Study On Purification Of Solar Grade Multicrystalline Silicon By Al-Si Solvent Refining

Solar-grade multi-crystalline silicon is the main feedstock for solar cell.Due to the development of photovoltaic industry,there is a huge demand for multi-crystalline Si feedstock with high purity.Based on the advantage of low cost,environment friendly and being simple in process,purification by alloy refining method is a promising technology.The Al-Si binary alloy is a good candidate.However,the alloy composition,annealing process and industrial processing technique should be investigated detailed.In this paper,the effect of Si content on the morphology and purity of primary Si has been analyzed systematically.The inoculation treatment was introduced into the solidification process of the Al-Si alloy.Moreover,the processing technology of multi-crystalline Si prepared by electromagnetic-directional solidification has been studied.Following results have been summarized.Firstly,the optimized alloy composition has been determined.The primary Si grains grow following twinning mechanism.The oriented growth direction was transferred from<112>to<112>and<111>as increasing Si content in Al-Si alloy,resulting to the increase of grain size of primary Si.Further increasing the Si content,the thickened primary grains probably collide to form "broken crystal",which decreases the recovery rate of primary Si.The optimized alloy composition is Al-50 wt.%Si,involving a recovery rate of 82.35%(almost equal to the theoretical value of 85.59%),impurity content of 2.31 ppmw for B,and 17.06 ppmw for P.The removal rate of metal impurities is higher than 94%,and the impurity content decrease from 484.9 ppmw to 4.486 ppmw,as well the content is below than lppmw for other metal impurities.Secondly,the inoculation treatment has been introduced into the solidification process.The results show that the nucleation rate of grains and grain size increased after inoculation treatment.The grown grains probably collided to form broken grains,leading to the decrease of recovery rate.Moreover,the bonded grains could separate easily in the sample with inoculation treatment,which decreases the impurity content of A1 and P in primary Si.The recovery rate for the sample with annealing at 900℃ for 2 hr is 83.41%,which is higher by 1.54%than that without inoculation treatment.The impurity content of Al is decreased of 48.8%in the obtained primary Si.Moreover,the oriented growth along<111>direction become obvious as decreasing the cooling rate,resulting to the increase of 50%in thickness of grains,thereby increasing the recovery rate.The impurity contents of primary Si decreased with decreasing the cooling rate,however,increased in the eutectic Si.Especially,the impurity content dramatically increased when the cooling rate is lower than 4℃/min.Consequently,the optimized cooling rate is 4℃/min considering the recovery rate and production cost.Thirdly,the industrial processing technique for multi-crystalline Si employing the electromagnetic-directional solidification has been studied.The results show that the primary Si was agglomerated at the bottom of ingot under the solidification with electromagnetic force.Meanwhile,the morphology of primary Si changed from plate-like to bulk-like.The plate-like grains appear oriented distribution due to the directional heat flow in the ingot The agglomeration was enhanced by decreasing the drawing rate of crucible.When the drawing rate is 10 mm/hr,the fraction of agglomeration layer was about 50%,as well the oriented distribution of the plate-like grains disappeared.On the other hand,the heat dissipation pass through crucible transferred from bottom to sidewall as decreasing the crucible position,resulting in the movement of agglomeration region to the corner of the crucible.In the solidification with lower crucible position,the morphology of primary Si is mainly plate-like,and the agglomeration was determined by the temperature gradient of crucible.Thus,the agglomeration of primary grains could be formed by both effect of electromagnetic force and temperature gradient in the crucible,it is beneficial for acid-leaching for the agglomeration induced by the former effect The optimized processing parameter is initial crucible position of 0 cm and drawing rate of 10 mm/hr,which is considered both the cost reduction and agglomeration effect.In conclusion,this paper is focusing on the solar-grade multi-crystalline Si prepared by Al-Si solvent refining method.A new processing technic has been proposed.The influence of Si content on the recovery rate of primary Si has been studied systematically,and the evolution of grain morphology has been elaborated.The recovery rate was increased by the effect of inoculation treatment Moreover,the agglomeration of grains has been characterized in the electromagnetic-directional solidification process.Based on the research background of Al-Si solvent refining,the obtained results in this work are innovative and practical,which would facilitate the industrial-scale production of multi-crystalline Si by solvent refining.