High-stability liquid nitrogenous fertilizer and preparation method thereof

Abstract

This invention belongs to the field of agricultural fertilizer technology, specifically relating to a highly stable liquid nitrogen fertilizer and its preparation method. Addressing the problems of existing liquid nitrogen fertilizers' tendency to crystallize at low temperatures and the need for the addition of chemical antifreeze or suspending agents, this invention provides a one-pot method for preparing highly stable liquid nitrogen fertilizer: a bio-based aldehyde source, urea, a catalyst, and water are mixed and reacted at 40-70°C for 2-8 hours, without separating intermediates during the reaction. The mixture is then cooled and filtered to obtain the final product. The bio-based aldehyde source is selected from at least one of furfural, 5-hydroxymethylfurfural, and glyoxylic acid. The product of this invention does not crystallize under low-temperature conditions, requires no chemical antifreeze, and has slow-release properties, making it suitable for drip irrigation, sprinkler irrigation, and other integrated water and fertilizer systems.

Description

Technical Field

[0001] This invention belongs to the field of agricultural fertilizer technology, specifically relating to a highly stable liquid nitrogen fertilizer and its preparation method. Background Technology

[0002] Liquid nitrogen fertilizers are widely used because they have high nutrient content, are easy to apply mechanically, and can be mixed with pesticides and trace elements. Common liquid nitrogen fertilizers on the market today include urea solution, ammonium nitrate solution, and urea-ammonium nitrate solution (UAN).

[0003] Urea exhibits low-temperature crystallization in aqueous solutions. When the ambient temperature drops below -5°C, urea solutions are prone to crystallization, leading to product clumping and pipeline blockage, severely impacting storage, transportation, and use. Existing technologies primarily employ two improvement approaches to address this issue: one is to add chemical antifreeze agents such as potassium formate, ethylenediamine, and dimethyl sulfoxide to lower the freezing point of the solution and inhibit crystallization; the other is to use suspending agents to disperse urea in a suspended state. However, chemical antifreeze agents pose potential hazards to crops, while suspended systems carry risks of stratification and sedimentation, and the product is a milky white, opaque form.

[0004] In recent years, bio-based materials have attracted attention in the fertilizer field. CN104892298A discloses a liquid nitrogen fertilizer containing biochemical humic acid, which is prepared by reacting concentrated biochemical humic acid obtained from furfural residue fermentation with ammonium nitrate and urea at a certain temperature. In this technology, furfural residue is converted into humic acid through microbial fermentation and then compounded with nitrogen. The fertilizer stability is mainly improved by the chelation and adsorption effects of humic acid, without involving the direct condensation modification of aldehyde source and urea.

[0005] On the other hand, basic research has reported on the condensation reaction of furfural and urea. Studies by Martinez-Garcia et al. have shown that furfural and urea can undergo a condensation reaction. However, this study used room temperature, acidic conditions for 72 hours, and the resulting solid powder was obtained after separation and purification. It did not involve agricultural applications, nor did it address the low-temperature stability of liquid nitrogen fertilizers.

[0006] Therefore, how to inhibit the crystallization of liquid nitrogen fertilizer under low-temperature conditions without adding chemical antifreeze is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] This invention provides a highly stable liquid nitrogen fertilizer and its preparation method, to solve the technical problem that existing liquid nitrogen fertilizers are prone to crystallization under low temperature conditions and require chemical antifreeze or suspending agents to maintain stability.

[0008] In a first aspect, the present invention provides a method for preparing the highly stable liquid nitrogen fertilizer.

[0009] The preparation method includes the following steps: (1) Mix 5-15 parts of bio-based aldehyde source, 10-40 parts of urea, 0.1-2 parts of catalyst and 20-40 parts of water, stir evenly to obtain a mixture; (2) Measure the pH of the mixture and adjust the pH of the mixture to 7-9 using a pH adjuster; the amount of the pH adjuster is 0.5-5 parts; (3) Stir the mixture at 40-70℃ for 2-8 hours to allow the bio-based aldehyde source to undergo a condensation reaction with urea, generating a condensate in situ; (4) After the reaction is complete, cool to room temperature to obtain the crude product liquid nitrogen fertilizer; (5) The crude product liquid nitrogen fertilizer is filtered to obtain clear and transparent liquid nitrogen fertilizer.

[0010] Preferably, in step (1), the bio-based aldehyde source is selected from at least one of furfural, 5-hydroxymethylfurfural (HMF), and glyoxylic acid. More preferably, the bio-based aldehyde source is furfural.

[0011] Preferably, in step (1), the catalyst is selected from at least one of ZnCl2, p-toluenesulfonic acid, phosphorous acid, and cation exchange resin. More preferably, the catalyst is ZnCl2. Using ZnCl2 as a catalyst can promote the uniform condensation reaction between furfural and urea, ensuring that the resulting fertilizer is stable for a long time without stratification or precipitation.

[0012] Preferably, in step (1), the molar ratio of the bio-based aldehyde source to urea in the mixed solution is 1:(2-6). More preferably, the molar ratio is 1:(3-4). Within this molar ratio range, furfural and urea can fully react to form furfural-urea condensate, making the nitrogen binding in the system more stable, effectively delaying nitrogen release, reducing leaching loss, and improving fertilizer utilization.

[0013] Preferably, in step (1), the stirring speed is 200-600 rpm and the stirring time is 5-15 minutes. More preferably, the stirring speed is 300-400 rpm and the stirring time is 10 minutes. Under these stirring conditions, furfural and urea can be quickly and evenly mixed, avoiding side reactions caused by excessively high local concentrations.

[0014] Preferably, in step (1), the mixing temperature is room temperature (15-30°C). Too high a mixing temperature may cause premature decomposition of urea or volatilization of furfural, while too low a mixing temperature will slow down the dissolution rate of urea.

[0015] Preferably, in step (2), the pH adjuster is selected from at least one of sodium hydroxide, potassium hydroxide, or ammonia. More preferably, the pH adjuster is potassium hydroxide. Using potassium hydroxide to adjust the pH not only provides the weakly alkaline environment required for the condensation reaction, but also introduces potassium ions (K+) as potassium nutrient, increasing the added value of the product.

[0016] Preferably, in step (2), the pH of the mixed solution is adjusted to 7-9. Within this pH range, the furfural resinification side reaction can be effectively suppressed, and urea hydrolysis can be avoided, ensuring the smooth progress of the condensation reaction.

[0017] Preferably, in step (3), the reaction temperature is 50-60℃ and the reaction time is 4-6 hours. Under these conditions, the condensation reaction between furfural and urea can proceed fully, generating a mixture mainly composed of furfural-urea condensate.

[0018] Preferably, in step (3), the stirring speed is 100-300 rpm. More preferably, the stirring speed is 150-200 rpm. At this stirring speed, the temperature of the reaction system can be kept uniform and mass transfer can be promoted, while the influence of excessive shear force on the condensate structure can be avoided.

[0019] Preferably, in step (3), the stirring reaction is carried out under normal pressure. Normal pressure reactions do not require pressurization equipment, are simple and safe to operate, and are suitable for industrial production.

[0020] Preferably, in step (4), the cooling is performed by natural cooling or water bath cooling, and the cooling time is 30-60 minutes. Natural cooling is suitable for small-scale production, in which the reaction vessel is placed in a room temperature environment to cool down naturally; water bath cooling is suitable for occasions that require rapid cooling, in which the reaction vessel is placed in flowing cold water to accelerate cooling.

[0021] Preferably, in step (4), the cooling time is 30-60 minutes. More preferably, the cooling time is 40-50 minutes. Excessive cooling may lead to localized supersaturation, causing precipitation of the condensate; excessive cooling will affect production efficiency. Slow cooling to room temperature within 30-60 minutes ensures homogeneous product stability.

[0022] Preferably, in step (4), the product temperature after cooling is 15-30℃.

[0023] Preferably, in step (5), the filtration is performed using a 200-400 mesh filter cloth or filter screen. More preferably, a 300 mesh filter cloth is used. Filtration can remove trace amounts of insoluble substances generated during the reaction, such as unreacted furfural polymers and catalyst residues, to obtain a clear and transparent liquid nitrogen fertilizer.

[0024] Preferably, in step (5), the filtration is performed using plate and frame filtration, vacuum filtration, or centrifugal filtration. More preferably, vacuum filtration is used, with a filtration pressure of 0.05-0.1 MPa.

[0025] Preferably, in step (5), the filtrate after filtration is a finished liquid nitrogen fertilizer, and the filter residue can be recycled.

[0026] Secondly, the present invention provides a highly stable liquid nitrogen fertilizer prepared by the above-described preparation method.

[0027] The liquid nitrogen fertilizer contains a condensate generated by the condensation reaction of a bio-based aldehyde source and urea.

[0028] Preferably, the liquid nitrogen fertilizer is a homogeneous and transparent solution.

[0029] Preferably, the total nitrogen content of the liquid nitrogen fertilizer is 15-30 wt%.

[0030] Preferably, the liquid nitrogen fertilizer does not crystallize after being stored at -10°C for 72 hours.

[0031] Preferably, the insoluble matter content of the liquid nitrogen fertilizer is less than 0.1%.

[0032] The beneficial effects of this invention are: 1. This invention uses a one-pot method to prepare liquid nitrogen fertilizer, eliminating the need for separating and purifying intermediates. The process is short, simple to operate, and low in cost. The preparation process is carried out without the addition of formaldehyde or chemical antifreeze, avoiding the toxicity and environmental risks of formaldehyde and the potential harm of antifreeze to crops.

[0033] 2. This invention utilizes the condensation reaction between a bio-based aldehyde source and urea to generate a condensate in situ, thereby inhibiting low-temperature crystallization of urea at the molecular level through an intermolecular hydrogen bond network. Experiments show that the product of this invention remains clear and transparent even after long-term storage at -15℃ and multiple freeze-thaw cycles, while ordinary urea solutions crystallize at -5℃, and even furfural-urea physical mixtures cannot achieve the same level of stability.

[0034] 3. The product of this invention is a homogeneous and transparent solution with low insoluble content and obvious slow-release characteristics. The nitrogen release rate is significantly lower than that of ordinary urea solution. No suspending agent needs to be added, and there is no risk of stratification or sedimentation. It is suitable for modern fertilization systems such as drip irrigation and sprinkler irrigation.

[0035] 4. The product of this invention exhibits excellent overall stability. After accelerated high and low temperature cycling tests at 60℃ / -30℃, the product of this invention remained clear and transparent after five consecutive cycles, while the comparative ordinary urea solution and furfural-urea physical mixture showed crystallization or turbidity after the first cycle. This indicates that the product of this invention can withstand extreme temperature shocks and remains stable over a wide temperature range from -15℃ to 60℃. Attached Figure Description

[0036] Figure 1 This is the reaction equation for the condensation reaction of furfural and urea in this invention.

[0037] Figure 2 This is a schematic diagram illustrating the mechanism by which the liquid nitrogen fertilizer of this invention inhibits low-temperature crystallization.

[0038] The left side is a schematic diagram of urea molecules arranging in an orderly manner to form crystals in a normal urea solution at low temperature; the right side is a schematic diagram of furfural-urea condensate molecules in the liquid nitrogen fertilizer of the present invention anchoring urea molecules through a hydrogen bond network, preventing them from arranging in an orderly manner and thus inhibiting crystallization. Detailed Implementation

[0039] To further illustrate the technical means and effects adopted by the present invention in order to achieve the purpose of the invention, the present invention will be described in detail below with reference to the embodiments.

[0040] In the following examples, all raw materials used were commercially available analytical grade or industrial grade products, and the water was deionized water. The equipment used included conventional reaction vessels, pH meters, and filtration devices. In the following examples, potassium hydroxide was added in solid form, and the amount used was based on pure KOH. For experiments, a 10wt% aqueous solution could be prepared as needed.

[0041] The condensation reaction equation of furfural and urea is as follows: Figure 1 As shown in the diagram. During the reaction, the aldehyde group of furfural undergoes nucleophilic addition with the amino group of urea to generate a hydroxymethylurea intermediate, which then condenses with another urea molecule, removing a water molecule to form a furfural-urea condensate.

[0042] Example 1 This embodiment provides a highly stable liquid nitrogen fertilizer, which is prepared from the following raw materials in parts by weight: 10 parts furfural, 25 parts urea, 0.5 parts ZnCl2, and 30 parts water.

[0043] The preparation method is as follows: (1) Add furfural, urea, ZnCl2 and water to the reaction vessel and stir at 300 rpm for 10 minutes to obtain a mixture; (2) Add 2 parts of potassium hydroxide (KOH) to adjust the pH of the mixture to 8.0; (3) Heat the mixture to 55°C and stir at 150 rpm for 5 hours to allow furfural and urea to undergo a condensation reaction and generate furfural-urea condensate in situ. (4) After the reaction is complete, allow it to cool naturally to room temperature (about 40 minutes) to obtain the crude product; (5) Filter with 300 mesh filter cloth to remove insoluble matter and obtain clear and transparent liquid nitrogen fertilizer.

[0044] Example 2 This embodiment provides a highly stable liquid nitrogen fertilizer, which is prepared from the following raw materials in parts by weight: 5 parts furfural, 15 parts urea, 0.5 parts ZnCl2, and 20 parts water.

[0045] The preparation method is as follows: (1) Add furfural, urea, ZnCl2 and water to the reaction vessel and stir at 300 rpm for 10 minutes to obtain a mixture; (2) Add 0.5 parts of potassium hydroxide (KOH) to adjust the pH of the mixture to 7.0; (3) Heat the mixture to 50°C and stir at 100 rpm for 6 hours to allow furfural and urea to undergo a condensation reaction and generate furfural-urea condensate in situ. (4) After the reaction is complete, allow it to cool naturally to room temperature (about 45 minutes) to obtain the crude product; (5) Filter with 300 mesh filter cloth to remove insoluble matter and obtain clear and transparent liquid nitrogen fertilizer.

[0046] Example 3 This embodiment provides a highly stable liquid nitrogen fertilizer, which is prepared from the following raw materials in parts by weight: 10 parts furfural, 30 parts urea, 0.1 parts p-toluenesulfonic acid, and 40 parts water.

[0047] The preparation method is as follows: (1) Add furfural, urea, p-toluenesulfonic acid and water to a reaction vessel and stir at 300 rpm for 10 minutes to obtain a mixture; (2) Add 2 parts of potassium hydroxide (KOH) to adjust the pH of the mixture to 8.0; (3) Heat the mixture to 40°C and stir at 200 rpm for 8 hours to allow furfural and urea to undergo a condensation reaction and generate furfural-urea condensate in situ. (4) After the reaction is complete, allow it to cool naturally to room temperature (about 30 minutes) to obtain the crude product; (5) Filter with 200 mesh filter cloth to remove insoluble matter and obtain clear and transparent liquid nitrogen fertilizer.

[0048] Example 4 This embodiment provides a highly stable liquid nitrogen fertilizer, which is prepared from the following raw materials in parts by weight: 10 parts of 5-hydroxymethylfurfural (HMF), 30 parts of urea, 0.5 parts of ZnCl2, and 30 parts of water.

[0049] The preparation method is as follows: (1) Add HMF, urea, ZnCl2 and water to the reaction vessel and stir at 300 rpm for 10 minutes to obtain a mixture; (2) Add 5 parts of potassium hydroxide (KOH) to adjust the pH of the mixture to 9.0; (3) Heat the mixture to 55°C and stir at 300 rpm for 5 hours to allow HMF and urea to undergo a condensation reaction and generate HMF-urea condensate in situ. (4) After the reaction is complete, allow it to cool naturally to room temperature (about 60 minutes) to obtain the crude product; (5) Filter with 300 mesh filter cloth to remove insoluble matter and obtain clear and transparent liquid nitrogen fertilizer.

[0050] Example 5 This embodiment provides a highly stable liquid nitrogen fertilizer, which is prepared from the following raw materials in parts by weight: 10 parts furfural, 40 parts urea, 2 parts ZnCl2, and 30 parts water.

[0051] The preparation method is as follows: (1) Add furfural, urea, ZnCl2 and water to the reaction vessel and stir at 300 rpm for 10 minutes to obtain a mixture; (2) Add 2 parts of potassium hydroxide (KOH) to adjust the pH of the mixture to 8.0; (3) Heat the mixture to 70°C and stir at 150 rpm for 2 hours to allow furfural and urea to undergo a condensation reaction and form a condensate in situ; (4) After the reaction is complete, allow it to cool naturally to room temperature (about 40 minutes) to obtain the crude product; (5) Filter with 400 mesh filter cloth to remove insoluble matter and obtain clear and transparent liquid nitrogen fertilizer.

[0052] Example 6 This embodiment provides a highly stable liquid nitrogen fertilizer, which is prepared from the following raw materials in parts by weight: 15 parts furfural, 18 parts urea, 0.8 parts ZnCl2, and 35 parts water.

[0053] The preparation method is as follows: (1) Add furfural, urea, ZnCl2 and water to the reaction vessel and stir at 300 rpm for 10 minutes to obtain a mixture; (2) Add 2 parts of potassium hydroxide (KOH) to adjust the pH of the mixture to 8.0; (3) Heat the mixture to 55°C and stir at 150 rpm for 5 hours to allow furfural and urea to undergo a condensation reaction and generate furfural-urea condensate in situ. (4) After the reaction is complete, allow it to cool naturally to room temperature (about 45 minutes) to obtain the crude product; (5) Filter with 300 mesh filter cloth to remove insoluble matter and obtain clear and transparent liquid nitrogen fertilizer.

[0054] Comparative Example 1 This comparative example provides a common urea solution, which is prepared by mixing 30 parts of urea and 30 parts of water.

[0055] The preparation method is as follows: Add urea and water to a reaction vessel and stir at 300 rpm for 10 minutes to completely dissolve the urea, thus obtaining a common urea solution.

[0056] Comparative Example 2 This comparative example provides a furfural-urea physical mixture prepared from the following raw materials in parts by weight: 10 parts furfural, 30 parts urea, and 30 parts water.

[0057] The preparation method is as follows: add furfural, urea and water into a reaction vessel, stir at 300 rpm for 10 minutes, add 2 parts of potassium hydroxide (KOH) to adjust the pH to 8.0, and a physical mixture is obtained (no heating reaction).

[0058] Comparative Example 3 This comparative example provides a urea solution with added chemical antifreeze, prepared from the following raw materials in parts by weight: 30 parts urea, 5 parts potassium formate, and 30 parts water.

[0059] The preparation method is as follows: Add urea, potassium formate and water into a reaction vessel, stir at 300 rpm for 10 minutes to completely dissolve urea and potassium formate, and obtain a urea solution containing chemical antifreeze.

[0060] Performance testing methods The following performance tests were performed on the samples prepared in Examples 1-6 and Comparative Examples 1-3.

[0061] 1. Low-temperature stability test Following industry-standard methods for evaluating the low-temperature stability of liquid fertilizers, each sample was placed in a constant-temperature freezer at -5℃, -10℃, and -15℃, and the sample status was observed after 24 hours and 72 hours, recording whether crystallization occurred. For samples that showed crystallization, the temperature and time of the first crystallization were recorded.

[0062] 2. Freeze-thaw cycle stability test Referring to the industry-standard method for evaluating the low-temperature stability of liquid fertilizers, the sample was frozen at -10℃ for 24 hours, then removed and allowed to thaw naturally to room temperature. This cycle was repeated three times to observe whether crystallization, stratification, or precipitation occurred.

[0063] 3. Determination of total nitrogen content The total nitrogen content was determined according to the requirements of NY / T 1107-2020 "Water-soluble Fertilizers with Macronutrients" and the methods specified in NY / T 1977-2010 "Determination of Total Nitrogen, Phosphorus, and Potassium Content in Water-soluble Fertilizers". The Kjeldahl method was used: an appropriate amount of liquid sample (containing approximately 0.05-0.1 g of nitrogen) was accurately weighed and placed in a digestion tube. Concentrated sulfuric acid and a mixed catalyst were added for digestion. After alkalization, the sample was distilled and absorbed with boric acid. The total nitrogen content was then calculated by titration with a standard hydrochloric acid titration solution.

[0064] 4. pH value measurement Referring to NY / T 1973-2021 "Determination of Water-Insoluble Matter Content and pH of Water-Soluble Fertilizers", after calibrating the pH meter with standard buffer solutions of pH 4.00 and pH 6.86, the electrode was inserted into the sample solution, and the stable pH value was read.

[0065] 5. Determination of water-insoluble matter content Referring to NY / T 1973-2021 "Determination of Water-Insoluble Matter Content and pH of Water-Soluble Fertilizers", the gravimetric method was used. The filter device (300-mesh filter cloth) was dried to constant weight and recorded as m1; 100 mL of sample was vacuum filtered; the filter cloth and insoluble matter were dried to constant weight and recorded as m2; the water-insoluble matter content was calculated as (m2-m1) / 100 × 100%. If the product is clear and transparent with no visible insoluble matter, this item can be reported as "<0.01%" or "Not Detected".

[0066] 6. Visual inspection Observe the appearance of each sample under natural light and record its color, transparency, and whether there is layering or sedimentation.

[0067] 7. High and low temperature cycling stability test A programmable high and low temperature test chamber was used to conduct high and low temperature cycling tests on the samples at 60℃ / -30℃. The samples were placed at 60℃ for 24 hours, then transferred to -30℃ for another 24 hours; this constituted one cycle. The cycle was repeated, and the sample condition was observed after each cycle, recording the number of cycles in which crystallization, stratification, precipitation, or solidification occurred. This test method referenced the industry-standard high and low temperature cycling stability evaluation method for liquid fertilizers.

[0068] The samples were tested according to the above testing methods, and the results are summarized in Table 1.

[0069] Table 1 Performance test results of each embodiment and comparative example

[0070] Note: Comparative Example 1 showed white crystals after being placed at -5℃ for 24 hours, with the first crystallization occurring at -5℃ and within 24 hours; Comparative Example 2 showed white crystals after being placed at -10℃ for 72 hours, with the first crystallization occurring at -10℃ and within 72 hours; Examples 1-6 remained clear and transparent after being placed at -15℃ for 24 hours, with no crystal precipitation; Comparative Example 1 showed a large amount of crystallization after the first 60℃ / -30℃ cycle; Comparative Example 2 showed turbidity after the first cycle and layered precipitation after the second cycle; Examples 1-6 remained clear and transparent after 5 consecutive cycles; "-" indicates that the sample has expired and was not tested further.

[0071] As can be seen from the data in Table 1: The liquid nitrogen fertilizers prepared in Examples 1-6 remained clear and transparent without any crystallization after being placed at -5℃ for 24 hours, -10℃ for 72 hours, and -15℃ for 24 hours. After three freeze-thaw cycles (freezing at -10℃ for 24 hours, thawing naturally at room temperature, repeated three times), the products still remained clear and transparent, without crystallization, stratification, or precipitation. Furthermore, in accelerated high-low temperature cycling tests at 60℃ / -30℃, Examples 1-6 remained clear and transparent after five consecutive cycles, demonstrating excellent resistance to temperature shock. The mechanism by which this invention inhibits low-temperature crystallization is as follows: Figure 2 As shown, furfural and urea undergo a condensation reaction to form a furfural-urea condensate. The carbonyl and amino groups in the condensate form a hydrogen bond network with free urea, which effectively prevents the orderly arrangement of urea molecules and inhibits crystallization within a wide temperature range (-15℃ to 60℃).

[0072] In contrast, the ordinary urea solution in Comparative Example 1 showed white crystals after being placed at -5℃ for 24 hours, and a large amount of crystals appeared on the first cycle of the 60℃ / -30℃ cycle; the furfural-urea physical mixture in Comparative Example 2 showed white crystals after being placed at -10℃ for 72 hours, and stratified precipitation appeared after 3 freeze-thaw cycles, and turbidity appeared on the first cycle of the 60℃ / -30℃ cycle; although the urea solution containing chemical antifreeze in Comparative Example 3 remained clear and transparent during low temperature and high temperature cycles, the product contained potassium formate, a chemical antifreeze, which poses a potential hazard to crops.

[0073] Examples 1-6 had a total nitrogen content of 18.5-26.5 wt%, a pH of 7.0-9.0, and an insoluble content of less than 0.1%, and were homogeneous, transparent solutions. Comparative Example 1 had a pH of 5.6, which was slightly acidic; Comparative Example 2 had an insoluble content of 0.15%, and the product had a slightly turbid appearance.

[0074] Nutrient release characteristic test Nutrient release characteristics were tested using Example 1 as a representative example. The cumulative nitrogen release rate of the sample in still water at 25°C was determined using the static water dissolution method. Specifically, an appropriate amount of the Example 1 sample (approximately 2.5 g of nitrogen) was weighed, placed in a dialysis bag, and immersed in 200 mL of distilled water. The sample was then allowed to stand at a constant temperature of 25°C. Water samples were taken on days 1, 3, 5, 7, 14, and 28. The nitrogen content in the water was determined according to the method specified in NY / T 1977-2010 "Determination of Total Nitrogen, Phosphorus, and Potassium Content in Water-Soluble Fertilizers," and the cumulative release rate was calculated. Fresh distilled water was used after each sampling. The nitrogen release rate of a common urea solution from Comparative Example 1 was determined using the same method as a control.

[0075] The results showed that the cumulative nitrogen release rate of the product in Example 1 was 18.5% on day 1, 52.3% on day 7, 72.6% on day 14, and 86.2% on day 28, exhibiting obvious slow-release characteristics. In contrast, the release rate of ordinary urea solution in Comparative Example 1 reached 95.2% on day 1, and was almost completely released by day 3. These results indicate that the product of the present invention has significant slow-release properties, effectively extending the nitrogen supply cycle and improving fertilizer utilization.

[0076] Since Examples 2-6 and Example 1 both contain furfural-urea condensate (Example 4 contains HMF-urea condensate), and their sustained-release mechanisms are the same, Examples 2-6 also have similar sustained-release properties.

[0077] In summary, this invention utilizes a one-pot preparation process, eliminating the need for separating and purifying intermediates, to directly obtain clear and transparent liquid nitrogen fertilizer under weakly alkaline conditions. Compared to existing technologies, the product of this invention remains stable at -15℃, solving the technical problems of liquid nitrogen fertilizer's tendency to crystallize at low temperatures and its reliance on chemical antifreeze agents. It also exhibits excellent slow-release properties, making it suitable for modern fertilization systems such as drip irrigation and sprinkler irrigation.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a highly stable liquid nitrogen fertilizer, characterized in that, Includes the following steps: Bio-based aldehyde source, urea, catalyst, pH adjuster and water are mixed and the pH is adjusted to 7-9. The mixture is reacted at 40-70℃ for 2-8 hours. After the reaction is completed, it is cooled and filtered to obtain a highly stable liquid nitrogen fertilizer.

2. The preparation method according to claim 1, characterized in that, By weight, the bio-based aldehyde source is 5-15 parts, urea is 10-40 parts, catalyst is 0.1-2 parts, pH adjuster is 0.5-5 parts, and water is 20-40 parts.

3. The preparation method according to claim 1, characterized in that, The bio-based aldehyde source is selected from at least one of furfural, 5-hydroxymethylfurfural, and glyoxylic acid.

4. The preparation method according to claim 1, characterized in that, The catalyst is selected from at least one of ZnCl2, p-toluenesulfonic acid, phosphorous acid, and cation exchange resin.

5. The preparation method according to claim 1, characterized in that, The pH adjuster is selected from at least one of sodium hydroxide, potassium hydroxide, and ammonia water.

6. The preparation method according to claim 1, characterized in that, The molar ratio of the bio-based aldehyde source to urea is 1:(2-6).

7. The preparation method according to claim 1, characterized in that, The reaction is carried out under stirring conditions at a speed of 100-300 rpm.

8. The preparation method according to claim 1, characterized in that, The cooling time is 30-60 minutes.

9. The preparation method according to claim 1, characterized in that, The filtration process uses 200-400 mesh filter cloth or filter screen.

10. A highly stable liquid nitrogen fertilizer prepared by the preparation method according to any one of claims 1-9.

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