Biochar-based granular fertilizer and preparation method thereof

Abstract

The invention discloses a charcoal-based granular fertilizer and a preparation method thereof, and relates to the technical field of fertilizers. According to the invention, bagasse is subjected to microwave expansion treatment to destroy cell walls of residue plant fibers, then metal loading is carried out on the bagasse, and metal ions and a carbon matrix of the bagasse form a stable compound in a high-temperature firing process. Secondly, bagasse carbon, sugarcane leaf compost and kaolin are physically mixed to form a compound, and kaolin layered silicate minerals can reduce leaching loss of the fertilizer. Phosphorus is combined with magnesium and calcium in bagasse carbon through phosphoric acid infiltration, stable phosphate is formed in a precipitation mode, the phosphate can react with silicate of kaolin to generate a complex, the degradation time of nutrients in soil is delayed, and long-acting nutrition is provided for crops. The organic fertilizer and the inorganic fertilizer supplement each other, and the compound fertilizer is stable in particle structure and can slowly release nutrients and prolong the fertilizer efficiency. The biochar-based granular fertilizer prepared by the invention has the effects of water retention and slow release.

Description

Technical Field

[0001] The invention relates to the technical field of fertilizers, and in particular to a biochar-based granular fertilizer and a preparation method thereof. Background Art

[0002] In terms of preparation methods, biochar-based granular fertilizers are mainly produced through doping, granulation and coating methods. Although the doping method is simple and quick, its molding rate is low and the slow-release effect needs to be improved. In contrast, the granulation method can significantly improve the mechanical properties and slow-release effect of the fertilizer by mixing biochar with fertilizer and then performing disc granulation or mechanical extrusion. The application effect of biochar-based granular fertilizers has also been widely verified. Studies have shown that this type of fertilizer can not only increase the content of nutrients such as nitrogen, phosphorus and potassium in the soil, but also improve the physical and chemical properties of the soil and the structure of the microbial community.

[0003] As a new type of agricultural input, biochar-based granular fertilizer has received widespread attention in recent years. The research on its preparation method and application effect has been continuously deepened, providing new ideas for the sustainable development of agriculture. Biochar itself has a rich pore structure and high stability, which can improve the physical properties of the soil, increase soil fertility and reduce nutrient loss. However, when biochar is used alone as a fertilizer, there are problems such as low nutrient content, high cost and possible dust pollution during application. Therefore, researchers began to combine biochar with chemical fertilizers to develop biochar-based compound fertilizers with slow-release properties and soil improvement functions. Summary of the invention

[0004] The object of the present invention is to provide a biochar-based granular fertilizer and a preparation method thereof, so as to solve the problems existing in the prior art.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: a biochar-based granular fertilizer, the fertilizer is prepared by combining bagasse charcoal, sugarcane leaves and kaolin; the bagasse charcoal is carbonized after microwave and metal ion loading pretreatment, and the preparation steps include:

[0006] (1) collecting bagasse from a sugar factory or agricultural waste, washing the bagasse with clean water for 3-5 times to remove sand and impurities, then drying the washed bagasse at 60-80° C. until the moisture content is less than 10%, and crushing the bagasse into particles of 1-5 mm using a crusher; wetting the crushed bagasse with water to make its moisture content reach 30-50%, evenly spreading the wet bagasse in a microwave reactor for microwave treatment, and naturally cooling the treated bagasse to room temperature after taking out; soaking the microwave-treated bagasse in a metal salt solution with a solid-to-liquid ratio of 1:10, stirring at 50-80° C. for 1-4 hours, so that the metal ions are fully adsorbed on the surface and pores of the bagasse, and the stirring speed is 120 rpm; then filtering the solid, washing it twice with deionized water, drying the bagasse loaded with metal ions at 60-80° C. for 3 hours, and grinding it to 100-200 mesh to obtain pretreated bagasse;

[0007] (2) Pre-carbonizing the pretreated bagasse, the pre-carbonizing temperature is controlled at 200-240°C, the time is 120-180min, then the temperature of the muffle furnace is adjusted to 450-600°C, and after reaching the set temperature, the constant temperature carbonization is continued for 3-5h, the obtained bagasse charcoal is washed with deionized water until the pH value is constant, and dried in an electric blast drying oven at 105°C for 1d to obtain bagasse charcoal;

[0008] Furthermore, a method for preparing biochar-based granular fertilizer comprises the following preparation steps:

[0009] S1. Add bagasse charcoal, diatomaceous earth and kaolin to a micro mixer, mix thoroughly for 5 minutes, take out, and then add to a 2wt% potassium dihydrogen phosphate aqueous solution, the solid-liquid ratio is 1:10, stir at 120-240rpm for 2-4h, centrifuge at 5000-8000rpm for 5-15min, and then dry at 80°C for 10h to obtain inorganic fertilizer particles; 0.5-1 part of humic acid, 0.5-1 part of bagasse slurry, 1-1.5 parts of filter mud, 5-6 parts of sugarcane leaves, and 2-3 parts of poultry manure are placed in a composting tank, stirred and mixed thoroughly, and straw is spread on the composting tank for fermentation for 7-14d, and then the compost is turned to promote ventilation and uniform decomposition, the compost temperature is controlled at 50-70°C, the moisture content of the compost is maintained at 50-60%, and the composting is completed after 2-3 months, and dried in a cool place to obtain organic fertilizer;

[0010] S2. Inorganic fertilizer and organic fertilizer are put into a disc granulator for granulation. After the granulation is completed, composite particles are obtained. The composite particles are then put into a blower dryer for drying. After the drying is completed, solid particles with a moisture content not exceeding 12% are obtained. After pulverization, the solid particles are obtained to obtain biochar-based granular fertilizers of 0.1-0.3 cm.

[0011] Furthermore, in the step (1), the microwave expansion treatment is performed for 1-5 min, and the microwave power is 300-500 W.

[0012] Furthermore, in step (2), Zn(NO3)2 and deionized water are used to prepare a metal ion aqueous solution with a concentration of 0.1-1 mol / L.

[0013] Furthermore, after the carbonization in step (2) is completed, the product is cooled and then taken out and ground into 80 mesh.

[0014] Furthermore, in step S1, the weight ratio of bagasse charcoal, diatomaceous earth and kaolin is 5:1:2-10:1:3.

[0015] Furthermore, in step S1, the sugarcane leaves are cut into pieces of 5-10 cm in size.

[0016] Furthermore, in step S1, the water content of poultry feces does not exceed 10%.

[0017] Furthermore, in step S1, the mixture is dried in a cool place to reduce the water content to 20-30%.

[0018] Furthermore, in step S2, the mass ratio of the inorganic fertilizer to the organic fertilizer is 4:6-5:5.

[0019] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0020] The composite fertilizer prepared by combining bagasse charcoal, sugarcane leaves and kaolin can achieve the effects of water retention and slow release.

[0021] First, the sugarcane bagasse is treated with microwave expansion to destroy the cell walls of the residual plant fibers, and then metal loading is performed on it. The pre-treated sugarcane bagasse has a larger specific surface area and stronger adsorption capacity after carbonization. The metal ions can form a stable complex with the carbon matrix of the sugarcane bagasse during the high-temperature firing process, which enhances the structural stability of the biochar. At the same time, it can provide the nutrients needed by plants and fix heavy metals in the soil. The carbonized sugarcane bagasse and sugarcane leaf compost still retain organic matter and minerals, such as potassium, calcium, magnesium and other nutrients, which can supplement nutrients for the soil and improve soil fertility.

[0022] Secondly, bagasse charcoal, sugarcane leaf compost and kaolin are physically mixed to form a composite. Kaolin is a layered silicate mineral with a large number of hydrophilic silanol groups on its surface. Its water retention properties can reduce fertilizer leaching and keep nutrients in the soil for a longer time for crop absorption. Phosphorus is combined with magnesium and calcium in bagasse charcoal through phosphoric acid infiltration to form stable phosphates by precipitation. This phosphate can react with silicates in kaolin to form complexes, delaying the time for nutrients to be degraded in the soil and providing long-term nutrition for crops. Organic fertilizers and inorganic fertilizers complement each other. This composite fertilizer not only has a stable particle structure, can slowly release nutrients and prolong fertilizer effectiveness, but also can effectively improve the physical and chemical properties of the soil, increase soil organic carbon content, cation exchange capacity and various plant nutrients. In addition, the biochar in the composite fertilizer can improve soil aeration and water retention, reduce nutrient loss and improve fertilizer utilization. DETAILED DESCRIPTION

[0023] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0024] In order to more clearly illustrate the method provided by the present invention, the following examples are used to describe in detail the various index testing methods of a biochar-based granular fertilizer prepared in the following examples as follows:

[0025] Sustained release: Take the same size of the embodiment and the comparative example and refer to GB / T23348 to detect the release rate of phosphorus.

[0026] Water retention: Take the embodiment and comparative example of the same size, mix them thoroughly with dry soil, spray them slowly with tap water until water seeps out from the bottom of the container, then stop spraying, and test the water retention rate of the samples in the soil within 36 days.

[0027] Example 1

[0028] (1) Collect bagasse from a sugar factory or agricultural waste, wash the bagasse with clean water for 3 times to remove sand and impurities, then dry the washed bagasse at 60°C until the moisture content is less than 10%, and use a crusher to crush the bagasse into 1 mm particles; wet the crushed bagasse with water to make its moisture content reach 30%, evenly spread the wet bagasse in a microwave reactor, and perform microwave expansion treatment for 1 min at a microwave power of 300 W. After the treated bagasse is taken out, it is naturally cooled to room temperature; Use Zn(NO3)2 and deionized water to prepare a metal ion aqueous solution with a concentration of 0.1 mol / L; soak the microwave-treated bagasse in the metal salt solution with a solid-liquid ratio of 1:10, stir at 50°C for 1 hour to allow the metal ions to be fully adsorbed on the surface and pores of the bagasse, and the stirring speed is 120 rpm; then filter the solid, wash it twice with deionized water, dry the bagasse loaded with metal ions at 60°C for 3 hours, and grind it to 100 mesh to obtain pretreated bagasse;

[0029] (2) Pre-carbonizing the pretreated bagasse at a temperature of 200°C for 120 min, then adjusting the temperature of the muffle furnace to 450°C, and maintaining the constant temperature carbonization for 3 h after reaching the set temperature, and then taking it out and grinding it to 80 mesh after cooling, washing the obtained bagasse charcoal with deionized water until the pH value is constant, and drying it in an electric heated blast drying oven at 105°C for 1 day to obtain bagasse charcoal;

[0030] (3) Add bagasse charcoal, diatomaceous earth and kaolin in a weight ratio of 5:1:2 to a micro mixer, mix thoroughly for 5 minutes, take out, and then add to a 2wt% potassium dihydrogen phosphate aqueous solution with a solid-liquid ratio of 1:10, stir at 120rpm for 2h, centrifuge at 5000rpm for 5min, and then dry at 80°C for 10h to obtain inorganic fertilizer particles; put 0.5 parts of humic acid, 0.5 parts of bagasse slurry, 1 part of filter mud, 5 parts of sugarcane leaves cut into 50cm in size, and 2 parts of poultry manure with a water content of no more than 10% in a composting tank, stir thoroughly, spread straw on the composting tank and ferment for 7 days, then turn the compost to promote ventilation and uniform decomposition, control the compost temperature at 50°C, maintain the water content of the compost at 50%, end the composting after 2 months, dry in a cool place to reduce the water content to 20%, and obtain organic fertilizer;

[0031] (4) Inorganic fertilizer and organic fertilizer are put into a disc granulator for granulation. After the granulation is completed, composite particles are obtained, wherein the mass ratio of inorganic fertilizer to organic fertilizer is 4:6. The composite particles are then put into a blower dryer for drying. After the drying is completed, solid particles with a moisture content not exceeding 12% are obtained, which are crushed to obtain 0.1 cm biochar-based granular fertilizer.

[0032] Example 2

[0033] (1) Collect bagasse from a sugar factory or agricultural waste, wash the bagasse with clean water for 4 times to remove sand and impurities, then dry the washed bagasse at 70°C until the moisture content is less than 10%, and crush the bagasse into 3 mm particles using a crusher; moisten the crushed bagasse with water to make its moisture content reach 40%, evenly spread the moistened bagasse in a microwave reactor, and perform microwave expansion treatment for 3 minutes at a microwave power of 400 W. After the treated bagasse is taken out, it is naturally cooled to room temperature; Zn(NO3)2 and deionized water were prepared into a metal ion aqueous solution with a concentration of 0.55 mol / L; the microwave-treated bagasse was immersed in the metal salt solution with a solid-liquid ratio of 1:10, and stirred at 65°C for 2.5 hours to allow the metal ions to be fully adsorbed on the surface and pores of the bagasse, and the stirring speed was 120 rpm; then the solid was filtered out and washed twice with deionized water, the bagasse loaded with metal ions was dried at 70°C for 3 hours, and ground into 150 mesh to obtain pretreated bagasse;

[0034] (2) Pre-carbonizing the pretreated bagasse at a temperature of 220°C for 150 min, then adjusting the temperature of the muffle furnace to 525°C, and maintaining constant temperature carbonization for 4 h after reaching the set temperature, and then taking it out and grinding it to 80 mesh after cooling, washing the obtained bagasse charcoal with deionized water until the pH value is constant, and drying it in an electric heated blast drying oven at 105°C for 1 day to obtain bagasse charcoal;

[0035] (3) Add bagasse charcoal, diatomaceous earth and kaolin in a weight ratio of 7.5:1:2.5 to a micro mixer, mix thoroughly for 5 minutes, take out, and then add to a 2wt% potassium dihydrogen phosphate aqueous solution with a solid-liquid ratio of 1:10, stir at 180 rpm for 3 hours, centrifuge at 6500 rpm for 10 minutes, and then dry at 80°C for 10 hours to obtain inorganic fertilizer particles; add 0.75 parts of humic acid, 0.75 parts of bagasse slurry, 1 25 parts of filter mud, 5.5 parts of sugarcane leaves cut into 7.5 cm in size, and 2.5 parts of poultry manure with a water content of no more than 10% are placed in a composting tank, fully stirred and mixed, and then straw is spread on the composting tank for fermentation for 10.5 days, and then the compost is turned to promote ventilation and uniform decomposition. The composting temperature is controlled at 60°C, and the water content of the pile is maintained at 55%. After 2.5 months, the composting is terminated and dried in a cool place to reduce the water content to 25%, thereby obtaining an organic fertilizer;

[0036] (4) Inorganic fertilizer and organic fertilizer are put into a disc granulator for granulation. After the granulation is completed, composite particles are obtained, wherein the mass ratio of inorganic fertilizer to organic fertilizer is 4.5:5.5. The composite particles are then put into a blower dryer for drying. After the drying is completed, solid particles with a moisture content not exceeding 12% are obtained, which are crushed to obtain 0.2 cm biochar-based granular fertilizer.

[0037] Example 3

[0038] (1) Collect bagasse from a sugar factory or agricultural waste, wash the bagasse with clean water for 5 times to remove sand and impurities, then dry the washed bagasse at 80°C until the moisture content is less than 10%, and use a crusher to crush the bagasse into 5 mm particles; wet the crushed bagasse with water to make its moisture content reach 50%, evenly spread the wet bagasse in a microwave reactor, and perform microwave expansion treatment for 5 minutes at a microwave power of 500 W. After the treated bagasse is taken out, it is naturally cooled to room temperature ; Use Zn(NO3)2 and deionized water to prepare a metal ion aqueous solution with a concentration of 1 mol / L; soak the microwave-treated bagasse in the metal salt solution with a solid-liquid ratio of 1:10, and stir at 80°C for 4 hours to allow the metal ions to be fully adsorbed on the surface and pores of the bagasse, with a stirring speed of 120 rpm; then filter the solid, wash it twice with deionized water, dry the bagasse loaded with metal ions at 80°C for 3 hours, and grind it to 200 mesh to obtain pretreated bagasse;

[0039] (2) Pre-carbonizing the pretreated bagasse, the pre-carbonizing temperature is controlled at 240°C for 180 min, then the temperature of the muffle furnace is adjusted to 600°C, and after reaching the set temperature, the constant temperature carbonization is continued for 5 h, and after cooling, the bagasse is taken out and ground into 80 mesh, and the obtained bagasse charcoal is washed with deionized water until the pH value is constant, and then placed in an electric heating blast drying oven at 105°C for 1 day to obtain bagasse charcoal;

[0040] (3) Add bagasse charcoal, diatomaceous earth and kaolin in a weight ratio of 10:1:3 to a micro mixer, mix thoroughly for 5 minutes, take out, and then add to a 2wt% potassium dihydrogen phosphate aqueous solution with a solid-liquid ratio of 1:10, stir at 240rpm for 4h, centrifuge at 8000rpm for 15min, and then dry at 80°C for 10h to obtain inorganic fertilizer particles; put 1 part of humic acid, 1 part of bagasse slurry, 1.5 parts of filter mud, 6 parts of sugarcane leaves cut into 10cm in size, and 3 parts of poultry manure with a water content of no more than 10% in a composting tank, stir thoroughly, spread straw on the composting tank and ferment for 14 days, then turn the compost to promote ventilation and uniform decomposition, control the compost temperature at 70°C, maintain the water content of the compost at 60%, end the composting after 3 months, dry in a cool place to reduce the water content to 30%, and obtain organic fertilizer;

[0041] (4) Inorganic fertilizer and organic fertilizer are put into a disc granulator for granulation. After the granulation is completed, composite particles are obtained, wherein the mass ratio of inorganic fertilizer to organic fertilizer is 5:5. The composite particles are then put into a blower dryer for drying. After the drying is completed, solid particles with a moisture content not exceeding 12% are obtained, which are crushed to obtain 0.3 cm biochar-based granular fertilizer.

[0042] Comparative Example 1

[0043] The difference between Comparative Example 1 and Example 2 lies in the difference in step (1), where step (1) is modified to: collect bagasse from a sugar factory or agricultural waste, rinse the bagasse with clean water 4 times to remove mud and impurities, then dry the washed bagasse at 70°C until the moisture content is less than 10%, and use a pulverizer to crush the bagasse into 3 mm particles; use Zn(NO3)2 and deionized water to prepare a metal ion aqueous solution with a concentration of 0.55 mol / L; soak the crushed bagasse in a metal salt solution with a solid-to-liquid ratio of 1:10, stir at 65°C for 2.5 hours to allow the metal ions to be fully adsorbed on the surface and pores of the bagasse, and the stirring speed is 120 rpm; then filter the solid to obtain the solid, rinse it with deionized water twice, dry the bagasse loaded with metal ions at 70°C for 3 hours, and grind it to 150 mesh to obtain pretreated bagasse; the remaining steps are the same as Example 2.

[0044] Comparative Example 2

[0045] The difference between Comparative Example 2 and Example 2 lies in step (1), where step (1) is modified to: collect bagasse from a sugar factory or agricultural waste, rinse the bagasse with clean water for 4 times to remove mud and impurities, then dry the washed bagasse at 70°C until the moisture content is less than 10%, and use a crusher to crush the bagasse into 3 mm particles; moisten the crushed bagasse with water to make its moisture content reach 40%, spread the moistened bagasse evenly in a microwave reactor, and perform microwave expansion treatment for 3 minutes at a microwave power of 400 W. After the treated bagasse is taken out, it is naturally cooled to room temperature, dried at 70°C for 3 hours, and ground into 150 mesh to obtain pretreated bagasse; the remaining steps are the same as those in Example 2.

[0046] Comparative Example 3

[0047] The difference between Comparative Example 3 and Example 2 is that step (3) is different. Step (3) is changed to: add bagasse charcoal and diatomaceous earth in a micro mixer at a weight ratio of 7.5:1, mix thoroughly for 5 minutes, take out, and then add to 2wt% potassium dihydrogen phosphate aqueous solution with a solid-liquid ratio of 1:10, stir at 180rpm for 3h, centrifuge at 6500rpm for 10min, and then dry at 80℃ for 10h to obtain inorganic fertilizer particles; add 0.75 parts of humic acid and 0.75 parts of sugarcane prepared by mixing Slag slurry, 1.25 parts of filter mud, 5.5 parts of sugarcane leaves cut into 7.5 cm in size, and 2.5 parts of poultry manure with a water content not exceeding 10% are placed in a composting tank, and after being fully stirred and mixed, straw is spread on the composting tank and fermented for 10.5 days, and then the compost is turned to promote ventilation and uniform decomposition, the compost temperature is controlled at 60°C, and the water content of the compost is maintained at 55%. After 2.5 months, the composting is completed, and the compost is dried in a cool place to reduce the water content to 25%, thereby obtaining an organic fertilizer; the remaining steps are the same as those in Example 2.

[0048] Comparative Example 4

[0049] The difference between Comparative Example 4 and Example 2 lies in the difference in step (3), whereby step (3) is modified as follows: bagasse charcoal, diatomaceous earth and kaolin are added into a micro mixer in a weight ratio of 7.5:1:2.5, mixed thoroughly for 5 minutes and then taken out to obtain inorganic fertilizer granules; 0.75 parts of humic acid, 0.75 parts of bagasse slurry, 1.25 parts of filter mud, 5.5 parts of sugarcane leaves cut into 7.5 cm in size and 2.5 parts of poultry manure with a water content of no more than 10% are placed in a composting tank, stirred and mixed thoroughly, and then straw is spread on the composting tank for fermentation for 10.5 days, and then the compost is turned to promote ventilation and uniform decomposition, the composting temperature is controlled at 60°C, the water content of the compost is maintained at 55%, the composting is terminated after 2.5 months, and the compost is dried in a cool place to reduce the water content to 25%, thereby obtaining an organic fertilizer; and the remaining steps are the same as those in Example 2.

[0050] Effect example

[0051] Table 1 below shows the performance analysis results of a biochar-based granular fertilizer using Examples 1 to 3 of the present invention and Comparative Examples 1 to 4.

[0052] Table 1

[0053]

[0054]

[0055] From the comparison of the experimental data of the sustained release rate of the embodiment and the comparative example, it can be found that the sugarcane bagasse of the present invention destroys the cell wall of the residual plant fiber through microwave expansion treatment, and then metal loading is performed on it. The pre-treated sugarcane bagasse has a larger specific surface area and stronger adsorption capacity after carbonization. The metal ions can form a stable complex with the carbon matrix of the sugarcane bagasse during the high-temperature firing process, thereby enhancing the structural stability of the biochar, while providing the nutrients required by the plants and fixing the heavy metals in the soil; the carbonized sugarcane bagasse and sugarcane leaf compost still retain organic matter and minerals, such as potassium, calcium, Nutrient elements such as magnesium can supplement nutrients for the soil and improve soil fertility. The present invention combines phosphorus with magnesium and calcium in bagasse charcoal through phosphoric acid infiltration to form stable phosphates by precipitation. This phosphate can react with silicates in kaolin to form complexes, delaying the time for nutrients to be degraded in the soil and providing long-term nutrition for crops. Organic fertilizers and inorganic fertilizers complement each other. This composite fertilizer not only has a stable particle structure, can slowly release nutrients, and prolong fertilizer effects, but also can effectively improve the physical and chemical properties of the soil, increase the soil organic carbon content, cation exchange capacity and various plant nutrients. From the comparison of the experimental data of water retention rate of the embodiment and the comparative example, it can be found that the present invention physically mixes bagasse charcoal, sugarcane leaf compost and kaolin to form a composite. Kaolin is a layered silicate mineral with a large number of hydrophilic silanol groups on its surface. Its water retention performance can reduce the leaching of fertilizer and keep nutrients in the soil for a longer time for crop absorption. In addition, the biochar in the composite fertilizer can improve the air permeability and water retention of the soil, reduce nutrient loss, and improve fertilizer utilization.

[0056] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations within the meaning and scope of the equivalent elements of the claims be included in the invention. Any marking in a claim should not be considered as limiting the claim to which it relates.

Claims

1. A biochar-based granular fertilizer, characterized in that: The fertilizer is prepared by combining bagasse charcoal, sugarcane leaves and kaolin; the bagasse charcoal is carbonized after microwave and metal ion loading pretreatment, and the preparation steps include: (1) collecting bagasse from a sugar factory or agricultural waste, washing the bagasse with clean water for 3-5 times to remove sand and impurities, then drying the washed bagasse at 60-80° C. until the moisture content is less than 10%, and crushing the bagasse into particles of 1-5 mm using a crusher; wetting the crushed bagasse with water to make its moisture content reach 30-50%, evenly spreading the wet bagasse in a microwave reactor for microwave treatment, and naturally cooling the treated bagasse to room temperature after taking out; soaking the microwave-treated bagasse in a metal salt solution with a solid-to-liquid ratio of 1:10, stirring at 50-80° C. for 1-4 hours, so that the metal ions are fully adsorbed on the surface and pores of the bagasse, and the stirring speed is 120 rpm; then filtering the solid, washing it twice with deionized water, drying the bagasse loaded with metal ions at 60-80° C. for 3 hours, and grinding it to 100-200 mesh to obtain pretreated bagasse; (2) Pre-carbonizing the pretreated bagasse at a temperature of 200-240°C for 120-180 min, adjusting the temperature of the muffle furnace to 450-600°C, and maintaining constant temperature carbonization for 3-5 h after reaching the set temperature. Washing the obtained bagasse charcoal with deionized water until the pH value is constant, drying it in an electric blast drying oven at 105°C for 1 day, and obtaining bagasse charcoal.

2. A method for preparing biochar-based granular fertilizer, characterized in that: The method comprises the following preparation steps: S1. Add bagasse charcoal, diatomaceous earth and kaolin to a micro mixer, mix thoroughly for 5 minutes, take out, and then add to a 2wt% potassium dihydrogen phosphate aqueous solution, the solid-liquid ratio is 1:10, stir at 120-240rpm for 2-4h, centrifuge at 5000-8000rpm for 5-15min, and then dry at 80°C for 10h to obtain inorganic fertilizer particles; 0.5-1 part of humic acid, 0.5-1 part of bagasse slurry, 1-1.5 parts of filter mud, 5-6 parts of sugarcane leaves, and 2-3 parts of poultry manure are placed in a composting tank, stirred and mixed thoroughly, and straw is spread on the composting tank for fermentation for 7-14d, and then the compost is turned to promote ventilation and uniform decomposition, the compost temperature is controlled at 50-70°C, the moisture content of the compost is maintained at 50-60%, and the composting is completed after 2-3 months, and dried in a cool place to obtain organic fertilizer; S2. Inorganic fertilizer and organic fertilizer are put into a disc granulator for granulation. After the granulation is completed, composite particles are obtained. The composite particles are then put into a blower dryer for drying. After the drying is completed, solid particles with a moisture content not exceeding 12% are obtained. After pulverization, the solid particles are obtained to obtain biochar-based granular fertilizers of 0.1-0.3 cm.

3. The method for preparing a biochar-based granular fertilizer according to claim 1, characterized in that: In the step (1), the microwave expansion treatment is carried out for 1-5 minutes, and the microwave power is 300-500W.

4. The method for preparing a biochar-based granular fertilizer according to claim 1, characterized in that: In the step (2), Zn(NO3)2 and deionized water are used to prepare a metal ion aqueous solution with a concentration of 0.1-1 mol / L.

5. The method for preparing a biochar-based granular fertilizer according to claim 1, characterized in that: After the carbonization in step (2) is completed, the product is cooled and then taken out and ground into 80 mesh.

6. The method for preparing a biochar-based granular fertilizer according to claim 2, characterized in that: In the step S1, the weight ratio of bagasse charcoal, diatomaceous earth and kaolin is 5:1:2-10:1:

3.

7. The method for preparing a biochar-based granular fertilizer according to claim 2, characterized in that: In step S1, the sugarcane leaves are cut into pieces with a size of 5-10 cm.

8. The method for preparing a biochar-based granular fertilizer according to claim 2, characterized in that: In step S1, the water content of poultry feces does not exceed 10%.

9. The method for preparing a biochar-based granular fertilizer according to claim 2, characterized in that: In step S1, the mixture is dried in a cool place to reduce the water content to 20-30%.

10. The method for preparing a biochar-based granular fertilizer according to claim 2, characterized in that: The mass ratio of inorganic fertilizer to organic fertilizer in step S2 is 4:6-5:5.

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