A method for preparing large pore volume spherical alumina

Abstract

The present invention provides a method for preparing large-pore spherical alumina, and relates to the technical field of spherical alumina. The preparation method of the present invention comprises the following steps: mixing pseudo-boehmite powder with water to obtain a suspension, adding a peptizing agent for peptization to prepare an aluminum hydroxide sol; adding cross-linking molecules to the aluminum hydroxide sol to form a system to be cross-linked; adding a cross-linking agent to the system to be cross-linked to obtain a sol containing a prepolymer; and subjecting the sol to oil-ammonia column molding, drying and roasting to obtain large-pore spherical alumina. The present invention adopts an oil-ammonia column molding method, which does not require the addition of a pore-expanding agent or new equipment, and the preparation process is simple and does not introduce new impurities, so it is suitable for industrial production. The large-pore spherical alumina prepared by the present invention has a pore volume of up to 0.60cm 3 / g or more, and can be used as a propane dehydrogenation catalyst carrier.

Description

Technical Field

[0001] The invention relates to the technical field of spherical alumina, and in particular to a preparation method of large-pore-volume spherical alumina. Background Art

[0002] Alumina has a high specific surface area, good chemical stability and thermal stability, and excellent mechanical strength. Its crystal phase and pore structure are diverse, suitable for a variety of catalytic reactions, and has been widely used in many fields such as petrochemicals, environmental protection, and automobile exhaust treatment. For example, in the field of petrochemicals, alumina carriers are used to prepare hydrogenation catalysts, cracking catalysts, etc.; in the field of environmental protection, it is used to prepare VOCs treatment catalysts; in the field of automobile exhaust treatment, it is used to prepare three-way catalysts, etc. In addition, the regeneration rate and safety of alumina catalysts are high, which is conducive to industrial applications. In industrial applications, spherical alumina has good particle fluidity, which is conducive to uniform distribution and mixing during the processing process. Its spherical structure reduces wear during mixing and processing, and extends the life of the equipment. It is currently the most widely used shaped alumina.

[0003] The methods for preparing spherical alumina carriers include rolling ball forming method, hot oil column forming method, oil-ammonia column ball forming method, and spray dispersion ball forming method. Among them, the oil-ammonia column forming method has simple forming operation and high production efficiency, and is one of the important methods for preparing high-quality spherical alumina in industry. The general process of the oil-ammonia column ball forming method is: dissolve the aluminum source in acid to prepare a mixed sol with a certain viscosity; the upper part of the forming column is an oil layer and the lower part is an ammonia water layer, and a surfactant is continuously injected between the two layers. The sol is dripped in, and the droplets are spherical due to surface tension. Under the action of gravity, they pass through the interface and solidify in the ammonia water layer; then the finished spherical alumina is obtained after washing, drying, and roasting.

[0004] The invention patent with the publication number CN108947568A discloses a method for preparing hollow alumina ball bricks using pore-forming agents. The method pre-mixes three kinds of micropowders, namely Al2O3 micropowder, clay fine powder and silicon micropowder, as the matrix part; fully stirs the spherical pore-forming agent of 50 to 150 μm with silica sol solution, so that the silica sol solution fully wets and adheres to the surface of the spherical pore-forming agent; then mixes and granulates the mixed powder of the three kinds of micropowders as the matrix with the spherical pore-forming agent attached with silica sol solution to obtain coated particles of the spherical pore-forming agent coated by the mixed powder; then fully mixes the hollow alumina ball with the binder, adds the coated particles and mixes again, and finally uses vibration and pressure to prepare various shaped products of required sizes, and obtains hollow alumina ball bricks after drying, heat preservation and firing. The obtained hollow alumina ball bricks have the characteristics of good high temperature creep resistance and excellent thermal shock resistance.

[0005] The invention patent with the publication number CN113563103A discloses a method for preparing gradient alumina porous ceramics by tape casting, using rice husk as a pore-forming agent, crushing and sieving the rice husk through a crusher to obtain rice husk powder of different specifications; 50 to 90 parts of spherical alumina, 1 to 40 parts of rice husk powder, 1 to 10 parts of sintering aids, 10 to 30 parts of binders and 1 to 10 parts of dispersants are dispersed in a solvent by mass, and ball milled into a uniform slurry; the slurry is vacuum-defoamed, and formed on a tape casting machine to obtain alumina porous ceramic green body, and the green body with different particle size pore-forming agents is stacked together in descending order according to the particle size of the pore-forming agent, and the green body of the superimposed alumina porous ceramic is pressurized in the thickness direction, and the secondary extrusion molding is performed, and the binder is removed and sintered to obtain gradient alumina porous ceramics. The porous ceramic sample obtained by this method has multi-level pores, uniform and easy-to-control pore size, the preparation method is simple and easy to operate, the raw materials are easy to obtain, and the full utilization of resources is achieved.

[0006] It can be seen that in the current spherical alumina preparation process, it is generally necessary to use a pore-forming agent or a pore-enlarging agent to expand the pores and adjust the macropore structure to ensure the comprehensive performance of the spherical alumina. However, after high-temperature calcination, the pore structure of the spherical alumina is prone to collapse, the product has poor stability and low strength, and seriously affects the use effect. Therefore, it is necessary to study and develop a preparation process for high-strength, large-pore volume spherical alumina that can still maintain a good pore structure after high-temperature calcination. Summary of the invention

[0007] In view of this, the present invention provides a method for preparing large-pore-volume spherical alumina, which introduces a cross-linked structure to form a network structure inside the small ball, and forms interconnected pores after high-temperature calcination, thereby finally obtaining high-strength large-pore-volume spherical alumina. The use of pore expanding agents and the introduction of impurity elements are avoided, and finally spherical alumina with the advantages of high strength and large pore volume is obtained.

[0008] The preparation method of the macroporous spherical alumina of the present invention comprises the following steps:

[0009] Pseudo-boehmite powder is mixed with water to obtain a suspension, and then a peptizing agent is added for peptization to obtain an aluminum hydroxide sol; a cross-linking molecule is added to the aluminum hydroxide sol to obtain a pre-cross-linking system; a cross-linking agent is added to the pre-cross-linking system to obtain a sol containing a prepolymer, and the sol is formed, dried, and calcined to obtain a large-pore spherical aluminum oxide;

[0010] The cross-linking molecule is at least one of soluble starch, cellulose, alginate, polyvinyl amide, and polyvinyl alcohol; the cross-linking agent is at least one of aldehydes such as formaldehyde, chloroacetaldehyde, propionaldehyde, and butyraldehyde.

[0011] Preferably, the solid content of the suspension is 10wt% to 30wt%.

[0012] Preferably, the peptizing agent is at least one of nitric acid solution, acetic acid solution and citric acid solution;

[0013] Preferably, the mass ratio of the peptizing agent to water is 1:(15-50).

[0014] Preferably, the mass ratio of the cross-linking molecules to the pseudo-boehmite powder is (0.1-1):1.

[0015] Preferably, the mass ratio of the cross-linking agent to the pseudo-boehmite powder is (0.1-1):1;

[0016] Preferably, the mass ratio of the cross-linking agent to the cross-linking molecules is (1.5-2.5):1.

[0017] Preferably, the drying temperature is 40° C. to 120° C., and the drying time is 6 to 24 hours; the roasting temperature is 600° C. to 1200° C., and the roasting time is 2 to 8 hours, and the roasting atmosphere is air.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are:

[0019] 1. The present invention adopts an oil-ammonia column molding method to prepare spherical alumina, adds a cross-linking compound to the sol, and optimizes the pore structure through the network structure formed by the cross-linking compound, so that the spherical alumina has a higher pore volume and specific surface area;

[0020] 2. The cross-linked compound used in the present invention can optimize the microporous structure and connect the pores. Compared with the currently commonly used pore-enlarging agents, the pore structure is better and the strength is higher;

[0021] 3. The present invention does not introduce impurity elements during the preparation of spherical alumina, and the added cross-linking compound can be completely removed during the high-temperature roasting process. The preparation process is simple and does not require the addition of new equipment. The large-scale industrial production of spherical alumina carriers can be quickly realized on the existing molding device;

[0022] 4. The spherical alumina prepared by the present invention has a pore volume of 0.60 cm 3 / g or more, and can be used as a catalyst carrier for propane dehydrogenation reaction. DETAILED DESCRIPTION

[0023] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0024] The invention provides a method for preparing large-pore spherical alumina. The method introduces a cross-linking structure to form a network structure inside a small ball, and then forms interconnected pores through high-temperature calcination, and finally obtains a high-strength large-pore spherical alumina.

[0025] The preparation method of the macroporous spherical alumina of the present invention comprises the following steps:

[0026] S1, adding water to pseudo-boehmite powder to prepare a suspension, adding a peptizing agent to the suspension for peptization to obtain an aluminum hydroxide sol;

[0027] S2, adding cross-linking molecules to the aluminum hydroxide sol of step S1 and stirring evenly to obtain a pre-cross-linking system;

[0028] S3, adding a crosslinking agent to the pre-crosslinking system of step S2, and mixing uniformly to obtain a sol containing a prepolymer;

[0029] S4, dripping the sol in step S3 into an oil-ammonia column to form it, and obtaining large-pore spherical alumina after drying and calcining.

[0030] The first step of the present invention is to prepare aluminum hydroxide sol. In some specific embodiments of the present invention, the solid content of the suspension is preferably 10wt% to 30wt%; the peptizing agent is at least one of nitric acid solution, acetic acid solution, and citric acid solution; the mass concentration of the solute in the peptizing agent is 50% to 70%; the mass ratio of the peptizing agent to water is 1: (15 to 50). In some preferred embodiments of the present invention, the peptizing agent is at least one of nitric acid and / or acetic acid solution. The present invention successfully prepares pseudo-boehmite powder into aluminum hydroxide sol with uniform performance by reasonably selecting the peptizing agent and its ratio, laying the foundation for subsequent cross-linking and the formation of pore structure.

[0031] The second step of the present invention is to prepare a pre-crosslinked system. In some specific embodiments of the present invention, the crosslinked molecules are at least one of soluble starch, cellulose, alginate, polyvinyl amide, and polyvinyl alcohol; the mass ratio of the crosslinked molecules to the pseudo-boehmite powder is (0.1-1):1. In some preferred embodiments of the present invention, the crosslinked molecules are soluble starch and / or polyvinyl amide. The present invention forms a uniform pre-crosslinked system by adding crosslinked molecules, providing a basis for the subsequent action of the crosslinking agent. The addition of crosslinked molecules helps to form a network structure during the subsequent calcination process, thereby generating uniformly connected large pores inside the alumina spheres and increasing the pore volume of the material. The ratio of crosslinked molecules to pseudo-boehmite powder affects the pore volume and material strength, so the mass ratio of the crosslinked molecules to the pseudo-boehmite powder of the present invention is preferably (0.1-1):1.

[0032] The third step of the present invention is to prepare a sol containing a prepolymer, that is, a system to be cross-linked. In some specific embodiments of the present invention, the cross-linking agent is at least one of formaldehyde, chloroacetaldehyde, propionaldehyde, butyraldehyde and other aldehydes; the mass ratio of the cross-linking agent to the pseudo-boehmite powder is (0.1-1):1; the mass ratio of the cross-linking agent to the cross-linking molecule is (1.5-2.5):1. The present invention adds a cross-linking agent to the aluminum hydroxide sol to promote a cross-linking reaction between the cross-linking molecules to form a more stable prepolymer structure. The cross-linking reaction constructs a preliminary network structure at the molecular level, laying the foundation for the connectivity of the pores in the subsequent molding and calcination. At the same time, the prepolymer formed by the cross-linking reaction is evenly distributed in the sol, which improves the bonding performance and molding stability of the material. In addition, the degree of the cross-linking reaction directly affects the retention of the network structure and the formation of large pores during the subsequent calcination process, thereby improving the pore volume and strength of the material. The mass ratio of the crosslinking agent to the pseudo-boehmite powder affects the formation of the network structure. Therefore, the present invention controls the mass ratio of the crosslinking agent to the crosslinking molecule to be (1.5-2.5):1 to ensure that the crosslinking molecules can fully react to form a uniform and stable prepolymer.

[0033] The fourth step of the present invention is the preparation of macroporous spherical alumina. In some specific embodiments of the present invention, the drying temperature is 40°C to 120°C, the drying time is 6 to 24 hours; the calcination temperature is 600°C to 1200°C, the calcination time is 2 to 8 hours, and the calcination atmosphere is air. In some preferred embodiments of the present invention, the drying temperature is 60°C to 100°C, the drying time is 6 to 12 hours; the calcination temperature is 800°C to 1000°C, and the calcination time is 4 to 6 hours. The present invention forms the sol in the oil phase by dripping the prepolymer-containing sol into the oil-ammonia column, solidifies it in ammonia water, and ensures that the size of the small balls is uniform, laying the foundation for subsequent drying and calcination. The drying step is used to remove moisture from the sphere, ensure the stability of the small ball structure and avoid cracks during calcination. The calcination step removes the organic matter remaining in the cross-linking agent and the cross-linking molecule, and at the same time strengthens the network structure of the spherical alumina through high temperature. During the high-temperature calcination process, the cross-linked network structure is further transformed into stable interconnected channels, forming a large pore volume structure. Calcination also improves the mechanical strength and thermal stability of the alumina spheres to meet the needs of practical applications.

[0034] In order to further illustrate the present invention, the following examples are provided for detailed description. The raw materials used in the following examples of the present invention are all commercially available.

[0035] Unless otherwise specified, all experiments were repeated 3 times and the results were expressed as mean values.

[0036] Example 1 A method for preparing large pore volume spherical alumina, the steps are as follows:

[0037] S1. Take 15g of pseudo-boehmite powder and add it into 78g of deionized water to form a suspension. Slowly add 2g of nitric acid (67%) to the suspension for peptization reaction for 1h. After the reaction is completed, aluminum hydroxide sol is obtained;

[0038] S2, adding 2 g of soluble starch to the aluminum hydroxide sol and stirring evenly to obtain a system to be cross-linked;

[0039] S3, adding 3g of formaldehyde to the system to be cross-linked to obtain a pre-cross-linked system;

[0040] S4. Add the pre-crosslinked system drop by drop into the oil-ammonia column to form the particles. Then place the formed particles in an oven at 60°C and dry for 6 hours. Then transfer them to a muffle furnace and calcine them at 800°C for 4 hours in an air atmosphere. After the calcination is completed, the product cools to room temperature to obtain large-pore spherical alumina.

[0041] Example 2 A method for preparing large pore volume spherical alumina, the steps are as follows:

[0042] S1. Take 20g of pseudo-boehmite powder and add it into 73g of deionized water to form a suspension. Slowly add 2g of nitric acid (67%) to the suspension for peptization reaction for 1h. After the reaction is completed, aluminum hydroxide sol is obtained;

[0043] S2, adding 2 g of soluble starch to the aluminum hydroxide sol and stirring evenly to obtain a system to be cross-linked;

[0044] S3, adding 3g of formaldehyde to the system to be cross-linked to obtain a pre-cross-linked system;

[0045] S4. Add the pre-crosslinked system drop by drop into the oil-ammonia column to form the particles. Then place the formed particles in an oven at 60°C and dry for 6 hours. Then transfer them to a muffle furnace and calcine them at 800°C for 4 hours in an air atmosphere. After the calcination is completed, the product cools to room temperature to obtain large-pore spherical alumina.

[0046] Example 3 A method for preparing large pore volume spherical alumina, the steps are as follows:

[0047] S1. Take 15g of pseudo-boehmite powder and add it into 78g of deionized water to form a suspension. Slowly add 2g of acetic acid (67%) to the suspension for peptization reaction for 1h. After the reaction is completed, aluminum hydroxide sol is obtained;

[0048] S2, adding 2 g of soluble starch to the aluminum hydroxide sol and stirring evenly to obtain a system to be cross-linked;

[0049] S3, adding 3g of formaldehyde to the system to be cross-linked to obtain a pre-cross-linked system;

[0050] S4. Add the pre-crosslinked system drop by drop into the oil-ammonia column to form the particles. Then place the formed particles in an oven at 60°C and dry for 6 hours. Then transfer them to a muffle furnace and calcine them at 800°C for 4 hours in an air atmosphere. After the calcination is completed, the product cools to room temperature to obtain large-pore spherical alumina.

[0051] Example 4 A method for preparing large pore volume spherical alumina, the steps are as follows:

[0052] S1. Take 15g of pseudo-boehmite powder and add it into 78g of deionized water to form a suspension. Slowly add 2g of nitric acid (67%) to the suspension for peptization reaction for 1h. After the reaction is completed, aluminum hydroxide sol is obtained;

[0053] S2, adding 2 g of polyvinyl amide to the aluminum hydroxide sol and stirring evenly to obtain a system to be cross-linked;

[0054] S3, adding 3g of formaldehyde to the system to be cross-linked to obtain a pre-cross-linked system;

[0055] S4. Add the pre-crosslinked system drop by drop into the oil-ammonia column to form the particles. Then place the formed particles in an oven at 60°C and dry for 6 hours. Then transfer them to a muffle furnace and calcine them at 800°C for 4 hours in an air atmosphere. After the calcination is completed, the product cools to room temperature to obtain large-pore spherical alumina.

[0056] Example 5 A method for preparing large pore volume spherical alumina, the steps are as follows:

[0057] S1. Take 15g of pseudo-boehmite powder and add it into 78g of deionized water to form a suspension. Slowly add 2g of nitric acid (67%) to the suspension for peptization reaction for 1h. After the reaction is completed, aluminum hydroxide sol is obtained;

[0058] S2, adding 2 g of soluble starch to the aluminum hydroxide sol and stirring evenly to obtain a system to be cross-linked;

[0059] S3, adding 3g of propionaldehyde to the system to be cross-linked to obtain a pre-cross-linked system;

[0060] S4. Add the pre-crosslinked system drop by drop into the oil-ammonia column to form the particles. Then place the formed particles in an oven at 60°C and dry for 6 hours. Then transfer them to a muffle furnace and calcine them at 800°C for 4 hours in an air atmosphere. After the calcination is completed, the product cools to room temperature to obtain large-pore spherical alumina.

[0061] Example 6 A method for preparing large pore volume spherical alumina, the steps are as follows:

[0062] S1. Take 15g of pseudo-boehmite powder and add it into 78g of deionized water to form a suspension. Slowly add 2g of nitric acid (67%) to the suspension for peptization reaction for 1h. After the reaction is completed, aluminum hydroxide sol is obtained;

[0063] S2, adding 2 g of soluble starch to the aluminum hydroxide sol and stirring evenly to obtain a system to be cross-linked;

[0064] S3, adding 3g of formaldehyde to the system to be cross-linked to obtain a pre-cross-linked system;

[0065] S4. Add the pre-crosslinked system drop by drop into the oil-ammonia column to form the particles. Then place the formed particles in an oven at 100°C and dry for 12 hours. Then transfer them to a muffle furnace and calcine them at 800°C for 4 hours in an air atmosphere. After the calcination is completed, the product cools to room temperature to obtain large-pore spherical alumina.

[0066] Example 7 A method for preparing large pore volume spherical alumina, the steps are as follows:

[0067] S1. Take 15g of pseudo-boehmite powder and add it into 78g of deionized water to form a suspension. Slowly add 2g of nitric acid (67%) to the suspension for peptization reaction for 1h. After the reaction is completed, aluminum hydroxide sol is obtained;

[0068] S2, adding 2 g of soluble starch to the aluminum hydroxide sol and stirring evenly to obtain a system to be cross-linked;

[0069] S3, adding 3g of formaldehyde to the system to be cross-linked to obtain a pre-cross-linked system;

[0070] S4. Add the pre-crosslinked system drop by drop into the oil-ammonia column to form the particles, then place the formed particles in an oven at 60°C and dry for 6 hours, then transfer them to a muffle furnace and calcine them at 1000°C in an air atmosphere for 6 hours. After the calcination is completed, the product cools to room temperature to obtain large-pore spherical alumina.

[0071] Comparative Example 1

[0072] S1. Take 15 g of pseudo-boehmite powder and add it into 83 g of deionized water to form a suspension. Slowly add 2 g of nitric acid (67%) to the suspension for peptization reaction for 1 hour. After the reaction is completed, aluminum hydroxide sol is obtained.

[0073] S2, add 5g of urea as a pore-enlarging agent to the aluminum hydroxide sol and stir evenly, then drip the mixture drop by drop into the oil-ammonia column to form;

[0074] S3. The formed particles are placed in an oven at 60°C and dried for 6 hours, and then transferred to a muffle furnace and calcined at 800°C for 4 hours in an air atmosphere. After the calcination is completed, the product is cooled to room temperature to obtain spherical alumina.

[0075] Comparative Example 2

[0076] S1. Take 20g of pseudo-boehmite powder and add it into 78g of deionized water to form a suspension. Slowly add 2g of nitric acid (67%) to the suspension for peptization reaction for 1h. After the reaction is completed, aluminum hydroxide sol is obtained;

[0077] S2, add 5g of urea as a pore-enlarging agent to the aluminum hydroxide sol and stir evenly, then drip the mixture drop by drop into the oil-ammonia column to form;

[0078] S3. The formed particles are placed in an oven at 60°C and dried for 6 hours, and then transferred to a muffle furnace and calcined at 800°C for 4 hours in an air atmosphere. After the calcination is completed, the product is cooled to room temperature to obtain large-pore spherical alumina.

[0079] Comparative Example 3

[0080] S1. Take 15 g of pseudo-boehmite powder and add it into 83 g of deionized water to form a suspension. Slowly add 2 g of acetic acid (67%) to the suspension for peptization reaction for 1 hour. After the reaction is completed, aluminum hydroxide sol is obtained.

[0081] S2, add 5g of urea as a pore-enlarging agent to the aluminum hydroxide sol and stir evenly, then drip the mixture drop by drop into the oil-ammonia column to form;

[0082] S3. The formed particles are placed in an oven at 60°C and dried for 6 hours, and then transferred to a muffle furnace and calcined at 800°C for 4 hours in an air atmosphere. After the calcination is completed, the product is cooled to room temperature to obtain large-pore spherical alumina.

[0083] The performance of the alumina products obtained in the examples and comparative examples was tested, and the results are shown in Table 1.

[0084] Table 1 Alumina product performance test results

[0085]

[0086]

[0087] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for preparing large pore volume spherical alumina, characterized in that: The following steps are involved: Pseudo-boehmite powder is mixed with water to obtain a suspension, and then a peptizing agent is added for peptization to obtain an aluminum hydroxide sol; a cross-linking molecule is added to the aluminum hydroxide sol to obtain a system to be cross-linked; a cross-linking agent is added to the system to be cross-linked to obtain a sol containing a prepolymer, and the sol is formed, dried, and calcined to obtain a large-pore spherical aluminum oxide; The cross-linking molecule is at least one of soluble starch, cellulose, alginate, polyvinyl amide, and polyvinyl alcohol; the cross-linking agent is at least one of aldehydes such as formaldehyde, chloroacetaldehyde, propionaldehyde, and butyraldehyde.

2. The preparation method according to claim 1, characterized in that: The solid content of the suspension is preferably 10 wt% to 30 wt%.

3. The preparation method according to claim 1, characterized in that: The peptizing agent is at least one of a nitric acid solution, an acetic acid solution and a citric acid solution.

4. The preparation method according to claim 1, characterized in that: The mass ratio of the peptizing agent to water is 1:(15-50).

5. The preparation method according to claim 1, characterized in that: The mass ratio of the cross-linking molecules to the pseudo-boehmite powder is (0.1-1):

1.

6. The preparation method according to claim 1, characterized in that: The cross-linking molecules are soluble starch and / or polyvinyl amide.

7. The preparation method according to claim 1, characterized in that: The mass ratio of the cross-linking agent to the pseudo-boehmite powder is (0.1-1):

1.

8. The preparation method according to claim 1, characterized in that: The mass ratio of the cross-linking agent to the cross-linking molecules is (1.5-2.5):

1.

9. The preparation method according to claim 1, characterized in that: The drying temperature is 40° C. to 120° C., and the drying time is 6 to 24 hours.

10. The preparation method according to claim 1, characterized in that: The calcination temperature is 600° C. to 1200° C., the calcination time is 2 to 8 hours, and the calcination atmosphere is air.

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