Uranium leaching from synthetic betafite: [(Ca,U)2(Ti,Nb,Ta)2O7

Abstract

The leaching of uranium from of a synthetic form of the naturally occurring pyrochlore group mineral betafite, was investigated in acid sulfate media. Uranium leaching curves obtained over a range of time, temperature, [H2SO4], [FeTOT] and redox potential conditions were similar with each having three discrete segments representing significantly different rates of uranium leaching. The first segment occurred in the initial min and involved extremely rapid leaching. This segment made up for the majority of the overall uranium leaching obtained over the test period (~2.10% U). The high rate of leaching in this segment was demonstrated to be attributed to liberation of surface oxidized uranium from the betafite structure. The second segment between 1 and 120min was characterised by slow uranium leach rates which could be partially influenced by altering the experimental conditions. Uranium leaching within this period was most likely due to oxidation of uranium by ferric iron which was progressively slowed by the competing diffusion reaction. The uranium leach rate in the third segment was negligible. This negligible leach rate was demonstrated to occur when approximately 2.10% U had leached from the sample and was shown to be due to passivation of the sample surface. Investigations into leaching betafite using various iron salts showed the uranium leaching more than doubling when iron fluoride was substituted for iron sulfate. The additional solubility was attributed to leaching of Nb, Ti, and Ta due to the in-situ formation of HF, where the HF either slowed the formation of a passivating layer or the HF was able to slowly dissolve the passivating layer.