Preparation method of multi-effect urea formaldehyde composite slow-release fertilizer

Abstract

The invention relates to the technical field of fertilizers, in particular to a preparation method of a multi-effect urea formaldehyde composite slow-release fertilizer, which comprises the following steps: adding a first batch of urea and a second batch of urea into an alkaline environment system of formaldehyde to react, adding a third batch of urea and NH2 (CH2) nNPO (OH) K to perform acid catalytic polymerization reaction under an acidic condition after the reaction is finished, finally, the pH value is adjusted to be neutral, the compound slow-release fertilizer is obtained, NH2 (CH2) nNPO (OH) K is added to react with hydroxymethylurea to produce slow-release cross-linked macromolecular substances, and the liquid slow-release fertilizer has the functions of quick-acting, medium-acting and long-acting slow-release fertilizers. Meanwhile, the problem of unstable storage of the liquid slow-release fertilizer is solved.

Description

Technical Field

[0001] The invention relates to the technical field of fertilizers, and in particular to a method for preparing a multi-effect urea formaldehyde composite slow-release fertilizer. Background Art

[0002] Urea formaldehyde slow-release fertilizer is a process in which urea and formaldehyde undergo hydroxymethylation and methyleneization reactions under the catalysis of alkali and acid to generate long-chain polymers. The greater the proportion of urea added, the longer the chain of the urea formaldehyde polymer, and the longer it takes to be decomposed into ammonium nitrogen that can be absorbed by plants in the soil, thus solving the problems of nitrogen loss and low fertilizer utilization rate caused by too rapid dissolution of urea.

[0003] Urea formaldehyde has the advantage of high nitrogen content due to the formation of long-chain ammonia nitrogen molecules, and is a fertilizer with good development and application prospects. Currently, there are three main forms of urea formaldehyde slow-release fertilizers on the market: solid particles, suspensions, and clear liquids. Among them, liquid slow-release fertilizers have slow-release properties that match the nitrogen release rate with the plant absorption rate, which can reduce the risk of nitrogen being leached or volatilized in the soil and protect the environment. At the same time, the liquid state helps to improve the distribution of soil moisture and nutrients, promote the formation of soil aggregate structure, and can be mixed with other nutrient solutions or pesticides in the actual production process to improve crop management efficiency and reduce the number of operations.

[0004] This type of fertilizer is seldom circulated in China and has been widely used mainly in agricultural countries such as North America, Europe and Asia. In the development of green agriculture, the demand for environmentally friendly fertilizers such as liquid urea formaldehyde slow-release fertilizers is increasing. At present, the domestic market urgently needs to solve the technical gap of liquid urea formaldehyde slow-release fertilizers for subsequent promotion and application.

[0005] CN 114634377 A discloses a clear liquid urea formaldehyde long-acting fertilizer and its preparation method and application. Urea, ammonia and formaldehyde are mixed and water is added under stirring conditions, and the mixture is heated to 50-85°C and a pressure of 0.05-0.15 MPa under stirring conditions for 1-5 hours, and then ammonia is added and stirred to make the materials in the system uniformly mixed until the pH is 8-11, and the mixture is cooled to obtain a clear liquid urea formaldehyde long-acting fertilizer.

[0006] Patent US 5449394 discloses a two-step method for preparing urea-formaldehyde liquid fertilizer. In the first step, urea and high-concentration formaldehyde are reacted under alkaline conditions to generate a multi-component mixture UFC85; in the second step, UFC85, NH3, and urea are mixed in a molar ratio of 3:2:1, and the pH is adjusted with NaHCO3. After the reaction is completed, the mixture is cooled to room temperature to obtain urea-formaldehyde liquid fertilizer.

[0007] CN 116789468 A discloses that urea is reacted with an excess of high-concentration formaldehyde solution in the presence of a small amount of ammonia for 30 minutes, and the pH is adjusted to 8.0 with NaHCO3 to generate water-soluble branched polymethylene urea. After the reaction is completed, excess water is removed by vacuum distillation and concentrated to obtain a stable storage polymer urea-formaldehyde slow-release liquid fertilizer.

[0008] The above patents CN114634377A, US5449394, and CN116789468 A provide methods for producing liquid urea formaldehyde. Although the production process is convenient and available, the instability of liquid urea formaldehyde in actual production and application is ignored. During storage, the reaction is likely to continue, resulting in a higher degree of polymerization, and the solution will become more turbid. At the same time, since the degree of polymerization of urea formaldehyde in liquid slow-release fertilizers is relatively low, the slow-release performance in the soil is average, and it is impossible to take into account the functions of rapid, medium, and long-term continuous nutrient supply to plants. Summary of the invention

[0009] In order to solve the above technical problems, an embodiment of the present invention provides a method for preparing a multi-effect urea formaldehyde composite slow-release fertilizer.

[0010] To achieve the above object, the embodiments of the present invention adopt the following technical solutions:

[0011] On the one hand, the present application provides a method for preparing a multi-effect urea formaldehyde composite slow-release fertilizer, comprising the following steps:

[0012] Add the first batch and the second batch of urea to the alkaline environment system of formaldehyde for reaction. After the reaction is completed, add the third batch of urea and NH2(CH2) under acidic conditions. n NPO(OH)K undergoes an acid-catalyzed polymerization reaction, wherein n is 1 to 6, and finally the pH value is adjusted to be neutral to obtain a composite slow-release fertilizer.

[0013] In some embodiments, the molar ratio of urea to formaldehyde in the first batch, the second batch, and the third batch is (1.1-5):1.

[0014] In some embodiments, the NH2(CH2) n NPO(OH)K accounts for 10-30% of the total mass of urea.

[0015] In some embodiments, the amount of urea added in the first batch is 40-80% of the total mass of urea, the amount of urea added in the second batch is 5-30% of the total mass of urea, and the amount of urea added in the third batch is 5-30% of the total mass of urea.

[0016] In some embodiments, the reaction process of the first batch and the second batch of urea is as follows:

[0017] Add the first batch of urea to the alkaline environment system of formaldehyde, stir and heat to 40-90°C, react for 1.5-3h, add the second batch of urea after the reaction, maintain the temperature at 40-90°C, and react for 0.5-1h.

[0018] In some embodiments, the pH value of the acid-catalyzed polymerization reaction is 3-5, the reaction temperature is 70-100° C., and the reaction time is 10-90 min.

[0019] In some embodiments, the NH2(CH2) n The preparation method of NPO(OH)K comprises the following steps:

[0020] Add ethylenediamine to the organic solvent to react with phosphorus pentachloride, then hydrolyze, add potassium hydroxide to the hydrolysis product system to react, and then obtain NH2(CH2) n NPO(OH)K.

[0021] In some embodiments, the molar ratio of ethylenediamine to phosphorus pentachloride is 1:1, the reaction temperature is 0°C, the reaction time is 15-30 min, and the hydrolysis temperature is 40-80°C.

[0022] In some embodiments, the molar ratio of the ethylenediamine to the potassium hydroxide is 1:1, the reaction temperature is 30-60° C., and the reaction time is 0.1 to 3 hours.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. A multi-effect liquid slow-release fertilizer that integrates quick-acting, medium-acting and long-acting effects

[0025] The reactants in the present invention are mainly urea and formaldehyde solution, wherein urea is added in three batches. The first step is to react, add the first batch of urea and formaldehyde solution, and react in an alkaline environment to cause hydroxymethylation, add the second batch of urea after a period of reaction, and continue the hydroxylation reaction to obtain hydroxymethylurea. The second step is to react, sequentially add the third batch of urea and NH2(CH2)nNPO(OH)K under acidic conditions, hydroxymethylurea and urea condensation reaction generate short-chain methylene urea, and cross-link with NH2(CH2)nNPO(OH)K to generate cross-linked macromolecular substances. Wherein the unreacted urea molecules in the system and urea and formaldehyde in the hydroxymethylation reaction react to produce hydroxymethylurea, provide quick-acting nitrogen to play a quick-acting role, and the methylene urea molecules with low polymerization degree generated in the methyleneation reaction under acidic conditions, their decomposition speed in the soil is relatively slow relative to urea, promote the growth of plants in the middle stage, and NH2(CH2)nNPO(OH)K is added to react with hydroxymethylurea to form a multi-element macromolecular substance, which can provide nutrition for the plant growth later stage.

[0026] 2. Liquid slow-release fertilizer with multiple nutrients and multifunctional stable storage

[0027] After the reaction, the stabilizer NH2(CH2)nNPO(OH)K is introduced into the system. The substance is neutral and can adjust the pH of the system to prevent excessive condensation between hydroxymethyl ureas under acidic conditions to form insoluble urea. The large number of hydroxyl and amino groups contained in the urea formaldehyde system form electrostatic and hydrogen bond interactions with nutrients such as the active sites of NH2(CH2)nNPO(OH)K, making each nutrient have higher solubility and stability in the system.

[0028] Take the phosphorus-containing substances such as NH2(CH2)nNPO(OH)K as an example, which provide phosphorus and nitrogen elements: phosphorus is an important nutrient for plant flowering and root development, while amino nitrogen helps plants synthesize protein and promote healthy growth.

[0029] By adding NH2(CH2)nNPO(OH)K and reacting with hydroxymethylurea to produce slow-release cross-linked macromolecular substances, the liquid slow-release fertilizer has the functions of quick-acting, medium-acting and long-acting slow-release fertilizers. At the same time, the problem of unstable storage of liquid slow-release fertilizers is solved. The addition of NH2(CH2)nNPO(OH)K maintains a neutral liquid environment, and the hydrogen-electric interaction formed causes the solution to stop polymerizing during storage and transportation. In the preparation method of the multi-effect liquid urea-formaldehyde composite slow-release fertilizer of the present invention, the carbon chain length of the urea-formaldehyde molecule is between 2 and 10, the insoluble matter is very small, and it has high stability. There is no precipitation after long-term storage, and it is easy to perform various spraying operations. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a graph of wheat growth after 15 days. DETAILED DESCRIPTION

[0031] The following will be combined with the accompanying drawings to clearly and completely describe the technical solutions in some embodiments of the present disclosure. Obviously, the described embodiments are only part of the embodiments of the present disclosure, rather than all the embodiments. Based on the embodiments provided by the present disclosure, all other embodiments obtained by ordinary technicians in this field belong to the scope of protection of the present disclosure.

[0032] At present, liquid urea formaldehyde is unstable in actual production and application, that is, it is easy to continue to react during storage to cause a higher degree of polymerization, and the solution becomes more turbid.

[0033] In view of the technical problems existing in the prior art, the present application provides a method for preparing a multi-effect urea formaldehyde composite slow-release fertilizer, comprising the following steps:

[0034] Add the first batch and the second batch of urea to the alkaline environment system of formaldehyde for reaction. After the reaction is completed, add the third batch of urea and NH2(CH2) under acidic conditions. n NPO(OH)K undergoes an acid-catalyzed polymerization reaction, wherein n is 1 to 6, i.e., n can be 1, 2, 3, 4, 5, 6, and finally the pH value is adjusted to be neutral to obtain a composite slow-release fertilizer.

[0035] NH2(CH2) n The structural formula of NPO(OH)K is shown below:

[0036] R1 is an unsubstituted C1-C6 alkyl group.

[0037] NH2(CH2)nNPO(OH)K is added to the system to form a macromolecule with hydroxymethyl urea, which prolongs the slow-release time of the fertilizer in the soil and increases the P and K elements to meet the different needs of plants. The structure of the macromolecule is shown below:

[0038]

[0039] Wherein R1 is an unsubstituted C1~C6 alkyl group.

[0040] In some embodiments, the molar ratio of urea to formaldehyde in the first batch, the second batch and the third batch is (1.1-5):1. By way of example, the molar ratio of urea to formaldehyde in the first batch, the second batch and the third batch may be 1.1:1, 1.2:1, 1.5:1, 2:1, 3:1, 4:1, 5:1, which is not specifically limited in the embodiments of the present application.

[0041] In some embodiments, the NH2(CH2) n NPO(OH)K is 10-30% of the total mass of urea. For example, NH2(CH2) n NPO(OH)K may be 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, or 30% of the total mass of urea, which is not specifically limited in the embodiments of the present application.

[0042] In some embodiments, the amount of urea added in the first batch is 40-80% of the total mass of urea, the amount of urea added in the second batch is 5-30% of the total mass of urea, and the amount of urea added in the third batch is 5-30% of the total mass of urea. Exemplarily, the amount of urea added in the first batch can be 40%, 50%, 60%, 70%, 80% of the total mass of urea, the amount of urea added in the second batch is 5%, 10%, 15%, 20%, 25%, 30% of the total mass of urea, and the amount of urea added in the third batch is 5%, 10%, 15%, 20%, 25%, 30% of the total mass of urea. The sum of the amount of urea added in the first batch, the amount of urea added in the second batch, and the amount of urea added in the third batch is 100%, that is, the total mass of urea is the sum of the amount of urea added in the first batch, the amount of urea added in the second batch, and the amount of urea added in the third batch.

[0043] In some embodiments, the first batch and the second batch of urea react as follows:

[0044] Add the first batch of urea to the alkaline environment system of formaldehyde, stir and heat to 40-90°C, react for 1.5-3h, add the second batch of urea after the reaction, maintain the temperature at 40-90°C, and react for 0.5-1h.

[0045] In some embodiments, the pH value of the acid-catalyzed polymerization reaction is 3-5, the reaction temperature is 70-100° C., and the reaction time is 10-90 min.

[0046] In some embodiments, it is characterized in that the NH2(CH2) n The preparation method of NPO(OH)K comprises the following steps:

[0047] Add ethylenediamine to the organic solvent to react with phosphorus pentachloride, then hydrolyze, add potassium hydroxide to the hydrolysis product system to react, and then obtain NH2(CH2) n NPO(OH)K, the organic solvent may be N-methylpyrrolidone.

[0048] Its NH2(CH2) n The preparation reaction equation of NPO(OH)K is as follows:

[0049] NH2(CH2)2NH2+PCl5→NH2(CH2)2NPCl3+2HCl

[0050] NH2(CH2)2NPCl3+3H2O→NH2(CH2)2NP=O(OH)2+3HClNH2(CH2)2NPCl3+3H2O

[0051] →NH2(CH2)2NP=O(OH)2+3HCl

[0052] NH2(CH2)2NP=O(OH)2+KOH→NH2(CH2)2NP=O(OH)K+H2O

[0053] In some embodiments, the molar ratio of ethylenediamine to phosphorus pentachloride is 1:1, the reaction temperature is 0°C, the reaction time is 15-30 min, and the hydrolysis temperature is 40-80°C.

[0054] In some embodiments, the molar ratio of the ethylenediamine to the potassium hydroxide is 1:1, the reaction temperature is 30-60° C., and the reaction time is 0.1 to 3 hours.

[0055] In order to make the present application easier to understand, the present invention will be described in detail below with reference to embodiments. These embodiments are merely illustrative and are not intended to limit the scope of application of the present invention.

[0056] Unless otherwise specified, the operations and processing methods involved in this application are conventional methods in the art. Unless otherwise specified, the instruments used in this application are conventional instruments in the art.

[0057] Example 1

[0058] A multi-effect urea formaldehyde composite slow-release fertilizer is provided. Under the condition of continuous stirring, 79.92 g of urea from the first batch and 100 g of a 37 wt% formaldehyde aqueous solution are added, and the mixture is heated to 60 DEG C under an environment where the pH value is adjusted to 9.5 by triethanolamine with continuous stirring, and the mixture is reacted for 1.5 hours. After the reaction, 33.3 g of urea from the second batch is added, and the mixture is heated to 80 DEG C and reacted for 0.5 hour to obtain a hydroxymethyl urea solution. The pH value is then adjusted to 5, and 19.98 g of urea from the third batch and 39.96 g of NH2(CH2)2NPO(OH)K are added. The mixture is heated to 70 DEG C and reacted for 30 minutes, wherein the hydroxymethyl urea and the urea undergo condensation reaction to generate short-chain methylene urea, and the hydroxymethyl urea and the NH2(CH2)2NPO(OH)K undergo cross-linking reaction to form a macromolecular polymer. Finally, the pH value is adjusted to be neutral, and the mixture is cooled to obtain a multi-effect urea formaldehyde composite slow-release fertilizer.

[0059] The synthesis method of NH2(CH2)2NPO(OH)K is as follows: 6g of ethylenediamine and 20.8g of phosphorus pentachloride are added to 50mL of N-methylpyrrolidone, stirred at 0°C for 15min, then stirred in a 60°C oil bath for 3h, 10mL of deionized water is added, the product is seen during hydrolysis, and 6g of potassium hydroxide is added to react and stirred in a 40°C oil bath for 1h.

[0060] The reaction equation of NH2(CH2)2NPO(OH)K and urea formaldehyde is as follows:

[0061]

[0062] Comparative Example 1

[0063] A multi-effect urea formaldehyde composite slow-release fertilizer comprises the following steps: adding 79.92 g of urea from the first batch and 100 g of a 37 wt% formaldehyde aqueous solution under a condition of continuous stirring, adjusting the pH to 9.5 with triethanolamine, and heating to 60 DEG C under continuous stirring for 1.5 hours; adding 33.3 g of urea from the second batch after the reaction; continuing to heat to 80 DEG C and reacting for 0.5 hours to obtain a hydroxymethyl urea solution; adjusting the pH to 5, adding 19.98 g of urea from the third batch, heating to 70 DEG C and reacting for 30 minutes, wherein the hydroxymethyl urea and the urea undergo condensation reaction to generate short-chain methylene urea; and finally adjusting the pH to be neutral and cooling to obtain the multi-effect urea formaldehyde composite slow-release fertilizer.

[0064] Example 1 and Comparative Example 1 were left to stand for one month at 25°C and 5°C, and the conditions of the fertilizers were observed. The results are shown in Table 1:

[0065] Table 1 Observation results of different formula slow-release fertilizers

[0066]

[0067] By comparing Example 1 with Comparative Example 1, it can be clearly observed that in Example 1, the slow-release fertilizer can stably exist at 5°C for more than 30 days. This is because the cross-linking reaction between the stabilizer NH2(CH2)2NPO(OH)K and hydroxymethyl urea forms a relatively stable macromolecular structure. This not only enhances the stability of the structure, but also successfully adjusts and maintains the pH value in the system within the neutral or weakly alkaline range, prevents the polymerization of free molecules under acidic conditions, provides ideal conditions for the long-term storage of the solution, and ensures its stability during storage. Based on the above in-depth comparative analysis, the preparation method proposed in the patent of the present invention effectively solves the storage problem faced by liquid urea formaldehyde slow-release fertilizers on the current market.

[0068] Example 2

[0069] A multi-effect urea formaldehyde composite slow-release fertilizer comprises the following steps: adding 53.28 g of urea from the first batch and 100 g of a 37 wt% formaldehyde aqueous solution under continuous stirring, adjusting the pH to 9.3 with triethanolamine, and heating to 60 DEG C with continuous stirring for 2.5 hours; adding 22.2 g of urea from the second batch after the reaction; continuing to heat to 80 DEG C and reacting for 0.5 hours to obtain a hydroxymethyl urea solution; then adjusting the pH to 4; adding 13.32 g of urea from the third batch and 26.64 g of NH2(CH2)2NPO(OH)K; heating to 90 DEG C and reacting for 10 minutes, wherein the hydroxymethyl urea and the urea undergo condensation reaction to generate short-chain methylene urea; and the hydroxymethyl urea and the NH2(CH2)2NPO(OH)K undergo cross-linking reaction to form a macromolecular polymer; and finally adjusting the pH to be neutral and cooling to obtain the multi-effect urea formaldehyde composite slow-release fertilizer.

[0070] The synthesis method of NH2(CH2)2NPO(OH)K is as follows: 6g of ethylenediamine and 20.8g of phosphorus pentachloride are added to 50mL of N-methylpyrrolidone, stirred at 0°C for 15min, then stirred in a 60°C oil bath for 3h, 10mL of deionized water is added, the product is seen during hydrolysis, and 6g of potassium hydroxide is added to react and stirred in a 40°C oil bath for 1h.

[0071] The reaction equation of NH2(CH2)2NPO(OH)K and urea formaldehyde is as follows:

[0072]

[0073] Comparative Example 2

[0074] A multi-effect urea formaldehyde composite slow-release fertilizer comprises the following steps: adding 53.28 g of urea from the first batch and 100 g of a 37 wt% formaldehyde aqueous solution under a condition of continuous stirring, adjusting the pH to 9.3 with triethanolamine, and heating to 60 DEG C under continuous stirring for 2.5 hours; adding 22.2 g of urea from the second batch after the reaction; continuing to heat to 80 DEG C and reacting for 0.5 hours to obtain a hydroxymethyl urea solution; adjusting the pH to 4; adding 13.32 g of urea from the third batch; heating to 90 DEG C and reacting for 10 minutes, wherein the hydroxymethyl urea and the urea undergo condensation reaction to generate short-chain methylene urea; and finally adjusting the pH to be neutral and cooling to obtain the multi-effect urea formaldehyde composite slow-release fertilizer.

[0075] Table 2 Composition of slow-release fertilizers with different formulations

[0076]

[0077] Comparative Example 2 The sustained-release material composition of Comparative Example 2 can be seen

[0078] The addition of NH2(CH2)2NPO(OH)K reduces the hydroxymethylurea in the composition of the slow-release fertilizer, promotes the formation of methylene urea, and contains cross-linking to form a macromolecular polymer, making the slow-release fertilizer more functionally diverse. The unreacted urea molecules in the system react with urea and formaldehyde in the hydroxymethylation reaction to produce hydroxymethylurea, providing quick-acting nitrogen and playing a quick-acting role. The low-polymerization methylene urea molecules generated in the methyleneation reaction under acidic conditions decompose in the soil more slowly than urea, promoting the mid-term growth of plants. The addition of NH2(CH2)nNPO(OH)K reacts with hydroxymethylurea to form a multi-element macromolecular substance, which can provide nutrition for the late stage of plant growth. Based on the above in-depth comparative analysis, the preparation method proposed in the present invention can provide a quick-acting, medium-acting and long-acting urea-formaldehyde slow-release fertilizer.

[0079] Table 3 Nitrogen content of slow-release fertilizer in soil (g / g)

[0080]

[0081] Note: Apply 1g of slow-release fertilizer to 1kg of soil, and the mass of nitrogen contained in 1kg of soil will increase over time.

[0082] Further exploring the efficacy of the slow-release fertilizer, it can be seen from Table 3 that in Comparative Example 2, the slow-release fertilizer achieves complete release in 30 days, while in Example 2, the release period can be extended to 90 days. The slow-release fertilizer in Example 2 can release nitrogen more persistently in the soil, indicating that the addition of NH2(CH2)2NPO(OH)K enables the slow-release fertilizer to form a high-molecular compound with a longer slow-release period, which can not only provide nutrients in the early stage of plant growth, but also meet the growth needs of plants in the later stage.

[0083] Based on the above analysis, the addition of NH2(CH2)2NPO(OH)K can prolong the sustained release time and provide a long-lasting effect.

[0084] Table 4 Soil fertility

[0085]

[0086] From Table 4, it can be seen that the use of slow-release fertilizer can effectively improve soil fertility and provide more nutrients for plants. For Example 2 and Comparative Example 2, the addition of NH2(CH2)nNPO(OH)K is more effective in increasing soil fertility, improving the physical properties of the soil, enhancing the root development of plants, significantly improving the water retention and nutrient retention capacity of the soil, and improving the utilization efficiency of fertilizers. Based on the above comparative analysis, it can be obtained that the preparation method of the present invention can provide plants with a multifunctional slow-release fertilizer with multiple nutrient elements.

[0087] Wheat growth Figure 1As shown, the growth of Example 2 and Comparative Example 2 is better than that of the blank group, indicating that the addition of slow-release fertilizer promotes the growth of wheat. Further, the growth of wheat in Example 2 is better. It can be seen from Table 5 that its stem length reaches 16 cm and its root length is 7 cm. It shows that the multi-effect slow-release fertilizer prepared by this patent can provide longer-lasting nutrients for plants due to the addition of NH2(CH2)2NPO(OH)K to form a macromolecular polymer, and provides multi-element nutrients, and the nutrient concentration is high, the slow-release effect is good, and it is beneficial to plant absorption and growth.

[0088] Table 5 Growth status of wheat

[0089] Stem / (cm) Root / (cm) Dry weight / (g) Fresh weight / (g) Example 2 16±0.35 7±0.55 236.35 491.1 Comparative Example 2 9±0.51 4±0.11 189.31 409.6

[0090] In the description of this specification, specific features, structures, materials or characteristics may be combined in an appropriate manner in any one or more embodiments or examples.

[0091] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for preparing a multi-effect urea formaldehyde composite slow-release fertilizer, characterized in that: The following steps are involved: Add the first batch and the second batch of urea to the alkaline environment system of formaldehyde for reaction. After the reaction is completed, add the third batch of urea and NH2(CH2) under acidic conditions. n NPO(OH)K undergoes an acid-catalyzed polymerization reaction, wherein n is 1 to 6, and finally the pH value is adjusted to be neutral to obtain a composite slow-release fertilizer.

2. The preparation method according to claim 1, characterized in that: The molar ratio of urea to formaldehyde in the first batch, the second batch and the third batch is (1.1-5):

1.

3. The preparation method according to claim 1, characterized in that: The NH2(CH2) n NPO(OH)K is 10~30% of the total mass of urea.

4. The preparation method according to claim 1, characterized in that: The amount of urea added in the first batch is 40-80% of the total mass of urea, the amount of urea added in the second batch is 5-30% of the total mass of urea, and the amount of urea added in the third batch is 5-30% of the total mass of urea.

5. The preparation method according to claim 4, characterized in that: The reaction process of the first batch and the second batch of urea is as follows: Add the first batch of urea to the alkaline environment system of formaldehyde, stir and heat to 40~90℃, react for 1.5~3h, add the second batch of urea after the reaction, maintain the temperature at 40~90℃, and react for 0.5~1h.

6. The preparation method according to claim 1, characterized in that: The pH value of the acid-catalyzed polymerization reaction is 3-5, the reaction temperature is 70-100° C., and the reaction time is 10-90 min.

7. The preparation method according to any one of claims 1 to 6, characterized in that The NH2(CH2) n The preparation method of NPO(OH)K comprises the following steps: Add ethylenediamine to the organic solvent to react with phosphorus pentachloride, then hydrolyze, add potassium hydroxide to the hydrolysis product system to react, and then obtain NH2(CH2) n NPO(OH)K.

8. The preparation method according to claim 7, characterized in that: The molar ratio of ethylenediamine to phosphorus pentachloride is 1:1, the reaction temperature is 0°C, the reaction time is 15-30 min, and the hydrolysis temperature is 40-80°C.

9. The preparation method according to claim 7, characterized in that: The molar ratio of the ethylenediamine to the potassium hydroxide is 1:1, the reaction temperature is 30-60° C., and the reaction time is 0.1-3 h.

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