Metallothermic reduction of Sc₂O₃ using Al–Ca alloys equilibrated with CaO–CaCl₂ flux

Through a fundamental study, the mechanism of metallothermic reduction of Sc₂O₃ in a CaO–CaCl₂ flux system was determined via a thermochemical equilibrium method, aiming to produce an Al–Sc–Ca alloy (Sc: ∼0.5–3.6 wt%; Ca: ∼0.8–1.8 wt%) at 1273–1623 K. The effect of the oxygen potential on Sc reduction was investigated to establish a general mechanism of Sc reduction. The effect of the CaO activity on the flux and content of Ca in the alloy was evaluated to confirm the role of Ca in the mechanism of metallothermic CaScO_2.5 reduction. Under CaO-saturated flux, the Sc recovery increased significantly. The oxygen potential established by the Ca–CaO equilibrium was the driving force for enhanced Sc recovery. The Sc recovery was positively correlated with the excess Ca (amount of Ca oxidized directly to CaO) in the alloy. The enthalpy and entropy of the metallothermic reduction were calculated from the temperature-dependence of the equilibrium constant using the van't Hoff equation. The equilibrium constant decreased with increasing temperature, indicating that the reaction is exothermic. An Al–Sc alloy with a maximum Sc content of 3.98 % was obtained via metallothermic reduction. The formation of the desired Al₃Sc compound was verified using X-ray diffraction. The uniform distribution of Sc in the alloy and faceted morphology of Al₃Sc were confirmed by scanning electron microscopy (SEM) and electron probe micro-analysis (EPMA).