MAS NMR study of the solidified cryolite systems with FeO addition

• The complete analysis of the chemical processes between cryolite (Na3AlF6) and FeO.
• The new data concerning to structural investigation of oxofluoroaluminate species.
• A new view on the behaviour of “oxygen” in cryolite melts.
• All the reaction products escape from the melts.
• The forming oxo-fluoro aluminates that evaporates at the melting.

Unexpected results were obtained during investigation of the chemical processes between cryolite (Na3AlF6) and FeO by multinuclear 27Al, 23Na and 19F MAS NMR spectroscopy in the system with different amount of FeO; (1−x)Na3AlF6–xFeO (x = 0.00, 0.05, 0.10, 0.14). Surprisingly, no reactions products are present in solidified fractions of samples after melting. NMR data of all the bulks proved the presence only of non-reacted cryolite and NaF, which is the product of known partial cryolite dissociation reaction. No FeO containing phases or other phases containing iron are present in the bulk of analyzed samples. On other hand all reaction products were identified in the condensed vapours that escape from the melts at reaction conditions. Two volatile (high temperature and low temperature) fractions were separated. In the high temperature condensate the presence of AlF3, Na5Al3F14, cryolite and Al2O3 was evidenced. The presence of AlF3 and Na5Al3F14 phases arises from decomposition of the vaporized NaAlF4 phase. The presence of Na3AlF6 and Al2O3 in condensed high temperature vapours seems to be ground-breaking result. The presence of these two phases was explained by the sequence of processes when at the beginning FeO was dissolved in cryolite, then oxygen was bounded to aluminium forming some fluorido-oxido-aluminates that are volatiles and during condensation they decompose to Na3AlF6 and Al2O3. Phases with remaining iron were identified in the low temperature condensed vapours.

The paper deals with the chemical behaviour of FeO in cryolite (Na3AlF6) melts, which are basic electrolytes for the industrial aluminium production. It presents mainly recent data concerning to structural investigation of fluorido-oxido-aluminate species. Surprisingly, no reactions products are present in solidified fractions of samples after melting. On other hand all reaction products were identified in the condensed vapours that escape from the melts at reaction conditions.Figure optionsDownload as PowerPoint slide