This study investigated the segregation behaviour of the phosphorus rich (P-rich) phase from the iron-rich (Fe-rich) phase in steelmaking slag from laboratory to industrial scale using computational fluid dynamics (CFD) in order to propose a suitable separation practice for the P-rich phase. Crucible experiments and corresponding CFD simulations using enthalpy-porosity approach were first performed to verify the concentration difference of phosphorus in the two phases. Both simulation and experimental results showed ~ 17 to 18 pct increase in phosphorus concentration in the top region of the crucible after solidification. The simulations were then scaled up to an industrial slag pot and slag pit. Reasonable agreement was obtained with published results for phosphorus concentration, and the total liquid amount in the 16-tonne slag pot, under practical cooling conditions. Simulations in the 30-tonne slag pit with in-ground insulation showed an increase of ~ 25 pct of the P-rich phase in the top region (while concentrating the Fe-rich phase in the bottom region). Differential concentration of the P-rich phase within the slag (as a result of heat transfer, micro/macro-segregation) suggested that separation of phosphorus in industrial scale slag pot and slag pit—as batch systems—is possible. Suggestions for separation in continuous operation are also discussed.