CN111362698A - Novel isotropic nuclear-grade graphite material and preparation method thereof - Google Patents
Novel isotropic nuclear-grade graphite material and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a novel isotropic nuclear-grade graphite material and a preparation method thereof, wherein the novel isotropic nuclear-grade graphite material is prepared by taking spheroidized natural crystalline flake graphite as an aggregate, the proportion of the spheroidized natural crystalline flake graphite in the aggregate is 50-100 wt%, and the specific preparation process comprises the following steps: spheroidization, purification, kneading, secondary crushing and screening, molding, primary roasting, impregnation, secondary roasting and graphitization. The isotropic nuclear-grade graphite material prepared by spheroidizing natural crystalline flake graphite has the characteristics of high density, high thermal conductivity, low expansion coefficient and excellent isotropy, and has a wide application prospect in the fields of nuclear energy, thermal management, machinery, chemical industry and the like.
Description
Technical Field
The invention belongs to the technical field of graphite materials, and relates to a novel isotropic nuclear grade graphite material obtained by taking spheroidized natural crystalline flake graphite as a raw material and a preparation method thereof.
Technical Field
Graphite has the following excellent properties: (1) high neutron scattering cross-section and low neutron absorption cross-section are excellent neutron moderators. (2) Good high temperature resistance and stable chemical properties. (3) High thermal conductivity and good radiation resistance. (4) The raw materials of the artificial graphite such as coke, natural graphite and the like are rich in sources and low in price, and various nuclear grade graphite with high purity, high strength and different particle size requirements can be easily prepared, so that the artificial graphite is widely used on nuclear reactors. Particularly, in high temperature gas cooled reactors and molten salt reactors which belong to fourth generation nuclear power technologies, graphite is used as a moderator material and a reflective layer material. However, since the graphite crystal has a layered structure, the bonding manner in the direction parallel to the sheet layer is different from that in the direction perpendicular to the sheet layer, and thus, it shows a remarkable anisotropy. Therefore, the artificial graphite is easily oriented preferentially in the production process, and is macroscopically anisotropic, and when anisotropic nuclear-grade graphite is irradiated, the irradiation life is shortened, and the problem of structural stability due to dimensional changes in different directions is caused, which is very disadvantageous for the application of the nuclear-grade graphite, and thus, the use of isotropic graphite is required. The graphite material of the reflecting layer in the ball bed type high temperature gas cooled reactor cannot be replaced, so the service life of the nuclear grade graphite directly influences the service life of the reactor. The life of currently used nuclear grade graphite is about 30-40 years, while the design life of future nuclear reactors is 50-60 years. Therefore, in order to extend the service life of the high temperature gas cooled reactor and to improve its economic competitiveness, it is necessary to develop isotropic nuclear grade graphite having more excellent properties. In addition, the isotropic graphite is also widely applied to industries such as machinery, metallurgy and the like, such as molds, crucibles, thermal field materials and the like.
Disclosure of Invention
The invention aims to provide a novel isotropic nuclear-grade graphite material, which is prepared by spheroidizing natural crystalline flake graphite and has the characteristics of high density, high thermal conductivity, low expansion coefficient and excellent isotropy.
In order to achieve the technical purpose, the invention adopts the following technical scheme:
a novel isotropic nuclear-grade graphite material is prepared by taking natural crystalline flake graphite containing sphericization as aggregate, wherein the proportion of the sphericized natural crystalline flake graphite in the aggregate is 50-100 wt%.
In a preferred scheme, the preparation process of the spheroidized natural crystalline flake graphite comprises the following steps: spheroidizing the flake graphite by using a spheroidizing classifier, wherein the D50 particle size of the spheroidized natural flake graphite is 1-300 mu m; more preferably 5 to 100 μm; most preferably 10-30 μm.
The aggregate of the invention can also contain other auxiliary raw materials, such as one or more of petroleum coke, pitch coke, secondary coke, unspherized natural crystalline flake graphite, natural microcrystalline graphite (also called soil graphite), mesocarbon microbeads or graphite return, and preferably 100 wt% of spheroidized natural crystalline flake graphite.
In a preferred embodiment, the preparation process of the isotropic nuclear grade graphite material comprises: spheroidization, purification, kneading, secondary crushing and screening, molding, primary roasting, impregnation, secondary roasting and graphitization; it should be noted that, the order of purification and spheroidization in the present invention can be interchanged, i.e., spheroidization can be performed first and then purification can be performed, or purification can be performed first and then spheroidization can be performed;
more preferably, the preparation process of the isotropic nuclear grade graphite material specifically comprises the following steps:
(1) spheroidizing: placing natural crystalline flake graphite with carbon content of more than 90% in a spheroidization classifier, and carrying out crushing, classification and spheroidization treatment; the natural crystalline flake graphite is placed in a sphericizing classifier to be crushed and classified, the natural crystalline flake graphite collides with a metal or ceramic module to be curled, the degree and the quality of sphericization can be controlled by adjusting the natural crystalline flake graphite through controlling and adjusting the collision speed, the collision angle and the collision frequency of the natural crystalline flake graphite and the module in the sphericizing classifier, and then the sphericization treatment can be completed;
(2) and (3) purification: the purity of the spheroidized natural crystalline flake graphite is improved to more than 99.9 percent by adopting high-temperature purification, and halogen and/or halogenated hydrocarbon gases such as chlorine, freon and the like can be introduced simultaneously in the high-temperature purification process, so that the purification effect is enhanced; or adopting chemical purification, and improving the purity of the spheroidized natural crystalline flake graphite to over 99.9 percent by using strong acid such as hydrochloric acid, hydrofluoric acid and the like or strong base such as sodium hydroxide and the like;
it should be noted that, the order of purification and spheroidization in the present invention can be interchanged, that is, the natural crystalline flake graphite with carbon content of more than 90% can be first spheroidized and then the spheroidized natural crystalline flake graphite can be purified, or the natural crystalline flake graphite with carbon content of more than 90% can be first purified and then the natural crystalline flake graphite after purification can be spheroidized;
(3) kneading: mixing and kneading aggregate containing the sphericized natural crystalline flake graphite and binder asphalt to obtain paste, wherein the binder asphalt can be medium-temperature asphalt or modified asphalt;
(4) secondary crushing and screening: secondarily crushing the kneaded paste, and sieving with a 50-200-mesh sieve to obtain powder;
(5) molding: carrying out compression molding or isostatic pressing on the sieved powder to obtain a graphite green body;
(6) primary roasting: placing the graphite green compact in a roasting furnace, heating to 800-;
(7) impregnation and secondary roasting: cooling the product after primary roasting, placing the product in an impregnation tank, and impregnating for 2-3h at the pressure of 6-15MPa for one time; then placing the mixture in a roasting furnace, raising the temperature to 800-;
(8) graphitization treatment: introducing halogen and/or halogenated hydrocarbon gas into the product after secondary roasting at the temperature of 2500-3000 ℃ for graphitization treatment to obtain the isotropic nuclear grade graphite material.
Preferably, in the step (1), the D50 particle size of the spheroidized natural flake graphite is 1-300 μm; more preferably 5 to 100 μm; most preferably 10-30 μm.
Preferably, in the step (1), the sphericity of the spheroidized natural crystalline flake graphite is not less than 0.6, and the sphericity is defined as the ratio of the minor axis to the major axis of the particle.
Preferably, in the step (3), the softening point temperature of the binder asphalt is 60-150 ℃, and the mass ratio of the aggregate containing the sphericized natural crystalline flake graphite to the binder asphalt is 2:1-10: 1.
Preferably, in the step (5), the pressure for isostatic pressing is 100 to 200 MPa.
The invention also provides a preparation method of the isotropic nuclear-grade graphite material, which comprises the following steps: spheroidization, purification, kneading, secondary crushing and screening, molding, primary roasting, impregnation, secondary roasting and graphitization; it should be noted that, the order of purification and spheroidization in the present invention can be interchanged, i.e., spheroidization can be performed first and then purification can be performed, or purification can be performed first and then spheroidization can be performed;
preferably, the preparation process of the isotropic nuclear-grade graphite material specifically comprises the following steps:
(1) spheroidizing: placing natural crystalline flake graphite with carbon content of more than 90% in a spheroidization classifier, and carrying out crushing, classification and spheroidization treatment; the natural crystalline flake graphite is placed in a sphericizing classifier to be crushed and classified, the natural crystalline flake graphite collides with a metal or ceramic module to be curled, the degree and the quality of sphericization can be controlled by adjusting the natural crystalline flake graphite through controlling and adjusting the collision speed, the collision angle and the collision frequency of the natural crystalline flake graphite and the module in the sphericizing classifier, and then the sphericization treatment can be completed;
(2) and (3) purification: the purity of the spheroidized natural crystalline flake graphite is improved to more than 99.9 percent by adopting high-temperature purification, and halogen and/or halogenated hydrocarbon gases such as chlorine, freon and the like can be introduced simultaneously in the high-temperature purification process, so that the purification effect is enhanced; or adopting chemical purification, and improving the purity of the spheroidized natural crystalline flake graphite to over 99.9 percent by using strong acid such as hydrochloric acid, hydrofluoric acid and the like or strong base such as sodium hydroxide and the like;
it should be noted that, the order of purification and spheroidization in the present invention can be interchanged, that is, the natural crystalline flake graphite with carbon content of more than 90% can be first spheroidized and then the spheroidized natural crystalline flake graphite can be purified, or the natural crystalline flake graphite with carbon content of more than 90% can be first purified and then the natural crystalline flake graphite after purification can be spheroidized;
(3) kneading: mixing and kneading aggregate containing the sphericized natural crystalline flake graphite and binder asphalt to obtain paste, wherein the binder asphalt can be medium-temperature asphalt or modified asphalt;
(4) secondary crushing and screening: secondarily crushing the kneaded paste, and sieving with a 50-200-mesh sieve to obtain powder;
(5) molding: carrying out compression molding or isostatic pressing on the sieved powder to obtain a graphite green body;
(6) primary roasting: placing the graphite green compact in a roasting furnace, heating to 800-;
(7) impregnation and secondary roasting: cooling the product after primary roasting, placing the product in an impregnation tank, and impregnating for 2-3h at the pressure of 6-15MPa for one time; then placing the mixture in a roasting furnace, raising the temperature to 800-;
(8) graphitization treatment: introducing halogen and/or halogenated hydrocarbon gas into the product after secondary roasting at the temperature of 2500-3000 ℃ for graphitization treatment to obtain the isotropic nuclear grade graphite material.
Preferably, in the step (1), the D50 particle size of the spheroidized natural flake graphite is 1-300 μm; more preferably 5 to 100 μm; most preferably 10-30 μm.
Preferably, in the step (1), the sphericity of the spheroidized natural crystalline flake graphite is not less than 0.6, and the sphericity is defined as the ratio of the minor axis to the major axis of the particle.
Preferably, in the step (3), the softening point temperature of the binder asphalt is 60-150 ℃, and the mass ratio of the aggregate containing the spheroidized natural crystalline flake graphite to the binder asphalt is 2:1-10: 1.
Preferably, in the step (5), the isostatic compaction pressure is 100-200 MPa.
The method utilizes a method for changing the shape of the aggregate to control the isotropy of the graphite product, and finally obtains the isotropic nuclear grade graphite material with high density, high thermal conductivity, low expansion coefficient and excellent isotropy. The key point of the invention is that the natural crystalline flake graphite is treated by spheroidization, the natural crystalline flake graphite has a flaky shape, if the natural crystalline flake graphite is not treated, flaky particles spontaneously form preferred orientation under the action of gravity during charging, and an anisotropic graphite product is obtained after molding. The inventor finds that only the natural flake graphite after the spheroidization treatment can be used as the aggregate to prepare the graphite product with high isotropy, and the principle is as follows: in the process of spheroidization, the flaky particles are curled and formed into spheres under the action of mechanical impact force. When the spherical particles are stacked, the orientation is randomly arranged, so that the product obtained after molding has no preferred orientation or has low preferred orientation. The effect of the spheronization treatment can be described by the degree of spheronization, which is defined as the ratio of the shortest axis to the longest axis length of the particle.
The advantages of the invention are as follows:
1. at present, isotropic coke, Gilsonite coke, needle coke, petroleum coke, pitch coke and the like are generally used as production raw materials of isotropic graphite in the carbon industry at home and abroad, and the invention adopts the spheroidized natural flake graphite as aggregate to produce a brand-new isotropic (nuclear grade) graphite which is a substantial improvement on the prior art route;
2. the invention takes natural crystalline flake graphite as a raw material, has sufficient source and low price, can be supplied continuously, and can avoid the situation that the structure and the performance of the coke raw material are changed due to the change of upstream industrial raw materials and processes, so that nuclear-grade graphite is forced to be researched and developed again;
3. the natural crystalline flake graphite powder has better pressing performance, so the product density is higher than that of the common artificial graphite under the same condition;
4. the product obtained by the invention has high thermal conductivity, and especially when 100% spheroidized natural crystalline flake graphite is used as a raw material, the thermal conductivity can reach more than 160W/m.k, thereby being beneficial to heat conduction.
5. The product obtained by the invention has low coefficient of thermal expansion which is less than 3 × 10-6the/K is 30 percent lower than that of commercial nuclear grade graphite.
6. The product obtained by the invention has excellent isotropy (the isotropy is the ratio of the thermal expansion coefficients in two mutually perpendicular directions, and the smaller the ratio, the more excellent the isotropy).
Drawings
FIG. 1 is a scanning electron micrograph of spheroidized natural crystalline flake graphite according to example 1 of the present invention;
FIG. 2 is a fracture morphology diagram of a spheroidized natural crystalline flake graphite-based isotropic nuclear grade graphite material prepared in example 1 of the present invention;
FIG. 3 is a thermal expansion diagram of the spheroidized natural flake graphite (SFG) -based isotropic nuclear grade graphite material obtained in example 1 of the present invention, wherein TR represents a horizontal direction in which a thermal expansion coefficient in a range from room temperature to 500 ℃ is 2.80 × 10-6AX represents an axial or vertical direction, and the coefficient of thermal expansion in the direction of room temperature to 500 ℃ is 2.90 × 10-6(ii)/K, accordingly, the article has an isotropy of 1.04;
fig. 4 is a polarization diagram of the spheroidized natural flake graphite-based isotropic nuclear-grade graphite material prepared in example 1 of the present invention.
Detailed Description
The present invention will be further described with reference to the following examples.
Example 1
In the embodiment, the mass ratio of the binder to the aggregate is 0.3:1, the aggregate is 100% of spheroidized natural crystalline flake graphite, the spheroidized natural crystalline flake graphite is a purified nuclear pure-grade raw material, the fixed carbon content is 99.99%, the D50 granularity is 21.0 microns, the spheroidization degree is 0.85, and an SEM image of the spheroidization method is shown in figure 1, which shows that the shape of the natural crystalline flake graphite is successfully converted from a sheet shape to a spherical shape after the spheroidization treatment of the natural crystalline flake graphite. The binder is modified asphalt and has a softening point of 115 ℃.
The preparation method comprises the following steps:
(1) spheroidizing: placing natural crystalline flake graphite with 97% of carbon content into a spheroidization classifier, and crushing, classifying and spheroidizing to obtain spheroidized natural crystalline flake graphite with D50 of 21.0 mu m and average spheroidization degree of 0.85;
(2) and (3) purification: purifying at high temperature of 3000 deg.C by continuous graphitizing furnace, introducing chlorine gas, and purifying to obtain pure-grade spheroidized natural crystalline flake graphite powder with carbon content of 99.99%;
(3) kneading: mixing and preheating the binder and the aggregate in a mass ratio of 0.3:1 in a mixing and kneading machine, mixing and kneading at the preheating temperature of 180 ℃ for 1h, adding the molten modified asphalt binder after preheating, starting mixing and kneading, and finishing after 2h, wherein the mixing and kneading temperature is 180 ℃;
(4) secondary crushing and screening: secondarily crushing the kneaded paste, and then sieving the paste by a 100-mesh sieve to obtain powder;
(5) molding: pressing the sieved powder into a graphite green body by isostatic pressing, wherein the isostatic pressing pressure is 150MPa, and the pressure maintaining time is 8 min;
(6) primary roasting: placing the graphite green body in a roasting furnace, heating to 950 ℃ at the heating rate of 0.05 ℃/min, and preserving heat for 2 hours to finish primary roasting;
(7) impregnation and secondary roasting: cooling the product after primary roasting, placing the product in an impregnation tank, and impregnating for 2 hours at the pressure of 8 MPa; then placing the mixture in a roasting furnace, heating to 950 ℃ at the heating rate of 0.05 ℃/min, and preserving heat for 2 hours to finish secondary roasting;
(8) graphitization treatment: introducing chlorine and freon into the product after secondary roasting at 2800 ℃ for graphitization treatment to obtain the isotropic nuclear grade graphite material. The fracture morphology is shown in fig. 2, which shows that the obtained graphite product has a compact microstructure and the aggregate and the binder are well combined. The polarization diagram is shown in fig. 4, the brightness of each crystal grain is obviously different, and the product has good anisotropy, so that the product has excellent isotropy in macroscopical aspect.
The obtained nuclear grade graphite material: bulk density 1.80g/cm3Ash content 40ppm, boron equivalent 2.0ppm, graphitization degree 91%, thermal expansion coefficient in TR direction 2.8 × 10-6K, coefficient of thermal expansion in AX direction of 2.9 × 10-6The isotropy is 1.04, the breaking strength is 26MPa, and the thermal conductivity is 160W/m.K.
Example 2
In the implementation, the mass ratio of the binder to the aggregate is 0.3:1, the aggregate is a mixture of spheroidized natural crystalline flake graphite and petroleum coke, the mass ratio of the spheroidized natural crystalline flake graphite to the petroleum coke in the mixture is 1:1, the spheroidized natural crystalline flake graphite is a purified nuclear pure-grade raw material, the fixed carbon content is 99.99%, the D50 granularity is 20.0 mu m, the spheroidization degree is 0.85, the fixed carbon content in the petroleum coke is 99.95%, the boron (B) and the gadolinium (Gd) are both less than 1ppm, the moisture content is less than or equal to 0.1%, the sulfur content is less than or equal to 0.05%, and the granularity is 15 mu m. The binder is modified asphalt with a softening point of 110 ℃.
The preparation method comprises the following steps:
(1) spheroidizing: placing natural crystalline flake graphite with 97% carbon content into a spheroidization classifier, and crushing, classifying and spheroidizing to obtain spheroidized natural crystalline flake graphite with D50 of 20.0 μm and average spheroidization degree of 0.85;
(2) and (3) purification: purifying at high temperature of 3000 deg.C by continuous graphitizing furnace, introducing chlorine gas, and purifying to obtain pure-grade spheroidized natural crystalline flake graphite powder with carbon content of 99.99%;
(3) kneading: uniformly mixing the spheroidized natural crystalline flake graphite powder and petroleum coke powder according to the mass ratio of 1:1 to obtain aggregate, mixing and preheating the binder and the aggregate with the mass ratio of 0.3:1 in a kneading machine, and kneading at the preheating temperature of 160 ℃ for 1 h. Adding the melted modified asphalt binder after preheating, beginning kneading, ending after 2h, and kneading at the temperature of 160 ℃;
(4) secondary crushing and screening: secondarily crushing the kneaded paste, and then sieving the paste by a 100-mesh sieve to obtain powder;
(5) molding: pressing the sieved powder into a graphite green body by isostatic pressing, wherein the isostatic pressing pressure is 150MPa, and the pressure maintaining time is 8 min;
(6) primary roasting: placing the graphite green body in a roasting furnace, heating to 950 ℃ at the heating rate of 0.05 ℃/min, and preserving heat for 2 hours to finish primary roasting;
(7) impregnation and secondary roasting: cooling the product after primary roasting, placing the product in an impregnation tank, and impregnating for 2 hours at the pressure of 8 MPa; then placing the mixture in a roasting furnace, heating to 950 ℃ at the heating rate of 0.05 ℃/min, and preserving heat for 2 hours to finish secondary roasting;
(8) graphitization treatment: introducing chlorine gas into the product after secondary roasting at 2800 ℃ for graphitization treatment to obtain the isotropic nuclear grade graphite material.
The obtained nuclear grade graphite material: bulk density 1.86g/cm3Ash content 40ppm, boron equivalent 2.0ppm, graphitization degree 89%, thermal expansion coefficient in TR direction 3.7 × 10-6K, coefficient of thermal expansion in AX direction of 3.9 × 10-6The isotropy is 1.06, the breaking strength is 36MPa, and the thermal conductivity is 130W/m.K.
Example 3
In the implementation, the mass ratio of the binder to the aggregate is 0.3:1, the aggregate is a mixture of spheroidized natural crystalline flake graphite and natural microcrystalline graphite, the mass ratio of the spheroidized natural crystalline flake graphite to the natural microcrystalline graphite in the mixture is 3:1, the spheroidized natural crystalline flake graphite is a purified nuclear pure-grade raw material, the fixed carbon content is 99.99%, the D50 granularity is 15.0 microns, the spheroidization degree is 0.7, and the fixed carbon content in the natural microcrystalline graphite is 99.99% and the granularity is 20 microns. The binder is modified asphalt with a softening point of 120 ℃.
The preparation method comprises the following steps:
(1) and (3) purification: the raw material is natural flake graphite with 97 percent of carbon content, and the natural flake graphite is purified by a continuous graphitization furnace at high temperature with the highest temperature of 3000 ℃ and purified by introducing chlorine gas to obtain nuclear pure grade natural flake graphite powder with 99.99 percent of carbon content;
(2) spheroidizing: putting the nuclear pure grade natural crystalline flake graphite powder into a spheroidization classifier, and crushing, classifying and spheroidizing the nuclear pure grade natural crystalline flake graphite powder to obtain spheroidized natural crystalline flake graphite powder with D50 of 15.0 mu m and an average spheroidization degree of 0.7, wherein the carbon content is 99.99%;
(3) kneading: uniformly mixing the spheroidized natural crystalline flake graphite powder and the natural microcrystalline graphite powder according to the mass ratio of 3:1 to obtain aggregate, and mixing and preheating the binder and the aggregate in the mass ratio of 0.3:1 in a kneading machine at the preheating temperature of 180 ℃ for 1 h. Adding the melted modified asphalt binder after preheating, beginning kneading, ending after 2h, and kneading at the temperature of 180 ℃;
(4) secondary crushing and screening: secondarily crushing the kneaded paste, and then sieving the paste by a 100-mesh sieve to obtain powder;
(5) molding: pressing the sieved powder into a graphite green body by isostatic pressing, wherein the isostatic pressing pressure is 150MPa and the time is 8 min;
(6) primary roasting: placing the graphite green body in a roasting furnace, heating to 950 ℃ at the heating rate of 0.05 ℃/min, and preserving heat for 2 hours to finish primary roasting;
(7) impregnation and secondary roasting: cooling the product after primary roasting, placing the product in an impregnation tank, and impregnating for 2 hours at the pressure of 8 MPa; then placing the mixture in a roasting furnace, heating to 950 ℃ at the heating rate of 0.05 ℃/min, and preserving heat for 2 hours to finish secondary roasting;
(8) graphitization treatment: introducing chlorine and freon into the product after secondary roasting at 2800 ℃ for graphitization treatment to obtain the isotropic nuclear grade graphite material.
The obtained nuclear grade graphite material: bulk density 1.81g/cm3Ash content 40ppm, boron equivalent 2.0ppm, graphitization degree 92%, thermal expansion coefficient in TR direction 2.7 × 10-6K, coefficient of thermal expansion in AX direction of 2.8 × 10-6The isotropy is 1.03, the breaking strength is 28MPa, and the thermal conductivity is 127W/m.K.
Comparative example 1
In the comparison, the mass ratio of the binder to the aggregate is 0.3:1, the aggregate is 100% of unspherized natural crystalline flake graphite, the natural crystalline flake graphite is a purified nuclear pure-grade raw material, the fixed carbon content is 99.99%, the D50 granularity is 20.0 mu m, and the sphericity is 0.2. The binder is modified asphalt with a softening point of 110 ℃.
The preparation method comprises the following steps:
(1) pretreatment: crushing and grading natural flake graphite with 97% of carbon content to obtain natural flake graphite with D50 granularity of 20.0 mu m and sphericization degree of 0.2, purifying at high temperature by adopting a continuous graphitization furnace, and introducing chlorine for purification to obtain nuclear pure grade natural flake graphite powder with 99.99% of carbon content;
(2) kneading: mixing and preheating the binder and the aggregate in an actual mass ratio of 0.3:1 in a kneading machine, wherein the preheating temperature is 180 ℃, and the preheating time is 1 h. Adding the melted modified asphalt binder after preheating, beginning kneading, ending after 2h, and kneading at the temperature of 180 ℃;
(3) secondary crushing and screening: secondarily crushing the kneaded paste, and then sieving the paste by a 100-mesh sieve to obtain powder;
(4) molding: pressing the sieved powder into a graphite green body by isostatic pressing, wherein the isostatic pressing pressure is 150MPa, and the pressure maintaining time is 8 min;
(5) primary roasting: placing the graphite green body in a roasting furnace, heating to 1000 ℃ at the heating rate of 0.05 ℃/min, and preserving heat for 2 hours to finish primary roasting;
(6) impregnation and secondary roasting: cooling the product after primary roasting, placing the product in an impregnation tank, preheating at 200 ℃, vacuumizing the impregnation tank to less than or equal to 0.02MPa, pouring molten asphalt, and then impregnating for 2 hours at the pressure of 8 MPa; then placing the mixture in a roasting furnace, raising the temperature to 1000 ℃ at the temperature rise rate of 0.05 ℃/min, and preserving the temperature for 2 hours to finish secondary roasting;
(7) graphitization treatment: graphitizing the product after the secondary roasting at 2800 ℃ to obtain the graphite product.
The obtained graphite product comprises the following components: bulk density 1.85g/cm3The graphitization degree is 91%, the isotropy degree is more than 2.0, and the product is anisotropic.
Comparative example 2
In the comparison, the mass ratio of the binder to the aggregate is 0.3:1, the aggregate is 100% of common petroleum coke, the content of fixed carbon in the common petroleum coke aggregate is 99.5%, the ash content is less than or equal to 0.2%, the moisture content is less than or equal to 0.1%, the sulfur content is less than or equal to 0.05%, and the granularity is 25 microns. The binder is modified asphalt with a softening point of 100 ℃.
The preparation method comprises the following steps:
(1) kneading: mixing and preheating the binder and the aggregate in an actual mass ratio of 0.3:1 in a kneading machine, wherein the preheating temperature is 160 ℃, and the preheating time is 1 h. Adding the melted modified asphalt binder after preheating, beginning kneading, ending after 2h, and kneading at the temperature of 160 ℃;
(2) secondary crushing and screening: secondarily crushing the kneaded paste, and then sieving the paste by a 100-mesh sieve to obtain powder;
(3) molding: pressing the sieved powder into a graphite green body by isostatic pressing, wherein the isostatic pressing pressure is 150MPa, and the pressure maintaining time is 8 min;
(4) primary roasting: placing the graphite green body in a roasting furnace, heating to 1000 ℃ at the heating rate of 0.05 ℃/min, and preserving heat for 2 hours to finish primary roasting;
(5) impregnation and secondary roasting: cooling the product after primary roasting, placing the product in an impregnation tank, preheating at 200 ℃, vacuumizing the impregnation tank to less than or equal to 0.02MPa, pouring molten asphalt, and then impregnating for 2 hours at the pressure of 8 MPa; then placing the mixture in a roasting furnace, raising the temperature to 1000 ℃ at the temperature rise rate of 0.05 ℃/min, and preserving the temperature for 2 hours to finish secondary roasting;
(6) secondary impregnation and tertiary roasting: cooling the product after secondary roasting, placing the product in an impregnation tank, preheating at 200 ℃, vacuumizing the impregnation tank to less than or equal to 0.02MPa, pouring molten asphalt, and performing secondary impregnation for 2 hours at the pressure of 8 MPa; then placing the mixture in a roasting furnace, heating to 1000 ℃ by adopting the same heating program as that of the primary roasting, and preserving heat for 2 hours to finish the tertiary roasting;
(7) graphitization treatment: graphitizing the product after the three times of roasting at 2800 ℃ to obtain the graphite product.
The obtained graphite product comprises the following components: bulk density 1.80g/cm3Graphitization degree 87%, thermal expansion coefficient in TR direction 3.4 × 10-6K, coefficient of thermal expansion in AX direction of 4.7 × 10-6The isotropy is 1.37, the breaking strength is 28MPa, and the thermal conductivity is 115W/m.K.
Claims (10)
1. A novel isotropic nuclear-grade graphite material is characterized in that: the spherical graphite material is prepared by taking spherical natural crystalline flake graphite as aggregate, wherein the proportion of the spherical natural crystalline flake graphite in the aggregate is 50-100 wt%.
2. The novel isotropic nuclear grade graphite material as claimed in claim 1, wherein: the preparation process of the spheroidized natural crystalline flake graphite comprises the following steps: spheroidizing the flake graphite by using a spheroidizing classifier, wherein the D50 particle size of the spheroidized natural flake graphite is 1-300 mu m.
3. The novel isotropic nuclear grade graphite material as claimed in claim 2, wherein: the D50 particle size of the spheroidized natural flake graphite is 5-100 μm.
4. The novel isotropic nuclear grade graphite material as claimed in claim 1, wherein: the aggregate containing the spheroidized natural crystalline flake graphite also comprises one or more of petroleum coke, pitch coke, secondary coke, non-spheroidized natural crystalline flake graphite, natural microcrystalline graphite, mesocarbon microbeads or graphite return material.
5. The method of making an isotropic nuclear grade graphitic material according to any one of claims 1-4, comprising:
the first scheme is as follows: spheroidization, purification, kneading, secondary crushing and screening, molding, primary roasting, impregnation, secondary roasting and graphitization; or
Scheme II: purification, spheroidization, kneading, secondary crushing and screening, molding, primary roasting, impregnation, secondary roasting and graphitization.
6. The method of preparing an isotropic nuclear grade graphite material as claimed in claim 5, wherein the process of preparing the isotropic nuclear grade graphite material specifically comprises the steps of:
the first scheme is as follows:
(1) spheroidizing: placing natural crystalline flake graphite with carbon content of more than 90% in a spheroidization classifier, and carrying out crushing, classification and spheroidization treatment to obtain spheroidized natural crystalline flake graphite;
(2) and (3) purification: the purity of the spheroidized natural crystalline flake graphite is improved to more than 99.9 percent by adopting high-temperature purification;
(3) kneading: mixing and kneading aggregate containing the sphericized natural crystalline flake graphite and binder asphalt to obtain paste, wherein the binder asphalt can be medium-temperature asphalt or modified asphalt;
(4) secondary crushing and screening: secondarily crushing the kneaded paste, and sieving with a 50-200-mesh sieve to obtain powder;
(5) molding: carrying out compression molding or isostatic pressing on the sieved powder to obtain a graphite green body;
(6) primary roasting: placing the graphite green compact in a roasting furnace, heating to 800-;
(7) impregnation and secondary roasting: cooling the product after primary roasting, placing the product in an impregnation tank, and impregnating for 2-3h at the pressure of 6-15MPa for one time; then placing the mixture in a roasting furnace, raising the temperature to 800-;
(8) graphitization treatment: introducing halogen and/or halogenated hydrocarbon gas into the product after secondary roasting at the temperature of 2500-3000 ℃ for graphitization treatment to obtain an isotropic nuclear grade graphite material; or
Scheme II:
(1) and (3) purification: the purity of the natural crystalline flake graphite with the carbon content of more than 90 percent is improved to more than 99.9 percent by adopting high-temperature purification;
(2) spheroidizing: placing the purified natural crystalline flake graphite in a spheroidization classifier, and carrying out crushing, classification and spheroidization treatment to obtain spheroidized natural crystalline flake graphite;
and (4) sequentially processing according to the steps (3) to (8) to obtain the isotropic nuclear grade graphite material.
7. The method of preparing an isotropic nuclear grade graphitic material according to claim 5, characterized in that: in the step (1) of the first scheme and the step (2) of the second scheme, the D50 particle size of the spheroidized natural flake graphite is 1-300 mu m.
8. The method of preparing an isotropic nuclear grade graphitic material according to claim 6, characterized in that: in the step (1) of the first scheme and the step (2) of the second scheme, the D50 particle size of the spheroidized natural flake graphite is 5-100 mu m.
9. The method of preparing an isotropic nuclear grade graphitic material according to claim 5, characterized in that: in the step (1) of the first scheme and the step (2) of the second scheme, the sphericity of the spheroidized natural crystalline flake graphite is not less than 0.6, and the sphericity is defined as the ratio of the minor axis to the major axis of the particle.
10. The method of preparing an isotropic nuclear grade graphitic material according to claim 5, characterized in that: in the step (3), the softening point temperature of the binder asphalt is 60-150 ℃, and the mass ratio of the aggregate containing the sphericized natural crystalline flake graphite to the binder asphalt is 2:1-10: 1;
in the step (5), the isostatic compaction pressurizing pressure is 100-200 MPa.
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