UNIT II PARTICLE SIZE REDUCTION
Lenin Prabhu SAssistant Professor
Dept. of Chemical Engineering
CH8451- Mechanical Operations
Even Sem. 2018 -2019
Definition
Objectives
Advantages & Disadvantages
Mechanism
Factor Affecting
Energy requirement
Methods
Size Reduction
Physical Decrement
Dimension or Decrease
of an object
So, Size Reduction means decreasing size of an object
Definition (1/2)
It refers to a mechanical process in which particles of
solids are cut or broken into small sizes by help of certain
external forces and without altering the state of aggregation
of solids.
Other terms: -
Comminution
Grinding
Milling
Pulverizing
Crushing
Definition (2/2)
Increase the surface area
Produce solid particles of desired shape, size
Separate unwanted particles effectively
Separate the valuable amongst the two constituents
Dispose solid wastes easily
Improve dissolution rate, solubility and binding strength,
dispersion properties
Mix solid particles more intimately in solid – solid
operations.
Improve the handling (storage and transportation)
characteristics.
Objectives
⧪ Content uniformity
⧪ Uniform flow
⧪ Effective drying
⧪ Increases surface area or viscosity
⧪ Uniform mixing and drying
⧪ Improve rate of absorption . Smaller the particles greater
is absorption.
⧪ Improve dissolution rate.
Advantages
1) Impact:
Size reduction occurs by a single rigid force.
The material is stationary and is hit by an object moving at high
speed or Moving particles strikes a stationary surface.
Gravity impact: impact occurs when the moving particle strikes a
stationary surface.
E.g.: Coal dropped onto a hard steel surface.
Dynamic impact: impact occurs when the material is more or
less stationary and is hit by an object moving at high speed.
E.g.: Materials dropping in front of a moving hammer.
Mechanism (1/5)
2) Compression:
• Material is crushed by application of a pressure. The particle is
broken by two rigid forces.
• The size reduction is done between two surfaces with the work
being done by one or both surfaces.
• It suitable for reducing extremely hard and abrasive rock.
• It is used for coarse reduction of hard solids to give relatively few
fines.
E.g.: Jaw crushers, Crushing rolls
Mechanism (2/5)
3) Attrition :
• Size reduction by rubbing or scrubbing of materials each other or
against two hard surfaces.
• Here, the material is subjected to pressure as in compression, but
the surfaces are moving relative to each other.
• It may be combined with shear and impact actions.
• It is preferred for crushing the less abrasive materials such as
pure limestone and coal
E.g.: Disc attrition mill, Hammer mills
Mechanism (3/5)
4) Shear :
• It consists of a trimming or cleaving action rather than the
rubbing action associated with attrition.
• It produced by a fluid or by particle–particle interaction.
• It is usually combined with other size-reduction actions like
impact and compression.
E.g.: single-roll crushers
5) Cutting:
• It gives a particle of definite size and shape, with few or no fines
by means of a sharp blade or blades.
• The material is cut into pieces.
E.g.: Knife, cutters, scissors Rotary knife cutter
Mechanism (5/5)
Mechanism (5/5)
Method Common exampleApprox. particle size
(μm)
Approx.
increase
in
fineness
of
the
product
Cutting
Scissors
Shears
Guillotine
Cutter mill
100 – 80,000
Compression
Pestle – Mortar
Nutcrackers
Roller mill
50 – 10,000
Impact Hammer mill
Disintegrator50 – 8,000
Attrition
Pestle – Mortar
File
Roller mill
Colloidal mill
1 – 50
Impact & Attrition • Ball mill
Fluid-energy mill1 – 2000
a) Hardness:
Harder the material difficult is the size reduction.
Mohs scale is commonly used to measure the hardness of particles
‐ Moh’s Scale = 1 is for graphite
‐ Moh’s Scale < 3 is for soft material
‐ Moh’s Scale > 7 is for hard material
‐ Moh’s Scale = 10 is for diamond
b) Structure:
Normal granular materials such as coal, ores and rocks can be
effectively crushed employing the normal forces of compression,
impact, and so on. With fibrous materials a tearing action is required.
Factor Affecting (1/5)
c) Toughness:
It is the resistance of a material to impact. It is the reverse of
friability or brittleness. It can be reduced by lowering the temp.
d) Stickiness:
It cause difficulty in size reduction. Material may stick to the
surface or mesh may get choked.
Solution: Dryness Addition of inert materials such as Kaolin to
sulphur
e) Friability:
The friability of the material is its tendency to fracture during
normal handling.
Factor Affecting (2/5)
f) Moisture content:
Materials do not flow well if the moisture content is higher (more
than 3 to 4% by weight). It influences properties such as Hardness,
Toughness & Stickiness. Less than 5 % moisture – Dry grinding. More
than 50 % -Wet grinding
g) Explosive Nature:
Explosive materials must be ground under wet conditions or in
the pressure of an inert environment.
h) Soapiness:
Soapiness is the measure of the coefficient of friction, μ, of the
surface of the material. If μ is low, the crushing will be more difficult.
Factor Affecting (3/5)
i) Crystallinity:
Every solid material has a specific crystalline pattern, breakage
occurs when sufficient pressure is applied on the solid
j) Abrasiveness:
If material is abrasive the final powder may be contaminated
with the metal worn out from the machine.
k) Temperature Sensitivity:
Heat creates problems with materials which tend to
melt/stick/decompose. Softening or melting may also be important -
leading to clogging. In some cases, cryogenic crushing may be
necessary using liquid nitrogen or, dry ice, e.g., in milling of spices or
size reduction of meat.
Factor Affecting (4/5)
l) Slipperiness:
material acts as lubricant.
m) Size of the starting material
n) Quantity of material to be reduced
o) Size, Shape, Flow & Bulk density of the product
p) Ratio of feed size to product size
Factor Affecting (5/5)
Size reduction may be achieved by two methods:
1) Precipitation
2) Mechanical process
Precipitation method: Substance is dissolve in appropriate solvent.
Mechanical process: Mechanical force is introduce by using different
equipments like ball mill, colloid mill etc.
Methods of Size reduction
In producing elastic deformation of the particles before
fracture occurs.
In producing inelastic deformation which results in size
reduction.
In causing elastic distortion of the equipment.
In friction between particles, and between particles and
the machine.
In noise, heat and vibration in the plant, and
In friction losses in the plant itself.
Energy utilization
Size reduction equipments Classifications
◂ Coarse crushers Large feed size to (50 - 5) mm product size
˃ Jaw crushers (Blake and Dodge), Gyratory crusher, Cone
crusher, crushing rolls (smooth and toothed rolls).
◂ Intermediate
crushers
(50 - 5) mm to (5 – 0.1) mm product size
˂ Roller mill, Cage mill, Granulator, Hammer mill, impactor,
vertical shaft impactor..
◂ Fine crushers/
Grinders
(5 – 2) to 200 mesh product size
˃ Ball mill, Pebble mill, Rod mill, Tube mill, Attrition
mill/Pulveriser.
◂ Ultrafine
Grinders
6 mm to (1– 50) μm product size
˂ Fluid energy or jet mill, hammer mill with internal
classification, colloid mill, fine impact mill.
◂ Cutting
machines
Definite size between 2 – 10 mm length
˃ Knife, cutters, scissors, dicers, slitters.
Size reduction equipments Classifications
A. Crushers (coarse and
fine)
B. Grinders (intermediate
and fine)
C. Ultrafine grinders
1. Jaw crushers
2. Gyratory crushers
3. Crushing rolls
1. Hammer mill, Impactors
2. Rolling compression mill
‐ Bowl mills
‐ Roller mills
3. Attrition mills
4. Tumbling mills
∶ Rod mills
∶ Ball mils, pebble mills
∶ Tube mills
1. Fluid energy mills
2. Agitated mills
3. Hammer mills with
internal classification
Crushers Grinders Ultrafine grinders Cutters
CompressionImpact and attrition, sometimes
combined with compressionAttrition
Characteristics
of cutter
Tumbling mills/Revolving mills
Characteristics Ball mill Rod mill Tube mill Pebble mill
Principles of
comminutionImpact
Rolling,
compression and
attrition
Impact Impact
Grinding media BallsSteel rods (metal
rods)
Small balls and
pebblesPebble
Material of
construction of
grinding media
Steel High carbon steel SteelCeramic pebbles made of flint
or porcelain
Diameter of
grinding media
12 – 125
mm50 mm - -
Feed sizeUpto 50
mmUpto 25 mm Upto 25 mm Upto 25 mm
Product size Fine Ultrafine Fine Fine
Length/diameter 1 to 1.5 : 1 1.5 to 3 : 1 3 to 4 : 1 1 to 2 : 1
Applications
Coal,
pigment,
feldspar for
pottery.
Particularly for
sticky materials
and not suitable
for tough
materials.
Same as ball
mill, but the
residence time
is more.
In paint, and pigment industries,
in cosmetic industries where
iron contamination is
objectionable. Also for high
specific gravity feed.