Swelling behavior of iron ore pellet reduced by H2–CO mixtures

•Swelling of pellet during reduction was examined by a consecutive imaging method.•Swelling index (SI) increased with temperature but weakened with addition of H2.•Formation of massive wüstite phase led to the swelling firstly.•Different iron precipitation caused difference in swelling states in the later.

Hydrogen and carbon monoxide mixtures are main reducing agents for gaseous direct reduction (DR) processes. In the present study, volume change of pellet reduced in H2–CO mixtures at 800–1000 °C was examined with a novel consecutive imaging method. Results showed that pellet expansion intensified with temperature and CO content in atmosphere and it reached the maximum at reduction degree of 20%–40% due to the formation of massive wüstite phase. Pellet in H2 rich atmospheres passed the wüstite stage rapidly and swelling consequently weakened. Iron whiskers that formed in the later reduction stage led to persistent swelling as pellet reduced by CO. However, pellets contracted obviously in H2 containing atmospheres for the collection of iron phase in the later. Compressive strength of pellets during reduction dropped to be the lowest in 10–20 min. And the time for pellets remained in the “low strength zone” (< 500 N/P) prolonged with temperature and CO proportion in reducing gas, which showed good correlation with their swelling behavior.

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