Sticking and Swelling of Iron Ore Pellets: Mechanisms and Controlling Factors

Abstract The sticking and breakage of iron ore pellets during high-temperature reduction pose significant challenges in shaft furnace ironmaking. As the steel industry transitions towards decarbonization, hydrogen-based shaft furnace processes are expected to play an increasingly important role in achieving low-emission steel production. One of the critical challenges in this transition is the global decline in iron ore quality. This review critically examines how degrading ore quality and changes in reduction mechanisms affect adverse pellet behaviors, such as sticking and swelling. The mechanisms of pellet sticking and swelling are discussed. Key contributors include the formation of iron whiskers, dense metallic iron, and low-melting eutectic phases. The impact of impurity oxides, such as SiO₂, Al₂O₃, MgO, CaO, TiO₂, and alkali oxides, on sticking and swelling indices is assessed, along with the mineralogy of iron ores. Lower-grade iron ores are tend to exhibit lower sticking and swelling. SiO₂ and MgO effectively mitigate these issues; SiO₂ forms a glassy silicate layer that inhibits iron whisker growth, while MgO enhances magnetite stability through the formation of high-melting spinel phases, promoting uniform distribution in the wüstite lattice. While the role of CaO is quite complex, a basicity (CaO/SiO₂) range of below 0.2 or above 0.8 is recommended to prevent adverse sticking and swelling behaviors. In contrast, alkali oxides have a detrimental effect on pellet performance. Furthermore, induration and reduction conditions including pellet size, induration temperature, reduction temperature, and reducing gas composition significantly affect these pellet behaviors. Higher reduction temperatures are correlated with increased sticking and swelling due to enhanced diffusion and crystallization of iron whiskers. Temperatures below 900 °C are recommended to prevent sticking in uncoated pellets. Importantly, the presence of hydrogen in the reducing atmosphere transitions the reduction mechanism from chemical-reaction control to diffusion control, thereby reducing iron whisker formation. Graphical Abstract