Slow-release coated fertilizer and preparation method thereof

Abstract

The invention relates to the technical field of fertilizers, and discloses a slow-release coated fertilizer and a preparation method thereof. Comprising the following raw materials in parts by weight: 57 to 77.6 parts of urea, 14.25 to 24.25 parts of diammonium phosphate, 0 to 5 parts of calcium superphosphate, 4.75 to 14.55 parts of potassium chloride, 0.03 to 0.2 part of chitosan, 0.0075 to 0.05 part of nano silicon dioxide, 0.001 to 0.015 part of glutaraldehyde, 2.96 to 4.94 parts of deionized water, 0.01 to 0.1 part of bacillus subtilis, 0.05 to 0.2 part of mesoporous silicon dioxide loaded nano selenium and 0.5 to 1 part of potassium fulvate. Chitosan, nano silicon dioxide and glutaraldehyde are matched with one another, and chitosan has pH responsiveness, so that the problems that a coating layer of a traditional slow-release fertilizer is prone to being affected by the environment and broken in advance, nutrients are rapidly released, and the slow-release effect is reduced are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of fertilizers, and specifically to a slow-release coated fertilizer and a preparation method thereof. Background Art

[0002] The slow-release coated fertilizer is a new type of fertilizer that can effectively improve fertilizer utilization rate, reduce environmental pollution, and achieve long-term and stable fertilizer supply for crops. It controls the nutrient release rate by coating a special material on the surface of fertilizer particles, such as sulfur, resin, polymer, etc. This kind of fertilizer has significant advantages. First, it greatly improves the fertilizer utilization rate. The nutrients of ordinary fertilizers are easily lost with rainwater or volatilized, and the utilization rate is only 30%-40%; while the slow-release coated fertilizer, relying on the coating, allows the nutrients to be slowly released, and the utilization rate reaches 60%-70%, reducing the fertilizer application cost. Second, it reduces the fertilization frequency. One application can continuously supply nutrients throughout the growth period of crops, avoiding frequent fertilization and saving labor and time costs. Third, it reduces environmental pollution. The slow-release coated fertilizer reduces nutrient leaching, volatilization and fixation, reduces pollution to soil, water body and atmosphere, and protects the ecological environment. It is applicable to a variety of crops and soil conditions. In the cultivation of vegetables and fruits, it can meet the nutrient requirements of crops at different stages, improving yield and quality; in sandy soil, it effectively reduces nutrient loss. With the development of agricultural modernization, this environmentally friendly and efficient slow-release coated fertilizer will play an increasingly important role in future agricultural production.

[0003] The coating layer of traditional slow-release fertilizers is easily affected by the environment and ruptures prematurely, resulting in rapid nutrient release and reducing the slow-release effect. Summary of the Invention

[0004] In view of the deficiencies of the prior art, the present invention provides a slow-release coated fertilizer and a preparation method thereof, which solve the problem that the coating layer of traditional slow-release fertilizers is easily affected by the environment and ruptures prematurely, resulting in rapid nutrient release and reducing the slow-release effect.

[0005] To achieve the above object, the present invention is realized through the following technical solutions: A slow-release coated fertilizer includes the following raw materials in parts by weight: 57-77.6 parts of urea, 14.25-24.25 parts of diammonium phosphate, 0-5 parts of superphosphate, 4.75-14.55 parts of potassium chloride, 0.03-0.2 parts of chitosan, 0.0075-0.05 parts of nano-silica, 0.001-0.015 parts of glutaraldehyde, 2.96-4.94 parts of deionized water, 0.01-0.1 parts of Bacillus subtilis, 0.05-0.2 parts of mesoporous silica supported nano-selenium, and 0.5-1 part of potassium humate.

[0006] A preparation method of a slow-release coated fertilizer includes the following steps: S1. Raw material mixing: Urea, diammonium phosphate, and potassium chloride are put into a twin-shaft mixer according to the formula ratio for mixing to obtain a raw material mixture; S2. Granulation and screening: After mixing, a disk granulator is used for granulation, and then a screening disk is used for screening to obtain fertilizer granules; S3. Coating solution preparation: Chitosan and deionized water are put into a magnetic stirrer for stirring to obtain a chitosan solution. The chitosan solution is poured into an ultrasonic disperser, and then nano-silica is added. The ultrasonic disperser is started to disperse the nano-silica to obtain a composite sol. The composite sol is poured into the magnetic stirrer, and glutaraldehyde is added to the composite sol. Then the magnetic stirrer is started for stirring to obtain a coating solution; S4. Fluidized bed spray coating: Hot air is introduced into the fluidized bed reactor for preheating, and then the fertilizer granules are put in to maintain a fluidized state. The solution pump is started to atomize the coating solution through a two-fluid nozzle to form a surface coating on the surface of the fertilizer granules. After spraying continuously for 30 - 45 minutes, the solution pump is closed, and fluidized drying is continued for 10 - 15 minutes; S5. Microbial inoculant loading: Bacillus subtilis is mixed with pure water through a drum mixer and sprayed onto the surface of the coated granules through a pressure sprayer; S6. Nano-selenium compounding: Mesoporous silica loaded with nano-selenium is loaded through a fluidized bed secondary spraying process; S7. Post-treatment and packaging: The finished product is cooled to room temperature by a vibrating fluidized bed, and a screening plate is used to screen out sticky granules. Then moisture-proof packaging materials are used and stored in a cool and dry place.

[0007] Preferably, in S1, the rotation speed of the twin-shaft mixer is controlled at 20 - 30 rpm, the mixing time is controlled at 10 - 15 minutes, and the particle size of the raw material mixture is controlled at 2 - 4 mm.

[0008] Preferably, in S2, while granulating with the disk granulator, pure water is used to humidify the fertilizer granules through a sprayer, and the moisture content of the fertilizer granules is controlled ≤2%. When screening with the screening disk, fine powder with a particle size <1.5 mm and large particles >4.5 mm are removed. When granulating with the disk granulator, the rotation speed is controlled at 20 - 30 rpm, the inclination angle of the disk is controlled at 45° - 60°, and the filling rate of the raw material mixture is controlled at 30% - 50%.

[0009] Preferably, in S3, when obtaining the chitosan solution, the deacetylation degree of chitosan ≥85%, the temperature in the magnetic stirrer is controlled at 30 - 40 °C, the stirring speed of the magnetic stirrer is controlled at 200 - 600 rpm, and the pH value of the chitosan solution is tested while stirring. When the pH value is adjusted to 5.5 - 6.5, stirring is stopped.

[0010] Preferably, in the step S3, when obtaining the composite sol, the particle size of the nano-silica is controlled within 10 - 50 nm, the power of the ultrasonic disperser is controlled within 100 - 300 W, the frequency is controlled within 20 - 80 kHz, the ultrasonic time is controlled within 10 - 15 minutes, the temperature is <50 °C, and the light transmittance of the composite sol is ≥90%.

[0011] Preferably, in the step S3, when obtaining the coating solution, the stirring speed of the stirrer is controlled within 300 - 600 rpm, and the stirring time is controlled within 5 - 6 minutes.

[0012] Preferably, in the step S4, when preheating, the temperature of the hot air is controlled within 30 - 50 °C, the air volume is controlled within 200 - 300 m 3 / h, and the preheating time is 5 minutes; The wind speed when the fertilizer particles are put in and kept in a fluidized state is controlled within 1.5 - 2.5 m / s; When forming a surface coating on the fertilizer particles, the atomization pressure is controlled within 0.2 - 0.4 MPa, and the spraying rate is controlled within 5 - 10 kg / h; after continuing fluidized drying for 10 - 15 minutes, the moisture content of the finished product is controlled ≤0.5%.

[0013] Preferably, in the step S5, the rotation speed of the drum mixer is set to 10 rpm, the mixing time is 5 minutes, and the working pressure of the pressure type sprayer is set within 1 - 5 MPa.

[0014] Preferably, in the step S6, the temperature of the fluidized bed is controlled within 30 - 50 °C, the wind speed is controlled within 1.2 - 2.0 m / s, the air volume is controlled within 150 - 250 m 3 / h, and the mesoporous silica - loaded nano - selenium solution is atomized into droplets using a two - fluid nozzle, the spraying pressure is controlled within 0.1 - 0.2 MPa, and the spraying rate is controlled within 3 - 8 kg / h.

[0015] The present invention provides a slow - release coated fertilizer and its preparation method. It has the following beneficial effects: 1. In the present invention, through the mutual cooperation of chitosan, nano - silica, and glutaraldehyde, by utilizing the pH - responsiveness of chitosan, the nutrient release can be controlled under different soil acid - base conditions. Nano - silica enhances the compactness of the coating, reduces water penetration, and stabilizes the coating structure at high temperatures. Glutaraldehyde promotes the cross - linking of chitosan to enhance the coating strength, jointly ensuring the stability of the slow - release effect, thereby solving the problem that the coating layer of traditional slow - release fertilizers is easily affected by the environment and ruptures in advance, resulting in rapid nutrient release and reduced slow - release effect.

[0016] 2. In the present invention, in the processes such as fluidized bed spray coating, parameters such as the temperature and air volume of the hot air are optimized and controlled. For example, when preheating, the temperature of the hot air is controlled within 30 - 50 °C, which effectively reduces the energy consumption during the coating process compared with the high temperature of the traditional process, and further reduces the production cost.

[0017] 3. The present invention uses relatively low-cost materials such as chitosan as the coating material, and precisely controls the dosage of each raw material, effectively controlling the cost while improving the sustained-release performance, thus facilitating large-scale application.

[0018] 4. The present invention promotes the decomposition of soil organic matter, improves soil fertility, inhibits the growth of harmful pathogenic bacteria, and enhances the disease resistance of crops by loading Bacillus subtilis. At the same time, by loading nano-selenium with mesoporous silica, it improves the absorption of selenium by crops, enhances the stress resistance of crops, and improves the nutritional value of agricultural products. Description of the Drawings

[0019] Figure 1 It is a schematic diagram of the preparation process steps of the present invention. Detailed Embodiments

[0020] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the drawings of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] Please refer to the attached Figure 1 , the embodiment of the present invention provides a slow-release coated fertilizer, which includes the following raw materials in parts by weight: 57 - 77.6 parts of urea, 14.25 - 24.25 parts of diammonium phosphate, 0 - 5 parts of superphosphate, 4.75 - 14.55 parts of potassium chloride, 0.03 - 0.2 parts of chitosan, 0.0075 - 0.05 parts of nano-silica, 0.001 - 0.015 parts of glutaraldehyde, 2.96 - 4.94 parts of deionized water, 0.01 - 0.1 parts of Bacillus subtilis, 0.05 - 0.2 parts of mesoporous silica loaded with nano-selenium, and 0.5 - 1 part of potassium humate.

[0022] Specifically, by using urea, diammonium phosphate, superphosphate and potassium chloride as the basic nutrient sources of the fertilizer, the essential nitrogen, phosphorus and potassium elements for plant growth can be provided. Urea has a high nitrogen content and is a quick-acting nitrogen source. Diammonium phosphate supplements nitrogen and phosphorus elements and promotes the root development of crops. Superphosphate provides water-soluble phosphorus. Potassium chloride supplements potassium elements and enhances the stress resistance of crops. The proportion of each component is reasonable to meet the nutrient requirements of different crops at each growth stage. Chitosan has pH responsiveness and controls nutrient release under different soil acid-base conditions. Nano-silica enhances the compactness of the coating, reduces water penetration, and stabilizes the coating structure at high temperatures. Glutaraldehyde promotes the cross-linking of chitosan and improves the coating strength. The three work together to achieve stable and long-term nutrient release and improve fertilizer utilization efficiency. Potassium humate is a kind of humic acid, which can chelate heavy metal ions in the soil and reduce their harm to crops. At the same time, it improves the soil aggregate structure, increases soil air permeability and water retention, and regulates soil pH. Bacillus subtilis, as a beneficial microorganism, propagates and metabolizes in the soil, secretes various enzymes and antibiotics, promotes the decomposition of soil organic matter, releases the fixed nutrients, improves soil fertility, and at the same time inhibits the growth of harmful pathogens and enhances the disease resistance of crops to ensure the healthy growth of crops. Mesoporous silica loaded with nano-selenium can improve the utilization efficiency of selenium element, enable crops to absorb more selenium element. Selenium has an antioxidant effect, which can not only enhance the stress resistance of crops, but also improve the nutritional value of agricultural products to meet people's needs for healthy agricultural products.

[0023] Please refer to the appendix Figure 1 , and a preparation method of a slow-release coated fertilizer comprises the following steps: S1. Raw material mixing: Urea, diammonium phosphate and potassium chloride are put into a twin-shaft mixer according to the formula ratio for mixing to obtain a raw material mixture; S2. Granulation and screening: After mixing, a disk granulator is used for granulation, and then a screening disk is used for screening to obtain fertilizer particles; S3. Coating solution preparation: Chitosan and deionized water are put into a magnetic stirrer for stirring to obtain a chitosan solution. The chitosan solution is poured into an ultrasonic disperser, and then nano-silica is added. The ultrasonic disperser is started to disperse the nano-silica to obtain a composite sol. The composite sol is poured into the magnetic stirrer, glutaraldehyde is added to the composite sol, and then the magnetic stirrer is started to stir to obtain a coating solution; S4. Fluidized bed spray coating: Hot air is introduced into the fluidized bed reactor for preheating, and then the fertilizer particles are put in to maintain a fluidized state. The solution pump is started to atomize the coating solution through a two-fluid nozzle to form a surface coating on the surface of the fertilizer particles. After spraying continuously for 30 - 45 minutes, the solution pump is closed, and fluidized drying is continued for 10 - 15 minutes; S5. Microbial inoculant loading: Bacillus subtilis is mixed with pure water through a drum mixer and sprayed onto the surface of the coated particles through a pressure-type sprayer; S6, Nano-selenium compounding: Loading nano-selenium supported on mesoporous silica is carried out by a fluidized bed secondary spraying process; S7, Post-treatment and packaging: The finished product is cooled to room temperature by a vibrated fluidized bed, and the agglomerated particles are removed by screening using a screening plate. Then, it is packaged with moisture-proof packaging materials and stored in a cool and dry place.

[0024] Specifically, through S1, raw material mixing can promote the uniform distribution of nitrogen, phosphorus, and potassium nutrients contained in urea, diammonium phosphate, and potassium chloride, ensure the stable proportion of various nutrients in the fertilizer, meet the balanced demand for multiple nutrients throughout the crop cycle, and form a raw material mixture with good fluidity and affinity through a double-shaft mixer, providing suitable materials for subsequent disk granulation and improving the molding rate and quality stability in the granulation process; through S2, granulation screening can process the mixed raw materials into granules of specific shapes and sizes, improve the physical stability of the fertilizer, prevent caking and stratification during storage and transportation, remove materials that do not meet the particle size requirements through a screening disk, ensure uniform finished product particle size, meet the strict requirements of mechanized fertilization for fertilizer particle size, and improve the convenience and accuracy of fertilization; through magnetic stirring, chitosan is fully dissolved in deionized water to form a uniform chitosan solution, laying a foundation for the subsequent preparation of composite sol. With the help of an ultrasonic disperser, nano-silica is evenly dispersed in the chitosan solution to prevent nanoparticle aggregation, ensure the dispersion stability of nano-materials in the composite sol, enhance the denseness and functionality of the coating. Magnetic stirring is used again to allow glutaraldehyde to react fully with the composite sol, promoting the cross-linking of chitosan molecules to form a stable coating solution, ensuring that the coating can exhibit good slow-release performance in subsequent processes; by introducing hot air to preheat the fluidized bed reactor, a suitable temperature environment can be created for the fluidization and coating of fertilizer particles. The fluidized state exposes the fertilizer particles in all directions, ensuring uniform coating. The coating solution is atomized through a two-fluid nozzle, and within 30 - 45 minutes, the droplets evenly adhere to the surface of the fertilizer particles. After cross-linking and curing, a continuous and dense coating layer is formed to achieve nutrient slow-release. By continuing fluidized drying for 10 - 15 minutes after turning off the solution pump, excess moisture in the coating can be removed, enhancing the binding force between the coating and the particles and improving the coating stability and weather resistance; through a drum mixer, Bacillus subtilis and pure water are fully mixed to ensure uniform dispersion of the bacterial agent, providing conditions for precise loading. The bacterial agent solution is evenly sprayed onto the surface of the coated particles through a pressure sprayer, enabling beneficial microorganisms to adhere to the fertilizer and play a role in improving the soil and promoting crop growth by entering the soil with the fertilizer application. By loading after coating, the bacterial agent is prevented from being damaged by high temperature and mechanical action during the previous processing, increasing the survival rate of the bacterial agent and ensuring its efficacy; through the fluidized bed secondary spraying process, nano-selenium loaded on mesoporous silica is evenly attached to the surface of the already coated fertilizer particles, realizing the compounding of nano-selenium and the fertilizer, adding selenium elements to the fertilizer, not only meeting the physiological requirements of plants for selenium, improving the antioxidant and stress resistance of crops, but also increasing the selenium content of agricultural products and enhancing the nutritional value of agricultural products. The fertilizer particles in the fluidized bed are in a fluidized state, increasing the contact area with the droplets, ensuring uniform nano-selenium loading, and improving the loading efficiency and effect;Quickly cool the finished product to room temperature through a vibrating fluidized bed, prevent the physicochemical properties of the fertilizer from changing due to high temperature, ensure the stable quality of the product, remove agglomerated particles through a screening plate, ensure the uniformity of product particles, meet the requirements of the market for the appearance and use performance of the fertilizer, and extend the product shelf life by using moisture-proof packaging and storing it in a cool and dry place to block external moisture and reduce the risks of the fertilizer getting damp, caking, and deteriorating.;

[0025] Please refer to the appendix Figure 1 In S1, the rotation speed of the twin-shaft mixer is controlled at 20 - 30 rpm, the mixing time is controlled at 10 - 15 minutes, and the particle size of the raw material mixture is controlled at 2 - 4 mm.

[0026] Specifically, by controlling the rotation speed of the twin-shaft mixer at 20 - 30 rpm, various raw materials can be moderately stirred and tumbled in the mixer, which not only ensures the full and uniform mixing of the raw materials but also avoids excessive friction, breakage, or generation of a large amount of heat of the raw materials due to too high a rotation speed, thus affecting the properties of the raw materials; the mixing time is controlled at 10 - 15 minutes to ensure that various raw materials have sufficient time for full mixing, enabling different components to be evenly distributed to form a mixture with stable properties and uniform composition, while avoiding energy waste and equipment wear caused by overlong mixing time; controlling the particle size of the raw material mixture at 2 - 4 mm enables the raw materials to better contact various solutions, inoculants, etc. in subsequent processes such as coating and loading, ensuring uniform coating and good loading effects, and also facilitating the storage, transportation, and use of the product, and avoiding problems such as caking and poor fluidity caused by too large or too small particle size.

[0027] Please refer to the appendix Figure 1 In S2, while granulating with a disk granulator, pure water is used to wet the fertilizer particles through a sprayer, and the moisture content of the fertilizer particles is controlled ≤ 2%. When screening with a screening disk, fine powder with a particle size < 1.5 mm and large particles > 4.5 mm are removed. The rotation speed of the disk granulator during granulation is controlled at 20 - 30 rpm, the inclination angle of the disk is controlled at 45° - 60°, and the filling rate of the raw material mixture is controlled at 30% - 50%.

[0028] Specifically, by humidifying with pure water and controlling the moisture content of fertilizer particles ≤ 2%, the raw materials can have appropriate viscosity, promoting granulation, preventing difficult granulation due to too little moisture or over-wet and caking-prone particles caused by too much moisture, which affects product quality and subsequent processing; removing fine powder with a particle size < 1.5 mm and large particles > 4.5 mm ensures uniform fertilizer particle size, meeting the product specification requirements, which is beneficial to improving the application effect and uniformity of fertilizers, and avoiding uneven fertilization caused by uneven particle size; controlling the rotational speed of the disk granulator at 20 - 30 rpm enables the raw materials to fully tumble and collide within the disk, facilitating the formation and growth of particles, while avoiding particle breakage due to too fast rotational speed or low granulation efficiency caused by too slow rotational speed; controlling the disk inclination angle at 45° - 60° helps the movement and tumbling of materials within the disk, enabling better mixing and agglomeration of raw materials into particles, while facilitating the control of the residence time and movement trajectory of particles within the disk, ensuring the granulation effect; controlling the filling rate of the raw material mixture at 30% - 50% can ensure sufficient space within the disk for the raw materials to fully tumble, mix, and granulate, avoiding overcrowding of materials within the disk due to too high filling rate, which affects particle formation and quality, or low granulation efficiency caused by too low filling rate.

[0029] Please refer to the attached Figure 1 In S3, when obtaining the chitosan solution, the deacetylation degree of chitosan ≥ 85%, controlling the temperature in the magnetic stirrer at 30 - 40 °C, controlling the stirring speed of the magnetic stirrer at 200 - 600 rpm, testing the pH value of the chitosan solution while stirring, and stopping stirring when the pH value is adjusted to 5.5 - 6.5; in S3, when obtaining the composite sol, controlling the particle size of nano-silica at 10 - 50 nm, controlling the power of the ultrasonic disperser at 100 - 300 W, the frequency at 20 - 80 kHz, the ultrasonic time at 10 - 15 minutes, the temperature < 50 °C, and the transmittance of the composite sol ≥ 90%; in S3, when obtaining the coating solution, controlling the stirring speed of the stirrer at 300 - 600 rpm and the stirring time at 5 - 6 minutes.

[0030] Specifically, by using chitosan with a deacetylation degree of ≥85%, its amino group content is ensured to be sufficient, which is conducive to subsequent cross-linking reactions, improving the stability and functionality of the coating. The temperature of the magnetic stirrer is controlled at 30 - 40°C, and the stirring speed is set at 200 - 600 rpm to accelerate the dissolution of chitosan, while avoiding the destruction of its molecular structure due to high temperature or high-speed stirring. The pH value of the chitosan solution is adjusted to 5.5 - 6.5 to enhance the solubility and stability of chitosan, preventing flocculation or precipitation, and providing a suitable environment for the subsequent addition of nano-silica. The particle size of nano-silica is controlled at 10 - 50 nm, and it is dispersed effectively by an ultrasonic disperser with a power of 100 - 300 W and a frequency of 20 - 80 kHz for 10 - 15 minutes to prevent agglomeration and ensure its uniform distribution in the chitosan solution. The temperature during the ultrasonic process is controlled <50°C to avoid the damage of the properties of nano-silica and chitosan due to high temperature, ensuring the quality of the composite sol, with its light transmittance ≥90%, forming a homogeneous and stable system. The stirrer stirs at a speed of 300 - 600 rpm for 5 - 6 minutes to promote the full contact between glutaraldehyde and the composite sol, accelerating the cross-linking of chitosan, stabilizing the performance of the coating solution, ensuring the smooth progress of the subsequent coating process, ensuring the uniform mixing of all components, and enabling the final coating to form a continuous, dense, and stable coating layer on the surface of the fertilizer particles, achieving a good slow-release effect.

[0031] Please refer to the appendix Figure 1 In S4, during preheating, the temperature of the hot air is controlled at 30 - 50°C, and the air volume is controlled at 200 - 300 m 3 / h, and the preheating time is 5 minutes; when the fertilizer particles are put in and maintained in a fluidized state, the wind speed is controlled at 1.5 - 2.5 m / s; when forming a surface coating on the surface of the fertilizer particles, the atomization pressure is controlled at 0.2 - 0.4 MPa, and the spraying rate is controlled at 5 - 10 kg / h; after continuing fluidized drying for 10 - 15 minutes, the moisture content of the finished product is controlled ≤0.5%.

[0032] Specifically, by controlling the hot air temperature at 30 - 50°C, the temperature of the fertilizer particles can be gently increased, enabling them to better combine with the coating material during the subsequent coating process, while avoiding damage to the fertilizer particles or premature reaction of the coating material due to excessive temperature. The air volume is controlled at 200 - 300 m 3 / h can make the hot air evenly blow on the fertilizer particles, ensuring uniform heating of the particles and creating good conditions for the subsequent fluidization and coating processes. The preheating time is 5 minutes, which ensures that the fertilizer particles reach an appropriate temperature without wasting energy or affecting production efficiency due to excessive preheating time. The wind speed is controlled at 1.5 - 2.5 m / s, keeping the fertilizer particles in a good fluidization state. The particles collide and roll with each other, which is conducive to the uniform wrapping of the coating material on the particle surface. At the same time, it can also enhance the contact between the particles and the hot air, improving the drying efficiency. The atomization pressure is controlled at 0.2 - 0.4 MPa, which can fully atomize the coating solution into fine droplets and evenly spray them on the surface of the fertilizer particles, forming a thin and uniform coating layer to improve the coating effect. The spraying rate is controlled at 5 - 10 kg / h, ensuring an appropriate and uniform supply of the coating material. It can not only form a complete coating on the surface of the fertilizer particles but also prevent the coating from being too thick or uneven due to too fast spraying. Continue fluidized drying for 10 - 15 minutes and control the moisture content of the finished product ≤ 0.5%, removing the excess moisture on the surface and inside of the fertilizer particles, curing the coating, improving the stability and storage properties of the fertilizer, and preventing the fertilizer from caking or deteriorating due to excessive moisture content during storage and transportation.

[0033] Please refer to the attachment Figure 1 , in S5, the rotation speed of the drum mixer is set at 10 rpm, the mixing time is 5 minutes, and the working pressure of the pressure sprayer is set at 1 - 5 MPa.

[0034] Specifically, by setting the rotation speed of the drum mixer at 10 rpm, the materials can tumble and mix in the drum at an appropriate speed. This rotation speed will neither cause uneven mixing of the materials or excessive friction and breakage due to being too fast, nor result in low mixing efficiency due to being too slow, ensuring uniform mixing of the materials. The mixing time is 5 minutes. During this time, the materials are fully contacted and mixed under the action of the drum mixer, achieving a uniform dispersion effect, while avoiding energy waste and reduced production efficiency caused by excessive mixing time. The working pressure of the pressure sprayer is set at 1 - 5 MPa. Within this pressure range, the liquid materials can be fully atomized. A lower pressure makes the atomized droplets larger, which is suitable for situations with higher requirements for droplet particle size. A higher pressure makes the droplets finer, increasing the contact area between the liquid and other materials and being conducive to subsequent reactions or drying processes.

[0035] Please refer to the attachment Figure 1 , in S6, the temperature of the fluidized bed is controlled at 30 - 50 °C, the wind speed is controlled at 1.2 - 2.0 m / s, and the air volume is controlled at 150 - 250 m 3 / h. Use a two-fluid nozzle to atomize the mesoporous silica-supported nano-selenium solution into droplets. The spray pressure is controlled at 0.1 - 0.2 MPa, and the spray rate is controlled at 3 - 8 kg / h.

[0036] Specifically, by controlling the fluidized bed temperature at 30 - 50 °C, it provides a suitable temperature environment for the interaction between the mesoporous silica-supported nano-selenium solution and other materials, which helps to improve the loading efficiency and stability, and at the same time avoids affecting the activity of nano-selenium and the performance of the solution due to too high or too low temperature; the wind speed is controlled at 1.2 - 2.0 m / s, so that the materials are in a good fluidization state in the fluidized bed, ensuring that after the mesoporous silica-supported nano-selenium solution is atomized, it can be evenly contacted and mixed with other materials, and at the same time promoting heat transfer to make the temperature distribution more uniform. The air volume is controlled at 150 - 250 m 3 / h, which can ensure that there is enough gas to carry heat and momentum, maintain the stable fluidization state in the fluidized bed, and provide guarantee for the full interaction between the atomized droplets and the materials; the spray pressure is controlled at 0.1 - 0.2 MPa, which can effectively atomize the mesoporous silica-supported nano-selenium solution into fine droplets, ensure the uniform dispersion of the solution in the fluidized bed, increase its contact area with other materials, and is conducive to improving the loading effect and reaction efficiency. The spray rate is controlled at 3 - 8 kg / h, ensuring that the mesoporous silica-supported nano-selenium solution is supplied at a suitable speed, which can not only ensure that enough solution participates in the reaction, but also will not cause too high local concentration or droplet aggregation due to too fast spraying, thus realizing uniform loading and good process control.

[0037] The following is a further introduction in combination with specific embodiments: Example 1: A slow-release coated fertilizer includes the following raw materials in parts by weight: 67.3 parts of urea, 19.25 parts of diammonium phosphate, 2.5 parts of superphosphate, 9.65 parts of potassium chloride, 0.2 part of chitosan, 0.05 part of nano-silica, 0.015 part of glutaraldehyde, 4.94 parts of deionized water, 0.055 part of Bacillus subtilis, 0.125 part of mesoporous silica-supported nano-selenium, and 0.75 part of potassium humate.

[0038] A preparation method of a slow-release coated fertilizer includes the following steps: S1. Raw material mixing: Urea, diammonium phosphate, and potassium chloride are put into a twin-shaft mixer according to the formula ratio for mixing to obtain a raw material mixture; S2. Granulation and screening: After mixing, granulation is carried out using a disk granulator, and then screening is carried out using a screening disk to obtain fertilizer granules; S3. Preparation of coating solution: Chitosan and deionized water are put into a magnetic stirrer for stirring to obtain a chitosan solution. The chitosan solution is poured into an ultrasonic disperser, and then nano-silica is added. The ultrasonic disperser is started to disperse the nano-silica to obtain a composite sol. The composite sol is poured into a magnetic stirrer, and glutaraldehyde is added to the composite sol, and then the magnetic stirrer is started for stirring to obtain a coating solution; S4. Fluidized bed spray coating: Preheat by introducing hot air into the fluidized bed reactor, then put in fertilizer particles to maintain the fluidized state. Start the solution pump to atomize the coating solution through a two-fluid nozzle to form a surface coating on the fertilizer particles. After spraying continuously for 30 - 45 minutes, turn off the solution pump and continue fluidized drying for 10 - 15 minutes; S5. Microbial inoculant loading: Mix Bacillus subtilis with pure water through a drum mixer and spray it onto the surface of the coated particles through a pressure sprayer; S6. Nano-selenium compounding: Mesoporous silica loaded with nano-selenium is loaded through a fluidized bed secondary spraying process; S7. Post-treatment and packaging: The finished product is cooled to room temperature by a vibrating fluidized bed, sieved using a sieve plate to remove sticky particles, and then stored in a cool and dry place using moisture-proof packaging materials.

[0039] In S1, the rotation speed of the double-shaft mixer is controlled at 20 - 30 rpm, the mixing time is controlled at 10 - 15 minutes, and the particle size of the raw material mixture is controlled at 2 - 4 mm.

[0040] In S2, while granulating with a disk granulator, wet the fertilizer particles with pure water through a sprayer, control the moisture content of the fertilizer particles ≤ 2%, remove fine powder with a particle size < 1.5 mm and large particles > 4.5 mm when sieving with a sieve plate. The rotation speed of the disk granulator during granulation is controlled at 20 - 30 rpm, the inclination angle of the disk is controlled at 45° - 60°, and the filling rate of the raw material mixture is controlled at 30% - 50%.

[0041] In S3, when obtaining the chitosan solution, the deacetylation degree of chitosan ≥ 85%, control the temperature in the magnetic stirrer at 30 - 40 °C, the stirring speed of the magnetic stirrer is controlled at 200 - 600 rpm, and test the pH value of the chitosan solution while stirring. Stop stirring when the pH value is adjusted to 5.5 - 6.5.

[0042] In S3, when obtaining the composite sol, the particle size of nano-silica is controlled at 10 - 50 nm, the power of the ultrasonic disperser is controlled at 100 - 300 W, the frequency is controlled at 20 - 80 kHz, the ultrasonic time is controlled at 10 - 15 minutes, the temperature < 50 °C, and the light transmittance of the composite sol ≥ 90%.

[0043] In S3, when obtaining the coating solution, the stirring speed of the stirrer is controlled at 300 - 600 rpm, and the stirring time is controlled at 5 - 6 minutes.

[0044] In S4, when preheating, the temperature of the hot air is controlled at 30 - 50 °C, the air volume is controlled at 200 - 300 m 3 / h, and the preheating time is 5 minutes; The wind speed when the input fertilizer particles are kept in a fluidized state is controlled at 1.5 - 2.5 m / s; When forming a surface coating on the surface of the fertilizer particles, the atomization pressure is controlled at 0.2 - 0.4 MPa, and the spraying rate is controlled at 5 - 10 kg / h; After continuous fluidized drying for 10 - 15 minutes, the moisture content of the finished product is controlled ≤ 0.5%.

[0045] In S5, the rotation speed of the drum mixer is set at 10 rpm, the mixing time is 5 minutes, and the working pressure of the pressure sprayer is set at 1 - 5 MPa.

[0046] In S6, the temperature of the fluidized bed is controlled at 30 - 50 °C, the wind speed is controlled at 1.2 - 2.0 m / s, the air volume is controlled at 150 - 250 m 3 / h, and the mesoporous silica - loaded nano - selenium solution is atomized into droplets using a two - fluid nozzle, the spraying pressure is controlled at 0.1 - 0.2 MPa, and the spraying rate is controlled at 3 - 8 kg / h.

[0047] Example 2: The difference between this example and the above Example 1 is as follows: A slow - release coated fertilizer, comprising the following raw materials in parts by weight: 67.3 parts of urea, 19.25 parts of diammonium phosphate, 2.5 parts of superphosphate, 9.65 parts of potassium chloride, 0.115 part of chitosan, 0.02875 part of nano - silica, 0.008 part of glutaraldehyde, 3.95 parts of deionized water, 0.055 part of Bacillus subtilis, 0.125 part of mesoporous silica - loaded nano - selenium, and 0.75 part of potassium fulvate.

[0048] Example 3: The difference between this example and the above Example 1 is as follows: A slow - release coated fertilizer, comprising the following raw materials in parts by weight: 67.3 parts of urea, 19.25 parts of diammonium phosphate, 2.5 parts of superphosphate, 9.65 parts of potassium chloride, 0.03 part of chitosan, 0.0075 part of nano - silica, 0.001 part of glutaraldehyde, 2.96 parts of deionized water, 0.055 part of Bacillus subtilis, 0.125 part of mesoporous silica - loaded nano - selenium, and 0.75 part of potassium fulvate.

[0049] Table 1: Comparison Example 1 Example 2 Example 3 Control Example Nitrogen cumulative release rate on the 30th day (%) 35 32 30 45 Nitrogen cumulative release rate on the 60th day (%) 65 64 62 70 Nitrogen cumulative release rate on the 90th day (%) 90 89 85 95 Coating integrity rate (%) 96 94 92 75 The comparison in the above table shows the differences in the nitrogen cumulative release rates of this slow-release coated fertilizer in soil leachate at 25°C and pH 6.5 at 30 days, 60 days, and 90 days respectively, and the differences in the integrity rate of the coating film after being stored for 7 days in a high-temperature and high-humidity environment (RH 90%) at 50°C. The comparative example is a traditional slow-release fertilizer without chitosan, nano-silica, and glutaraldehyde components. It can be seen from Table 1 that different amounts of chitosan, nano-silica, glutaraldehyde, and deionized water can affect the organizational structure and performance of the outer coating film of the slow-release coated fertilizer. Through the mutual cooperation of various elements, since chitosan has pH responsiveness, it can control nutrient release under different soil acid-base conditions; nano-silica enhances the denseness of the coating, reduces water penetration, and stabilizes the coating structure at high temperatures; glutaraldehyde promotes the cross-linking of chitosan to enhance the coating strength, jointly ensuring the stability of the slow-release effect, thus solving the problem that the coating layer of traditional slow-release fertilizers is easily affected by the environment and ruptures prematurely, resulting in rapid nutrient release and a decrease in the slow-release effect.

[0050] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A slow-release coated fertilizer, characterized in that, It comprises the following raw materials in parts by weight: 57 - 77.6 parts of urea, 14.25 - 24.25 parts of diammonium phosphate, 0 - 5 parts of superphosphate, 4.75 - 14.55 parts of potassium chloride, 0.03 - 0.2 parts of chitosan, 0.0075 - 0.05 parts of nano-silica, 0.001 - 0.015 parts of glutaraldehyde, 2.96 - 4.94 parts of deionized water, 0.01 - 0.1 parts of Bacillus subtilis, 0.05 - 0.2 parts of nano-selenium supported on mesoporous silica, and 0.5 - 1 part of potassium humate.

2. A preparation method of a slow-release coated fertilizer, characterized in that, It comprises the following steps: S1. Raw material mixing: Urea, diammonium phosphate, and potassium chloride are put into a twin-shaft mixer according to the formula ratio for mixing to obtain a raw material mixture. S2. Granulation and screening: After mixing, granulation is carried out using a disc granulator, and then screening is carried out using a screening tray to obtain fertilizer granules. S3. Preparation of coating solution: Chitosan and deionized water are put into a magnetic stirrer for stirring to obtain a chitosan solution. The chitosan solution is poured into an ultrasonic disperser, and then nano-silica is added. The ultrasonic disperser is started to disperse the nano-silica to obtain a composite sol. The composite sol is poured into the magnetic stirrer, and glutaraldehyde is added to the composite sol, and then the magnetic stirrer is started to stir to obtain a coating solution. S4. Fluidized bed spray coating: Hot air is introduced into the fluidized bed reactor for preheating, and then the fertilizer granules are put in to maintain a fluidized state. The solution pump is started to atomize the coating solution through a two-fluid nozzle to form a surface coating on the surface of the fertilizer granules. After spraying continuously for 30 - 45 minutes, the solution pump is closed, and fluidized drying is continued for 10 - 15 minutes. S5. Loading of microbial inoculant: Bacillus subtilis is mixed with pure water through a drum mixer and sprayed onto the surface of the coated granules through a pressure sprayer. S6. Nano-selenium compounding: Mesoporous silica supported nano-selenium is loaded through a fluidized bed secondary spray process. S7. Post-treatment and packaging: The finished product is cooled to room temperature by a vibrating fluidized bed, and screened using a screening plate to remove sticky granules, and then packaged with moisture-proof packaging materials and stored in a cool and dry place.

3. The preparation method of a slow-release coated fertilizer according to claim 2, characterized in that, In S1, the rotation speed of the twin-shaft mixer is controlled at 20 - 30 rpm, the mixing time is controlled at 10 - 15 minutes, and the particle size of the raw material mixture is controlled at 2 - 4 mm.

4. The preparation method of a slow-release coated fertilizer according to claim 2, wherein In S2, while granulating with the disc granulator, pure water is used to wet the fertilizer granules through a sprayer, and the moisture content of the fertilizer granules is controlled ≤2%. When screening with the screening tray, fine powder with a particle size <1.5 mm and large particles >4.5 mm are removed. When granulating with the disc granulator, the rotation speed is controlled at 20 - 30 rpm, the inclination angle of the disc is controlled at 45° - 60°, and the filling rate of the raw material mixture is controlled at 30% - 50%.

5. The preparation method of a slow-release coated fertilizer according to claim 2, characterized in that, In S3, when obtaining the chitosan solution, the deacetylation degree of chitosan ≥85%, the temperature in the magnetic stirrer is controlled at 30 - 40 °C, the stirring speed of the magnetic stirrer is controlled at 200 - 600 rpm, and the pH value of the chitosan solution is measured while stirring. When the pH value is adjusted to 5.5 - 6.5, stirring is stopped.

6. The preparation method of a slow-release coated fertilizer according to claim 2, characterized in that, In S3, when obtaining the composite sol, the particle size of nano-silica is controlled at 10 - 50 nm, the power of the ultrasonic disperser is controlled at 100 - 300 W, the frequency is controlled at 20 - 80 kHz, the ultrasonic time is controlled at 10 - 15 minutes, the temperature is <50 °C, and the light transmittance of the composite sol is ≥90%.

7. The preparation method of a slow-release coated fertilizer according to claim 2, wherein, In S3, when obtaining the coating solution, the stirring speed of the stirrer is controlled at 300 - 600 rpm, and the stirring time is controlled at 5 - 6 minutes.

8. The preparation method of a slow-release coated fertilizer according to claim 2, characterized in that, In S4, during preheating, the temperature of the hot air is controlled at 30 - 50 °C, the air volume is controlled at 200 - 300 m³ / h, and the preheating time is 5 minutes; The wind speed when the fertilizer particles are put in and kept in a fluidized state is controlled at 1.5 - 2.5 m / s; When forming a surface coating on the surface of the fertilizer particles, the atomization pressure is controlled at 0.2 - 0.4 MPa, and the spraying rate is controlled at 5 - 10 kg / h; After continuing fluidized drying for 10 - 15 minutes, the moisture content of the finished product is controlled to be ≤0.5%.

9. The preparation method of a slow-release coated fertilizer according to claim 2, characterized in that, In S5, the rotation speed of the drum mixer is set at 10 rpm, the mixing time is 5 minutes, and the working pressure of the pressure sprayer is set at 1 - 5 MPa.

10. The preparation method of a slow-release coated fertilizer according to claim 2, characterized in that, In S6, the temperature of the fluidized bed is controlled at 30 - 50 °C, the wind speed is controlled at 1.2 - 2.0 m / s, the air volume is controlled at 150 - 250 m³ / h, and the mesoporous silica-supported nano-selenium solution is atomized into droplets using a two-fluid nozzle, the spraying pressure is controlled at 0.1 - 0.2 MPa, and the spraying rate is controlled at 3 - 8 kg / h.

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