Electrical conductivity and viscosity of cryolite electrolytes for solar grade silicon (Si-SoG) electrowinning

Abstract Electrical conductivity of molten binary and ternary mixtures based on the system NaF–AlF 3 –SiO 2 was investigated by means of a tube–cell (made of pyrolytic boron nitride) with stationary electrodes. Viscosity of the binary system Na 3 AlF 6 –SiO 2 was measured by computerized torsion pendulum method. It was found that conductivity and viscosity varied linearly with temperature in all investigated mixtures. Obtained content dependence of electrical conductivity (isotherms) was divided into two parts. First, one represented the content region up to 10% (mole fraction) of SiO 2 ; second, the region was with a higher content of SiO 2 (from 10% up to 40%). While the conductivity considerably decreased with content of SiO 2 in the second part; it surprisingly rose in the low content range. A small addition of SiO 2 to the molten cryolite (up to 10%) could slightly increase viscosity, but had no influence on the slope of this dependence since it is responsible for a glassy-networks formation in the melt. Further addition of SiO 2 to the molten cryolite had a huge effect on the viscosity.