REEs and valuable metals recovery from the red muds of the Eurallumina site in Porto Vesme, Sardinia, Italy

Red Muds (RMs) or bauxite residues obtained by the Bayer process for alumina production are recognized to be a serious environmental hazard as well as a potential polymetallic source, as they are still rich in many valuable metals including rare earth elements (REEs), Sc, and Ga. To extract and recover the target elements from red mud the two essential steps are leaching/dissolution and recovery/adsorption from the leachate. The first leaching process evaluated is a sulfuric acid attack, while the recovery process is a solid-liquid adsorption onto sorbent substrates. The above approach was applied to the red muds from Porto Vesme disposal site, that previously underwent geochemical and mineralogical characterization (Mameli et al. 2025, Schingaro et al., 2025). The mineralogical composition includes hematite, goethite, ilmenite, gibbsite, boehmite, bayerite, anatase, rutile, quartz, sodalite, cancrinite, garnet, halite and a variable number of minor phases whereas geochemical analyses evidenced valuable critical transition metals and rare earth elements (REEs) associated to zircon (Hf), phosphate (Y) or ferrotitanate (LREEs). The leaching process was carried out through an acid attack with sulfuric acid, with the addition of hydrogen peroxide to suppress silica gel formation and allow the operation at a lower liquid to solid ratio (higher pulp concentration). The initial experiments show that the best operative conditions are high temperature and leaching agents concentration of 1M or 2M. No silica gel formation is observed at liquid to solid ratio of 10, while at a liquid to solid ratio of 5 silica gel forms after 2 hours of leaching. Recovery efficiencies vary for the different elements, from 70 to 90% (Sc, Ga) to 30 to 40% (La, V, Ce) to 15% (Y). The efficiency of recovery of the target elements improves when operating at a higher temperature, while the effect of the leaching agent concentration is only minor. Among the sorbent substrates initially tested, the best performances are obtained for materials derived from the alkaline activation of ladle steel slags. With these materials the adsorption efficiency remained higher than 60% (90% in the best case) for initial pulp concentrations up to 0.3 gCu/gsorbent.