Bioleaching of Scandium from Bauxite Residue using Fungus Aspergillus Niger

Bauxite residue (BR) is the main by-product of the alkaline production of alumina from bauxite containing significant amounts of valuable metals such as scandium that belongs to rare-earth elements (REEs), classified by the European Community as critical raw materials (CRMs). BR is considered a hazardous waste due to its huge volume and high alkalinity making its disposal a serious universal environmental problem. The recovery of scandium from Greek BR can be an excellent approach for waste management and resource efficiency of the waste using environmentally friendly biometallurgical methods. In this work, bioleaching of scandium from bauxite residue using the fungus Aspergillus niger was studied. Bioleaching experiments were performed using the Taguchi experimental design, in batch cultures with BR at various pulp densities (1, 5 and 10%, w/v), sucrose concentrations (40, 90 and 140 g/L) and fungus suspension of 2, 4, and 6% v/v under one-step bioleaching condition and subculturing. The highest Sc recovery equal to 46%, was achieved in 20 days at 1% pulp density. Biosorption phenomena were observed during the leaching process. Lactic, acetic, oxalic and citric were the main organic acids identified. Graphical Abstract