Structure Design and Phase Stability Analysis of Ca₂SiO₄-Based Solid-Solution Crystal in Molten Slag

Abstract

It is known that dephosphorization of molten iron is promoted by Ca₂SiO₄ precipitates in molten slag because they form a solid solution with Ca₃P₂O₈. Crystal structure of the Ca₂SiO₄ precipitate is important because it strongly affects phosphorus solubility. Although the α phase of the solid solution shows extremely high phosphorus solubility at high temperatures, its phase transition easily occurs to the α’ phase that has low phosphorus solubility when iron oxide is incorporated in slag. Phase stability of Ca₂SiO₄ crystal strongly depends on the solubility of foreign components, which influence the ionic configuration. To explore the optimum composition for enhancing structural stability of the α phase, this study conducted a structure design of Ca₂SiO₄-based solid-solution crystal by first-principles calculation based on density functional theory. The effect of foreign component solubility on the α phase stability was evaluated by free energy change of solid-solution formation. It was found that Ca²⁺ substitution with Fe²⁺ in α-Ca₂SiO₄ crystal makes the structure unstable, whereas Ba²⁺ incorporation enhances stability. In the latter case, oxygen ion configuration becomes distorted and the structure is relaxed. High-temperature in-situ X-ray diffraction analysis was performed to observe precipitation of the Ca₂SiO₄-based solid solution in a molten slag and its phase transition with decreasing temperature. The α phase of the Ca₂SiO₄-based solid solution initially precipitated at 2073 K, while the α→α’ phase transition and precipitation of the calcium ferrite phase occurred at temperatures lower than 1673 K. It was verified that the Ba-bearing Ca₂SiO₄ precipitates in slag contained significant phosphorus content.