Effects of a new acid mixture on extraction of the main impurities from metallurgical grade silicon

A room temperature refining process using a new acidic mixture was investigated for the purification of metallurgical grade-silicon (MG-Si) with an average particle size of 617 μm. MG-Si typically contains precipitated metal impurities, such as iron (Fe), aluminum (Al) and titanium (Ti), in the triple points of grain boundaries. These metal impurities tend to concentrate within the laminar structure at grain boundaries during solidification due to their different segregation coefficients and are leached with acid at different rates because of their different reactivities with acid. The addition of acetic acid (CH3COOH) to a conventional acid mixture composed of nitric acid (HNO3) and hydrofluoric acid (HF) resulted in improved extraction of the main metal impurities. This new acid mixture efficiently extracted Al, which is known to be particularly difficult to remove with conventional acid leaching agents, such as hydrochloric acid (HCl), nitric acid (HNO3), and hydrofluoric acid (HF). The recorded decreases of the Fe and Al concentrations in the material followed two distinct leaching events. The first decrease was due to the extraction of impurities located on the particle surface; the second occurred when the acid leaching time exceeded 15 h. This second leaching event may be due to the removal of metal impurities located deep in the material's pores and deep within the bulk structure of the MG-Si particles. After 25 h of leaching MG-Si with an acid mixture composed of HNO3, HF, and CH3COOH, the purity of the material increased from 99.74 to 99.99% with extraction efficiencies of 99.92 for Fe and 99.98% for Al and Ti.