Size reduction

Size Reduction
B. PHARM 2ND YEAR, III SEMESTER
SUBJECT:- PHARMACEUTICAL ENGINEERING
Prepared by:
Mrs. Kokare Pratima. S.

Contents
 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 by Kokare Pratima S.
2

Introduction
 Definition- Size reduction or Comminution
or Pulverization is the process of reducing
substances to smaller particles.
OR
 Process of reducing large solid unit
masses into small unit masses i.e. Coarse
or fine particle.
 When the particle size of solids is
reduced by mechanical means it is
known as milling.
 The size reduction operation can be
divided into two major categories
depending on whether the material is a
solid or a liquid.
 If the material is solid, the process is
called grinding and cutting, if it is liquid,
emulsification or atomization
3

Advantages:
Content
Uniformity
•With smaller particle size , number of particles per unit dose increases. Larger the
number of particle, better is the mixing. Therefore better content uniformity can
be achieved
Uniform flow
•Smaller and controlled particle size promotes the flow of powder,
this helps in tablet compression and capsule filling
Effective
extraction of
drugs
•Smaller particle allows rapid penetration of menstrum or solvent
into the tissue or cells, extraction becomes effective
Effective
drying
•Rate of drying increases as size reduction increases the surface
area
Improved
physical
stability
•Smaller particle size decreases the sedimentation rate, thus
increases the stability of suspension and emulsions
Improved
dissolution
Rate
•Size reduction increases the surface area, facilitate intimate
contact of solid particle
Improved rate
of absorption
•Smaller is the particle size, faster is the absorption
4

Disadvantages
Drug
Degradation
• It is possible due to heat produced during milling.
Thermolabile substances mostly affected. Increased Surface
area also facilitate decomposition.
Poor mixing
• Very small particles possess strong cohesive forces which
leads to aggregation which inhibits effective blending of
different additives.
Contamination
• During milling & grinding, the grinding surfaces of equipment's
wear off which may present as impurities in the powder.
5

Objectives
Improve dissolution rate, solubility, binding strength and
dispersion properties
Enhance stability of dispersed system for example,
stability of emulsions is increased by decreasing the size
of the oil globules
Increase the surface area to enhance the rate of a
physical or chemical process
Enhance flowability, improve compression and dose
uniformity
Improve physical appearance of products
Increase the therapeutic effectiveness of certain drugs
by reducing the particle size. Eg, Griseofulvin
6

Laws governing Size reduction
 Number of theories have been proposed to establish a relationship between energy input & the
degree of size reduction produced.
 Principle of size reduction: Energy required to produce a change dL in a particle of a typical
size dimension L is a simple power function of L:
dE
dL
= K Ln
Where,
dE is the differential energy required
dL is the change in a typical dimension
L is the magnitude of a typical length dimension
K and n are constants.
Theories of
Size
Reduction
Kick’s Law
Rittinger’s
Law
Griffith’s
Theory
Bond’s law
7

Kick’s Law
 According to this law, energy required to reduce size of particle is proportional to
ratio of initial size of a typical dimension to the final size of that dimension.
E = KK In (
𝒅 𝟏
𝒅 𝟐
)
Where,
E= energy required per mass of feed
KK= Kick’s constant
𝒅 𝟏 = Average initial size of pieces (m)
𝒅 𝟐 = Average size of ground particles (m)
𝒅 𝟏
𝒅 𝟐
= is size reduction ratio (RR) & is used to evaluate relative performance of
different type of equipments.
Coarse grinding has RR value below 8:1 while in fine grinding ratio can exceed 100:1.
 Application: Kick’s law gives reasonably good results for coarse grinding where
there is a relatively small increase in surface area per unit mass.
8

Rittinger’s Law
 This law states that the energy required for the size reduction of unit mass is
proportional to the new surface area produced.
E = KR (
𝟏
𝒅 𝟐
-
𝟏
𝒅 𝟏
)
Where,
KR= Rittinger’s constant
𝒅 𝟏 = Average initial size of pieces (m)
𝒅 𝟐 = Average size of ground particles (m)
1/d= s (Surface area)
So, equation will become
E= KR (Sn – S1)
S1= Initial specific surface area
Sn = New surface area
 Applications: Rittinger’s law gives better results with fine grinding where there is a
much greater increase in surface area
9

Bond’s law
 This law states that energy used for size reduction is proportional
to new crack length.
𝑬
𝑾
=
𝟏𝟎𝟎
𝒅 𝟐
−
𝟏𝟎𝟎
𝒅 𝟏
Where,
W= Bond work index work required to reduce a unit weight
𝒅 𝟏 = diameter of sieve aperture that allows 80% of mass of feed
to pass (in meters)
𝒅 𝟐 = diameter of sieve aperture that allows 80% of mass of
ground material to pass (in meters)
10

Mechanisms of Size Reduction
 The mechanism of size
reduction may vary with
material, therefore each
drug may require a
separate treatment.
 The general mechanism
may be described as
follows:
Force Principle Example of
equipment
Approx.
particle size
(mm)
Compression Nutcracker
Roller mill,
Pestle-Mortar
Crushing rolls
50 - 10,000
Impact Hammer Hammer mill 50 - 8000
Attrition File
Colloidal mill
roller mill
1 - 50
Cutting Scissors
Scissor,
shears cutter
mill, rotary
knife cutter
100 - 80,000
Combined
impact
and attrition
Ball Ball mill
1 - 2000
11

Mechanism
COMPRESSION
•The material is crushed
by application of
pressure.
•Compressive forces are
used for coarse crushing
of hard materials.
•Coarse crushing implies
reduction to a size of
about 3 mm.
•E.g. Roller mill
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 &
material is crushed in to
smaller pieces.
•E.g. Hammer mill, fluid
energy mill
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 fine
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 blade/s) by
dividing them into
smaller particles.
•Cutting is used to break
down large pieces of
material into smaller
pieces and definite
shape suitable for further
processing, such as in
the preparation of
powders and granules.
•E.g. Cutter mill
12

Factors affecting Size Reduction
1. HARDNESS-
 Surface property of the material.
 Harder the material more difficult to reduce size.
 Measured by Moh’s scale.
 Moh’s scale is from 1 to 10.
 1 to 3 are classified as soft (e.g. Talc, waxes)
 4 to 7 are intermediate (e.g. Limestone, bauxite)
 8 to 10 are hard (e.g. Quartz, diamond)
2. TOUGHNESS
 It’s more imp than hardness.
 Soft but tough material may present more problem in
reducing size than a hard but brittle substance.
 E.g. it is difficult to break the rubber than a
blackboard chalk stick.
13

3. ABRESIVENESS
 Property of hard material
 During the grinding of some very abrasive substances the final powder
may be contaminated with more than 0.1 % of metal worn from the
grinding mill.
4. STICKINESS
 Causes considerable difficulty in size reduction. This type of
materials may adhere to the grinding surfaces, or choke the meshes of
the sieve.
 E.g. Gummy or resinous substances
5. SLIPPERINESS
 Reverse of stickiness.
 This property also gives rise to size reduction difficulties, since the
material acts as a lubricant and lowers the efficiency of the grinding
surfaces.
14

6. SOFTENING TEMPERATURE
 During size reduction process sometimes heat is generated
which may cause some substances to soften.
 E.g. waxy substances such as stearic acid or drugs
containing oils or fats.
 This can be overcome by cooling the mill, either by a water
jacket or by passing a stream of cold air through the
equipment. Another alternative is to use liquid nitrogen.
7. MATERIAL STRUCTURE
 Mineral substances have lines of weakness. Along the lines
of weakness these materials splits in to forms like flakes.
 While vegetable drugs have a cellular structure that often
leads to long fibrous particles.
 Thus, the resulting product at particular operating
conditions may vary in their size. The energy required to
perform this operation may vary.
15

8. MOISTURE CONTENT
 These properties include hardness, toughness or stickiness etc.
 In general for size reduction materials should be dry or wet and
not entirely damp.
 Usually, less than 5 % moisture is suitable if the substance is to be
ground dry or more than 50 % if it is being subjected to wet
grinding.
9. PHYSIOLOGICAL EFFECT
 Some substances are very potent and small amounts of fines
generated have an effect on the operator’s health.
 To avoid these fines, mills must be enclosed; in addition exhaust
systems should be provided. If possible wet grinding is performed
to entirely eliminate the problem.
16

10. PURITY REQUIRED
 Some of the size reduction equipment's cause wear and tear of the grinding
surfaces.
 Use of these equipment's must be avoided whenever high degree of purity of
product is needed.
11. RATIO OF FEED SIZE TO PRODUCT RATIO
 Machines that produce a fine may need to carry out the size reduction in
several stages with different equipment's.
 E.g. preliminary crushing followed by coarse grinding and then fine grinding. In
such cases feed size is needed to be controlled in order to perform reduction
efficiently.
12. BULK DENSITY
 The capacities of most batch mills depend on volume.
 These mills usually demand solid materials by weight rather than volume. The
output of the mill is related to the bulk density of the substance.
 Higher the bulk density more is the product.
17

Principles, Construction, Working, Uses,
Merits & Demerits of Size Reduction
Equipments
Class I
•Application of
continuous
pressure
•Includes
equipment for
coarse
crushing.
Class II
•Blow or
impact
Class III
•Shearing
forces are
applied by
grinding or
abrasion
•Gives fine
grinding
Equipments are classified into three classes according to the nature
of the forces applied:
18

Hammer Mill
 Method of size reduction: Impact
 Principle: It operates on the principle of
impact between rapidly moving hammers
mounted on rotor and the stationary
powder bed.
 Construction & working:
1. Swinging hammers are attached to rotor
and covered with stout metallic casing.
2. Fed material is ejected by impact of high
speed hammers.
3. Screen of particular mesh size is attached
to outlet.
4. Material of larger size is retained in mill and
the product of required size comes out.
19

 Merits:
1. Rapid in action, and is capable of grinding many different types of
materials.
2. Easy to install and operate.
3. Little contamination of the product
 Demerits:
1. High speed of operation causes generation of heat that may
affect thermolabile materials or drugs containing gum, fat or resin.
2. The rate of feed must be controlled carefully as the mill may be
choked, resulting in decreased efficiency or even damage.
 Application: Fibrous materials, Brittle material is best fractured by
impact of blunt hammers.
20

Ball mill
 Method of size reduction: Impact and Attrition
 Principle: The size reduction in ball mill is a result
of fragmentation mechanisms (impact and
attrition) as the balls drop from near the top of
shell.
1. Rotating cylinder (metal, porcelain) containing
loose balls( metal-lead-antimony) rotates on
horizontal axis.
2. These balls occupy 30 to 50% of cylinder volume.
3. These balls lifted up and falls down in cascade.
4. At slow speed- balls slide over each other. High
speed- Due to high centrifugal force balls
pushed out toward wall and does not reduce
size much.
5. Therefore speed of balls should be optimum
speed so that balls will be lifted at top and falls
down.
21

 Merits:
1. Produces very fine powder (10 microns)
2. Suitable for milling toxic materials
3. It has a big crushing ratio and high production capacity
 Demerits:
1. Contamination of product may occur as a result of wear and tear
of the balls.
2. High machine noise level.
3. High production cost and high unit electricity consumption.
 Application: Used for dry and wet grinding. To produce fine powder
without dust (5 to 100 mm).
22

Fluid energy mill (Micronizers/ Ultra
fine grinders)
 Method of size reduction: Impact and attrition
 Principle: Based on principle of impact &
attrition. The size reduction takes place by high
velocity collision between particle.
1. Size reduction occurs due inter-particular
attrition and impact on turbulent particles.
2. Made up of loop of pipe internal diameter 2 to
20 cm and height 1-2 m.
3. Pipe allows free movement of particles in
steam of high velocity air.
4. High degree of turbulence causes the impact
and attrition.
5. The cyclone collector fitted at outlet gives
sufficiently fine powder.( 1 to 30 micron)
23

 Merits:
1. The particle size of the product is smaller
2. Little or no abrasion of the mill and so no contamination of the
product
3. To protect sensitive drugs from oxidative degradation this mill has
facility to use inert gases
 Demerits:
1. Energy consuming
2. High head space is required
3. Coarse feed size is not suitable
 Application: Suitable for heat sensitive drugs such as antibiotics
and vitamins
24

Edge Runner Mill (Roller stone mill)
 Method of size reduction: Attrition and
impact
 Principle: The principle of size reduction by
this mill are crushing due to heavy weight of
the stones or metal and shearing force.
Movement of stones or metal causes size
reduction. The Edge-runner mill has the pastel
equivalent mounted horizontally and rotating
against a bed of powders.
1. Consist of 2 heavy rollers mounted on central
& horizontal shaft which moves around the
bed in a shallow circular pan. The bed is
made of stone & iron.
2. Material to ground is placed on the bed. The
stones continuously evolve on its axis. Sie
reduction obtained by shearing along with
crushing.
3. Material is ground for definite period & then it
is passed through the sieves to get the
powder of required size.
25

 Merits:
1. Very fine particle sized materials can be obtained
2. Simple design, Utilizes less power
3. requires less attention during operation
 Demerits:
1. Not suitable for sticky materials
2. Produce lot of noise pollution
 Application: used to crush or grind all types of the drugs
26

End Runner Mill
 Method of size reduction: Crushing &
shearing
 Principle: Size reduction is done by
crushing due to weight of pestle &
shearing also involves due to pestle &
mortar movement.
1. Consists of pestle made of either stone or
metal, connected by a shaft.
2. The pastel rotates at its axis in a shallow
steel or porcelain mortar.
3. The material to be milled is fed into the
center of the circular mechanical mortar.
4. The pestle rotates against a bed of
powders. Mortar revolves at high speed
and causes the pestle to revolve.
5. Scrapers are employed in scraping the
material constantly from the bottom of the
wheel.
27

 Merits:
1. Simple design and thus cleaning and maintenance is easy.
2. Utilizes less electrical power
3. Produces fine and sometimes very fine particles
4. Requires less attention
 Demerits:
1. Not suitable for milling sticky materials
2. Machine noise causes lot of noise pollution
3. It runs only on batch operation
 Application:
1. Used to reduce fibrous crude drugs to a fine size.
2. Used for grinding semisolid preparations such as ointments and pastes to fine size.
3. Used for both wet and dry grinding of crude drugs.
28

1. What is size reduction? What are objectives of size reduction?
2. Discuss energy calculation theories involved in size reduction.
3. Discuss mechanisms involved in size reduction of material.
4. Discuss factors affecting size reduction.
5. Describe principles, construction, working, uses, merits and
demerits of:
A. Fluid energy mill,
B. Edge runner mill.
C. End runner mill
D. Ball mills
E. Hammer mill
Assignment Questions
29

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