A method for preparing large-size isostatic pressing graphite by using photovoltaic monocrystal thermal field waste graphite

Abstract

The present application relates to a kind of method for preparing large size isostatic pressing graphite by photovoltaic single crystal thermal field waste graphite, belong to waste graphite high-value utilization technical field.The present application can reduce the open pore produced in the process of baking by using photovoltaic single crystal thermal field waste graphite doping to prepare isostatic pressing graphite, so that the number of baking impregnation process flow can be effectively reduced;Waste graphite thermal field has smaller particle size, which can effectively enter the interstice of petroleum coke and needle coke during isostatic pressing forming as powder;And photovoltaic single crystal thermal field waste graphite has higher density and lower ash, which can improve the density of isostatic pressing graphite and reduce the ash content of isostatic pressing graphite when used for doping to prepare isostatic pressing graphite.

Description

Technical Field

[0001] This invention relates to a method for preparing large-size isostatically pressed graphite using waste graphite from photovoltaic single-crystal thermal fields, belonging to the technical field of high-value utilization of waste graphite. Background Technology

[0002] Isostatic graphite refers to graphite material produced by isostatic pressing, which involves applying uniform and constant liquid pressure during the molding process. In the isostatic pressing process, the larger the size of the graphite, the higher the requirements for raw materials and processes.

[0003] During the mixing process, a large amount of binder (usually asphalt or coal tar pitch) is added to the raw materials. After a long roasting process, the binder decomposes into a large amount of gas at high temperatures and is released, resulting in a large number of pores in the product after roasting. This leads to the following consequences: 1. Increased open porosity and decreased bulk density; 2. Reduced mechanical properties and machinability; 3. Weakened electrical conductivity and increased resistivity; 4. Reduced sealing performance; 5. Insufficient corrosion resistance and oxidation resistance. Isostatically pressed graphite after molding requires roasting. The roasting of graphite products refers to heating the pressed green body under air-isolated conditions, causing the binder to transform into coke. The asphalt contained in the green body is firmly wrapped in the transition layer between carbon particles. Therefore, when it is converted into coke at high temperatures, it forms an interfacial carbon mesh layer in the semi-finished product, which acts as a bridge and reinforcement. The performance of the roasted product is largely determined by the changes in the asphalt during the roasting process. During the calcination process, when the calcination temperature reaches approximately 300℃, complex chemical reactions occur between the aggregate and the binder. The binder decomposes, releasing a large amount of volatiles while undergoing a condensation reaction. In the low-temperature preheating stage, the green body expands due to heat, and then shrinks in volume due to the condensation reaction during the subsequent heating process. The larger the volume of the green body, the more difficult it is to release volatiles. Simultaneously, temperature differences and uneven shrinkage are more likely to occur between the surface and interior of the green body, all of which can lead to cracks. To avoid these issues and affect product quality, an impregnation process is often applied to the calcined graphite. This is usually done after the raw material has been calcined. The purpose of impregnation is to force a liquid impregnating agent into the pores of the porous product under certain temperature and pressure, thereby reducing the porosity, increasing its bulk density and mechanical strength, and improving its physical and chemical properties such as electrical and thermal conductivity.

[0004] In summary, the existing isostatic pressing (OSP) graphite production process is lengthy, requiring multiple calcination and impregnation processes, and demands high quality from the OSP graphite products. Utilizing waste graphite from photovoltaic monocrystalline hot zones for OSP preparation can reduce the number of open pores generated during calcination, thus effectively reducing the number of calcination-impregnation processes. The waste graphite from the hot zones has a small particle size, allowing it to effectively enter the voids of petroleum coke and needle coke during the OSP process. Furthermore, the waste graphite from photovoltaic monocrystalline hot zones has high density and low ash content, which, when used for doping in OSP preparation, can increase the density and reduce the ash content of the OSP graphite. Summary of the Invention

[0005] This invention addresses the problem of low yield of large-size isostatically pressed graphite in existing technologies. It proposes a method for preparing large-size isostatically pressed graphite using waste graphite from photovoltaic single-crystal thermal fields. The waste graphite from photovoltaic single-crystal thermal fields, which has undergone isostatic pressing and graphitization, has low ash and volatile matter content. Using this waste graphite for isostatic pressing reduces the number of open pores generated during the calcination process, thus effectively reducing the number of calcination-impregnation steps. The waste graphite from the thermal fields has a small particle size, allowing it to effectively enter the voids of petroleum coke and needle coke during isostatic pressing. Furthermore, the waste graphite from photovoltaic single-crystal thermal fields has high density and low ash content, which, when used for doping in the preparation of isostatically pressed graphite, can increase the density and reduce the ash content of the isostatically pressed graphite.

[0006] Waste graphite from photovoltaic monocrystalline hot zone can be used to produce isostatically pressed graphite with a height of 1500-1700mm, a length of 2500-2700mm, and a width of 2100-2300mm. The yield of isostatically pressed graphite is 5% higher than that of the original process.

[0007] A method for preparing large-size isostatically pressed graphite using waste graphite from photovoltaic single-crystal thermal fields, the specific steps of which are as follows:

[0008] (1) The waste graphite from the photovoltaic monocrystalline hot zone is crushed and sieved into low-particle-size, medium-particle-size, and high-particle-size waste graphite powder.

[0009] (2) Mix petroleum coke, needle coke and natural graphite evenly to obtain mixture A. Add waste graphite powder with low particle size, medium particle size and high particle size to mixture A and mix evenly to obtain mixture B.

[0010] (3) Add mixture B to a vertical roller mill and grind it until the average particle size is less than 20 μm to obtain mixed powder;

[0011] (4) Put the mixed powder and coal tar binder into a heated kneader and knead them to make the coal tar binder evenly adhere to the surface of the mixed powder. Then, let it cool naturally to room temperature to obtain a mixed paste.

[0012] (5) The mixed paste is added to a vertical roller mill for secondary grinding to 10-20μm to obtain a secondary mixed powder;

[0013] (6) The secondary mixed powder is filled into the rubber mold, compacted by high-frequency electromagnetic vibration, sealed, and then vacuumed to remove the air between the particles to obtain a large-size isostatic graphite blank.

[0014] (7) Place the large-size isostatic graphite blank in a calcining furnace, heat it at a constant rate to 250-270℃ in the first stage and hold it for 3-20 hours, heat it at a constant rate to 600-620℃ in the second stage and hold it for 3-20 hours, heat it at a constant rate to 1000-1020℃ in the third stage and hold it for 3-20 hours, and then cool it at a constant rate to room temperature to obtain the isostatic graphite calcined blank.

[0015] (8) Place the isostatically pressed graphite calcined green body into the impregnating agent and impregnate it for 40 to 150 minutes at a temperature of 130 to 200°C and a pressure of 0.5 to 5 MPa.

[0016] (9) Repeat steps (7) to (8) multiple times until the density of the isostatically pressed graphite green body reaches 1.691 g / cm³. 3 An isostatically pressed graphite precursor was obtained;

[0017] (10) The isostatically pressed graphite precursor is placed in a graphitization furnace for graphitization treatment to obtain large-size isostatically pressed graphite for photovoltaic applications; the isostatically pressed graphite has a height of 1500-1700 mm, a length of 2500-2700 mm, and a width of 2100-2300 mm. In step (1), the fixed carbon content of the waste graphite from the photovoltaic monocrystalline thermal field is not less than 93%, and the particle size of the waste graphite powder is 0.05-0.15 mm.

[0018] In step (2), the mixture A contains 30 wt.% petroleum coke, 40 wt.% needle coke, and 30 wt.% natural graphite.

[0019] The amount of waste graphite powder added to mixture B in step (2) is 10-30 wt.%.

[0020] In step (4), the mass ratio of the mixed powder and the coal tar pitch binder is 70:30 to 65:35.

[0021] In step (7), the first stage of uniform heating rate is 1 to 3.5℃ / h, the second stage of uniform heating rate is 0.5 to 4.5℃ / h, and the third stage of uniform heating rate is 1 to 5℃ / h.

[0022] The impregnating agent in step (8) is medium-temperature asphalt.

[0023] The graphitization process in step (10) is carried out at a temperature of 2600–3000℃ for 50–60 days.

[0024] The beneficial effects of this invention are:

[0025] (1) This invention utilizes waste graphite from photovoltaic single crystal hot zone to prepare isostatic graphite by doping, which can reduce the open pores generated during the calcination process, thereby effectively reducing the number of calcination-impregnation processes; the waste graphite hot zone has a small particle size, which can effectively enter the gaps of petroleum coke and needle coke during the isostatic pressing process as powder; and the waste graphite from photovoltaic single crystal hot zone has a high density and low ash content, which can be used to dope and prepare isostatic graphite to increase the density of isostatic graphite and reduce the ash content of isostatic graphite.

[0026] (2) The process of this invention is simple and does not require the introduction of additional reaction gases (such as Freon, chlorine, etc.), which can reduce economic costs and improve production safety.

[0027] (3) The large-size isostatic graphite prepared by the present invention has higher density and mechanical strength, and better resistivity. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0029] Example 1: A method for preparing large-size isostatically pressed graphite using waste graphite from a photovoltaic single-crystal thermal field, the specific steps of which are as follows:

[0030] (1) Waste graphite from photovoltaic monocrystalline hot zone is crushed and sieved to obtain waste graphite powder with a particle size of 0.05-0.075 mm;

[0031] (2) Mix petroleum coke, needle coke and natural graphite evenly to obtain mixture A, and add waste graphite powder to mixture A and mix evenly to obtain mixture B; with petroleum coke accounting for 30 wt.%, needle coke accounting for 30 wt.%, and natural graphite accounting for 30 wt.%, the amount of waste graphite powder added to mixture B is 10 wt.%.

[0032] (3) Add mixture B to a vertical roller mill and grind it until the average particle size is 18μm to obtain mixed powder;

[0033] (4) The mixed powder and coal tar binder are put into a heated kneader for kneading, so that the coal tar binder is evenly attached to the surface of the mixed powder. The mixture is then naturally cooled to room temperature to obtain a mixed paste. The mass ratio of the mixed powder and the coal tar binder is 70:30.

[0034] (5) The mixed paste is added to a vertical roller mill for secondary grinding to obtain a particle size of 15μm, thus obtaining a secondary mixed powder;

[0035] (6) The secondary mixed powder is filled into the rubber mold, compacted by high frequency electromagnetic vibration, sealed and vacuumed to remove the air between the particles to obtain a large-size isostatic graphite blank; the isostatic graphite blank has a height of 1500mm, a length of 2500mm and a width of 2300mm.

[0036] (7) Place the large-size isostatic graphite blank in a calcining furnace, and heat it at a rate of 2℃ / h to 250℃ and hold it for 10h, then heat it at a rate of 1℃ / h to 600℃ and hold it for 10h, then heat it at a rate of 3℃ / h to 1000℃ and hold it for 5h, and then cool it down to room temperature at a rate of 5℃ / h to obtain the isostatic graphite calcined blank.

[0037] (8) The isostatically pressed graphite calcined blank is placed in the impregnating agent in the impregnation tank and impregnated for 60 minutes at a temperature of 180℃ and a pressure of 1.5Mpa; the impregnating agent is medium-temperature asphalt.

[0038] (9) Repeat steps (7) to (8) multiple times until the density of the isostatically pressed graphite green body reaches 1.694 g / cm³. 3 An isostatically pressed graphite precursor was obtained;

[0039] (10) The isostatic graphite precursor is placed in a graphitization furnace and graphitized at a temperature of 2600℃ to obtain large-size isostatic graphite for photovoltaic use; the large-size isostatic graphite for photovoltaic use has a height of 1500mm, a length of 2500mm, and a width of 2300mm.

[0040] In this embodiment, the large-size isostatic graphite for photovoltaic applications has a flexural strength of 55 MPa, a compressive strength of 105 MPa, a resistivity of 8.5 μΩ / m, a thermal conductivity of 113 W / m·K, and a density of 1.90 g / cm³. 3 The content of metallic impurities is 28 ppm.

[0041] Example 2: A method for preparing large-size isostatically pressed graphite using waste graphite from photovoltaic single-crystal thermal fields, the specific steps of which are as follows:

[0042] (1) Waste graphite from photovoltaic monocrystalline hot zone is crushed and sieved to obtain waste graphite powder with a particle size of 0.075-0.1 mm;

[0043] (2) Mix petroleum coke, needle coke and natural graphite evenly to obtain mixture A, and add waste graphite powder to mixture A and mix evenly to obtain mixture B; with petroleum coke accounting for 25 wt.%, needle coke accounting for 25 wt.%, and natural graphite accounting for 25 wt.% in mixture A; and the amount of waste graphite powder added to mixture B is 25 wt.%.

[0044] (3) Add mixture B to a vertical roller mill and grind it until the average particle size is 18μm to obtain mixed powder;

[0045] (4) The mixed powder and coal tar binder are put into a heated kneader for kneading, so that the coal tar binder is evenly attached to the surface of the mixed powder. The mixture is then naturally cooled to room temperature to obtain a mixed paste. The mass ratio of the mixed powder and the coal tar binder is 70:30.

[0046] (5) The mixed paste is added to a vertical roller mill for secondary grinding until the particle size is 16μm to obtain a secondary mixed powder;

[0047] (6) The secondary mixed powder is filled into the rubber mold, compacted by high frequency electromagnetic vibration, sealed and vacuumed to remove the air between the particles to obtain a large-size isostatic graphite blank; the isostatic graphite blank has a height of 1600mm, a length of 2400mm and a width of 2300mm.

[0048] (7) Place the large-size isostatic graphite blank in a calcining furnace, and heat it at a rate of 2℃ / h to 250℃ and hold it for 10h, then heat it at a rate of 1℃ / h to 600℃ and hold it for 10h, then heat it at a rate of 3℃ / h to 1000℃ and hold it for 5h, and then cool it down to room temperature at a rate of 5℃ / h to obtain the isostatic graphite calcined blank.

[0049] (8) The isostatically pressed graphite calcined blank is placed in the impregnating agent in the impregnation tank and subjected to impregnation heat treatment at a temperature of 180℃ and a pressure of 1.5Mpa for 90min; the impregnating agent is medium-temperature asphalt.

[0050] (9) Repeat steps (7) to (8) multiple times until the density of the isostatically pressed graphite green body reaches 1.696 g / cm³. 3 An isostatically pressed graphite precursor was obtained;

[0051] (10) The isostatic graphite precursor is placed in a graphitization furnace and graphitized at a temperature of 2800℃ to obtain large-size isostatic graphite for photovoltaic use; the large-size isostatic graphite for photovoltaic use has a height of 1600mm, a length of 2400mm, and a width of 2300mm.

[0052] In this embodiment, the large-size isostatic graphite for photovoltaic applications has a flexural strength of 57 MPa, a compressive strength of 109 MPa, a resistivity of 9.3 μΩ / m, a thermal conductivity of 115 W / m·K, and a density of 1.91 g / cm³. 3 The content of metallic impurities is 26 ppm.

[0053] Example 3: A method for preparing large-size isostatically pressed graphite using waste graphite from a photovoltaic single-crystal thermal field, the specific steps of which are as follows:

[0054] (1) Waste graphite from photovoltaic monocrystalline hot zone is crushed and sieved to obtain waste graphite powder with a particle size of 0.05-0.075 mm;

[0055] (2) Mix petroleum coke, needle coke and natural graphite evenly to obtain mixture A, and add waste graphite powder to mixture A and mix evenly to obtain mixture B; the proportion of petroleum coke in mixture A is 25 wt.%, the proportion of needle coke is 25 wt.%, and the proportion of natural graphite is 35 wt.%; the amount of waste graphite powder added to mixture B is 20 wt.%.

[0056] (3) Add mixture B to a vertical roller mill and grind it until the average particle size is 15μm to obtain mixed powder;

[0057] (4) The mixed powder and coal tar binder are put into a heated kneader for kneading, so that the coal tar binder is evenly attached to the surface of the mixed powder. The mixture is then naturally cooled to room temperature to obtain a mixed paste. The mass ratio of the mixed powder and the coal tar binder is 75:25.

[0058] (5) The mixed paste is added to a vertical roller mill for secondary grinding until the particle size is 13μm to obtain a secondary mixed powder;

[0059] (6) The secondary mixed powder is filled into the rubber mold, compacted by high frequency electromagnetic vibration, sealed and vacuumed to remove the air between the particles to obtain a large-size isostatic graphite blank; the isostatic graphite blank has a height of 1700mm, a length of 2200mm and a width of 2300mm.

[0060] (7) Place the large-size isostatic graphite blank in a calcining furnace, and heat it at a rate of 2℃ / h to 250℃ and hold it for 10h, then heat it at a rate of 1℃ / h to 600℃ and hold it for 10h, then heat it at a rate of 3℃ / h to 1000℃ and hold it for 5h, and then cool it down to room temperature at a rate of 5℃ / h to obtain the isostatic graphite calcined blank.

[0061] (8) The isostatically pressed graphite calcined blank is placed in the impregnating agent in the impregnation tank and impregnated for 60 minutes at a temperature of 180℃ and a pressure of 1.5Mpa; the impregnating agent is medium-temperature asphalt.

[0062] (9) Repeat steps (7) to (8) multiple times until the density of the isostatically pressed graphite green body reaches 1.696 g / cm³. 3 An isostatically pressed graphite precursor was obtained;

[0063] (10) The isostatic graphite precursor is placed in a graphitization furnace and graphitized at a temperature of 2600℃ to obtain large-size isostatic graphite for photovoltaic use; the large-size isostatic graphite for photovoltaic use has a height of 1700mm, a length of 2200mm, and a width of 2300mm.

[0064] In this embodiment, the large-size isostatic graphite for photovoltaic applications has a flexural strength of 56 MPa, a compressive strength of 110 MPa, a resistivity of 8.7 μΩ / m, a thermal conductivity of 118 W / m·K, and a density of 1.89 g / cm³. 3 The content of metallic impurities is 26 ppm.

[0065] Example 4: A method for preparing large-size isostatically pressed graphite using waste graphite from a photovoltaic single-crystal thermal field, the specific steps of which are as follows:

[0066] (1) Waste graphite from photovoltaic monocrystalline hot zone is crushed and sieved to obtain waste graphite powder with a particle size of 0.1-0.15 mm;

[0067] (2) Mix petroleum coke, needle coke and natural graphite evenly to obtain mixture A, and add waste graphite powder to mixture A and mix evenly to obtain mixture B; with petroleum coke accounting for 25 wt.%, needle coke accounting for 25 wt.%, and natural graphite accounting for 25 wt.% in mixture A; and the amount of waste graphite powder added to mixture B is 25 wt.%.

[0068] (3) Add mixture B to a vertical roller mill and grind it until the average particle size is 12μm to obtain mixed powder;

[0069] (4) The mixed powder and coal tar binder are put into a heated kneader for kneading, so that the coal tar binder is evenly attached to the surface of the mixed powder. The mixture is then naturally cooled to room temperature to obtain a mixed paste. The mass ratio of the mixed powder and the coal tar binder is 70:30.

[0070] (5) The mixed paste is added to a vertical roller mill for secondary grinding until the particle size is 10μm to obtain a secondary mixed powder;

[0071] (6) The secondary mixed powder is filled into the rubber mold, compacted by high frequency electromagnetic vibration, sealed and vacuumed to remove the air between the particles to obtain a large-size isostatic graphite blank; the isostatic graphite blank has a height of 1500mm, a length of 2200mm and a width of 2300mm.

[0072] (7) Place the large-size isostatic graphite blank in a calcining furnace, and heat it at a rate of 2℃ / h to 250℃ and hold it for 10h, then heat it at a rate of 1℃ / h to 600℃ and hold it for 10h, then heat it at a rate of 3℃ / h to 1000℃ and hold it for 5h, and then cool it down to room temperature at a rate of 5℃ / h to obtain the isostatic graphite calcined blank.

[0073] (8) The isostatically pressed graphite calcined blank is placed in the impregnating agent in the impregnation tank and subjected to impregnation heat treatment at a temperature of 180℃ and a pressure of 1.5Mpa for 90min; the impregnating agent is medium-temperature asphalt.

[0074] (9) Repeat steps (7) to (8) multiple times until the density of the isostatically pressed graphite green body reaches 1.70 g / cm³. 3 An isostatically pressed graphite precursor was obtained;

[0075] (10) The isostatic graphite precursor is placed in a graphitization furnace and graphitized at a temperature of 2900℃ to obtain large-size isostatic graphite for photovoltaic use; the large-size isostatic graphite for photovoltaic use has a height of 1500mm, a length of 2200mm, and a width of 2300mm.

[0076] In this embodiment, the large-size isostatic graphite for photovoltaic applications has a flexural strength of 59 MPa, a compressive strength of 112 MPa, a resistivity of 8.8 μΩ / m, a thermal conductivity of 117 W / m·K, and a density of 1.94 g / cm³. 3+ The content of metallic impurities is 29 ppm.

[0077] Example 5: A method for preparing large-size isostatically pressed graphite using waste graphite from a photovoltaic single-crystal thermal field, the specific steps of which are as follows:

[0078] (1) Waste graphite from photovoltaic monocrystalline hot zone is crushed and sieved to obtain waste graphite powder with a particle size of 0.05-0.075 mm;

[0079] (2) Mix petroleum coke, needle coke and natural graphite evenly to obtain mixture A, and add waste graphite powder to mixture A and mix evenly to obtain mixture B; with petroleum coke accounting for 30 wt.%, needle coke accounting for 25 wt.%, and natural graphite accounting for 30 wt.% in mixture A; and the amount of waste graphite powder added to mixture B is 15 wt.%.

[0080] (3) Add mixture B to a vertical roller mill and grind it until the average particle size is 15μm to obtain mixed powder;

[0081] (4) The mixed powder and coal tar binder are put into a heated kneader for kneading, so that the coal tar binder is evenly attached to the surface of the mixed powder. The mixture is then naturally cooled to room temperature to obtain a mixed paste. The mass ratio of the mixed powder and the coal tar binder is 70:30.

[0082] (5) The mixed paste is added to a vertical roller mill for secondary grinding to obtain a particle size of 12μm to obtain a secondary mixed powder;

[0083] (6) The secondary mixed powder is filled into the rubber mold, compacted by high frequency electromagnetic vibration, sealed and vacuumed to remove the air between the particles to obtain a large-size isostatic graphite blank; the isostatic graphite blank has a height of 1500mm, a length of 2200mm and a width of 2300mm.

[0084] (7) Place the large-size isostatic graphite blank in a calcining furnace, and heat it at a rate of 2℃ / h to 250℃ and hold it for 10h, then heat it at a rate of 1℃ / h to 600℃ and hold it for 10h, then heat it at a rate of 3℃ / h to 1000℃ and hold it for 5h, and then cool it down to room temperature at a rate of 5℃ / h to obtain the isostatic graphite calcined blank.

[0085] (8) The isostatically pressed graphite calcined blank is placed in the impregnating agent in the impregnation tank and impregnated for 120 minutes at a temperature of 180℃ and a pressure of 1.5Mpa; the impregnating agent is medium-temperature asphalt.

[0086] (9) Repeat steps (7) to (8) multiple times until the density of the isostatically pressed graphite green body reaches 1.70 g / cm³. 3 An isostatically pressed graphite precursor was obtained;

[0087] (10) The isostatic graphite precursor is placed in a graphitization furnace and graphitized at a temperature of 2600℃ to obtain large-size isostatic graphite for photovoltaic use; the large-size isostatic graphite for photovoltaic use has a height of 1500mm, a length of 2200mm, and a width of 2300mm.

[0088] In this embodiment, the large-size isostatic graphite for photovoltaic applications has a flexural strength of 59 MPa, a compressive strength of 106 MPa, a resistivity of 9.0 μΩ / m, a thermal conductivity of 116 W / m·K, and a density of 1.91 g / cm³. 3 The content of metallic impurities is 27 ppm.

[0089] Comparative Example 1: The difference between this comparative example and Example 5 is that the particle size is 0.5–0.6 mm. The isostatically pressed graphite in this comparative example has a flexural strength of 48 MPa, a compressive strength of 92 MPa, a resistivity of 12.7 μΩ / m, a thermal conductivity of 110 W / m·K, and a density of 1.79 g / cm³. 3 The metal impurity content was 65 ppm. Although the isostatic graphite of this comparative example met the size requirements, its flexural strength, compressive strength, thermal conductivity and density were all lower than those of the large-size isostatic graphite for photovoltaic use in Example 5. Its resistivity and metal impurity content were higher than those of the large-size isostatic graphite for photovoltaic use in Example 5. Therefore, this comparative example did not achieve the goal of meeting the quality standards.

[0090] Comparative Example 2: The difference between this comparative example and Example 5 is that the amount of waste graphite powder added to mixture B is 2 wt.%. The isostatically pressed graphite of this comparative example has a flexural strength of 46 MPa, a compressive strength of 89 MPa, a resistivity of 14.2 μΩ / m, a thermal conductivity of 111 W / m·K, and a density of 1.81 g / cm³. 3 The metal impurity content was 47 ppm. Although the isostatic graphite of this comparative example met the size requirements, its flexural strength, compressive strength, thermal conductivity and density were all lower than those of the large-size isostatic graphite for photovoltaic use in Example 5. Its resistivity and metal impurity content were higher than those of the large-size isostatic graphite for photovoltaic use in Example 5. Therefore, this comparative example did not achieve the quality target.

[0091] The specific embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing large-size isostatically pressed graphite using waste graphite from a photovoltaic single-crystal thermal field, characterized in that, The specific steps are as follows: (1) The waste graphite from the photovoltaic monocrystalline hot zone is crushed and sieved into waste graphite powder; the particle size of the waste graphite powder is 0.05~0.15mm; (2) Petroleum coke, needle coke, and natural graphite are mixed evenly to obtain mixture A. Waste graphite powder is added to mixture A and mixed evenly to obtain mixture B. In mixture A, petroleum coke accounts for 20-30 wt.%, needle coke accounts for 20-30 wt.%, and natural graphite accounts for 20-30 wt.%; the amount of waste graphite powder added to mixture B is 10-30 wt.%. (3) Add mixture B to a vertical roller mill and grind it until the average particle size is less than 20 μm to obtain mixed powder; (4) Put the mixed powder and coal tar binder into a heated kneader for kneading, so that the coal tar binder is evenly attached to the surface of the mixed powder, and then cool it naturally to room temperature to obtain a mixed paste. (5) The mixed paste is added to a vertical roller mill for secondary grinding until the particle size is 10~20μm to obtain secondary mixed powder; (6) The secondary mixed powder is filled into the rubber mold, compacted by high frequency electromagnetic vibration, sealed and then vacuumed to remove the air between the particles to obtain a large-size isostatic graphite blank. (7) Place the large-size isostatic graphite blank in a calcining furnace, heat it at a constant rate to 250~270℃ in the first stage and hold it for 3~20h, heat it at a constant rate to 600~620℃ in the second stage and hold it for 3~20h, heat it at a constant rate to 1000~1050℃ in the third stage and hold it for 3~20h, and then cool it at a constant rate to room temperature to obtain the isostatic graphite calcined blank; the constant rate of heating in the first stage is 1~3.5℃ / h, the constant rate of heating in the second stage is 0.5~4.5℃ / h, and the constant rate of heating in the third stage is 1~5℃ / h. (8) The isostatically pressed graphite calcined blank is placed in an impregnating agent and subjected to impregnation heat treatment at a temperature of 130~200℃ and a pressure of 0.5~5Mpa for 40~150min; the impregnating agent is medium-temperature asphalt; (9) Repeat steps (7) to (8) multiple times until the density of the isostatically pressed graphite green body reaches 1.691 g / cm³. 3 The above yields an isostatically pressed graphite precursor; (10) The isostatic graphite precursor is placed in a graphitization furnace for graphitization treatment to obtain large-size isostatic graphite for photovoltaic use; the isostatic graphite has a height of 1500~1700mm, a length of 2500~2700mm, and a width of 2100~2300mm.

2. The method for preparing large-size isostatically pressed graphite using waste graphite from a photovoltaic single-crystal thermal field according to claim 1, characterized in that: In step (1), the fixed carbon content of the waste graphite in the photovoltaic monocrystalline thermal field is not less than 93%.

3. The method for preparing large-size isostatically pressed graphite using waste graphite from a photovoltaic single-crystal thermal field according to claim 1, characterized in that: In step (4), the mass ratio of the mixed powder and the coal tar pitch binder is 70:30 to 65:

35.

4. The method for preparing large-size isostatically pressed graphite using waste graphite from a photovoltaic single-crystal thermal field according to claim 1, characterized in that: Step (10) Graphitization treatment temperature 2600~3000℃, time 50~60d.