Abstract: A comprehensive effect of Al₂O₃ content and binary basicity B on the enrichment behavior of phosphates in rapidly quenched CaO-SiO₂-FeO-P₂O₅-Fe₂O₃-Al₂O₃ steelmaking slags was investigated through thermodynamic calculations and experimental determination. It is found that the determined mass percentage of P₂O₅ in the phosphate-enriched phase nC₂S-C₃P depends on the forming of free C₂S in the steelmaking slags and increasing the SiO₂ content can result in an obviously decreasing tendency of formed C₂S. Adding Al₂O₃ as a dilute agent for decreasing the viscosity and melting point of the steelmaking slags can be easily bonded with free C₂S to form a gehlenite(C₂AS) solid solution. Thus,adding Al₂O₃ in the steelmaking slags can decrease the amount of free C₂S in the steelmaking slags,which affects phosphate enrichment. In order to obtain a greater phosphate enrichment in the steelmaking slags,a coupling relationship between the binary basicity B and the mass percentage of Al₂O₃ in the initial steelmaking slags should be kept as(% Al₂O₃)=-27.70 + 21.62 B under the conditions of(Al₂O₃) 〈20.0% and the binary basicity B〉1.3.