Kinetic Analysis of Magnetite Reduction by Hydrogen in Temperature-Programmed System: Toward Green Ironmaking

The hydrogen-based direct reduction of magnetite presents considerable potential for achieving carbon neutrality. However, the complex reaction mechanisms have hindered establishment of predictive rate equations under non-isothermal conditions. In this study, the hydrogen reduction behavior and kinetics of reagent grade magnetite powder are investigated using thermogravimetric analysis (TG) combined with temperature-programmed reduction (TPR) methods at heating rates of 10–30 °C/min. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are employed to analyze the evolution of phase composition and microstructural during the reduction process, respectively. Results reveal that the reduction rate curve exhibits a distinct double-peak feature. Below 570 °C, wüstite exists as a metastable intermediate, and the reduction follows the pathway Fe₃O₄ → Fe₃O₄ + FeO + Fe → Fe. The formation of initial peaks may be attributed to the initial reduction of highly active surface sites. Above 570 °C, the reduction proceeds in two steps of Fe₃O₄ → FeO → Fe, with the majority of oxygen removal occurring within this temperature range. The apparent activation energy determined using isoconversional method ranges from 39.56 to 139.45 kJ/mol. Model fitting analysis reveals that the rate-controlling mechanism is temperature-dependent, sequentially transitioning from a chemical reaction model to a nucleation and growth model, then to a contracting volume model, and ultimately to a diffusion-controlled model. Additionally, mathematical deconvolution functions Fraser-Suzuki and Asym2Sig are used to separate the two overlapping reduction rate peaks. Based on the Šesták–Berggren model, reaction rate equations are constructed, enabling accurate prediction of the experimental TPR curves for the hydrogen reduction of magnetite.