The abundance of limonitic laterite ores in tropical and sub-tropical areas represents a large, and mostly unexploited, cobalt resource. Bioprocessing oxidised ores, and also waste materials such as tailings and processing
residues, using acidophilic microorganisms to catalyse the reductive dissolution of iron and manganese minerals, is an environmentally benign alternative approach of extracting valuable base metals associated with these deposits. This work describes results from laboratory-scale experiments in which five cobalt-bearing materials, three primary limonitic laterite ores and two processing residues (filter dust and slag), all sourced from mines and a processing plant in Greece, were bioleached under reducing conditions by a consortium of acidophilic bacteria (using elemental sulfur as electron donor) in stirred tank bioreactors at pH 1.5 and 35 °C. Whilst the target metal, cobalt, was successfully bioleached from all five materials (40–50% within 30 days) the extraction
of some other metals was more variable (e.g. between 2 and 48% of iron). Concentrations of soluble cobalt were highly correlated, in most cases, with those of manganese, in agreement with the finding that cobalt was primarily
deported in manganese (IV) minerals. Acid consumption also differed greatly between mineral samples, ranging between 3 and 67 moles H2SO4 g−1 cobalt extracted. Comprehensive mineralogical analysis of the three limonitic samples before and after bioprocessing revealed significant variations between the ores, and demonstrated that elemental and mineralogical variabilities can greatly impact their amenability for reductive bioleaching.