Nucleation and growth control for iron- and phosphorus-rich phases from a modified steelmaking waste slag

Recovering the iron (Fe) and phosphorus (P) contained in steelmaking slags not only reduces the environmental burden caused by the accumulated slag, but also is the way to develop a circular economy and achieve sustainable development in the steel industry. We had previously found the possibility of recovering Fe and P resources, i.e., magnetite (Fe₃O₄) and calcium phosphate (Ca₁₀P₆O₂₅), contained in steelmaking slags by adjusting oxygen partial pressure and adding modifier B₂O₃. As a fundamental study for efficiently recovering Fe and P from steelmaking slag, in this study, the crystallization behavior of the CaO–SiO₂–FeO–P₂O₅–B₂O₃ melt has been observed in situ, using a confocal scanning laser microscope (CLSM). The kinetics of nucleation and growth of Fe- and P-rich phases have been calculated using a classical crystallization kinetic theory. During cooling, a Fe₃O₄ phase with faceted morphology was observed as the 1st precipitated phase in the isothermal interval of 1300–1150°C, while Ca₁₀P₆O₂₅, with rod-shaped morphology, was found to be the 2nd phase to precipitate in the interval of 1150–1000°C. The crystallization abilities of Fe₃O₄ and Ca₁₀P₆O₂₅ phases in the CaO–SiO₂–FeO–P₂O₅–B₂O₃ melt were quantified with the index of (T_U − T_I)/T_I (where T_I represents the peak temperature of the nucleation rate and T_U stands for that of growth rate), and the crystallization ability of Fe₃O₄ was found to be larger than that of Ca₁₀P₆O₂₅ phase. The range of crystallization temperature for Fe₃O₄ and Ca₁₀P₆O₂₅ phases was optimized subsequently. The Fe₃O₄ and Ca₁₀P₆O₂₅ phases are the potential sources for ferrous feedstock and phosphate fertilizer, respectively.