The authors have found in their previous work that phosphorus exhibits remarkable segregations in the industrial hot metal pretreatment slag and it exists as Ca₃P₂O₈–Ca₂SiO₄ solid solution together with FeO–CaO–SiO₂–MnO–MgO matrix. Since their magnetic property of each phase is significantly different, it is possible to separate each phase with the aid of superconducting strong magnetic field. By applying strong magnetic field of 0.5 to 2.5 T to the crushed slag, more than 60% of phosphorus concentrated phase in the slag has been recovered. If the most of phosphorus can be removed from the slag, the residual slag is basically FeO–CaO–SiO₂–MnO–MgO with less P₂O₅ and thus it may be recycled to iron and steelmaking processes such as the sintering, hot-metal desiliconization, and hot-metal dephosphorization processes. In the present work, the recycling effect of the residual slag to the dephosphorization process is simulated based on the mass balance calculation. The significant reduction of total slag generation and CaO input has been demonstrated by the mathematical model considering phosphorus recovery and recycling of residual slag as a dephosphorization agent. It has also been indicated by the Waste Input–Output model that the phosphorus recovery from dephosphorization slag and the recycling of residual slag to hot metal dephosphorization process have great environmental and economical benefits.