A.V. Rudenko, A.A. Kataev, O.Yu. Tkacheva.
АBSTRACT
A technique for measuring the viscosity of cryolite melts by rotational method using a high-temperature rheometer FRS-1600 has been developed. The viscosity of molten sodium cryolite NaF-AlF3 with cryolite ratio (CR) of 2.1, 2.3, 2.5 and potassium cryolite KF-AlF3 with CR 1.5, which are used in the electrolytic production of aluminum and its alloys in industrial cells and laboratory facilities, was determined. The viscosity dependence on shear rate was found from the flow curves depicting the relationship between the shear stress and the shear rate, and from the viscosity curves, which represent the dependence of viscosity on shear rate, and then the shear rate parameter was chosen at which the melt behaves like a Newtonian fluid. For cryolite melts the area of laminar flow is in the range of shear rates of 10–16 s-1. The viscosity of sodium cryolite NaF-AlF3 was measured in the temperature range from the liquidus point to 1293 K at a constant shear rate of 12 s-1. The viscosity of cryolite melts decreases with decreasing CR. At 1283 K, the viscosity of NaF-AlF3 melts with CR 2.1, 2.3, and 2.5 is 2.35, 2.43, and 2.50 mPa⸳s, respectively. The obtained values exceed the known literature data by 25%, which can be explained by methodological difficulties in measuring the viscosity of cryolite melts by the oscillation method at temperatures of about 1273 K. The viscosity of potassium cryolite KF-AlF3 with CR = 1.5 was measured in the temperature range of 830–1040 K. It is 1.82 mPa⸳s at 1073 K. The viscosity temperature dependence of both cryolites is described by the linear equation ln(η) = a + b/T. The possibility of determining the phase transitions temperature in multicomponent systems from the viscosity temperature dependence is revealed.
KEY WORDS: dynamic viscosity, rotational viscometry, molten salts, sodium cryolite, potassium cryolite
DOI: 10.31857/S0235010622050073