CN109081695B

CN109081695B - Preparation method of high-density large-size ultrafine-aperture nuclear graphite material for molten salt reactor

Info

Publication number: CN109081695B
Application number: CN201810816334.4A
Authority: CN
Inventors: 黄岱; 杨辉; 李贺; 曹曙林
Original assignee: Sinosteel Corp New Material Zhejiang Co ltd
Current assignee: Symantec Advanced Materials Co ltd
Priority date: 2018-07-24
Filing date: 2018-07-24
Publication date: 2020-11-03
Anticipated expiration: 2038-07-24
Also published as: CN109081695A

Abstract

The invention provides a preparation method of high-density, large-size and ultrafine-pore nuclear graphite for a molten salt pile by using 1-5 mu m coke aggregate, which is characterized in that in the forming process, a paste is subjected to compacting treatment by vibration compaction and then is matched with control of the heating rate and the temperature difference in a furnace in the roasting and graphitization processes, so that the shrinkage rate of a graphite green body is controlled, the technical problems that the larger the specific surface area and the surface energy of particles are, the larger the average particle size of the traditional aggregate is, and the larger the specific surface area and the surface energy of the particles cannot be used for preparing a large-size nuclear graphite material are solved, and a plurality of halogen gases are introduced as purified gases and are synchronously purified in the graphitization process.

Description

Preparation method of high-density large-size ultrafine-aperture nuclear graphite material for molten salt reactor

Technical Field

The invention relates to the technical field of nuclear graphite production, in particular to a preparation method of a high-density large-size ultrafine-aperture nuclear graphite material for a molten salt reactor.

Background

Graphite is used in thermal neutron reactors, and also in fusion reactors, where it can act as a neutron moderator for the fuel region, a reflective layer material around the fuel region, and a structural material inside the core.

Nuclear graphite (nuclear graphite), a graphite material used in the nuclear industry.

The molten salt reactor is one of 6 candidates of a fourth-generation advanced nuclear energy system, and has the characteristics of high inherent safety, less nuclear waste, good diffusion resistance, good economy and the like. Nuclear graphite is used for the most core components within a molten salt reactor, primarily for the following reasons: (1) the molten salt diffusion and permeation resistant material is used for forming a reactor core reflecting layer structure and a moderator, provides a proper flow channel for molten salt coolant, is in direct contact with molten salt, and can diffuse and permeate on the graphite surface under the operating pressure to cause uneven neutron flux and influence the steady-state operation of a reactor, so that the diffusion resistance and the permeation resistance of the molten salt are core indexes of a molten salt reactor core graphite material, and under the condition that other performance indexes are certain, the average pore diameter and the most probable pore diameter of the molten salt reactor moderator graphite are required to be less than or equal to 1.0 mu m, and the smaller the average pore diameter is, the better the smaller the average pore diameter is. (2) The density of nuclear graphite determines the density of carbon atoms per unit volume and thus the moderating ability of the graphite. The lower the density, the poorer the moderating power, moderating the fast neutrons to the desired levelThe graphite volume of the energy increases and the reactor core becomes bulky, thereby increasing neutron leakage and construction costs. Further, the density of graphite is so low that the value as a slowing material is lost. In addition, the density of graphite is related to its strength, thermal conductivity, oxidation, etc., and generally, the lower the density, the poorer these performance criteria are. The power density of the molten salt reactor is generally high at a higher temperature, so the requirement on the volume density of the graphite body is higher than or equal to 1.85g/cm³. (3) Nuclear reactors generally require only possibly small volumes due to construction costs, and application diversity, but it is often the case that even smaller power reactors (e.g. 2MW thermal power) have greater dimensional requirements for graphite, with maximum dimensions exceeding 1000mm, especially for ultra-fine bore graphite materials for molten salt reactors, with the large dimensions being one of the major detents.

Aiming at the characteristics of high density, large size and superfine aperture of the nuclear graphite for the molten salt reactor, the existing graphite production technology is improved to meet the requirement of producing the nuclear graphite.

Nuclear graphite production has 4 major problems, namely stable mass production of large size, high purity, high density, high isotropy.

R8710 graphite produced by SiGerlicaceae in Germany as aggregate particles having an average particle size of 3 μm has a pore size distribution concentrated around 600 nm. It can be seen that it is effective to control the porosity and microstructure of the graphite material by changing the particle size and particle size distribution of the aggregate particles, however, the smaller the particle size of the aggregate, the larger the specific surface area and surface energy of the particles, and thus mutual agglomeration among the particles is easily caused; and the more binder needed to achieve a uniform coating structure increases the difficulty of preparing large-scale products, so this technical problem is not solved.

In Chinese patent No. CN201710468047.4 (2017 for short), a preparation method of fine-structure graphite is disclosed, which is characterized in that asphalt coke with the average particle size of 2-8 mu m is used as aggregate, high-softening-point asphalt is used as binder, paste is prepared in a liquid-phase dispersion mixing mode and is crushed to prepare pressing powder, and the pressing powder is prepared into fine-structure graphite with compact microstructure and pores through isostatic pressing, carbonization, impregnation and high-temperature graphitizationIsotropic graphite with fine diameter distribution, average pore diameter of 0.4-1.1 μm, and volume density of 1.8-1.9 g/cm³However, the preparation process is complicated, the preparation needs to be carried out in the direction of liquid phase mixing, and whether the preparation of the large-size nuclear graphite material can be carried out or not still needs to be checked.

In addition, the applicant discloses a large-size nuclear graphite material for a high-temperature gas-cooled reactor internals and a preparation method thereof in a patent with the patent number of CN201810272718.4 previously applied in 2018, 03, 29 and 29, wherein the large-size nuclear graphite material is used for the high-temperature gas-cooled reactor, the nuclear graphite material is prepared by adopting coke aggregates with the average particle size of 5-50 mu m through one-time impregnation or no impregnation, and the bulk density is 1.7-1.85 g/cm³。

In addition, the applicant discloses a preparation method of high-density ultrafine-pore graphite in a patent with the patent number of CN201810563807.4 previously applied in 2018, 06, 24 and 24, wherein a core graphite material is prepared by performing high-pressure impregnation twice on coke aggregate with the average particle size of 10-100 mu m, the average pore size is less than or equal to 20nm, the most probable pore size is less than or equal to 0.92 mu m, and the volume density is more than or equal to 1.90g/cm³。

However, the following problems still exist in the above prior art 1(CN201710468047.4), prior art 2(CN201810272718.4) and prior art 3(CN 201810563807.4):

1. aiming at the prior art 1, the specific surface area and the surface energy of particles are large due to the fact that aggregates with the average particle size of 2-8 mu m are adopted to manufacture the graphite material, the particles are agglomerated with each other, pores of the prepared and molded graphite material are unevenly distributed, defects exist in the interior of the prepared and molded graphite material, and the large-size nuclear graphite material cannot be prepared;

2. aiming at the prior art 2, the volume density and the pore size distribution of the nuclear graphite material prepared by adopting aggregate with the average particle size of 5-50 mu m do not reach the standard, so that the nuclear graphite has poor permeability resistance to molten salt when applied to a molten salt reactor;

3. aiming at the prior art 3, the nuclear graphite material prepared by adopting aggregate with the average particle size of 10-100 mu m has qualified index parameters in all aspects, but in the preparation process, high-pressure impregnation is needed twice, the preparation requirement is high, and the preparation time is long;

therefore, it is a problem to be solved to provide a method for preparing relatively simple graphite with high density, large size and ultrafine pore size, which is more suitable for use in molten salt reactor core.

Disclosure of Invention

In view of the above, the invention provides a preparation method of high-density, large-size and ultrafine-pore nuclear graphite for preparing a molten salt pile by using 1-5 μm coke aggregate, which aims to overcome the technical problems that the smaller the average particle size of the conventional aggregate is, the larger the specific surface area and the surface energy of the particle is, and the larger the large-size nuclear graphite material cannot be prepared.

The invention provides a preparation method of a nuclear graphite material with high density, large size and superfine aperture for molten salt reactor, which comprises the following steps:

1) selecting raw materials: the coke aggregate contains more than or equal to 98.5 percent of fixed carbon, less than or equal to 0.5 percent of ash, less than 0.1ppm of boron (B) and gadolinium (Gd), less than 0.5ppm of samarium (Sm), europium (Eu), cadmium (Cd) and lithium (Li), less than or equal to 0.5 percent of water and less than or equal to 0.1 percent of sulfur;

ash content in the binder is less than or equal to 0.5 percent, volatile matter is 35-60 percent, coking value is 40-70 percent, softening point is 80-200 ℃, quinoline insoluble substance is 8-25 percent, and toluene insoluble substance is 25-55 percent;

2) grinding raw materials: pulverizing coke aggregate to obtain coke aggregate with average particle size of 1-5 μm and upper limit of 30 μm;

3) material preparation and kneading: kneading 65-80 parts of coke aggregate and 35-20 parts of binder at the temperature of 150 ℃ for 80-150 min;

4) molding: loading the product obtained in step 3) into a membrane sleeve and vibrating on a vibrating table for 3-5min, wherein the vibration frequency is 20-50Hz, and the tap density is controlled to be 0.8-1.2g/cm³After the compaction is finished, the obtained product is molded, the molding pressure is 180MPa, the pressure maintaining time is 80-120min, the size length of the green body is 1500mm, the width is 1000mm, the height is 500mm, and the density of the molded green body is 1.7-1.75g/cm³；

5) Roasting: roasting the product obtained in the step 4), wherein the roasting temperature range is 800-1100 ℃, the heating rate is 0.5-3 ℃/h, the temperature difference in the furnace is less than or equal to 20 ℃, the roasting treatment time is 65-90 days, after the roasting is finished, the green body is cooled to 100 ℃, the cooling rate is 10-15 ℃/h, and the volume shrinkage is controlled to be 6-8%;

6) dipping: impregnating the product obtained in the step 5) with an impregnant, wherein the impregnation pressure is 8-12MPa, the temperature is 200-;

7) graphitization/nuclear purification treatment: graphitizing the product obtained in the step 6) at 2900-. The method comprises the following steps:

the large specification size is as follows: the length is 1000-.

In the step 1):

the coke is petroleum coke, pitch coke, mesophase carbon microspheres or metallurgical coke;

the coke is pretreated in addition to the fixed carbon and impurity levels, such as by calcining the coke feedstock at a temperature of 1100 ℃ and 1500 ℃.

The binder is coal pitch, petroleum pitch, artificial resin or sugar solution;

in the step 2):

the method of pulverization is a conventional pulverization method, preferably in the form of a Raymond mill, a rod mill, a ring roll mill, a jet mill or a combination thereof.

In the step 3):

the weight portions of the coke and the binder are 65-70 portions and 35-30 portions.

The kneading temperature is 150-;

in the step 4):

vibration time of 3-5min, tap density of 0.8-1.2g/cm³Preferably 1 to 1.2g/cm³；

The molding mode is isostatic pressing, vibration molding, compression molding and extrusion molding, and preferably isostatic pressing;

during molding: the pressure is 180MPa and the time is 80-120min, preferably, the pressure is 160MPa and the time is 100 MPa and 120 min;

in the step 5):

roasting can be carried out in a carbonization roasting furnace with high precision and temperature control requirements, wherein the carbonization roasting furnace comprises but is not limited to a car bottom type carbonization furnace, a ring type carbonization furnace, a tunnel and other similar carbonization furnaces;

preferably, the carbonization furnace is a car bottom type carbonization furnace.

The heating mode is natural gas, electricity and coal;

the roasting temperatures are respectively 800-1100 ℃, and preferably 850-900 ℃;

the temperature rising speed is 0.5-3 ℃/hour, and the temperature difference in the furnace is controlled to be less than 20 ℃. Preferably, the temperature rise speed is 0.5-1 ℃/h, the temperature difference in the furnace is controlled to be 10 ℃, and further preferably, the temperature rise speed is 0.5-0.6 ℃/h.

The roasting treatment time is 65 to 90 days, preferably 75 to 90 days.

In the step 6):

before the product obtained in the step 5) is impregnated, vacuumizing treatment is required, the vacuum degree is less than or equal to 1.5bar, and the treatment time is 5-10 h;

the impregnant is coal pitch, petroleum pitch, artificial resin and sugar solution;

the impregnation pressure is 8-12MPa, the temperature is 200-500 ℃, the pressure is 9-12MPa, and the temperature is 380-500 ℃.

In the step 7): graphitization can be carried out in a direct current Acheson graphitization furnace, an internal tandem graphitization furnace or other electric heating or induction heating furnaces;

the halogen is fluorine, chlorine, bromine, iodine, preferably chlorine, freon or similar chlorinated gases, fluorinated gases;

the graphitization temperature is 3000-3200 ℃, preferably 3000-3100 ℃;

another object of the present invention is to provide a nuclear graphite material prepared by the above method, which has the following indexes:

the volume density is more than or equal to 1.87g/cm³The flexural strength is more than or equal to 45MPa, the compressive strength is more than or equal to 90MPa, the tensile strength is more than or equal to 28MPa, and the thermal expansion coefficient is less than or equal to 5.5 multiplied by 10^-6The heat conductivity at room temperature is more than or equal to 100W/mK at 20-500 ℃, the ash content is less than or equal to 10ppm, the boron equivalent is less than or equal to 0.9ppm, the isotropy is 1.00-1.10, the elastic modulus is 8-15GPa, the average pore diameter (4V/A) is less than or equal to 20nm (mercury intrusion method, V is the volume-based median pore diameter, A is the area-based median pore diameter), and the most probable pore diameter is less than or equal to 1.0 mu m (mercury intrusion method).

The preparation method provided by the invention has the following advantages:

(1) the method comprises the steps of preparing fine-pore graphite by using coke aggregates with the average particle size of 1-5 mu m and an adhesive as raw materials, compacting the coke aggregates by using mechanical vibration of a vibration table in a forming process, eliminating intermittence among the coke aggregates, controlling a slow temperature rise rate and a low temperature difference in a furnace in a roasting process, enabling the internal temperature of a product to be uniform, ensuring that the adhesive is uniformly coked, enabling the volume of a green blank to be uniformly contracted by 6-8%, further eliminating gaps in the product, and avoiding stress concentration, so that the technical problems that the specific surface area and the surface energy of particles are large due to the fact that the average particle size of the coke aggregates is too small, the particles are mutually agglomerated, pores of the prepared and formed graphite material are not uniformly distributed, and defects exist in the interior are solved, and the preparation of the large-size ultrafine-pore;

(2) the method comprises the following steps of pretreating the coke aggregate, and calcining the coke aggregate at 1100-1500 ℃, so that the contents of ash, sulfur, impurity elements and the like in the raw materials are reduced, and the fixed carbon content is increased, so that the coke reaches the standard, and the product performance of a subsequent graphite blank is further improved;

(3) roasting can be carried out in a carbonization roasting furnace with high precision and temperature control requirements, wherein the carbonization roasting furnace comprises but is not limited to a car bottom type carbonization furnace, a ring type carbonization furnace, a tunnel type carbonization furnace and other similar carbonization furnaces, and the car bottom type carbonization furnace is preferred. Because the nuclear graphite blank can be carbonized at about 1000 ℃, the traditional carbonization roasting furnace has large heat loss and poor temperature uniformity control, the highest heating temperature needs to reach more than 1300 ℃, the roasting temperature in the furnace can meet the product carbonization requirement, and the energy consumption is high; the heat loss of the car bottom type carbonization furnace is small, the highest roasting temperature range is 800-1100 ℃, and compared with the traditional carbonization roasting furnace, the energy consumption of the car bottom type carbonization furnace is greatly reduced;

according to the invention, a large amount of volatile components are discharged in the roasting process of the isostatic pressing green body, the green body can shrink to a certain extent along with the carbonization of asphalt, and the larger the specification size is, the more internal defects are brought to the product by the volatilization and shrinkage, and the more easily the product is cracked in the roasting process. The method is also a process difficulty that large-size products are difficult to break through the roasting bottleneck, and an optimal roasting curve is obtained through a reasonable formula, temperature measurement, analysis and the like in the actual temperature rise process by controlling the temperature rise speed and the temperature difference in the furnace in the roasting process. The key innovation point is that the uniform discharge of asphalt volatile matters is ensured by controlling the temperature rise rate in the volatilization stage, and meanwhile, the temperature is quickly raised in the carbonization high-temperature stage, the carbon residue rate is improved, and the energy consumption is reduced. According to the invention, the roasting process strengthens the heating uniformity in the roasting process, and combines a fine roasting temperature rise curve design and a cooling process to control the shrinkage uniformity of the green body, so that the roasting yield of the large-specification size isostatic pressing product is improved to a great extent, and the manufacturing cost of the product is further reduced while the energy consumption is controlled;

(4) the superfine aperture graphite prepared by the method has the advantages that the problems of too low pressure, impermeable impregnation and incapability of blocking the aperture can easily occur in the impregnation process, a high-vacuum high-pressure (8-12 MPa) impregnation mode is adopted, the impregnation pressure is up to more than 8MPa by virtue of gas pressurization, the impregnation problem of a graphite product is broken through, and the practical guarantee is provided for the product performance;

(5) in the product graphitization/purification process, the rearrangement of carbon atoms is accompanied with the shrinkage of the volume, so that the internal defects of the product are easily caused, the special graphitization furnace is designed, the internal temperature difference of the graphitization furnace is ensured to be less than 200 ℃ by controlling the temperature difference in the key temperature section (1000-;

(6) it should be noted that the fuel of the nuclear reactor is natural uranium, which has strict requirements on the absorption cross section of graphite, and to ensure that the absorption cross section meets the requirements, the contents of impurity elements with high neutron absorption cross section, such as gadolinium (Gd), boron (B), samarium (Sm), europium (Eu), cadmium (Cd), lithium (Li), etc., should be as low as possible. Therefore, nuclear graphite cannot meet the requirement of a low neutron absorption cross section by only depending on graphitization temperature for high-temperature purification, because many impurity elements and carbon form high-melting-point compounds, and are difficult to volatilize at high temperature, especially the graphite needs a boron element which is controlled in a key way.

Table 1: molecular weight and boiling point of typical carbides

The results in table 1 show that the boiling points of the carbides of the graphite material without nuclear purification treatment, especially the boiling point of boron element, are all more than 3500 ℃, so that the graphite cannot meet the requirement of low neutron absorption cross section by only relying on graphitization temperature for high-temperature purification.

The molecular weight and boiling point parameters of the graphite material metal chloride after the nuclear purification treatment by introducing purified chlorine gas are shown in the following table 2.

Table 2: molecular weight and boiling point of typical metal chlorides

Reference documents:

CRC Handbook of Chemistry and Physics，75th ed.，1994

PlenumPress Handbook

as can be seen from Table 2, in the present invention, at the high temperature of the graphitization treatment, the boiling point of the metal chloride in the nuclear graphite material after the chlorine gas is introduced is far lower than the graphitization temperature, and the obtained graphite material has high purity. However, in order to control the boron content to 0.9ppm in the preferred state, it is necessary to further lower the boiling point of the boron compound and further lower the boron content by introducing Freon.

The molecular weight and boiling point parameters of the metal fluoride of the nuclear graphite material after nuclear purification treatment with purified freon are shown in table 3 below.

Table 3: molecular weight and boiling point of typical metal fluorides

As can be seen from table 3, in the present invention, at a high temperature of graphitization treatment, the boiling point of the metal fluoride in the graphite material after the freon is introduced is far lower than the graphitization temperature, and especially the boiling point of the boron element and the boron-like element with a high neutron absorption cross section is greatly reduced, so that the boron equivalent in the graphite material can be controlled specifically.

2. The inventors have conducted a literature search at the beginning of an experiment and found that, regarding a preparation method of ultrafine pore graphite, CN201710468047.4 (prior art 1) and CN201810272718.4 (prior art 2) and CN201810563807.4 (prior art 3) which were previously applied by the inventors, in order to obtain a high-density large-size ultrafine pore size nuclear graphite material for molten salt pile with a coke aggregate having an average particle size of 1 to 5 μm, the inventors have improved the following:

1) raw materials: the method is characterized in that only calcined asphalt coke is limited to be used as coke aggregate in the prior art 1, the average particle size of the calcined asphalt coke is 2-8 μm, the average particle size of the coke aggregate is limited to be 5-50 μm in the prior art 2, and the average particle size of the coke aggregate is limited to be 10-100 μm in the prior art 3, but the coke with the average particle size of 1-5 μm is used as the coke aggregate in the method, the average particle size is smaller than that in the prior art, and parameters of the average pore size and the most probable pore size (mercury pressing method) of the correspondingly prepared graphite material are optimized;

2) molding: the prior art 1, the prior art 2 and the prior art 3 directly carry out compression molding on the kneaded raw materials, but before molding, the invention carries out vibration compaction on the kneaded raw materials through a vibrating table, so that the density of the raw materials is controlled to be 0.8-1.2g/cm³The density of the raw material is higher, the gaps among the coke aggregates are smaller, the interior of the graphite material is tighter in the process of preparing the graphite material, the occurrence of the gaps in the graphite material is avoided, and the volume density of the obtained product reaches 1.7-1.75g/cm after molding because the coke aggregates with the particle size of 1-5 mu m are adopted in the invention³The density of the product is higher than that of the product obtained by the synchronous steps of the prior art 1 and the prior art 2;

3) baking: the roasting heating rate of the prior art 1 is 2-6 ℃/min, the temperature difference in a furnace is not required, the roasting heating rate of the prior art 2 is 1-5 ℃/h, the temperature difference in the furnace is 50-100 ℃, the roasting heating rate of the prior art 3 is 1-5 ℃/h, the temperature difference in the furnace is less than or equal to 150 ℃, the roasting heating rate of the invention is 0.5-3 ℃/h, the temperature difference in the furnace is less than 20 ℃, the heating is slower and more uniform, the binder is ensured to be uniformly coked, stress concentration is not generated, the volume shrinkage of a graphite green body is controlled to be 6-8 percent, the density of the graphite material can be further improved, internal gaps are eliminated, the high-density large-size ultra-fine-aperture nuclear graphite material for molten salt piles can be prepared, and because the average particle size of coke aggregate is small, when the product is cooled to 100 ℃, the two cooling rates are respectively controlled to be 10-15, the cooling rate is smaller than that of the prior art 1 and the prior art 2, and the cooling is more uniform;

4) dipping: the invention needs to carry out vacuum treatment on the roasted product, the vacuum degree of the vacuum treatment is less than or equal to 1.5bar because the product obtained by roasting has small open porosity and less adsorbed gas amount in unit volume, the treatment time is 5-10h, compared with the prior art 1 and the prior art 2, the vacuum degree is higher, and the treatment time is longer, and in the impregnation process, because the product has small open porosity and the amount of impregnant capable of permeating into the product is less, the impregnation pressure maintaining time is correspondingly reduced, and the weight gain rate is correspondingly reduced.

3. Compared with the conventional method

1) The raw material selection is more diversified, the average particle size of the particles is more fine, the parameters of the average pore diameter and the most probable pore diameter (mercury porosimetry) of the prepared graphite material are more optimized, the molten salt permeation resistance is more excellent, and the content of specific elements of impurity elements of the raw material is strictly controlled;

2) compared with the traditional forming process, the invention requires the uniformity of forming, the size of the product and the like to put forward specific control requirements, allows the forming mode to be diversified, and has strong operability and generalization;

3) compared with the traditional impregnation, the invention has high impregnation pressure, improves the consistency of the material to a certain degree, improves the density of the material and reduces the aperture of the material;

4) compared with the traditional graphitization/nuclear purification treatment, the method adopts the graphite purification/nuclear purification treatment at the same time, and adopts a halogen gas method, so that the process treatment steps are simplified, the purity of the nuclear graphite material is improved, and particularly the boron equivalent is less than 0.9ppm in an optimal state.

Generally, the method has the advantages of clear process, convenient operation, greatly reduced cost, convenient popularization and application, and suitability for large-scale production.

4. Compared with the prior art, the method introduces halogen or halogenated hydrocarbon purification gas in the graphitization process, and can greatly reduce the purification cost, thereby being beneficial to deep purification of graphite blocks and greatly reducing impurities and boron equivalent; the invention has simple process, convenient operation and less production equipment, thereby further reducing the cost, being convenient for popularization and application and being suitable for large-scale production.

Drawings

To more clearly illustrate the structural features and effects of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and together with the description, serve to explain the principles of the invention.

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without inventive exercise.

FIG. 1 is a schematic flow chart of the preparation process of the preferred embodiment of the present invention;

Detailed Description

The present invention is further described in detail with reference to specific embodiments, but the present invention is not limited thereto.

In the above examples, the contents are all by weight, if specifically indicated.

The preparation process is explained below with specific examples.

Example 1:

1. raw materials: 80 parts by weight of asphalt coke with the average particle size of 1 mu m and 20 parts by weight of asphalt;

2. the preparation method comprises the following steps:

2) grinding raw materials: pulverizing coke aggregate so that the average particle size does not exceed 1 μm;

3) material preparation and kneading: pouring the asphalt coke and the asphalt into a kneading machine for kneading at the temperature of 150 ℃ for 80 min;

4) molding: loading the product obtained in the step 3) into a membrane sleeve, vibrating for 3min on a vibrating table with the vibration frequency of 20Hz, and controlling the tap density to be 0.8g/cm³After the compaction is finished, the obtained product is molded, the molding pressure is 120MPa, the pressure maintaining time is 80min, the size length of a green body is 1000mm, the width is 500mm, and the height is 300 mm;

5) roasting: roasting the product obtained in the step 4), wherein the roasting high temperature range is 1100 ℃, the heating rate is 0.5 ℃/h, the temperature difference in the furnace is 10 ℃, the roasting treatment time is 90 days, and the volume shrinkage is controlled to be 8%;

6) dipping: impregnating the product obtained in the step 5) with an impregnant, keeping the impregnation pressure at 200 ℃ for 4 hours under the condition that the impregnation pressure is 10MPa, and the weight gain rate of the product reaches 8%, and then carrying out roasting treatment according to the step 5);

7) graphitization/nuclear purification treatment: graphitizing the product obtained in the step 6) at 3300 ℃, wherein the heating rate is 5 ℃/h, when the temperature is up to 1500 ℃, introducing halogen gas for purification, the flow rate is 80kg/h, when the temperature is up to 2500 ℃, introducing purified halogen gas, and the flow rate is 80kg/h, thus obtaining the final nuclear graphite blank.

The specific preparation flow chart is shown in figure 1.

Example 2:

1. raw materials: 70 parts of petroleum coke with the average particle size of 3 mu m and 30 parts of asphalt;

2. the preparation method comprises the following steps:

2) grinding raw materials: pulverizing coke aggregate so that the average particle size does not exceed 3 μm;

3) material preparation and kneading: pouring the asphalt coke and the asphalt into a kneading machine for kneading at the temperature of 300 ℃ for 120 min;

4) molding: loading the product obtained in step 3) into a membrane sleeve, vibrating for 5min on a vibrating table with the vibration frequency of 30Hz, and controllingTap density of 1.0g/cm³After the compaction, the obtained product is molded, the molding pressure is 160MPa, the pressure maintaining time is 100min, the size length of a green body is 1200mm, the width is 600mm, and the height is 400 mm;

5) roasting: roasting the product obtained in the step 4), wherein the roasting high temperature range is 850 ℃, the heating rate is 3 ℃/h, the temperature difference in the furnace is 20 ℃, the roasting treatment time is 80 days, and the volume shrinkage is controlled to be 7%;

6) dipping: impregnating the product obtained in the step 5) with an impregnant, keeping the impregnation pressure at 500 ℃ for 6 hours under the condition that the weight gain rate of the product reaches 12%, and then roasting according to the step 5);

7) graphitization/nuclear purification treatment: graphitizing the product obtained in the step 6) at 3100 ℃, wherein the heating rate is 7 ℃/h, the purified halogen gas is introduced at the temperature of 1800 ℃ at the flow rate of 50kg/h, and the purified halogen gas is introduced at the temperature of 2600 ℃ or above at the flow rate of 50kg/h, so as to obtain the final nuclear graphite blank.

The specific preparation flow chart is shown in figure 1.

Example 3:

1. raw materials: 65 parts by weight of asphalt coke with the average particle size of 5 mu m and 35 parts by weight of asphalt;

2. the preparation method comprises the following steps:

2) grinding raw materials: pulverizing coke aggregate so that the average particle size does not exceed 5 μm;

3) material preparation and kneading: pouring the asphalt coke and the asphalt into a kneading machine for kneading at the temperature of 400 ℃ for 150 min;

4) molding: loading the product obtained in the step 3) into a membrane sleeve and vibrating on a vibrating table for 4.5min,the vibration frequency is 50Hz, and the tap density is controlled to be 1.2g/cm³After the compaction is finished, the obtained product is molded, the molding pressure is 180MPa, the pressure maintaining time is 120min, the size length of a green body is 1500mm, the width is 1000mm, and the height is 500 mm;

5) roasting: roasting the product obtained in the step 4), wherein the roasting high temperature range is 900 ℃, the heating rate is 0.5 ℃/h, the temperature difference in the furnace is 10 ℃, the roasting treatment time is 65 days, and the volume shrinkage is controlled to be 6%;

6) dipping: impregnating the product obtained in the step 5) with an impregnant, keeping the impregnation pressure at 380 ℃ for 8 hours until the weight gain rate of the product reaches 16%, and then roasting according to the step 5);

7) graphitization/nuclear purification treatment: graphitizing the product obtained in the step 6) at 2900 ℃, wherein the heating rate is 10 ℃/h, the purified halogen gas is introduced at the temperature of 2200 ℃ at the flow rate of 30kg/h, and the purified halogen gas is introduced at the temperature of more than 2700 ℃ at the flow rate of 30kg/h, so as to obtain the final nuclear graphite blank.

The specific preparation flow chart is shown in figure 1.

Comparative example 1:

1. raw materials: 80 parts of asphalt coke with the average particle size of more than 5 mu m and 20 parts of asphalt;

2. the preparation method comprises the following steps:

2) grinding raw materials: grinding and crushing the coke aggregate to ensure that the average particle size exceeds the specified requirement;

3) material preparation and kneading: pouring the asphalt coke and the asphalt into a kneading machine for kneading at the temperature of 300 ℃ for 80 min;

4) molding: the product obtained in the step 3)Loading into membrane sleeve, vibrating on vibration table for 3min at vibration frequency of 50Hz, and controlling tap density to 1.3g/cm³After the compaction is finished, the obtained product is molded, the molding pressure is 180MPa, the pressure maintaining time is 120min, the size length of a green body is 1500mm, the width is 1000mm, and the height is 500 mm;

5) roasting: roasting the product obtained in the step 4), wherein the roasting high temperature range is 1100 ℃, the heating rate is 3 ℃/h, the temperature difference in the furnace is 20 ℃, the roasting treatment time is 65 days, and the volume shrinkage is controlled to be 6%;

6) dipping: impregnating the product obtained in the step 5) with an impregnant, keeping the impregnation pressure at 500 ℃ for 4 hours under the condition that the weight gain rate of the product reaches 16%, and then roasting according to the step 5);

The specific preparation flow chart is shown in figure 1.

Comparative example 2:

1. raw materials: 70 parts of petroleum coke with the average particle size of less than 1 mu m and 30 parts of asphalt;

2. the preparation method comprises the following steps:

4) molding: loading the product obtained in the step 3) into a membrane sleeve, vibrating for 5min on a vibrating table with the vibration frequency of 20Hz, and controlling the tap density to be 0.7g/cm³After the compaction is finished, the obtained product is molded, the molding pressure is 120MPa, the pressure maintaining time is 80min, the size length of a green body is 1000mm, the width is 500mm, and the height is 300 mm;

5) roasting: roasting the product obtained in the step 4), wherein the roasting temperature range is 800 ℃, the heating rate is 0.5 ℃/h, the temperature difference in the furnace is 10 ℃, the roasting treatment time is 90 days, and the volume shrinkage is controlled to be 8%;

6) dipping: impregnating the product obtained in the step 5) with an impregnant, keeping the impregnation pressure at 200 ℃ for 8 hours under the condition that the impregnation pressure is 12MPa, and the weight gain of the product reaches 8%, and then carrying out roasting treatment according to the step 5);

7) graphitization/nuclear purification treatment: graphitizing the product obtained in the step 6) at 3100 ℃, wherein the heating rate is 5 ℃/h, the purified halogen gas is introduced at the temperature of 1500 ℃ at the flow rate of 80kg/h, and the purified halogen gas is introduced at the temperature of more than 2500 ℃ at the flow rate of 80kg/h, so that the final nuclear graphite blank can be obtained.

The specific preparation flow chart is shown in figure 1.

Comparative example 3:

the control was graphite obtained in the example of prior art 1.

Comparative example 4:

the control was graphite obtained in the example of prior art 2.

Comparative example 5:

the control was graphite obtained in the example of prior art 3.

Table 1: graphite material grain size grading table

Table 1 shows the classification rules of the particle size classes of graphite materials in ASTM D7219 by the American society for testing materials, from which it can be seen that the graphite materials prepared from coke aggregates having an average particle size of 1-5 μm used in the present invention and R8710 graphite produced from coke aggregate particles having an average particle size of 3 μm by Germany, belong to the class of ultrafine particles, the average particle size of 2-8 μm of coke aggregates in the prior art 1 is below the class of ultrafine particles, the average particle size of 5-50 μm of coke aggregates in the prior art 2 is below the class of ultrafine particles, and the average particle size of 10-100 μm of coke aggregates in the prior art 3 covers two classes of ultrafine particles and fine particles.

Table 2: average particle size and specific surface area

As can be seen from table 2, the smaller the average particle size is, the larger the specific surface area is, and the larger the specific surface area is, the particles will agglomerate with each other, the pores of the prepared and molded graphite material are not uniformly distributed, and the inside of the graphite material has defects, so that the high-density large-size ultra-fine pore size nuclear graphite material for molten salt reactor cannot be prepared, which is also the main reason that the coke aggregate with small average particle size cannot be used for preparing the large-size graphite material in the conventional graphite material preparation process.

Table 3: parameter comparison table for preparing graphite material with or without tap in preparation and forming process of graphite material

As can be seen from table 3, the comparison of the parameter data of the 3 control groups set up, the volume density, average pore size and most probable pore size of the graphite material prepared from the paste after tapping are all better than those of the graphite material prepared from the paste without tapping, so that it is known that by tapping the molding paste, the air between the molding paste can be discharged, the gap between the molding paste can be eliminated, the void between the molding paste can be reduced, the density of the molding paste can be increased, and the density of the molding paste can be increased, so that the pore distribution of the produced graphite material can be more uniform, the defects inside the produced graphite material can be eliminated, and the paste can prepare graphite material with larger size, and this conclusion is reflected in the data of the maximum size of the graphite material product that can be prepared by the 3 control groups.

Table 4: temperature rise rate and in-furnace temperature difference parameter comparison table in graphite material preparation roasting process

As can be seen from table 4, through the contrast of 3 groups to the group data, the smaller the temperature difference in rate of rise and the stove, the volume shrinkage factor of graphite unburned bricks in the calcination process can be higher, the bulk density of the prepared graphite material can be improved, the average pore diameter and the most probable pore diameter can be smaller, and the reason for causing the phenomenon is because in the calcination process, through controlling the rate of rise and the temperature difference value in the stove in the calcination stove, can make the inside temperature infiltration of graphite unburned bricks more even in the calcination process, ensure that the binder in the thickener can receive the even coking of influence of temperature and carry out, make the volume of graphite unburned bricks can even internal contraction, the inside thermal stress of balanced product, it is inhomogeneous to have avoided inside coking, produce the thermal stress nonconformity, thereby lead to the inside fracture of product.

It is further illustrated that the volume shrinkage of the graphite green compact caused by coking of the binder is caused by that in the roasting process, when the graphite green compact is heated to 200-, the binder is unevenly coked, so that thermal stress concentration occurs in the graphite, and the graphite green body is unevenly bonded and internally cracked.

Table 5: main typical index data of nuclear graphite prepared in examples 1-3 and comparative examples 1-5

Table 5 the results show: the volume density of the nuclear graphite prepared in the examples 1 to 3 is more than or equal to 1.88 to 1.93g/cm³The anti-breaking strength is 45-50MPa, the compressive strength is 90-106MPa, the tensile strength is 28-38MPa, the thermal expansion coefficient is 4.3-multiplied by 10-6/K (20-500 ℃), the room temperature thermal conductivity is 100-113W/mK, the ash content is 8-12ppm, the boron equivalent is 0.6-0.9ppm, the isotropy is 1.04-1.08, the elastic modulus is 11-15GPa, the average pore diameter (4V/A) is 12-18nm (mercury intrusion method, V is the volume-based median pore diameter, A is the area-based median pore diameter), and the most probable pore diameter is 0.73-0.85 mu m (mercury intrusion method).

On the premise of satisfying the pore size distribution, the thermal expansion coefficient and the thermal conductivity coefficient data of the embodiment 3 are better, and the manufactured product is larger than other embodiments due to the larger particles, so that the product with larger size and stable quality can be manufactured more easily, because the embodiment 3 is a better embodiment.

It is further illustrated that comparing the performance parameters of the nuclear graphite prepared in examples 1-3 with that of comparative example 1, the most probable pore size (mercury intrusion method) of comparative example 1 was found to be 1.12 μm, which is much higher than those of examples 1-3, and the permeation resistance to molten salt was not satisfied, and comparing the performance parameters of the nuclear graphite prepared in examples 1-3 with that of comparative example 2, the thermal expansion coefficient and thermal conductivity of comparative example 2 were found to be much lower than those of examples 1-3, which are also important parameter indexes of the nuclear graphite material for molten salt reactor.

It is noted that the average pore diameter (mercury intrusion method) of 397-1132nm in comparative example 3 is much higher than that of examples 1-3, while the bulk density of 1.75-1.83g/cm3 in comparative example 4 is much lower than that of examples 1-3, and the parameter indexes of comparative example 5 are close to those of examples 1-3, but in the preparation process of comparative example 5, two times of high pressure impregnation and three times of calcination are required, and the production processing time is too long.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

The invention has been described above with a certain degree of particularity. It will be understood by those of ordinary skill in the art that the description of the embodiments is merely exemplary and that all changes that come within the true spirit and scope of the invention are desired to be protected. The scope of the invention is defined by the appended claims rather than by the foregoing description of the embodiments.

Claims

1. A preparation method of a high-density large-size ultra-fine pore size nuclear graphite material for a molten salt reactor is characterized in that the method is used for preparing the nuclear graphite material with the length of 1000-1500mm, the width of 500-1000mm and the height of 300-500mm, and comprises the following steps:

4) molding: loading the product obtained in step 3) into a membrane sleeve, vibrating on a vibrating table for 3-5min at a vibration frequency of 20-50Hz and controlling the tap density to be 0.8-1.2g/cm³After the compaction is finished, the obtained product is molded, the molding pressure is 180MPa, the pressure maintaining time is 80-120min, the size length of the green body is 1500mm, the width is 1000mm, the height is 500mm, and the density of the molded green body is 1.7-1.75g/cm³；

5) Roasting: roasting the product obtained in the step 4), wherein the roasting temperature range is 800-1100 ℃, the heating rate is 0.5-3 ℃/h, the temperature difference in the furnace is less than or equal to 20 ℃, the roasting treatment time is 65-90 days, the green body is cooled to 100 ℃ after the roasting is finished, the cooling rate is 10-15 ℃/h, and the volume shrinkage rate of the green body is controlled to be 6-8%;

6) dipping: vacuumizing the product obtained in the step 5), wherein the vacuum degree is less than or equal to 1.5bar, the processing time is 5-10h, impregnating the product by using an impregnant, the impregnation pressure is 8-12MPa, the temperature is 200-500 ℃, the pressure is maintained for 4-8 h, the weight gain of the product reaches 8-16%, then roasting the product according to the step 5), the temperature difference in the furnace in the roasting process is less than or equal to 30 ℃, cooling the green blank to less than or equal to 100 ℃ after roasting, and the cooling rate is 10-30 ℃/h;

7) graphitization/nuclear purification treatment: graphitizing the product obtained in the step 6) at the temperature of 2900-, introducing halogen gas for purification at the temperature of 1500-2200 ℃, the flow rate of 30-80kg/h, and ensuring that the temperature difference in the furnace is less than 200 ℃ at the temperature of 1000-2500 ℃, introducing purified halogen gas at the temperature of more than 2500 ℃, wherein the flow rate is 30-80kg/h, obtaining the final nuclear graphite blank, wherein the halogen gas is one or more than two of chlorine, freon or similar chlorinated gas and fluorinated gas, the volume density of the prepared final nuclear graphite blank is more than or equal to 1.87g/cm3, the average pore diameter 4V/A is less than or equal to 20nm measured by mercury intrusion method, wherein V is the volume-based median pore diameter, A is the area-based median pore diameter, and the most probable pore diameter is less than or equal to 1.0 μm measured by mercury intrusion method.

2. The preparation method as claimed in claim 1, wherein the coke aggregate is petroleum coke, pitch coke, mesophase carbon microsphere or metallurgical coke, and the coke aggregate needs to be pretreated, i.e. calcined at 1100-1500 ℃; the binder is coal pitch, petroleum pitch, artificial resin or sugar solution.

3. The method according to claim 1, wherein in the step 3): the kneading temperature is 300-400 ℃, and the kneading time is 120-150 min.

4. The method according to claim 1, wherein in the step 4): the vibration frequency is 30-50Hz, the vibration time is 4.5-5min, and the tap density is 1.0-1.2g/cm³The molding pressure is 160-180MPa, and the molding time is 100-120 min.

5. The method according to claim 1, wherein in the step 5): the roasting temperatures are respectively 850-900 ℃; the temperature rising speed is 0.5-0.6 ℃/hour, and the temperature difference in the furnace is controlled to be less than or equal to 10 ℃.

6. The method according to claim 1, wherein in the step 6): the impregnation pressure is 9-12MPa, the temperature is 380-.

7. The nuclear graphite material produced by the method of any one of claims 1 to 6, having the following characteristics: the breaking strength is more than or equal to 45MPa, the compressive strength is more than or equal to 90MPa, the tensile strength is more than or equal to 28MPa, and the thermal expansion coefficient is less than or equal to 5.5 multiplied by 10 measured at the temperature of 20-500 DEG C^-6The thermal conductivity at room temperature is more than or equal to 100W/mK, the ash content is less than or equal to 20ppm, the boron equivalent is less than or equal to 0.9ppm, the isotropy is 1.00-1.10, and the elastic modulus is 8-15 GPa.