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, 21, 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+/Fe-3 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+) = -0.00107X(2) + 0.0721X - 1.982.