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Comments:

[COM1] Mr Zheng-Yi Li goes to another university to study for PhD. So please add one institute 'b. National Center forMaterials Service Safety, University of Science and Technology Beijing , Beijing 10083' for Zhengyi Li. The later two institutesare 'c.Key Laboratory of Metallurgical Emission Reduction & Resources Recycling, Ministry of Education, Anhui University ofTechnology , Ma’anshan, China' 'd School of Metallurgy, Northeastern University , Shenyang, China' Zhan-xia Di ac Zheng-yiLiab Ru-fei Weiac Ying Liua Qing-min Mengac Tie-jun Chun ac Hong-ming Longac Jia-xin Liacd Ping Wangac

[COM2] Dr. Ru-fei Wei made a great contribution to facilitate communication in the completion of this work and the revision ofthe paper..To facilitate communication on this paperer, we add him to another corresponding author.

[COM3]Wu S, Xu J, Yang S, et al. Basic Characteristics of the Shaft Furnace of COREX Smelting Reduction Process Based onIron Oxides Reduction Simulation[J]. Isij International, 2010, 50(7):1032-1039.

Verso running head: Z.-X. DI ET AL.Recto running head: IRONMAKING & STEELMAKINGCopyright Line: © 2017 Institute of Materials, Minerals and MiningLicense:

Sticking behaviour and mechanism of iron ore pellets in COREXpre-reduction shaft furnace

Zhan-xia Dia,b, Zheng-yi Lia, Ru-fei Weia,b, Ying Liua, Qing-min Menga,b, Tie-jun Chuna,b, Hong-mingLonga,b, Jia-xin Lia,b,c and Ping Wanga,b

aSchool of Metallurgical Engineering, Anhui University of Technology, Ma’anshan, China[COM1]

bKey Laboratory of Metallurgical Emission Reduction & Resources Recycling, Ministry of Education, Anhui University ofTechnology, Ma’anshan, China

cSchool of Metallurgy, Northeastern University, Shenyang, China

CONTACT Rufei u [COM2]Ru-fei Wei un_rain@126.com, Hong-ming Long yaflhm@126.comSchool of MetallurgicalEngineering, Anhui University of Technology, Ma’anshan243002, China; Key Laboratory of Metallurgical Emission Reduction& Resources Recycling, Ministry of Education, Anhui University of Technology, Ma’anshan243002, China

(Received 19 March 2017; Accepted 11 July 2017)

ABSTRACTCOREX is a clean process releasing lower pollution and consuming fewer cokes than the blast furnace process. However,serious sticking phenomenon often occurs in COREX shaft furnace, causing many problems to the normal operation. In thisstudy, the loading reduction experiments of iron ore pellets were carried out under the simulating COREX reducingconditions. The influence of temperature and H2 content in the syngas on the sticking behaviour of the pellets was observedby scanning electron microscope, energy-dispersive spectrometer and X-ray diffraction. The results indicated that thesticking index increased from 6.7 to 90.43%, when the temperature increased from 750 to 950°C. The main composition ofsticking material was metallic iron, and the sticking behaviour depended upon the amount and morphology of precipitatediron on the pellets’ surface. The sticking mechanism was the interpenetrating diffusion mechanism of iron atoms between theadjacent pellets.

KEYWORDS: COREX; pre-reduction shaft furnace; pellets; sticking

IntroductionCOREX process, as a clean process releasing lower pollution than the blast furnace process, uses non-coking coal or smallamount of coke instead of normal coke, making metallic iron in the pre-reduction shaft furnace first and then melting it in thesmelter-gasifier furnace. It is shown that the [AQ2]worldwide COREX process overcomes many difficulties and makessignificant progress in saving energy and reducing emission, reducing cost and increasing production efficiency [1,2].However, there are still somemany[AQ3] disadvantages in the COREX process [2,3], such as the inadvertent sticking of ironore pellets in the shaft furnace [3-5].

The sticking of ironmaking burden [AQ4]has been a major problem in the ironmaking process, irrespective of fluidised bed orfixed bed;.hHowever, most previous studies are mainly on sticking problem with respect to fluidised bed [6,7]. The maininfluencing factors of sticking on iron ore are as follows [8-10]: reduction temperature, atmosphere and the gangue content.Normally, the sticking index (SI) of iron ore increases with the reduction temperature, H2 in the atmosphere can inhibit theproduction of iron whiskers and alleviate sticking phenomenon due to the precipitate morphology of porous iron appearing[AQ5][10-14]. In addition, Al2O3, CaO and MgO in the iron ores can also alleviate the sticking phenomenon [13,15]. Fourindexes describing the sticking are sticking tendency, sticking temperature, sticking time and SI. Earlier studies [16] also foundthe four factors that have different contributions to the sticking, and the sticking temperature is the biggest contributor. So far,there are three theories describing the sticking mechanism of the iron ores. The first one is high surface energy of new ironprecipitation [17,18]. When the new metal iron crystal forms in the reduction, the huge adhesive force generates on the ironcrystal surface, making the ferrous power or iron ore pellet cohere together. The second one is each hook of iron whisker onthe surface [19-21]. When the temperature is above 600°C, iron whisker grows on the surface and hooks with neighbouringwhisker, making the neighbouring pellets cohere. The third one is the formation of low melting point oxides between FexO andother high melting point material [19,20]. The melting point of FexO is the lowest among the gangue compounds, so FexO iseasy to form low melting point compound with other gangue compounds. When the temperature is above 850°C, low meltingpoint compounds (CaO–SiO2–FeO) appear in the pellet.

However, there are few studies on the sticking mechanism of iron ore pellet in COREX pre-reduction shaft furnace. In thisstudy, sticking behaviour of iron ore pellets under various temperatures and gas conditions was investigated. And the effect ofH2 content on the sticking behaviour and the morphology of precipitation iron was studied from the microcosmic angle.

Materials and methods

Experimental materialsThe oxidised pellets used in this experiment are provided by a Corp from Xinjiang province of China. Their particle size is 10–16 mm. Chemical composition of the pellets is shown in Table 1. According to the actual COREX furnace gas, the reducing gasflow is set as 0.88 m3 h−1, and its compositions are 68% CO, 23% H2 and 9% CO2. The protective gas is high purity N2

(99.99%) and its flow is 0.2 m3 h−1. The reduction experiments are carried out under 1.4 kg cm−2 load.

Table 1. Chemical compositions of pellets, %.

Fe2O3 SiO2 Al2O3 CaO MnO TiO2 MgO

88.22 8.48 1.25 0.75 0.33 0.23 0.22

Experimental methodsFive hundred grams of pellet samples[AQ6] were put into a tailor-made graphite crucible, and the graphite crucible was putinto the furnace, which can test withthe reduction softening behaviour, equipped with a load device tube (diameter of 90 mm × 1000 mm), as shown in Figure 1. The sample was heated to a pre-determined temperature under N2 protection at a heatingrate of 6°C min−1. After reaching an experimental temperature for 30 min, the gas was converted to the mixed reduction gas.Then, the reduction time was for 150 min under 1.4 kg cm−2 load conditions. After that, the gas was immediately converted toN2 again, cooling the sample until the room temperature.

Figure 1. Experimental apparatus for the reduction of pellets under load 1-load; 2-SiC guide bar; 3-flow metre; 4-graphitecrucible; 5-thermocouple; 6-SiC bearing; 7-furnace; 8-cylinder; 9-pressure sensor; 10-counterweight.

The reduction time is 4  h in industrial COREX installations [22], and the metallisation degree of pellets after this long-timereduction is 60–70%. In the laboratory experiments, the same metallisation degree of pellets is set as a basis to determine thereaction time. When the reduction time is 150 min in the laboratory, the metallisation degree is 65%. So, the reduction time inthe experiments was 150 min. The reduction temperature is 800–850°C in the COREX-3000 shaft furnace of Baosteel. Highertemperature than this will cause sticking. To study the behaviour of pellets, the experimental temperatures were set at 750,800, 850, 900 and 950°C. The different reduction gas compositions were studied, as shown in Table 2.

Table 2. The reduction gas composition, %.

CO H2 CO2 N2

1 68 23 9 0

2 63 28 9 0

3 58 33 9 0

4 53 38 9 0

5 48 43 9 0

CO H2 CO2 N2

The SI of different samples was determined by the drop test. First, the pellets after the reduction were weighed, and dropped10 times from a height of 1 m onto a steel plate surface. The weight of pellets still sticking together (cluster diameter > pelletsdiameter of 1.5 times [23]) after each dropping was recorded. The percentage of clusters remaining after each drop wascalculated as the mass percentage of sticking pellets. Then the mass percentage of sticking pellets against the drops numberwas plotted in a graph, shown in Figure 2. SI was defined as follows:

where S1 is the area under the curve,

%. The SI value is equal to zero when no clusters of two or more pellets remain and one hundred when all pellets remainclustered and do not disintegrate during the drop tests.

Figure 2. SI calculation of pellets after reduction.

Results and discussionThe iron ore pellets before and after reduction are shown in Figure 3. It can also be seen from Figure 3 that separating pelletsobviously formed a cluster after 150 min reduction. The influences of temperature and H2 content in the reduction gas onsticking were investigated and discussed in the following.

Figure 3. Iron ore pellets of before (a) and after (b) reduction.

Effect of temperature on stickingTo discuss the relation between sticking and the factors of sticking, temperature was examined first. According to the aboveexperimental steps, pellets were reduced for 150 min under from 750 to 950°C for sample atmosphere. SI under varioustemperatures is shown in Figure 4. The SI reaches 7 and 31% at 750 and 800°C, respectively. The SI at 800°C is more than25%[24]. With the increasing reduction temperature, the SI gradually increases, and the SI reaches the maximum (90%) at950°C. We can conclude that the effect of temperature is the main factor on the SI.

Figure 4. SI at different temperatures.

To study whether[AQ7] sticking was wWhether caused by precipitated plenty of iron, the phase composition of stickingmaterials was studied. It illustrates the X-ray diffraction (XRD) patterns of the sticking materials in Figure 5. It reflects thetendency of precipitated iron of reduced pellets. The intensity of spectral line of FexO decreases, while the intensity of spectralline of element Fe is up with increasing temperature, and the changing tendency of iron corresponds to sticking.

Figure 5. XRD patterns of the sticking material at different temperatures.

The next, the sticking phase of interface and the matrix of pellets were investigated by scanning electron microscope (SEM)and the energy dispersive spectrometer (EDS) analyse, as shown in Figure 6. A is the sticking phase, B and C are matrixcomponent points of two contiguous pellets. Analysis shows that the sticking phase mostly consists of iron, only a small amountof oxygen and other gangue phase material exist, the result corresponds to XRD analysis. And content of metallic iron insticking phase is more than matrix in pellet. Therefore, it is inferred that iron diffuses from the reaction interface containinghigh concentrations of precipitated iron to the pellet surface, sticking depends upon the amount of precipitated iron of surfaceof pellets.

Figure 6. SEM images of sticking interface and EDS spectra with element contents at 850°C.

The effect of H2 content in the reduction gas on stickingThe H2 content in the gas from the melting gasifier is about 28% in the industrial production of Baosteel Co Ltd [8], so in thiswork the hydrogen concentration is set from 23 to 43%, as shown in Table 2. The results of SI and metallisation degree underdifferent H2 contents at 850 and 900°C, respectively, as shown in Figure 7. When the content of H2 increases from 23 to 43%,the SI of pellets decreases from 30.72 to 15.99% at 850°C. The sticking tendency of 900°C corresponds to that of 850°C. TheSI decreases to 42.25% at 900°C. Comparing with SI under various temperatures, it is inferred that the temperature is thebiggest contributor to sticking [16]. With the increasing content of H2, the SI shows a downward trend. It can be concluded thatincreasing the proportion of H2 can inhibit the occurrence of sticking to a certain extent.

Figure 7. SI under different content of H2.

Figure 8 illustrates the XRD patterns of the sticking phase under various H2 contents. The diffraction peaks of iron obviouslystrengthen with increasing concentration of H2 contents, which is in accord with the SI changed with H2 concentration. And thepeak of iron increases very high with increasing H2 compared with them in different temperatures under the COREX gascondition.

Figure 8. XRD patterns of sticking material at different H2 content in the reduction gas.

With the increasing ratio of H2, the speed of precipitation iron was accelerated, and it generated a large amount of new iron ina short time in the reaction interface of pellet. Dussta’s etal.[AQ8] [7] [28] also found that the addition of hydrogen would makethe reaction speed increase, which was consistent with our result. When the temperature is greater than 810°C, reductionability of H2 is stronger than that of CO. Hydrogen will give diffusion priority to the surface of pellets; when the pellets arereduced, H2 will preferentially diffuse on the pellet surface [25], so rich hydrogen atmosphere is more convenient to acceleratethe precipitation of ferrous metal, and the increase of H2 content helps to speed up the process of reaction. In the process ofincreasing the ratio of H2, the precipitation of iron in the pellet increases greatly, and the adhesion of the new iron increases,because of the high surface energy. However, the SI is lower than that in actual COREX atmosphere. Therefore, the sticking ofthe pellet is related not only to the production speed of iron but also to the precipitation morphology of iron [6].

Mechanism of sticking caused by morphology of precipitated ironTo study the relation between sticking and morphology of precipitated iron, the microstructure of precipitated iron wasinvestigated by SEM at 850°C. As shown in the figure, Figure 9(a) is a picture of reduction in COREX gas containing 23% H2;Figure 9(b) is the microstructure in a hydrogen-rich atmosphere containing 33% H2. In Figure 9(a), precipitated iron includespyknotic layered iron and short whisker iron. In Figure 9(b), the form of precipitated iron is dominated by layered iron with lotsof pores, and whisker iron is rarely seen. Komatina and Gudenau [6] found that the sticking behaviour of iron ore was closelyrelated to the precipitation form of metal iron reduced on the particle surface, and.tThe precipitation form of metal iron mainlyincludes pyknotic layered iron, porous iron and fibrous whisker iron. T, the layered iron is mainly present in the state ofcontaining H2[AQ9]. WhichhisThis behaviour was also be [AQ10]found in our experiment, and qKomatina’s study verifies theobserved experimental results we found. The reason for the formation of these two different forms of iron should be differentatmosphere. In hydrogen-rich atmosphere, because H2 will accelerate the speed of precipitated metal iron, and it is too late foriron atoms to nucleate, so that crystallisation of Iron is not complete, and then it forms layered iron. It can be deduced that theprecipitated morphology of metallic iron is related to the content of gas. When the concentration of CO in the gas is high, themorphology of precipitated iron is mainly composed of whisker iron. When the gas is rich in hydrogen, the precipitated iron isdominated by pyknotic layered iron.

Figure 9. The microstructure of sticking in different hydrogen content.

Figure 10 shows the process of sticking; the oxygen atoms and hydrogen combined with CO forming H2O and CO2 which weretaken away, a large number of active metallic iron atoms are precipitated. First, the reducing gas reacts with the iron oxide onthe surface of the pellet, and the iron oxide continues to be reduced by hydrogen and CO. The reduction order of the ironoxide is Fe2O3–Fe3O4–FeO–Fe, and the iron is reduced according to the order. In the process, the previous oxygen vacancyis occupied by iron atoms, and iron atoms migrate to the surface constantly and are precipitated. The precipitation of ironcontinues to deposit together, and due to the different concentration gradient of iron atoms, the new consecutive precipitationof iron atoms migrates forward along the precipitation of the previous iron in a diffusion manner. When the iron contactseach[AQ11] pellet and under the action of the load, the precipitated iron will stick together, resulting in the sticking of pellets.As the new precipitation of iron has a huge surface energy [26], the surrounding precipitation of iron gathers togetherconstantly. High temperature makes the pellet soft ing[AQ12], and the pellet continues to be squeezed under the action of theload, as a consequence of which the SI in the pellet is much higher than the SI in blast furnace [4] and SI in FINEX. In theFINEX process, iron ore powder is used to smelt and it belongs to the fluidised bed process. Because the sticking is caused bythe diffusion of iron atoms, the diffusion of iron atoms can be achieved when irons are in contact. To inhibit sticking, thecontact area should not be too large, it means SI is not too high. Based on Wagner theory, Buffington etal. [27] studied theself-diffusion behaviour of iron in iron and iron oxides, and the results showed that the diffusion coefficient of iron in iron is 5–10 times of that in FeO. It can be concluded that metallic iron has higher sintering activity than FeO, which makes it easier forthe pellet to stick in the case of large amount of iron precipitation, and it is consistent with the experimental results. Iron atomsmigrate to the interface in a diffusive manner. Surface diffusion coefficient is in direct proportion to temperature, and when thetemperature is high, the surface diffusion coefficient is large, and the deposition speed of iron will be accelerated, and result inhigher sintering activity and the sticking will be strengthened [27].

Figure 10. The progress of causing sticking.

Conclusioni. The SI of pellets increases with the temperature. The SI increases from 6.7 to 90.43% when the temperature increases

from 750 to 950°C.ii. The SI of pellets decreases with the increase of H2 content. When the H2 increases from 23 to 43%, the SI decreases by

14.73% at 850°C and by 42.25% at 900°C.iii. The SI of pellets depends upon the amount of precipitated iron on the pellet surface and morphology of precipitation

iron. Iron atoms after reduction separate out and move out by diffusing, accumulating on the pellet surface. Newprecipitated iron with large surface energy continuously sinters together at high temperature, making the stickingphenomenon exacerbate. In the atmosphere containing hydrogen, the precipitation of whisker iron is suppressed andthe precipitation of layered iron is promoted. And the layered iron alleviates the occurrence of the sticking phenomenon.

iv. The sticking mechanism is the interpenetrating diffusion mechanism of iron atoms, interlocking of whisker iron and hightemperature promoted the occurrence of adhesion, pyknotic layered iron inhibits the sticking phenomenon.

Funding

This work was supported by National Science Foundation of China under Grant (No. 51404005 and[AQ1]National ScienceFoundation of China under Grant No. 51574002).

Disclosure statementNo potential conflict of interest was reported by the authors[AQ13].

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