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

Abstract

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 CO₂ 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 FeO₃³⁻ anion in the sodium carbonate-based melt.
2Na(CO₃)_0.5(l)+NaFeO₂(s)=Na₃FeO₃(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=−115000−10.33TlnT+98.32T(1283∼1423K)
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.