5. Conclusions

the bed temperature of 800C. As shown in Figure 12(a), the morphology of sample was porous when no MgO or CaO was added, and many tiny iron grains appeared on the surface. But the bed particles were covered by the local coating layer when adding MgO and CaO (Figure 12(b) and (c)). And no obvious iron whiskers and substance in molten state were found on the surface, which was inconsistent with the results suggesting cation additions promoted fibrous iron [33, 34]. The reason was that the growth of iron whiskers was suppressed due to the formation of coating layer. The EDS spot analysis (Figure 13(a) and (c)) shows that the compositions of this coating layer were not only Mg and Ca but also large amount of Fe. It was inferred that this layer consisted of some complex compounds where Fe2O3 were not reduced completely. However, unlike the coating layer, the uncoated surface appears the porous morphology. The EDS analysis (Figure 13(b) and (d)) show that the compositions of the uncoated surface were element Fe, suggesting that metallic iron was precipitated under the coating layer. This was because that the coating layer was porous and cracked, and thus the external/ internal diffusion for Fe oxides was easy. The metallization in bulk was slightly affected by surface coating. Therefore, it was inferred that the coating layer behaved like shell structure and inhibited the precipitated iron to expose on the surface of bed particle. The coating layer formed by adding MgO and CaO had a suppressive effect on defluidization and agglomera-

To further identify the formation of new phase of Mg or Ca compounds during the reduction, the dominant species in the agglomerates was analyzed by XRD. Figure 14 shows the phase composition with adding MgO and CaO before and after reduction. Before reduction the bed particles contained mainly Fe2O3 and a little MgOFe2O3. However, after reduction a great number of metallic irons were observed, and the Mg and Ca species were in the formation of MgOFeO and CaOFeO. Mg and Ca species can react with Fe2O3 to generate magnesium ferrite and calcium ferrite after pretreatment at 400700C [35], and these Fe compounds can

Figure 13. The EDS spot analysis of reduced particles (800C, 74–149 μm, 24.3 cm/s): (a) Point a; (b) Point b; (c) Point c;

tion.

118 Iron Ores and Iron Oxide Materials

and (d) Point d.


were in a good agreement with the experimental results. According to the operating phase diagram of fluidization obtained by this model, the stable fluidization and the defluidization region were determined.

3. Mg- and Ca-coating Fe2O3 particles were shown to significantly extend the defluidization time, and this inhibition effect was increased by increasing the addition amount. A coating layer on the surface was found to mainly contain magnesiowustite (MgOFeO) and calciowustite (CaOFeO) generated by the reactions between Mg/Ca oxides and Fe2O3/ FeO during reduction process, and this coating layer was effective in preventing the connection of precipitated iron. And compared with CaO, MgO was more effective in delaying defluidization at the same conditions, because the unstable calciowustite was reduced to metallic iron and cannot completely suppress the precipitation of iron.

Author details

Beijing, PR China

References

District, Beijing, PR China

\*, Jintao Gao<sup>1</sup>

\*Address all correspondence to: ywzhong@ustb.edu.cn

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Mechanism and Prevention of Agglomeration/Defluidization during Fluidized-Bed Reduction of Iron Ore

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1 State Key Laboratory of Advanced Metallurgy, University of Science and Technology,

2 Institute of Process Engineering, Chinese Academy of Sciences, Zhongguancun, Haidian

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