A mixed reactor, a preparation system and a preparation method of hexagonal boron nitride

Abstract

This invention discloses a mixing reactor, a preparation system for hexagonal boron nitride, and a preparation method, belonging to the field of boron nitride preparation. The mixing reactor includes a tank, inside which is a stirring shaft, comprising a first shaft and a second shaft that rotate coaxially and relatively independently. A double-ribbon stirrer and a dispersion disk are installed inside the tank; the dispersion disk is located at the bottom of the tank, the double-ribbon stirrer is fixed on the first shaft, and the dispersion disk is fixed on the second shaft. The top of the tank is connected to a vacuum pump through a vacuum extraction port. The top of the tank is also connected to a gas source through a gas inlet and has a gas outlet. A heating device is installed on the outer wall of the tank. The tank and the stirring shaft are connected by a sealing device. The first and second shafts are respectively connected to a drive device. This invention solves the problems in existing hexagonal boron nitride preparation methods, such as the need to complete raw material mixing, dehydration, and low-temperature reaction in different equipment, resulting in high material turnover, low efficiency, and high energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of boron nitride preparation, specifically relating to a mixing reactor, a preparation system for hexagonal boron nitride, and a preparation method. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Hexagonal boron nitride (h-BN) is an important high-performance ceramic material with a graphite-like layered structure. It has excellent thermal conductivity, electrical insulation, high-temperature stability, self-lubrication, and chemical inertness, and is widely used in thermally conductive fillers, high-temperature lubricants, mold release agents, cosmetics, and aerospace.

[0004] Currently, the large-scale industrial production of hexagonal boron nitride powder often employs the carbothermic reduction method, which commonly uses borax ( Hexagonal boron nitride is prepared by using boron as the boron source and urea or melamine as the nitrogen source through a high-temperature nitriding reaction. The production process is as follows: borax is mixed with urea or melamine in a certain proportion, ground and mixed in a ball mill, and then placed in the same calcining furnace for sequential low-temperature calcination and high-temperature calcination under an ammonia (or nitrogen) atmosphere. Following post-processing steps such as acid washing, water washing, drying, and pulverization, hexagonal boron nitride is obtained. This preparation method uses widely available and inexpensive raw materials, making it suitable for large-scale production.

[0005] However, in actual production, the process has the following problems: (1) The raw materials are difficult to mix evenly, and high-temperature sintering causes incomplete reaction, reduced product purity, reduced yield and poor batch consistency; (2) Borax crystal water is difficult to remove, and crystal water vaporizes rapidly during roasting and heating, which can easily cause material splashing and create safety hazards; (3) Urea will sublimate and decompose during low-temperature roasting, and will be carried away by the protective gas flow before participating in the reaction, resulting in nitrogen source waste (loss rate can reach 20-50%); (4) Agglomerated intermediates are easily generated during low-temperature roasting, which leads to incomplete reaction in subsequent high-temperature roasting.

[0006] To address the aforementioned issues, existing technologies often solve specific problems individually, such as: improving mixing uniformity by extending the ball milling time in the mixer or using liquid-phase mixing; removing borax crystal water by adding independent drying equipment before and after mixing; reducing nitrogen source loss by using sealed crucibles; and reducing urea sublimation loss and preventing intermediate formation by rapidly transitioning from a low-temperature calcination stage to a high-temperature calcination stage.

[0007] However, these solutions require the mixing, dehydration, and low-temperature reaction of raw materials to be completed in different equipment, resulting in high material turnover, low efficiency, high energy consumption, easy introduction of impurities, and limited improvement in actual effect. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a mixing reactor, a preparation system and method for hexagonal boron nitride, which dehydrates, dries, mixes and pre-reacts the raw materials in a single chamber, thus integrating a process that traditionally takes place in multiple devices into one apparatus.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] In a first aspect, a mixing reactor includes:

[0011] The tank body contains a stirring shaft, which includes a first shaft and a second shaft that are coaxially arranged and rotate relatively independently.

[0012] The tank is equipped with a double helical ribbon agitator and a dispersion disc. The dispersion disc is located at the bottom of the tank. The double helical ribbon agitator is fixed on the first shaft, and the dispersion disc is fixed on the second shaft.

[0013] The top of the tank is connected to a vacuum pump via a vacuum extraction port.

[0014] The top of the tank is also connected to a gas source via a gas inlet and has a gas outlet;

[0015] The outer wall of the tank is equipped with a heating device;

[0016] The tank body is connected to the stirring shaft via a sealing device; the first shaft and the second shaft are respectively connected to the drive device.

[0017] The mixing reactor of this invention creatively prepositions the precursor generation stage in the boron nitride preparation process, completing it in the same container as mixing and dehydration. Designed to address the multiphase changes of the materials processed during precursor preparation (solid powder in vacuum dehydration stage – liquid-solid slurry in melt wetting stage – high-viscosity semi-solid phase in high-shear pre-reaction stage – loose solid particles after cooling), it employs a coaxial, variable-velocity dual-stirring structure and staged temperature control. Macroscopic mixing and overall circulation are achieved through a dual-ribbon stirrer, while local dispersion is achieved through a dispersion disc. Combining vacuum dehydration, protective atmosphere melt wetting, and pre-reaction, mixing, dehydration, coating, pre-reaction, and cooling are continuously completed within the same reactor, resulting in a loosely granular core-shell structured precursor that can be directly used for high-temperature calcination without multiple transfers, and avoids precursor agglomeration leading to incomplete reaction.

[0018] In some embodiments, the twin-ribbon agitator includes two spiral ribbons rotating in opposite directions, one inner and one outer; the dispersion disc includes multiple layers of discs, with staggered dispersion teeth on the outer periphery of the discs.

[0019] In some implementations, the upper and lower layers of the dispersion disc are fixedly connected, and the dispersion teeth between adjacent layers are staggered to prevent materials from directly short-circuiting along the axial direction.

[0020] In some implementations, the dual ribbon agitator rotates at 20-80 rpm, and the dispersing disc rotates at 1000-6000 rpm.

[0021] In some implementations, the gas source is a nitrogen source or an ammonia source, and the gas outlet is connected to the exhaust gas treatment system.

[0022] In some implementations, the vacuum extraction port, gas inlet, and gas outlet are connected to the tank body via valves, and the tank body is connected to the stirring shaft via sealed bearings.

[0023] In some embodiments, the tank has a feed inlet at the top and a discharge outlet at the bottom, and both the feed inlet and discharge outlet are equipped with sealing structures.

[0024] In some embodiments, the heating device includes a jacket that covers the cylindrical portion of the tank and the bottom of the tank. The bottom of the jacket is provided with a heat transfer oil inlet, and the top is provided with a heat transfer oil outlet. A heater is connected between the heat transfer oil inlet and the heat transfer oil outlet.

[0025] In some implementations, a cooling device is also provided, which is connected in parallel with the heater via a three-way valve.

[0026] In some embodiments, a reaction endpoint determination device is also included, which includes a torque sensor disposed on the output shaft of the first drive device for determining the reaction endpoint.

[0027] In some embodiments, the inner wall of the tank and the surface of the twin-ribbon agitator are provided with an anti-stick coating, and the tips of the dispersion discs can be nitrided and hardened.

[0028] In some implementations, a temperature detection device is installed inside the tank.

[0029] In some embodiments, a controller is also included, which is electrically connected to the first drive unit, the second drive unit, the vacuum pump, the heating unit, the mass / flow controller, and each sensor.

[0030] Secondly, a preparation system for hexagonal boron nitride includes: the aforementioned mixing reactor, sieving device, and calcining furnace, wherein the mixing reactor and sieving device, and the sieving device and calcining furnace are all connected by a conveying device.

[0031] Thirdly, the present invention provides a method for preparing hexagonal boron nitride using the above-mentioned mixing reactor, comprising the following steps:

[0032] S1: Add the predetermined proportion of borax and urea into the mixing reactor, stir, and heat to 80-120℃ under negative pressure to remove the water of crystallization in the borax, so that the borax particles and urea are evenly mixed.

[0033] A protective atmosphere is introduced, and the mixture is heated to 180-220°C. The mixture is stirred and dispersed to melt the urea and react it with borax. When the borax reaction reaches 50-70%, the heating is stopped, and the mixture is cooled to obtain mixed particles containing borax, polyboronic acid ester and urea.

[0034] S2: Remove the above mixed particles, sieve them, and sinter them under a protective atmosphere to obtain crude boron nitride product.

[0035] This invention controls the reaction degree of borax to 50-70%, ensuring a sufficient number of "borax grinding balls" while preventing excessive cross-linking of the intermediate (polyboronate); the remaining unreacted borax can also serve as a flux or active center in subsequent calcination to promote the growth and crystallization of hexagonal boron nitride crystals.

[0036] In some embodiments, the crude boron nitride product is acid-washed, water-washed, and dried to obtain hexagonal boron nitride.

[0037] In some embodiments, in S2, the sintering temperature in the baking furnace is 1500-2100°C; preferably 1500-1800°C. By first preparing the precursor and then sintering to form boron nitride, the sintering temperature can be reduced by 100-200°C.

[0038] In some embodiments, in S1, the mixture is continuously stirred, a protective atmosphere is introduced, and the temperature is raised to 130-160°C to melt the urea and coat the borax particles. The temperature is then raised to 180-220°C to melt the urea and react it with the borax.

[0039] In some implementations, the molar ratio of borax to urea is 1:2 to 1:6.

[0040] The beneficial effects of this invention are as follows:

[0041] 1. This invention integrates three independent processes—raw material mixing, dehydration, and pre-reaction—into a single device, simplifying the process flow. Materials do not require intermediate handling, avoiding the risks of moisture absorption and impurity contamination. It also reduces labor and equipment footprint, shortening the production cycle. Utilizing the high-speed shearing of the dispersion disc and the solid-phase assisted dispersion of unreacted borax significantly inhibits the agglomeration of polyboronate intermediates, improving reaction uniformity and raw material utilization. This provides loose, controllable core-shell structured precursor particles for subsequent high-temperature calcination, reducing the required calcination temperature by 100-200°C and shortening the holding time by 20-30%, thus reducing energy consumption for h-BN production and improving the quality of the sintered product.

[0042] 2. This invention, by independently controlling the rotation speed of the two agitators, can flexibly adapt to the shear force requirements of different process stages, providing a structural basis for subsequent staged control. Compared with traditional devices, the device of this invention significantly improves mixing uniformity, raw material utilization, product purity, and process integration.

[0043] 3. This invention simultaneously performs negative pressure dehydration and stirring grinding in a sealed container. Meanwhile, molten urea uniformly coats borax particles under high-speed shearing, achieving near-molecular-level mixing and avoiding the problems of uneven mixing and incomplete local reactions inherent in traditional ball milling. The ten water molecules of borax are gently vaporized and discharged under controlled conditions, avoiding material splashing, furnace contamination, and safety accidents caused by the instantaneous vaporization of water molecules of borax in a high-temperature calcination furnace, as is common in traditional processes. Staged heating within a sealed cavity rapidly passes through the urea sublimation temperature range, improving urea utilization and reducing raw material costs.

[0044] 4. This invention determines the reaction endpoint in real time by monitoring the stirring power, ensuring that a sufficient amount of unreacted borax solid particles are always retained in the system. Driven by a high-shear dispersion disk, the high-speed moving solid particles collide with each other as micro-grinding media, continuously breaking down the viscous polyboronate intermediate and preventing it from agglomerating into clumps, thus avoiding incomplete sintering and poor boron nitride quality. Attached Figure Description

[0045] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0046] Figure 1 This is a schematic diagram of the structure of the mixing reactor according to an embodiment of the present invention.

[0047] Figure 2 This is a partial structural schematic diagram of the mixing reactor according to an embodiment of the present invention.

[0048] Figure 3 This is a partial structural diagram of the disk platter according to an embodiment of the present invention.

[0049] The components include: 1. Tank; 2. Stirring shaft; 21. First shaft; 22. Second shaft; 23. Angular contact ball bearing; 3. Double ribbon stirrer; 31. Outer ribbon; 32. Inner ribbon; 33. Connecting rod; 34. Scraper; 4. Dispersion disc; 41. Disc; 42. Dispersion teeth; 5. Heating device; 51. Jacket; 52. Heat transfer oil inlet; 53. Heat transfer oil outlet; 54. Heater; 55. Plate heat exchanger; 56. Three-way valve; 57. Circulating pump; 6. Vacuum extraction port; 7. Mass / flow controller; 8. Gas inlet; 81. Gas diffusion nozzle; 9. Gas outlet; 10. Vacuum pump; 11. Feed inlet; 12. Discharge outlet; 13. Sealing device; 14. First drive device; 15. Second drive device; 16. Gas source; 17. Shut-off valve; 18. Trace water analyzer. Detailed Implementation

[0050] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0051] Existing equipment suffers from problems such as uneven raw material mixing, insufficient drying of borax crystal water, and thermal decomposition of urea during boron nitride preparation, resulting in energy waste, nitrogen waste, and poor boron nitride quality. This invention addresses the multiphase changes of solid-liquid-solid during precursor preparation by completing mixing, drying, and pre-reaction in a single reactor, thoroughly simplifying and restructuring the traditional process. One core aspect of this invention is providing a mixing reactor; another core aspect is providing a system and method for preparing hexagonal boron nitride.

[0052] An embodiment of the present invention provides a mixing reactor, comprising:

[0053] The tank body 1 contains a stirring shaft 2, which includes a first shaft 21 and a second shaft 22 that are coaxially arranged and can rotate relatively independently. A double-ribbon stirrer 3 and a dispersing disc 4 are installed inside the tank body 1. The dispersing disc 4 is located at the bottom of the tank body 1. The double-ribbon stirrer 3 is fixed to the first shaft 21, and the dispersing disc 4 is fixed to the second shaft 22. The top of the tank body 1 is connected to a vacuum pump 10 via a vacuum extraction port 6. The top of the tank body 1 is also connected to a gas source 16 via a gas inlet 8 and has a gas outlet 9. A heating device 5 is installed on the outer wall of the tank body 1. The tank body 1 and the stirring shaft 2 are connected via a sealing device 13. The first shaft 21 and the second shaft 22 are respectively connected to a driving device.

[0054] This invention features a dual-ribbon structure and an independently driven, rotating dispersion disc 4. The low-speed zone focuses on mixing, while the high-speed zone focuses on dispersion, achieving a synergistic effect of "low-shear macro-mixing + high-shear micro-dispersion." Negative pressure assists in low-temperature dehydration (80-120℃, gauge pressure -0.08 to -0.095 MPa), causing the borax crystal water to be gently vaporized and extracted by a vacuum pump. Simultaneously, the dual-ribbon stirrer 3 achieves overall mixing, facilitating the release of water vapor. The dispersion disc 4 disperses and breaks up settled, agglomerated borax under high shear, accelerating dehydration and mixing. The heating device 5 heats the material for borax drying and dehydration. After losing its crystal water, the borax particles become loose and brittle, easily refined by the grinding mechanism (dispersion disc), accelerating the mixing process. The mixing and heating processes mutually promote each other, shortening the material processing time. Molten urea uniformly coats the borax particles under high-speed shear, achieving near-molecular-level mixing and avoiding the problems of uneven mixing and incomplete local reactions inherent in traditional ball milling. The temperature is increased to carry out the pre-reaction. The twin ribbon stirrer 3 and the dispersion disk 4 continue to rotate. Driven by the dispersion disk 4, the unreacted borax solid particles (a sufficient amount of unreacted borax solid particles are always maintained in the system) collide with each other as "micro-grinding media" to continuously disperse the viscous polyborate intermediate, prevent it from sticking together, and at the same time continuously destroy the polyborate shell that is forming to prevent it from becoming a barrier to diffusion and reaction.

[0055] This invention involves staged heating within a sealed cavity to rapidly pass through the urea sublimation temperature range, significantly reducing urea sublimation loss, improving nitrogen source utilization, and obtaining uniformly sized, non-adhesive solid particles. These particles are then subjected to high-temperature calcination to improve the quality of boron nitride.

[0056] In some embodiments, the top (upper end) of the tank body 1 is provided with a vacuum extraction port 6, a gas inlet 8 and a gas outlet 9; the vacuum extraction port 6 is connected to the vacuum pump 10 through a shut-off valve 17; the gas inlet 8 and the gas outlet 9 are respectively connected to the tank body 1 through the shut-off valve 17.

[0057] A gas quality / flow controller 7 is also installed upstream of the gas inlet 8 and connected to the gas source 16 to monitor and control the instantaneous flow rate of nitrogen or ammonia.

[0058] Preferably, the gas source 16 is a nitrogen gas source or an ammonia gas source, the gas inlet 8 is connected to the gas diffusion nozzle 81, and the nozzle of the gas diffusion nozzle 81 faces downward; the gas outlet 9 is equipped with a trace water analyzer 18, and when the measured water concentration is lower than the set value (e.g., ≤50 ppmV), the dehydration can be determined to be complete; the vacuum pump port 6 and the gas outlet 9 are connected to the exhaust gas treatment system.

[0059] In some embodiments, the rotation speed of the twin ribbon stirrer 3 is 20-80 rpm, and the rotation speed of the dispersing disk 4 is 1000-6000 rpm. The low-speed twin ribbon drives the overall circulation, while the high-speed dispersing disk 4 generates an extreme velocity gradient, tearing the viscous fluid into tiny droplets and dispersing the intermediate into small particles.

[0060] In some embodiments, the first shaft 21 and the second shaft 22 are configured as an upper and lower shaft structure, connected by a bearing. The first shaft 21 is positioned above the second shaft 22, and the two are connected by an angular contact ball bearing 23, suitable for bearing combined loads. This structure is simple, has fewer failures, and is easy to maintain.

[0061] The ends of the first shaft 21 and the second shaft 22 that are away from the tank 1 are respectively connected to the first drive device 14 and the second drive device 15, and the specific connection method is the prior art.

[0062] The connection and driving methods of the first shaft 21 and the second shaft 22 allow the twin ribbon stirrer 3 and the dispersion disk 4 to be driven by independent motors, running at different speeds and in different directions, so that the mixing / dispersion intensity can be switched at different stages, and the process window is wider.

[0063] In some embodiments, the tank body 1 and the stirring shaft 2 are connected by a sealed bearing. Specifically, the first shaft 21 is connected to the top of the tank body 1 by a sealed bearing, and the second shaft 22 is connected to the bottom of the tank body 1 by a sealed bearing. The sealing device 13 is equipped with a cooling and flushing device to extend the service life of the seal; this is prior art.

[0064] In some embodiments, the dispersion disk 4 includes multiple layers of disks 41, and the outer periphery of the disks 41 is provided with staggered dispersion teeth 42; preferably, the upper and lower layers of disks 41 of the dispersion disk 4 are fixedly connected, and the dispersion teeth 42 between adjacent layers of disks 41 are staggered to avoid the material from directly short-circuiting along the axial direction.

[0065] In some embodiments, the double-ribbon stirrer 3 includes two spiral ribbons, with the outer spiral ribbon 31 and the inner spiral ribbon 32 rotating in opposite directions.

[0066] The twin-ribbon agitator 3 generates strong axial convection and radial shear, causing the material to flow in three dimensions. This enables uniform mixing of both the solid before heating and the fluid after heating, forming the physical basis for achieving macroscopic mixing uniformity. When the multi-layer dispersion disk 4 rotates at high speed, the dispersion teeth 42 generate local extreme velocity gradients, shearing and breaking up agglomerates between the teeth. The teeth are staggered upwards and downwards and separated between layers, preventing direct axial short-circuiting of the material and extending the high-shear action time.

[0067] In some embodiments, the double-ribbon agitator 3 is configured with equal upper and lower diameters to ensure complete axial circulation of materials. Simultaneously, a scraper 34 is fixed to the outer edge of the outer ribbon to prevent material from accumulating on the wall and forming a "dead zone." The scrapers 34 are spaced apart along the spiral direction, ensuring axial flow of materials while scraping the wall. The cooperation between the scraper and the double-ribbon agitator 3 ensures that all materials in the tank undergo approximately the same number of cycles and shearing, improving the consistency of mixing and reaction, product yield, and equipment operational stability. Specifically, the diameter of the outer ribbon 31 is 0.8-0.9 times the inner diameter of the tank 1, and the diameter of the inner ribbon 32 is 0.55-0.65 times the diameter of the outer ribbon 31. The inner and outer ribbons are fixedly connected to the first shaft 21 via a connecting rod 33.

[0068] In some embodiments, the dispersion disk 4 is a multi-layer toothed dispersion disk 4 with 2-5 layers of disks 41 fixed on the second shaft 22. Each layer of disks 41 is directly fixed on the second shaft 22. Each layer of disks 41 has a plurality of dispersion teeth 42 evenly distributed along the circumferential direction on its outer peripheral edge. The dispersion teeth 42 are rectangular or trapezoidal. The two adjacent dispersion teeth 42 face opposite directions, one is inclined upward and the other is inclined downward. The dispersion teeth 42 of the two adjacent layers of disks 41 are staggered in the circumferential direction.

[0069] In some embodiments, a temperature detection device is also provided inside the tank 1. Specifically, a temperature sensor is provided on the inner wall of the tank 1, and the temperature sensor is located in the middle of the tank 1 to detect the temperature inside the tank 1.

[0070] The outer wall of tank 1 is provided with a jacket 51, which covers the cylindrical part of tank 1 and the lower end cap (bottom of tank 1). The bottom of the jacket 51 is provided with a heat transfer oil inlet 52, and the top is provided with a heat transfer oil outlet 53. The heat transfer oil outlet 53 is provided with a temperature sensor. A heater 54, a circulation pump 57, and a control valve are connected between the heat transfer oil inlet 52 and the heat transfer oil outlet 53. A spiral guide plate is provided inside the jacket 51 along the circumferential direction to force the heat transfer oil to flow along the spiral line. A cooling device (such as a plate heat exchanger 55) is also provided, which is connected in parallel with the heater 54 through a three-way valve 56.

[0071] In some embodiments, the top of the tank body 1 is provided with a feed inlet 11 and a pressure sensor, and the bottom is provided with a discharge outlet 12. The discharge outlet 12 is located on one side of the second shaft 22. The feed inlet 11 and the discharge outlet 12 are respectively provided with sealing structures (such as sealing flanges). At the same time, the tank body 1 is designed as a whole to be sealed, so that the tank body 1 can work in the range of negative pressure to slightly positive pressure.

[0072] In some embodiments, the bottom of the tank 1 is also provided with a hydraulically controlled liftable scraper 34 to clean up residual materials during discharge. This structure is existing technology.

[0073] In some embodiments, all components in contact with the material (such as the inner wall of tank 1, stirring shaft 2, double ribbon agitator 3, dispersion disc 4, seals, etc.) are made of stainless steel with a mirror-polished surface. The inner wall of tank 1 and the surface of double ribbon agitator 3 may be coated with a polytetrafluoroethylene (PTFE) anti-stick coating, and the tips of the teeth of dispersion disc 4 may be nitrided and hardened.

[0074] In some embodiments, a reaction endpoint determination device is also included. The reaction endpoint determination device includes a torque sensor, which is disposed on the output shaft of the first drive device 14, for detecting the torque of the stirring shaft 2 in real time, calculating the actual shaft power consumed by stirring (i.e., the net energy consumption of the material), and determining the reaction endpoint in real time by monitoring the stirring power.

[0075] The trace water analyzer, temperature sensor, torque sensor, pressure sensor, etc. are all electrically connected to the display to show the detection values. The operator can manually control the switches and various control valves according to the readings to realize the operation of the mixing reactor.

[0076] In some embodiments, a controller is also provided, which is electrically connected to the first drive device 14, the second drive device 15, the vacuum pump 10, the heating device 5, the mass / flow controller 7, and each sensor. The controller can be a programmable logic controller. The connection relationship between the controller and the various structures is prior art.

[0077] The controller controls the start and stop of the vacuum pump 10, the power output of the heating device 5, the flow rate of nitrogen (ammonia) and the opening and closing of each valve according to the preset program; it sends speed commands to the first drive device 14 and the second drive device 15, and receives the torque feedback signal from the first drive device 14; when the torque is detected to drop from the peak value to a predetermined threshold, the reaction endpoint is determined and heating is automatically stopped.

[0078] An embodiment of the present invention provides a preparation system for hexagonal boron nitride, comprising: the above-mentioned mixing reactor, wherein the outlet of the mixing reactor is connected to the inlet of the sieving device via a closed conveying device; and the outlet of the sieving device is connected to a calcining furnace via a conveying device.

[0079] Specifically, the sieving device is a vibrating screen with a mesh size of 5-8, used to sieve the mixed particles discharged from the above-mentioned mixing reactor; the discharge port of the undersize material of the sieving device is connected to the feed port of the roasting furnace via a conveyor belt. The roasting furnace is a pusher-plate type high-temperature roasting furnace with a maximum operating temperature of 2200℃. Nitrogen or ammonia gas can be introduced into the furnace for protection, and the sieved mixed particles are roasted at high temperature.

[0080] In addition to the above-described mixing reactor and preparation system, this invention also provides a method for preparing hexagonal boron nitride, which can employ the above-described mixing reactor or preparation system; the method for preparing boron nitride includes the following steps:

[0081] (1) Add borax and urea with a molar ratio of 1:2 to 1:6 into the above-mentioned mixing reactor, stir, and heat to 80-120°C under negative pressure to dry and remove the crystal water in the borax, and make the borax particles fine and uniformly mixed with urea.

[0082] Continue stirring, introduce a protective atmosphere, and continue heating to 130-160℃ to melt the urea and coat it with borax particles;

[0083] Continue heating to 180-220℃, maintain a slightly positive pressure in the mixing reactor (gauge pressure: 0.01-0.05MPa), and some borax reacts with urea to form polyboronic acid ester. At the same time, continue stirring to break up the agglomerated polyboronic acid ester, urea, and borax particles coated with polyboronic acid ester.

[0084] When the polyboronic acid ester reaction reaches 50-70%, stop heating, continue stirring and breaking it up, until it cools to below 50°C, to obtain mixed particles containing borax, polyboronic acid ester and urea;

[0085] (2) Place the mixed particles obtained in step (1) in a calcining furnace, heat to 1500-2100℃ under N2 (or NH3) atmosphere, and keep warm for 2-10 hours to obtain crude boron nitride product;

[0086] (3) The crude boron nitride product is acid washed, water washed and dried to obtain pure hexagonal boron nitride.

[0087] This invention creatively allows 30-50% of unreacted hard borax particles to always remain in the mixing reactor as "micro-grinding media" by controlling the degree of reaction (50-70%), actively assisting the dispersion disc in breaking up viscous substances.

[0088] The boron nitride apparatus and preparation method provided by this invention involve feeding a predetermined proportion of borax into tank 1 through the inlet, ensuring all valves are closed, starting the vacuum pump 10, opening the shut-off valve 17 of the vacuum extraction port 6, and drawing the pressure inside the tank to a negative pressure state, such as -0.09 MPa (gauge pressure). The drive device is then activated, causing the double-ribbon agitator 3 to rotate at 50 rpm; the dispersion disc 4 rotates intermittently at 1000 rpm (e.g., running for 2 minutes, stopping for 1 minute), while simultaneously starting the heating device 5 to heat tank 1. At this time, the crystal water in the borax rapidly vaporizes under negative pressure, and the water vapor is extracted through the vacuum extraction port 6. The double-ribbon agitator 3 rotates at low speed, continuously agitating the material, causing clumps to sink and be dispersed by the dispersion disc 4. The borax particles are dehydrated and refined, and then uniformly mixed with urea, while preventing localized overheating. When the material temperature reaches 80°C, it is kept at this temperature, and a trace moisture analyzer 18 detects the moisture content in the gas until the trace moisture analyzer reading falls below a set threshold, indicating that the material dehydration is complete.

[0089] Turn off vacuum pump 10 and close the valve of vacuum extraction port 6. Open the valves of gas source 16 and gas inlet 8, and introduce nitrogen (or ammonia) into the tank through mass / flow controller 7 to restore the pressure inside the tank to atmospheric pressure. Continue heating, rapidly raising the temperature to 130-140℃. The urea will begin to melt. Adjust the speed of the twin ribbon stirrer 3 to 60 rpm, and continue to maintain the dispersion disk 4 at an intermittent speed of 1200 rpm. The twin ribbon stirrer 3 causes the molten urea to flow rapidly and evenly coat each borax particle; the intermittent low-speed rotation of the dispersion disk 4 helps to break up any liquid-solid agglomerates that may form. Maintain the constant temperature for 0.5-1 hour to ensure that the urea completely impregnates the borax.

[0090] Continue heating, rapidly raising the temperature to 180-220℃, maintaining a slightly positive pressure inside the tank (gauge pressure 0.05MPa). During this process, increase the rotation speed of the dispersion disc 4 to 4000rpm and run it continuously, while maintaining the twin-ribbon agitator 3 at 60rpm. Urea decomposes to produce active gas, which reacts with borax to form a polyboronate intermediate. The high-speed rotation of the dispersion disc 4 generates extremely high shear force locally at the bottom of the tank, driving unreacted borax solid particles to move at high speed. The collision and friction between particles peel off and disperse the viscous polyboronate from the particle surface, preventing agglomeration. Simultaneously, the twin-ribbon agitator 3 continuously conveys the material to the dispersion disc 4 area, causing all materials to repeatedly undergo high shear forces.

[0091] During this stage, the operator (or controller) monitors the torque of the first drive device 14 in real time. As the reaction proceeds and the viscosity of the material changes, the monitored torque gradually increases. When the torque reaches its peak, it begins to decrease. When it drops to a predetermined value (e.g., 25 N·M), the degree of reaction reaches 50-70%. Heating is immediately stopped, and the heat transfer oil is switched to the cooling device to cool the material. When the material temperature drops below 50°C, the mixing reactor is stopped, and the discharge port 12 at the bottom of the tank 1 is opened to discharge the mixed particles.

[0092] The mixed particles are transported through a closed pipeline to a sieving device for sieving, and the undersize material is then transported to a calcining furnace for high-temperature sintering.

[0093] The core of this invention lies in using a coaxial, variable-speed dual-stirring system to control temperature and stirring mode in stages within the same tank, sequentially achieving vacuum dehydration, melt wetting, and high-temperature, high-shear pre-reaction. In the pre-reaction stage, unreacted borax particles are used as the grinding medium. High shear force is generated by the high-speed rotation of the dispersion disc, promptly breaking up the generated viscous polyboronate intermediate and inhibiting agglomeration. Simultaneously, by real-time monitoring of stirring power or torque, when the power decreases from its peak, the reaction degree is determined to have reached the target range of 50-70%, at which point heating is stopped and cooling is initiated. This yields core-shell structured precursor particles with unreacted borax as the core, polyboronate as the intermediate layer, and urea as the outer shell. These precursor particles are loosely granular and can be directly used in the subsequent high-temperature calcination step to obtain hexagonal boron nitride, simplifying the calcination process, reducing urea loss, and solving the problem of insufficient sintering caused by agglomerated intermediates generated during traditional calcination. This invention significantly improves safety, raw material utilization, product quality, energy saving, and process integration.

[0094] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing hexagonal boron nitride, characterized in that, Includes the following steps: (1) Add borax and urea with a molar ratio of 1:2-1:6 into a mixing reactor, stir, and heat to 80-120°C under negative pressure to dry and remove the crystal water in the borax, and make the borax particles fine and uniformly mixed with urea. Continue stirring, introduce a protective atmosphere, and continue heating to 130-160℃ to melt the urea and coat it with borax particles; Continue heating to 180-220℃, maintain a slight positive pressure in the mixing reactor, and some borax reacts with urea to generate polyboronic acid ester. At the same time, continue stirring to break up the agglomerated polyboronic acid ester, urea, and borax particles coated with polyboronic acid ester. When the polyboronic acid ester reaction reaches 50-70%, stop heating, continue stirring and breaking it up, until it cools to below 50°C, to obtain mixed particles containing borax, polyboronic acid ester and urea; (2) Place the mixed particles obtained in step (1) in a calcining furnace, heat to 1500-2100℃ under N2 or NH3 atmosphere, and keep warm for 2-10 hours to obtain crude boron nitride product; (3) The crude boron nitride product was acid washed, water washed and dried to obtain pure hexagonal boron nitride; The mixing reactor includes: The tank body contains a stirring shaft, which includes a first shaft and a second shaft that are coaxially arranged and rotate relatively independently. The tank is equipped with a double helical ribbon agitator and a dispersion disc. The dispersion disc is located at the bottom of the tank. The double helical ribbon agitator is fixed on the first shaft, and the dispersion disc is fixed on the second shaft. The top of the tank is connected to a vacuum pump via a vacuum extraction port. The top of the tank is also connected to a gas source via a gas inlet and has a gas outlet; The outer wall of the tank is equipped with a heating device; The tank body is connected to the stirring shaft via a sealing device; the first shaft and the second shaft are respectively connected to the drive device; The double-ribbon agitator includes two spiral ribbons with opposite directions of rotation; the dispersion disc includes multiple layers of discs, with staggered dispersion teeth on the outer periphery of the discs. The rotation speed of the twin ribbon agitator is 20-80 rpm, and the rotation speed of the dispersing disc is 1000-6000 rpm; It also includes a reaction endpoint determination device, which includes a torque sensor and is mounted on the output shaft of the first drive device to determine the reaction endpoint.

2. The method for preparing hexagonal boron nitride as described in claim 1, characterized in that, The upper and lower layers of the dispersion disc are fixedly connected, and the dispersion teeth between adjacent layers are staggered to prevent materials from passing directly through the axial direction.

3. The method for preparing hexagonal boron nitride as described in claim 1, characterized in that, The heating device includes a jacket that covers the cylindrical part of the tank and the bottom of the tank. The bottom of the jacket is provided with a heat transfer oil inlet and the top is provided with a heat transfer oil outlet. A heater is connected between the heat transfer oil inlet and the heat transfer oil outlet.

4. The method for preparing hexagonal boron nitride as described in claim 1, characterized in that, The tank is also equipped with a temperature detection device to monitor the temperature inside the tank.

5. The method for preparing hexagonal boron nitride as described in claim 1, characterized in that, In S2, the sintering temperature in the baking oven is 1500-2100℃.

6. A system for preparing hexagonal boron nitride, characterized in that, The method for preparing hexagonal boron nitride according to any one of claims 1-4 includes the mixing reactor, the sieving device, and the calcining furnace, wherein the mixing reactor and the sieving device, and the sieving device and the calcining furnace are all connected by a conveying device.

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