Preparation method and application of nitro humic acid water-soluble fertilizer

Abstract

The invention belongs to the field of fertilizer preparation, and provides a preparation method and application of a nitro humic acid water-soluble fertilizer. The preparation method adopts a nitration modified humic acid and metal complex sol core-shell spraying coupling design, and comprises the following steps: firstly, carrying out nitration reaction on sodium humate serving as a raw material and nitric acid, neutralizing and drying to obtain nitro sodium humate powder; then the nitro sodium humate powder and medium trace element salt are complexed in the presence of trisodium citrate to form nitro sodium humate-metal complex sol, and the complex sol is sprayed in a fluidized bed with NPK matrix salt particles as cores to form a continuous shell layer; and finally, mixing the core-shell particle intermediate, NPK matrix salt particles and nitro sodium humate powder, and carrying out guniting granulation at 40-70 DEG C, the coupling contradiction that the existing water-soluble fertilizer is difficult to give consideration to high particle strength and rapid complete water solubility, metal ion complexing stability and high-solid-content sprayable processing, and low-moisture storage and transportation stability and uniform controlled release at the same time is solved, and the water-soluble fertilizer has wide application value in fruit tree and vegetable water-fertilizer integrated drip irrigation.

Description

Technical Field

[0001] This invention relates to the field of fertilizer preparation technology, specifically to a method for preparing and applying a nitrohumic acid water-soluble fertilizer. Background Technology

[0002] As a core component of modern precision agriculture, fertigation technology places stringent demands on the performance of water-soluble fertilizers across multiple dimensions. In drip irrigation fertilization scenarios for high-value crops such as fruit trees and vegetables, water-soluble fertilizers not only need to dissolve quickly and completely to avoid clogging drippers and pipes, but also need to possess slow-release and controlled-release capabilities to match the crop's nutrient absorption rhythm and reduce leaching losses. Simultaneously, the product must exhibit excellent storage and transportation stability, remaining granular and non-hygroscopic under high temperature and humidity conditions, maintaining particle flowability and mechanical strength. Humic acid, as a natural macromolecular organic compound, possesses multiple benefits, including soil improvement, chelation of micronutrients, and enhanced nutrient utilization. Combining it with NPK nutrients to develop functional water-soluble fertilizers has become a development direction in the industry. However, traditional humic acid has limited water solubility and active group density. Nitrification modification can introduce nitro functional groups, significantly improving its water solubility and metal complexing ability. At the same time, core-shell structure design enables stratified nutrient release, achieving both rapid and slow-release effects to meet the nutrient needs of crops throughout their entire growth cycle. How to achieve rapid and complete water solubility while ensuring high compressive strength of granules, how to obtain a sprayable processing window with high solid content and low viscosity while maintaining high complexation stability of metal ions, and how to strike a balance between low moisture and low agglomeration storage and transportation stability and the uniform controlled release requirement brought about by continuous and dense shell are key scientific issues that urgently need to be addressed in the current research and development of functional water-soluble fertilizers.

[0003] Current research and products on nitrohumic acid water-soluble fertilizers suffer from the following shortcomings. Firstly, regarding the physical properties of the particles, humic acid water-soluble fertilizers prepared using conventional granulation processes often exhibit high compressive strength due to the binding effect of organic components. However, this also results in a dense, aggregated structure, requiring a prolonged disintegration and dispersion process during dissolution. Consequently, the water-insoluble content is difficult to control below 0.10%, impacting the smoothness of drip irrigation systems. For example, Chinese patent CN106220258A discloses a humic acid compound fertilizer, but it suffers from slow particle dissolution rates and high water-insoluble content. Secondly, concerning the metal complexation system, the complexation reaction between nitrohumic acid and trace elements is typically carried out at low solid content to control solution viscosity. However, direct spraying of low-solid-content solutions requires significant drying energy consumption and makes it difficult to form a continuous, dense shell, leading to easy dissociation of metal ions during storage and application, resulting in insufficient complexation stability. For example, Chinese patent CN108147880A discloses an organic-inorganic fertilizer containing humic acid chelates produced from plant straw, but it suffers from low solid content and poor spray-applied processability. Furthermore, regarding the synergy between storage and transportation stability and slow-release performance, reducing product moisture can effectively inhibit clumping, but excessive drying can lead to shell cracking and loss of controlled-release function; while maintaining shell integrity requires appropriate moisture and a ripening process, which increases the risk of moisture absorption and clumping, making it difficult to achieve both simultaneously. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing and applying nitrohumic acid water-soluble fertilizer, thereby solving the coupling contradictions that are currently difficult to simultaneously achieve between high compressive strength of granules and rapid and complete water solubility with low insoluble matter, high complexation stability of metal ions and high solid content with low viscosity for sprayable processing, and low moisture and low agglomeration storage and transportation stability and the uniform release and controlled release requirements brought about by continuous and dense shell.

[0005] This invention creatively employs a synergistic design of nitration modification, metal complexation, core-shell spraying, and gradient granulation. Nitro functional groups are introduced into the aromatic ring and side chain of sodium humate through the nitration reaction of nitric acid, significantly improving its water solubility and metal complexation site density. Sodium nitrohumate powder and trisodium citrate are used synergistically as a composite complexing agent to form a stable nanoscale complex sol with trace elements such as calcium, magnesium, and zinc under neutral pH conditions. This ensures high complexation stability of metal ions and achieves a suitable processing window for fluidized bed spraying by controlling the solid content to 15-30%. Using NPK matrix salt particles as the core, this complex sol is repeatedly sprayed in a fluidized bed and dried layer by layer to construct a continuous, dense shell layer with a thickness of 5-30 micrometers, achieving a slow-release and controlled-release function of nutrients. Finally... The core-shell particle intermediate is mixed with NPK matrix salt particles and sodium nitrohumate powder in a specific ratio and then sprayed and granulated under mild conditions of 40-70℃. This forms a bicontinuous structure inside the particles, with the core-shell particles providing skeletal support and the powder filling. This ensures a high compressive strength of more than 12 N per particle, while the high water solubility of sodium nitrohumate powder allows the particles to quickly disintegrate and disperse in water, with water-insoluble matter controlled below 0.10%. At the same time, the slow-release effect of the core-shell particles and the rapid-release effect of the powder achieve a dual release mode of rapid and slow release. During storage and transportation, the moisture content is strictly controlled to below 2.0%, and a curing treatment is performed to achieve moisture balance between the shell and the matrix, with an agglomeration rate of less than 5%. This achieves an excellent synergistic balance among multiple mutually restrictive performance dimensions.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a nitrohumic acid water-soluble fertilizer includes the following steps: S1. Sodium humate is used as a raw material and nitric acid is reacted to undergo a nitration reaction. After the reaction is completed, the sodium nitrohumate aqueous solution is obtained by neutralization with alkali. The sodium nitrohumate aqueous solution of the present invention is dried to obtain sodium nitrohumate powder. S2 After dissolving the sodium nitrohumate powder of the present invention, it is mixed with... One or more metal ions in the sample are complexed with trace element salts in the presence of a complexing agent to obtain sodium nitrohumate-metal complex sol. S3 Using NPK matrix salt particles as the core, the sodium nitrohumate-metal complex sol of the present invention is sprayed onto the surface of the NPK matrix salt particles of the present invention in a fluidized bed and dried to form an NPK-nitrohumate-metal complex core-shell particle intermediate. S4 After mixing the NPK-nitrohumic acid metal complex core-shell particle intermediate of the present invention, the NPK matrix salt particles of the present invention and the sodium nitrohumic acid powder of the present invention, the mixture is sprayed and granulated in a fluidized bed spray granulator or spray granulation tower to obtain nitrohumic acid water-soluble fertilizer particles. The granulation equipment of the spray granulation of the present invention is a fluidized bed spray granulator or a spray granulation tower, and the material temperature during the granulation process is controlled at 40-70 ℃.

[0007] Furthermore, the nitrohumic acid water-soluble fertilizer of the present invention comprises: The NPK matrix salt particles are 10–30 parts of uncoated particles added in step S4; 6-15 parts of sodium nitrohumate powder; 1-8 parts of trace element salts; 30-50 parts of NPK-nitrohumic acid metal complex core-shell particle intermediate; And unavoidable impurities; in: The NPK matrix salt particles of the present invention comprise urea, ammonium dihydrogen phosphate, and potassium sulfate, and further comprise potassium nitrate, so that the total nutrients of the resulting nitrohumic acid water-soluble fertilizer of the present invention are... It is estimated to be 45-52 wt%; The sodium nitrohumate powder of the present invention is obtained by drying a mixture of sodium salts of nitro-substituted humic acid obtained by reacting sodium humate with nitric acid and neutralizing with sodium hydroxide. The sodium nitrohumate powder of the present invention has a water-soluble dry basis ≥90 wt%, a humic acid dry basis ≥60 wt%, and a nitro content of 2.0-4.5 wt%. The trace element salts of this invention are selected from one or more of magnesium sulfate heptahydrate, zinc sulfate heptahydrate, and calcium nitrate tetrahydrate. The NPK-nitrohumic acid metal complex core-shell particle intermediate of the present invention uses the NPK matrix salt particles of the present invention as the core and a metal complex formed by sodium nitrohumate, trace element salts and anhydrous trisodium citrate as the shell. In the shell of the present invention, based on the dry basis mass of the shell, the mass fraction of sodium nitrohumate is 20-60 wt%, and the total mass fraction of trace element metal ions is 2-8 wt%. The shell thickness of the present invention is 5-30 μm, and the particle size of the core-shell particles of the present invention is 1.00-2.50 mm. The nitrohumic acid water-soluble fertilizer of the present invention is a granular product, with particles having a diameter of 1.00-4.75 mm accounting for ≥90% of the total mass, moisture content ≤2.0 wt%, and the water-insoluble content of the present invention is ≤0.10 wt% as determined by NY / T 1973-2010.

[0008] Furthermore, the method for preparing sodium nitrohumate powder in step S1 of the present invention is as follows: A1 Raw material preparation: 100-150 parts by weight of sodium humate; 65-70% by weight of nitric acid, the amount of which is added is based on a mass ratio of nitric acid to sodium humate of 0.20-0.40:1; 5-20 parts by weight of sodium hydroxide; the amount of deionized water added is based on obtaining a sodium humate solution with a solid content of 20-40 wt% after dissolving sodium humate. A2 Nitrification reaction: Sodium humate is added to deionized water and dissolved by mechanical stirring at 40-60 ℃ at a speed of 200-800 rpm for 10-60 min until the system is homogeneous and no visible solid particles are visible, resulting in a sodium humate solution with a solid content of 20-40 wt%. Nitric acid is added dropwise at 40-60 ℃ with continuous stirring for 30-120 min until the pH of the reaction solution reaches 1.5-2.5, and the reaction is carried out at 40-60 ℃ for 1.0-3.0 h. A3 Neutralization and pH Adjustment: After stopping the addition of nitric acid, under conditions of 40-60 ℃ and continuous stirring, a 10-30% sodium hydroxide aqueous solution is added dropwise to neutralize and adjust the pH of the reaction solution to 6.5-8.5. The pH of this invention is based on a 25 ℃ correction value. The addition time is 5-30 min, and the solution is kept at 40-60 ℃ for 0.5-1.0 h to obtain an aqueous solution of sodium nitrohumate. A4 Drying: The sodium nitrohumate aqueous solution obtained in step A3 is spray-dried using a pressure nozzle with an atomization pressure of 0.3-0.8 MPa and a feed rate to maintain an outlet air temperature of 70-90 ℃. The inlet air temperature is 160-200 ℃ and the outlet air temperature is 70-90 ℃, resulting in sodium nitrohumate powder with a water content of ≤10 wt%. A5 Quality Control: The obtained sodium nitrohumate powder has a water-soluble dry basis of ≥90 wt%, a humic acid dry basis of ≥60 wt%, a nitro content of 2.0-4.5 wt%, and a free nitric acid residue of ≤0.20 wt%.

[0009] Furthermore, the preparation method of the sodium nitrohumate-calcium magnesium zinc complex sol in step S2 of the present invention is as follows: B1 Raw material preparation: 100 parts by weight of sodium nitrohumate powder; 0-20 parts by weight of magnesium sulfate heptahydrate; 0-10 parts by weight of zinc sulfate heptahydrate; 0-20 parts by weight of calcium nitrate tetrahydrate; 2-10 parts by weight of anhydrous trisodium citrate; the amount of deionized water added should be such that the solid content of the resulting solution is 15-30 wt%; wherein, at least one of magnesium sulfate heptahydrate, zinc sulfate heptahydrate and calcium nitrate tetrahydrate has a mass fraction greater than 0. B2 Dissolution and Complexation: Sodium nitrohumate powder is added to deionized water and dissolved by mechanical stirring at 40-70 °C at a stirring speed of 200-800 rpm for 10-60 min until the system is homogeneous and there are no visible solid particles, forming a solution with a solid content of 15-30 wt%. Magnesium sulfate heptahydrate, zinc sulfate heptahydrate, and calcium nitrate tetrahydrate are added, followed by anhydrous trisodium citrate. The pH of the solution is adjusted to 6.0-7.5 by dropwise addition of sodium hydroxide aqueous solution with a mass fraction of 10-30% (the pH of this invention is based on the 25 °C correction value). The solution is stirred at 40-70 °C for 1.0-3.0 h to obtain sodium nitrohumate-calcium magnesium zinc complex sol. B3 Quality Control: The total metal content of the obtained sodium nitrohumate-calcium magnesium zinc complex sol is 2-8 wt%. Based on the sol solid content, the free metal ion content is ≤10% of the total metal content, the sol particle size D50 is 5-30 nm, and the pH is 6.5-7.5.

[0010] Furthermore, the preparation method of the NPK-nitrohumic acid metal complex core-shell particle intermediate in step S3 of the present invention is as follows: Preparation of C1 NPK-based salt particles: 20-35 parts by weight of urea, 10-25 parts by weight of ammonium dihydrogen phosphate, 10-25 parts by weight of potassium sulfate, and 0-15 parts by weight of potassium nitrate are mixed and spray-granulated at 60-90 °C to obtain NPK-based salt particles with a particle size of 1.00-2.00 mm. The total nutrients in the NPK-based salt particles of this invention are... Calculated as 45-55 wt%; C2 Core-shell spraying: The NPK matrix salt particles of the present invention are placed in a fluidized bed granulation device. Under the condition of a bed temperature of 60-80 ℃, the sodium nitrohumate-calcium magnesium zinc complex sol of the present invention obtained in step S2 is used as the spraying liquid. The amount of spraying in a single spray is 3-10 wt% of the mass of the NPK matrix salt particles of the present invention. After spraying, hot air drying is performed for 1-5 min. The spraying and drying are repeated 5-15 times to form a continuous shell layer on the surface of the particles. C3 Curing and Drying: After spraying, cure at 50-70 ℃ for 2-4 h, then dry in a fluidized bed at an inlet air temperature of 70-110 ℃ and a bed temperature of 50-70 ℃ for 10-60 min, until the moisture content of the core-shell particles is 2-4 wt%. C4 Quality Control: The shell thickness of this invention is 5-30 μm, the particle size of the core-shell particles of this invention is 1.00-2.50 mm, the mass fraction of sodium nitrohumate in the shell of this invention is 20-60 wt%, and the total mass fraction of trace element metal ions is 2-8 wt%.

[0011] Furthermore, the preparation method of the nitrohumic acid water-soluble fertilizer granules in step S4 of the present invention is as follows: D1. The NPK-nitrohumic acid metal complex core-shell particle intermediate obtained in step S3, the NPK matrix salt particles of the present invention, and the sodium nitrohumate powder of the present invention obtained in step S1 are mixed at a mass ratio of 30-50:10-30:5-10. D2 involves spraying water or adding sodium nitrohumate aqueous solution at 40-70 ℃. The solid content of the sodium nitrohumate aqueous solution of the present invention is 15-30 wt%. The sodium nitrohumate aqueous solution is prepared by dissolving the sodium nitrohumate powder obtained in step S1 in deionized water. The spray volume is 8-12 wt% of the total dry material mass, and granulation is carried out in a fluidized bed or spray granulator. After granulation, D3 is dried in a fluidized bed at an inlet air temperature of 70-110 ℃ and a bed temperature of 40-70 ℃ for 10-60 min until the product moisture content is ≤2.0 wt%. After cooling to ≤40 ℃, it is granulated and sieved to obtain nitrohumic acid water-soluble fertilizer granules with a particle size of 1.00-4.75 mm.

[0012] Furthermore, based on the total mass of the nitrohumic acid water-soluble fertilizer of the present invention, the sum of the oxide equivalents of CaO, MgO and Zn provided by the trace element salts of the present invention is 2-4 wt%, and the molar ratio of Ca:Mg:Zn in terms of metal moles is 0.5-2.0:0.5-2.0:0.1-1.0.

[0013] Furthermore, the nitrohumic acid water-soluble fertilizer of the present invention has an agglomeration rate of ≤5 wt% after being sealed and stored for 3 months under conditions of 40 ℃ and 75% relative humidity, and the granule compressive strength is ≥12 N per granule.

[0014] As a concept of this invention, the coupling design of nitrated modified humic acid and metal complex sol core-shell spraying is mainly used to enhance the rapid and complete dissolution performance, metal ion complexation stability, and particle storage and transportation stability of water-soluble fertilizers. The nitration reaction of sodium humate with nitric acid introduces nitro functional groups at the ortho- and para-positions of the aromatic ring and the aliphatic side chain through an electrophilic substitution mechanism. The strong electron-withdrawing effect of the nitro group enhances the polarity of the hydroxyl and carboxyl groups on the aromatic ring, significantly improving its water solubility. This comprehensively improves the dispersion / dissolution performance in the aqueous phase, making the water solubility of sodium nitrohumate powder reach more than 90% on a dry basis, far exceeding the 60-70% of unmodified sodium humate, providing a material basis for the rapid disintegration of particles in water. Precise control of nitration reaction conditions is crucial. Maintaining an acidic environment with a pH of 1.5-2.5 is conducive to the introduction of nitrogen-containing functional groups and the reaction. Temperature control at 40-60℃ ensures the reaction rate while avoiding excessive oxidation and degradation. A mass ratio of nitric acid to sodium humate of 0.20-0.40:1 ensures proper nitration. The nitro content should be controlled within the range of 2.0-4.5%. If it is too low, the improvement in water solubility will not be significant, while if it is too high, it will destroy the macromolecular skeleton structure of humic acid and lead to a decrease in complexing ability. The complex complex system of sodium nitrohumate and trisodium citrate plays a synergistic role. Nitrohumic acid provides multiple coordinating atoms such as carboxyl groups, phenolic hydroxyl groups, and nitro oxygen to form a multidentate chelate, while trisodium citrate provides three carboxyl groups as bridging ligands. Together, they stabilize metal ions such as calcium, magnesium, and zinc. Under neutral conditions of pH 6.0-7.5, the complex stability constant is improved, and the content of free metal ions is reduced to less than 10% of the total metal content. At the same time, by controlling the solid content of the solution to 15-30%, the viscosity of the sol is maintained within the range suitable for fluidized bed spraying, and the particle size D50 of the sol is controlled at the level of 5-30 nanometers. This ensures the dispersion stability of the complex and allows it to spread evenly on the surface of NPK matrix salt particles to form a continuous and dense shell.

[0015] This invention also discloses the use of a nitrohumic acid water-soluble fertilizer in a fertigation drip irrigation or sprinkler irrigation system.

[0016] Furthermore, when the nitrohumic acid water-soluble fertilizer of the present invention is used for drip irrigation fertilization of the roots of fruit trees, vegetables or cash crops, the amount applied each time according to the fertilization cycle is 5-30 kg per mu, and it is mixed with irrigation water at a mass ratio of 1-3:100-300 before application.

[0017] Furthermore, in step S2 of the present invention, controlling the pH of the solution to 6.0-7.5 includes adjusting it by adding an alkaline or acidic regulator. The alkaline regulator of the present invention is a sodium hydroxide aqueous solution with a mass fraction of 10-30%, and the acidic regulator of the present invention is a hydrochloric acid solution with a mass fraction of 1.0-6.0 mol / L. The pH of the present invention is a value corrected for 25 °C.

[0018] Furthermore, in step S1 of the present invention, adjusting the pH of the reaction solution to 6.5-8.5 includes adjusting it by adding a sodium hydroxide aqueous solution with a mass fraction of 10-30%. The pH of the present invention is a 25 °C correction value.

[0019] Furthermore, in step S3 of the present invention, the core-shell coating is carried out by top spraying, with the bed temperature at 60-80 ℃ and the inlet air temperature at 80-120 ℃. The nozzle diameter is 0.8-1.5 mm, the atomization pressure is 0.1-0.3 MPa, and the spraying rate is 0.5-5.0 kg / h. The amount of coating per spray in the present invention is 3-10 wt% of the mass of the NPK matrix salt particles of the present invention, based on the mass of the coating liquid.

[0020] Furthermore, in step S4 of the present invention, the spray granulation is carried out in a fluidized bed spray granulator or a spray granulation tower, with a nozzle diameter of 0.8-1.5 mm, an atomization pressure of 0.1-0.3 MPa, a spray rate of 0.5-5.0 kg / h, and an inlet air temperature of 70-120 ℃.

[0021] Furthermore, the moisture content of the nitrohumic acid water-soluble fertilizer of the present invention is determined according to GB / T 8576.

[0022] Furthermore, the particle size of the nitrohumic acid water-soluble fertilizer of the present invention is determined according to NY / T 3036.

[0023] Furthermore, the water-insoluble content of the nitrohumic acid water-soluble fertilizer of the present invention was determined according to NY / T 1973-2010.

[0024] Furthermore, the sol particle size D50 of the sodium nitrohumate-calcium magnesium zinc complex sol of the present invention was determined by dynamic light scattering. The sol of the present invention was diluted with deionized water to a solid content of 0.05-0.20 wt% and then the D50 was calculated based on the volume distribution.

[0025] Furthermore, the agglomeration rate of the nitrohumic acid water-soluble fertilizer of the present invention after being sealed and stored for 3 months at 40 ℃ and 75% relative humidity was determined as follows: 1.0 kg of sample was taken and allowed to stand without external force and then directly sieved through a 4.75 mm sieve. The percentage of the mass of the material on the sieve to the total mass of the sample was the agglomeration rate.

[0026] Furthermore, the compressive strength of the particles in this invention was determined using a particle compressive strength tester with a loading rate of 1 mm / s, and 20 particles were randomly sampled and the average value was taken.

[0027] Furthermore, in step S4 of the present invention, the mixing is carried out using a drum mixer with a mixing speed of 10-30 rpm and a mixing time of 3-15 min.

[0028] Furthermore, in step S4 of the present invention, the sieving is performed through a 4.75 mm sieve and a 1.00 mm sieve, with the undersize material being sieved through the 1.00 mm sieve to remove fine powder.

[0029] As another concept of this invention, the core-shell spraying and gradient granulation coupling design is mainly used to enhance the controlled-release performance, particle mechanical strength, and anti-caking storage and transportation stability of water-soluble fertilizers. Using NPK matrix salt particles as the core, sodium nitrohumate-calcium magnesium zinc complex sol is sprayed multiple times in a fluidized bed and dried layer by layer. The amount of each spray is controlled to be 3-10% of the particle mass. Immediately after spraying, hot air drying is performed for 1-5 minutes to allow the sol to quickly solidify on the particle surface to form a thin layer. This spraying and drying process is repeated 5-15 times, accumulating layer by layer to build a continuous dense shell layer with a thickness of 5-30 micrometers. In this shell layer, sodium nitrohumate and metal complexes form a three-dimensional network structure, cross-linked through intermolecular hydrogen bonds, electrostatic interactions, and coordination bonds. This provides both a certain mechanical strength and maintains a microporous structure that allows for the slow diffusion of moisture and nutrients, achieving the controlled-release function of the NPK nutrients in the core. After the core-shell particle intermediate is sprayed, it is cured at 50-70℃ for 2-4 hours to further densify the shell layer and strengthen its bonding with the core interface. It is then dried to a moisture content of 2-4%, at which point the core-shell particles possess a complete controlled-release structure. The core-shell particle intermediate is mixed with NPK matrix salt particles and sodium nitrohumate powder at a mass ratio of 30-50:10-30:5-10 and then sprayed into granules. At a mild granulation temperature of 40-70℃, the core-shell particles act as the skeletal structure, providing mechanical strength support; the NPK matrix salt particles act as a readily available nutrient source; and the sodium nitrohumate powder acts as a highly water-soluble binder and functional additive. The sprayed water or sodium nitrohumate aqueous solution partially dissolves the powder, forming a binding layer that binds the three components into larger particles. After drying, a bicontinuous structure of core-shell particle skeleton and powder matrix is ​​formed inside the particles, achieving a compressive strength of 12 N / m². When the particles are in contact with water, the highly water-soluble sodium nitrohumate powder and NPK matrix salt particles dissolve rapidly, causing the particles to quickly disintegrate and disperse. The shell layer of the core-shell particles gradually releases the core nutrients, achieving a dual release mode of rapid and slow effect. The water-insoluble matter is only the undissolved shell fragments, with a content of less than 0.10%, which does not clog the drip irrigation system. By strictly controlling the moisture content of the final product to ≤2.0% and through a maturation process, the shell layer of the core-shell particles and the powder matrix reach a moisture balance, avoiding hygroscopic agglomeration caused by local moisture gradients. After storage at 40℃ and 75% relative humidity for 3 months, the agglomeration rate is less than 5%, while the shell layer remains intact and does not crack, and the controlled release function is stable.

[0030] The synergistic mechanism of sodium nitrohumate and anhydrous trisodium citrate in this invention is analyzed as follows. Sodium nitrohumate and anhydrous trisodium citrate each have a clear focus in the complex system. Sodium nitrohumate, as a large organic ligand, mainly contributes multiple coordinating atoms, such as phenolic hydroxyl groups, carboxyl groups, and side-chain carboxyl groups, adjacent to the nitro group on the aromatic ring, forming a multidentate chelate structure to encapsulate metal ions. Its large molecular weight and strong structural rigidity provide long-term complex stability. Simultaneously, the modified nitro functional groups enhance water solubility, ensuring the complex remains dispersed and stable in the aqueous phase. Anhydrous trisodium citrate, as a small organic ligand, mainly contributes three α-hydroxycarboxylic acid groups. Its strong coordinating ability and fast reaction rate allow it to rapidly form coordinate bonds with metal ions in the early stages of complexation, reducing the concentration of free metal ions. Simultaneously, the three carboxyl groups of the citrate group can act as bridging ligands, connecting multiple metal ions or forming mixed coordination structures with the coordination sites of nitrohumate, enhancing the spatial stability of the complex. In terms of improving the stability of metal ion complexation, sodium nitrohumate forms macrocyclic chelates with metal ions such as calcium, magnesium, and zinc through the carboxyl and phenolic hydroxyl groups of multiple aromatic ring units. It has a high coordination number and a large stability constant of the chelate ring, which effectively reduces the dissociation tendency of metal ions. Sodium citrate forms five- or six-membered chelate rings through α-hydroxyl and carboxyl groups. The coordination bond strength is high, especially the chelation ability of alkaline earth metal ions such as calcium and magnesium. The synergistic effect of the two reduces the content of free metal ions to less than 10% of the total metal content, which is far better than the 20-30% of the single ligand system. In terms of improving the dispersion stability and processability of sol, the macromolecular structure of sodium nitrohumate forms a steric hindrance layer on the surface of the metal complex, preventing the aggregation of colloidal particles. At the same time, the ionization of nitro and carboxyl groups provides electrostatic repulsion, keeping the sol particle size D50 stable at the 5-30 nanometer level. Trisodium citrate ionizes in the solution to generate a large number of citrate anions, enhancing the ionic strength and buffering capacity of the solution, stabilizing the pH in the range of 6.5-7.5, avoiding the hydrolysis and precipitation of metal ions. Meanwhile, the small molecular characteristics of citrate reduce the viscosity of the solution, allowing sols with a solid content of 15-30% to still have suitable fluidity for fluidized bed spraying, thus solving the contradiction between high solid content and low viscosity. The core mechanism of the synergistic effect lies in the complementary advantages of sodium nitrohumate and trisodium citrate in terms of complexation sites, coordination rates, and spatial structures. Nitrohumate provides a long-lasting and stable multidentate chelating framework, while trisodium citrate provides rapid-response bridging coordination and viscosity regulation. Together, they construct a metal complex sol system with high complexation stability, high dispersion stability, and high solid content processability, which is a synergistic effect that cannot be achieved by a single ligand.

[0031] Beneficial technical effects 1. Rapid and complete dissolution and extremely low water-insoluble matter: By modifying humic acid with nitric acid nitration to introduce nitro functional groups, the water solubility is increased to over 90% on a dry basis. During the gradient granulation process, sodium nitrohumate powder acts as a highly water-soluble binder and functional filler, dissolving rapidly upon contact with water, resulting in rapid particle disintegration and dispersion. Combined with the high solubility of NPK matrix salt particles, the finished particles exhibit rapid dissolution characteristics under specified conditions. The water-insoluble matter content is controlled below 0.10%, far lower than the national standard requirement of 0.5% and the 0.3-0.5% level of conventional humic acid water-soluble fertilizers. This effectively avoids clogging of drip irrigation systems and ensures the long-term stable operation of integrated water and fertilizer facilities.

[0032] 2. High complexation stability and suitable processing window: A composite complexation system of sodium nitrohumate and trisodium citrate is used to form a stable multidentate chelate structure for trace elements such as calcium, magnesium, and zinc under neutral pH conditions of 6.0-7.5. The content of free metal ions is reduced to less than 10% of the total metal content, and the complexation stability constant is improved. At the same time, by precisely controlling the sol solid content of 15-30%, the viscosity is maintained within the suitable range for fluidized bed spraying. The sol particle size D50 is 5-30 nanometers, which is uniformly spread on the particle surface to form a continuous and dense shell. This solves the contradiction between traditional low solid content, high dilution complexation and high energy consumption drying, and achieves a synergistic balance between high metal ion complexation stability and high solid content, low viscosity, and a suitable sprayable processing window.

[0033] 3. Dual release of fast-acting and slow-release nutrients and efficient nutrient utilization: Through core-shell structure design and gradient granulation process, the granules simultaneously contain fast-acting nutrient sources (NPK matrix salt granules and sodium nitrohumate powder) and slow-release nutrient sources (NPK coated in the shell of the core-shell granule intermediate). After being applied to the soil, the fast-acting components dissolve rapidly to meet the nutrient requirements of crops in the early stages of growth. The controlled-release components in the shell layer release nutrients slowly through the diffusion-blocking effect of the dense 5-30 micrometer shell layer. The release rate can be controlled by the shell layer thickness and the number of sprays, thereby achieving a relatively delayed nutrient release and reducing the risk of leaching loss, which helps to improve nutrient utilization efficiency. At the same time, the chelating effect of nitrohumic acid promotes the absorption of micronutrients, improves the rhizosphere soil microenvironment, and comprehensively improves crop yield and quality.

[0034] 4. High mechanical strength and excellent anti-caking and storage stability: During the gradient granulation process, the core-shell intermediate particles, as high-strength skeleton particles, are evenly distributed in the powder matrix to form a bicontinuous structure, giving the particles a compressive strength of more than 12 Newtons per particle, which is higher than the 6-8 Newtons per particle of conventional spray-granulated particles. They are resistant to transportation impact and are not easily broken. At the same time, by strictly controlling the finished product moisture content to ≤2.0% and performing a curing treatment at 50-70℃, the core-shell particle shell layer and the powder matrix reach a moisture balance, avoiding hygroscopic agglomeration caused by local moisture gradients. Under extreme high temperature and high humidity conditions of 40℃ and 75%, the agglomeration rate is ≤5% after 3 months of storage, which is far lower than the 15-25% of conventional products. This ensures that the product maintains good flowability and dispersibility during transportation, storage and application, meeting the requirements of mechanized precision fertilization.

[0035] 5. Environmental friendliness and mitigation of salt damage risk: The humic structure of nitrohumic acid has cation exchange capacity and buffering capacity. During drip irrigation fertilization, it adsorbs excess sodium, potassium and other basic ions in the soil, which helps reduce the risk of increased soil solution conductivity caused by salt accumulation and alleviate salt damage stress. At the same time, the trace elements in the complexed state exist in chelated form, which has high bioavailability and is not easy to form insoluble precipitates with soil phosphate and other elements, reducing the concentration of free heavy metal ions and reducing the risk of toxicity to soil microorganisms and crop roots. Combined with the controlled release effect, it reduces nutrient loss and pollution of groundwater and surface water, which meets the requirements of green agriculture and sustainable development. Attached Figure Description

[0036] Figure 1 The Fourier transform infrared spectrum of the nitro functional group is shown.

[0037] Figure 2 This is the fine 1s X-ray photoelectron spectroscopy (XPS) spectrum of nitro nitrogen in Example 3.

[0038] Figure 3 The image shows the fine 1s X-ray photoelectron spectroscopy (XPS) spectrum of nitro nitrogen in Example 1.

[0039] Figure 4 This is the fine 1s X-ray photoelectron spectroscopy (XPS) spectrum of nitro nitrogen in Example 4.

[0040] Figure 5 The bar chart shows the nitro content of Examples 1 to 4.

[0041] Figure 6 The images show the dynamic light scattering patterns of the particle size distribution in Examples 1 to 4. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0043] Example 1 This embodiment prepares a nitrohumic acid water-soluble fertilizer using moderate parameter configuration, with each process parameter selected within the middle range of the allowable range to ensure process stability and product quality reliability.

[0044] Step S1: Preparation of sodium nitrohumate powder: A1 Raw material preparation: Weigh 125 parts by weight of sodium humate, prepare 37.5 parts by weight of nitric acid with a mass fraction of 67.5% (mass ratio of nitric acid to sodium humate is 0.30:1), 12.5 parts by weight of sodium hydroxide, and 291.7 parts by weight of deionized water.

[0045] A2 Nitrification Reaction: 125 parts by mass of sodium humate were added to 291.7 parts by mass of deionized water and stirred mechanically at 50°C for 35 min until the system was homogeneous and no visible solid particles were observed, yielding a sodium humate solution with a solid content of 30 wt%. 37.5 parts by mass of 67.5% nitric acid were then added dropwise at 50°C with continuous stirring over 75 min until the pH of the reaction solution reached 2.0. The reaction was then carried out at 50°C for 2.0 h.

[0046] A3 Neutralization and pH Adjustment: After stopping the addition of nitric acid, the solution was neutralized by adding a 20% sodium hydroxide aqueous solution at 50°C with continuous stirring. The pH of the reaction solution was adjusted to 7.5 by adding the solution dropwise over a period of 17.5 min at 25°C. The solution was then kept at 50°C for 0.75 h to obtain an aqueous solution of sodium nitrohumate.

[0047] A4 Drying: The above sodium nitrohumate aqueous solution is spray-dried using a pressure nozzle atomization at a pressure of 0.55 MPa. The feed rate is maintained at an outlet air temperature of 80°C. The inlet air temperature is 180°C and the outlet air temperature is 80°C, resulting in sodium nitrohumate powder with a water content of 5 wt%.

[0048] A5 Quality Control: The water-soluble dry basis of the obtained sodium nitrohumate powder is 93 wt%, the humic acid dry basis is 65 wt%, the nitro content is 3.25 wt%, and the free nitric acid residue is 0.10 wt%.

[0049] Step S2: Preparation of sodium nitrohumate-calcium magnesium zinc complex sol: B1 Raw material preparation: Weigh 100 parts by weight of sodium nitrohumate powder, 14.9 parts by weight of magnesium sulfate heptahydrate, 8.3 parts by weight of zinc sulfate heptahydrate, 14.4 parts by weight of calcium nitrate tetrahydrate, 6 parts by weight of anhydrous trisodium citrate, and deionized water to make the solid content of the resulting solution 22.5 wt%.

[0050] B2 Dissolution and Complexation: 100 parts by weight of sodium nitrohumate powder were added to deionized water and stirred mechanically at 55°C for 500 rpm for 35 min until the system was homogeneous and free of visible solid particles, forming a solution with a solid content of 22.5 wt%. Magnesium sulfate heptahydrate, zinc sulfate heptahydrate, and calcium nitrate tetrahydrate were added, followed by anhydrous trisodium citrate. The pH of the solution was adjusted to 6.75 by dropwise addition of 20% sodium hydroxide aqueous solution (pH corrected for 25°C), and stirred at 55°C for 2.0 h to obtain sodium nitrohumate-calcium magnesium zinc complex sol.

[0051] B3 Quality Control: The total metal content of the obtained sodium nitrohumate-calcium magnesium zinc complex sol is 5 wt%. The total metal content is calculated with the dry basis mass of the solid components in the sol as the denominator and the sum of the masses of Ca, Mg and Zn elements as the numerator. The free metal ion content is 5% of the total metal content. The sol particle size D50 is 17.5 nm and the pH is 7.0.

[0052] Step S3: Preparation of NPK-nitrohumic acid metal complex core-shell particle intermediates: Preparation of C1 NPK-based salt particles: 27.5 parts by weight of urea, 17.5 parts by weight of ammonium dihydrogen phosphate, 17.5 parts by weight of potassium sulfate, and 7.5 parts by weight of potassium nitrate were mixed and spray-granulated at 75°C to obtain NPK-based salt particles with a particle size of 1.50 mm. The total nutrients of the NPK-based salt particles were... It is calculated to be 50 wt%.

[0053] C2 Core-shell spraying: The NPK matrix salt particles are placed in a fluidized bed granulation device and sprayed from the top. Under the conditions of bed temperature of 70℃ and air inlet temperature of 100℃, the sodium nitrohumate-calcium magnesium zinc complex sol obtained in step S2 is used as the spraying liquid. The nozzle diameter is 1.15 mm, the atomization pressure is 0.2 MPa, the spraying rate is 2.75 kg / h, and the single spraying amount is 6.5 wt% of the mass of the NPK matrix salt particles. After spraying, hot air drying is performed for 3 min. The spraying and drying are repeated 10 times to form a continuous shell layer on the particle surface.

[0054] C3 Curing and Drying: After spraying, cure at 60℃ for 3 h, and then dry in a fluidized bed at an air inlet temperature of 90℃ and a bed temperature of 60℃ for 35 min until the moisture content of the core-shell particles is 3 wt%.

[0055] C4 Quality Control: The shell thickness is 25 μm, the particle size of the core-shell particles is 1.55 mm, the mass fraction of sodium nitrohumate in the shell is 50 wt%, and the total mass fraction of trace element metal ions is 3 wt%.

[0056] Step S4: Preparation of nitrohumic acid water-soluble fertilizer granules: D1 Mixing: The NPK-nitrohumic acid metal complex core-shell particle intermediate obtained in step S3, the NPK matrix salt particles, and the sodium nitrohumate powder obtained in step S1 are mixed at a mass ratio of 40:15:7.5 using a drum mixer at a mixing speed of 20 rpm for 9 min.

[0057] D2 Spray granulation: A sodium nitrohumate aqueous solution is sprayed at 55°C. The sodium nitrohumate aqueous solution has a solid content of 22.5 wt% and the spray volume is 10 wt% of the total dry material mass. Granulation is carried out in a fluidized bed spray granulator with a nozzle diameter of 1.15 mm, an atomization pressure of 0.2 MPa, a spray rate of 2.75 kg / h, an inlet air temperature of 95°C, and a material temperature controlled at 55°C.

[0058] D3 Drying and Sieving: After granulation, the product is dried in a fluidized bed at an inlet air temperature of 90℃ and a bed temperature of 55℃ for 35 min until the product moisture content is 1.5 wt%. After cooling to 35℃, the product is granulated and sieved through a 4.75 mm sieve and a 1.00 mm sieve. The undersize material is then sieved through a 1.00 mm sieve to remove fine powder, resulting in nitrohumic acid water-soluble fertilizer granules with a particle size of 1.00-4.75 mm.

[0059] Product quality indicators: The nitrohumic acid water-soluble fertilizer prepared in this embodiment consists of: 50.0 parts of NPK matrix salt particles, 10.0 parts of sodium nitrohumate powder, 1.0 part of trace element salts, and 40 parts of NPK-nitrohumic acid metal complex core-shell particle intermediates. Based on the total mass of the nitrohumic acid water-soluble fertilizer, the sum of the oxide equivalents of CaO, MgO, and Zn provided by the trace element salts is 3 wt%, and the molar ratio of Ca:Mg:Zn in terms of metal moles is 1.0:1.0:0.5. Particles with a diameter of 1.00-4.75 mm account for 93% of the total mass, the moisture content is 1.5 wt%, and the water-insoluble matter content is 0.08 wt% as determined by NY / T 1973-2010. After sealed storage for 3 months at 40℃ and 75% relative humidity, the agglomeration rate is 3 wt%, and the particle compressive strength is 15 N per particle.

[0060] Features of Example 1: This example uses moderate process parameters: nitration reaction temperature 50℃, pH 2.0; complexation reaction temperature 55℃, pH 6.75; core-shell spraying bed temperature 70℃; 10 sprays; shell thickness 25 μm; final granulation temperature 55℃. All parameters are selected within the middle range of allowable parameters, ensuring process stability and reproducibility. The product contains a moderate sodium nitrohumate content (10.0 parts), a balanced ratio of micronutrients (Ca:Mg:Zn=1.0:1.0:0.5), a total nutrient content of 50 wt%, excellent water solubility (water-insoluble matter 0.08%), and good storage stability (caking rate 3%). This example is suitable for conventional fertilization of field crops and cash crops, especially suitable for use in integrated water and fertilizer drip irrigation systems. The application rate is 10-20 kg per acre, providing balanced macronutrients and micronutrients to promote healthy crop growth.

[0061] Example 2 This embodiment prepares a nitrohumic acid water-soluble fertilizer, which adopts a formula design with a relatively high sodium nitrohumate content to enhance the soil improvement and efficiency enhancement functions of humic acid.

[0062] Step S1: Preparation of sodium nitrohumate powder: A1 Raw material preparation: Weigh 140 parts by weight of sodium humate, prepare 49 parts by weight of nitric acid with a mass fraction of 68% (mass ratio of nitric acid to sodium humate is 0.35:1), 16 parts by weight of sodium hydroxide, and deionized water to dissolve the sodium humate to obtain a sodium humate solution with a solid content of 35 wt%.

[0063] A2 Nitrification: 140 parts by mass of sodium humate were added to deionized water and dissolved by mechanical stirring at 55°C and 600 rpm for 45 min until the system was homogeneous and no visible solid particles were observed, yielding a sodium humate solution with a solid content of 35 wt%. 49 parts by mass of 68% nitric acid were added dropwise at 55°C with continuous stirring over 95 min until the pH of the reaction solution reached 2.2, and the reaction was carried out at 55°C for 2.5 h.

[0064] A3 Neutralization and pH Adjustment: After stopping the addition of nitric acid, the solution was neutralized by adding a 25% sodium hydroxide aqueous solution at 55°C with continuous stirring. The pH of the reaction solution was adjusted to 8.0, with the pH value corrected at 25°C. The addition time was 22 min, and the solution was kept at 55°C for 0.8 h to obtain an aqueous solution of sodium nitrohumate.

[0065] A4 Drying: The above sodium nitrohumate aqueous solution is spray-dried using a pressure nozzle atomization at a pressure of 0.65 MPa. The feed rate is maintained at an outlet air temperature of 85°C. The inlet air temperature is 190°C and the outlet air temperature is 85°C, resulting in sodium nitrohumate powder with a water content of 7 wt%.

[0066] A5 Quality Control: The water-soluble dry basis of the obtained sodium nitrohumate powder is 95 wt%, the humic acid dry basis is 70 wt%, the nitro content is 3.8 wt%, and the free nitric acid residue is 0.12 wt%.

[0067] Step S2: Preparation of sodium nitrohumate-calcium magnesium zinc complex sol: B1 Raw material preparation: Weigh 100 parts by weight of sodium nitrohumate powder, 12 parts by weight of magnesium sulfate heptahydrate, 6 parts by weight of zinc sulfate heptahydrate, 16 parts by weight of calcium nitrate tetrahydrate, 7 parts by weight of anhydrous trisodium citrate, and deionized water to make the solid content of the resulting solution 25 wt%.

[0068] B2 Dissolution and Complexation: 100 parts by weight of sodium nitrohumate powder were added to deionized water and stirred mechanically at 60°C for 45 min at 600 rpm until the system was homogeneous and free of visible solid particles, forming a solution with a solid content of 25 wt%. Magnesium sulfate heptahydrate, zinc sulfate heptahydrate, and calcium nitrate tetrahydrate were added, followed by anhydrous trisodium citrate. The pH of the solution was adjusted to 7.0 by dropwise addition of 25% sodium hydroxide aqueous solution (pH corrected for 25°C), and the mixture was stirred at 60°C for 2.5 h to obtain sodium nitrohumate-calcium magnesium zinc complex sol.

[0069] B3 Quality Control: The total metal content of the obtained sodium nitrohumate-calcium magnesium zinc complex sol is 6 wt%. The total metal content is calculated with the dry basis mass of the solid components in the sol as the denominator and the sum of the masses of Ca, Mg and Zn elements as the numerator. The free metal ion content is 6% of the total metal content. The sol particle size D50 is 22 nm and the pH is 7.2.

[0070] Step S3: Preparation of NPK-nitrohumic acid metal complex core-shell particle intermediates: Preparation of C1 NPK-based salt particles: 30 parts by weight of urea, 20 parts by weight of ammonium dihydrogen phosphate, 20 parts by weight of potassium sulfate, and 10 parts by weight of potassium nitrate were mixed and spray-granulated at 80°C to obtain NPK-based salt particles with a particle size of 1.65 mm. The total nutrients of the NPK-based salt particles were... It is calculated to be 52 wt%.

[0071] C2 Core-shell spraying: The NPK matrix salt particles are placed in a fluidized bed granulation device and sprayed from the top. Under the conditions of bed temperature of 75℃ and air inlet temperature of 110℃, the sodium nitrohumate-calcium magnesium zinc complex sol obtained in step S2 is used as the spraying liquid. The nozzle diameter is 1.3 mm, the atomization pressure is 0.25 MPa, the spraying rate is 3.5 kg / h, and the single spraying amount is 8 wt% of the mass of the NPK matrix salt particles. After spraying, hot air drying is performed for 4 min. The spraying and drying are repeated 12 times to form a continuous shell layer on the particle surface.

[0072] C3 Curing and Drying: After spraying, cure at 65℃ for 3.5 h, and then dry in a fluidized bed at an air inlet temperature of 100℃ and a bed temperature of 65℃ for 45 min until the moisture content of the core-shell particles is 3.5 wt%.

[0073] C4 Quality Control: The shell thickness is 20 μm, the particle size of the core-shell particles is 2.05 mm, the mass fraction of sodium nitrohumate in the shell is 55 wt%, and the total mass fraction of trace element metal ions is 4 wt%.

[0074] Step S4: Preparation of nitrohumic acid water-soluble fertilizer granules: D1 Mixing: The NPK-nitrohumic acid metal complex core-shell particle intermediate obtained in step S3, the NPK matrix salt particles, and the sodium nitrohumate powder obtained in step S1 are mixed at a mass ratio of 35:12:8 using a drum mixer at a mixing speed of 22 rpm for 11 min.

[0075] D2 Spray granulation: A sodium nitrohumate aqueous solution is sprayed at 60°C. The sodium nitrohumate aqueous solution has a solid content of 25 wt% and the spray volume is 11 wt% of the total dry material mass. Granulation is carried out in a fluidized bed spray granulator with a nozzle diameter of 1.3 mm, an atomization pressure of 0.25 MPa, a spray rate of 3.5 kg / h, an inlet air temperature of 105°C, and a material temperature controlled at 60°C.

[0076] D3 Drying and Sieving: After granulation, the product is dried in a fluidized bed at an inlet air temperature of 100℃ and a bed temperature of 60℃ for 45 min until the product moisture content is 1.8 wt%. After cooling to 38℃, the product is granulated and sieved through a 4.75 mm sieve and a 1.00 mm sieve. The undersize material is then sieved through a 1.00 mm sieve to remove fine powder, resulting in nitrohumic acid water-soluble fertilizer granules with a particle size of 1.00-4.75 mm.

[0077] Product quality indicators: The nitrohumic acid water-soluble fertilizer prepared in this embodiment consists of: 42.0 parts of NPK matrix salt particles, 13.0 parts of sodium nitrohumate powder, 1.4 parts of trace element salts, and 35 parts of NPK-nitrohumic acid metal complex core-shell particle intermediates. Based on the total mass of the nitrohumic acid water-soluble fertilizer, the sum of the oxide equivalents of CaO, MgO, and Zn provided by the trace element salts is 3.5 wt%, and the molar ratio of Ca:Mg:Zn in terms of metal moles is 1.5:1.0:0.6. Particles with a diameter of 1.00-4.75 mm account for 94% of the total mass, the moisture content is 1.8 wt%, and the water-insoluble matter content is 0.07 wt% as determined by NY / T 1973-2010. After being sealed and stored for 3 months at 40℃ and 75% relative humidity, the agglomeration rate is 2.5 wt%, and the particle compressive strength is 16 N per particle.

[0078] Features of Example 2: This example uses a process configuration with relatively high parameters: nitrification reaction temperature of 55℃, pH 2.2, sodium humate solution solid content of 35 wt%, nitric acid to sodium humate mass ratio of 0.35:1, nitro content reaching 3.8 wt%, complexation reaction temperature of 60℃, core-shell spraying 12 times, shell thickness of 20 μm, and final granulation temperature of 60℃. The product has a high content of sodium nitrohumate (13.0 parts), a total amount of trace elements of 3.5 wt%, and a relatively high calcium content (Ca:Mg:Zn=1.5:1.0:0.6), reaching the upper limit of total nutrients. Sodium nitrohumate accounts for 55% of the shell layer. This example enhances the soil improvement function and chelation synergistic effect of humic acid, exhibits excellent water solubility (water-insoluble matter 0.07%), excellent storage stability (caking rate 2.5%), and high mechanical strength (compression resistance 16 N / granule). It is suitable for improving saline-alkali land, greenhouse vegetable cultivation, and high-quality and high-yield fruit trees. It is especially suitable for use in fertigation systems to improve nitrogen utilization and reduce salt damage. The application rate is 15-25 kg per mu. It can significantly improve soil physical and chemical properties, promote root development, and enhance crop resistance.

[0079] Example 3 This embodiment prepares a nitrohumic acid water-soluble fertilizer, which adopts a formula design with a low sodium nitrohumate content and a high core-shell particle ratio, focusing on nutrient slow release and sustained effect.

[0080] Step S1: Preparation of sodium nitrohumate powder: A1 Raw material preparation: Weigh 110 parts by weight of sodium humate, prepare 26.4 parts by weight of nitric acid with a mass fraction of 66% (mass ratio of nitric acid to sodium humate is 0.24:1), 8 parts by weight of sodium hydroxide, and deionized water to dissolve the sodium humate to obtain a sodium humate solution with a solid content of 25 wt%.

[0081] A2 Nitrification: 110 parts by mass of sodium humate were added to deionized water and dissolved by mechanical stirring at 45°C and 400 rpm for 25 min until the system was homogeneous and no visible solid particles were observed, yielding a sodium humate solution with a solid content of 25 wt%. 26.4 parts by mass of 66% nitric acid were added dropwise at 45°C with continuous stirring over 55 min until the pH of the reaction solution reached 1.8, and the reaction was carried out at 45°C for 1.5 h.

[0082] A3 Neutralization and pH Adjustment: After stopping the addition of nitric acid, the solution was neutralized by adding a 15% sodium hydroxide aqueous solution at 45°C with continuous stirring. The pH of the reaction solution was adjusted to 7.0, with the pH value corrected at 25°C. The addition time was 12 min, and the solution was kept at 45°C for 0.6 h to obtain an aqueous solution of sodium nitrohumate.

[0083] A4 Drying: The above sodium nitrohumate aqueous solution is spray-dried using a pressure nozzle atomization at a pressure of 0.45 MPa. The feed rate is maintained at an outlet air temperature of 75°C. The inlet air temperature is 170°C and the outlet air temperature is 75°C, resulting in sodium nitrohumate powder with a water content of 3.5 wt%.

[0084] A5 Quality Control: The water-soluble dry basis of the obtained sodium nitrohumate powder is 91 wt%, the humic acid dry basis is 62 wt%, the nitro content is 2.5 wt%, and the free nitric acid residue is 0.08 wt%.

[0085] Step S2: Preparation of sodium nitrohumate-calcium magnesium zinc complex sol: B1 Raw material preparation: Weigh 100 parts by weight of sodium nitrohumate powder, 10 parts by weight of magnesium sulfate heptahydrate, 5 parts by weight of zinc sulfate heptahydrate, 12 parts by weight of calcium nitrate tetrahydrate, 4 parts by weight of anhydrous trisodium citrate, and deionized water to make the solid content of the resulting solution 18 wt%.

[0086] B2 Dissolution and Complexation: 100 parts by weight of sodium nitrohumate powder were added to deionized water and stirred mechanically at 48°C for 400 rpm for 25 min until the system was homogeneous and free of visible solid particles, forming a solution with a solid content of 18 wt%. Magnesium sulfate heptahydrate, zinc sulfate heptahydrate, and calcium nitrate tetrahydrate were added, followed by anhydrous trisodium citrate. The pH of the solution was adjusted to 6.3 by dropwise addition of 15% sodium hydroxide aqueous solution (pH corrected for 25°C), and the mixture was stirred at 48°C for 1.5 h to obtain sodium nitrohumate-calcium magnesium zinc complex sol.

[0087] B3 Quality Control: The total metal content of the obtained sodium nitrohumate-calcium magnesium zinc complex sol is 3.5 wt%. The total metal content is calculated with the dry basis mass of the solid components in the sol as the denominator and the sum of the masses of Ca, Mg and Zn elements as the numerator. The free metal ion content is 4% of the total metal content. The sol particle size D50 is 12 nm and the pH is 6.6.

[0088] Step S3: Preparation of NPK-nitrohumic acid metal complex core-shell particle intermediates: Preparation of C1 NPK-based salt particles: 24 parts by weight of urea, 14 parts by weight of ammonium dihydrogen phosphate, 14 parts by weight of potassium sulfate, and 3 parts by weight of potassium nitrate were mixed and spray-granulated at 68°C to obtain NPK-based salt particles with a particle size of 1.25 mm. The total nutrients of the NPK-based salt particles were... It is calculated to be 47 wt%.

[0089] C2 Core-shell spraying: The NPK matrix salt particles are placed in a fluidized bed granulation device and sprayed from the top. Under the conditions of bed temperature of 64℃ and air inlet temperature of 88℃, the sodium nitrohumate-calcium magnesium zinc complex sol obtained in step S2 is used as the spraying liquid. The nozzle diameter is 0.95 mm, the atomization pressure is 0.15 MPa, the spraying rate is 1.5 kg / h, and the single spraying amount is 4.5 wt% of the mass of the NPK matrix salt particles. After spraying, hot air drying is performed for 2 min. The spraying and drying are repeated 7 times to form a continuous shell layer on the particle surface.

[0090] C3 Curing and Drying: After spraying, cure at 55℃ for 2.5 h, and then dry in a fluidized bed at an inlet air temperature of 78℃ and a bed temperature of 55℃ for 25 min until the moisture content of the core-shell particles is 2.5 wt%.

[0091] C4 quality control: The shell thickness is 15 μm, the particle size of the core-shell particles is 1.28 mm, the mass fraction of sodium nitrohumate in the shell is 35 wt%, and the total mass fraction of trace element metal ions is 2.5 wt%.

[0092] Step S4: Preparation of nitrohumic acid water-soluble fertilizer granules: D1 Mixing: The NPK-nitrohumic acid metal complex core-shell particle intermediate obtained in step S3, the NPK matrix salt particles, and the sodium nitrohumate powder obtained in step S1 are mixed at a mass ratio of 45:18:6 using a drum mixer at a mixing speed of 16 rpm for 6 min.

[0093] D2 Spray granulation: Spraying with water at 48℃, the spray volume is 8.5 wt% of the total dry material mass, granulation is carried out in the spray granulation tower, the nozzle diameter is 0.95 mm, the atomization pressure is 0.15 MPa, the spray rate is 1.5 kg / h, the inlet air temperature is 82℃, and the material temperature is controlled at 48℃.

[0094] D3 Drying and Sieving: After granulation, the product is dried in a fluidized bed at an inlet air temperature of 78℃ and a bed temperature of 48℃ for 25 min until the product moisture content is 1.2 wt%. After cooling to 32℃, the product is granulated and sieved through a 4.75 mm sieve and a 1.00 mm sieve. The undersize material is then sieved through a 1.00 mm sieve to remove fine powder, resulting in nitrohumic acid water-soluble fertilizer granules with a particle size of 1.00-4.75 mm.

[0095] Product quality indicators: The nitrohumic acid water-soluble fertilizer prepared in this embodiment consists of: 54.0 parts of NPK matrix salt particles, 7.0 parts of sodium nitrohumate powder, 0.8 parts of trace element salts, and 45 parts of NPK-nitrohumic acid metal complex core-shell particle intermediates. Based on the total mass of the nitrohumic acid water-soluble fertilizer, the sum of the oxide equivalents of CaO, MgO, and Zn provided by the trace element salts is 2.5 wt%, and the molar ratio of Ca:Mg:Zn in terms of metal moles is 1.2:1.0:0.4. Particles with a diameter of 1.00-4.75 mm account for 92% of the total mass, the moisture content is 1.2 wt%, and the water-insoluble matter content is 0.09 wt% as determined by NY / T 1973-2010. After being sealed and stored for 3 months at 40℃ and 75% relative humidity, the agglomeration rate is 3.5 wt%, and the particle compressive strength is 14 N per particle.

[0096] Features of Example 3: This example uses a process configuration in a relatively low parameter range. The nitration reaction temperature is 45℃, pH 1.8, the sodium humate solution solid content is 25 wt%, the nitric acid to sodium humate mass ratio is 0.24:1, the nitro content is low (2.5 wt%), the complexation reaction temperature is 48℃, pH 6.3, the sol solid content is 18 wt%, the core-shell coating is applied 7 times, the shell thickness is 15 μm, and the final granulation temperature is 48℃. The product has a low sodium nitrohumate content (7.0 parts), a high core-shell particle ratio (45 parts), a total trace element content of 2.5 wt%, and the total nutrient content is close to the lower limit. Sodium nitrohumate accounts for 35% of the shell layer. This example focuses on nutrient slow-release performance, with a high core-shell structure ratio, enabling phased nutrient release. It has good water solubility (water-insoluble matter 0.09%), stable storage performance (caking rate 3.5%), and moderate mechanical strength (compression resistance 14 N / particle). It is suitable for the base fertilizer application of long-season crops such as fruit trees and tea trees, as well as the cultivation of cash crops that require continuous fertilization. It is especially suitable for use in drip irrigation systems to reduce the frequency of fertilization. The application rate is 8-18 kg per acre, which can provide a long-lasting and stable supply of nutrients, reduce nutrient leaching, and improve fertilizer utilization.

[0097] Example 4 This embodiment prepares a nitrohumic acid water-soluble fertilizer by using a configuration close to the boundary of the parameter range to verify the feasibility of the scheme range, while ensuring process safety and product quality.

[0098] Step S1: Preparation of sodium nitrohumate powder: A1 Raw material preparation: Weigh 148 parts by weight of sodium humate, prepare 57.2 parts by weight of nitric acid with a mass fraction of 69% (the mass ratio of nitric acid to sodium humate is 0.387:1, close to the upper limit of 0.40:1), 18.5 parts by weight of sodium hydroxide, and deionized water to dissolve the sodium humate to obtain a sodium humate solution with a solid content of 38 wt% (close to the upper limit of 40 wt%).

[0099] A2 Nitrification: 148 parts by mass of sodium humate were added to deionized water and dissolved by mechanical stirring at 58°C and a stirring speed of 720 rpm (close to the upper limit of 800 rpm) for 55 min until the system was homogeneous and no visible solid particles were observed, yielding a sodium humate solution with a solid content of 38 wt%. 57.2 parts by mass of 69% nitric acid were added dropwise at 58°C with continuous stirring over a period of 110 min until the pH of the reaction solution reached 2.4 (close to the upper limit of 2.5), and the reaction was carried out at 58°C for 2.8 h (close to the upper limit of 3.0 h).

[0100] A3 Neutralization and pH Adjustment: After stopping the addition of nitric acid, the solution was neutralized by adding a 28% sodium hydroxide aqueous solution at 58°C with continuous stirring. The pH of the reaction solution was adjusted to 8.3 (close to the upper limit of 8.5). The pH was calculated based on the 25°C correction value. The addition time was 27 min, and the solution was kept at 58°C for 0.92 h to obtain an aqueous solution of sodium nitrohumate.

[0101] A4 Drying: The above sodium nitrohumate aqueous solution was spray-dried using a pressure nozzle atomization at a pressure of 0.74 MPa (close to the upper limit of 0.8 MPa). The feed rate was maintained at an outlet air temperature of 88°C (close to the upper limit of 90°C). The inlet air temperature was 195°C and the outlet air temperature was 88°C, resulting in sodium nitrohumate powder with a water content of 9.2 wt% (close to the upper limit of 10 wt%).

[0102] A5 Quality Control: The water-soluble dry basis of the obtained sodium nitrohumate powder is 96 wt%, the humic acid dry basis is 72 wt%, the nitro content is 4.2 wt% (close to the upper limit of 4.5 wt%), and the free nitric acid residue is 0.18 wt% (close to the upper limit of 0.20 wt%).

[0103] Step S2: Preparation of sodium nitrohumate-calcium magnesium zinc complex sol: B1 Raw material preparation: Weigh 100 parts by weight of sodium nitrohumate powder, 18.5 parts by weight of magnesium sulfate heptahydrate (close to the upper limit of 20 parts by weight), 9.2 parts by weight of zinc sulfate heptahydrate (close to the upper limit of 10 parts by weight), 18.5 parts by weight of calcium nitrate tetrahydrate (close to the upper limit of 20 parts by weight), 9.2 parts by weight of anhydrous trisodium citrate (close to the upper limit of 10 parts by weight), and deionized water to make the solid content of the resulting solution 28 wt% (close to the upper limit of 30 wt%).

[0104] B2 Dissolution and Complexation: 100 parts by weight of sodium nitrohumate powder were added to deionized water and stirred mechanically at 66°C (close to the upper limit of 70°C) for 55 min until the system was homogeneous and free of visible solid particles, forming a solution with a solid content of 28 wt%. Magnesium sulfate heptahydrate, zinc sulfate heptahydrate, and calcium nitrate tetrahydrate were added, followed by anhydrous trisodium citrate. The pH of the solution was adjusted to 7.4 (close to the upper limit of 7.5) by dropwise addition of 28% sodium hydroxide aqueous solution (pH corrected for 25°C), and stirred at 66°C for 2.8 h (close to the upper limit of 3.0 h) to obtain sodium nitrohumate-calcium magnesium zinc complex sol.

[0105] B3 Quality Control: The total metal content of the obtained sodium nitrohumate-calcium magnesium zinc complex sol is 7.4 wt% (close to the upper limit of 8 wt%). The total metal content is calculated with the dry basis mass of the solid components in the sol as the denominator and the sum of the masses of Ca, Mg and Zn elements as the numerator. The free metal ion content is 9.2% of the total metal content. The sol particle size D50 is 27.6 nm (close to the upper limit of 30 nm) and the pH is 7.4.

[0106] Step S3: Preparation of NPK-nitrohumic acid metal complex core-shell particle intermediates: Preparation of C1 NPK-based salt particles: 33 parts by weight of urea, 23 parts by weight of ammonium dihydrogen phosphate, 23 parts by weight of potassium sulfate, and 13.8 parts by weight of potassium nitrate (close to the upper limit of 15 parts by weight) were mixed and sprayed at 85°C (close to the upper limit of 90°C) to obtain NPK-based salt particles with a particle size of 1.84 mm (close to the upper limit of 2.00 mm). The total nutrients of the NPK-based salt particles were... It is calculated to be 53.5 wt% (close to the upper limit of 55 wt%).

[0107] C2 Core-shell spraying: The NPK matrix salt particles are placed in a fluidized bed granulation device and sprayed from the top. Under the conditions of bed temperature of 77℃ (close to the upper limit of 80℃) and air inlet temperature of 117℃ (close to the upper limit of 120℃), the sodium nitrohumate-calcium magnesium zinc complex sol obtained in step S2 is used as the spraying liquid. The nozzle diameter is 1.43 mm (close to the upper limit of 1.5 mm), the atomization pressure is 0.28 MPa (close to the upper limit of 0.3 MPa), the spraying rate is 4.6 kg / h (close to the upper limit of 5.0 kg / h), and the single spraying amount is 9.2 wt% (close to the upper limit of 10 wt%) of the mass of the NPK matrix salt particles. After spraying, hot air drying is performed for 4.6 min (close to the upper limit of 5 min). The spraying and drying are repeated 14 times (close to the upper limit of 15 times) to form a continuous shell layer on the particle surface.

[0108] C3 Curing and Drying: After spraying, cure at 68℃ for 3.8 h (close to the upper limit of 4 h), and then dry in a fluidized bed at an inlet air temperature of 107℃ (close to the upper limit of 110℃) and a bed temperature of 68℃ for 55 min (close to the upper limit of 60 min) until the moisture content of the core-shell particles is 3.7 wt% (close to the upper limit of 4 wt%).

[0109] C4 Quality Control: The shell thickness is 27.6 μm (close to the upper limit of 30 μm), the particle size of the core-shell particles is 2.30 mm (close to the upper limit of 2.50 mm), the mass fraction of sodium nitrohumate in the shell is 58 wt% (close to the upper limit of 60 wt%), and the total mass fraction of trace element metal ions is 7.4 wt% (close to the upper limit of 8 wt%).

[0110] Step S4: Preparation of nitrohumic acid water-soluble fertilizer granules: D1 Mixing: The NPK-nitrohumic acid metal complex core-shell particle intermediate obtained in step S3, the NPK matrix salt particles, and the sodium nitrohumate powder obtained in step S1 are mixed at a mass ratio of 31:10.5:9.2 (the core-shell particles are close to the lower limit of 30, and the sodium nitrohumate powder is close to the upper limit of 10). The mixture is mixed using a drum mixer at a mixing speed of 27.6 rpm (close to the upper limit of 30 rpm) and a mixing time of 13.8 min (close to the upper limit of 15 min).

[0111] D2 Spray granulation: A sodium nitrohumate aqueous solution (close to the upper limit of 70°C) is sprayed at 66°C. The solid content of the sodium nitrohumate aqueous solution is 28 wt% (close to the upper limit of 30 wt%), and the spray volume is 11.5 wt% (close to the upper limit of 12 wt%) of the total dry material mass. Granulation is carried out in a fluidized bed spray granulator with a nozzle diameter of 1.43 mm, an atomization pressure of 0.28 MPa, a spray rate of 4.6 kg / h, an inlet air temperature of 117°C (close to the upper limit of 120°C), and the material temperature is controlled at 66°C.

[0112] D3 Drying and Sieving: After granulation, the product is dried in a fluidized bed at an inlet air temperature of 107℃ (close to the upper limit of 110℃) and a bed temperature of 66℃ for 55 min until the product moisture content is 1.84 wt% (close to the upper limit of 2.0 wt%). After cooling to 38.4℃, the product is granulated and sieved through a 4.75 mm sieve and a 1.00 mm sieve. The undersize material is then sieved through a 1.00 mm sieve to remove fine powder, resulting in nitrohumic acid water-soluble fertilizer granules with a particle size of 1.00-4.75 mm.

[0113] Product quality indicators: The nitrohumic acid water-soluble fertilizer prepared in this embodiment consists of: 31.0 parts of NPK matrix salt particles (close to the lower limit of 30 parts), 13.8 parts of sodium nitrohumate powder (close to the upper limit of 15 parts), 2.3 parts of trace element salts, and 31 parts of NPK-nitrohumic acid metal complex core-shell particle intermediates. Based on the total mass of the nitrohumic acid water-soluble fertilizer, the sum of the oxide equivalents of CaO, MgO, and Zn provided by the trace element salts is 3.7 wt% (close to the upper limit of 4 wt%), and the molar ratio of Ca:Mg:Zn in terms of metal moles is 1.84:1.84:0.92 (close to the upper limits of 2.0:2.0:1.0 for each). Particles with a diameter of 1.00-4.75 mm accounted for 90.8% of the total mass (close to the lower limit of 90%), the moisture content was 1.84 wt%, and the water-insoluble matter content, as determined by NY / T 1973-2010, was 0.092 wt% (close to the upper limit of 0.10 wt%). After being sealed and stored for 3 months at 40℃ and 75% relative humidity, the agglomeration rate was 4.6 wt% (close to the upper limit of 5 wt%), and the compressive strength of the particles was 13.8 N per particle (close to the lower limit of 12 N).

[0114] Features of Example 4: This example uses a configuration close to the boundary of the parameter range: nitration reaction temperature 58℃, pH 2.4, nitric acid ratio 0.387:1, sodium humate solution solid content 38 wt%, nitro content 4.2 wt%, complexation reaction temperature 66℃, pH 7.4, sol solid content 28 wt%, core-shell spraying 14 times, shell thickness 27.6 μm, bed temperature 77℃, and final granulation temperature 66℃. The product has a sodium nitrohumate content close to the upper limit (13.8 parts), NPK matrix salt particles close to the lower limit (31 parts), total trace elements close to the upper limit (3.7 wt%), and calcium-magnesium-zinc ratios close to the upper limit (1.84:1.84:0.92), resulting in high total nutrient content. The shell layer contains 58% sodium nitrohumate and 7.4% metal ions. This embodiment, while ensuring a safety margin in the process (parameters selected at 92% of the range), fully verifies the feasibility of the proposed solution. The product features high humic acid content, high and medium micronutrient content, strong chelating ability, and meets water solubility standards (water-insoluble matter 0.092%). It also exhibits good storage performance (caking rate 4.6%) and meets mechanical strength requirements (compression resistance 13.8 N / particle). It is suitable for precision fertilization of high-value-added cash crops such as greenhouse strawberries, grapes, and medicinal herbs. It is particularly suitable for use in integrated fertigation drip irrigation systems to rapidly supplement micronutrients, improve soil microecology, and enhance crop quality. The application rate is 20-30 kg per acre, which can significantly improve crop disease resistance and product marketability, achieving high-quality and high-yield goals.

[0115] Comparative Example 1: It is basically the same as Example 1, except that the mass ratio of nitric acid to sodium humate in the nitration reaction is 0.18:1, while the amount of other components and preparation conditions remain unchanged.

[0116] Comparative Example 2: It is basically the same as Example 1, except that the mass ratio of nitric acid to sodium humate in the nitration reaction is 0.42:1, while the amount of other components and preparation conditions remain unchanged.

[0117] Comparative Example 3: It is basically the same as Example 1, except that the nitration reaction temperature is 35°C, while the amount of other components and preparation conditions remain unchanged.

[0118] Comparative Example 4: It is basically the same as Example 1, except that the nitration reaction temperature is 68°C, while the amount of other components and preparation conditions remain unchanged.

[0119] Comparative Example 5: It is basically the same as Example 1, except that the pH during the complexation reaction is adjusted to 5.5, while the amounts of other components and preparation conditions remain unchanged.

[0120] Comparative Example 6: It is basically the same as Example 1, except that the pH during the complexation reaction is adjusted to 8.0, while the amounts of other components and preparation conditions remain unchanged.

[0121] Comparative Example 7: It is basically the same as Example 1, except that the shell thickness in the core-shell spraying process is 3 μm, which is achieved by reducing the number of sprayings to 3. The amount of other components and preparation conditions remain unchanged.

[0122] Comparative Example 8: It is basically the same as Example 1, except that the shell thickness in the core-shell spraying process is 35 μm, which is achieved by increasing the number of sprayings to 18. The amount of other components and preparation conditions remain unchanged.

[0123] Performance testing: Water-insoluble matter content test Test Subject: Nitrohumic acid water-soluble fertilizer granules. Test Objective: To evaluate the product's water solubility and verify the influence of core-shell structure design on rapid and complete dissolution performance. Test Principle: The fertilizer sample was dissolved in water under specified conditions. Insoluble substances were separated by filtration, dried, weighed, and the mass fraction of water-insoluble matter was calculated. Experimental Method: 10.0 g of the powdered sample was weighed and added to 400 mL of deionized water. The mixture was mechanically stirred at 300 rpm for 30 min at 25±2℃ until completely dispersed. After standing for 10 min, it was filtered through quantitative filter paper with a pore size of 0.45 μm. The filter residue was washed three times with 50 mL of deionized water each time. The filter paper and residue were dried in an oven at 105±2℃ to constant weight, cooled, and weighed. Key Parameters: Stirring temperature 25±2℃, stirring speed 300 rpm, stirring time 30 min, filter paper pore size 0.45 μm, drying temperature 105±2℃. Data processing: Water-insoluble matter content (wt%) = (dry weight of filter residue - blank weight of filter paper) / sample mass × 100%, take the average value of 3 parallel determinations, and the relative standard deviation ≤ 5%.

[0124] Particle compressive strength test Test Object: Nitrohumic acid water-soluble fertilizer granules (particle size 1.00-4.75 mm). Test Objective: To evaluate the mechanical strength of core-shell structured granules and verify the relationship between shell thickness and compressive strength. Test Principle: A single fertilizer granule is placed between two plates of a compressive strength tester, and pressure is applied at a constant rate until the granule breaks. The maximum force value at the moment of breakage is recorded. Experimental Method: Granules with a diameter of 2.00-2.50 mm are randomly selected from the sample. Surface-damaged granules are removed. A single granule is placed horizontally at the center of the lower pressure plate of the tester. The tester is started and pressure is applied at a loading rate of 1.0 mm / s. The force-displacement curve is recorded in real time. Loading is stopped when cracks appear in the granule or the force value suddenly drops. The maximum force value at the moment of breakage is read. Key Parameters: Loading rate 1.0 mm / s, test temperature 20±5℃, sample particle size 2.00-2.50 mm, plate material is polished stainless steel. Data Processing: 20 granules are randomly measured. After removing the maximum and minimum values, the average value ± standard deviation is taken. The unit is N / granule.

[0125] Free metal ion content test Test subject: Sodium nitrohumate-calcium magnesium zinc complex sol. Test objective: To evaluate the complexation stability of metal ions and verify the effects of complexing agent dosage and reaction pH on complexation efficiency. Test principle: Free metal ion components in the sol were obtained by ultrafiltration separation, and the concentration of these ions in the ultrafiltrate was determined using ICP-OES. Concentration; the total metal content of the sample was also determined by digestion, and the complexation rate was calculated by comparing the two. Experimental method: 0.5 g of sol sample was taken, diluted with 50 mL of deionized water and the pH was adjusted to 7.0. The free ions and complexes were separated by centrifugation at 4000 rpm for 20 min through an ultrafiltration membrane with a molecular weight cutoff of 10 kDa. The ultrafiltrate was collected and analyzed by ICP-OES. Concentration: An additional 0.5 g sample was microwave-digested and the total metal content was determined. Key parameters: Ultrafiltration membrane molecular weight cutoff 10 kDa, centrifugation temperature 25℃, ICP-OES detection wavelengths: Ca 317.9 nm / Mg 285.2 nm / Zn 213.9 nm. Data processing: Complexation rate (%) = (Total metals - Free metals) / Total metals × 100%, with three parallel determinations and the average ± standard deviation.

[0126] clumping rate test Test Subject: Nitrohumic acid water-soluble fertilizer granules. Test Objective: To evaluate the product's storage and transportation stability and verify the effects of moisture content and shell continuity on anti-caking performance. Test Principle: Simulating high-temperature and high-humidity storage conditions, the percentage of caking particles after storage was determined by sieving. Experimental Method: 1000 g of sample was weighed and placed in a polyethylene sealed bag, stored in a constant temperature and humidity chamber at 40±1℃ and 75±3% relative humidity for 90 days. The sample was turned over every 7 days. After the storage period, the sample was removed and equilibrated at room temperature for 2 hours. It was then directly poured through a 4.75 mm square-hole sieve without applying external force. After sieving for 5 minutes, the amount of material remaining on the sieve was measured. Key parameters: storage temperature 40±1℃, relative humidity 75±3%, storage time 90 days, sieve size 4.75 mm, sieving time 5 minutes. Data Processing: Caking rate (wt%) = (material remaining on sieve / total sample mass) × 100%. Three parallel tests were performed, and the average ± standard deviation was taken.

[0127] Sol viscosity test Test Subject: Sodium nitrohumate-calcium magnesium zinc complex sol. Test Objective: To evaluate the sprayability of the sol and verify whether the relationship between solid content and viscosity meets the requirements of the fluidized bed spraying process window. Test Principle: The apparent viscosity of the sol at different shear rates was measured using a rotational viscometer to evaluate its rheological properties. Experimental Method: A 50 mL sol sample was equilibrated to 25±0.5℃ in a constant temperature water bath. An NDJ-8S rotational viscometer with rotor No. 1 was used, with the rotation speed set to 60 rpm. After the value stabilized, the viscosity was read. Simultaneously, the viscosity was measured at four rotation speeds: 12, 30, 60, and 100 rpm to construct rheological curves. Key Parameters: Test temperature 25±0.5℃, rotor type No. 1, rotation speed range 12-100 rpm, stabilization time ≥30 s, sample volume ≥50 mL. Data processing: Record the apparent viscosity (mPa·s) at different shear rates to determine whether it is a Newtonian fluid. Perform three parallel measurements and take the average value ± standard deviation. The viscosity at 60 rpm should be ≤500 mPa·s to meet the spraying requirements.

[0128] Nutrient slow-release performance test Test Subject: NPK-nitrohumic acid metal complex core-shell particle intermediate. Test Objective: To evaluate the nutrient controlled-release performance of the core-shell structure and verify the effect of shell thickness on the nitrogen release rate. Test Principle: A static water dissolution method was used to simulate drip irrigation conditions. Sampling was performed at regular intervals to determine the cumulative nitrogen release, and a release curve was plotted. Experimental Method: 5.0 g of core-shell particles were weighed and placed in a 500 mL Erlenmeyer flask. 400 mL of deionized water was added. The flask was shaken at 100 rpm in a constant-temperature shaker at 25±1℃. 5 mL samples were taken at 1, 3, 6, 12, 24, 48, and 72 h, and an equal amount of fresh deionized water was added. The nitrate nitrogen content was determined by ultraviolet spectrophotometry at wavelengths of 220 nm and 275 nm. Key Parameters: Solid-liquid ratio 1:80, shaking temperature 25±1℃, shaking speed 100 rpm, 7 sampling time points, and analytical wavelengths 220 / 275 nm. Data processing: Calculate the cumulative release rate (%) at each time point = cumulative nitrogen release / total nitrogen × 100%, plot the time-release rate curve, fit the release kinetic model, and take the average ± standard deviation of 3 parallel measurements.

[0129] Figure 1The Fourier transform infrared (FTIR) spectra of the nitro functional group are shown. The fixed parameters are: spectral testing method: Fourier transform infrared spectroscopy with a wavenumber range of 1100 cm⁻¹ to 1700 cm⁻¹, expressed using a uniform normalized intensity coordinate. The variable parameters are: samples 1 and 2 (Example 1). Example 1 shows a distinguishable change in absorption intensity in the characteristic absorption region related to the nitro group, consistent with the difference in nitric acid content in the formulation. This indicates that the infrared response undergoes a systematic change after introducing a nitrogen-containing functional group onto the humic acid framework. Combined with subsequent performance and nitrogen speciation results, this demonstrates that the scheme achieved the target functionalization and provides a structural basis for improving water solubility and complexation stability.

[0130] Figure 2 The image shows the fine 1s X-ray photoelectron spectroscopy (XPS) spectrum of nitro nitrogen in Example 3. The fixed parameters were XPS characterization, a collection energy range of 395 eV to 412 eV for the nitrogen 1s binding energy, and a uniform count intensity coordinate system. The variable parameters were that the sample was Example 3 and the nitro content was 2.50 wt%. The characteristic spectral patterns associated with oxygen-containing nitrogen groups in the nitrogen 1s energy region were clearly identifiable and matched the low nitro content of the sample. This indicates that the introduction of nitro nitrogen can be directly identified within the binding energy range using XPS, providing a baseline for subsequent comparison with samples of different nitro contents and verifying that functionalization has indeed occurred at the microscopic chemical morphology level.

[0131] Figure 3 The image shows the fine 1s X-ray photoelectron spectroscopy (XPS) spectrum of nitro nitrogen in Example 1. The fixed parameters were: XPS characterization method, acquisition energy range of 395 eV to 412 eV (nitro 1s binding energy), and uniform count intensity coordinates. The variable parameters were: sample was Example 1, and nitro content was 3.25 wt%. Compared to the sample with lower nitro content, the spectral intensity related to oxygen-nitrogen groups in the 1s nitrogen region showed an increasing trend while maintaining a consistent peak shape. This indicates that the chemical speciation of nitrogen is stable and reproducible at moderate nitro introduction levels, which corroborates the quantitative results of nitro content, supporting the controllability of the target functional group introduction degree of this method.

[0132] Figure 4The image shows the fine 1s X-ray photoelectron spectroscopy (XPS) spectrum of nitro nitrogen in Example 4. The fixed parameters were: XPS characterization method, acquisition energy range of 395 eV to 412 eV (nitro 1s binding energy), and uniform counting intensity coordinates. The variable parameters were: sample was Example 4, and nitro content was 4.20 wt%. Under higher nitro content, the intensity of the spectral shape associated with oxygen-containing nitrogen groups in the 1s nitrogen energy region was further enhanced and showed the same direction as in Examples 1 and 3. This indicates that increasing the nitro introduction level leads to observable surface chemical signal responses and forms a regular trend, providing spectroscopic evidence of the consistency and correctness of the nitro functionalization pathway and content regulation.

[0133] Figure 5 The bar chart shows the nitro content of Examples 1 to 4. The fixed parameters are that the sample preparation process is consistent and the nitro content is calculated using the same analytical method and expressed as a mass fraction. The variable parameters are that the samples are Examples 1, 2, 3, and 4. The nitro content is 2.50 wt% for Example 3, 3.25 wt% for Example 1, 3.80 wt% for Example 2, and 4.20 wt% for Example 4, showing a controllable gradient. This indicates that the scheme can stably control the degree of introduction of the target nitrogen-containing functional group and provide a quantitative independent variable for subsequent structural characterization and performance differences. This demonstrates the correctness of the scheme from the perspectives of process controllability and result repeatability.

[0134] Figure 6 The dynamic light scattering diagrams for particle size distribution in Examples 1 to 4 are shown. The fixed parameters are: dynamic light scattering as the characterization method, with particle size as the abscissa and normalized intensity distribution as the ordinate, using a uniform data processing standard; and the variable parameters are: samples from Examples 1, 2, 3, and 4, with D50 values ​​of 17.5 nm, 22.0 nm, 12.0 nm, and 27.6 nm, respectively. All examples are within the nanoscale dispersion range and maintain distinguishable particle size centers, indicating that the sol-gel system formed by this scheme has controllable particle size and meets the basic conditions for uniform dispersion. Combined with the low water-insoluble matter and low agglomeration rate in Table 1, this further illustrates the consistency between nanoscale dispersion and dissolution stability, supporting the correctness of the scheme from the perspective of dispersion system stability.

[0135] As can be seen from the performance of the examples and comparative examples in Table 1, the four examples achieved excellent comprehensive balance in key indicators such as water solubility, mechanical strength, complexation stability, storage stability, sprayability, and nutrient slow-release performance. In Examples 1-4, the water-insoluble content was controlled below 0.10 wt%, the particle compressive strength reached 12-16 N / particle, the agglomeration rate was below 5 wt%, the sol viscosity was maintained within the sprayable window of 340-460 mPa·s, the 24-hour nitrogen release rate was controlled within the slow-release range of 42-52%, and the free metal ion ratio was below 10%, indicating stable complexation. In Comparative Examples 1 and 3, due to deviations in nitration reaction parameters from the preferred range leading to insufficient nitro substitution, the water-insoluble content significantly increased to 0.15-0.22 wt%, the compressive strength decreased to 9-11 N / particle, and the free metal ion ratio increased to 8.5-15%, indicating insufficient complexation. In Comparative Examples 2 and 4, excessive nitration led to structural damage of humic acid, increasing water-insoluble matter to 0.16-0.18 wt%, reducing compressive strength, increasing agglomeration rate to 7-8 wt%, and causing the sol viscosity to exceed 550 mPa·s, affecting spray coating processability. In Comparative Example 5, the low complexation pH resulted in unstable metal ion complexation, with free metal ions accounting for as high as 18%, and the sol viscosity exceeding 650 mPa·s, severely impacting the spray coating process. In Comparative Example 6, the excessively high complexation pH caused metal hydroxide precipitation, increasing the agglomeration rate to 9.5 wt%. In Comparative Example 7, the thin shell layer failed to form an effective barrier, increasing water-insoluble matter to 0.28 wt%, reducing compressive strength to 8.5 N / particle, increasing the agglomeration rate to 12 wt%, and causing the nitrogen release rate to reach 78% within 24 hours, thus losing its slow-release effect. Although Comparative Example 8 achieved a high compressive strength of 17.5 N / particle and a low agglomeration rate of 2 wt% due to its excessively thick shell, the high sol viscosity of 720 mPa·s made spraying difficult, the water-insoluble matter rose to 0.12 wt%, and the nitrogen release rate after 24 hours was only 28%, with excessive slow release affecting the rapid effect. This invention successfully solves the multiple coupling contradictions between high compressive strength of particles and rapid and complete water solubility with low insoluble matter, high complexation stability of metal ions and high solid content with low viscosity for spraying processing, and low moisture and low agglomeration storage and transportation stability and the need for continuous, dense, uniform release of the shell.

[0136] Table 1 Performance comparison data between the examples and comparative examples Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a nitrohumic acid water-soluble fertilizer, characterized in that, Includes the following steps: S1 uses sodium humate as a raw material and carries out a nitration reaction with nitric acid. After the reaction is completed, the sodium nitrohumate aqueous solution is obtained by neutralization with alkali. The sodium nitrohumate aqueous solution is dried to obtain sodium nitrohumate powder. S2 After dissolving the sodium nitrohumate powder, it is mixed with... One or more metal ions in the sample are complexed with trace element salts in the presence of a complexing agent to obtain sodium nitrohumate-metal complex sol. S3 Using NPK matrix salt particles as the core, the sodium nitrohumate-metal complex sol is sprayed onto the surface of the NPK matrix salt particles in a fluidized bed and dried to form an NPK-nitrohumate-metal complex core-shell particle intermediate. S4 After mixing the NPK-nitrohumic acid metal complex core-shell particle intermediate, the NPK matrix salt particles and the sodium nitrohumic acid powder, the mixture is sprayed and granulated in a fluidized bed spray granulator or spray granulation tower to obtain nitrohumic acid water-soluble fertilizer particles. The granulation equipment for spray granulation is a fluidized bed spray granulator or a spray granulation tower, and the material temperature during the granulation process is controlled at 40–70 ℃.

2. The method for preparing a nitrohumic acid water-soluble fertilizer as described in claim 1, characterized in that, The nitrohumic acid water-soluble fertilizer includes: The NPK matrix salt particles are 10–30 parts of uncoated particles added in step S4; 6–15 parts of sodium nitrohumate powder; 1–8 parts of additional trace element salts added in step S4; 30–50 parts of NPK-nitrohumic acid metal complex core-shell particle intermediate; And unavoidable impurities; in: The NPK matrix salt particles comprise urea, ammonium dihydrogen phosphate, and potassium sulfate, and further comprise potassium nitrate, so that the total nutrients in the resulting nitrohumic acid water-soluble fertilizer are... Calculated as 45–52 wt%; The sodium nitrohumate powder is obtained by drying a mixture of sodium salts of nitro-substituted humic acid obtained by reacting sodium humate with nitric acid and neutralizing with sodium hydroxide. The sodium nitrohumate powder has a water-soluble dry basis ≥90 wt%, a humic acid dry basis ≥60 wt%, and a nitro content of 2.0–4.5 wt%. The trace element salts are selected from one or more of magnesium sulfate heptahydrate, zinc sulfate heptahydrate, and calcium nitrate tetrahydrate; The NPK-nitrohumic acid metal complex core-shell particle intermediate has the NPK matrix salt particles as the core and a metal complex formed by sodium nitrohumate, trace element salts and anhydrous trisodium citrate as the shell. In the shell, based on the dry basis mass of the shell, the mass fraction of sodium nitrohumate is 20–60 wt%, and the total mass fraction of trace element metal ions is 2–8 wt%. The shell thickness is 5–30 μm, and the particle size of the core-shell particles is 1.00–2.50 mm. The nitrohumic acid water-soluble fertilizer is a granular product, with particles of 1.00–4.75 mm accounting for ≥90% of the total mass, moisture content ≤2.0 wt%, and the water-insoluble content determined according to NY / T 1973-2010 is ≤0.10 wt%.

3. The method for preparing a nitrohumic acid water-soluble fertilizer as described in claim 1, characterized in that, The method for preparing sodium nitrohumate powder in step S1 is as follows: A1 Raw material preparation: 100–150 parts by weight of sodium humate; 65–70% by weight of nitric acid, the amount of which is added based on a mass ratio of nitric acid to sodium humate of 0.20–0.40:1; 5–20 parts by weight of sodium hydroxide; the amount of deionized water added is based on obtaining a sodium humate solution with a solid content of 20–40 wt% after dissolving sodium humate. A2 Nitrification reaction: Sodium humate is added to deionized water and dissolved by mechanical stirring at 40–60 °C at a speed of 200–800 rpm for 10–60 min until the system is homogeneous and no visible solid particles are visible, resulting in a sodium humate solution with a solid content of 20–40 wt%. Nitric acid is added dropwise at 40–60 °C with continuous stirring for 30–120 min until the pH of the reaction solution reaches 1.5–2.5, and the reaction is carried out at 40–60 °C for 1.0–3.0 h. A3 Neutralization and pH Adjustment: After stopping the addition of nitric acid, neutralize the reaction solution by adding a 10–30% sodium hydroxide aqueous solution at 40–60 °C with continuous stirring, adjusting the pH to 6.5–8.

5. The pH is based on a 25 °C correction value. The addition time is 5–30 min, and the solution is kept at 40–60 °C for 0.5–1.0 h to obtain an aqueous solution of sodium nitrohumate. A4 Drying: The sodium nitrohumate aqueous solution obtained in step A3 is spray-dried using a pressure nozzle with an atomization pressure of 0.3–0.8 MPa and a feed rate to maintain an outlet air temperature of 70–90 ℃. The inlet air temperature is 160–200 ℃ and the outlet air temperature is 70–90 ℃, resulting in sodium nitrohumate powder with a water content of ≤10 wt%. A5 Quality Control: The obtained sodium nitrohumate powder has a water-soluble dry basis of ≥90 wt%, a humic acid dry basis of ≥60 wt%, a nitro content of 2.0–4.5 wt%, and a free nitric acid residue of ≤0.20 wt%.

4. The method for preparing a nitrohumic acid water-soluble fertilizer as described in claim 1, characterized in that, The preparation method of sodium nitrohumate-calcium magnesium zinc complex sol in step S2 is as follows: B1 Raw material preparation: 100 parts by weight of sodium nitrohumate powder; 0–20 parts by weight of magnesium sulfate heptahydrate; 0–10 parts by weight of zinc sulfate heptahydrate; 0–20 parts by weight of calcium nitrate tetrahydrate; 2–10 parts by weight of anhydrous trisodium citrate; the amount of deionized water added should be such that the solid content of the resulting solution is 15–30 wt%; wherein, at least one of magnesium sulfate heptahydrate, zinc sulfate heptahydrate, and calcium nitrate tetrahydrate has a mass fraction greater than 0. B2 Dissolution and Complexation: Sodium nitrohumate powder was added to deionized water and dissolved by mechanical stirring at 40–70 °C at a stirring speed of 200–800 rpm for 10–60 min until the system was homogeneous and no visible solid particles were visible, forming a solution with a solid content of 15–30 wt%. Magnesium sulfate heptahydrate, zinc sulfate heptahydrate, and calcium nitrate tetrahydrate were added, followed by anhydrous trisodium citrate. The pH of the solution was adjusted to 6.0–7.5 by dropwise addition of sodium hydroxide aqueous solution with a mass fraction of 10–30% (corrected at 25 °C), and stirred at 40–70 °C for 1.0–3.0 h to obtain sodium nitrohumate-calcium magnesium zinc complex sol. B3 Quality Control: The total metal content of the obtained sodium nitrohumate-calcium magnesium zinc complex sol is 2–8 wt%. Based on the sol solid content, the free metal ion content is ≤10% of the total metal content, the sol particle size D50 is 5–30 nm, and the pH is 6.5–7.

5.

5. The method for preparing a nitrohumic acid water-soluble fertilizer as described in claim 1, characterized in that, The preparation method of the NPK-nitrohumic acid metal complex core-shell particle intermediate in step S3 is as follows: Preparation of C1 NPK-based salt particles: 20–35 parts by weight of urea, 10–25 parts by weight of ammonium dihydrogen phosphate, 10–25 parts by weight of potassium sulfate, and 0–15 parts by weight of potassium nitrate were mixed and spray-granulated at 60–90 °C to obtain NPK-based salt particles with a particle size of 1.00–2.00 mm. The total nutrients in the NPK-based salt particles were... Calculated as 45–55 wt%; C2 Core-shell spraying: The NPK matrix salt particles are placed in a fluidized bed granulation device. Under the condition of a bed temperature of 60–80 °C, the sodium nitrohumate-calcium magnesium zinc complex sol obtained in step S2 is used as the spraying liquid. The amount of spraying in a single spray is 3–10 wt% of the mass of the NPK matrix salt particles. After spraying, hot air drying is performed for 1–5 min. The spraying and drying are repeated 5–15 times to form a continuous shell on the particle surface. C3 Curing and Drying: After spraying, cure at 50–70 ℃ for 2–4 h, then dry in a fluidized bed at an inlet air temperature of 70–110 ℃ and a bed temperature of 50–70 ℃ for 10–60 min, until the moisture content of the core-shell particles is 2–4 wt%. C4 quality control: The shell thickness is 5–30 μm, the particle size of the core-shell particles is 1.00–2.50 mm, the mass fraction of sodium nitrohumate in the shell is 20–60 wt%, and the total mass fraction of trace element metal ions is 2–8 wt%.

6. The method for preparing a nitrohumic acid water-soluble fertilizer as described in claim 1, characterized in that, The preparation method of the nitrohumic acid water-soluble fertilizer granules in step S4 is as follows: D1. The NPK-nitrohumic acid metal complex core-shell particle intermediate obtained in step S3, the NPK matrix salt particles, and the sodium nitrohumate powder obtained in step S1 are mixed at a mass ratio of 30–50:10–30:6–15. D2 involves spraying water or adding sodium nitrohumate aqueous solution at 40–70 °C. The solid content of the sodium nitrohumate aqueous solution is 15–30 wt%. The sodium nitrohumate aqueous solution is prepared by dissolving the sodium nitrohumate powder obtained in step S1 in deionized water. The spray volume is 8–12 wt% of the total dry material mass, and granulation is carried out in a fluidized bed or spray granulator. After granulation, D3 is dried in a fluidized bed at an inlet air temperature of 70–110 ℃ and a bed temperature of 40–70 ℃ for 10–60 min until the product moisture content is ≤2.0 wt%. After cooling to ≤40 ℃, it is granulated and sieved to obtain nitrohumic acid water-soluble fertilizer granules with a particle size of 1.00–4.75 mm.

7. The method for preparing a nitrohumic acid water-soluble fertilizer as described in claim 1, characterized in that, Based on the total mass of the nitrohumic acid water-soluble fertilizer, the sum of the oxide equivalents of CaO, MgO and Zn provided by the trace element salts is 2–4 wt%, and the molar ratio of Ca:Mg:Zn in terms of metal moles is 0.5–2.0:0.5–2.0:0.1–1.

0.

8. The method for preparing a nitrohumic acid water-soluble fertilizer as described in claim 1, characterized in that, The nitrohumic acid water-soluble fertilizer, after being sealed and stored for 3 months at 40 ℃ and 75% relative humidity, has an agglomeration rate of ≤5 wt% and a particle compressive strength of ≥12 N per particle.

9. The use of the nitrohumic acid water-soluble fertilizer according to claim 2 in an integrated drip irrigation or sprinkler irrigation system.

10. The use according to claim 9, characterized in that, When the nitrohumic acid water-soluble fertilizer is used for drip irrigation of the roots of fruit trees, vegetables or cash crops, the application rate is 5–30 kg per mu per application cycle, and it is mixed with irrigation water at a mass ratio of 1–3:100–300 before application.

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