Influence of Different MgO Coating Methods on ... - Science Direct

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ScienceDirect Materials Today: Proceedings 2S (2015) S332 – S341

Joint 3rd UK-China Steel Research Forum & 15th CMA-UK Conference on Materials Science and Engineering

Influence of different MgO coating methods on preventing sticking during reduction of Fe2O3 particles in a fluidized bed Lei Guo, Jingkun Tang, Huiqing Tang, Zhancheng Guo* State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, No. 30, Xueyuan Road, Haidian District, Beijing 100083, China

Abstract Coating MgO can inhibit sticking effectively during iron ore reduction in a fluidized bed, in order to explore the functional mechanism of coating, different MgO coating methods (briquetting-sintering method, high temperature injection method, powder method, slurry-sintering method) were tried in this experiment. The metallization ratio (MR) of the product increased slightly with the increase of the coating amount when the coating amount was below 1wt%. When the coating amount surpassed 1wt%, 3600 seconds of fluidization time and 95% of metallization ratio were achieved without sticking happening. The fundamental reason for inhibiting particle agglomeration was proved to be the formation of MgO·Fe2O3 after coating MgO, which caused surface modification of Fe2O3 particles during high temperature reduction process. That makes changes on the accumulation and growth characteristics of the new generating iron, then reduces the viscosity of the particles surface. Meanwhile, the relation between the coating effect and the coating mechanism is analysed in this work. © 2014 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the Chinese Materials Association in the UK (CMA-UK). This is an open (http://creativecommons.org/licenses/by-nc-nd/4.0/). the Chinese Materials Association in the UK (CMA-UK). Selectionarticle and Peer-review access under theunder CC responsibility BY-NC-NDoflicense (http://creativecommons.org/licenses/by-nc-nd/3.0/) Keywords: fluidized bed; sticking; magnesium ferrite; oxide coating; MgO; iron ore fines.

1. Introduction The increasing shortage of coking coal and iron ore of high quality, coupled with high environmental costs, are limiting the development of the traditional blast furnace iron-making process. Fluidized beds are satisfactorily suited to dispose the finely sized raw materials due to the good gas-solid mixing condition, high heat transfer speed, and large contact surface area. Many non-blast furnace iron-making processes in the world use fluidized bed as the

2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the Chinese Materials Association in the UK (CMA-UK). doi:10.1016/j.matpr.2015.05.047

Lei Guo et al. / Materials Today: Proceedings 2S (2015) S332 – S341

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reduction or pre-reduction device of iron ore fines, such as the Finmet and Fior methods (direct reduction processes) and Finex, Hismelt methods (smelting reduction processes), etc. However, the sticking problem is the drawback of the fluidized bed iron-making process. Based on former researches, the reason for sticking during fluidizing reduction of iron ore mostly caused by the newly generated iron with a high surface activity and stickiness[1], resulting in increased friction between the particles and localized agglomeration or even shutdown of the entire fluidization state. Currently, the methods for inhibiting the sticking problem include: lowering reduction temperature, coating methods[2-5], improving the gas flow velocity[6], stirring and agitating[7], etc. Jianhua Shao[8], Yiwei Zhong[9], Hayashi[3] and so on have studied the effects of coating MgO on sticking resistance of iron ore fines during fluidization. El-Geassy[10,11] found in the reduction experiments of MgO-doped Fe2O3 compacts that during firing of MgO containing iron oxides, MgO diffuses in the Fe2O3 lattice forming magnesioferrite (MgO·Fe2O3). The magnesioferrite has a spinel structure and melted at 1986 K. Magnesioferrite and its reduction product magnesioferrite can decreasing the stickiness on the surface of iron ore particles to some extent. The magnesioferrite is more difficult to be reduced into sticky newly generating metallic iron compared with ferrite, which makes great contribution to restrain sticking problem. To further explore the mechanism of coating MgO and improve its coating efficiency, the effects of four kinds of MgO coating methods on sticking prevention during hydrogen reduction of Fe2O3 particles in a fluidized bed at 1073 K were studied in this experiment. The characteristic of the formation of MgO·Fe2O3 with MgO and Fe2O3 powders under different sintering conditions was also studied, which explained the deep mechanism of different coating methods and provided the basis for further improving of coating efficiency. 2. Experimental 2.1. Experimental material and equipment The light calcinesia magnesia (MgO) powders whose density was 2.94 g/cm3 were used as the coating agent in this experiment. The median particle diameter X50 was 5.285 μm which was measured by a laser particle size analyser. 48.4% particles were under 5 μm and 11.5% were above 16.6 μm. In this study, self-made pure Fe2O3 particles, which can excluded the impact of gangue in iron ore particles, was used as the raw material. Pure Fe2O3 reagent powders were sintered in a muffle furnace at 1473 K under air atmosphere for 20 hours. Then, the Fe2O3 powders were agglomerated as a lump. To be broken and sieved after cooling to room temperature, the particles of size between 100-150 mesh (110-150 μm) were reserved for reduction test. The Fe2O3 powders produced by Beijing Chemical Reagent Company, was of chemical reagent grade with the purity greater than 99.5wt%. The fluidized bed apparatus used in this experiment was made of a transparent silica tube with an inner diameter of 0.03 m, the outer part is a preheating silica tube with an inner diameter of 0.07 m. As shown in Figure 1, an observation port was designed on the resistance furnace to monitor the fluidizing state. The reduction gas was mixture of 2 Nl/min N2 and 2 Nl/min H2. High purity nitrogen was used as the shielding gas during heating up and cooling stages.

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Fig. 1. The experimental setup

2.2. Experimental procedure and coating method Firstly, the heating program was started with nitrogen piping into the silica tube. When the fluidized bed went up to the set temperature the reduction was started with the reducing-gas valve switching on. If sticking occurred, switched to nitrogen gas immediately and started the cooling program. After the fluidized bed was cooled to room temperature took out the samples and prepared for following measurements. The metallization ratio was defined by the following equation: M Fe (1) M u100% R

TFe

MR: Metallization ratio, %; MFe: The total metallic iron, wt%; TFe: The total iron, wt%. The total iron and the metallic iron were measured by chemical analytical methods. The morphology analyzing was completed by the scanning electron microscope (SEM & EDS ZEISS EVO 18). In this experiment, batches of 20 g Fe2O3 particles were subjected with different coating methods. The MgO coating amount was the weight percentage of MgO accounting for the Fe2O3. Mixing the MgO powders and the Fe2O3 particles together directly at solid state was the powder method; Pressed the mixture of the MgO and the Fe2O3 powders with the pressure of 40 MPa, then put the briquettings into the muffle furnace and roasted for 3 hours at 1273 K. Finally crushed the sintered briquettings into powders and selected the powders with the diameter between 110-150 μm after being screened. That was the Briquetting-sintering method; Blew the MgO powders into the bottom of the iron ore with a powder injector when defluidization occurred, and then stirred to mix the MgO and the sticky VTM with an “L” shape stick at 1123 K with the rotating speed about 1 revolution per second for 3 mins, this is the so called high temperature injection method. 3. Results and discussion 3.1. Characterizations of the sticking behavior The development of defluidization process could be observed through the observation port of the fluidized bed during the experiment. A complete defluidization process was gradually spreading from the bottom up. When reduced to a certain extent, the bottom particles of the bed stopped fluidization at first. With the ongoing reduction the height of the fixed bed increased until all the fluidization was stopped. According to the theory of Geldart[12] on fluidized particles classification, the iron ore particles used in this experiment belonged to the class B and showed the bubbling fluidization state in gas-solid fluidized process. The static pressure in the bed was proportional to the


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depth and density of the bed, So the bottom of the bed material layer was subjected to the greatest pressure and the density there was also the largest. The turbulent properties of the bottom part were the worst, resulting in the sticking started from the bottom. As shown in Figure 2 (b), the particles with sticky surface sintered with the sieve plate. The sintered particles on the sieve caused the increase of gas resistance and the uneven flow distribution, so the occurrence of sticking was promoted. Some vertical pipelines with diameter of 3-5 millimeters emerged in the fixed bed after the total sticking as shown in Figure 2 (a). The flow of gas went through the pipelines without resistance caused by the fluidized material compared with the normal fluidization, so there would be a steep pressure drop after sticking[13]. Sometimes it fluidized normally apparently while some huge aggregated particles (as shown in Figure 2 (c)) had generated inside the fluidized bed.

Fig. 2. Schematic of the sticking behavior

3.2. Effects of different MgO coating methods on sticking As shown in Figure 3, for each coating methods in the experiment the fluidization duration did not increase much with the increase of the coating amount when the adding amount of MgO was under 1.5wt%; All these four kinds of coating method could achieve 3600 seconds of normal fluidization when the coating amount was above 1.5wt%. As shown in Figure 4, corresponding to the fluidization duration trend, the metallization ratio of the defluidized Fe2O3 particles did not increase significantly with the coating amount when the adding amount was under 1.5wt%; When the coating amount of MgO was more than 1.5wt%, all the four experimental coating methods can obtain the metallization ratios over 95%. To take the metallization ratio as the criterion, the order of the four kinds of coating method from good to poor should be: briquetting-sintering method, high temperature injection method, powder method and slurry-sintering method. The metallization ratios with briquetting-sintering method was much higher than that with the high temperature injection method when the adding amount of MgO was 1wt%, even though their total fluidization durations were near. The reason for this phenomenon was that after the completion of the high temperature coating method there required a certain time for filling the fluidized bed with the reduction gas, the reduction reaction stopped during this time but it was calculated into the reaction time of the high temperature injection method.

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Fig. 3. Influence of the MgO adding amount on the fluidization duration

Fig. 4. Influence of the MgO adding amount on the metallization ratio

Fig. 5. Influence of the MgO adding amount on the mean reduction rate

Divided the metallization ratio by the fluidization duration then the mean reduction rate was obtained, as shown in Figure 5. The longer the reduction time was, the higher the metallization ratio was and the lower the mean reduction rate was, that was due to the fast early reduction and the slow later reduction. The reduction of the Fe2O3 particles corresponds to the shrinking core model[14]. Firstly, fast phase of reduction: the rate is controlled by the transport of the reducing gas from the bubble phase of the fluidized bed to the iron ore particle and the transport of product gas from the particle to the bubble phase; Secondly, slow phase of reduction: the rate of reduction is controlled by solid state diffusion in the small grains of the iron ore particles[15]. As shown in Figure 6, with the improvement of the


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metallization ratio the porous morphology gradually develop from outside to the internal part of the particle.

Fig. 6. Cross section morphology of the Fe2O3 particles with different metallization ratio a: raw ore; b: MR 17.6%; c: MR 35.8%; d: MR 94.7%

3.3. The mechanism of coating

Fig. 7. Surface (a,c,e) and cross section (b,d,f) map by the EDS (High temperature injection method with 1wt% MgO coating amount, defluidized with metallization ratio of 35.8%)

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The element Mg was detected significantly in the outer periphery of the Fe2O3 particles, indicating a certain degree of coating effect was achieved. But most of the MgO adhered to the surface depressions of Fe 2O3 particles in the form of small particles. The adhesive force of the depression part was greater according to the Van der Waals force theory[16]ˈso the coating powders can be stuck in the holes and combined firmly with the matrix as shown in Figure 7. The coating effect was limited by the uneven distribution of the coating particles. So presumably, the use of a finer coating agent could help to improve the coating effect. The contact between MgO powders and Fe2O3 particles with briquetting method was more sufficient compared to the slurry method, which contributed to solid to solid reaction during the sintering process. So the briquettingsintering coating method got the best experimental results. The surface of the Fe 2O3 particles had the maximum stickiness when sticking happened and more depressions and holes generated after reduction, adding the coating agent at this point helped to improve the coating effect. So the sticking prevention effect of the high temperature method ranked only the second place following the briquetting-sintering method. The powder method was the easiest and the most cost-effective among the four coating methods, it also achieved 3600 seconds of normal fluidization duration with the coating amount of 1.5wt% as well as the three other coating methods; The sticking prevention effect of the high temperature injection method was better than that of the powder method, and it could be achieved by installing an ejection gun of coating agent onto the industrial scale fluidized bed apparatus; The improvement of the sticking prevention effect was limited for the briquetting-sintering method and the slurry-sintering method, and both of them confronted with a complicated process and high operating costs. 3.4. The mechanism of sticking prevention In the present study for the mechanism of the sticking prevention by coating MgO, the most convincing theory was that the MgO reacted with the oxides like Fe2O3 on the ore particle surface at high temperature to form into some substances of high melting point like MgFe2O4[11,17]. Those substances increased the softening temperature and decreased the content of the new generated iron on the ore particles’ surface, and then lowered down the sticking tendency of the highly reduced iron ore particles[18]. In order to grasp the characteristics of the formation reaction of MgFe2O4 with MgO and Fe2O3 more accurately, and thus to find a more economical and efficient way of coating treatment, the factors affecting the formation of the magnesium ferrite was studied in this experiment. As shown in Figure 8 and 9, the formation of MgFe2O4 at high temperature was proved by the XRD results. And the influence of different sintering temperatures as well as different sample preparation methods on the formation reaction of MgFe2O4 was further studied[19]. The sample preparation methods included the dry method (powder method, to mix the MgO and Fe2O3 powders derectly), the slurry method (to mix the MgO and Fe 2O3 powders with water at the state of slurry, and then oven dried at 393 K), the dry-briquetting method and the slurry-briquetting method. The pressure of the briquetting treatment was 200 MPa and the sample thickness was 2 millimeters. All the samples were roasted in a muffle furnace for 3 hours at the set temperature, and the heating rate was 10 K per minute from room temperature to the target temperature. After natural cooling to room temperature the samples were ground for XRD detecting.


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Fig. 8. Influence of temperature on the formation of magnesium ferrite (MgFe2O4)

As Figure 8 describes, all the four experimental samples were prepared by the dry method without briquetting. As the MgO had adsorbed water in the air and transformed into Mg(OH)2, the sample of 298 K was composed of αFe2O3 and Mg(OH)2 and there was no presence of MgFe2O4. It can be seen that the characteristic peaks of α-Fe2O3 changes from strong to weak with the increasing temperature from 1073 K, 1173 K to 1473 K. This proved that the generation amount of magnesium ferrite increased with the increase of temperature with the same holding time. The characteristic peaks of MgFe2O4 had emerged at 1173 K with 3 hours holding time, which reflected that the formation reaction of MgFe2O4 could happen with the dry method at this experimental conditions; The characteristic peaks of α-Fe2O3 became undetectable when the roasting temperature was 1473 K with the same 3 hours of holding time, so the formation of MgFe2O4 had reached a relatively complete degree at this condition.

Fig. 9. Influence of different sample preparation methods on the formation of magnesium ferrite

As shown in Figure 9, to determine the generating amount of MgFe2O4 by the characteristic peak intensity of αFe2O3, it could be inferred that the dry method and the slurry method made little influence on the formation of MgFe2O4 at 1273 K with 3 hours of holding time; The briquetting operations showed obvious promoting effect on the generation of MgFe2O4 in both samples with the dry and slurry methods.

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Fig. 10 Fracture morphology of MgO-Fe2O3 briquettes with dry (a,c,e) and slurry (b,d,f) mixing method (1:1 mole ratio) by EDS

As a, c, e in Figure 10 reflect, the MgO still existed in the form of particles after briquetting and roasting with the dry method according to the characteristics of the fracture surface morphology of the briquettes. The iron did not appear inside the MgO particles, indicating that there was no Fe3+ diffused into the MgO phase forming into MgFe2O4. The Mg element was evenly distributed in the matrix of Fe2O3 as detected in Figure 10 (e), considering with the XRD test results at 1373 K, it could be inferred that the MgFe2O4 had generated under this experimental condition. So the conclusion could be drawn that the Mg2+ could diffuse easily in the lattice of Fe2O3 while it was not the same for the diffusion of Fe3+ in MgO[20]. There was no MgO particles found in the sample treated with the slurry method and the Mg element showed a more even distribution characteristics as shown in Figure 10 (b, d, f). So the formation reaction of MgFe2O4 was more complete with this experimental method. 4. Conclusions Through the research on four different coating methods of briquetting-sintering method, high temperature injection method, powder method and slurry method, it helped to explore the mechanism of coating MgO on the ore particles surface and find an efficient coating method to improve the coating effect. The following conclusions were summarized according to the experimental results. 3600 seconds of fluidization duration and above 95% of metallization ratio were achieved when the coating amount of MgO surpassed 1.5wt% for all the four coating methods. The order of the four kinds of coating method from good to poor was: briquetting-sintering method, high temperature injection method, powder method and slurrysintering method.


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The MgO coating agent mainly adhered to the holes or depressions on the surface of the Fe2O3 particles in the form of particles, without good uniformity of distribution. The fundamental reason for the effect of sticking prevention was that the MgO and the the Fe2O3 on the ore surface reacted and formed into some substances of high melting point (e.g. MgFe2O4). It showed that higher temperature and the use of briquetting approach helped to generate MgFe2O4. Acknowledgements This study is supported by the National Natural Science Foundation of China (No.51234001) and the National Basic Research Program of China (973 Program, 2012CB720401). References [1] J. H. Shao,Z. C. Guo, H. Q. Tang, ISIJ Int. 51 (2011) 1290-1295. [2] J. F. Gransden, J. S. Sheasbu, Can Metall Quart. 13 (1974) 649-657. [3] S. Hayashi, S. Sawai, Y. Iguchi, ISIJ Int. 33(1993) 1078-1087. [4] M. H. Khedr, ISIJ Int. 40 (2000) 309-314. [5] U. F. Chinje, J. H. E. Jeffes, Ironmake Steelmake. 13 (1986) 3-8. [6] N. S. Srinivasan, Powder Technol. 124 (2002) 28-39. [7] T. M. Reed, M. R. Fenske, Ind Eng Chem Res. 47 (1955) 275-282. [8] J. H. Shao, Z. C. Guo, H. Q. Tang, Steel Res Int. 83 (2012) 1-8. [9] Y. W. Zhong, Z. Wang, Z. C. Guo, H. Q. Tang, Powder Technol. 241 (2013) 142-148. [10] A. A. El-Geassy, ISIJ Int. 36 (1996) 1344-1353. [11] A. A. EL-Geassy, ISIJ Int. 36 (1996) 1328-1337. [12] D. Geldart, Powder Technol. 7 (1973) 285-292. [13] Y. W. Zhong, Z. Wang, Z. C. Guo, H. Q. Tang, Powder Technol. 230 (2012) 225-231. [14] B. Weiss, J. Sturn, S. Voglsam, S. Strobl, H. Mali, F. Winter, J. Schenk, Ironmake Steelmake. 38 (2011) 65-73. [15] A. Habermann, F. Winter, H. Hofbauer, Z. Johann, J. L. Schenk, ISIJ Int. 40 (2000) 935-942. [16] Q. Li, V. Rudolph, W. Peukert, Powder Technol. 161 (2006) 248-255. [17] A. A. El-Geassy, Ironmake steelmake. 26 (1999) 41-52. [18] Y. W. Zhong, Z. Wang, Z. C. Guo, H. Q. Tang, Powder Technol. 241 (2013) 142-148. [19] S. F. Moustafa, M. B. Morsi, Mater Lett. 34 (1998) 241-247. [20] A. V. Zubets, Inorg Mater. 38(2002) 718-722.