PHARMACUTICAL UNIT OPEARTION:

SIZE REDUCTION

Pravin B. Awate ( M. Pharm Pharmaceutics)

Assistant Professor
RDCOP, BHOR
 Size Reduction

Content…
 Objectives,
 Mechanisms & Laws governing size reduction,
factors affecting size reduction,
 Principles, construction, working, uses, merits and demerits of
Hammer mill, ball mill, fluid energy mill, Edge runner mill & end runner
mill.
 Size Reduction
INTRODUCTION:

 Size reduction or comminution is the process of reducing drugs

(vegetable and chemical substances) into smaller pieces, coarse particles or
fine powder.

 Size reduction is the operation carried out for reducing the size of
bigger particles into smaller one of desired size and shape with the help of
external forces.

 COMMINUTION is another term used for size reduction.

 Size refers to physical Dimension of an object, whereas Reduction

refers to decrement or the process of decreasing the size.

Size reduction process is also termed as comminution or diminution 'or

pulverisation.

 Normally, size reduction may be achieved by two methods, namely

precipitation or mechanical process.
Size reduction may be achieved by two methods:
1] Precipitation
2] Mechanical process

 In the precipitation method, the substance is dissolved in an appropriate solvent.

Subsequently, it is finely precipitated by the addition of another solvent, which is
miscible with the first, but in the later the substance isinsolu.ble; This method is
suitable for the production of raw materials and bulk drugs.
 Inorganic chemicals, such as calcium carbonate, magnesium carbonate and· yellow
mercuric oxide, are prepared by precipitation Method.

 In the mechanical process, the substance is subjected to mechanical forces using

grincling equipment (ball mill, roller mill, colloid mill etc.). In general, dry grinding or
milling is used in the production of tablets and capsules, while wet grinding is used in
the preparation of suspensions, emulsions and ointments.
The method of milling is applied either in the production of raw materials or as a
part of the production cycle ·in the manufacture of dosage forms.
Pharmaceutical Applications/ 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 (menstrum), 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.
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.
 ADVANTAGES OF SIZE REDUCTION

1. Content uniformity:
Mixing of different ingredients can be effective, if the particle size is uniform and
small. Particles of optimum size are desirable for effective mixing.
2. Uniform flow:
Smaller particle size! and controlled size distribution, promote the flow of the. powder
into dies during compression of tablets The same principles are used in the production
of capsules.
3. Effective extraction of drugs:
Smaller particles\ allow rapid penetration of menstruum or solvent into the tissue or
cells of vegetable and animal origin (liver and pancreas). As a result, extraction or
leaching of active constituent; becomes effective and complete in preparation of
galenicals. The time required for extraction can be shortened.
4. Effective drying:
Drying of a granular mass can be· rapid and effective, if the size of granules is small
and uniform .
5. Improved physical stability:
In case of suspensions and emulsions, the rate of sedimentation decreases to a large
extent if particle size is small.
6. Improved dissolution rate :
Size reduction increases surface area, which facilitates intimate contact of solid
particles and gastric or intestinal juices. Thus, the rate of dissolution enhances.
7. Improved rate of absorption : The smaller the particle size, the faster is the
absorption, because of enhanced dissolution.
 DISADVANTAGES

1.Drug degradation:
Drug decomposition is possible due to the heat produced,· during milling.
Thermo-labile substances are the most affected. The increased surface
area also facilitates drug decomposition owing to enhanced dissolution.
Cooling support systems are provided to decrease the heat in milling
equipment. Drugs containing waxy materials become soft due to heat
generated during milling. Therefore the feed is chilled before milling.
2. Poor mixing :
Normally, very small particles possess strong cohesive forces, hence,
aggregation of particles is possible. Aggregation inhibits the effective
blending of different additives.
An increase in surface area may promote the adsorption of air, which may
inhibit wettability of the drug during production. Therefore, optimum·
particle size is desirable to improve blending and to avoid poor mixing.
3. Contamination :
During milling and grinding. the grinding surfaces wear off (examples are
ceramic or iron equipment), the particles of which are present as impurities
in the powder. Such type of mills should be avoided, when drugs of high
purity are required.
.
 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.
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
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.
 Factors Affecting 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 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.
 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.
3. Impact: It 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
This process involves breaking down of the material by rubbing action between two
surfaces, i.e., surface phenomena. Example is fluid energy ·mill
5. Combined impact and attrition
CLASS!FICATION OF SIZE REDUCTION EQUIPMENT
 HAMMER MILL
Principle .: Working :
The hammer mill operates on the principle· The material is put into the hopper which is
of impact between. rapidly moving hammers connected with the drum. These rotating
mounted on a rotor and· the powder hammers beat the material to yield smaller
material. particles The material is powdered to the
Construction: desired size, due to fast rotation of
It consists of a stout metal casing, hammers and is collected under the screen.
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.
 HAMMER MILL

Uses:
Particle size obtained from 10-400mm.
Also used to mill dry, wet and filter press cakes materials.
Advantage:
It is rapid in action, and is capable of grinding many different types of materials.
They are easy to install and operate, the operation is continuous.
There is little contamination of the product with metal abraded from the mill as no
surface move against each other.
The particle size of the material to be reduced can be easily controlled by changing the
speed of the rotor, hammer type, shape and size of the screen.
 Operated in a closed environment dust can. be reduced and explosion hazards can be
prevented.
Disadvantage:
 Heat build-up during milling is more, therefore, product degradation is possible.
 Hammer mills cannot be employed to mill sticky, fibrous and hard materials.
The screens may get clogged.
Wearing of mill and screen is more with abrasive materials.
 BALL MILL
These are also known as tumbling mills or Construction:
pebble mills.  It consists of a hollow cylinder which is
Principle: mounted on a metallic frame in such a way
The ball mill works on the principle of impact that it can be rotated on its longitudinal axis.
between rapidly moving balls  The length of the cylinder is slightly higher
and the powder material enclosed in a hollow than its diameter.
cylinder.  The cylinder contains balls that occupy 30
At low speed the balls roll over each other to 50% of the mill volume.
attrition will be mode of action thus in the  The ball size depends on the size of the
ball mill attrition and impact both mechanisms feed and the diameter of the mill.
takes place.  Balls are made up of steel, iron or
stoneware and act as grinding medium.
 BALLMILL
The drug to be ground is put into the cylinder of the  Advantages
mill and is rotated. The speed of rotation is very It can produce very fine powder. Ball mill is used for
important. both wet and dry grinding processes.
 At a low speed, the mass of balls will slide or roll Toxic substances can be ground, as the cylinder is
over each other and only a negligible amount of size closed system. Rod or bars can also be used as
reduction will occur. grinding media. (example: Sticky material are size
 At a high speed, the balls will be thrown out to reduced)
the walls by centrifugal force and no grinding will In ball mill, installation, operation and labour costs
occur. are low.
The compression by the balls against the wall will
not be sufficient for (effective comminution of the
substance.
 At correct speed, the centrifugal force just
occurs, as a result the balls are picked up by the Disadvantages
The ball mill is a very noisy machine.
mill wall and carried nearly to the top, where they
Ball mill is a slow process. Soft, tacky,
break contact with the wall and fall to the bottom
fibrous material cannot be milled by ball mill.
to be picked up. Uses:
In this manner, impact stress will also be induced For fine grinding with a particle size of 100
and the size reduction is made effective. to 5 mm or less.
 For production of ophthalmic and parenteral
products.
 For milling dyes, pigments and insecticides
at low speed
 FLUID ENERGY MILL
Principle: It consist of an elliptical pipe which has a
Fluid energy mill operates on the principle height of about 2 meters and diameter
of impact and attrition. Milling takes place may be ranging from 20 to 200 mm. The
because of high velocity collisions between mill surface may be made up of either soft
the suspended particles. stainless steel or tough ceramics.
Construction: Grinding nozzles are placed tangential and
opposed to the initial flow path of a
powder. Compressed air is used at 600
kilopascals to 1 megapascals. Venturi
feeder is provided in the path of the
airflow. An outlet with a classifier is
fitted to allow the escape of air.
 FLUID ENERGY MILL
Working:
Advantages of Fluid Energy Mill
The air or inert gas is introduced with a very
1. The mill is used to grind the material
high pressure through the nozzles. Solids are to fine powder.
introduced into air stream through inlet. Due 2. The particle size of powder can be
to high degree of turbulence, impact and controlled due to the use of a classifier.
attritional forces occurs between the 3. There is no wear of the mill and hence
particles. The fine particles are collected there is no contamination of the
product.
through a classifier. Fluid energy mill reduces
4. It is useful for grinding heat
the particles to 1 to 20 micron To get a very sensitive substances such as
fine powder, even up to five micron, the sulphonamides, vitamins and antibiotics
material is pre-treated 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.
 EDGE RUNNER MILL
Principle: Working:
The size reduction is done by crushing The material to be ground is put on the bed
(Compression) due to heavy weight of stones and with the help of the scrapers 'it is kept
and shearing force which is involved during in the path of the stone wheels. The material
the movement of these stones. is ground for a definite period and then it is
Construction passed through the sieves to get powder of
It consists of two heavy rollers and a. bed the required size.
made of stone or granite. The rollers have a Uses:
central shaft and they revolve on its axis. The Edge runner mill is used for grinding most of
rollers are mounted on a horizontal shaft and tough drugs to fine powder.
move around the bed.

Disadvantages:
Occupies more space
Contamination of product with roller
is possible
Time consuming process
Not employed for sticky materials
Energy consumption is high.
EDGE RUNNER MILL
 END RUNNER MILL
Principle: It is considered as mechanical mortar and
Size reduction is done by crushing pestle. It consist of steel mortar which is
(compression) due to heavy weight of steel fixedWorking : The. Material
to Flanged to be ground the
plate. Underneath
placed in mortar .The scrapper puts
pestle. Shearing stress is also involved flanged plate, a bevelled cog fitting is
the material in the path of pestle.
during the movement of mortar and pestle. attached to horizontal
The mortar revolvesshaft
at a bearing a pulley.
high speed .
HenceThethe plateisand
pestle mortar
placed can be
in the rotated
mortar.
Construction: with The
the high speed.
revolving mortar causes pestle to
The move.
pestle During
is dumbthis bell process,
shaped so sizethat
reduction
balancing is being
and efficient achieved
grinding by
by its weight
Shearing as well as crushing. The
can be achieved. The bottom of pestle is flat
material is being collected and passed
rather than round.
through a sieveThe pestle
to get carries
a the an arm
powder
whichof is hinged.
desired By this arrangement, the
size.
pestle can be raised from mortar to facilitate
Use:
It is suitable
emptying for fineThe
and cleaning. grinding.
narrow central
portion of pestle is longer than the band
Disadvantages:
around it. Hence pestle can rise and fall over
Not suitable for unbroken or slightly
material
brokenin the mortar.of drug.
condition
 END RUNNER MILL

. .
 THEORIES OF SIZE REDUCTION / MILLING

A number of theories have been proposed to establish a

relationship between energy input and the degree of size
reduction produced.

1. Rittinger’s theory
2. Bond’s theory
3. Kick’s theory
4. Walker’s theory
 RITTINGER’S THEORY

Rittinger’s theory suggests that energy required in a size reduction

process is proportional to the new surface area produced.

where,
E = energy required for size reduction
KR = Rittinger’s constant

Si = initial specific surface area

Sn = final specific surface area

Application:
It is most applicable in size reducing brittle materials undergoing
fine milling.
 BOND’S THEORY

Bond’s theory states that the energy used in crack propagation is

proportional to the new crack length produced.

where,
E = energy required for size reduction
KB = Bond’s work index

di = initial diameter of particles

dn = final diameter of particles

Application:
This law is useful in rough mill sizing. The work index is useful in
comparing the efficiency of milling operations.
 KICK’S THEORY

Kick’s theory states that the energy used in deforming (or

fracturing) a set of particles of equivalent shape is proportional to
the ratio of change of size, or:

where, E = energy required for size reduction

KK = Kick’s constant

di = initial diameter of particles

dn = final diameter of particles

Application:
For crushing of large particles Kick’s theory most useful.
 WALKER’S THEORY

Walker proposed a generalized differential form of the energy-

size relationship:
where
E = amount of energy (work done) required to produce a change
D = size of unit mass
K = Constant
n = constant
For n =1.0 Walker equation becomes Kick’s theory used for coarse
particles > 1 m.
For n =1.5 Walker equation becomes Bond’s theory. This theory is
used when neither Kick’s nor Rittinger’s law is applicable.
For n =2.0 Walker equation becomes Rittinger’s theory used for
fine particles < 1 m size.
THANKS!