Application of Microwave Heating in the Comprehensive Utilization of Titanium Resources 9782759826971

Table of contents :
Preface
Contents
Chapter 1 Microwave Absorbing Properties and Temperature Behaviour
Chapter 2 Microwave Pretreatment and Microwave Drying
Chapter 3 Microwave Carbothermic Reduction
Chapter 4 Microwave-Assisted Leaching and Intensification
Chapter 5 Microwave Roasting Process
Chapter 6 Microwave Heating Device

Citation preview

Current Natural Sciences

Guo CHEN, Lei GAO and Jin CHEN

Application of Microwave Heating in the Comprehensive Utilization of Titanium Resources

Printed in France

EDP Sciences – ISBN(print): 978-2-7598-2696-4 – ISBN(ebook): 978-2-7598-2697-1 DOI: 10.1051/978-2-7598-2696-4 All rights relative to translation, adaptation and reproduction by any means whatsoever are reserved, worldwide. In accordance with the terms of paragraphs 2 and 3 of Article 41 of the French Act dated March 11, 1957, “copies or reproductions reserved strictly for private use and not intended for collective use” and, on the other hand, analyses and short quotations for example or illustrative purposes, are allowed. Otherwise, “any representation or reproduction – whether in full or in part – without the consent of the author or of his successors or assigns, is unlawful” (Article 40, paragraph 1). Any representation or reproduction, by any means whatsoever, will therefore be deemed an infringement of copyright punishable under Articles 425 and following of the French Penal Code. The printed edition is not for sale in Chinese mainland. Ó Science Press, EDP Sciences, 2022

Preface

Titanium (Ti) is abundant in the crust, ranks fourth among metal elements, and is mainly concentrated in the form of ores such as ilmenite and rutile in nature. Thus, ilmenite and rutile are the origin of the modern titanium industry for the production of TiCl4, the intermediate products used for the manipulation of titanium sponge or titanium dioxide. In this industrial chain, the routes for the production of TiCl4 are flexible as shown in the following figure, where ilmenite is transferred to titanium slag or synthetic rutile. Meanwhile, ilmenite can also directly transfer to titanium dioxide with the assistance of acid leaching. Clearly, the demands for titanium and its related products will be continuously boosted in the future. Nevertheless, the traditional titanium production chain is facing challenges with the gradual depletion of high grade titanium resources and environmental concerns.

DOI: 10.1051/978-2-7598-2696-4.c901 Ó Science Press, EDP Sciences, 2022

IV

Preface

Thus, our research group in Yunnan Minzu University assisted by Kunming University of Science and Technology is working on the application of microwave heating in multiple production processes including drying, sintering and leaching, which are important for the production of titanium dioxide and related titanium products. Microwave heating, a multiphysics phenomenon that involves electromagnetic waves and heat transfer, is an important and powerful tool found in laboratories across the world, applied beyond reheating leftovers and across varying chemical applications. With the ability to heat efficiently, precisely, and safely, laboratory microwaves benefit chemical-synthesis, material-digestion, and now has semi-industrialization applications. In this book, the principles of microwave heating as applied to industrial processing are outlined and the basic design of the microwave enhancing processes is introduced and the book is divided into six chapters. Prof. Guo Chen has contributed on the design of the whole frame and the outline of the book, and also contributed chapter 1. Dr. Lei Gao has contributed chapters 2, 3, and 4. Prof. Jin Chen has contributed chapters 5 and 6. Prof. Wei Li has provided valuable assistance in the formation of chapters 1 and 3. Prof. Jinhui Peng has also contributed on the design of the whole frame of the book and provided significant guidance. In chapter 1, the microwave absorbing properties of various titanium resources including ilmenite, vanadium titano-magnetite and titanium slag are introduced. The corresponding temperature behaviours of these titanium resources under the radiation of microwave are systematically reported. In chapter 2, microwave pre-treatment technology and microwave drying technology in fascinating and attractive advanced inter-disciplinary fields of research are introduced as well. In chapter 3, the improvement on the carbothermic reduction in ilmenite and titanium-rich materials with microwave heating is reported, the purpose is to explore the possibility of further reduction in the energy consumption and environment issues. In chapter 4, research data related to microwave assisted leaching and intensification are introduced for process optimisation. In chapter 5, the preparation of rutile from different kinds of titanium slag by microwave roasting process is reported in addition to the optimised parameters suggested by response surface methodology. In chapter 6, life cycle assessment resulting from analytic hierarchy process and fuzzy comprehensive evaluation used for the optimisation of microwave heating devices is made. We hope these experimental data and the corresponding analysis can be helpful for the industrial application of microwave heating and thus further promote the development of titanium industry. We would like to acknowledge the National Natural Science Foundation of China (Grant No. U1802255, 51764052, 52104351), National Key R&D Program of China (2018YFC1900500), Yunnan Fundamental Research Projects (202101AU00088), Scientific Research Fund Project of Yunnan Education Department (No. 2021J0652), and the Kunming Key Laboratory of Energy Materials Chemistry, the Key Laboratory of Green-Chemistry Materials in University of Yunnan Province, the Innovative Research Team (in Science and Technology) in the University of Yunnan province for the financial support. Meanwhile, we also appreciate the financial support from School of Chemistry and the Environment, Yunnan Minzu University for the publication of this book.

Preface

V

The authors want to express their appreciation to their students Mr. He-wen Zheng, Miss. Qian-nan Li, Miss. Ye-qing Ling, Mr. Xian-dong Hao, Mr. Yu-xi Gui, Miss. Hong-ju Qiu, Miss. Yan-qiong Zhang, Mr. Si-rui Zhang, Miss. Jia-jia Lu, and Miss. Wei-wei Huang, for their contributions on the edition of this book. The authors pay deep respect and gratitude to those concerned for their suggestions and comments, and the authors are aware that the expertise is limited and there may be some errors in the book. If so, please do not hesitate to point them out.

Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

III

CHAPTER 1 Microwave Absorbing Properties and Temperature Behaviour . . . . . . . . . . . 1.1 Dielectric Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Measuring Instrumentation and Principle . . . . . . . . . . . . . . . . . . . . . 1.3 Microwave Heating Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Microwave-Absorbing Characteristics of Oxidised Ilmenite . . . . . . . . 1.4.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 Microwave-Absorbing Characteristics of Carbothermic Reduction Products of Ilmenite and Oxidised Ilmenite . . . . . . 1.4.3 Effect of Carbonaceous Reducing Agents on Microwave Absorbing Properties of Ilmenite . . . . . . . . . . . . . . . . . . . . . . 1.4.4 Effect of Catalyst on Microwave Absorbing Properties of Ilmenite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Microwave Absorbing Properties of Mechanically Activated Ilmenite . 1.5.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Effect of Mechanical Activated on Microwave Absorbing Properties of Ilmenite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Microwave Absorbing Characteristics of Mechanical Activated High Titanium Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.2 Dielectric Loss Factor of Mechanical Activated High Titanium Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.3 Dielectric Constant of Mechanical Activated High Titanium Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.4 Loss Tangent Coefficient of Mechanical Activated High Titanium Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Microwave Absorbing Properties of High Titanium Slag . . . . . . . . . . 1.7.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.2 Effect of Particle Size of High Titanium Slag on Microwave Absorbing Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.3 Effect of Mass Fraction of V2O5 on Microwave Absorbing Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . .

1 2 2 4 5 6

.

6

.

13

. . .

17 21 22

.

23

. .

26 26

.

28

.

28

. . .

29 30 30

.

31

.

33

Contents

VIII

1.8

Temperature Behaviour of Titanium Slag Under Microwave Heating . 1.8.1 Materials and Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . 1.8.2 Temperature Rise Characteristics of the Titanium-Rich Slag Using Microwave Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Microwave Absorption Properties and Thermal Behaviour of Vanadium Titano-Magnetite (VTM) . . . . . . . . . . . . . . . . . . . . . . . 1.9.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9.2 Thermal Behaviour of VTM During Microwave Heating . . . . . 1.9.3 Thermochemical Characteristics of VTM . . . . . . . . . . . . . . . . 1.9.4 Dielectric Properties of VTM . . . . . . . . . . . . . . . . . . . . . . . . . 1.9.5 Microwave Heating Characteristics of VTM . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. .

34 35

.

35

. . . . . . .

38 39 41 42 45 47 49

CHAPTER 2 Microwave Pretreatment and Microwave Drying . . . . . . . . . . . . . . . 2.1 Microwave Pretreatment of Ilmenite Ore . . . . . . . . . . . . . . . . 2.1.1 Materials and procedure . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Effects of Microwave Energy . . . . . . . . . . . . . . . . . . . . 2.1.3 Microwave Pretreatment Optimisation . . . . . . . . . . . . 2.2 Microwave Pretreatment of Titanium Slag . . . . . . . . . . . . . . 2.2.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Effects of Microwave Energy . . . . . . . . . . . . . . . . . . . . 2.3 High Effective Microwave-Assisted Drying of a Small Portion of Titanium Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Microwave Drying Behaviour . . . . . . . . . . . . . . . . . . . 2.3.3 Effects of Microwave Energy . . . . . . . . . . . . . . . . . . . . 2.3.4 Drying Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 High Effective Microwave-Assisted Drying of a Large Portion of Titanium Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Microwave Drying Behaviour . . . . . . . . . . . . . . . . . . . 2.4.3 Effects of Microwave Energy . . . . . . . . . . . . . . . . . . . . 2.4.4 Drying Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

53 54 54 58 64 72 72 73

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

76 77 78 80 81

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

82 83 83 86 90 91

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. 95 . 96 . 96 . 97 . 101

CHAPTER 3 Microwave Carbothermic Reduction . . . . . . . . . . . . . . . . . . . . . 3.1 Microwave Carbothermic Reduction of Ilmenite Ores . . . 3.1.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . 3.1.2 Effects of Microwave on Carbothermic Reduction . 3.1.3 Phase Diagram for the FeO–TiO2–TiO1.5 System . 3.2 Microwave Carbothermic Reduction of Ilmenite Ores with Sodium Silicate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Materials and Procedure . . . . . . . . . . . . . . . . . . .

. . . . .

. . . . .

. . . . .

. . . . . . . . . . 102 . . . . . . . . . . 103

Contents

Calibrations of Weight-Loss Fraction During Microwave Carbothermic Reduction . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Effects of Sodium Silicate on Microwave Carbothermic Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Microwave Carbothermic Reduction of Ilmenite Ores with NaCl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Calibrations of Weight-Loss Fraction During Microwave Carbothermic Reduction . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Effects of NaCl on Microwave Carbothermic Reduction . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IX

3.2.2

. . . . . 104 . . . . . 109 . . . . . 110 . . . . . 111 . . . . . 111 . . . . . 113 . . . . . 117

CHAPTER 4 Microwave-Assisted Leaching and Intensification . . . . . . . . . . . . . . . . . . . . . 4.1 Microwave-Assisted Leaching of Primary Titanium-Rich Materials . . 4.1.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Non-Isothermal Microwave Leaching Kinetics . . . . . . . . . . . . . 4.1.3 Microwave Absorption Characteristics During Leaching . . . . . 4.2 Microwave-Assisted Leaching of Titanium Slag Using Dilute Sulfuric Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Principle for the Simultaneous Removal of Cr(III) and V(V) . 4.2.3 Effects of Microwave Energy During the Process . . . . . . . . . . . 4.2.4 Effects of the Na2CO3/Slag Mass Ratio . . . . . . . . . . . . . . . . . 4.3 Microwave-Assisted Leaching of Titanium Slag Using Hydrochloric Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Characterisation by XRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Characterisation by Raman Spectroscopy . . . . . . . . . . . . . . . . 4.3.4 Characterisation by FI-IR . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.5 Characterisation by SEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Microwave-Assisted Leaching of High Titanium Slag Using Phosphoric Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Characterisation by XRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3 Characterisation by SEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4 Characterisation by FT-IR . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5 Characterisation by Raman . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Microwave Intensification for the Preparation of Rutile TiO2 from Panzhihua Sulphate Titanium Slag . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Characterisation by XRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3 Characterisation by SEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.4 Characterisation by Raman Spectroscopy . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . .

119 120 120 121 126

. . . . .

128 129 132 133 133

. . . . . .

146 147 149 151 153 154

. . . . . .

155 156 157 158 158 160

. . . . . .

161 161 164 165 165 167

X

Contents

CHAPTER 5 Microwave Roasting Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Preparation of Synthetic Rutile from Titanium Slag . . . . . . . . . . . . . 5.1.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2 Roasting Process with Conventional Heating . . . . . . . . . . . . . 5.1.3 Process Optimisation with Response Surface Methodology . . . 5.2 The Effect of Na2CO3/Slag Ratio and a Comparison Between Conventional and Microwave Heating for the Preparation of Rutile TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 The Effect of Na2CO3/Slag Ratio . . . . . . . . . . . . . . . . . . . . . . 5.2.2 The Comparison Between Different Heating Methods . . . . . . . 5.3 Optimisation of Microwave Roasting Process Using Response Surface Methodology (RSM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Process Optimisation with Response Surface Methodology . . . 5.3.3 Characterization of the Synthetic Rutile . . . . . . . . . . . . . . . . . 5.3.4 Another Optimisation Case . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Phase Transformation of Titanium Slag Using Microwave Irradiation 5.4.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 Systematical Study on the Influence of Microwave on the Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Preparation of Synthetic Rutile from Sulphate Titanium Slag with the Assistance of Microwave . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.2 Characterization of the Synthetic Rutile . . . . . . . . . . . . . . . . . 5.6 Preparation of Synthetic Rutile from High Titanium Slag with the Assistance of Microwave . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.2 Roasting Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.3 Weight Increase Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.4 Sulfur and Carbon Content Analysis . . . . . . . . . . . . . . . . . . . 5.6.5 TiO2 Content Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . .

171 172 172 175 180

. 186 . 186 . 189 . . . . . . .

194 195 196 201 203 209 209

. 210 . 214 . 215 . 216 . . . . . . .

222 222 224 226 226 227 228

CHAPTER 6 Microwave Heating Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Analytic Hierarchy Process and Fuzzy Comprehensive Evaluation of Microwave Tube and Shaft Furnace . . . . . . . . . . . . . . . . . . . . . 6.1.1 Hierarchical Structure of Assessment . . . . . . . . . . . . . . . . . 6.1.2 Details for the Calculation . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Features of the Microwave Tube Furnace . . . . . . . . . . . . . . 6.1.4 Features of Microwave Shaft Furnaces . . . . . . . . . . . . . . . . 6.2 Life Cycle Assessment on Microwave Leaching Process . . . . . . . . . 6.2.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Analytic Hierarchy Process . . . . . . . . . . . . . . . . . . . . . . . . .

. . . 231 . . . . . . . .

. . . . . . . .

. . . . . . . .

232 232 233 237 238 240 240 242

Contents

6.2.3 Pairwise Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.4 Life Cycle of Assessment of the Microwave-Assisted Leaching . 6.3 Life Cycle Assessment on Microwave Hot Air Systems . . . . . . . . . . . . 6.3.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Features of Microwave Hot Air Systems . . . . . . . . . . . . . . . . . 6.3.3 Life Cycle of Assessment of the Microwave Hot Air Systems . . 6.4 Numerical Modeling of the Microwave Heating Device . . . . . . . . . . . . 6.4.1 Materials and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 Details for Numerical Model . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.3 Temperature Rise Curve of High Titanium Slag by Microwave Irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.4 Characterisation of High Titanium Slag . . . . . . . . . . . . . . . . . 6.4.5 Distribution of the Microwave Fields . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

XI

. . . . . . . . .

242 243 243 243 249 250 250 250 251

. . . .

255 256 257 259

Chapter 1 Microwave Absorbing Properties and Temperature Behaviour With its ability to rapidly heat dielectric materials, the microwave energy is commonly used as a heat source and is an alternative to conventional conductive heating. In recent years, several potential applications of microwave in mineral and materials’ processing have been investigated, including microwave-assisted ore grinding, microwave-assisted carbothermic reduction of metal oxides, microwave-assisted drying and anhydration, microwave-assisted mineral leaching, microwave-assisted heating and smelting of sulphide concentrate, microwave-assisted pretreatment of refractory gold concentrate, microwave-assisted spent carbon regeneration, coke making and activated carbon production, and microwave-assisted waste management, microwave drying of coal, solid-state synthesis of inorganic materials and preparation of inorganic nanostructures in the liquid phase, etc. [1–12]. Advantages in utilising microwave technologies for the above processes include penetrating radiation, controlled electric field distribution, and selective and volumetric heating [3, 11, 13]. The study of the mechanism of microwave heating is very complicated. The complex permittivity and permeability of the absorbing material are the basic parameters that reflect the interaction between the microwave and materials [14]. However, to our best knowledge, there is little information on the microwave absorbing characteristics of materials in microwave irradiation, resulting in difficulties in investigating the interaction mechanism between microwave and materials, limiting the application of microwave heating in the industry [3]. Therefore, there is an urgent need to investigate microwave-absorbing characteristics of materials and minerals and collect their dielectric properties to promote microwave heating applications in different fields. We have recently investigated microwave-absorbing characteristics of oxidised ilmenite, mechanically activated ilmenite, mechanically activated high titanium slag, and high titanium slag. Additionally, we also investigated the thermal behaviour of titanium slag under microwave heating and vanadium titano-magnetite. The results of the mentioned studies are presented in sections 1.4, 1.5, 1.6, 1.7, 1.8, and 1.9, respectively. Section 1.1 has provided the fundamental knowledge related to DOI: 10.1051/978-2-7598-2696-4.c001 © Science Press, EDP Sciences, 2022

Application of Microwave Heating in the Comprehensive Utilization

2

dielectric properties, section 1.2 has introduced the instrumentation and principle used in the other section in chapter 1, and section 1.3 has introduced the microwave heating theory.

1.1

Dielectric Properties

A large variety of materials can be microwave heated, including dielectric materials. The materials’ dielectric properties help understand the materials’ structural information and benefit microwave heating devices’ design. A dielectric constant is a complex number, which can be expressed as follows, e
¼ e0  je00 0

ð1:1Þ

00

where e and e are the real part (dielectric loss factor) and the imaginary part (dielectric constant) of the complex dielectric constant, respectively. The dielectric constant (e0 ) is called relative permittivity or specific inductive capacity of the dielectric material, and it is an essential parameter in characterizing microwave capacitors, which determines the amount of incident energy reflected at the air-sample interface and the amount of the incident energy that enters the sample. The dielectric loss factor (e00 ) determines the microwave absorbing properties and is directly related to complex relative permittivity, measured through loss of energy in conduction, gentle polarisation currents, and other dissipative phenomena. According to the electromagnetic field theory, a dielectric in alternation electric field will be rotationally polarised, and it relaxes during polarisation. The electric displacement vector has phase hysteresis of an angle d, calculated from the following equation, tan d ¼

e00 e0

ð1:2Þ

The loss tangent coefficient calculates the ratio at any particular frequency between the real and imaginary parts of the dielectric materials. A significant loss tangent corresponds to high microwave absorption.

1.2

Measuring Instrumentation and Principle

A typical microwave resonant cavity perturbation system is used to measure the mentioned dielectric properties in section 1.1, which consists of microwave resonator, sweeping signal, detector and digital signal processor (DSP), multifunctional card, interface circuit, and computer. The microwave generated by a computer-controlled fast scanning microwave generator was transmitted to the microwave sensor. The microwave sensor’s output signals were picked up by the linear detector and DSP fed to a low-pass filter. The output signals of the low-pass filter were amplified and converted by the A/D converter. The computer performed

Microwave Absorbing Properties and Temperature Behaviour

3

FIG. 1.1 – Sketch of the microwave sensor system.

the data processing of the microwave resonant cavity perturbation system. The software control of the set-up was performed by the Windows XP operating system and programmed by Visual Basic 6.0. The schematic diagram of the microwave resonant cavity perturbation system is illustrated in figure 1.1. The microwave sensor system was based on microwave cavity perturbation technique and digital signal processing technique. From the theory of electric– magnetic field, the frequency shift and the output voltage of the microwave cavity can be given by, Z Dx ¼ x0 ðe0r  1Þ E0
 Edv=4W ð1:3Þ x Ve 1 1  ¼ 2e0 e00r Q Q0 Z W ¼ V

Z Ve

E0
 Edv=4W


 ðE0  D0 þ H0
 B0 Þ þ ðE0
 D1 þ H0
 B1 Þ dv

ð1:4Þ

ð1:5Þ

where W is the storage energy, x is the angular frequency, x0 is the angular frequency without the sample in a resonant cavity, Dx ¼ x  x0 is the shift of angular frequency, E is the electric field intensity of the sample in the resonant sensor, E0
, H0
are the hetero conjugations of electric field intensity and electromagnetic field intensity in the resonant sensor before perturbation, respectively, D0 , and B0 are the hetero conjugations of electric displacement and magnetic induction before perturbation, respectively, D1 and B1 are the increments of electric displacement and magnetic induction in samples after perturbation. mc and me are the volumes of the sample and the resonant sensor, respectively, dm is the volume of the element, Q0 and x0 are the quality factors (Q values) of the cavity unloaded (unperturbed condition) and loaded with the samples, respectively, e0 is the absolute permittivity of a vacuum (free space), e0r and e00r are the real and the imaginary part of the complex permittivity or dielectric loss of the sample, respectively.

4

1.3

Application of Microwave Heating in the Comprehensive Utilization

Microwave Heating Theory

In dielectric (electrically insulating) materials, the absorption (degree of interaction) of microwaves is related to the material’s complex permittivity, e ¼ e0 ðe0  ie00 Þ

ð1:6Þ −12

where e0 is the permittivity of free space (e0 = 8.86 × 10 F/m), the real part e0 is 00 the relative dielectric constant and the imaginary part e is the effective relative dielectric loss factor. The loss tangent tan d is also commonly used to describe these losses, which is defined as, tan d ¼

e00 r ¼ e0 2pf e0 e0

ð1:7Þ

where r is the total effective conductivity (S/m) caused by ionic conduction and displacement currents and f is the frequency. The power P that is absorbed per unit volume (W/m3) of the sample at any instant of time is described by P ¼ rjE j2 ¼ 2pf e0 e0 tan djE j2

ð1:8Þ

where jE j2 (V/m) is the magnitude of the internal electric field. It was assumed that the power is uniform throughout the volume and that thermal equilibrium has been achieved. This assumption is not always correct and also E, tan d, e0 and f are all interdependent. However, it does provide a valid approximation for the power absorbed and describes the fundamental relationships between the four variables. The power absorbed varies linearly with frequency, the relative dielectric constant, and the loss tangent and varies with the electric field’s square. The choice of the heating process has a significant impact on the morphology and surface properties of the products. The characteristics of the influence of heat transfer mechanisms on the experimental procedure, surface properties, and morphology of particles are schematically shown in figure 1.2. In the conventional heating process, the heat is transferred through temperature driving force from exterior to interior, and hence the external temperature is always higher than the interior. On the contrary, the heat transfer for microwave heating is from interior to exterior, and the surface temperature is lower than the internal temperature. In the alternating electric field, the polar molecules of a material change directions rapidly and wiggle at high speed to keep pace at the rate of microwave frequency [15, 16]. At the same time, those wiggle molecules collide with adjacent particles, which produces heat. In other words, the interactions of particles and microwave fields cause the energy conversion from microwave energy to thermal energy. However, the temperature rise depends on the microwave absorption properties of materials, including dielectric constant and thermal capacity [17]. During the microwave heating process, titanium minerals have a rapid temperature rising speed from decent microwave absorption properties. However, microwave energy slightly influences gangue minerals since their dielectric constant is low.

Microwave Absorbing Properties and Temperature Behaviour

5

FIG. 1.2 – Schematic diagram of the mechanism of microwave heating and conventional heating.

The difference in dielectric constants causes thermal stress resulting in the cracks on the interface between titanium minerals and gangue, increasing the sample’s specific surface area.

1.4

Microwave-Absorbing Characteristics of Oxidised Ilmenite

In this section, we introduce microwave-absorbing characteristics of oxidised ilmenite. Many studies show that ilmenite needs high reductive temperature or needs additives, such as pre-oxidisation, to improve reactivity [18–21] or leaching rate [22–27]. In the Panzhihua region, Sichuan Province of China, the ilmenite deposits 35% of the world’s titanium resource and approximately 92% of Chinese titanium resource [28]. Efficient utilization of the ilmenite resources is essential for the development of the titanium industry. Thus microwave heating is now utilised as an alternative heat source for conventional conductive heating to reduce ilmenite resources. To understand this application fundamental, microwave-absorbing characteristics of the mixtures between ilmenite and different carbonaceous reducing agents were studied using the microwave cavity perturbation technique. The relationships between dielectric loss and the microwave-absorbing characteristics of the ilmenite mixtures (mixed with coconut-based activated carbon, coke, and graphite) were studied.

6

Application of Microwave Heating in the Comprehensive Utilization

The results showed that the microwave-absorbing characteristic of these carbonaceous reducing agents (coconut-based activated carbon, coke, and graphite) were better than that of the oxidised ilmenite under the conditions of particle size of 175 µm–147 µm. The appropriate ratios of coconut-based activated carbon, coke, and graphite to oxidised ilmenite were 30%–80%, 30%–60%, and 10%–20%, respectively. Their optimal proportions of coconut-based activated carbon, coke, and graphite to the oxidised ilmenite were defined as 30%, 30%, and 10%, respectively, based on the actual production.

1.4.1

Materials and Procedure

The raw material, ilmenite, was obtained from Panzhihua (Sichuan province, P.R. China). The chemical compositions of ilmenite are as follows (% (w/w)): TFe (total Fe), 32.18; TiO2, 47.85; CaO, 1.56; MgO, 5.56; SiO2, 4.6; Al2O3, 2.16, respectively. Pre-oxidation was performed in a muffle furnace with an air stream continuously blown into the furnace (temperature 900 ± 5 °C, three hours). The particle size of the sample after oxidation is 175 µm–147 µm. The samples were prepared by mixing 5 g of ilmenite with a certain proportion of coconut activated carbon, coke, or graphite in an alumina crucible. Then the samples were thoroughly mixed for two min, and dried for two hours at 150 °C in the oven. 2 g of the mixed specimens was put into the microwave cavity with a microwave sensor to measure microwave absorbing characteristics.

1.4.2

Microwave-Absorbing Characteristics of Carbothermic Reduction Products of Ilmenite and Oxidised Ilmenite

Figure 1.3 shows the microwave spectra of the reductive product of oxidised ilmenite by microwave heating at different temperatures. Relative frequency shift, attenuation, and quality factors were obtained by analyzing microwave spectra’ changing trend. Parameters of microwave-absorbing characteristics are listed in table 1.1. The microwave-absorbing characteristics of the reduction product of oxidised ilmenite by microwave heating at different reductive temperatures are illustrated in figures 1.4 and 1.5, respectively. There is a tremendous change in microwave-absorbing characteristics of the reduction product of oxidised ilmenite by microwave heating at temperatures of 1000 °C, 1050 °C, and 1100 °C. Additionally, reduction products obtained at temperatures of 1000 °C, 1050 °C, and 1100 °C were characterised by XRD, as shown in figures 1.6 and 1.7, respectively. Figure 1.6 shows that the reduction product contains MgTi2O5, Fe2TiO5, and rutile TiO2, a small amount of α-Fe, not finding low-valent titanium oxides. Figure 1.7 shows that the diffraction intensity of the characteristic peak of TiO2 is 215 counts per 2nd (CPS), the intensity of the characteristic peak of Fe is 367 CPS, and the diffraction intensity of the characteristic peak of MgTi2O5 is 1300 CPS at a temperature of 1000 °C; the intensity of the characteristic peak of TiO2 is 218 CPS, the intensity of the characteristic peak of Fe is 76 CPS, but the diffraction intensity

Microwave Absorbing Properties and Temperature Behaviour

7

FIG. 1.3 – Microwave spectra of the reductive product of oxidised ilmenite by microwave heating.

TAB. 1.1 – Microwave-absorbing characteristic parameters of reduction product of oxidised ilmenite. No. Empty cavity O raw O750 O800 O850 O900 O970 O1000 O1050 O1100 O1130 O1200 O1250

Attenuation voltage (V) 2.2138 2.1244 1.9981 2.0876 2.0262 2.0764 2.0172 2.0229 2.1454 2.0681 2.0786 2.1430 2.1414

Frequency (GHz) 2.4759 2.4533 2.4378 2.4370 2.4359 2.4369 2.4418 2.4363 2.4446 2.4387 2.4456 2.4437 2.4458

Bandwidth (GHz) 0.0314 0.0349 0.0399 0.0359 0.0379 0.0353 0.0388 0.0382 0.0326 0.0359 0.0359 0.0325 0.0327

Quality factors 78.85 70.29 61.09 67.88 64.27 69.03 62.93 63.77 74.98 67.93 75.25 75.19 74.79

8

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.4 – Relationships between attenuations, microwave frequency, and different reductive

0.040

76

0.039

74

0.038

72

0.037

70

0.036

68

0.035

66

0.034

64

0.033

700

62

Bandwidth Qulity factor

0.032 800

900 1000 1100 Reductive temperature/ć

Quality factor

Bandwidth

temperatures.

60 1200

1300

FIG. 1.5 – Relationship between bandwidth, quality factor, and different reductive temperatures.

of the characteristic peak of MgTi2O5 is 1647 CPS at a temperature of 1050 °C; the diffraction intensity of the characteristic peak of TiO2 is 450 CPS, the diffraction intensity of the characteristic peak of Fe is 500 CPS, but the diffraction intensity of the characteristic peak of MgTi2O5 is 1745 CPS at a temperature of 1100 °C. Microwave-absorbing characteristics of the reduction product obtained at a temperature of 1050 °C are weak. However, microwave-absorbing characteristics of reduction product obtained at a temperature of 1100 °C became strong, the diffraction intensity of the characteristic peak of weak microwave-absorbing TiO2 was becoming more assertive. Still, the diffraction intensity of the characteristic peak of strong microwave-absorbing Fe was also becoming intense. The enhanced range of strong microwave absorbing material is greater than the reduced range of

Microwave Absorbing Properties and Temperature Behaviour

9

FIG. 1.6 – XRD pattern of the reductive product obtained at 1050 °C.

1100ć

Intensity(CPS)

1320 990

1050ć 660

1000ć

330 0

20

40

60

80

100

2-theta/deg

FIG. 1.7 – Comparison of XRD patterns of reductive products at 1000 °C, 1050 °C and 1100 °C.

weak microwave absorbing material. The microwave-absorbing characteristics of reduction products obtained at a temperature of 1100 °C are becoming vital. The reduction mechanism of ilmenite can explain the variation of diffraction intensity (diminishing and then enhancing). Strong microwave absorbing material Fe can form a solid solution with MgTi2O5 or pseudobrookite at a specific temperature, resulting in decreased metallic Fe’s content. When the temperature reaches 1050 °C, the smelting rate Fe in MgTi2O5 or pseudobrookite reaches the peak value. Then after the temperature of 1050 °C, the smelting rate gentles down.

10

Application of Microwave Heating in the Comprehensive Utilization

Figure 1.8 shows the microwave spectra of the reductive product of ilmenite by microwave heating at different temperatures, and parameters of microwave-absorbing characteristics are listed in table 1.2. The microwave-absorbing characteristics of

FIG. 1.8 – Microwave spectra of the reductive product of ilmenite by microwave heating.

TAB. 1.2 – Microwave-absorbing characteristic parameters of reduction product of ilmenite. No. Empty cavity MKN850 MKN900 MKN970 MKN1000 MKN1050 MKN1100 MKN1130 MKN1200

Attenuation voltage (V) 2.2138 1.7081 1.5943 1.6306 1.6413 1.3438 1.3895 1.4077 1.4099

Frequency (GHz) 2.4759 2.4315 2.4213 2.4162 2.4201 2.3850 2.3895 2.4009 2.3985

Bandwidth (GHz) 0.0314 0.0848 0.0832 0.0747 0.0776 0.0752 0.0772 0.0823 0.0811

Quality factors 78.85 28.67 29.10 32.35 31.18 31.72 30.95 29.17 29.57

Microwave Absorbing Properties and Temperature Behaviour 2.44

1.75

1.65

2.43 2.42

1.60 1.55

2.41 1.50 2.40

1.45

Microwave frequency/GHz

Attenuations microwave frequency

1.70

Attenuations/V

11

1.40 2.39 1.35 1.30 800

850

900

950

1000

1050

1100

1150

1200

2.38 1250

Reductive temperature/℃

FIG. 1.9 – Relationship between attenuations, microwave frequency, and different reductive temperatures.

FIG. 1.10 – Relationship between bandwidth, quality factor, and different temperatures.

ilmenite’s reduction product by microwave heating at different reductive temperatures are illustrated in figures 1.9 and 1.10, respectively. The previous results indicate that there are tremendous differences in microwave-absorbing characteristics of reduction products. To further understand these differences, reduction products obtained at temperatures of 850 °C, 900 °C, 1000 °C, 1050 °C, 1100 °C, 1130 °C and 1200 °C were characterised by XRD, as shown in figure 1.11. Reduction products at a temperature of 850 °C contain FeTiO3, salt silicates, a small amount of Fe, and TiO2 (figure 1.11(a)), where Fe is formed at a temperature of 850 °C by microwave heating. Reduction products at a temperature of 900 °C contain FeTiO3, salt silicates, Fe and TiO2, few FeTi2O5 (figure 1.11(b)), the diffraction intensity of the characteristic

12

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.11 – XRD patterns of reductive product at different temperatures. peak of FeTiO3 decreases, while, the diffraction intensity of the characteristic peak of Fe and TiO2 increases. The increase in powder Fe’s microwave-absorbing characteristics is more incredible than that of TiO2, causing a significant change in microwave-absorbing characteristics at a temperature range of 850 °C–900 °C. Figure 1.11(c) shows that the main phase of reduction product at a temperature of 1000 °C, 1050 °C, 1100 °C, and 1130 °C is FeTiO3 (diffraction angle 32.58°), powder metallic Fe (diffraction angle 44.68°), TiO2 (diffraction angle 27.58°) and FeTi2O5 (diffraction angle 25.26°), respectively. The diffraction intensity of the characteristic peak of FeTiO3 is decreasing in the temperature order of 1000 °C–1050 °C–1100 °C–1130 °C. The characteristic peak of FeTiO3 at a temperature of 1130 °C disappears. Thus, the reduction temperature by microwave heating is close to 1150 °C. The diffraction intensity of the characteristic peak of Fe is increasing in the temperature order of 1000 °C–1050 °C–1100 °C–1130 °C, and is 400 CPS, 650 CPS, 1050 CPS, 1200 CPS, respectively. With increasing temperature, the content of

Microwave Absorbing Properties and Temperature Behaviour

13

powder metallic Fe increases, indicating that the Fe phase hardly forms solid solutions with other phases. The diffraction intensity of the characteristic peak of FeTi2O5 at the temperature of 1000 °C, 1050 °C, 1100 °C, and 1130 °C is 850 CPS, 1250 CPS, 1342 CPS and 1248 CPS, respectively. FeTi2O can be mentioned as FeO2TiO2. The increasing diffraction intensity of the characteristic peak from 850 to 1250 CPS is the main reason for the significant change in microwave-absorbing characteristics of reduction product in the temperature range of 1000 °C–1050 °C. Figure 1.11(d) shows that the reduction product, received at a temperature of 1000 °C, has phases of FeTi2O5 (the isomorphic phase of MgTi2O5, which has a similar characteristic peak), Fe, FeTiO3, and a small amount of Ti3O5, few FeTiO3. This composition distribution is identical to the sample received at a temperature of 1130 °C (figure 1.11(c)), proposing that FeTi2O5 was formed at a temperature above 1000 °C.

1.4.3

Effect of Carbonaceous Reducing Agents on Microwave Absorbing Properties of Ilmenite

Figure 1.12 shows the mixtures’ microwave spectra with different ratios of carbonaceous reducing agents (coconut-based activated carbon, coke, and graphite) to oxidised ilmenite. Relationships between attenuation/frequency and ratios of carbonaceous reducing agents (coconut-based activated carbon, coke, and graphite) to oxidised ilmenite are illustrated in figures 1.13 and 1.14, respectively. The optimum ratio of carbonaceous reducing agents (coconut-based activated carbon, coke, and graphite) to oxidised ilmenite is obtained according to attenuations and relative frequency shifts. With increasing the reducing agent ratios, microwave spectra attenuate and move toward Red-Shift. The settled computer program computed attenuations and frequencies at the first wave crest of microwave spectra. Figure 1.13(a) shows that the attenuation order for a mixture of coconut-based activated carbon and oxidised ilmenite is U50 < U80 < U40 < U70 < U60 < U30 < U35 < U45 < U25 < U20 < U15 < U10 < U5 < U0 < Uair (U is attenuation; the subscript number is the ratio of carbonaceous reducing agents that were added to oxidised ilmenite, the air is the cavity without sample). Therefore, e00 50 > e00 80 > e00 40 > e00 70 > e00 60 > e00 30 > e00 35 > e00 45 > e00 25 > e00 20 > e00 15 > e00 10 > e00 5 > e00 0 > e00 air. The attenuation order for a mixture of coke and oxidised ilmenite shown in figure 1.13(b) is U60 < U55 < U45 < U50 < U40 < U35 < U30 < U25 < U20 < U15 < U10 < U5 < U0 < Uair. Therefore, e00 60 > e00 55 > e00 45> e00 50 > e00 40 > e00 35 > e00 30 > e00 25 > e00 20 > e00 15 > e00 10 > e00 5 > e00 0 > e00 air. The attenuation order for the mixture of graphite and oxidised ilmenite shown in figure 1.13(c) is U20 < U18 < U15 < U12 < U10 < U8 < U5 < U0 < Uair. Therefore, e00 20 > e00 18 > e00 15 > e00 12 > e00 10 > e00 8 > e00 5 > e00 0 > e00 air. Figure 1.14(a) shows that the relative frequency shift for a mixture of coconut-based activated carbon and oxidised ilmenite is Δω0 > Δω5 > Δω15 > Δω20 > Δω25 > Δω30 > Δω35 > Δω40 > Δω50 > Δω45 > Δω55 > Δω60 > Δω70 > Δω80

14

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.12 – (a) Microwave spectra of the mixtures of different ratios of coconut carbon to oxidised ilmenite; (b) Microwave spectra of the mixtures of different ratios of coke and oxidised ilmenite; (c) Microwave spectra of the mixtures of different ratios of graphite and oxidised ilmenite.

Microwave Absorbing Properties and Temperature Behaviour

15

FIG. 1.13 – (a) Relationship between attenuations and ratios of coconut-based activated carbon to oxidised ilmenite; (b) Relationship between attenuations and ratios of coke to oxidised ilmenite; (c) Relationship between attenuations and ratios of graphite to oxidised ilmenite.

16

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.14 – (a) Relationship between relative frequency shift and ratios of coconut-based activated carbon to oxidised ilmenite; (b) Relationship between relative frequency shift and ratios of coke to oxidised ilmenite; (c) Relationship between relative frequency shift and ratios of graphite to oxidised ilmenite.

Microwave Absorbing Properties and Temperature Behaviour

17

(Δω is relative frequency shift). Therefore, e0 0 > e0 5 > e0 15 > e0 20 > e0 25 > e0 30 > e0 35 > e0 40 > e0 50 > e0 45 > e0 55 > e0 60 > e0 70 > e0 80. Relative frequency shift order for the mixture of coke and oxidised ilmenite shown in figure 1.14(b) is Δω60 > Δω45 > Δω40 > Δω55 > Δω30 > Δω50 > Δω25 > Δω35 > Δω20 > Δω15 > Δω10 > Δω5 > Δω0 > Δωair. Therefore, e0 60 > e0 45 > e0 40 > e0 55 > e0 30 > e0 50 > e0 25 > e0 35 > e0 20 > e0 15 > e0 10 > e0 5 > e0 0 > e0 air. Relative frequency shift order for the mixture of graphite and oxidised ilmenite shown in figure 1.14(c) is Δω10 > Δω18 > Δω15 > Δω12 > Δω20 > Δω5 > Δω0 > Δω8. Therefore, e0 10 > e0 18 > e0 15 > e0 12 > e0 20 > e0 5 > e0 0 > e0 8. Figure 1.14 indicates that the appropriate range of the ratios of coconut-based activated carbon, coke, and graphite to oxidised ilmenite is 30%–80%, 30%–60%, and 10%–20%, respectively. Significantly, the lowest ratio is obtained when the ratio of coconut-based activated carbon, coke, and graphite to oxidised ilmenite is 50%, 60%, and 20%, respectively. The optimum ratios of coconut-based activated carbon, coke, and graphite to oxidised ilmenite are close to 30%, 30%, and 10%, respectively, resulting from production costs. In figure 1.13a and b, e0030 is the most appropriate value. In figure 1.13(c), e0010 is the most suitable value. During dialect heating, the equation of power absorbed by a given volume of material is P = ω e00r E2V, where ω is angle frequency of microwave field; E is the intensity of electric field in microwave field; V is effective volume of microwave-absorbing material. With fixed values of ω, E2, and V, the equation equals to P = k e00r (k is a fixed value). That is to say, P30, P30, and P10 are more appropriate than others in all measured values. Furthermore, it can be concluded from e0050 > e000 (figure 1.13(a)), e0060 > e000 (figure 1.13(b)) and e0020 > e000 (figure 1.13(c)) that the microwave-absorbing characteristics of coconut-based activated carbon, coke, and graphite are better than that of oxidised ilmenite. Thus 30% of coconut-based activated carbon, 30% of coke, and 10% of graphite to oxidised ilmenite are defined as the appropriate ratios for pilot plant experiments for carbo-thermic reduction of oxidised ilmenite in many subsequent cases.

1.4.4

Effect of Catalyst on Microwave Absorbing Properties of Ilmenite

For some of the reduction processes, catalysts including sodium silicate and sodium chloride also have non-ignorable influences. Figure 1.15 shows the microwave spectra of reduction products of ilmenite concentrates catalysed by sodium silicate, and table 1.3 lists the correspondence microwave absorbing characteristics parameters. Relative frequency shift, attenuations, and quality factors (Q) at the first wave crest of microwave spectra were computed by the computer’s program. From these parameters, the microwave-absorbing characteristics of reduction products at different conditions could be compared, the analysis results are shown in figures 1.15 and 1.16. By analyzing microwave-absorbing characteristics such as attenuation voltage, frequency, bandwidth, and quality factor, combined with table 1.3, figures 1.17, 1.7, and 1.8, significant changes were found for microwave-absorbing

Application of Microwave Heating in the Comprehensive Utilization

18

FIG. 1.15 – Microwave spectra of reduction product.

TAB. 1.3 – Microwave-absorbing characteristic parameters of reduction products. No. Empty cavity NZ800 NZ850 NZ900 NZ950 NZ1000 NZ1050 NZ1100 NZ1150 NZ1200

Attenuation voltage (V) 2.2135 1.9342 1.9241 1.9081 1.9303 1.9288 1.9406 1.7955 1.6351 1.6341

Frequency (GHz) 2.4755 2.4379 2.4373 2.4401 2.4391 2.4385 2.4418 2.4356 2.4234 2.4164

Bandwidth (GHz) 0.0320 0.0453 0.0459 0.0458 0.0454 0.0438 0.0438 0.0539 0.0852 0.0782

Quality factors (Q) 77.36 53.82 53.10 53.28 53.72 55.67 55.75 45.18 28.44 30.90

Microwave Absorbing Properties and Temperature Behaviour

19

FIG. 1.16 – Relationships between reduction temperature and attenuation, frequency of the microwave.

FIG. 1.17 – Relationships between reduction temperature and bandwidth, quality factor. characteristics of reduction products obtained at temperatures of 900 °C and 1100 °C. Figure 1.18 shows the microwave spectra of reduction products of ilmenite concentrates catalysed by sodium chloride, and table 1.4 lists the correspondence microwave absorbing characteristics parameters. From the related parameters, the microwave-absorbing characteristics of reduction products at different conditions were compared, the analysis results are shown in figures 1.19 and 1.20. By analyzing microwave-absorbing characteristics such as attenuation voltage, frequency, bandwidth, and quality factor, combined with table 1.4, figures 1.19 and 1.11, significant changes in microwave-absorbing characteristics of reduction products occurred at temperatures of 850 °C, 1100 °C, and 1150 °C.

Application of Microwave Heating in the Comprehensive Utilization

20

FIG. 1.18 – Microwave spectra of reduction products.

TAB. 1.4 – Microwave-absorbing-characteristic parameters of reduction products. No. k800 k850 k900 k950 k1000 k1050 k1100 k1150

Attenuation voltage (V) 1.8508 1.8254 1.8895 1.9075 1.8160 1.8158 1.8250 1.6017

Frequency (GHz) 2.4377 2.4337 2.4369 2.4378 2.4239 2.4278 2.4298 2.3988

Bandwidth (GHz) 0.0509 0.0529 0.0476 0.0452 0.0544 0.0499 0.0506 0.0658

Quality factors (Q) 47.89 46.00 51.19 53.93 44.56 48.65 48.02 36.45

Microwave Absorbing Properties and Temperature Behaviour

21

FIG. 1.19 – Relationships of reduction temperature, attenuation, and frequency of the microwave.

FIG. 1.20 – Relationships of reduction temperature, bandwidth, and quality factor.

1.5

Microwave Absorbing Properties of Mechanically Activated Ilmenite

Mechanical activation belongs to innovative processes to improve technological methods by creating new surfaces and solid-state structural defects [29–32], thus has advantages including size reduction, production of large surface area, and liberation of valuable minerals from their matrices [33–36]. In this section, the effects of mechanical activation on the microwave absorbing characteristics of ilmenite were systematically studied using the microwave cavity perturbation technique and the digital signal processing technique. The relationships between dielectric loss and the microwave absorbing characteristics of the

22

Application of Microwave Heating in the Comprehensive Utilization

mechanically activated ilmenite were studied. The crystal structures and surface chemical functional groups of ilmenite were characterised using XRD and FT-IR, respectively. The results indicated that the microwave absorbing characteristics of mechanically activated ilmenite are improved.

1.5.1

Materials and Procedure

The ilmenite was obtained from Panzhihua city, Sichuan province, China, with chemical compositions listed in table 1.5. TAB. 1.5 – Chemical composition of ilmenite. ΣFe 30.67

TiO2 15.71

SiO2 20.38

CaO 6.48

MgO 7.12

Al2O3 3.33

The ilmenite was characterised by an X-ray diffractometer (D/Max 2200, Rigaku, Japan) at a scanning rate of 0.25°/min with 2θ ranging from 5° to 100° using CuKα radiation (λ = 1.5418 Å) and a Ni filter. The voltage and anode current operated were 35 kV and 20 mA, respectively. The crystalline compounds of the ilmenite were performed using X-ray diffraction, and the results are shown in figure 1.21. Figure 1.21 shows that magnetite (Fe3O4) and ilmenite (FeTiO3) are the main crystalline compounds in the ilmenite ore; also, a minor amount of SiO2, CaO, MgO, and TiO2 is present.

FIG. 1.21 – The X-ray diffraction pattern of the ilmenite. Ilmenite’s infrared spectra were collected using an FT-IR spectrometer equipped (8700, Nicolet, USA). The angle of incidence of the I.R. beam was 45° and 100 scans were collected at a resolution of 4 cm−1 and averaged using the OMNIC spectroscopic software. The spectral range was 4000cm−1–400 cm−1. Ilmenite’s surface

Microwave Absorbing Properties and Temperature Behaviour

23

FIG. 1.22 – FT-IR spectra of ilmenite. chemical functional groups were characterised using Fourier transform infrared radiation (FT-IR), and the results are shown in figure 1.22, indicates that the broad band at 3434.2 cm−1 and 1067.1 cm−1 due to the stretching vibrations of O–H bonds, respectively. A weak peak at 1641.2 cm−1 is because the H–O–H bending vibrations of interlayer adsorbed H2O molecule at the surface of the mineral. Peaks at 976.9 cm−1 due to the stretching vibrations of Si–O and Si–O–Si of the SiO4 units. In the range of 400 cm−1–1000 cm−1, the I.R. bands of inorganic solids are usually because of the crystal lattice’s vibration of metallic ions. A certain amount of ilmenite was put into the conical ball mill to treat with different mechanical activation times (0, 30, 60, 90, 120, 150 s). Then, the treated ilmenite was dried at 105 °C for 2 h. During this process, a conical ball mill (XNQ-67, Wuhan, China) was employed for ilmenite’s mechanical activation.

1.5.2

Effect of Mechanical Activated on Microwave Absorbing Properties of Ilmenite

The microwave absorbing properties of mechanically activated ilmenite are assessed based on the microwave sensor’s output voltage and resonator frequency by comparing the case with treated samples and raw materials in the microwave cavity. Figure 1.23 shows the microwave spectra of ilmenite at different mechanical activation times. Relationships between attenuation/frequency and mechanical activation time of ilmenite are illustrated in figures 1.24 and 1.25, and listed in table 1.6, respectively. Ilmenite’s optimum mechanical activation time is obtained according to attenuations and relative frequency shifts. With changing the mechanical activation time of ilmenite, microwave spectra move toward red-shift and attenuate. The effect of ilmenite’s mechanical activation time on the frequency shift was studied in the range of 0–150 min. At the same time, the other experiment

24

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.23 – Microwave spectra of ilmenite at different mechanical activation times.

FIG. 1.24 – Relationship between frequency shift and mechanical activation time. parameters were fixed at a sampling weight of 100 g. The obtained results are shown in figure 1.24. Figure 1.24 shows that the frequency shift of ilmenite increases with the increase in the mechanical activation time from 15.4 to 63.3 MHz following the increase in the mechanical activation time from 0 to 120 min. In the mechanical activation time range from 0 to 120 min, the ilmenite frequency shift increases due to the rise in ilmenite’s particle size. Increasing the mechanical activation time to above 120 min is harmful to the frequency shift of ilmenite. The frequency shifts of ilmenite decrease obviously at the mechanical activation time of 150 min, mainly related to the dielectric loss of the ilmenite at a longer mechanical activation time.

Microwave Absorbing Properties and Temperature Behaviour

25

FIG. 1.25 – Relationship between the amplitude of voltage and mechanical activation time. Figure 1.25 indicates that ilmenite’s amplitude voltage decreases very quickly from 2.5117 to 2.1001 V with the increase in the mechanical activation time from 0 to 30 min. Increasing the mechanical activation time from 30 to 90 min has a slight adverse effect on ilmenite’s amplitude voltage. However, after 120 min, the increase in mechanical activation time has a slight impact on ilmenite’s amplitude voltage. This slight increase is mainly related to a smaller particle size of ilmenite. TAB. 1.6 – Microwave absorbing characteristics of ilmenite at different mechanical activation times. Mechanical activation time (min) Empty cavity 0 30 60 90 120 150

Frequency (GHz) 2.5000 2.4846 2.4551 2.4517 2.4460 2.4367 2.4418

Frequency shift (MHz) 0 15.4 44.9 48.3 54.0 63.3 58.2

Voltage attenuation (V) 0 2.5117 2.1001 2.0350 2.0014 1.8363 2.0871

Figure 1.24 shows that the frequency shift order for mechanically activated ilmenite is Dx120 [ Dx150 [ Dx90 [ Dx60 [ Dx30 [ Dx0 (Dx is frequency shift; the subscript number is the mechanical activated time). Therefore, e0120 [ e0150 [ e090 [ e060 [ e030 [ e00 . The e00 mechanically activated ilmenite keeps an inverse relationship with the attenuation of the microwave sensor. The order of voltage amplitude for mechanically activated ilmenite is U0 [ U30 [ U150 [ U60 [ U90 [ U120 (U is the amplitude of voltage). Therefore, e00120 [ e0090 [ e0060 [ e00150 [ e0030 [ e000 . Thus, optimum mechanical activated time is 120 min.

26

1.6

Application of Microwave Heating in the Comprehensive Utilization

Microwave Absorbing Characteristics of Mechanical Activated High Titanium Slag

The increasing use of the chloride or sulfate process for producing titanium dioxide pigments has motivated the search for more abundant and cheaper raw material than the presently used rutile resources [37–39]. Since available resources of high-grade natural rutile tend to diminish, the shortage of natural rutile has encouraged researchers to find an efficient method and way to convert anatase TiO2 of high titanium slag to rutile TiO2 of synthetic rutile [40–42]. Thus, attempts were made to investigate the influence of mechanical activation on structural performances and microwave absorbing properties of high titanium slag. The crystal structures and the molecular structure of high titanium slag before and after mechanical activation were also analysed using XRD and Raman spectra, respectively. The results showed that the application of mechanical activation techniques is effective and efficient for the treatment processing of high titanium slag. Ti2O3 is transformed partially from Fe3Ti3O10 under moderate mechanical activation, which can be better used for the production of synthetic rutile.

1.6.1

Materials and Procedure

The high titanium slags were received from Kunming city, Yunnan province, China. The chemical compositions of the high titanium slags are listed in table 1.7. TAB. 1.7 – Chemical composition of the high titanium slag. TiO2 72.33

Ti2O3 17.79

FeO 5.26

Al2O3 2.75

XRD instrument (D/Max 2200, Rigaku, Japan) was used to measure the crystal structure of the titanium slag and the treated samples. XRD pattern was acquired using an X-ray diffractometer with CuKα radiation and a Ni filter. The voltage and anode current operated were 35 kV and 20 mA, respectively. The continuously scanning rate with 0.25°/min of set time was used to record the XRD patterns of the samples. The Raman studies were performed with a confocal microprobe Raman system (Renishaw Ramascope System 1000, UK). A 514-nm argon laser was used for excitation. Backscattered Raman signals were collected through a microscope and holographic notch filters in the range of 100 cm−1−1000 cm−1 with a spectral resolution of 2 cm−1. The laser power on the sample surface was 20 µW, the spot diameter was 5 µm and the typical collection time of one measurement was 10 min. The infrared spectra of the titanium slag were collected using an FT-IR spectrometer equipped (8700, Nicolet, USA). The angle of incidence of the IR beam was 45° and 100 scans were collected at a resolution of 4 cm−1 and averaged using the OMNIC spectroscopic software. The spectral range was 4000 cm−1–400 cm−1.

Microwave Absorbing Properties and Temperature Behaviour

27

The XRD and Raman spectroscopy results are shown in figures 1.26 and 1.27, respectively. Figure 1.26 shows that Fe3Ti3O10 (JCPDS card No. 43-1011) and anatase TiO2 (JCPDS card No. 21-1272) are the main crystal structures in the high titanium slag. Besides, minor phases of rutile TiO2 (JCPDS card No. 65-0191) are also present.

FIG. 1.26 – XRD of the high titanium slag. Figure 1.27 is the Raman spectrum of high titanium slag, the peaks at 155.2 cm−1 are because of the asymmetric stretching vibration of O–Ti–O bonds of Ti3O5, while the bands at 195.8 cm−1 are because of the asymmetric stretching vibration of O–Ti–O bonds of TiO. The bands at 393.7 cm−1, 515.5 cm−1, and 637.3 cm−1 are because of anatase TiO2.

FIG. 1.27 – Raman spectrum of the high titanium slag.

28

Application of Microwave Heating in the Comprehensive Utilization

The raw material was placed in the conical ball mill and milled for the different duration (3, 6, 9, 12, 15, and 18 h). The microwave absorbing characteristics of milled high titanium slag and the heat-treated/dried (110 °C for 120 min) were assessed by placing the samples into the microwave resonant sensor in turn. The microwave absorbing properties of high titanium slag and mechanically activated material for different durations were considered using microwave resonant cavity perturbation techniques based on the microwave’s output voltage and resonator frequency sensor.

1.6.2

Dielectric Loss Factor of Mechanical Activated High Titanium Slag

The relationship between the dielectric loss factor and the mechanical activation duration of the high titanium slag was measured, and the results are shown in figure 1.28. The dielectric loss factor of mechanically activated high titanium slag decreased in the order of e0012 [ e006 [ e003 [ e0018 [ e009 [ e0015 (e00 is dielectric loss factor, the subscript number is the mechanically activated duration), and the maximum dielectric loss factor of the treated high titanium slag is received at the mechanical activation duration of 12 h.

FIG. 1.28 – The dielectric loss factor of the high titanium slag with mechanical activation duration.

1.6.3

Dielectric Constant of Mechanical Activated High Titanium Slag

Relationships between the dielectric constant and the high titanium slag’s mechanical activation duration are illustrated in figure 1.29. The dielectric constant of the mechanical activated high titanium slag decreased in the order of e012 [ e06 [ e09 [ e03 [ e018 [ e015 (e0 is dielectric constant, the subscript number is the

Microwave Absorbing Properties and Temperature Behaviour

29

FIG. 1.29 – Dielectric constant of the high titanium slag with mechanical activation duration. mechanical activated time). The maximum dielectric constant was observed at a mechanical activation duration of 12 h.

1.6.4

Loss Tangent Coefficient of Mechanical Activated High Titanium Slag

The relationships between the loss tangent coefficient and the high titanium slag’s mechanical activation duration are presented in figure 1.30. The loss tangent coefficient increased with the increase in the mechanical activation duration from 0.02069 to 0.02514 F/m, and the maximum value occurs at 12 h. The loss tangent

FIG. 1.30 – Loss tangent coefficient of the high titanium slag with mechanical activation duration.

30

Application of Microwave Heating in the Comprehensive Utilization

coefficient decreased in the order of tan d12 [ tan d18 [ tan d6 [ tan d15 [ tan d9 [ tan d3 (tan d is loss tangent coefficient, the subscript number is the mechanical activated time).

1.7

Microwave Absorbing Properties of High Titanium Slag

Equation (1.1) indicates that the real part of the permittivity is directly proportional to frequency shift. Equation (1.2) indicates that the permittivity’s imaginary part is inversely proportional to the amplitude of the voltage. In this section, microwave absorbing properties of high titanium slag were investigated using the microwave cavity perturbation technique. High titanium slag containing more than 90% TiO2 was prepared by carbothermal reduction of ilmenite. The temperature rise curve of high titanium slag in the microwave heating process was obtained. Crystalline compounds of high titanium slag before and after microwave irradiation were obtained and characterised by X-ray diffractometry (XRD). Effects of particle size of high titanium slag and mixtures of high titanium slag with different mass fractions of V2O5 on microwave absorbing properties were investigated systematically. High titanium slag shows good microwave absorption property; untreated high titanium slag mainly consists of crystalline compounds of anatase and iron titanium oxide. The microwave-irradiation treated one is mainly composed of crystalline compounds of rutile and iron titanium oxide. Synthetic anatase is transformed completely into rutile at about 1050 °C for 20 min under microwave irradiation. High-frequency shift and low amplitude of voltage make high titanium slag an ideal microwave absorbent. 180 µm of particle size and 10% mass fraction of V2O5 are the optimum conditions for microwave absorption.

1.7.1

Materials and Procedure

High titanium slag was obtained from Kunming City, Yunnan Province, China, and was prepared from ilmenite by a carbothermal reduction in an electric arc furnace. The chemical compositions of high titanium slag are shown in table 1.8. TAB. 1.8 – Chemical composition of high titanium slag (mass fraction, %). TiO2 90.12

FeO 5.26

Al2O3 2.75

P 0.014

S 0.049

C 0.049

The sample (100 g) was placed in the self-made microwave heating equipment and heated to 1050 °C for 20 min at 3 kW, and then naturally cooled in the furnace to room temperature. The high titanium slag with different mean particle sizes and the mixtures of different mass fractions of V2O5 were prepared. Each sample (2 g) was dried at 105 °C for 2 h. Finally, the samples’ microwave absorbing properties were obtained by putting the samples into the microwave resonant sensor in turn.

Microwave Absorbing Properties and Temperature Behaviour

1.7.2

31

Effect of Particle Size of High Titanium Slag on Microwave Absorbing Properties

The microwave spectra of particle size of high titanium slag are illustrated in figure 1.31.

FIG. 1.31 – Microwave spectra of different particle sizes of high titanium slag.

The microwave sensor with an empty chamber gives rise to the resonant curve with the highest resonant amplitude and the most considerable resonant frequency. The other resonant curves indicate lower resonant amplitude and smaller resonant frequency, resulted from the microwave sensor filled with different sizes of high titanium slag, respectively. The effect of particle size of high titanium slag on the frequency shift is illustrated in figure 1.32. The frequency shift increases gradually from about 47 to 84 kHz with increasing the particle size of high titanium slag from 120 to 180 µm, and then it decreases from 84 to 14 kHz with increasing the particle size of high titanium slag from 180 to 270 µm. Figure 1.33 shows the relationship between the amplitude of the voltage of the microwave sensor and particle size of high titanium slag. Similarly, the amplitude of voltage also decreases from about 1.868 to 1.701 V with increasing the particle size of high titanium slag from 120 to 180 µm, and increases from 1.701 to 2.317 V with increasing the particle size of high titanium slag from 180 to 240 µm. Finally, the amplitude of voltage decreases from 2.317 to 2.217 V with further increasing the particle size of high titanium slag from 240 to 270 µm. Therefore, the

32

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.32 – Relationship between frequency shift and particle size of high titanium slag.

FIG. 1.33 – Relationship between the amplitude of the voltage of microwave sensor and particle size of high titanium slag. particle size of 180 µm is the optimum size of high titanium slag in the microwave field. The perturbation technique is that the presence of high titanium slag in the resonant cavity causes a shift of resonant frequency and changes in the amplitude of the voltage of the resonant cavity. The changes in the amplitude of the cavity voltage exist because of the dielectric loss of the sample. The dielectric constant and loss tangent of high titanium slag can be calculated from the frequency and amplitude of the voltage.

Microwave Absorbing Properties and Temperature Behaviour

1.7.3

33

Effect of Mass Fraction of V2O5 on Microwave Absorbing Properties

Figure 1.34 shows the microwave spectra of high titanium slag mixtures with different mass fractions of V2O5. The microwave sensor with an empty chamber gives rise to the resonant curve with the highest resonant amplitude and the biggest resonant frequency. The other curves show different results for mixtures of high titanium slag with different mass fractions of V2O5.

FIG. 1.34 – Microwave spectra of mixtures of high titanium slag with different mass fractions of V2O5.

FIG. 1.35 – Relationship between frequency shift and mass fraction of V2O5.

34

Application of Microwave Heating in the Comprehensive Utilization

With increasing the mass fraction of V2O5 from 5 to 6% as shown in figure 1.35, the frequency shift decreases gradually from 8 to 3 MHz, and then it increases from 3 to 41 MHz when the mass fraction of V2O5 increases from 6 to 10%. Figure 1.36 shows the relationship between the amplitude of voltage and the mass fraction of V2O5.

FIG. 1.36 – Relationship between the amplitude of voltage and mass fraction of V2O5. The above discussion indicates that the mass fraction of V2O5 has a significant effect on the microwave absorbing properties of high titanium slag. The amplitude of voltage shows a negative relation with the mass fraction of V2O5. Yu et al. [24, 43] found that the optimum microwave absorbing property of mixtures is acquired with the mass fractions of V2O5 less than 10%. The explanation is that the phase transformation from anatase to rutile in high titanium slag is irreversible. V2O5 can enhance the phase transformation of TiO2, and lower the phase transformation temperature of TiO2 and increase the transformation ratio.

1.8

Temperature Behaviour of Titanium Slag Under Microwave Heating

Temperature rising characteristics of titanium-rich slag under microwave irradiation were obtained. The effects of microwave power, mean particle sizes of samples, and sample weight on temperature-rising characteristics of titanium-rich slag were systematically investigated. The crystalline compounds of the sample before and after

Microwave Absorbing Properties and Temperature Behaviour

35

microwave heating were characterised by X-ray diffractometry. The results show that the titanium-rich slag has a good microwave absorption property. The titanium-rich slag sample reached 1200 °C after 60 min at 3.0 kW in the microwave field. The XRD characterisation results show that untreated titanium-rich slag mainly consists of crystalline compounds of anatase TiO2 phase, while the microwave heating treated sample is mainly composed of crystalline compounds of rutile TiO2 phase. The results inferred that the anatase TiO2 phase converted to the rutile TiO2 phase in microwave heating.

1.8.1

Materials and Instrumentation

Titanium-rich slag was obtained from Kunming city, Yunnan Province, PR China. The raw materials were prepared from ilmenite by a carbothermal reduction in an electric arc furnace. The slag contains 72.33% TiO2, 17.79% Ti2O3, 5.26% FeO, 1.04% MnO, 2.75% Al2O3, 2.30% MgO, 2.57% SiO2, and minor elements such as S, P, and C. The titanium-rich slag was analysed for element content by the method by the recommended methods of the National Standard of the People’s Republic of China (GB/T). The microwave heating apparatus consists of a magnetron, a power controller, a matched load, a waveguide, and a cavity. A self-made microwave reactor has a multi-mode cavity, with a continuous controllable power capacity. The microwave power supply for the microwave reactor is made of 2 magnetrons, cooled by moisture circulation, at 2.45 GHz frequency and 1.5 kW power. A ceramics tube, 50 mm (outer diameter) × 80 mm (inner diameter) × 600 mm in length, is positioned at the center of the microwave oven, by drilling holes on the side faces, with ends projecting on both sides. The sample’s temperature was monitored using an infrared pyrometer (USA, Raytek, Marathon Series) with the circular crosswire focusing on the sample cross-section. The temperature was also measured using a thermocouple (China, Shengyun Company) as a reference. In case of any temperature discrepancy, the latter was used as the correct temperature. Titanium-rich slag was firstly crushed and sieved to prepare particles with a size less than 0.2 mm. Subsequently, titanium-rich slag was loaded on a ceramic boat placed inside a stainless steel tubular reactor with an internal diameter of 38 mm. Then the samples were heated to 393 K at a heating rate of 278 K/min in the drying oven and held for 120 min. After drying, the samples were cooled to room temperature. Certain amounts of titanium-rich slag (100, 200, 300, 400, 500 g) with different mean particle sizes (150, 180, 210, 240 µm) were heated at different target microwave power (1.5, 2.0, 2.5, 3.0 kW).

1.8.2

Temperature Rise Characteristics of the Titanium-Rich Slag Using Microwave Heating

The relationship between temperature and microwave heating time of the titanium-rich slag in the microwave is shown in figure 1.37, it shows that the

36

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.37 – Temperature rise characteristics of the titanium-rich slag at different microwave power in the microwave field. sample’s temperatures (100 g) have a positive relation with the microwave heating time. Simultaneously, the temperature of titanium-rich slag shares a positive relationship with microwave power. The temperature of the sample reached 1200 °C after 60 min at 3.0 kW. The temperature rise curve of the titanium-rich slag with different mean particle sizes in the microwave field is illustrated in figure 1.38. With the same microwave heating time, the temperature of samples (100 g) has a strong positive relationship with the mean particle size of titanium-rich slag from 150 to 180 µm, and

FIG. 1.38 – Temperature rise characteristics of the titanium-rich slag with different mean particle sizes in the microwave field.

Microwave Absorbing Properties and Temperature Behaviour

37

subsequently the value appears a negative relation with the particle size of high titanium slag in the range of 180–240 µm. Figure 1.39 presents the effect of sample weight on the temperature rise characteristics. The influence of sample weight and microwave heating time on the temperature rise characteristics has been described in figure 1.39. Titanium-rich slag is characterised by XRD, as shown in figure 1.40, indicating that the iron titanium oxide is the main crystalline compound in the titanium-rich

FIG. 1.39 – Temperature rise characteristics of the titanium-rich slag with a different weight in the microwave field.

FIG. 1.40 – XRD patterns of the titanium-rich slag.

38

Application of Microwave Heating in the Comprehensive Utilization

slag; besides, a minor amount of rutile is present. The iron titanium oxide has the most substantial diffraction peak at 2θ = 25.26°, which is close to the most substantial diffraction peak of anatase at 2θ = 25.30°, so the two peaks are overlapped. Titanium-rich slag treated after microwave heating is characterised by XRD as shown in figure 1.41, indicating that the diffraction peaks of the rutile TiO2 phase are broadened, and their intensities are decreased under the microwave heating. Compared with titanium-rich slag before microwave heating, the treated titanium-rich slag has peaked at 2θ = 27.44° and 54.33°, where the strongest and the 2nd strongest peaks of rutile TiO2 phase occur, respectively. The third strongest peak remained at 2θ = 36.09°. Clearly, the anatase TiO2 phase is completely converted to the rutile TiO2 phase during microwave heating.

FIG. 1.41 – XRD patterns of the titanium-rich slag obtained under microwave heating.

1.9

Microwave Absorption Properties and Thermal Behaviour of Vanadium Titano-Magnetite (VTM)

Vanadium titano-magnetite (VTM) contains high contents of vanadium and chromium with complex and dense structures. Microwave heating is expected to destroy the dense structure and further improve the extraction rate of vanadium and chromium. To understand the microwave heating process, exploring the dielectric properties of VTM is necessary. Microwave absorption properties and thermal behaviour of VTM were introduced in this section. VTM endowed great microwave absorption properties, with a minimum e0r value of 34.447 (F/M). Dielectric properties of VTM varied with temperature, which changing trend was matched to the three stages of microwave heating characteristics identified by heating rates, namely

Microwave Absorbing Properties and Temperature Behaviour

39

dehydration stage (30 °C–280 °C), oxidation decomposition of olivine phase and normal spinel phase (280 °C–650 °C), and oxidation decomposition of vanadium chromium spinel (650 °C–950 °C). Moreover, the maximum dielectric constant and highest microwave heating rate of VTM both appeared at the temperature regime of 500 °C–550 °C, which was also the main temperature regime for oxidation decomposition of olivine phase and normal spinel phase in VTM, demonstrating the appropriate process temperature for microwave heating technology to recycle VTM.

1.9.1

Materials and Procedure

As observed from table 1.9, the VTM has a vanadium content of 7.37% and a chromium content of 1.38%. Also, the VTM was obtained from oxidative blowing of vanadium-containing molten iron in the process of vanadium extraction; therefore, as presented in table 1.9, the VTM also contained a considerable content of iron, manganese, and titanium, which are worth recycling. TAB. 1.9 – Chemical compositions of VTM. Compositions Mass/w% Compositions Mass/w% Compositions Mass/w%

C 1.62 Mg 2.22 Zn 0.0072

O 37.1 Al 2.48 Ga 0.0087

Fe 27.4 Si 6.67 Sr 0.0060

Mn 5.24 P 0.0632 Zr 0.0104

V 7.37 S 0.0818 Nb 0.0222

Cr 1.38 Cl 0.0275 Others 0.0148

Ti 5.98 K 0.0892

Na 0.519 Ca 1.69

The analysis results are displayed in figure 1.42, in which the corresponding result was performed using the X-Ray Diffractometer (D/max-TTRIII, Rigaku, Japan) and FT-IR Spectroscopy (iS50 FT-IR, Nicolet, USA). As presented in figure 1.42(a), the VTM mainly contained (Mn, Fe)2SiO4 phase and (Mn, Fe)(V, Cr)2O4 phase; there also existed a minor amount of Fe2O3 phase, Fe2TiO4 phase, and SiO2 phase. As shown figure 1.42(b), the FT-IR absorption bands appeared at 468.09 cm−1, 578.52 cm−1, 876.59 cm−1, 962.88 cm−1, 1636.80 cm−1, and 3456.70 cm−1 for the VTM. The stretching and bending vibrations of –O–H. bond of moisture molecules on the surface of VTM cause the absorption bands at 1636.80 cm−1 and 3456.70 cm−1 [44, 45]; the vibrations of Si–O–Si bond in SiO2 phase and the stretching mode of Fe–O bond at the octahedral sites in (Mn, Fe)(V, Cr)2O4 phase result the absorption band at 468.09 cm−1, which is a superimposed bond [46], meanwhile, the stretching modes of metal-O bonds at the octahedral sites in (Mn, Fe)(V, Cr)2O4 phase arouse the absorption band at 578.52 cm−1 [47]; additionally, the asymmetric stretching vibrations of Si–O bond of [SiO4] tetrahedra bands in (Mn, Fe)2SiO4 phase cause the narrow absorption bands at 876.59 cm−1 and 962.88 cm−1 [48]. The received VTM was powder, with a particle size ranging from 80 to 180 mesh. Before testing, VTM was dried at 105 °C for 12 h. After drying, a quartz tube was filled with a certain weight of dried slag sample (about 4.4 g). After filling, the quartz tube loaded with the slag sample was subjected to the dielectric device to

40

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.42 – XRD pattern (a) and FT-IR spectra; (b) of raw VTM. measure the dielectric properties, measured under the microwave frequency of 2450 MHz from room temperature to 850 °C without a protective atmosphere. The substance’s dielectric properties were determined by the four test steps followed by cavity calibration, quartz empty tube calibration, charging and loading quartz tube test, and recalculation [49, 50]. Additionally, 50.0 g of the dried sample was introduced to the microwave furnace to investigate the microwave heating characteristics, measured under the same frequency of 2450 MHz and the temperature regime of room temperature to 950 °C without protective atmosphere, and at different microwave powers (including 600, 800, 1000, and 1200 W). Besides, 10.0 mg of the dried slag sample was introduced to the simultaneous thermal analyser to conduct the analysis of the thermogravimetric characteristics, measured at different heating rates (including 10 °C/min, 20 °C/min, and 30 °C/min) and the temperature regime of 30 °C–950 °C in an air atmosphere with a flow rate of 60 mL/min.

Microwave Absorbing Properties and Temperature Behaviour

1.9.2

41

Thermal Behaviour of VTM During Microwave Heating

Referring to the elemental compositions of VTM (table 1.9), the sample has a high content of O and various transition metal elements, including Mn, Fe, Cr, V, and Ti. With sufficient oxygen (O2) content and required thermodynamic conditions for the oxidation reactions, the mentioned transition metal elements will be oxidised during microwave heating process as presented in table 1.10. The above analysis related to the main metal elements and the corresponding valence states in the VTM, meanwhile, the corresponding dependency of standard Gibbs free energy (ΔGθ) on temperatures for those oxidation reactions is plotted in figure 1.43, wherein figure 1.43 (a) and (b) correspond to the first 7 reactions and the last 6 reactions numbered in table 1.10, respectively, and the thermodynamic data were obtained by FactSage thermodynamic software. TAB. 1.10 – Oxidation reactions probably appeared in VTM. No. (1) (2) (3) (4) (5) (6) (7)

Reactions 6FeO(s) + O2(g) = 2Fe3O4(s) 4Fe3O4(s) + O2(g) = 6Fe2O3(s) 6MnO(s) + O2(g) = 2Mn3O4(s) 4Mn3O4(s) + O2(g) = 6Mn2O3(s) 2Mn2O3(s) + O2(g) = 4MnO2(s) 2V2O3(s) + O2(g) = 4VO2(s) 4VO2(s) + O2(g) = 2V2O5(s)

No. (8) (9) (10) (11) (12) (13)

Reactions 2Cr2O3(s) + O2(g) = 4CrO2(s) 2CrO2(s) + O2(g) = 2CrO3(s) 4TiO(s) + O2(g) = 2Ti2O3(s) 2Ti2O3(s) + O2(g) = 4TiO2(s) C(s) + O2(g) = CO2(g) 2C(s) + O2(g) = 2CO(g)

According to the step-by-step conversion principle of metal oxides and table 1.10 [50–53], the VTM contains these phase conversions during microwave oxidation heating process: FeO → Fe3O4 → Fe2O3, MnO → Mn3O4 → Mn2O3 → MnO2, V2O3 → VO2 → V2O5, Cr2O3 → CrO2 → CrO3, and TiO → Ti2O3 → TiO2. Besides, figure 1.43 shows that the dependence of ΔGθ for the oxidation reactions of metal oxides in VTM on temperature all presented an increasing trend. With the rising heating temperature, these oxidation reactions proceed a decreasing trend. At temperatures above 93.1 °C, the oxidation reaction of 2Cr2O3(s) + O2(g) = 4CrO2(s) will gradually restrain, and the oxidation reaction of 2CrO2(s) + O2(g) = 2CrO3(s) will hardly occur; therefore, from room temperature to 950 °C, chromium (Cr) exist in the trivalent state (Cr3+) after finishing the microwave heating process of VTM. With temperatures above 529.0 °C, the reaction of 2Mn2O3(s) + O2(g) = 4MnO2(s) will be restrained; therefore, at the considered temperature regime of room temperature to 950 °C, manganese (Mn) exists in the form of Mn3O4 and Mn2O3 phases. Meanwhile, through the same analysis based on figure 1.43, at the temperature regime from room temperature to 950 °C, iron (Fe) is found to exist in the form of Fe2O3 and Fe3O4 phases, vanadium (V) will exist as VO2 and V2O5 phases, and titanium (Ti) is in the tetravalent state (Ti4+). Carbon (C) will react with oxygen (O2), which consumes some oxygen (O2) during heating process. The incomplete and complete oxidation reactions of carbon (C) will

Application of Microwave Heating in the Comprehensive Utilization

42

FIG. 1.43 – Dependence of ΔGθ for the oxidation reactions in VTM on temperature. provide some degree of inhibition and competition for other oxidation reactions of metal elements. Figure 1.43 shows that the dependency of ΔGθ on temperatures for the oxidation reactions has a decreasing trend, proving that improving temperature promotes the oxidation reaction, hence the degree of inhibition and competition originated from carbon (C) will become more evident and intense.

1.9.3

Thermochemical Characteristics of VTM

Thermogravimetric characteristics analysis helps explore the relationship between microwave absorption properties and thermal behaviour, thus reveals the thermochemical characteristics of VTM. Thermogravimetric measurements for VTM were conducted at different heating rates, as shown in figure 1.44.

Microwave Absorbing Properties and Temperature Behaviour

43

FIG. 1.44 – TG-DTG-DSC curves of VTM, (a) TG curves; (b) DTG curves; (c) DSC curves.

44

Application of Microwave Heating in the Comprehensive Utilization

TG curves are shown in figure 1.44(a), where a similar trend for the weight change in VTM with temperature and the thermochemical characteristics of VTM is considered to be composed of three stages: (1) the 1st stage (30 °C–280 °C), was the dehydration stage. Although the VTM was dried before testing, there still existed a certain amount of adsorption/bound moisture in the tested VTM sample; therefore, moisture will escape from the sample. Moreover, with a temperature rising from 30 °C to 280 °C, the slag sample’s weight changed slightly and finally fluctuated repeatedly. The weight change was resulting from superimposing the two processes: the removal of surface moisture and bound moisture and the partial oxidation reactions of free metal oxides, mainly FeO and MnO and a bit of V2O3 and Cr2O3, which was not wrapped and bound in the spinel phase (i.e. (Mn, Fe)(V, Cr)2O4) and olivine phase (i.e. (Mn, Fe)2SiO4). The removal of moisture rendered a decrease in weight, the oxidation reactions of metal oxides caused an increase in weight. Overall, the experiment presented a weight loss of VTM, which was 0.924%, 0.707%, and 0.847%, with the heating rate improving from 10 °C/min to 30 °C/min. (2) In the 2nd stage (280 °C–650 °C), the significant weight gain was caused by the olivine and normal spinel phases’ oxidation decomposition. Olivine phase was decomposed and oxidised into Mn2SiO4 and Fe2SiO4 phases at 300 °C, followed by Fe2+ and Mn2+ irons existed in the decomposed products were oxidised entirely into the corresponding high-valence metal oxides and quartz phase at 600 °C [53, 54], including Fe3O4, Fe2O3, Mn3O4, Mn2O3, MnO2, and SiO2, which rendered the significant weight increment for VTM. The normal spinel phase decomposed into FeV2O4, FeCr2O4, MnV2O4, and MnCr2O4 phases at temperatures above 500 °C, and the decomposed products of normal spinel phase were oxidised into Fe2VO4 and Fe2CrO4 at about 600 °C and completely decomposed and oxidized below 700 °C, accompanying with small amount of Fe2O3, (Fe0.6, Cr0.4)2O3, Fe2TiO5 phases, which also contributed to the weight gain of VTM [49, 51], namely 3.416%, 0.759%, and 1.610%, with the heating rate improved from 10 °C/min to 30 °C/min. (3) The 3rd stage (650 °C–950 °C), was the decomposition of V-Cr spinel, accompanied by the weight gain of VTM with 5.592%, 4.668% and 5.248% as the heating rate improved from 10 °C/min to 30 °C/min. Based on the above thermodynamics characteristics analysis, Cr will still exist in the Cr3+ state in the VTM. The thermodynamic characteristic of VTM was mainly represented by the oxidisation process of V3+ irons in vanadium chromium spinel into V4+, and V5+ irons, followed by the V4+ and V5+ irons were conjugated with Fe3+, Mn2+, and Ca2+ irons to form acid-soluble vanadate, including Mn2V2O7, FeVO4, CaVO3, CaV2O7, CrVO4, and Cr0.07V1.93O4 [52, 54]. Figure 1.44(b) displays the DTG curves, shows a similar changing trend with temperature, with 1 exothermic peak at the 1st stage, 3 exothermic peaks at the 2nd stage, and 1 exothermic peak, and 2 endothermic peaks at the 3rd stage. During the 1st stage, the exothermic peak was ascribed to the partial oxidation reactions of free metal oxides, at 104.9 °C, 104.9 °C, and 134.6 °C with the heating rate improved from 10 °C/min to 30 °C/min. During the 2nd stage, the 3 exothermic peaks near 520 °C, 580 °C, and 610 °C, were because of the oxidation decomposition of the olivine phase and normal spinel phase [54]. During the 3rd stage, the two

Microwave Absorbing Properties and Temperature Behaviour

45

endothermic peaks near 760 °C and 820 °C were because of the melt of vanadate generated by the V-Cr spinel’s oxidation decomposition. Figure 1.44(c) shows the DSC curves, where 2 epitaxial termination temperature peaks were noticed. The epitaxial termination temperature peak implies the intersection of the weight loss line and the DSC curve’s tangent, indicating the weight loss of VTM occurred. The 1st peak was because of the partial oxidation reactions of free metal oxides, at 80.18 °C, 131.59 °C, and 154.91 °C with the heating rate improved from 10 °C/min to 30 °C/min; and the 2nd peak was because of the oxidation decomposition of vanadium chromium spinel, at 804.87 °C, 807.96 °C, and 835.09 °C.

1.9.4

Dielectric Properties of VTM

The dielectric properties of VTM were measured at 2450 MHz, including the dielectric constant ðe0r Þ, dielectric loss factor ðe00r Þ, and loss tangent coefficient (tan δ). The obtained results are described in figure 1.45. Figure 1.45(a) illustrates the dielectric constant ðe0r Þ of VTM, referring to the ability of the material to absorb and store microwave energy [55, 56]. Figure 1.45(a) shows that the e0r value of VTM was 34.447 (F/M) at room temperature, indicating the VTM endows great microwave absorption properties. The high e0r value was resulting from the VTM contained a wide variety of metal elements and metal oxides (refer to table 1.9 and figure 1.42), which both show strong responsiveness to microwaves [56, 57]. As temperature improved to a value of 500 °C, the e0r presented a gentle increasing trend with the increase in temperature, and was from 34.447 (F/M) to 52.677 (F/M). The growth of e0r value resulted from superimposing the two stages’ influence: the dehydration stage at the 1st stage and the oxidation decomposition of the olivine phase at the 2nd stage. The removal of moisture rendered a decrease in e0r value of VTM, the oxidation decomposition of the olivine phase caused an increase. Moisture has strong microwave absorption properties. The dehydration of moisture in slag samples is bound to decrease the microwave absorption properties of VTM. The olivine phase was decomposed into Mn2SiO4 and Fe2SiO4 phases and oxidised into Fe2O3 and SiO2 at 300 °C. Fe2+ and Mn2+ irons in the decomposed products of the olivine phase were oxidised into the corresponding high-valence metal oxides, such as Fe3O4, Fe2O3 Mn3O4, Mn2O3, and MnO2 [54]. The order for the microwave absorption properties of different oxides is as following: Fe2O3 > Fe3O4 > FeO, and MnO2 > Mn2O3 > Mn3O4 > MnO [54, 58]; The oxidation reactions of low-valence metal oxides caused an increment in e0r value of VTM. Under the combined influence of the two processes, the e0r value presented a gently increasing trend on the whole. Once temperature exceeding 500 °C, the e0r value suddenly rose from 52.677 (F/M) to 117.959 (F/M) at 550 °C. The normal spinel phase decomposed into MnV2O4, FeV2O4, MnCr2O4, and FeCr2O4 phases at temperatures above 500 °C, and the decomposed products were oxidised into Fe2VO4 and Fe2CrO4, accompanying with Fe2O3, (Fe0.6, Cr0.4)2O3 and Fe2TiO5 phases, rendering the increase in the e0r value at this short temperature range [52, 54]. At a temperature above 550 °C, the normal spinel phase decomposed into

46

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.45 – Dielectric properties of VTM under microwave irradiation at 2450 MHz, (a) dielectric constant ðe0r Þ; (b) dielectric loss factor ðe00r Þ; (c) loss tangent coefficient (tan δ).

Microwave Absorbing Properties and Temperature Behaviour

47

FeV2O4, MnV2O4, FeCr2O4, and MnCr2O4 phases, the decomposed products were subsequently oxidised into Fe2VO4 and Fe2CrO4 at about 600 °C and completely oxidised at a temperature below 700 °C, providing Fe3+, Mn2+ and V3+ irons [52, 54]. The V3+ irons in vanadium chromium spinel oxidised into V4+ and V5+ irons, and followed by the V4+ and V5+ irons were conjugated with Fe3+, Mn2+, and Ca2+ irons to form acid-soluble vanadate, such as Mn2V2O7, FeVO4, CaVO3, CaV2O7, CrVO4, and Cr0.07V1.93O4 [52, 54], consuming the content of Fe3+, Mn2+ and V3+ irons and further to render the changing trend of e0r value at this temperature range. Figure 1.45(b) illustrates the dielectric loss factor ðe00r Þ of VTM, which indicates the substance’s ability to convert the absorbed microwave energy. Figure 1.45(b) shows that the e00r value was 1.47 (F/M) at room temperature, and then increased to 2.10 (F/M) at 100 °C, then decreased to 1.90 (F/M) at 500 °C. Subsequently, the value dropped to 1.35 (F/M) at 550 °C, kept constant with a temperature rising to 850 °C. Figure 1.45(c) illustrates the loss tangent coefficient (tan δ) of VTM, which presented the same changing trend with the ðe00r Þ curve, and was determined at 0.0430 at room temperature and improved to 0.0597 at 100 °C, then decreased to 0.0361 at 500 °C, 0.0114 at 550 °C, kept constant with a temperature rising to 850 °C.

1.9.5

Microwave Heating Characteristics of VTM

Microwave heating characteristics of VTM were measured at 2450 MHz with different microwave powers to assess the correctness and rationality of dielectric properties analysis, and the results are plotted in figure 1.46. Figure 1.46 shows that for the target temperature at near 950 °C, 14.0 min is required for VTM the heating process under microwave irradiation of 600 W, meanwhile with 800 W required 10.0 min, 1000 W required 7.0 min, and 1200 W required 7.0 min, respectively. For the heating results, heating the sample with the same heating time, namely 7.0 min, the final reached temperatures of VTM were different, with 950 °C under 1000 W and 957 °C under 1200 W. Based on the results, the microwave heating characteristics of VTM could be considered to contain three processes s consistent with the changes in the dielectric properties. The heating rate was 24 °C/min and 20 °C/min under 600 W, with 58 °C/min and 12 °C/min under 800 W, 42 °C/min and 48 °C/min under 1000 W, and 78 °C/min and 26 °C/min under 1200 W, respectively. The peak heating rates were determined with 185 °C/min at 427 °C under 600 W, 180 °C/min at 490 °C under 800 W, 250 °C/min at 553 °C under 1000 W, and 233 °C/min at 507 °C under 1200 W, respectively. The short heating times and the high heating rates claimed that the VTM showed significant responsiveness to microwaves and endowed excellent microwave absorption properties. The peak heating rates appeared at the temperature regime of 500 °C–550 °C, which matched the temperature range for the maximum dielectric constant appearance, suggesting the appropriate process temperature for microwave heating technology to recycle VTM.

48

Application of Microwave Heating in the Comprehensive Utilization

FIG. 1.46 – Microwave heating characteristics of VTM at different microwave powers, (a) 600 W; (b) 800 W; (c) 1000 W; (d) 1200 W.

Microwave Absorbing Properties and Temperature Behaviour

49

FIG. 1.46 – (continued).

References [1] Al H.M., Kingman S.W. (2004) Microwave-assisted leaching — a review, Hydrometallurgy 73, 189. [2] Haque K.E. (1999) Microwave energy for mineral treatment processes-A brief review, Int. J. Miner. Process. 57, 1. [3] Kitchen H.J., Vallance S.R., Kennedy J.L., et al. (2014) Modern microwave methods in solid-state inorganic materials chemistry: from fundamentals to manufacturing, Chem. Rev. 114, 1170. [4] Lester E., Kingman S., Dodds C., et al. (2006) The potential for rapid coke making using microwave energy, Fuel 85, 2057. [5] Li W., Peng J.H., Zhang L.B., et al. (2008) Pilot-scale extraction of zinc from the spent catalyst of vinyl acetate synthesis by microwave irradiation, Hydrometallurgy 92, 79. [6] Li W., Zhang L.B., Peng J.H., et al. (2008) Preparation of high surface area activated carbons from tobacco stems with K2CO3 activation using microwave radiation, Ind. Crops Prod. 27, 341. [7] Pickles C.A. (2009) Microwaves in extractive metallurgy: Part 1 - Review of fundamentals, Miner. Eng. 22, 1102. [8] Pickles C.A. (2009) Microwaves in extractive metallurgy: Part 2 - A review of applications, Miner. Eng. 22, 1112. [9] Standish N., Worner H. (1990) Microwave application in the reduction of metal oxides with carbon, J. Microw. Power Electromagn. Energy 25, 177. [10] Xia D.K., Pickles C.A. (1997) Applications of microwave energy in extractive metallurgy, a review, Cim Bull. 90, 96. [11] Zhu Y.J., Chen F. (2014) Microwave-assisted preparation of inorganic nanostructures in liquid phase, Chem. Rev. 114, 6462. [12] Kingman S.W. (2006) Recent developments in microwave processing of minerals, Int. Mater. Rev. 51, 1. [13] Ku H.S., Siu F., Sioures E., et al. (2001) Applications of fixed and variable frequency microwave (VFM) facilities in polymeric materials processing and joining, J. Mater. Process Technol. 113, 184.

50

Application of Microwave Heating in the Comprehensive Utilization

[14] National Research Council (NRC) (1994) Microwave Processing of Materials, National Materials Advisory Board, Commission on Engineering and Technical Systems. National Academy Press, USA. [15] Stuhl B.K., Yeo M., Sawyer B.C., et al. (2011) Microwave state transfer and adiabatic dynamics of magnetically trapped polar molecules, Phys. 85, 484. [16] Guo S.H., Li W., Peng J.H. et al. (2009) Microwave-absorbing characteristics of mixtures of different carbonaceous reducing agents and oxidized ilmenite, Int. J. Miner. Process. 93, 289. [17] Chen G., Chen J., Peng J.H. (2015) Effects of mechanical activation on structural and microwave absorbing characteristics of high titanium slag, Powder Technol. 286, 218. [18] Kucukkaragoz C.S., Eric R.H. (2006) Solid state reduction of a natural ilmenite, Miner. Eng. 19, 334. [19] Mo W., Deng G.Z., Luo F.C. (2002) Ti Metallurgy. Publishing Press of Metallurgical Industry, Beijing. [20] Gupta S.K., Rajakumar V., Grieveson P. (1987) Kinetics of reduction of ilmenite with graphite at 1000 to 1100°C, Metall. Mater. Trans. B 18, 713. [21] Xiao H.F., Guo Y.F., Qiu G.Z. (2002) A new technology for preparing rich titanium product from titanomagnetite concentrate, Multipurpose Util. Min. Resour. 4, 3. [22] Park E., Ostrovski O. (2004) Effects of preoxidation of titania-ferrous ore on the ore structure and reduction behaviour, Isij Int. 44, 74. [23] Roberts J.M.C. (1971) Ilmenite upgrading, Mineral. Mag. 125, 548. [24] Sarker M.K., Rashid A.K.M.B., Kurny A.S.W. (2006) Kinetics of leaching of oxidised and reduced ilmenite in dilute hydrochloric acid solutions, Int. J. Miner. Process. 80, 223. [25] Sun K. (1996) Nonisothermal behaviour of the oxidation of ilmenite in fluidised bed, GuangDong Nonferrous Met. 6, 20. [26] Zhang G.Q., Ostrovski O. (2002) Effect of preoxidation and heating on properties of ilmenite concentrates, Int. J. Miner. Process. 64, 201. [27] Zhou L.H. (2003) Study on the fluidizing oxidation mechanism of ilmenite, Nonferrous Met. 4, 12. [28] Xiong DH (2004) SLon Magnetic Separators Applied in the Ilmenite Processing Industry. Phys Sep Sci Eng 13, 119. [29] Welham N.J. (1998) Mechanically induced reduction of ilmenite (FeTiO3) and rutile (TiO2) by magnesium, J. Alloy Compd. 274, 260. [30] Sasikumar C., Rao D.S., Srikanth S., et al. (2004) Mehrotra: effect of mechanical activation on the kinetics of sulfuric acid leaching of beach sand ilmenite from Orissa, Hydrometallurgy 75, 1. [31] Kitamura M., Nagatani N., Senna M. (2002) Effects of mechanical activation on the electrorheological properties of a silicone oil-gibbsite suspension, Adv. Powder Technol. 13, 221. [32] Mityakin P., Rozental, et al. (1993) Mechanical activation as a direction of binding systems preparation, Adv. Powder Technol. 4, 267. [33] Choi H., Lee W., Kim S. (2009) Effect of grinding aids on the kinetics of fine grinding energy consumed of calcite powders by a stirred ball mill, Adv. Powder Technol. 20, 350. [34] Chen Y., Williams J.S., Campbell S.J., et al. (1999) Increased dissolution of ilmenite induced by high-energy ball milling, Mater. Sci. Eng. A 271, 485. [35] Li C., Liang B., Chen S.P. (2006) Combined milling-dissolution of Panzhihua ilmenite in sulfuric acid, Hydrometallurgy 82, 93. [36] Li C., Liang B., Guo L.H., et al. (2006) Effect of mechanical activation on the dissolution of Panzhihua ilmenite. Miner. Eng. 19, 1430. [37] Li C., Liang B. (2008) Study on the mechanochemical oxidation of ilmenite, J. Alloy Compd. 459, 354. [38] Rezaee M., Khoie S.M.M. (2010) Mechanically induced polymorphic phase transformation in nanocrystalline TiO2 powder, J. Alloy Compd. 507, 484. [39] Samal S., Mohapatra B.K., Mukherjee P.S., et al. (2009) Integrated XRD, EPMA and XRF study of ilmenite and titania slag used in pigment production, J. Alloy Compd. 474, 484. [40] Chen G., Peng J.H., Chen J. (2011) Optimizing conditions for wet grinding of synthetic rutile using response surface methodology, Miner. Metal. Process. 28, 44.

Microwave Absorbing Properties and Temperature Behaviour

51

[41] Chen G., Peng J.H., Song Z.K., et al. (2014) A new highly efficient method for the synthesis of rutile TiO2, J. Alloy Compd. 585, 75. [42] Chen G., Chen J., Peng J.H., et al. (2012) Application of response surface methodology for optimization of the synthesis of synthetic rutile from titania slag, Appl. Surf. Sci. 258, 4826. [43] Yu X.F., Wu N.Z., Xie Y.C., et al. (2001) The monolayer dispersion of V2O5 and its influence on the anatase-rutile transformation, J. Mater. Sci. Lett. 20, 319. [44] Chen D.L., Liu Z.S., Fan B.B., et al. (2014) Synthesis and characterisetion of TiN-coated cubic boron nitride powders, Int. J. Appl. Ceram Technol. 11, 946. [45] Mostafa N.Y., Kishar E.A., Abo-El-Enein S.A. (2009) FTIR study and cation exchange capacity of Fe3+ and Mg2+ substituted calcium silicate hydrates, J. Alloy Compd. 473, 538. [46] Hoex B., Peeters F.J.J., Creatore M. et al. (2006) High-rate plasma-deposited SiO2 films for surface passivation of crystalline silicon, J. Vac. Technol. Vac. Surf. Films 24, 1823. [47] Ishii M., Nakahira M., Yamanaka T. (1972) Infrared absorption spectra and cation distributions in (Mn, Fe)3O4, Solid State Commun. 11, 209. [48] Jeanloz R. (1980) Infrared spectra of olivine polymorphs: α, β phase and spinel, Phy. Chem. Miner. 4, 327. [49] Li K.Q., Chen J., Chen G., et al. Microwave dielectric properties and thermochemical characteristics of the mixtures of walnut shell and manganese ore, Biouresource Technol. 2019, 286. [50] Li K.Q., Chen G., Li X.T., et al. High-temperature dielectric properties and pyrolysis reduction characteristics of different biomass-pyrolusite mixtures in microwave field, Biouresource Technol. 2019, 294. [51] Gao H.Y., Jiang T., Xu Y.Z., et al. (2018) Change in phase, microstructure, and physical-chemistry properties of high chromium vanadium slag during microwave calcification-heating process, Powder Technol. 340, 520. [52] Li K.Q., Chen G., Chen J., et al. Microwave pyrolysis of walnut shell for reduction process of low-grade pyrolusite, Biouresource Technol. 2019, 291. [53] Zhang X.F., Liu F.G., Xue X.X. et al. (2016) Effects of microwave and conventional blank heating on oxidation behaviour, microstructure and surface morphology of vanadium slag with high chromium content, J. Alloy Compd. 686, 356. [54] Li X.S., Xie B., Wang G.E. et al. (2011) Oxidation process of low-grade vanadium slag in presence of Na2CO3, Trans. Nonferrous Met. Soc. China 21, 1860. [55] Li K.Q., Chen J., Peng J.H., et al. Dielectric properties and thermal behaviour of electrolytic manganese anode mud in microwave field, J. Hazard. Mater. 2020, 381. [56] Ye X.L., Guo S.H., Qu W.W. et al. (2019) Microwave field: high temperature dielectric properties and heating characteristics of waste hydrodesulfurisetion catalysts, J. Hazard. Mater. 366, 432. [57] He F., Chen J., Chen G. et al. (2019) Microwave dielectric properties and reduction behaviour of low-grade pyrolusite, JOM 11, 3909. [58] Su X.J., Mo Q.H., He C.L. et al. (2015) Microwave absorption characteristics of manganese compounds, Min. Metal. Eng. 35, 90.

Chapter 2 Microwave Pretreatment and Microwave Drying Microwave heating technology is applied for various reasons in fascinating and attractive inter-disciplinary research fields [1]. Microwave irradiation systems provide the microwave energy necessary to achieve high temperatures and finally reaching the efficient, energy-saving, and environmentally friendly industrial heating process, including drying [2, 3]. Among the drying technologies, conventional hot air drying has disadvantages, including low efficiency, energy-consuming, and inhomogeneous temperature gradient. Thus, the development of an alternative drying technology to provide solutions for the above issues is in urgent demand. As a novel drying technology, microwave drying is firstly noticed in food industries to provide fast drying and elaborate quality control of the final products. It now is spreading to more fields, including chemical process, metallurgy, and materials’ manipulation. The interaction between polar molecules in the material and microwave electromagnetic field makes the microwave energy heat materials on a macro level [4–8]. Thus, microwave heating technology shows many advantages, including fast drying rate, high product quality [9–12]; high efficiency and energy saving; selective heating; green cleaning, etc. Chen et al. [13] studied the microwave-assisted heating process to reduce low-grade pyrolusite, indicating that the manganese oxide powder was effectively prepared from the as-received material with the assistance of microwave energy at 600 °C for 40 min, and the reduction rate was 94.4%. Li et al. [14] prepared CaO-doped partially stabilised zircona (CaO-PSZ) with fused zirconia as raw material by microwave heating technology and determined the stable parameters for the preparation of Cao-PSZ. Thus, the application of microwave drying technology on the pretreatment shows a good prospect for a further improvement on the heating rates and the quality of drying products [15–18]. The advantages of microwave drying resulting from its unique heating mechanism are explained with the contrast between the heat and mass transfer models of microwave drying and conventional heating: the heat transfer direction of conventional drying is from outside to the inside of the material. The temperature gradient provides less assistance on the mass transfer of moisture from the inside to the outside [19]. Thus, in some cases, the mass DOI: 10.1051/978-2-7598-2696-4.c002 © Science Press, EDP Sciences, 2022

54

Application of Microwave Heating in the Comprehensive Utilization

transfer of moisture inside the material became a controlling step of the whole drying process. The only optimising method is to increase the drying temperature accompanied by high energy consumption. The moisture inside the material will preferentially absorb the microwave energy under microwave radiation, which subsequently converts into heating energy. Since the microwave can penetrate the material, the temperature gradient caused by microwave heating is slight. The moisture inside the material continuously diffuses from the inside to the surface for evaporation. Since the temperature gradient, heat transfer, and vapor pressure transfer direction inside the material provides few resistances on the water transfer direction, the diffusion process is encouraged, the cost for energy is reduced, and the dry efficiency is vastly improved [20]. This chapter introduces microwave energy applications on pretreatments of ilmenite ore and titanium slag and various drying processes for a different portion of titanium slags in sections 2.1–2.4, respectively.

2.1

Microwave Pretreatment of Ilmenite Ore

This section systematically investigates the influences of microwave irradiation on the surface characteristics of Panzhihua ilmenite. The crystal structures, surface morphology, and surface chemical functional groups of ilmenite were characterised before and after microwave irradiation and magnetic separation for different microwave treatment times using various methods, such as XRD, SEM, and FT-IR, respectively. XRD analysis showed that microwave treated ilmenite has the most substantial peaks of phase more than those of raw samples, indicating that the crystalline compound of ilmenite increased with the microwave irradiation time. SEM analysis showed the micro-cracking appeared at many grain boundaries of ilmenite after being pretreated by microwave treatment. The separations of ilmenite from gangue minerals were completed. They formed the micro-fissure within ilmenite minerals because of the different minerals and compounds’ microwave selective heating characteristics. The thermal stresses were caused by the uniform heat rate disturbed under microwave irradiation. The mineral processing results showed that microwave treated ilmenite samples’ magnetic separation characteristics and properties were better than those of microwave untreated ilmenite samples. Microwave irradiation is applied effectively and efficiently to the irradiation processes of Panzhihua ilmenite.

2.1.1

Materials and Procedure

Ilmenite ore utilised in this section was obtained from Panzhihua city, Sichuan province, China. The chemical compositions of ilmenite are shown in table 2.1. TAB. 2.1 – Chemical compositions of Panzhihua ilmenite ore (wt. %). ΣFe 30.67

TiO2 15.71

SiO2 20.38

CaO 6.48

MgO 7.12

Al2O3 3.33

Others 16.28

Microwave Pretreatment and Microwave Drying

55

(1) XRD analysis of ilmenite ore The mineralogical analysis of ilmenite was performed using X-ray diffraction. XRD pattern was acquired using an X-ray diffractometer with CuKα radiation and a Ni filter operated at 35 kV, 20 mA, and a scanning rate of 0.25°/min. Figure 2.1 shows the XRD pattern of ilmenite. Magnetite (Fe3O4) and ilmenite (FeTiO3) were the main crystalline compounds in the ore; also, a minor amount of SiO2, CaO, TiO2, MgO, and Al2O3 were present.

FIG. 2.1 – The X-ray diffraction pattern of ilmenite. (2) FT-IR spectra of ilmenite ore Figure 2.2 illustrates the FT-IR spectra of initial ilmenite samples. The FT-IR absorption showed broad bands at 3434.2 cm−1, 1067.1 cm−1, and a weak peak at 1641.2 cm−1 because of the asymmetric and symmetric stretching vibrations of weakly bound water at the minerals’ surface, respectively. Peaks at 976.9 cm−1 are due to the stretching vibrations of Si-O and Si-O-Si of the SiO4 units. In the range of 400 cm−1–1000 cm−1, the IR bands of inorganic solids were usually assigned to the crystal lattice’s vibration of metallic ions. (3) SEM and EDAX analyses of ilmenite ore A scanning electron microscope (SEM) characterised the raw materials’ SEM as shown in figure 2.3, indicating that the surface structures of ilmenite’s primary particles have regular and tighter crystal boundaries, with more minerals and gangues appearing on the surface. According to SEM analysis result, EDAX analysis of valuable minerals and gangue was carried out to know the ore’s composition. The EDAX spectrum revealed that the raw ore consists of a certain Fe and Ti (mainly within the light grey part). Subordinate amounts of Al, Mg, Ca, and Si also exist (mainly within the dark grey part). Most of the ores inlay in gangues as shown in figure 2.4(a–e).

56

Application of Microwave Heating in the Comprehensive Utilization

FIG. 2.2 – FT-IR spectra of ilmenite.

FIG. 2.3 – SEM of ilmenite: (a) 500×; (b) 200×; (c) 100×; (d) 50×.

Microwave Pretreatment and Microwave Drying

57

FIG. 2.4 – SEM image (a) of ilmenite ore and EDAX spectrum of regions 1(b), 2(c), 3(d) and 4(e).

58

Application of Microwave Heating in the Comprehensive Utilization

A scanning electron microscopy illustrated that the average particle size of mineral grains within the massive ore is approximately between 0.2 mm and 1 mm. If milled the ore to the size range of 80% below 0.074 mm, it could be dissociated effectively, as shown in figure 2.5. The mineral grains are disseminated consistently throughout the mineral, and ilmenite and magnetite grains (light grey areas) were inter-grown with larger grains of the matrix (dark grey areas).

FIG. 2.5 – The particle size of ilmenite ore.

2.1.2

Effects of Microwave Energy

The heating rates test proceeded with 40 g ore samples, and the microwave used was 1 kW of power and 2.45 GHz of frequency. The treated and untreated samples were all ground for 30 s using the sampling crusher to quantify changes. The size distribution of the ground specimen was determined by sieve analysis. Samples were heated at varying times between 10 and 60 s. Furthermore, the sample mass in the trials was ranging from 20 to 100 g. Subsequently, magnetite separation trials were carried out to prove the increase in liberation. In this trial, 40 g representative samples of the untreated and treated material were tested. The heating rate of ilmenite ore is shown in figure 2.6. Ilmenite ore could be heated effectively and attain 350 °C in one minute. Subsequently, the heating rate became slow. That is because of the low power density and sample mass. So in the present experiments, the microwave heating time was one minute. (1) XRD analysis of the microwave pretreated ilmenite ore The samples after microwave irradiation were characterised by XRD (D/Max 2200, Rigaku, Japan) at a scanning rate of 0.25°/min with 2θ ranging from 5° to 100° using CuKα radiation (λ = 1.5418 Å) and a Ni filter. The voltage and anode current operated were 35 kV and 20 mA, respectively. After scanning, the mineral

Microwave Pretreatment and Microwave Drying

59

FIG. 2.6 – Temperature rise curve of ilmenite ore by microwave irradiation. peaks of the samples were identified. XRD measurements were conducted to distinguish the crystal structure between the ilmenite ores and microwave treated samples as shown in figure 2.7. The main phase of figure 2.7(a) is ilmenite and several peaks of the XRD pattern match well with those of the magnetite reference XRD patterns. Figure 2.7(b) shows that the microwave treated ilmenite ore has a peak of phase more than that of raw ore. By comparing with figure 2.7(a), with the increase in microwave irradiation time, magnetite and ilmenite’s peak intensity increases. The peaks for impurities, mainly magnesium oxide, calcium oxide, and other gangues, appear [21]. The results

FIG. 2.7 – The XRD pattern of the ilmenite ore: (a) ilmenite ores; (b) microwave treated samples.

60

Application of Microwave Heating in the Comprehensive Utilization

indicate that after microwave irradiation at 3 kW for 30 s, the ilmenite ore structure has dissociated into the principal valuable minerals and more gangue monomer. (2) FT-IR analysis of the microwave pretreated ilmenite ore The samples’ surface chemical functional groups were characterised before and after microwave irradiation using FT-IR, and the results are shown in figure 2.8 using FT-IR spectrometer equipped (8700, Nicolet, USA). The angle of incidence of the IR beam was 45°, and 100 scans were collected at a resolution of 4 cm−1 and averaged using the OMNIC spectroscopic software. The spectra were collected within the range of 4000 cm−1–400 cm−1 wave number.

FIG. 2.8 – FT-IR spectra of the ilmenite ore: (a) ilmenite ores; (b) microwave treated samples. For the FT-IR spectra of the ilmenite ore in figure 2.8(a), absorption bands at 3434.2 cm−1, 1641.2 cm−1, 1067.1 cm−1, 976.9 cm−1, and 465.3 cm−1 are observed in the spectrum. The band at 3434.2 cm−1 and 1067.1 cm−1 have been assigned to the bending mode of hydroxyl (–OH) groups adsorbed on the sample’s surface. The band at 1624.3 cm−1 is assigned to H2O adsorbed on the surface of the ilmenite ores. The band at 465.3 cm−1 is assigned to the stretching vibrations of octahedral metal ions in the units [22, 23]. The FT-IR spectrum of the microwave treated ilmenite ore at a microwave power of 3 kW and microwave irradiation time of 30 s is shown in figure 2.8(b). Image 2(b) shows that the most apparent change in the spectrum is that the bands at 3434.2 cm−1, 1641.2 cm−1, and 1067.1 cm−1 become barely visible. This vibration mode compared with one of the ilmenite ores at 465.3 cm−1 c shows a slight blue-shift of the stretching vibrations of the band toward higher frequency [4]. (3) SEM and EDAX analyses of the microwave pretreated ilmenite ore Ilmenite ore, pretreated by microwave irradiation at a microwave power of 3 kW and the microwave heating time of 30 s, was characterised by SEM and EDAX techniques, and the results are shown in figures 2.9 and 2.10, respectively.

Microwave Pretreatment and Microwave Drying

FIG. 2.9 – SEM of the microwave treated ilmenite.

61

62

Application of Microwave Heating in the Comprehensive Utilization

FIG. 2.10 – EDAX of the microwave treated ilmenite: (a) SEM; (b) Areas1; (c) Areas 2; (d) Areas 3.

The SEM instrument (XL30ESEM-TMP, Philips, Holland) was operated at 20 kV in a low vacuum. Simultaneously, the energy dispersion scanner spectrometer (EDAX, USA) attached to the SEM was used for semi-quantitative chemical analysis. The results in the SEM image in figure 2.9, indicated that the crack in the grain boundaries of ilmenite ore appeared after being pretreated by microwave irradiation. The gangue inside the minerals was separated, caused by differential expansion between mineral and gangue under microwave irradiation. Additionally, another group data of EDAX analysis are represented in figure 2.11. Figure 2.11 shows that the light grey areas consist of a certain amount of Fe and Ti, and amounts of Al, Mg, Ca, and Si also exist. Titanium and iron content increases from gangue minerals to valuable minerals, in contrast to aluminum, magnesium, calcium, and silicon content decreases in a similar trend. Thus, ilmenite’s microwave irradiation gives rise to minerals with a higher grind ability, therefore easier accessibility to ilmenite’s separation.

Microwave Pretreatment and Microwave Drying

63

FIG. 2.11 – EDAX spectrum of the ilmenite ore after microwave irradiation: (a) SEM; (b) District 1; (c) District 2.

(4) Particle size distributions analysis of the microwave pretreated ilmenite ore Ilmenite and microwave treated ilmenite’s particle size distributions for various microwave irradiation times and uniform grinding time (60 s) are illustrated in figure 2.12 and table 2.2 using a laser diffraction analyser (Mastersize2000, Malvern, UK).

FIG. 2.12 – Particle size distributions of ilmenite before and after microwave irradiation for different exposure times: (1) 0 s; (2) 10 s; (3) 20 s; (4) 30 s. TAB. 2.2 – Results of particle size distributions of microwave irradiation treated ilmenite ores. Sample 1 2 3 4

d10 5.093 2.718 2.124 1.556

d50 54.755 29.657 26.191 16.758

d90 236.54 133.195 136.779 96.186

Application of Microwave Heating in the Comprehensive Utilization

64

Figure 2.12 shows the particle size distribution of the ilmenite ore and microwave treated ilmenite ore. The average particle diameter (d50) decreases gradually from about 54.75–16.76 µm with the increase in microwave irradiation time from 0 to 30 s. The results indicate that microwave irradiation techniques can be applied effectively and efficiently to the ilmenite ore’s pretreatment processing. The results signify that ilmenite treated by microwave irradiation method was more uniform and have a narrower distribution size. Ilmenite is composed of particles with a d50 around 54.755 µm, in which about 70% of the particles are below 100 µm. It could be seen that after 10 s of microwave irradiation time, the d50 decreased from 54.755 µm to 29.657 µm and longer exposure times up to the 30 s essentially had a minor effect on the distribution. The d50 slowly decreased to 16.758 µm. Generally, the particle size was found to decrease with the increasing microwave irradiation time. The results indicate that microwave irradiation techniques can be applied effectively and efficiently to the irradiation and grinding processing of ilmenite.

2.1.3

Microwave Pretreatment Optimisation

The main objective of this study is to investigate the influence of microwave pretreatment on magnetic separation of ilmenite using response surface methodology. Microwave power, time, and mass of sample are the main three dominant factors selected as independent variables while the recovery ratio of ilmenite is selected as a dependent variable. The optimum process conditions that result in the highest ilmenite recovery are identified using the Design-Expert software package. Microwave power (χ1), time (χ2), and mass of sample (χ3) were chosen as independent variables, while recovery ratio (Y) was the response (dependent variable). The range and the levels of the independent variables investigated in this study are given in table 2.3 based on preliminary experiments where the levels of independent variables were chosen to be 1000–2400 W, 15–40 min, and 30–70 g, respectively. The design matrix was generated using the design expert software (version 7.1.5, STAT-EASE Inc., Minneapolis, USA), which depicts the experimental conditions and the resultant output variable (recovery ratio) as shown in table 2.3 where the complete design consisted of 20 experimental points (8 factorial points, 6 axial points, and 6 center points), covering all combinations of the independent variables along with the repeat experimental runs.

TAB. 2.3 – Coded value of the independent variables and experimental ranges. Independent variables Microwave power (W) Time (min) Mass of sample (g)

Coded parameters −1.682 522.75 6.48 16.36

−1 1000.00 15.00 30.00

0 1700.00 27.50 50.00

1 2400.00 40.00 70.00

1.682 2877.25 48.52 83.64

Microwave Pretreatment and Microwave Drying

65

For statistical calculations, the chosen independent variables were coded according to equation (2.1) [24, 25]: Xi ¼

ð vi  v0 Þ Dv

ð2:1Þ

where Xi is a coded value of the variable, vi is the actual value of the variable, v0 is the actual value of the Xi at the center point level, and Dv is the step change of variable. The quadratic equation for predicting the optimal conditions can be expressed according to equation (2.2) [26, 27]: Y ¼ b0 þ

k X i¼1

b i vi þ

k X i¼1

bii v2i þ

n1 X n X

bij vi vj

ð2:2Þ

i¼1 j¼i þ 1

where b0 is a constant coefficient, bi is the linear coefficient, bii is the quadratic coefficients and bij is the interaction coefficients, k is the number of factors studied and optimised in the experiment, vi , vj are the coded values of independent variables, and the terms vi vj and v2i represent the interaction and quadratic terms, respectively. The software ‘Design Expert’ was used for the central composite design, experimental data analysis, quadratic model buildings, polynomial equations evaluation, and three-dimensional response surface and contour plotting. Table 2.4 provides the results of the experiments in terms of percentage recovery of ilmenite, which was found to vary from 57.00 to 72.00%. (1) Statistical analysis The ANOVA results of the quadratic model for the recovery ratio are summarised in table 2.5. According to Joglekar and May [28], the correlation coefficient of a good fit of a model should be at a minimum of 0.80, while a high R2 value illustrates better agreement between the calculated and observed results within the range of experiments [7, 29]. The correlation coefficient (R2 = 0.9410) indicates the proximity of the model equation with the experimental data. The adjusted determination coefficient (R2 = 0.8879) is also high advocating the significance of the model [30, 31]. The closer the value of adjusted R2 to 1, the better is the correlation between the experimental and predicted values [32]. The lack-of-fit F-value of 1.18 implies that it is not significant relative to the pure error. There is a 42.90% chance that a large lack-of-fit F-value could occur due to noise. The coefficient of variation (CV) indicates the degree of precision with which the experiments were conducted and is a good index of the reliability of the experiment [14, 33]. A lower CV means higher reliability of the experiment. The lower value of CV (1.93%) demonstrates that the performed experiments were highly reliable. Adequate precision measures of the signal-to-noise ratio greater than 4 are generally desirable [13, 14, 16]. The signal-to-noise ratio of 15.754, clearly indicates the suitability of the model to navigate the design space [8, 16].

66

Application of Microwave Heating in the Comprehensive Utilization TAB. 2.4 – Experimental design matrix and results.

Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Microwave power (W) 1000.00 2400.00 1000.00 2400.00 1000.00 2400.00 1000.00 2400.00 522.75 2877.25 1700.00 1700.00 1700.00 1700.00 1700.00 1700.00 1700.00 1700.00 1700.00 1700.00

Variables Time (min) 15.00 15.00 40.00 40.00 15.00 15.00 40.00 40.00 27.50 27.50 6.48 48.52 27.50 27.50 27.50 27.50 27.50 27.50 27.50 27.50

Mass of sample (g) 30.00 30.00 30.00 30.00 70.00 70.00 70.00 70.00 50.00 50.00 50.00 50.00 16.36 83.64 50.00 50.00 50.00 50.00 50.00 50.00

Recovery ratio (%) 58.00 66.00 60.00 66.00 57.00 62.00 60.00 65.00 61.00 72.00 60.00 64.00 62.00 61.00 66.00 65.00 66.00 68.00 65.00 65.00

TAB. 2.5 – Analysis of variance (ANOVA) for response surface quadratic model for recovery ratio. Source Degrees Sum Mean F-value p-value of variation of freedom of squares square Linear 11 93.66 8.51 6.23 0.0280 2FI 8 89.16 11.14 8.15 0.0167 Quadratic 5 8.09 1.62 1.18 0.4290 Cubic 1 6.55 6.55 4.79 0.0802 Residual error 10 14.92 1.49 Lack-of-Fit 5 8.09 1.62 1.18 0.4290 Pure error 5 6.83 1.37 Total 19 252.95 R2 = 0.9410; adj.R2 = 0.8879; CV = 1.93%; Adequate precision = 15.754 (>4.0)

Multiple regression coefficients obtained by employing a least square technique for the second-order polynomial model are summarised in table 2.6. The quadratic model F-value of 17.73 implies that the model is significant for the recovery ratio. The values of Prob > F > 0.0500 indicate that the model terms are significant [7, 14]. Table 2.6 presents the linear terms v1 , v2 , and the interaction terms v1 v3 and v2 v3 to be significant model terms based on the values of “Prob > F ” less than 0.050.

Microwave Pretreatment and Microwave Drying

67

TAB. 2.6 – Regression coefficients of predicated second-order polynomial model for the response variable. Regression analysis Source

Coefficient

Standard error of the coefficient

Degrees of freedom

Sum of squares

Mean squares

F-value

p-value

Model v1 v2 v3 v1 v2 v1 v3 v2 v3 v21

65.88 3.11 1.08 −0.56 −0.066 −1.66 −1.83 −0.25

0.50 0.33 0.33 0.33 0.32 0.32 0.32 0.43

9 1 1 1 1 1 1 1

238.03 132.26 15.88 4.32 0.064 39.59 48.48 0.50

26.45 132.26 15.88 4.32 0.064 39.59 48.48 0.50

17.73 88.65 10.64 2.90 0.043 26.53 32.50 0.34