Experiments were carried out to determine the ratio of Mn3+Mn2+ at equilibrium in CaO-SiO2-MruO-MgO slag under various slag compositions and oxygen partial pressures in temperature range of 1350-1450°C. As magnesia crucibles were used, the solubility limit of MgO could also be determined. Results indicate that the Mn3+Mn2+ ratio increases with slag basicity as well as with MniO content. The solubility of MgO, on the other hand, decreases as the slag basicity increases and also with increasing MruO content. The ratio (%Mn3+) /(%Mn2+) does not change in the oxygen partial pressure between 10-12atm and 10-6atm. At the constant oxygen partial pressure, the value of log(%Mn3') /(%Mn2+) does not change with the slag basicity less than 0. 9. On the other hand, the value of log(%Mn3) /(%Mn2+) decreases with increasing basicity of slag more than 0. 9. Although there is small deviation in the value of log (%Mn3') /(%Mn2+) , the value does not change in the slag composition containing more than 16% MntO. Temperature dependence of the ratio (%Mn3+) /(%Mn2+) at 5. 1×10-9atm in the temperature range of 1350 to 1450t; can be expressed as follows. From the expression, the reaction enthalphy was calculated to be -75kcal/mol. log (%Mn3+) /(%Mn2+) = 1. 64×104/T - 12. 3.
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