Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres

Abstract

The hydrogen reduction behavior of Fe 2 O 3 , Fe 3 O 4 and FeO is strongly influenced by time–pressure dependent process. The reduction of hematite takes place according to scheme: 3Fe 2 O 3 → 2 Fe 3 O 4 → 6 FeO → 6 Fe. The same pathway of hydrogen reduction for magnetite and wüstite is postulated. First, reduction of magnetite to wüstite Fe 3 O 4 → FeO and second step of wüstite disproponation 4FeO → Fe 3 O 4 + Fe. Only after the disappearance of Fe 2 O 3 phase, the reduction of Fe 3 O 4 to Fe can be observed. The appearance of FeO crystal phase as an intermediate compound of iron(III) oxide reduction was experimentally confirmed by XRD method above 560 °C. The complete reduction of hematite into metallic iron phase can be accomplished even at low temperature of up to 380 °C. The reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programmed reduction (TPR H2 and TPR CO ), 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-Fe 2 O 3 , goethite α-FeOOH, ferrihydrite Fe 5 HO 8 ·4H 2 O, magnetite Fe 3 O 4 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-Fe 2 O 3 → 2 Fe 3 O 4 → 6 Fe, three-stage mechanism 3Fe 2 O 3 → 2Fe 3 O 4 → 6FeO → 6Fe is postulated especially when temperature of reduction overlaps 570 °C. Up to this temperature the postulated mechanism may also involve disproportionation reaction, 3Fe 2+ ⇌ 2Fe 3+ + Fe, occurring at both the atomic scale on two-dimensional interface border Fe 3 O 4 /Fe or stoichiometrically equivalent and thermally induced, above 250 °C, phase transformation—wüstite disproportionation to magnetite and metallic iron, 4FeO ⇌ Fe 3 O 4 + 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–H 2 system, instead of one-step simple reduction FeO → Fe, a much more complex two-step pathway FeO → Fe 3 O 4 → Fe up to 570 °C or even the entire sequence of three-step process FeO → Fe 3 O 4 → 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/Fe 3 O 4 . Depending on TPR H2 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.