| With the rapid development of Photovoltaic industry, the demand of solar-grade silicon (SoG-Si) has been increased sharp because of its as the main photoelectric conversion material. Presently, the production of SoG-Si mainly depended on the Siemens process, which has restricted the development of Photovoltaic industry for its long process, high cost and energy consumption. So it is imminent to develop cheap, efficient and safe technology of the polysilicon. Metallurgy method for SoG-Si preparation, due to its cheap, efficient and safe, was considered a promising process. Slag refining and solvent refining have some advantages in removal of boron and phosphorus, but which still has some deficiencies in production of silicon materials only by one of them.Composite solvent refining was put forward based on the method of slag refining and solvent refining. Al-Si alloy and CaO-SiO2-Al2O3 slag were mixed and refined to product SoG-Si in this paper. The migration and distribution of key elements, includes Al and B, between the slag and metal phase was investigated. This new method was effectively to remove B and diminished the problem of solvent separation in the traditional composite solvent refining. Also, the removal effects of other impurities, includes P, Fe, Ca and Ti, were investigated. Furthermore, the effect and mechanism of B removal by electric field enhancement was investigated.(1) 60wt pct Al-Si alloy and 40wt pct CaO-40wt pct SiO2-20wt pct Al2O3 slag were mixed and refined by experiment and Factsage software calculation. The results showed thatthe best ratio of slag to metal is 5, the transformation rate of Al reached up to 96.52% and the B content in the refined Si decreased from 320.74ppmw to 23.37ppmw, getting the purification rate of B 92.28%. The migration patterns between slag and metal phase of Al and B were the same, both of which changed with time dependent stages. For Al, its migration bewteen metal/slag phase was most violent in the first 10min, while for B, the most violent migration into slag happened in the first 30min. And their migration rations were 12.32,10.96, respectively. After refined 90min, Al content droped from 60.00wt pct to 2.56wt pct and B content decreased from 302.74ppmw to 24.96ppmw.(2) The composition of the refined slag was changed with the oxidation of Al, and Al content would be enhanced with the increase of (CaO+SiO2)/Al2O3 under the same refining time. However, B content decrease at first and then increased, whose minimum value was 8.01ppmw when CaO-SiO2-Al2O3 was 1.24, and at that point the migration ratio and the purification ratio of B were 37.80 and 97.35%, respectively.(3) Better separation of slag and metal was achieved after refined, and the problems of separation and reclaim solvent during the traditional composite solvent refining process were be solved by aluminothermic reduction. The regularity of P removal was similar with B. It was found that it was helpful to remove P with increasingly ratio of slag to metal. When the ratio was 5, the P content decrease from 27.78ppmw to 2.46ppmw and the purification ratio was more than 90%. The new system would reach up to remove B and P simultaneously. The P purification changed as two different stages with refining time increasing. The first stage, within 30min after refining begin, P purification ratio increased and reached up to 87.5%. The second stage, after 30min refining, the P purification ratio became slowly because of the decrease of driving forces of the reaction and mass transfer. In a certain range, it was beneficial to enhance P purification ratio with the decrease of (CaO+SiO2)/Al2O3, whose maximum value was 94.24%.(4) The metal impurities were highly enriched in silicon grain boundaries. After acid leaching, the purification of Fe, Ca, Ti were 92.84%,76.07%and 96.20%, respectively.(5) B content decreased with the increase of electric voltage in the electric field condition. When the electric voltage was 3v, B content changed with three different stages with refining time increasing, which were the first 10min,10-60min,60-120min, respectively and the B purification speed were 17.77(ppmw/min),1.95(ppmw/min),0.07(ppmw/min). B content was much lower 20% by comparison beween 6v and Ov.(6) The mechanism of removal B in electric field was firstly investigated by cyclic voltammetry. It was found that, the reactions on the anode and cathode was listed as follows:Anode reaction:B+3O2-=(BO33-)+3e-Cathode reaction:Si4+4e-=Si... |
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