The sulfide capacity of the CaO-SiO2-MnO slag through the entire composition range was measured at 1 873 K using a gas-slag equilibration method and the effect of basicity and the activity coefficient of sulfide on the sulfide capacity of molten slag was investigated. Furthermore, the relationship between the sulfide capacity and the optical basicity of MnO-containing slags was evaluated in view of industrial applications. Sulfide capacity of the slag linearly increased by increasing the content of MnO not only at a given silica content but also at a fixed Vee ratio (=CaO/SiO2). The capacity and the modified Vee ratio (=(CaO+MnO)/SiO2) showed a good linear relationship. Assuming that the basicity and the stability of sulfide ion in the slag are proportional to the activity of MnO (aMnO) and the activity coefficient of MnS (γMnS), respectively, the composition dependency of sulfide capacity was well described by the changes in the ratio of aMnO to γMnS. The iso-sulfide capacity of the CaO-SiO2-MnO slag at 1 873 K was constructed in the present study. The capacity contours seemed to rotate clock-wisely from the CaO-SiO2 binary side to the MnO-corner. The sulfide capacity increased with increasing ratio of MnO to CaO at relatively acidic region in which silica content greater than about 40 mass%, while the substitution of CaO by MnO does not significantly affect the capacity in the low silica region. The sulfide capacity generally increased with increasing content of MnO regardless of changes in the content of other constituents in multicomponent MnO-containing slags. It is necessary to take different values for the theoretical optical basicity of MnO in order to estimate the sulfide capacity of MnO-containing slags depending on the content of silica.

The sulfide capacities of the CaO-SiO2-MnO-Al2O3-5 mass% MgO slags were measured at 1 873 K over a wide composition range using a gas-slag equilibration method. The effects of basicity and the activity coefficient of sulfide on the sulfide capacity of molten slag were also investigated based on the structural view of silicate melts. In the multicomponent silicate melts containing high MnO (up to about 50 mass%), the sulfide capacity mainly increased with increasing MnO content. The capacity and modified Vee ratio, i.e. (CaO+MnO+MgO)/ (SiO2+Al2O3), showed a good linear relationship. Assuming that the basicity and the stability of sulfide ions in the slag are proportional to the activity of basic oxides and the activity coefficient of sulfides, the composition dependency of the sulfide capacity is well described by changes in the aMO to γMS (M=Ca, Mn) ratio. The substitution of silica by alumina did not affect the sulfide capacity of the slags not only because of an increase in the activity of basic oxides but also because of a decrease in the stability of sulfides as Al2O3/SiO2 ratio increased. In the high silica melts of which silica content greater than about 30 mass%, the sulfide capacity increased with increasing MnO/CaO ratio, whereas it decreased by increasing the MnO/CaO ratio in the low silica melts (< about 30 mass%). This tendency of sulfide capacity resulted in the clock-wisely rotating iso-capacity contours in the CaO-SiO2-MnO-Al2O3-MgO system at 1 873 K. The dissolution mechanism of sulfur in the MnO-containing calcium silicate melts can be explained not only by the difference in the structural role of Ca 2+ and Mn 2+ ions but also by the changes in the content of O 2-ions according to the silica content.KEY WORDS: sulfide capacity; CaO-SiO2-MnO-Al2O3-MgO slag; basicity; sulfide stability; silicate structure; free oxygen.