1. Introduction

Fluidized-bed reactors can improve the reaction kinetics and realize better utilization of resource/ energy and lower pollutant emissions [1]. Therefore, as a trend in the industrial application, fluidized beds are ideally suited to the processing of these finely sized raw materials and have great competitiveness. However, fluidized beds were tested but failed because of the serious problem of particles agglomeration and subsequent defluidization [2]. The continuous operation and high productivity was often limited by partial or complete defluidization. It is, therefore, a critical problem to solve defluidization and particle agglomeration at high temperatures for the application of fluidized beds.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

Particle agglomeration in fluidized-bed systems has received considerable attention due to its close association with industrial processes. Gluckman [3] indicated that the generation of agglomerations depended on the cohesiveness of particles collisions. Seville and coworkers [4, 5] pointed that the defluidization phenomenon was attributed to an increased rate of sintering at elevated temperatures, and the tendency of particle to agglomerate depended strongly on their physical and chemical characteristics at high temperature. Two types of adhesion are considered [4–7]: (1) Visco materials cause sintering on glassy materials. Increasing the operating temperature can reduce the viscosity of the materials and cause a larger adhesive force. (2) Melting and chemical reaction produces liquid-phase materials. These liquid-phase materials can form a bridge between two particles and cause agglomeration and defluidization.

In the case of fluidized-bed reduction of iron ore, earlier works [8–10] indicated that sticking occurred mostly during metallization of ore. The defluidization tended to be preferred at a high fractional reduction and metallization degree. Some ore particles were precipitated by the metal iron with the fibrous shape on the particle surface. The sticking was initiated by the contact of the needles that hooked mechanically the particles together. Moreover, the work of Gransden et al. [9, 10] showed that the sticking was associated with the iron-iron contact regardless of formation of iron whiskers or not. They believed that the fresh precipitated iron had a high activity or surface energy, and thus appeared high adhesion energy to agglomeration. Zhong et al. [11] also reported agglomerates formed due to sintering of reduced iron, and nano/mircostructure on the particle surface had a promotive effect on particle agglomeration. Therefore, the sticking tendency depended strongly on iron precipitation of particles. With respect to adhesion of metallic iron, a sintering mechanism of iron particles has been reported involving the relationship between the bed temperature and the minimum gas fluidizing velocity required to prevent defluidization [12, 13]. However, most research studies focused on the metallic iron content and morphologies at the defluidization point [2, 8–11] and thus did not involve the evolution of particle properties during metallization. In the gas-solid reaction, new components were produced and thus caused the changes in surface structure and the particle properties. Therefore, the new phase formation can significantly affect the particle cohesiveness.

To determine the evolution of the real-time bed agglomeration tendencies and agglomeration potential, the controlled bed defluidization tests (CBD) were carried out, which were adapted from Öhman [14, 15]. Each experiment with a 5 g of iron oxide was started by a normal fluidized-bed reduction by CO at 700�C (1.0 NL/min, about 12.2 cm/s) to obtain a series of reduced samples with different metallization degrees (MFe). Preliminary reduction experiments indicated that when MFe was higher than 25%, the bed agglomeration would appear. Thus, MFe of all the reduced samples was controlled below 25%. And then at a point where a designated metallization degree was achieved by controlling the reducing time, the reduction was stopped and the fluidizing gas was switched to N2 atmosphere (1.0 NL/min, about 12.2 cm/s). Then, the bed was heated up at a rate of 3�C/min until a bed agglomeration was achieved. The bed defluidization temperature, Tdef, was determined by online analysis of the variations in the measured bed temperatures and differential pressures and was used to characterize bed agglomeration tendency at various metallization degrees. Defluidization is defined as any condition where a well-fluidized bed loses fluidization, whether partial or total [16]. A typical illustration of fluctuations in temperatures and differential bed pressures versus time in a controlled bed defluidization test is shown in Figure 2. Meanwhile, the controlled bed defluidization tests can also be carried out as a series of interrupted experiments to investigate the evolution of particles in

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

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The real-time agglomeration tendency of the reduced samples represented by the defluidization temperature Tdef was obtained by the controlled bed defluidization tests. As shown in Figure 3, the defluidization temperature decreases with the increase of the metallization degree, indicating an increase of agglomeration tendency. The analysis of XRD (X-ray diffraction) shows that all the reduced samples in the controlled defluidization tests only contain metallic iron and FeO (Figure 4). The diffraction peaks of metallic iron obviously strengthened with increasing reduction time, indicating the content of precipitated iron increased. Therefore, the agglomeration tendency depended strongly on the metallic iron content. At the metallization degree

the course of metallization.

Figure 1. Schematic diagram of fluidized-bed apparatus.
