Rheological and thermal properties of the KF-KCl-K2SiF6 electrolyte for electrolytic production of silicon

Viscosity and phase transitions were studied in the (KF-KCl)eut.-(10 mol. %)K2SiF6 system. Synchronous thermal analysis (STA) was used to study the phase transformations in the system in the temperature range of 298–1113 K. The (KF-KCl)eut.-(10 mol. %)K2SiF6 melt was found to be homogeneous at 903–993 K. The viscosity of the system under study was investigated by the rotational viscometer FRS 1600. The results may be described by linear function in coordinates ln(η) – 10 000/T at 898–984 K when viscosity changes from 2.8 to 2.1 mPa s. The melt's nonhomogeneity at 984–1055 K is manifested by the change of the temperature dependence slope. The correlation of viscosity and STA data allows determining the decomposition mechanism of K2SiF6 in the melt under study. The decomposition results in the formation of KF and SiF4. The viscosity of the investigated system was measured during the crystallization process at 876–898 K. The melt at the temperatures below 898 K was found to be in the form of suspension, which is illustrated by the deviation of the linear dependence in ln(η) – 10 000/T coordinates. At 876–898 K, the viscosity rises from 2.8 mPa s to 85.1 Pa s as temperature decreases.