The effect of molar ratio and flow velocity on the surface area required for pertraction of a Zr/Hf mixture

Abstract

Zirconium (Zr) and hafnium (Hf) need to be separated before being used in nuclear applications. Equilibria data have shown that Hf can be selectively extracted over Zr from a sulphate medium using organophosphorus extractants. This study investigated a modification of Zr and Hf solvent extraction whereby a hollow-fibre membrane contactor was used to separate the aqueous and organic phases, termed pertraction (PX). The aim of the study was to investigate the effects of the molar D2EHPA:Hf ratio and the volumetric flow rate on the estimated surface area required for an industrial PX extraction unit. Equilibrium experiments showed that the maximum separation factor (8.8) was found at D2EHPA:Hf ratio of 350:1. Mass transfer coefficients and kinetic selectivities of hafnium and zirconium were determined using a hollow fibre PX set-up. Variables investigated were: i) D2EHPA concentration, and ii) volumetric flow rate at a constant organic to aqueous ratio. The mass transfer coefficients of hafnium and zirconium and the separation ratio increased at higher flow rates. The best experimental conditions estimate that an industrial extraction unit capable of producing 500 ton per annum nuclear grade zirconium would require approximately 3 200 m2 membrane surface area, at a volumetric flow rate of 1.7 m3.h-1 when operated under similar conditions to those in this study.