Thermodynamic investigation of vanadium oxide in CaO–SiO₂–VO_x system at 1873 K

Owing to the complexity of the multivalence states of vanadium oxides in slag systems and experimental difficulties, thermodynamic properties of vanadium oxides have not been established yet. In the present study, the mixed-valence states and activities of the vanadium oxides in CaO–SiO₂–VO_x slag were investigated experimentally at 1873 K and oxygen partial pressures of 3.2 × 10^–9 and 3.1 × 10^–7 atm. After the CaO–SiO₂–VO_x slag had equilibrated with a platinum strip, the mixed-valence states of the vanadium oxides in the slag were estimated by performing X-ray photoelectron spectroscopy, and the activities of the vanadium oxides in the slag were calculated using the activity of vanadium in the platinum strip at equilibrium using thermodynamic equations. At an oxygen partial pressure of 3.2 × 10^–9 atm, V³⁺ was the dominant ion and V⁴⁺ was the second most abundant ion. With increasing VO_x content or basicity (CaO/SiO₂ ratio), the fraction of V³⁺ decreased, whereas that of V⁴⁺ increased. The activity of VO_1.5 was greater than those of the other vanadium oxides. On the other hand, when the oxygen partial pressure increased to 3.1 × 10^–7 atm, V⁴⁺ became the dominant ion. As the slag basicity increased, the fraction of V⁴⁺ increased further, whereas that of V³⁺ decreased to less than that of V⁵⁺. The activity of VO_1.5 was greater than those of the other vanadium oxides, limiting the effect of the slag basicity. Consequently, the valence state of vanadium oxide was affected by the slag basicity at a low oxygen partial pressure by acting as a network modifier. In contrast, at a higher oxygen partial pressure, the activity of vanadium oxide increased further but was not affected by the slag basicity because of its contribution to the network structure formation. The present findings can be applied to optimize the slag composition in steel refining or vanadium pentoxide production processes to increase the yield rate of vanadium.