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The sonochemical leaching of Chalcopyrite Abed, Nedam 2002

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THE SONOCHEMICAL LEACHING OF CHALCOPYRITE by  NED A M ABED B . Sc., Chemical Engineering, Jordan University of Science and Technology, Jordan, 1989 M . A . Sc., The University of British Columbia, 1999  A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in  THE FACULTY OF GRADUATE STUDIES Department of Metals and Materials Engineering We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH C O L U M B I A October 2001 ©Nedam Abed, 2001  In  presenting  degree at the  this  thesis  in  University of  partial  fulfilment  of  of  department  this thesis for or  by  his  or  scholarly purposes may be her  representatives.  permission.  Department The University of British Columbia Vancouver, Canada  for  an advanced  Library shall make it  agree that permission for extensive  It  publication of this thesis for financial gain shall not  DE-6 (2/88)  requirements  British Columbia, I agree that the  freely available for reference and study. I further copying  the  is  granted  by the  understood  that  be allowed without  head of copying  my or  my written  ABSTRACT A fundamental study of the sonochemical leaching of chalcopyrite in ferric ion media has been performed to understand and quantify the effects of sonication and other parameters on leaching reactions. The study covers sulfate and chloride media. The main leaching reaction: CuFeS ) + 4 F e 2(s  3+ (aq)  -> C u  2 + ( a q )  + 5Fe  2+ (aq)  + 2S  (s)  was found to be dependent on temperature and initial particle size, but less dependent on ferric ion concentration. Leaching kinetics were not affected by ferrous ion concentration or the amount of chalcopyrite added. Leaching was performed under a variety of sonication, thermodynamic and physical parameters. Reaction stoichiometry was confirmed through detailed wet chemistry analysis and mass balance calculations. The use of ultrasound activation showed a clear improvement in leaching kinetics and amount of copper extracted. Sonication has a catalytic-like effect on the leaching reaction. Compared to experiments without sonication, reaction rates were 2-3 times faster at the same temperature and can be faster by a factor of 20 for the same initial particle size. Regardless of initial particle size, the amount of copper extracted is comparable under sonication, and can be twice that under chemical leaching, implying the avoidance of fine particle grinding. Sonochemical leaching was found to be only temperature dependent, where the best copper extraction was at 75 °C. Leaching was further enhanced by the use of a grinding aid. The leaching behavior of chalcopyrite particles remained identical with and without ultrasound activation, implying that the known passivation mechanism in oxidative leaching was not eradicated, as evident from S E M and surface analysis techniques. Sonication did not contribute to particle passivation, as affirmed from different leaching procedures and sonication in aqueous and organic media. Parabolic leaching kinetics were established and confirmed from the estimation of different thermodynamic parameters and dependence on other physical parameters. The developed leaching models from this study are: For pure chalcopyrite: Sulfate media:  ii  l-3(l-X )  2 / 3  b  + 2 ( l - X ) =k.^ [Fe ] 3+  00 4  b  \2/3 z / J ^/  r i 7 „ 3 +J +- i 0u. 0 3  1-3(1-X ) +2(1-X )=k [Fe ] 1  1  b  b  2  28,950  exp  exp  without sonication  RT  | —  32,620 RT  with sonication  Chloride media: l-3(l-X )  2 / 3  b  +2(l-X )=^ [Fe ] R„ 3+  ai4  b  exp  l - 3 ( l - X ) + 2 ( l - X ) = k [ F e ^ f " exp 2/3  b  b  4  -24,840 v  RT  without sonication j  '-29,310^ RT J v  with sonication  For chalcopyrite concentrate: Sulfate media: l-3(l-X )  2 / 3  b  +2(l-X )=^ [Fe ] 3+  b  Rl  l-3(l-X ) +2(l-X )=k [Fe ] 2/3  3+  b  b  007  006  6  exp  exp  -26,990  I  RT  -31,580 RT  without sonication  with sonication  Chloride media: l-3(l-X )  2 / 3  b  +2(l-X )=^ [Fe ] R„ 3+  003  b  exp  l - 3 ( l - X ) + 2 ( l - X ) = k [ F e3+i0.05 ] exp 2/3  b  J+  b  8  UU3  -25,450 RT 29,950 RT  without sonication  with sonication  TABLE OF CONTENTS  Abstract  ii  List of Tables  vi  List of Figures  xii  Nomenclature  xxiii  Acknowledgment  xxvi  Chapter 1: Introduction  1  Chapter 2: Literature Review  3  2.1 Copper Hydrometallurgy  3  2.2 Ultrasound Science and Technology  15  2.2.1 Theory of Ultrasound  17  2.2.2 Bubble Dynamics  18  2.2.3 Acoustic Power  20  2.2.4 Sonication Systems  21  2.3 Ultrasound in Leaching Processes  23  Chapter 3: Objectives  36  Chapter 4: Experimental Procedures  38  4.1 Materials  38  4.2 Methods and Calculations  41  Chapter 5: Results and Discussion 5.1 Preliminary Sonochemical Leaching Experiments  ;  50 50  5.1.1 General Leaching Behavior  50  5.1.2 Effect of Sonication Parameters  56  I. Effect of Sonication Mode  57  II. Effect of Horn Depth of Immersion  61  III. Effect of Horn Assembly  65  5.2 Temperature Dependence  69  5.3 Particle Size Dependence  90  5.4 Ferric Ion Dependence  126  5.5 Ferrous Ion Dependence  144  5.6 Chalcopyrite Dependence  154  iv  5.7 Effect of Solids Content and Solution Volume (Scaling-up Experiments)  166  5.8 Effect of Leaching Procedure  175  5.9 Effect of Leaching Procedure with Sonication in Decane  192  5.10 Sonochemical Leaching with Tungsten Carbide Balls  209  5.11 Sonochemical Leaching at Controlled Redox Potential of 700 mV ..  222  Chapter 6: Conclusions  235  Chapter 7: Recommendations for Future Research  238  Bibliography  239  Appendices  246  Appendix I: Mass Balance Calculations  247  Appendix II: Leaching Kinetics Models  264  v  )  LIST OF TABLES  Table 2.1.1: Advantages and disadvantages of ferric sulfate leaching of chalcopyrite  5  Table 2.1.2: Summary of some published research on ferric sulfate leaching of CuFeS2 ..  13  Table 2.1.3: Summary of some published research on ferric chloride leaching of CuFeS2  14  Table 2.2.1: Summary of sonic spectra  17  Table 2.2.2: Selected results from published work on ultrasound use in leaching  35  Table 4.1.1: Detailed chemical analysis of the tested chalcopyrite concentrate  38  Table 4.1.2: The mineralogical composition of the tested chalcopyrite concentrate  38  Table 4.1.3: Detailed chemical analysis of the tested pure chalcopyrite  40  Table 4.1.4: The mineralogical composition of the pure chalcopyrite  40  Table 4.2.1: Measured redox potentials for the studied systems  43  Table 5.1.1: Behavior of leaching kinetics for pure chalcopyrite in sulfate and chloride media with and without ultrasound  activation, and related  KMnCu  consumption  52  Table 5.1.2: Effect of sonication mode on leaching kinetics for pure chalcopyrite in sulfate and chloride media  58  Table 5.1.3: Effect of horn depth of immersion on pure chalcopyrite conversion in sulfate and chloride media  62  Table 5.1.4: Effect of horn assembly on pure chalcopyrite conversion in sulfate and chloride media  66  Table 5.2.1: Temperature dependence of leaching kinetics for pure chalcopyrite in sulfate media with and without ultrasound activation  72  Table 5.2.2: Temperature dependence of leaching kinetics for chalcopyrite concentrate in sulfate media with and without ultrasound activation  73  vi  Table 5.2.3: Temperature dependence of leaching kinetics for pure chalcopyrite in chloride media with and without ultrasound activation 74 Table 5.2.4: Temperature dependence of leaching kinetics for chalcopyrite concentrate in chloride media with and without ultrasound activation  75  Table 5.2.5: Temperature dependence of reaction rates and related thermodynamic values for sulfate media  76  Table 5.2.6: Temperature dependence of reaction rates and related thermodynamic values for chloride media  76  Table 5.2.7: Parabolic leaching models for the studied systems showing the estimated activation energy  77  Table 5.3.1: Particle size dependence of leaching kinetics for pure chalcopyrite in sulfate media with and without ultrasound activation  96  Table 5.3.2: Particle size dependence of leaching kinetics for chalcopyrite concentrate in sulfate media with and without ultrasound activation  97  Table 5.3.3: Particle size dependence of leaching kinetics for pure chalcopyrite in chloride media, with and without ultrasound activation  98  Table 5.3.4: Particle size dependence of leaching kinetics for CuFeS2 concentrate in chloride media with and without ultrasound activation  99  Table 5.3.5: Particle size dependence of reaction rates for leaching in sulfate media, with and without ultrasound activation  100  Table 5.3.6: Particle size dependence of reaction rates for leaching in chloride media, with and without ultrasound activation  100  Table 5.3.7: Parabolic leaching models for the studied systems showing the dependence on particle size for non-sonicated systems and the  pseudo-zero-order  dependence for sonicated ones  101  Table 5.4.1: [Fe ] dependence of leaching kinetics for pure chalcopyrite in sulfate 3+  media with and without ultrasound activation  128  vii  Table 5.4.2: [Fe ] dependence of leaching kinetics for chalcopyrite concentrate in J+  sulfate media with and without ultrasound activation  129  Table 5.4.3: [Fe ] dependence of leaching kinetics for pure chalcopyrite in chloride 3+  media with and without ultrasound activation  130  Table 5.4.4: [Fe ] dependence of leaching kinetics for chalcopyrite concentrate in 3+  chloride media with and without ultrasound activation  131  Table 5.4.5: Ferric ion dependence of reaction rates for leaching in sulfate media, with and without ultrasound activation  132  Table 5.4.6: Ferric ion dependence of reaction rates for leaching in chloride media, with and without ultrasound activation  132  Table 5.4.7: Parabolic leaching models for the studied systems showing the dependence on temperature, particle size and ferric ion concentration  133  Table 5.5.1: [Fe ] dependence of leaching kinetics for pure chalcopyrite in sulfate 2+  media with and without ultrasound activation  146  Table 5.5.2: [Fe ] dependence of leaching kinetics for chalcopyrite concentrate in 2+  sulfate media with and without ultrasound activation  147  Table 5.5.3: [Fe ] dependence of leaching kinetics for pure chalcopyrite in chloride 3+  media with and without ultrasound activation  148  Table 5.5.4: [Fe ] dependence of leaching kinetics for chalcopyrite concentrate in 3+  chloride media with and without ultrasound activation  149  Table 5.6.1: Pure chalcopyrite effect on reaction kinetics for leaching in sulfate media, with and without ultrasound activation  156  Table 5.6.2: Copper released in solution at various pure chalcopyrite additions for leaching in sulfate media, with and without ultrasound activation  158  Table 5.6.3: Copper released in solution at various chalcopyrite concentrate additions for leaching in sulfate media, with and without ultrasound activation  159  Table 5.6.4: Copper released in solution at various pure chalcopyrite additions for leaching in chloride media, with and without ultrasound activation  160  Table 5.6.5: Copper released in solution at various chalcopyrite concentrate additions for leaching in chloride media, with and without ultrasound activation  161  Table 5.7.1: Effect of solids content and solution volume on pure chalcopyrite conversion in sulfate media (scale-up experiments)  167  Table 5.7.2: Effect of solids content and solution volume on pure chalcopyrite conversion in chloride media (scale-up experiments)  168  Table 5.7.3: Effect of solids content and solution volume on chalcopyrite concentrate conversion in sulfate media (scale-up experiments)  169  Table 5.7.4: Effect of solids content and solution volume on chalcopyrite concentrate conversion in chloride media (scale-up experiments)  170  Table 5.8.1: Results for two stages of pure chemical leaching mediated by 30 min of sonication in pure deionized water  176  Table 5.8.2: Results for two stages of pure chemical leaching mediated by 60 min of sonication in pure deionized water  179  Table 5.8.3: Results for two stages of pure chemical leaching preceded and mediated by 30 min of sonication in pure deionized water  181  Table 5.8.4: Results for single stage of pure chemical leaching preceded by 60 min of sonication in pure deionized water  184  Table 5.8.5: Results for two stages of pure chemical leaching preceded and mediated by a single stage of sonication, for 60 minutes, in pure deionized water  186  Table 5.8.6: Results for two stages of leaching preceded and then combined with 60 min of sonication in pure deionized water  188  Table 5.9.1: Results for sonication in decane (1 hour) followed by chemical leaching (2 hours) at 75 °C  193  Table 5.9.2: Results for sonication in decane (2 hours) followed by chemical leaching (2 hours) at 75 °C  197  ix  Table 5.9.3: Results for the procedure of sonication in decane (1 hour), leaching (1 hour), sonication again in decane for 1 hour and final chemical leaching for  200  another 1 hour Table 5.9.4: Results for the experimental procedure: chemical leaching (1 hour), sonication in decane (2 hours) and leaching (1 hour)  203  Table 5.9.5: Results for the procedure of sonication in decane (1 hour), leaching (1 hour) and combined sonication and leaching (1 hour)  206  Table 5.10.1: Summary of leaching results for the 13 um particle size under different conditions (pure chalcopyrite in sulfate and chloride media)  212  Table 5.10.2: Summary of leaching results for the 13 um particle size under different conditions (chalcopyrite concentrate in sulfate and chloride media) Table 5.10.3: Effect of using grinding media  213  (tungsten carbide balls) on pure  chalcopyrite conversion in sulfate and chloride media  214  Table 5.10.4: Effect of using grinding media (tungsten carbide balls) on chalcopyrite concentrate conversion in sulfate and chloride media  215  Table 5.11.1: Experimental results for leaching pure chalcopyrite in sulfate and chloride media at 700 mV and 25 °C  226  Table 5.11.2: Experimental results for leaching pure chalcopyrite in sulfate and chloride media at 700 mV and 75 °C Table 6.1: Developed parabolic leaching models for the studied systems  227 237  Appendix I Tables: Table 1.1: Sample material balance calculations for ferric sulfate leaching of pure chalcopyrite without ultrasound activation  248  Table 1.2: Sample material balance calculations for ferric sulfate leaching of pure chalcopyrite with ultrasound activation  250  Table 1.3: Sample material balance calculations for ferric chloride leaching of pure chalcopyrite without ultrasound activation  252  x  Table 1.4: Sample material balance calculations for ferric chloride leaching of pure chalcopyrite with ultrasound activation Table 1.5: Sample material balance calculations for ferric sulfate  254 leaching of  chalcopyrite concentrate without ultrasound activation Table 1.6: Sample material balance  calculations for ferric sulfate  256 leaching of  chalcopyrite concentrate with ultrasound activation-  258  Table 1.7: Sample material balance calculations for ferric chloride leaching of chalcopyrite concentrate without ultrasound activation  260  Table 1.8: Sample material balance calculations for ferric chloride leaching of chalcopyrite concentrate with ultrasound activation  262  xi  LIST OF FIGURES  Fig. 2.1.1: Eh-pH diagram for the copper-iron-sulfur-water system at 298.15 K  4  Fig. 2.1.2: Eh-pH diagram for the copper-iron-chloride-sulfur-water system at 298.15 K  9  Fig. 4.2.1: The ultrasonic processor and autotitrator  45  Fig. 4.2.2: The ultrasound generator and converter/horn assembly  46  Fig. 4.2.3: The reaction assembly during testing  47  Fig. 4.2.4: Schematic representation of the experimental set-up  48  Fig. 4.2.5: The experimental set-up during testing  49  Fig. 5.1.1: Plot of conversion and KMn04 consumption vs. time for leaching pure chalcopyrite in sulfate media, with and without ultrasound activation  53  Fig. 5.1.2: Plot of conversion and KMn04 consumption vs. time for leaching pure chalcopyrite in chloride media with and without ultrasound activation  53  Fig. 5.1.3: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C, with and without ultrasound activation  54  Fig. 5.1.4: Chemical control model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C, with and without ultrasound activation  54  Fig. 5.1.5: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C, with and without ultrasound activation  55  Fig. 5.1.6: Chemical control model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C, with and without ultrasound activation  55  Fig. 5.1.7: Effect of sonication mode on pure chalcopyrite conversion (sulfate media)  59  Fig. 5.1.8: Effect of sonication mode on pure chalcopyrite conversion (chloride media)  59  Fig. 5.1.9: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C under different sonication modes  60  Fig. 5.1.10: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C under different sonication modes  60 xii  Fig. 5.1.11: Effect of horn depth of immersion on pure chalcopyrite conversion (sulfate media) Fig.  63  5.1.12: Effect of horn depth of immersion on pure chalcopyrite conversion (chloride media)  63  Fig. 5.1.13: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C under different depths of horn immersion  64  Fig. 5.1.14: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C under different depths of horn immersion  64  Fig. 5.1.15: Effect of horn assembly on pure chalcopyrite conversion (sulfate media)  67  Fig. 5.1.16: Effect of horn assembly on pure chalcopyrite conversion (chloride media)  67  Fig. 5.1.17: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C under different horn assemblies  68  Fig. 5.1.18: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C under different horn assemblies  68  Fig 5.2.1: Plot of conversion vs. time at various temperatures for leaching pure chalcopyrite in sulfate media, without ultrasound activation  78  Fig 5.2.2: Plot of conversion vs. time at various temperatures for leaching chalcopyrite concentrate in sulfate media, without ultrasound activation  78  Fig 5.2.3: Plot of conversion vs. time at various temperatures for leaching pure chalcopyrite in sulfate media, with ultrasound activation Fig  5.2.4:  Plot  of  conversion  vs.  time  at  79 various  temperatures  for  leaching  chalcopyrite concentrate in sulfate media, with ultrasound activation  79  Fig. 5.2.5: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at various temperatures, without ultrasound activation  80  Fig. 5.2.6: Product layer model fitting of the conversion data for chalcopyrite concentrate in sulfate media at various temperatures, without ultrasound activation  <  80  Fig. 5.2.7: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at various temperatures, with ultrasound activation  81  xiii  Fig. 5.2.8: Product layer model fitting of the conversion data for chalcopyrite concentrate in sulfate media at various temperatures, with ultrasound activation  81  Fig. 5.2.9: Plot of reaction rates vs. inverse of temperature (Arrhenius plot) for pure chalcopyrite in sulfate media  82  Fig. 5.2.10: Plot of reaction rates vs. inverse of temperature (Arrhenius plot) for chalcopyrite concentrate in sulfate media Fig. 5.2.11: Plot of In (k/T) vs. inverse of temperature for pure chalcopyrite in sulfate media ..  82 83  Fig. 5.2.12: Plot of In (k/T) vs. inverse of temperature for chalcopyrite concentrate in sulfate media  :  83  Fig 5.2.13: Plot of conversion vs. time at various temperatures for leaching pure chalcopyrite in chloride media, without ultrasound activation  84  Fig 5.2.14: Plot of conversion vs. time at various temperatures for leaching chalcopyrite concentrate in chloride media, without ultrasound activation  84  Fig 5.2.15: Plot of conversion vs. time at various temperatures for leaching pure chalcopyrite in chloride media, with ultrasound activation  85  Fig 5.2.16: Plot of conversion vs. time at various temperatures for leaching chalcopyrite concentrate in chloride media, with ultrasound activation  85  Fig. 5.2.17: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at various temperatures, without ultrasound activation  86  Fig. 5.2.18: Product layer model fitting of the conversion data for chalcopyrite concentrate in chloride media at various temperatures, without ultrasound activation  86  Fig. 5.2.19: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at various temperatures, with ultrasound activation  87  Fig. 5.2.20: Product layer model fitting of the conversion data for chalcopyrite concentrate in chloride media at various temperatures, with ultrasound activation  87  Fig. 5.2.21: Plot of reaction rates vs. inverse of temperature (Arrhenius plot) for pure chalcopyrite in chloride media  88  xiv  Fig. 5.2.22: Plot of reaction rates vs. inverse of temperature (Arrhenius plot) for chalcopyrite concentrate in chloride media  88  Fig. 5.2.23: Plot of In (k/T) vs. inverse of temperature for pure chalcopyrite in chloride media  89  Fig. 5.2.24: Plot of In (k/T) vs. inverse of temperature for chalcopyrite concentrate in chloride media  89  Fig. 5.3.1: Plot of conversion vs. time at various size fractions for leaching pure chalcopyrite in sulfate media, without ultrasound activation  102  Fig. 5.3.2: Plot of conversion vs. time at various size fractions for leaching chalcopyrite concentrate in sulfate media, without ultrasound activation  102  Fig. 5.3.3: Plot of conversion vs. time at various size fractions for leaching pure chalcopyrite in sulfate media, with ultrasound activation  103  Fig. 5.3.4: Plot of conversion vs. time at various size fractions for leaching chalcopyrite concentrate in sulfate media, with ultrasound activation  103  Fig. 5.3.5: Plot of conversion vs. time at various size fractions for leaching pure chalcopyrite in chloride media, without ultrasound activation Fig.  5.3.6:  Plot of conversion  vs. time  at  various  104 size  fractions  for  leaching  chalcopyrite concentrate in chloride media, without ultrasound activation  104  Fig. 5.3.7: Plot of conversion vs. time at various size fractions for leaching pure chalcopyrite in chloride media, with ultrasound activation  105  Fig. 5.3.8: Plot of conversion vs. time at various size fractions for leaching chalcopyrite concentrate in chloride media, with ultrasound activation  105  Fig. 5.3.9: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at various size fractions, without ultrasound activation  106  Fig. 5.3.10: Product layer model fitting of the conversion data for CuFeS2 concentrate in sulfate media at various size fractions, without ultrasound activation  106  Fig. 5.3.11: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at various size fractions, with ultrasound activation  107  xv  Fig. 5.3.12: Product layer model fitting of the conversion data for CuFeS2 concentrate in sulfate media at various size fractions, with ultrasound activation  107  Fig. 5.3.13: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at various size fractions, without ultrasound activation  108  Fig. 5.3.14: Product layer model fitting of the conversion data for CuFeS2 concentrate in chloride media at various size fractions, without ultrasound activation  108  Fig. 5.3.15: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at various size fractions, with ultrasound activation  109  Fig. 5.3.16: Product layer model fitting of the conversion data for CuFeS2 concentrate in chloride media at various size fractions, with ultrasound activation  109  Fig. 5.3.17: Plot of reaction rates vs. inverse square of CuFeS2 mean particle diameter (pure chalcopyrite, sulfate media)  110  Fig. 5.3.18: Plot of reaction rates vs. inverse square of CuFeS2 mean particle diameter (chalcopyrite concentrate, sulfate media)  110  Fig. 5.3.19: Plot of reaction rates vs. inverse square of CuFeS2 mean particle diameter (pure chalcopyrite, chloride media)  Ill  Fig. 5.3.20: Plot of reaction rates vs. inverse square of CuFeS2 mean particle diameter (chalcopyrite concentrate, chloride media)  Ill  Fig. 5.3.21: S E M images for fresh pure chalcopyrite. Size fraction is -149 um +74 um (-100 mesh+200 mesh)  112  Fig. 5.3.22: S E M images for chemically leached pure chalcopyrite in sulfate media. Size fraction is -149 um +74 jam (-100 mesh +200 mesh), leached at 75 °C  113  Fig. 5.3.23: S E M images for sonochemically leached pure chalcopyrite in sulfate media. Size fraction is -149 um +74 um (-100 mesh +200 mesh), leached at 75 °C  114  Fig. 5.3.24: S E M images for chemically leached pure chalcopyrite in chloride media. Size fraction is -149 um +74 um (-100 mesh +200 mesh), leached at 75 °C  115  Fig. 5.3.25: S E M images for sonochemically leached pure chalcopyrite in chloride media. Size fraction is -149 um +74 um (-100 mesh +200 mesh), leached at 75 °C  116 xvi  Fig. 5.3.26: S E M images for chemically leached chalcopyrite concentrate in sulfate media. Size fraction is -180 um +149 um (-80 mesh +100 mesh)  117  Fig. 5.3.27: S E M images for the same solids in Fig. 5.3.26, magnified 3000X  118  Fig. 5.3.28: S E M images for sonochemically leached chalcopyrite concentrate in sulfate media. Size fraction is -180 um +149 um (-80 mesh +100 mesh)  119  Fig. 5.3.29: S E M images for the same solids in Fig. 5.3.28, magnified 3000X  120  Fig. 5.3.30: S E M images for chemically leached chalcopyrite concentrate in chloride media. Size fraction is -180 um +149 um (-80 mesh +100 mesh)  121  Fig. 5.3.31: S E M images for the same particles in Fig. 5.3.30, magnified 3000X  122  Fig. 5.3.32: S E M images for sonochemically leached chalcopyrite concentrate in chloride media. Size fraction is -180 um +149 um (-80 mesh +100 mesh)  123  Fig. 5.3.33: S E M images for the same solids in Fig. 5.3.32, magnified 3000X  124  Fig. 5.3.34: S E M images of the floating particles/globules  125  Fig. 5.4.1: Plot of conversion vs. time at various ferric sulfate concentrations for leaching pure chalcopyrite, without ultrasound activation  134  Fig. 5.4.2: Plot of conversion vs. time at various ferric sulfate concentrations  for  leaching chalcopyrite concentrate, without ultrasound activation  134  Fig. 5.4.3: Plot of conversion vs. time at various ferric sulfate concentrations for leaching pure chalcopyrite, with ultrasound activation  135  Fig. 5.4.4: Plot of conversion vs. time at various ferric sulfate concentrations for leaching chalcopyrite concentrate, with ultrasound activation  135  Fig. 5.4.5: Plot of conversion vs. time at various ferric chloride concentrations for leaching pure chalcopyrite, without ultrasound activation  136  Fig. 5.4.6: Plot of conversion vs. time at various ferric chloride concentrations for leaching chalcopyrite concentrate, without ultrasound activation  136  Fig. 5.4.7: Plot of conversion vs. time at various ferric chloride concentrations for leaching pure chalcopyrite, with ultrasound activation  137  xvii  Fig. 5.4.8: Plot of conversion vs. time at various ferric chloride concentrations for leaching chalcopyrite concentrate, with ultrasound activation  137  Fig. 5.4.9: Product layer model fitting of the conversion data for pure chalcopyrite at various ferric sulfate concentrations, without ultrasound activation  138  Fig. 5.4.10: Product layer model fitting of the conversion data for chalcopyrite concentrate at various ferric sulfate concentrations, without ultrasound activation  138  Fig. 5.4.11: Product layer model fitting of the conversion data for pure chalcopyrite at various ferric sulfate concentrations, with ultrasound activation  139  Fig. 5.4.12: Product layer model fitting of the conversion data for chalcopyrite concentrate at various ferric sulfate concentrations, with ultrasound activation  139  Fig. 5.4.13: Product layer model fitting of the conversion data for pure chalcopyrite at various ferric chloride concentrations, without ultrasound activation  140  Fig. 5.4.14: Product layer model fitting of the conversion data for CuFeS2 concentrate at various ferric chloride concentrations, without ultrasound activation  140  Fig. 5.4.15: Product layer model fitting of the conversion data for pure chalcopyrite at various ferric chloride concentrations, with ultrasound activation  141  Fig. 5.4.16: Product layer model fitting of the conversion data for chalcopyrite concentrate at various ferric chloride concentrations, with ultrasound activation  141  Fig. 5.4.17: Plot of In k vs. In [Fe ] for leaching pure chalcopyrite in sulfate media with and 3+  without ultrasound activation  142  Fig. 5.4.18: Plot of In k vs. In [Fe ] for leaching chalcopyrite concentrate in sulfate media 3+  with and without ultrasound activation  142  Fig. 5.4.19: Plot of In k vs. In [Fe ] for leaching pure chalcopyrite in chloride media with 3+  and without ultrasound activation  143  Fig. 5.4.20: Plot of In k vs. In [Fe ] for leaching chalcopyrite concentrate in chloride media 3+  with and without ultrasound activation  143  Fig. 5.5.1: Plot of conversion vs. time at various ferrous sulfate concentrations for leaching pure chalcopyrite, without ultrasound activation  150  xviii  Fig. 5.5.2: Plot of conversion vs. time at various ferrous sulfate concentrations for leaching chalcopyrite concentrate, without ultrasound activation  150  Fig. 5.5.3: Plot of conversion vs. time at various ferrous sulfate concentrations for leaching pure chalcopyrite, with ultrasound activation  151  Fig. 5.5.4: Plot of conversion vs. time at various ferrous sulfate concentrations for leaching chalcopyrite concentrate, with ultrasound activation  151  Fig. 5.5.5: Plot of conversion vs. time at various ferrous chloride concentrations for leaching pure chalcopyrite, without ultrasound activation  152  Fig. 5.5.6: Plot of conversion vs. time at various ferrous chloride concentrations for leaching chalcopyrite concentrate, without ultrasound activation  152  Fig. 5.5.7: Plot of conversion vs. time at various ferrous chloride concentrations for leaching pure chalcopyrite, with ultrasound activation  153  Fig. 5.5.8: Plot of conversion vs. time at various ferrous chloride concentrations for leaching chalcopyrite concentrate, with ultrasound activation  153  Fig. 5.6.1: Plot of conversion vs. time at various chalcopyrite additions for leaching pure chalcopyrite in sulfate media, without ultrasound activation  157  Fig. 5.6.2: Plot of conversion vs. time at various chalcopyrite additions for leaching pure chalcopyrite in sulfate media, with ultrasound activation  157  Fig. 5.6.3: Plot of amount of copper extracted vs. time at various pure CuFeS2 additions for leaching in sulfate media, without ultrasound activation  162  Fig. 5.6.4: Plot of amount of copper extracted vs. time at various CuFeS2 concentrate additions for leaching in sulfate media, without ultrasound activation  162  Fig. 5.6.5: Plot of amount of copper extracted vs. time at various pure CuFeS2 additions for leaching in sulfate media, with ultrasound activation  163  Fig. 5.6.6: Plot of amount of copper extracted vs. time at various CuFeS2 concentrate additions for leaching in sulfate media, with ultrasound activation  163  Fig. 5.6.7: Plot of amount of copper extracted vs. time at various pure CuFeS2 additions for leaching in chloride media, without ultrasound activation  164  xix  Fig. 5.6.8: Plot of amount of copper extracted vs. time at various CuFeS2 concentrate additions for leaching in chloride media, without ultrasound activation  164  Fig. 5.6.9: Plot of amount of copper extracted vs. time at various pure CuFeS2 additions for leaching in chloride media, with ultrasound activation  165  Fig. 5.6.10: Plot of amount of copper extracted vs. time at various CuFeS2 concentrate additions for leaching in chloride media, with ultrasound activation  165  Fig. 5.7.1: Effect of solids content and solution volume (scale-up experiments) on pure chalcopyrite conversion (sulfate media) without ultrasound activation  171  Fig. 5.7.2: Effect of solids content and solution volume (scale-up experiments) on pure chalcopyrite conversion (sulfate media) with ultrasound activation  171  Fig. 5.7.3: Effect of solids content and solution volume (scale-up experiments) on pure chalcopyrite conversion (chloride media) without ultrasound activation  172  Fig. 5.7.4: Effect of solids content and solution volume (scale-up experiments) on pure chalcopyrite conversion (chloride media) with ultrasound activation  172  Fig. 5.7.5: Effect of solids content and solution volume (scale-up experiments) on CuFeS2 concentrate conversion (sulfate media) without ultrasound activation  173  Fig. 5.7.6: Effect of solids content and solution volume (scale-up experiments) on CuFeS2 concentrate conversion (sulfate media) with ultrasound activation  173  Fig. 5.7.7: Effect of solids content and solution volume (scale-up experiments) on CuFeS2 concentrate conversion (chloride media) without ultrasound activation  174  Fig. 5.7.8: Effect of solids content and solution volume (scale-up experiments) on CuFeS2 concentrate conversion (chloride media) with ultrasound activation  174  Fig. 5.8.1: S E M image of the size fraction -180 um +149 um magnified 500X after 1 hour of chemical leaching as per Table 5.8.1  177  Fig. 5.8.2: S E M image of the same chemically leached sample in Fig. 5.8.1 but after sonication, magnified 500X Fig. 5.8.3: S E M image of the same particle surface in Fig. 5.8.2 magnified 3000X  178 178  xx  Fig. 5.8.4: S E M image of the size fraction -180 um +149 um magnified 3000X as per Table 5.8.2  180  Fig. 5.8.5: S E M image of the size fraction -180 um +149 um magnified 3000X as per Table 5.8.3  '.  182  Fig. 5.8.6: S E M image of the size fraction -180 urn +149 um magnified 3000X as per Table 5.8.4  185  Fig. 5.8.7: S E M image of the size fraction -180 um +149 um magnified 3000X as per Table 5.8.5  187  Fig. 5.8.8: S E M image of the size fraction -180 um +149 urn magnified 3000X as per Table 5.8.6  190  Fig. 5.9.1: S E M images of the size fraction -180 um +149 um (-80 mesh +100 mesh) sonicated in decane as per Table 5.9.1  195  Fig. 5.9.2: S E M images of the size fraction -180 um +149 um (-80 mesh +100 mesh) sonicated in decane as per Table 5.9.2  198  Fig. 5.9.3: S E M images of the size fraction -180 um +149 um (-80 mesh +100 mesh) sonicated in decane as per Table 5.9.3  201  Fig. 5.9.4: S E M images of the size fraction -180 um +149 um (-80 mesh +100 mesh) sonicated in decane as per Table 5.9.4  204  Fig. 5.9.5: S E M images of the size fraction -180 um +149 um (-80 mesh +100 mesh) sonicated in decane as per Table 5.9.5  207  Fig. 5.10.1: Plot of conversion vs. time for leaching pure chalcopyrite in sulfate media as per Table 5.10.3  216  Fig. 5.10.2: Plot of conversion vs. time for leaching pure chalcopyrite in chloride media as per Table 5.10.3  216  Fig. 5.10.3: Plot of conversion vs. time for leaching chalcopyrite concentrate in sulfate media as per Table 5.10.4  217  Fig. 5.10.4: Plot of conversion vs. time for leaching chalcopyrite concentrate in chloride media as per Table 5.10.4  217 xxi  Fig. 5.10.5: S E M images of chalcopyrite concentrate (size fraction -180 um +149 um) leached in chloride media with grinding aid as per Table 5.10.4  218  Fig. 5.11.1: Plot of conversion vs. time for leaching pure chalcopyrite in sulfate media at 700 mV and 25 °C  228  Fig. 5.11.2: Plot of conversion vs. time for leaching pure chalcopyrite in chloride media at 700mVand25°C  228  Fig. 5.11.3: Plot of conversion vs. time for leaching pure chalcopyrite in sulfate media at 700 mV and 75 °C  229  Fig. 5.11.4: Plot of conversion vs. time for leaching pure chalcopyrite in chloride media at 700 mV and 75 °C  229  Fig. 5.11.5: S E M images of pure chalcopyrite (size fraction -44 mm +38 mm) leached at 700 mV as per Table 5.11.2  230  xxii  NOMENCLATURE The following nomenclature was used in this research: English letters: A  : Vibrational amplitude, m  A  p  : Cross sectional area of transducer or probe tip in Eq. 2.15, m  2  b  : Stoichiometric factor  c  : Ultrasonic velocity; the speed of propagation of an ultrasound wave. In liquids, C (sound velocity) is ~1500 m/s while in air it is ~340 m/s  CM  : Bulk fluid concentration, M or mol/L  C  P  : Specific heat of reaction mixture (under isobaric conditions), J/kg per K  C  V  : Specific heat of reaction mixture (under isochoric conditions), J/kg per K  do  : Initial particle diameter, m  D  : Diffusivity, m / s  D  e  : Effective diffusivity, m /s  E  a  : Activation energy, J per mol  2  f  : Frequency, Hz, and is related to angular frequency GO by f = 00/71  h  : Planck's constant, 6.625 X 10" J s  I  : Ultrasound or acoustic intensity, W/m  Idiss  : Dissipated acoustic intensity; the intensity of ultrasound at the surface of the ultrasonic  34  2  device; delivered acoustic intensity to the medium, W/m k  : Parabolic leaching rate constant, min" , as defined by Eq. II.8. The subscripts and 1  superscripts refer to the designated systems, ka  : Parabolic leaching rate constant that includes the effective diffusivity (D ), molar e  volume (V), and concentration term (CAJ), min" . kdis defined in Eq. II.7. 1  k  : Parabolic leaching rate constant that includes the particle radius, min" per m. It is 1  d  defined as k, = Ro  kf  : Mass transport coefficient, m per min, as defined by Eq. II. 11  ki  : Linear leaching rate constant, min' , as defined by Eq. II.3  ko  : Pre-exponential factor, min"  k  : Surface chemical reaction rate constant, min'  s  1  1  1  xxiii  k  : Linear leaching rate constant that includes the particle radius, min" per m. It is defined 1  s  as  = —R 0  k  sc  : Specific rate constant for the surface reaction that includes the surface roughness factor, min"  K  1  : Polytropic index (a thermodynamic value). K ranges between 1 and y (see definition below).  m  : Mass of reaction mixture (solution), kg  P  : Gas pressure in the bubble at its maximum size, N / m  Pdiss  : Dissipated power due to heating the liquid medium by ultrasound irradiation, W  P  : Acoustic pressure at the initiation of collapse or liquid pressure at transient collapse,  m  2  N/m . The subscript m refers to the propagation medium. Pmax  : Maximum pressure developed within a bubble at the moment of transient collapsing, N/m  P  0  Poo  2  : Hydrostatic or ambient pressure of the liquid, N / m  2  : Pressure in the liquid far from the bubble, N/m . In an acoustic field, P» is estimated 2  from Eq. 2.7 r  : Coefficient of determination (a dimensionless statistical value ranging between 0 and 1)  2  R  : Bubble radius, m, or ideal gas constant, 8.314 J/mol per K  Rmax  '• Maximum radius a bubble can reach just before transient collapsing, m  R in  : Minimum radius a bubble starts with (usually just greater than the nucleation radius), m  Ro  : Initial particle radius or bubble radius at equilibrium, m  R  : First-order derivative of the bubble radius with respect to time t (radial velocity), m/s  R  : Second-order derivative of the bubble radius with respect to time t (radial acceleration),  m  m/s  2  t  : Time, s  T  : Recorded leaching temperature or temperature of the liquid system or medium, K  T  0  : Ambient reaction temperature, K  T  m a x  : Maximum temperature developed within a bubble at the moment of transient collapsing, K  V  : Molar volume, m /mol  Xb  : Conversion with respect to chalcopyrite. Xb is a fraction ranging between 0 and 1. xxiv  Greek letters: a  : Arbitrary constant for reaction rate dependence on ferric ion concentration. The subscripts refer to the systems under investigation as per Table 5.4.7.  P  : Constant defined by Eq. 11.15, m"  y  C : Ratio of specific heats of the gas within the bubble, y = — -. For isothermal  1  £  conditions (such as bubble growth phase), y is unity, and Eqs. 2.7 and 2.8 can be solved. 5  : Fluid boundary diffusion layer thickness, m  AG  0  : Free energy of formation, J per mol  AH  0  : Enthalpy of activation, J per mol  AS T|  0  : Entropy of activation, J per mol per K : Shear viscosity, m /s 2  S  T)B  : Bulk viscosity, m /s  K  : Boltzmann's constant, 1.38 X 10" J K"  Ko  : Transmission coefficient (dimensionless), 0.5 < K < 1.0. Ko is taken 1 in this research  u  : Viscosity of the bulk liquid medium, m /s  p  : Particle or fluid (liquid) medium mass density, kg/m . For incompressible fluids, p is  23  1  0  2  3  constant. pe  : Particle molar density, mol/m  0  : Some energy, J  a  : Surface tension of bulk liquid medium, N/cm or kg/s  T  : Time for complete conversion, min  x  m  : Time for complete transient collapse, s  co  : Angular frequency, Hz  %  : Constant defined by Eq. II. 15, m/min per kg  XXV  ACKNOWLEDGEMENT I would like to present thankfulness and express sincere respect and appreciation to my supervisor, Prof. David Bruce Dreisinger. His assistance and valuable suggestions, together with thoughtful supervision and constructive discussion lead ultimately to the successful completion of this research. For me, Prof. Dreisinger was not only a great mentor and advisor, but also a very dear friend who provided every moral and financial support expected from courteous persons. Several times he was there for help and never let me down. Prof. Dreisinger is the only person who gave me a lifelong opportunity to realize my childhood dreams and complete graduate studies. I do not know what to add, but it is the duty of this author and everyone who reads this thesis or uses it to thank Prof. Dreisinger for all what he has offered. Please make supplications to our Lord to be always with Prof. Dreisinger, wishing him and his family a good health and prosperous life. God bless you Dave, your wife, Bonnie, and children, Caleb, Olivia, Amanda and Marlayna. Next, I would like to thank Dr. Philip Horwitz of P G Research Foundation (Chicago, IL, USA) for the financial support provided throughout this research. I am really grateful for sponsoring my work. Special thanks also for Dr. Kenneth Suslick of the University of Illinois at Urbana- Champaign (UIUC) for the technical assistance and training provided during my stay at his research laboratory. The fruitful discussions with him are really invaluable. Thanks for the examination committee for their interest in this work, and important recommendations and feedback.  The assistance provided from my coworkers  in the  Hydrometallurgy Group at U B C is highly appreciated. Very special thanks for Dr. Berend Wassink for his assistance in chemical analysis and training. I am very grateful to my dear friends Mr. Jameel Al-Sakaji (Abu Ayman) and Mr. Mohammad Adili (Abu Habeeb), who provided every required financial and moral support for this great achievement: getting my Ph. D. degree. The same also goes to a countless number of friends all over the world, especially those who sacrificed their souls for the way of Allah. This great work is dedicated to my beloved parents for their support and patience. M y mom and dad offered every needed love, warm feelings, supplications and sincere efforts to ensure the full success of this research. Without my parents blessings this work would never be completed, which I really appreciate. A great word of "thank you" goes to my beloved brothers and sisters, and their families as well. xxvi  M y great thanks go to my beloved father-in-law, Sheikh Khader Abdul-Aziz, and compassionate mother-in-law U m Mohammad (Aziza Abidrabbu). Their supplications and prayers to Allah were very essential in finishing this work. Thanks also for my sister, Aya, and brothers, Mohammad, Moath, Mosab and Abdul-Aziz for their love, warm feelings and moral support. Last, but not least, very special thanks to my wife, Eshraq Khader Abdul-Aziz, for her patience and understanding during the course of this research. Her supplications and precious assistance were very vital in getting my Ph.D. degree. I am totally indebted and forever grateful to you, Eshraq. "Thanks, honey"  xxvii  CHAPTER 1 INTRODUCTION  The hydrometallurgical treatment of metal sulfides has long been suggested as an alternative to common pyrometallurgical treatment methods (Peters (1992)). Pyrometallurgical processes are attractive due to rapid reaction kinetics, but require high capital costs and have inherent environmental concerns. Hydrometallurgical methods are more suited for treating complex and/or low-grade ores, but also have some limitations that must be mitigated before being considered viable or feasible alternatives, thus finding wide applications. Examples of such limitations include: 1. The co-dissolution of metal components, necessitating the need for subsequent separation techniques, such as solvent extraction-electrowinning (SX-EW), to obtain high-purity products. In this context, selectivity remains a crucial issue. 2. The yield of elemental sulfur is not quantitative. A considerable portion of sulfidic sulfur is converted to sulfate sulfur or other forms. 3. The need for extreme treatment conditions, in particular high temperatures and pressures, very fine grinding, concentrated reagents, etc., to realize acceptable leaching rates and yield. In additions, these extreme conditions impose the use of expensive materials of construction. 4. Stream recycling which is needed to reduce reagent and energy consumption, as well as conforming to stringent environmental regulations. Nonetheless, there have been some successful applications of hydrometallurgy, such as those of zinc pressure leaching, nickel and cobalt recovery, pressure oxidation for gold production and others. There is a lot of opportunity for further research and development toward other successful applications in base metal production, especially copper sulfide concentrate hydrometallurgy. Novel developments in separation science and technology also open the way for such goals. Examples include resin-in-pulp technology, and new electrolytic cell designs and electrode materials. Depending on the type of leaching reaction, copper leaching is divided to three categories: oxidative, non-oxidative and reductive leaching (Peters (1976)). Oxidative leaching is most commonly investigated and currently in application. This type of reaction leads to the oxidation of metal components of the concentrate in solution. Common oxidants are ferric ion 1  and oxygen. In the case of ferric ion, leaching can be in sulfate or chloride media, under acidic conditions, and the general reaction is written as (for chalcopyrite as an example): CuFeS ) + 4Fe aq) -> C u 3+  2(s  (  2 + ( a q )  + 5Fe  2+ (aq  ) + 2S  (Eq. 1.1)  (s)  Ferric ion leaching of chalcopyrite in sulfate media is known to have two intrinsic problems: particle passivation and slow leaching kinetics, leading to low copper recovery. These are caused by the formation of product layers on the solid particles once leaching commenced (Hackl et al. (1995)). The reaction products form through a complex mechanism, and can be either a defect structure of CuFeS2 itself (Cu Fe S , where x, y and z do not correspond to x  y  z  chalcopyrite) or adherent elemental sulfur. Leaching kinetics are temperature dependent. Leaching is carried out at temperatures near the boiling point of solution (-100 °C), requiring autoclaves for higher temperatures. Due to limitations in temperature, fast leaching kinetics are difficult to achieve. To overcome these problems, several alternatives have been proposed, but these have found little real success or commercial application. Some of these alternatives were not investigated systematically. Such alternatives include: very fine grinding of particles (< 5 jam) to accelerate the leaching rates, using organic solvents to dissolve sulfur layers, and using complex leaching media (halide and sulfate mainly) to avoid passivation (Havlik and Kammel (1996)). Another interesting option is the leaching under extreme or non-classical conditions, such as ultrasound activation. Ultrasound activation has been in use for some applications in organic, inorganic and material chemistry. It has certain specific effects on reaction rates and mechanism. There have been some suggestions for its use in extractive metallurgy, and it is interesting to investigate its applications to hydrometallurgy. This work will first give a literature survey that covers ferric ion leaching of chalcopyrite, ultrasound science and technology, and selected studies on the use of ultrasound in extractive metallurgy, leaching in particular. This is followed by an outline of objectives and experimental procedures used in this research toward achieving such objectives. The results from the experimental  work will  then  be  discussed  before  presenting  the  final  conclusions.  Recommendations for future research are also given.  2  CHAPTER 2 LITERATURE SURVEY 2.1 Copper Hydrometallurgy Oxidative leaching of copper minerals, especially chalcopyrite, has been under investigation in an attempt to reach a viable and cost-effective treatment method for large-scale copper production. There are several published studies in this regard that have addressed all issues related, such as leaching medium, leaching mechanism, mineral composition, process technology and others (Peters (1992)). So far, few of these studies have been commercialized, specifically those under autoclaving conditions. According to Fig. 2.1.1, the stability diagram for copper-iron-sulfur-water system, and depending on the presence of oxidizing/reducing agents, solution composition and other prevailing thermodynamic conditions, chalcopyrite can react anodically or cathodically in aqueous media. In the presence of oxidizing solutions, hence oxidative leaching, leaching reaction leads to the formation of elemental sulfur and/or the release of sulfate ion in solution, depending on the oxidizing potential and the selected pH. The general leaching reaction in this case can be written as: CuFeS s) + 40x -> C u 2(  2 + ( a q )  + Fe  2+ (aq)  + 2S + 40x" (s)  (aq)  (Eq. 2.1)  Here Ox represents an oxidant (such as ferric ion, Eq. 1.1). This reaction is the most commonly observed one during chalcopyrite leaching under laboratory conditions. The formation of elemental sulfur (rhombic) is due to the fact that sulfur is oxidized to sulfate with great difficulty (at least in acid solutions) by a very irreversible path. Under certain oxidizing conditions, a new solid phase might form in addition to soluble species. As indicated earlier, some researchers have speculated that such a solid phase is merely a defect structure of chalcopyrite (Hackl et al. (1995)) Ferric ion leaching of chalcopyrite has long been recommended as an amenable method for copper extraction. This is attributed to the chemical reactivity of Fe  as a good oxidizing  agent. However, ferric ion leaching requires certain preparation steps to be efficient and suitable for commercial applications. These include heating to high temperatures, very fine grinding  3  (smaller than 10 um), and others. The necessity for fine grinding is largely required for achieving acceptable rates.  Fig. 2.1.1: Eh-pH diagram for the copper-iron-sulfur-water system at 298.15 K . Species activities as indicated (Peters (1976)).  4  The slowness of reaction in a short period of time due to the formation of product layers remains a major problem in this system, because such layers are adherent and relatively impervious, making it difficult to achieve high copper extraction. Lixiviant regeneration rates (i.e. oxidation of ferrous ions to ferric ions) may also be inadequate for commercial use because of mass transfer limitations, i f pressure oxidation is used or slow oxidation kinetics in acidic media (Dreisinger and Peters (1989)). Ferric ion leaching of chalcopyrite proceeds through an electrochemical mechanism that comprises an anodic and cathodic half reaction: Anodic reaction: CuFeS ) -> 2(s  Cu aq) 2+  (  + Fe  2+ (aq)  + 2S + 4e" (s)  E° = +0.41 V (vs. SHE)  (Eq. 2.2)  E° = +0.77 V (vs. SHE)  (Eq. 2.3)  AE° = +0.36 V  (Eq. 2.4)  Cathodic reaction: Fe  3+ (aq)  +  e"  ->  Fe aq) 2+  (  Giving the net reaction: CuFeS ) + 4 F e 2(s  -> C u  3+ (aq)  2 + ( a q )  + 5Fe  2+ (aq)  + 2S  (s)  Advantages of Fe2(SC»4)3 leaching of chalcopyrite: 1. Leaching is performed at atmospheric pressure and relatively low temperature 2. Lower costs compared to other reagents (such as ammonia) 3. Less corrosive action by ferric sulfate 4. No introduction of foreign constituents (such as nitrate and chloride species), which implies the avoidance of complex separation processes, such as solvent extraction or ion exchange 5. Sulfidic sulfur may be recovered as elemental sulfur 6. Sulfur gases are abated Disadvantages of Fe (SC»4)3 leaching of chalcopyrite: 2  1. Slowness of reaction in a short period of time due to the formation of product layers 2. Lixiviant regeneration rates may be inadequate for commercial use 3. Ultrafine grinding is needed to realize acceptable reaction rates and copper recovery 4. Requires high temperatures (> 95 °C) Table 2.1.1: Advantages and disadvantages of ferric sulfate leaching of chalcopyrite 5  Researchers commonly select ferric sulfate leaching due to some inherent advantages. These advantages are shown in Table 2.1.1. The disadvantages of this leachant are also included for comparison, which explains the little success in commercialization. With ferric sulfate, particle passivation occurs in the early stages of leaching (Beckstead et al. (1976)), unless extremely fine grinding is used. Very fine grinding also leads to faster leaching kinetics (shorter leaching time compared to others), but is economically less feasible. On the other hand, ferric sulfate leaching is desired because of the possible savings in reagent regeneration (H2SO4) through electrowinning (if used) or other unit operations. Sulfuric acid (H2SO4)  is normally added to prevent the hydrolysis of iron species.  Dutrizac (1989) and Dutrizac et al. (1969) studied the dissolution of sintered discs of synthetic chalcopyrite between 50 and 94 °C in acidic ferric sulfate solutions. Stoichiometric yields of sulfur and ferrous ion were obtained as per Eq. 2.4. The reaction displayed parabolic leaching kinetics and the rate was approximately an order of magnitude greater than that observed for natural chalcopyrite. The experimental activation energy was found to be 71 ±3 kJ/mol. Below ferric ion concentrations of 0.01 M , the rate controlling step was attributed to ferric sulfate diffusion through a thickening sulfur layer formed on the surface of chalcopyrite, even though a reaction order of 2 was observed with respect to ferric sulfate concentration. At higher ferric sulfate concentrations, the rate was independent of the ferric ion concentration and attributed to the outward diffusion of ferrous ions through the sulfur layer. The authors also found the rate to be insensitive to changes in acid concentrations and disk rotation speed. The unusual ferric ion dependence was attributed to the strong formation of ferric sulfate complexes in solution. Munoz-Castillo et al. (1979) studied the acid ferric sulfate leaching of chalcopyrite using monosized particles and attritor ground concentrate in an intensely stirred reactor at ambient pressure. Their results indicated that although the initial stage of reaction appears to be controlled by an electrochemical surface reaction, it contributes little to the overall extent of reaction. It was found that the reaction is controlled by a transport process through the reaction product, which was identified as the transport of electrons through formed sulfur layer. The reaction order was dependent on the inverse square of the initial particle diameter, and 94-  ^4-  94-  independent of Fe , Fe , Cu , and H2SO4 additions. The apparent activation energy from the experimental results was estimated to be 83.7 kJ/mol, which was shown to be approximately the same as the activation energy for transport of electrons through the elemental sulfur layer (96 6  kJ/mol), calculated from both conductivity and electron mobility measurements (Wagner's theory). Beckstead et al. (1976) studied the effect of particle size in the acid ferric sulfate leaching of attritor-ground chalcopyrite concentrates. According to the authors, the experimental results suggest that initial CuFeS2 particle diameter is the only controllable variable, which has a significant effect on copper extraction. Different size production procedures were tested and attrition grinding was found to be most useful. 90% copper extractions were possible by leaching particles of 0.5 um size at 93 °C in 3 hours. Enhanced leaching was not attributed to the "activation" or retained strained energy, rather to the increase in surface area. The leaching data were analyzed and found to follow the product layer control model, but the apparent activation energy was somewhat high (83.68 kJ/mol). Jones and Peters (1976) studied the leaching of chalcopyrite with ferric sulfate and chloride solutions. These researchers found different leaching chemistry in studied systems, and a substantial amount of sulfur was oxidized to sulfate in the ferric sulfate system. The authors noted that increasing ferric ion concentration enhanced copper extraction in the range 0.01-0.1 M . Beyond this range, copper extraction decreased. Additions of ferrous sulfate retarded the reaction, although it is a reaction product. The mixed potential measurements, around 0.61 V , indicated significant polarization of both the anodic and cathodic portions of the net leaching reaction (Eqs. 2.2-2.4). This means that chalcopyrite dissolution in ferric sulfate system is under mixed control. The authors found that chalcopyrite leaching rate was independent of particle sizes below 100 mesh (smaller than 149 um) in 0.1 M Fe2(S04)3 solution at 90 °C. Later, Dutrizac (1989) attributed this unusual independence to the use of non-monosized particles. Lowe (1970) studied the dissolution of ground and sized natural chalcopyrite in acidified ferric sulfate solutions over the temperature range 32 to 50 °C. Linear kinetics were observed and the apparent activation energy was found to be 75.3 kJ/mol. The rate of dissolution was independent of variation in sulfuric acid concentration over the range 0.05 to 0.78 M . The rate was also insensitive to changes in ferric ion concentrations greater than 0.02 M . The author interpreted the results as being indicative of rate control by a surface reaction with surface saturation of ferric sulfate. Some published work suggested the use of a pretreatment step to be combined with ferric sulfate leaching. The purpose was to introduce changes in the crystal structure and/or  7  composition of chalcopyrite, rendering it more amenable for chemical attack. A n example of such pretreatment steps is mechanical pretreatment or similar activation techniques. Rice et al. (1990) investigated the effects of turbomilling parameters on the simultaneous grinding and ferric sulfate leaching of chalcopyrite. The authors were able to induce crystalline changes in chalcopyrite by a specially designed turbomill. The results show that leaching at 25 °C was not enhanced by turbomilling. Rather, it was possible to completely dissolve the copper at around 90 °C by leaching for about 2 hours. 80% copper extraction was achieved with 1 hour of leaching time. The leaching rate was found to increase with increasing the operating mill speed and solid pulp content due to scrubbing-attrition effect on chalcopyrite, exposing more fresh surfaces to the lixiviant. However, the associated energy requirements were greater than other proposed leaching methods, but within the same order of magnitude. The electrochemical nature of copper sulfide leaching is more observed upon leaching in chloride media. Since aqueous leaching involves this halide ion, it is worth incorporating it in the stability diagram (shown in Fig. 2.1.2). The introduction of chloride ions to the leach solution causes the formation of new compounds or complex ions. It is clear from this thermodynamic figure that the chloride ion does not significantly alter the stability regions of sulfide phases. Almost all the effects are in the regions where dissolved copper is found (the effect of complexation), or its relevant oxides are found (formation of new phases). That is, its action is mainly in the leach solution and to prevent the formation of oxides that would otherwise slow down the leaching reaction, and so, leaching in chloride media is expected to be more effective and faster than that in sulfate media. Ferric chloride has been suggested as an alternative to ferric sulfate, because of the strong chemical reactivity of chloride ions and the high solubility of chloride salts in aqueous media. The presence of chloride ions can mitigate some of the difficulties associated with sulfate medium, especially the severe passivation on leached particles. This is depicted as altering the surface morphology of leached particles. However, chloride or halide media may cause severe corrosion of process equipment, necessitating expensive materials of construction. In ferric chloride leaching of chalcopyrite, copper and iron dissolve to their respective chloride species. The reaction between ferric chloride and chalcopyrite is written as: CuFeS s) + 4FeCi3(aq) -> CuCl (aq) + 5 F e C l 2(  2  2(aq)  + 2S  (s)  (Eq. 2.5)  8  Here, hydrochloric acid (HC1) is added to prevent the undesired hydrolysis of iron species. Compared to ferric sulfate leaching, higher copper extraction can be obtained, while the reaction (Eq. 2.5) has lower activation energy.  PH  Fig. 2.1.2: Eh-pH diagram for the copper-iron-chloride-sulfur-water system at 298.15 K . Species have the same activities as in Fig. 2.1.1, and the chloride ion presents at 1 M activity (Peters (1976)). 9  Ferron et al. (1995) presented the C U P R E X metal extraction process for copper recovery from chalcopyrite. The process is based on leaching with excess FeCh to produce a CuCb-rich solution and elemental sulfur. Chalcopyrite was leached at 95 °C and atmospheric pressure. Dissolved copper was mainly in the divalent state. The leach solution was cooled and subjected to a solid-liquid separation step to recover elemental sulfur (yield was ~65%). Excess iron was removed as goethite in a pressure oxidation stage that simultaneously generated part of the lixiviant (FeCl3). The remainder of the lixiviant was regenerated by chlorine oxidation of ferrous chloride. The C U P R E X process produced high purity copper powders at high current efficiency and an estimated power consumption of 2.66 kWhr/kg copper. However, the process flowsheet is complex and requires specially designed electrowinning cells. Everett (1994) presented the INTEC copper process. This process is a modernization of of the original Dextec copper process developed in the early 1980's. The Dextec process (Everett (1981)) is based on the direct electrolytic conversion of copper sulfides to copper and elemental sulfur. The process ran at atmospheric pressure and temperatures of 80-90 °C. The process chemistry was quite complex and it took a long time to gain an understanding of the parameters affecting the reaction rates. One of the major difficulties was the complex design of the electrolysis cell, making the patented radial diaphragm cell difficult to operate, especially with the continuous release of chlorine gas. The operating costs of the process made it difficult to produce copper at competitive prices. The INTEC copper process continued the use of complex halide chemistry, but the unique patented formation of the halogen species BrCL." effectively kept the chlorine in solution at all times. The process consists of leaching copper concentrates in an NaCl-NaBr solution (four stages) at 80-85 °C, with air blowing to precipitate iron as a goethite-type compound, followed by a two-stage purification process to remove impurities and recover precious metal values. The next stage is copper reduction in the same diaphragm cell. The process has some novel features like purification without solvent extraction and electrowinning at higher current densities. INTEC claimed its process will become the generic "process of choice" once its technical viability and economic feasibility are proven. Schweitzer and Livingston (1982) presented the C L E A R (Copper Leach, Electrolysis and Regeneration) process. Cupric chloride is the principal lixiviant but ferric chloride was added to serve as an indicator of the leach circuit balance. This process of Duval Corporation comprises four steps: concentrate leaching in two stages using a mixed K C l - N a C l brine as the leaching 10  solution, rejection of soluble iron as potassium jarosite in a pressure oxidation stage, copper electrowinning from cuprous chloride brine, and oxidizing the depleted solution to cupric chloride followed by recycling to the leaching stage. Although a facility was built, technical problems associated with electrolysis caused the C L E A R operation to shutdown in 1982. The electrolyte overvoltage associated with high current densities was a drawback over sulfate media. In addition, the purity of produced copper was not adequate, necessitating an electrorefining stage. Peters et al. (1981) patented the UBC-Cominco process for copper recovery from sulfide concentrates using ferric chloride leach route. The process utilizes FeCl3 leaching followed by cementation with metallic copper to reduce cupric ions to cuprous ions, where cuprous chloride is obtained via crystallization. The residual liquor is cemented with iron to produce cement copper seeds and ferrous chloride solution. The ferrous solution is oxygen pressure oxidized to regenerate the lixiviant: FeCl3, whereas excess iron is precipitated as hematite: Fe2C«3. Hydrogen reduction is used to produce metallic copper from CuCl crystals, while sulfidic sulfur reports almost quantitatively as elemental sulfur in the residue. Cominco diverted from this complex route to use a sulfate-based route in collaboration with Sherritt (Dutrizac (1992)). Kruesi et al. (1973) presented the Cymet process of the Cyprus Metallurgical Corporation. This was a process for converting the concentrates of base metal sulfides to the corresponding metals and elemental sulfur. The process uses two stages of leaching in a mixed FeCl3-CuCl2-NaCl solution to produce cuprous chloride while iron is rejected as jarosite. Vacuum crystallization was used to recover copper as CuCl followed by hydrogen reduction in a fluidized bed electrochemical reactor. The last step is smelting the precipitate to produce copper wire. The main technical problems for this process were the generation of HC1 in the reactor, corrosion problems and high capital cost in the vacuum crystallization unit. A pilot plant was built but shutdown in 1982 for economic reasons. The National Institute for Metallurgy (NIM) of the Republic of South Africa presented another process for ferric chloride leaching of chalcopyrite. According to Paynter (1973), the concentrate has to be ground to P95 -400 mesh (-38 um) for copper extraction to exceed 90%. Copper extraction was also found to depend on concentrate preparation stages, such as wet or dry storage, pretreatment with HC1, etc. Copper extractions of 95-97% were possible with a fine ground concentrate leached at 95 °C for about 8 hours. The prolonged leaching time was necessary to ensure complete sulfide sulfur oxidation to elemental sulfur (yield was ~95%). 11  Haver and Wong (1971) studied the ferric chloride leaching of chalcopyrite. These researchers investigated the effects of particle size, temperature and ratio of ferric chloride to chalcopyrite on the dissolution rate. For particle sizes of P98 minus 325 mesh (smaller than 44 um), it was possible to extract 99.5% of the contained copper in 2 hours at the boiling point of the solution (106 °C). Parabolic leaching kinetics were reported, which was attributed to the mass transport through a progressively thickening sulfur layer formed on chalcopyrite particles. The authors found that at a mass ratio of FeCl3/CuFeS2 smaller than 2.7, virtually all the dissolved copper was in the cuprous state and the overall reaction was: CuFeS s) + 3FeCl (aq) -> C u C l 2(  3  ( a q )  + 4FeCl ) + 2S 2(aq  (s)  (Eq. 2.6)  Ermilov et al. (1969) studied the leaching of chalcopyrite in FeCi3 solutions in the temperature range 60-106 °C. The authors reported an activation energy of 50 kJ/mol. The reaction rate was reported to be directly dependent on initial ferric ion concentration (in the range 1-3 M ) . According to the authors, the sulfur formed did not interfere with the dissolution kinetics or reaction mechanism as if being continuously chiseled from the particle surface. Sullivan (1930) was amongst the earliest researchers who investigated the behavior of ferric chloride leaching of natural chalcopyrite. In contrast to the results with ferric sulfate, the author found that stronger solutions of F e were beneficial. Fine grinding and high temperatures 3+  also increased the rate of extraction, as with any non-catalytic solid-fluid reaction. A temperature of 85 °C was reported to give good extraction amounts. Activation as a pretreatment step was also recommended  for FeCl3 leaching of  chalcopyrite. Jolly and Numeier (1991) studied the leaching of a sulfidized chalcopyrite concentrate. These researchers reported that more than 90% of contained copper could be extracted with less than 20% iron (i.e.: selective dissolution). The leaching temperature was set to be  106 °C. The authors claimed that by sulfidizing, the concentrate is more amenable to  leaching due to changes induced in the crystal structure of chalcopyrite. Tables 2.1.2 and 2.1.3 summarize the findings of several researchers who studied ferric ion leaching of chalcopyrite. More information can be found in Peters (1992), Dutrizac (1992) and Dutrizac and MacDonald (1974). 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CD  CO  u  O  o  §••8 3CO O  <u T3  o  CO  o  co CD  •s -a  GO  CO  3 00  (N r<i - t  2.2 Ultrasound Science and Technology From Tables 2.1.2 and 2.1.3 it can be seen that the previously mentioned limitations on wide adoption of copper hydrometallurgy are significant. The findings of these researchers explain why there is little application of hydrometallurgy to copper sulfides as a feasible and proven technology. Therefore it is necessary to search for new techniques or methods that can eliminate these drawbacks and lead to successful applications. In this regard, there is a growing interest in a new area of research designated as hydrometallurgy under extreme or non-classical conditions, which is concerned with the use of unconventional techniques (Havlik and Kammel (1996)). Examples include: 1. Mechanical or mechanochemical treatment coupled with leaching. Such activation aims at ultra-fine grinding (turbomilling, attrition grinding, oscillating milling) to increase the exposure to the leachant. 2. Leaching with supercritical fluids 3. Ultra-high frequency (UHF) enhanced leaching 4. Gamma radiation enhanced leaching 5. Thermal pretreatment other than roasting followed by leaching (e.g. microwave heating) 6. Ultrasound enhanced leaching Ultrasound has long been suggested as an enhancing technique in chemistry and chemical process industries, due to its inherent characteristics. It can produce a permanent chemical and/or physical change in the medium where it propagates. In this regard, ultrasound can have important effects on several solid-fluid reactions, which include: acceleration of reaction kinetics and improving amounts extracted. Its use can have another objective, which is mitigating the extreme conditions required for certain systems. These are caused by the action of ultrasonic waves on the solid particles, which include: particle breakage upon impingement, enhancement of transport rates in the vicinity of particles and others. The mechanism through which ultrasound induces its effects (Suslick et al. (1997)) is the so-called cavitational chemistry, which is the formation and growth of microbubbles and their subsequent intense implosion (collapse) that generates extreme localized conditions. Such conditions include high temperature and pressure gradients, fast cooling rates and high-speed collisions, leading to considerable enhancement in reaction kinetics. Other accompanying  15  phenomena include microstreaming, microjets, radiation pressure, acoustic pressure and degassing. In essence, ultrasound has a catalytic-like effect on reactions. The use of ultrasound in chemistry and other areas is now an established science and technology (Thompson and Doraiswamy (1999), Mason and Cordemans (1996) and Suslick (1988)). The term sonochemistry describes the use of ultrasound to enhance or alter chemical reactions. Currently, sonochemistry is widely used in the synthesis of chemicals whose production is impossible or difficult by classical methods. Examples include the reduction of methoxy aminosilane and the preparation of amorphous iron (Thompson and Doraiswamy (1999)). It has also other attractive features such as very fast reaction kinetics, high yield and high quality products. Another important feature is selectivity toward desired products and suppression of undesired reaction pathways. Examples are well known in organic, inorganic and polymer chemistry (Luche (1998) and Mason (1996 and 1993)). Other areas that have established applications of ultrasound include: the synthesis of nano-particles (Suslick et al. (1996)), because ultrasound also allows the flexible control of product size and morphology, food technology, such as milk homogenization or emulsification, metal welding, non destructive testing (NDT) and others (Mason (1991 and 1990)). In extractive metallurgy there are also some investigations in the fields of flotation, filtration, solvent extraction, ion exchange, electrowinning and electrorefining (Pesic and Zhou (1992), Walker (1979) and Polukhin (1969)). Ultrasound has also been suggested for usage in mineral leaching, such as bauxite leaching, zinc recovery from electric arc furnace dust, and others (McGrath and Farrar (1998) and Barrera-Godinez et al. (1992)). In terms of process intensification, ultrasound usage in leaching should lead to the following desired features: 1. Reduction of required leaching time, or faster reaction kinetics 2. Selective leaching. Selectivity arises from the fact that different materials have different responses to ultrasound. 3. High product yields. Selectivity leads to improved or high purity products and fewer byproducts, which are highly desired for technical and economic reasons. 4. Leaching at milder conditions, in particular lower temperatures and pressures As mentioned earlier, there are some studies on the use of ultrasound in leaching, but nothing is published on its use in the leaching of copper sulfide minerals. Hence, it is necessary 16  to study the possibility of using ultrasound in copper sulfide leaching and search for the conditions or combination of conditions that make such a method viable and feasible, and subsequently find commercial applications. Optimistically, it can be said that the successful application of ultrasound in chemical synthesis opens the way for new innovations and developments in metal sulfide leaching. 2.2.1 Theory of Ultrasound Sound is our experience of the propagation of pressure waves through some physical elastic medium, which is usually air. Liquids such as water do propagate such waves but space or vacuum does not. Sound is the conversion of mechanical energy to wave energy in a medium, resulting in some sort of disturbance. Such vibrating pressure waves have certain wavelength or frequency. If this vibrational frequency is too fast, too high a frequency, it cannot be heard, and this is ultrasound. It is sound waves that have vibrations too fast to hear (Ensminger (1988)). The part of the sonic spectrum, which ranges from about 20 kHz to 10 M H z , is called the ultrasound range. The following table summarizes sonic spectrum classification:  Range  Frequency range  Human hearing  16 Hz - 18 kHz  Conventional  power 20 kHz - 100 kHz  Category  Uses  High power - Low frequency  Cleaning, leaching  ultrasound Sonochemistry  20 kHz - 2 M H z  High power - Low frequency  Synthesis  Diagnostic ultrasound  5 M H z - 10 M H z  Low power - High frequency  Fetal development  Table 2.2.1: Summary of sonic spectra  The ultrasound waves carry energy. The induced disturbance or interaction with the medium is the result of the transmission of acoustic energy. Due to the differences in dimensions, there is no coupling of the acoustic field with medium molecules. The transmission of ultrasound waves through the medium induces also vibrational motion of molecules, which alternately compress and expand the medium molecular structure, resulting in variation in their spacing. If the intensity of ultrasound is sufficient, over-expanding of the liquid occurs and intramolecular forces will be unable to hold the structure intact. A s a result, the structure is disturbed and cavities or cavitational bubbles are created. 17  Every wave has a negative and a positive cycle. The negative cycle of the pressure wave is the rarefaction cycle, where bubble expansion or growth occurs without the addition of matter. Growth is due to efficient absorption of acoustic energy. The positive cycle is the compression cycle during which the bubble becomes oversized and can no longer absorb energy. Therefore bubble collapse (implosion) occurs. Gas compression generates heat. Since the absorbed energy is concentrated over a tiny amount of matter, the implosion is intense and results in extreme localized conditions: high temperature (-5000 K ) and pressures (-100 MPa) result. In addition, large shear forces, release of Shockwaves and liquid jets are also formed. Cavitational bubbles need nucleation sites, which are entrained gas bubbles, interstitial spaces or stagnant liquid crevices. Sometimes, non-wetted solid particles or transient bubbles from previous cavitational effect are also suitable. In summary, cavitation effectively concentrates energy by transforming the low energy density of sound into high energy density of a collapsing (imploding) microbubble.  2.2.2 Bubble Dynamics This topic has long been under investigation by several investigators. Recently, Colussi et al. (1998) published a detailed analysis of bubble dynamics. There are two types of cavitation: stable and transient. Stable cavitation means that the bubbles oscillate around their equilibrium position over several rarefaction/compression cycles. This type generates bubbles and the socalled microstreaming, which is a form of micro-turbulence. On the other hand, transient cavitation describes those bubbles capable of growing over one or more acoustic cycles, and then collapsing violently. Part of the transmitted acoustic energy is also absorbed by the molecules, leading to localized heating and alteration of the physical properties of the medium, such as viscosity and surface tension. Cavitation is controlled by several factors such as: 1. Ultrasonic frequency 2. Acoustic intensity (power) 3. Solvent type and physicochemical properties 4. Presence of gas bubbles: type and physicochemical properties 5. Ambient or external temperature and pressure  18  There are three different theories about the formation mechanism of cavitation: The hotspot, the electrical and the plasma theory, but the first one is widely adopted (Suslick (1990)). The fundamental equations of cavitation can be found in any textbook on ultrasonics (see Ensminger (1988) for example) and a brief review is given here. Bubble growth rate is dependent on cavitational type. Stable cavitation growth rate can be described by the Rayleigh-Plesset equation:  RR+!R = ^ 2  2OUR.  P„+ — R AR  3Y  2a  4^R  (Eq.2.7)  0  While transient cavitation growth rate is given by:  R  R  +  T. I R  1 =-  2  \ 3y  R  (Eq. 2.8)  max  V R J  Other developed equations to describe the extreme conditions upon cavitational collapse are  To  Pmax  P  "P (y-i)  (Eq. 2.9)  B  P (Y-D  Y-l  (Eq. 2.10)  m  3(y-l)  R„  (Eq. 2.11)  P (Y-l). m  It is widely accepted that bubble growth (rarefaction cycle) and the initial phase of the compression cycle are under isothermal conditions, while the rest of collapsing phase is under adiabatic conditions. This is valid since bubble life is only few microseconds. Prasad Naidu et al. (1994) estimated the bubble life to be 3 [is. The required time for complete collapse, under such assumptions, can be estimated from the relation: x  m  * 0.915R  n  IpJ  1+  pj  (Eq. 2.12)  There are several studies on estimating bubble dynamics. The introduction of software packages has made this task straightforward, but it is beyond the scope of this research.  19  2.2.3 Acoustic Power There are different types of ultrasound devices, but all of them consist of a generator and a transducer. The generator is used to convert the electricity at feed-line frequency to electrical energy at the ultrasonic frequency, while the transducer converts this energy into ultrasonic vibratory energy (ultrasound waves). Ultrasonic energy is transmitted via a probe (horn) or a vibrating plate. The latter is only suitable for liquid-solid systems. Transducers are generally of two types: piezoelectric and magnetostrictive. The latter is more developed and can endure extreme conditions, especially high temperatures. It can be operated "dry" with no damage (more robust). Piezoelectric transducers are less expensive, smaller and lighter. They can age considerably: a reduced power output with continuous operation, and structure will be damaged i f operated "dry". Magnetostrictive transducers are made of an iron or nickel tip surrounded by a coil. Hence vibration is due to a magnetic field. Piezoelectric transducers are made of quartz or lead zirconium titanate crystals that oscillate upon passing an alternating current. The fundamental equation of sonic energy is _  27t f f4 2  ©=  (Y-I)KI  2  i-r| +rt + s  pc  [3  v r  C  p  / t  .  (Eq.2.l3)  y  B  J  and shows the direct dependence on ultrasound frequency and the physicochemical properties of the medium. So far, there is no reliable method that can measure exactly the portion of acoustic power used in cavitation or reaction intensification. There are several procedures (Thompson and Doraiswamy (1999)), which are in two categories: physical (by calorimetry) or chemical (using dosimeter techniques such as a terephthalate dosimeter or similar radical traps). Calorimetry is the widely used method, where the solution temperature is monitored with time of sonication. In this case, the power dissipation is given as: diss = m C  p  The term  \ dt) t=0  p  y .  ( E q  - 2  H )  represents the initial slope of the plot of temperature rise of reaction mixture  versus time of exposure to ultrasound irradiation. Unfortunately, part of acoustic power delivered 20  is consumed in other events, such as heating the liquid medium through which ultrasonic waves propagate. Hagenson and Doraiswamy (1998) modified the previous equation to account for such losses and could get better estimations. The relation that relates ultrasonic intensity to dissipated power is: Idiss  = y :  A  (Eq.2.15)  § i  P  Ultrasonic intensity will decay exponentially with distance from transducer tip. Acoustic intensity is also related to the frequency via: I =0.5pcA f 2  2  (Eq. 2.16)  2.2.4 Sonication Systems The sonication set-up is commonly on three modes: the ultrasonic cleaning bath, the cuphorn sonicator (indirect sonication) and the probe (horn) sonicator (direct sonication). A study by Ratoarinoro et al. (1992) has shown that the probe configuration is the most effective sonication method. Ultrasonic cleaning is widely used for the purpose of cleaning in surface metal finishing applications. This system is simple, easily accessible and relatively inexpensive. The amount of sound energy that can be obtained is dependent on the bath age and design, liquid (water) level, and location of the reaction sample. Low acoustic intensity is possible. However, such systems do not offer reproducibility. The same bath type and design need to be used in all of the experiments. In addition, it is less flexible. The reaction sample needs to be accurately positioned and in the same location every time. Suslick et al. (1981) were the first researchers to propose the cup-horn sonicator (indirect sonicator). It is much like the previous type, except sonication is done with an inverted horn rather than bottom-based flat plates. The system has better frequency control and easier thermostating, in addition to complete control of atmosphere above the sample. According to these investigators, only the solution height (water level) in the reaction vessel alters the sonochemical yield. Other factors, such as off-center position, tilt angle, height of reaction vessel from the horn and coolant height in the cup, all have negligible effects on the yield. The ultrasonic probe (horn) is most widely used in sonochemical research, where the horn is directly immersed in the reaction mixture. This direct sonicator, also proposed by Suslick et al. (1981), has several advantages, including delivering a broad range of acoustic intensity, 21  flexible operation and maintenance, reproducibility, operability, and others. The ultrasound frequency is usually fixed, but the instrument (ultrasonic processor) is continuously and automatically adjusting the power input to the sonic horn. This ensures reproducibility. The probe is too expensive and tip erosion can contaminate reaction mixture. However, the tip is less expensive and replaceable. More robust designs are available. Reaction sample can be varied, and pulsed sonication is possible. According to the authors, neither the shape of the reaction vessel nor the horn depth of immersion significantly affects sonochemical yield. Other cautions to be considered are non-uniform distribution of ultrasonic intensity (higher near the horn tip than in the surrounding mixture) and potential noise hazard. The main question upon deciding the use of ultrasound is: How to generate and deliver the acoustic power properly? Ultrasonic power is relatively expensive, and scaling-up of sonochemically promoted processes is not well established. In addition, sonochemical reactor engineering is not developed. Most of the proposed sonochemical reactors are only bench-scale reactors, although new designs are increasingly published (Thompson and Doraiswamy (1999)). As with any other activation technique, ultrasound has some limitation and disadvantages. B y performing fundamental studies and constructive dialogue between the manufacturer and enduser(s), many of these drawbacks can be mitigated. There have been some trials to model sonochemical reactors. Gandhi (1997) has reviewed sonochemical reaction engineering with regard to bubble dynamics and chemical kinetics. Among their various studies, Keil and Dahnke (1997) presented numerical analysis and calculation of pressure fields in several different sonochemical reactors. These researchers were capable of establishing a reference research on solving many of the complex acoustic equations. Prasad Naidu et al. (1994) utilized bubble dynamics equations and simple material balance calculations to model a batch sonochemical reactor and describe the conditions upon cavitational collapse. The authors provided a procedure to estimate the minimum nucleation radius of a cavitational bubble, number of collapsing bubbles, temperature and pressure upon collapsing. Also the procedure allows the estimation of reaction rate constants for the selected system (sonication of potassium iodide). The effect of different gas atmospheres was also studied and comparative results were obtained.  22  2.3 Ultrasound in Leaching Processes There have been numerous publications in recent years on various aspects of sonochemistry and the use of ultrasound in other applications. Previously impossible or nonamenable reactions are now easily accessed with ultrasound, especially for pharmaceutical and chemical synthesis. The mechanism of interaction of ultrasound with any material or system depends on the latter type (physicochemical properties), wave frequency and acoustic intensity (power). Power ultrasound has chemical and physical effects. Distinguishing these effects is important for understanding the physical chemistry of sonicated leaching systems. Chemical effects include the formation of highly reactive species (free radicals) and their direct interaction with the reagents. In homogeneous systems, this role is well established, but in heterogeneous systems it is not yet clear i f there are any possible interactions with solid substrates. It is speculated that these radicals can cause pitting and wearing away of solid surface, by reaction with surface constituents. Physical effects include several events such as microstreaming: the microscopic turbulence created in the vicinity of particles, rooted to the momentum transferred to the fluid, and Shockwaves that cause surface erosion (removal of fine particles or product layers), cracking, fragmentation and fissure promoting. In addition to highly localized temperatures and pressures, the intense implosion of cavitational bubbles provides shear forces capable of intensifying the previous effects. Suslick et al. (1997) have concluded that ultrasonic stirring leads to interparticle collision and a fusion-like phenomenon that alters substrate size and morphology. They also showed that asymmetrical cavitational collapse forms a liquid microjet (inrush) that is responsible for most morphological changes. Most leaching reactions are dependent on reactive mineral surface area; particle size, leaching medium and reaction temperature. Such reactions are less dependent on solution concentration of products and agitation, as long as stirring is sufficient to keep particles in suspension. O f course, concentrate composition and structure, as well as the nature of the reaction itself, have direct effects on leaching. For these facts, ultrasound is expected to play a desired role in promoting leaching reactions. Ultrasound is envisioned to have two major effects in solid/liquid systems: 1. Improved mass and heat transport rates 23  2. Improved active reaction surface area To these, an improvement in certain properties of the solids is added. There has been some claim in the literature (Thompson and Doraiswamy (1999)) that ultrasound leads to supersaturation conditions, but this has not yet been established. Chalcopyrite is known to have low solubility in water (Weast (1976)), and this activation method might prove vital for the hard-to-leach CuFeS2 and consequently the response to chemical attack. Although it is known that mass transport is improved by ultrasound, there is no published study on its effect on species diffusivity. In the open literature there are some studies on the use of ultrasound in leaching. Much of the published work is qualitative, instead of being a systematic analysis of the factors that affect leaching and their interaction with acoustic parameters. According to Narayana et al. (1997), and as will be shown later, sonochemical leaching has several benefits, such as: 1. Faster reaction kinetics: As mentioned earlier, rates of hydrometallurgical reactions are slow at ambient conditions. Ultrasound effects can offer faster kinetics due to the accompanied phenomena. The prementioned chemical and physical effects are so important in any solid/liquid reaction. Sonochemical activity occurs in three distinct zones: within the bubble itself, the bubble/medium interface, and the bulk medium. From this, solid/liquid/bubble interface dynamics play a key factor in improving leaching. 2. Improved mass transfer rates through the effect of acoustic streaming. This is directly related to the previous benefit and results from the effect of ultrasound on concentration gradients. It implies increased solid dissolution rates. As will be shown later, ultrasound can also reduce reagent consumption, when molecular diffusion is enhanced. 3. Leaching at milder conditions, in terms of lower temperature, pressure and reagent concentration. This also implies a possible change to the leaching mechanism or reaction pathways. 4. Selectivity. Although this is highly desired, but less established. Selectivity implies high yield or metal extraction, quality products and fewer byproducts, which is also related to the reaction pathways. 5. Ultrasound can be an important tool in controlling product size and morphology  24  6. Minimizing the need for additional activating sources. Ultrasound itself is an activation method, and accompanied effects will negate the need for other accelerating sources. 7. As evident from published work, ultrasound can be applied to every part of a hydrometallurgical process. Its effect can also negate the need for additional processing stages. 8. Depending on the effect of ultrasound, ultrasound can be applied in a variety of modes to the leaching process. It can be applied to batch, semi-continuous or continuous processes. Ultrasound use is so flexible that new designs include on-line or in-loop sonication for a small volume. Such a feature is important for the purpose of increasing the productivity and cost reduction. From these points, it can be said that ultrasound can improve leaching reactions through three effects: hydrodynamic, kinetic and molecular effects. Although the results from the published work are encouraging, scaling-up attempts face several difficulties. These difficulties can be summarized as follows: 1. The proper generation of acoustic power: The generation of power ultrasound is known to be expensive and inefficient. This is mainly related to the method of sound generation. The cost of power generated and delivered by sonication or "acoustic watt" is currently higher than that of conventional sources, but improvements in designs and efficient techniques are being attained by which such a cost is progressively reduced. Sometimes, pulses of ultrasound are sufficient to produce cavitation, negating the need for continuous sonication, which is also desired. In addition, extra associated costs can be decreased by the possibility of eliminating other processing stages, or reducing the plant size. 2. The efficient delivery of generated acoustic power: The other related problem is the efficiency of using acoustic power to induce desired sonochemical effects. A considerable part of generated power is lost due to different factors. These include: efficiency of the ultrasound generator, partial heating of the transducer tip, heat absorbed by the medium, viscous dissipation or loss due to acoustic energy attenuation in the medium, energy scattering due to the large number of cavitational bubbles, sonoluminescence phenomenon, and others. Of course, reactor shape and its auxiliary fittings have an effect through sound reflection, absorption and reemission. New transducer designs, 25  such as the electromagnetic acoustic transducer (EMAT), are capable of mitigating such losses. 3. Measurement of required acoustic power or that used in cavitation: So far, there is no reliable method to achieve this important parameter. Measuring the acoustic power is directly related to the operating cost. There have been several proposed techniques for this purpose. The conventional one is through calorimetry. Other methods include chemical dosimeters. It is noted that conflicting results are obtained between these techniques, due to differences in instrument design and operation as well as the experimental procedures. The measurement of acoustic power is also related to measuring and estimating the energy distribution in the sonochemical reactor, to decide upon the correct design, which so far has not been addressed thoroughly. 4. Attainment of high sound intensity throughout a substantial volume of the reaction medium: Propagation of sound field to distances of practical interest into the reaction medium is a necessity for successful commercialization. In much of the published work, it is mentioned that ultrasound field cannot reach "remote" fluid regions due to rapid attenuation of the field. To overcome this there are several options possible. One is to generate a large amount of acoustic energy to cause cavitational phenomena in every region, which means excessive expenditures. The other one is to use acoustic "lenses", where the sound field is focused or driven in several directions. This wave-guiding technique is possible but difficult to adopt as it leads to complex reactor design (geometry and process integration). 5. The inherent limitations of the generator itself and its transducers/horn assembly. However, there are novel horn/probe designs that are very promising. New alloy materials can avoid the restriction of low power input, required to avoid rapid tip erosion. 6. Distinguishing chemical effects from physical effects: There have been few attempts to measure and model the sonochemical activation of different reactions. In the case of leaching reactions, it is necessary to understand and distinguish the chemical and physical effects of ultrasound. If the effect is chemical, it implies the need for continuous or pulsating sonication. If the effect is physical, sonication may be required right before leaching. In certain situations, it can be done on several disseminated stages. In either case, it is necessary to establish the dependence of leaching kinetics on sonication, as explained below. This can be achieved only through fundamental studies. 26  A recent study by Contamine et al. (1994) confirmed the presence of both effects. Their discrimination was possible by studying the reduction of ferric cyanide in a sonoelectrochemical reactor. The measurement of mass transfer coefficient indicated that ultrasound intensity is high at the reactor center, directly above the emitting surface and decreases in the radial direction. In spite of these and other associated difficulties, there have been several novel designs, especially for the purpose of chemical synthesis and pharmaceutical production. Thompson and Doraiswamy (1999) have reviewed several reactors with regard to geometry, process chemistry and industrial mode. For leaching with ultrasound, one can compare and assess possible or new designs by making use of published work on heterogeneous reactions, or reactors designed for multiphase systems, under the influence of ultrasound. A brief review of ultrasound application in extractive metallurgy is given here. Table 2.2.2 contains concise information from several studies. More information can be found in the designated references under Bibliography. McGrath and Farrar (1998) proposed the use of ultrasound for processing bauxite. Bauxite leaching by the Bayer process is known to be capital and energy intensive. Substantial amounts of reagents are needed, while extreme conditions (~237 °C and 2.8 MPa) are required in the autoclave. A feasible and successful modification to Bayer process is to convert the bauxite ore to a hydrate form that can be digested at lower temperatures, pressures and reagent concentrations. In addition to these goals, improving the productivity of the process, i.e. yield and production rates, is attained straightforwardly. The key to these developments is to alter and control the rate of alumina precipitation, and having better mass and heat transfer (thermal efficiency). Thus, ultrasound can be an important tool for these two factors. Ultrasound through cavitational phenomenon is envisioned to affect various steps in the Bayer process. In a three-phase reactor it can improve solid/liquid mixing and enhance leaching kinetics. If ultrasound has some effect on the solubility of bauxite, it can be a key in improving the precipitation rate and large particle yield of alumina oxide trihydrate (A1 0 .3H 0). 2  3  2  McGrath and his coworker proposed a patented hypermixer: ResonantSonics, which causes strong cavitation in liquids and slurries. This device causes rotation of an eccentric drive assembly at a rotational frequency. The resulting resonant cycloidal vibration causes cavitation and significant acoustic streaming. Experiments were conducted at ambient pressure and various 27  combinations of temperature, caustic concentration and leaching time. Acoustic parameters were fixed, but not given. Sonic agitation consistently gave higher recovery of aluminum trihydroxide. Much of the iron content was converted to separable hematite iron, releasing alumina for simple digestion. Acoustic streaming was found to be important because of its propensity to cause large velocity gradients and shearing stresses. Both of these effects provide the mechanical forces to remove diffusion layers/gradients during leaching. Strong mixing increased interparticle collision, while the high shear forces promoted surface microcracking. Sarveswara Rao et al. (1997) studied the influence of ultrasound on the ammoniacal leaching of copper oxides. Copper oxide ores generally contain less copper than copper sulfide ores and more difficult to upgrade by flotation. Thus, copper extraction from oxides requires very selective reagents and preferably well ground particles. Sarveswara Rao and his coworkers studied leaching with sonication in a cup horn set-up. Frequency was 20 kHz with an acoustic intensity of 3 W/cm . In general, it was found that 2  ultrasound decreased the required leaching time and increased copper extraction. As with other leaching reactions, copper extraction rate and recovery increased with decreasing the particle size up to a size fraction of +270 mesh (~53 um). Beyond this size, leaching required sonication to further increase copper extraction rate. However, ultrasound did not affect final copper recovery at such size fractions. Ammonia consumption was reported to decrease with the application of ultrasound, attributed by Sarveswara Rao and his coworkers to improved molecular diffusion at the solid/liquid/bubble interface. Unfortunately, it was noticed that sonication effect decreases with increasing solid content. Reaction rates and copper recovery at - 1 % SPD (solid pulp density) were greater than those at ~10%. Even in the absence of ultrasound, the same ultimate extractions were obtained, indicating that such systems are difficult to scale-up. The effect of ultrasound was said to be physical, through microscopic agitation, which was further supported by the decrease in leaching rates upon increasing the temperature. It is not clear how Sarveswara Rao and his coworkers concluded this, but possibly they leached at temperatures where the efficiency of sonication is lower. The employed acoustic intensity means increased power consumption. However, the authors used pulsed sonication instead of continuous sonication, which proved to be sufficient. 28  Kar et al. (1996) used ultrasound in the bacteria-enhanced leaching of laterites. Leaching of laterites is known to be energy intensive and costly, because the nickel content is low. Thus, conventional methods have several processing units and extreme conditions (autoclaving) are used. Leaching in the presence of microorganisms can be an alternative, especially for such lowgrade ores. In this case, however, leaching efficiency is related to the microorganism growth rate, which is known to be slow. Further, leaching time is too long to be adopted for scaling-up. The use of ultrasound was suggested as a method to enhance the growth rate of the microorganisms, by ensuring effective transport of nutrients through micromixing, and reduce the leaching time. Kar and his team used the method of indirect sonication. The test flasks were put in an ultrasonic cleaning bath, running at 43 kHz and having an acoustic intensity of 1.5 W/cm . 2  Continuous. sonication was used for 30 minutes. Low intensity sonication led to the selective leaching of nickel from the lateritic ore. Nickel extraction of 95% was possible in 14 days compared to 20 days without sonication. Dissolved iron was negligible in the leach liquor, making the liquor suitable for direct processing, i.e. selective leaching. Ultrasound was found to decrease the solution pH, ensuring suitable conditions for microbial growth and avoiding biomass contamination with precipitated iron species. The effect of ultrasound was attributed to improved liquid movement in the vicinity of microorganisms, resulting in efficient early growth (nutrients transport). A reduction in the lag period was reported, with more efficient production of bacterium secretions (organic acids). Cavitational collapse resulted in liquid jetting, which was capable of particle erosion, fragmentation and activation, as well as disruption of interfacial boundary layer. These effects offered more reactive surface area for leaching. The mechanism of ultrasound selectivity was not addressed, but it seems that, under the experimental conditions, nickel is more responsive to sonication. Jin et al. (1995) studied the extraction of silver from silver-rich complex sulfide ores. These researchers selected ferric ion-thiourea (TU) leaching due to possible rapid dissolution kinetics. As indicated below, T U consumption in precious metal leaching is high. The leaching mechanism with this system is complex, but leads to the formation of elemental sulfur and a complex T U - A g complex. Jin and his team used direct sonication, at 33 kHz and acoustic intensity of 0.2 W/ml, throughout leaching. External agitation was provided, using a magnetic stirrer. Leaching continued for 2 hours at 60 °C. The results indicated that silver recovery could be doubled upon 29  sonication. Ultrasonic vibrations were found to cause a decrease in the diffusion boundary layer thickness on ore particle, which also accelerated the leaching rate. The authors also reported selective leaching of silver over other constituents. Parabolic leaching kinetics were established. The controlling step was not identified, but probably was the diffusion of ferric or ferrous ions through a TU-passivated sulfide particle. Apparently ultrasonic vibrations removed such passivating layers. Temperature effect was found to be minor and activation energy was estimated to be -10.5 kJ/mol. Parabolic kinetics were further established by confirming the dependence on the inverse square of particle radius. Jin and his team developed the following leaching model: l-3(l-X ) +2(l-X ) = 9.22xl0" R ; [ T U ] 2/3  b  7  2  171  b  [Fe ] expf ° ^t RT J V 3+  a94  1 0 4 9  This model was used to maximize silver recovery to -96%. In a study by Tarasova et al. (1993), burst or pulsed sonication was used in the chemical and biological leaching of nickel laterites. Sonochemical leaching enhanced nickel extraction but was less selective. Experimental conditions were different from those of Kar et al. (1996). Ultrasonic frequency of 18-29 kHz, acoustic intensity of 1-9 W/cm , and pulsating sonication of 2  30 or 120 seconds duration were used. Tarasova and his team reported that sonobioleaching enhanced nickel extraction, and was slightly more selective. In this case, increasing the sonication period resulted in an increase in nickel extraction, but the acoustic intensity has almost no significant effect on nickel extraction. The efficiency of sonobioleaching was also dependent on the microorganism used. For example, Penicillium strains were more efficient than Aspergillus strains. In leaching with both strains, ultrasound effect was attributed to increased particle porosity and strong mixing. Ultrasound was said to open pore structures (micro-fissures), disrupt weakly associated particles and provide dispersion, all of which are important in solid/fluid reactions. It is not clear why increasing the ultrasonic intensity did not improve nickel extraction with these physical effects. Likely, higher intensities may have affected the bacterial culture performance. Barrera-Godinez et al. (1992) studied the effect of ultrasound on acidified brine leaching of double kiln-treated electric arc furnace (EAF) dust. This material contained a considerable amount of leachable zinc, but also contained iron. Selective recovery of zinc is desired due to the large tonnage of dust from steel industry. Conventional leaching techniques lead to the 30  codissolution of zinc and iron, with low recovery. Ultrasound was suggested as a method for selective recovery of zinc. In addition, iron removal requires oxidizing conditions to ensure the formation of ferric ions and their subsequent precipitation at pH > 2.8, thus producing a concentrated pregnant leach solution (PLS) suitable for direct electrowinning (EW). Oxidizing conditions require sufficient mixing of the particles with the leaching medium, which ultrasound can provide. Barrera-Godinez and his coworkers used the method of indirect sonication to study the effect of ultrasound on zinc dissolution. It was noticed that ultrasound enhanced the dissolution of zinc and retarded that of iron. Zinc recovery was doubled (from 40 to 80%) upon sonication at 55 kHz and 265 W/m . Sonication with air bubbling also enhanced zinc recovery and iron 2  removal. Ultrasound was found to enhance the solid/liquid/gas mass transfer necessary for efficient iron removal. The effect of ultrasound was attributed to different actions, such as: superficial particle disintegration, removal of weakly attached fine particles, microstreaming and cavitation. Cavitation provided extreme thermodynamic conditions that were said to retard iron dissolution or allow its precipitation. Particle disintegration allowed pore penetration by the leachant. Srikanth et al. (1992) presented a qualitative study on the leaching of layer silicates by acoustic-wave stimulation (AWS). Such layers contain potassium, and the leaching or removal of potassium at mild conditions is an important tool in understanding  low-temperature  geochemistry and soil mineralogy. The leaching of potassium is accompanied by lattice changes that also constitute a major aspect in the weathering (upgrading) of mica. Srikanth and his team used the method of indirect sonication at 20 kHz and an acoustic power of 375 W. Ultrasound was found to increase potassium dissolution in dilute NaCl solutions by an order of magmtude. The effect of ultrasound was attributed to particle disaggregation and surface cleaning. A good feature of their results is that ultrasound becomes ineffective below certain particle size. For particles with sizes less than 40 um, little improvement was achieved, and this should be considered and compared with the common trend of kinetics improvement with decreasing particle sizes. It is speculated that ultrasound coupling is less efficient with fine particles, in that there is a minimum size for particles to be able to receive and interact with ultrasonic Shockwaves. Apparently, the interaction of ultrasound with microsize particles is less significant.  31  Steensma (1988) studied the use of ultrasound in thiourea and cyanide leaching of gold and silver ores. Thiourea (TU) leaching has distinctive problems, such as: excessive reagent consumption, particle passivation, and undesired early precipitation of leached metal. Cyanide leaching is known to be slow (several days). The idea was to see i f ultrasound could mitigate these difficulties or not. Experimental conditions were not given, but the instrument was a two-nickel plate transducer (similar to a condenser) between which the slurry flows and being sonicated. The confinement of the treated pulp to the sound field is more effective than an ultrasound-cleaning bath. The results for thiourea leaching show that significant reduction in T U consumption (from 20 to 6 lb/ton) can be realized to obtain the same gold extraction. Leaching time was also reduced, but the system was less selective, as other metals were also leached. The effect of ultrasound was attributed to solid surface cleaning, causing better exposure to the lixiviant, improved solid-liquid contact and localized heat generation. It was stated that ultrasound induced a considerable change in the slurry energy state, through localized high temperatures and pressures, in addition to promoting interparticle collisions. The results for cyanide leaching also reflect enhancement of gold and silver recovery with ultrasound (-96% compared to -70% without sonication) and reduction of leaching time (from several days to less than six hours). For the tested ore, these results were attributed to the removal of the so-called "preg-robbing" phenomenon, which is the re-adsorption of dissolved gold on the clayish particles. Ultrasound was said to induce surface cleaning and particle dispersion. To further reduce the leaching time, the author suggested the use of fine grinding, a bonding chemical or activated carbon in the presence of ultrasound. Fairbanks et al. (1985) presented some qualitative results on the recovery of precious metals with the aid of ultrasound. Experimental conditions were not reported, but direct sonication promoted  leaching by removal of reaction products  (depassivation).  The  deagglomeration and cleaning action of ultrasound broke the surface tension of particles with water, resulting in better wetting and leachant penetration, for example by capillary effect. The use of ultrasound was also reported in filtration and particle separation. Khavskii and Bershitskii (1969) reviewed the use of ultrasound in autoclave soda leaching of scheelite for tungsten production. There are different methods for tungsten production. The hydrometallurgical methods include autoclaving with soda ash (alkaline) or hydrochloric acid leaching. Acid leaching is very slow, because scheelite has low solubility in 32  HC1, and thus extreme conditions are needed. Also, the formation of passivating product films, such as tungstic acid films, slows the interaction of scheelite with HC1. Alkaline leaching is preferred due to slightly better solubility, although carbonate films are also formed. Alkaline leaching is done at 180-220 °C, 5-18 atm, 400% excess soda ash and for six hours. It was desired to mitigate these conditions and improve the autoclaving process, hence the use of ultrasound. Experimental conditions were not presented, but the results were encouraging. Under continuous sonication, leaching time was reduced to 90 minutes, instead of 180 minutes without sonication, while tungsten extraction was increased from 30 to 50%. Heating is normally provided by pressurized steam. Khavskii and his coworker did not report the amount of steam consumed but required steam pressure was reduced from 15 to 7 atm. No information were reported on leaching temperature or reagent consumption. Khavskii and his coworker speculated the effect of ultrasound to alter the equilibrium conditions of the heterogeneous reaction at the solid/liquid interface, in particular speeding-up the diffusion process through the boundary film, and surface cleaning/destruction of passivating films. Ultrasound was also capable of producing a dispersion of concentrate particles in the leaching media. This dispersion is important to avoid colloidal phases and provide improved surface area. The problem of passivating films also appears in some neutralization reactions. Khavskii et al. (1969) studied the limestone neutralization of sulfuric acid. Neutralization of  H2SO4  or  acidic effluents is commonly practiced in hydrometallurgy. Sometimes, excessive amounts of limestone are needed to ensure effective neutralization, which implies increased sludge production. The neutralization can be done in a variety of designs, which require intensive grinding and mixing, making these vessels less operable. Ultrasonic oscillation can destroy such passivating films and increase the efficiency of the process. The authors found that neutralization time can be reduced to half its value, using stoichiometric amounts of reagents. Once again, ultrasound was able to clean the solid particle surface by removing the insoluble gypsum film. Passivating films can also be in elemental form. The oxidative leaching of several base metal sulfides, including chalcopyrite, is slowed because of the formation of a product layer of elemental sulfur. Such a layer hinders further leaching and limits the efficiency of the process. There have been several proposals to mitigate this problem, but few have received wide application. Possible techniques to depassivate the sulfur layer include the use of sulfur organic solvents, surfactants and simultaneous crushing and leaching. A possibility is to use ultrasound 33  to remove sulfur. Through its prescribed mechanism, ultrasound can disrupt the formed sulfur films. There is no published work on this possibility of removing inorganic sulfur, but there are some studies on removing organic sulfur from its substrates such as coal. From the previous brief review, it can be seen that ultrasound has a significant role in most of solid/liquid reactions. It can have two effects: chemical and physical, and the magnitude of every effect is dependent on the system being studied. Also, it is apparent that ultrasound has advantages and limitations, in that it can improve leaching kinetics and product yield, but may have limited efficiency below certain particle size or above certain solid content or temperature. Contradictory results can be obtained, which are largely affected by the selection of sonication system, experimental conditions and studied concentrate/material. Reproducibility thus arises as a strong factor for reaching final conclusions, prior to detailed studies for scale-up. The use of ultrasound in copper sulfide leaching is aimed at four main objectives: 1. Elimination of particle passivation 2. Acceleration of leaching kinetics 3. Improvement of copper extraction and 4. Mitigating the extreme leaching conditions required Deciding on the use of ultrasound in leaching is dependent on several factors. In the case of chalcopyrite (CuFeS2), for example, it was shown that under oxidative conditions, leaching is slow, or extreme conditions are required. Copper recovery is low, even under extreme chemical leaching conditions, while codissolution of metal values is unavoidable. The use of ultrasound is aimed at accelerating such slow kinetics and increasing the amount of copper extracted. Also, the passivation of mineral particles is prominent in sulfate systems (Chapter 1). The application of ultrasound to non-passivating systems might also lead to better copper extraction in a shorter period of time. Under the current technical and economical aspects of metal production, it is desired to treat chalcopyrite in a manner that leads to the production of a solution suitable for metal recovery by solvent extraction/electrowinning (SX/EW), rejection of iron in a suitable form and recovery of sulfidic sulfur as elemental sulfur. It is hoped that ultrasound can be combined with chemical leaching in a way that leads to these desired features.  34  CD  c  C M  co  O co  in m  T3  co  Cd  C  o  >  a  3 O  co  3  CD  S3 • M M  3"  o  •s O  3 O  H°  CO  00 3  "1  16 3«  •2 £ M M  i—i  (N m  CD  OH  a  M  1/3 >.  o  co 3 co O 3 o CD  '3  CD w  M  o  N  f1.1t  - M  CD Cd  3  OH i  .  '•8  , 3  CD  CD  CQ  3 X  CD  CD  o .2 o. co  3 C  w o o  3  Q m W oo in  T3  co  N< " <  2  M  g  >  00  c s  s OH  CO  CD  &  CD  CD  O •—  — o x <J  C*-H O  ffi o  «  3  -<* ^ -a -a ^ o .3  CD  IB  M  >  1  O co  3  a  3 O  oo cs  3  .2  r]  CO  b "S3 !r! M  >  • H  MH  CD  ^  m  ^  3 o  o  S3 3 N  (N m  m o  ^  -2 "2M 3 co  N  3  M  cd  CD  3 T3 CD  cd O "  -M  -3  <u .2  —  00 cd  00  -=  o S3 CD co  l<2  OS OS  • M  cd  S3  "S ^ k. Os  S3 os  cd ^-v  •J  in  CD OS 3 Os  i CD co  X!  3  3  ^  m  C M  3 co CD  CD . 3 <n -O i n  rt  8  OH  r  m  N  •si  III  3 2 c2 S3 5H 1 ra 3 H CL, m OH 00 oo CD CJ  co SD  ICQ S —  3  rt  kn  M  C M  .2  A  CD CD m  2 2 .5» 8 C "o .2'C O fl JH »  3  o CO o  co  23  w  3  CD ca » S CO S CD OH CD CD O 3 OH X> ^ O 3  8S  o  3  ts J  CD CM  § JS  .2 3  ^  <r> > .5  to =3  s»  S3 CN  Cd  c -a  CD  cd  3  rrn  CD  M  3 CD co o X3 C M  0  Z S i2  N  so  a  -3 8  CO  ^  00  3 3-1  e-j fN  00  -a -a a - £ Cd  CD  CO  CO  3  CD  S  cd  I-  CD -3  o>  S3  o  H—*  M  00  '1 , cd  00  3  3  CD  3  M  S ° 1 2 S3 6 .2 ' >  3  oo  -o  CD  o  CD  >3  J33 ^ S  3  l  CO  - M  D » « 2 CD C u co .2 VC  J2  3 O  CO  n  IT)  in OS  T3 *OH  CO  OH  ,  rH  13  S  3  TJ- i n  2  I  x  'co  CO OH  CD 0 co  /II  3 _>  U  cd  "3  c  N  CD ' JZ -2 OH  "H  00 3  11  J3  «  •4—»  CD  OH  CO  O  cd  M  OH  CD i N Cd CD  S3 .3 CN OS  i =3 o OS CQ O  M  -O CD  o  - M  CD  3 oo CN CN CN CD  CHAPTER 3 OBJECTIVES  As can be seen from the previous sections, there are some difficulties associated with the wide application of ferric ion leaching of chalcopyrite for copper extraction. These are mainly caused by the passivation of chalcopyrite particles once leaching is taking place, especially in sulfate systems. The passivation is caused by the formation of reaction products, which can be elemental sulfur or thin layers of other defect structures of chalcopyrite. Besides, extreme leaching conditions are needed in the form of elevated temperatures and very fine grinding. The use of ultrasound is depicted to mitigate or overcome the associated difficulties with ferric ion leaching of chalcopyrite. It is apparent from the published literature that ultrasonic waves have various effects on reaction systems, which can be physical or chemical. The physical effects are largely favored in this context to remove or avoid particle passivation, opening the way for increased copper extraction or recovery. It is also envisaged that sonication can be a useful tool in avoiding extreme leaching conditions. This research is concerned with studying the sonochemical leaching of copper sulfides by ferric ions. That is the application of ultrasound to oxidatively dissolving copper sulfides in sulfate and chloride media. Because this topic is not addressed in the open literature, it was necessary to perform a fundamental study to understand the effects of ultrasound on the leaching reactions. This study covers two different sulfide samples: a pure chalcopyrite and chalcopyrite concentrate. The experimental work is extended to cover leaching with and without ultrasound activation, in both the sulfate and chloride media. The main objectives of this research are: 1)  Developing engineering data on the use of ultrasound in copper sulfide leaching  2)  Studying the effects of thermodynamic parameters on leaching kinetics (temperature mainly)  3)  Studying the effects of physical parameters (particle size and solid content) on the leaching process  4)  Studying the effects of chemical parameters (redox potential, lixiviant concentration and solution composition) on the leaching reactions 36  Quantifying and understanding the effects of sonication parameters on the system. These include: sonication time, power input, sonication mode, horn assembly, leaching procedure, grinding aid, organic solvents and others.  37  CHAPTER 4 EXPERIMENTAL PROCEDURES  4.1 MATERIALS Two different chalcopyrite materials were used in this research. The first is a chalcopyrite concentrate and the second is a massive chalcopyrite mineral sample. The chalcopyrite concentrate was obtained from Gibraltar Mine, McLeese Lake, B . C , Canada. The detailed chemical analysis of the concentrate is given in the following table:  Element Copper Iron Sulfur (total)  Mass, % 26.0 24.0 28.6  Sulfur, (as sulfide, S ")  26.4  Sulfur, (as sulfate, S0 ")  2.14  Sulfur, (as elemental, S°)  <0.01  2  2  4  Acid insoluble* Element Aluminum Barium Bismuth Calcium Cobalt Lead  Mass, % 0.0253 0.0011 0.0255 0.0266 0.0024 0.0232  10.7 Mass, % 0.0165 0.4002 0.2001 0.0024 0.0022 0.0121  Element Magnesium Molybdenum Phosphorous Sodium Tungsten Zinc  * As siliceous gangue Table 4.1.1: Detailed chemical analysis of the tested chalcopyrite concentrate This chemical analysis was obtained using ICP (Inductively Coupled Plasma) method. The mineralogical composition of the concentrate is given in the following table: Mineral Chalcopyrite (CuFeS2) Pyrite (FeS ) Chalcanthite (CuS0 .5H 0) Siliceous gangue and other refractory oxides 2  4  2  Content, mass % 63.8 15.3 9.8 11.0  Molecular weight 183.513 119.967 249.677 N/E*  * N / E : not estimated Table 4.1.2: The mineralogical composition of the tested chalcopyrite concentrate 38  This composition was obtained using X-ray diffraction (XRD) and verified by the Rietveld method (O'Connor et al. (1992)). The sample analyzed was ground to Pioo -325 mesh +400 mesh (-44 um +38 jam). That is the entire sample was finer than this size for X R D analysis. The chalcopyrite concentrate used in the experiments was prepared as follow: The bulk concentrate was split first by coning and quartering. Then a sample weighing around 35 kg was split again using a riffle to obtain a representative sample for the fundamental study. The obtained sample, around 2 kg, was first wet screened to remove either extremely fine particles, which could bias initial leaching data by exhibiting faster kinetics in early stages of leaching, or coarse particle, which could exhibit slower kinetics at later stages. Rinsing with acetone and/or ethanol followed this to allow fast drying. The remaining sample was then subjected to a careful dry screening into discrete size fractions to obtain four different size fractions to be used in the experiments. These size fractions were: -80 mesh +100 mesh (-180 urn +149 jam), -100 mesh +200 mesh (-149 um +74 um), -240 mesh +270 mesh (-63 um +53 um) and -325 mesh +400 mesh (-44 um +38 um). Narrow sized particle samples were used because they allow better representation of the kinetic data. That is, estimated reaction rates will be constant at the same particle size, and a linear plot is obtained for the rate at various solution conditions. The individual size fractions were also analyzed by X R D and other wet chemistry methods (see below), and the results were consistent. For experiments with the concentrate, all the calculations were based on a mineral containing 63.8% chalcopyrite, 26.0% copper and 24.0% iron, all by mass. A l l samples were kept in tightly sealed glass bottles under an inert atmosphere of nitrogen to limit any oxidation to mineral surface. The pure chalcopyrite was obtained from the Strathcona Mine-Copper Zone in Levack Township, Levack, Ontario, Canada. The detailed chemical analysis of the mineral is given in Table 4.1.3. This chemical analysis was also obtained using ICP (Inductively Coupled Plasma) method. The mineralogical composition of the concentrate is given in Table 4.1.4. Once again, the Rietveld method was used to verify this composition. The same previous procedure was followed to prepare narrow sized samples of pure chalcopyrite. For experiments with pure chalcopyrite, the calculations were based on a mineral containing 97.5% chalcopyrite, 33.8% copper and 30.3% iron, all by mass.  39  Element Copper Iron Sulfur (total)  Mass, % 33.8 30.3 34.5  Sulfur, (as sulfide, S ' )  34.5  2  Sulfur (as sulfate, S0 ")  <0.01  Sulfur (as elemental, S°)  <0.01  2  4  Acid insoluble* Element Aluminum Calcium Lead Magnesium Nickel  Mass, % 0.0283 0.0132 0.0467 0.0212 1.3001  Element Phosphorous Potassium Sodium Zinc  < 1.0 Mass, % 0.2401 0.0197 0.0204 0.3943  * As siliceous gangue Table 4.1.3: Detailed chemical analysis of the tested pure chalcopyrite  Mineral Chalcopyrite (CuFeS2) Pentlandite (Fe,Ni) S Siliceous gangue and other refractory oxides 5  9  Content, mass % 97.5 1.5 1.0  Molecular weight 183.513 172.255 N/E*  * N / E : not estimated  Table 4.1.4: The mineralogical composition of the pure chalcopyrite  40  4.2 METHODS AND CALCULATIONS A l l the chemical reagents were of analytical grade and were used as received. Deionized water (DIW) was used in the experiments, which were performed under atmospheric pressure. A l l the experiments were performed batchwise. Every experiment with sonication was matched with one without sonication (control) for the purpose of comparison. Sonication set-up was of direct immersion type. The source of ultrasound was through an ultrasonic processor type Vibra Cell VCX-600 (Fig. 4.2.1), obtained from Sonics and Materials, Inc, Newtown, CT, USA. The instrument is capable of generating ultrasound waves at constant frequency (20 kHz) with variable power input. Ultrasonic waves were generated through an ultrasonic converter with vibrating piezoelectric PZT (lead zirconate titanium) crystals. The instrument is also fitted with a built-in thermometer (integrated temperature controller), connected to a teflon-covered thermocouple. This was used for solution temperature measurement. It also allowed precluding solution overheating by terminating the ultrasonics i f the leach solution temperature exceeds the predetermined limit. The power being delivered to the system was digitally displayed by a built-in wattmeter. Power input to the reaction mixture was kept constant (60 W at 20 kHz, unless otherwise stated). Ultrasonic waves were introduced to the leach mixture via a titanium alloy horn (Fig. 4.2.2). Standard horn size was 0.5" diameter with threaded end. This end enabled the use of replaceable tips or extenders (5" length). Other horn sizes used were 0.75" and 1" diameter. Erosion of the horn was not detected in any of the experiments. Unless otherwise stated, the ultrasonic intensity used in this research was 0.4 W/ml or -48 W/cm . The instrument also allows controlling the 2  mode of sonication by setting the on/off cycle. As a safety precaution, ultrasound produces high levels of noise. It is advised for those intending to work with sonication systems to use appropriate hearing protection and noise abatement systems. The glassware used was especially designed for sonochemical studies and obtained from Ace Glass, Inc., Vineland, NJ, USA. The type of borosilicate glass vessels used allowed the solid particles to gather directly under the horn tip. The tapered body (Fig. 4.2.3) is also capable of minimizing solution losses through evaporation by allowing adequate condensation of vapors. Thus, the tapered four-neck flasks allowed better control and operation of the experiments compared to conventional glass beakers. 41  Fig. 4.2.4 is a schematic representation of the experimental set-up. The required leach solution (volume is 150 ml, with a mass ratio of Fe /CuFeS > 2.7, unless otherwise stated) was 3+  2  put in the tapered glass flask. Sufficient acid was added to the leach solution to prevent undesired precipitation of iron species. It serves also as a background matrix for the leach system. No real effect was noticed for acid concentration beyond these purposes. The vessel was fitted with an oxidation-reduction probe (ORP) of the type Ag/AgCl electrode made of epoxy and connected to an autotitrator instrument (Radiometer TTA 88 titrator, Radiometer Copenhagen, Denmark, Fig. 4.2.1). A l l the experiments were performed at constant redox potential determined by solution composition and leaching conditions, unless otherwise specified. Fig. 4.2.5 shows the experimental set-up during testing. Sonication caused the solution to heat up at a rate of ~1 °C per 3 minutes of sonication. To maintain isothermal conditions, the reaction vessel was immersed in a circulating heating oil bath fitted with a thermocouple. Thus leach solution temperature was monitored by the ultrasonic processor's built-in thermometer, heating bath thermometer and externally by a digital display or mercury thermometer. Temperature measurements were always consistent by these methods. The reaction mixture temperature was kept within ± 0.5 °C of the predetermined value. The leach solution was continuously purged with nitrogen for at least 20 minutes to expel any entrained oxygen or air bubbles. Nitrogen has a low solubility in water (0.23 g/ml at 25 °C, decreasing to 0.14 g/ml at 40 °C, Weast (1976)) and is not expected to interfere with the leaching systems. It has also a sufficient polytropic index (K-1.4 at 70 °C, Eq. 2.13, Weast (1976)), which makes this less reactive gas suitable for the systems studied. Nitrogen purging was also maintained during the course of reaction to provide additional mixing of the solids. Combined with vigorous mixing by ultrasonic vibrations, the solids were kept suspended in the leach solution. That is, a homogeneous or well-mixed slurry was maintained with continuous exposure to the sonic field. Under the experimental conditions used, the flow rate of nitrogen has no real effect on leaching other than that needed for aiding in the suspension of the leach mixture and purging. Both solution temperature and potential readings were allowed to stabilize for 10 minutes before adding the solids. Once the required leaching conditions were reached, the solid sample was added in one batch and the reaction progress was monitored by keeping a constant redox potential throughout the experiment period (2 hours). The redox potential was kept constant by proper titration against potassium permanganate solution (0.3 M ) . KMn04 is a strong oxidizing 42  agent. Its usage was also to regenerate the lixiviant. Because sufficient acid is added, precipitation of M n 0 is avoided. Table 4.2.1 summarizes the measured redox potentials used in 2  this research. The correlation to SHE (standard hydrogen electrode) was obtained from Bratsch (1989). The electrode used was always tested via buffer and ferric/ferrous solutions at standard conditions: 1 M F e  3+  and 1 M F e  2+  at room temperature. In this case, the measured redox  potential was between 495-505 mV (vs. Ag/AgCl). As soon as the solids were added, the ultrasound source was turned on and signs of the reaction were noticed (change of solution potential, mainly, and floating of some reaction products). The reaction was allowed to take place for two hours and its potential and permanganate consumption were recorded.  Solution composition: 0.25 M Fe (S0 ) , 0.25 M F e S 0 and 1 M H S 0 2  4  Redox potential at temperature of vs. A g / A g S 0 vs. SHE 2  3  4  25 °C 400 625  4  2  65 °C 440 640  4  75 °C 455 645  85 °C 465 655  Solution composition: 0.5 M FeCl , 0.5 M F e C l and 1 M HC1 3  Redox potential at temperature of vs. Ag/AgCl vs. SHE  2  25. °C  65 °C  75 °C  85 °C  410 635  465 665  475 665  480 670  Table 4.2.1: Measured redox potentials for the studied systems  Sampling was done by taking 1-ml liquid samples from the reaction mixture at 15-minute periods. Sampling was by a 2-um fritted-glass tube to prevent any solid entrainment. The 1 ml sample was then diluted to the required volume prior to analysis for dissolved copper by atomic absorption spectroscopy (AAS). Although codissolution of copper and iron occurred, analysis for solution iron was not done since copper analysis is sufficient to track the reaction extent (Eq. 2.4) and iron concentration in solution is too high for accurate results. In this case, several dilution steps will be needed, making iron analysis less reliable, since frequent diluting can multiply the incorporated experimental errors. Iron analysis was done for the leach residue only (see below). 43  At the conclusion of the experiment, the reaction mixture was vacuum filtered and the filtrate volume was measured. In most experiments its volume was around 150 ml. A sample of the filtrate was also taken and analyzed by A A S . Every experiment was repeated twice, and thrice in some cases, to ensure consistency and validate the results obtained. This was also required for data reproducibility. The leach residue was extensively rinsed with deionized water. A sample of the wash solution was also taken and analyzed by A A S . Any dissolved copper was included in mass balance calculations. The rinsed residue was dried in an oven kept at 50 °C under nitrogen atmosphere, to prevent any oxidation of sulfide sulfur. After drying, the residue was weighed and a sample was taken for analysis by the digestion procedure (Bennewith and Hackl (1998)). The leach residue was kept in a tightly sealed glass bottles under an atmosphere of nitrogen. This was required for S E M (scanning electron microscopy) and other surface analysis techniques. Detailed chemical analyses (mass balance calculations) were done to study and confirm the reaction stoichiometry. Certain selected leach residues were subjected to detailed chemical analysis. First, the residue was rinsed and vacuum filtered with organic solvents (alcohols and carbon disulfide) to test for sulfur formation. The resulting filtrate was carefully vaporized to quantify the formed sulfur. The remaining sample was analyzed by the digestion procedure (bromine-water and aqua regia digestion). The digestion solution was analyzed for remaining copper and iron in order to establish mass balance calculations and account for reaction stoichiometry. Different organic solvents were used because only orthorhombic and monoclinic sulfur are soluble in CS2, while amorphous sulfur is not. Sulfur was detected in elemental form and was largely orthorhombic. The only seeming source for sulfur is from chalcopyrite, because pyrite is known to be almost inert in ferric sulfate or ferric chloride solutions. The term conversion (Xb) in this work is with respect to chalcopyrite and refers to the amount of copper extracted divided by the original content of copper (as chalcopyrite) in fresh solids. It is apparent from the net leaching reaction (Eq. 2.4) that oxidative leaching of chalcopyrite by ferric ion can be described by copper oxidation. This is made easy since the only source for leach copper is chalcopyrite, after accounting for copper from other sources (chalcanthite for example). The amount of KMnC«4 consumed through titration can also be used to track the extent of reaction. As will be shown in the next sections, the consumption of the titrant follows closely the amount of copper released.  44  Fig. 4.2.2: The ultrasound generator and converter/horn assembly  Fig. 4.2.3: The reaction assembly during testing. 1: Reaction vessel, 2: Temperature probe of the ultrasonic processor, 3: Ultrasonic converter and horn assembly, 4: Electrically-driven external agitator, 5: Redox probe and 6: Heating bath (with mineral oil)  00  a  I a,  X  o X -a o  i  o  a  CD  cm  CD  VI CO  >  CD  Io o o CD  Xi  s CD CO CO  c3 O  X  -*-»  CD  CD  co  x "o o CD  1  2 o  CD  s c  CD  o,  00  X  CD CD  .S x; CD  a  •s  X  o c o  •4—»  c  CD  co  2  a, 3> cd  £  CD  XJ  CD  00 CN  00  CJ  S-,  O  co  "g s  CHAPTER 5 RESULTS AND DISCUSSION  5.1 PRELIMINARY SONOCHEMICAL LEACHING EXPERIMENTS 5.1.1 GENERAL LEACHING BEHAVIOR Some diagnostic experiments were performed as an initial attempt to understand the system behavior during ferric ion leaching, with and without ultrasound activation. These experiments were performed in sulfate and chloride media, and for both pure chalcopyrite and chalcopyrite concentrate. The electrochemical mechanism for this system was discussed earlier (Section 2.1). The net leaching reaction is written as: CuFeS ) + 4 F e  3+  2(s  (aq)  -> C u  2 + ( a q )  + 5Fe  2+ (aq)  + 2S  AE° = +0.36 V  (S)  (Eq. 5.1)  To maintain this redox potential in the system, the system was titrated against potassium permanganate solution (0.3 M ) . The permanganate ion is a strong oxidizing agent making it suitable to regenerate the ferric ion while not interfering in the leaching process. The electrochemical reaction for the permanganate ion is: Mn0 " aq) 4  (  + 811^0 + 5e" -> M n  2 + ( a q )  + 4H 0 i) 2  E° = +1.49 V (vs. SHE)  (  (Eq. 5.2)  Giving the net reaction: CuFeS ) + Mn0 " q) + 8 H 2(S  4  (a  + (aq)  -> C u  2+ (aq  ) + Fe  3+ (aq)  + Mn  2 + ( a q )  + 2S + 4H 0 i) (s)  2  (  AE° = +0.31V (Eq. 5.3) In other words, the consumption of the permanganate ion can be used to track the reaction extent. Sulfuric or hydrochloric acid was added to the leach solution, as a background matrix, to prevent undesired precipitation of iron species. This was used in all experiments. Table 5.1.1 summarizes the results for leaching pure chalcopyrite at 75 °C, in both sulfate and chloride solution, with and without ultrasound activation. Solution volume was kept constant. The amount of KMn04 consumed is also shown and the data are plotted in Figs. 5.1.1 and 5.1.2.  50  As can be seen from these figures, copper extraction proceeds fast in the first hour of reaction time, and then slows down remarkably. Because of this trend, leaching kinetics are only significant in the first half of reaction period, which was set to two hours. The consumption of the titrant also follows the released copper in solution, confirming the stoichiometry of the reaction written in Eq. 5.1 for chalcopyrite dissolution. The trend of the curves in these figures suggests that ultrasound induces its effects on the particles as soon as it is introduced in the system. Increasing the sonication or leaching time beyond the set values did not give any significant improvement in amount of copper extracted. The well-known shrinking core model (SCM, Appendix II) was used to fit the leaching data in Table 5.1.1. In this regard, the reaction is assumed to be topochemical moving toward the center of the reacting particle. The rate of advancement of the reaction boundary can be controlled by: the diffusion of species through mass transfer boundary layer, the diffusion of species through the product layer or the rate of chemical reaction at the reaction boundary. Several types of controlling mechanisms were tried and Figs. 5.1.3-5.1.6 show the results for fitting by the product layer and chemical control models. Clearly the controlling mechanism is diffusion through the product layer (Figs. 5.1.3 and 5.1.5), which was subsequently used to fit other leaching data. Later, upon using S E M and other surface analysis techniques, it was demonstrated that a product layer forming on the leached particles, in the form of elemental sulfur or a defect structure of some copper sulfide is responsible for this behavior (see Appendix I). This layer or new solid phase is depicted to be semi-porous in nature and its volume is assumed to be equal to the volume of chalcopyrite consumed. The inward or outward aqueous phase diffusion of one or more of reaction species through the product layer can be considered the rate controlling mechanism. Alternately, solidstate diffusion processes may also require further consideration as a rate-controlling step. From the graphs presented and estimated values of parabolic leaching constants, it can be seen that the initial rates of leaching in the presence of ultrasound are faster than those under control conditions, which supports the argument that ultrasound induces its effects on the particles as soon as it is introduced in the system. As a final note, the conversion in Table 5.1.1 and that shown in other tables was calculated from copper concentration in solution after accounting for copper dissolved from other sources (if any), as was outlined in Section 4.2.  51  CN ir>  CN  3  o  3 *-M  3 O 43  3 SO  o co  2 3  5  3 O  3  o  3  o CO  O  3  3*  o U  CD  T3  OS  43 O  0) CM  3 co CD  CD  -4—»  o OH  o  o  O  3  CD  3 O  OH  CD  X!  o  3  CD  o  s 3. 00 m  +  s  3.  ^1-  CD +-»  u  'B  s  •d  >»  3  M  co  eg  O  _o  co  OH  O  o o 3 3 - o3 u  3o  *-M CD 3  OH  O,  00 3  CD OH  3  o  H-»  OO  c3  -M  <M CD .  %-» O  u  3 o  1  1 3 c  o  s-.  3  f M  o o  3  g  CD  CD  OH  3 O  3 O  3  3  3  CD  oo 3 3 < 3 0o0  3  3  *H  IU OH  3 CD H  O0 CD PH 3  u  o  OH  o" OH  00  o  o  o oo  O 3 CD  '§  3 •c CD  OH  X  X! CD  CQ  CD  co  CD CM  O CD OH  u  Time, min Fig. 5.1.1: Plot of conversion and KMnCU consumption vs. time for leaching pure chalcopyrite in sulfate media, with and without ultrasound activation. The data are from Table 5.1.1.  Time, min  Fig. 5.1.2: Plot of conversion and KMn04 consumption vs. time for leaching pure chalcopyrite in chloride media with and without ultrasound activation. The data are from Table 5.1.1. 53  • No sonication  0.16  Product layer control  • Sonication  0.14 0.12 X  1  0.10  of  + i  m i  0.08 0.06  Sonication: k = 0.0023 min" r = 0.98 2  0.04  Control: k = 0.0009 min •l  0.02  r = 0.98 2  0.00 15  30  45  60  Time, min  Fig. 5.1.3: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C, with and without ultrasound activation. The data are from Table 5.1.1. 0.30 0.25  • No sonication  Chemical control  • Sonication  Control: k = 0.0026 min'  1  r = 0.92  15  30  45  60  Time, min  Fig. 5.1.4: Chemical control model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C, with and without ultrasound activation. The data are from Table 5.1.1. 54  0.30 • No sonication • Sonication  0.25 X +  Product layer control  0.20 0.15  l  k = 0.0042 min  ><  r = 0.99 2  0.10  -o k = 0.0020 min"  0.05  r = 0.98 2  0.00 15  30  45  60  Time, min  5.1.5: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C, with and without ultrasound activation. The data are from Table 5.1.1. 0.40 0.35 -  o N o sonication  Chemical control  • Sonication  0.30 0.25 -  CO JO  .X  . 0.20 -  1 1  k = 0.0061 min"  1  r = 0.90 2  0.15 -  Control:  0.10 0.05 -  k = 0.0040 min"  1  nU.UU on < ' 0  r - 0.90 2  * * *  1  15  30  1  i  45  60  Time, min 5.1.6: Chemical control model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C, with and without ultrasound activation. The data are from Table 5.1.1. 55  5.1.2 E F F E C T OF SONICATION PARAMETERS It was necessary to study and understand the interaction of some sonication parameters prior to deciding on which ones to use. This was necessary to maximize the performance of the ultrasonic processor used and acquire all necessary information in this regard. To do so, several experiments were performed under a variety of sonication parameters in conjunction with chemical leaching. The parameters that were investigated include: sonication mode, horn assembly and depth of immersion. The ultrasonic processor controls the sonication power input to the leach solution. The frequency of ultrasonic waves was kept constant (20 kHz), while the power input was continuously adjusted, depending on horn size and solution resistance, to maintain supplying the waves at such a frequency. The term "solution resistance" refers to the difficulty with which ultrasonic waves face upon propagation in solution. It is directly related to solution contents and viscosity. The latter is temperature dependent. If viscosity increases, solution resistance increases, hence power input. Solution viscosity increases with decreasing temperature. The solution contents (solids content) have relatively minor effect on power input. To maintain constant power input, the instrument "amplitude control" or % loading was adjusted to give the same power input (primarily at 50% amplitude control). The maximum measured power input the instrument could produce was 0.4 W/ml. For 150 ml solution, this ultrasonic intensity refers to a power input of 60 W. If a 0.5" diameter sonication horn was used, this power input refers to an ultrasonic intensity of ~48 W/cm . 2  Regardless of solution volume or horn used, ultrasonic intensity was kept constant throughout this research and at its maximum value (0.4 W/ml), unless otherwise stated. Several horn sizes were available and used as necessary, and solution volume was changed for scale-up experiments, as will be shown later. The ultrasonic processor generates waves at fixed frequency (20 kHz), which corresponds to a wavelength of 75 mm, considering a sound speed of 1500 m/s in water. The height of reaction vessel assembly is around 200 mm, which means that the cavitational effects are taking place in the whole sonic field generated.  56  I. Effect of Sonication Mode The mode of sonication is important upon studying the effect of sonication on leaching. Selection of the mode will affect the total power delivered to the system, or energy cost. Also, the method of delivering the sonication power to the leach solution is important in obtaining the best effects. For example, pulsating mode allows better and sufficient exposure of the particles to the sonic field, which implies that ultrasound effects are fairly taking place in conjunction with chemical leaching. The sonication power can be delivered by various modes: continuously or in pulses. Several sonication modes were tried by changing the on/off cycle of the instrument. Results for three different modes are summarized in Table 5.1.2. They are for continuous sonication and pulsating sonication with 10 s - 5 s and 5 s - 10 s cycles. Figs. 5.1.7 and 5.1.8 are graphical representations of the results. Best leaching results were obtained with the mode 10 s on - 5 s off (hence 10 s-5 s cycle), and were comparable with those for continuous mode. Thus, the resulting exposure to the ultrasound field was sufficient to induce particle damage and other effects. Consequently, continuous sonication is not necessary. On the other hand, the mode 5 s-10 s is much less efficient than the 10 s-5 s mode. The obtained improvement over leaching without sonication (control) is relatively low. For these reasons, all the experimental work in this research was conducted using the 10 s-5 s cycle. A sonication time of 60 minutes was found to be adequate in obtaining the maximum possible effects of ultrasound. Increasing the sonication time to 90, 120 and even 180 minutes did not lead to any changes, under the experimental conditions employed. The effects of ultrasound were found to take place in a short period of time (< 30 minutes of sonication). This will be further investigated in future sections.  57  co  00  3  O o 3 o ,g o '•2 o c o o  U  CN  OS CO  00  ©  ©  OS SO © CO i n © ©  © CO so © ©  so in SO CO ©  CO 00 © © so o OS CN SO CN © CO i n SO SO O; © © © © ©' ©'  00  so  m  ©  in  CN 00  ©  O  CO  a  I'll OH  ii  CO  1  in  U  o  CD  •  o o o o o  in in © m ©  12 o  ^-^  p  m © O  co o O o i o i o CO in ©  © CO s o © in OS © CN in © © ©  00 s o s o 00 CN r-SO i n OS CO in i n s o s o SO s o © © © © ©  fa  IO  m ©  CD  Ii  o  c  o  -a CO CD  CD  fe  CD  CD  w  fa  '5•3  2 '-3  CTH  *o © © g © *-H © o © O  OS © i n CO in CN CO © ©  OS 00 CO i n i n CN in © ©  CO so i n in in in © ©  CO SO in in © ©  3 CO CD  OH  O CD X CD  id  CD OH  O,  o o  a o  2  * '8  o CD  o  2  "g o  CD  CD  , ii  s  co  +  O o in  3 in  o 'o H-J  CZ2  "C  >v OH  O  OH OH  o o  co Os  00  op ICD i 3  C o o  o  H-»  a CD CD O, X  fa  CD  1  CD N  CD' OH  >  OH  3 CD  CD  3  fa  ij-  s  fa  CD  3  u  3  o,  CO  C o  O  Ofi  H^  ii  CO  o  'o CO  CD  •s  CD  (-1  © © © co © in © o  so OS in ©  o CD  13 OH S-H  a © SO c o © © in © ©  in oo ^tin ©  in oo in in ©  © oo SO in ©  CO Os SO in in © ©  CD  oc fl -3 o  os so CN CN ©  00  t> CN CO ©  Os o OS CO ©  OS CO in  ©  ©  Os CN r- © OS  ©  ©  00 00  ©  ©  a 3 OH  o  '3 o  H  c  ©  *3  l<8  S-l 3 ,  I  1  CD  c o o co  CO  O OH  3  O  CD  CD  O  c o  3 'is a o o  3.  s  l-io  co  '3  o o c o CO  IE2  Ji  c o CD  T3 O  3 3  O  CN *o © i n in H © SO g © i CN CO o © U ©' ©' © ©  so in so CO ©  os © Os CO ©  OS 00 © Os s o © ©  ©  ©  ©  '3 CD  '3 o co  C+H  O o SO \< s o  CD  ,  fa  '13  OH  O o a  CD  O  OH OH  o  , ii  to  o s-b  3 fa  e  cd  |T3  O CN  CD  co  ? "3 CD  CD  00  O  CD  oo OS oo CO SO OS SO CO s o CN © f—1 i n oo OS m m in in m © © © © © ©'  Ice  H-»  Ii  <H-H  c  © © OS CN ©  o in  H-»  H-»  co © HI" i n © © o © CO O © ii W in CN  CD  CO  CD  CO so OS CN ©  fa  o  '3 o  o  O © 3 © © CN "2 © 3 © o U  O  CO  o  CD ,> '•M  "3  _o  fa  e  CD  CO 3  ©  m  © in CO  © in so  © in os ©  © CN  o  c  o W CN  S  m  CD*"  s  s T3 CD CO  9/ CO  W  T—H  CD  0.70 n  Time, min  Fig. 5.1.7: Effect of sonication mode on pure chalcopyrite conversion (sulfate media). The experimental conditions are as per Table 5.1.2.  0.90 i  Time, min  Fig. 5.1.8: Effect of sonication mode on pure chalcopyrite conversion (chloride media). The experimental conditions are as per Table 5.1.2. 59  0.16 O Control, k=0.0009/min  0.14 a-  • 5 s-10 s, k=0.0014/min  0.12  A 10 s-5 s, k=0.0024/min  0.10  • Continuous, k=0.0024/min  0.08 3  0.06 0.04 0.02 0.00 15  30  45  60  Time, min  Fig. 5.1.9: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C under different sonication modes. The data are from Table 5.1.2.  0.30  O Control, k=0.0020/min • 5 s-10 s, k=0.0027/min  0.25  A 10 s-5 s, k=0.0042/min  I  0.20  r<">  0.15  . +  • Continuous, k=0.0044/min  0.10 0.05 0.00 0  15  30  45  60  Time, min  Fig. 5.1.10: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C under different sonication modes. The data are from Table 5.1.2. 60  II. Effect of Horn Depth of Immersion As can be deduced from the literature survey, the method of producing and delivering ultrasonic power to the system has a direct effect on the performance of ultrasound as a viable activation method. In particular, solid particle exposure to the vibrations of the probe tip. The more the particles are exposed to the emitting surface of the horn, the more likely their residence time in the sonic field increases, leading to more interparticle collision. This also implies longer exposure to the ultrasonic vibrations, which for the system being studied means promoting particle fragmentation and breakage, thus allowing more copper extraction. So, efficient exposure to the ultrasonic field is important and must be investigated. Some experiments were performed with different depths of immersion. That is, the horn tip was allowed to be at different distances from the bottom of the sonochemical vessel (Fig. 4.2.4) and the reaction progress was monitored. The results are shown in Table 5.1.3. Clearly the method of delivering the sonication power has a significant effect on reaction extent. Half-immersion refers to the position where the distance between the horn tip and reaction vessel bottom is 60 mm. Full immersion is for 30 mm. Changing the depth of immersion was done using a special glass slide adapter. With half-immersion position, signs of less favorable reaction were noticed, such as less intense agitation and slow consumption of the titrant. With full immersion, the solids were close enough to the horn tip to interact and respond to ultrasonic waves. Intense agitation was observed, while continuous and/or rapid consumption of the titrant was obtained. In turn, this resulted in more particle breakage, explaining the higher conversion compared to half-immersion position. The effect in sulfate media is more pronounced, due to the higher propensity toward passivation once particle breakage was induced. Particle passivation will be discussed later in detail. The published work on homogeneous systems showed that the depth of immersion has no effect on solution sonication (Suslick et al. (1981)). For heterogeneous systems, this argument seems to be incorrect, specifically for the system under investigation. The graphs in Figs. 5.1.9 and 5.1.10 show that the position of the horn tip is very important in having efficient exposure to the sonication field and thus benefiting from ultrasound activation. This is translated to better copper extraction and leaching kinetics. For these findings, all the experiments in this research were performed with full horn immersion.  61  CN  so so o,  -a. u N  2 -3 CD  in  CD  3  o  3  o  T3 CD CM  3 CO 3  _o "co M  CD  > 3 O  CD  OH  O U  3  43  o  i  OH  CD  3 O  in  'co  O  CD OH  OH CD  T3  M  CD  cd  GO CD  O  <M  o  O  CD  CD  s  CD  3.  •5 >>  en +  s  in  N  -4->  •c  OH CD  >»  o. o o  OO  3  43 CD  '3  o  43  3  43  C M  o  CD CD  -4-»  03  in  3  3  T3 s o o  1  3  CD  O  9> 3 O  CD  X  w  s CD  03 CD  fa 3  u  3  1  CD  OH  OH  3 O  3 OH  3  o  'ob 3 3o < 3 03 03  in  00  kH  3  o  CD  E-  1  o  CD MH  o W  3 O  CD OH  3  _o  1  CO  etf  OH  3 O  3  o  3  o  03  T3  CD co  0.70 i  c Q  0  15  30  45  60  75  90  105  120  Time, min Fig. 5.1.11: Effect of horn depth of immersion on pure chalcopyrite conversion (sulfate media). The experimental conditions are as per Table 5.1.3.  0.90 i  Time, min  Fig. 5.1.12: Effect of horn depth of immersion on pure chalcopyrite conversion (chloride media). The experimental conditions are as per Table 5.1.3.  63  O Control, k=0.0009/rnin  0  15  30  45  60  Time, min 5.1.13: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C under different depths of horn immersion. The data are from Table 5.1.3. 0.30 i ,  ,  Time, min  5.1.14: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C under different depths of horn immersion. The data are from Table 5.1.3. 64  III. Effect of Horn Assembly The previous experiments were performed with the following horn assembly: a 0.5" diameter titanium alloy horn fitted with a 5" length extender. The titanium alloy extender was used to ensure sufficient propagation of the ultrasonic waves in solution and proper exposure by the solids to the ultrasonic field. To validate this selection, two different horn assemblies were tested: the horn with the extender and the horn with replaceable tips. The replaceable tips are made from the same horn material with the same size. They are widely used in homogeneous systems and for the merit of comparison they were used in this research. Table 5.1.4 summarizes the results obtained with different horn assemblies. With replaceable tips, the horn tip was at 90 mm distance from the vessel bottom. Upon sonication, solution agitation was noticed with both assemblies, but was much more vigorous with the extender. Some solid particles tended to settle down when replaceable tips were used so purging rate of nitrogen was increased slightly to provide sufficient mixing. As can be seen in Figs. 5.1.11 and 5.1.12, the results with the extender are better than those with replaceable tips, because the solid particles were kept in the vicinity of the emitting surface of the horn. There is some improvement in the amount of copper extracted with replaceable tips over control experiments, but that improvement is small compared to the one obtained with the extender, whether the latter was at half or full immersion (Table 5.1.3). Seemingly, solution cavitation was stronger with the extender compared to that with the replaceable tips. This was remarked as vigorous solution agitation and bubble formation. Because of sufficient exposure to the sonic field, more copper extraction was obtained with the extender. For these findings, all the experiments in this research were performed with a 0.5" diameter horn/ 5" length extender assembly with a 0.4 W/ml ultrasonic intensity (at 50% amplitude control). Total sonication time was set to 60 minutes. Frequency was 20 kHz with a 10 s-5 s on/off cycle, unless otherwise specified. As a final note on the results of the effects of sonication parameters, copper and iron appear to have similar response to ultrasound. This was concluded because upon performing mass balance calculations (Appendix I), selective dissolution was not obtained, under the experimental conditions used, and amount dissolved of every metal was as per reaction stoichiometry. 65  so so SO  <n CO  &  H-»  CD  I  CD  I  OH  2 "-3 CD  a  CD  -3 CD  OtH 03 CD  PH.  CD  C+H  u  3 c/3  3 c o o  3  *3  CD  T3  I  3 CD  H-H  X CD -3  X CD |H  tb  CD  OH OH  3 CD  o o 3 o  _o 'co *H  CD  o CD CD  OH  o CD CD  •s  O CD  ca  OH  CO CD  !-  o  C O  +  CD  _>  CD CO CO  ca  OD  *e3 o  '5b  H-» CD  OH  cC  o  S3 3  W  3 O  £3  _o ca CD  OH  CD  CD  OH  CD  OH  X  w  CD  3  13 f  3 O  S s s c s CD  3 O  OH  o o "os  1  OH  O -3 <+H  OH  o  ID  3  O O  CD |H 3  3  CD  N  s oo  M "o,  Pi  CD  CD  CD  EH  CD  CD  3 O H-J  'ob <  3 O  o  "3 o  03  00  o CD  »H  SI o OH  3 O O  "S o 03  CD  T3 CD CO  0.70  -i  Time, min  Fig. 5.1.15: Effect of horn assembly on pure chalcopyrite conversion (sulfate media). The experimental conditions are as per Table 5.1.4.  0.90  0  15  30  45  60  75  90  105  120  Time, min  Fig. 5.1.16: Effect of horn assembly on pure chalcopyrite conversion (chloride media). The experimental conditions are as per Table 5.1.4. 67  0.16 O Control, k=0.0009/min  0.14  • Replaceable tips, k=0.0009/min  0.12 CN  +  A Extender, k=0.0023/min  0.10 0.08 0.06 0.04 0.02 0.00 15  30  60  45  Time, min  5.1.17: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at 75 °C under different horn assemblies. The data are from Table 5.1.4.  0.30  • Control k=0.0020/min  0.25  • Replaceable tips, k=0.0021/min A Extender, k=0.0042/min  0.20 0.15 0.10 I  0.05 0.00 0  15  30  45  60  Time, min  5.1.18: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at 75 °C under different horn assemblies. The data are from Table 5.1.4. 68  5.2 Temperature Dependence It is known that ferric ion leaching of chalcopyrite is temperature dependent, especially in sulfate systems. As evident from the previous literature survey, several authors concluded that copper extraction and leaching kinetics are enhanced with increasing the leaching temperature. Therefore, it is necessary to analyze and understand the temperature dependence of leaching kinetics for the current systems, especially with ultrasound activation. To do so, a series of experiments was performed in the temperature range 25-85 °C. Several particle sizes have been tried for consistency and validity. Because experiments were also performed without sonication, it was really difficult to elucidate the temperature dependence of leaching kinetics for coarse particles (larger than 74 um). The experimental results for pure chalcopyrite and chalcopyrite concentrate in sulfate systems are summarized in Tables 5.2.1 and Table 5.2.2, respectively. During the experiments, it was noticed that below 65 °C no significant amounts of copper could be extracted, neither the leaching kinetics could be established properly. That is, a threshold temperature of 65 °C was found, below which temperature effects could not be effectively studied. Figs. 5.2.1-5.2.4 are graphical representation of the leaching data. These graphs show a gradual improvement in leaching kinetics and amount of copper extracted upon increasing the solution temperature. Distinctive leaching data are obtained at every temperature tested. It is also noticed that the improvement in leaching kinetics and amount of copper extracted upon increasing the temperature from 75 to 85 °C is not large. Because of the trends in Figs. 5.2.15.2.4, most of the experiments in this fundamental study were performed at 75 °C. It is also noted that the reaction proceeds well in the first half of total period, then slows down or reaches a plateau. The significance of the first 60 minutes of reaction time is clear, and so leaching data in this period were only considered. The parabolic leaching model was used to fit the leaching data in the previous graphs. Figs. 5.2.5-5.2.8 summarize the results of the fitting, and Table 5.2.5 shows the estimated parabolic leaching rate constants for the studied solids. The estimated thermodynamic values in Table 5.2.5 were derived from Arrhenius plots and those according to the transition state or absolute rate theory (Figs. 5.2.9-5.2.12). As can be seen, the use of ultrasound has a significant effect not only on reaction kinetics, but also on amount of copper extracted. Experiments with sonication lead to at least twice faster 69  leaching kinetics at the same temperature compared to control experiments, especially upon increasing the temperature to 75 °C. Around 50% improvement in amount of copper extracted was obtained, which can be reached in one hour with less than 40 minutes of sonication. Half of that amount of copper extracted can be reached with ultrasound in around 15 minutes. The leaching data in Tables 5.2.1 and 5.2.2 show that the use of ultrasound with leaching at temperatures greater than 75 °C did not improve the amount of copper extracted. That is, the efficiency of ultrasound as an activation method is less utilized with increasing the temperature beyond this value. This is explained by the loss of the intensity of cavitational implosion caused by the entrainment of excess amount of water vapor in the cavitational microbubbles. This excess vapor can cushion the implosion of bubbles, leading to less implosive intensity in the vicinity of leached particle surface and thus weaker action on the solids. Therefore, ferric sulfate leaching with ultrasound activation is best utilized at 75 °C. This is a significant reduction in required leaching temperature compared to those published in the open literature. As presented in Table 2.1.1, the suggested leaching temperature was near the boiling point of the leach solution (-106 °C). On the practical side this might imply that the use of ultrasound can lead to savings in heating requirements for process streams, which is desired. The efficiency of ultrasound as an activation method is also apparent upon comparing the results of chemical leaching to those with sonication. The results showed that leaching with sonication at 65 °C is equivalent or better than pure chemical leaching at 85 °C. This is also a reasonable saving in required heating for the system and implies a competitive option against other leaching techniques. The efficiency and possible heating savings are even better at 75 °C. The previous leaching experiments were repeated in chloride media under the same sonication and other experimental conditions. Tables 5.2.3 and 5.2.4 summarize the leaching data obtained from these experiments and the previous fitting was repeated. The results are shown in Figs. 5.2.13-5.2.24. Clearly, leaching in ferric chloride is more efficient than that in ferric sulfate. For example, pure chemical leaching at 75 °C gave around 50% more copper extraction than that in sulfate media for pure chalcopyrite. The reaction rates are also enhanced upon using chloride solutions as shown in Tables 5.2.5 and 5.2.6. With ultrasound activation, both reaction rates and amount of copper extracted are improved, but that improvement is more apparent in sulfate systems. This is attributed to the activity of the chloride ion, which might share directly in the 70  leaching process through the formation of active complexes (Section 2.1), compensating for the need to increase the leach temperature. The other apparent reason is that particle passivation by thin product layers is less severe in chloride media. This is caused by the activity or higher ionic strength of the chloride ions, which might also share in the leaching process by formation of complex ions. This might also imply the existence of different reaction paths. The apparent activation energy for the studied systems is in conformance with those controlled by diffusion through product layer. It is also comparable to those published in the open literature, although some discrepancies arise, possibly due to the experimental methods and conditions used. Because of the activity of the chloride ions, it is not unusual to find the activation energy in chloride media smaller than that in sulfate media. The use of the transition state theory also confirms these findings. The estimated value of entropy of change is smaller than zero. There is no apparent thermodynamic reason for this finding but it can be attributed to the extreme solution conditions created upon sonication. For those without sonication, this can be attributed to some sort of physical adsorption on the particle surface by certain complexes. Recalling that sufficient ferric ions are added to the leach solution, these ions can form different complexes, which can be adsorbed on chalcopyrite particles. This is more likely to be in chloride media. Similar reasoning was given for systems leached by complex ions or for leaching complex concentrates. More information can be found in Peters (1992) and Karagolge et al. (1992). The use of ultrasound in leaching did not alter the value of the activation energy significantly. This was expected and indicates that the effect of ultrasound is physical rather than chemical. The apparent decrease in activation energy is intelligible by the fact that leaching kinetics are largely accelerated with sonication. As apparent from the leaching graphs, 90% of amount of copper extracted can be obtained in less than 30 minutes of sonication. The estimated valued of activation energy show that chalcopyrite response to ferric ion leaching is similar whether it was in a pure form or as concentrate. The same also applies for chalcopyrite response to activation by ultrasound, confirming again the physical effects of ultrasound. With the estimated thermodynamic values for the systems studied, it is now possible to rewrite the parabolic leaching model (Appendix II) to include the apparent activation energy. 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The data are from Table 5.2.1.  0.50 0.45 0.40  O 25 deg. C A 65 deg. C o 75 deg. C  • 85 deg. C  0.35 S3  0.30  o  0.25  >  0.20 0.15 0.10 0.05 0.00 15  30  45  60  75  90  105  120  Time, min  Fig 5.2.2: Plot of conversion vs. time at various temperatures for leaching chalcopyrite concentrate in sulfate media, without ultrasound activation. The data are from Table 5.2.2. 78  Fig 5.2.3: Plot of conversion vs. time at various temperatures for leaching pure chalcopyrite in sulfate media, with ultrasound activation. The data are from Table 5.2.1.  0  15  30  45  60  75  90  105  120  Time, min  Fig 5.2.4: Plot of conversion vs. time at various temperatures for leaching chalcopyrite concentrate in sulfate media, with ultrasound activation. The data are from Table 5.2.2. 79  0.08 0.07 0.06 CN  +  a-  O 25 deg. C A 65 deg. C o 75 deg. C  • 85 deg. C  0.05 0.04 0.03  1-3(1-Xb)  +2(1-Xb) = kt  0.02 0.01 0.00  Fig. 5.2.5: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at various temperatures, without ultrasound activation. The data are from Table 5.2.1.  Fig. 5.2.6: Product layer model fitting of the conversion data for chalcopyrite concentrate in sulfate media at various temperatures, without ultrasound activation. The data are from Table 5.2.2. 80  0  15  30  45  60  Time, min  Fig. 5.2.7: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at various temperatures, with ultrasound activation. The data are from Table 5.2.1.  0.18 i  0  15  30  45  60  Time, min  Fig. 5.2.8: Product layer model fitting of the conversion data for chalcopyrite concentrate in sulfate media at various temperatures, with ultrasound activation. The data are from Table 5.2.2. 81  Control Sonication  2.75  2.85  2.95  3.05  3.15  r'xio , K 3  3.25  3.35  3.45  1  Fig. 5.2.9: Plot of reaction rates vs. inverse of temperature (Arrhenius plot) for pure chalcopyrite in sulfate media. The kinetic data are from Table 5.2.5.  -9.00  Control:  -8.50  lnk--3.25X10 T" +2.22  -8.00  r =0.99  3  • Control  1  • Sonication  2  -7.50 -7.00  Sonication:  -6.50  lnk=-3.80X10 T" +4.69  -6.00  r =0.99  3  1  -5.50 2.75  2.85  2.95  3.05 -1  3.15 3  3.25  3.35  3.45  -1  T X10 , K Fig. 5.2.10: Plot of reaction rates vs. inverse of temperature (Arrhenius plot) for chalcopyrite concentrate in sulfate media. The kinetic data are from Table 5.2.5. 82  -15.00  H  Control:  -14.50  In (k/T) =-3.16 X 10 T" -3.94  -14.00  r = 0.97  3  O Control  1  • Sonication  -13.50 -13.00 Sonication:  -12.50  In (k/T) = -3.60 X 10 T" -1.75 3  -12.00 -11.50 2.75  1  r = 0.98 2.85  2.95  3.05  3.15  T" X 10 , K" 1  3  3.25  3.35  3.45  1  Fig. 5.2.11: Plot of In (k/T) vs. inverse of temperature for pure chalcopyrite in sulfate media. The kinetic data are from Table 5.2.5.  Fig. 5.2.12: Plot of In (k/T) vs. inverse of temperature for chalcopyrite concentrate in sulfate media. The kinetic data are from Table 5.2.5. 83  • 25 deg. C A 65 deg. C O 75 deg. C • 85 deg. C  0.70 0.60 0.50  c o c o  0.40 0.30 0.20  15  0  30  60  45  75  90  105  120  Time, min 5.2.13: Plot of conversion vs. time at various temperatures for leaching pure chalcopyrite in chloride media, without ultrasound activation. The data are from Table 5.2.3. 0.70 0.60  O  25 deg. C  A  65 deg. C  O  75 deg. C  • 85 deg. C  0.50 a .2 VI  rH CD >  c o U  0.40 0.30 0.20 0.10 0.00 0  15  30  45  60  75  90  105  120  Time, min  5.2.14: Plot of conversion vs. time at various temperatures for leaching chalcopyrite concentrate in chloride media, without ultrasound activation. The data are from Table 5.2.4. 84  0.90 n • 25 deg. C A 65 deg. C  0.80  • 75 deg. C  0.70  a o e  > c o U  • 85 deg. C  0.60 0.50 0.40 0.30 0.20 0.10 0.00 0  15  30  45  60  75  Time, min  Fig 5.2.15: Plot of conversion vs. time at various temperatures for leaching pure chalcopyrite in chloride media, with ultrasound activation. The data are from Table 5.2.3.  c Vi > C  o O  Time, min Fig 5.2.16: Plot of conversion vs. time at various temperatures for leaching chalcopyrite concentrate in chloride media, with ultrasound activation. The data are from Table 5.2.4. 85  0.16  X>  0.14  A  65 deg. C  0.12  o  75 deg. C  0.10 0.08 X  O 25 deg. C  • 85 deg. C 1-3(1-Xb)  +2(1-Xb) = kt  0.06 0.04 0.02 0.00 30  15  45  60  Time, min Fig. 5.2.17: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at various temperatures, without ultrasound activation. The data are from Table 5.2.3. 0.16 0.14 0.12 CN  +  0.10 0.08  O  25 deg. C  A  65 deg. C  o  75 deg. C  • 85 deg. C 1-3(1-Xb)  +2(1-Xb) = kt  0.06 co  0.04 0.02 0.00 15  30  45  60  Time, min Fig. 5.2.18: Product layer model fitting of the conversion data for chalcopyrite concentrate in chloride media at various temperatures, without ultrasound activation. The data are from Table 5.2.4. 86  0.30  • 25 deg. C • 65 deg. C  0.25  • 75 deg. C 0.20  • 85 deg. C  CN  +  0.15 1-3(1-Xb)  co  +2(1-Xb) = kt  0.10 0.05 0.00 15  0  30  45  60  Time, min  Fig. 5.2.19: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at various temperatures, with ultrasound activation. The data are from Table 5.2.3. 0.35 i 0.30  A 65 deg. C  0.25 H CN  +  m  CO  I Jl*  • 25 deg. C • 75 deg. C • 85 deg. C  0.20 1-3(1-Xb)  0.15  +2(1-Xb) = kt  0.10 0.05 0.00 0  15  30  45  60  Time, min Fig. 5.2.20: Product layer model fitting of the conversion data for chalcopyrite concentrate in chloride media at various temperatures, with ultrasound activation. The data are from Table 5.2.4. 87  Control:  -8.00  O Control  In k = -2.99 X 1 0 T " + 2.25 3  !  • Sonication  r = 0.99 2  -7.00  Sonication:  -6.00  l n k = -3.53X 10 T 3  +4.54  !  r =0.99 -5.00  2.75  2.85  2.95  3.05  3.15  T ^ X I O , K" 3  3.25  3.35  3.45  1  Fig. 5.2.21: Plot of reaction rates vs. inverse of temperature (Arrhenius plot) for pure chalcopyrite in chloride media. The kinetic data are from Table 5.2.6. -8.50  O Control  Control:  • Sonication  In k = -3.06X 10 T" +2.53 3  1  r = 0.98  -7.50  -6.50 Sonication: l n k = -3.60X 10 T" + 4.82 3  -5.50  1  r = 0.98  -4.50 2.75  2.85  2.95  3.05  3.15  T^XIO ,^ 3  3.25  3.35  3.45  1  Fig. 5.2.22: Plot of reaction rates vs. inverse of temperature (Arrhenius plot) for chalcopyrite concentrate in chloride media. The kinetic data are from Table 5.2.6. 88  -13.50  o Control  Control:  • Sonication  ln(k/T) = -2.66X 10 ^ - 4 . 5 3 3  -13.00 -12.50 3a.  -12.00 Sonication: -11.50  In (k/T) = -3.20 X 10 T" - 2.24 3  1  r = 0.99  -11.00 -10.50 2.75  2.85  2.95  3.05  3.15  T" X 10 , K " 1  3  3.25  3.35  3.45  1  Fig. 5.2.23: Plot of In (k/T) vs. inverse of temperature for pure chalcopyrite in chloride media. The kinetic data are from Table 5.2.6. -14.00  Control:  O Control  In (k/T) = -2.74 X 10 T" - 4.26 3  -13.50  1  • Sonication  r = 0.97 -13.00 p  -12.50 -12.00 Sonication:  -11.50  In (k/T) =-3.28 X 10 T" - 1.96 3  -11.00  1  r = 0.98 2  -10.50 2.75  2.85  2.95  3.05  3.15  T'XIOIK"  3.25  3.35  3.45  1  Fig. 5.2.24: Plot of In (k/T) vs. inverse of temperature for chalcopyrite concentrate in chloride media. The kinetic data are from Table 5.2.6. 89  5.3. Particle Size Dependence The parabolic leaching kinetics have been established in the previous sections. To confirm the system behavior, it is necessary to investigate the dependence of leaching kinetics on particle size. Systems controlled by product layer diffusion show a clear dependence on particle size in that reaction rates are largely improved upon using smaller size fractions. To do so, a series of experiments was performed for various size fractions, with and without ultrasound activation. These experiments were also done for pure chalcopyrite and concentrate, in both sulfate and chloride media. Tables 5.3.1-5.3.4 summarize the results obtained for four different size fractions. The leach temperature was selected because no significant amounts of copper were extracted with coarse particles at lower temperatures and for the reasons presented in Section 5.2. As can be seen from Figs. 5.3.1 and 5.3.2, the system shows a clear dependence on chalcopyrite particle size. Leaching coarse particles in sulfate media without sonication did not give significant amounts of dissolved copper, neither the leaching kinetics were favorable. This is apparent from the leach data, although they were taken at 75 °C. With decreasing the particle size, both leaching kinetics and amount of copper extracted are improved. The use of the finest size fraction has almost tripled the final conversion, confirming the previous argument about the controlling mechanism, but the leaching kinetics are still slow. The use of ultrasound activation has a dramatic change to the system behavior. As can be seen from Figs. 5.3.3 and 5.3.4, both reaction kinetics and dissolved copper have increased. The obvious effect of ultrasound is that regardless of starting particle size, leaching kinetics are accelerated by the same order of magnitude leading to comparable amounts of dissolved copper, under the same experimental conditions. This is attributed to the "milling effect of ultrasound", which will cause intense impingement on solids leading to their breakage and subsequently dissolution. That is, ultrasound is the physical driving force behind the behavior observed. Its mechanism is largely by increasing the reaction surface area through particle milling. It can be seen that the enhancement by ultrasound milling is more pronounced with coarse particles. Based on Table 5.3.5, it can be seen that reaction rates were faster by more than 17 times compared to those without sonication. Most likely this is attributed to the sufficient interaction of coarse particles with ultrasonic waves or vibrations. The action of ultrasound occurs in a short period of time, -30 minutes of sonication, resulting in improved leaching 90  kinetics. Because of the trend of the conversion-time curves presented herein, it can be said that ultrasound has a "catalytic-like" effect on the mineral particles. This effect is physical as will be discussed shortly. Unfortunately, this strong effect of ultrasound did not lead to complete copper dissolution. As can be seen from Figs. 5.3.3 and 5.3.4, reaction kinetics were very fast in the first hour then reached a plateau, before the reaction slows completely. When such a kinetic behavior , is observed, it means that at certain stage there is a strong hindrance for the reaction to complete its progress. In the case of ferric ion leaching, such a hindrance is caused by the build-up of reaction products, which tend to "isolate" the particles from the surrounding medium. The reaction products can be a progressively thickening sulfur layer or very thin film of other solid products, as was outlined in Section 2.1. In other words, strong particle passivation occurs and is responsible for such a behavior. To elucidate on the events that are taking place, there are two explanations. Firstly, it seems as i f the particles were broken by sonication to smaller sizes, which reacted and released copper in solution, but later become passivated upon reaction with the leach solution. Secondly, it seems also that the action of sonication is taking place till reaching a minimum particle size beyond which the interaction of ultrasound waves with the solids is weak. This is attributed to the fact that at such small sizes, ultrasound cannot overcome the tensile forces of the particles to cause more breakage (Suslick et al. (1996)). The other possibility is that limiting particle size is reached because the fracture of the particles is determined by the magnitude of stress applied to them, which is inversely proportional to particle diameter (Tromans and Meech (2001)). It is possible that the stress generated by ultrasonic vibrations will no longer be able to interact with the defects in solid particles at such sizes. In this case, particle defects will no longer propagate or respond to ultrasonic vibrations, posing a limitation on ultrasonic effectiveness in leaching. In other words, the ultrasonic abrasion is taking place and the particles are being leached but below a certain particle size, the interaction with ultrasonic waves is less effective or of little benefit. As a result, ultrasonic milling ceases and copper extraction reaches a limiting value, explaining the plateau of the conversion-time curves. Physical inspection of the leach residue after filtration and washing showed very fine grinding by ultrasound, which was similar for all tested size fractions. Several samples of the leach residues were collected and subjected to sieving. The majority of the samples were smaller than 600 mesh size (< 10 um). Likely, ultrasonic vibrations cause only particle deformation or distortion below such sizes. 91  Because of the trend of the leaching curves presented above, it was necessary to search for some method to avoid this strong passivation or try new leaching media. Leaching in chloride media was performed and the results are included herein. Later, leaching at redox potentials where chalcopyrite passivation by a thin product layer is not expected will be presented for the merit of comparison. Tables 5.3.3. and 5.3.4 show the experimental results for leaching the solids in chloride media, with and without sonication. Figs. 5.3.5-5.3.8 summarize the results in graphical form. Clearly leaching in chloride media is more efficient than that in sulfate media. Even with coarse particles, leaching in chloride media can dissolve more copper. It can also be seen that leaching kinetics in chloride media are faster by approximately a factor of two compared to those in sulfate media, which is not unusual considering the activity of the chloride ions. The same trend of conversion data can be seen in chloride media, which implies that ultrasound effects are only limited by reaching certain particle size. Although chloride media proved to be more efficient than sulfate media, but the enhancement in the latter is greater. Comparison of the data in the tables shows that the amount of copper extracted in sulfate media was tripled upon using finer size fractions, suggesting a stronger dependence on particle size for this medium. That is, leaching in sulfate media requires fine grinding, which can be provided by sonication, explaining in turn the high values of enhancement ratios shown in Table 5.3.5. Combined with those in Table 5.3.6, the effectiveness of ultrasound as an activation method for leaching is evident. Table 5.3.6 also suggests that the problem of passivation in chloride media is less severe, explained by the reasons outlined above. As with leaching at different temperatures, the use of ultrasound has again improved the reaction rates. It can be seen from the tables that more than 50% of dissolved copper can be reached in about 15 minutes, indicating a strong acceleration of leaching rates. Since leaching kinetics were accelerated, it can now be concluded that the effect of ultrasound on particles is physical in terms of surface disruption, particle breakage and increasing available surface area for reaction by fragmentation. The remaining kinetics are dependent on the effectiveness of chemical leaching. Because particle passivation is still occurring, it was necessary to investigate i f ultrasound contributed to this problem or not. This was performed by changing the experimental procedure or using certain organic solvents, as will be addressed later. 92  The parabolic leaching model was used to fit the conversion data, which can be seen in Figs. 5.3.9-5.3.16. The strong dependence of reaction rates on particle size is demonstrated in Figs. 5.3.17-5.3.20. The rapidity of leaching kinetics in chloride media can easily be noticed by comparing the slopes of the lines in Figs. 5.3.17-5.3.20. Being so, the slopes of the lines are twice those in sulfate media, as indicated earlier. For experiments without sonication, the trend of the plot of reaction rates vs. inverse square of particle size is in conformance with theoretical considerations, in that a straight line is obtained with zero intercept. However, the rates under sonication do not conform to theory. Compared to experiments without sonication, the use of ultrasound seems to have shifted the lines upward, which is explained by the amount of energy transferred to solids by ultrasound waves. This energy is in the form of milling, explaining the trend of the lines. These lines for sonication also show that regardless of the initial particle size of chalcopyrite, similar leaching kinetics are obtained with comparable amounts of dissolved copper. Because the kinetics of sonochemically leached solids are independent of initial particle size, the leaching rates are pseudo-zero-order with respect to the initial size of chalcopyrite particle. Hence, the leaching models developed in Table 5.2.7 are now rewritten to include this finding as shown in Table 5.3.7. With regard to the findings obtained so far and those discussed in Section 2.1, it can be seen that the use of ultrasound has resulted in three major improvements. First, there is a significant reduction in the required leaching time to realize sufficient copper dissolution. Sonication made it possible to do so in less than an hour, compared to several hours or even days of continuous leaching set by other researchers (Section 2.1). Next is the lessening of the required leaching temperature. Most of the published work points at a temperature range of 95106 °C for reasonable leaching kinetics and copper extraction. With sonication, a temperature of 75 °C is adequate. Lastly is the avoidance of fine grinding. As evident from the literature review, most of the authors report that very fine grinding (< 5 um) is essential to cause significant copper dissolution. With ultrasound this prerequisite is entirely not necessary. Similar copper extraction was obtained with ultrasound activation whether coarse or fine particles were leached. On the practical side this implies that the use of ultrasound could lead to savings in milling or size reduction costs, through avoidance of intensive or fine grinding required for ferric ion leaching to be efficient.  93  The effects induced by ultrasound can be revealed by inspection of the surface morphology of fresh and leached particles. Several S E M images of various leached samples have been included in Figs. 5.3.21-5.3.33 to describe the physical effects of ultrasonic vibrations on solids under the experimental conditions used. Sufficient information is given with every image but it is worthwhile to point out the differences between leaching in sulfate and chloride media. Figs. 5.3.22 and 5.3.23 show the product layers formed on leached solids in sulfate and chloride media, respectively. These layers are mainly sulfur layers and possibly some very thin solid layer. As can be seen, there is a difference in the nature and shape of every layer. In chloride media, the product layer seems to be. porous or less dense compared to that in sulfate media. One can see this difference clearly by considering the images magnified 20,000 or 200,000 times. This nature of formed product layers will most likely affect the reaction extent, explaining the efficiency of chloride media. The product layer in this medium seems to have been punched or corroded by the attack of the active chloride ion, opening the way for the lixiviant to remain in contact with the chalcopyrite particles and extract more copper. This physical evidence can be used to support the arguments on the nature and shape of product layers formed during ferric ion leaching of chalcopyrite. The chloride ions attack the sulfur layer rendering it penetrable for leachant transport. The activation by ultrasound has caused significant changes to the nature and morphology of the product layers. As can be seen in Figs. 5.3.23 and 5.3.25, the product layers developed during chemical leaching are continuously being "cleaned-up" or removed by ultrasound waves. This has resulted in the exposure of the leached particle surface to the leach solution. The cleaning effect can be attributed to the collapse of microbubbles formed. These tiny bubbles (around 2.5 um in diameter) will implode asymmetrically in the vicinity of particles leading to liquid inrush, which is responsible for the surface morphology seen in the figures. Further, the intense implosion of these microbubbles is likely to create new cracks or micropores in the particles or deepen the already existing defect structures, as evident from the S E M images. The formed products look as i f agglomerating and being "chiseled" away or scrubbed from the reacting particle by the action of ultrasonic vibrations. This might explain the phenomenon observed during the experiments, which was the agglomeration and floating of some reaction products to the leach solution surface. Such a phenomenon was more prominent in chloride media, which is evident from Fig. 5.3.25.  94  This phenomenon was observed several times, especially upon sonochemical leaching at higher temperatures. The floating globules, which were expected to be sulfur or sulfur-coated particles, deflated upon cooling to room temperature. A n S E M image of these globules is shown in Fig. 5.3.34. The shape of these particles is similar to the identified build-up products shown in the previous S E M images, supporting their anticipated nature. To uncover the exact composition of these globules, different samples were collected and quantified using organic solvents as was explained in Section 4.2. The results confirmed that these globules are mainly sulfur particles (in elemental form as orthorhombic). This was also necessary to complete the mass balance calculations and establish the leaching reaction stoichiometry (as shown in Appendix I). The oxidative leaching of chalcopyrite will lead to a net change in molar volume of the particles. Assuming a basis of 1000 g chalcopyrite, sp. gr. is 4.2 and this mass will occupy 238.1 cm . The amount of elemental sulfur (orthorhombic, sp. gr. 2.07) that can be produced (assuming 3  100% yield) based on Eq. 5.1 is 349.5 g, which will occupy 168.8 cm . The net change in 3  particle volume will be: ' 168 8 ^ Volume change = 100 1 — 238.1 J corresponding to a net shrinkage of -30%, which will lead to the rejection of sulfur produced. The sulfur layer or particles produced seem to adhere loosely to the leached solids (i.e.: physically bonded), because sulfur globules were easily removed from the leached particles either by increasing the intensity of ultrasonic vibrations or rapid dissolution in organic solvents. The remaining passivation will be caused by an adherent thin layer of a defect structure of some copper sulfide, as was outlined previously (Chapter 1). The final conclusion from studying particle size dependence is that the leaching reaction (Eq. 2.4) is kinetically controlled by diffusion through product layer. Apparently, the transport of one or more of reaction species through a progressively thickening sulfur or other solid layer is rate limiting. Leaching kinetics were improved once the particles remained in contact with the lixiviant, as evident from leaching results in sulfate and chloride media. Ultrasound proved to be a good activation method for enhancing leaching kinetics and copper extraction through its physical effects, which include particle milling and fragmentation, as well as removal of product layers. 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The data are from Table 5.3.1. O -80mesh +100 mesh  Time, min Fig. 5.3.2: Plot of conversion vs. time at various size fractions for leaching chalcopyrite concentrate in sulfate media, without ultrasound activation. The data are from Table 5.3.2. 102  0.70  -I  0.60 -  Time, min  Fig. 5.3.3: Plot of conversion vs. time at various size fractions for leaching pure chalcopyrite in sulfate media, with ultrasound activation. The data are from Table 5.3.1. 0.70 -. 0.60 -  Time, min  Fig. 5.3.4: Plot of conversion vs. time at various size fractions for leaching chalcopyrite concentrate in sulfate media, with ultrasound activation. The data are from Table 5.3.2. 103  0.70 0.60 0.50  • -80 mesh+100 mesh A-100mesh+200iresh o -240 mesh+270 mesh o -325 mesh-r400 mesh  15  30  45  60  75  90  105  120  Time, rrrin  Fig. 5.3.5: Plot of conversion vs. time at various size fractions for leaching pure chalcopyrite in chloride media, without ultrasound activation. The data are from Table 5.3.3. 0.70 0.60 0.50 •|  O -80 mesh +100 mesh A-100 mesh+200 mesh o -240 mesh +270 mesh • -325 mesh +400 mesh  0.40  >  J  0.30 0.20 0.10 0.00  15  30  45  60  75  90  105  120  Time, min Fig. 5.3.6: Plot of conversion vs. time at various size fractions for leaching chalcopyrite concentrate in chloride media, without ultrasound activation. The data are from Table 5.3.4.  104  0.90 -. 0.80 -  Time, min  Fig. 5.3.7: Plot of conversion vs. time at various size fractions for leaching pure chalcopyrite in chloride media, with ultrasound activation. The data are from Table 5.3.3. 0.90 i  Time, min  Fig. 5.3.8: Plot of conversion vs. time at various size fractions for leaching chalcopyrite concentrate in chloride media, with ultrasound activation. The data are from Table 5.3.4. 105  0.060 i  0  15  30  45  60  Time,rnin Fig. 5.3.9: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at various size fractions, without ultrasound activation. The data are from Table 5.3.1.  0.060 0.050  O-80mesh+100 mesh A-100 mesh +200 mesh  l-3(l-X ) +2(l-Xb) = kodo" 1 2/3  2  b  o -240 mesh +270 mesh 0.040  • -325 mesh +400 mesh  0.030 CO  0.020 0.010 0.000 15  30  45  60  Time,min  Fig. 5.3.10: Product layer model fitting of the conversion data for CuFeS concentrate in sulfate media at 2  various size fractions, without ultrasound activation. The data are from Table 5.3.2. 106  0.16 • -80 mesh+100 mesh 0.14  A-100 mesh +200 mesh  0.12  • -240 mesh +270 mesh  0.10  • -325 mesh +400 mesh  1-3(1-Xb)  2/3  +2(1-Xb) = kt  0.08 0.06 0.04 0.02 0.00  Fig. 5.3.11: Product layer model fitting of the conversion data for pure chalcopyrite in sulfate media at various size fractions, with ultrasound activation. The data are from Table 5.3.1. 0.14 • -80mesh+100 mesh 0.12 £  0.10  ?  0.08  1-3(1-Xb)  +2(1-Xb) = kt  A-100 mesh +200 mesh • -240 mesh +270 mesh • -325 mesh +400 mesh  0.06 0.04  0  15  30  45  60  Time, min Fig. 5.3.12: Product layer model fitting of the conversion data for CuFeS concentrate in sulfate media at 2  various size fractions, with ultrasound activation. The data are from Table 5.3.2. 107  0.14 o -80 mesh +100 mesh  l-3(l-Xb) +2(l-Xb) - kodo" t 2/3  0.12  A-100 mesh+200 mesh  2  o -240 mesh +270 mesh  0.10  • -325 mesh +400 mesh 0.08 0.06 co  0.04  0  15  30  45  60  Time, min Fig. 5.3.13: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at various size fractions, without ultrasound activation. The data are from Table 5.3.3. 0.16 O -80 mesh +100 mesh 0.14  CN  +  CO  A -100 mesh +200 mesh  0.12  o -240 mesh +270 mesh  0.10  o -325 mesh +400 mesh  l-3(l-X ) +2(l-Xb) = kodo" t 2/3  2  b  0.08 0.06 0.04 0.02 0.00 15  30  45  60  Time, min Fig. 5.3.14: Product layer model fitting of the conversion data for CuFeS concentrate in chloride media 2  at various size fractions, without ultrasound activation. The data are from Table 5.3.4.  108  0.30  • -80 mesh+100 mesh A-100 mesh+200 mesh  0.25  15?  • -240 mesh +270 mesh  1-3(1-X )  2/3  b  +2(1-X )  :  b  • -325 mesh +400 mesh  0.20 0.15 0.10 0.05 0.00 0  15  30  45  60  Time, min  Fig. 5.3.15: Product layer model fitting of the conversion data for pure chalcopyrite in chloride media at various size fractions, with ultrasound activation. The data are from Table 5.3.3.  • -80 mesh +100 mesh  Time, min Fig. 5.3.16: Product layer model fitting of the conversion data for CuFeS concentrate in chloride media 2  at various size fractions, with ultrasound activation. The data are from Table 5.3.4. 109  25.00 O Control  20.00  • Sonication  Sonication: X  I CO  15.00  k = 0.62 do" +19.78 2  r = 0.97 2  10.00  o  c o  " Control:  o  CD  5.00  k=1.50 do"  2  2  r = 0.97  0.00 0.00  1.00  2.00  3.00 -2  4  4.00  5.00  6.00  -2  do X 10 , um 5.3.17: Plot of reaction rates vs. inverse square of CuFeS mean particle diameter. The data are from 2  Table 5.3.5 for pure CuFeS leaching with and without sonication (sulfate media). 2  25.00  20.00  X  c 03  15.00  • Control • Sonication  Sonication: k = 0.33 do" + 2  19.88  r = 0.99 10.00  CO  C O o  CD  Control:  5.00  *  k = 1.39 do"  2  r = 0.96 0.00 0.00  1.00  2.00  3.00 do X 10 , um' 2  4  4.00  5.00  6.00  2  5.3.18: Plot of reaction rates vs. inverse square of CuFeS mean particle diameter. The data are from 2  Table 5.3.5 for CuFeS concentrate leaching with and without sonication (sulfate media). 2  110  50.00  40.00 O Control X  30.00  Sonication: k = 1.06  £CO  c o o  20.00  • Sonication  do" + 2  36.56  r = 0.96 2  Control:  10.00  k = 3.44  d " 0  r = 0.96 2  0.00 0.00  1.00  2.00  3.00 -2  4.00  4  5.00  6.00  -2  X 10 , um  do  Fig. 5.3.19: Plot of reaction rates vs. inverse square of CuFeS mean particle diameter. The data are from 2  Table 5.3.6 for pure CuFeS2 leaching with and without sonication (chloride media).  50.00  40.00 o  X  30.00  o Control • Sonication  Sonication: k = 1.49 d o ' + 38.21 2  2CO  20.00  r = 0.96  C  o o ig  Control: 10.00  k = 3.73  d " 0  r = 0.97 0.00 0.00  1.00  2.00  3.00 do"  2  4.00  X 10 , um" 4  5.00  6.00  2  Fig. 5.3.20: Plot of reaction rates vs. inverse square of CuFeS mean particle diameter. The data are from 2  Table 5.3.6 for CuFeS concentrate leaching with and without sonication (chloride media). 2  Ill  X o o  CD  © o  CN O  X o o ©  d  CN  2)  x  13  o o  CD  C/3 CN CO  i/o'  C  £  ^  IO  + 60 O  as X o o o o" o  1 & o  "••3  CN  CD CD  .5  cd  3  IB  CD  CD N  £  bo co rn vi u  o  3  CO  CD DO  cd  CD  43  X o o  CD  ©"  •  a o _o  3 ft  -a  H-J  Cd CD  cd  -3 3 co  3 O  3 u o  in  ,  s  cd  w CN  co uo  CO  3 O s-  O  ft CD bH  CD  CD  T3  CD  43  CD cd CD  u  gp  CO  CD J>£  O  CD  X o ©  co  H 3 o is  CO  CD  -3  cd 3  X!  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Note the similarity with other images, in regard to products' building-up, and agglomeration. Experimental conditions are as per Table 5.3.3. 125  5.4 Ferric Ion Dependence: Ferric ion is a reactant in the main leaching reaction. It is also the chemical driving force behind copper extraction from chalcopyrite. This is obvious by recalling that the leaching reaction proceeds through an electrochemical mechanism, where ferric ion reduction is its cathodic part (Eqs. 2.2-2.4). Therefore, it is necessary to investigate the effect of ferric ion concentration on leaching kinetics. To unveil any influence of ferric ion concentration on leaching kinetics, a series of experiments has been performed with various Fe  concentrations. Initially, various low  concentrations (in the range 0.01-0.2 M) were tried to observe any response to the leachant by the solid particles. Due to the nature of the leaching system, it was not possible to determine any significant amounts of dissolved copper in this concentration range. After this observation and based on reaction stoichiometry, stoichiometric amounts of ferric ion were tried as an initial step. For the sulfate systems, the starting ferric ion concentration was 0.25 M . For the chloride systems, it was 0.5 M , but for the merit of comparison, the results at 0.25 M FeC^ are included. As can be seen in Tables 5.4.1 and 5.4.2, the ferric sulfate concentration does not have any significant effect on the amount of copper extracted, and consequently on leaching kinetics. Increasing its concentration shows no clear effect on reaction rates, which is clear from the graphs presented in Figs. 5.4.1-5.4.4. Even in the presence of ultrasound, leaching kinetics are insensitive to ferric ion concentration as apparent from Table 5.4.5. This result is consistent with the findings by other workers (Tables 2.1.2 and 2.1.3), who studied the system and obtained the same results. This behavior may be explained by the formation of complex iron-sulfate species in the system at higher concentrations, which tends to decrease the leaching reactivity by binding the ferric ion or causing iron precipitation. The same trend was also observed in chloride media (Tables 5.4.3, 5.4.4 and 5.4.6), suggesting that best ferric ion concentration to realize favorable kinetics is under stoichiometric additions. A s evident from Figs. 5.4.5-5.4.8, the presence of large concentrations of chloride ions does not alter the leaching kinetics, which again can be explained by the formation of strong complex ions. These ions tend to be more stable and less reactive to participate in the leaching process. Majima (1995), after performing several fundamental and electrochemical studies on ferric ion leaching of base metal sulfides, has concluded that the reaction rate for ferric ion 126  leaching of chalcopyrite is independent of ferric ion concentration for values > 0.1 M . His findings were further supported by several electrochemical measurements that correlated the leaching rate with corresponding electrochemical parameters. The author also found that leaching kinetics are independent of acid concentration. To complete the kinetic models developed from temperature and particle size dependence, the leaching data in Tables 5.4.1-5.4.4 were fitted by the parabolic leaching model and the results are summarized in Tables 5.4.5-5.4.6. Figs. 5.4.9-5.4.16 give graphical representations of the fittings and Figs. 5.4.17-5.4.20 show the dependence of reaction rates on ferric ion concentration. As can be seen, the straight lines obtained have a small slope, suggesting a slight dependence of leaching rates on ferric sulfate or ferric chloride concentration. The only exception, i f any, is that for pure chalcopyrite in chloride media, which can be attributed to experimental errors in analysis and others. As a final note on the fittings, the reaction rates obtained at 0.25 M F e C ^ were not considered in the fitting because of their very low value compared to those at other concentrations. The rates at this concentration are smaller by an order of magnitude than the others, making it difficult to be included in the linear fitting. The exponent a refers to the order of reaction rate with respect to F e  3+  concentration (see Nomenclature).  With these findings on ferric ion dependence, the leaching models developed in Table 5.3.7 are now rewritten in their final form to include the reaction rate order with respect to Fe . 3+  This is shown in Table 5.4.7. 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The data are from Table 5.4.1.  0.50 O  0.25 M  A0.5M  c o  o 0.75 M • 1.0 M  CO  > c o  o  15  30  45  60  75  90  105  120  Time, min  Fig. 5.4.2: Plot of conversion vs. time at various ferric sulfate concentrations for leaching chalcopyrite concentrate, without ultrasound activation. The data are from Table 5.4.2. 134  0.70 0.60 0.50 2  • 0.25 M A 0.50 M • 0.75 M • 1.0 M  0.40 -  <L> >  c o O  0.30 0.20 0.10 0.00 i 15  30  45  60  75  90  105  120  Time, min  Fig. 5.4.3: Plot of conversion vs. time at various ferric sulfate concentrations for leaching pure chalcopyrite, with ultrasound activation. The data are from Table 5.4.1.  0.70 0.60 0.50 c  Q > C  3  • • • •  0.25 M 0.50 M 0.75 M 1.0 M  0.40 0.30 -  ft/  0.20 0.10 0.00 i 15.  30  45  60  75  90  105  120  Time, min  Fig. 5.4.4: Plot of conversion vs. time at various ferric sulfate concentrations for leaching chalcopyrite concentrate, with ultrasound activation. The data are from Table 5.4.2.  135  0.70 0.60 0.50 H  o 0.25 M A 0.50 M o 0.75 M • 1.0 M  15  30  45  60  75  90  105  120  Time, min  Fig. 5.4.5: Plot of conversion vs. time at various ferric chloride concentrations for leaching pure chalcopyrite, without ultrasound activation. The data are from Table 5.4.3.  0.70 0.60 0.50  O0.25M A0.5M o 0.75 M o 1.0 M  c •§ 0.40  I  0.30  o  15  30  45  60  75  90  105  120  Time, min  Fig. 5.4.6: Plot of conversion vs. time at various ferric chloride concentrations for leaching chalcopyrite concentrate, without ultrasound activation. The data are from Table 5.4.4. 136  0.90 i  0  15  30  45  60  75  90  105  120  Time, min  Fig. 5.4.7: Plot of conversion vs. time at various ferric chloride concentrations for leaching pure chalcopyrite, with ultrasound activation. The data are from Table 5.4.3.  0.90 n  0  15  30  45  60  75  90  105  120  Time, min  Fig. 5.4.8: Plot of conversion vs. time at various ferric chloride concentrations for leaching chalcopyrite concentrate, with ultrasound activation. The data are from Table 5.4.4. 137  0.07 0.06 0.05  • A o a  0.25 M 0.50 M 0.75 M 1.0 M  1-3(1-Xb)  +2(1-Xb) = ko[Fe (S04)3] t al  2  0.04 0.03 co -  0.02 0.01 0.00 15  30  45  60  Time,rnin Fig. 5.4.9: Product layer model fitting of the conversion data for pure chalcopyrite at various ferric sulfate concentrations, without ultrasound activation. The data are from Table 5.4.1.  0.07 O 0.25 M 0.06 £  0.05  ?  0.04  £  0.03  .2/3 1-3(1-Xbr"+2(1-Xb) = ko[Fe2(S04)3] 1  A 0.50 M  a5  o 0.75 M • 1.0 M  CO  15  30  45  60  Time,min Fig. 5.4.10: Product layer model fitting of the conversion data for chalcopyrite concentrate at various ferric sulfate concentrations, without ultrasound activation. The data are from Table 5.4.2. 138  0.16  • 0.25 M  CN  +  l - 3 ( l - X ) + 2 ( l - X b ) = ko[Fe (S04)3] 2/3  0.14  A 0.50 M  0.12  • 0.75 M  0.10  • 1.0 M  b  2  a2  t  0.08 0.06  co  0.04 0.02 0.00 0  15  30  45  60  Time, min  Fig. 5.4.11: Product layer model fitting of the conversion data for pure chalcopyrite at various ferric sulfate concentrations, with ultrasound activation. The data are from Table 5.4.1. 0.16  3 CN  +  • 0.25 M  0.14  A 0.50 M  0.12  • 0.75 M  0.10  •  JII3  l - 3 ( l - X b r + 2 ( l - X b ) = ko[Fe (S04)3] 1 J  a6  2  1.0M  0.08 0.06  CO  0.04 0.02 0.00 0  15  30  45  60  Time, min  Fig. 5.4.12: Product layer model fitting of the conversion data for chalcopyrite concentrate at various ferric sulfate concentrations, with ultrasound activation. The data are from Table 5.4.2.  139  0.16 o 0.25 M 0.14  A 0.50 M  l-3(l-Xb) +2(l-Xb) = ko[FeCb] 1 2/3  o 0.75 M  0.12  a3  o 1.0 M  0.10 0.08 0.06 co  0.04 0.02 0.00 0  15  30  45  60  Time,min  Fig. 5.4.13: Product layer model fitting of the conversion data for pure chalcopyrite at various ferric chloride concentrations, without ultrasound activation. The data are from Table 5.4.3.  0.16 0.14 0.12 CN  +  0.10  • 0.25 M  l-3(l-Xb) +2(l-X ) = ko[FeCb] 1 2/3  a7  b  A 0.50 M o 0.75 M • 1.0 M  0.08 0.06  CO I  0.04 0.02 0.00  Time,min Fig. 5.4.14: Product layer model fitting of the conversion data for CuFeS concentrate at various ferric 2  chloride concentrations, without ultrasound activation. The data are from Table 5.4.4.  140  0.35 0.30  • 0.25 M A 0.50 M  0.25  • 0.75 M • 1.0 M  fa  l-3(l-Xb) +2(l-Xb) = ko[FeCb] 1 2/3  a4  0.20  m  0.15 i—i  0.10 0.05 0.00 0  15  30  45  60  Time, min Fig. 5.4.15: Product layer model fitting of the conversion data for pure chalcopyrite at various ferric chloride concentrations, with ultrasound activation. The data are from Table 5.4.3.  0.35 0.30  • 0.25 M A 0.50 M  • 0.75 M  3  0.25  l-3(l-Xb) +2(l-Xb) = ko[FeCb] t 2/3  a8  • 1.0 M  0.20 £  •  0.15  CO  Time, min  Fig. 5.4.16: Product layer model fitting of the conversion data for chalcopyrite concentrate at various ferric chloride concentrations, with ultrasound activation. The data are from Table 5.4.4. 141  -7.3 -1  -6.9  Control:  O Control  In k - 0.04 In [Fe2(S0 )3] - 7.01  • Sonication  4  x = 0.98  -6.5  Sonication: In k - 0.03 In [Fe2(S0 )3] - 6.01 4  r = 0.98 2  -6.1  i  0.0  -0.3  -0.6  -0.9  -1.2  -1.5  In [Fe (S04) ] 2  3  Fig. 5.4.17: Plot of In k vs. In [Fe ] for leaching pure chalcopyrite in sulfate media with and without 3+  ultrasound activation. The data are from Table 5.4.5. cti = 0.03 and a =0.04. 2  -7.3  A.  Control:  -6.9  O Control  In k = 0.07 In [Fe2(S04)3] - 7.03  • Sonication  r = 0.98 2  -6.5  Sonication: In k = 0.06 In [Fe2(S0 )3] - 6.04 4  r = 0.96 2  0.0  -0.3  -0.6  -0.9  •1.2  -1.5  In [Fe2(S0 )3] 4  Fig. 5.4.18: Plot of In k vs. In [Fe ] for leaching chalcopyrite concentrate in sulfate media with and 3+  without ultrasound activation. The data are from Table 5.4.5. ct = 0.07 and a = 0.06 5  6  142  -6.4 0  -6.2 Control: -6.0 44  M  O Control  Ink = 0.14In [FeCb]-6.14  • Sonication  r = 1.00  -5.8  Sonication: -5.6  Ink = 0.22In [FeCb]-5.32 r = 1.00 2  -5.4 -5.2 0.0  i  i  i  i  i  -0.2  -0.3  -0.5  -0.6  -0.8  In [FeCb] 5.4.19: Plot of In k vs. In [Fe ] for leaching pure chalcopyrite in chloride media with and without 3+  ultrasound activation. The data are from Table 5.4.6. cc = 0.14 and cx = 0.22. 3  4  -6.4  O Control • Sonication  -6.2 O  -6.0  - -  Control:  44  l n k = 0.031n[FeCb]-6.10  -5.8  r = 0.97 2  -5.6  Sonication:  H  l n k = 0.051n[FeCb]-5.32 r = 1.00 2  -5.4 -5.2 0.0  -0.1  -0.2  -0.3  -0.4  -0.5  -0.6  -0.7  -0.8  In [FeCb]  5.4.20: Plot of In k vs. In [Fe ] for leaching chalcopyrite concentrate in chloride media with and 3+  without ultrasound activation. The data are from Table 5.4.6. a = 0.03 and a = 0.05. 7  8  143  5.5 Ferrous Ion Dependence Ferrous ion is a reaction product. It is not expected to see any influence of its concentration on reaction kinetics. However, for the purpose of scaling-up or process development, it is necessary to investigate the effects of the presence of different amounts of ferrous ion in solution on the leaching process. To reveal any influence of [Fe ] on the amount of copper dissolved, several experiments were performed at various additions of ferrous sulfate or ferrous chloride. These salts are soluble in water and their presence is not expected to have any side effects, such as iron precipitation. The results are summarized in Tables 5.5.1-5.5.4 and graphically in Figs. 5.5.1-5.5.8. As can be seen, the addition of excess amounts of ferrous ion has no adverse effect on reaction kinetics or the amount of copper extracted. This agrees well with the findings by other workers (see Majima  (1995), for example) who confirmed from both chemical and  electrochemical studies that ferrous ion has no adverse effect on the leaching reaction. Copper extraction did not change in sonicated experiments compared to those under stoichiometric additions. This implies that ultrasound can be easily integrated into some existing process flowsheet, or adapted to variations in such flowsheets. The other point to be mentioned here is that the effect of ultrasound is now confirmed to be physical. The couple F e / F e is widely used in sonochemistry as a dosimeter to quantify the 3+  2+  efficiency of ultrasound waves. Ultrasound power is capable of oxidizing the ferrous ion through the formation of free radicals (primarily hydrogen and hydroxyl radicals by water sonolysis) or hydrogen peroxide in solution, which are trapped by this couple and can be used for efficiency estimation. Because the amount of copper extracted maintained its value under different ferrous ion concentrations, it is now concluded that, under the experimental conditions used, no chemical effects of ultrasound could be found. The only confirmed effect here is physical, which was observed in all the experiments by the very fine grinding of sonicated solids (i.e. extensive particle breakage). The other implication of the findings on the effect of ferrous ion concentration is that i f a process is to be developed, with or without sonication, the process can be run under a heavy recycle load of the leach solution. This also implies flexible operation in an integrated process flowsheet. 144  The fate of ferrous ion in a process flowsheet for ferric ion leaching of chalcopyrite is either for regenerating the lixiviant or discard as iron precipitate (hematite mainly). If regeneration was performed by pressure oxidation, the benefits of a heavy recycle load are immense in terms of reagent savings. Moreover, substantial amounts of acid could be regenerated, which will make the process, ideally, acid and reagent self-sufficient. 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The data are from Table 5.5.2. 150  0.70 0.60 0.50  c  .2  > c  o u  • 0.25 M A 0.375 M • 0.5 M • 0.75 M  0.40 0.30 0.20 0.10 0.00  Time, min  Fig. 5.5.3: Plot of conversion vs. time at various ferrous sulfate concentrations for leaching pure chalcopyrite, with ultrasound activation. The data are from Table 5.5.1.  0.70 0.60 0.50 o  e >  • 0.25 M A 0.375 M • 0.5 M • 0.75 M  0.40 0.30 0.20 0.10 0.00 15  30  45  60  75  90  105  120  Time, min  Fig. 5.5.4: Plot of conversion vs. time at various ferrous sulfate concentrations for leaching chalcopyrite concentrate, with ultrasound activation. The data are from Table 5.5.2. 151  0.70 0.60 0.50 "e  O0.5M A 0.625 M o 0.75 M o 1.0M  0.40 0.30 0.20 0.10 0.00 15  30  45  60  75  90  105  120  Time, min  Fig. 5.5.5: Plot of conversion vs. time at various ferrous chloride concentrations for leaching pure chalcopyrite, without ultrasound activation. The data are from Table 5.5.3.  0.70 0.60 0.50 c  o >  s o O  • A o •  0.5M 0.625 M 0.75 M 1.0 M  0.40 0.30 0.20 0.10 0.00 15  30  45  60  75  90  105  120  Time, min  Fig. 5.5.6: Plot of conversion vs. time at various ferrous chloride concentrations for leaching chalcopyrite concentrate, without ultrasound activation. The data are from Table 5.5.4. 152  0.90 0.80 0.70 S3 g  0.60  E  0.50  S3  0.40  u >  • • • •  0.5M 0.625 M 0.75 M 1.0M  0.30 0.20 0.10 0.00 15  30  45  60  75  90  105  120  Time, min  Fig. 5.5.7: Plot of conversion vs. time at various ferrous chloride concentrations for leaching pure chalcopyrite, with ultrasound activation. The data are from Table 5.5.3.  S3  .2 Su >  S3 O  O  15  30  45  60  75  90  105  120  Time, min  Fig. 5.5.8: Plot of conversion vs. time at various ferrous chloride concentrations for leaching chalcopyrite concentrate, with ultrasound activation. The data are from Table 5.5.4. 153  5.6 Chalcopyrite Dependence Chalcopyrite is one of the reactants. However, because the reaction is driven by ferric ions, as can be seen from the leaching mechanism in Eqs. 2.2-2.4, it is not expected that there will be any effect of CuFeS2 amount on leaching kinetics. Because of activation by ultrasound, the presence of more solids in solution implies more particle collision or impingement, which might lead to more reduction in particle size, and so more copper extraction. This depends largely on different factors, such as the intensity of sonication, reactor geometry, ability of the particles to interact with ultrasound waves and others. To see i f increasing the amount of solids would affect the leaching process or have any side effects, several experiments were performed using various amounts of chalcopyrite. Other reaction conditions were kept the same. Table 5.6.1 gives the experimental results for leaching pure chalcopyrite under different solids content. As with the previous results, the estimated conversion of chalcopyrite is shown. The results are also given in Figs. 5.6.1 and 5.6.2. Practically, increasing the amount of chalcopyrite means increasing the surface area of the reaction, hence more copper dissolution. The first impression from the previous table and figures is that chalcopyrite has an adverse effect on the amount of copper extracted. This in fact is a paradox, because it does not represent the actual situation. Since the amount of chalcopyrite is being increased, the theoretical copper content is also increased, which upon estimating the conversion gives the trend in the figures. A more accurate approach is to account for the amount of dissolved copper in solution in every run and make that the basis for comparison. In Tables 5.6.2-5.6.5, the amount of copper extracted is given for the same set of experiments and Figs. 5.6.3-5.6.10 are graphical representation of the leaching data. As evident, increasing the amount of chalcopyrite did not increase the amount of dissolved copper, even in the presence of ultrasound. Leaching kinetics were not affected. Physical inspection of the leach residue showed intensive particle breakage caused by ultrasound. Thus, the presence of excessive amounts of solids did not lead to the scattering or "straying" of ultrasonic waves. Because of this, the indifferent kinetic data are attributed to the deficiency in ferric ion concentration needed to extract contained copper. So, leaching kinetics are independent of chalcopyrite additions.  154  The other implication here is that sonicated systems can be run at higher solids content (i.e. scaling-up) without any difficulties. This in fact was tested later by running the system at different volumes with constant CuFeS2/Fe /Fe  ratios (solid/lixiviant ratios).  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O  3 3  CD  3  O  1 CD  CD  c  kH  X  W  a CD  OH X  W  u 3  3>  'B  kH  a  o fa  o  u CD  u CD  fa fa  a  "o H-»  izi  H-»  o H  3  CD  |  CD  CD  a OH  CN  3  O  3 CD  a  o  OO  O CD  3  CD  a kH  CD fa  X  CD  3  CD  T3  T3  O  CD  CD 'kH  fa  fa o, o O  _CD  CD  3 3 oo  43  o  &  w  0  15  30  45  60  75  90  105  120  Time, min Fig. 5.6.3: Plot of amount of copper extracted vs. time at various pure CuFeS additions for leaching in 2  sulfate media, without ultrasound activation. The data are from Table 5.6.2.  4000 n  0  15  30  45  60  75  90  105  120  Time, min Fig. 5.6.4: Plot of amount of copper extracted vs. time at various CuFeS concentrate additions for 2  leaching in sulfate media, without ultrasound activation. The data are from Table 5.6.3. 162  5000  Time, min Fig. 5.6.5: Plot of amount of copper extracted vs. time at various pure CuFeS additions for leaching in 2  sulfate media, with ultrasound activation. The data are from Table 5.6.2.  5000 -I  Time, min  Fig. 5.6.6: Plot of amount of copper extracted vs. time at various CuFeS concentrate additions for 2  leaching in sulfate media, with ultrasound activation. The data are from Table 5.6.3. 163  5000  n  Time, min 5.6.7: Plot of amount of copper extracted vs. time at various pure CuFeS additions for leaching in 2  chloride media, without ultrasound activation. The data are from Table 5.6.4.  6000  n  Time, min  5.6.8: Plot of amount of copper extracted vs. time at various CuFeS concentrate additions for 2  leaching in chloride media, without ultrasound activation. The data are from Table 5.6.5. 164  7000 i  0  20  40  60  80  100  120  Time, min  Fig. 5.6.9: Plot of amount of copper extracted vs. time at various pure CuFeS additions for leaching in 2  chloride media, with ultrasound activation. The data are from Table 5.6.4.  7000 -|  0  15  30  45  60  75  90  105  120  Time, min  Fig. 5.6.10: Plot of amount of copper extracted vs. time at various CuFeS concentrate additions for 2  leaching in chloride media, with ultrasound activation. The data are from Table 5.6.5. 165  5.7 Effect of Solids Content and Solution Volume (Scaling-up Experiments) The results obtained so far show that ultrasonic activation can be a good technique for enhancing the reaction kinetics of ferric ion leaching of chalcopyrite and improving the amount of copper extracted. As can be seen, two factors play an important role in the leaching process: particle size (physical factor) and leaching temperature (thermodynamic factor), in addition to sonication parameters. The use of ultrasound made it possible to obtain certain improvements in reaction rates and copper dissolution. The results reported previously were for a 150-ml solution. It was desired to perform some experiments at higher volumes as an indication for scale-up studies. Experiments were performed at 250, 500 and 750 ml and the results are given in Tables 5.7.1-5.7.4. Proper horn/extender assemblies were used. As can be seen, ultrasound can be applied to different solution volumes and still give comparable results. The performance of ultrasound was not affected by increasing the solution volume or amount of solids. Here the ratio of lixiviant/solids was kept constant and the reaction mixture volume was gradually increased. Ultrasonic intensity was kept constant as well (0.4 W/ml at the maximum power input of the ultrasonic processor). The difference from the experiments presented in Section 5.6 is that sufficient ferric ion is added here, so it is not unusual to obtain such conversion values. Physical testing of the leach residue showed again intensive particle breakage and fragmentation. The implication of the graphs in Figs. 5.7.1-5.7.8 is that the application of ultrasound for processing is straightforward. This activation method can thus be easily adopted for variations in production cycle (i.e. flexible operation). Higher solids contents (> 25%) were not tried due to design limitations of the ultrasonic processor. In regard of the results obtained herein and those at different additions of chalcopyrite (Section 5.6), ultrasound activation can be applied to systems with high solids content. Further work is required in this direction especially for commercialization, and must take into account the associated costs in using power ultrasound for leaching intensification. This is necessary since, under the experimental conditions used in this research, there appears to be a considerable sonication power consumption (estimated at ~35 kWhr/kg copper for the data in Tables 5.7.15.7.4), which is too excessive. 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The experimental conditions are as per Table 5.7.1.  0.70  n  0  15  30  45  60  75  90  105  120  Time, min  Fig. 5.7.2: Effect of solids content and solution volume (scaling-up experiments) on pure chalcopyrite conversion (sulfate media) with ultrasound activation. The experimental conditions are as per Table 5.7.1. 171  0.70 n  Time, min Fig: 5.7.3: Effect of solids content and solution volume (scaling-up experiments) on pure chalcopyrite conversion (chloride media) without ultrasound activation. The experimental conditions are as per Table 5.7.2  0.90 n  Time, min  Fig. 5.7.4: Effect of solids content and solution volume (scaling-up experiments) on pure chalcopyrite conversion (chloride media) with ultrasound activation. The experimental conditions are as per Table 5.7.2. 172  0.50 i  Time, min  Fig. 5.7.5: Effect of solids content and solution volume (scaling-up experiments) on CuFeS2 concentrate conversion (sulfate media) without ultrasound activation. The experimental conditions are as per Table 5.7.3.  0.70  Time, min  Fig. 5.7.6: Effect of solids content and solution volume (scaling-up experiments) on CuFeS concentrate conversion 2  (sulfate media) with ultrasound activation. The experimental conditions are as per Table 5.7.3. 173  0.70 -|  0  15  30  45  60  75  90  105  120  Time, min  Fig. 5.7.7: Effect of solids content and solution volume (scaling-up experiments) on CuFeS concentrate conversion 2  (chloride media) without ultrasound activation. The experimental conditions are as per Table 5.7.4.  0.90 -i  Time, min Fig. 5.7.8: Effect of solids content and solution volume (scaling-up experiments) on CuFeS concentrate conversion 2  (chloride media) with ultrasound activation. The experimental conditions are as per Table 5.7.4.  174  5.8 Effect of Leaching Procedure The leaching results obtained so far show that conversion in sulfate systems barely exceeds 45% under extreme chemical leaching conditions or 60% with ultrasound activation. These results were obtained repeatedly  under a variety of sonication, physical and  thermodynamic conditions. The trend of these results was attributed to strong particle passivation in sulfate systems. There was some doubt that ultrasound may have contributed to particle passivation or can only have very limited effects, caused by the nature of ultrasonic waves or leaching procedure. To verify any possibility of negative effect on leaching, the leaching procedure was changed, by switching between different leaching and sonication steps (sequence of sonication, leaching and combinations there-off). Later, organic solvents were also used for further assessment. On a theoretical basis, the extreme localized conditions established by ultrasound are less likely to retard the leaching reaction, in particular, the highly localized temperatures created from the implosion of cavitational bubbles. These temperatures provide localized solution heating in the vicinity of particles, which is desired since the system is clearly dependent on temperature. Particle breakage through impingement, caused by extreme ultrasonic vibrations, is also desired for this system, since the system is highly dependent on particle size. Therefore, it was necessary to examine the behavior of the system i f the experimental procedure was altered between pure chemical leaching and sonication. To do so, several experiments were performed under different scenarios of sonication and leaching steps. A l l of the experiments were performed under identical conditions. The leaching temperature was 75 °C, and the coarse size fraction -180 urn +149 um (-80 mesh +100 mesh) was selected for the reasons explained earlier. The chalcopyrite concentrate was selected. The experimental conditions were identical to those presented  in Table 5.3.2  (sonochemical leaching). Separate sonication was carried out in 150 ml of pure deionized water. Sonication was set at the same leach temperature. S E M images for combined leaching and sonication were presented in Figs. 5.3.26-5.3.29. The first experiment in this series has two stages of leaching where: 1) The solid particles were chemically leached in a fresh solution of 150 ml for one hour 175  2) The mixture was centrifuged and the remaining solid particles were sonicated in fresh 150 ml of deionized water. Sonication was performed for 30 minutes. 3) Again, the solution from the previous step was set to the required reagent concentration (fresh solution) and pure chemical leaching took place for another hour. Appropriate samples were taken at every stage and analyzed by standard methods for copper concentration. The following table summarizes the results of the analysis: Experimental objective  To study the effect of leaching procedure on copper extraction (CuFeS concentrate, narrow-sized fractions, sulfate media) 2  Experimental conditions Temperature Particle size range Solution volume [Fe (S0 ) ] [FeSOJ [H S0 ] 2  2  4  3  4  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time 2  75 °C -180 um +149 u r n (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79% 3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating ( 1 0 s - 5 s ) @ 50% amplitude control 30 min  Results First stage of leaching (fresh solution) Pure deionized water sonication Second stage of leaching (fresh solution) Accumulated copper extraction  A A reading, ppm of C u 1493 79 720 2292 2+  Conversion 0.1883 <0.01 0.0907 0.2889  Time, min 60 30 60 150  Table 5.8.1: Results for two stages of pure chemical leaching mediated by 30 minutes of sonication in pure deionized water From this table, it can be seen that pure chemical leaching caused copper dissolution, but to a very low extent, similar to that presented in Table 5.3.2. This is attributed to particle passivation, as was discussed earlier. Short sonication caused some improvement in copper extraction, which is depicted as particle fragmentation or surface disruption, instead of causing 176  passivation. The final conversion is about half that reported for sonochemical leaching of the same size fraction (Table 5.3.2), giving a primary indication of the benefit of ultrasound. Figs. 5.8.1-5.8.3 are S E M images of the leach residue. A s can be seen, surface topography has changed upon leaching or sonication. The figures show a gradual change in the nature of particle surface and the milling action of ultrasound. Compared to Figs. 5.3.26-5.3.29, the associated surface damage is less. The products of the final leach stage can be the cause of the surface topography seen here. It seems as i f there were cracks and fissures in the ultrasonically milled particles, which then were "filled" by reaction product. These figures show that the original concentrate particles were shattered to smaller particles by sonication. The damaged smaller particles lost their active reaction sites or surfaces (cracks) upon leaching (i. e.: were passivated).  .10  mm  20KU  i*€  Fig. 5.8.1: S E M image of the final leach product for the size fraction -180 um +149 um magnified 500X. The image was taken after chemical leaching for one hour. This was followed by sonication in pure deionized water for 30 minutes, before being chemically leached by another fresh solution for another hour. Experimental conditions are as per Table 5.8.1. 177  Fig. 5.8.2: S E M image of the same sample in Fig. 5.8.1 magnified 500X but after sonication  In the next experiment, sonication time was increased to 60 minutes and the same leach solution was added again to the sonicated particles. Table 5.8.2 summarizes the results of chemical analysis:  Experimental objective  To study the effect of leaching procedure on copper extraction (CuFeS concentrate, narrow-sized fractions, sulfate media) 2  Experimental conditions Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ] [H S0 ] 2  4  3  4  2  4  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time 2  75 °C -180 um +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79% 3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating (10 s - 5 s) @ 50% amplitude control 60 min  Results First stage of leaching (fresh solution) Pure deionized water sonication Second stage of leaching (same solution) Accumulated copper extraction  A A reading, ppm of C u 1585 236 2713 2713 2+  Conversion 0.1998 0.0297 0.3421 0.3421  Time, min 60 60 60 180  Table 5.8.2: Results for two stages of pure chemical leaching mediated by 60 minutes of sonication in pure deionized water  Seemingly, increasing sonication time did not result in much improvement in final copper extraction, but did cause more copper dissolution compared to Table 5.8.1. The use of a new leach solution (Table 5.8.1) seems to be of less benefit compared to using the same leach solution (Table 5.8.2). Fig. 5.8.4 is the S E M image of the leach residue.  179  Fig. 5.8.4: S E M image of the final leach product for the size fraction -180 urn +149 um magnified 3000X. The sample was chemically leached for one hour then sonicated in pure deionized water for 60 minutes before being chemically leached by the same leach solution for another hour. Experimental conditions are as per Table 5.8.2.  As can be seen, surface damage is much clearer than that shown in Fig. 5.8.3, due to increasing the sonication time. The previously noticed cracks and fissures are more visible, and the building-up of surface products contributes to surface nature. This figure gives a better idea of the events that have occurred. As in the previous experiments, reaction products easily filled the cracks or fissures in the damaged surface. Compared to Fig. 5.3.29c, it is obvious that combined leaching-sonication is better for this system. In the next experiment, sonication was used before chemical leaching. The solids were sonicated in deionized water for 30 minutes then chemically leached for one hour. The mixture was then centrifuged and the solids were again sonicated in deionized water for another 30  180  minutes. The same leach solution was then added to the solids and leaching was allowed to take place for another hour. Table 5.8.3 summarizes the results of chemical analysis.  Experimental objective  To study the effect of leaching procedure on copper extraction (CuFeS concentrate, narrow-sized fractions, sulfate media) 2  Experimental conditions Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ] [H S0 ] 2  4  3  4  2  4  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time 2  75 °C -180 um +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79% 3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating ( 1 0 s - 5 s ) @ 50% amplitude control 60 min  Results Pure deionized water sonication First stage of leaching (fresh solution) Pure deionized water sonication Second stage of leaching (same solution) Accumulated copper extraction  A A reading, ppm of C u 358 2093 60 2336 2336 2+  Conversion 0.0451 0.2638 <0.01 0.2945 0.2945  Time, min 30 60 30 60 180  Table 5.8.3: Results for two stages of pure chemical leaching preceded and mediated by 30 minutes of sonication in pure deionized water  As can be seen, more copper extraction occurred after chemically leaching the sonicated particles. This again shows that ultrasound is less likely to contribute to particle passivation. The amount of copper extracted implies a change in the nature of the sonicated solids, envisaged as milling or fragmentation. Second stage of sonication for leached particles did not make any significant difference. The final amount of copper extracted is smaller than that in the previous procedure. 181  Fig. 5.8.5 is the S E M image of the leach residue. There are some cracks and fissures in the damaged particles, but reaction products are seen to cover the milled particles after leaching. Surface pitting and the subsequent build-up of reaction products can be seen. Apparently, ultrasonic vibrations attacked the passivated particles causing such a surface. It is possible that disruption of the passivating layers was less achieved, caused by insufficient sonication time (medium stage of 30 minutes). It is not clear enough at this stage i f ultrasound caused any passivation, but since chemical analysis of the leach solution shows that copper concentration increases, the only seeming source of passivation is chemical leaching. It became evident from these experiments that sonication of solids between two stages of chemical leaching is either of little benefit, once particle passivation occurred, or may need to be prolonged to be beneficial.  Fig. 5.8.5: S E M image of the final leach product for the size fraction -180 um +149 um magnified 3000X. The sample was first sonicated in pure deionized water for 30 minutes then chemically leached for one hour. The sample was again sonicated in pure deionized water for 30 minutes before final chemical leaching for another hour. Experimental conditions are as per Table 5.8.3. 182  In the next experiment, the solids were first sonicated in pure deionized water for one hour, before allowing chemical leaching to take place. The solids were sonicated in 150 ml of deionized water then the leach solution was set to the required reagent concentration and allowed to leach for two hours. Table 5.8.4 summarizes the results of analysis. As can be seen from this table, sonication prior to chemical leaching gave a benefit in terms of more copper dissolution. Physical inspection of the sonicated particles showed them to be much finer than fresh ones. The better values for conversion compared to the previous experiments confirm that sonication did not lead to particle passivation. Instead, sufficient sonication leads to more particle damage opening the way for more copper dissolution. These results are better than those obtained in Table 5.8.3, with a very simple procedure. Fig. 5.8.6 shows the S E M image of the leached particles. The original surface damage can be seen easily, although the reaction products are again filling the cracks or fissures, causing particle passivation. That is, once particles were shattered with ultrasound, they again become passivated upon chemical leaching, which explains the plateau in the conversion-time curves. The passivating layers seem to be very thin and impervious making the particles less responsive to ultrasonic vibrations or lixiviant attack (i.e. less amenable to further leaching).  183  Experimental objective  To study the effect of leaching procedure on copper extraction (CuFeS concentrate, narrow-sized fractions, sulfate media) 2  Experimental conditions Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ] [H S0 ] 2  4  4  2  4  3  75 °C -180 um +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M  CuFeS Stoichiometric with respect to [Fe ] Mineral type Gibraltar chalcopyrite concentrate Total stoichiometric copper 7931.4 ppm Agitation N purging Solid pulp content 2.79% Sonication parameters Horn/flask configuration Conical flasks/Extender/Full immersion Sonication power input 60 W @ 20 kHz Sonication mode Pulsating ( 1 0 s - 5 s ) @ 50% amplitude control Total sonication time 60 min 3+  2  2  Results Pure deionized water sonication Single stage of chemical leaching Accumulated copper extraction  A A reading, ppm of C u 340 2873 2873 2+  Conversion 0.0429 0.3622 0.3622  Time, min 60 120 180  Table 5.8.4: Results for single stage of pure chemical leaching, preceded by 60 minutes of sonication in pure deionized water  184  Fig. 5.8.6: S E M image of the final leach product for the size fraction -180 um +149 um magnified 3000X. The sample was first sonicated in pure deionized water for 60 minutes then chemically leached for two hours. Experimental conditions are as per Table 5.8.4.  It became apparent from this experiment that sufficient sonication is required to realize adequate milling and eventually significant copper dissolution. Prolonged sonication between leaching stages is necessary if used, since particle disruption is more difficult once product layers are allowed to build up, or passivation has occurred. Because of the results of this experiment, it was decided to increase the sonication time between leaching stages and see i f a significant improvement in copper extraction is obtained. To do so, a sample of solids was first sonicated in deionized water for 60 minutes, before being chemically leached for one hour. The reaction mixture was then centrifuged and solid particles were sonicated again for another hour. Then the same leach solution was added and allowed to leach for another hour. Table 5.8.5 summarizes the results.  185  Experimental objective  To study the effect of leaching procedure on copper extraction (CuFeS concentrate, narrow-sized fractions, sulfate media) 2  Experimental conditions Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ] [H S0 ] 2  4  3  4  2  4  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time 2  75 °C -180 um +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79% 3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating (10 s - 5 s) @ 50% amplitude control 120 min  Results Pure deionized water sonication First stage of leaching (fresh solution) Pure deionized water sonication Second stage of leaching (same solution) Accumulated copper extraction  A A reading, ppm of C u 380 2375 110 3337 3337 2+  Conversion 0.0479 0.2994 0.0138 0.4207 0.4207  Time, min 60 60 60 60 240  Table 5.8.5: Results for two stages of pure chemical leaching preceded and mediated by a single stage of sonication, for 60 minutes, in pure deionized water  It is clear that more copper dissolution was obtained because more sonication time was allowed. These results confirm that ultrasound is not contributing to particle passivation. Rather, it enhances copper extraction, especially i f longer sonication time is used. However, this procedure is tedious, complex and too long. It shows that ultrasound is required after every stage of chemical leaching for at least 60 minutes to remove particle passivation once formed. The amount of copper extracted here is far less than that obtained with sonochemical leaching (Table 5.3.2), which was obtained in less than 60 minutes of sonciation. The benefit here of using ultrasound in decreasing the required leaching time is less readily achieved, especially knowing 186  that its effects will vanish in a short period of time upon milling to certain size range, as explained in Section 5.3. Fig. 5.8.7 is the S E M image of the leached residue. The residue particles are seen to have significant surface damage simply because of extra sonication time. The figure shows that ultrasound has induced deep cracks and fissures. The reaction products are seen to partially cover the damaged surface, indicating that these products are primarily responsible for any passivation phenomenon. Compared to Fig. 5.8.5, it is clear that more pitting of passivated particles has occurred by ultrasound. The attack on the passivation layers by ultrasonic vibrations explains the enhancement in amount of copper extracted.  Fig. 5.8.7: S E M image of the final leach product for the size fraction -180 um +149 um magnified 3000X. The sample was first sonicated in pure deionized water for 60 minutes then chemically leached for one hour. The sample was again sonicated in pure deionized water for 60 minutes before final chemical leaching for another hour. Experimental conditions are as per Table 5.8.5  It turned out that the best and simplest procedure is to combine sonication with chemical leaching and compare the results. In this case, the only variation to the sonochemical leaching 187  procedure (combined sonication-leaching, Table 5.3.2) is increasing the sonication time to 2 hours. By this method, particle passivation is being removed continuously as long as ultrasound is used and can interact with the solid particles. To do so, a solid sample was sonicated in deionized water for one hour then allowed to leach chemically for one hour. Leaching was then continued with sonication for another hour. Appropriate samples were taken and analyzed for dissolved copper. The results are shown in Table 5.8.6.  Experimental objective  To study the effect of leaching procedure on copper extraction (CuFeS concentrate, narrow-sized fractions, sulfate media) 2  Experimental conditions Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ] [H S0 ] 2  4  3  4  2  4  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time 2  75 °C -180 um +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79% 3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating ( 1 0 s - 5 s ) @ 50% amplitude control 120 min  Results Pure deionized water sonication First stage of leaching (fresh solution) Second stage of leaching (same leach solution) with sonication Accumulated copper extraction  A A reading, ppm of C u 333 2564 3450 2+  Conversion 0.0419 0.3232 0.4349  Time, min 60 60 60  3450  0.4349  180  Table 5.8.6: Results for two stages of leaching preceded and then combined with 60 minutes of sonication in pure deionized water  188  As can be seen in Table 5.8.6, the same results were obtained with this procedure and the final amount of copper extracted was still far smaller than that obtained with sonochemical leaching (Table 5.3.2). Thus, increasing the sonication time beyond 60 minutes did not improve copper dissolution as was expected. The only explanation here is that there is a little effect of ultrasound upon reaching certain particle size. Once ultrasonic milling started, its effects on copper dissolution are taking place until there is little or no interaction with ultrasonic vibrations (in less than 60 minutes). The use of ultrasound during leaching did enhance copper extraction, which is better than the results with other procedures, confirming that the chosen experimental procedure (Section 4.2) is the best approach. Physical testing of the leached sample in this experiment and those leached sonochemically showed significant particle damage and fragmentation. The leached particles were much finer than the fresh ones (smaller than 600 mesh or 10 um). Figs. 5.8.8 is the S E M image of the leach residue. As expected from chemical analysis of the leach solution, the application of ultrasound with leaching has led to the best results, in terms of particle damage and copper extraction. As can be seen in this figure, there is a significant surface damage and attrition to the leached particles. Compared with Fig. 5.8.7, for example, it can be seen that the presence of ultrasound was of importance in terms of removing product layers and surface "cleaning". The image in Fig. 5.8.8 is comparable to the one in Fig. 5.3.29c, confirming that combined leaching and sonication is the best procedure. Because of the morphology of the sonicated particles, ultrasound is not expected to be a source of passivation.  189  Fig. 5.8.8: S E M image of the final leach product for the size fraction -180 um +149 urn magnified 3000X. The sample was first sonicated in pure deionized water for 60 minutes then chemically leached for 1 hour. The sample was again leached for 1 hour with the application of ultrasound. Experimental conditions are as per Table 5.8.6.  With these findings, it is concluded that ultrasound effect is taking place continuously as long as it can interact with the particles, leading to the desired particle damage and product layer disruption (i.e.: depassivation). These experiments confirm that direct sonication of the reaction mixture is the most appropriate procedure, in terms of simplicity and ease of use. It can lead to the best possible copper dissolution (Table 5.3.2). The use of ultrasound is of significance in terms of enhancing leaching through particle milling. The size fraction used was the coarse one (-180 um +149 um or -80 mesh +100 mesh), and physical testing indicated that fragmentation was induced by ultrasound. Such effects were more obvious when ultrasound was combined with leaching (i.e.: sonochemical leaching). Particle passivation by ultrasound is less likely. Ultrasound was shown not to contribute to particle passivation. Rather, it improves copper extraction, especially when combined with chemical leaching. Altering the procedure for sonication (sonication then leaching, leaching then sonication, etc.) does not show leaching interruption. In several modes, the final copper 190  extraction is comparable, which implies that chemical leaching is the major source of particle passivation, by the formation of a sulfur layer or thin layers of a defect structure of some copper sulfide, as was outlined previously. When chemical leaching is the final single step, copper extraction is lower than that with sonication. Increasing the time of sonication beyond one hour under various conditions did not improve copper extraction in aqueous systems. The only seeming limiting factor is the size of the particles. Because coarse particles were used, ultrasound effects were obvious in the early stages of leaching (the first 60 minutes of total reaction time). This also agrees with the previous finding on the need for pulsating sonication instead of continuous sonication (that is, brief exposure to the ultrasonic field). Final conversion reached a plateau possibly because of reaching a certain minimum particle size, beyond which ultrasound has little effect (Section 5.3). It can also be attributed to re-passivation of the ultrasonically milled particles by the build-up of reaction products once these particles are broken. One of the suggestions in this regard is to use inert grinding media, such as industrial diamond or tungsten carbide. In this case ultrasound provides mixing and particle collision, while the grinding media provide more particle breakage. This will be addressed later. It was demonstrated that ultrasound can be applied in various modes and at different stages during leaching, leading to significant enhancement in metal dissolution. Since ultrasound effects are taking place continuously, one of the possible variations to the system is the use of flow-through reactor. This set-up allows sonicating small slurry volumes and can be run on a continuous basis, instead of a batchwise operation. The present experimental procedures have demonstrated the flexibility of using ultrasound as an activation method, and such a set-up is of importance for commercialization. This can be a good recommendation for future research.  191  5.9 Effect of Leaching Procedure with Sonication in Decane The results obtained from the previous section showed that ultrasound is less likely to cause particle passivation. Passivation remains a major problem for chalcopyrite oxidative leaching in sulfate media. As was shown, several procedure were tried to improve the amount of copper extracted and investigate i f ultrasound is a source for passivation. Under the scenarios used, it was concluded that ultrasound is not contributing to the slowness of leaching. Rather, it is a good activation method. The only apparent source of passivation was chemical leaching itself. One of the objectives of this research is to obtain basic engineering data on sonochemical leaching of base metal sulfides. To further verify the conclusions reached in the previous section and establish a complete systematic analysis, sonication of solids was performed in decane. Sonication in aqueous media generates hydroxyl radicals. If the experimental conditions are appropriate, these highly reactive species can react and end as a protective oxide layer on the particle surface (a passive oxide layer). This can hinder the progress of leaching. Sonication in decane is known not to release hydroxyl radicals, and so the possibility of oxide layer formation is avoided (Suslick et al. (1996)). With this theoretical background, experiments were performed with decane as the sonication media prior to leaching in ferric sulfate solution. The same experimental conditions employed in the previous section were used here. The chalcopyrite concentrate was again sonicated and leached under different scenarios. Several samples were taken for analysis and S E M images were taken to follow the changes in particle morphology. As an experimental note, viscosity of decane is smaller than that of water. Hence ultrasonic intensity is smaller. This was compensated by adjusting the instrument to the maximum possible ultrasonic intensity (0.4 W/ml) or by prolonging the sonication time. The first procedure was sonication followed by chemical leaching. A sample of the concentrate was sonicated for one hour in 150 ml decane. Through centrifuging, the solids were removed then dried and a leach solution was added. Chemical leaching was allowed to take place for 2 hours. The experimental results are summarized in Table 5.9.1.  192  Experimental objective  To study the effect of leaching procedure on copper extraction (CuFeS concentrate, narrow-sized fractions, sulfate media) 2  Experimental conditions Sonication medium Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ] [H S0 ] 2  4  3  4  2  4  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time 2  Decane 75 C -180 um +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M U  Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79% 3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating (10 s - 5 s) @ 50% amplitude control 60 min  Results Sonication in decane Chemical leaching (fresh solution) Accumulated copper extraction  A A reading, ppm of C u Not applicable 2362 2362 2 +  Conversion 0.2978 0.2973  Time, min 60 120 180  Table 5.9.1: Results for sonication in decane (1 hour) followed by chemical leaching (2 hours) at 75 °C  The decane sample was noticed to remain clear, indicating no copper dissolution. A sample of the solids after sonication was taken and tested using S E M . Fig. 5.9.1 contains the S E M images of the solids. The images reveal new features, different from those obtained with sonication in water. First, particle shattering is not taking place (Fig. 5.9.1a). It is apparent that decane, as an organic solvent, is not reacting with the solids (i.e.: no free radicals or other species to react). Chalcopyrite particles remained intact and only surface damage is induced (in the form of cracks and fissures), which can be seen very clearly in Fig. 5.9.1b. The sonicated particles are not broken but remained separated. These images confirm that ultrasound does change the surface morphology of the solids by creating several cracks and fissures, indicating that in the presence 193  of aqueous solutions these highly reactive sites are the source of reaction progress. Compared to Figs. 5.3.26-5.3.29, one can see the changes to particle surface upon sonication in water. In these figures the formed fissures and cracks seem to be altered (become less sharp), due to the building-up of reaction products. Fig. 5.9.1c shows that this is not the case with decane. Fig. 5.9.Id shows the sonicated particles before leaching magnified 3000X. It is apparent that sonication in decane leads to changes in surface morphology different from those in water. The cracks and fissures suggest also that particle damage can further be induced i f more sonication is used (possibly by increasing the sonication time). Fig. 5.9.le shows the particles after two hours of chemical leaching. The changes in particle surface are now similar to those obtained previously, suggesting again that ultrasound does not contribute to particle passivation. Fig. 5.9. If shows the disappearance of the cracks and fissures after chemical leaching, by the build-up of reaction products and/or particle dissolution. Apparently copper extraction is too low to be considered acceptable. The amount of copper extracted is smaller than those obtained with sonication in water (Table 5.3.2 for example). This is either caused by passivation through chemical leaching (the build-up of reaction products, Fig. 5.9.1) or insufficient sonication time. Physical testing of the sonicated particles by S E M supports these arguments. In an attempt to improve the results, more sonication time was used and the previous experiment was repeated with two hours of sonication in decane. The results are summarized in Table 5.9.2.  194  in  Experimental objective  To study the effect of leaching procedure on copper extraction (CuFeS concentrate, narrow-sized fractions, sulfate media) 2  Experimental conditions Sonication medium Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ]  Decane 75 °C -180 um +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M  [H S0 ]  1M  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time  Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79%  2  4  3  4  2  4  2  3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating ( 1 0 s - 5 s ) @ 50% amplitude control 120 min  Results Sonication in decane Chemical leaching (fresh solution) Accumulated copper extraction  A A reading, ppm of C u Not applicable 2634 2634 2 +  Conversion  0.3321 0.3321  Time, min 120 120 240  Table 5.9.2: Results for sonication in decane (2 hours) followed by chemical leaching (2 hours) at 75 °C As can be seen in Table 5.9.2, the amount of copper dissolved did not largely improve upon increasing the sonication time, suggesting that chemical leaching is controlled by the lixiviant leaching strength, rather than- by sonication. The slight improvement in copper extraction suggests that some physical effect took place as a result of increasing the sonication time. This can be perceived using S E M . Fig. 5.9.2 contains the S E M images of the sonicated and then leached particles. As can be seen and compared to Fig. 5.9.Id, more surface damage was obtained by increasing the sonication time. The sonicated particles retained their original shape, but a new pattern of sonication damage in the form of some shattering can be seen. The particles seem not to react upon sonication, confirming the previous statements. Leached particles show the same trend of surface change as in Fig. 5.9.If, confirming that the only source of particle passivation is after chemical leaching, caused by the building-up of reaction products. 197  (a)  (b) Fig. 5.9.2: S E M images of the size fraction -180 um +149 um (-80 mesh +100 mesh) under different conditions (Table 5.9.2): (a) Solids sonicated in decane for 2 hours, magnified 3000X (d) Solids after two hours of chemical leaching, magnified 3000X  198  As shown in Table 5.9.2, the improvement in copper extraction is small. Because of the prolonged sonication time, it was decided to change the previous experimental procedure in an attempt to have the best effects of sonication. Different leaching scenarios were tried and, in the same way, several samples were analyzed while different S E M images were taken. The procedures used herein are similar to those reported in Section 5.8. The first scenario was as follows: 1) Sonicating the solids for 1 hour in decane 2) Leaching the solids chemically for one hour 3) Sonicating the leached solids for another hour in decane 4) Leaching these solids again with the same leach solution for another hour As in the previous experiments, the solids were sonicated in 150 ml decane for one hour. Through centrifuging, the solids were removed, dried and a leach solution was added. After chemical leaching at 75 °C for one hour, centrifuging was again used to separate the leaching mixture. The partially leached solids were sonicated in decane for another hour, then centrifuged, dried and chemically leached as usual. The experimental results are summarized in Table 5.9.3. Apparently, final copper extraction under this scenario is comparable to that obtained in the previous experiment (2 hours of sonication in decane followed by 2 hours of leaching). The mediation of sonication between two leaching stages has given some improvement in copper extraction. Because of this result, it can be said that the effect of ultrasound on solids is physical. Its action is apparently to remove any reaction products built up on the particles. Furthermore, it was thought that i f the sonication time of the intermediate stage were increased, more copper extraction would be obtained. This was the scenario adopted in the next experiment. S E M images were also taken during the above experiment. Fig. 5.9.3 shows various images of solids during the course of the experiment. Fig. 5.9.3a shows the same solids after one hour of chemical leaching. Apparently, the cracks and fissures induced or created by ultrasound are now changed or filled with reaction products. It is now confirmed that ultrasound is less likely to contribute to leaching slowness or particle passivation. Fig. 5.9.3a confirms this finding, which is similar to that in Fig. 5.9.If, for example. Fig. 5.9.3b is the S E M image of the leached solids after being sonicated again in decane for one hour. Apparently, some sort of surface damage or disruption has been induced on the leached particles. This is supported by the increase in the amount of copper extracted upon performing the final leaching stage. This image also supports the previous indication that some 199  improvement in copper extraction can be obtained i f the intermediate sonication time is increased, in order to induce more surface damage. The little damage or activation obtained at this stage by ultrasound may be due to the smaller size of particles reached upon dissolution. Fig. 5.9.3c is the S E M image of the solids after the final leaching stage. The change in surface morphology is now clear and similar to that in Fig. 5.9.2b, for example. The original cracks or fissures caused by ultrasound are now covered or filled by reaction products. These images and those presented earlier show that this system follows the same pattern of passivation or leaching behavior under a variety of sonication conditions, which confirms that passivation is due to chemical leaching and can be considered the main reason for the observed slow kinetics and low copper extraction in sulfate systems.  Experimental objective  To study the effect of leaching procedure on copper extraction (CuFeS concentrate, narrow-sized fractions, sulfate media) 2  Experimental conditions Sonication medium Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ] [H S0 ] 2  4  3  4  2  4  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time 2  Decane 75 °C -180 um +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79% 3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating ( 1 0 s - 5 s ) @ 50% amplitude control 120 min  Results Sonication in decane Chemical leaching (fresh solution) Sonication in decane Chemical leaching (same leach solution) Accumulated copper extraction  A A reading, ppm of C u Not applicable 2366 Not applicable 2763 2763 2 +  Conversion 0.2983 0.3483 0.3483  Time, min 60 60 60 60 240  Table 5.9.3: Results for the procedure of sonication in decane (1 hour), chemical leaching (1 hour), sonication again in decane for 1 hour and final chemical leaching for another hour 200  The use of ultrasound prior to leaching showed that little improvement in copper extraction is obtained. Because of the improvement in copper extraction upon using sonication between two leaching stages, it was desired to see i f more copper could be dissolved i f the sonication time was increased. To do so, the following procedure was used: 1) Chemical leaching for 1 hour 2) Sonication of the leached solids in decane for 2 hours 3) Chemical leaching of the sonicated particles for another hour That is sonication is delayed until the build up of reaction products, then testing i f ultrasound can remove these products and improve the amount of copper extracted. In the same manner, a sample of the concentrate was chemically leached at 75 °C as shown in Table 5.9.4. The reaction mixture was then centrifuged and the solids were sonicated in 150 ml decane for 2 hours. The mixture was again centrifuged to separate the solids, which were dried and leached again by the same leach solution at 75 °C. The results are also shown in Table 5.9.4. As can be seen from this table, there is an improvement in copper extraction upon using this procedure compared to Table 5.9.2 or 5.9.3, for example. The increase in amount of copper released suggests that ultrasound has induced some physical effect on the leached particles, more likely to be surface damage or product layer disruption. S E M images in fact support this claim. Fig. 5.9.4a shows the chemically leached particles after being sonicated in decane for two hours. Compared to Fig. 5.3.27c, it is obvious that ultrasound has induced significant changes to particle surface. The reaction products can still be seen compared to fresh solids (see Fig. 5.3.27a). That is, the chemical reaction has altered the surface of the fresh concentrate, while ultrasonic vibrations caused product layer disruption or removal. Pitting of the particle surface can be seen, which implies more exposure of the particles to the lixiviant and more copper dissolution. Sonication in decane has disrupted or removed the reaction surface products, including sulfur, but no floating particles were observed, unlike the case with aqueous solutions. The prolonged sonication time proved beneficial in this case, which was expected because although smaller ultrasonic intensity is obtained i f decane is the medium, such a low intensity can be compensated i f sonication time is increased, as indicated earlier.  202  The value obtained for final conversion is similar to that obtained i f sonication was carried out in water, as reported in the previous section, which means that this system has the same behavior regardless of the sonication medium, provided that similar ultrasonic intensity or sufficient sonication time is used. Experimental objective Experimental conditions Sonication medium Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ] 2  4  3  4  [H2SO4]  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time 2  To study the effect of leaching procedure on copper extraction (CuFeS2 concentrate, narrow-sized fractions, sulfate media) Decane 75 °C -180 um +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79% 3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating (10 s - 5 s) @ 50% amplitude control 120 min  Results Chemical leaching (fresh solution) Sonication in decane Chemical leaching (same leach solution) Accumulated copper extraction  A A reading, ppm of C u Conversion 1324 . ' 0.1669 Not applicable 3263 0.4114 3263 0.4114 2 +  Time, min 60 120 60 240  Table 5.9.4: Results for the experimental procedure: chemical leaching (1 hour), sonication in decane (2 hours) and leaching (1 hour). Compare the results to those in Table 5.8.5. This same pattern followed by the system can again be proved by S E M images. Fig. 5.9.4b shows the solid particles at the end of the experiment. The sonicated particles were leached for another hour, and so surface products have formed again, giving the same particle morphology as in other images (Figs. 5.9.1f, 5.9.2b and 5.9.3c, for example). Because the amount of copper extracted is comparable to that obtained upon sonication in water, it can be said that ultrasound is not a source of particle passivation. Finally, this procedure can be considered the best approach if decane was selected as the sonication medium. 203  (a)  (b) Fig. 5.9.4: S E M images of the size fraction -180 um +149 um (-80 mesh +100 mesh) under different conditions (Table 5.9.4): (a) Leached solids after being sonicated for 2 hours in decane, magnified 3000X and (b) The same solids after additional chemical leaching for 1 hour, magnified 3000X 204  Another change to this system was performing sonication in two different media to see i f more copper can be extracted. The idea here is to see i f improvement can be obtained upon sonication under different conditions. Although this may make the procedure tedious and complex, especially since organic and inorganic solvents are used, it was tested to shed some light on the behavior and efficiency of ultrasound as an activation method in different media. To do so, the following experimental procedure was followed: 1) Sonication of the concentrate in decane for 1 hour 2) Leaching of the sonicated sample for 1 hour 3) Combined leaching and sonication of the sample for another hour in ferric sulfate solution In the same manner, after sonication in decane, the solids were centrifuged, dried and leached for one hour. Then sonication was applied to the reaction mixture and leaching was continued for another hour. Samples were taken during the experiment and Table 5.9.5 summarizes the results. Apparently, copper extraction did not improve significantly and, in regard to the previous procedure, the results are comparable. That is, using different sonication media will not cause dramatic changes to the system, but ultrasound itself can be considered a good activation method. The results are similar to those in Table 5.8.6. Regardless of the sonication medium, the effect of ultrasound will take place and its effectiveness is limited by the nature of the leaching reaction and particle size reached. Several S E M images were taken after every step of this experimental procedure and were similar to those previously presented for the same steps (Figs. 5.9.1d and 5.9.3a). Hence, the effect of ultrasound on solids is largely physical. This was further confirmed from S E M testing at the conclusion of the experiment (Fig. 5.9.5). This figure shows that little pitting is left on the particle surface due to the presence of ultrasound during leaching. It seems as i f there were two processes together: product build-up and their disruption or removal by ultrasound. The pitting or disruption on the particle surface in this figure is similar to that shown in Fig. 5.8.8 and confirms the physical effects of ultrasound. Because of the similar values obtained for copper extraction upon sonication in different media, ultrasound is unlikely to contribute to particle passivation. As indicated earlier, one of the  205  possibilities to improve further the experimental results is the use of grinding media, such as tungsten carbide, which will be discussed in the next section.  Experimental objective Experimental conditions Sonication medium Temperature Particle size range Solution volume [Fe (S0 ) ] [FeS0 ] 2  4  3  4  [H2SO4]  CuFeS Mineral type Total stoichiometric copper Agitation Solid pulp content Sonication parameters Horn/flask configuration Sonication power input Sonication mode Total sonication time 2  Results Sonication in decane Chemical leaching Sonochemical leaching Accumulated copper extraction  To study the effect of leaching procedure on copper extraction (CuFeS2 concentrate, narrow-sized fractions, sulfate media) Decane 75 °C -180 urn +149 um (-80 mesh +100 mesh) 150 ml 0.25 M 0.25 M 1M Stoichiometric with respect to [Fe ] Gibraltar chalcopyrite concentrate 7931.4 ppm N purging 2.79% 3+  2  Conical flasks/Extender/Full immersion 60 W @ 20 kHz Pulsating (10 s - 5 s) @ 50% amplitude control 120 min A A reading, ppm of C u Not applicable 2380 3046 3046 2 +  Conversion 0.3000 0.3840 0.3840  Time, min 60 60 60 180  Table 5.9.5: Results for the procedure of sonication in decane (1 hour), leaching (1 hour) and combined sonication and leaching (1 hour)  206  g. 5.9.5: S E M image of the size fraction -180 um +149 um (-80 mesh +100 mesh) under different conditions (Table 5.9.5): The solids at the end of the experiment (1 hour of sonication in decane, followed by 1 hour of chemical leaching, and finally another hour of combined chemical leaching and sonication, magnified 3000X  From these experimental findings, it can generally be deduced that decane can be used as a sonication medium, but it will make the procedure complex, since it is an organic solvent. Sonication intensity in decane is smaller than that in water, due to viscosity differences, but can be compensated by adjusting the ultrasonic processor to the required operating parameters or increasing the sonication time. This enables obtaining comparable results. Ultrasound is not contributing to particle passivation. Chemical leaching is the only source of passivation caused by the build-up of reaction products. The results for leaching the sonicated solids, whether sonication was done in decane or water, are comparable. Sonication in water is more efficient, in terms of amount of copper extracted and leaching kinetics observed. Sonication in aqueous systems does not lead to the formation of passive oxide layers, under the experimental conditions used. The effectiveness of ultrasound as an activation method is controlled by different factors, among which are: intensity and particle size. The ultrasonic processor in hand predetermined the 207  intensity level of the sonic waves. As evident, ultrasound effects are taking place as long as ultrasonic waves can interact with the solids (through breakage) and the leach medium (through mixing). The ultrasound effect is mainly physical (surface damage through disruption, pitting and particle impingement) and takes place regardless of solution type and conditions or experimental procedure. The remaining outcome is dependent on the efficiency of the leaching medium. Surface damage induced by sonication in decane differs from that in water. Particles sonicated in decane are not reacting and little shattering is observed, but significant cracks and fissures are created on the particle surface. The experimental procedure was changed several times to find the best mode or combination of sonication and leaching that will yield maximum copper extraction. With decane, the best procedure is likely sonicating the solids after being leached for some time. Sonication will remove or disrupt passivating layers and increase dissolved copper. In aqueous solutions, the best procedure is combining ultrasound with leaching, which means the removal of product layers and particle breakage, simultaneously and incessantly. S E M testing of the solids during the progress of the experiments supports the experimental findings. The presented S E M images gave useful hints on the behavior of the leaching systems and explained some of the differences in results obtained with sonication in water or decane.  208  5.10 Sonochemical Leaching with Tungsten Carbide Balls As was reported in the previous sections, copper extraction in most cases is not complete. The trend of the conversion-time curves was similar under various combinations of ultrasonic, thermodynamic and physical conditions. Ultrasound has been proved not to be a source of particle passivation. Passivation, especially in sulfate systems, is largely dependent on solution conditions, and caused by the build-up of reaction products. Because the system is dependent on the initial chalcopyrite particle size, it was thought that i f finer size fractions were used, there might be some improvement in the amount of copper extracted. Next, i f some inert grinding media were used, these, in combination with the physical effects of ultrasound, might lead to significant improvement in copper dissolution. As apparent from Section 2.1, some researchers have suggested the use of ultrafine particles (~0.5 um) to realize complete copper dissolution. In this context, and to benefit from the effects of ultrasound, a new size fraction was used. Solid particles of the size 13 um were prepared as outlined in Section 4.1. These particles were leached under a variety of sonication and chemical leaching conditions. In addition, an inert grinding medium composed of tungsten carbide (WC) balls was used to increase the effects of ultrasonic waves. The size of these balls was 1.14 mm (0.045") and their specific gravity was ~14. These balls do not interact or dissolve in the leach medium. The balls are hard enough to cause particle attrition, while not interfering with the leaching process/system. The balls were mixed with the solids prior to being added to the leach solution. Tables 5.10.1 and 5.10.2 summarize the experimental findings for pure chalcopyrite and the concentrate, respectively. The balls were used at a mass ratio of solids/balls of 2. The reaction with this size fraction was faster than with all the previously tested size fractions. Especially with ultrasound activation in chloride media, the reaction ceased in less than 20 minutes. The use of grinding media even accelerated the reaction. Samples taken in this period of time showed that significant amounts of copper were dissolved. Because it was difficult to follow the leaching kinetics, it sufficed to use the estimated final conversion as the basis for comparison. From the previous tables, encouraging results are obtained, specifically in chloride media. Compared to experiments without sonication, there is a clear improvement in the amount of copper extracted (around one-third improvement) with ultrasound activation. The use of the 209  grinding medium also increased the amount of copper extracted. Almost complete copper dissolution was possible. Later, these experiments were repeated with higher volumes (250, 500 and 750 ml), and the results were similar, giving indications of possible scaling-up. The improvement in sulfate media is not large. The increment in amount of copper extracted is less than one-fifth, suggesting that particle passivation has easily occurred with such fine sizes. However, the use of grinding aid has given some improvement in copper dissolution. This encouraged extending the use of grinding media to other size fractions. Several experiments with the grinding aid were performed under the best sonication and thermodynamic conditions established previously. The conditions are basically the same as those in Tables 5.3.1-5.3.4. For brevity, the results for the size fraction -180 um +149 um (-80 mesh +100 mesh) are reported here. Tables 5.10.3 and 5.10.4 summarize the results obtained. The improvement in amount of copper extracted with the grinding media is not very significant. However, the improvement in leaching kinetics is significant, as per Figs. 5.10.15.10.4. As can be seen from Figs. 5.10.1 and 5.10.3 for sulfate media, the reaction rate is faster with the grinding aid by about 50% compared to that with sonication alone, as can be revealed by a quick estimation using the parabolic leaching model. This enhancement translates to increasing the rate by 30 folds compared to leaching without ultrasound activation (control experiments). The improvement in chloride media, Figs. 5.10.2 and 5.10.4, is less observed, possibly because the problem of passivation is less severe compared to sulfate systems. Nonetheless, there is about 10% improvement in amount of copper extracted, while the leaching kinetics are about one-fourth faster with the grinding aid compared to those with sonication. Once again, compared to leaching without sonication, the reaction rate is faster by about 18 folds. So, the use of inert grinding aid, in the form of tungsten carbide balls, is justified under the current experimental conditions. During the sonication process, the balls were swinging under the horn tip and/or moving together as a single bed. The effect of the grinding aid is enhancing the abrasion removal of passivating or product layers on chalcopyrite. Physical testing of the leach residue confirmed this, where very fine particle milling was seen. The incomplete conversion is again attributed to the formation of product layers and the limitation on ultrasonic waves effectiveness imposed by the particle size reached, as explained earlier. Several S E M images were taken for different samples leached with the grinding aid. Sample images are shown in Fig. 5.10.5. Figs. 5.10.5a and b are for the fresh concentrate (size 210  fraction is -180 um +149 jam). Figs. 5.10.5c and d show the sample after chemical leaching in chloride media. It is noted that the particles are retaining their original shape, but the reaction products are clearly formed. The nature of the product layer is similar to that for pure chalcopyrite (relatively porous and less dense), as in Fig. 5.3.24. The use of ultrasound has caused dramatic changes to the solids. Clearly, from Figs. 5.10.5e and f, there is a significant damage to the particle surfaces, exposing them to the leachant. As can be seen, there are dramatic changes to both surface morphology (as a result of chemical reaction) and particle size (as a result of ultrasound milling). The ultrasonic milling was further enhanced by the use of grinding aid. As in Figs. 5.10.5g and h, the tungsten carbide balls caused extra damage to the particle surface. The solids appear to have a significant reduction in size or particle dissolution, explaining the high conversion obtained (Table 5.10.4). Likely, only reaction products appear.  211  o O  H-» CO  fa  1  CD  _o  43  3  CO  00  CO  © Os T 3 UO SO 3 oo 03 CO  JJ 3  43  CD  in  T3  fa fa  3  CD  d  U CD  bH  CD fa fa O CD  CD  u-> ©  Cl  O cd  |3 CD  CD  s 00  O cvo  a  g |3 g  id  O S-  3 o o  00  s o  3  CD !  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Time, min  Fig. 5.10.1: Plot of conversion vs. time for leaching pure chalcopyrite in sulfate media as per Table 5.10.3 0.90  -i  Time, min Fig. 5.10.2: Plot of conversion vs. time for leaching pure chalcopyrite in chloride media as per Table 5.10.3 216  0.70  n  0  15  30  45  60  75  90  105  120  Time, min 5.10.3: Plot of conversion vs. time for leaching chalcopyrite concentrate in sulfate media as per Table 5.10.4 0.90 -i  Q  Time, min  5.10.4: Plot of conversion vs. time for leaching chalcopyrite concentrate in chloride media as per Table 5.10.4 217  (c)  Fig. 5.10.5 (continued): The same size fraction leached without sonication as per Table 5.10.4 (chloride media), magnified (c) 500X and (d) 3000X  219  Fig. 5.10.5 (continued): The same size fraction leached with ultrasound activation 5.10.4 (chloride media), magnified (e) 500X and (f) 3000X  (g)  Fig. 5.10.5 (continued): The same size fraction leached with ultrasound activation and grinding aid as per Table 5.10.4 (chloride media), magnified (g) 500X and (h) 3000X 221  5.11 Sonochemical Leaching at Controlled Redox Potential of 700 mV So far, it was not possible to overcome the problem of passivation in the studied leaching systems. Several variations were employed that covered a wide range of experimental conditions without any real improvement, especially in sulfate systems. One of the suggestions in this regard was to perform the leaching at redox potentials where particle passivation is not expected (Pinches et al. (2001)). Specifically for chalcopyrite, this potential is around 700 mV (vs. SHE). As can be seen from the stability diagrams, Figs. 2.1.1 and 2.1.2, virtually soluble phases exist at this potential, hence it was necessary to study the system under such conditions in order to develop and obtain some basic and fundamental engineering data. To do so, the solution electrochemical potential was set at the required value (700 mV vs. SHE), and leaching conditions were as in the previous work. Tables 5.11.1 and 5.11.2 summarize the results for pure chalcopyrite leaching at 25 and 75 °C, respectively. The solution redox potential was set at the selected value by adding appropriate amounts of the oxidizing agent: KMn0 . 4  At 25 °C, no significant improvement was obtained. This in fact was expected, because the system is temperature dependent. Although comparable amounts of copper were extracted, leaching kinetics are still faster with ultrasound activation. With sonication, reaction rates are faster by a factor of two compared to those under control conditions. The trend of the conversion-time curves shown in Figs. 5.11.1-5.11.4 demonstrates the system dependence on temperature. The effects of leaching at pre-determined electrochemical potential appear upon leaching at 75 °C. Leaching at the selected potential seems to avoid the problem of passivation by a thin product layer. However, the trend of the leaching curves suggests that the leaching kinetics are still controlled by diffusion through product layer. This is recognizable if recalling that one of the reaction products is sulfur or a defect structure of chalcopyrite. The progressive thickening or build-up of sulfur layers or the formation of other thin layers is responsible for the observed kinetic behavior of the system. This can be the only explanation for the system's parabolic leaching behavior, under the experimental conditions employed. Because considerable amounts of copper were dissolved, there must be some change to the nature or state of the layers formed. It seems that the formed sulfur or product layers are 222  further being removed from the particle surface by ultrasonic vibration (abrasion), and/or subsequent dissolution or oxidation to sulfate caused by the strength or redox potential of the solution. This is the logical explanation for the improvement in the amount of copper extracted. S E M testing gave further information in this regard and supports these arguments as explained below. Complete copper dissolution was not obtained in sulfate media compared to that in chloride media, likely due to the nature of product layers formed and their subsequent slow dissolution under the experimental conditions employed. The dissolution of reaction products appears to be much slower in sulfate media compared to that in chloride media due to the nature of the ions. The product layers in sulfate media will isolate the reacted particle from the lixiviant for a longer time, giving slower kinetics and lower conversion. Although comparable amounts of copper were extracted with ultrasound activation alone or control leaching at 700 mV (vs. SHE), the leaching kinetics with ultrasound are still faster. Further, the leaching kinetics are twice faster with ultrasound activation at 700 m V (vs. SHE), as can be found by estimating the reaction rate constants. Moreover, the amount of copper extracted in the latter case was increased considerably, confirming again that ultrasound is not contributing to particle passivation. In chloride media, there is not much difference in leaching at 700 mV (vs. SHE) compared to that at other redox potentials. This is comprehensible since the problem of passivation by a thin product layer is less severe in chloride media. As was expected, leaching at 75 °C made it possible to obtain considerable copper dissolution, especially when leaching was carried out with ultrasound activation. The reaction progress was very fast in the first half period, confirming the efficiency of chloride ions as leaching media. The solid samples, especially in chloride media, almost dissolved while considerable precipitates formed at the conclusion of the first half of the reaction period. The precipitates were iron species, most likely hematite. As a matter of fact, centrifuging was needed to separate the filtrate from remaining leach residue (if any) and precipitates. The complete copper dissolution with ultrasound activation demonstrates again that particle passivation is not related to sonication. Finally, to ensure that the permanganate ions did not participate in copper extraction by direct reaction with chalcopyrite, the previous experiments were repeated with solutions 223  containing similar amounts of the permanganate ions. The same experimental conditions were employed. Analysis of the leach solution for dissolved copper showed that a very small amount was dissolved (-2.5%) at the highest level of MnCV addition, which makes the previous findings valid. Apparently, complete copper dissolution is possible in chloride media compared to that in sulfate media. Since the phenomenon of passivation by thin layers is avoided and similar sonication parameters are employed, the only determining factor in this context is the nature or morphology of product layers and the efficiency of the lixiviant. Previously, the product layer in chloride media was shown to be porous and less dense by S E M images (Figs. 5.3.24 and 5.3.25), compared to that in sulfate media (Figs. 5.3.22 and 5.3.23). The images were used to explain why complete copper extraction was not obtained in sulfate media. For sonochemical leaching at 700 mV (vs. SHE), S E M images were also taken to support the experimental findings. Sample S E M images are shown in Fig. 5.11.5. These were taken for pure chalcopyrite leaching in sulfate media at 75 °C, as per Table 5.11.2. Sulfate media was selected to track the changes in particle topology with regard to passivation. Figs. 5.11.5a and b are for the fresh pure chalcopyrite. Upon chemical leaching, particle passivation can easily be seen (Figs. 5.11.5c and d). The nature of the product layer is identical to the one shown in Fig. 5.3.22, in that it seems to be semi-impervious. Sonochemical leaching on the other hand, induced particle damage and surface pitting (Figs. 5.11.5e and f). As was demonstrated in the previous sections, the product layer seems to have been scrubbed or "corroded" by ultrasonic vibrations and the surface topology has been altered. The surface morphology here is identical to that shown in Fig. 5.8.8. Chemical leaching at 700 mV (vs. SHE) has introduced dramatic changes to the nature of the product layer and morphology of particle surface. As shown in Fig. 5.11.5g and h, reaction products tend to separate or move away from the leached particle. The particle itself tends to break away (collapse) or separates apart. This explains the improvement in the amount of copper extracted. Once the particle is broken or fragmented, by the strength of the leach solution, more particle surface is exposed to the lixiviant and so more copper dissolution occurs. Surface distortion is significant and distinguishes the leaching behavior at 700 mV (vs. SHE) from ordinary chemical leaching. This can be observed by comparing the images to those in Fig. 224  5.3.22, which shows the product layer to be relatively more impervious than the one shown in Fig. 5.11.5L These changes are further intensified upon applying ultrasound activation. As can be seen in Figs. 5.11.5i and j , more surface damage was induced. Milling is imposed on the particles, enhancing the tendency to separate apart or break away, which explains the good amount of copper extracted. Ultrasonic vibrations lead to the abrasion of formed products, causing more exposure to the lixiviant. The net result is finer particle sizes (increased surface area) and more copper extraction driven by the solution strength (redox potential). The original particles and leaching products have dissolved significantly.  225  3 O  > a  3 O oo  •a o 03  co  03 CD fa  o  u  '3 o  CD  03  3  s o H->  H3 CD fa  fa O  3  o O  o  s o  o  c  >  03  CD  H-»  o fa X o  S  o o  a  -o  co  CD fa  CD  o  3  •3 o  O  CD • CD kH fa  fa  in  o  co•  o O  3.  1.8 [3 CN^ >>  i"T3  a  CN  H—>  CD  03  CD CD fa co CD  oo co  +  co CD  a  o H  CD  H-»  CD  o o  N  'kH > .  •<s OfaO CD  a  3  4=  3  CD  o _3 a =) CD  o O  a.  3  in  03 Cl CD  co kH  -kJ  CD CD  3> O  1  -3 3  CD fa X  'C  a CD fa X  w  1 CD  I  CD  fa  CD  O  s a CD  cl  CD  CD fa  u  3  CD  kH  60 fa* Cl  o o fa  o 3 a, fa s s fa o o 3fa o 3 cccio 3 o o 3 3 3 '5b 3 o o CD  03  o  3 3  I  H-»  o o  CD  m  CD  H-»  03 03  03  CD  a 3 CD  -3 CD co  a & 60 3  3 3 CD fa  PQ  > a  o o  — [  3  C/3  CD  fa  <N  o  u  CD  fa  o 3  SO  CD  o C/3  G  _o  a  P  o o  "H3  3  -4—»  X  CD bH  CD  OH OH O  3 O  U  o 3 o  3  CD  CD  O  o  •H  OH  o  X  '3 o  o  C/3  T3 JD  3  O  CD  3  fa  bH H—»  3  OH  a  O  o I  B  CD CD  cd v «s  C/3  O  in  CD  o o  fa  C 3  s  i,g  £  00  So  co" CD  "O  co  +  O  3. -3-  3  H  3>  3  43 -3 CD  3  CO  CD  3  H-»  CN CD  fa  o 3* o U  OH  3  O  bH  CD  -3  1  1  CD  CD  CD  2  x w  oo  "3  co  O  CD  C/3  C/3  G O CD  bH  OH O CD  CD  3  o o  o  So  3.  3 -7!  CD _> *H3 O CD  >  bH  H->  CD  C/3  6-  OH  OH <H-H  O 3 C/3 CD CD '3 fa o  a c  CD  X  w  G CD  CD  fa  a  CD  fa  3 o  CD  | C/3  CD  fa 3  u  CD  fa  3 fa  3  bH  CD G  00  <  H—>  CD  13  3 |  1  %  3  3  ca fa O  CD  3 3 o o C/3 C/3  00  3  bH  o o fa CD  44  O  CD  3 o  C/3  T3  CD co  os  0.30  n  0  15  30  45  60  75  90  105  120  Time, min 5.11.1: Plot of conversion vs. time for leaching pure chalcopyrite in sulfate media at 700 m V and 25 °C. Experimental conditions are as per Table 5.11.1.  0.50 -i  0  15  30  45  60  75  90  105  120  Time, min 5.11.2: Plot of conversion vs. time for leaching pure chalcopyrite in chloride media at 700 mV and 25 °C. Experimental conditions are as per Table 5.11.1. 228  0.90 -|  Time, min  Fig. 5.11.3: Plot of conversion vs. time for leaching pure chalcopyrite in sulfate media at 700 mV and 75 °C. Experimental conditions are as per Table 5.11.2.  0  15  30  45  60  75  90  105  120  Time, min Fig. 5.11.4: Plot of conversion vs. time for leaching pure chalcopyrite in chloride media at 700 mV and 75 °C. Experimental conditions are as per Table 5.11.2. 229  (a)  (c)  232  (g)  00 Fig. 5.11.5 (continued): S E M images of pure chalcopyrite, chemically leached at 700 mV in sulfate media as per Table 5.11.2, magnified: (g) 500X and (h) 3000X  233  G) Fig. 5.11.5 (continued): S E M images of pure chalcopyrite, sonochemically leached at 700 mV in sulfate media as per Table 5.11.2, magnified: (i) 500X and (j) 3000X  234  CHAPTER 6 CONCLUSIONS  This research has laid the basis for basic and fundamental studies of sonochemical leaching of copper sulfides. The results and observations presented in this context can be used for further studies on the applications of ultrasound to base metals extractive metallurgy. The main conclusions from this research can be summarized as follows: 1) Ultrasound is an important method for accelerating leaching kinetics and improving amount of metal extracted. 2) Ultrasound effect is largely physical on studied minerals, in terms of particle breakage, fragmentation and disruption of surface products. Surface morphology is distinctively altered. 3) Ultrasound induces its effect on particles regardless of their initial sizes, leading to comparable leaching rates and metal extraction. Sonication can lead to significant mitigation in required leaching conditions, by virtue of lessening leaching temperature and the need for fine grinding. 4) Ultrasound efficiency is limited by the method of generating and delivering ultrasound power. Effective exposure of particles to the sonic field is the major factor in obtaining distinctive effects of ultrasound. The efficiency is directly affected by the prevailing physical and thermodynamic conditions, as well as sonication parameters. 5) Ultrasound is not contributing to particle passivation in aqueous media, under the experimental conditions used in this research, as confirmed from sonication in organic media and leaching at pre-controlled redox potentials. 6) Sonochemical leaching is largely temperature dependent. Best results were obtained by leaching at 75 °C using the maximum possible ultrasonic intensity (0.4 W/ml). Best sonication effects were obtained using the horn/extender assembly, 10 s-5 s cycles and 60 minutes of sonication at 20 kHz. Sonication beyond 1 hour is not necessary. 7) Copper extraction can be improved by sonochemical leaching in the presence of grinding media.  235  8) Sonication can be performed in aqueous or organic solutions. Similar results can be obtained although the latter require longer sonication time i f dissimilar ultrasonic intensity is used. The procedure in organic solutions is more complex than that in aqueous ones. 9) Ferric ion leaching of chalcopyrite is controlled by diffusion through product layer, as was confirmed from the estimated values of activation energy. Species transport through a thickening sulfur or other passivating layer is rate limiting. The slow kinetics are caused by the nature and morphology of product layers, as evident from S E M images. The leaching reaction is written as: CuFeS ) + 4 F e 2(s  3+ (aq)  -» Cu  2 + ( a q )  + 5Fe  2+ (aq)  + 2S  (S)  and reaction stoichiometry was confirmed. Sulfur was recovered in elemental form. 10) Leaching kinetics are temperature and particle size dependent. Kinetics are less dependent on initial concentration of ferric ion, ferrous ion or amount of chalcopyrite added. Best leaching temperature is 75 °C. Significant copper dissolution is obtained using particles of 13 um size in a 2-hour time frame. 11) Leaching in chloride media, with or without sonication, is more effective than that in sulfate media, in terms of faster leaching kinetics and more copper extraction. The nature and morphology of product layers play an important role in this regard. 12) Apparently, chalcopyrite has similar response to ultrasound activation whether its form was pure (massive) or concentrates. 13) Selective dissolution of copper or iron was not obtained as apparent from mass balance calculations. This implies that copper and iron have similar response to ultrasonic activation. Nonetheless, sulfur yield was quantitative. The developed leaching models under the experimental conditions employed in this research are shown in the following table:  236  O  OS  00 5 s  CN  , CD  CD  a C  "1 ccj CD  CD fa ,  CN SO CN ro  1  1  o in os oC  o 00 in  0 OS  Os SO" CN  ro  CN  1  1  1  1  fa  CD  CD  CD  CD  CD  CD  X CD  CD fa  CD fa  CD fa  CD fa  CD fa  CD fa  CD fa  X  CN O  m  ^  X CN  H rt  CO OS CN  rt  CN  rt  '1 ,1  O  0 00 H  o  in  +  +  X  X  o  CN O  4*  rt  X CN  IN  CN  X  XI X  CN  +  +  X  X  CN  X  G  o  CN  13  CD  co  aa -  G o o 3 CO  _> fa*  3  o 3 o 3o co  o  +  +  +  +  X  X  x> X  X  a  G  o 3 o 3o  G  O  3 CD  CO  3  3  43  43  2 ro  ro CD  O  O  43  G  1 M  CD  a CD  ll  |3  ca  ccj  CD  a CD  ro 'C  o u  CD CD  c3 fa  3  00  I  3  CD CD  > • > fa o o  O  '1  a3  CD  CD CO CO  fa  a CD  ro "C  o  3 u  CD  CD +->  "C  ro  ro  i-sa T3 2 iro  CN  CN  1 — 1 ro  G  X  X  I  ro  rt  1  G  CD CD  cSo  -4-*  fa  o CD 3 43 O  3o co  43  CHAPTER 7 RECOMMENDATIONS FOR FUTURE RESEARCH  There are several recommendations for the continuation of this fundamental study on the applications of ultrasound to non-catalytic solid-fluid reactions. As there is a need for developing basic  engineering  data  on  sonochemical leaching of metal  sulfides,  some  of the  recommendations are: 1) A complete economic evaluation of the use of ultrasound in ferric ion leaching, that comprises the cost of sonic energy generation with comparison to conventional sources. 2) Extending the research work to cover other copper sulfides. 3) Extending the research work to include leaching in other media, such as acid pressure leaching, ammonia, HNO3, etc. 4) Extending the research work to other base metal sulfides, such as sphalerite, pentlandite, nickel laterites, auriferous ores, etc. 5) Use of flow-through reactor. 6) Use of other powerful ultrasonic processors with other horns/horn assemblies. 7) Running the leaching systems at higher solids contents (> 20%) for the purpose of scalingup. 8) Changing the purge gas (nitrogen in this research) with those of higher values of polytropic index (such as helium, argon or krypton) and studying the effects of related factors, such as flow rate, physical properties, thermodynamic properties, etc.  238  BIBLIOGRAPHY 1.  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Tromans, D. and Meech J., "Enhanced Dissolution of Minerals: Stored Energy, Amorphism and Mechanical Activation", Minerals Engineering, V o l . 14, No. 11, November 2001 (in press). 70. Walker, R., "The Role of Ultrasound in the Electro winning and Electrorefining of Metals", Hydrometallurgy, V o l . 4, pp. 209-215,1979. 71.  Weast, R. C. (Ed.): C R C Handbook of Chemistry and Physics, 56th ed., Sections B and D , C R C Press, OH, 1976.  245  APPENDICES  Appendix I  : MASS BALANCE CALCULATIONS  Appendix II  : LEACHING KINETICS MODELS  APPENDIX I Mass Balance Calculations  In this appendix, sample mass balance calculations are given to demonstrate the general leaching reaction stoichiometry: CuFeS s) + 4 F e 2(  3+ (aq)  -> C u  2 + ( a q )  + 5Fe  2+ (aq)  + 2S  (s)  (Eq. 1.1)  The calculations were based on estimations made through various analysis techniques, as was discussed in Chapter 4. Tables 4.1.1-4.1.4 are the basis for the calculations presented here. To demonstrate the validity of the experimental work conducted in this research, 8 different mass balance calculations are presented, which cover leaching results for pure chalcopyrite and chalcopyrite concentrate in sulfate and chloride media, with and without ultrasound activation. The experimental conditions are the same as those in Tables 5.3.1-5.3.4. The estimations are straightforward and account for copper, iron and sulfur balance in fresh mineral, leach residue and leach solution, as applicable. As with any experimental work, there are some errors incorporated in the calculations. One can see some difference between experimental analysis and theoretical calculations, which is expected in such cases. However, the error magnitude is small and the results can be considered valid under the experimental conditions used. The experimental work and its related mass balance calculations were repeated three times to ensure consistency and accuracy in the estimations. Mass balance calculations confirmed the reaction stoichiometry and conversion estimation.  247  U 60  <:  CD  H-H  "C  < 60  So OH O  00  00  00  00  00  60  00  SO  >  3 x  CD CD  %  B  bH  3  -3 3 3  uo  CM  o  3 _> fa  CO  CD  3  O  co OS  3  o X  H-»  00  O  '-3  O  3  ca CD  •*-» O  3 O  fa  X  o  H-H  '5b o  OO  co 3  _3  s  X CD CD  CD  H-»  X  CD fa fa  o  -3 ~a Pi pi  O  o  "3 CD >  o  co co  CO  bH  3  X  CD  .2  3  3 O  bH  CD fa fa  CbH  CO CO  <+H  o CD  O  3  o  o  bH  o  ca  CD  I  CD  3 fa CO  a  ca  H-H  CD CD  o fa  CD  CD fa fa  o  >  O  co  CO  3 3  CD  3 3  3 8 o CD bH  CD  fa fa O  CD  -a CD >  o  CO CO  3  o  00  00  bH  CD CD - 3 0 0 CD  Cm  CD  3  CO  JD  00  o  3  op  o  CD  3  o  _CD  3  3 3 'to  o fa  So fa  co CD  ~o  -3 3  CD  > 00  00  CD bH  3  fa  CbH  O 00  3  3 u CD  3  H-H  O  x o  co O  ca 3 CD  In  co  CD fa  00  00  00  00  00  00  OO 0 0  3 O  I 3 u 3  u  CD CD CD  CD  CD  H->  CD  'bH  So fa  CD  So So fa O  fa  o  CD  cSo 3  H-J  o JfD  3  fa X o  CD  3  XI  CD  co  ca  CD  O  bH  u  OH X  00  CD  fa  (S CD fa  O O  oo oo CD fa  3 <D  So fa  •*—»  3  O  o  CD  3 U  CD 00  CD  co c3  3 CD 3 o  CD bH  CD  fa fa O CD  ca x  CD  fa fa  O CD  XI  CD  co  CD  CD  c3 XI CD  co c3  ca 3 CD  3 3 o > 3CD CD  -3  03  00  CD  CD O  co co  o  CD  3 O  fa  0.4237  1.81%  1.7795 1.0207 0.7250 1.7458 1.90%  Estimated conversion based on leach residue analysis Estimated conversion based on leach solution analysis (Experimental error for conversion  |Iron balance |Total iron in added mineral |Iron in leach residue |iron dissolved by reaction |Total iron dissolved and undissolved (Experimental error for iron balance  0.8325 0.7954  Theoretical sulfur formed Estimated sulfur formed via organic solvents dissolution Experimental error for sulfur formation  4.66%  2.0262 1.1797 0.7954 1.9751 2.52%  Total sulfur in added mineral Sulfur in leach residue (unreacted chalcopyrite) Sulfur formed by reaction Total sulfur formed and unreacted Experimental error for sulfur balance  Sulfur balance  DO  0.4161  1.9851 1.1439 0.8250 1.9689 0.81%'  |Total copper in added mineral |Copper in leach residue |Copper dissolved by reaction |Total copper dissolved and undissolved [Experimental error for copper balance  |Copper balance 60 60 60 60 DO  DO DO DO DO DO 60 60 60  o CN  U 00  «;  CD +->  < 00  c  CH  o o  00  CO  N? 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X  w  00 00 00 00  | bH  CD  M vi  OH  3 O  3CD a 3 CD 3 3 o o  CD  OH X  3  I o3 o CD  3 3  CD  o 3  3* 00  CD  c  3CD cI o Z  3  co  fe  3 3 CD 3  So  3  O  CD  OH O  CO  OH  m  m  00  oo oo  00  CN  co So  3 §  CD  3  3 CO  H J3  c3  -O bH  S3  CD  o  H3  OH O  o  03  CD  CD  bH  c2 bH  So  -o -a CD > o  03 l-H  CD OH OH  o  u  CO CO CD  OH O, O  OH O, O  3 O  T3  CD  co 03  _G  3 O  CO bH  CD  o  3  o co 03  CD bH  CD OH  1 CD  o 3 CD CD 3 O o o U H CD  CD  l-H  CD  OH OH O  bH  £3 O  CD  "3 CD  3  3  O  co bH  s CD O  o  3 O  bH  H-»  eg  CD  g bH  CD  3  bH  So  -O  T3  >  CD  3  o co  3  -3  CD CD  CD  CO  3 O  bH  CD OH X  w  •8  CO  CN  APPENDIX II LEACHING KINETICS MODELS  II.1 KINETICS OF SOLID - FLUID REACTIONS The basic principles of chemical reaction kinetics and those for leaching models can be found in any textbook on physical chemistry or kinetics (see, for example, Levenspiel (1972)). It is not the intention here to give a comprehensive description of leaching kinetics, rather to review such kinetics with respect to the main leaching reaction. The mathematical modeling of leaching kinetics has become an important tool in describing hydrometallurgical processes, and for the purpose of research and development. Leaching reactions are normally described as non-catalytic solid-fluid reactions, where one or more reaction steps are encountered. The early work on leaching modeling was applied for describing very simple laboratory reactions. Such models were developed assuming simple or elementary reactions (that is, one step mechanism). With progress and advances in leaching processes, such models were modified to account for various factors affecting leaching kinetics. For any solid-fluid reaction, the following mechanistic steps are considered: 1) Chemical reaction in solution 2) Boundary layer diffusion 3) Product layer diffusion 4) Chemical reaction at the solid-fluid interface with or without charge transfer 5) Diffusion of products away from the interface. 6) A combination of two or more of these steps Sample geometry also manifests itself in kinetic effects, because of variation in surface area during the reaction. Flat plates and disks react with a minimum variation in area, whereas isometric shapes, such as cubes, cylinders and spheres, react with considerable change in area. According to Levenspiel (1972), there are different leaching models for the stated mechanistic steps, which would account for the rate-determining step for a reaction of the type: A(i) + bB( ) -> products S  (Eq. II. 1)  264  For narrow-sized spherical particles, these models are: 1) Linear leaching model: When the rate-determining step (RDS) is a chemical reaction occurring on the particle surface, the surface reaction model is used to describe the experimental data. The general model is: l-(l-^L) V  £-t  b  (Eq. II.2)  J  Xb is the fraction of CuFeS2 reacted at time t, and Ro is the initial radius of chalcopyrite particle, ki is the linear rate constant, that includes the specific rate constant for the surface reaction and a surface roughness factor, represented by k , volume and cross sectional area of molecular sc  reactants, represented by the molar volume, V , stoichiometric factor, b (in the case of Eq. II. 1, b is the molar ratio of ferric ions to chalcopyrite), and the concentration term(s),CAf, defined as : ki =  (Eq. II.3)  C A f k s c  Vb  k If the reaction constant is written as k = — t h e n from this model a plot of s  R  (  1 - (1  v  0  X b  1/3 ^  -)  J  vs. time should yield a straight line having the slope k . The new version of this model is written s  as: - = 1-(1-X )  (Eq.II.4)  , / 3  b  T  where: =  ° bk C P  b  (Eq. II.5)  R  s  A f  P B is the particle molar density, Ro is the initial radius of chalcopyrite particle, b is the stoichiometric factor, k is the surface chemical reaction rate constant and CAf is the bulk fluid s  concentration. Xb is the conversion with respect to solids, while x represents the required time for complete conversion. This model is applicable to particles of changing and unchanging size. 2) Parabolic leaching model: When reaction products form on the reacting solid, the kinetics may be governed by the nature of the product layer. Generally, very porous coatings (product layers) will not inhibit the 265  reaction, and the rate will be controlled by other factors, possibly to include surface reaction or charge transfer. Less or non-porous (protective) layers will present a resistance to reagent reaching the reacting interface. The rate-controlling step for such a topochemical process (a reaction occurring on the particle surface) may involve the diffusion of one or more reactants through this layer (such as the case of ferric ion leaching of chalcopyrite studied in this research). In some cases, the diffusion of reaction products through such a layer may be limiting. The equation relating diffusion paths to sample geometry for reactions proceeding topochemically in spherical shapes is: (  1  v  i--x -a-x )' b  3  b  A ;  k  =  (Eq. II.6)  Again, Xb is the fraction of chalcopyrite reacted at time t, and Ro is the initial radius of chalcopyrite particle. The rate constant kd contains the effective coefficient of diffusion (D ); e  which is a combination of diffusion coefficient (D), porosity and tortuosity, molar volume of reactant (V), and concentration terms ( C A O , and is defined as: k  d  = 3 V b D  e  AC  A  Clearly a plot of  (Eq.  F  r  1--X -(1-X )' b  b  II.7)  vs. time should yield a straight line having the slope  The new version of this model is written as: - = 1-3(1-X )'+2(1-X ) x b  b  (Eq.II.8)  where: x=  ° 6bD C P  b  (Eq. II.9)  R  e  A f  P B is the particle molar density, Ro is the initial radius of chalcopyrite particle, b is the stoichiometric factor, D is the effective diffusivity of the fluid in the product layer (in this case it e  is a function of molecular diffusivity, tortuosity, shape factor, and roughness) and CAf is the bulk fluid concentration, x and Xb are the required time for complete conversion and the conversion of chalcopyrite, respectively, as above. The parabolic leaching rate constant (k in this research) is estimated from Eq. II.8. This model is only applicable to particles of unchanging size. 266  For these two models, surface reaction control and product layer diffusion control (or ash control model), one can distinguish reaction kinetics on the basis of a plot of reaction rates vs. inverse of initial particle size. A plot of the initial rates vs. — (Eq. II.5) gives a straight line for 1 linear kinetics and a plot of the initial rates vs. —-7 (Eq. II.9) gives a straight line for parabolic kinetics, where d is the initial diameter of chalcopyrite particle. 0  In this research, all the kinetic analysis by the selected leaching model is done with reference to chalcopyrite particles, and all generated rate expressions or leaching graphs are with respect to these solid particles, unless otherwise specified. So, Xb is related to chalcopyrite conversion. 3) Leaching model for diffusion through a boundary fluid film control: A less common case in leaching systems is when diffusion through a boundary fluid layer becomes the RDS. In this case the appropriate model is: - =X  (Eq. 11.10)  b  T  where:  T  =  T^V 3bk C f  (Eq. 11.11) A f  with kf being the mass transport coefficient between the fluid and solids (for the remaining symbols, the same definitions hold). This model is only applicable to particles of unchanging size. For particles with changing size, Eq. 11.10 is modified to read: - = 1-(1-X ) x  2 / 3  b  (Eq.II.12)  where T, as usual, is the time required for complete conversion, and defined as: PB O R  2bC D  (Eq. 11.13)  A f  267  with the same definitions for these symbols. This model is only applicable to small particles of changing size in the Stokes regime. 4) Mixed kinetics: Generalized rate equations can be developed to explain special cases of mixed kinetics, when one or more steps are rate determining. When both the interfacial area mechanism (chemical reaction control) and the diffusion mechanism are contributing to the control of the reaction, the mixed kinetic model will be: 1-  |x  b  -(1  -xj)  + p f 1 - ( 1 -xj)  =\  C  A f  t  (Eq. II. 14)  Xb is the fraction reacted at time t and CAf is the concentration of the fluid, A . The constants: P=^ _ R k 0  and  =^ R>p  s  (Eq. 11.15)  are determined empirically by the evaluation of selected data. The symbols are defined as above. k is the surface chemical reaction rate constant, and p is the particle mass density. It should be s  noted that the generalized expression shown above simplifies to linear kinetics model when k  s  «  k k kd or — « 0. Furthermore, when k » kd, or —- « 0, the resulting expression is identical to that 1  K  s  k  d  given for parabolic kinetics model. In the case where three controlling steps are involved, namely diffusion through boundary layer, surface chemical reaction and diffusion through product layer, the generalized model is: b8 D  3bR„  v b  2D  l-?X -(l-X )' b  b  +  - (1 - X ) ' ) = b  t  (Eq. 11.16)  e  with the same definitions for the symbols. 8 is the thickness of fluid boundary diffusion layer. For all the presented models, any convenient system of units can be used. More derivations for spherical and other geometrical shapes can be found in Levenspiel (1972). As a final note on linear fitting of the experimental data, a fit is statistically considered accurate and acceptable i f it has a coefficient of determination, r , greater than 0.96. r signifies the improvement or error reduction due to the straight line model (linear fitting). For r value of 268  1, the line obtained is a perfect fit and explains 100% of the variability. For r value of 0, the fit 2  represents no improvement.  II.2 THERMODYNAMIC DATA FROM T H E KINETIC ANALYSIS After deciding on a suitable kinetic model, the rate data can be recorded at various temperatures. The activation energy, E , is estimated from the slope of the curve of a plot of a  initial rate data (reaction constants) vs. inverse of temperature. This plot (Arrhenius plot) will normally result in a straight line from which E and other information can be obtained. a  According to Arrhenius equation, the reaction rate constant is defined as: -E k=k exp(-^)  (Eq. 11.17)  0  ko in some physical chemistry textbooks is called A . In logarithmic form, Eq. 11.17 is written as: lnk = ^ - + l n k  (Eq. 11.18)  0  1 - E Hence, a plot of In k vs. — should give a straight line of slope equals ——— and an intercept 1 K. equals In ko. From the slope, the apparent activation energy, E , can be estimated, while the a  intercept (at T" = 0) will give the pre-exponential (frequency) factor. Unless otherwise noted, all 1  logarithmic functions are Naperian or Natural logarithms. Generally, it can be said that for chemical reaction control systems, the activation energy is greater than 40 kJ per mol, while for diffusion control it is less than this value. Some authors prefer to give an activation energy value between 20 to 40 kJ per mol for mixed kinetic systems, but their claim is little of supporting evidence. The estimated activation energy would give different important information. If it was relatively small, then it will explain the rapidity of such a reaction, because, from the collision theory, i f the activation energy is large, only a small fraction of molecules will have enough energy to surmount the reaction barrier and react rather than rebound upon collision. The other important factor is the temperature sensitivity. A reaction with low activation energy is not very sensitive to temperature variations. High activation energy means that small variations in temperature, 5 to 10 degrees, would result in considerable change of reaction rates (at least by an  269  order of magnitude), which is suitable for chemical reaction controlled kinetics. Hence, it can be assumed that product diffusion kinetics are less temperature sensitive. The estimation of the activation energy would allow the estimation of the enthalpy of activation. According to the transition state theory, the enthalpy of activation of any reaction is estimated from the following equation:  k = K ^e  (  A  0  S  °  /  R  )  e-  (  A  °  H  /  R  T  (Eq. 11.19)  )  which results from replacing the activation energy with the definition of free energy of formation, AG = AH 0  0  -  T AS  (Eq. 11.20)  0  (see Nomenclature for the definitions of symbols). In logarithmic form:  ln(-) = — ^ - + ln { K % T RT I h 0  (  A  S  V  R  >  1 j  (Eq. 11.21)  So, the straight line obtained from plotting ln(—) vs. J_ may be used to estimate both the T T enthalpy and entropy of activation (from the slope and intercept, respectively). The value of the enthalpy of activation should be near to that of the energy of activation, and some discrepancy is normal in hydrometallurgical reactions. The entropy of activation 1  should have a net positive increase, that is AS ^ 0, for a reaction to take place. A demonstration 0  on related calculations can be found in Abed (1999).  270  

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