AN EVALUATION OF ANODE CORROSION AND GAS EMISSIONS DURING GOLD ELECTROWINNING by Amy Jo Chambers A thesis submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Master of Science Department of Metallurgical Engineering The University of Utah May 2014 Copyright © Amy Jo Chambers 2014 All Rights Reserved T h e University of Utah Graduate School STATEMENT OF THESIS APPROVAL The thesis of Amy Jo Chambers has been approved by the following supervisory committee members: Michael L. Free , Chair Oct. 21, 2013 Date Approved Sivaraman Guruswamy , Member Oct. 21, 2013 Date Approved Sanja Miskovic , Member Oct. 21, 2013 Date Approved and by Manoranjan Misra , Chair/Dean of the Department/College/School of Metallurgical Engineering and by David B. Kieda, Dean of The Graduate School. ABSTRACT Gold is an important metal for a variety of uses and applications. A primary method of refining gold is electrowinning; it is used throughout the world and is becoming more and more common not only for gold processing but for other metals as well. To better understand and mitigate the negative effects of gold electrowinning, specifically anode corrosion and emissions, the effects of electrolyte composition (cyanide, chloride, hydroxide, sulfate and silicate), current density, and temperature in electrowinning cells were examined. Stainless steel electrodes were used. The emissions for gold electrowinning were also studied; specifically, ammonia and hydrogen cyanide gas evolution were examined while varying temperature, current density, and concentration of cyanide and hydroxide. The data indicate that the greatest cause of increased anode corrosion rate is temperature. Lower temperatures examined did not cause significant corrosion except in one anode which had high cyanide and high silica. Silica is often used as a corrosion inhibitor and at low concentrations large corrosion rates were not observed, however, very high corrosion rates were observed with high concentrations of silica. At high temperature the greatest contributor to increased corrosion rate was large amounts of silica in the electrolyte. Low ion concentrations in the electrolyte showed sulfate as the greatest contributor to corrosion rate. Cyanide concentration had a small effect on corrosion rate. Corrosion was observed at varied current density, chloride, and hydroxide, however, there was not a significant linear relationship. Significant pitting was observed in the anodes exposed to high levels of chloride. The amount of ammonia evolved from the electrowinning cell was found to be directly proportional to the amount of cyanide loss in any given test. The largest contributor to cyanide destruction was temperature and cyanide concentration. The effects of hydroxide and current density were small and not statistically significant. The evolution of hydrogen cyanide was found to be negligible. iv TABLE OF CONTENTS ABSTRACT……………………………………………………………………………...iii LIST OF FIGURES……………………………………………………………………...vii LIST OF TABLES………………………………………………………………………...x ACKNOWLEDGMENTS……………………………………………………………….xii 1 INTRODUCTION………………………………………………………………...1 1.1 Background………………………………………………………………………..1 1.2 Electrowinning…………………………………………………………………….3 1.3 Corrosion…………………………………………………………………………..5 1.4 Emissions…...........................................................................................................12 2 EXPERIMENTAL PROCEDURES…………………………………………….21 2.1 Anode Corrosion Experiments…………………………………………………...21 2.2 Emissions Tests…………………………………………………………………..24 3 RESULTS………………………………………………………………………..30 3.1 Anode Corrosion………….……………………………………………………30 3.1.1 Effect of NaCN on Anode Corrosion……………...…………….………..….30 3.1.2 Effect of NaCl on Anode Corrosion…............….............……………...........31 3.1.3 Effect of Na2SO4 on Anode Corrosion………………….……………...…...31 3.1.4 Effect of NaOH on Anode Corrosion...………………….…………………..32 3.1.5 Effect of Na2SiO3 and Current Density on Anode Corrosion...………….....32 3.1.6 Effect of Lower Temperature (71° C) on Anode Corrosion...……….………33 3.1.7 Effect of Low Na2SiO3 and Na2SO4 on Anode Corrosion..………………..33 3.1.8 Intermediate Temperature Anode Testing...……………….………………...33 3.1.9 Low and Mixed Na2SiO3 and Na2SO4 Anodes......................................…...34 3.2 Emissions Testing……………………………..………………………………..34 4 DISCUSSION……………………………………………………………………53 4.1 Anode Corrosion……………………………………….………………………53 4.1.1 Effect of NaCN……………………………………………..………………..53 4.1.2 Effect of NaCl…………………………………………………..……………54 4.1.3 Effect of NaOH……………………………………………………...……….54 4.1.4 Effect of Current Density……..……………………………………………...55 4.1.5 Effect of Silica..……………………………………………………………...56 4.1.6 Effect of Sulfate...……………………………………………………………56 4.1.7 Effect of Mixed Sulfate and Silica..………………………………………….57 4.1.8 Effect of Temperature…..……………………………………………………58 4.2 Emissions Tests..………………………………………………………………..58 4.2.1 Ammonia Emission and Cyanide Loss………………………………..……..58 4.2.2 Cyanide Concentration vs. Cyanide Loss..…………………………………..59 4.2.3 Hydroxide Concentration and Cyanide Loss……………..………………….60 4.2.4 Current Density and Cyanide Loss..…………………………………………60 5 CONCLUSIONS………………………………………………………………...71 APPENDIX……………………………………...……………………………………….73 REFERENCES…………………………………………………………………………..75 vi LIST OF FIGURES Figure 1.1: Time versus energy of metals (adapted from Chandler 1998). .................................. 19 1.2: Corrosion of an anode in electrowinning (adapted from Chandler 1998). ................ 19 2.1: Anode corrosion setup. .............................................................................................. 26 2.2: Electrode before testing. ............................................................................................ 26 2.3: Three hour sample of electrowinning cell voltage data. ............................................ 27 2.4: Graph constructed during cleaning of anode 18. ....................................................... 27 2.5: Determination of optimum flow rate. ........................................................................ 28 2.6: Complete emissions test setup. .................................................................................. 28 2.7: Electrowinning cell without secondary containment. ................................................ 29 3.1: Anodes 0-3 correspond to results in Table 3.1. ......................................................... 35 3.2: Anodes 4-7 correspond to data in Table 3.1. ............................................................. 35 3.3: High variability of cyanide concentration. ................................................................ 36 3.4: Stable control of cyanide concentration. .................................................................... 37 3.5: Voltage-time plot showing anodes 0 and 1 over a two week period. ........................ 38 3.6: Anodes 8-15 correspond to data presented in Table 3.2. ........................................... 39 3.7: Anode 13 magnified................................................................................................... 39 3.8: Anodes 16-23 correspond to data presented in Table 3.3. ......................................... 40 3.9: Large pitting observed in cross section of anode 23. ................................................. 40 3.10: Anodes 24-31 correspond to the data in Table 3.4. ................................................. 41 3.11: Anodes 32-35 correspond to data in Table 3.5. ....................................................... 42 3.12: Cross section of anode 35. ....................................................................................... 42 3.13: Anodes 36-38 correspond to data in Table 3.5. ....................................................... 43 3.14: Anodes tested in electrolyte containing Na2SO4 at 71°C. ....................................... 44 3.15: Cross section of anode 42. ....................................................................................... 44 3.16: Anodes 45-47 correspond to data in Table 3.6. ....................................................... 45 3.17: Anodes 48-55 correspond to data in Table 3.7. ....................................................... 46 3.18: Anodes 56-63 correspond to data in Table 3.8. ....................................................... 46 3.19: Anodes 64-71 correspond to data in Table 3.9. ....................................................... 47 4.1: Cyanide vs. corrosion, data taken from Table 3.1 and test 48 in Table 3.7. .............. 61 4.2: The relationship between chloride and corrosion, data taken from Table 3.2. .......... 61 4.3: NaOH vs. corrosion, data taken from Table 3.4. ....................................................... 62 4.4: Current density vs. corrosion, data taken from Tables 3.2 and 3.5. .......................... 62 4.5: The relationship of Na2SiO3 with corrosion, data from Tables 3.5, 3.7, 3.9. ............ 63 4.6: The effect of Na2SO4 on corrosion, data from Tables 3.3, 3.7, and 3.9. ................... 63 4.7: Na2SO4 concentration values less than 0.5 g/L, data from Tables 3.7 and 3.9. ......... 64 4.8: The effect of Na2SO4 and Na2SiO3, data from Tables 3.3, 3.5, 3.7, and 3.9. ............ 64 4.9: Na2SO4 at various temperatures, data from Tables 3.3 and 3.8. ................................ 65 4.10: Na2SiO3 corrosion rate compared to temperature, data from Tables 3.4 and 3.8 .... 65 4.11: Cyanide loss vs. ammonia capture for all tests, data from Table 3.11. ................... 66 4.12: