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.

The solubility of TiO 2 in cryolite-alumina melts at 1020 ЊC was measured; it decreased with increasing alumina concentration up to ϳ3.5 wt pct total oxide and then increased at higher alumina concentrations. The solubility was found to be 3.1 wt pct Ti in cryolite, and 2.7 wt pct Ti in an aluminasaturated melt. Modeling indicated that the most probable titanium species are TiOF 2 and Na 2 TiO 3 , which coexist in the solution; the former dominates at low alumina concentrations and the latter at high alumina concentrations. Additional unknown amounts of fluoride may also be associated with these species. Determination of the solubility of TiO 2 in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt pct Ti at 975 ЊC to 2.8 wt pct Ti at 1035 ЊC, the apparent partial molar enthalpy of dissolution of TiO 2 being 88 Ϯ 4 kJ mol Ϫ1 .

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The solubilities of CuO (at 1 atm oxygen pressure) and of Cu 2 O (in equilibrium with Cu) were measured in cryolite-alumina melts at 1020°C as functions of the alumina content of the melt, and of the NaF/AlF 3 ratio in melts saturated with alumina. The results enable separation of the contributions of Cu(I) and Cu(II) species in solution, and are interpreted in terms of the solute species CuF, CuAlO 2 , and Na 2 CuF 4 . In alumina-saturated melts with NaF/AlF 3 = 3 the solubility of CuO is given by log (wt% Cu) = -9664/T + 7.0128, where T is temperature in kelvins, and of Cu 2 O by log (wt% Cu) = -8677/T + 6.173.