Sticking in Shaft Furnace and Fluidized Bed Ironmaking Processes: A Comprehensive Review Focusing on the Effect of Coating Materials

Production of direct reduced iron (DRI), particularly with green hydrogen, is a key pathway to the decarbonization of the iron and steel industry. However, the sticking tendency during the production of DRI creates serious operational issues and limits production outputs. Coating inert materials on the surface of iron ores can act as a barrier to effectively prevent the bonding between newly formed iron surfaces, and can interfere with the formation of iron whiskers. However, the principle of coating has not been systematically studied. This review covers the mechanism of sticking in both shaft furnaces and fluidized bed-based gaseous DRI production. The factors that influence the reduction kinetics and morphology, including physical and chemical ore properties, pellet induration conditions, and reduction conditions are summarized as well. Understanding the relationship between these factors and morphology change is critical to eliminating the sticking issues of DRI. Findings from this study suggest that coating with inert additives (e.g., metal oxides) can successfully prevent sticking in both shaft furnaces and fluidized bed processes. The types of additives and coating methods, the stage of reduction where the coating is applied, and reduction temperature will dramatically affect the coating performance. The outlook is discussed as well given the need for further work to improve the performance of coating (methods, timing, and cheaper alternatives), to further de-risk DRI technologies.