The effects of Al2O3 content on the morphology and reducibility of sinter were respectively investigated using confocal laser microscopy and thermogravimetric analysis at 1273 K under CO gas. To understand the effects of the sintering process, separate samples were prepared via the equilibrium and metastable reaction routes. In the equilibrium samples, the addition of Al2O3 led to the formation of the silico-ferrite of calcium and alumino phase and a decrease in the reduction rate due to the lowered reactivity of iron oxide. In contrast, in the metastable samples, the reduction rate increased after the addition of 2.5 mass% Al2O3. The addition of Al2O3 decreased the fraction of the liquid phase and increased the fraction of pores in the sample. As a result, the reduction rate is proportional to the Al2O3 content owing to the changes in the sinter morphology. In determining the reduction rate of the sinter, the influence of the microstructure on the diffusion of the reducing gas is more significant than that of the interfacial chemical reaction due to the formation of the SFCA phase. The microstructure changes of the sinter with the addition of Al2O3 and the corresponding reduction behaviors are further discussed.

The effect of extrinsic porosity on the reduction behavior of FeO was evaluated by thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and the Brunauer–Emmett–Teller (BET) technique and analyzed using the labyrinth coefficient of FeO. The extrinsic pore exhibited an abnormal effect on reduction behavior in the range of less than 50% reduction degree, despite the increase in apparent porosity. SEM and BET analysis indicated that the abnormal reduction behavior by extrinsic pores at the initial reduction stage was speculated to be due to the characteristic of extrinsic pore that is open only at one end. However, the overall porosity and reduction rate after a 40% reduction revert to the normal relationship. In addition, the experimental results indicated that the abnormal effect of the extrinsic pores in the initial stage was mitigated by an increase in the temperature. The abnormal effect of extrinsic porosity on FeO reduction was mathematically analyzed using the labyrinth coefficient. It can be summarized that not only the number of pores, but also their quality and distribution are important in determining the reduction rate.