Deposits in a coal fired grate-kiln plant for hematite pellet production: Characterization and primary formation mechanisms

Abstract

Deposit in the grate-kiln process is a well-known problem for iron ore pellet production. In this study, the deposits in a coal-fired grate-kiln plant for hematite pellet production were characterized to reveal their formation mechanisms. Scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and X-ray powder diffraction (XRD) analysis demonstrated that the main phase of the deposits was hematite with minor proportions of magnesioferrite, hedenbergite, quartz and anorthite. Comparisons of key elemental ratios showed that the silicate phases in the deposits mainly originated from the coal ash and partly from the pellet powder. The primary form of deposits formed in the grate and feeding area in the kiln was the liquid solidification. The hematite particles could be reduced to FeO by unburnt carbon from the uncompleted coal combustion, and then the sufficient liquids for deposit formation were generated with the presence of FeO. The primary manner to form the deposits near the burner zone in the rotary kiln was the hematite crystallization and diffusion, while the liquid phases played a secondary role in the deposit formation. The generated FeO caused by unburnt coal residue not only contributed to the formation of liquid phases at low-temperature parts but also was beneficial to the combination of solid particles at high-temperature parts in the grate and kiln.