Reduction kinetics of iron oxide particles in CO/CO 2 mixtures: Development of rate equation incorporating equilibrium limitation

Abstract

The reduction kinetics of iron oxides in CO/CO₂ gas mixtures in the temperature range 1073–1173 K were investigated, with emphasis on formulating a thermodynamically consistent rate equation that accounts for the presence of CO₂. The effect of gas composition (CO/CO₂ ratios) was evaluated across distinct reduction stages (Fe₂O₃ → Fe₃O₄ → FeO). Experimental data were analysed using the Avrami–Erofeev nucleation and growth model. Intrinsic kinetic parameters, including activation energies and reaction orders, were derived for the CO-driven reduction process. Results demonstrate that CO₂ affects the reduction rates due to equilibrium constraints. Activation energies for the reduction of haematite (Fe₂O₃) to magnetite (Fe₃O₄) and wustite (FeO) phases are 24.1 and 25.3 kJ·mol⁻¹, respectively, reflecting a strong Fe/O ratio dependence of CO reduction reactions. In contrast to prior studies, which overlooked equilibrium limitations, the rate equation derived in this work explicitly incorporates the thermodynamic effect of the product gas CO₂. The intrinsic activation energies are markedly lower than the apparent values reported in the literature, underscoring the necessity of equilibrium-adjusted kinetic analysis. The Avrami parameter (n = 1.6 and 1.2) and reaction orders (m = 1 and 1/2) together with the large activation energy and the reaction time scale indicate a particle-reaction-controlled mechanism for the fine particles of iron oxide investigated. The derived rate equation decouples intrinsic kinetics from equilibrium limitations, providing a unified rate equation for iron oxide reduction in CO/CO₂ mixtures.