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Size Size Reduction Reduction

Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction To produce smaller particles (in the

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Page 1: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

Size Size ReductionReduction

Page 2: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 3: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 4: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 5: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 6: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the
Page 7: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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).

Page 8: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 9: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 10: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 11: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 12: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 13: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 14: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 15: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 16: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 17: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 18: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 19: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 20: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 21: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 22: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 23: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 24: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 25: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 26: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 27: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

(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

Page 28: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 29: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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 .

Page 30: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 31: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 32: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 33: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 34: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 35: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

At high speeds, the balls

will be thrown out to the

wall by centrifugal force

and no

grinding will occur.

High speed with centrifuging

Page 36: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 37: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

Low speed with sliding

High speed with centrifuging

Correct speed with cascading

Page 38: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 39: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 40: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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:

Page 41: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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:

Page 42: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 43: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 44: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 45: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 46: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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

Page 47: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 48: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 49: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.

Page 50: Size Reduction. Size Reduction means the reduction of materials to smaller sizes. Objectives of Size Reduction  To produce smaller particles (in the

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.