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Synthesis of High Purity Zirconium Tetrafluoride for Nuclear Applications

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Synthesis of High Purity Zirconium Tetrafluoride for Nuclear Applications Synthesis of high purity zirconium tetrafluoride for nuclear applications Dissertation submitted in fulfilment of the requirements for the degree Master of Science in the Department of Chemistry Faculty of Agricultural and Natural Science at the University of the Free State Bloemfontein by ODUETSE SYDNEY MONNAHELA Supervisor: Prof. A. Roodt Co-Supervisors: Dr. J.B. Wagener and Dr. P.A.B. Carstens November 2008 Synthesis of high purity zirconium tetrafluoride for nuclear applications ABSTRACT The RSA has a great abundance of the potentially valuable zircon (Zr(Hf)SiO4) mineral, which is mostly exported in unexploited form, leading to huge losses in potential revenue. The purpose of this project was to attempt to purify Zr of Hf as well as 3d metals for nuclear applications. The fluoride route was used to exploit the difference in sublimation temperatures as a selective separation method for ZrF4 from HfF4, or vice versa, depending on the volatility of these species. The fluorination of ZrO2, HfO2 and plasma dissociated zircon (PDZ) samples and the identification of viable conditions for separation of ZrF4 and HfF4 in an anhydrous hydrogen fluoride and fluorine atmosphere were primarily investigated. The fluorination of these species was performed to understand the chemical behaviour of the tetrafluoride species prior to separation and whether this behaviour could not be manipulated to effect separation. Thermogravimetric analyses indicated that the formation of ZrF4 is possibly via the oxyfluoride. X-ray powder diffraction was used to confirm the results. The theoretical equilibrium composition calculations for separation of ZrF4 and HfF4 were simulated. The separation was further studied by theoretically calculating the separation coefficients of ZrF4 and the metal fluoride impurities. i To my son Atlegang whom I want to inspire by example ii Acknowledgements I would like to express my sincere gratitude and appreciation to God who has given me the talent to perform this research, my supervisors, Professor A. Roodt, University of the Free State, Dr. J.B. Wagener and Dr. P.A.B. Carstens, South African Nuclear Energy Corporation Limited (Necsa) for their support, suggestions, mentorship and guidance. I would also like to thank the following people: 1. My family for their support, encouragement and understanding. 2. Mr. B.M. Vilakazi for generating the thermogravimetric data and assistance with regard to the operation of a thermobalance. Thank you very much. 3. Mr. D. Moolman for the insightful discussions of the thermogravimetric results. 4. Mr. W.L. Retief for assistance with the literature survey and linguistic editing of the dissertation. 5. Mr J.P. le Roux for assistance with part of Chapter 3. 6. Dr. D. de Waal for assistance with Raman analyses and discussions. 7. Necsa and the University of the Free State for financial assistance. 8. Pelindaba Analytical Laboratories and the University of Pretoria for XRF and XRD analyses, respectively. iii LIST OF ABBREVIATIONS AND SYMBOLS Abbreviation Description ASTM American Society for Testing and Materials Ǻ Angstrom AMI Advanced Metals Initiative Bp Boiling point ca. Approximately DST Department of Science and Technology F2 Fluorine gas FG Fluoride glass GPa Gigapascal HSC Enthalpy, Entropy and Heat Capacity HF Hydrogen fluoride IR Infrared K Kelvin MIBK Metylisobutylketone Mp Melting point M Molecular weight N Mole fraction PDZ Plasma Dissociated Zircon PMC Palaborwa Mining Company PWR Pressure Water Reactor PBMR Pebble Bed Modular Reactor PAL Pelindaba Analytical Laboratories po Equilibrium vapour pressure ppb Parts per billion ppm Parts per million RSA Republic of South Africa REE Rare earth element SEM Scanning Electron Microscope TGA Thermogravimetric Analyzer Ts Sublimation Temperature TBP Tributylphosphate US United States $ United States Dollar UV Ultraviolet % Percent iv °C Degree Celsius kg Kilogram MW Megawatt nm Nanometer cm-1 Wavenumber mg Milligram g Gram ℓ Litre min Minute wt Weight tp Triple point sp Sublimation point v Table of Contents Abstract……………………………………………………………………………….i Dedication…………………………………………………………………………....ii Acknowledgements………………………………………………………………...iii List of abbreviations………………………………………………………………..iv Table of contents…………………………………………………………………...vi List of figures……………………………………………………………………….xii List of tables……………………………………………………………………….xvii Chapter 1 ..................................................................................... Introduction 1.1 THE ROLE OF THE AMI IN THE Necsa ZrF4 MANUFACTURING PROJECT .............................................................1 1.2 BACKGROUND AND HISTORY ...........................................................2 1.3 ZIRCONIUM ORES ...............................................................................4 1.4 CHEMISTRY OF ZIRCONIUM ..............................................................5 1.5 APPLICATIONS OF ZIRCON ...............................................................9 1.6 APPLICATIONS OF Zr METAL AND COMPOUNDS IN THE NUCLEAR INDUSTRY ..........................................................................9 1.7 NON-NUCLEAR APPLICATIONS OF Zr METAL AND THEIR COMPOUNDS ....................................................................................11 1.8 EXTRACTION OF ZIRCONIUM VALUES FROM ZIRCON ................. 12 1.9 AIM OF THE STUDY ..........................................................................12 Chapter 2 .............................................................................Literature survey 2.1 INTRODUCTION .................................................................................14 2.2 ZIRCONIUM METAL MINERAL FEEDSTOCK ................................... 15 2.3 INDUSTRIAL PROCESSES FOR SEPARATION OF ZIRCONIIUM FROM HAFNIUM ..........................................................17 vi 2.3.1 The TBP process ........................................................................18 2.3.2 The MIBK process ......................................................................19 2.3.3 Aliquat 336 process ....................................................................19 2.3.4 The CEZUS process ..................................................................19 2.4 Zr PRECURSOR FOR PURIFICATION STUDIES .............................. 20 2.5 PURIFICATION PROCESSES FOR THE PRECURSOR TO NUCLEAR GRADE .............................................................................22 2.5.1 Conventional aqueous routes ..................................................... 22 2.5.2 Novel anhydrous routes ............................................................. 23 2.6 PREFERRED Zr PRECURSOR ..........................................................27 2.6.1 ZrCl4 as precursor ......................................................................27 2.6.2 ZrF4 as precursor .......................................................................28 2.7 SUBLIMATION TEMPERATURES OF ZrF4 AND CONTAMINANT FLUORIDES ............................................................33 2.8 TECHNIQUES FOR ENHANCING THE EFFICIENCY OF THE ANHYDROUS SUBLIMATION PROCESS .......................................... 36 2.8.1 Improving the contaminant fluoride volatility by increasing the contaminant oxidation state ................................................ 37 2.8.2 Improving the separation by selective reduction of ZrF4 to non-volatile species such as ZrF3 ............................................. 37 2.8.3 Improving the separation by gettering ........................................ 37 2.8.4 Decreasing HfF4 concentration by metal fluoride selective sorption .....................................................................................39 2.9 RAMAN SPECTROSCOPY ................................................................40 2.10 CONCLUSIONS ..................................................................................43 Chapter 3 .................................... Thermodynamic equilibrium calculations 3.1 INTRODUCTION .................................................................................44 3.2 EQUILIBRIUM COMPOSITION CALCULATIONS .............................. 45 vii 3.3 EQUILIBRIUM COMPOSITION FOR THE REACTION BETWEEN ZrO2 AND HF ....................................................................47 3.3.1 The Zr-O-H-F system ................................................................. 47 3.3.2 The Zr-O-H-F-N system .............................................................48 3.4 EQUILIBRIUM COMPOSITION FOR THE REACTION BETWEEN ZrO2 AND F2 .....................................................................49 3.4.1 The Zr-O-F system .....................................................................50 3.4.2 The Zr-O-F-N system .................................................................51 3.5 EQUILIBRIUM COMPOSITION FOR SEPARATION OF ZrF4 AND HfF4 ............................................................................................52 3.5.1 The Zr-Hf-F system ....................................................................52 3.5.2 The Zr-Hf-F-N system ................................................................ 55 3.5.3 The Zr-Hf-F-O system ................................................................ 58 3.5.4 The Zr-Hf-F-O system with excess ZrO2 .................................... 59 3.6 EQUILIBRIUM COMPOSITION FOR REDUCTION OF ZrF4 .............. 60 3.6.1 The Zr-F-C-H system ................................................................. 60 3.6.2 The Zr-Hf-H system ....................................................................62
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