Preparation method of coal-based graphene with controllable size

Abstract

The application provides a preparation method of coal-based graphene with controllable size. The coal-based graphene with better controllability in size can be obtained by using the preparation method of the application with coal as raw material. The preparation method comprises the following steps: 1) performing graphitization treatment on raw coal to obtain graphitized material; 2) performing reaction on the graphitized material in a mixed solution of sulfuric acid and sodium nitrate; 3) continuously feeding potassium permanganate into the obtained reaction solution, and the mass ratio of the amount of potassium permanganate to the mass of the graphitized material in step 2) is preferably 2-8:1; 4) continuing the reaction of the reaction solution obtained in step 3) by heating; then adding an oxidizing agent to consume the remaining potassium permanganate; 5) performing gravity separation on the obtained reaction solution, and then performing ultrasonic treatment; 6) drying the slurry to obtain coal-based graphene oxide powder, and then performing high-temperature reduction; or performing chemical reduction on the slurry to obtain coal-based graphene.

Description

Technical Field

[0001] This invention relates to the field of graphene preparation technology, and in particular to a method for preparing coal-based graphene with controllable size. Background Technology

[0002] Graphene is a two-dimensional carbon material composed of one, two, or few layers of carbon atoms arranged in a periodically close-packed benzene ring structure (i.e., a hexagonal honeycomb structure). As a novel two-dimensional carbon material of the 21st century, graphene possesses unique structural characteristics and excellent electrical, mechanical, optical, and thermal properties, showing promising applications in energy storage, catalysis, and sensors, attracting widespread global attention. The inexpensive and controllable preparation of graphene is crucial to its development and application.

[0003] Currently, the main raw material for preparing graphene is natural flake graphite. There are approximately 800 million tons of natural flake graphite in the world, but only about 10-15% of it is graphitic carbon, which can be used to prepare graphene. With the rapid development and vigorous promotion of graphene technology, the raw materials for graphene will inevitably face a tight supply in the future, while coal resources are relatively abundant.

[0004] Although both coal and flake graphite are composed of carbon, they differ significantly. Coal has a three-dimensional cross-linked benzene ring structure, and how to exfoliate it to prepare a two-dimensional graphene structure remains unknown. Furthermore, considering downstream applications of graphene, the in-plane diameter of the graphene flakes is a core technical indicator. For example, the widely researched graphene thermal conductive films generally require graphene with a diameter of 300-500 nm or larger than 30 μm as raw material. Current methods for preparing graphene using flake graphite typically achieve a size retention rate of 10%-50% (for example, if the added raw material is 100 μm, the size of the exfoliated graphene is generally between 10-50 μm). Especially for 30-50 nm graphene, the flake graphite size is generally required to be between 1.5 μm and 5 μm. However, preparing flake graphite within this range presents significant challenges, and no particularly reliable technology is available. Control is generally achieved by manipulating the oxidation exfoliation process conditions, but this controllability is relatively poor and prone to random phenomena.

[0005] CN 104140144 A discloses a method for preparing a graphene oxide liquid crystal emulsion. This patent application uses natural graphite as a raw material to undergo an oxidation reaction to obtain graphene oxide, further adds carbon black, and then emulsifies it with vegetable oil to obtain the graphene oxide liquid crystal emulsion. The raw material in this patent application is natural graphite, not coal.

[0006] CN 110015654 A discloses a method for preparing coal-based graphene. This patent application mainly involves mixing concentrated phosphoric acid with coal-based graphite, perchloric acid, concentrated sulfuric acid, water, and hydrogen peroxide, filtering the mixture, and then further reducing it with hydrazine hydrate. This technical solution does not involve controlling the in-plane dimensions of the graphene. Summary of the Invention

[0007] This invention provides a method for preparing coal-based graphene with controllable size. Using coal as raw material, the preparation method of this invention can obtain coal-based graphene with better size controllability.

[0008] To achieve its objective, the present invention provides the following technical solution:

[0009] This invention provides a method for preparing coal-based graphene with controllable size, comprising the following steps:

[0010] 1) The raw coal is graphitized to obtain graphitized material, and the degree of graphitization of the graphitized material is ≥77%; and the particle size of the raw coal must meet the following requirements: D97 is 5-75μm and exhibits a normal distribution;

[0011] 2) The graphitized material is reacted in a mixed solution of sulfuric acid and sodium nitrate;

[0012] 3) Potassium permanganate is continuously added to the reaction solution obtained in step 2) at a feeding rate of 0.05-0.15 g / min, while stirring continuously, and the reaction temperature is controlled to not exceed 20±5℃ (i.e., not exceeding 20℃, with an allowable error of ±5℃). After the potassium permanganate is added, the reaction is stirred for 30-60 min; then the temperature is raised to 35-45℃ and the reaction is stirred for another 30 min. The preferred mass ratio of potassium permanganate to the graphitized material in step 2) is 2-8:1.

[0013] 4) Heat the reaction solution obtained in step 3) to 90-100℃ and continue the reaction. The heating rate is controlled at 1.5-5 min / ℃. Add deionized water while stirring continuously. The addition rate of deionized water is preferably controlled at 2 mL / min-20 mL / min. React for 10-50 min. Then add an oxidant to consume the remaining potassium permanganate.

[0014] 5) The material obtained in step 4) is subjected to multiple gravity separations, and is washed with deionized water during the multiple gravity separations until the separated liquid is neutral; then the separated material is diluted with deionized water and subjected to ultrasonic treatment to obtain coal-based graphene oxide slurry.

[0015] 6) The slurry is dried to obtain coal-based graphene oxide powder, and then reduced at high temperature to obtain coal-based graphene; or, the slurry is chemically reduced to obtain coal-based graphene.

[0016] This invention controls the particle size of raw coal to 5-75 μm with a D97 value and a normal distribution. Coal-based graphene is prepared according to the above process steps. In particular, in steps 3) and 4), with specific reaction conditions, potassium permanganate dosage, material input rate and heating rate, the oxidant can effectively peel off the raw material, and finally obtain coal-based graphene with better size controllability. For example, the size retention rate can be controlled between 5-30%, which is also conducive to obtaining graphene products with fewer layers.

[0017] In this paper, the formula for calculating the size retention rate is: the ratio of the in-plane dimension of the coal-based graphene to the size of the raw coal * 100%, where the in-plane dimension refers to the diameter of the graphene's circumscribed circle, and the size of the raw coal is its D50 value. The in-plane dimension of the coal-based graphene can be obtained by performing scanning electron microscopy (SEM) or transmission electron microscopy (TEM) on the coal-based graphene sample, acquiring multiple (more than 30) detection values ​​(i.e., the diameter of the graphene's circumscribed circle), and then averaging them as the in-plane dimension value used to calculate the size retention rate. If the number of detection values ​​obtained from a single sample detection is less than 30, the required number of detection values ​​are obtained by detecting multiple samples, and then averaging them. The in-plane dimensions in the following examples and comparative examples are obtained by averaging the values ​​obtained through SEM detection using the above method, and will not be described again hereafter.

[0018] In some embodiments, in step 1), the graphitization treatment can be high-temperature graphitization or catalytic graphitization technology; preferably, high-temperature graphitization is used, which can be carried out in a medium-frequency induction graphitization furnace. The conditions for the graphitization treatment include: first heating the raw coal to 1000-1100℃, then holding it at that temperature for 1-2 hours; then performing a second heating to 2800-3200℃ and holding it at that temperature for 2.5-4 hours;

[0019] Preferably, the heating rate of the first stage of heating is 10℃ / min, and the heating rate of the second stage of heating is 5℃ / min.

[0020] In some embodiments, in step 2), the amount of sulfuric acid used is 35-55 mL relative to 1 g of sodium nitrate; the sulfuric acid is preferably concentrated sulfuric acid with a mass concentration of not less than 95%. Specifically, in step 2), the sodium nitrate is dissolved in sulfuric acid, and the dissolution process is stirred with a stirrer for about 5-20 minutes to obtain the above-mentioned mixed solution of sulfuric acid and sodium nitrate.

[0021] In some embodiments, in step 2), the amount of the mixed solution used is 30-40 mL relative to 1 g of the graphitized material;

[0022] In some embodiments, in step 2), the reaction is carried out at 10°C ± 5°C, preferably with continuous stirring, and the reaction time is preferably 20-60 min. In step 2), continuous stirring ensures uniform mixing and prevents drastic changes in the reaction temperature.

[0023] In step 3), the preferred feeding rate of potassium permanganate is 0.05-0.15 g / min, for example, 0.05, 0.1, or 0.15 g / min. The inventors have found that using the above feeding rate for potassium permanganate facilitates the acquisition of coal-based graphene with better size controllability. If the feeding rate is too fast, the reaction will be violent, and the reaction temperature will rise sharply, resulting in a significant decrease in the size of the generated graphene flakes, making it impossible to obtain the product with the desired size retention rate. If the feeding rate is too slow, the interlayer van der Waals forces will prevent further intercalation reactions, making it difficult to obtain coal-based graphene.

[0024] Preferably, in step 3), the heating rate for the reaction at 35-45°C is controlled at 0.5-2°C / min, and the reaction time is 2-4 hours. Using this preferred heating rate facilitates better dimensional controllability.

[0025] Preferably, the volume ratio of the amount of deionized water added in step 4) to the volume ratio of the mixed solution of sulfuric acid and sodium nitrate in step 2) is 1:0.6-2.

[0026] Preferably, in step 4), the oxidant is hydrogen peroxide, preferably 30% hydrogen peroxide by mass. The ratio of the amount of hydrogen peroxide used to the potassium permanganate used in step 3) is preferably 1:0.6-2, in ml / g. In step 4), the addition rate of the hydrogen peroxide is preferably 2-8 ml / min. Using a preferred addition rate is beneficial for obtaining products with better size controllability.

[0027] In some embodiments, in step 5), the gravity separation is one or more of centrifugal separation, filtration, and static sedimentation; during multiple gravity separation processes, deionized water is continuously used for washing to make the coal-based graphene oxide slurry neutral. Through this gravity separation, it is beneficial to separate the particulate matter with incomplete oxidation reaction more thoroughly.

[0028] Preferably, when the gravity separation is centrifugal separation, the lower slurry obtained by centrifugation is diluted with deionized water and centrifuged again. This centrifugation is performed multiple times until the pH of the upper clear liquid obtained by centrifugation is neutral. At this time, the lower slurry obtained is the coal-based graphite oxide material.

[0029] Preferably, when the gravity separation is performed by vacuum filtration, the filter cake is continuously washed with deionized water during the vacuum filtration process, the extracted filtrate is neutral, and the obtained filter cake is the coal-based oxidized graphite slurry.

[0030] Preferably, when gravity separation is performed by static sedimentation, the supernatant obtained after static sedimentation is poured out, and then deionized water is added to the lower precipitate for static sedimentation again. This process is repeated multiple times until the supernatant obtained after static sedimentation is neutral. The lower precipitate obtained at this time is the coal-based oxidized graphite slurry.

[0031] In a preferred embodiment, in step 5), the dilution with deionized water is used to dilute the gravity-separated coal-based graphene oxide slurry to a concentration of 3-12 mg / ml; the ultrasonic power is 600-1000 W, and the ultrasonic time is 6-10 h; these preferred ultrasonic conditions facilitate better dimensional controllability. The inventors have found that using the above-mentioned preferred process conditions helps ensure that the final graphene dimensional controllability is between 5-30%. Increasing the ultrasonic power while using the same ultrasonic time significantly reduces the dimensional controllability. Furthermore, increasing the ultrasonic time while keeping the ultrasonic power constant also reduces the final graphene dimensional controllability. Increasing the solubility of the coal-based graphene oxide slurry, under the same ultrasonic power and time, will affect the number of layers in the final graphene product, resulting in low-quality coal-based graphene. Preferably, the temperature of the slurry should not exceed 50°C during ultrasonication.

[0032] Preferably, in step 6), the conditions for the high-temperature reduction include: conducting the process in an inert atmosphere, heating to 600-800°C at a heating rate of 2-15°C / min, and then holding the temperature for 1.5-3 hours. The inert atmosphere is, for example, nitrogen, with an oxygen content below 0.5%. The inventors have found that controlling the heating rate to 2-15°C / min during the high-temperature reduction process is beneficial for obtaining coal-based graphene with fewer than 10 layers, thus falling into the category of coal-based graphene. Increasing the heating rate significantly increases the number of layers in the final product, reaching approximately 20 layers, due to the influence of interlayer van der Waals forces. Conversely, a too-slow heating rate negatively impacts the overall process cost.

[0033] Preferably, in step 6), the conditions for chemical reduction include: using ascorbic acid as a reducing agent, with a mass ratio of ascorbic acid to the slurry of (5-20):1; preferably, the reaction temperature for the chemical reduction is 10-30℃, and the reaction time is preferably 5-15 hours. Specifically, continuous magnetic stirring is used during the chemical reduction process. The slurry obtained after reduction is dried, for example, by freeze-drying, to obtain coal-based graphene powder.

[0034] In a preferred embodiment, in step 1), the raw coal is obtained by steam pneumatic milling. For example, low-grade superheated steam can be used for pneumatic milling to obtain raw materials of the desired size. In some embodiments, the steam pneumatic milling is performed at, for example, a pressure of 0.5-1.2 MPa and a temperature of 180-320°C. In some embodiments, the milled raw coal is deashed before use and then graphitized; however, deashing is not always necessary, for example, if the particle size (D97) of the raw coal is 5 μm, deashing may not be required.

[0035] In this invention, there are no particular restrictions on the type of raw coal. It can be any type of coal that can be graphitized, such as anthracite, bituminous coal, or semi-coke. Anthracite is preferred, as it is beneficial to obtain products with better size controllability.

[0036] The technical solution provided by this invention has the following beneficial effects:

[0037] This invention uses coal as raw material, eliminating the need for flake graphite and effectively addressing issues such as flake graphite shortages and production volumes. This invention provides a coal-based graphene preparation process using coal as raw material. By controlling the size of the raw coal and combining it with an optimized graphene preparation process, it can effectively control the in-plane dimensions of the coal-based graphene product, achieving a size retention rate of 5-30%. Attached Figure Description

[0038] Figure 1 This is a particle size distribution diagram of the raw coal used in Example 1.

[0039] Figure 2 This is an electron microscope image of the coal-based graphene sample from Example 1.

[0040] Figure 3 The results are Raman spectroscopy findings for the coal-based graphene sample in Example 1.

[0041] Figure 4 The XRD results are for the coal-based graphene sample in Example 1. Detailed Implementation

[0042] To facilitate understanding of the present invention, the following description, in conjunction with embodiments, will further illustrate the invention. It should be understood that the following embodiments are merely for a better understanding of the invention and do not imply that the invention is limited to these embodiments.

[0043] Where specific experimental steps or conditions are not specified in the examples, the corresponding conventional experimental steps or conditions in this technical field can be followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0044] Example 1

[0045] The raw material coal (Taixi anthracite) used in this embodiment was obtained by steam pneumatic milling of coal with a particle size <1mm. The particle size D97 of this raw material coal was 58.95μm, and the particle size showed a normal distribution. The relevant detection results of its particle size are shown in Table 1 below. The particle size distribution is as follows: Figure 1 As shown.

[0046] Table 1

[0047] D75 = 29.68 μm D50 = 19.04 μm D84 = 35.88 μm D90 = 42.15 μm D97 = 58.95 μm

[0048] Coal-based graphene was prepared according to the following steps:

[0049] 1) The above-mentioned raw coal is subjected to graphitization treatment:

[0050] Raw coal was placed in a graphite crucible and then placed in a medium-frequency induction graphitization furnace. The temperature was increased from room temperature to 1000℃ at a rate of 10℃ / min and then held at that temperature for 1 hour. The temperature was then increased from 1000℃ to 2800℃ at a rate of 5℃ / min and held at that temperature for 3 hours. After that, the material was naturally cooled to room temperature to obtain graphitized material with a graphitization degree of 81%. Argon gas was used for protection throughout the experiment.

[0051] 2) The graphitized material obtained in step 1) is reacted in a mixed solution of sulfuric acid and sodium nitrate. The specific steps are as follows:

[0052] Sodium nitrate (NaNO3) was dissolved in 98% concentrated sulfuric acid at a ratio of H2SO4 / NaNO3 = 45 ml / g. The solution was stirred with a stirrer to ensure complete dissolution. The temperature of the sulfuric acid and sodium nitrate mixture was maintained at approximately 10°C (with an error of ±5°C). The aforementioned graphitized material was then added at a ratio of 1:34 g / ml to the sulfuric acid and sodium nitrate mixture. After adding the graphitized material, the reaction temperature was maintained at approximately 10°C (with an error of ±5°C) for 30 minutes, with continuous stirring to ensure uniform mixing and prevent drastic temperature fluctuations.

[0053] 3) Potassium permanganate is continuously and uniformly added to the reaction solution obtained in step 2) at a rate of 0.1 g / min, while stirring continuously throughout the entire feeding process, and the reaction temperature is controlled to not exceed 20℃ (with an error of ±5℃). After the potassium permanganate is added, the reaction is stirred for another 30 min; then the temperature is increased to 40℃ at a rate of 1℃ / min (with an error of ±5℃), and the reaction is stirred and kept at a constant temperature for 4 h; the mass ratio of the amount of potassium permanganate used to the mass ratio of the graphitized material in step 2) is 5:1.

[0054] 4) The reaction solution obtained in step 3) is heated to 98°C and the reaction continues at a rate of 2 min / °C. The reaction is carried out by adding deionized water while continuously stirring for 45 min. The ratio of deionized water to the aforementioned sulfuric acid and sodium nitrate mixed solution is 1:0.68 ml / ml, and the rate of addition of deionized water is controlled at 2 mL / min. Then, 30 wt% hydrogen peroxide is added at a rate of 2 ml / min. The ratio of hydrogen peroxide to the aforementioned potassium permanganate is 1:0.8 ml / g, thereby consuming the remaining potassium permanganate in the reaction.

[0055] 5) Centrifuge the obtained reaction solution, pour out the supernatant, collect the viscous slurry at the bottom of the centrifuge tube, add deionized water to wash and dilute it, and centrifuge again; repeat the centrifugation operation until the pH of the obtained supernatant is 7, and collect the bottom slurry at this time, which is the coal-based graphite oxide slurry.

[0056] Deionized water was added to the above slurry to obtain a slurry with a concentration of 8 mg / ml. The slurry was then subjected to ultrasonic treatment at a power of 700 W for 8 hours, while maintaining a temperature not exceeding 50°C.

[0057] 6) The slurry obtained by ultrasound is freeze-dried to obtain coal-based graphene oxide powder;

[0058] The aforementioned coal-based graphene oxide powder was loaded into a ceramic boat, and then the sample, along with the ceramic boat, was placed in a heating furnace for a high-temperature reduction reaction. Before starting the high-temperature reduction, nitrogen gas was first introduced to replace the air in the heating furnace, ensuring that the environment inside the furnace became inert (oxygen content was controlled below 0.5% by oxygen content detection). Then, the temperature was increased to 800°C at a rate of 5°C / min and held at this temperature for 2 hours, followed by natural cooling. Nitrogen gas was introduced throughout the process for protection. After complete cooling, coal-based graphene was obtained.

[0059] The obtained coal-based graphene was tested, among which... Figure 2 The image shows the electron microscopy results of a sample, which indicates the in-plane size of the coal-based graphene in the field of view. The image shows that the coal-based graphene is thin and wrinkled, exhibiting the morphology of graphene. Figure 3 The Raman spectroscopy results are shown in the figure. The result shows that the ID / IG ratio of the sample is 0.963, indicating that the defect peak D of the prepared sample is relatively low and within the range of graphene. Figure 4 The XRD results show typical broad graphene characteristic diffraction peaks around 25.58°, further confirming the formation of graphene. Furthermore, AFM analysis revealed that the product has fewer than 5 layers and a thickness of 1.123 nm.

[0060] The sample was subjected to SEM analysis and calculations showed that the size retention rate of the obtained coal-based graphene sample relative to the raw coal was 29.46%.

[0061] Example 2

[0062] The experiment was conducted in accordance with Example 1, except that the raw coal had a size of D50: 5 μm, D97: 11.82 μm, and the particle size was normally distributed.

[0063] The prepared graphene sample was analyzed by SEM and calculations showed that the size retention rate of the coal-based graphene sample relative to the raw coal was 17.28%. AFM analysis showed that the product had fewer than 5 layers and a thickness of 1.424 nm.

[0064] Example 3

[0065] The procedure was carried out in accordance with Example 1, except that the raw coal had a size of D50: 1.86 μm and D97: 5 μm, and the particle size was normally distributed; in step 6), a chemical reduction method was used instead of the high-temperature reduction in Example 1, and ascorbic acid and the coal-based graphene oxide slurry obtained in step 5) were mixed at a mass ratio of 10:1 and reacted at room temperature for 10 h; then the product was dried to obtain the coal-based graphene product.

[0066] The prepared graphene samples were subjected to SEM detection and calculation. The size retention rate of the obtained coal-based graphene samples relative to the raw coal was 20.06%.

[0067] AFM testing showed that the product has fewer than 10 layers and a thickness of 2.073 nm.

[0068] Comparative Example 1

[0069] The procedure was carried out in accordance with Example 1, except that the raw coal used was Shendong bituminous coal, with a size D97 of 74 μm, but not normally distributed.

[0070] The obtained graphene sample was tested and its size retention rate was 4%. AFM analysis of the sample showed that the product had more than 10 layers and a thickness of 8.37 nm.

[0071] Example 4 was carried out with reference to Example 1, except that: compared with Example 1, the raw material coal used was Shendong semi-coke, the size of the raw material coal D97: 74um, and the sample showed a normal distribution.

[0072] The obtained graphene sample was tested, and its size retention rate was 13.7%. AFM analysis of the sample showed that the product had 5 layers and a thickness of 1.68 nm. Due to the reduced coalification degree of the coal, the overall quality of the prepared graphene product was slightly lower than that of the coal-based graphene prepared in Example 1 using highly coalified coal (Taixi anthracite) as raw material; however, high-quality graphene could still be produced.

[0073] Comparative Example 2

[0074] The experiment was conducted in accordance with Example 1, except that the feeding rate of potassium permanganate was 0.3 g / min, compared to Example 1.

[0075] The obtained coal-based graphene sample was tested, and the size retention rate of the obtained coal-based graphene sample relative to the raw coal was 0.7%.

[0076] Comparative Example 3

[0077] The procedure was carried out in accordance with Example 1, except that, compared with Example 1, in step 5), the ultrasonic power was 2000W and the ultrasonic time was 8h during the ultrasonic treatment.

[0078] Experimental results: The size retention rate of the obtained coal-based graphene sample relative to the raw coal was 2.7%.

[0079] Comparative Example 4

[0080] The procedure was carried out in accordance with Example 1, except that, compared with Example 1, the heating rate was 25°C / min during the high-temperature reduction in step 6).

[0081] Experimental results: The size retention rate of the obtained coal-based graphene sample relative to the raw coal was 20.06%. AFM analysis showed that the product had far more than 10 layers, with a thickness of 8.586 nm.

[0082] It is readily understood that the above embodiments are merely illustrative examples for clear explanation and do not imply that the invention is limited thereto. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing size-controllable coal-based graphene, characterized in that, Includes the following steps: 1) The raw coal is graphitized to obtain graphitized material, and the degree of graphitization of the graphitized material is ≥77%; and the particle size of the raw coal must meet the following requirements: D97 is 5-75μm and exhibits a normal distribution; 2) The graphitized material is reacted in a mixed solution of sulfuric acid and sodium nitrate; 3) Potassium permanganate is continuously added to the reaction solution obtained in step 2) at a feeding rate of 0.05-0.15 g / min, while stirring continuously and controlling the reaction temperature to not exceed 20±5℃. After the potassium permanganate is added, the reaction is stirred for another 30-60 min; then the temperature is raised to 35-45℃ and the reaction is stirred for another 30 min. The mass ratio of the amount of potassium permanganate to the mass of the graphitized material in step 2) is 2-8:

1. 4) Heat the reaction solution obtained in step 3) to 90-100℃ and continue the reaction. The heating rate is controlled at 1.5-5℃ / min. Add deionized water while stirring continuously. The addition rate of deionized water is controlled at 2mL / min-20mL / min. React for 10-50min. Then add oxidant to consume the remaining potassium permanganate. 5) Perform multiple gravity separations on the material obtained in step 4), and wash with deionized water during the multiple gravity separations until the separated liquid is neutral; then dilute the separated material with deionized water and perform ultrasonic treatment, the ultrasonic power being 600-1000W, to obtain coal-based graphene oxide slurry. 6) The slurry is dried to obtain coal-based graphene oxide powder, and then heated to 600-800℃ at a heating rate of 2-15℃ / min for high-temperature reduction to obtain coal-based graphene; or, the slurry is chemically reduced to obtain coal-based graphene. The controllable size refers to the size retention rate being controllable between 5-30%; wherein, the size retention rate is calculated by the formula: the ratio of the in-plane size of the coal-based graphene to the size of the raw coal * 100%, where the in-plane size refers to the diameter of the circumscribed circle of the graphene, and the size of the raw coal is the D50 value of the raw coal.

2. The preparation method according to claim 1, characterized in that, In step 1), the graphitization treatment conditions include: heating the raw coal to 1000-1100℃ in the first stage and then holding it at that temperature for 1-2 hours; then heating it to 2800-3200℃ in the second stage and holding it at that temperature for 2.5-4 hours. The heating rate for the first stage is 10℃ / min, and the heating rate for the second stage is 5℃ / min.

3. The preparation method according to claim 1, characterized in that, In step 2), the amount of sulfuric acid used is 35-55 mL relative to 1 g of sodium nitrate; the sulfuric acid is concentrated sulfuric acid with a mass concentration of not less than 95%.

4. The preparation method according to claim 3, characterized in that, In step 2), the amount of the mixed solution used is 30-40 mL relative to 1 g of the graphitized material.

5. The preparation method according to claim 3, characterized in that, In step 2), the reaction is carried out at 10℃±5℃ with continuous stirring for 20-60 min.

6. The preparation method according to any one of claims 1-3, characterized in that, In step 3), the heating rate is controlled at 0.5-2℃ / min for the reaction at 35-45℃, and the reaction time is 2-4h.

7. The preparation method according to any one of claims 1-3, characterized in that, The volume ratio of the amount of deionized water added in step 4) to the volume ratio of the mixed solution of sulfuric acid and sodium nitrate in step 2) is 1:0.6-2.

8. The preparation method according to any one of claims 1-3, characterized in that, In step 4), the oxidant is hydrogen peroxide with a mass concentration of 30%; the ratio of the amount of hydrogen peroxide used to the potassium permanganate used in step 3) is 1:0.6-2, in ml / g; in step 4), the hydrogen peroxide is added at a rate of 2-8 ml / min.

9. The preparation method according to any one of claims 1-3, characterized in that, In step 5), the gravity separation is one or more of centrifugal separation, filtration, and static sedimentation.

10. The preparation method according to claim 9, characterized in that, When gravity separation is centrifugal separation, the lower slurry obtained by centrifugation is diluted with deionized water and centrifuged again. This centrifugation is repeated multiple times until the pH of the clear upper liquid obtained by centrifugation is neutral.

11. The preparation method according to claim 9, characterized in that, During gravity separation, specifically vacuum filtration, the filter cake is continuously washed with deionized water until the extracted filtrate is neutral.

12. The preparation method according to claim 9, characterized in that, When gravity separation is performed by settling, the supernatant obtained after settling is poured out, and then deionized water is added to the lower precipitate for settling again. This process is repeated multiple times until the supernatant obtained after settling is neutral.

13. The preparation method according to any one of claims 1-3, characterized in that, In step 5), the dilution with deionized water is used to dilute the material obtained by gravity separation to a slurry of 3-12 mg / ml. In step 5), the ultrasonication time is 6-10 hours; during ultrasonication, the temperature of the slurry should not exceed 50°C.

14. The preparation method according to any one of claims 1-3, characterized in that, In step 6), the conditions for the high-temperature reduction include: being carried out in an inert atmosphere and maintaining a constant temperature of 600-800℃ for 1.5-3 hours; Alternatively, in step 6), the conditions for the chemical reduction include: using ascorbic acid as a reducing agent, with the mass ratio of ascorbic acid to the slurry being (5-20):1; the reaction temperature for the chemical reduction is 10-30℃, and the reaction time is 5-15h.

15. The preparation method according to any one of claims 1-3, characterized in that, In step 1), the raw coal is obtained by steam pneumatic grinding; And / or, the raw coal is selected from one or more of anthracite, bituminous coal, and semi-coke.

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