Reductive bioleaching of limonitic laterites: Fe(III) bio-reduction kinetics in anaerobic batch and continuous stirred tank reactors

In the framework of the H2020 project CROCODILE, the recovery of Co from oxidized ores by reductive bioleaching has been studied. The objective was to reduce Fe(III) into Fe(II) to enhance the dissolution of Co from New-Caledonian limonitic laterites, mainly composed of goethite and Mn oxides. This study focused on the Fe(III) bioreduction which is the rate-limiting mechanism of this process. In a first step, biomass growth was sustained by aerobic bio-oxidation of elemental sulfur. In a second step, the biomass anaerobically reduced Fe(III) into Fe(II). This study aimed at assessing the Fe(III) bioreduction rate at 35 °C with a microbial consortium composed predominantly with Acidithiobacillus (At.) and Sulfobacillus (Sb.) species. It evaluated the influence of the biomass concentration on the Fe(III) bioreduction rate and yield, both in batch and continuous mode. It also aimed at evaluating the bioreduction rate at 46 °C using a slightly different microbial consortium. Finally, it determined the influence of the presence of the limonitic laterites on Fe(III) bioreduction rate. At 35 °C, a mean Fe(III) bioreduction rate of 1.7 mg.L-1.h-1 was measured, i.e. 13 times faster than the abiotic control (0.13 mg.L-1.h-1). An increase in biomass concentrations in liquid phase from 4.108 cell.mL-1 to 3.109 cell.mL-1 resulted in an increase of the mean Fe(III) bioreduction rate from 1.7 to 10 mg.L-1.h-1. Running the STR in continuous mode enabled further optimization of the operating conditions, to reach 20 mg.L-1.h-1. In batch mode, 46 °C bioreduction rate reached 8.4 mg.L-1.h-1 in the presence of ferric iron salt, but the addition of limonitic laterites revealed a complete inhibition of the microbial activity.