Multistep kinetic study of magnetite reduction by hydrogen based on thermogravimetric analysis

The utilization of hydrogen-based direct reduction methods for magnetite ore smelting is an important way to achieve the goal of reducing carbon emissions. At present, the coupling mechanism of the multi-step reaction during the reduction process is unclear, which restricts the understanding of the rate-limiting steps and the accurate description of kinetic parameters. In this paper, the reaction characteristics and multi-step kinetics of isothermal reduction of magnetite by hydrogen with varying concentrations are investigated at 600–900 °C. Results indicate that the coupling relationship between Fe₃O₄→FeO and FeO→Fe two steps is temperature-dependent. In the low-temperature range (600–700 °C), the coupling relationship between the two steps can be described by the parallel model. In the high-temperature range (750–900 °C), the coupling relationship can be described by the series model. Meanwhile, the first-step reaction Fe₃O₄→FeO is controlled by the nucleation or chemical reaction model, and the apparent activation energies are determined to range from 27.25 to 48.59 kJ/mol. The second-step reaction FeO→Fe is controlled by the combination of nucleation and diffusion models, and the apparent activation energies are determined to range from 30.70 to 45.14 kJ/mol.