Phosphorus Removal from Si-Fe Alloy Using SiO 2 -Al 2 O 3 -CaO Slag
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THE health and environmental issues associated with the use of fossil fuels along with their continuous depletion have led to a great interest in the development and increased production share of the world’s energy demand from renewable sources. Solar energy is among the main alternatives because of its abundance and versatility. Silicon is the key raw material in solar industry, and it is mainly produced through Siemens process in which solid silicon is converted to gaseous compounds. Producing solar grade silicon (SoG-Si) via Siemens process involves consuming ~ 130 kWh of energy per kg of SoG-Si. An alternative approach for the production of SoG-Si is employing metallurgical methods, which only consumes ~ 20 kW h of energy per kg of SoG-Si.[1] On the other hand, the lifetime carbon emissions related to photovoltaic is 21 gC/kWh using Siemens and Siemens-like processes for silicon purification. This figure is significantly reduced to 6.5 gC/kWh when metallurgical methods are employed.[2]
ALI HOSSEINPOUR and LEILI TAFAGHODI KHAJAVI are with the Materials Engineering Department, University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Contact e-mail: [email protected] Manuscript submitted October 12, 2018. Article published online April 24, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B
Metallurgical refining processes take place at molten state and do not require vaporization of silicon. Concentrations of impurities (such as B and P) in silicon need to be reduced to below 1 ppm to be applicable in solar industries. Numerous studies have recently been focused on purification of silicon through metallurgical methods because of the low operational cost of these processes.[3] Directional solidification is one of the processes that can be utilized for removal of most of the impurities from silicon. The Segregation coefficient of each impurity, which is defined as the ratio of the impurity concentration in solid silicon to that of liquid silicon, determines the efficiency of directional solidification for impurity removal. However, impurities such as P, which has substantially higher segregation coefficients than metallic impurities, cannot be efficiently removed by this method.[4] Electron beam melting is another metallurgical route which is mostly applied for removal of impurities with high vapor pressure.[5–7] The need for high vacuum levels in this method makes it an expensive route. Solvent refining is also a metallurgical route for purification of silicon. In this method, silicon is alloyed with a metal that has higher affinity for impurities. During solidification, most of the impurities are left in the alloy phase due to their higher solubility in the molten phase and their lesser affinity for the solid silicon.[8] Al,[9,10] Cu,[11] Ni,[12] and Fe[13,14] are the candidate metals that have been used as an alloying agent for removal of P from silicon. Based on the
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previous studies, segregation coefficient of P is reduced when silicon is alloyed with another metal.[13–16] Also, a previous
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