Particle classification, elutriation

Glassification. Classification (2,12,26,28) or elutriation processes separate particles by the differences in how they settle in a Hquid or moving gas stream. Classification can be used to eliminate fine or coarse particles, or to produce a narrow particle size distribution powder. Classification by sedimentation iavolves particle settling in a Hquid for a predetermined time to achieve the desired particle size and size distribution or cut. Below - 10 fim, where interparticle forces can be significant, gravitational-induced separation becomes inefficient, and cyclone and centrifugation techniques must be used. Classification also separates particles by density and shape. Raw material separation by differential sedimentation is commonly used in mineral processiag. 

For group B and D particles, nearly all the excess gas velocity (U — U,nj) flows as bubbles tnrough the bed. The flow of bubbles controls particle mixing, attrition, and elutriation. Therefore, ehitriation and attrition rates are proportional to excess gas velocity. Readers should refer to Sec. 17 for important information and correlations on Gel-dart s powder classification, minimum fluidization velocity, bubble growth and bed expansion, and elutriation. 

As the name implies, air classifiers operate on the basis of air flow. It is an elutriation process in which the separation occurs using air. Fine particles rise in the air stream and the coarse particles that are too heavy to be carried upward fall under the influence of gravity. The cut point in a classification system can be adjusted by the air velocity within the unit. 

Separation of kaolin and other clay minerals from the accompanying quartz and feldspar by elutriation and particle size classification in hydrocyclones... 

Special processes have been developed to manufacture spherical packing materials with a given particle size distribution. The size distribution is narrowed by a size classification process, e.g. by air elutriation. Typically, the average particle size of a packing for an analytical column lies between 3 and 5 xm, the dp of a packing for preparative columns ranges from 10 to 50 xm. 

Elutriation. In fluid classification, various forces act to control the separation of dispersed particles. As discussed earlier, the kinetic behavior of particles is influenced by gravitation (as with elutriators) or centrifugal or Coriolis force (in classifiers). The medium is usually water or air. In general, all fluid classifiers can be divided into two classes counterflow equilibrium (elutriation) and inverse flow separation. Elutriation is discussed briefly below (8). 

On the other hand, by means of fines elutriation, solids in the fluidized bed can be classified into fine and coarse products. Particles that are elutriated by the fluidized gas stream are known as fine products, whereas particles retained in the bed are known as coarse products. This process is called fluidized bed separation or classification or dedusting. For processes that require a certain degree of dedusting (removal of undesirable fine particles) or classification, operating gas velocity and location of gas exit should be chosen carefully in order to achieve the appropriate product cut size. Cut size refers to the critical size that separates the fine (elutriated) and coarse (remain in bed) particles. 

Further processing of the silicon carbide into the various macro- and microgrits depends upon such factors as the required particle size distribution [32] and shape, and involves breaking, grinding, sieving, air classification, and elutriation. 

Lights Removal Air classification methods are used to remove light contaminants such as dust, film and foam fi agments, and paper glass powder in a recycling operation in the absence of water [1, 10]. Two common types of air classifier systems, cyclone separators and multiaspirators, are shown in Figures 14.16 and 14.17. Other types of air classifiers are air knives, elutriators, zig-zag classifiers, and air tables. Air classifiers are rather simple equipment where control is often more art than science, and the equipment must be tuned for each stream of material. Separations of materials are based on differences in terminal velocities in an airstream and are highly dependent on particle size and shape. 

Classifiers work on the principle that the rate at which solid particles settle in a liquid depends on the particle size. The larger particles may be drawn off at the bottom and the smaller particles as an overflow at the top. A device of this type, used for the classification of very fine particles, is called an elutriator, and the operation of a series of such devices, elutriation. A suspension of about 10 per cent solids is fed into the side of a column against an upward current of water when smaller particles flow upwards and overflow into another vessel and larger particles settle out at the bottom. 

For elutriator systems, mechanical screen separators, and cyclone separators, various devices to perform the classification and removal function, include eludriator systems, mechanical system separators, and cyclone separators. Once parts have been ground, there is often a need to remove dust or oversize slivers of material from the regrind prior to its reintroduction into the process. Removal of fines can be accomplished by means of elutriation or air scalper systems which are shown in Fig. 7.24. Mechanical screen separators, such as Fig. 7.25, have the ability to remove both fines and oversize particles in one step. 

Using the elutriating stream concept, we can deduce the beneficial action of displacement washing versus dilution washing. The improved flow pattern enhances the classification of particles. Three examples are presented that show typical processes. 

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