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

For the first time, we have succeeded in directly identifying the crystalline phase precipitated from the fully liquid slag of the CaO–SiO2–FeOx–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% FeOx and 5 mass% P2O5, whereas nagelschmidtite precipitated for the molten slag with high P2O5 content. The α-2CaO·SiO2 precipitates contained much higher FeOx content than the reported solubility limit, which was supported by the diffraction angles positively deviated from those of the FeOx-free α phase in the CaO–SiO2–P2O5 system and chemical analysis of the quenched slag sample. This excess FeOx solute may influence the phosphorus distribution in the α-2CaO·SiO2 precipitates.