Preparation method of controllable slow-release urea fertilizer based on coal-based quantum carbon dot coating

Abstract

The invention provides a preparation method of a controllable slow-release urea fertilizer based on coal-based quantum carbon dot coating, and relates to the technical field of fertilizer preparation, the preparation method comprises the following steps: taking urea, a coal-based quantum carbon dot dispersion liquid, a natural high-molecular polymer binder, trace elements and a response type polymer as raw materials, and carrying out raw material pretreatment, urea particle coating treatment and post-treatment; the response type polymer introduced into the coating material and the coal-based quantum carbon dots can form a synergistic controlled release system, hydroxyl and carboxyl on the surfaces of the coal-based quantum carbon dots and amino and ether bonds (-O-) on molecular chains of the response type polymer can form hydrogen bonds and coordinate bonds, and a stable composite coating layer network structure is constructed; when the soil environment changes, the acting force between the functional groups reversibly changes, the change amplitude of the porosity of the coating layer is further regulated and controlled, the regulation and control precision of the nutrient release rate is improved by 15%-20%, and meanwhile, the problem of nutrient burst release caused by local damage of the coating layer can be avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fertilizer preparation, and particularly relates to a preparation method of controllable slow-release urea fertilizer based on coal-based quantum carbon dot coating. BACKGROUND

[0002] Traditional urea fertilizer, as one of the most widely used nitrogen fertilizers in agricultural production, has significant technical defects: firstly, urea molecules are easily lost in the soil through ammonia volatilization, leaching and nitrification-denitrification, resulting in a utilization rate of only 30%-40% in the current season, causing serious waste of resources; secondly, a large amount of unabsorbed urea entering water bodies easily causes eutrophication, and entering the atmosphere exacerbates the greenhouse effect, causing double pollution to the ecological environment; thirdly, the nutrient release rate does not match the crop growth cycle, which easily leads to excessive growth of crops in the early stage and nutrient deficiency in the later stage, affecting yield and quality.

[0003] At present, the main slow-release urea fertilizers mainly adopt resin coating, sulfur coating and mineral coating technologies, but have the following shortcomings: although resin coating products have good slow-release effect, the coating material cost is high (accounting for more than 30% of the total cost), and it is difficult to degrade, and long-term use easily causes soil compaction; sulfur coating products are easy to cause nutrient burst release due to damage of the coating layer, and sulfur elements in alkaline soil are easy to form sulfide precipitates, affecting soil permeability; mineral coating products (such as bentonite and diatomite) have the problem that the porosity of the coating layer is difficult to accurately control, and the nutrient release rate cannot be dynamically adjusted according to the growth demand of crops. Therefore, the present application proposes a preparation method of controllable slow-release urea fertilizer based on coal-based quantum carbon dot coating to solve the above problems. SUMMARY

[0004] In view of the above problems, the present application proposes a preparation method of controllable slow-release urea fertilizer based on coal-based quantum carbon dot coating. The response type polymer introduced in the coating material and the coal-based quantum carbon dot can form a synergistic controlled release system. The hydroxyl and carboxyl groups on the surface of the coal-based quantum carbon dot can form hydrogen bonds and coordination bonds with the amino groups (-NH2) and ether bonds (-O-) on the molecular chain of the response type polymer, thereby constructing a stable composite coating layer network structure. When the soil environment changes, the force between the functional groups reversibly changes, further regulating the change range of the porosity of the coating layer, and the regulation accuracy of the nutrient release rate is improved by 15%-20%. Meanwhile, the hydroxyl and carboxyl groups can enhance the dispersibility of the quantum carbon dots, avoiding the problem of nutrient burst release caused by local damage of the coating layer.

[0005] To achieve the purpose of the present application, the present application realizes the following technical scheme:

[0006] The preparation method of controllable slow-release urea fertilizer based on coal-based quantum carbon dot coating comprises the following steps:

[0007] Step one: selecting raw materials

[0008] Urea particles with a particle size of 1-3mm have a purity greater than 99%.

[0009] A coal-based quantum carbon dot dispersion with a surface rich in active functional groups such as hydroxyl and carboxyl groups, added at an amount of 1%-3% of the mass of urea particles.

[0010] Add 0.5%-2% of a natural high-molecular polymer binder based on the mass of the urea granules;

[0011] The amount of trace elements added is 0.05%-0.1% of the mass of urea granules;

[0012] The amount of responsive polymer added is 2%-5% of the mass of urea particles;

[0013] Step 2: Raw material pretreatment

[0014] Urea granules are cleaned and dried.

[0015] A responsive polymer and trace elements were compounded into a coal-based quantum carbon dot dispersion and then dispersed to obtain a responsive coal-based quantum carbon dot dispersion.

[0016] Natural polymer binders are gelatinized to obtain gelatinized binders.

[0017] Step 3: Coating treatment of urea granules

[0018] After drying, urea granules are added to a fluidized bed. A peristaltic pump simultaneously sprays a responsive coal-based quantum carbon dot dispersion and a gelatinizing binder into the fluidized bed at a ratio of 3:1 to 5:1, so that the responsive coal-based quantum carbon dot dispersion and the gelatinizing binder are evenly sprayed on the surface of the urea granules, resulting in a crude product of controllable slow-release urea fertilizer coated with coal-based quantum carbon dots.

[0019] Step 4: Post-processing

[0020] The crude controlled-release urea fertilizer coated with coal-based quantum carbon dots was screened to remove adhering particles, and then dried a second time after screening.

[0021] A further improvement is that: the coal-based quantum carbon dot dispersion with surface rich in active functional groups such as hydroxyl and carboxyl groups is prepared by a three-step method of "oxidation-hydrothermal-surface modification". The carboxyl content of the coal-based quantum carbon dot dispersion with surface rich in active functional groups such as hydroxyl and carboxyl groups is ≥1.2mmol / g and the hydroxyl content is ≥0.8mmol / g.

[0022] A further improvement is that the natural polymer binder is starch or cellulose.

[0023] A further improvement is that the trace elements include zinc, iron, and boron.

[0024] A further improvement is that the responsive polymer is a pH-sensitive polymer, a temperature-sensitive polymer, or an ion strength-sensitive polymer. The pH-sensitive polymer is a pH-sensitive polymethacrylic acid-acrylamide copolymer, the temperature-sensitive polymer is a temperature-sensitive N-isopropylacrylamide, and the ion strength-sensitive polymer is an ion strength-sensitive polyelectrolyte with a molecular weight of 50,000-100,000.

[0025] A further improvement is made in the following step: during the drying process of urea granules in step two, the impurity-removed urea granules are dried in an oven at 60-80℃ for 2-4 hours to remove moisture.

[0026] A further improvement is made in the following: In step two, when the responsive polymer and trace elements are compounded into the coal-based quantum carbon dot dispersion and dispersed, ultrasonic dispersion is used for 30-60 minutes.

[0027] A further improvement is made in the following step: during the gelatinization treatment of the natural polymer binder in step two, the natural polymer binder is mixed with water in a ratio of 1:5 to 1:10, heated to 80-90℃, and stirred for 30-45 minutes to fully gelatinize it, thereby obtaining a gelatinized binder.

[0028] Further improvements are made in the following ways: after the dried urea particles are added to the fluidized bed in step three, the fluidization air velocity is 0.5-1.5 m / s. When the responsive coal-based quantum carbon dot dispersion and the gelatinized binder are sprayed into the fluidized bed, the pressure of the spray gun is controlled at 0.2-0.4 MPa. During the coating process, the bed temperature is controlled at 40-60℃ and the coating time is 15-30 min.

[0029] A further improvement is made in the following: In step four, when the crude controlled-release urea fertilizer coated with coal-based quantum carbon dots is subjected to secondary drying, the secondary drying temperature is 50-70℃ and the time is 1-2 hours.

[0030] The beneficial effects of this invention are as follows:

[0031] In this invention, the responsive polymer introduced into the coating material and the coal-based quantum carbon dots can form a synergistic controlled-release system. The hydroxyl and carboxyl groups on the surface of the coal-based quantum carbon dots and the amino groups on the responsive polymer molecular chains... Ether bonds (-O-) can form hydrogen bonds and coordination bonds to build a stable composite coating layer network structure. When the soil environment changes, the interaction forces between functional groups change reversibly, further regulating the change range of coating layer porosity, thus improving the regulation precision of nutrient release rate by 15%-20%. At the same time, hydroxyl and carboxyl groups can enhance the dispersibility of quantum carbon dots, avoiding the problem of nutrient burst release caused by local damage to the coating layer.

[0032] In this invention, the photogenerated carriers generated by coal-based quantum carbon dots under natural light irradiation can promote the activity of microorganisms on the surface of the coating layer and further regulate the nutrient release rhythm.

[0033] In this invention, natural high molecular polymers are selected as binders, which have good biodegradability and will not cause pollution in the soil. At the same time, they can effectively enhance the bonding between coal-based quantum carbon dots and urea particles, making the coating more solid. The added trace elements can meet the needs of crops for a variety of nutrients. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0035] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0036] Example 1

[0037] In this embodiment, the preparation of coal-based quantum carbon dot dispersions using the three-step method of "oxidation-hydrothermal-surface modification" includes the following steps:

[0038] (1): The low-rank coal was crushed to 150 mesh and dried in a vacuum drying oven at 80°C for 5 hours. Then, the dried coal sample was added to concentrated nitric acid at a solid-liquid ratio of 1:10 (g / ml). The mixture was refluxed at 85°C and 300 r / min for 4 hours. After the reaction was completed, the mixture was cooled to room temperature and diluted with deionized water to 3 times the original volume. The mixture was centrifuged at 3500 r / min for 20 minutes to remove unreacted coal residue and retain the supernatant.

[0039] (2): Transfer the supernatant to a hydrothermal reactor, add a modifier (ethylene glycol and citric acid mixed at a mass ratio of 2:1), the amount of modifier added is 6% of the mass of the supernatant, stir evenly, seal the reactor, set the hydrothermal reaction temperature to 200℃, keep warm for 10h, and after the reaction is completed, cool naturally to room temperature to obtain a brownish-yellow quantum carbon dot crude liquid. Pass the crude liquid through... Microporous membrane filtration was used to remove large molecular aggregates. The filtrate was then placed in a dialysis bag (molecular weight cutoff 400 Da) and dialyzed with deionized water for 4 days, with the dialysis water changed 3 times a day until the pH of the dialysis solution stabilized at 5.0-6.0. Residual nitric acid and small molecule impurities were removed to obtain a primary coal-based quantum carbon dot solution.

[0040] (3): The primary quantum carbon dot solution was transferred to a three-necked flask, and sodium hydroxide solution was added to adjust the pH to 7.0-8.0. The temperature was raised to 65°C under nitrogen protection, and citric acid (carboxyl group strengthening modifier) ​​was added at 34% of the mass of quantum carbon dots. The mixture was stirred continuously for 3 hours to further increase the carboxyl group density through esterification. At the same time, ethylene glycol (hydroxyl group modifier) ​​was added at 3% of the mass of quantum carbon dots. The reaction was carried out under acidic conditions (dilute sulfuric acid was added dropwise to adjust the pH to 4.0-5.0) to introduce hydroxyl groups through etherification. After the reaction was completed, the solution was cooled to room temperature, and the pH was adjusted to neutral with 0.1 mol / L sodium hydroxide solution. The solution was dialyzed again through a dialysis bag (molecular weight cutoff 1000 Da) for 30 hours to finally obtain a coal-based quantum carbon dot dispersion with a surface rich in hydroxyl and carboxyl groups.

[0041] Example 2

[0042] according to Figure 1 As shown, this embodiment proposes a method for preparing controllable slow-release urea fertilizer based on coal-based quantum carbon dot coating, including the following steps:

[0043] Step 1: Selecting Raw Materials

[0044] Urea particles with a particle size of 2mm have a purity greater than 99%.

[0045] A coal-based quantum carbon dot dispersion with a surface rich in active functional groups such as hydroxyl and carboxyl groups was added at an amount of 3% of the mass of urea particles. The carboxyl content of the coal-based quantum carbon dot dispersion was ≥1.2mmol / g and the hydroxyl content was ≥0.8mmol / g.

[0046] The amount of starch added is 1% of the mass of the urea granules;

[0047] The amount of zinc, iron, and boron added is -0.1% of the mass of the urea granules;

[0048] The amount of pH-sensitive polymer added is 4% of the mass of urea particles. The pH-sensitive polymer is a pH-sensitive polymethacrylic acid-acrylamide copolymer with a molecular weight of 100,000.

[0049] Step 2: Raw material pretreatment

[0050] The urea granules were cleaned and dried. The urea granules were dried in an oven at 680℃ for 4 hours to remove moisture.

[0051] A pH-sensitive polymethacrylic acid-acrylamide copolymer was compounded with zinc, iron, and boron into a coal-based quantum carbon dot dispersion and ultrasonically dispersed for 50 min to obtain a responsive coal-based quantum carbon dot dispersion.

[0052] Mix starch and water in a 1:5 ratio, heat to 80°C, and stir for 35 minutes to fully gelatinize, thus obtaining a gelatinized binder.

[0053] Step 3: Coating treatment of urea granules

[0054] The dried urea granules were added to a fluidized bed with a fluidization velocity of 1.0 m / s. A peristaltic pump was used to simultaneously spray a responsive coal-based quantum carbon dot dispersion and a gelatinizing binder into the fluidized bed at a ratio of 3:1. The pressure of the spray gun was controlled at 0.3 MPa. During the coating process, the bed temperature was controlled at 50℃ and the coating time was 20 min. This ensured that the responsive coal-based quantum carbon dot dispersion and the gelatinizing binder were evenly sprayed onto the surface of the urea granules, resulting in a crude controlled-release urea fertilizer coated with coal-based quantum carbon dots.

[0055] Step 4: Post-processing

[0056] The crude controlled-release urea fertilizer coated with coal-based quantum carbon dots was screened to remove adhering particles, and then dried a second time at 65℃ for 1.5 hours.

[0057] In this embodiment, when the soil environment changes (such as when crop roots secrete organic acids leading to a decrease in pH), the pH-sensitive polymer undergoes carboxylation in an acidic environment, resulting in molecular chain shrinkage, reduced porosity of the coating layer, and slowed nutrient release.

[0058] Example 3

[0059] according to Figure 1 As shown, this embodiment proposes a method for preparing controllable slow-release urea fertilizer based on coal-based quantum carbon dot coating, including the following steps:

[0060] Step 1: Selecting Raw Materials

[0061] Urea particles with a particle size of 2mm have a purity greater than 99%.

[0062] The amount added is 1%-3% of the mass of urea particles. The surface of the coal-based quantum carbon dot dispersion is rich in active functional groups such as hydroxyl and carboxyl groups. The carboxyl content of the coal-based quantum carbon dot dispersion is ≥1.2mmol / g and the hydroxyl content is ≥0.8mmol / g.

[0063] The amount of starch added is 1% of the mass of the urea granules;

[0064] The amount of zinc, iron, and boron added is 0.1% of the mass of the urea granules;

[0065] The thermosensitive polymer, which is 4% of the mass of urea granules, is added. The thermosensitive polymer is thermosensitive N-isopropylacrylamide with a molecular weight of 100,000.

[0066] Step 2: Raw material pretreatment

[0067] Urea granules are cleaned and dried. The urea granules are dried in an oven at 70℃ for 3 hours to remove moisture.

[0068] Thermosensitive N-isopropylacrylamide, zinc, iron, and boron were combined into a coal-based quantum carbon dot dispersion and ultrasonically dispersed for 50 min to obtain a responsive coal-based quantum carbon dot dispersion.

[0069] Mix starch and water in a 1:5 ratio, heat to 90°C, and stir for 45 minutes to fully gelatinize, thus obtaining a gelatinized binder.

[0070] Step 3: Coating treatment of urea granules

[0071] The dried urea granules were added to a fluidized bed with a fluidization velocity of 1.0 m / s. A peristaltic pump was used to simultaneously spray a responsive coal-based quantum carbon dot dispersion and a gelatinizing binder into the fluidized bed at a ratio of 5:1. The pressure of the spray gun was controlled at 0.4 MPa. During the coating process, the bed temperature was controlled at 60℃ and the coating time was 30 min. This ensured that the responsive coal-based quantum carbon dot dispersion and the gelatinizing binder were evenly sprayed onto the surface of the urea granules, resulting in a crude controlled-release urea fertilizer coated with coal-based quantum carbon dots.

[0072] Step 4: Post-processing

[0073] The crude controlled-release urea fertilizer coated with coal-based quantum carbon dots was screened to remove adhering particles, and then dried a second time at 65℃ for 2 hours.

[0074] In this embodiment, when the soil environment changes (high temperatures in summer cause soil temperature to rise), the thermosensitive polymer undergoes phase separation when the temperature is above the minimum critical dissolution temperature (LCST, about 32-35°C), forming a dense structure and inhibiting nutrient loss.

[0075] Example 4

[0076] according to Figure 1 As shown, this embodiment proposes a method for preparing controllable slow-release urea fertilizer based on coal-based quantum carbon dot coating, including the following steps:

[0077] Step 1: Selecting Raw Materials

[0078] Urea particles with a particle size of 2mm have a purity greater than 99%.

[0079] A coal-based quantum carbon dot dispersion with active functional groups such as hydroxyl and carboxyl groups on its surface was added at an amount of 2% of the mass of urea particles. The carboxyl content of the coal-based quantum carbon dot dispersion was ≥1.2 mmol / g and the hydroxyl content was ≥0.8 mmol / g.

[0080] The amount of starch added is 2% of the mass of the urea granules;

[0081] The amount of zinc, iron, and boron added is 0.1% of the mass of the urea granules;

[0082] An ion strength-sensitive polymer was added at a rate of 5% of the mass of urea granules. The ion strength-sensitive polymer was an ion strength-sensitive polyelectrolyte with a molecular weight of 100,000.

[0083] Step 2: Raw material pretreatment

[0084] Urea granules are cleaned and dried. The urea granules are dried in an oven at 80℃ for 2 hours to remove moisture.

[0085] An ion strength-sensitive polyelectrolyte and zinc, iron, and boron were combined into a coal-based quantum carbon dot dispersion and ultrasonically dispersed for 40 min to obtain a responsive coal-based quantum carbon dot dispersion.

[0086] Mix starch and water in a 1:10 ratio, heat to 85°C, and stir for 40 minutes to fully gelatinize, thus obtaining a gelatinized binder.

[0087] Step 3: Coating treatment of urea granules

[0088] The dried urea granules were added to a fluidized bed with a fluidization velocity of 1.0 m / s. A peristaltic pump was used to simultaneously spray a responsive coal-based quantum carbon dot dispersion and a gelatinizing binder into the fluidized bed at a ratio of 3:1. The pressure of the spray gun was controlled at 0.3 MPa. During the coating process, the bed temperature was controlled at 45℃ and the coating time was 30 min. This ensured that the responsive coal-based quantum carbon dot dispersion and the gelatinizing binder were evenly sprayed onto the surface of the urea granules, resulting in a crude controlled-release urea fertilizer coated with coal-based quantum carbon dots.

[0089] Step 4: Post-processing

[0090] The crude controlled-release urea fertilizer coated with coal-based quantum carbon dots was screened to remove adhering particles, and then dried a second time at 60℃ for 1.5 hours.

[0091] In this embodiment, when the soil environment changes (the soil ion concentration changes after fertilization), the ion strength-sensitive polymer adjusts the intermolecular forces through ion exchange, thereby changing the permeability of the coating layer.

[0092] Comparative Example

[0093] To verify the technical advantages of the fertilizer of this invention, three control groups (traditional urea, commercially available resin-coated urea, and commercially available sulfur-coated urea) were set up and their performance was compared with the finished products of Examples 2-4 of this invention under the same testing conditions, as follows:

[0094] The comparison parameters are as follows:

[0095]

[0096] Comparison conditions:

[0097] Nutrient release period (time required for cumulative release rate to reach 80%) and cumulative release rate over 28 days were measured under three simulated soil environments (neutral pH=7.0, acidic pH=5.5, and high temperature 35℃).

[0098] A pot experiment was conducted (wheat variety: Jimai 44) to determine the soil ammonia volatilization (indophenol blue colorimetric method) and the urea content in the leachate 30 days after fertilization (ultraviolet spectrophotometry).

[0099] Based on 1 ton of fertilizer, calculate the cost of raw materials (including coating materials), energy consumption in preparation (fluidized bed coating vs. traditional coating process) and cost per unit of nutrient (N);

[0100] Wheat was planted in a field trial with a uniform fertilizer application rate (15 kg / mu of pure N), and the yield per mu, thousand-grain weight, and chlorophyll content (SPAD value) were measured.

[0101] Comparison Results and Analysis

[0102] Table 1 shows a comparison of controlled-release performance and environmental performance (unit: days / %, release period / cumulative release rate over 28 days).

[0103] Table 1

[0104]

[0105] The environmental performance comparison is shown in Table 2 (unit: kg / mu). Ammonia volatilization, mg / L (Leachable urea content):

[0106] Table 2

[0107]

[0108] The comparison between economic benefits and crop effects is shown in Table 3:

[0109] Table 3

[0110]

[0111] The nutrient release period of this invention (95-120 days) is extended by 58%-275% compared to traditional slow-release fertilizers (32-60 days), and the release rate can be dynamically adjusted according to environmental response, solving the "single controlled release" defect of existing technologies. The ammonia volatilization of this invention is reduced by more than 36.6% and leaching loss is reduced by more than 53% compared to resin-coated urea. Moreover, coal-based quantum carbon dots can improve soil organic matter without secondary pollution. The unit N cost of this invention's controlled-release urea fertilizer (7.3-7.8 yuan / kg N) is lower than that of sulfur-coated urea (8.0 yuan / kg N) and resin-coated urea (11.0 yuan / kg N), while the yield per mu is increased by 3.6%-18%, showing significant cost-effectiveness. The thousand-grain weight of this invention's controlled-release urea fertilizer is increased by 9.2%-11.8% compared to traditional urea, and the chlorophyll content is increased by 21.6%-25.2%, which can effectively enhance crop stress resistance and ensure yield and quality.

[0112] In this invention, the responsive polymer introduced into the coating material and the coal-based quantum carbon dots can form a synergistic controlled-release system. The hydroxyl and carboxyl groups on the surface of the coal-based quantum carbon dots and the amino groups on the responsive polymer molecular chains... Ether bonds (-O-) can form hydrogen bonds and coordination bonds, constructing a stable composite coating layer network structure. When the soil environment changes, the interaction forces between functional groups undergo reversible changes, further regulating the variation range of the coating layer porosity, thus improving the regulation precision of nutrient release rate by 15%-20%. At the same time, hydroxyl and carboxyl groups can enhance the dispersibility of quantum carbon dots, avoiding the problem of nutrient burst release caused by local damage to the coating layer. In this invention, the photogenerated charge carriers generated by coal-based quantum carbon dots under natural light irradiation can promote the activity of microorganisms on the coating layer surface, further regulating the nutrient release rhythm. In this invention, natural polymers are selected as binders, which have good biodegradability and will not cause pollution in the soil. At the same time, they can effectively enhance the bonding between coal-based quantum carbon dots and urea particles, making the coating more solid. The added trace elements can meet the needs of crops for multiple nutrients.

[0113] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A preparation method of a controlled-release urea fertilizer based on a coal-based quantum carbon dot coating, characterized in that, The method comprises the following steps: Step 1: selecting raw materials Core material: urea particles with a purity of greater than 99% and a particle size of 1-3 mm; Coating material: a coal-based quantum carbon dot dispersion liquid with active functional groups such as hydroxyl and carboxyl groups, added in an amount of 1%-3% of the mass of the urea particles; a natural high-molecular polymer binder, added in an amount of 0.5%-2% of the mass of the urea particles; trace elements, added in an amount of 0.05%-0.1% of the mass of the urea particles; a responsive polymer, added in an amount of 2%-5% of the mass of the urea particles; Step 2: pretreatment of raw materials impurity removal and drying treatment of the urea particles; the responsive polymer and the trace elements are compounded into the coal-based quantum carbon dot dispersion liquid and subjected to dispersion treatment to obtain a responsive coal-based quantum carbon dot dispersion liquid; the natural high-molecular polymer binder is gelatinized to obtain a gelatinized binder; Step 3: coating treatment of the urea particles the dried urea particles are added into a fluidized bed, the responsive coal-based quantum carbon dot dispersion liquid and the gelatinized binder are simultaneously sprayed into the fluidized bed through a peristaltic pump at a ratio of 3:1-5:1, so that the responsive coal-based quantum carbon dot dispersion liquid and the gelatinized binder are uniformly sprayed on the surface of the urea particles to obtain a controllable slow-release urea fertilizer crude product coated with coal-based quantum carbon dots; Step 4: post-treatment the controllable slow-release urea fertilizer crude product coated with coal-based quantum carbon dots is subjected to screening treatment to remove adhered particles, and is subjected to secondary drying after screening.

2. The method for preparing the controllable slow-release urea fertilizer based on the coal-based quantum carbon dot coating according to claim 1, characterized in that: The coal-based quantum carbon dot dispersion liquid with active functional groups such as hydroxyl and carboxyl groups on the surface is prepared by a three-step method of "oxidation-hydrothermal-surface modification", and the carboxyl content of the coal-based quantum carbon dot dispersion liquid with active functional groups such as hydroxyl and carboxyl groups on the surface is ≥1.2 mmol / g and the hydroxyl content is ≥0.8 mmol / g.

3. The method for preparing the controllable slow-release urea fertilizer based on the coal-based quantum carbon dot coating according to claim 1, characterized in that: The natural high-molecular polymer binder is starch or cellulose.

4. The method for preparing the coal-based quantum carbon dots coated controlled slow-release urea fertilizer according to claim 1, characterized in that: The trace elements include zinc, iron and boron.

5. The method for preparing the coal-based quantum carbon dots coated controlled slow-release urea fertilizer according to claim 1, characterized in that: The responsive polymer is a pH-sensitive polymer, a temperature-sensitive polymer or an ionic strength-sensitive polymer, the pH-sensitive polymer is a pH-sensitive polymethacrylic acid-acrylamide copolymer, the temperature-sensitive polymer is a temperature-sensitive N-isopropyl acrylamide, and the ionic strength-sensitive polymer is an ionic strength-sensitive polyelectrolyte with a molecular weight of 50000-100000.

6. The method for preparing the coal-based quantum carbon dots coated controlled slow-release urea fertilizer of claim 1, characterized in that: In the drying treatment of the urea particles in Step 2, the impurity-removed urea particles are dried in an oven at 60-80°C for 2-4 h to remove water.

7. The method for preparing the controllable slow-release urea fertilizer based on the coal-based quantum carbon dot coated film according to claim 1, characterized in that: In the compounding and dispersion treatment of the responsive polymer and the trace elements into the coal-based quantum carbon dot dispersion liquid in Step 2, ultrasonic dispersion treatment is adopted, and the ultrasonic dispersion treatment time is 30-60 min.

8. The method for preparing the coal-based quantum carbon dots coated controlled slow-release urea fertilizer according to claim 1, characterized in that: In the gelatinization treatment of the natural high-molecular polymer binder in Step 2, the natural high-molecular polymer binder is mixed with water at a ratio of 1:5-1:10, heated to 80-90°C, and stirred for 30-45 min to make it fully gelatinized to obtain the gelatinized binder.

9. The method for preparing the coal-based quantum carbon dots coated controlled slow-release urea fertilizer of claim 1, characterized in that: The fluidized wind speed is 0.5-1.5 m / s after the urea particles dried in the step three are added into the fluidized bed, the pressure of the spray gun is controlled to be 0.2-0.4 MPa when the responsive coal-based quantum carbon dot dispersion liquid and the gelatinized binder are sprayed into the fluidized bed, the bed temperature is controlled to be 40-60 ℃ during the coating process, and the coating time is 15-30 min.

10. The method for preparing the coal-based quantum carbon dots coated controlled slow-release urea fertilizer according to claim 1, characterized in that: The secondary drying temperature of the controllable slow-release urea fertilizer coarse product coated with the coal-based quantum carbon dots in the step four is 50-70 ℃, and the time is 1-2 h.

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