Acid and ferric sulfate bioleaching of uranium ores: A review

This review examines the acid and ferric sulfate bioleaching of uranium from low grade ores. The review traces back the progression of the technology from the time the role of microorganisms was recognized in the 1950’s and 1960’s. Some past and present uranium mining operations with active or potential microbial contribution are summarized. Experimental techniques and laboratory bioleaching experiments are described. Choice microorganisms have been iron- and sulfur-oxidizing acidophiles, comprising bacteria and archaea with mesophilic and thermophilic temperature ranges. Uranium is bioleached from ores in acidic ferric sulfate lixiviant. Ferric iron oxidizes tetravalent uranium to the hexavalent form and is thereby reduced to ferrous iron in this redox reaction. Microorganisms in the bioleaching process oxidize ferrous iron to the ferric form and thus regenerate ferric sulfate. Iron oxidation requires oxygen as the electron acceptor in the leach solution. Acidity ensures that ferric iron is soluble in the lixiviant and protons increase the solubilization of the oxidized, hexavalent uranium. Ancillary sulfide minerals such as pyrite enhance the bioleaching because their oxidation releases ferrous iron and reduced sulfur compounds for biological ferric iron and sulfuric acid generation. The main mining engineering approaches used for uranium leaching are heap, dump, stope, in situ, and in-place leaching. The efficiency of uranium bioleaching is affected by a number of mineralogical, physicochemical, microbial and process factors. Bioinformatics and synthetic biology are progressing the research on bioleaching microorganisms but these developments have not been materialized in the industrial practice of uranium mining. New applications of uranium bioleaching may focus increasingly on deposits where other products such as rare earth elements or base metals can be recovered in addition to uranium.