Synergistic Separation Behavior of Boron in Metallurgical Grade Silicon Using a Combined Slagging and Gas Blowing Refini
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ntly, fossil energy is drying up and the share of renewable energy resource, above all solar energy in the total world energy demand, is increasing yearly. Solar grade silicon (SoG-Si) purified from metallurgical grade silicon (MG-Si) is a base material in solar cells for photovoltaic (PV) application.[1] The solar grade silicon is mainly derived from the scrap of semiconductor grade silicon, which is produced by the chemical methods such as Siemens process, Silane process, and Fluid bed process. However, a metallurgical route with low energy consumption, low cost, and friendly environment is being paid more and more attention and becoming one of the research hot spots in the world.[2]
JIJUN WU, WENHUI MA, and BIN YANG are with the State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China, and also with the The National Engineering Laboratory for Vacuum Metallurgy and Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China. Contact e-mail: [email protected] YEQIANG ZHOU and MIN XU are with The National Engineering Laboratory for Vacuum Metallurgy and Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology. Manuscript submitted August 28, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
Metallurgical route composed of secondary refining,[3,4] acid leaching,[5] plasma treatment,[6] electron beam refining,[7] solvent refining,[8] and directional solidification[9] has been used to remove impurities from MG-Si and reduce the cost of SoG-Si production. The polycrystalline silicon produced by metallurgical route has reached a purity of 99.9999 pct (6N), but the optimization and integration of purification technology for MG-Si is still a challenge. Boron, a typical non-metallic impurity, is obstinate to be removed from MG-Si and badly affects the electric properties of silicon wafers. A little boron in silicon will combine into Fe-B pairs and B-O metastable complexes defects with interstitial oxygen and iron.[10] It will accelerate the auger recombination and reduce the mobility ratio of minority carriers in solar cells.[11] Similarly to the separation of sulfur, phosphorus, and carbon removal from molten iron in the steelmaking process, boron can be also separated and removed from molten MG-Si in the form of borates ( BO33 ) by adding calcium silicate slag. Johnston et al.[12] investigated that the separation efficiency of boron in MG-Si was higher than 80 pct by Al2O3-BaO-SiO2 and Al2O3-CaO-MgOSiO2 slag refining. What is more, the ternary and polynary slags with lower melting point and higher basicity were advantageous to boron removal. The research by Cai et al.[13] showed that boron in MG-Si was reduced to 1.11 ppmw by an addition of 10 pct CaF2 to CaO-SiO2 slag and the distribution coefficient of boron reached 4.61. The distribution coefficient of boron was improved to 5.81 by addin
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