Separation of Refined Silicon from Al-Si Alloys by Air Pressure Filtration
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rade silicon (SoG-Si) has increased continuously with the rapid development of the photovoltaic industry.[1] The Siemens process, which is the conventional method of producing SoG-Si, struggles to keep up with the demand due to its high cost and energy consumption. Therefore, processes based on low-cost metallurgical purification have been widely investigated, such as solvent refining,[2,3] directional solidification,[4] slag refining,[5] and other specialized purification methods.[6,7] High-purity silicon can be obtained by solvent refining, wherein the impurities segregate into the liquid phase during solidification. Alloying will decrease the segregation coefficients of impurities, which contribute to their removal from silicon. Several metals have been tested as solvents, including aluminum,[8,9] tin,[10] iron,[11] and copper.[12] The Al-Si system is a binary eutectic alloy that has a low melting point and does not form intermetallic compounds; therefore, the Al-Si system is a good potential candidate for solvent refining. After multiple refining and acid leaching cycles, the contents of impurities (including B and P) in refined silicon were reduced to the ppm level.[8,13] Combined TIANYANG LI, LEI GUO, ZHE WANG, and ZHANCHENG GUO are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted October 2, 2018. Article published online March 11, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B
with other processes such as directional solidification, solvent refining could prove to be potential for SoG-Si production. The separation of refined silicon is another key to the application of solvent refining. Chemical separation via acid leaching is effective,[14] but this approach results in substantial solvent losses and waste acid generation. Two kinds of physical separation methods (electromagnetic force and supergravity force methods) have been investigated for Al-Si alloys. An electromagnetic force agglomerates most silicon particles during solidification under an alternating magnetic field.[13] The electromagnetic force method could be further improved by combination with directional solidification,[15] but many solvents would remain in the enriched silicon. Both enrichment and filtration by supergravity are efficient,[16,17] but the supergravity force method is difficult to apply in practical production due to its high cost. Another potential separation method is melt filtration, wherein the solid-liquid separation is efficient with a suitable driving force. Filtration with mechanical extrusion is expensive and not suitable for Al-Si alloys; the silicon particles would accumulate and form a reticular structure that hinders the extrusion process. Filtration with air pressure has the potential for efficient separation at low cost. In this work, air pressure filtration was used to separate refined silicon particles from Al-Si alloy melts, providing an efficient separation method for solvent refining. Stainless steel sieves were
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