The essence of dehydration and clarification is the solid-liquid separation of suspensions; dehydration is to remove the liquid phase from the solid phase, and clarification is to remove the solid phase from the liquid phase. Concentration is to increase the concentration of the solid phase in the suspension, and classification is to divide the particles of different particle sizes in the suspension into two groups of particles larger than dkp and smaller than dkp based on a certain critical particle size dkp. Separation generally refers to the separation of two immiscible liquid phases. If the liquid-liquid two phases are continuous phases, the separation of the two phases is only related to their densities; if one phase is a continuous phase and the other phase is a dispersed phase, it constitutes the separation of emulsions. The two-phase separation of emulsions is not only related to the density difference, but also to the dispersed phase droplet size, surface tension and other factors, and the separation is difficult. Centrifugal separation is a physical process, and the difficulty of separation depends on the material properties of the material and the separation properties of the centrifuge.
1. Centrifugal dehydration
Centrifugal dehydration can be carried out in two types: filtering centrifuges and sedimentation centrifuges.
The filter centrifuge uses filter media, such as filter screens and screens, to retain solid particles on the filter screen under the action of centrifugal force, and the liquid is discharged through the filter screen to achieve solid-liquid separation. This type of machine has low energy consumption, washable filter cakes, and high solid particle dehydration rate. It is suitable for material particles with a close solid-liquid density difference and a particle size of microns or above, and the feed liquid solid content is allowed to be 30%~60%.
If the material particles such as crystals are allowed to break during the dehydration process, a scraper unloading centrifuge can be selected. If the material particles such as crystals are not allowed to break, piston unloading, manual pushing and centrifugal unloading centrifuges can be selected. In addition to the water absorption of the material itself, the dehydration performance is also related to the separation factor, separation time, filter mesh aperture, porosity, viscosity of the material, surface tension, hydrophobicity of the filter cake layer, etc. of the centrifuge.
For particles with low solid concentration, high viscosity, fine particle size or amorphous mycelium, filter centrifuges are generally not suitable. Because the particles are too fine, they are easy to leak through the filter and cause material leakage. If the filter is too fine, it will lead to poor hydrophobicity, and the machine processing capacity and particle dehydration performance will decrease. Amorphous mycelium and oily particles are easy to clog the filter. For these materials, it is recommended to use a sedimentation centrifuge without a filter.
The sedimentation centrifuge uses the density difference between the solid and liquid phases. In the centrifugal field, the solid phase has a large density and settles on the inner wall of the centrifuge drum. The sediment is discharged out of the machine through the spiral conveyor inside the drum. The liquid phase has a small density and tends to the center of the drum and flows out from the overflow port of the machine to achieve the purpose of liquid-solid separation and dehydration. This type of machine has a high separation factor and no filter. The solid-liquid separation uses the density difference between the two phases. It is particularly suitable for amorphous mycelium, oily sludge and fine particles with high viscosity.
This model does not require high solid content in the feed. The solid content of different materials can be between 1% and 40% (mass fraction). The particle size is micron or above. However, this type of machine has poor washing effect on sediment, higher energy consumption than filtering centrifuge, and worse dehydration effect than filtering centrifuge.
2. Centrifugal clarification
Clarification refers to removing a small amount of solid phase from a large amount of liquid phase. It is widely used in the pharmaceutical, food, beverage and other industries. Because the solid content of the material is low and the particle size is small, it is necessary to use a disc separator or tubular centrifuge with a higher separation factor for separation. Sometimes precision filter or membrane separation can also be used.
3. Centrifugal concentration
Concentration can increase the solid content in the suspension. For example, if the solid content in the original suspension is 0.5% ds (mass fraction, ds is dry solid, referring to absolute dry solid), the solid content is increased by 3%~5% ds through gravity sedimentation, centrifugal sedimentation or filtration. This process is the concentration process. The liquid phase is greatly reduced during the concentration process.
For example, sludge concentration in urban sewage treatment plants: if 100m3 of secondary sludge with a solid content of 0.5% ds is concentrated to 5% ds, 90㎥ of liquid must be removed from 100㎥ of secondary sludge to achieve a solid content of 5% ds, thereby greatly reducing the subsequent sludge dewatering volume, and the model and size of the selected dewatering machine are also reduced accordingly. Commonly used centrifugal concentrators include horizontal spiral unloading sedimentation centrifuges, disc nozzle slag separators and hydrocyclones, rotary screen concentrators, etc.
4. Centrifugal classification
Particle classification is based on the different particle sizes of particles in the suspension, and their sedimentation speeds are also different. The method of using different sedimentation speeds to divide the particles in the suspension into two or more particle groups according to the particle size is called wet classification; the method of using gas medium for particle classification is called dry classification.
Wet centrifugal classification, commonly used models include horizontal spiral unloading sedimentation centrifuges, the spiral discharge port of this type of machine is in the shape of a cross long groove, so that the outlet position of the machine feed pipe can be moved axially forward and backward to change the particle sedimentation time. According to the density difference between the solid and liquid phases and the critical particle size dkp required for classification, the appropriate separation factor, differential speed and outlet position of the feed pipe can be selected, so that particles larger than dkp are settled as sediment and discharged from the slag discharge port at the small end of the drum; particles smaller than dkp remain in the liquid phase and follow the liquid phase to be discharged from the overflow port at the large end of the drum, thereby achieving continuous particle classification. In order to achieve a good classification effect, the solid-liquid ratio of the suspension should be appropriate, and an appropriate amount of dispersant should be added during the classification process to prevent small particles from agglomerating.
For particle classification with a very small processing volume, a three-legged sedimentation centrifuge can be selected, but its separation factor is low, so it cannot classify smaller particles.
5. Centrifugal separation
Centrifugal separation refers to two-phase or three-phase separation of liquid-liquid, liquid-liquid-solid. However, the phases must be inhomogeneous systems that are immiscible with each other. The principle of separation is to use the density difference between the phases. Common ones include oil-water separation and oil-soap separation in food oil processing, and oil-water-slag separation in fuel oil and lubricating oil purification. The centrifuges used are disc-type manual slag separators and disc-type piston slag separation centrifuges. For oil-water-slag separation in the purification of palm oil, coal tar, and olive oil, horizontal spiral unloading three-phase centrifuges are mostly used. For liquid-liquid and liquid-liquid-solid separation in medicine, food, and beverages, tubular separators are generally used due to the small processing capacity.
