The speciation and solvent extraction of zirconium and hafnium : a computational and experimental approach

Abstract

Zirconium and hafnium have important applications within the nuclear industry, where zirconium is used to clad the uranium fuel rod tubes that are used during nuclear reactions and hafnium is used in the control rods which moderate these reactions. Since zirconium and hafnium occur together in nature, these metals have to be separated prior to use in nuclear reactors. A promising technology that can be used to separate these metals is solvent extraction. However, when evaluating the literature on the solvent extraction of zirconium and hafnium, it becomes apparent that these studies often entailed choosing a set experimental parameters on a trail-and-error basis and optimising those parameters, without paying attention to understanding the mechanisms which underpins these solvent extraction reactions. One of the reasons why these extraction mechanism is not understood is the lack of data pertaining to the speciation of zirconium and hafnium in aqueous phases. It this project, the mechanisms that underpin the solvent extraction of ZrF4 and HfF4 with phosphorus based extractants were investigated. Molecular modelling was used to investigate the aqueous speciation of ZrF4 and HfF4, and a combined molecular modelling and experimental approach was used to investigate the bonding and reactivity of the reactions between ZrF4 and HfF4 complexes and phosphorus based extractants. Concerning the modelling of the speciation, it was predicted that for pH ranges below 0, which is of interest in solvent extraction, the aqueous speciations of ZrF4 and HfF4 are dominated by Zr(H2O)2F4 and Hf(H2O)2F4. For pH ranges above 0, these complexes are hydrolysed to yield [Zr(H2O)F4(OH)]- and [Hf(H2O)F4(OH)]-. As the pH continues to increase, complexes which are further hydrolysed start to appear. Furthermore, it was predicted that the 𝐹− ligands do not dissociate. Concerning the investigations of the bonding and reactivity between the aqueous ZrF4 and HfF4 complexes and phosphorus based extractants, it was proposed that the extraction mechanisms involved the formation of hydrogen bonds between the extractants, which had to be protonated, and the aqueous zirconium and hafnium complexes. Furthermore, it was proposed that for these extractants to bind preferentially to either zirconium or hafnium, and therefore extract one of these metals selectively, the metals have to be coordinated to an acid anion, in such a way that the extractants can form hydrogen bonds to this acid anion. It was observed that when HClO4 was present in the aqueous phase, hafnium was extracted selectively; while aqueous phases for which HNO3 was present resulted in the selective extraction of zirconium or hafnium, depending on how many oxygen atoms were present on the extractants. Aqueous phases for which HCl was present did not result in selective extraction at all. Furthermore, phosphorus acid based extractants resulted in greater overall extraction compared to phosphorus oxide extractants. Overall, the greatest selectivity (30 %) that was observed involve the selective extraction of hafnium from an 8.0 M HClO4 medium.