Research On Enhanced Precipitation And Wet Removal Of Impurities In Industrial Silicon

Industrial silicon is the main raw material for the preparation of solar grade polysilicon.At the same time,in the out-of-furnace refining technology of industrial silicon,due to the large amount of oxyphilic impurity elements in the primary industrial silicon melt can be removed by ladle blowing refining,this process has become an essential purification process in the production process of industrial silicon.In this thesis,the effects of Ca,Al and other components changes in the primary industrial silicon melt caused by ladle blowing refining on impurity segregation in the solidification process of the industrial silicon melt are systematically studied,and the differences in impurity occurrence form,leaching behavior and leaching dynamic conditions between the primary industrial silicon and the refined industrial silicon are comparatively analyzed.Based on this,the enhanced precipitation and separation behavior of nonmetallic impurity P that can be realized in the industrial silicon production process are experimentally verified and characterized.The main conclusions are as follows:?1?While ladle refining changes the composition of the melt,it does not affect the co-precipitation trend of various impurity elements.The impurity phases such as Si-Ca,Si-Al-Ca,Si-Al-Fe-Ca,Si-Fe,Si-Fe-Ti,Si-V-Ti,etc.precipitated in the primary industrial silicon can also be formed in the refined industrial silicon,the Ca content is reduced,and the Si+Si₂Ca or Si+Si₂Al₂Ca composite phase appears in the refined industrial silicon.The inclusion phases of Si-Ca,Si-Al-Ca and Si-Fe-Ti have the ability to enrich nonmetallic impurity P.the content of P in the inclusion phase of Si-Fe-Ti is as high as 0.71%.The content of P in Ca-based impurity phase in primary industrial silicon is as high as 73.5%,while the solid solution of P in refined industrial silicon matrix is as high as 87.5%.?2?Due to the insolubility of CaF₂ and AlF₃,the optimum acid leaching process conditions for industrial silicon materials before and after refining with significant difference in Ca and Al contents are 4mol/L HCl+1mol/L HF and 4mol/L HCl+2mol/L HF respectively.Under the condition of 4mol/L HCl+2mol/L HF acid leaching,the removal rates of P in industrial silicon before and after refining are 85.06% and 16.44%respectively.The addition of KMnO4 is beneficial to the leaching behavior of Al and Ca.The leaching kinetics of impurity elements Ca and Fe in silicon was analyzed by using uniform reaction model.It was found that the change of impurity occurrence structure had obvious influence on the reaction order and activation energy.?3?Thermodynamic calculation shows that phosphate is much easier to form than binary metal Phosphide in ladle oxygen blowing refining process.Under vacuum condition,Ca₃P₂ is the most easily decomposed phosphorus compound that may exist in industrial silicon.Based on the fact that P in industrial silicon before and after refining mainly occurs in Ca₃P₂ and SiP phases respectively.The industrial silicon before and after refining is vacuum refined for 0.5h under the conditions of 1450?and 10^-2-10^-3Pa,the P content is respectively reduced to 31 and 49ppmw,and the volatilization rate of P in the primary industrial silicon is 2.09 times of that in the refined industrial silicon.Based on the phenomenon of poor separation effect of P from refined industrial silicon by acid leaching and combined with the P precipitation ability of Si-Fe-Ti phase,doping remelting experiments of ferrophosphorus,Fe,Ti and Mn were carried out respectively.The experiment found that after the trace impurities Mn and Ti are melted into the iron-rich phase,the ability of the iron-rich phase to precipitate P can be increased by nearly 10 times.After refining Si-3wt.%Fe-2wt.%Ti alloy and acid leaching,the removal rate of P in refined industrial silicon can be increased to 71.20%.