Effect of Pre-Reduced Structures on Deformation and Shrinkage Behavior of Iron Ore Pellets under CO and H₂ Reduction

In hydrogen-enriched blast furnace operations, the cohesive zone’s coke slits are expected to thin, requiring precise control. The softening behavior of lump ore, iron ore sinter, and pellets is influenced by factors such as reduction degree, gangue composition, and slag viscosity. This study investigates the softening and shrinkage mechanisms of hydrogen-reduced pellets, offering insights into cohesive zone behavior in hydrogen-enriched conditions. A load-softening test system with rapid heating and cooling capabilities was used to evaluate the softening temperature range of pre-reduced pellets in an inert N₂ atmosphere. Self-fluxed and acidic pellets were pre-reduced to 70% and 90% using CO and H₂ gases. Structural changes during softening and shrinkage were analyzed using optical microscopy on interrupted samples, with precise temperature control and force application to measure contraction rates. Results revealed distinct shrinkage behaviors between self-fluxed and acidic pellets under CO and H₂ reduction. Acidic pellets showed greater and more consistent shrinkage in H₂, while CO-reduced pellets exhibited gradual shrinkage at higher temperatures. Molten slag, forming around 1 050°C, significantly influenced shrinkage. Structural analysis highlighted variations in metallic iron and wüstite distributions, with CO-reduced pellets exhibiting a wider mixed region and pronounced sintering at elevated temperatures. Pellets with thicker metallic shells and higher reduction degrees showed greater deformation resistance under high-temperature loading. The mixed metallic iron and wüstite region’s thickness decreased during softening, correlating with molten slag formation. These findings underscore the critical role of mixed region deformation and slag generation in determining the softening behavior of reduced pellets.