A Multi-Company Sponsored Research Program to Study the Biologically Assisted Ferric Iron Leaching of Uranium Minerals

Presently, chemical leaching of uranium, particularly from an acidic gangue, involves acidic ferric iron solutions. The 6-valent uranium is soluble at the acidic pH values involved, whereas solubilization of 4-valent uranium involved oxidation to the 6-valent species with ferric iron as the primary oxidant. Under most commercial vat leaching conditions, the pH is modestly below the level at which microbiological leaching can occur. In heap or “in situ” leaching, microbiological leaching is responsible for a substantial proportion (in mine drainage situations for all) of the uranium solubilized. The bacterium, Thiobacillus ferrooxidans, oxidizes the sulfides of most metals to the corresponding sulfates, thus generating acidic solutions. Where iron sulfides are involved the iron moiety is solubilized and concomitantly oxidized to the ferric form. The acidic ferric solutions, thus generated, solubilize both the 4- and 6-valent forms of uranium. The pH values over which this microbiological action can normally proceed are in the approximate range of 1.8 to 2.6; the upper limit being the less absolute. As the 4-valent uranium is oxidized to the 6-valent form, the iron which is reduced to the ferrous form is quickly reoxidized to the ferric state by Thiobacillus ferrooxidans, thus enabling leaching to continue. Microbiological oxidation of ferrous to ferric iron is in no way dependent on the original source of the ferrous iron. Thus microbiological oxidation can interact with chemical leaching processes, where pH and other conditions permit.