Chromia-zirconia corundum composite brick for garbage incineration grate furnace

Abstract

The present application relates to the technical field of firebrick, in particular to a kind of chrome-zirconia corundum composite brick for waste incinerator grate furnace, the composite brick includes the following components by weight parts: white corundum powder 60-75 parts, chromium sesquioxide 7-14 parts, zirconia-silicon carbide composite powder 12-20 parts, magnesium oxide 2-5 parts, kaolin 3-6 parts, binder 1-4 parts;The zirconia-silicon carbide composite powder is made from the following raw materials by weight parts: tetraethyl orthosilicate 0.8-2 parts, anhydrous ethanol 2.5-6 parts, nano zirconia 0.8-1.5 parts, sucrose 2-3 parts, trimethoxysilane 0.4-1 part, deionized water 5.5-6.5 parts.The composite brick of the present application has high density, few internal defects, excellent refractory performance, small deformation under high temperature environment, excellent corrosion resistance, and the damage of incineration slag to the composite brick is small, which can effectively reduce the occurrence of fracture phenomenon and improve the service life of incinerator.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of firebrick, in particular to a chromium-zirconium corundum composite brick for waste incinerator grate furnace. BACKGROUND

[0002] At present, a large amount of garbage produced in life has seriously affected people's life and health, so it is necessary to treat these garbage. Since the garbage incineration method has the advantages of large volume reduction, timely treatment, high degree of harmlessness, and recyclable heat energy, etc., the garbage incineration method has become the current main garbage treatment method.

[0003] When incinerating garbage, an incinerator needs to be used. Since the working temperature in the incinerator is as high as 1300 DEG C, in order to improve the service life of the incinerator, a firebrick needs to be used for building. The existing firebrick has poor fire resistance, and in the long-term use process, the incineration slag will cause damage to the firebrick, so that the compressive property of the firebrick becomes poor, and eventually leads to fracture. SUMMARY

[0004] In order to solve the above problems, the present application provides a chromium-zirconium corundum composite brick for waste incinerator grate furnace.

[0005] The technical scheme of the present application is: a chromium-zirconium corundum composite brick for waste incinerator grate furnace, characterized in that the composite brick comprises the following components by weight: white corundum powder 60-75 parts, chromium sesquioxide 7-14 parts, zirconia-silicon carbide composite powder 12-20 parts, magnesium oxide 2-5 parts, kaolin 3-6 parts, and bonding agent 1-4 parts.

[0006] The zirconia-silicon carbide composite powder is made of the following raw materials by weight: tetraethyl orthosilicate 0.8-2 parts, anhydrous ethanol 2.5-6 parts, nano zirconia 0.8-1.5 parts, sucrose 2-3 parts, trimethoxysilane 0.4-1 part, and deionized water 5.5-6.5 parts.

[0007] Description: The composite brick of the above components has high density, few internal defects, excellent fire resistance, small deformation under high temperature environment, and excellent corrosion resistance, so that the damage of the composite brick by incineration slag is small, the fracture phenomenon can be effectively reduced, and the service life of the incinerator is improved.

[0008] Further, the white corundum powder is mixed by coarse powder and fine powder, wherein the coarse powder accounts for 60-75% of the total mass of the white corundum powder, and the particle size of the coarse powder is 0.1-4 mm, and the particle size of the fine powder is 15-60 microns.

[0009] Description: By using two kinds of white corundum powder with different particle sizes, the density of the composite brick is ensured, and the production cost is reduced.

[0010] Further, the adhesive is a 25-40% mass concentration aqueous dextrin solution.

[0011] Description: The adhesive of the above components has good effect, and can ensure the structural strength of the composite brick.

[0012] Further, the preparation method of the zirconia-silicon carbide composite powder comprises the following steps:

[0013] S1, according to the weight parts, tetraethyl orthosilicate is dissolved in anhydrous ethanol, and after stirring uniformly, a precursor solution is obtained; nano-zirconia and sucrose are added to deionized water, and after ultrasonic dispersion for 10-20 min, trimethoxysilane is added to obtain a dispersion liquid;

[0014] S2, the precursor solution is added to the dispersion liquid, and stirred for 30-60 min, then the dispersion liquid is heated to 40-60℃, and dilute hydrochloric acid is gradually added during the heating process, the pH value of the dispersion liquid is adjusted to 2, then it is placed at room temperature for 6-9 h to obtain a mixed gel;

[0015] S3, the mixed gel is dried, ground in sequence to obtain a mixed powder, then the mixed powder is calcined under argon protective atmosphere, the calcination temperature is 1300-1500℃, the calcination time is 1-2 h, and the zirconia-silicon carbide composite powder is obtained after calcination.

[0016] Description: The above preparation method attaches silicon gel to the surface of nano-zirconia through tetraethyl orthosilicate, and in-situ generates nano-silicon carbide through calcination, to obtain zirconia-silicon carbide composite powder, which is dispersedly distributed in the composite brick, improving the structural strength and corrosion resistance of the composite brick.

[0017] Further, in step S2, the mass concentration of the dilute hydrochloric acid is 10-15%.

[0018] Description: The above concentration of dilute hydrochloric acid is easy to prepare, and has good effect when adjusting the pH value.

[0019] Further, in step S3, the drying temperature is 100-120℃, and the drying time is 30-45 min.

[0020] Description: Defining the drying parameters can ensure the drying effect and avoid residual moisture in the mixed gel.

[0021] Further, in step S1, the particle size of the nano-zirconia is 300-500 nm.

[0022] Description: Defining the particle size of the nano-zirconia can ensure the particle size of the zirconia-silicon carbide composite powder, so that the zirconia-silicon carbide composite powder can effectively strengthen the performance of the composite brick.

[0023] Further, the preparation method of the composite brick comprises the following steps:

[0024] Step 1: Put white corundum powder, chromium sesquioxide, zirconia-silicon carbide composite powder, magnesium oxide and kaolin into a mixer and mix for 10-15 min to obtain a mixture;

[0025] Step 2: Add a binder and 30-40% of water based on the total weight of the mixture to the mixture, uniformly stir to obtain a mud, inject the mud into a mold, and use a molding press to press the mud in the mold into a green brick under a pressure of 20-40 MPa;

[0026] Step 3: Dry the green brick at 90-110℃ for 20-25 h, then put the green brick into a tunnel kiln for sintering, and obtain the composite brick after sintering.

[0027] Description: The composite brick prepared by the above preparation method has high density, few internal defects and excellent refractory performance, and can fully meet the daily use requirements of the incinerator.

[0028] Further, the temperature curve of the sintering process is as follows: increase the temperature to 500-650℃ at a temperature increasing rate of 15-20℃ / min, keep the temperature for 0.5-1 h; then increase the temperature to 800-900℃ at a temperature increasing rate of 10-14℃ / min, keep the temperature for 1-2 h; then increase the temperature to 1100-1200℃ at a temperature increasing rate of 6-10℃ / min, keep the temperature for 2-3 h; then increase the temperature to 1350-1450℃ at a temperature increasing rate of 2-5℃ / min, keep the temperature for 5-6 h; and then increase the temperature to 1500-1600℃ at a temperature increasing rate of 0.5-2℃ / min, keep the temperature for 35-40 h.

[0029] Description: The above temperature curve increases the sintering speed through multi-stage temperature increasing and keeping, and avoids cracks in the green brick caused by too fast temperature rising, thereby ensuring the strength of the composite brick.

[0030] The beneficial effects of the present application are:

[0031] (1) The composite brick of the present application has high density, few internal defects, excellent refractory performance, small deformation in a high temperature environment, excellent corrosion resistance, and small damage to the composite brick caused by incineration slag, thereby effectively reducing the occurrence of fracture and prolonging the service life of the incinerator.

[0032] (2) The present application attaches silicon gel to the surface of nano zirconia through tetraethyl orthosilicate, and generates nano silicon carbide in situ through calcination of the silicon gel to obtain zirconia-silicon carbide composite powder, which is dispersedly distributed in the composite brick to improve the structural strength and corrosion resistance of the composite brick. DETAILED DESCRIPTION

[0033] In order to further illustrate the means taken by the present application and the effects achieved, the technical solutions of the present application will be clearly and completely described below in conjunction with experiments.

[0034] Example 1: A kind of chrome zirconia corundum composite brick for waste incinerator grate furnace, the composite brick includes the following components by weight parts: white corundum powder 70 parts, chromium sesquioxide 10 parts, zirconia-silicon carbide composite powder 16 parts, magnesium oxide 3 parts, kaolin 5 parts, binder 3 parts;White corundum powder is mixed by coarse powder and fine powder;Among them, the coarse powder accounts for 60-75% of the total mass of white corundum powder, and the particle size of the coarse powder is 0.1-4mm, and the particle size of the fine powder is 15-60μm;The binder is a 30% mass concentration of dextrin aqueous solution;

[0035] The zirconia-silicon carbide composite powder is made of the following raw materials by weight parts: tetraethyl orthosilicate 1.4 parts, anhydrous ethanol 4.2 parts, nano zirconia 1.2 parts, sucrose 2.5 parts, trimethoxysilane 0.7 parts, deionized water 6 parts;

[0036] The preparation method of the zirconia-silicon carbide composite powder includes the following steps:

[0037] S1, according to the weight, dissolve tetraethyl orthosilicate in anhydrous ethanol, stir uniformly, then get the precursor solution;Nano zirconia and sucrose are added to deionized water, ultrasonic dispersion for 15min, then trimethoxysilane is added, and a dispersion liquid is obtained;Among them, the particle size of the nano zirconia is 300-500nm;

[0038] S2, add the precursor solution to the dispersion liquid, stir for 45min, then heat the dispersion liquid to 50℃, and gradually add dilute hydrochloric acid during heating, adjust the pH value of the dispersion liquid to 2, then place at room temperature for 8h, get mixed gel;Among them, the mass concentration of the dilute hydrochloric acid is 12%;

[0039] S3, the mixed gel is dried, ground in sequence to get mixed powder, then calcined under argon protection atmosphere, the calcination temperature is 1400℃, the calcination time is 1.5h, after calcination, the zirconia-silicon carbide composite powder is obtained;Among them, the drying temperature is 110℃, and the drying time is 40min;

[0040] The preparation method of the composite brick includes the following steps:

[0041] Step 1: Put white corundum powder, chromium sesquioxide, zirconia-silicon carbide composite powder, magnesium oxide and kaolin into a mixer, mix for 12min to get a mixture;

[0042] Step 2: the binder and 35% water of the total weight of the mixture are added into the mixture, and the mixture is stirred to obtain a mud, the mud is injected into a mold, and the mud in the mold is pressed into a brick body by a molding press under a pressure of 30 MPa;

[0043] Step 3: the brick body is dried at 100℃ for 22h, and then the brick body is put into a tunnel kiln for sintering, and a composite brick is obtained after the sintering is completed; wherein, the temperature curve of the sintering process is: the temperature is raised to 600℃ at a temperature raising speed of 18℃ / min, and the temperature is kept for 0.75h; then the temperature is raised to 850℃ at a temperature raising speed of 12℃ / min, and the temperature is kept for 1.5h; then the temperature is raised to 1150℃ at a temperature raising speed of 8℃ / min, and the temperature is kept for 2.5h; then the temperature is raised to 1400℃ at a temperature raising speed of 4℃ / min, and the temperature is kept for 5.5h; then the temperature is raised to 1550℃ at a temperature raising speed of 1℃ / min, and the temperature is kept for 38h.

[0044] Example 2: the example is basically the same as example 1, except that the composite brick comprises the following components by weight: white corundum powder 60 parts, chromium sesquioxide 7 parts, zirconia-silicon carbide composite powder 12 parts, magnesium oxide 2 parts, kaolin 3 parts, and binder 1 part.

[0045] Example 3: the example is basically the same as example 1, except that the composite brick comprises the following components by weight: white corundum powder 75 parts, chromium sesquioxide 14 parts, zirconia-silicon carbide composite powder 20 parts, magnesium oxide 5 parts, kaolin 6 parts, and binder 4 parts.

[0046] Example 4: the example is basically the same as example 1, except that the zirconia-silicon carbide composite powder is made of the following raw materials by weight: tetraethyl orthosilicate 0.8 parts, anhydrous ethanol 2.5 parts, nano zirconia 0.8 parts, sucrose 2 parts, trimethoxysilane 0.4 parts, and deionized water 5.5 parts.

[0047] Example 5: the example is basically the same as example 1, except that the zirconia-silicon carbide composite powder is made of the following raw materials by weight: tetraethyl orthosilicate 2 parts, anhydrous ethanol 6 parts, nano zirconia 1.5 parts, sucrose 3 parts, trimethoxysilane 1 part, and deionized water 6.5 parts.

[0048] Example 6: the example is basically the same as example 1, except that a precursor solution is added into the dispersion liquid, stirred for 30min, then the dispersion liquid is heated to 40℃, and dilute hydrochloric acid is gradually added during the heating process, the pH value of the dispersion liquid is adjusted to 2, then it is placed at room temperature for 6h to obtain a mixed gel.

[0049] Example 7: This example is basically the same as Example 1 except that the precursor solution is added to the dispersion, stirred for 60 min, then the dispersion is heated to 60°C, and dilute hydrochloric acid is gradually added during the heating process to adjust the pH of the dispersion to 2, then left to stand at room temperature for 9 h to obtain a mixed gel.

[0050] Example 8: This example is basically the same as Example 1 except that the calcination temperature is 1300°C and the calcination time is 1 h.

[0051] Example 9: This example is basically the same as Example 1 except that the calcination temperature is 1500°C and the calcination time is 2 h.

[0052] Example 10: This example is basically the same as Example 1 except that a binder is added to the mixture, and 30% of the total weight of the mixture is water.

[0053] Example 11: This example is basically the same as Example 1 except that a binder is added to the mixture, and 40% of the total weight of the mixture is water.

[0054] Example 12: This example is basically the same as Example 1 except that the green bricks are dried at 90°C for 20 h.

[0055] Example 13: This example is basically the same as Example 1 except that the green bricks are dried at 110°C for 25 h.

[0056] Example 14: This example is basically the same as Example 1 except that the temperature profile of the sintering process is as follows: heating to 500°C at a rate of 15°C / min, holding for 0.5 h; heating to 800°C at a rate of 10°C / min, holding for 1 h; heating to 1100°C at a rate of 6°C / min, holding for 2 h; heating to 1350°C at a rate of 2°C / min, holding for 5 h; heating to 1500°C at a rate of 0.5°C / min, holding for 35 h.

[0057] Example 15: This example is basically the same as Example 1 except that the temperature profile of the sintering process is as follows: heating to 650°C at a rate of 20°C / min, holding for 1 h; heating to 900°C at a rate of 14°C / min, holding for 2 h; heating to 1200°C at a rate of 10°C / min, holding for 3 h; heating to 1450°C at a rate of 5°C / min, holding for 6 h; heating to 1600°C at a rate of 2°C / min, holding for 40 h.

[0058] Experimental Example: The performance of the composite bricks prepared in each example is tested to explore the influence of the parameters of each example on the performance of the composite bricks, and the specific exploration is as follows:

[0059] 1. Explore the influence of the composition of the composite brick on the performance

[0060] Examples 1, 2, and 3 are used as experimental comparisons, and Example 1 is used as a reference. Pure zirconia powder is used to replace the zirconia-silicon carbide composite powder as Comparative Example 1. The performance of the composite bricks under different compositions is shown in Table 1 as follows:

[0061] Table 1 Performance of composite bricks under different compositions

[0062]

[0063]

[0064] From the data in Table 1, the load softening temperature of Examples 1, 2, and 3 is greater than 1850℃, indicating that the high-temperature resistance of the composite bricks is excellent, and the bulk density and compressive strength of the composite brick of Example 1 are the highest, indicating that the performance of the composite brick of Example 1 is better, and the composition of the composite brick of Example 1 is more optimal. Compared with Comparative Example 1, the bulk density, compressive strength, and load softening temperature of Example 1 are higher than those of Comparative Example 1, indicating that the performance of the composite brick decreases after using pure zirconia powder to replace the zirconia-silicon carbide composite powder.

[0065] 2. Explore the influence of the composition of the zirconia-silicon carbide composite powder on the performance of the composite brick

[0066] Examples 1, 4, and 5 are used as experimental comparisons, and the performance of the composite bricks under different compositions of the zirconia-silicon carbide composite powder is shown in Table 2 as follows:

[0067] Table 2 Performance of composite bricks under different compositions of zirconia-silicon carbide composite powder

[0068] Group Example 1 Example 4 Example 5 Bulk density (g / cm 3 )]]> 3.7 3.4 3.3 Room temperature compressive strength (MPa) 264 245 241 Load softening temperature (°C) >1850 >1850 >1850

[0069] From the data in Table 2, the bulk density and compressive strength of the composite brick of Example 1 are the highest, indicating that the performance of the composite brick of Example 1 is more optimal, and the composition of the zirconia-silicon carbide composite powder selected by Example 1 is the most optimal.

[0070] 3. Explore the influence of the preparation parameters of the mixed gel on the performance of the composite brick

[0071] Examples 1, 6, and 7 are used as experimental comparisons, and the performance of the composite bricks under different preparation parameters of the mixed gel is shown in Table 3 as follows:

[0072] Table 3 Performance of composite bricks under different preparation parameters of the mixed gel

[0073] Group Example 1 Example 6 Example 7 Bulk density (g / cm 3 )]]> 3.7 3.5 3.3 Room temperature compressive strength (MPa) 264 253 249 Load softening temperature (°C) >1850 >1850 >1850

[0074] From the data in Table 3, the volume density and compressive strength of the composite bricks of Example 1 are the highest, indicating that the performance of the composite bricks of Example 1 is better, and the mixed gel preparation parameters selected in Example 1 are optimal.

[0075] 4. Explore the effect of calcination parameters on the performance of composite bricks

[0076] Taking Examples 1, 8, and 9 as experimental comparisons, the performance of the composite bricks under different calcination parameters is shown in Table 4 as follows:

[0077] Table 4 Performance of composite bricks under different calcination parameters

[0078] Group Example 1 Example 8 Example 9 Bulk density (g / cm 3 )]]> 3.7 3.2 3.6 Room temperature compressive strength (MPa) 264 247 258 Load softening temperature (°C) >1850 >1850 >1850

[0079] From the data in Table 4, the volume density and compressive strength of the composite bricks of Example 1 are the highest, indicating that the performance of the composite bricks of Example 1 is better, and the calcination parameters selected in Example 1 are optimal.

[0080] 5. Explore the effect of water addition amount on the performance of composite bricks

[0081] Taking Examples 1, 10, and 11 as experimental comparisons, the performance of the composite bricks under different water addition amounts is shown in Table 5 as follows:

[0082] Table 5 Performance of composite bricks under different water addition amounts

[0083] Group Example 1 Example 10 Example 11 Bulk density (g / cm 3 )]]> 3.7 3.4 3.2 Room temperature compressive strength (MPa) 264 249 247 Load softening temperature (°C) >1850 >1850 >1850

[0084] From the data in Table 5, the volume density and compressive strength of the composite bricks of Example 1 are the highest, indicating that the performance of the composite bricks of Example 1 is better, and the water addition amount selected in Example 1 is optimal.

[0085] 6. Explore the effect of drying parameters on the performance of composite bricks

[0086] Taking Examples 1, 12, and 13 as experimental comparisons, the performance of the composite bricks under different drying parameters is shown in Table 6 as follows:

[0087] Table 6 Performance of composite bricks under different drying parameters

[0088] Group Example 1 Example 12 Example 13 Bulk density (g / cm 3 )]]> 3.7 3.3 3.6 Room temperature compressive strength (MPa) 264 252 260 Load softening temperature (°C) >1850 >1850 >1850

[0089] From the data in Table 6, the volume density and compressive strength of the composite bricks of Example 1 are the highest, indicating that the performance of the composite bricks of Example 1 is better, and the drying parameters selected in Example 1 are optimal.

[0090] 7. Explore the effect of sintering temperature curve on the performance of composite bricks

[0091] Take examples 1, 14, 15 as experimental comparison, at the same time, take example 1 as reference, the temperature rising speed is constant at 5℃ / min, the temperature is raised to 1550℃, and the temperature is kept for 38h as comparative example 2, the properties of the composite bricks under different sintering temperature curves are shown in table 7.

[0092] Table 7 Properties of composite bricks under different sintering temperature curves

[0093] Group Example 1 Example 14 Example 15 Comparative Example 2 Bulk density (g / cm 3 )]]> 3.7 3.5 3.3 3.0 Room temperature compressive strength (MPa) 264 254 251 229 Load softening temperature (°C) >1850 >1850 >1850 >1700

[0094] From the data in table 7, it can be seen that the volume density and the compressive strength of the composite brick of example 1 are the highest, compared with examples 14 and 15, which indicates that the performance of the composite brick of example 1 is better, and the sintering temperature curve selected by example 1 is better; compared with comparative example 2, the volume density, the compressive strength and the load softening temperature of the composite brick of example 1 are all higher than those of comparative example 2, which indicates that the sintering mode selected by example 1 is better.

Claims

1. A chrome zirconia corundum composite brick for a garbage incineration grate, characterized by, The composite brick comprises the following components in parts by weight: white corundum powder 60-75 parts, chromium sesquioxide 7-14 parts, zirconia-silicon carbide composite powder 12-20 parts, magnesium oxide 2-5 parts, kaolin 3-6 parts, and binder 1-4 parts; The zirconia-silicon carbide composite powder is prepared from the following raw materials in parts by weight: tetraethyl orthosilicate 0.8-2 parts, anhydrous ethanol 2.5-6 parts, nano zirconia 0.8-1.5 parts, sucrose 2-3 parts, trimethoxysilane 0.4-1 part, and deionized water 5.5-6.5 parts; The preparation method of the zirconia-silicon carbide composite powder comprises the following steps: S1. Dissolve the tetraethyl orthosilicate in the anhydrous ethanol, stir uniformly, and obtain a precursor solution; add the nano zirconia and sucrose to the deionized water, ultrasonically disperse for 10-20 min, add the trimethoxysilane, and obtain a dispersion liquid; S2. Add the precursor solution to the dispersion liquid, stir for 30-60 min, then heat the dispersion liquid to 40-60℃, gradually add dilute hydrochloric acid during the heating process, adjust the pH value of the dispersion liquid to 2, then place at room temperature for 6-9 h, and obtain a mixed gel; S3. Dry, grind the mixed gel in sequence, then calcine the mixed powder under an argon protective atmosphere, the calcination temperature is 1300-1500℃, the calcination time is 1-2 h, and the zirconia-silicon carbide composite powder is obtained after the calcination is completed.

2. The chrome zirconia corundum composite brick for a garbage grate furnace according to claim 1, characterized in that, The white corundum powder is mixed from coarse powder and fine powder; the coarse powder accounts for 60-75% of the total mass of the white corundum powder, and the particle size of the coarse powder is 0.1-4 mm, and the particle size of the fine powder is 15-60 μm.

3. The chrome zirconia corundum composite brick for a garbage grate furnace according to claim 1, characterized in that, The binder is a 25-40% mass concentration aqueous dextrin solution.

4. The chrome zirconia corundum composite brick for a garbage grate furnace according to claim 1, characterized in that, In step S2, the mass concentration of the dilute hydrochloric acid is 10-15%.

5. The chrome zirconia corundum composite brick for a garbage grate furnace according to claim 1, characterized in that, In step S3, the drying temperature is 100-120℃, and the drying time is 30-45 min.

6. The chrome zirconia corundum composite brick for a garbage grate furnace according to claim 1, characterized in that, In step S1, the particle size of the nano zirconia is 300-500 nm.

7. The chrome zirconia corundum composite brick for a garbage grate furnace according to claim 1, characterized in that, The preparation method of the composite brick comprises the following steps: Step 1: Put the white corundum powder, chromium sesquioxide, zirconia-silicon carbide composite powder, magnesium oxide, and kaolin into a mixer, mix for 10-15 min, and obtain a mixture; Step 2: Add the binder and water accounting for 30-40% of the total weight of the mixture to the mixture, stir uniformly, obtain a mud, inject the mud into a mold, and use a forming press to press the mud in the mold into a green brick under a pressure of 20-40 MPa; Step 3: Dry the green brick at 90-110℃ for 20-25 h, then put the green brick into a tunnel kiln for sintering, and obtain the composite brick after the sintering is completed.

8. The chrome zirconia corundum composite brick for a garbage incineration grate furnace according to claim 7, characterized in that, The temperature curve of the sintering process is as follows: heating to 500-650 DEG C at a heating rate of 15-20 DEG C / min, holding for 0.5-1 h; then heating to 800-900 DEG C at a heating rate of 10-14 DEG C / min, holding for 1-2 h; then heating to 1100-1200 DEG C at a heating rate of 6-10 DEG C / min, holding for 2-3 h; then heating to 1350-1450 DEG C at a heating rate of 2-5 DEG C / min, holding for 5-6 h; and then heating to 1500-1600 DEG C at a heating rate of 0.5-2 DEG C / min, holding for 35-40 h.

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