Liquid-Liquid Phase Separation and Phase Distribution in CaO–P2O5–FeO Slag for Phosphorus Recovery

Supposing to utilize the slag with high P2O5 content as a new phosphorus resource produced from a novel steelmaking process, a fundamental investigation was carried out with particular focus on spontaneous liquid-liquid phase separation occurring in the system CaO–P2O5–FeO for effective phosphorus recovery. At 1773 K, two types of phase separation, which is a double-layered or a dispersed structure of two liquid phases, were observed according to the bulk composition. The double layers separated vertically and consisted of the phase with higher P2O5 content over 40 mass%, and the phase with higher FeO content over 90 mass%. Such structure corresponds to the liquid-liquid separation in the compositional region which is located in higher P2O5 content (upper side) against the tie line between Ca3P2O8–FeO apex on the isothermal section of CaO–P2O5–FeO ternary system. At 1673 K, various separation was observed such as a double-layer or a dispersed structure of two liquid phases, and a coexistence of solid/liquid phases. The slag with the bulk composition lying in the upper two liquid region showed the double-layered structure, giving a promising result for phosphorus recovery through phosphate concentration. The condition for formation of double-layered structure was considered based on the index originally proposed for co-continuity in polymer blends processing, which consists of viscosity and volume fraction of the co-existing phases. The index was found to represent a specific condition for making co-continuous phases in this study and would be significant in view of effective phosphorus recovery.