Abstract
Metallurgical-grade silicon, widely used in various industries (as a steel deoxidizer or an alloying additive in alloys, for the semiconductor-grade silicon production, etc.), is obtained by melting silica-containing feeds in ore-thermal furnaces. This carbothermal process is slag-free. However, in the silicon production at Silicon JSC (Shelekhov, Irkutsk region), refinery slags are formed in the course of oxidizing silicon melt impurities in a ladle by blowing air (at ~1500-1600°C), performed to remove calcium, aluminum, and titanium impurities from the product. Due to high viscosity, this slag entraps silicon, the content of which reaches 42.1-65.3 % wt. thus requiring additional extraction to improve the general efficiency of production. The paper studies the mechanism of impurity inclusions generation in the chosen slag composition (based on CaO, Al2O3, SiO2) with ternary diagrams built using the Diatris 1.2 software. This software is based on a thermodynamic technique for calculating the ternary system liquidus parameters at a point with given component contents using a regular solution model. The crystallization way has been studied for the slag of the following composition, wt. fractions: CaO - 0.178, SiO2 - 0.785, Al2O3 - 0.0365. It is shown that the melt of the chosen composition stops crystallizing at the eutectic point with a melting point of 1271.83°C with the release of anorthite CaO·Al2O3·2SiO2, calcium monosilicate CaO·SiO2, and the CaO·7SiO2-L phase. It is planned to further study binary and ternary diagrams of other refinery slag components to choose the optimal temperature regime for extracting silicon.
Keywords:
metallurgical silicon, oxidation refining, slag, fusibility diagram, crystallization path.