Module 3: Size Separation

of Solids
• Screening - Screen analysis
• Screen efficiency and capacity
• Screening Equipment – Grizzlies – Trommels
• Vibrating screen, Gyratory screen, Banana screen
 Introduction
• Screening is effective method for separating
relatively coarse particle according to size.
• For very fine particles: according to terminal
falling velocity.
 Determining Particle Size
Various methods are used for measurement of
particle size. These depends on size range,
the physical properties and the condition of
dryness or wetness.
The following methods are used in laboratory.
1. Microscope
2. Screening
3. Sedimentation
4. Elutriation
5. Centrifuging
 Screening
• Screening is an operation used for the separation of
particles according to their sizes.
• Sieving and screening are distinguished by the fact
that sieving is a batch process used almost
exclusively for test purposes, whereas screening is a
continuous process and is used mainly on an
industrial scale.
• Sieves are manufactured with definite dimensions
and standard aperture sizes.
• Screens can be manufactured with any dimension
and any aperture sizes as per the requirement.
• In industrial screening, the particles of various
sizes are fed to the screen surface.
• The material passing through the screen
aperture is called underflow (undersize or
fines) while the material retained on the
screen surface is called overflow (oversize or
coarse).
 Dry and wet screening
• Screening is performed either dry or wet. Wet screening is
superior, adhering fines are easily washed off, and it avoids
the dust problem.
• But the cost of dewatering and drying the products is high.
• Screening is generally used for dry treatment of coarse
material.
• Dry screening can be done down to 10 mesh with
reasonable efficiency.
• Wet screening is usually applied to materials from 10 mesh
down to 30 mesh (0.5 mm).
• The recent developments in the Sieve Bend Screen have
made the wet screening possible at the 50 micron size.
 Screening (Sieving)

Importance of screening:
• Removes the fine from the feed material before a
reduction equipment.
• Prevents oversized material to enter into unit
operations.
• Produce a process grade material to meet specific feed
size.
• Removes fines from finished product before packing.
 Types of standard screens:
• Standard screen are of various types such as
Tyler standard screen series, U.S. sieve series
or Indian standard sieves.
• Testing sieves with square opening are
constructed of woven wire screens, the mesh
and dimensions of which are standardized
• Every screen is identified in meshes per inch
• In coarse screens, the term mesh refers to the
distance between adjacent wires or rods.
• In fine screens, the mesh is the number of
opening per linear inch counting from the
centre of any wire to a point exactly one inch
distance.
• Example: A 100 mesh screen will have 100
opening per linear inch.
• Minimum clear space between the edges of
the opening in the screening surface is termed
as screen aperture or screen size opening.
Classification of screens and their construction
• Screen surface is the medium containing the apertures for the
passage of the undersize material.
Type of screen surface Description Application
Parallel rods or profile Rod/bar cross section Used for lumpy and
bars Circular, triangular, wedge coarser sized particles
etc

Punched or perforated Openings Used for coarser and

plates Circular, In-line and small sizes
staggered openings

Square, In-line and Slotted opening are

staggered openings sometimes used for fine
particles
Slot-like, In-line and
staggered openings
Type of screen surface Description Application
Woven wires

Used for fairly coarse

Square particles

Rectangle Used for fine particles

Tripe shute elongated Used for fine particles

 Types of Screen/sieve analysis
1. Differential Analysis
The screen analysis in which the weight fraction of
the material retained on each screen is reported in
tabular or graphical form as a function of the mesh
size/screen opening.
2. Cumulative Analysis
The cumulative analysis is obtained from the
differential analysis by adding cumulatively, the
individual weight fractions of the material retained
on each screen, starting with the retained on the
largest mesh
 Screen analysis
• Fine particles are generally specified according to their
screen analysis.
• A screen analysis of a material is carried out using testing
sieves.
• A set of standard screens is arranged serially in a stack in
such a way that the coarsest of the screen is at the top
and the finest of the screen is at the bottom.
• The analysis is carried out by placing the sample on the
top screen and shaking the stack in a definite manner,
either, manually or mechanically for a definite length of
the time.
• The material remained in each screen is removed and
weighed.
 Screen analysis cont..
• For reporting the screen analysis, the amount of material retained
on each screen is expressed as the weight fraction of the total
sample as a function of the mesh size.
• The screen analysis of a sample is reported either in a tabular form
or graphs.
• As the particles retained on any one screen are passed through the
screen immediately above it, two numbers are needed to specify
the size, one for the screen through which the fraction passes and
the other for the screen on which that fraction retained.
• Hence, the notation 10/14 means through 10 mesh and on 14
mesh.
• An analysis reported in a tabular form in this manner is called a
differential analysis.
 Differential analysis
Mesh Screen Openings, μm Avg. Particle size, μm Weight fraction retained, mg
6/8 2362 2845 0.017
8/10 1651 2006 0.235
10/14 1168 1410 0.298
14/20 833 1000 0.217
20/28 589 711 0.105
28/35 417 503 0.062
35/48 295 356 0.028
48/65 208 252 0.017
65/100 147 178 0.010
100/150 104 126 0.005
150/200 74 89 0.002
Pan 0.004
1.0

The average size of the particle retained on any particular screen is

calculated as the arithmetic mean of two screen openings used to
obtain the fraction.
Cumulative

Mesh Screen Openings, Weight fraction

μm retained, mg
6 3327 0.00
8 2362 0.017
10 1651 0.252
14 1168 0.55
20 833 0.767
28 589 0.872
35 417 0.934
48 295 0.962
65 208 0.979
100 147 0.989
150 104 0.994
200 74 0.996
Pan 1.0
 Screen efficiency
• Screen efficiency (often called the effectiveness
of a screen) is a measure of the success of a
screen in closely separating oversize and
undersize materials.
• There is no standard method for defining the
screen efficiency.
• Screen efficiency can be calculated based on the
amount of material recovered at a given size.
• In an industrial screening operation, it is to be
specified whether the required material is
oversize or undersize or both.
 Effectiveness of Screens or Screen Efficiency
Consider that the feed to a screen consists of materials A and B. Where A
is the oversize and B is the undersize material.
Let F = Mass flow rate of feed, (kg/h)
D = Mass flow rate of overflow
B = Mass flow rate of underflow
xF = Mass fraction of material A in the Feed
xB = Mass fraction of material A in the underflow
xD Mass fraction of material A in the overflow

• Overall material balance over a screen

F=D+B -------(1)

• Material balance of 'A' over a screen

xF.F = xD.D + xB.B -------(2)
E = [(xF-xB) (xD-xF) xD(1-xB)] / [(xD-xB)2(1-xF) xF] -----(3)

• E = Overall efficiency of the screen

A dolomite mixture having the following screen analysis
through a standard 100 mesh screen. Calculate the
effectiveness of the screen and the mass ratio of
overflow and underflow to feed.
Mesh Feed Oversize (wt.%) Undersize (wt.%)
35 7.07 13.67 0.00
48 16.60 32.09 0.00
65 14.02 27.12 0.00
100 11.82 20.70 2.32
150 9.07 4.35 14.32
200 7.62 2.07 13.34
- 200 33.80 0.00 70.02
100 100 100

Mass ratio of overflow: 0.517, Mass ratio of underflow: 0.483, E=91.31

 Example problem
1. One tonne per hour of dolomite is produced by crushing and
the screening through a 14 mesh screen. According to the
screen analysis (in weight percent) given below, calculate
a) The total load of the crusher
b) The effectiveness of the screen

Mesh Feed (%) undersize (wt.%) oversize (wt.%)

4 on 14.3 - 20
8 on 20.0 - 28
14 on 20.0 0 28
28 on 28.5 40 24
48 on 8.6 30 0
100 on 5.7 20
100 through 2.9 10
100 100 100
 Factors Effecting the Performance of Screens
• Method of Feeding: Fed the material properly, Spread the material
evenly over a full width of the screening surface, Must fed at low
flow rate.
• Screening surfaces: Depends on speed and amplitude of vibration
for best performance
• Screen Slope: Depends on slope of the screen however slope cannot
be increased beyond a certain value because beyond that value
material will travel down the screen much faster without getting
screened and the screening efficiency reduces drastically.
• Vibration and Frequency: One has to select proper amplitude and
vibration to prevent blinding of the screening cloth and for long
bearing life. The frequency of vibration affects the capacity of the
screening equipment by regulating the number of contacts between
the material and the screening surface.
• Moisture in Feed: The moisture associated with feed material
adversely affects the screening operation and should be removed.
 Operation and maintenance of
different types of industrial screens
• The screens are used for size separations in
conjunction with crushing operations.
• In the mineral industry, screens are rarely
used for separations below 0.2 mm because
they have inadequate capacity.
• The sieve bends are used for separations as
low as 50 μm since these devices give sharper
separations than wet classifiers.
• Screens are classified as stationary and
dynamic screens as shown below:

Screen

Stationary Dynamic

Revolving Vibrating
Screening Equipment
STATIONARY SCREEN
 Grizzly
Equally spaced parallel rods
or bars running in flow
direction.
Sloped to allow gravity
transport.
Applications
• Lumpy or coarse
separations.
• Scalping before crushing.
• Dry separation.
 Divergator
Parallel rods running in flow
direction.
Fixed at one end.
Gap increases from fixed to free
end.
Alternate rods diverge at 5°–6°.
• Applications
• Separations in the range 400 to
25 mm size.
• Self-cleaning and blockage free.
• Dry separation.
 Sieve Bend
Stationary curved screen with
horizontal wedge bars at right
angles to slurry flow.
Feed slurry enters tangentially.
Imparts centrifugal action.
Applications
Separations in the range of
2 mm to 45 μm
Wet separation.
REVOLVING SCREEN
 Trommel
• Rotating, punched or woven
wire.
• Slightly inclined cylindrical shell.
• Applications
• Separations in the range of 10
to 60 mm.
• Dry if coarse, wet if fine.
• Also used for scrubbing lumpy
or coarse.
VIBRATING SCREEN
 Vibrating Grizzly
• Similar to stationary grizzly.
• Mechanical or Electrical
vibrations.
• Applications
• Coarse and Dry separations.
• Also used as feeders.
 Vibrating Screen
• High speed motion to lift
particles.
• Mechanical or Electrical
vibrations.
• Both horizontal and inclined
types.
• Applications
• Separations from 200 mm to
250 μm
• Dry if coarse, wet if fine.
 Operating conditions of various screens
Apparatus Particle size Efficiency Capacity
Grizzlies 20mm-300mm Low Very high
Trommels 6mm-55 mm Average Low
Vibrating screens 4 mesh-325 mesh High High
Gyratory screens 6 mm-40 mesh Very high Low
Banana screens 100mm high high

Banana screens Gyratory screens