Comparative study on the chemical stability of Fe3O4 and NiFe2O4 in molten salts

Tests on NixFe3-xO4 spinels, considered as promising materials in the aluminium production, were performed on Fe3O4 and NiFe2O4. First, FeAl2O4 aluminate was spontaneously formed by Fe3O4 immersion at 960 degrees C, due to Fe3+/Al3+ substitution into the spinel structure. Then, stability of aluminate compound was investigated into cryolite melt and results showed a rapid dissolution when the aluminate composition tends to FeAl2O4. Kinetic studies demonstrated that the spinel dissolution was around twentyfold higher than aluminate formation, confirming the micrographic observation. To prevent it, nickel was added in the spinel structure up to NiFe2O4 and no dissolution was observed.