| Solar energy is unlimited and clean,and it has become a new energy which is important for human to solve the problem of energy shortage.As for crystalline silicon is the main material of producing solar cell,the industrial demand of it is huge.Using low-cost raw materials to produce satisfactory silicon product can not only decrease the production cost of solar grade silicon(SoG-Si)by a large margin,but also can improve the market of photovoltaics.A new electrothermal metallurgy process to prepare upgraded metallurgical grade silicon(UMG-Si,purity of Si is above 99.3%and contents of impurities are lower than that in the MG-Si)was developed in this paper.Slagging agents were added into raw materials during the smelting process in order to improve the purity of the silicon products.This method combined the smelting process with slag refining process into one step.It not only simplified technological process,but also decreased the production cost of SoG-Si significantly.In this paper,different layers of the crucible were systematically analyzed to obtain the distribution and migration rules of impurity elements,and the reaction mechanism of impurity removal was studied.The main results of this research are as follows:(1)When adding SiO2-CaO slagging agent,the optimum pelletizing condition were:adding 3%of the binder and 15%of the water,the optimum pressure was 20 MPa,and the optimum holding time was 90 s.The optimum mixture proportion of SiO2:CaO was 8:2.Compared to the result without adding slagging agents,the concentration of elements Fe,Al,P and S were reduced by 90.71%,63.90%,44.52%and 56.93%with this new method.As a result,the purity of silicon product can reach up to 99.60%,and the silicon product match the standard of UMG-Si.(2)When adding SiO2-CaO-Al2O3 slagging agent,the optimum pelletizing condition were:adding 2%of the binder and 15%of the water,the optimum pressure was 25 MPa,and the optimum holding time was 120 s.The optimum mixture proportion of SiO2:CaO:Al2O3 was 7:2:1.Compared to the result without adding slagging agents,the concentration of elements Fe,P were reduced by 62.35%,39.98%with this new method,and the K was removed basically.As a result,the purity of silicon product can reach up to 99.66%,and the silicon product match the standard of UMG-Si.(3)When adding SiO2-CaO-CaF2 slagging agent,the optimum pelletizing condition were:adding 2%of the binder and 15%of the water,the optimum pressure was 25 MPa,and the optimum holding time was 120 s.The optimum mixture proportion of SiO2:CaO:CaF2 was 7:2:1.Compared to the result without adding slagging agents,the concentration of elements Al,P,Fe were reduced by 93.60%,52.38%and 49.51%with this new method,and K,S were removed basically.As a result,the purity of silicon product can reach up to 99.90%,and the silicon product match the standard of UMG-Si.(4)The model of reactions between slag and silicon were inferred.Slag and silicon were formed in skull zone and then flowed to product zone.Impurities in liquid Si would diffuse into Slag/Si interface and trend to react with slag.Then the reaction product would diffuse into melted slag phase from Slag/Si interface to form stable compound.The results indicate that adding slagging agent into the raw materials during smelting process is beneficial to remove impurity elements and improve the purity of silicon product. |
