For the application of Na₂CO₃ slag to the elimination of iron from molten copper, the thermodynamic properties of the Cu–O–Fe system and the distribution behavior of iron between the NaO_0.5–CO₂–FeO_1.5 slag and molten copper have been investigated at 1523 K. The first order interaction parameter in molten copper was determined as,
ε_Fe^O=−141 (1523 K)
by the equilibrium distribution experiments between the NaFeO₂-saturated slag and molten copper. By a combination of the value obtained above with the activity of FeO_1.5 determined as a function of the slag composition reported in the previous paper, the distribution ratios of iron between the slag and molten copper can be estimated over the composition range of 0.5<N<1{N=n_Na⁄(n_Na+n_Fe), n: the number of moles} under the partial pressure of CO₂ of 0.01∼0.08 MPa.
The lower CO₂ partial pressure and the higher oxygen partial pressure are more preferable for the removal of iron from molten copper. However, due to the strong affinity between oxygen and iron in molten copper, the iron distribution ratio has a maximum value when oxygen partial pressure is 0.5 Pa. The ratio decreases with decreasing FeO_1.5 content in the NaFeO₂-saturated slag. From the dependence of the ratio on the oxygen partial pressure in the homogeneous liquid slag, it was considered that iron exists as a trivalent form in the slag.