Direct Phosphoric Acid Leaching of Bauxite Residue for Selective Scandium Extraction

Bauxite residue, the high alkaline and fine-grained byproduct of Bayer process for alumina's production, is a material rich in numerous minerals and elements of high value and techno-economical interest in high tech applications such as rare earths, including scandium. Lately, the European Committee has characterized scandium as a critical element because of its risk supply chain. Scandium's high concentration in Greek bauxite residue classifies the waste as a candidate for low cost and high availability of the element, additionally improving its environmental fingerprint. For scandium recovery, hydrometallurgical treatment with inorganic acids is the most common, effective and simple method. In this study, the efficiency of phosphoric acid is investigated for scandium recovery by a direct leaching of bauxite residue without pretreatment. Multi leaching variables, such as acid molarity, solid/liquid ratio, process temperature and leaching time, were examined and optimized individually as well as in a comparative way aiming to scandium selective extraction (mainly with regard to iron) and process viability. A Sc selective recovery of 40% was obtained for phosphoric acid molarity 5 M, solid/liquid ratio 10%, leaching time, 60 min under ambient conditions with low iron leachability, no gel formation and no energy consumption.