Isostatic pressing special graphite preparation method and application

Abstract

The application discloses a kind of isostatic pressing special graphite preparation method and application, it is related to the technical field of special graphite preparation, the present application aims at solving the problem of graphite strength, the present application includes the following steps, the pre-preparation of raw material A and raw material B, the raw material A includes at least one of petroleum coke or pitch coke, the pre-preparation of raw material A adopts grinding process, the raw material B is modified pitch, and the pre-preparation of raw material B adopts high-temperature melting process;The raw material A and raw material B are mixed and kneaded to form semi-finished product.The scheme is by using petroleum coke or pitch coke instead of natural graphite as raw material, increase the selection of raw material for making isostatic pressing graphite, the particle size classification of graphite powder waste produced by processing and shaping is used, so that the product is fully utilized, by high-temperature melting of modified pitch under nitrogen protection.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of special graphite preparation, and in particular to a method for preparing isostatic special graphite and application thereof. BACKGROUND

[0002] Isostatic graphite is pressed from high-purity graphite. Isostatic graphite is a new product developed internationally in the past 50 years, and is closely related to today's high technology. With the rapid development of high-end industries such as nuclear power, semiconductors, and solar photovoltaics, the performance requirements for special graphite materials are becoming increasingly stringent. Traditional graphite preparation methods cannot meet the special requirements of high temperature, high pressure, and high purity. The traditional process uses low-melting-point modified graphite for impregnation before baking. Although the impregnation process can increase the hardness and compressive strength of isostatic graphite, it also increases the brittleness of isostatic graphite, significantly reduces the tensile strength and bending strength, and needs to avoid impact load, which cannot meet the use standards of high-temperature gas cooled reactors. Moreover, the impregnation process not only has a long production cycle and high cost, but also causes the internal pore structure to lose uniformity, easily causes stress imbalance, and easily cracks during baking or graphitization. It is only suitable for applications in the field of graphite electrodes. In order to produce high-end isostatic special graphite products, improve the uniformity of the internal structure and pores, and improve the comprehensive performance of isostatic special graphite, it has become an urgent task to overcome. SUMMARY

[0003] In order to solve the above problems, that is, to solve the problems raised in the above background, the present application proposes a method for preparing isostatic pressure special graphite and its application, which comprises the following steps: S1: pre-preparation of raw material A and raw material B, the raw material A comprising at least one of petroleum coke or pitch coke, the pre-preparation of the raw material A using a grinding process, first performing component quality inspection on the raw material A to ensure that there is no harmful impurity, and then using an air flow mill device to grind, the grinding being performed to a particle size of 15 μm, the raw material B being modified pitch, the pre-preparation of the raw material B using a high-temperature melting process, first performing component quality inspection on the raw material B to ensure that there is no harmful impurity, and then performing high-temperature melting under nitrogen protection, the melting temperature being controlled at 250-300°C; S2: mixing and kneading the raw material A and the raw material B to prepare a semi-finished product, in the mixing and kneading step, the raw material B being heated to 150°C in a heat conduction oil heating pot for preheating for 1-2 hours and gradually adding the raw material A, so that the softened paste-like raw material B layer by layer sticks to the raw material A to form a mass, the mixing and kneading being continuously performed until the raw material A is completely mixed into the mass, and then gradually increasing the temperature to 250°C through dynamic temperature control curve to mix and knead for 5-12 hours, so that the raw material B gradually reaches the melting temperature to be converted into a liquid state, and the raw material A and the raw material B in the mass are gradually and uniformly distributed in the temperature increasing and mixing process, and finally the temperature is decreased to 150°C for 1 hour for material curing and then discharged; the dynamic temperature control curve takes the mixing and kneading condition of the raw material A as the control standard, the temperature is increased at a rate of 15-20°C / h when there is no agglomeration of the raw material A, the temperature is stopped to maintain the viscosity of the raw material B when the raw material A appears to be agglomerated, the temperature is resumed to increase when the agglomerated raw material A is completely dispersed, and the temperature is decreased at a rate of 10°C / h to increase the viscosity of the raw material B when the agglomerated raw material A has not started to be dispersed for 0.5-1 hour after the temperature is stopped to increase, and the temperature is stopped to decrease when the agglomerated raw material A starts to be dispersed, and the temperature is resumed to increase after the raw material A is completely dispersed; S3: grinding the semi-finished product prepared in S2 into a powder with a particle size of 15 μm; S4: pressing the powder prepared in S3 into a mold and placing it into an isostatic press to be formed under a pressure of 150 MPa for 1 hour; S5: roasting the product prepared in S4 by heating it to 1200°C through a ring-type roasting furnace to prepare a carbonized product; S6: graphitizing the carbonized product prepared in S5 by gradually increasing the temperature to 3000°C through an Acheson graphitization furnace, the temperature increasing at a rate of 5°C / min from room temperature to 800°C, at a rate of 3°C / min from 800°C to 1200°C, and at a rate of 2°C / min from 1200°C to 3000°C; S7: machining and shaping the product prepared in S6, the particles with irregular shapes or surface burrs and edges being machined into isostatic pressure graphite particles and graphite powder in circular or oval shapes, and the particle size of the graphite powder being classified by sieving.

[0004] The further arrangement of the present application is that in the step S6, the carbonization product is purified by removing impurity elements by passing halogen gas during the process of heating to 3000 DEG C in the Acheson graphitization furnace, and the passing amount of the halogen gas is controlled to be 0.5%-1.5%.

[0005] The further arrangement of the present application is that the halogen gas includes at least one of chlorine gas or freon, but is not limited to them.

[0006] The further arrangement of the present application is that the isostatic pressing graphite particle is used as a nuclear power high temperature gas cooled reactor material, semiconductor manufacturing and solar photovoltaic manufacturing, and the particle size graded graphite powder is used for powder metallurgy, vacuum heat treatment and production of spherical fuel elements for high temperature gas cooled reactors.

[0007] The beneficial technical effect of the present application is that the present application uses petroleum coke or pitch coke instead of natural graphite as raw material, increases the selection of raw materials for producing isostatic pressing graphite, classifies the particle size of the graphite powder produced by processing and shaping, and fully utilizes the product, avoids oxidation by high temperature melting of the modified pitch under nitrogen protection, avoids agglomeration caused by too fine powder, and avoids affecting the molding density caused by too coarse powder by controlling the particle size of the raw material A powder, improves the mixing process, avoids the influence of mixing time and temperature on the distribution of the binder and the molding performance, also avoids the influence of long time high temperature on the binder volatilization, affects the molding effect, improves the graphitization process, further purifies the product, and improves the product strength. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1 The process flow chart of the present application is shown.

[0009] Figure 2 The SEM analysis 100 mu scale analysis diagram of the isostatic pressing graphite of the present application is shown.

[0010] Figure 3 The SEM analysis 10 mu scale analysis diagram of the isostatic pressing graphite of the present application is shown. DETAILED DESCRIPTION

[0011] The preferred embodiments of the present application are described below with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principles of the present application, and are not intended to limit the protection scope of the present application.

[0012] The present application proposes an isostatic pressing special graphite preparation method, which uses the characteristics of high carbon content and low ash content of petroleum coke or pitch coke as a graphite matrix material, and uses the characteristics of high temperature stability and fluidity of modified pitch as a binder.

[0013] The isostatic pressing graphite preparation method includes the following steps:

[0014] Raw material preparation stage: Before processing, the composition of raw material A (at least one of petroleum coke or pitch coke) needs to be inspected to ensure that there is no harmful impurity, and then the raw material A is ground to a particle size of 15 pm through a jet mill device, to avoid agglomeration caused by too fine, and to avoid affecting the molding density caused by too coarse. The composition of raw material B (modified pitch) is inspected to ensure that there is no harmful impurity, and then it is melted at high temperature under nitrogen protection, with the melting temperature controlled at 250-300°C to prevent the modified pitch from pyrolysis or oxidation.

[0015] Raw material processing stage:

[0016] (1) Kneading: Knead raw material A and raw material B in a heat transfer oil heating pot for 8-16 hours. Heat raw material B to 150℃ in the heat transfer oil heating pot for 1-2 hours to reach the softening temperature of raw material B and form a paste. Gradually add raw material A and knead at low temperature. The softened paste-like raw material B gradually adheres to raw material A to form clumps. With the stretching and shearing forces of the clumps during low-temperature kneading, the raw materials gradually diffuse to avoid agglomeration. By reducing the fluidity of raw material B and increasing its viscosity, raw material A and raw material B are initially mixed. Then, the temperature is raised to 250℃ and kneaded for 6 hours. -14 hours to improve the fluidity of raw material B so that it can be fully mixed with raw material A. At the same time, avoid prolonged high temperature to reduce the volatilization of raw material B and affect the bonding effect. During the heating process, the mixing time and heating rate are adjusted by dynamically controlling the temperature curve and online viscosity detection to ensure that the fluidity is within the optimal range. Raw material B gradually reaches the melting temperature and transforms into a liquid state. During the heating and mixing process, raw materials A and B in the lumps are gradually evenly distributed and combined. Finally, the temperature is reduced to 150 degrees Celsius and held for 1 hour to ensure that raw materials A and B are fully combined and produced as a semi-finished product. The dynamic temperature control curve uses the kneading condition of raw material A as the control standard. If raw material A does not agglomerate, the temperature is increased at a rate of 15℃ / h-20℃ / h. However, during the kneading stage, powdered raw material A often agglomerates, especially when molten high-temperature modified asphalt is directly kneaded with powdered raw material A. After the viscosity of the modified asphalt decreases and its fluidity increases, it is difficult to disperse and distribute the agglomerated raw material A evenly. This results in some microscopic particle structures. These particle structures show that the agglomerated raw material A is wrapped by raw material B. Even after grinding the kneaded product, some raw material A and raw material B powders are not fully combined. After calcination and graphitization, the overall internal structure and pores of the final product are uneven, affecting the overall performance of the product. The dynamic temperature control curve uses the mixing condition of raw material A as the control standard. If raw material A does not agglomerate, the temperature is increased at a rate of 20℃ / h. If raw material A agglomerates, the temperature is stopped to maintain the viscosity of raw material B until the agglomerated raw material A is completely dispersed. Then, the temperature is increased again at a rate of 20℃ / h. If the agglomerated raw material A does not begin to disperse within 0.5-1 hour after the temperature is stopped, the temperature is decreased at a rate of 10℃ / h to increase the viscosity of raw material B until the agglomerated raw material A begins to disperse. Then, the temperature is stopped. After raw material A is completely dispersed, the temperature is increased again at a rate of 20℃ / h.

[0017] (2) Molding: The semi-finished product is ground into powder with a particle size of 15μm, and the powder is pressed into a mold and placed into an isostatic press. It is pressed for 1 hour under a pressure of 150Mpa. The grinding equipment is made of stainless steel and is cleaned regularly to avoid introducing impurities and contaminating the powder during the grinding process. The isostatic press adopts a multi-directional pressure control system to ensure uniform pressure distribution.

[0018] (3) Carbonization: the shaped semi-finished product is heated to 1200 degrees Celsius by a ring-type baking furnace to remove volatile matter and form a carbon skeleton, thereby obtaining a carbonized product.

[0019] (4) Graphitization and purification: the carbonized product is subjected to graphitization by an Acheson graphitization furnace at a temperature of 3000 degrees Celsius. Gradient heating is used to avoid cracking or hidden cracking of the carbonized product. Hidden cracking not only reduces the overall performance of the product, but also causes leakage channels in the isostatic graphite, which affects the neutron slowing effect and the shielding effect of gamma rays. Therefore, gradient heating is used for graphitization treatment to ensure the stability of the carbonized skeleton. The heating rate is 5 degrees Celsius per minute from room temperature to 800 degrees Celsius, 3 degrees Celsius per minute from 800 degrees Celsius to 1200 degrees Celsius, and 2 degrees Celsius per minute from 1200 degrees Celsius to 3000 degrees Celsius. Halogen gas (chlorine or freon) is used to remove impurity elements and achieve purification during the gradual heating process in the Acheson graphitization furnace. An exhaust gas treatment system is also provided to avoid corrosion of the equipment caused by residual halogen gas.

[0020] (5) Finishing: the graphitized product is machined and shaped. Irregularly shaped or surface-burred particles are processed into circular or elliptical isostatic graphite particles and graphite powder. The graphite powder is classified by particle size through sieving.

[0021] Product performance verification (key parameters and detection standards):

[0022] Purity: detected by X-ray fluorescence spectroscopy (XRF), the content of impurity elements is 85 ppm (the total content of impurities in nuclear-grade graphite is usually less than 0.01%, i.e. 100 ppm).

[0023] Key elements: detected by ASTM C1233: neutron absorption performance test, the content of boron (B) is 0.3 ppm, and the neutron absorption cross-section is less than 3 mb (strictly controlled below 0.5 ppm, because boron has a very high neutron absorption cross-section, which may affect the efficiency of nuclear reactions).

[0024] Strength: detected by GB / T 3074.1: compression strength test (axial load method) and ISO 12985-1 high temperature tensile property test, the compression strength is 120 MPa, and the bending strength is 65 MPa (reaching the standard of high-density graphite).

[0025] Density: detected by GB / T 2997: carbon material volume density determination method, the true density is 1.88 g / cm³.

[0026] Pore uniformity: The porosity is 17%, the apparent porosity is 15%, and the pore size distribution is 3 nm-360 μm (complementary analysis by mercury intrusion method and gas adsorption method) by GB / T 25995: Pore rate analysis (mercury intrusion method) test.

[0027] Pore size variation coefficient (CV): The pore size CV value is 10% by ASTM E112 (image analysis method) test.

[0028] Microstructure uniformity: The controllable particle size of the particles is controlled in the range of 10 μm-15 μm.

[0029] Thermal stability: The thermal weight loss is <1% at 3000°C.

[0030] Nuclear application: The size change rate after neutron irradiation is <1%, and the gamma ray shielding rate is 95% (uniform pores can effectively absorb gamma rays) by gamma ray source and detector test (uniform pores can increase the collision probability of neutrons and carbon atoms, and can improve the neutron moderation efficiency by about 10%-15%).

[0031] The isostatic pressing graphite particles prepared in the scheme are mainly applied to nuclear power high-temperature gas cooled reactor materials (verified by neutron irradiation experiment, 4×10 6 Neutrons / m³ to verify the anti-irradiation performance, the uniform distribution of pores can increase the collision probability of neutrons and carbon atoms, and can improve the neutron moderation efficiency by about 10%-15%, if the pore distribution is uneven, the neutrons may form a low-resistance channel along the large pores, reducing the overall moderation effect, at the same time, the high-density solid carbon area corresponding to the uniform pores can effectively absorb gamma rays, and the pore aggregation area may become a weak point of radiation leakage, the uniform structure can reduce such risks, and for every 1% improvement in pore uniformity, the gamma ray shielding rate can increase by about 0.5%-1%, and in a nuclear reactor, the uniform pore structure helps to conduct heat uniformly, preventing local overheating and causing material performance degradation, thereby maintaining stable anti-radiation performance), semiconductor manufacturing (used for single crystal silicon growth crucible), and solar photovoltaic manufacturing (used for polycrystalline silicon ingot casting), the graphite powder produced is used for powder metallurgy, vacuum heat treatment, and production of spherical fuel elements for high-temperature gas cooled reactors after particle size grading.

[0032] Although the present application has been described with reference to the preferred embodiments, various modifications can be made to it without departing from the scope of the application, and equivalent substitutions can be made to the components thereof, especially, the technical features mentioned in each embodiment can be combined in any way as long as there is no structural conflict. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

[0033] In the description of the present application, the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like indicate directional or positional relationships based on the directions or positional relationships shown in the drawings, which are merely for the purpose of facilitating the description, and do not indicate or imply that the device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application. In addition, the terms "first", "second", "third" are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0034] In addition, it should also be noted that, in the description of the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connecting" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0035] The term "comprising" or any other similar word is intended to cover non-exclusive inclusion, so that the process, article or equipment / device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or also includes the elements inherent to the process, article or equipment / device.

[0036] So far, the technical solutions of the present application have been described in combination with the preferred embodiments shown in the drawings, but those skilled in the art can easily understand that the protection scope of the present application is obviously not limited to these specific embodiments. Those skilled in the art can make equivalent changes or replacements to the related technical features without departing from the principles of the present application, and the technical solutions after the changes or replacements will fall within the protection scope of the present application.

Claims

1. A method for preparing special isostatically pressed graphite, characterized in that: Includes the following steps, S1: Pre-processing of raw material A and raw material B, wherein raw material A includes at least one of petroleum coke or pitch coke, and the pre-processing of raw material A adopts a grinding process. First, raw material A is subjected to component quality inspection to ensure that there are no harmful impurities, and then it is ground using an air jet mill to grind to a particle size of 15μm. Raw material B is modified pitch, and the pre-processing of raw material B adopts a high-temperature melting process. First, raw material B is subjected to component quality inspection to ensure that there are no harmful impurities, and then it is melted at high temperature under nitrogen protection, with the melting temperature controlled at 250℃-300℃. S2: Mix raw material A and raw material B to make a semi-finished product. In the mixing step, heat raw material B to 150°C in a heat transfer oil heating pot for 1-2 hours and gradually add raw material A. The softened paste-like raw material B gradually adheres to raw material A to form a clump. Continue mixing until raw material A is completely mixed into the clump. Then, gradually raise the temperature to 250°C through dynamic temperature control curve and mix for 5-12 hours to allow raw material B to gradually reach the melting temperature and turn into a liquid state. This allows raw material A and raw material B in the clump to gradually distribute evenly during the heating and mixing process. Finally, cool down to 150°C and keep warm for 1 hour before discharging. The dynamic temperature control curve uses the mixing condition of raw material A as the control standard. If raw material A does not agglomerate, the temperature is increased at a rate of 15℃ / h-20℃ / h. If raw material A agglomerates, the temperature is stopped and the viscosity of raw material B is maintained until the agglomerated raw material A is completely dispersed. Then the temperature is increased again. If the agglomerated raw material A does not begin to disperse 0.5h-1h after the temperature is stopped, the temperature is decreased at a rate of 10℃ / h to increase the viscosity of raw material B until the agglomerated raw material A begins to disperse. Then the temperature is stopped and the temperature is increased again after raw material A is completely dispersed. S3: Grind the semi-finished product obtained in S2 into powder with a particle size of 15μm; S4: Press the powder obtained in S3 into a mold and place it in an isostatic press. Hold the pressure at 150 MPa for 1 hour to form the product. S5: The product obtained in S4 is roasted and heated to 1200 degrees Celsius in a ring roasting furnace to obtain a carbonized product. S6: The carbonized product obtained in S5 is graphitized by gradient heating to 3000℃ in an Atchison graphitization furnace. The heating rate is 5℃ / min from room temperature to 800℃, 3℃ / min from 800℃ to 1200℃, and 2℃ / min from 1200℃ to 3000℃. During the process of heating the carbonized product to 3000℃ in an Atchison graphitization furnace, halogen gas is introduced to remove impurity elements for purification, and the amount of halogen gas introduced is controlled at 0.5%-1.5%. S7: The product obtained in S6 is machined and shaped to process irregularly shaped particles or particles with burrs and sharp edges into round or elliptical isostatic graphite particles and graphite powder. The graphite powder is then classified by particle size through sieving.

2. The method for preparing isostatically pressed special graphite according to claim 1, characterized in that: The halogen gas includes chlorine or Freon.

3. The application of the isostatically pressed special graphite prepared according to any one of claims 1-2, characterized in that: The isostatically pressed graphite particles are used as materials for high-temperature gas-cooled reactors in nuclear power plants, semiconductor manufacturing, and solar photovoltaic manufacturing. The graphite powder after particle size classification is used for powder metallurgy, vacuum heat treatment, and the production of spherical fuel elements for high-temperature gas-cooled reactors.

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