Research On Removal Of Impurities In Metallurgical Grade Silicon By Metallurgical Method

People become aware of the importance of the research and use of new energy due to the gradual consumption of conventional energies and air pollution.Photovoltaic(PV) energy is widely considered as one of the most important new energies in 21st century since it has the merits such as cleanness and environmental protection,convenience and safety, inexhaustibility.Under the policies of different countries all over the world,PV industry has been in great development,however,the supply of solar grade silicon(SOG-Si) as the cornerstone of PV industry falls short of demand,which becomes the bottleneck of rapid development of PV industry.Compared with the conventional chemical routes such as improved Siemens method and new silane method,the preparation of SOG-Si directly from metallurgical grade silicon(MG-Si) by metallurgical method has the merits of low cost and consumption,especially,it may lower the cost of production further and specialize in providing feedstock for PV industry,which can solve the problem of the shortage of silicon feedstock.In this paper,the technical route of purification of MG-Si is designed and the related purification experiments in pilot scale are conducted by self-designed equipments including acid leaching tank,vacuum induction melting and first directional solidification furnace, electron beam melting and second directional solidification furnace.The parameters of acid leaching using single acid are determined and the ultrasonic field is proposed to impose on the process of acid leaching,the removal efficiency of metallic impurities may attain 88.9% through acid leaching.The multi-crystalline silicon ingots with large columnar grains are produced by directional solidification combining with heat exchange method and Bridgman method,the distributions of metallic impurity contents in the solidified ingot are analyzed and calculated,the decrease of pulling rate and cooling intensity of pedestal promote the removal of metallic impurities.Electron beam melting has a significant effect on the removal of high vapor pressure impurities such as Al,P,Ca,Mn and Zn,the distribution and removal laws of impurities in the molten silicon bath are studied during the electron beam melting.Through the above-mentioned purification route,the content of A1 in the local area of multi-crystalline silicon ingot may decrease form 2710 ppmw to 0.4 ppmw,the content of Fe may decrease from 2340 ppmw to less than 0.1 ppmw,the content of Ca may decrease from 576 ppmw to less than 0.1 ppmw,the content of Ti may decrease from 198 ppmw to less than 0.1 ppmw,the content of Mn may decrease from 125 ppmw to less than 0.1 ppmw,the content of Cu may decrease from 27 ppmw to less than 0.1 ppmw,the content of Zn may decrease from 0.6 ppmw to less than 0.1 ppmw,the content of P may decrease from 26 ppmw to 1.5 ppmw, which meet the requirement of SOG-Si basically.