Bioleaching of valuable elements from red mud using autochthonous biomass

Red mud (RM) is the main residue produced by the alkaline extraction of aluminum from bauxite, and it contains valuable metals (e.g., iron, aluminum, titanium, silicon). Aim of this research was to investigate the biologically induced leaching of valuable elements from RM using autochthonous biomass, in order to simultaneously reduce RM polluting potential and extract metals for their subsequent recovery. Such approach is challenging, since high alkalinity and pH, as well as the absence of sulphides, constrain the use of traditional bio-hydrometallurgical techniques. Bioleaching tests were performed at different temperatures (22°C, 28°C) and solid to liquid ratios (S/L: 2%, 5%), using a leaching medium containing glucose, yeast extract and ethanol. The best results were achieved at 28°C and S/L ratio of 5%: pH rapidly dropped from 9.7 to 6.25 and remained constant till the end of the test. Metal concentrations in the liquid phase were 2%-Fe, 8%-Al and 1%-Ti. As expected, iron was less available to leaching at such pH, as it was mostly present as hematite in RM. Abiotic contribution to metals leaching was negligible. Results are promising, and further investigations are needed to favor pH drop to lower values, with a consequent increase in metals bioleaching.