MO ch 2_screening_complete_10.12.2020

Screening
Presented by:-
Dhaval N. Yadav
Lecturer (GES CL II),
Chemical Engineering Department,
Shri K. J. Polytechnic, Bharuch
Email : dhaval.nyadav@gmail.com

Topics to be Covered
•Screening
• Screening Terminologies
• Ideal & Actual Screening
• Types of Screen Analysis
• Capacity & Effectiveness of Screen
• Screening Equipment
12-10-2020 D. N. Yadav (dhaval.nyadav@gmail.com) 2

Screening
•Unit Operation.
•In Principle it is separation of solid from solid.
•The separation is mainly on the basis of size alone.
•The objective of screening is size control.

Need of Screening
• Separation of valuable matter.
• To meet specific size requirement (specifications).
• Prevent an incompletely crushed particle (oversize) from
entering into other operations.
• Removal of fines or undersize particle before sending to
primary size reduction equipment's.
• Removal of fines from finished product prior to transport.

FEED SIZE REDUCTION
EQUIPMENT
MIXED SIZE
PRODUCT
CLASSIFIER
OVERSIZE
UNDERSIZE

Screening Terminologies
• Equipment used for solid separation is called as Screen.

• Industrial screen are made from
• Woven wire,
• Silk,
• Plastic cloth,
• Metal bars,
• Perforated or slotted plates etc.
• made of various metals but steel or stainless steel are
most common.

•Number of openings per
linear inch in a screen is
called Mesh No.
PAN
200 Mesh
100 Mesh
65 Mesh
48 Mesh
28 Mesh
14 Mesh
8 Mesh
4 Mesh
FEED

• The minimum clear space between the edges of the
opening in the screening surface Screen Aperture or
Screen size opening.

Oversize or Head
•Material which does not pass through screen is
called Oversize or Head. It is represented as
“Plus +”.

Undersize or Fine
•Material which passes through screen is
called Undersize or Fines. It is represented as
“Minus –”.

Oversize & Undersize Representation
•Material which passes through 14
Mesh Screen and gets retained on
28 Mesh Screen is represented as
14/28 or -14/+28
PAN
200 Mesh
100 Mesh
65 Mesh
48 Mesh
28 Mesh
14 Mesh
8 Mesh
4 Mesh
FEED

Screening Mechanism
Feed
Undersize or
Fines
Oversize or
Heads

Screening Mechanism
•The material to be screened is dropped from the
top onto a screening equipment.
•The undersize pass through the screen, whereas
oversize are collected on the screen.
•A single screen makes a single separation into two
fractions which are called as Unsized Fractions
(Oversize and Undersize).

Tyler Standard Screen Series
• Widely adopted means of screen standard.
• The Tyler Standard Screen Series are based on the opening
of the 200 mesh screen, which is established at screen
opening of 0.074mm.
• 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 actual mesh dimension of any screen to that
of the next smaller screen is √2 = 1.4112-10-2020 D. N. Yadav (dhaval.nyadav@gmail.com) 15

Mesh
No.
Wire
Diameter
(inch)
Screen Opening =
(inch)
Screen
Opening
(mm)
Screen Opening
Ratio
4 0.065
=
1
4
− 0.065 = 0.185
4.699
6 0.036
=
1
6
− 0.036 = 0.131
3.327
8 0.032
=
1
8
− 0.032 = 0.093
2.362
10 0.035
=
1
10
− 0.035 = 0.065
1.651
=
1.651
1.179
= 1.41
=
4.699
3.327
= 1.41
=
3.327
2.362
= 1.41
=
2.362
1.651
= 1.41
𝟏
𝑴𝒆𝒔𝒉 𝑵𝒐.
− 𝑾𝒊𝒓𝒆 𝑫𝒊𝒂.

Screening Procedure
• Screening is carried out by passing the size
reduced material through a screen with openings
of desired size.
• More than one number of screens of various sized
can be stacked on above the other.
• The arrangement of screens is such that a screen
of smaller mesh number (i.e. larger openings) is
at the top and the screen with largest mesh
number (i.e. smallest openings) is at the bottom.
PAN
200 Mesh
100 Mesh
65 Mesh
48 Mesh
28 Mesh
14 Mesh
8 Mesh
4 Mesh
FEED

• The mixed sized mixture is placed on the top most screen
in the stack and is then shaken mechanically (or manually)
for about 15-20 minutes.
• Because of this shaking mechanism the particles tumble
one over another and smaller size particle with respect to
screen opening fall through it and larger sized particles are
retained on scree.
• The particles retained on each screen are later weighed
and recorded in tabular form (differential & cumulative)
for analysis of mixture.

Representation of Screen Analysis
• Results of the screen analysis are represented in a tabular
manner as mass fraction of each screen increment as a
function of the mesh size range of increment.
• The results are tabulated as a function of Mesh Number,
Screen opening (Dpi, mm), Mass fraction retained (xi),
Average particle diameter in increment (Dpi, mm)

• Average size of crushed material, which has passed through
one screen and retained on a screen having smaller
openings, is the arithmetic average of two screen openings .
It is represented as Average Diameter or Average
Dimension.

Differential Analysis
• Samples of particles having various sizes
and densities, mixture is divided into
fractions.
• Each fraction is of constant density and
approximately constant size
• Each fraction is then weighed and noted in
a tabular form.
PAN
200 Mesh
100 Mesh
65 Mesh
48 Mesh
28 Mesh
14 Mesh
8 Mesh
4 Mesh
FEED

• For a sample size (N) consisting of equal sized particles of
Diameter (Dp):
• The Total Volume of particle is given as :
• Since volume of one particle is vp, The Total Number Of
Particles is calculated as
• If the surface area of one particle is sp, the Total Surface
Area Of Particles in sample size N is calculated as

• Equation (4) represents the total surface area (A) of particles
in a particular sample size (N).
• The Specific surface area (Aw) of all particles in different
sample size i.e. N1, N2, N3...Nn is sum of individual total
surface area(s) i.e. A1, A2, A3...An
: average particle diameter taken as arithmetic average
of smallest and largest particle diameters in increment

Sr. No Mesh No.
Screen
Aperture Dia.
Avg. Particle Size Dpi
(microns)
Mass fraction
(xi)
1 4 4.699 - 0
2 6 3.327 4.013 0.0351
3 8 2.362 2.8445 0.3
4 10 1.651 2.0065 0.25
5 14 1.168 1.4095 0.2
6 20 0.833 1.0005 0.13
7 28 0.589 0.711 0.05
8 35 0.417 0.503 0.02
9 48 0.295 0.356 0.009
10 65 0.208 0.2515 0.0057
11 100 0.147 0.1775 0.0038
12 150 0.105 0.126 0.0021
13 200 0.074 0.0895 0.0011
14 PAN 0 0.037 0.0005
TOTAL 1.007312-10-2020 D. N. Yadav (dhaval.nyadav@gmail.com) 24

Fractional/Differential Screening

Cumulative Analysis
• Obtained by consecutively adding individual increments
starting with that containing smallest particles; and plotting
cumulative sums against the max. particle diameter in the
increment

Sr. No
Mesh
No.
Screen
Aperture Dia.
Avg. Particle Size
Dpi (microns)
Mass fraction (xi)
Cumulative
Particle with
size greater
than Dpi
Cumulative
Particle with
size smaller
than Dpi
1 4 4.699 - 0 -
2 6 3.327 4.013 0.0351 0 0.9722
3 8 2.362 2.8445 0.3 0.0351 0.6722
4 10 1.651 2.0065 0.25 0.3351 0.4222
5 14 1.168 1.4095 0.2 0.5851 0.2222
6 20 0.833 1.0005 0.13 0.7851 0.0922
7 28 0.589 0.711 0.05 0.9151 0.0422
8 35 0.417 0.503 0.02 0.9651 0.0222
9 48 0.295 0.356 0.009 0.9851 0.0132
10 65 0.208 0.2515 0.0057 0.9941 0.6722
11 100 0.147 0.1775 0.0038 0.9998 0.0037
12 150 0.105 0.126 0.0021 1.0036 0.0016
13 200 0.074 0.0895 0.0011 1.0057 0.0005
14 PAN 0 0.037 0.0005
TOTAL12-10-2020 D. N. Yadav (dhaval.nyadav@gmail.com) 27
1.0
1.0
1.0

Ideal and Actual Screen
An ideal screen is a type
of screen which sharply
separate the feed mixture
in such a way that the
smallest particle in
oversize would be just
larger than the largest
particle in the underflow.
Mixed sized Feed
particle
Largest particle
in Undersize
Smallest particle
in Oversize

• An actual screen does not provide a precise separation as
obtained in ideal screen. Reason it is not possible to achieve
such sharp separation
• Cut diameter Dpc is defined as a point which clearly makes
the point of separation between undersize and oversize
particles and is nearly equal to mesh opening of screen.

Ideal Screen Actual Screen
Yields sharp separation. Does not yield sharp separation.
100% efficiency. Efficiency is never 100% and is
lesser.
Does not exist in reality. Exist in reality.
Overflow or head contains particles
with size greater than Cut Diameter.
Overflow or head may contain
particles with size smaller than Cut
Diameter.
Underflow or fines contains
particles with size smaller than Cut
Diameter.
Underflow or fines may contain
particles with size greater than Cut
Diameter.

Capacity and Effectiveness of Screen
• The capacity of a screen is measured by mass of material
which can be fed per unit time to unit area of a screen.
• It is defined as the amount or mass fed per unit area of
screen per unit time. (mass/area*time).
• The success of a screening operation is measured by its
efficiency which is also called as effectiveness of screen or
efficiency of screen.

Effectiveness of Screen
• Capacity and effectiveness of a screen are opposing factors as
maximum effectiveness is related to small capacity, whereas large
capacity is only attainable at the expense of efficiency.
• Thus a reasonable balance between capacity and effectiveness is
needed/desired in reality.
Effectiveness ∝
1
𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦

Factors affecting Effectiveness of
Screen
• Capacity of a screen.
• The chance of passing through the screen of an undersize
particle is a function of number of times the particle strikes
the screen surface.
• It also depends on the probability of passage of a particle
through the screen in a single contact. (Explanation : If a
screen is overloaded the number of contacts is small and
chance of passing on contact is reduced by particle
interference )

Effectiveness of Screen
In general the probability of particle passing through a screen depends
very much on the direction or configuration (irregular particle size and
shape) in which the particle approaches the screen.
For a particle of irregular shape its surface area exposed to the screen
opening is different in different directions. As a result it is possible that
when fed in one particular direction or configuration the particle may
pass through the screen, but when fed in another direction the same
particle maybe retained by the screen.

Derivation for Screen Effectiveness
Feed consists of material A (oversize) and B (undersize).
F : mass flow rate of feed (kg/hr)
D : mass flow rate of overflow (kg/hr)
B : mass flow rate of underflow (kg/hr)
xF : mass fraction of material A in feed
xD : mass fraction of material A in overflow Mass Fraction of Material A
xB : mass fraction of material A in underflow
1- xF : mass fraction of material B in feed
1- xD : mass fraction of material B in overflow Mass Fraction of Material
B12-10-2020 D. N. Yadav (dhaval.nyadav@gmail.com) 37

Overall material balance over the screen:
F = D + B Eqn (1)
Material balance of component A over the screen :
xF*F = xD*D + xB*B Eqn (2)
Substituting value of D from equation (1) in (2) we get:
xF*F = xD*(F-B) + xB*B
On Solving we get : Eqn (3)

•Similarly substitute value of B from eqn (1) in (2) :
Eqn (4)
•Screen Effectiveness based on the Oversize Material
is the ratio of the amount of Oversize Material A
that is actually in oversize to the amount of oversize
material in feed.
Eqn (5)

•Screen Effectiveness based on the Undersize
Material is the ratio of the amount of Undersize
Material B that is actually in oversize to the amount
of undersize material in feed.
…..Eqn (6)

• The overall effectiveness is thus given by :
Eqn (7)
• Substituting values of EA and EB from eqn (5) and (6) in eqn
(7) we get :
Eqn (8)

• Finally substituting values of (D/F) and (B/F) from eqn (3)
and (4) in eqn (8) we get :

Classification of Screen
Scree
n
Size of
Material to be
Handled
Coarse,
Intermediate
Fine Screen
Method of
Support
Stationary,
Moving Screen

Classification of Screen
• Stationary screen can be used for both coarse and fine
screening, but mostly for coarse screening as coarse
particles drop easily.
• Moving screens mainly used for fine screening are agitated
either mechanically or electrically to give linear, circular,
revolving or vibrating motions to the screens

Screening Equipment's:
Accordingly, the different types of screening equipment's are
as below:
• Grizzlies
• Vibrating Screens
• Gyratory Screens
• Trommels

Grizzlies
• Grizzlies consist of set of parallel bar usually made of steel
(MS) and of trapezoidal in cross-section where wider section
is placed at the top and smaller section between two bar will
available at the bottom.

• The bar are set with the slope of 30 to 45° with the
horizontal depending on the nature of material to be
treated.
• The length of bars is approx. 3 m with spacing ranging
from 50 to 200 mm.
• The material to be screened is introduced at the top of the
slope.
• Large pieces of solids rolls and slide to lower end, while
smaller pieces having size smaller than the bar opening fall
through the bars and are collected separately.

• The capacity of grizzly increases with angle but its efficiency
decreases.
• The largest application of grizzly is in the separation of under size
from a feed to primary crusher.
• These are employed in the separation of the undersize from the
feed to the primary crusher.

• A stationary grizzly is usually used for screening of dry
materials.
• It is simplest of all screening devices, least expensive and
required no power, easy to operate and maintain.
Drawback of Grizzly:
• There are chances that the openings of a grizzly getting
blocked by wedged shaped particles.
• The labour requirements in operating the grizzly are high
due to blocking of openings.

Trommel
FEED
COARSEST
PARTICLES
PARTICLE
SIZE DPi
PARTICLE
SIZE DPi
PARTICLE
SIZE DPi

Construction and Working
• Trommels are revolving screens
• They are usually cylindrical or conical in shape open at both ends
and are normally inclined at 5 - 10 ° with the horizontal .
• They are slow speed rotating machines and are rotated around 15
to 20 rpm.
• Trommels are constructed using a number of screen of gradually
increasing aperture size along the length of the cylinder.

• The feed to be screened is entered through a hopper at
one end.
• The machine is rotated slowly (manually or automatically),
and due to this rotation the particles move down and
screening takes place.
• The different size screens are placed in series. The
arrangement is such that the finest screen is first and the
coarsest the screens are arranged is at the last.

• The material to be screened is fed to the finest size screen.
• This is done so as the mixture of particles enter into the
finest zone the coarsest particle will retain on the screen
to be separated in the latter section and the finest particle
will pass in first section only.
• Because of this arrangement of sieves a number of
products of different sizes can be collected from a single
trommel.

• Trommels are thus both low capacity and low efficiency
machines when used as single aperture opening.
• However they are best suited for separation of coarse
particles, size ranging from 6 to 55 mm.
• The efficiency of a trommel is greatly affected by its length
and diameter, its speed of rotation, its inclination (slope
of a screen), screen opening, feed rate (kg/hr), percentage
of oversize in feed and percentage of moisture in feed.

Arrangement-1
• Largest perforation Trommel
is at the top in series.
• Most convenient arrangement
for solid separation.
• Coarsest of the particle is
obtained/separated first
• Finest of the particle is
obtained at the last
FEED
COARSET
PARTICLE
FINEST
PARTICLE

Arrangement-2
FEED
FINEST
PARTICLE
COARSEST
PARTICLE
• Largest perforation Trommel
is at the last/bottom in
series.
• Coarsest of the particle is
passes onto consecutive
Trommel in series
• Finest of the particle is
obtained first

Arrangement-3
• Concentric arrangement of
Trommel.
• Feed enters the centremost
coarsest screen.
• The smaller (fine) particles
will move from the coarsest
screen to the finest screen.
FigureTakenfromLecture6ScreeningEquipment's,EffectivenessandCapacityProf
NandaKishore,IIT,Guwhati

Grizzlies vs. Trommels
Grizzlies Trommels
Stationary Inclined Screen Revolving Screen
Screen is grid of metal bar. Screen is a perforated
cylindrical chamber.
Opening in screen are large. Opening in screen are small.
Can handle large feed size. Can handle small feed size.
High capacity. Less capacity.
Large labor requirement. Less labor requirement.
Cheap in construction. Costly in construction.12-10-2020 D. N. Yadav (dhaval.nyadav@gmail.com) 58

Vibrating Screen
12-10-2020 D. N. Yadav (dhaval.nyadav@gmail.com) 59Vibrating Screen supplied by K. C. Engineers, Ambala Cantt

Vibrating Screen
• Most widely used in chemical industry
• Can handle large tonnages of material (high capacity)
• High efficiency
• Provide good accuracy of sizing.
• Require less maintenance per ton of material handled, and
• Can handle a wide variety of materials from 480 mesh to 4
mesh.

• Special type of screens which are rapidly vibrated with
small amplitude because of which the material keeps
moving and screen does not get bind.
• Vibrations are produced either mechanically or electrically
with frequency of 1800 to 3600 per minute.
• The screens are arranged in a stack (one above other) such
that coarsest screen is at the top and finest screen is at the
bottom.

Vibrating Screen : Benefits
• It has a smooth and quiet operation,
• Exact sizing,
• High output,
• Simple structure,
• Easy maintenance,
• Low energy consumption,
• High screening efficiency and long life.

Gyratory Screen

Gyratory Screen
• Box like machines either square or round with series of decks
arranged one over the other such that the coarsest screen is
at top with finest at bottom.
• Each deck is having it’s individual screen as well as the outlet
and inclined at 16 - 30˚ with horizontal.
• The gyrations are in vertical plane about a horizontal axis
which are produced by eccentric shaft .

• It is designed specifically for high capacity separation by size
of dry material and for wet separation when oversize
material constitute a large percent of feed.
• The gyratory motion is up to 1450 cycles per minute.
• Gyratory motions are gentle and is ideal for more fragile
products.
• Application of these are in food, chemical, mineral,
pharmaceutical industries.

Factors affecting Screening
Presence of near mesh particles:
• These particles have size very close to the aperture size of
the screen. So these particles may partly pass through the
screen.
• The screen may be clog or blinded by these particles.
Screen Slope
• Slope of screening equipment is directly proportional to the
rate of fall of particles downwards.12-10-2020 D. N. Yadav (dhaval.nyadav@gmail.com) 66

Cohesion of particles :
• While dealing with a mixture of
particle it usually happens the
particle may clog (or join) with
each other and make a bigger
particle.
• As a result a finer particle gets
converted into a bigger, coarse
particle because of which it will
not pass through the screen
but will be retained on the12-10-2020 D. N. Yadav (dhaval.nyadav@gmail.com) 67

Moisture Content:
• The screening operation is highly influenced by the
moisture present in the screen.
• If particles are moist there are chances that they will
stick to the surface of the screen, as well as with
other particles.
• This will causing difficulties in screening operation.

Method of Feeding
• Manual or Automated.
• The material should be spread evenly over the screen and
must approach the screening surface in the direction
parallel to the longitudinal axis of the screen.
Screening Surfaces
• Use of single deck screen in series results into the most
efficient operations as in case of multiple desk screen

References
• McCabe, Warren L., and Julian C. Smith. 1967. Unit Operations of
Chemical Engineering. New York: McGraw-Hill.
• Gavhane K. A. 2015., Unit Operations -1, Nirali Prakashan, Pune India
• Dr. Shabina Khanam , “Mechanical Operations”, NPTEL Chemical
Engineering, 25 Sept 2016, https://nptel.ac.in/courses/103107123/
• Prof Nanda Kishore, “Mechanical Unit Operations”, NPTEL Chemical
Engineering, 27 May 2019,
https://nptel.ac.in/courses/103/103/103103155/

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