Dynamic Viscosity of KF–NaF–AlF₃ Cryolite Melts with Sc₂O₃ and Y₂O₃ Additions

Abstract—Molten mixtures of sodium and potassium cryolites KF–(10 wt %)NaF–AlF₃ with a cryolite ratio CR = 1.5 are promising electrolytes for the production of aluminum alloys with scandium, yttrium, zirconium, and boron using aluminothermic and electrochemical reduction of alloying component oxides at temperatures of 800–850°C. The viscosity is not only an important technological parameter, but also an information source for the structure and mechanism of viscous flow of a liquid. The dynamic viscosity of the KF–(10 wt %)NaF–AlF₃ (CR = 1.5) melt containing Sc₂O₃ (to 9.9 wt %) and Y₂O₃ (to 6.5 wt %) oxide additives has been measured at a constant shear rate of 12 s^–1 in the temperature range from the liquidus point to 930°C. The viscosity of the KF–(10 wt %)NaF–AlF₃ (CR = 1.5) melt is found to change from 1.8 to 1.3 mPa s in the temperature range 800–930°C, which is substantially lower than the viscosity of the KF–AlF₃ melt (CR = 1.5), which is 1.7–2.4 MPa s, in a lower temperature range of 800–730°C. Sc₂O₃ and Y₂O₃ oxide additives substantially increase the viscosity, which is related to the formation of complex REM ions and oxifluoroaluminates in the melt during dissolution of the oxides.