Properties, Handling and Mixing of Particulate Solids

By Sidra Jabeen Department of Chemical Engineering, University of Engineering & Technology Lahore

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SCREENING

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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.

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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

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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.

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Screening Terminology Mesh number of screen

o o o

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.

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Screening Terminology Screen Interval

o

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.

o

The ratio of actual aperture size of any screen to that of the next smaller screen is √2 = 1.41.

o

The area of openings in any one screen in the series is exactly twice that of the openings in the next smaller screen.

Screening Terminology Material flow through Screen Overflow Feed

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Overflow : The material that is rejected by the screen.

U nderflow : The material that is passed through the screen. Underflow

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Screening Terminology Material flow through Screen Overflow (oversize particles) Underflow (undersize 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 -14 + 20 The average size of the particles in the increment will be the arithmetic average of the aperture sizes of two screens.

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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 = 1.41

.



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

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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 – 1 i i + 1 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - D i – 1 D i D i + 1

  

Minimum particle diameter = D pi (min) Maximum particle diameter = D pi (max) = D i = D i-1 = √2 D i Particle Size Range = √2 D i – D i = 0.4142 D i Mean particle size = D pi, mean = (Di + Di-1) / 2 = (√2+1)D i / 2

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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

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The smallest mesh at the bottom and the largest at the top

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Materials are loaded at top and then shacked for a period of time



Materials are collected from every screen and weighed.

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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.

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Particle Sizing for top most screen 2.

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.

3.

If concentration is negligible , the top most screen may be neglecting for sizing

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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)

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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.

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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)

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Representation of results of Screen Analysis

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Differential Analysis

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Cumulative Analysis

Differential and Cumulative Analysis

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Mesh no. of Screen Aperture size of screen D pi (mm) Average Dia. of the particle D pi (Avg.) (mm) Mass Fraction x i Cumulative Mass Fraction smaller than D pi

4 6 8 10 14 20 28 Pan 4.50

3.19

2.26

1.6

1.13

0.8

0.57

3.84

2.72

1.93

1.36

0.96

0.68

0 0.02

0.05

0.1

0.18

0.25

0.25

0.15

1 0.98

0.93

0.83

0.65

0.4

0.15

0

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Factors affecting Screening 1.

2.

3.

4.

5.

6.

7.

Orientation of particle Presence of near mesh particles Shape of particles Size of Particles Motion Nature of particles Humidity or moisture

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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

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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.

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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)

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4. Size of particles



Coarse particles – screening is difficult

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Fine particles – screening is easy

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Ultrafine particles – loss as dust

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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

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6. Nature of feed particles

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Soft / porous particles – cohesive in nature – size enlargement

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Hard / rigid – better screening – impact may cause screen failure

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7. Humidity / moisture

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Greater the moisture – cohesiveness

 

size enlargement reduction in available screen surface

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Screen Blindness

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Bridging

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Cohesiveness



Size enlargement – reduction in available screen surface



Clogging



Irregular shape – reduction in flow area of screen

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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 D

pc

, 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