Centrifuge is the use of centrifugal force, separating liquid and solid particles or liquid and liquid mixtures of the components of the machinery. The centrifuge is mainly used to separate the solids particles from the suspension liquid in the emulsion, or to separate the two densities of different, and mutually insoluble liquids (for example, separating cream from milk); it can also be used to exclude liquids from wet solids. For example, use washing machine to dump dry and wet clothes; special Overspeed tube Separator can also separate the gas mixture of different density; using different density or particle size of solid particles in the liquid sedimentation rate of different characteristics, some sedimentation centrifuges can also be solid particles by density or granularity grading.
In ancient China, people tied the pottery jar with one end of the rope, holding the other end of the rope, rotating the clay jar, producing centrifugal force to squeeze the honey from the pottery jar, which is the early application of the principle of centrifugal separation.
Industrial centrifuges were born in Europe, such as the mid-19th century, successively appeared the three-legged centrifuge for the dehydration of textiles, and the top suspension centrifuge for separating the crystalline sugar from the sugar mill. These earliest centrifuges are intermittent and artificially drained.
Due to the improvement of slag unloading mechanism, the continuous operation of centrifuges was appeared in the 1930 of the 20th century, and the intermittent operation centrifuge was developed by automatic control.
Industrial centrifuge According to structure and separation requirements, can be divided into filter centrifuge, sedimentation centrifuge and separator three categories.
A centrifuge has a cylinder that rotates around its axis, called a drum, usually driven by a motor. After the suspension (or emulsion) joins the drum, it is quickly driven and rotating the drum with the same speed, in the centrifugal force of the components separated and discharged separately. Usually, the higher the drum speed, the better separation effect.
The principle of centrifugal separator has two kinds of centrifugal filtration and centrifugal sedimentation. Centrifugal filtration is the centrifugal pressure caused by suspending liquid in the centrifugal force field, and the effect on the filtration medium, so that the liquid through filtration medium become filtrate, the solid particles are trapped in the filtration media surface, thus realizing liquid-solid separation; centrifugal sedimentation is the principle of the rapid sedimentation stratification in centrifugal field by using different components of suspension (or emulsion) density, and the liquid-solid (or liquid-liquid) separation is realized.
There is also a kind of experimental analysis separator, can be liquid clarification and solid particles enrichment, or liquid-liquid separation, such separator has the normal pressure, vacuum, freezing conditions under the operation of different structural types.
The separation factor is an important index to measure the separation performance of centrifugal separator. It indicates the ratio of centrifugal force to the gravity of the separated material in the drum, the larger the separating factor, the quicker the separation effect is, the better the separation result is. The separation factor of centrifugal separator is generally 100~20000, the separation factor of overspeed tubular separator can be as high as 62000, and the separation factor of the speeding separator is up to 610000. Another factor determining the processing capacity of centrifugal separator is the working area of the drum and the large processing capacity of the working area.
Filter centrifuge and sedimentation centrifuge, mainly rely on enlarging drum diameter to enlarge the working face of the drum circumference; apart from the drum of the circular wall, there are additional working face, such as disc separator, disc and chamber separator inside the barrel, significantly increased the settlement working face.
In addition, the more difficult the separation of solids particles in suspension, the finer particles taken in the filtrate or separation fluid will increase, in this case, the centrifugal separator requires a high separation factor to effectively isolate the liquid viscosity of the suspension, the separation velocity slows down; the density difference of the suspension or emulsion components is large, the centrifugal sedimentation is favorable, and the suspension centrifugal filtration does not require the density difference of each component.
The selection of centrifugal separator shall be based on the size and concentration of solid particles in suspension (or emulsion), the density of solids and liquids (or two liquids), liquid viscosity, residue (or sediment) characteristics, as well as the requirements of separation, and so on comprehensive analysis to meet the residue (sediment) moisture content and filtrate (separation liquid) to clarify the degree of requirements, initially choose which type of centrifugal separator to use. Then the type and specification of centrifuges are determined according to the processing quantity and the automation requirements of the operation, and finally verified by actual test.
In general, for suspensions containing particles exceeding 0.01 mm. Filter centrifuge can be selected; for the particle small or compressible deformation of suspended liquid, the settling centrifuge is preferable to choose the separator for the low solids content, small particles and high degree of clarification of liquid in suspension.
The future development trend of centrifugal separator will be to strengthen separation performance, develop large centrifugal separator, improve slag unloading mechanism, increase special and combined rotary drum centrifuge, strengthen separation theory research and study the optimum control technology of centrifugal separation process.
The enhancement of separating performance includes increasing rotational speed, adding new impetus to centrifugal separation process, speeding up slag pushing, increasing the length of drum and prolonging the time of separating centrifugal sedimentation. The development of large centrifugal separator is mainly to increase drum diameter and use double-sided drum to improve processing capacity to reduce equipment investment, energy consumption and maintenance cost of unit volume materials. In the field of theoretical research, the fluid flow and the formation mechanism of the residue in the drum are studied, and the method of calculating the minimum separation degree and processing ability is studied.