Thermodynamic Properties of NaO_0.5-CO₂-FeO_1.5 Slag

Thermodynamic properties of NaO_0.5-CO₂-FeO_1.5 slag have been investigated for the application of sodium carbonate slag to the elimination of Fe from molten copper. The activities of NaO_0.5 and Na(CO₃)_0.5 were measured by the EMF method, using β-alumina as a solid electrolyte for various partial pressures of C02 at 1423 to 1523 K. There is a stable solid compound NaFeO₂ in this system, and the solubility measurement of this compound into sodium carbonate melt at 1523 K revealed that the solubility is highly limited and controlled by the partial pressure of CO_2. The dissolution reaction seems to proceed according to the following reaction and Fe exists as FeO^3- ₃ anion in the sodium carbonate-based melt. 2Na(CO₃)_0.5 (l) + NaFeO₂ (s)=Na₃ FeO₃ (l)+ CO₂ (g). The standard free energy change for the following reaction was determined as, NaO_0.5 (l)+FeO_1.5 (s)=NaFeO₂ (s) ΔG^°/J=-115000-10.33Tln T+98.32T(1283~1423 K) by the EMF method. The activity of FeO_1.5 in the NaFeO₂ (s)-saturated slag could be calculated by the above equilibrium relation from the activity of NaO_0.5 The activity of FeO_1.5 in the homogeneous liquid region of the slag was also evaluated by the distribution equilibrium measurements. The activity coefficient of FeO_1.5 in the homogeneous liquid region has a constant value for a constant CO₂ partial pressure, independent of the concentration.