阅读说明：本技术 一种脲甲醛缓释氮肥的生产方法 (Production method of urea formaldehyde slow-release nitrogen fertilizer ) 是由 陈文兴 周昌平 田娟 陈思帆 王瑶 虞江 于 2021-07-26 设计创作，主要内容包括：本发明属于肥料技术领域,尤其涉及一种脲甲醛缓释氮肥的生产方法,步骤如下：(1)将尿素和甲醛溶液混合,搅拌加入氢氧化钠固体调节混合溶液pH呈碱性,将混合溶液升温至一定温度,持续反应一定时间；(2)将步骤(1)的反应液连续排放到正常运行状态的输送带式聚合槽上,同步加入调节剂调节输送带式聚合槽上反应液pH值至酸性,聚合生成脲甲醛；(3)将步骤(2)生产的脲甲醛陈化12小时以上；(4)将陈化处理后的脲甲醛送到旋转闪蒸干燥器中干燥,采用沉降室收集产品,再经旋风分离除尘回收产品,尾气处理后排空。本发明提供的脲甲醛工业生产方法,其特点是提供了一个连续生产脲甲醛缓释肥的工艺技术,能直接生产出粉体脲甲醛缓释肥产品,生产工艺简单、质量稳定。(The invention belongs to the technical field of fertilizers, and particularly relates to a production method of a urea formaldehyde slow-release nitrogen fertilizer, which comprises the following steps: (1) mixing urea and formaldehyde solution, stirring, adding sodium hydroxide solid to adjust the pH value of the mixed solution to be alkaline, heating the mixed solution to a certain temperature, and continuously reacting for a certain time; (2) continuously discharging the reaction liquid obtained in the step (1) to a conveyor belt type polymerization tank in a normal operation state, synchronously adding a regulator to adjust the pH value of the reaction liquid on the conveyor belt type polymerization tank to acidity, and polymerizing to generate urea formaldehyde; (3) aging the urea formaldehyde produced in the step (2) for more than 12 hours; (4) and (3) conveying the aged urea formaldehyde into a rotary flash evaporation dryer for drying, collecting a product by adopting a settling chamber, performing cyclone separation for dedusting and recovering the product, and emptying after tail gas treatment. The industrial production method of urea formaldehyde is characterized by providing a process technology for continuously producing urea formaldehyde slow release fertilizer, and can directly produce powdered urea formaldehyde slow release fertilizer products, and its production process is simple and quality is stable.)

1. A production method of a urea formaldehyde slow-release nitrogen fertilizer is characterized by comprising the following steps:

(1) mixing urea and formaldehyde solution, adding sodium hydroxide solid while stirring to adjust the pH of the mixed solution to 8-10, heating the mixed solution to 40-60 ℃, and continuously reacting for 30-60 min;

(2) continuously discharging the reaction liquid obtained in the step (1) to a conveyor belt type polymerization tank in a normal operation state, synchronously adding a regulator to adjust the pH value of the reaction liquid on the conveyor belt type polymerization tank to acidity, and polymerizing to generate urea formaldehyde;

(3) aging the urea formaldehyde produced in the step (2) for more than 12 hours;

(4) and (3) carrying out spin flash drying on the aged urea formaldehyde, collecting the product by adopting a settling chamber, carrying out cyclone separation to remove dust and recover the product, and emptying the product after tail gas treatment.

2. The method for producing a urea formaldehyde slow-release nitrogen fertilizer as claimed in claim 1, wherein in the step (1), sodium hydroxide solid is added to adjust the pH of the mixed solution system to 8-9.

3. The method for producing the urea-formaldehyde slow-release nitrogen fertilizer as claimed in claim 1, wherein in the step (1), the concentration of the formaldehyde solution is 35-50%, and the molar ratio of urea to formaldehyde is 1_～2:1, heating to 45%_～Reaction at 60 ℃ of 30_～50min。

4. The method for producing the urea-formaldehyde slow-release nitrogen fertilizer as claimed in claim 3, wherein in the step (1), the molar ratio of urea to formaldehyde is 1.5:1, and the temperature is raised to 50 ℃ for reaction for 40 min.

5. The method for producing urea formaldehyde slow release nitrogen fertilizer as claimed in claim 1, wherein in step (2), the regulator includes but is not limited to monoammonium phosphate, urea phosphate, phosphoric acid, and the pH value of the reaction system is adjusted to 3.5_～4.5。

6. The method for producing a urea formaldehyde slow-release nitrogen fertilizer as claimed in claim 1, wherein in the step (2), the pH value of the reaction system is adjusted to 4.0.

7. The method for producing a urea formaldehyde slow-release nitrogen fertilizer as claimed in claim 1, wherein in the step (3), the polymerization aging is aging at room temperature for 12 hours.

8. The method for producing a urea formaldehyde slow release nitrogen fertilizer as claimed in claim 1, wherein in the step (4), the conditions of the spin flash drying are inlet temperature 280_～310℃。

Technical Field

The invention belongs to the technical field of fertilizers, and particularly relates to a production method of a urea formaldehyde slow-release nitrogen fertilizer.

Background

The urea-formaldehyde fertilizer is urea-formaldehyde polymer with relatively low solubility, which is obtained by two-step reaction of hydroxylation addition reaction and methylene condensation reaction of urea and formaldehyde under certain conditions. The urea formaldehyde fertilizer is a long-acting nitrogen fertilizer with good physical property and slow release property, and after being applied to soil, the urea formaldehyde fertilizer slowly releases nutrients under the dual effects of hydrolysis and microbial invasion, so that the fertilizer effect period is prolonged, and the utilization rate is high; but also can promote the soil to form a granular structure, improve the soil permeability and increase the root system penetrating power of crops. The popularization and application of the urea formaldehyde fertilizer have profound practical significance for upgrading the nitrogen fertilizer industry in China and relieving the energy and environmental pressure.

At present, urea formaldehyde fertilizers have several main problems, which limit the popularization and application of the urea formaldehyde fertilizers in agriculture, particularly in field crops: 1. the production cost of the urea formaldehyde slow release fertilizer is too high, which is generally 3-8 times of that of the conventional fertilizer, so that most of urea formaldehyde is only suitable for crops with high economic value, such as vegetables, flowers, lawns and the like; 2. compared with the traditional process, the production process of the urea formaldehyde slow release fertilizer is complex, the production period is long, the research on key equipment and process matching is relatively weak, and industrialization is difficult to form; 3. the product has unstable quality, easy agglomeration and poor slow release effect. The applicant also carries out related improvement research on the production process of the urea formaldehyde slow release fertilizer in the earlier stage.

The application number CN201410570901.4 discloses a preparation method of a urea-formaldehyde slow-release fertilizer, which comprises the steps of mixing urea and formaldehyde according to a certain mass ratio, adding sodium hydroxide to adjust the pH value, heating and stirring to enable the urea-formaldehyde to fully react, stopping heating after reacting for a period of time, adding monoammonium phosphate to adjust the pH value, mixing the obtained material and one or two of organic fertilizer and humic acid according to a certain mass ratio, then carrying out rotary flash evaporation drying, and then carrying out granulation. The patent adopts a spin flash drying method to solve the problems of long period and complex process existing in the traditional drying method, and solves the problem of agglomeration in the urea formaldehyde flash drying process by adding organic fertilizer and humic acid.

The patent with the application number of CN201510988064.1 discloses a urea-formaldehyde slow release fertilizer and a production method thereof, wherein the urea-formaldehyde slow release fertilizer is prepared by mixing formaldehyde and urea according to the molar ratio of 1: 2-2.5, continuing to react for a period of time after adding a sodium hydroxide regulating solution to be alkaline, continuing to react for a period of time after adding a phosphoric acid solution to be acidic, and then adding urea to perform mixed reaction, wherein the molar ratio of the added urea is 1: and 1, finally granulating and drying to obtain the urea formaldehyde slow release fertilizer. The urea formaldehyde fertilizer with stable mass production quality and good slow release effect is obtained by improving the synthesis process, and the slow release time of the urea formaldehyde fertilizer reaches 6-8 months.

The applicant further carries out deep research on the basis of earlier research and provides a production method of the urea-formaldehyde slow-release nitrogen fertilizer, which has the advantages of simple production process, better slow-release effect and stable quality.

Disclosure of Invention

The invention aims to provide a production method of a urea formaldehyde slow-release nitrogen fertilizer, which has the advantages of simple production process, better slow-release effect and stable quality, and is realized by the following technical scheme.

A production method of a urea formaldehyde slow-release nitrogen fertilizer comprises the following steps:

(1) mixing urea and formaldehyde solution, adding sodium hydroxide solid while stirring to adjust the pH of the mixed solution to 8-10, heating the mixed solution to 40-60 ℃, and continuously reacting for 30-60 min;

(2) continuously discharging the reaction liquid obtained in the step (1) to a conveyor belt type polymerization tank in a normal operation state, synchronously adding a regulator to adjust the pH value of the reaction liquid on the conveyor belt type polymerization tank to acidity, and polymerizing to generate urea formaldehyde;

(3) aging the urea formaldehyde produced in the step (2) for more than 12 hours;

(4) and (3) carrying out spin flash drying treatment on the aged urea formaldehyde, collecting the product by adopting a settling chamber, carrying out cyclone separation to remove dust and recover the product, and emptying the product after tail gas treatment.

Preferably, in the step (1), the pH value of the mixed solution system is adjusted to 8-9 by adding the sodium hydroxide solid.

Preferably, in the step (1), the mass concentration of the formaldehyde solution is 35-50%, the molar ratio of urea to formaldehyde is 1-2:1, and the temperature is raised to 45-60 ℃ for reaction for 30-50 min.

More preferably, in the step (1), the molar ratio of the urea to the formaldehyde is 1.5:1, and the temperature is raised to 40 ℃ for reaction for 40 min.

Preferably, in the step (2), the adjusting agent includes, but is not limited to, monoammonium phosphate, urea phosphate and phosphoric acid, and the pH value of the reaction system is adjusted to 3.5-4.5.

More preferably, in the step (2), the pH value of the reaction system is adjusted to 4.0.

Preferably, in the step (3), the polymerization aging is aging at normal temperature for 12 hours.

Preferably, the condition of the spin flash drying is that the inlet temperature is 280-310 ℃.

The invention has the beneficial effects that: the technical scheme focuses on the industrial production method of urea-formaldehyde, and is characterized in that urea-formaldehyde reaction liquid is continuously discharged onto a conveyor belt type polymerization tank in a normal operation state, a regulator is synchronously added to adjust the pH value of the reaction liquid on the conveyor belt type polymerization tank to be acidic, the reaction liquid is polymerized on the conveyor belt type polymerization tank to generate urea-formaldehyde, the urea-formaldehyde after polymerization is loosened by utilizing the operation characteristic of the conveyor belt type polymerization tank, the produced urea-formaldehyde has good dispersibility and does not need to be crushed before being dried. The produced urea formaldehyde is aged for more than 12 hours, and then can be directly conveyed to a rotary flash evaporation drier for drying to obtain the powder urea formaldehyde. The industrial production method of urea formaldehyde is characterized by providing a process technology for continuously producing urea formaldehyde slow release fertilizer, and can directly produce powdered urea formaldehyde slow release fertilizer products, and its production process is simple and quality is stable.

To better illustrate the advantageous effects of the invention, the following research procedures are also included in the present invention, which are intended to illustrate the advantageous effects of the invention, but are in no way limited to the scope of protection of the invention.

First, research on synthesis process

Urea and formaldehyde react to synthesize urea formaldehyde, the polymerization degree of urea formaldehyde determines the solubility of urea formaldehyde, and the solubility determines the fertilizer effect period. The solubility of urea-formaldehyde is directly related to the urea/formaldehyde molar ratio, the reaction temperature, the reaction time, the pH value, the feeding mode and the like. The invention researches the influence of factors such as material ratio, reaction temperature, pH value, feeding mode and the like on cold water insoluble nitrogen, hot water insoluble nitrogen, slow release effective nitrogen and activity coefficient of the urea formaldehyde slow release fertilizer, and optimizes the conditions of the synthesis process.

1. Effect of Urea/Formaldehyde molar ratio, reaction temperature, reaction time on Urea-Formaldehyde solubility

1.1 Experimental methods: and (2) respectively pouring urea and formaldehyde into a three-neck flask, adjusting the pH value of a mixed solution system to 8-9 by using sodium hydroxide when the reaction starts, adjusting the pH value of the system to 3.5-4.5 by using urea phosphate when the reaction finishes, and drying at 80 ℃ for 24 hours after the solution system is solidified to obtain a urea-formaldehyde sample. The influence of three factors of urea/formaldehyde molar ratio, reaction temperature and reaction time on the solubility of urea formaldehyde during reaction is researched by adopting an orthogonal experimental method. The synthesis experiment factor levels are shown in table 1.

TABLE 1 level table of synthesis experiment factors

And (3) carrying out an orthogonal experiment according to the synthesis experiment factor level table and a three-factor three-level orthogonal experiment table to synthesize the urea-formaldehyde sample. According to the standard urea formaldehyde slow release fertilizer (HG/T4137-2010), the content of total nitrogen, cold water insoluble nitrogen and hot water insoluble nitrogen of a urea formaldehyde sample obtained in an orthogonal experiment is measured according to the method of 'measuring the nitrogen content of a 2.5 urea formaldehyde sample', the slow release effective nitrogen and the activity coefficient are calculated according to the content of the cold water insoluble nitrogen and the hot water insoluble nitrogen, and the orthogonal experiment result of the solubility of the urea formaldehyde nitrogen is shown in Table 2.

TABLE 2 Quadrature experimental results for urea formaldehyde nitrogen solubility

1.2 analysis of the results of the experiment

The fertilizer effect period and the fertilizer effect of the urea formaldehyde are related to cold water insoluble nitrogen, hot water insoluble nitrogen and slow release effective nitrogen of the urea formaldehyde. And (3) carrying out statistical analysis on orthogonal experimental results of the cold water insoluble nitrogen content, the hot water insoluble nitrogen content and the slow-release effective nitrogen content of the urea formaldehyde in the table 2, and finding out the optimal process condition in the experimental range and the significant influence degree of experimental factors on the results. The results of the data analysis of the urea formaldehyde orthogonality experiment are shown in table 3.

TABLE 3 results of urea formaldehyde Quadrature Experimental data

Note k_ij-mean value of all index values of experimental results at the ith level of the ith factor; MAX ═ MAX (k)_1j，k_2j，k_3j)；MIN＝MIN(k_1j，k_2j，k_3j) (ii) a And the range R is MAX-MIN.

According to the analysis result of cold water insoluble nitrogen content orthogonal experiment data, the three factors have the extremely poor influence of A > B > C, the main influence factor in the experiment range is the molar ratio of A urea to formaldehyde, the second factor is the reaction temperature of B, and the influence of the reaction time of the factor C is the minimum.

According to the analysis result of the hot water insoluble nitrogen content orthogonal experiment data, the three factors have the extremely poor influence of A > C > B, the main influence factor in the experiment range is the molar ratio of A urea to formaldehyde, the second factor is C reaction time, and the influence of B reaction temperature is the minimum.

According to the analysis result of the slow-release effective nitrogen content orthogonal experiment data, the three factors have the extremely poor influence of A > B > C, the main influence factor in the experiment range is the molar ratio of A urea to formaldehyde, the second factor is the reaction temperature of B, and the influence of the reaction time of the factor C is the minimum.

1.3 analysis of influence trend of each factor on experimental results

Mean value (k) of the index values according to the experimental results of Table 3_ij) The influence and change rule trend lines of the cold water insoluble nitrogen content, the hot water insoluble nitrogen content and the slow release effective nitrogen content under three-factor horizontal experimental conditions are respectively drawn, and the results are shown in fig. 2, fig. 3 and fig. 4.

As can be seen from fig. 2, the urea/formaldehyde molar ratio shows significant solubility of cold water insoluble nitrogen, hot water insoluble nitrogen and slow release effective nitrogen of urea formaldehyde, and the cold water insoluble nitrogen content, the hot water insoluble nitrogen content and the slow release effective nitrogen content all increase with decreasing molar ratio. The molar ratio of urea to formaldehyde is 1.5-2:1, and the content of cold water insoluble nitrogen, hot water insoluble nitrogen and slow release effective nitrogen is higher, preferably 1.5: 1.

As can be seen from FIG. 3, the cold water insoluble nitrogen content of urea-formaldehyde increases slightly with decreasing reaction temperature, the hot water insoluble nitrogen content decreases with decreasing reaction temperature, and the slow-release effective nitrogen content increases with decreasing reaction temperature. The low temperature is favorable for increasing the content of slow-release effective nitrogen in urea formaldehyde, and the slow-release effective nitrogen content in formaldehyde is higher at the reaction temperature of 40-50 ℃, preferably 40 ℃.

As can be seen from FIG. 4, the cold water insoluble nitrogen content of urea-formaldehyde decreased slightly with the decrease of the reaction time, the hot water insoluble nitrogen content decreased first and then increased slightly with the decrease of the reaction temperature, and the slow-release effective nitrogen content increased first and then decreased slightly with the decrease of the reaction temperature. The reaction time is 30-50min, which is beneficial to increasing the content of slow-release effective nitrogen in the urea formaldehyde, and the preferable time is 40 min.

1.4 analysis of variance of orthogonal Cross Experimental results

In order to ensure the reliability of the value F of the statistical test quantity, the analysis results in the table 3 are analyzed by a variance analysis method to further judge the significance of the influence of each experimental factor on the cold water insoluble nitrogen, the hot water insoluble nitrogen and the slow-release effective nitrogen of the urea formaldehyde, and the analysis results are shown in the table 4.

TABLE 4 analysis of variance in orthogonal experiments for urea-formaldehyde synthesis

From the above table, it can be seen that the urea-formaldehyde cold water insoluble nitrogen is highly influenced by the urea/formaldehyde molar ratio, is influenced by the reaction temperature more remarkably, and is not influenced by the reaction time significantly; the urea-formaldehyde hot water insoluble nitrogen is highly influenced by the mole ratio of urea/formaldehyde, is not influenced significantly by the reaction temperature and is not influenced significantly by the reaction time; the slow release of effective nitrogen from urea-formaldehyde is significantly influenced by the molar ratio of urea to formaldehyde, is not significantly influenced by the reaction temperature, and is not significantly influenced by the reaction time.

The invention further researches the mechanism of the influence of the reaction factors on the urea-formaldehyde reaction:

the reaction product of urea and formaldehyde under basic conditions comprises mono-and dimethylol ureas. The larger the molar ratio of urea to formaldehyde is in the reaction process, the larger the excess coefficient of urea relative to formaldehyde is, and the higher the amount of generated hydroxymethyl urea is; as the urea/formaldehyde molar ratio decreases, the urea excess factor decreases, the amount of mono-methylol urea produced decreases, and the amount of dimethylol urea produced increases. Then under the acidic condition, the urea and the hydroxymethyl urea are subjected to condensation reaction to generate the polymethylene urea. In the condensation reaction, dimethylol urea participates in the condensation reaction more easily than monomethylol urea according to the structural characteristics of monomethylol urea and dimethylol urea, and a long-chain polymer can be formed only by the polymerization reaction in which dimethylol urea participates. During the condensation reaction, the polymerization reaction is terminated when the methylol urea participates in the polymerization reaction.

Therefore, in the synthesis process of urea formaldehyde, the higher the molar ratio of urea to formaldehyde, the more methylol urea is generated, the more soluble urea formaldehyde with low polymerization degree can be generated by condensation reaction, and the obtained urea formaldehyde has high solubility and reduced insoluble nitrogen; when the molar ratio of urea to formaldehyde is reduced, the more dimethylol urea is generated, more long-chain low-solubility urea formaldehyde is easily generated during condensation reaction, so that the cold water insoluble nitrogen content, the hot water insoluble nitrogen content and the slow-release effective nitrogen content are obviously increased.

In the synthesis process of urea formaldehyde, the change of reaction temperature and reaction time has no obvious influence on the amount of the hydroxymethyl urea and the dimethylol urea in a reaction system, but in the condensation process, the higher temperature can promote mass transfer, further promote the condensation reaction of urea formaldehyde, increase the polymerization chain length of urea formaldehyde and increase the content of cold water insoluble nitrogen of urea formaldehyde.

In conclusion, the synthesis process conditions are that the urea/formaldehyde molar ratio is 1.5-2:1, the reaction temperature is 40-50 ℃, and the reaction time is 30-50min, the urea formaldehyde has good slow release property; the synthesis process conditions are that the molar ratio of urea to formaldehyde is 1.5: 1. the reaction temperature is 40 ℃ and the reaction time is 40 min.

II, Effect of polycondensation reaction pH on Urea Formaldehyde solubility

The experimental method comprises the following steps: in a urea/formaldehyde molar ratio of 1.5: 1. the urea formaldehyde is synthesized at the reaction temperature of 40 ℃ for 40min, and condensed and polymerized into the urea formaldehyde under the conditions of different pH values so as to examine the influence of the pH value on the solubility of the urea formaldehyde.

As shown in fig. 5, it is understood from fig. 5 that as the pH value during the polymerization of urea-formaldehyde decreases, both the cold water insoluble nitrogen content and the hot water insoluble nitrogen content tend to increase as a whole; the slow-release effective nitrogen firstly rises along with the reduction of the polymerization pH value and then falls when the pH value is about 4. According to the polymerization reaction principle of urea/formaldehyde, the pH value of a condensation reaction system is low, the catalytic capability for the condensation reaction is strong, the condensation reaction is easy to occur in the reaction system, and more insoluble urea formaldehyde is generated; if the pH value of the condensation reaction system is increased, the condensation reaction capability of the hydroxymethyl urea and the dimethylol urea is reduced, and the generated insoluble urea formaldehyde is reduced. The polymerization chain length of the condensation reaction polymer of the urea formaldehyde is reduced along with the increase of the pH value of a condensation system, and the analysis data shows that cold water insoluble nitrogen and hot water insoluble nitrogen are reduced, and the reduction degree of the hot water insoluble nitrogen is larger, so that the content of the slow-release effective nitrogen is increased along with the increase of the pH value. When the pH value of the polycondensation reaction is 3.5-4.5, the content of slow-release effective nitrogen in the urea formaldehyde is higher, and the pH value of the polycondensation reaction is preferably 4.

Drawings

FIG. 1 is a flow chart of the production of urea formaldehyde according to the present invention.

FIG. 2 is a graph of urea/formaldehyde molar ratio versus urea formaldehyde solubility.

FIG. 3 is a graph of the relationship between the synthesis reaction temperature and urea formaldehyde solubility.

FIG. 4 is a graph of the relationship between synthesis reaction time and urea formaldehyde solubility.

FIG. 5 is a graph of polycondensation reaction pH versus urea formaldehyde solubility.

Detailed Description

The technical solution of the present invention is further limited by the following specific embodiments, but the scope of the claims is not limited to the description.

Example 1

The production method of the urea formaldehyde slow-release nitrogen fertilizer comprises the following steps:

(1) mixing urea and 35% formaldehyde solution, stirring, adding sodium hydroxide solid, and adjusting the pH value of the mixed solution to 8; wherein the molar ratio of urea to formaldehyde is 2: 1;

(2) heating the mixed solution to 50 ℃, continuously reacting for 30min, and adding monoammonium phosphate to adjust the pH value of the reaction system to 3.5;

(3) after the solution system obtained by the treatment in the step (2) is solidified, aging for 12h at normal temperature;

(4) and (3) drying the aged urea formaldehyde in rotary flash drying at the inlet temperature of 280-310 ℃, collecting the product by using a settling chamber, performing cyclone separation to remove dust and recover the product, and emptying the product after tail gas treatment.

Example 2

The production method of the urea formaldehyde slow-release nitrogen fertilizer comprises the following steps:

(1) mixing urea and 40% formaldehyde solution, stirring, adding sodium hydroxide, and adjusting the pH value of the mixed solution to 9; wherein the molar ratio of urea to formaldehyde is 1.5: 1;

(2) heating the mixed solution to 40 ℃, continuously reacting for 40min, and adding urea phosphate to adjust the pH value of the reaction system to 4.0;

(3) after the solution system obtained by the treatment in the step (2) is solidified, aging for 12h at normal temperature;

(4) and (3) conveying the aged urea formaldehyde to spin flash drying for drying, wherein the inlet temperature is 280-310 ℃, collecting the product by adopting a settling chamber, performing cyclone separation for dedusting and recovering the product, and emptying after tail gas treatment.

Example 3

The production method of the modified urea formaldehyde slow-release nitrogen fertilizer comprises the following steps:

(1) mixing urea and 50% formaldehyde solution, stirring and adding sodium hydroxide solid to adjust the pH value of the mixed solution to 9; wherein the molar ratio of urea to formaldehyde is 1.8: 1;

(2) heating the mixed solution to 45 ℃, continuously reacting for 50min, and adding phosphoric acid to adjust the pH value of the reaction system to 4.5;

(3) after the solution system obtained by the treatment in the step (2) is solidified, aging for 12h at normal temperature;

(4) and (3) conveying the aged urea formaldehyde to spin flash drying for drying, wherein the inlet temperature is 280-310 ℃, collecting the product by adopting a settling chamber, performing cyclone separation for dedusting and recovering the product, and emptying after tail gas treatment.

Example 4

The production method of the urea formaldehyde slow-release nitrogen fertilizer comprises the following steps:

(1) mixing urea and a formaldehyde solution with the mass concentration of 37%, stirring, adding sodium hydroxide solid, and adjusting the pH value of the mixed solution to 9; wherein the molar ratio of urea to formaldehyde is 2: 1;

(2) heating the mixed solution to 50 ℃, continuously reacting for 50min, and adding monoammonium phosphate to adjust the pH value of the reaction system to 4.5;

(3) after the solution system obtained by the treatment in the step (2) is solidified, aging for 12h at normal temperature;

(4) and (3) conveying the aged urea formaldehyde to spin flash drying for drying, wherein the inlet temperature is 280-310 ℃, collecting the product by adopting a settling chamber, performing cyclone separation for dedusting and recovering the product, and emptying after tail gas treatment.

Comparative example 1

The urea-formaldehyde fertilizer is prepared according to the first embodiment of the preparation method of the urea-formaldehyde slow release fertilizer disclosed in the patent with the application number of CN 201410570901.4.

Comparative example 2

The urea formaldehyde fertilizer is prepared according to the first embodiment of the urea formaldehyde slow release fertilizer and the production method thereof disclosed in the patent with the application number of CN 201510988064.1.

Experimental example 1

Urea formaldehyde solubility study: urea-formaldehyde fertilizers were prepared according to the methods of examples 1-3 and comparative examples 1-2, the total nitrogen, cold water-insoluble nitrogen and hot water-insoluble nitrogen contents of the samples were measured according to the standard urea-formaldehyde slow release fertilizer (HG/T4137-2010) by the method of "2.5 urea-formaldehyde sample nitrogen content measurement", and the slow release effective nitrogen and activity coefficient were calculated from the cold water-insoluble nitrogen and hot water-insoluble nitrogen contents. The results are shown in Table 5.

TABLE 5 solubility of different urea formaldehyde fertilizer samples

It should be noted that the above examples and test examples are only for further illustration and understanding of the technical solutions of the present invention, and are not to be construed as further limitations of the technical solutions of the present invention, and the invention which does not highlight essential features and significant advances made by those skilled in the art still belongs to the protection scope of the present invention.