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

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 alpha phase of the solid solution shows extremely high phosphorus solubility at high temperatures, its phase transition easily occurs to the alpha ' 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 alpha 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 alpha phase stability was evaluated by free energy change of solid-solution formation. It was found that Ca2+ substitution with Fe2+ in alpha-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 alpha phase of the Ca2SiO4-based solid solution initially precipitated at 2 073 K, while the alpha ->alpha ' phase transition and precipitation of the calcium ferrite phase occurred at temperatures lower than 1 673 K. It was verified that the Ba-bearing Ca2SiO4 precipitates in slag contained significant phosphorus content.