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

Abstract

The invention belongs to the technical field of fertilizer processing and discloses a method for preparing high-strength polyphosphate granular compound fertilizer by in-situ curing urea-formaldehyde. In this method, the in-situ solidification of urea-formaldehyde is to directly spray the pre-polymerization solution of urea-formaldehyde into the material of disc granulation. The solidification reaction occurs in the environment to form urea-formaldehyde; and the heating reaction of urea phosphate and urea produces polyphosphate, which has a certain slow-release effect on the phosphorus in the fertilizer, and at the same time greatly improves the strength of the granular fertilizer. The present invention conducts corn potted and field application tests on the prepared granular fertilizer, and the results show that the fertilizer can significantly reduce the pH of saline-alkali soil, improve the phosphorus supply intensity of the soil, and increase the yield of corn, and is suitable for popularization and application.

Description

A method for preparing high-strength polyphosphate granular compound fertilizer by in-situ curing urea-formaldehyde

technical field

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

Background technique

Urea-formaldehyde fertilizer (UF) is a long-chain polymer derived from the reaction between urea and formaldehyde. It is the main type of slow-release fertilizer currently used, and it is also a commonly used binder in disc granulation. Patent No. CN201310252232.1 discloses a preparation method of a low-formaldehyde urea-formaldehyde resin-gypsum-bentonite-based slow-release fertilizer. By putting urea and formaldehyde into the reaction vessel at one time, heating and stirring are started, and it is stopped when the temperature rises to 25°C. Heating, after the urea is completely dissolved, adjust the pH value to 4.0-5.0 with hydrochloric acid or sulfuric acid, react for 10-20 minutes, after reaching the turbidity point, adjust the pH value to 5.0-6.0 with aqueous sodium hydroxide solution, and react for 40-60 minutes, the temperature is slow Lower it to 40°C, adjust the pH to 7.0-8.0 with aqueous sodium hydroxide solution, cool to room temperature, and obtain a milky white jelly, which is then sprayed on a disc granulator for granulation. However, in the above-mentioned granulation process, the acid-base of the urea-formaldehyde suspension is not easy to control, the preparation process consumes a lot of energy and the urea-formaldehyde suspension is easy to block the nozzle, which will cause a certain loss of equipment.

Patent No. CN201510126249.1 provides a method for granulating powdery potassium chloride with synthetic urea-formaldehyde resin as a binder. The urea-formaldehyde suspension is synthesized by using urea and formaldehyde solution, and then the powdery potassium chloride and auxiliary substances are finely sieved. In the disc granulator, slowly spray urea-formaldehyde binder on the mixture of powdered potassium chloride and auxiliary substances, so that the fertilizer powder gradually becomes spherical particles of 2 to 5 mm, and is sieved and dried by drum; However, the urea-formaldehyde binder used therein also involves the difficulty of acid-base regulation and easy blockage of nozzles, and the strength of the prepared fertilizer granules is low, which is not conducive to transportation and application.

Patent No. CN201910559090.0 discloses a method for preparing a synergist for enhancing the strength of fertilizer granules from raw materials such as bentonite and modified metal silicate, but adding the fertilizer synergist during the fertilizer granulation process will reduce the The nutrient content of the fertilizer. Patent No. CN201210535114.7 discloses a water-soluble urea-formaldehyde glue, its manufacturing method and a method for improving the particle strength of water-soluble compound fertilizer by using it. Non-toxic urea-formaldehyde glue is sprayed on the surface of fertilizer granules, but the granule strength of this fertilizer is only about 20N, and it is easy to be damaged during transportation and application.

In addition, ammonium polyphosphate is a slow-release phosphate fertilizer. After being applied to the soil, the long-chain polyphosphate root will gradually be hydrolyzed into orthophosphate root, which can be absorbed and utilized by plants. It can be obtained by heating and reacting urea phosphate and urea. Urea phosphate is a strong acidic fertilizer with a pH of 1-2, and its nutrient content is as high as 60%. It is especially suitable for alkaline soil and can greatly improve the utilization rate of phosphorus. Patent CN201611122690.3 discloses a method for preparing humic acid compound fertilizer by using humic acid, urea phosphate, and ammonium polyphosphate. The steps are to react phosphoric acid and urea to form urea phosphate, and then react urea phosphate and urea to form polyphosphoric acid Ammonium, and then ammonium polyphosphate, urea and potassium chloride and other raw materials are pulverized and then granulated to obtain humic acid compound fertilizer; Difficult materials, not easy to break and other problems, the process is cumbersome, and the production efficiency is low.

Contents of the invention

In view of this, the purpose of the present invention is to provide a method for preparing high-strength polyphosphate granular compound fertilizer by solidifying urea-formaldehyde in situ.

It should be noted that urea-formaldehyde is divided into two parts during the synthesis process:

In the first step, urea and formaldehyde undergo an initial addition reaction under weakly alkaline (pH=7.5) conditions to generate urea-formaldehyde prepolymers such as monomethylolurea and dimethylolurea. The second step is to react under acidic conditions. down for curing.

In the present invention, urea and formaldehyde solution are first used to synthesize urea-formaldehyde pre-polymerization solution; powdery urea phosphate, urea, potassium chloride and montmorillonite are sieved and subdivided, and powdery urea phosphate, urea are sieved in a disc granulator , Potassium chloride and montmorillonite mixture is slowly sprayed with urea-formaldehyde prepolymer solution, and the strongly acidic urea phosphate can quickly solidify and react with urea-formaldehyde prepolymer to form urea-formaldehyde. The fertilizer powder gradually turns into spherical particles of 3-5 mm, and after sieving, it is heated and dried to prepare fertilizer particles with high particle strength. During the heating and drying process, urea phosphate and urea polymerize to form long-chain polyphosphate, which improves the particle strength of the fertilizer, facilitates mechanical fertilization, and facilitates coating. Problems such as clogged nozzle, low particle strength and complex preparation process.

In order to achieve the above object, the present invention adopts the following technical solutions:

A method for preparing high-strength polyphosphate granular compound fertilizer by solidifying urea-formaldehyde in situ, comprising the steps of:

1) Dissolving urea and formaldehyde after mixing, then adding triethanolamine to adjust pH, and reacting at constant temperature to obtain urea-formaldehyde prepolymerization solution for subsequent use;

2) crushing crystalline urea phosphate, urea, potassium chloride and montmorillonite and sieving, and placing the sieved material in a disc granulator to mix evenly;

3) Slowly spray the urea-formaldehyde prepolymer solution prepared in step 1) in the disc granulator equipped with sieved urea phosphate, urea, potassium chloride and montmorillonite mixture until the fertilizer granules become 3-5mm Particle size;

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

Preferably, in the step 1), the molar ratio of urea and formaldehyde is 1:1-1.2, the dissolution temperature is 60-70°C; and the pH is adjusted to 8-9, the constant temperature reaction temperature is 50-60°C, and the reaction The time is 1-1.5h.

Specifically, in the step 1), the molar ratio of urea and formaldehyde is 1:1.2, the dissolution temperature is 65°C; and the pH is adjusted to 8-9, the constant temperature reaction temperature is 50°C, and the reaction time is 1-1.5h .

It should be noted that the binder used in the present invention is urea-formaldehyde prepolymerization solution, and the disclosed preparation process is simple, low in production cost, can provide slow-release nitrogen, and will not block the nozzle of the atomizer of the disc granulator.

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

React phosphoric acid and urea at 80-90°C for 40-60 minutes, then crystallize at a constant temperature of 5-20°C for 10-24 hours, and then filter to obtain crystalline urea phosphate;

Wherein, the added molar ratio of phosphoric acid and urea is 1:0.9-1.1.

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

React phosphoric acid and urea at 80°C for 40 minutes, then crystallize at a constant temperature of 20°C for 24 hours, and then filter to obtain crystalline urea phosphate;

Among them, the molar ratio of phosphoric acid and urea is 1:1.

Preferably, in the step 3), the amount of urea-formaldehyde prepolymerization solution used is 3.0-10% of the weight of the mixed material, and the quality of the powdered urea phosphate, urea, montmorillonite and potassium chloride accounts for the total mass 5-20% of.

In addition, the urea-formaldehyde prepolymerization solution is sprayed in the mixed material. During the granulation process, the spraying rate (8-25mL/min) and atomization effect (atomization pressure of 0.5-1.2MPa) of the urea-formaldehyde prepolymerization solution should be controlled. ) and each granulation time (5-10min), so that the moisture content of the fertilizer granules is between 5-10%, and the fertilizer granules prepared within this moisture range have a smooth surface and are not easily deformed when heated and dried.

Furthermore, during the granulation process, when the proportion of urea phosphate and urea is gradually equal, the strength of granular fertilizer will be greatly increased, but the violent reaction between urea phosphate and urea will cause irregular fertilizer particles, so it is necessary to control the ratio of urea phosphate and urea , when the mass ratio of urea phosphate and urea is 1:1, the particle strength is the largest, and the particle fluidity of the fertilizer is the highest.

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

It is worth noting that during the drying and heating process, the temperature should not exceed 120°C, otherwise the loss of nitrogen nutrients in the fertilizer will be too high.

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

1. Preparation of urea-formaldehyde prepolymerization liquid: mix urea and formaldehyde according to the molar ratio of 1:1.2, dissolve urea at a temperature of 30-70°C, and then add triethanolamine solution to adjust the pH to 8-9 after the urea is completely dissolved. h obtains urea-formaldehyde prepolymer solution,

2. Granulation of powdered urea phosphate, urea and potassium chloride: sieve powdered urea phosphate, urea, potassium chloride and montmorillonite with a fineness of 60 mesh; Potassium and montmorillonite are mixed evenly, and the auxiliary substance accounts for 5-20% of the mass ratio of potassium chloride;

3) Slowly spray the urea-formaldehyde prepolymer solution prepared in step 1) into powdery urea phosphate, urea, potassium chloride and montmorillonite, and spray evenly and slowly to avoid falling loss of powdery materials. After the fertilizer particles gradually become 3-5mm in size, they are screened as balls;

4) Put the screened 3-5mm particles into the hot drum, heat and dry at 80°C, and return the rest to the disc to continue granulation to prepare slow-release compound fertilizer with higher particle strength.

Compared with the prior art, the present invention has the advantages of:

The binder used in the present invention is a urea-formaldehyde prepolymerized liquid prepared from urea and formaldehyde solution according to the set ratio and process conditions. The preparation process is simple and easy to master, it is not easy to block the nozzle and can provide slow-release nitrogen. It is applied to the granulation of urea phosphate, urea and potassium chloride. At the same time, a certain amount of montmorillonite is added to solidify urea-formaldehyde in situ to produce high-strength compound slow-release fertilizer. The compound slow-release fertilizer thus produced has high strength (50-65N), smooth surface, and good performance in storage, transportation and sowing, which is beneficial to later secondary processing such as coating, etc., and has broad market prospects.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only It is an embodiment of the present invention, and those skilled in the art can also obtain other drawings according to the provided drawings without creative work.

Accompanying drawing 1 is the technological process of the method for preparing high-strength polyphosphate granular compound fertilizer by in-situ curing urea-formaldehyde.

Accompanying drawing 2 is the urea-formaldehyde prepolymerization liquid (A) that the clarification of the present invention does not have the difficulty of clogging up the disc granulator nozzle and the urea-formaldehyde suspension (B) of the easily clogging disc granulator nozzle that uses at present.

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

Accompanying drawing 4 is the effect of the added quality of urea phosphate on the strength of fertilizer granules.

Accompanying drawing 5 is the impact of the added quality of urea phosphate on the fluidization of fertilizer granules.

Accompanying drawing 6 is as the particle hardness change of polyphosphate granule compound fertilizer as temperature rises.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some of the embodiments of the present invention, but not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

In-situ solidification urea formaldehyde as shown in Figure 1 prepares the method technical process of the polyphosphate granular compound fertilizer of high strength, wherein, comprises the following steps:

1. Weigh a certain proportion and quality of urea and formaldehyde solution (the molar ratio of urea/formaldehyde is 1/1.2) in the reactor;

2. Set the temperature of the reaction kettle to 30-70°C, add triethanolamine solution to adjust the pH to 8-9, and react at constant temperature for 1-1.5 hours to prepare the urea-formaldehyde prepolymerized liquid binder;

3. Transfer the urea-formaldehyde pre-polymerization liquid to the atomizer of disc granulation, and control the spraying rate (8-25mL/min) and atomization effect of spraying urea-formaldehyde pre-polymerization liquid (atomization air pressure is 0.5-1.2MPa ) and each granulation time (5-10min), so that the moisture content of the fertilizer granules is between 5-10%;

4. Weigh a certain quality of sieved powdered urea phosphate, urea, potassium chloride and montmorillonite in a disc granulator (diameter D=1-3m), adjust the inclination angle (θ=45°-55°) , The working speed is determined by the calculation formula (the working speed n is determined to be 0.8×32×√(Sinθ/D));

5. Slowly and evenly spray the pre-polymerization solution on the material in the disc granulator, wait until the fertilizer particles gradually become 3-5mm in particle size, sieve and return the remaining material to the disc for granulation;

6. Put the sieved 3-5mm compound fertilizer granules into the hot drum to heat and dry at 80-120°C.

For a better understanding of the present invention, the present invention will be further specifically described below through the following examples and comparative examples, but it cannot be interpreted as limiting the present invention, and some non-essential improvements made by those skilled in the art according to the above-mentioned content of the invention and adjustments are also deemed to fall within the protection scope of the present invention.

Example 1:

Weigh 0.120kg of urea and 0.1946kg of formaldehyde solution (the molar ratio of urea/formaldehyde is 1:1.2) in the reactor, set the temperature of the reactor to 55°C, and under the action of the stirrer, after the urea is completely dissolved, add Adjust the pH to 9 with triethanolamine, react at a constant temperature of 50° C. for 1 hour to prepare a urea-formaldehyde prepolymerization solution, and add the prepolymerization solution to the atomizer.

Sieve the powdered urea phosphate, urea and potassium chloride, and the fineness of the sieve is 60 mesh. Take by weighing 1.137kg urea phosphate, 2.861kg urea, 1.590kg potassium chloride and 1.396kg of montmorillonite in a disc granulator with a disc diameter of 1 meter, adjust the inclination angle of the disc granulator to be 45 degrees, and calculate The formula determines that the working speed is 0.8×32×√(Sinθ/D)=21.5r/min. The bentonite is sieved, and the fineness of the sieve is 60 mesh. Slowly and evenly spray the urea-formaldehyde prepolymerization solution on the material in the disc granulator, and when the fertilizer particles gradually become 3-5mm in size, sieve them according to the ball formation. Put the screened 3-5mm particles into the oven for heating and drying. The heating temperature is 100°C and return the remaining material to the disc to continue granulation. The strength of the prepared granules is 58N.

The content of each component in the dry matter weight of the compound slow-release fertilizer granule is: solid urea-formaldehyde content 9%, bentonite 18.33%, urea 37.57%, urea phosphate 14.93%, potassium chloride 20.88%, and the nutrient content is NP ₂ O ₅ -K ₂ O=19.85-6.57-13.10, pH 2.4.

Example 2:

Processing is as embodiment 1, but the material ratio that adds is different.

Weigh 1.137kg of urea phosphate, 2.861kg of urea, 1.347kg of potassium chloride and 1.396kg of montmorillonite that have passed through a 60-mesh sieve in a disc granulator with a disc diameter of 1 meter. The strength of the prepared fertilizer granules is 55N.

The content of each component in the dry matter weight of the compound slow-release fertilizer granule is: 9% of urea-formaldehyde prepolymerization solution, 19.00% of bentonite, 38.90% of urea, 15.48% of urea phosphate, and 18.34% of potassium chloride, and the nutrient content is NP ₂ O ₅ -K ₂ O = 20.54-6.81-11.50, pH 2.6.

Example 3:

Processing is as embodiment 1, but the material ratio that adds is different.

Weigh 1.137kg of urea phosphate, 2.861kg of urea, 1.59kg of potassium chloride and 0.621kg of montmorillonite that have passed through a 60-mesh sieve in a disc granulator with a disc diameter of 1 meter. The strength of the prepared fertilizer granules is 60N.

The content of each component in the dry matter weight of the compound slow-release fertilizer granule is: 9% of urea-formaldehyde prepolymerization solution, 9.17% of bentonite, 42.27% of urea, 16.80% of urea phosphate, 23.50% of potassium chloride, and the nutrient content is NP ₂ O ₅ -K ₂ O = 22.30-7.39-14.73, pH 2.1.

Example 4:

The ratio of the materials added is as in Example 1, but the temperature of drying and heating is different.

Put the screened 3-5mm particles into an oven for heating and drying at a temperature of 110°C, and return the remaining material to the disc to continue granulation. The strength of the prepared fertilizer granules is 50N.

The content of each component in the dry matter weight of the compound slow-release fertilizer granule is: 9% of urea-formaldehyde prepolymerization solution, 18.33% of bentonite, 37.57% of urea, 14.93% of urea phosphate, and 20.88% of potassium chloride. The nutrient content is NP ₂ O ₅ -K ₂ O = 19.85-6.57-13.10, pH 2.4.

In addition, it can be seen from accompanying drawing 2 that by the preparation conditions of the urea-formaldehyde prepolymerization solution in the present invention (the molar ratio of urea and formaldehyde is 1:1.2, the dissolution temperature is 65°C; and the pH is adjusted to 8-9, and the constant temperature reaction temperature is 50 ℃, the reaction time is 1-1.5h) the prepared urea-formaldehyde prepolymer liquid is a transparent liquid; and the currently used urea-formaldehyde suspension mentioned in the background technology has a layering phenomenon, so it is easy to block during use nozzles, damage equipment and reduce production efficiency.

And because it is mentioned in the above summary of the invention, polyphosphate can be generated by heating urea phosphate and urea during the fertilizer preparation process. In addition, the peak at 913 cm ⁻¹ in accompanying drawing 3 is the stretching vibration absorption peak of POP, indicating that during the preparation of fertilizer, polycondensation reaction of urea phosphate and urea occurs to generate polyphosphate.

And, when the quality of urea, potassium chloride and potassium chloride is 1:1:1, the quality of changing urea phosphate is 0.2, 0.5, 0.7, 0.8, 1, 2, 3, 4, 5, 6, and the fertilizer production After the granulation is completed and heated at 100°C for reaction and drying, the granule strength of the fertilizer first increases and then decreases, that is, when the mass ratio of urea phosphate, urea, potassium chloride and montmorillonite is 1:1:1:1, Fertilizers have the greatest granular strength.

When the quality of urea, potassium chloride and potassium chloride is 1:1:1, change the quality of urea phosphate to 0.2, 0.5, 0.7, 0.8, 1, 2, 3, 4, 5, 6, and the fertilizer granulation is completed After heating and drying at 100°C, the angle of repose of the fertilizer first decreases and then increases, that is, when the mass ratio of urea phosphate, urea, potassium chloride and montmorillonite is 1:1:1:1, the angle of repose The smallest, the best particle fluidization of the fertilizer.

When the mass ratio of urea phosphate, urea, potassium chloride and montmorillonite is 1:1:1:1, after the fertilizer granulation is completed, during the heating reaction and drying process, as the temperature rises, the fertilizer granules The strength increases gradually, because urea phosphate and urea generate more polyphosphate as the temperature increases during the heating process, which increases the particle strength of the fertilizer.

Example 5:

Pot experiment in saline-alkali land

In order to investigate the fertilizer effect of high-intensity polyphosphate granular compound fertilizer prepared by in-situ solidified urea-formaldehyde involved in the present invention, a saline-alkali soil corn pot test was carried out, as follows:

1. Test method:

The experiment was divided into five groups, among them, the PCK group was treated with no phosphate fertilizer; the DAP group was treated with ordinary phosphate fertilizer; the DAP80 group was treated with ordinary phosphate fertilizer with 20% reduction in phosphorus; the UPUK polyphosphate granular compound fertilizer group; %.

After first covering the bottom of the pot with 2kg of sand, then evenly mix all the fertilizers with 20kg of soil and put them into the pot. The sowing rate of wheat is 45 grains per pot. Each group was managed under the same conditions, and the yield of each group and the analysis of soil physical and chemical properties were counted after the corn was harvested.

2. Test results:

The results of the output of each group and the utilization rate of nutrients in the fertilizer are shown in Table 1. As can be seen from Table 1, after applying the high-intensity polyphosphate granular compound fertilizer of the present invention, the yield of field corn in saline-alkali land can be significantly improved, and the yield of field corn can be reduced. Soil pH, which increases the phosphorus supply intensity of the soil.

Table 1 Potted corn yield and soil physical and chemical properties

Note: In the table, the same letter after the average value of the same column indicates that there is no significant difference between treatments using Duncan's multiple comparison (P<0.05).

Embodiment 6:

Field experiment in saline-alkali land

In order to investigate the fertilizer effect of high-strength polyphosphate granular compound fertilizer prepared by in-situ solidified urea-formaldehyde involved in the present invention, a saline-alkali soil corn field test was carried out, specifically as follows:

1. Test method:

The experiment was divided into five groups, among them, the PCK group was treated with no phosphate fertilizer; the DAP group was treated with common phosphate fertilizer; the DAP50 group was treated with ordinary phosphate fertilizer with 50% reduction in phosphorus; the UPUK group was treated with polyphosphate granular compound fertilizer; %. The amount of nitrogen and potassium fertilizers used in each treatment was the same. Each treatment was repeated 3 times in a randomized block design. The planting density of corn is 4200 plants/mu. Each group was managed under the same conditions, and the yield and net income analysis of each group were counted after the corn was harvested.

2. Test results:

The results of the output of each group and the utilization rate of nutrients in the fertilizer are shown in Table 2. As can be seen from Table 2, the application of the high-strength polyphosphate granular compound fertilizer of the present invention can significantly improve the field corn yield and net income in saline-alkali land .

Table 2 Field Corn Yield, Yield Components and Net Income

Note: In the table, the same letter after the average value of the same column indicates that there is no significant difference between treatments using Duncan's multiple comparison (P<0.05).

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not 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)

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:

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 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:

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;

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.

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