explosives, corresponding to reduce the production of toxic gases and dust when blasting. (3)

Continuously improve the explosives formula

When studying explosives components, we should not only consider the oxygen equilibrium condition, but also consider their final reaction pathway and degree. When preparing the explosives, we should try to make the chemical activity similar, making the reaction rate close. Russia once developed a slurry explosive called Gramonite, and the toxic gas generated by the explosion of 1 kg of the explosive converted into CO is only 4.2 × 10–3 m3 . While the amount of toxic gas produced by conventional explosives can reach 0.24 m3 /kg. Therefore, improving the explosive formula is an effective measure to reduce the fume generation. (4)

Increase the water content of ore rock in the blasting area

The main measure is to have water injection in advance to the blasted ore rock to increase its water content. It has a lot of research and application by many countries, and China has its applications in coal mines. The specific method is to make eyelet work in the blasted ore rock, and then inject pressure water into these holes, so that it penetrates into the interior of the ore rock to improve the moisture content, thereby reducing the amount of blasting dust production during blasting. However, a large number of holes need to be drilled with large water consumption. (5)

Technology of covering the foam layer on the blasting area to reduce the blasting dust

It is easy for the dry fine dust deposited on the surface of the blasting area affected by the blasting shock wave enters into the blasting clouds together with the blasting gases, increasing the amount of blasting dust generated. To reduce this part of the dust, measures like spraying water on the surface, spraying covering agent curing dust or covering the foam layer and other measures can be taken. Before blasting, spraying water on the blasting area ground to increase the surface dust water content and the adhesion between the dust can reduce the blasting dust production. However, in dry climatic conditions, the water on the surface evaporates quickly, which will affect the effect of dust reduction. Only the former Soviet Union Kachach Institute had done the industrial trials about the application and research on the foam dust removal technology to control the blasting fume in Zuryanovsk open-pit mine. After loading and installation of the priming network, the 10-1 foam agent is used to issued 100 times air mechanical foam to the blasting area, with the foam layer thickness over the step surface 0.3– 0.6 m. Blasting 1 m3 ore rock needs foam consumption of 0.06–0.16 m3 . Foam dust removal technology is mostly used in areas with hot climates and water deficient. The dust reduction efficiency of covering the blast area with foam is up to 41.7%, and the ventilation time of open-pit mines can be reduced by 2/3–3/4. However, due to the problem of foam defoaming, the blasting area required to be covered with foam needs to be blasted within two hours.

44

3 Chemical Suppression Technology of Open-Pit Mine …

The generation of blasting dust can be reduced by spraying adhesive covering agents on the surface to solidify the dust. This measure is not limited by blasting time compared to spraying water and covering with a foam layer. (6)

Water infusion blasting

When a special non-toxic plastic bag filled with water (water stemming) is used to fill the blasthole instead of rock chips, the high temperature and pressure generated at the moment of blasting converts the water in the bag into superheated vapor or water mist. The atomized water reacts chemically with toxic gases to produce non-toxic or less toxic substances, and the basic reaction formula is as follows [1–3]: CO + H2 O = H2 + CO2

(3.1)

4CO + 2H2 O = CH4 + 3CO2

(3.2)

3NO2 + H2 O = HNO3 + HNO2 + NO

(3.3)

NO + H2 O = NO2 + H2

(3.4)

NO = N2 + O2

(3.5)

If the water is mixed with alkaline substances, the following reactions can also occur: + 2OH− + NO = NO− 3 + 2H

(3.6)

− 2OH− + 2NO2 = NO− 3 + NO2 + H2 O

(3.7)

2OH− + NO2 + NO = 2NO− 2 + H2 O

(3.8)

The reaction of Eqs. (3.1) and (3.4) is accelerated if catalyst substances such as Fe2 O3 are present in the ore rock. What’s more, the atomized water generated by blasting can condense the dust after contacting with it, which has a rapid dust reduction effect. Generally speaking, due to the hydrophobicity of dust, water infusion blasting has a low wetting capacity for suspended fine dust, and therefore has a correspondingly lower capacity for lowering fine dust. For this reason, the former Soviet Union carried out research on replace water with water enriched gelatin stemming and surfactant solution as blasthole filler.

3.1 Control Measures on Open-Pit Mine Blasting Dust

(7)

45

Technology of filling the blasthole with surfactant solution to reduce blasting dust and smoke

Fourteen relevant industrial experiments had been carried out with applying surfactant solutions in the former Soviet Union at the open-pit mining site of the Northern Mining Company. The blastholes are filled with 0.7–1.5% AB solution, when filled with 90–200 L of solution per hole, it can result in the coagulation rate of blasting dust increased by 50 times and a 27% reduction in atmospheric CO concentration and a 47% reduction in NOx concentration. The AB used in the experiments is a biological reagent with minimal impact on the environment [2]. However, the production of this substance is limited and the source of raw materials is not sufficient, which needs further development. Related studies have been conducted at University of Science and Technology Beijing. Tests on the surfactant solution developed by the Daye Iron Mine Application Laboratory to fill blastholes showed that the average efficiency of 58.45% for CO reduction, 56.15% for dust reduction, and the combined efficiency of 57.3% for the dust toxicity reduction compared to background blasting with geotechnical soil filling [4, 5]. (8)

Technology of filling the blasthole with water enriched gelatin stemming to reduce blasting dust and smoke

The experimental study on water enriched gelatin stemming, also called as gelatin stemming or soft clay paste used to fill the blastholes for suppressing blasting smoke was only carried out in the former Soviet Union, while domestic research in this field is still vacant. The basic component of water enriched gelatin stemming is water glass, and there are also salts which are used as gelatinizer. Made of water glass and gelatinizer, the water enriched gelatin stemming is semisolid state, and will release a large amount of free water after vigorous stirring, which is a thixotropic gel. The reason why it can reduce the blasting smoke and dust is that under the action of detonation wave, gelatin crushing releases 50–60% free water, and the water combines with the dust combination, making it wetting and condensing, which plays a role in dust reduction. The superheated steam reacts with the toxic gas, and the non-toxic substances produce to reduce toxicity. The former Soviet Union had eight experiments on the suppression of blasting smoke and dust with external water stemming in the Krivorog mining area. The test data showed that the dust reduction efficiency could reach 33–55% and smoke removal efficiency could reach 32–64%. When the water stemming or other similarity were filled in the blastholes, it can reduce the amount of dust discharged into the atmosphere by 44–55% and the harmful gas (in terms of CO) by 83–89%. It had also been reported that the dust removal efficiency of hole sealing with gelatin stemming could reach 60.2%, which is much more effective than that for respiratory dust, and the removal efficiency for CO is 54.1% [1–3]. As a new technology to suppress blasting dust and smoke, water enriched gelatin stemming has been studied abroad only in the former Soviet Union. The data show

46

3 Chemical Suppression Technology of Open-Pit Mine …

that this technology has significant effects in reducing the amount of blasting dust and smoke, so it is necessary to conduct in-depth research on it, which is important to fill the research gap in this field and reduce the harm of blasting dust and smoke in China. (9)

Catforming

Catforming is a process that uses the catalytic action of a catalyst to transform gaseous pollutants in exhaust gases into harmless or more easily removable substitutes than in their original state for the purpose of purification. Catforming of carbon monoxide is also called catalytic oxidation. Catalyst is a substance that can change the rate of a chemical reaction without being present in the product [6]. The current catalysts for catalytic oxidation are: precious metal, rare earth, and base metal oxide [7]. A typical catalytic oxidation reaction is: 1 CO + O2 catalysts CO2 (H = 2.8223 × 105 J/mol) 2 −−−−−→ (1)

(3.9)

Precious metal

Precious metal catalysts are the most widely used and effective type for CO oxidation. The precious metal elements are mainly Pt, Pd, and the carriers are porous substances AL2 O3 , Sn2 O2 , and activated carbon, etc. Precious metal elements are rare, so their catalyst preparation and cost price are high, and they only have the characteristics of high catalytic activity and low activity temperature on a few carriers. Huazhong University of Technology has invented a catalyst for catalytic oxidation of carbon monoxide Pd–Ni/Sn O2 [8], the active component (weight %) of which is Pd: 0.5–3; Ni: 1–10; Sn : 87–98. The catalytic conditions are room temperature or above room temperature and atmospheric pressure. The catalytic efficiency is 50– 98%, and the shape is granular, lumpy and spongy. The relation between catalytic efficiency of Pd/Sn O2 and Pd–Ni/Sn O2 and temperature is measured by performance testing of the catalyst on a normal chromatograph under 1:1 atmosphere of CO and O2 concentrations (where N contains about 88%) at an air speed of 1100−1 . The catalyst for normal temperature oxidation of carbon monoxide was developed at the Eleventh Institute of China State Shipbuilding Corporation Limited. Wang [9] et al. developed a mixed carbon and aluminum carrier and selected several outsourced AL2 O3 type carriers and carbon carriers. A series of activity comparison experiments and lifetime comparison experiments were carried out with various carriers prepared into catalysts, so that TL-2 type carriers have been selected as suitable carriers for the preparation of CO oxidation catalysts at room temperature. (2)

Rare earth

Although there is the shortage of precious metal resources in China, rare earth metals are abundant: they are inexpensive with better activity and stability. In recent years, China has preferred the research on rare earth metals as catalysts. Great achievements have been obtained in the research of purifying gas and diesel exhaust.

3.1 Control Measures on Open-Pit Mine Blasting Dust Table 3.1 Purifying effect of purifying agent on CO in coal furnace smoke

Experiment time (h)

47

CO concentration (ppm)

Purification rate (%)

Before purification

After purification

0

7200

200

97.2

48

5150

450

91.3

104

5300

550

89.6

Rare earth metal catalysts generally use Sn O2 , AL2 O3 , ceramics, etc. as carriers and rare earth oxides as active components. The inlet gas temperature requirement is high when the rare earth metal catalyst oxidizes CO. When the CO conversion is 90%, its catalytic humidity is generally above 250 °C. (3)

Base metal oxide

Since precious metal catalysts are expensive and rare earth metal catalysts require high catalytic temperature, it is necessary to find catalysts with low price and good activity at room temperature. Base metal oxide catalysts have great prospects for application because of the low cost and better activity. Institute of Hygiene and Environmental Medicine of Academy of Military Medical Sciences has developed a CO purifying agent. The components of this catalyst are mainly metal powder and metal oxides. After the observation of the purification effect of CO in the laboratory, coal-fired furnace and users (see Table 3.1), the results show that the CO purifying agent has good effect. Among the base metal oxide catalysts, hoplalite is widely used at present. The main components of hoplalite are manganese dioxide and copper oxide, which are classified into two types according to the components: binary and quaternary. Binary hoplalite is made up of 60% Mn2 O2 and 40% CuO, and quaternary hoplalite is made up of 5%Mn2 O2 , 30%CuO, 15%Co2 O3 and 5%Ag2 O. The main active component is Mn2 O2 and other components are co-catalysts. There is no significant difference in properties between binary and quaternary hoplalite at atmospheric pressure.

3.2 The Development of Water Enriched Gelatin Stemming 3.2.1 The Development of Water Enriched Gelatin Stemming (1)

The composition of water enriched gelatin stemming

According to the literature [1], water enriched gelatin stemming consists of water glass, salt gelling agents and water, among which water glass is the main component, which determines the formation of gelatin stemming and the toxicity performance of dust reduction. Therefore, it is necessary to analyze its physical and chemical properties.

48

3 Chemical Suppression Technology of Open-Pit Mine …

The chemical name of water glass is sodium polysilicate, the molecular formula is Na2 O·SiO2 , and the n is the gram molecule ratio of SiO2 and Na2 O, which is usually called the “modulus” of water glass. The smaller the “modulus” of water glass, the easier it is to dissolve in water, but the viscosity is smaller, and the ability to bond the dust is poor. The larger the “modulus” of water glass, the smaller the solubility, but the greater the viscosity and the better the ability to bond the dust [10]. Therefore, when selecting water glass for preparation of gelatin stemming, there should be appropriate modulus value. For water glass with n > 2, its aqueous solution undergoes a series of hydrolysis and ionization reactions, forming a complex colloid-molecule-ion system. The main reactions are as follows: When sodium silicate is dissolved in water, ionization reaction occurs: Na2 SiO3 = 2Na+ + SiO2− 3

(3.10)

When sodium silicate is in water, hydrolysis reaction occurs: H2 SiO3 = H+ + HSiO_3

(3.11)

HSiO3 = H+ + SiO2− 3

(3.12)

− SiO2− 3 + H2 O = HSiO3 + OH

(3.13)

HSiO3 = H2 SiO3 + OH−

(3.14)

The hydrolysate HSiO3 − v is easy to polymerize into dimers, which can further form polysilicates. For large modulus water glasses, the following hydrolysis reactions also occur: Na2 O · SiO2 + H2 O = nSiO2 + 2NaOH

(3.15)

This hydrolysis reaction occurs on the potentiometric ionic layer on the surface of the colloidal nucleus. A large amount of colloidal SiO2 is produced by the change of oxygen coupling reaction to the direction of self polymerization. The material forms a three-dimensional network structure by the silicon oxygen bond, and its structure is shown in Fig. 3.1 [11]. In the three-dimensional network structure, hydrated silica gel is formed by enclosing excess water in it, causing the water to lose its mobility. However, this water easily escapes and the gel dries out and cracks. For water glass with large modulus, the ionization equation of the colloidal SiO2 in its solution can be expressed as: 2m− [Na2 O · SiO2 ]x = Na+ + SiO2− 3 + [mSiO3 · n1 SiO2 ]

3.2 The Development of Water Enriched Gelatin Stemming

49

Fig. 3.1 Silicone structure diagram

+ [n1 SiO2 ] + [Na2 O · n3 SiO2 ]y

(3.16)

After diluting the solution, the complex silicate has further ionization: [mSiO3 · n1 SiO2 ]2m− = mSiO2− 3 + [n1 SiO2 ]

(3.17)

The SiO3 2− produced by Eq. (3.16) can be further hydrolyzed to form HSiO3 − , H2 SiO3 . Thus, the high modulus water glass solution contains multiple components. In Eqs. (3.16) and (3.17), when the pH of the solution decreases, the concentration of H+ ions increases and H+ interacts with SiO3 2− in the solution to form H2 SiO3 (H2 SiO3 has very little solubility in water), which binds SiO3 2− and reduces the concentration of SiO3 2− in the solution, disrupting the equilibrium of reaction Eq. (3.16), causing the reaction to shift to the right to produce more colloidal SiO2 . And the disruption of Eq. (3.17) equilibrium also causes the reaction of Eq. (3.15) to shift to the right. Therefore, when the water glass acidizes, both Eqs. (3.16) and (3.17) are made to move to the right, that is, move to the direction of producing more colloidal SiO2 . According to the above properties of water glass, it is known that if acids, strong acid and weak base salts or some substances that can produce acid under alkaline conditions are added to the solution of water glass, these substances can lower the pH of water glass solution, promoting the ionization and hydrolysis of water glass and generating a large amount of colloidal SiO2 . All these substances can be used as gelling agents for water glass solutions. Gelling agents can be inorganic compounds or organic compounds, and since inorganic compounds have the advantages of low cost and sufficient sources, strong acid and weak base salts of inorganic compounds are used as gelling agent for water glass in this study. In conclusion, the composition of gelatin stemming should include water, water glass, strong acid and weak base salts, etc. According to the literature, when highly valued weak alkaline ions such as Al3+ , 3+ Cr , Zn2+ , Cu2+ are used with water glass, OH− generated by the hydrolysis of water glass reacts with these highly valued weak alkaline ions to form alkalescence hydroxides, thus promoting the hydrolysis of water glass and producing more silicate

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colloids. Since these hydroxides are also in a colloidal state, they help to enhance the water–glass gelling effect. Therefore, strong acid salts of several of these metals, such as sulfates or hydrochloride, are tentatively identified. The solsolution containing water, water glass and strong acid and weak base salts as gelling agent is formulated, and the gelling agent can make the water glass hydrolyze and ionize to a greater extent to obtain a large amount of colloidal SiO2 . These colloidal SiO2 is connected by Si–O–Si bond, which makes the system have a certain rigidity and form a very open and continuous three-dimensional network structure. It encloses the solvent water in the sol, making the water lose its fluidity and be semi-solid, which is usually called gel or gelatin. The newly formed gel contains a large amount of liquid (liquid content is more than 50%) and the liquid contained is water, it is called hydrogel. When the hydrogel is used as blasthole filler, it is usually called water enriched gelatin stemming. (2) (1)

The Influencing Factors of Formation of Water Enriched Gelatin Stemming Adding order

As a kind of nonelastic gel, water enriched gelatin stemming can only be made by the solution method rather than the dry method. In other words, when preparing the stemming, water glass and gelling agent and other substances must be dissolved in water to make a solution, and then the solution is used to react to form a gel. When preparing the stemming, the order of adding each solution directly affects the nature of the stemming. The aqueous solution formulated by acidified gelling agent is regarded as A, water glass solution as B. If B is injected into A, the formed sol is acidic; while if A is injected into B, the sol is alkaline. Since the gelling time of sol is short under alkaline conditions, the gel particles have large pores, which is conducive to control blasting dust and toxic gases, so the order of addition used in the preparation of stemming is to inject A into B. In order to obtain a gel with homogeneous chemical composition, it is necessary to stir the B solution quickly and inject a fine stream of A solution into it while stirring, and the gel will be formed after standing for a period of time. It is found in the experiments that it is not the best method to add strongly acidic salts to the water glass solution, and the gel will be better adding a part overnight with good effect. (2)

PH value

The pH of the solution is a key factor to be considered in the preparation of gelatin stemming, and it not only affects the gel formation time, but also has a great influence on the gel structure. Under the condition that there is no added salt and the concentration of water glass is constant, the pH of water glass solution is adjusted by HCl, and the relationship between pH and gelling time is shown in curve 1 of Fig. 3.2. As can be seen from Fig. 3.2, the pH value that makes the shortest gelling time is between 8 and 10, and the pH value of water glass is higher or lower than such value, the gelling time increases.

3.2 The Development of Water Enriched Gelatin Stemming Fig. 3.2 Relationship between pH value and gelling time

51

4 3 2 1

1

0

2

-1 -2 0

(3)

2

4

6

8

10

12

PH

Effect of external salt concentration

As a gelling agent for water glass, salts have an effect on both the gelling time and the gelling structure of the water glass. Curve 2 of Fig. 3.2 shows the curves of pH versus gelling time with the addition of salts. It can be seen from the figure that the gelling time is shorter than that without the salt addition under the same concentration of water glass and pH value due to the addition of salt. (4)

Effect of water glass concentration

As the main substance for preparing gel, the concentration of water glass directly affects the gelling time and its strength. Experiments show that at a certain pH and temperature, the greater the concentration of water glass in the solution, the faster the gelation and the greater the gelation strength; conversely, the smaller the concentration, the slower the gelation; when the concentration of water glass is less than 1%, it is difficult to form a gel. It is found in the experiment that the prepared water enriched gelatin stemming is placed in the air, and with the continuation of time, the surface whitens and a part of the liquid contained in it is separated without basically changing the appearance, which is called syneresis. The syneresis is a spontaneous process and can occur even at low temperatures and in humid air, and elevated temperatures can accelerate the syneresis. Syneresis is one of the manifestations of gel aging, which can be explained as follows: after the formation of the gel with a mesh structure, the sol continues to act between the particles to make particles closer, sand the skeleton of the gel shrinks, squeezing some of the liquid (water) out of the particles, thus producing syneresis. In addition, there is also the aging phenomenon that the viscosity of gelatin stemming gradually decreases with time. The literature [12] explains it based on nanotechnology, arguing that the smaller the particles, the more suspended bonds on the surface, and the stronger the adsorption and chemical reaction capacity. In the initial stage, there are many small nano-particles within the gelatin stemming and its bonding is the strongest. While with the extension of time, there are fewer and fewer

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nano-particles and the bonding force gradually decreases, which enlightens us that we should try to avoid leaving the prepared stemming for a long time in the practical application. It also can be found in the experiment that the gel will release a large amount of free water after vigorous stirring, forming liquid sol. This is because as a non-elastic gel, there are long chain molecules of polymers in the gel, and these long chains are cross-linked to form a three-dimensional network structure, enclosing all the water mechanically within the structure. At this time, the system is in semi-solid state. If it is stirred vigorously, part of water surrounded by the long chain escapes to form a liquid sol due to the disruption of the long chain mesh structure. If little acid is added into the liquid sol, it can be restored to the gel state, and the gel strength is similar to that of the original gel. The biggest advantage of this gel is that the phenomenon of syneresis is not obvious, making it more suitable to be the blasthole filler when blasting to control pollution. Formula Optimization of Water Enriched Gelatin Stemming. The stemming formula is optimized according to the following methods: (1)

(2)

(3)

(4) (5)

The water glass with different modulus and different kinds of gelling agent substances are used as the basic raw materials for the preparation of stemming, and a part of the formula is initially prepared. In order to strengthen the cohesion of water glass and improve the degree of stability, the composition of certain acid and salt are changed to adjust the ratio, and change the order of each gelling agent injected into the water glass, and get another formula. A total of 35 formulas have been obtained from the above two steps, and the gelling time, gelling state and gelling performance of the 35 formulas are compared. The stemming formulas with better gelling is selected for blasting comparison experiments. Sand and water are used as blasthole filler respectively to obtain the background comparable experimental data of blasting dust and toxic gases reduction effect. Based on the experience gained from the preparation and experiments, the concentration and dose of the ingredients concerned are increased or decreased, and several new schemes are obtained. And the modulus of the water glass is changed for comparative blasting experiments.

There are 105 formulas in this study and 42 blasting comparable experiments, and finally three formulas for water enriched gelatin stemming with better dust and toxic gases reduction performance have been selected.

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3.2.2 Dust and Toxic Gases Reduction Effect of Water Enriched Gelatin Stemming (1)

The structure of blasting chamber and packing structure of blastholes

The blasting experiment is carried out in the blasting chamber and the structure is as shown in Fig. 3.3. The lower part of the chamber is a cylindrical section, which is 1.5 m high and 5.0 m in diameter; the upper part is a hemisphere. The total volume is 62.1 m3 and the total surface area is 82.4 m2 . The entrance/exit section has a rectangular shape with a cross-sectional area of 1.43 m2 and a connecting passage between the entrance and the atmosphere, which is 6 m in length. The bottom plate is loose sandy soil, mixed with a certain amount of dust from Daye Iron Mine, and the surface area of the sandy soil in the bottom plate is 19.6 m2 . In addition to the bottom plate and the entrance, the surrounding is supported by concrete, with a surface area of 61.4 m2 . There is a ventilation opening located on the wall of the chamber opposite the entrance and exit, with a surface area of 470 cm2 . The ventilation opening is connected to the fan by a duct and then to the atmosphere. The fan adopts exhaust ventilation. Depending on the needs of the experiment, the entrance and exit can be closed with a cloth curtain and the ventilation opening can be blocked with an iron plate. The experiment uses a single blasthole charge, and the blasthole is placed in the center of the chamber, with a depth of 840–900 mm and a hole diameter of 38.2 mm. Column charge of emulsion explosive is used for blasting, with a charge of about 0.35 kg, length of 430–490 mm, and a total filling length of 410–460 mm, including 370–400 mm of water enriched gelatin stemming. The arrangement of the blasthole

Fig. 3.3 Diagram of blasting chamber 1-closed curtain; 2-entrance; 3-sand; 4-ventilator; 5-air duct; 6-ventilation opening

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Fig. 3.4 Diagram of blasthole packing a filling length of sand; b filling length of agent or clear water; c length of explosive charging

A

B

C

loading and filling is shown in Fig. 3.4. The detonation method is a single 8# electrical detonator hole-bottom priming. (2)

Measuring instruments and measuring points arrangement

Kotze dust counter is used to measure the dust concentration. The concentration of dust is calculated by measuring the number of grains per unit area on the slide, and the unit is grains/cm3 . CO and NO2 are measured using a Draeger CO and NO2 monitoring and alarm instrument. In order to observe the distribution of blasting dust and smoke within the chamber, four measurement points, marked a, b, c and d, are selected, together with a auxiliary measuring point, marked e at the entrance to the chamber to observe air leakage and dispersion. The measuring instrument is generally placed within the breathing zone. The distribution of these measuring points is shown in Fig. 3.5. After 30 s of the blasting, three people enter the chamber, each with a CO and NO2 monitoring and alarm instrument and dust counter, in order to simultaneously measure the concentrations of CO, NO2 and dust. The measuring order of CO is a, b, c, d and e and the measuring order of NO2 is c, d, a, b and e. For the dust

Fig. 3.5 Diagram of measuring points arrangement of the blasting chamber

3.2 The Development of Water Enriched Gelatin Stemming

55

measurement, only points a and d are measured, representing the central and marginal concentrations respectively, but each point is sampled twice. Each measuring point is measured once in a cycle for every 6 min. After each blasting, the test lasts for generally 30 min, with a total of 5 cycles. (3)

Results of the blasting experiment

A total of 42 blasting experiments have been conducted with various stemming formulas prepared in the laboratory, and a large amount of data is obtained. The effect of blasting dust and toxic gases reduction of three better stemming formulas are compared and analyzed with the experimental data obtained from blasting with the sand and water as blasthole fillers. For the purpose of comparison, in the graphs, serial numbers “1” and “2” show the blasting experiments with sand and water as blasthole fillers, respectively, and serial numbers “3”, “4”, “5”, and “6” are the experimental data corresponding to the three stemming as blasthole fillers for blasting with better effect. “3” and “4” are the same formula, and the data of “1”, “2”, “5” and “6” are the mean of repeated experiments. (1)

Relevant data and result analysis of CO concentration test

The data from the four measuring points a, b, c, and d at the same moment is averaged to represent the CO concentration in the chamber at that moment. The relationship between time and the average CO concentration after blasting is shown in Fig. 3.6. As can be seen from Fig. 3.6, the overall trend of the curves corresponding to the six experiments is basically the same, starting with a sharp decline and then leveling off. Compared with the curves corresponding to “1” and “2”, the curves corresponding to “3”, “4”, “5” and “6” are below the curves corresponding to “1” and “2”. In the measured time range, the difference between the curves of “3”, “4”, “5” and “6” and “1” and “2” becomes larger and larger with time, which indicates two points: they produce less CO compared to the background blasting of “1” and Fig. 3.6 Relation curve of CO concentration and time with different filling materials when blasting

200

1 2 3 4 5 6

180 160 140

c/ppm

120 100 80 60 40 20 0

0

5

10

15

Time/min

20

25

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“2” under essentially the same conditions of the blasting experiment; the effect of stemming on CO not only acts at the moment of blasting to reduce the amount of CO production, but also continues to play a role in accelerating the adhesion or absorption of CO for some time after blasting. It should be noted that the concentration in “1” and “2” also decreases with time. This can be explained by the fact that the blasting medium, sand or water, has a certain adsorption capacity for CO. In addition, there is little leakage because the chamber cannot be absolutely sealed. Similar phenomena occur in other experiments. Because the experimental conditions are consistent, the effects of leakage cancel each other out in the comparison, which can be illustrated by the value of the concentration corresponding to the measuring point e. In the measurement cycle corresponding to the “1” and “2” experiments, the concentration values of the measuring point e are greater than the those of the “3”, “4”, “5” and “6” experiments. This is due to the high concentrations of “1” and “2”, and the equivalence of air leakage can be roughly seen. From the figure, it can be seen that the filler used in the curves of “3” and “4” is the same formula of stemming, and the curves almost overlap, which indicates that the blasting experiments have good repeatability. The curves of “3” and “4” are lower than the curves of “5” and “6”, which means that the former is more effective than the latter two in reducing CO concentration. Equation (3.18) is used to calculate the CO reduction efficiency of stemming relative to sand and clear water at a certain moment, and the relationship curve with CO efficiency at a certain moment after blasting has been drawn, as shown in Figs. 3.7 and 3.8. Efficiency expression is:   C1 j − Ci j  × 100% ηi j = C1 j Fig. 3.7 Efficiency—time curve of CO reduction relative to sand filling when blasting

80

2-1 3-1 4-1 5-1 6-1

70

Efficiency/%

(3.18)

60 50 40 30 20 10 0

5

10 15 T/(min)

20

25

3.2 The Development of Water Enriched Gelatin Stemming

80

Fig. 3.8 Efficiency—time curve of CO reduction relative to clear water filling when blasting

3-2 4-2 5-2 6-2

70 60

Efficiency/%

57

50 40 30 20 10 0

0

5

10

15

20

25

T/(min)

where i—blasting experiment serial number (1, 2, 3, 4, 5, 6); j—measuring time, min; Cij —the average value of CO concentration at the j-th moment in the i-th blasting experiment, ppm; ηij —CO reduction efficiency of stemming relative to blasthole filled with sand at the j-th moment in the i-th blasting experiment; C1j —average of measuring points at the j-th moment of blasting with sand-filled blasthole, ppm. In Fig. 3.7, “2–1” is the efficiency curve of clear water-filled versus sand-filled. It can be seen from the figure, although the blastholes filled with clear water instead of sand for blasting can reduce the generation of blasting dust and smoke, its efficiency is very low, the highest value is only 12.3%. The efficiency curve is nearly flat, and the most significant change in efficiency is only 4.7% from the beginning to the end, which indicates that the effect of clear water to reduce CO is mainly in the blasting moment, and the water on the effect of CO reduction is very small. Curve “6–1” is similar with “2–1”, which is also relatively straight. The fluctuation of CO reduction efficiency is small, indicating that the CO reduction effect of stemming formula corresponding to “6–1” is similar to that of water, mainly working in the blasting moment. But the difference between the stemming and water is that the CO reduction efficiency is much higher than that of water. Curve “3–1”, “4–1” and “5–1” are similar with low efficiency at the beginning. But the CO reduction efficiency increases with time, especially for the “3–1” and “4–1”, the CO reduction efficiency reach 58.1% and 56.3% after 13 min of blasting. Since the influencing factors of diffusion and adsorption are canceled out when calculating the efficiency, the reason for the increase of CO reduction efficiency of stemming with time must come from the stemming itself. According to the nature of the stemming, it can be explained as follows: the stemming is crushed during blasting, releasing a large amount of free water, and the high temperature and pressure make

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the water atomized and reacts with CO; additionally, the stemming is also dispersed into particles, which have a three-dimensional network structure and exist in the air in floating form. When the particles and CO meet in the air, they may have adhesion or absorption on CO, thus reducing the amount of CO. From the Fig. 3.8, it can be seen that each stemming-filled blasting also has CO reduction effect compared with clear water-filled, which is similar to sand. Different formulas have different changes of CO reduction efficiency-time curve. Curves “3– 2”, “4–2” and “5–2” are similar, but “3–2” and “4–2” are always higher than the “5–2”, indicating that the CO reduction efficiency of stemming used in experiment “3” and “4” is better than that of stemming used in experiment “5”. Curve “6–2” is relatively straight, and the CO reduction efficiency reaches 27.1% at the moment of blasting, which is higher than curves “3–2” and “4–2”. While the efficiency increases slightly with time, but the change is not obvious. At 7 min, the efficiency is lower than curves “3–2” and “4–2”, which indicates that the CO reduction of the stemming takes effect at the moment of blasting. (2)

Relevant data and result analysis of NO2 concentration test

The average value of NO2 concentration at four measuring points a, b, c and d at the same time is taken, which represents the NO2 concentration value in the chamber at the moment. The relationship between the average concentration of t-NO2 at the time after blasting is shown in Fig. 3.9. As can be seen from Fig. 3.9, the curves corresponding to the six experiments have similar shapes, starting with low concentration values, and reaching a maximum value over time, and then starting to show a decreasing trend thereafter. This is because a variety of forms of NOx such as NO, NO2 , etc. generate at the moment of blasting, and among these NOx , except for NO2 , which is more stable, all other NOx are non-stable substances and may be converted into NO2 when encountering O2 . In addition, NO2 is denser than air, and NO2 impacted by the blasting wave to high places has a tendency to sink. Thus, within the measurement range of breathing zone, Fig. 3.9 Relationship between NO2 concentration and time T with different filling materials after blasting

20

1 2 3 4 5 6

Concentration/(ppm)

18 16 14 12 10 8 6 4 2 0

0

5

10 15 T/(min)

20

25

3.2 The Development of Water Enriched Gelatin Stemming

59

NO2 concentration has an upward trend and eventually reaches a maximum value; thereafter, the NO2 concentration tends to decrease due to the absorption of NO2 by the sand and the dominant role of presence of microdiffusion. Even though the trends of the curves are the same, they differ in that the starting positions of curves “3”, “4”, “5” and “6” are lower than those of the “1” and “2”, and it takes less time to reach the peak. This indicates that the amount of NO2 produced is greatly reduced at the moment of blasting due to the action of the stemming, which continues to act on NO2 during the measuring time. The efficiency of stemming relative to sand and clear water at different moments is calculated by the same method used in the analysis of CO efficiency. Comparing with sand and clear water filling, the NO2 reduction efficiency-time curve for stemming filling when blasting is shown in Figs. 3.10 and 3.11. In Fig. 3.10, “2–1” is the NO2 reduction efficiency-time curve of clear water filling relative to sand, and it can be seen that its NO2 reduction effect is poor comparing with other curves with the highest efficiency of only 21.9%. Its efficiency does not change significantly with time. And the stemming has a high efficiency on NO2 , most of which are above 50%, and the highest reaches more than 80%. The trend of “3–1”, “4–1” and “5–1” is the same, with lower efficiency at the beginning, but the efficiency has an upward trend with time. This indicates that the NO2 reduction effect of the two stemmings used in the experiments of “3”, “4”, “5” not only occurs at the moment of blasting, but also has a very strong effect after blasting. But comparing with “5”, both the starting efficiency and the maximum efficiency of “3” and “4” are higher than “5”. Curve “6” is nearly straight, and the change with time is not obvious, which means that the NO2 reduction effect of the stemming used in “6” mainly occurs at the moment of blasting. While compared with the curve “2” corresponding to clear water, it is much more efficient and its NO2 reduction efficiency is more than 70%. 100

Fig. 3.10 Efficiency—time curve of NO2 reduction relative to sand filling when blasting

2-1 3-1 4-1 5-1 6-1

Efficiency/(%)

80

60

40

20

0

0

5

10

T/(min)

15

20

25

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Fig. 3.11 Efficiency—time curve of NO2 reduction relative to clear water when blasting

90

3-2 4-2 5-2 6-2

80

Efficiency/(%)

70 60 50 40 30 20 10 0

5

10

15

20

25

T/(min)

From Fig. 3.11, it can be seen that the stemming has NO2 reduction effect relative to clear water filling, but the NO2 reduction efficiency-time curve changes differently for different formulas. Curve “6–2” is nearly straight, and the NO2 reduction efficiency can reach 65.3% in the instant of blasting relative to clear water. Although there are slight fluctuations later, the maximum change is below 7.1%, which indicates that the NO2 reduction of the stemming is mainly in the instant of blasting with high efficiency. The changing trend of curve “3–2”, “4–2” and “5–2” is roughly the same, lower at the moment of blasting and significantly increase with the extension of time with the maximum change of up to about 50%, indicating that the stemming used in the experiments not only acts in the blasting instant, and the effect of NO2 reduction is still available for a period of time after blasting. By comprehensively analyzing the effects of CO, NO2 reduction with each stemming formula, it can be seen that the effect of NO2 reduction of stemming used in experiment 6 is obvious, reaching the maximum efficiency of up to 75% compared with sand, the maximum efficiency of over 69.5% compared with clear water. However, it has poor effect of CO reduction, the maximum efficiency of 39.8% relative to sand and the maximum efficiency of 33.6% relative to clear water. The stemming used in experiments 3 and 4 has higher efficiency on CO and NO2 , and the maximum efficiency of CO reduction relative to sand can reach 78.9%, and the maximum efficiency relative to clear water can reach 71.2%; the maximum efficiency of NO2 reduction relative to sand can reach 88.2%, and the maximum efficiency of NO2 reduction relative to clear water can reach 86.6%. Therefore, it is considered comprehensively that among the three formulas, the effect of the toxic gases reduction of the stemming used in experiments 3 and 4 is the best effect in blasting.

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61

3.2.3 Dust Reduction Effect of Water Enriched Gelatin Stemming The dust samples from each blasting experiment are observed under a microscope, and the number of dust grains on the slides is read for each sample. The average value of the two data measured at each point at each time is taken, and then divide it by 5cm3 of gas intake, which is the dust concentration at that point at that time, particles per cubic centimeter. Then the average of the data of the two measuring points is taken, that is, the dust concentration in the blasting chamber at that time. The curve of dust concentration with time is shown in Fig. 3.12. As can be seen from Fig. 3.12, the dust concentration–time curves for each experiment have the same trend with the maximum dust concentration at the moment of blasting and there is a gradual decrease in dust concentration as time increases. However, curves “3”, “4”, “5” and “6” corresponding to experiments 3, 4, 5 and 6 are much lower than the curve “1” corresponding to the sand filling and curve “2” corresponding to the water filling. This indicates that the stemming filler has a higher effect on reducing blasting dust compared with sand filler and water filler. The dust reduction efficiency-time curves of stemming filling relative to sand and clear water filling are shown in Figs. 3.13 and 3.14. In Fig. 3.13, curve “2–1” is the efficiency curve of the water filling relative to the sand filling. It can be seen from the figures that clear water also has a certain dust reduction effect relative to the sand filling, and its dust reduction efficiency decreases at the beginning and then increases with time. However, its dust reduction efficiency is much lower than those of the three stemming filling. The dust reduction efficiency of water enriched gelatin stemming relative to sand filling is very high, with the lowest efficiency more than 80%. The dust reduction efficiency of each stemming filling relative to sand fluctuates slightly with time, but the variation is 200 180

Concentration/(grain/cm3)

Fig. 3.12 Relation curve of dust concentration and time with different filling materials after blasting

1 2 3 4 5 6

160 140 120 100 80 60 40 20 0 0

5

10

15

Time/(min)

20

25

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Fig. 3.13 Efficiency—time curve of dust reduction relative to sand filling when blasting

100 90

Efficiency/(%)

80 70 60 50

2-1 3-1 4-1 5-1 6-1

40 30 20

0

5

10

15

20

25

20

25

T/(min)

Fig. 3.14 Efficiency—time curve of dust reduction relative to clear water when blasting

3-2 4-2 5-2 6-2

100

Efficiency/(%)

90 80 70 60 50 40 0

5

10

15 T/(min)

not significant. It can be concluded that the water enriched gelatin stemming has the effect of significantly reducing the amount of blasting dust relative to sand. From Fig. 3.14, it can be seen that each stemming has obvious dust reduction effect relative to clear water, as shown by the dust reduction efficiency of each stemming relative to clear water is more than 53.2%, and the highest efficiency can reach 98.4%.

3.2 The Development of Water Enriched Gelatin Stemming

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3.2.4 Dust and Toxic Gases Reduction Mechanism of Water Enriched Gelatin Stemming (1)

Theoretical analysis of dust and toxic gases reduction mechanism of water enriched gelatin stemming

The main components of water enriched gelatin stemming are water glass, gelling agent and water. Water glass is a viscous colloid. When coexisting with gelling agent and water, it can generate a large amount of colloidal SiO2 . Colloidal silicon dioxide is connected by Si–O–Si bond, forming a very open and continuous three-dimensional network structure, which encloses the solvent water in the sol. The water content in the newly formed gel can exceed 50%. When the gelatin stemming filling the balsthole instead of sand, the gel structure is broken under the action of the blasting wave, and it releases the free water. These water heats up, atomizes and rushes out of the orifice at a high speed, forming droplets with small volume in the air. These hightemperature droplets collide the dusts generated by the blasting, which can make the dust humidified and weighted, accelerating its settlement. As the water enriched gelatin stemming contains viscous colloidal material, when the stemming is crushed by the blasting wave, it will enter the blasting clouds together with the blasting dust and smoke. The blasting fine dust can be bonded with it with the viscosity of the stemming. Moreover, colloidal SiO2 has a three-dimensional network structure, which is porous of the inner part and it is larger than superficial area, so it has a strong adhesion capacity and can adsorb the fine dust generated by blasting to reduce dust. Therefore, the effect of dust reduction with gelatin stemming not only comes from the action of water, but also from the viscosity of stemming and the adsorption of dust reduction effect of colloidal SiO2 . These factors work together to make the high dust reduction efficiency of gelatin stemming compared with water when filling the blasthole. The water released from the gelatin stemming at the moment of blasting warms up and atomizes, and the absorption reaction occurs when the droplets come into contact with the CO and NO2 gas produced by blasting. This is the reason why clear water filling has a certain toxicity reduction effect. Compared with clear water, gelatin stemming contains Cu2+ and NH4 + , which can accelerate the reaction rate of water and CO, NO2 . In addition, SiO2 gelatinous material after releasing water has a strong adsorption effect due to its porous and large specific surface area, which can adsorb the toxic gases produced by blasting, thus playing a role in reducing toxicity. It can be seen that the reason why the gelatin stemming filling for blasting has better effect on blasting dust and toxic gases reduction than the water filling is mainly due to the adsorption of the stemming. (2)

Experimental analysis of dust and toxic gases reduction mechanism of water enriched gelatin stemming

An X-ray electron spectroscopy for chemical analysis is used to study the dust and toxic gases reduction mechanism of adsorption of water enriched gelatin stemming,

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which is a common method to study the surface properties of samples. X-rays are applied to the sample to be analyzed, and electrons of different energy levels in the atoms of the sample surface are excited into free electrons, which carry surface information and have energy characteristics. These electrons are collected and their energy distribution is studied for Electron Spectroscopy for Chemical Analysis, abbreviated as ESCA. The method can be used to measure the percent concentration of elements on the surface of the sample for semi-quantitative analysis, and to study the variation of electron binding energy data of elements on the surface of the sample. In order to study the dust and toxic gases reduction absorption mechanism of water enriched gelatin stemming, three samples are selected for the experiment: one is the 3# stemming with better dust and toxic gases reduction effect, which is marked as PaoNi; the second is the dust not affected by the stemming, which is marked as 0# (sand); the third is the dust collected from the blasting of the blasthole filled with 3# stemming, which is marked 3# (sand). ESCA is carried out on the three samples to measure the electron binding energy and atomic concentration of each element. Based on the changes of atomic concentration and electron binding energy of the elements contained, the working mechanism of water enriched gelatin stemming on dust is judged with reference to relevant data. The analysis is performed using a PHI5300ESCA system made in U.S. The system error of this instrument is 0.2 eV. During the test, numerical corrections are made for the elements under study. (1)

Test results and analysis of PaoNi sample

Full scan of X-ray photoelectron spectroscopy is done for the 3# stemming sample, as shown in Fig. 3.15. The horizontal coordinates in the figure indicate the electron binding energy values. Each element contained in the sample corresponds to a spectral 10

O1s

9 8 7

O(A)

6 5

Na(A)

4 3

Cls Si2s Si2p

2

Na2s

1 0 1000.0 900.0 800.0

700.0

Fig. 3.15 Scanning of PaoNi sample

600.0

500.0 400.0

300.0

200.0

100.0

0.0

3.2 The Development of Water Enriched Gelatin Stemming

65

Table 3.2 Electron binding energy and surface element concentration of PaoNi sample Element

C

Na

Si

O

N

Cl

Electron binding energy (ev)

284.6

1071.6

103.1

532.4

399.7

198.9

Surface element concentration (%)

25.94

7.34

16.32

49.34

0.4

0.56

peak, and the horizontal coordinate of the spectral peak is the electron binding energy of the element, the height of the spectral peak can reflect the concentration of the element on the surface. The peak is high, showing that the sample contains more of the element, and vice versa, the concentration is low. As can be seen from Fig. 3.15, the PaoNi sample mainly contains six elements, Si, O, Na, C, N, and Cl. By doing ESCA analysis on these elements, the electron binding energy and surface concentration values of each primary element in PaoNi can be obtained, as shown in Table 3.2. (2)

Analysis of ESCA results of 0# (sand) and 3# (sand) samples

Full scan of ESCA is carried out on 0# (sand) and 3# (sand), as is shown in Figs. 3.16 and 3.17. Comparing the two figures, it can be seen that the shapes of the two figures are very similar. The only difference is that the Cl element spectral peak of 3# sample is slightly higher than that of 0# sample, indicating that the Cl content of 3# sample is higher than that of 0# sample. The electronic binding energy and surface concentration of 3# and 0# samples are shown in Table 3.3. The two numbers in each cell of Table 3.3, the upper one indicates the electron binding energy of the element, and the number in the middle bracket is the concentration value of the element on the surface. Comparing the surface concentration values of the elements in the 0# (sand) and 3# (sand) samples, it can be seen that the

10

O1s

9 8

O(A)

7 6 5 4

Fe(A)

3

Cls

2

Si2s Si2p

1 0 1000.0 900.0 800.0 700.0 600.0 500.0 400.0 300.0 200.0 100.0 0.0

Fig. 3.16 Scanning of 0# sand

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3 Chemical Suppression Technology of Open-Pit Mine … 10 O1s

9 8 O(A)

7 6 5

Fe(A)

4

C1s

3 2

Ca2p

Si2s

Si2p

1 0 1000.0

900.0

800.0 700.0

600.0 500.0

400.0 300.0

200.0 100.0

0.0

Fig. 3.17 Scanning of 3# sand

Table 3.3 Electron binding energy and surface element concentration of 0# (sand) and 3# (sand) Sample

Electron binding energy (ev) (Surface concentration (%)) C

Na

Si

O

N

Cl

(sand)

284.6 (19.71)

1071.7 (0.7)

102.6 (15.49)

531.7 (53.63)

400.3 (0.48)

–

3# (sand)

284.6 (31.96)

1071.5 (0.79)

102.5 (14.96)

531.7 (49.28)

399.5 406.9 (3.21)

189.4 (0.34)

3# (washed)

284.6 (29.96)

1071.8 (0.44)

102.7 (17.00)

531.8 (53.22)

400.1 (1.19)

200.0 (0.09)

0#

amount of C, Na, N, and Cl on the surface of 3# (sand) sample is higher than that of the 0# (sand) sample, while the amount of Si and O is lower than that of 0# (sand) sample. What can be inferred is that there is a layer of material different from the surface of 0# (sand) adsorbed on the surface of 3# (sand), which contains less Si and O and more C, Na, N, and C1. Comparing the electron binding energies of C, Na, Si and O elements contained on the surface of 0# (sand) and 3# (sand) samples, it can be seen that the electron binding energy values of the same elements contained in the two samples are almost the same, and according to the theory of photoelectron spectroscopy, it can be assumed that these elements do not undergo chemical changes during the blasting experiments. The ESCA spectra of N element contained in 0# (sand) and 3# (sand) samples are shown in Fig. 3.18. As can be seen from Fig. 3.18 that the ESCA spectrum of 0# (sand) sample has only one spectral peak, which corresponds to a binding energy of 400.3 eV, and according to the literature [13], the presence state of N element is NHx . In the 3# (sand) sample, there are two spectral peaks with binding energies of 406.9

3.2 The Development of Water Enriched Gelatin Stemming Fig. 3.18 ESCA diagram of N element contained in 0# (sand) and 3# (sand) samples

10 9 8 7 6 5 4 3 2 1 0

67

407.0ev 399.5ev 3# 400.3ev 0# 414 412 410 408 406 404 402 400 398 396 394

and 399.5 eV, and the presence states of N element are NO2 and NHy . In order to determine whether the x and y values are the same, the values of the electron binding energy of N element must be compared. In NHx , the binding energy of N element is 400.3 eV, and the electron binding energy of N in NHy is 399.5 eV, the difference between which is 0.8 eV. While the systematic error of the instrument is 0.2 eV. According to the principle of spectral analysis, it is concluded that x is different from y, which means that NHx and NHy contained in 0# (sand) and 3# (sand) samples are different substances. It can be seen that the N element is chemically transferred in the blasting experiment and exists on the surface of 3# (sand) sample in NHy and NO2 forms. From the data in Table 3.3, it can be seen that the C1 element contained in the 3# (sand) sample is not present in 0# (sand) sample, while the presence of Cl element is shown in the PaoNi sample, which indicates that the components in PaoNi are adsorbed by the surface of 3# (sand) sample, which confirms the dust adsorption of stemming. The analysis of the above experimental results shows that a layer of material is adsorbed on the surface of 3# (sand) sample, which is different from that on the surface of 0# (sand) sample and is derived from the composition of the stemming. The function of stemming on dust can be explained as follows: at the moment of blasting, high temperature and high pressure gas acts on water enriched gelatin stemming and rocks within a certain range, making them crushed and thrown into high altitude. During the process of crushing, the stemming releases a large amount of free water, and atomized water acts with the dust in the air, making it wet and heavy and easy to settle. After releasing free water, the flying objects of the stemming still has the sticky property of gel, when it meets with the dust, it can bond and adsorb the dust, as a result, it will also make the dust heavier and play the role of dust reduction. In addition, after the dust and stemming flyings adsorb each other, the surface area of the dust increases and it is easier to adsorb the dust in the air, which also accelerates the dust settling. To sum up, water enriched gelatin stemming has the effect of dust reduction.

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Moreover, NO2 is also adsorbed on the surface of 3# (sand) sample. NO2 comes from the NO2 gas produced by blasting, which indicates that the dust or the flyings of stemming can also adsorb NO2 . (3)

Analysis of the experimental results of 3# (sand) sample surface adsorption characteristics

In order to further study the surface adsorption characteristics of water enriched gelatin stemming, the 3# (sand) sample is washed with water several times and then marked as 3# (washed) sample. The electron binding energy and concentration of each element are tested again by ESCA, and the results are shown in Table 3.3 for the data of 3# washed sample. Comparing the concentration values of each element on the surface of 3# (sand) sample and 3# washed sample in Table 3.3, it can be seen that the concentration values of Na, C, Cl, and N elements of the latter are smaller than those of the former, while the concentrations of Si and O increase. This can indicate that the material on the surface of 3# (sand) sample has been washed off partly after being rinsed with water several times, resulting in a decrease in the concentration of C, Na, N, and Cl elements, and an increase in the concentration of Si and O elements. Figure 3.19 shows the ESCA diagrams of the N element contained in the 3# (sand) sample and 3# washed sample. In Fig. 3.19, there is only one peak for the N element contained in the 3# washed sample, which corresponds to a binding energy of 400.1 eV, and its presence state is NHz , while the ESCA spectrum of the N element contained in the 3# (sand) sample has two peaks, which corresponds to binding energies of 406.9 eV and 399.5 eV, and its presence states are NO2 , NHy , respectively. The difference between the electron binding energy of N element in 10 9 8 7

3# sand

6 5 4 3 2

3# washed

1 0 414.0

411.9

409.8

407.7

405.6

403.5

401.5

399.4

397.3

395.2

393.1

Fig. 3.19 ESCA diagram of N element contained in 3# (sand) sample and 3# washed sample

3.2 The Development of Water Enriched Gelatin Stemming

69

NHy and that of N element in NHz is 0.6 eV > 0.2 eV, so z is different from y. And the electron binding energy of N element in 0# (sand) sample is 400.3 eV, and the presence state is NHx . The difference between the electron binding energy of N element in NHx and NHz is 0.2 eV, which is within the error range of the instrument, so it can be concluded that NHx and NHz are the same substance. It can be said that the NO2 and NHy adsorbed on the surface of 3# (sand) sample have been washed off by water, and the NHx substance inherent in 0# (sand) sample is revealed, so the adsorbed substance on the surface of 3# (sand) sample is attached by physical adsorption. The electron binding energy values of Cl element of 3# (sand) sample and 3# washed sample in Table 3.3 are 198.3 eV and 200.0 eV, respectively, which are known to exist in Cl− form and covalent state respectively according to the literature. The difference of electron binding energy of Cl element contained in these two samples is 1.7 eV > 0.2 eV, indicating that the existence form of Cl element in the two samples is not in the same. The reason for it is that the physically adsorbed Cl− is washed off the surface of the #3 (sand) sample when rinsing, and the chemically adsorbed covalent Cl is revealed and shown by the instrument in the test. Then comparing the electron binding energy values of Cl element contained in 3# (sand) sample, 3# washed sample and PaoNi, the electron binding energy of Cl element in PaoNi sample is 198.9 eV, and the difference between this value and the electron binding energy of C1 element contained in 3# (sand) sample and 3# washed sample is 0.6 eV and 1.1 eV respectively, both of which are greater than 0.2 eV, indicating that it is because that the element Cl in the stemming has undergone a chemical change in the blasting experiment and exists in the dust as Cl− and covalent state Cl, respectively. The Cl− form is adsorbed on the outer surface of the dust by physical adsorption, and the covalent state C1 is adsorbed on the inner surface of the dust by chemical adsorption rather than the instrument error. Comparing the concentration values of Cl element contained in the 3# washed sample with those of the 3# (sand) sample, it can be seen that Cl in the form of Cl− is in the majority, and only a very small amount exists in the form of covalent state. Therefore, the effect of physical adsorption is better than that of chemisorption, and it can also be said that the adsorption of dust surface and stemming is mainly by physical adsorption. Through the analysis of the results of three samples collected, the following conclusions can be drawn: (1)

The dust sample 3# (sand) collected when filling the blasthole with 3# water enriched gelatin stemming adsorbs a layer of substance different from 0# (sand) on the surface, which is derived from the composition of stemming, indicating that the gelatin stemming filled in the mouth of the blasthole is dispersed into fine particles by the blasting energy and thrown into the air together with the blasting dust and smoke at the moment of blasting, and these flying stemming is sticky and can be used with the dust generated by blasting, which makes the dust heavier and plays the role of dust reduction.

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In addition, there is NO2 gas adsorbed on the surface of 3# (sand), which is the product of blasting, indicating that the dust has adsorption effect on the toxic gases produced by blasting. This adsorption is because the silica gel in the gelatin stemming presents a three-dimensional network structure after the release of free water, which has the characteristics of porosity and large specific surface area, making the adsorption ability very strong, adsorbing the toxic and harmful gases and microfine dust generated by blasting, thus playing a role in reducing dust and smoke of blasting. (2)

Through comparing the surface composition of 3# (sand) and 3# (washed), it can be seen that both dust and stemming flying objects have physical and chemical adsorption, but physical adsorption is much larger than chemical adsorption, that is, the adsorption of dust and stemming is mainly physical adsorption.

3.3 Development of Blasting Dust and Smoke Inhibitors of Open-Pit Mine 3.3.1 Dust Reduction Mechanism of Blasting Dust and Smoke Inhibitors For ordinary water stemming, the additive of the water stemming bag is pure water, and the mechanism of its dust and toxic gases reduction is to use the plastic bag with water instead of or partly instead of stemming to fill in the blasthole. When the blasting ruptures, the water bag is boken, and under the action of high temperature and high pressure explosion wave, most of the water is vaporized, then condensed into very fine droplets which contact with the mine dust produced at the same time, forming the condensation nucleus of the droplets or being wetted by the droplets so as to reduce dust. Although ordinary water stemming can control the generation of blasting dust and smoke to some extent, it prevents the wetting of dust particles by water because mineral and rock powders have the degree of hydrophobicity and the surface tension of pure water is large [14]. What’s more, the nature of electrostatic charge carried by fine dust particles is the same as the charge carried by water mist particles, and they repel each other. Therefore, dust is generally not easy to be quickly and completely wetted by water, and it can not reach the allowed dust concentration as well, so the effect of dust and toxic gases reduction needs to be improved. For the new water stemming, it is the transformation of the ordinary water stemming, that is, adding some additives such as inorganic salts, sticky dust agents, surfactants, geling agents, etc. to reduce the surface tension of the water stemming, enhance the wetting ability and the strength of reaction with toxic and harmful gases, in order to improve the degree of atomization, dust capture effect and the ability of toxic and harmful gases reduction after the blasting of water stemming, so as to greatly enhance its dust and toxic gases reduction effect [15]. Although there is a wide range of new water stemming with various types and forms in the market, their

3.3 Development of Blasting Dust and Smoke Inhibitors …

71

mechanisms are similar [16, 17]. And the nature of water stemming dust and smoke inhibitors in actual experiments are basically summarized as follows: (1)

(2)

(3)

(4)

(5)

At the moment of blasting, the dust particles and toxic and harmful gases in the air collide with the vaporized or atomized water stemming solution particles, and the dust loses its ability to continue to fly after adsorption, weight gaining and sedimentation. It dissolves some of the soluble toxic gases, thus rapidly reducing the concentration of smoke and dust. The addition of inorganic salts and other additives in the inhibitor increase the density of the aqueous solution of water stemming, and the blasting has more bursting force. When the vaporized solution condenses again into very fine droplets and collides with dust particles, the density of the condensation nucleus formed or the dust wetted by the droplets is relatively large, and the relative speed is also large when colliding, so it is easier to settle than pure water. In addition, the gelling agent makes the solution sticky, which makes it easier to bond with the dust and makes the dust reduction effect more significant. After the addition of surfactants, the surface tension of the water stemming solution becomes smaller, and the dust is more easily wetted by the droplets, thus increasing the collision rate and enhancing the capturing ability. At the same time, the dust surface is densely filled with surfactant molecules of hydrophobic lattice to improve its hydrophilicity, and the wettability and permeability increase, mainly due to the combination of surfactant and additive, so that the hydrophobic lattice on the dust surface adsorbs surfactant to produce hydrophilicity; high-valent anions are adsorbed on the hydrophilic lattice on the dust surface to keep it hydrophilic. The dense filling effect of surfactant molecules of hydrophobic lattice on the surface of the dust and semi-micellar formation improve the hydrophilicity. In addition, surfactants can improve the adsorption of dust, inhibit the evaporation of water, and delay the time of moisture absorption, improving the effect of dust and smoke reduction. Water stemming solution can precipitate a large amount of free water when it is crushed into particles, and the water is in the state of steam under high temperature and high pressure, which can make the dust wetting, weighting and settling when it meets the dust in the air. Water vapor reacts with the toxic and harmful gases in the blasting smoke, especially the reaction is accelerated in the case of the catalysts such as Fe2 O3 , Al2 O3 , SiO2 and MgO in the ore rock, which play a part in reducing the toxicity. Toxicity reducing agent for toxic and harmful gases is added to the water stemming, which can react chemically with CO and nitrogen oxides in the blasting smoke under high temperature and pressure environment to generate non-toxic and harmless gases, which can achieve the purpose of rapid poison reduction.

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3.3.2 Component of Dust and Smoke Inhibitors (1)

Surfactant

Anionic surfactants are the most widely used surfactants with a long history of development, large production volume and variety, and their molecules generally consist of long-chain alkyl group (C10 to C20 ) and hydrophilic carboxylic acid groups, sulfonic acid groups, sulfate groups or phosphate groups. These surfactants have excellent wetting, dispersing and emulsifying properties, and all have high surface activity, so they are very widely used with extremely abundant market supply and types, so they are selected as one of the experimental objects in this subject. Cationic surfactants are mostly derivatives of organic amines, and when their solutions interact with low-energy mineral dust, the solutions become acidic and the organic amines are easily precipitated, thus losing their surface activity. At the same time, although the mixed solution of cationic surfactant and anionic surfactant has lower critical micelle concentration and surface tension than single component, and also has higher surface activity, phase separation often tend to occur above its critical micelle concentration. Then the solution becomes turbid or pearlescent, or even produces precipitation, which is very unfavorable to its performance. Therefore, there are less application of cationic surfactants in the field of chemical dust suppression. It is not used as an experimental object in this subject. Ampholytic surfactants are surfactants with two or more hydrophilic groups, namely anionic, cationic and nonionic, at the same time. Amphoteric ions are less soluble in water and have slightly poorer wettability, and these surfactants are more expensive and have a smaller practical application range than other types of surfactants. Considering the performance and the cost of dust and smoke inhibitor formula, it is excluded. The hydrophobic groups of nonionic surfactants are provided by hydrophobic compounds containing active hydrogen such as high-carbon fatty alcohols, alkyl phenols, fatty acids, fatty amines, etc. Their hydrophilic groups are provided by compounds containing ether groups that can form hydrogen bonds with water, free hydroxyl groups such as ethylene oxide, polyols, ethanolamines, etc. Due to its chemical composition of hydrophobic and hydrophilic groups, the nonionic surfactant has high surface activity with low surface tension of its aqueous solution, low critical micelle concentration, large micelle aggregation number and strong solubilization effect, which not only has good wetting, dispersing and solubilizing effects, but also has good corrosion inhibition, colloid protective, anti-corrosion and other multifaceted effects. Due to its superior performance and versatility, it is developing very rapidly and is currently the second largest type of surfactant after anionic surfactants in terms of production, taking a wide range of materials in experiments. Based on the effectiveness, wettability, solubility, safety, permeability, stability, compounding and biodegradability of surfactants in reducing surface tension, and taking into account the actual market situation, an effective and reasonable primary selection of surfactants is made, and the selected surfactants are as follows:

3.3 Development of Blasting Dust and Smoke Inhibitors …

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

73

Sodium dodecyl benzene sulfonate, abbreviated as LAS, is an anionic surfactant, and is the most consumed surfactant species at present, with excellent wetting ability and surface activity. It has excellent water solubility, good stability in acidic, alkaline and certain oxide solutions, has good biodegradability with sufficient raw material sources, low cost and mature manufacturing process. Sodium dodecyl sulfate, abbreviated as K12 , is an anionic surfactant, and it is easily soluble in water with good compatibility with anionic and nonionic complexes. It has excellent emulsification, penetration and dispersion properties with biodegradability of more than 90%. Sodium alpha-olefin sulfonate, abbreviated as AOS, was industrially produced in 1968 and is a high-foaming, hydrolytically stable anionic surfactant with excellent wetting properties, good biodegradability, and good compounding properties with both nonionic and other anionic surfactants. Secondary alkyl sulfonic acid, anionic surfactant, abbreviated as SAS-60, has similar surface activity to sodium alkylbenzene sulfonate, and has good stability in alkalis, weak acids and water, strong solubility, excellent wetting power with hard water resistance and oxidant resistance. It still has good wetting, emulsifying and dispersing ability in hard water, with better biodegradability than that of sodium alkylbenzene sulfonate, and its compatibility with anionic and nonionic surfactants is good, which can significantly improve the application effect of any system. Alkyl phosphate ester potassium salt, abbreviated as MAP-K, has lower surface tension and better biodegradability compared with LAS. Due to the excellent wetting, solubilization and corrosion inhibition properties of phosphate esters and good safety, it has attracted wide interest, and in recent years, phosphate ester surfactants have been developing rapidly in China with good compounding with other surfactants. Sodium fatty alcohol polyoxyethylene ether sulfate, abbreviated as AES, is easily soluble in water, non-toxic, and has superior solubility and surface activity, It has hard water resistance and good wetting ability, and has better compounding performance with sodium dodecylbenzene sulfonate. Fatty alcohol polyoxyethylene ether, referred to as penetrant JFC, fatty alcohol polyoxyethylene ether series surfactants have many varieties depending on the number of carbon atoms of the fatty alcohol and the number of ethylene oxide addition. The product of addition of low carbon alcohol C7 –C9 with 5–6 mol of EO (i.e. JFC) has excellent wetting ability and permeation properties. The fixed hydrophilic and lipophilic groups can be oriented on the surface of the solution which causes a significant decrease in surface tension. Octyl glucopyranoside, abbreviated as APG0810, is a new mild nonionic surfactant, which has the advantages of both common nonionic and anionic surfactants, such as high surface activity, which can significantly affect the surface properties of the system at relatively low concentrations, i.e., the surface tension can be significantly reduced; good water solubility, it has large solubility even in the presence of high concentrations without additives;

74

(9)

3 Chemical Suppression Technology of Open-Pit Mine …

good biodegradability, it has mild performance with fast and safe degradation and light pollution to the environment, which is a new green surfactant; excellent compounding performance, it has obvious synergistic effect on most surfactants. Lauryl glucoside, abbreviated as APG1214, has similar properties to octyl glucopyranoside, but has a larger molecular mass than the latter. It is an alkyl polyglucoside and is the reaction product of carbohydrate and fatty alcohol. It has good performance, solubility and stability, and has synergistic effect when compounding with anionic, nonionic and cationic surfactants to improve surface activity and reduce the irritation of other surfactants, with low toxicity and good biodegradation.

In conclusion, the experiment selects nine surfactants with good performance of sodium dodecyl benzene sulfonate (LAS), sodium dodecyl sulfate (K12 ), sodium alpha-olefin sulfonate (AOS), secondary alkyl sulfonic acid (SAS-60), alkyl phosphate ester potassium salt (MAP-K), sodium fatty alcohol polyoxyethylene ether sulfate (AES), fatty alcohol polyoxyethylene ether (penetrant JFC), octyl glucopyranoside (APG0810) and lauryl glucoside (APG1214) as the experimental primary subjects, and are replaced with A1 , A2 , A3 , A4 , A5 , A6 , A7 , A8 and A9 . (2)

Hygroscopic agents

For hygroscopic agents, the overall moisture absorption (anti-evaporation) performance of the formula is mainly taken into account, so inorganic salts with strong water absorption are generally selected, so as to improve the dust wettability and improve the wettability speed. Even after the dust is wetted, it can continue to absorb the moisture in the air, thus increasing the water content of the dust, to achieve the purpose of the dust reduction caused by the evaporation of water. At present, there are many hygroscopic materials in the domestic market, which can be roughly divided into adsorption hygroscopic materials and chemical hygroscopic materials. Adsorption hygroscopic materials are: silica gel (mSiO2 ·nH2 O), molecular sieve, alumina gel (Al2 O3 ), activated carbon, bone carbon, wood carbon (C) and bleaching earth (Al2 O3 ·4SiO2 ·nH2 O); chemical hygroscopic materials are: calcium chloride (CaCl2 ·nH2 O), magnesium chloride (MgCl2 ·nH2 O), quicklime (CaO), propanetriol (C3 H5 (OH)3 ), phosphorus pentoxide (P2 O5 ), lactam (NH(CH2 )5 OH), diethylene glycol (CH2 OHCH2 OCH2 OH). Among them, calcium chloride is colorless cubic crystals, and the general product is white solid or off-white solid particles or powder, slightly bitter in taste and odorless. Calcium chloride is produced from aqueous solution crystallized at room temperature as a hexahydrate salt, which becomes anhydrous salt after being heated to 260 °C. Studies show that the residual moisture in the air after CaCl2 drying is less than 0.36 × 10−3 mg-mL−3 for anhydrous calcium chloride and calcium chloride hexahydrate at a temperature of 25°C, so they have high hygroscopicity (deliquescence). Magnesium chloride anhydrous (MgCl2 ) can form 2, 4, 6, 8 and 12 crystalline hydrates with water at different temperatures. It usually presents magnesium chloride hexahydrate (MgCl2 ·6H2 O) at room temperature. Magnesium chloride loses all

3.3 Development of Blasting Dust and Smoke Inhibitors …

75

crystalline water at 118 °C and partially decomposes to release hydrogen chloride (HCl), and it decomposes to magnesium oxide (MgO) and hydrogen chloride (HCl) above 242 °C. Pure magnesium chloride is often yellowish brown, bitter in taste, with strong hygroscopicity, and is easily soluble in water and already alcohol. In view of the fact that the proportion of hygroscopic agents in the formula of water stemming inhibitor is great (more than 90%), and its cost price directly affects the total cost of water stemming inhibitor agent, therefore, calcium chloride and magnesium chloride, which are cheap, have their good prospects for promotion and application. In summary, by examining the chemical affinity between the above materials and water as well as the source of materials, price and other factors, this experiment considers to take calcium chloride and magnesium chloride as hygroscopic agents, which are more in line with the production reality of China’s mines, which are marked with B1 and B2 in the following part. (3)

Gelling agents

The main function of the gelling agent is water retention, which is generally polymeric substances with long molecular chains. It can polymerize fine dust particles into larger particles, and has affinity and adsorption on the inhibition object. The dust particles are combined into through the adhesion and penetration effect of coagulation factor, so as to improve the strength of the curing layer. The principle of its selection is to have a strong bonding, coagulation effect on the dust with a certain time limit, and it also should have low price and wide sources with good environmental and socio-economic benefits. According to these requirements, consulting the relevant literature, we have learned that water glass, whose chemical name is NaSiO3 ·9H2 O, is easily soluble in water, and has strong bonding ability, high strength with good acid resistance and heat resistance. It is an important binder in industry with low price, so it is chosen as one of the experimental objects of gelling agent. Polyacrylic amide, abbreviated as PAM, water-soluble polymer, is non-toxic and non-corrosive with good flocculation, and can make suspended substances through electrical neutralization; can play a bonding role through physical and chemical effects; has thickening effect under neutral and acidic conditions, and thickening will be more obvious when semi-reticulated structure is presented. Additionally, it can reduce the frictional resistance between liquids. Sodium polyacrylate, similar to polyacrylic amide, is also a macromolecular material with strong hygroscopicity and good stability. It can slowly dissolve in water to form a very viscous transparent liquid, and the viscosity of its 0.2% solution is about 2000–2500 mPa s. The viscosity of its solution is much larger than that of similar gelling agents such as sodium alginate and CMC, which is 15–20 times. The experiment primarily chooses gelling agents of water glass (sodium metasilicate nonahydrate), polyacrylic amide (PAM) and sodium polyacrylate as the objects, which are marked with C1 , C2 and C3 in the following part. (4)

Toxicity reducing agent

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Except for dust, the dust and smoke produced by blasting are mostly toxic and harmful gases such as CO and nitrogen oxides, so it is necessary to add the corresponding toxicity reducing agent to generate non-toxic gases by chemical reaction to minimize the concentration of toxic and harmful gases, making it reach the allowable range and not cause secondary pollution. (5)

Decreasing nitrogen oxides

Nitrogen oxides refer to oxides of nitrogen (as NO2 ) present in air in the form of nitric oxide and nitrogen dioxide. After referring to the relevant literature, ammonium salt is selected as the reagents for decreasing nitrogen oxides. Among them, ammonium nitrate (NH4 NO3 ) is colorless and odorless transparent crystals or white crystals, and is easily soluble in water. Pure ammonium nitrate is stable at room temperature and insensitive to blows, collisions or friction. However, under high temperature, high pressure and the presence of oxidizable substances (reducing agent) and electric sparks, it will explode. Ammonium nitrate can not explode with water containing 3% or more, but it is still decomposed at a certain temperature, and is very easy to absorb moisture for agglomeration. At 400 °C or more, it is easy to have violent decomposition and explosion, i.e. 4NH4 NO3 → 3N2 + 2NO2 + 8H2 O. Therefore, ammonium nitrate is not convenient for downhole transportation or preservation, it is left out of consideration. In addition, ammonium sulfate also produces toxic and harmful gases when it decomposes, so this experiment also does not consider ammonium sulfate as the toxicity reducing agent of nitrogen oxide. Ammonium chloride is stable in nature and decomposes into hydrogen chloride as well as ammonia gas when heated: NH4 Cl  NH3 + HCl. Among them, the solution of ammonia gas dissolved in water is alkaline, and has a certain effect on reducing the concentration of nitrogen oxides under high temperature and pressure at the moment of blasting. Ammonium acetate undergoes weak hydrolysis in aqueous solution, and the two products produced contribute to the hydrolysis, i.e. NH4 + + CH3 COO− + H2 O → NH3 ·H2 O + CH3 COOH, which helps to reduce the concentration of nitrogen oxides. To sum up, ammonium chloride and ammonium acetate are selected as the experimental objects of antidote in this project and they are marked with D1 and D2 in the following part. (2)

Decreasing carbon monoxide

For the reduction of carbon monoxide, this experiment considers the conversion of CO to non-toxic CO2 , and the addition of a catalyst in the high-temperature environment generated by the blasting can accelerate the reaction. Nowadays, catalysts are mostly used in the chemical field to reduce the concentration of carbon monoxide for achieving the purification of blasting smoke. The commonly used catalysts are precious metal catalysts (Au, Pd, etc.) and non-precious metal catalysts (CuO, CeO2 , etc.). Although precious metal catalysts have high catalytic efficiency, the price of gold, zirconium and other precious metals are expensive, while non-metallic catalysts have low catalytic temperature with excellent effect

3.3 Development of Blasting Dust and Smoke Inhibitors …

77

and moderate price. Therefore, taking the cost of water stemming dust and smoke inhibitor into account and after market survey, copper salt is selected as the toxicity reducing agent of carbon monoxide reduction agent. Under the action of high temperature, copper salt decomposes into copper oxide, which catalyzes carbon monoxide into carbon dioxide rapidly, thus reducing the concentration of carbon monoxide. In this experiment, copper sulfate and copper chloride are selected as the objects for the detoxification experiment and they are marked as E1 and E2 in the following part.

3.3.3 Monomer Experiment (1) (1)

Surfactant Surface tension

Making the nine surfactants A1 –A9 as the experimental subjects, the surface tensions of the nine surfactants at mass concentrations of 0.005, 0.05, 0.1 and 0.5% are measured using BZY-201 surface tension meter platinum plate method, and their wettabilities are measured by the reverse osmosis method. Each solution is measured three times, and the average value is finally taken. In addition, since surfactant A9 itself is in the form of a white paste, and its viscosity is greater than 4000 mPa s at room temperature. It has large molecular mass and the time to dissolve in water is long (measured at about 9 h), which is not suitable for practical use in underground mine engineering, so surfactant A9 is excluded. The surface tension values of surfactants A1 –A8 are measured and the specific values are shown in Table 3.4. The critical micelle concentration refers to the lowest concentration at which the surfactant molecules associate into micelles in the solvent. When the solution reaches the critical micelle concentration, the surface tension of the solution drops to the lowest value. At this time, when the surfactant concentration is increased again, the surface tension of the solution no longer decreases but a large number of micelles are formed, and the surface tension of the solution at this time is the minimum surface tension that the surfactant can achieve, expressed as cmc. The surface tension-concentration logarithmic graph is shown in Fig. 3.20, which shows that the surface tension of the aqueous solution of surfactant decreases sharply at the beginning with the increase of the solution concentration, and it changes slowly or no longer changes after reaching a certain concentration (i.e. cmc). The linear parts on both sides of the curve turning point are extended, and the concentration of the intersection point is the cmc of the surfactant in the system. From Table 3.4 and Fig. 3.20, the relationship between mass concentration and surface tension values is analyzed by taking representative surfactant A1 , surfactant A2 and surfactant A5 as examples. The surface tension of surfactant A1 decreases rapidly with increasing concentration, and then the concentration of the solution (not

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3 Chemical Suppression Technology of Open-Pit Mine …

Table 3.4 Surface tension values of surfactants A1 –A8 with different mass concentrations mN/m Frequency

Species A1 (%) 0.005

A2 (%) 0.05

0.10

0.50

0.005

0.05

0.10

0.50

Once

69.52

47.70

44.31

28.73

32.29

27.89

28.75

28.46

Twice

69.78

47.07

43.84

28.82

30.56

28.24

30.91

28.96

3 times

69.30

46.73

43.72

28.83

30.94

27.56

30.86

29.24

Average

69.53

47.17

43.96

28.79

31.26

27.90

30.17

28.89

Frequency

A3 (%) 0.005

0.05

0.10

0.50

0.005

0.05

0.10

0.50

Once

27.22

23.99

26.76

27.58

62.77

42.60

41.46

29.12

Twice

26.28

23.05

27.04

25.27

65.03

39.44

36.72

24.94

3 times

27.15

23.94

27.29

26.22

67.05

40.89

37.12

25.13

Average

26.88

23.66

27.03

26.36

62.77

42.60

36.72

29.12

Frequency

A5 (%) 0.005

0.05

0.10

0.50

0.005

0.05

0.10

0.50

Once

26.34

25.90

25.64

25.88

31.79

29.18

28.53

29.58

Twice

27.08

26.37

26.27

26.06

31.30

29.17

27.99

28.73

3 times

26.98

26.77

26.64

26.41

31.27

29.08

27.75

28.63

Average

27.03

26.14

26.18

26.12

31.45

29.14

28.09

28.98

Frequency

A7 (%) 0.005

0.05

0.10

0.50

0.005

0.05

0.10

0.50

Once

48.78

27.95

27.48

26.15

37.27

30.04

26.96

26.46

Twice

50.08

26.40

26.51

26.71

36.69

30.21

27.23

26.62

3 times

50.02

26.03

26.92

27.09

38.18

30.24

27.20

26.81

Average

49.63

26.79

26.97

26.65

37.38

30.16

27.13

26.63

A4 (%)

A6 (%)

A8 (%)

A1 A2 A3 A4 A5 A6 A7 A8

70 Surface tension/(mPa/m)

Fig. 3.20 Surface tension curve of surfactants A1 –A8 with different mass concentrations

60

50

40

30

20 0.0

0.1

0.2 0.3 0.4 Mass concentration/(%)

0.5

3.3 Development of Blasting Dust and Smoke Inhibitors …

79

Table 3.5 Critical micelle concentration of surfactants A1 –A8 Surfactant

A1

A2

A3

A4

A5

A6

A7

A8

Critical micelle concentration/%

0.5

0.1

0.05

0.1

0.05

0.05

0.05

0.05

listed) is increased, the surface tension no longer decreases and remains basically the same. Therefore, when reaching 0.5%, the surface tension basically tends to be constant because the lipophilic groups of the surfactant are most closely arranged in the solution, and this concentration is the critical micelle concentration, which is the optimal concentration of surfactant A1 with the lowest surface tension. Surfactant A2 has the highest surface tension at mass concentration of 0.005% (31.26 mN m−1 ) and the lowest surface tension at 0.05% (27.9 mN m−1 ). As the concentration increases, the surface tension values fluctuate slightly within the error range (within the error range), but overall trend is stable (27.90 mN m−1 , 30.17 mN m−1 , 28.89 mN m−1 ), so the critical micelle concentration of surfactant A2 is 0.05%. The surface tension of dilute solution of surfactant A5 decreases rapidly with increasing concentration (tension magnitudes are 27.03 mN m−1 , 26.14 mN m−1 in order) at concentrations between 0.005% and 0.05%. When reaching 0.05%, the surface tension basically tends to be constant (tension magnitudes are 26.14 mN m−1 , 26.18 mN m−1 , 26.12 mN m−1 in order), so the critical micelle concentration of surfactant A5 is 0.05%. The optimal concentrations of other surfactants (critical micelle concentration) are measured with the same method. The critical micelle concentrations of A1 –A8 are shown in Table 3.5. (2)

Wetting ability

The reverse osmosis method is used to measure the solution wetting rate, the relationship between wetting height and wetting time for different concentrations of A1 –A8 is shown in Figs. 3.21, 3.22, 3.23, 3.24, 3.25, 3.26, 3.27, 3.28, 3.29, 3.30, 3.31, 3.32, 3.33, 3.34, 3.35 and 3.36, where the time taken for water wetting to 6 cm is 529 s. From Fig. 3.21, in the initial stage of solution wetting dust, due to the air film on the surface of dust particles and water droplets, water droplets break through the air film and they can attach and condense each other only in the case that the two have high relative speeds. At this stage, due to the barrier of air film, the speed of solution wetting dust is relatively slow, and the speed of different concentrations of A1 wetting dust is not much different from that of clear water. With the increase of wetting height, i.e., more than 3 cm, the wetting speed gradually increases. Among them, the wetting speed of pure water gradually decreases and the time required becomes longer. In order to visualize the speed of dust wetting time of pure water and A1 with different concentrations, the higher the wetting height, the more obvious the difference in time required, so the relationship between the concentration of surfactant A1 and the time required to wet the dust to 6 cm is shown in Fig. 3.22. As can be seen from Fig. 3.22, after adding different concentrations of surfactant A1 , the wetting properties are all greatly improved compared with water. Among them, when the mass concentration is 0.005–0.05%, the wetting speed increases with

80

3 Chemical Suppression Technology of Open-Pit Mine …

Fig. 3.21 Dust wetting time of A1 with different mass concentrations

600

0.005% 0.05% 0.1% 0.5% Pure water

Wetting time/(s)

500 400 300 200 100 0 0

Fig. 3.22 Time of dust wetting to 6 cm of A1 with different mass concentrations

1

2

3 4 Wetting height/cm

5

6

530

Wetting time/s

520 510 500 490 480 470

0.0

0.1

0.2

0.3

0.4

0.5

Mass concentration/(%)

the increase of concentration (486, 500, 497, 481 s corresponding to the concentration in order), so the optimal wetting concentration is 0.5%. Meanwhile, the critical micelle concentration of A1 is also 0.5%, so the optimal concentration of surfactant A1 is chosen to be 0.5% by combining surface tension and wettability with Figs. 3.21 and 3.22. From Fig. 3.23, when the dust wetting to 1 cm, the difference of wetting time of A2 with different concentrations and pure water is small. With the elimination of dust air film, the wetting speed gradually accelerates, and at this time the wetting time changes significantly. The wetting time of 0.5% A2 is significantly higher than other solutions, followed by pure water. The wetting times of 0.005, 0.05, 0.1% A2 do not change significantly, and the relationship between concentration of surfactant A2 and its time required to wet the dust to 6 cm is shown in Fig. 3.24 below.

3.3 Development of Blasting Dust and Smoke Inhibitors … Fig. 3.23 Dust wetting time of A2 with different mass concentrations

0.005% 0.05% 0.1% 0.5% Pure water

600 500 Wetting time/(s)

81

400 300 200 100 0 0

1

2

3

4

5

6

Wetting height/m

Fig. 3.24 Time of dust wetting to 6 cm of A2 with different mass concentrations

620 600

Wetting time/s

580 560 540 520 500 480 460 440 0.0

0.1

0.2

0.3

0.4

0.5

Mass concentration/%

As shown in Fig. 3.24, within the A2 concentration of 0.005% to 0.1%, the wetting speed does not change significantly, and the time required for its wetting to 6 cm of the glass tube is 466, 470, and 461 s in order, which are basically close to each other with small fluctuations, but all of them are significantly faster than the wetting speed of pure water (529 s). When at 0.5%, the wetting speed is much slower than that of pure water, i.e. 604 s. Combined with its critical micelle concentration (0.1%) and Figs. 3.23 and 3.24, the concentration for the best performance of A2 is therefore taken as 0.1%. As shown in Fig. 3.25, the wetting time of A3 with different concentrations and that of water are the same when the solution touches the dust up to 1 cm, and the wetting time curves also basically overlap within 1–2 cm. After the wetting height is greater than 2 cm, the wetting time changes significantly, in which the wetting time

82

3 Chemical Suppression Technology of Open-Pit Mine …

Fig. 3.25 Dust wetting time of A3 with different mass concentrations

600

0.005% 0.05% 0.1% 0.5% Pure water

Wetting time/(s)

500 400 300 200 100 0 0

Fig. 3.26 Time of dust wetting to 6 cm of A3 with different mass concentrations

1

2

3 4 Wetting height/cm

5

6

580 570 Wetting time/(s)

560 550 540 530 520 510 500 0.0

0.1

0.2

0.3

0.4

0.5

Mass concentration/(%)

of 0.5% A3 is the longest, followed by pure water, and the relationship between the concentration of surfactant A3 and its time required to wet the dust to 6 cm is shown in Fig. 3.26. From Fig. 3.26 above, it can be seen that in the lower concentration range (0.005– 0.1%), comparing with water, the wettability of A3 solution is slightly improved and the difference is not very large, and the wetting time is 507 s, 498 s, and 519 s, respectively. However, when the concentration reaches 0.50%, the wettability of the solution decreases rapidly and takes the most time, i.e. 577 s, and is much slower than the wettability of water. And it is known that the critical micelle concentration of A3 is 0.05%, so 0.05% A3 is determined as the experimental surfactant. Analysis of Fig. 3.27 shows that when the solution touches the dust to 1 cm, the time for solution wetting dust is the same, similarly, when the dust is wetted

3.3 Development of Blasting Dust and Smoke Inhibitors … Fig. 3.27 Dust wetting time of A4 with different mass concentrations

83

600 0.005% 0.05% 0.10% 0.5% Pure water

Wetting time/(s)

500 400 300 200 100 0 0

2

4

6

8

10

Wetting height/(cm)

Fig. 3.28 Time of dust wetting to 6 cm of A4 with different mass concentrations

to 1.8 cm, the changes in wetting dust time with different concentrations gradually become obvious. And the wetting time of pure water is obviously the most, and the wetting speed of 0.1% A4 is slightly higher than that of other solutions. The relationship between the concentration of surfactant A4 and the time required to wet the dust to 6 cm is shown in Fig. 3.28. As shown in Fig. 3.28, the wetting rate of surfactant A4 is significantly higher than that of pure water in the range of mass concentration 0.005–0.5%, and the measured times are 470, 455, 432, 506 s in order. Within the concentration range of 0.005– 0.1%, the wetting rate is accelerated with the increasing concentration, and the best rate is at the concentration of 0.1%. The surface tension of surfactant A4 also reaches the lowest at 0.1%, and at the same time combined with Figs. 3.27 and 3.28, so the optimal performance concentration of A4 is 0.1%.

84

3 Chemical Suppression Technology of Open-Pit Mine …

Fig. 3.29 Dust wetting time of A5 with different mass concentrations

Fig. 3.30 Time of dust wetting to 6 cm of A5 with different mass concentrations

640 620

Wetting time/(s)

600 580 560 540 520 500 480 460 440 0.0

0.1

0.2

0.3

0.4

0.5

Mass concentration/(%)

From Fig. 3.29, it can be seen that the time for wetting to 1 cm of pure water and A5 with different mass concentrations is basically the same, and when the wetting height is greater than 1 cm, the wetting time of 0.5% A5 becomes significantly longer, which are higher than other concentrations, followed by pure water. While the difference in wetting time of 0.005, 0.05, 0.1% A5 is not large, and there is only a small interval of the curves, indicating that within the concentration of 0.005–0.1%, the concentration has little impact on the solution wetting performance. As shown in Fig. 3.30, the wetting speed of surfactant A5 does not change significantly within the concentration of 0.005–0.1%, which is basically close, but both are significantly faster than that of pure water. When the concentration is 0.5%, the wetting speed is much slower than that of pure water, which shows that the surfactant with high concentration does not mean good effects. Combined with its critical

3.3 Development of Blasting Dust and Smoke Inhibitors … Fig. 3.31 Dust wetting time of A6 with different mass concentrations

85

600 0.005% 0.05% 0.1% 0.5% Pure water

Wetting time/(s)

500 400 300 200 100 0 0

Fig. 3.32 Time of dust wetting to 6 cm of A6 with different mass concentrations

1

2

3 4 Wetting height/cm

5

6

0.2 0.3 0.4 Mass concentration/(%)

0.5

540 530

Wetting time/(s)

520 510 500 490 480 470 460

0.0

0.1

micelle concentration (0.05%), the concentration of A5 of the best performance is taken as 0.05%. The analysis of Fig. 3.31 shows that the difference in dust wetting time of the solution is very small when A6 solution with different mass concentrations contacts with dust to 0–2 cm. When the dust is wetted to 2 cm, the changes in wetting dust time with different concentrations gradually become obvious. The wetting time of pure water is obviously the most in the four stages after that, and the wetting time of 0.5% A6 is slightly faster than that of pure water, the time of A6 with remaining three concentrations is basically close. The relationship between the concentration of surfactant A6 and the time required to wet the dust to 6 cm is shown in Fig. 3.32. As can be seen from the analysis in Fig. 3.32, after adding A6 with different concentrations, comparing with water, the wetting speed of the solution is greatly

86

3 Chemical Suppression Technology of Open-Pit Mine … 600

Fig. 3.33 Dust wetting time of A7 with different mass concentrations

0.005% 0.05% 0.1% 0.5% Pure water

Wetting time/(s)

500 400 300 200 100 0 0

Fig. 3.34 Time of dust wetting to 6 cm of A7 with different mass concentrations

1

2 3 4 Wetting height/(cm)

5

6

7

530

Wetting time/(s)

520 510 500 490 480 0.0

Fig. 3.35 Dust wetting time of A8 with different mass concentrations

0.2 0.3 0.4 Mass concentration/(%)

0.5

0.005% 0.05% 0.1% 0.5% Pure water

600 500

Wetting time/(s)

0.1

400 300 200 100 0 0

1

2 3 4 Wetting height/(cm)

5

6

3.3 Development of Blasting Dust and Smoke Inhibitors … Fig. 3.36 Time of dust wetting to 6 cm of A8 with different mass concentrations

87

600

Wetting time/(s)

580 560 540 520 500 480 0.0

0.1

0.2 0.3 0.4 Mass concentration/(%)

0.5

improved. Among them, except for the longer wetting time (i.e., 502 s) at the concentration of 0.50%, the wetting time is similar in the range of 0.005–0.10%, and the wettability is better. And the critical micelle concentration of A6 is 0.05%. Therefore, the optimal performance concentration of surfactant A6 is 0.05%. As shown in Fig. 3.33, determined by the nature of the A7 solution itself, the time curves of the five concentrations basically overlap in the range of 0–4 cm of the wetting height of the dust, indicating that the air film on the surface of the dust particles and water droplets is not broken at this stage, and the solution rises relatively slowly, basically the same as pure water. After the wetting height is greater than 4 cm, the air film barrier disappears, the wetting time varies significantly due to the concentration of the solution itself, of which pure water has the longest wetting time. The wetting time of A7 with other four concentrations is not significantly different, and the relationship between the concentration of surfactant A7 and its time required to wet the dust to 6 cm is shown in Fig. 3.34. From Fig. 3.34, it can be seen that within the mass concentration of 0.005–0.5%, the wetting speed of surfactant A7 does not change significantly and is basically close, but all of them are significantly faster than the wetting speed of pure water with the wetting time of 482, 484, 483 and 482 s in order. This indicates that the concentration has little effect on the wetting performance of A7 within this concentration range. Combining its surface tension and critical micelle concentration (0.05%), it is concluded that the concentration of A7 with the best performance is taken as 0.05%. As can be seen from Fig. 3.35, the wetting time of surfactant A8 with different concentrations and that of pure water is the same when the solution touches the dust up to 1 cm, and the changes in wetting time become significant after the wetting height rises to 1 cm. The wetting time of 0.5% A8 is the longest, followed by pure water and 0.1% A8 . The relationship between the concentration of surfactant A8 and the time required to wet the dust to 6 cm is shown in Fig. 3.36.

88

3 Chemical Suppression Technology of Open-Pit Mine …

Table 3.6 Optimal performance concentration of surfactants A1 –A8 Surfactant

A1

A2

A3

A4

A5

A6

A7

A8

Optimal performance concentration/%

0.5

0.1

0.05

0.1

0.05

0.05

0.05

0.05

From Fig. 3.36, within the mass concentration range of 0.005–0.5%, for surfactant A8 , the time required to wet the dust increases with the increase in concentration (479, 483, 526, 584 s in order), that is, the wetting speed decreases in turn, and the wetting speed is not much different in the range of 0.005–0.05%. However, at a concentration of 0.5%, the wetting speed is much slower than that of pure water, combined with its optimal concentration of 0.05%, so the concentration of the best performance of A8 is taken as 0.05%. In summary, the optimum performance concentrations of the eight surfactants are obtained by combining the surface tension and wettability of surfactants A1 –A8 , which is as shown in Table 3.6. (2)

Hygroscopic agents

The hygroscopic agents are generally inorganic salts with relatively strong water absorption, so this experiment selects hygroscopic agent B1 and hygroscopic agent B2 as the experimental objects, and configure two hygroscopic solutions with 0.1, 0.5, 1, 3, 5, 8, 10% and other seven different mass concentrations, using BZY-201 surface tension meter platinum plate method to measure the surface tension. The reverse osmosis experiment is adopted to measure the wetting performance, the experimental results are shown in Table 3.7, Figs. 3.37 and 3.38. The surface tension value of pure water measured in the experiment is 72.43 mN/m, combined with Table 3.7 surface tension measurement results, it can be seen that with the mass concentration of 0.1–3%, there is little difference in the surface tension value of the hygroscopic agent B1 and B2 and water, so the impact on the surface tension of water is small; when the mass concentration is 3–10%, the surface tension begins to gradually increase and is greater than the surface tension value of water, which is not conducive to the capture of dust and smoke by the solution, so it is excluded. As can be seen from Fig. 3.37, similar to the nature of wetting dust of the surfactant, due to the barrier of air film, the wetting speed of wetting dust of hygroscopic agent B1 in 1–2 cm height is relatively slow, wetting time difference is not significant. With the increase in wetting height, air film is eliminated by the solution, and the wetting speed are gradually increased. The change in the time of wetting dust increased because Table 3.7 Surface tension of two hygroscopic agents mN/m Hygroscopic agent

Mass concentration/% 0.1

0.5

1

3

5

8

10

B1

72.70

72.80

72.64

72.32

73.89

75.06

75.77

B2

72.36

72.93

72.98

72.09

73.08

73.26

74.64

3.3 Development of Blasting Dust and Smoke Inhibitors … Fig. 3.37 Dust wetting time of hygroscopic agent B1 with different concentration

0.1% 0.5% 1.0% 3.0% 5.0% 8.0% 10.0% Pure water

600 500 Wetting time/(s)

89

400 300 200 100 0 0

1

2 3 4 Wetting height/(cm)

5

6

600

Fig. 3.38 Dust wetting time to 6 cm of hygroscopic agent B1 with different concentrations

580

Wetting time/(s)

560 540 520 500 480 460 0

2

4 6 8 Mass concentration/(%)

10

of the different concentration of the solution. When B1 concentration is 10%, its wetting time in different height are greatly higher than other solutions, and the time consumption for 0.5% B1 dust wetting to 3, 4, 5, 6 cm is the least. The relationship between the concentration of the hygroscopic agent B1 and its time required to wet the dust to 6 cm is shown in Fig. 3.38. It can be seen from Fig. 3.38 that the times for hygroscopic agent B1 with mass concentration of 0.1, 0.5, 1, 3, 5, 8, 10% to wet the dust in the glass tube to the 6 cm are 496, 468, 485, 491, 548, 543, 583 s in order, and it shows a trend that the wetting time first decreases and then increases with the increase of mass concentration. When the concentration is greater than 5%, the wetting speed is slower than that of pure water, and it has the fastest wetting speed at a concentration of 0.5%. Combined with the results of surface tension in Table 3.7 (the surface tension value of 0.1–0.3% is

90

3 Chemical Suppression Technology of Open-Pit Mine …

basically not much different), the best mass concentration of hygroscopic agent B1 is 0.5%. From Fig. 3.39, it can be seen that, similar to hygroscopic agent B1 , the time required for the initial stage of wetting the dust is not much different, and at the concentration of 10%, the wetting time of hygroscopic agent B2 in different height stages are much higher than that of other solutions. When the concentration is 0.5%, it has the least wetting time, and the relationship between the concentration of hygroscopic agent B2 and its time required for wetting the dust to 6 cm is shown in Fig. 3.40. According to the experimentally measured data and analysis of Fig. 3.40, it can be seen that the wetting time of hygroscopic agent B2 increases with the increase in mass concentration, that is, 515, 474, 490, 500, 522, 531 and 586 s in order. It is the same as the hygroscopic agent B2 , showing the trend of rising first and then falling, Fig. 3.39 Dust wetting time of hygroscopic agent B2 with different concentrations

600

0.1% 0.5% 1.0% 3.0% 5.0% 8.0% 10% Pure water

Wetting time/(s)

500 400 300 200 100 0 0

1

2 3 4 Wetting height/(cm)

5

6

7

600

Fig. 3.40 Dust wetting time to 6 cm of hygroscopic agent B2 with different concentrations

580

Wetting time/(s)

560 540 520 500 480 460

0

2

4 6 8 Mass concentration/(%)

10

3.3 Development of Blasting Dust and Smoke Inhibitors …

91

that is, the wetting speed first accelerates and then slows down. It also has the fastest wetting speed with the concentration of 0.5%. Similarly, according to the surface tension results of hygroscopic agent B2 in Table 3.7, the optimal mass concentration of B2 is 0.5%. To sum up, after comparing 0.5% hygroscopic agent B1 and 0.5% hygroscopic agent B2 , the surface tension value is similar, and the wettability of B1 is better than that of B2 . Therefore, the hygroscopic agent B1 is selected as the component of dust and smoke inhibitor with the optimal mass concentration of 0.5%. (3)

Gelling agents

According to the nature of gelling agents, three gelling agents C1 , C2 and C3 are initially selected for this experiment. Based on surfactants A2 , A5 and hygroscopic agent B1 , the surface tension and wettability of the solution after adding gelling agents with different concentrations are measured. In order to make the experimental data comparable under the same conditions, it is necessary to test the blank results under the same conditions. The experimental results are shown in Table 3.8. From the results in Table 3.8, it can be seen that the surface tension of the solution is 24.57 mN m−1 without the addition of the gelling agent and the wetting speed is 89.1 s cm−1 . After adding the three gelling agents with different concentrations, the changes in surface tension are not significant, and the maximum surface tension value is 30.14 mN m−1 (2% C1 ) and the minimum surface tension value is 24.16 mN Table 3.8 Solution surface tension and wetting rate after gelling agent Mass concentration/%

Serial number

Surface tension/mN m−1 Once

Twice

3 times

Average

Don’t add

0

1

24.43

24.76

24.53

24.57

89.1

C1

0.50

2

26.42

26.67

26.2

26.43

86.6

1

3

27.41

27.86

27.54

27.60

84.6

Gels

C2

C3

Wetting speed/s cm−1

2

4

30

30.18

30.24

30.14

88.7

0.10

5

24.69

25.23

25.26

25.06

454.5

0.20

6

24.29

25.1

25.81

25.07

783.3

0.30

7

24.3

24.12

25.06

24.49

0.03

8

–

–

–

–

112.1

0.005

9

–

–

–

–

92.4

0.01

10

–

–

–

–

95.3

0.02

11

–

–

–

–

111.1

0.05

12

24.65

25.24

25.29

25.06

82.9

0.10

13

23.95

23.82

24.21

23.99

84.4

0.20

14

24.22

23.94

24.33

24.16

86.9

0.0

92

3 Chemical Suppression Technology of Open-Pit Mine …

m−1 (0.2% C3 ), indicating that the gelling agents have minimal effect on the surface tension of the solution and all of them can significantly reduce the surface tension of water. In addition, after adding the gelling agent C2 , the speed of wetting dust is extremely slow with the longest wetting time of up to 4700 s and the shortest time of about 555 s. Considering the practical application in the excavation blasting of underground mine, the time should not be too long, so gelling agent C2 is excluded; After adding the gelling agent C3 , the wetting speed is slightly better than gelling agent C2 , but it is very easy to have agglomeration during the stirring and dissolving process (see Fig. 3.41 for the effect), resulting in uneven distribution of solution concentration, which is not conducive to dust suppression and toxicity reduction, so it is also not considered. The wetting performance after adding gelling agent C1 is shown in Fig. 3.42. Fig. 3.41 Effect picture after adding gelling agent C3

Fig. 3.42 Time of wetting dust after adding gelling agent C1 with different concentrations

600 0.0% 0.5% 1.0% 2.0%

Wetting time/(s)

500 400 300 200 100 0 0

1

2 3 4 Wetting height/(cm)

5

6

3.3 Development of Blasting Dust and Smoke Inhibitors … Fig. 3.43 Dust wetting time to 6 cm of gelling agent C1 with different concentration

93

535

Wetting time/(s)

530 525 520 515 510 505

0.0

0.5 1.0 1.5 Mass concentration/(%)

2.0

By analyzing the Fig. 3.42, it can be obtained that when the wetting height in the range of 0–2 cm, the time of dust wetting of gelling agent C1 with different concentrations and pure water is basically the same. When the wetting height is greater than 2 cm, the wetting time begins to slowly change, where the wetting time of pure water is slightly greater than the C1 solution when the wetting height is more than 2 cm after the four stages, the wetting time of 1% solution is the smallest, and the relationship between the concentration of gelling agent C1 and its time required to wet the dust to 6 cm relationship is shown in Fig. 3.43. As shown in Fig. 3.43, compared with no addition of gelling agent C1 , the wetting speed of the solution after the addition of C1 is slightly increased, of which the best wetting effect at a concentration of 1%, and after the addition of C1 , the solution is slightly gelatinous, which greatly increases the viscosity of the water stemming and has a strong adsorption force, which is more conducive to capture dust and improve the efficiency of dust reduction. (4) (1)

Toxicity reducing agent Decreasing nitrogen oxides

D1 and D2 have been selected as the toxicity reducing agents, and their toxin-reducing effects are measured. The solutions of two toxicity reducing agents with five different concentrations (0.4 mL) are sprayed into a glass bottle containing equal amount of nitrogen oxides (0.6 mL), and after 3 min, they are withdrawn and passed into the absorption liquid, and the absorbance of the absorption liquid is measured by spectrophotometer. The blank results are measured under the same conditions. The results are shown in Table 3.9 and Fig. 3.44. The absorbance of the nitrogen dioxide absorption liquid is 0.489, measured under blank conditions. From the analysis of Table 3.9 and Fig. 3.44, it is known that with the increasing concentration of the toxicity reducing agents D1 and D2 , the corresponding absorbance decreases continuously, and the toxicity reducing rates become higher. When the concentrations are 0.005–0.03%, the absorbance decreases

94

3 Chemical Suppression Technology of Open-Pit Mine …

Table 3.9 Absorbance of toxicity reducing agent with different concentrations

Reagent

Mass concentration/%

Absorbance

Nothing

0

0.489

D1

D2

Fig. 3.44 Absorbance curve of absorption liquid sprayed with toxicity reducing agent D with different mass concentrations

Detoxification rate/%

0.005

0.074

84.87

0.01

0.044

91.00

0.03

0.025

94.89

0.05

0.01

97.96

0.1

0.019

96.11

0.005

0.239

51.12

0.01

0.144

70.55

0.03

0.045

90.80

0.05

0.013

97.34

0.1

0.004

99.18

0.5 D1 D2

Absorbance

0.4 0.3 0.2 0.1 0.0 0.00

0.02 0.04 0.06 0.08 Mass concentration/(%)

0.10

significantly, indicating that the toxicity reducing rates increase significantly, and after their concentrations are greater than 0.03%, the change trend tends to level off and the toxicity reducing rates are relatively stable. In terms of toxicity reducing, the difference between D1 and D2 is not obvious. However, since D1 is more stable than D2 in terms of physicochemical properties, D1 is chosen as the toxicity reducing agent for NOx from the perspective of effectiveness and safety with the best mass concentration of 0.03%. (2)

Decreasing carbon monoxide

A certain amount of 99.99% CO is extracted with a syringe and passed into two solutions (E1 and E2 ) with three concentrations of 0.05, 0.1, and 0.2%. After 3 min, it is withdrawn into a sampling bag and detected with a Shimadzu GC-2014C gas chromatograph (argon as the carrier gas, and the detector is a thermal conductivity cell

3.3 Development of Blasting Dust and Smoke Inhibitors …

95

detector) to derive the remaining CO concentration, detecting its toxicity reducing effect. The experimental data as well as the curve trends are shown in Table 3.10 and Fig. 3.45. Under the same conditions, the concentration of CO is 9.04347% after the same amount of CO passes through the same volume of clean water as the toxicity reducing solution. It can be analyzed from Table 3.10 and Fig. 3.45 that when the concentration of E1 and E2 is 0.05–0.1%, the CO concentration decreases rapidly, indicating that the toxicity reducing rate increases significantly. When their mass concentration is 0.1–0.2%, the CO concentration does not change much, and the toxicity reducing rate does not increase again, so the optimal concentration of both toxicity reducing agents is 0.1%. No matter in which concentration range, the rangeability of CO concentration reduction of E1 is greater than that of E2 , that is, the toxicity reducing effect of the former is better than the latter, so E1 has been chosen as the toxicity reducing agent of CO with the optimal reduction concentration of 0.1%. Table 3.10 CO concentration

Type of disinfectant

Mass concentration/%

CO concentration/%

Nothing

0

9.04347

E1

0.05

2.99426

0.1

0.88786

0.2

0.57153

0.05

6.84986

0.1

1.21776

0.2

0.85224

E2

10

Carbon monoxide concentration/(%)

Fig. 3.45 Concentration curve of CO with toxicity reducing agent E with different concentrations

E1 E2

8 6 4 2 0 0.00

0.05 0.10 0.15 Mass concentration/(%)

0.20

96

3 Chemical Suppression Technology of Open-Pit Mine …

3.3.4 Compounding Experiments of Surfactants and Hygroscopic Agents (1)

Based on the 0.5% hygroscopic agent B1

Based on the hygroscopic agent B1 with the concentration of 0.5%, the surfactants A1 (0.5%), A2 (0.1%), A3 (0.05%), A4 (0.1%), A5 (0.05%), A6 (0.05%), A7 (0.05%), and A8 (0.05%) are compounded two by two to determine the surfactant type and optimal concentration according to the formula performance. Through 27 groups of experiments, the surface tension and wetting speed are measured. In order to visualize the magnitude of the two indicators in the 27 groups of formulas, the change trend graphs are made and shown in Figs. 3.46 and 3.47. 40

Surface tension/(mN/m)

Fig. 3.46 Surface tension of surfactants compounding based on the 0.5% hygroscopic agent B1

35

30

25

20

5

10 15 Serial number

20

25

90

Fig. 3.47 Wetting speed of surfactants compounding based on the 0.5% hygroscopic agent B1

88 Wetting velocity/(s/cm)

86 84 82 80 78 76 74 72 70 68

2

4

6

8

10 12 14 16 18 20 22 24 26

Serial number

3.3 Development of Blasting Dust and Smoke Inhibitors …

97

Through the 27 groups of compounding experiments based on 0.5% hygroscopic agent B1 , the results of surface tension of different formulas in Fig. 3.46 show that the surface tension fluctuates in the range 24.12–35.48 mN m−1 in different degrees. No matter what the compounding scheme, all greatly reduce the surface tension of water (72.43 mN m−1 ), and after sorting, the top five of surface tension from small to large are the 16th group (24.12 mN m−1 ), 6th groups (24.63 mN m−1 ), 7th group (25 mN m−1 ), 23rd group (25.59 mN m−1 ), 3rd group (25.61 mN m−1 ). The change in surface tension of these five groups is only 1.49 mN m−1 , which is not large, so all of them can be taken into account. Similarly, after analyzing the results of the wetting speed of each compounding scheme in Fig. 3.47, it shows that the fastest wetting speed is 69.5 s cm−1 (group 1) and the slowest is 87.4 s cm−1 (group 15) among the 27 groups of formulas, with a variation of 17.9 s cm−1 , indicating that different formulas have a relatively large effect on the wetting performance of the solutions. After sorting and comparing, the top five of wetting speed of compound solution based on hygroscopic agent B1 from fast to slow are 1st group (69.5 s cm−1 ), 11th group (71.1 s cm−1 ), 12th group 12 (71.4 s cm−1 ), 7th group (72.2 s cm−1 ), and 4th groups (74.1 s cm−1 ), of which the first four groups have basically no great fluctuations in wetting speed, therefore, they can be regarded as basically the same. Combining the top five of surface tension and wetting speed, it can be seen that 7th group has the best performance, namely surfactants A2 (0.1%) and A5 (0.05%). (2)

Based on the 0.5% hygroscopic agent B2

In order to ensure the accuracy of the experimental inhibitor formula, although the performance of hygroscopic agent B1 is better than that of B2 , taking the interaction and influence between hygroscopic agent type and surfactant into account, under permissible conditions, 0.5% B2 is regarded as the base to make the experimental data and results more accurate. The surfactants A1 –A8 are compounded two by two to determine the optimal hygroscopic agent most intuitively, and the results of surface tension and wetting speed are shown in Figs. 3.48 and 3.49. From Fig. 3.48, the maximum value of surface tension is 35.39 mN m−1 (19th group), the minimum value is 25.27 mN m−1 (17th group), and the surface tension of 27 groups formulas does not fluctuate much. In addition, the top five of the surface tensions in descending order are 17th group (25.27 mN m−1 ), 23rd group (25.97 mN m−1 ), 12th group (26.09 mN m−1 ), 7th group (26.28 mN m−1 ), and 22nd group (26.56 mN m−1 ), and the surface tension of these five groups varies very little. Similarly, as can be seen from Fig. 3.49 that after sorting and comparing, the top five of the wetting speed of the compound solution based on hygroscopic agent B2 from fast to slow are 3rd group (73.7 s cm−1 ), 8th group (74.3 s cm−1 ), 4th group (76.2 s cm−1 ), 7th group (77.3 s cm−1 ), and 5th group (77.6 s cm−1 ). Combining the results of surface tension and wetting speed, it can be seen that 7th group still has the best performance, i.e. surfactants A2 (0.1%) and A5 (0.05%). In conclusion, due to the same kind of surfactant of corresponding number in the two groups of experiments, combining with Figs. 3.48 and 3.49, the surface tension results of surfactants compounding do not change significantly after adding

98

3 Chemical Suppression Technology of Open-Pit Mine …

Fig. 3.48 Surface tension of surfactants compounding based on the 0.5% hygroscopic agent B2

B 36

Surface tension/(mN/m)

32

28

24

20

16

Fig. 3.49 Wetting speed of surfactants compounding based on the 0.5% hygroscopic agent B2

5

10 15 Serial number

20

25

92

Wetting velocity/(s/cm)

88 84 80 76 72 68

2

4

6

8

10 12 14 16 18 20 22 24 26 Serial number

hygroscopic agent B1 and hygroscopic agent B2 , indicating that the hygroscopic agent has little effect on the surface tension, but the wetting speed of the former is better than the latter, so the optimal hygroscopic agent is still 0.5% B1 .

3.3.5 Final Selection of Surfactant and Hygroscopic Agent Based on the hygroscopic agent B1 , the two-two compounding of surfactants can determine the types of surfactants A2 and A5 , but considering the mutual influence

3.3 Development of Blasting Dust and Smoke Inhibitors …

99

Table 3.11 Design of orthogonal experiment table Reagent number

Mass concentration/% B1

A2

A5

Level 1

0.3

0.05

0.03

Level 2

0.5

0.10

0.05

Level 3

0.7

0.20

0.08

between the size of A2 , A5 and B1 concentrations, a three-factor, three-level orthogonal experiment has been designed to ensure the experimental effect and quality. The design of orthogonal experiment table, experimental scheme and results are shown in Tables 3.11 and 3.12. The results of evaluating the formulas performance with surface tension and wetting speed as evaluation indexes are shown in Tables 3.13 and 3.14. As can be seen from Table 3.13, the magnitude of T1 , T2 and T3 reflects the effects of the three levels of concentration of B1 , A2 , and A5 on the surface tension of the solution. Since the smaller, the surface tension the better, it can be visualized that the combination with the lowest surface tension is the third level of B1 , the third level of A2 , and the first level of A5 , i.e., 0.7% B1 + 0.20% A2 + 0.03% A5 . Table 3.12 Scheme and results of orthogonal experiment Group

Reagent type and mass concentration (%)

Surface tension mN m−1

Wetting velocity s cm−1

B1

A2

A5

1

0.3

0.05

0.03

24.31

91

2

0.3

3

0.3

0.10

0.05

24.70

88.92

0.20

0.08

24.75

4

88.75

0.5

0.05

0.05

25.27

81.25

5

0.5

0.10

0.08

25.22

83

6

0.5

0.20

0.03

23.86

83.25

7

0.7

0.05

0.08

24.73

84.25

8

0.7

0.10

0.03

24.57

81.75

9

0.7

0.20

0.05

23.97

82.83

Table 3.13 Data analysis table with surface tension as evaluation index

Surface tension/mN m−1

B1

A2

A5

T1

24.59

24.77

24.25

T2

24.78

24.83

24.65

T3

24.42

24.19

24.90

R

0.36

0.58

0.65

100

3 Chemical Suppression Technology of Open-Pit Mine …

Table 3.14 Data analysis table with wetting speed as evaluation index

Wetting velocity/s cm−1  T1  T2  T3 

R

B1

A2

A5

89.62

85.22

85.38

82.55

84.62

84.38

83.00

85.00

85.38

7.07

0.6

1.0

Similarly, Table 3.14 shows that the formula having the best wetting performance with the wetting speed as an evaluation index is the second level of B1 , the second level of A2 , and the second level of A5 , i.e., 0.5% B1 + 0.10% A2 + 0.05% A5 . In order to more intuitively reflect the influence law and trend of the three factors of the orthogonal experiment on surface tension and wetting speed, the vertical  coordinates are set as the mean T and T1 , the horizontal coordinates are set as the factor level, and the factor level and index trend graphs are drawn, as shown in Figs. 3.50 and 3.51. When surface tension is used as the evaluation index, it can be seen from Table 3.13 and Fig. 3.50 that the ranges of the three levels of the three components B1 , A2 , and A5 are 0.36, 0.58, and 0.65, and the effects of the three levels of each factor on the index are extremely small. The surface tensions all fluctuate slightly around 24.6 mN m−1 , indicating that hygroscopic agent B1 , surfactant A2 and surfactant A5 have little effect on the surface tension of the solution, which can be basically ignored. A2

A5 25

24.8

24.8

24.8

24.6

24.6

24.6

T

25

T

T

B1 25

24.4

24.4

24.4

24.2

24.2

24.2

24

24

24

1

2

3

1

2

1

3

2

3

Fig. 3.50 Horizontal mean of each factor with surface tension as evaluation index

90

88

88

88

86 84

T

90

T

T

A5

A2

B1 90

86

84

84

82 1

2

3

86

82

82 1

2

3

1

Fig. 3.51 Horizontal mean of each factor with wetting speed as evaluation index

2

3

3.3 Development of Blasting Dust and Smoke Inhibitors …

101

When the wetting speed is used as the evaluation index, it can be analyzed from Table 3.14 and Fig. 3.51 that the ranges of each level of the three factors B1 , A2 and A5 are 7.07, 0.6 and 1.0, so the influence of the hygroscopic agent B1 is the largest, followed by the surfactant A5 , while the influence of A2 is the smallest, that is, the different concentrations of the components have a greater impact on the wetting speed of the solution. In summary, since the concentrations of B1 , A2 and A5 have very little effect on surface tension and a greater effect on wetting performance, the former can be ignored, and the formula is mainly based on wetting performance as an evaluation index. Therefore, the optimal concentrations of surfactant A2 , A5 and hygroscopic agent B1 in the orthogonal compounding experiments are 0.05%, 0.10% and 0.5%, which are consistent with the results of compounding, i.e. 0.05% A2 + 0.10% A5 + 0.5% B1.

3.3.6 The Optimal Formula of the Dust and Smoke Inhibitor (1)

Orthogonal experiment on the optimal formula

The formulas contain a variety of chemical components such as surfactants, hygroscopic agents, gelling agents, and toxicity reducing agents, etc. Although the optimal concentration of each component has been selected, the performance of the formulas is slightly affected by their possible interactions (It has been verified that the performance of hygroscopic agent and gelling agents after mixing with surfactant and toxicity reducing agent is relatively stable, which should not consider here), so an orthogonal experiment table of L9 (34 ) is designed according to a 4-factor, 3level scheme, and the optimal formula has been obtained. The design of orthogonal experiment table is shown in Table 3.15. The experiments are conducted according to the experimental combinations shown in the orthogonal experiment table, and the experimental scheme and the results of surface tension and wetting speed data are shown in Tables 3.16 and 3.17. (2) (1)

Data analysis and determination of the optimal formula. Data analysis with surface tension as the index.

Through the intuitive analysis of the data, the average values of the surface tension of three corresponding levels of each factor, i.e. T1 , T2 and T3 are calculated to reflect Table 3.15 Design of orthogonal experiment table Reagent number

Mass concentration/% A2

A5

D1

E1

Level 1

0.05

0.04

0

0

Level 2

0.10

0.05

0.03

0.05

Level 3

0.15

0.06

0.05

0.01

102

3 Chemical Suppression Technology of Open-Pit Mine …

Table 3.16 Scheme of orthogonal experiment Group

Reagent type and mass concentration (%) B1

C1

A2

A5

D1

E1

1

0.5

1

0.05

0.04

0

0

2

0.5

1

0.05

0.05

0.03

0.05

3

0.5

1

0.05

0.06

0.05

0.10

4

0.5

1

0.10

0.04

0.03

0.10

5

0.5

1

0.10

0.05

0.05

0

6

0.5

1

0.10

0.06

0

0.05

7

0.5

1

0.15

0.04

0.05

0.05

8

0.5

1

0.15

0.05

0

0.10

9

0.5

1

0.15

0.06

0.03

0

Table 3.17 Comprehensive table of surface tension and wetting speed of orthogonal experiment Group

Surface tension mN m−1

Wetting velocity s cm−1

1 time

2 time

3 times

Average

1

26.57

27.47

26.95

27.00

78.7

2

24.75

24.95

25.33

25.01

80.8

3

25.27

25.54

26.04

25.62

83.5

4

24.35

24.92

25.18

24.70

75.5

5

24.37

24.44

25.73

24.85

78.8

6

26.47

25.52

26.56

26.18

79.3

7

24.15

23.61

23.55

24.15

81.3

8

27.55

27.43

27.19

27.39

75.2

9

25.44

25.68

24.93

25.35

79.0

the superiority and inferiority of its performance, and the magnitude of the influence of each factor on the surface tension can be seen from the “range”, i.e. the difference between the maximum value and the minimum value of the average value of each level of the factor. Detailed calculation results are shown in Table 3.18. Table 3.18 Range analysis form with surface tension as evaluation index Surface tension/mN m−1

A2

A5

D1

E1

T1

25.88

25.28

26.86

25.73

T2

25.24

25.75

25.02

25.11

T3

25.63

25.72

24.87

25.90

R

0.64

0.47

1.99

0.79

3.3 Development of Blasting Dust and Smoke Inhibitors …

103

Since the smaller, the better the surface tension, it can be visualized from Table 3.18 that the formulas with the lowest surface tension of the solution are the second level of A2 , the first level of A5 , the third level of D1 and the second level of E1 , i.e. 0.1% A2 + 0.04% A5 + 0.05% D1 + 0.05% E1 . In order to more intuitively reflect the law and trend of the influence of the four factors of the comprehensive orthogonal experiment on the surface tension, the vertical coordinate is set as the mean value —T and the horizontal coordinate is set as the factor level, and the trend graph of the factor level and index is drawn, as shown in Fig. 3.52. By analyzing Table 3.18 and Fig. 3.52, it can be seen that the range of the three levels of the four components A2 , A5 , D1 , and E1 are 0.64, 0.47, 1.99, and 0.79, where the factor D1 has the greatest influence on the surface tension, and the three levels of the other factors A2 , A5 , and E1 have little influence on the index, as can be seen from the figure, the surface tensions are all in the range of about 25.5 mN m−1 . (2)

Data analysis with wetting speed as the index A2

A5

28

28

27.5

27.5

27

27

26.5

26.5

26

26

25.5

25.5

25

25

24.5

24.5

24

24 1

2

3

1

2

3

E1

D1 28

28

27.5

27.5

27

27

26.5

26.5

26

26

25.5

25.5

25

25

24.5

24.5

24

24 1

2

3

1

2

Fig. 3.52 Horizontal mean of each factor with surface tension as evaluation index

3

104

3 Chemical Suppression Technology of Open-Pit Mine …

Table 3.19 Range analysis form with wetting speed as evaluation index Wetting velocity/s cm−1  T1  T2  T3 

R

A2

A5

D1

E1

81.0

78.5

77.7

78.8

77.8

78.3

78.5

80.5

78.5

80.7

81.2

78.0

3.2

2.4

3.5

2.5

Same as above, the average values of the wetting speed of three corresponding    levels of each factor, i.e., T1 , T2 , T3 , are calculated and the results are shown in Table 3.19. From Table 3.19, it can be seen that since the faster the wetting rate, the better the effect, the formulas with the best solution wetting performance are the second level of A2 , the first (second) level of A5 , the first level of D1 , and the third level of E1 , i.e., 0.1% A2 + 0.04% A5 + 0% D1 + 0.1% E1 . Same as above, the vertical coordinate is set as the mean —T and the horizontal coordinate is set as the factor level, and the factor level and index trend chart is drawn, as shown in Fig. 3.53. Analysis of Table 3.19 and Fig. 3.53 shows that the ranges of the four components A2 , A5 , D1 and E1 at three levels are 3.2, 2.4, 3.5 and 2.5 with the wetting speed as the evaluation index, where also the toxicity reducing agent D1 has the greatest influence on the wetting performance, followed by surfactant A2 , and surfactant A5 and toxicity reducing agent E1 have the least influence on the index. In summary, D1 concentration has the greatest effect on both surface tension and wetting properties of water stemming dust and smoke inhibitor formulas. For surface tension, the surface tension of D1 in descending order is the second level (25.02 mN m−1 ), the third level (24.87 mN m−1 ), and the first level (26.86 mN m−1 ). For the wetting performance, the wetting speed of D1 in ascending order is the first level (77.7 s cm−1 ), the second level (78.5 s cm−1 ), and the third level (81.2 s cm−1 ). Combining the two evaluation indexes, the best quality concentration of the toxicity reducing agent D1 is the second level, i.e. 0.03% D1 . Since the concentrations of A2 , A5 and E1 have little effect on the surface tension of the formulas, the concentrations of A2 , A5 and E1 have the smallest effect on the wetting speed of the formulas with little difference. Given that the concentration of E1 has a greater effect on the wetting speed of the formulas, the wetting speed is the main factor, and the optimal concentration of the toxicity reducing agent E1 is chosen to be 0.1% (the third level). In addition, the selection results of the two indexes of A2 and A5 are the same, which are the second level of A2 and the first level of A5 , that is, 0.1% A2 + 0.04% A5 . Therefore, the best ratio of each component of the dust suppressing and toxicity reducing agent formula in the final excavation and blasting water stemming is A2 :A5 :B1 :C1 :D1 :E1 = 0.1%:0.04%:0.5%:1%:0.03%:0.1%.

3.3 Development of Blasting Dust and Smoke Inhibitors …

105

A2

A5

82

82

81

81

80

80

79

79

78

78

77

77

76

76

75

75

1

2

3

1

D1

2

3

E1

82

82

81

81

80

80

79

79

78

78

77

77

76

76 75

75 1

2

3

1

2

3

Fig. 3.53 Horizontal mean of each factor with wetting speed as evaluation index

(3)

Measurement of toxicity reduction of the optimal formula

From the above section, it can be known that the optimal dust and smoke inhibitor adding surfactants, hygroscopic agents, gelling agents and toxicity reducing agents has the lowest surface tension and the best wetting performance with high adhesive property, which can greatly improve the degree of atomization and the ability to capture dust after water stemming blasting. In order to ensure the toxicity reduction of the optimal formula, the NOx and CO reduction performance is measured separately. 0.4 ml of the optimal formulation of dust and smoke inhibitor solution is sprayed into 0.6 ml NOx in a glass bottle, and then it is withdrawn and passed into the absorption liquid after 3 min, and the absorbance of the absorption liquid is measured with a spectrophotometer, and the blank results are determined under the same conditions. Then 50 ml of 99.99% CO is slowly passed into 100 ml dust and smoke inhibitor solution, which is also detected by Agilent GC-2014C gas chromatograph to obtain

106 Table 3.20 Toxicity reduction of water stemming dust and smoke inhibitor solution of optimal formula

3 Chemical Suppression Technology of Open-Pit Mine … Detoxification index formula Blank Optimal formula Detoxification rate

Absorbance 0.576 0.021 96.4%

CO concentration/% 11.72901 1.20319 89.7%

the concentration of the remaining CO, detecting the toxicity reduction effect, and the measured data are shown in Table 3.20. As can be seen from Table 3.20, the optimal formula of dust and smoke inhibitor has a high ability to reduce nitrogen oxides and carbon monoxide with the toxicity reducing rate reaching more than 89%. Combined with its lowest surface tension and wettability, this dust and smoke inhibitor can greatly improve the dust reduction effect of water stemming, making the dust more easily wetted by liquid droplets, thus increasing the degree of atomization, and the ability of bonding dust and capturing dust after the blasting of water stemming.

3.4 Research on Blasting Dust Reduction with Foam Foam dust removal is a new dust removal technology, and foreign countries such as the United States, the former Soviet Union, Canada, West Germany and Eastern Europe focused on the research of wetting agent and foam dust removal agent (referred to as foaming agent) from the 1970s in order to improve the efficiency of respiratory dust removal. From the patent literature found abroad, it can be seen that the development of the foaming agent formulas, the structure of foaming generator and foam dust removal ways of different countries and different development units are different, which are developed for their own dust-producing conditions, especially the foaming agent formulas haven’t been fully disclosed. Even if the formulas have been disclosed, the chemicals it contain are not produced in domestic factories. If the chemicals are bought from abroad, it is bound to cost a lot of financial resources. There few data on foam dust removal in China. Therefore, we developed foaming agents and foaming generators (foamers for short) that can be used in the field, and studied the process and method of their application in open-pit mine blasting, in response to the conditions that domestic raw materials are widely available, cheap and easy to process and make.

3.4.1 Formula Requirement of Foaming Agent The effect of foam dust removal mainly depends on the formula of foaming agent, i.e., the selection and content of each chemical agent in the formula, and the general

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formula of foaming agent contains surfactants such as frother, wetting agents, stabilizers and solubilizers. From the structural point of view, all surfactant molecules are composed of two parts: polar hydrophilic group and non-polar lipophilic group. The hydrophilic group makes the molecule introduce water, while the oleophilic group makes the molecule leave water, i.e., introduce oil, so they are amphiphilic molecules. According to the structural classification of hydrophilic groups, surfactants are usually classified as ionic and nonionic surfactants, while ionic surfactants ionize in water to form cationically charged or anionically charged lipophilic agents, which can be further classified as cationic surfactants and anionic surfactants. Therefore, in the formula of foaming agent, cationic surfactants and anionic surfactants should not be mixed, and anionic surfactants or non-ionic surfactants are preferred. In addition, we should consider the wide source of surfactants with low price, easy processing and field application. The effects and requirements of various agents (surfactants) in the formula of foaming agent are analyzed combined with experimental analysis. (1)

Frother

In foam dust removal, the strength of the performance of the foaming agent directly affects the frothing volume and dust removal efficiency. Generally, the foaming agent is prepared by adding stabilizers and other additives with different performances in a certain proportion to the foaming agent with strong foaming property. Due to the different molecular structure of the frother, under the same conditions, the foam expansion is various. Generally speaking, it is considered that 1–20 times is low expansion foam, 20–200 times is medium expansion foam, 200–1000 times is high expansion foam, and the foam applied in dust removal is generally 100–400. The foaming ability of frother in the experiment can be determined by the foam height generated by shaking the container with a certain degree of frother solution. The foaming heights of different frothers are different under the same shaking times and speeds. This experiment uses the foam height h produced by a 100 ml glass container with 20 ml of aqueous solution of the frother after a sharp shaking, as shown in Fig. 3.54. Figure 3.55 shows the relationship between the foam height generated by Fig. 3.54 Measuring method of foam performance

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3 Chemical Suppression Technology of Open-Pit Mine …

Fig. 3.55 Relationship between the height and concentration of foam produced by frother

the three types of frothers and the concentration of the frothers solution, it can be seen that the foam height increases faster with the increase in the concentration of the frothers solution, and after increasing to a certain value (about 0.2–0.3%), the foam height decreases slowly with the increase in the concentration of the solution. (2)

Wetting agent

Wetting agent is a compound composed of groups of two different properties: hydrophilic group and hydrophobic group. When the wetting agent is dissolved in water, its molecules are completely surrounded by water molecules, one end of hydrophilic group is attracted by water molecules, introduced into the water; another end of hydrophobic group is dismantled by water molecules, stretched into the air. So the molecules of the wetting agent form a close directional arrangement layer on the aqueous solution surface, that is, the interfacial adsorption layer. Due to the existence of interfacial adsorption layer, it changes the contact state of the surface layer of water molecules and air, the contact area is greatly reduced, resulting in a reduction in the surface tension of water, while there is a adsorption between the hydrophobic group toward the air and dust particles, bringing the dust particles into the water and getting fully wet. The main indicators to measure the wetting effect of the wetting agent is the surface tension and wetting time, which are related to the nature of the wetting agent. Figure 3.56 shows the relationship curve between the surface tension and the concentration of the solution according to the surface tension measurement of different wetting agent aqueous solutions. It can be seen from the figure, the surface tension of water decreases with the increases in the concentration of wetting agent used and the test results show that a variety of wetting agents have a common law, that is, with the increase in the concentration of wetting agent, the surface tension decreases sharply in the initial stage. And to a certain concentration, the surface tension tends to be constant, the test value of about 29–37 mN/m. (3)

Stabilizer

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Fig. 3.56 Relationship between surface tension and concentration of wetting agent aqueous solution

Stabilizer (or foam stabilizer) refers to the certain additives which can stabilize the foam in the frother. There are many kinds of foam stabilizer, and it can be divided into inorganic and organic two according to the property, each of which has a certain range of choice. Experiments have proved that improperly-added stabilizer not only can not increase the stability of the foam, but also will reduce the original technical performance indicators of the frother. The stability of the foam depends on the formula, foaming mode and external factors of the foam fusion of the foaming agent, generally, the time for foam breaking is measured by the time for breaking 1/2 height of broken foam in the limited container, that is, the quality of foam stability; low surface tension of the solution is easy to generate foam, and the stability time is long. Figures 3.57, 3.58 and 3.59 show some of the phenomena found in the experiment after adding stabilizer in foaming agent formula B1 . Figure 3.57 shows the relationship between foam height and stabilizer content of aqueous solution of Fig. 3.57 Relationship between the foam height and stabilizer content

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3 Chemical Suppression Technology of Open-Pit Mine …

Fig. 3.58 Influence of different stabilizer content on foam height

Fig. 3.59 Influence of stabilizer content on surface tension

2.5/1000 5/1000 10/1000

34

s /（ mN/m）

32

30

28

26 0.00

0.05

0.10

0.15

0.20

0.25

C/（ %）

foaming agent with different concentrations (referred to as foam liquid) after adding 1:1 stabilizer. Figure 3.58 shows the effect of adding two types of stabilizers, 1:1 type and 1:2 type, on foam height in 1% foam liquid; Fig. 3.59 shows the effect of stabilizer content on surface tension. From Fig. 3.57, it can be seen that the foam height is maximum when the foam liquid concentration is certain and the stabilizer content is about 0.03–0.08%, and the foam height decreases with the increase of stabilizer content when it exceeds 0.08%. From the other two figures, it can be seen that different stabilizers have different effects on the foam height and the stabilizer content has little effect on the surface tension of the foam liquid. What’s more, it is found in the experiment that when the stabilizer content increases, the foam height decreases, which means that with the foam stabilization time increases. However, from the foam stability generated by the foaming generator, it can be observed that the foam stabilizer content increases to a certain value, the foam size generated is

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not the same, the foam film thickness is not the same, and the foam stabilization time decreases. This indicates that there is a suitable range for the stabilizer content which is added to the foaming agent formula, and the range of stabilizer content to this foaming agent formula is about 0.03–0.08%. (4)

Solubilizer

Surfactants have the ability to significantly increase the solubility of insoluble or slightly water-soluble organic substances after forming micelles in aqueous solution. At this time, the solution is transparent, which is called solubilization. The surfactant that can produce solubilization is called solubilizer, and the organic matter that is solubilized is called solubilized matter. The main factors affecting the solubilization effect are the molecular structure and properties, temperature, organic additives, electrolytes of solubilizer and solubilized. Solubilizer is an essential component in foaming agent formulation. (5)

Other additives

In order to avoid the insoluble dirt produced by foaming agent and the multivalent metal ions and ionic surfactants in the water or to prevent the foam liquid from freezing in winter, it need to add some complex agent or antifreeze agent in the foaming agent formula. Therefore, some other additives should be added to the foam agent formula according to the nature of water quality, climate conditions and other factors to facilitate the foam dust removal effect.

3.4.2 Development of Foaming Agent Formula (1)

Orthogonal experiment of foaming agent formula

It is impossible for any single agent to achieve the requirements of all aspects of performance, and it needs a variety of agents to be mixed in the foaming agent formula to achieve the desired purpose. As the role of each agent in the formula is different, and therefore the content of each agent is not necessarily the same, which needs to be determined by orthogonal experiment. In this experiment, four factors, namely frother A, wetting agent C, complex agent D and solubilizer E, are considered at three levels. The factors and levels are shown in Table 3.21, and L9(34 ) orthogonal table is selected. The design of the table and test results (surface tension, foam height) Table 3.21 Factor level table Factors

Foaming agent (A)

Wetting agent (C)

Complex agent (D)

Solubilizer (E)

Level 1

5

2

1

3

Level 2

20

6

7

10

Level 3

35

10

13

17

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3 Chemical Suppression Technology of Open-Pit Mine …

Table 3.22 Measurement values of surface tension and foam height Test number

1A

1 2 3 4 5 6 7 8 9

2C

1 (5) 1 (5) 1 (5) 2 (20) 2 (20) 2 (20) 3 (35) 3 (35) 3 (35)

3D

1 (2) 2 (6) 3 (10) 1(2) 2 (6) 3 (10) 1(2) 2 (6) 3 (10)

1 (1) 2 (7) 3 (13) 2 (7) 3 (13) 1 (1) 3 (13) 1 (1) 2 (7)

4E 1 (3) 2 (10) 3 (17) 3 (17) 1 (3) 2 (10) 2 (10) 3 (17) 1 (3)

Index σj

hj

32.02 31.57 34.37 33.65 31.75 32.02 32.38 32.29 33.65

13 21 39 44 56 16 71 28 56

are shown in Table 3.22, the intuitive analysis of surface tension and foam height are shown in Table 3.23, and the concentration of foam liquid in the experiment is 0.5%. As the purpose of the test is that the higher the foam height, the better the effect, the lower the surface tension, the better the effect. From the analysis of the range difference in Table 3.23, it can be seen that: for the foam height, the amount of foaming agent and complex agent has the greatest effect on the foam height, and it is best to take the third levels. The wetting agent and cosolvent have less effect on the Table 3.23 Intuitive analysis table of surface tension and foam height Factors

1A

2C

3D

4E

Note

Ii

97.96

98.05

96.33

97.42

IIi

97.42

95.61

98.87

95.97

Analysis of surface tension σ j = 1, 2, 3, 4

IIIi

98.32

100.04

98.5

100.31

Ii

32.65

32.68

32.11

32.47

IIi

32.47

31.87

32.96

31.99

IIIi

32.77

33.35

32.83

33.44

Ri

0.3

1.48

0.85

1.45

Ii

73

128

57

125

IIi

116

105

121

108

IIIi

115

111

166

111

Ii

24.33

42.67

19

41.67

IIi

38.67

35

40.33

36

IIIi

59

37

55.33

37

Ri

34.67

7.67

36.33

5.67

Analysis of foam height h j = 1, 2, 3, 4

Note 1 Ii = σ1i or h1i , IIi = σ2i , 或h2i , IIIi =σ3i , 或h3i , (i = 1, 2 … 9); 2 σ1i , σ2i , σ3i is σ1 , which is the surface tension corresponding to the first, second and third level respectively; 3 h1i , h2i , h3i is the foam height corresponding to first, second and third level respectively; Ii = 13 Ii , IIi = 1 3 IIi , IIIi

= 13 IIIi ; 4 range Rj is the maximum absolute value of the difference between I, II and III

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Fig. 3.60 Relationship between foam height and foam liquid concentration

foam height, which means that no matter what level is taken, the amount of wetting agent and cosolvent added has little effect on the foam height, and it should be taken the first level from the measurement results, so the formula that produces the best foam height is A3 C1 D3 E1 . For surface tension, the amount of wetting agent used has the greatest influence on the surface tension, the second level is the best, followed by the cosolvent with the second level. The dosage of complex agent should be taken the first level. The addition amount of foaming agent has little effect on the surface tension. From the measurement results, it should be taken the first level. Therefore, the formula with the lowest surface tension is A1 C2 D1 E2 . Based on the above analysis, it is concluded that each agent content (weight%) in foaming agent formula is: foaming agent A, 20–35; wetting agent C, 2–6; complex agent D, 4–10; cosolvent e, 3–10; stabilizer, 3–7; water, 40–60. (2)

Measuring results and analysis of foaming agent performance

According to the above requirements, the relationship between the foam height, surface tension and the concentration of foam liquid is shown in Figs. 3.60 and 3.61. It can be seen from these two figures that the surface tension of the foam liquid in these formulas is almost the same with the appropriate concentration of foam liquid is 0.5–1%. And the foaming performance of the formula B5 is the best. The property of the foam produced by the foaming generator is measured by the formula B5.

3.4.3 Experimental Study on Foaming Generator Performance (1)

The structure and principles of foaming generator

The foaming generator for dust removal is generally the water jet foaming device and air pressure foaming device, and the foaming device developed in the experiment

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3 Chemical Suppression Technology of Open-Pit Mine …

Fig. 3.61 Relationship between surface tension and foam liquid concentration

mainly consists of draught fan, foam screen, nozzle, force pump and liquid supply hose. The structure of foaming generator and measuring device are shown in Fig. 3.62. (1)

Working principle

With the draught fan as the air source, the aqueous solution of foaming agent is sprayed evenly on the foam screen in the special foaming generator. Under the action of the pressure airflow, it can continuously produce a large amount of high-expansion foam, which can be sprayed to the dust source through the pipeline or directly sprayed to the dust source, achieving the purpose of dust suppression and dust reduction. (2)

Device structure

Draught fan: Because of the small amount of foam required for the test, a miniature blower is used, with air volume of 0–3m3 /min and wind pressure of 10–30 mmH2 O. Liquid supply system: The liquid supply system is the foam liquid delivery part of the device, which is composed of electric force pump, pressure-resistant liquid

Fig. 3.62 Structure of foaming generator and measuring device 1-draught fan, 2-pitot tube, 3micromanometer, 4-foam segment, 5-nozzle, 6-flowmeter, 7-force pump, 8-foam screen, 9-foam outlet, 10-foam barrel, 11-bracket

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115

supply hose, pressure gauge, control valve and liquid barrel, etc. It draws the prepared foam liquid and supplies the nozzle of the foaming generator. Foam screen: It is an important part of the foam liquid, and the number of net layers, the size of the net hole and net material and other parts affect the quality of foaming. Five kinds of net material are used to carry out the test for foaming property under the same conditions, and the results show that the use of foam screen with double-layer nylon parabolic or spherical shape produces high quality foam with large frothering volume. (2) (1)

Parametric measurement Measurement of wind speed and volume

Test device is as shown in Fig. 3.62, with the Pitot tube and micropiezometer to measure the dynamic pressure value inside the pipeline (hd mmH2 0). Due to the small pipe diameter of the test, so the central point method is used, then. The wind speed inside the pipe V1 is  V1 = K

2gh d (m/s) ρg

(3.19)

The foam screen air quantity Qg is   Q g = V1 S1 m3 /s

(3.20)

The foam screen wind speed V is V =

Qg (m/s) S

(3.21)

In the above equation: k—correction coefficient of Pitot tube; hd —micromanometer reading, mmH2 O, ρg —gas density, kg/m3 ; S1 —sectional area of pipeline at the measuring point, m2 ; S—sectional area of pipeline where the foam screen is located, m2 . (2)

Measurement of foaming flow

There is a flowmeter installed between the force pump and the nozzle, and the flow rate of the foam liquid Q1 is read out directly. (3)

Measurement of frothering volume

A large container is put at the outlet of the foam, and the foaming time required to fill the container is traced. Then it is converted into the amount of foam produced per unit time, i.e. frothering volume Qf . (4)

Foam expansion

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Foam expansion refers to the ratio of free volume of certain amount of foam to volume of solution that is precipitated after the foam has burst. In the experiment, when the container with V容 volume is filled with foam, the net weight of the foam is measured. Then the volume V液 of the foam liquid is calculated, and the foam expansion n is: n=

Vcontainer Vliquid

(3.22)

3.4.4 Experimental Results (1)

Relationship between frothering volume and liquid supply volume

Figure 3.63 shows that the relationship curve between the frothering volume and the amount of liquid supply is drawn according to the measured results at different wind speeds of the foam screen, the test condition is the concentration of foam liquid of 0.5%. It can be seen from the curve in the figure, when the wind speed or wind volume of the foam screen is constant, the frothering volume increases with the increase of the liquid supply. The increase at the beginning is faster, and then gradually increases more slowly. When the expected liquid supply reaches a certain value, the frothering volume no longer increases. If the liquid supply increases again, the amount of residual liquid in the foaming generator will increase. (2)

Relationship between frothering volume and foam screen wind speed

Figure 3.64 shows that the relationship curve between the frothering volume and the wind speed of the foam screen is drawn according to the measured results at different Fig. 3.63 Relationship between frothering volume and liquid supply volume

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Fig. 3.64 Relationship between frothering volume and foam screen wind speed

liquid supply quantities, and the test condition is also the foam liquid concentration of 0.5%. It can be seen from the curve in the figure, when the amount of liquid supply is constant, as the wind speed of the foam screen increases within a certain range, the frothering volume also increases accordingly. Beyond the range, the wind speed of the foam screen increases, and the frothering volume decreases instead. It is observed that the wind speed of the foam screen increases too much, and there are flying bubbles and fine grained liquid beads splashing out of the foam outlet. Therefore, there is an optimal wind speed of the foam screen under the given structure of the foaming generator and the foam screen, and the optimal wind speed of the foaming generator is about 2.4-3 m/s. (3)

Relationship between frothering volume and foam liquid concentration.

The relationship between foam liquid concentration and foam height has been tested before, and the relationship between foam liquid concentration and frothering volume is determined by the foam produced by foaming generator. A lot of tests have carried out to prove that the law of change of the two is basically the same, that is, the higher the foam height, the larger the frothering volume. On the contrary, the lower the foam height, the smaller the frothering volume. Figure 3.65 shows that the relationship curve between frothering volume and foam liquid concentration is drawn according to the experimental results at different liquid supply volumes, with the test condition that the wind speed of the foam screen is 2.85 m/s. It can be seen from the curve that when the liquid supply volume is constant, with the increase of foam liquid concentration, the frothering volume increases faster at the beginning, and when it increases to about 8%, the frothering volume decreases when the foam liquid concentration increases again, which shows that there is a foam liquid concentration value with the maximum frothering volume, and the foam concentration of 5–8% is suitable for this test.

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Fig. 3.65 Relationship between frothering volume and foam liquid concentration

(4)

Relationship between stable time and foam liquid concentration

Adding stabilizer to the formula of the foaming agent is one of the important methods to increase the stability of foam, but it is found that the foam liquid concentration is related to the stabilization time by a large number of tests. Figure 3.66 shows that when the foaming wind speed of 2.85 m/s and the liquid supply volume of 351/h, the relationship between the stability time and foam liquid concentration based on the measured data is drawn. The foam stability time refers to the time required to break the foam height of 25 mm, not the single foam breaking time. The speed of breaking foam decreases with the increase of time, and it breaks foam faster at first, and then gradually decreases. As can be seen from the figure, the foam stabilization time increases with the increase of foam liquid concentration, and when the foam liquid concentration is 15%, the stabilization time is about 50 min. Fig. 3.66 Relationship between stable time and foam liquid concentration

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119

Fig. 3.67 Experimental device for measuring dust removal efficiency. 1. Draught fan, 2. Foaming generator, 3. Dust producing device, 4. Container with an open end, 5. Sampling head, 6. Dust meter

Moreover, it is found from the test that the more layers of foam screen, the less the frothering volume, while the foam stabilization time is increased. The area of the foaming generator outlet becomes smaller, the foam volume of the outlet is reduced, and the foam diameter becomes thinner. Therefore, the number of layers of foam screen and foaming generator outlet area should be determined according to the need.

3.4.5 Measurement of Foam Dust Removal Efficiency In order to determine the foam dust removal effect, we use the experimental device as shown in Fig. 3.67, the measurement method is: arrange the dust sampling head at a certain distance at the outlet of the wind flow, use talcum powder as the test dust, and measure the dust concentration by the filter membrane weighing method. First measure the dust concentration Co at the outlet of the container when the dust is generated by the dust generator, then put the foam and dust into the container at the same time, measure the dust concentration Cj at the outlet of the container, then calculate foam dust removal efficiency η according to the following equation. η=

Co − C j × 100% Co

(3.23)

In order to compare with the spraying dust removal effect, and the dust concentration Cw is also measured at the outlet of the container when spraying. The above equation can be used to calculate the efficiency of spraying dust removal efficiency as long as the Cj is changed to Cw . The results are shown in Table 3.24. As can be seen from Table 3.24, the dust concentration at the outlet is below 6.72 mg/m3 when using foam dust removal technology, with the dust removal efficiency is almost above 90%. When having dust removal with spraying, the dust removal efficiency is between 50 and 80%, and the water consumption of foam dust removal is 1/4 to 1/5 of that of spraying dust removal technology. To further determine the dust removal efficiency of foam on respiratory dust, the dispersity of the dust is measured. The relationship between dust removal efficiency and dust particle grain size is plotted as shown in Fig. 3.68 after calculating. The dust sample used for the test is number 3 in Table 3.24, and it can be seen from the figure that the dust

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Table 3.24 Measurement table of dust removal efficiency Test number

CO (mg/m3 )

Foam dedusting C j (mg/m3 )

Spray dedusting η (%)

Cw (mg/m3 )

ηw (%) 50.19

1

20.76

2.12

89.72

10.34

2

50.79

1.93

96.2

26.04

48.73

3

88.75

3.75

96.23

26.82

67.79

4

132.38

6.72

94.92

38.76

70.72

5

243.13

5.72

97.65

48.33

80.12

100

Dust removal efficiency/( %)

Fig. 3.68 Relationship between particle grain size and dust removal efficiency

80

water foam

60 40 20 0

0

5

10

15

20

d/( µm)

removal efficiency of foam on respiratory dust (dust particles less than 7um) is more than 85%, while the dust removal efficiency of spraying on respiratory dust is only 10–32%.

References 1. Gurin AA. Composition for dust and gas suppression in blasting Jobs. Chem Abstr 96558 w. 1992; 117(10):411. 2. Lu GR. Solution of ecological problems in open-air operation. Min Eng. 1991;9:43–5. 3. Lu GR. Method of reducing smoke and dust emission in the process of open pit and underground blasting. Min Eng. 1993;7:62–6. 4. Zhang XK, Li HY. Calculation and analysis of blasting dust emission in open pit mines. Metal Mine. 1996;3:41–4. 5. Du CF, Li HY. Field test of blasting dust and poison reduction in deep concave open pit. Chin J Eng. 2000; 22(4). 6. Wu Y. The catalytic chemistry. Beijing: Science Press; 1990. 7. Takashiro H. What is a catalyst. Beijing: Science Press; 1990.

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8. Wang DX. Preparation of CO oxidation catalyst at room temperature. Ships Chem Def. 1990; 2. 9. Wang DX. Preparation of carbon monoxide removal catalyst for gas mask. Ship Sci Technol. 2001;03:29–32. 10. U.S patent. Coal coating method. 4,613,523.1986.9 11. Jiang ZA, Du CF, He L. Study on application of fireproofing and fire-extinguishing technology using fire-retardant lime-gel in coal mine of high sulfur content. China Saf Sci J. 2003;13(10):31–3. 12. Wang HZ, Chen SL, Zhu WY. Nanometer technology unveil a very long period mystery of water glass’ ageing. New Chem Mater. 2003;31(3):38–40. 13. Mao JQ. Introduction to electron spectroscopy, vol. 2. Beijing: International Industry Press; 1992. pp. 64–75. 14. Wu GX. Mechanism of heteropolar wetting agents acted to the rock and ore dust particles. Ind Saf Environ Prot. 2005;6:1–3. 15. Copeland CR, Eisele TC, Kawatra SK. Choosing an effective dust suppressant. In: International mineral processing congress; 2010. 16. Jin LZ, Yu M, Liu JY. Dust-reduction mechanism and application research of efficient waterstemming. Chin J Eng. 2007;29(11):1079–82. 17. Ali RB, Hamed Z, Mohamad MM. Dust-reduction mechanism and application research of efficient water-stemming. Eng Village. 2007; 29(11):1079–82.

Chapter 4

Chemical Suppression of Dust Technology of Open Ore Stacking Yard

Abstract This chapter analyzes the mechanism of dust production in the storage yard, develops the dust suppressant for the storage yard by using single factor and orthogonal experiments. The performance of dust suppressant was characterized by surface curing effect, compressive strength, hygroscopicity and moisture releasability.

4.1 Mechanics of Dust Production of Open Ore Stacking Yard [1–6] According the theory of microscopic particles motion, when the average wind speed under the action of the wind is approximately equal to a certain critical value, individual prominent dust particles affected by turbulent flow velocity and pressure fluctuation begin to vibrate or swing back and forth, but do not leave the original position; particles in the yard only begin to dusting when reaching a certain wind speed, which is called the threshold velocity. The wind flows pass the undulating and concave convex surface of the material pile and form the extremely complex wind velocity gradient field and wind velocity rotation field in the material yard area, making the whole material yard area filled with vortexes of different scales and turbulence intensities. Among them, the local turbulence intensity increases at the back of the top of each material pile under strong wind conditions. When the wind velocity increases to reach or exceed the critical value, the vibration also intensifies, the dragging and rising forces increase accordingly and are sufficient to overcome the effect of gravity. And the running torque drives some unstable dust particles to roll or slide along the surface of the pile first. The microfine particles on the surface of the pile, especially on the top surface of the pile, have good turbulence following features, and enter the wind flow in large quantities under the push of strong wind and the wind-absorbing effect of strong vortex field. The fine particles in these particle clusters are in suspension in the atmosphere and are driven by the incoming flow and move together with the airflow, which is the process of dust production of the stacking pile.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Wang et al., Chemical Dust Suppression Technology and Its Applications in Mines (Open-pit Mines), https://doi.org/10.1007/978-981-16-9380-9_4

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4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard

After entering the wind current, the dust drifts far downstream with the wind and has a diffusion motion under the action of strong turbulence. When the dusty airflow crosses the stacking pile area, the wind speed decreases, the turbulence kinetic energy decreases, the gravitational effect becomes relatively large, and then the dust in the airflow starts to settle successively. Due to the diversity of dust particle geometry and spatial location and the variability of force conditions, part of the dust particles due to coarse particle size have large particle inertia force in the rolling process, and do not move with the drift after obtaining initial kinetic energy. They move along its own track, sliding and jumping in the air for a period of time. When encountering convex dust particles or being collided by other moving dust particles, they obtain part of the impulse. Dust particles that have obtained impulse quickly change their motions from horizontal motion to vertical motion and settle down after spreading a certain distance with the wind.

4.2 Measures of Dust Prevention of Open Ore Stacking Yard There are three reasons for dust from stacking pile: the stacking pile lacks of stickiness, and is loose and dry, fragile; lack of protective layer, and is in an exposed and naturally weathered state; no windproof retention device [7]. Now therefore, there are several measures of dust prevention: dust suppression by water spraying, dust suppression with enclosure structure, dust-proofing net covering, and chemical suppression of dust [8]. (1)

Dust suppression by water spraying

It is divided into traditional sprinkler, dust suppression by mist spray, ultrasonic dust suppression, dust suppression by spray electrostatic water and dust suppression by high pressure water. The main function of dust suppression by water spraying is to wet the fine dry dust particles, increase the relative density, and bond into larger particles, so that it can not fly under the action of external force [9, 10]. The method wastes a lot of water resources, and can not meet the long-term effective dust suppression demand for the season of large dust thickness or water shortage. What’ more, as a dust suppressant, water can be useful only when it does not evaporate. If the water sprinkling is not timely or sprinkling water is not appropriate, the surface of the pile is in alternating wet and dry conditions, and leads to the increased fine dust content, which causes more dust pollution [11]. At the same time, the duration of dust suppression water spraying in summer is short, and it is easy to freeze in winter, therefore, the method is not very effective. (2)

Dust suppression with enclosure structure

Setting up wind shield is of huge construction input and high cost. It cannot capture the dust in the air with poor dust suppression effect and poor practicality. It also has

4.2 Measures of Dust Prevention of Open Ore Stacking Yard

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limitations on the dust suppression ability of new dust generated by the construction of the project, and the removal of the fence will cause secondary pollution. (3)

Dust-proofing net covering [12–17]

Dust suppression with dust-proofing net or plastic cloth has high cost and is easy to wind blocking but it needs long time for installation and removal. The wastes of dust-proofing net or plastic cloth will cause new environmental pollution. (4)

Chemical suppression of dust

The emergence of chemical suppression of dust was highly appreciated in the 1930s [18]. Due to its effectiveness and novelty, the method has been widely adopted in many countries around the world with continuous innovation and developments, and is considered the best to solve dust pollution in scientific research and industrial operations [19]. Chemical suppression of dust is one of the most effective of all dust suppression methods. The mechanism is to wet or condense the fine dust with special chemicals to make the dust settle down to achieve the purpose of dust prevention and control, effectively reducing the pollution brought by fine dust to the natural environment and protecting people’s health to a large extent. Chemical suppression of dust began in the early twentieth century, as a new and high-efficient method of dust suppression, it has attracted a high degree of attention from researchers in various countries for its convenient use and excellent dust suppression effect. With the rapid development of the research and application of chemical dust suppressants in recent decades, there is a large number of studies related to chemical dust suppressants. According to the relevant information, the expression for dust settling velocity is as follows [20]:      1+3μ θr     vt = 2γ ρparticle ρparticle −ρfluid g 1 + 4μ θr + 6μ θr 2

(4.1)

where vt —dust settling velocity, m s−1 ; r —grain size, m; ρparticle —grain density, kg m−3 ; ρfluid —flow density, kg m−3 ; μ—kinematic viscosity coefficient. θr —coefficient of external friction. g—gravity coefficient, ms−2 . It can be seen from the expression for dust settling velocity, dust settling velocity is proportional to dust grain size and density. When ρparticle − ρfluid > 0, vt > 0, that is, the dust is in the state of constant settling. Therefore, increasing dust grain size or grain density are the two effective control methods to make dust settle rapidly. Chemical dust suppressants diluted evenly are sprayed on the surface of the material or dust area, which can make the surface of dust particles to maintain a certain degree of humidity, increasing the density of dust particles; some dust suppressants

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may agglomerate small dust particles into large particles, increasing the grain size, and the dust will settle quickly to achieve the purpose of suppressing the dust. Although chemical suppression of dust started late, it has developed rapidly in recent years. It is reported that a variety of chemical dust suppressants have been developed, which have the advantages of good dust suppression effect and longer retention time than water spraying dust suppression methods. Chemical suppression of dust has a broad application prospect in the future. At the same time, there are many problems and shortcomings which need to be further improved. At present, there are not many manufacturers engaged in production of chemical suppression of dust in China and there are fewer products, far from meeting the actual needs. Therefore, it is extremely important to have research and development on low-cost, multi-purpose, long-lasting, easy to use and environmentally friendly new dust suppression products and to promote to the whole country.

4.3 Development of Dust Suppressants of Open Ore Stacking Yard 4.3.1 The Dust-Settling Mechanism of Dust Suppressants of Open Ore Stacking Yard [21, 22] (1)

Film Former

The processed dust suppressant solution is dispersed into liquid droplets sprinkled on the surface of the material, the liquid droplets disperse the adsorbed air on the surface and contacts with the particle surface to wet the particles. Then the particles are filled with dust suppressant solution. Due to the water evaporation, the concentration and viscosity of the film former increases, and the film former trapped between the particles gradually solidifies, bonding the particles with the bonded dust suppressant to form a solid shell layer. The film formation mechanism is as follows: (1)

Physisorption Binding

Adsorption theory divides the bonding process into two stages. In the first stage, binder molecules disperse to the surface of the bonded object (sand grain) through Brownian Motion, making the two polar groups or molecular chain segments get close to each other. In this process, temperature rise, binder viscosity reduction and contact pressure increase are conducive to Brownian Motion, that is, increasing the temperature of the dust suppressant solution, reducing the viscosity of the solution and increasing the speed of the droplets are conducive to diffusion. Adsorption gravitation is generated in the second stage. When the intermolecular force between the binder and the particle surface reaches 1 nm, an intermolecular force, i.e. Van Der Waals Force, is generated, and its intermolecular force is as follows:

4.3 Development of Dust Suppressants of Open Ore Stacking Yard

E =−

  2 μ4 3 2 2 α + I + αμ (3kT ) R6 8

127

(4.2)

where μ—molecular dipole moment; α—polarizability; I —molecular ionization energy; R—intermolecular distance; k—Boltzmann constant; T —thermodynamic temperature. It is clear that the greater the polarity between the binder and the particles, the and the closer the contact between them, the more sufficient the adsorption, and the greater the bonding strength. In order to achieve the close contact between binder and particles, three aspects can be considered. Firstly, when spraying the solution, the droplets are energized by the pressurizing effect of the pump, and the high-speed droplets can better displace the gas adsorbed on the surface of the particles when contacting with the particles to achieve close contact. Secondly, adding surfactant can reduce solution surface tension, so as to achieve the full wetting of droplets to particles. Additionally, if the solution temperature is increased, it can also have the effect of enhancing penetration and wetting. As for applying contact pressure to increase diffusion, this phenomenon has been found during industrial tests. When spraying dust suppressant on the tailings surface with the pump, the solution penetrates quickly with large penetration depth. While using unpressurized drip sprinkling, it is obvious that there is runoff phenomenon of solution on the surface of the micro-fine tailings particle with slow penetration and small penetration depth. (2)

Mechanical Binding

The liquid film former not only fills the gap of the bonded particles, but also spreads to the depression of the particles surface. After curing, the film former forms mechanical interlocking force with the particle surface. (3)

Chemical Bonding

The chemical reaction between the film former and the bonded particles forms a chemical bond that firmly connects the two. Chemical bonds include hydrogen bond, ionic bond, covalent bond and coordinate covalent bond, of which hydrogen bond and covalent bond are the most prevalent and important forces acting on the bonding force. Coordinate covalent bond refers to the film former and the bonded particles in the bonding interface by film former to provide electron pairs, the bonded material to provide the acceptance of electron pairs of empty orbitals, thus forming the coordinate covalent bond of the bonding interface. Polar groups of the film former can be adsorbed with many substances to form hydrogen bond, the adsorbed particles form bridging group with the “bridging”. And a polymer can adsorb multiple particles, which increases the cohesion between the particles. Bonding dust suppressant relies on the above-mentioned bonding forces to achieve effective bonding between the binder and the particles, connecting the loose particles together, thus forming the ability to resist wind blowing.

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Crusting water resistance test shows that the reason for high crust stability and good resistance to rain erosion is also related to the film former. Film former is a kind of substance that can be solved in hot water but not in room temperature water. When rainwater falls on the crusted surface of the material, most rainwater will produce runoff on the surface of the hard shell because the film former does not dissolve in room temperature water, making the water through the hard shell greatly reduced, which is not only conducive to reducing material loss, but also playing a positive role in ensuring the moisture content of the underlying material. (2)

The Function of Filler

Filler is a kind of polysaccharide granular material, which is insoluble in cold water and partially soluble in high temperature water. Some particles swell and form a viscous suspension with large suspended particles. The role of filler in dust suppressants is reflected in the following aspects. (1)

Increase dust suppressant adhesion, and reduce costs

Viscosity of dust suppressant solution is mainly affected by the polymer film former concentration, the larger the concentration, the greater the viscosity. In order to make the solution reach a certain viscosity, it is bound to increase the cost and become less economical if it only relies on increasing the concentration of film former. Adding fillers can increase the viscosity of the film former solution, reducing the amount of film former and the cost of dust suppressants. (2)

Strengthening action

When the filler particles are sprayed on the material surface with the dust suppressant solution, the filler particles will fill in the gaps of the powder material particles, reducing the material particle gap, and then the film is more dense. The film-forming strength is low because of the weak interaction force between film former molecules and low cohesion energy. When filler with a certain particle is added to the film former solution, the active surface of the large particle filler and the polymer chain of the film former combine with each other to form a cross-linked structure. When a certain macromolecular chain is impacted by external force, the external force can be dispersed to other molecular chains through the cross-linking point. If one of the molecules breaks, the other molecular chains can act as usual without endangering the whole, so the film strength can be greatly improved. (3)

Reduce the shrinkage stress and cold and thermal stress

Due to the evaporation of water, dust suppressant solution sprayed on the surface of the material, the volume between the particles shrinks and decreases, and gradually cures to form a hard shell (film), so the curing process exists volume shrinkage stress. After the hard shell (film) is formed, the external temperature and humidity will change. Since the different thermal expansion or contraction coefficients of the polymer film former and the covered material, there will also be thermal expansion or cold contraction, which will generate stress concentration in the hard shell, causing the hard shell to crack and directly affecting the sealing effect. The filler can adjust

4.3 Development of Dust Suppressants of Open Ore Stacking Yard

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the shrinking percentage in the curing process to reduce the differences in expansion or contraction coefficients between film former and material particles when hot and cold alternate, which prevents cracks from extending, so as to ensure the continuous integrity of the cover layer. (3)

The Function of Hygroscopic Agent

As a hygroscopic agent, magnesium chloride has the function of moisture absorption and moisture retention, which makes the crust still maintain a certain elasticity and flexibility in dry climate, so that the cover layer is complete and continuous, and reduce the generation of cracks. In addition, the substance has a cementing effect. (4)

The Function of Surfactant

The effect of film former bonding particles is mainly related to the wetting of the solution and particles, the better the wetting, the closer the contact between the film former and the surface of the particles, the higher the shell strength. Therefore, adding the surfactant in the dust suppressant can reduce the surface tension of the solution to strengthen dust suppressant solution wetting and ensure the penetration of the solution, improving the thickness and strength of the crust.

4.3.2 Single-Factor Experiment The section focuses on the single-factor experiment of hygroscopic agent, singlefactor experiment of film former, single-factor experiment of surfactant and singlefactor experiment of filler to select the most effective complexant and the most suitable concentration of each complexant. (1)

Single-Factor Experiment of Hygroscopic Agent

Based on relevant literature, five hygroscopic agents of sodium polyacrylate, sodium carboxymethyl starch, propanetriol, triethanolamine, and trimethylolpropane are selected as the single factors of hygroscopic agents and they are named with A1 , A2 , A3 , A4 , A5 . The moisture absorption rate is measured to select the hygroscopic agent with the best moisture absorption and moisture release properties and its approximate concentration range. Soil samples have been milled, and dust samples are prepared through a 60 mesh standard sieve with impurities removed, and a certain amount of dust samples were weighed and placed in a culture dish (ϕ 75 mm). A certain amount of hygroscopic agent is put in water according to the target concentration, and stir continuously with a glass rod at the same time, and leave it to dissolve the solvent fully to make a solution. Take 10 ml of the prepared solution, spray it evenly on the surface of the dust samples, and make the dust sample completely soaked, the sample preparation is completed. Dry all samples in a high temperature blast oven at 105 °C to a constant weight and place it with closed cover in the dryer to cool naturally. Wash and number the

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4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard

culture dishes with lids, spread a thin layer (around 1 mm) of dried samples in each dish, and then dry at 105 °C to a constant weight. The samples are placed in the desiccator with the lid closed and cooled naturally, then record the weights. Because of the dry natural indoor conditions, it is necessary to provide an environment with sufficient humidity, so it needs homemade humidifiers. Put the wet towel which totally absorbed with 300 ml deionized water on the tray, place the samples and seal with cling film above the tray. Place the culture dish with the dried samples in the homemade humidifier, open the lid and put the thermohygrometer in the homemade humidifier. Weigh the weight of each sample every 1 h, measure the moisture absorption rate according to the Eq. (4.3) and record the temperature and relative humidity of the homemade humidifier at each test time point. Moisture absorption is characterized by the moisture absorption rate of the dust samples, and the equation for calculating the moisture absorption rate is shown in Eq. (4.3). Wi =

mi − ma ma

(4.3)

where Wi —moisture absorption rate of dust sample after several hours, %; m i —Weight of culture dishes and dust samples after several hours, g; m a —Original weight of culture dishes and dust samples, g. Five hygroscopic agents with certain amount are weighed and separately configured into solutions with three concentrations of 0.5, 1 and 3%. The concentration value selected in this article is an empirical value obtained by reviewing the relevant literature and understanding the physicochemical properties of various hygroscopic agents. Dust samples of 1#–15# are prepared according to the sample preparation method of the experimental protocol. The 16# dust sample is prepared by taking 10 ml deionized water as the control group. The sample concentration correspondence is shown in Table 4.1. The moisture absorption rate of the dust sample at each measurement time point is calculated according to Eq. (4.3), and the line graph of the variation of moisture absorption rate with time is drawn, as is shown in Fig. 4.1. Meanwhile, the line graph of temperature and relative humidity changes at each measurement time point is shown in Fig. 4.2. Table 4.1 Comparison table of reference number of hygroscopic agent dust sample Mass concentration

A1

A2

A3

A4

A5

0.5%

1#

4#

7#

10#

13#

1%

2#

5#

8#

11#

14#

3%

3#

6#

9#

12#

15#

Deionized water control

16#

4.3 Development of Dust Suppressants of Open Ore Stacking Yard

131

Fig. 4.2 Line chart of temperature and relative humidity with time

Temperature/℃

Fig. 4.1 Line chart of moisture absorption rate with time

22.0 21.5 21.0 20.5

Relative humidity/%

20.0 80 78 76 74 72 70 68 66

0

1

2

3

4

5

Time/h

As can be seen from Fig. 4.1 that the samples sprayed with A1 solution has the highest moisture absorption rate and the best moisture absorption effect. The samples that are sprayed with 3% concentration of A1 solution has the best hygroscopic effect, with the hygroscopic rate ranging from 1.06 to 1.26%. The samples sprayed with concentrations of 1 and 0.5% have poor hygroscopic effects, and the difference between the two is not significant, with the hygroscopic rate ranging from 0.84 to 1.02%; the samples sprayed with A2 solution, A3 solution and A5 solution have some hygroscopicity, but the hygroscopic effect is second to that of A1 solution. Among the samples sprayed with A2 solution, the moisture absorption effect is not much different between solutions with 0.5 and 3% concentrations, with the moisture absorption rate ranging from 0.73 to 0.87%. Solutions with 1% concentration has the worst moisture absorption effect, with the moisture absorption rate ranging from 0.53 to 0.62%. The best moisture absorption effect in the samples sprayed with A3 solution

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4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard

is that with 1% concentration, with moisture absorption rate ranging from 0.76 to 0.89%. The least effective solution is that with 0.5% concentration, with moisture absorption rates ranging from 0.58 to 0.71%. In the samples sprayed with the A5 solution, there is little difference in the moisture absorption between the solutions with 0.5 and 1% concentrations, with the moisture absorption rate ranging from 0.56 to 0.77%. Solution with 3% concentration has the worst moisture absorption effect, with moisture absorption rate ranging from 0.40 to 0.58%. The sample sprayed with A4 solution have the closest moisture absorption rate with the samples sprayed with deionized water, which is the least effective hygroscopic agent with the moisture absorption rate ranging from 0.38 to 0.57%. Meanwhile, combined with Figs. 4.1 and 4.2, when the moisture absorption rate of the samples is basically stable, the moisture absorption rate of most samples changes with the changes of relative humidity, and the moisture absorption of the hygroscopic agent has a greater correlation with the relative humidity of the environment. The relative humidity increases continuously from the beginning, peaked at 79% at the 3rd hour, and decreases to 76.2% at the 4th hour. All samples show an overall increasing trend of moisture absorption in the first 3 h, while the vast majority of samples show a decrease in moisture absorption in the 4th hour compared to the previous hour. This indicates that the samples sprayed with hygroscopic agent automatically absorb and release moisture according to the ambient relative humidity changes, and the samples will lose water when the ambient relative humidity is low and absorb water when the ambient relative humidity is high. The higher the moisture absorption rate of the sample, the greater the decline in moisture absorption rate when the relative humidity decreases. For example, the sample sprayed with 3% A1 solution decreases from 1.26 to 1.10% at the 4th hour, which is the sample with the largest decrease in moisture absorption rate, indicating that the better the hygroscopicity, the more the hygroscopic agent is affected by the relative humidity of the environment. Considering the experimental results comprehensively, it is concluded that hygroscopic agent A1 has the best moisture absorption and moisture release effects, and the moisture absorption rate of the samples sprayed with three different concentrations of A1 solution exceed that of all other samples. The hygroscopicity of A1 increases with the increase of concentration. However, from the aspect of economic cost, the cost of 0.5% A1 solution increased 6 times compared with 3% A1 solution, while the maximum hygroscopicity increased only 1.24 times. Meanwhile, there is not much difference of hygroscopicity between 0.5% A1 solution and 1% A1 solution, so 0.5% A1 solution is selected as the hygroscopicity monomer for the next experiment. (2)

Single-Factor Experiment of Surfactant

Based on relevant literature, three surfactants of sodium dodecylbenzene sulfonate, sodium lauryl sulfate and sodium lignosulfonate are selected as the single factors of surfactant and they are named with B1 , B2 , B3 . The best surfactant monomer and its approximate concentration range are selected through the measurement of surface tension value and reverse osmosis experiment.

4.3 Development of Dust Suppressants of Open Ore Stacking Yard

(1)

133

Measuring of Surface Tension Value

A certain amount of the three surfactants are weighed and configured into a solution according to the target concentration, and the surface tension of each solution is measured using a BZY-type surface tension meter. When measuring the surface tension of the surfactant solutions using the BZY-type surface tension meter, the solutions are poured into a glass dish to a height of approximately 80%, 1/3 of the sample is poured out to create a new surface, and then the measurement is started. The surface tension reduction effect is evaluated by comparing the surface tension values of different concentrations of each surfactant. The results of the measurements are shown in Table 4.2. Meanwhile, the average surface tension values of all solutions and the surface tension values of deionized water are plotted as line graphs as shown in Fig. 4.3. From the line graph of the average surface tension value with solution concentration in Fig. 4.3, it can be concluded that the average surface tension value of deionized water is 72.13 mN/m, and all three surfactant solutions are smaller compared with it. All of them have certain wettability to the dust. The B3 solution has the largest surface tension value compared to the remaining two surfactant solutions, which is dismissed, so surfactant B3 is excluded from the next experiment. The surface Table 4.2 Surface tension value measurement results Medicament

Concentration (%)

The surface tension was measured twice (mN/m) 1

2

B1

0.05

41.59

42.67

42.13

0.20

33.79

34.63

34.21

0.60

29.99

30.19

30.09

1

28.87

28.96

28.92

B2

B3

Deionized water

Average surface tension (mN/m)

2

28.37

28.42

28.40

0.05

29.05

29.68

29.37

0.20

26.94

27.53

27.24

0.60

29.64

30.07

29.85

1

30.25

30.30

30.28

2

30.37

30.57

30.47

0.05

53.97

56.51

55.24

0.20

49.90

52.89

51.39

0.60

48.63

49.64

49.13

1

44.32

43.70

44.01

2

39.06

39.26

39.16

71.75

72.50

72.13

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4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard

Fig. 4.3 Average surface tension value line graph

tension value of B2 do not change much with concentration, about 29–30 mN/m. The surface tension value of B1 solution decreases with the increasing concentration, and the decrease of surface tension value is flat after the concentration increases to 0.6%. When the concentration reaches 0.6%, the average surface tension value of B2 is 29.85 mN/m and the average surface tension value of B1 is 30.09 mN/m, the difference between which is very small. So the concentration gradient for the next experiment is developed at about 0.6%. (2)

Reverse Osmosis Experiment

The cacolumnity is used to measure the infiltration time of the surfactant on the dust sample by filling a thin glass tube with the dust sample and placing one end of the glass tube into the surfactant solution, the liquid rises in reverse. Different concentrations of surfactant solutions are prepared in glass tubes with size of 240 mm length and 8 mm inner diameter, and a 6-cm scale is marked on the lower end of each glass tube. The bottom of the glass tube is sealed with qualitative filter paper. Place 6 g of dust sample through a 60 mesh standard sieve and vibrate the dust sample up and down 20 times with the same force to ensure that the height of the dust sample is basically the same each time. The glass tube containing the dust sample is inserted vertically into the surfactant solution with a test tube clamp to a depth of 2 mm. The liquid rises in reverse by cacolumnity action, and the time interval required for the reverse osmosis rising to 1, 2, 3, 4, 5 and 6 cm is recorded with a stopwatch to compare the rate of infiltration. The reverse osmosis time is used to compare the wettability of the surfactant solution so as to select the optimal surfactant and its approximate concentration. The experimental results are shown in Fig. 4.4. From the statistical graph of the reverse osmosis experiments results in Fig. 4.4, it can be concluded that both surfactant solutions have a certain degree of reverse

4.3 Development of Dust Suppressants of Open Ore Stacking Yard

135

Fig. 4.4 Results of reverse osmosis experiment

osmosis feature. However, the time interval required for reverse osmosis of B1 solution is shorter and the infiltration speed is faster compared with B2 solution. Among the different concentrations of B1 solution, 1% solution has the shortest time interval required for reverse osmosis and the fastest infiltration rate, followed by the 0.2% solution. The difference between the two is extremely small. Therefore, from the perspective of economic cost, the 0.2% concentration of B1 solution is finally selected as the surfactant monomer for the next experiment. (3)

Single-Factor Experiment of Film Former

Based on relevant literature, three film formers of sodium carboxymethyl cellulose, Type 1788 polyvinyl alcohol and polyacrylamide are selected as the single factors of film former and they are named with D1 , D2 , D3 . The best film former monomer and its approximate concentration range are selected through the measurement of viscosity value, observation and comparison of film-forming properties, measurement of water corrosion resistance of cured layer and measurement of film mechanical properties. (1)

Separate Selection Experiments of Three Film Formers

Three film formers, D1 , D2 and D3 , are selected as single factors of film former in this experiment, and the best two film formers are selected by measuring the viscosity value, film-forming properties and film mechanical properties for the next two-two compounding experiments. A certain amount of three film formers is weighed and respectively configured into eight solutions of 0.1, 0.15, 0.2, 0.4, 0.8, 1.2, 1.6 and 2% concentrations. The dosage and concentration values of film formers are empirical values determined based on the physicochemical properties of each chemical and relevant literature. The viscosity of each solution is measured with a DV-II + PRO type rotational viscometer.

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4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard

Note: Since all three film-former solutions are non-Newtonian fluids, it is necessary to ensure the same rotor type and rotational speed for viscosity measurement for non-Newtonian fluids. Different rotors or rotational speeds may lead to a large difference in viscosity. However, under the same rotor and rotational speed conditions, the range of viscosity values that can be measured is limited. The rotational speed range of DV-II + PRO type rotational viscometer is 0–100% RPM, thus this experiment chooses the middle value of 50% RPM with the rotor model S1. At the same time, the viscometer continuously rotates in the fluid by means of a calibrated spring. The torsion degree of the spring measured by the rotary torque sensor is torque, which is proportional to the resistance formed by the viscous drag of the rotor immersed in the sample, and the torque is thus proportional to the viscosity of the fluid. The viscosity of the liquid can also be compared by comparing the torque. When using the viscometer, the torque needs to be in the range of 10–100%, so that the measured viscosity value will be informative. The torque in the range of 50–80% will have a higher accuracy. As there are differences in different agents and a wide range of viscosity, this article compares the viscosity and torque of different agents with different concentrations by rotating speed at 50% RPM, and the specific data are shown in Table 4.3. As can be seen from Table 4.3, the highest viscosity value among the three filmformer solutions is D1 solution, and the lowest viscosity value is D2 solution. The viscosity value of D1 solution increases with increasing concentration and increases exponentially from 0.4 to 1.2%, from 71.7 to 866.6 cp. The viscosity range decreases after 1.2%. At 2%, the viscosity value is the largest and exceeds the measurement range. D3 viscosity is smaller at low concentration, and the viscosity value increases with the increase of concentration, which is much smaller than D1 . D2 viscosity almost does not change with the change of solution concentration at low concentration, and the viscosity is very small, which is not much different from that of water. When the concentration reaches 0.8%, the viscosity starts to increase slightly with the solution concentration increase. But the viscosity value of D2 is the smallest when comparing the remaining two film-former solutions. (2)

Measurement of Film-forming Properties

Take 8 ml of the configured solutions and drop it on the surface of a dry culture dish, make it flow into a film and air drying under natural conditions. Observe the prepared films and compare and analyze the film-forming properties of the three film formers. Figure 4.5 shows the films made by D1 solution. As shown in Fig. 4.5, the thickness of the film made by D1 is not uniform enough, the leveling is not good enough, and it shrinks smaller and smaller with the evaporation of water. The shape of the film is irregular, and there is the phenomenon of shrinkage and warping at the edges after the film is formed. The film is brittle and not tough enough. Figure 4.6 shows the films made by D2 solution. As shown in Fig. 4.6, the film made by D2 has uniform thickness with the best leveling property among the three, and can be spread over a large area. After film

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Table 4.3 Film former viscosity values and torque statistics Film forming agent

Concentration (%)

Viscosity (cp)

Speed (%RPM)

Relative torque (%)

Remarks

D1

0.10

17.25

50

1.35

Low torque

0.15

23.65

50

1.85

Low torque

0.20

31.35

50

2.5

Low torque

0.40

71.70

50

5.5

Low torque

0.80

233.55

50

18.2

General accuracy measurement

1.20

866.60

50

68

High accuracy measurement

1.60

997.70

50

78.3

High accuracy measurement

2.00

EEEE

50

>100

The viscosity is too large to measure

0.10

1.28

50

0.1

Low torque

0.15

1.92

50

0.15

Low torque

0.20

1.92

50

0.15

Low torque

0.40

1.92

50

0.15

Low torque

0.80

3.20

50

0.2

Low torque

1.20

3.84

50

0.3

Low torque

1.60

5.12

50

0.35

Low torque

2.00

7.04

50

0.55

Low torque

0.10

2.56

50

0.2

Low torque

0.15

5.12

50

0.4

Low torque

0.20

7.68

50

0.6

Low torque

0.40

8.30

50

0.6

Low torque

0.80

14.70

50

1.2

Low torque

1.20

36.50

50

2.8

Low torque

1.60

40.95

50

3.2

Low torque

2.00

97.25

50

7.75

Low torque

D2

D3

formation, the shape is regular without edge upward, and the texture is more flexible (similar to plastic film). Figure 4.7 shows the films made by D3 solution. As shown in Fig. 4.7, D3 could not be made into a film with uniform thickness, and crystals are precipitated after drying. The solution with lower concentration (e.g. 0.4%) shows a layer of white adherent after drying, which is attached to the cling film and cannot be separated from the cling film; the solution with higher concentration (e.g. 2%) has obvious white crystalline particles precipitated after drying, which

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Fig. 4.5 D1 film

Fig. 4.6 D2 film

cannot form a film. Therefore, film former D3 is excluded in the next experiment which measures the mechanical properties of the film. (3)

Measurement of Film Mechanical Properties

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139

Fig. 4.7 D3 Film (left is a low concentration film, right is a high concentration film)

All the prepared films are cut into 30 mm * 10 mm strips, and the film thickness is measured with a digital thickness gauge at three different locations, and the average of the three measurement results is taken. The maximum tensile strength of the film is measured with a digital display push–pull gauge to calculate the tensile strength of the film and analyze the mechanical properties. The measurement results are shown in Table 4.4. Table 4.4 Tensile strength of monomer film (TS) Film forming agent

Concentration (%)

Average film thickness (FT)/mm

D1

0.10

0.0100

0.532

5.32

0.15

0.0250

1.39

5.56

0.20

0.0110

1.01

9.18

0.40

0.0237

3.2

13.52

0.80

0.0360

11.25

31.25

1.20

0.0275

10.5

38.18

D2

Maximum tensile force at break (Fm)/N

Tensile strength (TS)/Mpa

1.60

0.0485

10.01

20.64

0.10

0.0050

1.5

30.00

0.15

0.0094

2.95

31.26

0.20

0.0090

3.4

37.78

0.40

0.0113

4.45

39.26

0.80

0.0103

3.82

36.97

1.20

0.0240

11.49

47.88

1.60

0.0203

13.85

68.11

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According to the overall data in Table 4.4, the tensile strength of D2 films is larger than that of D1 films. The tensile strength of D1 films shows an overall trend of increasing and then decreasing, and the tensile strength of films with 1.2% concentration is the largest at 38.18 Mpa. The tensile strength of D2 films with 0.1–0.8% does not change greatly, ranging from 30 to 40 Mpa, and the tensile strength of films with 0.4% concentration is the largest at 39.26 Mpa. After 0.8%, the tensile strength increases with the increase of concentration. In conclusion, D3 has been ruled out in the next experiment because it is unable to form a film. However, D2 has better performance, while D1 solution has higher viscosity, and both of them have advantages and disadvantages. So the film former D1 and D2 are blended together for further experiments to investigate in which ratio the two substances are blended for the best viscosity, film-forming property and mechanical properties. (4)

Compounding Selection Experiment of Film Former D1 and D2

Comparing the previous experiments, it is found that D1 solution has the highest viscosity, while D2 solution has the best film-forming performance. Considering whether D1 and D2 have synergistic effect with each other and the mixed solution is more economical to save cost, the two solutions are blended for further study. (a)

Viscosity Measurement Experiment

Based on the results of the previous experiments and the relevant literature reviewing, the empirical value of D1 :D2 at the mass ratio of 1/1, 1/2 and 2/1 with the total concentration of the blended solution of 0.5% is determined for the next experiment. A certain amount of two film formers is weighed and configured into a solution with a total concentration of 0.5% by mass ratio of 1/1, 1/2 and 2/1, and the viscosity of each solution is measured with a DV-II + PRO type rotational viscometer with the rotational speed of 100% RPM. The rotor model is S1. The measurement results are shown in Table 4.5. As can be seen from Table 4.5, among the blends made of three different mass ratio film-former solution, the solution with D1 :D2 = 2:1 has the highest viscosity value of 59.2 cp at 100% RPM, while the solution with D1 :D2 = 1:2 has the lowest viscosity value of 26.9 cp, which is less than one-half of the former. Comparing with the difference of about 35 times in the monomer viscosity measurement experiment, the blending of the two film formers greatly reduces the difference in viscosity between them. Table 4.5 Viscosity values and torque statistics of co-mixed solutions of file former Film forming agent

Concentration (%)

Viscosity (cp)

Speed (%RPM)

Relative torque (%)

Remarks

D1 :D2 = 1:1

0.50

41.6

100

6.5

Low torque

D1 :D2 = 1:2

0.50

26.9

100

4.2

Low torque

D1 :D2 = 2:1

0.50

59.2

100

9.3

Low torque

4.3 Development of Dust Suppressants of Open Ore Stacking Yard

(a)

(b)

141

(c)

Fig. 4.8 Films of three different ratios of co-mixed solutions

(b)

Measurement of Film-forming Properties

Take 8 ml of the configured solution and drop it on the surface of a dry culture dish, make it flow into a film, and air drying at room temperature. Observe the prepared films and compare and analyze the film-forming properties of the three different ratios of solutions, and the films made are shown in Fig. 4.8. As shown in Fig. 4.8, (a): the film made by D1 :D2 = 1:1, (b): the film made by D1 :D2 = 1:2, and (c): the film made by D1 :D2 = 2:1. All three mass ratios can make films with uniform thickness and regular shape, which improve the problems of D1 films with better leveling. It can be spread over a larger area, and there is no edge warping phenomenon with better film toughness. It can be seen that the blending of the two film formers effectively improves the film-forming properties. (c)

Measurement of Film Mechanical Properties

All the prepared films are cut into 30 mm * 10 mm strips with two strips of each ration, and the film thickness is measured with a digital thickness gauge at three different locations, and the average of the three measurement results is taken. The maximum tensile strength of the film is measured with a digital display push–pull gauge to calculate the tensile strength of the film. The original length of the films and the length after stretching are also measured and the elongation is calculated. Two experiments are done for each ratio of film-former films to calculate the average tensile strength and average elongation to analyze the mechanical properties of the blended films. The measurement results are shown in Tables 4.6 and 4.7. From Tables 4.6 and 4.7, it can be seen that among the three blended films, the film with the highest tensile strength is the film made by D1 :D2 = 1:2, which is 29 Mpa, and the film with the highest elongation is also the film made by D1 :D2 = 1:2, which is 0.048%. Therefore, the film with D1 :D2 = 1:2 has the best mechanical properties among the blended films made of three ratios. (d)

Measurement of Water Corrosion Resistance of Cured Layer

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Table 4.6 Tensile strength of blend films (TS) Film forming agent

Membrane thickness (FT)/mm

Average film thickness (FT)/mm

Maximum tensile force at break (Fm )/N

Tensile strength (TS)/Mpa

Average tensile strength (TS)/Mpa

D1 :D2 = 1:1

0.016

0.021

3.98

18.95

16.16

0.026

3.475

13.37

0.012

3.52

29.33

0.018

5.16

28.67

0.012666667

1.52

12.00

0.014

1.85

13.21

0.026 0.021 0.022 0.038 0.018 D1 :D2 = 1:2

0.01

29.00

0.014 0.012 0.012 0.022 0.02 D1 :D2 = 2:1

0.011

12.61

0.015 0.012 0.019 0.009 0.014

Table 4.7 Elongation at break of blend films (E) Film forming agent

Original length of membrane (L0 )/mm

Length of film after stretching (L)/mm

Tensile length of membrane (L)/mm

Elongation (E)/%

Average elongation (E)/%

D1 :D2 = 1:1

23.2

24.1

0.9

0.0388

0.0366

14.5

15

0.5

0.0345

D1 :D2 = 1:2

20.5

21.3

0.8

0.0390

19.3

20.4

1.1

0.0570

22.7

23.1

0.4

0.0176

14.2

15.1

0.9

0.0634

D1 :D2 = 2:1

0.0480 0.0405

Through the above experiments, the ratio of film-former D1 :D2 is determined to be 1:2, so the total concentration of the blended solution will be determined by the water corrosion resistance of the cured layer of the film-former solution. Weigh a certain amount of film former and respectively configure the total concentration of 0.3, 0.5, 0.7 and 1% of the blended solution according to the ratio of D1 :D2

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143

Fig. 4.9 Water corrosion resistance curves of sand piles sprayed with different concentrations of film formers

= 1:2. The prepared solution is evenly sprayed on the surface of the dust sample to prepare a conical sand mold. The sand mold dries naturally at room temperature, and water is sprayed vertically onto the sand mold at a flow rate of 2 mL/(m2 s) to simulate rainfall. Each spraying lasts for 2 min with a total of 5 cycles to calculate the mass residue rate, and the change of mass residual rate with the number of water erosion is plotted into a broken line graph, as shown in Fig. 4.9. As shown in Fig. 4.9, the sand molds sprayed with film-former solution show significantly enhanced water corrosion resistance compared with those sprayed with deionized water. The sand pile with a concentration of 1% still has a mass residue rate of over 90% after 5 cycles. The sand pile with a concentration of 0.7% remains essentially unchanged in mass after 2 cycles, and the mass residue rate remains as high as 87.5% after 5 cycles. The consolidation layer of the sand pile with a concentration of 0.5% undergoes rupture after 5 cycles, and the mass residue rate is 81.3%; the mass loss of the sand pile mainly comes from the loose sand under the consolidation layer carried away by the water flow. The sand pile with 0.3% concentration breaks after 4 cycles with the solidified layer. Although the solidified layer breaks earlier, the mass residue rate is 65.3% after 5 cycles, which is much better than 47.5% for the sand pile sprayed with deionized water. This indicates that the selected blended solutions of the two film formers form a consolidation layer with considerable resistance to water corrosion. Comparing several different concentrations of film former solutions, it is found that 0.7 and 1% of the sand pile has the best water corrosion resistance, which are not comparable to each other, and the total concentration of the blended solution is finally chosen to be 0.7% for economic reasons. In conclusion, the blending of the two film formers improves the viscosity of D2 as well as the film-forming properties of D1 , and the overall performance is improved. Among the three blended films with different mass ratios, the film made of D1 :D2 of 1:2 has the best mechanical properties. Finally, it is concluded that the water corrosion resistance of the sand pile with a total concentration of 0.7% is

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4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard

comparable to that of 1% from the experiments on the water corrosion resistance of the sand pile consolidation layer of the film-former solution. Therefore, the solution with total concentration of 0.7% D1 :D2 is finally selected to be compounded at a mass ratio of 1/2, i.e., 0.23% D1 + 0.47% D2 as the film-former monomer for the next experiments. (4)

Single-Factor Experiment of Filler

Based on relevant literature, three fillers of guar gum, xanthan gum, sodium alginate and soluble starch are selected as the single factors of fillers and they are named with C1 , C2 , C3 , C4 . The best filler monomer and its approximate concentration are selected through the measurement of measurement of hardness value of solution crust and experiment of water resistance of film forming. (1)

Measurement of Hardness Value of Filler Crust

Weigh a certain amount of filler, add it to the mixed solution of the best type and concentration of compounding agents of the best type and concentration selected according to the previous single-factor experiment, and prepare the solution according to the target concentration. Weigh 40 g dust sample prepared by 60 mesh standard sieve and place it in the culture dish (ϕ 75 mm), then weigh 10 ml prepared mixed solution, and evenly spray it on the surface of 40 g dust sample. The samples dry and crust at room temperature. The indexes of evaluating the performance of the crust include the integrity, strength, hardness and toughness of the crust. In order to realize rapid measurement, the hardness is used to evaluate the performance of the crust. When measuring, a handheld LX-D Shore durometer is flat pressed on the prepared sample, and it must be read within 1 s after completely pressing on the sample. If there is other interval readings, it must be stated clearly. The hardness values are measured at different locations at least 6 mm apart from the samples for 5 times and the arithmetic mean of the hardness of these 5 points is regarded as the hardness value of the sample and record the data. Based on the results of the above single-factor experiments, 0.5% of A1 , 0.2% of B1 , 0.47% of D2 and 0.23% of D1 are selected, on which a certain amount of the four fillers are weighed to be configured into a mixed solution with different filler concentrations. The filler concentrations are respectively 0.1%, 0.2% and 0.3%. 10 ml configured mixed solution is weighed and sprayed evenly on the surface of 40 g dust samples. The prepared samples are left to dry and crust under natural conditions. The hardness of each sample is measured with an LX-D Shore durometer, and the results are shown in Table 4.8. The average hardness values of all samples are also plotted as line graph of Fig. 4.10. Analyzing the line graphs of the average hardness values of the crusts for each sample in Fig. 4.10, it can be concluded that the maximum hardness value of the sample made by adding C1 to the mixed solution is 23.6 HD for 0.2% concentration and the minimum value is 17.4 HD for 0.3% concentration. The maximum hardness value of the sample made by adding the mixture of C2 is 21.4HD for 0.2% concentration and the minimum value is 16.4HD for 0.1% concentration. The maximum

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145

Table 4.8 Results of hardness value Type

C1

C2

C3

C4

Concentration (%)

The hardness value was determined five times (HD)

Average hardness value (HD)

1

2

3

4

5

0.1

21

22

24

18

23

21.6

0.2

22

24

21

25

26

23.6

0.3

18

19

14

21

15

17.4

0.1

20

15

14

16

17

16.4

0.2

19

21

24

20

23

21.4

0.3

22

17

21

20

18

19.6

0.1

14

17

11

12

18

14.4

0.2

16

18

15

14

13

15.2

0.3

10

9

8

11

7

0.1

18

16

15

18

17

16.8

0.2

26

24

23

25

22

24

0.3

21

22

19

17

16

19

9

Fig. 4.10 Line graph of crust average hardness value

hardness value of the sample prepared by adding the mixture of C4 is 24HD for 0.2% concentration and the minimum value is 16.8HD for 0.1% concentration. The maximum hardness value of the sample made by adding C3 to the mixture is 15.2 HD for 0.2% concentration and the minimum value is 9 HD for 0.3% concentration. Compared to the other samples, the hardness value of the sample made by adding C3 is the smallest in all concentration gradients. Therefore, the filler C3 is excluded from consideration in the next experiments. It can be seen from the graph that the hardness values of the crusts of the samples made from the four filler mixtures are all maximum at a concentration of 0.2% as

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4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard

well, so the water resistance test is only conducted on the three films made from the mixtures of 0.2% C1 , C2 and C4 in the next experiment to test the water resistance of the formed films. (2)

Water Resistance Test of Formed Films

A mixture of 7 ml with different types and concentrations of fillers is dropped to the surface of a dry 75 mm culture dish, making it flow into a film, and then it airdries under natural conditions. After observing the states of the three films, they are respectively immersed in 200 ml of deionized water with the same temperature. The films with the best water resistance are compared and selected according to the length of time that it takes to absorb water and dissolve slowly in the deionized water, thus selecting the best filler and the approximate concentration. The films produced by adding a mixed solution of three different fillers are shown in Fig. 4.11. As shown in Fig. 4.11, (a) is the film made by adding 0.2% C1 filler in the mixed solution, (b) is the film made by adding 0.2% C4 in the mixed solution, and (c) is the film made by adding 0.2% C2 filler in the mixed solution. All the three films are thin in the middle and thick at the edges, and the film made by adding C2 has the highest tensile strength and is flat and uniform in thickness with the naked eye. The remaining two films have the rolling edges after film formation. (a)

0.2%C1

Figure 4.12 shows the dissolution process of the film made with 0.2% C1 filler. As shown in Fig. 4.12, (a) shows that the film is just put into deionized water and starts to absorb water and swell up. (b) shows that the thin middle part of the film gradually dissolves and disperses into a gel mass and starts to sink, and the whole film is no longer in the shape of a film. (c) shows that the gel mass continues to absorb water and swells until the whole gel mass settles at the bottom of the beaker. From (a) to (b), i.e., from the beginning of the immersion to the dissolution of the thin middle part of the film into a gel mass, which lasts for 4 min; from (b) to (c), i.e., from the beginning of the destruction of the complete film shape to the entire gel mass settling to the bottom, which lasts for 39 min.

(a)

(b)

Fig. 4.11 Films made by mixed solutions of three different fillers

(c)

4.3 Development of Dust Suppressants of Open Ore Stacking Yard

147

Fig. 4.12 Dissolution process of films made with 0.2% C1 filler

(b)

0.2%C4

Figure 4.13 shows the dissolution process of the film made with 0.2% C4 filler. As shown in Fig. 4.13, which is the same process as above, (a) shows the beginning of the film soaking, (b) shows that thinner part of the film is first expanded and dispersed, not shaping a film shape. (c) shows that the whole film absorbs water and swells, and becomes completely gelatinous, and sinks to the bottom of the beaker. From (a) to (b), i.e., from the beginning of soaking to the thinner part of the film swelling into a gel mass, which lasts for 2 min; from (b) to (c), i.e., from the beginning of the destruction of the complete film to a gel mass settling to the bottom, which lasts for 26 min. (c)

0.2%C2

Figure 4.14 shows the dissolution process of the film made with 0.2% C2 filler. As shown in Fig. 4.14, (a) shows that the film starts to soak and does not appear to start to swell and disperse in the middle first, and the integrity of the whole film is well maintained. (b) shows that whole film absorbs water and swells and is completely immersed in water, but still has an intact shape and does not appear to be dispersed into multiple micelles. (c) shows that the film continues to absorb water and swells, which loses its shape and settles at the bottom of the beaker in micelle shape. From

(a)

(b)

Fig. 4.13 Dissolution process of films made with 0.2% C4 filler

(c)

148

(a)

4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard

(b)

(c)

Fig. 4.14 Dissolution process of films made with 0.2% C2 filler

(a) to (b), i.e., from the beginning of immersion to the complete immersion of the whole film into water, which lasts for 26 min; from (b) to (c), i.e., from the complete immersion of the film into water to the settlement of the film as micelle shape to the bottom, which lasts for 109 min. By analyzing the above experimental results, it can be concluded that the films made by adding 0.2% C1 filler and 0.2% C4 filler to the mixed solution start to dissolve from the middle of the thinner part first, and the complete film shape is destroyed in a few minutes. In contrast, the film made by adding the 0.2% C2 filler to the mixed solution remain intact until it is completely immersed in water. What’s more, it takes 39 min for the film made of 0.2% C1 filler to swell and settle to the bottom of the beaker in the form of micelles, and that for the film made of 0.2% C4 filler is 26 min. While it takes 109 min for the film made of 0.2% C2 filler to settle at the bottom of the beaker, which is much longer than the former two. Therefore, the film made of mixed solution of C2 filler has the best water resistance. Combining with the results of the crust hardness test, C2 filler with the concentration of 0.2% is initially selected as the filler monomer for the next experiment.

4.3.3 Orthogonal Experiment Single-factor experiments investigate the influence of each factor on the comprehensive performance of dust suppressants, and the best type and concentration of each compounding agent in the process is initially selected. However, the results of singlefactor experiments have certain limitations and cannot fully present the interaction effects among the factors. The application of orthogonal experiment can handle with the problem of multi-factor and multi-level optimization design in the formula optimization design. Orthogonal experiment is a scientific experimental method to deal with multi-factor, multi-level experiments, which can be done through a smaller number of experiments, to achieve a more uniform level of factors with the correct conclusion. Orthogonal experiments are designed to analyze the interaction between

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149

the factors, fully explore the impact of each factor on the comprehensive performance of dust suppressants, so as to get the optimal formula. (1)

Design of the Orthogonal Table

The orthogonal experiment aims to select the concentration of the initial formulation of dust suppressant, which needs to select factors and fix level. The so-called factors refer to the different reasons or factors directly affecting test indexes. The so-called level, that is, the different values of the selected factors in the test. In this experiment, the factors refer to various compounding agents, i.e., hygroscopic agents, film formers, surfactants and fillers; the level is the concentration value selected for each compounding agent. This experiment selects four factors and the concentration level of the factors is set at 4. According to the number of factors and levels selected for this experiment, the standard orthogonal table L16(45 )is used for the experiment. The table allows a maximum of 4 levels and 5 factors, and this experiment only needs 4 of them. There is a blank column as the error column. And the random error is to be estimated based on the blank column when having variance analysis on the results of the orthogonal experiments. The grouping table for the designed orthogonal experiment formulation is shown in Table 4.9. (2)

Performance Evaluation Index

Table 4.9 Formulation grouping table of orthogonal experiment Experiment serial number

Factors A/%

B/%

C/%

D/%

1

1

1

1

1

2

1

2

2

2

3

1

3

3

3

4

1

4

4

4

5

2

1

2

3

6

2

2

1

4

7

2

3

4

1

8

2

4

3

2

9

3

1

3

4

10

3

2

4

3

11

3

3

1

2

12

3

4

2

1

13

4

1

4

2

14

4

2

3

1

15

4

3

2

4

16

4

4

1

3

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4 Chemical Suppression of Dust Technology of Open Ore Stacking Yard

For the comprehensive consideration of dust suppressant bonding characteristic and crust performance of the curing layer, orthogonal experiments simultaneously regard the viscosity and the hardness of the curing layer formed by spraying dust suppressant on the surface of dust sample after fully drying as the evaluation criteria for measuring experiments. (1)

(2)

Viscosity: A certain amount of agents are weighed in accordance with the orthogonal experimental formulation table, and 16 groups of dust suppressant solutions are prepared. It is notable that each solution needs to be fully dissolved and then it should be standed for defoaming. According to GB/T10247-2008 Method of Viscosity Measurement, NDJ-5S digital rotational viscometer is used to measure the viscosity value of the solution with No.3 rotor at the speed of 12 rpm. 5 viscosity values are read, the average value is taken as the final viscosity value, and record the experimental data. Hardness of the curing layer: A certain amount of agents are weighed in accordance with the orthogonal experimental formulation table, and 16 groups of dust suppressant solutions are prepared. Weigh 40 g dust samples prepared by 60 mesh standard sieve in a culture dish (ϕ 75 mm), take 10 ml of prepared dust suppressant solution evenly sprayed on the surface of the dust samples, and make the dust samples all soaked, making the sand mold samples.

All prepared samples are left to dry naturally at room temperature for the same period of time until the samples are completely dry and crusted. A handheld LX-D Shore durometer is flat pressed on the prepared sample, and it must be read within 1 s after completely pressing on the sample. The hardness values are measured at different locations at least 6 mm apart from the samples for 5 times and the arithmetic mean of the hardness of these 5 points is regarded as the hardness value of the sample [22]. The average hardness values of the 16 groups of experiments are recorded. (3)

Range Analysis of Orthogonal Experiment

(1)

Orthogonal Experiment for Viscosity

The viscosity value of the dust suppressant solution is examined as an evaluation index, the results are shown in Table 4.10. The numerical value of K 1 , K 2 , K 3 , K 4 reflects the impact of the four factors of four levels of A, B, C and D on the viscosity index. The greater the viscosity, the better the effect of dust suppressant on dust bonding and coagulation. As can be seen from the Table 4.10, A4, B1, C4, and D3 are the maximum value of the four factors A, B, C and D, that is, the optimal level, so A4 B1 C4 D3 is the optimal combination of orthogonal experiment for viscosity. The range RA , RB , RC , and RD are calculated for each column. Comparing each R value, it can be concluded that RA > RC > RD > RB , so the main order of the influence of the four test factors on viscosity is ACDB. In order to reflect the influence law and trend of four factors A, B, C and D on the viscosity value index more intuitively, the mean value K of the viscosity value index is set as the vertical coordinate, the four factors and four levels are set as

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Table 4.10 Range analysis form of results of orthogonal experiment for viscosity Experiment serial number

Factors A

Objective function B

C

D

Viscosity (mPa s)

1

1

1

1

1

2095

2

1

2

2

2

1737

3

1

3

3

3

2606

4

1

4

4

4

3051

5

2

1

2

3

3567

6

2

2

1

4

2951

7

2

3

4

1

3055

8

2

4

3

2

2845

9

3

1

3

4

3123

10

3

2

4

3

3566

11

3

3

1

2

2438

12

3

4

2

1

2672

13

4

1

4

2

4172

14

4

2

3

1

3681

15

4

3

2

4

3571 3111

16

4

4

1

3

K1

2372

3239

2649

2876

K2

3105

2984

2887

2798

K3

2950

2918

3064

3213

K4

3634

2920

3461

3174

R

1262

322

812

415

the horizontal coordinate, and the mean viscosity value response graph is drawn, as shown in Fig. 4.15. (2)

The crust hardness of the dust suppressant solution is examined as an evaluation index, the results are shown in Table 4.11.

The numerical value of K 1 , K 2 , K 3 , K 4 reflects the impact of the four factors of four levels of A, B, C and D on the hardness index. The greater the hardness, the better the compressive and rheumatic properties of the dust suppressant after crusting. As can be seen from the Table 4.11, A4, B2, C4, and D2 are the maximum value of the four factors A, B, C and D, that is, the optimal level, so A4 B2 C4 D2 is the optimal combination of orthogonal experiment for hardness. The range RA , RB , RC , and RD are calculated for each column. Comparing each R value, it can be concluded that RA > RB > RC > RD , so the main order of the influence of the four test factors on hardness is ABCD. In order to reflect the influence law and trend of four factors A, B, C and D on the hardness value more intuitively, the mean value K of the hardness value is set as the

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Fig. 4.15 Mean viscosity value response graph

vertical coordinate, the four factors and four levels are set as the horizontal coordinate, and the mean hardness value response graph is drawn, as shown in Fig. 4.16. (4)

Variance analysis used in the experiment helps further understand the influence of various factors on the dust suppressant comprehensive performance. Results of variance analysis of viscosity are shown in Table 4.12.

It can be concluded from Table 4.12 that with the viscosity as the performance evaluation index of the dust suppressant solution, Fcritical(0.10) = 5.36 after querying the F-distribution critical value table. For factor A, F value of 7.87 is larger than Fcritical(0.10) . Therefore, factor A has a significant influence on the viscosity of the dust suppressant solution. And the remaining three factors have less significant impact on the viscosity of the dust suppressant. The results of variance analysis of crust hardness are as shown in Table 4.13. It can be concluded from Table 4.13 that with the hardness after crusting as the performance evaluation index of the dust suppressant solution, F critical(0.10) = 5.36 after querying the F-distribution critical value table. For factor A, F value of 14.13 is larger than F critical(0.10) . Therefore, factor A has a significant influence on the crust hardness of the dust suppressant solution. For factor B, F value of 6.50 is larger than F critical(0.10) , which shows that factor B has obvious effect on the crust hardness, second only to factor A. And the remaining two factors have less significant impact on the crust hardness of the dust suppressant. According to the variance analysis of orthogonal experiments on viscosity and hardness, factors are ranked in order of priority by the magnitude of the range R. For the viscosity index, ACDB is ranked from primary to secondary, and the optimal level

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Table 4.11 Range analysis form of results of orthogonal experiment for hardness Experiment serial number

Factors

Objective function

A

B

C

D

Hardness (HD)

1

1

1

1

1

27.50

2

1

2

2

2

30.67

3

1

3

3

3

27.83

4

1

4

4

4

27.83

5

2

1

2

3

31.83

6

2

2

1

4

34.33

7

2

3

4

1

32.33

8

2

4

3

2

28.33

9

3

1

3

4

30.17

10

3

2

4

3

34.17

11

3

3

1

2

31.83

12

3

4

2

1

30.67

13

4

1

4

2

37.17

14

4

2

3

1

36.33

15

4

3

2

4

34.17

16

4

4

1

3

31.50

K1

28.46

31.67

31.29

31.71

K2

31.71

33.88

31.83

32.00

K3

31.71

31.54

30.67

31.33

K4

34.79

29.58

32.88

31.63

R

6.33

4.29

2.21

0.67

is A4 B1 C4 D3 . For the hardness index, ABCD is ranked from primary to secondary, and the optimal level is A4 B2 C4 D2 . The best formula of the two indexes is different. Since the roles of each factor for the performance of dust suppressants are various, the optimal formulations obtained by different measurement indicators differ. In order to determine the optimal formulation, it is proposed to use the integrated equilibrium method to analyze each factor of the dust suppressant. It can be concluded from the variance analysis, for factor A, it has a positive and significant impact on the indexes. From the impact degree of each factor on the indexes, factor A has the largest influence, which is the dominant factor to control the increase or decrease of the indexes. With the increase of the concentration of factor A, the positive effect of both indexes are enhanced, so A4 is chosen as the optimal level. For factor B, it has a positive and significant impact on the dust suppressant crust hardness, and the F value of 6.5 with hardness as the evaluation index is greater than the F value of 0.68 with dust suppressant viscosity as the evaluation index, so B2 is selected as the optimal level with the hardness as the evaluation index. For factor C, the F value of 3.42 with viscosity as the evaluation index is greater than the F value of 1.85 with dust

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Fig. 4.16 Mean hardness value response graph Table 4.12 Analysis of variance table of viscosity Experiment

Factor A

B

C

D

Freedom

3

3

3

3

Error

Total

3

15

Adjust the sum of squares of 3,232,988 deviations (Adj SS)

279,374 1,407,519 523,067 411,014 5,853,961

Sum of squares of mean 1,077,663 adjusted deviation (Adj MS)

93,125

469,173

174,356 137,005

F value

7.87

0.68

3.42

1.27

Fcritical (0.10)

5.36

5.36

5.36

5.36

Significance

Remarkable

Table 4.13 Analysis of variance table of hardness Experiment Freedom

Factor

Error

A

B

3

C

D 3

3

3

Adjust the sum of squares of 80.250 deviations (Adj SS)

36.931

10.514 0.903

Sum of squares of mean 26.7500 adjusted deviation (Adj MS)

12.3102

3.5046 0.3009 1.8935

F value

14.13

6.50

1.85

0.16

Fcritical (0.10)

5.36

5.36

5.36

5.36

Significance

Remarkable Remarkable

Total

3

15

5.681

134.278

4.3 Development of Dust Suppressants of Open Ore Stacking Yard Table 4.14 Orthogonal experiments to verify the experimental results

155

Experiment

Viscosity (mPa s)

Hardness (HD)

1

4182

36.25

2

4196

36.83

3

4150

39.29

Average value

4179

37.45

suppressant hardness as the evaluation index. With the increase in C concentration, the positive effect of both indexes are enhanced, so C4 is selected as the optimal level. For factor D, the F value of 1.27 with viscosity as the evaluation index is greater than the F value of 0.16 with dust suppressant hardness as the evaluation index. Therefore, D3 is selected as the optimal level with the viscosity as the evaluation index. The above analysis shows that the optimal dust suppressant ratio is A4 B2 C4 D3 , i.e. 0.6%A + 0.2%B + 0.28%C + 0.7%D. The differences of the results between the orthogonal experiment and the single-factor experiment indicate that there is a significant interaction between the factors, which further illustrates the necessity of conducting orthogonal experiments. (5)

Confirmatory Analysis

Validation experiments are carried out according to the optimal conditions derived from the orthogonal experiments, and dust suppressant solutions are prepared according to the formulations of 0.6%A, 0.2%B, 0.28%C, and 0.7%D (consisting of 0.46%D2 + 0.24%D1 ). Three parallel experiments are conducted to verify the experimental results, and the test results are shown in Table 4.14. The validation experiment indicates that the optimal formulation has higher viscosity and better hardness of the cured layer compared with the 16 groups of dust suppressant solutions made according to the orthogonal table, which has excellent overall performance.

4.4 Performance Study on Dust Suppressants of Open Ore Stacking Yard With the help of single-factor experiment and orthogonal experiments, a composite polymeric pile dust suppressant with excellent dust suppression performance and environmental friendliness prepared from five compounding agents has been finally obtained. In order to explore the performance of this compound polymer pile dust suppressant under the extreme conditions (such as rainfall, wind, frost, heat and other adverse weather conditions) that may be faced during the actual dust suppression work, this section analyzes the physicochemical properties and dust suppression performance of the optimal formulation of the dust suppressant itself.

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4.4.1 The Basic Physicochemical Property of Dust Suppressants (1)

Viscosity

As an important property index of dust suppressants, viscosity is closely related with dust suppressant stability and application performance. Generally speaking, the higher the viscosity of the dust suppressant, the higher the stability, which has a profound impact on the product from the synthesis stage to the subsequent transportation, storage and practical application, so the dust suppressants need to have the appropriate viscosity. Too high viscosity leads to the poor leveling of dust suppressants. When applied to the actual solid sand dust suppression, it will not only affect the product dispersion and infiltration effect, but also has an influence on the formation of dust suppressant solidification layer uniformity and depth of infiltration; too low viscosity reduces the stability of dust suppressants, and causes dust suppressants solidification components premature loss, reducing the life of the cured layer. The experiments refer to GB/T10247-2008 Method of Viscosity Measurement using NDJ-5S digital rotational viscometer to to measure the viscosity value of dust suppressant samples in the same environment. NDJ-5S digital rotational viscometer is equipped with 4 rotors (No. 1, 2, 3, 4) and has 4 speeds (6, 12, 30, 60 rpm), resulting in 16 combinations which allow the measurement of viscosity of various liquids within the range [23]. The completely dissolved dust suppressant solution beaker is placed in the constant temperature water tank for constant temperature. When the temperature is constant to 25 °C, NDJ-5S digital rotational viscometer is used to measure the viscosity value of the solution. No. 3 rotor is used in the experiment with the speed of 12 rpm. 6 best ratio of dust suppressant solution viscosity values are read, and the average value is taken as the final viscosity value, the measurement result of which is 4131 mpa s. Since the dust suppressant solution developed in this article is a non-Newtonian fluid, the viscosity values change significantly when the rotor as well as the rotational speed are changed. Therefore, there is no comparative value with Newtonian fluids. When compared with the viscosity values of the 16 groups of orthogonal experiments (with the same rotor and rotational speed), it is clear that the viscosity of optimal formulation has improved. In practical application, it can be properly diluted and then sprayed for the high viscosity of this dust suppressant solution, which improves the fluidity and reduces the cost of spraying as well. (2)

Surface Tension

Surface tension for dust suppressants is also a more important property index. Lower surface tension can enhance the wettability and permeability of the dust suppressant, not only can make the dust suppressant more easier to spread wetting when spraying, but also can improve the adsorption of the dust surface and extend the time of moisture absorption and water retention, maintaining the water content of the pile.

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157

BZY type surface tension meter is used in the experiment. As the measured solution is a higher viscosity liquid, it is necessary to wet the platinum plate at a height of 5 mm before gently wiping off some with facial tissue. When re-hanging the platinum plate, the displayed value shall be kept at 0–5 mN/m, and then the sample table shall be raised and the stable value shall be noted with the measurement result of 37.13 mN/m. Comparing with the surface tension value of 72.5 mN/m of deionized water, it can be concluded that the surfactant effectively reduces the surface tension of the dust suppressant. The surface tension of the dust suppressant solution adding with higher viscosity compounding agents such as film former and fillers is larger than that of surfactant monomer solution. (3)

pH Value

The dust suppressants are mostly applied to sand piles and material piles in urban habitats, and sandy soil layers threatened by desertification in the natural environment. Therefore, the chemical dust suppressants used need to be harmless and friendly to the environment. PH meter is used to measure the acidity of the dust suppressant in the experiment, and too acidic or too alkaline dust suppressant solution may change the material properties and may have an impact on the surrounding soil. The METTLER TOLEDO pH meter is used to measure the pH of the dust suppressant solution with the best proportion, and the result is 8.31, which is weakly alkaline. The rainwater is weakly acidic, and the material pile sprayed with dust suppressant can neutralize the weak acidity with rainwater, which shows that the dust suppressant prepared in this article is environmentally friendly.

4.4.2 Surface Curing Effect 10 ml of deionized water and the best formulation of the dust suppressant solution are uniformly sprayed on 40 g experimental sand held in a ϕ75mm culture dish, and the surface sand samples are sprayed with carbon after dried naturally. And then the sand surface of the two samples are observed with environmental scanning electron microscope and analyzed the samples. The sample surface is magnified to visually observe the bonding state between sand grains from the surface micromorphological perspective, and it is compared with the control sand mold surface samples after drying with deionized water spraying. Scanning electron micrographs of the surface of sand molds sprayed with deionized water and sprayed with dust suppressant solution are as shown in Fig. 4.17. Figure 4.17a shows the scanning electron microscope image of the surface of the sand mold after being dried by spraying with deionized water, and Fig. 4.17b shows the scanning electron microscope image of the surface of the sand mold after being dried and solidified by spraying with dust suppressant solution. From the comparison of the two images, it can be seen that the gap between sand grains on the surface

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Fig. 4.17 Scanning electron micrographs of the surface of sand molds sprayed with deionized water (a) and sprayed with dust suppressant solution (b)

(a)

(b) of the sand mold sprayed with deionized water is larger, the distribution of sand grains is more scattered with loose structure and it does not form a tight structure of agglomeration, which means that the compressive strength of this sand mold is low. From the surface of the sand mold sprayed with the dust suppressant solution, it can be clearly observed that there is an obvious bond formed after the drying of the dust suppressant between the sand grains, the sand grains agglomerating and linking together or even overlapping. It forms a state of solidification layer, the surface structure of which is highly compact, which indicates that the sand mold has a higher compressive strength. This microscopic bonding state also provides a conclusive microstructural basis for the stabilization of sandy soil by dust suppressants, as well as wind and water anti-erosion and the inhibition of open dust source escape

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4.4.3 Compressive Strength Compressive strength can reflect the reliability of consolidation layer formed after dust suppressant is mixed with sand at the macro level and can be used to measure the strength of adhesion of dust suppressant to sand particles at micro level. The stronger the compressive strength, the stronger the bonding effect. The increase in the strength of the solidification layer will also indirectly enhance the wind and water erosion resistance of the sand piles sprayed with dust suppressants. The experiment uses WDW-200D type microcomputer controlled universal material testing machine. At a constant room temperature of 25 °C, the sand column non standard parts prepared with the dust suppressant according to the optimal formulation are used for compression test using the universal material testing machine, keeping the compression speed at 2 mm/min until the first crack appeared on the sand column. The changes in the compressive strength of the sand column during the whole compression process are analyzed using the software, and then the curve between the compressive strength and displacement of the sand column is plotted and the maximum compressive strength is selected [24]. When the sand column reaches the maximum pressure using the universal material testing machine, the sand column ruptures, and the pressure measured at this time is the maximum compression strength in the experiment. The change curve of compression strength of the sand column sprayed with dust suppressant solution with the universal material testing machine compression displacement is shown in Fig. 4.18. As can be seen in Fig. 4.18, the compressive strength of the sand column sprayed with dust suppressant solution in the elastic strain stage of compression test (displacement 0–0.944 mm) remains in the plateau period, which may be voids between sand grains in the sand column after consolidation. These voids disappear slowly with the gradual increase of compressive displacement during the application of pressure by Fig. 4.18 Sand column displacement-compression strength curve

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the testing machine, so the compressive stress of the sand column should be maintained at a certain level. Then it enters the anelastic deformation stage (displacement 0.944–2.41 mm), the compression strength of the sand column tends to have a rapid rise. This compressive stress continues to climb and the sand column maintains a stable original state, which thanks to the strong adhesion of the dust suppressant filled between sand grains in sand column to the sand body and certain toughness of dust suppressant itself; Finally, in the yield stage (displacement 2.41–4.834 mm), the sand column reaches the yield point at the displacement of 2.41 mm, at this time the maximum compression strength is 0.326459 MPa. After crossing the yield point, the sand column cracks and quickly through the column, and compressive strength quickly decays with the main body displacement, which indicates that the consolidation effect of dust suppressant on the whole sand column at this time has been completely lost. However, the dust suppressant of the pile still has a good bonding effect on the sand particles from the lumpy sand column debris. In summary, the compression test shows that the dust suppressant of the pile studied in this article not only has good bonding ability and solidification effect on sand and dust, as well as a certain toughness and impact resistance, which is in line with the actual sand and dust suppression needs.

4.4.4 Wind Erosion Resistance Study on wind erosion resistance performance mainly explores whether the solidification layer formed by the dust suppressant spraying can still remain intact under windy natural conditions. If the consolidation layer remains intact under windy conditions, the sand under the consolidation layer cannot be moved, reducing the appearance of dusty, sandy and other bad weather. It not only reduces water and soil loss, but also contributes to environmental protection, which is one of the important indicators to test the performance of dust suppressant [25]. Conical sand molds sprayed with 15 ml of dust suppressant solution configured according to the optimal formulation and sprayed with deionized water are dried in a high temperature blast drying oven at 80 °C, weighed and the raw data recorded. Two wind scales are set respectively, 7 m/s and 14 m/s, and the conical sand molds are placed in the experimental device wind. SF-type axial flow blower are used to gradually increase the wind speed from low to high and parallel blowing are carried out on the conical sand molds sprayed with dust suppressant solution and deionized water. Digital anemometer is used to monitor the real-time wind speed on the sand molds surface, and the fixed wind speed is maintained for 10 min and 30 min respectively. Then observe the damage caused by different wind speed and different blowing time on the solidification layer, calculate the wind erosion rate W of the sand pile before and after the test, and evaluate the wind erosion resistance of the dust suppressant, the calculation equation as shown in Eq. (4.4).  W = (m1 − m2 ) m1

(4.4)

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161

Table 4.15 Statistics table sand mold wind erosion results Wind speed (m/s)

Blowing time (min)

Sand mould for spraying Wind dust suppressant erosion Sand Quality of rate (%) model sand model quality after wind before erosion (g) wind erosion (g) 103.0882

0.0049

Spraying deionized water sand model Sand model quality before wind erosion (g)

Quality of sand model after wind erosion (g)

102.2712

102.2325

Wind erosion rate (%)

7

10

103.0932

0.0378

30

103.0323

103.0292

0.0030

102.2185

101.8952

0.3163

14

10

103.0790

103.0768

0.0021

102.7069

102.2692

0.4262

30

102.8412

102.8358

0.0053

102.3780

96.1608

6.0728

where W—wind erosion rate, %; m1 —mass of sand mould before blowing, g; m2 —mass of sand mould after blowing, g. The wind erosion rate W of sand pile is calculated and the results are listed in Table 4.15. From Table 4.15, it can be seen that when the wind speed is 7 m/s, the wind erosion rate of the sand mold sprayed with deionized water is 0.0378% after 10 min of blowing with basically unchanged sand mold. And the wind erosion rate is 0.3163% after 30 min of blowing, which is expanded nearly 10 times, but still less than 1%. When the wind speed is 14 m/s, the wind erosion rate is still less than 1% after 10 min of blowing. However, when the wind erosion rate is 6.0728% after 30 min of blowing, which indicates that the consolidation layer on the surface of the sand mold is destroyed and there are small cracks. The sand under the consolidation layer is blown out from the destroyed cracks, which reduces the mass of the sand mold. For the sand mold sprayed with dust suppressant solution, regardless of wind speed of 7 or 14 m/s, blowing for 10 min or 30 min, the wind erosion rate is very small, basically 0%. When the wind speed is 7 m/s, wind erosion rate after blowing 30 min is greater than the that after blowing 10 min, which is because the sprayed dust suppressant has a certain moisture absorption and it absorbs the moisture in the air in the 30 min of blowing. Instead of being destroyed by the wind, the mass of the sand mold increases and the wind erosion rate decreases. There are clear international regulations on wind power rating, as shown in Table 4.16. Table 4.16 shows the wind rating scale, which is the current international standard for wind rating. Combined with Table 4.15, when the wind speed reaches 14 m/s, which is equivalent to seven strong gale, the solidification layer of the sand mold sprayed with deionized water is destroyed after blowing 30 min, while the wind erosion rate of the sand mold sprayed with dust suppressant is basically 0%, much smaller than that of the sand mold sprayed with deionized water. It is in line with the

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Table 4.16 Wind power rating scale Wind scale

Name

Wind speed (m/s)

Land surface image

Wave height (m)

0

No wind

0.0–0.2

Quiet, The smoke went up

0.0

1

Soft wind

0.3–1.5

Smoke indicating wind direction

0.1

2

Light wind

1.6–3.3

Feel the wind

0.2

3

Breeze

3.4–5.4

Banners spread out

0.6

4

Gentle wind

5.5–7.9

Blow up the dust

1.0

5

Cool breeze

8.0–10.7

The little tree swayed

2.0

6

Fierce wind

10.8–13.8

The wires are noisy

3.0

7

Wind

13.9–17.1

Walking difficulties

4.0

8

Gale

17.2–20.7

Break the branch

5.5

9

Strong Gale

20.8–24.4

Small loss house

7.0

10

Strong fierce wind

24.5–28.4

Pull up trees

11

Storm

28.5–32.6

Serious damage

11.5

12

Typhoon

>32.6

Devastating

14.0

9.0

dust suppressant wind erosion resistance requirements and meets the actual application conditions. In actual use, the dust suppressant solution can be properly diluted as needed and it can also resist very strong wind erosion, so that the dust of the pile is not raised and has a very good dust suppression effect.

4.4.5 Rain Resistance The purpose of rain resistance performance analysis is to investigate whether the material pile sprayed with dust suppressant can withstand the test of rain erosion without loss in natural environment. If the surface of the material pile remains the original shape after being washed with water, and there is no loosening or dissolution of dust suppressant. The covering performance of the consolidated layer is still maintained, and it is no longer to spray dust suppressant for consolidation. The conical sand mold sprayed with 15 ml of dust suppressant solution configured according to the optimal formulation and that sprayed with 15 ml of deionized water are placed in a high temperature blast drying oven at 80 °C until the mass no longer changes, weighed and the original mass of the sand mold is recorded. A spray nozzle is used to spray water vertically into the sand mold at a flow rate of 2 mL/s to simulate rainfall. The spraying continues for 2 min and then the molds are dried and weighed,

4.4 Performance Study on Dust Suppressants of Open Ore Stacking Yard

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Fig. 4.19 Mass residue rate curves of sand molds sprayed with dust suppressant solution and deionized water

which is one cycle. The spraying cycle is repeated 5 times and the mass residue rate L of the sand mold is calculated each time with the formula shown in Eq. (4.5). The mass residue rates of the 2 groups of sand molds are recorded.  L = m2 m1

(4.5)

where L—mass residue rate, %; m1 —mass of sand mould before spraying, g; m2 —mass of sand mould after spraying and drying, g. The change of mass residue rate of two groups of sand molds with spraying times is drawn into a broken line statistical chart, as shown in Fig. 4.19. Figure 4.19 shows the mass residue rates of the 2 groups of sand molds prayed with dust suppressant solution and deionized water after multiple water erosions. As can be seen from the figure that the mass loss of the sand mold sprayed with deionized water is more than 10% after the first spraying. After five spraying cycles, the mass residue rate is only 52.77% and the loss is close to 50%; while the sand mold sprayed with dust suppressant solution has almost no mass loss after five spraying cycles, and the mass residue rate is still as high as 99.89%. This shows that the consolidation layer formed by the dust suppressant has strong water erosion resistance, which can effectively prevent water and soil loss caused by rainwater erosion and meet the actual needs of sand fixation and dust suppression.

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4.4.6 Freeze–Thaw Resistance Property The actual using environment of the dust suppressants is harsh, which is in the areas of severe land degradation on the edge of deserts, especially in desertification areas. During the day, the temperature in the area directly under the sun may reach more than 60 °C, and at night, the temperature may plummet to below −15 °C. In addition, due to the man-made materials such as concrete and steel using extensively in the buildings, Urban Heat Island Effect further aggravates, and the temperature difference between day and night also significantly increases compared to the natural environment. Therefore, the actual application environment of the dust suppressant is often accompanied by drastic temperature changes, which has high requirements on freeze–thaw resistance of the dust suppressants. In this article, we want to examine the coping ability of the dust suppressant of the stockpile to the large span of temperature drastic changes occurring within 24 h. After several freeze–thaw cycles, the compressive strength of the sand column of the dust suppressant after experiencing different freeze–thaw cycles is measured, and the compressive strength of the sand column is regarded as an evaluation index to measure the consolidation performance of the dust suppressant and the fatigue degree of material. Five cylindrical sand columns sprayed with the dust suppressant with the best formulation are prepared, and these sand columns are kept at −20 °C for 12 h and then put at 80 °C for 12 h, which is one cycle for freezing and thawing. It needs 5 cycles. For each freeze–thaw cycle, with a sand column at a constant room temperature of 25 °C, the universal material testing machine is used to conduct a compression test, keeping the compression speed at 2 mm/min until the sand column cracks. The changes of the compression strength of the sand column throughout the compression process are analyzed using a software, and the maximum compression strength is selected. The changes of compression strength of the cylindrical sand mold after freeze–thaw experiment have been measured. The sand column made by spraying the dust suppressant with the best ratio is dried under natural conditions, and the freeze–thaw cycle experiment is conducted to measure the change of compression strength of the cylindrical sand mold after the freeze–thaw experiment. Figure 4.20 is the freeze–thaw resistance curve of the sand mold made by dust suppressant. As can be seen in Fig. 4.20, after one freeze–thaw test, the compression strength of the sand column decreases by 45.09%; after two freeze–thaw tests, the compression strength of the sand column increases by 2.23%; after three freeze–thaw tests, the compression strength of the sand column increases again by 11.48%. The sand column after a freeze–thaw test, the compression strength decreases dramatically, indicating that the limit conditions of drastic temperature change may destroy the structure of dust suppressant to some extent. After the second and third freeze–thaw cycles, the compression strength of the sand column increases slightly as the freeze– thaw test makes the dust suppressant penetrate into the cavities and grooves between the sand grains, leading to the tighter bonding between the dust suppressant and the

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Fig. 4.20 Compression strength curves of sand columns under different number of freeze–thaw cycles

sand grains and the larger compression strength. After the fourth freeze–thaw cycle, the compression strength of the sand column does not change significantly. After the fifth freeze–thaw cycle, the compressive strength of the sand column decreases, by 15.20%, which is consistent with the general law that polymer adhesives appears after several freeze–thaw tests. However, from the second freeze–thaw cycle, the rangeability of compression strength of the sand column becomes smaller, and the strength decay process is relatively slow under the severe temperature changes. This indicates that the compressive strength of the dust suppressant studied in this article does not continuously and dramatically decline with the increase of freeze–thaw cycle numbers. It basically remains stable after a severe temperature change with good freeze–thaw resistance, meeting the needs of outdoor practical applications.

References 1. Wang RS, Zhou XH. Researches on arising mechanism and influencing factors on diffusion of static coal pile dust with wind erosion. World Sci-Tech R & D. 2015;37(05):514–8. 2. Chen JB, Wang MY, Hu SC, et al. Influencing factors of dust in yard and application progress of dust suppression technology. Gansu Sci Technol. 2016;32(11):53–55+32. 3. He XY. Study on the coal and ore dust characteristic of Docks, Caofeidian. Hebei University of Science and Technology;2011. 4. Lei P. Research on dusting Rules for Bulk-cargo Yard of Iron Ore and Coal terminal. Tianjin university;2014. 5. Yang D. Research on Dusting and Migration Rule of the Dust at Open-air Coalyard. Liaoning Technical University;2007. 6. Wang XF, Liu Q, Tang ZG, et al. Wind tunnel test study on dust pollution. Environ Sci. 1987;06:21–5. 7. Liu X, Wang HN, Zhang YB, et al. Research status of road dust generation and controlling technologies in open-pit mines. Nonferrous Metals Sci Eng. 2016;7(3):100–6. 8. Simon WH. Road maintenance coherent strategies. World Min Equip. 1993;17(12):19–21.

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9. Wang K, Ma X, Jiang S, et al. Application study on complex wetting agent for dust-proof after gas drainage by outburst seams in coal mines. Int J Min Sci Technol. 2016;26(4):669–75. 10. De BM. A modern procedure for physical for physical soil Degraduation Pedologie. Soil Condit. 1993;43(1):157–95. 11. Han JJ. Synthesis and properties of acrylate copolymer emulsion as water-soluble dust depressor. Beijing University of Chemical Technology;2007. 12. Lee SJ, Park CW. The shelter effect of porous wind fences on coal piles in POSCO open storage yard. J Wind Eng Ind Aerodyn. 2000;84(1):101–18. 13. Kim RW, Lee IB, Kwon KS, et al. Design of a windbreak fence to reduce fugitive dust in open areas. Comput Electron Agri. 2017;S0168169917300224. 14. Gillies JA, Etyemezian, Nikolich G, et al. Effectiveness of an array of porous fences to reduce sand flux: Oceano Dunes, Oceano CA. J Wind Eng Indus Aerodyn. 2017;168:247–259. 15. Cheng JJ, Lei JQ, Li SY, et al. Disturbance of the inclined inserting-type sand fence to wind– sand flow fields and its sand control characteristics. Aeol Res. 2016;21:139–50. 16. Cong XC, Du HB, Peng ST, et al. Field measurements of shelter efficacy for installed wind fences in the open coal yard. J Wind Eng Ind Aerodyn. 2013;117:18–24. 17. Li B, Sherman DJ. Aerodynamics and morphodynamics of sand fences: a review. Aeol Res. 2015;17:33–48. 18. Zhou YP, Li YQ, He ZH. The research status and development of dust suppression agent. Guangzhou Chem Ind. 2015;43(7):48−49+69. 19. Wang Q, Chen X, Gong-li, et al. Study on characteristics of elements in PM2.5 during Haze-Fog weather in winter in urban Beijing. Spectroscopy Spect Anal. 2013;33(6):1441–1445(5). 20. Feng SW. New ionic liquids based on tertiary sulfoniums and perfluoroalkyltrifuoroborates: synthesis characterization and properties. Huazhong University of Science and Technology;2010. 21. Gu JY. Theory and foundation of bonding, vol. 6. Beijing: Science and Technology Press;2004. p. 12–22. 22. Cheng SY, Chen ZG. Adhesive production and application manual, vol. 4. Beijing: Chemical Industry Press;2003. p. 5–35. 23. Zhao WB, Dong QF, Bo FL, et al. Study on characteristics of composite fire extinguishing spray sealing material. Saf Coal Mines. 2018;49(03):24–8. 24. Cheng XQ. The preparation and research OFA new environmentally friendly sand-fixation dust suppressant by Xanthan gum grafted copolymerization. Beijing University of Chemical Technology;2017. 25. Zhang L. Study on Synthesis and properties of Anti-freezing dust Suppressant. Lanzhou Jiaotong University;2017.

Chapter 5

Chemical Dust Suppression Technology of Road Surface of Strip Mine

Abstract This chapter analyzes the mechanism of road-surface raise dust, and discusses factors and control measures of road-surface raise dust. Through site test, it analyzes the distribution law of raise dust concentration in parallel direction and vertical direction in the center of the road, and the influence of external factors on dust concentration on the road surface. Based on the law of road-surface raise dust, a dust suppressant featuring in compound, hygroscopic and moist was developed.

5.1 Mechanisms of Dust-Raising on Road Surface of Transportation Roads of a Strip Mine [1, 2] As one of the air pollution sources of mining enterprises, dust-raising on the road surface of strip mines is characterized by paroxysmal dust generation in the process of receiving, mixing, and feeding ores., The dust generation area is large, and is affected by meteorological conditions, especially wind speed and direction, thus causing heavy pollutes of the dust-raising generated on the road to the surrounding atmospheric environment. The dust-raising on the road surface of the strip mine is mainly caused by the condition of the road per se and the vehicle-mounted ore-bearing rock. The transportation roads of strip mines are generally gravel-paved roads, while some roads for driving face or refuse dumps are properly leveled soil roads with low construction standards and quality, which is easily deformed and broken to loose dust under the load of heavy vehicles. Meanwhile, the strong friction between the tires of the vehicles and the ground further smashes the dust. In addition, the repeated impact, rolling, and friction on the road surface caused by the frequent operation of vehicles lead to its sinking, cracking, and the subsequent formation of wrinkles and pits, making the mining transportation vehicles produce large bumps and vibrations when transporting, and thereby causing dusts and ore-bearing rocks attached to the rim to scatter, and particles on the ground to vibrate to loosen the dust at the same time. Such repeated compaction and kneading reduces the size of dusts as well as their weight and cohesive force, making them easy to overcome the force between particles under the action of the external dust anchoring load and rise. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Wang et al., Chemical Dust Suppression Technology and Its Applications in Mines (Open-pit Mines), https://doi.org/10.1007/978-981-16-9380-9_5

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5.1.1 Dust Anchoring Load Dust anchoring load refers to the force that can overcome the gravity, cohesive force and gravitational force between dust particles to cause dust to rise. The critical load is named as the limit anchoring force, which is determined by Eq. (5.1): Fa0 = C + W + G

(5.1)

where Fa0 —limit anchoring force, N; W—weight of dust particles, N; C—cohesive force between dust particles, N; G—gravitational force between dust particles, N. The critical anchoring force is mainly determined by the weight of the dust particles and the cohesive force between the particles. The cohesive force C between adjacent particles causing dusts to rise is related to the polarity of the dust charge, water content, material composition, whether there is a filling medium between the dust particles, etc. If the charge carried by dust particle is known, the charge gravitational force between the particles will be determined using physics equations. The weight of dust particles W is determined by three factors: water content, material composition and particle size. Gravitational force G between particles universally exists and can be determined through physical equations if the mass of dust particles and the center distance between particles are known. However, due to the small gravitational force between particles, it is generally ignored. Therefore, critical anchoring force is mainly determined by the weight of dust particles W and the cohesive force C between the particles. The critical condition for dust raising is shown in formula (5.2): Fa ≥ Fa0

(5.2)

5.1.2 Dust-Raising Generated by the Shear Friction of Automobile Tires on the Road Surface Since most transportation vehicles on the road surface of strip mines have large tonnage, their tires are embedded and squeezed into the ground during the transportation, generating strong friction, as well as shearing and scraping between the tires and the ground, deforming and breaking the ground, and thus causing the dustraising on the road surface under the action of induced airflow [3]. The dust on the road surface overcomes its limit anchoring force under the rolling action of the wheels, and the dust particles will be adhered to the tire. The dust particles are in a state

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of instantaneous force balance at that moment. Figure 5.1 shows the instantaneous force of the dust particles on the rotating tires of the moving heavy trucks. In Fig. 5.1, T f is the tangential component (friction force) of the tire cohesive force; Ts is the normal component of the tire cohesive force; A is the air resistance with always the opposite to the tire rotation direction; C is the centrifugal force which is related to the mass m of the dust particles, the radius r from the axle center to the rim and the angular velocity ω at the rim (C = mr ω2 ); and G is the gravity of the dust particles. In addition, the dust is also affected by the action of air buoyancy in the vertical direction, which however is ignored here since it is small. Compared with the other three forces, the cohesive force of tires is more difficult to determine. According to the empirical equation [4] (5.3), the cohesive force of tires correlates to the weight of the motor vehicle, air humidity, and vehicle speed. Tmax = mc0 (W + c1 )(H + c2 )(V + c3 )(ρd + c4 )(N + c5 )

(5.3)

where W is the weight of the motor vehicle; H—humidity of air; V—vehicle speed; ρd —particle density; N—number of dust particles on road surface; c1 , c2 , c3 , c4 , c5 —experience. In summary, the instantaneous equilibrium force state of the dust particles adhering to the tire is shown in Eq. (5.4). T + mg + A + C = 0

(5.4)

Therefore, the conditions for the dust to rise from the tire in the tangential and normal directions are: T f max − A − mg sin θ ≤ 0; and Ts max − C − mg cos θ ≤ 0 (θ is the angle between the centrifugal force C and the gravity G). Fig. 5.1 Instantaneous force on dust

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5.1.3 Dust-Raising Generated by the Mechanical Wind Load of the Vehicle The movement of transportation vehicles with a large load on the road surface of the strip mine inevitably causes ambient air flow. A huge negative pressure will be generated due to the squeezed space, thus forming an induced airflow. This phenomenon is usually called the “Bernoulli effect” [5]. In addition, due to the unevenness of the road surface, when the transportation vehicle passes, its wheels sink into the recessed road surface, causing the air inside squeezed out and new air replenished to form a powerful suction [6]. The strong induced air current will cause the broken dust on the ground and the fallen dust on both sides of the road to rise, forming secondary dust-raising. The mechanism of dust generation under the action of induced airflow is shown in Fig. 5.2. According to Bernoulli’s equation, the flowing air conforms to Eq. (5.5). 1 2 P0 1 2 Ph + gh + vah = + va0 ρa 2 ρa 2

(5.5)

where gh—gravitational energy per unit mass of fluid; P —pressure per unit mass of fluid; ρ 1 2 v —average kinetic energy per unit mass of fluid. 2 a It can be seen from Eq. (5.5) that when the height is h, the local pressure is the largest; when the height is less than h, the pressure gradient force does positive work on the dust-raising, and vice versa. If the outflow velocity is the highest at height h from the ground, the pressure difference will be formed at this height. According to the Law of Conservation of Energy, the pressure gradient force causes the dust to rise from the road surface. When any dust particle reaches the height h, it should conform to Eq. (5.6). 

h 0

1 f ds = ρd

 0

h

1 ∇ Pds ≈ ρd



h 0

∂ρ 1 dz = (Ph − P0 ) ∂z ρd

where f—pressure gradient force per unit mass of dust particles; s—mobile path of dust particles; Fig. 5.2 Mechanism of dust-raising of dust particles under the action of induced airflow

(5.6)

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z—the direction perpendicular to the ground. Equation (5.6) shows that the smaller the dust particle density, the easier it is to be raised. When the height is lower than h, there is a critical value to overcome the limit anchoring force and frictional resistance, and thus the maximum speed of the dust particles can be obtained. Equations (5.7) and (5.8) are obtained. 1 2 1 v = (Ph − P0 ) 2 max ρd  2 vmax = (Ph − P0 ) ρd

(5.7)

(5.8)

The maximum speed can be obtained from Eqs. (5.5) and (5.8), as equation formula (5.9).  vmax =

ρa ρd



1 2 1 2 va0 − vah − gh 2 2

 (5.9)

5.1.4 Dust Raised by Natural Wind Current on the Road Surface According to Brunt D’s estimation [7], when the wind speed exceeds 1 m/s, the air flow must be turbulent. Accordingly, the dust-raising caused by wind on the road of strip mine can be regarded as the movement of dust particles by turbulence. Under the action of turbulence, the forces that the airflow acts on a single dust particle mainly include: head-on resistance or drag force, raising force, impact force and the gravity of the dust particle. The force analysis is shown in Fig. 5.3. (1)

Drag force Fd

Fig. 5.3 Force analysis of dust particles

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This force is composed of two parts [8]: The first part is the friction Fd1 generated by the friction between the airflow and the surface of the dust particles, which does not pass through the center of gravity of the dust particles and has a different direction as the airflow direction because part of the surface of the dust particles indirectly contacts with the airflow. The second part is the wind pressure acting on the dust particles, that is, due to the separation of the streamlines on the top of the dust particles, vortexes are generated on the leeward side of the dust particles, thus generating resistance caused by pressure difference in the front and back of the particles, also known as the form resistance Fd2 . If the shape of dust particle is close to sphere, the form resistance will pass through its center of gravity. The general expression of drag force is shown in Eq. (5.10). Fd =

π purr2 d 2 C D 8

(5.10)

where ρ—air density, g/cm3 ; u r —relative velocity of airflow and dust particles, cm/s; D—dust particle size, cm; C D —drag coefficient is determined by the Reynolds number and particle shape. (2)

Raising force FL

This force comes from the rotation of dust particles and the shear of air velocity, as shown in Eq. (5.11). FL =

π u r × d 2 ρ 8

(5.11)

where —rotating speed of dust particles, r/s; u r —relative velocity of airflow and dust particles, g/cm3 ; D—dust particle size, cm; ρ—air density, g/cm3 . This force is related to the wind shear near the ground. (3)

The force Fi that hinders the movement of particles

The forces that hinder the movement of particles include weight W and forces between particles (van der Waals force, electrostatic force, capillary force, etc.). (4)

The influence of impact force Fm

In addition, the collision between particles will also affect the start of dust particles [9], and the collision force is caused by the collision of dust particles. According to the theorem of momentum, the momentum change of a particle equals the impulse of the acting force within that time in a certain time interval, as shown in Eq. (5.12).

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t S = mU2 − mU1 =

Fm dt

(5.12)

0

Considering that the impact force in the research is a constant force, equations (5.13) or (5.14) are obtained using the average theory. S = Fm dt = mu 2 − mu 1 Fm =

mu 2 − mu 1 S = t t

(5.13) (5.14)

where S—impulse; t—time when the force acts; m—particle mass; u2 , u1 —particle velocity before and after collision. Since the particle diameter of dust is generally small, the impact force between particles can be ignored. (5)

Thermophoresis, electrophoresis, photophoresis and other forces of dust particles

The thermophoresis, electrophoresis, and photophoresis of dust particles have significant effects only on ultrafine powder particles, which can be ignored in mineral powder particles, and the forces generated by unsteady flow fields are also ignored [10]. According to the relationship of the magnitude of these three forces, the condition for judging whether the particle is raising dust is: when F1 + FL > W +fi , the particle will raise dust; and when F1 + FL < W + fi , the particle will not raise dust. According to the theory of motion of microscopic particles, when the wind speed approaches a certain critical value under the action of wind, some individual particles are affected by turbulence and fluctuating pressure and begin to vibrate in their original position. When the wind speed exceeds the critical value, the vibration will increase accordingly, and the head-on resistance and rising force will also increase correspondingly, which is enough to overcome the gravity. Under the action of rotating torque, some unstable particles with high energy will first roll or slide along the surface. The geometry of particles and their spatial positions are diversified, and their force conditions are also changeable. Specifically, small particles in the particle swarm have good turbulence following features, which suspended in the atmosphere, and are driven by the incoming flow to move with the airflow. For some dust particles with relatively coarse particle diameter, their particle inertia force is relatively large. After obtaining the initial kinetic energy, they move along their own trajectory instead of drifting with the airflow, sliding and jumping in the air for a period of time. When these particles encounter bulging particles on the road surface or impacted by other moving particles, they will obtain a huge impulse, which will rapidly change

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the original horizontal movement direction to vertical direction, jumping up into the air flow and begin to move. In this way, dust-raising is formed and it spreads with the wind for a certain distance and then settles [11].

5.2 Influencing Factors and Control Measures of Dust-Raising on Transportation Roads in Strip Mines 5.2.1 Influencing Factors of Dust-Raising on the Road Surface of Strip Mines There are many spots to produce dust-raising on the road surface of strip mines, which are not fixed, and distributed in a large area. The road surface conditions (the nature and moisture content of the rock and soil on the road), the working status of the transportation vehicles (vehicle load, driving speed and density, etc.), and climatic conditions (wind speed, temperature, humidity, and the amount of dust in the air) will all affect the dust- raising concentration on the road surface of transportation road of the strip mine. (1)

Road conditions

(1)

Moisture content of dust on road surface

Compared with other influencing factors, the moisture content of the rock and soil on road surface is the main factor affecting the concentration of dust-raising [12], and the dust concentration decreases as the moisture content of the rock and soil on the road surface increases. The relationship between the two is shown in Fig. 5.4. It can be seen from the figure that when the moisture content is less than 0.5%, the dust concentration decreases faster with the increase of moisture content of rock and soil; when the moisture content reaches 2%, the reduction of dust concentration slows down as the moisture content of rock and soil increases; when the moisture content of the dust is about 4%, the dust concentration will be controlled within 2 mg/m3 ; when the moisture content is above 7%, there is no significant change in the dust concentration. Therefore, the optimal moisture content of rock and soil on road surface is about 4%, and the dust-raising concentration of the road surface can be kept in a low range at this time. (2)

Influence of dust load on road surface

When a transportation vehicle travels through the transportation road of a strip mine, it will inevitably drive the surrounding air flow to form an inducing airflow, which further causes the generation of secondary dust-raising of the dust on the road. The quality of dust deposition on road surface per unit area is called dust load on road surface, which is related to road conditions and cleanliness, and is also an important

5.2 Influencing Factors and Control Measures of Dust-Raising …

175

Fig. 5.4 Relationship between dust concentration and soil moisture content

factor affecting dust concentration on road surface. Based on actual measurement data and under the condition of a fixed vehicle speed at the same measuring point, when the dust load on road surface is below 0.1 kg/m3 , the dust concentration increases slowly, and it increases rapidly when the dust concentration on road surface is greater than 0.1 kg/m3 . It can be seen that by cleaning the road surface to keep the dust load on road surface below 0.1 kg/m3 , the secondary dust-raising from transportation vehicles can be reduced. (2)

Working conditions of transportation vehicles

Transportation vehicle models and tire models are generally different, which will also cause the differences in the number of wheels and load capacity of the vehicle and further affects the concentration of dust on the road surface, causing different environmental pollution. In addition, the excessively high vehicle speed will increase the inducing airflow, which will aggravate the secondary dust-raising. The former Soviet Union once used the Bellas 540 to test the relationship between the dust intensity and the running speed when running on dry gravel roads, and it was found that the dust intensity increased with the increase of vehicle speed, as well as the increasing trend. In addition, the frequent change of speed of transportation vehicles reduce the sedimentation speed of suspended dust particles around and increase the dust concentration in the surrounding air. (3)

Climatic conditions

Different climatic conditions have a significant impact on the generation and diffusion of secondary dust-raising from vehicles. Weather conditions such as temperature, humidity, and wind speed will all have a certain impact on the diffusion, sedimentation, suspension and aggregation of dust particles.

176

(1)

5 Chemical Dust Suppression Technology of Road Surface …

Atmospheric temperature

The air convection movement changes with the change of atmospheric temperature, promoting the diffusion of dust particles in the air. Generally, the higher the altitude is, the lower the temperature is. However, under certain special conditions, the opposite phenomenon happens, i.e., the temperature inversion. The temperature inversion will seriously hinder air convection movement, causing a large accumulation of dust particles near the ground, seriously polluting the air especially in winter [13]. The actual measurement data of the former Soviet Union [4] showed that the dust concentration on the transportation road of a strip mine was 750–800 mg/m3 at − 25 °C, while it dropped to 200–250 mg/m3 at −5 °C in the same location under the same conditions. It can be seen that when a negative temperature occurs, the lower the temperature is, the higher the road dust concentration is and the greater the air pollution is. (2)

Atmospheric humidity

Atmospheric humidity can reflect water content in the atmosphere. Under high air humidity, dust particles floating in the atmosphere will inevitably contact with water molecules in the air, which will increase the weight of the dust particles and cause their sedimentation. When it rains or snows, fine particles will be combined into large ones under the action of precipitation, dust particles are more likely to settle, thus reducing the air pollution index. (3)

Wind intensity

Dust particles suspended in the air are generally spread by wind. When the wind intensity is grade 2–3, large dust particles will naturally settle due to their own gravity, while floating dusts with a smaller mass (particle size is 0.l–10 μm) are easy to accumulate, deteriorating the air pollution; when the wind strength is greater than grade 4, the movement of the atmosphere is accelerated, which is beneficial to the diffusion of dust and the reduction of the dust concentration. Meanwhile, it can resuspend the large particles of dust deposited on the ground, and the strong wind can transmit dust over long distances, thus polluting the surrounding areas.

5.2.2 Control Measures for Dust-Raising on Road Surface Physical and chemical methods are mainly used to control the dust-raising on road surface. (1)

Physical dust suppression methods

At this stage, physical dust suppression methods mainly include wind barrier dust suppression and sprinkling dust suppression, which reduce air pollution by controlling dust concentration.

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(1)

177

Wind barrier dust suppression

Wind barrier dust suppression refers reducing wind speed and the amount of dust by using effective barriers such as building, green belts, windscreens or earth embankments to suppress the wind and dust. Building complexes and green belts are greatly restricted in practical applications due to their shortcomings of large construction area, high one-time investment, long construction period, low flexibility, and maintenance. Wind-proof and dust-suppression nets are widely used in wind barrier dust suppression. Jin [14] made a relatively systematic elaboration on the dust-proof mechanism, network parameters and material of the wind-proof dust-suppression net. The mechanism was to reduce the kinetic energy of the incoming wind by reducing its speed, avoiding its vortex, and reducing its turbulence to reduce dust. The comprehensive dust suppression effect of a single-layer wind and dust suppression net can reach 65–85%, while a double-layer one can reach 75–95%. (2)

Sprinkling dust suppression

Sprinkling is a traditional dust suppression method which is still used in many places due to its simplicity and convenience. The most commonly used dust suppression method in our country is sprinkling cars. This method is greatly affected by environment, which is not only costly, but also affects the transportation efficiency of production vehicles. In addition, using this method in northern China in winter will cause road icing, which is unreasonable and unsafe. (2)

Chemical dust suppression methods

Chemical dust suppression methods mainly include: dust suppression with wet dust suppressant, dust suppression with hygroscopic inorganic salt, dust suppression with super absorbent, dust suppression with bonding organic chemical dust suppressant and dust suppression with compound chemical dust suppressant [15]. (1)

Dust suppression with wet dust suppressant

Due to the high surface tension of water, when pure water is sprayed to suppress dust, fine dust particles are not easy to be wetted by water, thus results in a poor dust suppression effect. For example, the capture rate of 2 μm dust is only 1–28%. Due to the limitations of dust suppression directly using pure water [16], spraying water with additives to control dust-raising on road surface have been developed at home and abroad. The additive is mainly a wetting agent, which is basically consists of surfactants and certain inorganic salts. Dust suppression with wet dust suppressant mainly improves dust suppression efficiency by increasing the surface tension of water. (2)

Dust suppression with hygroscopic inorganic salt

In order to improve the efficiency of dust suppression, inorganic salts such as MgCl2 , CaCl2 , and NaCl are often added to water to increase the hygroscopicity, which are usually formulated to a 10–30% aqueous solution and sprayed on the surface of

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the dust-raising [17]. Their good evaporation resistance and moisture absorption can bring a better dust suppression effect. However, this kind of dust suppressant is corrosive to tires and parts of vehicles, and causes pollution to the environment if not properly used. Therefore, this method needs to be improved. (3)

Dust suppression with super absorbent

In the past two decades, after extensive research by domestic and foreign scholars, super absorbents such as polypropylene glycol, polyvinylamide resin [18], sodium polyacrylate, polyethylene glycol laurate [19] have gradually been developed and applied. Super absorbents have strong consolidation, viscosity and hygroscopicity [20]. However, they are expensive and still need improvement. (4)

Dust suppression with bonding organic chemical dust suppressant

The preparation of the bonding dust suppressant is as follows: a certain amount of water is added into the container to be heated, then polysaccharide polymer fillers, polymer film formers and other additives are added, and the solution is stirred at low speed for about 6–10 min to form a uniform solution, and cooled to below 50 °C and sprayed with a spray gun and a nozzle [21]. The shell formed by spraying the bonding dust suppressant has strong resistance to rain erosion and wind resistance, as well as a relatively low cost and good prospect. (5)

Dust suppression with compound chemical dust suppressant

In order to improve the effect of dust suppression, compound dust suppressants become the trend. A compound chemical dust suppressant refers to a dust suppressant composed of two or more dust suppressants under certain physical or chemical conditions [22], which integrates functions such as moisture absorption, condensation, and adhesion. This kind of dust suppressant has a good dust suppression effect and is widely applied, but domestic research is still relatively few.

5.3 Experimental Research on the Law of Dust-Raising Diffusion on Road Surface The experimental mine is located at the foot of Yanshan Mountain and on the shore of Luan River in Qian’an City, Tangshan District, Hebei Province, which is known as “Tie Qian’an”. The mine was established in 1969. By the end of 2004, the ore amount in the stope was about 250 million tons, the amount of rock was about 650 million tons, and the total amount was 900 million tons. The current mining capacity is 56 million tons of stripping and 10 million tons of iron ore. The mine is 200 km west to Beijing, 80 km southwest to Tangshan City, and 20 km southeast to Qian’an City. The geographical coordinates are: east longitude 118°32 –118°36 , north latitude 40°06 –40°09 . The transportation in the mining area is convenient. The highway directly reaches Beijing and Tianjin via Qianxi, Zunhua, Jixian and Sanhe, and is connected to the Beijing-Shenzhen main line. The special

5.3 Experimental Research on the Law of Dust-Raising Diffusion …

179

railway line runs from the Shuichang Concentrate Station through the Qian’an mining area and connects with the Jingshan Line at Beijiadian Station (Beishui Line), and connects with the Tongtuo Line at Shaheyi Station. It is in a warm temperate zone with a semi-humid monsoon climate, where summer is hot with plenty of rainfall, and winter is cold and dry. According to the data from the local weather station, the seasonality of precipitation in the region is quite obvious, which mostly concentrated in spring and summer, accounting for 58–72.7% of the annual total precipitation. Among them, May to August are the months of concentrated precipitation months, accounting for 37– 59.4% of the total precipitation. The annual average precipitation is 711.9 mm, the maximum is 1180.1 mm the minimum is only 499.8 mm, and the average relative humidity is 44.3%. The temperature varies greatly with seasons. The average annual temperature is 17 °C, the highest temperature is 31.2 °C (July); the lowest average temperature is only 2.1 °C (January); the highest temperature in summer is 36.7 °C, while the extreme lowest temperature in winter is −10.1 °C. The wind power in the mining area varies greatly from season to season, spring and summer are greater than autumn and winter. The average wind speed usually does not exceed 3.1 m/s. Due to local climate changes, the maximum local wind speed can reach 14 m/s. The northeast wind prevails in the spring and summer, and the southwest wind prevails in the autumn and east seasons. Combining with the topographic data of the mining area and field investigation, this study has presupposed 4 sections of roads for short-term measurement, as shown in Fig. 5.5. Road 1 is located in the southwest of the mining area with an average elevation of 30 m, which is mainly used to measure the dust concentration at the same level and different distances from the center of the dust source; Road 2 is located in the east of the mining area with an average elevation of −46 m, which is mainly used to measure the dust concentration of the same level and different heights from the ground; Road 3 is located in the west of the mining area with an average elevation of −123 m, which is mainly used to measure changes in dust concentration of the same dust source over time; Road 4 is located in the south of the mining area with Fig. 5.5 Distribution map of the measured road surface

180

5 Chemical Dust Suppression Technology of Road Surface …

an average altitude of −62 m, which is mainly used to measure the changes of dust concentration of the same dust source over time. The spatial distribution experiment of dust concentration on the road surface mainly measures the concentration of dust raised by vehicles passed by, and the distribution of dust concentration at different distances from the wind direction down the road to the dust source at different heights. The data was mainly collected on Road 1. There were 2 digital dust meters, and a total of 2 sampling spots could be arranged each time. The experiment on the variation of the dust concentration with time was also carried out on Road 1. Only one sampling spot was required by using a digital dust meter. (1)

Concentration distribution of dust-raising in the direction parallel to the centerline of the road surface

There was a total of 5 test points in the horizontal direction, and each test point was separated by 10–20 m. In the vertical direction, a background concentration measuring point was set at 25m from the center of the road to record the dust concentration in the natural environment. Due to the limited number of equipment, this set of data in this experiment was measured under the same natural conditions with different vehicles passed by. Each sampling time was 6 s. The layout of the experimental measurement points is shown in Fig. 5.6. Figure 5.7 showed the dust concentration at the same distance from the center of the road. It could be seen from the figure that, although there was a slight fluctuation in the concentration distribution of the dust in the x-axis direction parallel to the center line of the road, it could be regarded as equal expect for 3 points. Therefore, dust-raising on the road surface can be regarded as a continuous and uniform linear source of dust. (2)

Concentration distribution of the in the vertical direction of the centerline of the dust-raising road

The layout of the sampling points in the experiment was shown in Fig. 5.8. The sampling points were 1 m, 5 m, 10 m, 15 m, 20 m, and 25 m away from the edge Fig. 5.6 Experimental layout of the concentration distribution of dust-raising on a plane parallel to the centerline of the road

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181

Fig. 5.7 Dust concentration at the same distance from the center of the road

Fig. 5.8 Experimental layout of the concentration distribution of dust-raising on the plane perpendicular to the centerline of the road surface

of the car respectively, with a sampling height of 0.1 m. Measurement point 14, which was background, was set 25 m away from the edge of the vehicle, and the measurement result was shown in Fig. 5.8. It can be seen from Fig. 5.9 that the dust concentration decreases with the increase in its height. The concentration reached maximum at 0.5 m, and it attenuated greatly at the height of 0.8 m. At the height of 2.2 m, the concentration of the dust-raising was almost the same as that of the natural dust fall. Implying that the main part of

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Fig. 5.9 Dust concentration at different heights from the road surface

the dust-raising could not move here, and thus proving the pulsation mode of the dust-raising proposed earlier. (3)

Concentration distribution of dust-raising in the vertical direction

The layout of sampling points was shown in Fig. 5.10. The sampling point was 3 m away from the center line of the road, and the height was 0.1 m, 0.4 m, 0.8 m, and 1.2 m respectively. In addition, a sampler was arranged 15 m away from the center of the road to measure the concentration of natural dust fall. The measurement results were shown in Fig. 5.11. It could be seen from the curve in Fig. 5.11 that the dust concentration on the road surface was 1 m away from the center line of the road when the vehicle is moving, that was, the concentration at the wheel was the largest. The concentration

Fig. 5.10 Layout of experimental measurement points for the concentration distribution of dustraising on the road in the vertical ground direction

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183

Fig. 5.11 Dust concentration at different distances from the center of the road

was not much different from the that at 3 m. This may be that the reason that the dust-raising near the wheels were raised when the vehicle speed was high, and the dust-raising concentration would not be attenuated immediately. At 6 m, the dustraising concentration would be greatly reduced, while from 20 to 25 m from the center line, it hardly changed, where the dust-rasing concentration dropped to the background concentration. It could be inferred that the dust concentration measured at this time was suspended particles in the air, with a small flying angle, low movement height, and small diffusion distance. (4)

Influences of external factors on dust concentration on the road surface

(1)

The influences of the vehicle with no load or full load on dust concentration on the road surface

Combining the actual situation of the experimental mine, the trucks were all special mining trucks ordered by Liugong Group. The maximum load was 130 t, and the speed was basically constant at 4 m/s. The road surface was mainly divided into sprinkled roads and dry roads. Vehicles were divided into the vehicle with full load or no load. Therefore, the main factors affecting the dust concentration were whether the road surface was sprinkled with water and whether the vehicle was fully loaded. Measurement were carried on while vehicles were full loaded and unloaded, the curve of dust concentration with time was measured for a period of time, as shown in Figs. 5.12 and 5.13. It can be seen from the comparison of the two figures that when the vehicle was fully loaded and the dust concentration reached the peak, the concentration value was about 2–3 times of vehicles with no load, indicating that vehicle load had a great influence on the dust generation. The higher the load, the higher the dust concentration and the longer it took for the dust to spread. The main reason was that when the vehicle was moving on uneven roads, the compressed air suddenly

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Fig. 5.12 Dust concentration variation curve with time when the vehicle is with full load

Fig. 5.13 Dust concentration variation curve with time when the vehicle is with no load

expanded and raised the dust on the road surface. For further analysis, when the vehicle speed was slow, the amount of compressed air in the pit was less than the pressure difference caused at the same time when the vehicle was with no load. Therefore, there was relatively little dust generated when released instantaneously. On the contrary, when the vehicle was full loaded was, the pressure difference caused by the large load was relatively large. Therefore, the dust concentration generated was relatively large, and so did the height of the dust-raising.

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Fig. 5.14 Comparison of dust concentration before and after sprinkling the road

(2)

The influence of sprinkling and non-sprinkling on road dust concentration

The dust concentration of a mine car during normal driving was measured under the conditions of sprinkling and no sprinkling on the road, and the results were shown in Fig. 5.14. Through the comparison of the results, it could be found that after the road surface was sprinkled and the humidity of the road surface increases, the dust generated by a mine car during its normal movement was significantly reduced, and the concentration was reduced greatly, which was about 10% of the concentration of un-sprinkled road surface. The concentration value after sprinkling was close to the that of natural sedimentation, indicating that sprinkling on the road surface had a good dust suppression effect on automobile dust-raising.

5.4 Development of Compound Dust Suppressant Formula on the Road Surface 5.4.1 Performance Requirements of Dust Suppressants In order to suppress dust, the developed dust suppressant should have the following main properties: (1)

The dust suppressant solution has good water retention, moisture absorption and release properties [15]. The water retention performance of the dust suppressant solution is characterized by its anti-evaporation property, which can ensure less water evaporation under higher temperature. Meanwhile, dust

186

(2)

(3)

(4)

(5)

(6)

5 Chemical Dust Suppression Technology of Road Surface …

sprayed with the dust suppressant solution can change with the change of relative ambient humidity to maintain a certain humidity to prevent the generation of dust-raising. The dust suppressant solution should have good wetting and penetrating properties. The wettability of the solution is related to its surface tension. Generally speaking, the lower the surface tension, the better the dust moistening effect of the solution. Therefore, in order to effectively wet the particles, is the solution is required to have a surface tension as low as possible. Meanwhile, after the dust suppressant solution is sprayed on the road surface, it needs to quickly penetrate to a certain depth below the road surface. The dust suppressant solution needs to have a certain viscosity and good consolidation performance [23]. The road surface of the strip mine stope is sprayed with a dust suppressant solution. After the dust is fully wetted, the viscosity of the dust suppressant solution will increase due to the reduction of water caused by evaporation. The surface layer dust particles will condense under the cohesiveness of the dust suppressant solution. increasing the diameter of the particles, better resisting the wind and achieving a good dust suppression effect. The dust suppressant solution needs to have good antifreeze properties and not freeze at lower temperatures. If icing occurs when dust suppressant is sprayed on the road surface in winter, it will threaten driving safety and endanger life. Therefore, the dust suppressant should be able to withstand lower temperatures and have good antifreeze performance. The dust suppressant solution needs to be safe, non-toxic, and environmentfriendly. If rainfall occurs, surface runoff water or seepage water may be formed. The dust suppressant solution will eventually enter the environment with the precipitation. If it is harmful to the environment, animals and plants will be jeopardized. Therefore, each component of the dust suppressant solution should be harmless. The implementation process of the dust suppressant solution is simple, and the cost is economical. The area of road surface of strip mines is often very large. If the implementation process is too complicated, it will increase labor intensity and restrict the wide application of this technology. Therefore, the cost of dust suppressant is required to be economical.

5.4.2 Dust Suppression Mechanism of Dust Suppressant The industrial dust suppression test conducted on the road surface of Sijiaying Iron Mine with the hygroscopic dust suppressant developed in the laboratory showed that the road surface sprayed by the dust suppressant is knotted, smooth and hard, and can keep moist for a long time and has a significant dust suppression effect. Its excellent dust suppression performance mainly originates from the characteristics

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187

Table 5.1 Vapor pressure of magnesium chloride solution Temperature (°C)

Saturated water vapor pressure (Kpa)

Vapor pressure (kPa) of magnesium chloride solutions with different mass concentrations (%) 10

15

20

25

10

1.228

1.2

1.133

1.067

1.000

0.9333

20

2.338

2.266

2.133

2.066

2.000

1.876

30

4.242

4.133

4.000

3.733

3.6

3.333

40

–

12.199

6.993

6.666

6.266

5.773

5

of its components. The dust suppression mechanism of the dust suppressant will be analyzed below from the characteristics of each component. (1)

The Role of Hygroscopic Agent

As the main component of dust suppressant, magnesium chloride has multiple properties such as hygroscopicity, moisturizing and condensing dust. (1)

Moisturizing performance

Table 5.1 shows the vapor pressure values of saturated water and aqueous solutions of magnesium chloride with different concentrations. It can be seen from Table 5.1 that at the same temperature, the vapor pressure of magnesium chloride solution is lower than that of saturated water, and the greater the concentration of magnesium chloride, the lower its vapor pressure. The evaporation experiment data of magnesium chloride solution shows that the greater the concentration, the smaller its water loss rate, the stronger the anti-evaporation ability, and the higher the water retention rate. Therefore, the water retention capacity of the magnesium chloride solution is mainly related to the vapor pressure. The greater the concentration of the magnesium chloride solution, the lower the vapor pressure, the stronger the anti-evaporation and the better the water retention capacity. (2)

Hygroscopic performance

The hygroscopicity of magnesium chloride is also related to the vapor pressure of the solution. Air is composed of dry air and water vapor. The total pressure of air is equal to the sum of the partial pressure of dry air and water vapor. The evaporation rate of ground water and the hygroscopic effect of the hygroscopic agent are all related to the partial pressure of water vapor. The partial pressure of water vapor can be reflected by relative humidity. The greater the relative humidity, the greater the water content and vice versa. The content of water vapor varies greatly in different regions, seasons and time periods. When the air humidity is high, the partial pressure of water vapor in the air is large which is higher than the partial pressure of water vapor on the surface of the hygroscopic agent. Under the action of the pressure difference, the water vapor in the air is absorbed by the hygroscopic agent, keeping the dust on the road surface moist. When the air humidity decreases, the partial pressure of water vapor decreases, and when

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the partial pressure of water vapor on the surface of the hygroscopic agent is equal to the partial pressure of air vapor, the hygroscopic agent stops absorbing moisture [24]. When the partial pressure of water vapor on the surface of the hygroscopic agent is higher than the partial pressure of air vapor due to the increase in temperature, the hygroscopic agent releases the water in it. It was observed in the experiment that the humidity changes in a day in Luan County were roughly as follows: From about 6 o’clock in the evening to 9 o’clock in the morning of the next day, the air humidity was greater than 35%, which was the hygroscopic time of the hygroscopic agent. From 9 a.m. to 6 p.m., the air humidity was lower than 35%, the dust suppressant lost its moisture. It is just because the hygroscopic agent has the ability to continuously absorb moisture from the atmosphere that the dust sprayed with dust suppressant contains a certain amount of moisture for a long time, greatly increasing the dust wind speed to achieve the dust suppression. (3)

Condensed dust characteristics

When a dust suppressant solution containing magnesium chloride is sprayed on the road, Mg2+ enters the pores of the dust particles with the solution, which can exchange with ions in the dust particles, and is adsorbed on the surface of the dust particles. As a result, the electrostatic attraction between adjacent dust particles is increased, enhancing the bonding strengths of the particles and make them condense to large dust particles. (2)

The role of macromolecule polymers

(1)

The existence of moisture between particles

Water between dust particles appears in four forms, namely adsorbed water, film water, capillary water and gravity water. Fine dust has a large surface area and superfluous energy in the surface of the particles. The particle surface has a certain electric charge, which forms an electric field on the particle surface. The polarized water molecules and hydrated cations within the electric field range are adsorbed on the particle surface to form an adsorbed water layer. The adsorbed water dipole molecules are arranged in a directional arrangement, maintaining electrostatic attraction. Therefore, the adsorbed water has a very large viscosity and cannot move freely between dusts. Unbalanced van der Waals molecular forces still exists after the particle surface absorbs water, a film of water is thus formed around the adsorbed water layer, and the binding force between the film water and the particle surface is much weaker than that of adsorbed water. The adsorbed water and the film hydrate are combined to form the molecular bound water, which can be regarded as the outer shell of the particle and it deforms together with the particle under the action of external force. When the particles are wetted beyond the maximum molecular binding water, capillary water is formed. It is the moisture outside the range of the particle’s electric molecular gravity. The negative pressure in the capillary can pull the particles together and increase the cohesion between them. When there is a capillary liquid column in

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189

a particle group of uniform particle diameter, the cohesion of the formed aggregate is: τ=

1−ε 1 45σ × × 16 ε d

(5.15)

where: σ —surface tension of the liquid, mN/m; D—diameter of particles, m; ε—porosity of agglomerates. It can be seen that the cohesive force between the powder formed by the capillary is proportional to the surface tension of the liquid and is inversely proportional to the diameter of the particles. After the particles are fully moistened, there is still gravity water, which can move between dust particles relying on its own gravity. It can be seen that of for the four forms of water existing on the surface of dust, the latter two are free water that can move between dust particles and easily lose water when subjected to evaporation. (2)

Water retention of macromolecule polymers

Dust on the road surface sprayed with dust suppressants will gradually lose water due to evaporation, but this water loss rate is greatly lower than that sprayed with clean water. In addition to the moisturizing ability of the moisture absorbent, macromolecular substances in dust suppressants also plays a role. The macromolecular substance is a water-soluble macromolecule polymer with a linear long-chain structure and a large number of hydrophilic polar groups, which can retain more water molecules around, delay water infiltration, so that more water is available to be evaporated. In addition, the energy generated by the combination of water molecules and polar hydrophilic groups by hydrogen bonds is relatively large, thus the energy consumed to evaporate this part of the water is large that greatly lowers the evaporation rate, ensuring that the road dust maintains a certain amount of moisture for a long time. (3)

Condensed dust

Linear macromolecular substance has polar groups that can be adsorbed with many substances to form hydrogen bonds. The adsorbed particles form bridges through “bridging”. One macromolecule can adsorb multiple particles, increasing the cohesion between dusts. As the moisture evaporates, the moisture between the dust particles gradually decreases, and the concentration of macromolecule increases to form a network structure, which combines more dust particles and increases the dust particle diameter. (4)

Viscosity

After the road surface is sprayed with dust suppressant, the road surface becomes smooth and compacted and loses its viscosity due to the evaporation of water. When the humidity increases, the road surface begins to absorb moisture and its viscosity

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increases, which can bond the settled dust with other wet dust to form large dust particles, losing the ability to fly. Although the emulsified asphalt road surface or cement road surface has a good degree of compaction, it is inevitable that the road surface will be dusty due to the falling rock or the dust brought by tires. If the dust is not cleaned in time, there will still be a problem of flying dust. The dust suppressant developed in this research can continuously re-bond the dust, avoiding the dust problem caused by untimely cleaning of emulsified asphalt road surface or cement and other dust-proof road surfaces. In addition to vehicles on the road surfaces of the stope, crawler equipment is also usually seen, which will turn up the road surface when running on ordinary sprinkled roads, and thus aggravate the powdering of the road surface. After the dust suppressant is sprayed on the road surface, the degree of turned-up surface is greatly reduced when this type of equipment is running due to the high degree of compaction and increased strength. The small amount of turned-up surface, can be re-bonded with the surrounding road surface by the compaction effect of tires due to its viscosity after its moisture absorption. Therefore, compared with ordinary sprinkled roads, the road surface sprayed with the dust suppressant can not only reduce the amount of floating dust, but also greatly improve the level of road surface compaction, which helps to improve transportation efficiency. (3)

The effect of surfactants

According to the analysis of the particle size of the dust-raising on the road surface in industrial tests, the dust with a particle size of less than 65 mesh (0.25 mm) accounts for 45.19% of the total dust in the dust-raising on the road surface of the stope. The smaller the particle diameter of the dust, the larger the specific surface area, and the solution is not easy to be wetted when contact with it. The ability of the dust suppressant to wet the floating dust on the surface is a prerequisite for achieving dust suppression. The addition of surfactants to the dust suppressant solution can reduce the surface tension of the solution, facilitating the dust suppressant solution to better moisten the floating dust on the road surface and to increase the permeability of the solution.

5.4.3 Components of Dust Suppressant In order to meet the performance requirements, the main components of the dust suppressant should include: hygroscopic agent, coagulant and surfactant. (1)

Hygroscopic agent

Hygroscopic agents have the function of absorbing and releasing moisture, which can absorb water molecules in different states (liquid or gaseous). Under natural conditions, hygroscopicity is related to the relative humidity of the environment (or the vapor pressure of water). The higher the relative humidity, the better the

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191

hygroscopicity. The hygroscopic agent absorbs the moisture in the air and increases the moisture content of the dust on the road dust, increasing the bulk density of the dusts and suppress their ability to fly. At present, there are many kinds of hygroscopic materials in domestic market. Commonly used hygroscopic agents include inorganic salts, some organic solvents, and high-power absorbent resins appeared in recent years. Inorganic salt water absorbents include NaCl, MgCl2 , CaCl2 , K2CO3 , FeCl3 , active Al2 O3 , (NH4 )2 SO4 , silica gel, etc.; organic solvents include glycerol, ethylene glycol, caprolactam, sodium succinate, etc.; and high power absorbent resins include Sodium polyacrylate sol and starch grafted acrylate etc. After consulting a large number of literatures, and comprehensively combining the source, price, moisture absorption capacity and antifreeze performance of each agent, this study selected MgCl2 , CaCl2 , glycerol and ethylene glycol for experiments, which are denoted by A1 , A2 , A3 , A4 respectively thereinafter. (2)

Coagulant

The main function of the coagulant is to condense and retain water. The coagulant is generally a macromolecule material with a longer molecular chain, which binds fine dust particles into larger dust particles, increasing not only the weight of dust particles, but also their diameter. Meanwhile, the coagulant can also reduce the dispersion of dust, making it consolidate with the ground, improving the strength of the road surface, and playing a certain role in dust prevention. The selection principle of the coagulant is its strong binding and coagulation effect on dust and a certain aging effect. Its components have no negative effect on the environment, no harm to organisms and human, high temperature resistance, low price, and a wide range of sources. In addition, it is simple to use and has good environmental, economic and social benefits. Based on these requirements and referring to relevant literature, this experiment initially selected polyacrylamide, sodium polyacrylate, sodium carboxymethyl cellulose and carboxymethyl starch as aggregating agents for the test, which are denoted by B1 , B2 , B3 , B4 respectively thereinafter. (3)

Surfactant

As a prerequisite to achieve bonding, the dust suppressant solution should better wet the particles. In order to improve the wettability of the dust suppressant solution and for better permeability, surface active substances must be added to the solution to reduce its surface tension. The smaller the surface tension, the stronger its permeability. Surfactants can reduce the surface tension of the solution and enhance its wetting, dispersing, thickening and other properties. All liquids have a certain surface (interface) tension under certain conditions. Surfactant is on the two-phase surface (interface) of the entire system. By changing the properties of each interface, the surface tension value can be reduced, and the state of the surface (interface) can be changed at the same time. Surfactants have an asymmetric molecular structure, which contains not only hydrophilic groups but also certain lipophilic groups. It is a type of hydrophilic and lipophilic substance.

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Therefore, even the addition of a small concentration of surfactant will greatly reduce the surface tension of the solution and increase its permeability. The surfactant used in this experimental study should depend on the penetration of the dust suppressant. By consulting a large number of literatures, this experiment selected sodium dodecylbenzene sulfonate, sodium dodecyl sulfate and Tween-20 as the surfactant monomers, which are denoted by C1 , C2 respectively thereinafter.

5.4.4 Monomer Experiment (1)

Monomer experiment of hygroscopic agent

A certain amount of dust sample was weighed and placed in a petri dish (ϕ75 mm). A certain amount of hygroscopic agent was weighed and placed in a certain amount of water and continuously stirred with a glass rod to make the solvent stand still and to fully dissolve the solvent. 10 ml of the prepared solution is taken and evenly sprayed on the surface of the dust sample, to soak it. The sample preparation is completed. The dust sample preparation method is the same. In this experimental stage, the performance of the hygroscopic agent is evaluated by the moisture absorption of the dust sample under natural conditions and the anti-evaporation in the high-temperature drying box. Evaporation resistance (water retention) of the hygroscopic agent. During this phase of the experiment, the dust sample was placed in a high-temperature blast drying oven at a constant temperature of 45 °C for the anti-evaporation experiment, and the weight of the petri dish was measured every hour until there was no significant change in the value of the last two values. The temperature of the drying box was controlled at 55 °C, 65 °C, and 75 °C respectively, and the above experiment was repeated. The water retention rate of the dust suppressant was evaluated by the change rule of the water loss rate of the sample added with the dust suppressant. The calculation formula of water loss rate is shown in Eq. (5.16). η=

m0 − mi × 100% m

(5.16)

where, ϕ—moisture content of the dust sample, %; m0 —mass of the initial culture dish, g; mi —mass of the petri dish at an interval of i days, g; m—mass of 10 ml solution, g. The hygroscopicity of the hygroscopic agent under natural conditions. The dust sample under the condition of 75 °C was dried for two hours in a high temperature blast drying oven at a constant temperature of 110 °C, and taken out and weighed then. The temperature and relative humidity were recorded at that time, and the dust sample was placed in a laboratory environment at regular intervals, the weight of the petri dish was weighed to measure its hygroscopicity and moisture releasing property

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193

in natural environment, which was characterized by the hygroscopicity rate (moisture content) of the dust sample, and the calculation formula for the hygroscopicity rate is defined in Eq. (5.17). φ=

ωi − ω0 × 100% ω0 − ω1

(5.17)

where, φ—dust sample moisture content, %; ω0 —mass of the initial culture dish, g; ωi —mass of the petri dish at an interval of i hours, g; ω1 —mass of 10 ml solution, g. The hygroscopic agent experiment consisted of three parts. The monomer component with better hygroscopicity was selected through the separate selection experiment of four hygroscopic monomers. Through the pairwise compound selection experiment of four hygroscopic monomers, the components with better hygroscopicity were obtained. Then, the optimal ratio of components with better hygroscopicity was obtained through the compound selection experiment of hygroscopic agent A1 and A2 . This stage of the test was mainly to carry out analysis on the anti-evaporation and hygroscopicity of the four kinds of hygroscopic agent in the hygroscopic agent A1 , A2 , A3 and A4 through the following specific tests, and to find out one or two kinds of hygroscopic agent with the best anti-evaporation as the main component of the dust suppressor formula. The empirical dosage and concentration of hygroscopic agent were determined based on the physical and chemical properties of each monomer after consulting relevant domestic and foreign materials. The corresponding amounts of the four hygroscopic agents A1 , A2 , A3 , and A4 were weighed by using an electronic balance, and monomer solutions of the hygroscopic agent with a mass concentration of 15%, 20%, and 25% were respectively formulated to prepare 1#–12# dust samples 1#–12#, as shown in Table 5.2. 10 ml of clean water was taken to prepare 13# dust sample 13# as a control group. (1)

Hygroscopicity and moisture releasing property test under natural conditions

The weight of the 1#-13# dust sample is measured for 10 days at 9 a.m. and 6 p.m. every day, and the corresponding air temperature and relative humidity are recorded at each test time point. Table 5.2 Comparison table of dust sample numbers prepared by four hygroscopic agents of different concentrations Mass concentration (%)

A1

A2

A3

A4

15

1#

4#

7#

10#

20

2#

5#

8#

11#

25

3#

6#

9#

12#

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The measured data was processed according to the calculation equation of the water loss rate to calculate the water loss rate of the dust sample, and to draw the curve of the water loss rate over time shown in Fig. 5.15. The temperature and relative humidity changes at each measurement time point were shown in Fig. 5.16. It can be seen from Fig. 5.16 that the water loss rate of the 13# control dust sample sprayed with clear water was close to 100% after 2d, and maintained at 100% from

Fig. 5.15 Variation of water loss rate over time

Fig. 5.16 Changes in temperature and relative humidity over time

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195

2d to 10d. Its did not change with air temperature and relative humidity, indicating that pure water per se had no water retention capacity and poor anti-evaporation ability. While the dust sample sprayed with A1 , A2 , A3 , A4 solution had a water loss rate less than 100%. The water loss rate after 10 days could reach a minimum of 50% and a maximum of 90 %, indicating that the aqueous solutions of these four agents had a certain degree of water retention and can anti-evaporation under natural conditions. The dust samples sprayed with A1 , A2 , A3 , and A4 solutions lost water at a faster rate in about 3 days after the first measurement, and then the water loss rate gradually stabilized and water almost no longer lost. After reaching stability, the change trend of the water loss rate of the dust sample was basically the same as the relative humidity change trend in Fig. 5.16. It could be seen that the dust sample sprayed with a hygroscopic agent could absorb and release moisture. When the relative humidity in the air was high, it absorbed moisture in the air, and vice versa. The water loss rate of A2 solution (curve 6#) with a concentration of 25% was stable after spraying for about 3 days, the water loss rate was below 50%, and the water retention was good. It could be seen from the figure that when the solution concentration was the same, the water loss rate of A2 was the smallest and its water retention was the best, followed by A3 , then A1 , and the solution with the worst water retention was A4 . The same solution with different concentrations had different water retention effects. Generally, the water loss rate decreases with the increase of the concentration of the solution. (2)

Anti-evaporation test

The dust sample 1–13# were placed in a high-temperature blast drying oven at a constant temperature of 45 °C. The weight of each petri dish was weighed at an interval of one hour for continuous measurement for 7 h. The temperature of the hightemperature blast drying oven was controlled at 55 °C, 65 °C, and 75 °C respectively. The measured data was processed according to the equation of the water loss rate, the water loss rate of the dust sample was calculated, and its curve over time was drawn, as shown in Figs. 5.17, 5.18, 5.19 and 5.20. It could be seen from Fig. 5.17 that under the condition of 45 °C, the water loss rate of all dust samples within 7 h had an upward trend. The water loss rate was faster in the first 5 h and slower in the next two hours, and the water loss of the sprinkled dust sample (13#) was much greater than that of the rest of 12 dust samples. At hour 7, the water loss rate of dust sample 6# was the smallest, and the water loss rate of dust sample 4# and 5# was also below 50%, showing that A2 hygroscopic agent had the best anti-evaporation ability. As the concentration of the hygroscopic agent increased, the water loss rate of the dust sample decreased, indicating that the anti-evaporation property of the hygroscopic agent increased with the increase of the concentration. It can be seen from Fig. 5.18 that under the condition of 55 °C, the water loss rate of all dust samples within 7 h showed an upward trend. Different from 45 °C, the water loss rate of dust sample 5# and 6# decreased at hour 4. The water loss rate changed slowly since hour 4, the water loss rate of dust sample 13# increased sharply in the first 4 h, slowed down after reaching 4 h. At hour 5, it basically lost

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Fig. 5.17 The change of water loss rate at 45 °C

Fig. 5.18 The change of water loss rate at 55 °C

water completely and reached a constant value. For the remaining dust samples, the water loss rate was greater before hour 5, and slowed down after that. At hour 7, the water loss rate of dust sample 6# was still the smallest, followed by dust sample 5#, while the water loss rate of dust sample 3# and 4# remained basically the same. It could be seen that the low-concentration hygroscopic agent A2 had the same anti-evaporation performance as the high-concentration A1 . The water loss rate of different concentrations of hygroscopic agents A3 and A4 could also be kept below 90% at hour 7, which was better than that of sprinkled dust. Therefore, hygroscopic agents can enhance the anti-evaporation performance of dust sample. It could be seen from Fig. 5.19 that under the condition of 65 °C, the water loss rate of all dust samples within 7 h showed an upward trend. At hour 3, the water

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197

Fig. 5.19 The change of water loss rate at 65 °C

Fig. 5.20 The change of water loss rate at 75 °C

loss rate of dust sample 5# and 6# decreased, and slowly changed after that. The water loss rate of dust sample 13# increased sharply in the first 3 h and slowed down when reaching hour 3. At hour 4, it basically lost water completely and reached a constant value. The water loss rates of the rest of the samples were relatively high before hour 3, and slowed down after that. After hour 5, the water loss rate did not change significantly and basically reached a stable state. At hour 7, the water loss rate of dust sample 6# was still the smallest, followed by dust sample 5#. As could be seen in the figure, the anti-evaporation performance of dust sample 8# at 65 °C was obviously better than that at 55 and 45 °C. This might be the reason that the location of the dust sample in the high-temperature blast drying box was different,

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and the temperature in the drying box might be uneven, but it had little effect on the overall experimental results. It can be seen from Figs. 5.17, 5.18, 5.19 and 5.20 that under different temperature conditions, the water loss rates of the dust samples sprayed with each solution were basically the same, and the higher the temperature, the faster the water loss of the dust sample. The water loss rate of the dust sample became smaller as the drying time became longer, and the water loss rate of the dust sample sprayed with the hygroscopic agent gradually stabilized. When the temperature of the drying oven was 45 °C, after drying for 7 h, the water loss rate of the control dust sample sprayed with clean water was about 90%; while the water loss rate of the dust sample sprayed with A2 solution with a concentration of 25% was only about 34%. When the temperature was 55 °C, after drying for 5 h, the clean water control dust sample completely lost water. Its water loss rate reached 100%, and A2 solution with the best spray concentration of 25% with the best anti-evaporation performance had a water loss rate of only 50% after drying for 7 h. When the temperature was 65 °C, after drying for 3 h, the clean water control dust sample completely lost water. Its water loss rate reached 100%, and the A2 solution with the best anti-evaporation spraying concentration of 25% had a water loss rate of only 56% after drying for 7 h. When the temperature was 75 °C, after drying for 5 h, the clean water control dust sample completely lost water. Its water loss rate reached 100%, and the A2 solution with the best spray concentration of 25% with the best anti-evaporation performance had a water loss rate of only 62% after drying for 7 h. Consistent with the phenomenon observed in the anti-evaporation experiment at room temperature, the anti-evaporation of the solution of the same concentration is A2 , A3 , A1 , and A4 in descending order. (3)

Hygroscopicity test

Dust sample 1–13# were placed in a high-temperature blast drying box, and the temperature was set to 110 °C. The samples were dried for 2 h until their moisture was completely evaporated, and then they were placed under natural conditions. The dust samples were weighed at 9 a.m. and 6 p.m. every day for 5 consecutive days, and the daily ambient temperature and relative humidity were measured at the same time. The moisture content of the dust sample was calculated according to their weight change, and a curve of the moisture content of the dust sample over time was drawn in Fig. 5.21. The daily temperature and relative humidity changes over time were shown in Fig. 5.22. It could be seen from Fig. 5.21 that the moisture absorption effect of dust samples sprayed with A1 and A2 solutions (1 #–6 # curve) were better, and the peak moisture content of dust samples sprayed with A2 solution of 25% concentration could reach 10%. The dust sample sprayed with A4 solution had a certain but not obvious hygroscopicity. The peak moisture content of dust samples sprayed with A4 solution of 25% concentration was only 1.8%. However, dust samples sprayed with A3 solution and clear water had no hygroscopicity, and their moisture content remained unchanged. In the first 2–3 days, the moisture absorption speed of the dust sample was fast, and its moisture content was basically stable on the third day. After the

5.4 Development of Compound Dust Suppressant Formula …

199

Fig. 5.21 Changing situation of moisture content of dust samples with time

Fig. 5.22 Changes of temperature and relative humidity with time

moisture content of the dust sample was basically stable, it changed with the relative humidity of the surrounding environment. When the relative humidity was low, the dust sample lost water, and vice versa. showing that the dust sample sprayed with hygroscopic agent could automatically absorb and release moisture according to the changes of the relative humidity of the air.

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Table 5.3 Comparison table for numbers of dust samples prepared for four kinds of hygroscopic agents with different concentrations

(4)

Number

10%

15%

1

A1

A2

2

A1

A3

3

A1

A4

4

A2

A1

5

A2

A3

6

A2

A4

7

A3

A1

8

A3

A2

9

A3

A4

10

A4

A1

11

A4

A2

12

A4

A3

Selection experiment of four kinds of hygroscopic agent monomer by pairwise mixture

Considering whether there was synergistic effect among the four hygroscopic agents of A1 , A2 , A3 and A4 , they were mixed in pairs to observe the interaction between the absorbents. An electronic balance was used to weigh the corresponding amount of A1 , A2, A3 and A4 respectively, 18 kinds of mixed solutions of hygroscopic agents at a ratio of 1:1 and 1:1.5 were respectively prepared, and dust sample 1-18# were prepared in accordance with Tables 5.3 and 5.4. Dust sample 1–18# were placed in high-temperature blast drying oven with a constant temperature of 45 °C, each culture dish was weighed every hour for 7 consecutive hours. The above steps were repeated and the temperatures of hightemperature blast drying oven were controlled at 55 °C, 65 °C and 75 °C respectively. According to quality changes of dust samples, the changes of water loss rate of dust samples were calculated, and the change curve of water loss rate of dust samples with time was drawn, as shown in Figs. 5.23, 5.24, 5.25 and 5.26. Table 5.4 Proportion table in pairs for four kinds of solutions with the concentration of 1:1

Number

10%

10%

13

A1

A2

14

A1

A3

15

A1

A4

16

A2

A3

17

A2

A4

18

A3

A4

5.4 Development of Compound Dust Suppressant Formula …

201

Fig. 5.23 Changes of water loss rate with time at 45 °C

Fig. 5.24 Changes of water loss rate with time at 55 °C

It could be seen from Fig. 5.23 that the water loss rates of all dust samples showed an upward trend within 7 h, and the speed of water loss basically remained unchanged. The water loss rate of dust sample 4# remained the smallest in every hour, followed by dust sample 7# and 10#. It could be found that dust samples 4#, 7# and 10# were compounded from 15% of A1 , which showed that A1 played a dominant role in the anti-evaporation of hygroscopic agents. The higher the concentration of A1 , the stronger the anti-evaporation of the absorbent. The water loss rates of dust sample 1–18# were all below 60% at hour 7, showing that the anti-evaporation performance of the solutions in pairs of the four hygroscopic agents were all good. It could be seen from Fig. 5.24 that the water loss rates of all dust samples showed an upward trend within 7 h at 55 °C. The water loss rates of 18 dust samples were

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Fig. 5.25 Changes of water loss rate with time at 65 °C

Fig. 5.26 Changes of water loss rate with time at 75 °C

50–80% at the hour 7, but different from that at 45 °C, the speeds of water loss of dust sample 4#, 10#, 3# and 7# dropped off at the hour 4, and slowed down after that. The speeds of water loss of the rest dust samples were faster before the hour 5, but slowed down after that. The range of water loss rate was 50–80% at hour 7, which was larger than that at 45 °C. The range of water loss rate of dust sample 4# and 10# were basically the same, with a minimum value of 55% at hour 7. The content of A1 in dust sample 4# and 10# was 15%, but the difference was that dust sample 4# contained 10% A2 , while dust sample 10# contained 10% A3 . Therefore, it could be seen the important indicator influencing the anti-evaporation performance of the hygroscopic agent at 55 °C was the content of A1 , that was, the more the content of A1 , the better the anti-evaporation performance. The contents of the other three

5.4 Development of Compound Dust Suppressant Formula …

203

hygroscopic agents also had certain influences on the anti-evaporation performances of dust samples, but the influences were small. It could be seen from Fig. 5.25 that the water loss rates of all dust samples showed an upward trend within 7 h at 65 °C, and the change trend of water loss rates was basically the same with that at 55 °C. The water loss rates of 18 dust samples were 60–80% at hour 7, the speeds of water loss of dust sample 4#, 7# and 10# dropped off at the hour 4, and slowed down after hour 4, and the water loss curves of the three dust samples basically overlapped. The speeds of water loss of the rest dust samples were faster before the hour 5, and slowed down after that. It could be seen from the figure that the biggest change of water loss rate was dust sample 16# which meant that the water retention effect of 10% A2 mixing with 10% A3 was not good compared with other mixtures, and there was no synergistic effect between the two hygroscopic agents. Dust sample 4#, 7# and 10# with lowest water loss rates all contained 15% A1 , which was basically the same as that at 55 °C, that was, A1 played a leading role in the anti-evaporation performance of dust samples. It could be seen from Fig. 5.26 that the water loss rates of all dust samples showed an upward trend within 7 hours at 75 °C, and the change trend of water loss rate was to some extent different from that at 45 °C, 55 °C and 65 °C. The change of water loss rate of 18 dust samples was different in each hour, but the change rate gradually decreased, which reached a stable state at hour 5–7, and remained unchanged. The water loss rates of 18 dust samples were around 60–80% at the hour 7. It could be seen from the figure that the biggest change of water loss rate was dust sample 16# and 17#, which all contained 10% A2 . The difference was that dust sample 16# contained 15% A3 , and 17# contained 15% A4 . The water loss was greater when the content of A3 and A4 was more, indicating that these two hygroscopic agents had little effect on the anti-evaporation performance of dust samples, and no synergistic effect with A2 was found. For dust sample 1#, 4# and 10# with the lowest water loss rates, 4#, 7# and 10# all contained 15% A1 , and 1# contained 10% A1 and 15% A2 , which also showed that A1 played a leading role in the anti-evaporation performance of the dust samples. Moreover, A1 and A2 had a certain synergistic effect after mixing, and the effect was better. The change trend of water loss rate of dust samples was basically the same under different temperatures. The mixture with the best anti-evaporation was 10%A2 + 15%A1 (4# curve), and the water loss rates of dust samples were respectively 40%, 55%, 64% and 70% after being dried for 7 h when the oven temperatures were 45 °C, 55 °C, 65 °C and 75 °C respectively. The water loss rates were about 20% lower respectively compared with the mixture of 10%A2 + 10%A4 (16# curve) with the worst anti-evaporation. The speeds of water loss decreased with the increasing drying time, and the water loss rates tended to be stable finally. When the oven temperature was 75 °C, the water loss rates of dust samples tended to be stable after being dried for 5 h. ➀

Hygroscopicity experiment

Dust sample 1–18# were placed in the high-temperature blast drying oven, and the temperature was set at 110 °C. The samples were dried for 2 h until their moisture

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was completely evaporated, and then they were placed in natural conditions. The samples were weighed at 9 a.m. and 6 p.m. every day for 5 consecutive days, and the ambient temperature and relative humidity were also measured every day at the same time. The moisture contents of dust samples according to their weight changes were calculated, and the change curve of moisture contents of dust samples with time was drawn in Fig. 5.27. The daily changes of ambient temperature and relative humidity were shown in Fig. 5.28. It could be seen in Fig. 5.27 that dust samples sprayed with solution 1# to 18# all had certain hygroscopicity, and the changes of their moisture contents were basically consistent with that of relative humidity in Fig. 5.28, indicating that the hygroscopic agent could absorb moisture in the air and keep the dust samples moist when the relative humidity of the surrounding air was relatively high. The whole curve could be divided into three parts. The first part included dust sample 1#, 4#, 7#, 10# and 13#, whose moisture content were basically 6–11% after 3 days and the hygroscopicity was better under natural condition The dust samples sprayed with 10%A2 + 15%A1 solution (curve 4#) had the best hygroscopicity, whose moisture content could reach 9% after 3.5 days. The second part included dust sample 2#, 3#, 6#, 11#, 14#, and 17#, whose hygroscopicity were slightly worse compared to that of the former group and their moisture contents were basically 2–5% after 2 days. The third part included dust sample 5#, 9#, 12#, 16# and 18#, whose moisture content were basically less than 1% within 5 days and basically unchanged with poor water absorption effect. Dust sample 9#, 12# and 18# were composed of A3 and A4 with different proportions. Comparing the hygroscopicity of dust samples with single A3 and single A4 , their moisture absorption effects were not good whether the two hygroscopic agents act separately or mixed together. Sample 4# were composed of A3 and A2 with different

Fig. 5.27 Changes of moisture contents with time

5.4 Development of Compound Dust Suppressant Formula …

205

Fig. 5.28 Changes of temperatures and relative humidity with time

proportions, but there was good hygroscopicity if A2 was used alone. Therefore, it was indicated that the hygroscopicity decreased if the two hygroscopic agents were mixed, and no synergistic effect was found. To sum up, the anti-evaporation and hygroscopicity of the hygroscopic agents A3 and A4 were not as good as that of A1 and A2 . Therefore, further experiments were carried out on the hygroscopic agents A1 and A2 to investigate which proportion of the two substances mixture has the best anti-evaporation and hygroscopicity effect. (5)

Selection experiment for the mixture of hygroscopic agent A1 and A2

Compared with the previous two groups of experiments, the water loss rates of dust samples had stabilized after being dried for 7 h when the oven temperature was 75 °C. The water loss rate of dust samples sprayed with 25% A2 solution was about 60%, and that of dust samples sprayed with 10%A2 + 15%A1 mixed solution was about 70%, which all had good anti-evaporation performance. The hygroscopicity of the above two solutions was also good in the hygroscopicity experiment. The moisture content of dust samples was basically stable after being placed for 3.5 days under normal temperature, which was about 9%. Both of the above two solutions had better anti-evaporation and hygroscopicity, and the economic cost of the mixed solution was lower. Therefore, the mixed solution of A1 and A2 shall be further studied. An electronic balance was used to weigh the corresponding amount of hygroscopic agents A1 and A2 respectively, and 13 mixed solutions of hygroscopic agents were prepared according to Table 5.5.

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Table 5.5 Table for mixed concentration of hygroscopic agents A1 and A2

➀

Number

A1

A2

1

5%

20%

2

10%

15%

3

15%

10%

4

20%

5%

5

5%

15%

6

10%

10%

7

15%

5%

8

5%

10%

9

10%

5%

10

5%

5%

11

25%

0

12

20%

0

13

0

25%

Anti-evaporation experiment

Dust sample 1#–13# were put in a high-temperature blast drying oven with a temperature of 75 °C, each culture dish was weighed every hour a for 7 consecutive hours. The changes of water loss rate of dust samples were calculated according to the weight change, and the change curve of water loss rate of dust samples with time was drawn in Fig. 5.29. It could be seen from Fig. 5.29 that the water loss rates of all dusts showed an upward trend within 7 h at 75 °C. The water loss rates of 13 dust samples changed Fig. 5.29 Changes of water loss rate with time at 75 °C

5.4 Development of Compound Dust Suppressant Formula …

207

differently in each hour, and the speeds gradually decreased. The water loss rates change reached a stable state within 5–7 h, and remain unchanged. The water loss rates of the 13 dust samples were 50–80% at hour 7. It could be seen from the figure that the changes of water loss rate could be divided into two parts. The first part included dust samples of 1#, 2#, 3# and 13# with lower water loss rates, and their speeds of water loss were also slower than that of other samples. The water loss rates were 57–62% at hour 7, from small to large was 25% A2 (13# dust sample), 15% A1 + 10% A2 (3# curve), 5% A1 + 20% A2 (1# curve) and 10% A1 + 15% A2 (2# curve) successively. The second part included the rest dust samples, which were basically mixed by A1 and A2 with lower concentration, and the water loss rates were not as good as that of the first part. Therefore, it could be seen from the figure that the higher the concentration of the two hygroscopic agents, the lower the water loss rate, and the better the anti-evaporation performance. However, the anti-evaporation performance of A2 was better, and it had synergistic effect when mixed with A1 in a certain proportion. ➁

Hygroscopicity experiment

Dust sample 1#–13# were put in a high-temperature blast drying oven with a temperature of 110 °C. The samples were dried for 2 h until their moisture was completely evaporated, and then they were placed in natural conditions. The dust samples were weighed at 9 a.m. and 6 p.m. every day for 5 consecutive days, and the ambient temperature and relative humidity were also measured every day at the same time. The moisture contents of the samples were calculated according to their weight change, and the change curve of moisture content of dust sample with time was drawn in Fig. 5.30. The daily changes of ambient temperature and relative humidity were shown in Fig. 5.31.

Fig. 5.30 Changes of moisture content with time

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Fig. 5.31 Changes of temperatures and relative humidity with time

It could be seen in Fig. 5.30 that all dust samples sprayed with solution 1#–13# had certain hygroscopicity, and the change law of the curve as basically the same. The changes of their moisture content basically remained the same as that of relative humidity in Fig. 5.31, but the effects of moisture absorption were different, indicating that the hygroscopic agents A1 and A2 could absorb the moisture in air and keep the dust samples moist when the surrounding air had larger relative humidity whether A1 and A2 acted separately or mixed together, and both of two hygroscopic agents had the hygroscopicity. It could be seen from the figure that the dust sample1#, which was composed of 5%A1 and 20%A2, had the best moisture content, and the moisture content remained between 8 and 10%. The second was dust sample 13# composed of 25% A2 . It could also be seen that the hygroscopicity of A2 was better than that of A1 , but the two substances had synergistic effect when mixed, and the hygroscopicity was enhanced. Dust sample 10# and 12# had the worst moisture content below 4%, both A1 and A2 were 5% in dust sample 10#, and the hygroscopic effect was not good due to lower concentration. There was 20% A1 in dust sample 12#. Its lower hygroscopicity indicated that the hygroscopicity was poor when A1 acted alone. At the same time, the hygroscopicity of hygroscopic agent has a large correlation with relative humidity based on Figs. 5.30 and 5.31, and the relative humidity was smaller on the 4th day, which was 23%. The moisture contents of all 13 dust samples were lower than that of the previous day. The higher the moisture content, the greater the degree of reduction of the moisture content. For example, the moisture content of dust sample 1# was reduced from original 9.2% to 8.2%, while 10# was reduced from 3.2 to 2.8%, indicating that the solution with better the hygroscopicity was more affected by relative humidity.

5.4 Development of Compound Dust Suppressant Formula …

209

Considering the above experimental results, it could be inferred that the antievaporation and hygroscopicity of hygroscopic agents A1 and A2 were enhanced with the increase of their concentrations, and A2 was better. But when the hygroscopic agents A1 and A2 were mixed, synergistic effect occurred, and the anti-evaporation was enhanced. From the anti-evaporation experiment, it could be seen that the antievaporation performance of 15% A1 + 10% A2 was better, and its water retention was basically the same when 25% A2 acted alone. Its hygroscopicity could be maintained at about 5%, which can meet the actual demand. Moreover, the cost was greatly reduced after the two substances were mixed, thus 15% a1 + 10% A2 was selected for the subsequent experiment. (2)

Experiment of coagulant monomer

The viscosity of liquid is the nature of internal friction between flowing liquid molecules, which is characterized by viscosity. Viscosity is not only an important parameter to evaluate the coalescence effect of dust suppressant, but also the most basic macroscopic property characterization of coagulant solution [25]. First, the viscosity of each coagulant was measured. The viscosity range of these coagulants was primarily understood by consulting the literatures, and solutions with different concentrations were respectively prepared. The viscosity values were measured by NDJ-S digital viscometer, The data of the experiment was recorded and observed. Secondly, the cohesiveness of the coagulant was tested. 30 g of dust sample was weighed and put into a culture dish with a diameter of 75 mm. 10ml solution was taken to be sprayed on the dust sample evenly. The sample was dried in a hightemperature blast drying oven at 110 °C for 2 h after basically soaked. Then the culture dish was taken out, and dust samples were peeled off to the culture dish respectively to be mixed in JJ-5 cement mortar mixer for 3 min, and then sieved with 40 mesh and 80 mesh standard sieve. The proportions of oversize and undersize respectively with standard sieve of 40 mesh and 80 mesh were calculated, and the particle diameter of each dust sample was analyzed, thus the cohesiveness of the coagulant was evaluated. The following four kinds of coagulants named C1 , C2 , C3 and C4 were selected for the coagulant experiment through literature review. The coagulation effect of the monomer was evaluated through viscosity measurement experiment and cohesiveness experiment. (1)

Viscosity measurement experiment

The coagulants C1 , C2 , C3 and C4 with a concentration of 0.5, 1.0 and 1.5% were preliminarily selected. The viscosity of the above solutions were measured by NDJ-S digital viscometer. The results were shown in Table 5.6. It could be seen from Table 5.6 that C1 solution had higher viscosity at both low and high concentrations. When the solution concentration was 1%, the viscosity of C1 solution was the highest, which was 22 mpa s. When the concentration of C2 and C3 solutions was 1.5%, the viscosity was higher, but the viscosity was very low when the concentration was low. However, the viscosity of C4 solution almost did

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Table 5.6 Viscosity number of coagulant Concentration/%

C1 /mpa s

C2 /mpa s

C3 /mpa s

C4 /mpa s

0.5

11

1

1.1

1.1

1.0

22

11

3.1

1.3

1.5

39

33.3

48

1.2

not change with the solution concentrations. The viscosity was lower and slightly larger than that of water, Therefore, C4 was excluded in the next experiment. (2)

Cohesiveness experiment

The solutions of C1 , C2 , C3 and C4 coagulants with the concentration of 1.0% were respectively prepared, and the dust samples were prepared for cohesiveness experiment. Statistics on the particle diameter distribution of each dust sample were made, as shown in Table 5.7. It could be seen from Table 5.7 that dusts with particle diameter larger than 0.425 mm for dust samples sprayed with C1 solution accounted for 36%, that was to say, there were still many dust particles sticking together after the dust sample was stirred, indicating that the viscosity effect was good. For dust samples sprayed with C3 and C4 solutions, the viscosity effects were basically the same, and dusts with particle diameter larger than 0.425 mm accounted for 28%. For dust samples sprayed with C2 solution, the viscosity effect was slightly poor, and dusts with particle diameter larger than 0.425 mm only accounted for 23%. It could be seen that C1 had better cohesiveness. In conclusion, C1 was selected as the coagulant monomer for the next experiment, and the concentration range was 0.6–1.2%. (3)

Surfactant monomer experiment

Surface tension is an important performance indicator of dust suppressant. The spreading performance of a solution is inversely proportional to its surface tension. Surface tension is related to the type and temperature of the liquid. In general, the surface tension of pure water is relatively high, which is about 72.75 × 10−5 n/cm. If temperature rises gradually, the interaction between the molecules inside the liquid becomes smaller, the gravitational force of molecules inside the liquid to molecules on the surface becomes smaller, while the gravitational force of gaseous molecules to surface molecules becomes larger, making the surface tension of the liquid become Table 5.7 Particle size distribution of dust sample

Potion

>0.425 mm

0.425–0.18 mm

RD > RC > RA by comparing each R value, the Table 5.13 Range analysis table for results of orthogonal experiment Number

A

B

C

D

7 h water loss rate/%

1

13%

8%

0.6%

0.6%

77.52

2

13%

9%

0.8%

0.8%

76.61

3

13%

10%

1%

1%

75.25

4

13%

11%

1.2%

1.2%

73.22

5

14%

8%

0.8%

1%

76.96

6

14%

9%

0.6%

1.2%

75.47

7

14%

10%

1.2%

0.6%

73.60

8

14%

11%

1%

0.8%

70.53

9

15%

8%

1%

1.2%

75.69

10

15%

9%

1.2%

1%

73.97

11

15%

10%

0.6%

0.8%

72.75

12

15%

11%

0.8%

0.6%

77.62

13

16%

8%

1.2%

0.8%

73.82

14

16%

9%

1%

0.6%

72.97

15

16%

10%

0.8%

1.2%

71.45

16

16%

11%

0.6%

1%

76.61

K1

302.59

303.99

302.35

301.71

K2

296.56

299.01

302.64

293.70

1194.04 (y.)

K3

300.03

293.06

294.43

302.78

K4

294.85

297.97

294.61

295.84

K1

75.65

76.00

75.59

75.43

K2

74.14

74.75

75.66

73.43

K3

75.01

73.26

73.61

75.70

K4

73.71

74.49

73.65

73.96

R

1.94

2.73

2.05

2.27

74.63 (y)

–

5.4 Development of Compound Dust Suppressant Formula …

219

primary and secondary order of the influences of experiment factors on experiment indexes was BDCA. The values of K 1 ,K 2 , K 3 , and K 4 reflected the degree of influence on experiment indexes of A1 , A2 , A3 , A4 . Because a smaller water loss rate was better, it could be judged that A4 was the optimal level of factor A as K 4 was the smallest. Similarly, the optimal levels of factors B, C and D were B3, C3, and D2 respectively. The combination of optimal level of factors A, B, C and D was A4 B3 C3 D2, which was the optimal level combination of orthogonal experiment for anti-evaporation. In order to more directly reflect the influence law and trend of the four factors in anti-evaporation orthogonal experiment on the index of water loss rate, the ordinate was set as the average value K of the experiment index, the abscissa was set as the factor level, and the factor level and index trend chart were drawn, as shown in Fig. 5.37. The optimal combination of A4 B3 C3 D2 was taken as an example, the index of the water loss rate was estimated. To sum up, the optimal level combination of the experiment was A4 B3 C3 D2, namely 16% A + 10% B + 1.0% C + 0.8% D. (2)

Orthogonal experiment for hygroscopicity

Dust sample 1#–16# were put in the high-temperature blast drying oven with the temperature of 110 °C. The samples were dried for 2 h until their moisture was completely evaporated, and then placed under natural conditions. The weights of dust samples were measured at 9:00 a.m. and 6:00 p.m. every day for 10 consecutive days, the changes of moisture content with time were shown in Fig. 5.38, and the ambient temperature and relative humidity were shown in Fig. 5.39. 76.0

Filtration rate/m/s

75.5

75.0

74.5

74.0

73.5

1

2

3

4 1

2

3

4 1

2

Factor level

Fig. 5.37 Trend chart

3

4 1

2

3

4

220

5 Chemical Dust Suppression Technology of Road Surface …

Fig. 5.38 Changes of water loss rate of dust sample with time

Fig. 5.39 Changes of temperature and relative humidity with time

It could be seen from the figure that the moisture content of the dust sample changed with time. The moisture content was rising faster in the first three days, and slower on day 3–7. The moisture content reached the maximum on day 7, and had an obvious downward trend on day 7–10, indicating that the dust sample sprayed with dust suppressant solution had a certain moisture absorption and release. Among the 16 groups of dust samples, the water absorption capacity of dust sample 16# was the strongest, and the moisture content could reach 10% on day 7, while the water

5.4 Development of Compound Dust Suppressant Formula …

221

absorption capacity of 13# was the weakest, and its moisture content reached 7% on day 7. However, the moisture content of 16 groups of dust samples remained above 4% on day 3–10, which indicated that dust samples sprayed with dust suppressant still had a certain moisture retention performance. As shown in the figure, the relative humidity and temperature of dust samples also fluctuated with time, in which the change trend of relative humidity was basically the same as that of moisture content of dust samples. The moisture content increased correspondingly when the ambient humidity increased, and vice versa. However, the change of ambient temperature had nothing to do with the moisture content of dust samples. The moisture content after 9.5d was taken as the index of orthogonal experiment for range analysis, and the results were shown in Table 5.14. Table 5.14 Range analysis table for results of orthogonal experiment Number

A

B

C

D

Moisture content after 9.5 days

1

13%

8%

0.6%

0.6%

6.29

2

13%

9%

0.8%

0.8%

6.52

3

13%

10%

1%

1%

7.01

4

13%

11%

1.2%

1.2%

7.45

5

14%

8%

0.8%

1%

6.45

6

14%

9%

0.6%

1.2%

6.92

7

14%

10%

1.2%

0.6%

7.27

8

14%

11%

1%

0.8%

7.78

9

15%

8%

1%

1.2%

6.59

10

15%

9%

1.2%

1%

6.81

11

15%

10%

0.6%

0.8%

7.37

12

15%

11%

0.8%

0.6%

7.82

13

16%

8%

1.2%

0.8%

5.76

14

16%

9%

1%

0.6%

7.14

15

16%

10%

0.8%

1.2%

6.97

16

16%

11%

0.6%

1%

8.25

K1

27.276

25.090

28.828

28.528

–

K2

28.413

27.386

27.756

27.421

K3

28.584

28.621

28.518

28.515 27.929

K4

28.119

31.297

27.290

Number

A

B

C

D

Moisture content after 9.5 days

K1

6.819

6.272

7.207

7.132

–

K2

7.103

6.846

6.939

6.855

K3

7.146

7.155

7.130

7.129

K4

7.030

7.824

6.823

6.982

R

0.327

1.552

0.385

0.277

–

222

5 Chemical Dust Suppression Technology of Road Surface …

It could be seen from the table that B, C and D were the factors where the level only appeared once in the four groups of experiments (A1 , A2 , A3 , A4 ) for the examination of factor A. Since there was no interaction among B, C and D, the four groups of experiments had exactly the same experimental conditions for A1 , A2 , A3 , A4 . The values of K 1 ,K 2 ,K 3 and K 4 reflected the degree of influence of A1 , A2 , A3 , A4 on moisture content indexes. Because larger water content was better, it could be inferred that K 3 was the optimal level of factor A as it was the largest. Similarly, the optimal levels of factors B, C and D were B3, C3, and D2 respectively. The combination of optimal level of factors A, B, C and D was A4 B3 C3 D2, which was the optimal level combination of orthogonal experiment for anti-evaporation. According to the range formular R A = K 1 − K 4 = 0.327, the ranges RB , RC , RD in other columns could be obtained in a similar way, and the results were shown in the table. It could be seen that RB > RD > RC > RA by comparing each R value. Therefore, the primary and secondary order of the influences of experiment factors on experiment indexes was BDCA. In order to more directly reflect the influence law and trend of the four factors in orthogonal experiment for hygroscopicity on the indexes of moisture content, the ordinate was set as the average value K of the experiment index, the abscissa was set as the factor level, and the factor level and index trend chart were drawn, as shown in Fig. 5.40. According to the above two groups of orthogonal experiments of anti-evaporation and hygroscopicity, the primary and secondary relationship of each factor was arranged according to their range. For anti-evaporation, BDCA was arranged from primary to secondary, and the optimal level was A4 B3 C3 D2 . In terms of hygroscopicity, BCAD was arranged from primary to secondary, and the optimal level was A3 B4 C1 D1 . Each factor needed to integrate two indexes to select the optimal

7.15

Water absorption/%

7.10 7.05 7.00 6.95 6.90 6.85 6.80

1

2

3

4 1

2

Fig. 5.40 Trend chart of factor and index

3

2 4 1 Factor level

3

4 1

2

3

4

5.4 Development of Compound Dust Suppressant Formula …

223

level. Firstly, the level according to the index in the main contradiction position was selected, and then the level according to the index in the main position was selected. Factor A was in the secondary position in anti-evaporation but in the primary position in the hygroscopicity, thus A3 was selected; Factor B was in the most important position in the two indexes of anti-evaporation and hygroscopicity, while water loss rate was in the main position for factor B, and the hygroscopicity was secondary, thus B3 was selected; Factor C was important in the hygroscopicity and anti-evaporation, and anti-evaporation was more important than hygroscopicity for evaluating the performance index of dust suppressant, thus C3 was selected; Factor D was in an important position in anti-evaporation but in the secondary position in hygroscopicity, thus D2 was selected. Finally, the optimal proportion of dust suppressant A3 B3 C3 D2 was determined, namely 15% A + 10% B + 1.0% C + 0.8% D.

5.4.6 Formula Optimization Experiment The solutions of 1 # (15% A + 10% B), 2 # (15% A + 10% B + 1% C + 0.8% D), 3 # (15% A + 10% B + 1% C + 0.8% D + corrosion inhibitor), 4 # (clear water contrast), 5# (15% A), 6 # (10% B) were prepared respectively. The corrosion rate of each coupon in different solutions was calculated according to the formula of corrosion rate, and the results were shown in Table 5.15. It could be seen from the table that the corrosion rates of A3 iron and 20# carbon steel in 4 # clear water solution were very small, and the corrosion rates were 0.67 g m−2 h−1 and 0.38 g m−2 h−1 respectively. When the solution was 1 # (15% A + 10% B), the corrosion rates of A3 iron and 20# carbon steel were the highest, which were 3.26 g m−2 h−1 and 2.22 g m−2 h−1 respectively. It could be seen that the corrosion rate of 1 # (15% A + 10% B) solution on A3 iron and 20# carbon steel was almost 6 times of that in 4 # clear water solution. The table showed that the corrosion rate of 5 # (15% A) solution was less than that of 6 # (10% B) solution, and the corrosion rate of 1 # solution mixed with the two solutions was between the two. When solution 2# (15%A + 10%B + 1%C + 0.8%D) was selected, the corrosion rates of A3 iron and 20# carbon steel were 1.14 g m−2 h−1 and 1.26 g m−2 h−1 respectively, which was reduced by twice compared with that of 1# solution. The Table 5.15 Table for results of corrosion rate (g m−2 h−1 )

Number

A3 iron

20# carbon steel

1#

3.26

2.22

2#

1.14

1.26

3#

1.54

0.92

4#

0.67

0.38

5#

2.02

1.63

6#

3.23

2.75

224

5 Chemical Dust Suppression Technology of Road Surface …

reason might be that coagulant C had a certain viscosity, which was easy to adhere to the surface of the metal coupon, protecting the metal coupon to a certain extent and reducing the corrosion of metal coupon in salt solution. A certain amount of corrosion inhibitor was added into 3# solution on the basis of 2# solution, but the corrosion effect was not obvious from the perspective of corrosion rate. The corrosion rate of A3 iron was 1.54 g m−2 h−1 , which was higher than that of 2# solution, and the corrosion rate of 20# carbon steel was 0.92 g m−2 h−1 , which was lower than that of 2# solution. It could be seen than the corrosion inhibitor had a certain corrosion effect on 20# carbon steel. On the other hand, the corrosion rates of the four solutions on A3 iron were higher than that on 20# carbon steel, suggesting that the anti-corrosion performance of 20# carbon steel was slightly better than that of A3 iron.

5.4.7 Performance Characteristics of Dust Suppressant (1)

Moisture absorption and release

The changes of moisture content of 6 groups of dust samples with time were shown in Fig. 5.41 after measuring for 10 consecutive days. The change of moisture content of dust samples sprayed with dust suppressant were shown in Fig. 5.41, and the change trend and range of moisture content of 6 groups of dust samples were basically the same. The moisture content gradually increased from the first day, reached the maximum of 8.5% on the sixth day, obviously decreased to below 4% on day 7 and 8, then rose obviously again on day 9 and 10, and reached nearly 5% on day 10, indicating that the dust suppression of the dust suppressant was not weakened after day 10, which could still absorb and release moisture with the change of relative humidity in the air. The moisture content of the 10

Fig. 5.41 Changes of moisture content of dust samples with time

1 2 3 4 5 6

9

Moisture content/%

8 7 6 5 4 3 2 1 0

0

1

2

3

4

5

6

Time/d

7

8

9

10 11

5.4 Development of Compound Dust Suppressant Formula …

225

Fig. 5.42 Changes of moisture content with time

dust sample could be adjusted by its own moisture absorption and release to keep the dust sample moist. By averaging the moisture content of six groups of dust samples, the average curve for changes of moisture content of dust suppressant within 10 days was obtained. Connecting this curve with daily changes of temperature and relative humidity and Fig. 5.42 was obtained. In Fig. 5.42, (a) was the comparison chart of changes of the moisture content of dust sample with time and that of the ambient temperatures with time, and (b) was the comparison chart of changes of the moisture content of dust sample with time and that of the ambient humidity with time. It could be seen from chart (a) that the ambient temperature changed from 29 to 31 °C within 10 days. The temperature gradually decreased in the first four days, increased from the fifth day, reached the maximum temperature of 30.8 °C on the sixth day, and then gradually decreased and finally reached 30.1 °C on the tenth day. The temperature generally increased first and then decreased, which was basically consistent with moisture content, and the change trend of moisture content was not exactly the same as that of temperatures in the first four days. This might be the reason that the dust sample was put into the air after they were dried. The dust sample absorbed moisture from the air to maintain its own humidity because its moisture contents were low at the beginning. Therefore, the moisture content of the dust sample increased continuously in the first four days. Generally speaking, temperature had a certain effect on the change of moisture content. It could be seen from chart (b) that the change range of relative humidity was 35–65%, and the relative humidity was about 60% in the first six days, then started to drop to nearly 35% in the sixth to eighth days, and gradually increased to 60% in the nineth and tenth days. The moisture absorption started gradually in the

226

5 Chemical Dust Suppression Technology of Road Surface …

Table 5.16 Wind erosion results of dust samples The wind speed/ms−1

Water spraying dust sample

Spray dust samples with dust suppressor

Quality of dust samples before wind erosion/g

Quality of dust samples after wind erosion/g

Wind erosion rate/%

Quality of dust samples before wind erosion/g

Quality of dust samples after wind erosion/g

Wind erosion rate/%

15

29.9552

11.5429

61.47

34.3636

34.7075

0

20

29.4001

8.2355

71.98

34.7228

35.0611

0

first six days because the relative humidity in the air was relatively high, and the dust sample continuously absorbed the moisture in the air under high relative humidity on the sixth day, h thus its moisture content increased. When the relative humidity in the air dropped on the seventh day, the dust sample per se began to release moisture, and the moisture content dropped to about 4%, which kept a dynamic balance with the relative humidity in the air. When the relative humidity in the air increased again, the dust sample began to absorb moisture and the moisture content began to increase. It could be seen that the dust sample sprayed with the dust suppressant had a certain moisture absorption and release, and compared with the ambient temperature, the relative humidity of the environment had a greater impact on the moisture absorption and release of the dust suppressant. (2)

Weather-proof experiment

The wind erosion rate E of dust samples was calculated, the results were shown in Table 5.16. It could be seen from Table 5.16 that the wind erosion rate of dust sample sprayed with water was 61.47% when the wind speed was 15 m/s. The wind erosion rate of dust sample gradually increased with the increase of wind speed, reached 71.98% when the wind speed was 20m/s, and the dust samples were basically eroded by wind. However, the wind erosion rate of dust samples sprayed with dust suppressant was basically 0%, and the weight of dust sample before wind erosion was even less than that after wind erosion. The reason might be that dust sample sprayed with dust suppressant had certain hygroscopicity, which could absorb the moisture in the air and increase its moisture content. The moisture content of soil was an important factor affecting the wind erosion. Therefore, instead of being blown away, the dust samples had their weight increased when being eroded by wind. On the other hand, it could bond the dust samples together and prevent wind erosion because the dust suppressant contained some coagulant. When wind speed was 20 m/s, the wind erosion rate of the dust sample sprayed with dust suppressant was much lower than that of sprayed with clean water, and the wind erosion rate was less than 1%, which met the requirements. In addition, when the vehicle quickly passed the mine road, the instantaneous wind speed at the rear of the vehicle could reach 20 m/s. Therefore, the dust suppressant could resist

5.4 Development of Compound Dust Suppressant Formula … Table 5.17 Physical parameters of dust suppressant formula

227

Viscosity/mPa s

Surface tension/mN m-1

PH

The density/g cm−3

13.67

26.80

6.20

1.056

considerable wind erosion, and the road dust would not be raised when transport vehicles passed by, showing a good dust suppression effect. (3)

Toxicity and chemical properties

The viscosity, surface tension, PH and density of the dust suppressant formula were tested, and the results were shown in Table 5.17. It could be seen from Table 5.17 that the viscosity of dust suppressant solution is 13.67 mpa s by using NDJ-5S digital viscometer, indicating that the dust suppressant had a certain viscosity and good fluidity and was easy to spray. The surface tension of the optimized dust suppressant was 26.80 mN/m measured by BZY surface tension meter, which indicated that the surfactant composition in the dust suppressant had effectively improved the surface tension of dust suppressant. However, the surface tension of the dust suppressant solution was less than that of the surfactant monomer solution due to the influences of hygroscopic agent and coagulant. The PH value of dust suppressant solution was 6.20 measured by SevenEasy PH meter, which was slightly acidic, indicating that the dust suppressant solution still had weak corrosivity. It is necessary to develop a better corrosion inhibitor. The density of dust suppressant solution was 1.056 g cm−3 measured by densitometer, which was basically equivalent to the density of water. According to specification of technical indexes of dust suppressant in Technical Conditions for Dust Suppression of Railway Coal Transportation-Part 1: Dust suppressant (TBT-3210.1-2009), the viscosity, surface tension, PH and density of the dust suppressant formula had met the requirements of the technical indexes. In addition, according to the experimental detection of Analysis and Testing Center of Tsinghua University, the content of mercury, cadmium, lead, chromium, arsenic and other heavy metal ions in the formula had met the standard requirements, without any toxic effect. The analysis results were shown in Table 5.18. (4)

Determination of freezing point

The anti-freezing performance of dust suppressant can be characterized by freezing point. The freezing point of the optimal dust suppressant formula has been tested in this section. Table 5.18 Toxicity test results Sample

Hg

Cd

Pb

Cr

As

Liquid

50

>80

Configure the scale (%)

0.01

0.5

0.5

0.1

0.5

The amount of 1 m3 water 0.1 that is potion (kg)

5

5

1

5

The amount of 1 m gun holes used (kg)

0.002

0.113

0.113

0.023

0.113

5 m gun hole usage (kg)

0.01

0.567

0.567

0.113

0.567

6.1 Application of Blasting Dust Suppressant in Open-Pit Mine

(2)

255

(4) (5)

Put the plastic bag (flat width of 282.7 mm, thickness greater than 0.13 mm, non-toxic polyethylene) into the blasthole, bag length = 1.5 m + blasthole filling height. Put auxiliary 1# , auxiliary 2# , auxiliary 3# , and monomer A into plastic bags in turn; Add 4* auxiliaries while pouring water into the bag; Add water to individual bags half an hour before blasting.

(3)

Measurement results

(3)

The movement and diffusion of explosive smoke and dust are related to the state and speed of wind flow, so the position of each measurement point should be arranged as the following coordinate system: The movement and diffusion of explosive smoke and dust were related to the state and speed of wind flow, thus the position of each measurement point was arranged in the following coordinate system: Coordinate origin—the central area of the explosion; X axis—X axis-the positive direction was the main airflow direction, which was the horizontal axis. Y axis—the positive direction was the vertical main airflow direction, which was the horizontal axis. Z axis—the positive direction was away from the center of the earth, and was perpendicular to the plane of the explosion zone. (1)

Meteorological parameters

Meteorological parameters included temperature, humidity, air pressure, wind speed, etc. The measurement results were listed in Table 6.6. Table 6.6 Meteorological parameters for blasting Project

2nd

3rd

4th

5th

6th

Weather conditions Clear

1st

Clear

Clear

Clear

Clear

Clear

The point 85, 0, 3.1 coordinates (x, y, z)

70, 25, 0.5

20, 0, 0.5

25, 5, 0.5

110, 0, 0.5

45, 0, 0.5

Dry temperature (°C)

30.5

31.0

30.0

28.0

28.0

24.0

25.0

23.5

23.5

67.5

56.0

69.0

71.0

1023

1018

1015

1018

0.5–1.5

0.7–2.0

0.8–1.5

0.8–2.0

0.3–1.5

0.5–3.0

0.7–2.5

0.8–1.5

0.5–3.0

0.2–2.0

32

Wet temperature (°C) Humidity (%) Air pressure (hPa) Wind speed before explosion (m/s) Wind speed after explosion (m/s)

0.5–1.5

256

6 Field Application of Mine Dust Suppressant

Fig. 6.4 Changes in the direction of wind flow before and after the 4th test blast

The results have shown that the direction of air flow in the open-pit mine was unstable and the wind speed varied greatly. Specifically, at the moment of blasting, the action of blasting shock wave caused the short-time disorder of airflow direction. For example, in the fourth test, the direction of the main air flow before blasting was 150° from the direction of short-time air flow within 15 s after blasting. As shown in Fig. 6.4, the airflow direction turned back to the main airflow direction 15 s after blasting. During the 15 min measurement time after blasting, the wind speed was very unstable. In the sixth test, for example, the instantaneous change amplitude of the wind speed was 1.8 m/s, as shown in Table 6.7, and the wind speed at different locations varied greatly. (3)

Determination of CO and dust concentration

According to the relationship between the airflow direction of each test and the location of the explosion area, as well as the geographical conditions of the explosion area, the location to measure CO was determined. As shown in Table 6.8, the meanings of the symbols in the table were listed as follows: C—measurement point of CO instantaneous concentration; D—measurement point of instantaneous dust concentration; Table 6.7 Instant wind speed of the 6th blast Time after blasting (min)

1

2

3

4

5

6

Wind velocity (m/s)

1.0

0.9

1.2

0.15

0.2

1.3

Time after blasting (s)

7

8

9

10

11

12

Wind velocity (m/s)

0.3

1.1

0.65

0.5

2.0

0.4

6.1 Application of Blasting Dust Suppressant in Open-Pit Mine

257

Table 6.8 Locations of each measuring point Measure the point number

X0 (m)

Z0 (m)

t (s)

C1-1

80.0

Y0 (m) 0.0

3.0

420

C2-1

70.0

12.0

3.5

550

C2-2

30.0

13.0

1.0

150

D2-1

65.0

0.0

1.5

76

A2-1

100.0

0.0

0.0

600

C3-1

20.0

6.0

3.0

132

C3-2

80.0

30.0

25.0

140

D3-1

20.0

0.0

0.5

126

A3-1

70.0

5.0

0.0

600

A3-2

80.0

0.0

12.0

600

C4-1

25.0

10.0

1.5

550

A4-1

40.0

0.0

12.0

600

A4-2

20.0

0.0

0.0

600

C5-1

80.0

0.0

13.0

290

D5-1

150

0.0

13.0

290

A5-2

40.0

0.0

0.0

600

A5-3

50.0

0.0

12.0

600

C6-1

40.0

13.0

1.5

430

A6-1

50.0

20.0

0.0

600

A—measurement point of dust settlement; T—total measurement time; The first digit—serial number of the test; The second digit—serial number of the measuring point; X0 , Y0 , Z0 —the coordinate of the measuring point. In the six experiments, the change of CO concentration with time at each measuring point was shown in Fig. 6.5. The second and sixth measured data were listed for intuitive analysis, which were shown in Tables 6.9 and 6.10, respectively. It could be seen that there were several peaks in the change of CO concentration, indicating that CO continued to spill into the atmosphere from the blasting pile during a period of time after blasting, and the overflow speed was large. The dust particle diameter was graded by using the sampling filter membrane of the dust sampler and the sampling filter membrane of the barrel dust sampler (dust settlement per unit area was measured). A total of 11 representative filter membranes were analyzed, and the results were shown in Table 6.11. According to the analysis results, the number concentration, weight concentration and equivalent average settling particle size of blasting dust could be calculated. Total dust equivalent average settling particle size = 4.935 μm.

258

6 Field Application of Mine Dust Suppressant

Fig. 6.5 The CO concentration changes curve over time in the test

Table 6.9 CO concentration of point C2-1, 2nd test measurement point wind speed: 1.7 m/s Measure the point number

X0 (m)

Z0 (m)

t (s)

C1-1

80.0

Y0 (m) 0.0

3.0

420

C2-1

70.0

12.0

3.5

550

C2-2

30.0

13.0

1.0

150

D2-1

65.0

0.0

1.5

76

A2-1

100.0

0.0

0.0

600

C3-1

20.0

6.0

3.0

132

C3-2

80.0

30.0

25.0

140

D3-1

20.0

0.0

0.5

126

A3-1

70.0

5.0

0.0

600

A3-2

80.0

0.0

12.0

600

C4-1

25.0

10.0

1.5

550

A4-1

40.0

0.0

12.0

600

A4-2

20.0

0.0

0.0

600

C5-1

80.0

0.0

13.0

290

D5-1

150

0.0

13.0

290

A5-2

40.0

0.0

0.0

600

A5-3

50.0

0.0

12.0

600

C6-1

40.0

13.0

1.5

430

A6-1

50.0

20.0

0.0

600

6.1 Application of Blasting Dust Suppressant in Open-Pit Mine

259

Table 6.10 CO concentration of point C6-1 of the 6th test measurement point wind speed: 1.5 m/s Time (s)

CO concentration (ppm)

Time (s)

CO concentration (ppm)

Time (s)

CO concentration (ppm)

40

2

50

3

60

5

70

7

80

9

90

11

100

12

110

11

120

10

130

9

140

6

150

4

160

5

170

6

180

7

190

6

200

5

210

4

220

3

230

3

240

4

250

4

260

5

270

4

280

4

290

3

300

3

310

2

320

2

330

2

340

2

350

2

360

2

370

2

380

1

430

1

Table 6.11 Blast dust dispersion Particle size (μm)

≤1

1–2

2–5

5–10

>10

The second time of the 1-# film

103

54

35

6

2

A2-1

94

41

51

9

5

The 3rd time of the 1-# film

74

65

34

9

18

The 3rd time of the 2-# film

87

63

40

8

2

A3-1

25

71

59

31

14

A3-2

20

65

65

42

8

A3-3

11

42

68

43

36

A4-1

87

63

40

8

2

A4-4

83

76

24

9

8

A5-1

33

46

82

26

13

A6-2

25

30

66

51

28

642

616

564

242

136

Number of concentrations (%)

29.2

28.0

25.6

11.0

6.2

Weight concentration (%)

0.03

0.79

9.17

38.61

51.40

The equivalent average settling particle size of respirable dust with a particle size of less than 5 μm is 3.34 μm. The number concentration of dust is: Ndpj × 100% ηn j = n j=1 Ndpj

(6.27)

260

6 Field Application of Mine Dust Suppressant

Table 6.12 Blast dust dispersion for the 3rd blast Particle size (μm)

The sampling method

≤1

1–2

2–5

5–10

>10

Number of concentrations (%)

Sampler

40.3

32.0

18.5

4.2

5.0

Free settlement

9.3

29.7

32.0

19.3

9.7

Weight concentration (%)

Sampler

0.1

1.4

10.3

23.3

64.9

Free settlement

0.0

0.1

7.1

42.3

50.1

wherein ηn j —The number concentration of dust with a particle size of d pj ; N—the number of grades; Ndpj —The number of dust with particle size of d pj The weight concentration of dust is: Ndpj d 3pj ηwj = n 3 j=1 Ndpj d pj

(6.28)

The equivalent average settling particle size of dust is: n d 3p

=

j=1 n 

Ndpj d 3pj

(6.29)

Ndpj

j=1

Using the equations above, the number concentration, weight concentration and equivalent average settling particle diameter of blasting dust were calculated and listed in Table 6.12. The results of particle size analysis of blasting dust have shown that the dispersion of blasting dust in Daye Iron Mine was large, most of which were respirable dust with a particle size less than 10 μm. The dust collected by the dust sampler was from the atmosphere. In the three sets of filter membranes of the second blasting and the third blasting, the number concentration of dust with a particle size less than 10 μm was 96.3%, and the weight concentration was 40.6%. In addition, the dispersion of free settling dust was less than that of suspended dust in the atmosphere. The dust dispersion of two groups of sampler filter membranes in the third blasting was compared with that of three groups of free settling filter membranes, as shown in Table 6.9. For the number concentration of dust with a particle size less than 1 μm: the sampler was 40.4%, and the free settlement was 9.3%, with a difference of 31.1%. For the number concentration of dust with a particle size less than 5 μm: the sampler was 90.8%, and the free settlement was 71.0%, with a difference of 19.8%. The instantaneous dust concentrations in the atmosphere on the downwind side of the blasting pile were measured in the second, third and fifth blasting, and the measured results were shown in Tables 6.13, 6.14 and 6.15. Figure 6.6 showed the contrast curve of the concentration change of the third background test and the fifth solution test. It could be seen that the change rules of dust concentration at the fixed

6.1 Application of Blasting Dust Suppressant in Open-Pit Mine

261

Table 6.13 Dust concentration at point D2-1 of the 2nd test: 1.7 m/s Time (s)

15

21

30

36

42

48

54

76

Concentration (mg/m3 )

23.2

28.6

65.5

24.6

20.5

24.6

8.2

4.1

Table 6.14 Dust concentration of point D3-1, 3rd test: 1.6 m/s Time (s)

66

72

78

84

90

96

102

108

Concentration (mg/m3 )

45.1

267

221

258

111

73.7

28.7

16.4

Time (s)

114

120

126

Concentration (mg/m3 )

8.2

4.1

4.1

Table 6.15 Dust concentration at point D5-1 of the 5th test measurement: 2.0 m/s Time(s)

75

81

87

93

105

111

117

123

Concentration(mg/m3 )

16.4

24.6

12.3

12.3

12.3

8.2

4.1

4.1

Fig. 6.6 Curves of dust concentrations in the atmosphere that change over time

point on the wind side of the blasting pile were as follows: the dust concentration increased gradually after the blasting, reached a maximum value after a certain time, and then decreased with time. Under the meteorological, blasting and geological conditions of 6 explosions, the time for the dust concentration in the atmosphere to fall to the pre-blasting value was less than 150 s, that was, the dust concentration curve had only one extreme value point. Therefore, there was basically no dust escaping from the blasting pile, and the pollution source diffusion formed by blasting was in fact an instantaneous point source diffusion. The dust sampler measured the dust settlement. In the second to sixth blasting, the dust.

262

6 Field Application of Mine Dust Suppressant

Table 6.16 Dust settler measurements The point number

The effective diameter of the membrane (mm)

The amount of settlement (mg)

Settling per unit area (mg/m2 )

A2-1

55

0.45

189.5

A3-1

35

0.55

571.9

A3-2

35

0.55

571.9

A4-1

40

1.25

995.2

A4-2

40

1.25

995.2

A5-2

40

1.90

1512.7

A5-3

40

0.65

517.5

A6-1

40

0.60

477.7

The dust sampler measured the amount of dust settled. In the second to sixth blasting, the amount of settled dust was measured, and the results were shown in Table 6.16. The results showed that the amount of settled dust per unit area varied greatly due to the difference of the location, the distance from the blasting pile and the wind speed. The amount of settlement was related to the distance from the blasting area, the wind speed and stable state of the measuring point, meteorological conditions, blasting conditions and the geographical location of the measuring point. Therefore, the pollution situation of blasting dust could not be directly determined solely from the amount of settled dust per unit area. (4)

Intuitive evaluation of the toxic dust-reducing effect of the chemical liquid

According to the on-site observation, analysis of videos, photos and some data statistics, it could be seen that the filling liquid in the blasthole had the following intuitive effect on reducing blasting toxic dust than filling drilling slag. (1)

(2)

Fig. 6.7 has shown the photo comparison of smoke and dust pollution after blasting between the second test (above) and the fourth test (bottom). It could be seen from the figure that in the fourth blasting test with chemical charge, the amount of toxic gas in the dust cloud decreased significantly: the smoke cloud in the background test in the above figure was yellow because it contained a large amount of dust and toxic gases; the smoke cloud in the chemical test below appeared white, which was the manifestation of a large amount of water vapor. This indicated that at the moment of blasting, the soot production per unit blasting area had been greatly reduced, the concentration of pollutants in the smoke cloud was reduced, and the soot production per unit weight explosive or blasting per unit volume of ore and rock was much lower. The data measured in the experiment could also be compared intuitively. Tables 6.14 and 6.15 were the measured values of dust concentration in the downwind breathing zone of conventional filling and chemical filling. Table 6.14 showed that the dust concentration caused by the background test was as

6.1 Application of Blasting Dust Suppressant in Open-Pit Mine

263

Fig. 6.7 Comparison of soot contamination in the 2nd and 4th tests

high as 267v, while the highest dust concentration contributed by the chemical test represented in Table 6.15 was only 24.6 mg/m3 . This law was also seen in the change of CO concentration at the windward side of blasting. Taking Tables 6.6 and 6.7 as examples, it could be seen that the highest concentration of CO in the second test of background was 95 ppm, while in the sixth test with liquid added, the value was only 12 ppm. Based on different measuring points and meteorological factors, although the toxic dust-reducing effect of liquid filling blasthole could not be evaluated by simple efficiency calculation with the measured data, it could still give people a positive impression. If it was measured by the allowable concentration value stipulated in the mine safety regulations, for CO, it would take 5 min for conventional filling blasting to reduce the CO of the above measuring points to 24 ppm, while for the toxic dust-reducing liquid test, there was almost no time when the concentration of CO at the measuring point exceeded the standard. (5)

Quantitative evaluation of toxic dust-reducing effect of chemical liquid

The corresponding mathematical model was established based on the movement characteristics of blasting smoke and dust. Using computer, the amount of blasting dust and CO generated were calculated and the effect of reducing dust toxicity had been quantitatively evaluated. The diffusion of smock and dust in blasting dust could be used as instantaneous point source diffusion, and CO diffusion could be a stable point source diffusion in a

264

6 Field Application of Mine Dust Suppressant

certain time interval. The discharge amount of blasting dust and CO was calculated by the following mathematical model. Since it was calculated by the concentration in the air, the resulting emission was the total amount of pollutants emitted into the atmosphere. (1)

Using the instantaneous concentration of CO to calculate CO emissions Qz =

n 

C I (X 0 , Y0 , Z 0 , ti ) σY i σ Zi VX I

i=1

+

  1 Y02 exp[ 2 σY2i

(Z 0 + VZ iti + V f iti )2 ] (ti − ti−1 ) 2 σ Zi

(6.30)

wherein Q Z —Total emissions of toxic gases, kg; C—CO concentration measured, kg/m3 ; V —Wind speed in the main wind direction, m/s; σ —Mean square error of diffusion, m; N —Number of determinations; t—Measuring time, s0 and t0 are the time to start measuring after blasting; X0 , Y0 , Z0 — Coordinates of measured points, m; The subscript i represents the parameter measured for the i-th time. The mean variance σ is a function of the meteorological parameters and the cloud center O, (X, Y, Z). When these parameters and the position of the cloud center are determined, the mean variance σ value can be calculated referring to the table. (2)

Using the instantaneous concentration of dust at a certain point, the dust emission amount is calculated 3  1 C p (X 0 , Y0 , Z 0 , ti )σ Xi σY i σ Zi (2π) /2 2N i=1   

1 (X 0 − VX ti )2 Y02 (Z 0 − VZi ti − V P Z Li ti )2 exp + 2 + (ti − ti−1 ) 2 σ Xi2 σY i σ Zi2 (6.31) N

Qp =

wherein Q p —dust emission amount, kg; The subscript p represents the measurement result of dust concentration. (3)

Using the amount of dust settlement per unit area to calculate the amount of dust emission Qp =

3 1 (2π ) /2 q p / 2N

t 0

 ⎡ ⎛

⎞⎤ (X 0 − V X ti )2 Y02 1 (Z 0 − V Zi ti − V P Z Li ti )2 ⎠⎦ exp⎣ ⎝ + + · dt 2 σ2 σ2 σ2 Xi

Yi

Zi

wherein q p —Dust settlement amount per unit area of a given point, kg/m2 .

(6.32)

6.1 Application of Blasting Dust Suppressant in Open-Pit Mine

265

Table 6.17 Emissions and intensity of blast dust for measurement point C Project

1st 2nd

3rd

Emission (kg)

Ton explosive emission strength (kg/tonne explosives)

10,000 tons of ore rock emission intensity (kg/10,000 tons of ore rock) 4.601

C1-1

8.281

2.526

C2-1

11.354

2.536

5.407

C2-2

10.712

2.392

5.101

Arithmetic mean

11.033

2.464

5.254

C3-1

11.911

2.450

6.269

C3-2

18.685

3.845

9.834

Arithmetic mean

15.298

3.147

8.052

2.743

5.968

The bottom explosion is weighted

Average value

4th

C4-1

4.189

1.482

3.491

5th

C5-1

3.692

1.130

2.172

6th

C6-1

3.044

1.326

1.903

The bottom blasting is weighted

Average value

1.163

2.428

(6)

Comprehensive evaluation on the toxic dust-reducing effect of the chemical liquid

(1)

According to the established mathematical model, a computer program was written to have made a quantitative evaluation of the toxic dust-reducing effect of the solution. Tables 6.17 and 6.18 were respectively the emission amount and intensity of blasting dust and CO calculated by collecting the data of six blasting tests at different times. The toxic dust-reducing efficiency of liquid blasting relative to background blasting is calculated by:

(2)

  qy × 100% η = 1− qb

(6.33)

wherein η—The toxic dust-reducing efficiency of liquid chemical blasting relative to background blasting; q y —The emission intensity of blasting toxic dust during liquid blasting; qb —The emission intensity of blasting toxic dust during background blasting. (3)

Using computer to evaluate the efficiency of reducing the blasting toxic dust of liquid blasting compared to background blasting, the results were as follows:

266

6 Field Application of Mine Dust Suppressant

Table 6.18 Emissions and intensity of blast dust for measurement point A ,C and D Project

2nd

3rd The bottom blasting is weighted

Emission (kg)

10,000 tons of ore rock emission intensity (kg/10,000 tons of ore rock)

D2-1

275.082

61.430

130.991

A2-1

261.989

58.506

124.757

Arithmetic mean

268.536

59.968

127.874

D3-1

11.911

2.450

6.269

A3-1

301.597

62.057

158.735

A3-2

419.412

86.299

220.743

arithmetic mean

368.068

75.734

193.720

68.172

159.149

38.869

91.538

Average value

A4-1 4th

Ton explosive emission strength (kg/tonne explosives)

109.895

A4-2

48.529

17.172

40.441

Arithmetic mean

79.187

28.021

65.989

D5-1

110.626

33.851

65.074

A5-2

105.684

32.339

62.167

A5-3

58.921

18.030

34.659

Arithmetic mean

91.097

27.875

53.586

6th

C6-1

108.628

47.312

67.893

The bottom blasting is weighted

Average value

33.243

61.980

5th

According to the calculation of CO emission intensity per unit weight of explosives consumed, ηC O = 57.6%. According to the calculation of CO emission intensity of blasting unit weight of ore and rock, ηC O = 59.3%. According to the calculation of dust emission intensity per unit weight of explosive consumed, ηdust = 51.2%. According to the calculation of dust emission intensity of blasting unit weight of ore and rock, ηdust = 61.1%. The overall efficiency of liquid chemical blasting relative to background blasting, ηtotal = 57.3%. The comprehensive toxic dust-reducing efficiency meets the standards stipulated in the three contracts. (7)

Cost accounting

The cost was calculated based on the amount of materials, labor, water consumption, and blasting ore volume, as shown in Table 6.19.

6.2 Application of Dust Suppressant in Open-Pit Mine Yard Table 6.19 Costing of liquid testing

267

Project

Total usage

Unit price

Monomer

1.0 L

56.0 RMB/L

1# additives

49.9 kg

25.5 RMB/KG

2# additives

49.9 kg

1.0 RMB/KG

3# additives

10.0 L

1.0 RMB/L

10.0

4# additives

49.9 L

7.8 RMB/L

389.2

Plastic bag

57.0 kg

22.0 RMB/KG

Employment

15.4 people

4.0 RMB/people

Water use

10.0 m3

0.5 RMB/m3

Total

Cost (RMB) 56.0 1272.5 49.9

1254.0 61.6 5.0 3098.2

6.2 Application of Dust Suppressant in Open-Pit Mine Yard 6.2.1 Industrial Test of Hygroscopic Dust Suppressant in Suppressing Dust of Stockpile Hygroscopic dust suppressant showed an excellent moisturizing performance in laboratory and industrial tests, it was thus applied to control the coal pile dust. The experiment was carried out in October 2003. The tested coal pile was tapered with an area of 40 m2 and an average spraying amount of 3.25 kg/m2 . The spraying device was composed of a water pump, a spray blast and a sprinkler. The spraying test ended at 11:00 in the morning. The following phenomena were found after on-site observation the next morning: the coal pile sprayed with dust suppressant was moist and bright black before 10:00 in the morning. After that, due to evaporation, the surface coal gradually lost its moisture, and its color gradually changed from bright black to dark black. There was a crust with certain hardness formed on the surface of the coal pile. At about 5:00 in the afternoon, due to the moisture absorption of dust suppressants, the color of coal on the surface of the coal pile began to change from dark black to bright black again, and the coal became moist again. When coal was in a wet state, there was a liquid bridging force between the coal dust particles, which increased the cohesion between the dust and the wind speed to raise the dust, so that the coal dust was not easy to be raised. When the surface of the coal pile lost moisture, the adhesive effect of the dust suppressant not only increased the size of coal dust particles, but also connected the whole coal surface and increased the wind speed to raise the dust, making the dust not easy to be raised. In a word, the dust suppressant could restrain the rise of coal dust on the surface of coal pile, whether its was water absorption and moisture retention or consolidation. The coal pile was placed in the open air for 56 days and had experienced 6 times of rain and snow scouring. It was observed that the coal pile sprayed with hygroscopic dust suppressant was wet in the morning and hardened in the afternoon. When the

268

6 Field Application of Mine Dust Suppressant

Table 6.20 Water content determination data for raw coal in coal piles Sampling position (mm)

Coal-like quality before drying (g)

Coal-like quality after drying (g)

Water content (%)

0–5

100

91.6

8.4

5–10

100

91.5

8.5

temperature was lower than 0 °C, the surface of the coal stack did not freeze. The surface water content and wind resistance of coal pile were measured in the morning. The coal pile were sampled and weighed respectively at the position of 0–5 mm and 5–10 mm under its surface, dried on the electric furnace, then weighed again, and the surface moisture content of the coal pile was calculated, as shown in Table 6.20. According to Table 6.20, after sprayed with hygroscopic dust suppressant for 56 days, the moisture content of raw coal on the surface of coal pile could still reach 8.4%. According to relevant literatures, when the moisture content of the material was higher than 4%, dust was not able to be raised. A 7.5 KW axial fan was used to generate different wind speeds to simulate the natural blown to the dust on the coal pile. The wind speed could be adjusted by changing the distance between the fan outlet and the coal pile. At the position where the coal pile was blown by the wind, the wind speed was measured by the QDF-3 (range 0.05–30 m/s) hot ball anemometer. Meanwhile, a P5 digital dust meter was used to measure the amount of dust on the surface of the coal pile within one minute on the spot where the fan blew. In the raw coal yard, the atmospheric dust concentration was measured with a P5 dust meter. The results were shown in Tables 6.21 and 6.22. From the data of Tables 6.21 and 6.22, it could be seen that when the blowing speed reached 10 m/s, the dust concentration on the surface of the coal pile was basically the same as the environmental background concentration, and the average Table 6.21 Environmental Background Concentration Measurement Data

Atmospheric wind speed (m/s)

The correction factor

P5 Dust gauge reading

Dust concentration (mg/m3 )

2–4

0.017

20

0.34

Table 6.22 Dust concentration determination of coal piles Wind velocity (m/s)

The correction factor

P5 dust gauge reading

Dust concentration (mg/m3 )

Average concentration (mg/m3 )

10

0.017

24

0.41

0.38

20

0.34

10

6.2 Application of Dust Suppressant in Open-Pit Mine Yard

269

dust concentration was 0.38 mg/m3 , which was 9.62 mg/m3 less than the post dust concentration(10 mg/m3 ). The effect of the dust suppressant in restraining the dust from the material pile has been proved by industrial tests. By spraying hygroscopic dust suppressant, and after 56 days and 6 times of rain and snow scouring, the moisture content of raw coal was 8.4%, which still had the characteristics of moisturizing and hardening. When the blowing speed reached 10 m/s, the dust concentration was only 0.38 mg/m3 . The local area had entered winter by the time of the tests, and the coal pile sprayed with dust suppressant did not freeze, showing the ability of anti-freezing. This has overcome the problem that the normal extraction of coal was affected by freezing in winter when they were sprayed with water to reduce dust.

6.2.2 Dust Floating Test of Adhesive Dust Suppressant in Tailing Pond (1)

Test mine

The tested mine was a joint enterprise of mining and separation of 500 tons of ore per day. The dry tailing filling mining method was adopted in mining, and the flotation technology was adopted in mineral processing. About 500 tons of tailings were produced every day, some of which were used for underground filling, and the other part self-flowed to the front of the tailing dam through an 800 m pipeline to be evenly stacked. The tailing reservoir was 300 m long and 100 m wide, with a stacking elevation of +120 m and an effective storage capacity of 290 × 104 m3 . The tailings pond is surrounded by crops. When the climate is dry, the dust from the tailings pond seriously pollutes the surrounding environment. Figure 6.7 shows the dust-raising situation in summer when the wind force is level 4. In order to suppress dust from tailings, measures have been adopted to cover the surface with soil, as shown in Fig. 6.8. Although the dust suppression effect of this measure was remarkable, there were many problems. First, the cost was high. According to the data provided by the mine, it costed 40 yuan to buy 8 tons of soil, which could cover an area of 40 m2 tailing pond surface. The cost of manually spreading the ore was about 0.5 yuan per square meter, that was, 1.5 yuan was needed to cover 1 m2 of tailings, which did not include the transportation cost of the soil. On the other hand, since tailings needed to be constantly taken out to fill the mined-out area, the covering soil needed to be sifted out with a sieve when the tailings was removed, increasing the cost and making the process tedious; In addition, the thickness of each layer of soil must be more than 30 mm to effectively suppress the dust, occupying the storage capacity and reducing the service life of the tailings pond. Therefore, the use of dust suppressant in the mine to restrain the dust from the tailings pond had not only environmental benefits, but also economic benefits.

270

6 Field Application of Mine Dust Suppressant

Fig. 6.8 The covers soil and dust

Table 6.23 Particle size distribution of tailings Particle size (mm)

+0.15

+0.074

+0.038

– 0.038

Particle size distribution (%)

5.34

7.32

16.88

70.46

Cumulative distribution (%)

5.35

12.66

29.54

100

The tailings of Canzhuang Gold Mine were discharged from the pipe to the dam surface of the tailings pond in form of mud. The particle size distribution of the tailings was shown in Table 6.23. It could be seen from Table 6.23 that the particle size of the gold mine tailings was very small, and particles with a size less than 0.038 mm accounted for 70.46%. Once these fine particles were short of moisture, they were easy to enter the atmosphere with wind, causing pollution to the surrounding environment. The industrial test was carried out on March 27, 2004, in which two sites were selected. One site was located near the tailings discharge outlet named test site 1# , and the other site was located on the slope of the dam foundation named test site 2# . The area of each test site was about 50 m2 . During the experiment, the surface of site 1# was not completely dry, where a large amount of oily matter was deposited, and the matter was turned over to the lower layer with a shovel to increase the permeability of the solution, and a small part of it was left untreated. This part was used to compare the effect of surface treatment on dust suppression. Test site 2# was a slope made of loose tailings, which was almost at a right angle. Considering that the position was not affected by production and thus could be retained for a long time. In addition, the slope-fixing effect of the dust suppressant could also be observed. (2)

Processing and spraying technology of dust suppressant

According to the formula optimally selected by the laboratory, a certain quantity of water was added to the container to be heated to the boiling point. The film-forming

6.2 Application of Dust Suppressant in Open-Pit Mine Yard

271

Fig. 6.9 Bonding dust suppressant solution preparation process

agent was added first, and then the fillers and other auxiliaries were added after the agent was completely dissolved, and the preparation process was completed after an even stirring. When the solution was cooled below 50 °C, a pump was used as the power to spray on the surface of the coal pile through the sprinkler. The equipment used for spraying included a sprinkler, a spray blast and a water pump. The process was shown in Fig. 6.9. In this experiment, a total of 260 kg solution was prepared for a spraying area of about 100 m2 . The spraying amount was about 2.6 kg/m2 , which was less. During the spraying process, it was found that the untreated part of site 1# , had a poor solution permeability. Not only runoff existed, but also the solution accumulated on the surface and permeated slowly. On the other hand, the permeability of the solution treated at site 1# and site 2# was good, and there was no runoff phenomenon. The spraying ended in the morning. Through the on-site observation on the next afternoon, it was found that the crust was warped in the untreated part of site 1# , while the crust in site 1# and 2# was intact. Due to the less spraying, the crust was thin as 5 mm. This suggested that when spraying dust suppressant solution on the undried surface of tailings pond, the surface of the pile should be loosened to enhance its permeability. (3)

Measurement and analysis of test data

(1)

Test weather conditions

After the test, the on-site technicians made a detailed record of the weather, as shown in Table 6.24. According to the data in Table 6.24, there were 11 rainfalls within 65 days after the dust suppressant was sprayed, including 7 days of light rain, 1 day of heavy rain and 3 days of moderate rain. During this period, the highest temperature reached 32 °C, indicating that in these days, the shell had not only withstood strong sunlight, but also experienced a large number of rain erosion tests. (2)

Shell changes

After spraying dust suppressant on the surface of tailings, the test site was preserved until October 25, 2004, which was nearly 7 months. During this period, four on-site observations had been carried out and it was found that the shell was intact, without

272

6 Field Application of Mine Dust Suppressant

Table 6.24 Climate change during the trial Date

3.27–3.31

4.1

4.2–4.4

4.5

4.6

4.7–4.9

Weather

Sunny or cloudy

Light rain

Sunny

Light rain turned cloudy

Cloudy

Sunny

Date

4.10

4.11–4.12

4.13

4.14–4.22

4.23

4.24–4.25

Weather

Sunny and cloudy

Sunny

Cloudy and sunny

Sunny

Cloudy

Sunny and cloudy

Date

4.26

4.27–4.30

5.1

5.2

5.3

5.4–5.8

Weather

Raining

Sunny

Cloudy

Cloudy to light rain

Light rain turns to rain

Sunny or cloudy

Date

5.11

5.12

5.13–5.15

5.16

5.17

5.18

Weather

Light rain

Light rain cleared up

Sunny

Heavy rain

sunny and light rain

Sunny

Date

5.19

5.20–5.26

5.27

5.28–5.30

-

-

Weather

Sunny and light rain

Sunny or cloudy

Raining

Sunny

-

-

cracks, and the apparent change was not obvious, indicating that the dust suppressant had little influence on the natural environment and its degradability was not obvious. Figure 6.10 showed the shell when dust suppressant was sprayed for 147 days. In Fig. 6.10, the upper flat part was sprayed with dust suppressant, while the lower part was not. The crust sprayed with dust suppressant was intact, while the unsprayed area had large cracks. (3)

Shell thickness

Part of the shell was taken each time to measure the thickness change, as shown in Fig. 6.11. Fig. 6.10 Shelling when spraying dust suppressant 147 d

6.2 Application of Dust Suppressant in Open-Pit Mine Yard

273

Fig. 6.11 The thickness of the housing changes over time

According to the data in Fig. 6.11, the adhesive dust suppressant was sprayed on the surface of the tailings pond to form a shell, and the thickness of the shell changed over time. In the initial stage, the thickness increased with time, but in the later stage, it basically did not change. It could be explained that when rain water acted on the shell, the film-forming molecules swelled, the adhesive interface fell off, and the water molecules entered into the shell. The fillers and hygroscopic agents in the dust suppressants would move downward with the infiltration of water. Because these two substances also had a certain adhesive effect, when they were dried due to evaporation, they would bond, thus increasing the thickness. In the later stage, the dust suppressant components with adhesive activity gradually decreased, and thus its thickness did not change obviously. (4)

Shell blowing resistance

When the adhesive dust suppressant was sprayed in the tailings pond for 203 d, an axial flow fan of 5.5 KW was used to blow at the crust surface. When the wind speed reached 18 m/s, the shell did not break. The measured dust concentration was 0.45 mg/m3 , indicating that the surface of the tailings pond sprayed with adhesive dust suppressant was still intact and had a significant dust suppression effect after 203 days of wind blowing, high temperature, sun exposure and repeated rain. (4)

Economic analysis

The cost of adhesive dust suppressant is about 109 yuan. According to the spraying volume of 3 kg/m2 and the spraying of 333 m2 per 1 t of solution, the cost of spraying 1 m2 is 0.33 yuan, which is lower than the cost of existing domestic dust suppressants. The cost of adhesive dust suppressant is about 109 yuan. According to the spraying volume of 3 kg/m2 and 333 m2 per ton of solution, the cost of spraying 1 m2 is 0.33 yuan, which is lower than the cost of existing domestic dust suppressants.

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6 Field Application of Mine Dust Suppressant

For Canzhuang Gold Mine, the total area of tailings reservoir is nearly 10 × 10 ~ 4 m2 . At present, the cost of dust suppression is 1.5 yuan/m2 using the soil covering method to suppress dust. The cost of dust suppression is 300,000 yuan for the two times of dust suppression every year. When using adhesive dust suppressant, the cost of two sprays is 66,000 yuan, which is 234,000 (78%) yuan less. It can be seen that the adhesive dust suppressant has not only remarkable dust suppression effect, but also considerable economic benefits.

6.2.3 Dust Suppression Test of Binding Dust Suppressant in Open Coal Pile The original coal yard was located in the southwest of Zhongzhan District, Jiaozuo, with a designed annual coal preparation capacity of 30 × 104 t. Raw coal was from Gaoping, Shanxi Province, dominated by powder anthracite. After physical coal preparation, raw coal was supplied to WISCO and used for blast furnace coal injection. After the raw coal were transported to the plant, it was piled in the open air in the raw coal yard (covering an area of 5500 m2 ). Due to the dryness, less rain and windy in this area, the raw coal not only polluted the surrounding environment but also caused material loss in the process of storage, bringing certain economic losses to enterprises. In order to suppress the coal pile dust, the plant adopted the fixed sprinkler measure. The dustproof water was groundwater with a cost of about 2 yuan/t. Although the sprinkler measure was simple to implement, the effective dust suppression time was short, which was only 30 min high temperature weather in summer. In order to suppress dust effectively, it was necessary to sprinkle water frequently, a lot of groundwater was consumed and the cost was increased. On the northwest side of the original coal yard against the wall, a scraper was used to pile a conical coal pile with an area of 36 m2 , which was represented by contours as shown in Fig. 6.12. In the figure, a was a measuring platform with a size of 0.6 × 0.3 (m2 ), which was used to measure the compressive and shear strength of the adhesive dust suppressant crust. b was a steep slope close to 90°, which was used to observe the slope-fixing effect of dust suppressant. After the processed adhesive dust suppressant solution was sprayed through the water pump and sprinkler device, the fog droplets were small and uniform. The solution permeated quickly after being sprayed on the surface of the coal pile and there was no runoff phenomenon. The spraying experiment ended in the morning. The following phenomena were found after on-site observation the next morning: The coal pile sprayed with dust suppressant formed a hard shell with a thickness of about 20 mm on the second day, which was compact, continuous, and without cracks. Because there was almost no free fine coal dust on the surface of the coal pile, the dust-raising could be well suppressed. After spraying dust suppressant on

6.2 Application of Dust Suppressant in Open-Pit Mine Yard

275

Fig. 6.12 Coal heap contour chart

b

0.8 4.8 4 3.2 2.4

a

1.6 0.8 0

the steep slope, the slope consolidation effect was good, and the bulk material did not slide or collapse when the wind blew. Although the spray happened in autumn, it was winter in the late stage of the experiment, and the atmospheric temperature was below zero. Four rains and two snows happened during that period. At the time of determination, the crust was still continuous and intact and had a certain strength. The main measurement items included blowing test, shell compression test, shear strength test, etc. (1)

Test measurement and data analysis of wind blowing resistance

(1)

Test equipment

A 7.5 KW axial fan was used to create different wind speeds to simulate the natural wind-blown dust on the coal pile. (2)

Test equipment

QDF-3 hot bulb air meter, measuring range 0.05–30 m/s; FC-1A filter membrane dust sampler; P5 digital dust analyzer; (3)

Measuring method

The outlet of the fan was directed to the surface of the coal pile, and the blowing speed was measured by using the QDF- 3 hot ball anemometer at the position where the coal pile was blown by the wind. Meanwhile, the amount of dust in 1 min, where the fan blew on the surface of the coal pile, was measured by P5 digital dust detector, and the reading of the amount of dust at this wind speed was obtained. The wind

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6 Field Application of Mine Dust Suppressant

Table 6.25 Dust concentration determination of coal piles under different wind speeds Wind velocity (m/s) 5

The correction factor

P5 reading (1 min)

Dust concentration (mg/m3 )

0.017

25

0.425

13

37

0.629

23

41

0.697

speed was adjusted by changing the distance between the outlet of the fan and the coal pile, and the amount of dust under different wind speeds was measured. The results were shown in Table 6.25. The outlet of the fan was directed to the surface of the coal pile, and when the blowing speed reached 23 m/s, the shell was still intact and there was no sign of rupture, indicating that the shell formed by spraying binder dust suppressant would not be broken by wind blowing. It could be seen from the measured data of Table 6.25 that the dust concentration had increased with the increase of blowing speed. But even when the blowing speed reached 23 m/s, the dust-raising concentration of coal piles was only 0.697 mg/m3 . However, this value was still far from the post hygienic standard concentration of 10 mg/m3 , indicating that the binder dust suppressant sprayed on the coal pile could strictly control the concentration of the coal pile within the standard value in 56 days. The wind speed of 23 m/s was equivalent to a level 9 wind in meteorology, which was rare seen in inland areas. (2)

Pressure resistance test and data analysis

On the surface of the coal pile, a plane shell was taken to measure its area and thickness. The plane shell was placed on the rigid surface, and a rigid load-bearing block was placed on it. The weight was increased in turn until the surface of the coal shell was broken. The rigid load-bearing blocks were weighed, and then the ultimate compressive strength was calculated when the coal pile surface was broken, as shown in Table 6.26. The data in Table 6.26 were the compressive strength measured by removing the shell and placing it on the rigid surface. In fact, the compressive strength of coal pile plasma was much higher than that in the table. Before taking the flat shell as the compressive strength, two 24 × 12 (cm2 ) bricks were placed on plane a of the coal pile. A 68 kg experimenter stood with each foot on one brick, and no shell crushing was found. After that, the experimenter lifted a foot to put all his weight on one brick, and the shell was still uncrushed. It could be calculated that the compressive strength Table 6.26 Measurement of the pressure strength of coal piles Rigid blocks and heavy objects (kg)

Coal shell force area (m2 )

The thickness of the coal shell (mm)

Extreme pressure strength (Pa)

2.515

0.0063

20

3912.2

6.2 Application of Dust Suppressant in Open-Pit Mine Yard

277

Table 6.27 Measurement data on the shear strength of coal piles Heavy mass (kg)

The area of the force (m2 )

Extreme shear resistance (Pa)

1.884

0.0187

1007.5

Table 6.28 Measured particle size distribution of coal piles Particle size (mm)

10

Total

Quality (g)

15

28

21

36

100

Particle size distribution (%)

15

28

21

36

100

of the raw coal had reached 23.1 kPa at this time, and the shell was still not broken, indicating that the compressive strength of the coal pile was greater than this value and the shell strength was strong enough to support the weight of a normal human body. This is essential to ensure the integrity of the shell when the shell is disturbed by external forces. (3)

Shear resistance test of coal pile

In the position of the platform of the coal pile, the lower part of the pile was hollowed out, and heavy objects were gradually put on until the plane shell was crushed, and the shear force per unit shear area was calculated, as shown in Table 6.27. (4)

Particle size analysis of coal piles

The crushed coal samples mentioned above were further crushed and screened and weighed with sieves with pore diameters of 10 mm, 5 mm and 3 mm, respectively, and the particle size distribution was calculated, as shown in Table 6.28. It could be seen from the composition of the particle size in Table 6.28 that the particle size of the coal sample in the pressure test was large and the distribution was relatively dispersed. When the adhesive dust suppressant was sprayed on the surface of the coal pile, the coal particles were adhered by the adhesive force. In terms of fine particles, the larger the particle size, the smaller the specific surface area, the smaller the surface in contact with the dust suppressant, and the smaller the adhesive force. Therefore, the above values of compression and shear force were measured under the condition of the specific particle size composition. Compared with the laboratory data, the compressive strength measured by industrial test (3992 Pa) was nearly 60 times different from that measured by laboratory sand samples (232 kPa). Through analysis it was concluded that the material used in the laboratory was sand with uniform particle size, which was small and even, while the particle size of industrial test material was much larger, which was the main reason for the difference in compressive strength. In addition, laboratory samples were not scoured by rain water, while industrial material pile did, and there was the problem of loss and degradation of useful components of dust suppressant. Moreover, factors such as freezing and thawing caused by the environment would also affect its strength.

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6 Field Application of Mine Dust Suppressant

The test site was preserved until March 26, 2004, which was already 5 months from the date of spraying, the shell remained intact and the strength did not decrease significantly.

6.3 Application of Dust Suppressant on Pavement in Open-Pit Mine 6.3.1 Spring Road Dust Suppressant Industrial Experiment-Shougang Shuichang Iron Mine The tested mine was a large open-pit iron mine with an annual output of 800 × 104 tons, which was divided into north and south mining areas. Automobile was the main transportation mode of the mine, and the main models of vehicles were 42 t and 77 t mine cars. The stope pavement in the mining area was made of crushed stone from the concentrator and was a low-grade road surface. The total length of the semi-permanent road surface in the mining area was 10 km, and the average width was 25 m. The road section selected in this test was located in the south side of the north mining area, which was the arterial road for the transportation of ore and rock. The average traffic density in this section was 150 vehicles per hour. The tested section was 310 m long and 25 m wide. Before spraying, there was no loosening, leveling and other treatment carried out on the road surface, thus the road surface was hard and the solution permeability was poor. In order to ensure that the solution sprayed on the road did not cause runoff, the sprinkler was required to spray a small amount of solution each time. The location of the tested section and measuring point was shown in Fig. 6.13. The shaded part of the picture showed the road surface sprayed with dust suppressant. According to Fig. 6.13, there were four connections between this section and the road with no dust suppressant sprayed. When the vehicle ran from the road where the dust suppressant was not sprayed to the road where the dust suppressant was sprayed, its tires brought in a lot of dust. In the process of spraying dust suppressant, the tires brought out a large amount of dust suppressant solution that had not permeated in time because the road was not blocked. Therefore, the actual spraying amount was lower than the calculated spraying amount, thus the road section selected in the test was not ideal. (1)

Preparation process and spraying of dust suppressant solution

There were four kinds of substances in the dust suppressant formula: hygroscopic agent, high molecular polymer (polymer), penetrant and corrosion inhibitor (auxiliaries for short). The preparation process was shown in Fig. 6.14. In a container with a volume of 6.5 m3 , a certain amount of water was added in proportion, and then the polymer was added while stirring. When adding polymers,

6.3 Application of Dust Suppressant on Pavement in Open-Pit Mine

279

Fig. 6.13 Test road section and point distribution

Fig. 6.14 Dust inhibitor solution configuration process

Water High polymer

Predissolved polymers

Hygroscopic agent

Promoter

Stir to form a solution

Pump

Spray nozzle

Water

it was necessary to ensure that the polymers were uniformly dispersed in water to avoid agglomeration. The polymer solution formed should be kept for 1–2 h. After the solution was fully dissolved, water, auxiliaries and hygroscopic agents were added into it, and the dust suppressant solution had been completed after full stirring. The sprinkler sprayed the solution in the selected test section. Due to the limitation of container volume, the solution used in this experiment was prepared in three times, totaling 18.5 t, and the average spraying amount per unit area was 2.37 kg/m2 . The road surface of the test section had not been treated with any loosening and floating dust cleaning before spraying that the road surface was very hard in some spots. While in some spots a large amount of fine dust was deposited, resulting in poor permeability of the dust suppressant solution. Only by ensuring that the solution permeates to a certain depth can it have a dust suppression effect. During the test, in order to ensure the penetration of dust suppressant solution, on the one hand, the

280

6 Field Application of Mine Dust Suppressant

sprinkler sprayed a small amount of solution each time to avoid dust suppressant solution runoff. On the other hand, the road section was not blocked in the spraying process, and the ore and rock transport vehicles were allowed to drive normally on the test road section that their tires continued to roll over the road surface to play a stirring role to enhance the permeability of the road surface. Observation and determination began on the second day after spraying dust suppressant. The dry and wet temperature of the air were measured by dry and wet bulb thermometers, and the instantaneous dust concentration at the rear of vehicles was measured by a P5 photoelectric dust detector. (2)

Test data analysis

(1)

Meteorological conditions of the test

In the open pit, the dry temperature and wet temperature of the air were measured from 6:00 to 8:00 in the morning and 3:00 to 5:00 in the afternoon every day. The relative humidity of the air was obtained by looking up the table based on the difference of the temperatures. The meteorological conditions during the experiment were shown in Table 6.29. It could be seen from Table 6.29: A.

B.

(2)

During the experiment, the weather was fine and the temperature was high. The temperature in the afternoon was basically above 26 °C, and the highest temperature was 31.5 °C. The temperature difference between morning and afternoon was about 10 °C. The relative humidity difference between morning and afternoon on one day was relatively large. The relative humidity in the morning was more than 55%, while in the afternoon it was almost about 40%, and the lowest humidity was only 31%. The climatic conditions of high temperature and low humidity had resulted in a very strong water evaporation. According to the on-site observation, after sprinkling water on the road, due to the strong evaporation, the road dust can be seen obviously in less than 0.5 h, indicating that the effective dust suppression time of sprinkling water was very short. Analysis of penetration depth of road dust suppressant

Since the main component of the dust suppressant was magnesium chloride, the penetration depth of the dust suppressant could thus be determined by measuring the content of Mg2+ at different depths. Since there was no rain and no water was sprayed on the road during the test, the penetration depth of the dust suppressant hardly changed with time. The Mg2+ contents at different depths were obtained by averaging the multi-point measurements, as shown in Table 6.30. It could be seen from Table 6.30 that the Mg2+ content of the road sprayed with dust suppressant obviously decreased with the increase of depth. When the depth was more than 20 mm, the content of Mg2+ was basically the same as that of the original road without dust suppressant, indicating that the penetration depth of dust suppressant solution was only 20 mm.

6.3 Application of Dust Suppressant on Pavement in Open-Pit Mine

281

Table 6.29 Test weather conditions Measure the time

Temperature (°C)

Relative humidity (%)

The temperature is difference in the morning and afternoon (°C)

The relative humidity is difference in the morning and afternoon (%)

Weather conditions

5.25 morning 16

77

10

37

Sunny

5.25 afternoon

26

40

5.28 morning 17

55

11

28

Sunny

5.28 afternoon

37 12

34

Sunny

10.5

36

Sunny

10.5

54

The fog turned fine

10.5

31

Sunny to overcast

9.5

24

From cloudy to sunny

28

5.29 morning 19.5

65

5.29 afternoon

31.5

31

5.30 morning 19.5

70

5.30 afternoon

30

40

6.1 morning

17.5

96

6.1 afternoon 28 6.2 morning

19

42 81

6.2 afternoon 29.5

50

6.3 morning

74

20.5

6.3 afternoon 30

50

Table 6.30 Mg2+ plus content varies with depth The depth from the surface (mm)

0–2 2–5 5–10 10–20 >20

Spray the road surface Mg2+ plus content (mg/g)

135 85

The

Mg2+

plus content of the road surface is not sprayed (mg/g) 4.6

4.7

46

25

5.0

5.0

4.8

5.1

Through the test data, it could be known that after spraying dust suppressant, the salt content of surface dust was much higher than that of the original road surface. Since salt has the function of cementing dust, the dust wind speed of the road surface sprayed with dust suppressant will be greatly increased, thus the amount of dust will be reduced. (3)

Changes in the moisture content of road dust

The moisture content of road dust, even fine dust, was the main factor affecting the amount of road dust. Due to the liquid bridging force between the particles, the interaction force increased, and the dust with high moisture content was not easy to be raised under the same wind speed. For this reason, a continuous determination of

282

6 Field Application of Mine Dust Suppressant

Fig. 6.15 Flour dust moisture content changes curve over time

the moisture content of road dust was carried out during the test. The determination results were shown in Fig. 6.15. According to Fig. 6.15, the moisture content of road dust sprayed with dust suppressant was always maintained at about 4% during the measurement time. With the change of time, the moisture content had been maintained at a high level although there was some slight fluctuation. The road surface sprayed with water had a moisture content of less than 2% and fluctuates slightly over time, which was because the dust per se had a certain degree of hygroscopicity. (4)

Analysis of road dust dispersion

In the road sections where dust suppressant was sprayed and not sprayed, 10 representative units were selected, the dust was cleaned with a brush, dried and screened, and the particle size distribution was obtained, as shown in Table 6.31. According to Table 6.31, in the road sections where dust suppressants were sprayed, only 12% of the total dust particles with size of less than 100 mesh, and 88% of the dust particles are larger than 100 mesh. On the other hand, the dust on the road surface without dust suppressant was more than 100 mesh, accounting for only 33% of the total, while the dust less than 100 mesh accounted for 67% of the total. Table 6.31 Results of the particle size distribution of flour dust The type of road surface

Granularity distribution (%) >20 mesh

20–100 mesh

Spray dust suppressants

54

34

8

4

0

Dust inhibitors are not sprayed

10

23

42

21

4

100–200 mesh

200–300 mesh

1 mm

0.125–1

0.5 mm (%)

0.25–0.5 (%)