Co-solubility of aluminium and scandium oxides in molten sodium cryolite

During the electrolytic or metal-thermal obtaining of Al – Sc alloy, the molten sodium cryolite (Na3AlF6) is used as a dissolvent medium, which gives an opportunity to use the scandium oxide as an initial raw material for the second component. The value of Sc2O3 solubility in salt solution is an important characteristics of the process. The co-solubility of Sc2O3 and Al2O3 in the molten sodium cryolite NaF – AlF3 with a cryolite ratio of 2.3 was studied by thermal analysis and Raman spectroscopy. The liquidus temperature of the quazi-binary phase diagrams [(NaF – AlF3) – Sc2O3] – Al2O3 and [(NaF – AlF3) – Al2O3] – Sc2O3 was measured. All phase diagrams have an eutectic minimum, which shifts toward a decrease in the concentration of the added oxide as the concentration of the “fixed” oxide increases. The Sc2O3 solubility in the NaF – AlF3 melt is 4.5 mol.% at 980 оC and depends on the Al2O3 solubility at a certain electrolyte composition and temperature, but at the same time the co-solubility of scandium and aluminum oxides exceeds the solubility of each oxide individually. This phenomenon can be explained by the different mechanism of the Al2O3 and Sc2O3 interaction with the molten sodium cryolite. The Raman spectra of the quenched samples (NaF – AlF3) – Sc2O3 and (NaF – AlF3) – Sc2O3 – Al2O3 revealed that, unlike Al2O3, the chemical dissolution of Sc2O3 proceeds in two stages with the formation of the complex ions ScF63–, Al2OF62– and Sc2OF62–. However, at high concentrations of dissolved alumina, only oxyfluoride complexes of aluminum and scandium were detected in the spectra.