2. Mechanism of agglomeration and defluidization of iron/iron oxide particles

#### 2.1. Effect of metallization degree on agglomeration tendency

The fluidized-bed apparatus is shown in Figure 1, which is a bubbling fluidized bed consisting of a transparent silica tube with an inner diameter of 2.5 cm. The reactor is heated by a transparent electric resistance, and the fluidization state in the reactor can be observed at high temperature. Bed temperature is measured and controlled by a PID controller connected with a K-typed thermocouple. The gas flow rate and pressure drop across the bed is measured by a digital mass flow meter and a pressure transmitter, respectively. The pressure sensor is located at 1 cm below the gas distributor.

Mechanism and Prevention of Agglomeration/Defluidization during Fluidized-Bed Reduction of Iron Ore http://dx.doi.org/10.5772/intechopen.68488 107

Figure 1. Schematic diagram of fluidized-bed apparatus.

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.

2. Mechanism of agglomeration and defluidization of iron/iron oxide

The fluidized-bed apparatus is shown in Figure 1, which is a bubbling fluidized bed consisting of a transparent silica tube with an inner diameter of 2.5 cm. The reactor is heated by a transparent electric resistance, and the fluidization state in the reactor can be observed at high temperature. Bed temperature is measured and controlled by a PID controller connected with a K-typed thermocouple. The gas flow rate and pressure drop across the bed is measured by a digital mass flow meter and a pressure transmitter, respectively. The pressure sensor is located

2.1. Effect of metallization degree on agglomeration tendency

particle cohesiveness.

106 Iron Ores and Iron Oxide Materials

at 1 cm below the gas distributor.

particles

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

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 the course of metallization.

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

below 0.52%, no indication of defluidization is observed. This was because that the particle with lower amount of precipitated iron did not have enough adhesion force to form agglomerates and thus maintained a good quality of fluidization. When the metallization degree reached to 23.56%, the defluidization approached, indicating the defluidization was accompanied with the accumulation of precipitated iron. This result suggested that large quantities of metallic iron can increase the stickiness of particles by providing enough contact area of iron. This conclusion was in accord with that found by Gransden et al. [9, 10], who indicated that the agglomeration was caused by the iron-iron contact. Therefore, the reduced Fe2O3 particle

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Typical morphologies of reduced samples at various metallization degrees are shown in Figure 5. At lower metallization degree (i.e., <4.57%), numerous pits are formed on the oxide surface prior to iron nucleation, and the morphology presents smooth. But at higher metallization degree (i.e., >15.64%), the iron nuclei tend to appear (about 0.10.15 μm in a diameter), forming microconvexities on the surface. These iron nuclei with nano/microsize were prone to soften and sinter together due to a higher surface energy [11]. Thus, the reduced particles with a higher metallization degree had a stronger adhesive force for agglomeration. On the other hand, the particle surface becomes rough due to the formation of iron nuclei. Such rough surface caused the enhancement of friction force among bed particles to result in a poor fluidization quality.

To investigate the evaluation of particle cohesiveness responsible for agglomeration, the thermomechanical analysis (TMA) was carried out by a dilatometer (NETZSCH-DIL402C,

Figure 5. Evolution of surface morphology of Fe2O3 particles in the controlled defluidization tests: (a) MFe = 0.52%; (b)

with a higher metallization degree had a larger agglomeration tendency.

2.2. Effect of iron precipitation on particle cohesiveness

MFe = 4.57%; (c) MFe = 15.64%; and (d) MFe = 23.56%.

Figure 2. Illustration of a typical controlled defluidization test for Fe2O3 reduction.

Figure 3. Influence of metallization degree on the defluidization temperature in the controlled defluidization tests.

Figure 4. X-ray patterns of each sample from the controlled bed defluidization tests.

below 0.52%, no indication of defluidization is observed. This was because that the particle with lower amount of precipitated iron did not have enough adhesion force to form agglomerates and thus maintained a good quality of fluidization. When the metallization degree reached to 23.56%, the defluidization approached, indicating the defluidization was accompanied with the accumulation of precipitated iron. This result suggested that large quantities of metallic iron can increase the stickiness of particles by providing enough contact area of iron. This conclusion was in accord with that found by Gransden et al. [9, 10], who indicated that the agglomeration was caused by the iron-iron contact. Therefore, the reduced Fe2O3 particle with a higher metallization degree had a larger agglomeration tendency.

Typical morphologies of reduced samples at various metallization degrees are shown in Figure 5. At lower metallization degree (i.e., <4.57%), numerous pits are formed on the oxide surface prior to iron nucleation, and the morphology presents smooth. But at higher metallization degree (i.e., >15.64%), the iron nuclei tend to appear (about 0.10.15 μm in a diameter), forming microconvexities on the surface. These iron nuclei with nano/microsize were prone to soften and sinter together due to a higher surface energy [11]. Thus, the reduced particles with a higher metallization degree had a stronger adhesive force for agglomeration. On the other hand, the particle surface becomes rough due to the formation of iron nuclei. Such rough surface caused the enhancement of friction force among bed particles to result in a poor fluidization quality.

## 2.2. Effect of iron precipitation on particle cohesiveness

Figure 2. Illustration of a typical controlled defluidization test for Fe2O3 reduction.

108 Iron Ores and Iron Oxide Materials

Figure 4. X-ray patterns of each sample from the controlled bed defluidization tests.

Figure 3. Influence of metallization degree on the defluidization temperature in the controlled defluidization tests.

To investigate the evaluation of particle cohesiveness responsible for agglomeration, the thermomechanical analysis (TMA) was carried out by a dilatometer (NETZSCH-DIL402C,

Figure 5. Evolution of surface morphology of Fe2O3 particles in the controlled defluidization tests: (a) MFe = 0.52%; (b) MFe = 4.57%; (c) MFe = 15.64%; and (d) MFe = 23.56%.

Germany). The thermal expansion or contraction was measured to obtain the temperature at which sintering and surface softening became significant. The sample was heated up to 800�C at a rate of 10�C/min. The sample was in a flow of 50 ml/min of pure Ar, and the load on it was 30 cN.

measured at 800�C using the thermomechanical analysis as reported by Tardos [20]. Figure 5(b) shows that the surface viscosities gradually decreased as the metallization degree increased. When the metallization degree approaches to 23.56% (point of defluidization), the viscosity drops significantly, indicating a strong surface softening and stickiness of particles. This result suggested that the particle adhesion of iron oxide was enhanced with the increase of the amount

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For crystalline materials, the Huttig temperature is defined as the temperature where the lattices and surface atoms become appreciably mobile. For pure metals, this temperature was approximately 0.3Tm (about 330�C for iron) [22]. It was inferred that the Fe2O3 particle surface preformed viscosity because of the iron precipitation when the temperature was higher than 330�C. This fresh precipitated iron had high particle cohesiveness due to the higher activity and surface energy [8–11]. In the course of Fe2O3 reduction, numerous Fe vacancies were formed, and iron atoms were released from the oxide lattice due to oxygen removal. Consequently, the migration of iron atoms to the reducing front through Fe vacancies was accelerated due to the chemical potential gradient of O/Fe [23]. Therefore, the particle surface softened as metallic iron precipitating, resulting in a decrease of apparent surface viscosity.

Some of the agglomerates, sampled from controlled bed defluidization tests, were examined by SEM/EDS analysis. As seen in Figure 7, sintered necks instead of iron whiskers were observed between particles. The reduced particles are sticked together by the sintered neck, the diameter of which was roughly 0.8 μm. The EDS analysis showed that Fe was the dominant species (97 wt.%) in the connect position. Thus, the reduced particle was connected by a connective bridge composed of metallic iron. These results proved that the presence of iron, rather than iron oxide (FeO, Fe3O4), caused the formation of the sticky particle surfaces readily for agglomeration. In addition, it was noted that the agglomerates contained particles only several microns in diameter between coarse particles. These fine particles played a role of

of metallic iron on the surface.

"bridge" in the formation of agglomerates.

3. Model to predict agglomeration and defluidization

Figure 7. SEM image and EDS analysis of agglomerate sample at metallization degree of 15.64%.

The aim of this work is modeling the high-temperature defluidization behavior of iron powders involving the effects of gas velocity and gas properties. The calculation is focused on the

The samples from the controlled bed defluidization tests were examined. The greatest change in ΔL/ΔL<sup>0</sup> gradient occurs for each curve is a measure of the minimum sintering temperature (Ts) [4, 5]. As shown in Figure 6(a), the sample with a higher metallization degree has a lower value of Ts. This result showed that iron precipitation reduced significantly the minimum sintering temperature of the whole particle and thus enhanced the sintering activity of the reduced particle. The sintering rate depended on the diffusion coefficient (Ds) of materials. The value of Ds for α-iron self-diffusion was calculated to be approximately 10 times larger than that for the diffusion of Fe in FeO at 800�C according to the empirical correlations [17, 18]. This indicated that metallic iron had a higher sintering activity than FeO. Therefore, the tendency of the reduced particles to sinter together was intensified due to iron precipitation.

Many investigations showed the importance of the initial sintering temperature in fluidization quality [4, 5], because it is an indicator of the onset of agglomeration and is a softening point where the rate of sintering dramatically accelerated. Previous research studies [4, 5] have confirmed that the cohesiveness and sintering of the fluidized particles can lead to the uncontrolled particle agglomeration and subsequent defluidization at temperatures at or above the sintering point. A special class of agglomeration was due to the formation of new species on the surface of the solid particle during a chemical reaction. At temperatures well below the softening (sintering) points of both the reactants and the products, particle agglomeration can occur during the process of product formation [19]. Accordingly, in the case of reduction Fe2O3 to Fe, when metallic iron formed above sintering temperature, the adhesive force due to sintering was increased. Therefore, the sintering of metallic iron on the surface provided favorable conditions for agglomeration.

At the minimum sintering temperature (Ts), the surface of material began to soften and deform, and the surface stickiness began to appear [20, 21]. And the agglomeration and defluidization occurred as a result of having "sticky" bed materials. In this study, the surface viscosities of the reduced samples in the controlled bed defluidization tests at various metallization degrees were

Figure 6. Influence of iron precipitation on particle cohesiveness of reduced Fe2O3 particles: (a) the minimum sintering temperature; (b) (the surface viscosity of Fe2O3 particles at 800 �C).

measured at 800�C using the thermomechanical analysis as reported by Tardos [20]. Figure 5(b) shows that the surface viscosities gradually decreased as the metallization degree increased. When the metallization degree approaches to 23.56% (point of defluidization), the viscosity drops significantly, indicating a strong surface softening and stickiness of particles. This result suggested that the particle adhesion of iron oxide was enhanced with the increase of the amount of metallic iron on the surface.

Germany). The thermal expansion or contraction was measured to obtain the temperature at which sintering and surface softening became significant. The sample was heated up to 800�C at a rate of 10�C/min. The sample was in a flow of 50 ml/min of pure Ar, and the load on it was

The samples from the controlled bed defluidization tests were examined. The greatest change in ΔL/ΔL<sup>0</sup> gradient occurs for each curve is a measure of the minimum sintering temperature (Ts) [4, 5]. As shown in Figure 6(a), the sample with a higher metallization degree has a lower value of Ts. This result showed that iron precipitation reduced significantly the minimum sintering temperature of the whole particle and thus enhanced the sintering activity of the reduced particle. The sintering rate depended on the diffusion coefficient (Ds) of materials. The value of Ds for α-iron self-diffusion was calculated to be approximately 10 times larger than that for the diffusion of Fe in FeO at 800�C according to the empirical correlations [17, 18]. This indicated that metallic iron had a higher sintering activity than FeO. Therefore, the tendency of

Many investigations showed the importance of the initial sintering temperature in fluidization quality [4, 5], because it is an indicator of the onset of agglomeration and is a softening point where the rate of sintering dramatically accelerated. Previous research studies [4, 5] have confirmed that the cohesiveness and sintering of the fluidized particles can lead to the uncontrolled particle agglomeration and subsequent defluidization at temperatures at or above the sintering point. A special class of agglomeration was due to the formation of new species on the surface of the solid particle during a chemical reaction. At temperatures well below the softening (sintering) points of both the reactants and the products, particle agglomeration can occur during the process of product formation [19]. Accordingly, in the case of reduction Fe2O3 to Fe, when metallic iron formed above sintering temperature, the adhesive force due to sintering was increased. Therefore, the sintering of metallic iron on the surface

At the minimum sintering temperature (Ts), the surface of material began to soften and deform, and the surface stickiness began to appear [20, 21]. And the agglomeration and defluidization occurred as a result of having "sticky" bed materials. In this study, the surface viscosities of the reduced samples in the controlled bed defluidization tests at various metallization degrees were

Figure 6. Influence of iron precipitation on particle cohesiveness of reduced Fe2O3 particles: (a) the minimum sintering

the reduced particles to sinter together was intensified due to iron precipitation.

provided favorable conditions for agglomeration.

temperature; (b) (the surface viscosity of Fe2O3 particles at 800 �C).

30 cN.

110 Iron Ores and Iron Oxide Materials

For crystalline materials, the Huttig temperature is defined as the temperature where the lattices and surface atoms become appreciably mobile. For pure metals, this temperature was approximately 0.3Tm (about 330�C for iron) [22]. It was inferred that the Fe2O3 particle surface preformed viscosity because of the iron precipitation when the temperature was higher than 330�C. This fresh precipitated iron had high particle cohesiveness due to the higher activity and surface energy [8–11]. In the course of Fe2O3 reduction, numerous Fe vacancies were formed, and iron atoms were released from the oxide lattice due to oxygen removal. Consequently, the migration of iron atoms to the reducing front through Fe vacancies was accelerated due to the chemical potential gradient of O/Fe [23]. Therefore, the particle surface softened as metallic iron precipitating, resulting in a decrease of apparent surface viscosity.

Some of the agglomerates, sampled from controlled bed defluidization tests, were examined by SEM/EDS analysis. As seen in Figure 7, sintered necks instead of iron whiskers were observed between particles. The reduced particles are sticked together by the sintered neck, the diameter of which was roughly 0.8 μm. The EDS analysis showed that Fe was the dominant species (97 wt.%) in the connect position. Thus, the reduced particle was connected by a connective bridge composed of metallic iron. These results proved that the presence of iron, rather than iron oxide (FeO, Fe3O4), caused the formation of the sticky particle surfaces readily for agglomeration. In addition, it was noted that the agglomerates contained particles only several microns in diameter between coarse particles. These fine particles played a role of "bridge" in the formation of agglomerates.

Figure 7. SEM image and EDS analysis of agglomerate sample at metallization degree of 15.64%.
