Silicon Production via Reaction of Silicon Tetrachloride and Aluminum Subchloride

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TODAY, the Siemens-process is the basis of the solar grade (SOG) silicon industry. The process provides high-purity silicon production, but it is associated with low productivity and high energy consumption. For decades, many attempts were made to develop alternative routes for silicon production applicable in photovoltaic devices design. These alternative routes involve physical and chemical techniques and their combinations: metallurgical silicon upgrade (slag, electron beam refining, directional solidification, acid leaching),[1] electrochemical and metallothermic reduction.[2] Among the metallothermic processes, zincothermic reduction of silicon tetrachloride, developed in the middle of the twentieth century, was the first commercial technology for SOG-Si production. Na and Mg are also used as reductants for silicon compounds (halides and silica), but mainly on a laboratory scale.[1–4] Aluminum is considered a promising reducing agent because of its relatively low cost and mass production.[5] However, a

ROMAN A. ZAKIROV and OLEG G. PARFENOV are with the Institute of Chemistry and Chemical Technology of SB RAS, Akademgorodok 50/24, Krasnoyarsk, Russia 660036. Contact e-mail: [email protected] Manuscript submitted November 14, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS B

peculiarity of aluminum is its very low vapor pressure, even at a high temperature. Therefore, the reduction of silicon compounds by liquid aluminum proceeds only on the metal surface and results in silicon shell formation in the remaining aluminum core. Hence, this approach provides only Al-Si alloy production. The application of aluminum subchloride as a gasphase reductant has been mentioned in the literature.[6] The authors have recently published the results of TiAlalloy synthesis via reaction of AlCl and TiCl4.[7] The possibility of silicon production in the SiCl4-AlCl system was demonstrated by thermodynamic calculations and laboratory experiments, but without numerical characteristics.[8,9] Silicon halides reduction with alkali and alkali earth metals generates by-products with a high boiling point. These salts condense along with silicon; therefore, hydrothermal treatment of the product and leachate utilization are necessary. On the contrary, aluminum chloride has a low sublimation temperature of 453 K (180 C) and condenses separately from the silicon. Hence, it can be readily reused or utilized as a commercial product, for instance, as a wastewater treatment reagent. The quality of silicon depends strongly on the concentration of impurities in metal reductant, and, for SOG-Si production, at least 4-5N purity is required. Aluminum subchloride synthesis combines both gasphase reducing agent formation and in situ reductant

purification. Selective extraction of aluminum from Feand Si-rich crude alloy produced in direct electric smelting of bauxite was demonstrated early.[10,11] Thereby, the aluminum alloys (primarily, silumine) scrap may be considered an inexpensive reducing agent for silicon production. To sum it up, the adv

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