Phase Equilibria in High Phosphate-Containing Slag without CaO Saturation at Elevated Temperature

Abstract

Japan relies on imports for almost the entire amount of phosphorus, which is indispensable for human life and industrial materials. Therefore it is required to recover phosphorus from dephosphorization slag and sewage sludge which are promising unutilized phosphorus resources. Aiming wholesale recovery of phosphorus from the slag and sludge, a fundamental study on condensation of phosphorus in solid phase through high temperature phase separation was carried out. The experiments at 1 573 K on principal [CaO-SiO 2 -P 2 O 5 ] ternary model sample and Al 2 O 3 and Fe 2 O 3 added therein showed unsaturation with CaO and precipitation of solid 3CaO•P 2 O 5 as the phosphorus-concentrated phase in all the sample. The contamination of the other components in 3CaO•P 2 O 5 was low, especially less than 1 mass% for Al 2 O 3 added samples. The liquid phase was also formed with Al 2 O 3 or Fe 2 O 3 addition, and the liquidus composition in quaternary samples were consistent with those appeared in [CaO-SiO 2 -Fe 2 O 3 ] and [CaO-SiO 2 -Al 2 O 3 ] ternary diagram. P 2 O 5 content in the quaternary liquid phases were lower in Al 2 O 3 added samples than those in Fe 2 O 3 added samples. Such difference was discussed in terms of optical basicity and CaO activity for liquid phase. Due to the coexistence of solid SiO 2 phase at Fe 2 O 3 addition, the SiO 2 content in liquid phase became lower compared to Al 2 O 3 addition, and led to higher optical basicity, which would allow higher P 2 O 5 capacity in liquid phase. These considerations strongly suggest that an effective control of composition would be key technology for the phosphorus recovery through condensation and separation of phosphorus concentrated phase.