Hydrometallurgy in Extraction Processes, Volume I 9781351439640

Citation preview

1990

FOREWORD Hydrometallurgy is today one of the established and highly recognized branches of extractive metallurgy. The production and processing of nuclear metals and materials represents just one of many fields in which it has played a significant role. In fact, solventextraction and ion-exchange, the two elegant hydrometallurgical unit operations, were first commercially exploited in the metallurgy of uranium. This success story paved the road for their major introduction in the common metals extraction tlowsheets. The achievements of solvent extraction to process lean copper solution amenable for electrowinning needs hardly any introduction to extraction and process metallurgists. The Indian Atomic Energy program can rightly boast today about its well-founded materials program devoted primarily to wide-ranging materials production and processing activities. To give some specific examples, I refer to uranium concentrates production and refining, monazite resource processing to thorium and rare-earths intermediates, zircon processing, and columbite-tantalite resource processing. Extending the list, I also refer to the programs on reprocessing of irradiated fuel from our research and power reactors. All of these and many other related ones are truly representative and illustrative of the range and extent to which hydrometallurgy as a technology has entered in the nuclear energy program in the country. I have been very closely associated with and personally directed and pursued the research, development, and growth of some of these programs from their inception to translation to production levels. It has, therefore, truly been an immense pleasure for me to write a foreword to this text. The book has jointly been authored by Dr. Gupta and Dr. Mukherjee, who are formally attached to the Metallurgy Division of our center. They have to-date acquired very extensive specialization in chemical metallurgy, not only from their own research involvements in the Division, but also from their close interaction and collaboration with other divisions and with nuclear metals and materials production installations located elsewhere in the country. Both have, thus far, contributed prolifically to scientific and technical literature. Dr. Gupta has very recently completed writing a two-volume book entitled, Materials in Nuclear Energy Applications. The two volumes are presently available from CRC Press, Inc. I may sum up by saying that they are professionally quite mature and have the competence to take on this current task. The contents of this present set abundantly reflects the intent of the authors. They have, through the seven chapters, given complete coverage of almost all the aspects of the field of hydrometallurgy. The presentation has been organized very well. Each chapter includes an introductory section, extensive eaumeration of relevant physicochemical principles, and thorough descriptions of the applications in research and industry. Each chapter also carries a comprehensive reference list, which makes it possible for readers to get further detailed information, if necessary. I wish to compliment the authors for all the effort and initiative they have demonstrated in putting together this work. In my opinion, this publication is outstanding in its systematic treatment of the subject, in its depth of technical content, and in its comprehensive and upto-date coverage. These volumes are sure to gain wide readership. Researchers, professional scientists and engineers, and students of metallurgy and of other branches, such as chemistry and chemical engineering, will find the volumes extremely useful and very valuable as reference texts.

P. K. Iyengar Chairman Atomic Energy Commission India

PREFACE Hydrometallurgy has emerged as a leading technology in recent times. This branch of metallurgy has become extremely successful in a number of areas of extractive processes. It is also credited with tremendous potentialities. Current interest in the field of hydrometallurgy is profound. A voluminous body of literature and frequently held international symposia, conferences, and meetings bear ample testimony to this fact. We have been involved with this field professionally for many years and have long been nurturing the idea of writing an up-to-date treatise on the subject in its entirety. Hydrometallurgy in Extraction Processes, Volumes I and ll, is basically the outcome of this continued endeavor. There are, in all, seven chapters that constitute the two-volume set. An introductory appraisal of hydrometallurgy as a whole has been given in the first chapter. Subsequent chapters deal with the topic on leaching, the first major opening unit operation in hydrometallurgy. Leaching processes with mineral acids are treated in the second chapter, those with alkalies in the third, those with ferric and cupric ions in the fourth, and those with miscellaneous reagents such as chlorine, hypochlorite, cynide and dichromate, in the fifth. In hydrometallurgical process flowsheet, leaching is followed by the purification of the leached product - this subject is treated in the sixth chapter. Ion exchange, carbon adsorption, and solvent extraction processes which constitute the main solution purification techniques have been described therein. Hydrometallurgical process flowsheet is the final stage involving processing of the purified solution with the objective of recovery of the recoverables. This ultimate step is examined in the seventh chapter wherein processes involving crystallization, ionic precipitation, gaseous reduction, electrochemical reduction, and electrolytic reduction are described. We believe that metallurgists and materials scientists, in general, and chemical metallurgists, in particular, will fmd these volumes a worthwhile contribution and a very valuable addition to the existing literature. Unquestionable is its utility to chemists and chemical engineers. Students of metallurgy both of under-graduate and post-graduate levels and those electing to specialize should fmd it extremely useful. In addition, these volumes should provide a source of information to many others interested in this subject matter. We are very hopeful that this present publication will attract a wide readership. We have attempted to make it as comprehensive and exhaustive as possible. Our dear readers are, however, the best judge and we wholeheartedly welcome suggestions and criticisms from all. It is with a high sense of acknowledgment and deep appreciation that we dedicate the work to our wives and children- Sreejata, Anjana and Indranil; Chandrima and Chiradeep. They have played their sweet roles in seeing our work through to completion.

T. K. Mukherjee C. K. Gupta

THE AUTHORS C. K. Gupta, Ph.D., is the Head of the Metallurgy Division of the Bhabha Atomic Research Centre at Trombay, Bombay. Dr. Gupta obtained his B.Sc. and Ph.D. degrees in Metallurgical Engineering in 1961 and 1969, respectively, from the Department of Metallurgy, Banaras Hindu University, Varanasi. He is a recognized guide for M.Sc. (Tech.) and Ph.D. degrees in Metallurgical Engineering of the Bombay University, Bombay. Dr. Gupta is a council member of the Indian Institute of Metals, and Chairman of the Bombay Chapter of the Indian Institute of Metals . He was a member of the American Association for the Advancement of Science. He is an editor of the Transactions of the Indian Institute of Metals. He is also a member of the editorial boards of the journals, High Temperature Materials and Processes, Minerals Engineering, and Mineral Processing and Extractive Metallurgy Review. Dr. Gupta is the recipient of several awards for his many-sided contributions in Chemical Metallurgy. He has authored more than 150 papers and has been author or co-author/editor of four books. His current major research interest lies in hydrometallurgy of rare metals and secondary resources processing. T. K. Mukherjee, Ph.D., is the Group Leader, Hydrometallurgy, at the Metallurgy Division of the Bhabha Atomic Research Centre at Trombay, Bombay. Dr. Mukherjee received his B.E. degree from Calcutta University in 1967. He obtained his M.Sc. (Tech.) degree in 1973 and Ph.D. degree in 1985 from the University of Bombay and DIC in 1976 at the Imperial College of Science and Technology, London. Dr. Mukherjee is a life member of the Indian Institute of Metals. He has published more than 75 papers and won several medals for his papers. His interests and expertise are in hydrometallurgy of base metals and secondary resources processing.

ACKNOWLEDGMENTS The authors wish to thank the following sources for permission to reproduce specific figures and tables appearing in these volumes: The Minerals, Metals and Materials Society, Warrendale, Pa; The Electrochemical Society, Inc., Manchester, NH; The Australian Institute of Mining and Metallurgy, Victoria, Australia; Transactions of the Society of Mining Engineers of AIME, New York; Society of Chemical Industry, London, U.K.; The Institution of Mining and Metallurgy, London, U.K., The Canadian Institute of Mining and Metallurgy, Montreal, Quebec, Canada; Pergamon Press, Inc., Elmsford, NY; Elsevier Scientific Publishing Co., Amsterdam, The Netherlands; Academic Press, Inc., Orlando, Fl.; Gordon and Breach Science Publishers, New York; lnterscience Publishers, Inc., New York; Maclean Hunter Publishing Co., Chicago, IL; E. and F.N. Spon, Ltd., London, U.K.; Methuen and Co., Ltd., London, U.K., Society of Chemical Industry, London, U.K.; Harper and Row Publishers, Inc., New York; McGraw Hill, Inc., New York; National Institute for Metallurgy, Auckland Park, S.A.; The American Chemical Society, Washington, D.C., and the U.S. Bureau of Mines, Washington, D.C.

TABLE OF CONTENTS Volume I Chapter 1 Hydrometallurgy- An Introductory Appraisal ....................................... 1 I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 ll. Hydrometallurgical Processes ..................................................... 2 A. Advantages ................................................................ 2 B. Disadvantages ............................................................. 4 lll. Physicochemical Principles of Hydrometallurgical Processes ...................... 4 A. General .................................................................... 4 B. Dissolution Aspects ....................................................... 6 1. Solution Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Oxidation Potential ................................................ 6 3. Potential-pH Diagrams ............................................. 7 4. Chloride Metallurgy .............................................. 11 a. High Solubility of Metal Chlorides ........................ 11 b. Ferric and Cupric Chlorides ............................... 12 c. Chlorocomp1ex Ion Formation ............................. 13 d. Hydrochloric Acid ......................................... 14 5. Types of Dissolution Reactions ................................... 16 a. Physical Processes ......................................... 16 b. Chemical Processes ........................................ 16 c. Electrochemical Processes ................................. 18 d. Electrolytic Processes ..................................... 23 6. Pressure Hydrometal1urgy ......................................... 25 C. Separation Aspects ....................................................... 27 1. Crystallization .................................................... 27 2. Ionic Precipitation ..................... . .......................... 27 3. Electrochemical Reduction ........................................ 29 4. Reduction with Gas ............................................... 30 5. Carbon Adsorption and Ion Exchange ............................ 30 6. Solvent Extraction ................................................ 31 7. Electrolytic Process ............................................... 33 IV. Unit Processes in Hydrometallurgy .............................................. 35 A. Ore Preparation ........................................................... 36 B. Leaching ................................................................. 39 1. Leaching Reagents ................................................ 39 a. Acids ..................................... . ................ 39 b. Alkalies ................................................... 40 c. Salts ....................................................... 40 2. Leaching Methods ................................................ 40 a. In situ Leaching ........................................... 42 b. Heap Leaching ............................................ 42 c. Dump Leaching ........................................... 43 d. Vat Leaching .............................................. 46 e. Agitated Leaching ......................................... 47 C. Solid-Liquid Separation .................................................. 47 D. Back-End Operation ...................................................... 48

V. Summary ........................................................................ 52 References ............................................................................... 56 Chapter 2 Leaching with Acids ................................................................... 57 I. General .......................................................................... 57 II. Sulfuric Acid Leaching .......................................................... 59 A. Oxides .................................................................... 59 1. Copper Oxide ..................................................... 59 2. Zinc Oxide ........................................................ 62 3. Nickel Oxide ...................................................... 64 4. Aluminum Oxide ................................................. 66 5. Chromium Oxide ................................................. 67 6. Niobium and Tantalum Oxide .................................... 68 7. Titanium Oxide ................................................... 69 8. Uranium Oxide ................................................... 70 B. Sulfides .................................................................. 76 1. Copper Sulfide .................................................... 76 2. Nickel and Cobalt Sulfides ........................................ 83 3. Zinc Sulfide ....................................................... 87 C. Miscellaneous Primary Metal Resources .................................. 90 1. Aluminum Silicate ................................................ 92 2. Beryllium Silicate ................................................. 92 3. Cesium Aluminum Silicate ........................................ 92 4. Monazite Sand .................................................... 92 5. Phosphate Rock ................................................... 93 6. Manganese Nodules ............................................... 94 D. Secondary Metal Resources .............................................. 95 1. Anode Slime ...................................................... 96 Slags .............................................................. 96 2. 3. Ay Ash ........................................................... 97 4. Metal Scrap ....................................................... 97 ill. Nitric Acid Leaching ............................................................. 98 A. Sulfides .................................................................. 98 1. Copper Sulfide .................................................... 98 2. Nickel Sulfide ................................................... 102 3. Lead Sulfide ..................................................... 103 4. Molybdenum Sulfide ............................................. 103 B. Oxides and Silicates ..................................................... 105 1. Manganese Oxide ................................................ 105 2. Aluminum Silicate ............................................... 106 C. Secondary Metal Resources ...................•......................... 107 1. Colliery Waste ................................................... 107 2. Molybdenum Metal Scrap ........................................ 108 3. Irradiated Nuclear Fuel .......................................... 108 IV. Hydrochloric Acid Leaching .................................................... II 0 A. Oxides ................................................................... 110 1. Pyrochlore ....................................................... 110 2. Ilmenite .......................................................... Ill 3. Manganese Nodules .............................................. 112

B.

Sulfides ................................................................. 112 1. Iron Sulfide ...................................................... 112 2. Copper Sulfide ................................................... 114 3. Nickel Sulfide ................................................... 117 4. Complex Sulfide Concentrates ................................... 118 References .............................................................................. 121 Chapter 3 Leaching with Alkalies ................................................................ 127 I. General ......................................................................... 127 IT. Caustic Soda Leaching .......................................................... 129 A. Oxides ................................................................... 129 1. Aluminum Oxide ................................................ 129 2. Chromium Oxide ................................................ 132 3. Tungsten Oxide .................................................. 133 B. Phosphates .............................................................. 134 1. Thorium Phosphate .............................................. 134 C. Silicates .............................. . .................................. 135 1. Beryl. ............................................................ 135 D. Sulfides ................................................................. 135 1. Molybdenum Sulfide ............................................. 135 lll. Soda Ash Leaching ............................................................. 136 A. Molybdenum Oxide ..................................................... 136 B. Tungsten Oxide ......................................................... 137 C. Uranium Oxide .......................................................... 138 IV. Ammonia Leaching ............................................................. 142 A. Oxides ................................................................... 142 1. Nickel Oxide ..................................................... 142 2. Uranium Oxide .................................................. 149 3. Manganese Nodules .............................................. 149 B. Sulfides ................................................................. 150 1. Nickel Sulfide ................................................... 150 2. Copper Sulfide ................................................... 152 3. Copper-Zinc Sulfide ............................................. 158 4. Lead Sulfide ..................................................... 158 5. Sulfide Matte .................................................... 160 C. Secondary Resources .................................................... 162 References .............................................................................. 164 Chapter 4 Leaching with Ferric and Cupric Ions ............................................... 167 I. General ............................................ . ............................ 167 II. Ferric Sulfate Leaching .................. . ............ . .... . .................... 171 A. Copper Sulfides ......................................................... 171 1. Covellite ......................................................... 172 2. Chalcocite ....................................................... 173 3. Chalcopyrite ..................................................... 174 4. Bornite ........................................................... 176 5. Other Copper Minerals ........................................... 178 B. Nickel and Cobalt Sulfides .............................................. 180 C. Lead Sulfide ............................................................. 181

D. E.

Zinc Sulfide ............................................................. 183 Iron Sulfides ............................................................ 183 1. Pyrrhotite ........................................................ 184 2. Pyrite ............................................................ 184 F. Arsenic, Antimony, and Cadmium Sulfides ............................. 185 ill. Ferric Chloride Leaching ....................................................... 185 A. Copper Sulfides ......................................................... 185 B. Lead Sulfide ............................................................. 193 C. Zinc Sulfide ............................................................. 198 D. Iron Sulfides ............................................................ 199 E. Nickel Sulfides .......................................................... 200 F. Complex Metal Sulfides ................................................. 200 G. Metal Scrap ............................................................. 203 IV. Cupric Chloride Leaching ....................................................... 205 A. Copper Sulfides ......................................................... 205 B. Complex Metal Sulfides ................................................. 210 C. Copper Scrap ............................................................ 213 References .............................................................................. 214 Index ................................................................................... 221

TABLE OF CONTENTS Volume II Chapter 1 Other Leaching Processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 I. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 A. Cyanide Leaching ......................................................... 1 1. Basics of Cyanide Leaching of Au and Ag Ores.. . . . . . . . . . . . . . . . . . 1 a. Leaching Parameters and Methods ......................... 3 b. Cyanide Regeneration ...................................... 5 c. Effluent Treatment ......................................... 6 2. Basics of Cyanide Leaching of Copper Minerals ................... 6 a. Dissolution of Various Copper Minerals .................... 6 b. Copper Recovery from Cyanide Solutions .................. 8 c. General Comments ......................................... 8 B. Chlorine and Hypochlorite Leaching ...................................... 8 1. Chemistry of Cl2 Leaching ......................................... 9 2. Chemistry of Hypochlorous/Hypochlorite Leaching ................ 9 C. Dichromate Leaching ..................................................... 10 D. Electrochemical Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 IT. Cyanide Leaching ................................................................ 12 A. Gold ...................................................................... 12 1. Ores .............................................................. 12 a. Placer Deposits ............................................ 12 b. Free Milling Ores ......................................... 12 c. Refractory Ores ........................................... 12 2. Recovery Processes ............................................... 13 a. Free Milling Ores ......................................... 13 b. Refractory Ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3. Some Typical Plant Practices ..................................... 15 a. Heap Leaching Plants for Low-Grade Ores ................ 15 b. Plants for Processing Refractory Ores ..................... 18 B. Silver Ores ............................................................... 18 C. Copper Ores .............................................................. 21 1. Aotation Tailings ................................................. 21 2. Low-Grade Ores .................................................. 24 3. Leach Residue .................................................... 27 III. Chlorine Leaching ............................................................... 27 A. Copper Sulfide ........................................................... 27 B. Lead and Zinc Sulfides ................................................... 29 1. Chlorine Leaching ................................................ 29 2. Chlorine-Oxygen Leaching ........................................ 30 C. Nickel/Copper Sulfides ................................................... 32 D. Nickel and Zinc Oxides .................................................. 36 IV. Hypochlorite Leaching ........................................................... 37 A. Molybdenite .............................................................. 37 B. Mercury Ore ............................................................. 43 C. Gold Ore ................................................................. 44 V. Hypochlorous Acid Leaching .................................................. . . 47

VI. Vll.

Dichromate Leaching ............................................ ........... ..... 47 Electrochemical Leaching ........................................................ 49 A. White Metal .............................................................. 49 B. Nickel Matte ............................................................. 51 C. Complex Mattes .......................................................... 52 D. Sulfide Concentrates ................................................... .. . 54 l. Copper Sulfide .................................................... 54 2. Lead Sulfide ...................................................... 55 References ............................................................................... 57

Chapter 2 Solution Purification .............. ......... ...... ..... ........... ..... .. ............ ... 63 I. General .......................................................................... 63 A. Ion Exchange ............................................................. 63 l. Ion Exchange Processes ........................................... 63 2. Types of Exchanger ............................................... 64 3. Properties of Resins ............................................... 64 a. Exchange Capacity ........................................ 64 b. Swelling ................................................... 65 c. Selectivity ................................................. 66 4. Exchange Process Kinetics ........ .................... . ... ..... ... 66 5. Design and Operation of Ion Exchange Columns ................. 67 6. Ion Exchange Process Developments ............................. 71 7. Resin Poisoning and Its Regeneration ............................. 71 B. Carbon Adsorption ....................................................... 71 l. Adsorption Process ............................................... 71 2. Preparation and Properties of Activated Charcoal ................. 72 3. Modes of Adsorption/Desorption .................................. 72 a. Carbon-in-Column (CIC) .................................. 73 b. Carbon-in-Pulp (CIP) ....................................•. 73 c. Carbon-in-Leach (CIL) .................................... 73 C. Solvent Extraction ........................................................ 74 1. SX Process . ....................................................... 74 2. Choice and Types of Extractants .................................. 74 a. Cationic Extractants ....................................... 76 b. Anionic Extractants ..... ............. ...................... 77 c. Neutral Extractants ..................................... . .. 80 3. Additives in Solvent Systems ..... ........... ..................... 81 a. Diluent .................................................... 82 b. Modifiers .................................................. 83 4. Extraction Equilibria .............................................. 83 5. Extraction Isotherm ..............•................................ 86 6. Multiple Extractions .............................................. 87 7. Counter Current Extraction ....................................... 88 8. Equipments of SX ................................................ 90 a. Differential Contactors .................................... 91 b. Stage-Wise Contactors .................................... 92 ll. Application of Ion Exchange ..................................................... 92 A. Uranium .................................................................. 94 l. Chemistry ......................................................... 94 a. Acid Leach Liquors .....•................................. 95

III.

IV.

b. Carbonate Leach Liquors .................................. 95 c. Resin Regeneration ........................................ 96 2. Plant Practice ..................................................... 96 a. Fixed Bed ................................................. 96 b. Moving Bed ............................................... 97 c. Basket Resin-in-Pulp ...................................... 98 d. Continuous Resin-in-Pulp ................................. 98 e. Continuous Ion Exchange ................................. 99 B. Refractory Metals ........................................................ 99 1. Zirconium and Hafnium .......................................... 99 2. Vanadium ........................................................ 100 3. Niobium and Tantalum .......................................... 102 4. Molybdenum ..................................................... 102 5. Rhenium ......................................................... 103 6. Tungsten ......................................................... 104 7. Rare Earths ...................................................... 105 8. Common Metals ................................................ . 110 a. Copper ............................................... .... 110 b. Nickel and Cobalt ........................................ Ill 9. Precious Metals .................. ........ ........................ 112 Application of Carbon Adsorption .............................................. 114 A. Gold ..................................................................... ll4 1. U.S. Practices ................................................... 114 2. South African Practices .......................................... 116 B. Molybdenum ............................................................ 120 1. Leach Liquors ................................................... 120 2. Spent Acid ....................................................... 122 C. Rhenium ................................................................. 124 D. Vanadium ............................................................... 124 E. Uranium .... ... ........................... .. ............................. 124 Application of Solvent Extraction ............................................... 125 A. Copper ..... .. ............................ ........ ........ ......... ...... 125 1. Copper-Selective Extractants ..................................... 125 2. Large-Scale Practice ............................................. 126 B. Nickel, Cobalt, and Zinc ................................................ 127 1. Nickel from an Ammoniacal Solution ............................ 128 2. Nickel from an Acidic Solution .................................. 129 3. Cobalt from a Chloride Solution ................................. 129 4. Cobalt from a Sulfate Solution ................................... 130 a. Phosphoric Group (D2EHPA) ............................ 130 b. Phosphonic Group (M2EHPA) ........................... 132 c. Phosphinic Group (CYANEX272) ........................ 132 5. Recovery of Zinc ................................................ 132 C. Zirconium and Hafnium ................................................. 134 1. Chloride Solutions ............................................... 134 2. Nitrate Solutions ................................................. 135 3. Sulfate Solutions ................................................. 135 D. Vanadium, Molybdenum, and Tungsten ................................. l37 1. Vanadium ........................................................ 137 2. Molybdenum ......................... .. .......................... 139 3. Tungsten ......................................................... 140

E. F.

Niobium and Tantalum .................................................. 141 Uranium ................................................................. 144 1. Production of Uranium Concentrate .............................. 144 2. Production of Pure U02 from U Concentrate .................... 145 3. Processing of Wet Process Phosphoric Acid ..................... 146 4. Reprocessing of Irradiated Uranium Fuel ........................ 148 References .............................................................................. 149 Chapter 3 Metal Recovery Processes ............................................................. 157 I. General ......................................................................... 157 A. Crystallization ........................................................... 157 l. Attainment of Supersaturation ................................... 157 a. Simple SaJt ............................................... 160 b. Complex SaJt. ............................................ 161 2. Nucleation of Crystals ........................................... 161 3. Crystal Growth .................................................. 162 4. Crystallization Equipment ........................................ 163 a. Cooling Crystallizers ..................................... 163 b. Evaporating Crystallizers ................................. 163 c. Vacuum Crystal1izers ..................................... 164 B. Precipitation of Metal Compounds ...................................... 166 1. Hydroxides ....................................................... 166 2. Sulfides .......................................................... 167 3. Nature of the Precipitate ......................................... 168 C. Reduction with Gas ..................................................... 169 l. H2 Reduction .................................................... 169 a. Thermodynamics of H2 Reduction ........................ 169 b. Kinetics of H2 Reduction ................................. 172 2. CO Reduction .................................................... 173 3. S02 Reduction ................................................... 174 D. Cementation of Metals .................................................. 174 1. Thermodynamic Principle ........................................ 174 2. Kinetics .......................................................... 176 3. Form of the Precipitant .......................................... 177 4. Secondary Reactions ............................................. 178 5. Equipment ....................................................... 179 a. Gravity Launders ......................................... 179 b. Activated Launders ....................................... 179 c. Drum Precipitators ....................................... 179 d. Cone-Type Precipitators .................................. 180 E. E1ectrowinning of Metals ................................................ 181 l. Copper Winning ................................................. 182 2. Zinc Winning .................................................... 184 3. Gold Winning .................................................... 186 II. Application of Crystallization ................................................... 187 A. Aluminum Sulfate ....................................................... 187 B. Copper Sulfate .......................................................... 189 C. Nickel Sulfate ........................................................... 191 D. Ammonium SaJts of V, Mo, and W ..................................... 192

III.

IV.

V.

VI.

Application of Ionic Precipitation ............................................... 193 A. Aluminum ............................................................... 193 B. Alkali and Alkali Earth Metals .......................................... 193 C. Iron ...................................................................... 195 1. Sulfate Solutions ................................................. 195 a. Jarosite Process ........................................... 195 b. Goethite Process .......................................... 199 c. Hematite Process ......................................... 199 2. Chloride Solutions ............................................... 200 D. Copper, Nickel, and Cobalt ............................................. 200 E. Nuclear Metals ......................... ... .............................. 203 1. Uranium ......................................................... 203 a. Magnesium Diuranate .................................... 204 b. Ammonium Diuranate .................................... 204 c. Sodium Diuranate ........................................ 205 d. Uranium Peroxide ......... . .............................. 205 e. Uranium Phosphate ....................................... 205 2. Plutonium ........................................................ 206 a. Plutonium Peroxide ....................................... 206 b. Plutonium Oxalate ........................................ 206 c. Plutonium Fluoride ....................................... 206 3. Thorium ......................................................... 208 Application of Reduction with Gas ............................................. 209 A. Copper .................................................................. 209 1. Acid Solutions ................................................... 209 2. Basic Solutions ................................................. . 211 B. Nickel ................................................................... 213 C. Cobalt ................................................................... 218 D. Other Metals ............................................................ 218 Application of Cementation ..................................................... 219 A. Cementation of Copper .................................................. 219 1. Plant Practice .................................................... 219 2. Laboratory-Scale Investigations .................................. 223 B. Cementation of Gold and Silver ......................................... 224 1. Zinc as a Precipitant ............................................. 224 a. Zinc Boxes ............................................... 224 b. Merrill-Cro\Ye Equipment ................................ 225 2. Aluminum as a Precipitant. ...................................... 226 C. Cementation in Purification of Zinc Electrolyte ......................... 227 Advancement in Aqueous Electrowinning ....................................... 228 A. Winning of Copper ...................................................... 228 1. High Current Density ............................................ 228 a. Forced Circulation of Electrolyte ........................ . 228 b. Modified Anode Configuration ........................... 229 c. Ultrasonic Agitation ...................................... 229 d. Periodic Current Reversal ................................ 230 e. Air Sparging ............................................. 230 2. Reduction in Cell Voltage ....................................... 232 Oxygen Overvoltage ...................................... 232 a. b. Changes in Anode Reaction .............................. 234

3.

Electrowinning of Copper after Leaching and Solvent Extraction (L-SX-EW) ........................................... 236 4. Electrowinning of Copper from Dilute Leach Solutions .......... 237 a. Packed-Bed Cathode ............. . ....................... 237 b. Fluidized-Bed Electrode .................................. 238 5. Electrowinning of Copper from Chloride Solutions .............. 242 B. Winning of Zinc ......................................................... 245 1. Modernization of Cell Room Operation .......................... 245 2. Fluidized-Bed Electrolysis ....................................... 245 3. Nonconventional Electrolytes .................................... 246 a. Alkaline Electrolyte ...................................... 246 b. Chloride Electrolyte ...................................... 248 C. Conclusions ............................................................. 249 References .............................................................................. 250 Index ................................................................................... 257

Volume I

l

Chapter I

HYDROMETALLURGY -

AN INTRODUCTORY APPRAISAL

I. INTRODUCTION The extra~tion of metals from ores and/or concentrates is carried out either by pyrometallurgy or by hydrometallurgy. Pyrometallurgy encompasses the traditional high-temperature processes of roasting, smelting, converting, and refming. Hydrometallurgy is a relatively recent development compared with pyrometallurgy, the ancient art of metal production. Thousands of years ago, as history records, people learned to construct furnaces and use frre to melt rocks and extract metals. Much later came the use of water and aqueous solutions in place of dry, high-temperature methods for processing ores. Modem hydrometallurgy, in fact, can be traced back only to the end of the 19th century. Hydrometallurgy is essentially concerned with methods whereby metals, metal salts, or other metal compounds are produced by means of chemical reactions involving aqueous and organic solutions. The relationship between pyrometallurgy, hydrometallurgy, and prior mining and physical beneficiation processes is shown in Figure I. Hydrometallurgical processes nonnally operate in the temperature range of 25 to 250°C. The processes can operate at pressures of only a few kilopascals (vacuum) to as high as 5000 kPa. The late 1960s and early 1970s witnessed a great spurt in research and development of hydrometallurgical alternatives to conventional pyrometallurgical processes used to produce the bulk of nonferrous metals in the world. The strong points of hydrometallurgical processing lie in the wide and varied techniques and combinations of techniques that can be used to separate metals once they have been extracted into aqueous solutions. In principle, all the techniques of classic inorganic analytical chemistry are available to the hydrometallurgists and can be adapted for use in industrial processes. It is, indeed, true that it is the hydrometallurgists of recent times who are the main users of the vast amount of chemical knowledge built up by the classic inorganic chemists of the late 19th and early 20th centuries. Hydrometallurgists may aptly call themselves modem alchemists. There is a general awareness of the finite nature of resources around us. High-grade, traditional ore reserves face imminent threats of depletion. The need has arisen to devise ways and means to develop economic methods of processing the lean and complex ores that defy conventional processing. Apart from land-based resources, other unconventional resources such as those on the seabed (for example, manganese nodules) will be increasingly important as a source of a number of nonferrous metals. Secondary materials arising from industrial processes have received a great deal of attention. Secondary resources include complex materials discarded from pyrometallurgical processes, such as slags, dusts, sulfide mattes, and speisses; complex alloy scrap from fabrication processes; pickle liquors, sludges, and other industrial wastes. As far as such different materials are concerned, mention should also be made of the burgeoning electronics industry which, as we know today, is using increasingly rarer metals and semimetals. Recycling these materials seems to be a growing prospect. Similar is the situation regarding a number of other strategic materials. Table 1 presents a list of various raw materials that can be processed by hydrometallurgy. The extractive metallurgical industry is one of the prime targets for new laws introduced to combat environmental pollution. This is the overall picture against which the role of hydrometallurgy and its relative importance in extractive metallurgy must be assessed. While it is true that technically successful hydrometallurgical processes can be designed, they cannot compete commercially with the pyrometallurgical processes. The hydrometallurgy of copper typically illustrates this situation. In order to alleviate the sulfur dioxide

2

Hydrometallurgy in Extraction Processes

I I

PRECONCENTRATION

~----------------,

.----- --------,I

,..-------~-+----~

I

,---------.J

I

r---1

i I I

-----J

I

I I

I

I

I I I

I I I I I

:

I

REFINING

L..-------------- --------------- __ J

FIGURE I.

I

I I

----.1

Relationship between mining, physical beneficiation, pyromctallurgy, and hydromctallurgy. 1

problem associated with copper smelting technology, a number of elegant hydrobased flowsheets displaying masterly exposition of the capability of hydrometallurgy appeared on the scene for the treatment of relatively high-grade copper concentrates. Such processes, however, have failed to replace the pyroprocesses. In fact, pyrometallurgical processes themselves have been the subject of considerable development in response to the emission problem arising from the traditional smelting operation of copper. Intense smelting techniques developed by INCO, Noranda, and Mitsubishi have made significant inroads in copper metallurgy. The techniques have relegated copper hydrometallurgy to the treatment of secondary materials and low-grade ores. Hydrometallurgy is not the panacea for extractive metallurgy. It should be allowed to play a complementary role, not a competitive role, in operations of mineral beneficiation and pyrometallurgy. The more we are able to narrow the gap between geology, mineralogy, mining, engineering, metallurgical engineering, material science, and design engineering, the more we will successfully use hydrometallurgy in primary metal production and in other areas.

II. HYDROMETALLURGICAL PROCESSES Hydrometallurgy offers a number of significant advantages. There are also some disadvantages associated with hydrometallurgical operations. The limitations and advantages, however, need assessment for each individual application of hydrometallurgy. A. ADVANTAGES 1.

In comparing capital costs, expenditures of pyrometallurgy increase at a faster rate for

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TABLE 1 Some Different Forms of Raw Materials That Can Be Treated by Leaching Category Metals Oxides and hydroxides Complex oxides Sulfides Phosphates Silicates Selenides and tellurides Spent catalyst Slag and sludge

2.

3.

4.

5.

6.

7.

8.

Examples Precious metals, nuclear metals, oxide-reduced metals: copper, nickel and cobalt Bauxite, laterites, uranium orcs, zinc orcs and calcines, manganese orcs and nodules, rutile, copper oxide orcs Chromite, niobite-tantalite, pyrochlorc, ilmenite, wolframite, scheelite, titaniferrous magnetites Sulfides of the primary metals such as copper, nickel, lead or zinc, molybdenite Phosphate rock, monazite sand Beryl, spodumene, kaolinite, serpentine, zircon Anodic slimes from copper electrolysis Nickel, molybdenum, vanadium Copper converter slag, vanadium sludge, ferroalloy slag

a decreasing scale of operations than do hydrometallurgical processes. For example, at a capacity of 25,000 ton/year the capital cost of hydrometallurgy in most cases, becomes more favorable. Hydrometallurgical processes have the flexibility for treatment of complex ores and for production of a variety of by-product metals. Complex ores and concentrates in which a variety of recoverable metals are present can be effectively processed by hydrometallurgical routes. High overall revenues from by-products may have significant influence on overall process economics which would make the hydrometallurgical approach highly attractive. Penalties for hydrometallurgical production turndown are less severe. Economic penalty for production turndown favors the hydrometallurgical route. Since the labor component for hydrometallurgical plants can be as low as half that of pyrometallurgical plants, a significant unit price advantage can be realized when operating at less than full production, making these processes more competitive. There have been several instances where hydrometallurgical processing, totally or in part, has proved to be advantageous. For one thing, it can be used to treat low-grade ores that would be prohibitively expensive with pyrometallurgical refming methods. Low-grade uranium ores, for example, now are being exploited on a limited basis in Texas by methods that involve pumping sodium carbonate (leachant for uranium ore) directly into the deposit without bringing the ore to the surface. Field tests of this technique are also under way in Wyoming, New Mexico, and Colorado. Hydrometallurgical methods have successfully separated closely related metals, such as individual rare earths (zirconium from hafnium, and niobium from tantalum), from their ores. Hydrometallurgy is the only method for extracting metals that in some cases bypasses mineral during operations for enriching ores, such as crushing, grinding, and flotation. For instance, ore may simply be fractured and treated in place by aqueous solutions at a considerable cost savings. Hydrometallurgy can be less energy consuming when applied to low-grade ores or to ores at the mine site. Moreover, it operates at relatively low, often ambient, temperatures, compared with temperatures as high as 1500°C that are typical in pyrometallurgical furnaces. Much of the heat evolved in pyrometallurgy is difficult or impossible to recover. Hydrometallurgy has been suggested, too, as an alternative to traditional pyrometallurgical processes to reduce pollution, especially air pollution caused by smelter emissions of sulfur dioxide.

4 9.

Hydrometallurgy in Extraction Processes

Unlike smelting, hydrometallurgy offers high chemical specificity and flexibility that can be fme-tuned to take advantage of every nuance in chemical behavior. This is particularly true for chloride metallurgy.

10. In hydrometallurgical plants, solutions and slurries generally are transferred easily in closed pipeline systems. In pyrometallurgical processes, on the other hand, molten slags and mattes must be transferred from one furnace to another in heavy refractorylined ladles, a process which is inconvenient and costly. In addition, because of reduced gas solubility. the so2 that usually saturates such molten materials is emitted as the materials cool during transfer. B. DISADVANTAGES 1. 2. 3.

4. S.

Hydrometallurgical plants require sophisticated control schemes to maintain satisfactory operation. Hydrometallurgical plants operate more like chemical plants with the philosophy of control associated with chemical plants. There is no economic gain in substituting a pyrometallurgical plant, processing a reasonably high-grade resource, with a hydrometallurgical one. From the point of view of consumption of process materials, fuel, and electric power, hydrometallurgical processes are much more demanding than the pyrometallurgical ones because the latter use mostly oxygen of atmospheric air as a reagent and sulfur present in the ore as a source of heat. Engineering of hydrometallurgical plants is more complex and requires the full understanding of scaleup relationships as well as processing requirements. Hydrometallurgical processes can often generate significant amounts of liquid or solid wastes that may pose serious disposal problems. Table 2 compares the characteristics of pyro- and hydrometallurgical processes.

III. PHYSICOCHEMICAL PRINCIPLES OF HYDROMETALLURGICAL PROCESSES•-:zo A. GENERAL The difference between pyrometallurgy and hydrometallurgy can generally be appreciated on the basis of thermodynamic and kinetic considerations. It is true that thermodynamics is important as the foundation of all processes. In hydrometallurgy, thermodynamics can frequently be used to ascertain process limitations and can be quite useful in experimental planning and process selection and evaluation. A high degree of predictive accuracy is generally not needed at this stage of process development. There are a number of areas which can be treated by thermodynamic analysis. Some of the important areas are ( 1) calculation of the solubility of simple and complex salts and gases, including estimation of loading limits in leach liquors; (2) estimation of vapor pressures of volatile components; (3) determination of the extent of reaction under various conditions of temperature, pressure, and concentrations; and (4) calculation of distribution coefficients for ion-exchange processes. lbermodynamics, however, is particularly important for all high-temperature processes because in these temperature regimes the rates of reaction are generally fast enough not to constitute major governing parameters. Hydrometallurgy distinguishes itself from pyrometallurgy in being a low-temperature process. It is, therefore, much more susceptible to the relevant chemical kinetics than to thermodynamic considerations which dominate pyrometallurgy. Common ground between hydrometallurgy and pyrometallurgy does exist, however, as much of the chemistry carried out in both types of processes essentially involves systems that are heterogeneous. Mass transfer and phase separation are, therefore, common critical parameters with regard to space-time yields within processing equipment.

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TABLE 2 Comparison of Some Characteristics of Hydro- and Pyrometallurgical Processes

Terms of reference Treatment of high-grade orcs Treatment of low-grade orcs

Treatment of complex orcs

Pyrometallurgy More economical Unsuitable because large amounts of energy is required to melt associated gangues Unsuitable because separation is difficult

Process economics

Best suited for large-scale operations requiring a large capital investment

Treatment of secondary resources Separation of chemically similar metals Treatment of sulfide orcs

Unsuitable in most cases Not possible

Reaction rate Throughput Materials handling Environmental pollution

Solid residues

Toxic gases

Process feed

Operational feature

HydrometaUurgy

l...ess economical Suitable provided a selective leachant can be used Possesses the flexibility for treatment of complex orcs and for production of a variety of by-products Can be used for small-scale operations; it has a lower capital investment sensitivity to plant size in lower plant capacity ranges Suitable Possible

Sulfur dioxide is generated which in Can be treated without generating sulfur dioxide; the need to make high concentrations must be converted to sulfuric acid and in low and market sulfuric acid is elimiconcentrations must be disposed of nated; possibility exists to recover in other ways. For the former, a sulfur in elemental form market has to be found and for the latter, methods are available but are expensive Rapid as high temperatures are inGenerally slow as all operations are volved carried out at relatively low temps Very high unit throughput Small throughput for production unit Handling of molten metals, slags, Solutions and slurries can be easily mattes, etc. appears problematical transferred in closed pipe lines Problems of waste gas and noise; No atmospheric pollution problems. many processes emit large Problems of waste disposal, problems of wastewater; no dust probamounts of dusts which must be lem because materials handled recovered to abate pollution or because the dust itself contains valuusually are wet able metals Many residues, such as slags, are Most solid residues are in fmely dicoarse and harmless, so that they vided states: in their dried conditions create dust problems, in their can be stored in exposed piles without danger of dissolution; piles wet conditions release metal ions may be unsightly and esthetically in solutions; this may contaminate the environment unacceptable Many processes do not generate Many processes generate toxic gases; expensive systems are regases, and if they do, reactors can quired to combat their oonrclease be made gas-tight easily to the atmosphere Suitability for inhomogeneous feed Sensitivity to major variations instead; the process feed, in general, has to be more uniform and more attention to composition and mineralogy is required Represents uniquely metallurgical In general, more sophisticated conplant features; engineering is not trol schemes are required to maintain satisfactory operation; plants considered complex operate more lilce chemical plants with control philosophy associated with chemical plants; plant engineering is more complex

6

Hydrometallurgy in Extraction Processes

TABLE 3 List of Oxidizing Agents in the Order of Their Increasing Oxidation Potentials in Acidic Media5 Agent

Half cell reaction

Potential (V)

fel +

fel+ + e- -+ fe' + NO,- + 4 H• + 3 e- -+ NO + 2 H,O MnO, + 4 H• + 2 e- -+ Mn,. + 2 H,O O, + 4 H• + 4 e- -+ 2 H,O Cr,o,'- + 14 H• + 6 e- -+ 2 Cr'+ + 7 H,O Cl, + 2 e- -+ 2 a CIO, - + 6 H• + 6 e--+ Cl - + 4 H,O Mn04 - + 8 H• + 5 e- -+ Mn'• + 4 H,O H,O, + 2 H• + 2 e--+ 2 H,O so,'- + 2 H· + 2e- -+ so.z- + H,o

0.77 0.96 1.2 1.23 1.33 1.35 1.45 1.49 1.77 1.81 2 2.07

HNO, MnO,

o,

K,Cr,O,

ct,

NaCIO,• KMnO. H,O, H,SO, K,s,o.

o,

s,o.'- + 2e- -+ 2 so.'o, + 2 H• + 2 e--+ 0 2 + H,O

B. DISSOLUTION ASPECTS Leaching or dissolution is the frrst prerequisite of any hydrometallurgical process. Leaching is the term applied to the process of recovering a metal from an ore by a solvent or lixivant. In general, leaching is applied only to ores that are not adapted at an equal or greater profit by the longer established methods, such as gravity concentration, flotation, or smelting. Various aspects of dissolution are dealt with in the following sections. 1. Solution Chemistry The two fundamental parameters that hydrometallurgists can use to control the behavior of metals in aqueous solutions are pH and the oxidation potential of the solution. The thermodynamic behavior of an aqueous system is determined by these two parameters, together with the concentrations or activities of dissolved species. The pH of the solution, of course, determines the acid-base character of the system and is the main parameter controlling the solubility of oxidized or hydrolyzed metal species. A large number of hydrometallurgical processes are critically dependent on the control of pH for their successful operation. This control may be achieved by deliberately adding acid or base during a reaction to consume hydroxyl or hydrogen ions produced by the reaction or by designing the chemistry of the system so that it is self-buffering. An example is aluminum hydroxide. It is an amphoteric hydroxide and behaves as an acid. Al(OH)l(•> +:t AIO(OH)Z(-v

+

H+

=

log K 1

+

log K2

-

2 pH - log PHas

+

4 log &a- - log 'YPbC!Has

(15a)

Giving log K2 a value of 1. 7 and 4 log Ia- a value of 6. 7, the value of log (PbC:J/-) from Equation 15a, neglecting the last term, is 2.2, representing a ridiculously high lead concentration in solution. Of course, the solution would form PbC1 2 crystals instead, but the important factor would be that the combination of complex ion formation and high chloride ion activity would cause galena to decompose to 1 atm H2S and yield a PbC1 2 saturated solution in a brine at pH 0, while in the absence of chloride ions, this equilibrium would yield not more than 10- 6 M lead in solution. The impact of chloride complex ion formation is also felt under conditions of oxidative leaching. For example, in ferric or cupric chloride of chalcopyrite, the final reaction of leaching may be written:

At equilibrium, the copper concentration in solution will be determined by: log(CuC1 2-)

=

41 log K + 43 log Ia-

-

41 log ~p.z.

- log 11cua2-

(16a)

The value of K is calculated at about + 1.8. The object of the leach is to obtain a high concentration of CuC1 2 - at low Cu2+, i.e., most of copper in solution should be cuprous. This is thermodynamically favored only at high chloride ion activities; for example, under standard conditions (1 M Cl-) the equilibrium of Equation 16a is at a CuC1 2 -/Cu2 + ratio of about 2.8, but the solution is saturated in CuCl at a concentration of only 0.06 M CuCl 2 - . At a Cl- activity of about 50, CuCl is highly soluble and the equilibrium of Equation 16a is at a CuCl 2 -/Cu2 + activity ratio closer to 1()3 or 10". In principle, therefore, the leach could be carried out to the point of virtual disappearance of Cu2 +. A more significant application of complex formation by chloride ions lies in the leaching of gold. It has been established that gold is leached by ferric chloride solutions if ferrous chloride is essentially eliminated with 1 to 10 ppm of excess chlorine and no other chlorineconsuming metal is present to generate ferrous chloride (i.e., in the case of residues from leaching of copper concentrates by ferric chloride). Anionic chlorocomplexes account for the solubility of "insoluble" chlorides (PbC12 , AgCI, CuCI) in strong brines at both ambient and elevated temperatures. d. Hydrochloric Acid The properties of strong brines are of considerable importance to hydrometallurgists. Regarding strong brines, it is useful to consider hydrochloric acid fll'St. The importance of

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HCI to hydrometallurgy needs no special emphasis. While dilute solutions of 1M are normal with active coefficients of both ions around unity, strong solutions of 2 M have entirely different properties, with the mean activity coefficients of the ions rising to about 70 in saturated (19.2 M) HCI solutions. These numbers translate into mean hydrogen and chloride ion activities on the order of 1300. The same information is contained in a thermodynamic statement describing the reaction between HCI and a metal sulfide (i.e., ZnS): ZnS + 2 H+- -+ CuS Cu+ + HCN-+ CuCN + H+ Tio>• + 2 H20-+ Ti0(0H)2 + 2 H+ Ni>+ + H2 -+ Ni + 2 H+ eu>+ + Fe-+ Cu + Fe2 • Ni'+ + 2 e- -+ Ni 0

reaction is the discharge of sulfate ion, but the sulfate radical is not stable and immediately reacts with water in the electrolyte to form sulfuric acid and oxygen according to the following reactions: (64)

In the electrolytic decomposition of a solution of CuS04 , the solution will be depleted of one equivalent of copper for each equivalent of copper deposited at the cathode. The solution will also be enriched by the formation of one equivalent of sulfuric acid, and one equivalent of oxygen will be liberated at the anode. Since sulfuric acid is regenerated, the copper electrowinning method is adapted to a closed circuit. Table 5 shows at a glance the various methods available for the recovery of metals or their compounds from the leach liquors.

IV. UNIT PROCESSES IN HYDROMETALLURGY The physicochemical principles covered in the previous section made reference to hydrometallurgical processes without putting them in proper sequence of unit operations as they figure in a general hydrometallurgical process flowsheet starting from the ore to final product stage. This section does not, however, make a disjointed presentation. It treats the various hydrometallurgical processes or unit operations not in an isolated way but in the order in which they are fitted into the entire stream from the very beginning of the operation to the end. An articulate definition of the principle of unit operation came from Arthur D. Little. 24 "Any chemical process, on whatever scale conducted, may be resolved into a coordinated series of what may be termed unit actions, as pulverizing, mixing, heating, roasting, absorbing, condensing, lixivating, precipitating, crystallizing, filtering, dissolving, electrolyzing, and so on . The number of these basic unit operations is not very large, and relatively few of them are involved in any particular process. The complexity of chemical engineering results from the variety of conditions as to temperature, pressure, etc., under which the unit actions must be carried out in different processes and from the limitations as to materials of construction and design of apparatus imposed by the physical and chemical character of the reacting substances."

This definition clearly covers the field of extractive metallurgy in general and that of hydrometallurgy in particular. Hydrometallurgical processes can be divided into four main units of operations. In the first, the ore or feedstock is prepared and separated from the inert material. In the second,

36

Hydrometa/lurgy in Extraction Processes

dissolution of mineral-bearing fractions is effected through the use of an aqueous solvent. In the third, purification and concentration of the separated fractions are carried out so that in the final unit operation the desired form and purity of product can be obtained. Since no two extractive metallurgical projects are identical, it is not surprising that a variation in the combination of the unit operations selected is made from project to project. The hydroprocesses have been limited largely to those resource materials which because of grade or composition do not lend themselves to economic methods of concentration and pyrometallurgical extraction. As an example, reference is made to copper-bearing minerals. The hydrometallurgical methods would apply more often to these groups of copper-bearing materials classed as low-grade native coppers, the oxidized, or the mixed oxide-sulfide minerals. These, as a whole, do not lend themselves to other methods of concentration, either because treatment costs are too high or because the overall recovery of the contained copper is too low. In all hydrometallurgical processes, the feed used (e.g., ore) is a mixture of many components. The purpose of the process is to separate selectively one or more constituents of this mixture and then convert it into the required product. All hydrometallurgical processes then involve separation processes, and, as a broad classification, one can divide the various stages into separation and nonseparation processes. By defmition, a separation process transforms a mixture of components into two or more product streams which differ from one another in composition. Take, for instance, a case where separation of two key components, say A and B, in two product streams, X andY, is desired. In this case, the degree of separation between A and B that can be achieved by any particular separation process may be quantified by a separation factor, aAB, which can be defined as:

That is the ratio of the mole fractions of the key components in the product streams A and B and X and Y. Nonseparation processes include crushing, grinding, comminution, and conveying. It turns out in terms of the number of different processes involved (but not fmancial investment) that this is a minor class. The category of separation processes is by far the larger group and includes, for example, filtration, flotation, centrifugation, leaching, ion exchange, liquid extraction, precipitation, and electrowinning. The hydrometallurgical unit operations and their interconnections in a general hydrobased process flowsheet are shown in Figure 13.

A. ORE PREPARATION In terms of capital investment and operating costs, the ore preparation unit operation is frequently a major part of the process. The operating efficiency of this unit operation is a major factor in overall recovery and in subsequent solid entrainment problems. Ore preparation normally involves size reduction, solid-liquid separation, and, if flotation is used for separation, dewatering. An example involving all three operations is shown in Figure 14. The example typically represents the ore preparation for a sulfide copper ore containing about 2.0% copper. Size reduction in two or three stages is used frequently to obtain particles in suitable size range. In the given illustration, an intermediate washing stage is shown. This is used to separate slime and siliceous material. Gravity separation can be used. After washing, the ore passes through secondary and tertiary crushing stages. The crushed ore is then ground to reduce the particle size further so that a chemical separation using flotation can be achieved. In the example shown, a two-stage flotation is used with further size reduction between the stages; this is mainly done to improve the selectivity of the overall flotation process. The final tailings from the secondary flotation contain a very low con-

DOWN

PURE METAL COMPOUND

OR STREAM

BACK- END

t

FIGURE 13.

I

IMPURE METAL

Unit processes in hydrometallurgy; operational sequence.

METAL OR IT'S OXIDE

I

I

i

OR UP STREAM OPERATION

t

FRONT- END

:. ___________ -

.----------,

PURIFIED SOLUTION

METAL COMPOUND

ADSORPTION ON CHARCOAL

LEACH I LIQUOR

ORE

PURE METAL

CM

....

r.... ~

WATER

I

SECONDARY FLOTATION•

~

REGRIND

I· TAILINGS

OESLIMING

I

I

PRIMARY FLOTATION GRINDING

I

1:= WATERI

STORAGE

I FINE ORE

TERTIARY CRUSHING

l ____ ,,. .... ,, ..

H

SAND

SECONDARY CRUSHING

Schematic flow diagram for conversion of ore to concentralc.

WATER

I

SLIME

PRIMARY CRUSHING

+

FIGURE 14.

- - I CONCENTRATE I

FINAL ~ TAILINGS

RUNOF MINE ORE ~ STORAGE

~

§

"t:: ~

~... s· ::s

~

~ s·

i§=

s

t

~

1M 00

Volume I

39

centration of copper and are a waste stream from the process. The concentrate containing some 20% or so of copper is, in this case, dewatered and stored for subsequent processing. B. LEACHING After ore preparation, separation according to chemical composition usually follows. A variety of process routes are possible. In hydrometallurgical processes, the next stage is to dissolve the constituents of the ore to form a solution. The process is leaching, a solidliquid mass transfer process. Leaching may be carried out at ambient conditions or at elevated temperatures and/or under pressure. The process conditions will depend on chemical reactions taking place. The object in all cases is to produce metal ions or complexes which can then be extracted selectively from the solution. I. Leaching Reagents Any reagent meant to be used for leaching must have the following qualifications: 1.

2. 3.

It must dissolve the ore minerals rapidly enough to make commercial extraction possible, and it should show chemical inertness toward gangue minerals. In situations where gangue minerals are attacked, an excessive amount of the solvent is consumed and the leach liquor fouled with impurities to an undesirable extent. It must be cheap and readily obtainable in large quantities. If possible, it should be such that it can be regenerated in the subsequent processes following leaching.

Besides water, which is used to leach soluble sulfates or chlorides, many different solvents (see Table 6) have been used for leaching purposes. Some have been very successful; others have not. The following list includes some of the most important ones.

a. Acids Sulfuric, hydrochloric, and nitric acids have been widely used. Sulfuric acid is the cheapest. There are, however, some problems that are associated with the acid. When the raw material contains appreciable amounts of iron, formation of ferrous sulfate creates a disposal problem. It cannot be discharged into streams and therefore must be either neutralized or crystallized and decomposed; both are costly operations. A problem associated with the acid is that when it is used on ores that contain radium, such as uranium ores, the residue poses a health hazard because of its radium sulfate content. For this reason, nitric acid has been suggested as a replacement for sulfuric acid in uranium extraction operations. In dissolution with nitric acid, radium goes into solution along with uranium and can be disposed of in a controlled way so that the tailings from such operations will be radium free. Phosphate fertilizer producers face a similar situation. The phosphate rock typically contains 100 to 200 ppm of uranium which, during leaching by sulfuric acid to make wet-process phosphoric acid, is dissolved by the acid while radium stays with the residue gypsum. In Europe, nitric acid is used for leaching phosphate rock to produce nitrophosphate fertilizers, thus eliminating stockpiling of radium-containing gypsum. Processes that are under development will remove both uranium and radium from such fertilizer solutions, thereby preventing it from accompanying the fertilizer into the soil. Sulfuric acid also must compete with basic reagents such as ammonium hydroxide or sodium carbonate when ore contains appreciable amounts of acid-consuming gangue minerals, such as limestone or dolomite. It becomes more economical to use basic reagents despite their higher cost, because they are inert to such minerals. Hydrochloric acid has been used for the beneficiation of ilmenite (Fe0-Ti02) to obtain synthetic rutile from which a number of titanium products are produced. Nitric acid has gained prominence in processing of nuclear materials ranging from fuel (U, Pu) to structural

40

Hydrometallurgy in Extraction Processes TABLE 6 Classification of Leaching Reagents Reagent

Category Acids

Dilute H,S04 Dilute H2S04 plus oxidant Concentrated H,S04 Nitric acid

Alltalies

Salts

Aqueous chlorine Water

Hydrofluoric acid Hydrochloric acid Sodium hydroxide Sodium carbonate Ammonium hydroxide plus air Ferric chloride/sulfate Cupric chloride Sodium or potassium cyanide plus air Ferrous chloride plus air Aqueous chlorine, hypochlorous acid, hypochlorite Water

Examples Copper oxide, zinc oxide Sulfides of Cu, Ni, and Zn; uranium oxide ore Copper sulfide concentrate, laterites Sulfides of Cu, Ni, and Mo. Mo scrap, uranium concentrates, zirconium oxide Columbite-tantalite ore Ilmenite, nickel matte, reduced cassiterite Bauxite Uranium oxide, schcelite Nickel sulfide, copper sulfide, reduced laterite Base metal sulfide concentrates Base metal sulfide concentrates Gold and silver ores Nickel sulfide Sulfide concentrates of Cu, Ni, Zn, Pb, Hg, and Mo as well as reduced laterite Sulfates and chlorides, sodium vanadate, -molybdate, -tungstate, -stannate

(Zr) to reprocessing of spent fuel. The nitric acid can also be used for processing metal sulfides. Fi~ 15 illustrates its use for leaching chalcopyrite.

b. Allcalies Ammonia, sodium hydroxide, and sodium carbonate solutions are important examples of leaching with alkalies. Ammonium hydroxide plus oxygen or ammonium carbonate plus ammonium hydroxide is suitable for leaching native copper and copper carbonates. Sodium hydroxide is used in the Bayer process for the purification of bauxite. It is an excellent example of large-scale nonoxidative leaching. Sodium carbonate solutions are employed in leaching oxidized uranium ores.

c. Salts In the category of salts, special mention should be made of sodium and potassium cyanides, ferric and cupric salts. In the presence of oxygen, sodium or potassium cyanide dissolves gold and silver and provides an excellent example of oxidative leaching. Sulfate and chloride salts of iron and copper have found extensive applications, particularly in leaching operations involving sulfidic resources.

2. Leaching Methods Leaching methods obviously bear direct relevance to the nature of the mineral deposit. The flow of ore in leaching practice is illustrated in Figure 16. Mineral deposits by their very nature of occurrence either have to be left in place or can be transported for leaching. In the category that is transported for leaching, three different ore types can be identified: low-grade ore, direct leaching ore, and high-grade ore. Hydrometallurgy is unique in its application to low-grade ores which cannot be beneficiated economically. Direct leaching ores are those which have sufficient value that they can be sized and subjected to leaching. Bauxites treated by the Bayer process, gold ores suitable for slime leaching, nickel laterites for nickel and cobalt recovery, and acid and base leaching of uranium ores are among the important examples of direct leaching ores. Then there is the third category, high-grade ores

Volume I

41

CHALCOPYRITE NO LIQUID SOLID

ACID MOTHER LIQUOR

CELL ACID

WATER

AIR

Zn -SULFATE SOLUTION COPPER SlA...FATE

THERMAL HYDROLYSIS PRECIPITATION

SOLUTION COPPER METAL

GOETHITE

FIGURE 15. Flowsheet for nitric acid process. 23 MINERAL DEPOSIT

•

'

~NSITU ~EACHING I• _

t

LOW GRADE ORES AT MINES IU,Cul RECYCLED-I SOLUTION •

LEACH LIQUOR

I

SEPAtTION

t

ORES REQUIRING TRANSPORTATION TO LEACHING SITES

f

LOWER GRADE ORES IU,Cu,Ao,Aul

DIRECT LEACHING ORE (Cu,U,AI,Ni, Mo etc . }

HIGH GRADE ORE (Cu,Zn,AI,Ni,W, Mo etc.)

I

METAL CONCENTRATE METAL METAL METAL

FIGURE 16. Flow of ores in leaching practice.

which may be economically beneficiated to produce a concentrate prior to leaching. Examples of such concentrates are iron-nickel sulfide concentrate in the case of the Sherritt Gordon process, copper sulfide concentrate used in the CLEAR process, roasted zinc sulfide concentrates (standard roast-leach-electrowinning practice for zinc sulfides) in the case of the new Sherritt Gordon pressure leaching process, and flotations and gravity concentrates for gold recovery by cyanidation. Recovery of gold from concentrates and roast-leach-electrowinning applied to zinc represent commercial processes used throughout the world. The application of hydrometallurgy to the treatment of conventional base metal sulfide concen-

trates has not emerged as a promising new technology. Having discussed the various types of ores against the background of leaching, it is relevant to consider the various leaching methods as classified in Figure 17.

42

Hydrometallurgy in Extraction Processes LEACHING METHOD

I

'l

l

PERCOLATION LEACHING

IN SITU LEACHING

l

HEAP OR DUMP LEACHING

l

VAT LEACHING

FIGURE 17.

AGITATED LEACHING

~ THIN LAYER LEACHING

~

SUME(PULP) LEACHING

j PRESSURE LEACHING

l

BAKING PROCESS

Classification of leaching methods.

a. In situ Leaching In situ leaching is neither new nor unusual and is recorded to have been used on a small scale in Hungary during the 15th century. The in situ leaching method is known by other names such as leaching in place and solution mining. In this case, the ore is not mined at all, but is leached where it occurs. The ore is first fractured by explosives, and leaching is effected by alternate and intermittent circulation, first of air, followed by water and spent solution from the precipitation process. Natural drainage is relied on for accumulating the leach solution, either through the construction of drainage tunnels under the ore body or, in the case of exhausted or mined-out ore bodies, lower workings in the mine. The solution is pumped to the surface and processed for metal recovery. The efficiency of the process is difficult to evaluate in view of the variables of unknown tonnages and content either before or after leaching operations. In situ leaching has been applied successfully in a number of cases, both on mined-out ore bodies or on ore bodies too low in grade to be otherwise considered for economic treatment. The main difficulties in the process arise from channeling, which interferes with the even distribution of leach solution over the ore, and a later possibility of slimes and accumulated salts, in time filling the openings and thus interfering with solutionore contact. The advantages of solution mining are (I) less surface disturbance and environmental impact than conventional mining, beneficiation, and smelting; (2) lower capital and operating costs; (3) potential economic advantage for recovery of metals from materials that could not be treated by conventional methods; and (4) increased ore resources. 1be disadvantages are (l) complex technology relative to chemical and physical features, (2) testing short of field operation is difficult, (3) groundwater contaminants may result, and (4) a detailed data base has not been established commercially. In situ leaching of copper ores has received renewed interest, and the ore deposits amenable for treatment by this process have been classified into three general groups: (l) surface dumps or heaps located above the natural water table; (2) deposits located below the natural water table but accessible by conventional mining techniques; and (3) deposits located below the natural water table and too deep for economic mining by conventional methods. 1be first and second type of ore deposits have been practiced in commercial in situ leaching for the extraction of copper. For the third type of ore deposits, the technology is not available yet for in situ operations. b. Heap Leaching The heap-leaching method is probably one of the oldest, if not the oldest of the methods for recovery of copper from ores. It is said to have been used as early as 1752 in recovering copper from cupriferrous pyrite in Spain. This leaching method is very similar to in situ leaching in that natural oxidation by air, water, and ferric salts is relied on to convert sulfide minerals and oxide minerals to water-soluble sulfates. Actually, dilute H2S04 is sprayed, and as it percolates downward; the oxygenated environment begins the oxidation of metal

Volume I

43

sulfides. Eventually, the liquor becomes an acidic solution of ferric sulfate which is selfbuffering at about pH 2, due to precipitation and redissolution of basic ferric salts. The recovery of copper from low-grade sources is a typical example of application of heap leaching. Ores which must be removed from the mine but which are too low grade to treat at a profit by any other method are treated by heap leaching. Heap leaching differs from that of in situ leaching in that the ore is removed from the mine and is so arranged as to permit more effective contact of both oxidizing agents and lixivant with the minerals. Thus, maximum extraction of copper values can be obtained. Possibly, there is one limiting factor in that the ores should contain enough sulfide to oxidize readily, otherwise many years may be required before oxidation of the heap approaches completion. Since the oxidation reactions are exothermic, the heat liberated facilitates continued oxidation. Large tonnages are involved; consequently, mining costs have to be low, since the copper will necessarily be recovered over a long period. For heap leaching, a ground with a slight slope is selected. The site is cleared of any growth and is then rolled and packed with clay or slimes to make it as near waterproof as possible. Large boulders of ore are selected for building culverts for drainage and ventilation purposes, and the drainage is directed to a common point. The ore, without crushing or other preparation, is now carefully piled on this prepared area. In some cases, it may be found advantageous to classify the ore to the extent of placing the coarser ore on the bottom of the heaps and the fmer on the top, in order to expedite the circulation of air and solution. The top of the ore piles is provided with distributing trenches. Leach solutions are sprayed evenly over the section to be leached and are allowed to drain through the heap into the collecting basins. The copper, dissolving as copper sulfate, is recovered by cementation on detinned scrap iron. The spent solution, plus makeup water to take care of losses through evaporation, is recycled to the heap. The iron salts present in the precipitated solutions make them more effective than water alone. The cycle consists of alternate periods of leaching and oxidation. In a large heap, certain sections may be undergoing leaching action while other sections are permitted to oxidize. The method is quite simple, but the reaction is very slow. As years are required to obtain a commercial extraction, the heap-leaching method can be profitably applied only to very large tonnages.

c. Dump Leaching A leaching method which is very similar to heap leaching is dump leaching. It differs from heap leaching in that few if any special efforts are made to encourage recovery of copper from the rock. The difference between the two is also in the composition of the ore. The ore for the dump leaching is predominantly composed of sulfide-bearing minerals, while the ore for heap leaching is oxide-bearing. The low-grade ore is brought by trucks or by conveyer belt onto an impermeable site and deposited to form truncated cones. Most of the dumps are formed close to the mine site using the natural formation of the terrain. Often steep-sided valleys are filled or the waste ore is dumped on the hillside providing easier drainage. Larger dumps may be as high as 200 m, about 80 m wide at the top and 250 m at the bottom, containing 50,000 to 300,000 ton of the ore. The leach solution, water or dilute H2S04 solution, is sprinkled on the top of the heap; then it trickles through the ore body and is collected at the bottom of the heap. Figure 18 illustrates the general flow of the solution to the dump, to a holding basin, and to copper extraction. The conditions that are essential for leaching to occur and continue within the dump are (I) effective air circulation, (2) good bacterial activity, and (3) uniform solution contact with the particle. There are quite a few unknowns in dump leaching for any given dump. Some of the major ones are (I) a knowledge of air circulation relative to the dump configuration, (2) the hydrology in terms of channeling and bypass, (3) the effect of fines and precipitated salts, and (4) effect of weathering as a function of time. The heap- and dump-leaching techniques are

44

Hydrometallurgy in Extraction Processes

LEACH LIQUOR

STO~AGE BASIN

MAKE UP ACID AND WATER

COPPER FREE SOLUTION PURE COPPER SULFATE SOLUTION COPPER

FREE

SOLUTION

SPENT

COPPER METAL

ELECTROLYTE

IRON SALTS FIGURE 18.

Copper dump leaching.

highly effective and economic. The largest copper producer of the U.S., the Phelps Dodge Company, recently announced that they can produce 454 g of copper for about 30 cents from their waste ore by leaching-solvent extraction-electrowinning. In talking about heap- or dump-leaching technique, reference must be made to one of a group of techniques known generally as bacterial leaching processes. Bacterial leaching techniques have grown into a technology that is now known as biohydrometallurgy. Bacterial activity can be described by the direct and indirect leaching of metal sulfides. In the direct leaching process, the sulfide minerals are oxidized to metal sulfate, or it can be further given for pyrite oxidation: MS

+

2 02

Bacteria

MSO.

(65)

Both, F~(S04) 3 and H2S04 are participating in the indirect leaching process. Since Fel+ is an important oxidizing agent, it can oxidize metal sulfides to metal sulfates: MS

+ 2 Fel+ - M2 + + 2 Fe2+ + S

(67)

The role of bacteria in the indirect leaching mechanism is to reoxidize Fe2 + to Fe3 + and elemental sulfur set free in Equation 67 to H 2S04 : fe2+

Bacteria

fel +

+

e

(68)

Volume I S +

H20

+ I. 5 0 2 -

_B_ac_t_en_·a--+ H SO 2

•

45 (69)

If any metal oxide is associated with the ore, it gets solubilized by H2S04 : (70)

An additional way by which bacteria can assist leaching can be seen by referring to the galvanic mechanism of leaching. It is known that pyrite and chalcopyrite have rest potentials of 0.6 to 0.5 V, respectively. In acidic media, these sulfides in physical contact couple by forming galvanic cells, with chalcopyrite having lower potential as the anode and pyrite as the cathode. As a result of this interaction, chalcopyrite will be oxidized, and pyrite passivated, and as such, it will remain essentially unaffected: Anodic reaction: CuFeS 2 -+ Cu 2 + + Fe2 + + 2 S + 4 eCathodic reaction: 4 H + (on the surface of pyrite particles)

+

4 e- -+ 2 H2

(71) (72)

The role of bacteria here is to accelerate the galvanic reactions by continuously oxidizing the elemental sulfur film from the surface of the chalcopyrite particles (Equation 69). A typical example of the large-scale use of bacterial leaching is that of copper. All copper producers now have an integrated heap-dump or in situ leaching operation supplementing their mining and/or processing activities. Therefore, the relative importance of bacterial leaching in copper processing industries has increased in the past decade. Chalcopyrite leaching by bacterial proceeds according to the following reaction:

The above reaction is a typical example of bacterial direct mode of O)iidation. The indirect mode can be illustrated by the oxidizing power of ferric ion: (74)

The bacteria will reoxidize ferrous ion to ferric ion according to Reaction 68 and the elemental sulfur to sulfuric acid as shown in Equation 69. The microorganism, Thiobacillus fe"ooxidans, plays an important role in biohydrometallurgical treatment of sulfide-bearing minerals. In naturally occurring heap- and dump-leach processes, there are other microorganisms present. For example, T. thiooxidans which is morphologically similar toT. fe"ooxidans, is often found associated with leach solutions. The fundamental difference between the two species is recognized as the inability of T. thiooxidans to oxidize ferrous ion and insoluble heavy metal sulfides. Table 7 illustrates bacterial-assisted industrial copper-leaching operations. Another large-scale application is that of uranium extraction. Torma26 has suggested that uranium extraction can be expressed by the indirect mode of bacterial action, in which FeJ+ oxidizes the insoluble tetravalent uranium to its hexavalent state that is soluble in acid leach media.

46

Hydrometallurgy in Extraction Processes TABLE 7 Industrial Copper-Leaching Operations16 Mine (location)

Type of leaching

Total Cu ('ll>)

Cu-production Bacteria present

Duval, Copper Basin (U.S.) Duval Esperanga Mine (U.S.)

Dump Dump

0.31 0.15--0.2

Bluebird Mine, Miami (U.S.) Degtyansky (U.S.S.R.) Kosaka Mine (Japan) Rio Tinto, (Spain) Canaea (Mexico) Santa Domingo (Portugal)

Heap Dumplin situ

0.5

Unknown

Variable

In situ

0.15--0.25

Dump Dumplin situ

Variable Variable Variable

T. fe"ooxidons T. fe"ooxidons T. fe"ooxilllllrs T. fe"ooxilllllrs T. fe"ooxilllllrs

In situ

Unknown

Thiobacillus ferrooxidans

(tlyCU')

2300 2500 6800 900 800 8000 9000

670

The ferrous sulfate formed according to the above reaction will be reoxidized to ferric sulfate by bacteria according to Equation 68. Ferric sulfate is usually obtained from the oxidation of pyrite which is present in the uranium ores according to Equation 66. Studies have also shown that direct oxidation ofUH is also possible by T.ferrooxidans. Biohydrometallurgical processes for the extraction of uranium have been well established. For example, at Denison Mines in Canada, in situ leaching has been commercially practiced since the early 1960s.

d. Vat Leaching Vat leaching is a simple and efficient mode of effecting adequate contact between the solute and aqueous solvent. It utilizes the countercurrent principle, with the ore to be leached remaining stationary, as differentiated from most countercurrent methods, where all materials are moving. The ore, confined in a rectangular leaching vat is successively treated with an increasing concentration of leach solution, which may be added as a continuous flow or on a batch basis. Thus, maximum leaching strength first comes in contact with minimum metal concentration in the ore, and as its leaching potential diminishes, it comes in contact with increasing metal concentration in the ores. In the event of the leach solution entering the vat at the bottom and overflowing at the top, the system is known as upward percolation. It is more common if the flow of solution is continuous through a series of vats. If the solution enters either at the top or the bottom, but is withdrawn from the bottom, the system is known as downward percolation, which is the more common method if the batch method is used. A predetermined filling and soaking period, followed by a drain, complete a single cycle. This drained solution then advances to the next vat, and the process is repeated. Solutions are referred to as fust advance, second advance, etc., to designate the position in the complete leaching cycle. Dependent on the solubility of copper in the ore, a single charge of ore could be given any number of successive leaches with increasing concentrations of leach solution; if, for example, six cycles were necessary for optimum extraction, six charges of ore would be undergoing simultaneous leaching treatment. It is also important to separate the occluded leach solution from the final leach residue (it may represent as much as 30% of the total copper dissolved). A countercurrent principle of washing is employed. This method, essentially the same as that described for countercurrent leaching, may also be effected on either a continuous or batch basis. In either case, the residual gangue and its occluded solution are successively treated with wash solutions of decreasing copper concentration. The final step is effected with copper-free wash solution. The process consists, accordingly, of a series of dilutions, the effectiveness dependent on the degree of diffusion of the remaining occluded solution and incoming wash solution in each other.

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41

e. Agilaled Leaching The agitated leaching method is limited to those ores, or to that portion of those ores, that because of particle size do not permit free passage of leach solution through the interstices between the ore particles and, thus, cannot be effectively treated by heap or vat leaching. In this process, dissolution is effected by keeping the finely divided ore particles in suspension in the solvent. This is usually carried out in an agitated vessel, and the solids are dispersed in the liquid either by gas injection or by a rotating impeller. Dissolution of the ore may be either on a batch basis (a given charge of ore is agitated with required amount of leach solution until achieving optimum extraction) or on a continuous basis (ore and leach solution are added simultaneously. However, instead of dissolution in a single vessel, the ore-solution mixture passes from vessel to vessel in series until achieving optimum extraction. Pachuca vessels, with high lifts, are convenient for this type of agitation. A pachuca is a cylindrical tank with a conical bottom and contains a pipe that is coaxial with the leaching tank and is open at both ends. Compressed air is introduced, at the lower end of this pipe, which behaves as an air lift. The density of the pulp within the pipe is less than that of the pulp surrounding it, because of the column of air bubbles contained in the pipe; the pressure of the denser pulp causes the pulp in the central pipe to rise and overflow, thus circulating the entire charge. The pachuca vessel is extremely simple in design and has no moving parts. Essentially the only difference in the batch and continuous leach is in the feed and discharge mechanics, i.e., a central system vs. a number of individual circuits. In either method variations can be made in how, when, and where additions of either ore or leach solution are made. When the agitated leach method is used for pressure leaching operations, the leaching is carried out in an autoclave. Two major differences are evident between agitated and percolation leaching methods. First, in agitated leaching, the liquid is the continuous phase, and second, in this form of leaching turbulent flow conditions exist, whereas in percolation the flow is more usually laminar. There is, therefore, a considerable difference between the rates of mass transfer obtained in the two types. The higher transfer rates obtained under turbulent contact are achieved by a higher power input to sustain the turbulence. In the context of processing of low-grade copper ores, particularly oxide-sulfide mixed ores, which are difficult to concentrate via flotation or other techniques, a leaching process named thin layer leaching, originally proposed by Holmes and Narver, Inc. 29 has attracted attention. The process involves two principal steps: (I) a curing step in which the finely crushed moistened ore is attacked with strong sulfuric acid; the application of acid is followed by a curing or aging period; (2) the second step involves leaching with diluted sulfuric acid in a manner of normal heap leaching. This technology results in higher leaching rates than in normal vat leaching and a reduced acid consumption.

C. SOLID-LIQUID SEPARATION Solid-liquid separation is the basis of hydrometallurgy. In general, the methods of separation may be summarized as (I) multistage filtration with countercurrent washing, (2) countercurrent decantation and washing, depending on the leaching process used. For example, the separation of residual gangue and leach solution obtained in agitated leaching presents a somewhat different problem than is encountered in percolation methods of leaching, where simple draining suffices to separate leach solutions and leach residues. Filtration, while a convenient answer, is usually out of the question because of the tonnage and the nature of the residual gangue. Coupled with this, but of more importance, are the difficulties involved in getting a satisfactory wash. Adequate washing is even more important than indicated in the discussion of the percolation method of leaching, since an even higher percentage of occluded solution is tied up with the residual gangue. In general, the main factors influencing the selection of methods for solid-liquid separation are ( 1) the grade of

48

Hydrometallurgy in Extraction Processes

the ore, (2) the filterability and settling characteristics of the leached pulp, (3) the tonnage to be handled, (4) the cost and availability of equipment, and (5) local knowledge and preferences. The following sections briefly outline the processes. Filtration is widely used for the separation of leach liquors from solid residues. The layout of a typical circuit is shown in Figure 19. The circuit follows the single-stage acid leach and generally results in a soluble loss of the order of 1 to 2%. Incoming pulp has a liquid to solid ratio of approximately 0.55 to 0.66: I and the ratio of pregnant solution to solids leached may be in the range of 0. 75 to 0. 90: I. An advantage of this filtration is that the volume of pregnant solution is kept to a minimum with the result that excessive dilution of low-value pregnant solutions is avoided. The countercurrent decantation method is applicable where the ore has good settling characteristics or is readily flocculated by the addition of various reagents and the grade of ore is high. Thickeners have become the standard equipment for carrying out both the separation of residual gangue from leach solution and the washing of the occluded solution from the residual gangue. Thickeners by definition take a dilute pulp and resolve it into two products: a clear solution and a thickened pulp. When used in series, thickeners provide a convenient method for countercurrent washing. A typical countercurrent decantation washing circuit layout is shown in Figure 20. In general, four stages of washing in thickeners are employed to reduce the soluble loss to a figure of approximately 1% or less. The two methods may be used in conjunction, e.g., one stage of decantation may precede one or more stages of filtration. The decantation step lowers the value of the pulp to be filtered and also of the final residue obtained. It is also not uncommon to include a single stage of filtration at the end of a series of countercurrent thickeners. This assists in reducing the overall loss from the circuit. D. BACK-END OPERATION After solid-liquid separation, the back-end operation in hydrometallurgical processing takes hold. The leach solutions are taken either via purification and concentration or directly to the finishing operation involving precipitation to recover metals or their usable intermediates. In the field of solution purification and concentration, major processes are adsorption, ion exchange, and solvent extraction. The fact that certain metal-bearing ionic species are adsorbed rather effectively on activated charcoal and under suitable conditions desorbed to allow carbon to be reused have allowed carbon adsorption technology to establish itself in the extraction and process metallurgy of gold. The carbon-in-pulp (CIP) and carbon-in-leach (CIL) technologies are today widely adopted processes based on cyanidation for the recovery of gold from ores. The ion exchange and solvent extraction processes- the two powerful units of operations at the disposal of hydrometallurgists - have materially altered the complexion of extractive metallurgy discipline. The most common technique involves use of resin beads in fixed columns in the ionexchange process. Continuous ion-exchange technology is developing rapidly, and plants based on this have been built in Canada, the U.S. and South Africa. Ion exchange is uniquely suited to the extraction of values from very lean solutions where solvent losses using the solvent extraction process would be excessive. Solvent extraction or liquid-liquid extraction, a unit process defined earlier, is the process of transferring a substance in solution in one liquid into solution in a second liquid which is wholly or partially immiscible with the fli"St. The process is, in essence, a simple technique and may be illustrated by an example in which X and Y are the two substances that are present in an aqueous phase (L,), as shown in Figure 21. This solution is then mixed with an organic liquid (LJ lighter than the aqueous phase (L1) and not miscible with it. The L, and ~ are allowed to separate under gravity. The less dense 1...z floats on L 1 , and if Y is

I

AGURE 19.

DILUTION BEFORE LEACHING

----~~ ~~~~L_!'_E~- - - -

SOLUTION CLARIFICATION

FILTRATE TO PREGNANT

WASH

1

Layout of a typical two-stage filtration circuit.

SECONDARY FILTRATE

REPULPER

w~

FRESHWATER

1

j

BARREN SOLUTION DILUTION

... ~TO RESIDUE

FRESH WATER WASH

~

....

r ~

LEACHED PULP

PREGNANT

FIGURE 20.

Layout for a typical countercurrent decantation (CCD) washing circuit.

WASH WATER

~

"' ~

§"

!:l

~

~

:;·

~

a:::

I:::::

~

~

(A

=

Volume I L1 (AQUEOUS PHASE) CONTAINING X ANDY

51

2 . EXTRACTION L2 (ORGANIC PHASE)

y 'f y )( X X y

X

y

)(.

y

)C..

~

3. SEPARATION

14.

tl,NTAINING XONLY

' I I X '

m

'

SEPARATION OF L1 AND L2 PHASES

l

I

I~i!.TAINING

ruYONLY

~ 5. STRIPPING

~L3 ~L2+L3

'bd FIGURE 21.

L3 FOR RECOVERY OF Y

Solvent extraction, an elementary explanation.

more soluble in ~. it will contain Y but not X. By mixing ~ with another solution (L3 ), Y is stripped from ~ back into an aqueous phase, from which it can be recovered for use, and ~ can be recycled back to the beginning of the process. The given figure does not include a third process called scrubbing which is often adopted in the solvent extraction process for the removal of any coextracted impurities. The scrubbing step is incorporated after the extraction section and is accomplished by bringing the loaded solvent in contact with a fresh aqueous phase which takes away the impurities selectively. The essential step in solvent extraction is the intimate contact of two immiscible liquids for the purpose of mass transfer of constituents from one phase to the other followed by the separation of the two immiscible liquids. The device in which this is accomplished is known as a contactor. The simplest of the equipment is a packed column consisting of a vertical tube filled with metal or ceramic rings which break up the liquid phases (organic and aqueous) and force them into flowing through the column. The phases are separated solely by the difference in

52

Hydrometallurgy in Extraction Processes

density of the two phases, the lighter organic phase flowing up through the column and the heavier aqueous phase down the column. The packed column does not provide a vigorous mixing. The flow rates are comparatively low. As a consequence of these, the packed columns need to be quite tall for good extraction. The increased efficiency and lower height required by pulsing the feed to the column has given way to another column design called pulsed columns. The principal alternative to the packed/pulsed columns for solvent extraction is a device known as the mixer-settler. In this equipment the aqueous and organic phases are mixed in a mixing chamber. The separation between the two phases takes place in a longer settling chamber which is connected with the mixing chamber. A typical solvent extraction circuit illustrating extraction and stripping stages in mixer-settler equipment systems is shown in Figure 22. The back-end or downstream hydrometallurgical practice ends with unit operations for the recovery of metals and compounds. As shown in Figure 14, there are five different processes, namely, crystallization, ionic precipitation, chemical reduction, electrochemical reduction, and electrolytic reduction. The section dealing with physicochemical principles has outlined these processes.

V. SUMMARY Hydrometallurgy cannot be classed as antique and relegated to textbooks. Today, hydrometallurgy is well established as the principal method for extraction of many industrial metals. A number of important landmarks in the historical development of hydrometallurgy are given in Table 8. The major advances have been the development of the Bayer process for alumina, the use of cyanide leaching for gold and silver, the roast-leach-electrowinning process for zinc, dump and vat leaching of oxide copper ores, and the recovery of uranium from low-grade ores. This chapter opens with physicochemical principles of hydrobased processes. The behavior of any hydrometallurgical system is determined both by associated thermodynamics and by kinetic considerations, which determine the rate at which the thermodynamic equilibria are attained. The potential-pH diagrams are a very convenient way of showing the equilibria that occur in aqueous systems and the thermodynamically stable state at any given value of potential and pH. Although many reactions should, on a thermodynamic basis, proceed at room temperature, in practice kinetic factors often prevent the thermodynamically stable state from being achieved in a reasonable length of time. It is often necessary to resort to elevated temperatures to improve reaction rates, and in such a case thermodynamic equilibria calculated at room temperature are no longer applicable. In any system, the solid phases that are stable at elevated temperatures may differ from the corresponding stable phases at room temperature. For instance, hydrolysis of an iron chloride solution at 25°C yields the gelatinous iron hydroxide phase FeOOH, whereas hydrolysis at 200°C yields crystalline F~0 3 • In leaching and precipitation processes, there are complicated heterogeneous reactions at the solid-liquid phase boundaries in addition to homogeneous reactions in the solution, and a three-phase system in which gaseous reactants are used is even more complex. In a three-phase leaching system the reaction proceeds by the following consecutive steps: (I) transfer of the gas to the liquid phase, (2) transport of reactants to the solid-liquid phase boundary, (3) reaction at the solid-liquid phase boundary, (4) transport of products away from the reaction zone. The rate of the first step is dependent on gas solubility and efficiency of agitation. The solubility of gases depends on temperature and partial pressure used. The increased solubility of H2S at elevated pressure, together with enhanced reaction rates at elevated temperatures, has led to the commercial use of H2S as a precipitant for concentrating dilute nickel and cobalt solution. Kinetic factors are particularly important in the acid pressure leaching of sulfides, and it is found that, for metal sulfides, the oxidation products vary according to the temperatures and pH of the system.

..

·..

···:.

·::·:....

+++ +

+ + +

+ •.

I·

AQUEOUS

ORGANIC

BARREN AQUEOUS RAFFINATE

STAGE -I

FIGURE 22.

S- SETTLER

M- MIXER

STAGE- 2

EXTRACTION

ORGANIC

STAGE-I

STAGE-2

AQUEOUS STRIPPING SOLUTION

STRIPPING

CONCENTRATED AND PURIFIED AQUEOUS PRODUCT

L

I

Typical solvent exttaction circuit in mixer-settlers.

AQUEOUS FEED

STAGE-I

·I _I I

r ~

....

~

54

Hydrometallurgy in Extraction Processes

TABLE 8 Some Landmarks in the History of HydrometaUurgy 1 1700 1900 1900 1920 1926 1940 1953 1954 1956 1956 1958 1960 1964 1968 1968 1974 1975 1977 1981

Heap leaching of copper from weathered pyrite at Rio Tinto, Spain. Copper precipitated from solution with iron. Bayer process for aluminum. Pure alumina from caustic soda leach of bauxite followed by crystallization. Cyanide leaching of gold and silver ores. Precipitation of gold with zinc powder. Electrolytic zinc process. Sulfide ore roasted then leached in H;zS04 to produce electrolyte. Large-scale (6800 t/d of ore) H2S04 vat leaching of copper ore with electrowinning (Inspiration Consolidated Copper Co, U.S.). Carron process for nickel from oxide ores. Selective reduction followed by ammoniacal leach. Uranium recovery from low-grade ore. Acid leaching-ion exchange. Sherrin Gordon process for nickel from sulfide concentrates. Ammoniacal pressure leach followed by pressure hydrogen reduction to produce nickel powder (Canada) . Introduction of amine solvent extraction for uranium recovery from leach liquor. Aman, J. J. British Patent 793700 on the high temperature spray hydrolysis of metal chloride solutions to recovery of hydrochloric acid. Pressure sulfuric acid leach of oxide nickel ore, Moa Bay, Cuba. Expansion of copper production by bacteria leaching of low-grade ore dumps in U.S. First application of the Jarosite process for iron removed from zinc sulfate leach liquors (Asturiana de Zinc, Spain). Dump leaching of oxide copper ore with H;zS04 followed by solvent extraction and electrowinning (Ranchers: Exploration & Development Co. Ltd.: copper production 18,200 kgld . Falconbridge matte leach process for copper, nickel, cobalt, HCIIeach, and processing of chloride liquors by solvent extraction. Pachuca leach of oxide copper ore with H2S04 followed by solvent extraction and electrowinning (Nchanga Consolidated Copper Mines Ltd., Zambia); copper production- 182,800 kgld. Arbiter process - anunoniacal leach of copper sulfide concentrate followed by solvent extraction and electrowinning; plant at Montawa closed 1979. Successful piloting of direct pressure leaching of zinc concentrates by Corninco, Canada. Commercialization of Corninco's Trail operation for the processing of zinc concentrate.

The chapter then delves into the unit processes in hydrometallurgy. The front end or the upstream in a general hydrometallurgical process tlowsheet comprises ore preparation, leaching, and solid-liquid extraction. Leaching is a key step in hydrometallurgy. In leaching which may involve physical, chemical, electrochemical or electrolytic processes, many different raw materials (metals, oxides, and hydroxides, complex oxides, sulfides, phosphates, silicates, selenides, md tellurides, spent catalyst, slag, and sludge) can be treated using a wide variety of acids (sulfuric, nitric, hydrofluoric, and hydrochloric), alkalies (sodium hydroxide, sodium carbonate, ammonia solutions), aqueous salt solutions (ferric chlorides and sulfates, cupric chloride, sodium and potassium cyanide), aqueous chloride base lixivants (aqueous Cl2 , hypochlorous acid, hypochlorite) and water by employing simple or complex technology (solution mining, heap or dump leaching, vat leaching, agitated leaching under ambient and elevated pressures). The chemistry involved in the dissolution of minerals is essentially de