It is quite important to reveal the microscopic reaction mechanisms and the role of the solid and liquid phases in the solid CaO coexisting flux in the hot metal dephosphorization process. In the present study, solid CaO piece and FeOx–CaO–SiO2–P2O5 slag with various FeOx and P2O5 contents, and CaO/SiO2 ratios of the slag were reacted at 1573 and 1673K. The interface between solid CaO and molten slag was observed and analyzed by SEM/EDS. Microscopic reaction mechanisms between solid CaO and molten slag was discussed with changing reaction times, slag compositions and temperatures. The CaO–FeOx phase adjacent to solid CaO, and the CaO–SiO2 or CaO–SiO2–P2O5 solid phase coexisting with the FeOx–CaO–SiO2 liquid slag next to the CaO–FeOx phase were observed for all slag compositions, temperatures and reaction times. Phosphorus was condensed as 2CaO·SiO2–3CaO·P2O5 phase more easily in the case of higher CaO/SiO2 ratio and higher FeOx content in slag. There was a linear relationship between P2O5 content in 2CaO·SiO2–3CaO·P2O5 phase and the distance from CaO–FeOx phase to 2CaO·SiO2–3CaO·P2O5 phase. The P2O5 content increased from CaO–FeOx boundary toward bulk slag, and P2O5 content in the condensed phase near the CaO–FeOx phase increased with increasing reaction time. This P2O5 concentration gradient tended to diminish. These results suggest that the condensation of phosphorus as 2CaO·SiO2–3CaO·P2O5 phase was controlled by P2O5 diffusion from bulk slag to reaction interface, not by absorption of P2O5 into 2CaO·SiO2 particle.
