Separation of Zr and Hf via fractional crystallization of K2Zr(Hf)F6 : a theoretical and experimental study

Abstract

Due to the low absorption cross section for thermal neutrons of zirconium (Zr) in contrast to hafnium (Hf), Zr-metal must essentially be Hf free (<100 ppm Hf) to be suitable for use in nuclear reactors. However, Zr and Hf always occur together in nature, and due to similar chemical and physical properties, their separation is complicated. Separation can be achieved by traditional liquid-liquid extraction or extractive distillation processes, using Zr(Hf)Cl4 as feedstock. However, the production of K2Zr(Hf)F6 via the plasma dissociation route, developed by Necsa, could facilitate the development of an alternative separation process. The fractional crystallization of K2Zr(Hf)F6 from solution was investigated as a possible alternative for the separation of Zr and Hf. Both molecular modelling and experimental techniques were used in this study in which molecular modelling was applied to determine whether any thermodynamic limitations exists in terms of solid solution formation of the pure salts that could hinder the separation efficiency of fractional crystallization. Using the calculated thermodynamic properties of mixing, the separation efficiency was theoretically evaluated using a thermodynamic model to calculate the relative distribution coefficients. Crystallization was subsequently studied experimentally to investigate the validity of the separation efficiency predicted by the results obtained from molecular modelling. In addition, the influence of solvent composition and other physical parameters on the separation efficiency was investigated. A more efficient separation was obtained experimentally than predicted by the theoretical calculations; wherein the use of KF and HF as additives was found to influence the separation efficiency. Despite the limited correlation between the molecular modelling and the experimental results, the relative ease of solid solution formation predicted by the modelling results, which is not surprising owing to the close similarities between Zr and Hf might well restrict the separation efficiency by fractional crystallization. The disagreement between the experimental and the modelling results were attributed to different factors that could not be accounted for in the theoretical calculations. It was for example shown that the change in the vibrational entropy, which was initially neglected in the theoretical calculations, has a significant influence.