Inorganic pigment calcining equipment based on filler system

Abstract

The invention relates to the technical field of calcining equipment, and discloses inorganic pigment calcining equipment based on a filling system, which comprises a rotary furnace, a barrel for calcining an inorganic pigment is arranged in the rotary furnace, one end of the barrel is provided with a feed port for filling, and the other end of the barrel is provided with a discharge port for outputting the inorganic pigment. A spiral feeder extending into the inner wall of the barrel body is arranged at one end, close to the feeding hole, of the barrel body, the spiral feeder is in transmission connection with the barrel body, and the position, close to the feeding hole, of the barrel body can be cleaned while the barrel body rotates to calcine the inorganic pigment. The feeding piece is arranged at the feeding port, it is ensured that filler and pigment are evenly mixed and conveyed into the barrel, the materials entering the barrel are always kept in a uniform mixing state under conveying of the spiral feeder, it is ensured that the mixed materials are evenly conveyed into the barrel, and meanwhile the materials are prevented from being agglomerated and blocking the feeding port; and smooth conveying is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of calcination equipment, and particularly to an inorganic pigment calcination equipment based on a filler system. Background Art

[0002] Through calcination, the crystal structure of pigments can be improved, and organic substances, carbonates, moisture, metal impurities, etc. in raw materials can be effectively removed, thereby improving the purity of pigments, making their colors more pure, enhancing the stability of pigments, and changing the physical properties of pigments. For example, the color changes due to crystal form and impurity changes, and the particle size is regulated through particle agglomeration or shrinkage to meet the specific requirements of different application fields such as plastic coloring for color and particle size. Thus, the quality and applicability of inorganic pigments are comprehensively improved. At the same time, in order to achieve all-round improvement of pigment performance, inorganic pigments and fillers are ingeniously combined during the calcination process, so that the quality and application efficiency of inorganic pigments are greatly expanded and enhanced.

[0003] The cylindrical design of the calcination equipment enables the material to move in a circular motion along the cylinder wall during the rotation of the furnace bed. At the same time, due to a certain inclination angle of the furnace bed, the material will slowly move from the higher end to the lower end, thus realizing a continuous calcination process. However, when the material just entering the furnace bed has not had time to be evenly distributed under the action of the rotation of the furnace bed, it starts to move towards the bottom end due to the influence of the inclination angle. This will increase the thickness of the material near the feed port, while the material in other areas is relatively less. Moreover, after the pigment and the filler are mixed, they form an agglomerate with a certain viscosity. During the initial stage of calcination, the material shows a slight sintering phenomenon due to the increase in temperature, which will cause the material to agglomerate on the furnace bed and cannot move normally along the inclined furnace bed, resulting in local material accumulation at the feed port, and further causing uneven heating and incomplete reaction. The accumulation and excessive thickness of the material at the feed port will cause this part of the material not to be calcined synchronously with the material in other parts. Because the thermal conductivity of the accumulated material is affected, heat cannot be evenly transferred to the inside of the material, making this part of the material incompletely calcined, resulting in problems such as inconsistent crystal form transformation and incomplete impurity removal. Summary of the Invention

[0004] Technical Problems to be Solved In view of the above-mentioned disadvantages of the prior art, the present invention provides an inorganic pigment calcination equipment based on a filler system, which can effectively solve the problem of easy material accumulation at the feed port of the prior art.

[0005] To achieve the above object, the present invention is realized through the following technical solutions: The present invention provides an inorganic pigment calcination equipment based on a filler system, including: A rotary kiln, inside which there is a cylinder for calcining inorganic pigments. One end of the cylinder is provided with a feed port for filling materials, and the other end is provided with a discharge port for discharging inorganic pigments. A screw feeder is provided at one end of the cylinder near the feed inlet and penetrates into the inner wall of the cylinder. The screw feeder is connected to the cylinder in a transmission manner and can clean the space near the feed inlet of the cylinder while the cylinder rotates to calcine the inorganic pigment. A crushing member is provided at one end of the screw feeder near the cylinder to crush the agglomerated materials in the cylinder back and forth. Wherein, the other end of the crushing element is provided with a stirring element for avoiding stratification of materials during the calcination process and improving the cooling efficiency of the materials after calcination.

[0006] Furthermore, a transmission shaft is fixedly connected to the outside of the cylinder, and a linkage part is provided on the outer surface of the transmission shaft. The linkage part includes gear 1 meshingly connected to the lower surface of the transmission shaft, the other end of gear 1 is fixedly connected to a positioning shaft, a limit block is provided on the outside of the positioning shaft, the other end of the positioning shaft is fixedly connected to gear 2, and the upper surface of gear 2 is meshingly connected to gear 3, and gear 3 is located in the middle of the outer surface of the feed port.

[0007] Furthermore, the crushing part includes a fixed shaft fixedly connected to the middle part of the screw feeder, a movable groove is opened on the outer surface of the fixed shaft, a slider is slidably connected to the inner wall of the movable groove, the slider is embedded in the inner wall of the limiting groove, the limiting groove is arranged on the lower surface of the sleeve, the sleeve is sleeved on the outer wall of the fixed shaft, and a grinding block is fixedly connected to one end of the sleeve close to the cylinder.

[0008] Furthermore, the lower surface of the grinding block is configured to be a fan-shaped with inclined sides, and there is a gap between the bottom end of the grinding block and the inner wall of the cylinder in the vertical direction.

[0009] Furthermore, the stirring element includes a main shaft fixedly connected to one end of the fixed shaft, the main shaft surface is evenly provided with positioning areas, the outer surface of the positioning area is sleeved with a positioning cylinder, the inner wall of the positioning cylinder is provided with a threaded groove slidingly connected to the outer surface of the positioning area, and the outer surface of the positioning cylinder is symmetrically fixed with a stirring rod for stirring the mixed material in the cylinder.

[0010] Furthermore, the cylinder includes a preheating zone, a calcining zone and a cooling zone, which are arranged in sequence from left to right. The fixed shaft is located in the internal space of the preheating zone, and a groove 1 is provided on the inner wall of the preheating zone.

[0011] Furthermore, the other end of the preheating zone is fixedly connected to the calcining zone, the inner wall of the calcining zone is provided with a second groove, and the inner wall of the cooling zone is provided with a third groove.

[0012] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects: The present invention provides a feeding piece at the feed port to ensure that the filler and the pigment are mixed evenly and transmitted into the cylinder body at the same time. The material entering the cylinder body is always kept in a uniform mixed state under the transportation of the spiral feeder, ensuring that the mixed material is evenly transported into the cylinder body while avoiding material agglomeration and clogging at the feed port position, thereby ensuring smooth transportation.

[0013] The present invention sets three calcination spaces in the cylinder, which are a preheating zone, a calcination zone and a cooling zone from left to right. The pitch of the preheating zone is greater than that of the calcination zone and the cooling zone. The screw groove depths of the preheating zone and the calcination zone are equal. The screw pitches of the calcination zone and the cooling zone are equal, but the screw groove depth of the calcination zone is less than that of the cooling zone. When conveying materials, the materials move along the first groove to the second groove at a relatively stable speed under the push of the blades of the spiral feeder. When the materials just enter the preheating zone, due to the small pitch of the first groove, the pushing force received is relatively mild, which is convenient for the materials to smoothly enter the conveying state. As the material moves from channel 1 to channel 2, the pitch gradually increases, and the pushing distance of the spiral blade to the material becomes larger under the unit rotation angle, so that the moving speed of the material gradually increases, which is more conducive to the rapid entry of the material into the calcination state. For some materials with slightly higher viscosity, easy to adhere to the wall of the spiral groove or poor fluidity, the shallower depth of channel 1 and channel 2 is conducive to the initial entry and initial pushing of the material, avoiding the accumulation of materials due to excessive depth at the beginning. At the same time, the material at channel 2 will not be too thick, which is conducive to the uniform calcination of the material, avoiding the accumulation of too thick materials in the unit space, resulting in heat transfer obstruction, poor gas exchange, and uneven material mixing, which in turn causes uneven product temperature, incomplete reaction, abnormal particle size distribution, and ultimately causes color deviation, unstable performance and other quality problems. As the material moves forward, the gradually deepening spiral groove can accommodate more materials, so that more calcined materials can be stored in the unit space, improving space utilization, and at the same time, with the help of the material's own gravity and the continuous pushing effect of the spiral blade, the material is easier to flow to the discharge port.

[0014] The present invention provides a crushing piece at the preheating zone, and the screw feeder is connected to the transmission shaft that drives the cylinder to rotate through a linkage piece. When the cylinder rotates, gear three drives the screw feeder to rotate and transports the material into the preheating zone. At the same time, the fixed shaft at the preheating zone rotates, driving the crushing block to swing back and forth in the preheating zone, so as to crush the material gathered in the preheating zone.

[0015] The present invention provides a stirring member in the calcining zone, and the stirring rod of the stirring member rotates along with the fixed shaft. At the same time, due to the connection between the positioning zone and the threaded groove, the stirring rod rotates while reciprocating between the positioning zones, which can effectively stir the material in the calcining zone and avoid stratification of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0017] Figure 1 Schematic diagram of the overall structure of the embodiment of the present invention; Figure 2 Schematic diagram of the structure of the linkage member of the embodiment of the present invention; Figure 3 Schematic diagram of the split structure of the cylinder body of the embodiment of the present invention; Figure 4 Schematic diagram of the connection between the stirring member and the crushing member of the embodiment of the present invention; Figure 5 Schematic diagram of the split structure of the crushing member of the embodiment of the present invention; Figure 6 Schematic diagram of the split structure of the stirring member of the embodiment of the present invention.

[0018] The reference numerals in the figure respectively represent: 1, rotary kiln; 2, cylinder body; 21, preheating zone; 211, channel one; 22, calcination zone; 221, channel two; 23, cooling zone; 231, channel three; 3, feed inlet; 4, discharge opening; 5, transmission shaft; 6, linkage member; 61, gear one; 62, limit block; 63, positioning shaft; 64, gear two; 71, gear three; 72, screw feeder; 8, crushing member; 81, fixed shaft; 82, movable groove; 83, slider; 84, sleeve; 85, limit groove; 86, grinding block; 9, stirring member; 91, main shaft; 92, positioning area; 93, positioning cylinder; 94, stirring rod; 95, thread groove. Detailed implementation manners

[0019] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0020] The following will further describe the present invention with reference to the embodiments.

[0021] Embodiment:

[0022] Please refer to Figures 1-6The present invention provides a technical solution: a high-end inorganic pigment calcining equipment based on a filler system, comprising a rotary kiln 1, such as Figure 1 and Figure 2 The rotary kiln 1 shown is provided with a cylinder 2 for calcining inorganic pigments, one end of the cylinder 2 is provided with a feed port 3 for fillers, and the other end of the cylinder 2 is provided with a discharge port 4 for outputting inorganic pigments, and the end of the cylinder 2 close to the feed port 3 is provided with a screw feeder 72 that penetrates into the inner wall of the cylinder 2, and the screw feeder 72 is transmission-connected with the cylinder 2, and can clean the position of the cylinder 2 close to the feed port 3 while the cylinder 2 rotates to calcine the inorganic pigments, a transmission shaft 5 is fixedly connected to the outside of the cylinder 2, and a linkage member 6 is provided on the outer surface of the transmission shaft 5, and the linkage member 6 includes a gear 1 61 meshingly connected to the lower surface of the transmission shaft 5, and the other end of the gear 1 61 is fixedly connected to a positioning shaft 63, and a limit block 62 is sleeved on the outside of the positioning shaft 63, and the other end of the positioning shaft 63 is fixedly connected to a gear 2 64, and the upper surface of the gear 2 64 is meshingly connected to a gear 3 71, and the gear 3 71 is located in the middle of the outer surface of the feed port 3.

[0023] In the early stage of calcination, the raw materials have just entered the calcination equipment. At this time, the pigment is a mixed material containing pigments and fillers of different components. If a simple top feeding method is adopted, materials of different densities or particle sizes may begin to stratify when entering the calcining furnace under the action of gravity. In addition, if the raw materials have partially agglomerated or stratified during storage or transportation, and no effective pretreatment is performed before feeding, this state will continue to the early stage of calcination. Therefore, the present invention provides a feeding piece at the feed port 3 to ensure that the filler and pigment are mixed evenly and transferred to the cylinder 2. The material entering the cylinder 2 always maintains a uniform mixing state under the conveyance of the screw feeder 72, ensuring that the mixed material is evenly conveyed into the cylinder 2 while avoiding material agglomeration and clogging at the feed port 3, thereby ensuring smooth transportation.

[0024] refer to Figure 3, the inside of the cylinder body 2 is provided with three calcination spaces, which are the preheating zone 21, the calcination zone 22, and the cooling zone 23 in sequence from left to right. The pitch of the preheating zone 21 is greater than that of the calcination zone 22 and the cooling zone 23. The screw groove depths of the preheating zone 21 and the calcination zone 22 are equal. The pitches of the calcination zone 22 and the cooling zone 23 are equal, but the screw groove depth of the calcination zone 22 is less than that of the cooling zone 23. When conveying materials, the materials are pushed by the blades of the screw feeder 72 and move along the channel 211 towards the calcination zone 22 at a relatively stable speed. When the materials just enter the channel 211 of the preheating zone 21, due to the smaller pitch, the pushing force received is relatively gentle, which is convenient for the materials to smoothly enter the conveying state. As the materials move forward along the channel 211 to the channel 221, the pitch gradually increases, and the pushing distance of the blades of the screw feeder 72 on the materials becomes larger under a unit rotation angle, so that the moving speed of the materials gradually increases, which can effectively prevent the materials from accumulating at one end of the cylinder body 2 close to the feed port 3. For some materials with slightly higher viscosity, easy to adhere to the screw groove wall or poor fluidity itself, the shallower screw groove depth of the feed port 3 helps the initial entry and preliminary pushing of the materials, avoiding material accumulation caused by excessive depth at the beginning. At the same time, the materials at the channel 221 will not be too thick, which helps the uniform calcination of the materials, avoiding the problems of blocked heat transfer, poor gas exchange, uneven material mixing due to excessive accumulation of materials in a unit space, and further causing quality problems such as uneven product temperature, incomplete reaction, and abnormal particle size distribution, and finally resulting in color deviation and unstable performance of the product. As the materials move forward to the channel 231, the gradually deepening screw groove can accommodate more materials, enabling more calcined materials to be stored in a unit space, improving the space utilization rate. At the same time, with the help of the self-gravity of the materials and the continuous pushing action of the spiral blades, the materials are more likely to flow towards the discharge port.

[0025] At the same time, during the calcination process, the materials on the side close to the heat source heat up faster, and the physical properties of this part of the materials, such as viscosity and fluidity, may change first, resulting in stratification with other materials that have not yet heated up. The calcination temperature and heating rate are important influencing factors. Due to the too fast heating rate, the internal changes of the materials cannot be evenly adjusted in time, which will cause the changes of some components to be ahead of other components. Some low-melting components in the pigment may quickly melt and aggregate, while other components are still in a solid state, causing blockage at the position of the cylinder body 2 close to the feed port 3. Therefore, in this application, a crushing part 8 is provided at one end of the screw feeder 72. The screw feeder 72 is connected to the transmission shaft 5 that drives the rotation of the cylinder body 2 through a linkage part 6. While the cylinder body 2 rotates, the gear three 71 drives the screw feeder 72 to rotate and convey the materials into the preheating zone 21. At the same time, the fixed shaft 81 at the preheating zone 21 rotates, driving the grinding block 86 to swing back and forth at the position of the preheating zone 21, which can crush the materials accumulated in the preheating zone 21.

[0026] Reference Figure 4 andFigure 5 One end of the screw feeder 72 close to the cylinder body 2 is provided with a grinding member 8 for grinding the agglomerated materials in the cylinder body 2. The grinding member 8 includes a fixed shaft 81 fixedly connected to the middle part of the screw feeder 72. An activity groove 82 is formed on the outer surface of the fixed shaft 81. A slider 83 is slidably connected to the inner wall of the activity groove 82. The slider 83 is embedded in the inner wall of the limit groove 85. The limit groove 85 is arranged on the lower surface of the sleeve 84. The sleeve 84 is sleeved on the outer wall of the fixed shaft 81. One end of the sleeve 84 close to the cylinder body 2 is fixedly connected with a grinding block 86. The lower surface of the grinding block 86 is a fan shape with inclined sides, and there is a gap between the bottom end of the grinding block 86 and the inner wall of the cylinder body 2 in the vertical direction.

[0027] The rotation of the screw feeder 72 drives the rotation of the fixed shaft 81. The fixed shaft 81 and the sleeve 84 rotate under the restriction of the slider 83. The slider 83 slides on the inner wall of the limit groove 85. When the slider 83 slides to one end of the inner wall of the limit groove 85, the slider 83 applies a force to the sleeve 84, and the sleeve 84 drives the grinding block 86 to swing reciprocally. The grinding block 86 swings on the inner wall of the channel 211, and can grind the materials gathered in the preheating area 21 without disturbing the passage of the un-gathered materials.

[0028] The layering situation in the middle stage of calcination. As the calcination process progresses, the temperature gradually rises, and the physical and chemical changes of the materials intensify. Since the inorganic pigments and fillers are mixed into a composite material, when some pigment components undergo crystal form transformation, their density or volume may change, resulting in layering with other untransformed components. At the same time, for some pigments at high temperatures, the organic substances on their surfaces may melt or decompose, increasing the viscosity between the materials. At this time, the mixing of the materials only depends on the rotation of the calcination equipment itself, and the power for turning the materials is insufficient, and the more viscous parts will gather together to form layering.

[0029] The rotation of the fixed shaft 81 drives the rotation of the main shaft 91. The main shaft 91 drives the rotation of the positioning area 92. The outer surface of the positioning area 92 is slidably connected with the thread groove 95 of the positioning cylinder 93. As the main shaft 91 rotates, it can drive the positioning cylinder 93 to reciprocally slide along the positioning area 92 while rotating. The positioning cylinder 93 drives the stirring rod 94 to rotate, thereby stirring and turning the materials in the calcination area 22 to prevent the materials from gathering together and causing layering.

[0030] In a continuous calcination furnace, when the residence time of the materials in a certain area in the furnace exceeds the normal range, the materials in this area may be layered with the materials in other areas due to continuous reactions. Therefore, the stirring rod 94 in the present invention can reciprocally move under the restriction of the positioning area 92 to dynamically stir the materials in this area, improve the stirring effect, and reduce the probability of material layering.

[0031] In the later stage of calcination, the pigment has basically completed the main physical and chemical changes and begins to enter the cooling process. During the cooling process, natural cooling and forced air cooling are usually adopted, and the temperature distribution and shrinkage of the pigment will be different. Forced air cooling may cause the surface temperature of the pigment to drop rapidly while the internal temperature drops slowly. This temperature gradient will cause the pigment to delaminate. If the cooling rate is uneven, the shrinkage degree of the pigment will be different, resulting in delamination. Especially for some pigments containing multiple components, the thermal expansion coefficients and shrinkage rates of different components are different, and stress differences will be generated during the cooling process, causing the pigment to delaminate. The cooling method and speed have a significant impact on later delamination.

[0032] Therefore, in addition to playing the role of dispersing the delaminated materials like the stirring member 9 in the calcination zone 22, the partial structure of the stirring member 9 located in the cooling zone 23 can also stir the inorganic pigment. During the stirring process, the materials are continuously turned over, and the materials originally gathered together are dispersed. When the stirring member 9 rotates, it will drive the pigment to circulate and turn over. The hotter part of the material and the cooler part of the material continuously mix and exchange positions. In the area near the cooling wall surface, the material is quickly taken away after the temperature drops, and at the same time, the hotter material is brought to the vicinity of the cooling wall surface. This cycle is repeated to improve the cooling efficiency.

[0033] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the protection scope of the technical solutions of the embodiments of the present invention.

Claims

1. An inorganic pigment calcining device based on a filler system, characterized in that: include: A rotary kiln (1), wherein a cylinder (2) for calcining inorganic pigments is arranged inside the rotary kiln (1), a feed port (3) for filling material is arranged at one end of the cylinder (2), and a discharge port (4) for discharging the inorganic pigments is arranged at the other end of the cylinder (2); A screw feeder (72) is provided at one end of the cylinder (2) near the feed inlet (3) and penetrates into the inner wall of the cylinder (2). The screw feeder (72) is in driving connection with the cylinder (2) and can clean the space near the feed inlet (3) of the cylinder (2) while the cylinder (2) rotates to calcine the inorganic pigment. A crushing member (8) is provided at one end of the screw feeder (72) near the cylinder (2) for reciprocatingly crushing agglomerated materials in the cylinder (2); Wherein, the other end of the crushing element (8) is provided with a stirring element (9) for preventing material stratification during the calcination process and improving the cooling efficiency of the material after calcination.

2. The inorganic pigment calcining equipment based on the filler system according to claim 1, characterized in that: The outer side of the cylinder (2) is fixedly connected to a transmission shaft (5), and the outer surface of the transmission shaft (5) is provided with a linkage member (6), and the linkage member (6) comprises a gear 1 (61) meshingly connected to the lower surface of the transmission shaft (5), and the other end of the gear 1 (61) is fixedly connected to a positioning shaft (63), and the outer side of the positioning shaft (63) is provided with a limit block (62), and the other end of the positioning shaft (63) is fixedly connected to a gear 2 (64), and the upper surface of the gear 2 (64) is meshingly connected to a gear 3 (71), and the gear 3 (71) is located in the middle of the outer surface of the feed port (3).

3. The inorganic pigment calcining equipment based on the filler system according to claim 1, characterized in that: The crushing element (8) comprises a fixed shaft (81) fixedly connected to the middle part of the screw feeder (72); a movable groove (82) is provided on the outer surface of the fixed shaft (81); a slider (83) is slidably connected to the inner wall of the movable groove (82); the slider (83) is embedded in the inner wall of a limiting groove (85); the limiting groove (85) is arranged on the lower surface of a sleeve (84); the sleeve (84) is sleeved on the outer wall of the fixed shaft (81); and a grinding block (86) is fixedly connected to one end of the sleeve (84) close to the cylinder body (2).

4. The inorganic pigment calcining equipment based on the filler system according to claim 3 is characterized in that: The lower surface of the grinding block (86) is arranged in a fan shape with inclined sides, and a gap exists between the bottom end of the grinding block (86) and the inner wall of the cylinder (2) in the vertical direction.

5. The inorganic pigment calcining equipment based on the filler system according to claim 1, characterized in that: The stirring member (9) comprises a main shaft (91) fixedly connected to one end of a fixed shaft (81); a positioning area (92) is evenly arranged on the surface of the main shaft (91); a positioning cylinder (93) is sleeved on the outer surface of the positioning area (92); a threaded groove (95) is provided on the inner wall of the positioning cylinder (93) and is slidably connected to the outer surface of the positioning area (92); and stirring rods (94) for stirring the mixed material in the cylinder body (2) are symmetrically fixed on the outer surface of the positioning cylinder (93).

6. The inorganic pigment calcining equipment based on the filler system according to claim 3 is characterized in that: The cylinder (2) comprises a preheating zone (21), a calcining zone (22) and a cooling zone (23), wherein the preheating zone (21), the calcining zone (22) and the cooling zone (23) are arranged in sequence from left to right, the fixed shaft (81) is located in the internal space of the preheating zone (21), and the inner wall of the preheating zone (21) is provided with a groove 1 (211).

7. The inorganic pigment calcining equipment based on the filler system according to claim 6, characterized in that: The other end of the preheating zone (21) is fixedly connected to the calcining zone (22), the inner wall of the calcining zone (22) is provided with a second groove (221), and the inner wall of the cooling zone (23) is provided with a third groove (231).

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