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

Abstract

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.