某低品位铁矿回转窑还原结圈物特性及其形成机制

回转窑结圈一直以来是制约煤基回转窑直接还原工艺发展的重要因素,以某低品位铁矿回转窑还原结圈物为研究对象,深入研究回转窑结圈物的特性及其形成机制.从结圈物的宏观形貌、物化性能、软熔特性和微观结构入手对某低品位铁矿球团回转窑结圈物的特性进行分析,并结合热力学相图、化学物相及能谱分析研究了结圈物的形成机制.结果表明:结圈物由熔融物包裹球团形成,接近窑壁,其熔融包裹物增多,结圈物中MFe、CaO含量明显增大,软熔温度越低;由球团粉末中FeO与SiO₂形成的铁橄榄石及煤灰带入的CaO而形成的钙铁辉石低熔点相是造成结圈的主要原因;低熔点相的存在同时也促进了金属化球团间铁晶粒的相互扩散与迁移,从而加剧了结圈现象. 

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.