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 FeO
x–CaO–SiO
2–P
2O
5 slag with various FeO
x and P
2O
5 contents, and CaO/SiO
2 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–FeO
x phase adjacent to solid CaO, and the CaO–SiO
2 or CaO–SiO
2–P
2O
5 solid phase coexisting with the FeO
x–CaO–SiO
2 liquid slag next to the CaO–FeO
x phase were observed for all slag compositions, temperatures and reaction times. Phosphorus was condensed as 2CaO·SiO
2–3CaO·P
2O
5 phase more easily in the case of higher CaO/SiO
2 ratio and higher FeO
x content in slag. There was a linear relationship between P
2O
5 content in 2CaO·SiO
2–3CaO·P
2O
5 phase and the distance from CaO–FeO
x phase to 2CaO·SiO
2–3CaO·P
2O
5 phase. The P
2O
5 content increased from CaO–FeO
x boundary toward bulk slag, and P
2O
5 content in the condensed phase near the CaO–FeO
x phase increased with increasing reaction time. This P
2O
5 concentration gradient tended to diminish. These results suggest that the condensation of phosphorus as 2CaO·SiO
2–3CaO·P
2O
5 phase was controlled by P
2O
5 diffusion from bulk slag to reaction interface, not by absorption of P
2O
5 into 2CaO·SiO
2 particle.
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