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

With the growing demand for low-carbon ironmaking, understanding the hydrogen-based reduction behavior of iron ore pellets under varying gangue compositions and gas atmospheres is essential. In this work, fired hematite pellets with a basicity (CaO/SiO2) of 0.3 and SiO2 contents ranging from 1wt% to 4wt% were systematically investigated under three typical shaft furnace atmospheres (Midrex, HYL and COG) and 100% H2 conditions to elucidate their reduction behavior, kinetics, and structural evolution. The results indicate that increasing the hydrogen proportion significantly accelerates the reduction rate of the fired pellets, while an increase in SiO2 content generally leads to a reduction in the overall reaction rate. The influence of SiO2 content on the reduction performance exhibits clear divergence under different reducing atmospheres. For the fired pellets containing 1wt% and 2wt% SiO2, an increase in hydrogen concentration causes deterioration in reduction performance, as evidenced by the increase in reduction swelling index from 26.14% to 34.26% and the decrease in cold compressive strength from 110  N/P to 78  N/P. In contrast, 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  N/P to 271  N/P. Mechanism 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. The formation of slag phases helps alleviate internal stress caused by crystal structure transitions, improves structural stability during reduction, and promotes the transformation of metallic iron from whisker-like to layered morphology.