Modified attapulgite coated slow release fertilizer and preparation method thereof

Abstract

The invention discloses a modified attapulgite coated slow-release fertilizer and a preparation method thereof, and belongs to the technical field of slow-release fertilizers. Aiming at the problems of non-biodegradability, complex multi-layer coating process, high production cost, single hydrophilic and hydrophobic property and the like of the existing polyurethane coated slow-release fertilizer, the invention innovatively adopts heat modified attapulgite as a slow-release carrier, increases the specific surface area through calcination treatment, prepares the attapulgite into a coating liquid, and improves the slow-release effect of the coating liquid. The use amount of attapulgite can be effectively reduced, and a uniform and compact film is formed on the surface of the fertilizer to construct a slow-release composite layer. Through a simple, efficient and low-cost production process, the industrial production threshold is reduced, a first-stage release kinetic model is established, the experimental period can be effectively shortened by utilizing the mathematical model, and an efficient mathematical tool is provided for product mechanism research and product development. And a theoretical basis is provided for combination of a slow-release carrier and digital management, field intelligent fertility management and precise fertilization.

Description

Technical Field

[0001] This invention relates to the field of slow-release fertilizer technology, specifically to a coated slow-release fertilizer using modified attapulgite as a slow-release carrier and its preparation method. Background Technology

[0002] my country is the world's largest user of chemical fertilizers, accounting for over 50% of agricultural inputs. Since the 1980s, the growth rate of fertilizer consumption has consistently exceeded 90% of the global average. Long-term agricultural production has been plagued by a single fertilization structure and excessive application, with average fertilizer utilization rates only around 30% (30%–35% for N, 10%–20% for P2O5, and 35%–50% for K2O). This directly leads to severe nutrient losses: significant volatilization losses and a lack of synchronization between nutrient release patterns and crop nutrient absorption patterns; soil ecological degradation: over-reliance on chemical fertilizers leads to a decline in soil organic matter, causing soil acidification, soil compaction, and reduced crop quality; and water pollution is exacerbated: unused nitrogen and phosphorus enter water bodies through surface runoff and leaching, intensifying eutrophication.

[0003] Currently, slow-release fertilizers, as a key tool for improving agricultural productivity and a core technological support for achieving green agricultural transformation, not only provide long-lasting nutrition for crop growth, thereby increasing crop yield, but also reduce nutrient loss caused by volatilization and leaching, effectively improve the soil environment, are easy to apply, and are conducive to mobilizing farmers' enthusiasm and promoting sustainable and high-level agricultural development. Coated slow-release fertilizers are a new type of fertilizer that achieves slow nutrient release by coating fertilizer granules with one or more layers of functional film material, which can reduce environmental pollution and improve fertilizer utilization.

[0004] However, existing coating technologies still have significant drawbacks. First, they suffer from poor environmental compatibility: synthetic polymers such as polyurethane (PU) are non-biodegradable, leaving residues that pollute the soil. Production costs are high, and their raw material, polyisocyanate (MDI), relies on petrochemical products and is non-renewable, making it environmentally unfriendly. Second, there are economic and technological bottlenecks: multi-layer coatings are costly, and increasing the number of layers leads to a higher skin-to-core ratio, reducing the proportion of effective nutrients in the fertilizer and decreasing economic efficiency. Some processes require ultrasound or specialized equipment, resulting in high industrialization barriers. Third, they suffer from limited functionality: while pure polyurethane (PU) membranes have good mechanical properties, their hydrophilicity / hydrophobicity is limited, making it impossible to simultaneously achieve both slow release and water retention of fertilizer.

[0005] To solve the above problems, there is an urgent need for a new type of coated slow-release fertilizer that is environmentally friendly and has a simplified process. Summary of the Invention

[0006] The purpose of this invention is to solve the problems of poor degradability, short nutrient release period, insufficient water retention and high production cost of existing coated slow-release fertilizers, and to provide an environmentally friendly and simplified single-layer coated slow-release fertilizer.

[0007] The following technical solution is adopted:

[0008] In a first aspect, this invention provides a method for preparing a modified attapulgite-coated slow-release fertilizer. Specific steps include:

[0009] (1) Preparation of ordinary compound fertilizer granules: urea, superphosphate and potassium chloride are ground and mixed in a mass ratio, polyvinyl alcohol solution is added as a binder, granulated in a granulator, and dried to obtain compound fertilizer granules.

[0010] (2) Preparation of thermally modified attapulgite: Natural attapulgite is calcined at high temperature for 2-4 hours, ground and sieved to obtain modified attapulgite;

[0011] (3) Preparation of coating solution: Heat-modified attapulgite, coating binder, maleic anhydride-sodium p-styrene sulfonate copolymer solution and deionized water are mixed in proportion and subjected to high-speed stirring and ultrasonic treatment to form a homogeneous coating solution;

[0012] (4) Preparation of coated slow-release fertilizer: After surface activation treatment, the compound fertilizer granules are placed in a sugar coating machine and coated with coating liquid in 6-8 times. After spraying, they are dried with hot air. The final amount of coating liquid is 3%-5% of the total mass of the granules. After solidification, coated slow-release fertilizer granules are obtained.

[0013] Preferably, the mass ratio of urea, superphosphate, and potassium chloride in step (1) is 5:3:2;

[0014] Preferably, the specific steps of granulation in step (1) are as follows: dry mixing at 200-300 rpm for 1-3 minutes to ensure that the raw materials are fully and evenly dispersed; wet granulation at 1000-1200 rpm for 1-3 minutes to promote the adhesion of the binder (polyvinyl alcohol solution) to the particles through shearing force to form uniform particles.

[0015] Preferably, the specific steps of drying in step (1) are to dry in a dryer at 60-80°C for 30 minutes to remove moisture.

[0016] Preferably, the high-temperature calcination temperature in step (2) is 150-250℃.

[0017] Preferably, the coating solution formulation ratio in step (3) is as follows: by mass, 5-15 parts of thermally modified attapulgite, 1-5 parts of coating adhesive, 15-25 parts of maleic anhydride-sodium p-styrene sulfonate copolymer solution, and 55-80 parts of deionized water.

[0018] Preferably, the coating adhesive in step (3) is at least one of carboxymethyl cellulose, hydroxypropyl starch, starch acetate, gum arabic, guar gum, xanthan gum, ethyl cellulose, hydroxypropyl methyl cellulose, chitosan, sodium alginate, polyvinyl alcohol (PVA), polyethylene glycol (PEG), and polyacrylamide (PAM).

[0019] Preferably, the coating preparation conditions in step (4) are: rotation speed 600-800 RPM, tilt angle 30°, internal heating temperature 50-60°C, spray amount per spray is 0.5%-1% of particle mass, and hot air drying at 50-60°C for 5 minutes after spraying.

[0020] In a second aspect, the present invention provides a modified attapulgite-coated slow-release fertilizer obtained by the above preparation method.

[0021] Preferably, the coating thickness of the modified attapulgite soil-coated slow-release fertilizer is 80-110 μm.

[0022] Beneficial effects

[0023] This invention utilizes attapulgite-based materials to prepare a novel coated slow-release fertilizer. Attapulgite soil is formulated into a coating solution, effectively reducing the amount of attapulgite soil required. A uniform and dense film is formed on the fertilizer surface to construct a slow-release composite layer. This effectively solves problems such as short nutrient release period, poor slow-release performance, poor water retention, high production cost, complex production process, and environmentally unfriendly slow-release carriers in coated slow-release fertilizers. The simple, efficient, and low-cost production process lowers the threshold for industrial production. A first-order release kinetic model is established, which can effectively shorten the experimental cycle and facilitate researchers' rapid evaluation of the slow-release performance of coated slow-release fertilizers, providing an efficient mathematical tool for product mechanism research and product development. It also provides a theoretical basis for the integration of slow-release carriers with digital management, intelligent field fertilizer management, and precision fertilization. Attached Figure Description

[0024] Figure 1 Flowchart of the preparation process for coated slow-release fertilizer

[0025] Figure 2 The images are SEM (Scanning Electron Microscope) images of attapulgite before and after modification, where a and b are attapulgite before modification, and c and d are attapulgite after modification.

[0026] Figure 3 SEM image of the surface of the thermally modified attapulgite soil-coated slow-release fertilizer.

[0027] Figure 4 SEM image of a cross section of thermally modified attapulgite soil-coated slow-release fertilizer.

[0028] Figure 5This is a comparison chart of the cumulative nitrogen release rate of coated slow-release fertilizer before and after modification of attapulgite soil. Detailed Implementation

[0029] 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.

[0030] Example 1

[0031] A modified attapulgite soil-coated slow-release fertilizer

[0032] The specific preparation steps are as follows:

[0033] 1. Preparation of ordinary compound fertilizer granules

[0034] (1) Weigh urea, urea, superphosphate and potassium chloride in proportion (50g:30g:20g). The raw materials need to be ground and sieved, and added to the hopper in powder form.

[0035] (2) Dry mixing: Start the mixing paddle (250 rpm) and run it for 3 minutes to ensure that the raw materials are mixed evenly. Slowly and evenly add the pre-prepared polyvinyl alcohol solution into the mixing container while keeping the mixing paddle running at a low speed (250 rpm).

[0036] (3) Switch to the high-speed mode of the granulator blade (1100rpm) and run for 1-3 minutes to observe the particle formation. If the particles are loose, the time can be extended to 3 minutes; if the particles are too tightly packed, the time can be shortened to 1 minute.

[0037] (4) Transfer the wet granules to a dryer (60-80℃, 30 minutes) to remove moisture, and obtain the finished granules after cooling.

[0038] 2. Preparation of thermally modified attapulgite

[0039] (1) Take 500g of pretreated natural attapulgite clay and calcine it for 3 hours at 100℃, 150℃, 200℃, 250℃ and 300℃ respectively in a muffle furnace. After cooling to room temperature, crush and grind it through a 0.15mm sieve.

[0040] 3. Prepare the coating solution:

[0041] (1) Maleic anhydride was added to deionized water and stirred to disperse. Sodium p-styrene sulfonate was then added and mixed at a 1:1 molar ratio. 0.5% potassium persulfate initiator was then added, and the mixture was reacted at 85°C for 3 hours. After the reaction, 30% sodium hydroxide solution was added to adjust the pH to 7, followed by ethanol precipitation, filtration, and drying to obtain the maleic anhydride-sodium p-styrene sulfonate copolymer. 25g of the maleic anhydride-sodium p-styrene sulfonate copolymer was taken, and 75ml of water was added to ensure an effective content ≥25%. The solution was ultrasonically treated to ensure uniform solute dispersion, yielding a maleic anhydride-sodium p-styrene sulfonate copolymer solution.

[0042] (2) Weigh 10g of thermally modified attapulgite clay and 1g of carboxymethyl cellulose using an analytical balance. Weigh 20ml of maleic anhydride-sodium p-styrene sulfonate copolymer solution and 80ml of deionized water using a graduated cylinder. Pour the raw materials into a beaker and stir at high speed with a glass rod. Then, perform ultrasonic treatment to obtain a uniform coating solution.

[0043] 4. Preparation of coated slow-release fertilizer

[0044] (1) Material pretreatment

[0045] Use a vibrating screen (1.0mm aperture) to screen ordinary compound fertilizer granules, removing debris and oversized particles. Immerse the fertilizer granules in 0.5% dilute hydrochloric acid for 30 seconds, then rinse with deionized water until neutral. Afterward, place the fertilizer granules in an oven at 50℃ and dry for 5 minutes until the moisture content is <1%, to prevent the coating solution from penetrating.

[0046] (2) Set the coating pan speed to 600-800 RPM to ensure uniform tumbling of the particles, and adjust the pan tilt angle to 30°. Set the internal heating temperature to 50-60℃ and turn off the external heating to avoid local overheating that could cause the film to crack.

[0047] (3) Spread the pretreated fertilizer granules (about 100g) evenly into the coating pan, start the main unit to rotate, and at the same time turn on the air supply system to preheat for 5 minutes until the granule temperature reaches 40℃.

[0048] (4) Phased Impregnation: Spray the coating solution sequentially using a small spray gun or sprayer. The amount of solution added at one time is approximately 1-3 mL, which is 0.5%-1% of the granule mass. After each spray, turn on the internal heating to bring the granule temperature to 50°C and force-dry with hot air for 5 minutes. At this time, observe the granule surface. The film layer should be semi-transparent and uniform, with no liquid droplets remaining. The sound of the granules hitting the coating pan wall should change from dull to crisp. If the rotary drying time is too short, the coating solution may accumulate on the granule surface, causing the fertilizer granules to stick together. The total amount of coating solution should account for 3%-5% of the total granule mass. The actual amount used is approximately 5-10 g, corresponding to a film thickness of 80-110 μm.

[0049] (5) Repeat the cycle of "liquid addition → rotation → drying" 6-8 times until the target film thickness is achieved.

[0050] (6) Final curing: Turn off the air supply, raise the temperature to 60°C and rotate continuously for 30 minutes (to enhance the density of the membrane). Then turn off the internal heating, let the particles continue to rotate and allow the machine to cool down naturally to room temperature, turn off the main unit, take out the prepared fertilizer and remove debris and oversized particles. After screening, put the prepared coated slow-release fertilizer into a plastic bag and store it at room temperature.

[0051] Example 2

[0052] Performance testing of modified attapulgite soil-coated slow-release fertilizer

[0053] 1. Analysis of Attapulgite Modification

[0054] (1) The modified attapulgite soil at different temperatures was sent to a professional testing agency for analysis, and the following data were obtained:

[0055] Table 1 Comparison of various parameters of attapulgite at different temperatures

[0056]

[0057] Data analysis shows that when the calcination temperature is 200℃, the specific surface area, total pore volume, and micropore volume of attapulgite are all higher than at other temperatures. Therefore, 200℃ can be determined as the optimal calcination temperature.

[0058] (1) Samples before and after modification were sent to a testing institution for analysis. The results are as follows:

[0059] Table 2 Comparison of main components of attapulgite before and after modification

[0060]

[0061] The data in the table show that the content of palygorskite, the main component of attapulgite, increased significantly after this process.

[0062] (3) SEM images of attapulgite before and after modification are shown below. Figure 2 As shown, both unmodified and modified attapulgite soils exhibit a predominantly chain-like layered structure on their surfaces, along with abundant needle-like structures. The modified attapulgite soil shows a significant increase in both chain-like and needle-like structures, with a looser inter-assembly structure and larger pore gaps. This, to some extent, demonstrates that thermal modification of attapulgite soil can effectively remove impurities, and is consistent with the increased specific surface area of ​​the modified attapulgite soil.

[0063] Based on the above analysis, thermal modification of natural attapulgite can significantly improve its performance.

[0064] 2. Tests on the slow-release performance of attapulgite soil-coated slow-release fertilizer before and after modification

[0065] This invention strictly follows the static water extraction method in the People's Republic of China National Standard GB / T 23348-2009 to determine the nutrient release rate of the novel attapulgite soil-coated slow-release fertilizer before and after modification.

[0066] (1) Three experimental groups were set up before and after modification for parallel control. Therefore, six fertilizer samples were prepared. Three sets of experimental data were measured at each time point. The final selected data was the average value of the three samples.

[0067] Six groups of slow-release fertilizer samples, each weighing 10.00 ± 0.01 g, were accurately weighed using an analytical balance. Three groups of samples were then placed into 150 μm nylon mesh bags and sealed with a waterproof thread to ensure no particle leakage.

[0068] (2) Place the mesh bag into a 250mL glass bottle, add 200mL of deionized water to completely cover the sample. Seal the glass bottle with sealing film and check the seal to prevent moisture evaporation from affecting the volume.

[0069] (3) Constant temperature extraction: Place the three sets of glass bottles in a biochemical constant temperature incubator, set at 25±0.5℃, and store in the dark.

[0070] Sampling time points were selected at 24h, 3d, 5d, 7d, 10d, 14d, 28d, 42d, 56d, and 60d, and then every 10 days thereafter, until the cumulative release rate was ≥80%.

[0071] (4) Sampling and extract treatment: Before sampling, invert the glass bottle three times. Transfer the filtrate to a 250mL volumetric flask, dilute to the mark with deionized water, and shake well. Then, add 200mL of water to the glass bottle containing the sample bag, seal the bottle, and place it in a biochemical constant temperature incubator for further incubation.

[0072] (5) Test methods

[0073] (1) The total nitrogen content in fertilizer was determined by the NY / T525-2021-Kjeldahl method.

[0074] (2) The present invention uses the p-dimethylbenzaldehyde method (GB / T-23348-2009) to test the cumulative nitrogen release rate of fertilizer.

[0075] analyze Figure 3 It can be seen that the fertilizer surface exhibits a chain-like structure and a large number of needle-like structures, indicating that the thermoplastic attapulgite successfully adheres to the fertilizer surface and has a good effect.

[0076] The cross-sectional SEM image of the coated slow-release fertilizer is shown below. Figure 4As shown, there is a clear boundary between the coating and the fertilizer core, and the coating thickness is consistent with the expected results.

[0077] Table 1 Total nitrogen content of each sample in the unmodified fertilizer

[0078]

[0079] Table 2 Total nitrogen content of each sample in the modified fertilizer

[0080]

[0081] Table 3. Nitrogen release from samples at various time points before modification.

[0082]

[0083] Table 4. Cumulative nitrogen release rate of samples before modification

[0084]

[0085] Table 5. Nitrogen release from samples at various time points after modification.

[0086]

[0087] Table 6. Cumulative nitrogen release rate of the modified samples

[0088]

[0089] The time and cumulative nitrogen release rate were plotted, and then nonlinear fitting was performed using the first-order release kinetic equation, such as... Figure 5 As shown. Figure 5 This is a comparison chart of the cumulative nitrogen release rate of coated slow-release fertilizer before and after modification of attapulgite soil.

[0090] First-order release kinetics models describe the kinetics where the release rate is directly proportional to the remaining amount of a substance. These models are commonly used in diffusion-controlled release processes, such as drug release, fertilizer release, and material diffusion. R t =R ∞ ×(1-e -kt The Rt-ε is a core formula in first-order release kinetics describing the cumulative release amount. It is suitable for analyzing the change in release efficiency over time, where Rt represents the cumulative release amount at time t; R∞ represents the final maximum release amount (i.e., the total amount after complete release); k is the release rate constant, reflecting the speed of release; and e is the base of the natural logarithm. Through analysis... Figure 56. It can be concluded that the first-order release kinetic equation is a good fit for the cumulative nitrogen release rate of the new attapulgite soil-coated slow-release fertilizer, which conforms to the first-order kinetic model. The equations are: before modification, R∞ = 97.87 and k = 0.04692; after modification, R∞ = 99.99 and k = 0.02371.

[0091] First-order release kinetic equation R t =R ∞ ×(1-e -kt The form is simple and the physical meaning is clear, which makes it easy to quickly fit the experimental data and intuitively reflect the influence of the coating material properties (such as thickness and water permeability) on the release process.

[0092] The first-order release kinetic model shows a pattern of "rapid release in the early stage, gradual slowdown and approaching equilibrium in the later stage", which is consistent with the diffusion release characteristics of the membrane system. That is, the nutrient concentration gradient inside and outside the membrane is large in the early stage, the release rate is high, the gradient decreases over time, and the release tends to stabilize.

[0093] The k value obtained by fitting a first-order release kinetic model can quantify the release rate of different coating processes. For example, a larger k value indicates a faster release, which provides a theoretical basis for adjusting the formulation of coating materials.

[0094] First-order release kinetics models possess excellent predictive capabilities, allowing the extrapolation of release rates at any given time point using data from a limited set of time points. This aids in developing fertilization plans, such as matching the nutrient requirements of crops at different growth stages. In summary, first-order release kinetics models provide an efficient mathematical tool for characterizing the release performance, studying the mechanisms, and developing products using coated slow-release fertilizers. They also provide a theoretical basis for integrating slow-release carriers with digital management, intelligent field fertility management, and precision fertilization.

[0095] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a modified attapulgite soil-coated slow-release fertilizer, characterized in that, Includes the following steps: (1) Preparation of ordinary compound fertilizer granules: urea, superphosphate and potassium chloride are ground and mixed in a mass ratio, polyvinyl alcohol solution is added as a binder, granulated in a granulator, and dried to obtain compound fertilizer granules. (2) Preparation of thermally modified attapulgite: Natural attapulgite is calcined at high temperature for 2-4 hours, ground and sieved to obtain modified attapulgite; (3) Preparation of coating solution: Heat-modified attapulgite, coating adhesive, maleic anhydride-sodium p-styrene sulfonate copolymer solution and deionized water are mixed in proportion and subjected to high-speed stirring and ultrasonic treatment to form a homogeneous coating solution; (4) Preparation of coated slow-release fertilizer: After surface activation treatment, the compound fertilizer granules are placed in a sugar coating machine and coated with coating liquid in 6-8 times. After spraying, they are dried with hot air. The final amount of coating liquid is 3%-5% of the total mass of the granules. After solidification, coated slow-release fertilizer granules are obtained.

2. The method according to claim 1, characterized in that: The mass ratio of urea, superphosphate, and potassium chloride in step (1) is 5:3:

2.

3. The method according to claim 1, characterized in that: The specific steps of the granulation process in step (1) are as follows: dry mixing at 200-300 rpm for 1-3 minutes to ensure that the raw materials are fully and evenly dispersed; wet granulation at 1000-1200 rpm for 1-3 minutes to promote the adhesion of the binder (polyvinyl alcohol solution) to the particles through shearing force to form uniform particles.

4. The method according to claim 1, characterized in that: The calcination temperature in step (2) is 150-250℃.

5. The method according to claim 1, characterized in that: The coating adhesive in step (3) is selected from at least one of carboxymethyl cellulose, hydroxypropyl starch, starch acetate, gum arabic, guar gum, xanthan gum, ethyl cellulose, hydroxypropyl methyl cellulose, chitosan, sodium alginate, polyvinyl alcohol, polyethylene glycol, and polyacrylamide.

6. The method according to claim 1, characterized in that: The formulation ratio of the coating solution in step (3) is as follows: by mass, 5-15 parts of thermally modified attapulgite, 1-5 parts of coating adhesive, 15-25 parts of maleic anhydride-sodium p-styrene sulfonate copolymer solution, and 55-80 parts of deionized water.

7. The method according to claim 1, characterized in that: The coating preparation conditions described in step (4) are: rotation speed 600-800 RPM, tilt angle 30°, internal heating temperature 50-60°C, spray amount per spray is 0.5%-1% of particle mass, and hot air drying at 50-60°C for 5 minutes after spraying.

8. The method according to claim 1, characterized in that: The nutrient release pattern of the coated slow-release fertilizer conforms to the first-order release kinetic model R. t =R ∞ ×(1-e -kt ), where R t denoted as the cumulative nitrogen release rate on day t.

9. A modified attapulgite soil-coated slow-release fertilizer, characterized in that: Prepared by any one of the methods of claims 1-8, the coating layer has a thickness of 80-110 μm.

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