Gradient slow-release gypsum composite particle and preparation method thereof

Abstract

The invention belongs to the technical field of fertilizer preparation, and particularly relates to a gradient slow-release gypsum composite particle and a preparation method thereof, the gradient slow-release gypsum composite particle comprises a gypsum core, a humic acid slow-release layer, a phosphate transition layer and a response envelope from inside to outside; the composite particles are provided with response envelopes, and sensitive pore channels are distributed on the surfaces of the response envelopes. In different soil environments, the opening and closing states of the pore channels can be changed, and in acid soil, the pore channels are in a relatively open state, so that the release of nutrients is facilitated; and in neutral or alkaline soil, the pore channels can be partially closed to slow down the release rate of nutrients, and the response characteristic enables the composite particles to automatically adjust the release rate of the nutrients according to the actual pH condition of the soil, so that accurate nutrient supply in different soil environments is realized, and the utilization efficiency of the fertilizer is improved.

Description

Technical Field

[0001] This invention belongs to the field of fertilizer preparation technology, specifically a gradient slow-release gypsum composite granules and its preparation method. Background Technology

[0002] In agriculture and horticulture, fertilizer efficiency has always been a key research focus. Traditional fertilizers often release nutrients rapidly after application, leading to a large release of nutrients in a short period that plants cannot absorb in time, resulting in nutrient loss, environmental pollution, and resource waste. To improve fertilizer utilization, slow-release fertilizers have emerged. Slow-release fertilizers can control the rate of nutrient release, matching it to the plant's growth needs, reducing nutrient loss, and improving fertilizer efficiency.

[0003] A patent with publication number CN104447138A discloses an environmentally friendly sulfur-containing slow-release fertilizer coating. The coating is characterized by comprising a polymer sulfur-containing mixture film layer and a biodegradable polyester film layer. The biodegradable polyester film layer is the inner layer of the coating. The sulfur mass fraction in the polymer sulfur-containing mixture film layer is gradient-distributed, and there is a closed surface surrounding the fertilizer particles with a sulfur mass fraction of 90-99.9%. The key technical point is that the gradient distribution of sulfur mass fraction in the polymer sulfur-containing mixture film layer results in good impact resistance of the coating. As the slowest channel for controlling fertilizer permeation, it also has a longer release period.

[0004] However, the above technologies often have the following drawbacks: Currently, there are many types of slow-release fertilizers on the market, but most of them have some shortcomings; for example, some slow-release fertilizers rely solely on a single coating technology to control nutrient release, and their slow-release effect is greatly affected by environmental factors. Under different soil pH conditions, the nutrient release rate varies greatly, making it difficult to achieve precise nutrient supply; some slow-release fertilizers, although they have certain slow-release properties, have complex preparation processes and high costs, which limit their large-scale promotion and application.

[0005] Therefore, the present invention provides a gradient slow-release gypsum composite particle and its preparation method. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, at least one technical problem raised in the background art is solved.

[0007] The technical solution adopted by this invention to solve its technical problem is: a gradient slow-release gypsum composite particle, comprising, from the inside out:

[0008] The gypsum core is composed of 300-400 mesh anhydrous gypsum powder. Purity ≥ 98%, specific surface area 5000-5500 It has internal pores with a diameter of 50-200. It has a mesoporous structure with a porosity of 45±2%;

[0009] Humic acid slow-release layer: 80-100 mm thick It is composed of humic acid and nano-hydroxyapatite in a 7:3 mass ratio, wherein the fulvic acid content of the humic acid is ≥65%, and the particle size of the nano-hydroxyapatite is 20-30nm. Molar ratio 1.67;

[0010] Phosphate transition layer: 30-50 mm thick It contains a mixture of calcium pyrophosphate and ammonium polyphosphate in a mass ratio of 4:1, wherein the crystallinity of calcium pyrophosphate is 60-70%;

[0011] Response coating: The outermost layer is a chitosan-polylactic acid blend film, with a thickness of 150-200 mm. The chitosan has a degree of deacetylation ≥85%, the polylactic acid has a molecular weight of 100,000-120,000, and the membrane surface is distributed with... Sensitive channel, aperture in At 5.0, it is 50±5. , At 7.0, it was 120±10. .

[0012] A further improvement of the present invention is that the gypsum core further includes 1-2 Nano-sized zero-valent iron with a particle size of 50-80 μm Specific surface area ≥25m² / g;

[0013] The humic acid slow-release layer also includes 0.5-1.0... ammonium molybdate, in It exists in form.

[0014] A further improvement of the present invention is that the... The mass ratio of chitosan to polylactic acid in the response coating is 1:3, and the blending solvent is a mixture of ethyl acetate and dichloromethane with a volume ratio of 1:2.

[0015] The The inner surface of the sensitive channels is grafted with sodium carboxymethyl cellulose at a grafting density of 0.8-1.2 grafts. .

[0016] A further improvement of the present invention is that, In acidic soil with a pH of 5.0, the calcium release rate is 1.2-1.5. The phosphorus release rate is 0.8-1.0. ;

[0017] exist In neutral soil with a pH of 7.0, the calcium release rate decreases to 0.5-0.6. The phosphorus release rate decreased to 0.3-0.4%. ;

[0018] The release of molybdenum lags behind that of calcium and phosphorus by 15-20 days.

[0019] A method for preparing gradient slow-release gypsum composite particles includes the following steps:

[0020] S1. Core preparation: Anhydrous gypsum powder and nano-zero valent iron were ball-milled under nitrogen protection at a ball-to-material ratio of 10:1, a speed of 300 rpm, and a time of 2 hours.

[0021] The granulation process was carried out by spray drying, with an inlet temperature of 180±5℃ and an outlet temperature of 80±3℃, to obtain gypsum cores with a particle size of 1.0-1.5mm.

[0022] S2, Humic Acid Coating: Humic acid, nano-hydroxyapatite, and ammonium molybdate are mixed to form a 25%... The aqueous dispersion was then sonicated at 40 kHz for 30 min.

[0023] The coating was performed in a fluidized bed by bottom spraying, with an atomization pressure of 0.18 MPa, a bed temperature of 75±2℃, and a coating rate of 12-15 mL / min.

[0024] S3. Phosphate layer curing: Dissolve calcium pyrophosphate and ammonium polyphosphate in a 5% citric acid solution, with a solid content of 30%.

[0025] Centrifugal atomization spraying is adopted, with an atomizing disc rotation speed of 15,000 rpm and a particle bed temperature of 60±2℃.

[0026] S4. Preparation of responsive membrane: Chitosan and polylactic acid are dissolved in a mixed solvent, and 0.1 wt% Tween 80 is added as a pore-forming agent;

[0027] Films are formed using a fluidized bed top spraying process with an inlet temperature of 45℃, and the film thickness is controlled to be 150-200nm by the spraying time.

[0028] A further improvement of the present invention is that the atomization pressure of the spray drying in step S1 is 0.25. The atomizer uses a two-fluid nozzle, and the gas / liquid ratio is 1.2:1.

[0029] When the anhydrous gypsum powder is ball-milled under nitrogen protection, a dispersant comprising 0.1-0.5% of the total mass of the anhydrous gypsum powder and nano-zero ferric iron is added to the ball mill jar. The dispersant is sodium hexametaphosphate or a polycarboxylate superplasticizer.

[0030] A further improvement of the present invention is that, in step S2, the prepared 25 Before ultrasonic treatment, the aqueous dispersion is pre-dispersed by stirring at a speed of 500-1000 rpm. The stirring time is 10-20 minutes to allow humic acid, nano-hydroxyapatite and ammonium molybdate to be initially mixed evenly, and then subjected to ultrasonic treatment.

[0031] The power density of ultrasonic treatment is 50 The ultrasound interval ratio is .

[0032] A further improvement of the present invention is that, during the curing of the phosphate layer in step S3, after dissolving calcium pyrophosphate and ammonium polyphosphate in a 5% citric acid solution, a slow stirring method is used when the solid content reaches 30%, with a stirring speed of 200-500 rpm. This is to prevent localized overheating of the solution or the formation of bubbles that could affect the curing effect;

[0033] A further improvement of the present invention is that the formation of the pH-sensitive channel in step S4 includes:

[0034] After film formation, place in an environment with a relative humidity of 90% for 24 hours;

[0035] Passing in sequence 5.0 and Each was treated with 7.0 buffer solution for 1 hour;

[0036] Then, it was vacuum dried at 40°C until constant weight.

[0037] A further improvement of the present invention is that, in step S4, during the preparation of the response membrane, after dissolving chitosan and polylactic acid in a mixed solvent, before adding 0.1 wt% Tween 80 as a pore-forming agent, the solution is first filtered to remove insoluble impurities. The filtration process uses a 0.45 ppm solvent. Microporous filter membrane.

[0038] The beneficial effects of this invention are as follows:

[0039] 1. The composite particles of the present invention are provided with The response membrane has a surface distributed with Sensitive channels. In different In soil environments, the opening and closing state of pores changes. In acidic soils, the pores are relatively open, which is conducive to nutrient release; while in neutral or alkaline soils, the pores partially close, slowing down the rate of nutrient release. The responsive characteristics enable the composite particles to automatically adjust the nutrient release rate according to the actual pH conditions of the soil, achieving precise nutrient supply under different soil conditions and improving fertilizer utilization efficiency. Secondly, by setting up structures such as phosphate transition layers, an ion barrier is formed. This ion barrier can effectively control the release rate of nutrient molecules and avoid the release of large amounts of nutrients in a short period of time. That is, nutrient molecules such as calcium and phosphorus need to pass through the ion barrier to be released into the soil. The presence of the ion barrier makes the nutrient release process more stable and controllable, thereby reducing nutrient loss and waste.

[0040] 2. The components of the composite particles of this invention have a synergistic effect. Specifically, the gypsum core provides the basic nutrient source, while the humic acid slow-release layer not only further regulates nutrient release but also improves soil structure and enhances soil's water and fertilizer retention capacity. The responsive coating controls nutrient release while also regulating the soil microenvironment. The components work together to achieve gradient slow release, improve soil quality, and promote plant growth. Attached Figure Description

[0041] The invention will now be further described with reference to the accompanying drawings.

[0042] Figure 1 This is a flowchart of the preparation process of the present invention. Detailed Implementation

[0043] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] Please see Figure 1 This embodiment provides: a gradient slow-release gypsum composite particle, comprising, from the inside out:

[0045] The gypsum core is composed of 300-400 mesh anhydrous gypsum powder. Purity ≥ 98%, specific surface area 5000-5500 It has internal pores with a diameter of 50-200. It has a mesoporous structure with a porosity of 45±2%;

[0046] The plaster core also includes 1-2 Nano-sized zero-valent iron with a particle size of 50-80 μm , with a specific surface area ≥25m² / g, and mixed by mass ratio;

[0047] It should be noted that anhydrous gypsum powder is a carrier of nutrients such as calcium and sulfur. When it is ball-milled with nano-zero ferric iron under nitrogen protection, the nitrogen protection can prevent the nano-zero ferric iron from being oxidized and ensure its activity. The ball milling process can fully mix the anhydrous gypsum powder and nano-zero ferric iron, and the mechanical action of ball milling can change the particle size and surface properties of the material. The small size effect and surface effect of nano-zero ferric iron may help regulate the subsequent nutrient release behavior, and may also participate in some redox reactions in the soil, improving the soil microenvironment.

[0048] Humic acid slow-release layer: 80-100 mm thick It is composed of humic acid and nano-hydroxyapatite in a 7:3 mass ratio, wherein the fulvic acid content of the humic acid is ≥65%, and the particle size of the nano-hydroxyapatite is 20-30nm. Molar ratio 1.67;

[0049] The humic acid slow-release layer also includes 0.5-1.0... ammonium molybdate, in Form exists;

[0050] It should be noted that humic acid is a natural organic macromolecule with excellent ion exchange and adsorption capabilities. When formulated into an aqueous dispersion with nano-hydroxyapatite and ammonium molybdate, it coats the core surface. Nano-hydroxyapatite can exchange with some ions in the soil, regulating the nutrient release rate; humic acid can adsorb nutrient ions, slowing their release, while simultaneously improving soil aggregate structure and enhancing its water and fertilizer retention capacity; ammonium molybdate, as a micronutrient fertilizer, provides molybdenum to plants and may participate in nitrogen metabolism and other related biochemical processes in the soil. Ultrasonic treatment further disperses the components more evenly, enhancing the coating effect.

[0051] Phosphate transition layer: 30-50 mm thick It contains a mixture of calcium pyrophosphate and ammonium polyphosphate in a mass ratio of 4:1, wherein the crystallinity of calcium pyrophosphate is 60-70%;

[0052] It should be noted that the phosphate transition layer formed by calcium pyrophosphate and ammonium polyphosphate acts as an ion barrier. Calcium pyrophosphate has a certain crystalline structure, while ammonium polyphosphate can dissolve and provide phosphate ions. The layer formed after the two are mixed can control the diffusion rate of nutrient ions. When the composite particles are in the soil, nutrient ions such as calcium and phosphorus need to pass through this ion barrier to be released into the soil. The existence of the ion barrier makes the nutrient release process more stable and controllable, avoiding the loss and waste caused by the large release of nutrients in a short period of time.

[0053] Furthermore, the process by which calcium pyrophosphate and ammonium polyphosphate form an ion barrier includes:

[0054] When calcium pyrophosphate and ammonium polyphosphate are mixed in a specific solvent or environment, the ammonium polyphosphate partially dissolves, releasing phosphate ions, while the calcium pyrophosphate may exist in a slightly soluble or suspended form.

[0055] The phosphate ions produced by the dissolution of ammonium polyphosphate interact with the calcium ions of calcium pyrophosphate. This interaction includes adsorption, encapsulation, or the formation of new compounds or complexes.

[0056] As the mixture dries or solidifies, the mixture of calcium pyrophosphate and ammonium polyphosphate gradually forms a continuous or discontinuous film layer, i.e., an ion barrier, on the surface of the gypsum core.

[0057] It should be noted that the formation principle of the ion barrier is as follows: the dissolution products of calcium pyrophosphate particles and ammonium polyphosphate may form a physical barrier on the surface of the gypsum core. This barrier has a small pore structure, which can restrict the passage of larger ions or molecules.

[0058] Secondly, both calcium pyrophosphate and ammonium polyphosphate have certain ion exchange and adsorption capabilities. They can adsorb cations or anions in the soil and regulate the nutrient release rate through ion exchange.

[0059] Furthermore, calcium pyrophosphate is chemically stable and does not readily react with other substances in the soil. While ammonium polyphosphate can dissolve and release phosphate ions, these ions may combine with other substances in the soil to form stable compounds, thus slowing down nutrient release.

[0060] Meanwhile, the solubility of ammonium polyphosphate and the ion exchange capacity of calcium pyrophosphate may be affected by soil pH. In acidic soils, ammonium polyphosphate may dissolve more easily and release phosphate ions, while the ion exchange capacity of calcium pyrophosphate may be enhanced; in neutral or alkaline soils, the opposite may be true. This pH responsiveness helps the composite particles automatically adjust the nutrient release rate according to soil conditions.

[0061] Response coating: The outermost layer is a chitosan-polylactic acid blend film, with a thickness of 150-200 mm. The chitosan has a degree of deacetylation ≥85%, the polylactic acid has a molecular weight of 100,000-120,000, and the membrane surface is distributed with... Sensitive channel, aperture in At 5.0, it is 50±5. , At 7.0, it was 120±10. ;

[0062] The mass ratio of chitosan to polylactic acid in the response coating is 1:3, and the blending solvent is a mixture of ethyl acetate and dichloromethane with a volume ratio of 1:2.

[0063] The inner surface of the sensitive channels is grafted with sodium carboxymethyl cellulose at a grafting density of 0.8-1.2 grafts.

[0064] exist In acidic soil with a pH of 5.0, the calcium release rate is 1.2-1.5. The phosphorus release rate is 0.8-1.0. ;

[0065] exist In neutral soil with a pH of 7.0, the calcium release rate decreases to 0.5-0.6. The phosphorus release rate decreased to 0.3-0.4%. ;

[0066] The release of molybdenum lags behind that of calcium and phosphorus by 15-20 days.

[0067] It should be noted that chitosan-polylactic acid blend membranes are used as... The response membrane, its surface distribution Sensitive pores on soil The pores are sensitive to temperature; in acidic soils, changes in the chemical structure or charge of the pores cause them to remain relatively open, facilitating nutrient release. Conversely, in neutral or alkaline soils, alterations in the structure or charge of the pores lead to partial closure, slowing the nutrient release rate. The responsive characteristics enable the composite particles to adapt to the actual soil conditions. The system automatically adjusts nutrient release to meet the nutrient requirements of plants under different soil conditions and improves fertilizer utilization efficiency. At the same time, the mixed solvent of ethyl acetate and dichloromethane, along with Tween 80 as a pore-forming agent, helps to form a responsive membrane with specific pore size and properties.

[0068] Furthermore, the delayed release of molybdenum is due to the fact that ammonium molybdate readily forms chemical bonds or adsorption reactions with humic acid or other components in the humic acid slow-release layer. Humic acid contains various functional groups, such as carboxyl groups and phenolic hydroxyl groups, which can form coordinate bonds or other types of chemical bonds with molybdenum ions in ammonium molybdate, thus firmly fixing molybdenum in the humic acid slow-release layer. In contrast, calcium mainly exists in the gypsum core, and its binding with the surrounding matrix may be relatively weak, making it easier to release under the influence of the soil environment. Phosphorus is in the phosphate transition layer. Although the phosphate transition layer acts as a barrier, the dissolution and diffusion of components such as ammonium polyphosphate in the soil are relatively easy, allowing phosphorus to be released relatively quickly.

[0069] Furthermore, the strong interaction between molybdenum and components such as humic acid means that it requires a longer time and stronger external conditions, such as the action of soil microorganisms and chemical changes in the soil solution, to break this bond or adsorption state and thus achieve release.

[0070] Secondly, ammonium molybdate molecules are relatively large and have a complex structure, resulting in a relatively slow diffusion rate within the composite particles and in the soil. In contrast, calcium and phosphorus compounds are relatively small and move more rapidly through soil pores. For example, calcium ions migrate quickly in the soil solution via ion channels or diffusion, and phosphorus ions or compounds diffuse relatively quickly with water movement. Due to their larger size, ammonium molybdate molecules experience greater steric hindrance, leading to a slower diffusion process from the composite particles to the surface and from the surface into the soil solution, thus delaying the release of molybdenum.

[0071] On the other hand, soil pH, ionic strength, and microbial activity significantly influence molybdenum release. In soil environments, the release of calcium and phosphorus is relatively directly responsive to soil conditions; for example, in acidic soils, structural changes in the phosphate transition layer rapidly affect phosphorus release. The dissolution and migration of calcium in soil solution are also closely related to soil pH and other conditions. However, molybdenum release may be regulated by more complex soil factors. Certain soil microorganisms may participate in the transformation and release of molybdenum, but the growth and metabolic activities of these microorganisms require time, leading to a relatively delayed release. Furthermore, competition from other ions in the soil can also affect molybdenum release, making it take longer to reach a certain concentration in the soil solution.

[0072] It should be noted that the delayed release of molybdenum aligns with the nutrient requirements of plants: the amount and proportion of various nutrients required by plants differ at different growth stages. In the early stages of plant growth, the demand for macronutrients such as calcium and phosphorus is relatively high. The rapid release of these elements in the early stages can provide basic support for plant growth and promote physiological processes such as root development and cell division. As a micronutrient, molybdenum is required by plants in relatively small and stable amounts during growth. The delayed release of molybdenum ensures that at specific stages of plant growth, when the demand for molybdenum increases, such as during the reproductive growth stage, there is an adequate supply of molybdenum to meet the needs of plant growth and development, avoiding waste or potential adverse effects on plants due to excessive release in the early stages.

[0073] The delayed release of molybdenum facilitates improved fertilizer utilization efficiency: If all nutrients are released rapidly at the same time, some nutrients may not be fully absorbed by the plants in the short term and will be lost, resulting in low fertilizer utilization efficiency. The delayed release of molybdenum allows the nutrient release curve to better match the plant nutrient absorption curve. In this way, molybdenum can be absorbed and utilized by the plants at a more appropriate time, reducing the loss of nutrients such as fixation and leaching in the soil, improving the utilization efficiency of molybdenum, and also improving the overall utilization efficiency of compound granular fertilizers, thus reducing agricultural production costs.

[0074] Delayed release of molybdenum helps reduce the risk of environmental pollution: Excessive molybdenum entering soil and water bodies may cause environmental pollution. Delayed release of molybdenum can control its concentration in the soil and avoid excessively high concentrations in the short term. This can reduce the adverse effects of molybdenum on soil microbial communities, maintain the balance and stability of the soil ecosystem, and at the same time reduce the risk of molybdenum entering water bodies through runoff or seepage, thus protecting water quality.

[0075] A method for preparing gradient slow-release gypsum composite particles includes the following steps:

[0076] S1. Core preparation: Anhydrous gypsum powder and nano-zero valent iron were ball-milled under nitrogen protection at a ball-to-material ratio of 10:1, a speed of 300 rpm, and a time of 2 hours.

[0077] The granulation process was carried out by spray drying, with an inlet temperature of 180±5℃ and an outlet temperature of 80±3℃, to obtain gypsum cores with a particle size of 1.0-1.5mm.

[0078] Furthermore, the atomization pressure for spray drying is 0.25. The atomizer uses a two-fluid nozzle, and the gas / liquid ratio is 1.2:1.

[0079] Furthermore, during the ball milling process of anhydrous gypsum powder under nitrogen protection, a dispersant comprising 0.1-0.5% of the total mass of the anhydrous gypsum powder and nano-zero valent iron is added to the ball mill jar. The dispersant is sodium hexametaphosphate or a polycarboxylate-based water-reducing agent.

[0080] It should be noted that the dispersant is added to ensure that the anhydrous gypsum powder and nano-zero valent iron are mixed more evenly during the ball milling process, preventing agglomeration.

[0081] S2, Humic Acid Coating: Humic acid, nano-hydroxyapatite, and ammonium molybdate are mixed to form a 25%... The aqueous dispersion was then sonicated at 40 kHz for 30 min.

[0082] The coating was performed in a fluidized bed by bottom spraying, with an atomization pressure of 0.18 MPa, a bed temperature of 75±2℃, and a coating rate of 12-15 mL / min.

[0083] Furthermore, in step S2, the prepared 25 Before ultrasonic treatment, the aqueous dispersion is pre-dispersed by stirring at a speed of 500-1000 rpm. The stirring time is 10-20 minutes to allow humic acid, nano-hydroxyapatite and ammonium molybdate to be initially mixed evenly, and then subjected to ultrasonic treatment.

[0084] The power density of ultrasonic treatment is 50 The ultrasound interval ratio is ;

[0085] It should be noted that the pre-dispersion process allows humic acid, nano-hydroxyapatite, and ammonium molybdate to be initially and uniformly mixed in the aqueous dispersion, providing favorable preconditions for subsequent ultrasonic treatment. Sufficient pre-dispersion and appropriate ultrasonic treatment are crucial for ensuring a uniform humic acid layer coats the gypsum core surface. Only with uniform dispersion of the components can a uniform humic acid layer be formed during the coating process, thereby better leveraging the role of humic acid in regulating nutrient release and improving soil structure.

[0086] S3. Phosphate layer curing: Dissolve calcium pyrophosphate and ammonium polyphosphate in a 5% citric acid solution, with a solid content of 30%.

[0087] Centrifugal atomization spraying is adopted, with an atomizing disc rotation speed of 15,000 rpm and a particle bed temperature of 60±2℃.

[0088] Furthermore, during the curing of the phosphate layer in step S3, after dissolving calcium pyrophosphate and ammonium polyphosphate in a 5% citric acid solution, a slow stirring method is used when the solid content reaches 30%, with a stirring speed of 200-500 rpm. This is to prevent localized overheating of the solution or the formation of bubbles that could affect the curing effect;

[0089] It should be noted that a suitable stirring speed can ensure that the solution is mixed evenly, avoid local overheating or the generation of bubbles, thereby ensuring good solidification of the phosphate layer, forming an effective ion barrier, and controlling the release rate of nutrient ions.

[0090] S4. Preparation of responsive membrane: Chitosan and polylactic acid are dissolved in a mixed solvent, and 0.1 wt% Tween 80 is added as a pore-forming agent;

[0091] Films are formed using a fluidized bed top spraying process with an inlet temperature of 45℃, and the film thickness is controlled to be 150-200nm by the spraying time.

[0092] Furthermore, the formation of pH-sensitive channels in step S4 includes:

[0093] After film formation, place in an environment with a relative humidity of 90% for 24 hours;

[0094] Passing in sequence 5.0 and Each was treated with 7.0 buffer solution for 1 hour;

[0095] Then, vacuum dry at 40°C until constant weight is achieved;

[0096] Furthermore, in step S4, during the preparation of the responsive membrane, after dissolving chitosan and polylactic acid in a mixed solvent, the solution is filtered before adding 0.1 wt% Tween 80 as a pore-forming agent to remove insoluble impurities. The filtration process uses a 0.45 ppm solvent. Microporous filter membrane.

[0097] It should be noted that precise control The formation process of sensitive channels is crucial for ensuring the good performance of the response membrane. The key to responsive performance lies in filtration, which improves solution purity and ensures stable quality of the prepared responsive membrane, thereby accurately achieving the desired response based on soil conditions. It has the function of regulating nutrient release.

[0098] Example 1

[0099] A method for preparing gradient slow-release gypsum composite particles includes the following steps:

[0100] S1. Core preparation: Take 350g of anhydrous gypsum powder and 35g of nano zero-valent iron, and ball mill them at 300rpm for 2h under nitrogen protection at a ball-to-material ratio of 10:1 to obtain gypsum cores with a particle size of about 1.2mm.

[0101] S2, Humic Acid Coating: Weigh 19g of humic acid, 8.14g of nano-hydroxyapatite, and 0.34g of ammonium molybdate, and prepare a 25... The aqueous dispersion was first pre-dispersed by stirring at 750 rpm for 15 minutes, and then at a power density of 50 W / cm². The sample was treated with ultrasonic parameters for 30 min, followed by treatment in a 40 kHz ultrasonic environment for 30 min. Then, it was coated onto the core in a fluidized bed by bottom spraying, with an atomization pressure of 0.18 MPa, a bed temperature of 75 ℃, and a coating rate of 13.5 mL / min.

[0102] S3. Phosphate layer curing: Dissolve calcium pyrophosphate and ammonium polyphosphate in a 4:1 ratio in a 5% citric acid solution, with a solid content of 30%, and coat the humic acid layer with the solution while stirring at 350 rpm.

[0103] S4. Preparation of the responsive membrane: Chitosan and polylactic acid were dissolved in a 1:3 ratio in a mixed solvent of ethyl acetate and dichloromethane in a 1:2 volume ratio, and 0.1... Tween 80 film was formed using a fluidized bed top spraying process, with an inlet temperature of 45℃ and a film thickness controlled at 175 mm. .

[0104] Example 2

[0105] A method for preparing gradient slow-release gypsum composite particles includes the following steps:

[0106] S1. Core preparation: Take 320g of anhydrous gypsum powder and 32g of nano zero-valent iron, and ball mill them at 300rpm for 2h under nitrogen protection at a ball-to-particle ratio of 10:1 to obtain gypsum cores with a particle size of about 1.1mm.

[0107] S2, Humic Acid Coating: Weigh 19g of humic acid, 8.14g of nano-hydroxyapatite, and 0.34g of ammonium molybdate, and prepare a 25... The aqueous dispersion was first pre-dispersed by stirring at 750 rpm for 15 minutes, and then at a power density of 50 W / cm². The sample was treated with ultrasonic parameters for 30 min, followed by treatment in a 40 kHz ultrasonic environment for 30 min. Then, it was coated onto the core in a fluidized bed by bottom spraying, with an atomization pressure of 0.18 MPa, a bed temperature of 75 ℃, and a coating rate of 13.5 mL / min.

[0108] S3. Phosphate layer curing: Dissolve calcium pyrophosphate and ammonium polyphosphate in a 4:1 ratio in a 5% citric acid solution, with a solid content of 30%, and coat the humic acid layer with the solution while stirring at 350 rpm.

[0109] S4. Preparation of the responsive membrane: Chitosan and polylactic acid were dissolved in a 1:3 ratio in a mixed solvent of ethyl acetate and dichloromethane in a 1:2 volume ratio, and 0.1... Tween 80 film was formed using a fluidized bed top spraying process, with an inlet temperature of 45℃ and a film thickness controlled at 175 mm. .

[0110] Example 3

[0111] A method for preparing gradient slow-release gypsum composite particles includes the following steps:

[0112] S1. Core preparation: Weigh 330g of anhydrous gypsum powder and 33g of nano zero-valent iron, and ball mill them at 300rpm for 2h under nitrogen protection at a ball-to-powder ratio of 10:1 to obtain gypsum cores with a particle size of approximately 1.15mm.

[0113] S2. Humic Acid Layer Coating: 18g of humic acid, 7.71g of nano-hydroxyapatite, and 0.33g of ammonium molybdate were prepared into a 25wt% aqueous dispersion. Pre-dispersion was achieved by stirring at 700rpm for 18min. Subsequently, the dispersion was further prepared under conditions of 50W / cm² power density and an ultrasonic intermittent ratio of [missing value]. Under the conditions of ultrasonic treatment for 30 min, it was then treated for another 30 min in an ultrasonic environment of 40 kHz. After that, it was coated onto the gypsum core by bottom spraying in a fluidized bed. The atomization pressure was set to 0.18 MPa, the bed temperature was controlled at 75 ℃, and the coating rate was 13 mL / min.

[0114] S3. Phosphate layer solidification: Dissolve calcium pyrophosphate and ammonium polyphosphate in a 4:1 mass ratio in a 5% citric acid solution, with a solid content of 30%, and coat the humic acid layer with a stirring speed of 340 rpm.

[0115] S4. Preparation of the responsive membrane: Chitosan and polylactic acid were dissolved in a mixed solvent of ethyl acetate and dichloromethane at a mass ratio of 1:3 (v / v), and 0.1 g of the mixture was added. Tween 80 was used as a pore-forming agent to form a film via a fluidized bed top spray process, with an inlet temperature of 45°C and a film thickness controlled at 170 mm. .

[0116] Comparative Example 1

[0117] Prepared according to the method of Example 1, but without the responsive membrane preparation step, while keeping other steps unchanged.

[0118] Comparative Example 2

[0119] Prepared according to the method of Example 2, but the phosphate layer curing step was omitted in the preparation process, while the other steps remained unchanged.

[0120] Comparative Example 3

[0121] Prepared according to the method of Example 3, but the humic acid layer coating step was omitted during the preparation process, while the other steps remained unchanged.

[0122] The composite particles obtained in Examples 1, 2, 3, Comparative Example 1, Comparative Example 2, and Comparative Example 3 were respectively placed in... 5.0 acidic soil and Nutrient release rates were measured in neutral soil with a pH of 7.0, and the results are shown in Table 1.

[0123] Table 1

[0124]

[0125] Based on the data in the table above, we can see that:

[0126] The average release rate of Example 1 in both acidic and neutral soils was lower than that of Comparative Example 1, indicating that the presence of the responsive membrane can effectively slow down the release rate of nutrients, thereby providing a more sustained release effect.

[0127] Comparative Example 1, where the release rate of the non-pH-responsive membrane increased more significantly in acidic soil, indicates that the responsive membrane plays a more important role in controlling nutrient release under acidic conditions.

[0128] The average release rate of Example 2 was lower than that of Comparative Example 2 in both acidic and neutral soils, indicating that the presence of the phosphate layer helps to slow down the release of nutrients.

[0129] Comparative Example 2, which omitted the phosphate layer, showed that the release rate of nutrients increased in both acidic and neutral soils, especially in acidic soils. This suggests that the phosphate layer has a more significant effect on nutrient fixation in acidic environments.

[0130] The average release rate of Example 3 was lower than that of Comparative Example 3 in both acidic and neutral soils, indicating that the presence of the humic acid layer helps control the nutrient release rate.

[0131] The release rate of humic acid was higher in both acidic and neutral soils when the humic acid buffer layer was omitted (i.e., in Comparative Example 3), especially in acidic soils. This indicates that the buffering effect of the humic acid layer on nutrients is more important in acidic environments.

[0132] The release rates of Examples 1, 2, and 3 in acidic and neutral soils were relatively similar, indicating that gradient slow-release gypsum composite particles with different formulations can effectively control the release of nutrients.

[0133] In acidic soils, the average release rate of the samples was generally higher than that in neutral soils, which may be because the acidic environment promotes the dissolution and release of nutrients.

[0134] Conclusion: The design of gradient slow-release gypsum composite particles effectively controls the nutrient release rate through a multi-layer coating structure. The responsive membrane, phosphate layer, and humic acid layer each play an important role in slow release under different soil conditions. Omitting any of these layers will lead to an increase in the nutrient release rate, especially in acidic soils. Therefore, this design of gradient slow-release gypsum composite particles is reasonable and effective, and can provide sustained nutrient release according to soil conditions.

[0135] In summary: the composite particles of Examples 1-3 are in different... The release rates of calcium and phosphorus nutrients in soil differ significantly, with higher release rates in acidic soils and lower rates in neutral soils. This indicates that... The gradient slow-release mechanism formed by the combined action of the coating and phosphate transition layer can be adjusted according to soil conditions. Automatic regulation of nutrient release enables precise nutrient supply and improves fertilizer utilization efficiency;

[0136] Secondly, in Comparative Example 3, after omitting the humic acid slow-release layer, the release rates of calcium and phosphorus nutrients in both acidic and neutral soils were significantly higher than in Examples 1-3. This indicates that the humic acid slow-release layer can effectively regulate nutrient release, slow down the release rate of nutrients in the short term, and prevent nutrient loss. Humic acid also has the effect of improving soil structure and enhancing soil's ability to retain fertilizer and water. Its absence will affect the effect of the composite particles on improving soil quality.

[0137] The terms "front," "back," "left," "right," "top," and "bottom" all refer to the figures in the accompanying drawings. Figure 1 Based on the perspective of the observer, the side of the device facing the observer is defined as the front, the left side of the observer is defined as the left, and so on.

[0138] In the description of this invention, it should be understood that the terms "center", "longitudinal", "lateral", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the scope of protection of this invention.

[0139] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A gradient-released gypsum composite particle, characterized by: From inside to outside includes: Gypsum core: composed of 300-400 mesh anhydrous gypsum powder, Purity > 98%, specific surface area 5000-5500 , internal mesoporous structure with pore size 50-200 Porosity 45 ± 2%; Humic acid slow-release layer: thickness 80-100 The humic acid slow-release layer is composed of humic acid and nano-hydroxyapatite at a mass ratio of 7:3, wherein the content of fulvic acid in the humic acid is greater than or equal to 65%, and the particle size of the nano-hydroxyapatite is 20-30 nm. Molar ratio 1.67; Phosphate transition layer: thickness 30-50 comprising a mixture of calcium pyrophosphate and ammonium polyphosphate in a mass ratio of 4:1, wherein the calcium pyrophosphate has a crystallinity of 60-70%; Response envelope: the outermost layer is a chitosan-polylactic acid blend film, thickness 150-200 , wherein the degree of deacetylation of chitosan is greater than or equal to 85%, the molecular weight of polylactic acid is 100-120 thousand, and the surface of the film is distributed with sensitive channels, with a pore size of 50±5 when 5.0, 120±10 when 7.

0.

2. The gradient-released gypsum composite particle according to claim 1, characterized by: The gypsum inner core further comprises 1-2 nanometer zero-valent iron with a particle size of 50-80 m² / g and a specific surface area of ≥25 m² / g. The humic acid slow-release layer further comprises 0.5-1.0 of ammonium molybdate, in the form of .

3. The gradient-released gypsum composite particle according to claim 1, characterized by: The The mass ratio of the shell of the envelope chitosan to polylactic acid is 1:3, the blending solvent is a mixture of ethyl acetate and dichloromethane, and the volume ratio of the mixture of ethyl acetate and dichloromethane is 1:

2. The The inner surface of the sensitive channel is grafted with sodium carboxymethyl cellulose, and the grafting density is 0.8-1.2 .

4. The gradient-released gypsum composite particle according to claim 1, characterized by: In 5.0, the release rate of calcium element is 1.2-1.5 , the release rate of phosphorus element is 0.8-1.0 ; In 7. In neutral soil of 7.0, the release rate of calcium element is reduced to 0.5-0.6 , and the release rate of phosphorus element is reduced to 0.3-0.4 ; The release of molybdenum element lags behind calcium and phosphorus elements for 15-20 days.

5. A method for preparing the gradient-released gypsum composite particle according to any one of claims 1 to 4, wherein The method comprises the following steps: S1, core preparation: anhydrous gypsum powder and nano zero-valent iron are mixed under nitrogen protection by ball milling, the ball-to-material ratio is 10:1, the rotation speed is 300 rpm, and the time is 2 h; Granulation is performed by using a spray drying method, the inlet temperature is 180±5℃, the outlet temperature is 80±3℃, and the gypsum core with a particle size of 1.0-1.5 mm is obtained; S2, humic acid layer coating: humic acid, nano-hydroxyapatite and ammonium molybdate are mixed into 25 aqueous dispersion, and then ultrasonic treatment is performed for 30 min under the condition of 40 kHz; Coating is performed in a fluidized bed by using a bottom spraying mode, the atomization pressure is 0.18 MPa, the bed temperature is 75±2℃, and the coating rate is 12-15 mL / min; S3, phosphate layer solidification: calcium pyrophosphate and ammonium polyphosphate are dissolved in a 5% citric acid solution, and the solid content is 30%; Centrifugal atomization spraying is adopted, the atomization disc rotation speed is 15000 rpm, and the particle bed temperature is 60±2℃; S4, response film preparation: chitosan and polylactic acid are dissolved in a mixed solvent, and 0.1wt% Tween 80 is added as a pore-forming agent; Film formation is performed by using a fluidized bed top spraying process, the inlet temperature is 45℃, and the film thickness is controlled to be 150-200 nm by spraying time.

6. The method of claim 5, wherein the method is characterized by: The atomizing pressure of the spray drying in the S1 step is 0.25 The atomizer uses a two-fluid nozzle, and the gas / liquid ratio is 1.2:

1. When the anhydrous gypsum powder is mixed by ball milling under nitrogen protection, 0.1-0.5% of a dispersing agent accounting for the total mass of the anhydrous gypsum powder and the nano zero-valent iron is further added into the ball milling tank, and the dispersing agent is sodium hexametaphosphate or a polycarboxylate-based water reducing agent.

7. The method of claim 5, wherein the method is characterized by: The prepared 25 The water dispersion is stirred and pre-dispersed before ultrasonic treatment, the stirring speed is 500-1000 The stirring time is 10-20 min, so that the humic acid, nano-hydroxyapatite and ammonium molybdate are initially mixed uniformly, and then ultrasonic treatment is performed. The power density of the ultrasonic treatment was 50 , and the ultrasonic intermittent ratio was .

8. The method of claim 5, wherein the method is characterized by: In the S3 step of curing the phosphate layer, the calcium pyrophosphate and the ammonium polyphosphate are dissolved in a 5% citric acid solution, and in the process of reaching a solid content of 30%, a slow stirring method is adopted, with a stirring speed of 200-500 , to prevent local overheating of the solution or the generation of bubbles affecting the curing effect.

9. The method of claim 5, wherein the method is characterized by: The formation of the pH-sensitive pores in the S4 step comprises: After film formation, the product is placed in a relative humidity of 90% for 24 h; sequentially through 5.0 and 7.0 buffer for 1 h each; Then vacuum drying is performed at 40℃ until the constant weight is obtained.

10. The method of claim 5, wherein the method is characterized by: In the S4 step, in response to the membrane preparation, the chitosan and polylactic acid are dissolved in a mixed solvent, and before adding 0.1 wt% Tween 80 as a pore forming agent, the solution is first filtered to remove insoluble impurities, and the filtration uses a 0.45 microporous filter membrane. In the S4 step, in response to the membrane preparation, the chitosan and polylactic acid are dissolved in a mixed solvent, and before adding 0.1 wt% Tween 80 as a pore forming agent, the solution is first filtered to remove insoluble impurities, and the filtration uses a 0.45 microporous filter membrane.

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