Physical-Chemical Properties of the Naf-AlF3-Sc2O3-Al2O3 Molten System

The Al-Sc alloys are typically obtained by aluminothermy method.  However, the high alloy’s price is defined by the cost of a scandium-containing substance (such as ScF₃ or Sc, etc.) prepared from its raw material (Sc₂O₃) for the reaction with aluminum. Expenses for a multistage scandium extraction process are also substantial. Another way for producing the Al-Sc alloys can be the electrochemical co-reduction of Al and Sc from their oxides dissolved in a cryolite (NaF-AlF₃) melt. To develop the electrolysis process and to determine its operating parameters it is necessary to know some basic physical-chemical properties of the NaF-AlF₃-Sc₂O₃-Al₂O₃ molten system, such as liquidus temperature, Sc₂O₃ solubility in cryolite including a cross impact  of both Sc₂O₃ and Al₂O₃ oxides on their solubility and electrical conductivity of molten mixture. A thermo analysis (TA)  was used for liquidus temperature measurements. An oxide solubility was determined by the TA along with isothermic saturation method. An electrical conductivity was measured in the capillary (BN) cell  and in the cell with parallel Mo-electrodes using an impedance technique. The obtained results testified that the Sc₂O₃ behaved similar to the Al₂O₃. Its additions to the molten cryolite decreased the liquidus temperature and the electrical conductivity. The Sc₂O₃ solubility in the NaF-AlF₃ melt with molar ratio of NaF/AlF₃=2.3 was found to be about 10 wt% at 980 ᵒC.  The Al₂O₃ solubility in the scandium-containing cryolite decreased with the Sc₂O₃ concentration rise.  The total solubility of both oxides, in controversial to known data, was determined to be slightly higher than the solubility of each separate oxide in the NaF-AlF₃ melt.