3.2.2. Influence of gas properties

Na ¼ K � Nd ð11Þ

<sup>β</sup> <sup>ð</sup>12<sup>Þ</sup>

<sup>K</sup> <sup>¼</sup> <sup>α</sup> Ab<sup>2</sup>

where K is a regressive constant, representing the unknown errors in this equation. The variables in the model are as a function of temperature, and the correlations are nonlinear. Therefore, by combining Eqs. (9)–(12), the temperature to reach defluidization was obtained by a

Figure 8 presents the results obtained at different gas velocities. According to the definition of defluidization criterion, the temperature corresponding to the intersection of the curves of Na and K∙Nd is the defluidization temperature. As it can be seen, the temperature to reach defluidization increases with increasing the gas velocity for all the fluidizing gases. In previous studies [27–29], the generation of agglomeration and defluidization depended on the balance of the cohesive and breaking forces. And if the adhesive force between particles exceeded the breakage force, agglomeration and defluidization in the bed probably occur. As shown in

Figure 8. The variation of the calculated values of Na and K∙Nd with temperature: (a) N2; (b) Ar; and (c) H2.

3.2. Modeling results and comparison with experimental data

numerical method.

114 Iron Ores and Iron Oxide Materials

3.2.1. Influence of gas velocity

Figure 9(b) presents the effect of gas type on defluidization temperature. According to the calculated results, the defluidization temperature decreases when using the gas with greater viscosity and density as a fluidizing agent. As seen in Figure 9(b), at a constant gas velocity the adhesion force for different gases almost has no change, whereas the drag force is strongly dependent on the gas properties and increases with increasing the gas viscosity. Comparing the three fluidizing gases, the defluidization temperatures are in the following sequence: H2< N2< Ar. This was because the fluidizing gas with greater viscosity can produce a stronger drag force to resist agglomeration, which was in accord with the experimental results [21].

The calculated defluidization temperatures were in a good agreement with the experimental results in all experiment conditions, and thus confirmed the predicted modeling. The model successfully described the defluidization temperature as a function of gas velocity and gas property. According to the results above, the fluidizing phase diagram was obtained as shown in Figure 10, which was divided into the stable fluidization and the defluidization region. The fluidization state was maintained below the curve intersection of Na and K∙Nd, while the bed was defluidized above the intersection. This suggested that at a certain operating parameter,

Figure 9. Comparison of calculated defluidization temperature with experimental data: (a) Influence of gas velocity; and (b) influence of gas properties.

are higher than that of adding CaO. It was indicated that MgO species had a better effect to

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

http://dx.doi.org/10.5772/intechopen.68488

117

However, previous research studies suggested that some compounds with low melting points or iron whiskers were formed by adding MgO and CaO. These compounds and iron whiskers provided a favorable condition to form agglomeration of the Fe2O3 particles during reduction [8, 30]. With inconsistent results as compared to those of the experiment, this work was focused on investigating the relationship between the new phase formation and particle adhesion during Fe2O3 reduction. It has been confirmed that agglomeration at high temperature was attributed to the activity of metallic iron [10, 21]. The surface energy of precipitated iron may be deactivated or reduced by Mg/Ca oxide, and thus the surface cohesiveness was eliminated. On the other hand, MgO and CaO may react with Fe2O3 to generate some eutectics with high melting points or some stable compounds hard to be reduced to metallic iron. In these conditions, the formation of liquid phase and the connection of metallic iron on the surface can be avoided at high temperature. Therefore, MgO and CaO inhibited the formation

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

reduce the bed agglomeration tendency and inhibit the defluidization.

of agglomeration and delayed the defluidization time.

4.2. Behaviors of Mg and Ca species during reduction

(c) adding 2% CaO.

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

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 comprehensive model of agglomeration in a fluidized-bed reactor.
