Quality evaluation method of calcined needle coke for graphite electrode

Abstract

The invention relates to a quality evaluation method of calcined needle coke for a graphite electrode. The quality evaluation method comprises the following steps: S1, testing the true density, volatile component content, ash content and S and N content of a needle coke sample; if the test result meets the first condition, entering the step S2; the first condition is that the true density is not less than 2.12 g / cm < 3 >, the volatile component is not more than 0.4 wt%, the ash content is not more than 0.3 wt%, the S content is not more than 0.5 wt%, and the N content is not more than 0.5 wt%; s2, testing the particle length-width ratio of the needle coke sample; if the test result meets a second condition, the step S3 is executed, and the second condition is that the average length-width ratio of the particles is larger than or equal to 1.7; s3, testing the optical tissue structure content distribution of the needle coke sample, and if a test result meets a third condition, determining that the quality of the needle coke sample is qualified; the third condition is that the content of the fiber structure is greater than or equal to 35%, and the sum of the contents of the small pieces and the mosaic structure is less than or equal to 20%. The method provided by the invention can realize rapid evaluation of the quality of the needle coke so as to guide production.

Description

Technical Field

[0001] This invention relates to the field of needle coke for graphite electrodes, and more specifically, to a method for quality evaluation of calcined needle coke for graphite electrodes. Background Technology

[0002] To meet the huge market demand and the urgent requirements for energy conservation and emission reduction, the production of high-value-added carbonaceous materials such as needle coke has become a development strategy for the oil refining industry. Depending on the raw materials and operating parameters, delayed coking can produce various grades of coke with different physical properties. Among them, needle coke, a high-quality coke product, is widely used in the field of graphite electrodes, possessing excellent properties such as high thermal and electrical conductivity, high strength, high density, and low coefficient of thermal expansion (CTE). When needle coke is used as a raw material for high-power and ultra-high-power graphite electrodes, its quality must meet even higher requirements: low ash content, low sulfur and nitrogen content, low volatile matter, high true density, and low CTE. CTE is a key indicator for measuring the quality of needle coke, as it determines the coefficient of thermal expansion of the resulting graphite electrode. Furthermore, the electrode's operational performance in the circuit furnace depends on the coefficient of thermal expansion, making it a core indicator of graphite electrode quality. The coefficient of thermal expansion of needle coke directly affects product quality. A low CTE indicates a small volume change in needle coke with temperature variations, resulting in better heat resistance and higher quality, more stable graphite electrodes. The current method for testing the coefficient of thermal expansion (CTE) of needle coke involves a 7-day testing cycle. First, the coke is calcined to 1000-1400°C. It is then mixed with molten bitumen binder and extruded to form a green electrode. This electrode is subsequently baked to approximately 800-900°C, followed by heating to 2800-3400°C to achieve graphitization. The CTE is measured on this graphitized electrode using a dilatometer or capacitance method.

[0003] CN108680412A discloses a sample preparation method and a testing method for the thermal expansion coefficient of calcined coke, which includes sampling, crushing, kneading, molding, pre-calcination treatment, calcination, graphitization, sample processing, and thermal expansion coefficient determination, improving the repeatability and accuracy of thermal expansion coefficient determination. However, the sample preparation and testing methods are cumbersome.

[0004] CN 114460128A discloses a method for rapidly testing the thermal expansion coefficient of needle coke. Specifically, it includes: 1) determining the standard curve of a thermomechanical analyzer; 2) preparing the needle coke sample; 3) thermomechanical analysis; and 4) calculation: obtaining the MCTE value of the sample to be tested through a thermomechanical analyzer, substituting it into the linear formula (1) to calculate the thermal expansion coefficient of the sample after graphitization. The testing cycle is shortened from 7 days to 6-10 hours, which can quickly and accurately provide adjustment parameters for the production process, ensuring the stability of needle coke product quality and the production process.

[0005] To produce high-quality needle coke, it is necessary to guide the adjustment of delayed coking process parameters based on needle coke quality evaluation results. However, in actual production, the determination of the coefficient of thermal expansion (CTE) of needle coke samples is a cumbersome process, time-consuming, and costly. When process parameters need to be adjusted to improve product quality, it is difficult to quickly obtain quality evaluation results to guide production. When multiple needle coke samples are present, it is difficult to quickly determine the quality of the products, failing to meet the needs of downstream users. Therefore, how to rapidly evaluate the quality of needle coke, shorten the evaluation cycle, and guide production is an urgent problem to be solved. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a quality evaluation method for calcined needle coke for graphite electrodes, thereby shortening the evaluation cycle and reducing costs, and achieving rapid evaluation.

[0007] This invention provides a method for quality evaluation of calcined needle coke for graphite electrodes, comprising:

[0008] S1: Test the true density, volatile matter, ash content, and S and N content of the needle coke sample; if the test result of any one of these items does not meet the first condition, the needle coke sample is unqualified; if the test result meets the first condition, proceed to step S2; the first condition is: true density ≥ 2.12 g / cm³. 3 Volatile matter ≤ 0.4 wt%, ash content ≤ 0.3 wt%, sulfur content ≤ 0.5 wt%, nitrogen content ≤ 0.5 wt%;

[0009] S2: Test the aspect ratio of the needle coke sample particles; if the test result does not meet the second condition, the quality of the needle coke sample is unqualified; if the test result meets the second condition, proceed to step S3, where the second condition refers to an aspect ratio of particles greater than or equal to 1.7.

[0010] S3: Test the optical microstructure content distribution of the needle coke sample. If the test result does not meet the third condition, the needle coke sample is unqualified; if the test result meets the third condition, the coefficient of thermal expansion (CTE) ≤ 1.3 * 10⁻⁶. -6 / ℃, the quality of needle coke samples is qualified; the third condition is: in the optical structure content, the fiber structure content is ≥35%, and the sum of the content of small pieces and mosaic structure is ≤20%.

[0011] In the method provided by this invention, the needle coke refers to petroleum coke from a delayed coking process. The needle coke sample is calcined needle coke obtained by calcining raw coke at a temperature of 1250℃-1380℃ for 6-12 hours.

[0012] In this invention, "qualified" needle coke sample quality means meeting the YDDH-II grade of the graphite anode evaluation standard for oil-based calcined needle coke in the quality evaluation standard GB / T37308-2019, specifically, a true density ≥ 2.12 g / cm³. 3 Volatile matter ≤ 0.4 wt%, ash content ≤ 0.3 wt%, sulfur content ≤ 0.5 wt%, nitrogen content ≤ 0.5 wt%; coefficient of thermal expansion (CTE) value ≤ 1.3 × 10⁻⁶ -6 / ℃.

[0013] Compared with the prior art, the beneficial effects of the quality evaluation method for calcined needle coke for graphite electrodes provided by the present invention are as follows:

[0014] When needle coke is used as a raw material for high-power and ultra-high-power graphite electrodes, its quality must meet higher requirements. First, simple and quick testing indicators such as ash content, sulfur and nitrogen content, volatile matter, and true density are assessed. If these requirements are met, subsequent quality evaluation is conducted. CTE (Chemical Emission Tolerance) is a key indicator for measuring needle coke quality. Its determination requires a series of complex processes including batching, mixing, impregnation, molding, calcination, and graphitization, resulting in long testing times and high investment costs. CTE is closely related to the structure of needle coke. Quantitative analysis of optical microstructure and measurement of aspect ratio describe the microstructure of needle coke in data form, providing a simple and rapid way to obtain structural information and indirectly determine whether the CTE value meets the requirements. Using these methods can shorten the time required to evaluate needle coke quality, enabling rapid quality assessment and guiding production. Attached Figure Description

[0015] Figure 1 A flowchart of a method for evaluating the quality of calcined needle coke for graphite electrodes provided by the present invention. Detailed Implementation

[0016] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0017] This invention provides a method for quality evaluation of calcined needle coke for graphite electrodes, comprising:

[0018] S1: Test the true density, volatile matter, ash content, and S and N content of the needle coke sample; if the test result of any one of these items does not meet the first condition, the needle coke sample is unqualified; if the test result meets the first condition, proceed to step S2; the first condition is: true density ≥ 2.12 g / cm³. 3 Volatile matter ≤ 0.4 wt%, ash content ≤ 0.3 wt%, sulfur content ≤ 0.5 wt%, nitrogen content ≤ 0.5 wt%;

[0019] S2: Test the aspect ratio of the needle coke sample particles; if the test result does not meet the second condition, the quality of the needle coke sample is unqualified; if the test result meets the second condition, proceed to step S3, where the second condition refers to an aspect ratio of particles greater than or equal to 1.7.

[0020] S3: Test the optical microstructure content distribution of the needle coke sample. If the test result does not meet the third condition, the needle coke sample is unqualified; if the test result meets the third condition, the coefficient of thermal expansion (CTE) ≤ 1.3 * 10⁻⁶. -6 / ℃, the quality of needle coke samples is qualified; the third condition is: in the optical structure content, the fiber structure content is ≥35%, and the sum of the content of small pieces and mosaic structure is ≤20%.

[0021] Preferably, the needle coke samples are divided into two groups. The first group of needle coke samples is tested in step S1, and the second group of needle coke samples is tested in steps S2 and S3 simultaneously.

[0022] In the method provided by this invention, the needle coke refers to petroleum coke from a delayed coking process.

[0023] Preferably, the needle coke sample is calcined needle coke obtained by calcining raw petroleum coke at a temperature of 1250℃-1380℃ for 6-12 hours. More preferably, the calcination temperature is 1280℃-1350℃ and the calcination time is 8-10 hours.

[0024] One specific implementation method is as follows: The needle coke samples are divided into two groups. The first group of needle coke samples undergoes the detection in step S1; the second group of needle coke samples undergoes the detection in steps S2 and S3. The true density, volatile matter, ash content, and S and N content of the first group of samples are detected. It is determined whether the true density, volatile matter, ash content, and S and N content of the first group of samples all meet the required standards. If yes, the second group of needle coke samples proceeds to steps S2 and S3. In step S2, the aspect ratio of the particles in the second group of samples is tested. In step S3, the optical structure content of the second group of samples is tested. Based on the aspect ratio and the distribution of the optical structure content, it is determined whether the quality of the needle coke sample is qualified. If not, the quality of the needle coke sample is unqualified.

[0025] The true density of the needle coke sample was determined using the true density determination method for carbon materials - boiling method (GB / T 24203); the volatile matter of the needle coke was determined using the volatile matter determination method for carbon materials (YB / T 5189); the ash content of the needle coke sample was determined using the ash content determination method for carbon materials (GB / T 1429); and the S and N contents of the needle coke sample were determined using the total sulfur content determination method for coke (GB / T 2286) and the total sulfur content determination method for coke (GB / T 19227), respectively.

[0026] In this invention, the particle length-to-width ratio refers to the ratio of the length to the width of needle-shaped coke particles with a diameter of 0.5-1 mm. The analytical method involves crushing and sieving the needle-shaped coke sample to obtain needle-shaped coke particles with a diameter of 0.5-1.0 mm, measuring the length-to-width ratio of these particles, and taking the average value. A particle shape analyzer can be used to test the particle length-to-width ratio of the needle-shaped coke. The second condition in S2 of this invention refers to a particle length-to-width ratio greater than or equal to 1.7.

[0027] In this invention, the optical structure content distribution refers to the proportion of various optical structures in needle-shaped coke particles with a particle size of 0.2-1.0 mm observed under a polarizing microscope. The analytical method involves crushing and sieving the needle-shaped coke sample to obtain needle-shaped coke particles with a particle size of 0.2-1.0 mm. Under a polarizing microscope, coarse fibers, fine fibers, short fibers, large-scale structures, small-scale structures, and mosaic structures are identified, and the proportion of each optical structure is calculated. The fiber structure consists of coarse fibers, fine fibers, and short fibers. The short fibers have a length and width < 30 μm and an aspect ratio greater than 2.5. The fine fiber structure has an aspect ratio greater than 5, a length > 10 μm, and a width < 10 μm. The coarse fibers have a length > 30 μm and a width between 10 μm and 30 μm. The large-scale structures have a length and width both greater than 30 μm. The small-scale structures have a length and width between 10 μm and 30 μm. The mosaic structures have a length and width both less than 10 μm.

[0028] In this invention, the third condition mentioned in S3 is: the content of fiber structure is ≥35%, and the sum of the contents of small pieces and inlaid structure is ≤20%.

[0029] Preferably, the content of fiber structure is ≥40%, and the sum of the content of small pieces and inlaid structure is ≤16%.

[0030] In this invention, a qualified needle coke sample is defined as having a true density ≥ 2.12 g / cm³. 3 Volatile matter ≤ 0.4 wt%, ash content ≤ 0.3 wt%, sulfur content ≤ 0.5 wt%, nitrogen content ≤ 0.5 wt%; coefficient of thermal expansion (CTE) ≤ 1.3 × 10⁻⁶ -6 / ℃.

[0031] The optical microstructure of needle forks is influenced by the properties of the raw materials and processing techniques, including fine fibers, coarse fibers, short fibers, large flakes, small flakes, and mosaic structures. Different optical microstructure types exhibit different optical properties under a polarizing microscope, which are closely related to the performance of the needle forks. Therefore, optical microstructure analysis can be used for the quality evaluation of needle forks, including the types of optical microstructures contained in the sample and the content of different types of optical microstructures.

[0032] In a specific embodiment of the present invention, needle coke samples are divided into two groups. The second group is tested for its average aspect ratio and optical structure, provided that the true density, volatile matter, ash content, and S and N content of the first group meet the requirements. If any item in the first group fails to meet the requirements, the second group is not tested. Considering that the time required for testing true density, volatile matter, ash content, and S and N content is shorter than that required for particle aspect ratio and optical structure, unqualified needle coke products can be evaluated within 2-4 hours. A binary method is used to divide the needle coke samples into two groups to ensure random sampling. The particle aspect ratio testing time in step S2 of the present invention can be controlled within 3-5 hours. Preferably, the needle coke samples are divided into two groups, and the first and second groups of needle coke samples are tested simultaneously in steps S1 and S2. If the particle aspect ratio of the needle coke sample is less than 1.7, the needle coke sample can be evaluated as unqualified within 3-5 hours. The results of the optical tissue structure analysis and testing in step S3 can be controlled within 6 hours. It takes 6-15 hours to evaluate whether the needle coke sample is qualified or unqualified.

[0033] The optical morphological characteristics of needle coke samples can reflect their microscopic hierarchical structure. The fibrous structure within the optical microstructure gives needle coke high graphitizability. The fiber morphology undergoes sufficient plastic deformation, significantly alleviating internal stress. While promoting crystallite orientation, it also facilitates layer rearrangement within the crystallites, resulting in better thermal shock resistance. In contrast, mosaic and flake structures, with their small optical microstructure size and minimal plastic deformation, accumulate significant internal stress, which is detrimental to CTE performance. The particle aspect ratio (K-value) is a macroscopic manifestation of the microstructure. Due to anisotropy, needle coke generally has a large particle aspect ratio (K-value). A larger K-value indicates better needle-like texture and better fiber structure continuity. For qualified samples determined according to the above evaluation method, the CTE ≤ 1.3 * 10⁻⁶. -6 / ℃, and all indicators meet the YDDH-II grade coke standard for calcined oil-based needle coke used in graphite electrodes.

[0034] The following embodiments further illustrate the technical effects of the needle coke analysis and testing method provided by the present invention. However, the present invention is not limited thereto.

[0035] In the examples and comparative examples:

[0036] The needle coke samples were obtained from petroleum coke obtained under different delayed coking operating conditions at Maoming Petrochemical Branch of China Petroleum & Chemical Corporation, and were obtained after calcination.

[0037] The average particle aspect ratio test method is as follows:

[0038] After crushing the needle-shaped coke sample using a roller crusher, 100g of the sample was sieved on a mechanical vibrating screen to obtain coke particles with a diameter of 0.5-1 mm. Particle morphology analysis was performed using a QICPIC-GRI-DAS / L particle shape analyzer manufactured by Synpatek GmbH, Germany. The particle aspect ratio was calculated by taking the reciprocal of ASPECT_RATIO in the system. A sample was considered valid if more than 400 particles were measured. Finally, the average aspect ratio of the entire sample was calculated based on the aspect ratio of each particle. The method for testing the optical microstructure content distribution was as follows:

[0039] 4-5g of sample with a particle size of 0.2mm-1mm was sieved and used for sample preparation. This sample was poured into a mold coated with Vaseline, and appropriate amounts of resin and curing agent were added and stirred until homogeneous. After curing, the sample was removed and polished using sandpaper and a polishing disc. The polished sample surface was glossy and free of obvious scratches. The sample was observed under a polarizing microscope, revealing clear optical structures with distinct features. A dot-counting method was used to determine the step size (800μm recommended). 400 dots were selected with a dot spacing of 0.3mm-0.5mm and a row spacing of 0.5mm-0.8mm. Each dot was identified according to the classification method in Table 1, and the content of fine fibers, coarse fibers, short fibers, large flakes, small flakes, and mosaic structures was statistically analyzed. The classification criteria for optical structures are shown in Table 1. Analysis can be performed using computational software such as the MIVNT microscope image analysis system from Ningbo Yongxin Optics Co., Ltd.

[0040] Table 1

[0041]

[0042] Example 1

[0043] Sample A of calcined needle coke for production was selected and calcined at 1330℃ for 8 hours.

[0044] Step S1: The true density, volatile matter, ash content, and S and N content of needle coke sample A were tested, and the results are shown in Table 2. The test took 3 hours.

[0045] Needle coke sample A meets the first condition: true density ≥ 2.12 g / cm³. 3 Volatile matter ≤ 0.4 wt%, ash content ≤ 0.3 wt%, sulfur content ≤ 0.5 wt%, nitrogen content ≤ 0.5 wt%.

[0046] S2: The aspect ratio of needle coke sample A was tested, and the test took 3.5 hours. The results are shown in Table 3. Since the average aspect ratio of needle coke sample A is greater than 1.7.

[0047] S3: Continue testing the optical microstructure content distribution of needle coke sample A, which took 6 hours. The results are shown in Table 3.

[0048] Based on the particle aspect ratio K≥1.7 and the optical structure content distribution in Table 3, the fiber structure content is ≥35%, and the sum of the content of small pieces and mosaic structure is ≤20%, the needle coke sample is judged to be of qualified quality.

[0049] The CTE value of needle coke sample A was determined using the method described in GB / T 3074.4, "Determination of Coefficient of Thermal Expansion (CTE) of Graphite Electrode." Specifically, the sample was crushed and mixed with molten asphalt binder, and the asphalt / coke mixture was extruded to form a green electrode. This electrode was then calcined to approximately 800-900℃, followed by heating to 2800-3400℃ to achieve graphitization. The CTE was measured on the graphitized electrode using a dilatometer or capacitance method. After 62 hours, the CTE value was ≤1.30 × 10⁻⁶. -6 / ℃, meets the requirements.

[0050] Examples 2-6

[0051] Samples B, C, D, E, and F of calcined needle coke were taken for production and calcined under the following conditions: 1280℃ for 10h, 1300℃ for 9h, 1310℃ for 8h, 1290℃ for 9h, and 1320℃ for 8h, respectively.

[0052] Using the method of Example 1, S1: test the true density, volatile matter, ash content and S and N content of the needle coke sample. If the test results meet the first condition, S2: test the aspect ratio of the needle coke particles. If the test results meet the second condition, S3: test the optical microstructure content distribution of the needle coke sample. The test results are shown in Tables 2 and 3.

[0053] Table 2

[0054]

[0055]

[0056] Table 3

[0057]

[0058] Example 7

[0059] Take a sample G of calcined needle coke for production and calcine it at 1300℃ for 8 hours.

[0060] The needle coke samples were divided into two groups using the dichotomy method, denoted as G1 and G2 respectively.

[0061] Simultaneously, needle coke samples G1 and G2 were tested. For sample G1, true density, volatile matter, ash content, and S and N content were measured; the results are shown in Table 4. The testing time was 4 hours. For sample G2, the particle aspect ratio was measured; the results are shown in Table 5. The testing time was 4 hours. The CTE value of sample G2 was determined, taking 58 hours; the results are shown in Table 5.

[0062] Examples 8-9

[0063] Needle coke samples H and I, after calcination for production, were taken respectively. The calcination conditions were 1280℃ for 9 hours and 1290℃ for 10 hours, respectively. The needle coke samples H and I were treated according to the method in Example 7. The true density, volatile matter, ash content, and S and N content are shown in Table 4, with testing times of 2 hours and 3 hours, respectively. The aspect ratio of the needle coke samples was tested, and the results are shown in Table 4, with testing times of 4 hours and 5 hours, respectively. The CTE values ​​of needle coke samples H and I were measured, with testing times of 56 hours and 60 hours, respectively, and the results are shown in Table 4.

[0064] Table 4

[0065] Example 7 Example 8 Example 9 Needle coke sample number G H I <![CDATA[True density, g / cm 3 > 2.12 2.11 2.13 Volatile matter, wt% 0.5 0.32 0.33 Ash content, wt% 0.28 0.26 0.19 S content, wt% 0.61 0.35 0.3 Ni content, wt% 0.38 0.57 0.28 Particle aspect ratio K 1.68 1.66 1.65 Judgment result Unqualified Unqualified Unqualified <![CDATA[CTE value * 10 -6 / °C]]> 1.39 1.34 1.41

[0066] As shown in Table 4, needle coke sample G does not meet the requirement of volatile matter ≤ 0.4 wt%, and needle coke sample H does not meet the requirement of true density ≥ 2.12 g / cm³. 3 The needle coke samples G and H were directly deemed substandard. The aspect ratio of needle coke sample I was 1.65, which did not meet the requirement of ≥1.7, therefore needle coke sample I was deemed substandard.

[0067] The quality evaluation method for calcined needle coke for graphite electrodes provided by this invention takes 8 hours, 6 hours and 8 hours respectively in Examples 7-9. Compared with the method for testing CTE values, it takes 50 hours, 52 hours and 48 hours less. The quality evaluation method provided by this invention can quickly obtain the evaluation results of needle coke samples and adjust the needle coke production conditions in a timely manner.

Claims

1. A method for evaluating the quality of calcined needle coke for graphite electrodes, characterized in that, include: S1: Test the true density, volatile matter, ash content, and S and N content of the needle coke sample; if the test result of any one of these items does not meet the first condition, the needle coke sample is unqualified; if the test result meets the first condition, proceed to step S2; the first condition is: true density ≥ 2.12 g / cm³. 3 Volatile matter ≤ 0.4 wt%, ash content ≤ 0.3 wt%, sulfur content ≤ 0.5 wt%, nitrogen content ≤ 0.5 wt%; S2: Test the aspect ratio of needle coke particles; If the test result does not meet the second condition, the quality of the needle coke sample is unqualified; if the test result meets the second condition, proceed to step S3, where the second condition refers to a particle length-to-width ratio greater than or equal to 1.

7. S3: Test the optical microstructure content distribution of the needle coke sample. If the test result does not meet the third condition, the needle coke sample is unqualified; if the test result meets the third condition, the coefficient of thermal expansion (CTE) ≤ 1.3 * 10⁻⁶. -6 / ℃, the quality of needle coke samples is qualified; the third condition is: in the optical structure content, the fiber structure content is ≥35%, and the sum of the content of small pieces and mosaic structure is ≤20%.

2. The method for evaluating the quality of calcined needle coke for graphite electrodes according to claim 1, characterized in that, The needle coke samples were divided into two groups. The first group of needle coke samples was tested in step S1. The second group of needle coke samples was tested in steps S2 and S3 simultaneously.

3. The method for evaluating the quality of calcined needle coke for graphite electrodes according to claim 1 or 2, characterized in that, The needle coke mentioned refers to petroleum coke obtained through delayed coking.

4. The method for evaluating the quality of calcined needle coke for graphite electrodes according to claim 3, characterized in that, The needle coke sample is calcined needle coke obtained by calcining raw coke at a temperature of 1250℃-1380℃ and a calcination time of 6-12h.

5. The method for evaluating the quality of calcined needle coke for graphite electrodes according to claim 4, characterized in that, The calcination temperature is 1280-1350℃, and the calcination time is 8-10h.

6. The method for evaluating the quality of calcined needle coke for graphite electrodes according to claim 1 or 2, characterized in that, The particle length-to-width ratio refers to the ratio of the length to the width of needle coke particles with a diameter of 0.5-1.0 mm obtained by crushing and sieving the needle coke sample, and taking the average value.

7. The method for evaluating the quality of calcined needle coke for graphite electrodes according to claim 1 or 2, characterized in that, The optical structure content distribution refers to the process of crushing and sieving needle-shaped coke samples to obtain needle-shaped coke particles with a particle size of 0.2-1.0 mm, identifying coarse fibers, fine fibers, short fibers, large-scale structures, small-scale structures, and mosaic structures under a polarizing microscope, and calculating the proportion of each optical structure. The fibrous structure consists of coarse fibers, fine fibers, and short fibers. The short fibers have a length and width < 30 μm and an aspect ratio greater than 2.

5. The fine fiber structure has an aspect ratio greater than 5, a length > 10 μm, and a width < 10 μm. The coarse fibers have a length > 30 μm and a width between 10 μm and 30 μm. The large-scale structures have a length and width greater than 30 μm. The small-scale structures have a length and width between 10 μm and 30 μm. The mosaic structures have a length and width less than 10 μm.

8. The method for evaluating the quality of calcined needle coke for graphite electrodes according to any one of claims 1-7, characterized in that, In the optical tissue structure content described in S3, the content of fiber structure is ≥40%, and the sum of the contents of small pieces and mosaic structure is ≤16%.

9. The method for evaluating the quality of calcined needle coke for graphite electrodes according to any one of claims 1-7, characterized in that, The analytical method for the true density of the needle coke sample adopts the true density determination method for carbon materials - boiling method; the analytical method for the volatile matter of needle coke adopts the volatile matter determination method for carbon materials. The analytical method for the ash content of needle coke samples is the determination of the ash content of carbon materials; The S and N contents of needle coke samples were analyzed using the methods for determining the total sulfur content of coke and the methods for determining nitrogen in coal, respectively.

10. The method for evaluating the quality of calcined needle coke for graphite electrodes according to any one of claims 1-7, characterized in that, The quality qualification of the needle coke sample means that: true density ≥ 2.12 g / cm 3 , volatile matter ≤ 0.4 wt%, ash content ≤ 0.3 wt%, S content ≤ 0.5 wt%, N content ≤ 0.5 wt%; coefficient of thermal expansion CTE value ≤ 1.3*10 -6 / °C.

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