Mineralogical and Chemical Aspects of Brannerite Leaching

摘要

Of the uranium minerals considered refractory, brannerite is the most common one. Brannerite is a multiple oxide of uranium and titanium, ideally UTi_2O_6. However, the chemical composition of brannerite varies between deposits, with the general formula of (U,Ca,Y,Ce,La)(Ti,Fe)_2O_6.Brannerite is found as one of the major uranium minerals in many uranium and rare-earth element deposits including uranium deposits near Mount Isa in Queensland and in the Curnamona geological province in South Australia. Brannerite-rich uranium ores have been mined and processed in the Elliot Lake area of Ontario, Canada. Invariably, aggressive leaching conditions (>50 g/L H_2SO_4, >70°C for up to 48 hours) were required to extract uranium from brannerite. Pressure leaching has also been investigated for the processing of South African ores (Giiligan and Nikoloski, 2015a).A detailed study has been undertaken, aiming to identify reaction mechanisms for the leaching of brannerite in aqueous solutions and define a more efficient method of processing. A specimen of brannerite from Cordoba, Spain was leached in several lixiviants over a range of temperatures (25-96°C) and acid concentrations (10-200 g/L H_2SO_4). The rates of uranium and titanium dissolution were monitored. The leached residues were characterised in detail by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD) techniques.The results of this study have revealed that the rate of brannerite dissolution depends on the temperature and solution composition. Two separate modes of dissolution were identified. In the first, uranium initially dissolves ahead of titanium. In the second, both metals dissolve at similar rates. At higher temperatures, both uranium and titanium dissolved faster. Increases in the acid concentration result in faster uranium and titanium dissolution with variations in acid concentration having a greater effect on titanium dissolution than uranium dissolution. Above 50 g/L H_2SO_4 the second mode of dissolution becomes dominant (Gilligan and Nikoloski, 2015b).SEM-EDX analyses showed that the mineral specimen consisted of a mixture of uranium-titanium oxide (brannerite) and uranium-enriched titanium oxide. SEM-EDX analyses of the leached residues showed that the brannerite phase was more susceptible to leaching than the uranium-enriched titanium oxide phase. Some corroded, uranium depleted areas were identified in residues in which uranium was known to be dissolving ahead of titanium (Gilligan, Nikoloski and Deditius, in review). When the leaching kinetic data showed uranium and titanium dissolving at similar rates, brannerite particles appeared pitted and corroded with no sign of any uranium depleted areas. After leaching at higher temperatures (>75°C) brannerite was not detected in the residue, with residues consisting of uranium-enriched titanium oxide and insoluble gangue silicates. A secondary anatase phase was identified in residues leached above 75°C and below 50 g/L H_2SO_4. Leaching kinetics data for these high temperature/low acid leaching experiments showed the titanium concentration in solution decreasing after the first hour.