Effects of coal and iron ore blending on metallurgical properties of iron coke hot briquette

Powder Technology 328 (2018) 318–328

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Effects of coal and iron ore blending on metallurgical properties of iron coke hot briquette Hongtao Wang, Wei Zhao, Mansheng Chu ⁎, Zhenggen Liu, Jue Tang, Ziwei Ying School of Metallurgy, Northeastern University, Shenyang 110819, PR China

a r t i c l e

i n f o

Article history: Received 1 September 2017 Received in revised form 17 December 2017 Accepted 13 January 2018 Available online 6 February 2018 Keywords: Iron coke hot briquette Compressive strength Reactivity Post-reaction strength Low carbon emission blast furnace ironmaking

a b s t r a c t Utilization of iron coke hot briquette (ICHB), prepared by carbonizing coal-iron ore composite agglomerate, is considered as an effective countermeasure to achieve low carbon emission blast furnace (BF) ironmaking. ICHB shows high reactivity and thereby presents low post-reaction strength, which could have serious impact on the permeability of BF. In this paper, the effects of coal and iron ore blending on the metallurgical properties of ICHB were experimentally investigated and related mechanism were revealed based on Chinese raw material and fuel conditions. The results showed that with increasing coal B addition ratio, the compressive strength of ICHB is slightly improved. The gasification reaction ratio of ICHB is gradually decreased while the post-reaction strength is increased. Furthermore, with increasing coal C ratio, the compressive strength of ICHB tends to slightly increase firstly and then mildly decrease. The reaction ratio of ICHB is decreased firstly and then increased while the post-reaction strength and the crystallite size (Lc) tend to be enhanced initially and then reduced. Additionally, with increasing the addition ratio of iron ore from 0 to 20%, the compressive strength of ICHB is reduced from 5602 N to 4594 N while the porosity of ICHB is increased. The reaction ratio of ICHB is accelerated from 48.22% to 75.02% while the post-reaction strength is reduced from 74.08% to 26.84%. In terms of coal blending and iron ore blending, the reasonable preparation parameters of ICHB are conducted, namely 55% coal A, 10% coal C, 20% coal D, and 15% iron ore. Under the conditions, the compressive strength, the reaction ratio, and the post-reaction strength of ICHB are 4861 N, 63.00%, and 46.45%, respectively. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Currently, environmental problems such as global warming and air pollution have become increasingly serious and attracted more attention. Steel industry is supposed to be responsible for reducing CO2 emission since it consumes substantial fossil fuels. CO2 emission discharged from steel works accounts for 5%–7% of the global total CO2 emission [1]. Furthermore, blast furnace (BF)-converter process is dominant for steel production in the long term [2]. As a main sector in steel production, BF produces more than 94% of the total pig iron all over the world, whose energy consumption and CO2 emission account for about 80% of the whole steel works [3, 4]. Consequently, saving energy consumption and reducing CO2 emission in BF is greatly important for the sustainable development of steel industry. Coke is one of the most significant raw materials for BF ironmaking in terms of its effect on hot metal quality and BF operation [5]. Naito et al. [6] proposed that a reduction in coke consumption is an effective measure to mitigate CO2 emission and energy consumption in a BF, which could be realized by using highly reactive coke (reactivity of ⁎ Corresponding author. E-mail address: [email protected] (M. Chu).

https://doi.org/10.1016/j.powtec.2018.01.027 0032-5910/© 2018 Elsevier B.V. All rights reserved.

coke refers to the reaction ratio of coke with CO2) through decreasing the temperature of the thermal reserve zone (TRZ). Highly reactive coke can be prepared by adding minerals with catalytic effect [7]. Simultaneously, Naito et al. [8] reported that the utilization of highly reactive coke can decrease the reducing agent ratio (RAR) by approximately 25– 35 kg/ton of hot metal (kg/tHM). For C-CO2 reaction, alkali metal, alkaline earth metal, and transition metal have been known as high activity catalyst [9]. It was reported that there are two approaches for the production of highly reactive coke in conventional coke oven. The first one is adding iron- or calciuminorganic substance to the blend coals and to be carbonized in coke oven (the pre-addition method), while the second method is spraying aqueous solutions of iron- and calcium-salts to coke surface (the postaddition method) [10–12]. By these two methods, such highly reactive coke was successfully produced in commercial coke oven, and it was found that RAR of BF was reduced by 10 kg/tHM with utilizing 8% highly reactive coke [13]. Additionally, Yamamoto et al. [14–16] reported another approach to decrease RAR and CO2 emission of BF. They proposed and developed an innovative BF burden material, carbon iron composite (CIC), which was produced by carbonizing iron ore-coal briquette made up of iron ore and coal in a shaft furnace. During carbonization, iron oxides are quickly

H. Wang et al. / Powder Technology 328 (2018) 318–328

319

Table 1 Characterization data of four coals used in experiments. Coal

Fixed carbon/%

Ash/%

Volatile matter/%

Moisture/%

Final contraction value/mm

Max thickness of plastic layer/mm

Ash compositions/% CaO

SiO2

MgO

Al2O3

TFe

A B C D

61.52 72.77 77.91 80.63

8.75 9.45 9.48 9.25

28.25 16.26 11.38 8.81

1.48 1.52 1.23 1.31

22.9 26.1 18.5 n.a.

12 18 0 n.a.

8.13 1.59 3.77 4.02

40.03 49.68 45.96 58.96

2.46 0.49 0.93 0.80

27.85 39.31 33.51 21.36

6.96 2.75 5.88 3.64

n.a.: no data available.

reduced to FeO and metallic Fe. With catalytic effects of iron particles and the close arrangement of iron ore and carbonaceous materials, the reactivity towards CO2 of carbon inside the CIC is enhanced by increasing the addition ratio of iron ore. Moreover, blast furnace inner reaction simulator (BIS) test showed that with using CIC having 43% of total iron, the TRZ temperature could decrease about 186 °C and the shaft efficiency could increase 6.8% [17]. Furthermore, industrial test showed that BF operation with 43 kg/tHM charging could cause a decrease in RAR by 13–15 kg/tHM [18, 19]. Highly reactive coke generally shows low post-reaction strength due to its remarkably high reactivity compared with conventional coke, which could have negative effects on the permeability of BF. Nomura et al. [20] investigated the post-reaction strength of highly reactive coke under the condition of fixing two or three parameters (weight loss percentage, reaction time, and reaction temperature), and found that highly reactive coke produced by catalyst post-added method could achieve higher post-reaction strength at lower reaction temperature while catalyst pre-added coke has lower post-reaction strength. In terms of cold strength and post-reaction strength of CIC, Higuchi et al. [17] found that adding iron ore powder could cause a decrease in the cold strength of CIC because the dilatation of coal particles is prevented by iron ore particle, but with adding 30% iron ore powder, CIC has sufficient strength used as nut coke in BF. Some investigations about the cold strength and post-reaction strength of CIC have been made under different experimental conditions such as reaction temperature and reaction time, but the influences of the composition of coal and iron ore on the post-reaction strength of CIC have not been reported yet. In previous work [21], the preparation process of iron coke hot briquette (ICHB) was proposed depended on the Chinese raw material conditions and optimized. Under the optimized conditions, the compressive strength of ICHB is more than 5000 N. According to the testing method from Chinese National Standard (GB/T 4000-2008; equivalent to ASTM Standard D 5341) [22, 23], the reactivity of ICHB is considerably high while the post-reaction strength is considerably low compared with conventional coke, which could have negative effects on the permeability of BF. Furthermore, operating results from Nippon Steel Corporation (NSC) suggested that the post-reaction strength of coke depends about 70% on the parent coal or the blended coal [24]. In order to improve the post-reaction strength of ICHB simultaneously keeping the compressive strength and the reactivity at relatively higher level based on the previous investigation, the improvement of the post-reaction strength of ICHB by adjusting coal blending and iron ore blending were studied in this paper and other metallurgical properties of ICHB, such as compressive strength, pore structure, and carbon crystallite size (Lc), were also investigated. Simultaneously, the corresponding reaction mechanism was analyzed by means of scanning electron microscope (SEM) with energy dispersive X-ray spectrometer (EDS) examinations and X-ray diffraction (XRD)

Table 2 Chemical compositions of iron ore powder (mass fraction, %). Composition

TFe

FeO

SiO2

CaO

Al2O3

MgO

P

S

Content

66.69

26.40

5.31

0.18

0.31

1.80

0.02

0.05

analysis. This paper could provide technological support and theoretical basis for the practical application of ICHB. 2. Experimental 2.1. Raw materials Four types of coals were used in experiments. The characterizations of four coals come from Qitaihe City in China are listed in Table 1. All coals are single coal. Coal A and coal C are slightly caking coals. Coal B is a caking coal and coal D is an anthracite coal. The chemical composition of iron ore concentrate used in tests is shown in Table 2. The total iron content (TFe) and FeO content of iron ore are 66.69% and 26.40%, respectively. 2.2. Experimental procedure The raw materials were dried at 105 °C for 2 h in a draught drying cabinet. Then coals and iron ore were crushed to 90% b 75 μm and adequately mixed according to the predetermined ratio (as shown in Table 3) to form the mixtures of iron ore powder and coal fines. Afterwards, the mixtures were put into an ellipsoidal mold and heated to 300 °C in a heating furnace, then it was pressed to achieve coal-iron ore composite agglomerate (abbreviated as hot briquette hereinafter) by self-made hot press apparatus with 50 MPa pressure. Finally, the hot briquette was carbonized at 1000 °C for 4 h in a carbonization furnace (heating rate 3 °C/min) under Ar atmosphere. After carbonization, the carbonized product was cooled in N2 atmosphere (cooling rate 15 °C/min) and ICHB was achieved. The external morphologies of hot briquette (21 × 19 × 16 mm) and ICHB (18 × 16 × 14 mm) are shown in Fig. 1. Based on the previous work [21], to improve the metallurgical properties of ICHB, coal B or coal C were selected and added into the mixtures by partially replacing coal A (test 1–test 9). In terms of coking properties, coal A, B, and C were in the order of B N A N C. The proportions of other materials for test 1–test 9 were kept constant when coal B or coal C was added. Test 1 is the optimized process conditions in the

Table 3 Addition ratios of coals and iron for the preparation of ICHB (mass fraction, %). Test

1 2 3 4 5 6 7 8 9 10 11 12 13

Addition ratios of coal and iron ore Coal A

Coal B

Coal C

Coal D

Iron ore

65.00 60.00 55.00 50.00 45.00 60.00 55.00 50.00 45.00 64.71 61.47 58.24 51.76

0.00 5.00 10.00 15.00 20.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 5.00 10.00 15.00 20.00 11.76 11.18 10.59 9.41

20.00 20.00 20.00 20.00 20.00 20.00 20.00 20.00 20.00 23.53 22.35 21.18 18.82

15.00 15.00 15.00 15.00 15.00 15.00 15.00 15.00 15.00 0.00 5.00 10.00 20.00

320

H. Wang et al. / Powder Technology 328 (2018) 318–328

emission spectroscopy (ICP-AES, Optima 8300DV; PerkinElmer). The metallization degree was calculated by Eq. (3). Metallization degree ¼

Fig. 1. External morphologies of hot briquette and ICHB, (a) hot briquette; (b) ICHB.

previous work and thereby considered as a reference. Additionally, according to the results of coal blending, the effects of iron ore blending on the metallurgical properties of ICHB were investigated by adding iron ore into the mixtures at a ratio of 0 up to 20 mass% (test 7, and test 10–test 13). For test 7, and test 10–test 13, the proportions of coal A, C and D in blending coals were maintained unchanged at 64.71:11.76:23.53 mass% and the addition ratios of them were equidistantly decreased with increasing the proportion of iron ore powder. The gasification reaction tests of ICHB were performed according to GB/T 4000-2008. About 200 g ICHB was reacted with CO2 gas (5 L/min) at 1100 °C for 2 h in a vertical electric heating furnace. During heating process and cooling process, ICHB was protected by N2 atmosphere. After reaction, drum tests for gasified ICHB were carried out in the I-type drum tester (Φ130 × L700 mm). The reactivity index of ICHB is evaluated by the reaction ratio, which is calculated by formula (1). The post-reaction strength is assessed by I60010, which is the percentage of ICHB mass retained on a sieve with 10 mm apertures to the mass of the post-reaction ICHB after 600 revolutions (20 rpm × 30 min) in the I-type drum tester. The post-reaction strength of ICHB was calculated by formula (2). Reaction ratio m1  ½1−w1 ðAad Þ−w1 ðFeÞ−w1 ðFeOx Þ−m2  ½1−w2 ðAad Þ−w2 ðFeÞ−w2 ðFeOx Þ m1  ½1−w1 ðAad Þ−w1 ðFeÞ−w1 ðFeOx Þ  100% ð1Þ

where m1 and m2 are the mass of ICHB before and after reaction respectively, w1(Aad) is the ash content of ICHB before reaction and w2(Aad) is the ash content of ICHB after reaction, w1(Fe) and w2(Fe) are the metallic iron content of ICHB before and after reaction respectively, w1(FeOx) is the iron oxide content of ICHB before reaction and w2(FeOx) is the iron ore content of ICHB after reaction. Post  reaction strength ¼

m3  100% m2

Lc ¼

0:89λ B  cos θ

2.3. Characterization methods For carbon iron ore composite agglomerate, the compressive strength is one of the most important physical properties in view of preventing the agglomerate from becoming finely crushed during its handing, transportation, and subsequently charging into BF [25–27]. Twelve ICHB were used to measure the compressive strength by an electronic universal testing machine (WDW-QT10 type) according to Chinese National Standard [28]. Among 12 observed values, the maximum and the minimum were removed and the experimental value was equal to the average of the remaining values. The metallization degree of ICHB was analyzed by means of X-ray fluorescent (XRF, ZSXPrimus II; Rigaku, Japan) and inductively coupled plasma atomic

ð4Þ

where λ is the wave length of the X-ray radiation, B is the full width at half maximum intensity (FWHM) of the 002 carbon peak and θ is the 002 carbon peak position. Additionally, the microstructure of ICHB was evaluated by SEM/EDS (Zeiss, Ultra Plus) analysis. The surface of ICHB was ground on four different grades of abrasive paper (400, 800, 2000 and 3000 grit) with distilled water and burnished in a burnishing machine. The burnished samples were mounted on a conductive support and coated with a thin layer of gold-palladium alloy using a sputter coater. 3. Results and discussion 3.1. Effect of coal B addition ratio on metallurgical properties of ICHB 3.1.1. Compressive strength and porosity of ICHB Fig. 2 gives the effect of coal B addition ratio on the compressive strength of ICHB (test 1–test 5). It is clear that the compressive strength of ICHB is slightly improved from 5048 N to 5334 N with increasing coal B addition ratio from 0 up to 20%. On one hand, better coking property of coal B compared with coal A improve the solidification of particles and thereby increase the compressive strength with increasing coal B addition ratio. On the other hand, with adding coal B, the porosity of ICHB is decreased (as shown in Fig. 3, measured by liquid nitrogen adsorption

ð2Þ

where m3 is the mass of gasified ICHB with size more than 10 mm after I-type drum testing.

ð3Þ

where MFe and TFe represent the metallic iron content and total iron content in ICHB, respectively. Coke reactivity is related to its carbon structure and graphitization degree on the micro level, which can be measured by the X-ray diffraction [29, 30]. The graphitization degree could be well characterized by the 002 carbon peak in diffraction and the crystallite size (Lc) can be calculated by this peak [31, 32]. Higher Lc corresponds to higher graphitization degree and lower coke reactivity [33]. The XRD patterns of ICHB powder were achieved by using an X-ray diffractometer (MPDDY2094 type, manufactured by PANalytical B.V. Corporation in Netherlands). The copper Kα radiation (40 kV, 40 mA, wave length 0.154 nm) was used as the X-ray source and the scanned angular range varied from 5° to 90° with a scanning speed of 0.2°/s. The Lc value was calculated by the Scherrer's Eq. as follows.

5500

5000 Compressive strength /N

¼

MFe  100% TFe

4500

4000

3500

3000

0

5 10 15 Addition ratio of coal B /%

20

Fig. 2. Effect of coal B addition ratio on the compressive strength of ICHB.

H. Wang et al. / Powder Technology 328 (2018) 318–328

21

12 20 11 19 10 18

9

17

8 0

5 10 15 Addition ratio of coal B /%

77.05 39.11

76 Reaction ratio /%

Porosity of ICHB /%

78 Volatile matter of hot briquette /%

porosity of ICHB volatile matter of hot briquette

13

50

80 40.62

42.96

44.28

40

74

35 34.25

72 70

69.10

reaction ratio post reaction strength

30 25

68

20

66

63.64 63.04

64 62 60

15 61.91

61.04

10.65

10 5 0

58 0

20

45 Post-reaction strength /%

22

14

321

5 10 15 Addition ratio of coal B /%

20

Fig. 3. Effect of coal B addition ratio on the volatile matter of not briquette and the porosity of ICHB.

Fig. 5. Effects of coal B addition ratio on the reaction ratio and post-reaction strength of ICHB.

method). The decrease in the porosity could cause the acceleration of the compressive strength. Moreover, it can be seen from Fig. 3 that the porosity of ICHB is obviously reduced with increasing the addition ratio of coal B in the range from 0 up to 10%, and decreased slightly as coal B addition ratio sequentially increases. The amount of gas release during carbonization process is reduced since the volatile matter content of hot briquette is linearly decreased with increasing coal B addition ratio, which could be the main reason for the decrease of ICHB porosity. Additionally, the microstructure of ICHB with adding different ratios of coal B was given in Fig. 4. With adding coal B, the amount of pores is distinctly decreased and the microstructure of ICHB becomes compact.

the post-reaction strength is noticeably raised firstly and then slowly increased. Therefore, adding coal B in the mixtures could improve the hot performances of ICHB and the appropriate addition ratio is about 5% thoroughly considering the metallurgical properties and the production cost due to expensive prices of coal B compared with coal A. The variations for the reactivity and the post-reaction strength were related to some factors, such as porosity and carbon crystallite structure, which will be analyzed in following part. ICHB is a porous fissured material from the macro perspective and it microscopically consists of pores, microscopic cracks, and solid matrix. The reactivity of ICHB is considered to depend on its pore structure, carbon structure, matrix optical texture, and the internal mineral components [13]. It can be seen from Fig. 3 that the porosity of ICHB is gradually reduced with increasing coal B addition ratio. The decrease in porosity of ICHB could mitigate the internal diffusion of CO2 gas efficiently, and is favorable to decrease the reaction ratio and increase postreaction strength. XRD analyses of ICHB with different ratios of coal B were performed to investigate the carbon crystallite structure of ICHB, and the 002

3.1.2. Reaction ratio and post-reaction strength of ICHB The effects of coal B addition ratio on the reaction ratio and the postreaction strength of ICHB are shown in Fig. 5 (test 1–test 5, and adding 2.5% coal B is complementary later). With the addition ratio of coal B increasing from 0 to 5%, the reaction ratio of ICHB is dramatically decreased from 77.05% to 63.64%. The reaction ratio of ICHB tends to slightly decrease as the addition ratio is about 5%–20%. Furthermore,

(a)

(c)

(b)

(d)

(e)

Fig. 4. SEM for the microstructure of ICHB with adding different ratios of coal B, (a) without adding coal B, (b) 5%, (c) 10%, (d) 15%, (e) 20%.

322

H. Wang et al. / Powder Technology 328 (2018) 318–328

50

002 peak

45 Post-reaction strength /%

20% coal B 15% coal B 10% coal B 5% coal B 2.5% coal B

40 35 2

R =0.94402

30 25 20 15 10

without adding coal B

y = -2.00717 x + 167.71235

with adding coal B

5

25.5

26.0

26.5 27.0 2θ (deg.)

27.5

60

28.0

Fig. 6. Magnified 002 carbon peaks in XRD patterns of ICHB with adding different ratios of coal B.

carbon peaks in XRD patterns were magnified as shown in Fig. 6. According to these peaks, the Lc value of ICHB was calculated by formula (4), as shown in Fig. 7. The Lc value of ICHB is increased but tends to level off with increasing the addition ratio of coal B, which is one of the main reasons for the above variations of the reaction ratio and the post-reaction strength of ICHB. In addition, the relationship between the reaction ratio and the postreaction strength of ICHB with adding coal B is shown in Fig. 8. Under adding coal B, the post-reaction strength was decreased with the increase of the reaction ratio of ICHB. They show a better linear relationship and the linearity correlation coefficient (R2) is up to 0.944. Furthermore, since the amount of C consumed by solution loss reaction in a BF is nearly constant [12], the strength of ICHB after the reaction should be evaluated after a constant weight loss. Further study is necessary on the evaluation of post-reaction strength of ICHB. 3.2. Effects of coal C addition ratio on metallurgical properties of ICHB 3.2.1. Compressive strength and porosity of ICHB Fig. 9 displays the effects of coal C addition ratio on the compressive strength and the porosity of ICHB (test 1, and test 6–test 9). With increasing the addition ratio of coal C, the compressive strength of ICHB tends to slightly increase firstly and then mildly decrease. The maximum strength (5123 N) was achieved with adding 15% coal C. Moreover, the porosity of ICHB is reduced firstly and then raised. The

62

64

66 68 70 72 Reaction ratio /%

74

76

78

Fig. 8. Relationship between the reaction ratio and the post-reaction strength of ICHB with adding coal B.

decrease in the porosity could result in the acceleration of ICHB compressive strength with adding 0–15% coal C, while the increase in the porosity could lead to the subsequent reduction of the compressive strength with adding 20% coal C. Since the volatile matter of hot briquette is decreased due to the lower volatile matter of coal C compared with coal A, the porosity of ICHB is gradually reduced with adding 0–15% coal C. Besides, the reduction of iron ore is accelerated during carbonization and more pores are generated with adding 20% coal C, which causes the increase of ICHB porosity. The microstructure of ICHB with adding different ratios of coal C is shown in Fig. 10. It is clearly that the addition of coal C could reduce the generation of pores and the structure is increasingly compact with the addition ratio of coal C increasing from 0 up to 15%, which cause the decrease in the porosity and the increase in the compressive strength of ICHB, as shown in Fig. 9. In addition, an increase in pores of ICHB and more metallic iron (white spot in Fig. 10) can be observed with adding 20% coal C. The more metallic iron comes from the enhancement of reduction of iron ore, which leads to the generation of more pores. 3.2.2. Reaction ratio and post-reaction strength of ICHB Fig. 11 describes the effects of coal C addition ratio on the reaction ratio and the post-reaction strength of ICHB and their relationship (test 1, test 6–test 9, and adding 2.5% coal C is complementary later). With increasing the addition ratio of coal C from 0 to 10%, the reaction

14 compressive strength porosity

5200

13 12 11

4800 10 4600

9

4400

8 7

4200 4000

Fig. 7. Effect of coal B addition ratio on the carbon crystallite size of ICHB.

Porosity /%

Compressive strength /N

5000

6 0

5 10 15 Addition ratio of coal C /%

20

5

Fig. 9. Effects of coal C addition ratio on compressive strength and porosity of ICHB.

H. Wang et al. / Powder Technology 328 (2018) 318–328

323

(b)

(a)

(c) Pore

Pore Pore

(e)

(d)

Pore

Pore

Fig. 10. SEM for the microstructure of ICHB with adding different ratios of coal C, (a) without adding coal C, (b) 5%, (c) 10%, (d) 15%, (e) 20%.

77.05

46.45 42.43 39.27

Reaction ratio /%

76

38.87

74

50

55

45

50

40

45

35 32.69

72

reaction ratio post-reaction strength

70.57

70

30 69.63

25

67.79 68

20

66

15

66.21

64

10

63.00

10.65 62

(b)

Post-reaction strength /%

(a)

78

Post-reaction strength /%

80

60 5 10 15 Addition ratio of coal C /%

R =0.90529

30 25 20 with adding coal C

10

0 0

2

35

15

5

y = -2.59045 x +213.83964

40

62

20

64

66

68 70 72 Reaction ratio /%

74

76

78

Fig. 11. Effects of coal C addition ratio on reaction ratio and post-reaction strength of ICHB and their relationship, (a) reaction ratio and post-reaction strength, (b) relationship between reaction ratio and post-reaction strength.

80

17 16 Carbon crystallite size (Lc) /A

20% coal C 15% coal C 10% coal C 5% coal C 2.5% coal C without adding coal C

25.0

25.5

26.0

carbon crystallite size reaction ratio

77.05

15 14

12.97

76 74

13.15 12.56

13 12

78

72 69.63

70.57

70

11 10

10.02

10.54

66.21 67.79

9 8 8.76

66 64

63.00

62

7

26.5 27.0 2 theta /deg.

27.5

28.0

28.5

Fig. 12. 002 peak of carbon in XRD patterns of ICHB with adding different ratios of coal C.

68

60

6 0

5 10 15 Addition ratio of coal C /%

20

Fig. 13. Carbon crystallite size of ICHB with adding different ratios of coal C.

Reaction ratio /%

002 peak

324

H. Wang et al. / Powder Technology 328 (2018) 318–328

6000

90

53 5602

51.99

5000

4861

51

64.22

4594

4000

3000

0

5 10 15 Addition ratio of iron ore /%

20

Fig. 14. Effect of iron ore addition ratio on the compressive strength of ICHB.

16 14 12 10 8 6 4 0

5 10 15 Addition ratio of iron ore /%

20

Fig. 15. Effect of iron ore addition ratio on porosity of ICHB.

ratio of ICHB is decreased from 77.05% to 63.00% while the post-reaction strength of ICHB is increased from 10.65% to 46.45%, which is due to the lean effect of coal C containing low content of volatile matter and high content of inertinite component. Moreover, as the addition ratio of coal C is increased from 10% to 20%, the reactivity ratio of ICHB is raised from 63.00% to 69.63% while the post-reaction strength is slightly reduced from 46.45% to 38.87% mainly due to the lower coking capacity of coal C in comparison to coal A. Apart from the above reason, the reaction ratio and the post-reaction strength of ICHB are depended on its pore structure and carbon crystallite structure, which will be discussed

(a)

60

50.44 50.10

57.61 50

49 46.45

48

40

reactivity post-reaction strength

47

2000

70

50.27

50

46

Porosity of ICHB /%

80

74.08

46.74

30 26.84

Post-reaction strength /%

52 5023

Reactivity /%

Compressive strength /N

5324

20 10

45 0

5 10 15 Addition ratio of iron ore /%

20

Fig. 17. Effects of iron ore addition ratio on reactivity and post-reaction strength of ICHB.

later. Additionally, the reaction ratio shows good linear relationship on the post-reaction strength, and the R2 is about 0.905. Therefore, the hot performances of ICHB could be improved by adding coal C, and the reasonable addition ratio of coal C is about 10% from the perspective of compressive strength, reactivity, post-reaction strength, and the production cost because coal C is cheaper than coal A. The decrease in the porosity is not beneficial to the CO2 diffusion inside ICHB during the gasification reaction. So the reaction ratio of ICHB is decreased with adding 0–15% coal C. The porosity of ICHB achieves the minimum value with adding 15% coal C, but the reactivity of ICHB is not lowest at this point, which suggests that the porosity is not the only factor on determining the reaction ratio. Additionally, the carbon structure of ICHB is analyzed by means of XRD analysis and 002 peak of carbon of XRD patterns is shown in Fig. 12. Lc value was calculated by Eq. (4) and the results were shown in Fig. 13. Evidently, Lc value of ICHB is increased firstly and then decreased with increasing coal C addition ratio, which could mainly contribute to the above variation of the reaction ratio and the post-reaction strength of ICHB. The metallurgical properties of ICHB could be improved and optimized by adding coal B or coal C. Taking into full consideration of the quality of ICHB and the production cost, coal C is selected and added preferentially due to its lower price among the three coals (A, B and C), and the recommended addition ratio of coal C in the mixtures is about 10%. 3.3. Effects of iron ore addition ratio on metallurgical properties of ICHB 3.3.1. Compressive strength of ICHB The effect of iron ore addition ratio on the compressive strength of ICHB is given in Fig. 14 (test 7, and test 10–test 13). With increasing

(b) small pores large pores

matrix matrix Fig. 16. SEM photos of ICHB at 2000×, (a) without adding iron ore; (b) adding 10% iron ore.

H. Wang et al. / Powder Technology 328 (2018) 318–328

90

Post-reaction strength /%

80 y=-8.15741x+460.95984

70 2

60

R =0.66654

50 40 30

with adding iron ore

20 46

47

48

49 50 Reactivity /%

51

52

Fig. 18. Relationship between the reactivity and the post-reaction strength of ICHB with adding iron ore.

-C 20% iron ore 12800 - SiO2 9600 6400 3200 0 12800 15% iron ore 9600 6400 3200 0 12800 10% iron ore 9600 6400 3200 0 5% iron ore 12800 9600 6400 3200 0 12800 without adding iron ore 9600 6400 3200 0 10 20 30 40 50 60 2 theta /deg.

3.3.2. Reactivity and post-reaction strength of ICHB calculated by apparent weight loss The reactivity and the post-reaction strength of ICHB with adding different ratios of iron ore (test 7, and test 10–test 13) are calculated based on the apparent weight loss of ICHB during gasification reaction according to GB/T 4000–2008. The reactivity is the percentage of weight loss to the original ICHB mass after reaction (reactivity = (m1 − m2) / m1 × 100%, m1 and m2 same to equal (1)) and the post-reaction strength is calculated by formula (2). The results are shown in Fig. 17. The reactivity of ICHB is slightly increased with the increasing iron ore while the post-reaction strength is obviously decreased. Compared with the post-reaction strength, the variable amount of the reactivity is smaller in terms of quantity, which can be distinctly seen in Fig. 18. Evidently, the reactivity and post-reaction strength show lower linear correlation, which suggest that some problems could exist in these calculation methods. The related issues will be revealed in following part. In order to explain and illuminate the above phenomena, the XRD analyses of ICHB with adding iron ore powder (test 7, and test 10–test 13) before and after gasification reaction are performed and presented in Fig. 19. The main phases in ICHB before reaction consist of C, Fe, Fe3O4 and SiO2 with adding iron ore. Furthermore, the intensity of diffraction peaks corresponding to metallic iron is enhanced while that of carbon is reduced with increasing the iron ore addition ratio. After reaction, Fe3O4 phase disappear, and the diffraction peaks of FeO and 2FeO·SiO2 are detected and increased with adding 10%–20% iron ore. Simultaneously, the intensity of the peaks of metallic iron is decreased. FeO phase could be generated from the reduction of Fe3O4 or the oxidization of Fe during gasification reaction. According to the XRD analyses, the reduction of Fe3O4 to FeO can be confirmed, but the oxidization of Fe to FeO could not be conducted. Thermodynamic analysis during the gasification was investigated, as shown in Fig. 20. Obviously, the following reactions can thermodynamically occur due to their negative Gibbs free energy (ΔG) at 1100 °C. Reactions (5) and (7) result in the weight loss of ICHB while reaction (6) leads to the weight increment. The above two aspects will have a serious impact on the results if the calculation is just based on the

- Fe - Fe3O4

15000

- C -SiO2 - Fe - FeO - 2FeO·SiO2

20% iron ore

10000 5000 0 15000

15% iron ore

10000 5000

Intensity

Intensity

iron ore addition ratio from 0 to 20%, the compressive strength of ICHB is decreased from 5602 N to 4594 N. During the carbonization process, many pores emerged due to the release of volatile matter of coal and the reduction of iron ore, which result in the attenuation of pore wall and the increase in the porosity o ICHB, as seen in Fig. 15. Moreover, the SEM images of ICHB with adding iron ore powder (test 10 and test 12) are shown in Fig. 16. It is clearly that ICHB mainly consists of metallic iron, carbon matrix, and pore. The pores are mainly closed and small without adding iron ore while that change to be more and larger with adding 10% iron ore due to the acceleration of iron ore reduction. Additionally, iron ore powder, an inert substance, can decrease the total dilatation and maximum fluidity of blended coal [12]. The increase in the porosity of ICHB and the decrease in the total dilatation and maximum fluidity of blended coal contributed the mitigation of the compressive strength of ICHB.

325

0 15000

10% iron ore

10000 5000 0 15000

5% iron ore

10000 5000 0 15000

without adding iron ore

10000 (a) 70

80

90

5000 0 10

(b) 20

30

40 50 60 2 theta /deg.

70

80

90

Fig. 19. X-ray diffraction patterns of ICHB with adding different ratios of iron ore, (a) before gasification reaction and (b) after gasification reaction.

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H. Wang et al. / Powder Technology 328 (2018) 318–328

10

Fe þ CO2 ¼ FeO þ CO

ð6Þ

Fe3 O4 þ CO ¼ 3FeO þ CO2

ð7Þ

2FeO þ SiO2 ¼ 2FeO  SiO2

ð8Þ

0 -10

-30 C+CO2=2CO (reaction (5)) Fe+CO2=FeO+CO (reaction (6)) Fe3O4+CO=3FeO+CO2 (reaction (7)) FeO+SiO2=Fe2SiO4 (reaction (8))

-40

θ

ΔrG /(KJ/mol)

-20

-50 -60 -70 -80 1060

1070

1080

1090 1100 1110 Temperature /ć

1120

1130

1140

Fig. 20. Thermodynamic calculation of reactions possibly occurred during the gasification reaction.

Table 4 Metallization ratio of ICHB before and after gasification reaction. Addition ratio of iron ore/%

0 (test 10) 5 (test 11) 10 (test 12) 15 (test 7) 20 (test 13)

Before gasification reaction

After gasification reaction

TFe/%

MFe/%

Metallization degree/%

TFe/%

MFe/%

Metallization degree/%

0.69 7.80 10.01 12.73 14.39

0.36 4.45 7.62 10.33 11.96

52.17 57.09 76.08 81.16 83.09

0.95 12.09 21.50 33.60 37.16

0.48 5.90 10.03 14.18 13.50

50.53 48.80 46.66 42.20 36.32

apparent weight loss of ICHB before and after gasification. Additionally, reactions (5) and (7) are confirmed to occur during gasification, and the generated FeO could reacted with SiO2 to form fayalite (reaction (8)), which can be seen according to the XRD analysis of gasified ICHB in Fig. 19(b). C þ CO2 ¼ 2CO

ð5Þ

(a)

In order to clarify whether reaction (6) occurs or not during the gasification, the metallization degree of ICHB before and after reaction was calculated by Eq. (3) according to the metallic iron content (MFe) and the total iron content (TFe) of ICHB with adding iron ore, as listed in Table 4. It is clear that the metallization degree of ICHB after the reaction is obviously decreased compared with that of ICHB before reaction, which indicates the metallic iron in ICHB was oxidized during the reaction. Therefore, reaction (6) also happened during the gasification reaction. Fig. 21 gives the SEM/EDS analyses of ICHB with adding 10% (test 12) and 20% (test 13) iron ore after the gasification reaction. Compared with inner region of ICHB (Fig. 21(b)), outer region of ICHB (Fig. 21(a)) is largely eroded by CO2 gas stream due to generating more tiny particles, pits, cracks, and pores, which suggests that the gasification reaction of ICHB proceeds from the external to the internal. From Fig. 21(c) and the EDS results, Fe, FeO, C and other oxides exit in ICHB after the reaction. Metallic iron is reduced from iron oxide in the carbonization process of hot briquette, and FeO is produced by the reduction of Fe3O4 and the oxidization of Fe during the gasification reaction. 3.3.3. Reaction ratio of ICHB calculated based on the modified method To exclude the effects of metallic iron, iron ore, and ash on the gasification reaction of ICHB, the calculation method for the reactivity of ICHB was modified as formula (1) and the reactivity index was estimated by the reaction ratio. The calculation results are shown in Fig. 22. The reaction ratio of ICHB is evidently increased from 48.22% to 75.02% as the addition ratio of iron ore is increased from 0 up to 20%, which is mainly due to the catalytic effects of metallic iron and the increase in the porosity of ICHB. In terms of high reactivity and high post-reaction strength, the reasonable addition ratio of iron ore is about 10%–15%. Additionally, the relationship between the reaction ratio and the post-reaction strength is shown in Fig. 23. It can be seen that the reaction ratio shows high linear relationship on the post-reaction strength

(b)

(c)

pore P2 P1 P3

eroded (e) O

(f) C

Intensity C

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Energy /keV

Intensity

Fe

Intensity

(d) Fe

Si Al Fe C Mg

Fe

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Energy /keV

0.0

0.5

1.0 1.5 Energy /keV

2.0

Fig. 21. SEM/EDS analysis of ICHB after reactivity testing, (a) outer region of ICHB with adding 10% iron ore at 1000×, (b) inner region of ICHB with adding 10% iron ore at 1000×, (c) outer region of ICHB with adding 20% iron ore at 2000×, (d) EDS analysis of point P1, (e) EDS analysis of point P2, (f) EDS analysis of point P3.

H. Wang et al. / Powder Technology 328 (2018) 318–328

80 74.08

75.02

70

Reaction ratio /%

70 57.61

64.22

65

60 63.00

60

50 56.35

46.45

55

40 50.44 48.22

50

Post-reaction strength /%

reaction ratio post reaction strength

75

30 26.84

45

20 0

5 10 15 Addition ratio of iron ore /%

20

Fig. 22. Reaction ratio and post-reaction strength of ICHB with adding different ratios of iron ore.

(R2 is about 0.992), which indicates that the modified method for ICHB reactivity calculation is suitable. According to the above analysis and discussion, the suitable addition ratio of iron ore is about 15% for preparing highly reactive ICHB with high strength. The recommended preparation parameters are 15% iron ore, 55% coal A, 10% coal C, and 20% coal D. Under the conditions, the compressive strength, the reaction ratio, and the post-reaction strength of ICHB are about 4861 N, 63.00%, and 46.45% respectively. 4. Conclusions ICHB is considered as a new carbon iron composite to realize low carbon emission BF ironmaking. The effects of coal blending and iron ore blending on the metallurgical properties of ICHB were experimentally investigated. The following conclusions can be drawn: (1) With increasing coal B addition ratio from 0 to 20% in the mixtures, the compressive strength of ICHB is slightly improved. The reaction ratio of ICHB is gradually decreased from 77.05% to 61.04% while the post-reaction strength of ICHB is increased from 10.65% to 44.28%. (2) The compressive strength of ICHB tends to slightly increase firstly and then mildly decrease with increasing coal C addition ratio.

80

Post-reaction strength /%

70 60

y=-1.71024x+154.55048 2

R =0.99236

50 40 30 20 45

with adding iron ore after modification

50

55

60 65 Reaction ratio /%

70

75

Fig. 23. Relationship between the reaction ratio and the post-reaction strength with adding iron ore.

327

The reaction ratio of ICHB is decreased firstly and then increased while the post-reaction strength tends to be enhanced firstly and then reduced. The metallurgical properties of ICHB can be improved by adding coal C and the suitable addition ratio of coal C is about 10%. (3) With increasing the addition ratio of iron ore from 0 to 20%, the compressive strength of ICHB is decreased from 5602 N to 4594 N. The calculation method for ICHB reactivity should be modified since both the reduction of Fe3O4 and the oxidization of metallic Fe can occur during the gasification. After modification, the reaction ratio of ICHB is accelerated from 48.22% to 75.02% while the post-reaction strength is decreased from 74.08% to 26.84%, and they show higher linear relationship. (4) According to the coal blending and the iron ore blending, the recommended process parameters for ICHB preparation are 15% iron ore, 55% coal A, 10% coal C, and 20% coal D. Under the conditions, the compressive strength of ICHB is up to 4861 N, the reaction ratio is approximately 63.00% and the post-reaction strength is about 46.45%. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (50804008), the China Postdoctoral Science Foundation (No. 2016M601321), and the Fundamental Research Funds of the Central Universities of China (No. N162503003). References [1] C. Wang, C. Ryman, J. Dahl, Potential CO2 emission reduction for BF-BOF steelmaking based on optimized use of ferrous burden materials, Int. J. Greenhouse Gas Control 3 (2009) 29–38. [2] Z.L. Zhang, J.L. Meng, L. Guo, Z.C. Guo, Numerical study of the reduction process in an oxygen blast furnace, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 47B (2016) 467–484. [3] W.Q. Xu, W.J. Cao, T.Y. Zhu, Y.J. Li, B. Wan, Material flow analysis of CO2 emissions from blast furnace and basic oxygen furnace steelmaking system in China, Steel Res. Int. 86 (2015) 1063–1072. [4] K.D. Xu, Low carbon economy and iron and steel industry, Iron Steel 45 (2010) 1–12. [5] F.Y. Meng, S. Gupta, D. French, P. Koshy, C. Sorrell, Y.S. Shen, Characterization of microstructure and strength of coke particles and their dependence on coal properties, Powder Technol. 320 (2017) 249–256. [6] M. Naito, A. Okamoto, K. Yamaguchi, T. Yamaguchi, Y. Inoue, Improvement of blast furnace reaction efficiency by use of high reactivity coke, Tetsu-to-Hagané 87 (2001) 357–364. [7] I. Shimoyama, Research prospect on coke reactivity aiming at low RAR blast furnace operation, Tetsu-to-Hagané 96 (2010) 209–217. [8] M. Naito, A. Okamoto, K. Yamaguchi, T. Yamaguchi, Y. Inoue, Improvement of blast furnace reaction efficiency by temperature control of thermal reserve zone, Nippon Steel Technical Report 94 2006, pp. 103–108. [9] Y. Yamazaki, K. Hiraki, T. Kanai, X. Zhang, Y. Matsushita, M. Shoji, H. Aoki, T. Miura, The effect of metallic iron particle on coke-matrix after coke CO2 gasification reaction, J. Therm. Sci. Technol. 6 (2011) 278–288. [10] B.D. Flores, A. Guerrero, I.V. Flores, A.G. Borrego, M.A. Díez, E. Osório, A.C.F. Vilela, On the reduction behavior, structural and mechanical features of iron ore-carbon briquettes, Fuel Process. Technol. 155 (2017) 238–245. [11] S. Nomura, H. Kitaguchi, K. Yamaguchi, M. Naito, The characteristics of catalystcoated highly reactive coke, ISIJ Int. 47 (2007) 245–253. [12] S. Nomura, H. Terashima, E. Sato, M. Naito, Some fundamental aspects of highly reactive iron coke production, ISIJ Int. 47 (2007) 823–830. [13] S. Nomura, H. Ayukawa, H. Kitaguchi, T. Tahara, S. Matsuzaki, M. Natio, S. Koizumi, Y. Ogata, T. Nakayama, T. Abe, Improvement in blast furnace reaction efficiency through the use of highly reactive calcium rich coke, ISIJ Int. 45 (2005) 316–324. [14] T. Yamamoto, T. Sato, H. Fujimoto, T. Anyashiki, M. Sato, K. Takeda, Effect of raw materials on reaction behavior of carbon iron composite, Tetsu-to-Hagané 96 (2010) 683–690. [15] T. Anyashiki, K. Fukada, H. Fujimoto, Development of carbon iron composite process, JFE Technical Report 2009, pp. 1–6. [16] M. Sato, T. Yamamoto, M. Sakurai, Recent progress in ironmaking technology for CO2 mitigation at JFE Steel, JFE Technical Report 2014, pp. 103–109. [17] K. Higuchi, S. Nomura, K. Kunitomo, H. Yokoyama, M. Naito, Enhancement of lowtemperature gasification and reduction by using iron-coke in laboratory scale tests, ISIJ Int. 51 (2011) 1308–1315. [18] M. Sato, H. Matsuno, K. Ishii, Recent development of mid- and long-term CO2 mitigation technology at JFE Steel, Asia Steel International Conference 2015, pp. 12–13. [19] H.T. Wang, W. Zhao, M.S. Chu, C. Feng, Z.G. Liu, J. Tang, Current status and development trends of innovative blast furnace ironmaking technologies aimed to environmental harmony and operation intellectualization, J. Iron Steel Res. Int. 24 (2017) 751–769.

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