A method for preparing a large specific surface area, large pore volume alumina pellet

Abstract

The application provides a preparation method of alumina pellets with large specific surface area and large pore volume. 20 PO 10 EO 20 As a template inducer with large molecular weight and stable structure, it can spontaneously and orderly aggregate to form micelles in aqueous solution, and the aluminum source can self-assemble along the outer surface of the micelles after hydrolysis in the solution, so that the template induces the crystallization to control the morphology of crystal growth and regulate the size of the crystal, and promotes the increase of the order degree of the crystal and the uniform distribution of the crystal grains, and after the crystallization treatment, a stable and ordered mesoporous structure is formed. The powder obtained by the method has the characteristics of expanding the pore structure and the specific surface area, improving the loading performance and increasing the dispersion of the active component.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of alumina pellet preparation, and particularly relates to a preparation method of alumina pellets with large specific surface area and large pore volume. BACKGROUND

[0002] Active alumina is a widely used chemical product, which is widely used as a catalyst carrier due to its suitable specific surface area, rich pore volume and pore size, and good chemical stability. At present, in the field of chemical industry, especially in the field of petroleum chemical industry and biological chemical industry, high molecular weight raw materials are increasingly widely used, which requires the catalyst or catalyst carrier used to have large mesopores or macropores and large specific surface area. Therefore, it is a hotspot to study the improvement of the pore structure of active alumina.

[0003] Patent CN1160602A discloses a macroporous alumina carrier and a preparation method thereof, the pore volume of which is 0.80-1.20 mL / g, and the specific surface area is 110-200 m 2 / g. During the kneading of pseudo-boehmite and water or an aqueous solution, a physical pore expander such as carbon black and a chemical pore expander such as a phosphorus compound are simultaneously added. The specific surface area of the alumina carrier produced by the method still needs to be further improved. Patent CN104353502A discloses a preparation method of a macroporous spherical alumina, in which alumina gel, water and a seaweed acid salt solution are uniformly mixed, then pseudo-boehmite or alumina powder is added, a suspension slurry is prepared by high-speed stirring, and the slurry is dropped into an aqueous solution of a polyvalent metal cation to form gel pellets. After shaping, drying and calcination, the product is obtained. However, the addition of the aqueous solution of the metal cation may introduce other impurity ions, resulting in a decrease in the purity of the alumina. Patent CN103172097A discloses a preparation method of pseudo-boehmite, in which a met-alloaluminate solution and an aluminum salt solution are simultaneously poured into a rotating liquid film reactor for rapid mixing and nucleation. After the pseudo-boehmite is shaped and carried with active components, a catalyst is prepared, which can be widely used in catalytic hydrogenation reactions.

[0004] In the field of oil refining and chemical industry, with the deterioration and heavy quality of crude oil, there is an increasing need for alumina with high specific surface area and large pore volume as a catalyst carrier to improve the internal diffusion rate, promote the transfer of long-chain molecules, and improve the catalytic activity. The spatial structure of the material synthesized by the template-induced crystallization method depends on the structure guidance of the template agent, so that the skeleton and pore structure will change. Some template agents cause the collapse of the pore structure of pseudo-boehmite during peptization and heat treatment. Therefore, a template inducer with improved crystal order is needed to balance the basic properties and reaction performance of the pore material. In addition, the expansion of the pore structure of the synthesized pseudo-boehmite will inevitably lead to a decrease in the specific surface area, which is not conducive to the uniform dispersion of the active phase, thereby affecting the catalytic activity of the catalyst. SUMMARY

[0005] The present application aims to provide a method for synthesizing high-purity, large specific surface area, large pore volume pseudo-boehmite by alcohol aluminum hydrolysis method, and then preparing alumina pellets by combining the rotary liquid film reactor with template-induced crystallization, which has high specific surface area, rich pore structure, high crush strength and good stability. High specific surface area is beneficial to provide more active sites, increase the contact between active sites and reactants, and improve the reaction rate; large pore volume is beneficial to improve the dispersity of metal catalytic active centers and provide the required channels for the diffusion of metal active centers in the subsequent preparation of supported solid catalysts. The pore structure and specific surface area are expanded to improve the loading performance and increase the dispersity of active components. The alumina pellets are suitable for oxidation, dehydrogenation, reforming, polymerization and other reaction processes in the field of petroleum chemical industry, and are particularly suitable for continuous reforming, propane dehydrogenation and other reactions.

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

[0007] A method for preparing alumina pellets with large specific surface area and large pore volume, which specifically comprises the following steps:

[0008] (a) adding alcohol aluminum and an organic solvent into a rotary liquid film reactor at the same time, mixing, setting temperature and stirring to dissolve the alcohol aluminum;

[0009] (b) adding deionized water and a template inducer into the alcohol aluminum organic solution prepared in step (a) and stirring for a certain time to fully react;

[0010] (c) transferring the mixed slurry of step (b) to a hydrothermal crystallization reactor for crystal growth, setting a specific temperature and time for crystallization reaction;

[0011] (d) discharging, filtering and drying to constant weight after the crystallization is completed to obtain pseudo-boehmite powder;

[0012] (e) dissolving the pseudo-boehmite obtained in step (d) in water to obtain a pseudo-boehmite suspension, and adding acid solution for peptization;

[0013] (f) adding a solidifying agent into the aluminum sol prepared in step (e);

[0014] (g) dropping the slurry obtained in step (f) into a hot oil column to form spherical gel particles;

[0015] (h) taking out the gel pellets of step (g), and then aging, washing, drying and calcining to obtain alumina pellets.

[0016] Preferably, the alcohol aluminum in (a) is one or more of methanol aluminum, ethanol aluminum, isopropyl alcohol aluminum, sec-butyl alcohol aluminum, n-hexyl alcohol aluminum and isooctyl alcohol aluminum;

[0017] Preferably, the organic solvent in (a) is one of isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, n-pentyl alcohol, n-hexyl alcohol, and tetrahydrofuran;

[0018] Preferably, the molar ratio of the aluminum alcoholate to the organic solvent in (a) is 1:2-6;

[0019] Preferably, the dissolution temperature in (a) is 60-100 ℃, and the stirring speed is 300-600 rpm;

[0020] Preferably, the molar ratio of the aluminum alcoholate to the deionized water in (b) is 1:2-6; the rotating liquid membrane reactor is set to have a stirring time of 10 min-2 h;

[0021] Preferably, the template inducer in (b) is EO 20 PO 10 EO 20 ;

[0022] Preferably, the crystallization temperature in (c) is 100-160 ℃, and the crystallization time is 4-14 h;

[0023] Preferably, the drying temperature in (d) is 80-120 ℃;

[0024] Preferably, the mass percentage content of aluminum oxide in the pseudoboehmite suspension in (e) is 5-30%;

[0025] Preferably, the mass ratio of the acid solution to aluminum oxide in the pseudoboehmite suspension in (e) is 0.1-1, and the pH is controlled to be 2-5; the acid solution is one of aqueous solutions of nitric acid, hydrochloric acid, sulfuric acid, and formic acid with a mass percentage concentration of 3-30%;

[0026] Preferably, the mass ratio of the solidifying agent to aluminum oxide in the aluminum sol in (f) is 0.2-0.6; the solidifying agent is one or a mixture of both of hexamethylenetetramine and urea;

[0027] Preferably, the forming oil in the hot oil column in (g) is one of vacuum pump oil, mechanical oil, white oil, spindle oil, and mineral oil;

[0028] Preferably, the oil column temperature in (g) is 90-99 ℃;

[0029] Preferably, the aging temperature in (h) is 120-180 ℃, the aging time is 12-48 h; the drying temperature is 50-100 ℃, the drying time is 12-24 h; the calcination temperature is 550-1050 ℃, and the calcination time is 3-6 h.

[0030] The present application has the following advantages:

[0031] The method adopts the method of combining the rotary liquid film reactor with the template induced crystallization to prepare the pseudo-boehmite by hydrolyzing the aluminum alcoholate, and adopts the oil column forming to prepare the alumina pellets. The rotary liquid film reactor is used to promote the solid-liquid to rapidly and massively nucleate under the high shearing speed, increase the solid-liquid contact area, and increase the specific surface area of the pseudo-boehmite; the EO 20 PO 10 EO 20 As the macromolecular, structure-regular and stable template inducer, it spontaneously and orderly aggregates to form the micelles in the aqueous solution, the aluminum source is self-assembled along the outer surface of the micelles after the hydrolysis in the solution, the template induced crystallization controls the morphology of the crystal growth and regulates the size of the crystal, promotes the order degree of the crystal to increase and the grain distribution to be uniform, and the stable and orderly mesoporous structure is formed after the crystallization treatment. The alumina pellets prepared by the powder obtained by the method have the characteristics of expanding the pore structure and the specific surface area, improving the loading performance, and increasing the dispersion of the active component.

[0032] The method adopts the rotary liquid film reactor and the template induced crystallization to complete the hydrolysis process of the aluminum isopropylate. A large number of nucleation is promoted under the high shearing speed, the nucleation in the aging treatment process is prevented, the solid-liquid contact area is increased, and the specific surface area of the pseudo-boehmite is increased; compared with the pore expanding agents such as DMF and DMHA, the EO 20 PO 10 EO 20 The template induced crystallization controls the morphology of the crystal growth and regulates the size of the crystal, can promote the order degree of the crystal to increase and the grain distribution to be uniform; the pseudo-boehmite with the larger pore structure and specific surface area and the uniform grain is obtained, and the alumina pellets prepared by the further oil column forming have the larger specific surface area and pore structure. It is suitable for the oxidation, dehydrogenation, reforming, polymerization and other reaction processes in the petroleum chemical field, and is especially suitable for the continuous reforming, propane dehydrogenation and other reactions. DETAILED DESCRIPTION

[0033] In order to make the above features and advantages of the present application more obvious and easy to understand, the following examples are specifically described. The method of the present application is the conventional method in the field unless otherwise specified.

[0034] Example 1

[0035] A, 60 g of aluminum isopropylate and 65 g of isopropyl alcohol are added into the rotary liquid film reactor, the temperature is set to 80 ℃, the rotating speed is 300 rpm, and the operation is performed for 1 h to make the aluminum isopropylate completely dissolved; 5 g of EO 20 PO 10 EO 20The mixed solution was obtained by dissolving in 25 g of deionized water, and was added into the rotating liquid membrane reactor at a set flow rate of 20 g / h using a peristaltic pump, keeping the temperature at 80 °C, and after the water was added, stirring was continued for 30 min. The above mixed slurry was transferred to a hydrothermal crystallization reactor, and crystallization was carried out at 120 °C for 4 h. The crystallized slurry was filtered and dried at 120 °C for 12 h until the product mass no longer changed, to obtain a pseudoboehmite sample PB-1.

[0036] B. 10 g of the above self-made 100 mesh sieved pseudoboehmite PB-1 was taken, 30 g of deionized water and 1.5 g of 20 wt% nitric acid solution were added while stirring, to obtain an aluminum sol (pH 2-5). 1 g of 35 wt% hexamethylenetetramine solution was added to the aluminum sol, and after fully stirring and mixing, it was dripped into a 90 °C oil column filled with vacuum pump oil using a dispersion drop head to form, and the formed gel small balls were separated from the oil, and aged in an aging oven at 140 °C for 10 hours. The aged sol balls were washed, then dried at 60 °C for 12 hours to constant weight, and then calcined at 650 °C for 5 hours to obtain alumina small balls SALO-1.

[0037] Example 2

[0038] A. 60 g of aluminum isopropoxide and 65 g of isopropyl alcohol were added to a rotating liquid membrane reactor, the temperature was set to 80 °C, and the rotation speed was 600 rpm, and the aluminum isopropoxide was completely dissolved after running for 1 h; 5 g of EO 20 PO 10 EO 20 The mixed solution was obtained by dissolving in 25 g of deionized water, and was added into the rotating liquid membrane reactor at a set flow rate of 20 g / h using a peristaltic pump, keeping the temperature at 80 °C, and after the water was added, stirring was continued for 1 h. The above mixed slurry was transferred to a hydrothermal crystallization reactor, and crystallization was carried out at 120 °C for 4 h. The crystallized slurry was filtered and dried at 120 °C for 12 h until the product mass no longer changed, to obtain a pseudoboehmite sample PB-2.

[0039] B. 10 g of the above self-made 100 mesh sieved pseudoboehmite PB-2 was taken, 30 g of deionized water and 1.5 g of 20 wt% nitric acid solution were added while stirring, to obtain an aluminum sol (pH 2-5). 1 g of 35 wt% hexamethylenetetramine solution was added to the aluminum sol, and after fully stirring and mixing, it was dripped into a 90 °C oil column filled with vacuum pump oil using a dispersion drop head to form, and the formed gel small balls were separated from the oil, and aged in an aging oven at 140 °C for 10 hours. The aged sol balls were washed, then dried at 60 °C for 12 hours to constant weight, and then calcined at 650 °C for 5 hours to obtain alumina small balls SALO-2.

[0040] Example 3

[0041] A, 60 g aluminum isopropoxide and 65 g isopropyl alcohol were added into a rotating liquid film reactor, the temperature was set to 80 °C, the rotating speed was set to 300 rpm, and the reaction was run for 1 h to completely dissolve the aluminum isopropoxide; 5 g EO 20 PO 10 EO 20 was dissolved in 25 g of deionized water to obtain a mixed solution, and the rotating liquid film reactor was added with the mixed solution at a set flow rate of 20 g / h using a peristaltic pump, the temperature was kept at 80 °C, and after the water addition was completed, the stirring was continued for 30 min. The mixed slurry was transferred to a hydrothermal crystallization reactor, and the crystallization reaction was carried out at 140 °C for 4 h. The crystallized slurry was filtered and dried at 120 °C for 12 h until the product mass no longer changed, to obtain a pseudoboehmite sample PB-3.

[0042] B, 10 g of the above self-made 100-mesh sieved pseudoboehmite PB-3 was added into 30 g of deionized water and 1.5 g of a 20 wt% nitric acid solution, and continuously stirred to obtain an aluminum sol (pH 2-5). 1 g of a 35 wt% hexamethylenetetramine solution was added into the aluminum sol, and after the mixture was fully stirred, the dispersion drop head was used to drop the mixture into an oil column containing vacuum pump oil at 90 °C to form a gel ball. The formed gel ball was separated from the oil, and was aged in an aging oven at 140 °C for 10 h. The aged sol ball was washed, and then dried at 60 °C for 12 h until the mass was constant, and then calcined at 650 °C for 5 h to obtain an alumina ball SALO-3.

[0043] Example 4

[0044] A, 60 g aluminum isopropoxide and 65 g isopropyl alcohol were added into a rotating liquid film reactor, the temperature was set to 80 °C, the rotating speed was set to 300 rpm, and the reaction was run for 1 h to completely dissolve the aluminum isopropoxide; 5 g EO 20 PO 10 EO 20 was dissolved in 25 g of deionized water to obtain a mixed solution, and the rotating liquid film reactor was added with the mixed solution at a set flow rate of 20 g / h using a peristaltic pump, the temperature was kept at 80 °C, and after the water addition was completed, the stirring was continued for 30 min. The mixed slurry was transferred to a hydrothermal crystallization reactor, and the crystallization reaction was carried out at 140 °C for 4 h. The crystallized slurry was filtered and dried at 120 °C for 12 h until the product mass no longer changed, to obtain a pseudoboehmite sample PB-3.

[0045] B, take 10 g of the above self-made 100 mesh sieved after pseudo-boehmite PB-4, add 30 g of deionized water and 1.5 g of 20 wt% nitric acid solution after continuous stirring, obtain aluminum sol (pH 2~5). In the aluminum sol, add 1 g of 35 wt% hexamethyl tetramine solution, fully stir and mix, then drop into the oil column of 90 ℃ filled with vacuum pump oil by dispersion drop head, separate the shaped gel small balls from the oil, and age in the aging kettle of 140 ℃ for 10 hours. Wash the aged sol balls, then dry at 60 ℃ for 12 hours to constant weight, and then calcine at 650 ℃ for 5 hours, to obtain alumina small balls SALO-4.

[0046] Example 5

[0047] A, take 60 g of aluminum isopropoxide and 80 g of isopropyl alcohol into a rotating liquid membrane reactor, set the temperature to 80 ℃, the rotating speed to 600 rpm, and run for 1 h to completely dissolve the aluminum isopropoxide; dissolve 7.5 g of EO 20 PO 10 EO 20 in 25 g of deionized water to obtain a mixed solution, and add into the rotating liquid membrane reactor by using a peristaltic pump at a set flow rate of 20 g / h, keep the temperature at 80 ℃, and continue stirring for 30 min after the water addition is completed. Transfer the above mixed slurry to a hydrothermal crystallization reaction kettle, and crystallize at 140 ℃ for 4 h. Filter the crystallized slurry and dry at 120 ℃ for 12 h until the product mass no longer changes, to obtain a pseudo-boehmite sample PB-5.

[0048] B, take 10 g of the above self-made 100 mesh sieved after pseudo-boehmite PB-5, add 30 g of deionized water and 1.5 g of 20 wt% nitric acid solution after continuous stirring, obtain aluminum sol (pH 2~5). In the aluminum sol, add 1 g of 35 wt% hexamethyl tetramine solution, fully stir and mix, then drop into the oil column of 90 ℃ filled with vacuum pump oil by dispersion drop head, separate the shaped gel small balls from the oil, and age in the aging kettle of 140 ℃ for 10 hours. Wash the aged sol balls, then dry at 60 ℃ for 12 hours to constant weight, and then calcine at 650 ℃ for 5 hours, to obtain alumina small balls SALO-5.

[0049] Comparative Example 1

[0050] A, take 60 g of aluminum isopropoxide and 65 g of isopropyl alcohol into a three-necked flask, place in an 80 ℃ oil bath, and mechanically stir to dissolve the aluminum isopropoxide. Dissolve 5 g of EO 20 PO 10 EO 20The mixed solution was dissolved in 25 g of deionized water, and was added into a three-necked flask at a set flow rate of 20 g / h using a peristaltic pump, and the temperature was kept at 80 °C. After the water was added, the stirring was continued for 30 min. The mixed slurry was transferred to a hydrothermal crystallization reactor, and was aged at 120 °C for 4 h. The crystallized slurry was filtered and dried at 120 °C for 12 h until the product mass no longer changed, to obtain a pseudoboehmite sample CPB-1.

[0051] B. 10 g of the self-made 100-mesh sieved pseudoboehmite CPB-1 was taken, 30 g of deionized water and 1.5 g of a 20 wt% nitric acid solution were added, and stirring was continuously performed, to obtain an aluminum sol (pH 2-5). 1 g of a 35 wt% hexamethylenetetramine solution was added to the aluminum sol, and after the mixture was fully stirred, it was dripped into an oil column containing vacuum pump oil at 90 °C using a dispersion drop head to form a gel. The formed gel was separated from the oil, and was aged in an aging oven at 140 °C for 10 h. The aged sol was washed, and was dried at 60 °C for 12 h until the mass was constant. Then, the sol was calcined at 650 °C for 5 h, to obtain an alumina pellet CSALO-1.

[0052] Comparative Example 2

[0053] A. 60 g of aluminum isopropoxide and 65 g of isopropanol were added into a rotating liquid membrane reactor, and the temperature was set at 80 °C and the rotating speed was set at 300 rpm, to run for 1 h to completely dissolve the aluminum isopropoxide. 5 g of DMF and 25 g of deionized water were mixed, and were added into the rotating liquid membrane reactor at a set flow rate of 20 g / h using a peristaltic pump, and the temperature was kept at 80 °C. After the water was added, the stirring was continued for 30 min. The mixed slurry was transferred to a hydrothermal crystallization reactor, and was crystallized at 120 °C for 4 h. The crystallized slurry was filtered and dried at 120 °C for 12 h until the product mass no longer changed, to obtain a pseudoboehmite sample CPB-2.

[0054] B. 10 g of the self-made 100-mesh sieved pseudoboehmite PB-1 was taken, 30 g of deionized water and 1.5 g of a 20 wt% nitric acid solution were added, and stirring was continuously performed, to obtain an aluminum sol (pH 2-5). 1 g of a 35 wt% hexamethylenetetramine solution was added to the aluminum sol, and after the mixture was fully stirred, it was dripped into an oil column containing vacuum pump oil at 90 °C using a dispersion drop head to form a gel. The formed gel was separated from the oil, and was aged in an aging oven at 140 °C for 10 h. The aged sol was washed, and was dried at 60 °C for 12 h until the mass was constant. Then, the sol was calcined at 650 °C for 5 h, to obtain an alumina pellet CSALO-2.

[0055] Comparative Example 3

[0056] Take 60 g of isopropyl alcohol aluminum and 65 g of isopropyl alcohol into a three-necked flask which is placed in an 80 ℃ oil bath, and mechanically stir to dissolve the isopropyl alcohol aluminum. Use a peristaltic pump to add 25 g of deionized water into the three-necked flask at a set flow rate of 20 g / h, and keep the temperature at 80 ℃. After the water is added, continue to stir for 30 min. Transfer the above mixed slurry to a hydrothermal crystallization reactor, and age at 120 ℃ for 4 h. After crystallization, the slurry is filtered and dried at 120 ℃ for 12 h until the product mass no longer changes, to obtain a pseudoboehmite sample CPB-3.

[0057] The alumina pellets CSALO-3 are prepared by the same method as in Example 1.

[0058] The specific surface area and pore volume of the alumina pellets of the present application are determined by low-temperature liquid nitrogen adsorption. The physical property parameters of the above alumina pellets are shown in Table 1.

[0059] Table 1

[0060]

[0061] It is found from the experimental data that, when the hydrolysis process of pseudoboehmite is completed by using a rotating liquid membrane reactor, the specific surface area of the sample is greatly increased at high shear speed, because the solid-liquid contact area is increased during the reaction, which promotes a large number of nucleation. 20 PO 10 EO 20 Compared with DMF, the alumina pellets prepared by using the template inducing agent have a larger pore volume, and do not cause a decrease in the specific surface area. It can be seen that the pseudoboehmite prepared by combining the rotating liquid membrane reactor with the template-induced crystallization method has the characteristics of large specific surface area and large pore volume, and the alumina pellets prepared by oil column forming have the characteristics of large specific surface area and large pore volume.

[0062] The above description is only the preferred embodiment of the present application, and any equivalent changes and modifications made within the scope of the patent application of the present application shall be included in the scope of the present application.

Claims

1. A process for the preparation of large surface area, large pore volume alumina pellets, characterized in that, The method comprises the following steps: (a) mixing methanol aluminum and organic solvent in a rotating liquid film reactor, setting temperature and stirring to dissolve the methanol aluminum; (b) adding deionized water and template inducer to the methanol aluminum organic solution prepared in step (a) and stirring for a certain time to fully react; (c) transferring the mixed slurry of step (b) to a hydrothermal crystallization reactor for crystal growth, setting specific temperature and time for crystallization reaction; (d) discharging, filtering and drying to constant weight after the crystallization is completed to obtain pseudo-boehmite powder; (e) dissolving the pseudo-boehmite obtained in step (d) in water to obtain a pseudo-boehmite suspension, and adding acid to perform peptization; (f) adding a solidifying agent to the aluminum sol prepared in step (e); (g) dropping the slurry obtained in step (f) into a hot oil column to form spherical gel particles; (h) taking out the gel beads of step (g), and performing aging, washing, drying and calcination to obtain alumina beads. The molar ratio of the alcohol aluminum in step (b) to deionized water is 1:2-6; the stirring time is 10 min-2 h; the template inducer is EO 20 PO 10 EO 20 ; The crystallization temperature in step (c) is 100-160 ℃, and the crystallization time is 4-14 h.

2. The method of claim 1, wherein, The methanol aluminum in step (a) is one or more of methanol aluminum, ethanol aluminum, isopropanol aluminum, sec-butanol aluminum, n-hexanol aluminum and isooctanol aluminum; the organic solvent is one of isopropanol, n-butanol, sec-butanol, n-pentanol, n-hexanol and tetrahydrofuran; the molar ratio of methanol aluminum to organic solvent is 1:2-6; the dissolution temperature is 60-100 ℃, and the stirring speed is 300-600 rpm.

3. The method of claim 1, wherein, The drying temperature in step (d) is 80-120 ℃.

4. The method of claim 1, wherein, The mass percentage content of alumina in the pseudo-boehmite suspension in step (e) is 5-30%; the mass ratio of the acid to alumina in the pseudo-boehmite suspension is 0.1-1, and the pH is controlled at 2-5; the acid is one of 3-30% nitric acid, hydrochloric acid, sulfuric acid or formic acid aqueous solution.

5. The method of claim 1, wherein, The mass ratio of the solidifying agent to alumina in the aluminum sol in step (f) is 0.2-0.6; the solidifying agent is one or a mixture of the two of hexamethyltetramine and urea.

6. The method of claim 1, wherein, The molding oil in the hot oil column in step (g) is one of vacuum pump oil, mechanical oil, white oil, spindle oil and mineral oil; the oil column temperature is 90-99 ℃.

7. The method of claim 1, wherein, The aging temperature in step (h) is 120-180 ℃, the aging time is 12-48 h; the drying temperature is 50-100 ℃, the drying time is 12-24 h; the calcination temperature is 550-1050 ℃, and the calcination time is 3-6 h.

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