Solubility of Some Transition Metal Oxides in CryoliteAlumina Melts: Part I. Solubility of FeO, FeAl sub 2 O sub 4 , NiO, and NiAl sub 2 O sub 4

The solubilities of FeO, FeAl sub 2 O sub 4 , NiO, and NiAl sub 2 O sub 4 were measured in cryolite-alumina melts at 1020 deg C. FeO was found to be the stable solid phase at alumina concentrations below 5.0 wt pct, while FeAl sub 2 O sub 4 was stable above that. The corresponding figure for the nickel system was 3.0 wt pct Al sub 2 O sub 3 . These values correspond to Gibbs energies of formation of the aluminates (from the constituent oxides) of -17.6 and -29 kJ/mol, respectively. In alumina-saturated melts in the range 980 to 1050 deg C, the solubilities of both aluminates increased with increasing temperature, the apparent enthalpies of solution being 65 kJ/mol for FeAl sub 2 O sub 4 , and 249 kJ/mol for NiAl sub 2 O sub 4 . Investigation of the solubilities of the aluminates as a function of the NaF/AlF sub 3 ratio in alumina-saturated melts at 1020 deg C 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 sub 2 (liquid) and NiF sub 2 (solid) in dilute solution in cryolite are found to be 0.22 and 1.2, respectively.