Size reduction, process of size reduction, size reduction a topic of pharmaceutics, cutter mill, roller mill, hammer mill, ball mill, fluid energy mill, the disintegrator, mills used in pharmaceutical process

1. SIZE REDUCTION

2. Size Reduction
The process in which the particle size of a
substance is reduced from its smaller size to a
finer state of sub division ( coarse or powder
state) is known size reduction.
Milling is the process in which the particle size
of solid substance is reduced into coarse
particles by employing mechanical strength.

3. Advantages of size reduction
 Maintains content uniformity.
 Maintains uniform flow of materials.
 Enable extraction and drying of the drug in an
effective manner.
 Improve the absorption, dissolution rate and
physical stability of the product.
 Enhance the viscosity and surface area of drug.
 Improves the bioavailability.
 Aids in manufacturing of aerosols, inhalation,
ophthalmic & parenteral preparations.

4. Disadvantages of size reduction
Drug degradation is possible due to heat
produced during milling.
Poor mixing
Chances of contamination
May lead to some instrumental problems
like noisy environment

5. Importance of size reduction
 To increase the rate of solution in case of chemical
substances, because reduction of the particle size increases
the surface area for the action of solvent.
 To allow the rapid penetration of the solvent (menstruum),
in case of crude drugs for the extraction of active
constituents from vegetable and animal drugs.
 To increase the rate of absorption of a drug. The smaller
the particle size, the greater is the rate of absorption.
 To get a uniform powder because particle size reduction
helps in uniform mixing of drugs, required for
preparing different formulations for administration.
 To improve the stability of certain pharmaceutical
dosage form, such as suspensions. The rate of
sedimentation decreases to a large extent by reducing the
particle size of the drug.
 To help in the process of separation of solids from liquids
by filtration or by sedimentation. The rate of filtration or
sedimentation depends upon the particle size.

6. Factors affecting of size reduction
1. Hardness:- Easier for soft materials
2. Toughness:- Soft but tough material may present more problem in size reduction,
than a hard but brittle substance.
3. Stickiness:-Difficult to powder a drugs of having gummy or resinous nature
4. Material structure:- Materials with some special structure may cause problem
e.g. vegetable drugs which have cellular structure, generally produce long fibrous
particles on its size reduction. Similarly a mineral substance having lines of weakness,
produces flake like particles on its size reduction.
5. Moisture content:- 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 become softened during the
size reduction processes, if heat is generated.
7. Purity required:- Various mills used for size reduction often cause the grinding
surfaces to wear off and thus impurities come in the powder.
8. Physiological effect:-During their particle size reduction in a mill, dust is produced
which may have an effect on the operator.
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.
10. Bulk density:-The output of the size reduction of material in a machine,
depends upon the bulk density of the substance.

7. Mechanisms of size reduction
1. Cutting:- The material cut by means of sharp blades.
E.g. Cutter mill
2. Compression:- The material is crushed between
roller by the application of pressure. E.g. Roller mill
3. Impact:- this involves the operation of hammer or bars
at high speed.
Impact also occurs when the moving particles strike against a
stationary surface. E.g. Hammer mill
4. Attrition:- this process involves breaking down the
material by rubbing action between two surface. E.g. Fluid
energy mill

8. S. No. Mill Principle/
Action
Uses Not used for
1. Cutter mill Cutting Fibrous and crude drugs
(animal & vegetable
drugs)
Friable
material
2. Roller mill Compression Soft material Abrasive
material
3. Hammer
mill
Impact All most all drugs Abrasive
material
4. Ball mill Attrition and
Impact
Brittle drugs Soft material
5. Fluid
energy mill
Attrition and
Impact
Moderately Hard and
friable drugs
Soft and sticky
material
6. Edge
runner mill
Crushing and
Shearing
All most all drugs Sticky material
7. End runner
mill
Crushing and
Shearing
All most all drugs Sticky material

9. 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 casing
whereas the rotating knives are attached to the rotor. The upper part of casing also
contains a hopper inlet, whereas the lower part has a screen of desired aperture size.
Working: The material to be cut is put 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 cuts down the material into
small pieces.
Screen retains the material in the mill,
until the desired degree of size
reduction has been affected.
The product is collected which
comes out of the sieve.
Uses: The mill is used to obtain a coarse degree of size reduction of soft
materials such as roots, peels or wood before extraction.

10. 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.
Working:- The material to be crushed is fed from
the hopper into the gap between the two rollers.
Due to the rotation of these rollers, the material
is crushed. The gap between the rollers can be
adjusted to control the degree of size reduction.
Uses:-The roller mill is used for crushing and
cracking of seeds before extraction of fixed
oil from the same. It is also used to crush the
soft tissues, to help in the penetration of
solvent during the extraction process.
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.

11. HAMMER MILL
Principle:- It works on the principle of impact i.e. material is more or less stationary
and is hit by an object moving at a high speed.
Construction It consists of a stout metal casing, enclosing a central shaft,to which
four or more swinging hammers are attached. The lower part of the casing consists of
a screen, through which material can pass and collected in a suitable receiver, when
the desired degree of size reduction is reached.
Working The material is put into the hopper which is
connected with the drum. The material is
powdered to the desired size, due to fast rotation
of hammers and is collected under the screen.
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.
Advantage:This mill has the advantage of
continuous operation because the chance of
jamming is less as the hammers are not fixed.
The mill can produce coarse to moderately fine
powder.
Disadvantage: Due to high speed of operation, heat is generated which may affect
thermolabile drugs or materials. Moreover, high speed of operationalso causes
damage to the mill if foreign objects such as stone or metal ispresent in the feed.

12. BALL MILL
Principle: It works on the principle of impact and attrition.
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 cylinder contains balls that occupy
30-50% of the mill volume. The weight of the
balls is kept constant. The ball size depends
on the size of the feed and the diameter of the
mill. The cylinder and balls are made of metal
and are usually lined with chrome. In
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 and is rotated. 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 10 the walls by centrifugal force and no
grinding will occur. But at about 1/3rd of the 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.

13. Advantages of Ball Mill
Uses: The mill is used to grind brittle drugs to fine powder.
Disadvantages of Ball Mill
 It can produce very fine powders.
 It is used for batch operation.
 Ball mill is suitable for both dry & wet grinding process.
 Toxic substances can be ground, as the cylinder is a closed system.
 Since the ball mill is closed system, sterility can be achieved.
 In ball mill installation, operation and labour costs are low.
The ball mill is a very noisy machine.
Wear occurs from the balls as well as from the casing, which may result in
contamination of the product.
Ball milling is a slow process.
Soft & fibrous materials cannot be milled by this mill.

14. FLUID ENERGY MILL
Principle:- It works on the principle of impact and attrition.
Construction:- It consists of a loop of pipe, which has a
diameter of 20 to 200 mm, depending on the overall
height of the loop, which may be up to about 2 m. There
is an inlet for the feed and a series of nozzles for the
inlet of air or an inert gas. It also has an outlet with a
classifier which allows the air to escape but prevents the
particles to pass until they become sufficiently fine.
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 attritional forces occurs between the particles. The fine
particles are collected through a classifier.
Advantages of Fluid Energy Mill
 It has no moving parts, hence heat is not produced during milling.
 Rapid and efficient method for reducing powders to 30 minutes or less.
 Wear off chances is less so contamination is less.
Uses The mill is used to grind heat sensitive material to tine powder. The millis used
to grind those drugs in which high degree of purity is required.
Disadvantages of Fluid Energy Mill
Not suitable for soft & fibrous materials.
Expensive equipment.

15. 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 rev./min. The side and
upper inner surfaceof the drum is rough and
undulating. 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. Thematerial 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 dust bag is tied, which retains the fine particles of powder.
Advantage:-
 Rapid process & Continuous operation
Disadvantage:-
Not suitable for thermolabile materials due to high speed.
Can get choked.
Uses:-
Used for very hard drugs, vegetable drugs, for milling ointment.