Method for producing oil-based needle coke for low-swelling graphite electrode

Abstract

The application discloses a preparation method of oil-based needle coke with low air swelling performance for graphite electrodes, and belongs to the technical field of needle coke as raw materials of graphite electrodes. The oil-based needle coke with low air swelling performance is obtained by taking FCC oil slurry as raw material, and through the steps of vacuum cutting, hydrogenation impurity removal, delayed coking and high-temperature calcination. The application has the beneficial effects that: the needle coke with high orientation and high true density is prepared through the delayed coking process with multi-gradient heating, and then the needle coke is used as raw material to perform two-stage calcination, so that the needle coke has appropriate micropore and microcrack structures, and finally the oil-based needle coke with low air swelling performance is obtained.

Description

Technical Field

[0001] This invention belongs to the field of needle coke technology, a raw material for graphite electrodes, and more specifically relates to a method for preparing oil-based needle coke for graphite electrodes with low gas expansion performance. Background Technology

[0002] Needle coke, as an aggregate for ultra-high power graphite electrodes, is a high-quality carbon material with high graphitization, low coefficient of thermal expansion, excellent electrical conductivity, and thermal shock resistance. With the rapid development of electric arc furnace (EAF) steelmaking technology, the performance requirements for ultra-high power graphite electrodes are becoming increasingly stringent. Especially under continuous operation conditions with high current and high load, ultra-high power graphite electrodes must have good electrical conductivity and be able to withstand severe thermal shock and mechanical stress.

[0003] One of the key failure modes of electrodes during the production process is "gas expansion," which manifests as irreversible abnormal volume expansion of the electrode body during high-temperature graphitization. This leads to the formation of channels and a loose structure inside the electrode, reduced conductivity, and increased local heat generation. In severe cases, excessive heat generation can cause electrode breakage or connection failure, posing significant safety hazards and economic losses to steel companies. Therefore, reducing the gas expansion performance of needle coke used in graphite electrodes is particularly important.

[0004] Currently, published patents and literature on reducing the gas expansion characteristics of oil-based needle coke focus on single-faceted improvements, such as reducing the content of heteroatoms like S, N, and O in the raw materials used to produce needle coke, and conducting product research and design. However, due to the mutual influence between the indicators of needle coke products, reducing the heteroatom content in the coking raw materials to a lower level through hydrogenation will affect other indicators of needle coke. For example, when the sulfur content of the raw materials is reduced from 1.5%-2.5% to 0.35%-0.4%, a large number of high-quality tri- and tetra-cyclic aromatic hydrocarbon molecules, such as phenanthrene, pyrene, and phenanthrene, are also destroyed, ultimately leading to a decrease in the strength of the needle coke particles, making it difficult to meet customer requirements. Summary of the Invention

[0005] To address the aforementioned problems and overcome the shortcomings of existing technologies, this invention provides a production process for oil-based needle coke with low gas expansion characteristics. Specifically, needle coke with high orientation and high true density is first prepared through a delayed coking process with multi-gradient heating. Then, this material is used as raw material for two-stage calcination to give it an appropriate amount of microporous and microcracked structure, ultimately yielding an oil-based needle coke with low gas expansion characteristics.

[0006] The specific technical solution of the present invention to solve the above-mentioned technical problems is as follows: This method uses FCC oil slurry as raw material, and after vacuum cutting, hydrogenation to remove impurities, delayed coking, and high-temperature calcination, a type of oil-based needle coke with low gas expansion characteristics is obtained. Step 1: The FCC slurry is subjected to depressurization cutting to remove a large amount of ash impurities, gum asphalt and a small amount of light components from the slurry; The second step involves hydrogenating the oil slurry after vacuum cutting to remove impurities such as sulfur, nitrogen, and oxygen, yielding refined aromatic oil with the following specifications: sulfur content 0.35%-0.4%, aromatic content 65%-70%, and total content of tri-, tetra-, and penta-cyclic aromatics 40%-50%. The third step: The refined aromatic oil after hydrotreating is subjected to delayed coking. After the coking reaction is completed, it is cooled to room temperature and then the coke is removed directly to obtain needle coke. The process control technology is characterized by a uniform gradient temperature increase, as shown in the temperature curve below: 460℃ (holding temperature for 50 hours) → 10℃ / min increase → 490℃ (holding temperature for 10 hours). The characteristics of delayed coking are that the low-temperature zone is maintained for a longer period of time, the temperature difference between the high and low temperature zones is ΔT=30℃, the feeding time is t=60h, and the needle coke after decoking has a true density of 1.375g / cm³. 3 The fiber structure accounts for 38.9%, the sheet structure accounts for 59.5%, and the mosaic structure accounts for 1.6%. Step 4: The needle coke raw coke is subjected to step-by-step calcination treatment. The calcination temperature curves are as follows: ① 30℃→800℃→100℃→1400℃. The characteristic is that the heating rate is constant at 5℃ / min, the cooling rate from 800℃ to 100℃ is 60℃ / min, and the cooling from 1400℃ to room temperature is natural cooling.

[0007] After calcination in this manner, a needle-shaped coke with a suitable amount of micropores and microcracks is generated, exhibiting low gas expansion rate characteristics.

[0008] The beneficial effects of this invention are: This invention creatively discovers that during the calcination process of needle coke, a reasonable cooling process can induce the formation of a suitable amount and uniformly distributed micropore and microcrack structures within it. Furthermore, the internal structure of the needle coke itself also significantly influences the formation of these micropore and microcrack structures, and favorable raw material properties are a necessary condition for achieving this favorable internal structure. A creative production process for oil-based needle coke with low gas expansion characteristics was adopted. First, a delayed coking process with multi-gradient heating was used to prepare needle coke with high orientation and high true density. Then, this was used as raw material for two-stage calcination to give it an appropriate amount of micropore and microcrack structure, and finally, an oil-based needle coke with low gas expansion characteristics was obtained. Attached Figure Description

[0009] Appendix Figure 1 This is a scanning electron microscope image of the calcined needle coke prepared in Example 1; Appendix Figure 2This is a scanning electron microscope image of the calcined needle coke prepared in Example 2; Appendix Figure 3 This is a scanning electron microscope image of the calcined needle coke prepared in Example 3; Appendix Figure 4 This is a scanning electron microscope image of the calcined needle coke prepared in Comparative Example 1; Appendix Figure 5 This is a scanning electron microscope image of the calcined needle coke prepared in Comparative Example 2; Appendix Figure 6 This is a scanning electron microscope image of the calcined needle coke prepared in Comparative Example 3; Appendix Figure 7 This is a scanning electron microscope image of the calcined needle coke prepared in Comparative Example 4; Appendix Figure 8 This is a scanning electron microscope image of the calcined needle coke prepared in Comparative Example 5. Detailed Implementation

[0010] Specific embodiments of the present invention: To better understand the present invention, specific embodiments are described. It is worth emphasizing that the effects of these embodiments are not substantially different from those of various embodiments within the scope of protection of the present invention, including their respective reagents and reagent content ratios. All of them can achieve the effects described in the present invention and solve the above-mentioned problems. Other combinations are not described here. Example 1 This example provides a method for preparing oil-based needle coke for graphite electrodes with low gas expansion characteristics, including: using FCC oil slurry as raw material, and obtaining refined aromatic oil through depressurization and hydrogenation; In this embodiment, the total content of 3, 4, and 5-cyclic aromatic hydrocarbons in the refined aromatic oil is controlled at 40%. After delayed coking, a needle coke with high true density and high fiber structure ratio is obtained. The needle coke is then calcined using a stepwise calcination process to obtain a calcined oil-based needle coke for graphite electrodes with low gas expansion characteristics. Specific product indicators and calcination process are shown in Table 1.

[0011] Example 2 In this example, the total content of 3, 4, and 5-cyclic aromatic hydrocarbons in the refined aromatic oil was controlled at 45%. After delayed coking, a needle coke with high true density and high fiber structure ratio was obtained. The needle coke was calcined using a stepwise calcination process to obtain a calcined oil-based needle coke for graphite electrodes with low gas expansion characteristics. The specific product indicators and calcination process are shown in Table 1.

[0012] Example 3 In this example, the total content of 3, 4, and 5-cyclic aromatic hydrocarbons in the refined aromatic oil was controlled at 50%. After delayed coking, a needle coke with high true density and high fiber structure ratio was obtained. The needle coke was calcined using a stepwise calcination process to obtain a calcined oil-based needle coke for graphite electrodes with low gas expansion characteristics. The specific product indicators and calcination process are shown in Table 1.

[0013] To more intuitively demonstrate the technological advantages of this invention, a comparison is made between the preparation method of calcined oil-based needle coke for graphite electrodes with low gas expansion characteristics, and a method using equivalent substitution in the same process. Comparative Example 1: This comparative example prepared needle-shaped raw coke using the same method as in Example 2 above, except that: A one-step calcination process was used to calcine needle coke to obtain calcined oil-based needle coke for graphite electrodes. The specific product indicators and calcination conditions are shown in Table 1. Comparative Example 2 This comparative example prepared needle-shaped raw coke using the same method as in Example 2 above, except that: When needle coke is calcined using a stepwise calcination process, the cooling rate is different and too slow, resulting in calcined oil-based needle coke for graphite electrodes. The specific product indicators and calcination conditions are shown in Table 1. Comparative Example 3 This comparative example uses the same method as in Example 2 above to prepare needle coke raw coke. The difference is that a step-by-step calcination process is used to calcine the needle coke, the cooling rate is different and too fast, and calcined oil-based needle coke for graphite electrodes is obtained. The specific product indicators and calcination conditions are shown in Table 1. Comparative Example 4 This comparative example prepared needle coke using the same method as in Example 2 above, except that: FCC slurry oil was used as raw material, and the total content of 3, 4, and 5-cyclic aromatic hydrocarbons was different after depressurization and hydrogenation. During the calcination process, a reasonable cooling process can enable the formation of a suitable amount of uniformly distributed micropores and microcracks in needle coke. On the other hand, the internal structure of needle coke itself also has an important influence on the formation of micropores and microcracks, and good raw material properties are a necessary condition for the formation of a good internal structure.

[0014] This comparative example uses FCC oil slurry as raw material. After depressurization and hydrogenation, a refined aromatic oil with a total content of 3, 4, and 5-cyclic aromatics of 60% is obtained. After delayed coking, needle coke is obtained. The needle coke is calcined using the same stepwise calcination process as in Example 2 to obtain calcined oil-based needle coke for graphite electrodes. Specific product indicators and calcination process are shown in Table 1.

[0015] Comparative Example 5 This comparative example prepared needle coke using the same method as in Example 2 above, except that: FCC slurry oil was used as raw material, and the total content of 3, 4, and 5-cyclic aromatic hydrocarbons was different after depressurization and hydrogenation. This comparative example uses FCC oil slurry as raw material. After depressurization and hydrogenation, a refined aromatic oil with a total content of 30% of 3, 4, and 5-cyclic aromatics is obtained. After delayed coking, needle coke raw coke is obtained. The needle coke raw coke is calcined using the same stepwise calcination process as in Example 2 to obtain calcined oil-based needle coke for graphite electrodes. Specific product indicators and calcination process are shown in Table 1.

[0016] The true density test method is GB / T 24203-2009. The polarization structure division is carried out according to the standard described in reference 1. Yu Yinping, Zhao Yanan, Ma Chan, et al. "A needle-shaped polarized light microscopic quantitative analysis method" [J]. Carbon, 2021(2):40-43.

[0017] The gas expansion rate was measured according to the method used in reference 2. Raymond Perruchoud, Julien Wyss, Yan Xiujin. Pilot-scale evaluation of the applicability of graphite electrodes for electric arc furnace steelmaking in the production of raw coke and calcined coke [J]. Carbon Technology, 2015(06).

[0018]

[0019] As can be seen from Table 1, (1) Example 1 can prepare oil-based needle coke with low gas expansion characteristics and a porosity of 25.107%. Under the same formulation conditions, the graphite electrode rod prepared and graphitized at 3000℃ showed a corresponding product gas expansion rate of 1.82%, indicating good overall performance. This may be because the calcination process provided in Example 1 is 30℃→800℃→100℃→1400℃→natural cooling, wherein the heating rate is constant at 5℃ / min and the cooling rate from 800℃ to 100℃ is 55℃ / min. This cooling process causes thermal stress differences to form inside the needle coke, ultimately forming an appropriate amount of micropores and microcracks. Such structures can provide channels for heteroatoms in the material to escape during the subsequent graphitization process, thereby reducing gas expansion. Its microstructure was observed using a scanning electron microscope, such as... Figure 1 As shown, when observed under the 500μm standard, the product has a suitable amount of microcrack structure inside. The porosity is 25.107% when tested with a mercury porosimeter, and the corresponding electrode product has an air expansion rate of 1.82%.

[0020] (2) Example 2 can prepare oil-based needle coke with low gas expansion characteristics. The product porosity is 28.122%. Under the same formulation conditions, the graphite electrode rod prepared and graphitized at 3000℃ has a corresponding product gas expansion rate of 1.69%. The overall performance is good. This may be because Example 2 provides a calcination process of 30℃→800℃→100℃→1400℃→natural cooling, with a constant heating rate of 5℃ / min and a cooling rate of 60℃ / min from 800℃ to 100℃. During this process, the difference in thermal stress within the needle coke increases slightly, resulting in a slight increase in the number of micropores and microcracks, corresponding to increased porosity. This type of structure can provide more channels for the escape of heteroatoms from the material during subsequent graphitization, thereby further reducing the gas expansion rate. The microstructure was observed using a scanning electron microscope, such as... Figure 2 As shown, when observed under the 500μm standard, the product has a small number of micropores and microcracks. The porosity was measured to be 19.157% using a mercury porosimeter, and the corresponding electrode product gas expansion rate was 4.07%.

[0021] (3) Example 3 can prepare oil-based needle coke with low gas expansion characteristics and a porosity of 26.113%. The graphite electrode rod prepared under the same formulation conditions was graphitized at 3000℃, and the corresponding product gas expansion rate was 1.75%. Compared with Example 2, the porosity showed a decreasing trend and the gas expansion rate showed an increasing trend, but the overall indicators still met the requirements.

[0022] (4) Comparison Example 1 with Example 2: Comparative Example 1 was able to prepare oil-based needle coke with low gas expansion characteristics, and the product porosity was 13.146%. The graphite electrode rod prepared under the same formulation conditions, after graphitization at 3000℃, had a corresponding product gas expansion rate of 5.77%, which was poor overall. The reason is that the calcination process provided by Comparative Example 1 was 30℃→800℃→1400℃→natural cooling, with a constant heating rate of 5℃ / min. There was no cooling process from 800℃ to 1400℃. The internal thermal stress difference of the product was very small, and there were only a few micropores and microcracks. In the subsequent graphitization process, there was a lack of sufficient channels for heteroatoms in the material to escape, which eventually led to greater gas expansion. Its microstructure was observed using a scanning electron microscope, such as Figure 4 As shown, when observed under the 500μm standard, the product has only a small number of micropores and microcracks. The porosity was measured to be 13.146% using a mercury porosimeter, and the corresponding electrode product had an air expansion rate of 5.77%.

[0023] (5) Comparison Example 2 with Example 2: As shown in Table 1, Comparative Example 2 can prepare oil-based needle coke with low gas expansion characteristics. The porosity of the product is 19.157%. The graphite electrode rod prepared under the same formulation conditions, after graphitization at 3000℃, has a corresponding gas expansion rate of 4.07%, which is worse than the overall performance. The reason is that the calcination process provided by Comparative Example 2 is 30℃→800℃→100℃→1400℃→natural cooling, with a constant heating rate of 5℃ / min and a cooling rate of 8℃ / min from 800℃ to 100℃. During this process, the internal thermal stress difference of the product is insufficient to form a sufficient number of micropores and microcracks. In the subsequent graphitization process, the number of heteroatom escape channels in the material is insufficient, eventually resulting in large gas expansion. Its microstructure was observed using a scanning electron microscope, such as Figure 5 As shown, when observed under the 500μm standard, the product has a small number of micropores and microcracks. The porosity was measured to be 19.157% using a mercury porosimeter, and the corresponding electrode product gas expansion rate was 4.07%.

[0024] (6) Comparison Example 3 with Example 2: As shown in Table 1, Comparative Example 3 was able to prepare oil-based needle coke with low gas expansion characteristics, and the product porosity was 20.308%. The graphite electrode rod prepared under the same formulation conditions, after graphitization at 3000℃, had a corresponding product gas expansion rate of 3.86%, which was poor overall. The reason is that the calcination process provided by Comparative Example 3 was 30℃→800℃→100℃→1400℃→natural cooling, with a constant heating rate of 5℃ / min and a cooling rate of 90℃ / min from 800℃ to 100℃. During this process, the large difference in internal thermal stress caused the size of local pores and cracks in the internal structure to be too large, while some areas did not form pores and cracks, resulting in an overall non-uniform internal structure. During the subsequent graphitization process, the escape rate of heteroatoms in the material is uneven, eventually leading to significant gas expansion. The microstructure was observed using a scanning electron microscope, such as... Figure 6 As shown, under the 500μm standard, the product has localized large pores and cracks, while some areas do not have micropores or microcracks. The porosity measured by mercury porosimeter is 20.308%, and the corresponding electrode product has an air expansion rate of 3.86%.

[0025] (7) Comparison Example 4 with Example 2: As shown in Table 1, although Comparative Example 4 used the same calcination process as this invention, namely the calcination process of 30℃→800℃→100℃→1400℃→natural cooling, with a constant heating rate of 5℃ / min and a cooling rate of 60℃ / min from 800℃ to 100℃, the oil-based needle coke with low gas expansion characteristics produced had a porosity of 19.166%. Under the same formulation conditions, the graphite electrode rod prepared under these conditions, after graphitization at 3000℃, had a corresponding product gas expansion rate of 4.01%, indicating poor overall performance. This may be due to: The high total content of 3, 4, and 5-cyclic aromatic hydrocarbons in the raw materials used in Comparative Example 4 resulted in an excessive number of active sites and an excessively fast reaction rate during the mesophase reaction, leading to a rapid increase in system viscosity and a shortened mesophase solidification time. Ultimately, this resulted in a non-wide-range mesophase structure with poor orientation. Under the stress difference caused by the same calcination process conditions as in Example 2, this type of structure is more difficult to generate a suitable amount of micropores and microcracks. In the subsequent graphitization process, the amount of heteroatom escape channels in the material is insufficient, ultimately resulting in large gas expansion. Its microstructure was observed using a scanning electron microscope, such as Figure 7 As shown, under the 500μm standard, the product contains a small number of micropores and microcracks. The porosity measured by mercury porosimetry is 19.166%, and the corresponding electrode rod gas expansion rate is 4.01%. Therefore, it can be seen that during the calcination process, a reasonable cooling process can enable needle coke to form a suitable amount of uniformly distributed micropores and microcracks. On the other hand, the internal structure of the needle coke itself also has an important influence on the formation of micropores and microcracks, and good raw material properties are a necessary condition for the formation of a good internal structure.

[0026] (8) Comparison Example 5 with Example 2: As shown in Table 1, Comparative Example 5 used the same calcination process as this invention, namely, the calcination process was 30℃→800℃→100℃→1400℃→natural cooling, with a constant heating rate of 5℃ / min and a cooling rate of 60℃ / min from 800℃ to 100℃. However, the oil-based needle coke with low gas expansion characteristics produced a product with a porosity of 20.917%. Under the same formulation conditions, the graphite electrode rod prepared after graphitization at 3000℃ had a corresponding product gas expansion rate of 3.72%, indicating poor overall performance. This may be due to: The raw materials used in this comparative example had too low total content of 3, 4, and 5-cyclic aromatic hydrocarbons, resulting in too few active sites and too low mesophase formation efficiency during the coking reaction. Within the same curing cycle, the embedded structure increased but did not form a wide-area mesophase structure. Under the stress difference caused by the same calcination process conditions as in Example 2, this type of structure is difficult to generate an appropriate amount of micropores and microcracks. In the subsequent graphitization process, the amount of heteroatom escape channels in the material is insufficient, eventually leading to large gas expansion. Its microstructure was observed using a scanning electron microscope, such as Figure 8 As shown, when observed under the 500μm standard, the product has a small number of micropores and microcracks. The porosity was measured to be 29.917% using a mercury porosimeter, and the corresponding electrode product gas expansion rate was 3.72%.

[0027] In summary: This invention creatively discovers that during the calcination process of needle coke, a reasonable cooling process can enable the formation of a suitable amount and uniformly distributed micropores and microcracks within it. Furthermore, the internal structure of the needle coke itself also has a significant impact on the formation of micropores and microcracks, and good raw material properties are a necessary condition for the formation of a good internal structure.

[0028] A creative production process for oil-based needle coke with low gas expansion characteristics was adopted. First, a delayed coking process with multi-gradient heating was used to prepare needle coke with high orientation and high true density. Then, this was used as raw material for two-stage calcination to give it an appropriate amount of micropore and microcrack structure, and finally, an oil-based needle coke with low gas expansion characteristics was obtained.

Claims

1. A method for preparing oil-based needle coke for graphite electrodes with low gas expansion performance, characterized in that... The preparation method includes: using FCC oil slurry as raw material, obtaining refined aromatic oil through vacuum cutting and hydrogenation to remove impurities, subjecting the refined aromatic oil to delayed coking treatment, and directly removing coke after cooling to room temperature after the coking reaction is completed, thereby obtaining needle coke raw coke; and then subjecting the needle coke raw coke to stepwise high-temperature calcination and cooling to obtain oil-based needle coke.

2. The method for preparing oil-based needle coke for low-expansion graphite electrodes according to claim 1, characterized in that... The stepwise calcination process involves a calcination temperature curve of 30℃→800℃→100℃→1400℃, with a cooling rate of 55-65℃ / min from 800℃ to 100℃, and natural cooling from 1400℃ to room temperature.

3. The method for preparing oil-based needle coke for low-expansion graphite electrodes according to claim 1, characterized in that... The delayed coking process involves a uniform gradient temperature increase, with the following temperature curve: 460℃ constant temperature for 50 hours → 10℃ / min temperature increase → 490℃ constant temperature for 10 hours.

4. The method for preparing oil-based needle coke for low-expansion graphite electrodes according to claim 1, characterized in that... The total content of 3, 4, and 5-cyclic aromatic hydrocarbons in the refined aromatic oil is 40%-50%.

5. The method for preparing oil-based needle coke for low-expansion graphite electrodes according to claim 4, characterized in that... The refined aromatic oil contains 0.35%-0.4% sulfur and 65%-70% aromatics.

6. The method for preparing oil-based needle coke for low-expansion graphite electrodes according to claim 1, characterized in that... The porosity of the oil-based needle coke is 25.107%-28.122%.

7. The method for preparing oil-based needle coke for low-expansion graphite electrodes according to any one of claims 1-6, characterized in that... The oil-based needle coke is used to prepare graphite electrode rods through high-temperature graphitization.

8. The method for preparing oil-based needle coke for low-expansion graphite electrodes according to claim 7, characterized in that... The product air expansion rate of the graphite electrode rod is 1.69%-1.82%.

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