Free Access
Issue
Metall. Res. Technol.
Volume 117, Number 1, 2020
Article Number 108
Number of page(s) 7
DOI https://doi.org/10.1051/metal/2020001
Published online 27 January 2020

© EDP Sciences, 2020

1 Introduction

Benefitting from the policy of reform and opening-up in 1978, the development of Chinese iron and steel industry has entered a fast lane. Figure 1 presents the variation on crude steel output in China from 1950 to 2018 [1]. Since the 21st century, the increase in steel production becomes booming rapid. The gross in 2018 reaches up to 928.3 million tons, 29.2 times that of 1978, and accounts for nearly 51.3% of the total production all over the world.

Large blast furnace (BF) technology is one of the most important contents of iron-making industry development [2,3]. According to Chinese industry standards, BFs with effective volume larger than or equal to 4000 m3 are classified as large BF. Taking Baosteel No. 1 BF as the symbolic starting point, technical accumulation of large BF has been going on for 34 years in China. With Rizhao No. 2 BF from Shandong Steel putting into production, the amount of large BFs has grown into 24 (Tab. 1) in China [4,5]. However, the average BF volume of China still exists an evident disadvantage in contrast with Western Europe and Japan. Statistics show that the average volume of 1500 Chinese BFs in 2015 is only about 770 m3, while those of Western Europe and Japan have already got to 2063 m3 and 4157 m3 in 2008, respectively [6]. Moreover, the technical and economical indexes of Chinese large BFs are still at a moderate level and the growth is comparatively slow except the ones from Baosteel and Shougang [7].

In order to clarify the future direction of technical development for large BFs, the production status, total reductant consumption, raw materials and operation principles indexes of 22 large BFs from 2015 to 2016 have been comprehensively analyzed in this work. Furthermore, a detailed discussion has been made on various affecting factors in the development of large BF and the corresponding optimization methods.

thumbnail Fig. 1

Crude steel production in China from 1950 to 2018.

Table 1

BFs over 4000 m3 BFs in China.

2 Main indexes of large BF in China

2.1 Utilization coefficient and total reductant consumption

Figure 2 shows the utilization coefficient and total reductant consumption of 22 large BFs with an average volume of 4586.59 m3. The average utilization coefficient is 2.06 t · m−3 · d−1, lower than that of BFs less than 4000 m3. This is similar to the situation of Japanese BFs at the same period [8], manifesting that the operation concept of BF ironmaking in China is changing from blindly pursuing high yield and maximize benefits in short-term to long life, low consumption and stable production. It is quite advisable and commendable that BF operators pay more attention to long-term benefits brought by stable operation, high quality of hot metal and long-term campaign of BF. With the continuously increasing requirements for energy saving and environmental protection in China, total reductant consumption concept of large BFs turns into "one high and two low", namely, high coal ratio, low coke ratio and low reductant ratio, because of the big difference on price between coal and coke [9]. The average coke ratio, coal ratio and reductant ratio of 22 BFs are 349.99 kg · t−1, 156.09 kg · t−1 and 513.75 kg · t−1, respectively. Among all the BFs, 4 BFs from Baosteel characterize lower total reductant consumption and higher replacement ratio with an average coke ratio of 311.28 kg · t−1, coal ratio of 175.05 kg · t−1 and reductant ratio of 486.32 kg · t−1. Even though compared with the BFs from developed country, these indexes of BFs from Baosteel also has already reached the advanced level. On the other hand, statistics in Figure 2 reveal that the reductant mix and consumption of most large BFs in China remain to be improved to a higher level.

thumbnail Fig. 2

Utilization coefficient and total reductant consumption of large BFs.

2.2 Raw materials

Raw materials quality is one of the most important influence factors in BF stability. The maintenance of raw materials of high quality without cost increase represents a lasting challenge to ironmaking enterprises [10,11].

2.2.1 Coke

With the development of oxygen-coal injection, replacement ratio of large BF presents a gradual upward trend, and the skeleton role of coke becomes more prominent than before. This entails a higher requirement for coke quality [12]. The larger BF volume, the higher coke standard, including high strength, large particle size, low sulfur content and low ash. Feeding coke of high CRI, small CSR, small M40 and high M10 will deteriorate the hearth activity due to the decrease in coke size from furnace hearth and the permeability of blast furnace. Figure 3 shows the main indexes of coke fed into large BFs. The average ash is 11.94%, sulfur is 0.69%, M40 is 89.58%, M10 is 5.67%, CSR is 69.11%, CRI is 22.67%, particle size is 51.38 mm and nut coke ratio is 35.64 kg · t−1. These indexes above have been significantly improved compared with the past [13] and sufficiently satisfy the requirements for large BFs smelting in China. However, from the further comparative analysis of specific data, we find that several relational indexes of coke are divergent. For example, Baosteel Zhanjiang coke’s M40 is lower than the average while CSR is higher, indicating that the coke indexes should be adjusted according to its own situation within a reasonable range.

thumbnail Fig. 3

Main coke indexes of large BFs.

2.2.2 Burden structure and Fe content

Due to the continuous increase of steel production, the external dependence of iron ore for China has reached 84% in 2015 [14] and a huge amount of iron ore has to be imported from abroad. BF burden is generally composed of a high percent basicity sinters or pellets and a small percent of lump ore. Figure 4 shows the burden structure of large BFs. The average proportions of sinter, pellet and lump ore are 71.98%, 19.56% and 8.46%, respectively. The average Fe content of sinter is 57.25% and the average comprehensive Fe content is 59.16%. Iron ore used in large BFs is superior to that in BFs less than 4000 m3, but still inferior to that of large BFs from developed countries [8].

thumbnail Fig. 4

Burden structure and Fe content of large BFs.

2.3 Operation principle indexes

2.3.1 Operation level

The indexes shown in Figure 5 reflect the operation levels of various large BFs. The studied 22 BFs have the average air consumption of 1066.47 m3 · t−1, gas utilization rate of 48.53%, coke load of 5.1, first-grade hot metal of 76.07% and precipitator dust ratio 14.49 kg · t−1. These data manifest that the average operating level of Chinese large BFs evidently lags behind Europe, America, Japan and South Korea [15,16]. In particular, although air consumption and precipitator dust ratio both present higher, gas utilization, coke load and first-grade hot metal are all lower. Compare Figure 5 with Figure 2, it is not difficult to find that there is a strong correlation between gas utilization rate and reductant ratio. The reductant ratio decreases with the increase in gas utilization rate [17]. Take the 4 BFs from Baosteel as examples, their average reductant ratio of 486.32 kg · t−1 is highest among all the BFs in China, while the average gas utilization rate of 51.51% is lowest. Besides that, the utilization rate is closely related to coke load. And the two indexes of large BFs from Baosteel represent the highest level in China.

thumbnail Fig. 5

Main operation level indexes of large BFs.

2.3.2 Blast air principle

The average blast air temperature and oxygen enrichment rate of 22 large BFs are 1206.24 °C and 3.4%, respectively (Fig. 6). Although it is not as high as abroad, it is reasonable enough to ensure the safety and longevity of BF [18]. As we know, there is a reasonable position of tuyere raceway for each BF corresponding to the unique smelting situation. Hearth activity tends to decrease as tuyere raceway deviates away from the position [19]. Blast air speed plays an important role on the position of tuyere raceway [20]. The average blast air speed of large BFs in this study is 265 m · s−1. For large BF with a hearth diameter over 10 m, blast air speed should be in a range of 260–280 m · s−1 to blow through BF hearth center. The average pressure difference of 160 Kpa is lower than before but pretty well. It indicates that large BFs operation concepts have changed from high smelting strength to safe and stable production.

thumbnail Fig. 6

Main blast air principle indexes of large BFs.

2.3.3 Thermal principle

Thermal principle signifying the heat and temperature level of BF hearth regulates the equilibrium state of heat income and expense [21]. Stability of thermal principle is the premise to keep the production steady. Either too high or too low temperature of hearth will cause fluctuations in furnace, such as burden slipping or hanging. Hearth heat as the main thermal principle index consists of physical heat determined by the temperature of hot metal and chemical heat related to silicon content in hot metal. The average temperature and silicon content of hot metal from 22 BFs are 1502.38 °C and 0.43%, respectively. As shown in Figure 7, thermal principles of most BFs are far away from low-silicon smelting. The hot metal temperatures of some BFs are even less than 1490 °C, which is not conducive to the long-term stability and needed to increase 15–20 °C.

thumbnail Fig. 7

Main thermal principle indexes of large BFs.

2.3.4 Slag principle

Slag principle varied with the raw materials and hot metal composition involves slag fluidity, slag stability, slag sulphide capacity and position of soft melt zone [22]. As a main product from BF, slag plays an even more important role in BF smooth and hot metal quality than hot metal [23]. The slag indexes of various large BFs are presented in Figure 8. The average slag ratio, basicity, MgO content, Al2O3 content, MgO/Al2O3 are 301.71 kg · t−1, 1.18, 7.49%, 14.14%, 0.53, respectively and those of 4 BFs from Baosteel are 254.53 kg · t−1, 1.23, 6.65%, 15.03%, 0.44, respectively. By comparison, the latter are more reasonable. Although raw materials fed into each BF are different, slag ratio and Al2O3 content should be as low as possible and MgO/Al2O3 in an appropriate range according to the BF situation.

thumbnail Fig. 8

Main slag principle indexes of large BFs.

2.3.5 Charging principle

At present, there are two modes of charging principle for large BF in China: non-central coke charging and central coke charging [24]. The former takes the advantages of high gas utilization rate and low total reductant consumption, and the latter allows fluctuations in raw materials. Nevertheless, central coke charging always results in low gas utilization rate and high total reductant consumption. Gas utilization rates are generally in a range of 50.5–52.5% with non-central coke charging and decline to 46.5–48.5% with non-central coke charging. Four percent of gas utilization rate corresponded to more than 20 kg · t−1 total reductant consumption. In fact, the similarity between the two modes is to maintain central gas flow by placing some coke in BF center directly or indirectly.

To further illustrate this point, a detailed comparison of two domestic BFs with a same volume of 4747 m3 but different charging modes is made as below. The burden distribution matrices of the two BFs are shown in Table 2 and Figure 9.

As shown in Figure 9, one BF with central coke charging is marked as BF A and the other with non-central coke charging as BF B. The main production indexes of two BFs are shown in Table 3. It is clear that the performance of BF B is obviously better than BF A. The reductant ratio difference between the two is 33 kg · t−1. The production indexes are closely related to charging principle.

In order to reduce total reductant consumption, BF A has attempted to change the charging principle from central coke charging to non-central coke charging during June-July 2016. The practice experiment process is as follows: firstly, reduce the edge ores gradually; secondly, reduce the central coke and adjust the width of the coke platform; thirdly, completely cancel central coke. In the early stage of the process, the indexes have been evidently improved. The gas utilization rate increases to 50% and reductant ratio decreases to 495–500 kg · t−1. However, as the central coke have been completely canceled, BF A becomes volatile. Blast air pressure is too high to support normal blast air volume and central air flow decreases obviously. Also, the number of slipping and hanging increases. In order to maintain the stability and safety of BF A, central coke charging has to be reused. Thus, the practice experiment has been unsuccessful.

Reason for the failure is that the essences of the two charging principles are not clear. As shown in Figure 10, although non-central coke charging does not directly place coke in the BF center, large-grained coke is much easier to roll along the slope to the center funnel due to the crushing and rolling effects on coke from ore. Consequently, coke granularity in BF center is even larger than the middle and edge. Compared with central coke charging, central coke granularity with non-central coke charging is larger so that strong central gas flow is able to hold which is conductive to improve the breathability and permeability of deadman. From the above analysis, it is reasonable to adopt non-central coke charging to persue better hearth activity. The reason why non-central coke charging has been unsuitable to BF A is that the coke quality fed into BF A does not meet the corresponding requirements. The amount of large-grained coke is not adequate so as to form enough breathable and permeable channels.

Table 4 shows the main indexes of raw materials used in BF A and BF B. The quality of raw materials from BF B is better than BF A, especially in the amount of small granular coke and sinter. In this situation, a high rate of ore powder will arise in BF center if non-central coke charging is used. And Small granular ore or powder tends to mix in coke layer, which badly worsens the breathability and permeability of deadman. Based on the above analysis, we believe that non-central coke charging is not applicable to BF A unless the average granularity of coke increases to 51.50 mm, M10 reduces to 5% and sinter < 5 mm reduces to 4.0%. However, there is no large ground for material mixing in the company of BF A and quality of raw materials hardly keep long-term stable. Thus, it is too difficult for BF A to adopt non-central coke charging.

Table 2

Burden distribution matrices of BF A and BF B.

thumbnail Fig. 9

Burden distribution of BF A and BF B.

Table 3

Main production indexes of BF A and BF B.

thumbnail Fig. 10

Schematic diagram of coke crushing and rolling effect by non-central coke charging [25]. Note: Green was ore, blue was coke.

Table 4

Main raw materials of BF A and BF B.

3 Advice for large BF development

3.1 Maintaining stable

Stable state of BF has extremely important effects on utilization coefficient, total reductant consumption, gas utilization rate and other production indexes. It is not only the basic target of all operation principles, also the basis to realize high efficiency, high quality, low consumption and long campaign of large BF. To maintain stable state of large BF, in addition to the stability in raw materials, a comprehensive optimization on operation principles such as blast air, thermal, slag and charging should be continually performed.

3.2 Improving the quality of raw material

In order to decrease total reductant consumption and slag ratio, the quality of raw materials used in large BFs should be improved. The average pellet percent of Chinese large BFs is only 19.56% much lower than that of Europe and America large BFs. It is essential to increase the pellet percent in burden. On the other hand, coke quality also should be promoted for that of several large BFs below the nationwide average level.

3.3 Improving the operation principles

Blast air principle: for large BF with a hearth diameter over 10 m, blast air speed should be in a range of 260–280 m · s−1 to blow through BF hearth center. Blast air temperature of some large BFs lower than the average level should be increased by optimizing operation parameters of hot stove and redesigning its structure.

Thermal principle: low-silicon smelting is an effective way to reduce total reductant consumption, which requires silicon content of 0.15–0.25% and hot metal temperature of 1500–1520 °C. Several methods do work on increasing hearth heat, such as increasing slag basicity, raising blast air temperature and so on.

Although raw materials fed into each BF are different, slag ratio and Al2O3 content should be as low as possible and MgO/Al2O3 in an appropriate range according to the BF situation.

Charging principle: no doubt non-central coke charging represents the future direction of BF development because of its distinct advantages. Non-central coke charging requires high-quality and stable raw materials. Thus, it is essential to improve raw materials and conduct practice experiments before completely adopting non-central coke charging.

4 Summary

In this paper, the production situation, total reductant consumption, raw materials, operation principles indexes of large BFs in China from 2015 to 2016 have been elaborated. The developing actuality of Chinese large BFs has been quantitatively analyzed and the existing problems have been proposed clearly. Based on that, maintaining production stable, enhancing the quality of raw materials and improving the operation principles are suggested. Large BFs take advantages of high-quality hot meta, energy saving, cost-cutting, high mechanization and automation levels and are sufficient to meet the challenges of economic crisis, environmental pressure and security risk from the future.

Acknowledgements

The authors would gratefully express their gratitude to the operation indexes of large BFs from China Iron and Steel Association.

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Cite this article as: Bing Dai, Hong-ming Long, Yong-cai Wen, Yi-long Ji, Yun-cai Liu, Development and production of large blast furnaces from 2015 to 2016 in China, Metall. Res. Technol. 117, 108 (2020)

All Tables

Table 1

BFs over 4000 m3 BFs in China.

Table 2

Burden distribution matrices of BF A and BF B.

Table 3

Main production indexes of BF A and BF B.

Table 4

Main raw materials of BF A and BF B.

All Figures

thumbnail Fig. 1

Crude steel production in China from 1950 to 2018.

In the text
thumbnail Fig. 2

Utilization coefficient and total reductant consumption of large BFs.

In the text
thumbnail Fig. 3

Main coke indexes of large BFs.

In the text
thumbnail Fig. 4

Burden structure and Fe content of large BFs.

In the text
thumbnail Fig. 5

Main operation level indexes of large BFs.

In the text
thumbnail Fig. 6

Main blast air principle indexes of large BFs.

In the text
thumbnail Fig. 7

Main thermal principle indexes of large BFs.

In the text
thumbnail Fig. 8

Main slag principle indexes of large BFs.

In the text
thumbnail Fig. 9

Burden distribution of BF A and BF B.

In the text
thumbnail Fig. 10

Schematic diagram of coke crushing and rolling effect by non-central coke charging [25]. Note: Green was ore, blue was coke.

In the text

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