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. A model was developed to predict the respective sublimation rates of the two tetrafluorides and to use these rates to determine the separation within the sublimer as well as the partial pressures of the two tetrafluorides in the gas phase. The partial pressures were then used to further model the desublimation of the tetrafluorides on a water-cooled annular surface. The model sufficiently predicted the sublimation rates at lower temperatures, but over-predict at higher temperatures.
A major problem of the experiments was that the sublimation rate formed a plateau before complete sublimation had been achieved. This can be the formation of a crust-like surface or sintered cake which prevented further sublimation from occurring, which may be due to the presence of impurities originating from the zircon. The sublimation temperature remained the same, but sintering of the cake negatively influenced the sublimation kinetics.
The conclusion is that sublimation in an inert atmosphere may be feasible depending on the extent of separation achieved along the length of the desublimer as well as within the sublimer residue. A lower sublimation temperature resulted in better separation but with lower sublimation rates. Vacuum sublimation, however, results in higher fluxes and requires lower temperatures. It is therefore recommended that this work be repeated under vacuum conditions to determine the extent of separation that can be achieved along the length of the desublimer. To stress the importance of the recommendation, it must be noted that sublimation provides dry methods with no liquid waste. It may require fewer steps, which is less labour intensive and requires lower capital cost than other commercial technologies. The major drawback is the high energy requirements to obtain the high separation temperatures, which results in costly construction materials.
Dissertation (MEng)--University of Pretoria, 2018.