High purity scandium extraction from red mud by novel simple technology

Abstract Storage of red mud – bauxite processing waste – leads to serious environmental problems due to its high alkalinity and particle dispersity. Full or partial utilization or recycling of red mud could reduce the harmful effect on the environment. Scandium is the most valuable ingredient of red mud, yet it's extraction is poorly commercialized due to its high cost. The new efficient extraction technologies promise an ensured supply of scandium and a significant drop in cost. Here, scandium concentrate, extracted from leachate after carbonate treatment of red mud, was subjected to sulfatisation by H2SO4 to separate silica from water-soluble sulfates. To recover and selectively separate scandium from other impurity metals, the crystallization of two complex scandium and ammonium sulfates – NH4Sc(SO4)2 and (NH4)3Sc(SO4)3 – is proposed. The solubilities of these sulfatoscandiates in water, established by isothermal method, are 33.4 and 72.4 g/L, respectively. For the less soluble NH4Sc(SO4)2 a further considerable reduction of solubility has been observed in H2SO4 solutions of concentration above 3.5 М in the presence of 0.5 М NH4Cl at 20 ± 1 °C. More than 99% of scandium in the form of micron-sized NH4Sc(SO4)2 crystals has been recovered from a multicomponent liquid at 5–6 М H2SO4 and 0.5 М NH4Cl. The product contains extremely low levels of impurities. The precipitation of NH4Sc(SO4)2 offers a much higher selectivity in separation of Sc from the other main constituents, as demonstrated by the large separation coefficients between scandium and other metals βSc/M (e.g., for the couple with aluminum βSc/Al = 4280). The recrystallization product after calcination at 1000°С contains 99% Sc2O3.