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Size Size ReductionReduction
Size ReductionSize Reduction
Size Reduction means the reduction of materials to smaller sizes.
Objectives of Size ReductionTo produce smaller particles (in the preparation of
suspensions or to facilitate the mixing of powders).
To increase surface area (to increase adsorptive
properties).
In drugs that are crushed to expose cells prior to
extraction.
Reduce the bulk of a material, since shipping
charges may be based on volume.
Factors Affecting Size Factors Affecting Size ReductionReduction
The pharmaceutical industry uses a great
variety of materials, including chemical
substances, animal tissues, and vegetable
drugs which may be hard (seeds), fibrous (barks
or roots) or spongy (peels).
Thus, the methods of size reduction are
numerous, and selection of the suitable method
involves the material properties that may
influence the process.
The properties that affect The properties that affect size reduction include:size reduction include:
HARDNESS
TOUGHNESS
ABRASIVENESS
STICKINESS
SOFTENING TEMPERATURE
MATERIAL STRUCTURE
MOISTURE CONTENT
PHYSIOLOGICAL EFFECT
PURITY REQUIRED
RATIO OF FEED SIZE TO PRODUCT SIZE
BULK DENSITY
HARDNESSHARDNESS
As hardness is a surface property of the material
and it is differs than strength, thus, it is possible for
a material to be very hard, but if it is brittle, the size
reduction is easy.
Moh's Scale is a scale of hardness in which range the
hardness of a series of mineral substances has
hardness numbers between 1 and 10, ranging from
graphite to diamond.
In general, the harder the material the more difficult
to be reduced in size.
TOUGHNESSTOUGHNESSToughness is more important than hardness, so
that a soft but tough material may present more
problems in size reduction than a hard but brittle
substance;
For example, compare the ease with which a stick
of blackboard chalk can be broken and the difficulty
to break a rubber.
Toughness of many pharmaceutical materials, as in
fibrous drugs is often related to moisture content.
(the toughness of a `green' twig and the
brittleness of a dry one).
ABRASIVENESSABRASIVENESS
Abrasiveness is a property of hard materials,
particularly those of mineral origin, and limit the
type of machinery that can be used.
During the grinding of some very abrasive
substances the final powder has become
contaminated with more than
0.1 % of metal from the grinding mill.
It is a property that cause difficulty in size
reduction, because material may adhere to the
grinding surfaces, or the meshes of screens and
become choked.
Pharmaceutical substances that are gummy or
resinous may be troublesome, particularly if the
methods used for size reduction generate heat.
Solve this problem by complete dryness and the
addition of inert substances (as addition of kaolin to
sulphur)
STICKINESSSTICKINESS
Many of the size reduction processes result in the
generation of heat, which may soften some
substances example: Waxy substances, such as
stearic acid, and drugs containing oils or fats
It may be possible to cool the mill, either by a water
jacket or by passing a stream of air through the
equipment or to use liquid nitrogen.
SOFTENING TEMPERATURESOFTENING TEMPERATURE
MATERIAL STRUCTURE
Some substances are homogeneous in character, but
the majority show some special structure
Mineral substances may have lines of weakness
along which the material splits to form flake-like
particles
Vegetable drugs have a cellular structure often
leading to long fibrous particles.
MOISTURE CONTENTMOISTURE CONTENT
Moisture content influence a number of the
properties that affect size reduction as hardness,
toughness or stickiness.
In general, materials should be dry or wet but not
damp. Usually, less than 5 % of moisture is suitable
if the substance is to be ground dry, or more than
50 % if it is subjected to wet grinding.
PHYSIOLOGICAL EFFECTPHYSIOLOGICAL EFFECT
Some substances are very potent (as hormone
drugs) and small amounts of dust may have an
effect on the operators.
To solve this problem:
Use enclosed mills to avoid dust.
Special air extraction systems are desirable.
Wet grinding can eliminates the problem
entirely.
PURITY REQUIREDPURITY REQUIRED
Certain types of size reduction apparatus cause the
grinding surfaces to wear, thus such methods must
be avoided if a high degree of purity of product is
needed.
BULK DENSITYBULK DENSITY
When all other factors being equal, the output of
the machine is related to the bulk density of the
substance.
RATIO OF FEED SIZE TO PRODUCT SIZERATIO OF FEED SIZE TO PRODUCT SIZE
Machines that produce a fine product require a small
feed size. Thus, it may be necessary to carry out the
size reduction process in several stages with different
equipment;
for example, preliminary crushing, followed by coarse
grinding and then fine grinding.
Mechanisms of Size ReductionMechanisms of Size Reduction
There are four main methods of effecting size There are four main methods of effecting size
reduction, with different mechanisms:reduction, with different mechanisms:
Cutting. Cutting.
the material is cut by means of a sharp blade or the material is cut by means of a sharp blade or
blades.blades.
Compression. Compression.
the material is crushed by application of pressure.the material is crushed by application of pressure.
Impact. Impact. Occurs when the material is stationary and is hit Occurs when the material is stationary and is hit
by an object moving at high speed or when the by an object moving at high speed or when the
moving particle strikes a stationary surface. In moving particle strikes a stationary surface. In
either case, the material shatters to smaller pieces. either case, the material shatters to smaller pieces.
Attrition. Attrition. The material is subjected to pressure as in The material is subjected to pressure as in
compression, but the surfaces are moving relative compression, but the surfaces are moving relative
to each other, resulting in shear forces which break to each other, resulting in shear forces which break
the particles.the particles.
ApproximateApproximate
increase in increase in
FinenessFineness
of productof product
MethodMethodDiagramDiagramCommon Common example example
Cutting Cutting Scissors Scissors ShearsShears
Compression Compression NutcrackeNutcrackers rs
Impact Impact Hammer Hammer
Attrition Attrition (pressure and(pressure and
friction)friction)
File File
Mechanisms of size reduction Mechanisms of size reduction
METHODS OF SIZE REDUCTION METHODS OF SIZE REDUCTION
The term The term mill mill is used normally for machines for is used normally for machines for
size reduction. size reduction.
Cutting and compression have limited uses in Cutting and compression have limited uses in
pharmaceutical practice.pharmaceutical practice.
Impact and attrition are used much more Impact and attrition are used much more
widely, both separately and in combination, widely, both separately and in combination,
and there is a great variety in each type. and there is a great variety in each type.
CuttingCuttingCuttingCutting
On the small scaleOn the small scale, size reduction by cutting can , size reduction by cutting can
be affected by a knife or a cutter.be affected by a knife or a cutter.
On a large scaleOn a large scale,, a cutter mill is used. a cutter mill is used.
CUTTER MILLCUTTER MILL
The principle of operation:The principle of operation:knives are attached to a knives are attached to a
rotor and act against rotor and act against stationary knives in the stationary knives in the casing. casing. The lower part consists of The lower part consists of
a screen, so that material is a screen, so that material is retained in the mill until a retained in the mill until a sufficient degree of size sufficient degree of size reduction has been reduction has been effected.effected.Application:Application:The method is used to The method is used to
obtain a coarse size obtain a coarse size reduction of soft materials. reduction of soft materials. Examples:Examples:Roots, peels or woods, Roots, peels or woods,
prior to extraction.prior to extraction.
CUTTER MILLCUTTER MILLCUTTER MILLCUTTER MILL
CompressionCompressionCompressionCompression
On the small scaleOn the small scale size reduction size reduction by Compressionby Compression
carried out by a pestle and mortar. carried out by a pestle and mortar.
On a large scale, On a large scale, roller mill is used.roller mill is used.
The principle of operation:The principle of operation:The roller mill has two The roller mill has two
cylindrical rolls of stone or cylindrical rolls of stone or
metal, mounted horizontally, metal, mounted horizontally,
which rotate on their which rotate on their
longitudinal axes. Usually, one longitudinal axes. Usually, one
of the rolls is driven directly of the rolls is driven directly
while the second runs free, so while the second runs free, so
that whenthat when
ROLLER MILLROLLER MILL
ROLLER MILLROLLER MILLROLLER MILLROLLER MILL
material is placed above the rolls it is drawn in through material is placed above the rolls it is drawn in through
the the nip nip and the second roll is rotated by friction.and the second roll is rotated by friction.
The gap between the rolls can be adjusted to control The gap between the rolls can be adjusted to control
the degree of size reduction. the degree of size reduction.
Application:Application:
The roller mill is used for The roller mill is used for crushing seeds crushing seeds prior to prior to
extraction of fixed oils, or extraction of fixed oils, or bruising soft tissues bruising soft tissues after after
cutting to aid solvent penetration in extraction.cutting to aid solvent penetration in extraction.
ImpactImpact
On the small scaleOn the small scale size reduction size reduction by Impact by Impact
carried out by the shattering of brittle carried out by the shattering of brittle
substances with a hammer or with a pestle and substances with a hammer or with a pestle and
mortar. mortar.
HAMMER MILLHAMMER MILLHAMMER MILLHAMMER MILL
HAMMER MILLHAMMER MILL
The principle of operation:The principle of operation:
the the hammer mill hammer mill consists of consists of
a central shaft to which a central shaft to which
four or more four or more hammers hammers are are
attached. attached.
These are mounted with These are mounted with
swivel joints, so that the swivel joints, so that the
hammers swing out to a hammers swing out to a
radial position when the radial position when the
shaft is rotated. shaft is rotated.
The lower part of the casing The lower part of the casing
consists of a screen through consists of a screen through
which material can escape. which material can escape.
The screen can be changed The screen can be changed
according to the particle according to the particle
size required.size required.
AdvantagesAdvantages
(a)(a) It is rapid in action, and is capable of grinding It is rapid in action, and is capable of grinding
manymany
different types of materials.different types of materials.
(b) The product can be controlled by variation of (b) The product can be controlled by variation of
rotor speed, hammer type, and size and shape of rotor speed, hammer type, and size and shape of
mesh.mesh.
(c) Operation is continuous.(c) Operation is continuous.
(d) No surfaces move against each other, so that (d) No surfaces move against each other, so that
there is little contamination of the product with there is little contamination of the product with
metal abraded from the mill.metal abraded from the mill.
(a)(a)The high speed of operation causes generation of The high speed of operation causes generation of
heat that may affect thermolabile materials or heat that may affect thermolabile materials or
drugs containing gum, fat or resin. drugs containing gum, fat or resin.
The mill may be water-cooled, if necessary, to The mill may be water-cooled, if necessary, to
reduce this heat damage.reduce this heat damage.
(b) The rate of feed must be controlled carefully, (b) The rate of feed must be controlled carefully,
otherwise otherwise
the mill may be choked, resulting in decreased the mill may be choked, resulting in decreased
efficiency or even damage. efficiency or even damage.
(c) Because of the high speed of operation, the (c) Because of the high speed of operation, the
hammer mill is susceptible to damage by foreign hammer mill is susceptible to damage by foreign
objects such as stones or metal in the feed.objects such as stones or metal in the feed.
Magnets may be used to remove iron, but the Magnets may be used to remove iron, but the
feed must be checked visually for any other feed must be checked visually for any other
contamination. contamination.
DisadvantagesDisadvantages
Application:Application:
Applications include the powdering of barks, leaves, Applications include the powdering of barks, leaves,
roots, and crystals.roots, and crystals.
With cutting edges to the hammers, the method is With cutting edges to the hammers, the method is
useful for granulation of damp masses to produce useful for granulation of damp masses to produce
greater uniformity than is obtained by sieve greater uniformity than is obtained by sieve
granulation.granulation.
The hammer mill can’t used for sticky materials that The hammer mill can’t used for sticky materials that
choke the screen. choke the screen.
AttritionAttrition
Size reduction by attrition can be effected in
the laboratory by using pestle and mortar.
In small scale mechanical method is required
using the roller mill .
ROLLER MILLROLLER MILL
The rolls rotate at different speeds, so that the material
is sheared as it passes through the gap and is
transferred from the slower to the faster roll, from which
it is removed by means of a scraper.
Application:Used for size reduction and dispersion
of solids in suspensions, semisolids as
pastes; or ointments.
The principle of operation:The principle of operation:Two or three rolls in metal or
porcelain are mounted horizontally
with a very small adjustable gap in
between. ROLLER MILLROLLER MILL
Combined Impact and AttritionCombined Impact and AttritionCombined Impact and AttritionCombined Impact and Attrition
The mechanisms of impact and attrition can be
combined in two forms of mill.
In the ball mill the particles receive impacts from
balls or stones and are subjected to attrition as the
balls slide over each other.
In fluid energy mills the impacts and attrition occur
between rapidly moving particles.
BALL MILLBALL MILL
The ball mill consists of a hollow
cylinder mounted in such a way
that it can be rotated on its
horizontal axis with a rotational
frequency depend upon the
diameter of the mill ≈ 0.5 r.p.s
The principle of operation:The principle of operation:
The cylinder may be of metal, porcelain or of rubber,
to reduce abrasion. The balls may be of metal,
porcelain or stones (pebble mill).
The cylinder contains balls that occupy 30 to 50 per
cent of the mill volume.
BALL MILL
The ball size being dependent on
the size of the feed and the
diameter of the mill. Usually a
mill 1 m in diameter will use balls
of 75 mm, In practice, the balls
are damaged, so that a range of
sizes from 20 mm upwards are
used. This gives a better
product, since the larger balls
crush the feed and the smaller
ones form the fine product. The cylinder may be of metal, porcelain or of rubber,
to reduce abrasion. The balls may be of metal,
porcelain or stones (pebble mill).
BALL MILL
Importance factors in the operation of the ball
mill:
The amount of the material in the mill is of
importance:
too much exerting a reduced effect
too little leading to loss of efficiency and to
abrasion. The speed of rotation:
At low speeds, the mass of
balls will slide or roll over
each other and negligible
size reduction will occur.
Low speed with sliding
At high speeds, the balls
will be thrown out to the
wall by centrifugal force
and no
grinding will occur.
High speed with centrifuging
At about two-thirds of the
speed at which centrifuging
just occurs, movement
takes place as shown in Fig.
(c), that is, the balls are
carried almost to the top of
the mill
and then fall in a cascade
(tumble) across the
diameter of the mill. By this means, the maximum size reduction is
effected by impact of the particles between the
balls and by attrition between the balls.
Correct speed with cascading
Low speed with sliding
High speed with centrifuging
Correct speed with cascading
Advantages
A.It is capable of grinding a wide variety of materials
of
differing character and of different degrees of
hardness.
B. It can be used in a completely enclosed form;
which
makes it especially suitable for use with toxic
materials.
C. It can produce very fine powders.
D. It can be used for continuous operation, and a
classifier can be used in conjunction with the mill,
so that particles of suitable size are removed while
oversize particles are returned.
E. It is equally suitable for wet or dry grinding
processes.
Disadvantages
A. Wear occurs, principally from the balls, but
partially from the shell and this may result in the
contamination of the product; with abrasive
materials this may exceed 0.1%
B. Soft or sticky materials may cause problems by
caking on the sides of the mill or by holding the
balls in aggregates.
C. The ball mill is a very noisy machine, particularly
if the casing is of metal, but much less if rubber is
used.
Ball mills are applicable to a wide variety of
materials,
large ones being used for grinding ores prior to
manufacture of pharmaceutical chemicals
small versions for the final grinding of drugs or for
grinding suspensions.
Application:
FLUID ENERGY MILLFLUID ENERGY MILL
The fluid energy mill is
consist of a loop of pipe. A
fluid, usually air, is injected
at high pressure through
nozzles at the bottom of the
loop, giving rise to a high
velocity circulation in a very
turbulent condition. Fluid Energy Mill
The principle of operation:The principle of operation:
Solids are introduced into the
stream and, as a result of the
high degree of turbulence,
impacts and attritional forces
occur between the particles.
A classifier is incorporated in
the system, so that particles
are retained until sufficiently
fine.
The feed to the mill needs to
be pre-treated to reduce the
particle size to 100 mesh,
enabling the process to yield a
product as small as 5 μm or
less.
Fluid Energy Mill
Advantages
A.The particle size of the product is smaller than
that produced by any other method of size
reduction.
B.Expansion of gases at the nozzles leads to
cooling, counteracting the usual frictional heat
which can affect heat-sensitive materials.
C.Since the size reduction is by inter-particulate
attrition there is little or no abrasion of the mill
and so virtually no contamination of the product.
D.For special cases with very sensitive materials it
is possible to use inert gases.
E.Having a classifier as an integral part of the
system permits close control of particle size and
of particle size distribution.
F.The method is used where especially fine
powders are required, as antibiotics,
sulphonamides and vitamins.
Selection of Size Reduction Method
Different mills will give differing products from the
same starting material:
Particle shape may vary according to whether
size reduction is by impact or attrition
The proportion of fines may vary, so altering
colour, appearance, and properties.
Methods used for various size ranges
Degree of size
reduction
Typicalmethods
Examples
Large piecesCutter or
compressionmills
Rhubarb
Coarsepowders
Impact millsLiquorice,cascara
Fine powders
Combined impact and
attrition mills
Rhubarb,belladonna
Very finepowders
Fluid energy millsVitamins,antibiotics
Selection of Degree of Size ReductionThe use to which a powder is to be put, controls
the degree of size reduction.
The important factor is that the cost of size
reduction increases as the particle size
decreases. Thus, it is economically undesirable
to powder a substance to a finer degree than is
required.
Powder
If a substance is to be used directly in the form
of a powder as indigestion powders or dusting
powder, a fine powder is needed.
Suspension
When a drug is suspended in a liquid it should
be in the form of a very fine powder, as Light
Kaolin, and this is especially important if the
suspension is to be injected, as in the case of
procaine penicillin.
For extraction
Soft drugs such as Gentian need only be sliced
or crushed
Harder drugs such as Liquorice or Belladonna
should be in coarse to moderately coarse
powders.
Special characters may exert an effect:
The constituents of Ipecacuanha are of low-
solubility so that a fine powder is used to
improve the penetration of the solvent into
the particles.
Cascara is usually extracted with water and
swells when moistened, making a coarse
powder preferable as finer grades would
form an impenetrable mass.
The effect of degree of size reductionon the properties:
The viscosity of mucilage of tragacanth decreases with finer powders
The viscosity of mucilage of acacia increases with finer powders, where low grade gums may be improved by grinding.
Adsorption is a surface property, so that the adsorptive power of a substance is improved by grinding.
If a drug contains volatile oils, the greater surface area and the additional heat generated in grinding to a finer powder increase the loss of oil.
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