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0 Introduction: Centrifugal dehydrators are used in coal washing plants to treat slurry water, by removing the larger suspended particles in this slurry water (mainly coal slurry particles) through collection and concentration, so that the moisture content of the product obtained by centrifugation meets the required standards. To ensure that the moisture content of the product meets the requirements, mechanical dewatering methods must be used to remove as much moisture as possible from the coal slime water. Generally, the moisture content of the dehydrated product should be ≤20%, the recovery rate of coarse slime should be ≥80%, and the recovery rate of slime should be ≥70%. The efficiency of slime dewatering is related to the properties of the slime and the choice of dewatering machinery. The choice of the appropriate type of dewatering machinery should be based on a comprehensive analysis taking into account factors such as the sedimentation properties of slime water and slime, the particle size distribution of the slime, as well as site conditions. Technical, economic, environmental, and operational management aspects must all be considered in order to make a reasonable and appropriate decision. Currently, the mechanical dewatering methods used domestically and internationally include vacuum filtration, belt press filtration, plate and frame press filtration, and centrifugal dewatering. According to the information reviewed, 100% of the slime in countries around the world is subjected to dewatering treatment, with centrifugal dewaterers being used in 41.7% of cases, plate and frame filters in 25.8%, belt filters in 21.5%, and other dewatering machines in 11%. Here, we mainly introduce the use of centrifugal dehydrators in four coal washing plants. The slime water from the four coal washing plants mainly comes from the flushing water of the dewatering screens in the plant’s production systems and the tailings from the magnetic separators. After being concentrated by a cyclone, the slime water enters a centrifuge, with a high concentration of around 20% to 40%. It has been determined that the intrinsic moisture content of coal slime is low, it exhibits no significant hydrophilicity, and it is easy to remove water from coal slime through centrifugation. Furthermore, due to the limited space available at the four coal washing plants for slurry treatment, attention should be paid to the floor area occupied by the equipment when selecting it. Due to its numerous outstanding advantages, such as low space requirement, high degree of automation, the ability to operate continuously, easy operation and management, low civil engineering costs, low cost of slime treatment, and low moisture content in the resulting product, the centrifugal dewaterer is highly suitable for the actual conditions and requirements of these four coal preparation plants. Therefore, using a centrifugal dewaterer for slime dewatering is technically sound and economically viable. Based on the size of the slime particles in the slurry water, site constraints, and requirements regarding the level of automation, the LLL1150×600B type centrifugal dewatering machine is selected. The LLL1150×600B centrifugal dehydrator has a processing capacity of 40 t/h at an inlet concentration of 30 %. The diameter of the larger end of the sieve basket is 1,150 mm; the gaps in the sieve mesh are 0.25 mm or 0.35 mm (selectable as needed). The semi-cone angle of the sieve basket is 30°. The maximum rotation speed of the centrifuge’s sieve basket is 650 rpm, while its operating speeds are 600 rpm and 520 rpm. After deciding to use a centrifuge for dewatering, other major equipment was installed accordingly to meet the requirements of the coal slime water treatment process in the coal processing plant. 1 Structure and Working Principle of the Centrifuge. The LLL1150×600B centrifugal dehydrator is composed of a gear train, frame, sieve basket, spiral scraper rotor, bell cover, feeding cone, feed pipe, control box, frame, lubrication system, etc. Working principle of the centrifuge: When coal slurry concentrated in a coal processing plant, or coal slurry with polymer flocculants added, is fed into a centrifuge that rotates at high speed (about 620 r/min), the solid particles with higher density and larger size settle rapidly under the effect of centrifugal force and accumulate between the outer wall of the sieve basket and the spiral scraper rotor. Meanwhile, the liquid and coal slurry particles with lower density and smaller size pass through the gaps in the sieve basket and gather in the water collection tank. Inside the high-speed rotating centrifuge, there is a speed difference of 5 r/min to 12 r/min between the sieve basket and the spiral scraper rotor. The sludge accumulated on the outer wall of the sieve basket and between the spiral scraper rotor is scraped off by the spiral scraper and pushed to the bottom of the sieve basket from where it is discharged through the collection funnel. Liquids with low specific gravity and small particle size, as well as the sludge particles, are discharged through the liquid discharge pipes located on both sides of the machine base. As long as the slurry water is continuously and evenly fed into the high-speed rotating centrifuge, particles with a higher specific gravity and larger size will continuously settle, aggregate, and be discharged, while the separated water is also continuously removed, thereby processing the slurry and achieving solid-liquid separation. 2 Factors affecting the dewatering efficiency of slime centrifuges (1) Unadjustable mechanical factors: sieve basket length, sieve basket diameter, pitch length, cone angle. (2) Adjustable mechanical factors: rotating angular velocity of the sieve basket, speed of the spiral scraper rotor, and speed difference. (3) Process factors: slurry properties, feed rate, dosage of polymer flocculants, feed concentration, etc. With the centrifuge model and geometric dimensions fixed, certain mechanical or process parameters can be adjusted to achieve the desired solid-liquid separation effect for the sludge in the coal slime water being processed. It is possible to change the rotation speed of the centrifuge’s basket and adjust the force generating centrifugal acceleration, thereby increasing the separation factor, which facilitates solid-liquid separation ； Conversely, reducing the separation factor will lower the speed of the sieve basket, which is not favorable for solid-liquid separation. However, increasing the speed of the sieve basket too much will inevitably increase machine wear and generate loud noise. Based on the characteristics of the slime from the four coal washing plants, the author selected two different separation factors for the two centrifuges, with the calculations as follows: Separation factor: Fr = ω2r/9.8. In the formula: ω —— angular velocity, ω = n × 2π/60, rad/s ； n——rotational speed ； r —— radius of rotation, m. First centrifuge: n = 520 r/min, r = 0.5 m, Fr = 151.15. Second centrifuge: n = 600 r/min, r = 0.5 m, Fr = 201.2. In actual operation, the moisture content of the products produced by the first centrifugal dehydrator is higher than that of the products produced by the second centrifugal dehydrator. By selecting different sieve basket gaps, it is possible to achieve a balance between the optimal concentration of separated water and the optimal moisture content of the product. When the gaps in the sieve basket are reduced, the concentration of separated water decreases, while the moisture content of the product increases. The greater the speed difference, the shorter the residence time of the slurry in the centrifuge, the higher the moisture content of the slurry, and the higher the solid content in the separated water. Conversely, the smaller the speed difference, the longer the slurry stays in the centrifuge, resulting in more thorough solid-liquid separation; however, it is necessary to prevent the slurry from clogging. Once the properties of the coal slime are determined, it is possible to adjust the feeding rate and reduce the amount fed in, thereby improving the solid-liquid separation process ； Increasing the amount of flocculant added can accelerate the solid-liquid separation process and improve the separation efficiency. 3 Flocculant dosing equipment: The performance of the equipment used for dissolving and dosing flocculants has a direct impact on the effectiveness of PAM flocculants as well as on the actual amount applied. This project adopts a PAM coagulant preparation system with automatic drug absorption, automatic dissolution, and automatic dosing. The system consists of a vacuum material feeder, a storage cylinder, a twin-screw dosing pump, a water preparation pipeline system, a dissolution tank, a dosing tank, and other components. The bagged PAM flocculant solid particles are fed into the storage cylinder using a vacuum feeder. When preparing the solution, a twin-screw dosing pump accurately measures a certain amount of PAM solid particles, which are then delivered to the dissolution tank thanks to the suction force of a water jet pump installed in the water supply pipeline system. An agitator is installed in the dissolution tank to continuously stir the water, ensuring that the PAM solid particles dissolve evenly in the water. A set of level switches is installed in the dissolution tank; when the liquid level in the tank reaches the set minimum value, it automatically sends a signal to activate the equipment needed for preparing the working solution. Once completely dissolved, the PAM working solution is automatically discharged into the dosing tank via an electromagnetic drain valve. A level switch is installed in the dosing tank; when the liquid level in the tank reaches the set value, the electromagnetic drain valve is activated, and there is also a protection function to prevent the metering pump from running idle. The dosing of the chemical is carried out by 2 metering pumps (1 in use and 1 as a spare). A meter is installed at the outlet of the dosing tank to calibrate the PAM dosage. Behind the metering pump, there are a chemical dilution pipeline and a switching device. 4 Operating performance of centrifugal dehydrators To carry out solid-liquid separation of coal slime water in coal preparation plants using centrifugal dehydrators, it is necessary to select the optimal process parameters. It is necessary to study the concentration requirements and processing volume of the concentrated slurry fed into the centrifuge, as well as the differential speed and rotation rate of the centrifuge. Additionally, it is important to examine the impact of different amounts and concentrations of polyacrylamide (PAM) on the moisture content of the product after centrifugation, the concentration of the separated liquid, and the recovery rate of the slurry. 4.1 Amount of Cationic PAM added The relationship between the amount of cationic flocculant added and the product moisture, slime recovery rate, and concentration of the separated liquid is shown in Figures 1, 2, and 3. Operating conditions: Treatment capacity of 180 m3/h ； Sludge feed concentration: 25% ； Rotational speed: 600 r/min ； Differential speed: 5.0 r/min to 10 r/min. The following conclusions can be drawn from the experiments: (1) At a certain production level, as the amount of flocculant added increases, the recovery rate of coal slime also increases. The PAM dosing rate is 1.1 g/t, and the slime recovery rate can exceed 60% ； With a PAM addition of 0.9 g/t, the recovery rate of coarse slime can exceed 80%. (2) With a PAM dosing rate in the range of 0.7 g/t to 1.2 g/t, it can be ensured that the moisture content of the centrifuged products is <20%. (3) When cationic PAM is used, the water concentration at separation is lower, resulting in better sludge removal efficiency. 4.2 Amount of anionic PAM added The relationship between the amount of anionic flocculant added and the product moisture content, slime recovery rate, and concentration of the separated liquid is shown in Figures 4, 5, and 6. The upper curve in Figure 5 represents the coarse slime recovery rate, while the lower curve represents the slime recovery rate. Operating conditions: Treatment capacity of 180 m3/h ； Rotation speed: 600 r/min ； Sludge feed concentration: 25% ； Differential speed: 5.0 r/min to 10 r/min. The following conclusions can be drawn from the experiments: (1) At a certain yield, as the amount of flocculant added increases, the coal slime recovery rate also increases. The PAM dosing rate is 1.2 g/t, and the slime recovery rate can exceed 60% ； With a PAM addition of 1.0 g/t, the recovery rate of coarse slime can exceed 80%. With a PAM dosing rate in the range of 0.8 g/t to 1.2 g/t, it can be ensured that the moisture content of the centrifuged products is <20%. (2) When anionic PAM is used, the concentration of the separation liquid is high, resulting in poor sludge removal efficiency. 4.3 Processing Capacity The relationship between processing capacity, product moisture content, and slime recovery rate is shown in Figures 7 and 8. The upper curve in Figure 8 represents the coarse slime recovery rate, while the lower curve represents the slime recovery rate. Operating condition: PAM dosing rate of 1.1 g/t (cationic) ； Rotational speed: 600 r/min ； Differential speed: 5.0 r/min to 10 r/min ； Sludge feed concentration: 25 %. The following conclusions can be drawn from the experiments: (1) As the treatment amount increases, the moisture content of the product also increases. Good treatment results can still be achieved at a processing capacity of 180 m3/h. The typical processing capacity is 140 m3/h to 160 m3/h. (2) When the processing capacity is between 140 m3/h and 160 m3/h, the recovery rate of coarse slime remains above 80%. (3) When the processing capacity is between 140 m3/h and 160 m3/h, the moisture content of the product is <20%. 4.4 Feed Concentration The relationship between feed concentration, product moisture content, and slime recovery rate is shown in Figures 9 and 10. The upper curve in Figure 10 represents the coarse slime recovery rate, while the lower curve represents the slime recovery rate. Operating conditions: Sludge inflow rate is 180 m3/h ； Rotation speed: 600 r/min ； Differential speed: 5.0 r/min to 10 r/min ； The PAM dosing rate is 1.1 g/t (anionic). The following conclusions can be drawn from the experiments: The centrifuge does not require a high feed concentration; a feed concentration of over 24% is sufficient to ensure a high slurry recovery rate (≥70%) and an appropriate product moisture content (<20%). 4.5 Centrifuge differential speed: The author has only conducted a preliminary statistical analysis of the impact of differential speed on the moisture content of the product and the concentration of the separated liquid. Operating conditions: Feed rate 160 m3/h ； Rotational speed: 600 r/min ； Feed concentration 25 % ； PAM addition amount: 1.1 g/t. Tests conducted under these operating conditions allow the following conclusions to be drawn: (1) When the differential speed is between 5.0 r/min and 10 r/min, the moisture content of the centrifuge products is less than 20%, ranging instead from 15% to 17%. However, when the differential speed reaches 12 r/min, the moisture content of the centrifuge product increases to 28%, and the concentration of the separated liquid is high. (2) The differential speed has little impact on the moisture content of centrifuge products, but a suitable differential speed should be selected based on the feed concentration and processing volume. When the processing volume is high, too small a differential speed may cause clogging of the centrifuge, while too large a differential speed will increase the moisture content in the products produced by the centrifuge. 5 Optimal operating parameters for centrifugal dewatering machines: The optimal operating parameters for the four centrifuges used for treating coal slime in the coal preparation plant are such that the concentration of the coal slime fed into the centrifuges is above 24% ； Cationic and anionic PAM (polyacrylamide) agents can be used ； The PAM addition amount is 0.7 g/t to 1.3 g/t (per ton of raw coal used) ； Centrifuge speed: 600 r/min ； The differential speed of the centrifuge is 5 r/min to 10 r/min. The centrifuge is operated under the aforementioned process parameters to treat the slime water in coal preparation plants: (1) When the dosage of the cationic flocculant is 0.7 g/t to 1.2 g/t (based on the raw coal fed into the system), the slime recovery rate exceeds 60%, the recovery rate of coarse slime is ≥80%, and the moisture content of the product is <20%. (2) When the dosage of anionic PAM is 0.8 g/t to 1.3 g/t (per ton of feed coal), the slime recovery rate is >60%, the coarse slime recovery rate is ≥80%, and the moisture content of the product is <20%. (3) When a cationic flocculant is added, the concentration of the separated liquid is low ； When an anionic flocculant is added, the concentration of the separated liquid is high. (4) A centrifuge is used for the solid-liquid separation of concentrated sludge from water treatment plants; it offers convenient operation and management, achieves good solid-liquid separation results, results in low moisture content in the product, and a low concentration in the separated liquid