Centrifugation is the use of the powerful centrifugal force generated by the high-speed rotation of the centrifuge rotor to accelerate the settling speed of particles in a liquid, separating substances with different sedimentation coefficients and buoyancy densities in a sample. Therefore, it is necessary to use a centrifuge to generate powerful centrifugal force to force these particles to overcome diffusion and undergo sedimentation movement.
A centrifuge has a cylindrical drum that rotates at high speed around its own axis, called the bowl, usually driven by an electric motor. After the suspension (or emulsion) is added to the bowl, it is quickly driven to rotate at the same speed as the bowl. Under the action of centrifugal force, the components are separated and discharged separately. Generally, the higher the bowl speed, the better the separation effect.
Centrifugation is the use of the powerful centrifugal force generated by the high-speed rotation of the centrifuge rotor to accelerate the settling speed of particles in a liquid, separating substances with different sedimentation coefficients and buoyancy densities in a sample. Centrifuges are widely used in chemical, petroleum, food, pharmaceutical, mineral processing, coal, water treatment, and marine industries. What are the classifications of centrifuges?
Industrial centrifuges can be divided into three categories according to structure and separation requirements: filtering centrifuges, sedimenting centrifuges, and separators.
Principle of filtering centrifuge: There are many small holes on the bowl wall, and a filter medium (filter cloth) is inside the wall. The suspension rotates in the bowl, and the liquid phase is thrown out through the screen by centrifugal force, while the solid particles are retained by the screen to form a filter cake, thereby achieving solid-liquid separation.
Working sequence: start, feed, filter, wash, stop, discharge. Periodic, intermittent operation. The suspension is gradually added to the bowl after startup. Under the action of the centrifugal force field, the liquid is thrown out through the filter cloth (filter screen) and the bowl holes, while the solids are retained in the filter screen to form a filter cake.
The three-foot centrifuge is the most numerous and widely used type of filtering centrifuge. When using a centrifugal filter to separate suspensions containing very small particles, it is usually necessary to use a filter cloth as the filtering medium, and the filter cake must not move relative to the filter cloth. Obviously, the simplest type of centrifuge that meets these requirements is the three-foot centrifuge. It has a vertical shaft, a solid bottom, and a semi-enclosed top cylindrical bowl. The top edge is called the bowl rim. The bowl is supported on a three-foot frame, hence the name three-foot centrifuge.
In general, the bowl cover surface is drilled with many small holes, and the inner side is covered with one or more layers of large-pore metal filter screens as a support for the bag-shaped filter cloth installed in the bowl. Before or after startup, the suspension is introduced from the top into the bowl, so that the filtrate flows out through the cover surface, while the solid particles are retained on the filter cloth.
Generally, the filter cake is removed manually after shutdown or by replacing the filter cloth bag. A more advanced three-foot centrifuge can first reduce the bowl speed to a few revolutions per minute, then push a plow-shaped blade into the filter cake to remove the solid material. The plow blade turns the solids toward the bowl axis, causing the filter cake to fall out from the opening at the bottom of the bowl. With the help of a timer, all operations can be automated.
Structure: The bowl is fixed at the lower end of a slender shaft, the upper end of which has a bearing suspension mechanism connected to the motor, and the shaft drives the bowl to rotate.
Working characteristics: low-speed upper feeding, full-speed separation, washing, dewatering, low-speed lower discharge. The speed is continuous but changes periodically.
Advantages: ① Strong adaptability to materials, suitable for suspensions of different concentrations. ② Feeding and discharging without stopping the machine, continuous operation, relatively high efficiency. ③ Relatively simple structure.
Disadvantages: ① Speed reduction during feeding and discharging, periodic operation. ② The main shaft is long and prone to bending deformation.
Widely used in: chemical, pharmaceutical, light industry, food and other industries. Such as: salt, ammonium sulfate, ammonium bicarbonate, polyvinyl chloride, etc.
Disadvantages: The scraper wears quickly and is prone to impact. The scraper blade is subject to wear, impact, and corrosion during operation, so it is most easily damaged. Therefore, it should be made of wear-resistant, corrosion-resistant, high-strength materials.
Working characteristics: uniform continuous operation, automatic continuous feeding, hydraulic pulsating piston discharge, can achieve unattended automatic operation.
Structure: bowl, screen, pusher disc, feed distributor, hollow shaft, push rod, composite oil cylinder, casing, base, hydraulic system, etc.
Advantages: ① High efficiency, high output, stable operation. ② Automatic feeding and discharging, unattended operation. ③ Strong adaptability to materials, suitable for suspensions with coarse and medium particles.
Disadvantages: ① Not suitable for fine particle suspensions. ② Sensitive to concentration fluctuations, prone to leakage. ③ Relatively complex structure. The pusher disc reciprocates at a frequency of 20-30 times per minute; the stroke is 1/10 of the bowl length.
This centrifuge classification: ① Single-stage piston pusher centrifuge. ② Multi-stage piston pusher centrifuge - each stage has a short bowl, easy to push the cake, thin filter cake layer, long residence time of the filter cake, conducive to dewatering and washing.
Advantages: ① Simple structure, small size, low cost. ② High efficiency, low power consumption. ③ Automatic feeding and discharging, unattended operation.
Disadvantages: ① Sensitive to changes in material properties and concentration, poor adaptability. ② Material residence time is not easy to control, prone to liquid carryover.
Principle: The bowl has no holes and no filter screen. The suspension rotates at high speed with the bowl. Due to the different specific gravities of the solid and liquid phases, different centrifugal inertial forces are generated. The solid particles with greater centrifugal force deposit on the inner wall of the bowl, while the liquid phase settles in the inner layer, and then they are discharged from different outlets to achieve separation.
Widely used in: chemical, petroleum, metallurgy, pharmaceutical, light industry, food, wastewater treatment, etc., to handle suspensions with particle size d10μm.
Working characteristics: continuous feeding, separation, discharge, full-speed operation, high speed n=7000~8000rpm.
Working principle: The suspension enters from the center. Under the action of centrifugal force, an annular liquid pool is formed in the bowl. The heavy phase particles settle on the inner surface of the bowl to form sediment, which is pushed by the spiral blades to the small end of the bowl for discharge. There are circular overflow holes on the large end cover of the bowl, and the clarified liquid is discharged from the overflow holes.
The main operating parameters of the decanter centrifuge are: bowl speed, speed difference between the bowl and the conveying screw, overflow port position, and feed rate; the main structural parameters are: large end inner diameter D of the bowl, bowl length L, bowl length-to-diameter ratio L/D, half cone angle α of the bowl, and the number of screw heads and pitch of the conveying screw.
The main advantages of the decanter centrifuge: automatic continuous operation, no filter cloth required, long-term continuous operation, easy maintenance; wide application range, can be used for solid dewatering, liquid clarification, classification of solid particles by size, and liquid-liquid-solid separation; good adaptability to materials, can separate solid particles in the range of 0.005mm-2mm, and suspension volume concentration range of 2%-40%. Concentration fluctuations have little effect on separation performance; compact structure, easy to achieve structural sealing, sealed machines can operate under certain positive pressure; large single-machine production capacity (large machines can reach 190m3/h), low operating costs, small footprint.
The disadvantages of the decanter centrifuge are: the sediment generally has a high liquid content; although the sediment can be washed, the washing effect is not good; the structure is relatively complex, and the machine cost is high.
Function: The suspension enters the settling section for sedimentation separation, and the clarified liquid overflows from the overflow holes at the large end; the sediment is transported by the screw conveyor to the filtration section for filtration and dewatering.
Function: The sediment is filtered through the screen in this section, the liquid is separated, and the filter cake is discharged from the large holes at the left end. If the filter cake requires washing or drying treatment, washing liquid or steam is introduced through the center hole to wash the filter cake in the filtration section.
Function: The decanter centrifuge separates solid and liquid; the disc separator further processes the liquid to obtain a liquid product with high clarity.
Main separation chamber: It is a horizontal screw centrifuge, mainly for solid-liquid separation. Most solid particles are separated and discharged by the screw conveyor.
The suspension enters the screw centrifuge section from the central tube. Under high-speed centrifugal force, solid particles are separated from the liquid, and the sediment is discharged from the small end discharge holes by the conveyor.
The liquid enters the disc separator from the overflow holes at the large end of the centrifuge for further solid-liquid separation. Fine particles are discharged from the peripheral holes, and the high-clarity liquid is discharged from the central overflow hole (9).
Working principle: Based on the density difference between the liquid-liquid two phases, under a high-speed centrifugal force field, the liquid-liquid layers are separated, with the heavy phase in the outer layer and the light phase in the inner layer, and then discharged separately to achieve separation.
The separator is a type of sedimenting centrifugal separation equipment used to separate difficult-to-separate materials. Among separators, the disc separator is the most widely used sedimenting centrifuge. It can perform both separation and clarification operations.
The disc separator is a vertical centrifuge. The bowl is mounted on the upper end of the vertical shaft and is driven by an electric motor through a transmission device to rotate at high speed. Inside the bowl, there is a set of mutually nested disc-shaped parts - discs, with small gaps between the discs. The suspension (or emulsion) is added to the bowl from the feed pipe located at the center of the bowl. When the suspension (or emulsion) flows through the gaps between the discs, solid particles (or droplets) settle on the discs under centrifugal force to form sediment (or liquid layer). The sediment slides along the disc surface, detaches from the discs, and accumulates in the largest diameter part of the bowl. The separated liquid is discharged from the liquid outlet. The solids accumulated in the bowl are removed manually after the separator stops and the bowl is disassembled, or discharged from the bowl without stopping the machine through a slag discharge mechanism.
The main operating parameters of the disc separator are: bowl speed, light liquid speed, position of the interface between light and heavy liquids, feed rate, etc.; the main structural parameters of the disc separator are: bowl inner diameter, equivalent settling area, disc size and total number of discs, slag discharge method and mechanism.
The chamber separator is a widely used centrifugal sedimentation equipment for separating suspensions with a solid content of less than 2%. Its structure, except for the bowl, is basically the same as that of the disc separator (such as transmission mechanism, base, speed measuring device, brake, etc.).
In the bowl of the chamber separator, a set of concentric separation cylinders divides the inner space of the bowl into multiple chambers connected in series (i.e., annular spaces). The inner space of the bowl is generally divided according to the principle of equal radial spacing of each chamber or equal cross-sectional area of each chamber. The suspension to be clarified is added to the center of the bowl through the feed pipe, and then flows through each chamber one by one from the center outward. The flow velocity of the liquid in each chamber gradually decreases (when the radial spacing of each chamber is equal) or remains constant (when the cross-sectional area of each chamber is equal). From the center outward, due to the increasing radius, the separation factor of each chamber increases, so the largest solid particles deposit on the inner wall of the first chamber at the center, and finer solid particles settle in the outer chambers. The clarified liquid is discharged from the bowl by a centripetal pump. Because the chamber separator has concentric separation cylinders installed in the bowl, it shortens the settling distance of solid particles and expands the settling area. Compared with the disc separator, its structure is simpler, with fewer parts in the bowl, and the bowl has a larger space for sediment, so it can be used to separate suspensions with slightly higher solid content, and it is more convenient to remove sediment and clean the bowl than the disc separator.
Features: Speed up to 15000 r/min (high-speed type separator). Separation factor Fr=13205. Can separate emulsions with a density difference greater than 10 between the two phases; can also separate suspensions with solid particles d=0.01~1.0μm or solid concentration less than 1%.
Structure: tubular bowl, bowl inner diameter 105mm, bowl length 730mm, flexible main shaft, upper and lower bearing housings, casing, base, braking device, etc.
Working principle: The emulsion enters the bowl from the bottom, and under high-speed centrifugal force, the two liquids are separated. The heavy phase liquid is in the outer layer, and the light phase liquid is in the inner layer. The heavy phase liquid is discharged through the nozzle on the bowl, and the light phase liquid is discharged through the flow channel from the center of the separation head.
The tubular centrifuge has a very high separation factor, generally 13200-50000; it is compact and small in size, and is a widely used sedimenting high-speed centrifugal separation equipment. Its structure is shown in the figure. The tubular centrifuge has a vertical structure and can be divided into clarification type and separation type. The clarification type is used for clarifying suspensions containing a small amount of solids. The suspension enters the bowl from the lower part, and during the upward flow, the solid particles deposit on the inner wall of the bowl, and the clear liquid overflows from the upper part of the bowl. The separation type is used for separating emulsions. Under centrifugal force, the emulsion is separated into two liquid layers: heavy liquid and light liquid.
The tubular centrifuge is suitable for clarifying suspensions with a solid content of less than 1%, solid particles smaller than 5μm, and a small density difference between solid and liquid; it is also suitable for separating emulsions with a density difference between light and heavy liquids less than highly dispersed emulsions. The tubular centrifuge has a relatively simple structure, small size, stable operation, and a very high separation factor, capable of separating difficult-to-separate materials. Therefore, it is widely used in chemical, pharmaceutical, biological product production, paint manufacturing, vegetable oil refining, and mineral oil processing industries.
Main uses: separating viscous suspensions with low solid content, such as clarification of varnish, fruit juice, bacterial culture liquid; classifying solid particles in suspensions by size, such as using a tubular centrifuge to classify enamel raw materials to remove impurities; separating difficult-to-separate emulsions, such as separating water from vegetable oils, fish and animal fats, and mineral oils; and for separation and particle size distribution of highly dispersed and colloidal systems.
The main disadvantage of the tubular centrifuge is intermittent operation and small bowl volume. To avoid frequent shutdowns for sediment removal, the solid content of the material separated by a tubular centrifuge should not exceed 1%.
