Abstract
The acidic ferric sulphate system is the industrial standard for the acid leaching of uranium ores. Alternatives such as ferric chloride/hydrochloric acid have been investigated, but never used industrially. Selected ferric sulphate leaching experiments from previous work were repeated in acidic ferric chloride media to evaluate the effectiveness of this alternative system. Leaching experiments were run for 5 h and the effects of acid concentrations between 0.25 and 2.00 mol/L and temperatures between 25 and 96 °C were studied. A single experiment for each system was repeated with the addition of 10 g/L fluorapatite, known to inhibit uranium leaching in ferric sulphate media.
Ferric chloride media was less effective than ferric sulphate media for the extraction of uranium from brannerite at same temperature and acidity, except at high (2.00 mol/L) acid concentrations. Acidity had a significantly greater effect on the rate of dissolution in the chloride media. The rate of the dissolution reaction was first order with respect to acid concentration in the chloride system, compared to an order of approximately 0.5 with respect to acid concentration in the ferric sulphate system. This suggests that dissolution in chloride media takes place via a different mechanism in which the acid has a greater role in the leaching reaction.
Comparing the results from the chloride and ferric sulphate leaching suggests that the formation of stable uranyl complexes is a key component in the leaching process, as uranyl sulphate complexes are stronger than uranyl chloride complexes. Interestingly, the alternative system was less susceptible to interferences from phosphates, and therefore may be a viable alternative when dealing with high-phosphate refractory uranium ores.
