A method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient controlled evaporation granulation tower

Abstract

This invention provides a method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower, relating to the field of agricultural fertilizer preparation technology. The method includes setting up a high-temperature zone, a medium-temperature zone, and a low-temperature zone from bottom to top inside the evaporation granulation tower; mixing coal powder with supercritical water in the high-temperature zone to react and generate a quantum carbon dot precursor; purging the quantum carbon dot precursor with ammonia gas and then reacting it with a boric acid solution in the medium-temperature zone; mixing the carbon dot solution with molten urea; atomizing the two mixtures through a dual-fluid nozzle in the low-temperature zone to form compound fertilizer granules; and coating the compound fertilizer granules by spraying an oxidized sodium alginate / polyethylene glycol composite film solution into a fluidized bed coating machine. This invention integrates the three core processes of quantum carbon dot synthesis, surface modification, and fertilizer compounding within the same equipment by setting up high-temperature, medium-temperature, and low-temperature zones from bottom to top inside the evaporation granulation tower, avoiding the destruction of the quantum carbon dot structure caused by cross-equipment operation and ensuring product stability.

Description

Technical Field

[0001] This invention relates to the field of agricultural fertilizer preparation technology, and in particular to a method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient controlled evaporation granulation tower. Background Technology

[0002] Slow-release fertilizers are a type of fertilizer that uses special processes (such as coating technology and chemical synthesis) to "encapsulate" nutrients, allowing nutrients like ammonia, phosphorus, and potassium to be released slowly over a set time. Their core advantage lies in their ability to continuously provide balanced nutrition to plants, avoiding problems such as root burn and nutrient waste caused by excessive one-time fertilization. Coal-based quantum carbon dots, as a novel fluorescent nanomaterial, possess excellent photocatalytic performance and surface modifiability.

[0003] Existing slow-release fertilizers mostly rely solely on physical barriers of membranes for slow release, failing to dynamically regulate nutrient release based on environmental conditions (such as light and humidity). In arid regions with ample sunlight, nutrient release tends to be too rapid, while in cloudy or rainy areas, it is too slow. Quantum carbon dot preparation requires multi-stage synthesis with material loss rates of 8%-15% during inter-device transfer. Furthermore, repeated heating and cooling of each device consumes energy, with energy costs accounting for 30%-40% of the total cost, resulting in high fertilizer production costs and hindering large-scale promotion. Some slow-release fertilizers use non-degradable membrane materials, which can lead to decreased soil permeability and reduced microbial activity with long-term application. The synthesis of quantum carbon dots involves the extensive use of organic solvents, posing a risk of VOC emissions. Therefore, this invention proposes a method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower to address these problems. Summary of the Invention

[0004] To address the aforementioned issues, this invention proposes a method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower. By setting high-temperature, medium-temperature, and low-temperature zones from bottom to top inside the evaporation granulation tower, the three core processes of quantum carbon dot synthesis, surface modification, and fertilizer compounding are integrated into the same equipment, increasing material utilization to over 95% and avoiding quantum carbon dot structure damage caused by cross-equipment operation, thus ensuring product stability and significantly increasing quantum carbon dot yield. By coating the compound fertilizer granules with an oxidized sodium alginate / polyethylene glycol composite membrane solution, which has good air permeability and swelling properties, the fertilizer granules can be prevented from disintegrating too quickly while allowing nutrients to slowly penetrate.

[0005] To achieve the objectives of this invention, the invention is implemented through the following technical solutions:

[0006] A method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower includes the following steps:

[0007] Step 1: Set up a high-temperature zone, a medium-temperature zone, and a low-temperature zone inside the evaporation granulation tower from bottom to top. The temperature of the high-temperature zone is 300-400℃, the temperature of the medium-temperature zone is 150-250℃, and the temperature of the low-temperature zone is 50-100℃.

[0008] Step 2: Set up a first material transfer system between the high temperature zone and the medium temperature zone, set up a second material transfer system in the medium temperature zone to connect to the external mixing vessel, and set up a third material transfer system between the medium temperature zone and the low temperature zone;

[0009] Step 3: Powdered coal and supercritical water are mixed in a high-pressure reactor at a mass ratio of 1:5-1:10, and then transported to a high-temperature zone by a high-pressure pump to generate quantum carbon dot precursors.

[0010] Step 4: The quantum carbon dot precursor is transported to the intermediate temperature zone through the first material transfer system. Ammonia gas is introduced into the intermediate temperature zone for purging for 10-15 minutes. A 5%-10wt% boric acid solution is sprayed into the intermediate temperature zone at a volume ratio of quantum carbon dot precursor to boric acid solution of 10:1-15:1 and reacted for 20-30 minutes to modify the surface of the quantum carbon dot precursor and form a quantum carbon dot solution.

[0011] Step 5: The quantum carbon dot solution is transported to the external mixing vessel through the second material transfer system. Urea melt is added at a mass ratio of 1:20 to 1:30 to the quantum carbon dot solution and mixed and stirred to form a double mixture.

[0012] Step Six: The mixed liquid is transported to the low-temperature zone through the third material transfer system, atomized through a dual-fluid nozzle, and then cooled and solidified to form compound fertilizer granules with a particle size of 1-3mm.

[0013] Step 7: The compound fertilizer granules are fed into a fluidized bed coating machine and coated with an oxidized sodium alginate / polyethylene glycol composite membrane solution. After drying and cooling, the finished coal-based quantum carbon dot slow-release fertilizer is obtained.

[0014] Further improvements are made in the following aspects: In step three, the coal powder particle size is 50-100 mesh, the fixed carbon content is ≥70%, the supercritical water temperature is 380-400℃, the pressure is 22.1-25MPa, and when the coal powder and supercritical water are mixed in a high-pressure reactor and then transported by a high-pressure pump, the flow rate of the high-pressure pump is 5-10L / h. When the mixture of coal powder and supercritical water reacts in the high-temperature zone, the reaction time is 30-60min.

[0015] A further improvement is that the process of generating the quantum carbon dot precursor in step three also includes real-time monitoring of the particle size of the generated quantum carbon dot precursor using an online particle size analyzer. When the particle size of the quantum carbon dot precursor is greater than 10 nm, the temperature of the high-temperature zone is increased by 10 °C for every 1 nm increase in the particle size of the quantum carbon dot precursor. When the particle size of the quantum carbon dot precursor is less than 5 nm, the temperature of the high-temperature zone is decreased by 10 °C for every 1 nm decrease in the particle size of the quantum carbon dot precursor.

[0016] A further improvement is that the molar ratio of the carbon dot precursor to ammonia in step four is 1:2-1:3.

[0017] A further improvement is that, in step four, when the boric acid solution modifies the surface of the quantum carbon dot precursor, the reaction process also includes real-time monitoring of the solution pH value using a pH meter. When pH < 8, the ammonia gas flow rate is increased; when pH > 9, the ammonia gas flow rate is decreased to maintain the solution pH = 8-9.

[0018] A further improvement is that the temperature of the urea melt in step five is 130-140℃, and the stirring speed is 300-500 r / min when the quantum carbon dot solution is mixed with the urea melt.

[0019] A further improvement is that the compressed air pressure during atomization by the dual-fluid nozzle in step six is ​​0.4-0.6 MPa.

[0020] A further improvement is that: in step seven, the sodium alginate / polyethylene glycol composite membrane solution is formed by combining sodium alginate with a concentration of 2%-5wt% and polyethylene glycol with a molecular weight of 4000-6000 and a concentration of 1%-3wt% in a volume ratio of 2:1-3:1.

[0021] A further improvement is that the coating amount of the sodium alginate / polyethylene glycol composite film solution in step seven is 5%-8% of the mass of the compound fertilizer particles.

[0022] A further improvement is made in the following: In step seven, after the compound fertilizer granules are fed into the fluidized bed coating machine, the temperature of the fluidized bed coating machine is 60-70℃, the wind speed is 1-2m / s, the coating time is 15-20min, the drying temperature is 50-60℃, and the drying time is 10-15min.

[0023] The beneficial effects of this invention are as follows: By setting high-temperature zone, medium-temperature zone and low-temperature zone from bottom to top inside the evaporation granulation tower, this invention integrates the three core processes of quantum carbon dot synthesis, surface modification and fertilizer compounding into the same equipment, increasing the material utilization rate to over 95%, avoiding the destruction of quantum carbon dot structure caused by cross-equipment operation, ensuring product stability, significantly increasing the yield of quantum carbon dots, and significantly enhancing the photocatalytic activity of quantum carbon dots. Introducing supercritical water during the preparation of quantum carbon dot precursors can accelerate the carbonization reaction of coal powder, significantly improving efficiency.

[0024] This invention modifies the surface of quantum carbon dots by "nitrogen / boron dual doping" in the medium temperature range. Ammonia (gaseous) and boric acid solution (liquid) work synergistically to form a "gas-liquid two-phase doping system". The uniformity of dopant element distribution can be significantly improved, and boron can enhance the photocatalytic activity of quantum carbon dots, while nitrogen can improve their compatibility with fertilizers.

[0025] This invention applies coal-based quantum carbon dots to slow-release fertilizers. Under natural light irradiation, the quantum carbon dots generate photogenerated carriers, which can regulate the dissolution rate of nutrients inside fertilizer particles, achieving "light-controlled slow release". Compared with fertilizers without quantum carbon dots, the nutrient release cycle is extended by 2-3 times.

[0026] This invention coats compound fertilizer granules with a sodium alginate / polyethylene glycol composite membrane solution. The sodium alginate / polyethylene glycol composite membrane solution has good air permeability and swelling properties, which can prevent the fertilizer granules from disintegrating too quickly while allowing nutrients to penetrate slowly. Moreover, the composite membrane is biodegradable, avoiding secondary pollution. The coal-based quantum carbon dot slow-release fertilizer of this invention can adapt to different climatic conditions and can stably release nutrients in both arid and rainy environments. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the method flow 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

[0030] according to Figure 1 As shown in the figure, this embodiment proposes a method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient controlled evaporation granulation tower, including the following steps:

[0031] Step 1: Inside the evaporation granulation tower, a high-temperature zone, a medium-temperature zone, and a low-temperature zone are set from bottom to top. The temperature of the high-temperature zone is 350℃, the temperature of the medium-temperature zone is 200℃, and the temperature of the low-temperature zone is 60℃.

[0032] Step 2: Set up a first material transfer system between the high-temperature zone and the medium-temperature zone, set up a second material transfer system in the medium-temperature zone to connect to the external mixing vessel, and set up a third material transfer system between the medium-temperature zone and the low-temperature zone; the first material transfer system maintains a temperature of 200-250℃, the second material transfer system maintains a temperature of 150-200℃, and the third material transfer system maintains a temperature of 120-130℃. The temperature preservation method can be reasonably selected and set according to actual needs.

[0033] Step 3: Coal powder and supercritical water are mixed in a high-pressure reactor at a mass ratio of 1:8. The mixture is then pumped to a high-temperature zone for reaction to generate quantum carbon dot precursors. The coal powder has a particle size of 80 mesh and a fixed carbon content of ≥70%. The temperature is 380-400℃, and the pressure is 22.1-25MPa. This supercritical state is maintained until the reaction is complete. When the coal powder and supercritical water are mixed in the high-pressure reactor and pumped through the high-pressure pump, the pump flow rate is 6L / h. The reaction time of the coal powder and supercritical water mixture in the high-temperature zone is 50min. During the generation of quantum carbon dot precursors, the particle size of the generated quantum carbon dot precursors is monitored in real time using an online particle size analyzer. When the particle size of the quantum carbon dot precursors is greater than 10nm, the temperature of the high-temperature zone is increased by 10℃ for every 1nm increase in the particle size. When the particle size of the quantum carbon dot precursors is less than 5nm, the temperature of the high-temperature zone is decreased by 10℃ for every 1nm decrease in the particle size.

[0034] Step 4: The quantum carbon dot precursor is transported to the intermediate temperature zone via the first material transfer system. Ammonia gas is introduced into the intermediate temperature zone for purging for 15 minutes. The molar ratio of the carbon dot precursor to ammonia gas is 1:2. In this invention, ammonia gas... It is a nitrogen source, and its core function is to provide nitrogen atoms that can participate in doping for the quantum carbon dot precursor. An 8wt% boric acid solution is sprayed into the intermediate temperature zone at a volume ratio of quantum carbon dot precursor to boric acid solution of 10:1 and reacted for 20 minutes to modify the surface of the quantum carbon dot precursor. Sodium borohydride is then used. It is the source of boron element, used to provide boron atoms that can participate in doping. During the reaction, the pH value of the solution is monitored in real time by a pH meter. When pH < 8, the amount of ammonia gas introduced is increased; when pH > 9, the amount of ammonia gas introduced is reduced to maintain the solution pH = 8-9, thus forming a quantum carbon dot solution.

[0035] Step 5: The quantum carbon dot solution is transported to the external mixing vessel through the second material transfer system. Urea melt is added at a mass ratio of 1:25 (quantum carbon dot solution to urea melt (temperature 130℃)) and mixed and stirred at a stirring speed of 400 r / min to form a double mixture.

[0036] Step Six: The mixed liquid is transported to the low-temperature zone through the third material transfer system and atomized through a dual-fluid nozzle. The compressed air pressure during atomization is 0.5MPa. After cooling and solidification, it forms compound fertilizer granules with a particle size of 2mm.

[0037] Step 7: The compound fertilizer granules are fed into a fluidized bed coating machine at a temperature of 65℃ and a wind speed of 2m / s. A sodium alginate / polyethylene glycol composite film solution (formed by combining 2%-5wt% sodium alginate and 2wt% polyethylene glycol with a molecular weight of 5000 in a 2:1 volume ratio) is sprayed in for coating. After drying and cooling, the coal-based quantum carbon dot slow-release fertilizer product is obtained. The coating amount of the sodium alginate / polyethylene glycol composite film solution is equal to the weight of the compound fertilizer granules. The coating amount is 5%, the coating time is 15 min, the drying temperature is 55℃, and the drying time is 10-15 min. In this invention, the coating amount of sodium alginate / polyethylene glycol composite membrane solution is controlled by weighing method, that is: a dynamic weighing sensor is set at the inlet of the fluidized bed coating machine to monitor the total mass of particles entering the coating machine in real time; another dynamic weighing sensor is set at the outlet to monitor the total mass of particles after coating. The difference between the two is the coating amount. The coating amount is stabilized at 5%-8% by adjusting the spray flow rate of the composite membrane solution (0.5-1.0 L / min).

[0038] Verification Example

[0039] The coal-based quantum carbon dot slow-release fertilizer prepared by the method of the present invention is marked as an experimental example;

[0040] Comparative Example 1: Traditional resin-coated urea (commercially available, coating material is polyethylene, without quantum carbon dots).

[0041] Comparative Example 2: Slow-release fertilizer without quantum carbon dots (process is the same as in the example, but steps three to five are omitted, and the molten urea is directly atomized, granulated, and coated).

[0042] Comparative Example 3: Single nitrogen-doped quantum carbon dot slow-release fertilizer (process is the same as in the example, but boric acid solution is omitted, and only ammonia is used for nitrogen doping).

[0043] The verification results are shown in the table below:

[0044]

[0045] As shown in the table above, the nitrogen nutrient utilization rate (75.2%) of the coal-based quantum carbon dot slow-release fertilizer prepared by this invention is significantly higher than that of Comparative Example 1 (42.5%), Comparative Example 2 (58.3%) and Comparative Example 3 (65.1%), and the release period (82 days) is longer, indicating that the dual slow-release mechanism of "nitrogen / boron dual-doped quantum carbon dots + composite film" is effective.

[0046] The composite membrane degradation rate (88.5%) of the coal-based quantum carbon dot slow-release fertilizer prepared by this invention is much higher than that of Comparative Example 1 (15.3%), and the ammonia emission (35.2 mg / m³) is lower than that of all comparative examples, which meets the requirements of green agriculture.

[0047] This invention integrates the three core processes of quantum carbon dot synthesis, surface modification, and fertilizer compounding into a single device by setting high-temperature, medium-temperature, and low-temperature zones from bottom to top inside the evaporation granulation tower. This increases material utilization to over 95% and avoids quantum carbon dot structural damage caused by cross-device operations, ensuring product stability and significantly improving quantum carbon dot yield. Furthermore, the photocatalytic activity of the quantum carbon dots is significantly enhanced. Introducing supercritical water during the preparation of the quantum carbon dot precursor accelerates the coal powder carbonization reaction, significantly improving efficiency. In the medium-temperature zone, this invention performs "nitrogen / boron dual-doping" surface modification on the quantum carbon dot surface. The synergistic effect of ammonia (gaseous) and boric acid solution (liquid) forms a "gas-liquid two-phase doping system," significantly improving the uniformity of dopant element distribution. Additionally, boron enhances the photocatalytic activity of the quantum carbon dots. Nitrogen can enhance its compatibility with fertilizers. This invention applies coal-based quantum carbon dots to slow-release fertilizers. Under natural light, the quantum carbon dots generate photogenerated carriers, which can regulate the dissolution rate of nutrients inside the fertilizer particles, achieving "light-controlled slow release." Compared with fertilizers without quantum carbon dots (nutrient release cycle of 15-20 days), the nitrogen nutrient release cycle of the fertilizer of this invention is extended by 2-3 times. This invention coats the compound fertilizer particles with a sodium alginate / polyethylene glycol composite membrane solution. The sodium alginate / polyethylene glycol composite membrane solution has good air permeability and swelling properties, which can prevent the fertilizer particles from disintegrating too quickly while allowing nutrients to penetrate slowly. Moreover, the composite membrane is biodegradable, avoiding secondary pollution. The coal-based quantum carbon dot slow-release fertilizer of this invention can adapt to different climatic conditions and can stably release nutrients in both drought and rainy environments.

[0048] 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 preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower, characterized in that, Includes the following steps: Step 1: Set up a high-temperature zone, a medium-temperature zone, and a low-temperature zone inside the evaporation granulation tower from bottom to top. The temperature of the high-temperature zone is 300-400℃, the temperature of the medium-temperature zone is 150-250℃, and the temperature of the low-temperature zone is 50-100℃. Step 2: Set up a first material transfer system between the high temperature zone and the medium temperature zone, set up a second material transfer system in the medium temperature zone to connect to the external mixing vessel, and set up a third material transfer system between the medium temperature zone and the low temperature zone; Step 3: Powdered coal and supercritical water are mixed in a high-pressure reactor at a mass ratio of 1:5-1:10, and then transported to a high-temperature zone by a high-pressure pump to generate quantum carbon dot precursors. Step 4: The quantum carbon dot precursor is transported to the intermediate temperature zone through the first material transfer system. Ammonia gas is introduced into the intermediate temperature zone for purging for 10-15 minutes. A 5%-10wt% boric acid solution is sprayed into the intermediate temperature zone at a volume ratio of quantum carbon dot precursor to boric acid solution of 10:1-15:1 and reacted for 20-30 minutes to modify the surface of the quantum carbon dot precursor and form a quantum carbon dot solution. Step 5: The quantum carbon dot solution is transported to the external mixing vessel through the second material transfer system. Urea melt is added at a mass ratio of 1:20 to 1:30 to the quantum carbon dot solution and mixed and stirred to form a double mixture. Step Six: The mixed liquid is transported to the low-temperature zone through the third material transfer system, atomized through a dual-fluid nozzle, and then cooled and solidified to form compound fertilizer granules with a particle size of 1-3mm. Step 7: The compound fertilizer granules are fed into a fluidized bed coating machine and coated with an oxidized sodium alginate / polyethylene glycol composite membrane solution. After drying and cooling, the finished coal-based quantum carbon dot slow-release fertilizer is obtained.

2. The method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower according to claim 1, characterized in that: In step three, the coal powder has a particle size of 50-100 mesh and a fixed carbon content of ≥70%. The supercritical water temperature is 380-400℃ and the pressure is 22.1-25MPa. When the coal powder and supercritical water are mixed in a high-pressure reactor and then transported by a high-pressure pump, the flow rate of the high-pressure pump is 5-10L / h. When the mixture of coal powder and supercritical water reacts in the high-temperature zone, the reaction time is 30-60min.

3. The method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower according to claim 2, characterized in that: The process of generating the quantum carbon dot precursor in step three also includes real-time monitoring of the particle size of the generated quantum carbon dot precursor using an online particle size analyzer. When the particle size of the quantum carbon dot precursor is greater than 10 nm, the temperature of the high-temperature zone is increased by 10 °C for every 1 nm increase in the particle size of the quantum carbon dot precursor. When the particle size of the quantum carbon dot precursor is less than 5 nm, the temperature of the high-temperature zone is decreased by 10 °C for every 1 nm decrease in the particle size of the quantum carbon dot precursor.

4. The method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower according to claim 1, characterized in that: In step four, the molar ratio of carbon dot precursor to ammonia is 1:2 to 1:

3.

5. The method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower according to claim 4, characterized in that: In step four, when the boric acid solution modifies the surface of the quantum carbon dot precursor, the reaction process also includes real-time monitoring of the solution pH value using a pH meter. When pH < 8, the ammonia gas flow rate is increased; when pH > 9, the ammonia gas flow rate is decreased to maintain the solution pH = 8-9.

6. The method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower according to claim 1, characterized in that: In step five, the temperature of the urea melt is 130-140℃, and the stirring speed is 300-500 r / min when the quantum carbon dot solution is mixed with the urea melt.

7. The method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower according to claim 1, characterized in that: In step six, the compressed air pressure during atomization by the dual-fluid nozzle is 0.4-0.6 MPa.

8. The method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower according to claim 1, characterized in that: In step seven, the sodium alginate / polyethylene glycol composite membrane solution is formed by combining sodium alginate with a concentration of 2%-5wt% and polyethylene glycol with a molecular weight of 4000-6000 and a concentration of 1%-3wt% in a volume ratio of 2:1-3:

1.

9. The method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower according to claim 8, characterized in that: In step seven, the coating amount of the sodium alginate / polyethylene glycol composite film solution is 5%-8% of the mass of the compound fertilizer particles.

10. The method for preparing coal-based quantum carbon dot slow-release fertilizer using a temperature gradient-controlled evaporation granulation tower according to claim 9, characterized in that: In step seven, after the compound fertilizer granules are fed into the fluidized bed coating machine, the temperature of the fluidized bed coating machine is 60-70℃, the wind speed is 1-2m / s, the coating time is 15-20min, the drying temperature is 50-60℃, and the drying time is 10-15min.

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