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Direct reduction of fine iron ore concentrate in a conical fluidized bed
Direct reduction of fine iron ore concentrate in a conical fluidized bed Shengyi He, Haoyan Sun, Chaoquan Hu, Jun Li, Qingshan Zhu, Hongzhong Li PII: DOI: Reference:
S0032-5910(17)30213-9 doi:10.1016/j.powtec.2017.03.007 PTEC 12412
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Powder Technology
Received date: Revised date: Accepted date:
28 September 2016 11 February 2017 2 March 2017
Please cite this article as: Shengyi He, Haoyan Sun, Chaoquan Hu, Jun Li, Qingshan Zhu, Hongzhong Li, Direct reduction of fine iron ore concentrate in a conical fluidized bed, Powder Technology (2017), doi:10.1016/j.powtec.2017.03.007
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Direct reduction of fine iron ore concentrate in a conical fluidized bed
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese
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Shengyi He a,b, Haoyan Sun a, Chaoquan Hu a, Jun Li a, Qingshan Zhu a,b,*, Hongzhong Li a
Academy of Sciences, Beijing 100190, China
University of Chinese Academy of Sciences, Beijing 100049, China
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* Corresponding Author: Qingshan Zhu;
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E-mail:
[email protected]
Tel:
+86-010-62536108
Fax:
+86-010-62536108
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ACCEPTED MANUSCRIPT Abstract
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The direct utilization of fine iron ore concentrate in fluidized beds is of significance but has the
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problem of defluidization during operation. In this study, we have made a comparison of a conical
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fluidized bed and a cylindrical fluidized bed for the direct reduction process. The results showed that the conical fluidized bed was more versatile than the cylindrical fluidized bed in treating the fine iron
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ore concentrate. In the conical fluidized bed, with a high superficial gas velocity, defluidization was
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successfully inhibited without adding inert materials or causing elutriation issues. Simultaneously, the reduction efficiency was considerably improved and the obtained iron agglomerates exhibited a
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high metallization degree. This work also proposes a fluidization regime diagram for the conical
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fluidized bed at the temperature range from 650 to 800 oC. According to the fluidization regime
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diagram, the conical fluidized bed is able to considerably expand the operational temperature range for the direct reduction process. These results demonstrate that the conical fluidized bed is a clean,
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efficient alternative means for intensifying the direct reduction process.
Keywords: Conical fluidized bed; Cylindrical fluidized bed; Direct reduction; Defluidization; Agglomeration; High Temperature.
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ACCEPTED MANUSCRIPT 1 Introduction Fluidized beds are promising reactors for direct reduction (DR) processes because of their high
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mass and heat transfer efficiencies, possibility of continuous operation [1-3]. Actually, several DR
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processes based on fluidization technology have been established over the past several decades, such as the FINMET, Circored and FINEX processes [3]. The bottleneck in developing a fluidization DR
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process is the challenge associated with defluidization resulting from the adhesion of direct reduced iron (DRI) particles, which impedes continuous operation and hence lower the overall efficiency of
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the industrial production [2, 4-7]. A common solution for prevention of defluidization in the process
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is restricting the particle size distribution of the raw iron ore particles. For instance, the Circored
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process excludes the particles smaller than 0.10 mm in the raw materials before reduction in the fluidized bed [8]. This is reasonable since the defluidization issue tends to be exacerbated with the
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use of fine particles [9]. However, as high-grade lump ore resources are gradually depleted, low-grade iron ores are being utilized more often for iron-making, resulting in the generation of a
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greater amount of fine concentrate (<0.10 mm) [10]. Therefore, the prevention of defluidization during the DR of such fine iron ore concentrates is of great significance and has been extensively studied [2, 7, 9, 11].
Conical reactors appear to show promise with regard to solving the defluidization issue and are already widely utilized in the processing of adhesive particles. Based on the different designs, two gas-contact regimes in the conical reactors were distinguished, i.e., the conical fluidized bed regime and the conical spouted bed regime, which were both suitable for avoiding defluidization in chemical engineering processes. The conical spouted bed has been applied in the cases of waste plastic and
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ACCEPTED MANUSCRIPT biomass pyrolysis [12-15], gasification processes [16] and iron ore DR [17]. The vigorous movement in the central of the bed maintained the stable operation. Owaza [17] investigated the reduction of
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coarse iron ore particles (0.30 to 1.0 mm) in a conical spouted bed and found that the sticking
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tendency of such particles in the spouted bed was reduced compared with that in a conventional fluidized bed. However, the spouted bed usually processes coarse particles belonging to Class-D
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according to Geldart’s classification [18]. While, on the other hand, the conical fluidized bed is more favorable in treating fine particles or particles with wide size distribution. The conical fluidized bed
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has also been applied in the combustion/gasification of coal and biomass [19, 20], cohesive ultrafine
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particle fluidization [21, 22], and DR of iron ores [23, 24]. In conical fluidized bed, the high gas
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velocity (Ug) at the bottom of the vessel results in adequate fluidization of such coarse particles/agglomerates, while the low Ug at the top of the reactor avoids excessive elutriation of fine
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particles [25]. Several patents [23, 24] have claimed that a conical fluidized bed is adequate for the DR of iron ores having a wide size range, because the low gas velocity at the top of the bed
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considerably decreases the elutriation rate of fine particles. In contrast, little attention has been devoted to the DR of fine iron ore concentrates smaller than 0.10 mm in conical fluidized beds, although the conical fluidized bed showed great potential in processing iron ore fines. In the patent of Stephens, Jr. et al. [26], fine iron ore concentrate (<45 μm) was reduced in a conical fluidized bed together with coarse iron particles (250 to 900 μm). The fine particles stuck onto the surface of the coarse particles and consequently defluidization was prevented. However, the addition of such coarse particles tends to increase the bed weight without increasing productivity. In conclusion, a conical fluidized bed can exhibit excellent performance in the processing of sticky particles, but the direct
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ACCEPTED MANUSCRIPT utilization of fine iron ore concentrates in this type of bed without the use of additives still remains a
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challenge and requires further investigation.
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The objective of the present study was to investigate the possibility of the direct reduction of
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fine iron ore concentrate without any additions in a conical fluidized bed. Specifically, the fluidization and reduction behaviors of fine iron ore concentrate were studied. To assess the
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superiority of the conical fluidized bed approach, the fluidization and reduction behaviors in a
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cylindrical fluidized bed were also examined for comparison purposes. Finally, a regime diagram for
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the DR process in the conical fluidized bed is proposed.
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2 Experimental
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2.1 Materials
The fine iron ore concentrate used in the present study was sourced from Brazil. Its chemical
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composition is provided in Table 1, from which it can be seen that the main component was ferric oxide. The particle size distribution is shown in Fig. 1; the material had a volume-based mean particle size of 87 μm. The true density of the primary particles was 4.9 g/cm3 and the bulk density was 2.2 g/cm3. A typical SEM image of the raw iron ore particles is presented in Fig. 2. The particles were irregular and primarily flaky, thus had a greater tendency to stick during the DR process [27]. Notably, all the particles passed through a 150 mesh sieve, indicating that the shortest side of the particles was smaller than 100 μm. Since the particles were not perfectly round, however, the tested particle size was slightly larger than 100 μm.
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ACCEPTED MANUSCRIPT Both the reducing gas (H2) and the balancing gas (N2) used in this study were 99.999% purity
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and were supplied by the Beijing Beiwen Gas Chemical Industry Co. Ltd.
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2.2 Reactors
The main reactor to conduct the reduction experiments was a laboratory-scale quartz conical
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fluidized bed. The conical section had a height (H) of 170 mm and an initial inner diameter (D0) of 16 mm, while the inner diameter of the top cylindrical section (Dt) was 76 mm and the half cone angle of
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the conical section was 10o. A perforated quartz plate was used as the gas distributor and the diameter
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of each hole was 0.4 μm.
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For comparison purposes, direct reduction experiments were also conducted in a cylindrical fluidized bed. This bed had an inner diameter of 16 mm, equivalent to the value of D0, and an overall
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height of 270 mm.
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2.3 Apparatus and procedures
Fig. 3 shows a schematic diagram of the experimental apparatus. The apparatus was heated using an electric resistance furnace and the desired temperature was set with a temperature controller. The pressure drop of the fluidized bed reactor was monitored with a differential pressure sensor and recorded in real time by a computer. The gas flow volume was controlled by mass flow controllers. During one experimental trial, the fluidized bed was first pre-heated to the desired temperature under a N2 atmosphere, after which 8.0 g of the iron ore concentrate was fed into the bed from overtop. When the bed temperature was constant, the fluidization gas was switched to a 50%
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ACCEPTED MANUSCRIPT H2-50% N2 (by vol.) gas mixture with a specific Ug. When defluidization occurred or after a specific reduction time span, the fluidizing gas was abruptly switched back to N2 to immediately terminate
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the reduction. At this point, the reactor was removed from the furnace and quenched to room
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temperature by spraying water directly onto its outer surface. The reduced sample was decanted into a bag and sealed in preparation for further characterization.
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The reduction experiments in the conical fluidized bed were performed at 650-800 oC with an interval of 25 oC, while the Ug ranging from 0.1 m/s to 2.5 m/s with an interval of 0.1 m/s. The
Vg 0.015 D
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T 273.15 , 273.15
(1)
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Ug
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conversion from the gas volume rate to the Ug can follow equation (1),
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where Vg is the volume flow rate of the gas mixture, L/min; T is the operating temperature, oC. Since
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the superficial gas velocity in the conical fluidized bed varied with the bed height, the Ug value in the conical fluidized bed is assumed to have equaled the superficial gas velocity at the distributor. The
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gas velocity at the top of the conical section is referred to as Utop. The comparative studies in the cylindrical fluidized bed were performed at 775 oC with Ug ranging from 0.12 m/s to 0.36 m/s with an interval of 0.06 m/s.
2.4 Characterizations
The fluidization behavior during the reduction was characterized by the pressure drop profile and by direct visual observations. A dramatic fall of the pressure drop curve showed the occurrence of defluidization as illustrated by previous studies [2, 6, 7, 11]. The fluidization time in this paper represents the total reducing time in each experimental trial. Specifically, for the defluidized samples,
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ACCEPTED MANUSCRIPT the fluidization time represents the period of time from the beginning of the reduction to the occurrence of defluidization.
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The metallization degree (MD) in each trial was calculated as follows:
(2)
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MD=(M Fe /T Fe )×100%,
where MFe and TFe are the metallic iron content and the total iron content of the reduced sample,
Chinese National Standard GB 223.7-2002.
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respectively. Both the MFe and TFe values were determined by titrimetric methods according to the
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The DRI product was multi-sieved and weighed to assess any changes in particle size. The mean
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particle size (MPS) was calculated using the equation (3), (3)
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MPS=Σ(x i ×d i ),
where di is the average particle size of two adjacent sieves and xi is the mass fraction of particles
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with size di. The morphology of each sample was examined by scanning electron microscopy (SEM, JSM-7001F; JEOL).
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3 Results and discussion
3.1 Reduction and fluidization behaviors in the two types of fluidized bed
The direct reduction of the fine iron ore concentrate was conducted in the cylindrical fluidized bed at 775 °C. Fig. 4 plots the variations in both the fluidization time and the MD with Ug, and shows that increases in the value of Ug affect both the fluidization and reduction during the DR process. The MD value that triggers defluidization undergoes an obvious increase with increases in Ug, such that the MD of the defluidized sample at 0.36 m/s was approximately four times that at 0.12 m/s. In the
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ACCEPTED MANUSCRIPT previous studies, the fluidization behavior of the cohesive particles is mainly determined by the priority of two types of forces, i.e., the cohesive force and the breakage force [6, 11]. If the former is
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larger than the latter one, agglomeration and defluidization is probably to occur, while stable
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fluidization would be achieved if otherwise. In this study, the cohesive force is mainly the sintering force, which is in proportion to the metallic iron content on the particle surface [27]. While, the
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breakage force is mainly the collision force between two moving particles/agglomerates. The increase in Ug would enhance the momentum of the particles, thus increasing the breakage force
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during fluidization. These increases in the breakage forces could balance out larger cohesive forces,
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leading to enhanced MD values. Second, the fluidization time decreases with increasing Ug, thus it
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appears that defluidization occurs more readily at higher Ug values. In fact, the decreases in the fluidization primarily result from increases in the reduction efficiency. As an example, the MD at
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0.12 m/s was only 6.2% after 9.5 min, while the MD at 0.36 m/s reached 25.4% after only 6 min, a result that can be attributed to the enhanced external diffusion rate of the gas-solid reaction at higher
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Ug values, which in turn increases the reduction rate. Therefore, increasing Ug in a cylindrical fluidized bed can improve both the MD and the reduction efficiency. However, there is an upper limit to the extent to which Ug can be raised in a cylindrical fluidized bed, equivalent to the terminal gas velocity (Ut) of the fine particles. Thus, if a fluidized bed is operated at a gas velocity greater than the Ut, elutriation losses become a significant problem. In fact, in the present work, severe elutriation was observed at the outlet of the reactor at 0.36 m/s. According to calculations, this velocity is equivalent to the Ut of 77 μm fine iron ore particles under these conditions, and the particle size distribution of the ore used in this study (Fig. 1) shows that particles
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ACCEPTED MANUSCRIPT smaller than 77 μm accounted for 40% (by vol.) of the raw sample. It is therefore evident that, although further increasing Ug may have resulted in better fluidization, elutriation would have
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become more severe.
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Typically, an MD value of 90% is desirable for the DRI product, because this value is high enough to allow the material to be utilized for subsequent steel-making processes. For this reason, the
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main goal of the DR process is to prepare a DRI product having an MD higher than 90%. According to Fig. 4, in all trials, defluidization occurred within 10 min and the highest MD was just 25.4%, a
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result that is much lower than the desired value. Similar results were also achieved in previous
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studies [6, 7], indicating that defluidization occurs very rapidly when a cylindrical fluidized bed is
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employed for the DR of fine iron ore concentrate or iron oxide particles without the application of any anti-sticking methods. As an example, Zhang et al. [7] reported the onset of defluidization in a
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pure H2 atmosphere at 800 °C in conjunction with an MD below 25%. The above results demonstrate that a cylindrical fluidized bed is not suitable for the direct processing of fine iron ore concentrate.
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The DR of the same fine iron ore concentrate was subsequently conducted in the conical fluidized bed at the same reduction temperature, and Fig. 5 summarizes the variations in the fluidization time and the final MD with Ug in this bed. Notably, according to the theoretical prediction in Appendix B, the Umf and Umff of the primary particles in this condition are 0.0076 m/s and 0.010 m/s, respectively. Since the minimum operating Ug in the conical fluidized bed is 0.1 m/s, which is about 10 times of the Umff at this condition, indicating that a fully fluidization regime was obtained at the beginning of the reduction. Compared with the cylindrical fluidized bed, one obvious advantage of the conical fluidized bed is that a much wider gas velocity range can be employed.
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ACCEPTED MANUSCRIPT Since the bed diameter increases with the bed height, the gas velocity at the top of the conical bed determines whether or not the fine particles can be elutriated. If the gas is well distributed at the top
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of the conical bed, the Utop can be estimated by the equation (4).
(4)
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U top=U g× (D 0 /D t ) 2 .
In a typical instance, when the Ug at the bottom is as high as 2.5 m/s, the Utop is just 0.11 m/s,
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which is still much lower than the Ut of the fine particles.
As indicated in Fig. 5, based on the adjustability of the gas velocity, the conical fluidized bed
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has two main advantages over the cylindrical fluidized bed. First, the reduction efficiency in the
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conical fluidized bed is further increased at higher gas velocities. Efficiency improvements result
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from the additional acceleration of the external diffusion, as discussed previously, and the better gas-solid contact efficiency in the conical fluidized bed.
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Notably, when the conical fluidized bed was operated at low gas velocities, the gas-solid contact efficiency is no better than the cylindrical fluidized bed. Since the operating temperatures were the
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same in both reactors, the instinct reaction kinetics should be the same. Then the difference in the apparent reaction rate could indicate the mass transfer and gas-solid contact efficiency indirectly. At Ug = 0.4 m/s, the metallic iron generation rate in the conical fluidized bed is 3.12×10-3 mol/min. While, in the cylindrical fluidized bed, the value for generating metallic iron is 3.58×10 -3 mol/min with the Ug of 0.3 m/s. Though the Ug in the conical vessel was higher than the conventional one, the apparent
reaction rate was lower instead, which means the gas-solid contact efficiency in the conical fluidized bed is worse at this condition. The reason for this is closely associated with the conical geometry. Because the inner diameter increases with bed height, even the conical vessel has a higher Ug at the
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ACCEPTED MANUSCRIPT bottom, the Ug at the surface was only 0.21 m/s which was lower than the Ug in the cylindrical vessel. Besides, the bed height in the conical vessel during the fluidization process was lower than that of the
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cylindrical one, which also resulted from the geometry property of the conical vessel. Above facts
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indicated that the geometry of the conical vessel, at this level, had a negative effect on the gas-solid contact.
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However, the negative effect of the bed geometry can be remedied be applying much higher gas velocities in the conical reactor. For instance, at Ug = 0.8 m/s in the conical fluidized bed, the
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metallic iron generation rate became 12.54×10-3 mol/min, which is much higher than those at lower
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gas velocities. The increase in the apparent reaction rate mainly benefit from the vigorous movement
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of the particles at high gas velocities. According to previous study [28], in the fully fluidized bed regime, the particles go upwards along the axis while downwards near the wall in the conical vessel,
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which promotes the turbulence of the fluidized bed and enhances gas-solid contact. The analyses above show that, though the conical geometry and the Ug affect the gas-solid contact in different
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aspects, the positive influence of the Ug was dominant. Second, the fluidization time can be further prolonged at higher gas velocities. Typical pressure drop profiles used to identify the fluidization behavior are shown in Fig. 6. Defluidization occurred rapidly at gas velocities below 0.9 m/s, similar to the trends observed for a cylindrical fluidized bed. At 1.0 m/s, however, the fluidization time increased dramatically, to 51 min, and the final MD reached 90%. Thus additional increases in the Ug value can prolong the fluidization time to over 60 min. These reduction and fluidization behaviors indicate that, compared with a cylindrical fluidized
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ACCEPTED MANUSCRIPT bed, the conical fluidized bed is quite suitable for processing fine iron ore concentrate. The DRI products can be generated with high reduction efficiency and superior quality.
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However, despite all the advantages of the conical bed, one major concern is the stability of the
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DR process. As an example, although the MD of the product at 1.0 m/s reached the desired value, defluidization did eventually occur, meaning that the fluidization process was not stable. Fig. 6 also
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shows that the fluctuation of the pressure drop profiles gradually decreases with the reducing time. To obtain more information regarding the mechanism of the unstable fluidization and to achieve
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size evolution was carefully analyzed.
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stable fluidization, the fluidization time was prolonged to 2 h at higher gas velocities, and the particle
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Fig. 7 and Fig. 8 show the pressure drop and corresponding MPS variations at 1.2 and 1.3 m/s, respectively. The MPS value increased rapidly at both gas velocities during the initial 5 min, as a
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result of the formation of agglomerates. Over this time span, with the continued reduction of the iron ore particles, metallic iron deposits on the surfaces of the particles and thus increases the sticking
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force between particles, resulting in the agglomeration of primary particles. From 5 to 50 min, the fluidization was relatively stable and the MPS is seen to have slowly increased. The fluidization behaviors at these two gas velocities then exhibit obvious differences after 50 min. At this point, the MPS begins to grow uncontrollably at 1.2 m/s, such that the MPS increases to 550 μm and defluidization occurs at 105 min. Meanwhile, a transition in the pressure drop fluctuation is observed at approximately 50 min at 1.2 m/s, as shown in Fig. 7. In contrast, both the MPS and the pressure drop fluctuation were very stable at 1.3 m/s, as shown in Fig. 8. Fig. 9 presents the morphologies of the agglomerates after reduction for 60 min, and demonstrates that agglomerates larger than 800 μm
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ACCEPTED MANUSCRIPT were formed at 1.2 m/s, while no such agglomerates can be detected at 1.3 m/s. These analyses indicate that the unstable fluidization resulted from the pronounced size increase
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of the agglomerates, which in turn was caused by the coalescence of agglomerates (secondary
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agglomeration). Stable fluidization can thus be achieved at 1.3 m/s by preventing the agglomerates from further sticking to one another. This investigation demonstrates that stability can be maintained
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in the conical fluidized bed by further increasing the gas velocity.
The reason for the generation of secondary agglomerates was pretty confusing by traditional point of
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view. The high MD was suspected to be responsible to the secondary agglomeration behavior initially. Then
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the MD during the DR process was tested and shown in Fig. 10. According to Fig. 10, the MD for 1.3 m/s
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and 1.2 m/s was practically the same after 5 min reduction. The secondary agglomeration happened at 1.2
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m/s but did not at 1.3 m/s, indicating that there was no direct relationship in the MD and the agglomeration behavior. Besides, at the MD of over 90%, the particles surface was fully covered with metallic iron. The further increase in the MD resulted from the reduction of the unreacted core inside the particles, thus having
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minor influence on the cohesiveness of the DRI particles. Actually, the secondary agglomeration behavior may be related with the hydrodynamic transition during the DR process. After reduction for 50min, the proportion of the particles larger than 800 μm at 1.2 m/s and 1.3 m/s was 9.7 wt.% and 1.0 wt.%, respectively, according to the sieving results. The Umff of the agglomerates with 800 μm diameter is estimated to be 1.23 m/s by using the correlations presented in Appendix B. This indicates that the secondary agglomerates at 1.2 m/s are in a partially fluidized bed regime, while, the particles are still in a fully fluidized bed regime at 1.3 m/s. Previous study revealed that, in the partially fluidized bed regime, the particles on the surface of the bed were in a fixed bed state [28].
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ACCEPTED MANUSCRIPT The momentum of these particles became relatively small. So the breakage force during the contact of particles decreased sharply compared with that in the fully fluidized bed regime. Consequently, secondary
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agglomeration occurred. Accordingly, the DR process should be maintained in a fully fluidized bed
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regime.
The minimum gas velocity required to prevent defluidization of sticky particles is typically
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defined as the critical fluidization velocity (Uc). In this study, since an MD value of 90% was taken as the key indicator, we define the minimum Ug that can achieve a final MD greater than 90% as the
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Uc at that temperature. Because of the requirement for stable operation, gradual defluidization should
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also be prevented. Therefore, the minimum Ug at which stable fluidization can be maintained for at
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least 2 hours was defined as the minimum stable fluidization velocity (Umsf) at that temperature. For
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example, the Uc and Umsf at 775 °C in the conical fluidized bed were 1.0 and 1.3 m/s, respectively.
3.2 Regime diagram of direct reduction in a conical fluidized bed
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To further explore the stable operational region in the conical fluidized bed, reduction experiments were performed at different temperatures within the range of 650 to 800 °C. The pressure drop profiles were subsequently analyzed to identify the Uc and the Umsf at each temperature. Fig. 11 shows the fluidization regime diagram for the reduction process in a conical fluidized bed, in which three regimes are evident: the fast defluidization regime, the unstable fluidization regime, and the stable agglomeration fluidization regime. According to Fig. 11, both the Uc and Umsf increase with increases in temperature, in accordance with the results of prior research [2, 11, 29, 30] that found that the sticking tendency is in direct proportion to the operating temperature. Moreover, the gap
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ACCEPTED MANUSCRIPT between the two gas velocities enlarges with increasing temperature. The Uc and Umsf are equal at temperatures lower than 675 °C, indicating that unstable fluidization behavior is easier to eliminate at
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low temperatures, although the reduction efficiency is relatively low in this temperature range. At
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higher operating temperatures, such as 800 °C, where the reduction efficiency is quite high, excess gas is required to maintain stable fluidization. An increase in the difference between the two
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velocities (Umsf-Uc) can also result from an increase in the sticking force, which makes the coalescence of the agglomerates easier.
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Based on the theoretical and experimental results for the cylindrical fluidized bed, the Ut of the
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mean particle size at this temperature range is approximately 0.4 m/s. At temperatures below 675 °C,
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the Uc and Umsf values are both less than 0.3 m/s, suggesting that it is also possible to achieve stable fluidization in the cylindrical fluidized bed. However, above 700 °C, all the Uc and Umsf values are
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greater than 0.5 m/s, indicating that defluidization cannot be prevented. Therefore, the use of a conical bed can greatly expand the operational temperature range (up to 800 °C) during the reduction
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of fine iron ore concentrate.
To avoid defluidization in existing DR processes using cylindrical fluidized beds, only coarse iron ore particles (FINEX and FINMET: 0.05-8.0 mm [1]; Circored: 0.1-2.0 mm [9]) are processed, and at relatively low temperatures. Under these conditions, the advantages of a fluidized bed in terms of the mass and heat transfer would be inhibited by the internal diffusion and the low reaction temperature. As an example, in the second stage of the Circored process (in which the operational temperature is approximately 650 °C and the particle size is about 2 mm) the residence time of the iron ore particles in the fluidized bed is more than 3 h, while the MD only increases from 70% to
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ACCEPTED MANUSCRIPT 95% [3, 9]. The utilization of fine iron ores would decrease the internal diffusion restrictions to the gas-solid reaction and increase the reduction efficiency, and so a fluidized bed can exploit its
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advantages to the fullest only when fine particles are utilized. The use of low-grade iron ore also
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results in the enhanced production of fine iron ore concentrate. The utilization of fine iron ore concentrate is, therefore, also an objective of the fluidization of DR technology.
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Despite the above advantages, fine particles are more susceptible to sticking and defluidization such that the use of fine iron ores restricts the operating temperature. A previous study [4] found that
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fine particles (105 to 140 μm) tend to exhibit sticking above 600 °C. Zhu et al. [2] also reported that
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the sticking temperature of nanosized iron oxide particles was decreased to 450 °C. It is therefore
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evident that achieving high temperature operation requires additional anti-sticking measures. However, all the existing methods used to prevent defluidization, such as additive coatings [31],
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carbon deposition [7, 32-34], and granulation/pelletizing [2, 10], have their own limitations with regard to producing high quality DRI products. The additives introduce impurities into the final
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product, which must be separated by a subsequent smelting process. In addition, the carbon deposition method requires a critical carbon content above 10 wt.% to prevent defluidization [32]. Lei et al. [33, 34] optimized the carbon deposition process and decreased the critical carbon content substantially, to as low as 3.5 wt.%, although this is still greater than the general industry process demand of 3.0 wt.%. As a result, a subsequent process is also needed to lower the excess carbon content. Finally, the granulation/pelletizing method requires a sintering process prior to reduction, and this pre-treatment of the iron ore fines is time-consuming and increases the cost of the process. Compared with the existing methods, the conical fluidized bed is a viable means of preventing
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ACCEPTED MANUSCRIPT defluidization at high temperatures without any pre-treatment or the introduction of inert materials. This lack of defluidization results from the high Ug at the bottom of the fluidized bed, which
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maintains the fluidization of the agglomerates and prevents further coalescence of the agglomerates.
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Therefore, without the presence of defluidization, the conical fluidized bed is promising in achieving continuous operation, which is relevant for implementing the DR of fine iron ore concentrates at
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large scale. Besides, the high Ug can also enhance the external diffusion of the gas-solid reaction and further promote the reduction efficiency. To sum up, employing a conical fluidized bed can provide a
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new approach to the prevention of the defluidization problem, and so represents a clean and efficient
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approach to the intensification of the direct reduction process.
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4 Conclusions
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In this study, the direct reduction of a fine iron ore concentrate with an average particle size of 87 μm was investigated in a conical fluidized bed. The results verified that this type of concentrate
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can be utilized directly in a conical fluidized bed and that the desired DRI product (MD>90%) can be obtained. Compared with a cylindrical fluidized bed, the reduction efficiency of this process was significantly enhanced as the result of employing a high superficial gas velocity in the conical fluidized bed. In addition, it was found that defluidization was avoided even without adding any inert materials or causing elutriation issues. Moreover, stable fluidization was achieved in the conical fluidized bed through the formation of agglomerates and by preventing these agglomerates from further coalescing. Based on the fluidization behavior, the critical velocity and the minimum stable fluidization velocity at different temperatures were determined. As a result, a regime diagram over
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ACCEPTED MANUSCRIPT the temperature range of 650 to 800 °C was proposed and this diagram can be directly employed to
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guide the practical operation of DR in a conical fluidized bed.
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Appendix A: Nomenclature constants in the correlations
D0
bottom diameter of the conical fluidized bed (m)
D1
upper surface diameter of the solid particles in the conical fluidized bed (m)
Dt
top diameter of the conical fluidized bed (m)
dp
particle diameter (m)
g
gravity (m/s2)
H
height of the conical section (m)
MFe
metallic iron content (-)
TFe
total iron content (-)
T
temperature (oC)
Uc
critical velocity (m/s)
Ug
superficial gas velocity (m/s)
Umf
minimum fluidization velocity (m/s)
Umff
minimum fully fluidization velocity (m/s)
Umsf
minimum stable fluidization velocity (m/s)
Utop
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C1, C2
Vg
terminal velocity (m/s) superficial velocity at the top of the conical fluidized bed (m/s) gas volume flow rate (L/min)
Greek letters ε0
voidage of the fluidized bed
θ
cone angle of the conical fluidized bed (o)
μf
fluid viscosity (N·s/m2)
ρf
gas density (kg/m3)
ρs
particle density (kg/m3)
φs
sphericity of the solid particle
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DRI
direct reduced iron
MPS
mean particle size
MD
metallization degree
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direct reduction
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DR
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Abbreviations
Appendix B: Hydrodynamic analysis of the conical fluidized bed
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Previous studies [28, 35-37] indicated that, with the gas velocity was increased, three main regimes
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were discernable in the conical fluidized bed, i.e., the fixed bed regime, the partially fluidied bed regime, and the fully fluidized bed regime. The transtion gas velocities are defined as the minimum fluidization
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velocity (Umf) and the minimum fully fluidization velocity (Umff). The two key parameters, Umf and Umff
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could be estimated based on the dynamic balance of forces exerted on the fluidizing particles, as developed
C1U mf
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by Peng and Fan [28]. The correlations are listed below:
2
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(1 0 ) 2
3 0
f
d s
(5)
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D D 2 C1 0 U mff C2 0 U mff (1 0 )( s f ) g 0 D1 D1 where,
D02 D0 D1 D12 D0 2 C2 0 U mf (1 0 )( s f ) g 3D02 D1
2
(6)
(7)
p
and C2 1.75
1 0 f 03 s d p
(8)
The physical properties of the iron ore particles were listed in Table 2. The corresponding Umf and Umff were calculated and are listed in Table 3.
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ACCEPTED MANUSCRIPT Since H2 can reducing the iron ore particles, thus changing the particle physical properties, testing the real Umf and Umff at high temperatures at experimental conditions is impossible. To validate the
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accuracy of the correlations above, we compared the experimental and the estimated Umf and Umff for the
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iron ore particles at room temperature in pure N2 atmosphere. The tested Umf and Umff are 0.017 and 0.021 m/s, respectively, which are in good agreement with those calculated, indicating that the correlations are
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applicable in this case.
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ACCEPTED MANUSCRIPT Acknowledgments
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under grant no. 21325628 and by NSFC, under grant no. 51404228.
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This work was supported by the National Outstanding Youth Science Fund Project of NSFC,
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ACCEPTED MANUSCRIPT References [1] F. Plaul, C. Böhm, J. Schenk, Fluidized-bed technology for the production of iron products for
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steelmaking, J. S. Afr. I. Min. Metal., 109 (2009) 121-128.
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[2] Q. Zhu, R. Wu, H. Li, Direct reduction of hematite powders in a fluidized bed reactor, Particuology, 11 (2013) 294-300.
steelmaking, Particuology, 9 (2011) 14-23.
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[3] J.L. Schenk, Recent status of fluidized bed technologies for producing iron input materials for
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Can. Metall. Quart., 3 (1974) 649-657.
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[4] J.F. Gransden, J.S. Sheasby, The sticking of iron ore during reduction by hydrogen in a fluidized bed,
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[5] Z. Du, Q. Zhu, C. Fan, F. Pan, H. Li, Z. Xie, Influence of reduction condition on the morphology of
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newly formed metallic iron during the fluidized bed reduction of fine iron ores and its corresponding agglomeration behavior, Steel Res. Int., 87 (2016) 789-797. [6] B. Zhang, Z. Wang, X. Gong, Z. Guo, A comparative study of influence of fluidized conditions on
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sticking time during reduction of Fe2O3 particles with CO, Powder Technol., 225 (2012) 1-6. [7] T. Zhang, C. Lei, Q. Zhu, Reduction of fine iron ore via a two-step fluidized bed direct reduction process, Powder Technol., 254 (2014) 1-11. [8] D. Nuber, H. Eichberger, B. Rollinger, Circored fine ore direct reduction, Millen. Steel, 2006 (2006) 37-40. [9] B. G. Langston, F.M. Stephens Jr, Self-agglomerating fluidized-bed reduction, J. Met., 12 (1960) 312-316. [10] A.M. Nyembwe, R.D. Cromarty, A.M. Garbers-Craig, Prediction of the granule size distribution of
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ACCEPTED MANUSCRIPT iron ore sinter feeds that contain concentrate and micropellets, Powder Technol., 295 (2016) 7-15. [11] C. Lei, Q. Zhu, H. Li, Experimental and theoretical study on the fluidization behaviors of iron powder
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[12] G. Elordi, M. Olazar, G. Lopez, M. Artetxe, J. Bilbao, Product Yields and Compositions in the Continuous Pyrolysis of High-Density Polyethylene in a Conical Spouted Bed Reactor, Ind. Eng. Chem.
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Res., 50 (2011) 6650-6659.
[13] M. Artetxe, G. Lopez, M. Amutio, G. Elordi, M. Olazar, J. Bilbao, Operating Conditions for the
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Pyrolysis of Poly-(ethylene terephthalate) in a Conical Spouted-Bed Reactor, Ind. Eng. Chem. Res., 49
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(2010) 2064-2069.
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[14] R. Aguado, R. Prieto, M. José, S. Alvarez, M. Olazar, J. Bilbao, Defluidization Modeling of Pyrolysis
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of Plastics in a Conical Spouted Bed Reactor, Chem. Eng. Process, 44 (2005) 231-235. [15] R. Aguado, M. Olazar, M. José, G. Aguirre, J. Bilbao, Pyrolysis of sawdust in a conical spouted bed reactor. Yields and product composition, Ind. Eng. Chem. Res., 39 (2000) 1925-1933.
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[16] A. Erkiaga, G. Lopez, M. Amutio, J. Bilbao, M. Olazar, Syngas from steam gasification of polyethylene in a conical spouted bed reactor, Fuel, 109 (2013) 461-469. [17].M. Ozawa, Spouted bed reduction of iron ore, Tetsu-to-hagane 59 (1973) 361-371. [18] D. Geldart, Types of gas fluidization, Powder Technol., 7 (1973) 285-292. [19] P. Arromdee, V.I. Kuprianov, Combustion of peanut shells in a cone-shaped bubbling fluidized-bed combustor using alumina as the bed material, Appl. Energ., 97 (2012) 470-482. [20] W. Permchart, V.I. Kouprianov, Emission performance and combustion efficiency of a conical fluidized-bed combustor firing various biomass fuels, Bioresource Technol., 92 (2004) 83-91.
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ACCEPTED MANUSCRIPT [21] R. Deiva Venkatesh, J. Chaouki, D. Klvana, Fluidization of cryogels in a conical column, Powder Technol., 89 (1996) 179-186.
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[22] H. Li, H. Tong, Multi-scale fluidization of ultra fine powders in a fast-bed-riser/conical-dipleg CFB
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loop, Chem. Eng. Sci., 59 (2004) 1897-1904.
[23] I.O. Lee, Y.H. Kim, B.J. Jung, H.G. Kim, F. Hauzenberger, Fluidized bed type reduction apparatus for
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iron ore particles and method for reducing iron ore particles using the apparatus, U.S. Patent, 5785733, 1998.
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[24] Y.H. Kim, I.O. Lee, H.G. Kim, Fluidized bed type reducing system for reducing fine iron ore, U.S.
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Patents, US6224819, 2001.
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[25] M. Kwauk, Fluidization: idealized and bubbleless, with applications, Science Press, Beijing, 1992.
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[26] F.M. Stephens Jr, L.W. Coffer, B.J. Langston, Reduction of iron ore, U.S. Patent, 3053648, 1962. [27] S. Hayashi, Y. Iguchi, Factors aftecting the sticking of fine iron ores during fluidized bed reduction, ISIJ Int., 32 (1992) 962-971.
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[28] Y. Peng, L.T. Fan, Hydrodynamic characteristics of fluidization in liquid-solid tapered beds, Chem. Eng. Sci., 52 (1997) 2277-2290. [29] Y. Zhong, Z. Wang, Z. Guo, Q. Tang, Defluidization behavior of iron powders at elevated temperature: Influence of fluidizing gas and particle adhesion, Powder Technol., 230 (2012) 225-231. [30] J. Shao, Z. Guo, H. Tang, Influence of temperature on sticking behavior of iron powder in fluidized bed, ISIJ Int., 51 (2011) 1290-1295. [31] Y. Zhong, Z. Wang, Z. Guo, Q. Tang, Prevention of agglomeration/ defluidization in fluidized bed reduction of Fe2O3 by CO: The role of magnesium and calcium oxide, Powder Technol., 241 (2013)
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ACCEPTED MANUSCRIPT 142-148. [32] B. Zhang, Z. Wang, X. Gong, Z. Guo, Characterization of precipitated carbon by XPS and its
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prevention mechanism of sticking during reduction of Fe 2O3 particles in the fluidized bed, ISIJ Int., 53
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(2013) 411-418.
[33] C. Lei, G. Zhang, Q. Zhu, Z. Xie, Optimization of carbon deposition process during the pre-reduction
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of fine iron ore in a fluidized bed, Powder Technol., 296 (2016) 79-86.
[34] C. Lei, S. He, Z. Du, F. Pan, Q. Zhu, H. Li, Effects of gas composition and temperature on the
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fluidization characteristics of carbon-coated iron ore, Powder Technol., 301 (2016) 608-614.
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Eng. Process., 39 (2000) 379-387.
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[35] S. Jing, Q.Y. Hu, J.F. Wang, Y. Jin, Fluidization of coarse particles in gas-solid conical beds, Chem.
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[36] M. Rasteh, F. Farhadi, A. Bahramian, Hydrodynamic characteristics of gas-solid tapered fluidized beds: Experimental studies and empirical models, Powder Technol., 283 (2015) 355-367. [37] L. Gan, X. Lu, Q. Wang, Experimental and theoretical study on hydrodynamic characteristics of
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tapered fluidized beds, Adv. Powder Technol., 25 (2014) 824-831. [38] L. Guo, H. Gao, J. Yu, Z. Zhang, Z. Guo, Influence of hydrogen concentration on Fe 2O3 particle reduction in fluidized beds under constant drag force, Int. J. Min. Met. Mater., 22 (2015) 12-20.
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ACCEPTED MANUSCRIPT Table captions:
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Table 1. Chemical composition of the raw iron ore concentrate.
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Table 2. Physical properties of the iron ore particles and the DRI agglomerates
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Table 3. Umf and Umff calculated by the correlations for the iron ore particles and the DRI agglomerates
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Figure captions:
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Fig. 1. Particle size distribution of the raw iron ore concentrate.
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Fig. 2. SEM image of the raw iron ore concentrate.
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Fig. 3. Schematic diagram of the experimental apparatus.
Fig. 4. Variations in the fluidization time and the MD with Ug in the cylindrical fluidized bed
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operated at 775 °C.
Fig. 5. Variations in the fluidization time and the MD with Ug in the conical fluidized bed operated at 775 °C.
Fig. 6. Pressure drop profiles of the fluidization process at different gas velocities when operating at 775 °C.
Fig. 7. Variations in MPS and pressure drop with reducing time at 1.2 m/s and 775 °C.
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ACCEPTED MANUSCRIPT Fig. 8. Variations in MPS and pressure drop with reducing time at 1.3 m/s and 775 °C.
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Fig. 9. SEM images of the DRI particles reduced at 775 °C for 60 min at a) 1.2 m/s and b) 1.3 m/s.
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Fig. 10. Variations in MD with reducing time at 1.2 m/s and 1.3 m/s, 775 °C
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Fig. 11. Fluidization regime diagram for the DR process in the conical fluidized bed.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Figure 9
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Figure 11
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ACCEPTED MANUSCRIPT Table 1. Chemical composition of the raw iron ore concentrate.
TFe
Fe2O3
FeO
SiO2
CaO
68.94
96.80
0.72
1.98
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Compositio Al2O3
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n
MgO
0.10
0.10
0.30
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[Mass%]
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Table 2. Physical properties of the iron ore particles and the DRI agglomerates A
B
C
D
dp (×10-6m)
87
87
φs (-)
0.33
0.33
ε0 (-)
0.65
0.65
μg(×10-6
17.6
ρf (kg/m3)
1.36 16.0
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D1(×10-3m)
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D0 (×10-3m)
21.8
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4900
0.69
0.60
0.60
39.4
39.4
4900
2200
2200
0.175
0.175
0.175
16.0
16.0
16.0
21.8
23.4
23.4
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39.4
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ρs (kg/m3)
0.69
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N·s/m2)
800
250
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a
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Parameters
A: iron ore particles at room temperautre with pure N 2.
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B: iron ore particles at 775 oC with 50% H2+50%N2 (by vol.) mixture. C: Primary DRI agglomerates at 775 oC with 50% H2+50%N2 (by vol.) mixture. D: Secondary DRI agglomerates at 775 oC with 50% H2+50%N2 (by vol.) mixture. a: Viscosities of gas mixture at high temperatures are calculated from the literature of L. Guo et al. [38]
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Umf (m/s)
0.017
0.0076
0.090
0.882
Umff
0.022
0.010
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1.232
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0.125
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(m/s)
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Graphical Abstract
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ACCEPTED MANUSCRIPT Highlights The direct reduction of fine iron ore was studied in two types of fluidized bed.
The high gas velocity can considerably improve the reduction efficiency.
Defluidization was prevented without any additives in the conical fluidized bed.
Stable fluidization was achieved by forming stable agglomerates.
Stable operation temperature is enlarged to 800 oC in the conical fluidized bed.
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S0032-5910(17)30213-9 doi:10.1016/j.powtec.2017.03.007 PTEC 12412
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
28 September 2016 11 February 2017 2 March 2017
Please cite this article as: Shengyi He, Haoyan Sun, Chaoquan Hu, Jun Li, Qingshan Zhu, Hongzhong Li, Direct reduction of fine iron ore concentrate in a conical fluidized bed, Powder Technology (2017), doi:10.1016/j.powtec.2017.03.007
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Direct reduction of fine iron ore concentrate in a conical fluidized bed
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese
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Shengyi He a,b, Haoyan Sun a, Chaoquan Hu a, Jun Li a, Qingshan Zhu a,b,*, Hongzhong Li a
Academy of Sciences, Beijing 100190, China
University of Chinese Academy of Sciences, Beijing 100049, China
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b
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* Corresponding Author: Qingshan Zhu;
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E-mail:
[email protected]
Tel:
+86-010-62536108
Fax:
+86-010-62536108
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ACCEPTED MANUSCRIPT Abstract
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The direct utilization of fine iron ore concentrate in fluidized beds is of significance but has the
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problem of defluidization during operation. In this study, we have made a comparison of a conical
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fluidized bed and a cylindrical fluidized bed for the direct reduction process. The results showed that the conical fluidized bed was more versatile than the cylindrical fluidized bed in treating the fine iron
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ore concentrate. In the conical fluidized bed, with a high superficial gas velocity, defluidization was
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successfully inhibited without adding inert materials or causing elutriation issues. Simultaneously, the reduction efficiency was considerably improved and the obtained iron agglomerates exhibited a
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high metallization degree. This work also proposes a fluidization regime diagram for the conical
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fluidized bed at the temperature range from 650 to 800 oC. According to the fluidization regime
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diagram, the conical fluidized bed is able to considerably expand the operational temperature range for the direct reduction process. These results demonstrate that the conical fluidized bed is a clean,
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efficient alternative means for intensifying the direct reduction process.
Keywords: Conical fluidized bed; Cylindrical fluidized bed; Direct reduction; Defluidization; Agglomeration; High Temperature.
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ACCEPTED MANUSCRIPT 1 Introduction Fluidized beds are promising reactors for direct reduction (DR) processes because of their high
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mass and heat transfer efficiencies, possibility of continuous operation [1-3]. Actually, several DR
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processes based on fluidization technology have been established over the past several decades, such as the FINMET, Circored and FINEX processes [3]. The bottleneck in developing a fluidization DR
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process is the challenge associated with defluidization resulting from the adhesion of direct reduced iron (DRI) particles, which impedes continuous operation and hence lower the overall efficiency of
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the industrial production [2, 4-7]. A common solution for prevention of defluidization in the process
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is restricting the particle size distribution of the raw iron ore particles. For instance, the Circored
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process excludes the particles smaller than 0.10 mm in the raw materials before reduction in the fluidized bed [8]. This is reasonable since the defluidization issue tends to be exacerbated with the
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use of fine particles [9]. However, as high-grade lump ore resources are gradually depleted, low-grade iron ores are being utilized more often for iron-making, resulting in the generation of a
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greater amount of fine concentrate (<0.10 mm) [10]. Therefore, the prevention of defluidization during the DR of such fine iron ore concentrates is of great significance and has been extensively studied [2, 7, 9, 11].
Conical reactors appear to show promise with regard to solving the defluidization issue and are already widely utilized in the processing of adhesive particles. Based on the different designs, two gas-contact regimes in the conical reactors were distinguished, i.e., the conical fluidized bed regime and the conical spouted bed regime, which were both suitable for avoiding defluidization in chemical engineering processes. The conical spouted bed has been applied in the cases of waste plastic and
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ACCEPTED MANUSCRIPT biomass pyrolysis [12-15], gasification processes [16] and iron ore DR [17]. The vigorous movement in the central of the bed maintained the stable operation. Owaza [17] investigated the reduction of
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coarse iron ore particles (0.30 to 1.0 mm) in a conical spouted bed and found that the sticking
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tendency of such particles in the spouted bed was reduced compared with that in a conventional fluidized bed. However, the spouted bed usually processes coarse particles belonging to Class-D
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according to Geldart’s classification [18]. While, on the other hand, the conical fluidized bed is more favorable in treating fine particles or particles with wide size distribution. The conical fluidized bed
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has also been applied in the combustion/gasification of coal and biomass [19, 20], cohesive ultrafine
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particle fluidization [21, 22], and DR of iron ores [23, 24]. In conical fluidized bed, the high gas
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velocity (Ug) at the bottom of the vessel results in adequate fluidization of such coarse particles/agglomerates, while the low Ug at the top of the reactor avoids excessive elutriation of fine
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particles [25]. Several patents [23, 24] have claimed that a conical fluidized bed is adequate for the DR of iron ores having a wide size range, because the low gas velocity at the top of the bed
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considerably decreases the elutriation rate of fine particles. In contrast, little attention has been devoted to the DR of fine iron ore concentrates smaller than 0.10 mm in conical fluidized beds, although the conical fluidized bed showed great potential in processing iron ore fines. In the patent of Stephens, Jr. et al. [26], fine iron ore concentrate (<45 μm) was reduced in a conical fluidized bed together with coarse iron particles (250 to 900 μm). The fine particles stuck onto the surface of the coarse particles and consequently defluidization was prevented. However, the addition of such coarse particles tends to increase the bed weight without increasing productivity. In conclusion, a conical fluidized bed can exhibit excellent performance in the processing of sticky particles, but the direct
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ACCEPTED MANUSCRIPT utilization of fine iron ore concentrates in this type of bed without the use of additives still remains a
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challenge and requires further investigation.
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The objective of the present study was to investigate the possibility of the direct reduction of
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fine iron ore concentrate without any additions in a conical fluidized bed. Specifically, the fluidization and reduction behaviors of fine iron ore concentrate were studied. To assess the
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superiority of the conical fluidized bed approach, the fluidization and reduction behaviors in a
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cylindrical fluidized bed were also examined for comparison purposes. Finally, a regime diagram for
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the DR process in the conical fluidized bed is proposed.
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2 Experimental
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2.1 Materials
The fine iron ore concentrate used in the present study was sourced from Brazil. Its chemical
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composition is provided in Table 1, from which it can be seen that the main component was ferric oxide. The particle size distribution is shown in Fig. 1; the material had a volume-based mean particle size of 87 μm. The true density of the primary particles was 4.9 g/cm3 and the bulk density was 2.2 g/cm3. A typical SEM image of the raw iron ore particles is presented in Fig. 2. The particles were irregular and primarily flaky, thus had a greater tendency to stick during the DR process [27]. Notably, all the particles passed through a 150 mesh sieve, indicating that the shortest side of the particles was smaller than 100 μm. Since the particles were not perfectly round, however, the tested particle size was slightly larger than 100 μm.
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ACCEPTED MANUSCRIPT Both the reducing gas (H2) and the balancing gas (N2) used in this study were 99.999% purity
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and were supplied by the Beijing Beiwen Gas Chemical Industry Co. Ltd.
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2.2 Reactors
The main reactor to conduct the reduction experiments was a laboratory-scale quartz conical
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fluidized bed. The conical section had a height (H) of 170 mm and an initial inner diameter (D0) of 16 mm, while the inner diameter of the top cylindrical section (Dt) was 76 mm and the half cone angle of
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the conical section was 10o. A perforated quartz plate was used as the gas distributor and the diameter
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of each hole was 0.4 μm.
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For comparison purposes, direct reduction experiments were also conducted in a cylindrical fluidized bed. This bed had an inner diameter of 16 mm, equivalent to the value of D0, and an overall
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height of 270 mm.
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2.3 Apparatus and procedures
Fig. 3 shows a schematic diagram of the experimental apparatus. The apparatus was heated using an electric resistance furnace and the desired temperature was set with a temperature controller. The pressure drop of the fluidized bed reactor was monitored with a differential pressure sensor and recorded in real time by a computer. The gas flow volume was controlled by mass flow controllers. During one experimental trial, the fluidized bed was first pre-heated to the desired temperature under a N2 atmosphere, after which 8.0 g of the iron ore concentrate was fed into the bed from overtop. When the bed temperature was constant, the fluidization gas was switched to a 50%
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ACCEPTED MANUSCRIPT H2-50% N2 (by vol.) gas mixture with a specific Ug. When defluidization occurred or after a specific reduction time span, the fluidizing gas was abruptly switched back to N2 to immediately terminate
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the reduction. At this point, the reactor was removed from the furnace and quenched to room
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temperature by spraying water directly onto its outer surface. The reduced sample was decanted into a bag and sealed in preparation for further characterization.
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The reduction experiments in the conical fluidized bed were performed at 650-800 oC with an interval of 25 oC, while the Ug ranging from 0.1 m/s to 2.5 m/s with an interval of 0.1 m/s. The
Vg 0.015 D
2 0
T 273.15 , 273.15
(1)
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conversion from the gas volume rate to the Ug can follow equation (1),
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where Vg is the volume flow rate of the gas mixture, L/min; T is the operating temperature, oC. Since
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the superficial gas velocity in the conical fluidized bed varied with the bed height, the Ug value in the conical fluidized bed is assumed to have equaled the superficial gas velocity at the distributor. The
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gas velocity at the top of the conical section is referred to as Utop. The comparative studies in the cylindrical fluidized bed were performed at 775 oC with Ug ranging from 0.12 m/s to 0.36 m/s with an interval of 0.06 m/s.
2.4 Characterizations
The fluidization behavior during the reduction was characterized by the pressure drop profile and by direct visual observations. A dramatic fall of the pressure drop curve showed the occurrence of defluidization as illustrated by previous studies [2, 6, 7, 11]. The fluidization time in this paper represents the total reducing time in each experimental trial. Specifically, for the defluidized samples,
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ACCEPTED MANUSCRIPT the fluidization time represents the period of time from the beginning of the reduction to the occurrence of defluidization.
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The metallization degree (MD) in each trial was calculated as follows:
(2)
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MD=(M Fe /T Fe )×100%,
where MFe and TFe are the metallic iron content and the total iron content of the reduced sample,
Chinese National Standard GB 223.7-2002.
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respectively. Both the MFe and TFe values were determined by titrimetric methods according to the
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The DRI product was multi-sieved and weighed to assess any changes in particle size. The mean
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particle size (MPS) was calculated using the equation (3), (3)
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MPS=Σ(x i ×d i ),
where di is the average particle size of two adjacent sieves and xi is the mass fraction of particles
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with size di. The morphology of each sample was examined by scanning electron microscopy (SEM, JSM-7001F; JEOL).
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3 Results and discussion
3.1 Reduction and fluidization behaviors in the two types of fluidized bed
The direct reduction of the fine iron ore concentrate was conducted in the cylindrical fluidized bed at 775 °C. Fig. 4 plots the variations in both the fluidization time and the MD with Ug, and shows that increases in the value of Ug affect both the fluidization and reduction during the DR process. The MD value that triggers defluidization undergoes an obvious increase with increases in Ug, such that the MD of the defluidized sample at 0.36 m/s was approximately four times that at 0.12 m/s. In the
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ACCEPTED MANUSCRIPT previous studies, the fluidization behavior of the cohesive particles is mainly determined by the priority of two types of forces, i.e., the cohesive force and the breakage force [6, 11]. If the former is
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larger than the latter one, agglomeration and defluidization is probably to occur, while stable
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fluidization would be achieved if otherwise. In this study, the cohesive force is mainly the sintering force, which is in proportion to the metallic iron content on the particle surface [27]. While, the
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breakage force is mainly the collision force between two moving particles/agglomerates. The increase in Ug would enhance the momentum of the particles, thus increasing the breakage force
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during fluidization. These increases in the breakage forces could balance out larger cohesive forces,
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leading to enhanced MD values. Second, the fluidization time decreases with increasing Ug, thus it
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appears that defluidization occurs more readily at higher Ug values. In fact, the decreases in the fluidization primarily result from increases in the reduction efficiency. As an example, the MD at
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0.12 m/s was only 6.2% after 9.5 min, while the MD at 0.36 m/s reached 25.4% after only 6 min, a result that can be attributed to the enhanced external diffusion rate of the gas-solid reaction at higher
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Ug values, which in turn increases the reduction rate. Therefore, increasing Ug in a cylindrical fluidized bed can improve both the MD and the reduction efficiency. However, there is an upper limit to the extent to which Ug can be raised in a cylindrical fluidized bed, equivalent to the terminal gas velocity (Ut) of the fine particles. Thus, if a fluidized bed is operated at a gas velocity greater than the Ut, elutriation losses become a significant problem. In fact, in the present work, severe elutriation was observed at the outlet of the reactor at 0.36 m/s. According to calculations, this velocity is equivalent to the Ut of 77 μm fine iron ore particles under these conditions, and the particle size distribution of the ore used in this study (Fig. 1) shows that particles
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ACCEPTED MANUSCRIPT smaller than 77 μm accounted for 40% (by vol.) of the raw sample. It is therefore evident that, although further increasing Ug may have resulted in better fluidization, elutriation would have
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become more severe.
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Typically, an MD value of 90% is desirable for the DRI product, because this value is high enough to allow the material to be utilized for subsequent steel-making processes. For this reason, the
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main goal of the DR process is to prepare a DRI product having an MD higher than 90%. According to Fig. 4, in all trials, defluidization occurred within 10 min and the highest MD was just 25.4%, a
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result that is much lower than the desired value. Similar results were also achieved in previous
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studies [6, 7], indicating that defluidization occurs very rapidly when a cylindrical fluidized bed is
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employed for the DR of fine iron ore concentrate or iron oxide particles without the application of any anti-sticking methods. As an example, Zhang et al. [7] reported the onset of defluidization in a
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pure H2 atmosphere at 800 °C in conjunction with an MD below 25%. The above results demonstrate that a cylindrical fluidized bed is not suitable for the direct processing of fine iron ore concentrate.
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The DR of the same fine iron ore concentrate was subsequently conducted in the conical fluidized bed at the same reduction temperature, and Fig. 5 summarizes the variations in the fluidization time and the final MD with Ug in this bed. Notably, according to the theoretical prediction in Appendix B, the Umf and Umff of the primary particles in this condition are 0.0076 m/s and 0.010 m/s, respectively. Since the minimum operating Ug in the conical fluidized bed is 0.1 m/s, which is about 10 times of the Umff at this condition, indicating that a fully fluidization regime was obtained at the beginning of the reduction. Compared with the cylindrical fluidized bed, one obvious advantage of the conical fluidized bed is that a much wider gas velocity range can be employed.
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ACCEPTED MANUSCRIPT Since the bed diameter increases with the bed height, the gas velocity at the top of the conical bed determines whether or not the fine particles can be elutriated. If the gas is well distributed at the top
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of the conical bed, the Utop can be estimated by the equation (4).
(4)
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U top=U g× (D 0 /D t ) 2 .
In a typical instance, when the Ug at the bottom is as high as 2.5 m/s, the Utop is just 0.11 m/s,
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which is still much lower than the Ut of the fine particles.
As indicated in Fig. 5, based on the adjustability of the gas velocity, the conical fluidized bed
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has two main advantages over the cylindrical fluidized bed. First, the reduction efficiency in the
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conical fluidized bed is further increased at higher gas velocities. Efficiency improvements result
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from the additional acceleration of the external diffusion, as discussed previously, and the better gas-solid contact efficiency in the conical fluidized bed.
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Notably, when the conical fluidized bed was operated at low gas velocities, the gas-solid contact efficiency is no better than the cylindrical fluidized bed. Since the operating temperatures were the
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same in both reactors, the instinct reaction kinetics should be the same. Then the difference in the apparent reaction rate could indicate the mass transfer and gas-solid contact efficiency indirectly. At Ug = 0.4 m/s, the metallic iron generation rate in the conical fluidized bed is 3.12×10-3 mol/min. While, in the cylindrical fluidized bed, the value for generating metallic iron is 3.58×10 -3 mol/min with the Ug of 0.3 m/s. Though the Ug in the conical vessel was higher than the conventional one, the apparent
reaction rate was lower instead, which means the gas-solid contact efficiency in the conical fluidized bed is worse at this condition. The reason for this is closely associated with the conical geometry. Because the inner diameter increases with bed height, even the conical vessel has a higher Ug at the
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ACCEPTED MANUSCRIPT bottom, the Ug at the surface was only 0.21 m/s which was lower than the Ug in the cylindrical vessel. Besides, the bed height in the conical vessel during the fluidization process was lower than that of the
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cylindrical one, which also resulted from the geometry property of the conical vessel. Above facts
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indicated that the geometry of the conical vessel, at this level, had a negative effect on the gas-solid contact.
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However, the negative effect of the bed geometry can be remedied be applying much higher gas velocities in the conical reactor. For instance, at Ug = 0.8 m/s in the conical fluidized bed, the
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metallic iron generation rate became 12.54×10-3 mol/min, which is much higher than those at lower
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gas velocities. The increase in the apparent reaction rate mainly benefit from the vigorous movement
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of the particles at high gas velocities. According to previous study [28], in the fully fluidized bed regime, the particles go upwards along the axis while downwards near the wall in the conical vessel,
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which promotes the turbulence of the fluidized bed and enhances gas-solid contact. The analyses above show that, though the conical geometry and the Ug affect the gas-solid contact in different
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aspects, the positive influence of the Ug was dominant. Second, the fluidization time can be further prolonged at higher gas velocities. Typical pressure drop profiles used to identify the fluidization behavior are shown in Fig. 6. Defluidization occurred rapidly at gas velocities below 0.9 m/s, similar to the trends observed for a cylindrical fluidized bed. At 1.0 m/s, however, the fluidization time increased dramatically, to 51 min, and the final MD reached 90%. Thus additional increases in the Ug value can prolong the fluidization time to over 60 min. These reduction and fluidization behaviors indicate that, compared with a cylindrical fluidized
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ACCEPTED MANUSCRIPT bed, the conical fluidized bed is quite suitable for processing fine iron ore concentrate. The DRI products can be generated with high reduction efficiency and superior quality.
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However, despite all the advantages of the conical bed, one major concern is the stability of the
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DR process. As an example, although the MD of the product at 1.0 m/s reached the desired value, defluidization did eventually occur, meaning that the fluidization process was not stable. Fig. 6 also
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shows that the fluctuation of the pressure drop profiles gradually decreases with the reducing time. To obtain more information regarding the mechanism of the unstable fluidization and to achieve
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size evolution was carefully analyzed.
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stable fluidization, the fluidization time was prolonged to 2 h at higher gas velocities, and the particle
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Fig. 7 and Fig. 8 show the pressure drop and corresponding MPS variations at 1.2 and 1.3 m/s, respectively. The MPS value increased rapidly at both gas velocities during the initial 5 min, as a
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result of the formation of agglomerates. Over this time span, with the continued reduction of the iron ore particles, metallic iron deposits on the surfaces of the particles and thus increases the sticking
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force between particles, resulting in the agglomeration of primary particles. From 5 to 50 min, the fluidization was relatively stable and the MPS is seen to have slowly increased. The fluidization behaviors at these two gas velocities then exhibit obvious differences after 50 min. At this point, the MPS begins to grow uncontrollably at 1.2 m/s, such that the MPS increases to 550 μm and defluidization occurs at 105 min. Meanwhile, a transition in the pressure drop fluctuation is observed at approximately 50 min at 1.2 m/s, as shown in Fig. 7. In contrast, both the MPS and the pressure drop fluctuation were very stable at 1.3 m/s, as shown in Fig. 8. Fig. 9 presents the morphologies of the agglomerates after reduction for 60 min, and demonstrates that agglomerates larger than 800 μm
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ACCEPTED MANUSCRIPT were formed at 1.2 m/s, while no such agglomerates can be detected at 1.3 m/s. These analyses indicate that the unstable fluidization resulted from the pronounced size increase
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of the agglomerates, which in turn was caused by the coalescence of agglomerates (secondary
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agglomeration). Stable fluidization can thus be achieved at 1.3 m/s by preventing the agglomerates from further sticking to one another. This investigation demonstrates that stability can be maintained
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in the conical fluidized bed by further increasing the gas velocity.
The reason for the generation of secondary agglomerates was pretty confusing by traditional point of
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view. The high MD was suspected to be responsible to the secondary agglomeration behavior initially. Then
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the MD during the DR process was tested and shown in Fig. 10. According to Fig. 10, the MD for 1.3 m/s
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and 1.2 m/s was practically the same after 5 min reduction. The secondary agglomeration happened at 1.2
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m/s but did not at 1.3 m/s, indicating that there was no direct relationship in the MD and the agglomeration behavior. Besides, at the MD of over 90%, the particles surface was fully covered with metallic iron. The further increase in the MD resulted from the reduction of the unreacted core inside the particles, thus having
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minor influence on the cohesiveness of the DRI particles. Actually, the secondary agglomeration behavior may be related with the hydrodynamic transition during the DR process. After reduction for 50min, the proportion of the particles larger than 800 μm at 1.2 m/s and 1.3 m/s was 9.7 wt.% and 1.0 wt.%, respectively, according to the sieving results. The Umff of the agglomerates with 800 μm diameter is estimated to be 1.23 m/s by using the correlations presented in Appendix B. This indicates that the secondary agglomerates at 1.2 m/s are in a partially fluidized bed regime, while, the particles are still in a fully fluidized bed regime at 1.3 m/s. Previous study revealed that, in the partially fluidized bed regime, the particles on the surface of the bed were in a fixed bed state [28].
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ACCEPTED MANUSCRIPT The momentum of these particles became relatively small. So the breakage force during the contact of particles decreased sharply compared with that in the fully fluidized bed regime. Consequently, secondary
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agglomeration occurred. Accordingly, the DR process should be maintained in a fully fluidized bed
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regime.
The minimum gas velocity required to prevent defluidization of sticky particles is typically
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defined as the critical fluidization velocity (Uc). In this study, since an MD value of 90% was taken as the key indicator, we define the minimum Ug that can achieve a final MD greater than 90% as the
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Uc at that temperature. Because of the requirement for stable operation, gradual defluidization should
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also be prevented. Therefore, the minimum Ug at which stable fluidization can be maintained for at
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least 2 hours was defined as the minimum stable fluidization velocity (Umsf) at that temperature. For
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example, the Uc and Umsf at 775 °C in the conical fluidized bed were 1.0 and 1.3 m/s, respectively.
3.2 Regime diagram of direct reduction in a conical fluidized bed
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To further explore the stable operational region in the conical fluidized bed, reduction experiments were performed at different temperatures within the range of 650 to 800 °C. The pressure drop profiles were subsequently analyzed to identify the Uc and the Umsf at each temperature. Fig. 11 shows the fluidization regime diagram for the reduction process in a conical fluidized bed, in which three regimes are evident: the fast defluidization regime, the unstable fluidization regime, and the stable agglomeration fluidization regime. According to Fig. 11, both the Uc and Umsf increase with increases in temperature, in accordance with the results of prior research [2, 11, 29, 30] that found that the sticking tendency is in direct proportion to the operating temperature. Moreover, the gap
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ACCEPTED MANUSCRIPT between the two gas velocities enlarges with increasing temperature. The Uc and Umsf are equal at temperatures lower than 675 °C, indicating that unstable fluidization behavior is easier to eliminate at
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low temperatures, although the reduction efficiency is relatively low in this temperature range. At
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higher operating temperatures, such as 800 °C, where the reduction efficiency is quite high, excess gas is required to maintain stable fluidization. An increase in the difference between the two
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velocities (Umsf-Uc) can also result from an increase in the sticking force, which makes the coalescence of the agglomerates easier.
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Based on the theoretical and experimental results for the cylindrical fluidized bed, the Ut of the
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mean particle size at this temperature range is approximately 0.4 m/s. At temperatures below 675 °C,
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the Uc and Umsf values are both less than 0.3 m/s, suggesting that it is also possible to achieve stable fluidization in the cylindrical fluidized bed. However, above 700 °C, all the Uc and Umsf values are
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greater than 0.5 m/s, indicating that defluidization cannot be prevented. Therefore, the use of a conical bed can greatly expand the operational temperature range (up to 800 °C) during the reduction
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of fine iron ore concentrate.
To avoid defluidization in existing DR processes using cylindrical fluidized beds, only coarse iron ore particles (FINEX and FINMET: 0.05-8.0 mm [1]; Circored: 0.1-2.0 mm [9]) are processed, and at relatively low temperatures. Under these conditions, the advantages of a fluidized bed in terms of the mass and heat transfer would be inhibited by the internal diffusion and the low reaction temperature. As an example, in the second stage of the Circored process (in which the operational temperature is approximately 650 °C and the particle size is about 2 mm) the residence time of the iron ore particles in the fluidized bed is more than 3 h, while the MD only increases from 70% to
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ACCEPTED MANUSCRIPT 95% [3, 9]. The utilization of fine iron ores would decrease the internal diffusion restrictions to the gas-solid reaction and increase the reduction efficiency, and so a fluidized bed can exploit its
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advantages to the fullest only when fine particles are utilized. The use of low-grade iron ore also
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results in the enhanced production of fine iron ore concentrate. The utilization of fine iron ore concentrate is, therefore, also an objective of the fluidization of DR technology.
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Despite the above advantages, fine particles are more susceptible to sticking and defluidization such that the use of fine iron ores restricts the operating temperature. A previous study [4] found that
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fine particles (105 to 140 μm) tend to exhibit sticking above 600 °C. Zhu et al. [2] also reported that
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the sticking temperature of nanosized iron oxide particles was decreased to 450 °C. It is therefore
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evident that achieving high temperature operation requires additional anti-sticking measures. However, all the existing methods used to prevent defluidization, such as additive coatings [31],
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carbon deposition [7, 32-34], and granulation/pelletizing [2, 10], have their own limitations with regard to producing high quality DRI products. The additives introduce impurities into the final
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product, which must be separated by a subsequent smelting process. In addition, the carbon deposition method requires a critical carbon content above 10 wt.% to prevent defluidization [32]. Lei et al. [33, 34] optimized the carbon deposition process and decreased the critical carbon content substantially, to as low as 3.5 wt.%, although this is still greater than the general industry process demand of 3.0 wt.%. As a result, a subsequent process is also needed to lower the excess carbon content. Finally, the granulation/pelletizing method requires a sintering process prior to reduction, and this pre-treatment of the iron ore fines is time-consuming and increases the cost of the process. Compared with the existing methods, the conical fluidized bed is a viable means of preventing
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ACCEPTED MANUSCRIPT defluidization at high temperatures without any pre-treatment or the introduction of inert materials. This lack of defluidization results from the high Ug at the bottom of the fluidized bed, which
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maintains the fluidization of the agglomerates and prevents further coalescence of the agglomerates.
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Therefore, without the presence of defluidization, the conical fluidized bed is promising in achieving continuous operation, which is relevant for implementing the DR of fine iron ore concentrates at
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large scale. Besides, the high Ug can also enhance the external diffusion of the gas-solid reaction and further promote the reduction efficiency. To sum up, employing a conical fluidized bed can provide a
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new approach to the prevention of the defluidization problem, and so represents a clean and efficient
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approach to the intensification of the direct reduction process.
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4 Conclusions
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In this study, the direct reduction of a fine iron ore concentrate with an average particle size of 87 μm was investigated in a conical fluidized bed. The results verified that this type of concentrate
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can be utilized directly in a conical fluidized bed and that the desired DRI product (MD>90%) can be obtained. Compared with a cylindrical fluidized bed, the reduction efficiency of this process was significantly enhanced as the result of employing a high superficial gas velocity in the conical fluidized bed. In addition, it was found that defluidization was avoided even without adding any inert materials or causing elutriation issues. Moreover, stable fluidization was achieved in the conical fluidized bed through the formation of agglomerates and by preventing these agglomerates from further coalescing. Based on the fluidization behavior, the critical velocity and the minimum stable fluidization velocity at different temperatures were determined. As a result, a regime diagram over
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ACCEPTED MANUSCRIPT the temperature range of 650 to 800 °C was proposed and this diagram can be directly employed to
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guide the practical operation of DR in a conical fluidized bed.
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Appendix A: Nomenclature constants in the correlations
D0
bottom diameter of the conical fluidized bed (m)
D1
upper surface diameter of the solid particles in the conical fluidized bed (m)
Dt
top diameter of the conical fluidized bed (m)
dp
particle diameter (m)
g
gravity (m/s2)
H
height of the conical section (m)
MFe
metallic iron content (-)
TFe
total iron content (-)
T
temperature (oC)
Uc
critical velocity (m/s)
Ug
superficial gas velocity (m/s)
Umf
minimum fluidization velocity (m/s)
Umff
minimum fully fluidization velocity (m/s)
Umsf
minimum stable fluidization velocity (m/s)
Utop
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Ut
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C1, C2
Vg
terminal velocity (m/s) superficial velocity at the top of the conical fluidized bed (m/s) gas volume flow rate (L/min)
Greek letters ε0
voidage of the fluidized bed
θ
cone angle of the conical fluidized bed (o)
μf
fluid viscosity (N·s/m2)
ρf
gas density (kg/m3)
ρs
particle density (kg/m3)
φs
sphericity of the solid particle
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DRI
direct reduced iron
MPS
mean particle size
MD
metallization degree
IP
direct reduction
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DR
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Abbreviations
Appendix B: Hydrodynamic analysis of the conical fluidized bed
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Previous studies [28, 35-37] indicated that, with the gas velocity was increased, three main regimes
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were discernable in the conical fluidized bed, i.e., the fixed bed regime, the partially fluidied bed regime, and the fully fluidized bed regime. The transtion gas velocities are defined as the minimum fluidization
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velocity (Umf) and the minimum fully fluidization velocity (Umff). The two key parameters, Umf and Umff
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could be estimated based on the dynamic balance of forces exerted on the fluidizing particles, as developed
C1U mf
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by Peng and Fan [28]. The correlations are listed below:
2
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(1 0 ) 2
3 0
f
d s
(5)
4
D D 2 C1 0 U mff C2 0 U mff (1 0 )( s f ) g 0 D1 D1 where,
D02 D0 D1 D12 D0 2 C2 0 U mf (1 0 )( s f ) g 3D02 D1
2
(6)
(7)
p
and C2 1.75
1 0 f 03 s d p
(8)
The physical properties of the iron ore particles were listed in Table 2. The corresponding Umf and Umff were calculated and are listed in Table 3.
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ACCEPTED MANUSCRIPT Since H2 can reducing the iron ore particles, thus changing the particle physical properties, testing the real Umf and Umff at high temperatures at experimental conditions is impossible. To validate the
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accuracy of the correlations above, we compared the experimental and the estimated Umf and Umff for the
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iron ore particles at room temperature in pure N2 atmosphere. The tested Umf and Umff are 0.017 and 0.021 m/s, respectively, which are in good agreement with those calculated, indicating that the correlations are
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applicable in this case.
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ACCEPTED MANUSCRIPT Acknowledgments
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under grant no. 21325628 and by NSFC, under grant no. 51404228.
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This work was supported by the National Outstanding Youth Science Fund Project of NSFC,
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[26] F.M. Stephens Jr, L.W. Coffer, B.J. Langston, Reduction of iron ore, U.S. Patent, 3053648, 1962. [27] S. Hayashi, Y. Iguchi, Factors aftecting the sticking of fine iron ores during fluidized bed reduction, ISIJ Int., 32 (1992) 962-971.
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[28] Y. Peng, L.T. Fan, Hydrodynamic characteristics of fluidization in liquid-solid tapered beds, Chem. Eng. Sci., 52 (1997) 2277-2290. [29] Y. Zhong, Z. Wang, Z. Guo, Q. Tang, Defluidization behavior of iron powders at elevated temperature: Influence of fluidizing gas and particle adhesion, Powder Technol., 230 (2012) 225-231. [30] J. Shao, Z. Guo, H. Tang, Influence of temperature on sticking behavior of iron powder in fluidized bed, ISIJ Int., 51 (2011) 1290-1295. [31] Y. Zhong, Z. Wang, Z. Guo, Q. Tang, Prevention of agglomeration/ defluidization in fluidized bed reduction of Fe2O3 by CO: The role of magnesium and calcium oxide, Powder Technol., 241 (2013)
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ACCEPTED MANUSCRIPT 142-148. [32] B. Zhang, Z. Wang, X. Gong, Z. Guo, Characterization of precipitated carbon by XPS and its
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prevention mechanism of sticking during reduction of Fe 2O3 particles in the fluidized bed, ISIJ Int., 53
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(2013) 411-418.
[33] C. Lei, G. Zhang, Q. Zhu, Z. Xie, Optimization of carbon deposition process during the pre-reduction
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of fine iron ore in a fluidized bed, Powder Technol., 296 (2016) 79-86.
[34] C. Lei, S. He, Z. Du, F. Pan, Q. Zhu, H. Li, Effects of gas composition and temperature on the
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fluidization characteristics of carbon-coated iron ore, Powder Technol., 301 (2016) 608-614.
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Eng. Process., 39 (2000) 379-387.
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[35] S. Jing, Q.Y. Hu, J.F. Wang, Y. Jin, Fluidization of coarse particles in gas-solid conical beds, Chem.
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[36] M. Rasteh, F. Farhadi, A. Bahramian, Hydrodynamic characteristics of gas-solid tapered fluidized beds: Experimental studies and empirical models, Powder Technol., 283 (2015) 355-367. [37] L. Gan, X. Lu, Q. Wang, Experimental and theoretical study on hydrodynamic characteristics of
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tapered fluidized beds, Adv. Powder Technol., 25 (2014) 824-831. [38] L. Guo, H. Gao, J. Yu, Z. Zhang, Z. Guo, Influence of hydrogen concentration on Fe 2O3 particle reduction in fluidized beds under constant drag force, Int. J. Min. Met. Mater., 22 (2015) 12-20.
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ACCEPTED MANUSCRIPT Table captions:
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Table 1. Chemical composition of the raw iron ore concentrate.
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Table 2. Physical properties of the iron ore particles and the DRI agglomerates
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Table 3. Umf and Umff calculated by the correlations for the iron ore particles and the DRI agglomerates
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Figure captions:
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Fig. 1. Particle size distribution of the raw iron ore concentrate.
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Fig. 2. SEM image of the raw iron ore concentrate.
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Fig. 3. Schematic diagram of the experimental apparatus.
Fig. 4. Variations in the fluidization time and the MD with Ug in the cylindrical fluidized bed
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operated at 775 °C.
Fig. 5. Variations in the fluidization time and the MD with Ug in the conical fluidized bed operated at 775 °C.
Fig. 6. Pressure drop profiles of the fluidization process at different gas velocities when operating at 775 °C.
Fig. 7. Variations in MPS and pressure drop with reducing time at 1.2 m/s and 775 °C.
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ACCEPTED MANUSCRIPT Fig. 8. Variations in MPS and pressure drop with reducing time at 1.3 m/s and 775 °C.
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Fig. 9. SEM images of the DRI particles reduced at 775 °C for 60 min at a) 1.2 m/s and b) 1.3 m/s.
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Fig. 10. Variations in MD with reducing time at 1.2 m/s and 1.3 m/s, 775 °C
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Fig. 11. Fluidization regime diagram for the DR process in the conical fluidized bed.
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Figure 4
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Figure 9
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ACCEPTED MANUSCRIPT Table 1. Chemical composition of the raw iron ore concentrate.
TFe
Fe2O3
FeO
SiO2
CaO
68.94
96.80
0.72
1.98
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Compositio Al2O3
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MgO
0.10
0.10
0.30
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[Mass%]
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Table 2. Physical properties of the iron ore particles and the DRI agglomerates A
B
C
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dp (×10-6m)
87
87
φs (-)
0.33
0.33
ε0 (-)
0.65
0.65
μg(×10-6
17.6
ρf (kg/m3)
1.36 16.0
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D0 (×10-3m)
21.8
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0.69
0.60
0.60
39.4
39.4
4900
2200
2200
0.175
0.175
0.175
16.0
16.0
16.0
21.8
23.4
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ρs (kg/m3)
0.69
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N·s/m2)
800
250
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Parameters
A: iron ore particles at room temperautre with pure N 2.
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B: iron ore particles at 775 oC with 50% H2+50%N2 (by vol.) mixture. C: Primary DRI agglomerates at 775 oC with 50% H2+50%N2 (by vol.) mixture. D: Secondary DRI agglomerates at 775 oC with 50% H2+50%N2 (by vol.) mixture. a: Viscosities of gas mixture at high temperatures are calculated from the literature of L. Guo et al. [38]
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ACCEPTED MANUSCRIPT Table 3. Umf and Umff calculated by the correlations B
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0.0076
0.090
0.882
Umff
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0.010
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1.232
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0.125
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ACCEPTED MANUSCRIPT Highlights The direct reduction of fine iron ore was studied in two types of fluidized bed.
The high gas velocity can considerably improve the reduction efficiency.
Defluidization was prevented without any additives in the conical fluidized bed.
Stable fluidization was achieved by forming stable agglomerates.
Stable operation temperature is enlarged to 800 oC in the conical fluidized bed.
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