Thermodynamics of Phosphorus in Solvent Refining of Silicon Using Ferrosilicon Alloys

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th an acceptable purity level for solar applications through a cost-efficient process is a major challenge in the solar silicon industry. There is a growing body of literature on alternative metallurgical techniques that have been considered as energy and cost-efficient refining stages for producing solar grade silicon with a purity of 6-7 N. Considering different thermodynamic properties of various impurities in silicon, each refining step is appropriate for lowering the concentration of particular impurities. In other words, metallurgical techniques are selective in removing impurities; therefore, a succession of refining steps is required to achieve the purity requirements. Recrystallization of silicon from an alloy, also known as solvent refining, is one of the metallurgical processes that has been used for producing solar grade silicon from metallurgical grade silicon. The process is based on employing an alloying metal (M) as the solvent for crude silicon. Leaving the impurities in the molten phase,

LEILI TAFAGHODI KHAJAVI, Assistant Professor, is with the Department of Materials Engineering, University of British Columbia, 6350 Stores Road, room 309, Vancouver, British Columbia V6T 1Z4, Canada. Contact e-mail: [email protected] MANSOOR BARATI, Associate Professor, is with the Department of Materials Science and Engineering, University of Toronto, 184 College Street, Suite 140; Toronto, Ontario M5S 3E4, Canada. Manuscript submitted May 2, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B

refined silicon crystals are precipitated when the alloy melt is cooled below the liquidus temperature of the Si-M phase diagram. Unlike metallic impurities, the removal of phosphorus and boron during solidification refining is problematic due to their high distribution coefficient between solid and liquid silicon at the melting point of silicon. Multiple metallic elements such as Al,[1–10] Cu,[11–13] Ni,[14] Sb,[15] and Sn[16] have been employed as the alloying agent. In a recent study on metallurgical refining of silicon, effective phosphorus removal was obtained by combining solvent refining with iron and heavy media separation.[17–20] Iron is a suitable candidate because of its considerably lower cost and the smaller solid solubility in silicon compared with aluminum, copper, nickel, and the other metallic elements that have been used as the solvent.[21,22] This will contribute to lower costs and less remaining iron in the refined silicon. Furthermore, the by-product of the process, ferrosilicon, can be used in the steel industry. Considering these advantages, iron has been used as the alloying element for purification of silicon. The efficiency of solvent refining in purification of Si is largely determined by the thermodynamics of the ternary Si-P-M, namely, the relative affinity of silicon and alloying element toward the impurity. Therefore, investigating the thermodynamics of phosphorus distribution between solid silicon and iron-silicon alloy melt is essential to optimize phosphorus removal from silicon in the proposed solvent refining pr

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