Kinetics of iron reduction upon reduction/oxidation cycles

The reduction kinetics of iron ore with H2 and its re-oxidation with air are investigated by thermogravimetric analysis (TGA) over reiterated reduction/oxidation cycles. Using non-isothermal methods, three steps of reaction have been identified for reduction (activation energies of 133, 30 and 83 kJ/mol) and two for re-oxidation (30 and 70 kJ/mol). Isothermal analysis, instead, leads to wrong kinetic expressions. A remarkable “annealing” effect is observed: when the temperature of reduction is increased from 600 to 850°, the pre-exponential factors of the re-oxidation step decrease by an order of magnitude. Upon repeated cycles, reactivity decreases in the first three cycles, but is restored after 4–5 cycles and even increases upon further cycles. X-ray Diffraction (XRD) analysis shows changes in the crystalline phase of hematite upon reduction and re-oxidation, suggesting a possible spin-flip effect. After 4–5 reiterated cycles the opening up of porosity may counterbalance negative changes at the nanoscale.