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T-4046 ELECTROCHEMICAL ASPECTS OF MAGNESIUM DISSOLUTION IN AQUEOUS KCl-NaCl ELECTROLYTES by Jerrilyn P. McAndrew ARTHUR LAKES LIBRARY COLORADO SCHOOL OF MINES GOLDEN, CO 80401 ProQuest Number: 10783720 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10783720 Published by ProQuest LLC(2018). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 T—4046 A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Hines in partial fulfillment of the requirements for the degree of Master of Science (Metallurgical and Materials Engineering). Golden, Colorado Date:: A J w j . S T Signed: ]A1 L U Approved Dr. Gerapd-'F'T Martins Thesis Advisor Golden, Colorado Date: /&<r-<5r / ? 9 & un. J. J. Moore Professor and Head, Department of Metallurgical and Materials Engineering T—4046 ABSTRACT The dissolution of magnesium metal in equimolar sodium chloride/potassium chloride aqueous electrolytes was investigated. The focus of the work conducted was on the electrochemical aspects of the dissolution process, as it relates to the equilibria in the electrolyte and the dissolution kinetics. The research was initiated based on the perceived need (circa 1993) to remove the reactive metals - magnesium, aluminum, sodium and potassium - in a class of waste salts generated at Rocky Flats. These salts, because of Resource Conservation and Recovery Act (RCRA) regulations, could not be shipped for disposal because of the reactive metals cited that they contained. Today, 1996, this is no longer a high-priority RCRA regulations requirement. Magnesium, rather than sodium or potassium, was selected as the candidate metal to be investigated since its characteristics would define an "upper bound" as to the treatment strategy which would ultimately be developed. Aluminum was considered to be the least reactive, and not truly a hazardous material. The results of the study have delineated the corrosion characteristics of magnesium in high-concentration (5-7m) aqueous chloride-electrolytes. In T—4046 addition, an academic component of the research has developed a semi-empirical dissolution-rate model for the system. The model behavior appeared to be fundamentally consistent for ambient-temperature (21°C) dissolution, but at high temperature (70°C) the behavior was anomalous. The dissolution-rate was such that at 21°C, for an electrolyte volume to magnesium surface-area ratio of »200, the dissolution for this system could be achieved in approximately 10 minutes with an asymptotic magnesium concentration of approximately 40/iffi when the initial pH of the electrolyte was as-prepared («7.0) and 90MS when the initial pH was adjusted by hydrochloric-acid addition to &4.0. Employing an elevated temperature for the process does not appear to offer any clear benefit. T—4046 TABLE 07 CONTENTS PAGE ABSTRACT .............. iii TABLE OF CONTENTS..................................... v LIST OF FIGURES......................................vii LIST OF TABLES .......................................ix ACKNOWLEDGEMENTS..................................... X CHAPTER is INTRODUCTION....................... 1 1.1 Scope of the Research .............. 3 1.2 Objective of the R e s e a r c h .......... 4 1.3 Organization of Thesis ................. 5 CHAPTER 2: LITERATURE REVIEW ..................... 6 2.1 Electrochemical Corrosion-Behavior of M a g n e s i u m ........................ 7 CHAPTER 3: THEORETICAL CONSIDERATIONS.......... 12 3.1 Species Distribution (Speciation) .... 12 3.1.1 Model-Equations Development . 14 3.2 Electrode Reactions and Potentials .... 20 3.3 Dissolution-Rate Model Development . 3 4 CHAPTER 4: DETAILS OF EXPERIMENTS CONDUCTED.... 41 4.1 Materials Used in the Studies ...... 42 4.2 Details of Equipment ................... 45 4.3 Procedures Employed ................... 47 v T—4046 CHAPTER 5: RESULTS AND DISCUSSION.................. 50 5.1 Results of Experiments Conducted . .51 5.2 Model Predictions of the System Behavior . 60 CHAPTER 6: CONCLUSION ............................. 76 6.1 Discussion and Summary................... 76 6.2 Enumerated Conclusions .................. 79 6.3 Recommendations for Further Work ......... 80 REFERENCES 82 APPENDICES APPENDIX I: Definition or Equilibrium Quotients and Constants for the S y s t e m ...........84 APPENDIX II: QBASIC Computer Code for Determination of Species Distribution and Listing of Generated Output for Initial pH of 4.0 and 7 . 0 ........................... 86 APPENDIX III: Check of Solution ................. 91 APPENDIX IV: Summary of Corrosion Potentials Relative to the Standard Hydrogen Electrode . 94 APPENDIX V: QBASIC Code for Determination of Rate Constants and Corresponding Predicted pH Versus Time Behavior. (Listing of Measured Ph Versus Time Response Included with Generated Output for Each of the Eight Runs Processed.) . .96 AUTHOR'S BIOGRAPHICAL S K E T C H .........................110 T—4046 LIST OF FIGURES FIGURE TITLE PAGE 3.1 Electrode potential vs pH for reaction Mg(OH)2(s) + 2e* - Mg° + 20KT at 298°K .... 26 3.2 Electrode potential vs pH for reaction 2H* + 2e* - Hj(g) at 298#K 26 3.3 Electrode potential vs pH for reaction 21^0 + 2e" = 20H- + Hj at 298°K .............. 27 3.4 Electrode potential versus magnesium ion concentration for reaction Mg2* + 2e‘ = Mg° at 298°K. On the right axis is the pH versus magnesium concentration behavior, due to magnesium dissolution, for initial pH of 7 ............ 28 3.5 Electrode potential versus magnesium ion concentration for reaction Mg2* + 2e' - Mg° at 298°K. On the right axis is the pH versus magnesium concentration behavior, due to magnesium dissolution, for initial pH of 4 ............ 29 3.6 Current versus potential for reactions Mg2* + 2e* * Mg and 2HjO + 2e‘ = H2(g) predicted by the Butler-Volmer equation for hypothetical parameters, viz. exchange current density and the transfer coefficients ...................... 33 3.7 Hypothetical concentration profiles for Mg2* and OH" at two selected times during the dissolution process. The concentrations of Mg2* and OH* at the interface (distance-0) for both tt and tj are consistent with the saturated condition for Mg (OH) 2 (s) at the surface .................. 31 4.1 Cross-sectional and end view of the magnesium rotating-disc electrode ........... 39 4.2 Schematic of the electrode system used for electrochemical studies .................... 42 5.1 Electrode potential versus time for run JMEC3 . 56 vii Electrode potential versus time for run JMEC4 56 Electrode potential versus time for run JMEC5 57 Electrode potential versus time for run JMEC6 57 Electrode potential versus time for run JMEC7 58 Electrode potential versus time for run JMEC8 58 Electrode potential versus time for run JMEC9 59 Electrode potential versus time for run JMEC10 59 Predicted and measured pH versus time for run JMEC3 (Air contact; pH°=7.7; T=21°C) ....... 63 Predicted and measured pH versus time for run JMEC4 (Air contact; pH°=7.8; T=21°C) ....... 63 Predicted and measured pH versus time for run JMEC5 (Air contact; pH°=8.0; T = 2 1 ° C ) ....... 64 Predicted and measured pH versus time for run JMEC6 (Ar sparge; pH°«6.8; T=21°C) ......... 64 Predicted and measured pH versus time for run JMEC7 (Ar sparge; pH°=4.1; T=21°C) ......... 65 Predicted and measured pH versus time for run JMEC8 (Ar sparge; pH°=7.6; T=21°C) ......... 65 Predicted and measured pH versus time for run JMEC9 (Air contact; pH°=7.3; T=71°C) ....... 66 Predicted and measured pH versus time for run JMEC10 (Air contact; pH°=7.4; T=72°C) ....... 66 Predicted and measured pH versus time for run JMEC10 when optimized values of kt and kj from JMEC9 are used ............................ 70 viii T—4046 LIST OF TABLES TABLE TITLE PAGE 5.1 Matrix of Conditions Employed for Experiments . 52 5.2 Summary of Rate Coefficients, Mass Transfer Coefficients and Mass-Transfer Coefficient Ratios Obtained by the Computer-Aided Optimization Strategy, k, refers to Mg2+ and k, to OIT; AMp = 0.203 cm2; V„ = 40 cm2 ...... 7 68 5.3 Summary of Number-Averaged Mass-Transfer Coefficients and Corresponding Diffusion Coefficients for Runs JMEC3 and JMEC4. kt refers to Mg2+ and k2 to OH"........................ 74 T—4046 ACKNOWLEDGEMENTS I would like to thank Dr. Gerard P. Martins, for his contributions, guidance, patience, friendship, excellent memory, and perseverance during this course of study and thesis work. I cannot express sufficient gratitude to Dr. Martins for his flexibility and availability to meet with me outside of my work hours. Deep gratitude is extended to my family, especially my husband, Bill. Without their encouragement and support during my concurrent employment and studies, this work would have never been completed. Several students answered my questions while I worked to complete this thesis. Special thanks are given to Dr. Jae-Ho Lee for his assistance. I would also like to express my gratitude to Dr. John P. Hager and Dr. Rex Bull, who served as thesis committee members, for their counsel. x T—4046 1 CHAPTER 1 INTRODUCTION In the text which follows, a background on pyrochemical operations at the Rocky Flats Environmental Technology Site in Colorado is provided so that the reasons for conducting the research reported in this thesis can be placed in perspective. Pyrochemical operations have been performed within the Nuclear Weapons Complex in the United States and at Rocky Flats to purify plutonium for weapons production. The primary plutonium-americium separation process was molten salt extraction (MSE). In this pyrochemical process, americium-bearing plutonium is contacted with a molten sodium chloride/potassium chloride/magnesium chloride salt in a two-stage process.