Preparation method of trace elements of urea-based compound fertilizer

Abstract

The application discloses a preparation method of trace elements of urea-based compound fertilizer, relates to the production field of the urea-based compound fertilizer, and contains urea, monoammonium phosphate and potassium salt, and the method comprises the following steps: dissolving citric acid in water to obtain a citric acid solution in a pre-complexing reaction kettle independent of a urea melting device; heating to 60-80 DEG C, adding a salt containing divalent trace elements, and stirring to perform a complexing reaction; then adding the first part of urea, continuing to stir to react at 60-80 DEG C, and obtaining a trace element pre-complexing mother liquor; the preparation method of the trace elements of the urea-based compound fertilizer adopts industrial-grade citric acid as a main complexing agent, and uses urea as an auxiliary complexing agent to form a citric acid-urea synergistic complexing system. The market price of citric acid is about 1 / 4 to 1 / 3 of that of EDTA, and the production cost is significantly reduced.

Description

Technical Field

[0001] This invention relates to the production technology of urea-based compound fertilizer, specifically to a method for preparing trace elements in urea-based compound fertilizer. Background Technology

[0002] Urea-based compound fertilizer is a high-concentration compound fertilizer made by using urea as the nitrogen source, combined with basic fertilizers such as monoammonium phosphate and potassium salts, through melting, mixing, and granulation. To improve the absorption of micronutrients by crops, divalent micronutrients such as zinc, iron, and manganese are usually added to the fertilizer during the production process.

[0003] However, in existing urea-based compound fertilizer production, when inorganic divalent trace element salts (such as zinc sulfate and ferrous sulfate) are directly added to high-temperature urea molten slurry (typically 120℃–140℃), they react rapidly with phosphate ions in the slurry, forming insoluble phosphate precipitates. This process leads to a sharp decline in the content of water-soluble trace elements that can be absorbed by crops, resulting in a serious "degradation" problem. Even if trace elements are added in dry powder form during the later stages of granulation, the degradation problem is still difficult to effectively suppress due to the extremely fast reaction rate at high temperatures. Therefore, existing technologies often use synthetic chelating agents such as EDTA to protect trace elements, but these chelating agents are expensive, significantly increasing the manufacturing cost of compound fertilizers and making them difficult to promote and apply in conventional bulk fertilizer production. Therefore, how to effectively inhibit the combination of divalent trace element ions and phosphate ions to form precipitates in a low-cost manner during the production of urea-based compound fertilizers containing phosphates has become a pressing technical challenge in this field. Summary of the Invention

[0004] The technical problem this invention aims to solve is: how to effectively inhibit the combination of divalent trace element ions and phosphate ions to form insoluble phosphate precipitates in a low-cost manner during the high-temperature melting production process of urine-based compound fertilizer containing phosphate, thereby avoiding the deactivation of trace elements.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing trace elements in urea-based compound fertilizer, wherein the urea-based compound fertilizer comprises urea, monoammonium phosphate, and potassium salt, and the method includes the following steps:

[0006] S1: In a pre-complexing reactor independent of the urea melting equipment, citric acid is dissolved in water to obtain a citric acid solution; heated to 60-80°C, a salt containing divalent trace elements is added, and the mixture is stirred to carry out a complexation reaction; then the first portion of urea is added, and the mixture is stirred at 60-80°C to obtain a trace element pre-complexing mother liquor;

[0007] The ratio of the amount of citric acid added to the total molar amount of divalent trace elements in the salt containing divalent trace elements is 1.2:1 to 1.5:1; the first part of urea accounts for 10% to 15% of the total mass of urea in the urea-based compound fertilizer; the divalent trace element is at least one of zinc, iron, and manganese.

[0008] S2: Melt the remaining urea (excluding the first urea) at 120-135°C and mix it with the monoammonium phosphate and the potassium salt to obtain a basic slurry;

[0009] S3: The pre-complexed mother liquor of trace elements obtained in step S1 is sprayed onto the base slurry at a temperature of 90-100°C and granulated to obtain urea-based compound fertilizer granules containing complexed trace elements.

[0010] Further, in step S1, the salt containing divalent trace elements is at least one of zinc sulfate heptahydrate, manganese sulfate monohydrate, and ferrous sulfate heptahydrate.

[0011] Further, in step S1, the order in which the salt containing the divalent trace element is added is as follows: first add zinc sulfate heptahydrate, stir and react, then add manganese sulfate monohydrate, stir and react, and finally add ferrous sulfate heptahydrate.

[0012] Furthermore, in step S1, the temperature of the complexation reaction is controlled at 65–75°C, and the pH value of the system is controlled at 4.5–5.5.

[0013] Furthermore, in step S1, the citric acid is industrial-grade citric acid monohydrate, and the amount of water added is 3 to 4 times the mass of the citric acid.

[0014] Further, in step S1, the mass ratio of the first urea to the citric acid is 6:1 to 8:1.

[0015] Furthermore, in step S1, the total stirring time after adding the salt containing divalent trace elements is 40-50 minutes, and the stirring time after adding the first part of urea is 20-30 minutes.

[0016] Furthermore, in step S3, the trace element pre-complexing mother liquor is sprayed onto the base slurry in an atomized form using a metering pump and an atomizing nozzle.

[0017] Furthermore, in step S3, the granulation is carried out in a rotary drum granulator.

[0018] Furthermore, the method further includes, after step S3: drying the obtained urea-based compound fertilizer granules at 80-100°C until the moisture content is ≤1.5%, sieving out 2-4 mm particle size granules, and cooling to below 40°C to obtain the finished urea-based compound fertilizer.

[0019] The present invention also provides a urea-based compound fertilizer, which is prepared by the method described in any of the above-mentioned methods.

[0020] Compared with the prior art, the method for preparing trace elements in urea-based compound fertilizer provided by the present invention has the following beneficial effects:

[0021] This invention eliminates the need for expensive EDTA, instead using industrial-grade citric acid as the primary complexing agent and urea as an auxiliary complexing agent to form a citric acid-urea synergistic complexing system. The market price of citric acid is approximately one-quarter to one-third that of EDTA, significantly reducing production costs. Furthermore, both citric acid and urea are substances naturally present in fertilizers or beneficial to crops, eliminating the need for additional chemical additives.

[0022] This invention moves the complexation reaction to a pre-complexation reactor, independent of the urea melting equipment, and pre-prepares a trace element complexing mother liquor at a low temperature of 60-80°C to form a stable citric acid-urea complex. Because the complexation reaction is completed in an independent system, the trace elements are fully protected before contacting the base slurry, avoiding direct contact and reaction with phosphate ions during the high-temperature melting stage.

[0023] This invention involves spraying the pre-complexed mother liquor onto the base slurry using an atomization method when the temperature has been lowered to 90–100°C. Under these intermediate temperature conditions, the stability constant of the citric acid-urea complex remains significantly higher than the solubility product constant of the corresponding phosphate, preventing substantial replacement of the complex by phosphate ions. Furthermore, the spraying method ensures that the complex is primarily distributed on the particle surface, further reducing the depth of contact with phosphate ions and the driving force of the replacement reaction. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0025] Figure 1 This is a process flow diagram of the method of the present invention. Detailed Implementation

[0026] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0027] As attached Figure 1 As shown:

[0028] Example 1

[0029] A method for preparing trace elements in a urea-based compound fertilizer, wherein the urea-based compound fertilizer comprises urea, monoammonium phosphate, and potassium sulfate, the method comprising the following steps:

[0030] S1: In a pre-complexing reactor independent of the urea melting equipment, add 12.0 kg of water and heat to 55°C. Add 3.5 kg of industrial-grade citric acid monohydrate while stirring at 80 rpm and stir until completely dissolved to obtain a citric acid solution. Heat the solution to 70°C and add trace element salts in the following order for complexation reaction: First, add 2.0 kg of zinc sulfate heptahydrate (Zn≥21%) and stir for 20 minutes; then add 1.0 kg of manganese sulfate monohydrate (Mn≥31%) and stir for 15 minutes; finally, add 1.0 kg of ferrous sulfate heptahydrate (Fe≥19%) and stir for 15 minutes. After confirming complete dissolution, add 25.0 kg of granular urea (i.e., the first batch of urea) and continue stirring at 70°C for 25 minutes to obtain a trace element pre-complexing mother liquor.

[0031] The ratio of citric acid to the total molar mass of the three divalent trace elements (zinc, manganese, and iron) is 1.3:1; the first portion of urea accounts for 10% of the total mass of urea in the urea-based compound fertilizer.

[0032] S2: Melt the remaining 225.0 kg of urea at 130℃, mix it with 200.0 kg of monoammonium phosphate and 180.0 kg of potassium sulfate, stir evenly to obtain the basic slurry; after the basic slurry cools down naturally to 95℃, it is sent to the granulation process.

[0033] S3: The trace element pre-complexed mother liquor obtained in step S1 is uniformly sprayed onto the base slurry at a temperature of 95°C through a metering pump and atomizing nozzle, and granulated in a rotary drum granulator with a material residence time of 12 minutes to obtain urea-based compound fertilizer granules containing complexed trace elements.

[0034] S4: Dry the granulated material at 90℃ to a moisture content of 1.2%, sieve it with a vibrating screen, take particles with a diameter of 2-4mm, cool it to 35℃, and obtain the finished urea-based compound fertilizer.

[0035] The test data of the finished urea-based compound fertilizer prepared in this embodiment are as follows:

[0036] <![CDATA[Total nutrients (N+P₂O₅+K₂O), %]]> 42.5 Zinc content (Zn), % 0.15 Manganese content (Mn), % 0.08 Iron content (Fe), % 0.05 The percentage of water-soluble zinc in total zinc, % 93.2 Biuret content, % 1.1 Average compressive strength of particles, N 15

[0037] Example 2

[0038] A method for preparing trace elements in a urea-based compound fertilizer, wherein the urea-based compound fertilizer comprises urea, monoammonium phosphate, and potassium chloride, the method comprising the following steps:

[0039] S1: In a pre-complexing reactor independent of the urea melting equipment, add 10.5 kg of water and heat to 50°C. Add 3.0 kg of industrial-grade citric acid monohydrate while stirring at 80 rpm and stir until completely dissolved to obtain a citric acid solution. Heat the solution to 65°C and add 1.5 kg of zinc sulfate heptahydrate and 0.5 kg of manganese sulfate monohydrate sequentially, stirring each for 15 minutes. After confirming complete dissolution, add 22.5 kg of granular urea (i.e., the first batch of urea) and continue stirring at 65°C for 30 minutes to obtain a trace element pre-complexing mother liquor.

[0040] The molar ratio of citric acid to zinc and manganese is 1.5:1; the first portion of urea accounts for 12% of the total mass of urea in the urea-based compound fertilizer.

[0041] S2: The remaining 165.0 kg of urea is melted at 125°C and mixed with 150.0 kg of monoammonium phosphate and 120.0 kg of potassium chloride. The mixture is stirred evenly to obtain a basic slurry. The basic slurry is then transported and cooled to 90°C before being sent to the granulation process.

[0042] S3: The trace element pre-complexed mother liquor obtained in step S1 is evenly sprayed onto the base slurry at a temperature of 90°C through an atomizing nozzle, and granulated in a rotary drum granulator. The material residence time is 15 minutes to obtain urea-based compound fertilizer granules containing complexed trace elements.

[0043] S4: Dry the granulated material at 85℃ to a moisture content of 1.5%, sieve out 2-4mm particles, and cool to 38℃ to obtain the finished urea-based compound fertilizer.

[0044] The test data of the finished urea-based compound fertilizer prepared in this embodiment are as follows:

[0045] <![CDATA[Total nutrients (N+P2O5+K2O), %]]> 40.8 Zinc content (Zn), % 0.12 Manganese content (Mn), % 0.05 The percentage of water-soluble zinc in total zinc, % 91.5 Biuret content, % 0.9 Average compressive strength of particles, N 14

[0046] Example 3

[0047] A method for preparing trace elements in a urea-based compound fertilizer, wherein the urea-based compound fertilizer comprises urea, monoammonium phosphate, and potassium sulfate, the method comprising the following steps:

[0048] S1: In a pre-complexing reactor independent of the urea melting equipment, 17.2 kg of water was added and heated to 58°C. 4.3 kg of industrial-grade citric acid monohydrate was added while stirring at 80 rpm and stirred until completely dissolved to obtain a citric acid solution. The solution was heated to 75°C, and 2.5 kg of zinc sulfate heptahydrate, 1.2 kg of manganese sulfate monohydrate, and 1.2 kg of ferrous sulfate heptahydrate were added sequentially. After each salt was added, the mixture was stirred for 15 minutes. After confirming complete dissolution, 28.0 kg of granular urea (i.e., the first batch of urea) was added, and the mixture was stirred for another 20 minutes at 75°C to obtain a trace element pre-complexing mother liquor.

[0049] The ratio of citric acid to the total molar mass of the three divalent trace elements (zinc, manganese, and iron) is 1.2:1; the first portion of urea accounts for 15% of the total mass of urea in the urea-based compound fertilizer.

[0050] S2: The remaining 158.7 kg of urea is melted at 135°C and mixed with 220.0 kg of monoammonium phosphate and 200.0 kg of potassium sulfate. The mixture is stirred evenly to obtain a basic slurry. The basic slurry is then sent to the granulation process after being cooled naturally to 100°C.

[0051] S3: The trace element pre-complexed mother liquor obtained in step S1 is evenly sprayed onto the base slurry at a temperature of 100℃ through an atomizing nozzle, and granulated in a rotary drum granulator. The material residence time is 10 minutes to obtain urea-based compound fertilizer granules containing complexed trace elements.

[0052] S4: Dry the granulated material at 100℃ to a moisture content of 1.0%, sieve out 2-4mm particles, and cool to 32℃ to obtain the finished urea-based compound fertilizer.

[0053] The test data of the finished urea-based compound fertilizer prepared in this embodiment are as follows:

[0054] <![CDATA[Total nutrient (N+P₂O₅+K₂O), %]]><![CDATA[<]]> 43.1 Zinc content (Zn), % 0.20 Manganese content (Mn), % 0.11 Iron content (Fe), % 0.06 The percentage of water-soluble zinc in total zinc, % 92.8 Biuret content, % 1.3 Average compressive strength of particles, N 16

[0055] Comparative Example 1

[0056] This comparative example demonstrates the preparation of urea-based compound fertilizer using the traditional EDTA chelation high-temperature addition method. The specific steps are as follows: 250.0 kg of urea is melted at 130℃, and 1.8 kg of EDTA-zinc chelate (Zn content 15%) and 0.5 kg of EDTA-manganese chelate (Mn content 13%) are added. After stirring evenly, it is mixed with 200.0 kg of monoammonium phosphate and 180.0 kg of potassium sulfate to obtain a basic slurry. This slurry is then granulated in a rotary drum granulator, dried, sieved, and cooled to obtain the finished urea-based compound fertilizer.

[0057] The test data of the finished urea-based compound fertilizer prepared in this comparative example are as follows:

[0058] <![CDATA[Total nutrient content (N+P2O5+K2O), %]]> 42.3 Zinc content (Zn), % 0.14 Manganese content (Mn), % 0.06 The percentage of water-soluble zinc in total zinc, % 89.6 Biuret content, % 1.2 Average compressive strength of particles, N 15 Cost per ton of fertilizer chelating agent: yuan 162

[0059] Comparative Example 2

[0060] This comparative example demonstrates the preparation of urea-based compound fertilizer using a direct addition of inorganic salts. The specific steps are as follows: 250.0 kg of urea is melted at 130℃, and 2.0 kg of zinc sulfate heptahydrate and 1.0 kg of manganese sulfate monohydrate powder are directly added. After stirring evenly, the mixture is mixed with 200.0 kg of monoammonium phosphate and 180.0 kg of potassium sulfate to obtain a basic slurry. This slurry is then granulated in a rotary drum granulator, dried, sieved, and cooled to obtain the finished urea-based compound fertilizer.

[0061] The test data of the finished urea-based compound fertilizer prepared in this comparative example are as follows:

[0062] <![CDATA[Total nutrients (N+P2O5+K2O), %]]> 42.0 Zinc content (Zn), % 0.15 Manganese content (Mn), % 0.08 The percentage of water-soluble zinc in total zinc, % 16.3 Biuret content, % 1.1 Average compressive strength of particles, N 14 Cost per ton of fertilizer chelating agent: yuan 0

[0063] Comparative Example 3

[0064] This comparative example describes the preparation of urea-based compound fertilizer using a method of citric acid complexation without the addition of urea, and low-temperature complexation followed by high-temperature addition. The specific steps are as follows: In a pre-complexation reactor, 3.5 kg of citric acid is dissolved in water, and 2.0 kg of zinc sulfate heptahydrate is added to carry out a complexation reaction, obtaining a citric acid-zinc complex mother liquor (without adding urea); this mother liquor is directly added to molten urea at 130°C, and then mixed with monoammonium phosphate and potassium sulfate for granulation. The remaining steps are the same as in Example 1.

[0065] The test data of the finished urea-based compound fertilizer prepared in this comparative example are as follows:

[0066] <![CDATA[Total nutrients (N+P₂O₅+K₂O), %]]><![CDATA[]]><![CDATA[]]> 42.1 Zinc content (Zn), % 0.14 The percentage of water-soluble zinc in total zinc, % 55.8 Biuret content, % 1.2 Average compressive strength of particles, N 14 Cost per ton of fertilizer chelating agent: yuan 28

[0067] Data Analysis

[0068] Comparing Example 1 with Comparative Example 1, it can be seen that the water-soluble zinc content of the fertilizer prepared by the method of the present invention (93.2%) is higher than that of the EDTA chelation high-temperature addition method (89.6%), and the cost of chelating agent per ton of fertilizer is significantly reduced from 162 yuan to about 28 yuan, achieving a balance between better protection effect and lower cost.

[0069] Comparing Example 1 with Comparative Example 2, it can be seen that the water-soluble zinc in the direct addition method of inorganic salts accounts for only 16.3% of the total zinc, and most of the zinc has degraded and become ineffective, while the method of the present invention can reach 93.2%, which fully demonstrates the significant effect of the present invention on inhibiting phosphate precipitation.

[0070] Comparing Example 1 with Comparative Example 3, it can be seen that using only citric acid complexation without urea synergy and employing a high-temperature addition method, the proportion of water-soluble zinc in total zinc is only 55.8%, far lower than the 93.2% of the present invention. This indicates that the stability of citric acid complex alone is insufficient in high-temperature phosphate systems. The key to achieving high retention rates in the present invention lies in the combination strategy of enhancing stability through urea synergistic complexation and adding the pre-complexed mother liquor during medium-temperature coating rather than the high-temperature melting stage.

[0071] In summary, this invention, through a spatiotemporal separation process of "citric acid-urea synergistic low-temperature pre-complexation + medium-temperature coating", effectively solves the technical problem of degradation and failure of divalent trace elements in the production of urea-based compound fertilizers due to the reaction with phosphate to form precipitates, at a cost significantly lower than EDTA.

[0072] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A method for preparing trace elements in a urea-based compound fertilizer, wherein the urea-based compound fertilizer comprises urea, monoammonium phosphate, and potassium salt, characterized in that, The method includes the following steps: S1: In a pre-complexing reactor independent of the urea melting equipment, citric acid is dissolved in water to obtain a citric acid solution; heated to 60-80°C, a salt containing divalent trace elements is added, and the mixture is stirred to carry out a complexation reaction; then the first portion of urea is added, and the mixture is stirred at 60-80°C to obtain a trace element pre-complexing mother liquor; The ratio of the amount of citric acid added to the total molar amount of divalent trace elements in the salt containing divalent trace elements is 1.2:1 to 1.5:1; the first part of urea accounts for 10% to 15% of the total mass of urea in the urea-based compound fertilizer; the divalent trace element is at least one of zinc, iron, and manganese. S2: Melt the remaining urea (excluding the first urea) at 120-135°C and mix it with the monoammonium phosphate and the potassium salt to obtain a basic slurry; S3: The pre-complexed mother liquor of trace elements obtained in step S1 is sprayed onto the base slurry at a temperature of 90-100°C and granulated to obtain urea-based compound fertilizer granules containing complexed trace elements.

2. The method for preparing trace elements in a urea-based compound fertilizer according to claim 1, characterized in that, In step S1, the salt containing divalent trace elements is at least one of zinc sulfate heptahydrate, manganese sulfate monohydrate, and ferrous sulfate heptahydrate.

3. The method for preparing trace elements in a urea-based compound fertilizer according to claim 2, characterized in that, In step S1, the order in which the salt containing divalent trace elements is added is as follows: first add zinc sulfate heptahydrate, stir and react, then add manganese sulfate monohydrate, stir and react, and finally add ferrous sulfate heptahydrate.

4. The method for preparing trace elements in a urea-based compound fertilizer according to claim 1, characterized in that, In step S1, the temperature of the complexation reaction is controlled at 65–75°C, and the pH value of the system is controlled at 4.5–5.

5.

5. The method for preparing trace elements in a urea-based compound fertilizer according to claim 1, characterized in that, In step S1, the citric acid is industrial-grade citric acid monohydrate, and the amount of water added is 3 to 4 times the mass of the citric acid.

6. The method for preparing trace elements in a urea-based compound fertilizer according to claim 1, characterized in that, In step S1, the mass ratio of the first urea to the citric acid is 6:1 to 8:

1.

7. The method for preparing trace elements in a urea-based compound fertilizer according to claim 1, characterized in that, In step S1, the total reaction time after adding the salt containing divalent trace elements is 40-50 minutes, and the reaction time after adding the first part of urea is 20-30 minutes.

8. The method for preparing trace elements in a urea-based compound fertilizer according to claim 1, characterized in that, In step S3, the trace element pre-complexing mother liquor is sprayed onto the base slurry in an atomized form using a metering pump and an atomizing nozzle.

9. The method for preparing trace elements in a urea-based compound fertilizer according to claim 1, characterized in that, In step S3, the granulation is carried out in a rotary drum granulator.

10. A urea-based compound fertilizer, characterized in that, Prepared by the method according to any one of claims 1 to 9.

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