Evaluating: and Selecting: Chemical Equipment
G . F. ALLEN
SCREEN AND PNEUMATIC CLASS1FICATI0N
Material to be separated must be considered from two aspects: Flow characteristics and ability o f particles to act independently
ry separation can be accomplished in a number of D ways; however, discussion here is limited to those separations made on the basis of either particle size or density. Two types of equipment are used: screens which separate on the basis of size, and pneumatic classifiers which separate on the basis of either size or density. Some machines are made to incorporate both methods. I n this discussion, however, the two methods are treated as entirely separate techniques. Factors to Be Considered
Characteristics of the material, the most important factors involved, must be considered from two points of view: their effect on ability of the particles to act independently, and their effect on flow of the material into, through, and out of the separating machine. The most important properties include : Particle size Density Shape Surface Hardness
Porosity Friability Interparticle friction Surface moisture Angle of repose
Tendency to agglomerate Hygroscopicity Electrostatic charges Abrasiveness Bulk density
Particle Size
I€ all particles were spheres, their size could be designated in terms of diameter. However, particles of dry materials may range from spheres to long thin fibers. Therefore, size is usually designated in terms of range, based on two screens-one through which all the material will pass and the other through which none of the material will pass. Then between these two ex-
tremes, size is often specified further in terms of percentages of the material which can or cannot pass through screens having intermediate size openings. Thus, a typical product could be: 0% retained 48y0 retained 86% retained 99% retained 0% through
on 20 mesh on 40 mesh on 60 mesh on 100 mesh 720 mesh
This means that particle size, as normally specified, indicates the greatest dimension of the smallest crosssectional area of the particle. Particles that are extremely long in relation to their cross-sectional area will not always end up to pass through the screen; in such instances, particle size is often referred to in two dimensions determined by actual measurement, either linear or a combination linear for one dimension and a mesh designation for the other. O n extremely small particle sizes, the designation is often in terms of the aperture through which the particle will pass. A statement of efficiency does not consider differences in machine requirements such as power, screen area, or time except, of course, as they are translated into the final results. Efficiency requirements must be known in advance, because they affect not only the type of separating equipment considered, but also final design and specifications. Screen Separation
Separation of dry material on the basis of particle size by means of a screen is known by a number of terms, including sifting, screen grading, scalping, bolting, or VOL. 5 4
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separating. These terms have varied meaning in different industries, but they have no particular association with specific sifter designs. For example, scalping in most industries usually means the removal of lumps or foreign materials from a product without separation into fractions. Sometimes, however, this term is applied to the removal of large particles of the same material, usually for regrinding. Bolting is usually restricted to the milling industry and re-bolting signifies a scalping operation somewhat finer than average scalping. In this article, however, sifting, which is the most universal term, is used as a general term applicable to any separation on a screen or sieve. In production sifting, bulk material is passed over a screen so that particles smaller than the openings will pass through, but those larger will continue to pass over. The natural tendency of smaller particles to sink to the bottom of the mass when submitted to agitation contributes substantially to capacity and sifting economy. Speed and efficiency are determined largely by particle size and weight, interparticle friction, pressure exerted on the fine particles from above by larger particles, the amount and type of agitation, and the number and size of openings. In commercial operation, sifting is usually continuous and therefore provision must also be made for continuous passage of material through the sifter. The rate of material movement is largely determined by bulk den-
Gump Co.’s 440MA siJter as usually installed f o r continuous production with covered inspection openings on inlet and outlet spouts
sity, flow characteristics which are largely a matter of interparticle friction, amount and type of conveying motion, and the pitch of surfaces, including the screen on which the material is moved. An important factor is the ease with which fine particles pass through the screen openings. This in turn depends on the relationship between aperture size and particle size range. If most of the particles are substantially smaller than the screen opening, and most of those retained are substantially larger, then a large quantity of material can be sifted on a relatively small 40
INDUSTRIAL
A N D ENGINEERING CHEMISTRY
screen area. This is true In scalping lumps from a finely ground material. However, as particle size approaches aperture size, the longer the sifting time and the larger the screen area required. Furthermore, the proportion of material that will pass through the screen greatly- affects sifting capacity. For example, consider two samples of the same product, one coarsely ground of which 10yG will pass through the screen and the other finely ground of which 90% \vi11 pass through the same screen. The sifting capacity will be entirely different for these two samples. Passing only 10% of the material through the screen is not necessarily easier than passing 90y0. In the first place, the 10% may be only slightly smaller than screen aperture. Also it must be shaken down to the screen surface, and the 90% must be moved across and off the screen to make way for additional feed. Sometimes it is better to use a coarser screen first to remove a substantial portion of the oversize fraction before attempting finer sifting. Particle Shape and Characteristics. Particle shape is extremely important. It affects passage of undersize particles through the screen and also conveyance of oversize particles over the screen surface. Furthermore, particle shape affects interparticle friction and therefore the ease with which fine particles settle to the screen surface. Lt’here long thin particles are involved, it is sometimes desirable to retain the longer particles and pass only those whose length, width, and thickness are approximately equal. Then the problem becomes one of causing the long particles to lie flat so that the): !vi11 not pass through the screen. Irregularly shaped particles can lodge in the screen aperture and thus cause blinding. This problem is accentuated if the particles have a tall pyramid shape or irregular or long points. In such cases, rubber, neoprene, or nylon balls or plastic figure eights are used. Also, various types of brushes or cloth cleaners can be located so that they bounce against the underside of the screen and thus dislodge the particles. Many products that blind the openings of a woven metal wire screen can be sifted quite readily on woven silk screens. Flexibility of the openings of the silk allows the odd shaped particles to become dislodged more readily. If the particles tend to stick together, they must be separated. Material abrasiveness affects the selection of the screen material, and electrostatic charges built up by mechanical action of sifting will cause these particles to coat or blind the screen (particularly during low atmospheric humidity). Such particles cannot be dislodged by mechanical agitation. Grounding the screens may be helpful, and sometimes ozone generators or static eliminator bars are useful. Sifter Motion and Pitch. A number of motions can be applied to a screen, most of which have some degree of applicability to certain types of sifting operations. The purpose of screen motion is threefold: to spread the material over the entire screen surfacr, to
cause fine particles to settle to the screen surface, and to discharge oversize particles. Movement of material through the sifter can be facilitatad-by pitching surfaces, including not only that of the screen, but also those of trays or pans which collect fine fractions subsequent to screening. For extremely close sepgrations, material can be moved on a level screen by using Eights. This flow is particularly useful for force sifti@gand long sieve travel. Sioping a moving sereen from the inlet toward outlet wili also apsist in moving the material across the screen. The correct amount of pitch depends on the distance b e v e e ninlet and outlet and also on characteristics and quantity of material being sifted. Too much pitch may r e a h in the material reaching the discharge point before all d the fine particles have had a chance to pass through the'screen. However, when the material moves athe screen rapidly, the bed is thinner, and fine particles reach the screen surface more quickly. d n the other hand, a thin bed of material can result in less.material being sifted per square foot of screen area. For some materials, fine partides are aided in their passage through the screen weight' of material a b w e them. Also, some particles are so hard that when the bed is too thii,they tend to bounce on the screen instead of pming through it. For thick beds, perfectly level screens and sometimes a dam at the discharge edge is desirable. This may apply even though the percentage of oversize particles is relatively low. Flights to move material on a level screen are relatively small thin members, extending from one side of the
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HORIZONTAL MOTION 7
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However, except for special applications, reel-type sifters are no longer used because more screen area and therefore more floor space is required. Also, power requirements a m higher. cOMBU4ATlON VERTICAL-HORIZONTAL MOTION. Screens can be moved in an inclined plane with a reciprocating motion, for example, by means of an eccentric connecting rod (Figure 1). This motion then has a vertical component and also a horizontal component parallel to the horizontal projection of the path of moving material. Combined horizontal and vertical motion can also be obtained with a rotary drive attached directly to the screen and operating in a vertical plane parallel to the path of material flow. The horizontal motion is thc same as in Figure 1, but the vertical movement is of thi type:
The vertical component lifts the material from the screen part of the time and thus reduces the time available for fine particles to pas through the screen. On the other hand, this vertical motion can loosen the mass of material or break up agglomerates, so that dislodged line particles can settle to the bottom of the mass. The combined horizontal-vertical motion is also effective in moving large volumes of material rapidly over the screen surface with a combination of hop-and-slide movement. This technique is useful for coarse sifting, or where the screen openings are substantially larger than the particles which are to be passed through. HORIZONTAL RECIPROCATING MOTION.Horizontal reciprocating motion, often obtained with an eccentrir connecting rod, is usually in a path parallel to thf horizontal projection of the path of material as it move! from inlet to outlet thus:
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Figwe 7. An eclntric cennccting rodimpmts both vnticalondhariwntal m&n topariiclcs rls I@ mom oum thcpi&dsmm
material path. These move material, not only from inlet to discharge, but also they can pass material back and forth several times across different sections of the same screen. Types of Sifter Motion. The motion of the sifting screw can be either reciprocating or rotary or a combination of the two, giving an elliptical motion. The motiqn can be entirely in the horizontal plane, entirely in the vertical plane, or in both planes. In addition the screen can he wrapped .around an almost horizontal cyli+ler into which the material is fed at the high end.
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With a rather steep pitch, this motion can move large quantities of material. If the pitch is steep enough, the motion has a substantial component vertical to the screen surface, provides thus again the hop-and-slide movement. This screen works well where most of the particles that must pass through am much smaller than the screen opening. ~OUSINATXON HORIZONTAL RECIPROCA~WO-ROTARY MOTION. If an eccentric drive is used to change the horizontal recipmcating motion to a rotary motion in a horizontal plane at the inlet end of tbe sifting screen, while the other end continues to have a recipmcal motion because it slides in guides, them will he components of & m a t i n g motion both parallel and perpendicular to the directim of material flow at the inlet end. The c i m l a r motion at the inlet end gradually changes to an elliptical motion through the center section of the screen and IinaUy hmomes a Vue reciprocating motion at the discharge end in this manner:
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This action is effectivein spreading the material to the sides of the screen at the inlet end. The looping path of the material also presents more screen openings, because particles move not only back and forth but also from side to side across screen surface. HORIZONTAL-ROTARY MOTION. If the entire screen surface is caused to rotate in a horizontal plane, the material will travel in the overlapping loop path the entire distance from inlet to discharge in this manner:
This multidirectional motion overcomes interparticle friction and maximizes the number of openings available; thus, sifting can be accomplished on a relatively short screen. This technique is used for both coarse and extremely fine separations. Rotary motion is produced by two general methods: An off-center weight, attached to the screen through ita frame, is rotated at the proper speed and in the correct plane to impart the desired type and frequency of motion. The amplitude of movement is loosely controlled by the heaviness and eccentricity of the weight, as well as its position which affects its leverage on the screen. Sifters may also be driven by means of one or more direct connected eccentrics. The speed of the eccentric and the amount of eccentricity positively control the frequency and amplitude of motion. However, when the eccentric driving motion is applied at only one point, the flexible support may allow other portions of t h i s screen to take a different path of motion-usually an elliptical path. The erratic motion is caused by factors such as differences in material load at the inlet and discharge ends of the screen, and drag or inertia at the corners of the screen. This condition can of course be corrected by introducing synchronized driving motion at several points on the screen. In t h i s way, the function of eccentric supporting shafts, and eccentric driving shafts can be combined. Speed and Amplitude of Motion. Regardless of the type of motion, a maximum combmation of both amount and speed of movement seems desirable. However, there are two limiting factors. First, a point can be reached quickly where the material bounces too violently on the screen. Second, in designing the machine, the strengthweight-power requirement factors become practical limitations. The most desirable speed, throw, and pitch is the combination that imparts maximum particle movement which in turn induces fine particles to settle, and makes available the largest number of screen openings. Methods of Supporting Sifter Screens. Screens can be either suspended from above or supported from 42
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
below. Coil springs and leaf springs permit vertical motion. For suspended sifters, flexible reeds are widely used and for undersupported sifters, rods with flexible connections are used. Oscillating links and sliding supports can be used for reciprocal screens. Screens having a rotary motion in either the horizontal or the vertical plane can be supported on eccentric shafts. Amplitude and direction of motion can be entirely controlled when multiple eccentric support shafts are used. Screen movement should be entirely counterbalanced so that little or no vibration is transmitted to the support-
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Figure 2. In tha sm'es m a n g m t , p m t i c h which do no1 pass tluargh thcfirrr smtm are cm'ed to tha next mclbm, and so on moss oll tha m w
ing frame work. When this is impossible, shockabsorbing mounts are often used, but it is better to design so that such mounts are not needed. Sifter Flow Arrangement. Sifters may be required to make either a single separation into fractions or they may make several separations. Single separations may be made on either a single screen or on several screens. If several screens are used, they may be arranged in series, in parallel (Figures 2 and 3), or a Combination parallel-series arrangement. In the combination, any product that does not pass through the parallel screens is introduced to the top of an additional screen under them for a final clean-up. Multiple separations may be made either on a single level screen or on multiple screens arranged in a vertical
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(THROUGHS
OVERS Figwe 3. Tha pmolld m a n g m t splits the fnd of tka inlef ond introduccspml of it to m h smm
tack. When a single level screen is used, a section of the correct screen for the finest separation is used at the input end of the screen. This is followed by sections of the screen with progressively larger openings with a section of screen with the correct opening for the largest separation at the discharge end of the screen. The fine fraction is removed first, immediately after the material is introduced onto the screen surface. The progressively coarser fractions are removed in sequence as the material passes across the screen toward the discharge point. The coarsest fraction passes over the last section of the screen. When a multiple deck screen arrangement is used to obtain multiple separations, these are usually arranged with the screen with the largest openings on the top and the screen with the smallest openings at the bottom. I n some designs inclined trays may be installed between the screens to return the material that has passed through the screen above to the input end of the next screen so that it will have the opportunity for full screen travel. Sometimes this flow arrangement is reversed when pressure due to the weight of the coarser fractions is needed to cause the fine particles to pass through the screen openings. Screen Materials. For extremely abrasive products or for coarse separations, perforated metal screens are used. However, the amount of screen opening area in relation to the total screen area is relatively limited. Woven metal wire screen of a wide variety of wire and diameters is made of materials such as steel, stainless steel, Monel, copper, and bronze. Most are available with as many as 325 and in some cases 400 meshes to the inch. Coarse wire makes an extremely rugged screen suitable for handling abrasive materials. Finer wire increases substantially the per cent of open area and therefore sifting capacity. Woven silk and nylon are available in several weights, ranging from standard to triple extra heavy, and in sizes to 200 meshes per inch. Other Sifter Considerations. Several other factors are important in sifter design, particularly for dry materials that may be dusty. The screens should be easily interchangeable and the unit should be easy to clean, service, reassemble, and free of leaks between material fractions. Also, it is desirable to enclose the screens in a dust tight enclosure. Units should be operable for extended periods without excessive maintenance or downtime. They should require as little floor space and head room as possible and power requirements should not be excessive. Air Classification
Even though the particle size range may overlap, air is used widely to separate light materials from heavy
ones and also different particle sizes of the same material. Air classifiers can separate particles as small as about 5
B . G. Gump Co. Inc., Chicago, Ill., makers of equipment for the food, feed, and process industries.
AUTHOR G. F. Allen was formerly with the
microns, and as large as ten-mesh. However, their major use is for submesh sizes. Principles of Air Classification. Like water classification, air classification is based on differences of settling rates for different weight particles in a fluid. The particles are suspended in an air stream which then is introduced into a classification zone where the heavier particles settle and lighter particles continue with the air stream. Finally, the lighter particles are removed, and collected separately. Air classifiers can use the principle that drag force, exerted on the particle by the moving air stream, is directly proportional to the size of the particle. O n the other hand, gravitational, inertial, or centrifugal forces are proportional to the mass or weight of the particle, which in turn is a function of the cube of the particle diameter. Therefore, if these forces are made to act in opposition to the drag force of the air stream, separation of the particles on the basis of their weight-size ratio can be attained. If the forces are equal, the particle is held in equilibrium with an equal change of either remaining with the light fraction or of being removed with the heavier fraction. Theoretically, an air classifier should be designed so that particles are uniformly dispersed in an air stream of uniform velocity; thus the drag force on them will be directly proportional to their size. Further, the classification zone should be designed so that gravitational, inertial, or centrifugal forces act equally on each particle, and thus are exactly proportional to the particle mass. Also heavy particles should leave the air stream without colliding with light particles that continue to travel with the air stream. Then lOOyo removal of light particles should be attained. Factors AfFecting Air Classifier Efficiency. Designing an air classifier to meet all these requirements is extremely difficult, if not impossible. All particles do not behave alike in an air stream. Differences of shape and porosity or apparent specific gravity are material. Surface moisture may cause agglomeration, and flow characteristics affect distribution throughout the air stream. Particle hardness affects bouncing and electrostatic charges. I n general, these are the same characteristics that affect separation in sifters. Particle Size. When separating a single material strictly on the basis of particle size, efficiency is substantially affected by the distribution of particle sizes within the classifier. Particles near the separation size will be distributed between the coarse particles and the fine particles at the classifier outlets in accordance with the laws of probability. For example, in a typical air classifier operating at a separation point of 100-micron particle size, 90% of the particles twice this size or 200 microns will end up with the coarse particles and 10% will end up with the fine particles. Also, 90% of the 50-micron particles will end up with the fine particles and 10% with coarse particles. Types of Air Classifiers. Several types of air classifiers are made. Probably the best method of grouping is on VOL. 5 4
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Figurc 4. Graui6u6ionul clusn7kr. Matninl lo be scpmuttd is dropped in60 fhr lop of n hailonto1 m> rhmm- Hmov hmticles drop quickly. but Iightpmliclcr traocl far6hn
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Figwe 5. Innlid clarrifr. When direction of m i flow is
Figure 6. Cmrrifugd cImn$in. Hcnrry particles swirl down 6hc Eides of the collccfingh p p n , and light par6iclcs mc carried ou6 with 6hc air s b e m 44
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
the basis of the type of force used to counteract the drag force of the air stream. GRAVITATIONAL. In a typical gravitational classifier dry material is dropped into the top of a horizontal air stream. The heavier particles fall almost vertically through the air stream to a collecting hopper below. Lighter particles are carried progressively farther to other collecting hoppers and the lightest may be carried entirely out of the unit (Figure 4). In another type, a layer of solid material is mechanically conveyed horizontally through a porous section. A stream of air passing upward aerates the bed and lifts it from the surface of the conveyor. The lightest particles are carried away entirely with the air stream, hut the maximum weight of particles thus carried away can be controlled by velocity of the air stream. One unit of this type utilizes an inclined, vibratory conveyor to carry the dry material upward across a perforated plate through which a stream of air passes upward. The heaviest particles maintain close contact with the plate and are discharged at the highest point. The lighter materials, although still on the plate, are shaken loose a t the lower point. The gravitational classifier is used mostly for separating foreign materials from a product, because weight of the two materials usually diffen substantially. Also gravitational classifiers are sometimes used to obtain approximate particle size separations of relatively coarse products, ranging generally from 10 mesh down to 65 mesh. However, in this size range sifters are more efficient. INERTIAL TYPE. In these classifiers, the direction of air flow is changed abruptly (Figure 5). Thus, heavier particles will leave the air stream, whereas the lighter particles will negotiate the change in direction aqd continue on with the stream of air. Inertial classifiers are applicable to sizing from 48 down to 270 mesh. Actually a centrifugal separator which throws the dry material off the edge of a horizontal spinning plate is a form of inertial classifier. The heavier particles, thrown further, are collected farther out from the edge of the disk. The drag force resulting from velocity of the particles overcome inertial force; thus lighter particles drop more quickly. CENTRIFUGAL TYPE. These classifiers are based on the cyclone separation principle (Figure 6). Instead of operating the cyclone to obtain nearly 100% removal of the dry material, it is operated so that the light particles are carried out of the cyclone with the air. The heavier particles swirl to the outside and down the sides of the collecting hopper and are collected in the bottom. Additional air may be introduced near the bottom of the vertical section of the cyclone to control the particle weight at which material will remain in the air stream and be carried away by the upward air stream which discharges through the center of the cyclone at the top. Centrifugal-type classifiers are used for the finest particle size separations ranging from 270 to 325 mesh down to approximately 5 microns, and in some applications up to as large as 150 mesh.
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