Mechanism and Kinetics of Chalcopyrite Passivation and Depassivation During Ferric and Microbial Leaching
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MECHANISM AND KINETICS OF CHALCOPYRITE PASSIVATION AND DEPASSIVATION DURING FERRIC AND MICROBIAL LEACHING By ALAIN FUAMBA TSHILOMBO B.Eng., Faculte Polytechnique, University of Lubumbashi, 1994 M.Sc. , The University of Pretoria, 2000 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY In THE FACULTY OF GRADUATES STUDIES (Department of Materials Engineering) THE UNIVERSITY OF BRITISH COLUMBIA December 2004 ABSTRACT Chalcopyrite is known to be recalcitrant to conventional hydrometallurgical and biohydrometallurgical processes. Formation of passive layers on the chalcopyrite surface results in slow and incomplete leaching. The nature of how these passive layers are formed is the subject of much controversy. The most likely explanation is the formation of polysulphide compounds or copper-rich intermediate products on the chalcopyrite surface. The formation of these products depends mainly on temperature and solution potential. Based on these observations, electrochemical techniques were used to study the behaviour of chalcopyrite under a variety of conditions similar to ferric and microbial leaching. Electrochemical techniques have the advantage over other techniques of measuring properties at the solid-liquid interface. Slow chalcopyrite leaching was mainly observed under the following conditions: > low temperature (25°C) and low potential (0.45 to 0.6 V SCE) > high temperature (65°C) and high potential (above 0.6 V SCE) Leaching was accelerated at high temperature (65°C) under mildly oxidizing conditions (0.45 to 0.55 V SCE). The study also indicated that a polarized chalcopyrite surface inhibits ferric reduction and that the presence of pyrite during chalcopyrite leaching can be beneficial. The electrochemical study was validated in leaching tests carried out in a stirred-tank reactor with fine chalcopyrite particles. Leaching was retarded at low temperatures due to the presence of an induction period. The duration of the induction period decreased with increasing temperature. The addition of pyrite significantly increased both the rate and the extent of chalcopyrite leaching. Complete conversion of chalcopyrite was obtained within 16 hours at 65°C at a pyritexhalcopyrite mass ratio of 2:1. An electrochemical model that takes into consideration the galvanic interaction with pyrite and the "passivation" of chalcopyrite was proposed. The addition of microorganisms to the leaching system was investigated. Chalcopyrite was leached almost to completion (95%) within 30 days in the presence of thermophilic bacteria at low potentials and high temperatures. The bioleaching rate of chalcopyrite was further increased with the addition of pyrite. Finally, atmospheric leaching of chalcopyrite was carried out at 80°C under a range of conditions. Complete copper extraction was attained in 4 hours at a pyritexhalcopyrite ratio of 4:1. The present study has shown that chalcopyrite passivation can be prevented at low solution potentials, high temperatures and in the presence of moderate amounts of pyrite. TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS iv LIST OF FIGURES vii LIST OF TABLES xii LIST OF SYMBOLS xiii ACKNOWLEDGMENTS xv CHAPTER 1. INTRODUCTION 1 CHAPTER 2. LITERATURE REVIEW 5 2.1. Properties of chalcopyrite 5 2.2. Hydrometallurgical processes for copper sulphide minerals in sulphate media 6 2.2.1. Mt Gordon process 6 2.2.2. Activox process 8 2.2.3. MIM/Highlands Albion (Nenatech) process 8 2.2.4. AAC/UBC Hydrometallurgy process 9 2.2.5. Dynatec process 10 2.2.6. Total Pressure Oxidation process 10 2.2.7. CESL process 11 2.2.8. BacTech/Mintek process 11 2.2.9. BioCop process 12 2.2.10. GeoCoat process 12 2.2.11. Summary 13 2.3. Thermodynamic considerations 13 2.4. Retarding effect on the dissolution of chalcopyrite 15 2.4.1. Passivation of chalcopyrite during ferric leaching 16 2.4.2. Passivation of chalcopyrite during bioleaching 27 2.4.3. Passivation during the electrochemical dissolution of chalcopyrite 32 2.5. Methods for activating the passive film during the ferric leaching and bioleaching of chalcopyrite 45 2.5.1. Mechanical activation 45 2.5.2. Addition of silver 46 2.5.3. Beneficial effect of galvanic interactions between chalcopyrite and associated minerals 47 2.5.4. Thermophilic bioleaching 49 iv 2.6. Conclusions and focus of the present study 50 CHAPTER 3. METHODOLOGY AND EXPERIMENTAL PROCEDURE 52 3.1. Electrochemical measurements 53 3.1.1. Working electrodes 53 3.1.2. Apparatus 55 3.1.3. Procedure 57 3.2. Chemical leaching experiments 58 3.2.1. Chalcopyrite samples 58 3.2.2. Reagents 58 3.2.3. Apparatus 58 3.2.4. Procedure 60 3.2.5. Sampling and analysis 62 3.2.6. Oxidation of Fe(ll) to Fe(lll) by hydrogen peroxide 62 3.3. Bioleaching experiments 63 3.3.1. Material 63 3.3.2. Microorganisms 63 3.3.3. Equipment 64- 3.3.4. Procedure 64 3.3.5. Cell counting 65 3.4. Column tests 65 3.4.1. Material 66 3.4.2. Apparatus 67 3.4.3. Procedure.. 68 3.5. Atmospheric leaching 69 3.5.1. Chalcopyrite samples 69 3.5.2. Pyrite samples 70 3.5.3. Reagents 70 3.5.4. Apparatus 70 CHAPTER 4. RESULTS AND DISCUSSION 71 4.1. Electrochemical leaching 71 4.1.1. Anodic behaviour of chalcopyrite in acidic medium 71 4.1.2. Cathodic reduction of ferric ions on chalcopyrite 83 4.1.3. Mixed potential of chalcopyrite as a function of Fe(lll) and Fe(ll) concentrations 87 4.2. Chemical leaching 90 4.2.1. Effect of solution potential on reaction rate 90 v 4.2.2. Effect of temperature at constant Fe(lll):Fe(ll) ratio 101 4.2.3. Effect of pyrite on the ferric leaching of chalcopyrite 102 4.2.4. Development of an electrochemical model for the dissolution of chalcopyrite 114 4.2.5. Development of an electrochemical model for the galvanic interaction between pyrite and chalcopyrite 117 4.2.6. Electrochemical reaction and surface passivation model 121 4.2.7. Validation of the electrochemical-passivation model 122 4.3. Bioleaching of chalcopyrite 131 4.3.1. Mesophiles 131 4.3.2. Moderate thermophiles 135 4.3.3. Extreme thermophiles 136 4.3.4. Comparison of bioleaching with mesophiles and thermophiles 139 4.4. Column leaching 144 4.5. Atmospheric leaching 145 4.5.1. Effect of pyrite addition 146 4.5.2. Effect of initial solution potential 148 4.5.3. Effect of particle size 150 4.5.4. Effect of pyrite source 154 4.5.5. Effect of pulp density 156 4.5.6. Effects of impeller speed, choice of primary oxidant and acidity 156 4.5.7. Yield of elemental sulphur 160 CHAPTER 5. CONCLUSIONS 162 CHAPTER 6. FUTURE WORK AND RECOMMENDATIONS 165 References 166 Appendix A. Statistical analysis of experimental results 180 vi LIST OF FIGURES Figure 2.1 Crystal structure of chalcopyrite 5 Figure 2.2 Potential-pH diagram of the Cu-Fe-S-H20 system at 25°C: all solutes at 0.1 M activity except Cu2+ at 0.01 M 14 Figure 2.3 Concentrations of Fe(lll)-sulfato and bisulfato complexes in 0.2 M H2SO4 solutions having different Fe(S04)i.5 concentrations 18 Figure 2.4 Variation of reaction curves with temperature 19 Figure 2.5 Elemental sulphur-water Eh-pH diagram with extended sulfur stability 21 Figure 2.6 Leaching curves for the dissolution of various size fractions of natural chalcopyrite in sulfate solutions 24 Figure 2.7 Leaching rate curve of chalcopyrite with ferric sulfate 24 Figure 2.8 Stability of various iron precipitates as a function of pH and temperature 26 Figure 2.9 Scheme visualizing indirect leaching and direct leaching of metal sulphide 28 Figure 2.10 Effect of ferrous ion on the anodic polarization curve of Transvaal -1 CuFeS2 in 1 M H2S04, 40 mV min , 25°C 36 Figure 2.11 Anodic polarizarion curves for CuFeS2 from 6 different locations in -1 1 M H2S04l 30 mV min , 25°C 37 Figure 2.12 Schematic representation of galvanic interactions between chalcopyrite and intermediate phases formed during the dissolution of chalcopyrite , 39 Figure 2.13 Mixed potential for Type I leaching 42 Figure 2.14 Mixed potential for Type II leaching 42 Figure 2.15 Mixed potential for Type III leaching 43 Figure 2.16 Mixed potential for Type IV leaching 43 Figure 3.1 Electrochemical apparatus 56 Figure 3.2 Schematic representation of the controlled-potential chemical leaching system 59 Figure 3.3 Solution potential as a function of the Fe(lll):Fe(ll) ratio, pH 1.4, 25°C 61 Figure 3.4 Schematic drawing of a column 67 Figure 3.5 Column leach apparatus 68 vii Figure 4.1 Effect of temperature on the anodic dissolution of chalcopyrite, pH 1.5, scan rate = 1 mV s~1 73 Figure 4.2 Effect of scan rate on the anodic dissolution of chalcopyrite, pH 1.5, 25°C, de-aerated solutions 76 Figure 4.3 Effect of scan rate on the anodic dissolution of chalcopyrite, pH 1.5, 45°C, de-aerated solutions 76 Figure 4.4 Effect of scan rate on the anodic dissolution of chalcopyrite, pH 1.5, 65°C, de-aerated solutions 77 Figure 4.5 Effect of temperature on the anodic dissolution of chalcopyrite, pH 1.5, scan rate = 0.1 mV s_1 77 Figure 4.6 Constant potential experiments at 25°C on chalcopyrite in acidic solutions, pH 1.5 80 Figure 4.7 Anodic behaviour of fresh and polarized chalcopyrite surfaces in acidic solutions at 25°C, pH 1.5 80 Figure 4.8 Constant potential experiments at 45°C on chalcopyrite in acidic solutions, pH 1.5 81 Figure 4.9 Constant potential experiments at 65°C on chalcopyrite in acidic solutions, pH 1.5 81 Figure 4.10 Reduction of Fe(lll) on chalcopyrite as a function of applied potential at various concentrations of Fe(lll), pH 1.5, 25°C 84 Figure 4.11 Reduction of Fe(lll) on polarized chalcopyrite, pH 1.5, 0.001 M Fe(lll) 85 Figure 4.12 Reduction of Fe(lll)