Properties and formation mechanism of rings during rotary kiln reduction of low-grade iron ore

To ease the imbalance between the supply and demand for iron ore in the ferrous industry of China, a low-temperature reduction process via an ore-coal composite method was developed to recover iron from low-grade iron-ore resources (about 30%). In addition, industrial tests on this new reduction process were performed using a rotary kiln (φ1.5 m×15 m). However, rings were formed in the rotary kiln after some days of operation, and these rings affected normal operation. Ring formation during rotary kiln reduction has become a restraining factor for development of coal direct reduction processes using rotary kilns. Previous studies have mainly focused on the reduction process of high-grade ore (> 60%) for direct-reduced iron production. The manner in which highgrade ore reduction differs from low-grade ore reduction is unclear. So, the characteristics of the ring samples need to be studied primarily. Then, the characteristics that affect the formation mechanisms of the rings need to be investigated. Accordingly, relative operation may be developed and ring formation may be prevented. In this paper, ring samples formed in a rotary kiln during a low-grade iron-ore reduction process were studied. The characteristics and formation mechanisms of the ring samples were investigated in detail. The characteristics for ring samples collected from different positions in the rotary kiln were analyzed from the aspects of macro morphological, physical, and chemical compositions, softening and melting properties, and microstructural properties. Thermodynamic phase diagrams, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, X-ray diffraction, and chemical phase analyses were applied to reveal the ring formation mechanism during the rotary kiln reduction process. Results show that rings mainly comprise pellets and molten wrappage surroundings. The amount of molten wrappages and the proportions of MFe and CaO increase in the ring samples that are next to the kiln wall. The results also show that the ring samples exhibit lower softening and melting temperatures at this location. The main reason of ring formation is found to be the low melting phases including fayalite formed by FeO and SiO₂ in pelletizing powder and hedenbergite compounded by CaO (brought about by coal ash). Moreover, the existence of low melting point phases promotes the diffusion and migration of newly formed iron grains between metallized pellets, which exacerbates ring formation.