Sulfide Capacity of the CaO–SiO₂–MnO Slag at 1873 K

Abstract

The sulfide capacity of the CaO–SiO₂–MnO slag through the entire composition range was measured at 1873 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 (a_MnO) and the activity coefficient of MnS (γ_MnS), respectively, the composition dependency of sulfide capacity was well described by the changes in the ratio of a_MnO to γ_MnS. The iso–sulfide capacity of the CaO–SiO₂–MnO slag at 1873 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.