A High‐Temperature Multinuclear NMR Study of Na3AlF6–FeO andNa3AlF6–Fe2O3 Melts

Abstract In the Hall–Héroult process for the industrial production of aluminum, iron oxide impurities are known to lower the current efficiency as well as the metal quality. Iron can be present in the di‐ or trivalent form. The nature of dissolved Fe III and Fe II species and the reactions taking place in cryolitic melts have been investigated by high‐temperature NMR spectroscopy. The evolution of 27 Al, 23 Na, and 19 F NMR chemical shifts are reported in the Na 3 AlF 6 –FeO and Na 3 AlF 6 –Fe 2 O 3 systems for different iron oxide contents. They express the formation of oxofluoroaluminate species. For the Na 3 AlF 6 –FeO system, the 19 F signal appears only after 14 minutes at 1020 °C. The line position is shifted with heating time. This evolution is associated with a narrowing of the 27 Al signal due to a decrease in the content of paramagnetic compounds in the sample. This effect is probably caused by the evaporation of an Fe II compound, as already mentioned in the literature. For the Na 3 AlF 6 –Fe 2 O 3 system we have never observed the 19 F NMR spectra over the full heating time (ca. 30 min). This could be due to the influence of paramagnetic Fe III nuclei, probably in FeF 6 3– species present in the melt. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)