Natural graphite powder for high-temperature gas cooled reactor nuclear fuel element and preparation method of natural graphite powder

Abstract

The invention provides natural graphite powder for a high-temperature gas cooled reactor nuclear fuel element and a preparation method of the natural graphite powder. The preparation method comprises the following steps: performing screening, crushing, shaping and grading treatment on graphite fine powder to obtain powder with the particle size of 25-35 microns; carrying out primary purification and secondary purification on the powder by using a mixed acid solution, heating the obtained chemically purified graphite powder in vacuum, carrying out high-temperature purification, introducing a mixed gas of argon and carbon tetrafluoride when the temperature reaches 1800 DEG C, and starting a circular reaction; then continuously carrying out gradient heating to 2000 DEG C, 2200 DEG C and 2400 DEG C, and respectively carrying out circular reaction at each temperature; then cooling to 300-400 DEG C, introducing mixed gas of oxygen and argon, and carrying out oxidation treatment; and finally, scattering and homogenizing to obtain a natural graphite powder finished product. The natural graphite powder provided by the invention realizes lower total impurity element content and total boron equivalent and higher specific surface area.

Description

Technical Field

[0001] The present invention belongs to the technical field of natural graphite production, and particularly relates to natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements and a preparation method thereof. Background Art

[0002] High-temperature gas-cooled reactor nuclear fuel elements are composed of nuclear fuel particles and matrix graphite. The nuclear fuel particles are uniformly mixed with matrix graphite powder after being coated with multiple layers of carbon and silicon carbide, and then pressed into balls. A layer of matrix graphite layer with a thickness of 3 - 5 mm is further pressed on the surface of the mixture balls, and finally fuel balls with a diameter of 60 mm are made through processes such as carbonization, high-temperature purification, and processing. Among them, the matrix graphite powder is prepared from 64% natural graphite, 16% artificial graphite powder, and 20% phenolic resin. In nuclear fuel elements, the weight proportion of matrix graphite powder reaches more than 90%, which plays a decisive role in the physical properties of fuel pellets. And the proportion of natural graphite powder in the matrix graphite powder reaches 64%. Therefore, natural graphite powder is one of the key materials in the production and manufacturing process of nuclear fuel elements.

[0003] CN106006622A discloses a preparation method of natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements and the graphite powder. The specific preparation method is powder grinding, shaping and classification treatment, and high-temperature purification steps. When the graphite powder prepared by this method is used for nuclear fuel element process tests, since the graphite powder is all in a round or oval shape and the particle size distribution is reasonable, the pressed nuclear fuel elements will not form holes due to bridging phenomena inside, the density of fuel particles is large, the strength is high, the stability is good, and the service life in the reactor is long. However, in this technical solution, flake graphite with a fixed carbon content ≥ 99% and a particle size of 100 - 200 μm needs to be used as the raw material, the requirements for the raw material are relatively high, and the raw material cost is high; and it takes a lot of energy to crush large flakes to 30 - 40 μm, and the material yield is low; in addition, a large amount of chlorine gas and Freon need to be continuously introduced, resulting in large environmental pollution; the total impurity content of the final product is in the tens of ppm level, and the boron equivalent index is not easy to control.

[0004] Therefore, how to obtain natural graphite powder with low total impurity content and low boron equivalent through a more environmentally friendly method is an urgent problem to be solved currently. Summary of the Invention

[0005] To solve the above technical problems, the purpose of the present invention is to provide natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements and a preparation method thereof.

[0006] To achieve the above purpose, the present invention provides a preparation method of natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements, wherein the preparation method includes:

[0007] Step 1: Screen the graphite concentrate powder, and then crush, shape, and classify the screened material to obtain a powder with a particle size of 25 - 35 μm;

[0008] Step 2: Use a mixed acid solution to perform primary purification and secondary purification on the powder to obtain chemically purified graphite powder;

[0009] Step 3: Heat the chemically purified graphite powder under vacuum and perform high-temperature purification. When the temperature reaches 1800 °C, introduce a mixed gas of argon and carbon tetrafluoride, and start the cyclic reaction; then continue to increase the temperature in a gradient manner to 2000 °C, 2200 °C, and 2400 °C, and perform cyclic reactions at each temperature to obtain high-temperature purified graphite powder;

[0010] Step 4: Cool down to 300 - 400 °C, introduce a mixed gas of oxygen and argon, and perform oxidation treatment on the high-temperature purified graphite powder to obtain low-temperature oxidized graphite powder;

[0011] Step 5: Disperse and homogenize the low-temperature oxidized graphite powder to obtain the finished natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements.

[0012] According to a specific embodiment of the present invention, preferably, the carbon content of the graphite concentrate powder is 90% - 95%. The graphite concentrate powder refers to the graphite powder produced by flotation of graphite ore, and its particle size can be conventionally selected according to actual needs, generally with the content of particles larger than 100 mesh being 10% - 30%.

[0013] According to a specific embodiment of the present invention, preferably, the mixed acid solution is a mixture of sulfuric acid (H2SO4) and other acid solutions; the mass percentage of sulfuric acid in the mixed acid solution is 10% - 30%.

[0014] According to a specific embodiment of the present invention, preferably, the other acid solutions include one or a combination of two or more of hydrofluoric acid (HF), hydrochloric acid (HCl), and nitric acid (HNO3).

[0015] In some specific embodiments, preferably, the sulfuric acid is concentrated sulfuric acid with a concentration of 98%, the hydrofluoric acid is hydrofluoric acid with a concentration of 35% - 40%, the hydrochloric acid is hydrochloric acid with a concentration of 36%, and the nitric acid is nitric acid with a concentration of 68%.

[0016] In some specific embodiments, preferably, the process of primary purification using the mixed acid solution is: mix and purify the powder and the mixed acid solution at 60 - 65 °C, and the mass ratio of the powder to the mixed acid solution is 1:2 to 1:3; more preferably, during primary purification, the mixed acid solution is composed of sulfuric acid, hydrofluoric acid, and nitric acid, and the mass ratio of sulfuric acid, hydrofluoric acid, and nitric acid is (1 - 3):(4 - 7):(3 - 6).

[0017] In some specific embodiments, preferably, the process of secondary purification using a mixed acid solution is as follows: The powder and the mixed acid solution are mixed and purified at 65 - 70 °C, and the mass ratio of the powder to the mixed acid solution is 1:1 to 1:2; More preferably, during secondary purification, the mixed acid solution is composed of sulfuric acid, hydrofluoric acid, and hydrochloric acid, and the mass ratio of sulfuric acid, hydrofluoric acid, and hydrochloric acid is (1 - 3):(3 - 6):(4 - 8).

[0018] In some specific embodiments, preferably, the purity of the chemically purified graphite powder ≥ 99.9%, and the contents of B, Al, and Si elements are all < 1 ppm.

[0019] According to the specific embodiments of the present invention, preferably, in the third step, the cyclic reaction at each temperature is repeated 5 - 10 times.

[0020] According to the specific embodiments of the present invention, preferably, the process of the cyclic reaction includes: First, a mixed gas of argon and carbon tetrafluoride is introduced. When the furnace pressure reaches 30 - 60 Kpa, the gas supply is stopped. After reacting for 10 - 20 minutes, the vacuum is pumped. When the vacuum reaches below 100 Pa, the vacuum pumping is stopped, and the next cyclic reaction is entered.

[0021] According to the specific embodiments of the present invention, preferably, based on the volume of the mixed gas of argon and carbon tetrafluoride being 100%, the proportion of carbon tetrafluoride is 20 vol% - 50 vol%.

[0022] According to the specific embodiments of the present invention, preferably, the time for oxidation treatment is 1 - 4 hours.

[0023] According to the specific embodiments of the present invention, preferably, based on the volume of the mixed gas of oxygen and argon being 100%, the proportion of oxygen is 5 vol% - 20 vol%.

[0024] In some specific embodiments, preferably, the atmosphere for oxidation treatment is a mixed gas of high-purity oxygen and high-purity argon, the purity of the high-purity oxygen ≥ 99.999%; the purity of the high-purity argon ≥ 99.999%. The flow rate of the mixed gas of oxygen and argon is 5 - 10 L / min.

[0025] According to the specific embodiments of the present invention, preferably, in the third step, the power consumption for high-temperature purification is 8000 - 10000 KWh per ton of high-temperature purified graphite powder, and the consumption of carbon tetrafluoride is 6 - 8 kg per ton of high-temperature purified graphite powder.

[0026] According to the specific embodiments of the present invention, preferably, the third step further includes: After starting the cyclic reaction, steps of alkali solution absorption and alkali solution rinsing of the tail gas are carried out.

[0027] In some specific embodiments, preferably, the alkali solution absorption and alkali solution rinsing are achieved by starting the tail gas treatment device; the tail gas treatment device includes a primary alkali solution absorption device and a secondary alkali solution spraying device.

[0028] In some specific embodiments, preferably, in step one, the specifications of the upper sieve used for screening are 80 mesh to 100 mesh, and the specifications of the lower sieve are 240 mesh to 450 mesh.

[0029] In some specific embodiments, preferably, the pulverization is carried out in a mechanical mill, the mechanical mill is a 60-machine or an 80-machine, and the operating frequency of the main engine is 20 - 40 Hz. The average particle size of the pulverized powder is 25 - 35 μm.

[0030] In some specific embodiments, preferably, the shaping time is 10 - 30 minutes. In the present invention, putting the pulverized powder obtained into the shaping equipment can remove the edges and corners of the powder particles.

[0031] In some specific embodiments, preferably, the classification treatment removes the powder with small particle size through cyclone classification, the frequency of the classifier is 30 - 50 Hz, and the frequency of the fan is 20 - 40 Hz.

[0032] In some specific embodiments, preferably, the process of the dispersion and homogenization treatment is as follows: putting the low-temperature oxidized graphite powder into the homogenization equipment, stirring for 30 - 120 min, and obtaining the finished product of natural graphite powder after demagnetization by a demagnetizer.

[0033] In some specific embodiments, preferably, in the equipment for the homogenization treatment, the material in contact with the material is a ceramic material; more preferably, the ceramic material includes zirconia and / or alumina. In the present invention, the ceramic material can avoid introducing new metal impurities.

[0034] In the above preparation method, preferably, the preparation method of the natural graphite powder for the high-temperature gas-cooled reactor nuclear fuel element specifically includes the following steps:

[0035] (1) Screening: putting the graphite concentrate into a vibrating screen, screening out the particles with larger sizes and the particles with smaller sizes, and retaining the particles with intermediate sizes;

[0036] (2) Pulverization, shaping and classification: putting the screened material in step (1) into a mechanical mill for pulverization to obtain a powder with an average particle size of 25 - 35 μm; putting the powder into the shaping equipment to remove the edges and corners of the powder particles, and removing the powder with small particle size through cyclone classification to obtain a powder with a particle size distribution concentrated in 25 - 35 μm;

[0037] (3) Chemical purification: After subjecting the powder obtained in step (2) to primary purification and secondary purification with a mixed acid solution, chemically purified graphite powder with a purity ≥ 99.9% and the contents of B, Al, and Si elements all < 1 ppm is obtained;

[0038] (4) High-temperature purification: Load the chemically purified graphite powder obtained in step (3) into a crucible and place it in a high-temperature furnace for high-temperature purification. The high-temperature furnace maintains a vacuum state during the heating-up stage. When the furnace temperature reaches 1800 °C, a mixed gas of argon and carbon tetrafluoride is introduced, the cyclic reaction is started, and the tail gas treatment device is activated. After repeating the cyclic reaction 5 - 10 times, continue heating up. Repeat the cyclic reaction 5 - 10 times at 2000 °C, 2200 °C, and 2400 °C respectively to complete the high-temperature purification;

[0039] (5) Low-temperature oxidation: Lower the furnace temperature to 300 °C - 400 °C, introduce a mixed gas of high-purity oxygen and high-purity argon, and carry out oxidation treatment for 1 - 4 hours and then take it out of the furnace;

[0040] (6) Dispersing and homogenizing treatment: Put the graphite powder after low-temperature oxidation in step (5) into a homogenizing device, stir for 30 - 120 min, and obtain the finished product of natural graphite powder after demagnetization by a demagnetizer.

[0041] The present invention also provides a natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements, which is prepared by the above preparation method.

[0042] According to the specific implementation scheme of the present invention, preferably, the total impurity element content of the natural graphite powder < 10 ppm, the specific surface area is 4 - 8 m 2 / g, the boron element content < 0.1 ppm, and the total boron equivalent is within 1 μg / g; more preferably, the total impurity element content of the natural graphite powder < 5 ppm, the boron element content < 0.05 ppm, and the total boron equivalent is within 0.15 μg / g.

[0043] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0044] (1) The preparation method provided by the present invention can increase the surface defects of the graphite powder and improve the specific surface area of the graphite powder through low-temperature oxidation treatment after high-temperature purification, which is beneficial to better adhesion of the graphite powder to the resin during specific use.

[0045] (2) When the preparation method provided by the present invention is used for high-temperature purification, by means of intermittently introducing a small amount of gas through a cyclic reaction, compared with continuously introducing gas, the reaction efficiency between the reaction gas and impurities can be greatly improved, pollutant emissions can be reduced, and production costs can be lowered; at the same time, compared with chlorine gas and Freon, carbon tetrafluoride is more environmentally friendly. In addition, during the high-temperature purification process, by maintaining a low-pressure state through the initial vacuum in the furnace and subsequent circulation, impurity elements can be more easily overflowed, thereby reducing the reaction temperature and reaction time and reducing energy consumption; at the same time, by carrying out stepwise reactions at different temperature segments, the reaction time in the high-temperature segment can be reduced, further reducing the energy consumption of the product.

[0046] (3) In the prior art, it takes a lot of energy to crush large flake graphite with a particle size of more than 150 μm to the target finished product particle size of 25 - 35 μm. However, in the present invention, screening out a suitable particle size first and then performing crushing can reduce energy consumption and increase the yield, and the increase in yield improves the comprehensive utilization value of the raw materials; in addition, since the value of large flake graphite is relatively high, the large flake graphite removed in the screening step of the present invention can continue to be used for other purposes, further improving the comprehensive utilization value of the raw materials.

[0047] (4) The present invention uses graphite fine powder as the raw material and adopts a method of chemical purification after crushing to a qualified particle size, which reduces the requirements for the graphite fine powder raw material, greatly reduces the quantity of the purified material, and further reduces the usage amount of acid solution during the purification process, thereby reducing pollutant emissions and lowering production costs. In the prior art, the carbon content requirement for flake graphite is relatively high, reaching more than 99%, and chemical purification also needs to be carried out subsequently, increasing pollution.

[0048] (5) In the chemical purification stage of the present invention, H2SO4 is added to remove B impurities in the raw materials, which can treat specific impurities in advance, reduce the contents of B, Al, and Si elements to below 1 ppm, and ensure that the purity and boron equivalent of the finished product meet the requirements.

[0049] (6) Through the homogenization treatment of the present invention, it is beneficial to disperse the agglomerated particles during the high-temperature purification process, increase the specific surface area of the product, and improve the batch stability of the product.

[0050] (7) The natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements provided by the present invention achieves lower total impurity element content and B element content, as well as a higher specific surface area, and can ensure that the total boron equivalent of each batch of products is within a low range, with high product stability. Description of the Drawings

[0051] Figure 1 It is a scanning electron microscope image of the finished product of the natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements prepared in Example 1. Detailed Embodiments

[0052] For a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the following provides a detailed description of the technical solution of the present invention, but it should not be construed as a limitation on the scope of implementation of the present invention.

[0053] Example 1

[0054] This example provides a preparation method for natural graphite powder used in high-temperature gas-cooled reactor nuclear fuel elements, and the specific steps are as follows:

[0055] (1) Screening: Put graphite concentrate with a carbon content of 94% into a vibrating screen to screen out large particles with a particle size greater than 100 mesh and small particles with a particle size less than 325 mesh, and retain graphite particles with a particle size of 44μm - 150μm;

[0056] (2) Crushing, shaping, and classification: Put the screened material in step (1) into a mechanical mill of 60 machines for crushing, with the main machine running frequency of 35Hz to obtain powder. After testing, its D50 is 31.2μm; put this powder into a shaping device, with a shaping time of 15 minutes to remove the edges and corners of the powder particles; then, remove small-particle-size powder through cyclone classification to leave the required powder. After testing, its D50 is 29.8μm;

[0057] Among them, the frequency of the classifier is 40Hz, and the frequency of the fan is 25Hz.

[0058] (3) Chemical purification: Use a mixed acid solution to perform a primary purification and a secondary purification process on the powder obtained in step (2). After testing the chemical element content by ICP, the chemical purification graphite powder with a B element content of 0.6ppm, an Al element content of 0.9ppm, an Si element content of 0.7ppm, and a carbon content of 99.96% is obtained;

[0059] Among them, in the primary purification, the mixed acid solution is a mixture of sulfuric acid with a concentration of 98%, hydrofluoric acid with a concentration of 40%, and nitric acid with a concentration of 68%. The mass ratio of sulfuric acid, hydrofluoric acid, and nitric acid is 2:4:4, the reaction temperature is 60°C, and the mass ratio of the powder to the mixed acid solution is 1:2.5;

[0060] In the secondary purification, the mixed acid solution is a mixture of sulfuric acid with a concentration of 98%, hydrofluoric acid with a concentration of 40%, and hydrochloric acid with a concentration of 36%. The mass ratio of sulfuric acid, hydrofluoric acid, and hydrochloric acid is 1.5:3.5:5, the reaction temperature is 70°C, and the mass ratio of the powder to the mixed acid solution is 1:1.5.

[0061] (4) High-temperature purification: Load the chemically purified graphite powder from step (3) into a crucible and place it in a high-temperature furnace for high-temperature purification. The high-temperature furnace maintains a vacuum state during the heating-up stage. When the furnace temperature reaches 1800 °C, introduce a mixed gas of argon and carbon tetrafluoride, start the cyclic reaction, and activate the tail gas treatment device. After repeating the cyclic reaction 8 times, continue heating up, and perform the cyclic reaction 8 times at each of the three temperatures of 2000 °C, 2200 °C, and 2400 °C to complete the high-temperature purification;

[0062] Among them, the process of the cyclic reaction includes: First, introduce a mixed gas of argon and carbon tetrafluoride. Taking the volume of the mixed gas of argon and carbon tetrafluoride as 100%, the proportion of carbon tetrafluoride is 40 vol%. Stop gas injection when the furnace pressure reaches 30 - 60 Kpa, react for 10 - 20 minutes and then evacuate. Stop evacuating when the vacuum reaches below 100 Pa and enter the next cyclic reaction;

[0063] The power consumption for high-temperature purification per ton of product is 8500 KWh, and the consumption of carbon tetrafluoride is 6.8 kg.

[0064] (5) Low-temperature oxidation: Lower the furnace temperature to 350 °C, introduce a mixed gas of high-purity oxygen and high-purity argon, and take it out of the furnace after 2 hours of oxidation treatment;

[0065] Among them, taking the volume of the mixed gas of high-purity oxygen and high-purity argon as 100%, the proportion of high-purity oxygen is 10 vol%, and the flow rate of the mixed gas is 5 L / min.

[0066] (6) Dispersing and homogenizing treatment: Put the graphite powder obtained in step (5) into a homogenizing device, stir for 60 min, and obtain the finished natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements after demagnetization by a demagnetizer.

[0067] The scanning electron microscope image of the finished natural graphite powder prepared in this example is as Figure 1 shown. Analyze its quality, and the results are shown in Table 1. Each analysis item in Table 1 (such as laser particle size D50) is the test result of the final finished natural graphite powder.

[0068] Example 2

[0069] This example provides a preparation method of natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements, and the specific steps are as follows:

[0070] (1) Screening: Put the graphite concentrate with a carbon content of 91% into a vibrating screen, screen out large particles with a particle size greater than 100 mesh and small particles with a particle size less than 240 mesh, and retain graphite particles with a particle size of 63 μm - 150 μm;

[0071] (2) Crushing, shaping and classification: Put the screened material in step (1) into a mechanical mill of 60 machine for crushing, with the main machine running frequency of 40 Hz to obtain powder. After testing, its D50 is 30.4 μm. Put the powder into a shaping device for 10 minutes to remove the edges and corners of the powder particles. Then, remove the powder with small particle size through cyclone classification, and leave the required powder. After testing, its D50 is 29.2 μm;

[0072] Among them, the frequency of the classifier is 45 Hz, and the frequency of the fan is 25 Hz.

[0073] (3) Chemical purification: Use a mixed acid solution to carry out the first purification and the second purification process on the powder obtained in step (2). After testing the chemical element content by ICP, the chemical purified graphite powder with B element content of 0.52 ppm, Al element content of 0.67 ppm, Si element content of 0.48 ppm, and carbon content of 99.98% is obtained;

[0074] Among them, in the first purification, the mixed acid solution is a mixture of sulfuric acid with a concentration of 98%, hydrofluoric acid with a concentration of 40%, and nitric acid with a concentration of 68%. The mass ratio of sulfuric acid, hydrofluoric acid, and nitric acid is 2.5:4:3.5, the reaction temperature is 65 °C, and the mass ratio of the powder to the mixed acid solution is 1:2;

[0075] In the second purification, the mixed acid solution is a mixture of sulfuric acid with a concentration of 98%, hydrofluoric acid with a concentration of 40%, and hydrochloric acid with a concentration of 36%. The mass ratio of sulfuric acid, hydrofluoric acid, and hydrochloric acid is 1:3:6, the reaction temperature is 65 °C, and the mass ratio of the powder to the mixed acid solution is 1:1.5.

[0076] (4) High-temperature purification: Put the chemically purified graphite powder in step (3) into a crucible and put it into a high-temperature furnace for high-temperature purification. The high-temperature furnace maintains a vacuum state during the heating stage. When the furnace temperature reaches 1800 °C, a mixed gas of argon and carbon tetrafluoride is introduced, the cyclic reaction is started, and the tail gas treatment device is started. After repeating the cyclic reaction 8 times, continue to heat up. At three temperatures of 2000 °C, 2200 °C, and 2400 °C, the cyclic reaction is carried out 6 times each to complete the high-temperature purification;

[0077] Among them, the process of the cyclic reaction includes: first introduce a mixed gas of argon and carbon tetrafluoride. Taking the volume of the mixed gas of argon and carbon tetrafluoride as 100%, the proportion of carbon tetrafluoride is 30 vol%. When the furnace pressure reaches 30 - 60 Kpa, stop ventilation. After reacting for 10 - 20 minutes, evacuate again. When the vacuum reaches below 100 Pa, stop evacuating and enter the next cyclic reaction;

[0078] The power consumption per ton of products for the high-temperature purification is 8100 KWh, and the consumption of carbon tetrafluoride is 5.2 kg.

[0079] (5) Low-temperature oxidation: Lower the furnace temperature to 300 °C, introduce a mixed gas of high-purity oxygen and high-purity argon, and take the product out of the furnace after 3 hours of oxidation treatment;

[0080] Among them, based on the volume of the mixed gas of high-purity oxygen and high-purity argon being 100%, the proportion of the high-purity oxygen is 10 vol%, and the flow rate of the mixed gas is 5 L / min.

[0081] (6) Dispersion and homogenization treatment: Put the graphite powder obtained in step (5) into a homogenization device, stir for 60 min, and obtain the finished natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements after demagnetization by a demagnetizer.

[0082] Analyze the quality of the finished natural graphite powder prepared in this example. The results are shown in Table 1. Each analysis item in Table 1 (such as laser particle size D50) is the test result of the final finished natural graphite powder.

[0083] Example 3

[0084] This example provides a method for preparing natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements. Compared with Example 1, the difference is only:

[0085] (1) The number of cycles at three temperatures of 2000 °C, 2200 °C, and 2400 °C in the high-temperature purification stage is different: the 2000 °C cyclic reaction is carried out 10 times, the 2200 °C cyclic reaction is carried out 8 times, and the 2400 °C cyclic reaction is carried out 5 times. The power consumption per ton of product is 8300 KWh, and the consumption of carbon tetrafluoride is 6.5 kg;

[0086] (2) The parameters in the low-temperature oxidation stage are different: the low-temperature oxidation treatment time is 4 hours, the proportion of high-purity oxygen in the mixed gas of high-purity oxygen and high-purity argon is 20 vol%, and the flow rate of the mixed gas is 10 L / min;

[0087] The remaining steps remain unchanged, and the finished natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements is obtained.

[0088] Analyze the quality of the finished natural graphite powder prepared in this example. The results are shown in Table 1. Each analysis item in Table 1 (such as laser particle size D50) is the test result of the final finished natural graphite powder.

[0089] Comparative Example 1

[0090] This comparative example provides a method for preparing natural graphite powder. Compared with Example 1, the difference is that in the chemical purification stage, acid-base method is used for purification, and the remaining steps remain unchanged;

[0091] The steps of purification by the acid-base method are as follows: Graphite concentrate powder and sodium hydroxide powder are subjected to high-temperature alkali fusion at 780 °C for 2 h, and the mass ratio of graphite concentrate powder to sodium hydroxide powder is 5:1. After the alkali fusion is completed, it is cooled with the furnace, dispersed, and washed. The obtained product is acid-leached with a concentrated hydrochloric acid solution (mass concentration of 20%) at a mass ratio of 1:1. The acid-leaching temperature is 90 °C. After the acid-leaching is completed, it is washed with water until neutral, filtered, and dried to obtain graphite powder;

[0092] After purification by the acid-base method, the carbon content of the graphite powder is 99.24%, the B element content is 1.2 ppm, the Al element content is 44 ppm, and the Si element content is 65 ppm;

[0093] It can be seen from this that the purity of the graphite powder obtained by purification with the acid-base method is lower than that of the graphite powder obtained by purification with the mixed acid method in step (3) of Example 1.

[0094] This comparative example continues to carry out the subsequent steps (4) to (6) according to Example 1 to obtain the final natural graphite powder product, and its quality analysis is shown in Table 1.

[0095] Comparative Example 2

[0096] This comparative example provides a method for preparing natural graphite powder. Compared with Example 1, the difference is that after high-temperature purification, no dispersion and homogenization treatment are carried out, and the remaining steps remain unchanged.

[0097] The quality of the natural graphite powder product prepared in this comparative example was analyzed, and the results are shown in Table 1.

[0098] Comparative Example 3

[0099] This comparative example provides a method for preparing natural graphite powder. Compared with Example 1, the difference is that a mixed gas of chlorine and argon is introduced during the high-temperature purification stage, and the remaining steps remain unchanged.

[0100] Among them, based on the volume of the mixed gas of chlorine and argon being 100%, the proportion of chlorine is 30 vol%.

[0101] The quality of the natural graphite powder product prepared in this comparative example was analyzed, and the results are shown in Table 1.

[0102] Comparative Example 4

[0103] This comparative example provides a method for preparing natural graphite powder. Compared with Example 1, the difference is that after high-temperature purification, it is directly cooled and taken out of the furnace without undergoing low-temperature oxidation treatment, and the remaining steps remain unchanged.

[0104] The quality of the natural graphite powder product prepared in this comparative example was analyzed, and the results are shown in Table 1. Among them, the purity of the natural graphite powder product = 100% - the content of total impurity elements.

[0105] Table 1

[0106]

[0107] As can be seen from Table 1, the total boron equivalent and boron content of the natural graphite powder prepared in Comparative Example 1 and Comparative Example 3 are relatively high and not controlled within the ideal range; although the purity of the natural graphite powder prepared in Comparative Example 2 and Comparative Example 4 has been improved, the specific surface area is relatively small; in contrast, the natural graphite powder prepared in Example 1 not only has a low total impurity element content and a low total boron equivalent, but also has a relatively high specific surface area at the same time.

Claims

1. A preparation method of natural graphite powder for nuclear fuel elements of a high-temperature gas-cooled reactor, wherein, The preparation method includes the following steps: Step 1: Screen the fine graphite powder, and then crush, shape, and classify the screened material to obtain a powder with a particle size of 25 - 35 μm; Step 2: Use a mixed acid solution to perform primary purification and secondary purification on the powder to obtain chemically purified graphite powder; Step 3: Heat the chemically purified graphite powder under vacuum and perform high-temperature purification. When the temperature reaches 1800 °C, introduce a mixture of argon and carbon tetrafluoride, and start the cyclic reaction; then continue to increase the temperature in gradients to 2000 °C, 2200 °C, and 2400 °C, and perform cyclic reactions at each temperature respectively to obtain high-temperature purified graphite powder; Step 4: Cool down to 300 - 400 °C, introduce a mixture of oxygen and argon, and perform oxidation treatment on the high-temperature purified graphite powder to obtain low-temperature oxidized graphite powder; Step 5: Disintegrate and homogenize the low-temperature oxidized graphite powder to obtain the finished natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements.

2. The preparation method according to claim 1, wherein, The carbon content of the fine graphite powder is 90% - 95%.

3. The preparation method according to claim 1, wherein The mixed acid solution is a mixture of sulfuric acid and other acid solutions; the mass percentage of sulfuric acid in the mixed acid solution is 10% - 30%.

4. The preparation method according to claim 3, wherein, The other acid solution includes one or a combination of two or more of hydrofluoric acid, hydrochloric acid, and nitric acid.

5. The preparation method according to claim 1, wherein, In Step 3, the cyclic reaction at each temperature is repeated 5 - 10 times.

6. The preparation method according to claim 5, wherein, The process of the cyclic reaction includes: first introduce a mixture of argon and carbon tetrafluoride, stop ventilation when the furnace pressure reaches 30 - 60 Kpa, react for 10 - 20 minutes and then evacuate to vacuum, stop evacuating when the vacuum reaches below 100 Pa, and enter the next cyclic reaction.

7. The preparation method according to claim 1, wherein Based on the volume of the mixture of argon and carbon tetrafluoride being 100%, the proportion of carbon tetrafluoride is 20 vol% - 50 vol%.

8. The preparation method according to claim 1, wherein The time for the oxidation treatment is 1 - 4 hours; Based on the volume of the mixture of oxygen and argon being 100%, the proportion of oxygen is 5 vol% - 20 vol%.

9. The preparation method according to claim 1, wherein In Step 3, the power consumption for high-temperature purification is 8000 - 10000 KWh per ton of high-temperature purified graphite powder, and the consumption of carbon tetrafluoride is 6 - 8 kg per ton of high-temperature purified graphite powder.

10. The preparation method according to claim 1, wherein, Step 3 also includes: after starting the cyclic reaction, performing the steps of alkali solution absorption and alkali solution rinsing on the tail gas.

11. A natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements, which is prepared by the preparation method described in any one of claims 1 - 10.

12. The natural graphite powder for high-temperature gas-cooled reactor nuclear fuel elements according to claim 11, wherein, The total impurity element content of the natural graphite powder is < 10 ppm, the specific surface area is 4 - 8 m 2 / g, the boron element content is < 0.1 ppm, and the total boron equivalent is within 1 μg / g.

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