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Sulfur dispersing agents for nickel sulfide leaching above the melting point of sulfur Tong, Libin 2009

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SULFUR DISPERSING AGENTS FOR NICKEL SULFIDE LEACHING ABOVE THE MELTING POINT OF SULFUR by Libin Tong  B.Sc., Northeastern University, China, 1997 M.Sc., Northeastern University, China, 2000  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Materials Engineering  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  January 2009 © Libin Tong, 2009  ABSTRACT The effects of sulfur dispersing agents (SDAs) in the oxygen pressure leaching of nickel concentrate at medium temperature were investigated. Liquid sulfur-aqueous solution interfacial tensions and liquid sulfur-sulfide mineral contact angles were measured at 140ºC, 690 kPa overpressure by nitrogen, and 1.0 mol/L NiSO4. The effects of SDAs including lignosulfonate, Quebracho, o-phenylenediamine (OPD), and humic acid were evaluated by the calculation of the work of adhesion in the liquid sulfur-sulfide mineral-aqueous solution systems. It was found that the sulfide mineral surface is sulfophobic at pH from 4.1 to 4.5 due to the hydrolysis of nickel (II) ions to nickel hydroxide and the deposition of nickel hydroxide on the mineral surface. These findings apply to four different sulfide mineral systems, including pentlandite, nickeliferous pyrrhotite, pyrrhotite, and chalcopyrite. Lignosulfonate, Quebracho, and humic acid were found to significantly reduce the work of adhesion indicating they should be effective SDAs. OPD is ineffective in changing the work of adhesion of sulfur on the mineral sulfides indicating that it is not a good candidate for sulfur dispersion.  The adsorption behavior of SDAs, including lignosulfonate, Quebracho, OPD, and humic acid on elemental sulfur and on nickel sulfide concentrate was investigated. Lignosulfonate, Quebracho, and humic acid were characterized by their infrared spectra. The charge changes on elemental sulfur surface were characterized by the measurement of the electrokinetic sonic amplitude (ESA) in the absence or presence of SDAs. The adsorption of lignosulfonate on molten sulfur surface was calculated by the Gibbs Equation. The adsorption of lignosulfonate, Quebracho, and humic acid on the nickel concentrate was investigated at ambient temperature. The adsorption of lignosulfonate, Quebracho, and humic acid on the nickel concentrate was found to be monolayer adsorption, which was fitted to the Langmuir adsorption isotherm. Electrostatic interaction and ii  ion-binding are the possible mechanisms for the adsorption of lignosulfonate and humic acid on the nickel concentrate. Quebracho is adsorbed on the nickel concentrate through hydroxyl and sulfonate groups. OPD cannot adsorb on the molten sulfur surface. OPD undergoes chemical change in aqueous solution in the presence of ferric at ambient temperature.  Oxygen pressure leaching experiments were performed at 140 or 150ºC under 690 kPa oxygen overpressure. The particle size of the nickel concentrate was found to be an important factor in leaching. During the leaching of nickel concentrate with P80 of 48 µm, the SDAs were believed to be fully degraded before nickel was fully extracted. At most 66% nickel was extracted in the presence of 20 kg/t OPD. Fine grinding (P80 of 10 µm) was sufficient for 99% nickel recovery at low pulp density while at high pulp density, the nickel extraction increased from 95% to 99% with addition of SDAs. Based on the leaching results on a nickel concentrate sample (-44 µm), OPD had the effect of increasing the nickel extraction to about 99%, followed by Quebracho (83%), lignosulfonate (72%), and humic acid (61%). It is suggested that the oxidation product of OPD is effective in solving the sulfur wetting problem in leaching. 97% nickel was recovered in the presence of 5 g/L chloride ion. Chloride ion has an effect to enhance the performance of lignosulfonate under leaching conditions.  iii  TABLE OF CONTENTS Abstract…………..……………………………………………………………………………...ii Table of Contents…………….………………………………………………………………..iv List of Tables…………………………………………………………………………………….x List of Figures………………………………………………………………………..…………xv List of Symbols…….…..…………………………………………………...…………………xxi Acknowledgements………………………...………………..………………………………xxiv 1.0 Background ............................................................................................................................. 1 2.0 Literature Review ................................................................................................................... 6 2.1 Properties of Pentlandite ....................................................................................................... 6 2.1.1 Crystal Structure ............................................................................................................ 7 2.1.2 Thermodynamic Aspects of Leaching ........................................................................... 8 2.1.3 Pentlandite Mineral Surface Chemistry ......................................................................... 9 2.2 Elemental Sulfur ................................................................................................................. 11 2.2.1 Properties of Elemental Sulfur ..................................................................................... 11 2.2.2 The Behavior of Sulfur in Oxidative Leaching of Sulfide Ores .................................. 12 2.3 Sulfur Dispersing Agents .................................................................................................... 13 2.3.1 Characteristic Features of Sulfur Dispersing Agents ................................................... 14 2.3.2 Properties of Individual Sulfur Dispersing Agents ...................................................... 15 2.4 Oxygen Pressure Leaching in Sulfate Media ...................................................................... 19 2.4.1 Pressure Leaching of Zinc Sulfide Concentrates ......................................................... 19 2.4.2 Pressure Leaching of Refractory Gold Ores and Concentrates ................................... 20 2.4.3 Pressure Leaching of Copper Sulfide Concentrates..................................................... 21 2.4.4 Pressure Leaching of Pyrrhotite Concentrates ............................................................. 22 iv  2.5 Sulfur Dispersing Agents in Leaching ................................................................................ 22 2.5.1 Sulfur Dispersing Agents in Oxygen Pressure Leaching of Zinc Concentrates .......... 23 2.5.2 Sulfur Dispersing Agents in Oxygen Pressure Leaching of Copper Sulfide Minerals 27 2.5.3 Sulfur Dispersing Agents in Pressure Oxidation of Refractory Gold Ores and Concentrates .......................................................................................................................... 27 2.5.4 Sulfur Dispersing Agents in Oxygen Pressure Leaching of Pyrrhotite Concentrates . 28 2.5.5 Sulfur Dispersing Agents in Oxygen Pressure Leaching of Pentlandite Concentrates 29 2.6 Liquid Sulfur-Aqueous Solution Interface.......................................................................... 29 2.6.1 Surface Tension of Pure Liquid and Binary Systems .................................................. 30 2.6.2 Interfacial Excess at Liquid Sulfur-Aqueous Solution Interface ................................. 31 2.7 Mineral-Liquid Sulfur Interface .......................................................................................... 32 2.7.1 Mineral-Liquid Sulfur Contact Angle .......................................................................... 32 2.7.2 Work of Adhesion ........................................................................................................ 34 2.7.3 Interfacial Energy Change ........................................................................................... 35 2.8 Methods for the Measurement of Interfacial Properties ..................................................... 37 2.8.1 The Measurement of Interfacial Tension ..................................................................... 37 2.8.2 The Measurement of Contact Angle ............................................................................ 39 2.9 Electroacoustic Characterization of Mineral Suspensions .................................................. 40 2.9.1 Zeta Potential ............................................................................................................... 40 2.9.2 Electroacoustic Method ............................................................................................... 40 2.9.3 Zeta Potential of Pentlandite ........................................................................................ 41 2.10 Adsorption of Sulfur Dispersing Agents on Mineral Surface ........................................... 41 2.10.1 Beer-Lambert Equation .............................................................................................. 41 2.10.2 Quantitative Determination of Sulfur Dispersing Agents in Solution ....................... 42 2.10.3 Adsorption Mechanisms ............................................................................................ 43 v  2.10.4 Adsorption Isotherms ................................................................................................. 43 2.11 Infrared Spectroscopy ....................................................................................................... 45 2.11.1 Infrared Spectroscopy Techniques ............................................................................. 45 2.11.2 Sample Preparation Methods ..................................................................................... 46 2.12 Kinetic Study of Nickel Sulfide Concentrate.................................................................... 46 2.12.1 Sulfate Leaching of Nickel ........................................................................................ 46 2.12.2 Chloride Leaching of Nickel ...................................................................................... 47 2.13 Objectives of the Present Study ........................................................................................ 49 3.0 Interfacial Studies ................................................................................................................. 51 3.1 Materials ............................................................................................................................. 51 3.1.1 Reagents ....................................................................................................................... 51 3.1.2 Minerals ....................................................................................................................... 51 3.1.3 Sulfur Dispersing Agents ............................................................................................. 52 3.2 Experimental Procedures .................................................................................................... 54 3.3 Results and Discussion ....................................................................................................... 54 3.3.1 Interfacial Properties without Sulfur Dispersing Agent............................................... 56 3.3.2 Influences of Sulfur Dispersing Agents at High pH .................................................... 63 3.3.3 Influence of Sulfur Dispersing Agents at Low pH ...................................................... 73 3.3.4 Interfacial Energy Change ........................................................................................... 84 3.4 Conclusions ......................................................................................................................... 86 4.0 Characterization of Sulfur Dispersing Agents ................................................................... 87 4.1 Materials and Apparatus ..................................................................................................... 87 4.1.1 Materials ...................................................................................................................... 87 4.1.2 Equipment .................................................................................................................... 88 4.2 Experimental Procedure ...................................................................................................... 88 vi  4.3 Results and Discussion ....................................................................................................... 88 4.3.1 Characterization of Lignosulfonate.............................................................................. 88 4.3.2 Characterization of Quebracho .................................................................................... 90 4.3.3 Characterization of Humic Acid .................................................................................. 91 4.4 Conclusions ......................................................................................................................... 93 5.0 Surface Charge Characterization ........................................................................................ 94 5.1 Materials and Apparatus ..................................................................................................... 95 5.1.1 Materials ...................................................................................................................... 95 5.1.2 Apparatus ..................................................................................................................... 96 5.2 Experimental Procedure ...................................................................................................... 96 5.3 Results and Discussion ....................................................................................................... 97 5.3.1 The Influence of Different Kinds of Lignosulfonate ................................................... 97 5.3.2 The Influence of Different Kinds of Sulfur Dispersing Agents ................................... 98 5.3.3 The Isoelectric Point of Nickel Concentrate .............................................................. 101 5.4 Conclusions ....................................................................................................................... 102 6.0 Sulfur Dispersing Agent Adsorption on Nickel Concentrate .......................................... 103 6.1 Materials and Apparatus ................................................................................................... 103 6.1.1 Reagents ..................................................................................................................... 103 6.1.2 Mineral ....................................................................................................................... 104 6.1.3 Sulfur Dispersing Agents ........................................................................................... 104 6.1.4 Apparatus ................................................................................................................... 104 6.2 Experimental Procedures .................................................................................................. 105 6.2.1 Quantitative Determination of Lignosulfonate and Humic Acid ............................... 105 6.2.2 Quantitative Determination of Quebracho ................................................................. 108 6.2.3 Quantitative Determination of OPD .......................................................................... 110 vii  6.2.4 Calculation ................................................................................................................. 116 6.3 Results and Discussion ..................................................................................................... 116 6.3.1 Langmuir and Freundlich Adsorption Isotherms ....................................................... 117 6.3.2 Adsorption of Lignosulfonate on Nickel Concentrate ............................................... 120 6.3.3 Adsorption of Humic Acid on Nickel Concentrate.................................................... 127 6.3.4 Adsorption of Quebracho on Nickel Concentrate ...................................................... 128 6.4 Conclusions ....................................................................................................................... 129 7.0 Oxygen Pressure Leaching of Nickel Concentrate .......................................................... 131 7.1 Materials and Apparatus ................................................................................................... 132 7.1.1 Nickel Sulfide Concentrate ........................................................................................ 132 7.1.2 Reagents ..................................................................................................................... 134 7.1.3 Sulfur Dispersing Agents ........................................................................................... 134 7.1.4 Apparatus ................................................................................................................... 134 7.2 Experimental Procedure .................................................................................................... 135 7.2.1 Experimental Procedure ............................................................................................. 135 7.2.2 Analytical Methods .................................................................................................... 137 7.3 Leaching Chemistry .......................................................................................................... 139 7.3.1 Low Pulp Density Leaching....................................................................................... 139 7.3.2 High Pulp Density Leaching ...................................................................................... 140 7.4 Results and Discussion ..................................................................................................... 141 7.4.1 Low Pulp Density Leaching of Nickel Concentrate (P80 of 48 µm) .......................... 141 7.4.2 Effect of Fine Grinding at Low Pulp Density ............................................................ 151 7.4.3 Effect of Fine Grinding at High Pulp Density ........................................................... 153 7.4.4 High Pulp Density Leaching of Nickel Concentrate (-44 µm) .................................. 157 7.4.5 Effect of Chloride at High Pulp Density .................................................................... 161 viii  7.4.6 Effect of Chloride on the Voisey’s Bay Nickel Concentrate ..................................... 166 7.5 Conclusions ....................................................................................................................... 169 8.0 Conclusions and Recommendations .................................................................................. 171 8.1 Conclusions ....................................................................................................................... 171 8.2 Recommendations ............................................................................................................. 174 References .................................................................................................................................. 176 Appendix 1 Measurement of Interfacial Tension and Contact Angle ................................. 197 Appendix 2 Calibration Curves and Experimental Data for the Adsorption Studies........ 202 Appendix 3 Determination of Ferrous Iron and Free Acid .................................................. 209 Appendix 4 Mass Balance and Assays Results in Leaching Studies .................................... 213  ix  LIST OF TABLES  Table 2.1 Selected physical constants of sulfur [43] .................................................................... 12 Table 3.1 List of sulfur dispersing agents evaluated in the interfacial studies ............................. 53 Table 3.2 Results of surface tension measurement on water at 22ºC (different drops), the value from literature is 72.4 mN/m [149] ....................................................................................... 55 Table 3.3 Results of interfacial tension between liquid sulfur and aqueous solution (different drops): 140ºC, 690 kPa N2, NiSO4 1.0 mol/L, BorrePAL U 0.5 g/L, pH: 4.8. ..................... 55 Table 3.4 Contact angle between liquid sulfur and sulfide mineral (different drops): 140ºC, 690 kPa N2, NiSO4 1.0 mol/L, 8.90×10-4 mol/L H2SO4, No sulfur dispersing agent addition. ... 55 Table 3.5 The influence of nickel sulfate concentration on the interfacial properties: 140ºC, 690 kPa N2, nickeliferous pyrrhotite ............................................................................................ 63 Table 3.6 Molecular area of lignosulfonate adsorbed on the molten sulfur surface, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4 ...................................................................................................... 68 Table 3.7 The influence of different kinds of lignosulfonate on the interfacial properties: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4, nickeliferous pyrrhotite ................. 74 Table 3.8 The influence of sulfur dispersing agents on interfacial properties: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4, pentlandite (from Voisey’s Bay) ....................... 81 Table 3.9 The influence of lignosulfonate (BorrePAL U) on interfacial properties: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4, pentlandite (from Voisey’s Bay) .......... 82 Table 3.10 The influence of sulfur dispersing agents on interfacial properties, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4, pentlandite (from Sudbury) ................................ 82 Table 3.11 Interfacial energy change for complete sulfur wetting: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0 mol/L H2SO4, no sulfur dispersing agent .............................................................. 84 Table 3.12 Interfacial energy change for complete sulfur wetting: 140ºC, 690 kPa N2, 1.0 mol/L x  NiSO4, 0.18 mol/L H2SO4, no sulfur dispersing agent ......................................................... 85 Table 6.1 The metal content in solution after the nickel concentrate was conditioned at ambient temperature with (acidic) water for 1 h............................................................................... 105 Table 6.2 Calibration equations for the UV analysis of different sulfur dispersing agents ........ 111 Table 6.3 Langmuir and Freundlich isotherms and their linear regression methods [172] ........ 117 Table 6.4 The parameters of the Langmuir model for the adsorption of BorrePAL U on the nickel concentrate at ambient temperature (natural pH)................................................................ 118 Table 6.5 The parameters of the Freundlich model for the adsorption of BorrePAL U on the nickel concentrate at ambient temperature (natural pH) ..................................................... 118 Table 6.6 The parameters of Langmuir-1 adsorption model for the adsorption of sulfur dispersing agents on nickel concentrate at ambient temperature ......................................................... 121 Table 6.7 Molecular area of lignosulfonate adsorbed on the nickel concentrate........................ 124 Table 7.1 Metal extractions in the presence or absence of sulfur dispersing agents: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 48 µm), 2 h........ 142 Table 7.2 Properties of leach solution at ambient temperature, leaching conditions: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 48 µm), 2 h........ 143 Table 7.3 Nickel concentrate A and leach residue, leaching conditions: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 48 µm), 2 h ........................... 146 Table 7.4 Metal extraction in the absence or presence of sulfur dispersing agents: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 10 µm), 2 h............... 152 Table 7.5 Properties of leach solution at ambient temperature: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 10 µm), 2 h ........................... 153 Table 7.6 Metal extraction in the absence or presence of sulfur dispersing agents: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (P80 of 10 µm), 2 h............. 155 Table 7.7 Properties of leach solution at ambient temperature: 140ºC, 690 kPa oxygen xi  overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (P80 of 10 µm), 2 h ......................... 156 Table 7.8 Metal extraction in the absence or presence of sulfur dispersing agents: 150ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (-44 µm), 2 h ..................... 158 Table 7.9 Properties of leach solution at ambient temperature: 150ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (-44 µm), 2 h .................................. 159 Table 7.10 Metal extraction in the presence of sulfur dispersing agent: 150ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate (-44 µm), 2 h ....................................... 163 Table 7.11 Properties of leach solution at ambient temperature: 150ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate (-44 µm), 2 h ....................................... 165 Table 7.12 Oxygen pressure leaching of the Voisey’s Bay nickel concentrate (P80 of 50 µm): 150ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate, 1.5 h, 5 kg/t lignosulfonate, 10 g/L chloride ........................................................................................... 168 Table 7.13 Metal extraction in the presence of sulfur dispersing agent: 150ºC, 690 kPa oxygen overpressure, 49 g/L H2SO4, 250 g/L Voisey’s Bay Concentrate (P80 of 50 µm), 1.5 h .... 169 Table 7.14 Properties of leach solution at ambient temperature: 150ºC, 690 kPa oxygen overpressure, 49 g/L H2SO4, 250 g/L Voisey’s Bay Concentrate (P80 of 50 µm), 1.5 h .... 169 Table A2.1 pH values of BorrePAL U and BorrePAL N solution after conditioning with the nickel concentrate. A: initial pH 4.8; B: initial pH 3.5. ...................................................... 206 Table A2.2 pH values of BorrePAL S and humic acid solution after conditioning with the nickel concentrate. A: initial pH 4.8; B: initial pH 3.5. ................................................................. 207 Table A2.3 pH values of Quebracho solution after conditioning with the nickel concentrate. A: initial pH 4.8; B: initial pH 3.5. .......................................................................................... 208 Table A4.1 Mass balance of leaching experiments: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L nickel concentrate (P80 of 48 µm), 2 h ............................................. 214 Table A4.2 ICP assay results (ppm) and S species (%) of nickel concentrate A (P80 of 48 µm) 215 xii  Table A4.3 Assay results (ICP) of leach solution (mg/L), wash solution (mg/L), and leach residue (ppm): 140°C, 690 kPa O2, 0.5 mol/L H2SO4, 20 g/L nickel concentrate (P80: 48 µm), 2 h ............................................................................................................................... 216 Table A4.4 Mass balance of leaching experiments: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L nickel concentrate B (P80 of 10 µm), 2 h ......................................... 221 Table A4.5 Assay results (ICP) of leach solution (mg/L), wash solution (mg/L) in the pressure leaching experiments: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L nickel concentrate B (P80 of 10 µm), 2 h, SDA: 5 kg/t ....................................................... 221 Table A4.6 Assay results (AA) of leach residue (ppm) and nickel concentrate (%), experimental conditions: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L nickel concentrate B (P80 of 10 µm), 2 h, SDA: 5 kg/t, *: repeated analysis ................................ 223 Table A4.7 Mass balance of leaching experiments: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L nickel concentrate B (P80 of 10 µm), 2 h ....................................... 223 Table A4.8 Assay results (AA & ICP) of leach residue samples, experimental conditions: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L nickel concentrate B (P80 of 10 µm), 2 h, SDA: 5 kg/t ......................................................................................................... 224 Table A4.9 Mass balance of leaching experiments: 150°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L nickel concentrate C (P100 of 44 µm), 2 h ...................................... 225 Table A4.10 Assay results (ICP) of leach residue and nickel concentrate C: 150°C, 690 kPa O2, 0.5 mol/L H2SO4, 250 g/L nickel concentrate C (P100 of 44 µm), 2 h, SDA: 5 kg/t ........... 226 Table A4.11 Mass balance of leaching experiments in mixed sulfate and chloride media: 150°C, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L nickel concentrate C (P100 of 44 µm), 2 h........................................................................................................................................ 227 Table A4.12 Assay results (ICP) of leach residue, experimental conditions: 150°C, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L nickel concentrate C (P100 of 44 µm), 2 h .. 229 xiii  Table A4.13 Mass balance of leaching experiments: 150°C, 690 kPa oxygen overpressure, 250 g/L Voisey’s Bay concentrate D (P80 of 50 µm), 1.5 h ....................................................... 231 Table A4.14 Assay results (ICP) of leach residue, experimental conditions: 150°C, 690 kPa oxygen overpressure, 40 or 49 g/L H2SO4, 250 g/L Voisey’s Bay concentrate D (P80 of 50 µm), 1.5 h ............................................................................................................................ 232 Table A4.15 Quantitative phase analysis of one nickel concentrate sample (XSTRATA Nickel Limited, Strathcona Mill) using the rietveld method and X-ray powder diffraction data .. 233 Table A4.16 Analytical results (%) of Voisey’s Bay nickel concentrate ................................... 234  xiv  LIST OF FIGURES Figure 2.1 The structure of pentlandite showing the “cube cluster” of tetrahedral cations which are coordinated to one S1 and three S2 atoms [36] ................................................................ 7 Figure 2.2 Eh-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulfur species = 0.1 mol/L. Activities of aqueous iron and nickel species = 0.1 mol/L (bold line) and 10-6 mol/L (fine line) [37]. ............................................................................................... 8 Figure 2.3 Eh-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulfur species = 0.1 mol/L. Activities of aqueous iron and nickel species = 0.1 mol/L [37]. ......... 10 Figure 2.4 Surface functional groups on the mineral surface [38] ............................................... 11 Figure 2.5 Chemical structure of a typical lignosulfonate segment [54] ...................................... 16 Figure 2.6 Speciation diagram of functional groups for the lignosulfonate-water system [55] ... 16 Figure 2.7 The basic repeating unit in condensed tannins [57] .................................................... 17 Figure 2.8 The structure of O-phenylenediamine ......................................................................... 18 Figure 2.9 Speciation diagram for the OPD-H2O system [59] ..................................................... 18 Figure 2.10 Building block of humic acids [62] ........................................................................... 19 Figure 2.11 General flowsheet of Cominco’s zinc pressure leach plant [66] ............................... 20 Figure 2.12 Liquid sulfur-mineral contact angle in aqueous solution (a) in the absence of a sulfur dispersing agent and (b) in the presence of a sulfur dispersing agent [93]. .......................... 33 Figure 2.13 The three stages of wetting of sulfide mineral by liquid sulfur [93] ......................... 37 Figure 2.14 Profile of a pendant drop ........................................................................................... 39 Figure 3.1 Liquid sulfur-aqueous solution interfacial tension versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4 ................................................................................... 57 Figure 3.2 Liquid sulfur-sulfide mineral contact angle versus acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4 ............................................................................................................. 57 xv  Figure 3.3 Work of adhesion versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4 .................................................................................................................................... 58 Figure 3.4 Liquid sulfur-aqueous solution interfacial tensions versus the acidity of aqueous solution: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.01 mol/L FeSO4 ..................................... 59 Figure 3.5 Liquid sulfur-chalcopyrite contact angles versus the pH of aqueous solution 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.01 mol/L FeSO4 ................................................................ 60 Figure 3.6 Work of adhesion versus the pH of aqueous solution 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, and 0.01 mol/L FeSO4 .............................................................................................. 61 Figure 3.7 Eh-pH diagram of NiSO4-H2O system at 140ºC. Activities of nickel species is 1.0 mol/L [150] ........................................................................................................................... 62 Figure 3.8 Liquid sulfur-aqueous solution interfacial tension versus nickel sulfate concentration: 140ºC, 690 kPa N2, Aldrich LS CA: 0-0.7 g/L. .................................................................... 64 Figure 3.9 Liquid sulfur-aqueous solution interfacial tension versus lignosulfonate dosage 140ºC, 690 kPa N2, 1.0 mol/L NiSO4 ............................................................................................... 65 Figure 3.10 Liquid sulfur-aqueous solution interfacial tension versus concentration curves of lignosulfonate: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4 .......................................................... 67 Figure 3.11 Liquid sulfur-aqueous solution interfacial tension versus sulfur dispersing agent dosage: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, no sulfuric acid ........................................... 70 Figure 3.12 Liquid sulfur-aqueous solution interfacial tension versus temperature, the influence of different kinds of lignosulfonate: 0.3 g/L lignosulfonate, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, no sulfuric acid ......................................................................................................... 71 Figure 3.13 Liquid sulfur-aqueous solution interfacial tension versus temperature: 0.3 g/L SDA, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, no sulfuric acid ........................................................ 72 Figure 3.14 Liquid sulfur-aqueous solution interfacial tension versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 M NiSO4, 0.5 g/L sulfur dispersing agent ...................................... 75 xvi  Figure 3.15 Liquid sulfur-aqueous solution interfacial tension versus sulfur dispersing agent concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4 ..................... 76 Figure 3.16 Liquid sulfur-nickeliferous pyrrhotite contact angle versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.5 g/L sulfur dispersing agent ........ 77 Figure 3.17 Work of adhesion versus sulfuric acid concentration, liquid sulfur-nickeliferous pyrrhotite-aqueous solution system, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.5 g/L sulfur dispersing agent .................................................................................................................... 77 Figure 3.18 Liquid sulfur-nickeliferous pyrrhotite contact angle and work of adhesion versus sulfur dispersing agent dosage, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4 ............................................................................................................................................... 78 Figure 3.19 Liquid sulfur-pyrrhotite contact angle versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.5 g/L sulfur dispersing agent ................................................... 79 Figure 3.20 Work of adhesion versus sulfuric acid concentration, liquid sulfur-pyrrhotiteaqueous solution system: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.5 g/L sulfur dispersing agent ...................................................................................................................................... 79 Figure 3.21 Liquid sulfur-pyrrhotite contact angle and work of adhesion versus sulfur dispersing agent dosage, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4 ...................... 80 Figure 4.1 Infrared spectrum of sodium lignosulfonate (BorrePAL U) ....................................... 89 Figure 4.2 Infrared spectra of different kinds of lignosulfonate ................................................... 90 Figure 4.3 Infrared spectrum of sulfited Quebracho (Orfom® Grade 2 Tannin) ......................... 91 Figure 4.4 Infrared spectrum of humic acid-potassium salt (modified lignite) ............................ 93 Figure 5.1 Particle size distribution of elemental sulfur powder .................................................. 95 Figure 5.2 Electroacoustic effect of sulfur in the presence of lignosulfonate at pH 2.5 ............... 98 Figure 5.3 Electroacoustic effect of sulfur in the presence of SDA at pH 2.5 .............................. 99 Figure 5.4 ESA signal of nickel concentrate-water suspensions as a function of pH, the acidity of xvii  the solution was adjusted by sodium hydroxide, hydrochloric acid or sulfuric acid .......... 102 Figure 6.1 Absorbance spectra of BorrePAL U in aqueous solution, at the concentration of 150 mg/L, 125 mg/L, 100 mg/L, 75 mg/L, 50 mg/L, 25 mg/L, 10 mg/L, 5 mg/L, from top to the bottom at pH 3.5; at the concentration of 100 mg/L, 50 mg/L at natural pH ..................... 106 Figure 6.2 Absorbance spectra of BorrePAL N in aqueous solution at pH 3.5 .......................... 107 Figure 6.3 Absorbance spectra of BorrePAL S in aqueous solution at pH 3.5........................... 107 Figure 6.4 Absorbance spectra of humic acid in aqueous solution at pH 3.5 ............................. 108 Figure 6.5 Absorbance spectra of Quebracho: (1) 100 mg/L pH 2.2; (2) 50 mg/L pH 2.2; (3) 2 g/L Quebracho, butanol reagent: supernatant = 7:1; (4) 2 g/L Quebracho, butanol reagent: supernatant = 14:1; (5) butanol reagent: supernatant = 7:1, without Quebracho ............... 109 Figure 6.6 Absorbance spectra of OPD: 1: pH 6.1; 2: pH 4.5; 3: pH 3.2; 4: pH 2.4; 5: pH 5.8; 6: pH 4.0; 7: pH 3.0; 8: pH 2.5 (1-4, 100 mg/L; 5-8, 50 mg/L) .............................................. 110 Figure 6.7 The influence of metal content (9.6 mg/L FeSO4, 8.5 mg/L NiSO4) and time on the absorbance spectra of 50 mg/L OPD at pH 3.5. From top to the bottom: Line 1-3 with iron and nickel; 5 h, 4 h, 3 h; Line 4-6 without metal; 5 h, 4 h, 3 h. Time recording started once OPD was put into acidic water to dissolve ......................................................................... 112 Figure 6.8 The influence of metal content (9.6 mg/L FeSO4, 8.5 mg/L NiSO4) on the absorbance spectra of 50 mg/L OPD at pH 3.5 at 5 h: 1 with iron; 2 without iron and nickel; 3 with nickel. Time recording started once OPD was put into acidic water to dissolve................ 113 Figure 6.9 Influence of conditioning time on the sulfur dispersing agent adsorption on the nickel concentrate at natural pH .................................................................................................... 114 Figure 6.10 Influence of time on the stability of OPD when conditioning with the nickel concentrate: 140 mg/L OPD, 4.00 g nickel concentrate, 100 mL OPD solution ................ 115 Figure 6.11 Adsorption isotherm for BorrePAL U adsorption on the nickel concentrate at natural pH - linear regression method ............................................................................................. 119 xviii  Figure 6.12 Adsorption isotherm for BorrePAL U adsorption on the nickel concentrate at natural pH - non-linear regression method ..................................................................................... 120 Figure 6.13 The adsorption of BorrePAL U on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation ............................................................... 122 Figure 6.14 The adsorption of BorrePAL N on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation ............................................................... 123 Figure 6.15 The adsorption of BorrePAL S on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation ............................................................... 123 Figure 6.16 The adsorption of humic acid on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation ............................................................... 127 Figure 6.17 The adsorption of Quebracho on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation ............................................................... 129 Figure 7.1 Particle size distributions of the nickel concentrate samples. Concentrate A: XSTRATA Nickel concentrate sample; concentrate B: reground XSTRATA Nickel concentrate sample .............................................................................................................. 133 Figure 7.2 Diagram of the setup for oxygen pressure leaching experiment ............................... 135 Figure 7.3 A scanning electron micrograph of the XSTRATA Nickel concentrate sample (-100+200 mesh): 1, pentlandite; 2, pyrite; 3, pyrrhotite; 4, chalcopyrite; 5, gangue ........ 137 Figure 7.4 Leach residue in the absence of sulfur dispersing agent (> 125μm fraction):  140ºC,  690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80: 48 µm), 2 h .... 145 Figure 7.5 EDX Mapping of leach residue (> 125 μm): 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 48 µm), 2 h ........................................................ 145 Figure 7.6 Chemistry of the acid-butanol reaction. Note that the reaction involves oxidation and that the terminal unit does not give a colored anthocyanidin product structure [184, 57] . 149 Figure 7.7 The scheme for the formation of one-dimensional nanobelts by direct mixing of OPD xix  and FeCl3 aqueous solution at ambient temperature [186] ................................................. 150 Figure 7.8 Cumulative oxygen consumption versus time: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (P80 of 10 µm), 2 h, 5 kg/t additive ....................... 156 Figure 7.9 Cumulative oxygen consumption versus time: 150ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (-44 µm), 2 h, 5 kg/t additive................................ 158 Figure 7.10 The influence of chloride concentration on the cumulative oxygen consumption: 150ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate (-44 µm), 2 h, 5 kg/t lignosulfonate sealed in an ampoule, * lignosulfonate was not sealed in ampoule ..... 164 Figure 7.11 Cumulative oxygen consumption versus time: 150ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate (-44 µm), 2 h, 5 g/L chloride, 5 kg/t sulfur dispersing agent .................................................................................................................................... 166 Figure A1.1 Cross-sectional view of Owusu’s high temperature-high pressure apparatus for generating pendant and sessile drops of liquid sulfur ......................................................... 198 Figure A2.1 Calibration curves of BorrePAL U for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 281 nm ......................................................... 202 Figure A2.2 Calibration curves of BorrePAL N for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 280 nm ......................................................... 203 Figure A2.3 Calibration curves of BorrePAL S for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 278 nm ......................................................... 203 Figure A2.4 Calibration curves of humic acid for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 254 nm ......................................................... 204 Figure A2.5 Calibration curves of Quebracho by acid-butanol assay for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 550 nm ...... 204 Figure A2.6 Calibration curves of OPD for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 266 nm ......................................................................... 205 xx  LIST OF SYMBOLS  absorbance; the molecular area occupied by one lignosulfonate molecule  A  A S saturated amount of adsorption  adsorption on the nickel concentrate surface, mg/m2  Abs  concentration of solute, mg/L  c  c RX Z  concentration of ionic surfactant, mol/L  or C Eq equilibrium concentration of solution, mg/L  C  C in initial sulfur dispersing agent concentration, mg/L  path length through the sample, cm  d  d e equatorial diameter of drop, cm d s diameter of drop at a distance d e from apex, cm  acceleration due to gravity, 9.81 m/S2  g  G    specific surface free energy   G W surface free energy change associated with the mineral wetting by liquid sulfur H  dimensional parameter as a function of drop shape  I 0 intensity of the light entering the sample I  K  intensity of the light emerging from the sample Langmuir adsorption constant  K  a measure of the adsorbent capacity, or adsorption constant  m  coefficient of the Gibbs adsorption; or weight of nickel concentrate sample, g  M  molecular weight of the adsorbate xxi  N A is Avogadro constant, 6.022×10  23  mol-1  the intensity of adsorption  1/ n  P80 80% passing particle size, μm  adsorption density, mg/m2  q  q m maximum adsorption density, mg/m  ionic surfactant  RX  Z  R  universal gas constant  S  shape ratio of pendant drop  S BET  2  specific surface area of nickel concentrate, m2/g  SDA sulfur dispersing agent transmittance or temperature, °C, K  T  2  W  work of adhesion; mJ/m  2  W C work of cohesion, mJ/m D  Wa  dispersion force component of work of adhesion  DA  donor-acceptor bonding component of W aP  DD  dipole-dipole interactions component of W aP  Wa  Wa  E  electrostatic interactions component of W aP  P  polar force component of work of adhesion  H  hydrogen bonding force component of W aP  Wa Wa Wa  I  Wa    Wa  dipole-induced dipole interactions component of W aP  - bonding component of W a  P  xxii  W T total liquid sulfur-mineral interfacial energy change, mJ/m  2  W I liquid sulfur-mineral energy change due to adhesional wetting, mJ/m W II liquid sulfur-mineral energy change due to spreading wetting, mJ/m  2  2  W III liquid sulfur-mineral energy change due to immersional wetting, mJ/m  2    liquid sulfur-mineral contact angle; degree    surface tension of molten sulfur; interfacial tension between two bulk phases; mN/m   A the surface tension of aqueous solution, mN/m  MA mineral-aqueous solution interfacial tension, mN/m  MS mineral-liquid sulfur interfacial tension, mN/m  S the surface tension of molten sulfur, mN/m   SA liquid sulfur-aquous solution interfacial tension, mN/m   i  adsorption of component i at liquid sulfur-aqueous solution interface   max the maximum amount of lignosulfonate adsorbed at the liquid sulfur-aqueous solution  interface  RX Z  adsorption of ionic surfactant at liquid sulfur-aqueous solution interface   i chemical potential of component i  SA attractive cohesive energy between liquid sulfur and aqueous solution   constant or surface area of mineral in contact with an equal surface of the liquid sulfur after  adhesion  molar absorptivity 3  3    difference in density between the liquids under test, kg/m or g/cm  xxiii  ACKNOWLEDGEMENTS  I would like to thank the following people, without whom I could not have completed this work: Professor David Dreisinger, my supervisor, for his support and guidance. Berend Wassink, Jianming Lu for their help and assistance in the lab, as well as other group members. Guy Kluck, Borregaard LignoTech, for the lignosulfonate and humic acid samples. And many thanks for the free Quebracho sample from Chevron Phillips Chemical Company LP. Shiyi Xie, my wife, for her support from year 2003 to 2008.  xxiv  1.0 Background Pressure hydrometallurgical techniques have been in use for about 150 years, with applications such as extraction of aluminium, tungsten, nickel and cobalt, copper, zinc, gold, and uranium. Pressure oxidation with oxygen has been applied industrially to treat sulfide ores or concentrates, such as nickel, zinc, copper and refractory gold ores [1-2].  Pentlandite is the most common nickel sulfide mineral. Pentlandite coexists with other sulfide minerals, such as pyrrhotite, chalcopyrite, and pyrite, and may contain cobalt and possibly gold, silver, and Platinum Group Metals. Sulfide minerals in nickel concentrates are often present in complex mineralogies and may be intergrown with one another and with gangue [3]. The choice of hydro or pyrometallurgical methods to treat nickel sulfide concentrates depends on the complexity of minerals, as well as the content of cobalt and Platinum Group Metals [4]. Nickel sulfide concentrates are mainly (>90%) treated by pyrometallurgical methods to form nickel matte, and nickel matte can be further treated to produce nickel products, such as nickel cathode, nickel pellets/powder, or nickel oxide [5].  Several hydrometallurgical nickel matte refining methods have been developed and commercialized, such as the ammonia pressure leaching process, atmospheric acid leaching process, acid pressure leaching process, and chloride leaching process. The ammonia pressure leaching process developed by Sherritt Gordon was commercialized at Fort Saskatchewan, Alberta, Canada in 1954 and at Kwinana in Western Australia in 1970 [6-9]. It was the first integrated hydrometallurgical method to treat nickel sulfide concentrates. Today it is mainly utilized to treat nickel matte in Australia and nickel sulfide precipitate in Canada. The atmospheric acid leaching process was developed by Outokumpu for the Harjavalta refinery in 1960 and was also adopted by Bindura and Empress nickel refineries in Zimbabwe [5, 10-12]. Outokumpu upgraded this process in 1981 and 1996. Sherritt Gordon developed an acid pressure 1  leaching process in the 1960s. Three major platinum producers in South Africa and Stillwater in the USA adopted the acid pressure leaching technology [5, 10-17]. The chloride leaching process was developed and successfully commercialized by Falconbridge at Kristiansand in Norway and was also adopted at refineries in France and Japan [10, 18-19].  Pyrrhotite concentrates represent another major source of nickel although their nickel content is usually less than 1%. To treat pyrrhotite concentrates, INCO and Falconbridge tried to use the roast-leach process, but neither succeeded. The first commercial hydrometallurgical method was successfully applied at the Nadezhda metallurgical plant in Norilsk in 1979 [20-24].  A problem with the pyrometallurgical method for the treatment of nickel concentrates is sulfur dioxide pollution. However, the direct leaching process can solve the problem, which in turn, is potentially economically attractive. The only two integrated hydrometallurgical methods for nickel concentrate which were commercially operated were the Outokumpu HIKO process and Sherritt’s ammonia leaching process. In both cases, all the sulfur in the feed was converted to ammonium sulfate. The Outokumpu HIKO process was carried out in 1991 at the Outokumpu Chemicals’ Kokkola plant [25-26]. Nickel concentrates and secondary nickel raw materials were leached in an autoclave with an oxygen pressure of 500 kPa at 110°C. During this process, nickel was obtained in the form of nickel sulfate. No further information is publicly available for this process. Sherritt’s process was based on ammoniacal oxygen pressure leaching and the recovery of nickel powder by hydrogen reduction. Its drawbacks were high nickel loss and intensive energy consumption.  Seven hydrometallurgical processes were developed to treat nickel sulfide ores and concentrates, including the BioNIC Process, Intec Nickel Process, CESL Process, Activox® Process, INCO Process, OUTOKUMPU Process, and PLATSOL Process. The BioNIC Process aimed to extract nickel from low-grade sulfide ores, and its feasibility was proved by laboratory and pilot scale 2  experiments [27]. Intec Nickel Process was based on the leaching by the Halex  oxidant (BrCl2-), and it was deemed as a potential method to treat low-grade and complex nickel concentrates at low capital and operating costs [28].  The CESL process was developed to treat nickel-copper-cobalt sulfide concentrates. The process consisted of oxygen pressure leaching in a mixed sulfate and chloride medium at about 150°C, in which chloride ions acted as a catalyst. The metal components, such as copper, nickel, and cobalt, were leached into solution, while iron remained in the solid phase; most of the sulfide sulfur was converted to the elemental form. The pilot plant test work conducted by CESL on Mesaba concentrates (14% Cu, 2% Ni) demonstrated that the CESL process was a feasible method of treating complex bulk concentrates [29-30].  The Activox® process was developed to treat sulfide concentrates using a combination of fine grinding and a low temperature, low pressure leaching. Generally, the sulfide concentrate was ground to a P80 of 10 µm. The leaching temperature was below 110°C with the total pressure set to about 1 MPa at a slurry density of 10-30% solids, and a residence time of 1-2 h. Most of the sulfide sulfur was converted to elemental sulfur. The Activox® process has been successfully tested at a demonstration plant [31].  INCO proposed a method to treat nickel concentrates, which involved fine grinding, atmospheric acid chlorine leaching, and oxidative pressure leaching [32]. Copper and cobalt were recovered from solution by solvent extraction or precipitation, and nickel was recovered by direct electrowinning. In INCO’s mini-pilot plant, 97-98% of nickel was extracted from concentrates. During the pressure leaching stage, a sulfur dispersing agent was used to disperse molten sulfur to ensure the complete extraction of base metals.  3  OUTOKUMPU invented a process for recovering nickel from nickel sulfide ores or concentrates by an oxidative atmospheric pressure leaching process using a sodium chloride solution containing hydrochloric acid [33]. The leaching step was carried out by multistage counter current leaching between 95ºC and 110ºC. Most of the sulfide sulfur in the feed was converted to elemental sulfur, which reduced the sulfate content in the leachate.  The PLATSOL Process used total oxidation autoclave leaching with addition of chloride salts to treat copper-nickel-cobalt-PGE/PGM concentrates. The leaching step was operated at 225ºC and about 690 kPa of oxygen overpressure. Copper, nickel, and cobalt extraction were high under total oxidation conditions. The effect of chloride is to extract PGM and Au as chloride complexes [34].  As an alternative to the above seven processes, oxidative pressure leaching of sulfide concentrates in sulfuric acid media at medium temperatures (140-150ºC) is a potential method of nickel extraction. This leaching process has already been commercialized for zinc sulfide concentrates, and similar processes were developed for copper sulfide concentrates. Leaching at temperatures below the melting point of elemental sulfur is another way to avoid the formation of sulfuric acid; however the operating cost maybe high due to the cost of fine grinding. Leaching results in the transformation of sulfide sulfur to sulfate at temperatures above 200°C. The higher temperatures and pressures of this strategy require more robust engineering and more frequent maintenance, which means higher capital and operating cost. The problem of the pressure leaching at medium temperatures is that molten sulfur could occlude unreacted minerals and lower leaching kinetics and nickel recovery. However, the addition of sulfur dispersing agents into the leaching solution is a possible way to solve this problem.  4  Research on the nature of molecular interactions in the liquid sulfur-nickel sulfide mineral-nickel sulfate aqueous system is required to advance the study and commercial design of processes operating at 150ºC. Few studies have been performed on the behavior of SDAs in the direct oxygen pressure leaching of nickel concentrates at medium temperatures. This research focuses on the following aspects:   To understand the reason why SDAs are effective in solving the sulfur wetting problem during leaching    To understand how SDAs adsorbed on both sulfur and nickel concentrate surfaces    To understand the effect of SDAs in the pressure leaching of nickel concentrate.  5  2.0 Literature Review Sulfur dispersing agents in hydrometallurgy (also called surface active agents or surfactants in the literature) have an effect on the surface chemistry in the liquid sulfur-sulfide mineral-aqueous solution system. Previous investigations on the effect of sulfur dispersing agents were focused on interfacial phenomena and/or leaching kinetics. The effect of sulfur dispersing agents on the oxygen pressure leaching of sphalerite, chalcopyrite, and pyrite was thoroughly studied, while research on the interfacial properties in the liquid sulfur- pentlandite-nickel sulfate system is limited.  In hydrometallurgy, studies of the adsorption of sulfur dispersing agents on sulfide minerals at low pH are rare. In contrast, it is a subject of great interest in mineral flotation, but under basic pH conditions. The methodology of interfacial studies in the liquid sulfur-sulfide mineral-aqueous solution system was reviewed. In addition, the theory and methods on adsorption isotherm and the leaching kinetics of nickel concentrates in both sulfate and chloride media were reviewed. The results of the literature review are presented in the following sections.  2.1 Properties of Pentlandite Pentlandite has the chemical formula of (Fe,Ni)9S8. Pentlandite has a yellowish bronze color, with a Mohs scale hardness of 3.5-4.0 and a specific gravity of 4.6-5.0. Pentlandite is non-magnetic and melts at or above 1000ºC [35].  6  2.1.1 Crystal Structure  The crystal structure of pentlandite is shown in Figure 2.1. Pentlandite can form isometric crystals, but it is usually found in massive granular aggregates. The metal to sulfur ratio or the nickel to iron ratio ranges widely in natural pentlandite. The chemical formulas for nickel-rich and for iron-rich pentlandites are Ni9-xFexS8, Fe9-xNixS8, respectively. The structure of natural pentlandites is similar to that of synthetic Co9S8 and can be stabilized by metal-metal bonding. Three metal-metal bonds around each tetrahedral cation result in the formation of a metallic cube cluster [36].  Figure 2.1 The structure of pentlandite showing the “cube cluster” of tetrahedral cations which are coordinated to one S1 and three S2 atoms [36]  7  2.1.2 Thermodynamic Aspects of Leaching  The Fe4.5Ni4.5S8-H2O system Eh-pH diagram helps to provide understanding of the leaching methods for pentlandite. Figures 2.2 and 2.3 are the Eh-pH diagrams for the Fe-Ni-S aqueous system at 298 K. In acid media, pentlandite decomposition could occur according to reaction (2-1): Fe 4 .5 Ni 4 .5 S 8  4 . 5 O 2  18 H     4 . 5 Fe  2   4 . 5 Ni   G Re action ( 298 K )   1881 . 83 0  2   9 H 2O  8 S  (2-1)  kJ mol-1  Figure 2.2 Eh-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulfur species = 0.1 mol/L. Activities of aqueous iron and nickel species = 0.1 mol/L (bold line) and 10-6 mol/L (fine line) [37]. 8  The sulfide sulfur is oxidized directly to elemental sulfur. The sulfur product layer can block the mass transport of oxidant and metal products between the bulk solution and the mineral interface.  2.1.3 Pentlandite Mineral Surface Chemistry  Eh-pH diagrams of the Fe-Ni-S aqueous system may help explain how pentlandite surface chemistry is influenced by pH and potential. Sulfide minerals have at least two functional groups:  Me  OH  and  S  H . The possible surface functional groups on sulfide mineral  surfaces are shown in Figure 2.4 [38]. A sulfur rich surface is formed below pH 4 on the pentlandite mineral surface due to the formation of elemental sulfur [39]. When natural pentlandites are exposed to de-ionized water, the mineral surface is rich in iron oxyhydroxide species and depleted in nickel and sulfur [40]. These aspects are consistent with the Eh-pH diagram showing elemental sulfur at low pH and the increased presence of iron and nickel oxides and hydroxides at high pH.  9  Figure 2.3 Eh-pH diagram for the Fe-Ni-S aqueous system at 298 K. Activities of aqueous sulfur species = 0.1 mol/L. Activities of aqueous iron and nickel species = 0.1 mol/L [37].  10  Figure 2.4 Surface functional groups on the mineral surface [38]  2.2 Elemental Sulfur Sulfur is a soft, bright yellow solid at ambient temperature and is insoluble in water. Carbon disulfide is a commonly used solvent of sulfur. Sulfur can form stable compounds with all elements except noble gases [35].  2.2.1 Properties of Elemental Sulfur  Sulfur is the element with the largest number of allotropes. Currently about thirty sulfur allotropes have been characterized. In solid and liquid states the main sulfur allotropes are Sλ (crown-shaped octatomic sulfur ring), Sμ (long, polymerized chains of elemental sulfur), and Sπ (octatomic sulfur chain). Sλ, the only stable sulfur allotrope in the solid state, crystallizes either as orthorhombic α-S8, monoclinic β-S8 or monoclinic γ-S8. At atmospheric pressures, α-S8 is stable below 95.5°C; it transforms to β-S8 above this temperature. β-S8 is stable up to 114.5°C [41, 42]. Some physical properties of sulfur are given in Table 2.1 [43].  11  Liquid sulfur is a combination of Sλ, Sµ, and Sπ: below 160°C, the proportion of Sλ is more than 90%; above 160°C, the proportion of Sµ increases greatly to over 25% at 180°C; at any temperatures, the proportion of Sπ is low [44]. Liquid sulfur dynamic viscosity exhibits a minimum of 0.007 Pa s at 157°C but increases dramatically in the temperature range of 157-190°C, reaching to a maximum of 93.2 Pa s at 187°C. If the temperature rises further, the viscosity decreases from 190°C and reaches to 0.1 Pa s at the boiling point, 444.6ºC.  Table 2.1 Selected physical constants of sulfur [43] Property  Ideal Value  Natural Value  112.8  110.2  119.3  114.5  Freezing point of solid phase, °C Orthorhombic monoclinic Density of solid phase, 20°C, g/cm  3  Orthorhombic  2.07  Monoclinic  1.96  Amorphous Density of liquid, g/cm  1.92 3  125°C  1.7988  130°C  1.7947  140°C  1.7865  150°C  1.7784  2.2.2 The Behavior of Sulfur in Oxidative Leaching of Sulfide Ores  The behavior of sulfur is complex in the oxygen pressure leaching of zinc sulfide minerals at moderate temperatures. Four steps are needed to recover elemental sulfur from zinc concentrates, including pressure leach, flotation, melting & filtration, and purification [45]. The formation of 12  elemental sulfur is unfavorable with an increase of pH or temperature [46]. The leaching temperature, <157C, is important to improve the leaching kinetics and obtain a minimum viscosity of molten sulfur [47].  The formation of sulfuric acid in the oxygen pressure leaching process greatly depends on temperature and oxygen partial pressure. Below the melting point of sulfur the rate is extremely slow, while above this temperature it is appreciable and increases rapidly with rising temperature [44, 48]. However, sulfur was found to be rather stable during leaching. Peters extended the sulfur stability region on Eh-pH diagram by increasing the standard free energy of formation of sulfate and bisulphate to 314 kJ mol-1, in order to express the overvoltage necessary to oxidize sulfur to sulfate [49]. In the zinc industry sulfate ion formation in solution is balanced by sulfate loss in residue and bleed solutions. Sulfate ion is formed by oxidation of pyrite and a small fraction of the elemental sulfur formed during leaching [45].  2.3 Sulfur Dispersing Agents Sulfur dispersing agents in hydrometallurgy have been called surface active agents or surfactants in previous studies. Any material with the ability to disperse molten sulfur from sulfide mineral surfaces could be viewed as sulfur dispersing agents. Surfactants have an ability to lower the free energy of the phase boundary while sulfur dispersing agents must have the ability to increase the contact angle between liquid sulfur and sulfide mineral under oxygen pressure leaching conditions.  Dispersion is defined as a homogeneously mixed state of small particles in a liquid medium [50]. Dispersants are compounds used to maintain particles suspended in a liquid medium by lowering the coagulation rate. DLVO theory is a comprehensive theory of the interaction potential 13  between colloidal particles. The interaction potential between two colloidal particles is the sum of the electrostatic repulsive and the van der Vaals attractive interactions. In this study, the term "sulfur dispersing agent" will be used to describe the role of the chemical additive in removing sulfur from unreacted sulfides during leaching. The mode of dispersing sulfur is not the same as in classical DLVO theory but rather involves changing in the wetting behavior of liquid sulfur on the mineral surface. Nevertheless, the overall result is dispersion of sulfur as fine droplets into the bulk slurry and hence we use the nomenclature "sulfur dispersing agent". As a short form in this thesis "SDA" is used to represent sulfur dispersing agent.  2.3.1 Characteristic Features of Sulfur Dispersing Agents  A surfactant can be absorbed onto the surfaces or interfaces of a system and can change the surface or interfacial free energies when it exists in low concentrations. The characteristic molecular structure of a surfactant contains a hydrophobic group and a hydrophilic group. Based on the nature of the hydrophilic group surfactants can be categorized into four groups: anionic, cationic, zwitterionic, and nonionic. The structures of the hydrophobic group can be divided into nine groups [51]: branched-chain, long alkyl groups (C8-C20); straight-chain, long alkyl groups (C8-C20); alkylnaphthalene residues (C3 and greater-length alkyl groups); long-chain (C8-C15) alkylbenzene residues; high-molecular-weight propylene oxide polymers (polyoxypropylene glycol derivatives); long-chain perfluoroalkyl groups; rosin derivatives; polysiloxane groups; lignin derivatives. Lignosulfonate was classified as an anionic surfactant by Rosen [51].  The sulfur dispersing agents used in the oxygen pressure leaching of sulfides must have the ability to disperse the molten sulfur from the mineral surface. The requirements for SDA are as follows [52]:   Not to introduce undesirable impurities into slurry    Are soluble in the solution phase 14    Do not enter into any reactions that change the essential chemical nature of the constituents in slurry    No negative impact on downstream operations.  2.3.2 Properties of Individual Sulfur Dispersing Agents  Lignosulfonate and Quebracho are commercial SDAs in the oxygen pressure leaching of zinc. O-phenylenediamine (OPD) was proved effective in the pressure leaching of sulfide minerals, including sphalerite, chalcopyrite, as well as pyrite gold ores and concentrates. Humic acid (modified lignite), recommended by LignoTech USA, was investigated in this study.  (1) Lignosulfonate  Lignosulfonate is a complex anionic polyelectrolyte which is obtained as a byproduct from the production of wood pulp using sulfite pulping. It is difficult to characterize the chemical and physical properties of lignosulfonate due to natural variability. The wide variety of physical properties makes industrial lignosulfonate applicable as a dispersant and a binder [53]. Lignosulfonate is water soluble and has a molecular weight range from several hundred to several million [50]. Its structure contains sulfonic, free phenolic, primary and secondary alcoholic, and carboxylate groups [51]. The chemical structure of a typical lignosulfonate segment is shown in Figure 2.5 [54]. The dissociation of functional groups in lignosulfonate is shown in Figure 2.6 [55].  15  O CH HOH2C  SO3Na CH  OCH3 OH  Figure 2.5 Chemical structure of a typical lignosulfonate segment [54]  Figure 2.6 Speciation diagram of functional groups for the lignosulfonate-water system [55]  16  (2) Quebracho  Quebracho is the traditionally important commercial source of condensed tannins (or proanthocyanidins, PAs). The formation of condensed tannin involves simple polyphenols, such as flavan-3-ols, flavan-3-, and -4-diols [56]. Quebracho extract acts as a mud thinner, as well as a depressant and defoaming agent in the mining industry. The basic repeating unit in condensed tannins is shown in Figure 2.7 [57].  OH OH 8  HO  A  B O 2 R3  C  6 4  3 R2  R1 R1 = OH, R3 = H R1 = OH, R3 = OH R1 = H, R3 = H R1 = H, R3 = OH R1 = R2 = OH, R3 = H  Proanthocyanidin Prodelphinidin Profisetinidin Prorobinetinidin (-)-epicatechin  Figure 2.7 The basic repeating unit in condensed tannins [57]  (3) O-phenylenediamine  OPD, also called 1, 2-benzenediamine, has the chemical formula C6H4(NH2)2 and belongs to the aromatic amine family. The structure of OPD is shown in Figure 2.8. Pure OPD is a white solid but rapidly becomes colored on exposure to air. OPD’s molecular weight is 108.1, and its melting point and boiling point are 102°C and 256-258°C, respectively. OPD is easily soluble in hot water but only dissolves 4 g in 100 mL water at ambient temperature. OPD is useful to form heterocyclic compounds and to identify a variety of functional groups as an analytical reagent 17  [58]. OPD is positively charged at low pH in solution as shown in Figure 2.9 [59].  NH2 NH2  Figure 2.8 The structure of O-phenylenediamine  Figure 2.9 Speciation diagram for the OPD-H2O system [59]  (4) Humic Acid  Humic acid (HA), exists widely in soil and natural water. It is a polyelectrolyte formed from plants and animals by decay and biological action [60]. HA can also be extracted from low rank coal and oxidized coal with aqueous alkali solution. In lignite, humic acid is so abundant that 20% or more can be extracted. The HA molecular structure is variable, and strongly depends on 18  the source of the humic acid. The molecular weight is reported in a wide range 600-10,000 [61]. Carboxyls, hydroxyls and carbonyls are the major oxygen-containing functional groups in humic acid. A proposed building block of humic acid is shown in Figure 2.10 [62].  HO O  OH O  O  O  COOH H2N OH HO HOOC  COOH OH  Figure 2.10 Building block of humic acids [62]  2.4 Oxygen Pressure Leaching in Sulfate Media Oxygen pressure leaching techniques have been commercialized to treat pyrrhotite concentrates and zinc sulfide concentrates at medium temperatures and copper sulfide concentrates and refractory gold ores/concentrates at high temperatures. More hydrometallurgical processes are approaching or entering the commercial stage [63-64].  2.4.1 Pressure Leaching of Zinc Sulfide Concentrates  The Sherritt Zinc Pressure leaching process for the treatment of zinc sulfide concentrates has succeeded in commercial operation since 1981, and subsequently was adopted by several other zinc producers to replace the traditional roast-leach technology. Zinc concentrates were leached in zinc spent electrolyte at about 150°C, under 700 kPa oxygen overpressure, and 1 h retention 19  time. Zinc extraction exceeded 98%, and most of the sulfide sulfur was converted to elemental sulfur [65]. Figure 2.11 shows the general flowsheet of Cominco’s zinc pressure leach plant [66]. In most commercial zinc pressure leach processes the lignosulfonate concentration is 0.3 g/L.  Figure 2.11 General flowsheet of Cominco’s zinc pressure leach plant [66]  2.4.2 Pressure Leaching of Refractory Gold Ores and Concentrates  Sherritt Gordon’s pressure oxidation technology was applied to the treatment of refractory gold ores. Its first commercial application was commenced at McLaughlin Gold Mine, Homestake 20  Mining Company in 1983. In this technology, pressure oxidation was operated at temperatures between 190 and 225°C with an oxygen partial pressure of 350-700 kPa. The above operating parameters ensured an effective oxidation of refractory sulfide phase allowing for gold cyanidation of the leach residue [67-69].  2.4.3 Pressure Leaching of Copper Sulfide Concentrates  The pressure leaching processes for copper hydrometallurgy include the total pressure oxidative process and medium temperature leaching process. The total pressure oxidative process was commercialized by Phelps Dodge Mining Company in 2003. All the sulfide sulfur in the copper concentrates was converted to sulfate and sulfuric acid at 200-230°C, and more than 99% of the copper was dissolved in the pressure leaching stage [70-71].  Dynatec’s copper process was based on its zinc leach process. For this process, the concentrate was ground to about 30-50 µm and oxidized at 150°C. About 25% of sulfur was oxidized to sulfate. The addition of low rank coal was found effective at dispersing elemental sulfur during leaching [72]. The AAC/UBC (Anglo American Corporation/ University of British Columbia) method for the treatment of copper concentrates was a combination of regrinding (5-20 µm) and moderate pressure leaching at 150°C. A SDA was added to disperse the molten sulfur formed during the leaching process [73-74]. Both medium temperature processes are not yet industrialized. However, Phelps Dodge is currently commissioning a medium temperature leaching process for chalcopyrite at Morenci.  21  2.4.4 Pressure Leaching of Pyrrhotite Concentrates  In the Norilsk process the typical composition in the pyrrhotite concentrates was: 2-3% Ni, 0.5-1% Cu, 0.1-0.15% Co, and up to 10-15 g/t of platinoids. The concentrate was leached with sulfuric acid at 130-150C, under 1.0 MPa oxygen partial pressure with the production of elemental sulfur and iron oxides. During the process nickel and copper were precipitated back into the residue from the leach pulp; elemental sulfur and iron oxides were separated from the residue by flotation and pressure filtration. Although 65% of sulfide sulfur in the feed was converted to elemental sulfur, only about 30% of the elemental sulfur was recovered as final product. The primary product was enriched nickel concentrate containing 8.5-10% Ni, 2-3% Cu, and 0.3-0.5% Co, which was further treated by the pyrometallurgical method. In this process, 15-20% of the base metals and 30-60% of the platinoids were lost in high-iron pulps [10, 20-23, 69].  2.5 Sulfur Dispersing Agents in Leaching Elemental sulfur, formed during the oxygen pressure leaching of sulfide minerals, can coat or encapsulate unreacted sulfide particles and hinder the transfer of reagents between the bulk solution and the mineral surface. Several methods were tried to solve this sulfur coating problem as follows [75-76]:   Using an organic sulfur solvent, e.g., CCl4 was added into the leach solution to dissolve the sulfur layer on the mineral surface as leaching proceeded.    Simultaneously grinding and leaching, e.g., an attritor was used as the grind-leach reactor so that sulfur layer could be cracked and removed from the mineral surface.    Recycling unleached sulfide after sulfur removal.    Using an inert solid recycle stream, e.g., a fine fraction of leach residue was added to 22  increase the solids content in the autoclave.   Alternative operating conditions, such as fine grinding and higher temperatures.    Using sulfur dispersing agents.  The first two methods mentioned above were proposed to solve sulfur problem at low temperatures. Lignosulfonate and Quebracho are currently used as SDAs in the zinc pressure leaching process, and the former is also applied commercially to the oxidative pressure leaching of pyrrhotite concentrates in Russia.  2.5.1 Sulfur Dispersing Agents in Oxygen Pressure Leaching of Zinc Concentrates  Forward and Veltman [77] found that molten sulfur occluded unreacted zinc sulfide particles, resulting in low zinc extraction at temperatures higher than the melting point of elemental sulfur. Leaching at low temperatures (below 119°C) was chosen to prevent the wetting problem but more time was needed for complete zinc extraction. The leaching at higher temperatures (above 120°C) plus the recycling of the unleached sulfide minerals suffered from high separation costs and slow leaching kinetics.  In the 1970s Sherritt Gordon Mines Limited disclosed a rapid and complete zinc extraction method from zinc sulfide concentrates at 125-175°C with the addition of SDAs [52]. The efficient SDAs were lignin, tannin compounds, or alkylaryl sulphonates. Lignosulfonate or Quebracho aimed to prevent zinc sulfide particles from being coated by the molten sulfur, thus prevented the inhibition of the reaction. The zinc extraction increased from 63.3% to 97.8% from a finely ground ore with the addition of only 0.2 g/L Quebracho. Based on the usage of SDA, a two-stage pressure leaching process for zinc sulfide concentrates was invented [78].  23  Lignosulfonate and Quebracho, the first two SDAs, have been commercially applied in Sherritt’s direct oxidative pressure leaching technology. Alternative additives have been investigated for improving the zinc pressure leaching processes, e.g., sulfite-cellulose liquor and OPD were found effective to increase the zinc extraction in other investigations [79-80]. Batch and miniplant tests proved that low rank coal was a cheap and effective additive with the potential to replace lignosulfonate and Quebracho. A solid state nuclear magnetic resonance (NMR) study indicated that obvious differences existed in the relative amount of aliphatic and aromatic carbon among coal samples in different grades. Coal was effective to disperse liquid sulfur in the pressure leaching experiment when more than 35% of the carbon was in the aliphatic form [81-82].  The influence of SDAs on zinc extraction, sulfur behaviors and zinc electrowinning was further investigated in previous studies. The addition of SDA did not always increase zinc extraction [83]. A large amount of Quebracho was found to hinder the reaction rate. Zinc recovery around 98% was obtained with the moderate addition of Quebracho, while zinc extraction decreased to 40% with an increased addition of Quebracho. The SDA adsorbed on the mineral surface and was suspected of creating a barrier to mass transfer across the mineral-solution interface. SDA was found to suppress zinc extraction from sulfides at pH 0.5-1.5. This is due to the formation of less amount of sulfur during leaching so that the sulfur wetting problem is less pronounced affect leaching than the adverse effect of SDA on the mass transfer across the mineral-solution interface [46].  SDA could adversely impact elemental sulfur flotation after leaching. Lignosulfonate and Quebracho favored the formation of fine sulfur particles which were difficult to recover through flotation [45]. Lignosulfonate was harmful to sulfur flotation due to its dispersing function [84]. The effects of OPD and sodium lignosulfonate on zinc electrowinning were investigated: OPD had a very negative effect on the power consumption of the subsequent zinc electrowinning 24  process. Sodium lignosulfonate had no negative impact on this process or on the quality of the zinc deposits [85].  Xia [84] tried to explore the relationship between leaching behavior and the adsorption of lignosulfonate on the mineral surface. The presence of lignosulfonate increased the surface potential of the mineral and decreased the surface tension of the leach solution. The adsorption of lignosulfonate on the mineral surface increased with the concentration of SDA, which resulted in a higher leaching rate until the adsorption reached saturation. The adsorption mechanism discovered by Owusu [86] showed that lignosulfonate was adsorbed both physically and chemically by sphalerite while OPD was adsorbed chemically through the interaction of the C-N functional group with metal ions, forming metal-amine complexes. The previous pressure leaching studies reflected the behavior of SDAs in leaching while the adsorption study reflected the behavior of additives at low temperatures but provided a way to understand the mineral leaching behavior. However, considering the harsh leaching environment, the behavior of SDA is so complex that the adsorption mechanism might change with time and leaching conditions. Lignosulfonate was degraded by ferric ions to produce ferrous and degraded lignosulfonate, with a half-life of approximately 10 min under oxidative pressure leaching conditions [87].  Pioneering work was performed on the behavior of SDAs under the zinc oxygen pressure leaching conditions. Owusu [88-92] studied the interfacial phenomena in the liquid sulfur-sphalerite-aqueous solution system in the absence or presence of SDAs, including lignosulfonate and OPD. A high temperature, high pressure apparatus was constructed to measure the effect of the additives on the liquid sulfur-aqueous solution interfacial tensions and on the liquid sulfur-mineral contact angles. It was found that the interfacial tensions decreased with the addition of effective SDAs while the contact angles increased in the presence of effective additives. A significant reduction of the work of adhesion was found by the interfacial studies. Owusu observed that the interfacial tension reduction was not the only standard to select 25  SDAs. The other criteria include: the ability to increase the contact angle and the ability to remain stable under pressure leaching conditions.  Owusu stated that lignosulfonate and OPD were effective SDAs due to their ability to enhance zinc extractions, which was consistent with the behavior of additives found in the interfacial studies. Under the conditions of: 140°C, 690 kPa pressure by nitrogen, 1 mol/L zinc sulfate solution, 0.1 mol/L sulfuric acid, in the absence of SDA, the liquid sulfur-aqueous solution interfacial tension was 54 mN/m; in the presence of at least 0.3 g/L lignosulfonate it decreased to 27-30 mN/m. The contact angle between liquid sulfur and sphalerite increased from 80° to 148°. The changes of interfacial tensions and contact angles resulted in the changes to the work of adhesion from 63.7 mJ/m2 to 5.3 mJ/m2. While under the same conditions with addition of 0.3 g/L OPD, there was a small change of the interfacial tension from 54.0 mN/m to 52.4 mN/m. The contact angle increased to 127°, and the work of adhesion decreased to 23.4 mJ/m2.  Depending on the measurement of interfacial tensions and contact angles, both the interfacial energy change in the liquid sulfur-sphalerite-aqueous solution system and the interfacial excess of SDAs at the liquid sulfur-aqueous solution interface were estimated [93]. The driving force for complete sulfur occlusion of unreacted sulfide particles was a decrease of the liquid sulfur-mineral interfacial energy. The presence of SDAs increased the interfacial energy from a negative value in the absence of additives to a positive value which did not favor the sulfur occlusion process. Without the addition of SDA the interfacial energy was -56.6 mJ/m2, while with the addition of lignosulfonate or OPD the interfacial energy increased to 142.5 mJ/m2 or 187.8 mJ/m2. The estimated interfacial excesses of SDAs were 0-1.4 μmol/m2, depending on the concentration of lignosulfonate and naphthalene sulfonic acid-formaldehyde condensates.  26  2.5.2 Sulfur Dispersing Agents in Oxygen Pressure Leaching of Copper Sulfide Minerals  Hackl investigated the feasibility of using SDAs to solve sulfur wetting problem and to enhance copper extraction from the chalcopyrite mineral in the temperature range 125-155˚C [94-96]. Most of the SDAs decomposed quickly. The best results were obtained with addition of 50 kg/t OPD continuously at 125˚C. Under these conditions, 80% copper was extracted into leach solution in 6 h. Chalcopyrite leached slowly even when molten sulfur was prevented from wetting the mineral surfaces. Hackl postulated that the reaction rate was ultimately controlled by a passivating mechanism unrelated to the elemental sulfur formation. It was suggested that chalcopyrite is passivated by copper polysulfide layer formed during leaching. Based on this study and two integrated pilot-plant campaigns the AAC/UBC process was developed. In this process the passivation problem was overcome by fine grinding and the sulfur wetting problem was solved by the addition of SDAs, such as lignosulfonate, Quebracho, or OPD.  The Dynatec process was developed to extract copper from sulfide concentrates. Low rank coal was added to the leaching solution instead of water-soluble SDAs. Dynatec advocated regrind and recycle of residue after elemental sulfur removal [97-99].  2.5.3 Sulfur Dispersing Agents in Pressure Oxidation of Refractory Gold Ores and Concentrates  Sulfur in the pyrite autoclave leach system can encapsulate unreacted pyrite particles and slow or stop the oxidation process at temperatures of 190-230ºC. Dreisinger et al. [100] investigated the effect of SDA in the pressure oxidation of pyrite gold ores and concentrates. They found that OPD was better than lignosulfonate to decrease the interfacial tension between molten sulfur and 27  sulfuric acid solutions and to increase the contact angle between molten sulfur and the pyrite mineral surface. The work of adhesion decreased greatly in the presence of OPD. The pyrite oxidation rate increased in the presence of OPD as measured by the oxygen consumptions in the oxygen pressure leaching experiments. OPD was fully degraded during the high temperature oxidation leaching processes since no residual OPD or degradation products of OPD were detected in the solution or leach residue.  2.5.4 Sulfur Dispersing Agents in Oxygen Pressure Leaching of Pyrrhotite Concentrates  Kunda et al. [101] investigated the aqueous oxidation of artificial pyrrhotite and found that the rate of oxidation was improved considerably by adding 1.0 g/L SDAs at 110ºC. The additives used in the study were Acrysol A-3, Aerosol C-61, Gum Arabic, and Duofol T. The addition of Aerosol C-61 prevented the formation of sulfur granules at about 150ºC. No further information was provided about the properties of the additives.  In the 1970s researchers at INCO worked on a pyrrhotite concentrate leach process and found that the addition of certain organic materials to the leach solution allowed the operation to work smoothly at medium temperatures [102-103]. The sulfur coating problem was solved and more base metals such as nickel were recovered in the presence of recommended additives, including mixtures of water-soluble aliphatic amines, polyalkylene glycol esters, polyoxyethylene lauryl ethers, and sulfonated aromatics. In order to reduce the cost of additives tall oil pitch, an effective SDA depending on the leaching experiments, was recommended to replace the previously suggested materials.  Although lignosulfonate was the major SDA used in the Norilsk process, the new reagent TsIATIM-208, extracted from selective purification of petroleum oil fractions and sulfite-yeast 28  mash concentrates, was investigated in a kinetic study [104]. SDA addition methods were investigated to improve the above process [105]. The feeding of lignosulfonate distributed along the length of the autoclave required less reagent addition than the single-point addition method did.  2.5.5 Sulfur Dispersing Agents in Oxygen Pressure Leaching of Pentlandite Concentrates  The influence of lignosulfonate on the interfacial properties was investigated in the liquid sulfur-pentlandite-aqueous solution system under the pressure leaching conditions of the INCO Process [106]. Calcium and sodium lignosulfonate were found to decrease the liquid sulfur-aqueous solution interfacial tensions and to increase the liquid sulfur-pentlandite contact angles. Although no leaching experiments were performed, molten sulfur was believed to be easier to remove from the mineral surface during leaching in the presence of lignosulfonate. The presence of chloride ions in the aqueous solution reduced the work of adhesion in the liquid sulfur-pentlandite-aqueous solution system, which meant liquid sulfur would be easier to remove from the pentlandite mineral surface if the leaching solutions contained chloride ions.  2.6 Liquid Sulfur-Aqueous Solution Interface Interface is the boundary between two immiscible phases. Surface is the interface where one phase is a gas, usually air [51]. At the interface between liquid sulfur and aqueous solution molecules have higher potential energies than those in the two bulk phases. Usually the presence of SDA tends to decrease the interfacial tension of the system.  29  2.6.1 Surface Tension of Pure Liquid and Binary Systems  For a one-component system the surface tension is equal to the surface free energy per unit area [107]. The surface tension of most liquids decreases linearly with the increase of temperature, reaching a value of 0 at the critical temperature. The following equations indicate the relationship between the surface tension of liquid sulfur and temperature [108]. The temperature of 159°C is the transition temperature of liquid sulfur, beyond which it starts to polymerize [41].   73 . 4  0 . 105 T (T < 159°C)  (2-2)    65 . 7  0 .0566 T (T > 159°C)  (2-3)  Where  is the surface tension, mN/m; T is the temperature, °C.  For a two-phase system, the interfacial tension is defined by the equation below: G        i    i  (2-4)  i  Where G  is the specific surface free energy;  is the interfacial tension between two bulk phases;  i is the chemical potential of component i ;  i is the adsorption of component i at the interface.  The interfacial tension between liquid sulfur and an aqueous solution can be represented by the following equation [50]:  SA   S   A  2 SA  (2-5)  Where  S is the surface tension of molten sulfur;  A is the surface tension of aqueous solution;  SA is the attractive cohesive energy between the liquid sulfur and the aqueous solution. 30  The effect of pressure on surface tension has a relationship with the change in molar volume when molecules move from the interior of a liquid to the surface region. Owusu found that the influence of pressure on the interfacial tension between liquid sulfur and aqueous solution could be neglected. This meant the molar volume changes of the two phases were very small under leaching conditions [88].  2.6.2 Interfacial Excess at Liquid Sulfur-Aqueous Solution Interface  The SDA is adsorbed at the interface between liquid sulfur and aqueous solution when it is added to the liquid sulfur-aqueous solution system. The hydrophilic group is believed to interact with the aqueous phase while the hydrophobic group interacts with the sulfur phase [93].  The interfacial excess of SDA, a thermodynamic number, is an estimate of the concentration of SDA adsorbed at the interface in excess of its concentration in bulk solution per unit area. For the adsorption of an ionic surfactant, the interfacial excess can be calculated by the Gibbs equation [109]:  RX Z    1    mRT    ln c RX Z  (2-6)  Where RX Z is the ionic surfactant; R is the universal gas constant = 8.32 J/(mol K); T is temperature, K; m is coefficient of the Gibbs adsorption, which can be calculated according to the assumed model for the electrical double layer formed at the interface between liquid sulfur and aqueous solution. The estimation of m from Gouy’s electrical double layer was adopted by Owusu [93] and it was assumed that both SDA (such as lignosulfonate) and zinc sulfate dissociate completely. Equation (2-7) was used to calculate the interfacial excess at the interface between liquid sulfur and aqueous solution.  RX z    1 RT      ln c RX z  (2-7) 31  When the interfacial tension is reduced with the addition of a compound the interfacial excess is positive; when the interfacial tension is elevated with the addition of a compound the interfacial excess is negative. According to Owusu [88], in the liquid sulfur-aqueous solution system, the interfacial excess of zinc sulfate was negative since the interfacial tension increased with the concentration of zinc sulfate increasing; the interfacial excess of OPD was near zero because of small changes of the interfacial tension; the interfacial excess of lignosulfonate was positive due to the lowering of interfacial tension.  2.7 Mineral-Liquid Sulfur Interface In the three-phase system of the liquid sulfur-mineral-aqueous solution wetting is the replacement of aqueous solution by liquid sulfur. Dispersing, the opposite process of wetting, is the replacement of liquid sulfur by aqueous solution. The standard to determine if a wetting process takes place is to calculate the driving force:  MA  (  MS   SA ) (refer to Figure 2.12). However, the interfacial tension of solids cannot easily be measured directly. This difficulty may be solved by the measurement of the contact angle between liquid sulfur and the mineral in the presence of an aqueous solution.  2.7.1 Mineral-Liquid Sulfur Contact Angle  The contact angle between liquid sulfur and mineral in aqueous solution with relation to interfacial tension can be represented by Young’s equation [107, 109]:  MA   MS   SA cos   Where  MA  is the mineral-aqueous solution interfacial tension, mN/m;  MS  (2-8) is the  32  mineral-liquid sulfur interfacial tension, mN/m;  SA is the interfacial tension, mN/m between liquid sulfur and aqueous solution;  is the liquid sulfur-mineral contact angle, degree.  In the absence of SDA the wetting of mineral particles by liquid sulfur (small value of contact angle) is due to the attraction between mineral and molten sulfur. A high value of contact angle between mineral and molten sulfur means that liquid sulfur has a low affinity for the mineral particles [90]. Figure 2.12 shows the contact angle increases owing to the addition of SDA in the liquid sulfur-mineral-aqueous solution system. The effect of SDA on contact angle can be explained by the following statement. SDA is adsorbed at the interfaces of liquid sulfur-aqueous solution and mineral-aqueous solution to lower their interfacial tensions. The interfacial tension between mineral and liquid sulfur does not change or changes subtly. The contact point of the liquid sulfur-mineral-aqueous solution is pulled by the relatively large interfacial tension  MS . This deformation in the shape of liquid sulfur is called rolling-up [50].  Figure 2.12 Liquid sulfur-mineral contact angle in aqueous solution (a) in the absence of a sulfur dispersing agent and (b) in the presence of a sulfur dispersing agent [93].  33  2.7.2 Work of Adhesion  In the liquid sulfur-mineral-aqueous solution system work of adhesion is the reversible work required to separate a unit area of liquid sulfur from a mineral surface. Mathematically, work of adhesion is expressed by Dupré [51]: W a   MA   SA   MS    G W /   (2-9)  Where W a is work of adhesion, unit: mJ/m2;  is the surface area of mineral in contact with an equal surface of the liquid sulfur after adhesion;  G W is the surface free energy change associated with the wetting by the liquid sulfur. Equations (2-8) and (2-9) simplify to Equation (2-10) W a   SA (1  cos  )  (2-10)  The work of adhesion between the two dissimilar phases can also be expressed as the sum of the different intermolecular forces acting between liquid sulfur and the solid mineral such as: Wa  Wa  Wa D  P  (2-11)  Wa  Wa  Wa P  H  DD     Wa  Wa  Wa I  DA   Wa   E  (2-12)  Where W aD is due to the dispersion force component of the intermolecular forces which is always present in all systems; W aP is due to the polar component of the interaction forces depending on the polar nature of the phases and occurs to different degrees depending on the system; W aH is hydrogen bonding force component; W aDD is bonding force component due to dipole-dipole interactions; W aI dipole-induced dipole interactions; W a   - bonds; W a  DA  donor-acceptor bonds; W aE electrostatic interactions. For non-polar systems, only dispersive forces are present and hence such a phase interacts with polar systems only through W aD [86, 34  109].  In the liquid sulfur-mineral-aqueous solution system, at the interfaces between liquid sulfur and mineral, liquid sulfur and aqueous solution, and aqueous solution and mineral the adjacent interfacial macromolecular layers are subjected to an asymmetric force field. This results in interfacial tensions between any two phases. The work of adhesion is represented by equation (2-10). Generally, in the presence of effective SDA, the interfacial tension between liquid sulfur and aqueous solution decreases and the contact angle between the liquid sulfur and the mineral increases. Consequently, the work of adhesion decreases, which means less work is needed to separate liquid sulfur from the mineral surface. Therefore, molten sulfur can be easily removed from sulfide mineral surfaces by agitation in leaching.  2.7.3 Interfacial Energy Change  Interfacial energy is defined as the work required to increase the unit area of a surface isothermally and reversibly [110]. It is the sum of free energy of all the molecules present at the interface between different phases and a function of the interfacial tension. Interfacial energy change was used by Owusu [93] to explain the effect of SDAs in the leaching system. Figure 2.13 shows the three stages that a mineral particle undergoes from aqueous solution into liquid sulfur under pressure leaching conditions. The mineral particles are considered to be a cube of unit volume or dimensions. When a particle moves from I to II, adhesional wetting takes place; when the particle moves from II to III, spreading wetting takes place; when the particle moves from III to IV, immersional wetting takes place. The energy changes during the initial three stages are given by the following equations: W I    SA (1  cos  )  W a  (2-13)  W II   4  SA cos   (2-14)  35  W III    SA (cos   1)  (2-15)  For complete wetting of the particle by liquid sulfur, the total liquid sulfur-mineral interfacial energy change, W T (mJ/m2) is: W T  W I  W II  W III   6  SA cos   (2-16)  W I is always negative based on the equation 2-13. Thermodynamically, the adhesional wetting  process is spontaneous. Work must be done in the system to avoid the mineral wetting by liquid sulfur. SDA is used to decrease the extent of first stage wetting. Agitation is a necessary condition to help sulfur leave the mineral surface.  When  is less than 90°, W II is negative and immersion wetting is spontaneous. If the contact angle is greater than 90° under leaching conditions, the immersional wetting process will be undermined. W III is positive when   0 . The spreading process is weakened with the increase of  .  When  is less than 90°, W T is negative and the wetting process is spontaneous. To avoid this process, equal or greater energy input is required. When  is greater than 90°, W T is a positive value and the dispersion of liquid sulfur from the mineral surface is spontaneous.  36  Figure 2.13 The three stages of wetting of sulfide mineral by liquid sulfur [93]  2.8 Methods for the Measurement of Interfacial Properties Many methods have been proposed to measure the interfacial tension. The capillary rise, Wilhelmy slide, pendant or sessile drop, and bubble methods are methods available for quiescent surfaces. Flow methods and capillary waves can be used to investigate dynamic interfaces [107]. Tilting plate and sessile drop techniques were introduced to measure the contact angle between liquid and solid surface [111].  2.8.1 The Measurement of Interfacial Tension  The pendant drop method is suitable for the liquid-liquid interfacial tension measurement under controlled environment at conditions such as high pressure and high temperature [112]. It was first used by Owusu [88] to measure the interfacial tension between liquid sulfur and aqueous 37  solution under leaching conditions which are also used in this study.  Method of the Selected Plane  The method of the selected plane is a procedure to determine interfacial tension from pendant drop images. This procedure was used to measure liquid sulfur-aqueous solution interfacial tensions by Owusu [88]. The interfacial tension was calculated from the following formula.   gd e  2     (2-17)  H  Where   is the difference of density between the liquid sulfur and aqueous solution; g is acceleration due to gravity; d e is equatorial diameter of the drop; H is a function of drop shape; S  d s / d e . Tables of values of H as a function of S are available in the literature [113].  A drop of liquid sulfur in aqueous solution is illustrated in Figure 2.14. Three parameters d s , d e , and capillary tip diameter were measured. The density of liquid sulfur at high temperature  was obtained from Table 2.1 and the influence of pressure on the sulfur density was not considered in Owusu’s study. The densities of aqueous sulfate solution at 20-100ºC, under atmospheric pressure could be measured from experiments or obtained from literature [114]. The density of aqueous sulfate solution at higher temperatures was estimated by assuming the solutions had the same coefficient of expansion as water.  38  Figure 2.14 Profile of a pendant drop  Axisymmetric Drop Shape Analysis  Axisymmetric drop shape analysis-profile (ADSA-P) was developed to determine liquid-fluid interfacial tensions and contract angles by fitting the Laplace equation of capillarity to an arbitrary array of coordinate points selected from the drop profile [112, 115].  2.8.2 The Measurement of Contact Angle  The sessile drop method is widely used for the measurement of contact angle by aligning a tangent with the drop profile at the point of contact with the surface. Owusu measured the contact angle on a photograph by using a protractor while Hackl used ADSA method to determine contact angles.  39  2.9 Electroacoustic Characterization of Mineral Suspensions Surface charge is an important character of surfactant containing mineral suspensions. The charge density on the mineral surface changes due to the adsorption of SDA. The magnitude and sign of the surface potential are conveniently described by zeta potential which is important in explaining particle interaction phenomena such as selective aggregation, rheological behavior, and reagent interactions. In addition, zeta potential is diagnostic of the mineral surface chemistry and purity.  2.9.1 Zeta Potential  When a sulfide mineral particle immerses in the aqueous solution an electrical double layer is formed around the particle. Surface of shear within which the fluid is stationary is an imaginary surface close to the mineral surface. The average potential on the surface of shear is the zeta potential [116]. All four kinds of electrokinetic effects, including electrophoresis, streaming potential, electro-osmosis, sedimentation potential, can be used to evaluate zeta potential.  2.9.2 Electroacoustic Method  When a high frequency sound wave passes through a colloidal dispersion it creates a macroscopic potential difference called the ultrasonic vibration potential or UVP effect; when an alternating electric field is applied to a colloidal suspension a sound wave can be generated which is called electrokinetic sonic amplitude or ESA effect [117-118]. The ESA magnitude is usually proportional to the zeta potential so that ESA signal and zeta potential have the same isoelectric point when both are measured in a system.  40  2.9.3 Zeta Potential of Pentlandite  Pure pentlandite mineral is required to measure the zeta potential. Synthetic pentlandite was used by researchers in the measurement due to the difficulty in obtaining pure natural pentlandite mineral. Surface oxidation of pentlandite increased the isoelectric point. The changes of zeta potential in the presence of metal ions or surfactant indicated their adsorption behavior [119-122].  2.10 Adsorption of Sulfur Dispersing Agents on Mineral Surface The adsorption behavior of SDAs on sulfide mineral surfaces under leaching conditions was mainly determined by interfacial phenomena studies. Four aspects of surfactant adsorption at the solid-liquid interface have been investigated, such as the concentration of surfactant, the orientation and packing of the surfactant, the rate of adsorption, and the system energy change. This study concentrates on the first aspect which requires the determination of the appropriate concentration of SDA in solution for pentlandite leaching. The UV-Vis spectroscopy determination of SDAs, the adsorption mechanisms of surfactant and the adsorption isotherms are reviewed in this section.  2.10.1 Beer-Lambert Equation  The Beer-Lambert equation is the basis for the determination of the concentration of SDAs in aqueous solution, which can be written as equation (2-18) [123]  I   100 A   cd  log 10    log 10   T (%)  I0       (2-18)  41  Where A is absorbance;  is molar absorptivity; c is the concentration of solute; d is the path length through the sample, cm; T is the transmittance; I 0 is the intensity of the light entering the sample; I is the intensity of the light emerging from the sample.  2.10.2 Quantitative Determination of Sulfur Dispersing Agents in Solution  Quantitative Determination of Lignosulfonate  Generally, a wavelength around 280 nm was chosen as the detection wavelength of lignosulfonate because it has a characteristic absorption peak, and a detailed measurement procedure was given in the literature [124].  Quantitative Determination of Humic Acid  The study of adsorption of lignite-derived humic acid on other solids than sulfide minerals was investigated in previous research [60]. The concentration of humic acid can be determined by its UV/Vis spectrum at a wavelength of 254 nm.  Quantitative Determination of Quebracho  Condensed tannin is complicated to analyze because of its diversity of structures due to its origin. Its analysis methods were reviewed by many researchers [125-127]. Acid-butanol assay is the most common method to determine proanthocyanidin in plant tissues. Quebracho tannin is a commercially available source of condensed tannins and widely used as standard material when analyzing the concentration of condensed tannins from different sources [126-127].  42  Quantitative Determination of OPD  There are at least three photometric methods available to determine the concentration of OPD. The main methods for the determination of OPD include diazotization-coupling procedures, spectrophotometric quantification after a specific color-generating reaction, and direct UV-Vis spectroscopy method [128-132].  2.10.3 Adsorption Mechanisms  The adsorption of surfactant at the solid-liquid interface is strongly influenced by the nature of the structure groups on the solid surface, the molecular structure of the surfactant, and the environment of the aqueous phase. The mechanisms of adsorption are listed below, and more detailed information can be found in literature [51].   Ion exchange    Ion pairing    Acid-base interaction    Adsorption by polarization of π electrons    Adsorption by dispersion forces    Hydrophobic bonding.  2.10.4 Adsorption Isotherms  Langmuir and Freundlich adsorption isotherms are used to fit surfactant adsorption. The Langmuir model assumes (1) the surface consists of adsorption sites, (2) all sites are equivalent, (3) the adsorbed species interact only with a site and not with each other; (4) the adsorption is limited to a monolayer [107, 133]. The Langmuir equation is shown below [134]:  43  q  qm K  C 1  KC  (2-19)  Where q is adsorption density, mg/m2; q m is the maximum adsorption density, mg/m2; K  is the Langmuir adsorption constant; C is an equilibrium concentration of solution, mg/L. The Freundlich adsorption isotherm (Equation 2-20) is the most important multisite adsorption isotherm [134]. It assumes that the heat of adsorption decreases with increasing surface coverage because of particle interactions or surface heterogeneity [135]. q  KC  1/ n  (2-20)  Where K is a measure of the adsorbent capacity; 1 / n is the intensity of adsorption.  Influence of Molecular Weight  The effect of molecular weight of a polymer on its adsorption on solid surface is represented by the following equation [136-137]: A S  KM  a  (2-21)  Where A S is the saturated amount of adsorption; M is the molecular weight of the adsorbate; K  and   are constants. Generally, the amount of polymer adsorption increases with  increasing the molecular weight.  Adsorption Isotherm Studies on Sulfide Minerals  Xia [84] showed that the adsorption of lignosulfonate on the ZnS concentrate was higher than other materials in the system and the loading capacity of ZnS concentrate did not change within the temperature range from 20ºC to 80ºC. The loading capacity for sulfur was slightly temperature dependent. Ansari [138] found that lignosulfonate adsorbed onto chalcopyrite surfaces by about 2 mg/m2 in 0.001 mol/L KCl solution at the ambient temperature and the 44  adsorption process was controlled by electrostatic forces and chemical interactions between lignosulfonate and metal-hydroxy sites on the mineral surface.  2.11 Infrared Spectroscopy Attenuated Total Reflection-Fourier transform infrared technique was employed to investigate the nature of SDA adsorption at the sphalerite-aqueous solution interface, which could distinguish physical and chemical adsorption mechanisms [86]. Physical adsorption is a reversible process, and the adsorption efficiency and capacity decrease with the increase of environment temperature.  2.11.1 Infrared Spectroscopy Techniques  All spectra arise from the absorption or emission of radiation that occurs between definitely quantized energy levels. UV-Vis spectra result from transitions between electronic levels. Vibrational and rotational transitions in a molecule give rise to infrared spectra [139].  The position and intensity of infrared spectra are useful characteristics. The Beer-Lambert law is the normal basis of quantitative analysis. Specific functional groups in molecules absorb infrared radiation over typical frequency ranges. So there is a good correlation between a certain wavenumber and molecular structure [140].  Physical adsorption, the result of van der Waals forces, results in a slight change of the spectra of minerals; chemical adsorption, a stronger chemical interaction between surfactant and mineral, results in a new infrared spectrum and greater band shifts and intensities [139].  45  2.11.2 Sample Preparation Methods  Three groups of sampling techniques were introduced by Smith, including transmission techniques, reflectance techniques, and photoacoustic spectroscopy [140]. Transmission techniques are popularly used to obtain infrared spectra, and were used in this study. KBr acts as a support and a diluent for solid samples. Generally, the solid sample is diluted to about 1% in KBr. The advantages of transmission techniques include high signal-to-noise ratio, relatively low expenditure, and universality; while the disadvantages include the limitation of sample thickness (1-20 µm) and time-consuming sample preparation.  2.12 Kinetic Study of Nickel Sulfide Concentrate From the thermodynamic point of view, nickel sulfide decomposition could occur by oxidation in acid solution with the formation of elemental sulfur [37]. Nevertheless, from the process point of view, it is important to know the leaching kinetics of sulfide minerals under different conditions, such as temperature, solution composition, oxygen partial pressure, particle size, and so on.  2.12.1 Sulfate Leaching of Nickel  Boateng and Phillips [141] investigated the leaching kinetics of pentlandite concentrate (13% Ni) at temperatures between 45-125°C. The pentlandite concentrate was leached in kerosene with 1 mol/L bis(2-ethylhexyl)phosphoric acid (DEHPA), and the pH of the solution was adjusted to the 1 to 2 range by adding sulfuric acid. The shrinking core model was used to interpret the data, and the leaching rate was shown to be controlled by both chemical reaction and mass transfer 46  processes. The reaction rate was approximately first order with respect to oxygen pressure for the 377-791 kPa pressure range. The main reactions occurring in the solution are as follows: O2  4 H    NiS  Ni Fe  2   4 e  2 H 2O 2   Fe  (2-22)   S  2e  3  (2-23)  e  (2-24)  Vezian [142] investigated the influence of particle size on acid pressure leaching of chalcopyrite-pentlandite-pyrrhotite concentrate. The composition of the concentrate was 6.85% Ni, 8.83% Cu, 0.55% Co, 29.15% S, and 36.5% Fe. When the mineral particles were finer than 20 µm, using a pulp density of 30% solids, oxygen pressure of 550 kPa, a temperature of 110°C, and a retention time of 8 h, almost complete dissolution of the nickel and the cobalt was achieved. The decomposition of chalcopyrite was over 96%.  Pandey et al. [143] studied the pressure sulfuric acid leaching of a sulfide concentrate to recover copper, nickel, and cobalt. The concentrate contained 10.85% Ni, 15.0% Cu, 0.37% Co, 26.6% Fe, and 33.3% S. They found that the recovery of the valuable metals generally increased with the increase of temperature from 100-145°C and pressure from 1085 to 5195 kPa. The nickel recovery was 64-78%. The low nickel recovery was due to the presence of nickel in the pyrite phase as well as the coating of mineral particles with iron oxide and sulfur. The overall reaction of pentlandite oxidation was represented by the following reaction: NiS  FeS  H 2 SO 4  1 . 5 O 2  0 . 5 H 2 O  NiSO  4   Fe ( OH ) 3  2 S  (2-25)  2.12.2 Chloride Leaching of Nickel  The effect of low chloride addition (0.5-10 g/L) on the oxygen pressure leaching of nickel sulfide concentrate was performed at 110ºC and 550-2100 kPa oxygen partial pressure [144]. The 47  composition of the nickel concentrate was 7.05% Ni, 1.80% Cu, 38.2% Fe, and 27.8% S. The addition of low concentrations of chloride could not influence the dissolution of pyrrhotite and pentlandite, but higher copper extraction from chalcopyrite was achieved. Compared with a pure sulfate system, the presence of chloride ion in the leaching solution could maximize the conversion of sulfide sulfur to the elemental form and reduce iron levels in solution.  Lu et al. [145] studied the kinetics and mechanism of pentlandite leaching in mixed oxygenated acidic chloride-sulfate solutions. The composition of the concentrate was 18.5% Ni, 0.95% Cu, 0.28% Co, 36.8% Fe, and 34.0% S. The oxygen partial pressure, chloride ion concentration, and temperature played important roles in the experiments. 96% nickel extraction could be achieved through the 10 h leaching at 85°C and a 103 kPa oxygen partial pressure. The addition of small amount of NaCl (0.5 mol/L) increased the leaching rate, and chloride fostered the formation of a more porous sulfur product which allows the reactants to diffuse through the sulfur film to the reaction surface. The main reactions can be described as follows: Fe 4 .5 Ni 4 .5 S 8  4 . 5 O 2  18 H  4 Fe  2   O2  4 H  Fe 4 .5 Ni 4 .5 S 8  18 Fe    3     4 Fe   9 H 2 O  4 . 5 Ni 3  2   4 . 5 Fe   2 H 2O   22 . 5 Fe  2   4 . 5 Ni  2   8S  (2-26) (2-27)  2   8S  (2-28)  Hubli et al. [146] examined the ferrous chloride-oxygen leach process to recover nickel and copper from the nickel-copper sulfide concentrate (8% Ni). The leaching was operated at 110ºC and 377 kPa oxygen pressure for 8 h. The theoretical amount of ferrous chloride referred to the amount of ferrous chloride that would provide the stoichiometric amount of chloride ion to form dichlorides of nickel and copper. When the ferrous chloride addition increased from the theoretical amount of 60-100%, nickel recovery increased from 86% to near 100% linearly. Copper recovery was lower than that of nickel when the ferrous chloride was less than 80% theoretical amount, after that the copper extraction was faster than nickel resulting in a slightly 48  higher recovery. The reactions occurring during the leaching process are shown as follows: 3 FeCl  2    3 4  1  O2   ( NiFe ) S  4 FeCl  Fe 2 O 3  2 FeCl  2   NiCl  3  2   FeCl  (2-29)  3  2  S  (2-30)  The cupric chloride-oxygen leach process was also investigated [147]. The copper extraction was fast and 99% extraction could be acquired in about 2 h while more time was needed for the extraction of nickel. In both studies hydrochloric acid was used to adjust the pH of the slurry. The reactions occurring during the leaching process are as follows: ( Ni , Fe ) S  4 CuCl ( Ni , Fe ) S  4 CuCl  6 FeCl  2  2     NiCl  10 Cl    2   FeCl  2   4 CuCl   4 CuCl  S 2 3   Ni  2   FeCl  (2-31) 2  S   12 CuCl  4 . 5 O 2  3 H 2 O  6 FeO  OH  12 CuCl  2  (2-32) (2-33)  Maurice and Hawk [148] examined the ferric chloride leaching of a mechanically activated pentlandite-chalcopyrite concentrate. Mechanical activation improved the kinetics of both copper extraction from the chalcopyrite and nickel extraction from the pentlandite.  2.13 Objectives of the Present Study Based on the literature review, little is known on a molecular level about the interaction between nickel sulfide minerals and SDAs in the leaching systems. Oxygen pressure leaching of nickel concentrate at medium temperature in acid sulfate media is not an industrial process. It is the objective of this study to understand the effect of SDAs in the pressure leaching of nickel concentrate above the melting point of sulfur. Oxygen pressure leaching of nickel concentrate in sulfuric acid medium at medium temperature is a potential method of nickel extraction. This study supplies information on the possibility of developing new leaching process. 49  The scope of the study involved three parts:   To understand the reason why SDAs are effective in solving the sulfur wetting problem. It was achieved by interfacial studies (Chapter 3). The work of adhesion was used to evaluate the effect of SDAs under leaching conditions.    To understand how SDAs adsorbed on both sulfur and nickel concentrate surface. It was achieved by adsorption studies (Chapter 4-6). The functional groups of SDAs were characterized by Fourier transform infrared (FTIR) spectroscopy. The adsorption of SDAs on the elemental sulfur surface was investigated by an electroacoustic method. The amount of SDAs adsorbed on the nickel concentrate surface was determined by the measurement of the concentration of SDAs in the aqueous solution.    To understand the effect of SDAs in the pressure leaching of nickel concentrate. It was achieved by leaching studies (Chapter 7). The leaching study concentrated on the effect of SDAs on the extraction of nickel.  Interfacial studies were investigated under low acid and high acid conditions. This study is used to understand the leaching behavior of SDAs in the system with high acid addition as well as to understand the leaching behavior of sulfide minerals under low acid conditions. Information acquired from interfacial studies is useful to understand the present pressure leaching processes and also helpful for the future commercial process design. The adsorption study is new which supplies basic knowledge on the behavior of SDAs in the mineral system. The adsorption mechanisms of SDAs on both sulfur and nickel concentrate surfaces were investigated. Through this study, a better understanding was acquired on the behavior of SDAs (especially OPD) under pressure leaching conditions. The methods used in the adsorption studies were found useful in understanding the stability of SDAs. The leaching study showed that the leaching system containing nickel concentrate is more complex than the system containing zinc concentrate. Information was obtained and found useful in understanding both the available process (INCO process) and the potential process (oxygen pressure leaching process) of nickel extraction. 50  3.0 Interfacial Studies The nature of molecular interactions in the liquid sulfur-sulfide mineral-aqueous solution system was investigated in the absence and presence of SDAs. This study is trying to answer why SDAs are effective in solving the sulfur wetting problem in leaching. Liquid sulfur-aqueous solution interfacial tensions were measured by pendent drop method. Liquid sulfur-sulfide mineral contact angles were measured by sessile drop method. The work of adhesion was used to evaluate the effect of SDAs. The efficiency and effectiveness of four different kinds of SDAs were also evaluated by the interfacial studies.  3.1 Materials  3.1.1 Reagents  Nickel sulfate and ferrous sulfate solutions were prepared with reagent grade chemicals and deionized water. The chemicals used in interfacial experiments are listed below: (1) Sulfur powder, 99.999%, from Alfa Aesar; (2) Nickel (II) sulfate hexahydrate, 99%, from Sigma Aldrich; (3) Sulfuric acid, 95.0-98.0%, from Fisher; (4) Iron (II) sulfate heptahydrate, from Fisher.  3.1.2 Minerals  The mineral specimens used in the measurement of contact angles are as follows: (1) Pyrrhotite, from Essex County, N.Y. 51  (2) Pyrrhotite (nickeliferous), from XSTRATA Nickel, Ont. (3) Pentlandite, from Vale INCO, Voisey’s Bay mine, drill core samples of massive sulfide, and received from The University of Toronto. (4) Pentlandite in pyrrhotite, from Sudbury, Ont. (5) Chalcopyrite, from Messina, Transvaal Republic of South Africa. The mineral samples were cut into small cubes using a diamond saw. Samples with obvious color and grain differences were discarded. The cubes were polished as smooth as possible by sequentially grinding on successively finer grit grinding wheels and finishing with a 1µm alumina/water suspension polish. The polished face was washed with a stream of water to remove loosely attached alumina particles, followed by cleaning in an ultrasonic bath. Next the sample was washed with a stream of water once again and finally, rinsed with deionized water and aqueous solution. The aqueous solution is the solution prepared for the measurement of interfacial tension and contact angle.  3.1.3 Sulfur Dispersing Agents  The water-soluble SDAs investigated in the interfacial studies are shown in Table 3.1. All four kinds of SDAs were used without further treatment.  (1) Lignosulfonate Two kinds of lignosulfonate were obtained from Aldrich, including sodium lignosulfonate (Aldrich LS NA) and calcium lignosulfonate (Aldrich LS CA) with MW 18000; five kinds of lignosulfonate were obtained from Borregaard LignoTech USA, including BorrePAL U, BorrePAL N, BorrePAL S, Borresperse NA, and D-1929. 100% water soluble BorrePAL series additives were recommended by Borregaard LignoTech USA. BorrePAL U is a sodium-based, softwood lignosulfonate with MW 45000-75000; BorrePAL N is a calcium-based, softwood lignosulfonate with MW 20000-40000; BorrePAL S is a calcium-based lignosulfonate with MW 52  10000-15000. (2) Quebracho Two kinds of Quebracho were used in this study: one was obtained from Anglo American, and the other, Orfom® grade 2 Tannin, was obtained from Chevron Phillips Chemical Company LP, which contained 85-95% sulfited Quebracho and 5-15% water. (3) OPD, 98%, was obtained from Acros Organics. (4) Humic acid-potassium salt, the modified lignite, was obtained from Borregaard LignoTech USA.  Table 3.1 List of sulfur dispersing agents evaluated in the interfacial studies Generic Name  Trade Name/Supplier  MW (g/mol)  Sodium Lignosulfonate  BorrePAL U  45000-75000 Anionic  Calcium Lignosulfonate  BorrePAL N  20000-40000 Anionic  Calcium Lignosulfonate  BorrePAL S  10000-15000 Anionic  Sodium Lignosulfonate  Borresperse NA  Anionic  Sodium Lignosulfonate  D-1929  Anionic  Calcium Lignosulfonate  Aldrich  Sodium Lignosulfonate  Aldrich  OPD  Acros Organics  Quebracho  Anglo American  Nonionic  Quebracho  Orfom® Grade 2 Tannin  Nonionic  Humic Acid-K salt (modified lignite)  D-616  Anionic  18000  Type  Anionic Anionic  108.1  Cationic  53  3.2 Experimental Procedures Interfacial experiments were performed using the high temperature high pressure apparatus. The apparatus was heated externally with a heating tape wrapped around the body, and pressurized with nitrogen gas. The temperature was controlled by a PID temperature controller and pressure was measured by an Omega Engineering pressure meter. The primary procedures for the measurement of interfacial tension and contact angle were provided in the literature [94, 102]. The apparatus and experimental procedures are listed in Appendix 1.  3.3 Results and Discussion To check for inaccuracies in the measured interfacial parameters that may arise from possible optical distortion effects the apparatus was tested by measuring the surface tension of deionized water at ambient temperatures. The results are given in Table 3.2. The results indicate that the apparatus is suitable for the measurement of surface and interfacial tension by pendent drop method.  The accuracies of interfacial tensions were influenced by the speed of the liquid sulfur drop formation, which influences the timing of photos taken. Table 3.3 shows some repeated measurements of interfacial tensions between liquid sulfur and aqueous solution at 140ºC. The average values of interfacial tension are repeatable. The results of repeated experiments were within ±2%. The accuracies of contact angles could be influenced by the heterogeneity of the mineral surface, the impurities that may introduce to the surface, etc. Table 3.4 shows the repeated measurements of contact angles between liquid sulfur and sulfide mineral. The results of repeated experiments were within ±3%.  54  Table 3.2 Results of surface tension measurement on water at 22ºC (different drops), the value from literature is 72.4 mN/m [149] Experiments  Surface Tension  Average  Standard Deviation  No.  (mN/m)  (mN/m)  (mN/m)  1  71.3, 71.7, 71.4  71.5  0.2  2  72.6, 72.4, 71.5, 72.6, 71.6, 71.7  72.1  0.5  Table 3.3 Results of interfacial tension between liquid sulfur and aqueous solution (different drops): 140ºC, 690 kPa N2, NiSO4 1.0 mol/L, BorrePAL U 0.5 g/L, pH: 4.8. Experiments  Interfacial tension  Average  Standard Deviation  No.  mN/m  mN/m  mN/m  1  26.5, 26.7, 26.7, 26.1, 26.8, 27.1  26.7  0.3  2  26.8, 26.7, 26.1, 26.8, 26.9, 26.6, 26.5  26.6  0.2  3  27.2, 26.2, 26.0, 26.4, 26.2, 26.8, 26.4  26.5  0.4  4  27.0, 26.9, 27.2, 26.8, 26.7, 26.9  26.9  0.2  5  25.8, 27.0, 27.2, 27.0, 26.7, 26.1, 26.7  26.8  0.3  Table 3.4 Contact angle between liquid sulfur and sulfide mineral (different drops): 140ºC, 690 kPa N2, NiSO4 1.0 mol/L, 8.90×10-4 mol/L H2SO4, No sulfur dispersing agent addition. Mineral  Nickeliferous pyrrhotite  Pyrrhotite  Experiments  Contact Angle  No.  (º)  1  110  2  109  1  117  2  110  55  3.3.1 Interfacial Properties without Sulfur Dispersing Agent  In this section the influence of sulfuric acid concentration on interfacial properties in different mineral systems was investigated. The experiments were performed at 140 º C, 690 kPa overpressure of nitrogen, 1.0 mol/L nickel sulfate. The influence of nickel sulfate concentration on the interfacial properties in the liquid sulfur-nickeliferous pyrrhotite-acidic aqueous solution system were also investigated at 140ºC, 690 kPa, at pH 1.14-1.23.  (1) The Influence of Sulfuric Acid Concentration on Interfacial Properties  Figure 3.1 shows the influence of sulfuric acid concentration on the interfacial tension between liquid sulfur and the nickel sulfate aqueous solution. Sulfuric acid has a very small influence on interfacial tensions. The interfacial tension remains at about 51.5 mN/m when the acid concentration increases from 0 to 0.18 mol/L.  Figure 3.2 shows the influence of sulfuric acid concentration on the contact angle between liquid sulfur and sulfide minerals. Sulfuric acid has a great influence on the contact angle between liquid sulfur and nickeliferous pyrrhotite. The contact angle decreases from 156º to 72º when acid concentration increases from 0 to 0.04 mol/L. The contact angle remains at 70º-73º when the sulfuric acid concentration increases from 0.04 to 0.18 mol/L.  To further test the findings that liquid sulfur cannot wet sulfide mineral in the high pH solution, three other mineral samples, including pyrrhotite, pentlandite from Voisey’s Bay, and pentlandite from Sudbury, were investigated in the measurement of contact angles, and the same result was obtained. Without acid addition, contact angles between liquid sulfur and sulfide minerals are between 152º and 159º. The contact angles range from 80 to 88º when 0.18 mol/L sulfuric acid is present in the aqueous solution. 56  Figure 3.1 Liquid sulfur-aqueous solution interfacial tension versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4  Figure 3.2 Liquid sulfur-sulfide mineral contact angle versus acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4 57  Figure 3.3 shows the influence of sulfuric acid concentration on the work of adhesion. The work of adhesion was calculated from the equation 3-1 [50]. Sulfuric acid concentration had a great impact on the work of adhesion in the liquid sulfur-sulfide mineral-aqueous solution system. The work of adhesion increased from 3-6 mJ/m2 to 53-69 mJ/m2 in different systems when the sulfuric acid concentration increased from 0 to 0.18 mol/L. The work of adhesion equals the work required to separate a unit area of liquid sulfur from a mineral surface. Agitation supplies the work during leaching. Therefore, the small values of the work of adhesion indicate that the sulfur wetting problem does not exist. W a   SA (1  cos  )  (3-1)  Figure 3.3 Work of adhesion versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4  From the above research, it is concluded that sulfuric acid has a great influence on the surface chemistry of sulfide mineral. To investigate the influence of various aqueous solutions on interfacial properties three kinds of aqueous solution (deionized water, 1.0 mol/L NiSO4 solution, 58  and 0.01 mol/L FeSO4 solution) were tested. Sulfuric acid was used to adjust the pH of the above aqueous solutions. In the measurement of contact angle the sulfide mineral (i.e. nickeliferous pyrrhotite, pyrrhotite, and pentlandite) surface was found uneven and easily broken, which indicated some change of the mineral surface group or the composition of the aqueous solution. Therefore, chalcopyrite instead of pyrrhotite or pentlandite was used because the mineral surface remained smooth during the entire process, indicating that chalcopyrite was not reactive and did not obviously affect the aqueous solution composition.  Figure 3.4 shows the influence of pH on the interfacial tensions in three different aqueous solution systems. For all the systems, pH has very small influence on the interfacial tension in the pH range from 1.92 to 4.53. The interfacial tensions in both ferrous sulfate and nickel sulfate systems are higher than those in the acidic water system. Comparing with the interfacial tensions obtained in the acidic water system, the interfacial tension differences range from 0.3 to 1.4 mN/m for the ferrous sulfate system and from 2.6 to 3.8 mN/m for the nickel sulfate system.  Figure 3.4 Liquid sulfur-aqueous solution interfacial tensions versus the acidity of aqueous solution: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.01 mol/L FeSO4 59  Figure 3.5 shows the influence of pH on contact angles in different aqueous solution systems. With the solution pH increasing from 1.92 to 4.53, the contact angle increases from 80º to 85º, from 86ºto 97º, and from 87º to 147º in the water system, the ferrous sulfate system, and the nickel sulfate system, respectively.  Figure 3.5 Liquid sulfur-chalcopyrite contact angles versus the pH of aqueous solution 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.01 mol/L FeSO4  Figure 3.6 shows the influence of pH on the work of adhesion in three different aqueous solution systems. When the pH increases from 1.92 to 4.53, the work of adhesion decreases from 56.3 mJ/m2 to 52.4 mJ/m2 in the water system, from 52.5 mJ/m2 to 43.0 mJ/m2 in the ferrous sulfate system, and from 54.0 mJ/m2 to 8.3 mJ/m2 in the nickel sulfate system. Sulfuric acid has a great influence on the liquid sulfur-sulfide mineral contact angle. The sulfur wetting problem does not appear in a high pH solution (pH > 4). There are two possibilities leading to the phenomenon as follows: one is the influence of aqueous solution on the mineral surface, e.g., the influence of 60  nickel sulfate or ferrous sulfate on the surface properties of the sulfide mineral; the other is the oxidation of sulfide mineral during the experiment, e.g., the influence of sulfuric acid on the surface properties of the sulfide mineral.  Figure 3.6 Work of adhesion versus the pH of aqueous solution 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, and 0.01 mol/L FeSO4  In the liquid sulfur-chalcopyrite-acidic water system, the contact angle remains relatively stable with increasing the pH of solution (water). This indicates that mineral surface oxidation is not the reason why the contact angle is very high. In the liquid sulfur-chalcopyrite-ferrous sulfate system, ferrous sulfate oxidation, as shown by reaction 3-2, occurred during this experiment and formed a red precipitate. The contact angle increases with decreasing the acidity of aqueous solution; however the changes of the contact angle with acidity are very small. This indicates that even if a small amount of iron (< 0.01 mol/L) leached from pyrrhotite or pentlandite during the experiments as shown by reactions (3-3 and 3-4), it is not the iron species that makes the sulfide 61  mineral surface sulfophobic. In the liquid sulfur-chalcopyrite-nickel sulfate system, nickel sulfate has a great influence on the contact angle at the solution pH up to 4.5. This indicates that the hydrolysis of nickel ions, as shown in reaction 3-5, has a great influence on the sulfide mineral surface, and the nickel hydroxide interaction with mineral surface makes the mineral sulfophobic. Figure 3.7 shows the Eh-pH diagram of NiSO4-H2O system at 140ºC. 2 Fe  2   3 H 2 O  Fe 2 O 3  6 H  Fe 4 .5 Ni 4 .5 S 8  32 H 2 O  4 . 5 Ni Fe 1 x S  4 H 2 O  (1  x ) Fe  NiSO  4  2    2   2e     4 . 5 Fe 2  (3-2) 2   SO 4  8 H   H 2 O  Ni ( OH ) 2  H 2 SO 4  2   8 SO 4  64 H    8e       66 e    (3-3) (3-4) (3-5)  Figure 3.7 Eh-pH diagram of NiSO4-H2O system at 140ºC. Activities of nickel species is 1.0 mol/L [150]  62  (2) The Influence of Nickel Sulfate Concentration on Interfacial Properties  Table 3.5 shows the influence of nickel sulfate concentration on the interfacial properties in the liquid sulfur-nickeliferous pyrrhotite-aqueous solution system at low pH (1.14-1.23). When the concentration increases from 0.1 mol/L to 1.5 mol/L, the changes of interfacial properties are recorded as follows: the interfacial tension varies from 47.3 mN/m to 52.1 mN/m; the contact angle varies from 72° to 75º; the work of adhesion varies from 61.9 mJ/m2 to 67.0 mJ/m2. The results indicate that nickel sulfate has no great influences on the interfacial properties when the nickel sulfate concentration is greater than 0.5 mol/L.  Table 3.5 The influence of nickel sulfate concentration on the interfacial properties: 140ºC, 690 kPa N2, nickeliferous pyrrhotite NiSO4  pH  mol/L  Interfacial Tension  Contact Angle  Work of Adhesion  (mN/m)  (ºC)  (mJ/m2)  0.1  1.14  47.3  72  61.9  0.2  1.16  48.4  72  63.4  0.5  1.20  51.2  72  67.0  0.8  1.23  51.5  73  66.6  1.0  1.22  51.6  73  66.7  1.2  1.23  52.1  75  65.6  1.5  1.22  51.5  73  66.6  3.3.2 Influences of Sulfur Dispersing Agents at High pH  In this section, the influences of several factors on the liquid sulfur-aqueous solution interfacial tensions were studied: 63    Nickel sulfate concentration    The dosage of SDAs    Operating temperature.  (1) The Influence of Nickel Sulfate Concentration  Figure 3.8 shows the influence of nickel sulfate concentration on the liquid sulfur-aqueous solution interfacial tension at 140ºC, 690 kPa pressure by nitrogen. In the absence of SDA, the interfacial tension increases from 47.7 to 51.4 mN/m when the nickel sulfate concentration increases from 0.1 to 1.5 mol/L. The interfacial tensions decrease quickly due to the introduction of the Aldrich LS CA. The interfacial tensions drop down to 25.8-27.3 mN/m with 0.5-0.7 g/L lignosulfonate concentrations. Any further increase in the lignosulfonate concentration over 0.5 g/L does not greatly influence the interfacial tensions in the system.  Figure 3.8 Liquid sulfur-aqueous solution interfacial tension versus nickel sulfate concentration: 140ºC, 690 kPa N2, Aldrich LS CA: 0-0.7 g/L. 64  (2) The Influence of Sulfur Dispersing Agents Dosage  Figure 3.9 shows the influences of seven kinds of lignosulfonate dosage on the interfacial tension in the absence of sulfuric acid. Due to the addition of lignosulfonate, the interfacial tensions decrease quickly to their minimum values range from 25.7 to 28.5 mN/m at the 0.7 g/L dosage. The maximum difference among interfacial tensions is 3.2 mN/m when the dosage is greater than 0.3 g/L. Depending on the effect to decrease the interfacial tension at 0.1-0.7 g/L, BorrePAL U and D-1929 are the most effective lignosulfonate to decrease interfacial tension and the most ineffective lignosulfonate is Aldrich LS NA which is not used in the further study.  Figure 3.9 Liquid sulfur-aqueous solution interfacial tension versus lignosulfonate dosage 140ºC, 690 kPa N2, 1.0 mol/L NiSO4  When the SDA is added to solution, it is adsorbed at the liquid sulfur-aqueous solution interface: the hydrophilic part interacts with the aqueous solution, and the hydrophobic part interacts with liquid sulfur. With the increase of the SDA concentration, the amount of additive adsorbed at the 65  interface increases until a saturation state is reached. A layer or layers of the additive’s molecules completely cover the interface. In the saturated state, the additive adsorption could not be affected by further increasing the concentration, but the hydrophobic part of the SDA could possibly interact to form aggregates.  That cations in lignosulfonate do not play any active role in both the interfacial tension reductions and contact angle increases is possibly due to the presence of large amount of nickel (II) in solution. In addition, other cations, such as iron leached from mineral, can play the same role as calcium. The dispersing effect depends on the adsorption of anionic charge of lignosulfonate. The findings mentioned above are consistent with previous studies [84, 151]. For BorrePAL series additives, the influence of molecular weight on interfacial properties is very small, possibly because all of them are effective SDAs to decrease the work of adhesion. Different kinds of lignosulfonate cannot be distinguished by the small differences among their work of adhesion.  The effective decrease of the interfacial tension by SDA means that the additive adsorbs strongly at the liquid sulfur-aqueous solution interface. The amount of SDA adsorbed at the interface can be calculated by the Gibbs’ adsorption isotherm if both SDA and nickel sulfate dissociate completely. An effective SDA is positively adsorbed at the interface while nickel sulfate is negatively adsorbed at the liquid sulfur-aqueous solution interface.  Figure 3.10 shows the relationship between the liquid sulfur-aqueous solution interfacial tension and lignosulfonate concentration. The average molecular weight was used in the calculation. The limiting concentration of lignosulfonate can be estimated from Figure 3.10, i.e., the relationship between interfacial tension and ln C . For example, the limiting concentration of BorrePAL N is approximately 0.5 g/L. The limiting concentration here is the limiting lignosulfonate concentration above which the interfacial tension does not decrease dramatically any further. The 66  limiting concentration of BorrePAL U and BorrePAL S is about 0.5 g/L. The adsorption reaches the highest at the limiting concentration.  The maximum amount of lignosulfonate accumulated at the interface between elemental sulfur and nickel sulfate solution was estimated by the Gibbs equation (equation 3-6). As illustrated in Figure 3.10, two adjacent data points are connected by a straight line, and the slope of the line equals to d  / d ln C . The values of d  / d ln C , R and T are put into the Gibbs equation. Then the adsorption density can be calculated. The maximum adsorption is obtained when the d  / d ln C value is the largest.  Figure 3.10 Liquid sulfur-aqueous solution interfacial tension versus concentration curves of lignosulfonate: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4  67  The maximum amount of lignosulfonate accumulated at the liquid sulfur-aqueous solution interface was estimated by the Gibbs equation as shown in Table 3.6. The molecular area A occupied by one lignosulfonate molecule was calculated from the equation 3-7 (monolayer adsorption was assumed), where, N A is Avogadro constant, 6.022×1023 mol-1 [152]. The values of A are presented in Table 3.6. The radius of lignosulfonate ranges from 0.7 to 7 nm when the molecular weight is in the range of 1325 to 73196 [153]. The molecular area calculated in this study is inconsistent with previous studies. The influence of the ionic strength and the adsorption mechanism could be used to explain the differences. (1) The size of the lignosulfonate molecule decreases with increasing the ionic strength. Gupta [153] found that for a lignosulfonate with a particular molecular weight, the radius was estimated to be up to two times smaller in 0.1 mol/L NaCl solution than in water. (2) The adsorption of lignosulfonate on the elemental sulfur surface is not monolayer adsorption.  max    A  1    RT    (3-6)   ln C  1  (3-7)  N A max  Table 3.6 Molecular area of lignosulfonate adsorbed on the molten sulfur surface, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4 Lignosulfonate  Average MW  Adsorption  10  8  mol / m  2  Molecular Area  Radius   20  nm   10  m  2  BorrePAL U  60000  172  97  0.55  BorrePAL N  30000  165  101  0.57  BorrePAL S  12500  196  85  0.52  Aldrich LS CA  18000  286  58  0.43  Aldrich LS CA*  18000  148  112  0.60  *: The concentration of nickel sulfate is 0.1 mol/L. 68  The major component of the aqueous solution is lignosulfonate (0-0.7 g/L) and nickel sulfate (1.0 mol/L). It is assumed that both lignosulfonate and nickel sulfate dissociate completely when applying the Gibbs equation. No sulfuric acid was added into the aqueous solution. The influence of acid concentration (0-0.1 mol/L) on the interfacial excess is very small due to its small influence on the interfacial tension (Figure 3.1). The influence of nickel sulfate concentration (0.1-1.0 mol/L) on the interfacial excess is also small due to its small influence on the interfacial tension (Figure 3.8). The error in the measurement of interfacial tension may introduce error on the maximum adsorption which makes the radius of the calculated molecular area ranges from 0.4 to 0.6 nm. The error may come from the lack of experimental data, e.g., the lack of interfacial tension values when the lignosulfonate concentration is less than 0.1 g/L. This is the reason why the molecular radius of Aldrich LS CA is 0.6 nm when the nickel sulfate concentration is 0.1 mol/L.  Figure 3.11 shows the influences of OPD, Quebracho, and humic acid concentration on the interfacial tensions in the absence of sulfuric acid. With the introduction of Quebracho, the interfacial tensions decrease to the value ranges from 34.4 to 37.9 mN/m at the 0.5-0.7 g/L dosage. Any further increase in the dosage over 0.5 g/L has no great effect on the interfacial tensions. Two kinds of Quebracho were studied. The difference between their interfacial tensions is less than 3.2 mN/m when the dosage ranges from 0.1 to 0.7 g/L.  OPD has no obvious effect on interfacial tension decreases with the 0.1-0.7 g/L dosage. This indicates that OPD cannot adsorb on the liquid sulfur surface. With addition of humic acid, precipitation appears during the measurement process, and its amount increases with the increase of dosage. Further studies were performed on humic acid in solution with the 8.9×10-2 mol/L sulfuric acid, and no precipitate was found. This demonstrates that the precipitate’s formation depends on the acidity of solution, and this precipitate is possibly nickel hydroxide as suggested by the Eh-pH diagram in Figure 3.7. The interaction between humic acid and nickel hydroxide 69  results in only a fraction of humic acid being available to interact with the liquid sulfur-aqueous solution interface.  Figure 3.11 Liquid sulfur-aqueous solution interfacial tension versus sulfur dispersing agent dosage: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, no sulfuric acid  (3) The Influence of Temperature  Generally, the interfacial tension decreases with the increase of temperature. Figure 3.12 shows the influences of temperature on the interfacial tension in the presence of lignosulfonate. For any lignosulfonate sample, the difference among the interfacial tensions at temperature ranges from 130 to 150ºC is less than 3.3 mN/m. The difference among different kinds of lignosulfonate on the interfacial tensions at any operating temperature ranges from 2.2 to 3.2 mN/m. BorrePAL U and D-1929 are the most effective lignosulfonate depending on the effect of interfacial tension decrease. 70  Figure 3.12 Liquid sulfur-aqueous solution interfacial tension versus temperature, the influence of different kinds of lignosulfonate: 0.3 g/L lignosulfonate, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, no sulfuric acid  Figure 3.13 shows the influence of temperature on the interfacial tensions in the absence or presence of SDAs, including Quebracho and OPD. For Quebracho, the interfacial tension difference is less than 2.6 mN/m when the temperature ranges 130-150ºC. The difference between different kinds of Quebracho on interfacial tension at any indicated temperature ranges as small as 1.2-1.6 mN/m. Thus, the Orfom® Grade 2 Tannin (a kind of Quebracho) instead of the other one was used in the further study. The other reason to select the Orfom® Grade 2 Tannin is that detailed information is available for this product. For OPD, the difference among the interfacial tensions is less than 1.9 mN/m when the temperature ranges from 130 to 150ºC. At any temperature, OPD has almost no effect on the decrease of interfacial tension.  71  Figure 3.13 Liquid sulfur-aqueous solution interfacial tension versus temperature: 0.3 g/L SDA, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, no sulfuric acid  Surface tension decreases with increasing temperature because the cohesive energy between molecules decreases with increasing temperature [50]. The interfacial tension between the liquid sulfur and the aqueous solution is represented by equation 3-8. According to the experiments the interfacial tension  SA decreases with temperature in the presence of SDA. This shows that the decrease of  S   A is greater than that of 2 SA .  SA   S   A  2 SA  (3-8)  72  3.3.3 Influence of Sulfur Dispersing Agents at Low pH  In this section, the influence of different kinds of lignosulfonate was studied. In addition, the interfacial properties were investigated in three systems:   Liquid sulfur-nickeliferous pyrrhotite-aqueous solution system    Liquid sulfur-pyrrhotite-aqueous solution system    Liquid sulfur-pentlandite-aqueous solution system  Four kinds of SDAs, including lignosulfonate, Quebracho, humic acid, and OPD, were studied where lignosulfonate and Quebracho refer to BorrePAL U and Orfom® Grade 2 Tannin, respectively. The pH of aqueous solution ranges from 4.8 to 1.2 when the sulfuric acid concentration increases from 0 to 0.09 mol/L.  (1) The Influence of Different Kinds of Lignosulfonate  Table 3.7 shows the influence of six different kinds of lignosulfonate on interfacial properties. The differences in both interfacial tensions and contact angles are small. The contact angle ranges from 146º to 150º, and the work of adhesion ranges from 3.5 to 4.7 mJ/m2 in the presence of different kinds of lignosulfonate. BorrePAL U was chosen to be investigated in further because it is most effective in decreasing interfacial tension and increasing contact angle. In addition, BorrePAL U was recognized by Borregaard LignoTech to produce the highest metal recoveries in the pressure leaching applications in the BorrePAL series additives.  73  Table 3.7 The influence of different kinds of lignosulfonate on the interfacial properties: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4, nickeliferous pyrrhotite Lignosulfonate  Interfacial Tension  Contact Angle  Work of Adhesion  (0.5 g/L)  (mN/m)  (º)  (mJ/m2)  Sodium LS, BorrePAL U  26.0  150  3.5  Calcium LS, BorrePAL N  26.9  147  4.4  Calcium LS, BorrePAL S  27.1  150  3.6  Sodium LS, Borresperse NA  27.2  146  4.7  Sodium LS, D-1929  25.9  146  4.4  Calcium LS, Aldrich  26.7  147  4.3  *LS is lignosulfonate  (2) Liquid Sulfur-Nickeliferous Pyrrhotite-Aqueous Solution System  Figure 3.14 shows the influence of sulfuric acid on interfacial tensions in the absence or presence of SDA. For OPD, lignosulfonate, and Quebracho, the influences of sulfuric acid on interfacial tension are very small. For humic acid (D-616), the interfacial tension drops from about 41.1 to 31.5 mN/m when the sulfuric acid concentration increases from 0 to 0.18 mol/L.  The pKa of sulfonate group in lignosulfonate is 1.5 [55]. In the absence of sulfuric acid, the value of pH of 1.0 mol/L nickel sulfate solution is 4.8. 100% sulfonate groups dissociate at pH 4.8. In the presence of 0.09 mol/L sulfuric acid, the value of pH of 1.0 mol/L nickel sulfate solution is 1.2. About 33% sulfonate groups dissociate at pH 1.2. The dissociation of sulfonate groups has no effect on the liquid sulfur-aqueous solution interfacial tension. This indicates that the adsorption of lignosulfonate or Quebracho on the liquid sulfur is not determined by the electrostatic interaction. Hydrophobic bonding is a possible adsorption mechanism. OPD is not influenced by the acidity because it cannot adsorb on the sulfur surface. The interaction between 74  humic acid and nickel hydroxide results in only a fraction of humic acid being available to interact with the liquid sulfur-aqueous solution interface.  Figure 3.14 Liquid sulfur-aqueous solution interfacial tension versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 M NiSO4, 0.5 g/L sulfur dispersing agent  Figure 3.15 shows the influence of the SDA dosage on interfacial tensions in the presence of 8.9×10-2 mol/L H2SO4. The behavior of lignosulfonate, Quebracho, and OPD is almost the same with the results illustrated in Figure 3.9 and Figure 3.11 since sulfuric acid has no great influences on the interfacial tensions in the presence of the above three kinds of SDAs. The effect of humic acid is different with the results illustrated in Figure 3.11. The addition of sulfuric acid in the aqueous phase makes the nickel sulfate stable and less humic acid interact with nickel species. Therefore, more humic acid is available to adsorb onto the liquid sulfur-aqueous solution interface.  75  Figure 3.15 Liquid sulfur-aqueous solution interfacial tension versus sulfur dispersing agent concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4  Figure 3.16 and Figure 3.17 show the influence of the sulfuric acid concentration on the contact angle and the work of adhesion in the liquid sulfur-nickeliferous pyrrhotite-aqueous solution system. Lignosulfonate, Quebracho, and humic acid have an effect to increase the contact angles and decrease the work of adhesion.  Figure 3.18 shows the influence of the SDA dosage on the contact angles and the work of adhesion. Generally, lignosulfonate, Quebracho, and humic acid increase contact angles and decrease the work of adhesion. The contact angle increases greatly when the SDA dosage increases from 0 to 0.1 g/L, and varies in a small range when the dosage is over 0.1 g/L.  76  Figure 3.16 Liquid sulfur-nickeliferous pyrrhotite contact angle versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.5 g/L sulfur dispersing agent  Figure 3.17 Work of adhesion versus sulfuric acid concentration, liquid sulfur-nickeliferous pyrrhotite-aqueous solution system, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.5 g/L sulfur dispersing agent 77  Figure 3.18 Liquid sulfur-nickeliferous pyrrhotite contact angle and work of adhesion versus sulfur dispersing agent dosage, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4  In the presence of OPD, the liquid sulfur drop on the mineral surface changes too quickly to be recorded completely for the measurement of contact angles. It was observed that the angle decreases from a value greater than 90º to less than 90º. The maximum value recorded is 126º with addition of 0.5 g/L OPD. Therefore, lignosulfonate, Quebracho, and humic acid are effective SDAs at decreasing the work of adhesion in the liquid sulfur-nickeliferous pyrrhotite-aqueous solution system. OPD is ineffective at decreasing the work of adhesion.  (3) Liquid Sulfur-Pyrrhotite-Aqueous Solution System  Figure 3.19 and Figure 3.20 show the influence of sulfuric acid concentration on the contact angle and the work of adhesion in the liquid sulfur-pyrrhotite-aqueous solution system. Lignosulfonate, Quebracho, and humic acid increase contact angles and decrease the work of adhesion. 78  Figure 3.19 Liquid sulfur-pyrrhotite contact angle versus sulfuric acid concentration: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.5 g/L sulfur dispersing agent  Figure 3.20 Work of adhesion versus sulfuric acid concentration, liquid sulfur-pyrrhotiteaqueous solution system: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.5 g/L sulfur dispersing agent 79  Figure 3.21 shows the influence of the SDA dosage on the contact angle and the work of adhesion. Generally, lignosulfonate, Quebracho, and humic acid increase contact angles and decrease the work of adhesion. Contact angles increase dramatically when the SDA dosage increases from 0 to 0.1 g/L.  Figure 3.21 Liquid sulfur-pyrrhotite contact angle and work of adhesion versus sulfur dispersing agent dosage, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4  The effect of OPD on the contact angle between liquid sulfur and pyrrhotite was investigated. The liquid sulfur drop on the mineral surface changes too quickly to be recorded completely by taking photos during the measurement of contact angles. It was observed that the angles decreases from a value greater than 90º to even less than 73º which is the value in the absence of SDA. Therefore, lignosulfonate, Quebracho, and humic acid are effective SDAs at decreasing the work of adhesion in the liquid sulfur-pyrrhotite-aqueous solution system. OPD is ineffective at decreasing the work of adhesion. 80  (4) Liquid Sulfur-Pentlandite-Aqueous Solution System  Table 3.8 shows the influence of different SDAs on interfacial properties in the liquid sulfur-pentlandite (from Voisey’s Bay)-aqueous solution system. Lignosulfonate, Quebracho, and humic acid are effective at increasing contact angles and decreasing the work of adhesion. For OPD, the contact angle reported in Table 3.8 is a stable value.  Table 3.8 The influence of sulfur dispersing agents on interfacial properties: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4, pentlandite (from Voisey’s Bay) Sulfur Dispersing Agent  Interfacial Tension  Contact Angle  Work of Adhesion  0.5 g/L  (mN/m)  (º)  (mJ/m2)  None  51.6  96  46.2  OPD  51.1  102  40.5  Quebracho  35.9  134  11.0  Lignosulfonate  26.0  150  3.5  Humic Acid  32.3  138  8.3  Lignosulfonate was found to be the most effective SDA at decreasing the work of adhesion in the pentlandite system. The effect of SDA dosage was investigated and the results are shown in Table 3.9. The contact angle increases dramatically when the SDA dosage increases from 0 to 0.1 g/L, and remains relatively stable with further increasing the dosage.  81  Table 3.9 The influence of lignosulfonate (BorrePAL U) on interfacial properties: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4, pentlandite (from Voisey’s Bay) Concentration  Interfacial Tension  Contact Angle  Work of Adhesion  (g/L)  (mN/m)  (º)  (mJ/m2)  0.0  51.6  96  46.2  0.1  29.9  137  8.0  0.3  27.2  152  3.2  0.5  26.0  150  3.5  0.7  25.8  150  3.5  Pentlandite from Sudbury was used to repeat the experiments due to the shortage of pentlandite from Voisey’s Bay. These results are shown in Table 3.10. Lignosulfonate, humic acid, and Quebracho are effective at decreasing interfacial tension, increasing contact angle, and decreasing the work of adhesion. OPD is ineffective at changing interfacial properties. This is consistent with the findings shown in Table 3.8.  Table 3.10 The influence of sulfur dispersing agents on interfacial properties, 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 8.9×10-2 mol/L H2SO4, pentlandite (from Sudbury) SDA  Interfacial Tension  Contact Angle  Work of Adhesion  0.5g/L  (mN/m)  (º)  (mJ/m2)  None  51.6  93  48.9  OPD  51.1  92  49.3  Quebracho  35.9  144  6.9  Lignosulfonate  26.0  149  3.7  Humic Acid  32.3  145  5.8  82  (5) The Role of Sulfur Dispersing Agents - Decreasing the Work of Adhesion  The SDA accumulates or adsorbs at the liquid sulfur-aqueous solution interface and lowers excess free energy  SA at this interface. As shown in Figure 2.12, the shape of liquid sulfur on the sulfide mineral surface is determined by the balance of three interfacial tensions. The driving force (negative value):  MA  (  MS   SA ) makes the contact angle increase.  As a result, the work of adhesion decreases in the presence of effective SDA. Therefore, less agitation energy, which acts to separate liquid sulfur from sulfide mineral surface, is required in the leaching step with the addition of SDA. Lignosulfonate, Quebracho, and humic acid are effective SDAs, which are recommended for leaching studies. OPD appears to be an ineffective SDA based on results of these interfacial studies.  In theory, if the agitation energy is big enough, the concept of the work of cohesion W C should be mentioned. Work of cohesion is the work required to produce two unit areas of interface. In this study, work of cohesion is the work needed to separate one unit area of liquid sulfur to two unit areas as represented by equation 3-9. Equation 3-10 shows the relationship between work of adhesion and work of cohesion. The contact angle between liquid sulfur and sulfide minerals is greater than 0ºand smaller than 180º, so W A  W C . Therefore, a sulfur drop is more easily removed from a mineral surface than separated from some broken surface within the sulfur drop. W C  2  SA WA WC    (3-9)  1  cos  2  (3-10)  83  3.3.4 Interfacial Energy Change  When a mineral particle goes from aqueous solution into liquid sulfur, three stages of wetting happen: adhesional wetting, spreading wetting, and immersional wetting. There are the three situations in this study.  (1) In the Absence of Sulfur Dispersing Agent - Low Acidity  The contact angle between liquid sulfur and sulfide mineral is higher than 90º as illustrated in Figure 3.2. The energy changes during the three stages wetting process are calculated and presented in Table 3.11. The total energy change W T  0 , and the work of adhesion range over 3.4-6.0 mJ/m2 without acid addition. Spreading wetting and immersional wetting cannot happen under these conditions.  Table 3.11 Interfacial energy change for complete sulfur wetting: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0 mol/L H2SO4, no sulfur dispersing agent Work  Nickeliferous  mJ/m2  Pyrrhotite  WI  -4.4  WII  Pyrrhotite  Pentlandite  Pentlandite  (Voisey’s Bay)  (Sudbury)  -6.0  -4.8  -3.4  187.5  181.2  186.0  191.6  WIII  98.2  96.6  97.8  99.2  WT  281.2  271.8  279.0  287.4  84  (2) In the Absence of Sulfur Dispersing Agent - High Acidity  In the presence of 0.18 mol/L sulfuric acid, the contact angle range over 70º-88ºin different mineral systems as illustrated in Figure 3.2. The energy changes during the three stages of the wetting process are calculated and presented in Table 3.12. The total energy change W T  0 and the work of adhesion range over 53.0-68.7 mJ/m2. The sulfur wetting problem exists, and removal of molten sulfur from the sulfide mineral surface is unattainable by mechanical agitation.  The contact angle between liquid sulfur and pentlandite mineral is greater than 90º in the presence of 8.9×10-2 mol/L H2SO4, which means that the immersional wetting cannot happen. The contact angle is determined by the nature of pentlandite mineral and possibly influenced by: (1) surface chemistry of the sulfide mineral during the measurement, nitrogen instead of oxygen was used in the study; (2) surface chemistry of the sulfide mineral before the measurement, no surface protection method was applied on the mineral surface after it was polished; (3) the heterogeneous structure of the pentlandite mineral, no pure pentlandite mineral was utilized in this study.  Table 3.12 Interfacial energy change for complete sulfur wetting: 140ºC, 690 kPa N2, 1.0 mol/L NiSO4, 0.18 mol/L H2SO4, no sulfur dispersing agent Work  Nickeliferous  mJ/m2  Pyrrhotite  WI  -68.7  WII  Pyrrhotite  Pentlandite  Pentlandite  (Voisey’s Bay)  (Sudbury)  -60.1  -53.0  -53.9  -70.0  -35.6  -7.1  -10.7  WIII  33.7  42.3  49.4  48.5  WT  -105.1  -53.3  -10.7  -16.1 85  (3) In the Presence of Sulfur Dispersing Agent - High Acidity  The liquid sulfur-sulfide mineral contact angles are greater than 134º in the presence of 0.5 g/L lignosulfonate, Quebracho, or humic acid as illustrated in Figure 3.16, Figure 3.19, Table 3.8, and Table 3.10. The total energy change W T  0 and the work of adhesion range over 2.1-11.0 mJ/m2. Under these conditions molten sulfur can be removed from the sulfide mineral surface by mechanical agitation.  3.4 Conclusions   The sulfide mineral surface is sulfophobic at pH from 4.1 to 4.5 (and probably higher pH) due to the hydrolysis of nickel (II) ions to nickel hydroxide and the deposition of nickel hydroxide on the mineral surface. The sulfide minerals affected by this phenomenon include pentlandite, nickeliferous pyrrhotite, pyrrhotite, and chalcopyrite.    In three different mineral systems, including liquid sulfur-pentlandite-aqueous solution, liquid  sulfur-nickeliferous  pyrrhotite-aqueous  solution,  and  liquid  sulfur-pyrrhotite-aqueous solution system. Lignosulfonate, Quebracho, and humic acid were found to significantly reduce the work of adhesion indicating they are effective SDAs. OPD was an ineffective SDA under these conditions.   The adsorption of lignosulfonate on the molten sulfur surface was calculated by the Gibbs Equation. The adsorption is multilayer adsorption. The limiting lignosulfonate concentration was found to be 0.5 g/L.    OPD cannot adsorb on the molten sulfur surface.  86  4.0 Characterization of Sulfur Dispersing Agents The factors that influence the adsorption of SDAs onto minerals consist of the nature of structure groups on the mineral surface, the molecular structure of SDA, and the environment of the aqueous phase. To understand the adsorption behavior of SDA the first step is to understand the functional groups of SDAs. The surface groups of sulfide minerals and the functional groups of SDAs were introduced in Chapter 2. In this chapter, the infrared spectra of SDAs (including lignosulfonate, Quebracho, and humic acid) are characterized. OPD has the chemical formula C6H4(NH2)2 and -NH2 is the major functional group. Therefore, the infrared spectrum of OPD was not measured.  4.1 Materials and Apparatus  4.1.1 Materials  Potassium bromide of infrared quality was used. SDAs are indicated below: (1) BorrePAL U, sodium lignosulfonate, was obtained from Borregaard LignoTech. (2) BorrePAL N, calcium lignosulfonate, was obtained from Borregaard LignoTech. (3) BorrePAL S, calcium lignosulfonate, was obtained from Borregaard LignoTech. (4) Quebracho, Orfom® Grade 2 Tannin, was obtained from Chevron Phillips Chemical Company LP. (5) Humic acid-potassium salt (modified lignite) was obtained from Borregaard LignoTech.  87  4.1.2 Equipment  The infrared spectra were obtained using a Perkin-Elmer System 2000 Fourier transform infrared (FTIR) spectrophotometer. Fifty scans were run to collect the infrared spectra in the wavenumber range from 4000 to 400 cm-1. The spectral resolution was 0.5 cm-1.  4.2 Experimental Procedure The absorption spectra of the mineral sample and SDAs were recorded by the KBr transmission technique. About 3 mg of the material and 300 mg of KBr were mixed thoroughly and ground in an agate mortar. A disc was formed by pressing the mixture at a pressure of about 28000-34000 kPa. The disc samples were mounted in a holder in the path of the radiation beam.  4.3 Results and Discussion  4.3.1 Characterization of Lignosulfonate  Figure 4.1 shows the infrared spectrum of lignosulfonate obtained with the KBr pellet transmission technique. The qualitative analysis of the lignosulfonate’s infrared spectrum is based on band assignments to functional groups, as provided in previous investigations [86, 154-157]. (1) The strong peak at 3426 cm-1 is assigned to the asymmetric and symmetric stretching vibrations of the OH functional group. (2) The peak at 2938 cm-1 is due to CH stretching of methyl or methylene group. 88  (3) The shoulder band at 2843 cm-1 indicates the stretching vibrations of the –OCH3 group. (4) The bands at 1604 and 1513 cm-1 are assigned to the skeletal vibrations of the molecule. (5) The bands at 1465 and 1420 cm-1 come from CH bonds of -OCH3 group. (6) The peak at 1384 and 1267 cm-1 are assigned to the phenolic OH bending vibrations. (7) The peaks at 1211, 1036, and 654 cm-1 are due to asymmetric or symmetric stretching vibrations of the sulfonate group.  Figure 4.1 Infrared spectrum of sodium lignosulfonate (BorrePAL U)  Figure 4.2 shows the infrared spectra of different kinds of lignosulfonate. The peaks at 1711 cm-1 (BorrePAL N), 1717 cm-1 (BorrePAL S) arise from carboxyl groups. The peaks at 1331, 1120 cm-1 (BorrePAL S) are due to OH groups. Lignin character is influenced by its origin, and the amount of methoxyl, hydroxyl, and carboxyl groups are influenced by the pulping temperature and pulping time during the production of lignin [156]. The infrared spectra of lignosulfonate indicate the presence of sulfonate, hydroxyl, and methoxyl groups in all of the three different 89  kinds of lignosulfonate. Carboxyl group was found in the BorrePAL N and BorrePAL S.  Carboxylic acid groups are characterized by bands at about 1660-1740 cm-1, 1350-1450 cm-1, 1200-1300 cm-1, or 880-950 cm-1 [157]. Possibly very small amount of carboxyl groups exist in BorrePAL U which cannot be detected by its IR spectrum. The carboxylic acid content in lignosulfonate was not determined by other methods in this study.  Figure 4.2 Infrared spectra of different kinds of lignosulfonate  4.3.2 Characterization of Quebracho  Figure 4.3 shows the infrared spectrum of Quebracho. The qualitative analysis of infrared spectrum of Quebracho is based on band assignments to functional groups in previous studies [157-159]. (1) The strong peak at 3407 cm-1 is due to the vibrations of OH groups. 90  (2) The peak at 2923 cm-1 and the shoulder at 2849 cm-1 are due to CH groups. (3) The peaks at 1616, 1521, and 1453 cm-1 are due to aromatic skeletal vibrations. (4) The band at 1373 cm-1 is assigned to the phenolic OH bending vibrations. (5) OH and ether groups contribute to the bands ranging from 1285 to 1038 cm-1. (6) The peak at 975 cm-1 is due to OH out-of-plane bending. (7) The peaks at 1038 and 651 cm-1 are characteristic of sulfonate groups. The infrared spectrum of Quebracho indicates the presence of hydroxyl and sulfonate groups.  Figure 4.3 Infrared spectrum of sulfited Quebracho (Orfom® Grade 2 Tannin)  4.3.3 Characterization of Humic Acid  Figure 4.4 shows the infrared spectrum of humic acid. The qualitative analysis of infrared spectrum of humic acid is based on band assignments to functional groups in the literature. (1) The band centered at 3437 cm-1 is due to the H-bonded OH stretching of carboxyl, phenol 91  and alcohol vibration [60, 160]. (2) Two bands centered at 2915 cm-1 and 2849 cm-1 are due to C-H stretching of methyl and methylene groups of aliphatic chains [60, 160]. (3) The weak shoulder at 1653 cm-1 is due to the C-C stretching of aromatic rings and to the C=O stretching of conjugated carbonyl groups [160-161]. (4) The band centered at 1587 cm-1 is assigned to protonated carboxylic (-COOH), carboxylate anion (-COO-) and ester carbonyl (-COOR) groups [162]. (5) The peak at 1386 cm-1 is attributed to the symmetrical C-H bending vibrations from aliphatic CH3 [160]. (6) The band centered at 1190 cm-1 is due to hydroxyl group, aromatic ethers, or esters [157]. (7) The peak at 1115 cm-1 is assigned to ring breathing and C-O stretching [161]. (8) The band centered at 1041 cm-1 may due to C-O stretching of carbohydrate groups, aromatic C-H deformation, and the sulfonate group [157, 161]. (9) The peak at 617 cm-1 is attributed to the sulfonate group [157]. The presence of sulfonate group is consistent with the presence of sulfur content in humic acid. The structure of humic acid is complicated and heterogeneous. The infrared spectrum indicates the presence of carboxyl, sulfonate, and hydroxyl group.  92  Figure 4.4 Infrared spectrum of humic acid-potassium salt (modified lignite)  4.4 Conclusions   The infrared spectra of lignosulfonate indicate the presence of sulfonate, hydroxyl, methoxyl groups in BorrePAL U, BorrePAL N, and BorrePAL S. Carboxyl group was found in the BorrePAL N and BorrePAL S.    The infrared spectrum of Quebracho indicates the presence of hydroxyl and sulfonate groups.    The infrared spectrum of humic acid indicates the presence of carboxyl, sulfonate, and hydroxyl groups.  93  5.0 Surface Charge Characterization The isoelectric point (iep) is the pH at which the zeta potential is zero. Nickel sulfide mineral and elemental sulfur are positively charged at pH < pHiep. All the reactions, including mineral oxidation, metal ion hydrolysis, the oxidation of sulfur-rich surface, could modify the surface chemistry of mineral and further affect its zeta potential [163]. When a SDA is introduced into the nickel sulfide mineral-aqueous solution system, the mineral surface charge change is due to the adsorption of SDA. This leads to the change of measurable values, such as zeta potential or electrokinetic sonic amplitude (ESA).  Potentiometric titration is commonly used to investigate the surface chemistry of minerals, which is performed in a pre-determined pH range. Volumetric titration can be used to study the influence of additive concentration on the surface charge change at a fixed pH value. The isoelectric point of a nickel concentrate sample was determined by potentiometric titration. Nickel sulfide concentrate is reactive at low pH and no pure pentlandite mineral was available. Elemental sulfur instead was used to indicate the surface charge of SDAs. Most of the experiments were performed to investigate the behavior of SDAs at a fixed pH.  94  5.1 Materials and Apparatus  5.1.1 Materials  (1) Elemental Sulfur  Elemental sulfur was obtained from Alfa Aesar, at a purity of 99.5%. The original particle size of sulfur was -100 mesh (150μm). The sulfur sample was ground by a ring-and-puck pulverizer for about 2 min and stored in double bags at 4ºC (or below). The particle size distribution of the sulfur powder was determined using the Malvern Mastersizer 2000 and is shown in Figure 5.1.  SDAs investigated in this study included: lignosulfonate (BorrePAL U, BorrePAL N, and BorrePAL S), Quebracho (Orfom® Grade 2 tannin), humic acid (D-616), and OPD (Acros Organics). All the SDAs were used without pretreatment.  Figure 5.1 Particle size distribution of elemental sulfur powder 95  (2) Nickel Concentrate  The nickel concentrate slurry was obtained from XSTRATA Nickel Limited (Strathcona Mill). A sample was washed with deionized water and sieved to pass a 325 mesh sieve, then washed with pH 2.0 sulfuric acid solution for 5 min. The sample was washed repeatedly with deionized water and dried under vacuum. The nickel concentrate sample was put into polyethylene bags and stored in a refrigerator to avoid oxidation.  5.1.2 Apparatus  The experiments were carried out using a Zeta Probe (Colloidal Dynamics, Warwick, RI) which allows the instrument to measure the particle size range of 1 nm to 10 µm. For coarse materials the equipment cannot obtain zeta potentials without input of the size distribution such as d50 and d85 (assuming a log-normal size distribution) [164]. Considering that the sulfur samples contain very coarse particles, the ESA signal instead of zeta potential was reported.  5.2 Experimental Procedure (1) Volumetric Titration  10 g sulfur powder and 70 g 0.01 mol/L NaCl background solution were put in a 250 mL conical flask, and conditioned in a shaker for 24 h. This sulfur slurry was further treated in a laboratory blender for 5 min and then transferred into a measuring cup. All suspensions were de-aerated in a Baxter vacuum oven at room temperature by gradually lowering the pressure to about 0.06 atm and quickly returning back to the atmospheric pressure [164]. The decrease of pressure results in the formation of a thick layer of froth which collapsed when the air goes back into the chamber. 96  Additional background solution was added to make sure that the total solution addition is 250 g. The measuring cup was placed on the Zeta Probe mixer, and the suspension was mixed at 160 rpm for 50 min. The pH of suspension was adjusted by addition of 2.0 mol/L HCl. Then the Zeta Probe was ready to do measurements. Volumetric titration mode of Zeta Probe was adopted in this study. Once the measuring mode was selected the measurements were performed by the apparatus automatically.  (2) Potentiometric Titration  10 g nickel concentrate and 250 g distilled water were added into a measuring cup. The measuring cup was placed on the Zeta Probe mixer and the suspension was mixed at 170 rpm for 25 min. The potentiometric titration mode of Zeta Probe was adopted in this study. The pH of suspension was adjusted to pH 10 by addition of 2.0 mol/L NaOH. The end pH of suspension was pH 2.5, with pH increment 0.5. The acidity was adjusted by addition of 2.0 mol/L HCl or 0.5 mol/L H2SO4.  5.3 Results and Discussion  5.3.1 The Influence of Different Kinds of Lignosulfonate  The influence of different kinds of lignosulfonate on the ESA signal is shown in Figure 5.2. The ESA signal decreases dramatically from 0.04-0.05 mPa*m/V to -0.30 mPa*m/V, -0.26 mPa*m/V, or -0.25 mPa*m/V in the presence of BorrePAL U, BorrePAL N, or BorrePAL S, when the SDA dosage increases from 0 to 60 mg/L. The ESA signal remains relatively stable with the additive dosage increasing further.  97  The ESA signal of sodium lignosulfonate, BorrePAL U, is larger than that of the calcium lignosulfonate, including BorrePAL N and BorrePAL S, which possibly arises from the higher degree of dissociation of sodium lignosulfonate. It was reported that 41% sodium lignosulfonate dissociates in an infinitely dilute aqueous solution. For calcium lignosulfonate the value is 8.5% [165].   R  SO 3 Na  R  SO 3  Na R  SO 3 Ca         R  SO 3  Ca  (5-1) 2  (5-2)  Figure 5.2 Electroacoustic effect of sulfur in the presence of lignosulfonate at pH 2.5  5.3.2 The Influence of Different Kinds of Sulfur Dispersing Agents  The influence of SDA concentration on the ESA signal is illustrated in Figure 5.3. Sulfur is positively charged at pH 2.5 in the absence of additive. In the presence of OPD, sulfur is still 98  positively charged and the ESA signal remains stable at 0.03-0.05 mPa*m/V. In the presence of 60 mg/L Quebracho, humic acid, or lignosulfonate (BorrePAL U) respectively, the ESA signal decreases dramatically from 0.04 mPa*m/V to -0.13 mPa*m/V, -0.21 mPa*m/V, or -0.30 mPa*m/V, and it remains relatively stable with further addition of these additives.  Lignosulfonate  The surface charge of sulfur changed from positive to negative indicating the adsorption of lignosulfonate. The larger ESA signal implies a higher charge density on the shear plane of sulfur. Lignosulfonate is negatively charged at pH 2.5 in the aqueous solution, and its surface charge principally comes from the dissociation of the sulfonate groups.  Figure 5.3 Electroacoustic effect of sulfur in the presence of SDA at pH 2.5  99  OPD  With the addition of OPD the surface charge or more precisely the charge density on the shear plane of sulfur is not changed. According to the speciation diagram of OPD in water, at pH 2.5 there are three species in solution: 98.2% HOPD+, 1.24% H2OPD2+, and 0.56% OPD [59]. OPD is positively charged in the acidic solution due to the following reaction:  NH  2   H      NH   3  (5-3)  Without considering the oxidation of OPD, two possibilities could happen and result in the stable state of the surface charge or charge density: -NH3+ cannot adsorb on the sulfur surface or -NH3+ groups replace the same amount of positive charges on the sulfur surface. The former is more likely to happen depending on the relationship between the liquid sulfur-aqueous solution interfacial tension and the OPD concentration. The oxidation of OPD in the aqueous solution is discussed in Chapter 6.  Quebracho  The surface charge of sulfur changed from positive to negative due to the adsorption of Quebracho. The pKa of phenolic hydroxy group in polyphenols is 9.2-9.9 [166]. Quebracho is nonionic at pH 2.5 if the phenol group is the only functional group that can dissociate. The negative charge of Quebracho is due to the presence of sulfonate groups, which is also indicated by its IR spectrum.  Humic Acid  The surface charge of sulfur changed from positive to negative due to the adsorption of humic acid. The negative charge partly comes from the dissociation of potassium from humic acid instead of carboxyl and hydroxyls groups. It is sure that the charge comes from the functional 100  group with relatively low pKa ranges from 0.5 to 4.5. The infrared spectrum indicates that sulfonate group supplies the negative charge for humic acid.  5.3.3 The Isoelectric Point of Nickel Concentrate  Figure 5.4 shows the ESA signal of nickel concentrate-water suspensions as a function of pH. The isoelectric point of the nickel concentrate is pH 5.25. Nickel concentrate is positively charged at pH lower than 5.25 while negatively charged at pH higher than 5.25. The isoelectric point of the nickel concentrate sample cannot be obtained when the pH of the aqueous solution was adjusted by sulfuric acid. Figure 5.4 indicates that the nickel concentrate surface is negatively charged due to the adsorption of SO42- at pH between 3.5 and 4.8, which is the pH range studied in Chapter 6. The adsorption of sulfate on the surface hydroxyl groups can be represented by reaction 5-4 and reaction 5-5 [167], which assume that protonation of the surface hydroxyl group on one plane and sulfate complexation on another plane. MOH  H     MOH  MOH  H     SO 4  MOH  2   2  (5-4)  2  2  SO 4  (5-5)  101  Figure 5.4 ESA signal of nickel concentrate-water suspensions as a function of pH, the acidity of the solution was adjusted by sodium hydroxide, hydrochloric acid or sulfuric acid  5.4 Conclusions   Sodium lignosulfonate has higher charge density than calcium lignosulfonate. This possibly arises from the higher degree of dissociation of sodium lignosulfonate than that of the calcium lignosulfonate.    At pH 2.5, lignosulfonate has the highest charge density, followed by humic acid and Quebracho.    OPD cannot adsorb onto elemental sulfur (solid) surface.    The isoelectric point of the nickel concentrate is pH 5.25.  102  6.0 Sulfur Dispersing Agent Adsorption on Nickel Concentrate The adsorption of SDAs including lignosulfonate, humic acid, Quebracho, and OPD on nickel sulfide concentrates was investigated. The adsorption mechanisms were discussed. To decrease the influence of cations leached from the nickel concentrate during the experiment, the conditions were confined to pH 3.5-4.8 and ambient temperature. Although the conditions of the study in this chapter are different from those in the pressure leaching studies, the information collected from this chapter is helpful to understand the behavior of SDAs during oxygen pressure leaching. The methods utilized to determine the concentration of SDAs have potential applications in studying the degradation of SDAs during leaching. OPD was found to be unstable in the nickel concentrate-(acidic) water system, as discussed in this chapter.  6.1 Materials and Apparatus  6.1.1 Reagents  The SDA solution was prepared in deionized water, and its acidity was adjusted by sulfuric acid. The chemicals used in the solution preparation and sample treatment are shown as follows: (1) Sulfuric acid, 95.0-98.0%, from Fisher; (2) 1-butanol, 99.9%, from Fisher; (3) Iron (II) sulfate heptahydrate, from Fisher; (4) Hydrochloric acid, 36.5-38.0%, from Fisher.  103  6.1.2 Mineral  The nickel concentrate slurry was obtained from XSTRATA Nickel Limited (Strathcona Mill). A sample was washed with deionized water and sieved to pass -325 mesh (-44 μm), then washed with pH 2.0 sulfuric acid solution for 5 min. The sample was washed repeatedly with deionized water and dried under vacuum. The nickel concentrate sample was put into polyethylene bags and stored in a refrigerator to avoid oxidation.  The specific surface area of the nickel concentrate was analyzed by the Quantachrome Autosorb-1MP BET analyzer under vacuum after the outgassing at 40ºC for 17 h. The surface area of the nickel concentrate sample is 1.62 m2/g.  6.1.3 Sulfur Dispersing Agents  The adsorption study involves the following SDAs: lignosulfonate (BorrePAL U, BorrePAL N, and BorrePAL S), Quebracho (Orfom® Grade 2 tannin), humic acid (D-616), and OPD (Acros Organics).  6.1.4 Apparatus  A UV-2401PC UV-Vis recording spectrophotometer by Shimadzu was used in the study. For uniform conditioning in the adsorption experiments, a Lab-line Orbit Environ Shaker was used at a moderate setting for the indicated amount of time. A centrifuge was used to separate aqueous solution from nickel concentrate.  104  6.2 Experimental Procedures To investigate the behavior of SDAs in the nickel concentrate-aqueous solution systems, the adsorption isotherm experiments were carried out at natural pH and at pH 3.5. Natural pH means that the nickel concentrate was treated in deionized water. An initial pH 3.5 solution was prepared by adding diluted sulfuric acid solution into deionized water. A small amount of cations could be leached into solution when the nickel concentrate was conditioned with (acidic) water, as shown in Table 6.1. Considering the stability of SDA, the presence of ferrous ions is not a problem in the UV/Vis determination of lignosulfonate, humic acid, and Quebracho. The stability of OPD is discussed later.  Table 6.1 The metal content in solution after the nickel concentrate was conditioned at ambient temperature with (acidic) water for 1 h pH  [Fe], mg/L  [Ni], mg/L  [Cu], mg/L  3.00  18.4  9.7  2.0  3.50  9.6  8.5  0.7  4.00  6.4  8.3  0.4  4.76  4.5  8.1  0.3  6.2.1 Quantitative Determination of Lignosulfonate and Humic Acid  The quantitative determination method of SDAs in aqueous solution was reviewed in Chapter 2. The general procedure to determine the concentration of lignosulfonate and humic acid in aqueous solution is shown below: a) 4.00 g of nickel concentrate was weighed and put into a 250 mL conical flask. b) 100 mL SDA solution was added into the conical flask. For a “blank” sample, 100 mL 105  deionized water (or acidic water) was added into the conical flask. c) The sample was conditioned on a shaker table. The conditioning time was determined to be when the absorbance readings did not significantly change, indicating that the adsorption equilibrium was achieved. d) After conditioning, the sample was transferred to a 50 mL centrifuge tube and centrifuged. e) The supernatant of the sample was then poured into a clean quartz UV 1 cm cell, and absorbance was obtained at specific wavelengths for each kind of SDA.  Figures 6.1, 6.2, and 6.3 illustrate the absorbance spectra of BorrePAL U, BorrePAL N, and BorrePAL S, respectively. The UV spectra of lignosulfonate are not influenced by the aqueous solution acidity under this study’s conditions. The calibration equations of lignosulfonate at different pH are shown in Table 6.2. The calibration curves of SDAs are shown in Appendix 2.  Figure 6.1 Absorbance spectra of BorrePAL U in aqueous solution, at the concentration of 150 mg/L, 125 mg/L, 100 mg/L, 75 mg/L, 50 mg/L, 25 mg/L, 10 mg/L, 5 mg/L, from top to the bottom at pH 3.5; at the concentration of 100 mg/L, 50 mg/L at natural pH 106  Figure 6.2 Absorbance spectra of BorrePAL N in aqueous solution at pH 3.5  Figure 6.3 Absorbance spectra of BorrePAL S in aqueous solution at pH 3.5 107  Figure 6.4 illustrates the absorbance spectra of humic acid at 50 mg/L and 100 mg/L at pH 3.5. The influence of pH on the humic acid’s spectra can be ignored. The calibration equations of humic acid at different pH are shown in Table 6.2.  Figure 6.4 Absorbance spectra of humic acid in aqueous solution at pH 3.5  6.2.2 Quantitative Determination of Quebracho  Butanol Reagent Preparation  The acid-butanol assay was used to determine the concentration of Quebracho in aqueous solution. The preparation of butanol solution reagent includes three steps as follows: a) 50 mL concentrated hydrochloric acid was added into a 1 L volumetric flask; b) 0.7 g ferrous sulfate heptahydrate was added into the flask and fully dissolved in the concentrated acid; c) 1-butanol was added up to the mark of the 1 L volumetric flask. 108  Quantitative Determination of Quebracho  The procedure stated in 6.2.1 was used to measure the concentration of Quebracho except step e). The supernatant needs a special treatment before step e). 2 mL supernatant sample and 14 mL butanol reagent were added into a heavy walled test tube, capped and mixed. For a “blank” sample, 2 mL deionized water and 14 mL butanol reagent were added into the heavy wall test tube. The mixed samples were conditioned in a water bath at 95ºC for 1 h. After cooling the sample was transferred into a clean quartz UV 1 cm cell, and the absorbance measurement was obtained at a wavelength of 550 nm. Figure 6.5 shows the absorbance spectra of Quebracho at concentration of 50 mg/L and 100 mg/L as well as the spectra after the treatment by butanol-HCl reagent. The calibration equation for Quebracho is shown in Table 6.2.  Figure 6.5 Absorbance spectra of Quebracho: (1) 100 mg/L pH 2.2; (2) 50 mg/L pH 2.2; (3) 2 g/L Quebracho, butanol reagent: supernatant = 7:1; (4) 2 g/L Quebracho, butanol reagent: supernatant = 14:1; (5) butanol reagent: supernatant = 7:1, without Quebracho 109  6.2.3 Quantitative Determination of OPD  Direct spectrophotometric determination of OPD in water is a method recommended by Isaev [131] which does not require expensive or rare reagents and strict control of temperature, medium, time, or other conditions. Figure 6.6 shows that the spectra of OPD are influenced by pH due to the protonation of –NH2 group. Wavelength 266 nm and 427 nm are pH independent. Wavelength 266 nm was used to draw the calibration line of OPD. The calibration equation is shown in Table 6.2.  Figure 6.6 Absorbance spectra of OPD: 1: pH 6.1; 2: pH 4.5; 3: pH 3.2; 4: pH 2.4; 5: pH 5.8; 6: pH 4.0; 7: pH 3.0; 8: pH 2.5 (1-4, 100 mg/L; 5-8, 50 mg/L)  110  Table 6.2 Calibration equations for the UV analysis of different sulfur dispersing agents Sulfur Dispersing Agent  pH  Wavelength  Calibration Equation  nm  c: mg/L  R2  BorrePAL U  Natural  281  A = 0.01209 c  1.000  BorrePAL U  3.5  281  A = 0.01203 c  1.000  BorrePAL N  Natural  280  A = 0.00961 c  1.000  BorrePAL N  3.5  280  A = 0.00949 c  1.000  BorrePAL S  Natural  278  A = 0.00529 c  1.000  BorrePAL S  3.5  278  A = 0.00525 c  1.000  Humic Acid  Natural  254  A = 0.02172c  1.000  Humic Acid  3.5  254  A = 0.02131c  1.000  Quebracho  Natural  550  A = 0.00036c  0.997  3.5  266  A = 0.0137c  0.997  OPD  Figure 6.7 and Figure 6.8 illustrate the influences of metal content on the spectra of 50 mg/L OPD at pH 3.5. The increase of absorption at wavelength 266 nm and 427 nm is possibly due to the chemical transformation of OPD as illustrated in Figure 6.7. Figure 6.8 shows that nickel sulfate alone cannot increase the adsorption of OPD and ferrous sulfate is helpful to the oxidation of OPD. It is unsuitable to determine the concentration of OPD by the direct spectrophotometric method in the nickel concentrate-aqueous solution system. The diazotization-azo coupling method is also unsuitable due to the autooxidation of OPD and the influence of iron on the OPD absorption in the spectrum range from 400 to 500 nm.  111  Figure 6.7 The influence of metal content (9.6 mg/L FeSO4, 8.5 mg/L NiSO4) and time on the absorbance spectra of 50 mg/L OPD at pH 3.5. From top to the bottom: Line 1-3 with iron and nickel; 5 h, 4 h, 3 h; Line 4-6 without metal; 5 h, 4 h, 3 h. Time recording started once OPD was put into acidic water to dissolve  112  Figure 6.8 The influence of metal content (9.6 mg/L FeSO4, 8.5 mg/L NiSO4) on the absorbance spectra of 50 mg/L OPD at pH 3.5 at 5 h: 1 with iron; 2 without iron and nickel; 3 with nickel. Time recording started once OPD was put into acidic water to dissolve  Figure 6.9 shows the influence of conditioning time on the adsorption of different SDAs. For BorrePAL U, BorrePAL N, and BorrePAL S, the conditioning time is 80 min; for humic acid and Quebracho the conditioning time is 120 min and 100 min, respectively.  113  Figure 6.9 Influence of conditioning time on the sulfur dispersing agent adsorption on the nickel concentrate at natural pH  OPD is unstable in aqueous solution and the following equation shows the possible oxidation of OPD by 1-electron oxidation to form s-BQDI and follows the formation of BQDI due to removal of a second electron [168]:  NH2  NH  -e -  -2H+ NH2 (OPD)  NH  -e  NH (s-BQDI)  NH (BQDI)  (6-1)  OPD is easily oxidized by common oxidants, such as Ag2O, PbO2, and MnO2, and forms 2,3-diaminophenazine (DAP) and other by-products. DAP is a common product of the oxidation of OPD in the presence of transition metal ions. OPD can be catalytically oxidized by molecular oxygen in the presence of cobalt (II) salts (equation 6-2) [169]. OPD is oxidized by ferric to form 114  ferrous and DAP at pH 2 [170]. The concentration of DAP was measured by UV-Vis absorption spectra at 425 nm after the reaction between OPD and hydrogen peroxide [171]. DAP has two absorption peaks at 257 nm and 425 nm.  NH2  N  NH2  N  NH2  OPD, O2 Co2+ NH2  (DAP)  (6-2)  Figure 6.10 demonstrates the chemical transformation of OPD with time during OPD conditioning with the nickel concentrate. The absorption peak at 438 nm increases with conditioning time increasing. This indicates the formation of OPD oxidation product or products.  Figure 6.10 Influence of time on the stability of OPD when conditioning with the nickel concentrate: 140 mg/L OPD, 4.00 g nickel concentrate, 100 mL OPD solution  115  6.2.4 Calculation  The amount of SDA adsorbed on the nickel concentrate surface can be calculated from the formula given below: Abs   ( C in  C Eq )  V m  S BET  (6-3)  Where Abs is the amount of SDA adsorbed in mg/m2; C in is the initial SDA concentration in mg/L; C Eq is the equilibrium SDA concentration as measured in mg/L; S BET is the specific surface area for nickel concentrate in m2/g; m is the weight of the nickel concentrate sample in g.  6.3 Results and Discussion Adsorption isotherms are introduced in the literature and the adsorption behavior can be classified into five general forms [133]. The adsorption of lignosulfonate on ZnS and sandstone was investigated [84, 134]. Langmuir and Freundlich adsorption isotherms were used to fit the adsorption isotherms. In this study, the amount of SDA adsorbed on the nickel concentrate surface increases with increasing dosage and then saturates at about monolayer coverage. Langmuir and Freundlich adsorption isotherms were used to fit the adsorption of SDAs. Langmuir adsorption gave the best fit, with the relative coefficient square (R2) of 0.990-0.998.  Langmuir adsorption isotherm is a useful approximation of the adsorption behavior of SDAs on the nickel concentrate surface. However, it can not fully explain what is observed. The Langmuir adsorption isotherm assumes that all the adsorption sites are equivalent and the adsorbed species do not interact with each other. In this study, nickel concentrate could contain different sites on a 116  given surface. The adsorbate-adsorbate interactions could be significant. The experimental data that deviations from the Langmuir adsorption isotherm found in this study are possibly due to the above two reasons.  6.3.1 Langmuir and Freundlich Adsorption Isotherms  The Langmuir and Freundlich adsorption isotherms were used to fit SDAs adsorption, though except for OPD, SDAs have macromolecular structure. Linear regression was usually used to determine the adsorption isotherms. Four different Langmuir linear regression methods and the Freundlich linear regression method are shown in Table 6.3. For the non-linear method the isotherm parameters were determined using the solver add-in with Microsoft Excel by minimizing the respective coefficient of determination between experimental data and the calculated value. Detailed information about Langmuir linear regression methods is available in the literature [172].  Table 6.3 Langmuir and Freundlich isotherms and their linear regression methods [172] Isotherm  Linear form  Langmuir-1  C    q  Langmuir-2  1 q   Langmuir-3  qm K  C  qm (  q  1  C  K  qm  q  1  1  1  qm  q  C  1  )    K  qm C  vs. C  vs.  1  KC q  qm  ( q  Langmuir-4  C  Freundlich  1  Plot  q  KC  1/ n  1  )  q  q  K C   K  qm  K  q  log( q )  log( K )  1 / n log( C )  q C  vs.  1 C q C  vs. q  log( q ) vs. log( C )  117  The adsorption parameters were obtained according to the linear form of the adsorption isotherm. Figure 6.11 shows the adsorption isotherm for BorrePAL U adsorption on the nickel sulfide concentrate at natural pH. Langmuir-1 gave the best fit, as shown by the relatively higher R2 values as shown in Table 6.4 and Table 6.5. In addition, Langmuir-1 is not sensitive to the data error although it has some bias toward fitting the data. Langmuir-2 is very sensitive to data error in the low concentration range indicated by the linear regression equation shown in Table 6.3.  Table 6.4 The parameters of the Langmuir model for the adsorption of BorrePAL U on the nickel concentrate at ambient temperature (natural pH) qm (mg/m2)  Ka  R2  Langmuir-1  1.02  0.0778  0.996  Langmuir-2  1.06  0.0622  0.979  Langmuir-3  1.05  0.0643  0.942  Langmuir-4  1.06  0.0606  0.942  Non-linear  1.05  0.0662  -  Table 6.5 The parameters of the Freundlich model for the adsorption of BorrePAL U on the nickel concentrate at ambient temperature (natural pH) K  1/n  R2  Linear  0.250  0.278  0.899  Non-linear  0.255  0.274  -  118  Figure 6.11 Adsorption isotherm for BorrePAL U adsorption on the nickel concentrate at natural pH - linear regression method  Figure 6.12 shows the adsorption isotherm of BorrePAL U on the nickel concentrate based on the non-linear regression method. The higher R2 value suggests that the Langmuir isotherm is more appropriate isotherm than the Freundlich isotherm for the adsorption of BorrePAL U on the nickel concentrate. Similar Langmuir adsorption parameters were acquired using different regression methods. Langmuir-1 linear regression method was used in this study due to its high R2 value.  119  Figure 6.12 Adsorption isotherm for BorrePAL U adsorption on the nickel concentrate at natural pH - non-linear regression method  6.3.2 Adsorption of Lignosulfonate on Nickel Concentrate  The Langmuir adsorption isotherm was used to fit the adsorption isotherm of three different kinds of lignosulfonate. The parameters of Langmuir adsorption model are shown in Table 6.6. The adsorption isotherms of BorrePAL U are illustrated in Figure 6.13, and the maximum value is 0.91 mg/m2 at natural pH and 1.02 mg/m2 at pH 3.5 when the initial concentration ranges from 0 to 200 mg/L. The adsorption isotherms of BorrePAL N are illustrated in Figure 6.14, and the maximum value is 1.17 mg/m2 at natural pH and 1.22 mg/m2 at pH 3.5 when the initial concentration ranges from 0 to 200 mg/L. The adsorption isotherms of BorrePAL S are illustrated in Figure 6.15, and the maximum value is 1.06 mg/m2 at natural pH and 1.37 mg/m2 at 120  pH 3.5 when the initial concentration ranges from 0 to 200 mg/L. For all the three kinds of lignosulfonate mentioned above, more lignosulfonate is adsorbed onto minerals at lower pH.  Table 6.6 The parameters of Langmuir-1 adsorption model for the adsorption of sulfur dispersing agents on nickel concentrate at ambient temperature Sulfur Dispersing Agent BorrePAL U  BorrePAL N  BorrePAL S  Humic Acid  Quebracho  pH  qm (mg/m2)  Ka  R2  Natural pH  1.02  0.0778  0.996  pH 3.5  1.13  0.0941  0.996  Natural pH  1.51  0.0295  0.990  pH 3.5  1.43  0.0538  0.993  Natural pH  1.11  0.0880  0.992  pH 3.5  1.55  0.0548  0.995  Natural pH  1.56  0.1019  0.993  pH 3.5  1.88  0.0918  0.991  Natural pH  4.23  0.0339  0.998  pH 3.5  4.08  0.0314  0.997  The pH values of the conditioned solution were measured and presented in Appendix 2. For BorrePAL U, when the initial pH of the solvent solution is 4.8 and 3.5, the pH values of the conditioned solution range from 4.4 to 4.5 and from 3.9 to 4.0, respectively. For BorrePAL N, when the initial pH of the solvent solution is 4.8 and 3.5, the pH values of the conditioned solution range from 4.1 to 4.3 and from 3.9 to 4.0, respectively. For BorrePAL S, when the initial pH of the solvent solution is 4.8 and 3.5, the pH values of the conditioned solution range from 4.1 to 4.2 and from 3.9 to 4.0, respectively.  121  The pH values of conditioned solution were useful in evaluating the adsorption mechanism. The nickel concentrate sample was pretreated with sulfuric acid solution and washed with deionized water for many times. The pH of the conditioned solution indicates that the residue acid (if exists) has no great effect on the adsorption environment. The pH of different kinds of lignosulfonate solution (3%) varies greatly: pH 8.5 for BorrePAL U, pH 5 for BorrePAL N, and pH 3-4.5 for BorrePAL S. The pH values of the conditioned solutions indicate that lignosulfonate have no great influence on the adsorption environment.  Figure 6.13 The adsorption of BorrePAL U on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation  122  Figure 6.14 The adsorption of BorrePAL N on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation  Figure 6.15 The adsorption of BorrePAL S on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation 123  (1) Influence of Molecular Weight on Lignosulfonate Adsorption  The surface active and polyelectrolytic properties of lignosulfonate are determined by three factors, including the size and the shape of molecule, the degree of sulfonation of the molecule, the degree of branching and cross-linking within the core of molecule [165].  The difference among lignosulfonates in the study is molecular weight. Lignosulfonate molecules are approximately spherical in aqueous solution. The molecular diameter is influenced by the acidity of aqueous solution [55]. The molecular areas of lignosulfonate calculated from Langmuir adsorption isotherm are shown in Table 6.7. BorrePAL U takes up the largest area on the mineral surface, while BorrePAL S takes up the smallest area, which is consistent with their molecular weight. The radius of lignosulfonate ranges from 0.7 to 7 nm when the molecular weight is in the range of 1325 to 73196 [153]. The molecular area calculated in this study is consistent with previous studies.  Table 6.7 Molecular area of lignosulfonate adsorbed on the nickel concentrate Lignosulfonate  Average Mw  BorrePAL U  BorrePAL N  BorrePAL S  60000  30000  12500  Adsorption  10  8  mol / m  2  Molecular Area  Radius   20  nm   10  m  2  Initial pH  1.77  9399  5.5  Natural  1.97  8444  5.2  3.5  5.37  3094  3.1  Natural  4.63  3584  3.4  3.5  9.36  1774  2.4  Natural  10.72  1549  2.2  3.5  124  (2) Influence of Cation on Lignosulfonate Adsorption  The adsorption capacity of lignosulfonate is higher at lower pH. Iron, nickel, and copper are leached from the nickel concentrate during the conditioning period and more cations are found in the acidic solution. Two factors could influence the adsorption isotherm depending on the amount of cation in the solution: one is the cation bridging effect; the other is the molecular weight effect [137]. The bridging effect increases the SDA adsorption, while the influence of molecular weight on the adsorption of polymer is complex. The amount of polymer adsorption increases with increasing the molecular weight. The molecular shape may change with the changes of molecular weight, and the spherical molecule adsorbed on the solid surface through minimal points of attachment results in more chances for desorption than adsorption.  Sulfide minerals surfaces have at least two functional groups include  Me  OH  and  S  H .  In the presence of a cation like Fe2+, the following reactions take place [38]:  S  H  Fe   2   S  Fe  OH     S  Fe  H      S  Fe  OH  (6-4) (6-5)  Based on Table 6.1, the cation content in aqueous solution after conditioning with the nickel concentrate at pH 3.5 is 1.72×10-4 mol/L iron, 1.45×10-4 mol/L nickel, and 1.10×10-5 mol/L copper. Considering the low cation concentration, the increase of adsorption at lower pH possibly arises from the bridging effect of cation between the mineral surface and SDA.  (3)The Influence of pH on Lignosulfonate Adsorption  The behavior of lignosulfonate is influenced by its functional groups: sulfonate (pKa = 1.5), carboxylate (pKa = 5.1), and phenolic hydroxyl groups (pKa = 10.5) [55]. The isoelectric point of nickel concentrate was found to be pH 5.25 by the potentiometric titration. Therefore, the electrostatic interaction is a possible mechanism for the adsorption of lignosulfonate on the 125  nickel concentrate surface.  (4) The Influence of Environment on Lignosulfonate Adsorption  The surface charge of a polyelectrolyte is influenced by metal ions in solution, pH, and temperature. Lindstrom [173] investigated the interaction between simple ions and lignosulfonate and found that divalent ion interacts more strongly with the polymer than monovalent ions do. The interaction between metal ions and lignosulfonate is determined by the electrostatic interaction. Khvan [174] studied the interaction between lignosulfonate and transition metal ions and found that in the lignosulfonate-Ni2+ systems, a metal ion displaces one proton (equation 6-6). R  SO 3 H  Ni  2   R  SO 3 Ni     H    (6-6)  Kontturi [175] found that the effective charge numbers of lignosulfonate decrease with increasing salt concentration and that lignosulfonate is positively charged in the presence of 1.0 mol/L BaCl2. The effective charge numbers are almost zero in the presence of 1.0 mol/L MgCl2. Lignosulfonate is not ionized in the highly acidic solution [176], and the degree of dissociation is around zero at pH 1, 20-30% at pH 5 and 35-80% at pH 11. The effective charge numbers of lignosulfonate are also influenced by temperature, dropping the effective charge number to zero at about 40ºC in the 0.1 mol/L NaCl solution.  The adsorption process is viewed as physical adsorption if electrostatic attraction and hydrophobic bonding are the major driving forces [177]. Considering the influences of temperature, ionic strength, and the pH of the environment in interfacial studies, lignosulfonate is 126  most likely to adsorb chemically onto the sulfide mineral surface.  6.3.3 Adsorption of Humic Acid on Nickel Concentrate  The Langmuir adsorption isotherm was used to fit the adsorption isotherm of humic acid, and the results are shown in Figure 6.16 and Table 6.6. The maximum adsorption is 1.42 mg/m2 at natural pH and 1.69 mg/m2 at pH 3.5 when the initial concentration of humic acid ranges from 0 to 200 mg/L. More humic acid is adsorbed onto minerals at the initial pH 3.5. The pH values of the conditioned solution are measured and listed in Appendix 2. The pH values of the conditioned solutions range from 4.5 to 5.1 and from 4.0 to 4.4, when the initial pH of the solvent solution is 4.8 and 3.5, respectively.  Figure 6.16 The adsorption of humic acid on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation 127  The adsorption capacity of humic acid is higher at lower pH. Considering the low cation concentration, the increase of adsorption at lower pH possibly arises from the bridging effect of cation between the mineral surface and SDA. Sulfonate, carboxylic, and hydroxyl groups are the major functional groups of humic acid. The isoelectric point of nickel concentrate was found to be pH 5.25 by the potentiometric titration. Therefore, the electrostatic interaction is a possible mechanism for the adsorption of humic acid on the nickel concentrate surface.  6.3.4 Adsorption of Quebracho on Nickel Concentrate  The Langmuir adsorption isotherm was used to fit the adsorption isotherm of Quebracho, and the results are shown in Figure 6.17 and Table 6.6. The maximum adsorption is 4.02 mg/m2 at natural pH and 3.92 mg/m2 at pH 3.5 when the initial concentration of Quebracho ranges from 0 to 800 mg/L. The pH values of the conditioned solution are measured and listed in Appendix 2. The pH values of the conditioned solution range from 4.1 to 4.2 and from 3.7 to 4.0 when the initial pH of the solvent solution is 4.8 and 3.5, respectively.  The negative charge of Quebracho is likely to come from the dissociation of sulfonate group. The electrostatic interaction is a possible mechanism for the adsorption of Quebracho on the nickel concentrate surface. The interaction between Quebracho and the nickel concentrate is possibly effected by chemical adsorption through sulfonate groups and by the formation of hydrogen bonds through hydroxyl groups. Last [178-179] and Iskra [158] proposed two mechanisms to explain the adsorption of Quebracho onto the mineral surface: the adsorption happened through free acid groups; metal cations act as bridges between Quebracho and the sulfide mineral. In this study, the adsorption of Quebracho was determined by Acid-Butanol method. The concentration of iron in the solution influences the adsorption at 550 nm. Therefore, the bridging effect cannot be investigated by the Acid-Butanol method.  128  Figure 6.17 The adsorption of Quebracho on the nickel concentrate at ambient temperature, the adsorption data was fitted by Langmuir equation  6.4 Conclusions   The adsorption of lignosulfonate, humic acid, and Quebracho on nickel concentrate is monolayer adsorption, and the Langmuir adsorption isotherm was used to fit the adsorption isotherm.    Electrostatic interaction and ion-binding are the possible mechanisms for the adsorption of lignosulfonate or humic acid on the nickel concentrate surface.    Electrostatic interaction is a possible mechanism for the adsorption of Quebracho on the nickel concentrate surface. The interaction between Quebracho and the nickel concentrate is possibly effected by chemical adsorption through sulfonate groups and by the formation of hydrogen bonds through hydroxyl groups. 129    OPD is unstable in the presence of ferric in the aqueous solution. The chemical transformation of OPD during its conditioning with the nickel concentrate was proved by the UV/Vis spectra changes.  130  7.0 Oxygen Pressure Leaching of Nickel Concentrate SDAs, including lignosulfonate, Quebracho, humic acid, and OPD, were evaluated in the interfacial and the adsorption studies. Lignosulfonate, Quebracho, and humic acid adsorbed onto the nickel sulfide mineral surface, decreasing the work of adhesion in the liquid sulfur-nickel sulfide mineral-aqueous solution system. OPD was unstable at ambient temperature when added to acidic nickel concentrate slurry and had the least effect on increasing the contact angle and decreasing the work of adhesion. However, OPD was used in the leaching tests because OPD showed good sulfur dispersion effects in the pressure leaching of sphalerite, chalcopyrite, and pyrite as stated in Chapter 2.  The studies in previous chapters provided useful information about SDAs, elemental sulfur, and nickel sulfide minerals under different conditions: (1) Interfacial studies: medium temperature (140ºC), wide pH range (sulfuric acid concentration: 0-0.18 mol/L), high ionic strength (nickel sulfate concentration: 0.1-1.5 mol/L) (2) Surface charge determination: ambient temperature, pH 2.5, low ionic strength. (3) Adsorption study: ambient temperature; pH ranging 3.5-4.8, low ionic strength. (4) Functional group determination: no aqueous solution. SDAs undergo degradation during pressure leaching, so the information obtained by infrared spectra indicates the structure and functionality of the SDAs prior to leaching.  The leaching environment is the most severe one in this study: moderate temperature, low pH, high ionic strength, and the presence of an oxidizing gas. This leaching study concentrates on the effect of four kinds of SDAs on the extraction of nickel. This study is useful in understanding both the available nickel concentrate leaching process (INCO process) and the potential process (oxygen pressure leaching process) of nickel extraction.  131  7.1 Materials and Apparatus  7.1.1 Nickel Sulfide Concentrate  Nickel Concentrate from XSTRATA Nickel  Nickel sulfide concentrate was obtained from XSTRATA Nickel (Strathcona Mill). The pH of the as-received slurry ranged from 11 to 12. A trace amount of free cyanide (< 0.02 mg/L) was found in the solution phase. The nickel concentrate sample was washed with deionized water and dried at ambient temperature overnight. Quantitative X-ray diffraction shows that the received nickel concentrate contained: 36.3% pentlandite, 30.6% pyrrhotite, 11.0% pyrite, 5.0% chalcopyrite, and 17.1% gangue. Three samples were acquired from the XSTRATA Nickel concentrate and used in the leaching studies.  Nickel concentrate sample (A). This is the nickel concentrate from XSTRATA Nickel, used without further treatment. The particle size distribution is shown in Figure 7.1, and 80% of the nickel concentrate was less than 48 µm. Chemical assay showed the content of metals and sulfur: 16.0% Ni, 44.5% Fe, 32.3% STotal, and 2.6% Cu.  Nickel concentrate sample (B). The nickel concentrate sample A was reground to P80 of 10 µm. Fine grinding was achieved using a stirred media detritor (SMD) and confirmed by the particle size analysis. The particle size distribution is shown in Figure 7.1. Chemical assay showed the content of metals: 13.8% Ni, 36.6% Fe, and 2.5% Cu. The metal contents in sample A and sample B were analyzed by different methods as discussed in 7.2.2.  Nickel concentrate sample (C). High purity pentlandite can, in principle, be acquired by gravity concentration to remove the gangue, followed by magnetic separation to remove the magnetic 132  pyrrhotite, and by flotation to remove pyrite [180]. An attempt to get high purity pentlandite concentrate was made in the lab. The XSTRATA Nickel concentrate was passed over a gravity concentration table and was sieved to pass a 325 mesh sieve (-44 µm). This is the nickel concentrate sample (C) used in the leaching studies. Chemical assay showed the content of metals: 11.6% Ni, 44.4% Fe, and 1.8% Cu. The nickel concentrate sample passing the 325 mesh screen was not upgraded.  Figure 7.1 Particle size distributions of the nickel concentrate samples. Concentrate A: XSTRATA Nickel concentrate sample; concentrate B: reground XSTRATA Nickel concentrate sample  Nickel Concentrate from Voisey’s Bay  A high grade nickel concentrate sample was obtained from Vale INCO, Voisey’s Bay mine. Pentlandite, pyrrhotite, chalcopyrite, and cobalt sulfide are the major sulfide minerals in the 133  sample. Chemical assay showed the content of metals and sulfur: 20.8% Ni, 37.4% Fe, 31.4% STotal, 1.68% Cu, and 0.92% Co. The P80 of the fresh, high grade nickel concentrate is expected to be 50 µm. All the analytical results were supplied by INCO.  7.1.2 Reagents  All solutions were prepared with reagent grade chemicals and deionized water. The reagents used in this study were: (1) Sulfuric acid with 95-98% purity (2) Medical grade pressurized oxygen with minimum 99.5% purity (3) Sodium chloride, reagent grade.  7.1.3 Sulfur Dispersing Agents  The oxygen pressure leaching study involved the following SDAs: lignosulfonate (BorrePAL U), Quebracho (Orfom® Grade 2 tannin), humic acid (D-616), and OPD (Acros Organics). All the organics were used without pretreatment.  7.1.4 Apparatus  Oxygen pressure leaching experiments were carried out in a standard 2 L Parr titanium autoclave equipped with a PID temperature controller. Oxygen consumption was measured by a mass flowmeter. The more oxygen is consumed, the greater the extent of sulfide oxidation. Figure 7.2 shows the setup for oxygen pressure leaching experiments.  134  Temperature & Agitator Controller  System  Magnetic Gas Release  Data Acquisition  Stirrer  Valve Rupture Disc Assembly  Cooling In Cooling Out  Pressure Gage  Mass Flowmeter Gas Inlet Value  Cooling In Cooling Out Stirring Assembly  Autoclave Bomb Thermowell  Heating Coil  Oxygen Dip Tube  Source  Cooling Coil  Figure 7.2 Diagram of the setup for oxygen pressure leaching experiment  7.2 Experimental Procedure  7.2.1 Experimental Procedure  Oxygen pressure leaching experiments were carried out on a 0.5 L scale. The SDA was dissolved in deionized water before adding into the autoclave. For some of the experiments, SDA was 135  sealed in an ampoule to make sure it came into contact with the nickel concentrate at time zero. Other leaching conditions were: (1) Temperature: 140 or 150ºC (2) Nickel concentrate: 20 g/L or 250 g/L (3) Sulfuric acid concentration: 49 g/L or 40 g/L (4) Oxygen over pressure: 690 kPa (5) Leaching time: 1.5 h or 2 h.  The experimental procedures were presented as follows: a) The desired amount of nickel concentrate, SDA solution, sodium chloride, and deionized water were added into the autoclave. b) The desired amount of sulfuric acid was sealed in an ampoule to prevent nickel concentrate sample oxidation during heating up of the autoclave. For some experiments, the desired amount of SDA was sealed in an ampoule. The ampoule with sulfuric acid and the ampoule with SDA were put into the autoclave. c) The autoclave was sealed, then heated. The head space was vented once the temperature exceeded 100ºC. d) At the desired temperature the autoclave was pressurized with oxygen. A PC was used to record the flow rate of oxygen. At the same time, the agitator was switched on and the agitator blades broke the ampoules to release the contained acid and the contained SDA. e) When the experiment ended, oxygen was shut off. The slurry was rapidly cooled down to ambient temperature. f) The pulp was filtered. The volume of leach solution was measured and the filtrate was stored in plastic bottles. The residue was washed with deionized water, and the volume of wash solution was measured, and sample collected and stored in a plastic bottle. The residue was dried in air overnight at 50ºC in an oven.  136  7.2.2 Analytical Methods  Nickel Concentrate and Residue Analysis  The particle size of nickel concentrate was analyzed using the Mastersizer 2000 in the Department of Mining and Mineral Process Engineering, UBC. Wet screening was utilized in the lab to examine the sulfide agglomeration of leach residue. The Quantitative X-ray diffraction (QXRD) analysis of the received nickel concentrate sample was completed in the Department of Earth and Ocean Sciences, UBC. The XRD (qualitative) method was used to characterize leach residue in the department of Materials Engineering, UBC. A thin section of XSTRATA Nickel concentrate sample (-100+200 mesh) was prepared by Vancouver Petrographics Limited. A scanning electron micrograph is illustrated as Figure 7.3.  Figure 7.3 A scanning electron micrograph of the XSTRATA Nickel concentrate sample (-100+200 mesh): 1, pentlandite; 2, pyrite; 3, pyrrhotite; 4, chalcopyrite; 5, gangue  137  Sample Analysis  Total sulfur in the concentrate was determined by LECO induction furnace. Nickel concentrate and leach residue were digested by a multi-acid digester and analyzed by the inductively coupled plasma (ICP). The finely ground nickel concentrate sample B and the leach residue (low pulp density leach) were digested by aqua regia and analyzed by atomic adsorption method. All the analyses mentioned above were conducted by International Plasma Labs Ltd. (IPL), a local analytical company. The nickel, iron, and copper content in leach solution and wash solution were analyzed by atomic adsorption method in the lab. Ferrous iron was determined by potassium dichromate titration [181]. The pH of filtrate samples were measured by a glass pH probe. The concentration of free acid in the filtrate samples was determined by a method used by McDonald [182]. The analytical methods for ferrous and free acid are described in Appendix 3.  Determination of Base Metal Extraction  The extent of nickel and copper extraction was determined from the residue analysis. The extent of iron extraction was determined from the residue analysis for the leaching experiments without addition of sodium chloride. The percent of base metal extraction was calculated by equation 7-1. Head  Me %   m Me   m Me  Re s  Head   100 %  m Me  (7-1)  Head Where Me % is the percent of base metal extraction; m Me is the mass of base metal in the  Re s head; m Me is the mass of base metal in the residue.  The extent of iron extraction was calculated from the leach solution for the leaching experiments with addition of sodium chloride (equation 7-2).  138  Sol  Fe %   m Fe   100 %  Head  (7-2)  m Fe  Where Fe % is the extent of iron extraction; m FeSol is the mass of iron solubilized; m FeHead is the mass of iron in the head. With addition of sodium chloride the lowest iron content in the leach solution is about 0.1 g/L. Therefore it is more reasonable to calculate the iron extraction from leach solution instead of leach residue. The mass balance of base metals and the analytical results were attached in Appendix 4.  7.3 Leaching Chemistry  7.3.1 Low Pulp Density Leaching  The leaching studies indicate that the oxidation of nickel concentrate consumes oxygen and acid, and forms elemental sulfur. Nickel and iron are leached into aqueous phase. Ferrous accounts for a small fraction in the total iron. The oxygen pressure leaching of pentlandite can be represented by the following three reactions [145]. Fe 4 .5 Ni 4 .5 S 8  4 . 5 O 2  18 H  4 Fe  2     O2  4 H  Fe 4 .5 Ni 4 .5 S 8  18 Fe  3     4 Fe   9 H 2 O  4 . 5 Ni 3  2   4 . 5 Fe   2 H 2O   22 . 5 Fe  2  2   8S  (7-3) (7-4)   4 . 5 Ni  2   8S  (7-5)  Pyrrhotite, pyrite, and chalcopyrite oxidation can be represented by the following reactions [90, 95]. 2 FeS  1 . 5 O 2  6 H 2 FeS  2     2 Fe  3   7 . 5 O 2  H 2 O  2 Fe   2 S  3 H 2O  3  2   4 SO 4  2 H  (7-6)   (7-7)  139  2 CuFeS  2   10 H     2 . 5 O 2  2 Cu  2   2 Fe  3   4 S  5 H 2O  (7-8)  The oxidation of elemental sulfur to sulfate is represented by reaction 7-9: S  1 . 5 O 2  H 2 O  H 2 SO 4  (7-9)  7.3.2 High Pulp Density Leaching  During high pulp density leaching, there was not enough sulfuric acid to keep iron in the aqueous phase. It is anticipated that iron undergoes hydrolysis during leaching based on the following reactions [182]. Ferric sulfate undergoes hydrolysis to form hematite (reaction 7-10), hydronium jarosite (reaction 7-11), basic ferric sulfate (reaction 7-12), or sodium jarosite (reaction 7-15) depending upon the acidity.  Fe 2 ( SO 4 ) 3  3 H 2 O  Fe 2 O 3  3 H 2 SO 4  (7-10)  3 Fe 2 ( SO 4 ) 3  14 H 2 O  2 ( H 3 O ) Fe 3 ( SO 4 ) 2 ( OH ) 6  5 H 2 SO 4  (7-11)  Fe 2 ( SO 4 ) 3  2 H 2 O  2 Fe ( OH ) SO 4  H 2 SO 4  (7-12)  2 ( H 3 O ) Fe 3 ( SO 4 ) 2 ( OH ) 6  3 Fe 2 O 3  4 H 2 SO 4  5 H 2 O  (7-13)  2 Fe ( OH ) SO 4  H 2 O  Fe 2 O 3  2 H 2 SO 4  (7-14)  3 Fe 2 ( SO 4 ) 3  2 NaCl  12 H 2 O  2 NaFe 3 ( SO 4 ) 2 ( OH ) 6  5 H 2 SO 4  2 HCl  (7-15)  140  7.4 Results and Discussion  7.4.1 Low Pulp Density Leaching of Nickel Concentrate (P80 of 48 µm)  The Effect of Agitation Speed  Nickel concentrate (P80 of 48 µm) was used in the low pulp density leaching studies. The weight percent of solids in the pulp is 2%. The effect of agitation speed on nickel extraction was studied in the absence of SDAs at 600, 700, and 800 rpm under the leaching conditions: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate, and 2 h.  The nickel extraction increased from 27% to 32%, when the agitation speed increased from 600 rpm to 700 rpm. The agitation speed had no great effect on the nickel extraction when it was greater than 700 rpm. The agitation speed of 700 rpm was adopted in the low pulp density leaching and high pulp density leaching of the finely ground nickel concentrate sample.  The Effect of Sulfur Dispersing Agents on Metal Extraction  Table 7.1 shows the metal extraction in the absence or presence of SDAs. The nickel extraction increased from 32% to 56-57% in the presence of 5 kg/t lignosulfonate, Quebracho, or OPD. There was no great influence for lignosulfonate and Quebracho on the nickel extraction when the dosage was 20 kg/t. Humic acid and OPD were more effective on the nickel extraction when the dosage was 20 kg/t. 66% nickel was extracted in the presence of 20 kg/t OPD. In the presence of humic acid, the nickel extraction increased from 42% to 55% when the dosage increased from 5 kg/t to 20 kg/t. No great increase of nickel extraction was found when the SDA concentration in the leach solution increased from 0.1 g/L to 0.4 g/L. High SDA dosage (20 kg/t) is not recommended in the further study considering its ability to increase nickel extraction, cost, and 141  the possible negative impact on downstream operations.  The extraction of iron and copper is also shown in Table 7.1. For each experiment more iron was extracted than nickel. This is due to the oxidation of pyrrhotite. More iron and copper were extracted into aqueous phase in the presence of lignosulfonate, Quebracho, OPD, or humic acid. SDAs have an effect to increase the nickel, copper, and iron extraction based on low pulp density leaching experiments.  Table 7.1 Metal extractions in the presence or absence of sulfur dispersing agents: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 48 µm), 2 h Sulfur Dispersing Agent  Ni (%)  Fe (%)  Cu (%)  None  32  55  43  Lignosulfonate: 5 kg/t  57  71  64  Lignosulfonate: 20 kg/t  53  68  58  Lignosulfonate: 50 kg/t  66  73  67  Quebracho: 5 kg/t  56  70  62  Quebracho: 20 kg/t  56  72  58  OPD: 5 kg/t  57  73  63  OPD: 20 kg/t  66  75  50  Humic Acid: 5 kg/t  42  62  61  Humic Acid: 20 kg/t  55  69  62  Properties of Leach Solution  Total iron, ferrous, and sulfuric acid concentration in the leach solution are shown in Table 7.2. For all the experiments ferrous accounted for about 10% of total iron. The free acid concentration decreased from 49 g/L to about 40 g/L after leaching. For sulfide sulfur, high acid 142  concentration favors the formation of elemental sulfur instead of the formation of sulfate. Nickel, copper, and iron was extracted into aqueous phase under high acid conditions.  More nickel was extracted into leach solution when the lignosulfonate and Quebracho dosage increased from 5 kg/t to 20 kg/t as shown in Table 2. However the nickel extraction calculated from leach residue showed no increase. This is possible due to sampling error. Concentrated sulfuric acid was sealed in an ampoule which was put into autoclave. The broken glass was not separated from leach residue. The broken glass and leach residue were ground together before sending for residue analysis.  Table 7.2 Properties of leach solution at ambient temperature, leaching conditions: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 48 µm), 2 h Sulfur Dispersing Agent  Ni  Cu  Fe  H2SO4  Ferrous  g/L  g/L  g/L  g/L  g/L  None  1.04  0.24  4.74  39.4  0.35  Lignosulfonate: 5 kg/t  1.44  0.30  5.59  40.3  0.50  Lignosulfonate: 20 kg/t  1.66  0.31  6.03  40.1  0.59  Lignosulfonate: 50 kg/t  1.92  0.34  6.20  39.0  0.67  Quebracho: 5 kg/t  1.56  0.30  5.81  39.8  0.62  Quebracho: 20 kg/t  1.76  0.29  6.18  40.3  0.58  OPD: 5 kg/t  1.56  0.31  6.06  39.7  0.63  OPD: 20 kg/t  1.95  0.24  6.48  40.8  0.76  Humic Acid: 5 kg/t  1.04  0.30  5.08  39.5  0.51  Humic Acid: 20 kg/t  1.57  0.31  5.77  39.3  0.56  143  More copper was extracted into leach solution when the lignosulfonate dosage increased from 5 kg/t to 20 kg/t as shown in Table 2. However the copper extraction calculated from leach residue showed no increase. This is possible due to sampling error. Less copper was extracted into leach solution when the Quebracho and OPD concentration increased from 5 kg/t to 20 kg/t. No repeated pressure leaching experiment was made to test the influence of SDAs on copper extraction.  Leach Residue  Nickel concentrate A was wet sieved, and 1.7% of the feed could not pass a 120 mesh sieve (125 μm). In the absence of SDA, the leach residue was washed thoroughly and wet sieved. 78.0% of leach residue could not pass the 120 mesh sieve. Figure 7.4 shows the leach residue over the 120 mesh in the absence of SDA. Figure 7.5 shows the EDX mapping of leach residue (> 125 μm), which indicates that sulfide minerals were covered by sulfur. The XRD analysis was performed on the coarse residue and the main components were pyrite, pentlandite and sulfur, as shown in Table 7.3. Sulfide minerals and sulfur were not found in the residue passing the 120 mesh sieve. The XRD analysis shows that pentlandite, pyrrhotite, pyrite, and chalcopyrite are the main sulfide minerals in the nickel concentrate. Based on the leach residue’s XRD analysis, most of pyrrhotite in the feed was oxidized which results in high iron extraction at low pulp density leaching.  144  Figure 7.4 Leach residue in the absence of sulfur dispersing agent (> 125μm fraction):  140ºC,  690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80: 48 µm), 2 h  Figure 7.5 EDX Mapping of leach residue (> 125 μm): 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 48 µm), 2 h 145  Table 7.3 Nickel concentrate A and leach residue, leaching conditions: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 48 µm), 2 h Nickel Concentrate A  Leach Residue  (P80 of -48 µm)  Weight % Minerals  +125 µm  -125 µm  +125 µm  -125 µm  1.7  98.3  78.0  22.0  Pentlandite, pyrrhotite, pyrite,  Pentlandite  No sulfide  chalcopyrite, and gangue  Pyrite  No sulfur  Sulfur  The final leach residue (with or without SDA addition) contained a large proportion of aggregated material. This indicates that a large portion of sulfide minerals un-oxidized during leaching. However, no quantitative analysis was made to correlate the nickel extraction with the amount of aggregated leach residue. The leach residue was easy to be separated into aggregated fraction and fine fraction. The fine fraction of leach residue was added into autoclave to increase the solids content (surface area) in previous study [76]. However inert solid recycle stream has no effect to solve the sulfur wetting problem. The useful solid that can increase base metal extraction is low rank coal. From this study useful information was obtained on the effect of coal in leaching, suppose humic acid is extracted from coal during leaching, humic acid alone can not solve the sulfur wetting problem. In addition, humic acid (un-sulfited) is not soluble in water which may also influence its ability to disperse molten sulfur from sulfide mineral surface.  The low nickel recovery partly arises from the sulfur wetting problem, and this can be solved by the addition of effective SDAs. More than 34% of nickel was not extracted during leaching of non-reground concentrate, which may be due to the following reasons: (1) the formation of an iron-containing product layer; (2) nickel being locked in an inert iron-sulfur mineral matrix; (3) slow leaching kinetics of pentlandite; (4) fast degradation rate of SDAs. An iron product layer 146  cannot form because of the presence of 38-49 g/L sulfuric acid in the leachate. Pentlandite was liberated based on the SEM-EDX study of the thin section sample of the XSTRATA Nickel concentrate sample (75-150 µm). The low nickel recovery mainly results from the slow leaching kinetics of pentlandite or other nickel containing minerals under the conditions in this study or the fast degradation speed of SDAs. The obvious experiment is to try a longer leaching time with addition of fresh SDA. However, this method was not adopted because 2 h is long enough for most oxygen pressure leaching experiments. And an easier method is to regrind the concentrate.  Lignosulfonate, Quebracho, OPD, and humic acid showed their effects on removing molten sulfur from sulfide mineral surface in the leaching of non-reground concentrate at low pulp density. Significant effects of SDAs to increase nickel extraction were not found. The particle size of nickel concentrates is an important factor in leaching. More time is needed to leach larger particles for 100% nickel extraction. All four kinds of SDAs investigated in this study experienced degradation during leaching. It is believed that the SDAs are fully degraded before nickel was fully extracted.  The Degradation of Sulfur Dispersing Agents  Molten sulfur formed during the leaching process will attach onto the sulfide mineral surface due to its hydrophobic property. SDAs have the ability to solve the sulfur wetting problem. However, SDAs degrade during the leaching process before the job is done. The degradation of OPD was proofed and introduced in Chapter 6.  In the pressure leaching of chalcopyrite, SDA was found not to affect the subsequent solvent extraction process due to the decomposition of SDA during pressure oxidation [74]. Lignosulfonate was degraded by ferric ions to produce ferrous and degraded lignosulfonate, with a half-life of approximately 10 min under oxidative pressure leaching conditions [87]. However, 147  detailed information about the degradation of SDA under leaching conditions is not available. Lignosulfonate is harmful to sulfur flotation due to its dispersing function or due to its function in the formation of fine sulfur particles which were difficult to recover through flotation [45, 84].  To understand SDA degradation, future work needed to study the functional group transformation, MW changes, and new material formation. To understand the influence of SDA degradation, future work needed to investigate the interfacial properties of degraded products. The most important issue is to understand the influence of the degraded SDA on nickel extraction, residue aggregation, and downstream operations, such as sulfur flotation, solvent extraction, and electrowinning. The methods proposed in Chapter 6 to determine the concentration of lignosulfonate, Quebracho, OPD, and humic acid in solution can also be used to study the degradation of SDAs. The available information on the degradation of SDA in the literature was introduced.  (1) Lignosulfonate  An investigation on the electro-degradation of sodium lignosulfonate presented useful information on the lignosulfonate degradation [183]. Two aspects were presented: (1) the molecular weight of lignosulfonate changes. Sodium lignosulfonate was degraded into a low molecular fraction and high molecular weight fraction. The formation of the high molecular weight fraction indicated the condensation occurred simultaneously with degradation. (2) the functional group of lignosulfonate changes. Phenolic hydroxyl and carboxyl contents increased to their maximum values and then decreased as the reaction proceeded. The increase of the hydroxyl groups is possibly due to the cleavage of aryl ether linkages, and further oxidation resulting in the decrease of the hydroxyl groups. The formation of carboxyl groups is due to the oxidation reaction, while the cleavage of carbon-carbon bond leads to the reducing of this functional group. 148  (2) Quebracho  The degradation of Quebracho under pressure leaching conditions is not available in the literature. The interflavan links in Quebracho may be broken under acid conditions. Quebracho is unstable under leaching conditions indicated by the butanol-HCl method which was used to measure the concentration of Quebracho in solution. Quebracho (proanthocyanidins) was depolymerized to yield anthocyanidins in acidic solution with addition of ferric. The reaction is presented in Figure 7.6 [184, 57]. Ferric iron catalyzes the oxidation of Quebracho. However, excess ferric could inhibit Quebracho depolymerization or reduce the absorbance of the oxidation product. The oxidation of Quebracho under leaching conditions could yield materials other than anthocyanidin. This problem deserves further study.  Figure 7.6 Chemistry of the acid-butanol reaction. Note that the reaction involves oxidation and that the terminal unit does not give a colored anthocyanidin product structure [184, 57]  (3) OPD  OPD undergoes a protonation reaction in the aqueous solution as a function of the pH. The protonation can be represented by equation 7-16 or equation 7-17. In the presence of an oxidant, the chemical transformation of OPD was discussed in Chapter 6. 2,3-diaminophenazine (DAP) is 149  a common product of the oxidation of OPD. A possible oxidation reaction of OPD during leaching is the formation of DAP. However, the oxidation of OPD during leaching is believed to be far more complex than one single reaction. Camurri [185] investigated the initial stages (up to the formation of the dimer) of the electropolymerization mechanism of OPD. Five different dimer species could produced through nine reactions by considering ten monomers as obtained from the combination of oxidation and protonation/deprotonation reactions.  NH  2   H  OPD  H        NH   3   [OPDH ]  (7-16)   (7-17)  DAP is not the final oxidation product of OPD under the pressure leaching conditions. One-dimensional nanobelts were obtained by direct mixing of OPD and FeCl3 aqueous solution at ambient temperature [186]. The formation of the nanobelts involves two stages: (1) the oxidation of OPD by FeCl3 resulting in the formation of DAP molecules; and (2) self-assembly of the DAP molecules, forming the one-dimensional nanobelts as indicated by Figure 7.7. The chemical transformation of OPD under pressure leaching conditions is very complex and warrants further study.  Figure 7.7 The scheme for the formation of one-dimensional nanobelts by direct mixing of OPD and FeCl3 aqueous solution at ambient temperature [186] 150  (4) Humic Acid  No studies on the oxidation of humic acid under leaching conditions were found. The degradation of humic acid was utilized to study its structural moieties. Usually, the study involves degradation digests of humic acid and the analysis of the oxidation products including solvent extractable (lipophilic) products and water soluble (hydrophilic) products. The H2O2 oxidation of lignite humic acid (8% H2O2; pH 5-6; 70ºC; 16 h) indicates that hydrophilic fraction dominate the oxidation products.  The hydrophilic fractions consisted of aliphatic  (poly)carboxylic acids related to carbohydrate moieties and benzene polycarboxylic acids. The lipophilic fraction mainly consisted of high molecular weight compounds such as long chain alkanoic acids and alkanols [187].  7.4.2 Effect of Fine Grinding at Low Pulp Density  From the above leaching experiments, more than 34% nickel was not extracted from the concentrate. It is necessary to find a way to extract all the nickel in the feed. Regrinding the nickel concentrate to P80 of 10 µm is a possible way to increase the nickel extraction. The leaching experiments were operated under the following conditions: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 10 µm), 2 h, and 700 rpm. The SDA dosage was 5 kg/t. The weight percent of solids in the pulp is 2%.  Table 7.4 shows the nickel extraction in the absence or presence of SDA. Almost all the nickel (99%) in the feed was extracted in the absence of SDA. Fine grinding supplies a large surface area for sulfide mineral to come into contact with the oxidant. Therefore, the nickel extraction is high in the beginning of the leaching experiment. More and more sulfur formed as the leaching experiments proceeds. However, the surface area of liquid sulfur is much less than the surface area of nickel sulfide mineral. Sulfur wetting is not a problem during leaching. The SDA 151  addition is unnecessary in the low pulp density leaching of the finely ground nickel concentrate.  Table 7.4 Metal extraction in the absence or presence of sulfur dispersing agents: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 10 µm), 2 h Sulfur Dispersing Agent  Ni  5 kg/t  %  None  99  Lignosulfonate  99  Quebracho  100  OPD  100  Humic Acid  98  Table 7.5 shows the properties of leach solution at ambient temperature. Compared with the data in Table 7.2, more iron was extracted into aqueous solution and more ferrous was oxidized to ferric. The sulfuric acid concentration remains at about 40 g/L. The oxidation of pentlandite, pyrrhotite, and ferrous consumes acid, while the oxidation of pyrite and elemental sulfur generate acid. If more sulfur was oxidized to sulfate, it also favors the nickel extraction. Assuming the ICP analysis of the leach solution is accurate, the calculated nickel content in the head is 14.1%. The nickel content in the leach residue ranges from 0.16% to 0.53% (Appendix 4). Even if the assay error is as large as 10%, the error on the nickel extraction is within ±1%.  152  Table 7.5 Properties of leach solution at ambient temperature: 140 ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L concentrate (P80 of 10 µm), 2 h Sulfur Dispersing Agent  Ni  Cu  Fe  H2SO4  Ferrous  g/L  g/L  g/L  g/L  g/L  None  2.76  0.37  7.43  40.4  0.08  Lignosulfonate  2.80  0.48  7.75  38.0  0.27  Quebracho  2.80  0.49  8.04  39.2  0.26  OPD  2.82  0.48  8.05  39.4  0.30  Humic Acid  2.83  0.46  7.58  38.1  0.23  5 kg/t  7.4.3 Effect of Fine Grinding at High Pulp Density  Most nickel was extracted from the reground concentrate in the low pulp density leaching experiments. It is important to know the leaching behavior at high pulp density. Nickel concentrate B (P80 of 10 µm) was utilized in the high pulp density leaching studies. The weight percent of solids in the pulp is 20%. The leaching experiments were operated under the following conditions: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate, 2 h, and 700 rpm. The SDA dosage was 5 kg/t.  Table 7.6 shows the metal extraction in the absence or presence of SDAs. The nickel extraction is about 95% in the absence of SDA, increased to about 99% in the presence of lignosulfonate, Quebracho, OPD, or humic acid. The copper extraction is about 95% in the absence of SDA, and increased up to 99% in the presence of lignosulfonate or Quebracho. Very little benefit was obtained from the addition of the SDAs in the high pulp density leaching of the finely ground nickel concentrate.  153  During the high pulp density leaching studies, the instantaneous oxygen consumptions were monitored and showed in Figure 7.8. In the absence of SDAs, the oxygen flow rate stopped completely after about 100 min of leaching while the oxygen flow rate stopped after about 90 min of leaching in the presence of humic acid and about 83 min in the presence of lignosulfonate, Quebracho, or OPD. After 120 min of leaching, the total oxygen consumed is 60.6 L in the absence of SDA; the oxygen consumption ranges from 59.6 L to 60.6 L in the presence of humic acid, lignosulfonate, or Quebracho; the oxygen consumption is 56.8 L in the presence of OPD. This indicates that OPD influences oxygen consumption reactions during leaching. Table 7.7 shows the properties of leach solution at ambient temperature in the absence or presence of SDAs.  When the pulp density is higher, more sulfur could form during leaching. The high nickel extraction without addition of SDA indicates that the surface area of molten sulfur is much less than the surface area of sulfide minerals. Therefore, sulfur wetting is not a problem after fine grinding at pulp density about 2-20%.  The interfacial properties of molten sulfur could be influenced by the acidity of leach solution. Based on the reaction 7-3, 1.0 mole nickel in the pentlandite mineral consumes 1.0 mole sulfuric acid. Assuming the behavior of iron is represented by reaction 7-3, 7-4, and reaction 7-10, 1.0 mole iron in the sulfide mineral consumes 1.5 moles sulfuric acid when iron is oxidized to ferric. 1.0 mole ferric generates 1.5 moles sulfuric acid when ferric transforms to hematite. Therefore, pyrrhotite does not consume the sulfuric acid added into the autoclave. The sulfuric acid in the autoclave is not enough for 100% pentlandite oxidation suppose there is no pyrite and sulfur oxidation. When the acid was run out of the sulfide mineral surface became sulfophobic/hydrophilic based on the findings in Chapter 3. Therefore, there is no sulfur wetting problem under low acid conditions. As leaching experiments proceeds, low acid conditions favors the oxidation of sulfur to sulfate. In addition, more pyrite could be oxidized to generate 154  acid. The low pH of leach solution could make the sulfide mineral surface become sulfophilic. The surface area of molten sulfur should be much less than the surface area of sulfide mineral, so the leaching experiments could continue. This is the possible reason why 95% nickel was extracted in the absence of SDA.  There are at least three advantages to regrinding the nickel concentrate. (1) High nickel extraction. Small amount of SDA (< 5 kg/t) is possibly enough for 99% nickel extraction. (2) Reduced leaching time. The rate of oxygen consumption during leaching indicates the rate of oxidation of the sulfide minerals. Leaching time is reduced with addition of SDA. Similar finding was reported by analyzing the oxygen consumption data. Leaching time was found greatly reduced when the nickel concentrate was first treated in an atmospheric acid chlorine leach than the direct oxygen pressure leach [32]. (3) High copper extraction. The regrinding of the nickel concentrate increases the copper extraction from chalcopyrite mineral.  Table 7.6 Metal extraction in the absence or presence of sulfur dispersing agents: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (P80 of 10 µm), 2 h Sulfur Dispersing Agent  Ni  Cu  Fe  5 kg/t  %  %  %  None  95  95  73  Lignosulfonate  99  99  70  Quebracho  99  99  67  OPD  99  96  59  Humic Acid  99  98  68  155  Figure 7.8 Cumulative oxygen consumption versus time: 140ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (P80 of 10 µm), 2 h, 5 kg/t additive  Table 7.7 Properties of leach solution at ambient temperature: 140 ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (P80 of 10 µm), 2 h Sulfur Dispersing Agent  Ni  Cu  Fe  Ferrous  5 kg/t  g/L  g/L  g/L  g/L  None  29.2  5.1  3.4  1.08  Lignosulfonate  30.2  5.2  57.9  0.90  Quebracho  30.4  5.2  57.7  0.92  OPD  30.4  5.0  51.1  1.40  Humic Acid  30.3  5.4  59.3  0.47  156  7.4.4 High Pulp Density Leaching of Nickel Concentrate (-44 µm)  The behavior of SDAs in the pressure leaching of nickel concentrate is not clear. The particle size plays an important role in leaching. It is better to study the effect of SDAs using concentrate with different size fraction. Nickel concentrate C (P100 of 44 µm) was utilized in the high pulp density leaching studies. The weight percent of solids in the pulp is 20%. The leaching experiments were operated under the following conditions: 150ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate, 2 h, and 800 rpm.  Table 7.8 shows the metal extraction in the absence or presence of 5 kg/t SDA. The nickel extraction is 15% in the absence of SDA. The nickel extraction is 72%, 83%, 99%, 61% in the presence of lignosulfonate, Quebracho, OPD, or humic acid. More copper and iron are extracted in the presence of SDAs. The highest nickel extraction was obtained in the presence of OPD, while the copper extraction is less than optimal in the presence of OPD.  Figure 7.9 shows the instantaneous oxygen consumptions versus time during the high pulp density leaching studies. In the absence of SDA, the oxygen flow rate stopped completely after about 30 min of leaching whereas in the presence of SDA oxygen keeps flowing into the autoclave after 30 min leaching. This indicates that in the absence of SDA, all oxygen consuming reactions stopped completely after about 30 min. The oxygen consumption after 120 min leaching increased from 28.3 L in the absence of SDA, to about 41.5 L, 45.5 L, 48.7 L and 51.5 L in the presence of humic acid, lignosulfonate, Quebracho or OPD. The oxygen consumption is consistent with the nickel extraction data shown in Table 7.8.  157  Table 7.8 Metal extraction in the absence or presence of sulfur dispersing agents: 150ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (-44 µm), 2 h Sulfur Dispersing Agent  Ni  Cu  Fe  5 kg/t  %  %  %  None  15  18  13  Lignosulfonate  72  52  25  Quebracho  83  82  27  OPD  99  67  23  Humic Acid  61  33  22  Figure 7.9 Cumulative oxygen consumption versus time: 150ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (-44 µm), 2 h, 5 kg/t additive  158  Table 7.9 shows the properties of the leach solution at ambient temperature. For all the experiments ferrous accounted for about 4-5% of total iron. The free acid concentration decreased from about 49 g/L to the range of 28 to 35 g/L in the presence of different kinds of SDAs. This indicates that 40 g/L sulfuric acid may be sufficient in leaching.  Table 7.9 Properties of leach solution at ambient temperature: 150 ºC, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L concentrate (-44 µm), 2 h Sulfur Dispersing Agent  Ni  Cu  Fe  H2SO4  Ferrous  5 kg/t  g/L  g/L  g/L  g/L  g/L  None  4.1  0.8  11.2  31.1  0.47  Lignosulfonate  20.1  2.7  33.9  28.1  1.17  Quebracho  23.0  3.5  29.4  32.1  0.98  OPD  25.6  2.9  26.0  35.7  1.20  Humic Acid  16.2  1.6  22.6  31.2  0.93  There is only 15% nickel leached in the absence of SDA due to the sulfur wetting problem. Assume no sulfur was oxidized during leaching and all the sulfur formed by pentlandite oxidation (equation 7-3), the amount of sulfur formed is 2.1 g. The surface area of the nickel concentrate after acid washing (Chapter 6) is 1.62 m2/g. Therefore the sulfur film formed on the sulfide mineral surface is very thin. Another factor that influenced leaching is pulp density. The interactions between particles could possibly make the molten sulfur wetting problem worse. Generally, SDAs have the ability to adsorb on the sulfide mineral surface. SDAs and the oxidation product of OPD have the ability to decrease the work of adhesion in the liquid sulfur-sulfide mineral-aqueous solution system. Therefore more nickel was extracted with addition of SDAs. However, the extent of pentlandite oxidation is different with addition of different kind of SDAs. The sulfur wetting is influenced by the properties of SDAs, the stability of SDAs, and the properties of oxidation product of SDAs. 159  The pH of the final leachate was less than 1, measured at ambient temperature. At such a low pH, most sulfonate groups are combined with protons or cations such as ferric. Carboxylic and hydroxyl groups were not dissociated at all. Although lignosulfonate showed a high negative charge density at pH 2.5 when adsorbed onto the elemental sulfur surface, the effective surface charge was near zero at pH 1 [176]. Electrostatic attraction is not the major driving force for the adsorption.  Sulfonate groups were found in the Quebracho molecule from its infrared spectrum. But Quebracho had less surface charge density than lignosulfonate did base on the electroacoustic study. Considering the influence of the solution pH on the dissociation of sulfonate and hydroxyl groups, electrostatic interaction is not the main mechanism for the Quebracho absorption on the sulfide mineral surface.  Humic acid has the least effect on increasing nickel extraction in leaching, though it was found effective to decrease the work of adhesion in the liquid sulfur-sulfide mineral-aqueous solution system. This indicates the faster degradation of humic acid under leaching conditions.  OPD showed the strongest capability of removing molten sulfur among the four additives, though it was found ineffective to decrease the work of adhesion in the interfacial studies. OPD was found unstable in the acidic nickel concentrate slurry at ambient temperature. The effect of OPD during leaching is totally dependent on its degradation product. Without considering the adsorption of SDAs on the nickel concentrate, the initial concentration of OPD is 1.25 g/L. OPD experienced chemical transformation with ferric in the aqueous solution. The oxidation product of OPD was effective in solving the sulfur wetting problem in leaching. DAP is a possible oxidation product of OPD. However, DAP is not the only oxidation product of OPD due to the complexity of the leaching environment. More work on the transformation of OPD is required.  160  Based on Moudgil [177], the adsorption process is characterized as physical adsorption if electrostatic and hydrophobic interactions are the major driving forces. If adsorbed physically, SDAs may not bind to the mineral surface but exist in the double layer region with great mobility. Under leaching conditions, electrostatic interaction is not the major driving force for the adsorption. Considering the polar nature of pentlandite, hydrophobic bonding cannot be the major driving force. Therefore, chemical adsorption is the major driving force for the adsorption of SDAs on the sulfide mineral surface under leaching conditions.  7.4.5 Effect of Chloride at High Pulp Density  In the pressure leaching of nickel concentrate C in acid sulfate media, the highest nickel extraction was achieved with addition of OPD. The nickel extraction is only 72% with addition of lignosulfonate. Lignosulfonate is commercial SDA used in the pressure leaching of sphalerite. It is important to know the possible application of lignosulfonate. Nickel concentrate C was utilized in the high pulp density leaching studies. The weight percent of solids in the pulp is 20%. The leaching experiments were operated under the following conditions: 150ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate, 2 h, 800 rpm. SDA was sealed in an ampoule to prevent the oxidation before leaching. Chloride was added into the autoclave as sodium chloride.  Lignosulfonate was sealed in an ampoule to prevent oxidation before leaching. The oxygen consumption indicates that the oxidation of lignosulfonate during the heating stage has little effect on the oxygen consumption during leaching. The nickel and copper extraction as shown in Table 7.10 and Table 7.11 indicates that the change of lignosulfonate during the heating stage has little effect on the nickel and copper recovery. However, SDA was sealed in an ampoule to make sure it came into contact with the nickel concentrate at leaching time zero.  161  The Influence of Sodium Chloride  Table 7.10 shows the influence of chloride concentration on the metal extraction in the absence or presence of SDAs. The nickel extraction is 68% in the presence of 5 kg/t lignosulfonate and 0 g/L chloride. The nickel extraction is 97% in the presence of 0 kg/t SDAs and 5 g/L chloride. The role of chloride to increase the leaching kinetics of nickel is not clear and the following effects of chloride should be considered [188, 189]: (1) faster leaching kinetics due to the enhanced proton activity; (2) a reagent similar to SDA to solve the sulfur wetting problem during leaching; (3) a complexing agent for cuprous ions which makes sodium chloride act as a catalyst; (4) the formation of Cu(II)(OH)Cl0 species which accelerates the leaching process; (5) a reagent which increases the surface area and the porosity of the product layer on the sulfide mineral surface. The first two points are possible effects of sodium chloride in the pressure leaching experiments.  Figure 7.10 shows the influence of chloride concentration on the cumulative oxygen consumption. This indicates that the major oxygen consumption reactions finished in 1 h. The oxygen consumption decreased greatly when the chloride concentration increased from 0 g/L to 5 g/L. The oxygen consumption decreased to a lesser extent when the chloride concentration increased from 5 g/L to 10 g/L. This indicates that the chloride ion has an effect to inhibit the oxidation of sulfide or elemental sulfur to sulfate. Table 7.11 shows that the sulfuric acid concentration in the leach solution after 2 h leaching decreased from 34.7 g/L to 21.5 g/L when the chloride concentration changed from 0 g/L to 5 g/L. The sulfuric acid concentration is 15.4 g/L when the chloride concentration increased to 10 g/L. This is consistent with the oxygen consumption data.  162  Table 7.10 Metal extraction in the presence of sulfur dispersing agent: 150ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate (-44 µm), 2 h Cl-  Ni  Cu  Fe  g/L  %  %  %  Lignosulfonate*: 5 kg/t  0  68  65  19.4  Lignosulfonate: 5 kg/t  0  68  67  18.7  Lignosulfonate: 10 kg/t  5  99  90  1.7  Lignosulfonate: 5 kg/t  5  99  92  1.3  Lignosulfonate: 2.5 kg/t  5  98  91  0.8  Lignosulfonate: 5 kg/t  10  98  90  0.7  Quebracho: 5 kg/t  5  97  88  0.8  OPD: 5 kg/t  5  99  90  1.4  Humic Acid: 5 kg/t  5  98  83  0.4  None  5  97  83  0.4  Sulfur Dispersing Agent  * Lignosulfonate was not sealed in an ampoule  The sulfuric acid in the autoclave (40 g/L) is not enough for 100% pentlandite oxidation suppose there is no pyrite and sulfur oxidation. When the acid was run out of the sulfide mineral surface became sulfophobic/hydrophilic based on the findings in Chapter 3. Therefore, there is no sulfur wetting problem under low acid conditions. As leaching experiments proceeds, low acid conditions favors the oxidation of sulfur to sulfate. However, the addition of sodium chloride favors the formation of sulfur instead of sulfate. Therefore, less acid generated as shown by reaction 7-9. The addition of sodium chloride favors the hydrolysis of iron (reaction 7-15) during leaching. Less acid can form when ferric transforms to sodium jarosite (reaction 7-15) instead of hematite (reaction 7-10). The concentration of iron is very low in the leach solution. This indicates the low acid concentration during leaching. Another source of acid formation during leaching is the oxidation of pyrite (reaction 7-7). 163  Figure 7.10 The influence of chloride concentration on the cumulative oxygen consumption: 150 ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate (-44 µm), 2 h, 5 kg/t lignosulfonate sealed in an ampoule, * lignosulfonate was not sealed in ampoule  The Effect of Different Kinds of Sulfur Dispersing Agents  Based on the nickel extraction, very little benefit was obtained from the addition of the SDAs as indicated by Table 7.10 and Table 7.11. Figure 7.11 shows the cumulative oxygen consumption in the presence of different kinds of SDAs. This indicates that the major oxygen consumption reactions finished in 60 min. The oxygen consumption after 120 min leaching was 38.1 L, 39.3 L, 39.2 L and 35.7 L in the presence of lignosulfonate, Quebracho, OPD or humic acid respectively. There is no great difference on the oxygen consumption with addition of different kinds of SDAs. This indicates that SDAs has no great effect to increase the major oxidation reactions during leaching or to reduce the leaching time.  164  Table 7.11 Properties of leach solution at ambient temperature: 150ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate (-44 µm), 2 h Cl-  Ni  Cu  Fe  H2SO4  g/L  g/L  g/L  g/L  g/L  Lignosulfonate*: 5 kg/t  0  18.3  2.8  21.4  33.9  Lignosulfonate: 5 kg/t  0  17.8  2.8  20.7  34.7  Lignosulfonate: 10 kg/t  5  25.5  3.8  1.8  19.9  Lignosulfonate: 5 kg/t  5  25.6  3.9  1.4  21.5  Lignosulfonate: 2.5 kg/t  5  26.1  3.9  0.8  18.8  Lignosulfonate: 5 kg/t  10  25.3  3.8  0.7  15.4  Quebracho: 5 kg/t  5  24.6  3.2  0.7  16.7  OPD: 5 kg/t  5  25.3  3.3  1.7  19.5  Humic Acid: 5 kg/t  5  25.0  3.1  0.4  12.6  None  5  25.0  3.5  0.3  11.2  Sulfur Dispersing Agent  * Lignosulfonate was not sealed in an ampoule  However, there is at least one advantage to add SDAs into the mixed sulfate and chloride media. In the absence of SDAs, the copper extraction is 83%. Lignosulfonate and OPD increase the copper extraction from 83% to about 90% with addition of 5 kg/t dosage. Further study indicates that 2.5 kg/t lignosulfonate is enough to enhance the copper extraction. The final leach residue (without SDA addition) contained aggregated material. Sulfur is one of the major mineral in the aggregated fraction. Possibly, sodium chloride has the ability to decrease the work of adhesion of molten sulfur by enhancing the performance of lignosulfonate. However, sodium chloride has no ability to disperse sulfur in the slurry. The leach residue contained less aggregated material when the addition of lignosulfonate increased from 2.5 kg/t to 10.0 kg/t. Therefore, SDA influences the behavior of sulfur and possibly has negative impact on the downstream operation such as sulfur flotation. 165  Figure 7.11 Cumulative oxygen consumption versus time: 150ºC, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate (-44 µm), 2 h, 5 g/L chloride, 5 kg/t sulfur dispersing agent  7.4.6 Effect of Chloride on the Voisey’s Bay Nickel Concentrate  The purpose of the study in this part is to understand the influence of both sodium chloride and lignosulfonate on the nickel extraction. However, no more nickel concentrate A (P80 of 48 µm) is available in the lab. The Voisey’s Bay nickel concentrate (P80 of 50 µm) was utilized in the high pulp density leaching studies. The weight percent of solids in the pulp is 20%. The leaching experiment was operated under the conditions: 150ºC, 690 kPa oxygen overpressure, 40 or 49 g/L H2SO4, 250 g/L concentrate, 1.5 h, 800 rpm. SDA was dissolved in deionized water and added into the autoclave directly. The chloride was added into the autoclave as sodium chloride.  166  Table 7.12 shows the leaching results of the Voisey’s Bay nickel concentrate. The particle size of nickel concentrate is critical in leaching which is proved by the oxygen pressure leaching of the Voisey’s Bay nickel concentrate (P80 of 50 µm). Lignosulfonate was fully degraded before pentlandite was totally leached during the pressure leaching of nickel concentrate (P80 of 48 µ). It was suggested that the particle size in the range of 15 to 30 µm is optimum for a high nickel and copper extraction as well as the solid/liquid separation [32].  The pH of the leach solution is 1.9, which indicates there is insufficient sulfuric acid in the autoclave assuming pentlandite and pyrrhotite are the only acid consuming minerals in the Voisey’s Bay nickel concentrate. Pentlandite consumed all the sulfuric acid in the ampoule. When the acid was run out of the sulfide mineral surface became sulfophobic/hydrophilic, there is no sulfur wetting problem under low acid conditions. The sulfide sulfur was oxidized to elemental sulfur during leaching and a portion of elemental sulfur was oxidized to sulfuric acid based on the reaction 7-9. With the acid concentration increasing the sulfide mineral surface became sulfophilic/hydrophobic. Lignosulfonate has the ability to solve the sulfur wetting problem until it was destroyed in the autoclave.  The pH of the leach solution can be adjusted by the amount of sulfuric acid added into the autoclave. The final pH value of the leach solution was suggested between 1.8 and 3.3 in the literature [32]. Under such conditions, nickel was found stable in the aqueous solution, while the leached copper can precipitate as basic sulfate salt, antlerite (CuSO4·2Cu(OH)2) when the pH value is greater than 2.5. Nickel hydroxide disperses molten sulfur according to interfacial studies.  The  influence  of  antlerite  on  the  interfacial  properties  in  the  liquid  sulfur-pentlandite-aqueous solution system deserves further study.  167  Table 7.12 Oxygen pressure leaching of the Voisey’s Bay nickel concentrate (P80 of 50 µm): 150 º C, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L concentrate, 1.5 h, 5 kg/t lignosulfonate, 10 g/L chloride pH of the leach solution  1.9  Fe, g/L  0.1  Ni extraction, %  85%  Cu extraction, %  47%  Table 7.13 and Table 7.14 show the influence of lignosulfonate and chloride ion on the pressure leaching of the Voisey’s Bay nickel concentrate. The nickel extraction is 45% with addition of 5 kg/t lignosulfonate, without addition of sodium chloride. 26% nickel is extracted with addition of 10 g/L sodium chloride, without addition of lignosulfonate. The nickel extraction is 74% in the presence 5 kg/t lignosulfonate and 10 g/L sodium chloride.  The addition of sodium chloride to the leach solution enhanced the performance of lignosulfonate. It is consistent with the findings of the interfacial study by Brown [106]. In Brown’s study, chloride ion enhanced the performance of lignosulfonate, lowering the work of adhesion by about 20% in the liquid sulfur-pentlandite-aqueous solution system. In the autoclave, iron facilitates the transfer of oxygen in the oxidative pressure leaching [32]. However, lignosulfonate can be degraded by ferric ions, with a half-life of about 10 min [87]. It is assumed that the addition of sodium chloride influences the oxidation of SDA by the removal of ferric and the formation of sodium jarosite.  168  Table 7.13 Metal extraction in the presence of sulfur dispersing agent: 150ºC, 690 kPa oxygen overpressure, 49 g/L H2SO4, 250 g/L Voisey’s Bay Concentrate (P80 of 50 µm), 1.5 h Experiment  Lignosulfonate  Chloride  Ni  Cu  Fe  No.  kg/t  g/L  %  %  %  1  5  0  45  28  21  2  0  10  26  24  0.9  3  5  10  74  34  1.1  Table 7.14 Properties of leach solution at ambient temperature: 150ºC, 690 kPa oxygen overpressure, 49 g/L H2SO4, 250 g/L Voisey’s Bay Concentrate (P80 of 50 µm), 1.5 h Experiment  Lignosulfonate  Chloride  Ni  Cu  Fe  H2SO4  No.  kg/t  g/L  g/L  g/L  g/L  g/L  1  5  0  25.9  1.6  17.9  33.5  2  0  10  17.2  1.2  0.4  14.3  3  5  10  36.7  1.9  0.6  6.2  7.5 Conclusions   The particle size of nickel concentrate is an important factor in leaching. The SDAs are fully degraded before the acceptable amount of nickel was leached from the XSTRATA Nickel concentrate (P80 of 48 µm). The finding was proved by the oxygen pressure leaching of the Voisey’s Bay nickel concentrate (P80 of 50 µm).    Fine grinding (P80 of 10 µm) is sufficient for 99% nickel recovery at low pulp density. At high pulp density the nickel recovery increased from 95% to 99% with addition of SDAs. The addition of SDAs accelerates the oxygen consuming reactions. 169    The oxygen pressure leaching of the nickel concentrate (-44 µm) indicates that 99% nickel can be extracted in the presence of 5 kg/t OPD. OPD is the best SDA based on the nickel extraction followed by Quebracho (83%), lignosulfonate (72%), and humic acid (61%). The chemical transformation product of OPD was effective in solving the sulfur wetting problem in leaching.    The oxygen pressure leaching of the nickel concentrate (-44 µm) indicates that 97% nickel was extracted in the presence of 5 g/L chloride which was added as sodium chloride. Lignosulfonate and OPD increase the copper extraction from 83% to about 90%.    The sodium chloride inhibits the oxidation of sulfide or elemental sulfur to sulfate. The addition of sodium chloride favors the hydrolysis of iron. Sodium chloride increases nickel extraction in the pressure leaching of Voisey’s Bay Concentrate (P80 of 50 µm). The effect of sodium chloride is influenced by the particle size of nickel concentrate.  170  8.0 Conclusions and Recommendations 8.1 Conclusions The effect of SDAs, including lignosulfonate, Quebracho, humic acid, and OPD, on decreasing the work of adhesion was investigated in the interfacial studies. The functional groups of lignosulfonate, Quebracho, and humic acid were characterized by their infrared spectra. The charge density of SDAs was characterized by the measurement of electrokinetic sonic amplitude (ESA) in the sulfur-acid water system. The adsorption of lignosulfonate, Quebracho, and humic acid on the nickel concentrate was investigated at ambient temperature. The effect of SDAs in the oxygen pressure leaching was studied at both low pulp density and high pulp density.  Interfacial Studies  (1) The sulfide mineral surface is sulfophobic at pH from 4.1 to 4.5 (and probably higher pH) due to the hydrolysis of nickel (II) ions to nickel hydroxide and the deposition of nickel hydroxide on the mineral surface. The sulfide minerals affected by this phenomenon include pentlandite, nickeliferous pyrrhotite, pyrrhotite, and chalcopyrite. The work of adhesion in the liquid sulfur-mineral-aqueous system ranges from 3.4 to 8.8 mJ/m2  (2) The effect of SDAs in the oxygen pressure leaching of nickel concentrate at moderate temperature was investigated by the interfacial studies. Three different mineral systems were  looked  at,  including  liquid  sulfur-pentlandite-aqueous  solution,  liquid  sulfur-nickeliferous pyrrhotite-aqueous solution, and liquid sulfur-pyrrhotite-aqueous solution system. Lignosulfonate, Quebracho, and humic acid were found to significantly reduce the work of adhesion indicating they are effective SDAs. OPD was an ineffective SDA under these conditions. 171  Infrared Spectrum, Surface Charge Characterization, and Adsorption Studies  (1) The infrared spectrum of lignosulfonate indicates the presence of sulfonate, hydroxyl, and methoxyl groups in BorrePAL U, BorrePAL N, and BorrePAL S. Carboxyl group was found in the BorrePAL N and BorrePAL S. The infrared spectrum of Quebracho shows the presence of hydroxyl and sulfonate groups. The infrared spectrum of humic acid indicates the presence of carboxyl, sulfonate, and hydroxyl groups.  (2) The adsorption of lignosulfonate on the molten sulfur surface was calculated by the Gibbs equation. The adsorption of SDAs on elemental sulfur was investigated by the measurement of the ESA signal at pH 2.5 and ambient temperature. Sodium is easier to dissociate from sodium lignosulfonate than the dissociation of calcium from calcium lignosulfonate. The study indicates that electrostatic interaction is a possible mechanism for the adsorption of lignosulfonate and humic acid onto elemental sulfur surface at ambient temperature; OPD cannot adsorb onto elemental sulfur surface.  (3) The adsorption of lignosulfonate, humic acid, and Quebracho on nickel concentrate is monolayer adsorption. The Langmuir adsorption isotherm was used to fit the adsorption isotherm. Electrostatic interaction and ion-binding are the possible mechanisms for the adsorption of lignosulfonate or humic acid on the nickel concentrate surface. The interaction between Quebracho and the nickel concentrate is possibly effected by chemical adsorption through sulfonate groups and by the formation of hydrogen bonds through hydroxyl groups. OPD is unstable in the presence of ferric in the aqueous solution. The oxidation of OPD during its conditioning with the nickel concentrate was proved by the UV/Vis spectra changes.  172  Oxygen Pressure Leaching Studies  (1) The particle size of nickel concentrate is an important factor in leaching. It was suggested that the SDAs are fully degraded before an acceptable amount of nickel was leached from the XSTRATA Nickel concentrate (P80 of 48 µm). This finding was proved by the oxygen pressure leaching of the Voisey’s Bay nickel concentrate (P80 of 50 µm). Fine grinding (P80 of 10 µm) is sufficient for 99% nickel recovery at low pulp density while at high pulp density, the nickel recovery increased from 95% to 99% with addition of SDAs. The addition of SDAs accelerates the oxygen consuming reactions.  (2) The oxygen pressure leaching of the nickel concentrate (-44 µm) indicates that 99% nickel can be extracted in the presence of 5 kg/t OPD. OPD is the best SDA based on the nickel extraction followed by Quebracho (83%), lignosulfonate (72%), and humic acid (61%). Copper recovery is much less than the nickel recovery in the presence of different SDAs. It is suggested that the oxidation product of OPD is effective in solving the sulfur wetting problem in leaching.  (3) In the oxygen pressure leaching of the nickel concentrate (-44 µm) 97% nickel was extracted in the presence of 5 g/L chloride added as sodium chloride. The addition of SDAs was used to increase copper extraction. Lignosulfonate and OPD increase the copper extraction from 83% to about 90%.  (4) Sodium chloride inhibits the oxidation of sulfide or elemental sulfur to sulfate. The addition of sodium chloride favors the hydrolysis of iron. Sodium chloride increases nickel extraction in the pressure leaching of Voisey’s Bay Concentrate (P80 of 50 µm). Sodium chloride has an effect to enhance the performance of lignosulfonate under leaching conditions. 173  8.2 Recommendations (1) The adsorption behavior of lignosulfonate, humic acid and Quebracho on the nickel concentrate is influenced by the environmental conditions. The influence of temperature, ionic strength, and pH on the adsorption behavior of SDA deserves further study. In addition, the stability of SDAs also needs to be evaluated under these conditions.  (2) OPD shows different behavior compared with other SDAs. OPD cannot adsorb on the elemental sulfur surface; it is unstable in the presence of the oxidant even at ambient temperature; it is ineffective at decreasing the work of adhesion in the liquid sulfur-sulfide mineral-aqueous solution system. However, it is the most effective SDA for the nickel extraction in a pure sulfate system. The behavior of OPD under leaching conditions deserves further study in the future.  (3) The sulfide mineral surface is sulfophobic at pH from 4.1 to 4.5 due to the hydrolysis of nickel (II) ions to nickel hydroxide and the deposition of nickel hydroxide on the mineral surface. At pH 2.5-3.0, the leached copper can precipitate as basic copper sulfate. The effect of basic copper sulfate on the interfacial properties in the liquid sulfur-sulfide mineral-aqueous solution system deserves further study.  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Furnica, M., Polarographic, Amperometric and Spectrophotometric Study on the Reaction between O-phenylenediamine and Iron (III), Buletinul Institutului Politehnic din lasi, Sectia 2: Chimie (English Abstract Only), 24(3-4), pp. 35-39, 1978. 171. Zhang, K., Cai, R., Chen, D. and Mao L., Determination of Hemoglobin Based on its Enzymatic Activity for the Oxidation of O-phenylenediamine with Hydrogen Peroxide, Analytica Chimica Acta, 413(1-2), pp. 109-113, 2000. 172. Kumar, K.V., and Sivanesan, S., Comparison of linear and non-linear method in estimating the sorption isotherm parameters for safranin onto activated carbon, Journal of Hazardous Materials, 123(1-3), pp. 288-292, 2005. 173. Lindström, T., Interaction between Lignosulfonates and Simple Metal Ions, Colloid and Interface Science Vol. V: Biocolloids, Polymers, Monolayers, Membranes, and General Papers, Edited by M. Kerker, Academic Press Inc., New York, pp. 217-230, 1976. 174. Khvan, A.M. and Abduazimov, K.A., Interaction of Lignosulfonate with Certain Metal Ions, Chemistry of Natural Compounds, 26(5), pp. 575-577, 1991. 175. Kontturi, A.-K., Determination of Diffusion Coefficients and Effective Charge Numbers of Lignosulfonate: Influence of Ionic Strength and the Valency of the Counter-ion, Journal of the Chemical Society, Faraday Transactions 1, 84(11), pp. 4033-4041, 1988. 176. Kontturi, A.-K., Diffusion Coefficients and Effective Charge Numbers of Lignoulphonate: 194  Influence of Temperature, Journal of the Chemical Society, Faraday Transactions 1, 84(11), pp. 4043-4047, 1988. 177. Moudgil, B.M., Soto, H. and Somasundaran P., Adsorption of Surfactants on Minerals, Reagents in Mineral Technology, Edited by P. Somasundaran and M. Moudgil, Marcel Dekker, Inc., New York, pp. 79-104, 1988. 178. Last, G.A. and Cook, M.A., Collector-Depresssant Equilibria in Flotation. I. 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Chen F., Lu, Z., and Tu B., Electro-Degradation of Sodium Lignosulfonate, Journal of Wood Chemistry and Technology, 23(3-4), pp. 261-277, 2003. 184. Haslam, E., Plant Polyphenols, Cambridge University Press, Cambridge, 1989. 185. Camurri, G., Ferrarini, P., Giovanardi, R., Benassi, R., and Fontanesi, C., Modelling of the Initial Stages of the Electropolymerization Mechanism of O-Phenylenediamine, Journal of Electroanalytical Chemistry 585(2), pp. 181-190, 2005. 186. He D., Wu, Y., and Xu, B., Formation of 2,3-Diaminophenazines and Their Self-Assembly 195  into Nanobelts in Aqueous Medium, European Polymer Journal 43(9), pp. 3703-3709, 2007. 187. Allard, B. and Derenne, S., Oxidation of Humic Acids from an Agricultural Soil and a Lignite Deposit: Analysis of Lipophilic and Hydrophilic Products, Organic Geochemistry 38(12), pp. 2036-2057, 2007. 188. Senanayake, G., Chloride Assisted Leaching of Chalcocite by Oxygenated Sulphuric Acid Via Cu(II)-OH-Cl, Minerals Engineering, 20(11), pp. 1075-1088, 2007. 189. Senanayake, G., Review of Theory and Practice of Measuring Proton Activity and pH in Concentrated Chloride Solutions and Application to Oxide Leaching, Minerals Engineering, 20(7), pp. 634-645, 2007.  196  Appendix 1 Measurement of Interfacial Tension and Contact Angle A1.1 Apparatus  Interfacial experiments were performed using the high temperature high pressure apparatus shown in Figure A1.1. The apparatus was heated externally with a heating tape wrapped around the body, and pressurized with nitrogen gas. The temperature was controlled by a PID temperature controller and pressure was measured by an Omega Engineering pressure meter.  A1.2 Formation of Pendant Drops  The primary procedures of the measurement have been provided in the literature [1, 2]: a) Solution preparation. An appropriate amount of SDA and nickel sulfate was dissolved in the deionized water. Alternatively the SDA was dissolved in diluted sulfuric acid solution which was prepared from reagent sulfuric acid. b) The optical cell (O) was filled with about 3 mL of the aqueous solution prepared in step a). c) The optical cell was mounted into the seat (K). d) The base of the bomb was carefully screwed into the bomb. e) The syringe (A) was partially filled with sulfur and screwed into the bomb head. f) The bomb was pressurized with nitrogen. g) The RTD probe was inserted into the thermowell, and heating was initiated. h) The light (L) and camera (C) were positioned in preparation for viewing and photographing liquid sulfur drops. i) Measurement. At stable operating temperatures the syringe plunger (D) was screwed downwards until a pendant drop of sulfur was formed at the tip which was immersed in the 197  solution. The drop was made as large as possible without detaching from the tip. The drop was then photographed at least three times within 3 min. j) Clean up. To terminate a test, the heating tape was turned off, and the bomb was cooled down and then depressurized. The syringe and cell were cleaned in 10% w/v aqueous nitric acid and rinsed with deionized water.  Figure A1.1 Cross-sectional view of Owusu’s high temperature-high pressure apparatus for generating pendant and sessile drops of liquid sulfur A: drive-screw syringe with plunger D; B: stainless-steel bomb; C: camera for taking pictures; I: gas inlet; K: adjustable height seat; L: light source for backlighting; M: mineral; O: rectangular optical glass well; P: converging lens; S: sapphire windows; T: thermowell for thermocouple; V: replaceable Pyrex glass tubing tip; AS: aqueous solution; LS: liquid sulfur.  198  A1.3 Formation of Sessile Drops  The procedures for the measurement of pendant drops and sessile drops are the same except for procedures a), b), and i): a) Solution and mineral preparation. An appropriate amount of SDA and nickel sulfate was dissolved in the deionized water or in the diluted sulfuric acid solution prepared from concentrated sulfuric acid. The top face of the mineral sample was carefully polished with a 1 µm alumina/water suspension and cleaned with the deionized water. b) The mineral sample was mounted in the optical cell, and the cell was filled with aqueous solution prepared in step a). i) Measurement. At stable operating temperatures the syringe plunger was screwed downwards. The sulfur drop was formed and detached from the syringe tip, and then fell onto the mineral surface. Once the drop movement ceased, the drop was photographed. Photographs of the drop were taken within 3 min.  A1.4 Calculation of Interfacial Tension  The interfacial tension was calculated from the measured profile dimensions d e and d s and using the following equations [3]: S  ds / de  (A1-1)    gd e  2     (A1-2)  H  An example is given below to show the measurement of the interfacial tension between liquid sulfur and nickel sulfate solution at 140°C, 690 kPa, NiSO4 1.0 mol/L, BorrePAL U 0.5 g/L, pH: 4.8 (refer to Table 3.3). (1) The density of 1.0 mol/L nickel sulfate solution was measured at ambient temperature (20°C) 199  by a 25 cm3 pyknometer. The density of solution at 20°C is 1.1546 g/mL. (2) The density of 1.0 mol/L nickel sulfate solution at 140°C was estimated by assuming the solution had the same coefficient of expansion as water [4]. The density of solution at 140°C is 1.0625 g/mL. (3) The density of molten sulfur at 140°C is 1.7865 g/mL [5]. (4) The density difference    0 .7240 g/mL. (5) Acceleration due to gravity g  980 .665 cm/S2 [4]. (6) The diameter of the glass tip was measured by a micrometer. The diameter of the glass tip is 0.1238 cm. (7) Three parameters were measured from the magnified photos taken during the experiment by the ruler tool of the software Adobe Photoshop. The diameter of the glass tip in the photo equals 2.543 cm. The value of d e in the photo equals 4.362 cm. The value of d s in the photo equals 3.007 cm. (8) The magnification factor was calculated. The parameter d e = 0.212 cm. (9) S  d s / d e  0 . 689 . (10) The value of 1 / H is a function of S . 1 / H = 0.83792, which is available in the literature [3]. (11) The value of interfacial tension is calculated from equation A1-2.   26 .8 mN/m.  References 1. Owusu, G., The Role of Surfactants in the Leaching of Zinc Sulphide Minerals at Temperature Above the Melting Point of Sulphur, PhD thesis, Department of Materials Engineering, The University of British Columbia, pp. 208, 1993. 2. Hackl, R.P., The Leaching and Passivation of Chalcopyrite in Acid Sulfate Media, PhD thesis, Department of Materials Engineering, the University of British Columbia, 1995. 200  3. Ambwani, D.S. and Fort Jr. T., Pendant Drop Technique for Measuring Liquid Boundary Tensions, Surface and Colloid Science, Edited by R.J. Good and R.R. Stromberg, Plenum, New York, 11, pp. 93-119, 1979. 4. Weast, R.C., Astle, M.J. and Beyer, W.H., CRC Handbook of Chemistry and Physics (69th Edition), CRC Press, Inc., Boca Raton, Florida, 1988. 5. Bixby, D.W., Fike, H.L., Shelton, J.E. and Wiewiorowski, T.K., Sulfur, Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Volume 23, pp. 232-266, 1992.  201  Appendix 2 Calibration Curves and Experimental Data for the Adsorption Studies A2.1 Calibration Curves of Sulfur Dispersing Agents  The calibration curves of SDAs are shown from Figure A2-1 to Figure A2-6.  Figure A2.1 Calibration curves of BorrePAL U for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 281 nm  202  Figure A2.2 Calibration curves of BorrePAL N for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 280 nm  Figure A2.3 Calibration curves of BorrePAL S for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 278 nm 203  Figure A2.4 Calibration curves of humic acid for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 254 nm  Figure A2.5 Calibration curves of Quebracho by acid-butanol assay for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 550 nm 204  Figure A2.6 Calibration curves of OPD for the UV-Vis determination in the adsorption studies at ambient temperature, wavelength: 266 nm  205  A2.2 The pH Values of Aqueous Solution after Conditioning with Nickel Concentrate  Table A2.1 pH values of BorrePAL U and BorrePAL N solution after conditioning with the nickel concentrate. A: initial pH 4.8; B: initial pH 3.5. Concentration  BorrePAL U  BorrePAL N  mg/L  A  B  A  B  20  4.42  3.90  4.08  3.99  40  4.43  3.90  4.19  3.93  60  4.43  3.87  4.21  3.92  80  4.49  3.88  4.22  3.95  100  4.51  3.89  4.24  3.95  120  4.52  3.91  4.25  3.94  140  4.50  3.92  4.14  3.97  160  4.50  3.94  4.10  3.95  180  4.40  3.95  4.15  3.96  200  4.42  3.95  4.18  3.97  206  Table A2.2 pH values of BorrePAL S and humic acid solution after conditioning with the nickel concentrate. A: initial pH 4.8; B: initial pH 3.5. Concentration  BorrePAL S  Humic acid  mg/L  A  B  A  B  20  4.09  3.85  4.54  4.02  40  4.15  3.88  4.58  4.07  60  4.15  3.91  4.50  4.14  80  4.16  3.91  4.50  4.12  100  4.16  3.95  4.58  4.19  120  4.15  3.93  4.65  4.19  140  4.16  3.93  4.70  4.20  160  4.16  3.93  4.82  4.28  180  4.15  3.95  4.95  4.32  200  4.15  3.97  5.05  4.38  207  Table A2.3 pH values of Quebracho solution after conditioning with the nickel concentrate. A: initial pH 4.8; B: initial pH 3.5. Concentration  Quebracho  mg/L  A  B  100  4.14  3.79  200  4.09  3.72  300  4.16  3.80  400  4.11  3.73  500  4.18  3.85  600  4.11  3.94  700  4.20  3.89  800  4.12  4.00  208  Appendix 3 Determination of Ferrous Iron and Free Acid A3.1 Determination of Ferrous Iron by Potassium Dichromate Titration  Reagents  Sulfuric (H2SO4) / Phosphoric (H3PO4) acid mixture 1. Wash all glassware with 10% HNO3 and distilled water. 2. Place a 1L beaker in a cold-water bath and add 300 mL of distilled water. 3. While mixing, slowly add 300 mL sulfuric acid (reaction is exothermic and requires the slow addition of sulfuric acid). 4. Let cool. 5. Slowly add 300 mL of phosphoric acid, Mix. 6. Transfer to a plastic storage vessel.  Sodium diphenylamine sulphonate indicator solution (C6H5NHC6H4SO3Na) 1. Dissolve 0.2 g sodium diphenylamine sulphonate into 100 mL of distilled water. 2. Transfer to a storage/dispensing vessel.  Standard potassium dichromate solution (0.09N K2Cr2O7) 1. Into a clean 250 mL beaker weigh 4.3899 g of potassium dichromate. 2. Add 100 mL of distilled water and mix to dissolve. 3. Quantitatively transfer solution to a 1000 mL volumetric flask and bring to volume with distilled water and mix. 4. This solution has an iron value of about 5 mg/mL. 209  Procedure  1. Aliquot a suitable volume of unknown sample, typically 1.00 mL to 10.0 mL, into a clean 250 mL beaker. If sample is high in iron take a smaller aliquot of 0.5 mL - 1.0 mL. 2. Dilute the unknown sample to a working volume of approximately 150 mL. 3. Add 10 mL of sulfuric/phosphoric acid mixture. 4. Add a few drops of sodium diphenylamine sulphonate indicator solution. 5. Titrate with standard potassium dichromate solution until a permanent color change from green to violet. 𝐾2 𝐶𝑟2 𝑂7 + 6𝐹𝑒𝐶𝑙2 + 14𝐻𝐶𝑙 = 2𝐾𝐶𝑙 + 2𝐶𝑟𝐶𝑙3 + 6𝐹𝑒𝐶𝑙3 + 7𝐻2 𝑂  (A3-1)  Calculations  𝐹𝑒 2+ 𝑔 𝐿 = 𝑉1 × 0.005 × 1000/𝑉2  (A3-2)  Where 𝑉1 is the volume of titrant, mL; 𝑉2 is the sample volume, mL.  References: 1. Lenahan, W.C., and Murray-Smith, R. de L., Assay and Analytical Practice in the South African Mining Industry, The South African Institute of Mining and Metallurgy, Johannesburg, pp. 176-178, 1986.  210  A3.2 Determination of Free Acid  Reagents  Calcium EDTA solution (0.05 mol/L) 1. Weight out required Na2H2EDTA into a volumetric flask, and dissolve in deionized water. 2. Add 3% excess calcium nitrate (the molar ratio of calcium to EDTA is 1.03:1). 3. Adjust solution pH to 5.7 by adding 1.00 mol/L NaOH standard solution, add slowly with stirring to ensure no solids persist, make up to volume. 4. Check initial pH prior to titration. Adjust the pH if it is vary different from pH 5.7.  Potassium Persulfate  1. Dissolve 1.0 g potassium persulfate into 100 mL of deionized water. 2. Transfer to a storage/dispensing vessel.  Procedure  1. Aliquot a suitable volume of leachate sample, typically 5.00 mL, into a clean 250 mL beaker. 2. Dilute the leachate sample to a volume of approximately 70 mL. 3. Adjust the Eh of the solution to over 600 mV with potassium persulfate solution. 4. Add a 20% excess of calcium EDTA solution into the beaker. 5. Titrate with standard 1.00 mol/L sodium hydroxide solution to an endpoint pH of 5.7.  Calculations 𝐻2 𝑆𝑂4 𝑔 𝐿 = 98.075 × 𝑉𝑁𝑎𝑂𝐻 × 𝐶𝑁𝑎𝑂𝐻 /(2𝑉𝐻2 𝑆𝑂4 )  (A3-3)  Where 𝑉𝑁𝑎𝑂𝐻 is the volume of sodium hydroxide, mL; 𝐶𝑁𝑎𝑂𝐻 is the concentration of sodium 211  hydroxide, 0.1 mol/L; 𝑉𝐻2 𝑆𝑂4 is the volume of leachate sample, mL.  References 1. McDonald, R.G. and Muir, D.M., Pressure Oxidation Leaching of Chalcopyrite Part I. Comparison of High and Low Temperature Reaction Kinetics and Products, Hydrometallurgy, 86(3-4), pp. 191-205, 2007.  212  Appendix 4 Mass Balance and Assays Results in Leaching Studies In the Tables of mass balance in this appendix the following abbreviations are utilized: Element: Ele; Leach solution: LSol; Wash solution: WSol; Leach residue: LRes; Concentration: Conc; Extraction: Ext; lignosulfonate: LS; Quebracho: Que; o-phenylenediamine: OPD; Humic acid: HA; Sulfur dispersing agent: SDA. In the leaching experiments, sulfuric acid was sealed in an ampoule. Broken glasses were not picked out in some experiments. Glasses were ground together with leach residue and samples containing glasses were sent for analysis.  All samples (In Table A4.1 - Table A4.6): nickel concentrate, leach solution, wash solution, leach residue were sent to IPL for analysis. Most leach residue samples in Table A4.7 were sent to IPL for Ni, Fe, and Cu AA analysis. One leach residue sample was sent to IPL for ICP analysis. Therefore the leaching behavior of Co can be checked from the ICP results. In Table A4.7 as well as the following Tables leach solution and wash solution samples were analyzed by AA in the lab. Leach residue samples were sent to IPL for ICP analysis.  The Quantitative X-ray diffraction results of a XSTRATA Nickel concentrate was shown in Figure A4.15. The analytical results of Voisey’s Bay nickel concentrate were supplied by Vale INCO and the results were attached in the Appendix.  213  Table A4.1 Mass balance of leaching experiments: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L nickel concentrate (P80 of 48 µm), 2 h SDA  LSol  LSol  WSol  WSol  LRes  LRes  Conc  Conc  Out/In  Ext  g/L  L  g/L  L  %  g  %  g  %  %  Ni  1.043  0.489  0.024  0.665  5.33  20.50  15.97  10.00  101  32  Fe  4.745  0.489  0.097  0.665  9.84  20.50  44.61  10.00  99  55  Cu  0.238  0.489  0.007  0.665  0.72  20.50  2.60  10.00  103  43  LS  Ni  1.436  0.486  0.035  0.640  4.09  17.00  15.97  10.01  89  57  5kg/t  Fe  5.594  0.486  0.107  0.640  7.69  17.00  44.61  10.01  92  71  Cu  0.297  0.486  0.006  0.640  0.55  17.00  2.60  10.01  93  64  LS  Ni  1.658  0.488  0.037  0.740  4.41  17.10  15.97  10.02  99  53  20kg/t  Fe  6.026  0.488  0.085  0.740  8.37  17.10  44.61  10.02  99  68  Cu  0.311  0.488  0.004  0.740  0.64  17.10  2.60  10.02  102  58  LS  Ni  1.916  0.488  0.035  0.734  2.92  18.80  15.97  10.01  94  66  50kg/t  Fe  6.201  0.488  0.089  0.734  6.33  18.80  44.61  10.01  96  73  Cu  0.341  0.488  0.006  0.734  0.45  18.80  2.60  10.01  98  67  Que  Ni  1.563  0.484  0.033  0.626  4.06  17.40  15.97  10.01  93  56  5kg/t  Fe  5.807  0.484  0.101  0.626  7.73  17.40  44.61  10.01  95  70  Cu  0.299  0.484  0.006  0.626  0.57  17.40  2.60  10.01  95  62  Que  Ni  1.760  0.489  0.026  0.738  4.11  17.10  15.97  10.02  99  56  20kg/t  Fe  6.177  0.489  0.077  0.738  7.32  17.10  44.61  10.02  97  72  Cu  0.288  0.489  0.004  0.738  0.64  17.10  2.60  10.02  97  58  OPD  Ni  1.559  0.489  0.028  0.600  4.11  16.90  15.97  10.01  92  57  5kg/t  Fe  6.061  0.489  0.089  0.600  7.17  16.90  44.61  10.01  95  73  Cu  0.305  0.489  0.005  0.600  0.57  16.90  2.60  10.01  96  63  None  Ele  214  SDA  Ele  LSol  LSol  WSol  WSol  LRes  LRes  Conc  Conc  Out/In  Ext  g/L  L  g/L  L  %  g  %  g  %  %  OPD  Ni  1.952  0.491  0.034  0.701  3.34  16.40  15.97  10.02  96  66  20kg/t  Fe  6.480  0.491  0.084  0.701  6.90  16.40  44.61  10.02  98  75  Cu  0.239  0.491  0.004  0.701  0.79  16.40  2.60  10.02  96  50  HA  Ni  1.040  0.488  0.022  0.655  4.79  19.50  15.97  10.01  91  42  5kg/t  Fe  5.085  0.488  0.088  0.655  8.72  19.50  44.61  10.01  95  62  Cu  0.299  0.488  0.006  0.655  0.52  19.50  2.60  10.01  97  61  HA  Ni  1.571  0.486  0.038  0.671  3.72  19.40  15.97  10.01  94  55  20kg/t  Fe  5.773  0.486  0.108  0.671  7.04  19.40  44.61  10.01  95  69  Cu  0.308  0.486  0.007  0.671  0.51  19.40  2.60  10.01  97  62  Table A4.2 ICP assay results (ppm) and S species (%) of nickel concentrate A (P80 of 48 µm) Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  ICP  6307  <5  96  <2  <2  <0.2  5119  77  4390  25972  ICP*  6466  <5  95  <2  <2  <0.2  5326  77  4380  26066  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  ICP  451178  <2  84  14071  175  <3  10  162951  <100  1749  ICP*  440964  <2  87  13579  183  <3  10  156304  <100  2089  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  ICP  2  15.0  1557  34  <2  596  <5  48  686  10  ICP*  2  14.6  1555  35  <2  607  <5  47  705  10  Assay  S(tot)  S(ele)  S(-2)  S(SO4)  Sulfur  32.28  0.58  31.47  0.13  Sulfur*  32.47  0.58  31.70  0.13  *: repeated analysis 215  Table A4.3 Assay results (ICP) of leach solution (mg/L), wash solution (mg/L), and leach residue (ppm): 140°C, 690 kPa O2, 0.5 mol/L H2SO4, 20 g/L nickel concentrate (P80: 48 µm), 2 h SDA  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  LSol  101.7  <0.1  <0.2  <0.01  <0.1  <0.01  77.5  9.0  27.7  237.6  WSol  1.7  <0.1  <0.2  <0.01  <0.1  0.01  2.1  0.79  0.78  6.68  LRes  22494  <5  <5  <2  <2  <0.2  2978  33  1744  7243  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  LSol  4745  <0.05  1.02  84.4  2.34  <0.05  0.48  1043  <0.1  34.5  WSol  96.71  0.21  <0.05  2.8  0.04  <0.05  <0.02  23.96  <0.1  31  LRes  98405  <2  50  5159  58  <3  12  53348  <100  14398  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  <0.02  18.5  0.43  <0.2  46.8  <0.1  0.61  15.65  <0.01  WSol  <0.01  <0.02  4  <0.01  <0.2  26.1  <0.1  <0.01  0.44  <0.01  LRes  <1  15.8  29946  13  <2  52632  6  39  66  94  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  LSol  95.5  <0.1  <0.2  <0.01  <0.1  <0.01  73.4  1.93  32.49  297.2  WSol  1.2  <0.1  <0.2  <0.01  <0.1  <0.01  1.6  <0.01  0.82  5.87  LRes  28098  <5  <5  <2  <2  <0.2  3553  20  1577  5480  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  LSol  5594  <0.05  1.56  80.9  2.22  <0.05  0.26  1436  <0.1  29  LS  WSol  107.4  <0.05  <0.05  1.5  <0.01  <0.05  <0.02  34.66  <0.1  <2  5kg/t  LRes  76906  <2  20  4307  52  <3  8  40856  <100  17469  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  0.22  20  0.38  <0.2  37.9  <0.1  0.57  17.20  <0.01  WSol  <0.01  <0.02  <1  <0.01  <0.2  4.6  <0.1  <0.01  0.39  <0.01  LRes  <1  4.7  32576  13  <2  557  7  5  12  123  None  SDA  216  SDA  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  LSol  100.0  <0.1  <0.2  <0.01  <0.1  <0.01  76.1  3.99  38.08 310.84  WSol  1.6  <0.1  <0.2  <0.01  <0.1  <0.01  3.3  <0.01  0.91  4.38  LRes  28088  <5  22  <2  <2  <0.2  3503  23  1662  6418  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  1.03  83.1  2.37  <0.05  0.35  1658.46  <0.1  27  LSol  6025.90 <0.05  Co  Cu  LS  WSol  84.89  0.16  <0.05  2.3  <0.01  <0.05  <0.02  37.16  <0.1  27  20kg/t  LRes  83705  <2  27  4973  57  <3  11  44064  <100  19251  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  0.20  40  0.41  <0.2  27.6  <0.1  0.61  17.24  <0.01  WSol  <0.01  <0.02  3  <0.01  <0.2  2.1  <0.1  <0.01  0.35  <0.01  LRes  <1  3.6  42314  14  <2  978  16  6  38  114  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  LSol  98.5  <0.1  <0.2  <0.01  <0.1  <0.01  75.1  4.2  46.2  341.3  WSol  1.8  <0.1  <0.2  <0.01  <0.1  <0.01  3.9  0.10  0.95  5.77  LRes  25423  30  <5  3  <2  <0.2  3343  25  1228  4538  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  LSol  6201.2  <0.05  0.8  81.1  2.3  <0.05  0.2  1916.2  <0.1  25.0  LS  WSol  89.18  0.14  <0.05  2.3  <0.01  <0.05  <0.02  35.31  <0.1  23  50kg/t  LRes  63298  <2  45  5310  61  <3  14  29179  <100  17055  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  <0.02  75.0  0.4  <0.2  41.3  <0.1  0.6  18.1  <0.01  WSol  <0.01  <0.02  4  <0.01  <0.2  10.2  <0.1  <0.01  0.38  <0.01  LRes  <1  13  39478  14  <2  45875  8  40  16  107  SDA  217  SDA  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  LSol  95.7  <0.1  <0.2  <0.01  <0.1  <0.01  72.7  1.29  37.98  298.7  WSol  1.6  <0.1  <0.2  <0.01  <0.1  <0.01  1.7  <0.01  0.91  6.00  LRes  26961  <5  <5  <2  <2  <0.2  3728  26  1600  5728  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  LSol  5807  <0.05  1.22  78.8  2.17  <0.05  0.21  1563  <0.1  18  Que  WSol  101.3  <0.05  <0.05  2.3  <0.01  <0.05  <0.02  33.27  <0.1  21  5kg/t  LRes  77311  <2  26  5114  65  <3  9  40627  <100  16092  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  <0.02  16  0.37  <0.2  45.4  <0.1  0.59  17.7  <0.01  WSol  <0.01  <0.02  <1  <0.01  <0.2  8.6  <0.1  <0.01  0.44  <0.01  LRes  <1  6.9  31596  15  <2  10079  12  11  8  118  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  LSol  101.1  <0.1  0.3  <0.01  <0.1  <0.01  77.5  1.37  42.66  287.9  WSol  1.6  <0.1  <0.2  <0.01  <0.1  <0.01  3.0  <0.01  0.79  4.07  LRes  29077  <5  <5  <2  <2  <0.2  3611  19  1476  6402  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  LSol  6176.6  <0.05  0.97  83.6  2.28  <0.05  0.17  1759.87  <0.1  28  Que  WSol  76.64  0.19  <0.05  2.5  <0.01  <0.05  0.15  26.10  <0.1  31  20kg/t  LRes  73237  <2  30  4700  54  <3  11  41077  <100  20349  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  0.16  25  0.44  <0.2  28.6  <0.1  0.60  18.18  <0.01  WSol  <0.01  <0.02  3  <0.01  <0.2  3.0  <0.1  <0.01  0.32  <0.01  LRes  <1  3.0  43464  14  <2  1284  11  6  19  124  SDA  218  SDA  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  LSol  94.8  <0.1  <0.2  <0.01  <0.1  <0.01  72.5  1.20  39.54  305.1  WSol  0.9  <0.1  <0.2  <0.01  <0.1  <0.01  1.2  <0.01  0.75  4.76  LRes  28725  <5  <5  <2  <2  <0.2  3759  20  1558  5734  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  LSol  6061  <0.05  1.56  78.5  2.16  <0.05  0.23  1559  <0.1  22  OPD  WSol  88.90  <0.05  <0.05  1.5  <0.01  <0.05 <0.02  28.41  <0.1  <2  5kg/t  LRes  71695  <2  23  4492  57  <3  8  41053  <100  17217  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  0.14  14  0.37  <0.2  54.6  0.6  0.58  17.99  <0.01  WSol  <0.01  <0.02  <1  <0.01  <0.2  1.7  <0.1  <0.01  0.39  <0.01  LRes  <1  2.8  33682  14  <2  1351  11  6  <1  124  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  LSol  99.1  <0.1  0.4  <0.01  <0.1  <0.01  75.5  1.17  48.88  239.0  WSol  1.1  <0.1  <0.2  <0.01  <0.1  <0.01  2.6  <0.01  0.76  3.66  LRes  29736  <5  10  3  <2  <0.2  3835  27  1309  7891  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  LSol  6480  <0.05  1.19  80.0  2.25  <0.05  0.27  1952  <0.1  31  OPD  WSol  84.18  <0.05  <0.05  1.6  <0.01  <0.05 <0.02  34.03  <0.1  19  20kg/t  LRes  69040  <2  27  5712  64  <3  12  33407  <100  20356  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  <0.02  15  0.41  <0.2  34.8  <0.1  0.61  18.04  <0.01  WSol  <0.01  <0.02  3  <0.01  <0.2  1.9  <0.1  <0.01  0.31  <0.01  LRes  <1  12.4  43575  15  <2  1466  12  6  8  123  SDA  219  SDA  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  LSol  95.8  <0.1  <0.2  <0.01  <0.1  <0.01  74.8  5.44  26.27  298.8  WSol  1.4  <0.1  <0.2  <0.01  <0.1  <0.01  1.8  0.65  0.64  6.18  LRes  23907  <5  <5  <2  <2  <0.2  3301  30  1791  5232  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  LSol  5085  <0.05  1.54  78.9  2.22  <0.05  0.30  1040  <0.1  35  HA  WSol  87.86  0.14  <0.05  2.1  <0.01  <0.05  <0.02  21.73  <0.1  21  5kg/t  LRes  87154  <2  46  5089  61  <3  12  47891  <100  14827  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  <0.02  16  0.40  <0.2  47.7  <0.1  0.62  17.9  <0.01  WSol  <0.01  <0.02  1  <0.01  <0.2  9.8  <0.1  <0.01  0.41  <0.01  LRes  <1  11.1  28608  13  <2  35932  11  28  6  103  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  LSol  98.6  <0.1  <0.2  <0.01  <0.1  <0.01  79.4  2.33  37.9  307.8  WSol  2.1  <0.1  <0.2  <0.01  <0.1  <0.01  3.0  0.08  1.04  7.34  LRes  25357  25  27  <2  <2  <0.2  3402  23  1392  5085  Assay  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  LSol  5773  <0.05  0.93  80.7  2.26  <0.05  0.21  1571  <0.1  94  HA  WSol  107.7  0.20  <0.05  3.0  <0.01  <0.05  <0.02  37.72  <0.1  29  20kg/t  LRes  70386  <2  50  5310  63  <3  13  37153  <100  16994  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  LSol  <0.01  <0.02  26  0.40  <0.2  37.0  <0.1  0.60  18.3  <0.01  WSol  <0.01  <0.02  3  <0.01  <0.2  11.9  <0.1  <0.01  0.48  <0.01  LRes  <1  11.2  38753  14  <2  45056  10  40  10  106  SDA  220  Table A4.4 Mass balance of leaching experiments: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L nickel concentrate B (P80 of 10 µm), 2 h SDA  Ele  5kg/t  LSol  LSol  WSol  WSol  LRes  LRes  Conc  Conc  Out/In  Ext  g/L  L  g/L  L  %  g  %  g  %  %  0.29  4.33  13.65  10.00  102  99  None  Ni  2.758  0.490  0.025  1.326  LS  Ni  2.800  0.488  0.0256  1.206  0.24  4.45  13.65  10.01  103  99  Que  Ni  2.796  0.490  0.0246  1.130  0.16  2.18  13.65  10.00  103  100  OPD  Ni  2.821  0.490  0.0227  1.270  0.20  2.13  13.65  10.01  104  100  HA  Ni  2.827  0.488  0.0318  1.186  0.53  5.47  10.00  106  98  13.65  Table A4.5 Assay results (ICP) of leach solution (mg/L), wash solution (mg/L) in the pressure leaching experiments: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L nickel concentrate B (P80 of 10 µm), 2 h, SDA: 5 kg/t SDA  Assay  Al  Sb  As  Ba  Bi  Cd  Ca  Cr  Co  Cu  None  LSol  107.8  <0.1  1.9  <0.01  <0.1  <0.01  67.7  3.74  82.13 366.96  LS  LSol  111.0  <0.1  2.0  <0.01  <0.1  <0.01  69.0  2.99  84.44 481.91  Que  LSol  111.0  <0.1  1.9  <0.01  <0.1  <0.01  69.5  1.84  85.31 486.23  OPD  LSol  109.7  <0.1  2.2  <0.01  <0.1  <0.01  69.7  2.66  85.32 483.68  HA  LSol  110.1  <0.1  1.7  <0.01  <0.1  <0.01  70.9  4.35  82.19 463.69  None  WSol  0.9  <0.1  <0.2  <0.01  <0.1  <0.01  1.0  0.22  0.84  7.55  LS  WSol  <0.2  <0.1  <0.2  <0.01  <0.1  <0.01  0.8  <0.01  0.80  6.82  Que  WSol  <0.2  <0.1  <0.2  <0.01  <0.1  <0.01  0.9  <0.01  0.85  6.32  OPD  WSol  <0.2  <0.1  <0.2  <0.01  <0.1  <0.01  0.7  <0.01  0.74  5.72  HA  WSol  0.9  <0.1  <0.2  <0.01  <0.1  <0.01  0.9  0.15  0.98  6.45  None*  LSol  110.7  <0.1  1.8  <0.01  <0.1  <0.01  68.6  3.76  83.54 370.02  221  SDA  Assay  None  LSol  LS  Fe  La  Pb  Mg  Mn  Hg  Mo  Ni  P  K  7414.35 <0.05  2.89  78.7  2.39  <0.05  0.45  2739.73  <0.1  <2  LSol  7747.75 <0.05  2.63  79.3  2.39  <0.05  0.58  2799.61  <0.1  <2  Que  LSol  8037.38 <0.05  2.57  81.3  2.43  <0.05  0.63  2795.93  <0.1  <2  OPD  LSol  8048.53 <0.05  3.15  80.2  2.48  <0.05  0.77  2821.09  <0.1  <2  HA  LSol  7584.97 <0.05  2.71  80.0  2.42  <0.05  0.56  2827.02  <0.1  29  None  WSol  55.61  <0.05 <0.05  1.2  0.04  <0.05  <0.02  25.13  <0.1  <2  LS  WSol  57.77  <0.05 <0.05  0.9  0.06  <0.05  <0.02  25.59  <0.1  <2  Que  WSol  59.19  <0.05 <0.05  1.5  0.04  <0.05  <0.02  24.59  <0.1  15  OPD  WSol  55.02  <0.05 <0.05  0.9  0.04  <0.05  <0.02  22.73  <0.1  <2  HA  WSol  71.44  <0.05 <0.05  1.1  0.04  <0.05  <0.02  31.75  <0.1  <2  None*  LSol  2.72  79.7  2.42  <0.05  0.56  2775.32  <0.1  <2  SDA  Assay  Sc  Ag  Na  Sr  Tl  Ti  W  V  Zn  Zr  None  LSol  <0.01  0.52  11  0.53  <0.2  151.1  <0.1  0.89  18.65  1.70  LS  LSol  <0.01  0.17  17  0.56  <0.2  55.2  <0.1  0.90  19.13  1.08  Que  LSol  <0.01  0.40  13  0.58  <0.2  63.4  <0.1  0.88  19.06  2.84  OPD  LSol  <0.01  0.13  11  0.58  <0.2  41.9  <0.1  0.96  18.92  2.98  HA  LSol  <0.01  <0.02  14  0.58  <0.2  42.5  <0.1  0.85  19.24  0.91  None  WSol  <0.01  <0.02  <1  <0.01  <0.2  17.8  <0.1  <0.01  0.55  <0.01  LS  WSol  <0.01  <0.02  <1  <0.01  <0.2  5.6  <0.1  <0.01  0.30  <0.01  Que  WSol  <0.01  <0.02  <1  <0.01  <0.2  3.4  <0.1  <0.01  0.30  <0.01  OPD  WSol  <0.01  <0.02  <1  <0.01  <0.2  3.3  <0.1  <0.01  0.29  <0.01  HA  WSol  <0.01  <0.02  <1  <0.01  <0.2  8.8  <0.1  <0.01  0.31  <0.01  None*  LSol  <0.01  0.48  11  0.55  <0.2  152.6  <0.1  0.90  18.86  1.72  7439.19 <0.05  *: repeated analysis  222  Table A4.6 Assay results (AA) of leach residue (ppm) and nickel concentrate (%), experimental conditions: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 20 g/L nickel concentrate B (P80 of 10 µm), 2 h, SDA: 5 kg/t, *: repeated analysis SDA  Assay  Ni  SDA  Assay  Ni  None  LRes  2869  OPD  LRes  2046  LS  LRes  2408  HA  LRes  5336  Que  LRes  1602  Assay  Ni  Cu  Fe  Conc  13.65  24534  36.58  Conc*  13.79  24503  36.54  Table A4.7 Mass balance of leaching experiments: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L nickel concentrate B (P80 of 10 µm), 2 h SDA  Ele  LSol  LSol  WSol  WSol  LRes  LRes  Conc  Conc  g/L  L  g/L  L  %  g  %  g  %  %  Ni  29.200  0.461  0.691  1.815  1.97  39.59  13.65  125.01  91  95  Fe  63.425  0.461  1.559  1.815  31.18  39.59  36.58  125.01  97  73  Cu  5.060  0.461  0.114  1.815  0.39  39.59  2.45  125.01  88  95  Ni  30.184  0.420  1.713  1.685  0.26  46.62  13.65  125.08  92  99  Fe  57.888  0.420  3.349  1.685  29.69  46.62  36.58  125.08  96  70  Cu  5.180  0.420  0.294  1.685  0.07  46.62  2.45  125.08  88  99  Ni  30.392  0.450  0.849  2.200  0.21  48.46  13.65  125.06  92  99  Fe  57.650  0.450  2.216  2.200  31.18  48.46  36.58  125.06  100  67  Cu  5.172  0.450  0.144  2.200  0.07  48.46  2.45  125.06  87  99  Ni  30.384  0.438  0.934  2.180  0.40  57.72  13.65  125.03  91  99  Fe  51.063  0.438  1.886  2.180  32.56  57.72  36.58  125.03  99  59  Cu  5.048  0.438  0.152  2.180  0.24  57.72  2.45  125.03  88  96  Ni  30.264  0.463  0.822  1.858  0.39  45.34  13.65  125.03  92  99  Fe  59.310  0.463  1.633  1.858  32.33  45.34  36.58  125.03  99  68  Cu  5.352  0.463  0.146  1.858  0.15  45.34  2.45  125.03  92  98  5kg/t  None  LS  Que  OPD  HA  Out/In Ext.  223  Table A4.8 Assay results (AA & ICP) of leach residue samples, experimental conditions: 140°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L nickel concentrate B (P80 of 10 µm), 2 h, SDA: 5 kg/t SDA  Assay  Ni  Cu  Fe  %  ppm  %  None  LRes  1.97  3917  31.18  LS  LRes  0.26  720  29.69  Que  LRes  0.21  654  31.18  OPD  LRes  0.4  2350  32.56  SDA  Assay  Ni*  Ag  Cu  Pb  Zn  As  Sb  Hg  Mo  Tl  %  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  0.39  59.8  1468  225  50  <5  <5  <3  21  <2  Bi  Cd  Co  Ni  Ba  W  Cr  V  Mn  La  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  34  <0.2  179  3738  105  11  171  72  302  <2  Sr  Zr  Sc  Ti  Al  Ca  Fe  Mg  K  Na  ppm  ppm  ppm  %  %  %  %  %  %  %  71  8731  5  1.03  0.98  0.73  32.33  2.48  0.43  0.68  HA  LRes  P % <0.01 *: repeated analysis  224  Table A4.9 Mass balance of leaching experiments: 150°C, 690 kPa oxygen overpressure, 0.5 mol/L H2SO4, 250 g/L nickel concentrate C (P100 of 44 µm), 2 h SDA  Ele  LSol  LSol  WSol  WSol  LRes  LRes  Conc  Conc  Out/In  Ext  g/L  L  g/L  L  %  g  %  g  %  %  Ni  4.056  0.422  0.273  1.530  9.59  128.67  11.54  125.04  100  15  Fe  11.192  0.422  1.050  1.530  37.51  128.67  44.44  125.04  98  13  Cu  0.777  0.422  0.054  1.530  1.40  128.67  1.75  125.04  101  18  Ni  20.104  0.433  0.494  2.048  3.76  109.48  11.54  125.01  96  72  Fe  33.910  0.433  0.864  2.048  38.03  109.48  44.44  125.01  105  25  Cu  2.690  0.433  0.064  2.048  0.97  109.48  1.75  125.01  108  52  Ni  23.016  0.425  0.967  1.910  2.36  101.38  11.54  125.08  97  83  Fe  29.430  0.425  1.203  1.910  40.04  101.38  44.44  125.08  100  27  Cu  3.486  0.425  0.145  1.910  0.38  101.38  1.75  125.08  98  82  Ni  25.584  0.413  1.152  1.968  0.15  107.12  11.54  125.05  90  99  Fe  25.960  0.413  1.159  1.968  40.20  107.12  44.44  125.05  101  23  Cu  2.932  0.413  0.130  1.968  0.69  107.12  1.75  125.05  100  67  Ni  16.208  0.443  0.483  2.003  5.20  109.33  11.54  125.03  96  61  Fe  22.610  0.443  1.009  2.003  39.53  109.33  44.44  125.03  99  22  Cu  1.646  0.443  0.050  2.003  1.34  109.33  1.75  125.03  104  33  5kg/t  None  LS  Que  OPD  HA  225  Table A4.10 Assay results (ICP) of leach residue and nickel concentrate C: 150°C, 690 kPa O2, 0.5 mol/L H2SO4, 250 g/L nickel concentrate C (P100 of 44 µm), 2 h, SDA: 5 kg/t SDA  Assay  Ni*  Ag  Cu  Pb  Zn  As  Sb  Hg  Mo  Tl  %  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  Conc  ---  11.55  15.9  17535  93  675  <5  <5  <3  18  <2  None  LRes  9.59  16.5  13957  89  92  <5  <5  <3  14  <2  LS  LRes  3.76  21.8  9680  94  47  <5  <5  <3  17  <2  Que  LRes  2.36  22.2  3847  94  58  <5  <5  <3  17  <2  OPD  LRes  0.15  21.4  6856  79  46  <5  <5  <3  18  <2  HA  LRes  5.20  18.5  13354  91  66  <5  <5  <3  18  <2  Conc*  ---  11.53  15.0  17529  96  668  <5  <5  <3  17  <2  SDA  Assay  Bi  Cd  Co  Ni  Ba  W  Cr  V  Mn  La  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  Conc  ---  <2  <0.2  3158  113092  <2  <5  97  51  164  <2  None  LRes  21  <0.2  2787  94680  <2  8  78  45  103  <2  LS  LRes  11  <0.2  1315  36895  33  12  108  58  136  <2  Que  LRes  16  <0.2  348  15528  50  6  116  61  137  <2  OPD  LRes  20  <0.2  81  1415  57  <5  109  58  132  <2  HA  LRes  36  <0.2  1848  51915  21  8  94  53  124  <2  Conc*  ---  <2  <0.2  3179  112939  <2  5  98  54  165  <2  SDA  Assay  Sr  Zr  Sc  Ti  Al  Ca  Fe  Mg  K  Na  ppm  ppm  ppm  %  %  %  %  %  %  %  Conc  ---  22  17  1  0.06  0.44  0.40  44.45  1.18  0.05  0.16  None  LRes  17  27  1  0.11  0.30  0.25  37.51  0.78  0.09  0.28  LS  LRes  26  28  1  0.85  0.41  0.32  38.03  1.12  0.11  0.37  Que  LRes  26  19  1  0.71  0.50  0.33  40.04  1.13  0.09  0.30  OPD  LRes  26  15  1  0.54  0.48  0.32  40.20  1.12  0.07  0.21 226  SDA  Assay  Sr  Zr  Sc  Ti  Al  Ca  Fe  Mg  K  Na  ppm  ppm  ppm  %  %  %  %  %  %  %  HA  LRes  25  14  1  0.12  0.33  0.29  39.53  0.93  0.16  0.23  Conc*  ---  22  12  1  0.06  0.44  0.40  44.43  1.19  0.05  0.16  SDA  Assay  P %  Conc  ---  <0.01  None  LRes  <0.01  LS  LRes  <0.01  Que  LRes  <0.01  OPD  LRes  <0.01  HA  LRes  <0.01  Conc*  ---  <0.01  *: repeated analysis  Table A4.11 Mass balance of leaching experiments in mixed sulfate and chloride media: 150°C, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L nickel concentrate C (P100 of 44 µm), 2 h SDA  Cl-1  Ele  g/L LS# 5kg/t  0  LS 5kg/t  0  LSol  LSol  WSol  WSol  LRes  LRes  Conc  Conc  Out/In  Ext  g/L  L  g/L  L  %  g  %  g  %  %  Ni  18.312 0.435  0.646  1.975  4.01  115.14 11.54  125.05  96  68  Fe  21.448 0.435  0.728  1.975  40.47  115.14 44.44  125.05  103  19  Cu  2.820  0.435  0.102  1.975  0.66  115.14  1.75  125.05  100  65  Ni  17.800 0.434  0.602  1.978  4.17  111.11 11.54  125.08  94  68  Fe  20.664 0.434  0.728  1.978  40.70  111.11 44.44  125.08  100  19  Cu  2.818  0.103  1.978  0.66  111.11  125.08  98  67  0.434  1.75  227  SDA  Cl-1  Ele  g/L LS 10kg/t  5  LS 5kg/t  5  LS 2.5kg/t  5  LS 5kg/t  10  Que 5kg/t  5  OPD 5kg/t  5  HA 5kg/t  None  5  5  LSol  LSol  WSol  WSol  LRes  LRes  Conc  Conc  Out/In  Ext  g/L  L  g/L  L  %  g  %  g  %  %  Ni  25.536 0.403  1.171  2.030  0.12  135.93 11.54  125.00  89  99  Fe  1.819  0.403  0.110  2.030  40.06  135.93 44.44  125.00  100  1.7  Cu  3.810  0.403  0.145  2.030  0.16  135.93  1.75  125.00  93  90  Ni  25.576 0.416  1.061  1.870  0.14  133.97 11.54  125.04  89  99  Fe  1.369  0.416  0.087  1.870  41.27  133.97 44.44  125.04  101  1.3  Cu  3.922  0.416  0.140  1.870  0.13  133.97  1.75  125.04  94  92  Ni  26.080 0.418  0.919  1.883  0.17  135.06 11.54  125.01  89  98  Fe  0.828  0.418  0.044  1.883  40.43  135.06 44.44  125.01  99  0.8  Cu  3.930  0.418  0.139  1.883  0.14  135.06  1.75  125.01  96  91  Ni  25.272 0.415  1.073  1.880  0.26  138.03 11.54  125.03  89  97  Fe  0.672  0.415  0.049  1.880  39.17  138.03 44.44  125.03  98  0.7  Cu  3.836  0.415  0.163  1.880  0.16  138.03  1.75  125.03  96  90  Ni  24.608 0.405  1.172  1.860  0.31  139.08 11.54  125.00  87  97  Fe  0.733  0.405  0.066  1.860  38.56  139.08 44.44  125.00  97  0.8  Cu  3.184  0.405  0.178  1.860  0.19  139.08  1.75  125.00  86  88  Ni  25.344 0.385  1.454  1.866  0.16  139.89 11.54  125.01  88  98  Fe  1.712  0.385  0.059  1.866  39.64  139.89 44.44  125.01  101  1.4  Cu  3.252  0.385  0.222  1.866  0.15  139.89  1.75  125.01  86  90  Ni  24.968 0.422  0.974  1.885  0.23  138.80 11.54  125.04  88  98  Fe  0.426  0.422  0.034  1.885  40.24  138.80 44.44  125.04  101  0.4*  Cu  3.122  0.422  0.146  1.885  0.26  138.80  1.75  125.04  89  83  Ni  25.020 0.421  0.897  1.810  0.29  139.32 11.54  125.05  87  97  Fe  0.278  0.421  0.047  1.810  41.74  139.32 44.44  125.05  105  0.4  Cu  3.494  0.421  0.132  1.810  0.27  139.32  125.05  95  83  1.75  #: lignosulfonate was not sealted in an ampoule; *: the extraction of iron was calculated from solution 228  Table A4.12 Assay results (ICP) of leach residue, experimental conditions: 150°C, 690 kPa oxygen overpressure, 40 g/L H2SO4, 250 g/L nickel concentrate C (P100 of 44 µm), 2 h Cl-  Ag  Cu  Pb  Zn  As  Sb  Hg  Mo  Tl  Bi  g/L  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  LS#:5kg/t  0  18.3  6621  98  160  <5  <5  <3  10  <2  <2  LS:5kg/t  0  19.5  6576  100  174  <5  <5  <3  11  <2  <2  LS:10kg/t  5  12.7  1593  79  50  <5  <5  <3  7  <2  4  LS*:10kg/t  5  12.9  1561  81  59  <5  <5  <3  7  <2  4  LS:5kg/t  5  15.9  1310  85  62  <5  <5  <3  10  <2  <2  LS:2.5kg/t  5  14.1  1407  76  104  <5  <5  <3  11  <2  <2  LS:5kg/t  10  10.5  1558  77  150  <5  <5  <3  6  <2  <2  Que:5kg/t  5  11.9  1938  68  198  <5  <5  <3  8  <2  10  OPD:5kg/t  5  11.1  1549  87  173  <5  <5  <3  9  <2  19  HA:5kg/t  5  12.2  2635  68  173  <5  <5  <3  8  <2  13  None  5  <0.5  2610  <2  50  5  <5  <3  <1  <2  13  None*  5  <0.5  2744  <2  52  5  <5  <3  <1  <2  13  SDA  Cl-  Cd  Co  Ni  Ba  W  Cr  V  Mn  La  Sr  g/L  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  LS#:5kg/t  0  <0.2  963  40071  57  <5  94  65  117  <2  23  LS:5kg/t  0  <0.2  1271  41745  56  <5  96  63  118  <2  23  LS:10kg/t  5  <0.2  212  1275  76  <5  88  54  99  <2  19  LS*:10kg/t  5  <0.2  210  1243  76  <5  86  53  100  <2  19  LS:5kg/t  5  <0.2  238  1374  91  <5  94  58  103  <2  20  LS:2.5kg/t  5  <0.2  312  1730  76  <5  93  57  97  <2  19  LS:5kg/t  10  <0.2  219  2632  73  <5  89  54  99  <2  19  Que:5kg/t  5  <0.2  291  3067  71  6  86  51  98  <2  18  OPD:5kg/t  5  <0.2  274  1567  73  <5  86  51  100  <2  18  SDA  229  Cl-  Cd  Co  Ni  Ba  W  Cr  V  Mn  La  Sr  g/L  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  HA:5kg/t  5  <0.2  383  2309  73  <5  89  53  102  <2  18  None  5  <0.2  364  2835  44  <5  70  11  79  <2  20  None*  5  <0.2  372  2892  46  <5  71  11  82  <2  21  SDA  Cl-  Zr  Sc  Ti  Al  Ca  Fe  Mg  K  Na  P  g/L  ppm  ppm  %  %  %  %  %  %  %  %  LS#:5kg/t  0  14  <1  0.92  0.33  0.28  40.47  0.93  0.08  0.19  <0.01  LS:5kg/t  0  17  <1  0.87  0.37  0.28  40.70  0.94  0.10  0.26  <0.01  LS:10kg/t  5  12  <1  0.06  0.36  0.25  40.53  0.79  0.05  0.86  0.01  LS*:10kg/t  5  11  <1  0.06  0.35  0.24  40.06  0.79  0.05  0.87  0.01  LS:5kg/t  5  13  <1  0.56  0.37  0.25  41.27  0.81  0.06  0.55  0.01  LS:2.5kg/t  5  18  <1  0.71  0.44  0.25  40.31  0.79  0.11  0.59  0.01  LS:5kg/t  10  18  <1  0.21  0.45  0.25  39.17  0.79  0.10  0.92  0.01  Que:5kg/t  5  12  <1  0.06  0.37  0.24  38.56  0.78  0.06  0.98  0.01  OPD:5kg/t  5  11  <1  0.06  0.35  0.24  39.64  0.79  0.05  0.96  0.01  HA:5kg/t  5  13  <1  0.06  0.39  0.25  40.24  0.80  0.13  0.80  0.01  None  5  338  <1  0.05  0.39  0.23  42.13  0.71  0.04  0.76  <0.01  None*  5  356  <1  0.05  0.38  0.24  41.35  0.70  0.04  0.76  <0.01  SDA  #: lignosulfonate was not sealted in an ampoule; *: repeated analysis  230  Table A4.13 Mass balance of leaching experiments: 150°C, 690 kPa oxygen overpressure, 250 g/L Voisey’s Bay concentrate D (P80 of 50 µm), 1.5 h Experiment  Ele  LSol  LSol  WSol  WSol  LRes  LRes  Conc  Conc  Out/In  Ext  g/L  L  g/L  L  %  g  %  g  %  %  No.1:LS:5kg/t  Ni  40.47  0.454  1.085  1.890  3.39 116.59  20.80  125.02  94  85  Cl-1:10g/L  Fe  0.072  0.454  0.057  1.890  39.17 116.59  37.40  125.02  98  0.3  H2SO4:40g/L  Cu  1.722  0.454  0.054  1.890  0.95 116.59  1.68  125.02  95  47  No.2:LS:5kg/t  Ni  25.87  0.462  0.673  1.984  13.86 102.68  20.80  125.00  106  45  Cl-1:0g/L  Fe  17.90  0.462  0.704  1.984  42.16 102.68  37.40  125.00  113  21  H2SO4:49g/L  Cu  1.616  0.462  0.038  1.984  1.47 102.68  1.68  125.00  111  28  No.3:LS:0kg/t  Ni  17.23  0.440  0.662  1.680  15.28 125.67  20.80  125.07  107  26  Cl-1:10g/L  Fe  0.409  0.440  0.150  1.680  41.11 125.67  37.40  125.07  111  0.9  H2SO4:49g/L  Cu  1.152  0.440  0.034  1.680  1.27 125.67  1.68  125.07  103  24  No.4:LS:5kg/t  Ni  36.74  0.435  1.125  2.093  5.54 124.09  20.80  125.07  97  74  Cl-1:10g/L  Fe  0.551  0.435  0.125  2.093  41.85 124.09  37.40  125.07  112  1.1  H2SO4:49g/L  Cu  1.946  0.435  0.061  2.093  1.12 124.09  1.68  125.07  112  34  231  Table A4.14 Assay results (ICP) of leach residue, experimental conditions: 150°C, 690 kPa oxygen overpressure, 40 or 49 g/L H2SO4, 250 g/L Voisey’s Bay concentrate D (P80 of 50 µm), 1.5 h Ag  Cu  Pb  Zn  As  Sb  Hg  Mo  Tl  Bi  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  No.1  17.2  9528  104  148  <5  <5  <3  8  <2  <2  No.2  28.6  14730  120  77  12  <5  <3  20  <2  <2  No.3  17.3  12695  102  73  18  <5  <3  39  <2  <2  No.4  21.0  11166  89  46  <5  <5  <3  24  <2  <2  No.2 Repeat  27.6  14662  121  74  11  <5  <3  20  <2  <2  Cd  Co  Ni  Ba  W  Cr  V  Mn  La  Sr  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  ppm  No.1  <0.2  1484  33877  34  8  5  25  12  <2  3  No.2  <0.2  6074  138943  <2  <5  2  21  10  <2  3  No.3  <0.2  6518  152833  <2  9  32  19  10  <2  3  No.4  <0.2  1918  55446  <2  11  19  22  11  <2  3  No.2 Repeat  <0.2  5976  138193  <2  <5  3  21  10  <2  3  Zr  Sc  Ti  Al  Ca  Fe  Mg  K  Na  P  ppm  ppm  %  %  %  %  %  %  %  %  No.1  29  <1  0.06  0.30  0.04  39.17  0.03  0.17  0.54  <0.01  No.2  15  <1  0.84  0.24  0.03  42.22  0.02  0.14  0.32  <0.01  No.3  6  <1  0.79  0.06  0.02  41.11  0.01  0.03  0.69  <0.01  No.4  9  <1  1.05  0.08  0.02  41.85  0.01  0.04  0.73  <0.01  No.2 Repeat  16  <1  0.86  0.26  0.03  42.09  0.02  0.15  0.33  <0.01  Experiment*  Experiment  Experiment  *Experimental conditions, refer to Table A4.13  232  Table A4.15 Quantitative phase analysis of one nickel concentrate sample (XSTRATA Nickel Limited, Strathcona Mill) using the rietveld method and X-ray powder diffraction data Minerals in the sample  Ideal Formula  Quartz  SiO2  1.3  Clinochlore  (Mg,Fe2+)5Al(Si3Al)O10(OH)8  5.1  Plagioclase  NaAlSi3O8 – CaAl2Si2O8  2.3  Actinolite  Ca2(Mg,Fe2+)5Si8O22(OH)2  3.4  Talc  Mg3Si4O10(OH)2  2.9  Gypsum  CaSO4·2H2O  1.4  Pyrite  FeS2  11.0  Chalcopyrite  CuFeS2  5.0  Pyrrhotite  Fe1-xS  30.6  Pentlandite  (Fe,Ni)9S8  36.3  Sulfur, elemental  S  0.7  Total  Weight, %  100.0  The Quantitative X-ray diffraction (QXRD) analysis of the received nickel concentrate sample was completed in the Department of Earth and Ocean Sciences, UBC.  233  Table A4.16 Analytical results (%) of Voisey’s Bay nickel concentrate Al  As  Bi  Ca  Cd  Co  Cr  Cu  0.0161  <0.00632  <0.00632  0.180  <0.00632  0.916  <0.00126  1.68  Fe  Mg  Mn  Na  Ni  P  Pb  S  37.4  0.0164  0.0122  <0.00632  20.8  <0.0126  0.0162  31.4  Se  Si  Te  Zn  Total  <0.0126  0.0379  0.140  0.00971  92.52  CaRaw  Co  Co  Ni  Ni  Ni  Se  Se  228.615  237.863  216.555  222.486  227.021  196.026  203.985  0.916  0.908  20.8  20.8  22.4  <0.0126  0.0159  0.180  The major sulfide minerals in the Voisey’s Bay nickel concentrate sample are (%WT): chalcopyrite (0.25-1.5), pentlandite (85-95), pyrrhotite (5-15), cobalt sulfide (1-1.5). The 80% passing size of fresh high grade nickel concentrate is expected to be 50 µm based on pilot plant results (range from 25-90 µm). The high grade nickel concentrate is expected to age and oxidize during storage thus forming agglomerates and lumps.  234  

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