4.2. Behaviors of Mg and Ca species during reduction

there was a limiting operating temperature and gas velocity, above which the defluidization occurred. Therefore, the model can be considered as a theory reference to avoid defluidization in the actual fluidizing operations. However, the model cannot simulate the agglomeration involving chemical reaction and phase transformation. More work is needed for a comprehen-

4. Prevention of agglomeration by surface coating of Mg and Ca oxides

Figure 11 shows the effects of the addition of MgO and CaO on the defluidization time of Fe2O3 particles at various operating temperatures. It was found that adding Mg and Ca had the similar effect on prolonging the defluidization time. As the addition content of MgO and CaO increased, the defluidization time was delayed. The defluidization time of adding MgO

Figure 11. Influence of operating temperature on the defluidization time (50–74 μm, 36.5 cm/s): (a) adding MgO and (b)

sive model of agglomeration in a fluidized-bed reactor.

116 Iron Ores and Iron Oxide Materials

Figure 10. The fluidization phase diagram of iron powders at elevated temperatures.

4.1. Effect of MgO and CaO addition on defluidization

adding CaO.

In previous studies [31, 32], the formation of coating layer and connective bridge among the bed particles had been found based on the SEM (scanning electron microscopy)/EDS (energy dispersive spectrometry) analysis. However, in the case of no liquid phases, the physicochemical behavior of Mg/Ca species on the surface and their effects on the bed particles has not been determined yet. Therefore, the focus was the role of Mg and Ca species in the formation of the coating layer. The surfaces of bed particle samples were analyzed by SEM/EDS after the test at

Figure 12. The SEM images of reduced particles (800�C, 74–149 μm, 24.3 cm/s): (a) no additive; (b) adding 2% MgO; and (c) adding 2% CaO.

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 agglomeration.

be reduced to low-valent oxides, whereas the reaction rate was much lower than that of pure Fe2O3. Therefore, the rate of surface metallization was decreased by adding MgO and CaO. Because the defluidization occurred at a critical metallization degree, agglomeration/ defluidization was delayed by reducing the time to reach the critical metallization degree by

Figure 14. The XRD patterns of Fe2O3 particles before and after reduction (800C, 74–149 μm, 24.3 cm/s): (a) adding 2%

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The composition and reducibility of the coating layers formed by adding MgO and CaO were different. For adding MgO, MgOFe2O3 was observed, which was reduced to MgOFeO based on XRD analysis. But the reduction of MgOFeO to Fe hardly happens below 1100C [36]. And the surface not only contains Fe and Mg, but also considerable O (Figure 13(a)), indicating the outer layer was mainly composed of oxides. Therefore, an unreduced coating layer in the formation of MgOFeO generated on the surface, and thus prevented the contact of iron precipitated. As a result, the defluidization was inhibited. However, for adding CaO, the Fe oxides in the calciowustite coating can be reduced thermodynamically to metallic iron [37]; and the outer layer (Figure 13(c)) contains Fe, Ca, and a little O, suggesting that the phases of Fe were mainly metallic iron and a little oxide. It indicated that the calciowustite was reduced finally to iron. Therefore, the inhibition effect of Ca species can only temporarily inhibit defluidization. When metallic iron appeared on the surface, the defluidization occurred again. Thus, the inhibition effect of CaO on defluidization was less than that of MgO, especially at high temperatures

1. Particle cohesiveness and agglomeration tendency were initiated by metallization and depended strongly on the amount of iron precipitation. As the metallization degree increased, the fluidization behavior of Fe2O3 particles evolved from cohesiveness to sticky, and thus agglomeration appeared. The precipitation of metallic iron with submicro size was clearly identified as the necks on the Fe2O3 surfaces, which caused the formation of agglomerates.

2. Based on force balance, a quantitative model for the fluidization characteristics of iron powders was developed to describe the defluidization behavior at elevated temperatures. The theoretical model successfully predicted the defluidization temperature as a function of fluidizing gas velocity and gas properties. The simulated defluidization temperatures

adding MgO and CaO.

MgO and (b) adding 2% CaO.

5. Conclusions

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; and (d) Point d.

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Figure 14. The XRD patterns of Fe2O3 particles before and after reduction (800C, 74–149 μm, 24.3 cm/s): (a) adding 2% MgO and (b) adding 2% CaO.

be reduced to low-valent oxides, whereas the reaction rate was much lower than that of pure Fe2O3. Therefore, the rate of surface metallization was decreased by adding MgO and CaO. Because the defluidization occurred at a critical metallization degree, agglomeration/ defluidization was delayed by reducing the time to reach the critical metallization degree by adding MgO and CaO.

The composition and reducibility of the coating layers formed by adding MgO and CaO were different. For adding MgO, MgOFe2O3 was observed, which was reduced to MgOFeO based on XRD analysis. But the reduction of MgOFeO to Fe hardly happens below 1100C [36]. And the surface not only contains Fe and Mg, but also considerable O (Figure 13(a)), indicating the outer layer was mainly composed of oxides. Therefore, an unreduced coating layer in the formation of MgOFeO generated on the surface, and thus prevented the contact of iron precipitated. As a result, the defluidization was inhibited. However, for adding CaO, the Fe oxides in the calciowustite coating can be reduced thermodynamically to metallic iron [37]; and the outer layer (Figure 13(c)) contains Fe, Ca, and a little O, suggesting that the phases of Fe were mainly metallic iron and a little oxide. It indicated that the calciowustite was reduced finally to iron. Therefore, the inhibition effect of Ca species can only temporarily inhibit defluidization. When metallic iron appeared on the surface, the defluidization occurred again. Thus, the inhibition effect of CaO on defluidization was less than that of MgO, especially at high temperatures
