Uranium extraction from a pure natural brannerite mineral by acidic ferric sulphate leaching

• Purest form of natural brannerite (35.8% U, 20.1% Ti) studied to date in acid leaching.
• Almost complete U extraction achieved under practical conditions.
• Effects of temperature, Fe3+ and H2SO4 concentrations, ORP and particle size measured.
• U extraction impeded by precipitation of TiO2 diffusion layer on surface of brannerite grains.

Brannerite is a refractory uranium mineral from which it is very difficult to liberate the uranium. Hence in commercial mineral processing operations, brannerite often reports to the residue. This paper will show that for a pure form of natural brannerite nearly complete extraction of uranium (∼99%) is achievable under practical conditions. The efficient extraction of uranium from ores containing brannerite requires a detailed understanding of the fundamental mechanisms governing the rate and extent of dissolution. These mechanisms are often complicated by the presence of gangue minerals which consume reagents and impact on the solution chemistry. In this study, the acidic ferric sulphate leaching of an exceptionally pure, natural brannerite mineral (35.8% U, 20.1% Ti) was investigated under atmospheric conditions. Hence the variation in mineral composition was not present as a complicating factor and the results were able to identify some of the inhibiting mechanisms, and also the preferred conditions for the leaching of brannerite in an acidic ferric sulphate system. The effects of temperature (40–80 °C), ferric ion concentration (0–100 g/L), H2SO4 concentration (10–200 g/L), redox potential (424–752 mV vs. Ag/AgCl), and particle size on uranium and titanium extractions were studied for leach times up to 48 h. Under relatively mild conditions (40 °C, 24 h leach time, 40 g/L H2SO4), the extent of uranium extraction was 94.4%. The extractions improved with the use of a higher temperature, a finer particle size, and a longer leach time. The presence of ferric iron was essential for enhanced dissolution rates, but had only a minor effect on the final uranium extractions, particularly at 60 °C and 80 °C. All of the leach residues studied had some crystalline anatase (TiO2) and lead sulphate (anglesite) present. A strong correlation was found between the concentrations of unleached uranium and the amount of titanium precipitated in the residues, which could be explained by the observation of a Ti-enriched diffusion layer on the surface of the dissolving grains of brannerite, which hindered the extraction process. These findings further the current understanding of the extraction process and lead a step closer to elucidation of the mechanism of the extraction process.