2. Introduction
Definition- Size reduction 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.
4. Objectives
Improvement in dissolution and absorption of drug
Effective extraction of drug
Increased therapeutic effectiveness of drug
Effective mixing of material
Improved physical appearance
Improved physical stability
5. objectives
Content Uniformity- Mixing becomes effective if particle size
is uniform and small. 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 drying- Rate of drying increases as size
reduction increases the surface area.
8. Mechanism
Impact —particle concussion by a single rigid
force/pressure (hammer). Moving objects strikes particle
surface- particle strikes stationary surface- Hammer mill
Attrition —arising from particles collision/scraping against
one another or against a rigid surface. Caught between
two moving surface – Fluid energy mill
Cutting —produced when the particle is compressed
between the edges of two hard surfaces moving
tangentially. Strong force by sharp object on a narrow
area. Lab scale- blade, knife. Large scale- Cutter mill.
Waxy substances- during semisolid preparations.
Compression—particle disintegration by two rigid forces
(nutcracker). Crushed by application of pressure- Mortar
pestle
9.
10. Theory and laws governing size
reduction
Griffith theory
Rittinger’s theory
Kicks theory
Bonds theory
Relationship between Stress and
Strain
11. Griffith theory
According to Griffith theory, all solids have some structural
weakness or flaws in them.
A flaw developes into crack under strain
Amount of force to be applied depends on crack length &
focus of stress at the atomic bond of crack apex.
12. laws governing size
Energy required to reduce the size of particles
is inversely proportional to the size raised to
the some power.
dE
d
D
=
C
-D n
Where E= Amount of energy required to produce a
change
D = Size of unit mass
C, n = Constants
13. Integrating equation (1),
E= C ln(di/dn)
(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
14. Rittinger 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
Si = initial surface area
Sn = new specific surface area
KR = Rittinger’s constant.
E= amount of energy.
15. Bonds theory
States that energy used in crack propagation is
proportional to new crack length produced.
crack propagation- deforming or fracturing set of
particles
new crack length produced- change in particle
dimension
KB – work required to reduce unit weight from infinite size to 100
17. Factors Affecting Size
Reduction
Factors related
to the nature of
raw material
Hardness
Toughness
Abrasiveness
Stickiness
Softening
temperature
Material
structure
Ratio of feed size
to product size
Factors related to
the nature of
Finished product
Particle Size
Ease of Sterilization
Contamination of
milled material
Factors related to
safety and economy
18. Hardness
Hardness:
An arbitrary scale of hardness has been devised known as
Moh’s Scale;
Moh’s Scale =1 is for graphite
Moh’s Scale <3is for soft
material
Moh’s Scale >7 is for hard
material
Moh’s Scale =10is fordiamond
The harder the material the
more difficult it is to reduce in
size
19. Toughness and Abrasiveness
Toughness:
Toughness of material is more important
Fibrous drugs: moisture content
Abrasiveness:
Property of Hard material (particularly those of mineral
origin)
Grinding of abrasive material leads to contamination of
finished product.
20. Stickiness
Material may adhere to grinding surfaces
The meshes of the Screen may be choked
If the material used for size reduction produces heat then
substance that are gummy or resinous may be
troublesome to the size reduction process
Sometimes the addition of inert substance may
sometimes assistance such as kaolin to sulphur.
21. Softening temperature &
Slipperiness
Softening temperature:
Heat generated during grinding cause some substance to be
soften.
Waxy substances, such as stearic acid, or drugs containing
oils or fat
Slipperiness:
The reverse of stickiness
The material acts as a lubricant and lowers the efficiency to
the grinding surfaces.
This also give rise to size reduction difficulties
22. Material structure & Moisture
content
Material Structure:
Some substances are homogeneous in character, but the
majority shows special structure.
eg. Mineral substances, vegetable drugs.
Moisture content:
Moisture content influence the hardness, toughness,
stickiness.
Usually, < 5% of moisture is suitable if substance is to be
ground dry or
> 50% if it is being subjected to wet grinding.
23. Physiological effect and purity
Physiological effect:
Some substances are very potent
Small amount of dust may have effect on operator
Enclosed mills used to avoid dust
Purity required:
If high degree of purity needed then cleaning between
the batches is to be done
24. Ratio of feed size to product
size
Machines that produces a fine product require a small
feed size
25. Factors related to nature of finished
product
Particle Size: Griseofulvin
Ease of Sterilization: Parentrals and ophthalmics
Contamination of milled materials: For potent and low
dose drugs
Factors related to safety and economy: Safety,
irritability and toxicity
* Cost
* Power consumption
* Space occupied
* Labour Cost
27. General parts of size reduction
equipment
Hopper
Milling chamber
Receiver
Other accessories
Sieves/ Screens
Cyclone separators
Dust Collectors
Cooling devices
28. Hammer Mill
Principle: Impact
Range :40 to 8000 µm
Construction:
Hopper
Jacket wall
Central shaft
Hammers ( 4 or more)
Screen
Receiver
Working
29. Hammer mills
In a hammer mill, swinging hammer heads are attached to a
rotor that rotates at high speed inside a hardened casing.
The hammer mill consists of a steel casing in which a
central shaft is enclosed to which a number of swinging
hammers are attached.
When the shaft is rotated the hammers swing out to a radial
position.
On the lower part of the casing a screen of desired size is
fitted which can be easily replaced according to the particle
size required.
The material is crushed and between the hammers and the
casing and remains in the mill pulverizeduntil it is fine
enough to pass through a screen which forms the bottom of
30. 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 of the cross freely or fixed to the
central rotor. The rotor is rotates at a high speed (8000-15000rpm)
inside the drum while material is fed into a feed hopper.
The material is impacted by the hammer bars and expelled
through screens in the drum of a selected size.
Both brittle and fibrous materials can be handled in hammer mills,
though with fibrous material, projecting sections on the casing may
be used to give a cutting action.
The hammer mills are available in various designs and shapes.
In pharmaceutical industry they are used for grinding dry materials,
wet filter cakes, ointments and slurries
31. Advantages
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,
32. Disadvantages
The high speed of operation causes generation of heat
that may affect thermolabile materials or drugs containing
gum, fat or resin. The mill may be watercooled to reduce
this heat damage.
The rate of feed must be controlled carefully as the mill
may be choked, resulting in decreased efficiency or even
damage.
Because of the high speed of operation, the hammer mill is
susceptible to damage by foreign objects such as stones
or metal in the feed. Magnets may be used to remove iron,
but the feed must be checked visually for any other
33. Ball mill
Principle: Impact & Attrition
Range :1 to 100 µm
Construction –
Hollow cylinder
Metallic frame
Inner wall-
Balls
Working
34. Working
a) slow- low
centrifugal force–
balls roll over each
other– only attrition
b) very high-high
centrifugal force–
balls thrown to wall-
only compression
c) correct- moderate
centrifugal force- balls
lifted to top & cascade
at bottom- both impact
& attrition
Ball mill depend on Speed
35. Uses and advantages
Use:
for fine grinding, 100 to 5 mm or less
for abrasive material
for dyes & pigments
Advantages:
Installation, operation and labour costs are low
Efficient for dry & wet grinding
Inert gas, unstable & explosive material
Suitable for grinding toxic substances
for ophthalmic and parental products sterility can be achieved
can be adopted to continuous process
Installation & operation - economical
36. Disadvantages:
Noisy
Not Suitable For Sticky
Wear- Contamination
Slow Process
Soft, fibrous material cannot be milled
37. Fluid energy mill
Principle: Impact & Attrition
Range : 1 to 20 µm
Construction
• Hollow elliptical tube-
Height 2 m,
Diameter – 20-200 mm,
Inner surface- stainless steel or ceramic.
• Feeder – Near bottom
• Fluid inlet jets – compressed air- 600 kilopascal to 1 mega
pascal
• Classifier (Bag filters / cyclone separator) – Air escape /
fine particles
38. WORKING
Powder is introduced through the solid inlet
fluid enters, the suspended particles colloid
with each other and break.
Attrition (particle collide with each other) &
Impact (particle collide with casing of mill)
Small particles carried to outlet and
removed by cyclone separator or filters
The coarse particles undergo re-circulation
in the chamber until size reduced sufficiently
39. Uses:
For hard particles
for ultrafine grinding
Strict control of particle size
Examples are antibiotics, sulphonamides and vitamins.
40. Advantages Disadvantages
Smaller particle size produced
than other methods
Expensive
Upto 6000 kg of feed is
milled/hour
Not suitable for milling soft,
tacky and fibrous material
No wear of the mill so no
contamination
-
Thermolabile substance can be
ground
-
Less maintenance -
41. Edge runner mill
Principle: compression
Range: 50 to 10000 µ
Construction:
2 heavy rollers
Bed containing feed of material
Shaft
Scraper
42. Working
Material placed on bed (mortar)
Wheels (rollers) are rotated and move along the bed
Material crush by compression
Scrappers redirect in zone
Effective size reduction takes place
43. USE:
• For tough material & those obtained from natural sources.
Advantages:
• Material produced- high density - not segregate easily
• Easy to operate
• Material produced- spherical
Disadvantages:
• Not useful for sticky material
• Mill requires more space
• Slow milling/ Time Consuming
• Energy consumption is quite high
• Contamination possible
44. End Runner Mill
Principle: Compression & attrition
Range : 5 to 10000 µ
Construction
Mortar & pestle – steel or porcelain
Pestle- Heavy – weight - crushing
Scrapper
45. WORKING
Material placed at bottom of mortar
The pestle is placed in the mortar
Mortar revolves with high speed
Cause pestle to revolve- attrition
Weight of pestle - compression
Scrapper- keep material in zone
Powder is collected and passed
through sieves to get desired size.
46. USE:
For fine grinding
For natural products
Advantages:
The mill is suitable for dry as well as wet grinding.
Disadvantages:
Not useful for sticky material
Mill requires more space
Slow milling
