Thermodynamic Properties of the Chloride and Fluoride Salts of Zirconium and Hafnium Dissolved in Molten Salt Solvents
Total Page：16
File Type：pdf, Size：1020Kb
AN ABSTRACT OF THE THESIS OF JOSEPH ALLEN MEGY for the degree DOCTOR OF PHILOSOPHY (Name) (Degree) In CHEMISTRY presented on 2.1 sf IS- (Major Department) (Date) Title: THERMODYNAMIC PROPERTIES OF THE CHLORIDE ANT) FLUORIDE SALTS OF ZIRCONIUM AND HAFNIUM DISSOLVED IN MOLTEN SALT SOLVENTS Abstract approved:_Redacted for privacy Dr .I Stephen J. Hawkes Zirconium and hafnium, which invariable exist together in nature, are separated commercially by an aqueous-organic, liquid- liquid countercurrent extraction i.n order to provide pure zirconium for use in nuclear reactors.The anhydrous chemistry of zirconium and hafnium was studied to provide background for the development of an anhydrous process for the separation of these very similar elements. Accurate thermodynamic tables for hafnium IV fluorides, chlorides, and oxides, were developed as existing tables had been partially derived from estimates of thermodynamic properties, which recent experimental work showed in some cases to be quite inaccurate. The existing electrochemical,vapor pressure, and calorimetric studies of zirconium and hafnium chlorides and fluorides complexed with alkali chloride and(or) fluoride salts have been reviewed and analyzed for consistency. An experimental study of the oxidation-reduction equilibrium of zirconium and hafnium between a molten salt phase and molten zinc phase via the Zr(IV) + Hfmolten Zrmolten molten salt zinc= Hf(IV)moltensalt+ zinc reaction was made. The reaction went strongly to the right with an equilibrium constant of about 200, thus showing promise as an anhydrous process for separating zirconium and hafnium. The rate of approach to chemical equilibrium was studied ina baffled stirred batch reactor.The equilibration rate was found to be mass transfer controlled.The mass transfer rate could be calcu- lated approximately from the physical properties of the molten metal and salt phases, using the Lewis correlation, which was developed empirically from data taken from low temperature aqueous-organic systems. Thermodynamic Properties of the Chloride and Fluoride Salts of Zirconium and Hafnium Dissolved in Molten Salt Solvents by Joseph Allen Megy A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy June 1975 APPROVED: Redacted for privacy Associate Professor of Chemistry in charge of major Redacted for privacy Chairman of Department of Chemistry Redacted for privacy Dean of Grad ate School Date thesis is presented FA:1915 Typed by Clover. Redfern for Joseph Allen Megy TABLE OF CONTENTS Chapter Page I.INTRODUCTION 1 II.BACKGROUND 8 Basic Chemistry 8 Electrochemistry 10 Electrowinning Studies 11 Electrochemical Studies 18 Vapor Pressure Studies 25 Chloride s 25 Fluorides 30 Calorimetric Stud ies 31 Activity of Zirconium and Hafnium in Molten Zinc 32 III.CORRELATION OF THE LITERATURE DATA 35 Introduction 35 Vapor Pressure and Calorimetry Correlation 37 Electrochemical Data for Zr(IV) 39 Electrochemical Data for Zr(II) 44 Relative Stabilities of the Zirconium and Hafnium Chlorides in Molten Salts 47 Comparison of the Half-Cell Potentials for Zirconium and Hafnium Chlorides 50 Partitioning of Zirconium and Hafnium between a Molten Zinc Phase and a Molten NaCl-KC1 Salt Phase 50 Conclusions 55 IV. EXPERIMENTAL 59 Introduction 59 Experimental Design Considerations 61 Apparatus 62 Procedure 62 Containment 64 Charging and Dumping the Reactor 71 Sampling 73 Dry Box 79 Phase Diagrams 81 Mass Transfer Experiments 81 Introduction 81, Apparatus 85 Reagent Preparation 95 Experimental Procedure 95 Chapter Page Data 99 Discussion 99 Equilibrium Experiments 108 Apparatus 109 Reagent Preparation 113 Procedure 114 Chemical Analysis 115 Data 117 Discussion 118 V. CONCLUSIONS 127 BIBLIOGRAPHY 131 APPENDICES 144 Appendix I:Discussion of Calculations 144 Appendix II:The Thermodynamics of Hafnium Tetrachloride, Hafnium Tetra- fluoride, and Hafnium Oxide 148 LIST OF TABLES Table Page 1.Vapor pressure of zirconium and hafnium tetrahalides over their solid phases and complexes with alkali chlorides. 28 2.The free energy of mixing of gaseous zirconium tetra- chlorides in NaC1 and KCl solutions. 40 3.Free energy of formation of ZrC1 4(melt)at infinite dilution in NaC1 (melt)and KC1(melt) 41 4.Comparison of electrochemical data. 45 5.Disproportionation of zirconium chlorides in chloride melts at 1013°K. 49 6.Disproportionation of hafnium chlorides in chloride melts at 1013°K. 51 7.Relative concentration of zirconium species in salt melt. 51 8.Analysis of interface from all chloride reductive extraction experiment. 75 9.Salt preparation for fused salt mass transfer experiments. 96 10.Analysis of salt mix for mass transfer experiments. 97 11.Zinc impurities. 97 12.Analysis of zirconium and hafnium used inmass transfer experiments. 100 13.Salt and metal input to mass transfer experiments. 100 14.Data and derived information from non-dispersed mass transfer experiment. 101 15.Data and derived information from dispersed mass transfer experiment. 103 Table Page 16.Hafnium concentration vs. time in each phase. 105 17.Comparison of mass transfer values. 107 18.Physical properties of the salt and metal phases of the mass transfer experiments. 108 19.Analysis of secondary standards for neutron activation analysis. 118 20.Data and derived results of equilibrium experiment #1. 119 21.Data and derived results of equilibrium experiment #2. 120 22.Data and derived results of equilibrium experiment #3. 121 Appendix Tables I. Hf(s). 153 II. HfOz(s) 154 III. HfC1 4(s). 154 IV. HfC14(g) 155 V. HfF4(s). 155 VI. HfF 4(g). 156 VII.Hf(s) + O2(g) = Hf0 (s) 156 VIIIHf(s) + 2C12 = HfC1 4(s). 157 IX. Hf(s) + 2C1(g) = HfC1 2 4(g) 157 X. Hf(s) +2F2(g) =HfF4(s). 158 XI. Hf(s) + 2F2(g) = HfF4(g). 158 XII. MO (s)ZrC14 (g) = Zr0 (s) + HfC14(g). 159 Table Page XIII. HfC14(g) + Z F4(g) = I-IfF4(g) + ZrC14(g). 159 XIV. Free energy of reaction for the oxygen-chloride and chloride-fluoride metathesis reactions of zirconium and hafnium. 160 LIST OF FIGURES Figure Page 1.Activity coefficients of X2ZrC16 in XCl melts (X Li, Na, and K). 29 2.Comparison of values for free energy of formation of ZrC14 in molten alkali halides obtained byvapor pressure-calorimetric and electrochemical measure- ments. 42 3.Half cell potential for Zr(II) in molten alkali chloride melts. 46 4.E*cen values for zirconium and hafnium chlorides in NaCl-KC1 melts obtained by Sukakura. 52 5.Equilibrium coefficient for Hfmetal+ Zr(IV) melt Zrmetal+ HIV)meltV as calculated from the data of Ivanovskii. 56 6.Reactor for preliminary experiments. 63 7.Microphoto of metal phase. 78 8.Equilibrium points for a chloride salt phase. 83 9.Equilibrium points for a fluoride-chloride salt phase. 84 10.Furnace for reductive-extraction experiments. 87 11. Frame for supporting furnace. 88 12.Method of attaching studs. 89 13.Tungsten baffle. 90 14.Stainless steel heat shield. 91 15.Thermocouple well. 93 16.Tungsten stir bar. 94 17.Modified baffle assembly. 110 Figure Page 18.Agitator for equilibrium experiments. 111 19.Carbon receiver for equilibrium experiment. 112 20.Variation of equilibrium constant with temperature. 125 THERMODYNAMIC PROPERTIES OF THE CHLORIDE AND FLUORIDE SALTS OF ZIRCONIUM AND HAFNIUM DISSOLVED IN MOLTEN SALT SOLVENTS I. INTRODUCTION Approximately seven million pounds of zirconium metal were made in the United States in 1973 (4).The principal use for zirconium metal is as a structural component of nuclear reactors. Other applications include use as a corrosion resistant container material for the chemical processing industry, as pyrophoric material in ordnance and flashbulbs, and as a component in ferrous and non-ferrous alloys. Zirconium forms 0.028% of the lithosphere (1), and is the eighteenth most abundant element. The principal ore, zircon (ZrO2 SiO2), is especially resistant to weathering and is often found concentrated in beach sands (2).All known deposits of zircon contain from 1-5W% of (Hf0SiO2) (3).Hafnium is found immedi- 2 ately below zirconium in family IV B of the periodic table.Even though the atomic number of hafnium, element 72, is substantially greater than zirconium, element 40, the ionic radii is decreased by the "lanthanide contraction" to nearly that of zirconium. Thus, the chemical properties of zirconium and hafnium compounds are very similar. Early in the development of nuclear power reactors, zirconium metal was selected as the material of choice for construc- tion of reactor core components. This selection was based on its excellent corrosion resistance and low affinity for thermal neutrons. Unfortunately, hafnium has a high affinity for thermal neutrons and can not be tolerated in reactor cores.Therefore, the separation of hafnium from zirconium is required for material to be used in nuclear reactors. A crash development program was successfully carried out in the early fifties at the Bureau of Mines in Albany, Oregon, for pro- ducing high-purity, hafnium-free, zirconium metal. The process developed involved the following steps (5): 1) Zircon is combined with carbon at high temperatures in a carbon arc furnace, volatilizing SiO from the product dur- ing the process. The product of this step, zirconium oxycarbonitride, is reacted with chlorine at about 600° C to form impure zirconium tetrachloride. 2) The impure zirconium tetrachloride is dissolved in an aqueous solution containing hydrochloric acid and ammonium thiocyanate. The hafnium is removed from this solution into methyl isobutyl ketone by countercurrent liquid-liquid