Interactions of gases in molten salts: Carbon dioxide and oxygen in cryolite alumina melts

Abstract

A radiotracer method has been developed to measure the solubility of CO2 in cryolite-based systems at temperatures above 1273 K and also a method to measure the solubility of oxygen. The high solubility of CO2 in cryolite (0.34 pct A12O3, 1.68 pct CaF2), and the highly negativeAH of solution, suggest complex formation. In the presence of 5.33 pct excess A12O3, the solubility of CO2 is several times greater than in cryolite. In contrast, oxygen shows a solubility about a thousandth that of CO2. The entropy of CO2 in cryolite indicates a loss of translational degrees of freedom in a manner consistent with complexing. However, in the presence of increased A12O3 (in spite of the increase of solubility) the entropy change upon solution appears to indicate little loss of translational freedom. It is suggested that CO2 is complexed in both systems, but more strongly in the cryolite system. Complexing in cryolite produces the small carbonate (CO- 3)ion, and this compensates the loss of translational freedom arising from the CO2 complexing with aluminofluorides. Measured solubilities for CO2 were 1.0 X 10-3 moles liter−1 atm-1 in cryolite at 1273 K, 4.6 x 10-3 moles liter−1 atm-1 in (NaF)2:3 A1F3 + 5.33 wt pct A12O3 at 1233 K. At 1230 K, oxygen showed a solubility of only 3.0 X 106 moles liter−1 atm-1 in the latter melt.