Separation of hafnium from zirconium using membrane based solvent extraction

Abstract

The need for purified Zr and Hf has become imperative with the development of the nuclear industry. Solvent extraction (SX) processes, such as the TBP and MIBK processes have been successfully applied for the industrial separation of Zr and Hf, however, the crud and emulsion formation of SX processes have prompted efforts to find alternative separation methods. Membrane based solvent extraction (MBSX) is one such alternative, where a porous membrane between the aqueous and organic phases creates an immobilised liquid-liquid (L-L) interface, thus avoiding phase dispersion, while still allowing for mass transfer between the phases. In view of the limited data on this novel technology, the aim of this study was to develop an approach to predict the technical feasibility of an MBSX unit. For this purpose, the Hf selective separation from Hf/Zr solutions using a D2EHPA-H2SO4 system was selected. To attain this, the partition coefficients of Zr and Hf, as well as the effect of the fluid velocity on either side of the membrane was determined experimentally, followed by the development of a process model to technically evaluate an industrial-scale unit. Liquid-liquid equilibria (LLE) and MBSX experimental data were collected. The LLE data showed that a high separation factor was obtained for the D2EHPA-H2SO4 system, while also the corresponding partition coefficients of Zr and Hf were determined. A separation factor of 7.5 was achieved when contacting an aqueous phase of 16 g.L-1 Zr, 0.36 g.L-1 Hf, and 98 g.L-1 H2SO4 with an organic phase consisting of 230 g.L-1 D2EHPA, and 41 g.L-1 1-octanol in Shellsol 2325. The partition coefficients of Zr and Hf were 20.7 and 157 respectively. According to the MBSX experiments, the non-selective diffusion through the shell-side boundary layer dominated the mass transfer resulting in a separation ratio below 2.0 with overall mass transfer coefficients in the order of 9.4 x10-8 and 1.3 x10-7 m.s-1 for Zr and Hf respectively. The lumen did not offer significant resistance to mass transfer and was negligible for Re numbers higher than 0.61. A process model and design methodology were proposed, and in combination with Sherwood relations that were fitted to the experimental data, and industrial-scale design was proposed. These calculations confirmed that the shell-side dominated the overall rates of mass transfer, and through this, that the low separation ratio made the industrial-scale application of MBSX illadvised as the membrane surface area that was predicted to be in the order of a trillion m2, and the organic flow rates in the order of 100 m3.s-1. It was finally concluded that this technology can only be considered for further exploration if the shell-side mass transfer coefficient increased at least with a factor 1000 to 1 x10-4 m.s-1.