Analysis of the Coke Particle Size Distribution and

ISIJ International, Vol. 59 (2019), No. 11

© 2019 ISIJ1997

ISIJ International, Vol. 59 (2019), No. 11, pp. 1997–2004

* Corresponding author: E-mail: [email protected]: https://doi.org/10.2355/isijinternational.ISIJINT-2019-253

1. Introduction

The hearth is occupied by coke, slag and hot metal and is a critical zone of blast furnace.1,2) Hearth coke takes up about 70% of the space in the hearth.1,2) The hot metal and the slag are filled in the void of the coke bed. In the hearth, the coke residence time is much longer, especially in the central part called deadman.2) The hot metal quality, draining conditions of liquid phases, the campaign life and the blast furnace operation are determined by the deadman permeability and its state (sitting or floating).1–5) The perme-ability of the deadman is directly related to the coke size dis-tribution and its porosity. Therefore, it is important to study the coke particle size distribution and deadman porosity for understanding the blast furnace hearth performance. On the tuyere level core drilling technique has enabled technolo-gists to effectively extract deadman coke samples from an operating blast furnace thereby providing a lot of potentially useful information about various important phenomena that occur.6–9) However, there is no guarantee that the deadman behaviors on the tuyere level reflect the condition of the deadman in the lower part of the hearth. At present, the

Analysis of the Coke Particle Size Distribution and Porosity of Deadman Based on Blast Furnace Hearth Dissection

Qun NIU,1)* Shusen CHENG,1) Wenxuan XU,1) Weijun NIU2) and Yaguang MEI1)

1) School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing, 100083 P. R. China.2) Ironmaking Plant, Henan Fengbao Special Steel Co. Ltd., Lingyang Town, Linzhou, Henan, 456561 P. R. China.

(Received on April 16, 2019; accepted on June 3, 2019; J-STAGE Advance published date: July 31, 2019)

Changes in particle size distribution, mineral yield and strength of coke samples from various locations of two Chinese blast furnaces as well as deadman porosity were investigated in the present study for an in-depth understanding about the blast furnace hearth phenomenon. It was found that the percentage of <10 mm coke fines varied from 20% to 49% in majority of the hearth-level regions. The average size of hearth coke was about 20 mm–31 mm. Compared with the feed coke, the hearth coke size was observed to decrease by 43%–63%. The average size of hearth coke particles of a 2 800 m3 blast furnace in diam-eter direction distributed in “M-shape” in majority of the hearth-level regions while that of a 5 500 m3 blast furnace distributed in inversed “V-shape”. The hearth coke mass was 1.43–2.21 times of the feed coke under the same conditions. The M10 of hearth coke with size larger than 40 mm after drum test was about 11%–18% and the M40 was 75%–79%. The M10 increased with the increasing distance to the tuyere level while the M40 decreased with the distance. Due to the catalytic effect of hot metal on coke graphitization, the M10 of hearth coke in the lower part was increased by 63.6% compared with the coke in the upper part. The average porosity of the edge, the middle and the center areas was 0.334, 0.299 and 0.250, respectively. The average porosity of deadman decreased with the increase of distance to the cen-ter line of the taphole and the increasing distance to the furnace wall.

KEY WORDS: blast furnace; hearth; deadman; coke; size particle distribution; porosity.

deadman coke below the tuyere level can only be obtained by blast furnace dissection or overhaul. Previous studies on hearth coke behaviors have focused on coke graphitization, mineral transformation, mineral yield, open porosity and microstructure, etc., through the dissection of furnace after blow-out.10–14) Although the information of the particle size distribution of deadman coke and deadman porosity is criti-cal for understanding the blast furnace hearth phenomenon, the relevant study is rare and insufficient. Jiao et al. investi-gated the mean size of hearth coke using image-processing technique and found the hearth coke size was about 32 mm–34 mm.15) But, the effect of the coke fines on the mean size of hearth coke cannot be accurately considered in the image processing process. In addition, the particle size dis-tribution of deadman coke was not studied.15) Therefore, the particle size distribution of hearth coke need further study to gain a thorough understanding of the hearth conditions.

In this paper, the particle size distribution, ash yield and cold strength of hearth coke as well as deadman porosity were investigated in detail with samples obtained from two Chinese blast furnaces during its overhaul or medium maintenance period. The aim of this paper is to develop an understanding of the behavior and characteristic of the hearth coke in the large blast furnace.


© 2019 ISIJ 1998

2. Samples and Methods

In the present study, two large Chinese blast furnaces were surveyed, which are called BF A and BF B, respec-tively. The relevant parameters are listed in Table 1. The deadman coke samples were extracted from BF A and BF B when it was shut down for overhaul and medium main-tenance, respectively. The vertical positions of the samples obtained from BF A were at 1.8 m (V1), 2.5 m (V2), 3.3 m (V3) and 5.0 m (V4) away from the center line of tuyere, and the horizontal position of sample locations were 0.1 m (H1), 2.95 m (H2), 5.8 m (H3), 2.95 m (H4) and 0.1 m (H5) from hearth sidewall towards the furnace center, as shown in Fig. 1. The vertical positions of the samples

obtained from BF B were at the distance of 1.5 m (V5) and 2.0 m (V6) under center line of tuyere, and the horizontal position of sample locations was 2.5 m (H6), 4.0 m (H7) and 7.2 m (H8) from hearth sidewall towards the furnace center. The coke samples obtained from BF A were sieved using standard sieves with size of 10 mm, 25 mm, 40 mm, 60 mm and 80 mm in turn. The photos of coke samples taken from hearth of BF A at V1-H3 was shown in Fig. 2. The coke samples extracted from BF B were sieved using standard sieves with size of 5 mm, 10 mm, 25 mm and 40 mm in turn. Then the mass of different coke samples was weighed, and the corresponding proportion was calculated at the identical level, respectively. The average particle size was calculated according to Eq. (1)

Table 1. The parameters of BF A and BF B.

Parameter Volume Hearth diameter

Tuyere number

Depth of salamander

Blow in date

Blow out date

Tap hole number

Coke ratio

Coal ratio

Blast Temperature

Utilization coefficient

Hearth height M10 M40 CSR CRI

Unit m3 m – m – – – kg/t kg/t °C t/(m3·d) m % % % %

BF A 2 800 11.6 30 2.400 2007-6-28 2016-9-20 3 347 160 1 160 2.10 4.1 5.8 87.9 66 22

BF B 5 500 15.5 42 3.463 2010-6-26 2017-11-17 4 315 183 1 217 2.16 4.6 5.5 90.4 72 19

Fig. 1. Schematic of the hearth coke sampling positions. (a) Vertical positions; (b) horizontal positions. (Online version in color.)

Fig. 2. Photos of coke samples taken from hearth of BF A at V1-H3. (a) 60–80 mm; (b) 40–60 mm; (c) 25–40 mm; (d) 10–25 mm; (e) <10 mm. (Online version in color.)


© 2019 ISIJ1999

R Ravg i i

i

n

��

1

.............................. (1)

where Ravg is the average size of coke samples at some positions, mm; Ri is the average size of sieve size range i, mm; ωi is the mass percentage of coke samples with sieve size range i.

The hearth lump cokes of blast furnace center at different heights were crushed to passing less than 74 microns and then 4–5 g coke fines were ashed by heating in air at 1 088 K (815°C) to remove carbon.16) Afterwards, the ash yield was obtained. Cold strength of hearth coke was character-ized by a tumbler test.2) For this test, all of the >40 mm coke of BF A located at H1 to H5 of the identical hearth height was tumbled in a rotating drum for 100 rotations at a constant speed of 25 r/min under standard atmospheric pressure. Afterwards, the size distribution after tumbling was determined and used as a coke strength indicator. The weight percentage of coke larger than 40 mm after 100 rotations was called M40 and the weight percentage of coke smaller than 10 mm was called M10.2)

The deadman was dissected in this study by using rope saw after blow-out with the salamander tapping. Deadman images with the residual solidified blast furnace slag and hot metal were acquired manually from digital camera. Try one best to make the camera lens perpendicular to the deadman section when taking pictures. The deadman porosity can be analyzed by digital image processing technique. Matlab is one of the digital image processing softwares developed to compute the deadman porosity from digital images. For

a given deadman image, particular characteristics of gray level are present because the intensity of the reflected light of coke and blast furnace slag is different. Then the coke and blast furnace slag can be distinguished by adjusting the threshold using matlab software to obtain the binary image from the acquired digital deadman image. Finally, the deadman porosity is obtained by calculating the ratio of the solidified hot metal and slag area to the selected area.

3. Results and Discussion

3.1. Coke Size Distribution of BF AParticle size distribution of deadman coke at different

positions was shown in Fig. 3. The percentage of feed coke particles larger than 40 mm is almost 85.68%, while that of feed coke particles smaller than 25 mm is about 4.54%. However, the coke particles smaller than 40 mm were the main component of hearth coke. The percentage of deadman coke particles smaller than 40 mm varied from 64% to 94%. A small percentage (<15%) of coke particles in the hearth coke samples were 60 mm–80 mm in size. Compared with the charged coke, the percentage of deadman coke smaller than 25 mm as well as that within 25–40 mm increased remarkably. It indicated that a serious degradation occurred when the feed coke descended to hearth level. The high percentage of coke fines in the hearth region has been often related to the state of bed permeability.8) In these extracted samples, the percentage of <10 mm fines remains larger than 20% in majority of the hearth-level regions and the maximum percentage can reach 44%. It is worth noting that

Fig. 3. Particle size distribution of deadman coke at different distance under the tuyere. (a) V1 (1.8 m); (b) V2 (2.5 m); (c) V3 (3.3 m); (d) V4 (5.0 m). (Online version in color.)


© 2019 ISIJ 2000

no obvious trend can be observed for the samples in radial and vertical direction. This may be because of the complex-ity and inhomogeneity of the hearth. It is understandable due to the segregation of burden distribution and difference of tuyere size. Thus, it is almost impossible to obtain the complete homogeneity of the hearth in all positions.

The average particle size of deadman coke at differ-ent positions was shown in Fig. 4. It can be seen that the average size of coke particles in deadman was not larger than 40 mm. The average coke size of H1, H2, H3 and H5 decreased sharply first and then increased with the increase of distance to tuyere level, while the average coke size of H4 increased with the distance. Deadman coke size of H2 and H4 was larger than the coke size of H1, H5 and H3 at the same hearth height. That is, the average size of deaman coke particles in diameter direction distributes in “M-shape” in majority of the hearth-level regions except for the V1 region.

The average particle size of deadman coke at different hearth levels was shown in Fig. 5. The average size of charged coke was 54.9 mm. The average size of deadman

coke at the distance of 1.8 m, 2.5 m, 3.3 m and 5.0 m under the tuyere level was 30.2 mm, 25.6 mm, 27.6 mm and 31.3 mm, respectively. The feed coke size was observed to decrease by about 43%–53% in the descent to the hearth in all the studied hearth level locations in the present study.

3.2. Cold Strength of Hearth Coke of BF AParticle size distribution of deadman coke at different

hearth heights before and after drum test was shown in Fig. 6. It can be seen clearly that the percentage of coke size within 60–80 mm and 40–60 mm as well as that larger 80 mm decreased after drum test. A lot of small coke pieces (<40 mm) was produced, especially the coke size less than 10 mm, due to the fragmentation of larger coke after drum test. There was no difference in the amount of coke size within 25–40 mm at 1.8 m (V1), 2.5 m (V2) and 5.0 m (V4) from the tuyere level after drum test. However, the amount of coke size less than 10 mm increased with the increase of distance to the tuyere level, while that of coke size within 10–25 mm decreased with the distance.

The M40 and M10 of deadman coke after drum test was presented in Fig. 7. The M10 and M40 of the feed coke was 88% and 6%, respectively. However, the percentages of coke less than 10 mm after drum test (M10) at 1.8 m, 2.5 m and 5.0 m from the tuyere level was 11%, 15% and 18%, respectively, while that of coke larger than 40 mm after drum test (M40) was 79%, 76% and 75%, respectively. Compared with feed coke, the M40 of hearth coke was observed to decrease by about 13% and the M10 increased by 144%. The M10 increased with the increasing distance to the tuyere level while the M40 decreased with the distance. During the graphitization of coke, the crystalline state of coke changed from almost amorphous carbon to very well ordered graphitic carbon.14,17,18) Accordingly, the deteriora-tion of coke structure was intensified and coke powdering was more obvious. The graphitization of coke can be pro-moted by high temperature, long time of high temperature treatment and hot metal.14,17–19) Hearth coke was immersed in high temperature blast furnace slag and molten iron for a long time, which promotes its graphitization, reduces its strength and aggravates its pulverization. Therefore, it

Fig. 5. Average particle size of deadman coke at different layers.Fig. 6. Particle size distribution of deadman coke before and after

drum test. (Online version in color.)

Fig. 4. Average particle size of deadman coke at different posi-tion. (Online version in color.)


© 2019 ISIJ2001

results in the M10 of hearth coke being much larger than that of feed coke. Due to the catalytic effect of hot metal on coke graphitization,19) the M10 of deadman coke in the iron layer (V4) was increased by 63.6% comparing with the coke in the upper part (V1). Coke was highly graphitized due to long-term exposure to high temperature and hot metal immersion. It was inevitable that hearth coke in the iron layer was easy to pulverize, which is not conducive to the permeability of deadman. The graphitization of coke in the hearth started from the coke surface and the graphitiza-tion process led to the formation of coke fines. Therefore, the hearth should be kept active so that molten iron could pass through the gap between coke particles to dissolve the graphitized carbon on the hearth coke surface or coke powder into the molten iron. In this way it can reduce the amount of coke fines and promote the renewal of the dead-man to maintain good liquid permeability of the deadman.

3.3. Coke Size Distribution of BF BThe percentage of feed coke particles larger than 40 mm

and within 25–40 mm as well as that smaller than 25 mm was about 81%, 17% and 2%, respectively. Coke size dis-tribution of deadman at 1.5 m and 2.0 m below the tuyere level were presented in Fig. 8. The proportion of deadman coke larger than 40 mm was far less than that of feed coke, while the proportion of less than 25 mm deadman coke was obviously larger than that of feed coke, indicating a serious degradation occurs at these positions during its descend to the hearth level. The percentage of most deadman coke particles smaller than 40 mm varied from 67% to 89%. The percentage of <10 mm fines remained larger than 24% in majority of the hearth-level regions and the maximum per-centage could reach 49%. The percentage of hearth coke larger than 40 mm increased sharply while that of hearth coke particles smaller than 5 mm and within 5–10 mm as well as that within 10–25 mm tended to decrease from the sidewall to the blast furnace center in the radial direction.

The average particle size of deadman coke at differ-ent positions were shown in Fig. 9. As can be seen from the figure, the average size of coke at 1.5 m and 2.0 m below the centerline of the tuyere was 26 mm and 21 mm,

Fig. 7. M40 and M10 of deadman coke after drum test. (Online version in color.)

Fig. 8. Particle size distribution of deadman coke at different lay-ers. (Online version in color.)

decreased by about 51%, 63% compared with that of feed coke (53 mm), respectively. The particle size of deadman coke decreased with the increase of distance to tuyere level and distance to the center in the radial direction. The aver-age particle size of deadman coke in the center of the blast furnace was the largest. That is, the average size of deaman coke particles in diameter direction distributed in inversed “V-shape”.

3.4. Ash Yield of Hearth Coke of BF A and BF BThe ash yield of deadman coke at different locations

were shown in Fig. 10. A-DM1.8, A-DM2.5, A-DM3.3, A-DM5.0 and A-DM>5.0 stood for the coke obtained from 1.8 m, 2.5 m, 3.3 m, 5.0 m below the tuyere level and deadman bottom at BF A center, respectively. B-DM1.5, B-DM3.0, B-DM4.5 and B-DM4.8 stood for the coke obtained from 1.5 m, 3.0 m, 4.5 m and 4.8 m below the tuyere level at BF B center, respectively. The ash content of the coke at 1.8 m from the tuyere level of BF A and that at 1.5 m from the tuyere level of BF B was 10.75% and 7.81%, which decreases by 29.5% and 37.0% compared with the feed coke, respectively. As the distance from the centerline of the tuyere continues to increase (>=2.5 m), the content of minerals in the deadman coke increases significantly. The

Fig. 9. The average particle size of deadman coke at different positions.


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mineral content of deadman coke in BF A and BF B was 3.15–4.04 times and 3.12–4.21 times as much as that of feed coke, respectively. The hearth coke mass is 1.43–2.21 times of the feed coke under the same conditions in the majority of the hearth-level locations. The remarkable increase in the mineral content of the coke hearth greatly increased the gravity of the deadman, so that the floating height of the deadman was lowered compared with the hearth coke without the increase of ash content, and even the deadman sat on the hearth bottom. The designed salamander depth (h) of the most blast furnaces in China is about 20% to 23% of the hearth diameter, which is much lower than that of the Japanese huge blast furnace (about 25% to 30% of the hearth diameter).20–23) Since most Chinese designers do not consider the density variation between the hearth coke and the feed coke, it may make the deadman which is designed to float sit on hearth bottom. When the deadman is sitting on the bottom, the hot metal circulation is faster and the wall shear stress is larger near the bottom corners leading to serious erosion.5,24) Besides, the sitting deadman is not conducive to the regeneration and stable existence of the protective layer. The shallow salamander depth may be one of the main reasons for the short life of Chinese large blast furnace (about ten years). The depth of salamander after blow out of the blast furnace (hmax) at home and abroad is mostly about 28%–33% of hearth diameter,15,20–22,25,26) as shown in Table 2, indicating that the depth of salamander in this range is reasonable in this range. The increasing degree

of salamander depth (R=100(hmax−h)/h) after blow out of the blast furnace decreased with the increase of the ratio of the designed salamander depth (h) to hearth diameter (d), as shown in Fig. 11. When the ratio of the designed salamander depth (h) to hearth diameter (d) was greater than 24%, the increasing degree of salamander depth after blow out of the blast furnace was less than 23%. Consider-

Fig. 10. Ash yield of deadman coke at different locations (DM0.0: feed coke).

Table 2. Investigation on the depth of salamander after blow out of the blast furnace.15,20–22,25,26)

Item Campaign life/year d/m h/m hmax/m h/d/% hmax/d/% R/%

Baosteel No. 1 BF (2nd) 11.25 13.40 2.60 3.80 19.40 28.36 46.15

Baosteel No. 2 BF (1st) 15.17 13.40 1.80 4.10 13.43 30.60 127.78

Baosteel No. 3 BF (1st) 19.00 14.00 2.99 4.58 21.32 32.69 53.33

Wusteel No. 4 BF (3rd) 9.75 11.20 2.00 3.34 17.86 29.82 67.00

Wusteel No. 5 BF (1st) 15.67 12.20 1.90 3.70 15.57 33.04 112.12

Ansteel No. 11 BF (3rd) 6.00 11.50 2.00 3.90 17.39 31.97 83.81

Kimitsu No. 3 BF (1st) 11.00 13.60 1.00 4.50 7.35 33.09 350.00

Kokura No. 2 (2nd) 20.00 9.60 1.82 3.50 19.00 36.46 91.89

Germany BF 9.00 10.30 2.76 3.38 26.80 32.82 22.46

Fig. 11. Relationship between the increasing degree of salaman-der depth after blow out and the ratio of the designed sal-amander depth to hearth diameter. (Online version in color.)


© 2019 ISIJ2003

ing the increased content of deadman coke, maintaining the stability of the salamander depth, floating the deadman and reducing the circulation of molten iron, it is suggested that the salamander depth should be appropriately increased to more than 23% of the hearth diameter.

3.5. Deadman Porosity of BF AThe digital deadman image of BF A for calculation of

deadman porosity and its binary image were shown in Fig. 12. In the digital image, the black phase was coke and the grey phase was blast furnace slag. In the binary image, the black phase was coke and the white phase was blast fur-nace slag. The porosity of the deadman at different regions in the hearth was shown in Table 3. The overall average porosity of the deadman was 0.297. The average porosity of the upper area and the bottom area was 0.309 and 0.281, respectively. It indicated that the average porosity of dead-man decreased with the increase of the distance to the center line of the taphole. The porosity of the middle area and the centre area decreased with the increase of the distance to the center line of the taphole, while the porosity of the edge area increased with the increase of the distance. It is consistent with the previous findings.15) In the radial direc-tion, the average porosity of the deadman in the edge (E) region, middle (M) region and blast furnace center region (C) was 0.334, 0.299 and 0.250, respectively. The porosity of the deadman decreased with the increasing distance from furnace wall. The uneven distribution of the porosity of the deadman led to non-uniform distribution of velocity of mol-ten iron. That is, the velocity in the furnace center zone was low. In contrast, the velocity of molten iron near the side wall was high. In actual production, the length of raceway (0.8 m–2.5 m27,28)) was limited and the diameter of hearth was huge (the hearth diameter of some huge blast furnaces has reached 15.5 m20)). Accordingly it was difficult for the high temperature gas to reach the center zone of the hearth. Then, the temperature of the deadman in the furnace center zone was low and central deadman was easy to be inactive. Thus it further widened the small porosity area of the dead-man. At last, the circumferential flow was more remarkable, leading to the increasing sidewall abnormal erosion.

Therefore, improving the porosity of the central dead-man and make high temperature gas in the tuyere level pass through the center of the blast furnace play an important role in the longevity of the blast furnace. The porosity of

Fig. 12. Image for calculation of deadman porosity. (a) digital image; (b) binary image. (Online version in color.)

Table 3. Porosity of the deadman at different regions in the hearth.

Item Total region Edge (E) Middle (M) Center (C)

Upper 0.309 0.312 0.352 0.258

Bottom 0.281 0.356 0.243 0.241

Total region 0.297 0.334 0.299 0.250

the deadman was directly determined by the coke particle size distribution and the content of coke fines. Alkalis could destroy coke structure, reduce coke strength and break it into small pieces and fines.29,30) Thus, improving the quality of feed coke, reducing the alkalis load and making the high-temperature gas pass through the center of the blast furnace using the reasonable burden charging program were the core operation of extending the life of blast furnace hearth.

4. Conclusion

(1) The percentage of most hearth coke particles smaller than 40 mm varied from 64% to 94%. The percentage of <10 mm fines ranged from 20% to 49% in majority of the hearth-level regions.

(2) The average size of hearth coke was about 20 mm–31 mm. Compared with the feed coke, the hearth coke size was observed to decrease by 43%–63%.

(3) The average particle size distribution of coke in dif-ferent blast furnaces varied greatly. The average size of dea-man coke particles of BF A in diameter direction distributed in “M-shape” in majority of the hearth-level regions except for the V1 region. However the average size of deaman coke particles of BF B distributed in inversed “V-shape”.

(4) The M10 of hearth coke was about 11%–18% and the M40 was 75%–79%. The M10 increased with the increasing distance to the tuyere level while the M40 decreased with the distance. Compared with feed coke, the M40 of hearth coke was observed to decrease by about 13% and the M10 increase by 144%. Due to the catalytic effect of hot metal on coke graphitization, the M10 of hearth coke in the lower part was increased by 63.6% comparing with the coke in the upper part.

(5) The mineral content of the hearth coke in the upper part (below the tuyere level) was about 8%–11% which decreases by 29.5%–37.0% compared with the feed coke.


© 2019 ISIJ 2004

However, the mineral content of hearth coke in the lower part (below the tuyere level) was 3.15–4.21 times as much as that of feed coke. The hearth coke mass was 1.43–2.21 times of the feed coke under the same conditions in the majority of the hearth-level locations.

(6) The overall average porosity of the deadman was 0.297. The average porosity of the upper area and the bot-tom area was 0.309 and 0.281, respectively. It indicated that in the height direction, the average porosity of deadman decreased with the increase of the distance to the center line of the taphole. In the radial direction, the average porosity of the deadman in the edge region, middle region and blast fur-nace center region was 0.334, 0.299 and 0.250, respectively. The porosity of the deadman decreased with the increasing distance from furnace wall.

AcknowledgmentThis work was financially supported by the National

Key R&D Program of China (2017YFB0304300&2017YFB0304302).

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