by CC Huang · Cited by 3 — information on how air classification works, how feed affects air classifier performance, and how to evaluate classification efficiency,
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Powder and Bulk Engineering, December 1996 69 Air classifiers: How they work and how to select one C.C. Huang Hosokawa Micron Powder Systems Do you know how air classifiers operate and how to select one for your application? Read this article for information on how air classification works, how feed affects air classifier performance, and how to evaluate classification efficiency, throughput, and energy requirements. Final sections discuss typical air classifiers and applications and how to select one. Related information provides some technical back- ground. n air classifier is the most commonly used equip- ment for separating large dry particles from small A ones. It can perform classification in the gravita- tional or centrifugal field. A gravity unit typically classi- fies powders from 20 microns to several millimeters; a centrifugal unit classifies powders from 1 to 100 microns. A typical air classifier application is in closed-circuit dry grinding to reduce overgrinding and increase throughput. Another is in open-circuit milling to produce various powder size classes from the same process. By separating oversize, the unit can prevent coarse particles from spoil- ing the surface finish of plastics, paints, and coatings or causing electronic components to fail. By separating ul- trafhe particles, the air classifier can improve a powder™s flow and performance and eliminate airborne dust that™s harmful to workers. When discussing air classification, it™s helpful to under- stand two terms: cut size and cut sharpness. Cut size is the particle size at which classification occurs -that is, the particle size that is found with equal probability in the fine and coarse fractions. The final product can be either the fine or coarse fraction, depending on your application. Cut sharpness indicates how much the fractions™ particle size distributions overlap after classification; in a sharp cut, few wrong-sized particles remain in each fraction. How air classification works Air classification is a complex process that™s based on air- flow. In an air classifier, particles are fed into an air (or gas) stream. Two dominant forces act on the particles – a drag force caused by the air flowing around the particles and a mass force in the gravitational or centrifugal field. Precisely evaluating all the forces acting on the particles is often not possible, but it™s possible to obtain a simpli- fied model to describe the air classifying principle. [Edi- tor™s note: For more information, see the related sidebar, fiSome technical background on air classification,fl else- where in this article.] Various airflow concepts – elutriation, free vortex, and forced vortex – can be used separately or in combination in an air classifier to achieve the desired classification. Elutriation provides gravity-based particle classification in which the particles are air-washed in the gravitational field. It™s a crude method that classifies most of a feed™s fine and coarse particles by introducing the feed into flowing air. The airflow raises the fines against gravity to a fines outlet, while the heavier coarse particles deceler- ate and fall with gravity against the airflow to a coarse outlet. Increasing or decreasing the classifier™s airflow velocity adjusts the cut size. Elutriation is seldom used alone but can provide preseparation to improve a centrifu- gal air classifier™s efficiency (described in the following information). A centrifugal air classifier uses free or forced vortex air- flow or both. The classifier™s cut size depends on the sep- aration radius, which defines the classifier™s actual size. 0 0 73 ra 5 2. d 0 cn 0 -0 0 Q d % P, 3 Q m r_ 7i
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70 Powder and Bulk Engineering, December 1996 Thus a large centrifugal classifier can™t achieve as small a cut size as a smaller unit of the same type. In a free vortex unit, afree vortex moves in a decaying cir- cular pattern toward an outlet, as in a cyclone. The feed and air enter the housing tangentially. Aided by a set of adjustable guide vanes, the circular airflow throws coarse particles to the housing™s periphery, where they flow to a coarse outlet, and draws fines inward with the airflow to- ward an air-and-fines outlet. Varying three parameters – 0 0 73 ra 5 2. d 0 cn 0 the airflow, the classifier™s dimensions, and the guide vane angles -controls the cut size. Compared with the free vortex unit, which produces a rel- atively imprecise classification, aforced vortex air classi- fier is more complex and achieves a more precise classification. The airflow is drawn from the housing™s outer edges, and a driven vaned rotor called a clusslfying wheel forces the airflow with entrained feed particles into a circular motion (the forced vortex) around the wheel.
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72 The resultant centrifugal force throws the coarse particles to the periphery, but the fines pass through the wheel™s vanes and escape to the outlet with the airflow. Varying the airflow velocity and wheel speed controls the cut size and allows an infinitely adjustable classification within the classifier™s limits because the radial air velocity and the particle™s tangential velocity can be varied independently. Powder and Bulk Engineering, December 1996 Another air classifier combines gravitational and cen- trifugal forces so that elutriation preseparates the feed to improve the vortex™s efficiency. A classifying wheel and air-and-fines outlet are at the classifier™s top. Air entrain- ing the feed particles flows from below. The classifying wheel creates a forced vortex that carries the fines through the wheel™s vanes; the fines then pass out of the wheel with the air to the air-and-fines outlet. Coarse parti- cles fall with gravity along the housing walls toward the bottom outlet. A secondary air source below the wheel generates a free vortex, which backwashes the coarse, picks up any fines that settle out with the coarse, and re- cycles the fines to the wheel. A unit that depends almost entirely on the forced vortex is a high-energy dispersion air classifier. The unit has a small classification zone (the area where separation oc- curs -in this case, inside the disk-like classifying wheel) and a small gap between the wheel edges and the housing. These factors and the wheel™s high speed create high-ve- locity air at the wheel edges that disperses or washes the fines from the coarse just before the feed enters the vor- tex, ensuring the fines can™t re-entrain or reagglomerate with the coarse. This increases the classification effi- ciency by providing a finer cut size and sharper cut. How feed affects air classifier performance Your feed™s properties and particle size and feedrate have a big effect on the air classifier™s performance. Under- standing these factors can help you select an air classifier that effectively handles your feed. Feedproperties. For air classification, the ideal feed is uniform, homogeneous, spherical, smooth, dry, and eas- ily dispersible. Unfortunately, few powders have these properties. For instance, a feed with high-aspect-ratio particles in which one aspect or dimension is much larger or smaller than the others (such as flat, thin mica particles) can trick the air classifier. The classifier can permit over- sized particles to slip through depending on which parti- cle dimension – major or minor – is presented to the classifier. Porous particles or ones with protruding fiten- taclesfl or sharp edges can also impair classification. Fines can hide in a coarse particle™s pores, and tentacles can tan- gle and agglomerate with other particles. The feed™s dispersibility is often the most important prop- erty for air classification. If the particles can™t be evenly, 0 0 73 ra 5 2. individually dispersed in air, the air classifier can™t do its water, oil, or fat or are hygroscopic (tending to pick up classification. S To overcome these problems, you can use various preclas- drying or drying-blending, solvent removal, extraction, dispersion, homogenizing, and deagglomeration. d Particle size and feedrate. An air classifier™s perfor- mance can change with variations in the feed particle size distribution and feedrate. Under the same operating con- ditions, almost all air classifiers will produce a finer prod- uct and smaller cut size when the feed is finer. When the feedrate increases, which increases the classi- fier load, each type of air classifier behaves differently. A forced vortex air classifier has a hurdpe$omzance -that is, the classifier can maintain a constant particle size dis- tribution of fines with the increasing load. The fines can even become slightly finer as the feedrate increases, while the coarse fraction can contain more fines, resulting in a distinct drop in cut sharpness. Hard performance is desirable to guarantee the fines quality against feedrate variations. An excessive feedrate will ultimately overload and stop the classifier. d job. For instance, particles that contain moisture such as and hold moisture) are likely to agglomerate and resist cn 0 -0 E s -. cn 3 -0 0 Q P, Q m r_ sification techniques besides grinding, including vacuum % 7i 3 3 (D 3 rn E. 2. ra A free vortex air classifier has a softperformance – that is, when the feedrate increases and overloads the classification zone, the cut size increases and the fines distribution shifts toward the coarser range. Such soft performance can be desirable in a closed-circuit grinding and classification process to prevent system breakdown. How to evaluate classification efficiency, throughput, and energy requirements Before discussing types of air classifiers and how to select one, it™s helpful to understand how to evaluate classifica- tion efficiency, throughput, and energy requirements. Classification efficiency. After classification, the fine and coarse fractions™ particle size distributions more or less overlap. That is, the fine fraction contains particles larger than the cut size, and coarse fraction contains parti- cles smaller than the cut size. The fewer wrong-sized par- ticles in the fractions, the sharper the cut. In a processing plant, high cut sharpness means the best- quality fine fraction and most economical processing be- cause oversize in the final product is sharply limited, the fine fraction is large, and little material is lost because the coarse fraction contains few fines. [Editor™s note: For more information on evaluating cut size and sharpness,
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74 see the related sidebar, fiSome technical background on air classification,fl elsewhere in this article.] Powder and Bulk Engineering, December 1996 Throughput. Throughput is the mass flowrate of material (feed, fines, or coarse) through the air classifier and strongly affects cut sharpness. Cut sharpness typically re- mains constant up to a certain throughput and then contin- uously decreases. Throughput also depends on the airflow and classifier size, because more airflow can carry more material and a larger classifier can handle greater airflow.™ Energy requirements. Completely dispersing the feed will provide an efficient classification with high cut sharpness. For a very fine classification (for which the feed is typically fine), the classifier must have enough power to fully disperse the feed, which requires more en- ergy. The specific energy each air classifier requires for producing a defined final product quantity at a specified cut size and sharpness can vary widely. Typical air classifiers and applications Many air classifiers with classification capabilities rang- ing from very coarse to extremely fine are available, and each unit depends on elutriation or centrifugal force or both. Typical examples are the gravity air classifier, spi- ral separator, combination elutriation-vortex separator, cyclone separator, turbine classifier, multiwheel turbine classifier, and high-energy dispersion classifier. Gravity air classifier. The gravity air classifier, shown in Figure 1, is based almost entirely on elutriation. It has a zigzag-shaped classifying channel to improve classifica- tion. Air enters at the bottom, and the feed enters at the middle. The air carries the fines upward against gravity and out the top, and the coarse drop by gravity and exit the bottom. The unit provides gross classification in a size range also covered by sieving, but the unit™s classification principle also separates materials that can™t be effectively sieved, such as rocks from minerals and valuable metals from scrap. The classifier also dedusts feeds such as plastic granules, coke, chalk, fertilizers, and bauxite. Spiralseparator. The spiral separator, shown in Figure 2, has internal guide vanes and a rotating chamber wall and uses the free vortex concept. The unit uses an internal fan to induce its own airflow. The tangential airflow is di- rected into the unit™s classification zone by the guide vanes, which creates a free vortex. The free vortex sepa- rates the fines, which exit with the air at the unit™s side, from the coarse, which exit the front. The unit classifies particles from 3 to 80 microns. Chang- ing the airflow volume and guide vane angles adjusts the cut size. A typical application is producing fine iron-free ceramic powders. Combination elutriation-vortex separator. In the combi- nation elutriation-vortex separator, shown in Figure 3, feed and air enter at the bottom and are drawn upward to a
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0 0 73 2. 76 Powder and Bulk Engineering, December 1996 Figure 4 Example cyclone separator I I vertically mounted classifying wheel, which creates a forced vortex. Airflow draws fines through the wheel to- ward the exit at the housing™s top. The coarse are thrown by centrifugal force toward the housing™s periphery, then fall by gravity as they™re backwashed by the secondary air, and exit the bottom. The unit classifies particles from 10 to 150 microns and often operates as part of a grinding process to classify the grinder™s output. The separator can be efficient and cost- effective at relatively high throughputs as long as the sep- arator™s airflow is balanced to accommodate the grinder™s airflow requirement. The unit can remove oversized par- ticles from powder coatings, alumina, and minerals. Cyclone separator. The cyclone separator is a cyclone equipped with a vertically mounted classifying wheel, as shown in Figure 4, and combines free and forced vortex concepts. Air and feed enter at the side. The cyclone (which creates the free vortex) spatially separates the coarse flowing near the wall from the fines flowing in the separator™s central region (where the classifying wheel creates the forced vortex), improving the wheel™s classi- fication efficiency. The air and fines exit the top, and the coarse exit the bottom. The unit can provide precise size classification in the 8- to 300-micron range. It™s typically used for producing pow- der coatings with a narrow particle size distribution and exact top size control. Turbine classifier. A turbine classifier, as shown in Fig- ure 5a, is equipped with a horizontally mounted classify- ing wheel and combines elutriation with free and forced vortex concepts. Air enters the housing™s lower section, and feed enters the upper section. Coarse flow downward by gravity, aided by elutriation and the free vortex in the housing below the wheel, and exit the bottom. The vortex flow created inside the wheel entrains the fines, which exit with the air through the classifier™s side. The unit classifies particles from 5 to 150 microns. It pro- vides top cuts (removing the coarse) and bottom cuts (re- moving the fines) as well as handles some bandwidth applications (removing the coarse or fines to achieve a 0 cn 0 77
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80 very narrow size range), such as bottom-cutting an ultra- fine product at about 5 microns. Powder and Bulk Engineering, December 1996 Multiwheel turbine classifier. Another variation of the turbine classifier is the multiwheel unit: which has sev- eral horizontally mounted classifying wheels around a central air-and-fines outlet that leads vertically upward, as shown in Figure 5b. Operation and applications are similar to that of the turbine classifier, but the multiwheel 0 0 73 ra 5 2. turbine unit permits scaling up throughput without having – to increase the cut size. 0 cn 0 -0 High-energy dispersion classifier. The high-energy dis- persion classifier, which relies almost entirely on the forced vortex concept, has a high-speed, high-energy classifying wheel, as shown in Figure 6. Air enters at the bottom, and feed enters at the side. Classification occurs S cn 3 E -. s ca 8 almost entirely inside the wheel. Coarse exits the side, and fines and air exit the wheel™s center. The unit can classify particles from 1 to 50 microns. It can provide tight-bandwidth particle size distributions for chromatographic materials such as silica, aluminum, and polystyrene beads, which must be within a few microns and contain very few particles outside the range. The clas- sifier can provide clean top cuts for alumina and provide extremely efficient classifications in bottom-cut applica- tions for toners at 4 to 5 microns and powder coatings at 5 to 10 microns. The unit is often used for classifying high- aspect-ratio materials like silicon carbide whiskers be- cause the classifier™s high-energy dispersion makes it easier to separate the particles by their major dimension. P, 3 Q m r_ 7i 3 3 (D 3 ra rn cn. 2. How to select an air classifier Work with an air classifier manufacturer to select a unit that will meet your needs. In addition to considering your feed™s properties, particle size, and feedrate, consider the
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