Method for manufacturing mullite - high silica glass composite material using hard clay

Abstract

The present invention belongs to the technical field of refractory material preparation, and specifically relates to a method for manufacturing mullite-high silica glass composite materials using hard clay. The preparation method consists of the following steps: (1) Mix hard clay, topaz, molybdenum tailings, and lithium mica lithium slag to obtain a mixture, and then perform crushing and classification screening to prepare mixture granular materials with different particle sizes; (2) Mix the mixture granular materials with different particle sizes, and then add a mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide for grinding to prepare a powder; (3) Press into a green body and then dry; (4) Fire to prepare mullite-high silica glass composite materials. The mullite-high silica glass composite materials prepared by the method of the present invention have excellent refractoriness and a low coefficient of thermal expansion.

Description

Technical Field

[0001] The present invention belongs to the technical field of refractory material preparation, and particularly relates to a method for manufacturing mullite-high silica glass composite materials using hard clay. Background Art

[0002] Mullite-high silica glass composite materials are a new type of refractory raw material composed of mullite and amorphous high silica glass (a glass phase mainly composed of SiO 2 This composite material has excellent properties such as high refractoriness, low expansion coefficient, high hardness, and wear resistance. At high temperatures, the high silica glass phase transforms into a high-viscosity liquid phase, which is beneficial to the high-temperature performance. At low temperatures, due to the low thermal expansion coefficient of the high silica glass material, the phase composition does not contain free quartz, and the mullite forms an intersecting network structure, which is beneficial to improving the thermal shock resistance.

[0003] Mullite is a high-quality refractory raw material and is the most stable binary solid solution under normal pressure in the Al 2 O 3 -SiO 2 binary system. The natural mullite with the chemical formula 3Al 2 O 3 ·2SiO 2 is very rare and is usually artificially synthesized by sintering or electrofusion methods. Due to the excellent properties of mullite, it is widely used and can be applied to industries such as refractories, ceramics, metallurgy, casting, and electronics. It has characteristics such as high temperature resistance, high strength, low thermal conductivity, and remarkable energy-saving effects, and is suitable for the inner linings of petroleum cracking furnaces, metallurgical hot blast stoves, ceramic roller kilns, tunnel kilns, electroceramic drawer kilns, glass crucible kilns, and various electric furnaces.

[0004] With the rapid development of modern industry and the continuous increase in the production of steel and non-ferrous metals, the production of refractories is also increasing rapidly. The amount of tailings (small materials) discharged from refractory raw material mines is increasing day by day. At the same time, with the increasing depletion of rich ore resources and the increasing proportion of poor ore and tailings resources mined, the scale of tailings use is expanding day by day. Therefore, the comprehensive utilization of tailings can not only reduce the stacking of tailings but also improve the environmental hygiene of the mining area.

[0005] At present, there are many types of mullite in China. According to the synthesis method, it can be divided into sintered mullite and fused mullite. If classified according to the raw materials used in production, it can be divided into coal gangue-based mullite, bauxite-based mullite (including all-natural), alumina-based mullite (high purity), etc., but there is no mullite produced using hard clay as a raw material. Therefore, how to prepare mullite-high silica glass composite materials using small materials such as hard clay has become an urgent technical problem to be solved. Summary of the Invention

[0006] The objective of the present invention is to provide a method for manufacturing mullite-high silica glass composite materials using hard clay. The mullite-high silica glass composite materials prepared by this method have excellent refractoriness and a low coefficient of thermal expansion.

[0007] The method for manufacturing mullite-high silica glass composite materials using hard clay according to the present invention comprises the following steps:

[0008] (1) Mix hard clay, topaz, molybdenum tailings, and lithium mica lithium slag to obtain a mixture, and then perform crushing, classification, and screening to prepare mixture granular materials of 5 - 8 mm, mixture granular materials of 3 - 5 mm, and mixture granular materials of 2 - 3 mm; wherein, in the mixture, by weight, 71 - 72.5 parts of hard clay, 26 - 28 parts of topaz, 5 - 7.5 parts of molybdenum tailings, and 5 - 7 parts of lithium mica lithium slag;

[0009] (2) Mix the mixture granular materials of 5 - 8 mm, the mixture granular materials of 3 - 5 mm, and the mixture granular materials of 2 - 3 mm prepared in step (1) evenly, and then add a mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide for grinding to prepare a powder;

[0010] (3) Press the powder into a green body, and then dry it;

[0011] (4) First, heat it at a rate of 4.5 °C / min to 1000 °C, then heat it at a rate of 2.3 °C / min to 1230 - 1250 °C for heat preservation, and finally cool it to room temperature with the furnace to prepare mullite-high silica glass composite materials.

[0012] Among them:

[0013] The hard clay described in step (1), by mass percentage, has the following chemical composition: Al 2 O 3 39.09%, SiO 2 44.56%, Fe 2 O 3 1.62%, TiO 2 0.89%, CaO 0.28%, MgO 0.24%, loss on ignition 13.32%.

[0014] The topaz described in step (1), by mass percentage, has the following chemical composition: Al 2 O 3 53.69%, SiO 2 34.64%, MgO 0.01%, CaO 0.22%, Fe 2 O 3 0.29%, TiO2 0.06%, K 2 O 0.03%, Na 2 O 0.01%, F 11.05%.

[0015] The molybdenum tailings described in step (1), in terms of mass percentage, have the following chemical composition: SiO 2 75.68%, Al 2 O 3 12.50%, K 2 O 5.54%, Na 2 O 2.48%, CaO 0.75%, MgO 0.56%, loss on ignition 2.49%.

[0016] The lepidolite lithium slag described in step (1), in terms of mass percentage, has the following chemical composition: SiO 2 55.79%, Al 2 O 3 21.21%, Fe 2 O 3 0.87%, CaO 3.21%, MgO 0.15%, K 2 O 4.54%, Na 2 O 10.59%, TiO 2 2.97%, SO 3 0.03%, loss on ignition 0.64%.

[0017] The mass ratio of the 5 - 8 mm mixture pellets, 3 - 5 mm mixture pellets, and 2 - 3 mm mixture pellets described in step (1) is 22:35:43.

[0018] In step (2), the mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide accounts for 3.5 - 3.8% of the total mass of the 5 - 8 mm mixture pellets, 3 - 5 mm mixture pellets, and 2 - 3 mm mixture pellets.

[0019] In step (2), the mass ratio of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide is 1.38:1:1.

[0020] In step (2), the grinding time is 13 - 15 h. After grinding, it is sieved through a 300 - mesh sieve to prepare the powder.

[0021] In step (3), the pressing pressure is 80 MPa, the drying temperature is 105 - 108 °C, and the drying time is 24 h.

[0022] In step (4), the heat preservation time is 3.3 - 3.5 h.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) In the method for manufacturing a mullite-high silica glass composite material using hard clay according to the present invention, hard clay and topaz are used as the main raw materials, and molybdenum tailings and lepidolite lithium slag are used as auxiliary raw materials. Among them, the sintering of topaz at high temperature can generate mullite phase, SiF 4 and HF gas, and the presence of SiF 4 and HF gas can promote the formation of mullitization. In addition to containing a large amount of silicon dioxide and alumina, molybdenum tailings and lepidolite lithium slag have a high alkali metal content. Their addition can ensure the formation of mullite densification at high temperature, inhibit the transformation of cristobalite crystal form at high temperature, and convert all the excess silicon dioxide into high silica glass phase. The glass phase has a high viscosity, so that the prepared composite material has excellent high-temperature performance. At low temperature, cristobalite is eliminated, and mullite forms a cross-linked network structure, making it have a low thermal expansion coefficient.

[0025] (2) In the method for manufacturing a mullite-high silica glass composite material using hard clay according to the present invention, the mixture of hard clay, topaz, molybdenum tailings and lepidolite lithium slag is crushed, decomposed and screened. Lattice defects are generated by mechanical external force to improve the reaction activity. A mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide and tungsten trioxide is added as a sintering promoter to reduce the reaction temperature during firing. After being pressed into shape, the chemical reaction between materials is promoted. Finally, sintering is carried out by controlling the heating rate and holding time to obtain a mullite-high silica glass composite material with excellent performance.

[0026] (3) In the method for manufacturing a mullite-high silica glass composite material using hard clay according to the present invention, a mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide and tungsten trioxide is used as a sintering promoter. Sodium dihydrogen phosphate dihydrate can act as a glass phase to promote the diffusion rate of Al 3+ and Si 4+ at the interface, which is beneficial to the formation of mullite phase; the addition of yttrium oxide can significantly reduce the firing temperature and promote the densification of mullite phase; the addition of tungsten trioxide can promote the nucleation of mullite and the formation of high silica glass phase at high temperature. The synergistic effect of the three ensures the performance of the mullite-high silica glass composite material prepared at a lower firing temperature.

[0027] (4) The mullite-high silica glass composite material prepared by the method of the present invention has excellent refractoriness and a low thermal expansion coefficient. Specific Embodiments

[0028] The present invention will be further described below in conjunction with embodiments.

[0029] Example 1

[0030] The method for manufacturing mullite - high silica glass composite material using hard clay in Example 1 consists of the following steps:

[0031] (1) Mix hard clay, topaz, molybdenum tailings, and lithium mica lithium slag to obtain a mixture, and then perform crushing, classification, and screening to prepare mixture granular materials of 5 - 8 mm, 3 - 5 mm, and 2 - 3 mm; wherein, in the mixture, by weight, there are 71 parts of hard clay, 28 parts of topaz, 5 parts of molybdenum tailings, and 7 parts of lithium mica lithium slag;

[0032] (2) Mix the mixture granular materials of 5 - 8 mm, 3 - 5 mm, and 2 - 3 mm prepared in step (1) evenly, and then add a mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide for grinding to prepare a powder;

[0033] (3) Press the powder into a green body and then dry it;

[0034] (4) First, heat it to 1000 °C at a rate of 4.5 °C / min, then heat it to 1240 °C at a rate of 2.3 °C / min for heat preservation, and finally cool it to room temperature with the furnace to prepare the mullite - high silica glass composite material.

[0035] Among them:

[0036] The hard clay described in step (1), in terms of mass percentage, has the following chemical composition: Al 2 O 3 39.09%, SiO 2 44.56%, Fe 2 O 3 1.62%, TiO 2 0.89%, CaO 0.28%, MgO 0.24%, loss on ignition 13.32%.

[0037] The topaz described in step (1), in terms of mass percentage, has the following chemical composition: Al 2 O 3 53.69%, SiO 2 34.64%, MgO 0.01%, CaO 0.22%, Fe 2 O 3 0.29%, TiO 2 0.06%, K 2 O 0.03%, Na 2 O 0.01%, F 11.05%.

[0038] The molybdenum tailings described in step (1), in terms of mass percentage, have the following chemical composition: SiO 2 75.68%, Al 2 O 3 12.50%, K 2 O 5.54%, Na 2 O 2.48%, CaO 0.75%, MgO 0.56%, loss on ignition 2.49%.

[0039] The lepidolite lithium slag described in step (1), in terms of mass percentage, has the following chemical composition: SiO 2 55.79%, Al 2 O 3 21.21%, Fe 2 O 3 0.87%, CaO 3.21%, MgO 0.15%, K 2 O 4.54%, Na 2 O 10.59%, TiO 2 2.97%, SO 3 0.03%, loss on ignition 0.64%.

[0040] The mass ratio of the 5 - 8 mm mixture granular material, 3 - 5 mm mixture granular material, and 2 - 3 mm mixture granular material described in step (1) is 22:35:43.

[0041] In step (2), the mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide accounts for 3.6% of the total mass of the 5 - 8 mm mixture granular material, 3 - 5 mm mixture granular material, and 2 - 3 mm mixture granular material.

[0042] In step (2), the mass ratio of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide is 1.38:1:1.

[0043] In step (2), the grinding time is 14 h. After grinding, it is sieved through a 300 - mesh sieve to prepare the powder material.

[0044] In step (3), the pressing pressure is 80 MPa, the drying temperature is 106 °C, and the drying time is 24 h.

[0045] In step (4), the heat preservation time is 3.4 h.

[0046] The mullite - high silica glass composite material prepared in Example 1 is tested for performance. The bulk density is 2.65 g / cm 3 , the refractoriness is 1830 °C, and the thermal expansion coefficient at 1300 °C is 3.5×10 -6 / °C.

[0047] Example 2

[0048] The method for manufacturing mullite - high silica glass composite material using hard clay in this Example 2 consists of the following steps:

[0049] (1) Mix hard clay, topaz, molybdenum tailings and lithium mica lithium slag to obtain a mixture, and then perform crushing, classification and screening to prepare mixture granular materials of 5 - 8 mm, mixture granular materials of 3 - 5 mm, and mixture granular materials of 2 - 3 mm; wherein, in the mixture, by weight, 71.8 parts of hard clay, 27 parts of topaz, 7.5 parts of molybdenum tailings, and 6 parts of lithium mica lithium slag;

[0050] (2) Mix the mixture granular materials of 5 - 8 mm, 3 - 5 mm, and 2 - 3 mm prepared in step (1) evenly, and then add a mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide and tungsten trioxide for grinding to prepare a powder;

[0051] (3) Press the powder into a green body and then dry it;

[0052] (4) First, heat it to 1000 °C at a rate of 4.5 °C / min, then heat it to 1250 °C at a rate of 2.3 °C / min for heat preservation, and finally cool it to room temperature with the furnace to prepare the mullite - high silica glass composite material.

[0053] Among them:

[0054] The hard clay described in step (1), in terms of mass percentage, has the following chemical composition: Al 2 O 3 39.09%, SiO 2 44.56%, Fe 2 O 3 1.62%, TiO 2 0.89%, CaO 0.28%, MgO 0.24%, loss on ignition 13.32%.

[0055] The topaz described in step (1), in terms of mass percentage, has the following chemical composition: Al 2 O 3 53.69%, SiO 2 34.64%, MgO 0.01%, CaO 0.22%, Fe 2 O 3 0.29%, TiO 2 0.06%, K 2 O 0.03%, Na 2 O 0.01%, F 11.05%.

[0056] The molybdenum tailings described in step (1), in terms of mass percentage, have the following chemical composition: SiO 2 75.68%, Al 2 O 3 12.50%, K 2 O 5.54%, Na 2 O 2.48%, CaO 0.75%, MgO 0.56%, loss on ignition 2.49%.

[0057] The lepidolite lithium slag described in step (1), in terms of mass percentage, has the following chemical composition: SiO 2 55.79%, Al 2 O 3 21.21%, Fe 2 O 3 0.87%, CaO 3.21%, MgO 0.15%, K 2 O 4.54%, Na 2 O 10.59%, TiO 2 2.97%, SO 3 0.03%, loss on ignition 0.64%.

[0058] The mass ratio of the 5-8 mm mixture granular material, 3-5 mm mixture granular material, and 2-3 mm mixture granular material described in step (1) is 22:35:43.

[0059] In step (2), the mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide accounts for 3.8% of the total mass of the 5-8 mm mixture granular material, 3-5 mm mixture granular material, and 2-3 mm mixture granular material.

[0060] In step (2), the mass ratio of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide is 1.38:1:1.

[0061] In step (2), the grinding time is 15 h, and after grinding, it is sieved through a 300-mesh sieve to prepare a powder material.

[0062] In step (3), the pressing pressure is 80 MPa, the drying temperature is 108 °C, and the drying time is 24 h.

[0063] In step (4), the heat preservation time is 3.3 h.

[0064] The mullite-high silica glass composite material prepared in Example 2 was subjected to performance testing. Its bulk density is 2.64 g / cm 3 , the refractoriness is 1830 °C, and the thermal expansion coefficient at 1300 °C is 3.7*10 -6 / °C.

[0065] Example 3

[0066] The method for manufacturing mullite - high silica glass composite material using hard clay described in this Example 3 consists of the following steps:

[0067] (1) Mix hard clay, topaz, molybdenum tailings, and lithium mica lithium slag to obtain a mixture, and then perform crushing, classification screening to prepare mixture granular materials of 5 - 8 mm, mixture granular materials of 3 - 5 mm, and mixture granular materials of 2 - 3 mm; among them, in the said mixture, by weight, 72.5 parts of hard clay, 26 parts of topaz, 6.5 parts of molybdenum tailings, and 5 parts of lithium mica lithium slag;

[0068] (2) Mix the mixture granular materials of 5 - 8 mm, 3 - 5 mm, and 2 - 3 mm prepared in step (1) evenly, and then add a mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide for grinding to prepare a powder;

[0069] (3) Press the powder into a green body, and then dry it;

[0070] (4) First, heat it to 1000°C at a rate of 4.5°C / min, then heat it to 1230°C at a rate of 2.3°C / min for heat preservation, and finally cool it to room temperature with the furnace to prepare mullite - high silica glass composite material.

[0071] Among them:

[0072] The hard clay described in step (1), by mass percentage, has the following chemical composition: Al 2 O 3 39.09%, SiO 2 44.56%, Fe 2 O 3 1.62%, TiO 2 0.89%, CaO 0.28%, MgO 0.24%, loss on ignition 13.32%.

[0073] The topaz described in step (1), by mass percentage, has the following chemical composition: Al 2 O 3 53.69%, SiO 2 34.64%, MgO 0.01%, CaO 0.22%, Fe 2 O 3 0.29%, TiO 2 0.06%, K 2 O 0.03%, Na 2O 0.01%, F 11.05%.

[0074] The molybdenum tailings described in step (1), by mass percentage, have the following chemical composition: SiO 2 75.68%, Al 2 O 3 12.50%, K 2 O 5.54%, Na 2 O 2.48%, CaO 0.75%, MgO 0.56%, loss on ignition 2.49%.

[0075] The lepidolite lithium slag described in step (1), by mass percentage, has the following chemical composition: SiO 2 55.79%, Al 2 O 3 21.21%, Fe 2 O 3 0.87%, CaO 3.21%, MgO 0.15%, K 2 O 4.54%, Na 2 O 10.59%, TiO 2 2.97%, SO 3 0.03%, loss on ignition 0.64%.

[0076] The mass ratio of the 5 - 8 mm mixture granular material, 3 - 5 mm mixture granular material, and 2 - 3 mm mixture granular material described in step (1) is 22:35:43.

[0077] In step (2), the mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide accounts for 3.5% of the total mass of the 5 - 8 mm mixture granular material, 3 - 5 mm mixture granular material, and 2 - 3 mm mixture granular material.

[0078] In step (2), the mass ratio of sodium dihydrogen phosphate dihydrate, yttrium oxide, and tungsten trioxide is 1.38:1:1.

[0079] In step (2), the grinding time is 13 h, and after grinding, it is passed through a 300 - mesh sieve to prepare a powder material.

[0080] In step (3), the pressing pressure is 80 MPa, the drying temperature is 105 °C, and the drying time is 24 h.

[0081] In step (4), the heat preservation time is 3.5 h.

[0082] The mullite - high silica glass composite material prepared in Example 3 was subjected to performance testing, and its bulk density was 2.62 g / cm 3, the refractoriness is 1820 °C, and the coefficient of thermal expansion at 1300 °C is 3.7×10 -6 / °C.

[0083] Comparative Example 1

[0084] The method for manufacturing the mullite-high silica glass composite material using hard clay described in this Comparative Example 1 is the same as that in Example 1. The only difference is that in step (2), the mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide and tungsten trioxide is not added.

[0085] Perform performance tests on the mullite-high silica glass composite material prepared in Comparative Example 1. Its bulk density is 2.45 g / cm 3 , the refractoriness is 1600 °C, and the coefficient of thermal expansion at 1300 °C is 5.6×10 -6 / °C.

[0086] Comparative Example 2

[0087] The method for manufacturing the mullite-high silica glass composite material using hard clay described in this Comparative Example 2 is the same as that in Example 1. The only difference is that in step (1), molybdenum tailings and lepidolite lithium slag are not added. In the mixture described in step (1), by weight, it consists of 71 parts of hard clay and 28 parts of topaz.

[0088] Perform performance tests on the mullite-high silica glass composite material prepared in Comparative Example 2. Its bulk density is 2.52 g / cm 3 , the refractoriness is 1670 °C, and the coefficient of thermal expansion at 1300 °C is 6.8×10 -6 / °C.

Claims

1. A method for manufacturing a mullite-high silica glass composite material using hard clay, characterized in that: It consists of the following steps: (1) Mixing hard clay, topaz, molybdenum tailings and lithium mica slag to obtain a mixture, and then crushing, grading and screening to prepare 5-8 mm mixed granular materials, 3-5 mm mixed granular materials and 2-3 mm mixed granular materials; wherein, in the mixture, by weight, hard clay 71-72.5 parts, topaz 26-28 parts, molybdenum tailings 5-7.5 parts, lithium mica slag 5-7 parts; (2) The 5-8 mm mixed material particles, 3-5 mm mixed material particles and 2-3 mm mixed material particles prepared in step (1) are mixed evenly, and then a mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide and tungsten trioxide is added and ground to prepare a powder; (3) Pressing the powder into green compacts and then drying them; (4) firstly, heating to 1000°C at a rate of 4.5°C / min, then heating to 1230-1250°C at a rate of 2.3°C / min, and then cooling to room temperature with the furnace to prepare a mullite-high silica glass composite material; in: The molybdenum tailings described in step (1) have the following chemical composition, calculated by mass percentage: SiO2 75.68%, Al2O3 12.50%, K2O 5.54%, Na2O 2.48%, CaO 0.75%, MgO 0.56%, and loss on ignition 2.49%; The lithium mica slag in step (1) has the following chemical composition, in terms of mass percentage: SiO2 55.79%, Al2O3 21.21%, Fe2O3 0.87%, CaO 3.21%, MgO 0.15%, K2O 4.54%, Na2O 10.59%, TiO2 2.97%, SO3 0.03%, and loss on ignition 0.64%; The mass ratio of the 5-8 mm mixed material particles, the 3-5 mm mixed material particles, and the 2-3 mm mixed material particles in step (1) is 22:35:43; In step (2), the mass ratio of sodium dihydrogen phosphate dihydrate, yttrium oxide and tungsten trioxide is 1.38:1:

1.

2. The method for manufacturing mullite-high silica glass composite material using hard clay according to claim 1, characterized in that: The hard clay in step (1) has the following chemical composition, in terms of mass percentage: Al2O3 39.09%, SiO2 44.56%, Fe2O3 1.62%, TiO2 0.89%, CaO 0.28%, MgO 0.24%, and loss on ignition 13.32%.

3. The method for manufacturing mullite-high silica glass composite material using hard clay according to claim 1, characterized in that: The topaz described in step (1) has the following chemical composition, calculated by mass percentage: Al2O3 53.69%, SiO2 34.64%, MgO 0.01%, CaO 0.22%, Fe2O3 0.29%, TiO2 0.06%, K2O 0.03%, Na2O 0.01%, and F1 1.05%.

4. The method for manufacturing mullite-high silica glass composite material using hard clay according to claim 1, characterized in that: In step (2), the mixture of sodium dihydrogen phosphate dihydrate, yttrium oxide and tungsten trioxide accounts for 3.5-3.8% of the total mass of the mixed material particles of 5-8 mm, the mixed material particles of 3-5 mm and the mixed material particles of 2-3 mm.

5. The method for manufacturing mullite-high silica glass composite material using hard clay according to claim 1, characterized in that: The grinding time in step (2) is 13-15 hours, and after grinding, the mixture is sieved through a 300-mesh sieve to obtain a powder.

6. The method for manufacturing mullite-high silica glass composite material using hard clay according to claim 1, characterized in that: In step (3), the pressing pressure is 80 MPa, the drying temperature is 105-108° C., and the drying time is 24 h; and in step (4), the holding time is 3.3-3.5 h.