Study On The Influence Mechanism Of Iron On The Purification Of Solar Grade Polycrystalline Silicon By Al-Si Alloy Solvent Refining Solution

Solvent refining with Al-Si alloy is one of the most promising technologies to produce solar grade polysilicon at low cost,which has been commercialized on a small scale.In this processing route,the cheap raw materials contain more Fe impurities.Meanwhile,considering employing the Al-Si-Fe alloy prepared by thermal reduction of coal gangue as starting material for purification,the production cost can be further reduced.However,the content of Fe in this alloy is very high(?8 wt.%).Therefore,it is very important to clarify the effect of Fe on the purification process.Unfomately,no systematic investigation has been carried out until now.Based on the above research background,this paper studies the purification process of high-purity polysilicon production using Al-Si-Fe alloy with high Fe content(4?11 wt.%),aiming to clarify the role of Fe on the growth of primary silicon and the removal of impurities,thereby to obtain the purification solution for Al-Si-Fe alloy solvent refining.The main conclusions are as follows:Firstly,Al-Si-Fe ternary alloy was prepared by adding Fe to Al-50 wt.%Si alloy,which was employed as raw material to purification.The influence of Fe on the growth of the primary silicon grain and the removal of impurities was analyzed by characterization of the morphology,size,recovery rate and the content and distribution of impurities in the purified silicon grains.It was found that the Fe rich phase formed after solidification.The content of large size grains(>1mm)was reduced,so that the recovery rate of primary silicon decreased significantly.After solidification,Fe element mainly exists in the Fe-rich phase,and B atoms tend to converge in this phase,showing a certain impurity absorption effect.The viscosity of the alloy is reduced by Fe addition,resulting in the decrease of removal rate of impurities.However,the formed Al-Fe-Si phase changes the growth pattern of some primary silicon grains,leading to the reduction of contamination of inclusions in primary silicon.Based on the effect of alloy viscosity and the control of grain morphology,employing the alloy with low Fe content(<4wt.%)or high Fe content(>10 wt.%)to purify could obtain polysilicon with low impurity content.Secondly,the influence of the cooling method for alloy ingots on the purification process of Al-50Si alloy and Al-Si-Fe alloy was studied respectively.It was found that during the purification process of Al-50Si alloy,the fraction of large primary silicon grains decreases slightly after rapid cooling of the alloy ingot;But the removal rate of impurities is significantly improved.However,the recovery rate of primary silicon decreases greatly in the Al-Si-Fe alloy ingot with rapid cooling treatment.Based on the above research results,it can be concluded that the processing solution of solvent refining with Al-Si-Fe alloy to prepare solar-grade polysilicon is as follows:the alloy composition with low Fe content(?4%)is selected,and the rapid cooling method should be adopted for cooling treatment followed the ingot solidified at 600? for 2h.Finally,the composition of Fe-rich phase was determined;thereafter its growth mode and its effect on the primary silicon grains were analyzed.The results showed that the Fe-rich phase is Al13Fe4Si8,which grew later than the primary silicon grain and can grow lying on the surface of the primary silicon grain.Al-Fe-Si phase has two typical morphologies,one is lamellar and the other has tip morphology.With the increase of Fe content in the alloy,the lamellar phase grows rapidly along the length direction.The growth direction of the lamellar phase is either parallel to or at an Angle with the preferred<112>growth direction of the primary silicon,which can inhibit the lateral lamellar growth of the primary silicon grains,providing a new solution for reducing the Al inclusion.Moreover,the Al-Fe-Si phase,which has the advantage of growing at the tip,turns into a disk shape after growing up and can occupy the intergranular space between the primary silicon grains.Due to the consumption of a large amount of Si atoms in the growth process,a super cooling zone derived from composition decrease of Si is formed in the growth front of the primary silicon grains,which drives the secondary dendrite growth of primary Si grains,therefore appears a wavy morphology.