Effects of solidification rate on the leaching behavior of metallic impurities in metallurgical grade silicon

Abstract

Precipitation of the impurity phase during the solidification of metallurgical grade silicon melt determines the distribution law and occurrence state of impurities in silicon, while also determining the removal efficiency of impurities during the leaching process. In this research, distribution and occurrence states of inclusions in silicon at different solidification rates were detected via an electron probe microanalyzer (EPMA). Results have shown that the size of the inclusions becomes larger following electric resistance remelting, and some new impurity phases are generated, such as the VSi2 phase, TiSi2 phase, Si-Fe-Al-Ni-Zr phase, and Si25FeAl2Ca16NiCu2 phase. It is seen that there are clear boundaries between different phases. In addition, based on agitation leaching of 4.0 mol·L−1 HCl and 2.0 mol·L−1 HF mixed acid, it was found that most of the impurities in industrial silicon were effectively removed during this process. The impurities in the industrial silicon decreased from 5233.0 ppmw to 181.7 ppmw. The precipitation of the Si-Fe-Al-Ni-Zr (Cu) phase was the main factor providing improved efficiency of impurity removal, which exposed the impurity elements to the greatest extent. Industrial silicon was subjected to resistance remelting at different solidification rates, and the removal effects of impurities were also different. The impurity removal effect was optimal at a solidification rate of 1 °C/min, as the impurities were reduced to 131.9 ppmw.