PRE-TREATMENT OF TANTALUM AND NIOBIUM ORES FROM DEMOCRATIC REPUBLIC OF CONGO (DRC) TO REMOVE URANIUM AND THORIUM Elie KABENDE BSc (Applied Physics) GradDip (Extractive Metallurgy) 2020 This thesis is presented for the degree of Masters of Philosophy at Murdoch University DECLARATION I declare that this thesis is my own account of my research contains as its main content work which has not previously been submitted for a degree at my tertiary education institution. E …………………………………………. Elie kabende ABSTRACT Tantalum (Ta) and niobium (Nb) have applications in high-technology electronic devices and steel manufacturing, respectively. Africa, South America and Australia collectively provide about 80% of the global supply of Ta-Nb concentrates. In Africa, Sociéte Minière de Bisunzu (SMB) located in the Democratic Republic of Congo (DRC) remains the major supplier of Ta- Nb concentrates containing about 33 wt % Ta2O5 and 5 wt % Nb2O5 as the two oxides of main economic values, associated with uranium (0.14 wt %) and thorium (0.02 wt %). The presence of U and Th complicates the transportation logistics of tantalite to international markets, due to stringent regulation on an allowable limit of U and Th at 0.1 wt %. High radiation levels also hinder further primary beneficiation of the Ta-Nb concentrates at the Bisunzu mine to an extent of at least 50 wt % Ta2O5 and Nb2O5 combined. Digestion of the concentrate using HF is the conventional method to remove U and Th followed by chemical treatment to recover Ta2O5 and Nb2O5. The HF digestion process is hazardous and requires large investments. The main objective of this study is the mineral identification in the ore and concentrates of SMB mine sites, to investigate the possibilities of upgrading the Ta2O5 and the Nb2O5 by weight using physical separation and concentration, and removing U and Th using chemical treatment. The mineral identification analysis conducted on eight concentrates sourced from different mining locations at the Bisunzu mine in DRC has shown that manganocolumbite, manganotantalite and ferrotantalite are the major phases with cassiterite, as well as pyrochlore and microlite group of minerals hosting uranium as minor components. The rare earth minerals, quartz and lepidolite were also identified in the ore and concentrates. The quantitative analysis revealed a chemical composition range of 20 – 47 wt % Ta2O5 and 5.4 – 22 wt % Nb2O5 as the principal oxides, associated with 0.005 – 0.802 wt % U and 0.001 – 0.072 wt % Th as radioactive elements. Due to high content of iron in concentrate, the magnetic separation of selected sample i has shown an increase of Ta2O5 and Nb2O5 to 50 wt % and 19 wt %, respectively, in the non- magnetic portion. Samples with low content of iron have shown weak magnetic properties. Sieving of selected concentrates also increased 18 wt % Ta2O5 and 9 wt % Nb2O5 in coarse size fraction of –2800+1000 μm by about 100%. It was also observed that U and Th were more concentrated in the fines (–1000 μm) i.e., 4844 g/t U and 135 g/t Th compared to 1132 g/t U and 50.5 g/t Th in coarse particles. The sulfuric acid bake of fine particles (–75 μm) at 300 oC for 2 h followed by leaching o with H2SO4 and K2S2O8 at 90 C for 3 h lowered the concentration of U and Th in the feed from 0.8 wt % and 0.03 wt %, respectively, to ≤ 0.01 wt %. The measured concentrations of dissolved metal ions (U, Th, Ta and Nb) show reasonable agreement with published solubility diagrams. An average of 7% Ta and 9% Nb dissolved in the bake-leach process with leaching efficiencies of 100% U and 60% Th. The reliability of the analytical methods and bake-leach results have been confirmed by the elemental mass balance calculation. The H2SO4 and CaF2 leaching at 90 oC dissolved 100% U and 90% Th along with 81% Ta and 43% Nb which warrants further investigations. ii ACKNOWLEDGMENTS First and foremost, words cannot express my gratitude towards my mentor and principal supervisor Associate Professor Gamini Senanayake through my MPhil study and successful completion of my thesis. Nevertheless, this is extended to Doctor Artur Deditius for accompanying me to collect samples from DR Congo mine sites. I would like to thank Professor Bogdan Dlugogorski, Doctor Hans Oskierski and Associate Professor Mohammednoor Altarawneh for their support and encouragement to complete my thesis. To the laboratory staff especially Ken Seymour and Stewart Kelly your technical supports are a blessing in my quest to reach my goals. To Doctor Ibukun Oluwoye, James Mulwanda and Sean Hunt from College of Science, Health, Engineering and Education your experience and knowledge sharing completes my studies and this thesis is a testimony for all your kind assistance. To my beloved brothers, sisters and my fiancée (Joyeuse Nziza) thank you for all the love, prayers and support throughout my entire journey. A special word of gratitude goes to the one whom I consider as my father Edouard Hizi Mwangachuchu for his unmeasurable love and care to me and his invaluable contribution to my growth in general, and my studies in particular. Last but not least I am truly grateful for the MPhil Scholarship from Sociéte Minière de Bisunzu (SMB Sarl) which provided the opportunity to pursue my study in Murdoch University/Perth. iii TABLE OF CONTENTS DECLARATION i ABSTRACT i ACKNOWLEDGMENTS iii TABLE OF CONTENTS iv LIST OF FIGURES viii CHAPTER 1 INTRODUCTION 1 1. 1. Background 1 1.2. Mining challenges at SMB 8 CHAPTER 2 LITERATURE REVIEW 14 2.1. Geology, mineralogy, composition and properties of tantalum and niobium ores 14 2.2. Radioactivity and mineralogy of U and Th in tantalum and niobium ores 18 2.3. Mineralogy of tantalum-niobium concentrates from Africa 23 2.4. Beneficiation routes for upgrading tantalum-niobium concentrates 26 2.5 Physical separation and concentration at Niobec and Greenbushes 30 2.6. Chemical decomposition of tantalum and niobium concentrate 34 2.7. Dissolution of metals from columbite-tantalite 36 2.8. Potential-pH and solubility diagrams 40 CHAPTER 3 MATERIALS AND METHODS 47 3.1. Materials 47 3.1.1. Concentrates 47 3.1.2. Chemical reagents 48 iv 3.2. Methods 49 3.2.1. Sample preparation 49 3.2. Analytical techniques 52 3.2.1. X-ray Fluorescence 52 3.2.2. Geiger-Muller Counter 52 3.2.3. Scanning electron microscopy 53 3.2.4. X-ray diffraction 54 3.2.5. Inductive coupled plasma mass spectrometry (ICP-MS) 54 3.3. Batch baking and leaching experiments 55 3.3.1. Eh - pH measurements 58 3.3.2. Characterisation and elemental mass balance 58 CHAPTER 4 RESULTS AND DISCUSSIONS 61 4.1. Characterisation of ore and concentrate from SMB deposit 61 4.1.1. Geology and mineralogy of Bisunzu mines 61 4.1.2. Tantalite concentrates of different colours from SMB mines 65 4.1.3. Chemical composition of ore and concentrate from SMB deposit 68 4.1.4. XRD analysis of SMB ore and concentrates 79 4.1.5. SEM observations and EDS analysis of sample C 83 4.1.6. SEM observations and EDS analysis of sample G 87 4.1.7. Correlation of elemental assays 89 4.1.8. Comparison between different ores 91 4.1.9. Summary of characterisation 93 4.2. Radioactivity of tantalite samples collected at Bisunzu 94 4.3. Physical separation and concentration 96 v 4.3.1. Sieving to upgrade tantalum and niobium oxides and remove uranium and thorium 96 4.3.2. Magnetic separation and concentration of Ta-Nb oxides 99 4.3.3. Summary of physical separation and concentration 103 4.4. Leaching to remove uranium and thorium from tantalum and niobium concentrate 104 4.4.1. Summary of leaching results and elemental mass balance 104 4.4.2. Direct leaching with H2SO4 111 o o 4.4.3. Leaching with K2S2O8 at 40 C after roasting at 600 C 114 o 4.4.4. Leaching with K2S2O8 at 40 C after alkaline bake 115 o o 4.4.5. Leaching with K2S2O8 at 40 C after baking with H2SO4 at 400 C 118 o o 4.4.6. Leaching with K2S2O8 at 90 C after baking with H2SO4 at 400 C 121 4.4.7. Effect of baking conditions on uranium and thorium leaching with K2S2O8 at 90 oC 127 (a) Effect of bake time on uranium leaching 127 (b) Effect of particle size on uranium leaching 128 (c) Effect of bakesolid mass to acid volume ratio on uranium leaching 130 (d) Effect of bake conditions on thorium leaching 130 4.4.8. Effect of leaching conditions on uranium and thorium leaching with K2S2O8 after baking at 400 oC 133 (a) Effect of leaching temperature 133 (b) Effect of acid concentration 136 (c) Effect of solid mass to liquid volume ratio 137 (d) Effect of oxidant mass to liquid volume ratio 138 (e) Effect of agitation 139 4.4.9. Effect of leaching conditions on minor elements after baking at 400 oC 140 4.4.10. Effect of baking at 100 – 300 oC and leaching at 90 oC 144 vi (a) Major elements 144 (b) Minor elements 147 (c) Most suitable leach conditions 148 4.4.11. Correlation between leaching efficiencies 148 4.4.12. Characterisation of bake product and leach residue of sample C 152 4.4.13. Acid decomposition and leaching of tantalite concentrate with CaF2 161 4.4.14. Summary 165 CHAPTER 5 SUMMARY, CONCLUSIONS AND FUTURE WORK 167 5.1. Summary and conclusions 167 5.2. Recommendations for future work 171 REFERENCES 173 APPENDIX 184 Appendix A 1 184 Appendix A 2 186 Appendix A 3 188 Appendix A 4 190 Appendix A 5 195 Appendix A 6 198 Appendix A 7 215 Appendix A 8 239 vii LIST OF FIGURES Figure 1.