Impurities Removal from Metallurgical-Grade Silicon by Combined Sn-Si and Al-Si Refining Processes

The purification of metallurgical-grade silicon (MG-Si) by combined solvent refining processes has been studied. The final high-purity silicon was recovered through Sn-Si refining and Al-Si refining processes in sequence after acid leaching, and the removal mechanism of impurities was explored. Inductively coupled plasma (ICP) chemical analysis revealed the concentrations of main impurities including B and P, and typical metallic impurities except for solvents Sn and Al were reduced to below 1 ppmw. The final removal efficiencies of B and P were 97.7 pct and 99.8 pct, respectively, and those of most metallic impurities were above 99.9 pct. SEM analysis showed that P-containing phases (Al-Ca-Mg-Si-P and Al-Si-P) formed on the surface of refined Si after Sn-Si refining and Al-Si refining, which was confirmed to be the main approach for P removal. It was also found that the formation of binary silicide such as Fe₃Si₇ and Mn₁₁Si₁₉ or multicomponent phases such as Ca-Mg-Si phase occurred during the solvent refining process, and they segregated on the grain boundaries in Si or attached to the surface of Si, which led to high removal efficiency of metallic impurities by the solvent refining process.