Indirect bioleaching of low-grade nickel limonite and saprolite ores using fungal metabolic organic acids generated by Aspergillus niger

The objective of the present study was to investigate the indirect bioleaching of low-grade nickel limonite and saprolite ores using fungal metabolic organic acids generated by Aspergillus niger. The indirect bioleaching experiments were carried out in a 2000 ml five-neck glass vessel containing 500 ml of the culture filtrate comprising fungal metabolic organic acids (which were obtained from organic acid biogeneration experiments after filtration) supplemented with limonite or saprolite ores. For the organic acid biogeneration experiments, the highest generation of organic acids by A. niger was achieved for cassava starch as carbon source in the presence of KH2PO4, (NH4)2SO4 and MgSO4 under aeration condition, which were subsequently employed for the indirect bioleaching experiments. The highest nickel leaching efficiencies from saprolite and limonite ores were 88.9% and 84.9%, respectively, which were obtained at leaching temperature of 95 °C, stirring speed of 400 rpm, ore particle size of -177 + 149 μm, and leaching time of 4 h (for saprolite) and 24 h (for limonite). Most importantly, the leaching using the biogenerated organic acids in this study is highly selective to magnesium (Mg) and is relatively selective to iron (Fe), thus supporting their potential industrial applications with respect to economic and environmental concerns.