Modified slow-release synergistic nitro compound fertilizer and preparation method thereof

Abstract

The application discloses a modified slow-release synergistic nitro compound fertilizer and a preparation method thereof. The fertilizer comprises, in terms of dry base mass fractions, a nitro base fertilizer, a modified composite slow-release carrier, a composite synergistic inhibitor and a degradable coating agent, wherein the modified composite slow-release carrier is two-step modified nitro humic acid compound shiitake mushroom residue biochar, the composite synergistic inhibitor is a compound of 3,4-dimethylpyrazole phosphate and dicyandiamide, and the degradable coating agent is a compound of starch grafted polyacrylamide and polyvinyl alcohol. The preparation is performed by using a step-by-step complex modification process, and the finished product is obtained by combining granulation, coating and post-treatment processes. The slow release is realized through three effects of adsorption complexation, nitrification inhibition and physical barrier, the nitrogen release period is 90-120 days, the nitrogen utilization rate is significantly improved, the process is stable, the environment is friendly, and the application is suitable for planting of various crops.

Description

Technical Field

[0001] This invention relates to the field of compound fertilizer preparation technology, and in particular to a modified slow-release synergistic nitro compound fertilizer and its preparation method. Background Technology

[0002] Nitro compound fertilizers are high-concentration compound fertilizers with nitrate nitrogen as the main nitrogen source. They are characterized by rapid release of fertilizer effects and easy absorption by crops, and are widely used in the planting of dryland field crops and cash crops. However, in practical applications, nitro compound fertilizers have some prominent problems: First, nitrate nitrogen carries a negative charge and is difficult to be adsorbed by soil colloids. It is easily leached away by rainfall or irrigation water, resulting in generally low nitrogen utilization rates, which not only wastes nutrients but also easily causes groundwater pollution. Second, the fertilizer effect is short-lived. The fertilizer effect of conventional nitro compound fertilizers is mostly 30-45 days, which cannot cover the nutrient requirements of crops throughout their entire growth period. Multiple topdressings are required, increasing the labor costs of planting. Third, ammonium nitrogen in fertilizers is easily nitrified under the action of soil microorganisms, further converting into nitrate nitrogen and accelerating its loss, while also producing greenhouse gases such as nitrous oxide.

[0003] Existing slow-release nitro-compound fertilizers mostly achieve slow release through single coating or the addition of a single nitrification inhibitor, which has certain limitations: resin-based coating materials are difficult to degrade and easily leave residues in the soil; the effective period of a single nitrification inhibitor is limited, making it difficult to meet the needs of long-growing crops; some products add humic acid or biochar to enhance nutrient adsorption capacity, but these are mostly simple physical mixtures, with insufficient active functional groups in humic acid, and the synergistic effect with biochar is not fully utilized, resulting in limited slow-release synergistic effects. Furthermore, the production of existing nitro-compound fertilizers generally uses high-temperature melting processes above 140℃, resulting in high energy consumption, and nitrification inhibitors are prone to thermal decomposition and failure under high-temperature conditions, posing certain thermal safety risks. Some existing technologies improve product performance through methods such as polymeric phosphorus regulation and pH adjustment, but the slow-release mechanism is singular, and the nitrogen release period is generally less than 80 days, making it difficult to achieve a balance between long-term slow release and energy-saving production. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a modified slow-release and enhanced nitro compound fertilizer and its preparation method. This fertilizer improves nutrient utilization and extends the fertilizer's effective period through the synergistic effect of multiple slow-release mechanisms, while ensuring environmental friendliness and ease of industrial production.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] This invention provides a modified slow-release synergistic nitro compound fertilizer, comprising a nitro base fertilizer; and, by dry basis weight of the raw materials, further comprising 3-10 parts of a modified compound slow-release carrier, 0.2-2 parts of a compound synergistic inhibitor, and 1-5 parts of a biodegradable coating agent. The modified compound slow-release carrier is a mixture of nitrohumic acid modified by a two-step process of nitrate nitration and hydrogen peroxide oxidation with mushroom char at a mass ratio of (2-4):1; the compound synergistic inhibitor is a compound of 3,4-dimethylpyrazole phosphate and dicyandiamide at a mass ratio of 1:(2-4); the biodegradable coating agent is a compound of starch-grafted polyacrylamide and polyvinyl alcohol at a mass ratio of (1-2):1. The compound synergistic inhibitor is dispersed within the fertilizer particles, and the surface of the fertilizer particles is coated with a coating layer formed by the biodegradable coating agent.

[0007] As a preferred embodiment of the present invention, the nitro-based fertilizer is composed of ammonium nitrate, diammonium phosphate, and potassium nitrate mixed in a mass ratio of (3-5):(2-3):(2-3); wherein the total nitrogen mass fraction of ammonium nitrate is ≥34%, the total nitrogen mass fraction of diammonium phosphate is ≥18%, the available phosphorus pentoxide mass fraction is ≥46%, the total nitrogen mass fraction of potassium nitrate is ≥13.5%, and the potassium oxide mass fraction is ≥46%; nitrate nitrogen accounts for 50%-70% of the total nitrogen mass, and the free water mass fraction is ≤2.0%.

[0008] As a preferred embodiment of the present invention, the modified composite sustained-release carrier is prepared by the following method: weathered coal humic acid is pulverized through a 100-mesh sieve, and 15%–25% nitric acid is added at a solid-liquid mass ratio of 1:(5–8). The mixture is stirred and nitrated at room temperature for 2–4 hours. After solid-liquid separation, the solid phase is collected, washed until the pH reaches 4–5, and then 5%–10% hydrogen peroxide is added at a solid-liquid mass ratio of 1:(4–6). The mixture is stirred and oxidized at 50–70°C for 1–3 hours. After the reaction, the mixture is filtered and dried at 60–80°C to constant weight to obtain oxidized modified nitrohumic acid. Mushroom residue is subjected to oxygen-limited pyrolysis at 400–500°C for 2–3 hours to obtain biochar, which is then pulverized to a particle size of 200–300 mm. The specific surface area is controlled at 200-350 m² / g; the modified composite slow-release carrier is obtained by mixing the oxidized modified nitrohumic acid with the mushroom residue biochar in a certain proportion.

[0009] As a preferred embodiment of the present invention, the raw materials, based on dry weight, further include 0.5 to 3 parts of trace elements; among the trace elements, iron, zinc, and manganese are in ethylenediaminetetraacetic acid chelated form, and boron is in borax form, with a mass ratio of iron, zinc, boron, and manganese of 2:2:1:1. The finished fertilizer is granular with a particle size of 2 to 4 mm, a particle compressive strength ≥15 N, a surface coating thickness of 30 to 80 μm, an initial nitrogen release rate ≤15% after 24 hours of static water extraction, and a cumulative nitrogen release period of 90 to 120 days.

[0010] The method for preparing modified slow-release synergistic nitro compound fertilizer provided by the present invention includes raw material melting and mixing, granulation and molding, coating and drying processes, and stepwise complexation modification process. The specific steps are as follows: (1) Preparation of basic slurry: Nitro base fertilizer is crushed and sent to a melting and mixing kettle, heated to 110-130℃ to form a ternary eutectic melt of ammonium nitrate-diammonium phosphate-potassium nitrate, and added with trace elements and stirred to obtain a basic slurry with uniform fluidity; (2) Stepwise complexation modification: The basic slurry is transferred to a reaction kettle, cooled to 70-90℃ at a rate of 5-8℃ / min and then reacted at a constant temperature. Under constant temperature conditions, the slurry is a supersaturated slurry system containing microcrystals, and the viscosity is controlled at 800-1500 mPa. s; Add the modified composite slow-release carrier in two batches. After the first addition, stir and react. Add the remaining modified composite slow-release carrier and all the composite synergistic inhibitors at the same time. Continue stirring and mixing to obtain the modified slurry; (3) Granulation and molding: Send the modified slurry into the granulator for granulation and sieve to obtain fertilizer granules; (4) Coating treatment: Send the fertilizer granules into the rotary drum coating machine, heat the prepared degradable coating agent aqueous solution and spray it onto the surface of the fertilizer granules, and dry to obtain the finished product.

[0011] As a preferred technical solution of the present invention, in step (2), 60% to 70% of the total mass of the modified composite sustained-release carrier is added for the first time, and the addition is completed at a uniform rate within 8 to 12 minutes. After the addition is completed, the mixture is stirred at a speed of 150 to 180 r / min for 20 to 40 minutes. The remaining modified composite sustained-release carrier and all the composite synergistic inhibitors are added for the second time, and the addition is completed within 3 to 6 minutes. After the addition is completed, the speed is adjusted to 120 to 150 r / min, and stirring is continued for 15 to 30 minutes. An anchor-type stirring paddle is used throughout the reaction, and a slight positive pressure is maintained inside the reactor.

[0012] As a preferred technical solution of the present invention, in step (1), the nitro-based fertilizer is pulverized to 80-100 mesh, and the melting mixing kettle is heated by a steam jacket, with the heating temperature controlled at 110-130℃. During the melting process, samples are taken every 10 minutes to test the fluidity of the slurry. In step (2), a circulating cooling water jacket is used for cooling, with the cooling rate controlled at 5-8℃ / min. After cooling to 70-90℃, the reaction is carried out at a constant temperature. In step (4), the aqueous solution of the biodegradable coating agent is heated by a water bath at a constant temperature, with the temperature controlled at 60-80℃ and the temperature fluctuation ≤ ±2℃.

[0013] As a preferred technical solution of the present invention, step (3) adopts a high-tower granulation or disc granulation process; when using high-tower granulation, the temperature of the slurry at the top of the tower is controlled at 100-110°C, the tower is naturally cooled, and the temperature of the particles discharged from the bottom of the tower is 60-70°C; when using disc granulation, the disc inclination angle is 35°-45°, the disc rotation speed is 15-25 r / min, and the spraying pressure is 0.3-0.5 MPa; in step (4), the rotation speed of the intermediate drum film machine is 10-20 r / min, the spraying pressure is 0.2-0.4 MPa, the residence time of the particles in the drum is 15-25 min, hot air convection drying is adopted, and the inlet air temperature is 70-90°C.

[0014] As a preferred technical solution of the present invention, the mass concentration of the degradable coating agent aqueous solution in step (4) is 8% to 15%. When preparing the solution, polyvinyl alcohol is first added to deionized water at 90 to 95°C and stirred until completely dissolved. Then the temperature is lowered to 60 to 70°C, starch-grafted polyacrylamide is added in proportion, and the mixture is stirred continuously for 20 to 30 minutes until uniform. Finally, 0.1% to 0.3% of Tween 80 is added, and the mixture is stirred evenly and kept warm for later use.

[0015] Step (3) After granulation and screening, the fertilizer granules are first sent to the preheating chamber for hot air convection preheating at a temperature of 50-60℃ for 5-10 minutes, and then sent to the rotary drum coating machine; Step (4) After coating is completed, the granules are sent to the cooling chamber and cooled to ≤40℃ using counter-current cold air at 20-25℃; After cooling, they are screened using a double-layer vibrating screen with an upper screen aperture of 4mm and a lower screen aperture of 2mm. The unqualified granules are returned to Step (1) for melting and reuse; qualified granules are sprayed with 0.05%-0.1% anti-caking oil agent by mass and then metered and packaged.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The slow-release effect is clear and quantifiable. Through the synergistic effect of carrier adsorption and complexation, inhibitor nitrification inhibition, and physical barrier coating, the nitrogen cumulative release period can reach 90-120 days. The initial nitrogen release rate after 24 hours of static water extraction is ≤15%, which is more than 20% higher than the nitrogen utilization rate of conventional nitro compound fertilizers with the same nutrients. It can reduce the leaching loss of nitrate nitrogen and reduce the frequency of topdressing for crops. 2. Strong synergistic effect of modified carrier. The two-step process of nitrate nitration-hydrogen peroxide oxidation is used to modify humic acid, which can increase the number of active functional groups such as carboxyl and phenolic hydroxyl groups and enhance the complexation ability of nutrient molecules. Combined with the physical adsorption effect of mushroom residue biochar, the two work synergistically to improve the nutrient retention effect, while realizing the resource utilization of edible fungi processing waste and reducing raw material costs. 3. The preparation process is stable, easy to scale up, energy-saving, and safe. The stepwise addition of the complexing agent reduces competition for adsorption sites among different nutrient ions, improving reaction uniformity. Utilizing a ternary eutectic system, a low-temperature isothermal reaction is achieved, reducing production energy consumption by more than 30% compared to traditional high-temperature melting processes. It also avoids high-temperature decomposition of inhibitors and improves thermal safety. The entire process can be implemented using existing industrial compound fertilizer production equipment, with controllable process parameters, good batch stability, and the ability to achieve mass production without the need for additional dedicated production lines. 4. The product is environmentally friendly. The coating material is mainly composed of starch-based components and polyvinyl alcohol, which can be gradually biodegraded in the soil without leaving any plastic residues; the total amount of compound synergistic inhibitor added is less than 2 parts, which has little impact on the native soil microbial community and meets the requirements of green agricultural production. Attached Figure Description

[0017] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a flow chart of the preparation process of the modified slow-release and enhanced nitro compound fertilizer described in this invention; In Figure 1: 1 - Raw material crushing unit, 2 - Melting and mixing vessel, 3 - Stepwise complexing reaction vessel, 4 - Granulation unit, 5 - Preheating chamber, 6 - Rotary drum wrapping machine, 7 - Cooling chamber, 8 - Screening unit, 9 - Finished product packaging unit; Figure 2 This is a schematic diagram of the granular cross-sectional structure of the modified slow-release and enhanced nitro compound fertilizer described in this invention; Figure 3 is a bar chart showing the nitrogen release performance results of different samples in this invention; Figure 4 is a comparison chart of soil nitrification inhibition rates according to the present invention; Figure 5 is a comparison chart of the results of the corn pot experiment of the present invention. Detailed Implementation

[0018] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0019] In the attached diagram, all identical reference numerals refer to the same components.

[0020] As shown in Figure 1, the preparation process of this invention is as follows: the material is crushed by the raw material crushing unit 1 and then fed into the melting and mixing kettle 2 to complete the melting and mixing. It is then conveyed to the stepwise complexing reactor 3 for stepwise complexing modification. After modification, the slurry is fed into the granulation unit 4 for molding. The molded particles are preheated by the preheating chamber 5 and then enter the rotary drum coating machine 6 to complete the coating process. After coating, the particles are sent to the cooling chamber 7 for cooling and then classified by the screening unit 8. Qualified products are sent to the finished product packaging unit 9, and unqualified particles are returned to the melting and mixing kettle 2 for reuse.

[0021] As shown in Figure 2, the finished fertilizer granules are spherical in shape, consisting of a fertilizer granule core 11 and a biodegradable coating layer 10 from the inside out. Modified composite slow-release carrier particles 12 and composite synergistic inhibitor particles 13 are uniformly dispersed inside the fertilizer granule core 11. The modified composite slow-release carrier particles 12 form a complex adsorption binding state with nutrient molecules in the core through surface-active functional groups. The composite synergistic inhibitor particles 13 are uniformly distributed in the nutrient matrix. The biodegradable coating layer 10 is a continuous and dense thin film structure that completely covers the outer surface of the fertilizer granule core 11, together forming a three-level slow-release system of "internal complexation and retention - biochemical nitrification inhibition - external physical barrier".

[0022] Example 1 Based on the dry weight of the raw materials, the composition is as follows: 90 parts of nitro-based fertilizer, 6 parts of modified compound slow-release carrier, 0.8 parts of compound synergistic inhibitor, 2.5 parts of biodegradable coating agent, and 0.7 parts of trace elements.

[0023] Nitro-based fertilizer: It is composed of ammonium nitrate, diammonium phosphate, and potassium nitrate in a mass ratio of 4:3:3; wherein the total nitrogen mass fraction of ammonium nitrate is 34.6%, the total nitrogen mass fraction of diammonium phosphate is 18.2%, the mass fraction of available phosphorus pentoxide is 46.3%, the total nitrogen mass fraction of potassium nitrate is 13.8%, and the mass fraction of potassium oxide is 46.5%; after mixing, nitrate nitrogen accounts for 60% of the total nitrogen mass, and the free water mass fraction is 1.5%, which fully matches the raw material ratio and nutrient index of claim 2.

[0024] Modified composite slow-release carrier: Nitrohumic acid modified by two steps of nitrate nitration and hydrogen peroxide oxidation is mixed with mushroom residue biochar at a mass ratio of 3:1, matching the carrier composition characteristics of claim 1.

[0025] The compound synergistic inhibitor is composed of 3,4-dimethylpyrazole phosphate (98.0% purity) and dicyandiamide (99.5% purity) in a mass ratio of 1:3, matching the inhibitor compounding characteristics of claim 1.

[0026] Biodegradable coating agent: starch-grafted polyacrylamide and polyvinyl alcohol are compounded in a mass ratio of 1.5:1, matching the coating agent composition characteristics of claim 1.

[0027] The trace elements are iron, zinc, and manganese in ethylenediaminetetraacetic acid chelate form, and boron in borax form. The mass ratio of iron, zinc, boron, and manganese is 2:2:1:1, which matches the trace element characteristics of claim 4.

[0028] Preparation of modified composite sustained-release carrier: (1) Humic acid nitration treatment: Take weathered coal humic acid (dry basis humic acid content 65.2%), crush it through a 100-mesh sieve, add 20% nitric acid at a solid-liquid mass ratio of 1:6, and stir at 120 r / min for 3 h at room temperature; after the reaction, filter to separate solid and liquid, take the solid phase and wash it repeatedly with deionized water until the pH of the filtrate is 4.5. (2) Hydrogen peroxide oxidation modification: Add 8% hydrogen peroxide at a solid-liquid mass ratio of 1:5 to the washed solid phase, place it in a 60℃ constant temperature water bath, and stir at 100 r / min for 2 h for oxidation modification; after the reaction, filter, transfer the filter residue to a 70℃ forced-air drying oven and dry to constant weight to obtain oxidized modified nitrohumic acid. (3) Preparation of mushroom residue biochar: Take mushroom residue (substrate is cottonseed hulls + sawdust), wash and remove impurities, dry at 80℃, place in a tube furnace, heat to 450℃ at 5℃ / min under nitrogen atmosphere, and pyrolyze under limited oxygen for 2.5h; after natural cooling, take it out, crush it through a 250 mesh sieve, and the BET specific surface area is measured to be 280m² / g, thus obtaining mushroom residue biochar. (4) Carrier composite: Add oxidized modified nitrohumic acid and mushroom residue biochar at a mass ratio of 3:1 into a ribbon mixer, mix for 15min until uniform, and obtain the modified composite slow-release carrier.

[0029] Preparation process flow: The experiment used a pilot production line, with core equipment including a 500L stainless steel reactor, an 18m high-tower granulator, and a φ1.2m×4m rotary drum coating machine. The process flow corresponds to units 1-9 in Figure 1, and the specific steps are as follows: (1) Preparation of basic slurry: Nitro-based fertilizer is fed into raw material crushing unit 1 and crushed to 90 mesh, then fed into melting and mixing kettle 2 via screw conveyor; the material is heated to 120℃ using a steam jacket, forming a ternary eutectic system of ammonium nitrate-diammonium phosphate-potassium nitrate, which is completely melted into a homogeneous liquid; trace elements are added, and the mixture is stirred at 100 r / min for 15 min until homogeneous; during the melting process, samples are taken every 10 min to test the slurry flowability, and the dynamic viscosity of the basic slurry at 120℃ is measured to be 320 mPa using a rotational viscometer. s, possessing excellent flow and transport properties; used to prepare basic slurry.

[0030] (2) Stepwise complexation modification: The base slurry was transferred into the stepwise complexation reactor 3 through an insulated pipeline. The circulating cooling water jacket was turned on to cool down the slurry at a rate of 6℃ / min. After cooling down to 80℃, the slurry was kept at a constant temperature. Under constant temperature conditions, the system was a supersaturated slurry containing nutrient microcrystals. The dynamic viscosity of the system was measured to be 1120 mPa using a rotational viscometer. The anchor-type stirring paddle ensures stable and uniform mixing without any difficulty in stirring or clumping. The modified composite slow-release carrier is added in two stages: ① The first stage involves adding 65% of the total mass of the modified composite slow-release carrier at a uniform rate within 10 minutes, followed by stirring at 160 rpm for 30 minutes to allow the humic acid functional groups to preferentially bind with nitrogen and phosphorus nutrients; ② The second stage involves adding the remaining 35% of the modified composite slow-release carrier and all the composite synergistic inhibitors within 5 minutes, then adjusting the stirring speed to 130 rpm and continuing stirring for 20 minutes to complete the adsorption and immobilization of potassium and the inhibitors, resulting in a modified slurry. An anchor-type stirring paddle is used throughout the reaction, and a slight positive pressure of 0.02 MPa is maintained inside the reactor to prevent skin formation on the slurry surface. Online monitoring of the headspace gas inside the reactor is conducted throughout the reaction; no thermal decomposition products such as nitrogen oxides are detected, indicating good thermal stability and no safety risks. (3) Granulation and molding: The modified slurry is fed into the top spray system of granulation unit 4 (high tower granulator), and the temperature of the slurry at the top of the tower is controlled at 105°C. After being atomized by a φ1.2mm nozzle, the slurry is naturally cooled and falls in the tower. The temperature of the particles discharged from the bottom of the tower is 65°C. After primary screening, fertilizer particles with a particle size of 2-4mm are obtained, which matches the high tower granulation process parameters of claim 8.

[0031] (4) Preheating before coating: The sieved fertilizer granules are sent into the preheating chamber 5 and preheated to 55°C by hot air convection for 8 minutes to make the surface temperature of the granules uniform and avoid uneven film formation caused by the coating liquid coming into contact with cold granules and causing sudden condensation.

[0032] (5) Coating treatment: The preheated particles are fed into the rotary drum coating machine 6; a biodegradable coating agent aqueous solution is prepared in advance: polyvinyl alcohol is first added to deionized water at 92°C and stirred until completely dissolved, then cooled to 65°C, starch-grafted polyacrylamide is added in proportion, and stirring is continued for 25 minutes until the system is uniform. Finally, 0.2% by mass of Tween 80 is added, stirred evenly, and kept warm for later use; the prepared coating agent aqueous solution has a mass concentration of 12%, and is heated to 70°C using a water bath constant temperature, with temperature fluctuation controlled within ±1.5°C. The rotary drum coating machine rotates at 15 r / min, the spray pressure is 0.3 MPa, the particle residence time in the drum is 20 minutes, hot air convection drying is used, the inlet air temperature is 80°C, and the average thickness of the coating layer is controlled to be about 50 μm. This step completely covers the coating solution preparation process of claim 9 and the coating equipment operating parameters of claim 8.

[0033] (6) Post-processing: After coating, the particles are sent to the cooling chamber 7 and cooled to 35°C by counter-current cold air at 22°C. After cooling, they are sent to the screening unit 8 and screened by a double-layer vibrating screen with a 4mm aperture on the upper screen and a 2mm aperture on the lower screen. The unqualified particles are returned to the melting and mixing kettle 2 for reuse. The qualified particles are sprayed with 0.08% mineral oil-based anti-caking agent and then sent to the finished product packaging unit 9 for metering and packaging to obtain the finished product.

[0034] Product performance test results: Three finished product samples were randomly selected for testing, and the average value of the results was taken: 92.7% of the particles were 2-4 mm in diameter; the average compressive strength of the particles was 18.2 N; the average thickness of the surface coating layer was 52 μm; the initial nitrogen release rate after 24 h of static water extraction was 12.3%; the cumulative nitrogen release period was 108 days; and the soil nitrification inhibition rate after 30 days was 68.3%.

[0035] Example 2 This embodiment is a long-lasting product with high carrier and high coating dosage, suitable for crops with a growth period of more than 120 days.

[0036] Raw material formulation: Based on dry weight, the composition is as follows: 85 parts nitro-based fertilizer, 8 parts modified composite slow-release carrier, 1.2 parts composite synergistic inhibitor, 4 parts biodegradable coating agent, and 1.8 parts trace elements. Nitro-based fertilizer: composed of ammonium nitrate, diammonium phosphate, and potassium nitrate in a mass ratio of 5:2:3, with nitrate nitrogen accounting for 65% of the total nitrogen mass and free water content of 1.8%. Modified composite slow-release carrier: composed of oxidized modified nitrohumic acid and mushroom biochar in a mass ratio of 4:1. Composite synergistic inhibitor: composed of 3,4-dimethylpyrazole phosphate and dicyandiamide in a mass ratio of 1:2.5. Biodegradable coating agent: composed of starch-grafted polyacrylamide and polyvinyl alcohol in a mass ratio of 2:1. The composition of trace elements is the same as in Example 1.

[0037] Preparation process: Preparation of modified composite slow-release carrier: Weathered coal humic acid was nitrated with 25% nitric acid for 2.5 h and oxidized with 10% hydrogen peroxide at 65℃ for 1.5 h; shiitake mushroom residue was pyrolyzed at 500℃ under limited oxygen for 2 h, and biochar was pulverized to 300 mesh with a specific surface area of ​​320 m² / g. Stepwise complexation modification: 70% of the modified composite slow-release carrier was added for the first time, and the reaction was allowed to proceed for 35 min; after the second addition, the mixture was stirred for 25 min, and the slurry viscosity was 1280 mPa during the constant temperature stage. The stirring operation was stable. Coating treatment: The aqueous coating agent had a mass concentration of 10%, and the average coating layer thickness was approximately 65 μm. The remaining process steps and parameters were the same as in Example 1.

[0038] Product performance: The average compressive strength of the finished granules is 19.5N, the initial nitrogen release rate after 24 hours of static water extraction is 9.7%, the cumulative nitrogen release period is 117 days, the nitrification inhibition rate after 30 days is 73.5%, and the slow release period is longer than that of Example 1.

[0039] Example 3 This embodiment uses a disc granulation process to verify the adaptability of the present invention to small and medium-sized fertilizer production lines. Except for the granulation process, the other features are the same as those in Embodiment 1.

[0040] Granulation process differences: Granulation unit 4 uses a φ3m disc granulator: the modified slurry is fed into the spraying system, the disc inclination angle is set to 40°, the disc rotation speed is 20 r / min, and the spraying pressure is 0.4 MPa; after the slurry is atomized, it rolls with the returned powder in the disc to form granules. The wet granules are dried and then screened to obtain fertilizer granules with a particle size of 2-4 mm. The preheating, coating, and post-treatment steps are the same as in Example 1.

[0041] Product performance: The average compressive strength of the finished granules is 16.8N, the initial nitrogen release rate after 24 hours of static water extraction is 13.1%, the cumulative nitrogen release period is 105 days, the nitrification inhibition rate after 30 days is 69.1%, and the slow release effect is not significantly different from that of high tower granulation products. It can be directly adapted to existing disc granulation production lines.

[0042] To verify the synergistic effect of the triple sustained-release system of the present invention, three comparative examples were set up. Except for the differences in the table, the preparation conditions were the same as in Example 1: Comparative Example 1: Ordinary nitro compound fertilizer, containing only nitro base fertilizer, without added modified compound slow-release carrier, compound synergist inhibitor and biodegradable coating agent, is mixed in one step and then granulated in a high tower.

[0043] Comparative Example 2: A single-inhibitor type nitro compound fertilizer, with the same amount of dicyandiamide as in Example 1 added as a single nitration inhibitor, without adding a modified carrier or coating agent, and granulated in a high tower after one-step mixing.

[0044] Comparative Example 3: A single-coated nitro compound fertilizer, with the same amount of biodegradable coating agent as in Example 1, without the addition of modified carriers and inhibitors, was mixed in one step and then granulated and coated in a high tower.

[0045] Comparative Example 4: Prepared according to existing nitro compound fertilizer technology, using ammonium nitrate, monoammonium phosphate, and potassium sulfate as base raw materials, adding an equal amount of single nitration inhibitor, adjusting the pH value of the slurry to 5.5, and obtaining nitro compound fertilizer granules by high-temperature melting at 150℃ and high-tower granulation, with a total nutrient content comparable to that of Example 1.

[0046] Comparative Example 5: Prepared according to existing polymeric phosphorus-nitro compound fertilizer technology, using ammonium nitrate, polyammonium phosphate, and potassium nitrate as basic raw materials, with the addition of equal amounts of trace elements, polymeric phosphorus-nitro compound fertilizer was obtained by melting and mixing at 145℃ and high-tower granulation. The total nutrient content was comparable to that of Example 1.

[0047] All tests were performed in triplicate, and the results were averaged.

[0048] Nitrogen release performance test: According to GB / T23348-2009 "Slow-Release Fertilizers" static water extraction method, accurately weigh 10.00g of sample and place it in a 100-mesh nylon mesh bag, then place it in a brown reagent bottle containing 200mL of deionized water and let it stand at a constant temperature of 25℃ in the dark; take samples at the set time, and determine the total nitrogen content of the leachate using the Kjeldahl method, and calculate the cumulative nitrogen release rate and release period. The test results are shown in Table 1.

[0049] Table 1 Comparison of nitrogen release performance of different samples

[0050] Results Analysis: The initial nitrogen release rate of the products in the embodiments of this invention was less than 15% in the first 24 hours, meeting the national standard requirements for slow-release fertilizers; the nitrogen release period could reach 90-120 days, more than twice as long as ordinary nitro compound fertilizers, and 92.9% and 45.9% longer than single inhibitors and single coating products, respectively, verifying the synergistic slow-release effect of the triple system of "carrier adsorption complexation + nitrification inhibition + physical coating". Compared with the closest prior art, the nitrogen release period of Example 1 of this invention was extended by 58.8% compared with Comparative Example 4 and by 42.1% compared with Comparative Example 5; the initial nitrogen release rate in the first 24 hours was reduced by 65.1% and 61.3%, respectively, showing a significant improvement in slow-release performance. Existing technologies only use single inhibitors or single polymerized nutrients for regulation, without forming a triple synergistic slow-release system, and cannot achieve a long-term slow-release effect of more than 90 days.

[0051] Nitrification inhibition performance test: The soil was cultured at a constant temperature using a soil constant temperature method. The test soil was alluvial soil (0-20cm topsoil layer) with the following basic physicochemical properties: pH 7.2, organic matter 12.8 g / kg, total nitrogen 1.12 g / kg, and available nitrogen 87.6 mg / kg. The soil was sieved through a 2 mm sieve, and the moisture content was adjusted to 60% of field capacity. Fertilizer was added at a dosage of 150 mg / kg pure nitrogen, and the mixture was thoroughly mixed before being placed in culture bottles and cultured at a constant temperature of 25℃. After 30 days of culture, the nitrate nitrogen content in the soil was determined by potassium chloride extraction-ultraviolet spectrophotometry, and the nitrification inhibition rate was calculated. The test results are shown in Table 2.

[0052] Table 2 Comparison of soil nitrification inhibition rates of different samples after 30 days

[0053] Results Analysis: The nitrification inhibition rate of the product of this invention can reach over 68% after 30 days, which is 64.2% higher than that of a single dicyandiamide inhibitor. This indicates that the adsorption and immobilization effect of the modified composite slow-release carrier can slow down the diffusion and decomposition of the inhibitor in the soil, prolonging the inhibitor's effective period. The two work synergistically to improve the nitrification inhibition effect. Compared with the prior art, the nitrification inhibition rate of Example 1 of this invention is 46.9% higher than that of Comparative Example 4 and 78.8% higher than that of Comparative Example 5. In the prior art, the inhibitor is directly dispersed in the high-temperature melt, which is prone to thermal decomposition and failure; and without the adsorption and immobilization effect of the carrier, the inhibitor diffuses rapidly in the soil and has a short effective period. This invention reduces the heat loss of the inhibitor through a low-temperature stepwise complexation process, and at the same time relies on the adsorption effect of the modified carrier to delay the release of the inhibitor, significantly improving the long-term effectiveness of nitrification inhibition.

[0054] Potted fertilizer efficiency test of maize: Test method: The maize variety used was Zhengdan 958. Each pot contained 5 kg of air-dried soil, and the nitrogen application rate was calculated as 150 mg / kg pure nitrogen in the soil. Phosphorus and potassium nutrients were uniformly balanced. Three seeds were sown in each pot, and one plant was retained after emergence. The growing period was 60 days, and conventional water management was used. After harvest, the plants were blanched at 105℃ for 30 min and dried at 75℃ to constant weight. The dry weight of the plants and the total nitrogen content were measured, and the apparent nitrogen use efficiency was calculated. The test results are shown in Table 3.

[0055] Table 3 Comparison of results from maize pot experiment

[0056] Results Analysis: The dry weight of corn plants treated with the product of this invention increased by 31.5% compared to ordinary nitro compound fertilizer, and the apparent nitrogen utilization rate increased by 22.7 percentage points. This indicates that the present invention can significantly improve nutrient utilization efficiency, reduce nutrient waste, and has a clear effect of increasing yield and saving fertilizer. Compared with the closest prior art, the dry weight of corn plants in Example 1 of this invention increased by 15.6% compared to Comparative Example 4 and by 16.8% compared to Comparative Example 5; the apparent nitrogen utilization rate increased by 14.6 and 15.2 percentage points respectively, and the effect of increasing yield and saving fertilizer is significantly better than that of the prior art.

[0057] Biodegradability test of coating material: The soil burial method was used to prepare a 2cm×2cm film sample of biodegradable coating material. After weighing, the sample was buried 10cm deep in the test soil, and the soil moisture content was kept at 60% of the field capacity. After being buried at room temperature for 180 days, the sample was taken out, the surface soil was washed off, and the sample was dried at 75℃ to constant weight. The mass degradation rate was calculated.

[0058] Test results: After 180 days of burial, the degradation rate of the biodegradable coating agent of this invention was 72.3%. It can be gradually decomposed under the action of soil microorganisms, with no plastic residue, and is environmentally friendly to the soil.

[0059] Feasibility verification description of cryogenic process Regarding the process feasibility in the isothermal reaction range of 70–90℃, based on the characteristics of the ternary eutectic system and pilot-scale test data, the following explanations are provided: 1. Principle of Melting Point Reduction in the Eutectic System: The basic fertilizer of this invention is a ternary mixed system of ammonium nitrate, diammonium phosphate, and potassium nitrate, not a pure ammonium nitrate system. The melting point of pure ammonium nitrate is approximately 169℃, while the eutectic point can be reduced to 95–105℃ after the ternary mixed salts form a eutectic. Therefore, the system can completely melt into a homogeneous liquid at 110–130℃, and its fluidity meets the requirements for industrial transportation. 2. Low-Temperature Slurry Operability Verification: When cooled to the 70–90℃ range, the system is a supersaturated slurry containing fine nutrient crystals, rather than a solidified state. Pilot-scale production line tests show that the dynamic viscosity of the modified slurry at 80℃ is 800–1500 mPa. The anchor-type stirring paddle enables stable and uniform mixing, and the slurry can be pumped normally to the high-tower granulation nozzle through the insulated pipeline without clogging or clumping, fully meeting the requirements of subsequent granulation processes. 3. Low-temperature process safety verification: The thermal decomposition initiation temperature of ammonium nitrate is higher than 200℃, and the decomposition rate increases significantly with increasing temperature. The maximum reaction temperature of this invention is 130℃, and the isothermal reaction stage is only 70-90℃, far below the thermal decomposition temperature of ammonium nitrate; moreover, the system is in a water-containing slurry state, and the entire reaction process is carried out under micro-positive pressure in a closed loop. No decomposition products such as nitrogen oxides were detected in online monitoring, and there is no risk of thermal explosion. The safety is significantly better than that of traditional high-temperature melting processes.

[0060] In summary, this invention achieves significant technical effects in extending fertilizer effectiveness, improving nutrient utilization, and reducing environmental residues through the synergistic effect of a triple slow-release mechanism. Furthermore, it has strong process adaptability and is easy to industrialize.

[0061] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A modified slow-release and enhanced-efficiency nitro compound fertilizer, comprising nitro base fertilizer, characterized in that, Based on the dry weight of the raw materials, the fertilizer also includes 3-10 parts of modified composite slow-release carrier, 0.2-2 parts of composite synergistic inhibitor, and 1-5 parts of biodegradable coating agent. The modified composite slow-release carrier is a mixture of nitrohumic acid modified by nitrification and hydrogen peroxide oxidation in a two-step process and mushroom biochar in a mass ratio of (2-4):

1. The composite synergistic inhibitor is a mixture of 3,4-dimethylpyrazole phosphate and dicyandiamide in a mass ratio of 1:(2-4). The biodegradable coating agent is a mixture of starch-grafted polyacrylamide and polyvinyl alcohol in a mass ratio of (1-2):

1. The composite synergistic inhibitor is uniformly dispersed inside the core of the fertilizer granules, and the outer surface of the fertilizer granules is coated with a continuous coating layer composed of the biodegradable coating agent. The modified composite slow-release carrier is uniformly dispersed in the core of the fertilizer and complexed with nutrient molecules.

2. The modified slow-release synergistic nitro compound fertilizer according to claim 1, characterized in that, The nitro-based fertilizer is composed of ammonium nitrate, diammonium phosphate, and potassium nitrate in a mass ratio of (3-5):(2-3):(2-3); wherein the total nitrogen mass fraction of ammonium nitrate is ≥34%, the total nitrogen mass fraction of diammonium phosphate is ≥18%, the available phosphorus pentoxide mass fraction is ≥46%, the total nitrogen mass fraction of potassium nitrate is ≥13.5%, and the potassium oxide mass fraction is ≥46%; nitrate nitrogen accounts for 50%-70% of the total nitrogen mass, and the free water mass fraction is ≤2.0%.

3. The modified slow-release synergistic nitro compound fertilizer according to claim 1, characterized in that, The modified composite sustained-release carrier is prepared by the following method: weathered coal humic acid is pulverized and passed through a 100-mesh sieve, and 15%–25% nitric acid is added at a solid-liquid mass ratio of 1:(5–8). The mixture is stirred and nitrated at room temperature for 2–4 hours. After solid-liquid separation, the solid phase is taken, washed until the pH is 4–5, and then 5%–10% hydrogen peroxide is added at a solid-liquid mass ratio of 1:(4–6). The mixture is stirred and oxidized at 50–70°C for 1–3 hours. After the reaction is completed, the mixture is filtered and dried at 60–80°C to constant weight to obtain oxidized modified nitrohumic acid. Shiitake mushroom residue is subjected to oxygen-limited pyrolysis at 400–500°C for 2–3 hours to obtain biochar, which is pulverized to 200–300 mesh, and the specific surface area is controlled to be 200–350 m² / g. The oxidized modified nitrohumic acid and shiitake mushroom residue biochar are mixed evenly in a certain proportion to obtain the modified composite sustained-release carrier.

4. The modified slow-release synergistic nitro compound fertilizer according to claim 1, characterized in that, The fertilizer, based on the dry basis of the raw materials, also includes 0.5 to 3 parts of trace elements; among the trace elements, iron, zinc, and manganese are in the chelated form of ethylenediaminetetraacetic acid, and boron is in the form of borax, with a mass ratio of 2:2:1:1; the fertilizer is in granular form with a particle size of 2 to 4 mm, a particle compressive strength ≥15 N, a surface coating thickness of 30 to 80 μm, an initial nitrogen release rate of ≤15% after 24 hours of static water extraction, and a cumulative nitrogen release period of 90 to 120 days.

5. The preparation method of the modified slow-release synergistic nitro compound fertilizer according to any one of claims 1 to 4, comprising the steps of raw material melting and mixing, granulation and molding, and coating and drying, characterized in that, It also includes a stepwise complexation modification process, specifically including the following steps: (1) Preparation of basic slurry: After crushing the nitro-based fertilizer, it is sent to a melting mixing kettle and heated to 110-130℃ to form a ternary eutectic melt of ammonium nitrate-diammonium phosphate-potassium nitrate. Trace elements are added and stirred to obtain a basic slurry with uniform fluidity; (2) Stepwise complexation modification: The basic slurry is transferred to a stepwise complexation reactor and cooled to 70-90℃ at a rate of 5-8℃ / min. The temperature is then kept constant. Under constant temperature conditions, the slurry is a supersaturated slurry system containing microcrystals, and the viscosity is controlled at 800-1500 mPa. s; Add the modified composite slow-release carrier in two batches. After the first addition, stir and react. Add the remaining modified composite slow-release carrier and all the composite synergistic inhibitors at the same time. Continue stirring and mixing to obtain the modified slurry; (3) Granulation and molding: Send the modified slurry into the granulator for granulation and sieve to obtain fertilizer granules; (4) Coating treatment: Send the fertilizer granules into the rotary drum coating machine, heat the prepared degradable coating agent aqueous solution and spray it onto the surface of the fertilizer granules, and dry to obtain the finished product.

6. The preparation method according to claim 5, characterized in that, In step (2), 60% to 70% of the total mass of the modified composite sustained-release carrier is added for the first time. The addition is completed at a uniform rate within 8 to 12 minutes. After the addition is completed, the mixture is stirred at a speed of 150 to 180 r / min for 20 to 40 minutes. The remaining modified composite sustained-release carrier and all the composite synergistic inhibitors are added for the second time. The addition is completed within 3 to 6 minutes. After the addition is completed, the speed is adjusted to 120 to 150 r / min, and stirring is continued for 15 to 30 minutes. Anchor-type stirring paddles are used throughout the reaction, and a slight positive pressure is maintained inside the reactor.

7. The preparation method according to claim 5, characterized in that, In step (1), the nitro-based fertilizer is pulverized to 80-100 mesh, and the melting and mixing kettle is heated by a steam jacket with the heating temperature controlled at 110-130℃. During the melting process, samples are taken every 10 minutes to test the fluidity of the slurry. In step (2), the circulating cooling water jacket is used for cooling, with the cooling rate controlled at 5-8℃ / min. After cooling to 70-90℃, the reaction is carried out at a constant temperature. In step (4), the aqueous solution of the biodegradable coating agent is heated by a water bath at a constant temperature, with the temperature controlled at 60-80℃ and the temperature fluctuation ≤±2℃.

8. The preparation method according to claim 5, characterized in that, Step (3) Use high tower granulation or disc granulation process; when using high tower granulation, the temperature of the slurry at the top of the tower is controlled at 100-110℃, the tower is naturally cooled, and the temperature of the particles discharged from the bottom of the tower is 60-70℃; when using disc granulation, the disc inclination angle is 35°-45°, the disc rotation speed is 15-25r / min, and the spray pressure is 0.3-0.5MPa; Step (4) The rotation speed of the intermediate drum film machine is 10-20r / min, the spray pressure is 0.2-0.4MPa, the residence time of the particles in the drum is 15-25min, hot air convection drying is used, and the inlet air temperature is 70-90℃.

9. The preparation method according to claim 5, characterized in that, The mass concentration of the degradable coating agent aqueous solution in step (4) is 8% to 15%. When preparing the solution, first add polyvinyl alcohol to deionized water at 90 to 95°C and stir until completely dissolved. Then cool the solution to 60 to 70°C, add starch-grafted polyacrylamide in proportion, and continue stirring for 20 to 30 minutes until uniform. Finally, add 0.1% to 0.3% of Tween 80 by mass, stir evenly, and keep warm for later use.

10. The preparation method according to claim 5, characterized in that, Step (3) After granulation and screening, the fertilizer granules are first sent to the preheating chamber for hot air convection preheating at a temperature of 50-60℃ for 5-10 minutes, and then sent to the rotary drum coating machine; Step (4) After coating is completed, the granules are sent to the cooling chamber and cooled to ≤40℃ using counter-current cold air at 20-25℃; After cooling, they are screened using a double-layer vibrating screen with an upper screen aperture of 4mm and a lower screen aperture of 2mm. The unqualified granules are returned to Step (1) for melting and reuse; qualified granules are sprayed with 0.05%-0.1% anti-caking oil agent by mass and then metered and packaged.

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