CN114605190B

CN114605190B - Method for preparing high-strength polyphosphate granular compound fertilizer by in-situ curing of urea formaldehyde

Info

Publication number: CN114605190B
Application number: CN202210199722.9A
Authority: CN
Inventors: 刘之广; 杨贵婷; 张民; 赵洪猛; 陈琪; 李泽丽; 李明洋
Original assignee: Shandong Agricultural University
Current assignee: Shandong Agricultural University
Priority date: 2022-03-02
Filing date: 2022-03-02
Publication date: 2023-04-18
Anticipated expiration: 2042-03-02
Also published as: CN114605190A

Abstract

The invention belongs to the technical field of fertilizer processing, and discloses a method for preparing a high-strength polyphosphate granular compound fertilizer by in-situ curing of urea formaldehyde. In the method, the in-situ solidification of the urea-formaldehyde is to directly spray a pre-polymerization liquid of the urea-formaldehyde into a material granulated by a disc, and the pre-polymerization liquid of the urea-formaldehyde is subjected to a solidification reaction under an acid environment provided by urea phosphate to form the urea-formaldehyde because the material contains a strongly acidic phosphate fertilizer-urea phosphate; the urea phosphate and the urea are heated to react to generate the polyphosphate, so that a certain slow release effect is achieved on the phosphorus in the fertilizer, and the strength of the granular fertilizer is greatly improved. The application test of the prepared granular fertilizer in potted corn plants and fields shows that the fertilizer can obviously reduce the pH value of saline-alkali soil, improve the phosphorus supply intensity of the soil and increase the corn yield, and is suitable for popularization and application.

Description

Method for preparing high-strength polyphosphate granular compound fertilizer by in-situ curing of urea formaldehyde

Technical Field

The invention belongs to the technical field of fertilizer processing, and relates to a method for preparing a high-strength polyphosphate granular compound fertilizer by in-situ curing urea formaldehyde, in particular to a method for preparing a high-strength polyphosphate granular compound fertilizer by taking urea, potassium chloride, urea phosphate and montmorillonite as raw materials.

Background

Urea formaldehyde fertilisers (UF) are long chain polymers derived from the reaction between urea and formaldehyde, are the main class of slow release fertilisers currently in use, and are also the binders commonly used in disc granulation. The patent No. CN201310252232.1 discloses a preparation method of a low-formaldehyde urea-formaldehyde resin-gypsum-bentonite-based slow release fertilizer, urea and formaldehyde are put into a reaction container at one time, heating and stirring are started, heating is stopped when the temperature is raised to 25 ℃, after the urea is completely dissolved, the pH value is adjusted to 4.0-5.0 by hydrochloric acid or sulfuric acid, the reaction is carried out for 10-20min, after the turbidity point is reached, the pH value is adjusted to 5.0-6.0 by sodium hydroxide aqueous solution, after the reaction is carried out for 40-60min, the temperature is slowly reduced to 40 ℃, the pH value is adjusted to 7.0-8.0 by sodium hydroxide aqueous solution, the temperature is cooled to room temperature, milky white jelly is obtained, and then the milky jelly is sprayed into a disc granulator for granulation. However, in the granulation process, the acid and alkali of the urea-formaldehyde suspension are not easy to regulate, the energy consumption in the preparation process is high, and the urea-formaldehyde suspension easily blocks the spray nozzle, so that certain equipment loss is caused.

Patent No. CN201510126249.1 provides a powdery potassium chloride granulation method using synthetic urea-formaldehyde resin as a binder, urea and formaldehyde solution are used for synthesizing urea-formaldehyde suspension, then powdery potassium chloride and auxiliary substances are sieved and subdivided, urea-formaldehyde binder is slowly sprayed on the mixture of the powdery potassium chloride and the auxiliary substances in a disc granulator, so that fertilizer powder is gradually changed into spherical particles with the particle size of 2-5 mm, and the particles are sieved and dried by a rotary drum; however, the urea-formaldehyde binder applied in the method also relates to the problems that the acid and the alkali are not easy to regulate and control and the spray head is easy to block, and the prepared fertilizer particles have low strength and are not beneficial to transportation and application.

Patent No. CN201910559090.0 discloses a method for preparing a synergist for enhancing the strength of fertilizer particles from bentonite, modified metal silicate and other raw materials, but the addition of the fertilizer synergist in the fertilizer granulation process can reduce the nutrient content of the fertilizer. Patent No. CN201210535114.7 discloses a water-soluble urea-formaldehyde glue, a manufacturing method thereof and a method for improving the strength of water-soluble compound fertilizer particles by using the water-soluble urea-formaldehyde glue, wherein the method is to spray the water-soluble urea-formaldehyde glue on the surfaces of the fertilizer particles after a granulation, drying and screening process and before a cooling process, but the particle strength of the fertilizer is only about 20N and is easy to break in the transportation process and the application process.

In addition, the ammonium polyphosphate is a slow-release phosphate fertilizer, and after the ammonium polyphosphate is applied to soil, long-chain polyphosphate roots can be gradually hydrolyzed into orthophosphate roots which are absorbed and utilized by plants, and the orthophosphate roots can be obtained after urea phosphate and urea are subjected to a heating reaction. The urea phosphate is a strong acid fertilizer with pH of 1-2, has a nutrient content of about 60%, is particularly suitable for alkaline soil, and can greatly improve the utilization rate of phosphorus. Patent CN201611122690.3 discloses a method for preparing a humic acid compound fertilizer by using humic acid, urea phosphate and ammonium polyphosphate, which comprises the steps of reacting phosphoric acid and urea to generate urea phosphate, then reacting the urea phosphate with the urea to generate ammonium polyphosphate, and then crushing raw materials such as the ammonium polyphosphate, the urea and potassium chloride and then granulating to obtain the humic acid compound fertilizer; however, in the process of obtaining ammonium polyphosphate by reacting urea phosphate with urea, the ammonium polyphosphate has the problems of difficult continuous discharging, difficult crushing and the like in the production process, and has complex process and low production efficiency.

Disclosure of Invention

In view of the above, the invention aims to provide a method for preparing a high-strength polyphosphate granular compound fertilizer by in-situ curing urea formaldehyde.

It is noted that urea formaldehyde is divided into two parts during the synthesis:

in the first step, urea and formaldehyde are subjected to initial addition reaction under the condition of weak alkalinity (pH = 7.5) to generate urea formaldehyde prepolymers such as hydroxymethyl urea, dimethylol urea and the like, and in the second step, the urea formaldehyde prepolymers are cured under the acidic condition.

In the invention, urea and formaldehyde solution are used to synthesize urea formaldehyde pre-polymerization solution; sieving powdered urea phosphate, urea, potassium chloride and montmorillonite, and slowly spraying urea formaldehyde pre-polymerization liquid onto the mixture of powdered urea phosphate, urea, potassium chloride and montmorillonite in a disc granulator to obtain urea formaldehyde, wherein the urea phosphate with strong acidity can be rapidly solidified and reacted with the urea formaldehyde prepolymer. The fertilizer powder is gradually changed into spherical particles with the particle size of 3-5mm, and the fertilizer particles with high particle strength can be prepared by heating and drying after screening. The urea phosphate and urea are polymerized to form long-chain polyphosphate in the heating and drying process, so that the particle strength of the fertilizer is improved, the mechanical fertilization is facilitated, the coating is facilitated, and the problems that a urea-formaldehyde suspension used in the existing disc granulation is easy to block a spray head, the particle strength is low, the preparation process is complex and the like are solved.

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for preparing a high-strength polyphosphate granular compound fertilizer by in-situ curing urea formaldehyde comprises the following steps:

1) Mixing and dissolving urea and formaldehyde, then adding triethanolamine to adjust the pH value, and reacting at constant temperature to obtain urea formaldehyde pre-polymerization liquid for later use;

2) Crushing and sieving crystal urea phosphate, urea, potassium chloride and montmorillonite, and placing the sieved materials in a disc granulator to be uniformly mixed;

3) Slowly spraying the urea formaldehyde pre-polymerization solution prepared in the step 1) into a disc granulator filled with sieved urea phosphate, urea, potassium chloride and montmorillonite mixed materials until fertilizer particles are 3-5mm in particle size;

4) And (3) placing the screened 3-5mm particles in a drying oven for heating reaction and drying to obtain the high-strength polyphosphate particle compound fertilizer.

Preferably, in the step 1), the molar ratio of urea to formaldehyde added is 1:1-1.2, the dissolving temperature is 60-70 ℃; and adjusting the pH value to 8-9, keeping the reaction temperature at 50-60 ℃ and the reaction time at 1-1.5h.

Specifically, in the step 1), the molar ratio of urea to formaldehyde is 1:1.2, the dissolving temperature is 65 ℃; and adjusting the pH value to 8-9, keeping the reaction temperature at 50 ℃ and the reaction time at 1-1.5h.

The invention has the advantages that the binder used in the invention is urea formaldehyde pre-polymerization liquid, the disclosed preparation process is simple, the production cost is low, the slow-release nitrogen can be provided, and the spray head of the atomizer of the disc granulator cannot be blocked.

Preferably, the preparation steps of the crystal urea phosphate in the step 2) are as follows:

reacting phosphoric acid and urea at 80-90 ℃ for 40-60min, crystallizing at 5-20 ℃ for 10-24h, and filtering to obtain crystal urea phosphate;

wherein the adding molar ratio of the phosphoric acid to the urea is 1:0.9-1.1.

Specifically, the preparation steps of the crystal urea phosphate in the step 2) are as follows:

reacting phosphoric acid and urea at 80 ℃ for 40min, crystallizing at 20 ℃ for 24h at constant temperature, and filtering to obtain crystal urea phosphate;

wherein the adding molar ratio of the phosphoric acid to the urea is 1:1.

preferably, in the step 3), the usage amount of the urea formaldehyde prepolymer solution is 3.0-10% of the weight of the mixed material, and the mass of the powdery urea phosphate, the urea, the montmorillonite and the potassium chloride accounts for 5-20% of the total mass.

In addition, the urea formaldehyde prepolymer solution is sprayed into the mixed material, and in the granulation process, the spraying rate (8-25 mL/min), the atomization effect (the atomization air pressure is 0.5-1.2 MPa) and the granulation time (5-10 min) of the sprayed urea formaldehyde prepolymer solution are controlled, so that the moisture content of the fertilizer particles is 5-10%, and the fertilizer particles prepared in the moisture range have smooth surfaces and are not easy to deform by heating, drying and drying.

Furthermore, in the granulation process, the strength of the granulated fertilizer is greatly increased when the addition ratio of urea phosphate to urea is gradually equal, but the fertilizer granules are irregular due to the violent reaction of urea phosphate and urea, so the ratio of urea phosphate to urea needs to be controlled, and when the addition ratio of urea phosphate to urea is 1: at 1, the granule strength is the greatest and the granule fluidization of the fertilizer is the highest.

Preferably, the drying temperature in the step 4) is 90-120 ℃, and the time is 30-180min.

It is worth pointing out that the temperature should not exceed 120 ℃ during the drying and heating process, otherwise the loss of nitrogen nutrient in the fertilizer is too high.

Specifically, the method for preparing the high-strength polyphosphate granular compound fertilizer by in-situ curing of urea formaldehyde comprises the following steps:

1. preparation of urea formaldehyde pre-polymerization liquid: urea and formaldehyde are mixed according to a molar ratio of 1:1.2 mixing, dissolving urea at 30-70 ℃, adding triethanolamine solution to adjust the pH value to 8-9 after the urea is completely dissolved, reacting for 1-1.5h at constant temperature to obtain urea formaldehyde pre-polymerization solution,

2. granulating powdered urea phosphate, urea and potassium chloride: sieving powdered urea phosphate, urea, potassium chloride and montmorillonite with 60 mesh sieve; uniformly mixing powdered urea phosphate, urea, potassium chloride and montmorillonite, wherein the auxiliary substance accounts for 5-20% of the mass ratio of the potassium chloride;

3) Slowly spraying the urea formaldehyde pre-polymerization solution prepared in the step 1) into powdery urea phosphate, urea, potassium chloride and montmorillonite, and uniformly and slowly spraying to avoid the falling loss of powdery materials. Sieving the fertilizer granules according to the balling condition when the fertilizer granules gradually become 3-5mm in particle size;

4) Adding the sieved particles with the particle size of 3-5mm into a hot rotary drum, heating at 80 ℃, drying, returning the residual materials to a disc, and continuing to granulate to prepare the slow-release compound fertilizer with high particle strength.

Compared with the prior art, the invention has the advantages that:

the binder used in the invention is urea formaldehyde pre-polymerized liquid prepared by urea and formaldehyde solution according to a set proportion and process conditions, the preparation process is simple and easy to master, the spray head is not easy to block, and slow release nitrogen can be provided. The produced compound slow release fertilizer has high strength (50-65N), smooth surface and good storage, transportation and sowing performances, is beneficial to secondary processing in the later period such as coating and the like, and has wide market prospect.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.

FIG. 1 is a process flow of a method for preparing a high-strength polyphosphate granular compound fertilizer by in-situ curing of urea formaldehyde.

FIG. 2 shows urea formaldehyde pre-polymerized liquid (A) used for clarifying suspension-free substances and not easily blocking a spray head of a disc granulator and urea formaldehyde suspension (B) used for easily blocking the spray head of the disc granulator.

FIG. 3 is an infrared spectrogram of polyphosphate generated by heating urea phosphate and urea in the polyphosphate granular compound fertilizer.

FIG. 4 shows the effect of the quality of urea phosphate addition on the strength of fertilizer granules.

FIG. 5 shows the effect of the quality of urea phosphate addition on the fluidization of fertilizer granules.

Figure 6 shows the particle hardness change of the polyphosphate particle compound fertilizer along with the temperature rise.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

As shown in fig. 1, the process flow of the method for preparing the high-strength polyphosphate granular compound fertilizer by in-situ curing of urea formaldehyde comprises the following steps:

1. weighing urea and formaldehyde solution (the molar ratio of urea to formaldehyde is 1/1.2) in a certain proportion and mass in a reaction kettle;

2. setting the temperature of a reaction kettle at 30-70 ℃, adding a triethanolamine solution to adjust the pH value to 8-9, and reacting at constant temperature for 1-1.5h to prepare a urea formaldehyde prepolymer adhesive;

3. transferring the urea formaldehyde prepolymer solution into an atomizer for disk granulation, and controlling the spraying rate (8-25 mL/min), the atomizing effect (the atomizing air pressure is 0.5-1.2 MPa) and the granulation time (5-10 min) of spraying the urea formaldehyde prepolymer solution, so that the moisture content of fertilizer particles is 5-10%;

4. weighing a certain mass of sieved powdery urea phosphate, urea, potassium chloride and montmorillonite in a disc granulator (the diameter D =1-3 m), adjusting the inclination angle (theta = 45-55 degrees), and determining the working rotation speed by a calculation formula (determining the working rotation speed n to be 0.8 multiplied by 32 multiplied by the square root (Sin theta/D));

5. slowly and uniformly spraying the pre-polymerization solution into the materials in a disc granulator, screening and returning the rest materials to the disc granulator for granulation when fertilizer particles gradually become 3-5mm in particle size;

6. adding the screened compound fertilizer particles with the size of 3-5mm into a hot rotary drum, heating at the temperature of 80-120 ℃, and drying.

In order to better understand the present invention, the following examples and comparative examples further illustrate the present invention, but should not be understood as the definition of the invention, and the technicians in this field according to the invention content of some non-essential modification and adjustment, also considered in the protection of the scope of the invention.

Example 1:

weighing 0.120kg of urea and 0.1946kg of formaldehyde solution (the molar ratio of urea to formaldehyde is 1.2) in a reaction kettle, setting the temperature of the reaction kettle to be 55 ℃, adding triethanolamine to adjust the pH value to be 9 after the urea is completely dissolved under the action of a stirrer, reacting at the constant temperature of 50 ℃ for 1 hour to obtain urea formaldehyde pre-polymerization liquid, and adding the pre-polymerization liquid into an atomizer.

Sieving powdered urea phosphate, urea and potassium chloride to obtain 60 mesh sieve. 1.137kg of urea phosphate, 2.861kg of urea, 1.590kg of potassium chloride and 1.396kg of montmorillonite are weighed into a disc granulator with the disc diameter of 1 meter, the inclination angle of the disc granulator is adjusted to be 45 degrees, and the operating rotating speed is determined to be 0.8 multiplied by 32 x (Sin theta/D) =21.5r/min by a calculation formula. Sieving bentonite to obtain 60 mesh sieve. Slowly and uniformly spraying the urea formaldehyde prepolymer solution on the materials in the disc granulator, and screening the fertilizer granules according to the condition of balling when the fertilizer granules gradually become 3-5mm in diameter. And adding the sieved particles with the particle size of 3-5mm into an oven for heating and drying, wherein the heating temperature is 100 ℃, returning the residual materials to a disc for continuous granulation, and the strength of the prepared particles is 58N.

The dry matter weight of the composite slow release fertilizer particles comprises the following components: 9 percent of solid urea-formaldehyde, 18.33 percent of bentonite, 37.57 percent of urea, 14.93 percent of urea phosphate, 20.88 percent of potassium chloride and N-P as nutrient ₂ O ₅ -K ₂ O =19.85-6.57-13.10, ph 2.4.

Example 2:

the process was as in example 1, but with different proportions of the materials added.

1.137kg of urea phosphate, 2.861kg of urea, 1.347kg of potassium chloride and 1.396kg of montmorillonite which had been sieved through a 60-mesh sieve were weighed into a disk pelletizer having a disk diameter of 1 meter, and the strength of the fertilizer granules prepared was 55N.

The dry matter weight of the composite slow release fertilizer particles comprises the following components: 9 percent of urea formaldehyde pre-polymerization liquid, 19.00 percent of bentonite, 38.90 percent of urea, 15.48 percent of urea phosphate, 18.34 percent of potassium chloride and N-P as nutrient ₂ O ₅ -K ₂ O =20.54-6.81-11.50, ph 2.6.

Example 3:

1.137kg of urea phosphate, 2.861kg of urea, 1.59kg of potassium chloride and 0.621kg of montmorillonite which have been sieved with a 60-mesh sieve are weighed into a disk granulator with the diameter of a disk of 1 meter, and the strength of the prepared fertilizer granules is 60N.

The dry matter weight of the composite slow release fertilizer particles comprises the following components: 9 percent of urea formaldehyde pre-polymerization liquid, 9.17 percent of bentonite, 8978 percent of urea, 16.80 percent of urea phosphate, 23.50 percent of potassium chloride and N-P with the nutrient content ₂ O ₅ -K ₂ O =22.30-7.39-14.73, ph 2.1.

Example 4:

the materials were added as in example 1, but the temperature of the drying heat was different.

And adding the screened particles of 3-5mm into an oven for heating and drying, wherein the heating temperature is 110 ℃, returning the residual materials to a disc for continuous granulation, and the strength of the prepared fertilizer particles is 50N.

The dry matter weight of the composite slow release fertilizer particles comprises the following components: 9 percent of urea formaldehyde pre-polymerization liquid, 18.33 percent of bentonite, 37.57 percent of urea, 14.93 percent of urea phosphate, 20.88 percent of potassium chloride and N-P as nutrient ₂ O ₅ -K ₂ O =19.85-6.57-13.10, ph 2.4.

In addition, as can be seen from the attached figure 2, the urea formaldehyde prepolymer solution prepared by the preparation conditions of the urea formaldehyde prepolymer solution (the molar ratio of urea to formaldehyde is 1.2, the dissolution temperature is 65 ℃, the pH is adjusted to 8-9, the constant temperature reaction temperature is 50 ℃, and the reaction time is 1-1.5 h) is a transparent liquid; the urea-formaldehyde suspension liquid used at present in the background art has a layering phenomenon, so that the spray head is easy to block in the using process, the equipment is damaged, and the production efficiency is reduced.

And because it was mentioned in the above summary of the invention that the heating of urea phosphate and urea during the fertilizer production process can generate polyphosphate. And 913cm in FIG. 3 ^-1 The peak is a stretching vibration absorption peak of P-O-P, which shows that urea phosphate and urea are subjected to polycondensation reaction in the preparation process of the fertilizer to generate polyphosphate.

And, the mass ratio of urea, potassium chloride and potassium chloride is 1:1:1, changing the mass of the urea phosphate to be 0.2, 0.5, 0.7, 0.8, 1, 2, 3, 4, 5 and 6, heating the granulated fertilizer at 100 ℃ for reaction and drying after the fertilizer granulation is finished, and then reducing the particle strength of the fertilizer, namely the mass ratio of the urea phosphate to the urea to the potassium chloride to the montmorillonite is 1:1:1: at 1, the granular strength of the fertilizer is maximum.

The mass of urea, potassium chloride and potassium chloride is 1:1:1, changing the mass of the urea phosphate to be 0.2, 0.5, 0.7, 0.8, 1, 2, 3, 4, 5 and 6, heating the granulated fertilizer at 100 ℃ for reaction, drying, reducing the angle of repose of the fertilizer and then increasing the angle of repose of the fertilizer, namely the mass ratio of the urea phosphate to the urea to the potassium chloride to the montmorillonite is 1:1:1:1, the angle of repose is the smallest and the granule fluidization of the fertilizer is the best.

The mass ratio of urea phosphate, urea, potassium chloride and montmorillonite is 1:1:1:1 hour, after the fertilizer granulation is finished, the strength of the fertilizer granules is gradually increased along with the increase of the temperature in the heating reaction and drying processes, because urea phosphate and urea generate more polyphosphate along with the increase of the temperature in the heating process, so that the strength of the fertilizer granules is increased.

Example 5:

saline-alkali soil pot experiment

In order to investigate the fertilizer efficiency of the high-strength polyphosphate granular compound fertilizer prepared by in-situ curing urea formaldehyde, a saline-alkali soil corn pot experiment is carried out, and the fertilizer efficiency comprises the following concrete steps:

1. the test method comprises the following steps:

the test is divided into five groups, wherein the PCK group is not treated by phosphate fertilizer; DAP group common phosphate fertilizer; reducing the phosphorus of the DAP80 group common phosphate fertilizers by 20 percent; a UPUK polyphosphate granular compound fertilizer group; the phosphate reduction of the UPUK80 polyphosphate granular compound fertilizer is 20 percent.

After the bottom of the pot is fully paved with 2kg of sand, all fertilizers and 20kg of soil are uniformly mixed and then are put into the pot. The seeding rate of the wheat is 45 grains per pot. And managing each group under the same condition, and counting the yield of each group after harvesting the corns and analyzing the physicochemical properties of the soil.

2. And (3) test results:

the results of the yield of each group and the utilization rate of nutrients in the fertilizer are shown in table 1, and it can be seen from table 1 that after the high-strength polyphosphate granular compound fertilizer disclosed by the invention is applied, the yield of field corn in saline-alkali soil can be obviously improved, the pH of the soil is reduced, and the phosphorus supply strength of the soil is improved.

TABLE 1 potted corn yield and soil physicochemical Properties

Note that the mean values in the table are followed by the same letter meaning that the differences between the multiple comparison treatments using Duncan are not significant (P < 0.05).

Example 6:

field test of saline-alkali soil

In order to investigate the fertilizer efficiency of the high-strength polyphosphate granular compound fertilizer prepared by in-situ curing urea formaldehyde, a field test of saline-alkali soil corns is carried out, and the method specifically comprises the following steps:

1. the test method comprises the following steps:

the test is divided into five groups, wherein the PCK group is not treated by phosphate fertilizer; DAP group common phosphate fertilizer; reducing phosphorus by 50% for DAP50 group common phosphate fertilizers; UPUK group polyphosphate granular compound fertilizer; the phosphorus of the UPUK50 polyphosphate granular compound fertilizer is reduced by 50 percent. The application amount of the nitrogen fertilizer and the potassium fertilizer is the same. Each treatment was repeated 3 times, randomized block design. The planting density of the corn is 4200 plants/mu. Each group was managed under the same conditions and the yield and net profit analysis of each group was counted after maize harvest.

2. And (3) test results:

the results of the yield of each group and the utilization rate of nutrients in the fertilizer are shown in table 2, and as can be seen from table 2, the yield and the net income of the field corn in the saline-alkali soil can be obviously improved by applying the high-strength polyphosphate granular compound fertilizer disclosed by the invention.

TABLE 2 field corn yield, yield contributors and net earnings

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a high-strength polyphosphate granular compound fertilizer by in-situ curing of urea formaldehyde is characterized by comprising the following steps:

3) Slowly spraying the urea formaldehyde pre-polymerization solution prepared in the step 1) into a disc granulator filled with sieved urea phosphate, urea, potassium chloride and montmorillonite mixed materials until the fertilizer particles are 3-5mm in particle size;

4) Placing the screened 3-5mm particles in an oven for heating reaction and drying to obtain the high-strength polyphosphate particle compound fertilizer;

in the step 1), the molar ratio of the added urea to the added formaldehyde is 1:1-1.2, and the dissolving temperature is 60-70 ℃; adjusting the pH value to 8-9, keeping the reaction temperature at 50-60 ℃ and the reaction time at 1-1.5 h;

the preparation steps of the crystal urea phosphate in the step 2) are as follows:

wherein the adding molar ratio of the phosphoric acid to the urea is 1:0.9-1.1;

in the step 3), the usage amount of the urea formaldehyde pre-polymerization liquid is 3.0-10.0% of the weight of the mixed material, and the urea phosphate, the urea, the montmorillonite and the potassium chloride account for 5-20% of the total mass; spraying the urea-formaldehyde prepolymer liquid into the mixed material, and controlling the spraying rate of the sprayed urea-formaldehyde prepolymer liquid to be 8-25mL/min, the atomization air pressure to be 0.5-1.2MPa and the granulation time to be 5-10min each time in the granulation process, so that the moisture content of the fertilizer particles is 5-10%, and the fertilizer particles prepared in the moisture range have smooth surfaces and are difficult to deform by heating and drying;

the drying temperature in the step 4) is 90-120 ℃, and the time is 30-180min.