Transcript of "Separation, screening and classification"
Dated : 13th January, 2014
• Techniques used to separate one material
from the other are called separation.
• Separations are extremely in chemical
• In fact, much processing equipment is
devoted to separate one phase or one
material from the other
Types of Separation
• Diffusional Separation
• Mechanical Separation
• Diffusional separation is a technique used for the
separations of homogeneous mixtures.
• This separation involves the transfer of the
material between the phases.
• Following are the well adopted methods for the
• The techniques used for the separation of the
• These are based on the physical differences
between the particles such as size, shape or
• It can be applied for separating solids from
solids, solids from liquids and also solids from
• There are further two types of mechanical
• Classification is applied for the separation
of solids from the gases and solids from
• Screening is used for the separation of
solids from solids
• Separation of mixture of particles of
various sizes into two or more fractions by
a screening surface is called screening.
• This method is only based on the size of
• Over size material
Material that retain on the screening surface is
called over size material.
• Under size material
Material that passes through the screening
surface is called under size material.
• Intermediate material
When two screens are used for screening, the
material that retain on the second screen is
called the intermediate material.
• Unsized function
A single screen can make a single separation
into two fractions i.e. under size and over size.
Such type of functions is called the unsized
• Sized function
When the material is passed through the series
of screens then it is divided into many fractions.
Such type of function is known as sized function
Hole of the screen is called the mesh.
• Mesh number
It is defined as the numbers of holes per
• Aperture of screen
Aperture is the maximum clear space
between the edges of the screen opening.
It is usually given in inches or mm.
• Clear and actual opening
Actual opening of the screen is always
smaller than corresponding mesh number
because wire diameter is also included.
There is a formula for expressing the
mesh number and clear opening
1/mesh# = clear opening + wire
Construction of screens
Perforated or slotted metal plates
Silk or plastic clothes
• U.S. Tyler
These are the U.S standard sieves and are
available in mesh number 4 – 325.
• U.S. ASTM
It is “American Society of Testing Materials”
standard sieves series and are available in
mesh number 4 – 325.
British standard sieve as available in mesh
number 5 – 300.
“Institute of Mining and Metallurgy”
standard sieves are available in mesh
number 5 – 200.
“French standard sieves” are available in
mesh number 17 – 38.
Material Balance Over Screen
Consider two materials ‘a’ & ‘b’ is feed for
screening. Let F, D and B be the mass
flow rates of feed, overflow and underflow
respectively. Let xF, xD and xB be the mass
fractions of material ‘a’ in three streams,
therefore mass fractions of the material ‘b’
are 1 – xF, 1 – xD and 1 – xB in the three
• Overall material balance:
• Material balance of component ‘a’
FxF = DxD + BxB
• Overflow to feed ratio:
D = (xF – xB)
F (xD – xB)
• Underflow to feed ratio:
B = (xF – xD)
F (xB – xD)
Screen Effectiveness /
• Effectiveness of screen
It is the measure of the success of the screen in
closely separating the materials.
• Consider two materials ‘a’ & ‘b’ is feed for
screening. Let F, D and B be the mass flow rates
of feed, overflow and underflow respectively. Let
xF, xD and xB be the mass fractions of material ‘a’
in three streams, therefore mass fractions of the
material ‘b’ are 1 – xF, 1 – xD and 1 – xB in the
three streams. It is supposed that ‘a’ is larger
in size than ‘b’ material.
• Screen effectiveness based on the
Ea = DxD/FxF
• Screen effectiveness based on the
Eb = B(1 – xB)/F(1 – xF)
• Combined overall efficiency
E = E a × Eb
E = xD(1 – xB)(xF – xB) (xD – xF)
xF(1 – xF)(xD – xB)2
Factors Affecting the
Mesh size and wire diameter
Direction of approach of particle to
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