Thermodynamic Activity of MnO in CaO–SiO<sub>2</sub>–MnO–FeO–MgO Molten Slags

In order to understand the thermodynamic characteristics of smelting process for high manganese twinning-induced plasticity (TWIP) steel, the activity coefficient of MnO in molten slag of MnO–CaO–SiO2–FeO–MgO slag system with up to 52 mass% MnO was measured at 1450°C by the experiments of the manganese equilibrium between liquid silver and molten slag under the mixed gas atmosphere of CO, CO2 and Ar. The distribution ratio of manganese LMn between the molten silver and molten slag, and the equilibrium concentration quotient K' were also measured. The effects of MnO, FeO and basicity on the activity coefficient of MnO, the distribution ratio of manganese and the equilibrium concentration quotient were discussed. The activity coefficient of MnO, the distribution ratio of manganese and the equilibrium concentration quotient as a function of the concentration of the components in molten slag were investigated by regression analysis method. The results show that: (a) The Raoultian activity coefficients of MnO in molten slag are less than unity. (b) When the basicity B = 0.77–1.15, MnO = 15.8–52.9 mass% and FeO = 7–20 mass%, the activity coefficient of MnO increases as MnO content increases, but the LMn and the K' decrease as MnO content increases. (c) When B = 0.89–1.08, MnO = 43.6–46.9 mass% and FeO = 7–20 mass%, the activity coefficient of MnO increases as FeO content increases, but the LMn and the K' decrease as FeO content increases. (d) When MnO = 40–45 mass%, FeO = 9–14 mass% and B = 0.7–1.8, the activity coefficient of MnO increases as the basicity increases, but the LMn and the K' decrease as the basicity increases.