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Relation between Sticking and Metallic Iron Precipitation on the Surface of Fe2O3 Particles Reduced by CO in the Fluidized Bed
Ben ZHANG, Zhi WANG and Zhancheng GUO.
(Chinese Academy of Sciences, Beijing, China.)
(May 6, 2011)
The Fe2O3 particles (150–224 ì m diameter) were reduced in a laboratory fluidized bed with CO–N2 mixture gas at 700–900°C to investigate the relation between sticking and iron precipitation. As a result, the sticking tended to occur with acceleration of the reduction rate, judging from the fluidization time. The sticking depended strongly on the metallization ratio signifying the probability of iron-iron contact that estimated the contact area of precipitated iron when particles collide together, whereas the reduction degree had indirect influence on it. Many tiny iron grains with the diameter of approximately 20–40 nm were found on the surface of particles by SEM and EDS. According to theory of microcrystal melting point, the grains reached Tammann temperature easily, leading to higher surface energy of iron, producing higher adhesion force among the reduced particles.
The dominating technology for ironmaking is blast-furnace process. Nevertheless, it is strongly dependent upon coke. The price of coke is always rising due to lack of coking coal, resulting in increase of the ironmaking cost.
Many new ironmaking processes as alternatives to the blast furnace have been developed in order to get rid of limit of cokes. Due to no need prior treatment for fine iron ores, uniform temperature in the reactor and excellent heat and mass transport, the fluidized bed had been used as reduction or pre-reduction reactor in many processes. However, defluidization due to sticking occurred easily during reduction of fine iron ores in the fluidized bed, impeding the industrialization of fluidized bed reactors during ironmaking process.
Three different types of sticking were recognized by many former investigations on reduction of fine iron ore in the fluidized bed. The first type of sticking was caused by freshly precipitated iron with high surface energy. The iron resulted in high adhesion force between reduced iron ore particles. B. Zhang and Z. Wang further studied on relation between sulfur pressure and sticking of fine iron ores in fluidized bed reduction. The iron saturated by atoms of sulfur had high surface energy, tending to cause sticking.
The second kind of sticking appeared when some iron whiskers precipitated on the particle surface, being initiated by the contact of the whisker that hooked mechanically the particles together. Z.Guo also reported that the iron whisker precipitated on the surface of particle permitting iron-iron contact and thereby causing sticking. And the iron whisker was one type of morphology of precipitated iron with high surface energy. The third type of sticking occurred accompanying with a liquid phase within the overheated zone, which was stimulated by presence of gangue, because there was a build up of low melting eutectic phase (CaO–SiO2–FeO) that sticked together in an iron ore particle. The scientist reported that the ores had a strong sticking tendency at temperature above 810°C. When temperature increased, some compounds of the iron ore started to soften and melt.
Since the iron ore has complicated chemical composition, leading many factors affecting sticking of ore particles to entangle together, it was difficult to clarify the sticking mechanism of each factor. To simplify the chemical composition of iron ores, chemical grade Fe2O3 particles were selected as materials for reduction in the fluidized bed. The most former researches mentioned above indicated the sticking was associated with the precipitated iron, but the relation between sticking and the reduction degree found by them was unreasonable, because the reduction degree could not describe exactly the quantity of precipitated iron. Moreover, it was suggested that the sticking was the result of sintering of metallic iron. However, they discussed defluidization resulted from neck growth by keeping the contact of particles for one hour.
Obviously, the time of particles remaining in the fluidized bed was far less than one hour.
Based on these former findings, this paper discussed the relation between sticking and precipitated iron by metallization ratio, and explored the sticking mechanism of precipitated iron by analysis of the surface energy of iron
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