Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres

The reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programmed reduction (TPRH2 and TPRCO), thermo-gravimetric and differential temperature analysis (TG-DTA-MS), and conventional and “in situ” XRD methods. Five different compounds of iron oxides were characterized: hematite α-Fe2O3, goethite α-FeOOH, ferrihydrite Fe5HO8·4H2O, magnetite Fe3O4 and wüstite FeO. In the case of iron oxide-hydroxides, goethite and ferrihydrite, the reduction process takes place after accompanying dehydration below 300 °C. Instead of the commonly accepted two-stage reduction of hematite, 3 α-Fe2O3 → 2 Fe3O4 → 6 Fe, three-stage mechanism 3Fe2O3 → 2Fe3O4 → 6FeO → 6Fe is postulated especially when temperature of reduction overlaps 570 °C. Up to this temperature the postulated mechanism may also involve disproportionation reaction, 3Fe2+ ⇌ 2Fe3+ + Fe, occurring at both the atomic scale on two-dimensional interface border Fe3O4/Fe or stoichiometrically equivalent and thermally induced, above 250 °C, phase transformation—wüstite disproportionation to magnetite and metallic iron, 4FeO ⇌ Fe3O4 + Fe. Above 570 °C, the appearance of wüstite phase, as an intermediate of hematite reduction in hydrogen, was experimentally confirmed by “in situ” XRD method. In the case of FeO–H2 system, instead of one-step simple reduction FeO → Fe, a much more complex two-step pathway FeO → Fe3O4 → Fe up to 570 °C or even the entire sequence of three-step process FeO → Fe3O4 → FeO → Fe up to 880 °C should be reconsidered as a result of the accompanying FeO disproportionation wüstite ⇌ magnetite + iron manifesting its role above 150 °C and occurring independently on the kind of atmosphere—inert argon or reductive hydrogen or carbon monoxide. The disproportionation reaction of FeO does not consume hydrogen and occurs above 200 °C much easier than FeO reduction in hydrogen above 350 °C. The main reason seems to result from different mechanistic pathways of disproportionation and reduction reactions. The disproportionation reaction wustite ⇌ magnetite + iron makes simple wüstite reduction FeO → Fe a much more complicated process. In the case of thermodynamically forced FeO disproportionation, the oxygen sub-lattice, a closely packed cubic network, does not change during wüstite → magnetite transformation, but the formation of metallic iron phase requires temperature activated diffusion of iron atoms into the region of inter-phase FeO/Fe3O4. Depending on TPRH2 conditions (heating rate, velocity and hydrogen concentration), the complete reduction of hematite into metallic iron phase can be accomplished at a relatively low temperature, below 380 °C. Although the reduction behavior is analogical for all examined iron oxides, it is strongly influenced by their size, crystallinity and the conditions of reduction.