Research progress on the preparation and application of needle coke

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Abstract: Needle coke is a carbon material with excellent properties such as highly graphitizable and good thermal shock resistance. It is widely used in the fields of graphite electrodes and lithium ion battery anode materials. This article introduces the preparation methods and application status of needle coke, and prospects for future research directions.

Needle coke is a silver-white solid carbon material with metallic luster, which has obvious needle-like texture structure when observed under a microscope. Because of its low thermal expansion coefficient, high electrical conductivity, and high chemical stability, it is widely used in high-performance graphite electrode materials, lithium ion batteries, electrochemical capacitors and other fields. At present, only the United States, Japan and other countries can produce high-quality needle coke on a large scale. In recent years, my country's needle coke preparation technology has made major breakthroughs and has basically realized industrialized production, but the quality is still far behind foreign high-quality needle coke.

1 Preparation of needle coke

The preparation of needle coke is divided into three stages: raw material pretreatment, delayed coking and calcination. The main task of raw material pretreatment is to remove the quinoline insolubles in the raw materials that are not conducive to the development of the mesophase to obtain refined raw materials; delay the process of coking that is to carbonize the refined raw materials to generate mesophase. The quality of needle coke is directly affected by the formation process of the mesophase and its structural morphology. Therefore, by controlling delayed coking conditions, obtaining a wide-area streamlined mesophase with stable performance is the key to preparing high-quality needle coke; calcination is a process of high temperature treatment of the mesophase produced by delayed coking at around 1450 ℃. High temperature treatment can remove the volatile matter and moisture, and at the same time increase the degree of graphitization and then increase the density, conductivity and chemical stability. The needle coke obtained through this process can be used as the raw material for high-power graphite electrodes.

1.1 No physical disturbance method

1.1.1 Single raw material preparation

The raw materials of traditional synthetic needle coke are mainly coal-based and oil-based. Coal-based raw materials mainly refer to coal tar pitch, coal tar, etc.; oil-based raw materials mainly include thermal cracking residue, ethylene cracking residue, etc. In traditional needle coke production processes, a single coal-based or oil-based raw material is often used for pretreatment to produce needle coke. In recent years, laboratory studies have discovered methods for obtaining high-quality needle coke and mesophases such as modification methods.

(1) Direct refining method

The requirements for raw materials of needle coke are shown in Table 1. Aromatics or heavy hydrocarbons that easily produce aromatics are essential components for preparing needle coke raw materials. Among them, the linearly connected three to four ring short side chain aromatic hydrocarbons are high-quality raw materials for the preparation of needle coke. Impurities such as ash and quinoline insolubles affect the reaction rate of the coking process and form a mosaic structure; sulfur atoms not only affect the growth of mesophase spheres during delayed coking, but also escape during the calcination stage, resulting in needle coke crystals. Irreversible expansion, lower graphitization degree, lower needle coke performance. Traditional needle coke raw materials need to be refined before they can be used in the preparation of needle coke. The oil-based needle coke raw materials have less impurities and are generally removed by sedimentation and separation.

The main purpose of refining coal-based raw materials is to remove quinoline insolubles. Quinoline insolubles will affect the fusion of mesophase spheres, causing the optical structure of the product to become mosaic structure and the performance of needle coke is reduced. The methods for removing quinoline insolubles mainly include solvent method, vacuum distillation method, and modification method.

Sun Yanrui and others used the mixed solvent method to successfully reduce the quinoline insoluble content in the medium temperature coal pitch to below 0.1%. Tang Shibo et al. successfully reduced the quinoline-insoluble content of high-temperature coal tar to 0.178% by solvent sedimentation method using xylene and mineral spirits as solvents at a solvent ratio of 1, an aromatic-to-lipid ratio of 0.6, and sedimentation at 75 ℃ for 2 h. The removal rate reached 87.7%.

After the coal tar pitch was fully dissolved by the mixed solvent, Xing Guozheng et al. inserted the electrode sheet into the solution and applied high-voltage direct current. The insoluble materials such as quinoline insolubles and ash were deposited on the electrode sheet under the action of the electric field. The solution is distilled off to obtain refined coal pitch. Cao et al. used a high-voltage electrostatic field to remove quinoline insolubles and ash from coal tar pitch, and successfully obtained a refined coal pitch with no quinoline insolubles and low ash content. Comparing this method with the commonly used centrifugal method, it is found that the activity of the products of the two methods is almost the same, but the energy consumption of the high-voltage electrostatic field method is 1/12 of that of the centrifugal method, which has extremely high application value. Sun Zhenxing and others used solvent centrifugation to treat medium-temperature coal pitch and obtained refined pitch with a quinoline insoluble content of 0.03% and a yield of more than 84%.

(2) Modification method

Using other substances to modify the asphalt-based raw materials can increase the reactivity of the raw materials, promote the formation of the mesophase streamline optical microstructure, and ultimately improve the performance of needle coke. Yang Haixiao et al. used thermal bromination/debromination polymerization to convert refined low-temperature coal tar pitch into high softening point pitch, and prepared 95% optically anisotropic wide-area mesophase from the debrominated pitch.

Chai Yunjie and others used trichloronitrobenzene as modifier and toluene sulfonic acid as catalyst to modify refined coal tar pitch. The aromatics polymerization reactivity of the raw material was successfully improved, and the order of the obtained semi-coke and needle coke was significantly improved.

Su Lei et al. used chlorinated aromatic hydrocarbons to modify the refined medium temperature coal pitch under the catalysis of methylbenzene sulfonic acid, so that the fiber composition in the optical structure was improved and the carbonization yield was significantly improved.

(3) Preparation of special raw materials

With the continuous advancement of research, the raw materials for the preparation of needle coke are no longer just pitch materials. Mochida et al. discovered that pure aromatic hydrocarbons, such as naphthalene and phenanthrene, can also be converted into needle coke under the catalysis of aluminum chloride. Qin Zhihong et al. found that the dense intermediate mass obtained by separating coal according to the extraction and stripping method is a high-quality raw material for the preparation of mesophase small spheres. Shan Liang et al. used the coal dense intermediate mass to successfully prepare a mesophase with excellent optical properties under normal pressure. Semi coke provides a new solution for the preparation of needle coke.

1.1.2 Co-carbonization method

The content of quinoline insolubles in coal pitch is too high, and the content of macromolecular aromatic hydrocarbons is too high, which will cause the reaction viscosity to be too high during the coking process, which is not conducive to the reaction and the orderly arrangement of molecules, and finally leads to mosaic structure; in petroleum pitch The higher content of alkanes is beneficial to adjust the viscosity of the reaction system, but the ash and sulfur content exceeds the standard due to the addition of catalysts in the production process of petroleum products and the carryover of petroleum production. Therefore, a single coal pitch or petroleum pitch must be pretreated by raw materials to meet the requirements of raw materials for needle coke preparation, so the co-carbonization of raw materials with complementary properties is an effective means to solve the problem.

(1) Co-carbonization of traditional raw materials

Mochida et al. used FCC slurry and petroleum low-sulfur vacuum residue for co-carbonization, and obtained high-quality needle coke with a thermal expansion coefficient of only 0.1×10-6/℃. Dong Yawei and others co-carbonized brominated petroleum pitch (BPP) and refined coal pitch (RCTP) in the presence of benzoyl chloride to produce semi-coke. Ma Wenming et al. co-carbonized FCC slurry and coal tar pitch, used the rich aromatic components in coal tar pitch as the main raw material for coking, and used naphthenic and fatty side chains in FCC slurry to reduce system viscosity and provide coking. The raw material of air flow successfully produced needle coke with dense structure and obvious fiber characteristics, as shown in Figure 1.

(2) Polymer method

Part of the polymer can effectively provide alkyl components for the reaction system, reduce the viscosity of the reaction system, provide a bridged alkyl structure, and promote the formation of streamlined optical structures. Cheng et al. used ethylene tar pitch and waste polystyrene as raw materials for co-carbonization. Polystyrene provided a large amount of naphthenic and other alkyl components for the system, changing the optical structure of the finished product from a rough mosaic type to a streamlined type, and at the same time the thermal expansion coefficient decreased. It is 1/10 of the product without polystyrene, and successfully produced high-quality needle coke. Hu et al. used waste butadiene rubber to modify petroleum pitch to increase the content of alkanes, olefins and aromatics in petroleum pitch and promote the formation of mesophase. In the process of co-carbonization using petroleum pitch and waste polyethylene, Cheng et al. interacted to produce a large number of naphthenic structures and methylene bridges, which promoted the reaction and obtained a mesophase with a streamlined structure content of 100%. Lou et al. used alkyl bitumen and polystyrene to co-carbonate to prepare a mesophase with good optical structure and regular crystal structure.

(3) Nucleating agent method

The addition of mesophase carbon materials can significantly improve the coking process. These carbon materials may act as "nucleating agents" in the mesophase formation process, and promote the formation of mesophase spheres. Zhang et al. added the naphthyl mesophase pitch to the refined coal pitch for co-carbonization. After the appropriate amount was added, the formation of small spheres was accelerated during the cracking process, which improved the yield and aromaticity of the mesophase. Qin Bin et al. added an appropriate amount of mesocarbon microspheres (MCMB) to the refined coal tar pitch. The MCMB reacted with the polycyclic aromatic hydrocarbon molecules in the refined coal tar pitch to increase the MCMB. After that, the MCMB merged and finally broke. Form an ordered layer structure. The final prepared needle coke has good optical structure performance, the thermal expansion coefficient and resistivity are greatly reduced, and the graphitization degree is significantly improved.

(4) Separation and coordination method

Firstly, the raw materials are separated by extraction family components, and then carbonization or co-carbonization of different family components is carried out to explore the suitable raw material composition for preparing needle coke. Commonly used reagents are benzene, toluene, n-heptane and so on. Mochida et al. used benzene to divide the pitch into two groups of benzene-soluble and benzene-insoluble components, and then carried out co-carbonization. The study found that the ratio of the two has a greater impact on the product performance, and the higher benzene-soluble components are beneficial to the formation of needle coke. , But too little benzene insoluble content will cause the yield to decrease.

Li et al. used toluene and n-heptane to distinguish alkyl petroleum pitch into n-heptane-soluble matter, n-heptane-insoluble-toluene soluble matter, and toluene-insoluble matter. First, compare the carbonized products of the 3 race components with the carbonized products of naphthenic petroleum pitch. Through calculation, it is found that the sum of the mesophase content of the carbonized products of the 3 race components is not equal to the mesophase content of the carbonized products of the naphthenic petroleum pitch. There is an interaction between the two groups, and then the two-by-two co-carbonization of the group components is carried out. Finally, it is found that the co-carbonization products of the other two group components have a strong interaction except for the small interaction between n-heptane and toluene insolubles. , Promote the formation of high-quality mesophase. At the same time, by analyzing the components of each group, it is determined that the cycloalkyl component has a greater effect on the coking process than the chain alkanes. Figure 2 shows the polarization comparison of the group components respectively carbonized and co-carbonized.

1.2 Physical disturbance assisted method

In recent years, various physical disturbances such as mechanical stirring, magnetic field, and electric field have been added to the delayed coking process, as an auxiliary means to successfully obtain high-quality needle coke. Mechanical stirring can make the mesophase fully contact the molecules in the reaction process, and at the same time, it can draw the mesophase fiber and improve the coking effect. Electric and magnetic fields are the use of the characteristics of macromolecular aromatics that can be arranged under the influence of electromagnetic fields, so that the molecules are arranged in an orderly manner, thereby increasing the needle-like structure and improving product performance. Xie Xiaoling et al. used refined coal pitch as a raw material and added electric field during the delayed coking process. They found that adding electric field under suitable conditions can significantly increase the number of mesophase spheres. After that, manual stirring was carried out at about 450 ℃ to successfully make the fiber structure orientation more orderly. Wang Ying et al. mechanically agitated the sample, and the texture of the obtained mesophase semi-focus became obvious and clear.

Zhao Shigui and others used coal tar pitch as a raw material to prepare needle coke under magnetic field induction and ultrasonic cavitation conditions. Ultrasonic cavitation can effectively remove impurities in the pyrolysis process, and magnetic field induction greatly improves the order of needle coke fibers. Xing Guozheng et al. added mechanical stirring during the preparation of semi-coke and found that stirring can significantly improve the fiber structure, and found that the effect of intermittent stirring is significantly better than that of continuous stirring.

2 Application of needle coke

In industry, the main application areas of needle coke are graphite electrodes for electric furnace steelmaking and anode materials for lithium-ion batteries, both of which account for more than 90% of market demand; in laboratory research, it is widely used in electrochemical capacitors, graphene preparation and other fields.

2.1 High-power and ultra-high-power graphite electrodes

Electric furnace steelmaking has become an important development direction of steelmaking enterprises in recent years due to its advantages of low energy consumption and low pollution emissions. Electric furnace steelmaking uses scrap steel as the main raw material, three-phase alternating current as the power source, and uses electric current to heat, melt, and refine the metal through the high temperature of the arc generated between the graphite electrode and the metal material. High-power and ultra-high-power graphite electrodes are necessary consumables for electric furnace steelmaking. Graphite electrodes prepared from needle coke have the advantages of high electrical conductivity, small thermal expansion coefficient, and high mechanical strength, which can meet the quality of large-capacity electric arc furnaces. Claim. High-power and ultra-high-power graphite electrodes have also become the main application direction of needle coke. With the increase in research and development, the quality of needle coke in my country has steadily improved. With the strengthening of social environmental protection awareness and the development of power supply, the output and proportion of electric furnace steelmaking will steadily increase, and the demand for needle coke will gradually increase.

2.2 Anode materials for lithium batteries

Since the birth of lithium-ion batteries, they have quickly occupied the power supply market for electronic devices due to their high energy density, low cost, and long life. Needle coke has become a high-quality material for the negative electrode of lithium batteries because of its high conductivity, easy graphitization, and low price. Niu Pengxing et al. compared the properties of pitch coke and needle coke after graphitization and found that needle coke is easier to graphitize than pitch coke under the same graphitization conditions, and the charge and discharge performance is also better than pitch coke. He Detao and others successfully conducted a pilot study on needle coke for lithium battery negative electrodes, and the prepared needle coke has reached the index requirements. The charge and discharge test of lithium battery negative electrodes shows that the reversible capacity reaches 355.5 mAh/g, and it has a high High rate performance and better cycle performance. In recent years, the rapid development of new energy vehicles has driven the demand for lithium-ion batteries to rise rapidly. According to data from Zhiyan Consulting and Founder Securities Research Institute, since the outbreak of the new energy automobile industry, the demand for negative electrode materials for lithium batteries has accounted for 30%-40% of the total demand for needle coke, and has gradually become one of the main application areas of needle coke.

2.3 Other applications

Needle coke has been developed and applied in a variety of new fields due to its excellent performance. Zhong et al. innovatively used needle coke instead of carbon black and added it to the carbon slurry of the perovskite battery to effectively improve the battery power conversion efficiency. Im et al. used a two-stage steam activation method to activate needle coke to obtain an activated carbon material with a specific surface area of ​​1134 m2/g and a mesoporosity of 78%. Qiao et al. used KOH to activate needle coke to prepare an electric double-layer capacitor with high conductivity. Wang Jiuzhou and others successfully prepared supercapacitors with excellent rate performance by oxidizing and calcining needle coke with mixed acid. Fu et al. doped the catalyst with needle coke and obtained good photocatalytic activity. Xing et al. used needle coke as the raw material to successfully prepare graphene by the oxidation-exfoliation-thermal reduction method. The graphene oxide adsorbs malachite green better than graphene prepared from natural graphite, providing a new method for graphene preparation .

3 Outlook

The preparation process of needle coke has made great breakthroughs with the efforts of researchers, but there are still many problems in the large-scale domestic production of high-quality needle coke:

Raw materials such as coal tar pitch often require raw material pretreatment, and the pretreatment process is very complicated and costly, which is not conducive to industrial production. Therefore, looking for raw materials that do not require or only require simple pretreatment is of great help to the next expansion of needle coke production. At the same time, exploring low-cost and simple raw material pretreatment methods is also an effective solution to this problem.

Existing physical disturbance process research is still relatively macro, with few investigation factors, and it is difficult to carry out quantitative control and expand production. Increasing physical disturbance will inevitably require designing and updating equipment, as well as increasing operating costs. Therefore, the quantitative analysis of physical disturbances and the expansion and energy-saving design of reaction equipment should also be the next research direction.

The current delayed coking reaction mechanism is not perfect, which makes it difficult to innovate the selection and preparation methods of high-quality needle coke raw materials. The exploration of the mechanism will help to expand the choice of raw materials and effectively control the parameters of the delayed coking process. Therefore, the reaction mechanism needs to be further explored.

The high temperature of the calcination process, excessive energy consumption, and high requirements for equipment and materials directly lead to an increase in preparation costs. Searching for new suppressing expansion agents or catalysts, reducing energy consumption and calcination equipment requirements are important directions that researchers need to explore.

Although needle coke has been used in industry, its application fields are few and the demand is unstable, which restricts the expansion of needle coke production scale. Therefore, other application areas of needle coke still need to be developed by researchers and practitioners.

In the future, with the deepening of research and the deepening of the understanding of reaction mechanism and raw materials, the mass production of high-quality needle coke is just around the corner.

Source: Carbon Technology Author: Bing Yang, Qin Zhihong, Yang Xiaoqin Lin Zhe