Treatment system for copper smelting waste refractory material

Abstract

The invention belongs to the field of smelting, and particularly relates to a copper smelting waste refractory material treatment system which is characterized in that a copper smelting waste refractory material enters a crusher, discharged materials of the crusher are conveyed to a screening device through a first belt conveying device, a coarse material bin and a fine material bin are arranged at a discharge port of the screening device, and discharged materials of the coarse material bin are transferred to a converting furnace batching site; and the discharged material of the fine material bin is transferred to a smelting furnace batching site. According to the scheme, the copper smelting waste refractory material is crushed through the crusher, the copper sheet blocks are separated from the refractory bricks, then the crushed bricks and the copper sheet blocks are separated through the screening device, the copper sheet blocks and the coarse refractory bricks are returned to the converting furnace for circular treatment, the fine brick blocks are returned to the smelting furnace for circular smelting, and therefore effective treatment of the copper smelting waste refractory material is achieved.

Description

Technical Field

[0001] The invention belongs to the field of smelting, and in particular relates to a processing system for copper smelting waste refractory materials. Background Art

[0002] The recycling of smelting waste is not only an urgent need for resource reuse, but also a basic task to meet environmental protection regulations. In the process of pyrometallurgical copper smelting, some refractory linings are usually used, including metallurgical furnace linings, chute linings, etc. The types and material compositions of refractory linings in different parts are different, such as magnesia-chrome bricks, high-alumina bricks, silicon carbide bricks, chrome corundum bricks, and temperature-resistant casting materials. These refractory materials are in contact with high-temperature melts during use, and the matte or crude copper melt will inevitably penetrate into them. Therefore, when these linings are repaired and replaced, a large amount of copper metal will inevitably be taken away. In order to recover this part of copper, people have tried to use the grinding-ore dressing method, but due to the influence of impurities such as magnesium, the recovery rate is not high and the copper loss is large. Therefore, most smelters currently still use manual sorting to pick out the obvious copper-containing parts and return them to the smelting furnace, and the copper-containing parts are not obvious as waste. This not only has a large workload for manual sorting, but also a poor on-site dust environment, and it is difficult for manual sorting to distinguish and pick out fine particles of copper, and there is still some copper loss. Since a considerable part of the furnace bricks are sintered into blocks under high temperature, the blocks are too large to be directly returned to the furnace. Therefore, the large blocks of slag are usually crushed by jaw crusher and then the coarse particles are returned to the refining furnace. Since the material usually contains some large pieces of copper, in order to ensure the normal operation of the crusher, this part must be manually picked out in advance, which is labor-intensive and inefficient. In addition, the large copper that is missed often enters the jaw crusher and dies in the crusher cavity or even damages the crusher. At the same time, if fine powder is directly added to the refining operation, it will be lost with the volatilization of smoke and dust. If it is manually screened, not only is the labor intensity too high and the efficiency is low, but the working environment is also very bad. Summary of the invention

[0003] The object of the present invention is to provide a processing system for copper-smelting waste refractory materials, which can effectively separate the waste refractory materials and the copper pieces mixed therein, thereby improving the processing efficiency.

[0004] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a processing system for copper-smelting waste refractory materials, the copper-smelting waste refractory materials enter the crusher, the discharge of the crusher is transported to the screening device by the first belt conveyor, the discharge port of the screening device is provided with a coarse material bin and a fine material bin, the discharge of the coarse material bin is transported to the charging site of the blowing furnace, and the discharge of the fine material bin is transported to the charging site of the smelting furnace.

[0005] The above scheme first uses a crusher to crush the copper-smelting waste refractory materials and separate the copper sheets and refractory bricks, and then enters a screening device to separate the broken bricks from the copper sheets. The copper sheets and large coarse-grained refractory bricks are returned to the blowing furnace for circulation treatment, and the fine-grained bricks are returned to the smelting furnace for circulation smelting, thereby realizing the effective treatment of copper-smelting waste refractory materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 This is a schematic diagram of processing the copper-smelting waste refractory material of the present invention;

[0007] in: Figure 1 Middle markings: 10-annular cutter, 11-cutting part, 12-rolling part.

[0008] Figure 1 Markings: 1-scraper, 2-hopper, 3-feeder, 4-dust collection unit, 5-crusher, 6a-first belt conveyor, 6b-second belt conveyor, 7-screening device, 8-coarse material bin, 9-fine material bin.

[0009] Figure 2 The schematic diagram of the internal three-dimensional structure of the crusher; Figure 3 It is a top view of the crusher; Figure 4 is a three-dimensional diagram of a first disc cutter in the present invention; Figure 5a , 5b 5 and 5c are schematic diagrams of the working process, wherein 5b shows the housing of the device, and the housing is omitted in other views; Figure 6 It is a schematic diagram of the matching state of the roller sleeve during the working process; Figure 7a , 7b 7c are three specific implementation schemes of the roll-pressed sleeve. DETAILED DESCRIPTION

[0015] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

[0016] A system for processing copper smelting waste refractory materials, wherein the copper smelting waste refractory materials enter a crusher 5, and the discharge of the crusher 5 is conveyed to a screening device 7 by a first belt conveyor 6a. A coarse material bin 8 and a fine material bin 9 are arranged at the discharge port of the screening device 7. The discharge of the coarse material bin 8 is transported to a charging site for a blowing furnace, and the discharge of the fine material bin 9 is transported to a charging site for a smelting furnace.

[0017] Specifically, a feeder 3 with a hopper 2 is arranged upstream of the crusher 5, and the scraper 1 transfers the waste refractory materials to the hopper 2 of the feeder 3. The feeder 3 is connected to the crusher 5 through the second belt conveyor 6b.

[0018] The system further comprises a dust collecting unit 4 , and dust collecting pipelines connected to the dust collecting unit 4 are arranged at or beside the crusher 5 and the screening device 7 .

[0019] A dust collection pipeline is provided above or beside the feeder 3 and connected to the dust collection unit 4 .

[0020] The specific process is as follows: the waste refractory materials replaced during the copper smelting process are transported to the feed hopper 2 through the scraper 1, fed to the second belt conveyor 6b through the feeder 3, and then transported to the crusher 5 through the second belt conveyor 6b. The crushed materials are transported to the screening device 7 for screening through the first belt conveyor 6a. The materials on the screen enter the coarse material bin 8, which is returned to the blowing furnace for processing, and the materials under the screen enter the powder bin 9, which is returned to the smelting furnace for processing with the copper concentrate raw materials. A dust collection system 4 is set up in the scheme, and the dust at each drop point is sucked from the pipeline to the dust treatment system by the fan to eliminate environmental pollution. The collected dust is returned to the smelting furnace for processing together with the powder.

[0021] In order to realize automatic management, a chain control system can also be set up to carry out chain monitoring of the start, stop and failure of each device in the system.

[0022] After the waste refractory material is transported to the crusher 5, see Figure 2 , 3 5b, the crusher 5 comprises a housing, wherein a first rotating shaft 10A and a second rotating shaft 10B are arranged in parallel and spaced apart from each other, and the axis cores of the first rotating shaft 10A and the second rotating shaft 10B are coplanar and located in a horizontal plane;

[0023] The first rotating shaft 10A is provided with a first disc cutter 20A and a first roller pressing sleeve 30A, which form a circumferential synchronous rotation and axial limit fit with the first rotating shaft 10A; the second rotating shaft 10B is provided with a second disc cutter 20B and a second roller pressing sleeve 30B, which form a circumferential synchronous rotation and axial limit fit with the second rotating shaft 10B;

[0024] The blades of the first disc cutter 20A and the second disc cutter 20B at the corresponding positions on the first rotating shaft 10A and the second rotating shaft 10B form a shearing fit, and the circumferential roller surfaces of the first rolling sleeve 30A and the second rolling sleeve 30B at the corresponding positions on the first rotating shaft 10A and the second rotating shaft 10B form a first rolling surface 31A and a second rolling surface 31B for extruding the material;

[0025] During the rotation of the first rotating shaft 10A and the second rotating shaft 10B, the trajectories of the first disc cutter 20A and the second roller pressing sleeve 30B, and the trajectories of the first roller pressing sleeve 30A and the second disc cutter 20B avoid each other.

[0026] The copper-smelting waste refractory materials enter from the upper feed port of the crusher shell. During the rotation of the first rotating shaft 10A and the second rotating shaft 10B, the large-sized materials are first broken into medium-sized materials under the shearing action of the first disc knife 20A and the second disc knife 20B, and the broken medium-sized materials are directly moved to the rolling area composed of the first rolling surface 31A and the second rolling surface 31B on the side to be further broken into small materials, thereby ensuring the effective separation of the metallurgical furnace lining waste adhering to and mixed with crude copper or matte or anode copper blocks, and the copper blocks can be separated from the furnace lining waste by a simple screening operation of the subsequent screening device. The device not only solves the separation of copper blocks from furnace lining waste, but also realizes the crushing of furnace lining waste, significantly improves the treatment efficiency of metallurgical furnace lining waste adhering to and mixed with copper blocks, and improves the copper recovery rate.

[0027] In order to improve the separation effect of copper sheet blocks and furnace lining waste, the present application is realized by a first disc cutter 20A and a second disc cutter 20B that cooperate with each other. Preferably, a first toothed tool 21A is circumferentially distributed at the edge of the first disc cutter 20A, and a second toothed tool 21B is circumferentially distributed at the edge of the second disc cutter 20B. The first toothed tool 21A and the second toothed tool 21B have the same shape and size, and the first toothed tool 21A and the second toothed tool 21B are mirror-arranged on the symmetry plane of the first rotating shaft 10A and the second rotating shaft 10B. The tooth shapes of the first toothed cutter 21A and the second toothed cutter 21B that cooperate with each other can form a bayonet-shaped constraint area, ensuring that the material arriving between the first toothed cutter 21A and the second toothed cutter 21B is first clamped and constrained between the first toothed cutter 21A and the second toothed cutter 21B, and the material blocks will not maintain a tumbling motion state when the first rotating shaft 10A and the second rotating shaft 10B rotate. During the relative rotation of the first toothed cutter 21A and the second toothed cutter 21B, the copper sheet blocks adhered to and mixed with the metallurgical furnace lining waste will be twisted and deformed, and the metallurgical furnace lining waste will be squeezed and crushed, thereby ensuring the complete separation of the metallurgical furnace lining waste and the copper sheet blocks. It should be noted that the shearing cooperation formed by the first toothed tool 21A and the second toothed tool 21B does not strictly require seamless cooperation like the blades of daily scissors that fit each other, but the rotating surfaces of the blades that cooperate with each other on the first toothed tool 21A and the second toothed tool 21B maintain a suitable spacing, because as long as the copper sheet block is deformed, the mutual misalignment phenomenon between the bonding surface of the metallurgical furnace lining waste will occur, and the separation between the two is inevitable. The copper sheet block only needs to be deformed without being sheared into fragments, and the service life of the first toothed tool 21A and the second toothed tool 21B can be increased. In addition, the design of the same specifications and sizes of the first toothed tool 21A and the second toothed tool 21B can significantly reduce the processing cost of spare parts and reduce the reserve of spare parts.

[0028] Since the first toothed cutter 21A and the second toothed cutter 21B have the same shape and size, only the specific structure of the first toothed cutter 21A will be described in detail below.

[0029] like Figure 3 As shown, the cutting edge of the first toothed cutter 21A includes a first radial cutting edge 211A, and the cutting edge of the first toothed cutter 21A includes a first axial cutting edge 212A.

[0030] The first radial cutting edge 211A is the intersection line between the tooth front side 22A and the disc surfaces on both sides of the first disc cutter 20A.

[0031] The first axial cutting edge 212A is the intersection line between the tooth front surface 213A and the tooth top surface 214A or the tooth back surface 215A.

[0032] The first disc cutter 20A is made of tool steel plate. Figure 3 It can be seen that the first toothed tool 21A has 5 teeth, each tooth has 2 first radial blades 211A. When the first radial blade 211A on one side is damaged, it is replaced with the first radial blade 211A on the other side for cooperation, which can significantly improve the service life of the first disc cutter 20A; the main function of the first axial blade 212A is to initially clamp the material block, especially the relatively smooth surface of the copper sheet block faces the first axial blade 212A. The first axial blade 212A can ensure that it is clamped to prevent it from rolling and falling into the area where the straight roller sleeve 30 is located and difficult to be broken in time.

[0033] See also Figure 2 , 5a As shown in Figures 5a, 5b, 5c and 5d, the first rotating shaft 10A and the second rotating shaft 10B rotate in the opposite direction with the linear velocity of the adjacent side being downward. More specifically, the first rotating shaft 10A and the second rotating shaft 10B rotate in the opposite direction with the linear velocity of the adjacent side being downward. When the blade ends of the first radial blade 211A and the second radial blade 211B located at the top of the tooth are close to overlap, the first radial blade 211A and the second radial blade 211B enclose an angular area A with the opening downward. During the rotation of the rotating shaft 10, the angle between the first radial blade 211A and the second radial blade 211B changes from small to large. The clamping opening between the first radial blade 211A and the second radial blade 211B changes from large to small to clamp the material block. As the first radial blade 211A and the second radial blade 211B gradually approach, the material block is clamped. Figure 5a , 5b As shown, if there is material containing copper flakes in the angular area A, the copper flakes will inevitably be folded and accompanied by further fragmentation of the furnace lining waste, and the separation of the copper flakes and the furnace lining waste will become inevitable.

[0034] like Figure 7a , 7bAs shown in 7c, the rolling sleeve 30 is a single cam, a double cam, a triple cam, a quadruple cam or a petal-shaped cam. In order to adapt to the number of teeth of the first toothed tool 21A, the number of cams of the rolling sleeve 30 is selected accordingly. It should be noted that the long diameter circumferential surface of the cam on the rolling sleeve 30 is located in the area between adjacent teeth of the first toothed tool 21A in the circumferential direction.

[0035] More specifically, the circumferential surface of the first rolling sleeve 30A constitutes the first rolling surface 31A and the first rolling surface 31A is parallel to the axial direction of the first rotating shaft 10. During the rotation of the first rotating shaft 10A and the second rotating shaft 10B, the spacing between the first rolling sleeve 30A and the first rolling surface 31A and the first rolling surface 31B adjacent to the second rolling sleeve 30B at the same axial position on the first rotating shaft 10A and the second rotating shaft 10B is equal. In this way, the crushing degree of the furnace lining waste can be basically consistent, avoiding a large amount of powder when it is broken.

[0036] In addition, the rolling surface 31 is evenly distributed with small conical teeth 311, which can improve the anti-slip performance between the rolling surface 31 and the material. The sliding of the material between the two rolling surfaces 31 will seriously affect the breakage efficiency.

[0037] The axial dimension of the first disc cutter 20A is smaller than the axial dimension of the first roller-pressing sleeve 30A, and the axial dimension of the second disc cutter 20B is smaller than the axial dimension of the second roller-pressing sleeve 30B. This scheme firstly ensures that the first disc cutter 20A and the second disc cutter 20B at the corresponding axial positions cooperate with each other and avoid interference with other first disc cutters 20A or second disc cutters 20B. At the same time, there is enough space between the cooperating first roller-pressing sleeve 30A and the second roller-pressing sleeve 30B beside the cooperating first disc cutter 20A and the second disc cutter 20B to receive materials in time.

[0038] The first disc cutters 20A and the first rolling sleeves 30A are alternately arranged on the first rotating shaft 10 and the axial end surfaces of the first disc cutters 20A and the first rolling sleeves 30A abut against each other.

[0039] An axial limiting fit is formed between the outer end surface of the first disc cutter 20A or the first rolling sleeve 30A at the outermost end in the axial direction and the first rotating shaft 10A, and the limiting fit at at least one end is a detachable fit.

[0040] The above solution solves the problems of axial positioning and installation and disassembly of the first disc cutter 20A and the first rolling sleeve 30A.

[0041] Preferably, the radius of the tooth top circle of the first toothed cutter 21A is 20-30 mm greater than the radius of the large diameter circle of the first rolling sleeve 30A, and the spacing between the rotating surfaces of the first radial blade 211A and the second radial blade 211B on the first toothed cutter 21A and the second toothed cutter 21B is 2-3 mm. The spacing between the rotating surfaces of the first radial blade 211A and the second radial blade 211B defined in this solution ensures effective folding of the copper block without cutting or shearing the copper block, thereby achieving separation of the copper block from the furnace lining waste and eliminating the need for frequent tool repair operations.

[0042] The notable feature of this application is that the crushed materials are graded and classified, the coarse particles enter the blowing furnace for treatment, and the fine particles and powder are combined with the copper concentrate raw materials to enter the smelting furnace for treatment, so as to avoid the powder directly entering the blowing furnace and being lost with the smoke and dust, better match the adaptability of different furnace types to the materials, so that both coarse and fine materials can be returned to the furnace for treatment, and the copper recovery rate in the materials is improved.

[0043] At the same time, in terms of the classification of coarse and fine materials, a double-layer linear vibrating screen is used to mechanically screen and grade the crushed materials. The upper screen hole size of the vibrating screen is 30mm, and the lower screen hole size is 3mm. The use of a double-layer vibrating screen for screening and the reasonable setting of the screen hole size not only greatly reduces the labor intensity of manual screening, but also effectively avoids dust pollution and improves the working environment with the setting of the dust collection system. In addition, the collected dust and powder are returned to the smelting process together to avoid material loss.

[0044] The innovation of the present invention lies in:

[0045] 1. In view of the characteristics of copper smelting refractory materials, a set of fully automated processing procedures has been set up. Except for the loading and unloading of materials, no manual labor is required in the process, which greatly reduces the labor intensity, improves the working environment, and increases labor productivity.

[0046] 2. This scheme uses a special crusher to crush copper-containing refractory materials. In particular, according to the differences in material properties, the cutter is redesigned to not only meet the crushing needs of different materials, but also effectively avoid the material from being squeezed too finely, thereby avoiding the increase in metal loss in the powder due to the generation of secondary dust. At the same time, through multiple tests, the relevant parameters of the equipment are optimized, and the relevant parameters of the equipment suitable for the crushing of copper-containing refractory materials are designed, thereby eliminating the pre-sorting process of conventional crushing, reducing labor intensity and improving labor productivity.

[0047] 3. The crushed materials are graded and classified. The coarse-grained materials enter the blowing furnace for processing, and the fine-grained materials enter the smelting furnace for processing. This prevents the powder from entering the blowing furnace directly and being lost with the smoke and dust. It better matches the adaptability of different furnace types to the materials, so that both coarse and fine materials can be returned to the furnace for processing, greatly improving the recovery rate of copper in the materials.

[0048] 4. A double-layer linear vibrating screen is used to mechanically screen and grade the crushed materials, and the appropriate screen hole size is determined through experiments. It can not only improve screening efficiency and greatly reduce the labor intensity of manual screening, but also effectively avoid dust pollution with the setting of the dust collection system, improve the working environment, and further recover the dust.

[0049] Example

[0050] A certain factory processes 2,476 tons of copper-containing waste refractory materials annually, with an average copper content of 13.08%.

[0051] The copper-containing waste refractory materials are loaded into the feed hopper by the loader, and enter the special shredder (the drum speed is 74r / min, the cutter is made of wear-resistant alloy, the hardness is HRC65, the cutter shape is a "peak-spine type" with peaks and spines, the spine is 25mm lower than the peak tooth tip, the blade thickness is 150mm, and the tooth spacing is 2mm) through the chain plate feeder (GB650)-belt conveyor. After crushing, the material enters the double-layer vibrating screen (2ZS1530) by the belt conveyor, and the screened material (3-50) mm enters the coarse material bin, and then returns to the rotary furnace for processing, and the screened material (0-3) mm enters the powder bin and then returns to the flash furnace with the copper concentrate raw material. The main data are shown in Table 1.

[0052] Table 1: Processing of copper-containing waste refractory materials in a certain factory

[0053] Weight (tons) Copper content (%) Copper metal content (tons) raw material 2476 13.08 323.92 Oversize 1689 15.32 258.75 Screened material 787 8.82 65.16

[0054] From the above data, it can be seen that if the powder is not recycled or sold, 65 tons of copper metal will be lost each year. It can be seen that the use of the shredder and processing system of the present invention to process copper smelting waste refractory materials greatly improves the copper recovery rate, reduces the waste of metal resources, and also brings objective economic benefits to the enterprise.

Claims

1. A system for processing copper smelting waste refractory materials, characterized by: Copper smelting waste refractory materials enter the crusher (5), and the output of the crusher (5) is transported to the screening device (7) by the first belt conveyor (6a). The output port of the screening device (7) is provided with a coarse material bin (8) and a fine material bin (9). The output of the coarse material bin (8) is transported to the charging site of the blowing furnace, and the output of the fine material bin (9) is transported to the charging site of the smelting furnace.

2. The system for processing copper-smelting waste refractory materials according to claim 1, characterized in that: A feeder (3) with a hopper (2) is arranged upstream of the crusher (5), and the scraper (1) transfers the waste refractory material to the hopper (2) of the feeder (3). The feeder (3) is connected to the crusher (5) through a second belt conveyor (6b).

3. The system for processing copper-smelting waste refractory materials according to claim 1, characterized in that: The system further comprises a dust collection unit (4), and dust collection pipelines connected to the dust collection unit (4) are arranged at or beside the crusher (5) and the screening device (7).

4. The system for processing copper-smelting waste refractory materials according to claim 2, characterized in that: A dust collection pipeline is provided above or beside the feeder (3) and is connected to the dust collection unit (4).

5. The system for processing copper-smelting waste refractory materials according to claim 1, characterized in that: The crusher (5) comprises a first rotating shaft (10A) and a second rotating shaft (10B) which are parallel to each other and arranged at intervals, and the axis cores of the first rotating shaft (10A) and the second rotating shaft (10B) are coplanar and located in a horizontal plane; A first disc knife (20A) and a first roller pressing sleeve (30A) are arranged on the first rotating shaft (10A), and the first disc knife (20A) and the first roller pressing sleeve (30A) form a circumferential synchronous rotation and axial limit fit with the first rotating shaft (10A); a second disc knife (20B) and a second roller pressing sleeve (30B) are arranged on the second rotating shaft (10B), and the second disc knife (20B) and the second roller pressing sleeve (30B) form a circumferential synchronous rotation and axial limit fit with the second rotating shaft (10B); The blades of the first disc knife (20A) and the second disc knife (20B) at corresponding positions on the first rotating shaft (10A) and the second rotating shaft (10B) form a shear fit. The circumferential roller surfaces of the first roller pressing sleeve (30A) and the second roller pressing sleeve (30B) at corresponding positions on the second rotating shaft (10B) constitute a first roller pressing surface (31A) and a second roller pressing surface (31B) for extruding the material; During the rotation of the first rotating shaft (10A) and the second rotating shaft (10B), the trajectories of the first disc cutter (20A) and the second roller pressing sleeve (30B), and the trajectories of the first roller pressing sleeve (30A) and the second disc cutter (20B) avoid each other.

6. The copper-smelting waste refractory processing system according to claim 1 is characterized by: A first toothed tool (21A) is circumferentially distributed at the edge of the first disc cutter (20A), and a second toothed tool (21B) is circumferentially distributed at the edge of the second disc cutter (20B). The first toothed tool (21A) and the second toothed tool (21B) have the same shape and size, and are arranged in a mirror image on the symmetry plane of the first rotating shaft (10A) and the second rotating shaft (10B).

7. The copper-smelting waste refractory material processing device according to claim 6 is characterized by: The blade of the first toothed cutter (21A) includes a first radial blade (211A) or a first axial blade (212A).

8. The copper-smelting waste refractory processing system according to claim 5, 6 or 7, characterized in that: The first rotating shaft (10A) and the second rotating shaft (10B) rotate in opposite directions with the linear speed of the adjacent side being downward.

9. The copper-smelting waste refractory treatment system according to claim 5, 6 or 7, characterized in that: The first rotating shaft (10A) and the second rotating shaft (10B) rotate in opposite directions downward at a linear speed on the adjacent side. When the blade ends of the first radial blade (211A) and the second radial blade (211B) that cooperate with each other and are located at the top of the teeth are close to overlapping, the first radial blade (211A) and the second radial blade (211B) enclose an angular area (A) with an opening downward. During the rotation process of the rotating shaft (10), the angle between the first radial blade (211A) and the second radial blade (211B) changes from small to large.

10. The copper-smelting waste refractory treatment system according to claim 5, 6 or 7, characterized in that: The radius of the tooth top circle of the first toothed tool (21A) is 20 to 30 mm greater than the radius of the large diameter circle of the first rolling sleeve (30A), and the spacing between the rotating surfaces of the first radial blade (211A) and the second radial blade (211B) on the first toothed tool (21A) and the second toothed tool (21B) is 2 to 3 mm.

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