Different type of screening equipment and their application in chemical industry

Different type of screening equipment and their application in chemical industry. Screening equipment consists of a drive that induces vibration, a screen media that causes particle separation, and a deck that holds the screen media and the drive and is the mode of transport for the vibration. It is used during the mechanical screening processes, designed to separate one material from another. As the second part of the material handling process, screening equipment is used to separate raw material from a crusher or quarry into even finer grades, coming closer to an end product. There are two types of screens [wet and dry], totally dependent on the raw material. Wet screens utilize spray nozzles and water along with screen vibration in the sorting process, while dry screens use vibration only.

1. What is screening A method of separating solid particles according to size alone is called screening. Screening is an operation for separating solids on the basis of size alone. It refers to the separation of solid materials on the basis of size using screens of known openings. Screening is the separation of a mixture of solid particles of various sizes into two or more fractions by means of a screening surface.

2. Screening terminogy Aperture size of screen The size of square opening(length of clear space between individual wires) is called aperture size of screen. Mesh number of screen mesh no indicate the number of Aperture/opening per linear inch of the screen. Mesh size of screen it is a size of 1 opening in a given screen, mesh size=1/Mesh number

3. Different type of screening equipment Grizzly Screens Trommels Screens Shaking Screens Gyrating Screens Oscillating Screens

4. Grizzly Screens

5. Construction A grizzly is a grid of parallel metal bars set in an inclined stationary frame, with a slope of 30 to 45o. The slope, and therefore the path of the material is parallel to the length of the bars. The length of the bar may be up to 3 m and the spacing between the bars is 50 to 200 mm. The material of construction of the bars is manganese steel to reduce wear. Usually, the bar is shaped in such a way that its top is wider than bottom, and hence the bars can be made fairly deep for strength without being choked by material passing partway through them. A stationary grizzly is usually used for a dry free flowing material and is not satisfactory for a moist and sticky material.

6. working A coarse feed (say from a primary crusher) is fed at the upper end of the grizzly. Large chunks roll and slide to the lower end (the tail discharge), whereas small lumps having size less than the opening in the bars fall through the grid into a separate collector. If the angle of inclination to the horizontal is greater, greater is the output (throughput) but the lower is the screen efficiency. Stationary inclined woven-metal screens operate in the same way that separate particles 12 to 100 mm in size.

7. working A grizzly finds its greatest application in the separation of the undersize (fines) from the feed to a primary crusher. A stationary grizzly is the simplest of all separating devices. It requires no power and is the least expensive to install and maintain. As the openings in the grizzly have a tendency to get blocked by wedge shaped particles . The labor requirement for operating the grizzly is high and it is difficult to change the openings in the bars. Grizzlies are used for only the coarsest and roughest separations.

8. advantage Simple in construction In expensive Low-maintenance

9. Disadvantage Difficult to adjust opening size. There is a problem of separating moisture of sticky material.

10. application Screening of gross grain. Separation of gross grain before classifying. Separation of find grain from crusher. Protection of screening in front of conveyor belts.

11. Trommels Screens

12. Construction A trommel is a revolving screen consisting of a cylindrical frame surrounded by wire cloth or perforated plate (which acts as a screening surface). It is open at one or both ends and inclined at a slight angle to the horizontal so that the material is advanced by the rotation of the cylinder. These units revolve at relatively low speeds of 15 to 20 rpm. A trommel is a mechanically operated screen consisting of a slowly rotating perforated cylinder (or cylindrical frame) surrounded by wire cloth (or perforated plate) with its axis at a slight angle to the horizontal.

13. Construction The perforations in the screening surface may be of the same size throughout (i.e., over the whole length of the cylinder) or may be of different size in which case the small size perforation section is near the feed end. It is driven at the feed end through a gear mechanism. It has a feed point at the upper end, an undersize product discharge below the screening surface and a oversize discharge at the opposite end (lower end). Fig shows a schematic diagram of trommel having sections of different size perforations.

14. working The material to be screened is fed at the upper end and gradually moves down the screening surface towards the lower end. In doing so the material passes over the apertures of gradually increasing size A s the single cylinder is provided with perforations ranging from the finest desired at the feed end to the coarsest at the discharge end. If the single cylinder is provided with the screen having three different size perforations then we get four fractions. The finest material is collected as the underflow in the compartment near the feed end and the oversize material (coarsest) is withdrawn from the discharge end. Such type of arrangement is usually used for smaller capacities.

15. Working With this type of trommel, there is a tendency of blockage of the apertures by the large material and the screen with the finest opening being the weakest it is subjected to the largest wear. The operating speed of a trommel is 30 to 50% of the critical speed (the critical speed is the one at which the material is carried completely round in contact with the screening surface).

16. advantage Wide adaptability of material Simple and diverse feeding method High screening efficiency Large processing capacity Low consumption Low noise Long service life Environmental protection Easy maintenance

17. Disadvantage Low efficiency Difficult to produce multiple fractions Low capacity Screen plugging Screen blinding Drum damage

18. application Screening of blasted rocks and granite Application of sticky and wet material Primary processing General recycling Wood waste Demolition waste

19. Shaking Screens

20. construction In some situations, the screen is rapidly vibrated with small amplitude to keep the material moving and prevent blinding as far as possible. Vibrating screens are commonly used in industry where large capacity and high efficiency are desired. The vibrations may be produced mechanically or electrically, accordingly we have mechanically vibrated screens and electrically vibrated screens. The vibrations may be produced either mechanically or electrically with frequency of 1800 to 3600 or even more per minute.

21. construction Mechanical vibrations are generally passed on from high speed eccentrics to the casing and from there to inclined screens so that the whole assembly is vibrated. Electrical vibrations are generally passed on from heavy duty solenoids directly to the screens so that only screens are vibrated. Vibrating screens may be mounted in a multideck fashion (not more than three decks) with the coarsest screen at the top, either horizontally or inclined up to 45o. Electrically vibrated screens are widely used in the chemical industry. The vibrating screens have accuracy of sizing, increased capacity per square meter and low maintenance cost per ton of material.

22. working In case of a single screen, the vibrations are given to the screen to effect the separation of solid particles into two size fractions. In case of vibrating screen (generally consists of three decks), the material to be separated is fed to the top screen and simultaneously the screens are vibrated either electrically or mechanically at a frequency of 1000 to 3500 per minute (1000-3500 vibrations per minute). Due to vibrations the particles on the screen are kept moving and due to inclination given to the screens, the oversize material travels along the screen and is collected separately. The undersize material passes through the screen and is collected. Four fractions are obtained with a three deck screen.

23. advantage Can be cleaned quickly Low noise level Easy to assemble Very low maintenance , Because there is nothing to lubricate. Fast screen change High reliability with low power consumption Good price / Performance ratio Maintenance free, long-lasting anti-vibration mountings

24. Disadvantage High cost of maintenance of screens and its supporting structures. Screen has low capacity Time required to handle batch wise operation is very large for heavy duty operations

25. Application In wood processing industry (Pelletizing) In Recycle industry In chemical industry like fertilizer industry, plastic industry, in foundries Animal feed industry Glass and ceramic industry Mining and construction material industry

26. Gyrating Screens

27. construction Gyratory screens which are gyrated vertically contain several decks of screens arranged one above the other and held in a box or casing. The screens are arranged such that the coarsest screen is at the top and the finest at the bottom. Discharge ducts are provided for the screens to permit removal of the several fractions. The casing is inclined at an angle ranging from 16 and 30 ° with the horizontal.

28. construction The gyrations are in a vertical plane about a horizontal axis and are produced by an eccentric shaft fixed in the floor of the casing halfway between the feed location and discharge. The screens are rectangular and fairly long. The speed of gyration, the amplitude of throw and the angle of tilt can be adjusted as per requirements.

29. working  The feed is inserted from the top and gyratory motion triggers the penetration of particles into the next deck through screen openings. Casings are inclined at relatively low angles (< 15°) to the horizontal plane, with gyrations occurring in the vertical plane. The eccentric masses can be varied in such as the increase of top eccentric mass leads to an increase in horizontal throw, promoting the discharge of oversize materials. Increment in bottom eccentric mass boosts the material turn over on the screen surface, maximizing the quantity of undersize-material penetration. Oversize materials are discharged via tangential outlet.

30. working The option to select number of decks enables gyratory equipment to accurately separate materials consisting particles that are very close in size. This advantage is unrivalled and proves to be significant in the powder processing industry where fine materials are involved. High separating efficiency and ease of maintenance puts gyratory screening ahead compared to other processes in terms of product quality. Existing gyratory equipment designs are already on the market, more to come with further development. Recent studies have shown that potential improvements are available for cost-saving and effective separation process.

31. advantage Low running cost Ideal for multi-fraction separation Flexible range of application Good efficiency and quality of separation Easy to maintained Low screen blinding

32. Disadvantage Large amount of floor space Relatively difficult to operate Susceptible to lumps and agglomerates in the feed

33. application Process Industry processing of ceramic, pulp and paper mil, paints, sand, starch slurry Food industry screening of refined table salt, papaya cubes, turmeric pigment, clarification of alkaline extracts Chemical industry screening of hydrate lime, classification of polyesters beads, anhydrous aluminum chlorides

34. Oscillating Screens

35. construction The oscillating screening machine (oscillating screen) type ASM was specially developed for separating and sorting bulk solid materials. Thanks to the wide, orbital and vibration-free horizontal oscillation, the greatest screening results can be achieved. Our many years of experience in the manufacturing of screening machines are reflected in the long-lifetime and low-maintenance of our products, which can also be attributed to the sturdy construction of the machines. The “ASM” series screening machines (also called: woodchip screens) are available with screen sizes from 0.5 to 24 square meters and particle size from 0.1 to 150 mm

36. working The material is fed evenly into the feed hopper of the screening machine or gyratory screen with the aid of a dosing system, conveyor belt or the like. In the feed area, the material to be screened is spread out on its way to the screening area. Through the rapid and orbital oscillations, the loaded material is kept constantly in motion as it flows over the screen surface in the direction of the discharge area.

37. working The orbital movement of the screen causes the material to distribute itself horizontally over the surface, so that it is almost impossible for long thin pieces to stand upright on the screen or drop through it. The material to be screened, which is smaller than the apertures in the screen elements, falls through. From there, it moves on to the next screening stage, if required. This procedure is repeated until the material has been completely screened. The screened material is then discharged through the corresponding discharge outlets.

38. advantage High screening capacity since the entire surface area of the installed screen is put to use Highly economical, as up to six fractions can be handled with low space requirement Low noise level Low maintenance and repair Compact construction Highly flexible under changing conditions Any fraction can be selected Also available as dust-proof model

39. Disadvantage Suitable for vertical arrangement of screens not suitable for horizontal arrangements. Screen is preferred for fine material screening only for mean particle diameter less than 4mesh size Iterating cost is high due to provision of electrical or magnetic field based oscillation arrangements. Suitable for batch wise operation with medium to smaller feed rate. Not suitable for screening of wet materials

40. application Pulp and paper industry Waste wood incineration plant Biomass power station Recycling industry Plastic industry Brick-works Fiber board industry

41. Reference UNIT OPERATION-1 , by K.A GAVHANE https://www.sf-gmbh.de/en/oscillating-screens.html https://encyclopedia.che.engin.umich.edu/Pages/SeparationsMechanical/Screeners/Screeners.html

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