Thermodynamics of NaO0.5-CO2-SnO2 Slag and Estimation of Distribution Ratio of Sn between the Slag and Molten Copper

In view of the application of sodium carbonate slag to the elimination of Sn from molten copper, the thermodynamic properties of NaO_0.5-CO₂-SnO₂slag have been studied by conducting the following experiments: (1) Activities of NaO_0.5in the slag were determined by the EMF method using β“ -alumina as a solid electrolyte over the composition range of 1>N≥ 0.795{N=n_Na+n_sn)>,n: the number of moles) under the partial pressures of CO₂of 0.001~0.08 MPa at 1423 ~ 1523 K. It was found that the activity of NaO _0.5did not depend on the slag composition for the constant partial pressure of C02 and the slag was saturated with Na₂SnO₃(s). (2) Then the solubility of Na_2Sn03(s) into sodium carbonate melt was measured under the various partial pressures of CO₂and oxygen at 1523 K. With decreasing partial pressure of CO₂the solubility increased, but the partial pressure of oxygen had no effect. (3) Activities of SnO₂were determined by the distribution equilibrium measurements for Sn between the Na₂SnO₃(s)-saturated slag and molten copper. From the activity of SnO₂combined with that of NaO_0.5, the standard free energy change for the following reaction was determined as: 2NaO_0.5(l) + Sn02(s)=Na₂SnO₃(s) ΔG°/kJ=-159(1523 K) The activity of SnO₂in the homogeneous liquid region of the slag was also evaluated by the distribution equilibrium measurements. On the basis of the results obtained, the distribution ratios of Sn between the slag and molten copper were estimated. The ratios decrease with decreasing SnO₂content in the Na₂SnO₃(s)-saturated slag. The higher oxygen partial pressure and the lower CO₂partial pressure are more preferable for the removal of Sn from molten copper.