No abstract

The uranium titanate mineral brannerite, UTi2O6 is the most common of the uranium minerals which is considered refractory. Ore containing brannerite mineralisation has been mined and processed in several locations around the world. Under typical uranium ore processing conditions, brannerite is often lost to tailings. In order to design an effective process for the leaching of high-brannerite uranium ores, it is first necessary to understand the mechanism of the chemical processes through which brannerite dissolves in the absence of interferences from the host rock. In the present study, a specimen of brannerite obtained as a single crystal was leached in sulphuric acid (10-200 g/L) and ferric sulphate (2.8 g/L Fe 3 + ) solution at 25-96 °C for 5 h. The rate of titanium dissolution was monitored along with uranium. Comparisons between the rates at which these two elements dissolved and the morphological changes that were observed to take place during the dissolution process indicated two different sets of leaching reaction mechanisms. At low temperatures, uranium dissolved at a much higher rate than titanium initially, leaving titanium rich areas on the brannerite particles similar to observations reported in earlier investigations which suggest incongruent dissolution. The calculated activation energies for uranium and titanium dissolution were 36 and 48 kJ/mol respectively. At higher temperatures, uranium and titanium dissolved at similar rates in constant proportions suggesting congruent dissolution. The calculated activation energy for this reaction was 23 kJ/mol. The transition between incongruent and congruent dissolution took place at lower temperatures when the acid concentration was higher. Titanium appeared to undergo hydrolysis after dissolution, forming anatase. This side reaction was most favourable at lower acid concentrations and high temperatures.

Refractory uranium ores containing uranium as multiple oxides typically require leaching at elevated temperatures (>75 °C) and acid concentrations (>50 g/L H 2 SO 4) in order to effectively extract the uranium. Some uranium ores contain large amounts of acid-soluble gangue, such as carbonates or soluble silicates and when the carbonate content exceeds approximately 8%, the acid consumption by gangue may render acid leaching uneconomical. In these situations, leaching in alkalinecarbonate is a potential option. The advantages of this approach include improved selectivity for uranium over other elements, reduced gangue dissolution and the ability to recycle the lixiviant. One disadvantage is slower rates of leaching and hence the need for finer grinding. However, when a uranium ore contains significant amounts of acid-soluble gangue with refractory uranium mineralisation (a double refractory ore), the process options are more limited. Refractory uranium minerals, such as brannerite, are often reported as slow to dissolve in alkaline carbonate media. If a double refractory uranium ore is to be processed effectively, it is necessary to understand the behaviour of minerals like brannerite in the alkaline leaching system. In the present study, a sample of brannerite was leached in a sodium carbonate solution (1 mol/L total carbonate) for 24 hours at temperatures from 50 to 90 °C. The effect of potassium ferricyanide (25 mmol/L) added as an oxidant was also examined. All residues were characterised by XRD and SEM-EDX techniques, and both uranium and titanium dissolution rates were monitored. Uranium extraction reached 83% after 24 hours of leaching. The leaching rate showed a high dependence on temperature with an activation energy of 45 kJ/mol. The residue was pitted, similar to what has been observed previously after leaching in acidic media. At 80 °C and 90 °C, the titanium re-precipitated within the pits, potentially slowing the dissolution process.

No abstract

No abstract