Evaporation Behavior Of Impurities In Molten Metallurgical Grade Silicon

With the development of global economy, traditional energies are confronted with depletion. In order to solve environmental and energy problems, as a kind of clean renewable energy, solar energy has a rapid progress in research and development.However, with the explosive growth of Photovoltaic industry, it has become increasingly difficult to secure an adequate supply of solar grade multicrystalline silicon (SOG-Si), which are mainly applied for solar cells. Compared with the conventional chemical routes for preparing SOG-Si, such as improved Siemens method, metallurgical method with the merits of low consumption and less pollution may lower the cost of production further and specialize in providing feedstock for PV industry.Because of the low vapor pressure of silicon, the pure elements with higher vapor pressure than that of silicon can be expected to be removed during the melting of silicon. Therefore, as important processes of metallurgical method, vacuum refining and electron beam refining have been widely applied. In this research the purification of metallurgical silicon was carried out by vacuum refining and low pressure refining. The effect of melting parameters on the impurities removal and the evaporation behavior of impurities in molten silicon during refining were also investigated. Simultaneously, the effect of P removal by electron beam refining was investigated, and then, it was compared with the results obtained by vacuum refining. Based on the research, the conclusions can be drawn in the following.Vacuum refining is effective in the removal of P, Al, Ca in metallurgical silicon. The contents of impurities decreased with the increase of time and the removal rates increased with the increase of temperature. The removal rates of P, Al, Ca can be expressed by the first order equation. The rate constants were expressed respectively a lnκP = -4.81-12220/T, lnκA1=-0.36 -22361/T, lnκCa= - 2.04-18329/T. The activation energy of removing P, Al, Ca from molten silicon were estimated to be 102,186,152 kJ/mol respectively. The overall removal rates of P, Al, Ca are controlled by the diffusion in molten silicon and the evaporation from the molten silicon surface.In low pressure refining, the removal rate of P was also expressed by the first order equation but did not change with the increase of temperature. The rate constant was lower than that of vacuum refining due to the change of atmospheric pressure. The rate-determining steps of dephosphorization were diffusion in molten silicon and gas phase.In electron beam refining, the P content, which is below 0.1 ppm could directly satisfy the purity requirements of SOG-Si. The removal rate of P was higher than that of vacuum refining. The rate constant was expressed as lnκP=-4.40-12955/T and the activation energy of removing P was estimated to be 107 kJ/mol. The experimental result is in consistence with that under vacuum refining.