| Currently, the production processes of solar grade silicon (SoG-Si), via traditional chemical methods, are fairly energy intensive and environment-unfriendly. Therefore,it is necessary to develop a new type of production approaches with low energy consumption and low cost to purify Si. Al-Si solvent refining is a very promising process to produce SoG-Si at large scale with low cost. This process can remove B and P effectively at the same time. In this dissertation, systematic researches based on the basic properties of impurities in the metallurgical grade silicon (MG-Si) have been carried out. A series of samples were cast by a self-designed electromagnetic induction high temperature refining furnace. By analyzing the ingots structure, impurities content,segregation behavior, impurities removal mechanism, etc., the following results can be obtained.(1) Distribution of Al in Al-Si alloys with different aluminum contents during electromagnetic continuous solidification was investigated. The macrostructure of the Al-Si alloy ingots reveals that the content of primary silicon plates in hypereutectic alloys decreases from the bottom to the top of the ingots and no obvious primary silicon plate exists in eutectic alloy. The fitting results of A1 distribution in the alloys reveal two types of Al distribution patterns in hypereutectic Al-Si alloys: Scheil model for high temperature solidification and initial transient model for low temperature one. An apparent segregation coefficient of Al is proposed to characterize the solidification process, which is approximately 0.83 in the hypereutectic alloys. But Al distributes evenly in the eutectic Al-Si alloy.(2) The highest removal rates of impurities B, P, Al, Fe and Ti during continuous solidification of Al-Si alloys are 74.06%, 84.31%, 60.68%, 99.56% and 99.44%,respectively. Macrostructure of the Al-Si alloy ingots and concentration profile of the impurities reveal that the impurites segregate to eutectic Al-Si melt during the gro"wth of primary Si flakes, and P, Al, Fe and Ti gradually segregates to the top of the ingots during directional solidification. An apparent segregation coefficient is introduced to describe the segregation behavior of the impurities. The apparent segregation coefficients of all the impurities decrease with increase of solidification temperature,and are much higher than the theoretical equilibrium coefficients.(3) The effect of Ti addition and composition of Al-Si alloy on B removal during silicon refining by directional solidification of Al-Si alloy have been studied. It was found from structure observation that the primary Si flakes were agglomerated at the bottom part of the ingot after the directional solidification. So the primary Si flakes would be efficiently collected with less hydrochloric acid consumption by cutting away the eutectic part of the ingot. The content of B in the primary Si flakes could be reduced to less than 1 ppmw with excess Ti in Al-30Si. An apparent segregation coefficient is used to characterize the B removal during the refining process. The mechanism of the high B removal rate in Al-30Si alloys with small amount of Ti addition and the effect of Si content on B removal in Al-Si-Ti system during directional solidification were revealed.(4) In the preliminary study on the impurities removal by directional solidification of MG-Si, it was found that the volume of the columnar grain zone in the Si ingots and the impurities removal rate became higher when the solidification rate decreased. The highest removal rate of impurities B, P, Al, Fe and Ti after directional solidification are 60%, 56%, 99.8%, 99.4% and 99.9%, respectively. In addition, the content of B was further reduced by moist H2 gas blowing during the refining process. Finally, a feasible low-cost industrial route to manufacture SoG-Si by Al-Si solvent refining is proposed according to the conclusions in this dissertation. |
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