Method for preparing coal-based quantum carbon dot slow-release urea through sectional drying-fluidized bed granulation
By using segmented drying-fluidized bed granulation technology, the problems of uneven distribution and thermal decomposition of coal-based quantum carbon dots in urea particles have been solved, achieving uniform distribution and stable binding of coal-based quantum carbon dot slow-release urea, extending the slow-release period and reducing production costs.
Patent Information
- Application Number
- CN202511617661.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-11-06
- Publication Date
- 2026-01-16
AI Technical Summary
In traditional slow-release urea preparation processes, coal-based quantum carbon dots are unevenly distributed and prone to agglomeration, and high-temperature drying can easily lead to thermal decomposition of urea, making it impossible to form a stable long-term slow-release mechanism.
A segmented drying-fluidized bed granulation method is adopted, which removes moisture through low-humidity pre-drying, forms covalent bonds through medium-temperature shaping, and locks the structure through low-temperature curing. Combined with ultrasonic atomization and airflow suspension technology, the uniform distribution of MCQDs on the surface and internal pores of urea masterbatch is achieved.
It improves the uniformity of MCQDs distribution and binding stability in urea granules, prolongs the sustained-release period, reduces production costs, and enhances granule compressive strength and yield.
Smart Images
Figure CN121342590A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of slow-release fertilizer preparation technology, and in particular to a method for preparing coal-based quantum carbon dot slow-release urea by segmented drying-fluidized bed granulation. Background Technology
[0002] The preparation of sustained-release urea often employs the traditional process of "drum granulation + coating," where urea particles are rolled by rotating a drum while a coating layer is formed by spraying resin, inorganic minerals, or other sustained-release agents. Some processes incorporate fluidized bed granulation technology, but the drying process often uses a single high-temperature parameter without considering the loading characteristics of nano-sustaining agents such as quantum carbon dots. In recent years, coal-based quantum carbon dots (MCQDs) have been explored for use in the preparation of sustained-release urea due to their wide availability, low cost, and high oxygen content from coal-based waste materials such as coal gangue and fly ash. However, they are often directly mixed with urea particles without functionalizing their surface.
[0003] Traditional processes have the following drawbacks: First, in rotary drum granulation, the slow-release agent is prone to agglomeration due to uneven particle rolling, resulting in uneven distribution of the slow-release agent on the urea surface and in the internal pores, ultimately leading to large fluctuations in the slow-release effect. Second, in a single high-temperature drying process, when the temperature approaches or exceeds the urea decomposition temperature of approximately 132.7℃, thermal decomposition of urea is easily triggered; if the temperature is too low, the surface moisture of the particles cannot be completely removed, leading to particle agglomeration. Third, unfunctionalized coal-based quantum carbon dots are chemically inert and cannot form a stable bond with the urea matrix. Physical mixing alone can easily cause them to detach during transportation or application, making it impossible to construct a long-term slow-release mechanism. Therefore, this invention proposes a segmented drying-fluidized bed granulation method for preparing coal-based quantum carbon dot slow-release urea to solve the problems existing in the prior art. Summary of the Invention
[0004] To address the aforementioned problems, this invention proposes a segmented drying-fluidized bed granulation method for preparing coal-based quantum carbon dot slow-release urea. This method achieves the following: In the pre-drying stage, a low-humidity environment thoroughly removes surface moisture from the urea masterbatch, preventing particle agglomeration caused by moisture during MCQDs spraying; in the medium-temperature setting stage, within a range below the urea decomposition temperature, it promotes the formation of covalent bonds between MCQDs and the urea crystal facets through a silane coupling agent, enhancing bonding stability; and in the low-temperature curing stage, it locks in the formed bonded structure, preventing structural damage caused by temperature fluctuations, thus solving the dilemma of decomposition or agglomeration inherent in traditional single-stage drying.
[0005] To achieve the objectives of this invention, the invention is implemented through the following technical solution: a method for preparing coal-based quantum carbon dot slow-release urea by segmented drying-fluidized bed granulation, comprising the following steps:
[0006] S1: Coal-based quantum carbon dots (MCQDs) were prepared by hydrothermal method using coal-based waste as raw material. Silane coupling agent was added for surface functionalization modification to obtain functionalized MCQDs dispersion.
[0007] S2: Industrial urea is crushed and sieved to obtain urea masterbatch with a particle size of 1.0-3.0 mm;
[0008] S3: Feed the urea masterbatch into the fluidized bed granulator, turn on the ultrasonic atomization device to spray the functionalized MCQDs dispersion onto the surface of the urea masterbatch, and at the same time execute the three-stage drying process: the pre-drying stage controls the temperature T1 to 40-60℃ and the humidity ≤30%R, the medium-temperature setting stage controls the temperature T2 to 80-120℃, and the low-temperature curing stage controls the temperature T3 to 25-40℃.
[0009] S4: After cooling and sieving, coal-based quantum carbon dot slow-release urea product is obtained.
[0010] A further improvement is that, in S1, the silane coupling agent is 3-aminopropyltriethoxysilane (KH550). - Glycidyl etheroxypropyltrimethoxysilane (KH560) One or more of the following: - Methacryloxypropyltrimethoxysilane (KH570).
[0011] A further improvement is that, in S1, the mass ratio of silane coupling agent to MCQDs is 1:5-1:20, and the conditions for preparing MCQDs by hydrothermal method are: the mass ratio of coal-based waste to deionized water is 1:10-1:30, the hydrothermal temperature is 160-220℃, and the hydrothermal time is 4-8h.
[0012] A further improvement is made in S3, where the atomization pressure of the ultrasonic atomizing device is 0.2-0.5 MPa and the atomized particle size is 5-20 mm. The spraying rate of the MCQDs dispersion is 5-15 mL / min.
[0013] A further improvement lies in: in S3, the airflow velocity v of the fluidized bed granulator is related to the particle size d and density of the functionalized MCQDs. The following collaborative matching formula is satisfied:
[0014]
[0015] Where v is the airflow velocity (m / s), k is the airflow coordination coefficient 22-26, and d is the MCQDs particle size ( ), The density of MCQDs is (g / cm³).
[0016] A further improvement lies in the fact that the particle size d of the functionalized MCQDs is 0.003-0.01. ,density The value is 1.2-1.5 g / cm³, and after substituting it into the formula, the range of the airflow velocity v is 1.2-1.5 m / s.
[0017] A further improvement is made in S3, where, in the three-stage drying process, the medium-temperature setting temperature T2, the time t2, and the low-temperature curing temperature T3 satisfy the following synergistic control formula:
[0018] T2×t2=C×(T2-T3),
[0019] Where T2 is the medium-temperature setting temperature (°C), t2 is the medium-temperature setting time (min), C is the drying synergy constant (5-8), and T3 is the low-temperature curing temperature (°C).
[0020] Further improvements include: the pre-drying stage time t1 is 10-20 min, the low-temperature curing stage time t3 is 20-30 min, and the medium-temperature setting time t2 is calculated to be 5-15 min using a formula.
[0021] Further improvements are made in the following aspects: In S4, the cooling temperature is 20-25℃, the sieving uses a 2.0-4.0mm sieve, and the finished product particle size qualification rate is ≥95%.
[0022] A further improvement is made in step S4, where fine particles with a diameter less than 2.0 mm obtained after sieving are crushed to 0.1-0.5 mm and then mixed with urea masterbatch from step S2 at a mass ratio of 1:8-1:12 before being fed back into the fluidized bed granulator; coarse particles with a diameter greater than 4.0 mm are crushed to 1.0-3.0 mm and then directly returned to the fluidized bed granulator in step S3 to continue granulation.
[0023] The beneficial effects of this invention are as follows:
[0024] 1. In the pre-drying stage, the present invention thoroughly removes the surface moisture of urea masterbatch through a low-humidity environment, avoiding particle agglomeration caused by moisture during MCQDs spraying; in the medium-temperature setting stage, within a range below the urea decomposition temperature, it promotes the formation of covalent bonds between MCQDs and urea crystal surfaces through silane coupling agents, thereby improving the bonding stability; in the low-temperature curing stage, it locks the formed bonded structure, avoiding structural damage caused by temperature fluctuations, thus solving the dilemma of decomposition or agglomeration in traditional single drying.
[0025] 2. This invention refines the MCQDs dispersion into finer particles using an ultrasonic atomization device. The tiny droplets, combined with the airflow suspension effect of the fluidized bed at 1.2-1.5 m / s, allow the droplets to uniformly contact the outer surface and internal pores of the urea masterbatch. Compared with the rolling spraying of traditional drum granulation, this combination improves the distribution uniformity of MCQDs, increases the qualified particle size, and improves the compressive strength of the particles by uniform airflow.
[0026] 3. This invention transforms the originally inert MCQDs into active structural units through silane functionalization modification. The -COOH and -OH on their surface can form a hydrogen bond network with urea molecules. The nanoparticles of the MCQDs themselves form a physical barrier, and the coal-based carbon material can respond to the degradation of soil microorganisms. This triple mechanism prolongs the slow-release period, and the raw material is coal-based waste, which reduces production costs, thus combining long-term effectiveness with economic benefits. Attached Figure Description
[0027] Figure 1 This is a flowchart of the present invention. Detailed Implementation
[0028] 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.
[0029] Example 1
[0030] according to Figure 1 As shown in the figure, this embodiment proposes a method for preparing coal-based quantum carbon dot slow-release urea by segmented drying-fluidized bed granulation, including the following steps:
[0031] S1: Coal-based quantum carbon dots (MCQDs) were prepared from coal-based waste using a hydrothermal method. Surface functionalization was performed by adding a silane coupling agent to obtain a functionalized MCQDs dispersion. The silane coupling agent was 3-aminopropyltriethoxysilane (KH550). - Glycidyl etheroxypropyltrimethoxysilane (KH560) One or more of the following: -methacryloyloxypropyltrimethoxysilane (KH570). The mass ratio of silane coupling agent to MCQDs is 1:5-1:20. The conditions for preparing MCQDs by hydrothermal method are: mass ratio of coal-based waste to deionized water 1:10-1:30, hydrothermal temperature 160-220℃, and hydrothermal time 4-8h. Using coal-based waste as raw material realizes the resource utilization of solid waste and significantly reduces the preparation cost of MCQDs. The precise ratio and modification conditions of the silane coupling agent can introduce sufficient active functional groups on the surface of MCQDs, laying the foundation for the subsequent formation of stable covalent bonds with urea crystal faces.
[0032] S2: Industrial urea is crushed and sieved to obtain urea masterbatch with a particle size of 1.0-3.0mm. Uniform particle size urea masterbatch can ensure uniform stress during subsequent MCQDs dispersion spraying and reduce uneven load caused by particle size differences. The particle size range of 1.0-3.0mm can provide sufficient space for the internal pore load MCQDs and avoid agglomeration caused by masterbatch that is too small or insufficient spraying caused by masterbatch that is too large.
[0033] S3: Feed the urea masterbatch into a fluidized bed granulator, turn on the ultrasonic atomizing device to spray the functionalized MCQDs dispersion onto the surface of the urea masterbatch, and simultaneously execute a three-stage drying process: Pre-drying stage: temperature T1 controlled at 40-60℃, humidity ≤30%R; medium-temperature setting stage: temperature T2 controlled at 80-120℃; low-temperature curing stage: temperature T3 controlled at 25-40℃; atomization pressure of the ultrasonic atomizing device is 0.2-0.5MPa, and atomized particle size is 5-20 mm. The spraying rate of the MCQDs dispersion is 5-15 mL / min. The airflow velocity v of the fluidized bed granulator is related to the particle size d and density of the functionalized MCQDs. The following collaborative matching formula is satisfied:
[0034]
[0035] Where v is the airflow velocity (m / s), k is the airflow coordination coefficient 22-26, and d is the MCQDs particle size ( ), The density of MCQDs is expressed in g / cm³. The particle size d of the functionalized MCQDs is 0.003-0.01. ,density The value is 1.2-1.5 g / cm³, and after substituting it into the formula, the range of the airflow velocity v is 1.2-1.5 m / s.
[0036] In the three-stage drying process, the medium-temperature setting temperature T2, the time t2, and the low-temperature curing temperature T3 satisfy the following synergistic control formula:
[0037] T2×t2=C×(T2-T3),
[0038] Wherein, T2 is the medium-temperature setting temperature (°C), t2 is the medium-temperature setting time (min), C is the drying synergy constant (5-8), and T3 is the low-temperature curing temperature (°C). The pre-drying stage time t1 is 10-20 min, the low-temperature curing stage time t3 is 20-30 min, and the medium-temperature setting time t2 is calculated to be 5-15 min using a formula. The synergistic effect of ultrasonic atomization and a specific airflow velocity allows the MCQDs dispersion to uniformly cover the surface and internal pores of the urea masterbatch, solving the problem of uneven distribution of the slow-release agent in traditional drum granulation; the three-stage drying temperature-time synergistic control can not only remove surface moisture through pre-drying to avoid agglomeration, but also promote covalent bond formation through medium-temperature setting, while avoiding thermal decomposition of urea through low-temperature curing, achieving precise matching of multi-stage requirements.
[0039] S4: After cooling and sieving, the coal-based quantum carbon dot slow-release urea product is obtained. The cooling temperature is 20-25℃, and the sieving uses a 2.0-4.0mm sieve. The finished product particle size qualification rate is ≥95%. The fine particles with a particle size less than 2.0mm obtained after sieving are crushed to 0.1-0.5mm and then mixed with the urea masterbatch from step S2 at a mass ratio of 1:8-1:12 before being fed back into the fluidized bed granulator. The coarse particles with a particle size greater than 4.0mm are crushed to 1.0-3.0mm and then directly returned to the fluidized bed granulator in step S3 to continue granulation. The cooling temperature of 20-25℃ can lock the bonding structure formed between MCQDs and urea, avoiding the destruction of the slow-release network caused by temperature fluctuations. The recycling design of fine and coarse particles increases the raw material utilization rate to over 98%, significantly reducing material loss during the production process.
[0040] Example 2
[0041] according to Figure 1 As shown in the figure, this embodiment proposes a method for preparing coal-based quantum carbon dot slow-release urea by segmented drying-fluidized bed granulation, including the following steps:
[0042] Preparation and modification of MCQDs: 10g of coal gangue (coal-based waste) was mixed with 200mL of deionized water and hydrothermally reacted at 180℃ for 6h; centrifuged (8000r / min, 15min), 0.22 MCQDs were obtained by membrane filtration and dialysis with a 350 Da dialysis bag for 48 h; KH550 was added at a mass ratio of 1:10, and the mixture was stirred at 60 °C for 2 h to obtain a functionalized MCQDs dispersion (d=0.005). , ρ=1.3g / cm³).
[0043] Preparation of urea masterbatch: After industrial urea is crushed and sieved, urea masterbatch with a diameter of 1.0-2.0 mm is selected.
[0044] Granulation and drying: Urea masterbatch is fed into a fluidized bed and ultrasonic atomization is started (0.3MPa, atomized particle size 10). Spraying rate 10mL / min); Airflow velocity calculated according to algorithm 1: v=24×(0.005 / 1.3)^(1 / 2)=24×0.062≈1.49m / s; Medium temperature time calculated according to algorithm 2: C=6, T2=100℃, T3=30℃, t2=6×(100-30) / 100=4.2min (take 4min); Three-stage drying: T1=50℃ (t1=15min, RH=25%)→T2=100℃ (t2=4min)→T3=30℃ (t3=25min).
[0045] Finished product collection: Cool to 22℃, sieve through a 2.0-3.0mm sieve, finished product particle size qualification rate 96.2%, compressive strength 26.5N, slow release period 105 days.
[0046] Example 3
[0047] according to Figure 1 As shown in the figure, this embodiment proposes a method for preparing coal-based quantum carbon dot slow-release urea by segmented drying-fluidized bed granulation, including the following steps:
[0048] Preparation and modification of MCQDs: 10g of fly ash was mixed with 150mL of deionized water and hydrothermally reacted at 200℃ for 5h; MCQDs were obtained by centrifugation, filtration, and dialysis; KH560 was added at a mass ratio of 1:15, and the mixture was stirred at 55℃ for 2.5h to obtain a functionalized MCQDs dispersion (d=0.008). , ρ=1.4g / cm³).
[0049] Preparation of urea masterbatch: Select urea masterbatch with a diameter of 1.5-2.5 mm.
[0050] Granulation and drying: Ultrasonic atomization (0.4MPa, atomized particle size 15μm, spraying rate 12mL / min); airflow velocity: v=23×(0.008 / 1.4)^(1 / 2)=23×0.075≈1.72m / s (adjust k=21, v=21×0.075≈1.58m / s, close to the upper limit); medium temperature time: C=7, T2=110℃, T3=35℃, t2=7×(110-35) / 110≈4.8min (take 5min); three-stage drying: T1=55℃ (t1=12min, RH=28%)→T2=110℃ (t2=5min)→T3=35℃ (t3=28min).
[0051] Finished product collection: Cool to 23℃, sieve through a 2.0-3.5mm sieve, finished product particle size qualification rate 95.8%, compressive strength 25.8N, slow release period 100 days.
[0052] Example 4
[0053] according to Figure 1 As shown in the figure, this embodiment proposes a method for preparing coal-based quantum carbon dot slow-release urea by segmented drying-fluidized bed granulation, including the following steps:
[0054] Preparation and modification of MCQDs: 10g of coal slime was mixed with 250mL of deionized water and hydrothermally reacted at 190℃ for 7h; MCQDs were obtained by centrifugation, filtration, and dialysis; KH570 was added at a mass ratio of 1:8, and the mixture was stirred at 65℃ for 1.5h to obtain a functionalized MCQDs dispersion (d=0.003μm). =1.35g / cm³).
[0055] Preparation of urea masterbatch: Select urea masterbatch with a diameter of 2.0-3.0 mm.
[0056] Granulation and drying: Ultrasonic atomization (0.25MPa, atomized particle size 8μm, spraying rate 8mL / min); airflow velocity: v=26×(0.003 / 1.35)^(1 / 2)=26×0.047≈1.22m / s; medium temperature time: C=6, T2=90℃, T3=32℃, t2=6×(90-32) / 90≈3.9min (take 4min); three-stage drying: T1=45℃ (t1=18min, RH=22%)→T2=90℃ (t2=4min)→T3=32℃ (t3=22min).
[0057] Finished product collection: Cool to 21℃, sieve through a 2.5-4.0mm sieve, finished product particle size qualification rate 97.1%, compressive strength 27.2N, slow release period 115 days.
[0058] The performance of the finished products from Examples 2, 3, and 4 was compared with that of conventional rotary drum granulation coated urea (control group, using resin coating, drum speed 25 r / min, continuous drying at 130℃). The results are as follows:
[0059]
[0060] This segmented drying-fluidized bed granulation method for preparing coal-based quantum carbon dot slow-release urea (MCQDs) thoroughly removes surface moisture from urea masterbatch in a low-humidity environment during the pre-drying stage, preventing particle agglomeration caused by moisture during MCQDs spraying. In the medium-temperature setting stage, within a range below the urea decomposition temperature, it promotes the formation of covalent bonds between MCQDs and urea crystal faces via silane coupling agents, enhancing bonding stability. The low-temperature curing stage locks in the formed bonded structure, preventing structural damage caused by temperature fluctuations, thus solving the dilemma of decomposition or agglomeration inherent in traditional single-stage drying. Furthermore, this invention uses an ultrasonic atomization device to refine the MCQDs dispersion into 5-20 particles. The tiny droplets, combined with the 1.2-1.5 m / s airflow suspension in a fluidized bed, allow the droplets to uniformly contact the outer surface and internal pores of the urea masterbatch. Compared to the rolling spraying of traditional rotary drum granulation, this combination improves the distribution uniformity of MCQDs, resulting in a higher particle size qualification rate. Simultaneously, the uniform force of the airflow enhances the compressive strength of the particles. Furthermore, this invention transforms the originally inert MCQDs into active structural units through silane functionalization modification. The -COOH and -OH groups on their surface can form a hydrogen bond network with urea molecules. The nanoparticles of the MCQDs themselves form a physical barrier, and the coal-based carbon material can respond to the degradation by soil microorganisms. This triple mechanism extends the slow-release period, and the use of coal-based waste as raw material reduces production costs, combining long-term effectiveness with economic efficiency.
[0061] 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 method for the preparation of coal-based quantum carbon dots slow-release urea by sectional drying-fluidized bed granulation, characterized in that: The method comprises the following steps: S1: taking coal-based waste as raw material, preparing coal-based quantum carbon dots MCQDs by a hydrothermal method, adding a silane coupling agent for surface functionalization modification, and obtaining a functionalized MCQDs dispersion liquid; S2: crushing and screening industrial urea to obtain urea master particles with a particle size of 1.0-3.0 mm; S3: feeding the urea master particles into a fluidized bed granulator, starting an ultrasonic atomization device to spray the functionalized MCQDs dispersion liquid on the surface of the urea master particles, and simultaneously performing a three-stage drying process: the pre-drying stage controls the temperature T1 to be 40-60 DEG C, the humidity is ≤30% R, the medium-temperature shaping stage controls the temperature T2 to be 80-120 DEG C, and the low-temperature curing stage controls the temperature T3 to be 25-40 DEG C; S4: after cooling and screening, coal-based quantum carbon dots slow-release urea finished products are obtained.
2. The process for the preparation of coal-based quantum carbon dots slow release urea by sectional drying-fluidized bed granulation as claimed in claim 1 wherein: In the S1, the silane coupling agent is one or more of 3-aminopropyltriethoxysilane (KH550), γ-glycidoxypropyltrimethoxysilane (KH560), 3-methacryloxypropyltrimethoxysilane (KH570). - one or more of 3-aminopropyltriethoxysilane (KH550), γ-glycidoxypropyltrimethoxysilane (KH560), 3-methacryloxypropyltrimethoxysilane (KH570).
3. The process for the preparation of coal-based quantum carbon dots slow release urea by sectional drying-fluidized bed granulation as claimed in claim 2 wherein: In the S1, the mass ratio of the silane coupling agent to the MCQDs is 1:5-1:20, and the hydrothermal method for preparing the MCQDs has the following conditions: the mass ratio of the coal-based waste to deionized water is 1:10-1:30, the hydrothermal temperature is 160-220 DEG C, and the hydrothermal time is 4-8 h.
4. The process for the preparation of coal-based quantum carbon dots slow release urea by sectional drying-fluidized bed granulation as claimed in claim 1 wherein: In the S3, the atomization pressure of the ultrasonic atomization device is 0.2-0.5 MPa, the atomization particle size is 5-20 μm, and the spraying rate of the MCQDs dispersion liquid is 5-15 mL / min.
5. The process for the preparation of coal-based quantum carbon dots slow release urea by sectional drying-fluidized bed granulation as claimed in claim 4 wherein: In the S3, the airflow velocity v of the fluidized bed granulator and the particle size d and density of the functionalized MCQDs satisfy the following formula: satisfy the following synergistic matching formula: ， Where v is the airflow velocity (m / s), k is the airflow coordination coefficient 22-26, and d is the MCQDs particle size ( ), where ρ is the density of MCQDs (g / cm³).
6. The process for the preparation of coal-based quantum carbon dots slow release urea by sectional drying-fluidized bed granulation as claimed in claim 5 wherein: The functionalized MCQDs have a particle size d of 0.003-0.01 , a density of 1.2-1.5 g / cm3, and a value range of the air flow velocity v of 1.2-1.5 m / s after substitution.
7. The process for the preparation of coal-based quantum carbon dots slow release urea by segmented drying-fluidized bed granulation as claimed in claim 1 wherein: In the S3, in the three-stage drying, the medium-temperature shaping temperature T2 and time t2 and the low-temperature curing temperature T3 satisfy the following cooperative control formula: T2 x t2 = C x (T2-T3), wherein T2 is the medium-temperature shaping temperature (DEG C), t2 is the medium-temperature shaping time (min), C is the drying cooperative constant (5-8), and T3 is the low-temperature curing temperature (DEG C).
8. The process for the preparation of coal-based quantum carbon dots slow release urea by sectional drying-fluidized bed granulation as claimed in claim 7 wherein: The pre-drying stage time t1 is 10-20 min, the low-temperature curing stage time t3 is 20-30 min, and the medium-temperature shaping time t2 is calculated by the formula to be 5-15 min.
9. The process for the preparation of coal-based quantum carbon dots slow release urea by sectional drying-fluidized bed granulation as claimed in claim 1 wherein: In the S4, the cooling temperature is 20-25 DEG C, the screening uses a screen with a mesh size of 2.0-4.0 mm, and the finished product particle size qualified rate is ≥95%.
10. The process for the preparation of coal-based quantum carbon dots slow release urea by segmented drying-fluidized bed granulation as claimed in claim 1 wherein: In the S4, the fine particles with a particle size less than 2.0 mm obtained after screening are crushed to 0.1-0.5 mm, mixed with the urea master particles in the step S2 at a mass ratio of 1:8-1:12, and then fed into the fluidized bed granulator again; and the coarse particles with a particle size greater than 4.0 mm are crushed to 1.0-3.0 mm, and then directly returned to the fluidized bed granulator in the step S3 for further granulation.