Aluminum electrolysis cell anode scrap waste heat recovery device

Abstract

The utility model relates to the technical field of equipment for obtaining metal by an electrolytic method, in particular to a structure of an electrolytic bath. According to the technical scheme, the aluminum electrolysis cell anode scrap waste heat recovery device comprises a box body and is characterized in that a side door is arranged on one side of the box body, a connector connected with a pipeline is arranged on the other side of the box body, a long-strip-shaped window is formed in the top of the box body, a plurality of openings are distributed in one side of the window, and a supporting structure is arranged at the bottom of the box body. And an anode material is placed on the supporting structure. The utility model provides a residual anode waste heat recovery device for an aluminum electrolysis cell, which solves the problems that the cell condition is influenced due to long heating time of a new anode, the waste heat recovery efficiency is low, the energy consumption waste is serious and the use is inconvenient, and achieves the purposes of high waste heat recovery efficiency, energy conservation, low cost and convenience in use.

Description

Technical Field

[0001] The utility model relates to the technical field of equipment for obtaining metals by electrolysis, in particular to the structure of an electrolytic cell. Background Art

[0002] Electrolytic aluminum is obtained by electrolysis. Industrial production adopts cryolite-alumina molten salt electrolysis. The aluminum electrolysis process is carried out in an aluminum electrolytic cell. The raw material used for electrolysis is alumina, the electrolyte is molten cryolite, and a carbon anode is used. The electrolysis operation is carried out at 950-980°C. Alumina is supplied to the tank feed box by a dense phase conveying system and is uniformly added to the electrolyte through a point feeder at a computer-controlled rate through shelling. The result of electrolysis is molten aluminum on the cathode and CO2 on the anode. The molten aluminum is regularly sucked out of the tank with a vacuum ladle, and the ladle containing metal aluminum is transported to the casting workshop. For example, the Chinese patent with the publication number CN114016086A discloses a waste heat boiler type aluminum electrolytic cell structure, and provides the following technical solutions: on the basis of the existing aluminum electrolytic cell cathode melting pool furnace structure and the anode carbon block steel claw group conductive structure unchanged, with reference to the waste heat boiler structure design principle, the anode operation space of the existing aluminum electrolytic cell that is integrally connected is divided into several relatively independent, relatively small anode insulation operation compartment spaces that can accommodate several anode carbon blocks by using the inner flue baffle, partition isolation steel plate, outer insulation furnace wall and upper horizontal insulation cover plate. And on the inner flue baffle and partition isolation steel plate of the built-in intermediate flue of the improved aluminum electrolytic cell, negative pressure wind output pipes and heat conduction pipe devices are arranged to absorb and output the heat loss energy flue gas and the emitted heat energy in the aluminum electrolytic cell to the waste heat utilization device of the aluminum electrolytic cell for efficient waste heat utilization. However, the above-mentioned waste heat boiler type aluminum electrolytic cell structure does not store the new electrode and the residual electrode together, which wastes the heat radiation between them and makes it inconvenient to store and access the new electrode and the residual electrode. Utility Model Content

[0003] In response to the problems in the background technology that the long heating time of the new electrode affects the cell condition, the waste heat recovery efficiency is low, the energy waste is serious and it is inconvenient to use, the utility model proposes a waste heat recovery device for the residual electrode of an aluminum electrolytic cell, which achieves the purposes of high waste heat recovery efficiency, energy saving, low cost and easy use.

[0004] To achieve the above-mentioned purpose, the utility model adopts the following technical scheme: a waste heat recovery device for residual anodes of an aluminum electrolytic cell, comprising a box body, characterized in that it also includes: a side door is provided on one side of the box body, and an interface for connecting a pipeline is provided on the other side; a long strip window is provided on the top of the box body, and a plurality of openings are distributed on one side of the window; a support structure is provided at the bottom of the box body, and anode materials are placed on the support structure.

[0005] The advantage of this design is that the side doors and interfaces facilitate the operation and maintenance of the equipment. The openings on the long windows can further promote air flow and improve the heat dissipation effect. The support structure can stably place the anode material.

[0006] Preferably, a guide rod passes through the opening, a size of the opening is larger than a size of the guide rod, and a guide rod seal is filled between the opening and the guide rod for sealing.

[0007] The advantage of this design is that the guide rod in the opening can play a supporting role, and the filled guide rod seal can ensure the sealing performance of the device, avoid the occurrence of air leakage, and improve the thermal insulation performance.

[0008] Preferably, the support structure is composed of a frame structure composed of a plurality of support beams, and the support beams are covered with a layer of heat-conducting steel plate, and the heat-conducting steel plate is detachable.

[0009] The advantage of this design is that the support beams of the frame structure can provide stable support, and the covered heat-conducting steel plate can improve the efficiency of heat conduction. At the same time, the detachable design makes it easy to change the insulation effect according to the actual application.

[0010] Preferably, the window is flipped outwards, and a phase-change energy storage material is provided at the bottom of the box body, and the phase-change energy storage material is located below the support beam and the heat-conducting steel plate.

[0011] The advantage of this design is that the window is opened outward to facilitate the placement of the anode and increase the utilization rate of the internal space. At the same time, the phase change energy storage material is arranged at the bottom of the box to improve the storage and release effect of thermal energy and improve the energy utilization rate.

[0012] Preferably, a steel claw is fixed under the guide rod, and the steel claw grasps the anode material.

[0013] The advantage of this design is that by fixing the steel claws, the anode material can be stably grasped to prevent it from moving or tilting, thereby improving the stability of the device.

[0014] Preferably, the interface is circular, and the connecting pipes of the interface include an exhaust pipe for collecting residual anode flue gas and a preheating pipe for introducing flue gas into the electrolytic cell.

[0015] The advantage of this design is that the setting of the connecting pipe can not only collect the flue gas from the spent poles to prevent the collection of flue gas from affecting the working environment and causing pollution, but also allow the flue gas from the electrolytic cell to be introduced to preheat the entire device, thereby forming a joint preheating of the new pole by the flue gas and the spent poles.

[0016] Preferably, the guide rod seal is made of silicone rubber or carbon fiber high temperature resistant material.

[0017] The advantage of this design is that the guide rod seal made of silicone rubber or carbon fiber high temperature resistant material has good high temperature resistance and sealing performance, which can effectively prevent gas leakage and energy loss.

[0018] Preferably, the outer layers of the sides and the front of the box are made of metal insulation material, and the inner layer is made of flexible insulation material.

[0019] The advantage of this design is that the metal insulation material can provide good structural strength and high temperature resistance, and the flexible insulation material can provide better insulation effect and maintain the internal temperature of the device stable.

[0020] Preferably, the thickness of the heat-conducting steel plate is 3 mm to 7 mm.

[0021] The advantage of this design is that the appropriate thickness of the heat-conducting steel plate can provide good heat conduction effect and has sufficient strength and stability.

[0022] Compared with the prior art, the utility model has the following beneficial effects.

[0023] 1. The device designed by the utility model greatly simplifies the operation process and maintenance of the equipment by setting a side door on one side of the box body and an interface for connecting the pipeline on the other side. The design of the side door allows operators to easily load and unload anode materials and inspect the equipment, while the setting of the interface facilitates connection with the external pipeline system to achieve effective collection and preheating of flue gas. This design not only improves work efficiency, but also reduces the difficulty of operation, making equipment maintenance more convenient.

[0024] 2. The new anode is placed in the device to conduct heat with the phase change material at the bottom, and also conduct convection heat with the air in the box. At the same time, the remaining anode is placed in the device to reduce heat radiation to the outside. When the remaining anode and the new anode are placed in the device at the same time, there will be heat radiation between the remaining anode and the new anode, and the preheating effect of the new anode will be better.

[0025] 3. By fixing the steel claws under the guide rod, the device can stably grasp the anode material to prevent it from moving or tilting, thereby improving the stability of the entire device. At the same time, the guide rod seal made of silicone rubber or carbon fiber high temperature resistant material ensures the sealing performance of the device, prevents gas leakage and energy loss, and enhances the safety of the device. In addition, the box body adopts a double-layer structure of metal insulation material and flexible insulation material, which not only provides good structural strength and high temperature resistance, but also ensures good insulation effect, ensures the stability of the internal temperature of the device, and further improves the safety and reliability of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The utility model is a schematic structural diagram of an embodiment.

[0027] Figure 2 This is a schematic diagram of the internal structure of an embodiment of the utility model from the front.

[0028] Figure 3 The figure is a schematic diagram of the internal structure of an embodiment of the utility model from the side.

[0029] Figure 4 The figure is a schematic diagram of the internal structure of an embodiment of the utility model when viewed from above.

[0030] Illustration description: supporting structure 1, box bottom 2, box side 3, side door 4, box front 5, interface 6, box top 7, opening 8, window 9, support beam 10. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical scheme and advantages of the present disclosure clearer, the following will further describe the embodiments of the present disclosure in detail with reference to the accompanying drawings. The proportions of the components are not drawn according to the actual proportions, and the proportions and sizes shown in the accompanying drawings should not limit the substantial technical scheme of the present utility model. These embodiments do not describe all the details in detail, nor do they limit the present utility model to only the specific embodiments described.

[0032] See also Figure 1-4 As shown, a waste heat recovery device for residual anodes of an aluminum electrolytic cell includes a box body, and is characterized in that it also includes: a side door 4 is provided on one side of the box body, and an interface 6 for connecting a pipeline is provided on the other side, a long strip window 9 is provided on the top 7 of the box body, and a plurality of openings 8 are distributed on one side of the window 9, and a support structure 1 is provided on the bottom 2 of the box body, and anode materials are placed on the support structure 1.

[0033] During the aluminum electrolysis production process, the anode is continuously consumed as the electrochemical reaction proceeds, and the carbon anode needs to be replaced regularly. The anode replacement work has become an important operation in the aluminum smelting process. At present, there are the following problems with the anode replacement process:

[0034] 1) The residual anode takes away a lot of heat from the electrolytic cell:

[0035] The temperature of the replaced hot anode scrap is generally around 800 degrees Celsius, which takes away a lot of heat from the electrolytic cell. If it is placed in the electrolysis workshop for natural cooling, the heat will be released into the environment. At the same time, the fluoride contained in the anode scrap will react with water vapor in the air to generate HF gas, which is highly toxic. Studies have shown that the fluoride produced by the anode scrap in the electrolysis workshop accounts for about 1 / 3 of the fluoride emitted by the electrolysis workshop, which is equivalent to about 0.2kgF / t-Al. Preliminary calculations show that the heat taken away by the unit amount of original aluminum anode scrap is equivalent to about 6kg of standard coal.

[0036] 2) The new electrode takes a long time to heat up, which affects the tank conditions:

[0037] After the cold new anode is placed in the electrolytic cell, it needs the heat in the cell to heat it to a sufficient temperature to conduct electricity, which requires additional energy consumption. However, during the pole replacement process, after the anode enters the electrolytic cell, it gradually rises from room temperature to the normal production temperature of electrolysis, which requires a large amount of heat to be absorbed from the electrolytic cell, which has a serious negative impact on the stability, thermal balance, current efficiency, anode current distribution, etc. of the electrolytic cell, and greatly interferes with the normal production of the electrolytic cell. Improve the working efficiency of aluminum electrolytic cells.

[0038] In summary, the utility model aims to design a heat storage heat exchanger for making full use of the waste heat of the aluminum electrolytic waste poles. The purpose is to reduce the large amount of energy waste in the aluminum electrolysis process by recovering the waste heat of the waste poles and heating new poles, improve the production stability of the electrolytic cell and increase energy efficiency, reduce energy consumption and improve economy, and to a certain extent reduce the impact of harmful gases and dust released by the open waste poles in the workshop on the environment.

[0039] At present, some scholars use the high-temperature flue gas in the electrolytic cell to preheat the new anode. For example, Wang Hongyong et al. designed an anode block preheating exchange device. The device is a box-type structure. The box walls on each side of the box are made of insulation materials. A row of residual anode tray brackets are provided on the bottom surface of the box bottom. The new anode tray brackets are arranged above every two adjacent residual anode tray brackets. The new anode carbon block and the residual anode carbon block are preheated and exchanged in the device. Wu Deguang et al. designed a two-stage preheating system for aluminum electrolysis carbon anode blocks. The first stage preheating is carried out in a special preheating room. The high-temperature flue gas from the electrolytic cell is used for heating. The water vapor of the new anode block is removed, and the temperature of the new anode is increased to a certain extent, which can reach 80-100°C; the second stage preheating is carried out in a special transport trolley. The high-temperature residual anode, carbon slag and alumina agglomerates are used for heating. The temperature of the new anode will be further increased, and ideally it can reach about 200°C, thereby reducing thermal shock. Gui Jingyu et al. designed a residual anode flue gas recovery and sealed hauling device for aluminum electrolysis. The anode scraps just replaced by aluminum electrolysis are enclosed in a sealed space to achieve fixed-point fume recovery and sealed transportation of the anode scraps. The anode scraps that have been recovered and cooled can be directly transported to the anode assembly workshop by trailer for loading and unloading and chaining, and there is no dust or fly ash during the entire transportation process.

[0040] However, these designs have complicated operation processes and require secondary transformation of the flue gas collection system of the aluminum electrolytic cell, which increases the investment cost. At the same time, the switching of the device will lead to the emission of harmful gases. Some scholars designed the device to only recover the flue gas from the spent pole, but did not utilize the large amount of waste heat in the spent pole. Some scholars used the waste heat of the spent pole to preheat the new pole, but it was a self-heating in the device, which could not store heat, the heat was not fully utilized, the preheating distribution at the bottom of the new pole was uneven, and the upper tank would cause the resistance at different places at the bottom of the new pole to be different, resulting in uneven current distribution. The operation process is complicated when the new and old poles are stacked.

[0041] The utility model improves the anode tray into a heat exchange device, utilizes phase change energy storage materials to recycle and store the waste heat of the anode waste, has large recovered heat, stable performance, and uses the recovered heat to stably preheat the new anode, thereby improving the comprehensive utilization rate of energy. It reduces the negative effects of splashing and depolarization caused by the moisture of the new anode, and reduces the energy consumption caused by the fluctuation of internal heat exchange during the pole change, improves the stability and working efficiency of the aluminum electrolytic cell, can also collect the anode waste flue gas for the next step of utilization, and to a certain extent reduces the impact of harmful gases and dust released by the open anode waste in the workshop on the environment.

[0042] like Figure 1 , Figure 2 and Figure 3 In one embodiment shown, Figure 1 This is a schematic diagram of the structure of an embodiment of the utility model. Figure 2 This is a schematic diagram of the internal structure of an embodiment of the utility model from the front. Figure 3 The figure is a schematic diagram of the internal structure of an embodiment of the utility model. The utility model is designed to recover the residual heat of the residual anode of an aluminum electrolytic cell with exquisite design and comprehensive functions. The main body of the device is a box body provided with a side door 4 and a connecting pipe interface 6. The top 7 of the box body is equipped with a long strip window 9. A plurality of openings 8 are evenly distributed on one side of the window. The size of these openings is slightly larger than the guide rod passing through, so as to ensure that the guide rod seal filled between the guide rod and the opening can be effectively sealed. The supporting structure 1 at the bottom 2 of the box body is supported by a frame composed of multiple support beams 10, on which a layer of detachable heat-conducting steel plate is laid, and the thickness of the steel plate is between 3mm and 7mm. The window is designed to be a structure that can be opened outward, which is convenient for operation and maintenance. Phase change energy storage material is also configured at the bottom of the box body, located under the support beam and the heat-conducting steel plate, for efficient storage and release of heat energy. Steel claws are fixed under the guide rod to firmly grasp the anode material and ensure stable operation of the device. The interface 6 is circular, and the connected pipes include the exhaust pipe for collecting residual anode flue gas and the preheating pipe for introducing flue gas into the electrolyzer, so as to realize the effective recovery and preheating of flue gas. The guide rod seal is made of high temperature resistant silicone rubber or carbon fiber material to ensure the sealing performance. The side 3 and the front 5 of the box body adopt a double-layer insulation structure, with the outer layer of metal insulation material and the inner layer of flexible insulation material, which not only ensures the structural strength but also provides a good insulation effect. The overall design takes into account the convenience of operation, heat recovery efficiency, device stability and safety, and embodies the concept of high efficiency and energy saving.

[0043] The utility model device is a waste heat recovery device for residual poles of aluminum electrolytic cells, which embodies the design concept of high efficiency, energy saving and environmental protection. First, through the design of the side door and the window that can be opened on the top, the device provides great convenience in operation and maintenance, so that operators can easily carry out daily maintenance and use. Secondly, the heat recovery function of the equipment effectively collects and utilizes the waste heat generated during the electrolysis process by using phase change energy storage materials and heat-conducting steel plates, thereby significantly reducing energy consumption and operating costs. In addition, the design of the supporting structure and seals inside the device also ensures the stability and safety of the entire system, reducing heat loss and environmental pollution. Overall, this device not only improves the efficiency of energy utilization, but also helps enterprises reduce production costs and meet environmental protection requirements. It is an economical and environmentally friendly technical solution.

[0044] like Figure 4 In one embodiment shown, Figure 4 The schematic diagram of the internal structure of an embodiment of the utility model is a top view. The utility model is designed to recover the waste heat of the residual pole of an aluminum electrolytic cell. The main body includes a box body. The side 3 and the front 5 of the box body are closed by a structure with an outer layer of metal insulation material and an inner layer of flexible insulation material. A circular interface 6 is provided on the back plate of the side of the box body to connect the pipeline, which can collect the flue gas of the residual pole to prevent the collection of the flue gas from affecting the working environment and causing pollution. At the same time, the flue gas of the electrolytic cell can be introduced to preheat the entire device, forming a joint preheating of the flue gas and the residual pole to the new pole. Phase change energy storage material is added to the bottom 2 of the box body, and the whole device is placed near the electrolytic cell. An opening 8 for the residual pole guide rod to pass through is provided at the top 7 of the box body. The opening size is slightly larger than the residual pole guide rod, so as to minimize the gap between the two after the window is closed. A window 9 that can be opened upward is provided on the top of the box body. When the residual pole or new pole needs to be placed in or taken out, the side door 4 is first opened to both sides, and then the window 9 is turned upward, and the anode is placed horizontally using an overhead crane. The open area formed at the top of the box body after the window 9 is opened is convenient for placing the anode, and is closed during heat storage and heat exchange to avoid heat loss and escape of smoke.

[0045] One opening can be provided at the top of the box for the guide rod to pass through, or multiple openings can be provided side by side according to the size of the box to accommodate multiple residual poles or new poles at the same time. A guide rod seal that cooperates with the guide rod is provided at the opening. The guide rod seal can be made of high-temperature resistant materials such as silicone rubber or carbon fiber. After the residual pole is placed in the box, the guide rod seal made of high-temperature resistant material fills the gap between the guide rod and the opening to form a seal. A support structure 1 can be provided at the bottom of the box, and the area outside the support beam is open to promote flow, so that the phase change material can flow at the bottom to better exchange heat.

[0046] In addition, the utility model transforms the tray and designs it into a waste heat recovery energy storage device to recycle the waste heat of the residual pole. A heat storage heat exchanger is designed to use phase change composite materials to perform gradient heating of the new pole from bottom to top. Phase change energy storage materials are used at the bottom of the tray to recover and store the waste heat of the residual pole. The heat storage density is large and the performance is stable. The new anode is preheated stably, which alleviates the problem of mismatch between energy supply and demand in time and space and improves the comprehensive utilization rate of energy. Asbestos is laid around the entire device (for thermal insulation) and the asbestos is fixed with screws. The screws can be controlled to increase or decrease the thickness of the asbestos. Phase change energy storage materials and thermal insulation materials are laid on the bottom. There is an inevitable opening at the top, and steel brushes are used for insulation. The overall thermal insulation measures of the device are very complete.

[0047] At the same time, it can reduce the negative effects such as splashing and de-polarization caused by the moisture of the new anode, and reduce the energy consumption caused by the fluctuation of internal heat exchange when changing the pole. Improve the stability and working efficiency of the aluminum electrolytic cell. The function of the device of the utility model is to reduce energy consumption and improve economy by recovering the waste heat of the residual pole and heating the new pole, improve the production stability of the electrolytic cell and increase energy efficiency, and to a certain extent reduce the impact of harmful gases and dust released by the open residual pole in the workshop on the environment (through the sealed insulation box to reduce the fluoride and dust released by the residual pole, improve the working environment of the electrolytic workshop, and reduce the emission of pollutants). The feasibility of the step-by-step utilization of waste heat of the device of the utility model is high. According to the characteristics of the step-by-step utilization of waste heat temperature, the heat of the anode rod can be used to prepare steam for the alumina workshop or refrigeration air conditioning after preheating the anode rod, reducing the power consumption of the air conditioner and the lighting of the factory.

[0048] Among them, the new anode is placed in the device to conduct heat with the phase change material at the bottom, and can also conduct convection heat transfer with the air in the box. At the same time, the residual pole is placed in the device to reduce heat radiation to the outside world. When the residual pole and the new pole are placed in the device at the same time, there will be heat radiation between the residual pole and the new pole, and the preheating effect of the new pole will be better. A circular interface is provided on the back panel on the side of the device to connect the pipe, which can collect the flue gas of the residual pole to prevent the collection of flue gas from affecting the working environment and causing pollution. The flue gas of the electrolytic cell can also be introduced to preheat the entire device, forming a joint preheating of the flue gas and the residual pole to the new pole. The heat-conducting steel plate is designed to be detachable, and the phase change energy storage material is also designed to be changeable. The phase change energy storage material is laid on the bottom of the device in different ways, and the thickness of the thermal insulation cotton can be increased or decreased, and the preheating effect of the device can be flexibly changed.

[0049] In summary, the utility model improves the anode tray into a heat exchange device, and uses phase change energy storage materials to recycle and store the waste heat of the residual anode, so that the recovered heat is larger and the performance is more stable; the new anode can be stably preheated; the operation is simpler, and the heat exchange process is more reasonable and standardized, which conforms to the workshop operation rules and will not add extra workload to the workshop. This device is a closed box, and its function is to reduce energy consumption and improve economy by recycling the waste heat of the residual anode and heating the new anode, improve the stability of electrolytic cell production and increase energy efficiency, and to a certain extent reduce the impact of harmful gases and dust released by the open residual anode in the workshop on the environment.

[0050] The present invention is not limited to the above-mentioned embodiments. Regardless of any changes in shape or material composition, any structural design provided by the present invention is a variation of the present invention and should be considered within the protection scope of the present invention.

Claims

1. A waste heat recovery device for aluminum electrolytic cell residual anode, comprising a box, characterized in that: Also includes: The box body is provided with a side door (4) on one side and an interface (6) for connecting a pipeline on the other side. The box body top (7) is provided with a long strip window (9), and a plurality of openings (8) are distributed on one side of the window (9). The box body bottom (2) is provided with a support structure (1), and anode materials are placed on the support structure (1).

2. The device for recovering residual heat from residual anode of an aluminum electrolysis cell according to claim 1, characterized in that: A guide rod passes through the opening (8); the size of the opening (8) is larger than the size of the guide rod; and a guide rod seal is filled between the opening (8) and the guide rod for sealing.

3. The aluminum electrolysis cell residual heat recovery device according to claim 1 or 2, characterized in that: The support structure (1) is composed of a frame structure composed of a plurality of support beams (10); the support beams (10) are covered with a layer of heat-conducting steel plate, and the heat-conducting steel plate is detachable.

4. The device for recovering residual heat from residual anode of an aluminum electrolysis cell according to claim 3, characterized in that: The window (9) is opened outwards, and a phase-change energy storage material is provided at the bottom (2) of the box body, wherein the phase-change energy storage material is located below the support beam (10) and the heat-conducting steel plate.

5. The device for recovering residual heat from residual anode of an aluminum electrolysis cell according to claim 2, characterized in that: A steel claw is fixed under the guide rod, and the steel claw grasps the anode material.

6. The device for recovering residual heat from residual anode of an aluminum electrolysis cell according to claim 1, characterized in that: The interface (6) is circular, and the connecting pipe of the interface (6) comprises an exhaust pipe for collecting spent anode flue gas and a preheating pipe for introducing flue gas into the electrolytic cell.

7. The device for recovering residual heat from residual anode of an aluminum electrolysis cell according to claim 5, characterized in that: The guide rod seal is made of silicon rubber or carbon fiber high temperature resistant material.

8. The device for recovering residual heat from residual anode of an aluminum electrolysis cell according to claim 1, characterized in that: The outer layers of the side surfaces (3) and the front surface (5) of the box body are made of metal thermal insulation material, and the inner layers are made of flexible thermal insulation material.

9. The device for recovering residual heat from residual anode of an aluminum electrolysis cell according to claim 4, characterized in that: The thickness of the heat-conducting steel plate is 3 mm to 7 mm.

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