Size reduction (Communition)

1. COMMINUTION
 The process of reducing the particle size of a
substance to a finer state or powder is known
as comminution.
 It is done with the help of mortar and pestle
or mills.
 Different mills used for the reduction of
particle size are hammer mill, ball mill, colloid
mill, fliud energy mill, etc.

2. Objectives/Advantages of Size
Reduction
1. To produce smaller particles.
2. To increase surface area.
3. To achieve proper mixing.
4. To improve handling of powders.
5. To prepare pharmaceutical dispersions.
6. To improve the flow of powder.
7. To improve physical stability.
8. To improve dissolution rate of powders.
9. To improve rate of absorption.
10. To improve the extraction process.

3. Principals of size reduction
1. Cutting
The material is cut into small pieces by means of shape
blade. e.g: cutter mill
2. Compression
The material is crushed by the application of pressure.
e.g: mortar and pestle, colloid mill
3. Impact
The material is stationery and hit by a moving object
at high speed or vice versa. e.g: hammer mill
4. Attrition
The material is in motion with the surface and the
pressure is applied to the material. e.g: roller mill, fluid
energy mill

5. Factors affecting size reduction
1. Hardness
Hardness is the resistance of a material to deformation. It
is measured using Moh’s scale (1-10) from graphite to
diamond.
2.Toughness
Toughness is the ability of a material to absorb energy
and resist impact. Soft but tough material is more
difficult to reduce size than hard but brittle. e.g:
toughness-rubber & hardness-chalk
3. Abrasiveness
It is the property of hard materials especially minerals
that causes scraping or wearing of machine. Abrasive
substance when reduced in size may contain 0.1% metal
from machine.

6. 4. Stickiness
It is the property of a material to stick to the
machine parts during size reduction and block the
parts. Drying or addition of inert substance may help
in sticky materials. e.g: gum, resins
5. Softening temperature
Temperature required to soften or melt materials is
important in size reduction. So, temperature should
be kept low to avoid softening of materials and
reduce their size. e.g: waxy substance can melt at
high temperature
6. Moisture content
Moisture content of a material can affect its
hardness, toughness or stickiness. Generally
materials should be dry and contain less than 5%
moisture.

7. 7. Bulk density
Bulk density of powder is the defined as the mass
per unit volume. Higher density means less volume
occupied and more batch capacity.
8. Physiological effects
Some substances are very potent (podophyllum,
hormones, griseofulvin) and dust may effect person.
So, such substances should be grinded on closed
mills.
9. Ratio of feed size to product size
The amount of material feed affects the product
size. So, size reduction may be done in several
stages with different equipments. e.g: coarse
grinding followed by fine grinding

8. Cutter mill
 It works on the principle of cutting.
 Stationery and rotating blades are attached
to the mill and screen of desired size is
present at the lower part.
 The material is feed through hopper and size
of material is reduced by blades moving at
high speed.
 Coarse powder is obtained through this mill.

10. Colloid mill
 It works on the principle of shearing.
 Colloid mill consists of a conical rotor and stator.
The distance between rotor and stator is adjusted
between 0.005 to 0.075 cm. The rotor is connected
to a high speed motor which can revolve at a
speed of 3000 to 20000 rpm.
 The material is feed to hopper and size of material
is reduced as it passes through moving rotor and
stator.

12. Advantages of colloid mill:
1. Products with particle size less than 1 µ can
be obtained.
2. Size reduction is carried out in the presence
of liquid.
3. Useful for preparing suspensions, emulsions,
lotions, ointments and creams.

13. Hammer mill
 It works on the principle of impact.
 Hammer mill consists of central shaft
attached with number of hammers in a steel
case. When shaft is rotated hammers swing in
radial position. Screen of desired size is fitted
on the lower portion.
 The material is feed to hopper and size of
material is reduced by the impact of
hammers.

16. Advantages of hammer mill:
1. They are simple to install and operation is
continuous.
2. They are fast in action and can reduce the
size of different types of materials.
3. There is no chances of contamination due to
abrasion of metal.
4. The particle size of the product can be
controlled by changing speed, hammer or
screen.

17. Roller mill
 It works on the principle of attrition.
 Roller mill consists of two metal rolls with
adjustable gap between them. The speed of
the rollers and gap between rollers
determines the size of particles.
 The material is feed to hopper and the size of
the particles is reduced by the moving rollers.

19. Ball mill
 This mill works on the principle of impact and
attrition.
 It consists of cylindrical container filled with
number of balls made up of steels. The balls
acts as grinder. The size of the balls
determines the particle size. The cylindrical
container is rotated at slow speed.
 The material is feed to the hopper and the
moving balls reduces the size of the material.

22. Advantages of ball mill
1. It produces very fine powders.
2. It can grind different types of materials from
wet to dry.
3. It can be used for continuous operation.
4. They are simple to operate and clean.
5. It is used in completely closed form so
suitable for toxic materials.

23. Fluid energy mill
 It is also known as jet mill, ultrafine grinding
mill or micronizer.
 It works on principle of impact and attrition.
 A fluid (usually air) is passed through the
nozzle at high pressure which will move the
materials at high velocity with turbulence.
 Coarse particle are feed into the mill and
turbulence causes impact and attrition to
produce fine particles.

27. Advantages of fluid energy mill:
1. It can produce very fine particles.
2. It is suitable for heat sensitive products.
3. No contamination due to abrasion.
4. It does not contain any moving parts.