Details of Size Reduction Process, Equipment used in it.

1. MRS. ASHWINI P. SHEWALE
PRINCIPAL
(PDEA’S COLLEGE OF
PHARMACY,HADAPSAR, PUNE
Size Reduction

2. Size Reduction
Definition : Size reduction or comminution is the process of
reducing drugs (vegetable and chemical substances) into
smaller pieces, coarse particles or fine powder.
Importance of Particle Size Reduction
The process of size reduction is commonly employed in
pharmaceutical industries due to the following reasons:
1. 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.
2. 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.
3. To get a uniform powder because particle size reduction
helps in uniform mixing of drugs, required for preparing
different formulations for administration.

3.  4. To increase the rate of absorption of a drug. The smaller
the particle size, the greater is the rate of absorption.
 5. 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.
 6. 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.
 Factors Affecting Size Reduction
 The following factors affect the process of 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.

4. 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 powder 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 substance 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.

5. 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
materials 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.

6. METHODS OF SIZE REDUCTION
The following are the methods of size
reduction, in which different
mechanisms are involved:
1. Cutting The material is cut on a small scale
by means of a sharp blade, knife)root cutter
or any other sharp instrument. On a large
scale, a cutter mill)is used. Cutting of drug is
usually done to hasten the drying of drugs.
2. Compression In this method, the material is
crushed by the application of pressure.On a
small scale, size reduction is carried out by
using pestle and mortar, whereas on a large
scale, roller mill is used.

7. 3. Impact occurs when the material is more
or less stationary and is hit by an object
moving at high speed or when the moving
particle strikes a stationary surface. In
either case, the material breaks into small
pieces. There is no apparatus which can
be used on a small scale to affect size
reduction by impact. But on a large scale,
hammer mill and disintegrator are used
when size reduction of material is done by
impact.
4. Attrition
5. Combined impact and attrition

8. CUTTER MlLL
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.

9. 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. 2. Compression
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 centimetres upto a meter
in diameter. Generally, one of the roller is driven directly, while the second
one runs freely.
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.

11.  Impact
 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. 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. ue to high speed of operation, heat is generated which may affect I
 thermolabile drugs or materials. Moreover, high speed of operation also!
 causes damage to the mill if foreign objects such as stone or metal is I
 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.

13. THE DISINTEGRATOR
Principle The size reduction in disintegrator is done by impact.
Construction It consists of a steel drum having a shaft in the centre.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 surface of
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. 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 dust
bag is tied, which retains the fine particles of powder.

14. BALL MILL
Principle It works on the principle of impact and attrition.
Conduction 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.

15. 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 (as shown
in Fig. 5-6A). At a high speed, the balls will be thrown out 10 the
walls by centrifugal force and no grinding will occur (Fig. 5-6B).
But at about fi/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.

16.  Uses The mill is used to grind brittle drugs to fine powder.
 Advantages of Ball Mill
 1. It can produce very fine powders.
 2. It can be used for continuous operation, if sieve or
classifier is attached to the mill.
 3. It is capable of grinding a large variety of materials of
different character and of different degree of hardness.
 4. It is suitable for both wet or dry grinding processes.
 5. It can Jbe used to grind toxic materials as it can be used
in a completely enclosed form.
 Disadvantages of Ball Mill
 1. The ball mill is a very noisy machine.
 2. Wear occurs from the balls as well as from the casing,
which may result in contamination of the product

19. 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 upto 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 untilthey 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.

20. Fluid energy mill reduces the particles to 1 to 20 micron To get a very fine
powder, even upto five micron, the material is pretreated
to reduce the particle size to the order of 100 mesh and then passed through
fluid energy mill.
Uses The mill is used to grind heat sensitive material to tine powder. The mill is
used to grind those drugs in which high degree of purity is required.
Advantages of Fluid Energy Mill
1. The mill is used to grind the material to fine powder.
2. The particle size of powder can be controlled due to the use of a classifier.
3. There is no wear of the mill and h?n<*e there is no contamination of the
product.
4. It is useful for grinding heat sensitive substances such as sulphonamides,
vitamins and antibiotics.

22. MISCELLANEOUS MILLS
EDGE RUNNER MILL
Principle The size reduction .is done by crushing due to heavy weight of
stones and shearing force which is involved during the movement of these
stones.
Construction It consists of two heavy rollers and a. bed made of
stone or granite. The rollers have a central shaft and they revolve on its
axis (Fig. 5-8). The rollers are mounted on a horizontal shaft and move
around the bed.
Working The material to be ground is put on the bed and with the
help of the scrapers'it is kept in the path of the stone wheels. The
material is ground for a definite period and then it is passed through the
sieves to get powder of the required size.
Uses Edge runner mill is used for grinding most of the drugs to fine
powder, but it requires more floor space than other commonly used
mills. The major advantage of this mill is that it requires less attention
during operation

25. The basic principles governing selection of degree of size reduction are as
follows:
1. Drugs required to be administered in powders and suspension in mixtures
are reduced to fine powder.
2. Drugs to be injected in suspension form are reduced to very fine powder.
3. Drugs which are to be extracted, need size reduction, which depends upon
the following factors:
(a) Solvent or menstruum : Water has more penetration power than other
organic solvents such as alcohol (60% or more). Therefore, a higher
degree of size reduction is required with alcoholic as menstruum than with
water.
(b) Drug/menstruum ratio : The rate of solution is greater in case of a larger
than a small volume of solvent. Moreover, the size reduction of a solute
also hastens the solution formation. Therefore, in preparing a weak
preparation, the entire drug or coarse grade of drug can be used but for a
stronger preparation, fine degree of size reduction is required.
(c) Physical character of the drug and localisation of the constituents : The
menstruum is slowly penetrated into hard woody drugs than into soft
drugs. Therefore in the case of hard drugs it becomes necessary to reduce
them to a fine powder than in the case of soft drugs for the same process
of extraction and menstruum. The localisation of active principles is an
important factor in deciding the degree of size reduction of drugs e.g.
active constituents of digitalis leaves are localised chiefly in the harder
tissues of leaves and hence require a fine degree of size reduction.

26.  (d) Process : In infusion and decoction processes the menstruum
used is water and the drug/menstruum ratio is large. As such
there is no need of a higher degree of size reduction. In the case
of maceration and percolation processes, the drugs/ menstruum
ratio is comparatively smaller than for infusion and decoction.
Therefore, a higher degree of size reduction is required. The
same is also very essential in case the menstruum is alcohol and
extraction process is percolation.

27. LEVIGATION AND ELUTRIATION
Levigation is the process of wet grinding. The material is converted
into paste with water and then grinding of paste is done in a mortar,
using a flat headed pestle. In case of large scale grinding, it is done
by using colloidal mill or a mill resembling an edge-runner mill. The
process is used for the preparation of light kaolin, chalk, red and
yellow mercuric oxide, calamine and camphor etc. After the
process of levigation, the paste obtained contains fine particles
along with a small proportion of coarse particles. The fine particles
are separated from the coarse particles by the process of
elutriation.
Elutriation is the process of separation of fine particles and coarse
particles from a paste obtained after levigation. The paste is mixed
with a large volume of water and the mixture is allowed to stand for
a short period during which the heavy coarse particles with high
density settle down on the bottom of the vessel. The fine particles
with low density remain suspended in upper layers of the liquid.
The upper
liquid is poured off and the fine particles are allowed to settle
down on the bottom. The wet paste containing fine particles is then
dried. In small scale grinding, conical flask can be used but on a
large sale, elutriation tanks are used (For detail see chapter on size
separation).

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