1. Properties, Handling and Mixing of Particulate SolidsBy
Sidra Jabeen
Department of Chemical Engineering,
University of Engineering & Technology Lahore

2. SCREENING

3. Screen / Sieve
Screen is an open container usually cylindrical with uniformly spaced openings at the base. It is normally made of wire cloth, the wire diameter and interspacing between wires is carefully controlled.

4. Screening and its Terminology
Screening is a method of separating particles on the basis of their size.
Aperture size of screen
Mesh number
Screen interval
Material flow through screen

5. Screening Terminology Aperture Size of screen
The size of a square opening (length of clear space between individual wires) is called the aperture size of screen.

6. Screening Terminology
Mesh number of screen
Screens are designated by their mesh number. Mesh no. indicated the number of apertures/openings per linear inch of the screen.
E.g. A screen having 10 square openings per inch is said to have mesh no. 10
Higher the mesh no., smaller the aperture size of screen.

7. Screening Terminology
Screen Interval
Screen interval is a factor by which aperture size of a screen is to be divided to get the aperture size of next successive screen.
The ratio of actual aperture size of any screen to that of the next smaller screen is √2 =
The area of openings in any one screen in the series is exactly twice that of the openings in the next smaller screen.

8. Screening Terminology Material flow through Screen
Feed
Underflow
Overflow
Overflow: The material that is rejected by the screen.
Underflow: The material that is passed through the screen.

9. Screening Terminology Material flow through Screen
Overflow (oversize particles)
Oversize particles -> Plus material Undersize particles -> Minus Material
Two numbers are needed to tell the size range of the increment; one for the screen through which the material passes and the other on which it is retained.
E.g. 14 / 20 or
The average size of the particles in the increment will be the arithmetic average of the aperture sizes of two screens.
Underflow (undersize particles)

10. Tayler Standard Screen Series
Square shape opening
The area of the openings in any one screen in the series is exactly twice that of the openings in the next smaller screen.
The ratio of the actual mesh dimension of any screen to that of the next smaller screen is = √2 =
Based on 200 mesh No. screen
Usually 5-6 screens are arranged in a stack
for closer sizing intermediate screens are available (with mesh dimension 1.189)
Shaken mechanically for a definite time

11. Tayler Standard Screen Series
Size of the upper screen must be larger by a factor √2 and that of lower screen must be smaller by a factor √2
i – Di – 1 
i Di 
i Di 
Minimum particle diameter = Dpi (min) = Di
Maximum particle diameter = Dpi (max) = Di-1 = √2 Di
Particle Size Range = √2 Di – Di = Di
Mean particle size = Dpi, mean = (Di + Di-1) / 2 = (√2+1)Di / 2

12. How Screen / Sieve Analysis is done?
Screen analysis is carried out using number of screens so that aperture size reduces for lower sieve
Screens are arranged serially in a stack
The smallest mesh at the bottom and the largest at the top
Materials are loaded at top and then shacked for a period of time
Materials are collected from every screen and weighed.

13. Particle Sizing for top most screen
1. If the particles are large enough with appreciable concentration (mass fraction) so that their average size could easily be measured with the help of thread and meter rod. Then few representative particles are chosen 5 to 6 prominent dimensions of each particle is measured so that its average size is known.

14. Particle Sizing for top most screen
If concentration (mass fraction) is appreciable and particles are small, then imaginary sieve immediately above the screen under consideration in T.S.S.S is used and the arithmetic mean of clear opening of two screens is used as representative size of material present over top most screen.
If concentration is negligible , the top most screen may be neglecting for sizing

15. Particle Sizing for top most screen
4. If concentration is large enough with relatively wide variation in sizes of particles then 2 to 3 imaginary screens are assumed and then material is distributed over those screen
(equal weight distribution, experience based distribution, graphical approach, computer simulation)

16. Particle Sizing for bottom most screen (Pan)
1. If concentration is negligible , the bottom most screen may be neglecting for sizing
2. If concentration is small, however particles are nearly of same size then arithmetic mean of clear opening of the screen above pan and imaginary screen below it is taken.

17. Particle Sizing for bottom most screen (Pan)
3. If concentration is large enough with relatively wide variation in sizes of particles then 2 to 3 imaginary screens are assumed and then material is distributed over those screen
(equal weight distribution, experience based distribution, graphical approach, computer simulation)

18. Representation of results of Screen Analysis
Differential Analysis
Cumulative Analysis

19. Differential and Cumulative Analysis
Mesh no. of Screen
Aperture size of screen
Dpi (mm)
Average Dia. of the particle
Dpi (Avg.) (mm)
Mass Fraction
x i
Cumulative Mass Fraction smaller than Dpi 4
4.50 - 1 6
3.19
3.84
0.02
0.98 8
2.26
2.72
0.05
0.93 10
1.6
1.93
0.1
0.83 14
1.13
1.36
0.18
0.65 20
0.8
0.96
0.25
0.4 28
0.57
0.68
0.15
Pan

23. Factors affecting Screening
Orientation of particle
Presence of near mesh particles
Shape of particles
Size of Particles
Motion
Nature of particles
Humidity or moisture

24. 1. Orientation of Particle
Overall probability of passage of one particle
No. of times particle strike
Probability of passage during single strike
Angle of approach
Perpendicular - larger the chance
Endwise – min. contact area –high probability of screening
Sidewise – max. contact area – low probability of screening

25. 2. Presence of near mesh particles
Particles having size very close to the aperture size of the screen.
They may pass through the screen in any particular configuration.
They may cause clogging or blinding of the screen.

26. 3. Shape of particles
Regular shape (spherical) – screening is easy and efficient
Irregular shape – screening is difficult (they may pass through the screen in one particular direction or may retain on the screen in any other direction)

27. 4. Size of particles Coarse particles – screening is difficult
Fine particles – screening is easy
Ultrafine particles – loss as dust

28. 5. Motion
The purpose of induced motion is to enhance the probability of particles to strike on screen surface
Jolting action
Sifting action
Too high – reduces efficiency – as particle bounce back
It reduces the screen blindness
High vibration at high feed rate is used

29. 6. Nature of feed particles
Soft / porous particles – cohesive in nature – size enlargement
Hard / rigid – better screening – impact may cause screen failure

30. 7. Humidity / moisture Greater the moisture – cohesiveness
size enlargement
reduction in available screen surface

31. Screen Blindness Bridging Cohesiveness
Size enlargement – reduction in available screen surface
Clogging
Irregular shape – reduction in flow area of screen

32. Comparison of IDEAL & ACTUAL screen
An ideal screen would sharply separate the feed mix in such a way that the smallest particle in overflow would be just larger than the largest particle in underflow
Ideal separation defines a cut diameter Dpc , the point of separation between oversize and undersize fractions and is equal to aperture size of the screen.
Actual screens don’t perform a perfect separation about the cut diameter