Multistep reduction kinetics of Fe3O4 to Fe with CO in a micro fluidized bed reaction analyzer

The multiple step reduction kinetics of Fe3O4 to Fe with CO was investigated to understand the reaction mechanism for the production of metallic iron from hematite. The experiments were carried out in a micro fluidized bed reaction analyzer at different temperatures and CO concentrations using mass spectrometer analysis to determine the composition of the off gas. The overall kinetic parameters were then investigated based on the conversion obtained as a function of the oxygen loss, which can be calculated from the concentrations of CO and CO2 in the off gas. According to the analysis of the activation energy, a transformation of the reaction mechanism was identified at the conversion of 0.1. Considering that the reduction involves reactions in series, i.e., Fe3O4→FeO→Fe, it could be concluded that the FeO→Fe step was only involved beyond this transfer point, while the Fe3O4→FeO step occurred during the entire lifetime of Fe3O4. Meanwhile, the reaction rate for the reduction of Fe3O4 to FeO was controlled by the phase boundary, whereas the reduction of FeO to Fe was controlled by the phase boundary and diffusion. The reaction rate of the overall reduction process was determined by FeO→Fe, and the rate increased with increasing temperature from 600 °C to 720 °C and then decreased at approximately 720 °C until it reached a minimum value, and then increased again from 760 °C to 800 °C. Furthermore, the apparent activation energies of Fe3O4→FeO at 600–800 °C, FeO→Fe at 600–720 °C, and FeO→Fe at 760–800 °C were determined to be 35.01 ± 3.53 kJ/mol, 42.83 ± 0.25 kJ/mol, and 14.27 ± 1.69 kJ/mol, respectively.