Modified single-component geopolymer curing agent for treating sludge and sludge soil curing method thereof

Abstract

The invention discloses a modified single-component geopolymer curing agent for treating sludge and a sludge soil curing method thereof. The curing agent comprises the following substances in parts by weight: 50-60 parts of fly ash, 30-40 parts of granulated blast furnace slag, 8-12 parts of a solid alkali activator, 1-5 parts of nano silicon dioxide and 1-5 parts of an ionic admixture. The sludge soil solidification method comprises the following steps: weighing dredged sludge dry soil, fly ash, slag, solid sodium hydroxide and sodium silicate pentahydrate according to a ratio, and carrying out dry mixing; adding nano silicon dioxide and an ionic admixture into the obtained mixed dry material, and continuously carrying out dry mixing until the mixture is uniform; adding water into the obtained mixed dry material, stirring at a low speed to primarily infiltrate the mixed dry material, and then stirring at a high speed until the mixed dry material is uniformly mixed to obtain solidified soil; and compacting and molding the obtained solidified soil, and sealing and curing. According to the invention, through compounding modification of nano SiO2 and ionic adulteration, the mechanical property and water stability are effectively improved.

Description

Technical Field

[0001] This invention pertains to curing agents and curing methods, specifically a modified single-component geopolymer curing agent for treating silt and a method for curing silt soil. Background Technology

[0002] In recent years, port dredging and river regulation projects have generated a large amount of dredged silt. Dredged silt is characterized by high water content, low strength, and poor water stability, making it unsuitable for direct use as filler material in engineering projects. Although traditional cement-based curing agents can effectively improve the mechanical properties of silt, their production process is energy-intensive and generates significant carbon emissions, which contradicts the current concept of sustainable development.

[0003] Geopolymers are green cementitious materials with a three-dimensional network structure formed from silica-alumina-containing solid wastes (such as fly ash and blast furnace slag) under alkaline activation conditions. With their high strength, good durability, and environmental friendliness, they are considered a substitute for cement. In solidifying sludge, geopolymers exhibit advantages such as high strength and good durability, meeting the requirements of resource utilization and low-carbon development. However, geopolymer-solidified sludge also suffers from slow early strength development, insufficient water stability, and relatively low tensile strength, which limits its engineering application effectiveness.

[0004] To improve the overall performance of geopolymer-cured materials, researchers typically introduce modifying materials to regulate the reaction process and enhance mechanical properties. For example, nanomaterials, with their high specific surface area and reactivity, can act as nucleation sites to accelerate the formation of the geopolymer gel phase and refine the pore structure through micro-filling, thereby effectively improving the early strength and compressive strength of the material. Fiber-based materials or some organic compounds can promote the formation of gel products and improve the interfacial transition zone, thus enhancing the tensile strength and other indicators of geopolymer materials. Traditional geopolymers often employ liquid-solid two-component systems, which suffer from inconvenient construction, strong corrosivity, and high risks during storage and transportation. Although single-component geopolymer systems have emerged, improving the convenience of on-site construction, current research on their modification, especially the synergistic enhancement mechanisms involving the composite incorporation of multiple modifying materials, remains relatively weak, hindering the large-scale application of these materials in engineering. Traditional cement-based curing agents suffer from high energy consumption and high carbon emissions, while geopolymer materials, although possessing green potential, often exhibit defects such as unstable mechanical properties and high water sensitivity in the cured soil. Summary of the Invention

[0005] Purpose of the invention: In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a modified single-component geopolymer solidifier with good stability for treating silt. Another purpose of this invention is to provide a method for solidifying silt soil using a modified single-component geopolymer solidifier for treating silt that improves the mechanical properties of the solidified soil.

[0006] Technical solution: The present invention provides a modified single-component polymer curing agent for treating sludge, comprising the following components in parts by weight: 50-60 parts fly ash (FA), 30-40 parts granulated blast furnace slag (GGBS), 8-12 parts solid alkali activator, 1-5 parts nano silica, and 1-5 parts ionic admixture.

[0007] Furthermore, the ionic dopant is an LSS-1 type ionic dopant.

[0008] Furthermore, 4-5 parts of ionic external dopant.

[0009] Furthermore, the solid alkali activator is prepared by mixing solid sodium hydroxide and sodium silicate pentahydrate in a mass ratio of 1:5 to 9. Preferably, the mass ratio of sodium hydroxide to sodium silicate pentahydrate is 1:7.

[0010] Furthermore, the mass ratio of fly ash to granulated blast furnace slag is 6:4~5. Preferably, the mass ratio of fly ash to granulated blast furnace slag is 6:4.

[0011] Furthermore, the particle size of the nano-silica is 25~35nm.

[0012] Furthermore, the specific surface area of ​​fly ash ranges from 400 to 600 m². 2 / kg.

[0013] The method for solidifying silt using a modified single-component geopolymer solidifier for treating silt, as described in this invention, includes the following steps:

[0014] Step 1: Weigh out the dredged silt dry soil, fly ash, slag, solid sodium hydroxide and sodium silicate pentahydrate according to the proportions, and dry mix them.

[0015] Step 2: Add nano-silica and ionic admixture to the dry mixture obtained in Step 1, and continue to dry mix until homogeneous;

[0016] Step 3: Add water to the mixed dry material obtained in Step 2, stir at low speed to initially wet it, and then stir at high speed until it is evenly mixed to obtain solidified soil;

[0017] Step four: Compact the solidified soil obtained in step three into shape and seal it for curing.

[0018] Furthermore, in step one, the mass ratio of dredged silt and dry soil to fly ash is 900:50~70.

[0019] Furthermore, in step three, the low-speed stirring speed is 30~60 r / min and the stirring time is 120~180 seconds, to ensure the formation of uniform solidified soil.

[0020] The static compaction method was used for layered compaction, with the surface roughened after each layer to ensure good interlayer bonding. After molding, the specimen was immediately wrapped and sealed with plastic film to prevent moisture evaporation and ensure the polymerization reaction proceeded under constant humidity conditions. Subsequently, the sealed specimen was placed in a standard curing room and cured to the predetermined age. After curing, the specimen was removed and tested according to relevant standards.

[0021] Preparation Principle: This innovative approach combines nano-SiO2 with LSS-1 type ionic admixtures to synergistically enhance performance. Nano-SiO2, with its high specific surface area and reactivity, provides abundant nucleation sites for the geopolymer gel phase, significantly accelerating the formation and growth of the aluminosilicate network. Furthermore, the micro-filling effect of nanoparticles effectively densifies the porous structure. Simultaneously, the ionic admixture induces ion exchange at the soil particle-geopolymer gel interface and acts as a cementing agent, enhancing interfacial chemical adhesion and mechanical continuity. The single-component geopolymer is prepared by pre-mixing solid waste raw materials such as fly ash and slag, solid alkali activators, and modifying materials into a solid powder, completely avoiding the use of strongly alkaline liquids. When nano-SiO2 and LSS-1 are combined, nano-SiO2 accelerates the formation of aluminosilicate gel and fills micropores, improving structural density. The LSS-1 ionic admixture undergoes ion exchange and adsorption on the soil particle surface, enhancing interfacial adhesion and improving overall stability. The combined effect of these two admixtures enhances the interfacial bonding between the geopolymer gel structure and soil particles, resulting in a significant synergistic enhancement.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following significant features:

[0023] 1. Through the compound modification of nano-SiO2 and ionic admixture, the two materials produce a synergistic enhancement effect of "1+1>2" in terms of their working mechanism. The mechanical properties and water stability of the solidified soil are effectively improved. Nano-SiO2, with its high reactivity and micro-filling effect, mainly acts as a reinforcing phase, which can significantly promote the formation of geopolymer gel and compact the microstructure, thereby effectively improving the unconfined compressive strength of the solidified soil. Ionic admixture significantly improves the tensile strength and water stability of the solidified soil. When the two are mixed, they show a synergistic effect, which not only balances various mechanical properties, but also completely solves the core problem of insufficient water stability of geopolymer solidified soil, enabling the water stability coefficient of solidified soil to exceed 1.0 after 28 days.

[0024] 2. The product of this invention is in dry powder form, which is convenient for transportation and storage. It avoids the hidden dangers caused by the liquid alkali activator used in traditional two-component geopolymers. On-site construction only requires mixing with sludge and water. The process is simple and suitable for large-scale sludge solidification projects. It is both environmentally friendly and engineering-applicable.

[0025] 3. The raw materials of the product are mainly industrial solid wastes such as fly ash and slag, which significantly reduces the dependence on traditional cement and reduces related energy consumption and carbon emissions.

[0026] 4. Although LSS-1 type ionic admixture is liquid, it can be effective with a small amount. After stirring, the droplets of ionic admixture are encapsulated in solid powder, and the modified polymer material as a whole remains solid. This characteristic of single-component polymers significantly improves the safety of material transportation and storage and the convenience of on-site construction, eliminates the safety risks of alkaline solutions, simplifies the mixing process, and better meets the requirements of actual engineering for ease of operation, environmental friendliness and application reliability. Attached Figure Description

[0027] Figure 1 The images show a comparison of scanning electron microscopy (SEM) images of unmodified and modified geopolymer-solidified dredged sludge. In the image, a is the SEM image of unmodified geopolymer-solidified soil, and b is the SEM image of modified geopolymer-solidified soil. Detailed Implementation

[0028] Unless otherwise specified, all materials and reagents used in the following embodiments are commercially available. Experimental methods not specifically described in the embodiments are generally performed under conventional conditions or as recommended by the manufacturer. The dredged silt was dredged soil from the Qinhuai River in Nanjing, with a plasticity index Ip = 19.8, a liquid limit of 61.9%, and a clay content of 14.13%. The fly ash and blast furnace slag used to prepare the geopolymer were from Zhengzhou Huifeng New Materials Co., Ltd. in Henan Province. The selected nano-SiO2 was purchased from Xuancheng Jingrui New Materials Co., Ltd., with a particle size of 25-35 nm. The ionic admixture LSS-1 was purchased from Chengdu Sengda New Materials Co., Ltd., and is a compound of anionic surfactant and inorganic electrolyte, appearing as a yellowish-brown homogeneous liquid; specific parameters are shown in Table 1. The main components of fly ash include SiO2 and Al2O3, with SiO2 accounting for 35-60% by mass and Al2O3 accounting for 20-40% by mass. Its specific surface area ranges from 400 to 600 m². 2 / kg. The specific surface area of ​​granulated blast furnace slag ranges from 350 to 450 m². 2 / kg, including SiO2 and CaO, with SiO2 accounting for 20~30% by mass and CaO accounting for 30~50% by mass.

[0029] Table 1 Basic Characteristics of LSS-1 Type Ion Additives

[0030]

[0031] Example 1

[0032] A method for solidifying silt using a modified one-component geopolymer solidifier for treating silt includes the following steps:

[0033] Step S1, Raw material weighing: Weigh the following raw materials according to the following weight proportions: 900 parts of dredged silt, 54 parts of fly ash, 36 parts of granulated blast furnace slag, 10 parts of solid alkaline activator (where the mass ratio of NaOH to Na2SiO3·5H2O is 1:7), 140 parts of water, 2 parts of nano silica, and 2 parts of LSS-1 ionic admixture.

[0034] Step S2: Pour the weighed dredged sludge, fly ash, granulated blast furnace slag, sodium hydroxide solid, and sodium silicate pentahydrate into a mixing pot. Start the mixer and stir at a low speed of 45 r / min for 90 seconds to ensure that all dry powder components are mixed evenly.

[0035] Step S3: Add the modified material (nano SiO2 + ionic admixture LSS-1) to the mixing pot and continue stirring at low speed for 90 seconds to ensure that the modified material is evenly dispersed in the dry powder to obtain the mixed dry material.

[0036] Step S4: Slowly add a predetermined amount of water to the obtained dry mixture, while stirring continuously during the water addition process. After the water is added, stir at a low speed of 45 r / min for 150 seconds, and then increase the stirring speed to 150 r / min and continue stirring for 90 seconds to ensure the formation of a uniform solidified soil.

[0037] Step S5: Fill the mixed solidified soil into a cylindrical mold with a diameter of 50mm and a height of 50mm. Compact the soil in layers using a static compaction method, and roughen the surface after each layer to ensure good bonding between layers.

[0038] Step S6: After molding, immediately wrap and seal the specimen with plastic film to prevent moisture evaporation and ensure that the geopolymerization reaction proceeds under constant humidity conditions. Then, place the sealed specimen in a standard curing room and cure it to the predetermined age (7 days, 28 days). After curing, remove the specimen and proceed with testing according to the "Specifications for Geotechnical Testing of Highways" (JTG 3430-2020).

[0039] Example 2

[0040] A method for solidifying silt using a modified one-component geopolymer solidifier for treating silt includes the following steps:

[0041] Step S1: Weighing raw materials: Weigh the following raw materials according to the following weight proportions: 900 parts of dredged silt, 50 parts of fly ash, 40 parts of granulated blast furnace slag, 8 parts of solid alkaline activator (where the mass ratio of NaOH to Na2SiO3·5H2O is 1:9), 140 parts of water, 1 part of nano silica, and 1 part of LSS-1 ionic admixture.

[0042] Step S2: Pour the weighed dredged sludge, fly ash, granulated blast furnace slag, sodium hydroxide solid, and sodium silicate pentahydrate into a mixing pot. Start the mixer and stir at a low speed of 30 r / min for 90 seconds to ensure that all dry powder components are mixed evenly.

[0043] Step S3: Add the modified material (nano SiO2 + ionic admixture LSS-1) to the mixing pot and continue stirring at low speed for 90 seconds to ensure that the modified material is evenly dispersed in the dry powder to obtain the mixed dry material.

[0044] Step S4: Slowly add a predetermined amount of water to the obtained mixed dry material, while stirring during the water addition process. After the water is added, stir at a low speed of 30 r / min for 180 seconds, and then increase the stirring speed to 120 r / min and continue stirring for 120 seconds to ensure the formation of uniform solidified soil.

[0045] Step S5: Fill the mixed solidified soil into a cylindrical mold with a diameter of 50mm and a height of 50mm. Compact the soil in layers using a static compaction method, and roughen the surface after each layer to ensure good bonding between layers.

[0046] Step S6: After molding, immediately wrap and seal the specimen with plastic film to prevent moisture evaporation and ensure that the geopolymerization reaction proceeds under constant humidity conditions. Then, place the sealed specimen in a standard curing room and cure it to the predetermined age (7 days, 28 days). After curing, remove the specimen and proceed with testing according to the "Specifications for Geotechnical Testing of Highways" (JTG 3430-2020).

[0047] Example 3

[0048] A method for solidifying silt using a modified one-component geopolymer solidifier for treating silt includes the following steps:

[0049] Step S1: Weighing raw materials: Weigh the following raw materials according to the following weight proportions: 900 parts of dredged silt, 60 parts of fly ash, 30 parts of granulated blast furnace slag, 12 parts of solid alkaline activator (where the mass ratio of NaOH to Na2SiO3·5H2O is 1:5), 140 parts of water, 5 parts of nano silica, and 5 parts of LSS-1 ionic admixture.

[0050] Step S2: Pour the weighed dredged sludge, fly ash, granulated blast furnace slag, sodium hydroxide solid, and sodium silicate pentahydrate into a mixing pot. Start the mixer and stir at a low speed of 60 r / min for 90 seconds to ensure that all dry powder components are mixed evenly.

[0051] Step S3: Add the modified material (nano SiO2 + ionic admixture LSS-1) to the mixing pot and continue stirring at low speed for 90 seconds to ensure that the modified material is evenly dispersed in the dry powder to obtain the mixed dry material.

[0052] Step S4: Slowly add a predetermined amount of water to the obtained mixed dry material, while stirring during the water addition process. After the water is added, stir at a low speed of 60 r / min for 120 seconds, and then increase the stirring speed to 180 r / min and continue stirring for 60 seconds to ensure the formation of a uniform solidified soil.

[0053] Step S5: Fill the mixed solidified soil into a cylindrical mold with a diameter of 50mm and a height of 50mm. Compact the soil in layers using a static compaction method, and roughen the surface after each layer to ensure good bonding between layers.

[0054] Step S6: After molding, immediately wrap and seal the specimen with plastic film to prevent moisture evaporation and ensure that the geopolymerization reaction proceeds under constant humidity conditions. Then, place the sealed specimen in a standard curing room and cure it to the predetermined age (7 days, 28 days). After curing, remove the specimen and proceed with testing according to the "Specifications for Geotechnical Testing of Highways" (JTG 3430-2020).

[0055] Comparative Example 1

[0056] A method for preparing single-component geopolymer-stabilized soil modified with nano-SiO2 includes the following steps:

[0057] Step S1, Raw material weighing: Weigh each raw material according to the ratio. The raw materials and their weight percentages are as follows: 900 parts of dredged silt, 54 parts of fly ash, 36 parts of slag, 10 parts of solid alkaline activator (where the mass ratio of NaOH to Na2SiO3·5H2O is 1:7), 140 parts of water, and 2 parts of nano silica.

[0058] Step S2: Pour the weighed dredged sludge, fly ash (FA), granulated blast furnace slag (GGBS), sodium hydroxide solid, and sodium silicate pentahydrate into a mixing pot. Start the mixer and stir at low speed for 90 seconds to ensure that all dry powder components are mixed evenly.

[0059] Step S3: Add the modified material (nano SiO2) to the mixing pot and continue stirring at low speed for 90 seconds to ensure that the modified material is evenly dispersed in the dry powder to obtain a mixed dry material.

[0060] Step S4: Slowly add the predetermined amount of water to the obtained mixed dry material, while stirring during the water addition process. After the water is added, stir at a low speed for 150 seconds, then increase the stirring speed to a high speed and continue stirring for 90 seconds to ensure the formation of a uniform solidified soil.

[0061] Step S5: Fill the mixed solidified soil into a cylindrical mold with a diameter of 50mm and a height of 50mm. Compact the soil in layers using a static compaction method, and roughen the surface after each layer to ensure good bonding between layers.

[0062] Step S6: After molding, immediately wrap and seal the specimen with plastic film to prevent moisture evaporation and ensure that the geopolymerization reaction proceeds under constant humidity conditions. Then, place the sealed specimen in a standard curing room and cure it to the predetermined age (7 days, 28 days). After curing, remove the specimen and proceed with testing according to the "Specifications for Geotechnical Testing of Highways" (JTG 3430-2020).

[0063] Comparative Example 2

[0064] A method for preparing a single-component geopolymer-modified solidified soil with a single ionic admixture LSS-1 includes the following steps:

[0065] Step S1, Raw material weighing: Weigh each raw material according to the ratio. The raw materials and their weight percentages are as follows: 900 parts of dredged silt, 54 parts of fly ash, 36 parts of slag, 10 parts of solid alkaline activator (where the mass ratio of NaOH to Na2SiO3·5H2O is 1:7), 140 parts of water, and 2 parts of LSS-1 ionic admixture.

[0066] Step S2: Pour the weighed dredged sludge, fly ash (FA), granulated blast furnace slag (GGBS), sodium hydroxide solid, and sodium silicate pentahydrate into a mixing pot. Start the mixer and stir at low speed for 90 seconds to ensure that all dry powder components are mixed evenly.

[0067] Step S3: Add the modified material (ionic admixture LSS-1) to the mixing pot and continue stirring at low speed for 90 seconds to ensure that the modified material is evenly dispersed in the dry powder to obtain the mixed dry material.

[0068] Step S4: Slowly add the predetermined amount of water to the obtained mixed dry material, while stirring during the water addition process. After the water is added, stir at a low speed for 150 seconds, then increase the stirring speed to a high speed and continue stirring for 90 seconds to ensure the formation of a uniform solidified soil.

[0069] Step S5: Fill the mixed solidified soil into a cylindrical mold with a diameter of 50mm and a height of 50mm. Compact the soil in layers using a static compaction method, and roughen the surface after each layer to ensure good bonding between layers.

[0070] Step S6: After molding, immediately wrap and seal the specimen with plastic film to prevent moisture evaporation and ensure that the geopolymerization reaction proceeds under constant humidity conditions. Then, place the sealed specimen in a standard curing room and cure it to the predetermined age (7 days, 28 days). After curing, remove the specimen and proceed with testing according to the "Specifications for Geotechnical Testing of Highways" (JTG 3430-2020).

[0071] Comparative Example 3

[0072] A single-component geopolymer-stabilized soil, the preparation method of which includes the following steps:

[0073] Step S1, Raw material weighing: Weigh each raw material according to the ratio. The raw materials and their weight percentages are as follows: 900 parts of dredged silt, 54 parts of fly ash, 36 parts of slag, 10 parts of solid alkaline activator (where the mass ratio of NaOH to Na2SiO3·5H2O is 1:7), and 140 parts of water.

[0074] Step S2: Pour the weighed dredged sludge, fly ash (FA), granulated blast furnace slag (GGBS), sodium hydroxide solid, and sodium silicate pentahydrate into a mixing pot. Start the mixer and stir at low speed for 90 seconds to ensure that all dry powder components are mixed evenly.

[0075] Step S3: Slowly add the predetermined amount of water to the obtained mixed dry material, while stirring during the water addition process. After the water is added, stir at a low speed for 150 seconds, then increase the stirring speed to a high speed and continue stirring for 90 seconds to ensure the formation of a uniform solidified soil.

[0076] Step S4: Fill the mixed solidified soil into a cylindrical mold with a diameter of 50mm and a height of 50mm. Compact the soil in layers using a static compaction method, and roughen the surface after each layer to ensure good bonding between layers.

[0077] Step S5: After molding, immediately wrap and seal the specimen with plastic film to prevent moisture evaporation and ensure that the geopolymerization reaction proceeds under constant humidity conditions. Then, place the sealed specimen in a standard curing room and cure it to the predetermined age (7 days, 28 days). After curing, remove the specimen and proceed with testing according to the "Specifications for Geotechnical Testing of Highways" (JTG 3430-2020).

[0078] For the above comparative examples and embodiments, the solidified soil was cured before testing, and the experimental data are shown in Table 1. As shown in Table 2, the modification schemes provided by this invention all improved the solidification effect on silt soil in different aspects. Comparative Example 1 (single nano-doped) showed the best performance in terms of compressive strength and CBR; Comparative Example 2 (single ion-doped) showed good improvement in terms of tensile strength and water stability; Example 1 (double doping) showed the best water stability (28d water stability coefficient > 1), achieving a balance and improvement in comprehensive performance. Figure 1 The SEM images show that the microstructure of the modified geopolymer-cured soil (Example 1) is more compact than that of the unmodified sample (Comparative Example 3).

[0079] Table 2 Performance Test Results

[0080]

Claims

1. A modified one-component geopolymer curing agent for treating sludge, characterized in that, The substance comprises the following components in parts by weight: 50-60 parts fly ash, 30-40 parts granulated blast furnace slag, 8-12 parts solid alkali activator, 1-5 parts nano silica, and 1-5 parts ionic admixture.

2. The modified single-component geopolymer curing agent for treating sludge according to claim 1, characterized in that: The ionic admixture is an LSS-1 type ionic admixture.

3. The modified single-component geopolymer curing agent for treating sludge according to claim 1, characterized in that: The ionic admixture is 4-5 parts.

4. The modified single-component geopolymer curing agent for treating sludge according to claim 1, characterized in that: The solid alkali activator is composed of solid sodium hydroxide and sodium silicate pentahydrate mixed in a mass ratio of 1:5~9.

5. A modified single-component geopolymer curing agent for treating sludge according to claim 1, characterized in that: The mass ratio of fly ash to granulated blast furnace slag is 6:4~5.

6. A modified single-component geopolymer curing agent for treating sludge according to claim 1, characterized in that: The particle size of the nano-silica is 25~35nm.

7. A modified single-component geopolymer curing agent for treating sludge according to claim 1, characterized in that: The specific surface area of ​​the fly ash is in the range of 400~600 m². 2 / kg.

8. A method for solidifying silt using a modified single-component geopolymer solidifier for treating silt according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step 1: Weigh out the dredged silt dry soil, fly ash, slag, solid sodium hydroxide and sodium silicate pentahydrate according to the proportions, and dry mix them. Step 2: Add nano-silica and ionic admixture to the dry mixture obtained in Step 1, and continue to dry mix until homogeneous; Step 3: Add water to the mixed dry material obtained in Step 2, stir at low speed to initially wet it, and then stir at high speed until it is evenly mixed to obtain solidified soil; Step four: Compact the solidified soil obtained in step three into shape and seal it for curing.

9. The method for solidifying silt using a modified single-component geopolymer solidifier for treating silt according to claim 8, characterized in that: In step one, the mass ratio of dredged silt and dry soil to fly ash is 900:50~70.

10. The method for solidifying silt using a modified single-component geopolymer solidifier for treating silt according to claim 8, characterized in that: In step three, the low-speed stirring speed is 30~60 r / min and the stirring time is 120~180 seconds.

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