Low-temperature high-porosity high-efficiency denitration catalyst and preparation method thereof

Abstract

This invention discloses a low-temperature, high-porosity, high-efficiency denitrification catalyst and its preparation method, comprising MnO x - It is composed of a CeO2 heterojunction and WO3, in which MnO x The molar ratio of WO3 to CeO2 is 1:0.8-1.2, and the amount of WO3 added is 2-5%. It also includes mesoporous TiO2 and 3D graphene to form a bimodal distribution structure of 50-200 nm mesopores and 1-5 μm macropores; it also includes a hierarchical pore structure formed by a dual-mode pore-forming agent. This invention provides a low-temperature, high-porosity, high-efficiency denitrification catalyst and its preparation method using MnO2. x -CeO2 heterojunction replaces vanadium-based materials, through Mn 3+ / Mn 4+ With Ce 3+ / Ce 4+ Redox cycling enhances low-temperature activity, and WO3 inhibits SO2 adsorption poisoning. Mesoporous TiO2 (30-50nm) provides a high active specific surface area, while 3D graphene is used to construct a macroporous framework (1-5μm) to enhance mass transfer efficiency and compressive strength (≥3.5MPa). The pore distribution is synergistically regulated by a dual-mode pore-forming agent and gradient calcination technology (segmented temperature control from 250-550℃) to avoid pore structure collapse caused by high-temperature sintering.

Description

Technical Field

[0001] This invention relates to the field of denitrification catalyst technology, specifically to a low-temperature, high-porosity, high-efficiency denitrification catalyst and its preparation method. Background Technology

[0002] Nitrogen oxides (NOx) are a major source of air pollution, primarily originating from vehicle exhaust and industrial emissions. High concentrations of NOx lead to problems such as photochemical smog and acid rain, severely impacting the environment and human health. Therefore, reducing NOx emissions has become a crucial task for environmental protection. Currently, the market for denitrification catalysts is booming against the backdrop of increasingly stringent environmental policies and intensified efforts to control air pollution. Through selective catalytic reduction (SCR), NOx is reduced to harmless nitrogen and water, significantly lowering the concentration of pollutants in the atmosphere. SCR denitrification technology is widely used in flue gas treatment systems across various industries, including power, steel, cement, and glass, and is a key means for nations and enterprises to achieve ultra-low industrial emissions.

[0003] When using denitrification catalysts, the activation temperature of traditional vanadium-titanium system catalysts needs to be ≥180℃, which cannot be directly used for denitrification treatment of low-temperature flue gas (below 150℃) in steel sintering, coking and other processes. This results in the need to add heat exchange devices to the process, which significantly increases energy consumption and cost. In addition, the reaction mass transfer efficiency of conventional catalysts is limited, and the honeycomb structure is prone to breakage due to thermal stress, which reduces the redox reaction of the denitrification catalyst and further reduces the efficiency of environmental treatment. Summary of the Invention

[0004] The purpose of this invention is to provide a low-temperature, high-porosity, high-efficiency denitrification catalyst and its preparation method, so as to overcome the above-mentioned shortcomings in the prior art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: containing MnO x - It is composed of a CeO2 heterojunction and WO3, in which MnO x The molar ratio of WO3 to CeO2 is 1:0.8-1.2, and the amount of WO3 added is 2-5%. It also includes the construction of mesoporous TiO3 and 3D graphene to form a bimodal distribution structure of 50-200nm mesopores and 1-5μm macropores; It also includes a hierarchical porous structure formed by a dual-mode pore-forming agent to enhance surface acidity and low-temperature activity.

[0006] As a further description of the above technical solution: The catalyst has a specific surface area of ​​180-220 m² / g, a porosity >65%, and a compressive strength ≥3.5 MPa, and can withstand temperatures of 150℃ and a space velocity of 4000 h⁻¹. -1 NO under certain conditions xConversion rate ≥83%, and conversion rate ≥95% at 250℃.

[0007] As a further description of the above technical solution: The dual-mode pore-forming agent is a composite system of polymethyl methacrylate microspheres (particle size 5-10 μm, addition amount 5-8%) and soluble starch (addition amount 3-5%).

[0008] As a further description of the above technical solution: The dual-mode pore-forming agent is a composite system of polymethyl methacrylate microspheres (particle size 5-10 μm, addition amount 5-8%) and soluble starch (addition amount 3-5%).

[0009] As a further description of the above technical solution: A method for preparing a low-temperature, high-porosity, and high-efficiency denitration catalyst, wherein the low-temperature, high-porosity, and high-efficiency denitration catalyst described in any one of the above methods is characterized by comprising the following steps: S1: Slurry preparation: The active component precursor (manganese nitrate, cerium nitrate, etc.), carrier, pore-forming agent and deionized water are ball-milled and mixed for 2-4 hours to form a homogeneous slurry with a solid content of 40-45%. S2. Low temperature extrusion: Using an extruder, the die head temperature is controlled at ≤40℃, the screw speed is 10-15rpm, and the structure is formed into a honeycomb structure (pore diameter 3×3mm, wall thickness 0.6mm). S3. Drying and shaping: First stage: 40℃ / 12h, humidity 80% to slow release moisture and prevent cracking; Second stage: 80℃ / 6h for rapid shaping. The extruder includes an extruder head, a one-way screw is rotatably arranged inside the extruder head, and a gradient filter assembly is arranged inside the extruder head for filtering incompletely melted raw materials and impurities. The gradient filter assembly includes a coarse filter screen, a fine filter screen, and a nano-coated fine filter screen. The fine filter screen is equipped with an adjustment component to adjust the pore size of the fine filter screen to suit the viscosity of different materials.

[0010] As a further description of the above technical solution: The fine filter screen is composed of a first filter screen and a second filter screen with the same pore size. The first filter screen is rotatably disposed inside the extruder head, and the second filter screen is fixed inside the extruder head. The adjustment component includes a plurality of first sliding rods slidably disposed inside the first filter screen, and a plurality of second sliding rods slidably disposed inside the second filter screen and in active contact with each of the first sliding rods.

[0011] As a further description of the above technical solution: Each of the second sliding rods extends to the outside of the first filter screen and is fixedly connected to the same U-shaped ring plate. Two fixed rings are fixedly connected to the outside of the first filter screen. Both fixed rings are slidably disposed within the U-shaped ring plate. Multiple arc-shaped grooves are opened on the outside of any of the fixed rings. Multiple fixed blocks that are slidably disposed within each arc-shaped groove are fixedly connected to the inner wall of the U-shaped ring plate.

[0012] As a further description of the above technical solution: It also includes multiple scrapers rotatably disposed on the surface of the gradient filter assembly, which are used to clean impurities and air bubbles on the surface of the gradient filter assembly. The scrapers are connected to a one-way screw via a drive assembly. The drive assembly includes a connecting rod fixedly connected to the one-way screw. Each scraper is sleeved on the connecting rod. A worm gear is fixedly connected to the outside of the connecting rod. A worm wheel is driven and meshed on the outer surface of the worm gear. A driving bevel gear is fixedly connected to the worm wheel via a universal joint. A driven bevel gear that meshes with the driving bevel gear is sleeved on the connecting rod. The scraper is fixed to the outside of the driven bevel gear.

[0013] As a further description of the above technical solution: It also includes multiple vibrating balls, which are used to vibrate the gradient filtration assembly to vibrate impurities and bubbles to the top of the gradient filtration assembly. The multiple vibrating balls are slidably disposed on the connecting rod, and the multiple vibrating balls are in active contact with the coarse filter screen, the fine filter screen, and the nano-coated fine filter screen, respectively.

[0014] As a further description of the above technical solution: The vibrating ball is fixedly connected to a fixed tube, which is sleeved over the connecting rod. The connecting rod extends into the fixed tube and is fixedly connected to a guide block. The fixed tube has a spiral groove adapted to the guide block. The connecting rod is covered with multiple protective covers, each of which is rotatably connected to the gradient filter assembly.

[0015] In the above technical solution, the low-temperature, high-porosity, high-efficiency denitrification catalyst and its preparation method provided by the present invention have the following beneficial effects: This invention: by using MnO x -CeO2 heterojunction replaces vanadium-based materials, through Mn 3+ / Mn 4+ With Ce 3+ / Ce 4+ Redox cycling enhances low-temperature activity, and WO3 inhibits SO2 adsorption poisoning. Mesoporous TiO2 (30-50nm) provides a high active specific surface area, while 3D graphene is used to construct a macroporous framework (1-5μm) to enhance mass transfer efficiency and compressive strength (≥3.5MPa). The pore distribution is synergistically regulated by a dual-mode pore-forming agent and gradient calcination technology (segmented temperature control from 250-550℃) to avoid pore structure collapse caused by high-temperature sintering.

[0016] It should be understood that the foregoing general description and the following detailed description are exemplary and illustrative only, and are not intended to limit this disclosure.

[0017] This application provides an overview of various implementations or examples of the technology described in this disclosure, and is not a full disclosure of the entire scope or all features of the disclosed technology. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0019] Figure 1 This is a three-dimensional structural schematic diagram provided for an embodiment of the present invention; Figure 2 This is a schematic diagram of the longitudinal section structure of the extruder head provided in an embodiment of the present invention; Figure 3 A three-dimensional structural diagram of the gradient filtering component provided in an embodiment of the present invention; Figure 4 This is a partial three-dimensional structural diagram of the driving component provided in an embodiment of the present invention; Figure 5 A partial longitudinal section diagram of the fine filter screen provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of the internal structure of the U-shaped ring plate and the fixing ring provided in an embodiment of the present invention; Figure 7 Provided for embodiments of the present invention Figure 6 The enlarged view at point B is shown below; Figure 8 This is a schematic diagram of the internal structure of the connection between the vibrating ball and the connecting rod provided in an embodiment of the present invention.

[0020] Explanation of reference numerals in the attached figures: 1. Extruder head; 11. One-way screw; 21. Coarse filter screen; 22. Fine filter screen; 221. First filter screen; 222. Second filter screen; 23. Nano-coated fine filter screen; 31. First sliding rod; 32. Second sliding rod; 33. U-shaped ring plate; 34. Fixed ring; 35. Arc groove; 36. Fixed block; 4. Scraper; 51. Connecting rod; 52. Worm gear; 53. Worm wheel; 54. Universal joint; 55. Driving bevel gear; 56. Driven bevel gear; 57. Protective cover; 6. Vibrating ball; 61. Fixed tube; 62. Spiral groove; 63. Guide block. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the described embodiments of this disclosure without creative effort are within the scope of protection of this disclosure.

[0022] Please see Figures 1-8 This embodiment provides a low-temperature, high-porosity, high-efficiency denitrification catalyst containing MnO. x - It is composed of a CeO2 heterojunction and WO3, in which MnO x The molar ratio of TiO2 to CeO2 is 1:0.8-1.2, and the amount of WO3 added is 2-5%. Mesoporous TiO2 is constructed with 3D graphene to form a bimodal distribution structure of 50-200nm mesopores and 1-5μm macropores. The hierarchical pore structure formed by the dual-mode pore-forming agent is used to enhance surface acidity and low-temperature activity.

[0023] In a further embodiment of the present invention, the catalyst has a specific surface area of ​​180-220 m². 2 / g, porosity > 65%, compressive strength ≥ 3.5MPa, at 150℃ and space velocity 4000h -1 NO under certain conditions x Conversion rate ≥83%, and conversion rate ≥95% at 250℃.

[0024] Furthermore, the dual-mode pore-forming agent is a composite system of polymethyl methacrylate microspheres (particle size 5-10 μm, addition amount 5-8%) and soluble starch (addition amount 3-5%).

[0025] Furthermore, the dual-mode pore-forming agent is a composite system of polymethyl methacrylate microspheres (particle size 5-10 μm, addition amount 5-8%) and soluble starch (addition amount 3-5%).

[0026] In a further embodiment of the present invention, a method for preparing a low-temperature, high-porosity, high-efficiency denitrification catalyst, based on any one of the above-mentioned low-temperature, high-porosity, high-efficiency denitrification catalysts, includes the following steps: S1: Slurry preparation: The active component precursor (manganese nitrate, cerium nitrate, etc.), carrier, pore-forming agent and deionized water are ball-milled and mixed for 2-4 hours to form a homogeneous slurry with a solid content of 40-45%. S2. Low temperature extrusion: Using an extruder, the die head temperature is controlled at ≤40℃, the screw speed is 10-15rpm, and the structure is formed into a honeycomb structure (pore diameter 3×3mm, wall thickness 0.6mm). S3. Drying and setting: First stage: 40℃ / 12h, humidity 80% to slow release moisture and prevent cracking; Second stage: 80℃ / 6h for rapid setting. The extruder includes an extruder head 1, a one-way screw 11 is rotatably installed inside the extruder head 1, and a gradient filter assembly is installed inside the extruder head 1 to filter incompletely melted raw materials and impurities. The gradient filter assembly includes a coarse filter screen 21, a fine filter screen 22, and a nano-coated fine filter screen 23. The fine filter screen 22 is equipped with an adjustment component to adjust the pore size of the fine filter screen 22 to match the viscosity of different materials.

[0027] In the embodiments provided by the present invention, the fine filter screen 22 is composed of two first filter screens 221 and second filter screens 222 with the same pore size. The first filter screen 221 is rotatably disposed inside the extruder head 1, and the second filter screen 222 is fixed inside the extruder head 1. The adjustment component includes a plurality of first sliding rods 31 slidably disposed inside the first filter screen 221. A plurality of second sliding rods 32 are slidably disposed inside the second filter screen 222 and are in active contact with each of the first sliding rods 31. The ends of the second sliding rods 32 are arc-shaped. When the second sliding rods 32 push the first sliding rods 31 to move and rotate, it is ensured that the first sliding rods 31 are always in active contact with the second sliding rods 32 after rotation.

[0028] Specifically, each of the second sliding rods 32 extends to the outside of the first filter screen 221 and is fixedly connected to the same U-shaped ring plate 33. Two fixing rings 34 are fixedly connected to the outside of the first filter screen 221. Both fixing rings 34 are slidably disposed within the U-shaped ring plate 33. The inner wall of the U-shaped ring plate 33 slides and seals against the opposite side of the two fixing rings 34. The two fixing rings 34 are embedded. The first sliding rod 31 slides within the two rings. Multiple arc-shaped grooves 35 are opened on the outside of any fixing ring 34. Multiple fixing blocks 36 are fixedly connected to the inner wall of the U-shaped ring plate 33 and slidably disposed within each arc-shaped groove 35.

[0029] The present invention further includes multiple scrapers 4 rotatably disposed on the surface of the gradient filter assembly, which are used to clean impurities and air bubbles on the surface of the gradient filter assembly. The scrapers 4 are connected to a one-way screw 11 through a drive assembly. The drive assembly includes a connecting rod 51 fixedly connected to the one-way screw 11. Each scraper 4 is sleeved on the outside of the connecting rod 51. A worm gear 52 is fixedly connected to the outside of the connecting rod 51. A worm wheel 53 is driven and meshed on the outer surface of the worm gear 52. A driving bevel gear 55 is fixedly connected to the worm wheel 53 through a universal joint 54. A driven bevel gear 56 that drives and meshes with the driving bevel gear 55 is sleeved on the connecting rod 51. The scrapers 4 are fixed to the driven bevel gear 56. By setting the drive assembly, the rotation speed transmitted from the connecting rod 51 to the scraper 4 is reduced, thereby ensuring the normal filtration of the coarse filter screen 21, the fine filter screen 22, and the nano-coated fine filter screen 23.

[0030] In this invention, a plurality of vibrating balls 6 are also included, which are used to vibrate the gradient filter assembly, vibrating impurities and bubbles to the top of the gradient filter assembly, and then extracting the bubbles and impurities by a vacuum pumping device. The vacuum pumping device is set outside the extruder head 1 and is interconnected with the channels opened on the top of the coarse filter screen 21, the fine filter screen 22, and the nano-coated fine filter screen 23. The plurality of vibrating balls 6 are slidably set on the connecting rod 51, and the plurality of vibrating balls 6 are in active contact with the coarse filter screen 21, the fine filter screen 22, and the nano-coated fine filter screen 23 respectively.

[0031] In this invention, a fixed tube 61 is fixedly connected to the vibrating ball 6. The fixed tube 61 is sleeved on the connecting rod 51. The vibrating ball 6 is connected to the first filter screen 221 through a guide rod, which restricts the vibrating ball 6 to slide relative to the connecting rod 51. The connecting rod 51 extends into the fixed tube 61 and is fixedly connected to a guide block 63. A spiral groove 62 adapted to the guide block 63 is opened in the fixed tube 61. Multiple protective covers 57 are sleeved on the connecting rod 51. Each protective cover 57 is rotatably connected to the gradient filter assembly. An abutment spring is provided between the fixed tube 61 and the connecting rod 51. After the vibrating ball 6 moves to vibrate the gradient filter assembly, the guide block 63 is reset through the straight groove with the assistance of the abutment spring. The protective cover 57 is used to cover the drive assembly to ensure the stable operation of the drive assembly. The connecting rod 51 is rotatably set on the inner wall of the protective cover 57.

[0032] In use, the active component precursor, carrier, pore-forming agent and composite binder are mixed to form a solid homogeneous slurry, which is then injected into the extruder head 1. The homogeneous slurry is extruded by the unidirectional screw 11. The homogeneous slurry is filtered through the gradient filter assembly to remove air bubbles and impurities. During the filtration process, the rotation of the one-way screw 11 drives the connecting rod 51 to rotate synchronously, thereby driving the worm gear 52 to rotate, which in turn causes the worm wheel 53 to rotate at low speed. Through the universal joint 54, the driving bevel gear 55 is driven to rotate, which causes the driven bevel gear 56 sleeved outside the connecting rod 51 to rotate synchronously, driving the scraper 4 to rotate on the surface of the gradient filter assembly, thereby cleaning the impurities and bubbles on the surface of the gradient filter assembly. When the connecting rod 51 rotates, it drives the guide block 63 to rotate synchronously, which in turn drives the spiral groove 62 to move. Since the spiral groove 62 is composed of a rotating groove and a straight groove, it drives the spiral groove 62 to slide back and forth outside the guide block 63, so that the vibrating ball 6 moves synchronously, vibrates the gradient filter component, performs exhaust treatment, and then is extracted by a vacuum pump. When the homogeneous slurry flows in the extruder, due to differences in slurry viscosity, the high-viscosity slurry will cause filter pores to become clogged when passing through the filter screen. At this time, the pressure difference on both sides of the fine filter screen 22 is inconsistent. When the pressure difference on both sides of the filter screen is inconsistent, the side with the larger pressure difference will push the second sliding rod 32 to move, causing the first sliding rod 31 to move synchronously. Therefore, it will push the U-shaped ring plate 33 to move, causing the fixed block 36 to slide in the arc groove 35, thereby driving the fixed ring 34 to rotate, driving the first filter screen 221 to rotate, so that the second... The first filter screen 221 and the second filter screen 222 rotate to be aligned, thereby increasing the pore size of the fine filter screen 22, reducing the pressure difference, avoiding inconsistent pressure differences on both sides of the filter screen, which would embed impurities into the filter screen and affect filtration, thus improving the extrusion efficiency. Similarly, when the viscosity of the homogeneous slurry is low, and the pressure difference between the fine filter screen 22 and the nano-coated fine filter screen 23 is large, the U-shaped ring plate 33 is pushed to move, so that the pore size of the first filter screen 221 and the second filter screen 222 is reduced. During cleaning, the filter screen is maximized, which can assist in the cleaning of the extruder head.

[0033] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a low-temperature, high-porosity, high-efficiency denitration catalyst, characterized in that, The above-mentioned denitrification catalyst contains MnO x - It is composed of a CeO2 heterojunction and WO3, in which MnO x The molar ratio of WO3 to CeO2 is 1:0.8-1.2, and the amount of WO3 added is 2%-5%. It also includes the construction of mesoporous TiO2 and 3D graphene to form a bimodal distribution structure of 50-200nm mesopores and 1-5μm macropores; It also includes a hierarchical porous structure formed by a dual-mode pore-forming agent to enhance surface acidity and low-temperature activity; It also includes the following steps: S1: Slurry preparation: The active component precursor, carrier, pore-forming agent and deionized water are ball-milled and mixed for 2-4 hours to form a homogeneous slurry with a solid content of 40-45%. The active component precursor includes manganese nitrate and cerium nitrate. S2. Low-temperature extrusion: Using an extruder, the die head temperature is controlled at ≤40℃, the screw speed is 10-15rpm, and the structure is formed into a honeycomb structure with a pore diameter of 3×3mm and a wall thickness of 0.6mm; S3. Drying and shaping: First stage: 40℃ / 12h, humidity 80% to slow release moisture and prevent cracking; Second stage: 80℃ / 6h for rapid shaping. The extruder includes an extruder head (1), a one-way screw (11) is rotatably arranged inside the extruder head (1), and a gradient filter assembly is arranged inside the extruder head (1) for filtering incompletely melted raw materials and impurities. The gradient filter assembly includes a coarse filter screen (21), a fine filter screen (22), and a nano-coated fine filter screen (23). The fine filter screen (22) is provided with an adjustment component for adjusting the pore size of the fine filter screen (22) to match the viscosity of different materials; The fine filter (22) is composed of two first filter (221) and second filter (222) with the same pore size. The first filter (221) is rotatably disposed in the extruder head (1), and the second filter (222) is fixed in the extruder head (1). The adjustment component includes a plurality of first sliding rods (31) slidably disposed in the first filter (221), and a plurality of second sliding rods (32) slidably disposed in the second filter (222) in contact with each of the first sliding rods (31).

2. The method for preparing a low-temperature, high-porosity, high-efficiency denitrification catalyst according to claim 1, characterized in that, Each of the second sliding rods (32) extends to the outside of the first filter screen (221) and is fixedly connected to the same U-shaped ring plate (33). Two fixed rings (34) are fixedly connected to the outside of the first filter screen (221). Both fixed rings (34) are slidably disposed in the U-shaped ring plate (33). Multiple arc-shaped grooves (35) are opened on the outside of any of the fixed rings (34). Multiple fixed blocks (36) are fixedly connected to the inner wall of the U-shaped ring plate (33) and are slidably disposed in each arc-shaped groove (35).

3. The method for preparing a low-temperature, high-porosity, high-efficiency denitrification catalyst according to claim 1, characterized in that, It also includes multiple scrapers (4) rotatably disposed on the surface of the gradient filter assembly, which are used to clean impurities and air bubbles on the surface of the gradient filter assembly. The scrapers (4) are connected to a one-way screw (11) through a drive assembly. The drive assembly includes a connecting rod (51) fixedly connected to the one-way screw (11). Each scraper (4) is sleeved on the outside of the connecting rod (51). A worm gear (52) is fixedly connected to the outside of the connecting rod (51). A worm wheel (53) is driven and meshed on the outer surface of the worm gear (52). The worm wheel (53) is fixedly connected to a driving bevel gear (55) through a universal joint (54). A driven bevel gear (56) is sleeved on the connecting rod (51) and driven and meshed with the driving bevel gear (55). The scrapers (4) are fixed on the outside of the driven bevel gear (56).

4. The method for preparing a low-temperature, high-porosity, high-efficiency denitrification catalyst according to claim 3, characterized in that, It also includes multiple vibrating balls (6) for vibrating the gradient filter assembly, which vibrates impurities and bubbles to the top of the gradient filter assembly. The multiple vibrating balls (6) are all slidably disposed on the connecting rod (51), and the multiple vibrating balls (6) are in active contact with the coarse filter screen (21), the fine filter screen (22), and the nano-coated fine filter screen (23), respectively.

5. The method for preparing a low-temperature, high-porosity, high-efficiency denitrification catalyst according to claim 4, characterized in that, The vibrating ball (6) is fixedly connected to a fixed tube (61), which is sleeved on the outside of the connecting rod (51). The connecting rod (51) extends into the fixed tube (61) and is fixedly connected to a guide block (63). The fixed tube (61) has a spiral groove (62) adapted to the guide block (63). The connecting rod (51) is covered with multiple protective covers (57), and each of the protective covers (57) is rotatably connected to the gradient filter assembly.

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