Enhanced Precipitation And Wet Removal Of Impurity P In Industrial Silicon

Hydrometallurgy purification,which is the pretreatment process in the metallurgical method,is preliminarily used to purify metallurgical grade silicon(MG-Si)via acid leaching.This process has many advantages,including effective removal of impurities,simple equipment,and easy scalability.However,the removal efficiency of non-metal impurity P in MG-Si by acid leaching is very low,which is related to the distribution and morphology of phosphorus impurity in silicon.In this paper,according to the physical characteristics of phosphorus impurity in MG-Si,the effect of impurity content and composition on the precipitation of P impurity and the theoretical principle of removing phosphorus by acid leaching,the mixture leaching system and the strengthen effect of additive were studiedsystematically.A good purification effect was obtained.Besides,the effect and mechanism of thermal pretreatment on the removal phosphorus from MG-Si by HCl-HF leaching was discussed.(1)Remelting of MG-Si with master alloy addition,when the contents of Al and Ca impurities are higher in silicon,the SEM-EDS results shown that phosphorus dissolution in Si-Al-Ca and Si-Al-Fe-Ca phases.(2)By adjusting the composition and impurity content of the silicon melt using secondary refining,two types of silicon are formed:crude metallurgical-grade silicon(C-MG-Si)and secondary refined metallurgical-grade silicon(S-MG-Si).The SEM-EDS results show that the phosphorus element only be detected in the C-MG-Si grain boundaries,the Si2Al2Ca phase becomes enriched with phosphorus.(3)By studying the influence of various factors on phosphorus removal efficiency,the optimum operation conditions for the removal of phosphorus from C-MG-Si and S-MG-Si were obtained:acid concentration:4 mol·L-1,particle size of raw material 75?106?m,leaching time:6 h,leaching temperature:65?,and liquid-solid ratio:4:1.Using these conditions,the mass fractions of P in C-MG-Si and S-MG-Si were reduced from 105 and 76 ppmw to 48 and 61 ppmw,respectively,with removal efficiencies of 54.3%and 19.7%,respectively.(4)After the addition of HF acid to HCl solution,the removal rate of P could be improved.By investigating the effect of HF concentration on the removal rate of P impurity in HC1-HF mixed acid,it was found that with the increase of HF concentration,the removal rate of P increased first,then stabilized,and finally decreased.In the 4 mol· L-1 HC1-3 mol·L-1 HF leaching,the concentration of P in C-MG-Si and S-MG-Si were reduced to 46 and 58 ppmw,respectively,with removal efficiencies of 56.2%and 23.7%,respectively.At the same time,the purity of C-MG-Si and S-MG-Si were increased from 97.55 and 99.31%to 99.91 and 99.87%,respectively,with extraction efficiencies of 96.14%and 81.59%,respectively.The obtained removal efficiency of phosphorus and metal impurities can be improved by adopting C-MG-Si as the raw material.Using C-MG-Si as raw material for acid leaching pretreatment to prepare solar grade polysilicon can reduce the process of secondary refining and save production cost.(5)The impurity phase in MG-Si was studied by in situ etching.The etching results showed that Si-Ca,Si-Al-Ca and Si-Al-Fe-Ca phases could be dissolved in HC1 solution,while Si-Fe and Si-Fe-Al could not be dissolved by HC1,but could be completely dissolved in HC1,HF solution.(6)Thermal oxidation pretreatment enables to segregate the impurities to the surface of silicon powder.When the MG-Si powders treated with thermal oxidation at 700? and 80? for 2 h,then leaching in 4 mol· L-1 HCl and 3 mol L-1 HF solution,the removal rate of P was increased by a small margin;However,after thermal oxidation at 900?1100?,it plays a certain role in preventing the precipitation of phosphorus in the silicon,which is not conducive to the subsequent acid leaching.