The extractive metallurgy of brannerite: Leaching kinetics, reaction mechanisms and mineralogical transformations

Brannerite, ideally UTi2O6 is a refractory uranium mineral found in many uranium and rare earth element ore deposits around the world, including many in Australia. As brannerite is refractory, ores containing brannerite require more intense leaching conditions than typical uranium ores. Brannerite is the most common refractory uranium mineral, and the most important uranium ore mineral after uraninite (UO2) and coffinite (U(SiO4)1-x(OH)4x). Several high-brannerite uranium deposits in Australia remain un-developed, despite being discovered as early as the 1950s. The aim of this study was to understand the leaching chemistry of brannerite in a variety of systems, starting with the conventional acidic ferric sulphate system and alternatives including the ferric chloride-hydrochloric acid system and the alkaline-carbonate system. The principal assumption is that an improved understanding of the leaching chemistry of brannerite will lead to more effective extraction processes, improving the extractions at existing mines, and enabling the development of new ones. Brannerite was found to undergo congruent dissolution in acid, contrary to the often-reported mechanism in which a titanium oxide coating forms on the surface. Phosphate released by gangue minerals such as apatite can cause the formation of this layer however. When leaching with acidic ferric solutions, sulphate media is superior to chloride media. Alkaline carbonate leaching was also found to be effective for brannerite leaching, albeit much slower than acid leaching. These same alkaline leaching conditions were applied to a sample of refractory uranium ore from Queensland high in acid soluble gangue and shown to be effective. These findings are discussed in detail below. A sample of brannerite from the Dieresis deposit in the Sierra Albarrana region of Spain was characterised in detail by XRD and SEM-EDX methods. The brannerite was found to be altered and metamict (rendered amorphous by self-irradiation), as is typical for brannerite. Many brannerite particles contained linear zones of titanium oxide surrounded by silicon enriched and uranium depleted brannerite, consistent with descriptions of naturally altered brannerite. These altered zones were more susceptible to leaching, regardless of the leaching conditions. All leached residues were analysed by the same methods to understand the changes taking place in the solid phase during leaching. This suggests that the extent of natural alteration influences the leachability of a particular brannerite. The leaching of brannerite was studied in acidic ferric sulphate media (0.05 mol/L or 2.8 g/L Fe3+) over a range of temperatures (25-96°C) and acid concentrations (10-200 g/L H2SO4) for five hours. Leached brannerite was pitted and corroded. The rate of leaching was strongly dependent on temperature and weakly dependent on acid concentration. At lower temperatures, brannerite dissolved incongruently in the early stages of leaching. At higher temperatures brannerite dissolved congruently for the entirety of the leaching experiment. The transition between these two mechanisms happened at lower temperatures when the acid concentration was higher. In the incongruent dissolution reaction, the activation energies for uranium and titanium release were 36 and 48 kJ/mol respectively. In the congruent dissolution process, the activation energy was 23 kJ/mol for both uranium and titanium dissolution. At high temperatures (>75°C) and low acid concentrations (<25 g/L H2SO4), the concentration of titanium dropped after the first hour of leaching and some secondary anatase (TiO2) formed. This anatase was distinct from the anatase in the original material in that it contained iron and did not contain uranium, confirming that it formed during leaching. Ferric chloride and cupric sulphate lixiviants were studied over a similar range of temperatures and acid concentrations. As with the ferric sulphate leaching tests, the oxidising cation concentration was kept constant at 0.05 mol/L. The leaching behaviour of brannerite in cupric sulphate media was quite similar to what was observed in ferric sulphate media; the rate of leaching was slightly lower than what was observed in ferric sulphate media under comparable conditions. In chloride media, the rate of leaching was slow compared to sulphate media at the same temperature and acid concentration. This suggests that the formation of stable uranium complexes is an important part of the dissolution process. Uranyl sulphate complexes are much stronger than uranyl chloride complexes. Certain leaching experiments were repeated with the addition of minerals commonly associated with brannerite to gain a clearer understanding of the effects of deleterious gangue. These experiments were run at the extremes and middle of the range of temperatures and acid concentrations studied. Ilmenite accelerated the precipitation of anatase while fluorite significantly increased the rate of uranium and titanium dissolution. Fluorapatite greatly reduced the rate of brannerite dissolution. These results showed a previously unknown interaction between phosphorus and titanium. Phosphate helped to initiate the formation of a titanium oxide coating on the leached brannerite, inhibiting the leaching reaction. Higher concentrations of sulphuric acid reduced these negative effects. Interestingly, phosphate improved the rate of leaching in chloride media, suggesting that chloride leaching may be a viable option when processing high-phosphate refractory uranium ores. Alkaline leaching may be an effective alternative processing option. While it is often reported that brannerite and similar minerals will not readily dissolve in alkaline media, leaching experiments with sodium carbonate based lixiviants showed that alkaline leaching of brannerite is possible. Compared with acid leaching, it is slow however. Uranium extractions of 83% were achieved over 24 hours of leaching at 90°C in sodium carbonate media. These leaching experiments were repeated with a high-carbonate refractory uranium ore from Queensland and resulted in comparable extractions. Alkaline leaching is a viable alternative when dealing with high-acid consuming ores that contain brannerite. This study has shed more light on the reaction mechanisms involved in brannerite leaching in typical industrial leaching systems, resulting in a much clearer understanding of brannerite leaching chemistry, potentially enabling the extraction of uranium from overlooked ore deposits. Mineral texture and alteration were also found to influence brannerite leaching. The negative and positive effects of certain gangue minerals have been understood in greater detail, and ways of mitigating or utilising these effects have been devised. Finally, alkaline leaching has been tested and shown to be effective for the leaching of brannerite and refractory uranium ores. Further work is needed to establish the most effective range of conditions and reagent dosages for the leaching of refractory uranium ores and develop economically viable processes based on this new information