Continuous production method of coal-based quantum carbon dot in-situ coated slow-release urea in evaporation granulation tower

Abstract

The invention provides a continuous production method of coal-based quantum carbon dot in-situ coated slow-release urea in an evaporation granulation tower, and relates to the technical field of slow-release fertilizer production.The continuous production method comprises the following steps that S1, coal liquefaction residues serve as a carbon source, electrolysis is conducted after carbonization and polyvinylidene fluoride forming are conducted, and coal-based quantum carbon dot dispersion liquid is prepared; s2, transforming an evaporation granulation tower, arranging three layers of coaxial atomizing nozzles in the height direction of a tower body, and constructing a gradient temperature field in the tower; s3, urea melt liquid, coal-based quantum carbon dot dispersion liquid and a modified silane coupling agent are sprayed into the tower through three layers of coaxial atomization nozzles; the three steps of granulation, coating and curing are continuously completed in the same evaporation granulation tower, the production period is shortened, meanwhile, particle transfer damage is avoided, the yield is increased, the industrial production cost is greatly reduced, the quantum carbon dots are prepared with the industrial by-product coal liquefaction residues as the carbon source, the solid waste treatment problem of the coal liquefaction residues is solved, and the production cost is reduced. The carbon source cost is also reduced.

Description

Technical Field

[0001] This invention relates to the field of slow-release fertilizer production technology, and in particular to a continuous production method for in-situ coating of slow-release urea with coal-based quantum carbon dots in an evaporation granulation tower. Background Technology

[0002] Current slow-release urea production primarily employs the traditional process of granulation followed by coating. The core process involves first atomizing and cooling molten urea into urea granules using an evaporation granulation tower, then transferring the granules to specialized equipment such as fluidized beds or spray coating devices for surface coating with resin, sulfur, or nanomaterials. However, the evaporation granulation tower serves only as a granulation device, with a limited function. Nanomaterial coating technology, requiring separate equipment, is often used in laboratories or small-scale production, making it unsuitable for the continuous production demands of industrial-grade urea. Furthermore, the preparation of quantum carbon dots in existing technologies often uses biomass or petrochemical byproducts as carbon sources, resulting in high raw material costs and unstable sourcing, limiting their large-scale application in the slow-release fertilizer field.

[0003] Existing technologies have the following drawbacks: First, the granulation-coating process requires multiple machines to operate in tandem, resulting in numerous process interruptions and a production cycle of 8-12 hours. Furthermore, the particles are prone to breakage during transport, with a yield of only 85%-90%. Second, the preparation of nanoscale coatings relies on precision laboratory equipment, making it difficult to achieve precise control of the microstructure in macroscopic industrial devices. This results in poor coating uniformity and unstable sustained-release performance. Third, quantum carbon dots prepared from traditional carbon sources have low quantum yields and weak interfacial bonding with urea particles, making them prone to coating detachment and failing to meet long-term sustained-release requirements. Therefore, this invention proposes a continuous production method for in-situ coating of sustained-release urea with coal-based quantum carbon dots in an evaporation granulation tower to solve the problems existing in the prior art. Summary of the Invention

[0004] To address the aforementioned issues, this invention proposes a continuous production method for in-situ coating of coal-based quantum carbon dots with slow-release urea within an evaporation granulation tower. This method continuously completes the three-step processes of granulation, coating, and solidification within the same evaporation granulation tower, shortening the production cycle, avoiding particle breakage during transport, improving yield, and significantly reducing industrial production costs.

[0005] To achieve the objectives of this invention, the invention is implemented through the following technical solution: a continuous production method for in-situ coating of coal-based quantum carbon dots with slow-release urea in an evaporation granulation tower, comprising the following steps:

[0006] S1: Coal-based quantum carbon dot dispersion is prepared by carbonizing and molding coal liquefaction residue as a carbon source, followed by electrolysis after carbonization and polyvinylidene fluoride molding.

[0007] S2: Modify the evaporation granulation tower by installing three layers of coaxial atomizing nozzles along the height of the tower and constructing a gradient temperature field inside the tower.

[0008] S3: Urea molten liquid, coal-based quantum carbon dot dispersion, and modified silane coupling agent are sprayed into the tower through three layers of coaxial atomizing nozzles. Combined with a gradient temperature field, urea granulation, in-situ quantum carbon dot coating, and coating layer solidification are continuously completed in the tower to obtain a slow-release urea product.

[0009] A further improvement is that in S1, the carbonization temperature is 1000-1400℃ and the carbonization time is 2-3h.

[0010] A further improvement is that, in S1, the electrolyte is a 0.5 mol / L sulfuric acid solution, the electrolysis current is 1-1.5 A, and the electrolysis time is 10-40 h.

[0011] A further improvement is that, in S1, the concentration of the coal-based quantum carbon dot dispersion is 5-8 wt%, and the quantum yield is controlled at 45%-55%.

[0012] A further improvement is made in S1, where the thickness d of the coal-based quantum carbon dot coating layer is controlled by the electrolysis time t, satisfying the following formula:

[0013] d = a × t + b,

[0014] Where: d is the coating thickness in nm; t is the electrolysis time in h; a is the thickness coefficient, which is 4.5-5.0 nm / h; b is the base thickness, which is 5-10 nm.

[0015] A further improvement lies in the fact that the sustained-release period T of the sustained-release urea is controlled by the coating layer thickness d, satisfying the following formula:

[0016]

[0017] Where: T is the sustained-release period in days; d is the coating thickness in nm; c is the period coefficient, which ranges from 0.5 to 0.6 days / nm; The base cycle is 5-10 days.

[0018] A further improvement is made in S2, where the three coaxial atomizing nozzles are arranged from top to bottom along the tower body as an upper nozzle, a middle nozzle, and a lower nozzle; the lower nozzle is used to spray molten urea, the middle nozzle is used to spray coal-based quantum carbon dot dispersion, and the upper nozzle is used to spray modified silane coupling agent.

[0019] Further improvements are made in that: the temperature of the urea melt is 130-140℃, the temperature of the coal-based quantum carbon dot dispersion is 80-90℃, and the amount of the modified silane coupling agent is 2-3% of the mass of the coal-based quantum carbon dots.

[0020] A further improvement is that, in S2, the gradient temperature field is divided into a low-temperature zone at the top of the tower, a high-temperature zone in the middle of the tower, and a high-temperature zone at the bottom of the tower from top to bottom; the temperature of the low-temperature zone at the top of the tower is 15-20℃, the temperature of the high-temperature zone in the middle of the tower is 40-50℃, and the temperature of the high-temperature zone at the bottom of the tower is 80-90℃.

[0021] A further improvement is that, in S3, the injection pressures of the three coaxial atomizing nozzles are different: the pressure of the lower nozzle spraying urea melt is 0.3-0.4 MPa, the pressure of the middle nozzle spraying coal-based quantum carbon dot dispersion is 0.2-0.3 MPa, and the pressure of the upper nozzle spraying modified silane coupling agent is 0.15-0.25 MPa.

[0022] The beneficial effects of this invention are as follows:

[0023] 1. This invention completes the three-step process of granulation, coating and curing continuously in the same evaporation granulation tower, shortens the production cycle, avoids particle breakage during transport, improves the yield, and significantly reduces industrial production costs.

[0024] 2. This invention uses coal liquefaction residue, an industrial byproduct, as a carbon source to prepare quantum carbon dots. This not only solves the solid waste treatment problem of coal liquefaction residue, but also reduces the cost of carbon sources. The quantum yield of the prepared coal-based quantum carbon dots can reach 45%-55%, which is far superior to traditional biomass carbon sources. The interfacial bonding force is strong, and the coating layer peeling rate is less than 2%.

[0025] 3. This invention achieves quantitative control of coating thickness by precisely regulating the electrolytic preparation parameters of coal-based quantum carbon dots and the gradient temperature field inside the tower, thereby designing different slow-release cycles to meet the nutrient requirements of different crops such as wheat, corn, and fruit trees throughout their entire growth period; at the same time, it realizes the precise control of micro-nano structures by macro-industrial devices, providing a new paradigm for the cross-integration of process engineering and nanotechnology. Attached Figure Description

[0026] Figure 1 This is a flowchart of the present invention. Detailed Implementation

[0027] 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.

[0028] Example 1

[0029] according to Figure 1As shown in the figure, this embodiment proposes a continuous production method for in-situ coating of coal-based quantum carbon dots with slow-release urea in an evaporation granulation tower, including the following steps:

[0030] S1: Using coal liquefaction residue as a carbon source, a coal-based quantum carbon dot dispersion is prepared by carbonization, molding with polyvinylidene fluoride, and then electrolysis. The carbonization temperature is 1000-1400℃, and the carbonization time is 2-3 hours. The electrolyte is a 0.5 mol / L sulfuric acid solution, the electrolysis current is 1-1.5 A, and the electrolysis time is 10-40 hours. The concentration of the coal-based quantum carbon dot dispersion is 5-8 wt%, and the quantum yield is controlled at 45%-55%. The thickness d of the coal-based quantum carbon dot coating layer is controlled by the electrolysis time t, satisfying the following formula:

[0031] d = a × t + b,

[0032] Where: d is the coating thickness in nm; t is the electrolysis time in h; a is the thickness coefficient, which is 4.5-5.0 nm / h; b is the base thickness, which is 5-10 nm.

[0033] The release period T of sustained-release urea is controlled by the coating thickness d, satisfying the following formula:

[0034]

[0035] Where: T is the sustained-release period in days; d is the coating thickness in nm; c is the period coefficient, which ranges from 0.5 to 0.6 days / nm; The base cycle is 5-10 days. Using coal liquefaction residue as a carbon source, this method achieves the resource utilization of industrial solid waste, reducing carbon source costs (40%-50% lower than traditional biomass carbon sources) and minimizing solid waste stockpiling pollution, thus combining environmental protection and economic efficiency. The synergistic effect of carbonization at 1000-1400℃ and specific electrolysis parameters results in a quantum yield of 45%-55% for coal-based quantum carbon dots, far exceeding that of traditional biomass carbon dots (typically <30%), enhancing the interfacial bonding between the coating layer and urea particles, and reducing the shedding rate (<2%). By quantitatively linking electrolysis time, coating layer thickness, and slow-release cycle through formulas, precise design of slow-release performance is achieved, allowing for flexible adjustment (30-120 days) according to the needs of different crops throughout their entire growth cycle, thus offering wider adaptability.

[0036] S2: The evaporation granulation tower is modified by installing three layers of coaxial atomizing nozzles along the height of the tower, and constructing a gradient temperature field within the tower. The three layers of coaxial atomizing nozzles, from top to bottom along the tower, are the upper nozzle, middle nozzle, and lower nozzle. The lower nozzle is used to spray molten urea, the middle nozzle is used to spray coal-based quantum carbon dot dispersion, and the upper nozzle is used to spray modified silane coupling agent. The temperature of the molten urea is 130-140℃, the temperature of the coal-based quantum carbon dot dispersion is 80-90℃, and the amount of modified silane coupling agent is 2-3% of the mass of the coal-based quantum carbon dots. The gradient temperature field, from top to bottom along the tower, is divided into a low-temperature zone at the top, a high-temperature zone in the middle, and a high-temperature zone at the bottom. The temperature of the low-temperature zone at the top is 15-20℃, the temperature of the high-temperature zone in the middle is 40-50℃, and the temperature of the high-temperature zone at the bottom is 80-90℃. The layered design of the three-layer coaxial atomizing nozzle integrates urea granulation, carbon dot coating, and coupling agent curing functions into a single tower, breaking away from the traditional "multi-equipment, step-by-step" model and reducing equipment investment and floor space by more than 30%. A gradient temperature field (80-90℃ high-temperature zone → 40-50℃ medium-temperature zone → 15-20℃ low-temperature zone) is precisely matched with the material temperature (urea 130-140℃, carbon dots 80-90℃), providing the optimal reaction environment for each stage of "nucleation-coating-curing" and improving the uniformity of the coating layer (thickness deviation <5%). The modified silane coupling agent dosage is controlled at 2-3% of the carbon dot mass, ensuring sufficient cross-linking and curing of the coating layer while avoiding increased costs and environmental risks caused by excessive coupling agent, achieving a balance between performance and economy.

[0037] S3: Urea molten liquid, coal-based quantum carbon dot dispersion, and modified silane coupling agent are injected into the tower through three layers of coaxial atomizing nozzles. Combined with a gradient temperature field, urea granulation, in-situ quantum carbon dot coating, and coating layer solidification are continuously completed within the tower to obtain a slow-release urea product. The injection pressure of the three layers of coaxial atomizing nozzles is different: the pressure of the lower nozzle injecting urea molten liquid is 0.3-0.4 MPa, the pressure of the middle nozzle injecting coal-based quantum carbon dot dispersion is 0.2-0.3 MPa, and the pressure of the upper nozzle injecting modified silane coupling agent is 0.15-0.25 MPa. Different nozzles use differentiated injection pressures (0.15-0.4 MPa) to adapt to the material characteristics of urea molten liquid (high viscosity), carbon dot dispersion (medium viscosity), and coupling agent (low viscosity), ensuring uniform atomized particle size (deviation <10%), laying the foundation for the uniformity of the subsequent coating layer. The process of granulation, coating, and curing is completed continuously within the same tower, avoiding breakage caused by particle transport in traditional processes (breakage rate reduced from 8-10% to <2%), increasing the yield to 96%-98%, and shortening the production cycle to 3-4 hours (more than 60% shorter than traditional processes). During the in-situ coating process, urea particle nucleation and carbon dot coating occur simultaneously. The temperature gradient induces co-crystallization of carbon dots and urea molecules, enhancing the interfacial bonding between the coating layer and the particles. The coating layer detachment rate during the slow-release process is <2%, far lower than the traditional post-coating process (8-10%).

[0038] Example 2

[0039] according to Figure 1 As shown in the figure, this embodiment proposes a continuous production method for in-situ coating of coal-based quantum carbon dots with slow-release urea in an evaporation granulation tower, including the following steps:

[0040] Preparation of coal-based quantum carbon dots: Coal liquefaction residue was carbonized at 1000℃ for 2 hours and mixed with polyvinylidene fluoride at a mass ratio of 1:0.3 to form a mold; the molded body was placed in 0.5 mol / L sulfuric acid electrolyte, and the electrolysis current was controlled at 1A for 10 hours to prepare a coal-based quantum carbon dot dispersion with a concentration of 5 wt% and a quantum yield of 45%.

[0041] Evaporation granulation tower parameter settings: the lower nozzle sprays 130℃ molten urea liquid (spray pressure 0.3MPa), the middle nozzle sprays the above-mentioned carbon dot dispersion liquid at 80℃ (spray pressure 0.2MPa), and the upper nozzle sprays modified silane coupling agent (dosage is 2% of the carbon dot mass, spray pressure 0.15MPa); the temperature field inside the tower is set as follows: low temperature zone at the top of the tower 15℃, high temperature zone in the middle of the tower 40℃, and high temperature zone at the bottom of the tower 80℃.

[0042] Product performance: The prepared slow-release urea coating has a thickness of 50nm, a slow-release period of 30 days, a nutrient release rate of 85% after soaking in water for 30 days, a production cycle of 3 hours, and a yield of 96%.

[0043] Example 3

[0044] according to Figure 1 As shown in the figure, this embodiment proposes a continuous production method for in-situ coating of coal-based quantum carbon dots with slow-release urea in an evaporation granulation tower, including the following steps:

[0045] Preparation of coal-based quantum carbon dots: Coal liquefaction residue was carbonized at 1200℃ for 2.5h and mixed with polyvinylidene fluoride at a mass ratio of 1:0.3 to form a mold; the molded body was placed in 0.5mol / L sulfuric acid electrolyte, and the electrolysis current was controlled at 1.2A for 25h to prepare a coal-based quantum carbon dot dispersion with a concentration of 6.5wt% and a quantum yield of 50%.

[0046] Evaporation granulation tower parameter settings: the lower nozzle sprays 135℃ molten urea liquid (spray pressure 0.35MPa), the middle nozzle sprays the above-mentioned carbon dot dispersion liquid at 85℃ (spray pressure 0.25MPa), and the upper nozzle sprays modified silane coupling agent (dosage is 2.5% of the carbon dot mass, spray pressure 0.2MPa); the temperature field inside the tower is set as follows: low temperature zone at the top of the tower 18℃, high temperature zone in the middle of the tower 45℃, and high temperature zone at the bottom of the tower 85℃.

[0047] Product performance: The prepared slow-release urea coating has a thickness of 120nm, a slow-release period of 75 days, a nutrient release rate of 88% after soaking in water for 75 days, a production cycle of 3.5 hours, and a yield of 97%.

[0048] Example 4

[0049] according to Figure 1 As shown in the figure, this embodiment proposes a continuous production method for in-situ coating of coal-based quantum carbon dots with slow-release urea in an evaporation granulation tower, including the following steps:

[0050] Preparation of coal-based quantum carbon dots: Coal liquefaction residue was carbonized at 1400℃ for 3 hours and mixed with polyvinylidene fluoride at a mass ratio of 1:0.3 to form a mold; the molded body was placed in 0.5 mol / L sulfuric acid electrolyte, and the electrolysis current was controlled at 1.5 A for 40 hours to prepare a coal-based quantum carbon dot dispersion with a concentration of 8 wt% and a quantum yield of 55%.

[0051] Evaporation granulation tower parameter settings: the lower nozzle sprays 140℃ molten urea liquid (spray pressure 0.4MPa), the middle nozzle sprays the above-mentioned carbon dot dispersion liquid at 90℃ (spray pressure 0.3MPa), and the upper nozzle sprays modified silane coupling agent (dosage is 3% of the carbon dot mass, spray pressure 0.25MPa); the temperature field inside the tower is set as follows: the low temperature zone at the top of the tower is 20℃, the temperature zone in the middle of the tower is 50℃, and the high temperature zone at the bottom of the tower is 90℃.

[0052] Product performance: The prepared sustained-release urea coating has a thickness of 200nm, a sustained-release period of 120 days, a nutrient release rate of 90% after soaking in water for 120 days, a production cycle of 4 hours, and a yield of 98%.

[0053] Validation data

[0054] The table below compares the performance of Examples 2, 3, and 4 with the traditional granulation-coating process:

[0055]

[0056] This continuous production method for in-situ coating of slow-release urea with coal-based quantum carbon dots within an evaporation granulation tower completes the three-step process of granulation, coating, and solidification continuously within the same tower, shortening the production cycle, avoiding particle breakage during transport, improving yield, and significantly reducing industrial production costs. Furthermore, this invention uses coal liquefaction residue, an industrial byproduct, as a carbon source to prepare quantum carbon dots, solving the solid waste treatment problem of coal liquefaction residue and reducing carbon source costs. The quantum yield of the prepared coal-based quantum carbon dots can reach 45%-55%, far exceeding that of traditional biomass carbon sources, with strong interfacial bonding and a coating layer detachment rate of less than 2%. In addition, by precisely controlling the electrolytic preparation parameters of the coal-based quantum carbon dots and the gradient temperature field within the tower, this invention can achieve quantitative control of the coating layer thickness, thereby designing different slow-release cycles to meet the nutrient requirements of different crops such as wheat, corn, and fruit trees throughout their entire growth cycle. Simultaneously, it achieves precise control of micro-nano structures from a macro-industrial device, providing a new paradigm for the cross-integration of process engineering and nanotechnology.

[0057] 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 continuous production method of in-situ coated slow-release urea with coal-based quantum carbon dots in an evaporation prilling tower, characterized in that, The method comprises the following steps: S1: coal liquefaction residue is used as a carbon source to prepare a coal-based quantum carbon dot dispersion liquid through carbonization and polyvinylidene fluoride molding and electrolysis; S2: a vaporization prilling tower is modified, three layers of coaxial atomizing nozzles are arranged in the height direction of the tower body, and a gradient temperature field is constructed in the tower; S3: urea melt, coal-based quantum carbon dot dispersion liquid and modified silane coupling agent are sprayed into the tower through the three layers of coaxial atomizing nozzles respectively, and the urea prilling, in-situ coating of quantum carbon dots and curing of the coating layer are continuously completed in the tower in combination with the gradient temperature field, so that a slow-release urea product is obtained.

2. The continuous production process of in-situ coated slow-release urea with coal-based quantum carbon dots in an evaporation prilling tower according to claim 1, characterized in that: In S1, the carbonization temperature is 1000-1400℃, and the carbonization time is 2-3h.

3. The continuous production process of in-situ coated slow release urea with coal-based quantum carbon dots in an evaporation prilling tower according to claim 2, characterized in that: In S1, the electrolyte for electrolysis is a 0.5mol / L sulfuric acid solution, the electrolysis current is 1-1.5A, and the electrolysis time is 10-40h.

4. The continuous production process of in-situ coated slow-release urea with coal-based quantum carbon dots in an evaporation prilling tower according to claim 3, characterized in that: In S1, the concentration of the coal-based quantum carbon dot dispersion liquid is 5-8wt%, and the quantum yield is controlled to be 45%-55%.

5. The continuous production process of in-situ coated slow release urea with coal-based quantum carbon dots in an evaporation prilling tower according to claim 4, characterized in that: In S1, the thickness d of the coal-based quantum carbon dot coating layer is regulated by the electrolysis time t, and the following formula is met: d=a×t+b, wherein d is the coating layer thickness, the unit is nm, t is the electrolysis time, the unit is h, a is the thickness coefficient, the value is 4.5-5.0nm / h, and b is the basic thickness, the value is 5-10nm.

6. The continuous production process of in-situ coated slow release urea with coal-based quantum carbon dots in an evaporation prilling tower according to claim 5, characterized in that: The slow-release period T of the slow-release urea is regulated by the coating layer thickness d, and the following formula is met: ， Wherein: T is the slow-release period, unit is day; d is the coating layer thickness, unit is nm; c is the period coefficient, the value is 0.5-0.6 day / nm; is the basic period, the value is 5-10 days.

7. The continuous production process of in-situ coated slow release urea with coal-based quantum carbon dots in an evaporation prilling tower according to claim 1, characterized in that: In S2, the three layers of coaxial atomizing nozzles are sequentially arranged from top to bottom along the tower body as an upper nozzle, a middle nozzle and a lower nozzle; the lower nozzle is used for spraying the urea melt, the middle nozzle is used for spraying the coal-based quantum carbon dot dispersion liquid, and the upper nozzle is used for spraying the modified silane coupling agent.

8. The continuous production process of in-situ coated slow release urea with coal-based quantum carbon dots in an evaporation prilling tower according to claim 7, characterized in that: The temperature of the urea melt is 130-140℃, the temperature of the coal-based quantum carbon dot dispersion liquid is 80-90℃, and the amount of the modified silane coupling agent is 2-3% of the mass of the coal-based quantum carbon dot.

9. The continuous production process of in-situ coated slow release urea with coal-based quantum carbon dots in an evaporation prilling tower according to claim 1, characterized in that: In S2, the gradient temperature field is divided into a tower top low-temperature zone, a tower middle temperature zone and a tower bottom high-temperature zone from top to bottom along the tower body; the temperature of the tower top low-temperature zone is 15-20℃, the temperature of the tower middle temperature zone is 40-50℃, and the temperature of the tower bottom high-temperature zone is 80-90℃.

10. The continuous production process of in-situ coated slow release urea with coal-based quantum carbon dots in an evaporation prilling tower according to claim 7, characterized in that: In S3, the spraying pressures of the three layers of coaxial atomizing nozzles are different: the pressure of the lower nozzle for spraying the urea melt is 0.3-0.4MPa, the pressure of the middle nozzle for spraying the coal-based quantum carbon dot dispersion liquid is 0.2-0.3MPa, and the pressure of the upper nozzle for spraying the modified silane coupling agent is 0.15-0.25MPa.

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