The MgO solubility in the CaO-MgO-Fe_2O_3-FeO-SiO_2-(MnO)-(Al_2O_3) 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/SiO_2 mass ratios of the main slag were 1, 1.5, 2, 3 and 4, and total iron content was in the range of 10 to 35 percent. Moreover, 1 to 13 percent of MnO and 2 to 12 percent al Al_2O_3 were added to the melt to evaluate their effects on the MgO solubility. The effect of slag composition on the MgO solubility was discussed and quantified by means of a newly developed formula. As the basicity in slag increases, the MgO solubility decreases. The effect of iron oxide content is observed to be dependent on the basicity of slag. An increase in iron oxide content makes the MgO solubility higher for basic slag but lower for acidic slag. It is revealed that the MgO solubility in steelmaking slag is controlled by the complex anion formation reaction of iron oxide. Both Al_2O_3 and P_2O_5 increase the MgO solubility by diluting the basic oxides as SiO_2 does, while manganese oxide affects the MgO solubility in a similar manner as iron oxide. The MgO solubility can be described as a function of slag Composition, X = (percent CaO) + 0.45(percent Fe_2O_3 + percent FeO) + 0.55 (percent MnO), in the equation of (percent MgO) =0.00816X~2-1.404X + 62.31. Based on the results, the guidance for addition of MgO-containing material could be suggested for best slag coating practice.
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