An investigation on the dissolution of synthetic and natural uraninite

Nuclear power has rapidly come to the forefront as a cheaper, more reliable fuel source to solve the world’s energy crisis. In order to meet future demands for nuclear fuel there has been increased interest in optimizing the extraction of uranium from its various minerals. One of the major uranium minerals used to produce nuclear fuel is tetravalent uraninite (UO2). The majority of processes used worldwide to extract uranium from uraninite, involve an oxidation/acid dissolution step (sulphuric acid is predominantly used). The rates of the aforementioned reactions involved in the dissolution of UO2 can be influenced by a number of factors. These include standard parameters (such as temperature and Fe concentration), the presence of foreign ions in the leach slurry (due to the dissolution of the gangue minerals), location of the UO2 within the ore (liberation) and the dopants associated with UO2. In this thesis, the chemistry of uraninite dissolution was investigated in detail, with a major focus being on dissolution of this mineral under similar conditions to those used in the uranium minerals processing industry. These investigations involved studies on the dissolution of different forms of uraninite, which included synthetic uraninite, natural uraninite and Pb and Th doped synthetic uraninites. The investigation on the effect of standard parameters (temperature, acid, oxidation-reduction potential, total iron etc.) on the dissolution of synthetic UO2 determined that the dissolution of UO2 was found to most closely follow 1st order kinetics with an approximately linear dependence of the UO2 dissolution rate on [Fe]TOT for each of the system ORPs tested with the rate orders indicating a step change between an ORP ≥420 and ≤460 mV. The studies on the effects of foreign ions found that F- had a detrimental effect on UO2 dissolution at low–medium concentration (0-0.3 g/L) and a positive effect on UO2 dissolution when present in medium–high concentrations (0.5-1 g/L). With the oxo-anions, PO43- was found to have a negative effect when present at concentrations >5.26×10-2 M and the SO42- had a negative effect when present at medium – high concentrations. The investigation on the influence of cationic substitution on the structure and composition was then conducted. The decrease in dissolution compared to pure UO2 (85%) observed with the doped UO2’s, was postulated to be caused by a combination of factors including; (a) Pb/Th reprecipating onto the surface of the UO2 as PbSO4/ThSO4; (b) formation of several new Pb/Th-uranyl secondary minerals; and or (c) the consumption of leachant caused by complexation with Pb/Th to form the sulphate. The concluding study focused on the effect of standard parameters on natural uraninite, and the changes in dissolution rates between the natural ores and the synthetic uraninite was attributed to the effect caused by lead impurities within the natural uraninites being preferentially leached and then re-precipiatated as a sulphate.