Copper distribution between FeS-NaS_0.5 flux and carbon saturated liquid iron has been studied at 1673 K. Copper distribution ratio, L_Cu=(mass%Cu)_flux/[mass%Cu]_iron, was about 9 when FeS only was used as sulfide flux. The addition of NaS_0.5 to FeS increased the copper distribution ratio. The L_Cu showed the maximum value of about L_Cu=24, at around X_NaS0.5=0.4, and became nearly constant at the range of higher NaS_0.5 content than 0.4 of NaS_0.5. The addition of NaS_0.5 also lowered sulfur content in liquid iron from 1.9 mass% at X_NaS0.5=0 to 0.04 mass% at X_NaS0.5=0.8. CuS_0.5 dissolved in flux decreased L_Cu at high concentration range of NaS_0.5.
In order to discuss the effect of NaS_0.5 and CuS_0.5 on the copper distribution, the activity coefficient of CuS_0.5 in flux was evaluated. It was found that NaS_0.5 decreased the activity coefficient of CuS_0.5 in flux, so that the L_Cu increased in spite of the significant decrease of sulfur potential in the system by the addition of NaS_0.5.