Hydrometallurgy is considered a promising method to produce solar-grade silicon (SOG-Si) from metallurgical-grade silicon (MG-Si) due to its advantages of low cost, simple operation, and easy control. In this paper, the effects of particle size of MG-Si, type of acid, leaching time, temperature, and liquid-to-solid ratio on the purification efficiency were investigated in an external field environment with the addition of ultrasound. The purification efficiencies of the two acid-leaching methods were compared. It was found that the simultaneous use of HCl + HNO3 was more effective than using only HCl for impurity removal in MG-Si: the extraction efficiencies of impurities Fe and Al were increased by 2.2% and 13.4%, respectively. The impurity contents in MG-Si before and after calcination pretreatment were also compared. This paper is expected to help researchers select the appropriate hydrometallurgical technique to reduce the impurity content in MG-Si.