Coarse coal slime in coal preparation products is incorporated into mixed coal as the final product, and its moisture directly affects the index of mixed coal, so controlling the moisture of coarse coal slime is of great help to improve the overall quality of commercial coal. Commercial coal users are far away from the coal preparation plant, water is too high, long-distance transportation will cause invalid transportation. At the same time, due to the great influence of winter weather, it is easy to freeze in the carriage during transportation, and the greater the moisture content, the more serious the freezing, which results in the unsafe driving of railway and the difficulty of unloading.On the one hand, wet coal preparation precipitation ensures that the quality of commercial coal meets the needs of customers, on the other hand, it can recycle and utilize water resources, reduce transportation costs, and meet the requirements of subsequent treatment operations. This paper carries on the standard sampling, the test, the data arrangement to the incoming, the discharge and the centrifuge of the centrifuge in the field operation, calculates the process performance of the equipment, whether meets the field request. For equipment that does not meet the requirements, find out the reasons for the transformation.
2. Factors of Dehydration Effect of Ring Coal Mud Centrifuge
2.1 Effect of Process Conditions on Working Effect of Centrifugal Dehydration Machine
2.1.1 Material properties
(1) Particle shape: spherical, cubic or irregular polygonal formation of the material layer of large voids, easy to dehydrate, while flat particle resistance is large, not conducive to dehydration.
(2) Particle size characteristics: generally, the larger the fine grain content, the worse the dehydration effect, the higher the moisture content of the product. The large change of feed particle size will cause the sieve basket to lose its balance in a short period of time and make the body produce additional irregular vibration. If this situation is only short time, it has no effect on the equipment; if the degree of vibration is increased and the feed is not gone when the feed stops, the cause of occurrence must be found, possibly the adhesion of the human material at the end of the sieve basket. This will produce additional vibration, resulting in uneven distribution of materials on the sieve basket.
(3) Ingredients: If the clay mineral content in the human material is large, because of its easy muddy, it will produce a lot of fine small particles, which greatly increases the content of fine particles in the material. These fine-grained clay minerals adhere to the sieve basket and remain in the centrifuge solution, resulting in difficult dehydration, which affects the next coal slime water treatment work.
(4) Moisture content of human material: when the moisture content of human material is 25%~30%, the particle size composition has a great influence on the moisture content of the product. Because of the large surface area of fine grained materials, carrying more moisture. The fine-grained material did not follow the centrifugal liquid flow, resulting in high moisture content of the product. But when the feed moisture reaches 40% to 50%, the fine grain content increases in the centrifuge liquid, while the fine grain content left in the product is relatively reduced, so the product moisture is relatively stable.
2.1.2 process conditions
（1） Effect of feed quantity:According to the statistics of the scraper discharging centrifuge currently used in ST washing and selection center, its single production capacity range is about 30~100 t/h and the load per unit area is 45-167 t/ (m2 "). When the feed quantity of centrifuge is in the above range, the change of human feed quantity has little effect on the water content of product and the loss amount of material in centrifuge liquid. When the amount of feed exceeds the above range, the amount of solid loss in the centrifugal liquid is also small, but the product moisture will rise sharply.
（2） Material speed also has an effect on the moisture content of the product: the feed speed should be kept as stable as possible, do not suddenly big and small, should be kept uniform. If the speed is not uniform and the thickness of the material layer on the sieve basket is uneven, it will lead to the imbalance of the sieve basket and the poor dehydration effect, which can not guarantee the moisture content of the product.
（3） It is absolutely not allowed to give material when the centrifuge is still. If the material into the static centrifuge, will be all from the solid end of the discharge, some of the remaining materials will be stuck in the sieve basket or block, resulting in the opening of the sieve basket out of balance, damage to the centrifuge.
（4） Appropriate centrifuge rotation number: increasing the speed of the centrifuge can reduce the moisture of the final product, by changing the diameter of the main drive tape wheel to complete. Scraper discharge centrifuge used in the washing center range 500~800 r/min.
（5） Production class to strengthen daily inspection, check fence, net, cover and other safety protection facilities, centrifuge vibration situation, attached chute, pipeline smooth, triangle belt complete, intact, tension appropriate, oil level suitable, oil quality in accordance, bearing temperature is not more than 75℃, motor temperature meets the requirements.At the same time, check the moisture of the product, check the particle condition in the centrifugal liquid, and judge whether there is a sieve basket broken. Meet problems, report and deal with them in advance. Maintenance class to maintain the centrifuge, check the transmission and lubrication, tubing, joint sealing condition, grounding protection and other safety protection device status, to ensure the normal operation of the centrifuge. Regular lubrication and maintenance work to ensure that the centrifuge for a long time without trouble safe operation.
2.2 Effects of mechanical structural parameters
(1) Separation factor: The separation factor represents the size of the centrifugal force of the centrifuge. the rotation number is fast, the centrifugal force is large, and the moisture content of the coarse slime after dehydration is low. According to the data inquiry, when the product moisture requirement is more than 10%, increasing the number of centrifuge rotation is useful to reduce the product moisture. However, the number of centrifuge sieve basket rotation too fast will speed up the wear of the sieve basket, also not conducive to product dehydration. As a result, horizontal vibration centrifuge must select the appropriate separation factor to achieve the best, the general value range of 60~100.
(2) Structural parameters of sieve basket: the height of sieve basket, the height of sieve basket of horizontal scraper centrifuge is relatively fixed, and the moving speed of material on sieve surface depends on the relative speed of sieve basket and scraper.
(3) Cone angle of sieve basket:Refers to the included Angle between the sieve basket bus and the sieve basket axis. The smaller the cone angle, the lower the solid content of centrifugal liquid, the smaller the effect on water. If the cone angle is too large, the material sliding speed difference before and after the sieve basket causes the material layer thickening at the end of the discharge, resulting in the uneven distribution of the material along the length of the sieve basket, which makes the dehydration effect of the centrifuge worse.
(4) Sieve basket diameter: mainly affects the processing capacity. In the case of constant diameter, the sudden increase in the amount of material, resulting in the sieve surface material layer too thick, affecting the smooth discharge of centrifugal liquid, affecting the moisture of the product.
(5) Height of the sieve basket: the height of the general sieve basket determines the residence time of the material on the sieve basket. when selecting the height of the sieve basket, it should be ensured that the material has enough dehydration time.The material movement speed in the scraper discharging centrifuge is determined by the relative speed of the sieve basket and the scraper, and the height of the sieve basket can be lower. Usually the height of the basket is 1 m, the dehydration time is about 2 s.
(6) Screen gap: sieve basket is the working face of centrifuge dehydration, which plays a vital role in centrifuge. the gap of the sieve surface of the slit is the main factor affecting the dehydration efficiency. the size of the gap of the screen is related to the particle size, viscosity, concentration and other factors of the treated material, especially the particle size of the material.Generally, the gap is 0.25~0.5mm, under the premise of ensuring the dehydration efficiency of the product, reducing the sieve seam can reduce the wear of the sieve strip and reduce the solid content in the centrifugal liquid; conversely, the solid content in the centrifugal liquid increases and the wear speed of the sieve strip accelerates. When other parameters are certain, the length of the sieve basket determines the residence time of the material in the sieve basket, and the longer the material residence time, the lower the moisture content of the product.
(7) Screen basket and scraper clearance: the gap between the screen basket of the scraper discharge centrifugal dehydrator and the rotor of the scraper has a great influence on the work of the centrifugal dehydrator. As the gap decreases, the amount of coal retained on the sieve surface decreases, and the load of the centrifugal dehydrator decreases, which will reduce the clogging of the screen and facilitate dehydration. If the gap increases, the adhesive material on the sieve surface will slide along the material layer. The clearance requirement is 1~3 mm. for the scraper discharge centrifuge Increase or decrease the gasket between the scraper and the hub disc, adjust the gap according to the 4:1 relationship, that is, the thickness of the gasket changes 1 mm, proportionally to the gap between the screen basket and the scraper for each change of 4 mm, so as to adjust the gap between the screen basket and the scraper. When the wear is too serious to adjust, the scraper should be replaced.Sieve basket with the differential between the blade rotor, due to the effect of blade, so dehydrated material were forced to move according to the design of the scraper helix, moving coal flow and sieve seam Angle cutting, make the effective screen seam width is big, but coal flow along the spiral movement speed, increase the processing capacity of the centrifugal dehydrator, but the moisture content increases, the centrifugal dehydration after the products dehydration effect is reduced.
3. Evaluation Standard of Dehydration Performance of Coal Mud Centrifuge
3.1 Standard sampling
(1) Coarse slime centrifuge feed: sampling before arc sieve. The dehydration effect of the arc screen directly affects the feeding situation of the centrifuge entering coarse coal slime, so it is necessary to ensure that most of the water is removed from the arc screen.The cinder should be retained on the discharge end of the arc screen, if there is no cinder, the cinder flow directly into the centrifuge, to flip or replace the arc screen. In the sampling process, we should pay attention to take the material from the end of the arc screen, it is forbidden to sample on the surface of the arc screen.
(2) Coarse slime centrifuge discharge: centrifuge under the chute or on the belt conveyor sampling. The appropriate sampling tool should be selected for the sampling of the chute under the centrifuge. the sample is sealed to write the label. When sampling on belt conveyor, full section sampling should be done to achieve standard accuracy.
(3) Coarse slime centrifuge centrifuge centrifuge liquid: centrifuge centrifuge liquid pipe sampling. Because the centrifugal liquid flow rate is fast, it is advisable to adopt a deeper sampling barrel to prevent spatter, and stipulate that the centrifugal liquid sampling should be at least 10 L.
3.2 Evaluation of Process Effect of Dehydration Equipment
(1) Evaluation Index: Industry Standard MT/ IV95-2006 stipulates that the external moisture of the product and the solid yield of the product are the main indexes to evaluate the process effect of dehydration equipment.The dehydration rate was used as an auxiliary evaluation index. But because the thermal coal preparation plant finally takes the commercial coal total moisture as the final index. So it's all moisture when you' re testing the slime.
The solid yield of the product is the percentage of the solid mass in the product to the solid mass in the feed. The formula is:
Dehydration rate is the percentage of the quantity of water removed to the water quantity of human material, and its calculation formula is:
Formula: Ys- product solid yield,%;
Yw- Dehydration rate,%
a - Solid mass concentration in man,%
b - Percentage of solid mass in centrifuge (or sieve water, filtrate),%
c - Percentage of solid mass in the product, %
(2) Production and measurement: take the feed of dehydration equipment, sieve water (or centrifugal liquid, filtrate) and dehydration products, determine their mass percent concentration for small screening test, dehydration products to determine the total moisture.
(3) Recording and determination: recording and measuring equipment model specifications, uses and handling capabilities. also, measured and recorded as needed. The main elements are:
Material properties--Coal grade, particle size composition, ash;
Equipment characteristics--Centrifuge diameter, speed, centrifugal strength, screen aperture, etc.
Operational condition-- The way of human material, the variety and amount of flocculant added in the filter feed, etc.
(1) Fill in the process effect test report form of dehydration equipment.
4 Evaluation of Dehydration Effect of Coarse Cement Centrifuge in a Coal Preparation Plant
4.1 Evaluation and Analysis of Process Effect of Horizontal Scraper Centrifuge
4.2 Process evaluation and improvement of scraper discharge centrifuge
It can be concluded from the test report that the comprehensive index evaluation of a factory in April is better than that of March. The total moisture content of the product has decreased, the solid recovery rate and dehydration efficiency of the product have been improved, and the product has reached the assessment index of 21.0%, which meets the requirements of commercial coal. The reasons for the improvement of centrifuge effect are summarized as follows:
(1) In order to reduce the content of fine particles and ensure the stability and uniformity of the material, the main measures are as follows:
① Replace the bottom flow of the hydrocyclone, from the original diameter of 80 mm to 100 mm, to ensure that the overflow of the hydrocyclone does not run coarse, the bottom flow does not appear too thin phenomenon. so as to reduce fine mud production and further improve the recovery rate of clean coal
② Adjust the feed pressure of the hydrocyclone, adjust the feed pressure of the hydrocyclone according to the moisture of the centrifuge product and the feed condition of the centrifuge to meet the feed concentration and particle size composition requirements of the centrifuge. Through the opening and closing size and the opening group number of the coal slime man-material frequency conversion pump and the man-material valve, the influence of the man-material pressure fluctuation is reduced.
③ Improve the feeding box on the arc screen, by increasing the volume of the feeding box, set up a baffle in the feeding box, so that the bottom flow from the hydrocyclone slow down, reduce spatter, in front of the feeding box by drilling the coal mud water overflow evenly flow to the arc screen, so that it evenly covered with the curved screen surface, while the arc screen added water screen to prevent the arc screen running water, to help effectively remove moisture.
④ The curved screen of running water is treated by adjusting the angle or turning the screen surface to ensure that the material passes evenly on the curved screen. Check the sieve seam of centrifuge sieve basket in each shift to ensure that the sieve basket is not blocked and damaged, and the wear of the sieve seam does not exceed the standard. Turn direction every 15 d, clean the arc screen surface with nylon rod every 5 h in operation, ensure the effect of the arc screen dehydration and desliming.
(2) Sew the sieve seam of the centrifuge sieve basket from the o.4mm to 0.35mm. if the sieve basket sieve seam is too large, the solid recovery rate will be reduced, too small dehydration effect is not good, the factory will screen basket sieve seam by 0.4mm to 0.35mm to meet the field process requirements.
(3) Professional training of sampling workers to improve the accuracy of sampling.
(4) Routine inspection to be in place. When the heart machine starts feeding, the feed quantity should be guaranteed to be half of the normal feed quantity as far as possible, and then gradually increase to the required feed quantity. In order to realize the best operation of centrifugal dehydration equipment, the quality of human material, the speed of human material, the composition of particle size and the moisture content of feed should be kept stable as far as possible. Routine inspections require the following:
① The sieve basket must be rinsed with water before starting. Heaps may fall on the net basket at downtime. when the machine starts, the uneven load generated on the basket causes the centrifuge to swing violently, and this violent swing causes the sieve basket to be damaged too early.
② The centrifuge must be fully flushed before shutdown. Always check the sieve basket to ensure that the sieve basket is not blocked and not broken. Pipes do not "leak ", centrifuge oil pressure stability, clear oil quality.
③ It is strictly prohibited for external water to enter the slime system. Strengthen the professional management of cleaning, clean water strict control, prevent the entry of external water.
④ Do a good job of coal slime moisture detection. Inspection post every hour to check the product moisture, daily coal slime sampling test, once found that the moisture is too high, to report to deal with.
⑤ Strengthen process equipment maintenance. The post carries on the comprehensive inspection to the whole system coal slime equipment every day, reports the inspection problems in time, handles in time, the processing effect does not meet the production request must carry on the renewal transformation.
In general, the factors that affect the dehydration effect of centrifuge include material properties, feed concentration, particle size, feed uniformity, feed quantity, structure parameters of centrifuge, etc. When these conditions are relatively stable, the normal operation of the centrifuge can be realized. At the same time, the safety inspection and fine maintenance of the equipment can greatly improve the performance of the equipment, reduce the number of maintenance times and maintenance intensity, which is a cost saving and benefit creation, whether from the production time, the manpower consumption of maintenance and maintenance, or the replacement and use of equipment accessories. Centrifuge is an effective equipment to control the moisture of the product. During the routine inspection and inspection and maintenance, we should pay attention to the use of the centrifuge, adjust the operation parameters in time, and make the centrifuge in the best working condition to ensure the qualified product.
Any inquiry for HCC-Intelligent Centrifuge , please contact us.
Tel/WA: +86 13185581224