The Olympic Dam (OD) mine site in South Australia has the world’s largest deposit of recoverable uranium. Three main types of uranium minerals are found in the OD deposit- tetravalent and hexavalent uraninite [UO2, UO3]; coffinite [U(SiO4)1-x(OH)4x]; and brannerite [U(Ti, Fe)2O6]. Detailed knowledge of the factors that influence the dissolution of these minerals (i.e. the mechanisms of dissolution) are important in achieving optimum processing of the uranium bearing ore at OD. The mechanism of UO2 dissolution is of particular interest as this mineral usually comprises a significant portion of the uranium bearing minerals in the OD ore. Although there have been previous studies on the dissolution of UO2 there is conjecture regarding the dissolution mechanism under acidic conditions, particularly the oxidation step(s) which are important as the U4+ ion is insoluble.
In this study the influence of a number of parameters on the rate of UO2 dissolution were investigated to explore the dissolution mechanism. Pure synthetic crystalline UO2 was obtained by first heating uranyl acetate in air to produce pitchblende (U3O8), which was then reduced in [95%/5%] Ar/H2 at 1000 °C to form UO2. The parameters that were investigated were; initial [UO2], temperature, particle size, [H2SO4] and oxidant concentration ([Fe3+]). Of these parameters oxidant concentration was found to have the most significant effect on the rate and overall extent of UO2 dissolution. A proposed mechanism for UO2 was determined based on the experimental results obtained.