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This research focuses on the development of a biological treatment process to enhance silver recovery from a low grade, silver-bearing manganese ore. These manganese oxide ores have been classified as refractory and thereby characterized noncommercial for the recovery and production of silver by conventional methods. A species of Bacillus polymyxa, which was isolated from the Crystal Hill Mine in southern Colorado, facilitated the reductive dissolution of manganese dioxide. Insoluble Mn(IV) was reduced to soluble Mn(II), and silver ions contained within the manganese oxide lattice were released and/or exposed, making them amenable to cyanide extraction. A direct relationship was observed between the biological transformation of Mn(IV) to a lower valence and increased silver recovery following cyanidation. Batch studies, with ore passing minus 10 mesh (0.14 inch diameter), had silver recoveries that were twice the amount extracted by chemical treatment alone over a 120-hour period. However, rates of reductive dissolution diminished significantly as ore particle size increased and specific surface area decreased. |
