Coupled effect of SiO₂ content and hydrogen-enriched atmospheres on the reduction behavior and microstructural evolution of fired hematite pellets

As demand grows for low-carbon ironmaking, it is essential to understand how the hydrogen reduces iron ore pellets under varying gangue compositions and gas atmospheres. In this work, fired hematite pellets with a basicity (mass ratio of CaO to SiO₂) of 0.3 and SiO₂ contents ranging from 1wt% to 4wt% were systematically investigated under three typical shaft furnace atmospheres (Midrex, HYL, and coke oven gas (COG)) as well as under 100% H₂, to clarify the reduction kinetics, reaction mechanism, and microstructural evolution of the fired pellets. The results indicate that higher hydrogen proportion significantly accelerates the reduction rate of the fired pellets, while an increase in SiO₂ content generally leads to a decrease in the overall reaction rate. However, the effect of hydrogen concentration on the reduction behavior of the pellets varied markedly with their silicon content. For the fired pellets containing 1wt% and 2wt% SiO₂, an increase in hydrogen concentration causes deterioration in reduced pellet characteristics, as evidenced by the increase in reduction swelling index from 26.14% to 34.26% and the decrease in cold compressive strength from 110 to 78 N. In contrast, fired pellets with 3wt% and 4wt% SiO₂ exhibit the opposite trend, with the reduction swelling index decreasing from 15.26% to 9.23% and cold compressive strength improving from 179 to 271 N. Kinetics analysis indicates that under 100% H₂, the reduction of fired pellets with 1wt% SiO₂ is governed by a mixed gas-diffusion and uniform reaction model, whereas fired pellets with 4wt% SiO₂ follow an unreacted core model. These differences in reduction kinetics, reduction behavior, and post-reduction properties are closely associated with the formation of more Al-bearing calcium silicate slag phases in high-SiO₂ pellets, which strengthen intergranular bonding, buffer phase-transformation-induced stress, and promote the evolution of metallic iron from whisker-like to granular or layered morphologies.