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
• As 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/Screene
rs/Screeners.html