Leaching experiments were carried out by varying acid concentrations, liquid-to-solid ratios, hydrogen peroxide addition, time and temperature.
The optimized conditions ([H₂SO₄] = 1.6 M, [H₂O2] = 1.6 M; solid/liquid 10%; T = 90 °C; t = 240 min) provided fast extraction and high recoveries of scandium (91%) and lanthanum (90%), with additional recoveries of gallium (61%), strontium (44%) and vanadium (42%).
Mineralogical analyses of the solid residues revealed the dissolution of aluminosilicates (e.g. sodalite, Na4(Si3Al3)O12Cl and cancrinite, Na8(AlSiO4)6(CO3)(H2O)2) and carbonates (e.g. calcite, CaCO3), partial depletion of hematite (Fe2O3) relative to the original bauxite residues and the precipitation of sulphate phases induced by sulfuric acid.
The results of the present study prove that wet leaching is a viable route for valorization of Portovesme red muds while contributing to mitigation of long-term storage impacts.