Research On Preparation Of High Purity Silicon By Electrothermal Process And Secondary Refining

With the traditional energy sources gradually exhausted in the world, developed countries are developing renewable energy. Solar cell energy has great prospect because of its characteristics of rich resources, widely distributed, clean and safety and so on. The rapid development of solar energy industry makes the demand of solar grade silicon increased rapidly. Owing to high energy consumption, high cost, the main production technology of solar grade silicon, Siemens method, is severely restrict the development of China’s Photovoltaic(PV) industry, the research of preparation of solar grade silicon with low cost will be impotent for developing China’s PV industry.The main production technology of solar grade silicon, improved-Siemens method, is monopolized by United States, Japan, Germany ect. Solar grade silicon in China mainly relies on imports, since the enterprises in China failed to grasp the core technology of improved-Siemens method. Even though, during the crystalline silicon rod cutting process, about50wt%of it was loss since crystalling silicon is cut into high purity silicon powder and become a part of slurry. If high purity silicon in slurry can be recycled and used as raw materials for preparing solar-grade silicon, it will be impotent for both resource recycling and reducing the import amount of solar-grade silicon in our country.In this paper, the beneficiation silicon sample of the solar grade crystal silicon kerf loss slurry as raw material, the high purity silicon is prepared and refined by electrothermal process and secondary refining, which laid the foundation of preparing solar grade silicon. The main contents of the dissertation are:the properties research of kerf loss slurry, acidwashing and impurity removing of the beneficiation silicon, the preparation of high purity silicon by electrothermal process using the beneficiation silicon sample and the research of secondary refining. The obtained results as follows: (1) The properties research of kerf loss slurry:the XRD, XRF, chemical quantitative analysis and particles size analysis have been used for the properties research of kerf loss slurry, results shows:the content of silicon is30.50wt%, silicon carbide is34.90wt%, water and PEG is26.63wt%, iron and iron oxide is5.36wt%, other is2.61wt%, particle size mainly concentrated in1.0～23.8μm.(2) Acidwashing and impurity removing of the beneficiation silicon:the leach time, leach temperature and hydrochloric acid concentration have been studies for the influence of iron leach ratio in beneficiation silicon. The optimum process conditions of acidwashing and impurity removing obtained by orthogonal experiment are:the leach time is3h, leach temperature is70℃, concentration of leach liquid is15wt%, stirring speed is150r/min and the liquid-solid ratio is4:1. The iron leach ratio can reach97.00wt%at those conditions, the iron content in beneficiation silicon sample fall from6.84wt%to0.22wt%.(3) The preparation of high purity silicon by electrothermal process:the high purity silicon has been obtained by electric arc melting from the beneficiation silicon sample. The means of XRD, SEM and XRF is used for the sample of product, result shows:the product deposited in the crucible bottom, the structure was compact, the mainly phase is silicon, the impurity content is very low such as Fe, Si, Al, B and P, the impurity phase of Fe and Si mainly distributed in grain boundary.(4) The research of impurity removing by secondary refining:the effect of removing B and P has been studied through inlet the high purity argon, the mixed gases of high purity argon and water vapor separately, The results show that the effects on removing the boron with bubbles of high purity Ar is not significant but phosphorus is very obvious.The removing rates of B and P are4%and29%respectively. On the condition of high purity Ar and20℃water vapor, the content of boron and phosphorus are decrease. The lowest removing rates of B and P are53%and43%respectively when in150min. On the condition of high purity Ar and90℃water vapor, the content of boron and phosphorus are decrease. The lowest removing rates of B and P are72%and69%respectively.Through these research shows that the high purity silicon with low content of B and P can be obtained by acidwashing and impurity removing, electrothermal process and secondary refining. The purity of high purity silicon will be further improved by vacuum refining, unidirectional solidification and other methods, the purity requirement of solar grade poly-silicon can be reached in future.