Bioleaching of secondary and critical raw materials from Red Mud by a mixed culture in a semi-continuous reactor

Red Mud (RM) is a byproduct of the Bayer Process, used to extract alumina from bauxite. This residue contains valuable metals and rare earth elements (REEs), which can be bioleached under acidic conditions and later recovered, eventually reducing the RM pollution potential. The focus of this study was to extract metals through bioleaching using biostimulated biomass. A sequencing batch reactor (SBR) was operated at two different solid to liquid ratios (S/L, 2% and 1.5%) under aerobic conditions, using a feeding strategy based on pH evolution over time. The lowest pH obtained at 2% S/L was 3.8. Metals extraction efficiencies were 68 ± 18% for Mn, 58 ± 4.5% for Mg, 51 ± 2.9% for Al, 0.6 ± 0.07% for Ti and < 0.1% for Fe. At 1.5% S/L, a slightly lower pH was achieved (3.4), with metals extraction efficiencies similar to those obtained at 2% S/L, but more stable over time. At 1.5% S/L, REEs (i.e., Nd, Ce, Y, Sc and La) extraction efficiencies were also evaluated, and the highest value was obtained for Nd (around 65%). The results were promising and provided useful information for further enhancing the efficiency of metals extraction, in the perspective of process scale-up.