An investigation on the effects of several anions on the dissolution of synthetic uraninite (UO₂)

With the increase in the demand for energy, there is an increased interest in nuclear power sourced from a number of uranium minerals, primarily tetravalent UO₂. During the leaching of uranium from its ores, a number of gangue minerals are leached which lead to a number of different ions in the leach slurry. The current work undertaken focused on the effect of anions that typically leach from gangue minerals found in uranium bearing ore bodies on the dissolution of synthetic UO₂. The anions studied were F ^-, Cl^-, Br^-, SO₄^{2 -}, PO₄^{3 -} and NO₃^-. Cl^- and Br^- were found to have no significant influence on UO₂ dissolution over the concentration range studied (0-1.8 g/L Cl^-; 0-4 g/L Br^-), whilst F^- was found to have 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), when compared to the dissolution obtained in the presence of the other halides studied. Of the oxo-anions studied NO ₃^- had a positive effect on UO₂ dissolution over the entire concentration range studied and PO₄^{3 -} had a negative effect when present at medium-high concentrations. This was due to precipitation of FePO₄ at medium-high PO₄^{3 -} concentrations. SO₄^{2 -} had a negative effect when present at medium-high concentrations. This was most likely due to the formation of less effective Fe^III based complexes, where the Fe^III species present in solution play an important role in the UO₂ oxidation/dissolution mechanism. Results obtained on the influence of all anions investigated simultaneously are also presented and discussed.