Fly ash-based slow-release fertilizer and preparation method and application thereof

Abstract

The application discloses fly ash-based slow-release fertilizer and a preparation method and application thereof, and comprises the following process: fly ash hollow microspheres are mixed with sodium hydroxide solids and subjected to calcination treatment to obtain a calcination product; the calcination product is mixed with deionized water and then filtered to obtain washed residues; the washed residues are mixed with an acid solution and subjected to drying to obtain a fly ash slow-release carrier; the fly ash slow-release carrier is placed in a negative pressure environment, saturated urea solution is introduced into the negative pressure environment, and the urea-loaded fly ash is obtained through filtration separation and drying treatment; the surface of the urea-loaded fly ash is sprayed with a binder, and inorganic mineral powder is added for coating treatment, and the fly ash-based slow-release fertilizer is obtained through drying treatment. The slow-release time of urea is prolonged, and the cost is greatly reduced.

Description

Technical Field

[0001] This invention belongs to the field of slow-release fertilizers, and relates to a fly ash-based slow-release fertilizer, its preparation method, and its application. Background Technology

[0002] Nitrogen is one of the most critical nutrients for crop growth and plays a vital role in agricultural production. Among the many types of nitrogen fertilizers, urea has become the most widely used nitrogen fertilizer globally due to its high nitrogen content (46%) and low production cost. However, after traditional urea is applied to the soil, it is rapidly hydrolyzed by soil urease, resulting in excessively high local concentrations of ammonium nitrogen in the soil. This makes nitrogen easily lost through ammonia volatilization, nitrate leaching, and surface runoff. Statistics show that the nitrogen utilization rate of traditional urea is typically only 20% to 35%. This not only causes a huge waste of resources but also seriously leads to a series of environmental problems such as eutrophication of water bodies, soil acidification, and the emission of greenhouse gases (such as N2O).

[0003] To improve nitrogen use efficiency and reduce environmental pollution, the development of efficient and environmentally friendly controlled-release fertilizers has become an important direction in this field. Controlled-release fertilizers slow down the release rate of nutrients in the soil through mechanisms such as physical coating, chemical binding, or matrix retardation, thus matching the nutrient absorption patterns of crops.

[0004] Current slow-release fertilizer preparation strategies mainly include coated and matrix-composite types. While polymer-coated fertilizers (such as those using polyolefins and polyurethanes) exhibit excellent slow-release performance and good controlled-release characteristics, their preparation processes are complex and production costs are high. Furthermore, the coating materials are mostly petroleum-based polymers, which are difficult to degrade naturally in soil. Long-term, large-scale use can lead to the accumulation of microplastic residues in the soil, damaging soil structure and posing potential ecological risks, thus limiting their large-scale promotion and application in field crops.

[0005] In contrast, matrix-based slow-release fertilizers that utilize natural inorganic minerals or industrial solid waste as carriers have attracted much attention in recent years due to their advantages such as wide availability of raw materials, low cost, environmental friendliness, and soil improvement functions. In existing technologies, researchers have used non-metallic minerals such as attapulgite and bentonite as adsorbent carriers or coating materials for urea, utilizing the interlayer structure or surface adsorption of the minerals to inhibit nutrient release.

[0006] Fly ash is a major solid waste emitted by coal-fired power plants, and it is rich in a silica-alumina material with a hollow spherical structure, thin walls, light weight, and stable chemical properties—fly ash hollow microspheres. Currently, the utilization of FACs is mainly focused on their use as simple physical fillers (such as plastic and rubber fillers) or building materials (such as concrete admixtures), failing to fully realize their unique microcapsule hollow structure and its application potential in the agricultural field.

[0007] Regarding the application of fly ash in fertilizer technology, existing technologies mainly employ the following methods: Geopolymer Coating Method: Some technologies use fly ash to form geopolymers through alkali activation, which are then directly used as coating materials to encapsulate urea particles. While this method utilizes the chemical reactivity of fly ash, it essentially forms a dense shell, failing to utilize the cavities of microspheres, and the coating layer is prone to cracking, leading to nutrient bursts. Sintering Method: Some technologies prepare slow-release fertilizers primarily composed of metastable silicate minerals by sintering fly ash mixed with potassium salts. This method is energy-intensive and damages the original physical morphology of the fly ash. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a fly ash-based slow-release fertilizer, its preparation method and application, which improves the slow-release time of urea and greatly reduces the cost.

[0009] To achieve the above objectives, the present invention employs the following technical solution: A method for preparing a fly ash-based slow-release fertilizer includes the following steps: S1. Mix fly ash hollow microspheres with solid sodium hydroxide and calcine them to obtain the calcined product; S2. The roasted product is mixed with deionized water and then filtered to obtain water-washed residue. S3. Mix the water-washed residue with an acidic solution, filter and dry to obtain a fly ash slow-release carrier; S4. Place the fly ash slow-release carrier in a negative pressure environment, introduce a saturated urea solution into the negative pressure environment, and obtain urea-loaded fly ash after filtration, separation and drying. S5. The fly ash loaded with urea is coated onto the surface of the urea fertilizer core with a binder, then sealed with paraffin wax, and after drying, fly ash-based slow-release fertilizer is obtained.

[0010] Optionally, in S1, the mass ratio of fly ash hollow microspheres to sodium hydroxide solid is 1:1-5:1, the calcination temperature is 200-350℃, and the calcination time is 2-10h.

[0011] Optionally, in S2, the solid-liquid ratio of the calcined product to deionized water is 1g:10ml~1g:20ml, and the settling time is 2-5h.

[0012] Optionally, in S3, the acidic solution is hydrochloric acid solution, the solid-liquid ratio of the water washing residue to the hydrochloric acid solution is 1:10~1:20, and the settling time is 2-5 hours.

[0013] Optionally, in S4, the temperature of the saturated urea solution is maintained at 45-80℃, and the fly ash slow-release carrier is placed in a vacuum device to perform a vacuuming operation to form a negative pressure environment. After the saturated urea solution is introduced, the vacuuming operation is repeated 3-5 times.

[0014] Optionally, before S1, the process may also include: preparing fluidized bed fly ash into a slurry and subjecting it to gravity sedimentation classification, collecting the gravity sedimentation classification products and subjecting them to filtration and drying, ultimately obtaining hollow microspheres with enriched particle size.

[0015] Optionally, in S5, the adhesive is a polyvinyl alcohol solution with a mass fraction of 2%-8%.

[0016] Optionally, in S5, the urea fertilizer cores are put into a round pot granulator for rolling. After the urea fertilizer cores are sprayed with binder until they stick together, fly ash powder loaded with urea is added at a weight ratio of 1:1 to 1:5 for rolling and coating treatment. Finally, a paraffin wax layer is applied.

[0017] A fly ash-based slow-release fertilizer is prepared using the above-mentioned method. It has a granular morphology, with a urea fertilizer core at the center. The urea fertilizer core is coated with a layer of fly ash loaded with urea, and the outermost layer is a paraffin sealing layer. The fly ash loaded with urea is made of hollow fly ash microspheres, which are filled with urea. The outer wall of the hollow fly ash microspheres is composed of silicon oxide and aluminum oxide, and is covered with urea.

[0018] Application of a fly ash-based slow-release fertilizer in the field of fertilizers.

[0019] Compared with the prior art, the present invention has the following beneficial effects: This invention breaks through the traditional single-coating structure of slow-release fertilizers, constructing a multi-level core-shell structure with urea fertilizer core as the center, fly ash loaded with urea as the coating layer, and paraffin as the outermost sealing layer. Among these, the hollow fly ash microspheres, as the core carrier of the coating layer, play an irreplaceable supporting and synergistic role in the entire product, specifically in the following three aspects: Firstly, at the level of microstructure and physical load-bearing capacity, fly ash hollow microspheres possess a natural hollow spherical structure. Not only are their internal cavities filled with crystalline urea, acting as miniature fertilizer storage chambers, but their outer walls are also covered with silica, alumina, and urea. These high-strength, inorganic, rigid microspheres adhere to the urea fertilizer core, forming a robust coating layer that maintains extremely high mechanical stability in the complex stress environment of the soil. This effectively avoids the nutrient bursting phenomenon caused by compression damage during transportation or application of traditional polymer-coated slow-release fertilizers, fundamentally ensuring the long-term integrity of the slow-release structure.

[0020] Secondly, in terms of ecological recycling and soil improvement, fly ash hollow microspheres, as a major industrial solid waste from coal combustion, can be transformed into high-value-added coating materials. This not only truly turns waste into treasure and significantly reduces the production cost of slow-release fertilizers, but also completely avoids the secondary pollution problem caused by the slow degradation of traditional resin coating materials in the soil. Furthermore, the inorganic components (silicon oxide and aluminum oxide) covering the outer wall of the microspheres slowly weather and release into the soil, providing multiple agronomical synergistic effects, including improving soil compaction, regulating soil pH, and promoting crop root development.

[0021] Finally, at the level of synergistic adaptation of the multi-stage sustained-release mechanism, the inorganic components such as silica and alumina covering the outer wall of the hollow microspheres have excellent interfacial affinity, which not only enables the microspheres to bind tightly to the central urea fertilizer core, but also ensures the dense adhesion of the outermost paraffin seal layer. This intrinsic interfacial compatibility of the material constitutes a unique long-lasting, tiered sustained-release system.

[0022] Furthermore, the technical solution of this invention achieves excellent cascaded slow-release effects from two dimensions: microscopic pore interception and macroscopic multi-layer physical barrier. Firstly, the cavities inside the hollow microspheres provide ample storage space for urea. When the urea crystallizes after entering the cavity and is released outwards, it must overcome the physical resistance of the microsphere wall pores, forming the first slow-release barrier at the microscopic level. Secondly, the dense silica and alumina on the outer wall of the hollow microspheres, together with the attached urea, constitute the second physical barrier encapsulating the fertilizer core. Thirdly, the outermost paraffin sealing layer effectively repels water, completely blocking the channels for initial water intrusion. After application to the soil, environmental moisture must first slowly degrade / penetrate the paraffin sealing layer, then pass through the fly ash-loaded coating layer filled with microporous resistance, and finally reach and dissolve the central urea fertilizer core. This spatiotemporal blocking mechanism, which penetrates layer by layer from the surface to the interior and releases nutrients in stages from the inside out, greatly extends the nutrient release cycle, enabling the nitrogen release pattern to match the crop's growth cycle needs in a long-term and stable manner, ultimately significantly improving fertilizer utilization. Attached Figure Description

[0023] Figure 1 This is a flowchart of the method for preparing fly ash-based slow-release fertilizer according to the present invention; Figure 2 A scanning electron microscope image of the fly ash slow-release carrier obtained in Example 1 of the present invention; Figure 3 A cross-sectional scanning electron microscope image of the fly ash carrier-coated slow-release fertilizer obtained in Example 1 of the present invention; Figure 4 This is a graph showing the 24-hour slow-release results of the fly ash carrier-coated slow-release fertilizer obtained in Example 1 of the present invention. Figure 5A scanning electron microscope image of the fly ash hollow microsphere carrier obtained in Example 3 of the present invention; Figure 6 This is a scanning electron microscope image of the fly ash hollow microsphere carrier obtained in Example 4 of the present invention. Detailed Implementation

[0024] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. 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 should fall within the scope of protection of the present invention.

[0025] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0026] This embodiment provides a method for preparing fly ash-based slow-release fertilizer, such as... Figure 1 As shown, the process includes the following: (1) Using the gravity sedimentation separation method for hollow fly ash microspheres in Chinese patent CN118807962A,

A Gravity Sedimentation Separation Column, System and Method for Hollow Fly Ash Microspheres

[0027] The sorting process removes transition metals such as iron and manganese that are harmful to the environment and organisms, eliminating the biotoxicity of the materials. The resulting hollow microspheres are mainly composed of silicates and other substances, making them environmentally friendly.

[0028] Table 1 Gravity Settlement Sorting Parameters

[0029] (2) Sodium hydroxide solid was ground in a mortar to ensure full contact with the wall of the hollow microspheres. Sodium hydroxide solid and hollow microspheres of different sizes were uniformly mixed in a mass ratio of 1:1-5:1 and then compacted into a quartz boat. The mixture was then calcined in a muffle furnace at 200-350℃ for 2-10 hours. The etching effect of alkaline solution under normal pressure is limited, while the alkaline reaction under high pressure is too violent and usually destroys the spherical structure of the hollow microspheres. In order to effectively penetrate the outer glass wall of the hollow microspheres and retain the spherical structure of the hollow microspheres, a solid-phase alkali was chosen to perform point-to-point contact etching to remove the amorphous silicon-aluminum glass phase on the surface of the microspheres. Sodium hydroxide solid begins to melt at 315℃. At this temperature and above, the hollow microspheres are prone to phase transition and destruction of the spherical structure. At low temperatures, the reactivity is low. Therefore, a solid-phase alkali calcination of sodium hydroxide at 250℃ was chosen to etch the hollow microspheres.

[0030] (3) Mix the calcined product with deionized water at a solid-liquid ratio of 1g:10ml to 1g:20ml, stir, let stand for 2-5 hours, and then filter under vacuum to obtain the water-washed residue. Water washing is used to remove unreacted sodium hydroxide, as well as soluble sodium silicate, sodium aluminate, etc. generated by alkali calcination, and at the same time removes some soluble silica and aluminum to prevent these substances from interfering with urea loading or affecting the sustained-release performance in subsequent steps.

[0031] (4) Mix the water-washed residue with hydrochloric acid at a solid-liquid ratio of 1:10 to 1:20, stir evenly, let stand for 2-5 hours, and then vacuum filter and dry to obtain the fly ash slow-release carrier. Acid washing is used to remove the aluminosilicate glass phase that was activated in alkali roasting but not completely dissolved in water, some mullite / quartz, and a large number of metal impurity ions, so as to avoid these impurities from clogging the pores or reducing the specific surface area of ​​the carrier.

[0032] (5) Prepare a saturated urea solution at 45-80℃. Place the fly ash slow-release carrier in a vacuum device, evacuate to remove air from the pores, and draw in the saturated urea solution under negative pressure. Maintain the vacuum and let it stand for 10-50 minutes. Repeat the vacuuming process 3-5 times. Filter to separate the solid and dry to obtain urea-loaded fly ash. The principle of vacuum impregnation is to remove air from the pores under negative pressure, making it easier for the saturated urea solution to enter the microsphere cavity; the saturated urea solution can increase the solubility of urea and increase the loading.

[0033] (6) Put the urea fertilizer core into a round pot granulator, spray 2-8% PVA (polyvinyl alcohol) binder, and wait for the urea particles to stick together. Then, add fly ash loaded with urea at a weight ratio of 1:1-1:5. As a water-soluble binder, PVA can uniformly coat the urea particles and the fly ash loaded with urea to form a dense coating layer; a concentration of 5% is the optimal range that balances bonding strength and flowability.

[0034] (7) Then, evenly sprinkle 5-15% paraffin powder and wait for the paraffin powder to completely melt and cover the surface of the urea granules as a protective coating. Then turn off the radiant heating device and the disc granulator, and pass the granules through a sieve to obtain fly ash coated slow-release fertilizer. Paraffin acts as a hydrophobic layer to prevent moisture from entering the coating layer and delay the release of urea; a 10% ratio is the critical value to ensure complete coverage without increasing costs too much.

[0035] Hollow microspheres of different sizes are effectively separated using a sorting device. Through a solid-phase alkali calcination-water washing-acid washing process, hollow microspheres with different morphologies—porous, sheet-like, and nanoparticle-like—are obtained. This effectively increases the specific surface area of ​​the hollow microspheres, allowing for effective penetration through the microsphere walls and creating effective urea loading channels and spaces. This results in a urea loading capacity of 40%. The manufactured inorganic fly ash-coated slow-release fertilizer, by controlling the ash ratio, achieves a release rate of less than 50% within 24 hours, with an effective release period of 4 days. This effectively extends the fertilizer release period, and the release characteristics conform to those of slow-release fertilizers.

[0036] The fly ash coated slow-release fertilizer prepared by the above method has a granular morphology with a urea fertilizer core at the center. The urea fertilizer core is coated with a layer of fly ash loaded with urea, and the outermost layer is a paraffin seal. This layer of fly ash loaded with urea is made of etched porous fly ash hollow microspheres. The interior of the fly ash hollow microspheres is filled with urea (urea that crystallizes after entering the cavity). The outer wall is mainly composed of silicon oxide and aluminum oxide, and a small amount of urea is covered on the outer wall. The outer wall has a large content of silicon oxide and aluminum oxide, which is formed by etching, while the urea content is small and is formed by hydrogen bonding adsorption.

[0037] The aforementioned fly ash coated slow-release fertilizer is used in the fertilizer industry.

[0038] Example 1: (1) Prepare a 2% concentration fluidized bed fly ash slurry. Set the unit time water supply and ore feed rate inside the six separation pipes. The specific parameters are shown in Table 1. Then start the water pump and feed pump in sequence to complete the equipment startup. After waiting 48 hours, the classification is completed. Collect each gravity sedimentation classification product, filter, dry, weigh, and bag for later use. The product numbers are named H1, H2, H3, and H4 respectively.

[0039] (2) Hollow microspheres of different sizes were mixed with sodium hydroxide solid at a mass ratio of 1.5:1 and then calcined at 250°C for 5 hours.

[0040] (3) The roasted product was mixed with deionized water at a solid-liquid ratio of 1g:20ml, stirred and allowed to stand for 2 hours, and then vacuum filtered to obtain the water-washed residue.

[0041] (4) The water-washed residue and hydrochloric acid were mixed at a solid-liquid ratio of 1:10. After stirring evenly, the mixture was allowed to stand for 2 hours and then vacuum filtered and dried to obtain the fly ash slow-release carrier.

[0042] (5) Prepare a saturated urea solution at 60℃. Place the fly ash slow-release carrier in a vacuum device, evacuate to remove air from the pores, and draw in the saturated urea solution under negative pressure. Maintain the vacuum and let it stand for 30 minutes. Repeat the evacuation process 3 times. Filter to separate the solid and dry to obtain fly ash loaded with urea.

[0043] (6) Put the urea fertilizer core into the round pot granulator, spray 5% binder, and wait for the urea particles to stick together before adding fly ash loaded with urea at a ratio of 1:1.

[0044] (7) Then, a certain proportion of paraffin powder is evenly sprinkled on the urea granules. After the paraffin powder has completely melted, it covers the surface of the urea granules as a protective coating. Then, the radiant heating device and the disc granulator are turned off, and the granules are sieved to obtain fly ash coated slow-release fertilizer.

[0045] Figure 2 The image shows a scanning electron microscope image of the fly ash hollow microsphere carrier obtained in Example 1. As can be seen from the image, the originally smooth and dense outer wall of the glass was etched, exposing unreacted three-dimensional interlocking rod-shaped crystals. This is a typical morphology of the mullite and quartz skeleton of hollow microspheres. The obtained etched fly ash hollow microspheres have a rough surface and a certain porosity that is connected to the cavity.

[0046] Figure 3 The image shown is a cross-sectional scanning electron microscope image of the etched fly ash hollow microsphere-coated slow-release fertilizer obtained in Example 1. It can be seen that the surface of the slow-release fertilizer is a paraffin layer, the middle layer is etched fly ash hollow microspheres loaded with urea, and the core is the urea fertilizer core.

[0047] Figure 4The image shows the 24-hour cumulative release rate of the etched fly ash hollow microsphere-coated slow-release fertilizer obtained in Example 1. The 24-hour cumulative release rate is 32%, and the release period is 7 days. Compared with existing technologies, most existing fly ash-coated slow-release fertilizers use unmodified fly ash for direct coating or simple mixing. Because the microsphere cavities are not opened, nutrients are only adsorbed on the surface, resulting in problems such as excessively rapid initial release (24-hour release rate often >80%) and extremely short effective period. This invention successfully constructs microporous channels on the microsphere wall through solid-phase alkaline etching, realizing the migration and loading of urea into the microsphere cavity. Comparative experiments show that the slow-release performance of Example 1 is significantly better than that of the unmodified fly ash-coated fertilizer, effectively solving the problem of poor slow-release performance of inorganic fly ash carriers.

[0048] Example 2: (1) Prepare a 1% slurry from the fluidized bed fly ash. Set the unit time water supply and feed rate inside the six separation pipes as shown in Table 1. Then, start the water pump and feed pump in sequence to start the equipment. After waiting 48 hours, the classification is completed. Collect each gravity sedimentation classification product, filter, dry, weigh, and bag for later use. The product numbers are named H1, H2, H3, and H4 respectively.

[0049] (2) Hollow microspheres of different sizes were mixed with sodium hydroxide solid at a mass ratio of 1.5:1 and then calcined at 300°C for 4 hours.

[0050] (3) The roasted product was mixed with deionized water at a solid-liquid ratio of 1g:15ml, stirred and allowed to stand for 4 hours, and then vacuum filtered to obtain the water-washed residue.

[0051] (4) The water-washed residue and hydrochloric acid were mixed at a solid-liquid ratio of 1:10. After stirring evenly, the mixture was allowed to stand for 4 hours and then vacuum filtered and dried to obtain the fly ash slow-release carrier.

[0052] (5) Prepare an 80℃ saturated urea solution, place the fly ash slow-release carrier in a vacuum device, evacuate to remove air from the pores, draw in the saturated urea solution under negative pressure, maintain the vacuum and let stand for 10 minutes, repeat the vacuuming process 3 times. Filter to separate the solid and dry to obtain urea-loaded fly ash.

[0053] (6) Put the urea fertilizer core into the round pot granulator, spray the binder, and wait for the urea particles to stick together before adding fly ash loaded with urea at a ratio of 2:1.

[0054] (7) Then, a certain proportion of paraffin powder is evenly sprinkled on the urea granules. After the paraffin powder has completely melted, it covers the surface of the urea granules as a protective coating. Then, the radiant heating device and the disc granulator are turned off, and the granules are sieved to obtain fly ash coated slow-release fertilizer.

[0055] The cumulative release rate of the fly ash hollow microsphere-based slow-release material obtained in this embodiment is 67% over 24 hours, and the effective release period is 4 days.

[0056] Example 3: (1) Prepare a 3% concentration fluidized bed fly ash slurry. Set the unit time water supply and ore feed rate inside the six separation pipes. The specific parameters are shown in Table 1. Then start the water pump and feed pump in sequence to complete the equipment startup. After waiting for 48 hours, the classification is completed. Collect each gravity sedimentation classification product, filter, dry, weigh, and bag for later use. The product numbers are named H1, H2, H3, and H4 respectively.

[0057] (2) Hollow microspheres of different sizes were mixed with sodium hydroxide solid at a mass ratio of 2:1 and then calcined at 200°C for 10 hours.

[0058] (3) The roasted product was mixed with deionized water at a solid-liquid ratio of 1g:15ml, stirred and allowed to stand for 3 hours, and then vacuum filtered to obtain the water-washed residue.

[0059] (4) The water-washed residue and hydrochloric acid were mixed at a solid-liquid ratio of 1:20. After stirring evenly, the mixture was allowed to stand for 3 hours and then vacuum filtered and dried to obtain the fly ash slow-release carrier.

[0060] (5) Prepare a 50°C saturated urea solution. Place the fly ash slow-release carrier in a vacuum device and evacuate to remove air from the pores. Under negative pressure, draw in the saturated urea solution, maintain the vacuum and let it stand for 20 minutes. Repeat the evacuation process 5 times. Filter to separate the solid and dry to obtain urea-loaded fly ash.

[0061] (6) Put the urea fertilizer core into the round pot granulator, spray 5% binder, and wait for the urea particles to stick together before adding fly ash loaded with urea at a ratio of 1:1.

[0062] (7) Then evenly sprinkle 10% paraffin powder, and wait for the paraffin powder to completely melt and cover the surface of the urea granules as a protective coating. Then turn off the radiant heating device and the disc granulator, and pass it through a sieve to obtain fly ash coated slow-release fertilizer.

[0063] Figure 5 The image shows a scanning electron microscope (SEM) image of the fly ash hollow microsphere carrier obtained in Example 3. As can be seen from the image, the originally smooth and dense outer wall of the vitreous body has undergone effective reaction, transforming into sheet-like stacked hollow microspheres. The cumulative release rate of the fly ash hollow microsphere-based slow-release material obtained in this example is 37% over 24 hours, with an effective release period of 6 days. Urea diffuses from the interlayer gaps; the tortuous pores increase diffusion resistance, resulting in a slower release rate.

[0064] Example 4: (1) Prepare a 5% concentration fluidized bed fly ash slurry. Set the unit time water supply and feed rate inside the six separation pipes as shown in Table 1. Then start the water pump and feed pump in sequence to start the equipment. After waiting 48 hours, the classification is completed. Collect each gravity sedimentation classification product, filter, dry, weigh, and bag for later use. The product numbers are named H1, H2, H3, and H4 respectively.

[0065] (2) Hollow microspheres of different sizes were mixed with sodium hydroxide solid at a mass ratio of 5:1 and then calcined at 350°C for 2 hours.

[0066] (3) The roasted product was mixed with deionized water at a solid-liquid ratio of 1g:10ml, stirred and allowed to stand for 5 hours, and then vacuum filtered to obtain the water-washed residue.

[0067] (4) The water-washed residue and hydrochloric acid were mixed at a solid-liquid ratio of 1:15. After stirring evenly, the mixture was allowed to stand for 5 hours and then vacuum filtered and dried to obtain the fly ash slow-release carrier.

[0068] (5) Prepare a saturated urea solution at 45℃. Place the fly ash slow-release carrier in a vacuum device, evacuate to remove air from the pores, and draw in the saturated urea solution under negative pressure. Maintain the vacuum and let it stand for 50 minutes. Repeat the evacuation process 4 times. Filter to separate the solid and dry to obtain fly ash loaded with urea.

[0069] (6) Put the urea fertilizer core into the round pot granulator, spray the binder, and wait for the urea particles to stick together before adding fly ash loaded with urea at a ratio of 2:1.

[0070] (7) Then, a certain proportion of paraffin powder is evenly sprinkled on the urea granules. After the paraffin powder has completely melted, it covers the surface of the urea granules as a protective coating. Then, the radiant heating device and the disc granulator are turned off, and the granules are sieved to obtain fly ash coated slow-release fertilizer.

[0071] Figure 6 The image shows a scanning electron microscope (SEM) image of the fly ash hollow microsphere carrier obtained in Example 4. As can be seen from the image, the originally smooth and dense outer wall of the vitreous body has undergone effective reaction, transforming into hollow microspheres composed of stacked nanospheres. The cumulative release rate of the fly ash hollow microsphere-based slow-release material obtained in this example is 64% over 24 hours, with an effective release period of 2.5 days.

[0072] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0073] In the above embodiments of this application, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0074] In the several embodiments provided in this application, it should be understood that the disclosed technical content can be implemented in other ways. The device embodiments described above are merely illustrative; for example, the division of units can be a logical functional division, and in actual implementation, there may be other division methods. For instance, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the displayed or discussed mutual coupling, direct coupling, or communication connection may be through some interfaces; the indirect coupling or communication connection between units or modules may be electrical or other forms.

[0075] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0076] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

[0077] It should be understood that the above description is for illustrative purposes and not for limitation. Many embodiments and applications beyond the provided examples will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of this patent should not be determined by reference to the above description, but rather by reference to the foregoing claims and the full scope of their equivalents. For purposes of completeness, all articles and references, including patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended as a waiver of that subject matter, nor should it be construed as an indication that the applicant has not considered that subject matter as part of the disclosed inventive subject matter.

Claims

1. A method for preparing a fly ash-based slow-release fertilizer, characterized in that, Includes the following processes: S1. Mix fly ash hollow microspheres with solid sodium hydroxide and calcine them to obtain the calcined product; S2. The roasted product is mixed with deionized water and then filtered to obtain water-washed residue. S3. Mix the water-washed residue with an acidic solution, filter and dry to obtain a fly ash slow-release carrier; S4. Place the fly ash slow-release carrier in a negative pressure environment, introduce a saturated urea solution into the negative pressure environment, and obtain urea-loaded fly ash after filtration, separation and drying. S5. The fly ash loaded with urea is coated onto the surface of the urea fertilizer core with a binder, then sealed with paraffin wax, and after drying, fly ash-based slow-release fertilizer is obtained.

2. The method for preparing fly ash-based slow-release fertilizer according to claim 1, characterized in that, In S1, the mass ratio of fly ash hollow microspheres to sodium hydroxide solid is 1:1-5:1, the calcination temperature is 200-350℃, and the calcination time is 2-10h.

3. The method for preparing fly ash-based slow-release fertilizer according to claim 1, characterized in that, In S2, the solid-liquid ratio of the calcined product to deionized water is 1g:10ml~1g:20ml, and the settling time is 2-5h.

4. The method for preparing fly ash-based slow-release fertilizer according to claim 1, characterized in that, In S3, the acidic solution is hydrochloric acid solution, the solid-liquid ratio of the water washing residue to the hydrochloric acid solution is 1:10~1:20, and the settling time is 2-5 hours.

5. The method for preparing fly ash-based slow-release fertilizer according to claim 1, characterized in that, In S4, the temperature of the saturated urea solution is maintained at 45-80℃. The fly ash slow-release carrier is placed in a vacuum device to perform a vacuuming operation to create a negative pressure environment. After the saturated urea solution is introduced, the vacuuming operation is repeated 3-5 times.

6. The method for preparing fly ash-based slow-release fertilizer according to claim 1, characterized in that, Before S1, the process also includes: preparing fluidized bed fly ash into a slurry and performing gravity sedimentation classification, collecting the gravity sedimentation classification products and performing filtration and drying treatment, and finally obtaining hollow microspheres with enriched particle size.

7. The method for preparing fly ash-based slow-release fertilizer according to claim 1, characterized in that, In S5, the binder is a polyvinyl alcohol solution with a mass fraction of 2%-8%.

8. The method for preparing fly ash-based slow-release fertilizer according to claim 1, characterized in that, In S5, the urea fertilizer cores are put into a round pot granulator for rolling. After the urea fertilizer cores are sprayed with binder until they stick together, fly ash powder loaded with urea is added at a weight ratio of 1:1 to 1:5 for rolling and coating treatment. Finally, paraffin wax is used for sealing.

9. A fly ash-based slow-release fertilizer, characterized in that, The product is prepared by any one of the preparation methods described in claims 1-8. It has a granular morphology, with a urea fertilizer core at the center, a layer of fly ash loaded with urea on the outside of the urea fertilizer core, and a paraffin sealing layer on the outermost layer. The urea-loaded fly ash uses hollow fly ash microspheres, which are filled with urea. The outer wall of the fly ash hollow microspheres consists of silicon dioxide and aluminum oxide, and is covered with urea.

10. An application of the fly ash-based slow-release fertilizer according to claim 9 in the field of fertilizers.

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