Investigation of the Influence of Alkali Metal Compounds on the Durability of the Grate Bar Field of Conveyor Machines During Iron Ore Pellet Firing
DOI:
https://doi.org/10.62911/ete.2025.03.02.06Keywords:
alkali metals; sodium compounds; pellet firing; grate bar field; high-temperature corrosion; chromates; ferrates; bentonite binder; conveyor induration machine.Abstract
This investigation analyzes the influence of alkali metal compounds introduced into the firing process through the bentonite binder contained in iron ore pellets on the durability of grate bar assemblies in conveyor-type induration machines. During pellet firing, sodium- and potassium-bearing components inherent to montmorillonitic bentonites transform into reactive forms and interact at high temperatures with the grate bar material. In the presence of chromium in the steel, a complex oxide scale develops at the metal–oxide interface, incorporating low-melting chromate phases. These phases significantly reduce the protective capacity of the oxide film, promote intense scale spalling, and accelerate high-temperature chemical erosion, ultimately resulting in rapid and catastrophic grate bar degradation. To evaluate the degradation mechanisms, laboratory tests were carried out on samples of cast steel 40Kh24N12SL, which were oxidized and exposed to sodium-containing components at 1070 °C for six hours. Scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed the formation of multilayer oxide scales enriched in sodium and chromium and the presence of reaction products characteristic of chromates, confirming the intensification of corrosion processes in the presence of alkali metals. The obtained results clarify the origins of severe grate bar wear under pellet induration conditions when bentonite is used as the binder. Since the alkali metal content is defined by the mineralogical nature of bentonite and is technologically necessary for achieving adequate pellet strength, its reduction is not feasible. Therefore, mitigating the destructive effect of alkali metals must rely on selecting more corrosion-resistant alloys for grate bars and optimizing the temperature–oxidation parameters of the firing process.
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