Basic Evaluation of Phase Relation in a Phosphorus-Containing System Saturated with CaSiO₃ at Elevated Temperatures for the Utilization of Steelmaking Slag and Sewage Sludge as Phosphorus Resources

In view of obtaining fundamental information on phosphorus recovery from steelmaking slag and sewage sludge, a laboratory experiment using the model specimen of a slag/sludge mixture prepared at 1573 K was carried out to investigate phase relation in a [CaO-SiO2-P2O5]-based system. The triangular compositional region, comprising of apices CaO·SiO2 (CS), 3CaO·P2O5 (C3P), and 2CaO·SiO2-3CaO·P2O5 solid solution (C2S-C3Pss), was considered with particular interest. In this region, using SEM-EDX observation it was found that solid saturated CS and the solid C2S-C3Pss with a relatively high phosphorus content can coexist. With the addition of Al2O3 or Fe2O3 to the same specimens, the liquidus phase appeared as a third phase; however, CS and C2S-C3Pss phases were still observed for up to 5mass% addition. The further addition of Al2O3 or Fe2O3 to 10mass% resulted in dissolution of the solid CS phase, although C2S-C3Pss remained as the phosphorus concentrated phase. These results show that phase equilibria based on the ternary system would be stable and be beneficial for phosphorus recovery