Sublimation/deSublimation separation of ZrF4 and HfF4

Abstract

This dissertation details research aimed at the separation of Zr and Hf in the tetrafluoride form. Separation of ZrF4 and HfF4 was achieved using sublimation followed by desublimation. The separation involves the sublimation of the tetrafluorides in an inert atmosphere under controlled parameters. The sublimed mass (700 C to 800 C) diffuses into nitrogen which then flows across a water-cooled desublimer (annulus) with the aim of desubliming the one metal fluoride in preference to the other. This implies that separation was achieved in both the sublimer and the desublimer, due to differences in both the sublimation and desublimation rates. The aim was for the sublimer residue to be Hf-rich and the desublimer content to be Zr-rich. The Zr/Hf content was determined by means of ICP-OES analysis. It must be noted that the work reported in this thesis is based on only a first sublimation step. The reason for this is that the equipment used was on laboratory scale and that the mass collected from the desublimer was not sufficient to allow a second or third step to be carried out. Little information was available in the literature on the sublimation separation of Zr and Hf, especially in the fluoride form, most of it being sublimation under vacuum conditions. On an industrial scale, only vacuum sublimation of ZrF4 has been introduced into the industry, and no information was found for sublimation of ZrF4 in an inert atmosphere on an industrial scale. There was also limited information on the sublimation rate of ZrF4 or HfF4 in an inert atmosphere. In the process described in this dissertation, optimal temperature selection is crucial since low temperatures result in a low sublimation rate, and high temperatures not only increase the level of impurities in the sublimed product, but also increase the cost of construction material and energy consumption. The aim was to determine the experimental conditions, i.e. sublimation time, temperature and position on the desublimer, which will provide optimal separation conditions. These conditions must, however, be compared with the operating cost, as this will be higher at a higher temperature and for longer sublimation runs.