The present study describes anoxic Mn bioleaching of low grade ferromanganese ores using a dissimilatory Mn reducing consortia (MRC). Effect of external addition of electron shuttles such as humic acid and anthraquinone sulphonic acid was studied under same conditions. The MRC were found to be capable of utilizing MnO2 as electron acceptor as evidenced by Mn dissolution in media. In the presence of electron shuttles, Mn extraction was found to be enhanced by 2 to 4 times compared to the case without. A maximum of 58 (+/- 3%) Mn was found to be leached from an amorphous ore while 38 (+/- 2%) Mn was leached from a crystalline ore, in 50 days in presence of 100 mu M electron shuttle at 20 g/L S:L ratio. A significant finding was consistently low leaching of Fe under all conditions studied (< 50 mg/L or < 5%) and in-situ oxidation of Fe(II) by MnO2 to form insoluble ferric compounds as observed in residues. The presence of electron shuttle enhances reductive dissolution by MRC which favors dissolution of Mn more than Fe under anoxic conditions. X-ray fluorescence spectroscopy and X-ray diffraction peaks of residues confirm that MnO2 is preferentially dissolved than Fe2O3. These findings indicate that anoxic bioleaching can help to selectively recover Mn from low grade ferromanganese ores.
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