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

#### 4.1. Effect of MgO and CaO addition on defluidization

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) adding CaO.

are higher than that of adding CaO. It was indicated that MgO species had a better effect to reduce the bed agglomeration tendency and inhibit the defluidization.

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 of agglomeration and delayed the defluidization time.
