| Owing to the rapid development of iron and steel industry, the demand of iron ore has been increasing greatly in recent years. Because of the poor resource endowment, the slow reserves growth, the limited production ability and the shortage of self-subsistence, the prominent contradiction between supply and demand appears, leading to the excessive dependence on imported iron ore in the iron and steel industry in China. Thus, it has great strategic significance to improve the utilization of domestic refractory iron ore to meet the domestic demand.Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences collaborating with Northeastern University researched and developed a continuous type suspension roaster, which is based on the fluidization technology. The device has many advantages such as low roasting temperature, short roasting time, well-distributed products and low energy consumption. This research takes pure hematite and oolitic hematite of western Hubei as raw material. The process and transformation mechanism of the reduction of Fe2O3 and the oxidation of Fe3O4 have been researched to provide a theoretical basis for the design and application of the new equipment.The results of hematite magnetic roasting tests show that the main factors that affect the magnetic roasting are roasting temperature and time, while the particle size and reducing atmosphere have lower effects. The magnetism of the two kinds of roasting products being cooled by water quenching or in a nitrogen gas protection atmosphere is preferable. After being cooled to 200℃ in the nitrogen gas protection atmosphere, roasting products were exposed to the air for hours, and their magnetism remain unchanged. But if the temperature were over 250℃, the magnetism of the roasting products would decline sharply. The thermodynamic studies have shown that the thermodynamic trend of hematite reducing to magnetite is obvious. The roasting products were easy to be oxidized when cooled in the air. Their magnetism depends on the temperature at which they were exposed to the air in the cooling process. The temperature range of Fe3O4 transforming to y-Fe2O3 was 450-570K. If the temperature is higher than 570K, the crystal lattice of the y-Fe2O3 may be changed and its magnetism would descend. Hematite magnetic roasting is a heterogeneous reaction process, accompanied by a growth of magnetic domains and the conversion of the lattice. The reducing process transforms at the order of α-Fe2O3â†'γ-Fe2O3â†'Fe3O4.Suitable roasting conditions for oolitic hematite was feed size-0.3mm, roasting temperature 550～575℃, CO concentration 70%～90% and total gas flow 5-7m3/h. Under these conditions, roasting products were obtained and treated by grinding-magnetic separation. The concentrate with a TFe grade of 56.22% and a recovery of 88.84% was obtained. After magnetic roasting, weakly magnetic iron minerals were transformed into strong magnetic magnetite and maghemite, resulting in an easy separation. But the elements distribution of the ore hasn’t been changed in the magnetic roasting and it still needs grinding to dissociation for separation. Lattice transformation exists among the iron minerals in the magnetic roasting. During the cooling process, the temperature should be less than 600K when the roasting products were exposed to the air. The temperature of magnetite transforming into maghemite was 500 to 600K during the cooling process, but the conversion rate is low. There was little difference in the magnetism between the roasting products cooled by water quenching and cooled in the air. But the later can recycle large amount of heat.This research has analyzed the iron mineral transformation mechanism in the process of magnetic roasting, laying a theoretical foundation for the design and application of the new continuous type suspension roaster. |
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