Magnetite reduction kinetics under H2 and CO atmospheres at high temperature

Magnetite reduction using H2-CO-N2 mixtures with varying H2/CO molar ratios (9:1, 5:5, and 1:9) was investigated over a temperature range of 1400-1550 oC by thermogravimetric analysis (TGA). The results show that both increasing the hydrogen content in the reducing gas mixture and elevating the reaction temperature significantly accelerate the reduction process. Analysis of peak distribution characteristics and interruption experiments indicates that the overall reduction proceeds in two distinct steps: Fe3O4 → FeO followed by FeO → Fe. Kinetic modeling demonstrates that these steps are governed by different mechanisms: the Fe3O4 → FeO step is best described by a nucleation and growth model, while the FeO → Fe step follows a phase-boundary controlled (contracting cylinder) model. Using the model-fitting method, the apparent activation energies (Ea) for Fe3O4 → FeO were determined to be 73.65, 81.43, and 108.46 kJ/mol, and for FeO → Fe were 114.35, 118.32, and 130.08 kJ/mol, corresponding to H2/CO ratios of 9:1, 5:5, and 1:9, respectively. In addition, the model-free (iso-conversional) method was applied to evaluate the variation of Ea with conversion. The obtained activation energies for Fe3O4 → FeO were 72.94, 89.62, and 97.27 kJ/mol, while those for FeO → Fe were 115.06, 118.86, and 122.74 kJ/mol under the same H2/CO ratios. The close agreement between values derived from both methods confirms the reliability of the kinetic analysis and provides robust insight into the reduction mechanisms of magnetite under mixed H2–CO atmospheres.