Coal-based needle coke

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Production process of coal-based needle coke. Needle coke is the main raw material for manufacturing high-grade graphite electrodes. 　　It is divided into oil-based and coal-based types depending on the raw material. Acicular coke produced from petroleum heavy oil is of the oil-based type, while acicular coke produced from coal tar pitch and its fractions is of the coal-based type. The two processes for producing needle coke are not exactly the same, but their uses are basically identical. 　　The United States was the first to master the production technology for petroleum-based needle coke in the late 1950s. As oil processing moves toward lighter-grade, advanced processing methods such as catalytic cracking, the supply of oil-based needle coke raw materials has decreased. Coupled with the two oil crises in the 1970s, this further heightened concerns about the instability of raw material supplies. Thus, since the 1970s, countries such as Japan and Germany have **been committed to developing coal-based needle coke technology. In 1979, the industrial production of coal-based needle coke in Japan led to a coexistence of the oil-based and coal-based needle coke markets. The development of needle coke technology in China started relatively late. In recent years, with the development of the domestic electric arc furnace steelmaking industry and advancements in electrode production technology, the demand for needle coke has been increasing year by year, and significant progress has also been made in needle coke production technology. In the mid-1990s, industrial plants for coal-based needle coke and petroleum-based needle coke were successively built and put into operation. The coastal chemical coal-based acicular coke utilizes patented technology from Anshan Coking Refractory Materials Design and Research Institute. The installation was first constructed in April 1992, and by June 1994, its construction was completed; thereafter, testing began along with research on industrialization techniques based on the original patented technology. After going through processes such as production trials, plant modifications, and technical improvements, in 1998 industrial production of coal-based needle coke overcame the challenges related to smooth operation, enabling continuous production; the product was used to manufacture high-grade graphite electrodes. The coal-based needle coke production process is briefly introduced below. 　　1 Process Flow The process for producing needle coke in coastal chemical industries includes three steps: raw material pretreatment, delayed coking, and calcination (the process flow is shown in Figure 1). The material selected is coal tar pitch and its fractions from Ansteel’s coking plant. 　　 　　Figure 1 Schematic diagram of the coal-based needle coke production process 1.1 Raw material pretreatment Under certain heating conditions, the asphalt molecules undergo oligomerization and dehydrogenation polycondensation reactions, thereby forming polycondensation products of planar macromolecules. The larger the molecular weight, the stronger the intermolecular van der Waals forces. These planar macromolecules undergo aggregation and nucleation processes to form larger spheres. Spherulites are optically anisotropic, whereas the bitumen matrix from which they form is isotropic; once formed, spherulites can be observed under a polarizing microscope. The surface energy of a newly formed sphere system is minimal. Such small spheres have a higher surface tension than the parent pitch, whose isotropic molecular weight gives rise to them. Therefore, when the two small spheres meet, the planar macromolecular layers interpenetrate into each other, and the small spheres merge, bringing the system to a more stable thermodynamic state. Even after merging, they remain spherical in order to maintain the system’s minimum surface free energy. As the balls merge multiple times, their diameter gradually increases. When the diameter becomes large enough that surface tension can no longer maintain its spherical shape, the sphere begins to disintegrate, forming clusters of continuously flowing tissue, a process known as phase transition. This substance formed after the disintegration of asphalt globules is called the mesophase. The substance that makes up the spherules is also an intermediate-phase substance, which is why they are called intermediate-phase spherules. Mesophase spherulites are the basic substances through which needle coke is oriented to form coke. However, coal tar pitch contains certain impurities (including native QI), which adhere to the periphery of the mesophase and hinder the growth and merging of spherical crystals. Even after coking, needle coke with a good fiber structure cannot be obtained. Therefore, the purpose of pre-treating coal tar pitch raw materials is to first remove the impurities that facilitate the growth of harmful spherical particles, and then to modify its composition through heat treatment in order to obtain raw materials suitable for the production of needle coke; this is both the goal of raw material pre-treatment and a necessary condition for producing needle coke from coal tar pitch. 　　1.2 Raw material pretreatment methods
In the production of needle coke by coastal chemical plants, a upgrading process is employed to heat-treat the raw materials without adding any solvents. Impurities in the raw materials (including native quinoline-insoluble substances) are removed through flash distillation. Subsequently, the purified raw material oil undergoes further thermal treatment to obtain an appropriate composition that meets the requirements for producing needle coke. At the same time, products such as carbon black feedstock oil or electrode pitch are also produced. 　　However, the key to this technology lies in determining appropriate heat treatment conditions during production—conditions that can effectively control the degree of reaction, adjust the content of the effective component in the raw material—the β-component—to the desired level, while also ensuring continuous and smooth production. Due to the high aromaticity of coal tar pitch and its fractions (over 90%), their reactivity is low; as a result, their reaction behavior is difficult to control during heat treatment. At low temperatures, reactions are less likely to occur. Once the reaction conditions are met, the reaction rate becomes very fast, and the viscosity of the material increases sharply, leading to a deterioration of the heat treatment conditions and preventing the production system from operating for extended periods of time. During the heat treatment process, the variation of material properties with temperature has a certain impact on the continuous operation of the system, as shown in Table 1. 　　Table 1 Effect of Heat Treatment Temperature on Material Properties and Operating Cycle
Performance Parameters
Heat Treatment Temperature/°C: ≤410, 410–420, 430–440, 450
β-Component Content/%: Below 2.0, 3.0–7.0, 10, 20
System Border Operating Cycle: 20 days, 10 days, 1 week
E100 (Engler viscosity): ≤1.70, ≤2.50
Softening Point/°C: 10, 20, 1.3
Delayed Coking Conditions
The maximum temperature at the outlet of the tubular furnace is generally kept at no more than 510 °C. The temperature inside the tower must remain above 460 °C for more than 6 hours. Since the coking mechanisms of needle coke and pitch coke are not exactly the same, it is necessary to select operating parameters correctly to meet the thermal conversion process of mesophase spheres and the conditions for the formation of needle coke. Table 2 shows the changes in the quality of three batches of needle coke made from the same raw materials under different process conditions. 　　As can be seen from Table 2, the needle coke of the three batches had basically the same raw material conditions, but there were significant differences in quality. This shows that, under basically identical raw material conditions, the key to the quality of needle coke lies in the optimal selection of process parameters. In coking production, it is necessary to take into account the effects of various factors such as temperature, pressure, circulation ratio, and heating rate on the quality of the coke, as well as the directional influence of the gas velocity within the tower on the structure of the coke. 　　Table 2 Quality characteristics of needle coke from different batches using the same raw materials Index I 9-5 Index II 9-7 Index III 9-8 True density/(g/cm3): 2.11, 2.12, 2.12–2.13 Ash content/%: 0.26, 0.22, 0.22 Volatiles/%: 0.6–0.75, 0.6–0.7, 0.4–0.55 CTE/(10-6/°C) (room temperature–600°C): 2.13, 1.82, 1.64 Appearance (distinct needle structure)/%: 50, 70, 85 Preliminary experimental studies show that, during the initial stage of the coking reaction, operating at a relatively high pressure and then reducing the pressure in the coking tower at a certain rate during the later stages results in needle coke of better quality, along with higher yields of coke. Analysis suggests that maintaining a high pressure in the tower during the initial stages of coking is beneficial for the anisotropic development of the mesophase. Under such conditions, more volatile substances remain in the coking tower, and these substances help to moderate the coking reaction through dissolution or hydrogen transfer, thereby keeping the viscosity of the coking material low. This facilitates the full growth and merging of mesophase particles. In the later stages of coking, reducing the pressure at a certain rate encourages large mesophase molecules to release gas along specific pathways during solidification. By achieving a uniform gas flow rate for the gas removal process, needle coke with high crystallinity can be produced. 　　The recycle ratio (R) is also a major process parameter in delayed coking production. The choice of R size is related to the properties of the raw material. The value of R varies depending on the choice of raw material; therefore, it is only possible to compare the impact of R on coking production when considering materials of the same type. 　　Furthermore, based on the production experience of Mitsubishi in Japan, the concentration of coking substances in the crude oil fed into the coking tower (Conradson carbon number) affects the crystallization rate, which in turn influences the quality of needle coke. Therefore, it is necessary to control the Conradson carbon value of the feed material entering the coking tower; generally, it should be no more than 30%. Secondly, variations in the conditions throughout the tower lead to differences in the reaction of the feed materials inside it, resulting in uneven quality of the coke formed at different locations within the tower. Some research has also been conducted on this. Generally, the coke quality is best in the middle or upper-middle part of the tower, where the fiber structure is clear and the pores are uniform. The coking quality at the lower part of the tower is the worst, accounting for about 15% of the total coking amount in each tower. 　　1.4 Study on the calcination process The raw coke produced in the coking tower has a true density of 1.40–1.42 g/cm3, with a volatile content of 7%–9%. In this life, needle coke needs further heating treatment to ensure that its various physical and chemical properties as well as its electrical conductivity meet the requirements for raw materials used in graphite electrodes. 　　Experimental studies indicate that during the calcination of carbon materials, the combined effect of the emission of volatiles and changes in molecular structure leads to an improvement in the electrical conductivity of the calcined material. The increase in the true density of the calcined material is mainly due to the continuous release of volatiles from the material at high temperatures, along with decomposition and polycondensation reactions, which result in structural rearrangement and volume contraction. Therefore, for the same amount of char formation, the higher the calcination temperature, the lower the volatiles in the calcined coke, the higher its true density, and the better the quality of the needle coke (see Table 3). 　　Table 3 Changes in volatiles and true density of needle coke with temperature Calcination temperature/°C: 1100–1200, 1200–1300, 1350±50, 1450±50 Volatiles/%: 0.75–0.90, 0.7–0.80, 0.6–0.7, ≤0.55 True density/g/cm³: 2.04–2.08, 2.09–2.10, 2.11, 2.12–2.13 However, if the calcination temperature is too high, it is limited by the quality of the refractory materials in the calcination furnace. Therefore, when calcining needle coke in a tank furnace, it is necessary to take into account both the requirement to ensure the quality of the needle coke and the service life of the calcination equipment. The calcination temperature should not exceed 1,500 °C; it can generally be strictly controlled at (1450±50) °C, although this requires considerable skill to achieve. The residence time of needle coke in the calcination zone is more than twice that of pitch coke. On the premise that the quality of raw coke remains stable, the quality of needle coke generally stays stable as well. 　　2 Prospects for domestic coal-based needle coke Comparison with the quality of Japanese coal-based needle coke (see Table 4). A comparison with the microstructure of Japanese coal-based needle coke is shown in Figure 2. Table 5 shows the X-ray diffraction results for the (002) crystal plane. The quality of the fabricated electrodes is shown in Table 6. 　　Taking all the above factors into account, compared with Japanese needle coke, the main problems of domestically produced coal-based needle coke are its higher CTE and resistivity. 　　Table 4 Comparison of Quality Parameters with Japanese Coal-Based Needle Coke Item Japanese Needle Coke* Coastal Chemical Needle Coke Nippon Steel Mitsubishi True Density/g/cm3 2.10–2.12 2.12–2.14 2.12–2.13 Sulfur Content/% 0.3–0.4 0.2–0.3 0.2–0.4 Ash Content/% 0.20–0.50 0.08≤0.25 Volatiles/% 0.6–0.9 0.6–0.9 0.4–0.6 CTE/(10-6/°C) △1.70–1.90 1.70–1.90 2.60–2.80 Note: *Actual measurement results for imported needle coke in 1998 and 1999 ； △Baked at 1100℃, tested at room temperature to 600℃. 　　Table 5 Results of X-ray diffraction measurements for the (002) crystal plane
Serial No. 2θ/° Crystal Plane (nm) Half-width of diffraction peak/° Lc/nm
1# 25.75 30.3456 2.67 35.0874
2# 25.6160 0.3475 2.69 63.1531
3# 25.7140 0.3465 2.08 34.0808

Table 6 Results of X-ray diffraction measurements for the (002) crystal plane
Specifications: Volume density/g/cm³, Flexural strength/MPa, Elastic modulus/GPa, Ash content/%, CTE/(10-6/℃), True density/g/cm³, Resistivity/μΩ·m. Source of data:
Φ250mm: 1.71, 18.08, 11.79, 5.86 – Datong Carbon Factory
Φ300mm: 1.71, 14.04, 9.91, 10.05, 2.13, 2.22, 7.2 – Shanghai Carbon Factory
Φ350mm: 1.39, 9.86, 7.0, 0.08, 1.75, 5.4 – Fushun Carbon Factory
Φ500mm: 1.66, 11.87, 8.64, 0.05, 2.13, 2.24, 7.6 – Lanzhou Carbon Factory

Figure 2 X-ray diffraction analysis spectra of coal-based needle coke
1#, 3# – Haian Chemical Needle Coke ； 2# – Nippon Steel needle coke Preliminary studies suggest that reducing CTE and resistivity should start with adjusting the structure and properties of the raw materials. By matching more suitable raw materials with optimized process conditions, the crystallinity of needle coke can be further improved, resulting in needle coke with lower CTE and resistivity. 　　Furthermore, in addition to being directly related to the quality of needle coke used for the electrodes, the resistivity is also dependent on the conditions of the graphitization process. If too much electrode material is loaded into the furnace, the current density at the furnace core will be low ； Poor furnace insulation, high heat loss, and uneven filling of the resistive material can lead to current imbalance, resulting in insufficient temperature inside the furnace; these factors can also cause an elevated resistivity. Therefore, in the carbon industry’s production of high-quality graphite electrodes, it is necessary not only to conduct research on and select the appropriate quality of needle coke used, but also to study the electrode manufacturing processes, technologies, and equipment, as well as their compatibility with domestic raw materials, in order to accelerate the localization of raw materials for high-grade graphite electrodes.