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

Potential vs. pH diagrams for the systems Fe–Si–(Cl−)–(F−)–H2O were generated using the HSC5.0 software and applied to purification of metallurgical-grade silicon (MG-Si) by hydrometallurgical methods. The diagrams for the Fe–Si–H2O system show that with increasing electrochemical potential (Eh) the order of appearance of Fe silicides is FeSi2, FeSi, and Fe3Si. Further, from a thermodynamic standpoint, Si in the Fe silicides (FeSi2, FeSi, and Fe3Si) would be easier to oxidize than Fe, due to the lower potentials of the corresponding silicides, compared with Fe. The early formation of SiO2 during dissolution may prohibit or retard 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−–H2O system indicate that dissolution of Fe silicides with relatively low Si content, such as FeSi and Fe3Si, 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−–H2O system suggest that the dissolution of Fe silicides would be enhanced by decreasing temperature or removing FeF2(s) (e.g., via ultrasonication) during the leaching process.

► Plotting of potential-pH diagrams for the systems of Fe-Si-(Cl)-(F)-H2O.
► Prediction of Fe silicides speciation and stability.
► Dissolution and passivation behavior of Fe and Si.