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On the Leaching Behavior of Uranium- Bearing Resources in Carbonate- Bicarbonate Solution by Gaseous Oxidants

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ON THE LEACHING BEHAVIOR OF URANIUM- BEARING RESOURCES IN CARBONATE- BICARBONATE SOLUTION BY GASEOUS OXIDANTS by Erik Hunter A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Mining and Earth Systems Engineering). Date ___________________ Signed: _________________________ Erik Hunter Signed: _________________________ Dr. Linda Figueroa Thesis Advisor Signed: _________________________ Dr. Mark Kuchta Thesis Advisor Golden, Colorado Date ________________________ Signed: _________________ Dr. Hugh Miller Professor and Head Department of Mining Engineering ii ABSTRACT Uranium is recognized to be a critical commodity in the context of satisfying the global energy-demands for the twenty-first century and beyond. In 2013, a significant supply gap of 17% between worldwide production and consumption of uranium was identified. Consequently, low-grade uranium-bearing limestone rock, including mining waste, may represent a significant future uranium-resource, provided an economically-viable method of extraction is developed. Alkaline leaching of uranium from limestone media has been practiced extensively in the past; however, cutting-edge research during the last 20 years has been rather meager. The research reported in this thesis is an investigation into the use of an alternative gaseous oxidant (oxygen/ozone), which may serve to improve the economics of alkaline leaching; specifically, heap leaching. Components of the research include the employment of computational software (PHREEQ), which allowed the equilibrium distribution of the aquo-species in solution to be determined, as well as solubility behavior of candidate condensed-species (precipitates). Also, two suites of leaching experiments were performed, one with synthetic UO2 particles, which served as a model system, and the other with comminuted uranium-bearing Todilto limestone. The aqueous-phase lixiviant for each was comprised of 40 gpl Na2CO3 and 15 gpl NaHCO3. The invariant leaching-configuration was: continuously-stirred batch-reactor with the ratio of uranium-bearing particulate-source to aqueous-phase such that a hypothetical-maximum uranium concentration in solution of 1.0 gpl (0.0040 molar) could be achieved. The duration of the two suites of experiments was 48-hours and 72-hours, respectively. A robust two-parameter non- linear function, Recovery (%) = 100 [1− exp(−a •(t^b)], was formulated to regress (smooth) the data acquired from the leaching experiments. The regressed-equations (containing optimal values for the parameters “a” and “b”) served as a quantitative tool for relative assessment of the leaching-rate characteristics of each member-experiment within a suite of experiments performed. The work reported in this thesis is considered to be a significant contribution to the knowledge-base required for planning cost-effective strategies for uranium recovery from low- grade alkaline uranium-bearing resources. Additionally, it provides process-engineers with information for developing appropriate recovery-methods (flowsheets), while at the same time being able to assess potential environmental impacts. iii TABLE OF CONTENTS ABSTRACT …………………………………………………………….……………………… iii LIST OF FIGURES ………………………………………………...…………………………... ix LIST OF TABLES .…………………………………………………………………………..... xiv ACKNOWLEDGEMENTS …………………………………………………………………… xix CHAPTER 1 INTRODUCTION ……………………………………………………………….1 1.1 Executive Summary ………………………………………………………….......1 1.2 Introduction …………………………………………………………................... 2 1.3 Justification for the Research Reported in this Thesis ……………………...…... 4 1.4 Objective of the Research Undertaken ………………………...……….………. 5 1.5 Scope of Research ……………………………………………………….....…… 6 1.6 Research Conducted ………………………...………………………………….. 7 1.7 Original Contributions ………………………………………………………..… 9 1.8 Research Methodology ………………………………………………………... 10 1.9 Organization of Thesis ………………………………………………………… 11 CHAPTER 2 LITERATURE SURVEY AND REVIEW …………………………….……... 12 2.1 Market Forces and Uranium Demand ……………...……………………….… 13 2.2 Colorado Uranium Production …………………………………………...…… 13 2.3 Extractive Metallurgy of Uranium-Vanadium Ores ……………………..…… 15 2.3.1 Conventional Alkaline Leaching of Uranium ………………..….…... 16 2.3.2 Ore Pretreatment …………………………………………….…….… 16 iv 2.3.3 Concentration and Purification …………………………………...… 18 2.3.4 Uranium Acid Heap Leaching ……………………………………… 19 2.3.5 Pilot Scale Alkaline Heap Leaching of Uranium …………………… 20 2.3.6 Laboratory Scale Uranium Leaching Experiments at the Colorado School of Mines ………………………………………..… 21 2.3.7 Grants Ridge/Armijo Project Setting …………………………..…… 22 2.3.8 Autoradiography ………...…………………………………….…… 28 CHAPTER 3 THEORETICAL CONSIDERATIONS OF ALKALINE URANIUM LEACHING …………………………………………………………………… 32 3.1 Reactions that occur in the Alkaline Leaching of Uranium Dioxide ……….…. 32 3.2 Side Reactions in Alkaline Leaching of Uranium Bearing Materials ……….… 34 3.3 Choice of Oxidants for Alkaline Leaching…………………………………..… 39 3.4 Leaching Experiments Involving Synthetic Uranium Dioxide and Uranium Ores ………………………………………………………………….. 39 3.5 Electrochemical Model for Uranium Leaching ……………………………..… 45 3.6 Evaluation of Various Oxidants Used in Uranium Leaching …………...….… 54 3.7 Ozone Mechanics and Decomposition ………………………………………… 57 3.8 Two Widely Accepted Theories of Ozone Decomposition …………………… 57 3.8.1 The Staehelin-Bühler-Hoigné Model ………………………….…… 57 3.8.2 The Tomiyasu, Fukutomi, and Gordon Model (TFG Model) of Ozone Decomposition ……………………………...…………….… 59 3.9 A History of the Commercial Use of Ozone ………………………………...… 60 3.10 Methods for Ozone Generation ……………………………………………...… 62 3.11 Commercial Ozone Production Technology – Capital Costs ……………….… 64 3.12 Commercial Ozone Production Technology – Operating Costs ………….…… 66 v 3.13 Plasma Microchip Reactor Ozone Generator Technology ……………….…… 67 3.14 Modeling of the Synthetic UO2 and Todilto Limestone Leach Solutions……... 68 CHAPTER 4 DETAILS OF EXPERIMENTS CONDUCTED ………………………………73 4.1 Chemical and Physical Characteristics of the Uranium-Bearing Materials ……74 4.1.1 Synthetic Uranium Dioxide ………………………………………..… 74 4.1.2 Characterization of Todilto Limestone ………………………………. 74 4.1.3 Characterization of Comminuted Todilto Limestone …………………77 4.2 Reagents Employed for Leaching Experiments ……………………………… 80 4.3 Details of Apparatus Utilized for Leaching Experiments ………….………… 81 4.3.1 Apparatus Utilized for the Leaching of Synthetic Uranium Dioxide ………………………………………………….....82 4.3.2 Methods for Determination of Gas-phase and Aqueous-phase Ozone Concentration…………………………………………………………. 85 4.3.3 Todilto Limestone Leaching Apparatus …………………………..… 88 4.4 Procedures for Leaching of Uranium-Bearing Materials ……………….……. 89 4.4.1 Synthetic Uranium Dioxide Leach Experiment Procedures ……….… 89 4.4.2 Todilto Limestone Leaching Procedures ……………………….……. 93 4.5 Leachate Analysis Procedures…………………………………………….….. 96 CHAPTER 5 RESULTS AND DISCUSSION ………………………………...…..…….… 99 5.1 Simulation of Equilibrium Species-Distribution for the U-CO3- H2O System Generated by PHREEQ Software………………………………….…...……. 99 vi 5.1.1 Equilibrium Species Distribution for the (Synthetic) UO2- Carbonate system………………………………………………...… 100 5.1.2 Equilibrium Species Distribution for the Todilto Limestone, Uranium Carbonate System ……………………………….........…. 101 5.2 Results and Discussion of Leach-Experiments Conducted ……………...… 101 5.2.1 Regression Analysis Strategy for Correlation of Data Derived from Leach Experiments ……………………………...…………… 102 5.2.2 Graphical Display and Interpretation/Discussion of Leaching Performance for Synthetic UO2 and Todilto Limestone ……...…… 105 CHAPTER 6 CONCLUSION …………………………………………………………...… 138 6.1 Retrospective and Discussion ………………………………….…………… 138 6.2 Enumerated Conclusions …………………………………………………… 142 6.2.1 PHREEQ Modeling ……………………………………………...… 142 6.2.2 UO2 Leaching …………………………………………….….……... 143 6.2.3 Todilto Limestone Leaching …………………………...……...…… 143 6.2.4 World Limestone Uranium Deposit Resource and Historical Production ………………………………..………….… 144 6.2.5 Cost Estimates of Commercial-Scale Ozone Generation in Heap Leaching ………………………………………………………...…. 145 6.3 Recommendations for Further Research ……………………………..……… 148 REFERENCES CITED …………………………………………………………………..…… 150 APPENDIX A NUCLEAR POWER BASICS ………………………………………....…… 162 APPENDIX B INTERNAL CONSISTENCY VERIFICATION OF PHREEQ …………..………………………………………………..…… 173 APPENDIX C PHREEQC CALCULATIONS– SYNTHETIC UO2 SOLUTION …..……………………………………….………….…… 174 vii APPENDIX D ICP-MS ANALYSIS OF TODILTO LIMESTONE O3 INJECTED LEACH 9 SOLUTION ………………….…………………...… 178 APPENDIX E PHREEQ CALCULATIONS; TODILTO LIMESTONE O3 INJECTED LEACH 9 SOLUTION ………….…………………………...… 180 APPENDIX F ME-MS 41U ANALYSIS OF < 149 μm TODILTO LIMESTONE ………………………………………….…….……198 APPENDIX G INNOV-X X-RAY FLUORESCENCE SPECTROMETER SPECIFICATIONS AND CALIBRATION EXPERIMENTS…….……….. 202 APPENDIX H DISSOLVED OXYGEN, PH, ORP, AND TEMPERATURE DATA …………………………………….…………..… 205 APPENDIX I ENVIRONMENTAL CONDITIONS DATA ………………………………………………………….…………… 220 APPENDIX J TELEDYNE OZONE MONITOR CALIBRATION CERTIFICATE ……………………………………………………….…….. 233 APPENDIX K CROSS-CORRELATION OF ANALYSIS METHODS ………….……..… 234 APPENDIX L MASS BALANCE CALCULATIONS: TOLDILTO LIMESTONE O3 INJECTED LEACH EXPERIMENTS 5 AND 7 ……………...…….… 238 APPENDIX M EVALUATION OF ‘ZUN ZUN’ REGRESSION ANALYSIS ……….…… 241 APPENDIX N RECOVERY COMPUTATIONS FOR TODILTO LIMESTONE O3 (H-REL)
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