Bioprocessing of refractory oxide ores by bioreduction: Extraction of silver, molybdenum, and copper

Although much research has been conducted in the area of biooxidation of sulfide ores, few investigations in the bioreduction of oxide ores have been published. We report high recoveries of silver, molybdenum, and copper from refractory oxide ores through bioreduction. Metal-reducing Bacilli solubilize manganese and iron. Crystals of pyrolusite, goethite, and hematite are bacterially dissolved and entrapped target metal ions are released. Zinc and copper are solubilized by metabolic end products such as organic acids. Bioreductive leaching of oxide ores was more successful than chemical leaching. A 0.1% cyanide bottle roll on an Arizona manganiferous silver ore resulted in 13.7% silver extraction. Bioreductive leaching extracted 86% of the silver into the growth medium. Also, 99.8% of the manganese, >2;99% of the copper, and 91% of the zinc were solubilized. A cyanide bottle roll of the bioleached solids extracted an additional 8.5% of the silver for a total recovery of 94.5%. A 5% sulfuric acid bottle roll of a Nevada iron oxide molybdenum ore recovered 1.8% of the moly. Bioreduction solubilized 93% of the molybdenum into the growth medium. A follow-up acid bottle roll recovered no additional moly. Bioreductive leaching can be used to recover target metals from refractory oxide ores without the use of cyanide or sulfuric acid.