Mechanism of surface morphology evolution in the reduction of fine iron ore in a conical fluidized bed reactor

Journal Pre-proofs Mechanism of surface morphology evolution in the reduction of fine iron ore in a conical fluidized bed reactor Xu Zhang, Shengyi He, Haoyan Sun, Qingshan Zhu, Jun Li, Hongzhong Li PII: DOI: Reference:

S0009-2509(19)30958-3 https://doi.org/10.1016/j.ces.2019.115468 CES 115468

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Please cite this article as: X. Zhang, S. He, H. Sun, Q. Zhu, J. Li, H. Li, Mechanism of surface morphology evolution in the reduction of fine iron ore in a conical fluidized bed reactor, Chemical Engineering Science (2020), doi: https://doi.org/10.1016/j.ces.2019.115468

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Mechanism of surface morphology evolution in the reduction of fine iron ore in a conical fluidized bed reactor Xu Zhanga,b1, Shengyi Hea,b1, Haoyan Suna,b, Qingshan Zhua,b*, Jun Lia,b*, Hongzhong Lia,b a

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

b

School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: Surface morphology evolution in the reduction of fine iron ore in a conical fluidized bed reactor under various conditions was investigated. Regime diagrams of morphology on iron ore reduction with H2 were obtained for choosing appropriate operating condition. The result indicates that it is beneficial to inhibit the formation of iron whiskers under high H2 content, high gas velocity and low reduction temperature. Further, the formation mechanism of the whiskers in the iron ore reduction with H2 atmosphere was revealed by the quantitative analysis of the relative rate between the iron diffusion and reduction. The result indicates that iron diffusion rate is the dominant factor that brings about whiskers during the reduction. Based on the regime diagrams of morphology, the formation of whiskers could be inhibited with the guidance of a whisker formation tendency value below 2.51 m4·mol-1. This provide a method of guidance for the application on iron ore reduction process by adjusting the relative rate between diffusion of iron and reduction. Keywords: surface morphology; conical fluidized bed; fine iron ore; whisker

1

Xu Zhang and Shengyi He contribute equally to the article.

*Corresponding authors: Qingshan Zhu ([email protected]), and Jun Li ([email protected]) 1

1. Introduction As an important method of the non-blast furnace technology, fluidized direct reduction (DR) process has the advantages of direct access to fine ore (Gaines, H.P. et al., 2007; Sibakin, 1962; Schenk, 2011), high reduction efficiency (Pang et al., 2015) and extensive adaption to complex minerals (Sun et al., 2016; Sun et al., 2017) compared to shaft furnaces and rotary kilns processes. Moreover, several fluidized DR processes have been developed to produce reduced irons, such as FIOR, FINMET, Circored and FINEX (Schenk, 2011). However, defluidization due to sticking of iron nano/micro-particle remains the bottleneck of these processes for the treatment of fine iron ores with particle size less than 100 μm (Zhang et al., 2011; Nuber et al., 2006). In order to avoid the defluidization problem, harsh technological conditions like using coarse iron ore particles, lowering the reduction temperature and reducing the metallization ratio must be adopted to inhibit the excessive agglomeration of reduced iron ore particles (Hillisch and Zirngast, 2001), which decreased the efficiency of the reduction process. It has been proved that the fluidization behaviors and reduction efficiency of iron ores are greatly influenced by the surface morphology of newly formed iron nanoparticles during the reduction process (Du et al., 2016; Komatina and Gudenau, 2004; Mikami et al., 1996). Previous studies indicated that three different surface morphologies can be formed during the fluidized DR process, i.e., the dense iron, the fibrous iron (growing whiskers) and the porous iron (Li et al., 2018; Zhong et al., 2012). The microstructure of porous morphology is beneficial for reducing gas

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diffusion and reduction, while the dense structure morphology makes it difficult to diffuse into the inner part of iron gain, thus leading to a negative effect on final reduction degree (Li et al., 2017; Hayashi et al., 1990). The formation of fibrous whisker morphology on the particle surface, however, is the worst case, where the needle-like fibrous whiskers tend to stick with each other to form large agglomerations rapidly, resulting in an occurrence of defluidization (Komatina and Gudenau, 2004; Zhong et al., 2012; Hayashi et al., 1990; Wong et al., 1999). The formation mechanism of direct reduction iron (DRI) whiskers has drawn extensive attention under both CO and H2 reductive atmospheres. Zhong (Zhong et al., 2016) and Lu (Lu et al., 2018) employed density functional theory (DFT) calculation to investigate the adsorption of H2 and CO on FeO (111) phase and further predicted the metallic iron growth orientation. They concluded that the collision and squeeze of CO-CO2 led to the non-uniformed metallic iron and promoted the formation of whiskers. Besides, reduction in H2 had the advantage of better gas diffusion to improve dynamic reduction condition. Hence, whiskers grew more easily in CO. Du (Du et al, 2016) claimed that the formation of whiskers was related to the phase angle of Fe/Fe1-xO interface and the amount of metallic iron on the surface. They found that the phase angle of Fe/Fe1-xO interface formed in H2 atmosphere was acute, while the formed phase angle in CO atmosphere was obtuse, where the vertical iron growth orientation on grain surface contributed to occurrence of whisker. Wagner (Wagner, 1952) claimed that both the reduction potential of CO/ (CO+CO2) and temperature markedly influenced the DRI (direct reduction iron) morphology. Schiller

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further clarified the morphology zone at different temperatures and reduction potentials (Schiller, 1987). According to Schiller’s morphology zone, both the dense iron and the porous iron structures are formed under high reduction potential, of which the former occurs at the lower temperature. The fibrous structure appears at the intermediate temperature and the low reduction potential. Several groups reported the morphology in H2-N2 reducing gas, but drew contradictory conclusions on whether the whiskers can be formed (Matthew et al., 1990; Gransden and Sheasby, 1974; Guo et al., 2015; Shao, 2012). Matthew found different porous morphologies in a wide range of H2 contents (5-100 vol.%) and temperature (600-1100C) at dominantly high gas velocities (Matthew et al., 1990). The porous morphology changed with the different H2 contents and temperatures, and concluded that it was determined by the relative rate of iron growth and decomposition of wüstite. Guo also indicated that porous morphologies were formed during the reduction of a sintering regent Fe2O3 in a fluidized bed at 800-1000°C and 5-100 vol.% H2 content (Guo et al., 2015). However, Gransden and Sheasby found that whiskers were formed under the mixture of H2-N2 within the temperature range of 630-680 oC while the whisker disappeared when the temperature was at 595 °C (Gransden and Sheasby, 1974). Shao found an obvious growth of whiskers during the reduction of a Brazil iron ore at 700 °C with a gas velocity of 0.15 m/s in a fluidized bed (Shao, 2012). In conclusion, there is no consensus on the surface morphology of DRI in the H2 atmosphere and its qualitative analysis, which is indicative of the necessity to carry out an in-depth study on the surface morphology of DRI particles

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and its formation mechanism. Aimed at this, the surface morphology evolution of DRI particles at the H2 atmosphere was studied in a conical fluidized bed reactor. The effects of gas velocity (Ug), H2 content and reducing temperature on the surface morphology were investigated. The whisker formation tendency was also calculated to show the corresponding mechanism for DRI whiskers growth. 2. Experimental 2.1. Materials The iron ore concentrate used was Brazil fine iron ore with a volume-based mean particle size of 86.9 µm in the range of 20-200 μm measured by a laser particle size analyzer (Beckman Coulter LS13320) in Fig.1. The Brazil fine iron ore showed flat and smooth surfaces (Fig. 1). The chemical composition of the Brazil fine iron ore is provided in Table 1. It can be seen that the main component is ferric oxide. To distinguish the value of iron element, the content of FeO and total iron were measured by the titration methods according to the Chinese standard of GB/T6730.8-2016 and GB/T6730.65-2009, respectively. The physical properties of the particle are presented in Table 2. The particle density and the bulk density of the raw iron ore particle were 4900 kg/m3 and 2200 kg/m3, as measured by pycnometer and funnel methods, respectively. The particle sphericity was 0.28, as calculated by the pressure drop data using the Ergun equation, where the gas velocity, bed height and pressure drop tested in a cylinder fluidized bed were substituted to the Ergun equation. The fluidization behavior was tested in the conical fluidized bed at room 5

temperature. The results are shown in Fig. 2. The value of Umf for the Brazil iron ore at room temperature in the N2 atmosphere were 0.020, which is much higher than the theoretical values of 0.012m/s. The theoretical value of Umf at room temperature was calculated by the equation (Kunii et al., 1977):

U mf =

r02  (  s - g )  g 1650  

(Re  20)

(1)

The voidage tested at minimum fluidization is 0.65. This result indicated that it is difficult to fluidize the Brazil iron ore due to its flake particle. In addition, measured pressure drop at minimum fluidization was 276 Pa, which is less than the theoretical pressure drop (390 Pa), indicating the responsibility of the taper wall of conical fluidized bed. Both the H2 (99.99%) and N2 (99.99%) were provided by Beijing Beiwen Gas Chemical Industrial Co.Ltd. 2.2. Apparatus and procedures The fluidized reduction experiments were conducted in a laboratory-scale quartz conical fluidized bed. The schematic diagram of the fluidized apparatus is shown in Fig. 3. The conical section of the conical fluidized bed has a height (H) of 170 mm with the initial inner diameter (D0) of 16 mm and upper inner diameter (Dt) of 76 mm in the conical section. The cone angle of the conical fluidized bed was referred to the extension cord angle which was formed by the conical fluidized bed symmetrical side wall. The conical fluidized bed adopted in the experiment has a half cone angle (α/2) measure of 10 °. A porous quartz plate was used as the gas distributor at the bottom of

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the tapered segment, with a uniform pore diameter (Φ) of 0.4 mm made by laser drilling. The pores in the distributor formed regular triangle cell with the side length (D1) of 1.8 mm. Most of the reduction of iron ore used are the cylinder fluidized bed, which is usually used for treating the coarse iron ore particles with particle size larger than 0.25 mm (Pang et al., 2015; Nesibe et al.¸2015). However, the cylinder fluidized bed is not difficult to operate for the reduction of fine iron ore particles with particle size less than 0.2 mm due to sintering of the reduced iron ore that causes defluidization. To prevent the defluidization, the conical fluidized bed based on two gas-contact regimes (the conical fluidized bed regime and the conical spouted bed regime) was proposed. Ozawa et al found the spouted fluidized bed with a conical angle of 60° could overcome the undesirable sintering of the reduced iron ore at 800-900 °C (Ozawa et al., 1973), while Kim and Lee indicated that the optimum angle of the conical fluidized bed was within 3-25° for reducing fine iron ore of wide grain range (Kim et al., 2001; Lee et al., 1998). In our previous work, a conical fluidized bed with the angle of 10 ° was adopted to reduce Brazil iron ore and showed good fluidization quality compared to a cylinder fluidized bed (He et al., 2017). In the conical fluidized bed, the gas velocity can be expressed as Eq. (2): U t = U g  (D 0 / D t ) 2

(2)

where the D0 and the Dt represent the initial inner diameter and upper inner diameter in the conical section of the fluidized bed, respectively; the Ut and the Ug represent the superficial gas velocity located in the conical section corresponding to the inner

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diameters of the Dt and the D0, respectively. It can be inferred from Eq. (2) that the gas velocity is in inverse proportion to the square of diameter along the axis, thus the value of Ut is lower than that of Ug, which can effectively prevent the fine particles from blowing out of the bed. A vertical tube furnace was adopted as the heat source for the fluidized bed and sustained the desired bed temperature with a temperature controller. The bed pressure drop was monitored by a differential pressure gauge sensor and recorded by a computer. The gas flow was controlled by mass flowmeter controllers. For all the experiments, the fluidized bed was firstly preheated to a desired temperature under the N2 atmosphere, after which 8.0 g of the iron ore concentrates were fed into the bed from the top. After the bed temperature reached the desired temperature within 2 minutes, the fluidization gas was switched and mixed in a gas holder with a desired ratio before flowing through the fluidized bed with a specific Ug. The H2 content refers to the volume ratio of the H2 to the gas mixture, which can be expressed as Eq. (3): (3)

CV H 2 = [V H 2 / (V H 2 + V N 2 )]  100%

where the CVH2 is the H2 content in the gas mixture; the VH2 and the VN2 represent the volumetric gas flow rates of hydrogen and nitrogen in each trial, respectively. When defluidization occurred (pressure drop decreased suddenly) or after the specific reduction time (fixed time for kinetics analysis), the gas flow was promptly switched back to N2 to terminate the reduction immediately. The fluidized bed was then removed from the furnace and quenched to room temperature by spraying water

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directly onto its outer surface. The sample was decanted to a bag and sealed for the further characterization. The fluidized reduction of Brazil fine iron ore was conducted in the temperature range of 600-800 oC. The operating Ug ranged from 0.2 m/s to 1.5 m/s under the reduction temperature with the interval of 0.1 m/s. The Ug can be calculated by the gas volume rate as follow:

Ug =

Vg 2 0

0.015πD



T 283.15

(4)

where Vg is the volume flow rate of the gas mixture at 283.15 K and normal atmosphere, L/min; T is the operating temperature, K; D0 is the initial inner diameter in the conical section of the fluidized bed, 16 mm. Since the superficial gas velocity in the conical fluidized bed varied with the bed height, the Ug value in the conical fluidized bed refers to the superficial gas velocity at the distributor. Reduction degree (RD) is defined as the ratio of current weight loss to the theoretical maximum weight loss (mT), which can be expressed as: RD =

mC mT

(5)

where mC is current weight loss and mT is theoretical maximum weight loss after the reduction process. The theoretical maximum weight loss (mT) can be calculated by supposing the ferrous and ferric in the iron ore are completely reduced to metallic iron:

mT =

MO (2w 0,Fe2+ + 3w 0,Fe3+ ) 2MFe

(6)

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where MO and MFe are the molar mass of O and Fe, respectively, g/mol; W0,Fe2+ and W0,Fe3+ are the weight content of ferrous and ferric in the raw iron ore, respectively. The current weight loss (mC) can be calculated by the following equation:

mC =

w Fe 2 + - w 0, Fe 2 + 2M Fe + w Fe 2 + ) ( MO

(7)

where WFe2+ is the weight content of the ferrous ion after the reduction. By substituting equation 2 and 3 into equation 1, we have the following equation: RD =

w Fe2 + + 3w Fe0 - w 0, Fe2 +

(2w 0, Fe2 + + 3w 0,Fe3+ )[1 + w Fe2 + ´ M O / (2 * M Fe )]

´100%

(8)

where WFe2+ and WFe0 represent weight contents of the ferrous and metallic iron in the reduced sample, respectively. The metallic iron and ferrous iron can be measured based on the chemical titration standards methods (GB/6730.6-2016 and GB/6730.8-2016). Before the titration, chloride ferric solution and hydrochloride acid were added to dissolve metallic iron and iron oxides of the reduced sample in sequence. W0,Fe2+ and W0,Fe3+ are weight contents of the ferrous and ferric in the raw material, respectively. Theoretically, the iron containing phase in the reduction experiences the transition of hematite, magnetite, wüstite and metallic iron in sequence. To calculate the reduction rate from wüstite to metallic iron, reduction degree (RDw) is defined as the ratio of the current weight loss (RD) minus the weight loss assigned to theoretical maximum weight loss of wüstite iron (RDwt) to the theoretical maximum weight loss from wüstite iron to metallic iron. In this case, the RDw can be expressed as equation (9): 10

RDw =

RD  RDwt 100% 1  RDwt

(9)

Based on the definition of RDw, the values of RDw varies from -0.5 to 1. When RDw is below zero, the corresponding phase contains magnetite or hematite. When RDw is zero, the iron ore is completely converted to wüstite phase and no metallic iron is formed. The morphology of raw and reduced samples was examined by scanning electron microscopy (SEM, JSM-7001F; JEOL). The phase structure of the reduced products was examined by X-ray diffractometer (XRD, X’PERT-PRO, PANalytical) with Cu Kα radiation (λ=1.5408 Å) at 40 kV and 40mA. 3. Results and discussion 3.1. Morphology of DRI Fig. 4 illustrates the morphology of DRI at 700 C with various Ug in the mixture of H2 (50 vol.%) and N2. The result indicates that the Ug has an evident effect on the surface morphology of the DRI. At the low Ug of 0.2 m/s, the surface of DRI was covered with slender whiskers that hooked between the particles, which easily lead to agglomeration and defluidization. The whiskers became much shorter with the Ug increasing. When the Ug reached 0.8 m/s, there was no distinct whisker but numerous bumps on the particle surface. With a further increasing of Ug, sponge iron structure with flat and small pores was formed on the particles surface. Subsequently, the morphology of the DRI at the temperature 700 °C and Ug 0.8 m/s with H2 contents of 10 vol.%, 30 vol.%, 70 vol.% and 90 vol.% was investigated. 11

The results are presented in Fig. 5. At the low H2 content of 10 vol.%, a beneficial condition was created for whiskers growth and plenty of conical whiskers appeared on the particle surface. With an increase to 30 vol.% in the H2 content, the whiskers became inconspicuous and a few pores were formed on the surface of particle. When the H2 content was improved to above 70 vol.%, there was no whisker but plenty of small pores on the surface of particle. The result indicates that the morphology of the DRI evolves from whiskers structure to smooth porous structure with the increase of H2 content. Further, the effect of temperature on the morphology evolution was also investigated at the different reduction temperatures of 600 °C, 650 °C, 750 °C and 800 °C and the Ug 0.8 m/s with 50 vol.% H2 content. Fig. 6 shows that the particle surface is flat and the pore is unconspicuous at the lower reduction temperature of 600-650 °C, indicating the formation of dense shape DRI particle. At the elevated temperature of 750 °C and 800 °C, many conical whiskers grow on the particle surface. It should be noted that defluidization occurred at 750 °C and 800 °C within 93 s and 46 s, respectively. This result indicates that elevating temperature favors for whiskers, which leads to a rapid defluidization. According to the above results, besides the ore properties (Komatina and Gudenau, 2004), the DRI morphology is to a large extent determined by reduction temperature, H2 content and Ug. The favorable conditions for the whisker growth are low H2 content, low Ug and elevated temperature. 3.2. Morphology diagram of DRI 12

To further investigate the joint effect of Ug and H2 content on the surface morphology, reduction was conducted at 775 °C within the H2 content of 10-100 vol.% and the Ug value of 0.6-1.5m/s (H2-N2). A morphology diagram was obtained to plot the relationship of DRI morphologies between Ug values and H2 contents at 775 °C (Fig. 7). Generally, the porous morphology was formed at the high Ug and high H2 content, while the whiskers are prone to be developed at the low Ug and low H2 content. Moreover, a critical operation line about Ug and H2 content was obtained, lower than which the whiskers are formed on the surface. The Us point on the operation line decreases with the increase of H2 content. Noted that only the whiskers iron rather than porous iron can be obtained when the H2 content was less than 30 vol.% below Ug 1.5 m/s. The morphology diagram to the relationship of DRI morphologies between temperature and Ug at the 60 vol.% H2 content is shown in Fig. 8. The whiskers are usually formed at elevated temperature, while it is effectively inhibited by increasing the Ug. Similar to Fig. 7, a critical operation line about Ug and temperature was also obtained, lower than which the whiskers are formed on the surface. However, when the operation point is above the operation line, the DRI morphology can be divided into two different regions: dense shape and porous shape according to the temperature 700 oC. As mentioned above, several groups reported contradictory conclusions on the morphology in H2-N2 reducing gas (Matthew et al., 1990; Gransden and Sheasby, 1974; Guo et al., 2015; Shao, 2012). The morphology diagrams of Fig. 7 and Fig. 8

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can combine the contradictive results in the mixture of H2-N2. The formation conditions of whiskers (Shao, 2012; Gransden and Sheasby, 1974) and dense structures (Gransden and Sheasby, 1974) can be included in the whiskers zone and dense structure zone respectively (Fig. 7 and Fig. 8), indicating the validity of the morphology evolution regimes of the present study. In addition, Mathew (Matthew et al., 1990) and Guo (Guo et al., 2015) only found pourous structure with the dominant of Ug, which is in agreement with the trends of the morphology evolution in Fig. 7 and Fig. 8. Although the reduction condition in the lab-scale experiment is different from that of large-scale fluidized bed reactors in industry, the morphology evolution regimes obtained in the lab-scale experiments are similar to the one obtained in large-scale fluidized bed reactors. Thus, the morphology regimes obtained in the lab-scale experiment would provide a method of guidance for their application by adjusting the reduction conditions of temperature, Ug and the H2 content. 3.3. Mechanism of DRI morphology formation Brenner and Sears (Brenner and Sears, 1956) attributed the growth of whisker to the supersaturation of metallic iron on the solid surface rather than the reducing gas type. When the supersaturation was in low level, whiskers began to be formed due to the screw dislocation effect. On the contrary, with the high level of iron supersaturation, the flat surface appeared. Wagner (Wagner, 1952) analyzed the reduction process and proposed that the morphology was concerned with the dispersion of iron supersaturation. Specifically, when the process was controlled by 14

chemical reaction, the iron supersaturation on the surface was homogeneous, and iron crystal nucleus developed into whiskers. On the contrary, when the reduction process was controlled by the iron atom diffusion, the supersaturation was inhomogeneously distributed, and the dense iron was formed. Based on the theory of Wagner, Nicolle and Rist (Nicolle and Rist, 1979) built the mathematical model expressed as the Eq. (10):

w

Ds r r0(p / p0 -1 )

(10)

where w is the tendency of whisker formation; Ds is the iron atoms diffusion rate; r is the reaction rate; r0 is the particle radius; p is the metallic iron content in the wüstite; p0 is the saturated content of iron in the wüstite. The above model comprehensively associates the whisker formation with the iron diffusion rate, reduction rate, supersaturation and ore particle size. Although it is applicable to whisker growth under the CO-rich atmosphere (Gong et al., 2014; Degel, 1996), it is seldom applied to analyze the morphology in the H2. One possible reason is that the reaction rate constant in the H2 is 40 times than that in the CO at 800-1000 °C (Nicolle and Rist, 1979). Thus, the reaction in the H2 tends to be controlled by the diffusion rate of metallic iron, which significantly impedes the whisker growth. Herein, the morphology evolution of iron particles was supposed to depend on the relative rate between diffusion and nucleation of metallic iron crystals during the reduction process. It is worth distinguishing the gas diffusion and iron diffusion

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during the reduction as they have opposite effects on whiskers growth. Under the iron diffusion domination, whiskers are likely to form (Yamashita et al., 2007; Matthew et al., 1990; Nicolle and Rist, 1979; Elmoujahid and Rist, 1988; John and Hayes, 1982), while whiskers can be inhibited under the domination of nucleation of metallic iron condition, which is enhanced by the gas diffusion. Reduction of wüstite in H2 is widely generalized as a one order reaction (Liu et al., 2014; Hou et al., 2012), and the reaction rate reported by Liu in the fluidized reduction is expressed as Eq. (11):

r = k0 (CH2 -

CH2O k

) exp(-

E0 ) RT

(11)

where r is the reaction rate, mol/(m3·s); CH is the H2 concentration, mol/m3; CH O is 2 2 the H2O concentration, mol/m3; k is the equilibrium constant of reduction; E0 is the activation energy of reaction at 800-950 °C, 96 ± 7kJ/mol; k0 is the rate constant of reaction, s-1. The H2 conversion X can be expressed as Eq. (12): X=

CH 2O

(12)

CH 2O + CH 2

The Eq. (10) can be derived from the simultaneous equations of Eq. (11) and Eq. (12):

 E0 X  r = k0CH2 1)  exp(RT  k 1-X  

(13)

From Eq. (13), it can be inferred that at the specific temperature, the reduction rate can be accelerated by both the high H2 concentration CH and low H2 conversion 2

X, and a high CH can bring about low X because of the corresponding excess H2 flow 2

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within the gas residence time. On one hand, the reaction rate is in proportion to H2 concentration. On the other hand, the factor 1-X /[k 1-X ] in the Eq. (13) of reduction rate increases with the decrease of X value within the domain of X  (0, k/(1+k)). For instance, the reduction rate in pure H2 is even greater than 10 times that in the H2 content of 10 vol.%, thus inhibiting whiskers in pure H2 reduction. Besides, the Ug has an enormous influence on the reaction rate. Mathematically, Eq. (13) indicates that the dominant reduction rate at the elevated Ug brings about a short residence time of H2 and a corresponding minor value of H2 conversion X with other condition equaling. Similarly to the H2 content, the factor 1-X /[k 1-X ] in the Eq. (13) of reduction rate increases with the decrease of X value under the domain of X  (0, k/(1+k)), thus inhibiting whiskers. According to the previous literatures (Kawasaki et al., 1962; Parisi and Labored, 2004), the reduction process of iron ore particle is a typical gas solid reaction that can be described using an unreacted core model and expressed as Eq. (14) (Li et al., 2017): t

r0 0 r0 0 r02 0  RD   [1  (1  RD)(1/3) ]   [1/ 2  RD / 3  (1  RD)(2/3) / 2] kg (C0  Cq ) k0 (C0  Cq ) (C0  Cq ) De

(14)

where t is the reaction time, s; RD is the reduction degree, %; r0 is the characteristic initial radius of ore particle, m; 0 is the density of oxygen of solid phase, mol/m3;C0 is the initial volume concentration of reducing gas, mol/m3;Cq is the balance volume concentration of reacting gas, mol/m3; kg is the mass transfer coefficient of gas phase in the phase boundary layer, m/s; k0 is the reaction rate constant, m/s; and De is effective diffusion coefficient, m2/s;

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The first term on the right hand of Eq. (14) refers to the contribution of external diffusion resistance. The second term refers to the contribution of internal diffusion resistance, and the last term shows the interfacial chemical reaction resistance. For the relationship between kg and Ug of the first term, Foka provided the calculation of mass transfer diffusion coefficient kg, which applies to Geldart A and Geldart B particles in a turbulent fluidized bed as Eq. (15) (Foka et al., 1996): (15)

k g a1 =1.631Sc 0.37U g

The Sc stands for Schmidt number and is expressed as below:

Sc 

  De

(16)

where a1 is the phase boundary area per volume, 1/m. ρ is fluid density, kg/m3, De is the effective diffusion coefficient, m2 /s; μ is the kinetic viscosity of the fluid, N·s/m2. In Eq. (16), Schmidt number (Sc) is a dimensionless scalar, defined as the ratio of kinetic viscosity coefficient to the product of diffusion coefficient times fluid density, and it can be regarded as a constant under a specific temperature. From Eq. (14) and Eq. (15), it can be concluded the kg is in direct proportion to the Ug, while it is inversely proportional to the contribution of external diffusion resistance. With other conditions fixed, when the Ug is doubled, the contribution of external diffusion resistance is halved, thus accelerating the reduction and inhibiting the whiskers. Although it is widely acknowledged that the external diffusion resistance is negligible in the fluidized reaction for the beneficial mass transfer between the gas and solid phase, the slow Ug of the upper conical section contributes to the external diffusion resistance and it is rational to take the external diffusion resistance into consideration. 18

Additionally, the high Ug can improve the H2 concentration gradient (C0 - Cq ) and decrease the external diffusion resistance, the internal diffusion resistance and the interfacial chemical reaction resistance simultaneously on the right hand of Eq. (14) because the C0 is improved during the fluidized reduction. From a macro point of view, at the same reduction temperature and bed height, the higher Ug brings about the shorter residence time of the gas and the less formed H2O with the same flow of H2. Thus, it improves the reaction average driving force and the reduction rate. On the other hand, the high Ug can effectively purge the produced H2O out of the reaction interface. Then the concentration gradient is improved, which boosts the reduction. Therefore, at the low H2 concentration and low Ug, the reaction rate slows down, thus leading to the fibrous iron. The analytical result accords with the experimental fact. The effect of temperature on the morphology is more complicated than the reducing gas concentration and Ug. Because the increase in temperature can elevate both the iron diffusion rate and the reaction rate. Simultaneously, it promotes the reaction driving force by improving the equilibrium constant of the reaction in H2. According to the research, in the reduction process under H2, the temperature can be considered as the only influencing factor on the surface diffusion rate of iron atoms. Matsumura (Matsumura, 1971) concluded the diffusion rate in Eq. (14) below 912 oC. Ds = Di exp(-

Ea ) RT

(17)

where D i is the diffusion constant, 2.4m2 /s; Ea is the diffusion activation energy, approximately 243 kJ/mol; R is the gas constant, 8.314 J/(mol·K); T is the reaction temperature, K.

19

Combine Eq. (10), Eq. (13) and Eq. (17), the tendency of whisker formation can be obtained as Eq. (18): Di

w

X p r0  k0  CH2 [1 ]( -1) k (1-X ) p0

exp(

E0 - Ea ) RT

(18)

where w is the tendency of whisker formation; Di is the diffusion constant, 2.4m2 /s; E0 is the activation energy of reaction at 800-950 °C , 96 ± 7kJ/mol; Ea is the diffusion activation energy at 700-1050 °C, approximately 243 kJ/mol; R is the gas constant, 8.314 J/(mol·K); T is the reaction temperature, K; CH is the H2 2 concentration, mol/m3; X is the H2 conversion; k is the equilibrium constant of reduction; k0 is the reaction rate constant; r0 is the particle radius, m; p is the metallic iron content in the wüstite; p0 is the saturated content of iron in the wüstite. Eq. (18) associates the w with equilibrium constant of reaction, reduction rate and iron diffusion rate. For the contribution factor of the equilibrium constant of reduction expressed as 1/ [1 -

X )] , supposing the H2 conversion to be 0.2, the value k (1-X )

drops from 9.3 (600 ℃ and k equaling 0.28) to 2.0(800 ℃ and k equaling 0.51) with the increasing of temperature. Actually, improving the temperature can significantly enhance reduction rate and the H2 conversion in the industry, which decreases the 1/[1-

exp(

X )] k (1-X )

decrement value to less than 3.65 times. However, the factor

E0 - Ea ) increases to 43.5 times greater at the same condition. Therefore, w value RT

increases to even greater than 9.4 times with the equaling values of the k0, the p/p0 and the CH2, accelerating the whisker growth.

20

3.4. Calculation of whisker formation tendency Eq. (18) presents the relative rate between the iron diffusion and reduction. For a specific iron ore, with the domination of the former, whiskers grow on the surface. Unfortunately, it is short of the detail report about this value. To further investigate the rates of reduction and iron diffusion, isothermal reduction at 775 °C was carried out by varying Ug and H2 content based on the operation line of whisker formation in Fig. 7. The reduction conditions in whisker formation region are Ug values of 0.84 m/s, 1.05 m/s, 1.26 m/s, 1.47 m/s with the corresponding H2 contents of 60 vol.%, 50 vol.%, 40 vol.%, 30 vol.%, respectively. The reduction conditions in porous formation region are Ug values of 0.84 m/s, 1.05 m/s, 1.26 m/s, 1.47 m/s with the corresponding H2 contents of 70 vol.%, 60 vol.%, 50 vol.%, 45 vol.%, respectively. Fig. 9 presents the phase transformation within the reduction time of 0-240 s at the Ug 1.47 m/s. At the reduction time of 60 s, the XRD peaks assigned to the hematite in Brazil iron ore were mainly transformed into wüstite with extremely weak XRD peaks assigned to magnetite as the intermediate product. It can be concluded that the reduction of Brazil iron ore experiences the phase transformation from hematite to magnetite and wüstite in the first 60 s before the precipitation of iron. Subsequently, the wüstite is reduced to iron dominantly. Schematic illustration of the phase transformation processes and morphology evolution mechanism is shown in Fig. 10. At the first stage, hematite was reduced to magnetite and wüstite in sequence, during which the unreacted core was formed, resulting a resistance of inner diffusion. At the second stage, the formed hematite and

21

magnetite were further reduced to wüstite. It should be noted that metallic iron is rarely formed due to the dominantly lower equilibrium constant of reducing wüstite. Finally, the iron phase was precipitated on the particle surface, where the morphology of iron phase depends on the reaction and diffusion limitations. With the domination of iron diffusion (reaction-limited), a lack of supersaturation creates the beneficial condition for the whisker growth due to the screw dislocation effect. When the reduction takes the dominant role (diffusion-limited), a high level of supersaturation is provided that favors for the iron nucleation and inhibits whiskers to contribute sponge iron. With the two factors equaling, the compromise of the two factors takes place and the sponge iron with bumps on particle surface forms. This can be classified into the porous morphology when excessive agglomerations are inhibited. Fig. 11 and Fig. 12 show the reduction degree in the formation region of whisker and porous, respectively. There was a rapid increase in RD value under various Ug in both whisker region and porous region at the first 60 s, after which the increment slowed down. All of these RD values at the reduction time of 60 s were closed to 33.15%, which is corresponding to the theoretical maximum weight loss of wüstite (RDwt). Therefore, it is reasonable to take the 60 s as the initial point of whisker formation. To investigate the reduction of wüstite to metallic iron, a defined RDw is proposed based on RD and the theoretical maximum weight loss of wüstite (RDwt) as is presented in Eq. (6). Fig. 13 and Fig. 14 plot the RDw in the formation region of whisker and porous, respectively. It should be noted that the reduction time in Fig. 13 and Fig. 14 lagged behind that in Fig. 11 and Fig. 12 for 60 s, respectively.

22

The first order reaction model was adopted to fit the reduction rate in both whisker region and porous region plotted in Fig. 15 and Fig. 16, respectively. The corresponding fitting results are shown in Table 3 and Table 4, respectively. The linearity of all the curves suggests that the reduction in both whisker region and porous region is in good accordance with the first order reaction model. The slope of the linear fitting represents the value of the corresponding reaction rate constant of first order kinetics model k0, which was listed in Table 5. In Eq. (18), the H2 conversion (X) can be obtained by the calculation of RD value, the sample weight and the flow rate of H2. Noted that the H2 conversion (X) was calculated with the reduction time of 60-300 seconds under the domination of wüstite transforming to metallic iron. The particle size (r0) in this work is 86.9 μm. The supersaturation (p/p0) adopted is 1.04, which was reported at the close temperature of 800 °C (Iguchi et al., 1994). At the same temperature of 775 °C, both the iron diffusion rate and the equilibrium constant (0.475) can be regarded as invariant constants. Therefore, the whisker formation tendency can be calculated by the isothermal reduction and the result is shown in Table 5. It can be concluded the w in whiskers region was higher than that in porous region. Whiskers occurred with the w value higher than 3.06 m4·mol-1, while it was inhibited when w value was lower than 2.51 m4·mol-1. The unit of whisker formation tendency can also be expressed as (m2·s-1)/[mol·(m2·s) -1], which is directly proportional to the ratio of metallic iron diffusion rate (m2·s-1) to the metallic iron formation rate on the unite surface [mol·(m2·s) -1], thus determining whisker growth.

23

Previous researchers attributed the morphology of reduced iron ore to the influence of reduction conditions such as the type of reducing gas (Nicolle and Rist, 1979, Gong et al., 2014; Degel, 1996), the gas content (Shao, 2012) and the temperature (Gransden, J. F., Sheasby, J. S., 1974). In this work, it was found that the morphology was also dependent on the relative rate between the diffusion and reduction. From Eq. 18, when the relative rate was lower than 2.51 mol/m4, the whiskers growth can be inhibited. When the relative rate was higher than 3.06 mol/m4, the whiskers appeared. Aimed at revealing the morphology evolution and the mechanism for promoting the fluidization quality, three DRI morphologies were achieved in the conical fluidized bed by controlling the Ug, temperature and H2 content, indicative of the possibility to modify DRI morphology. Effect of the H2 content, temperature and Ug on the morphology can be explained by the analysis of relative rate of metallic iron diffusion and reduction. Further, the whisker formation tendency was calculated for its prediction. 4. Conclusion The influences of Ug, H2 content, and reduction temperature on the DRI morphology were investigated. The regime diagrams of morphology on iron ore reduction under different operating conditions were obtained. The formation mechanism of the whisker was analyzed. The following conclusions can be drawn from this work: 1. In the mixture of H2-N2, whiskers can develop on the surface of DRI particles. The 24

Ug, H2 content and temperature have substantially influence on the surface morphology. With low Ug, low H2 content and high temperature, the whiskers are likely to be formed. 2. The formation tendency of DRI whiskers is determined by the relative rate between the iron diffusion and reduction of iron ore. At the specific temperature, increasing the Ug and the H2 content accelerates the reduction rate, thus inhibiting the whiskers. Although the increase of temperature promotes both the reaction rate and the iron diffusion rate, the iron diffusion rate is predominant factor, thus leading to whiskers. 3. When the whisker formation tendency value is above 3.06 m4·mol-1, whiskers will form, while it was prevented when the whisker formation tendency value is below 2.51 m4·mol-1. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant No. 21736010, 21978300 and 51974287.

25

Nomenclature and abbreviations a1 CH2O CH2 C0 Cq CV H2 D0 D1 Dt Di Ds De Ea E0 WFe2+ W0,Fe2+ W0,Fe3+ WFe0 k kg k0 MFe MO p0 p R RD RDwt RDw

Phase boundary area per volume, 1/m Concentration of produced H2O in fluidized gas, mol/m3 Concentration of H2 in fluidized gas, mol/m3 Initial volume concentration of reducing gas, mol/m3 Balance volume concentration of reacting gas, mol/m3 Content of H2 in fluidized gas Initial inner diameter of the conical section, mm Distance of closed pores in the distributor, 1.8 mm Top inner diameter of the conical section, mm Diffusion constant, 2.4m2 /s Iron diffusion rate, m2 /s Effective diffusion coefficient, m2/s Diffusion activation energy at 700-1050 °C, 243 kJ/mol Activation energy of reaction at 800-950 °C, 96 ± 7kJ /mol ferrous weight contents of ferrous ion in the reduced sample ferrous weight contents of raw material ferric weight contents of raw material Metallic iron weight contents of ferrous ion in the reduced sample Equilibrium constant of reduction Mass transfer coefficient of gas phase in the phase Boundary layer, m/s Reaction rate constant, s-1 Molar mass of iron, 55.85 g/mol Molar mass of oxygen, 16.00 g/mol Saturated content of iron in the wüstite Metallic iron content in the wüstite the gas constant, 8.314 J/(mol·K) Reduction degree from ferric oxide to metallic iron Theoretical reduction degree value of ferric oxide to wüstite based on RD Reduction degree from wüstite to metallic iron 26

r0 Sc t T Ug Umf Ut Vg VH2 VN2 w X μ ρ ρ0 Φ φs ε0 ρg ρs θ

Characteristic initial radius of ore particle, m Schmidt number Reaction time, s Temperature, K Gas velocity on the bottom of conical section, m/s Minimum fluidized gas velocity, m/s Gas velocity on the top of conical section, m/s Volume flow rate of the gas mixture, L/min H2 volume, L/min N2 volume, L/min Whisker formation tendency, m4·mol-1 H2 conversion in the fluidized gas Dynamic viscosity of the fluid, N·s/m2 Fluid density, kg/m3 Density of oxygen of solid phase, mol/m3 Pore diameter of the distributor, 0.4 mm Sphericity of the solid particle Voidage of the fluidized bed Gas density , kg/m3 Particle density, kg/m3 Cone angle of the conical fluidized bed, °

27

REFERENCES Brenner, S.S., Sears, G.W., 1956. Mechanism of whisker growth-III nature of growth sites. Acta Metall. Sin. 4, 268-270. Degel, R., 1996. Eisenerzreduction in Der Wirbelschiht Mit Wasserstoffreichem Gas: Sticking Und Ansatze (Dissertation) Aachen: RWTH Aachen. Du, Z., Zhu, Q.S., Fan, C.L., Pan, F., Li, H.Z., Xie, Z.H., 2016. Influence of Reduction Condition on the Morphology of Newly Formed Metallic Iron During the Fluidized Bed Reduction of Fine Iron Ores and its Corresponding Agglomeration Behavior. Steel Res. Int. 87, 789-797. Elmoujahid, S., Rist, A., 1988. The Nucleation of Iron on Dense Wustite-A Morphological Study. Metall. Mater. Trans. B 19, 787-802. Foka, M., Guy, C., Klvana, D., Chaouki, J., 1996. Gas phase hydrodynamics of a gas-solid turbulent fluidized bed reactor. Chem. Eng. Sci. 51, 713-723. Gaines, H.P., Kopfle, J.T., Ravenscroft, C.M, 2007. Overview of MIDREX Direct Reduction technology 2006 project update. Iron and Steel Rev. 51, 176-184. Gong, X.Z., Zhang, B., Wang, Z., Guo, Z.C., 2014. Insight of iron whisker sticking mechanism from iron atom diffusion and calculation of solid bridge radius. Metall. Mater. Trans. B 45, 2050-2056. Gransden, J. F., Sheasby, J. S., 1974. The sticking of iron ore during reduction by hydrogen in a fluidized bed. Can. Metall. Q. 4, 649-657. Guo, L., Gao, H., Yu, J., Zhang, Z., Guo, Z., 2015. Influence of hydrogen concentration on Fe2O3 particle reduction in fluidized beds under constant drag

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type reduction apparatus for iron ore particles and method for reducing iron ore particles using the apparatus, U.S. Patent, 5785733. Li, J., Kong, J., He, S.Y., Zhu, Q.S., Li, H.Z., 2018. Self-agglomeration mechanism of iron nanoparticles in a fluidized bed. Chem. Eng. Sci. 177, 455-463. Li, J., Kong, J., Zhu, Q.S., Li, H.Z., 2017. Efficient synthesis of iron nanoparticles by self-agglomeration in a fluidized bed. AIChE J. 63, 459-468. Li, W., Fu, G.Q., Chu, M.S., 2017. Reduction behavior and mechanism of Hongge vanadium titanomagnetite pellets by gas mixture of H2 and CO. J. Iron Steel Res. Int. 26, 34-42. Liu, W., Lim, J.Y., Marco A.S., Allan N.H., Stuart A.S., Dennis, J.S., 2014. Kinetics of the reduction of wustite by hydrogen and carbon monoxide for the chemical looping production of hydrogen. Chem. Eng. Sci. 120, 149-166. Lu, F., Wen, L.Y., Zhong, H., Xu, J., Zhang, S.F., Duan, H.M., Yang, Z.Q., 2018. Microscopic behavior and metallic iron morphology from reduction of iron oxide by CO/H2 in a fluidized bed. J. Appl. Crystallogr. 51, 1641-1651. Matsumura, G., 1971. Sintering of iron wires. Acta Metall. Sin. 19, 851-855. Matthew, S.P., Cho, T.R., Hayes, P.C., 1990. Mechanisms of porous iron growth on wustite and magnetite during gaseous reduction. Metall. Mater. Trans. B 21, 733-741. Mikami, T., Kamiya, H., Horio, M., 1996. The mechanism of defluidization of iron particles in a fluidized bed. Powder Technol. 89, 231-238. Nesibe, D., Sedat, Y., Sahin, M.G., 2015. Investigation of Direct Reduction

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Mechanism of Attepe Iron Ore by Hydrogen in a Fluidized Bed, Metall. Mater. Trans. B 46:2278-2287 Nicolle, R., Rist, A., 1979. The mechanism of whisker growth in the reduction of wustite. Metall. Mater. Trans. B 10, 429-438. Nuber, D., Eichberger, H., Rollinger, B., 2006. Circored fine ore direct reduction. Millen Steel, 37-40. Ozawa M., 1973. Spouted bed reduction of iron ore, ISIJ Int. 59, 361-371. Pang, J.M., Guo, P.M., Zhao, P., 2015. Reduction Kinetics of Fine Iron Ore Powder in Mixtures of H2-N2 and H2-H2O-N2 of Fluidized Bed. J. Iron Steel Res. Int. 22, 391-395. Parisi, D.R., Laborde, M.A., 2004. Modeling of counter current moving bed gas-solid reactor used in direct reduction of iron ore. Chem. Eng. J. 104, 35-43. Schenk, J.L., 2011. Recent status of fluidized bed technologies for producing iron input materials for steelmaking. Particuology 9, 14-23. Schiller, M., 1987. Mikromorphologie der Eisenphase als Fogle der Reduktion von Eisenoxiden (Dissertation). Aachen: RWTH Aachen. Shao, J.H., 2012. Basic research on the bonding mechanism of fluidized iron particles and the key technology of inhibiting bonding (Dissertation in Chinese). Beijing: Beijing University of Technology. Sibakin, J., 1962. Development of the SL direct reduction process. Blast Furnace and Steel Plant 50, 977-989. Sun, H.Y., Adetoro, A.A., Pan, F., Wang, Z., Zhu, Q.S., 2017. Effects of

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High-temperature preoxidization on the titanomagnetite ore structure and reduction behaviors in fluidized bed. Metall. Mater. Trans. B 48, 1898-1907. Sun, H.Y., Adetoro, A.A., Wang, Z., Pan, F., Li L., 2016. Direct reduction behaviors of titanomagnetite ore by carbon monoxide in fluidized bed. ISIJ Int. 56, 936-943. Wagner, C., 1952. Mechanism of the reduction of oxides and sulphides to metals. AIME TRANS. 194, 214-216. Wong, P.L., Kim, M.J., Kim, H.S., Choi, C.H., 1999. Sticking Behaviour in Direct Reduction of Iron Ore. Ironmak. Steelmak. 26, 53-57. Yamashita, T., Nakada, T., Nagata, K., 2007. In-Situ Observation of Fe0.94O Reduction at High Temperature with the Use of Optical Microscopy. Metall. Mater. Trans. B 38, 185-191. Zhang, B., Gong, X.Z., Wang, Z., Guo, Z.C., 2011. Relation between sticking and metallic iron precipitation on the surface of Fe2O3 particles reduced by CO in the fluidized bed. ISIJ Int. 51, 1403-1409. Zhong, H., Wen, L., Li, J., Xu, J., Hu, M., Yang, Z., 2016. The adsorption behaviors of CO and H2 on FeO surface: A density functional theory study. Powder Technol. 303, 100-108. Zhong, Y.W., Wang, Z., Gong, X.Z., Guo, Z.C., 2012. Sticking behavior caused by sintering in gas fluidisation reduction of hematite. Ironmak. Steelmak. 39, 38-44.

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Tables Table 1. Chemical composition of the Brazil fine iron ore Composition

Fe2O3*

FeO*

SiO2

CaO

MgO

Al2O3

[wt.%]

96.80

0.72

1.98

0.10

0.10

0.30

*The corresponding chemical components were measured by chemical titration, and others were measured by X Ray Fluorescence analysis.

33

Table 2. Physical properties of the Brazil fine iron ore Parameters r0

φs(-)

ε0(-)

(μm)

ρs

ρf

μ

Umf

(kg·m3)

(kg·m3)

(×10-6N·s/m2) (m/s)

A

86.9

0.28

0.65

4900

1.36

17.6

0.020

B

86.9

0.28

0.65

4900

0.33

40.9

0.025

A: iron ore particles at room temperature with pure N2. B: iron ore particles at 775 °C with pure N2

34

Table 3. Linear fitting of Fig. 15 Equation: y = ax + b Ug/(m·s-1)

Parameter

Value

Standard

Pearson’s r

error 0.84

1.05

1.26

1.47

Intercept

-0.102

0.0302

Slope

0.00357

2.06×10-4

Intercept

-0.0940

0.018

Slope

0.00497

1.22×10-4

Intercept

-0.137

0.0468

Slope

0.00446

3.19×10-4

Intercept

-0.164

0.0299

Slope

0.00409

2.04×10-4

35

Adj. R-Square

0.995

0.987

0.999

0.998

0.992

0.980

0.996

0.990

Table 4. Linear fitting of Fig. 16 Equation: y = ax + b Ug/(m·s-1)

Parameter

Value

Standard

Pearson’s r

error 0.84

1.05

1.26

1.47

Intercept

-0.0315

0.0635

Slope

0.00474

4.32×10-4

Intercept

-0.0217

0.0495

Slope

0.00442

3.37×10-4

Intercept

-0.066

0.0638

Slope

0.00536

4.34×10-4

Intercept

-0.0718

0.0266

Slope

0.0052

1.81×10-4

36

Adj. R-Square

0.988

0.968

0.991

0.977

0.990

0.974

0.998

0.995

Table 5. The calculated whisker formation tendency for whisker and porous morphology Morphology

Whisker

Porous

k0 (s-1)

w (m4·mol-1)

21.36

0.00357

3.06

50

25.00

0.00497

3.78

1.26

40

24.45

0.00446

4.93

1.47

30

27.48

0.00409

11.29

0.84

70

20.07

0.00474

1.79

1.05

60

18.45

0.00442

2.02

1.26

50

19.63

0.00536

2.15

1.47

45

19.91

0.00520

2.51

Ug

H2 content

H2 conversion ratio X

(m/s)

(vol.%)

(%)

0.84

60

1.05

37

Figures

Fig. 1. SEM image and laser size distribution of the fine iron ore concentrate

38

Fig. 2. Relationship between Ug and bed pressure drop

39

Fig. 3. Schematic diagram of the experimental apparatus

40

Fig. 4. Effect of Ug on the surface morphology of the DRI at 700 ºC with 50 vol.% H2 content. (a): 0.2 m/s; (b): 0.6 m/s; (c): 0.8 m/s; (d): 1.0 m/s

41

Fig. 5. Effect of H2 content on the surface morphology of the DRI at 700 ºC and 0.8 m/s. (a): 10 vol.%; (b): 30 vol.%; c: 70 vol.%; d: 90 vol.%

42

Fig. 6. Effect of temperature on the surface morphology of the DRI at Ug 0.8 m/s with 50 vol.% H2 content. (a): 600 ºC; (b): 650 ºC; (c): 750 ºC; (d): 800 ºC

43

Fig. 7. Morphology diagram at different H2 contents and Ug values (775 ºC)

44

Fig. 8. Morphology diagram at different temperatures and Ug values at H2 content of 60

vol.%

45

Fig. 9. XRD patterns of Brazil fine iron ore within the reduction time of 0-240 s

46

Fig. 10. Sketch of phase transformation and morphology evolution in reduction

47

Fig. 11. Reduction degree (RD) from ferric oxide to iron at different Ug values and H2 contents at 775 °C in whiskers area (Ug 0.84 m/s with 60 vol.% H2 content; Ug 1.05 m/s with 50 vol.% H2 content; Ug 1.26 m/s with 40 vol.% H2 content; Ug 1.47 m/s with 30 vol.% H2 content;)

48

Fig. 12. Reduction degree (RD) from ferric oxide to iron at different Ug values and H2 contents at 775 °C in porous area (Ug 0.84 m/s with 70 vol.% H2 content ; Ug 1.05 m/s with 60 vol.% H2 content; Ug 1.26 m/s with 50 vol.% H2 content; Ug 1.47 m/s with 45 vol.% H2 content;)

49

Fig. 13. Reduction degree (RDw) from wüstite to iron at different Ug values and H2 contents at 775 °C in whiskers area (Ug 0.84 m/s with 60 vol.% H2 content; Ug 1.05 m/s with 50 vol.% H2 content ; Ug 1.26 m/s with 40 vol.% H2 content; Ug 1.47 m/s with 30 vol.% H2 content;)

50

Fig. 14. Reduction degree (RDw) from wüstite to iron at different Ug values and H2 contents at 775 °C in porous area (Ug 0.84 m/s with 70 vol.% H2 content; Ug 1.05 m/s with 60 vol.% H2 content; Ug 1.26 m/s with 50 vol.% H2 content; Ug 1.47 m/s with 45 vol.% H2 content;)

51

Fig. 15. Linear fitting of first order reaction in the whisker region during the phase change from wüstite to iron under at different Ug values

52

Fig. 16. Linear fitting of first order reaction in the porous region during the phase change from wüstite to iron under different Ug values

53

Highlight:  Surface morphology of fine iron ore particles was investigated in a conical fluidized bed.  Regime diagrams of morphology were obtained to rationally choose the operating condition.  Morphology of iron ore was dependent on the compromise of reduction rate and iron diffusion.  Iron diffusion rate is the dominant factor that brings about whiskers during the reduction.  Whiskers were inhibited in the dominance of reduction rate.

Abstract figure:

54

Author contributions Xu Zhang and Shengyi He performed the experimental investigation, writing-original draft and methodology. They contribute equally to the article. Qingshan Zhu and Jun Li performed the supervision, writing-review & editing and conceptualization. Haoyan Sun performed the experiment and frame design. Hongzhong Li performed the revision and validation methodology.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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