A dry grinding method of heavy calcium carbonate for breathable film

Abstract

The application discloses a dry grinding method of heavy calcium carbonate special for air permeable film, and relates to the technical field of pore-forming agents for air permeable film. The steps are as follows: raw material preparation; raw material cleaning; raw material crushing: the crushing system is connected with the online cleaning, and the raw material is crushed into sand and stone materials with gradation; vertical mill grinding: no water or grinding aid is added, the mill temperature is above 85 DEG C, then the material is fed, the returned material and the feeding material are fully mixed in the vertical mill, the final gradation of the total feeding material of the vertical mill is realized, the dense material layer is reached, the grinding pressure of the vertical mill, the linear speed of the classifier wheel of the classifier and the static pressure increment of the fan are controlled; and the modified material is conveyed. The application has the following advantages: a brand-new process is adopted, no grinding aid is added, the crushing into sand and stone materials with gradation, the feeding, the returned material, the grinding pressure, the linear speed of the classifier wheel of the classifier and the air pressure and air volume of the fan are controlled, a coordination effect is produced, the requirement of narrow particle size distribution is realized, the dry powder obtained by grinding has the characteristics of low moisture and high activity, the process flow is obviously simplified, and the cost is greatly reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pore-forming agents for breathable films, and specifically relates to a dry grinding method for heavy calcium carbonate for breathable films. BACKGROUND

[0002] Breathable films are widely used in the production of baby diapers, adult paper diapers, feminine sanitary napkins and medical protective clothing, and other personal hygiene protection products. They have the functions of liquid and bacteria barrier, and due to their breathability, they provide a dry and comfortable experience for the use of these protective products.

[0003] Breathable films with liquid and bacteria barrier properties are made of modified heavy calcium carbonate particles with a narrow particle size distribution. The particles are stretched into a film to give the finished film breathability. Therefore, the particle size distribution of heavy calcium carbonate particles is one of the key indicators of the breathability and liquid and bacteria barrier properties of breathable films, and is characterized by air permeability and water pressure test.

[0004] At present, the mature process on the market is wet grinding, followed by drying and modification, to prepare breathable films that meet the application of personal hygiene protection products. The special wet grinding process successfully achieves the technical goal of narrow particle size distribution, meeting the technical needs of the application. However, due to the complexity of the manufacturing process and high cost, it restricts the promotion of market application. In recent years, some domestic heavy calcium carbonate manufacturers have adopted dry grinding plus secondary classification technology to control particle size distribution, and have made great progress. However, during the grinding process, grinding aids need to be added for grinding aid, which may cause over-grinding, resulting in excessive content of ultra-fine particles less than 0.5 microns. Compared with the particle size distribution of the product of wet grinding, there is still a technical bottleneck of excessive ultra-fine particles less than 0.5 microns, i.e. the particle size distribution is not narrow enough. The present application improves the vertical grinding process to achieve the technical goal of obtaining the same narrow particle size distribution as wet grinding through vertical grinding. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a dry grinding method for heavy calcium carbonate for breathable films, which can solve the problems of complex manufacturing process, high cost of wet grinding process, the need to add grinding aids for grinding aid in dry grinding plus secondary classification technology, excessive content of ultra-fine particles less than 0.5 microns caused by over-grinding, and not narrow enough particle size distribution.

[0006] To solve the above technical problems, the technical solution of the present application is as follows:

[0007] S1, raw material preparation: selecting high-whiteness coarse-crystal marble raw material;

[0008] S2, raw material cleaning: fully online cleaning of the raw material to wash off the secondary pollution of foreign matter received by the raw material during transportation and storage, cleaning pressure ≥ 4 kg;

[0009] S3, raw material crushing: the crushing system is connected with the online cleaning, and the raw material after online cleaning directly enters the crushing system to realize crushing while cleaning and avoid recontamination after cleaning. The raw material is crushed by a heavy hammer crusher to be ≤3 cm of sand and stone material with grading, realizing the primary stone grading of the vertical mill feed, making the vertical mill layer have a dense layer formed by grading, avoiding vibration and overgrinding. If the grading is not good, there will be gaps, so the layer is not dense, causing vibration. The uniform layer formed by grading makes the layer more uniform, and the mill roller is not easy to shake when passing through the uniform layer.

[0010] S4, vertical mill grinding: the crushed raw material is conveyed to the vertical mill feed bin by a belt conveyor and a bucket elevator. No water or grinding aid is added during the grinding process, and the grinding temperature is maintained above 85℃ to control the moisture content of the powder in the grinding stage below 1‰ and excite the surface active points of the powder. After the grinding temperature meets the requirements, the feeding starts. Quantitative return material is carried out during the vertical mill process. The quantitative return material and the feed are fully mixed by a double-shaft stirring auger to adjust the grading of the total feed, realize the ultimate grading of the total feed of the vertical mill, achieve a dense layer, control the grinding pressure of the vertical mill, the linear speed of the classifier wheel of the vertical mill classifier, and the static pressure increment of the vertical mill fan, and avoid overgrinding.

[0011] The vertical mill grinding produces a coordinated effect through the control of feeding, return material, grinding pressure, classifier wheel linear speed of the classifier, and fan pressure and air volume, realizing the requirements of low sieve residue, small top cut particle size, and few ultra-fine particles with a particle size of ≤0.5μm in narrow particle size distribution.

[0012] S5, heat preservation and conveying modification: the dry powder after grinding directly enters the feed bin of the subsequent modification process through the conveying system for timely modification under heat preservation conditions to improve the modification effect.

[0013] Further, in the step S4, the return material quantitative control is 2.0 tons / hour.

[0014] Further, in the step S4, the vertical mill grinding pressure is ≤6.0MPa.

[0015] Further, in the step S4, the linear speed of the classifier wheel of the vertical mill classifier is ≥60m / s.

[0016] Further, in the step S4, the static pressure increment of the vertical mill fan is ≥13kPa.

[0017] Further, in the step S4, after 30 minutes of grinding, sampling and detection are carried out, and after fine tuning according to the detection results, the vertical mill enters the stable process production stage.

[0018] Further, in the step S4, when the crushed raw material is conveyed by the belt conveyor, a permanent magnet iron remover installed on the belt conveyor removes the iron impurities in the raw material, and the magnetic flux of the permanent magnet iron remover reaches 12000 Gauss.

[0019] Further, in the step S4, the feeding is started for 10 minutes to make the vertical mill have a starting layer of grinding, and the feeding is stopped after 10 minutes, and the roller production is started.

[0020] Further, in the step S2, when the raw material is cleaned on line, the block stones are fed into a cleaning feed hopper by a forklift, and then fed into a vibrating batten screen for cleaning by a plate chain machine, and the block stones are uniformly laid on the batten screen and vibrated to move forward by vibration, and enter a 2-meter cleaning zone.

[0021] The cleaning zone is provided with a row of washing pipes every 25 cm, each washing pipe is provided with 10 high-pressure cleaning nozzles, the high-pressure cleaning nozzles are installed above the vibrating batten screen, and the cleaning water with a pressure of ≥4.0 kg is used to fully clean each surface of each block stone passing through the cleaning zone, and the water, foreign matters and fine ores after cleaning are discharged from the gaps between the batten screens, and the gap between the batten screens is 2.0 cm.

[0022] The advantages of the present application are that: a new process is adopted, the raw material is crushed into sand and stone materials with a grading once, the primary stone grading of the vertical mill feeding is realized, the final grading of the total feeding of the vertical mill is realized by adjusting the total feeding grading through feeding and return material, the dense layer is achieved, the vibration and over-grinding are avoided, the grinding pressure, the classification wheel linear speed of the classifier and the fan pressure and air volume are controlled, the coordination effect is produced, the narrow particle size distribution requirements of low sieve residue, small top cut particle size and few ultra-fine particles with a particle size of ≤0.5 μm are realized, the grinding aid is not needed, the over-grinding is avoided by only using the dry grinding process of the vertical mill, the technical requirements of the air permeable membrane pore-forming agent are met, and the dry powder ground by the vertical mill has the characteristics of low moisture and high activity by controlling the grinding temperature.

[0023] The process of the present application can significantly simplify the process flow, does not need the drying process matched with the wet grinding process, greatly reduces the cost, does not need the secondary classification process of the dry grinding process and avoids the defects of the particle size distribution of the secondary classification process, reduces the cost for the application and popularization of the personal protection air permeable membrane, and has significant economic benefits. Embodiment

[0024] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with specific embodiments. The following embodiments can make the skilled in the art more fully understand the present application, but the present application is not limited in the scope of the embodiments.

[0025] The specific embodiment of the present application adopts the following technical scheme: comprising the following steps:

[0026] S1, raw material preparation: select high whiteness of coarse-grained marble raw material;

[0027] S2, raw material cleaning: the raw material is fully cleaned online to wash off the secondary pollution of foreign matter received by the raw material during transportation and storage, and the cleaning pressure is greater than or equal to 4 kg;

[0028] When the raw material is cleaned online, the block stone is fed into the cleaning feed hopper by the forklift, and then fed into the vibrating batten screen for cleaning by the plate chain machine. Through vibration, the block stone is evenly laid on the batten screen and vibrates forward, entering the 2-meter cleaning area.

[0029] Every 25 cm in the cleaning area is equipped with a row of washing pipes, each of which is equipped with 10 high-pressure cleaning nozzles. The high-pressure cleaning nozzles are installed above the vibrating batten screen, and the cleaning water with a pressure greater than or equal to 4.0 kg is used to fully clean each surface of each block stone passing through the cleaning area. The water, foreign matter and powder after cleaning are discharged from the gap between the batten screen, and the gap between the batten screen is 2.0 cm.

[0030] S3, raw material crushing: the crushing system is connected with the online cleaning, and the raw material after online cleaning directly enters the crushing system to realize cleaning and crushing at the same time, avoid re-pollution after cleaning, and the raw material is crushed by a heavy hammer crusher to be ≤3 cm of sand and stone material with grading, realizing the primary stone grading of the vertical mill feed, making the vertical mill material layer dense, avoiding vibration and over-grinding;

[0031] S4, vertical mill grinding: the crushed raw material is conveyed to the vertical mill feed bin by the belt conveyor and the bucket elevator. When the belt conveyor conveys the crushed raw material, the permanent magnet iron remover installed on the belt conveyor removes the iron impurities in the raw material, and the magnetic flux of the permanent magnet iron remover reaches 12000 Gauss.

[0032] No water or grinding aid is added during the grinding process, and the grinding temperature is maintained above 85℃ to control the moisture content of the powder in the grinding stage below 1‰ and stimulate the surface active points of the powder. After the grinding temperature meets the requirements, the feeding starts. After 10 minutes of feeding, the vertical mill has a starting material layer, and the roller production starts after 10 minutes of feeding.

[0033] Quantitative return material is carried out in the vertical mill process, the quantitative control of return material is 2.0 tons / hour, the quantitative return material and the feed are fully mixed through the double-shaft stirring auger to adjust the gradation of the total feed, realize the ultimate gradation of the total feed of the vertical mill, achieve the dense layer, control the grinding pressure of the vertical mill ≤6.0 MPa, the linear speed of the classifier wheel of the vertical mill classifier ≥60 m / s, the static pressure increment of the fan matched with the vertical mill ≥13 kPa, produce the coordination effect, and avoid over-grinding; after grinding for 30 minutes, sampling detection is carried out, and after fine adjustment according to the detection result, the vertical mill enters the stable process production stage.

[0034] The grinding of the vertical mill produces the coordination effect through the control of the feed, return material, grinding pressure, linear speed of the classifier wheel of the classifier and the wind pressure and air volume of the fan, and realizes the requirements of low sieve residue, small top cut particle size, few ultra-fine particles with a particle size of ≤0.5 μm and narrow particle size distribution.

[0035] S5, heat preservation and conveying modification: the dry powder after grinding is directly fed into the feed bin of the subsequent modification process through the conveying system, and is timely modified under the heat preservation condition to improve the modification effect. Embodiment

[0036] The production process of the modified product of the professional heavy calcium carbonate of the breathable film with the product brand GT2 / 3X produced by the application is taken as an example for description:

[0037] S1, the marble block with a size of ≤500 mm and high whiteness is selected from the imported Vietnam.

[0038] S2, the block stone is fed into the cleaning feed hopper through the forklift feeding, and is fed to the vibrating slat screen for cleaning through the plate chain machine; the high-pressure cleaning nozzle is installed above the vibrating slat screen, the block stone is uniformly laid on the slat screen and vibrates forward through vibration, and enters the 2-meter cleaning area; a row of flushing pipes are arranged every 25 cm in the cleaning area, 10 high-pressure cleaning nozzles are arranged on each flushing pipe; each surface of each block stone passing through the cleaning area is fully cleaned through the cleaning water with a pressure of ≥4.0 kg, and the water, foreign matters and powder ore after cleaning are discharged from the gap between the slats of the slat screen, and the gap between the slats is designed to be 2.0 cm.

[0039] S3, the cleaned block stone directly falls into the heavy hammer crusher to realize the cleaning and crushing at the same time, avoid the pollution after cleaning again, is crushed into the stone material with a size of ≤3.0 cm, the dense layer of the vertical mill material layer is formed by the gradation, and the vibration is avoided to avoid over-grinding.

[0040] S4, the stone material is sent into the vertical mill raw material bin through the belt conveyor and the bucket elevator, and the permanent magnet iron remover with a magnetic flux of 12000 Gauss is installed on the belt conveyor to remove the iron impurities in the raw material.

[0041] The vertical mill is started and the heater is turned on. When the air temperature of the system reaches 85°C, the feeding is started for 10 minutes to make the vertical mill have a starting layer of grinding. After 10 minutes of feeding, the roller production is started. No water or grinding aid is added during the grinding process. The moisture content of the powder in the grinding stage can be controlled to be lower than 1‰. The surface active points of the powder are excited. The dry powder ground by the vertical mill has the characteristics of low moisture content and high activity.

[0042] The grinding pressure is set to 6.0 MPa, the speed of the classifier wheel of the classifier is set to 2250 rpm, and the opening of the fan is set to 100%. The coordinated effect can be achieved to avoid overgrinding. After 30 minutes of grinding, the sample is detected. After the fine adjustment according to the detection result, the vertical mill enters the stable process production stage. The quantitative return material is carried out during the vertical mill process. The quantitative control of the return material is 2.0 tons / hour. The quantitative return material and the feeding are fully mixed by the double-shaft stirring auger to adjust the gradation of the total feeding, realize the ultimate gradation of the total feeding of the vertical mill, achieve the dense layer, and avoid vibration and overgrinding.

[0043] S5, modification under heat preservation: the dry powder after grinding is directly fed into the feeding bin of the subsequent modification process through the conveying system, and is timely modified under heat preservation to improve the modification effect.

[0044] The market mainstream modified heavy calcium carbonate special for breathable film is divided into two products with a typical value of 2.0 μm of average particle size (D50) and a typical value of 2.5 μm of average particle size (D50). The particle size distribution of the product with a typical value of 2.5 μm of average particle size (D50) is taken as an example to illustrate the technical effect achieved by the present application.

[0045] The technical index of the particle size distribution is subject to the detection value of the Malven MS3000 instrument. The sieve residue of ≥45 μm and ≥20 μm involved in the particle size distribution is subject to the sieving result of the 325 mesh and 625 mesh screen of the American VWR laboratory. The particle size distribution and the sieve residue meet the following technical indexes:

[0046] 1) D98 (top cut particle size): 9.0 ± 0.5 μm;

[0047] 2) D50 (average particle size): 2.50 ± 0.15 μm;

[0048] 3) D10: ≥0.675 microns;

[0049] 4) Specific surface area (BET detection): 4.25 ± 0.25 m2 / g;

[0050] 5) Sieve residue of ≥45 μm: ≤10 ppm;

[0051] 6) Sieve residue of ≥20 μm: ≤50 ppm;

[0052] 7) Spacing: 2.30±0.05, calculation formula: (D90-D10) / D50.

[0053] The measured values ​​of the heavy calcium carbonate prepared in Example 1 are as follows: D50 is 2.52 μm; D98 is 9.08 μm; D10 is 0.705 μm; and the specific surface area (BET) is 4.25 m². 2 / g; span is 2.260; residue on ≥45μm sieve is 8ppm; residue on ≥20μm sieve is 22ppm.

[0054] The specialized vertical mill process described in Example 1 achieved particle size distribution and sieve residue that met the above technical specifications.

[0055] Comparative Example 1:

[0056] Comparative Example 1 is the same as Example 1 in other steps, except that in step S3, when the raw material is crushed by a heavy hammer crusher, the raw material is crushed into 5 cm sand and gravel in one go.

[0057] Comparative Example 2:

[0058] Comparative Example 2 is the same as Example 1 in other steps, except that in step S4, when the vertical mill is heated and the system air temperature reaches 50°C, the feeding begins, the grinding pressure is set to 6.0 MPa, the classifier wheel speed is set to 1200 rpm, the fan opening is set to 100%, and no quantitative return material is performed during the vertical mill process to extend the grinding time.

[0059] Comparative Example 3:

[0060] The wet process for preparing heavy calcium carbonate includes the following steps: Dry grinding of the selected raw ore; preparation of the heavy calcium carbonate slurry to a concentration of 45%; preheating the prepared slurry to 40°C to obtain mixture 1; pouring mixture 1 into a vertical stirred mill, using zirconia beads with a diameter of 0.7–1.0 mm as the grinding medium, and grinding at 300 r / min for 2 hours to obtain mixture 2; sieving coarse particles in mixture 2 in a centrifuge to obtain mixture 3; dehydrating mixture 3 by pressure filtration, followed by drying, pulverizing, grading, and packaging to obtain heavy calcium carbonate.

[0061] Comparative Example 4:

[0062] The conventional dry process for preparing heavy calcium carbonate includes the following steps: crushing the raw ore; grinding the crushed raw material using a horizontal ball mill with zirconia rods as the grinding media, the rods having a diameter of 30mm to 50mm and a length of 60mm to 100mm, and a grinding time of 40 minutes to 60 minutes; grinding after classification; and then grinding again after secondary classification to obtain heavy calcium carbonate.

[0063] The particle size distribution and the sieve residue and moisture of the heavy calcium carbonate prepared in Example 1 and Comparative Examples 1-4 were compared, and the comparison results are shown in Table 1.

[0064] Table 1: Comparison results of the particle size distribution and the sieve residue and moisture of the heavy calcium carbonate prepared in Example 1 and Comparative Examples 1-4

[0065]

[0066] The heavy calcium carbonates in Example 1 and Comparative Examples 1-4 were respectively applied to the preparation of the breathable film, and the air permeability of the prepared breathable film was compared, and the comparison results are shown in Table 2.

[0067] Table 2: Comparison results of the air permeability of the breathable film prepared by using the heavy calcium carbonate prepared in Example 1 and Comparative Examples 1-4

[0068]

[0069] From the above comparison results, it can be seen that, compared with Comparative Example 4, the heavy calcium carbonate prepared in Example 1 has less ultra-fine particles with a particle size of ≤0.5 μm and a narrower particle size distribution, and the desired effect can be achieved by the preparation method of Example 1; compared with Comparative Example 3, the preparation process of Example 1 is simpler and does not need to add grinding aids, which meets the technical requirements of the pore-forming agent of the breathable film, improves the processing efficiency, greatly reduces the production cost, and the particle size distribution of Example 1 is also narrower, the content of ultra-fine particles is less, and the air permeability of the breathable film prepared from the heavy calcium carbonate prepared in Example 1 is better.

[0070] From the comparison of Example 1 and Comparative Examples 1-2, it can be seen that, according to the method of Example 1, the raw material is crushed into stone material with a size of ≤3.0 mm, the stand mill material layer has a grading to form a dense material layer, and the system air temperature reaches 85℃ when the material is fed, so that the stand mill has a starting material layer for grinding, and the quantitative return material is carried out during the grinding process, the quantitative return material and the feed are fully mixed, the grading of the total feed is adjusted, the final grading of the total feed of the stand mill is realized, the dense material layer is reached, and the grinding pressure, the speed of the classifier wheel of the classifier, and the opening degree of the fan are controlled to produce a coordinated effect, which can significantly avoid vibration and over-grinding. The heavy calcium carbonate prepared in Example 1 has less ultra-fine particles with a particle size of ≤0.5 μm, a narrower particle size distribution, and lower moisture, and the air permeability of the breathable film prepared from the heavy calcium carbonate prepared in Example 1 is better.

[0071] The above shows and describes the basic principles and main features of the present application and the advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A dry grinding method for heavy calcium carbonate in a vertical mill specifically for breathable membranes, characterized in that: Includes the following steps: S1. Raw material preparation: Select high whiteness coarse-grained marble raw materials; S2. Raw material cleaning: Thoroughly clean the raw materials online to remove secondary contamination from foreign matter that may have been acquired during transportation and storage. The cleaning pressure is ≥4 kg. S3. Raw material crushing: The crushing system is connected to the online cleaning system. The raw materials after online cleaning directly enter the crushing system to achieve crushing while cleaning, avoiding re-contamination after cleaning. The raw material crushing uses a heavy hammer crusher to crush the raw materials into graded sand and gravel with a size of ≤3 cm in one go, realizing the primary stone gradation of the vertical mill feed, so that the vertical mill material layer has a dense material layer formed by gradation, avoiding vibration and over-grinding. S4. Vertical Mill Grinding: The crushed raw materials are transported to the vertical mill feed hopper via belt conveyor and bucket elevator. No water or grinding aids are added during the grinding process, and the grinding temperature is maintained above 85℃ to control the moisture content of the powder to be below 1‰ during the grinding stage and to activate the active points on the powder surface. Feeding begins after the grinding temperature reaches the required level. Quantitative return material is carried out during the vertical mill process. Quantitative return material and feed material are fully mixed by a twin-shaft stirring auger to adjust the gradation of the total feed material and achieve the ultimate gradation of the total feed material in the vertical mill, resulting in a dense material layer. The grinding pressure of the vertical mill, the linear speed of the classifying wheel of the vertical mill classifier, and the static pressure increment of the vertical mill's matching blower are controlled to avoid over-grinding. The return material quantity is controlled at 2.0 tons / hour; the grinding pressure of the vertical mill is ≤6.0MPa; the linear velocity of the classifying wheel of the vertical mill classifier is ≥60m / s; the static pressure increment of the vertical mill's matching blower is ≥13kPa; Vertical mill grinding achieves a coordinated effect by controlling the feeding, return, grinding pressure, classifier wheel speed, and fan pressure and volume, thus meeting the requirements of a narrow particle size distribution with low sieve residue, small top cut particle size, and low content of ultrafine particles ≤0.5μm. S5. Insulated Conveying Modification: The ground dry powder is directly fed into the feed hopper of the subsequent modification process through the conveying system, and is modified in a timely manner under insulated conditions to improve the modification effect.

2. The dry grinding method for heavy calcium carbonate in a vertical mill for breathable membranes according to claim 1, characterized in that: In step S4, after grinding for 30 minutes, samples are taken for testing. After fine-tuning based on the test results, the vertical mill enters the stable process production stage.

3. The dry grinding method for heavy calcium carbonate in a vertical mill for breathable membranes according to claim 1, characterized in that: In step S4, when the crushed raw material is conveyed by the belt conveyor, the permanent magnet separator installed on the belt conveyor removes iron impurities from the raw material. The magnetic flux of the permanent magnet separator reaches 12,000 gauss.

4. The dry grinding method for heavy calcium carbonate in a vertical mill for breathable membranes according to claim 1, characterized in that: In step S4, the material is fed for 10 minutes to prepare the vertical mill with an initial material layer for grinding. After 10 minutes of feeding, the mill begins production.

5. The dry grinding method for heavy calcium carbonate in a vertical mill for breathable membranes according to claim 1, characterized in that: In step S2, when the raw materials are cleaned online, the stones are fed into the cleaning feed hopper by a loader, and then fed into the vibrating slat screen for cleaning by a plate chain conveyor. Through vibration, the stones are evenly spread on the slat screen and vibrate forward into the 2-meter cleaning zone. The cleaning zone is equipped with a row of flushing pipes every 25cm. Each flushing pipe has 10 high-pressure cleaning nozzles installed above the vibrating slat screen. The cleaning water with a pressure of ≥4.0 kg thoroughly cleans all surfaces of each stone block that passes through the cleaning zone. The cleaned water, foreign objects and ore powder are discharged from the gaps between the slats of the slat screen, which are 2.0cm apart.

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