Transition of Blast Furnace Slag from Silicate Based to Aluminate Based: Sulfide Capacity

The effect of Al₂O₃ and Al₂O₃/SiO₂ ratio on the sulfide capacity of the molten aluminosilicate CaO-SiO₂-Al₂O₃-MgO-TiO₂ slag system with high Al₂O₃ content was measured at 1773 K (1500 °C) using a metal-slag equilibration method. The sulfide capacity between silicate-based and aluminate-based slag was also compared based on the thermodynamic analysis and structural characteristics of melts. At a fixed CaO/SiO₂ ratio of 1.20, the sulfide capacity decreases with increasing Al₂O₃ content primarily due to the decrease of free oxygen (FO) and the activity of O^2–. Increasing the Al₂O₃/SiO₂ ratio from 0.47 to 0.79 causes a significant increase in the sulfide capacity of the slags, and a slight increase is found when the Al₂O₃/SiO₂ ratio is more than 0.79. The effect of the substitution of silica by alumina on the sulfide capacity of the slags was not only due to an increase in the activity of basic oxides (\( a_{{{\text{O}}^{2 - } }} \)) but also to a decrease in the stability of sulfide (\( \gamma_{{{\text{S}}^{2 - } }} \)). Moreover, \( a_{{{\text{O}}^{2 - } }} \) and \( \gamma_{{{\text{S}}^{2 - } }} \) increase in a similar degree, and the weaker binding electronegativity of Al³⁺ with oxygen atoms results in a slight increase in the final sulfide capacity in the aluminate-based slag system with Al₂O₃ ↔ SiO₂ substitution. Five different sulfide capacity models were employed to predict the sulfide capacity, and the iso-sulfide capacity distribution diagram based on the Young’s model was obtained in the high Al₂O₃ corner of the diagram.