Preparation method of large-particle urea surface treatment liquid and application of large-particle urea surface treatment liquid in polyurethane coating

Abstract

The invention discloses a preparation method of large-particle urea surface treatment liquid and application of the large-particle urea surface treatment liquid in polyurethane coating, and belongs to the technical field of chemical fertilizer industry. The large-particle urea surface treatment liquid is prepared from nano powder, a dispersing agent, a film-forming high polymer, a wetting agent, a defoaming agent and water. The large-particle urea for surface treatment is coated with polyurethane, so that the large-particle urea with high smoothness can be obtained under the conditions of high efficiency and low energy consumption. Tests show that the nitrogen nutrition accumulative release rate in 24 hours is 0.10-0.33%, the nitrogen nutrition accumulative release rate in 7 days is 3.59-5.02%, the nitrogen nutrition accumulative release rate in 28 days is 14.95-16.37%, the nitrogen nutrition accumulative release rate in 60 days is 30.46-32.09%, and the polyurethane coated urea is excellent in controlled release rate.

Description

Technical Field

[0001] This invention relates to the field of fertilizer industry technology, specifically to a method for preparing a surface treatment solution for large-particle urea and its application in polyurethane coating. Background Technology

[0002] With social development and rising labor costs, long-acting fertilizers are gaining increasing acceptance among users, with a growing demand for polyurethane controlled-release coated fertilizers. However, large-particle urea (2-5mm in diameter) produced in general factories has a rough surface, making it prone to incomplete coating during polyurethane coating, leading to a significant increase in initial nutrient release. To avoid this problem, urea needs to be coated multiple times, which increases coating costs and reduces productivity for film coating technology. Therefore, if large-particle urea has a smooth and rounded appearance, it can not only improve productivity and reduce production costs but also improve the appearance quality and enhance market competitiveness. Thus, how to improve the surface smoothness of large-particle urea during pretreatment to reduce coating dosage is an urgent problem to be solved.

[0003] Chinese patent CN101659583A discloses a post-processing method for granular fertilizer, including the following steps: 1) preheating the granular fertilizer to a certain temperature and placing it in a rotating drum to make the fertilizer particles flow; 2) applying a coating liquid to the surface of the granular fertilizer and drying it; the solvent of the coating liquid is water, and the solute includes urea and / or ammonium nitrate, with a mass concentration of 30%-80% for urea and / or ammonium nitrate in the coating liquid; the total coating amount on the surface of the granular fertilizer, on a dry basis, is 0.5-10% of the mass of the granular fertilizer. However, this patent uses a large amount of coating liquid, and the improvement of surface smoothness mainly relies on the frictional deformation between the granular fertilizer particles, resulting in relatively low grinding efficiency. Especially for fertilizers with large particle size and poor shape regularity, the bulk density is low, and the contact area between particles is small, resulting in a small frictional area between particles. It takes a long time to obtain a good surface smoothness, which is inefficient. Chinese patent CN104496566A discloses a method for reconstituted surface of large-particle urea. First, large-particle urea is placed in a drum equipped with lifting plates and heated air ducts, causing it to move clockwise or counterclockwise. Then, a reconstituted liquid is slowly and evenly sprayed onto the surface of the large-particle urea. The friction between the particles and the drum, lifting plates, and the particles themselves softens the raised parts of the urea surface or dissolves them in the reconstituted liquid under external force. During the drying process, the urea dissolved in the reconstituted liquid recrystallizes and fills the low-lying parts of the large-particle urea surface. Finally, the large-particle round urea becomes smooth and round, and after drying and hardening with a hot air blower, the surface treatment is complete. The reconstituted liquid involved in this patent is water, formaldehyde aqueous solution, or urea-formaldehyde aqueous solution. Formaldehyde aqueous solution or urea-formaldehyde aqueous solution contains formaldehyde, a highly toxic substance, posing significant environmental problems. Furthermore, the hot air treatment temperature of the urea particles in this patent is as high as 70-180℃, resulting in an inherent disadvantage in terms of energy consumption. It is evident that the current surface treatment of polyurethane-coated large-particle urea still suffers from significant problems such as poor smoothness, high energy consumption, and low efficiency. Therefore, developing a surface treatment liquid and method for large-particle urea to obtain high-smoothness large-particle urea under efficient and low-energy conditions is of great economic significance for promoting the overall reduction of polyurethane-coated large-particle urea. Summary of the Invention

[0004] In view of the above-mentioned prior art, the purpose of this invention is to provide a method for preparing a surface treatment solution for large-particle urea and its application in polyurethane coating. This invention prepares a surface treatment solution for large-particle urea that can achieve high-smoothness large-particle urea under efficient and low-energy conditions. After polyurethane coating, it can produce polyurethane-coated urea with excellent controlled-release rate.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a surface treatment liquid for large-particle urea, comprising the following raw materials in parts by weight: 20-60 parts of nanoparticle powder, 1-3 parts of dispersant, 0.5-1.5 parts of film-forming polymer, 0.5-2 parts of wetting agent, 0.1-0.5 parts of defoamer, and 700-1200 parts of water; The nanopowder is one of nano-calcium carbonate, nano-calcium sulfate, and nano-silica. The dispersant is one of polyethylene glycol and polyvinylpyrrolidone K30; The film-forming polymer is one of sodium polyacrylate, polyacrylamide, and sodium alginate; The wetting agent is one of alkyl glycosides and fatty alcohol polyoxyethylene ethers.

[0006] Furthermore, the particle size of the nanoparticles is 1~100nm.

[0007] Furthermore, the number average molecular weight of polyethylene glycol is 200~600 g / mol.

[0008] Furthermore, the number-average molecular weight of the film-forming polymers is 50,000 to 200,000 g / mol.

[0009] Furthermore, the defoamer is a polyether-modified polysiloxane.

[0010] In a second aspect, the present invention provides a method for preparing the large-particle urea surface treatment solution, wherein the raw materials are taken in parts by mass, and the nanoparticles, dispersant, wetting agent, defoamer and water are mixed and dispersed at a dispersion rate of 9500~11000 rpm for 5~8 hours. Then the dispersion rate is increased to 12000~15000 rpm, and a film-forming polymer is added and dispersed for 4~7 hours to obtain the large-particle urea surface treatment solution.

[0011] A third aspect of the present invention provides the application of the aforementioned large-particle urea surface treatment solution in the preparation of polyurethane-coated fertilizers.

[0012] In a fourth aspect, the present invention provides a coated fertilizer, wherein the core of the coated fertilizer is sprayed with the aforementioned large-particle urea surface treatment liquid.

[0013] Furthermore, the mass ratio of the large-particle urea surface treatment solution to the fertilizer core is (0.01-1):100.

[0014] In a fifth aspect, the present invention provides a method for preparing the coated fertilizer, comprising the following steps: preheating the fertilizer core to 35-45°C, spraying a surface treatment liquid of large-particle urea, and after the smoothness meets the requirements, drying it with hot air at 55-65°C for 15-25 minutes, and after air drying, spraying a polyurethane coating liquid to obtain the coated fertilizer.

[0015] The beneficial effects of this invention are: This invention utilizes nanoparticles, dispersants, film-forming polymers, wetting agents, defoamers, and water to prepare a surface treatment solution for large-particle urea. The large-particle urea used for surface treatment is then coated with polyurethane, resulting in highly smooth large-particle urea under efficient and low-energy conditions. Experiments show that the cumulative nitrogen release rate is 0.10–0.33% after 24 hours, 3.59–5.02% after 7 days, 14.95–16.37% after 28 days, and 30.46–32.09% after 60 days, demonstrating excellent controlled-release rate of the polyurethane-coated urea. Detailed Implementation

[0016] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0017] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.

[0018] This invention designs a surface treatment liquid for large-particle urea using nanoparticles as the main grinding aid. Through the optimized combination of auxiliary substances such as dispersants, wetting agents, and film-forming polymers, it significantly promotes the grinding process of large-particle urea. This is mainly because after the nanoparticles are dispersed into nanoscale microparticles, they have excellent flowability and wetting and dispersibility, forming a highly fluid film on the surface of large-particle urea. In this film, the highly wear-resistant inorganic nanoparticles with excellent grinding aid properties achieve a grinding effect through rapid relative sliding between urea particles. During the grinding process, the treatment liquid composed of nanoparticles has two main effects. First, it fills the large gaps between urea particles caused by uneven surfaces through rapid flow and surface wetting. This increases the bulk density of urea particles during grinding, ensuring that the irregular urea particles are always in close contact. This close contact also increases the friction between particles. Second, the irregular protrusions on the surface of urea particles are subjected to strong friction from the highly wear-resistant inorganic nanoparticles, which quickly smooths out the protrusions on the surface of the urea particles and rapidly improves the smoothness of the urea particle surface.

[0019] The dispersants added in this invention, namely polyethylene glycol and polyvinylpyrrolidone K30, have a very strong anchoring and dispersing effect on inorganic nanoparticles. By coating and physically isolating the surface of inorganic powders, they reduce the agglomeration between individual inorganic nanoparticles, thereby achieving nanoscale dispersion of inorganic nanoparticles.

[0020] The film-forming polymer added in this invention mainly acts as a delay agent for the nanoparticles on the surface of urea particles, increasing the contact friction between the nanoparticles and the urea particle surface, thereby making it easier to improve the smoothness of the urea particle surface. Secondly, during the drying process, the nanoparticles adhered to the urea surface by the film-forming polymer can form a relatively smooth and dense film on the urea particle surface by utilizing its film-forming effect. This film layer can effectively avoid the problems of decreased smoothness caused by the detachment of nanoparticles from the surface of urea particles after drying, as well as dust pollution caused by the detachment of nanoparticles.

[0021] The test materials used in the embodiments of this invention, unless otherwise specified, are all conventional test materials in the art and can be purchased through commercial channels. The alkyl glycoside (APG) used is manufactured by Shandong Huage Group, product name APG0810. The polyether-modified polysiloxane used has CAS number 27306-78-1, manufactured by Anhui Mingyi Silicon Industry Co., Ltd., product name MY 2000; the sodium polyacrylate used has CAS number 9003-04-7, manufactured by Xumei Biotechnology Co., Ltd.; the nano-calcium carbonate used has CAS number 471-34-1, manufactured by Zibo Zengsheng Chemical Co., Ltd.; the nano-calcium sulfate used has CAS number 99400-01-8, manufactured by Changzhou Guangwei New Material Technology Co., Ltd.; and the nano-silica used has CAS number 7631-86-9, manufactured by Hubei Huifu Nanomaterials Co., Ltd., product category or brand name HL-200.

[0022] Example 1: A method for preparing a surface treatment solution for large-particle urea The large-particle urea surface treatment solution of the present invention comprises the following components: nanoparticles, dispersant, film-forming polymer, wetting agent, defoamer, and water.

[0023] The nanoparticles used are nano-calcium carbonate with a particle size of 50 nm; The dispersant used is polyethylene glycol, with a number average molecular weight of 200 g / mol; The film-forming polymer used is sodium polyacrylate, with a number-average molecular weight of 100,000 g / mol; The wetting agent used is an alkyl glycoside; the defoamer used is a polyether-modified polysiloxane.

[0024] The preparation method of large particle urea surface treatment solution is as follows: First, 50g of nanoparticles, 2g of dispersant, 1g of wetting agent, 0.4g of defoamer, and 1000g of water are placed in a high-speed dispersion vessel and dispersed at a dispersion rate of 10,000 rpm for 7 hours. Then, the dispersion rate is increased to 13,000 rpm, and 1g of film-forming polymer is added. After dispersing for 6 hours, the material is discharged to obtain a large-particle urea surface treatment solution.

[0025] Example 2: A method for preparing a surface treatment solution for large-particle urea The large-particle urea surface treatment solution of the present invention comprises the following components: nanoparticles, dispersant, film-forming polymer, wetting agent, defoamer, and water.

[0026] The nanoparticles used are nano-calcium sulfate with a particle size of 1 nm; The dispersant used is polyethylene glycol, with a number average molecular weight of 600 g / mol; The film-forming polymer used is polyacrylamide with a number-average molecular weight of 50,000 g / mol; The wetting agent used is fatty alcohol polyoxyethylene ether; the defoamer used is polyether-modified polysiloxane.

[0027] The preparation method of large particle urea surface treatment solution is as follows: First, 20g of nanoparticles, 1g of dispersant, 0.5g of wetting agent, 0.1g of defoamer, and 700g of water are placed in a high-speed dispersion vessel and dispersed at a dispersion rate of 9500 rpm for 5 hours. Then, the dispersion rate is increased to 12000 rpm, and 0.5g of film-forming polymer is added. After dispersing for 4 hours, the material is discharged to obtain a large-particle urea surface treatment solution.

[0028] Example 3: A method for preparing a surface treatment solution for large-particle urea The large-particle urea surface treatment solution of the present invention comprises the following components: nanoparticles, dispersant, film-forming polymer, wetting agent, defoamer, and water.

[0029] The nanoparticles used are nano-silica with a particle size of 100 nm; The dispersant used was polyvinylpyrrolidone K30; The film-forming polymer used is sodium alginate, with a number-average molecular weight of 200,000 g / mol; The wetting agent used is fatty alcohol polyoxyethylene ether; the defoamer used is polyether-modified polysiloxane.

[0030] The preparation method of large particle urea surface treatment solution is as follows: First, 60g of nanoparticles, 3g of dispersant, 2g of wetting agent, 0.5g of defoamer, and 1200g of water were placed in a high-speed dispersion vessel and dispersed at a dispersion rate of 11000 rpm for 8 hours. Then, the dispersion rate was increased to 15000 rpm, and 1.5g of film-forming polymer was added. After dispersing for 7 hours, the material was discharged to obtain a large-particle urea surface treatment solution.

[0031] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that no dispersant is added; instead, 2 parts of dispersant are replaced with 2 parts of water in equal amounts. The preparation method is as follows: First, 50g of nanoparticles, 1g of wetting agent, 0.4g of defoamer, and 1002g of water are placed in a high-speed dispersion vessel and dispersed at a dispersion rate of 10,000 rpm for 7 hours. Then, the dispersion rate is increased to 13,000 rpm, and 1g of film-forming polymer is added. After dispersing for 6 hours, the material is discharged to obtain a large-particle urea surface treatment solution.

[0032] The nanoparticles used are nano-calcium carbonate with a particle size of 50 nm. The film-forming polymer used is sodium polyacrylate, with a number-average molecular weight of 100,000 g / mol; The wetting agent used is an alkyl glycoside; the defoamer used is a polyether-modified polysiloxane.

[0033] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that no film-forming polymer is added; instead, 1 part of the film-forming polymer is replaced with 1 part of water. The preparation method is as follows: First, 50g of nanoparticles, 2g of dispersant, 1g of wetting agent, 0.4g of defoamer, and 1000g of water are placed in a high-speed dispersion vessel and dispersed at a dispersion rate of 10,000 rpm for 7 hours. Then, the dispersion rate is increased to 13,000 rpm, and 1g of water is added. After dispersing for 6 hours, the material is discharged to obtain a large-particle urea surface treatment solution.

[0034] The nanoparticles used are nano-calcium carbonate with a particle size of 50 nm; The dispersant used is polyethylene glycol, with a number average molecular weight of 200 g / mol; The wetting agent used is an alkyl glycoside; the defoamer used is a polyether-modified polysiloxane.

[0035] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that no wetting agent is added; instead, 1 part wetting agent is replaced with 1 part water. The preparation method is as follows: First, 50g of nanoparticles, 2g of dispersant, 0.4g of defoamer, and 1001g of water were placed in a high-speed dispersion vessel and dispersed at a dispersion rate of 10,000 rpm for 7 hours. Then, the dispersion rate was increased to 13,000 rpm, and 1g of film-forming polymer was added. After dispersing for 6 hours, the material was discharged to obtain a large-particle urea surface treatment solution.

[0036] The nanoparticles used are nano-calcium carbonate with a particle size of 50 nm; The dispersant used is polyethylene glycol, with a number average molecular weight of 200 g / mol; The film-forming polymer used is sodium polyacrylate, with a number-average molecular weight of 100,000 g / mol; The defoamer used is polyether-modified polysiloxane.

[0037] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that no dispersant, film-forming polymer, or wetting agent is added; instead, an equal amount of water is used. The preparation method is as follows: The preparation method of large particle urea surface treatment solution is as follows: First, 50g of nanoparticles, 0.4g of defoamer, and 1003g of water were placed in a high-speed dispersion vessel and dispersed at a dispersion rate of 10,000 rpm for 7 hours. Then, the dispersion rate was increased to 13,000 rpm, and 1g of water was added. After dispersing for 6 hours, the material was discharged to obtain a large-particle urea surface treatment solution.

[0038] The nanoparticles used are nano-calcium carbonate with a particle size of 50 nm; The defoamer used is polyether-modified polysiloxane.

[0039] Experimental Example 1 Large particle urea surface treatment, polyurethane coating, and controlled release rate testing: The surface treatment solution obtained in Examples 1-3 and Comparative Examples 1-3 was used to treat large-particle urea. Specifically, the large-particle urea (4mm particle size) was preheated to 40°C and then placed into a rotating drum. The drum contained evenly distributed lifting plates to aid in fertilizer agitation. A large beam with several nozzles distributed within it was used to spray the surface treatment solution onto the rotating fertilizer particles. The amount of surface treatment solution used was 0.1% of the mass of the urea particles being treated. The rolling of fertilizer granules disperses the surface treatment liquid on the surface of all fertilizer granules, forming a liquid film. The nanoparticles in this liquid film have excellent grinding aid effect. In addition, the surface of urea granules softens in a moist state. Under the friction between granules and the grinding aid effect of nanoparticles in the surface treatment liquid, the smoothness of the granule surface is rapidly improved, and the sphericity of the granules is also rapidly improved. The urea granules are transferred to the drying tunnel and dried with hot air at 60℃ for 20 minutes to obtain large granule urea with a smooth surface and suitable for polyurethane coating. Weigh 4.0 kg of the smooth, polyurethane-coated large-particle urea obtained above, and heat it to 50°C in a coating machine. Then, prepare the A and B two-component polyurethane coating solution. The A component coating solution has the following formula by weight: 30 g of 4110 polyether polyol, 40 g of polybutylene adipate diol, 20 g of castor oil, and 10 g of polypropylene adipate as a plasticizer. The B component is isocyanate PM-200, also called polymeric MDI (polymethylene polyphenyl polyisocyanate), purchased from Wanhua Chemical Group, product number Wanhua PM-200. Take 11 grams of A and 11 grams of B, and spray them onto the surface of urea particles in the coating machine. The two-component polyurethane coating liquid mixes with the continuously moving large urea particles and is quickly and evenly distributed onto the surface of the large urea particles. After about 4 minutes, the two-component polyurethane coating liquid solidifies into a dense, tough and smooth film layer on the surface of the large urea particles. Repeat the above coating process twice more to coat a total of 3 polyurethane film layers. The product is then discharged as polyurethane-coated urea.

[0040] Since there are no specific applicable standards for the surface smoothness and roughness of fertilizer particles, the nitrogen nutrient controlled release rate of the polyurethane-coated urea product is used to indirectly reflect the surface smoothness of large-particle urea after surface treatment. Large-particle urea treated with the surface treatment solutions obtained in Examples 1-3 and Comparative Examples 1-4, i.e., the corresponding polyurethane-coated urea, were tested for nitrogen nutrient cumulative release rate according to the provisions of the national standard "GB / T 23348-2009 Slow-Release Fertilizers". The cumulative release rate of nitrogen nutrient was continuously monitored, and the time to release 80% of the nitrogen nutrient was recorded as the fertilizer slow-release performance index; the longer the time, the better the performance. The test results are shown in Tables 1 and 2.

[0041] Table 1. Cumulative release rate of nitrogen nutrients Table 1. Cumulative release days of nitrogen nutrients As can be seen from the data in Tables 1 and 2, by continuously monitoring the cumulative release rate of nitrogen nutrients and recording the time for releasing 80% of nitrogen nutrients as an indicator of fertilizer slow-release performance, the data of Examples 1-3 are better than those of Comparative Examples 1-4. That is, the combined use of dispersant, film-forming polymer and wetting agent in the preparation of large-particle urea surface treatment solution has a promoting effect on the cumulative release of nitrogen nutrients. In Examples 1-3, the cumulative nitrogen release rate over 24 hours was less than 0.5%, the cumulative nitrogen release rate over 7 days did not exceed 6%, and the cumulative nitrogen release rates over 28 days and 60 days remained stable within 40% and 80% of the total nitrogen content, respectively. This indicates that the large urea particles treated with the treatment solution prepared in this invention, after polyurethane coating, exhibit excellent controlled-release performance. In Comparative Example 1, no dispersant was added, and the cumulative nitrogen release rate of Comparative Example 1 reached the highest value among all examples and comparative examples. This may be because without a dispersant, the nanoparticles are difficult to disperse to the nanoscale. Over-agglomerated nanoparticles do not promote the grinding process of urea particles; on the contrary, after drying, the over-agglomerated nanoparticles adsorb onto the surface of urea particles, exacerbating the surface unevenness of the particles, ultimately resulting in very poor controlled-release effect after polyurethane coating. In Comparative Example 2, no film-forming polymer was added, and the cumulative nitrogen release rate of Comparative Example 2 increased significantly. This indicates that the film-forming polymer plays a crucial role in the urea particle grinding process. This may be because the film-forming polymer has a certain retarding effect on the nanoparticles on the urea particle surface during the grinding process, increasing the contact friction between the nanoparticles and the urea particle surface, thus making it easier to improve the surface smoothness of the urea particles. Ultimately, it has a positive effect on the polyurethane coating process, resulting in coated urea with a better sustained-release effect. In Comparative Example 3, no wetting agent was added. The cumulative nitrogen nutrient release rate of Comparative Example 3 at each time period was significantly greater than that of Example 1. This may be because the wetting agent can promote the wetting effect of the nanoparticles on the urea particle surface in the treatment liquid, improve the surface contact effect between the nanoparticles and the urea particles, and thus maximize the contact friction between the nanoparticles and the urea particle surface, thereby improving the grinding efficiency. At the same time, it also plays a very significant role in rapidly improving the surface smoothness of the urea particles.

[0042] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A surface treatment solution for large-particle urea, characterized in that, The raw materials include the following parts by weight: 20-60 parts of nanoparticles, 1-3 parts of dispersant, 0.5-1.5 parts of film-forming polymer, 0.5-2 parts of wetting agent, 0.1-0.5 parts of defoamer, and 700-1200 parts of water; The nanopowder is one of nano-calcium carbonate, nano-calcium sulfate, and nano-silica. The dispersant is one of polyethylene glycol and polyvinylpyrrolidone K30; The film-forming polymer is one of sodium polyacrylate, polyacrylamide, and sodium alginate; The wetting agent is one of alkyl glycosides and fatty alcohol polyoxyethylene ethers.

2. The large-particle urea surface treatment solution according to claim 1, characterized in that, The particle size of the nanoparticles is 1~100nm.

3. The large-particle urea surface treatment solution according to claim 1, characterized in that, The number average molecular weight of polyethylene glycol is 200~600 g / mol.

4. The large-particle urea surface treatment solution according to claim 1, characterized in that, The number-average molecular weight of the film-forming polymers is 50,000 to 200,000 g / mol.

5. The large-particle urea surface treatment solution according to claim 1, characterized in that, The defoamer is a polyether-modified polysiloxane.

6. The method for preparing the large-particle urea surface treatment solution according to any one of claims 1-5, characterized in that, Take the raw materials in parts by weight, first mix the nanopowder, dispersant, wetting agent, defoamer and water, and disperse at a dispersion rate of 9500~11000 rpm for 5~8 hours. Then increase the dispersion rate to 12000~15000 rpm, add the film-forming polymer, and disperse for 4~7 hours to obtain large particle urea surface treatment solution.

7. The application of the large-particle urea surface treatment solution according to any one of claims 1-5 in the preparation of polyurethane coated fertilizer.

8. A coated fertilizer, characterized in that, The core coating of the coated fertilizer is sprayed with the large-particle urea surface treatment liquid as described in any one of claims 1-5.

9. The coated fertilizer according to claim 8, characterized in that, The mass ratio of large-particle urea surface treatment solution to fertilizer core is (0.01-1):

100.

10. The method for preparing the coated fertilizer according to claim 8 or 9, characterized in that, The process includes the following steps: preheating the fertilizer core to 35-45℃, spraying a surface treatment liquid of large-particle urea, and after the smoothness meets the requirements, drying it with hot air at 55-65℃ for 15-25 minutes, and after air drying, spraying a polyurethane coating liquid to obtain a coated fertilizer.

Images