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Synthesis and solubility of arsenic tri-sulfide and sodium arsenic oxy-sulfide complexes in alkaline… Li, Wen 2013

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Synthesis and Solubility of Arsenic Tri-sulfide and Sodium Arsenic Oxy-sulfide Complexes in Alkaline Sulfide Solutions by Wen Li B. Sc., (Materials Engineering), Hebei University of Technology, China, 2006  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in  THE FACULTY OF GRADUATE STUDIES (Materials Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  June 2013 © Wen Li, 2013  Abstract Alkaline sulfide leaching (ASL) at approximately 100 ºC has been used to selectively extract arsenic and antimony from enargite and tetrahedrite concentrates. Sodium thioarsenate has been postulated to crystallize from alkaline sulfide leaching solutions upon cooling. However, literature data on the solubility of sodium thio-arsenate as well as proof of its crystallization from ASL solutions is scant. In this thesis, the solubility of leach-produced and synthetic sodium thio-arsenate is studied. To determine arsenic solubility in ASL solutions, sodium thio-arsenate and sodium arsenic oxide sulfide complexes are synthesized by various means and characterized by EDX, QXRD, and ICP. The synthesis of amorphous As2S3, sodium arsenic oxy-sulfide complexes, and sodium thio-arsenate is first presented. For amorphous As2S3 synthesis, the effect of concentration of sodium sulfide (0.1 M) and hydrochloric acid (1 M), temperature (40 ~ 60 ºC), and aging time (48 hours) was optimized. The solubility of synthetic sodium arsenic oxy-sulfide complexes and sodium thio-arsenate in ASL solutions increases significantly as temperature is increased to 95 ºC. More importantly, the solubility of sodium thio-arsenate at certain temperatures is significantly affected by the concentration of sodium hydroxide and sulfide in solution. Due to the common ion effect, if NaOH and HS- concentrations are very high, the solubility of sodium thio-arsenate decreases. Enargite leaching tests were done to characterize the precipitate that occurred upon cooling and to verify the arsenic saturation point, which should be between 38.5 ~ 58 g/L (0.51 M ~ 0.78 M) As depending on the NaOH and HS- concentration. Comparison with i  solubility experiments of pure sodium thio-arsenate shows that arsenic solubility in ASL solutions is supersaturated. However, direct comparison of saturation in ASL solutions and the solubility as obtained by the synthetic solutions/crystallites prepared here is not possible given the complex nature of the ASL crystallites that appear not to contain the often discussed “sodium thio-arsenate”.  ii  Preface This dissertation is original, unpublished, independent work by the author, W. Li.  iii  Table of Contents Abstract ..................................................................................................................................................................i Preface ................................................................................................................................................................. iii Table of Contents.................................................................................................................................................iv List of Tables ..................................................................................................................................................... viii List of Figures .......................................................................................................................................................x Acknowledgments ..............................................................................................................................................xiv Chapter 1 Introduction ........................................................................................................................................1 Chapter 2 Literature Review ...............................................................................................................................7 2.1 Copper arsenic minerals ...............................................................................................................................7 2.2 Metallurgical processing of enargite .......................................................................................................... 11 2.2.1 Pyrometallurgical treatment ............................................................................................................... 14 2.2.2 Alkaline sulfide leaching of enargite .................................................................................................. 17 2.2.3 Arsenic removal and precipitate analysis ........................................................................................... 20 2.2.4 Sodium thio-arsenate solubility .......................................................................................................... 24 2.3 Synthesis of amorphous As2S3 ................................................................................................................... 25 2.4 Synthesis of sodium thio-arsenate .............................................................................................................. 26 2.5 Solubility of amorphous As2S3 ................................................................................................................... 30 Chapter 3 Objectives .......................................................................................................................................... 33 Chapter 4 Experimental Procedures ................................................................................................................ 34 4.1 Preliminary solubility study on arsenic trioxide ......................................................................................... 34 4.2 Synthesis of amorphous As2S3 ................................................................................................................... 34 4.2.1 Concentration effect ............................................................................................................................ 37 4.2.2 Temperature effect .............................................................................................................................. 39 4.2.3 Effect of hydrochloric acid addition rate ............................................................................................ 39 4.2.4 Aging time effect ................................................................................................................................. 40 4.3 Solubility of amorphous As2S3 in sodium sulfide solution ......................................................................... 40 iv  4.4 Synthesis of sodium thio-arsenate .............................................................................................................. 40 4.4.1 Synthesis of precursor sodium arsenate by hydrogen peroxide oxidation of As (III) ......................... 40 4.4.1.1 Synthesis of sodium thio-arsenate from sodium arsenate and sodium sulfide solution ................................ 42 4.4.1.2 Synthesis of sodium thio-arsenate from sodium arsenate and solid sodium sulfide ..................................... 42 4.4.1.3 Synthesis of sodium thio-arsenate from sodium arsenate and hydrogen sulfide gas ..................................... 43  4.4.2 Synthesis of sodium thio-arsenate by reaction of arsenic tri-sulfide with elemental sulfur ................ 45 4.5 Solubility of synthetic sodium arsenic oxy-sulfide complexes and sodium thio-arsenate.......................... 46 4.6 Solubility accuracy calibration test ............................................................................................................ 47 4.7 Precipitation from an enargite leaching test ............................................................................................... 48 Chapter 5 Results and Discussion ..................................................................................................................... 52 5.1 Solubility of As2O3 in Na2S and NaOH solution ........................................................................................ 52 5.2 Characterization of synthesized amorphous As2S3 ..................................................................................... 55 5.2.1 Effect of reagent concentrations ......................................................................................................... 56 5.2.2 Effect of temperature .......................................................................................................................... 57 5.2.3 Effect of hydrochloric acid solution flow rate..................................................................................... 59 5.2.4 Effect of aging time ............................................................................................................................. 60 5.3 Solubility of As2S3 in Na2S solution........................................................................................................... 61 5.4 Characterization of synthetic crystalline products ...................................................................................... 63 5.4.1 Attempts to synthesize sodium arsenic oxy-sulfide from sodium arsenate precursor ......................... 63 5.4.2 Synthesis of sodium thioarsenate by reaction of As2S3 with elemental sulfur ..................................... 67 5.4.3 Arsenic content determination by ICP analysis .................................................................................. 69 5.5 Solubility of synthesized sodium thio-arsenate products ........................................................................... 70 5.5.1 Sodium arsenate precursor synthesized by oxidation of As (III) with hydrogen peroxide .................. 70 5.5.1.1 Sodium thio-arsenate synthesized by the addition of sodium sulfide solution to dissolved sodium arsenate precursor (product #1) .............................................................................................................................................. 70 5.5.1.2 Sodium thio-arsenate synthesized by the addition of solid sodium sulfide to dissolved sodium arsenate precursor (product #2) .............................................................................................................................................. 70 5.5.1.3 Sodium thio-arsenate synthesized by the addition of hydrogen sulfide to dissolved sodium arsenate precursor (product #3) .............................................................................................................................................. 72  5.5.2 Synthesis of thio-arsenate by oxidation of As (III) with elemental sulfur ........................................... 73 v  5.5.2.1 Solubility of sodium thio-arsenate (product #4) in leach solution under air atmosphere and argon atmosphere ............................................................................................................................................................... 73 5.5.2.3 Solubility of synthetic crystal #4 (sodium thio-arsenate) at constant hydroxide (2.5 M/100 g/L) and varying hydrosulfide content (0 M ~ 1.0 M/0 ~ 56 g/L) in argon .......................................................................................... 75  5.6 Solubility accuracy calibration test ............................................................................................................ 78 5.7 Solubility of As in ASL solutions originating from an enargite leaching test ............................................ 78 Chapter 6 Conclusion ......................................................................................................................................... 82 Chapter 7 Future Work ..................................................................................................................................... 83 References ........................................................................................................................................................... 84 Appendix ............................................................................................................................................................. 90 Part A. Tables of solubility data by experiment and OLI systems software for amorphous As 2S3 and synthetic crystals ............................................................................................................................................................. 90 A.1 Solubility data of orpiment in literature and amorphous As2S3 by experiment and OLI systems software ....................................................................................................................................................... 90 A.2 Solubility data of synthetic crystal #2, #3, #4 (sodium thio-arsenate) by experiment ........................... 96 A.2.1 Solubility of synthetic crystal #2 in air ........................................................................................................... 96 1.  In 100 ml 2.5 M sodium hydroxide and 1.0 M sodium sulfide solution ..................................................... 96  2.  In 100 ml 2.5 M sodium hydroxide and 0.5 M sodium sulfide solution ..................................................... 96  3.  In 100 ml 1.0 M sodium hydroxide and 1.0 M sodium sulfide solution ..................................................... 96  A.2.2 Solubility of synthetic crystal #3 in air ........................................................................................................... 97 1.  In 10 ml 2.5 M sodium hydroxide and 1.0 M sodium sulfide solution....................................................... 97  2.  In 10 ml 2.5 M sodium hydroxide and 0.5 M sodium sulfide solution....................................................... 97  3.  In 10 ml 1.0 M sodium hydroxide and 1.0 M sodium sulfide solution....................................................... 98  A.2.3 Solubility of synthetic crystal #4 (sodium thio-arsenate) in air ...................................................................... 98 1.  In 20 ml 2.5 M sodium hydroxide and 1.0 M sodium sulfide solution under air atmosphere .................... 98  2.  In 20 ml 2.5 M sodium hydroxide and 0.5 M sodium sulfide solution under air atmosphere .................... 98  3.  In 20 ml 1.0 M sodium hydroxide and 1.0 M sodium sulfide solution under air atmosphere .................... 99  A.2.4 Solubility of synthetic crystal #4 (sodium thio-arsenate) in argon.................................................................. 99 1.  In 10 ml 2.5 M sodium hydroxide and 1.0 M sodium sulfide solution under argon atmosphere ............... 99 vi  2.  In 10 ml 2.5 M sodium hydroxide and 0.5 M sodium sulfide solution under air atmosphere .................. 100  3.  In 10 ml 1.0 M sodium hydroxide and 1.0 M sodium sulfide solution under air atmosphere .................. 100  A.2.5 Solubility of synthetic crystal #4 (sodium thio-arsenate) at constant hydroxide and varying hydrosulfide content in argon ...................................................................................................................................................... 101 1.  In 10 ml 2.5 M sodium hydroxide and 1.0 M sodium hydrosulfide solution ........................................... 101  2.  In 10 ml 2.5 M sodium hydroxide and 0.5 M sodium hydrosulfide solution ........................................... 101  3.  In 10 ml 2.5 M sodium hydroxide and 0 M sodium hydrosulfide solution .............................................. 101  Part B. EDX analysis of synthetic complexes ................................................................................................ 102 Part C. Quantitative x-ray diffraction result of arsenic precipitate from enargite leaching ............................ 105  vii  List of Tables TABLE 1.1 MAJOR ARSENIC MINERALS OCCURRING IN NATURE (WTO ARSENIC COMPOUNDS, 2001) ....................1 TABLE 1.2 ARSENIC DEPOSITS OF THE WORLD (WTO ARSENIC COMPOUNDS, 2001) ..............................................2 TABLE 1.3 SUMMARY OF TOXIC EFFECTS IN HUMANS AFTER OCCUPATIONAL INHALATION EXPOSURE TO INORGANIC ARSENIC COMPOUNDS (EUROPEAN COMMISSION 2001) ............................................................... 4  TABLE 2.1 THE MOST COMMONLY FOUND (OR ANTICIPATED) ARSENIC COMPOUNDS IN AIR (EUROPEAN COMMISSION 2001) ........................................................................................................................................8 TABLE 2.2 THE MOST COMMON CU-AS-SB-S MINERALS AND THEIR PHYSICAL PROPERTIES (BARTHELMY 2004) ...9 TABLE 4.1 CONDITIONS OF AS2S3 SYNTHESIS CONCENTRATION TESTS .................................................................. 37 TABLE 4.2 CONDITIONS OF AS2S3 SYNTHESIS TEMPERATURE TESTS ...................................................................... 39 TABLE 4.3 CONDITIONS OF AS2S3 SYNTHESIS FLOW RATE TESTS ........................................................................... 39 TABLE 4.4 CONDITIONS OF AS2S3 SYNTHESIS AGING TIME TESTS .......................................................................... 40 TABLE 4.5 THREE DIFFERENT LEACH SOLUTIONS FOR SOLUBILITY TESTS.............................................................. 47 TABLE 4.6 THREE CONDITIONS FOR VARYING HYDROSULFIDE AND CONSTANT NAOH ......................................... 47 TABLE 4.7 ELEMENTAL ANALYSIS OF THE ENARGITE SAMPLE ............................................................................... 49 TABLE 4.8 QUANTITATIVE PHASE ANALYSIS OF THE ENARGITE SAMPLE ............................................................... 49 TABLE 5.1 EDX RESULT FOR EFFECT OF CONCENTRATION .................................................................................... 56 TABLE 5.2 EDX RESULT FOR EFFECT OF TEMPERATURE........................................................................................ 58 TABLE 5.3 EDX RESULT FOR EFFECT OF HYDROCHLORIC ACID FLOW RATE .......................................................... 59 TABLE 5.4 EDX RESULT FOR EFFECT OF AGING TIME ............................................................................................ 60 TABLE 5.5 ELEMENTAL ANALYSIS OF SYNTHETIC SODIUM ARSENATE BY EDX .................................................... 63 TABLE 5.6 EDX ANALYSIS FOR SYNTHETIC CRYSTAL #1, #2, AND #3 ................................................................... 64 TABLE 5.7 ELEMENTAL ANALYSIS OF SYNTHETIC CRYSTALLINE PRECIPITATE #4 BY EDX ................................... 67 TABLE 5.8 QXRD RESULT AND CRYSTAL PARAMETERS OF SYNTHETIC SODIUM THIO-ARSENATE ......................... 67 TABLE 5.9 ICP ANALYSIS FOR ARSENIC CONTENT IN PRECIPITATES #2, 3 AND 4 ................................................... 69 TABLE 5.10 SOLUBILITY DATA FOR ACCURACY CALIBRATION .............................................................................. 78 TABLE 5.11 ARSENIC CONCENTRATION OF SOLUTION A. AND B. BY ICP ............................................................... 79 TABLE 5.12 QXRD RESULT FOR ARSENIC PRECIPITATE FROM ENARGITE LEACHING TESTS (SEE XRD SPECTRA IN APPENDIX FIGURE C-1, 2, 3)......................................................................................................................... 80 TABLE A-1 ORPIMENT SOLUBILITY IN SOLUTIONS TO WHICH NO SULFIDE HAS BEEN ADDED (WEBSTER 1990) ..... 90 TABLE A-2 ORPIMENT SOLUBILITY IN SOLUTIONS TO WHICH SULFIDE HAS BEEN ADDED (WEBSTER 1990) .......... 91 viii  TABLE A-3 SOLUBILITY PREDICTION OF AS2S3 IN 1 WT% OF 1 L NA2S SOLUTION................................................. 93 TABLE A-4 SOLUBILITY PREDICTION OF AS2S3 IN 2 WT% OF 1 L NA2S SOLUTION................................................. 94 TABLE A-5 SOLUBILITY PREDICTION OF AS2S3 IN 3 WT% OF 1 L NA2S SOLUTION................................................. 95 TABLE A-6 SOLUBILITY OF AS2S3 IN 1 WT%, 2 WT%, AND 3 WT% OF 80 ML NA2S SOLUTION............................... 95 TABLE A-7 SOLUBILITY OF IN 2.5 M NAOH AND 1.0 M NA2S SOLUTION .............................................................. 96 TABLE A-8 SOLUBILITY OF IN 2.5 M NAOH AND 0.5 M NA2S SOLUTION .............................................................. 96 TABLE A-9 SOLUBILITY OF IN 1.0 M NAOH AND 1.0 M NA2S SOLUTION .............................................................. 97 TABLE A-10 SOLUBILITY OF IN 2.5 M NAOH AND 1.0 M NA2S SOLUTION ............................................................ 97 TABLE A-11 SOLUBILITY OF IN 2.5 M NAOH AND 0.5 M NA2S SOLUTION ............................................................ 97 TABLE A-12 SOLUBILITY OF IN 1.0 M NAOH AND 1.0 M NA2S SOLUTION ............................................................ 98 TABLE A-13 SOLUBILITY OF NA3ASS4·8H2O IN 2.5 M NAOH AND 1.0 M NA2S SOLUTION .................................. 98 TABLE A-14 SOLUBILITY OF NA3ASS4·8H2O IN 2.5 M NAOH AND 0.5 M NA2S SOLUTION .................................. 99 TABLE A-15 SOLUBILITY OF NA3ASS4·8H2O IN 1.0 M NAOH AND 0.5 M NA2S SOLUTION .................................. 99 TABLE A-16 SOLUBILITY OF NA3ASS4·8H2O IN 2.5 M NAOH AND 1.0 M NA2S SOLUTION ................................ 100 TABLE A-17 SOLUBILITY OF NA3ASS4·8H2O IN 2.5 M NAOH AND 0.5 M NA2S SOLUTION ................................ 100 TABLE A-18 SOLUBILITY OF NA3ASS4·8H2O IN 1.0 M NAOH AND 0.5 M NA2S SOLUTION ................................ 100 TABLE A-19 SOLUBILITY OF NA3ASS4·8H2O IN 2.5 M NAOH AND 1.0 M NAHS SOLUTION .............................. 101 TABLE A-20 SOLUBILITY OF NA3ASS4·8H2O IN 2.5 M NAOH AND 0.5 M NAHS SOLUTION .............................. 101 TABLE A-21 SOLUBILITY OF NA3ASS4·8H2O IN 2.5 M NAOH AND 0 M NAHS SOLUTION ................................. 101  ix  List of Figures FIGURE 2.1 CONVENTIONAL PROCESSING OF PRIMARY COPPER SULFIDE ORES (SAFARZADEH ET AL. 2012).......... 11 FIGURE 2.2 A CLASSIC HYDROMETALLURGICAL ROUTE FOR COPPER PRODUCTION FROM OXIDE ORES .................. 12 FIGURE 2.3 A HYDROMETALLURGICAL ROUTE FOR ENARGITE OR TENNANTITE ................................................... 13 FIGURE 2.4 A COMPARISON OF WORLDWIDE COPPER PRODUCTION BY COMBINED METHODS (TOTAL) AND SOLVENT EXTRACTION/ELECTRO-WINNING (SX/EW) BETWEEN 1980 AND 2005 (REPLOTTED FROM MOATS 2005) .... 14  FIGURE 2.5 A SIMPLIFIED FLOWSHEET OF THE EL INDIO ENARGITE ROASTER DRAWN WITH DATA ........................ 16 BY SMITH 1986...................................................................................................................................................... 16  FIGURE 2.6 SIMPLIFIED EH-PH DIAGRAM FOR THE CU-AS-S-H2O SYSTEM WITH 1 MOL/L DISSOLVED SPECIES AT 25 ºC AND 1 ATM (FILIPPOU ET AL. 2007) ..................................................................................................... 18 FIGURE 2.7 SIMPLIFIED EH-PH DIAGRAM FOR THE CU-S-AS-H2O SYSTEM WITH 1 MOL/L DISSOLVED SPECIES AT 25 ºC AND 1 ATM (FILIPPOU ET AL. 2007) ..................................................................................................... 19 FIGURE 2.8 AN ARSENIC DISPOSAL OPTION 1 BY ADDING H2SO4 (NADKARNI ET AL. 1975) .................................. 21 FIGURE 2.9 ARSENIC DISPOSAL OPTIONS 2 AND 3 BY ADDING CO2 AND SO2 (NADKARNI ET AL. 1975) ................ 22 FIGURE 2.10 FINAL ARSENIC ROUTE BY SCORODITE PRECIPITATE (NADKARNI ET AL. 1975) ................................. 23 FIGURE 2.11 SOLUBILITY OF SODIUM THIOARSENATE AND STIBNITE IN AQUEOUS SODIUM SULFIDE (2.2~2.4M) (REPLOTTED FROM NADKARNI AND KUSIK, 1988) ........................................................................................ 25 FIGURE 2.12 SCHEMATIC ILLUSTRATION OF THE CU3ASS4-NAHS-NAOH LEACHING AND NA3ASS4 PRECIPITATION PROCESS FOR ARSENIC REMOVAL AND CIRCULATION OF WASTE WATER TO LEACHING AND PRECIPITATION STAGES FOR RE-USE (TONGAMP ET AL. 2010A, 2010B) ................................................................................. 27  FIGURE 2.13 THE SOLUBILITY OF ORPIMENT IN 3.43 WT % NA2S SOLUTIONS AT 52, 100, 150 AND 200 ºC, AS A FUNCTION OF PRESSURE (REPLOTTED FROM WEISSBERG ET AL. 1966) ......................................................... 32  FIGURE 4.1 THE DELTA GIBBS ENERGY OF AS2S3 SYNTHESIS REACTIONS BY HSC ............................................... 35 FIGURE 4.2 AS2S3 SOLUBILITY PREDICTED BY OLI AT PH RANGE 0~12 AT 25 ºC .................................................. 36 FIGURE 4.3 EXPERIMENT SETUP FOR AS2S3 SYNTHESIS ......................................................................................... 37 FIGURE 4.4 SYNTHETIC ARSENIC TRI-SULFIDE POWDER ......................................................................................... 38 FIGURE 4.5 SYNTHETIC SODIUM ARSENATE ........................................................................................................... 41 FIGURE 4.6 CRYSTALLINE PRECIPITATE #1 SYNTHESIZED BY ADDITION OF SODIUM SULFIDE SOLUTION TO SODIUM ARSENATE SOLUTION .................................................................................................................................... 42  FIGURE 4.7 CRYSTALLINE PRECIPITATE #2 SYNTHESIZED BY ADDITION OF SOLID SODIUM SULFIDE TO SODIUM ARSENATE SOLUTION .................................................................................................................................... 43  x  FIGURE 4.8 EXPERIMENTAL SETUP FOR SYNTHESIS OF SODIUM ARSENIC OXY-SULFIDE COMPLEXES BY H2S ......... 44 FIGURE 4.9 CRYSTALLINE PRECIPITATE #3 SYNTHESIZED BY H2S GAS ADDITION TO SODIUM ARSENATE SOLUTION ..................................................................................................................................................................... 45 FIGURE 4.10 CRYSTALLINE PRECIPITATE #4 SYNTHESIZED BY REACTION OF ELEMENTAL SULFUR (12.8 G) WITH ARSENIC TRISULFIDE (50 G) IN 200 ML AT 3M SODIUM SULFIDE ................................................................... 46  FIGURE 4.11 CRYSTALLINE PRECIPITATE #4 SYNTHESIZED BY REACTION OF ELEMENTAL SULFUR WITH ARSENIC TRISULFIDE AFTER FILTRATION..................................................................................................................... 46  FIGURE 4.12 ENARGITE MINERAL SAMPLES ....................................................................................................... 48 FIGURE 4.13 CRYSTAL PRECIPITATED FROM PREGNANT ASL SOLUTION AFTER ENARGITE LEACHING .................. 50 FIGURE 4.14 SIZE OF CRYSTALLITES OBTAINED UPON COOLING OF ASL PREGNANT LEACH SOLUTION ................. 51 FIGURE 5.1 SOLUBILITY OF AS2O3 IN 0.5 M SODIUM SULFIDE SOLUTIONS ............................................................. 52 FIGURE 5.2 SOLUBILITY OF AS2O3 IN 0.5 M SODIUM HYDROXIDE SOLUTIONS ....................................................... 54 FIGURE 5.3 PSD RESULT FOR EFFECT OF CONCENTRATION ON THE PRECIPITATION OF AS2S3 FROM AS2O3-NA2S SOLUTION ..................................................................................................................................................... 57  FIGURE 5.4 PSD RESULT FOR EFFECT OF TEMPERATURE ON THE PRECIPITATION OF AS2S3 FROM AS2O3-NA2S SOLUTION ..................................................................................................................................................... 58  FIGURE 5.5 PSD RESULT FOR EFFECT OF HYDROCHLORIC ACID FLOW RATE ON THE PRECIPITATION OF AS2S3 FROM AS2O3-NA2S SOLUTION ................................................................................................................................ 59 FIGURE 5.6 PSD RESULT FOR AS2S3 SAMPLES OBTAINED BY VARYING AGING TIME .............................................. 61 FIGURE 5.7 PREDICTED (OLI SOFTWARE) AND EXPERIMENTAL SOLUBILITY OF AS2S3 IN NA2S SOLUTIONS ........... 62 FIGURE 5.8 RIETVELD REFINEMENT PLOT OF SAMPLE “SYNTHETIC SODIUM ARSENATE” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES ....................... 65  FIGURE 5.9 RIETVELD REFINEMENT PLOT OF SAMPLE “SYNTHETIC PRODUCT #3” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES ....................... 66  FIGURE 5.10 RIETVELD REFINEMENT PLOT OF SAMPLE “SYNTHETIC SODIUM THIO-ARSENATE, PRODUCT #4” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE  xi  BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES. ... 68  FIGURE 5.11 SOLUBILITY OF SYNTHETIC PRODUCT #2 IN SODIUM HYDROXIDE AND SULFIDE SOLUTIONS EXPOSED TO AMBIENT AIR ........................................................................................................................................... 71  FIGURE 5.12 SOLUBILITY OF SYNTHETIC PRODUCT #3 IN SODIUM HYDROXIDE AND SULFIDE SOLUTIONS EXPOSED TO AMBIENT AIR ........................................................................................................................................... 72  FIGURE 5.13 SOLUBILITY OF PRODUCT #4 (SODIUM THIO-ARSENATE (NA3ASS4·8H2O)) IN SODIUM HYDROXIDE AND SULFIDE SOLUTION EXPOSED TO AMBIENT AIR ...................................................................................... 73  FIGURE 5.14 SOLUBILITY OF PRODUCT #4 (SODIUM THIOARSENATE (NA3ASS4·8H2O)) IN SODIUM HYDROXIDE AND SULFIDE SOLUTION IN ARGON ATMOSPHERE  ......................................................................................... 74  FIGURE 5.15 SOLUBILITY OF SODIUM THIO-ARSENATE (PRODUCT #4, NA3ASS4·8H2O) IN CONSTANT SODIUM HYDROXIDE (2.5 M) AND VARYING HYDROSULFIDE CONCENTRATION (0 ~ 1.0 M) SOLUTIONS IN ARGON ATMOSPHERE ................................................................................................................................................ 76  FIGURE 5.16 THE SOLUBILITY OF SODIUM THIO-ARSENATE IN CONSTANT HYDROXIDE AT 20ºC (2.5 M NAOH) AND VARYING HYDROSULFIDE SOLUTION COMPARIED WITH THAT OBTAINED BY TONGAMP ET AL. (2010) AT 30ºC  ..................................................................................................................................................................... 77 FIGURE B-1 EDX ANALYSIS OF SYNTHETIC AMORPHOUS ARSENIC TRI-SULFIDE ................................................. 102 FIGURE B-2 EDX ANALYSIS OF SYNTHETIC SODIUM ARSENATE .......................................................................... 102 FIGURE B-3 EDX ANALYSIS OF SYNTHETIC CRYSTALLINE COMPLEX #1 .............................................................. 103 FIGURE B-4 EDX ANALYSIS OF SYNTHETIC CRYSTALLINE COMPLEX #2 .............................................................. 103 FIGURE B-5 EDX ANALYSIS OF SYNTHETIC CRYSTALLINE COMPLEX #3 .............................................................. 104 FIGURE B-6 EDX ANALYSIS OF SYNTHETIC CRYSTALLINE #4 (SODIUM THIO-ARSENATE) ................................... 104 FIGURE C-1 RIETVELD REFINEMENT PLOT OF SAMPLE “AS PRECIPITATE FROM ENARGITE LEACHING WITH NA2S:ENARGITE ~ 0.130” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES. .......................................................................................................................... 106  FIGURE C-2 RIETVELD REFINEMENT PLOT OF SAMPLE “AS PRECIPITATE FROM ENARGITE LEACHING WITH NA2S:ENARGITE ~ 0.324” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES;  xii  VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES. .......................................................................................................................... 107  FIGURE C-3 RIETVELD REFINEMENT PLOT OF SAMPLE “AS PRECIPITATE FROM ENARGITE LEACHING WITH NA2S:ENARGITE ~ 0.648” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES. .......................................................................................................................... 108  xiii  Acknowledgments  This thesis would not have been possible without the generous financial support of Newmont Mining Corporation and the Natural Sciences and Engineering Research Council of Canada.  Also, I’d like to thank everyone who generously offered very kind help during the research, especially my supervisor Dr. Edouard Asselin and instructor Dr. Berend Wassink. And without my parents’ supports, it’ll be much more difficult to finish this work in two years.  xiv  Chapter 1 Introduction Enargite (Cu3AsS4) usually associates with copper sulfide minerals, such as chalcopyrite (CuFeS2), which is the main mineral source for copper. El Indio in Chile, Lepanto in the Philippines, and Chelopech in Bulgaria are three examples of such copper deposits (Filippou et al. 2007) (refer to Tables 1.1 and 1.2 for arsenic mineralization and a regional distribution of arsenical ores). Table 1.1 Major arsenic minerals occurring in nature (WTO Arsenic Compounds, 2001) Mineral  Composition  Occurrence  Native arsenic  As  Hydrothermal Veins  Proustite  Ag3AsS3  Generally one of the late Ag minerals in the sequence of primary deposition  Rammelsbergite  NiAs2  Commonly in mesothermal vein deposits  Safflorite  (Co,Fe)As2  Generally in mesothermal vein deposits  Seligmannite  PbCuAsS3  Occurs in hydrothermal veins  Smaltite  CoAs2  -  Niccolite  NiAs  Vein deposits and norites  Realgar  AsS  Vein deposits, often associated with orpiment, clays and limestones, also deposits from hot springs  Orpiment  As2S3  Hydrothermal veins, hot springs, volcanic sublimation product  Cobaltite  CoAsS  High-temperature deposits, metamorphic rocks  Arsenopyrite  FeAsS  The most abundant As mineral, dominantly mineral veins  Tennantite  (Cu,Fe)12As4S13  Hydrothermal veins  Enargite  Cu3AsS4  Hydrothermal veins  Arsenolite  As2O3  Secondary mineral formed by oxidation of arsenopyrite, native arsenic and other As minerals  Claudetite  As2O3  Secondary mineral formed by oxidation of realgar, arsenopyrite and other As minerals  Scorodite  FeAsO4·2H2O  Secondary mineral 1  Table 1.1 Major arsenic minerals occurring in nature (WTO Arsenic Compounds, 2001) continued Mineral  Composition  Occurrence  Annabergite  (Ni,Co)3(AsO4)2·8H2O  Secondary mineral  Hoernesite  Mg3(AsO4)2·8H2O  Secondary mineral, smelter wastes  Haematolite  (Mn,Mg)4Al(AsO4)(OH)8  -  Conichalcite  CaCu(AsO4)(OH)  Secondary mineral  Adamite  Zn2(OH)(AsO4)  Secondary mineral  Domeykite  Cu3As  Found in vein and replacement deposits formed at moderate temperatures  Loellingite  FeAs2  Found in mesothermal vein deposits  Pharmacosiderite  Fe3(AsO4)2(OH)3·5H2O  Oxidation product of arsenopyrite and other As minerals  Table 1.2 Arsenic deposits of the world (WTO Arsenic Compounds, 2001) Type of deposits  As mineral(s)  Average As  Location -1  conc. (mg kg ) Enargite-bearing  Enargite  1000 (0.1%)  United States, Argentina, Chile,  copper-zinc-lead  Peru, Mexico, Republic of the  deposits  Philippines, Spain, Yugoslavia, USSR  Arsenical pyrite  Arsenopyrite, tennantite  40,000 (4%)  United States, Sweden, Federal Republic of Germany, Japan,  copper deposits  France, USSR Native silver and  Smaltite, domeykite,  25,000 (2.5%)  Canada, Norway, Germany,  nickel-cobalt  safflorite, rammelsbergite,  Democratic Republic,  arsenide bearing  cobaltite, niccolite,  Czechoslovakia  deposits  loellingite, arsenopyrite, et al  Arsenical gold deposits  Arsenopyrite, loellingite  <5000 (0.5%)  United States, Brazil, Canada, Republic of Sounth Africa, Australia, USSR  2  Table 1.2 Arsenic deposits of the world (WTO Arsenic Compounds, 2001) continued Type of deposits  As mineral(s)  Average As conc.  Location  -1  (mg kg ) Arsenic sulfide  Realgar, orpiment  2000 (0.2%)  United States, People’s Republic of China  and arsenic sulfide gold deposits  Arsenical tin  Arsenopyrite  2000 (0.2%)  United States, Bolivia, Australia, Indonesia, Malaysia, Republic of  deposits  South Africa Arsenical  Arsenopyrite  6000 (0.6%)  United States, Canada, et al  quartz, silver and lead-zinc deposits  Since arsenic is toxic (massive poisoning and multiple cancer cases in the past), increasing environmental concern has arisen for the disposal of arsenic compounds by environmental protection organizations all over the world. Related environmental regulations are starting to limit the application of roasting, smelting and converting of high-arsenic content copper sulfide concentrates, due to the generation of As2O3 or As2S3 dusts. After chronic exposure to arsenic dusts in the air, effects on several human organs/systems (neurological effects and cardiovascular effects) are found to start at an arsenic concentration of 50 µg/m3 (See Table 1.3). Thus, arsenic receives a high penalty charge from smelters, which is typically 2.5 to 3.0 USD for every 0.1% over 0.1% ~ 0.2% arsenic in the concentrate (Peacey et al. 2010). The conventional processing options for copper-arsenic concentrates are therefore increasingly difficult to justify.  3  Table 1.3 Summary of toxic effects in humans after occupational inhalation exposure to inorganic arsenic compounds (European Commission 2001) Health  Effect  endpoints  Lowest observed effect level (concentration)  Respiratory  Irritation of upper respiratory tract in  effects  workers exposed to arsenic dusts  Neurological  After inhalation of inorganic arsenicals,  effects  peripheral neuropathy inworkers  Gastrointestinal  Nausea and anorexia in one worker (but  effects  not in another) exposed to arsenic trioxide  0.1-1 mg As/m3 0.05 mg As/m3 0.11 mg As/m3  dust; no other studies with quantitative exposure data available Cardiovascular  Smelter workers exposed to arsenic  effects  trioxide dusts have a higher incidence of  0.05-0.5 mg As/m3  Raynaud’s disease and increased constriction of blood vessels in response to cold Skin effects  Hyperkeratosis and hyperpigmentation are  No data  common in people exposed by the oral route, but have very rarely been reported after inhalation exposure Haematological  Anaemia is often noted in humans exposed  effects  by the oral route, but has not been observed  No data  after inhalation exposure  One option to make a clean concentrate (i.e. with low arsenic) for sale to a smelter is to leach enargite in sodium sulfide solution at 95 ºC~105 ºC, with pH adjustment above 13 with sodium hydroxide: the so-called Alkaline Sulfide Leach (ASL) (Balaz et al. 2000, Delfini et al. 2003, Curreli et al. 2003, Tongamp et al. 2009). After leaching of enargite, a chalcocite-type (Cu2S) residue and soluble arsenic complexes in leach solution are produced, and thus copper and arsenic are successfully separated. The solid copper-containing residue 4  can be forwarded for further processing to a smelter and a white arsenic-containing crystal is precipitated (or crystallized) as the leach solution is cooled. Indeed, Nadkarni and Kusik (1988) showed that most of the soluble sodium thio-arsenate (presumed to be Na3AsS4 in their work) can be precipitated by cooling the leach solution from leach temperature to room temperature. With its low cost and easy implementation, cooling and precipitation appears to be one of the best options for arsenic separation from the leach liquor. However, quantitative x-ray diffraction of the white arsenic-containing precipitate, which separates from the enargite leaching solution upon cooling, shows that the complexes do not have the exact stoichiometry of sodium thio-arsenate (Na3AsS4) – see Chapter 5. Since the stoichiometry of the arsenic-bearing precipitates is highly variable (Parada Torres 2011) and appears to vary based on the prevailing leach conditions, it is more useful from a fundamental perspective to determine how much arsenic can be separated by cooling solutions of sodium thio-arsenate. In effect, the solubility of stoichiometric sodium thioarsenate should be determined. However, throughout the relevant literature there is very little, if any, report on the solubility of Na3AsS4. It is very difficult to find Na3AsS4 or any other similar chemical (e.g. hydrate) for purchase. Thus it must be synthesized. Arsenic trioxide and arsenic tri-sulfide are the main reactants that can be used to synthesize sodium thio-arsenate complexes, thus their solubility in alkaline sulphide solutions are also of interest. As an example, sodium thio-arsenate is a light yellow acicular crystal, which grows over a period of 8-10 hours. A slight change in arsenic concentration might destroy the growth of the pure crystal. Hence, the concentration of arsenic tri-sulfide is a key parameter for sodium thio-arsenate synthesis.  5  The arsenic tri-sulfide ordered from chemical supply companies (e.g. Alfa Aesar) is an amorphous product synthesized at very high temperature and it is likely polymerized. Its solubility in sodium sulfide solutions is too low to complete the sodium thio-arsenate synthesis process. Thus, a new method to synthesize arsenic tri-sulfide in aqueous system is also required. This thesis is organized as follows:   Chapter 2 presents a review of the literature relevant to treating arsenic-bearing minerals including a discussion on the alkaline sulfide leach and relevant arsenic chemistry.    Chapter 3 presents the objectives of the research    Chapter 4 presents the experimental methods used to synthesize arsenic precipitates and then to measure their solubility    Chapter 5 presents the results of the work and discusses their importance    Chapters 6 and 7 present the conclusions from the study and the proposed future work  6  Chapter 2 Literature Review 2.1 Copper arsenic minerals Arsenic is a ubiquitous element that comprises about 0.00005% of the earth’s crust and ranks 20th in abundance (Gulledge et al. 1973). It has always had connections to- and uses in medicine, agriculture, livestock, electronics and various other industries including metallurgy (Nriagu and Azcue 1990). There have been several arsenic poisoning episodes reported around the world. The arsenic contamination incident in well water on the south-west coast of Taiwan (1961 ~ 1985) is well known: chronic arsenicism was observed in 40,421 people from 37 villages, which resulted in 7418 cases of hyperpigmentation, 2868 of keratosis, 360 of blackfoot disease, and some cases of cancer (liver, lung, skin, prostate, bladder, kidney (Chen et al. 1988, Tsai et al. 1998, Guo et al. 1997, Bates et al. 1992, Lu et al. 1975). See Table 2.1 for the most commonly found arsenic compounds in air that were investigated by the European Commission in 2001. Arsenic toxicity is a function of physical state (gas, solution, or powder), the rate of absorption into cells, the rate of elimination, the nature of chemical substituents in the toxic compound and various other factors. The toxicity of arsenical complexes decreases in the following order, arsines (AsH3) > inorganic As (III) > arsenoxides (organic As (III)) > inorganic As (V) > arsonium compounds > native As (Whitacre and Pearse 1972, Hindmarsh and McCurdy 1986). Thus, it is generally concluded that the soluble inorganic arsenicals are more toxic than the organic ones, and that inorganic As (III) is more toxic than inorganic As (V) (Maeda 1994). Under oxidizing conditions, arsenates (inorganic As (V)) are the stable species and can be strongly sorbed onto clays, iron and manganese oxides/hydroxides as well 7  as various organic matters. Under reducing conditions, arsenites (inorganic As (III)) are the predominant species, which are more toxic, soluble, and mobile than arsenates (inorganic As (V)) (Reichert and Trelles 1921). Ferric arsenate is the least soluble and least toxic form of arsenic for disposal in the environment (Harris 2000). Table 2.1 the most commonly found (or anticipated) arsenic compounds in air (European Commission 2001)  Name Arsenic trioxide, As oxide Arsenic pentoxide  Chemical  State of  Physical  Melting  Solubility in  Formula  Oxidation  Condition  point  water  As2O3, (AsO3)3-  +3  312.3 ºC  37 g/l (20 ºC)  Decomposes  1500 g/l (16  at 315 ºC  ºC)  As2O5, (AsO3)-  Solid (gaseous)  +5  Solid  H3AsO4  +5  Solid  35.5 ºC  (CH3)AsO(OH)2  +5  Solid  161 ºC  Soluble  (CH3)2AsO(OH)  +5  Solid  195~196 ºC  660 g/l (25 ºC)  AsCl3  +3  Liquid  131 ºC  Soluble  As2S3  +3  Solid  320 ºC  Insoluble  As4S4  0/+3  Solid  307 ºC  Insoluble  As2S5  +5  Solid  Arsin  AsH3  -3  Gaseous  Metallic As  As  0  Solid  Arsenic Acid Monomethylarson ic acid (MMA) Dimethylarsinic acid (DMA) Other trivalent As compounds  Realgar Other pentavalent As compounds  3-  /(AsO4) /(As2O7)  4-  Decomposes at ~500 ºC -117 ºC 817 ºC (28 atm)  3020 g/l (12.5 ºC)  Insoluble 20% (20 ºC) Insoluble  (a) Due to its high vapor pressure, it can be present in the vapor phase. (b) The arsenate species are defined by the pH on the medium (liquid or liquid film on a particle). (c) This arsenic halide is liquid at room temperature, but can be air-dispersed either in liquid or in vapor state.  8  In metallurgy, arsenic is usually found in sulfide ores, along with copper, nickel, lead, and cobalt (Mandal and Suzuki 2002). The two main copper-arsenic minerals are enargite (Cu3AsS4) and tennantite (Cu12As4S13), and enargite, particularly, is the principal mineral in many deep epithermal copper deposits. There also exist in smaller proportions such arsenicbearing minerals as luzonite (Cu3AsS4), lautite (CuAsS), sinnerite (Cu6As4S9), domeykite (Cu3As), and algodonite (Cu6As) (Filippou et al. 2007, Maske and Skinner 1971). Table 2.2 the most common Cu-As-Sb-S minerals and their physical properties (Barthelmy 2004)  Name  Ideal composition  Crystal system  Color steel gray/  Density -3  Hardness  (g cm )  (Mohs)  4.46  3  4.48  3.5  4.64  3.5-4  Enargite  Cu3AsS4  Orthorhombic  Luzonite  Cu3AsS4  Tetragonal  Tennantite  Cu12As4S13  Cubic  Sinnerite  Cu6As4S9  Triclinic  steel gray  4.44  5  Lautite  CuAsS  Orthorhombic  black/steel gray  4.82  3-3.5  Famatinite  Cu3SbS4  Tetragonal  4.69  3-4  Tetrahedrite  Cu12Sb4S13  Cubic  4.95  3.5-4  violet black gray/pink brown/red gray/brown/ cherry red  grayish black/ steel black black/steel gray  Several copper-arsenic mineral rich deposits have been found all over the world, especially in South America. In Chile, chalcopyrite and pyrite are the primary sulfide ores in the Andean Porphyry Copper deposits, but tennantite and enargite are also present (Pinchiera et al. 1998). For the El Indio deposit, enargite and tennantite presents as major minerals, along with minor quantities of chalcopyrite, arsenopyrite, and tetrahedrite (Smith 1986). Several areas in Mexico are rich in enargite, for example, the Cananea copper deposit has a 9  high arsenic content, and it is reported that enargite accounts for 80% of the arsenic in its concentrate (Perez-Segura and Zendejas-Mendivil 1991). At the Chelopech mine in Bulgaria, copper-arsenic sulfosalts are the majority of the copper ore body, and 93% of arsenic in the copper-arsenic sulfosalt can be recovered during the flotation process (Baltov et al. 2008). Considerable quantities of copper-arsenic minerals are also found at the Northparkes and (Smith and Bruckard 2007) the Tampakan deposit in the Philippines (Senior et al. 2006), as well as the Gortdrum deposit in the Republic of Ireland (Stuart and Down 1973). According to Table 2.2, enargite (Cu3AsS4) is an orthorhombic cuprous thioarsenate, and luzonite has the same ideal chemical composition but different structure, which is tetragonal dimorph. The inversion temperature from luzonite to enargite is about 280ºC (Maske and Skinner 1971). Fillippou reported that another difference in chemical composition between luzonite and enargite is the antimony content. Springer (1969) believed that natural enargite only contains up to 6 wt% Sb, which corresponds to Cu3As0.8Sb0.2S4. But for luzonite, much of the arsenic is replaced by antimony, which makes the chemical composition falling between Cu3AsS4 and Cu3As0.5Sb0.5S4 (Luce et al. 1977). In natural deposits, enargite usually co-exists with pyrite (FeS2), lunzonite, tennantite, and some other copper sulfides which are predominant at deeper levels (Filippou et al. 2007). Tennantite (Cu12As4S13) is the second most common copper-arsenic mineral, but it has a more complex composition than enargite. Generally, there are two explanations about the composition of tennantite. One explains that tennantite is a cubic cuprous-cupric thioarsenite. The other says that tennantite is the end member of a solid solution series, where arsenic and antimony replace each other in the mineral structure Cu12+x(As,Sb)4+yS13, where 0 ≤ x ≤ 1.99 and -0.02 ≤ y ≤ 0.37 (Skinner et al. 1972, Luce et al. 1977). 10  2.2 Metallurgical processing of enargite Conventional pyrometallurgical processing (Figure 2.1) is applied for more than 70% of the world’s copper production, most of which originates from chalcopyrite concentrates containing small amounts of enargite (Safarzadeh et al. 2012). However, with the depletion of conventional copper ores, the copper industry is clearly interested in the development and utilization of copper-arsenic deposits, such as those generating concentrates high in enargite and tennantite. However, the deportment and stabilization of arsenic in a benign form that meets all the environmental regulations tends to be a major challenge for industrial processes.  FIGURE 2.1 CONVENTIONAL PROCESSING OF PRIMARY COPPER SULFIDE ORES (SAFARZADEH ET AL. 2012) 11  The hydrometallurgical route (See Figure 2.2) for low-grade copper oxide ores consists of heap leaching, solvent extraction (or ion exchange) and electrowinning. This route has been well-developed for almost 50 years, especially in South America, and approximately 25% of copper is produced through this process (See Figure 2.2). However, this route is generally not amenable to the processing of enargite or tennantite (See Figure 2.3) although the Newmont Mining Corporation appears to be using a heap bio-leach for an enargite-bearing ore at Yanacocha.  FIGURE 2.2 A CLASSIC HYDROMETALLURGICAL ROUTE FOR COPPER PRODUCTION FROM OXIDE ORES  12  FIGURE 2.3 A HYDROMETALLURGICAL ROUTE FOR ENARGITE OR TENNANTITE  13  18 16  Total Solvent Extraction/Electrowinning  Worldwide Production (million tons/year)  14 12 10 8 6 4 2 0 1980  1985  1990  1995  2000  2005  Year  FIGURE 2.4 A COMPARISON OF WORLDWIDE COPPER PRODUCTION BY COMBINED METHODS (TOTAL) AND SOLVENT EXTRACTION/ELECTRO-WINNING (SX/EW) BETWEEN 1980 AND 2005 (REPLOTTED FROM MOATS 2005)  2.2.1 Pyrometallurgical treatment  Due to the relatively low melting point of copper-arsenic-sulfur phases, enargite can be smelted at around 640 ~ 690ºC (Muller and Blachnik 2002). Under relatively high SO2 partial pressure and relatively low O2 partial pressure at 550~700ºC, enargite transforms into tennantite, and subsequently into digenite (Cu9S5), chalcocite (Cu2S), and nonstoichiometric chalcocite (Cu1.96S) (Padilla et al. 1997, 1999, 2001). This series of reactions is shown as follows: 14  8Cu3AsS4 (s) → 2Cu12As4S13 (s) + 3S2 (g)  (2.2.1)  2Cu12As4S13 (s) → 12Cu2S (s) + 4As2S3 (g) + S2 (g)  (2.2.2)  Overall reaction: 4Cu3AsS4 (s) → 6Cu2S (s) + 2As2S3 (g) + 2S2 (g)  (2.2.3)  Various laboratory and pilot studies (Secco et al. 1988, Holmstrom 1989, Padilla et al. 1999, 2001) show that reaction 2.2.3 can only occur at an appreciable rate above 600 ºC. According to Secco et al. (1998), the arsenic sulfide products (As2S3 and AsS) melt before volatilization at 600 ºC. Therefore, roasting proceeds at 650 ºC for a considerable time to avoid the formation of sticky products. To separate arsenic from Cu as much as possible, 720 ºC is required, and the oxygen concentration in the roaster must be less than 0.3% v/v to maintain the efficiency of arsenic elimination (Filippou et al. 2007). The arsenic sulfide gases coming out from roasting can be burnt with excess air at 750ºC to produce gaseous arsenic trioxide and sulfur dioxide: 2As2S3 (g) + 9O2 (g) → 2As2O3 (g) + 6SO2 (g)  (2.2.4)  S2 (g) + 2O2 (g) → 2SO2 (g)  (2.2.5)  The oxidized gases are cooled down to 390 ºC and then to 120 ºC. And most of gaseous As2O3 desublimes as “gray arsenic trioxide” at 120 ºC, which is captured in baghouse filters. At El Indio (Smith 1986, Smith and Paredes 1988), a similar industrial route (see Figure 2.5) was used and the crude arsenic trioxide captured in bag filters contained 97~97.5% As2O3 and about 1.0% Sb. It was packaged and sold until 2002, due to the lack of a market for such an arsenic product (Safarzadeh et al. 2012). 15  FIGURE 2.5 A SIMPLIFIED FLOWSHEET OF THE EL INDIO ENARGITE ROASTER DRAWN WITH DATA BY SMITH 1986  Since the arsenic-bearing gases released during mineral processing cannot go directly into the atmosphere, facilities to capture, separate, oxidize, and stabilize the gases are needed. However, the cost for these facilities makes the pyrometallurgical process quite expensive. Furthermore, the considerable amount of SO2 and toxic metals in fugitive gas emissions from smelters present an ongoing challenge. At present, the maximum allowable arsenic content in copper concentrates for smelting is set at 0.5%, and this is expected to decrease to 0.3% in future (Baxter et al. 2010). Industrial ore processing of arsenic-bearing sulfides, ideally results in the final disposal of arsenic as scorodite (FeAsO4·2H2O) due to its long term stability. However, in certain jurisdictions, and for various reasons such as disposal site characteristics (humidity or  16  rain fall), crystallinity, the presence of oxygen, sulfides and complexing agents, other arsenic mineralizations have historically been sent to tails (Riveros et al. 2001). 2.2.2 Alkaline sulfide leaching of enargite  Enargite is a refractory mineral, compared to the other common copper rich minerals, because it is hard to dissolve in most aqueous media. Based on Figure 2.6 and 2.7 (Filippou et al. 2007), enargite is thermodynamically stable in acid solution (please note that the highly toxic arsine gas, AsH3, does not appear on Figures 2.6 or 2.7 despite its thermodynamic stability under reducing conditions). Enargite also does not dissolve easily in alkaline solution. In fact, enargite is the last in the order by which copper sulfides dissolve in ammonia solutions (Kuhn 1974): Chalcocite > covellite > bornite > chalcopyrite > enargite  17  FIGURE 2.6 SIMPLIFIED EH-PH DIAGRAM FOR THE CU-AS-S-H2O SYSTEM WITH 1 MOL/L DISSOLVED SPECIES AT 25 ºC AND 1 ATM (FILIPPOU ET AL. 2007)  Considerable research has focused on the processing of enargite concentrates by hydrometallurgical methods, either for the preparation of a copper-sulfur concentrate that can then be smelted (Mihajlovic 2007) or for the direct extraction of copper from enargite concentrates (Baxter et al. 2010).  18  FIGURE 2.7 SIMPLIFIED EH-PH DIAGRAM FOR THE CU-S-AS-H2O SYSTEM WITH 1 MOL/L DISSOLVED SPECIES AT 25 ºC AND 1 ATM (FILIPPOU ET AL. 2007)  Alkaline sulfide leaching, also referred to as the “Sunshine Process”, consists of dissolution of the arsenic (as well as antimony, mercury, gold and tin) from the enargite source mineral in an alkaline sodium sulfide solution (pH > 13) at 95 ~ 105 ºC. The residue from this process is a copper sulfide, which, in theory is amenable for shipment and eventual smelting. The general reaction formula for this process is given here: 2Cu3AsS4 (s) + 3Na2S (aq) → 2Na3AsS4 (aq) + 3Cu2S (s)  (2.2.6)  It has also been proposed that NaHS and NaOH can be used effectively to leach enargite as shown in reaction 2.2.7. The rationale given to justify this reaction is that 19  sufficient caustic is required to ensure that S2- is predominant. However, several studies have indicated that the pKa for the HS-/S2- equilibrium actually lies near pH 18, and there is significant evidence that both OH- and HS- play an active role in the dissolution of enargite (Wilkomirsky et al. 1998). 2Cu3AsS4 (s) + 3NaHS (aq) + 3NaOH (aq) → 2Na3AsS4 (aq) + 3Cu2S (s) + 3H2O (l)  (2.2.7)  2.2.3 Arsenic removal and precipitate analysis  After alkaline sulfide leaching, there are generally two ways to eliminate arsenic from the arsenic-pregnant solution: acidification and crystallization. Arsenic tri-sulfides will be precipitated by acidification (2.2.8) (Parada Torres 2011). For further processing, arsenic sulfides can be oxidized with oxygen (or by other means) to form hydrogen arsenate (2.2.9). Scorodite (insoluble ferric arsenate) would be the final disposal form obtained via this method through the addition of ferric sulfate (2.2.11) with proper pH control (2.2.10): 2AsS43- (aq) + 6H+ (aq) → As2S3 (s) + 3H2S (g)  (2.2.8)  As2S3 (s) + 7O2 (g) + 6H2O (l) → 2H3AsO4 (aq) + 3H2SO4 (aq)  (2.2.9)  H2SO4 (aq) + Ca(OH)2 (aq) → CaSO4 (aq) + H2O (l)  (2.2.10)  2H3AsO4 (aq) + Fe2(SO4)3 (aq) → 2FeAsO4 (s) + 3H2SO4 (aq)  (2.2.11)  The hydrogen ions in (2.2.8) can originate from sulfuric acid, sulfurous acid or carbonic acid etc. For example, Holmes (1973) provides a two-step acidification via SO2, wherein gold complexes (pH~10) and arsenic tri- or penta-sulfides (pH~3, 4) can be precipitated separately.  20  Crystallization of sodium thio-arsenate occurs via cooling of process liquors. Nadkarni et al. (1975) presented several routes for handling the arsenic crystals after they have been crystallized and these are presented in Figures 2.8 and 2.9. The removal of arsenic by cooling provides a chance to reduce costs as reagents are theoretically not consumed. As is removed from the circuit in Figure 2.8 as the pentasulfide after reaction with sulfuric acid.  FIGURE 2.8 AN ARSENIC DISPOSAL OPTION 1 BY ADDING H2SO4 (NADKARNI ET AL. 1975)  21  FIGURE 2.9 ARSENIC DISPOSAL OPTIONS 2 AND 3 BY ADDING CO2 AND SO2 (NADKARNI ET AL. 1975) 22  FIGURE 2.10 FINAL ARSENIC ROUTE BY SCORODITE PRECIPITATE (NADKARNI ET AL. 1975)  It is interesting to note that in Figure 2.8 there are three different proposed ways to regenerate the Na2S solution required for further leaching. (1). Using barium sulfide Na2SO4 (aq) + BaS (s) → BaSO4 (s) + Na2S (aq)  (2.2.12)  BaSO4 (s) + 2C (s) → BaS (s) + 2CO2 (g) (700~1000ºC)  (2.2.13)  (2). Using carbon Na2SO4 (aq) + 2C (s) → Na2S (aq) + 2CO2 (g)  (2.2.14)  (3). Using calcium sulfate Na2SO4 (aq) + Ca(OH)2 (aq) → CaSO4 (s) + 2NaOH (aq)  (2.2.15) 23  2NaOH (aq) + H2S (g) → Na2S (aq) + 2H2O (l)  (2.2.16)  2.2.4 Sodium thio-arsenate solubility  The solubility of sodium thioarsenate in ASL solutions is very important for arsenic removal and process design. However, there is a lack of experimental data on the solubility of sodium thio-arsenate in ASL solutions in general. In particular, there is very little data as to the solubility at elevated temperatures, which would indicate maximum solution loading of arsenic during leaching. The only available data appears to be presented by Nadkarni and Kusik in 1988 (cf. Figure 2.11) and it reports solubility of arsenic in sodium sulfide solutions from ~20 to 45 ºC. Further, it should be noted that Nadkarni and Kusik’s work did not confirm that Na3AsS4 was the precipitated arsenic species. The only study that appears to positively identify the arsenic precipitate from cooled ASL solutions appears to be that of Tongamp et al. (2010a, 2010b) where Na3AsS4 was identified by XRD and ICP (to verify total As content) after precipitation from leached orpiment and oxidation with elemental S at ~30 ºC and from solutions containing 2.5 M NaOH and ~3-4.5 M HS-. The solubility curve of sodium thio-arsenate and thio-stibnite from Nadkarni and Kusik (1988) as shown in Figure 2.11 is quite steep and is adequate for cooling and crystallizing the arsenic species from solution.  24  Arsenic/Antimony Concentration (M)  0.45  [Na3AsS4] [Na3SbS4]  0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 20  25  30  35  40  45  o  Temperature ( C) FIGURE 2.11 SOLUBILITY OF SODIUM THIOARSENATE AND STIBNITE IN AQUEOUS SODIUM SULFIDE (2.2~2.4M) (REPLOTTED FROM NADKARNI AND KUSIK, 1988)  2.3 Synthesis of amorphous As2S3 Orpiment, chemical formula As2S3, is a common monoclinic arsenic sulfide mineral and may be used as a precursor for the synthesis of sodium thio-arsenate. Natural orpiment is an orange to yellow mineral that can be found worldwide, and occurs as a sublimation product in volcanic fumaroles, low temperature hydrothermal veins, hot springs and as a byproduct of the decay of other arsenic minerals. Synthesized orpiment occurs both in crystalline and amorphous forms, which feature polymeric structures consisting of trigonal pyramidal arsenic (III) centers linked by sulfide centers. The sulfide centers are two-fold coordinated to two arsenic atoms. In crystalline form, orpiment adopts a ruffled sheet 25  structure (Wells 1984). The bonding between the sheets consists of Van der Waals forces. Amorphous As2S3 does not possess a layered structure but is more highly cross-linked (Wells 1984). Amorphous As2S3 may be obtained via fusion of elemental sulfur and elemental arsenic at 390 ºC, which gives a polymeric As2S3 with the adamantine geometry. Rapid cooling of the reaction melt ensures the disordered arrangement of the bonds, resulting in the glass. The reaction can be shown as following reaction 2.3.1: 2As (s) + 3S (s) → As2S3 (s)  (2.3.1)  Amorphous As2S3 also can be formed by hydrogen sulfide purging through an arsenic tri-chloride solution (reaction 2.3.2). Weissberg et al. (1966) precipitated amorphous As2S3 from a slightly acidic aqueous solution of As2O3 by slowly bubbling bottled H2S through the solution (reaction 2.3.3). 2AsCl3 (aq) + 3H2S (g) → As2S3 (s) + 6HCl (aq)  (2.3.2)  As2O3 (aq) + 3H2S (g) → As2S3 (s) + 3H2O (l)  (2.3.3)  2.4 Synthesis of sodium thio-arsenate As mentioned earlier, sodium thio-arsenate is a likely arsenic-containing complex that forms during enargite leaching but very little research has studied sodium thio-arsenate synthesis. Tongamp et al. (2010a, 2010b) added arsenic trisulfide to alkaline leaching solution after enargite leaching, which was continuously recycled until the arsenic concentration reached over 35 g/L. The solution was then treated in a precipitation process 26  where elemental sulfur and antimony where added to precipitate 50 ~ 60% of the dissolved arsenic as Na3AsS4. The filtrate was recirculated to the enargite leaching process. This process is shown in Figure 2.12:  FIGURE 2.12 SCHEMATIC ILLUSTRATION OF THE CU3ASS4-NAHS-NAOH LEACHING AND NA3ASS4 PRECIPITATION PROCESS FOR ARSENIC REMOVAL AND CIRCULATION OF WASTE WATER TO LEACHING AND PRECIPITATION STAGES FOR RE-USE (TONGAMP ET AL. 2010A, 2010B)  A practical method for sodium thio-arsenate synthesis can be achieved from Tongamp’s study. As2S3 dissolves in high concentrations of Na2S (50 ~ 250 g/L) and NaOH (50 ~ 100  27  g/L) solution at 90 ºC, and Na3AsS4 can be precipitated with the addition of elemental sulfur as an oxidant followed by cooling to 30 ºC. These reactions are shown as follows: As2S3 (s) + 3Na2S (aq) → 2Na3AsS3 (aq)  (2.4.1)  Na3AsS3 (aq) + S (s) → Na3AsS4 (aq)  (2.4.2)  Orpiment dissolution increases linearly from acidic to alkaline conditions (Lengke et al. 2009, Lengke and Tempel, 2002), and a series of thio-arsenate species can be formed. Orpiment dissolution is controlled by weak van der Waals bonding, which further weakens at alkaline pH when hydroxide adsorption occurs at the surface of the orpiment (Mamedov and Mikhailov 1997, Floroiu et al. 2004). In sulfide deficient solutions, model calculations and laboratory studies show that arsenite (AsO33-) is the dominant aqueous species with 70 ~ 90% of total arsenic at pH 2 ~ 8 (Eary 1992, Floroiu et al. 2004). However, in anoxic, sulfide-rich solutions, the formation of thio-arsenite (AsO3-xSx3-) species has been supported by Helz et al. (1995, 2008). Free sulfide ions are suggested to react with orpiment to form and release thioarsenite. A similar chemical method is provided by Anderson and Twidwell in 2008. As mentioned in their work, a mixture of sodium sulfide and sodium hydroxide is a unique hydrometallurgical system that is very selective for tin, gold, antimony, arsenic and mercury. Thus, when the system is applied to an arsenic containing material like orpiment, a solution of sodium thio-arsenite is formed. The series of chemical reactions is illustrated as: Na2S (aq) + As2S3 (s) → 2NaAsS2 (aq)  (2.4.3)  NaAsS2 (aq) + Na2S (aq) → Na3AsS3 (aq)  (2.4.4) 28  And arsenic trioxide can also be a reactant to form thio-arsenite. The reactions are: 4Na2S (aq) + As2O3 (s) + 3H2O (l) → 2NaAsS2 (aq) + 6NaOH (aq)  (2.4.5)  NaAsS2 (aq) + Na2S (aq) → Na3AsS3 (aq)  (2.4.6)  A certain amount of elemental sulfur reacted with sodium sulfide will form polysulfides, which have sufficient oxidizing power to form the arsenate. This can be explained as follows: (x-1)S0 (s) + Na2S (aq) → Na2Sx (where x= 2 to 5)  (2.4.7)  Na2Sx (aq) + (x-1)Na3AsS3 (aq) → (x-1)Na3AsS4 (aq) + Na2S (aq)  (2.4.8)  A more specific study by Suess and Planer-Friedrich (2012) shows that thio-arsenates can form as dominant species in the leaching of orpiment (As2S3) and arsenopyrite (FeAsS), and the mobility of arsenic needs to be considered in studies, especially at neutral to alkaline pH. Under acidic conditions, no thio-arsenates formed in orpiment dissolution. Under neutral to alkaline conditions, mono-, di-, trithioarsenate species accounted for up to 50% of total arsenic in orpiment leaching solution. Furthermore, X-ray Absorption Spectroscopy based analysis showed that thio-arsenites are necessary precursor species for thio-arsenate formation, which are formed through the oxidation of thio-arsenite at highly alkaline conditions in the presence of trace amounts of oxygen (Planer-Friedrich et al. 2010). Another way to form thio-arsenate species is presented by Rochette et al. (2000). The species of thio-arsenate (AsO3S-, AsO2S2-, and AsOS3-) can be formed by controlling the amount of hydrogen sulfide gas, which is purged to react with arsenate. H2AsO4- (aq) + H2S (g) → H2AsO3S- (aq) + H2O (l)  (2.4.9)  29  H2AsO3S- (aq) + H2S (g) → H2AsO2S2- (aq) + H2O (l)  (2.4.10)  AsO43- (aq) + 4H2S (g) → AsS43- (aq) + 4H2O (l)  (2.4.11)  Zhang (2004) also showed that monothio-arsenate (AsO3S3-) and dithio-arsenate (AsO2S23-) can be formed by the recombination of arsenite and disulfide: AsO33- (aq) + S22- (aq) → AsO3S3- (aq) + S2- (aq)  (2.4.12)  AsO33- (aq) + S22- (aq) + H2O (l) → AsO2S23- (aq) + 2OH-(aq)  (2.4.13)  2.5 Solubility of amorphous As2S3 Arsenic is commonly present in sulfide ores, concentrates, and tailings, and As2S3 is an important As-S complex during hydrometallurgical processing. To synthesize sodium arsenic oxy-sulfide complexes, a determination of amorphous As2S3 solubility in sulfide-rich solutions is needed. Due to the difference in structure, amorphous As2S3 may be expected to be more soluble than orpiment (Eary 1992). The solubility of orpiment in water was determined to be 2.1x10-6 mol/L at 16 ~ 18 ºC by Biltz (1907), and 3.6x10-6 mol/L at 0ºC by Holtje (1929). Ringbom (1953) used the 0ºC solubility data (Holtje 1929) to obtain an equilibrium constant for the following reaction (2.5.1): As2S3 (s) + S2- (aq) → 2AsS2- (aq), K=100  (2.5.1)  Wunschendoref (1929) determined the mole ratios of As2S3/Na2S in 1.0 mol/L Na2S solutions saturated with As2S3, ranging from 0.990 at 18 ºC to 1.009 at 100 ºC. Babko and 30  Lisetskaya (1956) studied the equilibria involved in the system of thiosalts with Sn, Sb, and As, by determining the solubility and pH of various buffered solutions which were saturated with SnS2, Sb2S3, and As2S3 at 20 ºC. The reactions for As2S3 dissolution in these solutions was concluded to be: As2S3 (s) + 2OH- (aq) → AsS2- (aq) + AsS(OH)2 (aq), K=1.4x102  (2.5.2)  As2S3 (s) + 2S2- (aq) → AsS2- (aq) + AsS33- (aq), K=8.7x1012  (2.5.3)  Srivastava and Ghosh (1957) studied the dissolution of As2S3 and As2S5 by determining the solubility, conductivity, and potential in dilute aqueous solutions of constant H2S and variable NaOH concentrations at room temperature. The mechanism of As2S3 dissolution with HS- ions was: As2S3 (s) + 5HS- (aq) → 2AsS45- (aq) + 5H+ (aq)  (2.5.4)  Weissberg et al. (1966) measured the solubility of orpiment in Na2S-H2O solutions under conditions ranging from 0.55 to 3.43 wt% Na2S, temperatures from 25 to 200 ºC, and pressures from 100 to 1500 bar. Orpiment solubility increases linearly with increasing Na2S concentration, and decreases non-linearly with increasing pressure. Temperature below 120 ºC shows slight effect on the orpiment solubility. And the orpiment solubility grows at an increasing rate as the temperature goes above 120 ºC.  31  o  52 C o 100 C o 150 C o 200 C  As 2S 3 concentration (wt%)  12.0  11.5  11.0  10.5  10.0  0  200  400  600  800  1000  1200  1400  1600  Pressure (bar)  FIGURE 2.13 THE SOLUBILITY OF ORPIMENT IN 3.43 WT % NA2S SOLUTIONS AT 52, 100, 150 AND 200 ºC, AS A FUNCTION OF PRESSURE (REPLOTTED FROM WEISSBERG ET AL. 1966)  Webster (1990) determined the solubility of synthetic, crystalline As2S3 in O2-free, dilute and sulfide-bearing solutions at temperature of 25 and 90 ºC.  The predominant  dissolved arsenic species were H3AsO3 and H2As3S6- at pH 2 ~ 6.  32  Chapter 3 Objectives Based on the literature review it is clear that the solubility of As in ASL solutions requires further study. To generate data relevant to ASL systems, the present work is based on the following objectives: 1. To do a preliminary study on arsenic trioxide solubility in sodium sulfide and sodium hydroxide solutions for the synthesis of amorphous As2S3 and sodium arsenic oxy-sulfide complexes. 2. To determine the solubility of synthetic amorphous As2S3 in sodium sulfide solutions. 3. To synthesize a series of sodium thio-arsenate complex. 4. To determine the solubility of sodium thio-arsenate in alkaline sulfide solutions. 5. To characterize the arsenic precipitates that result from enargite leaching in alkaline sulfide solutions.  33  Chapter 4 Experimental Procedures 4.1 Preliminary solubility study on arsenic trioxide Four groups of tests were done with the intention of exploring arsenic dissolution behavior in sodium sulfide and sodium hydroxide solution as well as to establish procedures for synthesis of arsenic tri-sulfide and sodium arsenic oxy-sulfide complexes. Individual experiments at constant temperature (20 ºC, 40 ºC, 60 ºC, 80 ºC, 95 ºC) were done, each over 48 hours, with magnetic stirring. 300 ml of sodium sulfide and sodium hydroxide solutions with 0.5 M concentration were prepared in a 500 ml jacketed cell. The temperature was maintained by water circulation through the cell jacket from a Cole-Parmer temperature controller/water bath. Over 48 hours, a constant amount of arsenic trioxide was added every 20 ~ 30 minutes until complete dissolution of arsenic trioxide was observed by visual inspection. Addition of arsenic trioxide stopped when no further dissolution was observed. Two different conditions were studied separately: with oxygen addition and without oxygen oxidation. For the oxygen-free solubility tests, argon gas was purged through the solution for more than fifteen minutes before arsenic trioxide addition.  4.2 Synthesis of amorphous As2S3 Arsenic tri-sulfide provided by the Alfa-Aldrich Company was first used in the synthesis of sodium arsenic oxy-sulfide complexes, but its low solubility in sodium sulfide and sodium hydroxide solutions was observed in several experiments. This As2S3 may have been polymerized at high temperature thus affecting its solubility. Synthesis of amorphous arsenic tri-sulfide was then done in an aqueous system via a new method. This method is shown in reaction 4.2.1 as follows: As2O3 (s) + 3Na2S (aq) + 6HCl (aq) → As2S3 (s) + 6NaCl (aq) + 3H2O (l)  (4.2.1) 34  The Delta Gibbs Free Energy (∆Gº) of this reaction (4.2.1) at temperatures ranging from 0 ºC to 90 ºC was calculated thermodynamically by the HSC software and it is shown in Figure 4.1:  -695  -700  G (KJ)  -705  -710  -715  -720  -725  0  20  40  60  80  100  o  Temperature ( C)  FIGURE 4.1 THE DELTA GIBBS ENERGY OF AS2S3 SYNTHESIS REACTIONS BY HSC  As predicted by HSC, the formation of arsenic tri-sulfide in reaction 4.2.1 is very favorable. And as temperature increases, the reaction should proceed more favorably. Furthermore, the predicted solubility of As2S3 (by OLI software simulation) in acidic (HCl) and alkaline (Na2S) solutions through the pH range 0~12 shows that As2S3 is highly soluble in the alkaline sulfide system. OLI software simulation uses a basic electrolyte thermodynamic model - the AQ model, which is based on a true speciation model, a predictive equation of state (Helgeson EOS), an activity coefficient model, and convergence 35  heuristics. The model is based upon published experimental data. The model uses data regression wherever possible and estimation and extrapolation where required. Also it provides general simulation capability giving accurate prediction for almost any water chemistry mixture over a very wide range. Figure 4.2 demonstrates that As2S3 is barely soluble in HCl and at low pH. OLI was not able to calculate the solubility above pH 12.  -1.5 -2.0 -2.5  log [As2S3]  -3.0 -3.5 -4.0 -4.5 -5.0 -5.5 -6.0 4  6  8  10  12  pH  FIGURE 4.2 AS2S3 SOLUBILITY PREDICTED BY OLI AT PH RANGE 0~12 AT 25 ºC  Based on these data, the synthesis of As2S3 could occur through the use of the following experimental setup.  36  FIGURE 4.3 EXPERIMENT SETUP FOR AS2S3 SYNTHESIS  Arsenic trioxide powder was dissolved in sodium sulfide solution at different concentrations at different temperatures, and HCl solution was pumped slowly into the reactor. The inflow of hydrochloric acid solution stopped when pH dropped below 5, at which point virtually all of the arsenic was precipitated as yellow As2S3. The yellow precipitate was kept in solution for varying aging times to allow for particle growth. 4.2.1 Concentration effect  Since the effect of the concentrations of sodium sulfide and hydrochloric acid solutions plays a large role in the synthesis of As2S3, optimized concentrations of these reagents were first determined. 0.1 M, 0.2 M, 0.3 M sodium sulfide solution and 1 M, 2 M, 3 M hydrochloric acid were tested with different combinations as shown in Table 4.1. Table 4.1 Conditions of As2S3 synthesis concentration tests Experiment  Temperature  [Na2S]  [HCl]  Flow rate  Aging time  No.  (ºC)  (M)  (M)  (ml/min)  (hr)  1  50  0.1  1  3.2  16  2  50  0.1  2  3.2  16  3  50  0.1  3  3.2  16  37  Table 4.1 Conditions of As2S3 synthesis concentration tests continued Experiment  Temperature  [Na2S]  [HCl]  Flow rate  Aging time  No.  (ºC)  (M)  (M)  (ml/min)  (hr)  4  50  0.2  1  3.2  16  5  50  0.2  2  3.2  16  6  50  0.2  3  3.2  16  7  50  0.3  1  3.2  16  8  50  0.3  2  3.2  16  9  50  0.3  3  3.2  16  FIGURE 4.4 SYNTHETIC ARSENIC TRI-SULFIDE POWDER  The synthetic yellow precipitates were vacuum filtered, and dried in an oven overnight (below 100 ºC). After drying the solid was dissolved and ground with a mortar and pestle in DI water to remove any residual NaCl and HCl, and then filtered again. The solid 38  was again dried overnight (below 100 ºC) and ground again with a mortar and pestle into finer powder. The yellow powder samples (Figure 4.4) were analyzed by EDX (Hitachi Model S3000N VPSEM) and the PSD was measured by Malvern Mastersizer 2000 Laser Diffraction Particle Size Analyser. 4.2.2 Temperature effect  The As2S3 synthesis experiments were operated at different temperature (25 ºC, 40 ºC, 60 ºC, and 80 ºC) with the same procedure as described above, and the optimized concentrations of sodium sulfide and hydrochloric acid (as determined above) were applied. The detailed experimental conditions for the effect of temperature are presented in Table 4.2. Table 4.2 Conditions of As2S3 synthesis temperature tests Experiment  Temperature  [Na2S]  [HCl]  Flow rate  Aging time  No.  (ºC)  (M)  (M)  (ml/min)  (hr)  10  25  0.1  1  3.2  2  11  40  0.1  1  3.2  2  12  60  0.1  1  3.2  2  13  80  0.1  1  3.2  2  4.2.3 Effect of hydrochloric acid addition rate  The rate of HCl solution addition was also studied, and the experimental parameters are presented in Table 4.3. Table 4.3 Conditions of As2S3 synthesis flow rate tests Experiment  Temperature  [Na2S]  [HCl]  Flow rate  Aging time  No.  (ºC)  (M)  (M)  (ml/min)  (hr)  14  50  0.1  1  3.2  2  15  50  0.1  1  5.9  2  39  4.2.4 Aging time effect  The main impact of different aging times was expected to be on particle size. 2, 8 and 48 hours aging times were applied in experiment detailed in Table 4.4. Table 4.4 Conditions of As2S3 synthesis aging time tests Experiment  Temperature  [Na2S]  [HCl]  Flow rate  Aging time  No.  (ºC)  (M)  (M)  (ml/min)  (hr)  16  50  0.1  1  3.2  2  17  50  0.1  1  3.2  8  18  50  0.1  1  3.2  48  4.3 Solubility of amorphous As2S3 in sodium sulfide solution To prepare for sodium thioarsenate synthesis, the solubility of amorphous arsenic trisulfide in sodium sulfide solutions was determined by a series of experiments. Similar to the previous arsenic trioxide solubility tests, fifteen experiments were run at constant temperature (20 ºC, 40 ºC, 60 ºC, 80 ºC, 95 ºC) at three different concentrations (1 wt% or 0.13 M, 2 wt% or 0.26 M, and 3 wt% or 0.39 M) of sodium sulfide solution. Over 48 hours, the 50 ml sodium sulfide solution was stirred, and a constant amount of synthetic arsenic trisulfide powder was periodically added until dissolution stopped as determined by visual inspection.  4.4 Synthesis of sodium thio-arsenate 4.4.1 Synthesis of precursor sodium arsenate by hydrogen peroxide oxidation of As (III)  White sodium arsenate crystals (Figure 4.5) were first produced by dissolving 30 g arsenic trioxide in 250 ml sodium hydroxide solution (4 M), followed by oxidation with 300 ml hydrogen peroxide (3 M) at 50 ºC. Since the solubility of sodium arsenate decreases dramatically with decreasing temperature, sodium arsenate crystals precipitated as the solution was cooled. The reactions are as follows: 40  As2O3 (s) + 6NaOH (aq) → 2Na3AsO3 (aq) + 3H2O (l)  (4.4.1)  Na3AsO3 (aq) + H2O2 (aq) → Na3AsO4 (s) + H2O (l)  (4.4.2)  FIGURE 4.5 SYNTHETIC SODIUM ARSENATE  Different sulfide sources were then used (solid sodium sulfide, pre-dissolved sodium sulfide, hydrogen sulfide gas) throughout the sodium thio-arsenate synthesis attempts. The anticipated sodium thio-arsenate production reaction was (the original concentration of sodium arsenate was 3.2 M): Na3AsO4 (aq) + 4S2-(aq) → Na3AsS4 (aq) + 4O2-(aq)  (4.4.3)  41  4.4.1.1 Synthesis of sodium thio-arsenate from sodium arsenate and sodium sulfide solution  32.5 g As2O3 was dissolved in 0.5 L of sodium hydroxide solution (6 M) at room temperature, and hydrogen peroxide (0.5 mol) was added for the oxidation of As (III) to As (V). 2 L of concentrated sodium sulfide solution (2 M) was then pumped in to the mixture and stirred overnight. A white acicular crystal was found to precipitate (Figure 4.6). After filtration, washing and drying, the crystalline product (#1) was analyzed by Energy Dispersive X-Ray Spectroscopy (EDX).  FIGURE 4.6 CRYSTALLINE PRECIPITATE #1 SYNTHESIZED BY ADDITION OF SODIUM SULFIDE SOLUTION TO SODIUM ARSENATE SOLUTION  4.4.1.2 Synthesis of sodium thio-arsenate from sodium arsenate and solid sodium sulfide  The As2O3-H2O2-NaOH solution was prepared as before, but 960 g or 4 mol solid sodium sulfide nona-hydrate (Na2S∙9H2O) was added to the sodium arsenate solution with  42  overnight stirring (As:S ratio ~ 0.25). A white crystal was precipitated (Figure 4.7). After filtration, washing and drying, the crystalline product (#2) was analyzed by EDX.  FIGURE 4.7 CRYSTALLINE PRECIPITATE #2 SYNTHESIZED BY ADDITION OF SOLID SODIUM SULFIDE TO SODIUM ARSENATE SOLUTION  4.4.1.3 Synthesis of sodium thio-arsenate from sodium arsenate and hydrogen sulfide gas  Different amounts (10 g, 20 g, 100 g, and 300 g) of synthetic sodium arsenate were added to sodium sulfide solutions (2 M) separately at room temperature. A pH meter was used during the hydrogen sulfide gas sparging process (see setup in Figure 4.8). The temperature of the solution was increased to 70 ºC to ensure that the entire sodium arsenate solid was dissolved. Hydrogen sulfide gas was then slowly sparged through the Na3AsO4Na2S solution (with stirring) at a rate of approximately 30 bubbles per minute until the pH stopped changing, which meant the reaction had reached equilibrium. 43  Na3AsO4 (aq) + 4H2S (g) → Na3AsS4 (aq) + 4H2O (l)  (4.4.4)  Following that, the solution was naturally cooled to room temperature. Only the sodium sulfide solution with 300 g of added sodium arsenate resulted in the precipitation of a crystal (Figure 4.9). After filtration, washing and drying, the crystalline product #3 was analyzed by EDX.  FIGURE 4.8 EXPERIMENTAL SETUP FOR SYNTHESIS OF SODIUM ARSENIC OXY-SULFIDE COMPLEXES BY H2S  44  FIGURE 4.9 CRYSTALLINE PRECIPITATE #3 SYNTHESIZED BY H2S GAS ADDITION TO SODIUM ARSENATE SOLUTION  4.4.2 Synthesis of sodium thio-arsenate by reaction of arsenic tri-sulfide with elemental sulfur  To eliminate the oxygen content of the product, synthetic arsenic tri-sulfide can be used in the synthesis of sodium thio-arsenate by reaction with elemental sulfur. The yellow As2S3 powder (50 g) was dissolved in concentrated sodium sulfide solution (200 ml at 3 M) at 60 ºC, and various amounts of elemental sulfur were added until the complete dissolution of As2S3. The solution was cooled down naturally from 60 ºC to room temperature until the solution became homogeneous and the color became dark yellow. The anticipated reactions were as follows: As2S3 (s) + 3Na2S (aq) → 2Na3AsS3 (aq)  (4.4.5)  Na3AsS3 (aq) + S (s) → Na3AsS4 (aq)  (4.4.6)  45  FIGURE 4.10 CRYSTALLINE PRECIPITATE #4 SYNTHESIZED BY REACTION OF ELEMENTAL SULFUR (12.8 G) WITH ARSENIC TRISULFIDE (50 G) IN 200 ML AT 3M SODIUM SULFIDE  FIGURE 4.11 CRYSTALLINE PRECIPITATE #4 SYNTHESIZED BY REACTION OF ELEMENTAL SULFUR WITH ARSENIC TRISULFIDE AFTER FILTRATION  As temperature decreased, an acicular crystal was precipitated (Figure 4.10). The resulting synthetic crystal #4 (Figure 4.11) was analyzed by EDX and Quantitative X-Ray Diffraction (QXRD).  4.5 Solubility of synthetic sodium arsenic oxy-sulfide complexes and sodium thioarsenate The solubility of synthetic crystals #2, #3 and #4 was measured separately in three different leach solution (Table 4.5) at 20 ºC, 40 ºC, 60 ºC, 80 ºC and 95 ºC.  46  Table 4.5 Three different leach solutions for solubility tests Experiment  [NaOH] (M)  [Na2S] (M)  1  2.5  1.0  2  2.5  0.5  3  1.0  1.0  No.  To limit the formation of polysulfide species, argon gas was sparged during the solubility experiments to avoid oxidation by ambient oxygen.  Some experiments were  performed at constant sodium hydroxide concentration where the concentration of the hydrosulfide ion was varied from 0 M to 1.0 M (Table 4.6). For these experiments, the solubility of synthetic sodium thioarsenate crystals was again studied in an argon atmosphere from 20 ºC to 95 ºC. Table 4.6 Three conditions for varying hydrosulfide and constant NaOH Experiment  [NaOH] (M)  [NaHS] (M)  1  2.5  0  2  2.5  0.5  3  2.5  1.0  No.  4.6 Solubility accuracy calibration test Since the dissolution of synthetic complexes in sodium sulfide and hydroxide solutions is very quick and obvious to the naked eye, the saturation point was determined by visual inspection. Nonetheless, to calibrate the accuracy of this saturation point, as determined by visual inspection, two more solubility tests were performed. For the 2.5 M NaOH & 1.0 M Na2S solution at 20 ºC in an argon atmosphere and for the 2.5 M NaOH & 0.5 M NaHS solution at 60 ºC in argon, the following procedure was done:  47    0.1 ml of the saturated solution was taken at the point of visual saturation, diluted 500 times, and the solution was analyzed for arsenic by ICP    The mixture (now containing two phases) was then allowed to reach equilibrium over at least 12 hours. 0.1 mL of the solution phase was then again taken, diluted and sent for ICP analysis.    These two ICP analyses were compared to ascertain the accuracy of the visual inspection method.  4.7 Precipitation from an enargite leaching test An enargite sample (Figure 4.12, Tables 4.7 and 4.8) was leached in mixed sodium hydroxide (2.5 M) and sodium sulfide solution (0.5 M) at 95 ºC for 24 hours, after which time the leaching solution was rapidly vacuum filtered at high temperature in less than ten minutes.  FIGURE 4.12 ENARGITE MINERAL SAMPLES 48  Table 4.7 Elemental analysis of the enargite sample Element  Content (wt %)  Cu  28.30  Fe  15.03  As  9.44  Sb  0.40  Bi  0.26  Ag  0.04  Zn  2.55  Ni  0.00  Al  0.00  Ca  0.05  Pb  1.25  Table 4.8 Quantitative phase analysis of the enargite sample Mineral  Formula  Content (wt %)  Enargite  Cu3AsS4  44.1  Tennantite  (Cu, Ag, Fe, Zn)12As4S13  4.6  Luzonite  Cu3(As, Sb)S4  2.3  Pyrite  FeS2  34.6  Chalcopyrite  CuFeS2  6.5  Sphalerite  (Zn, Fe)S  4.2  Galena  PbS  1.2  Quartz  SiO2  2.5  Large yellow needle-like crystalline precipitates appeared after the leach solution had cooled overnight (cf. Figure 4.13 and Figure 4.14). To determine the As content in the crystal, 1 gram of the crystal was dissolved in 1 liter of DI water and sent for ICP. The As content in the crystals as determined by ICP was ~ 0.38 wt%. This number is not accurate as a large  49  amount of water was likely still associated with the crystals. Nevertheless, since very low arsenic was found in the crystals, most of the arsenic was left in the leach solution. In order to further understand why the arsenic did not precipitate upon cooling, another arsenic precipitation test was done by separating the leach solution (without crystals) into two equal volumes (referred to as solution a. and b.). One half of the mass of precipitated low As crystals was put into solution a., while nothing was added to solution b. Both solutions were then heated to 90 ºC with stirring. Sulfur was then added to both solutions at an As:S ratio of 1 and the mixture was equilibrated for 2 hours. The solutions were then cooled, but no crystalline precipitate was found. Finally, 0.1 ml of solutions a. and b. was sampled, diluted 1000 times and sent for ICP analysis.  FIGURE 4.13 CRYSTAL PRECIPITATED FROM PREGNANT ASL SOLUTION AFTER ENARGITE LEACHING  50  FIGURE 4.14 SIZE OF CRYSTALLITES OBTAINED UPON COOLING OF ASL PREGNANT LEACH SOLUTION  51  Chapter 5 Results and Discussion 5.1 Solubility of As2O3 in Na2S and NaOH solution In the absence of oxygen, the solubility of arsenic in 0.5 M sodium sulfide is constant at ~1.0 M from 20 ºC to 80 ºC. At 95 ºC, the solution turns to black when the arsenic concentration reaches 0.75 M. In the presence of oxygen, arsenic solubility is constant from 20 ºC to 40 ºC, and then increases linearly from 0.8 M at 40 ºC to 2.0 M at 95 ºC (cf. Figure 5.1) 2.2  Arsenic Concentration (M)  2.0  Ar de-aerated conditon O2 sparging condition  1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 10  20  30  40  50  60  70  80  90  100  o  Temperature ( C) FIGURE 5.1 SOLUBILITY OF AS2O3 IN 0.5 M SODIUM SULFIDE SOLUTIONS  When dissolved in sodium sulfide solution, arsenic exists as AsO33-, AsO2S3-, AsOS23-, and AsS33-. According to Kim (2000), the kinetics of As (III) to As (V) oxidation at low 52  temperatures are slow. For example, it takes 2 ~ 5 days for pure oxygen and 4 ~ 9 days for air to oxidize more than 50% of the As (III) in groundwater to As (V). Thus, the function of oxygen may be to push As species transformation from AsS33- to AsO33-. This can be shown as follows:  (5.1.1) The solubility curves presented in Figure 5.1 likely represent the solubility of a mixture of arsenic (III) oxy-sulfide ions. Oxygen sparging may result in oxygen reduction and the formation of hydroxyl through reaction 5.1.2: O2 (g) + 2H2O (l) + 4e- → 4OH- (aq)  (5.1.2)  The accompanying anodic reaction could be sulphide oxidation. Thus the equilibrium in 5.1.1 would be pushed to the left, and the solubility of this mixture with more oxygen increases dramatically with increasing temperature, possibly indicating that the As (III) oxyanions are more soluble than the As (III) oxy-sulfide ions. On the other hand, without oxygen from outside the system, the solubility stays stable from 25 to 80 ºC, and decreases from 1 M to 0.75 M at elevated temperature. The solubility of As2O3 in 0.5 M NaOH (i.e. without sulfide) is presented in Figure 5.2. Without oxygen (or sulfide) present, the arsenic solubility in 0.5 M sodium hydroxide solution increases significantly from 20 ºC to 95 ºC, especially from 60 ºC to 95 ºC. It is of note that the de-aerated condition in Figure 5.2 is similar in profile to that of the oxygen saturated conditions in Figure 5.1. The arsenic solubility in the presence of oxygen but absence of sulfide also appears to be similar. These findings further corroborate the idea that 53  added oxygen does not significantly contribute to oxidation of As (III) during the experiments presented herein but, rather, it shifts the speciation from As (III) sulfides to As (III) oxy-anions. 2.0  Arsenic Concentration (M)  1.8  Ar de-aerated condition O2 sparging condition  1.6 1.4 1.2 1.0 0.8 0.6 0.4 10  20  30  40  50  60  70  80  90  100  o  Temperature ( C) FIGURE 5.2 SOLUBILITY OF AS2O3 IN 0.5 M SODIUM HYDROXIDE SOLUTIONS  For both conditions in Figure 5.2, arsenites (AsO33-) exist in the solutions, according to reactions 5.1.2 and 5.1.3 (Zheng Y.J. et al., 2008). As2O3 (s) + 2OH- (aq) → 2AsO2- (aq) + H2O (l)  (5.1.3)  As2O3 (s) + 6OH- (aq) → 2AsO33- (aq) + 3H2O (l)  (5.1.4)  54  However, it should be noted that the solubility of As quickly (T > 20 ºC) exceeded the added hydroxide in Figure 5.2 indicating that reactions 5.1.3 and 5.1.4 may no longer be valid at higher temperature. As observed from Figure 5.2, the solubility of As2O3 in NaOH with oxidation at 80 ºC is slightly lower than the solubility of As2O3 without oxidation. This may be because high temperatures have a positive effect on the oxidation of As (III) with oxygen (Wang et al. 2000).  5.2 Characterization of synthesized amorphous As2S3 Since the solubility of arsenic trioxide is relatively high, white arsenic trioxide is dissolved very quickly in sodium sulfide solution. When dilute hydrochloric acid solution (1 M, 2 M, and 3 M) was slowly pumped into the As2O3-Na2S solution an interesting phenomenon was observed: as soon as the hydrochloric acid solution droplet touched the As2O3-Na2S solution, a yellow precipitate formed at the droplet surface, which then quickly disappeared as the acid and alkaline solution mixed. The yellow precipitate re-appeared at the bottom of the cell as bulk pH was lowered and it was found to be arsenic tri-sulfide (As2S3) after characterization. The reason for its disappearance after its first precipitation at the HCl solution droplet surface is due to arsenic tri-sulfide’s high solubility in sodium sulfide solution and its low solubility in hydrochloric acid solution. To optimize the conditions of arsenic tri-sulfide synthesis, different concentrations of sodium sulfide and hydrochloric acid solution, temperature, flow rate of hydrochloric acid solution, and aging time were applied. The products were compared according to their EDX spectra and PSD results.  55  5.2.1 Effect of reagent concentrations  For different concentrations of sodium sulfide (0.1 M, 0.2 M, and 0.3 M) and hydrochloric acid (1 M, 2 M, 3 M), the characterization results are as follows: Table 5.1 EDX result for effect of concentration EDX Result  Experiment  Temperature  [Na2S]  [HCl]  Flow rate  Aging time  #  (°C)  (M)  (M)  (ml/min)  (h)  As wt%  S wt%  1  50  0.1  1  3.2  16  61.8  38.2  2  50  0.1  2  3.2  16  61.93  38.07  3  50  0.1  3  3.2  16  67.04  32.96  4  50  0.2  1  3.2  16  59.7  40.3  5  50  0.2  2  3.2  16  69.93  30.07  6  50  0.2  3  3.2  16  67.22  32.78  7  50  0.3  1  3.2  16  61.95  38.05  8  50  0.3  2  3.2  16  65.95  34.05  9  50  0.3  3  3.2  16  63.57  36.43  Theoretically, the EDX result for arsenic tri-sulfide should be close to 61 wt% As and 39 wt% S.  56  7.0 -6  6.5  #1 (P80=10.209 10 m) -6 #2 (P80=17.560 10 m) -6 #3 (P80=14.662 10 m) -6 #4 (P80=16.779 10 m) -6 #5 (P80=19.776 10 m) -6 #6 (P80=19.254 10 m) -6 #7 (P80=19.925 10 m) -6 #8 (P80=21.667 10 m) -6 #9 (P80=19.932 10 m)  6.0 5.5 5.0  Volume In (%)  4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5  0  100  200  300  400  500  600  -6  Particle Size (10 m) FIGURE 5.3 PSD RESULT FOR EFFECT OF CONCENTRATION ON THE PRECIPITATION OF AS2S3 FROM AS2O3-NA2S SOLUTION  The chemical composition of the samples from experiments #1, #2, #4, #7 is close to the theoretical value. The particle size from experiments #1 and #2 had mean values of approximately 10.21 µm and 17.56 µm. In conclusion, the more diluted solution used for both sodium sulfide and hydrochloric acid solutions, the better the chemical composition and the coarser the particle size distribution. The latter makes solid-solution separation easier. 5.2.2 Effect of temperature  Different Temperatures (25 °C, 40 °C, 60 °C, and 80 °C) were used, and the EDX results and PSD results are shown in Table 5.2 and Figure 5.4. 57  Table 5.2 EDX result for effect of temperature EDX  Experiment  Temperature  [Na2S]  [HCl]  Flow rate  Equilibrium  #  (°C)  (M)  (M)  (ml/min)  time (h)  As wt%  S wt%  10  25  0.1  1  3.2  2  56.34  43.66  11  40  0.1  1  3.2  2  59.19  40.81  12  60  0.1  1  3.2  2  59.36  40.64  13  80  0.1  1  3.2  2  59.11  40.89  8 -6  #10 (P80=11.838 10 m) -6 #11 (P80=13.333 10 m) -6 #12 (P80=24.453 10 m) -6 #13 (P80=19.176 10 m)  7  Volume In (%)  6 5 4 3 2 1 0 0  100  200  300  400  500  600  -6  Particle Size (10 m) FIGURE 5.4 PSD RESULT FOR EFFECT OF TEMPERATURE ON THE PRECIPITATION OF AS2S3 FROM AS2O3-NA2S SOLUTION  Arsenic tri-sulfide samples synthesized from 40 °C to 80 °C have similar chemical compositions. A better particle size distribution for solid-solution separation can be achieved at lower temperature (25 °C ~ 40 °C). At 60 °C and 80 °C, the PSD is bi-modal and a portion of the particles are distributed around 200 µm to 400 µm. Thus, a temperature of around 40 °C to 60 °C would appear to be optimal. 58  5.2.3 Effect of hydrochloric acid solution flow rate  Two different flow rates of hydrochloric acid solution (3.2 ml/min, 5.9 ml/min) were applied. The EDX and PSD results for these experiments are listed in Table 5.3. Table 5.3 EDX result for effect of hydrochloric acid flow rate EDX  Experiment  Temperature  [Na2S]  [HCl]  Flow rate  Equilibriu  #  (°C)  (M)  (M)  (ml/min)  m time (h)  As wt%  S wt%  14  50  0.1  1  3.2  2  61.44  38.56  15  50  0.1  1  6.4  2  59.35  40.65  7  -6  #14 (P80=13.763 10 m) -6 #15 (P80=15.512 10 m)  6  Volume In (%)  5 4 3 2 1 0 0  100  200  300  400  500  600  -6  Particle Size (10 m) FIGURE 5.5 PSD RESULT FOR EFFECT OF HYDROCHLORIC ACID FLOW RATE ON THE PRECIPITATION OF AS2S3 FROM AS2O3-NA2S SOLUTION  59  The EDX result and PSD result showed similar chemical composition and particle size distribution. Thus, the effect of hydrochloric acid solution addition rate was not significant. 5.2.4 Effect of aging time  The aging time after the reaction was also varied: 2 h, 8 h, and 24 h aging times were applied. The EDX and PSD results are shown in Table 5.4. Table 5.4 EDX result for effect of aging time EDX  Experiment  Temperature  [Na2S]  [HCl]  Flow rate  Equilibrium  #  (°C)  (M)  (M)  (ml/min)  time (h)  As wt%  16  50  0.1  1  3.2  2  59.50  40.50  17  50  0.1  1  3.2  8  52.63  47.37  18  50  0.1  1  3.2  48  58.84  41.16  S wt%  60  8 -6  #16 (P80=18.197 10 m) -6 #17 (P80=14.978 10 m) -6 #18 (P80=15.185 10 m)  7  Volume In (%)  6 5 4 3 2 1 0 0  100  200  300  400  500  600  -6  Particle Size (10 m) FIGURE 5.6 PSD RESULT FOR AS2S3 SAMPLES OBTAINED BY VARYING AGING TIME  Experiments #16 and #18 had the closest chemical composition to the theoretical composition of arsenic tri-sulfide. All three experiments had similar PSDs with a concentrated particle distribution around 15 µm ~ 18 µm. In conclusion, the aging time should be short (2 hours) or very long (48 hours) so that an ideal result in terms of chemistry can be achieved.  5.3 Solubility of As2S3 in Na2S solution Figure 5.7 presents the solubility of As2S3 in Na2S solution was as predicted by OLI systems software and as measured. According to OLI, arsenic tri-sulfide dissolves linearly  61  with the concentration of sodium sulfide. Temperature has almost no influence on the solubility. 0.6 in 0.13M Na2S solution by experiment in 0.26M Na2S solution by experiment  0.5  in 0.39M Na2S solution by experiment in 0.13M Na2S solution by OLI  Arsenic Concentration (M)  in 0.26M Na2S solution by OLI in 0.39M Na2S solution by OLI  0.4  0.3  0.2  0.1  0.0 10  20  30  40  50  60  70  80  90  100  o  Temperature ( C) FIGURE 5.7 PREDICTED (OLI SOFTWARE) AND EXPERIMENTAL SOLUBILITY OF AS2S3 IN NA2S SOLUTIONS  Generally, experimental data for the solubility of As2S3 agrees well with the predicted solubility by the OLI systems software. The solubility of arsenic tri-sulfide is about 0.11 M (1 wt% Na2S solution), 0.25 M (2 wt% Na2S solution), and 0.35 M (3 wt% Na2S solution) at 20 °C, and increases to 0.14 M (1 wt% Na2S solution), 0.29 M (2 wt% Na2S solution), and 0.39 M (3 wt% Na2S solution) at 95 °C. The obvious difference between predicted solubility and experimentally measured solubility is the effect of temperature. Temperature increments did modestly improve the measured solubility of arsenic tri-sulfide. The effect of sodium 62  sulfide concentration is generally consistent between the model and measurements shown here. The solubility of amorphous As2S3 linearly increases with the concentration of sodium sulfide solution. Referring to the data of Weissberg (1966); the solubility of synthetic crystalline As2S3 is 0.4280 M at 52 °C and 0.4309 M at 100 °C in 3.43 wt% Na2S solution. The solubility of amorphous As2S3 can be predicted from our work (using the assumption of a linear relationship between Na2S concentration and As2S3 solubility) as 0.4046 M at 20 °C, 0.4216 M at 40 °C, 0.4335 M at 60 °C, 0.4493 M at 80 °C, and 0.4612 M at 95 °C in 3.43 wt% sodium sulfide solutions. Compared with the Weissberg data presented these values are reasonably close.  5.4 Characterization of synthetic crystalline products 5.4.1 Attempts to synthesize sodium arsenic oxy-sulfide from sodium arsenate precursor  Synthetic sodium arsenate was synthesized first and analyzed by EDX (cf. Table 5.5). This complex was then used for sodium arsenic oxy-sulfide complex synthesis. Table 5.5 Elemental analysis of synthetic sodium arsenate by EDX Element  Concentration (wt %)  Oxygen  41.56  Sodium  20.34  Arsenic  38.10  Synthetic crystal #1, #2, and #3 were synthesized by using sodium sulfide solution, solid sodium sulfide, and hydrogen sulfide gas, respectively. They were analyzed by EDX for elemental composition and these data are presented in Table 5.6. 63  Table 5.6 EDX analysis for synthetic crystal #1, #2, and #3 Sample Synthetic crystal #1 Synthetic crystal #2 Synthetic crystal #3  Sodium (wt%) 20.38 27.33 23.12  Arsenic (wt%) 34.10 10.67 31.27  Oxygen (wt%) 45.10 47.27 40.60  Sulfur (wt%) 0.43 14.73 5.02  Comparing the EDX results of these three synthetic crystals, sodium sulfide solution is not a good choice for the replacement of oxygen by sulfur in sodium arsenate, because the sulfur content in crystal #1 is very low (0.43 %). However, for crystal #2 and #3, the oxygen in the original sodium arsenate is replaced by sulfur from solid Na2S or H2S gas to some extent. Synthetic sodium arsenate (cf. Table 5.5 and Figure 5.8) and product #3 (cf. Table 5.6 and Figure 5.9) were characterized by QXRD, but an unknown crystalline phase (not in the database) was detected for both samples. For synthetic sodium arsenate, there was only a minor amount of this unknown crystalline phase; but for synthetic product #3, the unknown crystalline phase was the main species. Thus we can only estimate the chemical composition of these synthetic products by EDX for O:S ratio and ICP for arsenic content.  64  3AL_SynthSodiumArsenateComplex.raw_1  Na3.25(OH)0.25(H2O)12)(AsO4) 100.00 %  26,000  24,000  22,000  20,000  18,000  Counts  16,000  14,000  12,000  10,000  8,000  6,000  4,000  2,000  0 5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  80  2Th Degrees  FIGURE 5.8 RIETVELD REFINEMENT PLOT OF SAMPLE “SYNTHETIC SODIUM ARSENATE” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES  65  50,000  2AL_SynthSodiumArsenicOxy-SulfideComplex.raw_1  Na3.25(OH)0.25(H2O)12)(AsO4) 100.00 %  48,000 46,000 44,000 42,000 40,000 38,000 36,000 34,000 32,000 30,000  Counts  28,000 26,000 24,000 22,000 20,000 18,000 16,000 14,000 12,000 10,000 8,000 6,000 4,000 2,000 0 5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  80  2Th Degrees  FIGURE 5.9 RIETVELD REFINEMENT PLOT OF SAMPLE “SYNTHETIC PRODUCT #3” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES  66  5.4.2 Synthesis of sodium thioarsenate by reaction of As2S3 with elemental sulfur  Precipitate #4 showed the highest elemental sulfur content of all the synthetic sodium thioarsenate-type precipitates (cf. Table 5.7). Precipitate #4 was also characterized by QXRD for quantitative phase determination, as shown in Table 5.8 and Figure 5.10. Both Table 5.8 and Figure 5.8 show that a relatively pure crystal of sodium thioarsenate was produced. Table 5.7 Elemental analysis of synthetic crystalline precipitate #4 by EDX Element  Concentration (wt %)  Sodium  16.4  Sulfur  48.5  Arsenic  35.1  Table 5.8 QXRD result and crystal parameters of synthetic sodium thio-arsenate Synthetic  Standard parameters  Na3AsS4·8H2O  in database  Space group  P121/c1  P121/c1  Cell volume (Å3)  1506.04(5)  1490.65  100.000  100.000  1.765  1.85  a (Å)  8.6927(2)  8.651(3)  b (Å)  12.9878(2)  12.995(4)  c (Å)  13.7126(2)  13.596(5)  Beta (°)  103.393(1)  102.77(7)  Phase name  Purity (wt%) 3  Crystal density (g/cm ) Lattice parameters  67  1AL_SynthSodiumArsenicSulfurComplex.raw  Na3AsS4 8H2O 100.00 %  20,000  18,000  16,000  14,000  Counts  12,000  10,000  8,000  6,000  4,000  2,000  0  -2,000 5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  80  2Th Degrees  FIGURE 5.10 RIETVELD REFINEMENT PLOT OF SAMPLE “SYNTHETIC SODIUM THIO-ARSENATE, PRODUCT #4” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES.  68  5.4.3 Arsenic content determination by ICP analysis  The synthetic precipitates #2, #3, and #4 were analyzed by EDX and QXRD (sodium thio-arsenate), but neither of these methods can give an exact bulk arsenic concentration. Hence, for each synthetic crystalline complex, one gram of the solid was dissolved in one liter of DI water. The average arsenic content was then measured by ICP-OES (detection range for ICP analysis: 0.2 ~ 10000.0 mg/L) as shown in Table 5.9: Table 5.9 ICP analysis for arsenic content in precipitates #2, 3 and 4  As (mg/L) Sample  2 g of precipitate in 1 L  As (wt%) wt% in the solid by  As (wt%)  calculation from ICP  by EDX  solution assay  Synthetic crystal #2  161.7  8.1  10.7  Synthetic crystal #3  377.8  18.9  31.3  Synthetic crystal #4  367.5  18.4  35.1  The theoretical concentration of As in sodium thio-arsenate octa-hydrate (Na3AsS4∙8H2O) is 18.03%. Thus, according to QXRD and ICP results, the chemical and structural composition of synthetic sodium thio-arsenate, product #4, is very pure. For synthetic crystals #2 and #3, no valid QXRD spectra were obtained indicating that the precipitates were amorphous. Compared with the As concentration by ICP, the As concentration by EDX is much higher for all three crystals. This is because during EDX analysis crystalline water was likely lost due to evaporation in the vacuum chamber of the microscope. For products #2 and #3, the degree of hydration cannot be determined by drying because they are very stable at room temperature and melt at around 60 ~ 80 ºC. Hence, their crystalline water number are estimated as 12.5 (24.3%) and 8.7 (39.5%) according to the As concentration determined by ICP and EDX.  69  5.5 Solubility of synthesized sodium thio-arsenate products 5.5.1 Sodium arsenate precursor synthesized by oxidation of As (III) with hydrogen peroxide 5.5.1.1 Sodium thio-arsenate synthesized by the addition of sodium sulfide solution to dissolved sodium arsenate precursor (product #1)  Since the content of sulfur and arsenic in precipitate #1 is very low, it was not used for the determination of solubility. 5.5.1.2 Sodium thio-arsenate synthesized by the addition of solid sodium sulfide to dissolved sodium arsenate precursor (product #2)  The solubility of synthetic crystal #2 is presented in Figure 5.9. Since there’s no valid QXRD result, water content cannot be determined quantitatively. Instead, the degree of hydration is calculated by comparison of EDX (without crystalline water) and ICP (with crystalline water). There are 24.3 wt% of water in crystal #2 by theoretical calculation, which might be higher than the real water content by compare with crystal #4 (sodium thioarsenate). Thus, the solubility of synthetic crystal #2 will be slightly lower than the exact solubility data.  70  1.8  Arsenic Concentration (M)  1.6 1.4  in 2.5M NaOH and 1.0M Na2S solution in 2.5M NaOH and 0.5M Na2S solution in 1.0M NaOH and 1.0M Na2S solution Nadkarni's data in 2.2~2.4M Na2S solution (1988)  1.2 1.0 0.8 0.6 0.4 0.2 0.0 10  20  30  40  50  60  70  80  90  100  o  Temperature ( C) FIGURE 5.11 SOLUBILITY OF SYNTHETIC PRODUCT #2 IN SODIUM HYDROXIDE AND SULFIDE SOLUTIONS EXPOSED TO AMBIENT AIR  Compared with Nadkarni and Kusik’s data in 2.2 ~ 2.4 M sodium sulfide solution, the solubility of synthetic crystal #2 is a little lower than that of Na3AsS4 by Nadkarni and Kusik (1988) in Na2S solution. As temperature is increased, the solubility of synthetic crystal #2 increases quickly from 0.05 M (20 °C) up to 1.25 M (95 °C) in 2.5 M NaOH and 0.5 M/1.0 M Na2S solution. In 1.0 M NaOH and 1.0 M Na2S solution, the solubility of synthetic crystal #2 is slightly greater than for the other two concentration conditions in the range of 20 °C to 60 °C, and then reaches a saturation point (of ~ 0.85 M As) at 80 °C.  71  5.5.1.3 Sodium thio-arsenate synthesized by the addition of hydrogen sulfide to dissolved sodium arsenate precursor (product #3)  Arsenic Concentration (M)  6.0 5.5  in 2.5M NaOH and 1.0M Na2S solution  5.0  in 1.0M NaOH and 1.0M Na2S solution  4.5  in 2.5M NaOH and 0.5M Na2S solution Nadkarni's data in 2.2~2.4M Na2S solution (1988)  4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 10  20  30  40  50  60  70  80  90  100  o  Temperature ( C) FIGURE 5.12 SOLUBILITY OF SYNTHETIC PRODUCT #3 IN SODIUM HYDROXIDE AND SULFIDE SOLUTIONS EXPOSED TO AMBIENT AIR  The water content in crystal #3 was estimated to be 39.5%. Since this estimate may result in error, it is possible that the actual solubility for As may be slightly higher than the experimental data presented in Figure 5.12. The solubility of synthetic crystal #3 in 2.5 M NaOH and 0.5 M/1.0 M Na2S solution fit very well with the solubility of “Na3AsS4” reported by Nadkarni and Kusik in 2.2 ~ 2.4 M sodium sulfide solution. Thus, there is a possibility that the sodium thioarsenate mentioned by Nadkarni and Kusik (1988) from enargite leaching in sodium hydroxide (0.25 M) and sulfide (2.2 ~ 2.4 M) solution is in fact a sodium arsenic oxy-sulfide complex, rather than stoichiometric sodium thio-arsenate. 72  5.5.2 Synthesis of thio-arsenate by oxidation of As (III) with elemental sulfur 5.5.2.1 Solubility of sodium thio-arsenate (product #4) in leach solution under air atmosphere and argon atmosphere  The solubility of sodium thio-arsenate was first measured in air. To assess the degree of polysulfide interference, the same solubility experiments were repeated under argon atmosphere. The sulfide and hydrosulfide ions may act as reducing agents for As (V), after the dissolution of Na3AsS4 in NaOH and Na2S solution. Since the solubility of As (III) is greater than As (V), the reduction of As (V) to As (III) should be accompanied by an increase in measured solubility.  4.5 4.0  in 2.5M NaOH & 1.0M Na2S solution in 2.5M NaOH & 0.5M Na2S solution  Arsenic Concentration (M)  in 1.0M NaOH & 1.0M Na2S solution  3.5  Nadkarni's data in 2.2~2.4M Na2S solution (1988)  3.0 2.5 2.0 1.5 1.0 0.5 0.0 10  20  30  40  50  60  70  80  90  100  o  Temperature ( C) FIGURE 5.13 SOLUBILITY OF PRODUCT #4 (SODIUM THIO-ARSENATE (NA3ASS4·8H2O)) IN SODIUM HYDROXIDE AND SULFIDE SOLUTION EXPOSED TO AMBIENT AIR  73  4.5 4.0  in 2.5M NaOH & 1.0M Na2S solution in 2.5M NaOH & 0.5M Na2S solution  Arsenic Concentration (M)  in 1.0M NaOH & 1.0M Na2S solution  3.5  Nadkarni's data in 2.2~2.4M Na2S solution (1988)  3.0 2.5 2.0 1.5 1.0 0.5 0.0 10  20  30  40  50  60  70  80  90  100  o  Temperature ( C) FIGURE 5.14 SOLUBILITY OF PRODUCT #4 (SODIUM THIOARSENATE (NA3ASS4·8H2O)) IN SODIUM HYDROXIDE AND SULFIDE SOLUTION IN ARGON ATMOSPHERE  After comparison, the solubility data of synthetic sodium thioarsenate under air and argon atmospheres are very close. Thus, the oxygen in the air does not affect the crystal dissolution much. This may indicate that negligible amounts of sulfide or hydrosulfide were oxidized to elemental sulfur, which means that almost all of arsenic in the solution exists as As (V). Thus the solubility experiments should theoretically be accurate. Rochette et al. (2000) found that in an acidic environment (pH ~ 4), reduction of As (V) to As (III) by sulfide is a rapid process, with a rate constant of 3.2 x 102 M/h. However, for the conditions used here, the solution pH is well above 13, and S2- and HS- are highly stable. Thus the oxidation of S2- to S0 and the attendant increase in measured solubility is likely not a problem for our work.  74  By comparison, Nadkarni and Kusik’s Na3AsS4 solubility data is ~ 55 % lower than the experimental data at 40 ºC for synthetic Na3AsS4·8H2O (product #4) presented here. According to Figure 5.10 above and the QXRD analyses from enargite leaching tests presented in Table 5.11, a possible explanation for this is that Nadkarni and Kusik is actually reporting the solubility of a mixture of sodium arsenic oxy-sulfide complexes. The solubility of synthetic sodium thioarsenate (product #4) grows rapidly from around 0.17 M at 20 °C to almost 4.0 M at 95 °C. Hence, based on solubility data, about 93.75% of synthetic sodium thio-arsenate can be separated from solution by cooling, which means that near quantitative arsenic removal from the leach solution can be achieved. However, it was observed through the course of this study that if the arsenic concentration in solution is too high, the solution can become a solid or gel-like material after cooling. According to Tongamp et al., (2010), an initial As concentration of 0.51 ~ 0.79 M can fall to 0.18 ~ 0.26 M through cooling from 90 ºC to 30 ºC, thus reaching 60% ~ 73% As removal (note that the current study cooled to 20 ºC). Thus there should be an ideal high temperature As concentration point where high arsenic removal is obtained but also where gel-formation is avoided. 5.5.2.3 Solubility of synthetic crystal #4 (sodium thio-arsenate) at constant hydroxide (2.5 M/100 g/L) and varying hydrosulfide content (0 M ~ 1.0 M/0 ~ 56 g/L) in argon  To study the solubility of sodium thio-arsenate with increasing hydrosulfide concentration, several experiments were done in solutions with constant sodium hydroxide concentrations (2.5 M) at different temperatures.  75  3.2  o  at 20 C o at 40 C o at 60 C o at 80 C o at 95 C  Arsenic Concentration (M)  2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0  0.0  0.2  0.4  0.6  0.8  1.0  -  HS Concentration (M) FIGURE 5.15 SOLUBILITY OF SODIUM THIO-ARSENATE (PRODUCT #4, NA3ASS4·8H2O) IN CONSTANT SODIUM HYDROXIDE (2.5 M) AND VARYING HYDROSULFIDE CONCENTRATION (0 ~ 1.0 M) SOLUTIONS IN ARGON ATMOSPHERE  As seen from Figure 5.13, there is an obvious decrease in solubility as hydrosulfide concentration is increased, especially at lower temperature 20 ~ 60 ºC. By increasing the total solute in the original solution, it is likely that the common ion effect results in lower sodium thio-arsenate solubility. A similar trend was found in the previous solubility experiments for products #3 and #4 (Na3AsS4·8H2O) in sodium hydroxide and sulfide solutions. For example, the solubility of products #3 and #4 is higher in 1.0 M NaOH and 1.0 M Na2S solution than in 2.5 M NaOH and 1.0 M Na2S solution. To verify this assumption, Tongamp’s (2010) experimental data are presented with ours in Figure 5.14.  Tongamp’s data appears to  maintain the trend of lowering solubility as HS- concentration increases although a plateau does appear to exist around 0.2 M As.  76  0.7 Solubility Experimental Data of Na3AsS4 8H2O in 0~1M NaHS and 2.5M NaOH solution Tongamp's Data in 3.2~4.5M NaHS and 2.5M NaOH solution (2010)  Arsenic Concentration (M)  0.6 0.5 0.4 0.3 0.2 0.1 0.0  0  1  2  3  4  5  -  HS Concentration (M) FIGURE 5.16 THE SOLUBILITY OF SODIUM THIO-ARSENATE IN CONSTANT HYDROXIDE AT 20ºC (2.5 M NAOH) AND VARYING HYDROSULFIDE SOLUTION COMPARIED WITH THAT OBTAINED BY TONGAMP ET AL. (2010) AT 30ºC  For all three synthetic complexes (#2, #3 and #4), solubility increases with temperature. The main difference observed is the importance of the temperature effect which is, from low to high: crystal #2 < crystal #4 (sodium thio-arsenate) < crystal #3. Compared with Nadkarni and Kusik’s solubility data for “Na3AsS4”, the best agreement with the experimental data presented herein is for the solubility of synthetic crystal #3 (also #2 to some extent) between 20 °C to 45 °C. In contrast, the solubility of relatively pure synthetic sodium thio-arsenate (product #4) is obviously greater than that presented by Nadkarni and Kusik. The QXRD results of the arsenic precipitate obtained after cooling of solution used for enargite leaching (refer to Chapter 5.7), show that the “industrial” precipitate should be a mixture of sodium arsenic oxy-sulfide complexes (Na3AsO2S2, Na3AsOS3, and Na3AsO3S etc.). Hence, the solubility of synthetic crystal #3, instead of pure sodium thio-arsenate, appears to be a better reference for the solubility of arsenic in ASL solutions. 77  5.6 Solubility accuracy calibration test Two solubility tests were done for solubility accuracy calibration, and the two conditions are: in 2.5M NaOH & 1.0M Na2S at 20 ºC under argon atmosphere (experiment a.) and in 2.5M NaOH & 0.5M NaHS at 60 ºC under argon atmosphere (experiment b.). When the solution was determined to be saturated according to visual observation, the first solution sample was taken and diluted 500 times in DI water. Then, after equilibration overnight (more than 12 hours), the second sample was taken in the same way. The four solution samples were sent for ICP analysis and the results are shown in Table 5.9. Table 5.10 Solubility data for accuracy calibration As Conc. determined by  As Conc. in 1st sample  As Conc. in 2nd sample  visual inspection (M)  by ICP (M)  by ICP (M)  a  0.178  0.169  0.179  b  1.482  1.275  1.332  Experiment  According to Table 5.9, the solubility data achieved by visual observation (which involves calculation of dissolved arsenic based on the mass of sample added) and by ICP of solutions both at the time of visual saturation and after overnight exposure are reasonably close. Since the error obtained between the visual saturation method and the 1st sample ICP measurement is actually larger than that obtained when comparing visual inspection with the 2nd sample (overnight saturation), the error in the methodology lies not in the ability to “see” the point of saturation but rather in the ability to generate a completely homogenous head sample or to get reproducible assays.  5.7 Solubility of As in ASL solutions originating from an enargite leaching test The enargite concentrate was leached in 2.5 M NaOH and 0.5 M Na2S solution at 95 ºC for 24 hours, but very low arsenic content was detected by ICP analysis of the crystallite obtained after cooling of the solution (Table 4.9). This was initially thought to be due to speciation i.e. that the As was preferentially speciated as As (III) instead of As (V). Thus elemental sulfur was added into solution portion a. (with ½ of the crystallites also added to the solution) and solution portion b. (no crystal) at 90 ºC with the intention of complete 78  oxidation of arsenic in the leach solution. However, upon cooling of this “oxidized” solution, still no crystalline precipitate was obtained. The Arsenic concentration determined by ICP analysis of solutions a. and b. is presented in Table 5.10. Table 5.11 Arsenic concentration of solution a. and b. by ICP Sample  As conc. (g/L)  As conc. (M)  Volume (L)  As mass (g)  a.  38.5  0.51  0.35  13.48  b.  29.9  0.40  0.35  10.47  According to elemental and phase analyses of the concentrate, the total arsenic in the head sample was 28.32 g. Referring to the ICP results in Table 5.10, the arsenic in the crystals and in the leach solution were 6.02 g ((13.48 – 10.47) × 2) and 20.93 g, respectively. Thus, the arsenic extraction during leaching was approximately 95.16%. The reason why no precipitate was observed, even after oxidation, may be gleaned from the solubility data presented above. The solubility of synthetic product #4 at room temperature is around 0.25 M As (cf. Figure 5.11 and Table A-14) in 2.5 M NaOH and 0.5 M Na2S solution. However, much of the sodium hydroxide and sulfide is consumed during leaching, which can increase the arsenic solubility such as in 1.0 M NaOH and 1.0 M Na2S solution (cf. Figure 5.10 and Table A-12). It seems likely, then, that the 0.4 M As concentration (Table 5.10) was below the solubility limit (which was apparently above 0.51 M As here) for this particular ASL solution. This is corroborated to a certain extent by Tongamp et al. (2010) who mention that it is necessary to increase the arsenic concentration to at least > 35 g/L or 0.47 M in 50 ~ 250 g/L or 0.89 ~ 4.46 M NaHS and 50 ~ 100 g/L or 1.25 ~ 2.5 M NaOH solution before precipitation by cooling. The nature of the crystallite is also clearly important. The crystallite analyzed in Table 5.10 was found to be too unstable to analyze by QXRD and three other enargite leaching tests performed during this thesis in 3.9 M NaOH and Na2S concentration varying from 0.34 to 0.86 to 1.72 M, revealed that the predominant crystallized As complex was found to be sodium arsenic oxide sulfide hydrate (Na3AsO2S2·11H2O) (Table 5.11), which represented 89.3%, 83.4%, and 87.1% of the crystalline phases (XRD-detectable) in the 79  precipitate, respectively. Sodium thio-arsenate was never detected. Hence, we can conclude that the arsenic precipitate that occurs after cooling of ASL solutions is a mixture of sodium arsenic oxy-sulfide complexes (Na3AsO2S2, Na3AsO3S, Na3AsOS3, and perhaps Na3AsS4), rather than only sodium thio-arsenate as previously reported in the literature (Nadkarni and Kusik 1988, Anderson et al 2008). It is also important to note that because the leaching system was exposed to an air atmosphere, carbon dioxide was likely involved in some of the reactions as it produced Trona (Na3H(CO3)2∙2H2O) in the precipitate. The leaching reactors in future work should be de-aerated to simulate industrial conditions where the surface to volume ration would be much lower. Table 5.12 QXRD result for arsenic precipitate from enargite leaching tests (See XRD spectra in Appendix Figure C-1, 2, 3)  Phase  Sodium Arsenic Oxide Sulfide Hydrate  Sodium Sulfide Arsenate Hydrate Sodium Aluminum Silicate Hydrogen Carbonate Hydrate  #1  #2  #3  3.9M NaOH &  3.9M NaOH &  3.9M NaOH &  0.34 M Na2S  0.86 M Na2S  1.72 M Na2S  solution  solution  solution  Na3AsO2S2(H2O)11  89.3%  83.4%  87.1%  Na3AsO3S(H2O)7  4.1%  0.6%  1.0%  2.6%  4.8%  4.2%  Ideal Formula  Na7.67(Al6Si6O24)(HCO 3)(CO3)0.5(H2O)4.19  Arsenolite  As2O3  0.5%  0.7%  0.7%  Trona  Na3H(CO3)2∙2H2O  3.5%  10.5%  6.9%  100.0%  100.0%  99.9%  Total  For the three enargite leaching tests presented in Table 5.11, the arsenic concentration in solution was around 58.5 g/L (0.78 M). Compared with the measured solubility of 80  products #2, #3, and #4, the As concentration in these ASL solutions (~ 0.78 M) is above the highest arsenic solubility (0.57 M) at room temperature (solubility of As from synthetic Na3AsS4·8H2O in 2.5 M NaOH solution in Table A-21). Thus to precipitate arsenic from ASL solutions, we need to increase the arsenic concentration above the arsenic saturation point at room temperature for the synthetic solutions tested in this work. This finding may be related, as discussed above, to the amount of sodium hydroxide and sulfide that remains unreacted after leaching. In aggregate, these data indicate that solubility studies of “pure” crystallites in simulated solutions (such as those reported here) can only yield order of magnitude estimates of what one might expect to see in an industrial process. The chemistry of the ASL is so complicated that As solubility almost certainly has to be determined on a case by case basis.  81  Chapter 6 Conclusion 1. The optimum conditions for As2S3 synthesis are low concentration of sodium sulfide (0.1 M) and hydrochloric acid (1M), temperatures of 40 ºC ~ 60 ºC, and long aging time. 2. The solubility of synthetic amorphous arsenic tri-sulfide in sodium sulfide solution increases linearly with the concentration of sodium sulfide, but changes only slightly with temperature. 3. A good way to synthesize sodium arsenic oxide sulfide complexes is to add solid sodium sulfide or purge hydrogen sulfide gas into sodium arsenate solution, but it is difficult to control the O:S ratio in the final product. 4. Relatively pure sodium thio-arsenate crystals can be synthesized by dissolving amorphous As2S3 in sodium sulfide solution with the addition of elemental sulfur at high temperature (~ 70 ºC). After slowly cooling the solution, a cluster of faint yellow crystal is formed. 5. In sodium hydroxide and sulfide solutions, the solubility of synthetic sodium thioarsenate increases dramatically with temperature, and a similar trend was found for the solubility of synthetic sodium arsenic oxide sulfide products. This gives an effective path to separate arsenic from leach solution. 6. There are two ways to enhance the crystallization of the arsenic precipitate: increase the sodium hydroxide and sodium sulfide concentrations or increase the arsenic concentration in leach solution before cooling. 7. According to the QXRD analyses of the arsenic precipitates resulting from cooling of ASL solutions, pure “sodium thio-arsenate” cannot be considered a likely crystallite in industrial processes. The precipitated complexes are comprised of sodium arsenic oxide sulfides (Na3AsO2S2, Na3AsOS3, and Na3AsO3S). 8. The ASL solutions originating from enargite leaching are able to maintain a certain degree of supersaturation over the solubilities measured in this work. However, direct comparison is difficult as each solution appears to generate a different type of crystallite.  82  Chapter 7 Future Work A good way to decrease sulfide or hydrosulfide consumption during arsenic precipitation by cooling may be to increase the ratio of NaOH:Na2S in the leach solution. Ongoing work at UBC has demonstrated that leaching in sub-stoichiometric sulphide solutions is possible. The crystalline arsenic precipitates would then be preferentially present as sodium arsenic oxy-sulfide species with a high O:S ratio. This may save money as in some jurisdictions sulphide is more expensive than hydroxide but this may also help to improve the recirculation od the sulfide source. Thus not only could more sulfides be recirculated, but more arsenic removal could be achieved due to the common ion effect.  This thesis provides some evidence to support the contention that the As oxy-sulfides are more stable than the sulphides. For example, in the case of sodium thioarsenate synthesis by method I, hydrogen sulfide gas was used according to the following anticipated reaction (4.4.4): Na3AsO4 (aq) + 4H2S (g) → Na3AsS4 (s) + 4H2O (l)  (4.4.4)  Sodium thioarsenate formation should have been more favorable in this case than when adding Na2S as the sulphide source. However, EDX results showed that after complete reaction, the O:S ratio in the final product was still as high as 8:1. Moreover, in the three enargite leaching tests presented in Table 5.11 it was observed that the lowest molar O:S ratio was 1:1 (in Na3AsO2S2(H2O)11) and that the product with a 3:1 O:S ratio (Na3AsO3S(H2O)7) was most prevalent as the OH-:S2- ratio was highest. 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Tables of solubility data by experiment and OLI systems software for amorphous As2S3 and synthetic crystals A.1 Solubility data of orpiment in literature and amorphous As2S3 by experiment and OLI systems software Table A-1 Orpiment solubility in solutions to which no sulfide has been added (Webster 1990) At 25 ºC pH  mSkt (x 105)  Ipart*  As (mg/kg)  1.40  1.7  0.444  0.82  1.42  1.8  0.444  0.89  1.55  0.9  0.400  0.46  2.14  1.3  0.040  0.64  2.23  1.5  0.040  0.76  2.58  1.0  0.011  0.50  2.58  1.3  0.011  0.64  3.02  0.9  0.004  0.44  3.07  1.1  0.004  0.54  3.84  2.4  0.001  1.2  4.28  2.4  0.000  1.2  5.16  6.1  0.000  3.0  5.98  2.9  0.000  1.4  6.06  5.7  0.000  2.9  6.28  6.0  0.000  3.0  6.45  6.1  0.050  3.0  7.16  81  0.000  41  At 90 ºC pH  mS (x 104)  Ipart  As (mg/kg)  1.16  3.3  0.200  17  1.27  3.6  0.200  18  2.61  2.8  0.006  14  2.61  2.0  0.006  9.4  3.00  3.0  0.002  15  3.46  5.4  0.010  27 90  At 90 ºC continued pH  mS (x 104)  Ipart  As (mg/kg)  3.60  3.5  0.020  17  3.84  4.2  0.100  21  4.02  3.5  0.200  17  4.29  3.4  0.000  17  4.29  3.0  0.000  15  5.11  4.7  0.000  24  5.11  3.8  0.000  19  5.18  3.9  0.000  20  5.93  5.8  0.000  30  5.93  2.7  0.000  14  6.09  8.9  0.010  44  6.27  7.2  0.000  36  6.85  11  0.000  52  Table A-2 Orpiment solubility in solutions to which sulfide has been added (Webster 1990) At 25 ºC pH  mSkt  Ipart  As (mg/kg)  1.45  0.0016  0.392  0.64  2.21  0.0294  0.036  0.009  2.23  0.0282  0.036  0.02  3.05  0.0320  0.005  0.04  3.36  0.0168  0.011  0.28  4.25  0.0239  0.000  2.3  4.35  0.0170  0.000  0.71  4.50  0.0170  0.000  0.84  4.51  0.0133  0.001  1.1  4.82  0.0212  0.001  2.1  5.56  0.0283  0.002  16  5.59  0.0228  0.000  20  6.46  0.0180  0.026  110  6.64  0.0197  0.079  620  6.75  0.0180  0.057  280 91  At 90 ºC pH  mSkt  Ipart  As (mg/kg)  1.57  0.0168  0.066  0.06  1.64  0.0184  0.066  0.07  2.05  0.0255  0.022  0.08  2.07  0.0218  0.020  0.05  2.08  0.0194  0.020  0.03  2.10  0.0206  0.020  0.08  2.54  0.0225  0.006  0.30  2.57  0.0188  0.006  0.36  3.07  0.0214  0.002  0.76  3.07  0.0214  0.002  0.88  3.70  0.0184  0.006  0.97  3.70  0.0185  0.006  3.9  4.12  0.0294  0.000  1.8  4.23  0.0184  0.000  4.6  4.23  0.0184  0.000  3.4  4.27  0.0192  0.000  4.1  4.30  0.0171  0.000  1.2  4.41  0.0192  0.000  6.3  4.55  0.0087  0.000  6.6  4.56  0.0087  0.000  4.3  4.58  0.0172  0.000  6.6  4.76  0.0026  0.000  4.5  4.77  0.0039  0.000  6.4  5.10  0.0350  0.000  170  5.18  0.0039  0.000  5.2  5.21  0.0178  0.002  35  5.21  0.0178  0.002  35  5.23  0.0204  0.000  69  5.25  0.0310  0.000  50  5.38  0.0221  0.001  54  5.46  0.0158  0.002  23  5.57  0.0158  0.002  24  5.64  0.0339  0.000  30  5.75  0.0338  0.000  24 92  At 90 ºC continued pH  mSkt  Ipart  As (mg/kg)  5.99  0.0196  0.001  69  6.07  0.0242  0.001  130  6.07  0.0242  0.001  120  6.10  0.0240  0.001  120  6.22  0.0011  0.020  55  6.27  0.0195  0.020  64  6.31  0.0015  0.100  77  6.35  0.0011  0.200  56  6.59  0.0231  0.028  250  6.59  0.0246  0.028  330  6.75  0.0308  0.100  440  6.87  0.0315  0.036  700  6.88  0.0367  0.040  900  6.97  0.0285  0.036  550  7.00  0.0340  0.080  710  7.05  0.0319  0.040  660  7.10  0.0591  0.055  1400  7.11  0.0055  0.100  210  7.24  0.0051  0.050  150  7.26  0.0567  0.072  720  7.28  0.1328  0.144  1000  7.31  0.0588  0.072  850  Table A-3 Solubility prediction of As2S3 in 1 wt% of 1 L Na2S solution Temperature  Mole of As2S3  Concentration of  (ºC)  (mol)  As2S3 (M)  20  0.125339  0.125339  25  0.125837  0.125837  30  0.126152  0.126152  35  0.126349  0.126349  40  0.126467  0.126467  45  0.12653  0.12653  50  0.126554  0.126554 93  Table A-3 Solubility prediction of As2S3 in 1 wt% of 1 L Na2S solution continued Temperature  Mole of As2S3  Concentration of  (ºC)  (mol)  As2S3 (M)  55  0.126549  0.126549  60  0.126525  0.126525  65  0.126487  0.126487  70  0.126442  0.126442  75  0.126395  0.126395  80  0.126353  0.126353  85  0.126322  0.126322  90  0.12631  0.12631  95  0.126322  0.126322  Table A-4 Solubility prediction of As2S3 in 2 wt% of 1 L Na2S solution Temperature  Mole of As2S3  Concentration of  (ºC)  (mol)  As2S3 (M)  20  0.256194  0.256194  25  0.256847  0.256847  30  0.257254  0.257254  35  0.257506  0.257506  40  0.257655  0.257655  45  0.257732  0.257732  50  0.257757  0.257757  55  0.257745  0.257745  60  0.257707  0.257707  65  0.257651  0.257651  70  0.257585  0.257585  75  0.257516  0.257516  80  0.257453  0.257453  85  0.257404  0.257404  90  0.257377  0.257377  95  0.257381  0.257381  94  Table A-5 Solubility prediction of As2S3 in 3 wt% of 1 L Na2S solution Temperature  Mole of As2S3  Concentration of  (ºC)  (mol)  As2S3 (M)  20  0.390506  0.390506  25  0.391252  0.391252  30  0.391714  0.391714  35  0.391997  0.391997  40  0.392163  0.392163  45  0.392246  0.392246  50  0.39227  0.39227  55  0.392251  0.392251  60  0.392201  0.392201  65  0.39213  0.39213  70  0.392046  0.392046  75  0.39196  0.39196  80  0.391878  0.391878  85  0.391811  0.391811  90  0.391768  0.391768  95  0.391759  0.391759  Table A-6 Solubility of As2S3 in 1 wt%, 2 wt%, and 3 wt% of 80 ml Na2S solution Concentration  Temperature  Weight of As2S3  Mole of As2S3  Concentration of  of Na2S  (°C)  added (g)  (mol)  As2S3 (M)  1 wt%  20  2.2  8.94E-3  0.112  Na2S  40  2.5  0.010  0.127  Solution  60  2.6  0.011  0.132  80  2.6  0.011  0.132  95  2.7  0.011  0.137  2 wt%  20  5.0  0.020  0.254  Na2S  40  5.1  0.021  0.259  Solution  60  5.3  0.022  0.269  80  5.6  0.023  0.285  95  5.8  0.024  0.295  3 wt%  20  6.8  0.028  0.346  Na2S  40  7.2  0.029  0.366  Solution  60  7.4  0.030  0.376  80  7.6  0.031  0.386  95  7.8  0.032  0.396 95  A.2 Solubility data of synthetic crystal #2, #3, #4 (sodium thio-arsenate) by experiment A.2.1 Solubility of synthetic crystal #2 in air  For synthetic crystal #2, the experimental solubility data is shown as follows: 1.  In 100 ml 2.5 M sodium hydroxide and 1.0 M sodium sulfide solution Table A-7 Solubility of in 2.5 M NaOH and 1.0 M Na2S solution  2.  Temperature  Weight of product #2  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (Molar)  20  5  5.39E-3  0.053  40  15  0.016  0.156  60  77  0.083  0.700  80  146  0.157  1.162  95  160  0.173  1.243  In 100 ml 2.5 M sodium hydroxide and 0.5 M sodium sulfide solution Table A-8 Solubility of in 2.5 M NaOH and 0.5 M Na2S solution  3.  Temperature  Weight of product #2  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  20  5  5.39E-3  0.053  40  23  0.025  0.235  60  89  0.096  0.789  80  153  0.165  1.203  95  159  0.172  1.237  In 100 ml 1.0 M sodium hydroxide and 1.0 M sodium sulfide solution  96  Table A-9 Solubility of in 1.0 M NaOH and 1.0 M Na2S solution Temperature  Weight of product #2  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  20  7.5  8.09E-03  0.079  40  22.5  0.024  0.230  60  92.5  0.100  0.815  80  97.5  0.105  0.850  95  97.5  0.105  0.850  A.2.2 Solubility of synthetic crystal #3 in air  For synthetic crystal #3, the experimental solubility data is shown as follows: 1.  In 10 ml 2.5 M sodium hydroxide and 1.0 M sodium sulfide solution Table A-10 Solubility of in 2.5 M NaOH and 1.0 M Na2S solution  2.  Temperature  Weight of product #3  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  20  0.25  6.30E-04  0.062  40  0.75  0.002  0.183  60  2.75  0.007  0.625  80  31  0.078  3.510  95  78  0.196  4.814  In 10 ml 2.5 M sodium hydroxide and 0.5 M sodium sulfide solution Table A-11 Solubility of in 2.5 M NaOH and 0.5 M Na2S solution Temperature  Weight of product #3  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  20  0.3  7.56E-04  0.075  40  0.9  0.002  0.219  60  2.6  0.007  0.594  80  59  0.149  4.462  95  89.6  0.226  4.972  97  3.  In 10 ml 1.0 M sodium hydroxide and 1.0 M sodium sulfide solution Table A-12 Solubility of in 1.0 M NaOH and 1.0 M Na2S solution Temperature  Weight of product #3  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  20  0.5  1.26E-03  0.123  40  2.4  0.006  0.552  60  6.4  0.016  1.287  80  88.4  0.223  4.957  95  123.4  0.311  5.291  A.2.3 Solubility of synthetic crystal #4 (sodium thio-arsenate) in air  Since the QXRD result is very ideal (100% purity), the solubility of sodium thio-arsenate (Na3AsS4·8H2O) should be relatively accurate. For synthetic crystal #4 (sodium thioarsenate), the experimental solubility data is shown as follows: 1. In 20 ml 2.5 M sodium hydroxide and 1.0 M sodium sulfide solution under air atmosphere Table A-13 Solubility of Na3AsS4·8H2O in 2.5 M NaOH and 1.0 M Na2S solution Temperature  Weight of Na3AsS4·8H2O  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  1.4  3.43E-03  0.168  5.2  0.013  0.588  14.8  0.036  1.466  29.2  0.072  2.439  58.2  0.143  3.693  20 40 60 80 95  2. In 20 ml 2.5 M sodium hydroxide and 0.5 M sodium sulfide solution under air atmosphere  98  Table A-14 Solubility of Na3AsS4·8H2O in 2.5 M NaOH and 0.5 M Na2S solution Temperature  Weight of Na3AsS4·8H2O  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  2.1  5.15E-03  0.249  6.6  0.016  0.731  19.7  0.048  1.835  39.7  0.097  2.975  63.3  0.155  3.854  20 40 60 80 95  3. In 20 ml 1.0 M sodium hydroxide and 1.0 M sodium sulfide solution under air atmosphere Table A-15 Solubility of Na3AsS4·8H2O in 1.0 M NaOH and 0.5 M Na2S solution Temperature  Weight of Na3AsS4·8H2O  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  2.5  6.13E-03  0.295  7.5  0.018  0.821  18.5  0.045  1.749  36.5  0.089  2.824  71  0.174  4.073  20 40 60 80 95  A.2.4 Solubility of synthetic crystal #4 (sodium thio-arsenate) in argon To avoid the oxygen in air, same experiments were done under argon atmosphere. The experimental solubility data is as follows: 1. In 10 ml 2.5 M sodium hydroxide and 1.0 M sodium sulfide solution under argon atmosphere  99  Table A-16 Solubility of Na3AsS4·8H2O in 2.5 M NaOH and 1.0 M Na2S solution Temperature  Weight of Na3AsS4·8H2O  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  0.75  1.84E-03  0.179  2.5  0.006  0.567  7  0.017  1.402  14  0.034  2.369  28.5  0.070  3.653  20 40 60 80 95  2. In 10 ml 2.5 M sodium hydroxide and 0.5 M sodium sulfide solution under air atmosphere Table A-17 Solubility of Na3AsS4·8H2O in 2.5 M NaOH and 0.5 M Na2S solution Temperature  Weight of Na3AsS4·8H2O  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  1  2.45E-03  0.237  3.2  0.008  0.711  9.5  0.023  1.785  19  0.047  2.896  29  0.071  3.686  20 40 60 80 95  3. In 10 ml 1.0 M sodium hydroxide and 1.0 M sodium sulfide solution under air atmosphere Table A-18 Solubility of Na3AsS4·8H2O in 1.0 M NaOH and 0.5 M Na2S solution Temperature  Weight of Na3AsS4·8H2O  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  1  2.45E-03  0.237  3.5  0.009  0.771  9.5  0.023  1.785  17.5  0.043  2.749  34  0.083  3.991  20 40 60 80 95  100  A.2.5 Solubility of synthetic crystal #4 (sodium thio-arsenate) at constant hydroxide and varying hydrosulfide content in argon 1.  In 10 ml 2.5 M sodium hydroxide and 1.0 M sodium hydrosulfide solution Table A-19 Solubility of Na3AsS4·8H2O in 2.5 M NaOH and 1.0 M NaHS solution Temperature  Weight of Na3AsS4·8H2O  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  0.75  1.84E-03  0.295  2.5  0.006  0.721  7  0.017  1.319  14  0.034  2.310  28.5  0.070  2.943  20 40 60 80 95  2.  In 10 ml 2.5 M sodium hydroxide and 0.5 M sodium hydrosulfide solution Table A-20 Solubility of Na3AsS4·8H2O in 2.5 M NaOH and 0.5 M NaHS solution Temperature  Weight of Na3AsS4·8H2O  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  1.5  3.68E-03  0.351  3.5  0.009  0.771  7.5  0.018  1.482  13  0.032  2.250  18.5  0.045  2.848  20 40 60 80 95  3. In 10 ml 2.5 M sodium hydroxide and 0 M sodium hydrosulfide solution Table A-21 Solubility of Na3AsS4·8H2O in 2.5 M NaOH and 0 M NaHS solution Temperature  Weight of Na3AsS4·8H2O  Mole of  Concentration of  (°C)  added (g)  Arsenic (mol)  Arsenic (M)  2.5  6.13E-03  0.567  4.5  0.011  0.964  10  0.025  1.857  16  0.039  2.593  22  0.054  3.164  20 40 60 80 95  101  Part B. EDX analysis of synthetic complexes  FIGURE B-1 EDX ANALYSIS OF SYNTHETIC AMORPHOUS ARSENIC TRI-SULFIDE  FIGURE B-2 EDX ANALYSIS OF SYNTHETIC SODIUM ARSENATE  102  FIGURE B-3 EDX ANALYSIS OF SYNTHETIC CRYSTALLINE COMPLEX #1  FIGURE B-4 EDX ANALYSIS OF SYNTHETIC CRYSTALLINE COMPLEX #2  103  FIGURE B-5 EDX ANALYSIS OF SYNTHETIC CRYSTALLINE COMPLEX #3  FIGURE B-6 EDX ANALYSIS OF SYNTHETIC CRYSTALLINE #4 (SODIUM THIO-ARSENATE) 104  Part C. Quantitative x-ray diffraction result of arsenic precipitate from enargite leaching  105  35,000  1AL_PrecipTestAs1.raw_1  Na3AsO2S2(H2O)11 Na3(AsO3S)(H2O)7 Arsenolite Trona Na7.67(Al6Si6O24)(HCO3)(CO3)0.5 (H2O)4.19  89.35 % 4.11 % 0.48 % 3.48 % 2.58 %  30,000  25,000  Counts  20,000  15,000  10,000  5,000  0  -5,000  5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  80  2Th Degrees  FIGURE C-1 RIETVELD REFINEMENT PLOT OF SAMPLE “AS PRECIPITATE FROM ENARGITE LEACHING WITH NA2S:ENARGITE ~ 0.130” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES.  106  2AL_PrecipTestAs2.raw_1  Na3AsO2S2(H2O)11 Na3(AsO3S)(H2O)7 Arsenolite Trona Na7.67(Al6Si6O24)(HCO3)(CO3)0.5 (H2O)4.19  83.45 % 0.56 % 0.70 % 10.51 % 4.78 %  25,000  20,000  Counts  15,000  10,000  5,000  0  -5,000  5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  80  2Th Degrees  FIGURE C-2 RIETVELD REFINEMENT PLOT OF SAMPLE “AS PRECIPITATE FROM ENARGITE LEACHING WITH NA2S:ENARGITE ~ 0.324” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES.  107  3AL_PrecipTestAs3.raw_1  Na3AsO2S2(H2O)11 Na3(AsO3S)(H2O)7 Arsenolite Trona Na7.67(Al6Si6O24)(HCO3)(CO3)0.5 (H2O)4.19  30,000  87.12 % 1.04 % 0.73 % 6.93 % 4.19 %  25,000  20,000  Counts  15,000  10,000  5,000  0  -5,000 5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  80  2Th Degrees  FIGURE C-3 RIETVELD REFINEMENT PLOT OF SAMPLE “AS PRECIPITATE FROM ENARGITE LEACHING WITH NA2S:ENARGITE ~ 0.648” (BLUE LINE - OBSERVED INTENSITY AT EACH STEP; RED LINE - CALCULATED PATTERN; SOLID GREY LINE BELOW - DIFFERENCE BETWEEN OBSERVED AND CALCULATED INTENSITIES; VERTICAL BARS - POSITIONS OF ALL BRAGG REFLECTIONS). COLOURED LINES ARE INDIVIDUAL DIFFRACTION PATTERNS OF ALL PHASES.  108  

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