Thermodynamic properties of NaO0.5-CO2-FeO1.5 slag have been investigated for the application of sodium carbonate slag to the elimination of Fe from molten copper. The activities of NaO0.5 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 CO2. The dissolution reaction seems to proceed according to the following reaction and Fe exists as FeO3- 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, NaO0.5 (l)+FeO1.5 (s)=NaFeO2 (s) ΔG°/J=-115000-10.33Tln T+98.32T(1283~1423 K) by the EMF method. The activity of FeO1.5 in the NaFeO2 (s)-saturated slag could be calculated by the above equilibrium relation from the activity of NaO0.5 The activity of FeO1.5 in the homogeneous liquid region of the slag was also evaluated by the distribution equilibrium measurements. The activity coefficient of FeO1.5 in the homogeneous liquid region has a constant value for a constant CO2 partial pressure, independent of the concentration.
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