The thermodynamics of manganese oxide and iron oxide and the phosphate capacity of CaO-SiO 2 -MnO-Fe t O-P 2 O 5 -MgO sat slags with high MnO contents relevant to the smelting of MnO ores in steelmaking were investigated. Previous data were limited to about 5 pct MnO, whereas in this study MnO contents up to 25 pct were studied. The activity of MnO showed positive deviation from ideal behavior and increased with basicity, while that of Fe t O decreased with basicity. This is reflected in that the manganese distribution ratio at a given Fe t O content decreases with basicity and high Mn in the metal is favored by high basicity. CaF 2 additions up to 4 pct did not affect the activities of MnO and Fe t O. The activity coefficient of P 2 O 5 decreases and the phosphate capacity increases with basicity. There was not adverse effect of high MnO contents on the dephosphorizing abilities of the slags.
In bath smelting, the FeO activity of the slag must be known to predict the equilibrium of slag-metal reactions and for effective control of the rate of reduction in the system. Also, knowledge of the solubility of MgO in these slags is useful for reducing refractory consumption. A series of measurements of the FeO activity in simulated bath smelting slags (CaO-SiO 2 -Al 2 O 3 -MgO sat -FeO) were conducted by the electromotive force (EMF) technique. The influence of the slag composition on the relationship between the FeO activity coefficient and FeO content was studied. It has been found that the measured FeO activity coefficient decreases with increasing FeO content in the slag and increases slightly with increasing slag basicity, which is defined as (CaO ϩ MgO)/(SiO 2 ϩ Al 2 O 3 ) on a mole fraction basis.The measured values of the FeO activity coefficient are in reasonable agreement with previously published data. The solubility of MgO was also measured and found to range from 16 to 30 pct and decrease with increasing basicity.
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