Impact of SiO2 content on the hydrogen based direct reduction performance of high-grade hematite pellets

The gangue composition is one of the key factors influencing the reduction behavior of iron ore pellets, particularly the SiO 2 content, which exerts a pronounced effect on the reduction performance of acidic pellets under hydrogen-rich atmospheres. In this study, high-grade hematite oxidized fired pellets with a basicity (CaO/SiO 2 ) of 0.3 were prepared, and the SiO 2 content was systematically varied from 1.0% to 4.0%. Reduction experiments were conducted under a simulated hydrogen-rich gas atmosphere, representative of the Midrex shaft furnace process. The results show that with increasing SiO 2 content, the internal structure of the fired pellets becomes more compact, resulting in a decline in both the reduction rate and the metallization degree –from 98.88% to 95.01%. At the same time, the reduction swelling index significantly decreases from 21.56% to 7.26%, and the clustering index drops markedly from 19.56% to 8.56%. A moderate increase in SiO 2 improves reduction disintegration resistance and suppresses carbon deposition. However, when SiO 2 content reaches 4.0%, a slight increase in reduction pulverization is observed. Mechanistic analysis reveals that a moderate increase in SiO 2 content contributes to enhancing the structural stability of fired pellets, suppressing reduction swelling and clustering tendency, and thereby optimizing the overall reduction behavior. However, an excessive SiO 2 content leads to increased slag phase formation, which hinders the reduction process, resulting in a lower metallization degree and aggravated reduction disintegration. This study provides a theoretical basis for the preparation and performance optimization of high-quality fired pellets suitable for hydrogen-based shaft furnace direct reduction processes.