Screening is the passing of material through definite and uniform apertures is the only true and accurate means of grading to a required particle size. Air separation and hydraulic classification depend upon gravity and particle shape, and result in the segregation and retention of material of higher specific gravity and lower surface area irrespective of size.
The use of Screens increases with the education and civilization of a people and with the improving and perfecting of an art. In our advanced civilization practically everything that we eat, wear and use has been in contact with, or dependent upon screens in some phase of its growth, development or processing. In this treatise, we are only concerned with the sorting, grading or sizing as accomplished with a mechanical screening device.
Some materials such as beach sands, clays, native chemicals, etc., occur in nature in a closely graded state resulting from a mechanical water sorting, precipitation or gravity deposition. They require only scalping or some form of treatment for removal of tramp coarse foreign elements. Others such as salt, sugar and various chemicals are crystallized or precipitated in their processing to fairly close limits of size. They require only such sorting or grading as is dictated by market preference and conditions of use.
In mechanical mixtures such as raw cement, finished fertilizers, stock feeds, etc., the ingredients are blended, ground and screened to a definite fineness. This maintains the intimate relationship by preventing segregation of a coarse constituent through automatic sorting. We have all noted how by piling an ungraded material the fines will segregate in the center of the pile and the coarse will automatically run to the outside and bottom. Metallic and non-metallic ores, stone and other aggregates, coal and coke, various furnace products, chemicals, cerealsetcetera, must be crushed, ground, disintegrated or pulverized before they can go on to further processing and ultimate use. In these fields screens are used for sorting into definite grades, top scalping for removal of coarse oversize and foreign material, bottom scalping for elimination of fines and dirt, and to return oversize to a crusher or grinder until it is reduced to a size finer than the opening of the screen. This latter practice is known as closed circuit crushing or grinding.
A nest of standard brass framed screens, with a definite ratio between openings, is used to sort a representative sample into the clean fractions retained on each screen. The tabulated resulting sieve analysis graphically shows the percentages of given sizes present in the sample. (Table I, p. 347). It indicates just what is available for recovery by screening through and over certain openings in a commercial production screening operation and also shows the reduction obtained by passage through crusher or grinding mill.
Another important factor in commercial screening that will be revealed by a sieve analysis is the percentage of near-mesh material present in the screen feed. If, for instance, it is observed that 40 percent of the sample had passed through the 8-mesh testing screen and was retained on 10- mesh and another 40 percent had passed through 10-mesh and was retained on 14-mesh, an efficient productionscreening operation at 10-mesh would require the maximum in screen area, particularly as to length. This preponderance of near-mesh, or go and no-go size of particle, obviously makes a difficult separation condition. In such cases unless the proper care is taken in the selection of the type of screening device and the specification of the wire cloth used on it, the openings may fill up and blind to a point where no separation is obtained.
In addition to the necessary sieve analysis, other factors must be known before a proper and intelligent recommendation can be made on any but the simplest of screening problems. Many cases require a laboratory test, simulating actual operating conditions, before the size and type of the screen can be determined and proper specification of screen cloth selected. The screen doctor must have the answer to the following questions before he can make proper diagnosis and prescribe treatment:
Capacity required in tons or gallons per hour? This should be expressed in both average and maximum, because peak loads, even of short duration, may result in spoiling of products previously graded or may upset subsequent steps in the operation, due to the drop in screening efficiency. Sufficient screen area should be provided to handle the maximum load.
Type of screening, wet or dry? How much water can be added? In the case of wet screening it is necessary to know if a definite density of the through screen product must be maintained and how much spray water can be added to rinse the oversize.
Percentage of moisture present in the feed? The maximum figure should be given here because different materials become unscreenable at varying degrees of moisture. To effect a separationat a given fineness it may be necessary to dry the material or add water and wash it through the screen.
Is material free-screening? An affirmative answer here obviates practically all other questions. Sticky? As clay, some food products, chemicals, etc. This determines if screening is practical and type of wire cloth recommended.
By closed circuit crushing or grinding it is meant that the product from a crusher or grinder is fed to a screen. The material that has been reduced to sufficient fineness passes through the openings and the oversize is returned to the breaker for further reduction. Escape from the circuit can only be through the screen so this product, the undersize, is equal in tonnage to the initial feed to the crusher or mill. The oversize returned for further work is known as the circulating load. It is a most important factor and can be extremely insidious. If the screen is inefficient and rejects finished material or if the crusher will not reduce the oversize fast enough, this load may build up, and rapidly, to a point beyond the capacity of the breaker, the screen or the conveying equipment, whicheverproves to be the neck of the bottle.
For greatest economy and efficiency, fines should be removed by means of a screen just as fast as they are created in each successive stage of crushing or grinding. Most every case must be handled on its own individual merits and proper balance worked out. In some cases a circulating load as high as 1,000 percent is considered economical. Picture how this would affect the requirement in screening capacity with eleven tons of material handled for every ton produced.
The percentage of circulating load can be readily determined from the sieve analyses of the screen feed, the oversize and the undersize (See Table 1). Samples should be taken simultaneously after circulating load has reached its peak. Conditions and analyses will be similar to those set forth in flowsheet at right. The formula can be expressed:
PercentCirculating Load=100 (B-C/A-C -1) A=Percent finer than required sizein the screen feed. B=Percent finer than required sizein the screen undersize. C=Percent finer than required sizein the screen oversize.
In the example, A equals 35.0,B equals 95.0, and C equals5.0. The value of 1 in the formula represents the initial feed to the circuit which is equivalentto the undersize, or product removed through the screen.
Percent Efficiency=100(100 F-D/AF) A=Percent finer than required size in the screen feed. D=Percent coarser than required size in the screen feed. F =Percent coarser than required size in the screen oversize.
There are different schools of thought on this subject and other formulae. Some operators are satisfied to simply use the percentage coarser than the screen opening in the overscreen product as the efficiency figure. This would be F in the above formula and 95 percent instead of 90.22 percent.
Dependent on the nature of the material and type of operation, screening may be accomplished through bars, perforated plate or woven wire screen. The bar screen is used for scalping extremely coarse material where definite sizing is of secondary importance and abrasion is severe. Perforated plate offers a smooth surface upon which heavy oversize will slide very easily, often too easily for good screening. Under some conditions it blinds less readily than woven wire screen. Objections to it are the fact that the openings wear gradually larger and larger, and the percentage of blank area is so high.
For most purposes woven wire screen, or wire cloth, is the best medium. With it the maximum in open area can be obtained. Various weights, metals and alloys, and shapes of openings are available to satisfy conditions of heavy load, abrasion, corrosion, screenability and capacity. Mesh in wire cloth is the number of openings per lineal inch and means nothing unless accompanied by the decimal designation of the wire diameter or the actual opening of the screen. It is best to specify the required screen opening as this can then be obtained in several meshes, dependentupon the weight of wire that is used. Obviously, for a given opening, the greater the mesh count and the finer the wire diameter, the higher will be the percentage of open area in the fabric.
Much as we might like to do so, we cannot have our cake and eat it, too. Therefore, the selection of a screen specification is usually a compromise. Dependent upon conditions, screen life is constantly being sacrificed for screenability and vice versa. For instance, a heavy and abrasive material suggests an extra heavy wire to secure maximum life. It is found, however, that the low percentage of open area restricts capacity and that the large wire diameter promotes blinding and lowers efficiency. A compromise is, therefore, made by easing off on the weight of the wire. Conversely, another material may, for instance, be damp and sticky, dictating the use of an extremely fine diameter of wire to minimize the surface upon which it may build up. Such a screen specification may last only a few hours and capacity and efficiency must be sacrificed in the interest of longer screen life.
Rectangular and elongated screen openings assist greatly in increasing capacity and eliminating blinding. The opening in a square mesh screen is shaped similar to a funnel and particles can be wedged into it to bear on all four sides. The rectangular opening limits this contact to three sides and thus minimizes the possibility of wedge blinding. When this slot is further elongated to many times the opening width, a springing of the long wires is possible and permanent blinding is eliminated. Naturally, these long openings can not be used for true sizing of anything but cubical or granular materials. Where flakes and slivers are present and cannot be tolerated in the screen under-size, square mesh cloth must be used at the sacrifice of capacity.
For abrasion resistance, high- carbon spring steel wire is available. Stainless steel and the non- ferrous alloys give a selection where rust and corrosion are a factor. The difference between success and failure of a screening operation may rest with the selection of the proper screen clothspecifications and this subject requires considerable thought and study, plus experience.
Reviewing the foregoing, it is readily understandable that a fixed table of screen capacities would be misleading and dangerous. There are so many variables that two neighbouring plants, working on the same deposit, may have entirely different screening conditions, due, for instance, to a difference in crushing practice. Larger tonnages can be handled on scalping operations, and in some cases with closed circuit crushing, than on close grading into specific fractions. On some materials a scalping deck over the sizing screen increases capacity by breaking and distributing the load and opening- up the mat of material. Washing increases capacity materially over so-called dry screening.
From the grizzly and trommel we have seen the development of screening devices through the shaking, knocking and bumping stages to the high speed vibrating screen of today. This development ran the range of eccentric head motions; knockers; cams; air, cam and electric vibrators; unbalanced shafts and eccentric flywheels; grasshopper motions, etc., up to the present positive-drive, high-speed, circle-throw, eccentric- shaft screen.
In this type the throw and speed must be properly specified and coordinated to secure the best screening action. Bearings should not be under shock and design should not be complicated with compensators and adjustments to eat power and tempt experimentation. The loading of the bearings should be so minimized that the equipment manufacturer evidences his confidence in his design by extending a generous guarantee.
In closing, it is recommended that the screen user select a proved and simple machine that will give uniform, continuous, care-free operation. Your supplier should qualify to consult with you on installation, operation, and selection of proper screen cloth specifications. Do not overlook this important service feature.
A simple definition of a screen is a machine with surface(s) used to classify materials by size. Screening is defined as The mechanical process which accomplishes a division of particles on the basis of size and their acceptance or rejection by a screening surface.
Knowledge of screening comes mainly from experience. However, through experiment, test facilities and compilation of field data, reliable criteria have been developed by screen manufacturers. This factual data is now tabulated for use in selecting the type and size of screen best suited for the job.
The most common application of a vibrating screen is to separate an unconfined conglomerate of materials into different size fractions. Other popular uses of screens are scalping, washing, dewatering and dedusting. A review of the duty is essential to know the type of screen to recommend. When this is established, the capacity chart is then used to determine the size of unit required.
Nordberg-Lokomo supplies different types of screens, each designed for a specific range of duty. Occasionally there is a choice between the types we offer. In these cases when there is doubt, you can rely on Nordberg-Lokomo experience to help you make the selection.
COARSE FRACTION Particles which pass over the screen deck, FINE FRACTION Particles which pass through the screen deck. SEPARATION SIZE/ SPLIT SIZE Particle size at which feed separates into two products (coarse fraction and fine fraction). OVERSIZE Material larger than the hole size. UNDERSIZE Material smaller than the hole size. HALF SIZE Material smaller than half of the hole size. SCREENING CAPACITY (Q)Amount of material passing through the screen deck in tonnes/hour FEEDING CAPACITY Amount of material fed to the screen deck in tonnes/hour EFFICIENCY OF SCREENING (EFFICIENCY OF UNDERSIZE RECOVERY)Amount of material smaller than the hole size in undersize compared to the total amount of material smaller than the hole size in the feed.
The particle distribution of the feed has an essential impact on purity. See three examples in figure 1. In each one of them the efficiency is 90 %, but the undersize proportion of the coarse fraction varies (3.2 %, 9.1 %, 23 %).
Factors effecting the screening can roughly be divided into three groups: characteristics of material (B, C, F, K, L) characteristics of screen (D, E, AF) characteristics of screening element (A, G, H, J)
[t/h] (passing through) A = Nominal capacity [(m/h)/m] (passing through) B = Oversize factor C = Halfsize factor D = Deck location factor E = Wet screening factor F = Material weight [t/m] (bulk density) G = Efficiency factor H = Shape factor for mesh holes J = Factor for proportion of holes in the mesh K = Factor for crushed stone and gravel L = Factor for humidity content AF = Effective screening area [m]
The factors are obtained from diagrams based on relationships observed empirically. Since these factors are known, it is consequently possible to calculate the specific capacity of the screen in tons/h per square meter.
The amount of oversize describes the amount of the particles of the limit size. Particles which are considerably larger than the hole do not make screening difficult. Large stones push stones of limit size through the screening element.
The quantity of the half size is used to inform / present the quantity of the fine material. Material smaller than half the hole size passes through the screening element very easily. If a feed contains a lot of fine material, it can be fed in large quantities onto a screen. If there is little fine material the screening capacity falls. This is due to the fact that there are a lot of particles of limit/critical size. The throughput of particles of limit size (0.5 1.0 x hole) is very poor.
Flaky (thickness is small, relative to the other two dimensions) and elongated (length is larger than other two dimensions) stones are the most difficult to screen. They pass over the screen deck laying on their widest side. At worst they become wedged in holes and thus block the whole screen deck.
When the humidity is under 3 % it has no significant importance. The problems start at 4 5 %. At 9 30 % screening is very difficult. When there is more water the screening gets easier, and it is close to separation of water screening.
Stroke length, rotation speed, stroke angle, and screen inclination form together parameters which affect the operation of the screen. These fundamental factors have to be in proportion to each other. Stroke length and material amplitude have an effect on:
how the holes of the element stay unblocked. If the stroke length is too small also the material amplitude stays too small and the element gets blocked. The problem arises when the hole size is large (50 mm or more).
Acceleration of the screen box can be calculated by the stroke and rotation speed. When stroke angle and inclination are taken into the calculation, the vertical acceleration can be found. Vertical acceleration has an effect on the screening efficiency and the rate of travel.
Acceleration should be 4.5-5.5 x G (G=9.81m/s) with horizontal screens to reach a good screening result. To avoid structural damage for the screening unit, no acceleration greater than 6-7 times G are allowed.
Stroke angle has an effect on the material amplitude and the rate of travel. The most suitable stroke angle for horizontal screens is 55-60 degrees. Too upright a position can reduce the rate of travel. Horizontal stroke angle can improve the rate of travel but reduce screening efficiency. It also increases the wear rate of the mesh.
Speed of travel can be increased by inclining the screening surface. If the surface is greatly inclined, the stroke must be short to prevent material sliding over the mesh too quickly. Inclination of the surface can keep the mesh holes open more easily.
The bed of material may not exceed a height more than 3-5 times the size of the mesh hole on the discharge side of the screening surface. A higher bed of material will reduce the screening efficiency. Feeding capacity for each mesh size depends on the width of the screen. To get efficient screening results the depth of material bed must be at least 2 times the mesh hole diameter on the end side of the surface. Then volume of oversize will determine the width of the screen.
The depth of material bed should be within allowable limits on the beginning and the end of the surface when choosing the screen.Screening area is not theonly dominant parameter while choosing the screen. In practice the length is 2- 3 times the width.
A deck factor should be used when calculating lower decks in muitideck screens. In lower decks the feed drops not only at the beginning of the deck, but also later in the direction of the flow. That is why material close to separation size will not be screened out.
Effective screening area is the area where material can drop down through the surface. Effective surface area is about 0.7-0.9 times the whole area. The whole area is determined by the inside parameters of the screening unit: length times width.
Figure 3. Schematic diagram showing how the screening effect varies along the screen deck. Stratification takes place within zone 1, screening of fine undersize particles (75% of the size of the screen apertures) takes place within zone 2 and screening of critical undersize particles, i.e. particles of a size close to the size of the screen apertures, takes place within zone 3.
The amount of loading influences screening efficiency. In practice it is impossible to reach 100% efficiency. Maximum efficiency is about 95%. In most of the cases 90% is achieved and the screen can be said to be under 100% loading.
The greater the open area of the mesh, the more effective is the throughput. When determining the open area of the mesh, the diameter of the wire between holes in different meshes differs, and has to be taken into consideration.
The type of the mesh will have an effect on screening efficiency. The most significant difference will be in special screening cases. For example while screening elongated material, mesh should be of the vibrating type (rubber or harpmesh)
By scalping it is meant screening of coarse material in order to remove the undersize, typically before a primary crusher. Because of the coarse feed the top deck, which may be the only one, is often of a grizzly type. i.e. grizzly bars as opposed to mesh. This type of screening calls for a robust construction whilst there is no requirement for screening efficiency.
Leaving out the ancient trommel screens, stationary grids, and similar types, the following means are used to make the screen vibrate. All screens today are vibrated by various methods to pass the undersize through the apertures of the screen mesh or grizzly bars.
By freely vibrating screens one means screens that are supported on springs, and the box is vibrated by a vibrating mechanism (also called an exciter) which vibrates the screen box in various ways, depending on the type of vibrating unit.
Screens with a circular motion are the most common type. The vibration is circular because of a single eccentric shaft mechanism. This movement would not move the material forward, unless the screen is inclined in the direction of the material flow. This in turn means that the screening efficiency is not quite as good as a horizontal screen. The capacity as such is often higher as this screen is able to transport the material more quickly. The higher the inclination, the greater the transport ability. Inclination is typically 12 20. The inclination also helps to prevent pegging.
The depth of this material layer is more critical with a circular motion screen than with a horizontal screen. The inclination reduces screening efficiency. This type of screen may be used for almost any application. They are also cheaper to produce.
The vibration of this type screen is created by two eccentric shafts, rotating in opposite directions. This gives the box a linear motion. The stroke angle would depend on the relation of the eccentric weights of the two shafts to each other. Because of the linear stroke the material is moved in the direction of the stroke and the screen may be installed horizontally. That is why they are often called horizontal screens. The inclination would be typically 0 5.
The horizontal screen gives high screening efficiency, and they are often used for final and fine screening. Another advantage over inclined screens is their lower profile and therefore, horizontal screens mean lower structures and buildings, and shorter conveyors.
Elliptical motion can be achieved by various means. One method is by using three eccentric shafts, two of which would create the long axis and the third, the short one. These screens are used in special applications where the aim is to gain advantages of both circular and linear motion screens. It is a compromise however, there would also be a measure of disadvantages. These screens are typically installed at an 0 5 angle.
The eccentric shaft(s) of this screen type are connected both to the screen box and the foundation. The two shaft type would give a circular motion whilst the single shaft type would give this near the vibrating unit, and differ with the loading, depending on the action at each end.
These screens are used mainly for screening coarse material. The screens become heavy, and the dynamic forces which the foundation has to absorb, are a disadvantage. Brute force screens are installed 12 20 inclined.
The vibration of this type of screen is created by the resonance between the under-frame and the screen box or decks, and because of the resonance little energy is needed to vibrate the box. These screens are always installed horizontally.
The advantage of this type is the high efficiency as the screen can be very long, and therefore are mainly used for fine screening. They also have a low profile which can be advantageous. However they have very heavy and expensive structures.
Sizers are generally small and equipped with multi decks to assist screening. The products of two or more decks are often blended in the chute work of the screen. They have very high capacity because of the inclination. The apertures of the meshes need to be considerably larger than the cut, and thus affect the efficiency. This is compensated for by the blending. The advantage of this type is that it can be used for difficult material with less blinding than with other types.
The steep angle at the feed end gives the material a high velocity, some 3 4 m/s. Later the angle levels out and slows down the material to 1 1.5 m/s in the middle and 0.5 0.8 m/s at the discharge end. This is where the screening efficiency is achieved. These screens are generally large and used in high tonnage plants, particularly in mining where fewer fractions are separated.
There are a number of special screens, of which the flip flop is an example. The special narrow rubber mesh strips are installed perpendicularly between two separate frames. The meshes being attached to one frame on one side and to the other at the other side the bulk receives extremely high accelerations. This helps screening of wet, dirty and other difficult materials.
This table is a guide only to the parameters of a horizontal screen. When solving screening problems, take also into account the size parameters of the material, screen cloths and physical screening conditions.
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Critical factors in the selection of appropriate screening procedures to detect different phenotypic responses to nutrient-deficiency stress are discussed. Various morphological, anatomical, and physiological plant factors responsible for adaptations to nutrient deficiency, particularly low-P stress, are reviewed. Also, the relative effectiveness of various screening culture techniques for detecting phenotypic efficiencies based on specific plant features are considered.
The relative ineffectiveness of liquid culture media in detecting plant factors critical in P acquisition from low-P natural environments is recognized, and a culture medium that is effective under these conditions is described. P adsorbed onto alumina, after mixing with coarse sand, serves as a P source in nutrient cultures. Buffered P concentrations approximating soil solution concentrations are maintained in this system, and P availability at the root surface seems diffusion-limited. With this system, significant differences in the growth of tomato strains under P stress were detected.
The desirability of screening phenotypes at the same degree of depression from maximum yield (equivalent deficiency stress) is discussed. The need for evaluations at equivalent stress is associated with the capacities of plants in general to respond to deficiency stress with morphological and physiological changes that may not be under genetic control, for example an increase in root:shoot ratio. Additional capacity to adjust the same plant factors often are characteristic of specific phenotypes. The relative growth of the same tomato strains under equivalent and non-equivalent P-deficiency stress is compared. Significant strain differences were observed under both conditions. However, the relative responses among strains for several efficiency parameters were very different under the two types of stress.
Breeze V G, Canway R J, Wild A, Hopper M J and Jones L H P 1982 The uptake of phosphate by plants from flowing nutrient solution. I. Control of phosphate concentration in solution. J. Exp. Bot. 33, 183189.
Chapin F S III and Bieleski R L 1982 Mild phosphorus stress in barley and a related low phosphorus-adapted barley grass: Phosphorus fractions and phosphate absorption in relation to growth. Physiol. Plant. 54, 309317.
Coltman R, Gerloff G and Gabelman W 1982 Intraspecific variation in growth, phosphorus acquisition and phosphorus utilization in tomatoes under phosphorus-deficiency stress.In Proc. of Ninth Int. Plant Nutrition Colloq. Ed. A Scaife, pp. 117122.
Coltman R R, Gerloff G C and Gabelman W H 1986 Equivalent stress comparisons for evaluating physiological and morphological differences among tomato strains differentially tolerant to P deficiency. J. Am. Soc. Hort. Sci. Submitted.
O'Sullivan J, Gabelman W H and Gerloff G C 1974 Variations in efficiency of nitrogen utilization in tomatoes (Lycopersicon esculentum Mill.) grown under nitrogen stress. J. Am. Soc. Hort. Sci. 99, 543547.
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