Reduction Degradation Behavior and Mechanisms of Lump Ore in Hydrogen-Based Shaft Furnaces

Lump ore reduction generates fines that can induce furnace hanging or slipping. Reducing this degradation is key to maintaining stable operation. To address the degradation problem of lump ores during direct reduction in hydrogen-based shaft furnaces, this study systematically investigated the effects of reduction temperature and reduction time on the degradation behavior of three lump ores with distinct crystal structures under HYL atmosphere. The phase evolution, thermal decomposition characteristics and microstructural damage mechanisms during reduction were elucidated. The results showed that all three lump ores exhibited the most severe degradation after 60 min of reduction at 700 °C, with reduction degradation index values below 3.15 mm (RDI−3.15mm) of 32.50%, 15.17%, and 22.42% for Lump Ore 1, Lump Ore 2, and Lump Ore 3, respectively. During isothermal reduction at 500 °C, pronounced degradation occurred in all three ores at around 20 min (RDI−3.15mm: 33.09%, 18.09%, and 8.85% for Lump Ore 1, Lump Ore 2, and Lump Ore 3, respectively). Microscopic mechanism analysis revealed that dehydration of lump ores with high crystal water content caused structural damage, which was coupled with phase transformation stresses during reduction and led to severe degradation. In porous lump ore, stress was more readily dissipated and crack propagation was effectively buffered. In contrast, dense lump ore was prone to stress accumulation caused by locally nonuniform reduction, ultimately resulting in penetrating cracks. This study clarifies the degradation behavior of the three lump ores under different reduction regimes and reveals the evolution of phases and microstructures during reduction, providing a theoretical basis for stable operation of hydrogen-based shaft furnaces.