Solutions of Iron Oxides in Molten Cryolite

All the iron oxides (FeO, Fe3O4, Fe2O3, and FeAl2O4) dissolve in cryolite-alumina melts to give solutions containing both Fe(II) and Fe(III). The factor controlling the Fe(II)/Fe(III) ratio is the oxygen pressure, and experimental results are interpreted on that basis. Predictions are made of the variation of solubility with oxygen pressure, and the standard potential of the Fe2+/Fe3+ redox couple is calculated. The anode and anode gas of an industrial Hall-Heroult cell appear to be insufficiently oxidizing to cause significant conversion of Fe(II) to Fe(III). An anomaly in the liquidus diagrams for FeF2 - Na3AlF6 and FeO Na3AlF6 is accounted for in terms of solid solution of FeF2 in cryolite.