Impact of SiO2 Content on the Hydrogen-Based Direct Reduction of Acidic High-Grade Fired Hematite Pellets

This study investigated the effect of different SiO2 contents on the direct reduction performance of high-grade fired hematite pellets by hydrogen-rich gas. The experimental results indicated that as SiO2 content increased in the range of 1.10-4.13%, the reduction degree index slightly decreased, while the reduction swelling index significantly decreased and the cold compressive strength of reduced pellets was improved. The fired pellets with 4.13% SiO2 had a reduction swelling of 18.16% and a Cold Compressive Strength of 289 N/P, whereas pellets with 1.10% SiO2 showed a reduction swelling of 26.1% and the lowest cold compressive strength of 182 N/P. Higher SiO2 content promoted the formation of low-melting silicate liquid phases, which enhanced the bonding and interaction between iron grains, reduced crack and pore formation caused by stress concentration during reduction, minimized structural differences between grains, and improved the reduction performance of the fired pellets. Microstructural analysis revealed that the fired pellets with higher SiO2 content exhibited a denser iron product layer after reduction and fewer cracks, and possessed more stable structure leading to lower reduction swelling and higher cold compressive strength. The results demonstrate that the amount of liquid phase plays multiple roles in enhancing the reduction performance and structural stability of the fired pellets during the reduction process.