Reduction Behavior of Iron Oxides in Hydrogen and Carbon Monoxide Atmospheres - 2007

Geochemistry & Hydrochemistry

Jozwiak, and 4 co-authors, reported in 2007 that the reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programed 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 a-Fe₂O₃, goethite a-FeOOH, ferrihydrite Fe₅HO₈ x 4H₂O,
magnetite Fe₃O₄ and wurstite 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 a-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—wurstite disproportionation to magnetite and metallic iron, 4FeO ⇒ Fe₃O₄ + Fe.

Above 570 °C, the appearance of wurstite 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 wurstite ⇒ 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 wurstite 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 wurstite ⇒ 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 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.