Particle Size reduction

SIZE REDUCTION
Shivanee Vyas
Assistant Professor
School of Pharmaceutical Technology & Management, NMIMS

Content
• Size reduction –Introduction
• Objectives and factors affecting size reduction
• Methods of size reduction
• Equipment's used for Size reduction

Introduction
Size reduction is a process of reducing large solid unit masses - vegetables or chemical
substances into small unit masses, coarse particles or fine particles. Size reduction is
commonly employed in pharmaceutical industries. Size reduction process is also referred to
as Comminution and Grinding.

• Objectives of Understanding Size Reduction
1. Size reduction leads to increase of surface area.
2. To increase the therapeutic effectiveness of certain drugs by reducing the particle size.
3. To get the uniform powder because particle size reduction helps the uniform mixing of
drugs for the preparation of dosage forms.
4. To increase the rate of absorption of a drug. The smaller the particle size, the greater the
rate of absorption.
5. Pharmaceutical suspensions require finer particle size. It reduces rate of sedimentation.
6. The stability of emulsions is increased by decreasing the size of the oil globules.
7. All the ophthalmic preparations and preparations meant for external application to the
skin must be free from gritty particles to avoid irritation of the area to which they are
applied.
8. The physical appearance of ointments, pastes and creams can be improved by reducing its
particle size

Factors affecting size reduction
1 Hardness: The hardness of the material affects the process of size reduction .It is easier to
break soft material to a small size than hard material.
2 Toughness: The crude drugs of fibrous nature or those having higher moisture content ,are
generally tough in nature. A soft but tough material may present more problem in size
reduction , than a hard but brittle substance.
3 Stickiness: Stickiness causes a lot of difficulty in size reduction. This is due to the fact
that material adheres to the grinding surfaces or sieve surface of the mill. It is difficult to
power a drugs of having gummy or resinous nature, if the method used for size reduction
generates heat. Complete dryness of material may help to overcome this difficulty.
4 Material structure: Materials which show some special structure may cause problem
during size reduction e.g. vegetable drugs which have cellular structure, generally produce
long fibrous particles on its size reduction. Similarly a mineral substances having lines of
weakness, produces flake like particles on its size reduction.

5. Moisture content: The presence of moisture in the material influences a number of its properties
such as hardness, toughness or stickiness which in its turn affects the particle size reduction. The
material should be either dry or wet. It should not be damp. The material having 5% moisture in
case of dry grinding and 50% moisture in wet grinding does not create any problem.
6. Softening temperature: Waxy substances such as stearic acid or drugs containing oils or fats
,become softened during the size reduction processes if heat is generated. This can be avoided by
cooling the mill.
7. Purity required: Various mills used for size reduction often cause the grinding surfaces to wear
off and thus impurities come in the powder .If a high degree of purity is required, such mills must
be avoided. Moreover, the mills should be thoroughly cleansed between batches of different
material in order to maintain purity.
8. Physiological effect: Some drugs are very potent. During their particle size reduction in a mill
dust is produced which may have an effect on the operator .In such cases, the enclosed mills may
be used to avoid dust.
9. Ratio of feed size to product size: To get a fine powder in a mill, it is required that a fairly small
feed size should be used. Hence it is necessary to carry out the size reduction process in several
stages, using different equipment e.g. preliminary crushing followed by coarse powder and then
fine grinding.
10. Bulk density: The output of the size reduction of material in a machine, depends upon the bulk
density of the substance.

Methods of size reduction
The following are the methods of size reduction, in which
different mechanism are involved;
1. CUTTING: The material is cut on a small scale by
means of sharp blade, knives etc. On a large scale, a
cutter mill is used.
CUTTER MILL
• Principle: The size reduction is done by cutting with the
help of sharp knives.
• Construction: there are two types of knives which are
fitted in this mill. These are stationary knives and rotating
knives. The stationary knives are fixed in the casting
whereas the rotating knives are attached to the rotor. The
upper part of casting also contains hopper inlet, whereas
the lower part has a screen of desire aperture size.

• Working: the material to be cut into the hopper of the mill.
The rotor rotates at a high speed. Due to the rotation of the
rotor, the material comes very close between stationary
knives and rotating knives. It cut down the material into
small pieces. Screen retains the material in the mill, until
the desire degree of size reduction has been affected. The
product is collected which comes out of the sieves.
• Uses: This mill is used for the size reduction of soft
materials such as roots, peels or wood.

2. COMPRESSION: In this method, the material is crushed by the
application of pressure. Example is Roller Mill.
ROLLER MILL
• Principle: The material is crushed by the application of pressure. The
mill works on the principle of compression of material by applying a
pressure on it.
• Construction: It consists of two cylindrical rollers made of stone or
metal, which are mounted horizontally. These rollers are capable of
rotation on their longitudinal axis. These rollers may be from a few
centimeters up to a meter in diameter. Generally, one of the roller is
driven directly, while the second one runs freely. The gap between the
rollers can be controlled to obtain the desired particle size.
• Working: The rollers are allowed to rotate. The material is fed from
the hopper into the gap between the two rollers. The material is
crushed while passing through the rollers can be adjusted to control
the degree of size reduction. The product is collected into a receiver.
• Uses: The roller mill is used for crushing and cracking of seeds before
extraction of fixed oil. It is also used to crush the soft tissues, to help
in the penetration of solvent during the extraction process.

3. IMPACT: This involves the operation of hammers at high speed.
When a lump of material strikes the rotating hammers, the
material splits apart. This action continue until particle of desired
size are obtained. Example is hammer mill.
HAMMER MILL
• Principle: The hammer mill operate on the principle of impact
between rapidly moving hammers mounted on a rotor and the
powder material.
• Construction: The hammer can be either horizontal or vertical
shaft type. Hammers are usually made of hardened steel,
stainless steel with impact surface. This unit is enclosed with a
chamber containing a removable screen through which the
material must pass. Screen are prepared using metal sheet of
varying thickness with perforated holes.

Working: The hammers are allowed to be in continuous motion (8000-15000RPM)
the feed material is placed into the hopper, which flows vertically down and then
horizontally, while hammers are in continuous motion. These rotating hammers
beat the material to yield smaller particle. Then, these pass through the screen.
Due to the high speed of operation, heat is generated which may affect thermolabile
drugs or materials. Moreover, high speed of operation also causes damage to the
mill if foreign objects such as stone or metal is present in the feed.
Uses The hammer mill is used for producing intermediate grades of powder from
almost all types of substances except sticky materials that choke the screen. The
expected particle size may vary from 10 to 400mm

THE DISINTEGRATOR
• Principle: The size reduction in disintegrator is done by
impact.
• Construction: It consists of a steel drum having a shaft in the
center. The shaft contains a disc, on which four beaters are
fixed. The shaft rotates with a speed of 5000-7000 RPM. The
side and upper inner surface of the drum is rough. The lower
part of the drum has a detachable screen or sieve. The sieve
has a definite pore size.
• Working: The beaters are mainly responsible for grinding but
are helped by the undulation of the inner surface and
roughness of drum.
• The material is fed to beaters, through hopper which is fitted
to the drum. The material is broken into small particles by
impact of the beaters. Due to high velocity of beaters the air
velocity inside the chamber is increased. The air is allowed to
pass through an outlet on which the dust bag is tied, which
retains the fine particles of powder.
• Uses: The mill is used to powder all types of drugs including
very hard drugs. The drug should be dried before feeding into
the disintegrator to get a fine powder. To avoid the jamming
of the beaters of the disintegrator, use moderately small
pieces.

4. ATTRITION: This process involves breaking down of the material
by rubbing between two surfaces, Example is ball mill.
BALL MILL
These are also known as tumbling mills.
• Principle: It works on the principle of impact and attrition both for
the size reduction.
• Construction: It consists of a hollow cylinder which is mounted on a
metallic frame in such a way, that it can be rotated on its
longitudinal axis. The length of the cylinder is slightly greater than
its diameter.
The cylinder and balls are made of metal and are usually lined with
chrome. In the pharmaceutical industry, sometimes the cylinder of the
ball mill is lined with rubber or porcelain. The balls used in these
mills are also made of rubber or porcelain.

• Working: The drug to be ground is put into the cylinder of the
mill in such a quantity that it is filled to about 60% of the
volume. A fixed number of balls are introduced and the cylinder
is closed. The mill is allowed to rotate on its longitudinal axis.
The speed of rotation is very important. At a low speed, the mass of
balls will slide or roll over each other and only a negligible amount
of size reduction will occur.
At a high speed, the balls will be thrown out to the walls by
centrifugal force and no grinding will occur.
But at correct speed, the centrifugal force just occurs with the
result that the balls are carried almost to the top of the mill and
then fall in. By this way the maximum size reduction is effected by
the impact of particles between the balls and by attrition between
the balls.
After a suitable time, the material is taken out and passed through
a sieve to get powder of the required size.
• Uses: The mill is used to grind brittle drugs to fine powder.
High speed
Low speed
Correct speed

• Advantages:
1. It can produce very fine particles.
2. It can be used for continuous operation.
3. Ball mill is suitable for both wet and dry grinding process.
4. Toxic substance can be ground, as the cylinder is a closed system.
5. Since the mill is closed system, sterility can be achieved.
6. In the ball mill, installation, operation and labor costs are low.
7. It is capable of grinding a large variety of materials of different character and
of different degree of hardness.
• Disadvantages:
1. The ball mill is a very noisy machine.
2. Soft, sticky, fibrous material cannot be milled by ball mill.

Fluid Energy Mill
• Principle: fluid energy mill works on the principle of impact and attrition.
• Construction: It consist of a loop of pipe, which has a height about 2 meter and diameter
may be ranging from 20-200 millimeter. The mill surface may be made of stainless steel.
There is an inlet for the feed and a series of nozzles for the inlet of air or inert gas. An
outlet with a classifier (cyclone separator or bag filter) is fitted to allow the escape of air.

• Working: The air or inert gas is introduced with a very high pressure through the
nozzles. Solids are introduced into air stream through inlet. Due to high degree of
turbulence, impact and attrition forces occurs between the particles the fine particles
are collected through a classifier. Fluid energy mill reduces the particles to 1-20 micron.
The powder remains in the mill, until its size is reduced sufficiently.
• Uses: fluid energy mill is used to reduce the particle size of most of the drugs such as
antibiotics and vitamins. Ultra fine grinding can be achieved.
• Advantages:
1. It has no moving parts, hence heat is not produced so it is useful for heat sensitive
substance such as vitamins, antibiotics and sulphonamides.
2. The particle size of powder can be controlled due to the use of a classifier.
3. It is rapid and an efficient method for reducing powder to 30mm or less.
• Disadvantages:
1. It is not suitable for milling of soft, sticky and fibrous material.
2. The equipment is expensive.

Particle Size reduction

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