Reasons are outlined for thinking that accepted values for the activity of Al 2 O 3 in solution in cryolite are in error, and a new analysis is made of literature cryoscopic data. A general treatment is given for the effect of incongruent melting caused by the nonstoichiometry of solid cryolite. The effect is substantial, and accurate calculations require a knowledge of the effect of the solute on the activity of NaF in the solution, since that controls the composition of the solid separating. The results in the case of alumina as solute show that the slope of the log (activity) vs log (concentration) plot is about 3 in dilute solution and 1.5 in concentrated solution, corresponding to species containing 1 and 2 atoms of oxygen, respectively. Any calculations employing the earlier relation ϰ (wt pct a Al O 2 3 Al 2 O 3 ) 2.77 , which was based on electrochemical measurements of activity, need to be redone.
The vapor pressures of liquid mixtures of sodium chloride and aluminum chloride have been measured over a range of compositions from 50 to 67 mole % aluminum chloride. The results have been correlated with the published phase diagram. A compound, presumed to be NaAICl<, exists in the gas phase above mixtures containing around 50 mole % aluminum chloride.
The solubilities of FeO, FeAl 2 O 4 , NiO, and NiAl 2 O 4 were measured in cryolite-alumina melts at 1020 ЊC. FeO was found to be the stable solid phase at alumina concentrations below 5.0 wt pct, while FeAl 2 O 4 was stable above that. The corresponding figure for the nickel system was 3.0 wt pct Al 2 O 3 . These values correspond to Gibbs energies of formation of the aluminates (from the constituent oxides) of Ϫ17.6 and Ϫ29 kJ/mol, respectively. In alumina-saturated melts in the range 980 to 1050 ЊC, the solubilities of both aluminates increased with increasing temperature, the apparent enthalpies of solution being 65 kJ/mol for FeAl 2 O 4 , and 249 kJ/mol for NiAl 2 O 4 . Investigation of the solubilities of the aluminates as a function of the NaF/AlF 3 ratio in alumina-saturated melts at 1020 ЊC showed maxima at a molar ratio of around 5. The results are discussed in terms of the species apparently existing in the solution, and are consistent with the solute species being fluorides, not oxyfluorides. The activity coefficients of FeF 2 (liquid) and NiF 2 (solid) in dilute solution in cryolite are found to be 0.22 and 1.2, respectively.
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