Double-coating coated controlled-release fertilizer and preparation method thereof

Abstract

The invention relates to a double-coating coated controlled-release fertilizer and a preparation method thereof. A double-coating enveloping mode is adopted, a controlled-release fertilizer technology which is low in cost, environmentally friendly, high in release precision and capable of meeting the requirements of various crops is provided, the fertilizer utilization rate can be increased, the agricultural production cost can be reduced, the ecological environment load can be reduced, and wide application prospects are achieved; the selected water-based polyacrylate has the characteristics of being relatively low in price, relatively good in film-forming property, relatively proper in viscosity, relatively easy to degrade and the like; the selected carnauba wax has good adhesiveness, frictional property and hardness, also has the characteristics of strong hydrophobicity and no toxicity, and is very suitable for being used as an outer coating of the water-based polymer coated controlled-release fertilizer.

Description

Technical Field

[0001] This invention belongs to the field of controlled-release fertilizer preparation, specifically relating to a double-coated controlled-release fertilizer and its preparation method. Background Technology

[0002] In agricultural production, fertilizers, as core inputs for ensuring crop growth and increasing yield, have their application efficiency directly impacting agricultural economic benefits and ecological sustainability. Traditional chemical fertilizers (such as urea and diammonium phosphate) are prone to multiple problems after application due to their rapid nutrient release and easy solubility: Firstly, the nutrient release rhythm is severely mismatched with the needs of crops at different growth stages. Seedlings may suffer from "seedling burn" due to concentrated nutrient release, while critical growth stages may experience insufficient fertilizer supply due to premature nutrient loss, resulting in generally low fertilizer utilization rates. Secondly, unabsorbed nitrogen, phosphorus, and other nutrients enter the environment through surface runoff, underground leaching, and ammonia volatilization, causing a series of ecological problems such as soil compaction, eutrophication of water bodies, and atmospheric nitrogen deposition, which contradicts the current needs of green agricultural development. To address these challenges, controlled-release fertilizers have emerged. Their core principle involves coating or modifying fertilizer nutrients through physical, chemical, or biological means to achieve slow, controlled nutrient release. Currently, most controlled-release fertilizers on the market are coated types. These fertilizers control the release rate by adjusting the coating thickness or porosity. However, they generally suffer from high coating material costs, poor biodegradability, and soil pollution from residual coatings. Furthermore, the coating layer is easily damaged during agricultural machinery operations, leading to sudden nutrient release, which hinders their widespread application in agricultural production.

[0003] In addition, existing controlled-release fertilizers have a prominent shortcoming of "poor versatility": different crops (such as grasses and cash crops) and different growth stages have vastly different requirements for the proportions of nitrogen, phosphorus, potassium and micronutrients, but existing products are mostly fixed formulas and release modes, which cannot achieve "crop-customized" fertilization; at the same time, in complex field environments, natural factors such as drastic temperature fluctuations and uneven rainfall can easily cause the controlled-release system to fail, further limiting its large-scale application. Summary of the Invention

[0004] The purpose of this invention is to provide a double-coated controlled-release fertilizer and its preparation method, which has the advantages of low cost, environmental friendliness, high release precision and adaptability to the needs of a variety of crops. It can improve fertilizer utilization, reduce agricultural production costs and reduce the ecological burden.

[0005] Therefore, the technical solution adopted by the present invention is: a double-coated controlled-release fertilizer, comprising a fertilizer core and a water-based polymer coating wrapped around the fertilizer core, and a wax coating sprayed onto the surface of the water-based polymer coating. By weight: fertilizer core granules: water-based polymer: wax = 9:0.7-0.8:1-2.

[0006] Furthermore, the water-based polymer is water-based polyacrylate, and the wax is carnauba wax.

[0007] A method for preparing a double-coated controlled-release fertilizer is as follows: using a fluidized bed coating device, fertilizer core granules and water-based polyacrylate coating solution are first placed into the fluidized bed coating device. The fluidized bed coating control parameters are as follows: Coating solution flow rate: 2-3 rpm; Inlet air temperature: 45-50℃; Air outlet temperature: 40-45℃; The initial coating process takes 1.5-2 hours to complete. After the coating is completed and dried, the prepared carnauba wax coating emulsion is added to the inlet, and the coating process is started again. It takes 2-2.5 hours to complete. After cooling, the double-coated controlled-release fertilizer is obtained. The control parameter ranges for secondary fluidized bed coating are as follows: Coating solution flow rate: 2-3 rpm; Inlet air temperature: 45-50℃; Air outlet temperature: 40-45 ℃.

[0008] Furthermore, the method for preparing the carnauba wax emulsion is as follows: 1) Oil phase preparation Add carnauba wax to an oil phase pot with a stirrer, heat to 90-95°C, turn on the stirrer until completely melted at a speed of 250-300 rpm, add polysorbate-80, vitamin E, and acetate, continue stirring for 5-10 minutes, then reduce the stirring speed to 100 rpm and maintain the temperature, set aside. By weight: palm wax: polysorbate-80: vitamin E: acetate = 70-75: 10-15: 1-3: 8-12; 2) Aqueous phase preparation Add deionized water and glycerin to the aqueous phase pot, stir to dissolve, heat to 90-95°C, add phenoxyethanol, stir until completely dissolved, stirring speed is 150-200 rpm for later use; The proportions of each material by weight are: deionized water: glycerol: phenoxyethanol = 94-97: 4-6: 0.2-0.3; 3) Emulsification stage Keep the oil phase and water phase at the same temperature of about 90-95°C, with a temperature difference of ±2°C. Slowly pour the water phase into the oil phase pot, turn on the high-speed shear mixer at 1800-2000 rpm, and continue shearing for 10-15 minutes. Observe the state of the emulsion during this period until no stratification or particles appear. 4) Heat preservation and cooling Turn off the high-shear mixer and switch to a paddle mixer. Maintain the temperature and allow the emulsifier to be fully adsorbed at the interface to reduce particle agglomeration. Slowly cool to room temperature at a rate of 0.5-1℃ / min, while continuing to stir at low speed during the cooling process, until the emulsion is a uniform milky white liquid. Based on the prepared carnauba wax emulsion, deionized water is added to the emulsion and stirred evenly to prepare a coating solution with a solid content of 10%.

[0009] The advantages of this invention are: This invention adopts a double-coating method, which has the characteristics of low cost, environmental friendliness, high release precision and adaptability to the needs of a variety of crops, thus improving fertilizer utilization, reducing agricultural production costs and reducing the ecological burden, and has broad application prospects.

[0010] The water-based polyacrylate selected in this invention has the characteristics of relatively low price, good film-forming properties, suitable viscosity, and easy degradation; the selected carnauba wax has good adhesion, friction and hardness, and also has the characteristics of strong hydrophobicity and non-toxicity, making it very suitable as the outer coating of water-based polymer-coated controlled-release fertilizers. Attached Figure Description

[0011] Figure 1 Cumulative release curves of coated fertilizers with different palm wax contents.

[0012] Figure 2 Cumulative release curves of coated fertilizers under different wax types. Detailed Implementation

[0013] Example 1: Add 70g of carnauba wax to the oil phase vessel equipped with a stirrer, heat until completely dissolved, and start stirring. Add 10g of polysorbate-80, 2g of vitamin E, and 10g of acetate, continue stirring for 5-10 minutes, then stop stirring and maintain the temperature at approximately 90-95°C. Set aside. Add 190ml of deionized water and 10g of glycerin to the aqueous phase vessel, stir to dissolve, and heat to 90-95°C. Add 0.4g of phenoxyethanol. Set aside. Keep the oil and aqueous phases at the same temperature (±2°C), approximately 90°C, and slowly pour the aqueous phase into the oil phase vessel. Turn on the high-shear mixer to begin emulsification, continuing shearing for 10 minutes, observing the emulsion state during this time: if separation or particles appear, extend the homogenization time or add more emulsifier. Turn off the homogenizer and switch to a paddle mixer, maintaining the temperature to allow the emulsifier to fully adsorb at the interface and reduce particle aggregation. Slowly cool to room temperature at a rate of 0.5-1°C / min. Its emulsion solids content is approximately 29%-33%.

[0014] A coating solution with a solid content of 10% was prepared by adding deionized water to a pre-prepared carnauba wax emulsion with known solid content and stirring until homogeneous. Coated fertilizer C1 was then prepared using a fluidized bed reactor, in which polyacrylate coating accounted for 8% of the total content and wax coating accounted for 1%. Example 2:

[0015] The rest of the process is the same as in Example 1, except that the wax coating amount is 2% of the total amount, and coated fertilizer C2 is prepared. Example 3:

[0016] The rest of the process is the same as in Example 1, except that the oil phase is changed to the addition of paraffin wax, and the amount of wax coating is 2% of the total amount, to prepare coated fertilizer P2. Example 4:

[0017] The remaining steps are the same as in Example 1, except that calcium stearate is added to the carnauba wax coating solution to prepare coated fertilizer C3. The addition of calcium stearate reduces the viscosity of the wax, thus reducing the possibility of adhesion and caking of the coated fertilizer during production and storage.

[0018] The core of the wax emulsification required for the fertilizer coating of this invention is to uniformly disperse the wax in the aqueous phase to form a stable emulsion, so as to more easily form a continuous and dense film on the surface of fertilizer particles, rather than pursuing extreme particle fineness (the core advantage of high shear). After the two phases are compatible in the shear mixer and the required particle fineness (≤3μm) is achieved, the mixing is switched to paddle stirring. Through its low-speed and wide-range material circulation, the water and oil phases are gently mixed, avoiding local overheating caused by high shear that leads to wax oxidation and degradation, or damage to the stability of the emulsion due to excessive shear force (such as demulsification and stratification). It can also ensure the overall uniformity of large-volume materials and avoid problems such as insufficient local emulsification and uneven temperature distribution caused by the limited stirring range of high shear mixers.

[0019] The palm wax used in this invention is a wax with high hardness and good abrasion resistance, as well as a certain degree of resistance to mechanical damage, thus playing a good role in preventing coating damage. In actual storage, traditional coated fertilizers will exhibit varying degrees of adhesion after being stored for more than a year. Separating the adhered fertilizer particles can lead to coating damage. However, after the wax reprocessing, the adhesion phenomenon is greatly reduced. Even if adhesion occurs, the inner membrane remains intact during separation due to the double-layer film.

[0020] This invention adjusts the amount of water-based polyacrylate coating and the loading of carnauba wax to ensure the nutrient content of the fertilizer core while achieving a customized controlled-release cycle for different crops, making it highly compatible with the crop's nutrient requirements during growth. In contrast, existing single-coating fertilizers require an increased coating amount to achieve a longer controlled-release period, which reduces the corresponding nutrient content.

[0021] refer to Figure 1 , Figure 2 The static water dissolution test showed that the cumulative nutrient release rate of the double-coated fertilizer was only 1.16% after 24 hours, far below the 15% threshold stipulated by the industry standard, effectively avoiding the risk of fertilizer damage during the seedling stage. The outer layer is a hydrophobic layer, and the inner layer is a controlled-release layer. The initial release is slow, but the release rate accelerates when the outer layer is depleted or when the environment (temperature, microorganisms) triggers the inner layer, forming an "S"-shaped inflection point. In the 25℃ constant temperature simulation test, its nutrient release curve showed an "S"-shaped characteristic, with a fit of more than 90% to the nutrient requirement pattern of rice throughout its entire growth period, significantly better than single-coated fertilizers (fit ≤85%). Furthermore, by adjusting the amount of water-based polyacrylate coating (7%-10%) and the loading of carnauba wax (0.5%-2%), the release cycle can be continuously adjusted from 30 days to 120 days, making it suitable for crops with various growth cycles.

Claims

1. A double-coated controlled-release fertilizer, characterized in that, It includes a fertilizer core and a water-based polymer coating that wraps around the fertilizer core, and a wax coating is sprayed onto the surface of the water-based polymer coating. By weight: fertilizer core granules: water-based polymer: wax = 9:0.7-0.8:1-2.

2. The double-coated controlled-release fertilizer according to claim 1, characterized in that, The water-based polymer is water-based polyacrylate, and the wax is carnauba wax.

3. A method for preparing a double-coated controlled-release fertilizer, characterized in that, Preparation is carried out as follows: Using a fluidized bed coating device, the fertilizer core granules and water-based polyacrylate coating solution are first placed into the fluidized bed coating device. The fluidized bed coating control parameters are as follows: Coating solution flow rate: 2-3 rpm; Inlet air temperature: 45-50℃; Air outlet temperature: 40-45℃; The initial coating process takes 1.5-2 hours to complete. After the coating is completed and dried, the prepared carnauba wax coating emulsion is added to the inlet, and the coating process is started again. It takes 2-2.5 hours to complete. After cooling, the double-coated controlled-release fertilizer is obtained. The control parameter ranges for secondary fluidized bed coating are as follows: Coating solution flow rate: 2-3 rpm; Inlet air temperature: 45-50℃; Air outlet temperature: 40-45 ℃.

4. The method for preparing a double-coated controlled-release fertilizer according to claim 3, characterized in that, The method for preparing the carnauba wax emulsion is as follows: 1) Oil phase preparation Add carnauba wax to an oil phase pot with a stirrer, heat to 90-95°C, turn on the stirrer until completely melted at a speed of 250-300 rpm, add polysorbate-80, vitamin E, and acetate, continue stirring for 5-10 minutes, then reduce the stirring speed to 100 rpm and maintain the temperature, set aside. By weight: palm wax: polysorbate-80: vitamin E: acetate = 70-75: 10-15: 1-3: 8-12; 2) Aqueous phase preparation Add deionized water and glycerin to the aqueous phase pot, stir to dissolve, heat to 90-95°C, add phenoxyethanol, stir until completely dissolved, stirring speed is 150-200 rpm for later use; The proportions of each material by weight are: deionized water: glycerol: phenoxyethanol = 94-97: 4-6: 0.2-0.3; 3) Emulsification stage Keep the oil phase and water phase at the same temperature of about 90-95°C, with a temperature difference of ±2°C. Slowly pour the water phase into the oil phase pot, turn on the high-speed shear mixer at 1800-2000 rpm, and continue shearing for 10-15 minutes. Observe the state of the emulsion during this period until no stratification or particles appear. 4) Heat preservation and cooling Turn off the high-shear mixer and switch to a paddle mixer. Maintain the temperature and allow the emulsifier to be fully adsorbed at the interface to reduce particle agglomeration. Slowly cool to room temperature at a rate of 0.5-1℃ / min, while continuing to stir at low speed during the cooling process, until the emulsion is a uniform milky white liquid. Based on the prepared carnauba wax emulsion, deionized water is added to the emulsion and stirred evenly to prepare a coating solution with a solid content of 10%.

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