Hydrogen Reduction of Synthetic Fe ₂ O ₃ Pellets: The Role of Calcium Oxide in Porosity Evolution and Reduction Kinetics

To determine how CaO affects pore evolution and hydrogen reduction kinetics, cylindrical pure Fe ₂ O ₃ pellets and 0.9 wt% CaO‐containing pellets were pressed, sintered, and reduced in H ₂ at 800 °C. Thermogravimetric data were analyzed with a mixed‐control formulation of the unreacted‐core model, and cross‐sectional SEM image analysis estimated porosity and pore surface area. CaO addition decreased the time to 95% reduction by 25% on average and increased the reduction rate by a factor of 1.2–1.4 throughout reduction. From 33% to 95% reduction, mixed‐control analysis indicated that interfacial reaction resistance accounted for 68%–85% of modeled resistance. The CaO‐containing pellet had a 19% higher apparent interfacial reaction rate constant and a 43% higher effective diffusivity than the pure pellet. Relative to their sintered volumes, pure pellets contracted by 27%, whereas CaO‐containing pellets swelled by 8% after reduction. Compared with pure pellets, CaO‐containing pellets developed a coarser, more porous metallized layer that densified less beyond 50% reduction, despite generally lower pore surface area. CaO promoted intraparticle porosity and lowered the tendency to form a dense Fe layer. Kinetic and microstructural results indicate that both lower apparent interfacial reaction resistance and improved intrapellet gas transport contributed to faster reduction of the CaO‐containing pellets.