Preparation method of water-soluble fertilizer containing humic acid

Abstract

The invention relates to a preparation method of a water-soluble fertilizer, in particular to a preparation method of a humic acid-containing water-soluble fertilizer. Aiming at the water-soluble fertilizer containing the humic acid, a strategy of firstly wrapping a layer of chitosan is innovatively adopted, and active groups such as carboxyl on the surface of the humic acid are effectively prevented from directly reacting with hydroxyl of polyvinyl alcohol, so that the stability of a foaming film is remarkably improved. Chitosan is used as a common additive of the water-soluble fertilizer, not only has good water solubility and biocompatibility, but also can be used as an isolating layer to prevent humic acid from damaging a polyvinyl alcohol foaming film structure, and the moisture-proof and anti-caking effects of the water-soluble fertilizer in the storage and transportation processes are ensured.

Description

Technical Field

[0001] This invention relates to a method for preparing a water-soluble fertilizer, specifically a method for preparing a water-soluble fertilizer containing humic acid. Background Technology

[0002] Solid water-soluble fertilizers, as an important type of fertilizer in modern agriculture, are highly favored for their high nutrient concentration, easy solubility, and good compatibility. These fertilizers can be flexibly applied through various methods such as fertigation and foliar spraying, and can also be combined with drip irrigation technology to achieve integrated water and fertilizer management, greatly improving water resource utilization efficiency and fertilizer absorption rate. Among them, humic acid water-soluble fertilizers, as a special type of solid water-soluble fertilizer, have unique soil-improving and crop-growth-promoting effects due to their rich humic acid content, making them a popular product in modern agriculture.

[0003] A common technical challenge in the preparation, storage, and transportation of solid water-soluble fertilizers is the problem of water absorption and clumping. Because the main components of water-soluble fertilizers, such as urea and phosphates, are highly hygroscopic, they readily absorb moisture from the air in humid environments, causing fertilizer particles to clump together. This problem is particularly pronounced for humic acid water-soluble fertilizers. The abundant hydrophilic groups, such as carboxyl groups, on the surface of humic acid further enhance its hygroscopicity, making humic acid water-soluble fertilizers more prone to clumping during storage and transportation. This not only affects the physical properties and application efficacy of the fertilizer but may also lead to fertilizer waste and increased application difficulty.

[0004] To address the problem of hygroscopic agglomeration in solid water-soluble fertilizers, existing technologies propose forming a coating film on the crystal surface. However, the water-blocking properties of this coating film reduce its water solubility. To further address this issue, existing technologies propose coating the fertilizer granules with a hydrophilic modified polyvinyl alcohol (PVA) foam film. This coating technology utilizes the excellent film-forming and hydrophilic properties of PVA, combined with a foaming process, to form a porous microporous foam film on the fertilizer granule surface. This foam film effectively isolates the fertilizer granules, preventing direct contact between them, thus significantly improving the fertilizer's anti-caking performance. Simultaneously, due to the porous structure of the foam film, water can quickly penetrate during fertilizer dissolution, ensuring the fertilizer's dissolution rate and efficiency. However, for water-soluble fertilizers containing humic acid, the above method has significant shortcomings. The surface of humic acid is rich in active groups such as carboxyl groups. These groups easily react chemically with the hydroxyl groups of PVA in a humid environment, leading to decreased stability of the foam film and even damage to the coating structure, thereby losing its original anti-caking and dissolution-promoting effects. Therefore, for water-soluble fertilizers containing humic acid, there is an urgent need to develop a new preparation process that can effectively prevent moisture absorption and clumping while ensuring the stability of the coating. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes a solution to the issue of humic acid-containing water-soluble fertilizers absorbing water and clumping. By first loading a chitosan film as an intermediate layer on the outside of the base fertilizer raw material, and then loading an aldehyde-modified polyvinyl alcohol foam film, the problem of direct contact between humic acid and polyvinyl alcohol is solved. Moreover, the aldehyde-modified polyvinyl alcohol and chitosan form covalent bonds, thereby improving the compatibility between polyvinyl alcohol and chitosan.

[0006] Specifically, this invention provides a method for preparing a humic acid-containing water-soluble fertilizer, which includes the following steps: Step 1: Mix the base fertilizer raw materials evenly, and then extrude and granulate the evenly mixed base fertilizer raw materials. During the process, spray water appropriately, with the amount of water being 2wt% to 4wt% of the total weight of the base fertilizer raw materials. Screen out particles with a particle size of 2mm to 3mm. Step 2: Dissolve chitosan powder in an acidic aqueous solution and stir until completely dissolved to obtain a chitosan solution with a concentration of 1wt% to 3wt%. Put the particles into a coating machine and use the chitosan solution to uniformly spray the surface of the particles. The spraying amount is 1% to 3% of the total particles to form a chitosan film. Dry and cure the particles coated with the chitosan film. Step 3: Disperse polyvinyl alcohol in water, heat and stir to prepare a polyvinyl alcohol solution of 11wt% to 12wt%, add 0.5% to 2% of the weight of polyvinyl alcohol aldehyde oxidizing agent to the polyvinyl alcohol solution, and react for 2 to 4 hours at a pH of 4 to 6 and a temperature of 50℃ to 70℃ to introduce aldehyde groups into the polyvinyl alcohol molecular chain, thereby obtaining an aldehyde-modified polyvinyl alcohol solution. Step 4: Add a foaming agent and a foam stabilizer to the aldehyde-modified polyvinyl alcohol solution. The foaming agent includes sulfonate polyether diol, glycerol, butanediol and sodium dodecylbenzene sulfonate. The foam stabilizer is a mixture of ammonium stearate and lauryl alcohol polyoxyethylene ether. After stirring evenly, a foaming liquid is obtained. Step 5: Spray the foaming liquid onto the surface of the chitosan-coated base fertilizer granules to form an aldehyde-modified polyvinyl alcohol foam film. The coating amount of the foam film is 3% to 5% of the total granule weight. Step 6: Dry the coated granules at a temperature of 50℃ to 70℃ for 4 to 8 hours to obtain a water-soluble fertilizer containing humic acid.

[0007] Specifically, the base fertilizer raw materials include humic acid, nitrogen-phosphorus-potassium compound fertilizer, trace element fertilizer and organic matter, wherein the weight ratio of humic acid is 5% to 20% of the total weight of the base fertilizer raw materials.

[0008] Specifically, the chitosan solution uses an aqueous acetic acid solution with a concentration of 0.1N to 1N as the solvent; the drying and curing step is carried out at a temperature of 40°C to 60°C for 1 to 3 hours to ensure that the chitosan film is completely cured on the surface of the base fertilizer particles.

[0009] Specifically, the nitrogen, phosphorus and potassium compound fertilizer is composed of urea, diammonium phosphate and potassium sulfate mixed in a weight ratio of 1.2:1:0.8 to 1.8:1.2:1.

[0010] Specifically, the micronutrient fertilizer includes boric acid, zinc sulfate, manganese sulfate, and ammonium molybdate, each accounting for 0.1% to 1% of the weight of the base fertilizer raw materials; the organic matter is amino acids, each accounting for 1% to 5% of the weight of the base fertilizer raw materials; the foaming agent contains sulfonate polyether diol, glycerol, butanediol, and sodium dodecylbenzene sulfonate in weight ratios of 1% to 3%, 0.5% to 1.5%, 0.5% to 1.5%, and 0.2% to 0.6% of the weight of polyvinyl alcohol, respectively; and the foam stabilizer contains ammonium stearate and lauryl alcohol polyoxyethylene ether in weight ratios of 1:1 to 1:2.

[0011] Specifically, the humic acid is pretreated to improve its compatibility and bioactivity with the base fertilizer raw materials. The pretreatment includes at least one of acidification, alkalization or oxidation treatment.

[0012] Specifically, the base fertilizer raw materials also include mineral fillers such as bentonite, vermiculite, or perlite to improve the fertilizer's water retention and slow-release properties; the drying process also includes ventilation measures to promote rapid evaporation of moisture and improve drying efficiency; the preparation process also includes screening the base fertilizer raw materials to remove impurities and unqualified particles, thereby improving the fertilizer's uniformity and quality.

[0013] Specifically, the aldehyde-modifying agent is selected from one or more of formaldehyde, acetaldehyde, and propionaldehyde.

[0014] Specifically, an appropriate amount of acid or base catalyst is added during the reaction process to promote the aldehyde reaction and improve the efficiency of aldehyde group introduction.

[0015] Specifically, the acid catalyst is selected from one or more of hydrochloric acid, sulfuric acid, or phosphoric acid, and the base catalyst is selected from one or more of sodium hydroxide, potassium hydroxide, or sodium carbonate. The amount of catalyst used is 0.1% to 1% of the weight of polyvinyl alcohol.

[0016] Compared with the prior art, the present invention has the following advantages: Firstly, for water-soluble fertilizers containing humic acid, this invention innovatively employs a strategy of first coating with a layer of chitosan. This effectively prevents the active groups such as carboxyl groups on the surface of humic acid from directly reacting with the hydroxyl groups of polyvinyl alcohol, thereby significantly improving the stability of the foamed film. Chitosan, a commonly used additive in water-soluble fertilizers, not only possesses excellent water solubility and biocompatibility but also acts as a barrier layer, preventing humic acid from damaging the structure of the polyvinyl alcohol foamed film and ensuring the moisture-proof and anti-caking effect of the water-soluble fertilizer during storage and transportation.

[0017] Secondly, this invention further modifies polyvinyl alcohol with aldehyde groups, enabling it to form stable covalent bonds with chitosan, thereby enhancing the structural stability and compatibility of the entire coating layer. Moreover, the specific amount of aldehyde group modification also improves the biodegradability of polyvinyl alcohol. Detailed Implementation

[0018] Example 1 A method for preparing a humic acid-containing water-soluble fertilizer includes the following specific steps and points: first step: The base fertilizer raw materials include humic acid (10% by weight of the total base fertilizer raw materials), nitrogen, phosphorus and potassium compound fertilizer (mixed with urea, diammonium phosphate and potassium sulfate in a weight ratio of 1.5:1.1:0.9, accounting for 85% by weight of the total base fertilizer raw materials), micronutrient fertilizer (including boric acid, zinc sulfate, manganese sulfate and ammonium molybdate, each accounting for 0.5% by weight of the base fertilizer raw materials), and organic matter (amino acids, each accounting for 3% by weight of the base fertilizer raw materials).

[0019] Mix the base fertilizer raw materials evenly, then extrude and granulate the mixed base fertilizer raw materials, spraying water appropriately during the process, with the water amount being 3wt% of the total weight of the base fertilizer raw materials, and screen out particles with a particle size of 2.5mm.

[0020] Step Two: Chitosan powder was dissolved in a 0.5N aqueous acetic acid solution and stirred until completely dissolved to obtain a 2wt% chitosan solution.

[0021] The granules are placed into a coating machine, and a chitosan solution is used to uniformly spray the surface of the granules. The amount of coating is 2% of the total granules, forming a chitosan film.

[0022] The particles coated with chitosan film were dried and cured at 50°C for 2 hours to ensure that the chitosan film was completely cured on the surface of the fertilizer particles.

[0023] Step 3: Polyvinyl alcohol was dispersed in water and heated and stirred to prepare an 11.5 wt% polyvinyl alcohol solution.

[0024] Add 1% formaldehyde by weight of polyvinyl alcohol as an aldehyde-modifying agent and 0.5% hydrochloric acid as an acid catalyst to a polyvinyl alcohol solution. React for 3 hours at pH 5 and temperature 60℃ to introduce aldehyde groups into the polyvinyl alcohol molecular chain, thus obtaining an aldehyde-modified polyvinyl alcohol solution.

[0025] Step 4: A foaming agent and a foam stabilizer are added to an aldehyde-modified polyvinyl alcohol solution. The foaming agent comprises sulfonate polyether diol, glycerol, butanediol, and sodium dodecylbenzenesulfonate (at weight ratios of 2%, 1%, 1%, and 0.4% of the polyvinyl alcohol), and the foam stabilizer is a mixture of ammonium stearate and lauryl polyoxyethylene ether (at a weight ratio of 1:1.5). After stirring until homogeneous, a foaming solution is obtained.

[0026] Step 5: The foaming liquid is sprayed onto the surface of the base fertilizer granules coated with chitosan to form an aldehyde-modified polyvinyl alcohol foam film. The amount of the foam film is 4% of the total granule weight.

[0027] Step 6: The coated granules were dried at 60°C for 6 hours to obtain a water-soluble fertilizer containing humic acid.

[0028] Example 2 The difference between Example 2 and Example 1 is that acetaldehyde is used instead of formaldehyde in the third step.

[0029] Example 3 The difference between Example 2 and Example 1 is that sodium hydroxide is used instead of hydrochloric acid in the third step.

[0030] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the second and third steps are missing, namely, the direct spraying of polyvinyl alcohol foam film onto the outer surface of the humic acid-containing base fertilizer.

[0031] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the third step is missing, namely, the outer surface of the humic acid-containing base fertilizer is coated with chitosan and then sprayed with polyvinyl alcohol foam film.

[0032] Comparative Example 3 The difference between Comparative Example 3 and Example 1 lies in the third step, where 0.2% by weight of formaldehyde is added to the polyvinyl alcohol solution as an aldehyde-modifying agent.

[0033] Comparative Example 4 The difference between Comparative Example 4 and Example 1 lies in the third step, where 5% by weight of formaldehyde is added to the polyvinyl alcohol solution as an aldehyde-modifying agent.

[0034] Characterization test 1. Dissolution rate test Test objective: To evaluate the dissolution rate of products prepared in different examples and comparative examples under unstirred conditions.

[0035] Test conditions: Sample preparation: Weigh 4g of each of the products prepared in Examples 1-3 and Comparative Examples 1-4, accurate to 0.01g.

[0036] Test water: 1L of purified water that meets the quality standards for agricultural irrigation water.

[0037] Test temperature: Set a constant test temperature of 25℃.

[0038] Test procedure: Place the sample evenly in the test water without stirring, and record the time required from the time the sample is added to the water until it is completely dissolved (with no obvious solid particles suspended).

[0039] Repeated testing: To ensure the accuracy of the test results, it is recommended to perform multiple repeated tests and take the average value as the final result.

[0040] 2. Determination of water-insoluble matter content Test objective: To evaluate the content of water-insoluble impurities in the products prepared in different examples and comparative examples.

[0041] Test conditions: Sample preparation: Weigh appropriate amounts of the products prepared in Examples 1-3 and Comparative Examples 1-4 (determined according to NY / T1973-2021 standard), accurate to 0.01g.

[0042] Test water: Purified water that meets the quality standards for agricultural irrigation water.

[0043] Test procedure: The test shall be conducted in accordance with the NY / T 1973-2021 standard, including steps such as dissolving, filtering, washing, drying and weighing.

[0044] Result calculation: The insoluble content is calculated based on the final mass of the filter residue and filter paper.

[0045] 3. Anti-caking test Test objective: To evaluate the anti-caking performance of products prepared in different embodiments and comparative examples after pressure and slamming treatment.

[0046] Test conditions: Sample preparation: Select multiple samples of humic acid-containing water-soluble fertilizer prepared according to the technical solution of this invention, with each sample weighing 500 grams, to ensure its representativeness.

[0047] Test environment: A high humidity environment was set up, with relative humidity controlled between 80%RH and 90%RH, and the temperature maintained at room temperature (approximately 25℃). A constant temperature and humidity chamber was used for testing to ensure the stability of the test environment.

[0048] Test duration: The sample was placed in the test environment for 14 consecutive days to simulate long-term storage conditions.

[0049] Sieving test: After the test, the sample is sieved using a standard sieve. The sieve aperture is selected according to the fertilizer particle size to ensure accurate assessment of the degree of agglomeration. The mass of agglomerated material that cannot pass through the sieve is recorded, and the agglomeration rate is calculated.

[0050] The specific characterization results are shown in Table 1. Table 1

[0051] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a humic acid-containing water-soluble fertilizer, characterized in that... Includes the following steps: Step 1: Mix the base fertilizer raw materials evenly, and then extrude and granulate the evenly mixed base fertilizer raw materials. During the process, spray water appropriately, with the amount of water being 2wt% to 4wt% of the total weight of the base fertilizer raw materials. Screen out particles with a particle size of 2mm to 3mm. Step 2: Dissolve chitosan powder in an acidic aqueous solution and stir until completely dissolved to obtain a chitosan solution with a concentration of 1wt% to 3wt%. Put the particles into a coating machine and use the chitosan solution to uniformly spray the surface of the particles. The spraying amount is 1% to 3% of the total particles to form a chitosan film. Dry and cure the particles coated with the chitosan film. Step 3: Disperse polyvinyl alcohol in water, heat and stir to prepare a polyvinyl alcohol solution of 11wt% to 12wt%, add 0.5% to 2% of the weight of polyvinyl alcohol aldehyde oxidizing agent to the polyvinyl alcohol solution, and react for 2 to 4 hours at a pH of 4 to 6 and a temperature of 50℃ to 70℃ to introduce aldehyde groups into the polyvinyl alcohol molecular chain, thereby obtaining an aldehyde-modified polyvinyl alcohol solution. Step 4: Add a foaming agent and a foam stabilizer to the aldehyde-modified polyvinyl alcohol solution. The foaming agent includes sulfonate polyether diol, glycerol, butanediol and sodium dodecylbenzene sulfonate. The foam stabilizer is a mixture of ammonium stearate and lauryl alcohol polyoxyethylene ether. After stirring evenly, a foaming liquid is obtained. Step 5: Spray the foaming liquid onto the surface of the chitosan-coated base fertilizer granules to form an aldehyde-modified polyvinyl alcohol foam film. The coating amount of the foam film is 3% to 5% of the total granule weight. Step 6: Dry the coated granules at a temperature of 50℃ to 70℃ for 4 to 8 hours to obtain a water-soluble fertilizer containing humic acid.

2. The method for preparing a humic acid-containing water-soluble fertilizer according to claim 1, characterized in that, The base fertilizer raw materials include humic acid, nitrogen-phosphorus-potassium compound fertilizer, trace element fertilizer and organic matter, wherein the weight ratio of humic acid is 5% to 20% of the total weight of the base fertilizer raw materials.

3. The method for preparing a humic acid-containing water-soluble fertilizer according to claim 1, characterized in that, The chitosan solution uses an aqueous acetic acid solution with a concentration of 0.1N to 1N as the solvent; the drying and curing step is carried out at a temperature of 40°C to 60°C for 1 to 3 hours to ensure that the chitosan film is completely cured on the surface of the base fertilizer particles.

4. The method for preparing a humic acid-containing water-soluble fertilizer according to claim 1, characterized in that, The nitrogen, phosphorus and potassium compound fertilizer is composed of urea, diammonium phosphate and potassium sulfate in a weight ratio of 1.2:1:0.8 to 1.8:1.2:

1.

5. The method for preparing a humic acid-containing water-soluble fertilizer according to claim 1, characterized in that, The micronutrient fertilizer includes boric acid, zinc sulfate, manganese sulfate, and ammonium molybdate, each accounting for 0.1% to 1% of the weight of the base fertilizer raw materials; the organic matter is amino acids, each accounting for 1% to 5% of the weight of the base fertilizer raw materials; the foaming agent contains sulfonate polyether diol, glycerol, butanediol, and sodium dodecylbenzenesulfonate in weight ratios of 1% to 3%, 0.5% to 1.5%, 0.5% to 1.5%, and 0.2% to 0.6% of the weight of polyvinyl alcohol, respectively; and the foam stabilizer contains ammonium stearate and lauryl alcohol polyoxyethylene ether in weight ratios of 1:1 to 1:

2.

6. A method for preparing a humic acid-containing water-soluble fertilizer according to any one of claims 1 to 5, characterized in that, The humic acid is pretreated to improve its compatibility and biological activity with the base fertilizer raw materials. The pretreatment includes at least one of acidification, alkalization or oxidation treatment.

7. A method for preparing a humic acid-containing water-soluble fertilizer according to any one of claims 1 to 6, characterized in that, The base fertilizer raw materials also include mineral fillers such as bentonite, vermiculite or perlite to improve the fertilizer's water retention and slow release properties; the drying process also includes ventilation measures to promote rapid evaporation of moisture and improve drying efficiency; the preparation process also includes screening the base fertilizer raw materials to remove impurities and unqualified particles, thereby improving the uniformity and quality of the fertilizer.

8. The method for preparing a humic acid-containing water-soluble fertilizer according to claim 4, characterized in that, The aldehyde-modifying agent is selected from one or more of formaldehyde, acetaldehyde, and propionaldehyde.

9. The method for preparing a humic acid-containing water-soluble fertilizer according to claim 1, characterized in that, During the reaction, an appropriate amount of acid or base catalyst is added to promote the aldehyde reaction and improve the efficiency of aldehyde group introduction.

10. The method for preparing a humic acid-containing water-soluble fertilizer according to claim 9, characterized in that, The acid catalyst is selected from one or more of hydrochloric acid, sulfuric acid, or phosphoric acid, and the base catalyst is selected from one or more of sodium hydroxide, potassium hydroxide, or sodium carbonate. The amount of catalyst used is 0.1% to 1% of the weight of polyvinyl alcohol.

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