A magnetic composite adsorbent for one-step separation and adsorption of emulsified oil and a preparation method and application thereof

Abstract

The application discloses a kind of magnetic composite adsorbent for emulsified oil one-step separation, adsorption, raw material includes oil phase and water phase;Wherein, the oil phase includes: by weight parts, octadecyl methacrylate 8-10 parts, butyl acrylate 1-3 parts, benzyl methacrylate 1-3 parts, modified nano ferric oxide 0.5-1.5 parts and initiator 0.5-1.0 parts, crosslinking agent 0.5-1 parts and pore forming agent 3-7 parts;The water phase includes: by weight parts, dispersing agent 0.5-1 part, water 30-80 parts.The application introduces inorganic magnetic material modified ferric oxide in acrylate polymerization system, so that the magnetic composite adsorbent can realize the destruction of emulsified oil oil-water interface layer, so as to realize the separation and adsorption of emulsified oil;In addition, due to the special magnetism of ferric oxide, the magnetic composite particles after separation and adsorption can also be conveniently recycled under the action of magnetic field.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer materials, and in particular to a magnetic composite adsorbent for one-step separation and adsorption of emulsified oil and application thereof. BACKGROUND

[0002] Emulsified oil is a stable oil-water mixture with strong stability. Due to its special physical and chemical properties, it is widely used in many fields such as lubrication, cleaning, cooling and defoaming. However, the pollution of emulsified oil can cause harm to the ecological environment and human health.

[0003] The sources of emulsified oil pollution mainly include industrial wastewater, waste from automobile repair, shipbuilding and other industries, and food processing and catering industries. These wastes can be discharged into groundwater, soil and water bodies, causing water pollution and soil pollution; in addition, the volatility of emulsified oil can also cause air pollution. The harm of emulsified oil to the environment includes destruction of aquatic ecosystems, impact on crop and plant growth, pollution of groundwater and soil, etc. It also has a great impact on human health. Long-term exposure to emulsified oil pollution environment can cause skin allergies, respiratory diseases, nervous system diseases, etc. At the same time, if emulsified oil is ingested or inhaled, it can cause poisoning and even endanger life. Therefore, reducing the unreasonable discharge and pollution prevention and control of emulsified oil is of great importance to protect the environment and human health.

[0004] Therefore, it is a problem to be solved by those skilled in the art to provide a solid adsorbent for the separation and adsorption of emulsified oil and a preparation method thereof. SUMMARY

[0005] The present application aims to provide a magnetic composite adsorbent for one-step separation and adsorption of emulsified oil and a preparation method and application thereof.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] A magnetic composite adsorbent for one-step separation and adsorption of emulsified oil, the raw materials including an oil phase and a water phase;

[0008] The oil phase includes, by weight, 8-10 parts of methacrylate octadecyl ester, 1-3 parts of butyl acrylate, 1-3 parts of benzyl methacrylate, 0.5-1.5 parts of modified nano-tetrairon oxide, 0.5-1.0 parts of initiator, 0.5-1 parts of crosslinking agent and 3-7 parts of pore-forming agent.

[0009] The water phase includes, by weight, 0.5-1 parts of dispersant and 30-80 parts of water.

[0010] The prepared emulsified oil adsorption magnetic composite adsorbent has fast oil absorption rate, high oil-water separation efficiency and can be conveniently recovered under the action of a magnetic field.

[0011] Under the conditions of a surfactant content of 7500 mg / L, an oil content of 25 g / L and a separation time of 4 h, the separation efficiency of the solid adsorbent on CTAB-stabilized water-in-toluene emulsified oil under the action of a magnetic field is 92.8%, and the separation efficiency is still 32.1% after the emulsified oil is continuously separated for 5 times.

[0012] In addition, the solid adsorbent can be aggregated under the action of a magnetic field after separation, and can be conveniently recovered, and the solid adsorbent still has good magnetic responsiveness after being continuously separated for 5 times.

[0013] Preferably, the modified nanometer ferroferric oxide is a product obtained by hydrophobic modification of hydrophilic nanometer ferroferric oxide particles by a silane coupling agent.

[0014] Preferably, the specific preparation steps of the modified nanometer ferroferric oxide are as follows: the silane coupling agent is added into a nanometer ferroferric oxide suspension, and after stirring and reacting for 4-6 h, the modified nanometer ferroferric oxide is obtained by washing and drying.

[0015] Preferably, the concentration of the nanometer ferroferric oxide suspension is 10 g / L.

[0016] The addition amount of the silane coupling agent is 2% of the volume of the nanometer ferroferric oxide suspension.

[0017] The silane coupling agent is KH-570; compared with other silane coupling agents, KH-570 has a double bond structure, so that the inorganic nanoparticles modified by the silane coupling agent can participate in the polymerization reaction of acrylate; similarly, vinyltrimethoxysilane can be used instead.

[0018] The stirring speed is 500 r / min, which ensures that the nanometer ferroferric oxide particles can be chemically bonded with the silane coupling agent in a dispersed and non-aggregated form.

[0019] The reaction temperature is 45-50℃; the temperature for hydrophobic modification of the nanometer ferroferric oxide should not be too high, otherwise the nanometer ferroferric oxide will be oxidized, the magnetic response surface effect will be weakened or even lost, and thus the modification effect and use performance will be affected.

[0020] Preferably, the preparation method of the nanometer ferroferric oxide suspension is as follows: the nanometer ferroferric oxide is added into a dispersion medium, the supernatant is removed after ultrasonic treatment, and the dispersion medium is continuously added until the concentration of the suspension is 10 g / L.

[0021] Preferably, the dispersion medium is a mixed solution of anhydrous ethanol and deionized water in a volume ratio of 1:1.

[0022] The ultrasonic treatment temperature is room temperature, the frequency is 35 kHz, and the time is 1-2 hours.

[0023] The nano-magnetic iron oxide has a particle size of 200 nm.

[0024] The ferroferric oxide nanoparticles are low in cost, widely available, good in chemical stability, surface modifiability and biocompatibility, and are an ideal magnetic material. The magnetic response surface effect and superparamagnetism of the ferroferric oxide nanoparticles enable the nanoparticles to be separated from a complex multi-phase system under the action of a magnetic field. The particle size of the ferroferric oxide nanoparticles should not be too small or too large. When the particle size is too large, the pore channels in the polymer for oil adsorption will be blocked, resulting in a decrease in the adsorption and separation performance of the polymer. When the particle size is too small, the magnetic response surface effect is weak, and the modified polymer cannot have the magnetic responsiveness.

[0025] Preferably, the initiator is benzoyl peroxide.

[0026] The crosslinking agent is N,N-dimethyl bisacrylamide.

[0027] The pore-forming agent is ethyl acetate. The pore-forming agent is usually selected from solvents that are immiscible with monomers and various types of additives and do not participate in the polymerization reaction, and can enable the chain growth process of the polymerization reaction to grow and solidify in a certain spatial structure. After the polymerization is completed, the pore-forming agent that is not reacted and remains on the surface and inside of the polymer can be removed by solvent extraction cleaning, so as to obtain a resin product with pore channels. In addition, toluene and cyclohexane and other organic substances that do not participate in the reaction can be used instead.

[0028] The dispersing agent is polyvinyl alcohol, which is an amphiphilic surfactant. After being added to the synthesis system, the oil phase is stably suspended in the dispersed phase water in the form of spherical droplets under the action of mechanical stirring, and the polymerization occurs in the unit place of the droplets. In addition, cetyltrimethylammonium bromide and other organic substances with amphiphilic properties that do not participate in the polymerization of the acrylate system and can be easily removed by physical or chemical methods after polymerization can be used instead.

[0029] According to the preparation method of the magnetic composite adsorbent for one-step separation and adsorption of emulsified oil, the method comprises the following specific steps:

[0030] (1) The raw materials are weighed according to the weight parts: 8-10 parts of methacrylic acid octadecyl ester, 1-3 parts of butyl acrylate, 1-3 parts of benzyl methacrylate, 0.5-1.5 parts of modified nano-magnetic iron oxide particles, 0.5-1.0 parts of an initiator, 0.5-1 part of a crosslinking agent, 3-7 parts of a pore-forming agent, 0.5-1 part of a dispersing agent, and 30-80 parts of water;

[0031] (2) mixing the octadecyl methacrylate, the butyl acrylate, the benzyl methacrylate, the modified ferroferric oxide nanoparticles, the initiator, the crosslinking agent and the pore-forming agent uniformly to obtain an oil phase;

[0032] (3) dissolving the dispersant in the water to obtain an aqueous phase by stirring and dissolving;

[0033] (4) adding the oil phase dropwise into the aqueous phase, stirring to form uniform droplets of the oil phase in the aqueous phase, and after 6-8 hours of reaction, sequentially performing aging, cooling, washing and drying to obtain a solid adsorbent.

[0034] Preferably, the dissolving temperature in step (3) is 40℃.

[0035] The reaction temperature in step (4) is 75-80℃ under a nitrogen environment.

[0036] The aging in step (4) is aging at 80-90℃ for 0.5-1h.

[0037] The drying in step (4) is drying at 55-65℃ until the product mass reaches a constant weight.

[0038] The application of the magnetic composite adsorbent as described above in emulsion oil separation and adsorption.

[0039] Preferably, the use method is: immersing the magnetic composite adsorbent in emulsion oil, applying a magnetic field with a magnetic field strength of 0.5T every 15min time interval, and taking out the magnetic composite adsorbent under the action of the magnetic field after the separation is completed.

[0040] Preferably, the emulsion oil is an O / W type emulsified toluene / water mixture.

[0041] Compared with the prior art, the present application has the following beneficial effects:

[0042] The present application discloses a magnetic composite adsorbent with emulsion oil separation and adsorption functions and a preparation method thereof. In the present application, inorganic magnetic material modified ferroferric oxide is introduced into an acrylate polymerization system, so that the magnetic composite adsorbent can realize the destruction of the emulsion oil water interface layer, thereby realizing the separation and adsorption of emulsion oil. In addition, due to the special magnetism of ferroferric oxide, the magnetic composite particles after separation and adsorption can also be conveniently recycled under the action of a magnetic field. BRIEF DESCRIPTION OF DRAWINGS

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced. The drawings in the present description are only embodiments of the present application.

[0044] Figure 1 SEM image of the magnetic composite adsorbent prepared for the application example embodiment 1;

[0045] Figure 2 Emulsified oil droplet content chart before demulsification of the toluene-in-water emulsified oil stabilized by CTAB for the magnetic composite adsorbent of the application example embodiment 1;

[0046] Figure 3 Emulsified oil droplet content chart after demulsification of the toluene-in-water emulsified oil stabilized by CTAB for the magnetic composite adsorbent of the application example embodiment 1. DETAILED DESCRIPTION

[0047] Embodiments of the application are described below, examples of which are shown in the accompanying drawings, the embodiments described with reference to the drawings are exemplary and are intended to explain the application, and are not understood as a limitation of the application.

[0048] Embodiment 1

[0049] A preparation method of a magnetic composite adsorbent for one-step separation and adsorption of emulsified oil, specifically comprising the following steps:

[0050] (1) precisely add 50 mL of distilled water and 0.05 g of polyvinyl alcohol in a three-necked flask, heat to 40℃ and stir to dissolve to obtain an aqueous phase;

[0051] (2) uniformly mix 7.2 g of stearyl methacrylate, 1.4 g of butyl acrylate, 1.4 g of benzyl methacrylate, 0.07 g of benzoyl peroxide, 0.05 g of N,N-methylene bisacrylamide and 0.1 g of modified ferric oxide to obtain an oil phase;

[0052] (3) add the oil phase to the aqueous phase, heat to 75℃ under nitrogen atmosphere, and form uniform liquid droplets in the water under the condition of 200 r / min mechanical stirring, and react for 6 h; after the reaction is completed, heat to 80℃ and age for 0.5 h; after the product is cooled to room temperature, wash with anhydrous ethanol and deionized water, and vacuum dry in a 65℃ oven until the weight is constant, to obtain the magnetic composite adsorbent;

[0053] The preparation method of the modified ferric oxide is specifically as follows:

[0054] 1) prepare 1000 mL of anhydrous ethanol-water solution with a ratio of 1:1 as a dispersion medium;

[0055] 2) weigh 5 g of ferric oxide nanoparticles (200 nm) into 500 mL of the dispersion medium, ultrasonic treat at room temperature and 35 kHz for 1 h to remove excess impurities on the surface of the particles, remove the supernatant after ultrasonic treatment, and continue to add the dispersion medium to 500 mL to obtain a ferric oxide suspension;

[0056] 3) Add 10 mL of KH-570 to the magnetite suspension obtained in step 2), and react at 50°C under constant temperature, closed, and strong stirring at 500 r / min for 5 h. After cooling, solid-liquid separation is performed, and the obtained solid particles are washed with anhydrous ethanol three times and dried at 100°C for 12 h to obtain modified nano-magnetite particles.

[0057] Figure 1 The scanning electron microscope images of the prepared magnetic composite adsorbent, wherein (a) is the adsorbent surface morphology image with a magnification of 100X, (b) is the adsorbent cross-section morphology image with a magnification of 100X, (c) is the adsorbent surface morphology image with a magnification of 800X, and (d) is the adsorbent surface morphology image with a magnification of 1.5kX. As can be seen from the images, the polymer has a large number of pores and holes of various types.

[0058] Example 2

[0059] A preparation method of a magnetic composite adsorbent for one-step separation and adsorption of emulsified oil, specifically comprising the following steps:

[0060] (1) precisely add 40 mL of distilled water and 0.15 g of polyvinyl alcohol into a three-necked flask, heat and stir to dissolve at 40°C to obtain an aqueous phase;

[0061] (2) uniformly mix 6.2 g of octadecyl methacrylate, 3.2 g of butyl acrylate, 0.6 g of benzyl methacrylate, 0.05 g of benzoyl peroxide, 0.07 g of N,N-methylenebisacrylamide, and 0.06 g of modified magnetite to obtain an oil phase;

[0062] (3) add the oil phase to the aqueous phase, heat to 75°C under a nitrogen environment, and form uniform droplets of the oil phase in the water under mechanical stirring at 200 r / min for 6 h; after the reaction is completed, heat to 80°C, and age for 0.5 h; after the product is cooled to room temperature, wash with anhydrous ethanol and deionized water, and vacuum dry in an oven at 65°C until the weight is constant to obtain a magnetic composite adsorbent;

[0063] The preparation method of the modified magnetite is specifically as follows:

[0064] 1) prepare 2000 mL of a 1:1 ratio of anhydrous ethanol-water solution as a dispersion medium;

[0065] 2) weigh 10 g of magnetite nanoparticles into 1000 mL of the dispersion medium, and ultrasonically treat at room temperature and 35 kHz for 1 h to remove excess impurities on the surface of the particles; after ultrasonic treatment, remove the supernatant, continue to add the dispersion medium to 1000 mL, and obtain a modified magnetite suspension;

[0066] 3) Add 20 mL of KH-570 to the magnetite suspension obtained in step 2), and react at 50°C for 5 h under constant temperature, closed, and strong stirring at 500 r / min. After cooling, solid-liquid separation is performed, and the obtained solid particles are washed with anhydrous ethanol for three times, and dried at 100°C for 12 h to obtain modified magnetite powder.

[0067] Example 3

[0068] A preparation method of a magnetic composite adsorbent for one-step separation and adsorption of emulsified oil, specifically comprising the following steps:

[0069] (1) precisely add 60 mL of distilled water and 0.06 g of polyvinyl alcohol into a three-necked flask, heat and stir to dissolve at 40°C to obtain an aqueous phase;

[0070] (2) uniformly mix 6.9 g of stearyl methacrylate, 1.4 g of butyl acrylate, 1.7 g of benzyl methacrylate, 0.05 g of benzoyl peroxide, 0.04 g of N,N-methylenebisacrylamide, and 0.18 g of modified magnetite (preparation method same as Example 1) to obtain an oil phase;

[0071] (3) add the oil phase into the aqueous phase, heat to 75°C under nitrogen environment, and form uniform droplets of the oil phase in the water under mechanical stirring at 200 r / min for 6 h. After the reaction is completed, heat to 80°C, and age for 0.5 h. After the product is cooled to room temperature, wash with anhydrous ethanol and deionized water, and vacuum dry in an oven at 65°C until constant weight to obtain the magnetic composite adsorbent.

[0072] Comparative Example 1

[0073] A preparation method of a solid adsorbent, specifically comprising the following steps:

[0074] (1) precisely add 50 mL of distilled water and 0.07 g of polyvinyl alcohol into a three-necked flask, heat and stir to dissolve at 40°C to obtain an aqueous phase;

[0075] (2) obtain an oil phase by uniformly mixing 7.1 g of stearyl methacrylate, 1.4 g of butyl acrylate, 1.4 g of benzyl methacrylate, 0.05 g of benzoyl peroxide, and 0.05 g of N,N-methylenebisacrylamide;

[0076] (3) add the oil phase into the aqueous phase, heat to 70°C under nitrogen environment, and form uniform droplets of the oil phase in the water under mechanical stirring at 200 r / min for 6 h. After the reaction is completed, heat to 80°C, and age for 0.5 h. After the product is cooled to room temperature, wash with anhydrous ethanol and deionized water, and vacuum dry in an oven at 65°C until constant weight to obtain the solid adsorbent.

[0077] Application Example

[0078] 5 parts of surfactant cetyltrimethylammonium bromide (CTAB) was dispersed in 250 parts of water respectively, after complete dissolution, 1 part of toluene was added, and continuous stirring was carried out at room temperature for 2-3 h, the stirring speed was 1000-2000 rpm, to prepare surfactant-stabilized O / W toluene / water emulsified oil;

[0079] 0.5 g of the adsorbent prepared in Examples 1-3 and Comparative Example 1 was respectively loaded into a non-woven fabric bag, completely immersed in 100 mL of oil-water mixture, a magnetic field with a magnetic field strength of 0.5 T was applied every 15 min, while Example 1 was set as Control Group 1 without applying an external magnetic field, and the solid adsorbent was quickly taken out under the action of the magnetic field after 4 h of shaking separation. The oil content of the emulsified oil after separation was determined by ultraviolet spectrophotometry; at the same time, the separation efficiency of the solid adsorbent for the above two O / W type emulsified oil-water mixtures after continuous separation for 5 times was studied, and the related results are as follows:

[0080] The separation efficiency of the magnetic composite adsorbent in Control Group 1 for CTAB-stabilized toluene-in-water emulsion without applying an external magnetic field was 92.4%, and the saturation separation time was 18 h; the separation efficiency of the magnetic composite adsorbent in Example 1 for CTAB-stabilized toluene-in-water emulsion under the action of a magnetic field was 92.8%, and the saturation separation time was 4 h. In addition, after the magnetic composite adsorbent was continuously separated for 5 times for the above O / W type emulsified oil, the separation efficiency was still 32.1%, and after desorption by soaking in anhydrous ethanol, the separation and adsorption capacity of the magnetic composite adsorbent for O / W type emulsified oil could be restored to 59.3% of the initial separation efficiency; wherein, as Figure 2 and 3 The change in the content of emulsified oil droplets before and after the demulsification of the magnetic composite adsorbent for CTAB-stabilized toluene-in-water emulsion is shown in the figure, as can be seen from the figure, the number of toluene droplets is greatly reduced after the separation of the emulsified oil-water mixture by the magnetic composite adsorbent.

[0081] The separation efficiency of the magnetic composite adsorbent in Example 2 for CTAB-stabilized toluene-in-water emulsion was 74.6%, and the saturation separation time was 6 h. In addition, after the magnetic composite adsorbent was continuously separated for 5 times for the above O / W type emulsified oil, the separation efficiency was still 25.7%, and after desorption by soaking in anhydrous ethanol, the separation and adsorption capacity of the magnetic composite adsorbent for O / W type emulsified oil could be restored to 47.3% of the initial separation efficiency.

[0082] The separation efficiency of the magnetic composite adsorbent for CTAB-stabilized water-in-toluene emulsion was 62.7%, and the saturated separation time was 6 h. In addition, after the magnetic composite adsorbent was used for continuous separation of the O / W emulsified oil for 5 times, the separation efficiency was still 23.6%, and after desorption by soaking in anhydrous ethanol, the separation and adsorption capacity of the magnetic composite adsorbent for the O / W emulsified oil could be restored to 45.1% of the initial separation efficiency.

[0083] The separation efficiency of the solid adsorbent for CTAB-stabilized water-in-toluene emulsion was 16.9%, and the saturated separation time was 4 h. In addition, after the solid adsorbent was used for continuous separation of the O / W emulsified oil for 5 times, the separation efficiency was 3.6%, and after desorption by soaking in anhydrous ethanol, the separation and adsorption capacity of the solid adsorbent for the O / W emulsified oil could be restored to 87.9% of the initial separation efficiency.

[0084] The above examples are only used to illustrate the technical solutions of the present application, and 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.

Claims

1. The application of a magnetic composite adsorbent in the separation and adsorption of emulsified oil, characterized in that, Used for one-step separation and adsorption of emulsified oil; The raw materials of the magnetic composite adsorbent include an oil phase and an aqueous phase; The oil phase comprises, by weight, 8-10 parts of octadecyl methacrylate, 1-3 parts of butyl acrylate, 1-3 parts of benzyl methacrylate, 0.5-1.5 parts of modified nano-ferric oxide, 0.5-1.0 parts of initiator, 0.5-1 parts of crosslinking agent and 3-7 parts of pore-forming agent. The aqueous phase comprises, by weight, 0.5-1 parts of dispersant and 30-80 parts of water; The preparation method of the magnetic composite adsorbent includes the following specific steps: (1) Weigh the raw materials by weight: 8-10 parts of octadecyl methacrylate, 1-3 parts of butyl acrylate, 1-3 parts of benzyl methacrylate, 0.5-1.5 parts of modified nano iron oxide particles, 0.5-1.0 parts of initiator, 0.5-1 parts of crosslinking agent, 3-7 parts of pore-forming agent, 0.5-1 parts of dispersant, and 30-80 parts of water; (2) The octadecyl methacrylate, butyl acrylate, benzyl methacrylate, modified iron oxide nanoparticles, initiator, crosslinking agent and pore-forming agent are mixed evenly to obtain an oil phase; (3) Dissolve the dispersant in the water and stir to obtain an aqueous phase; (4) The oil phase is added dropwise to the aqueous phase and stirred to form uniform droplets in the aqueous phase. After reacting for 6-8 hours, the oil phase is aged, cooled, washed and dried in sequence to obtain the solid adsorbent.

2. The application according to claim 1, characterized in that, The modified nano-ferric oxide is a product obtained by hydrophobically modifying hydrophilic ferric oxide nanoparticles with a silane coupling agent.

3. The application according to claim 2, characterized in that, The specific preparation steps of the modified nano-ferric oxide are as follows: add silane coupling agent to the nano-ferric oxide suspension, stir and react for 4-6 hours, and then wash and dry to obtain the modified nano-ferric oxide.

4. The application according to claim 3, characterized in that, The concentration of the nano-ferric oxide suspension is 10 g / L; The amount of the silane coupling agent added is 2% of the volume of the nano-ferric oxide suspension; The silane coupling agent is KH-570 or vinyltrimethoxysilane; The stirring speed is 500 r / min; The reaction temperature is 45-50℃.

5. The application according to claim 4, characterized in that, The preparation method of the nano-ferric oxide suspension is as follows: nano-ferric oxide is added to a dispersion medium, ultrasonically treated and the supernatant is removed, and the dispersion medium is added until the concentration of the suspension is 10 g / L.

6. The application according to claim 5, characterized in that, The dispersion medium is a mixture of anhydrous ethanol and deionized water in a volume ratio of 1:

1. The ultrasonic treatment was performed at room temperature, at a frequency of 35 kHz, for a duration of 1-2 hours. The nano-iron oxide has a particle size of 200 nm.

7. The application according to claim 1, characterized in that, The initiator is benzoyl peroxide; The crosslinking agent is N,N-dimethylbisacrylamide; The pore-forming agent is any one of ethyl acetate, toluene, and cyclohexane; The dispersant is polyvinyl alcohol or hexadecyltrimethylammonium bromide.

8. The application according to claim 1, characterized in that, The dissolution temperature mentioned in step (3) is 40℃; The reaction temperature described in step (4) is 75-80℃, under a nitrogen atmosphere; The aging process described in step (4) is aging at 80-90℃ for 0.5-1h; The drying in step (4) is to dry the product at 55-65℃ until the product reaches a constant weight.

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