Process for the production of isotropic coke for nuclear graphite

Abstract

A preparation method of isotropic coke for nuclear graphite, distillation of high-temperature coal tar to obtain fractional oil; the fractional oil is mixed with a solvent, and then flocculation separation is performed to obtain light phase oil and heavy phase oil; the light phase oil is subjected to distillation to recover the solvent, and then light purified tar is obtained; the heavy phase oil is subjected to distillation to recover the solvent, and then heavy tar is obtained; the heavy tar and the solvent are subjected to extraction separation to obtain a soluble phase and an insoluble phase; the soluble phase is subjected to distillation to recover the solvent, and then heavy purified tar is obtained; the light purified tar is subjected to multi-stage series hydrogenation to obtain fractional oil A; the heavy purified tar is subjected to hydrocracking to obtain fractional oil B; the fractional oil B is subjected to vacuum distillation to obtain fractional oil C; the fractional oil A is subjected to asphaltization reaction to obtain pre-polymerized asphalt; the fractional oil C and the pre-polymerized asphalt are mixed and subjected to liquid-phase carbonization to obtain mosaic structure green coke; the mosaic structure green coke is crushed and subjected to calcination to obtain isotropic coke for nuclear graphite. The coke has excellent physical and chemical properties, low manufacturing cost and short manufacturing period.

Description

Technical Field

[0001] This invention belongs to the field of nuclear graphite raw material preparation technology, and relates to a method for preparing isotropic coke for nuclear graphite. Background Technology

[0002] Nuclear energy has low environmental pollution and low energy material requirements, making it an effective means of solving my country's energy problem. High-temperature gas-cooled reactors (HTGRs) have advantages such as high safety, high power generation efficiency, high coolant outlet temperature, good neutron economy, and flexible fuel cycle, making them a recognized advanced reactor type and a potential effective energy source for energy-scarce areas in remote regions and the southeastern coastal areas of my country. However, HTGRs operate under harsh conditions, are highly corrosive, and involve high reaction temperatures, placing very high demands on materials.

[0003] Carbon materials possess excellent properties such as light weight, high toughness, high specific strength, high modulus, high stability, corrosion resistance, and non-toxicity, making them important structural, functional, and biomedical materials with broad application prospects in aerospace, marine, and sports fields. Due to their low coefficient of thermal expansion, excellent thermal shock resistance, and low neutron activation performance, carbon materials are crucial moderators in nuclear reactors. High-purity graphite is an excellent neutron moderator material for nuclear reactors, with a moderation ratio second only to heavy water, but at a lower cost and allowing the utilization of natural uranium energy. Generally, carbon-graphite materials used in nuclear reactors are called nuclear reactor graphite, or simply nuclear graphite. Nuclear graphite is a polycrystalline material with a high degree of graphitization, requiring high purity to prevent thermal neutrons from adsorbing onto the moderator block, and its isotropic nature ensures stability in three-dimensional space under high neutron radiation conditions.

[0004] For safety, nuclear reactors require densely structured components with consistent performance. While graphite possesses excellent nuclear reactivity, ideal graphite crystals have a two-dimensional sheet-like structure with high strength along the carbon layers and only weak molecular bonds between the layers, exhibiting significant anisotropy. Therefore, the directionality of material properties can be adjusted by manufacturing isotropic graphite or through composite materials.

[0005] Nuclear graphite is a crucial material in the construction of nuclear power plants, and its development and research are key to the success of such projects. Given the vital role of nuclear graphite in nuclear energy development and the unique working environment it requires, in order to build and develop my country's high-temperature gas-cooled reactor, it is essential to independently solve the supply problem of nuclear graphite and research and develop our own.

[0006] The properties of graphite are related to the properties of its raw materials, especially coke aggregates. Traditionally, suitable raw materials for manufacturing nuclear graphite are selected from various industrial petroleum cokes or pitch cokes. Under fast neutron irradiation, graphite properties change. This is a result of structural changes caused by the dislocation of carbon atoms in the graphite lattice due to high-energy neutron bombardment, followed by diffusion and recombination of these dislocated atoms. Graphite with good properties before irradiation may not necessarily have good properties after irradiation, and graphite with poor properties before irradiation usually will also have poor irradiation properties. Changes in the microstructure of graphite are the intrinsic cause of irradiation damage. Fast neutron flux, irradiation temperature, and irradiation time are the external conditions for irradiation damage. There are complex relationships between raw materials, manufacturing processes, fast neutron flux, and irradiation temperature; different graphites often exhibit different irradiation behaviors under the same working conditions, and the same graphite may show different irradiation behaviors under different working conditions. Currently, the only way to evaluate whether a coke aggregate is suitable for manufacturing graphite for high-temperature gas-cooled reactors (HTGRs) is to create an environment as close as possible to the working environment of graphite in a predetermined HTGR, and to conduct irradiation tests on graphite made from that coke aggregate to actually examine its irradiation behavior. Irradiation tests are accelerated tests, and the fast neutron fluence rate of the test reactor is typically 1-2 orders of magnitude higher than that of the actual reactor. Irradiation test equipment is complex, time-consuming (1-3 years), and expensive (a single test curve, such as the graphite size versus fast neutron fluence at a certain temperature, costs over $1 million). In Germany alone, from the early 1970s to the late 1980s, the cost of developing nuclear graphite exceeded 50 million marks, not including the main cost of irradiation tests: neutron costs. As mentioned above, the decade-long stagnation resulted in the loss of the developed typical nuclear graphite, one of the main reasons for which was the disappearance of the coke raw materials used to manufacture these typical nuclear graphites. Because upstream industries constantly innovate their processes due to changes in raw materials or for economic reasons, the composition, microstructure, and properties of the byproduct coke change accordingly. This lesson shows that a detailed assessment of raw material supply should be conducted at the beginning of nuclear graphite research and development. Coke aggregate must not only meet the performance requirements for manufacturing high-temperature gas-cooled reactor graphite but also be available in a long-term, sustainable manner. If changes in upstream raw materials or processes lead to changes in the structure and properties of coke, nuclear graphite production will need to be redeveloped from scratch, making the development of high-temperature gas-cooled reactors unsustainable, or at least significantly reducing its economic competitiveness.For the sustainable and healthy development of high-temperature gas-cooled reactors (HTGRs), the initial research and development of nuclear graphite should focus on selecting varieties of conventional coke that can be adequately supplied for a considerable period. If the long-term supply of coke cannot be guaranteed, the research and development of nuclear graphite should be extended forward, starting with feedstock oils. In-depth research should be conducted on the process and mechanism of coke formation, mastering the technology to produce coke with the same (or at least similar) structure and performance that meets the requirements of HTGRs from different feedstock oils. This would eliminate the dependence on raw materials for nuclear graphite production. Nuclear graphite, as a crucial material in nuclear power plant construction, has not yet been domestically produced. One important reason is that high-performance, cost-effective, and cycle-shortening raw materials for nuclear graphite production, such as isotropic coke, binder pitch, and impregnating agent pitch, are mainly imported. Therefore, to accelerate the development of my country's nuclear power industry and achieve sustainable energy, the primary task is to independently solve the problem of raw material supply for nuclear graphite production, thereby resolving the overall supply issue of nuclear graphite. Summary of the Invention

[0007] This invention provides a method for preparing isotropic coke for nuclear graphite, aiming to produce an isotropic coke with high graphitization, high purity, and low heteroatom content. It also possesses high purity, high density, high strength, high thermal conductivity, high corrosion resistance, high wear resistance, high radiation stability, low elastic modulus, low coefficient of thermal expansion, and the highest possible isotropy. This isotropic coke for nuclear graphite exhibits excellent physicochemical properties, low manufacturing cost, and short manufacturing cycle. This invention successfully solves the problem of producing structurally and performance-stable coke that meets the requirements of high-temperature gas-cooled reactors, eliminating the dependence on raw materials for nuclear graphite production. Furthermore, it provides a new process route for the research and preparation of nuclear-grade carbon materials.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A method for preparing isotropic coke for nuclear graphite includes the following steps:

[0010] 1) Distillate oil is obtained from high-temperature coal tar through vacuum distillation. The vacuum distillation process is characterized by controlling the bottom temperature of the column at 220-340℃, the top temperature at 130-230℃, and the vacuum degree at 0.01-0.09MPa.

[0011] The density of the distillate oil is 0.9–1.1 g / cm³. 3 The content of oxygen-containing compounds existing in the form of phenols and their derivatives is ≤1%.

[0012] 2) After mixing the distillate oil obtained in step 1) with solvent A, the light phase oil and heavy phase oil are obtained by flocculation separation. The conditions for flocculation separation are: flocculation mixing temperature is 20-80℃, flocculation reaction temperature is 60-140℃, and flocculation time is 1-8h.

[0013] Solvent A includes one or a mixture of several of petroleum ether, gasoline, kerosene, light diesel oil, cyclohexane, and pyridine; the distillate oil and solvent are mixed in a mass ratio of distillate oil:solvent = 10:(1-12).

[0014] The yield ratio of the light phase oil to the heavy phase oil is 1:(0.1~1), and the quinoline insoluble content of the light phase oil is ≤0.1%.

[0015] 3) The light phase oil obtained in step 2) is distilled to recover the solvent and obtain light purified tar.

[0016] The quinoline insoluble content of the light purified tar is ≤0.1%.

[0017] The distillation conditions were as follows: the bottom temperature was controlled at 240–380℃, the top temperature was controlled at 140–300℃, and the vacuum degree was controlled at 0.01–0.09 MPa.

[0018] 4) The heavy phase oil obtained in step 2) is distilled to recover the solvent and obtain heavy tar.

[0019] The distillation conditions were as follows: the bottom temperature was controlled at 240–380℃, the top temperature was controlled at 140–300℃, and the vacuum degree was controlled at 0.01–0.09 MPa.

[0020] 5) The heavy tar obtained in step 4) is mixed with solvent B and extracted to obtain a soluble phase and an insoluble phase; the soluble phase is then distilled to recover the solvent and obtain purified heavy tar.

[0021] The distillation conditions are as follows: the bottom temperature is controlled at 120–360℃, the top temperature is controlled at 40–220℃, and the vacuum degree is controlled at 0.01–0.09 MPa.

[0022] The solvent B is one or a mixture of several of toluene, xylene, pyridine, heptane, pentane, crude benzene, quinoline, and isoquinoline; the heavy tar and the solvent are mixed in a mass ratio of heavy tar: solvent B = 1:(1~5).

[0023] The extraction and separation conditions are: extraction temperature 100-320℃, sieve mesh size 300-1200 mesh.

[0024] The quinoline insoluble content of the soluble phase is ≤0.1%.

[0025] The quinoline insoluble content of the heavy purified tar is ≤0.1%.

[0026] 6) The light purified tar obtained in step 3) is subjected to a multi-stage series hydrogenation impurity removal process to obtain distillate oil A.

[0027] The multi-stage series hydrogenation and impurity removal process is a fluidized bed, fluidized bed, or fixed bed multi-stage series hydrogenation and impurity removal process, in which one stage is a water-resistant hydrogenation catalysis, the second or more stages are hydrogenation refining catalysis, the reaction temperature of the first stage is 180-320℃, and the reaction temperature of the second stage or subsequent stages is 220-400℃.

[0028] The distillate oil A is a full-range oil obtained through a multi-stage series hydrotreating process, with a density of 0.88–1.0 g / cm³. 3 The content of the distillate below 350℃ is ≥60%, and the content of the distillate below 500℃ is ≥95%; the sulfur content is ≤0.2%, and the nitrogen content is <0.3%.

[0029] 7) The heavy purified tar obtained in step 5) is subjected to hydrocatalytic cracking to obtain distillate oil B.

[0030] The hydrocatalytic cracking process is a fixed-bed catalytic hydrogenation process with a reaction temperature of 280–440°C.

[0031] The distillate oil B is a full-range oil obtained by fixed-bed hydrocatalytic cracking of heavy purified tar. Its content of fractions below 500℃ is ≥90%; quinoline insoluble content is ≥3%; aromatic content in fractions above 370℃ is ≥50%; sulfur content is ≤0.2%; and nitrogen content is <0.3%.

[0032] 8) After removing the 370℃ front-range oil from fraction B obtained in step 7) by vacuum distillation, the resulting heavy oil is fraction C. The distillation conditions are: bottom temperature controlled at 380-480℃, top temperature controlled at 280-360℃, and vacuum degree controlled at 0.01-0.09MPa.

[0033] The distillate oil has a quinoline insoluble content ≥6%, an aromatic content ≥65%, a sulfur content ≤0.2%, and a nitrogen content <0.3%.

[0034] 9) The distillate oil A obtained in step 6) is subjected to an asphaltification reaction to obtain prepolymerized asphalt.

[0035] The asphaltification reaction requires nitrogen or an inert gas for purging protection. The inert gas is high-purity nitrogen or high-purity argon, etc. After purging, the initial pressure of the reaction is 0.01-0.5 MPa, the final pressure is 0.1-1.0 MPa, the reaction temperature is 260-400℃, the isothermal time is 1-6 h, and the heating rate is 3-10℃ / min.

[0036] The prepolymerized asphalt has a softening point of 20–80℃, a toluene-insoluble content of 6–18%, a quinoline-insoluble content of ≥3%, an ash content of ≤0.05%, a sulfur content of ≤0.2%, and a nitrogen content of <0.3%.

[0037] 10. After mixing the distillate oil C obtained in step 8) with the prepolymerized asphalt obtained in step 9), the mixture is subjected to a liquid phase carbonization process to obtain a semi-coke with an embedded structure. The liquid phase carbonization process is protected by inert gas or nitrogen.

[0038] In step 10) above, the distillate oil C and prepolymer asphalt are mixed in a mass ratio of distillate oil C: prepolymer asphalt = 1:(0.1~5).

[0039] The liquid-phase carbonization process conditions are as follows: reaction temperature is 460–540℃, isothermal time is 0.5–4h, and heating rate is 3–10℃ / min.

[0040] The true density of the inlaid structure semi-coke is ≥1.6 g / cm³. 3 Volatile matter ≤8%, ash content ≤100ppm, and the average particle size of the mosaic structure is 4~28μm.

[0041] 11) After crushing the mosaic structure semi-raw coke obtained in step 10), isotropic coke for nuclear graphite is obtained by calcination.

[0042] The average particle size of the semi-coke after crushing in the mosaic structure is 10–30 mm.

[0043] The calcination process uses a rotary kiln, with a calcination temperature of 1200–1600℃, an oxygen content of 2–15% in the kiln, and a residence time of 1–4 hours for the calcined material.

[0044] The isotropic coke used for nuclear graphite in this invention has an average particle size of 1–8 mm and a true density ≥ 2.12 g / cm³. 3 Powder resistivity ≤150μΩ·m, vibration bulk density ≥0.85g / cm³ 3 Porosity ≤28%, Hastelloy grindability index 12-15, sulfur content ≤0.2%, nitrogen content <0.3%, ash content ≤100ppm, neutron-absorbing impurities <2ppm, particle stability ≥92%, graphitization degree ≥65%, isotropy 1-1.05.

[0045] All percentage contents in this invention are mass percentages.

[0046] Compared with the prior art, the beneficial effects of the present invention are:

[0047] 1) The isotropic coke for nuclear graphite prepared by the method of the present invention has the characteristics of high graphitization degree, high purity and low heteroatom content. It also has properties such as high purity, high density, high strength, high thermal conductivity, high corrosion resistance, high wear resistance, high radiation resistance, low elastic modulus, low expansion coefficient and as high as possible isotropy.

[0048] The nuclear graphite obtained by this invention has a purity ≥99% and a graphitization degree ≥95%. The boron content is ≤0.5×10⁻⁶. -6 %, compressive strength > 60 MPa, flexural strength > 35 MPa, density ≥ 1.70 g / cm³ 3 Isotropicity <1.05, total ash content <0.2%, total sulfur and nitrogen content <0.3%, coefficient of thermal expansion (room temperature to 600℃) ≤1.0×10⁻⁶ -6 / ℃.

[0049] 2) The isotropic coke for nuclear graphite prepared by the method of the present invention has the advantages of excellent physical and chemical properties, low manufacturing cost and short manufacturing cycle.

[0050] 3) The raw materials of this invention are abundant and inexpensive, and can be used to produce various types of nuclear graphite with high purity, high strength and different density requirements. It can realize the large-scale, stable quality, good thermal stability and low coefficient of thermal expansion of nuclear graphite in the cold state.

[0051] 4) This invention successfully solves the problem of producing coke with stable structure and performance that meets the requirements of high-temperature gas-cooled reactors, eliminates the dependence of coke for nuclear graphite production on raw materials, and provides a new process route for the research and development and preparation of nuclear-grade carbon materials. Attached Figure Description

[0052] Figure 1 This is a microscopic structure diagram of the mosaic structure of the semi-focal polarized light microscope according to Embodiment 1 of the present invention.

[0053] Figure 2 This is a microscopic image of the nuclear graphite of Embodiment 1 of the present invention using an isotropic char polarizing microscope. Detailed Implementation

[0054] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0055] Example 1:

[0056] High-temperature coal tar was distilled in a vacuum distillation column, with the top temperature controlled at 140℃, the bottom temperature at 250℃, and the vacuum degree at 0.08MPa. The bottom product was a distillate oil with a density of 0.98 g / cm³. 3 The content of oxygen-containing compounds in the form of phenols and their derivatives was 0.92%. Distillate oil and kerosene were mixed at a mass ratio of 10:7.5 and thoroughly stirred at 80℃. The mixture was then introduced into a flocculation reactor with a length-to-diameter ratio of 3:1 for flocculation separation at 100℃ for 3 hours. Light phase oil and heavy phase oil were then separated at a mass ratio of 1:0.25, with the light phase oil containing 0.09% quinoline-insoluble matter. The light phase oil was poured into a distillation kettle, with the top temperature controlled at 140℃, the bottom temperature at 240℃, and the vacuum degree controlled at 0.07 MPa. Kerosene was recovered by distillation, and light purified tar was obtained at the bottom of the kettle, containing 0.085% quinoline-insoluble matter. The heavy phase oil was poured into a distillation kettle, with the top temperature controlled at 150℃, the bottom temperature at 260℃, and the vacuum degree controlled at... Kerosene was recovered by distillation at 0.07 MPa, yielding heavy tar at the bottom of the still. The heavy tar and toluene were then introduced into an extractor at a mass ratio of 1:3. A 400-mesh sieve was used, and the extraction temperature was controlled at 120℃ for 2 hours. The extraction separated the soluble and insoluble phases, with the quinoline-insoluble content of the soluble phase ≤0.06%. The soluble phase was then poured into a distillation still, and the top and bottom temperatures were controlled at 80℃ and 150℃, respectively. Toluene was recovered by distillation under a vacuum of 0.07 MPa, yielding purified heavy tar at the bottom of the still. The quinoline-insoluble content of the purified heavy tar was ≤0.03%. The purified light tar was then subjected to two-stage hydrogenation in a fluidized bed to remove impurities, yielding distillate oil A. The first stage reaction temperature was 180℃, and the second stage reaction temperature was 320℃. The density of distillate oil A was 0.93 g / cm³. 3The fraction with a temperature below 350℃ has a content of 60.2%, the fraction with a temperature below 500℃ has a content of 96.8%, a sulfur content of 0.12%, and a nitrogen content of 0.13%. Heavy purified tar is subjected to fixed-bed catalytic hydrogenation to obtain fraction oil B at a reaction temperature of 380℃. Fraction oil B has a quinoline-insoluble content of 3.3%, an aromatic content of 65% in the fraction above 370℃, a sulfur content of 0.172%, a nitrogen content of 0.193%, and a fraction with a temperature below 500℃ of 96%. Fraction oil B is then passed through a vacuum distillation column with the top temperature controlled at 280℃, the bottom temperature controlled at 390℃, and the vacuum degree controlled at 0.09MPa. The bottom of the column yields fraction oil C, which has a quinoline-insoluble content of 6.7%, an aromatic content of 70.5%, a sulfur content of 0.162%, and a nitrogen content of [missing information]. The amount was 0.173%; distillate oil A underwent asphaltification reaction in a polymerization reactor. First, high-purity nitrogen was used to purge the reactor, setting the initial pressure to 0.05 MPa. The pressure was then increased from room temperature to 360℃ at a rate of 3℃ / min, and held at this temperature for 6 hours. The final pressure inside the reactor was maintained at 0.45 MPa, yielding prepolymerized asphalt. The softening point of the prepolymerized asphalt was 55℃, the toluene-insoluble content was 14.2%, the quinoline-insoluble content was 6.8%, the ash content was 0.04%, the sulfur content was 0.13%, and the nitrogen content was 0.143%. Distillate oil C and prepolymerized asphalt were mixed at a mass ratio of 1:5 and then introduced into a coking reactor for liquid-phase carbonization reaction. High-purity nitrogen was used to purge the reactor, the reaction temperature was 520℃, the holding time was 2 hours, and the heating rate was 10℃ / min, yielding a semi-coke with an embedded structure. (See...) Figure 1 This is a polarizing microscope image of the mosaic-structured semi-focal microstructure. The true density of the mosaic-structured semi-focal microstructure is 1.63 g / cm³. 3 The volatile matter content was 7.8%, the ash content was 100 ppm, and the average particle size of the mosaic structure was 12 μm. After crushing the obtained mosaic structure semi-raw coke to an average particle size of less than 25 mm, the calcination temperature was controlled at 1600℃, the oxygen content in the kiln was 5%, and the residence time of the calcined material was 1 hour. Isotropic coke for core graphite was obtained through rotary kiln calcination. The average particle size of the isotropic coke for core graphite was 4 mm, and the true density was 2.124 g / cm³. 3 The powder resistivity is 140 μΩ·m, and the vibration bulk density is 0.88 g / cm³. 3 The porosity is 24%, the Hastelloy grindability index is 12, the sulfur content is 0.112%, the nitrogen content is 0.123%, the ash content is 98 ppm, the neutron-absorbing impurities are 1.92 ppm, the particle stability is 93.5%, the degree of graphitization is 75%, and the isotropy is 1.02. See [link / reference]. Figure 2 The image shows the microstructure of the nuclear graphite using an isotropic polarized coke microscope. The nuclear graphite obtained using this invention has a purity of 99.7%, a degree of graphitization of 95.5%, and a boron content of 0.45 × 10⁻⁶. -6It has a compressive strength of 65 MPa, a flexural strength of 39 MPa, and a density of 1.74 g / cm³. 3 The isotropy is 1.02, the total ash content is 0.12%, the total sulfur and nitrogen content is 0.3%, and the coefficient of thermal expansion (from room temperature to 600℃) is 0.98 × 10⁻⁶. -6 / ℃.

[0057] Example 2:

[0058] High-temperature coal tar was distilled in a vacuum distillation column, with the top temperature controlled at 230℃ and the bottom temperature at 340℃, under a vacuum of 0.03 MPa. The bottom product was a distillate oil with a density of 1.08 g / cm³. 3 The content of oxygen-containing compounds in the form of phenols and their derivatives was 0.92%. The distillate oil was mixed with cyclohexane at a mass ratio of 10:3, stirred thoroughly at 60℃, and then introduced into a flocculation reactor with a length-to-diameter ratio of 4:1 for flocculation separation at 70℃ for 2 hours. The light phase oil and heavy phase oil were then separated at a mass ratio of 1:0.3, with the light phase oil containing 0.01% quinoline-insoluble matter. The light phase oil was poured into a distillation kettle, with the top temperature controlled at 180℃, the bottom temperature at 280℃, and the vacuum at 0.04 MPa. Cyclohexane was recovered by distillation, and light purified tar was obtained at the bottom of the kettle, with a quinoline-insoluble matter content of 0.01%. The heavy phase oil was poured into a distillation kettle, with the top temperature controlled at 200℃, the bottom temperature at 300℃, and the vacuum at 0.04 MPa. Cyclohexane was recovered by distillation at 0.05 MPa, and heavy tar was obtained at the bottom of the still. The heavy tar and xylene were introduced into an extractor at a mass ratio of 1:2. The sieve mesh size was 800 mesh, and the extraction temperature was controlled at 145℃ for 2 hours. The soluble and insoluble phases were separated by extraction, and the quinoline insoluble content of the obtained soluble phase was 0.05%. The soluble phase was poured into a distillation still, and the top temperature of the column was controlled at 120℃, the bottom temperature at 250℃, and the vacuum degree at 0.06 MPa. Xylene was recovered by distillation, and heavy purified tar was obtained at the bottom of the still, with a quinoline insoluble content of 0.03%. The light purified tar was subjected to two-stage hydrogenation in a fluidized bed to remove impurities, yielding distillate oil A. The first stage reaction temperature was 280℃, and the second stage reaction temperature was 390℃. The density of distillate oil A was 0.91 g / cm³. 3The fraction with a temperature below 350℃ has a content of 72.2%, the fraction with a temperature below 500℃ has a content of 98.7%, a sulfur content of 0.18%, and a nitrogen content of 0.243%. Heavy purified tar is subjected to fixed-bed catalytic hydrogenation to obtain fraction oil B at a reaction temperature of 420℃. Fraction oil B has a quinoline-insoluble content of 4.5%, an aromatic content of 73% in the fraction above 370℃, a sulfur content of 0.172%, a nitrogen content of 0.183%, and a fraction with a temperature below 500℃ of 98%. Fraction oil B is then distilled in a vacuum distillation column with the top temperature controlled at 290℃, the bottom temperature controlled at 400℃, and the vacuum degree controlled at 0.07MPa. The bottom of the column yields fraction oil C, which has a quinoline-insoluble content of 7.8%, an aromatic content of 85%, a sulfur content of 0.165%, and a nitrogen content of 0.175%. Oil A undergoes an asphaltification reaction in a polymerization reactor. First, the reactor is purged with high-purity nitrogen to achieve an initial pressure of 0.01 MPa. The pressure is then increased from room temperature to 380°C at a rate of 5°C / min, and held at this temperature for 2 hours. The final pressure inside the reactor is maintained at 0.3 MPa, yielding prepolymerized asphalt. The prepolymerized asphalt has a softening point of 65°C, a toluene-insoluble content of 14.6%, a quinoline-insoluble content of 6.8%, an ash content of 0.02%, a sulfur content of 0.174%, and a nitrogen content of 0.223%. Distillate oil C and the prepolymerized asphalt are mixed at a mass ratio of 1:5 and then introduced into a coking reactor for liquid-phase carbonization. The reactor is purged with high-purity nitrogen, the reaction temperature is maintained at 460°C, the holding time is 4 hours, and the heating rate is 10°C / min, yielding a semi-coke with an embedded structure. The true density of the semi-coke with the embedded structure is 1.65 g / cm³. 3 The volatile matter content was 7.8%, the ash content was 96 ppm, and the average particle size of the mosaic structure was 4.5 μm. After crushing the obtained mosaic structure semi-raw coke to an average particle size of less than 10 mm, the calcination temperature was controlled at 1500℃, the oxygen content in the kiln was 8%, and the residence time of the calcined material was 2 h. Isotropic coke for core graphite was obtained through rotary kiln calcination. The average particle size of the isotropic coke for core graphite was 6 mm, and the true density was 2.13 g / cm³. 3 The powder resistivity is 140 μΩ·m, and the vibration bulk density is 0.90 g / cm³. 3 The porosity is 19%, the Hastelloy grindability index is 12, the sulfur content is 0.154%, the nitrogen content is 0.183%, the ash content is 84 ppm, the neutron-absorbing impurities are 1.62 ppm, the particle stability is 95%, the degree of graphitization is 75.5%, and the isotropy is 1.01. The purity of the nuclear graphite obtained by this invention is 99.6%, the degree of graphitization is 96%, and the boron content is 0.42 × 10⁻⁶. -6 It has a compressive strength of 73 MPa, a flexural strength of 46 MPa, and a density of 1.79 g / cm³. 3The isotropic degree is 1.01, the total ash content is 0.14%, the total sulfur and nitrogen content is 0.23%, and the coefficient of thermal expansion (from room temperature to 600℃) is 0.94 × 10⁻⁶. -6 / ℃.

[0059] Example 3:

[0060] High-temperature coal tar was passed through a vacuum distillation column, with the top temperature controlled at 230℃ and the bottom temperature at 340℃, and the vacuum degree controlled at 0.02MPa. The bottom product was a distillate oil with a density of 0.103 g / cm³. 3 The content of oxygen-containing compounds in the form of phenols and their derivatives was 0.72%. The distillate oil and light diesel oil were mixed at a mass ratio of 10:6 and thoroughly stirred at 80℃. The mixture was then introduced into a flocculation reactor with a length-to-diameter ratio of 3:1 for flocculation separation at 130℃ for 2.5 hours. The light phase oil and heavy phase oil were then separated at a mass ratio of 1:0.25. The quinoline-insoluble content of the light phase oil was 0.03%. The light phase oil was poured into a distillation kettle, with the top temperature controlled at 160℃, the bottom temperature at 320℃, and the vacuum degree controlled at 0.05 MPa. Light diesel oil was recovered by distillation, and light purified tar was obtained at the bottom of the kettle. The quinoline-insoluble content of the light purified tar was 0.012%. The heavy phase oil was poured into a distillation kettle, with the top temperature controlled at 160℃, the bottom temperature at 320℃, and the vacuum degree controlled at 0.05 MPa. Light diesel oil was recovered by distillation at 0.07 MPa, and heavy tar was obtained at the bottom of the still. The heavy tar and quinoline were introduced into an extractor at a mass ratio of 1:3. The sieve mesh size was 400 mesh, and the extraction temperature was controlled at 245℃ for 2 hours. The extraction separated the soluble and insoluble phases, with the quinoline insoluble content in the soluble phase being 0.02%. The soluble phase was poured into a distillation still, and the column top temperature was controlled at 220℃, the column bottom temperature at 350℃, and the vacuum degree at 0.07 MPa. Quinoline was recovered by distillation, and heavy purified tar was obtained at the bottom of the still, with a quinoline insoluble content of 0.01%. The light purified tar was then subjected to two-stage hydrotreating in a fluidized bed to obtain distillate oil A. The first stage reaction temperature was 315℃, and the second stage reaction temperature was 390℃. The density of distillate oil A was 0.893 g / cm³. 3The fraction with a temperature below 350℃ has a content of 80.2%, the fraction with a temperature below 500℃ has a content of 98.8%, a sulfur content of 0.092%, and a nitrogen content of <0.193%. Heavy purified tar is subjected to fixed-bed catalytic hydrogenation to obtain fraction oil B at a reaction temperature of 430℃. Fraction oil B has a quinoline-insoluble content of 5.6%, an aromatic content of 75% in the fraction above 370℃, a sulfur content of 0.12%, a nitrogen content of 0.181%, and a fraction with a temperature below 500℃ of 98.5%. Fraction oil B is then distilled in a vacuum distillation column with the top temperature controlled at 280℃, the bottom temperature controlled at 390℃, and a vacuum degree controlled at 0.08 MPa. The bottom of the column yields fraction oil C, which has a quinoline-insoluble content of 8.6%, an aromatic content of 80.5%, a sulfur content of 0.11%, and a nitrogen content of 0.18%. Distillate oil A undergoes asphaltification in a polymerization reactor. First, the reactor is purged with high-purity nitrogen to achieve an initial pressure of 0.01 MPa. The pressure is then increased from room temperature to 260°C at a rate of 5°C / min, and held at this temperature for 6 hours. The final pressure inside the reactor is maintained at 0.3 MPa, yielding prepolymerized asphalt. The prepolymerized asphalt has a softening point of 30°C, a toluene-insoluble content of 8.2%, a quinoline-insoluble content of 3.1%, an ash content of 0.02%, a sulfur content of 0.092%, and a nitrogen content <0.193%. Distillate oil C and prepolymerized asphalt are mixed at a mass ratio of 1:2 and introduced into a coking reactor for liquid-phase carbonization. The reactor is purged with high-purity nitrogen, the reaction temperature is maintained at 540°C for 4 hours, and the heating rate is 8°C / min, yielding a semi-coke with an embedded structure. The true density of the semi-coke with the embedded structure is 1.86 g / cm³. 3 The volatile matter content was 5.8%, the ash content was 92 ppm, and the average particle size of the mosaic structure was 8 μm. After crushing the obtained mosaic structure semi-raw coke to an average particle size of less than 12 mm, the calcination temperature was controlled at 1450℃, the oxygen content in the kiln was 3%, and the residence time of the calcined material was 3 h. Isotropic coke for core graphite was obtained through rotary kiln calcination. The average particle size of the isotropic coke for core graphite was 4 mm, and the true density was 2.132 g / cm³. 3 The powder resistivity is 122 μΩ·m, and the vibration bulk density is 0.105 g / cm³. 3 The porosity is 18%, the Hastelloy grindability index is 12.4, the sulfur content is 0.092%, the nitrogen content is <0.193%, the ash content is ≤90ppm, the neutron-absorbing impurity is 1.32ppm, the particle stability is 94.5%, the degree of graphitization is 85%, and the isotropy is 1.01. The purity of the nuclear graphite obtained by this invention is 99.4%, the degree of graphitization is 96.2%, and the boron content is 0.41×10⁻⁶. -6 It has a compressive strength of 67 MPa, a flexural strength of 48 MPa, and a density of 1.82 g / cm³. 3The isotropic degree is 1.01, the total ash content is 0.09%, the total sulfur and nitrogen content is 0.23%, and the coefficient of thermal expansion (from room temperature to 600℃) is 0.96×10⁻⁶. -6 / ℃.

[0061] Example 4:

[0062] High-temperature coal tar was distilled in a vacuum distillation column, with the top temperature controlled at 130℃, the bottom temperature at 240℃, and the vacuum degree at 0.09MPa. The bottom product was a distillate oil with a density of 0.105 g / cm³. 3 The content of oxygen-containing compounds in the form of phenols and their derivatives was 0.51%. The distillate oil was mixed with petroleum ether at a mass ratio of 10:11 and stirred thoroughly at 40°C. The mixture was then introduced into a flocculation reactor with a length-to-diameter ratio of 3.5:1 for flocculation separation at 60°C for 4 hours. The light phase oil and heavy phase oil were then separated at a mass ratio of 1:1. The quinoline-insoluble content of the light phase oil was 0.021%. The light phase oil was poured into a distillation kettle, with the top temperature controlled at 180°C, the bottom temperature at 270°C, and the vacuum degree controlled at 0.07 MPa. Petroleum ether was recovered by distillation, and light purified tar was obtained at the bottom of the kettle, with a quinoline-insoluble content of 0.02%. The heavy phase oil was poured into a distillation kettle, with the top temperature controlled at 190°C, the bottom temperature at 300°C, and the vacuum degree controlled at... Petroleum ether was recovered by distillation at 0.07 MPa, and heavy tar was obtained at the bottom of the still. The heavy tar and pyridine were introduced into an extractor at a mass ratio of 1:1. The sieve mesh size was 400 mesh, and the extraction temperature was controlled at 120℃ for 2 hours. Soluble and insoluble phases were separated by extraction, and the quinoline insoluble content of the obtained soluble phase was 0.05%. The soluble phase was poured into a distillation still, and the top temperature of the column was controlled at 80℃, the bottom temperature at 150℃, and the vacuum degree at 0.08 MPa. Pyridine was recovered by distillation, and heavy purified tar was obtained at the bottom of the still, with a quinoline insoluble content of 0.045%. The light purified tar was subjected to two-stage hydrogenation in a fluidized bed to remove impurities, yielding distillate oil A. The first stage reaction temperature was 230℃, and the second stage reaction temperature was 370℃. The density of distillate oil A was 0.96 g / cm³. 3The fraction with a content of 73% below 350℃, 97.8% below 500℃, 0.153% sulfur, and 0.23% nitrogen was obtained by catalytic hydrogenation of heavy purified tar in a fixed bed at 400℃. The quinoline-insoluble content of fraction B was 3.8%, the aromatic content of the fraction above 370℃ was 63.8%, the sulfur content was 0.174%, the nitrogen content was 0.193%, and the content of the fraction below 500℃ was 97.5%. Fraction B was then distilled in a vacuum distillation column with the top temperature controlled at 280℃, the bottom temperature at 390℃, and the vacuum degree at 0.09 MPa. The bottom of the column yielded fraction C, which had a quinoline-insoluble content of 7.2%, an aromatic content of 78.5%, a sulfur content of 0.172%, and a nitrogen content of 0.194%. %; Distillate oil A undergoes asphaltification reaction in a polymerization reactor. First, the reactor is purged with high-purity nitrogen to achieve an initial pressure of 0.5 MPa. The pressure is then increased from room temperature to 400℃ at a rate of 8℃ / min, held at this temperature for 1 hour, and the final pressure inside the reactor is maintained at 1.0 MPa, yielding prepolymerized asphalt. The prepolymerized asphalt has a softening point of 45℃, a toluene-insoluble content of 13.6%, a quinoline-insoluble content of 5.3%, an ash content of 0.02%, a sulfur content of 0.151%, and a nitrogen content of 0.21%. Distillate oil C and prepolymerized asphalt are mixed at a mass ratio of 1:3.5 and introduced into a coking reactor for liquid-phase carbonization reaction. The reactor is purged with high-purity nitrogen, the reaction temperature is 490℃, the holding time is 4 hours, and the heating rate is 7℃ / min, yielding a semi-coke with an embedded structure. The true density of the semi-coke with the embedded structure is 1.81 g / cm³. 3 The volatile matter content was 5.3%, the ash content was 92 ppm, and the average particle size of the mosaic structure was 8 μm. After crushing the obtained mosaic structure semi-raw coke to an average particle size of less than 15 mm, the calcination temperature was controlled at 1500℃, the oxygen content in the kiln was 5%, and the residence time of the calcined material was 2.5 h. Isotropic coke for core graphite was obtained through rotary kiln calcination. The average particle size of the isotropic coke for core graphite was 4 mm, and the true density was 2.134 g / cm³. 3 The powder resistivity is 115 μΩ·m, and the vibration bulk density is 0.92 g / cm³. 3 The porosity is 22%, the Hastelloy grindability index is 12, the sulfur content is 0.15%, the nitrogen content is 0.2%, the ash content is 96 ppm, the neutron-absorbing impurities are 1.8 ppm, the particle stability is 96%, the degree of graphitization is ≥84%, and the isotropy is 1.01. The purity of the nuclear graphite obtained by this invention is 99.6%, the degree of graphitization is 97.1%, and the boron content is 0.44 × 10⁻⁶. -6 It has a compressive strength of 70 MPa, a flexural strength of 43 MPa, and a density of 1.83 g / cm³. 3The isotropy is 1.01, the total ash content is 0.19%, the total sulfur and nitrogen content is 0.27%, and the coefficient of thermal expansion (from room temperature to 600℃) is 0.93 × 10⁻⁶. -6 / ℃.

[0063] Example 5:

[0064] High-temperature coal tar was distilled in a vacuum distillation column with the top temperature controlled at 150℃, the bottom temperature controlled at 310℃, and the vacuum degree controlled at 0.08MPa. The bottom product was a distillate oil with a density of 0.998 g / cm³. 3 The content of oxygen-containing compounds in the form of phenols and their derivatives is 0.32%. Distillate oil and kerosene are mixed at a mass ratio of 10:7 and stirred thoroughly at 70℃. The mixture is then introduced into a flocculation reactor with a length-to-diameter ratio of 5:1 for flocculation separation at 90℃ for 2.5 hours. Light phase oil and heavy phase oil are then separated at a mass ratio of 1:0.3, with the light phase oil containing 0.02% quinoline-insoluble matter. The light phase oil is poured into a distillation kettle, with the top temperature controlled at 160℃, the bottom temperature at 280℃, and the vacuum degree controlled at 0.07 MPa. Kerosene is recovered by distillation, and light purified tar is obtained at the bottom of the kettle, containing 0.01% quinoline-insoluble matter. The heavy phase oil is poured into a distillation kettle, with the top temperature controlled at 170℃, the bottom temperature at 300℃, and the vacuum degree controlled at... Kerosene was recovered by distillation at 0.07 MPa, and heavy tar was obtained at the bottom of the still. The heavy tar and toluene were introduced into an extractor at a mass ratio of 1:2. The sieve mesh size was 400 mesh, and the extraction temperature was controlled at 120℃ for 2 hours. Soluble and insoluble phases were separated by extraction, and the quinoline insoluble content of the obtained soluble phase was 0.03%. The soluble phase was poured into a distillation still, and the top temperature of the column was controlled at 80℃, the bottom temperature at 150℃, and the vacuum degree at 0.07 MPa. Toluene was recovered by distillation, and heavy purified tar was obtained at the bottom of the still, with a quinoline insoluble content of 0.02%. The light purified tar was subjected to two-stage hydrogenation in a fluidized bed to remove impurities, yielding distillate oil A. The first stage reaction temperature was 220℃, and the second stage reaction temperature was 340℃. The density of distillate oil A was 0.92 g / cm³. 3The fraction with a temperature below 350℃ has a content of 71.2%, the fraction with a temperature below 500℃ has a content of 98.8%, a sulfur content of 0.17%, and a nitrogen content of 0.19%. Heavy purified tar is subjected to fixed-bed catalytic hydrogenation to obtain fraction oil B at a reaction temperature of 380℃. Fraction oil B has a quinoline-insoluble content of 4.2%, an aromatic content of 75% in the fraction above 370℃, a sulfur content of 0.142%, a nitrogen content of 0.143%, and a fraction with a temperature below 500℃ of 98%. Fraction oil B is then distilled in a vacuum distillation column with the top temperature controlled at 280℃, the bottom temperature controlled at 390℃, and a vacuum degree controlled at 0.09 MPa. The bottom of the column yields fraction oil C, which has a quinoline-insoluble content of 7.6%, an aromatic content of 80.5%, a sulfur content of 0.141%, and a nitrogen content of 0.14%. Distillate oil A undergoes asphaltification in a polymerization reactor. First, the reactor is purged with high-purity nitrogen to achieve an initial pressure of 0.01 MPa. The pressure is then increased from room temperature to 360°C at a rate of 3°C / min, and held at this temperature for 4 hours. The final pressure inside the reactor is maintained at 0.4 MPa, yielding prepolymerized asphalt. The prepolymerized asphalt has a softening point of 55°C, a toluene-insoluble content of 13.8%, a quinoline-insoluble content of 6.3%, an ash content of 0.01%, a sulfur content of 0.17%, and a nitrogen content of 0.19%. Distillate oil C and prepolymerized asphalt are mixed at a mass ratio of 1:2 and then introduced into a coking reactor for liquid-phase carbonization. The reactor is purged with high-purity nitrogen, the reaction temperature is maintained at 520°C for 2 hours, and the heating rate is 5°C / min, yielding a semi-coke with an embedded structure. The true density of the semi-coke with the embedded structure is 1.76 g / cm³. 3 The volatile matter content was 4.8%, the ash content was 88 ppm, and the average particle size of the mosaic structure was 13 μm. After crushing the obtained mosaic structure semi-raw coke to an average particle size less than 12 mm, the calcination temperature was controlled at 1300℃, the oxygen content in the kiln was 3%, and the residence time of the calcined material was 2.5 h. Isotropic coke for core graphite was obtained through rotary kiln calcination. The average particle size of the isotropic coke for core graphite was 4.5 mm, and the true density was 2.124 g / cm³. 3 The powder resistivity is 140 μΩ·m, and the vibration bulk density is 0.95 g / cm³. 3 The porosity is 20%, the Hastelloy grindability index is 14, the sulfur content is 0.152%, the nitrogen content is 0.173%, the ash content is 90 ppm, the neutron-absorbing impurities are 1.4 ppm, the particle stability is 96%, the degree of graphitization is 83%, and the isotropy is 1.01. The purity of the nuclear graphite obtained by this invention is 99.7%, the degree of graphitization is 95.2%, and the boron content is 0.46 × 10⁻⁶. -6 It has a compressive strength of 73 MPa, a flexural strength of 41 MPa, and a density of 1.84 g / cm³. 3The isotropy is 1.01, the total ash content is 0.16%, the total sulfur and nitrogen content is 0.21%, and the coefficient of thermal expansion (from room temperature to 600℃) is 0.93×10⁻⁶. -6 / ℃.

Claims

1. A method for preparing isotropic coke for nuclear graphite, characterized in that, The methods and steps include the following: 1) Distillate oil is obtained from high-temperature coal tar through vacuum distillation. The vacuum distillation process is characterized by controlling the bottom temperature of the column at 220~340℃, the top temperature at 130~230℃, and the vacuum degree at 0.01~0.09MPa. 2) After mixing the distillate oil obtained in step 1) with solvent A, the light phase oil and heavy phase oil are obtained by flocculation separation. The conditions for flocculation separation are: flocculation mixing temperature is 20~80℃, flocculation reaction temperature is 60~140℃, and flocculation time is 1~8h. 3) The light phase oil obtained in step 2) is distilled to recover the solvent and obtain light purified tar; 4) The heavy phase oil obtained in step 2) is distilled to recover the solvent and obtain heavy tar; 5) The heavy tar obtained in step 4) is mixed with solvent B and extracted to separate the soluble and insoluble phases; the soluble phase is then distilled to recover the solvent and obtain purified heavy tar. 6) The light purified tar obtained in step 3) is subjected to a multi-stage series hydrogenation impurity removal process to obtain distillate oil A; 7) The heavy purified tar obtained in step 5) is subjected to hydrocatalytic cracking to obtain distillate oil B; 8) After removing the 370℃ front-range oil from the distillate oil B obtained in step 7) by vacuum distillation, the resulting heavy oil is distillate oil C. 9) The distillate oil A obtained in step 6) is subjected to an asphaltification reaction to obtain prepolymerized asphalt; the prepolymerized asphalt has a softening point of 20~80℃, a toluene-insoluble content of 6~18%, and a quinoline-insoluble content of ≥3%; 10) After mixing the distillate oil C obtained in step 8) with the prepolymerized asphalt obtained in step 9), the mixture is subjected to a liquid phase carbonization process to obtain a semi-coke with an embedded structure; the liquid phase carbonization process conditions are: reaction temperature at 460~540℃, constant temperature time at 0.5~4h, and heating rate at 3~10℃ / min. 11) After crushing the mosaic structure semi-raw coke obtained in step 10), isotropic coke for nuclear graphite is obtained by calcination.

2. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, In step 1) above, the density of the distillate oil is 0.9~1.1 g / cm³. 3 The content of oxygen-containing compounds existing in the form of phenols and their derivatives is ≤1%.

3. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, In step 2) above, solvent A includes one or a mixture of several of petroleum ether, gasoline, kerosene, light diesel oil, cyclohexane, and pyridine; the distillate oil and solvent are mixed in a mass ratio of distillate oil:solvent = 10:(1~12); The yield ratio of the light phase oil to the heavy phase oil is 1:(0.1~1), and the quinoline insoluble content of the light phase oil is ≤0.1%.

4. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, The quinoline insoluble content of the light purified tar described in step 3) above is ≤0.1%; The distillation conditions in steps 3 and 4 above are as follows: the bottom temperature of the column is controlled at 240-380℃, the top temperature of the column is controlled at 140-300℃, and the vacuum degree is controlled at 0.01-0.09MPa.

5. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, The distillation conditions in step 5) above are: the bottom temperature of the column is controlled at 120-360℃, the top temperature of the column is controlled at 40-220℃, and the vacuum degree is controlled at 0.01-0.09MPa; The solvent B is one or a mixture of several of toluene, xylene, pyridine, heptane, pentane, crude benzene, quinoline, and isoquinoline; the heavy tar and the solvent are mixed in a mass ratio of heavy tar:solvent B = 1:(1~5); The extraction and separation conditions are: extraction temperature 100-320℃, sieve mesh 300-1200 mesh; The content of quinoline-insoluble matter in the soluble phase is ≤0.1%; The content of quinoline insoluble matter in the heavy purified tar is ≤0.1%.

6. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, In step 6) above, the multi-stage series hydrogenation and impurity removal process is a multi-stage series hydrogenation and impurity removal process of boiling bed, fluidized bed or fixed bed, wherein the reaction temperature of the first stage is 180~320℃, and the reaction temperature of the second stage or subsequent stages is 220~400℃. The density of the distillate oil A is 0.88~1.0 g / cm³. 3 The content of the distillate below 350℃ is ≥60%; the content of the distillate below 500℃ is ≥95%; the sulfur content is ≤0.2% and the nitrogen content is <0.3%.

7. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, In step 7) above, the hydrocatalytic cracking process is a fixed-bed catalytic hydrogenation process with a reaction temperature of 280~440℃; The distillate oil B has the following characteristics: a content of ≥90% of the fraction below 500℃; a content of ≥3% of quinoline insoluble matter; an aromatic content of ≥50% of the fraction above 370℃; a sulfur content of ≤0.2%; and a nitrogen content of <0.3%.

8. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, The distillation conditions in step 8) above are: the bottom temperature of the column is controlled at 380-480℃, the top temperature of the column is controlled at 280-360℃, and the vacuum degree is controlled at 0.01-0.09MPa; The distillate oil has a quinoline insoluble content of ≥6%, an aromatic content of ≥65%, a sulfur content of ≤0.2%, and a nitrogen content of <0.3%.

9. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, The asphaltification reaction in step 9) above requires nitrogen or inert gas purging protection. The initial reaction pressure is 0.01~0.5MPa, the final reaction pressure is 0.1~1.0MPa, the reaction temperature is 260~400℃, the isothermal time is 1~6h, and the heating rate is 3~10℃ / min. The prepolymerized asphalt has an ash content of ≤0.05%, a sulfur content of ≤0.2%, and a nitrogen content of <0.3%.

10. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, In step 10) above, the distillate oil C and prepolymer asphalt are mixed in a mass ratio of distillate oil C: prepolymer asphalt = 1: (0.1~5); The true density of the inlaid structure semi-coke is ≥1.6 g / cm³. 3 Volatile matter ≤8%, ash content ≤100ppm, and the average particle size of the mosaic structure is 4~28μm.

11. The method for preparing isotropic coke for nuclear graphite according to claim 1, characterized in that, The average particle size of the embedded structure semi-raw coke after crushing in step 11) above is 10~30mm; The calcination process uses a rotary kiln, with a calcination temperature of 1200~1600℃, an oxygen content of 2%~15% in the kiln, and a residence time of 1~4 hours for the calcined material.

12. An isotropic coke prepared using the method for preparing isotropic coke for nuclear graphite as described in any one of claims 1-11, characterized in that, Its average particle size is 1~8mm, and its true density is ≥2.12g / cm³. 3 Powder resistivity ≤150μΩ·m, vibration bulk density ≥0.85g / cm³ 3 Porosity ≤28%, Hastelloy grindability index 12~15, sulfur content ≤0.2%, nitrogen content <0.3%, ash content ≤100ppm, neutron-absorbing impurities <2ppm, particle stability ≥92%, graphitization degree ≥65%, isotropy 1~1.05.

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