An environmentally friendly method for preparing water-based few-layer graphene and graphene nanosheets

Abstract

This invention relates to the field of graphene material preparation and environmentally friendly chemical technology, and discloses an environmentally friendly preparation method for water-based few-layer graphene and graphene nanosheets, including the following steps: Step 1: Raw material screening, selecting natural graphite particles with a particle size ≤20μm and a purity ≥99.5%, wherein the particle size distribution of the graphite particles satisfies D10≥2μm, D50=8~12μm, and D90≤18μm. This invention achieves precise separation of unexfoliated coarse graphite particles, graphene nanosheets (GNP), and few-layer graphene (FLG) through two-stage continuous centrifugation. The purity of the FLG dispersion is ≥95%, and the purity of the GNP solid is ≥98%. The two products can be obtained simultaneously without separate preparation processes, making them suitable for different application scenarios. The FLG dispersion can be directly used in water-based coatings and biomedical carriers, while the GNP solid can be used in composite materials and battery electrodes. Moreover, the products exhibit excellent dispersion stability. The FLG dispersion shows no stratification after standing for 48 hours, the GNP water content is ≤1%, and the specific surface area reaches 300~600 m² / g, meeting the performance requirements of graphene materials in multiple fields.

Description

Technical Field

[0001] This invention relates to the fields of graphene material preparation and environmentally friendly chemical technology, specifically to an environmentally friendly preparation method for water-based few-layer graphene and graphene nanosheets. Background Technology

[0002] Traditional graphene preparation methods suffer from four major drawbacks, severely restricting their environmental friendliness, scalability, and product quality, making it difficult to meet the demands of industrial applications: Severe pollution and poor environmental performance: The mainstream preparation methods mostly rely on organic solvents (such as N-methylpyrrolidone and ethanol) as dispersion media. Organic solvents are easy to volatilize and pollute the air. Wastewater treatment costs are high (accounting for more than 30% of the total production cost). Moreover, residual solvents will affect the purity of the product and the safety of its application, which does not meet the environmental protection production standards.

[0003] Low exfoliation efficiency and uneven product quality: Traditional exfoliation often uses a single ball milling or ultrasonic process, which makes it difficult to effectively weaken the interlayer forces of graphite, resulting in incomplete exfoliation. The yield of few-layer graphene (2~5 layers) is ≤70%, and the proportion of multilayer impurities (≥15 layers) in the product exceeds 20%. In addition, the sheet size distribution is disordered (CV value ≥25%), and the number of layers fluctuates greatly, which cannot meet the requirements of high-precision applications for product uniformity.

[0004] Separation is difficult and multiple products cannot be prepared simultaneously: Traditional methods are difficult to obtain two high-value products, few-layer graphene and graphene nanosheets, at the same time. They require separate process design to prepare them separately, which doubles the equipment investment and production costs. Moreover, the separation process often uses single centrifugation or filtration, and it is difficult to completely remove the incompletely peeled graphite coarse particles, resulting in a product purity of ≤90%.

[0005] Insufficient scale and high oxidation risk: Laboratory-level methods (such as ultrasonic exfoliation) have low yields (daily output ≤1kg) and are difficult to scale up; industrial-level methods are prone to graphite oxidation (D / G peak ratio ≥0.5) due to uncontrolled process parameters, which destroys the graphene crystal structure, reduces the conductivity and mechanical properties of the product, and has poor dispersion stability (separation rate exceeds 30% after standing for 24 hours). Summary of the Invention

[0006] The purpose of this invention is to provide an environmentally friendly method for preparing water-based few-layer graphene and graphene nanosheets, so as to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: an environmentally friendly method for preparing water-based few-layer graphene and graphene nanosheets, comprising the following steps: Step 1: Raw material screening. Select natural graphite particles with a particle size ≤20μm and a purity ≥99.5%. The particle size distribution of the graphite particles should meet the following requirements: D10≥2μm, D50=8~12μm, and D90≤18μm. Step 2: Preparation of colloidal suspension. The materials are prepared according to the mass ratio of graphite particles:deionized water:dispersant = 1000:797:3, that is, 797 grams of deionized water and 3 grams of dispersant are added for every kilogram of graphite particles. The dispersant is selected from at least one of sodium deoxycholate (SDC), sodium dodecylbenzenesulfonate (SDBS), stearoylcholine (SC), BYK series additives, sodium carboxymethyl cellulose (CMC) or water-soluble polymer. The dispersant is added to the deionized water and stirred at 300~500 rpm for 10~20 min until completely dissolved. Then the graphite particles are slowly added and the stirring is continued for 30~60 min to form a stable colloidal suspension. The absolute value of the zeta potential of the suspension is ≥30mV and there is no obvious stratification or sedimentation after standing for 24 h. Step 3: Shear emulsification pretreatment. The colloidal suspension is fed into a high-shear emulsifier. The shearing speed is set to 8000~12000 rpm and the shearing time is 20~40 min. During the shearing process, the system temperature is controlled to be ≤30℃ through a cooling water bath. This allows the graphite particles to be initially dispersed under the action of shear force, forming a colloidal solution in which the graphite particles are stably suspended. At this time, the average particle size of the graphite particles in the colloidal solution is reduced to 5~10 μm. Step 4: Micron-level wetting in sand milling. The sheared colloidal solution is introduced into a horizontal sand mill. The grinding media of the sand mill is zirconia beads. The grinding chamber pressure is 0.2~0.4MPa, the circulation flow rate is 10~20L / h, and the grinding speed is set to 2000~3000rpm and the grinding time is 30~120min. Step 5: Mixing and wetting the nanoparticles. Transfer the sand-milled colloidal solution into a high-speed mixer. Set the mixing speed to 8000 rpm and the mixing time to 4 hours. Nitrogen gas is continuously introduced during the mixing process. The dispersion uniformity of the colloidal solution is characterized by the coefficient of variation (CV), with a CV value ≤15%. Step 6: High-pressure emulsification. The mixed colloidal solution is fed into a high-pressure homogenizer. The working pressure is set to 1500~2000 bar, and 30~40 high-pressure homogenization cycles are performed. The feed rate for each cycle is 5~10 L / h. Under high pressure, the graphite layers are exfoliated to form a graphene slurry composed of few-layer graphene and graphene nanosheets. The solid content of the slurry is 10%~15%. Step 7: Continuous centrifugation separation. A horizontal continuous centrifuge is used to perform two-stage centrifugation separation on the graphene slurry. The first stage centrifugation is set at a speed of 500 rpm and a centrifugation time of 20-30 min to allow the incompletely peeled coarse graphite particles to settle and separate, and the supernatant (containing FLG and GNP) is collected. The second stage centrifugation is set at a speed of 2500 rpm and a centrifugation time of 20-30 min to allow the graphene nanosheets (GNP) to settle, and the upper liquid is a dispersion of few-layer graphene (FLG), thus achieving effective separation of the two products. Step 8: Vacuum freeze drying. The solid graphene nanosheets obtained from the second-stage centrifugation are transferred into a vacuum freeze dryer. The cold trap temperature is set to -50 to -40°C, the vacuum degree to 10 to 20 Pa, and the drying time to 24 to 48 hours. A solid GNP product with a moisture content of ≤1% is obtained. The specific surface area of ​​this product is 300 to 600 m² / g, and the bulk density is 0.05 to 0.1 g / cm³.

[0008] Preferably, in the raw material screening step, the purity of the graphite particles is characterized by a combination of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), with a carbon content ≥99.5%, a total impurity element content (such as silicon, iron, and calcium) ≤0.5%, and the interlayer spacing of the graphite particles is 0.335~0.340 nm, with moderate interlayer forces, which is conducive to subsequent exfoliation. In the preparation of the colloidal suspension, the choice of dispersant can be flexibly adjusted according to the application scenario of the product. When used in the biomedical field, sodium deoxycholate (SDC) or stearoylcholine (SC) with good biocompatibility are preferred; when used in the composite material field, SDBS or BYK series additives with excellent dispersion stability are preferred; when used in the water-based coating field, CMC or water-soluble polymers with both dispersing and thickening effects are preferred. The stability of the colloidal suspension is evaluated by the sedimentation rate equation, the sedimentation rate formula is: .

[0009] Preferably, in the shear emulsification pretreatment step, the high-shear emulsifier adopts a stator-rotor structure, with a stator-rotor gap of 0.1~0.3mm, and the formula for the shear force generated during the shearing process is: By adjusting the shearing speed, the shearing rate can be achieved. The shear force generated is sufficient to break the initial agglomeration of graphite particles, while avoiding excessive shearing that could damage the graphite particle structure. The particle size distribution of the colloidal solution after shearing was monitored by dynamic light scattering (DLS), requiring D90≤12μm and D50=6~8μm; the temperature of the cooling water bath during shearing was precisely controlled at 25~30℃ by a thermostatic circulator, and the pH value of the solution was maintained at 6~8.

[0010] Preferably, in the micron-level wetting step of the sand mill, the grinding media filling rate of the sand mill is 70%~80%, the sphericity of the grinding media is ≥0.95, and the hardness is ≥HRA85. During the grinding process, the particle size change of the colloidal solution is monitored in real time by an online particle size analyzer. When the average particle size drops to 1~5μm and remains stable for 10min, the sand milling is stopped. The maximum grinding time does not exceed 2h to prevent excessive grinding from causing the structural collapse of the graphite particles. The micron-level wetting degree of the colloidal solution after sand milling is characterized by a contact angle test, and the contact angle between the graphite particles and water is ≤30°.

[0011] Preferably, the mixed nano-wetting step specifically includes: The high-speed mixing device features a twin-shaft stirring structure with helical impellers. The mixing speed is strictly controlled at 8000 rpm, and the mixing time is 4 hours. This parameter combination was optimized through orthogonal experiments, enabling the dispersant adsorption capacity on the graphite particle surface to reach over 90% of the saturated adsorption capacity. An electrostatic repulsion layer is formed on the graphite surface by a saturated adsorbed dispersant, which further promotes nanoscale exfoliation. The nitrogen gas introduced during the mixing process has a purity of ≥99.99% to prevent the oxygen in the solution from reacting with the graphite particles in an oxidation reaction. The degree of oxidation of the mixed colloidal solution is characterized by the D / G peak ratio of Raman spectroscopy, with a D / G peak ratio ≤0.2. The particle size variation coefficient (CV) of the mixed colloidal solution is tested by a laser particle size analyzer, with a CV value ≤15%.

[0012] Preferably, the high-pressure emulsification step specifically includes: The high-pressure homogenizer adopts a plunger structure, and the working pressure can be continuously adjusted within the range of 1500~2000 bar, achieving precise separation between graphite layers through pressure regulation; When the pressure is below 1500 bar, the peeling efficiency decreases significantly, and the FLG yield is ≤70%; when the pressure is above 2000 bar, it is easy to cause graphene sheets to break, and the product defect rate increases. The number of high-pressure homogenization cycles was set to 30-40, and the number of cycles was determined by the stripping efficiency model. Calculations show that when When the coefficient of friction is 30, the peeling efficiency is ≥90%. After more than 40 cycles, the improvement in peeling efficiency is ≤5%, resulting in poor economic benefits. During high-pressure emulsification, the material temperature is controlled to ≤40℃ by a cooling jacket to avoid the heat generated by high pressure causing dispersant decomposition or graphene oxidation. In the emulsified graphene slurry, the number of layers of few-layer graphene (FLG) is characterized by transmission electron microscopy (TEM), with 2~5 layers accounting for ≥85% and sheet diameter of 500~2000nm; the number of layers of graphene nanosheets (GNP) is 5~10 layers accounting for ≥90% and sheet diameter of 1~5μm. The absolute value of the zeta potential of the slurry is ≥40mV, and there is no stratification after standing for 48h.

[0013] Preferably, the continuous centrifugation separation step specifically includes: A horizontal spiral sedimentation centrifuge was used, with the first stage centrifugation speed strictly controlled at 500 rpm and the centrifugation time at 20-30 min. This combination of parameters ensures that incompletely exfoliated coarse graphite particles (particle size ≥10 μm) settle completely, with a sedimentation efficiency ≥99%. The mass ratio of FLG to GNP in the supernatant after the first-stage centrifugation is 3:7 to 4:6. Separation is achieved through a second-stage centrifugation (2500 rpm, 20-30 min). The sedimentation rate of GNP at this speed is... The calculations show that the settling velocity of GNP is 5 to 8 times that of FLG. During centrifugation, the differential speed of the centrifuge is controlled at 5~10 rpm to ensure continuous discharge of solid particles. The purity of the FLG dispersion after centrifugation is ≥95%, and the purity of the GNP solid is ≥98%. The small amount of wastewater generated by centrifugation can be treated by an ultrafiltration membrane recovery device, and the recovered dispersant can be recycled.

[0014] Preferably, in the vacuum freeze-drying step, the drying process of the graphene nanosheets is divided into three stages: freezing, sublimation drying, and desorption drying, specifically: During the freezing stage, the GNP solid is frozen at -50°C for 2-4 hours until it is completely frozen into a solid state. During the sublimation drying stage, the vacuum level is set to 10~20Pa and the cold trap temperature is -50℃, and the process is continued for 20~30h, so that solid water is directly sublimated into water vapor and captured by the cold trap. During the drying stage, the temperature is raised to 20~30℃ and drying continues for 4~8 hours to remove adsorbed water inside the GNP, finally obtaining a solid GNP product with a moisture content of ≤1%.

[0015] This invention provides an environmentally friendly method for preparing water-based few-layer graphene and graphene nanosheets. It has the following beneficial effects: 1. This invention uses deionized water as the sole dispersion medium in the preparation process, without using any organic solvents, and without toxic or harmful gas emissions, thus eliminating pollution at the source; the small amount of wastewater generated by centrifugation is treated by ultrafiltration membrane, resulting in a dispersant recovery rate of ≥80%, a water resource recycling rate of ≥90%, and a 70% reduction in environmental treatment costs, which meets the national green chemical production standards.

[0016] 2. This invention employs a streamlined process of "shear emulsification-sand milling-high pressure emulsification-continuous centrifugation," with precise and controllable parameters at each stage, enabling 24-hour continuous production. The daily output of a single production line is ≥50kg, which is more than 50 times higher than the capacity of traditional laboratory methods. Furthermore, the process is highly automated, requiring no frequent manual intervention, and reducing the cost of large-scale production by 40%, thus solving the bottleneck of "low capacity and high cost" in the industrial production of graphene.

[0017] 3. This invention employs a three-stage progressive exfoliation process involving "shear emulsification pretreatment → sand milling and micron-level wetting → high-pressure emulsification," which gradually weakens the interlayer forces of graphite. In few-layer graphene (FLG), the proportion of 2-5 layers is ≥85%, and in graphene nanosheets (GNP), the proportion of 5-10 layers is ≥90%, improving the uniformity of the number of layers by 60% compared to traditional methods. The product has a concentrated sheet size distribution and a particle size variation coefficient (CV) ≤15%, avoiding the problems of "disordered number of layers and uneven sheet size" in traditional methods.

[0018] 4. This invention achieves precise separation of unexfoliated coarse graphite particles, graphene nanosheets (GNP), and few-layer graphene (FLG) through two-stage continuous centrifugation. The purity of the FLG dispersion is ≥95%, and the purity of the GNP solid is ≥98%. The two products can be obtained simultaneously without separate preparation processes, making them suitable for different application scenarios. The FLG dispersion can be directly used in water-based coatings and biomedical carriers, while the GNP solid can be used in composite materials and battery electrodes. Moreover, the products exhibit excellent dispersion stability. The FLG dispersion shows no stratification after standing for 48 hours, the GNP water content is ≤1%, and the specific surface area reaches 300~600m² / g, meeting the performance requirements of graphene materials in multiple fields. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall process flow of the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention. Example 1

[0022] A preferred embodiment of the environmentally friendly preparation method of water-based few-layer graphene and graphene nanosheets provided by the present invention is as follows: Figure 1The method for environmentally friendly preparation of water-based few-layer graphene (FLG) and graphene nanosheets (GNP) includes eight core steps: raw material screening, colloidal suspension preparation, shear emulsification pretreatment, sand milling and micron-level wetting, mixed nano-level wetting, high-pressure emulsification, continuous centrifugation, and vacuum freeze-drying. The entire process uses deionized water as the sole dispersion medium and does not employ any organic solvents, achieving environmentally friendly, large-scale, and high-quality co-preparation of FLG and GNP with a yield ≥90%. In the raw material screening step, natural graphite particles with a particle size ≤20μm and a purity ≥99.5% are selected. The particle size distribution of the graphite particles meets the requirements of D10≥2μm, D50=8~12μm, and D90≤18μm (tested by a laser particle size analyzer) to ensure the uniformity of the subsequent exfoliation process. In the preparation of the colloidal suspension, the materials are prepared according to the mass ratio of graphite particles:deionized water:dispersant = 1000:797:3, that is, 797 grams of deionized water and 3 grams of dispersant are added for every kilogram of graphite particles. The dispersant is selected from at least one of sodium deoxycholate (SDC), sodium dodecylbenzenesulfonate (SDBS), stearoylcholine (SC), BYK series additives, sodium carboxymethyl cellulose (CMC) or water-soluble polymers. The dispersant is added to the deionized water and stirred at 300~500 rpm for 10~20 min until completely dissolved. Then the graphite particles are slowly added and the stirring is continued for 30~60 min to form a stable colloidal suspension. The absolute value of the zeta potential of the suspension is ≥30mV (tested by a zeta potential meter), and there is no obvious stratification or sedimentation after standing for 24 h. In the shear emulsification pretreatment step, the colloidal suspension is fed into a high-shear emulsifier, and the shearing speed is set to 8000~12000 rpm and the shearing time is 20~40 min. During the shearing process, the system temperature is controlled to be ≤30℃ by a cooling water bath, so that the graphite particles are initially dispersed under the action of shear force to form a colloidal solution in which the graphite particles are stably suspended. At this time, the average particle size of the graphite particles in the colloidal solution is reduced to 5~10 μm. In the micron-wetting step of sand milling, the sheared colloidal solution is introduced into a horizontal sand mill. The grinding media of the sand mill is zirconia beads (particle size 0.1~0.3mm), the grinding chamber pressure is 0.2~0.4MPa, the circulation flow rate is 10~20L / h, the grinding speed is set to 2000~3000rpm, and the grinding time is 30~120min (not exceeding 2h). Through the impact and friction of the grinding media, the graphite particles are further refined to the micron level of 1~5μm, realizing the micron-wetting of graphite particles and improving the efficiency of subsequent nano-exfoliation. In the nano-wetting step, the colloidal solution after sand milling is transferred to a high-speed mixer. The mixing speed is set to 8000 rpm and the mixing time is 4 hours. Nitrogen gas (flow rate 5~10 L / min) is continuously introduced during the mixing process to prevent oxidation of the solution. Through high-speed shearing and dispersion, the surface of the graphite particles is fully wetted and nanoscale dispersion units are formed. The dispersion uniformity of the colloidal solution is characterized by the coefficient of variation of particle size (CV), and the CV value is ≤15%. In the high-pressure emulsification step, the mixed colloidal solution is fed into a high-pressure homogenizer, with a working pressure of 1500~2000 bar, and 30~40 high-pressure homogenization cycles are performed. The feed rate for each cycle is 5~10 L / h. Under high pressure, the graphite layers are exfoliated to form a graphene slurry composed of few-layer graphene (FLG, 2~5 layers, 500~2000 nm in diameter) and graphene nanosheets (GNP, 5~10 layers, 1~5 μm in diameter). The solid content of the slurry is 10%~15%. In the continuous centrifugation separation step, a horizontal continuous centrifuge is used to perform two-stage centrifugation separation on the graphene slurry. The first stage centrifugation is set at a speed of 500 rpm and a centrifugation time of 20-30 min to allow the incompletely peeled coarse graphite particles to settle and separate, and the supernatant (containing FLG and GNP) is collected. The second stage centrifugation is set at a speed of 2500 rpm and a centrifugation time of 20-30 min to allow the graphene nanosheets (GNP) to settle, and the upper liquid is a dispersion of few-layer graphene (FLG), thus achieving effective separation of the two products. In the vacuum freeze-drying step, the solid graphene nanosheets obtained from the second-stage centrifugation are transferred into a vacuum freeze dryer. The cold trap temperature is set to -50 to -40°C, the vacuum degree to 10 to 20 Pa, and the drying time to 24 to 48 hours, resulting in a solid GNP product with a moisture content of ≤1%. The specific surface area of ​​this product is 300 to 600 m² / g, and the bulk density is 0.05 to 0.1 g / cm³. Example 2

[0023] Please see Figure 1 Furthermore, based on Example 1, the following was obtained: In the raw material screening step, the purity of the graphite particles was characterized by a combination of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), with a carbon content ≥99.5%, a total impurity element content (such as silicon, iron, and calcium) ≤0.5%, and an interlayer spacing of 0.335~0.340 nm (2 nm in the XRD pattern). =26.5° Calculated interlayer spacing of graphite (002) peak), the interlayer force is moderate, which is conducive to subsequent exfoliation; In the preparation of colloidal suspension, the choice of dispersant can be flexibly adjusted according to the application scenario of the product. When used in the biomedical field, sodium deoxycholate (SDC) or stearoylcholine (SC) with good biocompatibility are preferred; when used in the composite material field, SDBS or BYK series additives with excellent dispersion stability are preferred; when used in the water-based coating field, CMC or water-soluble polymers with both dispersing and thickening effects are preferred; The stability of colloidal suspension is evaluated by the sedimentation rate equation, and the sedimentation rate (in Where is the radius of the graphite particle. The density of graphite, The density of water, It is the acceleration due to gravity. (The solution viscosity is used to control the graphite particle radius and solution viscosity, so that the sedimentation rate is ≤0.1mm / h, thus ensuring the stability of the suspension in subsequent processes.)

[0024] In the shear emulsification pretreatment step, the high-shear emulsifier adopts a stator-rotor structure with a stator-rotor gap of 0.1~0.3mm. The shear force generated during the shearing process... (in The dynamic viscosity of the solution. For shear rate, (This refers to the shearing area), and the shearing rate is achieved by adjusting the shearing speed. The shear force generated is sufficient to break the initial agglomeration of graphite particles, while avoiding excessive shearing that could damage the graphite particle structure. The particle size distribution of the colloidal solution after shearing is monitored by a dynamic light scattering (DLS) instrument, requiring D90≤12μm and D50=6~8μm to ensure uniform dispersion of graphite particles. During the shearing process, the temperature of the cooling water bath is precisely controlled at 25~30℃ by a thermostatic circulator to prevent excessive temperature from causing dispersant failure or graphite particle oxidation. The pH value of the solution is maintained at 6~8 (monitored by an online pH meter) to avoid extreme pH values ​​affecting dispersion stability.

[0025] In the micron-level wetting step of sand milling, the grinding media filling rate of the sand mill is 70%~80%, the sphericity of the grinding media is ≥0.95, and the hardness is ≥HRA85, ensuring that the wear of the media during the grinding process is ≤0.1% / h, avoiding the introduction of impurities. During the grinding process, the particle size change of the colloidal solution is monitored in real time by an online particle size analyzer. When the average particle size drops to 1~5μm and remains stable for 10min, the sand milling is stopped. The maximum grinding time does not exceed 2h to prevent over-grinding from causing the structural collapse of graphite particles. The micron-level wetting degree of the colloidal solution after sand milling is characterized by a contact angle test. The contact angle between graphite particles and water is ≤30°, ensuring that the dispersant and water can fully penetrate into the interlayer of graphite during the subsequent nano-wetting process, weakening the interlayer van der Waals forces. In addition, the circulation system of the sand mill is made of titanium alloy to avoid the introduction of metal impurities during the grinding process and ensure product purity.

[0026] In the nano-wetting step, the high-speed mixing device is a biaxial stirring structure with a helical impeller design to ensure strong turbulence in the colloidal solution during mixing, thus improving dispersion uniformity. The mixing speed is strictly controlled at 8000 rpm, and the mixing time is 4 hours. This parameter combination was optimized through orthogonal experiments, enabling the dispersant adsorption on the graphite particle surface to reach over 90% of the saturated adsorption capacity. (in This represents the saturation adsorption capacity. This refers to the dispersant concentration. The dispersant, which is the adsorption equilibrium constant, forms an electrostatic repulsion layer on the graphite surface through saturated adsorption, further promoting nanoscale exfoliation. The nitrogen gas introduced during the mixing process has a purity of ≥99.99% to prevent the oxygen in the solution from reacting with the graphite particles in an oxidation reaction. The degree of oxidation of the mixed colloidal solution is characterized by the D / G peak ratio of Raman spectroscopy. The D / G peak ratio is ≤0.2, ensuring that the crystal structure of the graphite is not destroyed. The particle size variation coefficient (CV) of the mixed colloidal solution is tested by a laser particle size analyzer. The CV value is ≤15%, indicating that the solution is uniformly dispersed, laying the foundation for efficient exfoliation by subsequent high-pressure emulsification.

[0027] In the high-pressure emulsification step, the high-pressure homogenizer adopts a plunger-type structure, and the working pressure can be continuously adjusted within the range of 1500~2000 bar. Precise exfoliation between graphene layers is achieved through pressure regulation. When the pressure is below 1500 bar, the exfoliation efficiency decreases significantly, and the FLG yield is ≤70%. When the pressure is above 2000 bar, it easily leads to graphene sheet breakage, increasing the product defect rate. The number of high-pressure homogenization cycles is set to 30~40 times, determined by the exfoliation efficiency model. (in For peeling efficiency, The rate constant is Calculated as the number of iterations, when At a temperature of 30°C, the peeling efficiency is ≥90%. After more than 40 cycles, the peeling efficiency improvement is ≤5%, resulting in poor economic benefits. During high-pressure emulsification, the material temperature is controlled to ≤40°C by a cooling jacket to prevent the heat generated by high pressure from causing dispersant decomposition or graphene oxidation. In the emulsified graphene slurry, the number of layers of few-layer graphene (FLG) is characterized by transmission electron microscopy (TEM), with 2~5 layers accounting for ≥85% and sheet diameter of 500~2000nm. The number of layers of graphene nanosheets (GNP) is 5~10 layers accounting for ≥90% and sheet diameter of 1~5μm. The absolute value of the zeta potential of the slurry is ≥40mV, and there is no delamination after standing for 48h.

[0028] In the continuous centrifugation separation step, a horizontal spiral sedimentation centrifuge is used. The first-stage centrifugation speed is strictly controlled at 500 rpm, and the centrifugation time is 20-30 min. This parameter combination ensures that incompletely exfoliated coarse graphite particles (particle size ≥10 μm) settle completely, with a sedimentation efficiency ≥99%, avoiding the influence of coarse particles on the purity of subsequent products. In the supernatant after the first-stage centrifugation, the mass ratio of FLG to GNP is 3:7-4:6. Separation is achieved through a second-stage centrifugation (speed 2500 rpm, time 20-30 min). At this speed, the sedimentation rate of GNP is... (in GNP particle size, GNP density, The density of water, Angular velocity, Where is the centrifugal radius, (Based on solution viscosity), the sedimentation rate of GNP is calculated to be 5-8 times that of FLG, thus enabling effective separation. During centrifugation, the differential speed of the centrifuge is controlled at 5-10 rpm to ensure continuous discharge of solid particles. The purity of the FLG dispersion after centrifugation is ≥95% (excluding particles with a diameter ≥1μm), and the purity of the GNP solid is ≥98% (excluding coarse particles with a diameter ≥10μm). The small amount of wastewater generated during centrifugation (mainly containing unadsorbed dispersant) can be treated by an ultrafiltration membrane recovery device. The recovered dispersant can be recycled, and the water resource recovery rate is ≥80%, meeting environmental protection requirements.

[0029] In the vacuum freeze-drying process, the drying of graphene nanosheets is divided into three stages: freezing, sublimation drying, and desorption drying. In the freezing stage, the GNP solid is frozen at -50℃ for 2–4 hours to completely solidify it. In the sublimation drying stage, a vacuum of 10–20 Pa and a cold trap temperature of -50℃ are maintained for 20–30 hours, allowing the solid water to directly sublimate into water vapor and be captured by the cold trap. In the desorption drying stage, the temperature is raised to 20–30℃, and drying continues for 4–8 hours to remove adsorbed water from the GNP, ultimately obtaining a GNP solid product with a water content ≤1%. The specific surface area of ​​the dried GNP product, measured by the BET method, is 300–600 m² / g, and the bulk density is 0.0. The particle size distribution is 5~0.1 g / cm³, and the interlayer spacing is 0.340~0.345 nm, which is slightly larger than that of the original graphite, indicating good exfoliation effect. The liquid form of FLG dispersion can be directly used in water-based coatings, inks, composite materials and other fields, with a solid content of 5%~8% and a stability of ≥6 months. The solid form of GNP product can be further ground to the required particle size and used in battery electrodes, thermal conductive materials and other fields. The entire preparation process does not use organic solvents, has no toxic or harmful gas emissions, and the wastewater can be recycled after treatment, which meets the requirements of environmental protection production. Moreover, the production process can be carried out continuously, with a daily output of ≥50 kg per production line, which meets the needs of large-scale mass production.

[0030] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0031] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An environmentally friendly method for preparing water-based few-layer graphene and graphene nanosheets, characterized in that, Includes the following steps: Step 1: Raw material screening. Select natural graphite particles with a particle size ≤20μm and a purity ≥99.5%. The particle size distribution of the graphite particles should meet the following requirements: D10≥2μm, D50=8~12μm, and D90≤18μm. Step 2: Preparation of colloidal suspension. The materials are prepared according to the mass ratio of graphite particles:deionized water:dispersant = 1000:797:3, that is, 797 grams of deionized water and 3 grams of dispersant are added for every kilogram of graphite particles. The dispersant is selected from at least one of sodium deoxycholate (SDC), sodium dodecylbenzenesulfonate (SDBS), stearoylcholine (SC), BYK series additives, sodium carboxymethyl cellulose (CMC) or water-soluble polymer. The dispersant is added to the deionized water and stirred at 300~500 rpm for 10~20 min until completely dissolved. Then the graphite particles are slowly added and the stirring is continued for 30~60 min to form a stable colloidal suspension. The absolute value of the zeta potential of the suspension is ≥30mV and there is no obvious stratification or sedimentation after standing for 24 h. Step 3: Shear emulsification pretreatment. The colloidal suspension is fed into a high-shear emulsifier. The shearing speed is set to 8000~12000 rpm and the shearing time is 20~40 min. During the shearing process, the system temperature is controlled to be ≤30℃ through a cooling water bath. This allows the graphite particles to be initially dispersed under the action of shear force, forming a colloidal solution in which the graphite particles are stably suspended. At this time, the average particle size of the graphite particles in the colloidal solution is reduced to 5~10 μm. Step 4: Micron-level wetting in sand milling. The sheared colloidal solution is introduced into a horizontal sand mill. The grinding media of the sand mill is zirconia beads. The grinding chamber pressure is 0.2~0.4MPa, the circulation flow rate is 10~20L / h, and the grinding speed is set to 2000~3000rpm and the grinding time is 30~120min. Step 5: Mixing and wetting the nanoparticles. Transfer the sand-milled colloidal solution into a high-speed mixer. Set the mixing speed to 8000 rpm and the mixing time to 4 hours. Nitrogen gas is continuously introduced during the mixing process. The dispersion uniformity of the colloidal solution is characterized by the coefficient of variation (CV), with a CV value ≤15%. Step 6: High-pressure emulsification. The mixed colloidal solution is fed into a high-pressure homogenizer. The working pressure is set to 1500~2000 bar, and 30~40 high-pressure homogenization cycles are performed. The feed rate for each cycle is 5~10 L / h. Under high pressure, the graphite layers are exfoliated to form a graphene slurry composed of few-layer graphene and graphene nanosheets. The solid content of the slurry is 10%~15%. Step 7: Continuous centrifugation separation. A horizontal continuous centrifuge is used to perform two-stage centrifugation separation on the graphene slurry. The first stage centrifugation is set at a speed of 500 rpm and a centrifugation time of 20-30 min to allow the incompletely peeled coarse graphite particles to settle and separate, and the supernatant (containing FLG and GNP) is collected. The second stage centrifugation is set at a speed of 2500 rpm and a centrifugation time of 20-30 min to allow the graphene nanosheets (GNP) to settle, and the upper liquid is a dispersion of few-layer graphene (FLG), thus achieving effective separation of the two products. Step 8: Vacuum freeze drying. The solid graphene nanosheets obtained from the second-stage centrifugation are transferred into a vacuum freeze dryer. The cold trap temperature is set to -50 to -40°C, the vacuum degree to 10 to 20 Pa, and the drying time to 24 to 48 hours. A solid GNP product with a moisture content of ≤1% is obtained. The specific surface area of ​​this product is 300 to 600 m² / g, and the bulk density is 0.05 to 0.1 g / cm³.

2. The environmentally friendly preparation method of water-based few-layer graphene and graphene nanosheets according to claim 1, characterized in that, In the raw material screening step, the purity of the graphite particles is characterized by a combination of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The carbon content is ≥99.5%, the total content of impurity elements (such as silicon, iron, and calcium) is ≤0.5%, and the interlayer spacing of the graphite particles is 0.335~0.340nm, with moderate interlayer forces, which is conducive to subsequent exfoliation. In the preparation of the colloidal suspension, the choice of dispersant can be flexibly adjusted according to the application scenario of the product. When used in the biomedical field, sodium deoxycholate (SDC) or stearoylcholine (SC) with good biocompatibility are preferred; when used in the composite material field, SDBS or BYK series additives with excellent dispersion stability are preferred; when used in the water-based coating field, CMC or water-soluble polymers with both dispersing and thickening effects are preferred. The stability of the colloidal suspension is evaluated by the sedimentation rate equation, the sedimentation rate formula is: .

3. The environmentally friendly preparation method of water-based few-layer graphene and graphene nanosheets according to claim 1, characterized in that, In the shear emulsification pretreatment step, the high-shear emulsifier adopts a stator-rotor structure, with a stator-rotor gap of 0.1~0.3mm. The formula for the shear force generated during the shearing process is: The shearing rate was achieved by adjusting the shearing speed to 1×10⁻⁶. 5 ~5×10 5 s -1 The shear force generated is sufficient to break the initial agglomeration of graphite particles, while avoiding excessive shearing that could damage the graphite particle structure. The particle size distribution of the colloidal solution after shearing was monitored by dynamic light scattering (DLS), requiring D90≤12μm and D50=6~8μm; the temperature of the cooling water bath during shearing was precisely controlled at 25~30℃ by a thermostatic circulator, and the pH value of the solution was maintained at 6~8.

4. The environmentally friendly preparation method of water-based few-layer graphene and graphene nanosheets according to claim 1, characterized in that, In the micron-level wetting step of the sand mill, the grinding media filling rate of the sand mill is 70%~80%, the sphericity of the grinding media is ≥0.95, and the hardness is ≥HRA85. During the grinding process, the particle size change of the colloidal solution is monitored in real time by an online particle size analyzer. When the average particle size drops to 1~5μm and remains stable for 10min, the sand milling is stopped. The maximum grinding time does not exceed 2h to prevent over-grinding from causing the structural collapse of graphite particles. The micron-level wetting degree of the colloidal solution after sand milling is characterized by a contact angle test, and the contact angle between graphite particles and water is ≤30°.

5. The environmentally friendly preparation method of water-based few-layer graphene and graphene nanosheets according to claim 1, characterized in that, The specific steps of the mixed nanowetting process are as follows: The high-speed mixing device features a twin-shaft stirring structure with helical impellers. The mixing speed is strictly controlled at 8000 rpm, and the mixing time is 4 hours. This parameter combination was optimized through orthogonal experiments, enabling the dispersant adsorption capacity on the graphite particle surface to reach over 90% of the saturated adsorption capacity. An electrostatic repulsion layer is formed on the graphite surface by a saturated adsorbed dispersant, which further promotes nanoscale exfoliation. The nitrogen gas introduced during the mixing process has a purity of ≥99.99% to prevent the oxygen in the solution from reacting with the graphite particles in an oxidation reaction. The degree of oxidation of the mixed colloidal solution is characterized by the D / G peak ratio of Raman spectroscopy, with a D / G peak ratio ≤0.

2. The particle size variation coefficient (CV) of the mixed colloidal solution is tested by a laser particle size analyzer, with a CV value ≤15%.

6. The environmentally friendly preparation method of water-based few-layer graphene and graphene nanosheets according to claim 1, characterized in that, The high-pressure emulsification step specifically includes: The high-pressure homogenizer adopts a plunger structure, and the working pressure can be continuously adjusted within the range of 1500~2000 bar, achieving precise separation between graphite layers through pressure regulation; When the pressure is below 1500 bar, the peeling efficiency decreases significantly, and the FLG yield is ≤70%; when the pressure is above 2000 bar, it is easy to cause graphene sheets to break, and the product defect rate increases. The number of high-pressure homogenization cycles was set to 30-40, and the number of cycles was determined by the stripping efficiency model. Calculations show that when When the coefficient of friction is 30, the peeling efficiency is ≥90%. After more than 40 cycles, the improvement in peeling efficiency is ≤5%, resulting in poor economic benefits. During high-pressure emulsification, the material temperature is controlled to ≤40℃ by a cooling jacket to avoid the heat generated by high pressure causing dispersant decomposition or graphene oxidation. In the emulsified graphene slurry, the number of layers of few-layer graphene (FLG) is characterized by transmission electron microscopy (TEM), with 2~5 layers accounting for ≥85% and sheet diameter of 500~2000nm; the number of layers of graphene nanosheets (GNP) is 5~10 layers accounting for ≥90% and sheet diameter of 1~5μm. The absolute value of the zeta potential of the slurry is ≥40mV, and there is no stratification after standing for 48h.

7. The environmentally friendly preparation method of water-based few-layer graphene and graphene nanosheets according to claim 1, characterized in that, The continuous centrifugation separation step specifically includes: A horizontal spiral sedimentation centrifuge was used, with the first stage centrifugation speed strictly controlled at 500 rpm and the centrifugation time at 20-30 min. This combination of parameters ensures that incompletely exfoliated coarse graphite particles (particle size ≥10 μm) settle completely, with a sedimentation efficiency ≥99%. The mass ratio of FLG to GNP in the supernatant after the first-stage centrifugation is 3:7 to 4:

6. Separation is achieved through a second-stage centrifugation (2500 rpm, 20-30 min). The sedimentation rate of GNP at this speed is... The calculations show that the settling velocity of GNP is 5 to 8 times that of FLG. During centrifugation, the differential speed of the centrifuge is controlled at 5~10 rpm to ensure continuous discharge of solid particles. The purity of the FLG dispersion after centrifugation is ≥95%, and the purity of the GNP solid is ≥98%. The small amount of wastewater generated by centrifugation can be treated by an ultrafiltration membrane recovery device, and the recovered dispersant can be recycled.

8. The environmentally friendly preparation method of water-based few-layer graphene and graphene nanosheets according to claim 1, characterized in that, In the vacuum freeze-drying step, the drying process of graphene nanosheets is divided into three stages: freezing, sublimation drying, and desorption drying, specifically: During the freezing stage, the GNP solid is frozen at -50°C for 2-4 hours until it is completely frozen into a solid state. During the sublimation drying stage, the vacuum level is set to 10~20Pa and the cold trap temperature is -50℃, and the process is continued for 20~30h, so that solid water is directly sublimated into water vapor and captured by the cold trap. During the drying stage, the temperature is raised to 20~30℃ and drying continues for 4~8 hours to remove adsorbed water inside the GNP, finally obtaining a solid GNP product with a moisture content of ≤1%.

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