Research On Wet Purification And Semi-industrialization In Metallurgical Preparation Of Solar-grade Silicon

Large-scale development of photovoltaic (PV) industry requires the polysilicon production technology with low cost and environment friendly. At present, the primary technology for production of polysilicon is improvement Siemens method. It restricts the development of PV industry because of its high cost and environmental pressure. With economical and low energy consumption and environmental friendly? metallurgical method for preparation of solar grade silicon (SoG-Si) is a new technology. Hydrometallurgical process is an indispensable part for metallurgical methodes, which can remove the most of impurities in the metallurgical grade silicon (MG-Si). Hydrometallurgical method for purification of MG-Si has been studied long time, but only on the processes in laboratory, being lack of systematic research and semi industrialization. Hydrometallurgical process, the mechanism and semi industrialization for purification of MG-Si have been studied in this paper. The MG-Si produced in Yunnan of china has been used as raw material. The main contents as follows:(1) Deportmentes of impurity elements in MG-Si were investigated, and the results show that metal impurities Fe, Al, Ca and Ti was mainly distributed in binary intermetallic FeSi2,CaSi2and ternary FeAl3Si2, FesAl8Si7, FeTiSi2. The SiP phase was first time found in MG-Si.(2) Me-Si-(F)-H2O potential-pH diagram for Al, Ca, Ti, Fe and P at25?have been calculated or quoted. According to those, leaching agents and specific conditions for removal the impurities of Fe, Al, Ca, Ti and P from MG-Si were analyzed under the viewpoint of thermodynamics.(3) The AfGm, of FeTiSi2and FeAl3Si2were calculated through two-parameter model, and the results of?fGm of FeTiSi2and FeAl3Si2were-94.23kJ·mol-1and-240.052kJ·mol-1respectively. Based on those, chemical reaction mechanism of the removal of FeAl3Si2, CaSi2, FeSi2and FeTiSi2through leaching by HF or HC1were discussed and proposed.(4) Experiments confirmed the theoretical results that the mixed acid system HF-HCl is the most effective leaching agent for removal of metal impurities Fe, Al, Ca, Ti in MG-Si, but the system under normal pressure and temperature is not obvious on the removal of non metal impurities B, P. By leaching MG-Si with3%HF and2%HC1at70?for5h and using two stage leaching, the content of Fe, Al, Ca and Ti in products were less5ppmw, less60ppmw, less lOppmw and less5ppmw respectively.Hydrofluoric acid pressure leaching process for removing the non metal impurities B and P was researched, in the process temperature has the greatest influence on the removal of non metal impurities B, P. When the extraction temperature is200?, the HF content is5%, the ratio of liquid to solid5:1, powder particle size80%<200mesh, leaching after5h, B and P content in the product are respectively from23and81ppmw decreased to9.5and18.8ppmw.(5) Those chemical mechanism, which proposed in this paper, of the removal of impurities phases FeSi2, CaSi2, FeAl3Si2and FeTiSi2by HC1or HF were verified by experiments from three aspects as follows:the effect of HC1or HF on removal impurities phases in MG-Si, the composition of gas generated in the leaching process and the ionic form of iron in leaching solution.(6) Semi industrialization experiments with annual output of500ton UMG-Si have been studied. The results showed that the optimal conditions for HF-HCl system were1.5%HF,1.5%HC1, silica fume particle size<75microns, liquid to solid ratio5:1, leaching time for4h. The Fe, Al, Ca and Ti in product were respectively1OOppm or less,200ppmw or less,150ppmw or less,5ppmw or less, and the product quality has reached3.5N after leaching by a two-stage contained under optimum conditions. This process is a technology with low cost and good investment prospect.