Extraction of Al and rare earths (Ce, Gd, Sc, Y) from red mud by aerobic and anaerobic bi-stage bioleaching

Red mud (RM) is a typical mass quantity of solid waste generated from industrial alumina production and usually consists of valuable metals such as Al and rare earths (REs). This work mainly investigated the extraction of Al and REs from RM via aerobic and anaerobic bi-stage bioleaching by Acidianus manzaensis with the addition of pyrite, by monitoring the morphology, phase and speciation transformations of Al, Si, S and Fe using primarily SEM-EDS, XRD, XPS, and XANES. The results showed that during the aerobic bioleaching stage with increase in mass ratios of pyrite/RM, the extraction rates of Al and REs increased; the highest rates for Al (85.1%), Ce (82.4%), Gd (86.8%), Y (85.3%) and Sc (78.6%) were achieved at pyrite/RM mass ratio 2:1 after 22 d of bioleaching. A slight decrease then occurred because that the complexes formed from jarosites, hematite, diaspore and SiO2-gel can adsorb small amounts of the dissolved elements. Under anaerobic conditions, the additive S0 gradually oxidized to sulfate, and the jarosites gradually dissolved into Fe2+ and disappeared at day 10 with maximal bioleaching rate for Al, Ce, Gd, Y and Sc as 52.5%, 86.3%, 93.7%, 90.2% and 74.9%, indicating that the residues from the first stage were dissolved with the dissimilatory reduction of jarosites by A. manzaensis using S0 as an electron donor. These results suggested a new method for the efficient recovery of valuable metals from RM.