Thermodynamics and Kinetics of Boron Removal from Metallurgical Grade Silicon by Addition of High Basic Potassium Carbon
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IN recent years, solar energy has been paid much attention due to its potential advantages and applications in commercial products. The photovoltaic (PV) industry has experienced a booming economic growth, between 15 and 30 pct annually, in the past two decades alone.[1] Silicon-based photovoltaic industry specifically plays a crucial role in meeting the worldwide demand for solar energy. Solar grade silicon (SoG-Si, 99.9999 pct) is in especially high demand for application in solar cells.[2] The Siemens process, a traditional technique for producing semiconductor grade silicon (EG-Si, 99.9999999999 pct), has been commercially applied to SoG-Si; however, the process includes chlorination, distillation, and reduction of Si, which consumes large amount of energy and discharges toxic pollutants. In effort to decrease the energy consumption,
JIJUN WU, WENHUI MA, and BIN YANG, Professors, are with the State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming University of Science and Technology, Kunming 650093, P.R. China, and also with the National Engineering Laboratory for Vacuum Metallurgy/Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, P.R. China. Contact e-mail: [email protected] FANMAO WANG, Postgraduate of Master, is with the National Engineering Laboratory for Vacuum Metallurgy/Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology. YUN LEI, Research Scientist, is with the State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming University of Science and Technology. Manuscript submitted October 19, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
researchers have explored a less expensive silicon purification method from metallurgical grade silicon (MG-Si, 99 pct) to SoG-Si called the ‘‘metallurgical route.’’ These processes include ladle refining,[3] acid leaching,[4] solvent refining,[5] electron-beam refining,[6] directional solidification,[7] and others. Several refining methods such as the ELKEM Solar Silicon (ESS) process in Norway, the NS Solar process in Japan, the Shanghai Propower and Ningxia processes in China, and the Photosil process in France have successfully been applied to produce SoG-Si. The metallurgical route is expected to completely replace the Siemens process and become the primary method of SoG-Si production owing to its low cost and low energy consumption.[8,9] Boron, one of the typical non-metallic impurities in metallurgical grade silicon, is difficult to remove due to its high segregation coefficient and low vapor pressure.[10,11] Oxidizing refining is a relatively efficient method for boron removal based on the thermodynamic properties of boron in silicon.[12] Researchers[13,14] found that boron can be removed using a calcium silicate slag reagent. Slag refining is considered a promising method of removing boron from MG-Si and has been extensively studied and commercialized in the ELKEM S
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