Improving the quality of metallurgical-grade silicon by acid leaching

Abstract

This work deals with the problem of hydrometallurgical refining of metallurgical-grade silicon. Samples of metallurgical-grade silicon after oxidative refining from JSC Silicon, RUSAL (Shelekhov, Irkutsk Oblast, Russia) were subjected to X-ray fluorescence and electron microprobe analysis. The conducted elemental analysis determined their following composition, wt%: Al – 0.53, Fe – 0.6094, Ti – 0.0491, Ca – 0.0628, V – 0.0066, Cr – 0.002, Mn – 0.014, Cu – 0.003, P – 0.010, Ba – 0.007, Ni – 0.007, and Zn – 0.002. The examined samples were found to comprise the following intermetallic compounds: AlFeSi2 (with an admixture of Ca), FeSi2 (with an admixture of Al), and FeSi2Ti (with an admixture of Zr). In order to purify silicon from impurities, 10% H2SO2, HCl, HNO3, as well as 4% HF in different ratios were used as solvents. The feasibility of interactions between the intermetallic compounds and the selected solvents was assessed by calculating changes in the Gibbs energy, which had negative values. Experiments on impurity leaching were carried out using silicon samples with a particle size of –200 µm under constant stirring with a magnetic stirrer under the temperature of 600С, the liquid-to-solid ratio of 5:1, and the leaching duration of 60 min. The highest degree of silicon purification (86.85%) was achieved under leaching with a mixture of sulfuric and hydrofluoric acids in a ratio of 1:1. The use of a mixture of sulfuric and hydrochloric acids at a ratio of 1:3 resulted in the silicon purification of 41.48%. Thus, optimal solvents allowing the maximum purification of silicon from impurities were determined.