SOLID-SOLID SEPARATION:
SCREENING
SCREENING
• A separation method used to separate solids based
on their particle size alone.
• Different screening surfaces:
§Interwoven Wire Mesh
§Cloth
§Perforated Plates
§Bars
SEPARATION THEORY
Basic design features in screening:
1. Surface and aperture
2. Types of screens
3. Screen movement
FEED
Screen
OVERSIZE (Tails)
UNDERSIZE (Fines)
Important Characteristics of a Particle
1. Composition
q made up of one kind of solid? or mixture of two or more distinct
kinds of solids?
q defines properties such as density, conductivity, etc.
2. Size
q diameter of the particle, surface area per volume of the particle
q affects properties such as settling properties, and reactivity
3. Shape
q regular in shape – dimensional properties can be defined easily,
thus surface area and volume can be computed easily as well.
q irregular in shape – use of a shape factor such as sphericity
PARTICLE SIZE
1. Equidimensional Particles
q In general “diameter”, Dp may be specified
q Example: spherical particle
2. Non-Equidimensional Particles
q Dp is the second longest major dimension
q Example: cubic particle
Units of Dp depend on the size of the particles:
a. Coarse particles: inches or millimeters
b. Fine particles: screen size (e.g. Mesh No.)
c. Very fine particles: micrometers or nanometers
d. Ultra fine particles: surface area per unit mass, m2 / g
MESH SCREENS
Mesh Number – number of openings per linear inch
MESH SCREENS
q Mesh size, M and the wire diameter, w determine
the aperture size, a.
Example: In a 4-Mesh screen
MESH SCREENS
qMesh screens are arranged with increasing mesh
number, thus decreasing size of opening, from top
to bottom.
SIEVE SCALE
q A sieve scale is a series of testing sieves having
openings in a fixed succession.
q The ratio of aperture of a given sieve to the
aperture of the next one in a sieve series is a
constant.
Three types:
1. Tyler Standard Sieve Series (√2 progression)
2. US Sieve Series
3. International Test Sieve Series
Methods of Indicating Particle Size
1. Unsized Particles
q obtained from passing a feed into a single screen
q either upper or lower size limit can be indicated
Example:
Feed
Mesh 4
Oversize: +4, +4.76 mm
Undersize: -4, -4.76 mm
Methods of Indicating Particle Size
2. Sized Particles
q intermediate obtained from passing a feed into a series
of screens
q both upper and lower size limits are known
Feed
Mesh 4
Mesh 6
+4
-4
-4+6 or 4/6
-6
PARTICLE SIZE DISTRIBUTION
1) Differential Screen Analysis
• Data consist of Mesh Number (n) vs. weight fraction or %
retained on the screen (∆𝜙# )
100 kg
fraction of feed
Mesh 6
10 kg ; 0.10
Mesh 8
Mesh 10
Mesh 14
Mesh 20
Pan
90 kg
50 kg
26 kg
14 kg
40 kg ; 0.40
24 kg ; 0.24
12 kg ; 0.12
8 kg ; 0.08
6 kg
6 kg ; 0.06
PARTICLE SIZE DISTRIBUTION
2) Cumulative Screen Analysis (Larger than Dp)
• Data consist of Mesh number (n) vs. Cumulative fraction
larger than n (𝜙# )
100 kg
fraction of feed
Mesh 6
10 kg ; 0.10
Mesh 8
Mesh 10
Mesh 14
Mesh 20
Pan
90 kg
50 kg
26 kg
14 kg
40 kg ; 0.40
24 kg ; 0.24
12 kg ; 0.12
8 kg ; 0.08
6 kg
6 kg ; 0.06
Cumulative Screen Analysis
(Larger than Dp)
PARTICLE SIZE DISTRIBUTION
3) Cumulative Screen Analysis (Smaller than Dp)
• Data consist of Mesh number (n) vs. Cumulative fraction
smaller than n (1-𝜙# )
100 kg
fraction of feed
Mesh 6
10 kg ; 0.10
Mesh 8
Mesh 10
Mesh 14
Mesh 20
Pan
90 kg
50 kg
26 kg
14 kg
40 kg ; 0.40
24 kg ; 0.24
12 kg ; 0.12
8 kg ; 0.08
6 kg
6 kg ; 0.06
Cumulative Screen Analysis
(Smaller than Dp)
SCREENING EQUIPMENT
1) Grizzly Screens
ØSet of parallel metal bars in an inclined stationary frames
ØOpening is large
ØCapacity is large
ØCheap construction
SCREENING EQUIPMENT
2) Vibrating Screens
ØFrequency of the screen is mainly controlled by an
electromagnetic vibrator which is mounted above and
directly connected to the screening surface.
ØHigher chance of separation due to “popcorn effect”
SCREENING EQUIPMENT
3) Rotary Screening
ØComposed of a rotating perforated drum set in an
inclined position.
ØMovement of feed is through the hollow cylindrical drum
with lateral surface composed of different mesh sizes
arranged from smallest to largest openings
MATERIAL BALANCE
Example: It is desired to remove particles smaller than mesh 4.
Feed: F, xF
Mesh 4
where:
x = fraction of desired material
(1 – x) = fraction of undesired material
OMB: F = P + R
DMB: FxF = PxP + RxR
UMB: F(1-xF)= P(1-xP) + R(1-xR)
Product: P, xP
Reject: R, xR
SCREEN EFFECTIVENESS
q may be due to blinding, rupture, or blockage (by other
particles) of the screen opening/s and improper orientation
of the particle as it hits the screen
q calculated by the product of recovery of desired material in
the product and recovery of undesired material in the reject
or
ILLUSTRATIVE PROBLEM 1
It is desired to remove small particles from a crushed
stone mixture by screening through a 10-mesh
screen. The screen analysis of feed, overflow and
underflow are given in the table.
a) Calculate the mass ratios of the overflow and underflow
to feed.
b) Find the effectiveness of the screen.
ILLUSTRATIVE PROBLEM 1
Screen Analysis of the Feed:
ILLUSTRATIVE PROBLEM 2
It is desired to separate 1000 kg of a mixture of
crushed solids into three fractions, a coarse fraction
retained on an 20 mesh screen; a middle fraction
passing through a 20 mesh screen and retained on a
65 mesh screen; and a fine fraction passing through
a 65 mesh screen. Two Tyler Standard Screens are
used to remove particles 20/65. Screen analysis of
the feed, coarse, medium and fine fractions are
given.
What are the effectiveness of the 20 and 65 screens,
given the following screen analysis?
ILLUSTRATIVE PROBLEM 2
Screen Analysis:
ILLUSTRATIVE PROBLEM 3
It is desired to separate a mixture of sugar crystals
into two fractions, a coarse fraction retained on
an 8-mesh screen, an a fine fraction passing
through it.
Mass fraction of +8 particles in feed = 0.46
Mass fraction of +8 particles in coarse = 0.88
Mass fraction of +8 particles in fine = 0.32
What is the effectiveness of the screen?
ILLUSTRATIVE PROBLEM 4
1800 lbs of dolomite per hour is produced by
crushing and screening through a 14-mesh screen.
Calculate the total load to the
What is the effectiveness of the screen?
Screen analysis is given.
crusher.
ILLUSTRATIVE PROBLEM 5
Limestone is crushed by six units operating in
parallel and the products separated by six 35mesh screens also in parallel, into two fractions.
The effective dimensions of each screen is 6 ft x 20
ft. The common undersize from the screens come
out at the rate of 50 tons/h. Assume no losses.
What is the effectiveness of each screen?
Calculate the capacity of each screen in lb/day-ft2.
Screen analysis is given.
ILLUSTRATIVE PROBLEM 6
A quartz mixture is screened on a 10-mesh screen.
The cumulative screen analysis of feed, overflow,
and underflow are given. Calculate the following:
1. The mass ratio of overflow to feed
2. The mass ratio of underflow to feed
3. Overall effectiveness of screen
ILLUSTRATIVE PROBLEM 7
Granular feldspar is produced by beneficiation of high alumina
river sand. After screening, drying, and magnetic separation,
the recovery is only 21%. The screen analysis of this product is
shown.
In a specific application, this product must be reprocessed to
remove all grains finer than 100 mesh screen. 2 MT of
reprocessed product is required.
The MT of river sand with 10% moisture that would have to be
beneficiated is ______?
SEATWORK
Find the effectiveness of
Mesh 35 and 100 if the
product desired is the
middle fraction.