High-thermal-shock-resistance high-purity corundum brick and preparation method thereof

Abstract

The application relates to a high-thermal-shock-resistance high-purity corundum brick and a preparation method thereof, and belongs to the technical field of refractory materials. The high-thermal-shock-resistance high-purity corundum brick comprises the following raw materials in parts by mass: tabular corundum particles 55-65 parts, tabular corundum fine powder 20-25 parts, modified fused zirconium corundum particles 8-12 parts, nano-alumina powder 5-8 parts, rho-alumina powder 2-3 parts, a toughening agent 0.3-1 part, and a binding agent 0.3-1.5 parts. The toughening agent causes multiple strengthening mechanisms such as crack deflection, crack branching and micro-cracks, increases the fracture toughness of the high-thermal-shock-resistance high-purity corundum brick, and makes the high-thermal-shock-resistance high-purity corundum brick of the application have high thermal shock resistance and mechanical properties. Meanwhile, the fused zirconium corundum has a short fiber structure through modification, the fiber can hinder the straight-line expansion of the main crack, force the crack to deflect and form crack branching, and further enhance the thermal shock resistance and mechanical properties of the high-thermal-shock-resistance high-purity corundum brick.

Description

Technical Field

[0001] This invention belongs to the field of refractory materials technology, specifically relating to a high-purity corundum brick with high thermal shock resistance and its preparation method. Background Technology

[0002] High-purity corundum bricks with high thermal shock resistance refer to high-grade refractory products made from corundum and alumina powder, with an alumina content of 99%. These bricks have extremely low impurity content, containing less than 0.3% SiO2 and less than 0.2% Fe2O3. Therefore, they can be used in harsh environments, such as the Midrex reduction furnace for reducing iron from natural gas, coal chemical kilns for coal gasification, and hazardous waste kilns containing HF atmospheres. However, Fe2O3 is generally reduced to Fe3O4, FeO, and Fe in C and CO reducing atmospheres, causing cracks in the refractory bricks, loss of structural strength, and spalling. SiO2 is reduced to SiO gas in high-temperature reducing atmospheres containing a certain amount of H2, or forms SiF4 in HF atmospheres, causing structural loosening and loss of strength in the refractory bricks.

[0003] Chinese patent application CN117362015A discloses a high-purity corundum brick and its preparation method. This high-purity corundum brick contains, by weight percentage of the raw material oxides: 0.08–1.0% SiO2, 0.15–0.45% Na2O, 0.05–0.15% Li2O, 0.05–0.25% P2O5, 0.05–0.15% B2O3, 1.2–3.0% Cr2O3, and the balance being Al2O3 and impurities. Although the thermal shock resistance and mechanical properties of this high-purity corundum brick are improved through the compounding of the raw materials, its thermal shock resistance remains relatively weak in some harsh working conditions. Summary of the Invention

[0004] The first objective of this invention is to provide a high-purity corundum brick with high thermal shock resistance to solve the technical problem of poor thermal shock resistance of existing corundum bricks.

[0005] The second objective of this invention is to provide a method for preparing high-purity corundum bricks with high thermal shock resistance.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A high-purity corundum brick with high thermal shock resistance comprises the following raw materials by weight: 55-65 parts of tabular corundum particles, 20-25 parts of fine tabular corundum powder, 8-12 parts of modified fused zirconia corundum particles, 5-8 parts of nano-alumina powder, 2-3 parts of p-alumina powder, 0.3-1 parts of toughening agent, and 0.3-1.5 parts of binder.

[0008] Furthermore, the toughening agent is prepared by mixing YAlO3, SiAlON, and Al2O3 to obtain a mixture, mixing the mixture with a solvent to obtain a slurry, and then ball-milling, drying, calcining, and pulverizing the slurry to obtain the final product.

[0009] Furthermore, the mass ratio of YAlO3 to SiAlON is 1:15-20; the mass ratio of YAlO3 to Al2O3 is 1:1-5; the amount of solvent added accounts for 30-35% of the total mass of the mixture; and the solvent is ethanol.

[0010] Furthermore, the calcination step is as follows: under inert gas, the temperature is raised to 1200-1500℃ at a heating rate of 100-120℃ / h, and held for 2-3 hours.

[0011] Furthermore, the ball milling speed is 350-500 r / min, the ball milling time is 2-4 h, and the drying temperature is 80-120℃.

[0012] Furthermore, the modified fused zirconia corundum particles are prepared by mixing fused zirconia corundum, binder, and water evenly, then extruding and granulating them into columnar particles, and then firing the columnar particles at 1200-1500℃ to obtain the desired product; the diameter of the columnar particles is 20-30 μm and the length is 40-60 μm.

[0013] Furthermore, the mass ratio of the fused zirconia alumina to the binder is 20:0.05 to 0.1, and the mass ratio of the fused zirconia alumina to water is 20:0.8 to 1; the binder is carboxymethyl cellulose.

[0014] Furthermore, the toughening agent has a particle size of 25–40 μm; the binder is a sol; the tabular corundum particles comprise 23–27% coarse aggregate, 55–70% medium aggregate, and the remainder fine aggregate; the coarse aggregate comprises tabular corundum with a particle size of 5–3 mm; the medium aggregate comprises 25–30% tabular corundum with a particle size of 3–1 mm, 20–25% tabular corundum with a particle size of 1–0 mm, and 10–15% fine tabular corundum powder with a particle size of 1–0.088 mm; the fine aggregate comprises micro-powdered tabular corundum with a particle size <0.088 mm.

[0015] A method for preparing a high-purity corundum brick with high thermal shock resistance includes the following steps: mixing the prescribed amounts of tabular corundum particles, tabular corundum fine powder, modified fused zirconia corundum particles, nano-alumina powder, and p-alumina powder with water; continuously stirring with hydrogen chloride gas for 10-20 minutes; then filtering and drying to obtain substance A; mixing substance A, toughening agent, and binder; then adding 3-7% of the total mass of water from the raw materials; stirring evenly; then pressing in a mold to remove air bubbles to form a green body; and finally firing the green body to obtain the final product.

[0016] Furthermore, the firing process is as follows: first, the temperature is raised to 200-400℃ at a heating rate of 60-80℃ / h and held for 1-3 hours; then, the temperature is raised to 900-1000℃ at a heating rate of 150-180℃ / h and held for 2-4 hours; finally, the temperature is raised to 1600-1650℃ at a heating rate of 30-60℃ / h and held for 8-12 hours.

[0017] The beneficial effects of this invention are:

[0018] The toughening agent of this invention increases the fracture toughness of high-thermal-shock-resistant high-purity corundum bricks through multiple strengthening mechanisms, including crack deflection, crack branching, and microcrack formation. This results in high thermal shock resistance and mechanical properties in the high-thermal-shock-resistant high-purity corundum bricks of this invention. Simultaneously, this invention modifies fused zirconia corundum to have a short-fiber structure. These fibers can hinder the linear propagation of the main crack, forcing crack deflection and crack branching, further enhancing the thermal shock resistance and mechanical properties of the high-thermal-shock-resistant high-purity corundum bricks. The p-alumina added in this invention can be converted to α-alumina by calcination at high temperatures. α-Alumina significantly increases the thermal shock resistance and mechanical strength of the high-thermal-shock-resistant high-purity corundum bricks of this invention.

[0019] Impurities such as Fe2O3 in high-purity corundum bricks are reduced to Fe3O4, FeO, and Fe in a reducing atmosphere of C and CO, causing cracks in the refractory bricks, resulting in loss of structural strength and spalling. The high thermal shock resistant high-purity corundum bricks of this invention incorporate hydrogen chloride gas during preparation. The hydrogen chloride gas reacts with Fe2O3 to form water-soluble FeCl3, preventing the reduction of Fe2O3 in the high-purity corundum bricks in a reducing atmosphere that could lead to cracking. Furthermore, the raw materials used in this invention are all free of silicon dioxide, preventing the silicon dioxide in the high thermal shock resistant high-purity corundum bricks from being corroded by HF gas or reduced in a strong reducing atmosphere, thus avoiding structural damage to the refractory bricks. Detailed Implementation

[0020] The present invention will be further described below with reference to embodiments thereof.

[0021] The content of Al2O3 in the tabular corundum particles is ≥99.2%, Fe2O3 ≤0.2%, and SiO2 ≤0.2%. The particle size of the nano-alumina powder is 50–100 nm. The particle size of the fine tabular corundum powder is ≤0.074 mm. The particle size of the ρ-alumina powder is ≤0.1 μm.

[0022] Example 1

[0023] Example 1, a high-purity corundum brick with high thermal shock resistance, comprises the following raw materials: 55 kg of tabular corundum, 23 kg of fine tabular corundum powder, 9 kg of modified fused zirconia corundum particles, 5 kg of nano-alumina powder, 2 kg of p-alumina powder, 0.3 kg of toughening agent, and 0.3 kg of sol. The toughening agent has a particle size of 30 μm. The tabular corundum includes coarse aggregate, medium aggregate, and the remainder fine aggregate. The coarse aggregate includes 25% tabular corundum with a particle size of 5–3 mm; the medium aggregate includes 28% tabular corundum with a particle size of 3–1 mm, 22% tabular corundum with a particle size of 1–0 mm, and 15% fine tabular corundum powder with a particle size of 1–0.088 mm; the fine aggregate includes micro-powdered tabular corundum with a particle size <0.088 mm.

[0024] The toughening agent is prepared by mixing 0.5 kg of YAlO3 powder, 10 kg of SiAlON powder, and 0.5 kg of Al2O3 powder to obtain a mixture. 3.5 kg of ethanol is added to the mixture to obtain a slurry. The slurry is ball-milled at 350 rpm for 2 hours and dried at 100°C to obtain a solid. The solid is heated to 1200°C in nitrogen at a heating rate of 120°C / h and held at that temperature for 2 hours. After cooling at room temperature, it is pulverized to obtain the final product.

[0025] The modified fused zirconia alumina particles are prepared as follows: 20 kg of fused zirconia alumina, 0.05 kg of carboxymethyl cellulose, and 0.8 kg of water are mixed evenly, then extruded and granulated into columnar particles. During the extrusion granulation process, continuous drying is carried out to prevent particle adhesion. After drying, the columnar particles are calcined at 1200℃ to obtain the final product. The columnar particles have a diameter of 20 μm and a length of 50 μm.

[0026] The preparation method of high-purity corundum bricks with high thermal shock resistance is as follows: The formula amounts of tabular corundum, fine tabular corundum powder, modified fused zirconia corundum particles, nano-alumina powder, and p-alumina powder are mixed with a small amount of water. HCl gas is passed through the water and the mixture is stirred continuously for 10 minutes. The mixture is then filtered and dried to obtain substance A. Substance A, toughening agent, sol, and 4 kg of water are mixed and stirred evenly to obtain a clay mixture. The clay mixture is placed in a mold and pressed to remove air bubbles, forming a blank. After firing the blank, the desired product is obtained.

[0027] The firing process is as follows: heat the temperature to 200℃ at a rate of 60℃ / h and hold for 2 hours; then heat the temperature to 900℃ at a rate of 180℃ / h and hold for 2 hours; finally heat the temperature to 1630℃ at a rate of 30℃ / h and hold for 8 hours.

[0028] Example 2

[0029] Example 2's high thermal shock resistant high-purity corundum brick comprises the following raw materials: 65 kg of tabular corundum, 25 kg of fine tabular corundum powder, 8 kg of modified fused zirconia corundum particles, 5 kg of nano-alumina powder, 3 kg of p-alumina powder, 0.8 kg of toughening agent, and 1.2 kg of sol. The toughening agent has a particle size of 40 μm. The tabular corundum includes coarse aggregate, medium aggregate, and the remainder fine aggregate. The coarse aggregate includes 27% tabular corundum with a particle size of 5–3 mm; the medium aggregate includes 30% tabular corundum with a particle size of 3–1 mm, 20% tabular corundum with a particle size of 1–0 mm, and 10% fine tabular corundum powder with a particle size of 1–0.088 mm; the fine aggregate includes micro-powdered tabular corundum with a particle size <0.088 mm.

[0030] The toughening agent is prepared by mixing 1 kg of YAlO3 powder, 15 kg of SiAlON powder, and 3 kg of Al2O3 powder to obtain a mixture. 6.5 kg of ethanol is added to the mixture to obtain a slurry. The slurry is ball-milled at 500 rpm for 2 hours and dried at 120°C to obtain a solid. The solid is heated to 1350°C in nitrogen at a heating rate of 100°C / h and held at that temperature for 2 hours. After cooling at room temperature, it is pulverized to obtain the final product.

[0031] The modified fused zirconia corundum particles are prepared as follows: 20 kg of fused zirconia corundum, 0.1 kg of carboxymethyl cellulose, and 1 kg of water are mixed evenly, and then extruded and granulated into columnar particles. During the extrusion granulation process, continuous drying is carried out to prevent particle adhesion. After drying, the columnar particles are calcined at 1400℃ to obtain the final product. The diameter of the columnar particles is 30 μm and the length is 40 μm.

[0032] The preparation method of high-purity corundum bricks with high thermal shock resistance is as follows: The formula amounts of tabular corundum, fine tabular corundum powder, modified fused zirconia corundum particles, nano-alumina powder, and p-alumina powder are mixed with a small amount of water. HCl gas is passed through the water and the mixture is continuously stirred for 20 minutes. The mixture is then filtered and dried to obtain substance A. Substance A, toughening agent, sol, and 7 kg of water are mixed and stirred evenly to obtain a clay mixture. The clay mixture is placed in a mold and pressed to remove air bubbles, forming a blank. After firing the blank, the desired product is obtained.

[0033] The firing process is as follows: the temperature is increased to 350℃ at a rate of 80℃ / h and held for 1 hour; then the temperature is increased to 1000℃ at a rate of 150℃ / h and held for 2 hours; finally, the temperature is increased to 1600℃ at a rate of 50℃ / h and held for 12 hours.

[0034] Example 3

[0035] Example 3's high thermal shock resistant high-purity corundum brick comprises the following raw materials: 60 kg of tabular corundum, 20 kg of fine tabular corundum powder, 12 kg of modified fused zirconia corundum particles, 8 kg of nano-alumina powder, 2.5 kg of p-alumina powder, 1 kg of toughening agent, and 1.5 kg of sol. The toughening agent has a particle size of 25 μm. The tabular corundum includes coarse aggregate, medium aggregate, and the remainder fine aggregate. The coarse aggregate includes 23% tabular corundum with a particle size of 5–3 mm; the medium aggregate includes 25% tabular corundum with a particle size of 3–1 mm, 20% tabular corundum with a particle size of 1–0 mm, and 10% fine tabular corundum powder with a particle size of 1–0.088 mm; the fine aggregate includes micro-powdered tabular corundum with a particle size <0.088 mm.

[0036] The toughening agent is prepared by mixing 1 kg of YAlO3 powder, 16 kg of SiAlON powder, and 5 kg of Al2O3 powder to obtain a mixture. 7 kg of ethanol is added to the mixture to obtain a slurry. The slurry is ball-milled at 400 rpm for 4 hours and dried at 100°C to obtain a solid. The solid is heated to 1500°C in nitrogen at a heating rate of 120°C / h and held at that temperature for 2 hours. After cooling at room temperature, it is pulverized to obtain the final product.

[0037] The modified fused zirconia corundum particles are prepared as follows: 20 kg of fused zirconia corundum, 0.07 kg of carboxymethyl cellulose, and 0.9 kg of water are mixed evenly, then extruded and granulated into columnar particles. During the extrusion granulation process, continuous drying is carried out to prevent particle adhesion. After drying, the columnar particles are calcined at 1500℃ to obtain the final product. The columnar particles have a diameter of 20 μm and a length of 60 μm.

[0038] The preparation method of high-purity corundum bricks with high thermal shock resistance is as follows: The formula amounts of tabular corundum, fine tabular corundum powder, modified fused zirconia corundum particles, nano-alumina powder, and p-alumina powder are mixed with a small amount of water. HCl gas is passed through the water and the mixture is continuously stirred for 10 minutes. The mixture is then filtered and dried to obtain substance A. Substance A, toughening agent, sol, and 5 kg of water are mixed and stirred evenly to obtain a clay mixture. The clay mixture is placed in a mold and pressed to remove air bubbles, forming a blank. After firing the blank, the desired product is obtained.

[0039] The firing process is as follows: heat the temperature to 400℃ at a rate of 70℃ / h and hold for 1 hour; then heat the temperature to 1000℃ at a rate of 180℃ / h and hold for 2 hours; finally heat the temperature to 1650℃ at a rate of 60℃ / h and hold for 12 hours.

[0040] Comparative Example 1

[0041] The raw materials for the high-purity corundum brick in Comparative Example 1 are the same as those in Example 1.

[0042] The preparation method of the high-purity corundum brick of Comparative Example 1 is as follows: the formula amounts of tabular corundum, tabular corundum fine powder, modified fused zirconia corundum particles, nano alumina powder, ρ-alumina powder, toughening agent, sol and 4 kg of water are mixed and stirred evenly to obtain mud. The mud is placed in a mold and pressed to remove air bubbles to form a blank. After firing the blank, the brick is obtained.

[0043] The firing process is as follows: heat the temperature to 200℃ at a rate of 60℃ / h and hold for 2 hours; then heat the temperature to 900℃ at a rate of 180℃ / h and hold for 2 hours; finally heat the temperature to 1630℃ at a rate of 30℃ / h and hold for 8 hours.

[0044] Comparative Example 2

[0045] The preparation method of the high-purity corundum brick in Comparative Example 2 is roughly the same as that in Example 1. The difference between the preparation method of the high-purity corundum brick in Comparative Example 2 and Example 1 is that no toughening agent was added in Comparative Example 2.

[0046] Comparative Example 3

[0047] The preparation method of the high-purity corundum brick in Comparative Example 3 is roughly the same as that in Example 1. The difference between the preparation method of the high-purity corundum brick in Comparative Example 3 and Example 1 is that the modified fused zirconia corundum particles in Example 1 are replaced with fused zirconia corundum particles of equal mass in Comparative Example 3.

[0048] Experimental Example 1

[0049] Performance testing

[0050] 1. Thermal shock resistance test: Place the sample in a resistance furnace and keep it at 1100℃ for 20 minutes. Remove it and quench it in cold water for 5 minutes. Place it in the air for 5 minutes. Then put it back into the resistance furnace and keep it at 1100℃ for 5 minutes. Repeat this process 10 times.

[0051] 2. Mechanical property testing;

[0052] 3. Iron oxide content test: The Fe2O3 content in high-purity corundum bricks was determined by sodium thiosulfate titration.

[0053] 4. Silica content test;

[0054] 5. Porosity test;

[0055] 6. Bulk density test.

[0056] Table 1. Performance test results of high-purity corundum bricks in Examples 1-3 and Comparative Examples 1-3

[0057]

[0058]

[0059] As shown in Table 1, the high-purity corundum brick with high thermal shock resistance prepared by this invention exhibits good thermal shock resistance and mechanical properties, making it suitable for use under conditions of large temperature fluctuations. Furthermore, the high-purity corundum brick with high thermal shock resistance prepared by this invention has a low iron oxide content, preventing reduction in a reducing atmosphere that could affect its structural strength. In addition, the high-purity corundum brick with high thermal shock resistance of this invention has a very low silica content, preventing silica from being corroded by HF gas or reduced in a strong reducing atmosphere, thus avoiding structural damage to the refractory brick.

[0060] Experimental Example 2

[0061] The high-purity corundum bricks from Examples 1-3 and Comparative Examples 1-3 were placed in a high-temperature furnace at 1100℃ and kept at that temperature for 20 minutes. Reducing gases such as CO, H2, and water vapor were continuously introduced into the high-temperature furnace. At the same time, Al2O3 particles with a particle size of 50-100μm were sprayed at high speed onto the surface of the high-purity corundum bricks from Examples 1 and Comparative Examples 1. After 20 minutes, the high-purity corundum bricks from Examples 1 and Comparative Examples 1 were removed and placed in cold water for rapid cooling. They were then placed in air for 5 minutes and then placed back into the high-temperature furnace for 5 minutes. Reducing gases such as CO, H2, and water vapor, as well as Al2O3 particles, were continuously introduced into the high-temperature furnace. This process was repeated 50 times. The flexural strength of Examples 1-3 and Comparative Examples 1-3 after testing is shown in Table 2.

[0062] Table 2. Room temperature flexural strength of high-purity corundum bricks in Examples 1-3 and Comparative Examples 1-3

[0063] sample Flexural strength at room temperature (MPa) Flexural strength at room temperature after testing (MPa) Example 1 28.7 18.3 Example 2 30.2 20.5 Example 3 29.5 19.7 Comparative Example 1 26.9 9.5 Comparative Example 2 25.7 9.8 Comparative Example 3 24.5 10.1

[0064] As shown in Table 2, the flexural strength of the high-purity corundum bricks in Examples 1-3 remained relatively high compared to their room-temperature flexural strength, while the flexural strength of the high-purity corundum bricks in Comparative Examples 1-3 decreased significantly. This is because the high-purity corundum brick in Comparative Document 1 had a higher Fe2O3 content, and the Fe2O3 was reduced by the reducing gas, affecting the structural strength of the high-purity corundum brick. Comparative Documents 2 and 3 did not contain toughening agents or modified fused zirconia corundum particles, resulting in poorer fracture toughness.

Claims

1. A method for producing a high-purity corundum brick having high thermal shock resistance, characterized by comprising: High thermal shock resistance high purity corundum brick includes the following raw materials by mass fraction: tabular corundum particles 55-65 parts, tabular corundum fine powder 20-25 parts, modified fused zirconia corundum particles 8-12 parts, nano alumina powder 5-8 parts, p-alumina powder 2-3 parts, toughening agent 0.3-1 part, binder 0.3-1.5 parts; ​ The preparation method of high thermal shock resistance high purity corundum brick includes the following steps: mixing tabular corundum particles, tabular corundum fine powder, modified fused zirconia corundum particles, nano alumina powder, p-alumina powder and water in formula amount, bubbling hydrogen chloride gas in water for 10-20 min, then filtering and drying to obtain material A, mixing material A, toughening agent and binder, then adding 3-7 % of water of total raw material mass, stirring uniformly, then placing in a mold to press out bubbles to form a green body, and firing the green body to form a product. The preparation method of the toughening agent is: mixing YAlO3, SiAlON and Al2O3 to obtain a mixture, mixing the mixture with a solvent to obtain a slurry, ball milling, drying, calcining and crushing the slurry to obtain the toughening agent. The preparation method of the modified fused zirconia corundum particles is: mixing fused zirconia corundum, a binder and water uniformly, extruding and granulating into columnar particles, and firing the columnar particles at 1200-1500 ℃ to obtain the modified fused zirconia corundum particles; the diameter of the columnar particles is 20-30 μm, and the length is 40-60 μm.

2. The method of producing high thermal shock resistance high purity corundum bricks according to claim 1, characterized in that, The mass ratio of YAlO3 to SiAlON is 1:15-20, the mass ratio of YAlO3 to Al2O3 is 1:1-5, the addition amount of the solvent is 30-35 % of the total mass of the mixture, and the solvent is ethanol.

3. The method of producing high thermal shock resistance high purity corundum bricks according to claim 1, characterized in that, In the preparation method of the toughening agent, the calcining step is: heating to 1200-1500 ℃ at a heating rate of 100-120 ℃ / h under an inert gas, and keeping the temperature for 2-3 h.

4. The method of producing high thermal shock resistance high purity corundum bricks according to claim 1, characterized in that, In the preparation method of the toughening agent, the rotating speed of the ball mill is 350-500 r / min, and the ball milling time is 2-4 h; the drying temperature is 80-120 ℃.

5. The method of producing high thermal shock resistance high purity corundum bricks according to claim 1, characterized in that, In the preparation method of the modified fused zirconia corundum particles, the mass ratio of fused zirconia corundum to the binder is 20:0.05-0.1, and the mass ratio of fused zirconia corundum to water is 20:0.8-1; the binder is carboxymethyl cellulose.

6. The method of producing high thermal shock resistance high purity corundum bricks according to claim 1, characterized in that, The particle size of the toughening agent is 25-40 μm; the binder is sol; the tabular corundum particles include 23-27 % coarse aggregate, 55-70 % medium aggregate and the balance of fine aggregate; the coarse aggregate includes tabular corundum with a particle size of 5-3 mm; the medium aggregate includes 25-30 % tabular corundum with a particle size of 3-1 mm, 20-25 % tabular corundum with a particle size of 1-0 mm and 10-15 % tabular corundum fine powder with a particle size of 1-0.088 mm; and the fine aggregate includes tabular corundum micropowder with a particle size of less than 0.088 mm.

7. The method of producing high thermal shock resistance high purity corundum bricks according to claim 1, characterized in that, The preparation method of the high-thermal shock resistance high-purity corundum brick comprises the following steps: firing, wherein the firing comprises the following steps: firstly, heating at a heating rate of 60-80 ℃ / h to 200-400 ℃, and keeping the temperature for 1-3 h; secondly, heating at a heating rate of 150-180 ℃ / h to 900-1000 ℃, and keeping the temperature for 2-4 h; and finally, heating at a heating rate of 30-60 ℃ / h to 1600-1650 ℃, and keeping the temperature for 8-12 h.

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