Improving the Quality of Metallurgical-Grade Silicon by Acid Leaching

The aim is to conduct research in the field of hydrometallurgical refining of metallurgical silicon. The object of research was metallurgical silicon after oxidative refining from AO Kremnii, part of the United Company RUSAL (Shelekhov, Irkutsk oblast, Russia). The chemical composition of the obtained samples was studied by X-ray fluorescence analysis and X-ray spectral microanalysis. According to elemental analysis, metallurgical silicon contains (wt %) 0.53Al, 0.6094Fe, 0.0491Ti, 0.0628Ca, 0.0066V, 0.002Cr, 0.014Mn, 0.003Cu, 0.010P, 0.007Ba, 0.007Ni, 0.002Zn. It is shown that intermetallic compounds of the following composition were recorded in the studied samples: AlFeSi₂ (with an admixture of Ca), FeSi₂ (with an admixture of Al), FeSi₂Ti (with an admixture of Zr). To reduce the content of impurities in silicon, we selected the following acids as solvents: 10% H₂SO₄, HCl, and HNO₃, as well as 4% HF in various ratios. To study the possibility of reactions of interaction of intermetallic compounds with selected solvents, the values of the Gibbs energy change, which had negative values, were calculated. Experimental leaching of impurities was conducted on silicon samples with particle sizes of –200 μm under constant stirring using a magnetic stirrer. The process parameters included a temperature of 60°C, a liquid-to-solid ratio of 5 : 1, and a purification duration of 60 min. It was found that using a solvent mixture of sulfuric and hydrofluoric acids in a 1 : 1 ratio achieved the highest degree of silicon purification, with an impurity removal rate of 86.85%. It is shown that when using a mixture of sulfuric and hydrochloric acids in a ratio of 1 : 3, the degree of purification of metallurgical silicon was 41.48%. In this way, solvents that enable maximum purification of silicon from impurity elements were identified.