Application of eutectic solvent in graphite purification and graphite purification method

Abstract

The invention relates to the field of deep processing of graphite materials, in particular to application of a deep eutectic solvent in graphite purification and a graphite purification method. According to the preparation method, the eutectic solvent and the graphite oxide are subjected to mixed reaction, and eutectic solvent molecules and oxygen-containing functional groups on the surface of the graphite oxide form strong hydrogen-bond interaction through rich hydrogen bond sites in the reaction process, so that on one hand, a graphite oxide sheet layer can be further stripped; on the other hand, impurities such as metal cations (such as K, Na, Ca and Mg) embedded between graphite layers can be effectively complexed and extracted, a soluble complex is formed, and therefore efficient separation of the impurities and a graphite framework is achieved. The fixed carbon content of the graphite material obtained through the technical means can reach 98% or above, and preferably, when hydrogen bond donors in the eutectic solvent are ethylene glycol and urea, the fixed carbon content can reach 99% or above.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of deep processing of graphite materials, and in particular to application of a eutectic solvent in graphite purification and a graphite purification method. BACKGROUND

[0002] Graphite, as an important mineral resource, is widely used in high-tech fields such as metallurgy, chemical industry, machinery, aerospace, new energy (such as lithium ion battery) and nuclear industry due to its excellent electrical conductivity, thermal conductivity, high temperature resistance, corrosion resistance and lubrication performance. However, the naturally mined graphite ore usually contains a large amount of impurities such as silicate minerals, feldspar, mica, pyrite and compounds of potassium, sodium, calcium, magnesium and other elements, and the fixed carbon content is usually between 70%-90%. The presence of these impurities seriously reduces the electrical conductivity, thermal stability and chemical stability of graphite, greatly limiting its application in high-end fields. Therefore, efficient purification of graphite to obtain high-purity or ultra-high-purity graphite is a key link to improve its added value and expand its application range.

[0003] Currently, the main methods for graphite purification in industry are flotation method, hydrofluoric acid method, alkali-acid method and high-temperature purification method.

[0004] The flotation method mainly utilizes the natural hydrophobicity of the surface of graphite to separate graphite and its associated hydrophilic minerals by adding flotation aids and water. However, the flotation method can only increase the fixed carbon content of graphite to about 90%-95%, and it is difficult to remove impurities or fine particles in the form of isomorphism or inclusions. The purification precision is limited and cannot meet the requirements of producing high-purity graphite above 99%, and it is usually used as a pretreatment method for subsequent deep purification.

[0005] The hydrofluoric acid method mainly utilizes the reaction of hydrofluoric acid with silicon-containing impurities in graphite minerals to generate volatile silicon tetrafluoride or soluble fluorosilicic acid, so that silicon, calcium, iron and other impurities in graphite minerals are separated from the graphite system. The hydrofluoric acid method can effectively remove silicon, calcium, iron and other impurities in graphite minerals, and is one of the most efficient methods for graphite purification. However, hydrofluoric acid is highly corrosive and toxic, and will interact with human bones for a long time after entering the human body. The discharge of fluorine-containing wastewater will cause significant ecological damage to the surrounding environment. The requirements for waste collection devices for fluorine-containing wastewater and waste gas are extremely high, and the treatment process is extremely difficult, and the cost of waste treatment is very expensive.

[0006] The alkali-acid method is to mix graphite with a strong base such as sodium hydroxide and then react under high-temperature melting conditions to convert silicates and other impurities into soluble sodium silicate; then the metal ions and oxides are dissolved by treating with hydrochloric acid, and then high-purity graphite is obtained after washing, dewatering and drying. However, the alkali-acid method consumes a large amount of strong acid and strong base in the production process, generates a large amount of high-salinity wastewater, has a high treatment cost, and requires a large amount of energy to provide the reaction temperature in the melting process. In addition, the process flow of the alkali-acid method is long, involves multiple washing and neutralization, and has a fast equipment depreciation and a high maintenance cost.

[0007] The high-temperature purification method is to take advantage of the fact that the melting point of graphite is much higher than that of impurities, and to heat natural graphite to above 2500℃ in a high-temperature graphite furnace filled with an inert gas atmosphere, so that most of the impurities are evaporated or reacted and separated from the graphite. However, the high-temperature purification method is a typical high-energy consumption process, and the huge power consumption required to maintain the ultra-high temperature makes it difficult to reduce the production cost. Due to the ultra-high temperature reaction environment, a special high-temperature graphitization furnace is required for the reactor, which requires a large investment in equipment. This method is currently only suitable for high-value-added special graphite products and is difficult to popularize in the market of ordinary high-purity graphite.

[0008] Therefore, a green and efficient graphite purification method is needed in the art. SUMMARY

[0009] Based on the above reasons, the present application provides an application of a deep eutectic solvent (DES) in graphite purification and a graphite purification method. By combining the oxidation graphite technology with the green chemical tool of deep eutectic solvent, a series of interrelated technical problems such as environmental pollution, safety hazards, high energy consumption, and limited purification precision faced by existing graphite purification technologies are comprehensively solved, thereby providing a new clean, efficient and economic path to high-purity graphite.

[0010] To achieve the above purpose, the first technical solution of the present application discloses an application of a deep eutectic solvent in graphite purification. The purification is achieved by mixing and reacting the deep eutectic solvent with the oxidation graphite to form strong hydrogen bonds on the surface of the oxidation graphite, thereby separating the impurities from the oxidation graphite framework.

[0011] Further, the mass ratio of the oxidation graphite to the deep eutectic solvent is 1: (10-100).

[0012] Further, the mixing reaction temperature is 15-30℃, and the treatment time is 4-8h.

[0013] The second technical solution of the present application discloses a graphite purification method, which comprises the following steps:

[0014] S1. After mixing and reacting the oxidation graphite with the deep eutectic solvent, washing and solid-liquid separation are performed to obtain a solid product;

[0015] S2. Heat reduction of the solid product to obtain high purity graphite.

[0016] Further, the hydrogen bond acceptor in the deep eutectic solvent of S1 is selected from any one or combination of quaternary ammonium salt, and the hydrogen bond donor is selected from urea, ethylene glycol, citric acid, and glucose.

[0017] Further, the mixing reaction temperature is 15-30℃, and the treatment time is 4-8h.

[0018] Further, the heat reduction of S2 is to heat the solid product to 500-1500℃ under a protective atmosphere, and the heat preservation time is 0.5-5h.

[0019] Further, the heating rate is 8℃ / min.

[0020] Beneficial effects: the present application mixes the deep eutectic solvent with the graphite oxide, and in the reaction process, the molecules of the deep eutectic solvent form strong hydrogen bond with the oxygen-containing functional groups on the surface of the graphite oxide through the rich hydrogen bond sites, which on one hand helps to further exfoliate the graphite oxide layers, and on the other hand can effectively complex and extract the impurities (such as K⁺, Na⁺, Ca²⁺, Mg²⁺, etc.) embedded in the graphite interlayer, forming a soluble complex, so as to realize the efficient separation of impurities and graphite skeleton. The fixed carbon content of the graphite material obtained by the technical means of the present application can reach more than 98%, and preferably when the hydrogen bond donor in the deep eutectic solvent is ethylene glycol and urea, the fixed carbon content can reach more than 99%. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the description of the embodiments of the present application. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0022] Figure 1 It is the process flow chart of the graphite purification method of the present application;

[0023] Figure 2 It is the XRD spectrum comparison chart of different materials;

[0024] Figure 3 It is the infrared spectrum comparison chart of different materials;

[0025] Figure 4 It is the ultraviolet-visible absorption spectrum of different materials;

[0026] Figure 5 It is the fixed carbon content test chart of graphite. DETAILED DESCRIPTION

[0027] In order to make the technical problems, technical solutions and beneficial effects solved by the present application clearer, the present application will be further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0028] The first embodiment of the present application discloses an application of a deep eutectic solvent in graphite purification. The purification is achieved by mixing and reacting the deep eutectic solvent with the graphite oxide slurry, so that the deep eutectic solvent forms strong hydrogen bonds on the surface of the graphite oxide, and the impurities are separated from the graphite oxide framework.

[0029] As shown in Figure 1 The second embodiment of the present application discloses a method for purifying graphite using the above-mentioned deep eutectic solvent, which comprises the following steps:

[0030] S1. After mixing and reacting the graphite oxide with the deep eutectic solvent, the solid product is obtained by washing and solid-liquid separation;

[0031] S2. The solid product is heat-reduced to obtain high-purity graphite.

[0032] In the present embodiment, the graphite oxide is obtained by oxidizing the graphite to be purified. For example, after oxidizing the graphite to be purified with a strong acid and a strong oxidizing agent, the graphite oxide is obtained by washing and purifying. Alternatively, the graphite oxide is directly obtained from the market, and the purity of the graphite oxide cannot meet the requirements.

[0033] The graphite to be purified can be natural graphite or graphite with relatively low purity in the prior art (for example, graphite with purity that cannot meet the requirements after being purified by the prior art means).

[0034] For example, when the graphite to be purified is natural graphite, the present embodiment provides a specific method for preparing graphite oxide: uniformly drying and grinding the natural graphite sample to obtain graphite powder; mixing and oxidizing the graphite powder, concentrated sulfuric acid and potassium permanganate to obtain graphite oxide slurry; and obtaining pure graphite oxide solid by washing, filtering and drying the graphite oxide slurry.

[0035] In the present embodiment, the deep eutectic solvent (Deep Eutectic Solvents) refers to a multi-component liquid formed by mixing a hydrogen bond donor (such as urea, carboxylic acid) and a hydrogen bond acceptor (such as a quaternary ammonium salt) in a specific molar ratio, which has rich hydrogen bond sites. In the present embodiment, the deep eutectic solvent is preferably a hydrogen bond acceptor selected from a quaternary ammonium salt, and a hydrogen bond donor selected from any one or a combination of urea, ethylene glycol, citric acid and glucose.

[0036] The method for preparing the eutectic solvent described in the present application is a conventional method in the art, for example, mixing the hydrogen bond acceptor and the donor in a certain ratio (such as 1:1-4), and heating and stirring to obtain. In the preferred embodiment, when the hydrogen bond acceptor in the eutectic solvent is choline chloride and the hydrogen bond donor is urea, the preferred method for preparing the eutectic solvent is provided in a specific embodiment of the present application: mixing choline chloride and urea in a molar ratio of 1:(1-2), continuously stirring at 80°C until the mixture becomes a clear and transparent liquid to obtain the eutectic solvent.

[0037] It should be noted that the above method for preparing the eutectic solvent is only an example, and those skilled in the art can prepare it according to the needs.

[0038] In a further embodiment, the mass ratio of the eutectic solvent to the graphene oxide is 1:(10-100); the mixing reaction temperature is 15-30°C, and the treatment time is 4-8h. In this process, the eutectic solvent molecules form strong hydrogen bonds with the oxygen-containing functional groups on the surface of the graphene oxide through their abundant hydrogen bond sites. On the one hand, it helps to further exfoliate the graphene oxide layers, and on the other hand, it can effectively complex and extract impurities (such as K⁺, Na⁺, Ca²⁺, Mg²⁺, etc.) embedded in the interlayer of the graphene, forming a soluble complex, thereby realizing efficient separation of impurities and graphene framework.

[0039] In the present embodiment, the reaction product needs to be washed and separated after the reaction described in S1, and the purpose is to obtain a purified solid product, which is a solid graphene oxide; wherein the washing and solid-liquid separation are conventional methods in the art, for example, a specific embodiment discloses that the washing and solid-liquid separation can be carried out by centrifugation or filtration after the reaction, and then the obtained solid is washed with organic solvents (such as ethanol, etc.) or deionized water for multiple times to remove the residual eutectic solvent and extracted impurity ions.

[0040] In the present embodiment, the thermal reduction is to heat the solid product to 500-1500°C under a protective atmosphere for 0.5-5h. At this temperature, the oxygen-containing functional groups in the graphene oxide are rapidly decomposed and escape in the form of gas, realizing the reduction of graphene oxide to graphite. At the same time, the residual trace amount of eutectic solvent or its decomposition products are also completely removed. The protective atmosphere is an inert gas such as nitrogen or argon. The heating rate of the thermal reduction is preferably 8°C / min.

[0041] The fixed carbon content of the graphite material obtained by the embodiments of the present application can reach more than 99%.

[0042] The technical means and technical effects of the present application will be described in detail below through specific examples.

[0043] Example 1 Purification of graphite

[0044] In this example, a natural graphite sample obtained from Luobei graphite mine was purified. Before purification, the ash content of the natural graphite sample was determined by GB / T 212-2008 "Method for industrial analysis of coal", and the fixed carbon content (i.e. purity %) of the natural graphite sample was determined to be 94%.

[0045] (1) Raw material pretreatment: The natural graphite was dried to remove the surface adsorbed water. Then, it can be mechanically ground or sieved to obtain a natural graphite powder with uniform particle size distribution.

[0046] (2) Preparation of graphite oxide: 2g of natural graphite powder was mixed with 46mL of 98% concentrated sulfuric acid and 6g of potassium permanganate in a reaction vessel to carry out an oxidation reaction, and graphite oxide slurry was obtained;

[0047] The oxidation reaction is carried out in three stages:

[0048] Low temperature stage: 2g of natural graphite was added to a three-necked flask equipped with a condensation reflux device, and 46mL of concentrated sulfuric acid was added dropwise under stirring, followed by the slow addition of 6g of potassium permanganate (KMnO4) powder;

[0049] Medium temperature stage: After the raw materials were added, the mixture was controlled at a temperature of 35°C in the medium temperature reaction stage, and was stirred vigorously for 30min;

[0050] High temperature stage: After the medium temperature stage reaction was completed, a certain amount of ionized water was slowly added, and the temperature was raised to about 90°C in the high temperature reaction stage, and was maintained for 30min. The reaction was completed to obtain graphite oxide slurry.

[0051] (3) Washing and purification of graphite oxide: The graphite oxide slurry was washed by adding a certain amount of deionized water, and after mixing evenly, a certain amount of 3% H2O2 was added. Hot filtration was carried out, and the filter cake was washed with deionized water until there was no SO4 2- in the filtrate. According to the need, it was dried in an oven at 100°C for 12h, and was sealed and stored for use, to obtain pure graphite oxide solid.

[0052] (4) Preparation of deep eutectic solvent: Choline chloride (quaternary ammonium salt) was used as hydrogen bond acceptor, and urea (Urea) was used as hydrogen bond donor. After mixing according to a molar ratio of 1:1, the mixture was continuously stirred at 80°C until it became a clear transparent liquid, and a deep eutectic solvent was obtained;

[0053] (5) Deep eutectic solvent cooperated impurity separation: the solid graphite oxide obtained in (3) and the deep eutectic solvent obtained in (4) are mixed in a mass ratio of 1:20, and stirring treatment is performed to allow the deep eutectic solvent to fully penetrate into the interlayer of the graphite oxide and react with the impurities; the treatment temperature is maintained at 15-30°C, and the treatment time is 6h.

[0054] (6) Separation and washing: the mixture after the reaction in (5) is subjected to solid-liquid separation to obtain the purified solid graphite oxide. The separation can be performed by centrifugation or filtration. Subsequently, the obtained solid is washed with an organic solvent (such as ethanol, etc.) or deionized water for multiple times to remove the residual deep eutectic solvent and the extracted impurity ions, thereby obtaining a solid product.

[0055] (7) Thermal reduction: the solid product after washing in (6) is dried, and then subjected to thermal treatment under a protective atmosphere (such as nitrogen, argon, etc.) to obtain the final high-purity graphite product; in the thermal treatment process, the temperature is raised to 600°C at a rate of 8°C / min and maintained for 1h.

[0056] Example 2 Purification of graphite

[0057] The purification method is the same as that in Example 1, except that the deep eutectic solvent in Example 2 uses choline chloride (quaternary ammonium salt) as the hydrogen bond acceptor and glucose (Glc) as the hydrogen bond donor.

[0058] Example 3 Purification of graphite

[0059] The purification method is the same as that in Example 1, except that the deep eutectic solvent in Example 3 uses choline chloride (quaternary ammonium salt) as the hydrogen bond acceptor and citric acid (Cit) as the hydrogen bond donor.

[0060] Example 4 Purification of graphite

[0061] The purification method is the same as that in Example 1, except that the deep eutectic solvent in Example 4 uses choline chloride (quaternary ammonium salt) as the hydrogen bond acceptor and ethylene glycol (EG) as the hydrogen bond donor.

[0062] Comparative Example 1 Purification of graphite

[0063] The purification method is the same as that in Example 1, except that no deep eutectic solvent is added in Comparative Example 1, and steps (4), (5) and (6) are not performed, and the graphite oxide (GO) is directly reduced.

[0064] Test Example 1 Characterization analysis of graphite oxide

[0065] The solid product (graphite oxide treated with deep eutectic solvent) obtained after step (6) in Examples 1-4, and the graphite oxide treated in step (3) without deep eutectic solvent treatment in Comparative Example 1 are subjected to X-ray diffraction analysis (XRD), respectively.Figure 2 ), Fourier infrared spectroscopy (FT-IR) analysis ( Figure 3 ), ultraviolet-visible spectroscopy (UV) analysis ( Figure 4 ), and the results are as follows.

[0066] Figure 2 is a comparison of XRD patterns of different materials. As can be seen from the figure, the graphite oxide shows a sharp (001) crystal face characteristic diffraction peak near 10° in 2θ value, and the interlayer spacing is about 0.88 nm according to the Bragg equation, which is much larger than the interlayer spacing of ideal graphite (0.3354 nm), indicating that the insertion of oxygen-containing functional groups during the oxidation process effectively expands the graphite interlayer structure. The characteristic peak of the graphite oxide is sharp and has no significant broadening, indicating that the layered crystal structure is complete and the crystallinity is good. The characteristic peak of the original graphite at 26.4° is not observed in the spectrum, indicating that the graphite raw material is fully oxidized and the product has high purity. In contrast, ethylene glycol, glucose, citric acid and urea all show broadened diffuse diffraction peaks, which is a typical characteristic of amorphous structure, further highlighting the regular layered structure of graphite oxide as a crystal material.

[0067] Figure 3 is a comparison of infrared spectra of different materials. The wide absorption peak in the wave number range of 3679-2998 cm⁻¹ is attributed to the stretching vibration effect of hydroxyl and adsorbed water molecules in the material. The characteristic absorption peak at 1600 cm⁻¹ corresponds to the stretching vibration of C=C bond, which is an important characterization of carbon skeleton structure. The absorption peak at 1037 cm⁻¹ wave number is due to the stretching vibration of ether bond (C-O-C), while no obvious absorption peak of epoxy or peroxide group is observed at 856 cm⁻¹ wave number. By comparing the spectra of each material, it is found that the characteristic absorption peak positions of graphite oxide, ethylene glycol, glucose, citric acid and urea are basically the same, but the peak shape and relative intensity are different, which reflects the difference in the type and content of functional groups of each material, and at the same time indicates that these organic matters and graphite are mainly combined through physical interactions such as hydrogen bonding.

[0068] Figure 4 is the ultraviolet-visible absorption spectrum of GO, EG, Glc, Cit, Urea five kinds of materials. Graphite oxide shows a characteristic absorption peak near 380 nm caused by π-π transition of C=C bond, and a wide and gentle shoulder peak caused by n-π transition of C=O bond, which is consistent with its typical spectral characteristics. Ethylene glycol has no obvious characteristic absorption in the ultraviolet region, showing a slow rise in the background absorption with decreasing wavelength. Glucose, citric acid and urea all show similar end absorption characteristics in the same range, and the absorption intensity increases with decreasing wavelength, which is caused by the absorption characteristics of the functional groups such as carbonyl and hydroxyl in the molecule. The spectral characteristics of the five materials are obviously different, effectively reflecting the different chemical structures and electronic energy level characteristics of each material.

[0069] Characterization of purified graphite

[0070] The purified graphite obtained from Examples 1-4 and Comparative Example 1 was subjected to ash content determination according to GB / T 212-2008 "Method for Industrial Analysis of Coal", and then the fixed carbon content (i.e. purity %) of the graphite species was determined. The determination results are shown in Table 5: compared with natural graphite, the fixed carbon content of the oxidized graphite (Comparative Example 1) and the purified graphite after reduction of the oxidized graphite treated by the deep eutectic solvent (Examples 1-4) all increased; among them, the fixed carbon content of the oxidized graphite after reduction of the oxidized graphite treated by the deep eutectic solvent increased significantly. Combined with the test results of Test Example 1, it is indicated that the strong hydrogen bond effect formed between the abundant hydrogen bond sites of the deep eutectic solvent molecules and the oxygen-containing functional groups on the surface of the oxidized graphite can further exfoliate the graphite layers, and effectively complex and extract the metal cations and other impurities (such as K+, Na+, Ca2+, Mg2+ and the like) embedded in the graphite interlayers to form soluble complexes, thereby realizing efficient separation of the impurities and the graphite framework. It can be seen that, among the above deep eutectic solvents, the best purification effect can be achieved when the graphite is purified by urea.

[0071] From the above analysis, it can be seen that the present application combines oxidized graphite with a green medium, i.e. a deep eutectic solvent, to construct a brand new and non-traditional purification path for graphite. The specific embodiment is as follows in three core aspects:

[0072] Interface interaction: the stable and multiple hydrogen bond network formed between the deep eutectic solvent molecules and the large number of oxygen-containing functional groups (such as -COOH, -OH) on the surface of the oxidized graphite is used as the main separation driving force. This force not only can effectively intercalate and exfoliate the graphite layers, expose the wrapped impurities, but also can specifically complex and extract the metal ion impurities, realizing precise separation at the molecular level.

[0073] Integrated process design of "separation-reduction": the two steps of "impurity separation" and "structural thermal reduction", which are traditionally separate or harsh in conditions, are ingeniously integrated into a coherent and mild process by the present application. The intermediate product after treatment by the deep eutectic solvent can directly complete purification and graphitization through one-step medium-low temperature heat treatment, greatly simplifying the process and reducing energy consumption.

[0074] Alternative application of green chemical solvent: the low-toxic, biodegradable and recyclable deep eutectic solvent is used as a revolutionary substitute for traditional strong acid, strong base or highly toxic hydrofluoric acid, which fundamentally solves the high pollution and high risk problems faced by the graphite purification industry.

[0075] Solvent circulation process: in the present application, the impurity-containing eutectic solvent eluted by ethanol in Example (1) Step 6 can be separated by rotary evaporation under reduced pressure to realize the reuse of DES, which is an important link to improve the economy and environmental protection of the method.

[0076] The above-described examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application.

Claims

1. Use of a deep eutectic solvent in the purification of graphite, characterized in that, The purification is achieved by mixing and reacting the eutectic solvent with the oxidized graphite, so that the eutectic solvent forms strong hydrogen bonding on the surface of the oxidized graphite, and separates the impurities from the oxidized graphite framework.

2. Use according to claim 1, characterized in that, The mass ratio of the oxidized graphite to the eutectic solvent is 1: (10-100).

3. Use according to claim 1, characterized in that, The mixing and reacting temperature is 15-30℃, and the processing time is 4-8h.

4. A method of purifying graphite, characterized by, The method comprises the following steps: S1. After mixing and reacting the oxidized graphite with the eutectic solvent, washing, solid-liquid separation, and obtaining the solid product; S2. Heating reduction of the solid product to obtain high-purity graphite.

5. The method of claim 4, wherein, The hydrogen bond acceptor in the eutectic solvent in S1 is selected from quaternary ammonium salt, and the hydrogen bond donor is selected from any one or combination of urea, ethylene glycol, citric acid, and glucose.

6. The method of claim 4, wherein, The mixing and reacting temperature is 15-30℃, and the processing time is 4-8h.

7. The method of claim 4, wherein, The heating reduction in S2 is heating the solid product to 500-1500℃ under a protective atmosphere, and keeping the temperature for 0.5-5h.

8. The method of claim 7, wherein, The heating rate is 8℃ / min.

Images