Mass transfer of P₂O₅ between liquid slag and solid solution of 2CaO · SiO₂ and 3CaO · P₂O ₅

Hot metal dephosphorization slags in the BOF can be considered to be within the CaO-SiO₂-FeO-P₂O₅ system, and are usually in the dicalcium silicate (C₂S) saturated composition range. It is well known that C₂S forms a pseudo-binary solid solution with tricalcium phosphate (C₃P) over a wide composition range at the dephosphorization treatment temperature. To increase the reaction efficiency of dephosphorization, it is important to increase the mass transfer rate of P ₂O₅ from the liquid slag to the solid solution. In order to clarify the mechanism of mass transfer of P₂O₅ between the solid solution and liquid slag, an artificially made C₂S-C ₃P solid solution rod was dipped into the C₂S-C ₃P saturated slag and the interface was observed. When the activity of P₂O₅ in liquid slag was higher than that in solid solution, a reaction layer was formed at the interface, and its width increased with immersion time. A concentration gradient of P₂O₅ was observed in the solid solution. When the activity of P₂O₅ in liquid slag was lower than that in solid solution, no reaction layer was formed, and P₂O₅ did not transfer to the liquid slag. In this case, P₂O₅ in the solid solution was quite stable. The reason for these phenomena was discussed. The mass transfer of CaO and SiO₂ must occur simultaneously with the mass transfer of P ₂O₅ in order to maintain the pseudo-binary relation of the solid solution.