Feed Particle Classification

Nonopaque materials, those that allow the transmission of light, can be optically classified as isotropic or anisotropic. Isotropic materials (e.g., oil, water, epoxy, crystals in the isometric system) transmit light in all directions at the same velocity. Anisotropic materials (e.g., alite, calcite, quartz, all crystals in systems other than the isometric) divide the light into 

Sand and sandstone are assemblages of quartz, chert, feldspar, mica, fragments of feldspar-rich igneous, sedimentary, and metamorphic rocks, fossils, clay, and other constituents in various percentages. Sandstones contain cements, normally calcite or quartz, or a clay matrix, that bind the grains together. These rocks, therefore, typically contain a mixture of monocrystalline and polycrystalline particles. 

After one becomes familiar with the microscopical aspects of the quarry rocks and the raw feed, evaluation of the feed with the use of burnability equations or other routine microscopical procedures can be initiated. Burnability equations help us to organize om thought about the suitability of various raw materials — that is, how easily they combine in the kiln — and can provide a basis for feed comparison and improvement, even though the equations are not used to determine free lime produced in a laboratory test. 

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In sohd—sohd separation, the soHds are separated iato fractions according to size, density, shape, or other particle property (see Size reduction). Sedimentation is also used for size separation, ie, classification of soHds (see Separation, size separation). One of the simplest ways to remove the coarse or dense soHds from a feed suspension is by sedimentation. Successive decantation ia a batch system produces closely controUed size fractions of the product. Generally, however, particle classification by sedimentation does not give sharp separation (see Size MEASUREMENT OF PARTICLES). 

The product withdrawal pipe is located in the annulus halfway up the draft tube where the vector of the circulation velocity of the suspension and that of the withdrawal velocity of the product point in the same direction. Using this arrangement ensures low particle classification effects. Furthermore, the feed tubes and withdrawal tube can be exactly positioned in the reactor and scaled... 

The Matsuzaka Elbow-Jet classifier (Fig. 11) is based on a transverse flow principle (26). The stream of feed particles are accelerated to minimize the effect of gravity, and introduced into an air jet at right angles. The particles are fanned out in the classification zone with the trajectories for particles of the same hydrodynamic behavior, ie, size and shape, being the same. Classification is achieved by mounting one or more cutters in the classification zone, thus dividing the feed into two or more fractions. A stream of fine particles of less than 5 Jm can be produced in this manner. 

Let /Ji(yj,Xi,tn) denote the total amount of particles in the (/,/)th cell at time tn. Then, the governing equations of the system [6,7] because of the boundary conditions at the inlet and outlet of the mill depending on the feed and classification conditions can be divided into three groups. Namely,... 

If the dry milling equipment does not include internal classification, or if a separate classifier is not part of the equipment train, feed particle size may affect milled particle size. Additionally, as one main mechanism for particle breakage in these mills is interparticle collisions, the density of solids in the mill or the solids feed rate can be an important parameter to control to obtain reproducible milling results. 

Where such a read in size exists, and vhere doth blinding effects are prevalent, particle classification of the feed prior to filtration may prove barefidal As outlined in Chapter 4, an ahemative is to allow passage of the fine matoial in the eatfy stages of a s aration these fore-runnings may then be recycled to the filter. 

The principle of a cross flow system is shown in Fig. 1, which may either be used for the measurement of settling rate distributions, the technical classification in fluids, or the separation of particles from fluids. It consists, for example, of a rectangular channel which is flown through with a fluid from left to right and an average fluid velocity. The feed particles enter the flow with a certain ini-... 

Whereas Geldart s classification relates fluidized-bed behavior to the average particle size in a bed, particle feed sizes maybe quite different. For example, in fluidized-bed coal (qv) combustion, large coal particles are fed to a bed made up mostly of smaller limestone particles (see Coal conversion processes). 

Particle Size Distribution. Almost every feed slurry is a mixture of fine and coarse particles. Performance depends on the frequency of distribution of particle size ia the feed. Figure 5 shows that whereas all of the coarse particles having a diameter greater than some are separated, fewer of the very fine particles are, at any given feed rate. The size distribution frequency of particles ia feed and centrate for a fine and coarse feed are quite different. More coarse particles separate out than fine ones. Classification of soHds by size is often done by centrifugal sedimentation. 

It is quite common ia the designs for fine classification to recontact the coarse stream transversely or ia counterflow with air before dischargiag it (see Fig. 9). This removes dry fine particles not removed ia the primary classification. That is, these particles are swept back iato the feed and given another chance to exit with the fine particles. Such an arrangement iacreases the overall sharpness iadex and reduces the overall apparent bypass. Another variation is to reenter the air from the sohd/gas separation of the coarse stream. 

Spray Dryers A spray diyer consists of a large cyhndrical and usu ly vertical chamber into which material to be dried is sprayed in the form of small droplets and into which is fed a large volume of hot gas sufficient to supply the heat necessary to complete evaporation of the liquid. Heat transfer and mass transfer are accomphshed by direct contact of the hot gas with the dispersed droplets. After completion of diying, the cooled gas and solids are separated. This may be accomplished partially at the bottom of the diying chamber by classification and separation of the coarse dried particles. Fine particles are separated from the gas in external cyclones or bag collectors. When only the coarse-particle fraction is desired for fini ed product, fines may be recovered in wet scrubbers the scrubber liquid is concentrated and returned as feed to the diyer. Horizontal spray chambers are manufactured with a longitudinal screw conveyor in the bottom of the diying chamber for continuous removal of settled coarse particles. 

Increase pore size to increase rate of fluid penetration. Decrease pore size to increase extent of fluid penetration. Modify particle size distrihiition of feed ingredients. Alter milling, classification or formation conditions of feed if appropriate to modify particle size distrihiition. 

In determining the proper size and number of cyclones required for a given application, two main objectives must be considered. The first is the classification or separation that is required, and the second is the volume of feed slurry to be handled. In the case of hydroclones, before determining whether these objectives can be achieved, it is necessary to establish a base condition as follows Feed liquid - water at 20 C. Feed solids - spherical particles of 2.65 specific gravity Feed concentration - less than 1 % solids by volume Pressure drop - 69 kPa (10 psi) Cyclone geometry - "standard cyclone" as described above. 

Particles with the lowest specific gravity are carried with the water towards the outside wall of the spiral. The spiral separates at its greatest efficiency when used in the size range of 10 to 200 mesh. Some particles will be recovered both above and below these size ranges, but occasionally, ultrafine and very coarse heavies will be lost in the tailings, as will be middlings or unliberated ore particles. The spiral will benefit, therefore, from the use of hydraulic classification as a feed preparation step. 

Thickeners, thickeners are primarily used for liquid-solid separation (see Section 10.4). When used for classification, the feed rate is such that the overflow rate is greater than the settling rate of the slurry, and the finer particles remain in the overflow stream. 

In processes where classification or separation of particles is required, the efficiency of separation will be a function of one or more distributed properties of the particles. The function which describes the efficiency with which particles are separated by size (d) is usually termed the grade efficiency, G(d). For particles in a narrow size interval between d and d + Ad, G(d) is defined as the mass ratio of such particles in the underflow to that in the feed. The overall separation efficiency E corresponds to the particle size d for which G(d) equals E. 

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