BP304T : SIZE REDUCTION

By
Ms. Akshita
Assistant Professor
ISF College of Pharmacy, MOGA
akshita@isfcp.org
 CONTENTS
• Introduction of size reduction
• Objectives of size reduction
• Disadvantages and disadvantages of size reduction
• Factors affecting size reduction
• Mechanism of size reduction
• Laws governing size reduction
• Size reduction equipments
• Hammer mill
• Cutter mill
• Ball mill
• Fluid energy mill
• Edge runner mill
• End runner mill
 SIZE REDUCTION
• Also known as comminution, diminution and pulverization.
• It is defined as the process of reducing large solid particles into smaller particles.
• It is necessary if the starting material is too coarse and the final product needs to
be fine powder.
• Degree of size reduction depends upon the machinery, methods, duration to
which the size of particle is reduced.

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 OBJECTIVES OF SIZE REDUCTION
• Increase Surface Area: By reducing the particle size, the surface area of the
material increases, which can enhance the rate of chemical reactions,
dissolution, and other surface-related processes.
• Improve Mixing and Blending: Smaller and uniform particle sizes improve
the homogeneity of mixtures, leading to better consistency in final products.
• Facilitate Drying: Smaller particles have a larger surface area-to-volume
ratio, which can enhance the rate of drying processes.
• Enhance Bioavailability: In pharmaceutical applications, reducing particle
size can increase the bioavailability of drugs by improving solubility and
absorption.
• Ease of Handling and Transport: Reduced particle size can make materials
easier .to handle, transport, and store due to more uniform and manageable
particle sizes
 ADVANTAGES OF SIZE REDUCTION
• Reducing the size of particles increases their total surface
area.
• It improves the homogeneity of mixtures, leading to better
product consistency.
• Size reduction leads to uniform particle size distribution,
ensuring consistent mixing and formulation
• Enhanced drying efficiency is achieved through reduced
particle size
• Increase rate of absorption of drug
• Increase therapeutic effectiveness of drug
• Size reduction simplifies the handling, transport, and storage
of materials. It contributes to improved product quality and
consistency.
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 DISADVANTAGES OF SIZE REDUCTION
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.
 DISADVANTAGES OF SIZE REDUCTION
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 wettability of the drug during production.
• Therefore, optimum particle size is desirable to improve blending and
to poor mixing.
CONTAMINATION
• During milling and grinding, the grinding surfaces wear off, 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
• Hardness:- Hardness is the surface property of the
substance. It is easy to break the soft material than hard
material which is determined by a device known as Mohs
scale, scale range 1-10.
• Toughness:- The crude drugs having fibrous nature are
tough. The tough materials create more difficulty in size
reduction process.
• Abrasiveness:- As material cause wear and tear, special
equipments are require their size reduction.
• Stickiness:- Stickiness causes lots of difficulty in size
reduction as the sticky materials may adheres to the grinding
machine altering the process.
 FACTORS AFFECTING SIZE REDUCTION
• Moisture content: The presence of moisture in the material
influences a number of its propertie such as hardness,
toughness or stickiness which in its turn affects the particle
size reduction. T 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.
• 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.
 FACTORS AFFECTING SIZE REDUCTION
• 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 ther fine grinding.
• Bulk density: The output of the size reduction of material in a
machine, depends upon the bulk density of the substance.
 MECHANISM/ MODES OF SIZE REDUCTION
Impact

• It occurs when a material is more or less

stationary and is hit by a fast-moving object
or when a moving particle collides with a
stationary surface, impact occurs. The
material is crushed in both cases into smaller
pieces.

COMPRESSION

• In this, the material is crushed by the

application of pressure

• Compressive forces are used for the coarse

crushing of hard materials.

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 MECHANISM/ MODES OF SIZE REDUCTION
ATTRITION
• The material is subjected to pressure as in
compression, but the surfaces are moving relative to
each other, resulting in shear forces which break the
particles.
• Shear or attrition forces are applied in ine
pulverization when the size of products can reach the
micrometer range.
• E.g. Fluid energy mill
CUTTING
• Cutting reduces the size of solid materials by
mechanical action (sharp blades) by dividing them
into smaller particles.
• E.g. Cutter mill

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 LAWS GOVERNING IN SIZE REDUCTION
Various theories are there to establish relationship between energy input and size
reduction
 LAWS GOVERNING IN SIZE REDUCTION
Rittinger’s law
• According to Rittinger's hypothesis, energy, E required for size reduction of unit
mass is directly proportional to the new surface are produced.
• It is expressed as:
E = KR (Sn-Si)
Where,
E = amount of energy (work done), kW.h
Si = initial specific surface area, um2 / μm3
Sn = new specific surface area, μm2 / μm3
KR=Rittinger's constant, energy per unit area, kW.h/um3
E = amount of energy (work done), kW.h

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 LAWS GOVERNING IN SIZE REDUCTION
Kick’s Law
According to this law, the energy used in deforming or fracturing a set of particles
of equivalent shape is proportional to the ratio of size change.
E = Kx In di/dn
Where,
E= energy required per mass of feed
Kx= Kick's constant
di= diameter of the particle in the initial stage, μm
dn= diameter of the particle in the final stage , μm

Application: Kick's law gives reasonably good results for coarse grinding where
there is a relatively small increase in surface area per unit mass.

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 LAWS GOVERNING IN SIZE REDUCTION
Bond’s law
• The energy used in crack propagation proportional to the new crack length
produced.
• It states that the energy used for deforming or fracturing a set of particles of
equivalent shape is proportional to the change in particles dimensions.

E = 2KB 𝟏
−
𝟏

𝒅𝒏 𝒅𝒊

Where,
KB= Bond's work index, energy per unit mass, Kw.h.vum
di = Initial diameter of particles,μm
Dn= New diameter of particles, μm
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SIZE REDUCTION EQUIPMENTS
 HAMMER MILL
Principle: Hammer Mill is based on the principle of Impact between hammer and
powder material that is more or less stationary.
Construction
• Hammer Mill consists of a metal casing, enclosing high speed rotor turning inside
cylindrical casing.
• The rotor is mounted on a shaft which is usually horizontal
• It has 4 to 8 hammers are attached to enclosed central shaft.
• The hammers are rectangular bars of metal with plain or enlarged ends.
• When the shaft is rotated with the help of motor, the hammer swing out to a radial
position.
• The lower part of the casing consists of a screen, through which material can
pass.
• It is than collected in a suitable receiver, when the desired degree of size
reduction is reached.
• On the side of the casing there is a feed hopper.
Working
• The feed material is placed into the
hopper
• The material from the hopper flows
vertically and then horizontally.
• The hammers are in continuous motion
and rotating at a high speed of 8000 to
15000 revolutions per minute.
• When the feed material strikes with the
rotary hammer, the material breaks down
into smaller pieces.
• The hammers also act as a centrifugal fan,
that large amount of air is drawn through
mill
• Then these particles pass through the
screen.
• The finess of product can be regulated by
altering
• Rotor speed, feed rate, number of hammer,
type of hammer, size of discharge
Advantages
• Easy to install, dismantle and clean up.
• Capable of grinding many different types of materials.
• Hammer mill occupies small space.
• It is versatile speed and screen can be changed rapidly.
Disadvantages
• Heat buildup during milling is more.
• Hammer mills cannot be employed to mill sticky, fibrous and materials.
• The screens may get clogged.
Uses
• Food product grinding like legumes, nutshells, groundnuts, and beans.
• Various types of hammer mills are used for powder making in the chemical and
pharmaceutical industries.
 CUTTER MILL
Principle
• In the cutter mill, size reduction involves cutting or shearing the feed
material with the help of sharp knives.
Construction
• Cutter mill consists of a metal casing, enclosing a high-speed rotor turning
inside a cylindrical casing.
• The milling chamber consists of two types of knives, namely rotating
knives and stationary knives
• A horizontally mounted rotor disc consists of 2 to 12 knives spaced
uniformly. The hopper is placed above it.
• The bottom of the casing holds a screen through which Product
outlet/discharge can pass
• The lower part of the casing consists of a screen, through which material
can pass.
• It is then collected in a suitable receiver when the desired degree of size
reduction is reached.
CUTTER MILL
Working :
• The rotor disc is allowed to rotate at speeds from 200 to 900 revolutions per
minute.
• The feed material is loaded into the hopper, which flows down by the force of
gravity.
• During the rotation of disc, the material comes very close between the stationary
and rotating knives, thereby the material is cut into small pieces.
• Smaller particles pass through the screen.
• The knives lift the coarser particles up while rotating and promote further size
reduction.
• The product is collected into a receiver.
• The particle size and shape are determined by the rotor size, gap between the
rotating
Uses:
• Cutter mills are used for the size reduction (finer than 80 to 100 mesh) of tough
and fibrous materials.
• Medicinal plants, plant parts and animal tissue are normally converted into small
parts.
• Soft materials such as roots, peels and wood are cut before extraction.
• It is also used in the manufacture of rubber, plastics, recycling of paper waste and
plastic materials.
 BALL MILL
It is also known as pebble mill or tumbling mill.
PRINCIPLE:
• The ball mill works on the principle of Impact between the rapidly
moving balls and the powder material. At low speed, balls will roll
over each other and attrition will be predominant.
CONSTRUCTION:
• Ball mill 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 is made of a metal and is usually lined with chrome.
• The cylinder contains balls that occupy 30 to 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 diameter of the balls varies from 2cm to 15cm.
• Balls are made of steel, iron or stoneware.
WORKING:
• The material to be ground is kept in a hollow cylinder
up to 60% of the volume.
• A fixed number of balls is placed in the cylinder and
then the cylinder is closed.
• The mill is allowed to rotate. Speed of rotation is an
important point of consideration.
• At low speed, the mass of balls will slide or roll up one
over another and will only produce an insignificant
amount of size reduction.
• At high speeds, the balls are thrown to the cylinder wall
due to centrifugal force and no grinding will occur.
• At corrected speed, centrifugal forces occurs, thereby,
the balls are carried almost to the top of the mill and
then fall in a cascade across the diameter of the mill. In
this way, the maximum size reduction is obtained by the
impact of the particles between the balls and by attrition
between the balls.
ADVANTAGES
• It can be used in a completely enclosed form.
• It can produce very fine powders.
• It can be use for continuous operations.
• Very suitable for wet or dry process.
DISADVANTAGES
• The ball mill is very noisy machine.
• It is a slow process.
• Wear occurs from the balls and casing, result
in contamination of the product.
• Soft, tacky, fibrous material cannot be milled
by ball mill.
USES
• Fine grinding with a a particle size of 100 to
5 mm or less.
• Production of opthamic and parentral
products.
 Fluid energy mill
• It is also known as pulverizer, ultrafine grinders, micronizer, and jet mill
• The particles are reduced using an attrition and impact mechanism involving
air or inert gas injected through the nozzles in the chamber.
• This mill is primarily used to finely grind heat sensitive material
PRINCIPLE
• Fluid energy mill operates on the principle of impact and attrition.
• In this equipment, the feedstock is suspended within a high velocity air stream.
Milling takes place because of high velocity collisions between the suspended
particles.
CONSTRUCTION
• Fluid energy mill consists of an elliptical pipe.
• The mill surface may be made of either soft stainless or tough ceramics.
• Grinding nozzles may be placed tangential or opposed to the initial flow path of
powder.
• Inert gases can be used to minimize or eliminate the oxidation of susceptible
compounds.
• Venturi feeder is provided in the path of the airflow.
• An outlet is provided with classifier to allow escape of air.
WORKING
• A nozzle at the bottom of the loop injects air at
very high pressure.
• Compressed air with a pressure between 600
kilopascals and 1.0 megapascals commonly
used.
• Due to the high velocity of the air, turbulence is
produced.
• By feeding the solids into the stream,
turbulence is created.
• Turbulence causes particles to collide and
attrition between each other.
• By incorporating a classifier into the process,
the system collects only fine particles
• for processing as products, while larger
particles are sent back into the air stream for
further reduction in size.
• To feed the mill, raw materials have been
reduced in size and screened to meshes or
smaller product is produced.
ADVANTAGES
• The machine has no moving parts and thus the tendency of contamination
due to wear of parts is minimized.
• The equipment is easily sterilized.
• Small particle size is usually obtained at the end of milling.
• Thermolabile materials can be milled with little degradation since the heat
produced by the process is nullified by the cooling effect of the expansion of
the compressed gas
DISADVANTAGES
• Tendency of forming aggregates or agglomerates after milling.
• Generation of amorphous content due to high energy impact.
• Formation of ultra-fine particles
USES
• Fluidized energy is used in milling thermolabile materials
• It is the choice of mill when a higher degree of drug purity is required
• Fluidized energy mill is used for the fine grinding of frits, Kaolin, Zircon,
titanium, and calcium, alumina.
 Edge Runner Mill
• It is also known as a roller stone mill.
• A machine consisting of one or more heavy steel rolls or grindstones set
on a horizontal shaft rotating around a pan or trough that is used for
crushing stone, fibrous matter for papermaking, or other material.
PRINCIPLE
• The size reduction is done by crushing (compression) due to the heavy
weight of stones.
• Shearing force is also involved during the movement of the stones.
CONSTRUCTION
• It consists of two heavy rollers and may weigh several tons.
• The rollers move on a bed, which is made of stone or granite.
• Each roller has a central shaft and revolves on its axis.
• Further, the rollers are mounted on a horizontal shaft and move around the
bed of steel or granite.
• The stones may vary from 0.5 to 2.5 m in diameter.
Edge Runner Mill
WORKING
• The material to be ground is placed on the bed.
• With the help of a scrapper, it is kept in the path of the stone wheel.
• The stones revolve on their axes, and at the same time travel around the
shallow stone bed.
• The outer part of the wheel has to travel a greater distance so that size
reduction is achieved by shearing as well as the inner, as crushing
• The material is ground for a definite period.
• The powder is collected and passed through a sieve to get powder of the
required size.
• It is a batch process.
ADVANTAGES
• It produces fine particles
• It does not require required attention during operation.
DISADVANTAGES
• Edge runner mill occupies more space than other commonly used mills.
• Contamination of the product with roller material is possible.
• The milling process is time-consuming.
• It is not used for sticky material.
• Energy consumption is quite high.
USES
• Edge runner mill is used for grinding tough materials to fine powder.
• It is still used for plant-based products, while more sophisticated mills use
chemicals and drugs
 End Runner Mill
• An automated version of mortar and pestle size reduction equipment.
• The friction of material between the mortar and the pestle causes a heavy-
weight pestle to revolve when the mortar is rotated by a motor at the
PRINCIPLE
• Size reduction is done by crushing due to the heavy weight of the steel
pestle.
• Shearing stress is also involved during the movement of mortar and
pestle.
CONSTRUCTION
• This grinding apparatus consists of a stone or metal pestle connected by a shaft.
• In a shallow steel or porcelain mortar the pastel revolves on its axis.
• The pestle is generally in the shape of a dumbbell.
• At the bottom, the mortar is secured by a flanged plate.
• Scrappers are located in the center and on either side of the circular pan.
• The pestle is horizontally positioned and rotates against a powdered bed.
WORKING
• The material to be milled is fed into the center of the circular mechanical mortar
(pan) and is worked outwards by the action of the wheels and mill is operated
• The pestle rotates against a bed of powders
• Mortar revolves at high speed and causes the pestle to revolve. Scrapers are
employed in scraping the material constantly from the bottom of the wheel and
are feed back to the wheel were it gets crushed further.
• Finally, pestle is raised from the mortar manually or automatically to facilitate
emptying and cleaning
ADVANTAGES
• It has a simple design and thus cleaning and maintenance is easy.
• It utilizes less electrical power.
• It produces fine and sometimes very fine particles.
• Requires less attention during the milling operation
• It has no problem with choking or clogging as it has no sieves for size
separation.
DISADVANTAGES
• It runs only on batch operation.
• It is not suitable for milling sticky materials.
• Unsuitable for drugs that are hard.
USES
• It is used to reduce fibrous crude drugs to a fine size.
• It is used for wet and dry grinding of crude drugs.
MILL ACTION PRODUCT USES MATERIAL
SIZE NOT USED
Cutter mill Cutting 20 to 80 mesh Fibrous, crude Friable material
(animal and
friable material,
vegetable drug)

Hammer mill Impact 4 to 325 mesh Almost all Abrasive

drugs material

Ball mill Attrition and 20 to 200 mesh Brittle drugs Soft material
impact

Fluid energy Attrition and 1 to 30μm Moderately Soft and sticky

mill impact hard and friable material
material

End runner mill Crushing & 20 to 80 mesh Almost all Stick material
shearing drugs
Edge runner Crushing & 20 to 80 mesh Almost all Stick material
mill shearing drugs
Thank you!