Purification of metallurgical grade silicon by acid leaching

Abstract

The purification of metallurgical grade silicon (≈98% Si) from Companhia Portuguesa de Fornos Eléctricos by acid leaching is studied as a function of the particle size, time, temperature and concentration of leaching agents (HNO 3, H 2SO 4, HCl and HF). It was found that using only hydrochloric acid (16%, 5 h, 80°C) and a relatively coarse fraction (116 μm) it is possible to remove ∼85% of the impurities and to obtain 99.9% pure Si after further leaching with hydrofluoric acid (2.5%, 2 h, 80°C). All the other acids and their mixtures, including aqua regia, gave results poorer than those for hydrochloric acid. The finer fractions were more difficult to purify. These results differ from other previously reported on purification studies of silicon by acid leaching and are ascribed to the unusual composition of the MG-Si used in this study.