Preparation of micaceous iron oxide by the oxidation of iron with pressurized oxygen in concentrated sodium hydroxide solution at elevated temperatures

Iron powders were oxidized in NaOH solutions of 5-25 mol kg^-1 at 403-563 K and 5 MPa of oxygen partial pressure. Various types and morphologies of iron compounds such as fine particles of Fe₃O₄, micaceous α-Fe₂O₃, and coagulated particles of α-NaFeO₂ were formed depending on the experimental conditions. The observed critical concentrations of NaOH above which α-NaFeO₂ was formed was in good agreement with those thermodynamically calculated for the hydrolysis equilibrium of α-NaFeO₂.