Extraction and recovery of valuable elements from Red Mud through biohydrometallurgy

Among all the materials, aluminum played a key role in the industrial development of the last two centuries, thanks to the breakthrough of the Bayer process. This consists in alumina extraction from bauxite by chemical dissolution. The main waste produced through the Bayer process is called Red Mud (RM). RM might be environmentally dangerous due to its high alkalinity (the pH in fact ranges between 10 and 12.5) and the presence of toxic elements. Nevertheless, the contemporary approach to waste streams could turn this aspect into an advantage. Indeed, RM can be considered today as a secondary resource, perfectly matching the concept of circular economy. Bioleaching is a biohydrometallurgical process based on the ability of microorganisms (i.e., bacteria and/or fungi) to leach metals via direct and indirect mechanisms, limiting the use of chemicals and requiring low energy supply. Among all the possible ways to extract metals from mineral/ solid waste, bioleaching is characterized by several advantages from an ecological and economical point of view. Metals can be bioleached under acidic conditions and later recovered, thus drastically reducing the RM pollution potential. The acidic environment can be produced by heterotrophic microorganisms when supplied with an organic substrate. The innovative approach followed during the experimentation was that of favouring the enrichment of the heterotrophic biomass living on RM without any external inoculum (bioaugmentation). Several temperatures and Solid-Liquid ratios on two RMs have been tested, showing variable results depending on target metals. The results of these first tests proved the possibility of applying a bioleaching process without any external inoculum addition: it was possible not only to neutralize the RM, but also to accomplish metals release that also has great potential of optimization. To enhance even more the bioleaching process an SBR was started. The enriched heterotrophic biomass so obtained showed a promising bioleaching potential. All the elements of interest were released, and the highest extraction efficiency was achieved for Al, Mn and Mg a few times higher than the results obtained in batch tests. To reduce the operational cost related to chemical reagents supply for the leaching medium composition, an organic waste (residues from beer production) was used to provide the carbonium content necessary to sustain the biological activity. Once leached, metals have to be recovered from the solution. Among different ways to recover metals from aqueous solution, adsorption is an efficient and economic treatment to eliminate and recover metals from residual water. It is called biosorbent any biological material that presents very good adsorption properties. To consider a material as biosorbent, the surface chemical property is the most important aspect to evaluate. This process was selected to recover Neodymium (Nd), one of the rare earth elements extracted from RM, from aqueous solution. Sugar beet pulp was demonstrated to perform well as biosorbent for Nd recovery.