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Graphite raw material – Isotropic coke. Abstract: Due to its excellent physicochemical properties, isotropic graphite is widely used in fields such as machinery, metallurgy, nuclear energy, and aluminum electrolysis. This article briefly describes the raw materials and preparation methods for isotropic graphite materials, and focuses on a new type of raw material, isotropic coke, as well as its advantages in producing isotropic graphite. Isotropic graphite is an important industrial material that has emerged in recent years. It is a high-quality special type of graphite possessing many excellent physicochemical properties, such as identical physical characteristics in all directions, with its properties being independent of size, shape, and the direction from which the sample is taken ； The material has a dense microstructure; the surface of the products exhibits high hardness and mechanical strength ； The material has good thermal shock resistance and is not prone to cracking under conditions of rapid cooling and heating ； High temperature resistance and strong antioxidant properties. It has been widely used in fields such as machinery, metallurgy, nuclear energy, aluminum electrolysis, chemicals, aerospace, and biology, and has become an essential high-performance engineering material in the industrial development of the modern world. Currently, isotropic graphite is mainly prepared using vibration molding, compression molding, and isostatic pressing techniques. The raw materials used include petroleum coke, pitch coke, anthracite, ultra-fine graphite powder, secondary coke, mesophase carbon microballs (McMB), etc. This article provides an overview of the raw materials and preparation methods for isotropic graphite materials, focusing on a new type of raw material called isotropic coke and the advantages of using it to produce isotropic graphite. 1 Isotropic graphite 1.1 Methods for preparing isotropic graphite The main methods for preparing isotropic graphite materials are traditional methods and self-sintering methods. The traditional method is a two-stage approach that combines fine particle powder with isostatic pressing technology: the raw materials (petcoke, pitch coke, or anthracite) are ground into ultra-fine powders, mixed uniformly with an appropriate amount of binder, and then ground again into compacted particles with particle sizes ranging from several tens to several hundred micrometers. After calcination, this results in isotropic coke with a microgranular structure in which the ultra-fine powder solids serve as the structural units; upon graphitization, isotropic graphite is obtained. The method used in industry is to mix the aggregate with a binder, shape it, bake it, impregnate it, and graphitize it. Usually, to achieve sufficient bulk density, the calcination and impregnation process needs to be repeated 2–3 times. Fang Dengke et al. used calcined petroleum coke as an aggregate and high-temperature coal tar pitch as a binder. Through cold mixing, kneading, and ball milling, followed by a cold isostatic pressing process at various pressures, isotropic graphite materials were produced. The effect of molding pressure on the microstructure and mechanical properties of the resulting materials was investigated ； Increasing the molding pressure, reducing the number of large-sized flaky structural particles in the aggregate petroleum coke, and improving the uniformity of the mixing between the aggregate and the binder all contribute to enhancing the physical properties of the isotropic graphite materials produced ； Using high-temperature coal tar pitch as a binder helps to improve the thermal conductivity and electrical properties of isotropic graphite materials. Li Zhengcao and others conducted a comparative study on the isotropy, pore diameter, pore morphology, and microcrystalline structure of foreign graphite types IG-11 and NBG-18, as well as the domestic graphite type HSM-SC. Their findings showed that when fine particles are used as raw materials, the average diameter of the coke particles has a significant impact on the size of the pores within the graphite. Under identical processing conditions, graphite made from fine particles exhibits better mechanical properties, and the quality of isotropic graphite produced from fine particles is closely related to improved mixing uniformity. Traditional methods require the use of binders. In the production of isotropic graphite using binders, more than one-third of the binder’s mass decomposes and volatilizes during the baking process. Additionally, due to the differences in volume contraction between the filler and the binder, the resulting graphite material has a high porosity and poor structural uniformity, which leads to low mechanical strength in the final material. The self-sintering method uses self-sintering single-phase powders as raw materials to produce isotropic materials; its principle is similar to that of powder metallurgy. No binder needs to be added during the production process, and during sintering the particles bond to each other and contract uniformly, enabling the material to achieve high density and isotropy. Therefore, it holds great potential for development. Green coke powder possesses self-sintering properties; it contains a certain amount of volatiles, which enables the particle surfaces to bond together with each other. This prevents the formation of a loose structure during high-temperature heat treatment, as would occur due to significant differences in the degree of shrinkage between the filler and the binder, thereby improving the density and mechanical properties of the final product. The process of producing high-strength, high-density graphite using binderless green coke is simple and can basically meet the production requirements for products of relatively high quality. However, the volatile matter in green coke has a significant impact on the product properties, thus requiring special selection. He Chihuan and others prepared high-density carbon materials using raw petroleum coke as a starting material, and carried out screening and optimization of the preparation process. McMB possesses inherent adhesiveness; after a single firing and graphitization process, the green body achieves a high density without the need for impregnation. Its geometric shape is spherical, and the particles are arranged randomly during shaping, which ensures the isotropy of the material. A great deal of research has been conducted on the preparation of isotropic graphite using MCMB. Gao Yanshan et al. prepared high-density isotropic carbon using McMB with different contents of B-resin. They found that as the B-resin content increased, the density and flexural strength of the product first increased and then decreased. This indicates that to obtain integral carbon blocks with good properties, there must be an optimal B-resin content. Lu Yonggen et al. used MCMB as the raw material and carried out compression molding at room temperature, while investigating the effects of different raw materials, molding pressures, heat treatment temperatures, and heating rates on the properties of the products. Shenke et al. used MCMB with two different degrees of pre-oxidation as raw materials and employed a cold isostatic pressing technique to produce isotropic graphite. The degree of pre-oxidation has a significant impact on the properties of the samples; raw materials that have been slightly oxidized can be used to produce isotropic graphite with a bulk density of 1.82 g/cm3, a flexural strength of 34 MPa, a thermal expansion coefficient of 4.48×10^-3 K^-1, and an isotropy index of 1.00. 1.2 Applications of isotropic graphite Isotropic graphite possesses a range of excellent properties such as good lubricity, electrical and thermal conductivity, heat resistance and thermal shock resistance, high mechanical strength at high temperatures, and good machinability; these characteristics enable its wide use in industries such as metallurgy, electricity, electronics, and chemicals. Such as heating elements used in the semiconductor industry, molds for integrated circuit assembly, metal continuous casting molds used in the metallurgy industry, and electrodes for electrical discharge machining. Isotropic graphite possesses excellent slowing and reflecting properties, along with ideal structural strength, good and uniform thermal conductivity, and high purity. These characteristics make it suitable as a neutron slowing and reflecting material in high-temperature gas-cooled reactors, thus rendering it one of the indispensable materials in the nuclear industry. In the field of electrolytic aluminum production, graphitized cathode carbon blocks are a type of isotropic graphite that are used as the bottom and side linings of aluminum electrolysis cells, as well as as the cathode material in these cells. The electrolytic aluminum industry is a high-energy-consuming sector, and its transformation and development toward lower carbon emissions, sustainability, and environmental protection have become an inevitable trend. Using isotropic graphite as a lining can improve the lifespan of the electrolyzer, reduce power consumption, and increase the production capacity per unit area of the electrolyzer. Liao Xian’an pointed out that isotropic coke is the most suitable raw material for producing graphitized cathode carbon blocks. Ecocarbon produces high-quality graphitized cathode carbon blocks using isotropic coke as the raw material, a fact that has been confirmed through industrial trial production. Developed countries such as the United States, Japan, and Germany attach great importance to the development and application of isotropic graphite. Nearly a hundred different types of isotropic graphite are currently available, providing a wide range of new graphite products for use in other high-tech fields. At present, the development of isotropic graphite in China is progressing rapidly. However, compared with those in developed countries, domestic isotropic graphite still lags behind in terms of quality, variety, and performance. The high-quality isotropic graphite required in China still has to be imported; therefore, developing high-performance isotropic graphite materials for use in advanced fields is the goal that researchers are striving to achieve. 2 Isotropic Focals 2.1 Methods for Preparing Isotropic Focals An isotropic focal refers to a material whose physical and chemical properties do not change depending on the direction in space; in other words, parameters such as the coefficient of thermal expansion (CTE) and specific resistance are roughly the same in all directions. At the microscale, the optical structure in all directions is a fine mosaic structure without any single orientation. The isotropy of isotropic foci can be characterized by the CTE ratio, which is the ratio of radial to axial CTE; it is also referred to as the degree of isotropy, with values generally ranging from 1.0 to 1.1. Research on isotropic foams is currently limited in China and remains at the laboratory stage, whereas there are relatively more studies on this topic abroad. The research on the preparation methods of isotropic coke focuses mainly on the following three aspects: (1) Selecting appropriate raw materials. Utaklawatam proposed a method for preparing isotropic coke using the liquid products or pitch generated by the rapid pyrolysis of coal; the pyrolysis temperature for coal is 750°C, while 600°C is considered a more suitable temperature. At 650°C, with a holding time of less than 5 seconds, the liquid products or pitch obtained from the rapid pyrolysis of coal are carbonized to yield isotropic coke. Koelling Georg chose tar pitch that had undergone low-temperature carbonization as the raw material, controlled its hydrogen and carbon contents as well as its softening point, and heated it to 900–1300°C to produce isotropic coke with lower isotropy and fewer impurities ； Selecting raw materials with appropriate molecular structures and compositions to produce isotropic coke is the most direct method, with relatively low production costs; however, the availability of such raw materials may be limited or even difficult to obtain. Qu Bin and others used inexpensive coal tar pitch as a raw material, adjusted the quinoline-insoluble content to 5%–25%, and controlled the number and particle size of the small spheres by regulating the heating rate, reaction temperature, and holding time during the polymerization and carbonization reactions. The isotropic coke produced exhibited a dense and uniform fine interlocking structure; macroscopically, its CTE ratio was close to 1, and the particle stability was not less than 90% ； This method is simple, economical, and highly controllable, making it suitable for mass production. (2) The air oxidation method: Patent CN1306070A, filed by Ling Licheng and others from the Shanxi Coal Chemistry Research Institute, describes a method for producing isotropic coke using coal tar or petroleum coke residue distillate as raw material. The coal tar or petroleum coke residue distillate is placed in a reactor and heated; once the temperature reaches 120°C, stirring begins and an oxygen-containing gas is introduced at a certain flow rate. The reaction temperature ranges from 260 to 430°C, and after reaching this temperature, the mixture is held at that temperature for 5 to 15 hours to obtain the oxidized coal tar or petroleum residue distillate. This product is then processed at 460–500°C and under a pressure of 0.1. Isotropic cokes with a fine or extremely fine embedded microstructure, obtained by coking at 1 MPa under nitrogen protection. Patent number uS5066385 discloses a method for producing isotropic coke. Using petroleum residue as the raw material, air oxidation at 400–600°C is carried out to raise the softening point to 120–240°C; thereafter, this material is mixed with pyrolytic tar (which has a lower sulfur content than the petroleum residue feedstock). The resulting mixture is then subjected to delayed coking to produce isotropic coke with low sulfur and low cTE values. Patent No. US3960704 discloses a method for producing isotropic petroleum coke, in which air is blown (oxidized) into a residue, such as the residues from the bottom of a feed fractionation tower, to raise its softening point to 120. At 240°C, the residue resulting from air oxidation was further processed via delayed coking to produce isotropic coke with a CTE ratio of less than 1.5. Wang Lin and others obtained a series of oxidized asphalts with different softening points by air oxidation of refined asphalt. Analysis of these materials showed that air blowing promotes the dehydration condensation reaction and cross-linking reactions. As the degree of oxidation increases, the aromatic hydrogen content, TI—QS, true density, softening point, and hydrocarbon ratio of the oxidized asphalts increase, while the alkyl hydrogen content and Ts decrease significantly ； Mild oxidation has little effect on the microstructural organization of the intermediate phase during the coking process. Severe oxidation can promote the development of this intermediate phase structure, resulting in isotropic coke with a high degree of optical isotropy; however, excessive oxidation is detrimental to the formation of isotropic coke. Therefore, by controlling the degree of oxidation, it is possible to regulate the final optical microstructure, thereby obtaining high-quality isotropic coke with a low cTE value, high bending strength and true density, as well as low electrical resistivity and ash content. (3) Additive method: The additives are preferably from groups 4A and 5A of the periodic table as well as sulfides; there are no restrictions, but tin Sn, arsenic As, red phosphorus, and lead Pb are particularly useful. The mixed oil feedstock with additives is subjected to delayed coking to obtain isotropic coke. Zhu Yifei found that the heteroatoms in the crude oil and asphalt used to produce isotropic coke can accelerate pyrolytic condensation, lowering the temperature required for thermal conversion. By the time the temperature at which the mesophase forms is reached, the viscosity of the system is already high, which hinders further development of the mesophase and thus facilitates the formation of an isotropic structure. 2. 2 Advantages of using isotropic coke to produce isotropic graphite: The properties of graphite products depend to a large extent on the properties of the filler coke, and the properties of coke are primarily determined by its microstructure. For isotropic foams, a finer micro-embossed structure results in better macroscopic isotropy. To meet the key quality indicators in Table l, with a macroscopic isotropy within the range of 1.0–1.1 and a particle stability of over 90%, it is essential to control its microscopic fine interlocking structure. Patent CNl0384941lA further proves this. Key quality indicators of isotropic coke: Serial number, Item, Unit, Indicator, Remarks. 1. True specific gravity, g/cm3: ≥2.12; 2. Specific resistivity of powder, μΩm: ≤150; 3. Vibratory bulk density, g/cm3: ≥0.85 for 2–4 mm particles; 4. Particle size stability, %: ≥92; 5. Graphitization degree, %: ≥65; 6. Harrow hardness grindability index: 12–15; 7. Porosity, %: ≤28. Isotropic coke is an excellent raw material for isotropic graphite materials, with petroleum coke and coal tar pitch coke being commonly used. Isotropic fused graphite has high mechanical strength and good density; furthermore, the isotropy of the raw materials provides a technical foundation for the production of isotropic graphite materials. Traditional methods for producing isotropic graphite using anisotropic coke require crushing the raw material into sufficiently small particles, followed by processes such as kneading, crushing and screening, mixing, isostatic pressing, calcination, impregnation, secondary calcination, graphitization, and processing for purification. However, using isotropic coke as the raw material greatly simplifies the process. During the preparation process, the degree of isotropy of the coke particles, as well as the shaping and heat treatment processes, all influence the isotropy of the resulting graphite. When using isotropic coke as raw material to produce isotropic graphite, a high degree of isotropy in the coke particles means that it is not necessary to grind the raw material to a very fine particle size; it is possible to use coarser particles, which shortens the production process, reduces investment and energy consumption, and improves both production efficiency and product quality. Vibration molding can also be used as an alternative to the expensive isostatic pressing equipment, **reducing costs. It has been shown that by using isotropic coke as an aggregate, it is possible to keep the isotropy of isotropic graphite below 1.10 even with compression molding. Therefore, the development of high-quality isotropic foci is one of the foundations for producing high-quality isotropic graphite, and using isotropic foci as aggregates in high-strength and high-density isotropic graphite offers significant advantages. 3 Conclusion Isotropic coke possesses excellent properties such as high mechanical strength, good density, and isotropy, making it the ideal aggregate for producing isotropic graphite. Graphite products manufactured using isotropic coke exhibit significantly better characteristics compared to those made from anisotropic coke. Moreover, vibration molding can be used in place of the expensive isostatic pressing process, resulting in a simpler manufacturing process and reduced costs. Furthermore, our country is a major producer of coal tar. If coal tar pitch can be processed into isotropic coke, it will not only provide raw material for isotropic graphite but also offer new ways to utilize coal tar pitch. We should pay more attention to isotropic coke and systematically develop new carbon products using it as a raw material, so as to better serve our country’s low-carbon economy.