Silica gel free region and rare earth metal extraction correlations in reprocessing bauxite residue

The solid waste, bauxite residue, is the main sustainable development challenge for the alumina industry as large volumes of the material must be maintained indefinitely in long-term storage facilities unless complete site remediation can be achieved. Efforts in bauxite residue reutilization have recently shifted from its use as an additive in cement and ceramics manufacturing to development of processes for the extraction and production of metals and metal compound products. A hydrometallurgical approach is considered here and factors such as reaction temperature, time and acid concentration were examined to systematically characterize the leaching behavior of critical rare earth elements and how they correlate among each other and with major elements in the bauxite residue. The study showed that, within the reaction time, low temperature and low acid concentrations primarily caused release of Al, Si and a minority of La, while a majority of Fe coupled with Ce, Sc or V dissolved when higher temperatures and acid concentrations were applied. The statistical significance of the correlations was confirmed by analysis of correlation coefficients among elemental concentrations in leachates using machine learning mathematical models. Management of silica gel formation is a challenge in the hydrometallurgical treatment of bauxite residue, therefore, the silica gel free formation region was identified. Based on the stability field of silica gel formation, a stepwise acid leaching process for bauxite residue is proposed to facilitate downstream processing of the critical metals.