The equilibrium relation of the phase separation in the Fe-Cu-B system is investigated at 1873 K for potential use in the separation of copper from iron in steel scrap recycling. Liquid immiscibility is confirmed even at low boron content in iron, [mass%B]_{(in Fe)}=0.006. By combining the Taylor series equation proposed by Wagner and that of the quadratic formalism proposed by Darken, the thermodynamic interaction parameters of copper for boron in molten iron, which can be applied with precision over the wide concentration range, are derived. The first and the second order interaction parameters of copper for boron in molten iron at 1873 K are:
ε_Cu^B=−ρ_Cu^Cu,B=12.1(±0.6),ρ_Cu^B=−18.9(±11.0) (N_{B(in Fe)}<0.34)
e_Cu^B=0.254(±0.014),r_Cu^B=−0.032(±0.014) ([mass%B]_{(in Fe)}<8.9)
Moreover, the immiscibility range in the Fe-Cu-B system is also investigated by applying a symmetrical and regular solution model.