Dissolution windows for hydrometallurgical purification of metallurgical-grade silicon to solar-grade silicon: Eh-pH diagrams for Fe silicides

摘要

Potential vs. pH diagrams for the systems Fe-Si-(Cl~- )-(F~-)-H_2O were generated using the HSC5.0 software and applied to purification of metallurgical-grade silicon (MG-Si) by hydrometaliurgical methods. The dia grams for the Fe-Si-H_2O system show that with increasing electrochemical potential (Eh) the order of ap pearance of Fe silicides is FeSi_2, FeSi, and Fe_3Si. Further, from a thermodynamic standpoint. Si in the Fe silicides (FeSi_2, FeSi, and Fe_3Si) would be easier to oxidize than Fe, due to the lower potentials of the corresponding silicides, compared with Fe. The early formation of SiO_2 during dissolution may prohibit or re tard further dissolution of Fe, as verified by previous experimental results. This inhibitive effect is expected to increase with increasing Si content in the silicides. The diagrams for the Fe-Si-Cl~- -H_2O system indicate that dissolution of Fe silicides with relatively low Si content, such as FeSi and Fe_3Si, is enhanced by adding chloride ions to acidic solutions. In the presence of HF, formation of fluoro-complexes enlarges the stability domains of dissolved iron and silicon, which increase with increasing {F} and/or decreasing temperature. The trends obtained for the Fe-Si-F~- -H_2O system suggest that the dissolution of Fe silicides would be enhanced by de creasing temperature or removing FeF_(2(S)) (e.g., via ultrasonication) during the leaching process.