Unit -1 B

Meaning of size reduction ?

 Size refers to physical Dimension of an object.

 Reduction refers to decrement or the process of decreasing
the size
 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.
 OBJECTIVES
• In the materials processing industry, size reduction or
comminution is usually carried out to-
• Increase the surface area because, in most reactions
involving solid particles, the rate of reactions is
directly proportional to the area of contact with
a second phase.
• Break a material into very small particles in order
to separate the valuable amongst the two
constituents.
• Achieve intimate mixing.
• To dispose solid wastes easily .
• To improve the handling characteristics.
• To mix solid particle more intimately.
Disadvantages of size
reduction
• Drug degradation
• Contamination

Advantages of size reduction

 Content uniformity

 Uniform flow

 Effective drying

 Increases surface area or viscosity

 Uniform mixing and drying

 Improve rate of absorption .

Smaller the particles greater is absorption.
 Improve dissolution rate.
 Mechanisms of size reduction

• Impact —particle concussion by a

single rigid force (hammer).
• Compression—particle disintegration
by two rigid forces (nutcracker).
• Shear —produced when the
particle is compressed between
the edges of two hard surfaces
moving tangentially.
• Attrition —arising from particles scraping
against one another or against a rigid surface
(a file).
 If material is hard
When
compressio If material is abrasive
n forces
needed- If the material is not stick

When material will break

cubically
Where the finished product
is to be relatively coarse in
size
 SIZE REDUCTION THEORIES
 The energy requirement for particle size reduction is a
function of input and output of particle size,
hardness, strength and other properties of solids.

Various theories for energy requirement are:-

 Rittinger’s theory
 Kick’s theory
 Bond’s theory.


Integrating equation (1),

E= C ln(di/dn) ….
(2)
(di/dn) = reduction ratio.
If n=1.0 equation (2) becomes Kick’s theory.

If n=1.5 equation (2) becomes Bond’s theory.

If n=2.0 equation (2) becomes Rittinger’s
theory.
 Rittinger’s theory
According to this theory energy E required for size reduction
of unit mass is directly proportional to the new surface area
produced.

E=KR (Sn – Si) …. (3)

Where Applications:-
Si = initial surface area Applicable to brittle materials
undergoing fine milling.
Sn = new specific surface This
areatheory ignore deformation
K = Rittinger’s constant. before fracture
R

E= amount of energy
 Bond’s Theory

1.Bond’s work index is the work required to reduce unit weight

from a theoretical infinite size to 80% passing 100µm.
2. This theory is useful for rough mill sizing.
3.The work index is useful for comparing efficiency of milling
operations
 Kick’s Theory

For compression of large particles kick’s theory is usef

 Rittinger’s theory:- (n=2.0) Energy α new surface area

formed.

 Bond’s theory:- (n=1.5) Energy used in crack propagation α

Crack length produced.

 Kick’s theory:- (n=1.0) Energy α Ratio of change in size.

Factors affecting size reduction
1. Hardness:

 It is a surface property of the material.

 It is frequently confused with a property named strength.
 Thus, it is possible for a material to be very hard, but if it is
 brittle also then size reduction may present no special problems.
 An arbitrary scale of hardness has been devised known as Moh’s
Scale;
 Moh’s Scale = 1 is for graphite
 Moh’s Scale < 3 is for soft material
 Moh’s Scale > 7 is for hard material
 Moh’s Scale =10 is for diamond
 The harder the material the more
difficult it is to reduce in size
2. Material structure:
 Some substances are homogeneous in character.
 Mineral substances may have lines of weakness along which
 the materials splits to form flake-like particles.
 Vegetable drugs have a cellular structure often leading to long fibrous
particles.

3. Abrasiveness:
 Abrasiveness is a property of hard materials
(particularly those of mineral origin).
 It may limit the type of machinery that can be
used.
 During the grinding of some very abrasive substances the
final powder may be contaminated with more than
 0.1 percent of metal worn from the grinding mill
4.Moisture content:
It is found that materials do not flow well if they contain
between about 5 and 50 per cent of moisture. Under
these conditions the material tends to cake together in
the form of balls. In general, grinding can be carried out
satisfactorily outside these limits.
5. Crushing strength:
The power required for crushing is almost directly
proportional to the crushing strength of the material
6. Softening temperature:
 During size reduction process sometimes heat is
generated which may cause some substances to soften,
and the temperature at which this occurs can be
important.
 Some methods can be used to overcome this like
cooling the mill, either by a water jacket or by passing
a stream of air through the equipment.
 Waxy substances, such as stearic acid, or drugs
containing oils or fats are examples that may be
affected.
7.Friability:
The friability of the material is its tendency to fracture during
normal handling. In general, a crystalline material will break
along well-defined planes and the power required for
crushing will increase as the particle size is reduced.
8.Stickiness:
A sticky material will tend to clog the grinding equipment
and it should therefore
be ground in a plant that can be cleaned easily.
9.Soapiness:
In general, this is a measure of the coefficient of friction of
the surface of the material. If the coefficient of friction is
low, the crushing may be more difficult.
10.Explosive:
Such materials must be ground wet or in the presence of
an inert atmosphere.
11.Materials yielding dusts that are harmful to the
health:
Such material must be ground under conditions where the
dust is not allowed to escape.
Energy utilization
 One of the first important investigations into the
distribution of the energy fed into a crusher was
carried out by OWENS who concluded that energy
was utilized as follows:
 In producing elastic deformation of the particles before
fracture
occurs.
 In producing inelastic deformation which results in size
reduction.
 In causing elastic distortion of the equipment.
 In friction between particles, and between
particles and the machine.
 In noise, heat and vibration in the plant, and
 In friction losses in the plant itself.
 Owens estimated that only about 10 per cent of the
VARIOUS EQUIPMENTS
USED INREDUCTION
SIZE
Size reduction is a process of reducing large solid unit
masses, coarse particles or fine particles.
Size reduction may be achieved by two methods:
1 Precipitation
2 Mechanical process

1.Precipitation method: Substance dissolve in appropriate

solvent.

2.Mechanical process: Mechanical force is introduce

by using different equipments like ball mill, colloid mill
etc.
 CLASSIFICATION OF SIZE REDUCTION EQUIPMENT

D. Cutting machine,
A. Crusher ex- cutter mill.
Ex-edge runner mill,
end runner mill.

B. Grinder
(1)Impact mill. ex-hammer mill.
(2)Rolling-compression. ex-roller mill.
(3)Attrition mills, ex-attrition mill.
(4)Tumbling mills. ex-ball mill.

C. Ultrafine grinder,
ex-fluid energy mill.
 MORTAR AND PESTLE

 This is the classical and the simplest equipment for grinding.

 It is work on application of attrition and pressure.

 In this equipment both mortar and pestle are rotating.

This equipment cannot be provided with a sieve for.

continuous removal of fines.

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 Construction & Working
• A hammer mill is essentially a
steel drum containing a vertical
or horizontal rotating shaft or
drum on which hammers are
mounted.
• The hammers swings on the
ends or fixed to the central
rotor.
• The rotor is rotates at a
high speed inside the drum
while material is fed into a
feed hopper.
• The material is put into the
hopper which is connected
with the drum.
• The material is powdered
to the desired size due to
 ADVANTAGES OF HAMMER MILL
• 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.
DISADVANTAGES
• Heat buildup 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
• These are also knows as tumbling mills.
MILL
• Principle: The ball mill works on the principle of impact
between the rapidly moving balls and the powder material,
both enclosed in a hollow cylinder.
• Working: At low speeds, the ball roll over each other and
attrition (rubbing action) will be the predominate mode of
action. Thus, in the ball mill, impact or attrition or both are
responsible for the size reduction.
 Advantages
• It can produce very fine powder.
• Ball mill is used for both wet and dry grinding
processes.
• Toxic substances can be ground, as the cylinder is
closed system.
• Rod or bars can also be used as grinding media.
• (example: Sticky material are size reduced) In ball
mill,
• Disadvantages
installation, operation and labour costs are low.

• The ball mill is a very noisy machine.

• Ball mill is a slow process.
• Soft, fibrous material cannot be milled by ball
mill.
 Working

 Powder is introduced through the inlet of venture.

Air introduce through the grinding nozzles transport the
powder in the circular track of the mill.
The turbulent air stream break the particles colloids with
each other and break.
Particles are carried out to outlet and the coarse particle
undergo recirculation.

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Fluidized bed jet Mill
21
 Working:

The material to be ground is

placed in the mortar. The mortar
revolves at a high speed. The
revolving mortar causes the pestle
to revolve during this process,
size reduction is achieved.
 General characteristics of various types of mills

MILL ACTION PRODUCT SIZE USE FOR NOT USED FOR

Hammer Mill Impact 4 to 325 mesh almost all drugs Abrasive material

Ball Mill Attrition and 20 to 200 mesh Brittle drugs soft materials
impact
Fluid Energy Mill Attrition and 1 to 30 microns Moderately hard soft and sticky
impact and friable materials
materials
Edge & end Crushing and 20 to 80 mesh
Runner Mill shearing almost all drugs sticky materials