A biological drying method for sludge

Abstract

This invention discloses a method for the biological drying of sludge, comprising the following steps: baking biomass materials, mixing the resulting baked product with sludge, biochar materials, and a conditioner for biological drying, thereby completing the sludge drying process. In this invention, by biologically drying the mixture of baked product, sludge, biochar materials, and conditioner, not only can the self-generated heat within the system be used to effectively dewater the sludge in a very short time, but the bioavailability of the microbial carbon source can also be improved, effectively reducing the generation of polluting gases. This method can effectively reduce the emission of polluting gases while achieving rapid dewatering. It has advantages such as simple process, low treatment cost, low energy consumption, high drying efficiency, good dewatering effect, low polluting gas production, and low content of toxic and harmful components in the dried product. It can solve the problems of biomass resource waste and pollution, achieve sludge reduction and resource utilization, and has high returns and good application prospects.

Description

Technical Field

[0001] This invention belongs to the field of organic solid waste disposal and relates to a biological drying method for sludge, specifically a biological drying method for sludge based on low-temperature baking. Background Technology

[0002] In recent years, the production of urban sewage sludge in my country has increased dramatically year by year. Sludge, a byproduct of urban wastewater treatment, is a complex heterogeneous substance composed of organic matter, inorganic matter, microbial cells, inorganic particles, and colloids. Furthermore, even after mechanical dewatering, the moisture content of sludge remains as high as 80%, severely restricting its subsequent treatment, disposal, and transportation. Therefore, the primary task is to efficiently dewater and dry the sludge, effectively reduce greenhouse gas emissions during the process, and obtain a stable, dried product in a short time to provide an effective method for sludge disposal.

[0003] Biological drying, as a low-energy-consumption method for efficiently utilizing high-moisture-content organic solid waste, can leverage the abundant microorganisms inherent in sludge mixed with agricultural waste straw. Under suitable oxygen conditions, aerobic microorganisms metabolize and decompose organic matter, releasing a large amount of energy. The generated bioheat is used to remove moisture, achieving water removal in a relatively short time while partially stabilizing the product, and has received increasing attention in recent years. However, existing biological drying technologies suffer from problems such as demanding processing conditions, excessively long fermentation cycles, short periods of high-temperature maintenance, excessive emissions of pollutants during drying, and significant material loss after drying, leading to a reduction in the quality of sludge biomass. For example, some researchers have proposed a biological drying method of "deep dehydration + composting." This method first uses iron salts, aluminum salts, or calcium oxide as conditioners to deeply dehydrate the sludge, reducing its moisture content from over 97% to below 65%. Then, the deeply dehydrated sludge with a moisture content below 65% is composted, reducing the moisture content from 65% to below 35%. However, this method requires the sludge to undergo deep dehydration and reduce its moisture content to below 65% before composting, which involves harsh processing conditions and drawbacks such as a long fermentation cycle, short high-temperature maintenance time, and high cost. Furthermore, the composting process generates odors and greenhouse gas pollution, which is detrimental to environmental protection and the reduction of the greenhouse effect. At the same time, the conditioners used in the deep dehydration process are prone to... The introduction of toxic and harmful components (such as chlorine, sulfur, and iron) can result in poor quality of dried sludge products, hindering their subsequent resource utilization. For instance, some researchers have proposed inoculating sludge with microbial agents for biological drying, aiming to improve efficiency through the introduction of specific microorganisms and control of stirring, ventilation, and exhaust conditions. However, this method requires complex screening, acclimatization, and compounding of specific microorganisms, leading to a complex drying process with demanding conditions and a long cycle. Furthermore, the growth of microorganisms consumes large amounts of organic pollutants and releases significant amounts of polluting gases (such as hydrogen sulfide, methane, carbon dioxide, and nitrogen oxides), posing a risk of generating large quantities of polluting gases. Furthermore, in existing biological drying technologies, when biomass materials are used as sludge conditioners, the biomass materials must first be sun-dried or oven-dried. Then, accurate calculations of the mass ratio are needed to obtain a suitable C / N ratio sludge mixture before composting or biological drying can proceed. This process is complex and suffers from problems such as high conditioner requirements, low efficiency, and high levels of toxic and harmful substances in the drying products. Therefore, obtaining a biological drying method for sludge that is simple in process, low in treatment cost, high in drying efficiency, good in dewatering effect, low in pollutant gas production, and low in toxic and harmful components in the dried products is of great significance for realizing the subsequent resource utilization of sludge. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to address the shortcomings of the existing technology by providing a biological drying method for sludge that is simple in process, low in processing cost, high in drying efficiency, good in dewatering effect, low in pollutant gas production, and low in toxic and harmful components in the dried product.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0006] A method for biological drying of sludge, comprising the following steps:

[0007] S1. Baking the biomass materials;

[0008] S2. Mix the baked product obtained after baking treatment in step S1, sludge, biochar material and conditioner to obtain a mixture.

[0009] S3. The mixture obtained in step S2 is subjected to biological drying to complete the sludge drying treatment.

[0010] In a further improvement to the above-mentioned biological drying method, in step S1, the baking process is carried out at a temperature of 200°C and the baking time is 10 minutes.

[0011] In a further improvement to the above-mentioned biological drying method, the heating rate during the baking process in step S1 is 10℃ / min.

[0012] In a further improvement to the above-mentioned biological drying method, in step S1, the baking process is carried out under anaerobic conditions.

[0013] In a further improvement to the above-mentioned bio-drying method, in step S1, the biomass material is at least one of camellia oleifera fruit, reed stalks, and sawdust; the biomass material is further treated before use by washing, drying, crushing, and sieving to obtain biomass material powder; the particle size of the biomass material powder is 10mm to 20mm.

[0014] In a further improvement to the above-mentioned biological drying method, in step S2, the mass ratio of the baked product to the sludge is 1 to 2:1; and the moisture content of the sludge is ≥90%.

[0015] In a further improvement to the above-mentioned biological drying method, in step S2, the amount of biochar material added is 2% to 5% of the total mass of the mixture; the particle size of the biochar material is 20 mesh to 25 mesh; and the biochar material is at least one of bamboo charcoal, camellia shell biochar, and wood charcoal.

[0016] In a further improvement to the above-mentioned biological drying method, the amount of the conditioning agent added is 2% to 5% of the total mass of the mixture; the conditioning agent is at least one of ferric chloride, magnesium sulfate, magnesium chloride, potassium carbonate, and sodium hydroxide.

[0017] In a further improvement to the above-mentioned biological drying method, step S2, after mixing, further includes the following treatment: adjusting the moisture content of the mixture to 55% to 60%.

[0018] In a further improvement to the above-mentioned biological drying method, step S3 further includes intermittent aeration of the mixture during the biological drying process; during the biological drying process, the total aeration volume is controlled at 0.5 L / min·kg for the first three days, and the aeration time is 20 min; after the fourth day, the total aeration volume is controlled at 0.2 L / min·kg, and the aeration time is 10 min.

[0019] In a further improvement to the above-mentioned biological drying method, step S3 further includes the following treatment: when the temperature of the mixture is below 55°C, the mixture is turned over; the frequency of turning over is 1 to 2 times per day.

[0020] Compared with the prior art, the advantages of the present invention are as follows:

[0021] (1) In view of the shortcomings of existing sludge biological drying methods, such as harsh treatment conditions, long fermentation cycles, short high-temperature maintenance time, excessive emissions of pollutants during drying, and excessive material loss after drying leading to a decrease in the quality of sludge biomass, this invention creatively provides a sludge biological drying method. First, the biomass material is baked. Baking improves the physical and chemical properties of the biomass, significantly improving its energy density and bulk density. Therefore, when used as a raw material to construct the pile, it helps improve the pile structure and adjust its porosity, maintaining a sufficiently oxygen-rich environment under the influence of aeration, thus promoting biological drying and enabling the pile to maintain a longer high-temperature time. Simultaneously, the baking process reduces the oxygen content of the biomass and allows for a certain degree of pyrolysis and carbonization of cellulose, hemicellulose, and lignin. This effectively removes moisture and light volatiles from the biomass while increasing its absorption and utilization potential. Therefore, using it as a raw material to construct the pile not only improves the nutrient distribution of the pile, facilitating microbial absorption and utilization, but also... This method reduces the generation of polluting gases in the heap, particularly significantly reducing the generation of oxygen-containing greenhouse gases (such as CO2 and N2O). Based on this, the baked products, sludge, biochar materials, and conditioners obtained after baking are mixed to form a mixture, which is then subjected to biological drying. During this process, under the combined action of the baked products, sludge, biochar materials, and conditioners, microorganisms rapidly multiply and continuously and efficiently generate heat. This allows for effective dewatering of the sludge within a very short time using the system's self-generated heat, enabling the sludge to maintain a moisture content of up to 11 days. The internal content is reduced to less than 30%. At the same time, under the combined action of biochar materials and conditioners, oxidation is promoted by changing the porosity and increasing the surface area of ​​the reactor space. The suitable slightly acidic and low moisture content environment disturbs the anaerobic environment, thereby inhibiting denitrification and the activity of methanogenic bacteria, improving the bioavailability of microbial carbon sources, and thus effectively reducing the generation of polluting gases. In particular, it can significantly reduce greenhouse gas emissions, greatly reduce the secondary pollution that may be caused to the environment during the drying process, and will not introduce too many toxic and harmful components into the dried product. Compared with conventional biological drying methods, this invention, for the first time, adds baked biomass materials, biochar materials, and conditioners to sludge together. This not only eliminates the conditioning process but also effectively reduces the emission of polluting gases, especially greenhouse gases, while achieving rapid dehydration. It has advantages such as simple process, low treatment cost, low energy consumption, high drying efficiency, good dehydration effect, low polluting gas production, and low content of toxic and harmful components in the dried product. It can solve the resource waste and pollutants caused by the direct combustion of large amounts of biomass materials, and is also conducive to the reduction and resource utilization of sludge. It has high returns and good application prospects.

[0022] (2) In this invention, the baking process can not only reduce the gas emission during baking while ensuring a certain dehydration effect, but also ensure a high solid yield and energy yield, thereby improving product quality and reducing energy consumption.

[0023] (3) In this invention, by optimizing the mass ratio of baking product to sludge to 1-2:1, the baking product and the material can be mixed with mechanical stirring to obtain the optimal C / N ratio and the optimal moisture content of the mixture. The operation is very simple. In addition, if the mass ratio of the two is too high, the carbon content of the pile will be higher than the nitrogen content, which will reduce the decomposition rate of organic matter during the sludge biological drying process and reduce the drying efficiency. It will also increase the emission of CO2 and CH4 during the subsequent biological drying process. In addition, if the mass ratio of the two is too low, the nitrogen content of the pile will be higher than the carbon content, which will lead to excessive nitrogen being decomposed by microorganisms, and at the same time produce odor and ammonia and other harmful substances, thus affecting the quality of the biological drying product. By optimizing the addition amount of biochar material to 2% to 5% of the total mass of the mixture, it is beneficial to rapid dehydration and reduce the release of polluting gases. This is because excessive biochar addition will increase the pH value of the pile, resulting in a large amount of NH4. + The accumulation of NH3 can affect the degradation rate and drying rate of organic matter during bio-drying, and also increase NH3 emissions. On the other hand, too low a biochar content will amplify the water retention capacity of the biochar itself, thereby reducing water removal and hindering rapid dehydration. By optimizing the amount of conditioner added to 2% to 5% of the total mass of the mixture, the system contains an appropriate amount of additives (conditioner), which can promote the maximization of nitrogen retention. If the conditioner content is too high, it will increase the salt ion concentration in the pile, thereby affecting the microbial activity during the bio-drying process.

[0024] (4) In this invention, by including intermittent aeration of the mixture during the biological drying process, and optimizing the total aeration amount and aeration time, a suitable aeration amount can be provided according to different stages of biological drying. This not only improves the drying effect but also helps to reduce the treatment cost. For example, a larger aeration amount is provided during the heating and high-temperature periods of the pile, thereby providing a continuous and sufficient amount of oxygen to make the aerobic microorganisms in the pile more active, thereby maximizing the increase of pile temperature and decomposition of organic matter, thus greatly enhancing the water removal effect of the process. During the cooling period, a reasonable reduction in the aeration amount can control the energy consumption and economic cost of the entire biological drying process. Attached Figure Description

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0026] Figure 1 This is a process flow diagram of the biological drying method for sludge in Examples 1-3 of the present invention.

[0027] Figure 2 The graph shows the changes in water content of the system during the biological drying process of Examples 1-3 and Comparative Example 1 of the present invention.

[0028] Figure 3 The graph shows the temperature changes of the system during the biological drying process in Examples 1-3 and Comparative Example 1 of this invention.

[0029] Figure 4 The graph shows the changes in N2O accumulation and emissions during the bio-drying process of Examples 1-3 and Comparative Example 1 of this invention. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention.

[0031] The materials and instruments used in the following examples are all commercially available.

[0032] Example 1:

[0033] A biological drying method for sludge, the process flow diagram of which is shown below. Figure 1 As shown, it includes the following steps:

[0034] (1) Wash the camellia fruit and place it in a 105℃ oven for 24 hours to dry. Pour the dried camellia fruit into a pulverizer and pulverize it, then sieve it to obtain camellia fruit fragments with a particle size of 10mm. Place the obtained camellia fruit into a self-heating roasting reactor and, under anaerobic conditions, raise the temperature from room temperature to 200℃ at a rate of 10℃ / min, hold for 10 minutes, and after roasting, lower the temperature to room temperature to obtain the roasted product. Pulverize and sieve the bamboo charcoal to obtain biochar powder with a particle size of 20-25 mesh.

[0035] (2) Mix sludge with a moisture content of 91.8% after mechanical dewatering from the wastewater treatment plant and roasted camellia fruit and cattail fragments at a weight ratio of 1:1, and pour the mixture into a biological drying reactor. Add ferric chloride granules and biochar powder, each accounting for 5% of the total material weight, to the biological drying reactor. Stir thoroughly for 5 minutes using a mechanical stirrer until homogeneous to obtain the reaction material (mixture). In this step, after homogeneous stirring, the moisture content of the mixture can be adjusted to 55%–60% according to actual needs.

[0036] (3) The mixture is subjected to biological drying treatment. During the biological drying process, air is intermittently introduced into the bottom of the biological drying reactor to aerate the mixture. For the first 3 days, the aeration rate is controlled at 0.5 L / min·kg and the ventilation time is 20 min. From the 4th to the 11th day, the aeration rate is controlled at 0.2 L / min·kg and the ventilation time is 10 min to complete the biological drying treatment of the sludge. During the biological drying process, the mixture is turned over as needed. Specifically, when the temperature of the mixture is below 55℃, the mixture is turned over once to twice a day.

[0037] Comparative Example 1:

[0038] A method for biological drying of sludge, comprising the following steps:

[0039] (1) Wash the camellia fruit and put it in an oven at 105℃ for 24 hours to dry. Take the dried camellia fruit and put it into a pulverizer to crush it and sieve it to obtain camellia fruit fragments with a particle size of 10mm. Crush the bamboo charcoal with a pulverizer and sieve it to obtain biochar powder with a particle size of 20-25 mesh.

[0040] (2) At a weight ratio of 1:1, sludge with a mechanical dewatering content of 92.4% (from the same batch as the dewatered sludge used in Example 1, with only a reasonable difference in moisture content) from a wastewater treatment plant is mixed with crushed camellia fruit pieces and poured into a biological drying reactor. Ferric chloride granules and biochar powder, each accounting for 5% of the total material weight, are then added to the biological drying reactor. The mixture is thoroughly stirred for 5 minutes using a mechanical stirrer until homogeneous, yielding the reaction material (mixture). In this step, after homogeneous stirring, the moisture content of the mixture can be adjusted to 55%–60% according to actual needs.

[0041] (3) The mixture is subjected to biological drying treatment. During the biological drying process, air is intermittently introduced into the bottom of the biological drying reactor to aerate the mixture. For the first 3 days, the aeration rate is controlled at 0.5 L / min·kg and the ventilation time is 20 min. From the 4th to the 11th day, the aeration rate is controlled at 0.2 L / min·kg and the ventilation time is 10 min to complete the biological drying treatment of the sludge. During the biological drying process, the mixture is turned over as needed. Specifically, when the temperature of the mixture is below 55℃, the mixture is turned over once to twice a day.

[0042] Example 2:

[0043] A biological drying method for sludge, the process flow diagram of which is shown below. Figure 1 As shown, it includes the following steps:

[0044] (1) Wash the reed stalks and dry them in a 105℃ oven for 24 hours. Then, crush the dried reed stalks in a pulverizer and sieve them to obtain reed stalk fragments with a particle size of 10mm. Place the obtained reed stalk fragments into a self-heating roasting reactor and, under anaerobic conditions, raise the temperature from room temperature to 200℃ at a rate of 10℃ / min, hold for 10 minutes, and after roasting, lower the temperature to room temperature to obtain the roasted product. Crush the bamboo charcoal in a pulverizer and sieve it to obtain biochar powder with a particle size of 20-25 mesh.

[0045] (2) At a weight ratio of 1:1, sludge with a mechanical dewatering content of 91.2% (from the same batch as the dewatered sludge used in Example 1, with only a reasonable difference in moisture content) from a wastewater treatment plant and roasted reed straw fragments are mixed and poured into a biological drying reactor. Potassium carbonate granules and biochar powder, each accounting for 2.5% of the total material weight, are then added to the biological drying reactor. The mixture is thoroughly stirred for 5 minutes using a mechanical stirrer until homogeneous, yielding the reaction material (mixture). In this step, after homogeneous stirring, the moisture content of the mixture can be adjusted to 55%–60% according to actual needs.

[0046] (3) The mixture is subjected to biological drying treatment. During the biological drying process, air is intermittently introduced from the bottom of the biological drying reactor to aerate the mixture. For the first 3 days, the aeration rate is controlled at 0.5 L / min·kg and the ventilation time is 20 min. From the 4th to the 11th day, the aeration rate is controlled at 0.2 L / min·kg and the ventilation time is 10 min to complete the biological drying treatment of the sludge. During the biological drying process, the mixture is turned over as needed. Specifically, when the temperature of the mixture is below 55℃, the mixture is turned over once to twice a day.

[0047] Example 3:

[0048] A biological drying method for sludge, the process flow diagram of which is shown below. Figure 1 As shown, it includes the following steps:

[0049] (1) Wash the reed stalks and dry them in a 105℃ oven for 24 hours. Then, crush the dried reed stalks in a pulverizer and sieve them to obtain reed stalk fragments with a particle size of 10mm. Place the obtained reed stalk fragments into a self-heating roasting reactor and, under anaerobic conditions, raise the temperature from room temperature to 200℃ at a rate of 10℃ / min, hold for 10 minutes, and after roasting, lower the temperature to room temperature to obtain the roasted product. Crush the bamboo charcoal in a pulverizer and sieve it to obtain biochar powder with a particle size of 20-25 mesh.

[0050] (2) At a weight ratio of 1:2, sludge with a mechanical dewatering content of 91.9% (from the same batch as the dewatered sludge used in Example 1, with only a reasonable difference in moisture content) from a wastewater treatment plant and roasted reed straw fragments are mixed and poured into a biological drying reactor. Sodium hydroxide granules and biochar powder, each accounting for 2.5% of the total material weight, are then added to the biological drying reactor. The mixture is thoroughly stirred for 5 minutes using a mechanical stirrer until homogeneous, yielding the reaction material (mixture). In this step, after homogeneous stirring, the moisture content of the mixture can be adjusted to 55%–60% according to actual needs.

[0051] (3) The mixture is subjected to biological drying treatment. During the biological drying process, air is intermittently introduced into the bottom of the biological drying reactor to aerate the mixture. For the first 3 days, the aeration rate is controlled at 0.5 L / min·kg and the ventilation time is 20 min. From the 4th to the 11th day, the aeration rate is controlled at 0.2 L / min·kg and the ventilation time is 10 min to complete the biological drying treatment of the sludge. During the biological drying process, the mixture is turned over as needed. Specifically, when the temperature of the mixture is below 55℃, the mixture is turned over once to twice a day.

[0052] During the bio-drying process of Examples 1-3 and Comparative Example 1, the changes in water content, temperature, and gas concentration of the system were monitored in real time, and the results are as follows: Figure 2 , Figure 3 , 4 As shown.

[0053] like Figure 2-4 The results show that: in Example 1, after 11 days of bio-drying treatment, the final moisture content of the dried product was 26.9%; the highest temperature of 65.2°C was reached on the 3rd day of bio-drying; compared with the control group (Comparative Example 1) of biomass without roasting treatment, the N2O emission in the bio-drying system of Example 1 can be reduced by 21.41%.

[0054] like Figure 2-4 The results show that: in Example 2, after 11 days of bio-drying, the final moisture content of the dried product was 30.6%; the highest temperature of 59.8°C was reached on the 3rd day of bio-drying; compared with the control group (Comparative Example 1) of biomass without roasting treatment, the N2O emission in the bio-drying system of Example 2 can be reduced by 87.58%.

[0055] like Figure 2-4The results show that in Example 3, after 11 days of bio-drying, the final moisture content of the dried product was 34.9%; the highest temperature of 60.3°C was reached on the 3rd day of bio-drying; compared with the control group (Comparative Example 1) of biomass without roasting treatment, the N2O emission in the bio-drying system of Example 3 can be reduced by 45.99%.

[0056] In addition, such as Figure 2-4 The results show that in Comparative Example 1, after 11 days of biological drying treatment, the final moisture content of the dried product was 30.1%; the highest temperature of 48.7℃ was reached on the 3rd day of biological drying.

[0057] The results above show that, compared with conventional biological drying methods, this invention, for the first time, adds baked biomass materials, biochar materials, and conditioners to sludge. This not only eliminates the conditioning process but also effectively reduces the emission of polluting gases, especially greenhouse gases, while achieving rapid dehydration. It has advantages such as simple process, low treatment cost, low energy consumption, high drying efficiency, good dehydration effect, low polluting gas production, and low content of toxic and harmful components in the dried product. It can solve the resource waste and pollutants caused by the direct combustion of large amounts of biomass materials, and is also conducive to the reduction and resource utilization of sludge. It has high returns and good application prospects.

[0058] The above embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for biological drying of sludge, characterized in that, Includes the following steps: S1. Baking treatment of biomass materials; the baking treatment is carried out at a temperature of 200℃; the baking treatment time is 10 min; the heating rate during the baking treatment is 10℃ / min; the baking treatment is carried out under anaerobic conditions; S2. Mix the baked product obtained after baking treatment in step S1, sludge, biochar material and conditioner to obtain a mixture. S3. The mixture obtained in step S2 is subjected to biological drying to complete the sludge drying treatment. The biological drying process also includes intermittent aeration of the mixture. During the biological drying process, the total aeration volume is controlled at 0.5 L / min·kg for the first three days, and the aeration time is 20 min. After the fourth day, the total aeration volume is controlled at 0.2 L / min·kg, and the aeration time is 10 min. The biological drying process also includes the following treatment: when the temperature of the mixture is below 55℃, the mixture is turned over. The frequency of turning over is 1 to 2 times per day.

2. The biological drying method according to claim 1, characterized in that, In step S1, the biomass material is at least one of camellia oleifera fruit, reed stalks and sawdust; the biomass material is further processed before use as follows: the biomass material is washed, dried, crushed and sieved to obtain biomass material powder; the particle size of the biomass material powder is 10 mm to 20 mm.

3. The biological drying method according to claim 1, characterized in that, In step S2, the mass ratio of the baked product to the sludge is 1 to 2:1; the moisture content of the sludge is ≥90%.

4. The biological drying method according to claim 1, characterized in that, In step S2, the amount of biochar material added is 2% to 5% of the total mass of the mixture; the particle size of the biochar material is 20 mesh to 25 mesh; the biochar material is at least one of bamboo charcoal, camellia shell biochar, and wood charcoal; the amount of conditioning agent added is 2% to 5% of the total mass of the mixture; the conditioning agent is at least one of ferric chloride, magnesium sulfate, magnesium chloride, potassium carbonate, and sodium hydroxide.

5. The biological drying method according to claim 4, characterized in that, In step S2, after mixing, the following treatment is also included: adjusting the moisture content of the mixture to 55% to 60%.

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