Viscosity of Conventional Cryolite-Alumina Melts

Viscosity of conventional cryolite-alumina melts NaF-AlF3-CaF2-Al2O3 with a cryolite ratio (xNaF/xAlF(3), mol/mol) 2.3 was studied in dependence on the CaF2, Al2O3 content and temperature. The viscosity of cryolite-alumina electrolyte samples prepared under lab conditions and electrolyte samples of industrial baths was measured by the rotary method using an FRS 1600 rheometer (Anton Paar). The region of laminar flow of the melt, determined according to the dependence of viscosity on the shear rate at a constant temperature, was 10-15 s(-1) for all studied samples. The viscosity temperature dependence of cryolite-alumina melts was measured at a shear rate of 12 +/- 1 s(-1) in the temperature range from liquidus to 1020 degrees C. The change in the viscosity of all samples in the investigated temperature range (50-80 degrees) can be described by a linear equation. The average temperature coefficient of linear equations describing the viscosity of cryolite-alumina electrolytes prepared in laboratory conditions is 0.005 mPa s/degrees C, which is 2 times less than the temperature coefficient of industrial electrolytes. Thus, the change in the viscosity of electrolytes in industrial baths with increasing temperature is more significant. Additives of both alumina and calcium fluoride increase the viscosity of the cryolite melt. The viscosity of the prepared samples of conventional composition NaF-AlF3- (5 wt %) CaF2- (4 wt %) Al2O3 (CR = 2.3) is equal to 3.11 +/- 0.04 mPa s at an electrolysis operating temperature of 960 degrees C. However, the viscosity of industrial baths with the same cryolite ratio exceeds the viscosity of the prepared electrolytes by 10-15% and is in the range of 3.0-3.7 mPa s, depending on the composition of the electrolyte.

Automated summary

The viscosity of conventional cryolite-alumina melts was studied in relation to the content of CaF2, Al2O3, and temperature. It was found that the viscosity of electrolytes increases with temperature, and the coefficients of temperature dependence differ between laboratory and industrial conditions.