Reduction mechanism of high-chromium vanadium ... - Springer Link

International Journal of Minerals, Metallurgy and Materials Volume 22, Number 6, June 2015, Page 562 DOI: 10.1007/s12613-015-1108-9

Reduction mechanism of high-chromium vanadium–titanium magnetite pellets by H2–CO–CO2 gas mixtures Jue Tang, Man-sheng Chu, Feng Li, Ya-ting Tang, Zheng-gen Liu, and Xiang-xin Xue School of Material and Metallurgy, Northeastern University, Shenyang 110819, China (Received: 31 October 2014; revised: 24 December 2014; accepted: 1 March 2015)

Abstract: The reduction of high-chromium vanadium–titanium magnetite as a typical titanomagnetite containing 0.95wt% V2O5 and 0.61wt% Cr2O3 by H2–CO–CO2 gas mixtures was investigated from 1223 to 1373 K. Both the reduction degree and reduction rate increase with increasing temperature and increasing hydrogen content. At a temperature of 1373 K, an H2/CO ratio of 5/2 by volume, and a reduction time of 40 min, the degree of reduction reaches 95%. The phase transformation during reduction is hypothesized to proceed as follows: Fe2O3 → Fe3O4 → FeO → Fe; Fe9TiO15 + Fe2Ti3O9 → Fe2.75Ti0.25O4 → FeTiO3 → TiO2; (Cr0.15V0.85)2O3 → Fe2VO4; and Cr1.3Fe0.7O3 → FeCr2O4. The reduction is controlled by the mixed internal diffusion and interfacial reaction at the initial stage; however, the interfacial reaction is dominant. As the reduction proceeds, the internal diffusion becomes the controlling step. Keywords: magnetite; ore reduction; phase transformation; reaction mechanism; kinetics

1. Introduction High-chromium vanadium–titanium magnetite (HCVTM) containing 0.61wt% Cr2O3 with a complex mineral composition and special material characteristics is a special vanadium–titanium magnetite (VTM). The HCVTM used in this work was produced in one district in Russia. Given the increasing supply and demand gap of domestic iron ore in China, this type of iron ore is imported in large quantities because of its low price and high comprehensive utilization value of iron, vanadium, titanium, and chromium [1–2]. At present, VTM is usually smelt in a blast furnace (BF) and most of the titanium is concentrated into the slag [3–4]. However, the utilization rate of valuable components is low and the BF process is long and difficult to control in a flexible manner. In the past few decades, the direct reduced iron (DRI) process has been adopted as a relatively practical and effective way to utilize VTM [4–6]. However, the solid-state reduction by the reaction of titaniferous ore and carbon is relatively slow, consumes large amounts of energy, and is limited to a narrow range of operating temperatures [4,7–8]. Corresponding author: Man-sheng Chu

Moreover, the titanium-bearing slag is contaminated by the solid reductant. Thus, gas-based reduction appears to be a promising method to process VTM [9–10]. Studies on the reduction reaction and mechanism of the gas-based reduction of VTM or titaniferous ores have been previously conducted using CO, H2, and gas mixtures [11–15]. The reduction of VTM is known to be slower than that of normal hematite or magnetite iron ore. Merk and Pickles [8] reduced ilmenite with CO at temperatures from 773 to 1373 K; they demonstrated that both the reaction rate and the degree of reduction depended on the formation of a metallic shell of iron and that the reduction rate accelerated with increasing temperature. Park and Ostrovski [7] reported that the titanomagnetite was a magnetite–ulvospinel solid solution ((Fe3O4)1−x(FeTi2O4)x, x = 0.27 ± 0.02) and was reduced to metallic iron and titanium sub-oxides by CO–CO2–Ar gas mixtures. Sun et al. [4] investigated the mechanism of titanomagnetite reduction by H2 in a fixed-bed reactor at temperatures from 1123 to 1323 K; their results indicated that the reduction was mainly controlled by interfacial chemical reactions during the whole reduction process. Si et al. [13] deduced the optimal process parame-

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© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2015

J. Tang et al., Reduction mechanism of high-chromium vanadium–titanium magnetite pellets by H2–CO–CO2 gas mixtures

ters for the reduction of ilmenite concentrate under H2–CO mixtures as follows: H2 content greater than 50vol%, reduction temperature of 1423 K, reduction time of 90 min. Based on the technological superiority of gas-based reduction and for the purpose of utilizing the special and valuable HCVTM efficiently, the research on the gas-based reduction of HCVTM was desired. However, the information about the reduction of HCVTM by H2–CO–CO2 gas mixtures remained unclear and insufficient. In this work, the isothermal reduction of HCVTM by H2–CO–CO2 was conducted in a laboratory gas-based shaft furnace. During the reduction process, the reaction mechanism, phase transformation, and kinetics were investigated systematically.

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((Cr0.15V0.85)2O3 and (Cr1.3Fe0.7O3)). The results of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) are presented in Fig. 2. Combining the elemental proportions of point A obtained from the EDS spectrum and the results of XRD analysis, the phase composition of point A is initially identified as a mixture of Fe9TiO15 and Fe2Ti3O9. Table 1. Chemical composition of the oxidized HCVTM pellet wt% TFe FeO Cr2O3 V2O5

TiO2

59.4 0.50 0.589 0.911 4.485

Al2O3

SiO2

MgO CaO

3.07

2.034

1.03

0.21

2. Experimental 2.1. Materials The green pellet was prepared from HCVTM and roasted for 20 min at 1573 K in an air atmosphere. The oxidized pellet was obtained after roasting and was then used in the reduction. The chemical composition of the oxidized HCVTM pellet is listed in Table 1. The key phases containing Fe, Ti, V, and Cr, as identified by X-ray diffraction analysis, are shown as Fig. 1. The titanium is observed to be present in the form of a hematite solid solution (Fe9TiO15 = 4Fe2O3·FeTiO3) and pseudorutile (Fe2Ti3O9). The vanadium and chromium generate their own solid solution

Fig. 1. X-ray diffraction patterns of the oxidized HCVTM pellet.

Fig. 2. SEM and EDS analyses of the oxidized HCVTM pellet.

2.2. Methods and procedure The reduction of oxidized HCVTM pellets by H2–CO–CO2 gas mixtures was studied in a laboratory gas-based shaft furnace, which is schematically shown in Fig. 3. The furnace was heated to each experiment temperature with N2 gas flowing through the reactor before the reduction reaction was initiated. Then, approximately 20 oxidized HCVTM pellets with a diameter of (12.5 ± 0.5) mm were placed into the crucible and positioned in the constant-temperature zone

of the furnace. When the samples and furnace reached the same temperature, the reduction gas mixtures were introduced into the reactor and the reduction started. Notably, the total gas flow rate was controlled at 4 L/min (the critical gas flow rate of the gas-based furnace used in the work) to ensure a sufficiently large ratio of gas to solid. Consequently, the influence of gas flow on the reduction process was diminished. After the reduction, the crucible was removed from the furnace and quenched quickly under the rapidly flowing nitrogen gas. The detailed experimental scheme is

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Int. J. Miner. Metall. Mater., Vol. 22, No. 6, June 2015

given in Table 2. The reduction temperature was varied from 1223, 1273, 1323, to 1373 K and the volume ratio of H2 to CO was changed from 5/2, 1/1, to 2/5. In the reduction tests, the status where the degree of reduction did not change was defined as the finish of reduction. Finally, the reduction degree was calculated as the mass fraction of oxygen in iron oxides removed during the course of reduction. The samples in the course of reduction were analyzed by XRD (SIEMENS D5000) and SEM-EDS (AMERICIAN).

Fig. 3. Experimental gas-based reduction setup.

Table 2. Reduction test scheme Parameter

Temperature / K

Experimental value 1223, 1273, 1323, 1373

Volume ratio of H2 to CO 2/5, 1/1, 5/2

Note: H2% + CO% + CO2% = 100% and CO2% = 5%.

3. Results and discussion 3.1. Effects of temperature and atmosphere on the reduction of HCVTM pellets (1) Effects of temperature and atmosphere on the degree of reduction. The effects of atmosphere on the degree of reduction at different temperatures are presented in Fig. 4. As evident in the figure, the change rules of reduction are similar. Initially, the rate of reduction is rapid. However, as the reaction proceeds, the layer of products becomes thicker, which leads to the decreased gas diffusion; the rate of reaction consequently decreases. At the same temperature, the reduction is accelerated and the degree of reduction increases when the H2 concentration in the gas mixture is increased. However, the reduction trends at different temperatures differ. The acceleration of reduction reaction with increasing H2 content is more obvious at higher temperatures (Figs. 4(a)–(d)). Under

Fig. 4. Effects of atmosphere on the degree of reduction: (a) 1223 K; (b) 1273 K; (c) 1323 K; (d) 1373 K.


the conditions of 1273 K and H2/CO = 5/2, 1323 K and H2/CO = 5/2, 1373 K and H2/CO = 5/2, and 1373 K and H2/CO = 1/1, the degree of reduction reaches or exceeds 90% at reduction times of 45, 32, 30, and 40 min, respectively. In particular, when H2/CO = 5/2, the degree of reduction reaches 93% and 95% after 40 min of reduction at 1323 K and 1373 K, respectively. The reasons why H2 content accelerates the reduction of HCVTM pellets are summarized as follows: H2, whose molecules are much smaller than those of the other mixture gases, exhibits more efficient diffusion to the inner part of the HCVTM pellet. In addition, some of the H2 molecules, instead of taking part in the reduction, may have served as catalysts that greatly accelerate the reduction [16]. In general, a high H2 content not only accelerates the reduction reaction but also improves the dynamic conditions, which both contribute to the advanced degree of reduction of the HCVTM pellets. The effects of temperature on the degree of reduction are given as Fig. 5, where the degree of reduction is observed to

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be obviously improved with increasing temperature. The reduction degree of the HCVTM pellets after reduction 40 min at 1373 K is greater than 95%. The reduction degree of the pellets increases under the conditions of a higher reduction temperature and higher H2 content, because the reducing capacity of H2 is higher than that of CO under high temperatures. In addition, the reduction reaction involving H2 is endothermic, whereas the reaction of CO is exothermic. Thus, the thermodynamics and kinetics of H2 reduction are simultaneously improved when the reduction temperature is increased. However, in the case of CO reduction, the kinetics is enhanced but the thermodynamics is worsened with increasing temperature [17]. Compared with ordinary iron ore [18], the HCVTM pellet was more difficult to be reduced and the final degree of reduction could not reach 100%. These results were perhaps due to the special material characteristics and more complex mineral composition of HCVTM, compared to those of iron ore.

Fig. 5. Effects of temperature on the degree of reduction: (a) H2/CO = 5/2; (b) H2/CO = 1/1; (c) H2/CO = 2/5.

(2) Effects of temperature and atmosphere on the reduction swelling index (RSI). The reduction swelling index (RSI) of the HCVTM pellets was measured, as shown in Fig. 6. As evident in the figure, the RSI is less than 20% and no baleful swelling is observed under the experimental conditions. However, the RSI is worsened with increasing reduction temperature and in-

creasing CO content. In previous studies [19–20], when the pellets were reduced in an atmosphere with high CO content, metallic iron was reported to be flocculent or fibrous and increase in directivity. Then, when it came across the hindering from adjacent particles, high inner stress was generated and many irregular lacunas emerged, which led to both poor strength and low RSI.

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of isothermal reduction (1373 K, H2/CO = 5/2); the results are shown in Fig. 8 and Table 3. The main phases in the initial oxidized HCVTM pellet were hematite (Fe2O3), titanohematite solid solution (TTH, Fe9TiO15 = 4Fe2O3·FeTiO3), pseudorutile (Fe2Ti3O9), and solid solutions ((Cr0.5V0.85)2O3 and (Cr1.3Fe0.7O3)). After 5 min of reduction, metallic iron (Fe), magnetite (Fe3O 4), and titanomagnetite (TTM, Fe0.25Ti0.25O4 = 1/4 3Fe3O4·Fe2TiO4) were detected; in addition, the solid solution of vanadium and chromium was transformed into coulsonite (Fe 2 VO 4 ) and chromite (FeCr2O4) when the degree of reduction was 29.49%.

Fig. 6. Effects of temperature and atmosphere on the reduction swelling index (RSI).

(3) Effects of temperature and atmosphere on the compressive strength after reduction. The compressive strength (CS) of HCVTM pellets after reduction under different conditions is described in Fig. 7. With increasing temperature and CO content, the CS after reduction decreases; however, the influence of temperature is more obvious. In the experimental ranges, HCVTM pellets after reduction exhibited good strength, exceeding the requirement for BF production in Japan (141 N) [21]. Compared with the BF, the gas-based shaft furnace was shorter and smaller, and the standing time of the pellets in the furnace was shorter. Thus, the strength after reduction obtained in this work was sufficient to satisfy the requirement for gas-based BF production. Fig. 8. XRD patterns of the pellets reduced for different times (1373 K). Table 3. Phase composition of the reduction products reduced for different times Reduction time / min

Phase identified by XRD

Degree of reduction / %

30

Fe2O3, TTH, Fe2Ti3O9, (Cr0.15V0.85)2O3, Cr1.3Fe0.7O3 Fe, Fe3O4, FeO, TTM, Fe2VO4, FeCr2O4 Fe, Fe3O4, FeO, TTM, Fe2VO4, FeCr2O4 Fe, Fe3O4, FeO, TTM, Fe2VO4, FeCr2O4 Fe, FeO, FeTiO3, Fe2VO4, FeCr2O4

3.2. Phase transformation in the HCVTM pellets during reduction

40

Fe, FeTiO3, Fe2VO4, FeCr2O4

95.38

50

Fe, TiO2

95.38

Pellet samples were analyzed by XRD during the process

60

Fe, TiO2

95.38

0 5 10 Fig. 7. Compressive strength of the pellets after reduction under different conditions.

20

0 29.49 45.99 72.71 90.35


During the reduction from 5 min to 30 min, the peak of metallic iron became dominant in the XRD patterns but the peaks for wustite (FeO) gradually decreased in intensity. This result was attributed to the reduction of TTM to wustite being presumably slower than that of wustite to metallic iron, resulting in the rapid reduction of wustite. Magnetite and titanomagnetite disappeared, and the new phase of ilmenite (FeTiO3) was observed after 30 min of reduction, when the degree of reduction was 90.35%. Wustite peaks were not detected after 40 min of reduction; however, the unreduced ilmenite still remained. In contrast, the peaks of both coulsonite and chromite were less intense than before. When the reduction proceeded for 50 min, the degree of reduction was greater than 95% and the reduction from ilmenite to rutile (TiO2) was nearly completely achieved. When the reduction time was extended further, although the intensity of diffraction peaks for each phase increased, the species composition remained the same. The final reduced HCTVM pellet contained iron and rutile. The V-bearing and Cr-bearing phases were not detected after 40 min of reduction, which might be attributable to their lower content in the raw material and the limitations of the analysis method. The peaks of V-bearing and Cr-bearing phases became less intense as the reduction proceeded. In this work, the transformation rules for V and Cr were analyzed before 40 min of reduction. V and Cr remained in the forms of coulsonite (Fe2VO4) and chromite (FeCr2O4), respectively. According to the previous literature about phase relations in the Fe–Ti–O ternary system, the CO–CO2 gas mixtures were used in the experiment. The reduction of TTM in the Fe–Ti–O system would proceed as [22–23] Fe3−xTixO4→FeO+Fe2TiO4→Fe+Fe2TiO4→ Fe+FeTiO3→Fe+FeTi2O5→Fe+TiO2. (1) In this study, the reductions of Fe-bearing and Ti-bearing phases in oxidized HCVTM pellets were also illustrated by the Fe–Ti–O ternary diagram shown in Fig. 9. The reduction path was described as follows: Fe2O3→Fe3O4→FeO→Fe (2) Fe9TiO15+Fe2Ti3O9→Fe2.75Ti0.25O4→FeTiO3→TiO2 (3) The phases not detected in the present reduction experiments were the intermediate products of ulvospinel (Fe2TiO4) and pseudobrookite (FeTi2O5), as shown in dashed lines  and  in Fig. 9, respectively. Because of the high content of H2 in the reduction atmosphere, the reactions were rapid, which resulted in the rapid transformation of the intermediate products, making them difficult to detect. On the basis of XRD analysis and phase diagram description, phase transformations in the HCVTM pellets during the reduction by H2–CO–CO2 gas mixtures were sum-

567

marized as the following equations. Here, the reduction by CO was given as an example. In fact, the reduction by H2 was similar to the following reactions and analyses. The reduction of titanohematite solid solution (TTH) and pseudorutile (Fe2Ti3O9) with the formation of titanomagnetite (TTM) and wustite in the first 5 min of reduction proceeds as 34Fe2O3 + 2Fe2Ti3O9 + 2Fe9TiO15 + 20CO = 8(3Fe3O4·Fe2TiO4) + 20CO2 + 2FeO. (4) Eq. (4) can occur through the following equations. 31Fe2O3 + 2Fe2Ti3O9 + 15CO = 6(3Fe3O4·Fe2TiO4) + 15CO2 (4a) 3Fe2O3 + 2Fe9TiO15 + 5CO = 2(3Fe3O4·Fe2TiO4) + 2FeO + 5CO2 (4b) The reduction of titanomagnetite (TTM) with the formation of ilmenite (FeTiO3) after 30 min of reduction proceeds as 3Fe3O4·Fe2TiO4 + 13CO = 10Fe + FeTiO3 + 13CO2 (5) and Eq. (5) can occur through the following equations. Fe2TiO4·3Fe3O4 + 3CO = 9FeO + Fe2TiO4 + 3CO2 (5a) 9FeO + 9CO = 9Fe + 9CO2 (5b) Fe2TiO4 + CO = Fe +FeTiO3 + CO2 (5c) In the end, the final reduction from ilmenite to rutile proceeds as FeTiO3 + CO = Fe + TiO2 + CO2. (6) Thus, the overall reduction reaction of the HCVTM pellets is represented by Eq. (7). 17Fe2O3 + Fe2Ti3O9 + Fe9TiO15 + 67CO = 45Fe + 4TiO2 + 67CO2 (7) Eqs. (4), (5), and (6) correspond to the solid lines [1], [2], and [3] in Fig. 9, respectively. In the equilibrium state, the structure transformation of Ti-bearing phase in HCVTM pellets during the reduction is represented by the given sequence.

Fig. 9. Fe–Ti–O ternary diagram system.

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TTH (rhombohedral) + pseudorutile (hexagonal) → TTM (spinel cubic) → ilmenite (rhombohedral) → rutile


(8)

3.3. Morphology changes in the HCVTM pellet during reduction Morphology changes of HCVTM pellets during the re-

duction at 1373 K and H2/CO = 5/2 are presented in Fig. 10. In the initial stage, particles in the pellets were loosened, and less metallic iron was generated as a result of insufficient reduction. As the reduction time increased, the hematite was reduced to metallic iron, and the titanomagnetite was reduced by the removal of oxygen. The bright-white grains representing metallic iron were refined and increased in

Fig. 10. Morphology changes of HCVTM pellets during the gas-based reduction (H2/CO = 5/2) at 1373 K: (a) 0 min; (b) 5 min; (c) 10 min; (d) 20 min; (e) 30 min; (f) 40 min; (g) 50 min; (h) 60 min.


number. Large pores in the pellets were dispersed into numerous small pores. After 30 min of reduction, XRD patterns in Fig. 8 indicated that the hematite was reduced completely and the ilmenite was obtained from the gradual reduction of titanomagnetite; in addition, metallic iron in the pellets was relatively pure, which facilitated the nucleation and growth. When the reduction time was prolonged further, the iron grains grew and agglomerated; meanwhile, the number of pores decreased and the metallic iron was contiguous, which provided the compelling evidence that the pellets were sufficiently reduced. 3.4. Kinetic analysis during reduction The reduction of pellets by reducing gas proceeds via a typical gas-solid reaction interface. The gas reduction in a pellet with lower porosity proceeds via a topochemical reaction path and is described by the unreacted core model [23]. Therefore, the unreacted core model was applied to study the reduction kinetics of HCVTM pellets in this work. To simplify the problem, the following assumptions were made as: (1) the volume of a pellet did not change during the reduction process; (2) the reduction was a reversible first-order process and the layer of intermediate product was thin. Because of the critical value of gas flow rate used in the experiments, the restriction of external diffusion on the reduction reaction was avoided. The results indicated that no linear relationship existed between the degree of reduction and the reaction time, which further indicated that the external diffusion could be neglected. Therefore, the unreacted core model applied in the work is presented as Eq. (9). The relationship between the reduction time and the restriction step is listed in Table 4. Table 4. Relationship between the reduction time and the restriction step Reduc- t = a[1 − t = b[1 − 3(1 − t = a[1 − (1 − f)1/3] + b[1 tion time (1 − f)1/3] f)2/3 + 2(1 − f)1/3] − 3(1 − f)2/3 + 2(1 − f)] Restric- Chemical Internal diffuChemical reaction and tion step reaction sion internal diffusion Note: a and b is a constant.

r0 1 1 − 3(1 − f ) 2 / 3 + 2(1 − f )  + ⋅ 6 De  c −c* K 1 ⋅ 1 − (1 − f )1/ 3  = 0 ⋅t 1+ K k  r0 ⋅ ρ

(9)

where r0 is the characteristic initial pellet radius, m; De the effective diffusion coefficient, m2/s; f the degree of reduc-

569

tion, %; K the equilibrium constant of reaction; k the reaction rate constant, m/s; c0 and c* the reduction gas concentration at the granule surface and in equilibrium, respectively, mol/m3; ρ the initial oxygen concentration in pellet, mol/m3; and t the reduction time, s. Fig. 11 shows the comparisons between the chemical reaction-controlled model and the internal diffusion-controlled model in different reducing atmospheres at 1323 K. No linear relationships existed between the reduction time and the chemical reaction or internal diffusion. Thus, the reduction of HCVTM pellets was a multi-step and complex process throughout the whole reduction process and was controlled neither by a single chemical reaction nor by a single internal diffusion process. Simplification of Eq. (9) gives Eq. (10) as the following equation. t  1/ 3 2/3 1 − (1 − f )1/ 3 = tD 1 + (1 − f ) − 2(1 − f )  + tC   ρ r02 tD = 6 De (c0 − c*)   ρ r0 K  tC = k (1 + K )(c0 − c*) 

(10)

where tD and tC are the complete reduction time when the reduction is controlled by internal diffusion and chemical reaction, respectively. A mixed controlled model under different conditions is given in Fig. 12. By plotting the linear regression relationship between t/[1−(1−f)1/3] and 1+(1−f)1/3−2(1−f)2/3, tD and tC were obtained from the intercept and slope, respectively. Finally, the k and De values under different conditions are listed in Table 5. As evident in the table, both k and De increased with increasing temperature and H2 content. The resistance of internal diffusion (FD) and interfacial reaction (FK) were calculated as Eq. (11) and Eq. (12), respectively. With FΣ defined as FΣ= FD + FK, the relative resistance of internal diffusion (FD/FΣ) and interfacial reaction (FK/FΣ) were calculated. Here, the results for two series of experimental conditions are shown in Fig. 13, which illustrates that the changes in relative resistance are approximately the same. At the initial stage of reduction, the reduction was under mixed control; however, the interfacial reaction was dominant. As the reduction proceeded, the internal diffusion was the restriction step and became dominant gradually. Both low internal diffusion resistance and superior kinetics conditions in the two cases of high reduction temperature and high H2 content were conducive to obtaining a high final degree of reduction.

570


Fig. 11. Comparisons between the chemical-reaction-controlled model and the internal-diffusion-controlled model in different reducing atmospheres at 1323 K: (a) H2/CO = 5/2; (b) H2/CO = 1/1; (c) H2/CO = 2/5.

Fig. 12. Mixed-control model under different conditions: (a) H2/CO = 5/2; (b) H2/CO = 1/1; (c) H2/CO = 2/5 (R is the linearly dependent coefficient).


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Table 5. Values of k and De under different conditions Parameter

H2/CO by volume

k / (m·s−1)

De / (m2·s−1)

Temperature / K 1223

1273

1323

1373

5/2

1.167×10−2

1.288×10−2

1.552×10−2

1.538×10−2

1/1

1.059×10−2

1.175×10−2

1.403×10−2

1.406×10−2

2/5

−3

9.384×10

−2

1.116×10

−2

1.227×10

1.469×10−2

5/2

3.929×10−5

3.882×10−5

5.625×10−5

5.898×10−5

1/1

3.094×10−5

3.738×10−5

3.955×10−5

4.719×10−5

2/5

−5

−5

−5

3.000×10−5

1.948×10

2.385×10

2.857×10

Fig. 13. Relative resistance for the reduction of HCVTM pellets: (a) 1323 K; (b) H2/CO = 5/2.

FD =

r0  (1 − f ) −1/ 3 − 1 De 

(11)

FK =

K (1 − f ) −2 / 3 k (1 + K )

(12)

temperature of 1373 K, a reduction time of 40 min, and an H2/CO volume ratio of 5/2, the degree of reduction reaches 95%.

The reaction rate constant (k) was followed the Arrhenius formula as a function of temperature.

 −ΔE  k = A exp    RT 

(13)

where A is a frequency factor, R the gas constant (J/(mol·K)), T the temperature (K), and ΔE the apparent activation energy (kJ/mol). Using the values of k obtained from the aforementioned linear regression calculation, the relationship between lnk and 1/T (1223–1373 K) is plotted as shown in Fig. 14. The calculated values of ΔE and A are obtained from slope and intercept, respectively, and the data are shown in Table 6. As shown in the table, with increasing H2 content, ΔE decreases, resulting in the acceleration of reduction reaction. The equations for k for temperatures from 1223 to 1373 K in different atmospheres are presented in Table 6.

Fig. 14. Relationship between lnk and 1/T (R is the linearly dependent coefficient). Table 6. Relationship between k and T in different reducing atmospheres (1223–1373 K) H2/CO by ΔE / Frequency volume (kJ·mol−1) factor

k

5/2

36.792

0.655

 −36.792 × 103  k = 0.655exp    8.314T 

4. Conclusions

1/1

36.868

0.569

 −36.868 × 103  k = 0.655exp    8.314T 

(1) The increases of temperature and H2 content facilitate the reduction of HCVTM pellets from 1223 to 1373 K. At a

2/5

37.280

0.504

 −37.280 × 103  k = 0.655exp    8.314T 

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(2) The phase transformations of Fe-bearing, Ti-bearing, V-bearing (40 min), and Cr-bearing (40 min) species in HCVTM pellets during the reduction by H2–CO–CO2 are summarized as follows: Fe2O3 → Fe3O4 → FeO → Fe; Fe9TiO15 + Fe2Ti3O9 → Fe2.75Ti0.25O4 → FeTiO3 → TiO2; (Cr0.15V0.85)2O3 → Fe2VO4; Cr1.3Fe0.7O3 → FeCr2O4. (3) The reduction of HCVTM pellets by H2–CO–CO2 gas mixtures proceeds topochemically, and the reduction is a multi-step and complex process. During the initial stage, the reduction is under mixed control; however, the interfacial reaction is dominant. As the reduction proceeds, internal diffusion becomes the restriction step.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51090384), the National High-Tech Research and Development Program of China (No. 2012AA062302), and the Fundamental Research Funds for the Central Universities of China (Nos. N110202001 and N130602003).

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