Abstract: Composition and phase transformation mechanism in the oxidization process of CaO-FeOx-SiO2 (CFS) system with high iron content were investigated by comprehensive thermodynamic calculation and deduction. Variation of the iron valence states with oxygen equilibrium fractional pressure at different temperatures was calculated and phase microstructure was observed and confirmed by metallographic microscope, SEM, EDX, and XRD. Grain size and crystallizing quantity of magnetite were determined by imagine analyzer, and contents of iron in different valence states were obtained by chemical analysis. Thermodynamic regularity of oxidation process and magnetite (Fe3O4) precipitation were studied by calculation and confirmed by experiment data. Effects of CaO addition on enrichment of magnetite were also discussed. The results showed that, with increasing the oxygen partial pressure, content of magnetite grew up rapidly, became saturation and precipitate, iron was enriched into the magnetite phase. Main phases in the cooling down system were magnetite, fayalite (Fe2SiO4), and glass state silicate ((Fe, Ca)SiO4). In oxidizing process, contents of fayalite declined while those of magnetite increased. Above 1423 K, while keeping oxygen partial pressure lg(pO2 /p0)>-7.89, most iron was enriched into the magnetite phase and precipitated after cooling down, magnetite was always the first precipitated crystal phase. When molecule ratio n(Fe3+)/n(Fe2+) in the system was 1/4, initialmagnetite precipitate temperature was about 1640 K. As n(Fe3+)/n(Fe2+) ratio increased, crystal precipitated temperature became higher, and it was about 1720 K while n(Fe3+)/n(Fe2+)=1.8/1. Increasing the content of CaO in the systempromoted the enrichment of iron into magnetite phase during oxidizing process.

Key words: Oxidizing, Magnetite, Thermodynamics, Phase transformation, CaO-FeOx-SiO2 system