Iron redox equilibria in BOF slag equilibrated with ambient air

The Fe²⁺/Fe³⁺ ratio in the CaO‐MgO‐Fe₂O₃‐FeO‐SiO₂ based slag was measured under the condition of equilibrium with the ambient air at 1873 K as a fundamental study for precise slag coating control in BOF operation. The CaO/SiO₂ mass ratios of the main slag were 1, 1.5, 2, 3 and 4, and total iron mass content was in the range of 10 to 35 %. Moreover, mass contents of 1 to 13 % of MnO and 2 to 12 % of Al₂O₃ were added to the melt to evaluate their effects on the Fe²⁺/Fe³⁺ ratio. The effect of slag composition on the Fe²⁺/Fe³⁺ ratio was discussed and quantified into a form of formula. As the basicity in slag increases, the Fe²⁺/Fe³⁺ ratio decreases. The effect of iron oxide mass content is observed to be dependent on the basicity of slag. An increase in iron oxide mass content makes the Fe²⁺/Fe³⁺ ratio higher for basic slag but lower for acidic slag. It is revealed that the redox reaction of iron oxide in steelmaking slag under the ambient air is controlled by the complex anion formation reaction of iron oxide, and that the iron oxide in basic slag exists in the form of 2 or more kinds of complex anion controlling the oxygen anion content. Both Al₂O₃ and P₂O₅ increase the Fe²⁺/Fe³⁺ ratio by diluting the basic oxides as SiO₂ does, while manganese oxide lowers the Fe²⁺/Fe³⁺ ratio enormously down to nearly zero. The Fe²⁺/Fe³⁺ ratio can be described as a function of slag composition, X = (%CaO) + 0.38(%Fe₂O₃ + %FeO)+3.2(%MnO), in the equation of log(Fe²⁺/Fe³⁺) = ‐0.00107X² + 0.0721X ‐ 1.982.