Phase Identification of Crystal Precipitated from Molten CaO–SiO₂–FeO_x–P₂O₅ Slag by High Temperature In-situ X-ray Diffraction

Abstract

For the first time, we have succeeded in directly identifying the crystalline phase precipitated from the fully liquid slag of the CaO–SiO₂–FeO_x–P₂O₅ system by high-temperature in-situ X-ray diffraction analysis. Dephosphorization from molten iron can be promoted by 2CaO·SiO₂ precipitates in molten P₂O₅-containing slag because they form a solid solution with 3CaO·P₂O₅. Knowledge of the crystal structure of the 2CaO·SiO₂ precipitate is important because it strongly affects the phosphorus solubility. Although it is believed that the α phase of the 2CaO·SiO₂–3CaO·P₂O₅ solid solution precipitates from the molten slag, the crystal structure of the precipitate has not been identified because the crystal structure of the 2CaO·SiO₂ compound rapidly changes by phase transition when cooled from high temperature. In this study, slag samples were aerodynamically levitated and completely melted by laser heating under an Ar atmosphere, and then the diffraction patterns were obtained by transmitting a high-energy and high-intensity X-ray beam into the molten slag. We verified that the α-2CaO·SiO₂ phase precipitated as the primary crystal phase from molten slag containing 10–30 mass% FeO_x and 5 mass% P₂O₅, whereas nagelschmidtite precipitated for the molten slag with high P₂O₅ content. The α-2CaO·SiO₂ precipitates contained much higher FeO_x content than the reported solubility limit, which was supported by the diffraction angles positively deviated from those of the FeO_x-free α phase in the CaO–SiO₂–P₂O₅ system and chemical analysis of the quenched slag sample. This excess FeO_x solute may influence the phosphorus distribution in the α-2CaO·SiO₂ precipitates.