Screening is the most important unit operation in any industry. It is the operation involve before the process as well as after the process. It is related to the separation based on the size. so screening mechanism is necessary to understand.

1. SCREENING
By Mrs Anita Singh
B.E. Chemical
M.E. Petrochemical

2. CONTENT
 Definition
 Purpose
 Types of screening
 Screening Mechanism
 Screening Surface

3. What is
Screening ???
 It is process of separating a mixture of particles of
different size into two or more no. of fractions.
 It is accomplished by passing the material in an open
cylindrical container with uniformly spaced opening
of the desired size at the base called screen.

4. Screening
 Each fraction is more uniform in particle size than
the original mixture.
 Followed in variety of industries, such as mining
and mineral processing, plastics, recycling and
pharmaceuticals.

5. Purpose
To remove coarse particles for further size
reduction
To remove fine particles from crusher feeds to
save power by preventing over.
To grade the crushed products into
commercial sizes

6. Limiting Screen/ Undersize/ minus material
The screen through material passed
Retaining Screen/ Oversize/Plus material
which has retained

7. Types of Screening
Dry Screening Wet Screening
 Treatment of
material containing a
natural amount of
moisture
 Material that has
dried before screening
 Cement, talcum
powder, or aluminum
powder
 Water is added to the
material on the screen
to remove undesirable
material, mostly clay
and fine particles
Used with moist
product whose
screening is difficult
without washing.

8. Screening mechanism
The particles must be brought to the screen
opening and be presented at such a velocity
and in such a direction the passage of
undersize particle is not affected by the edges
of the openings.
Due to tonnage requirements, thicker
screening surfaces are used and some kind of
motion is given.

9.  The whole operation is divided into two parts.
1) Stratification:
Material delivered to the screen, it loses its vertical
component of velocity and change direction.
Due to some kind of vibration large particle rise to the
top of bed while smaller sift through the void and find
way to reach the screen opening.

10. 2) Separation probability
 particle reach the screen opening and rejected if they
are larger than the opening and pass if they are smaller
than the opening
It is a Function of particle size and screen opening. If
the difference is large easy separation.

11. Screening Surfaces
 Three types
1) Parallel bars/rods
2) Punched plate
3) Woven Wires

12. 1) Parallel bars/rods
• Made of steel bars,
discarded rails or cast
iron bars
•Fixed in a parallel
positions by cross bars
and spacers
•Used for coarsest
works, not for fine
grinding

13. 2) Punched plates
• Made of metal/ plastic sheets punched by dies of various
patterns- circular, square, rectangular, hexagonal and slotlike.
•For coarse separation  Circular opening
• For Fine separation  slotted opening, lesser tendency to
blind
• Disadvantage : Low % of open area

14. • woven of gauged wires, generally made up of steel to
produce either square or rectangular opening intended
to prolong their life.
•Made up of any length and in width up to 5 feet.
•Square opening  Coarse and Intermediate separation
Rectangular opening  Fine separation
3) Woven- wire Screen

15. THANK YOU