Size reduction is the process of reducing larger particles into smaller particles using external forces. The key mechanisms of size reduction are cutting, compression, impact, attrition, and a combination of impact and attrition. Different types of mills use these mechanisms, including hammer mills, ball mills, fluid energy mills, edge runner mills, and end runner mills. Factors like hardness, toughness, stickiness, softening temperature, and moisture content affect how easily a material can undergo size reduction. Laws of Rittinger, Kick, and Bond govern the energy requirements for size reduction.

1. SIZE REDUCTION
Size reduction is the operation carried out for reducing the size of bigger
particles into smaller one of desired shapes & size with the help of external
forces.
Comminution is another term used for size reduction. The row materiel is
generally present in the large size which cannot be used by the industries so, the
materiel needs to be converted into smaller particle or powder.
Conversion of large materiel into smaller sized particles is called as “size
reduction”.
OBJECTIVES OF SIZE REDUCTION:
In the materiel process industries size reduction or commination is useful carried
out in order to-
01. Increases the surface area because in most reactions involving solid particles
the rate of reaction is directional to the ratio area with the second phase.
02. A materiel into very small particles in ordered to separate the valuable
substance among the two constituents.
03. Pharmaceutical capsules, insufflation & suppositories & ointments require
smaller particle size.
04. To allow the rapid penetration of solvent in crude drugs to extract
phytochemicals.
05. In case suspensions, fine particles size is important because it reduce the
rate of sedimentation.
06. To improve the handling characteristics, to mixed solid particle more
ultimately.

2. MECHANISM OF SIZE REDUCTION
Size reduction involves the following mechanisms:
01. Cutting. 04. Attrition.
02. Compression. 05. Combination of impact & attrition.
03. Impact.
01. Cutting = The materiel is cut by sharp blade. It is useful for size reduction of
fibrous or very solid.
Ex. Cutter mill.
02. Compression = In this mechanism the size reduction is achieved by
crushing the materiel by application of pressure.
Ex. Roller mill.
03. Impact = Impact occurs when the materiel is kept stationary & is hit by an
object moving at the high speed or when the materiel is kept moving at
high speed against a stationary object.
Ex. Hammer mill.
This action continuous until particles of required size are obtain.
04. Attrition = It involve collision between the two-particle having high kinetic
energy or a high velocity particle with a stationary phase.
Ex. Roller mill & fluid energy mill.
05. Combination of impact & attrition = this type of mechanism involves both
effects i.e. impact & attrition.
Ex. Ball mill, is used in the combination of impact & attrition.

3. LAWS GOVERNING SIZE REDUCTION:
They are used to predict energy requirement for size reduction.
01. Rittinger’s law.
02. Kick’s law.
03. Bond’s law.
04. Holmes law.
05. Harris law.
01]. Rittinger’s law –
The energy required in crushing is proportional to the new surface crated as a
result of particle fragmentation.
i.e. Energy  New surface
Energy = [ K .1/D2 – 1/D1]
Where, D1 & D2 is the particle size at start & end of the process respectively.
K = KR . Kc
Where, KR is Rittinger’s constant & fc is crushing strength of material.
02]. Kick’s law =
The energy required to reduce the size of particle is proportional to the square
root of the diameter of the product particles.
E = Kk In [d1/d2]
E = energy required.
KK = kick’s constant.
D1 = average initial size of particle.
D2 = final particle size.

4. 03. Bond’s law =
The total work input required to reduce particle size is proportional to the square
root of the diameter of the produce particles.
W = 10 Wi 1/d2 – 1/d1
Where,
W = is energy consumed.
D1 = is size of feed.
D2 = is the size of product.
WI = is the bond index.

5. Factor Affecting Size Reduction:
01). Hardness – It is the surface property of the materiel. It is possible for a
materiel to be very hard but if it is brittle also then size reduction may have no
special problem.
“Moh’s scale” is also known as scale of hardness.it is used which has been given
the hardness number between 1-10. Up to 3 are known as soft & above 7 are
hard. while it is between 3-7 are intermediate.
02). Toughness – Some time it is more important than harness, a soft but tough
materiel may cause more problem in size reduction then a hard but brittle
substance break.
Ex: It is difficult to rubber then a chock.
03). Stickiness – That causes difficulty in size reduction for the materiel that may
adhere to the grinding surface or the meshes of the screen may become chop.
If the materiel used for size reduction produce heat then substance that are
gummy may be problem some to the size reduction process.
04). Softening temperature – During the size reduction process some time
heat is generated which may cause some substance to soften waxy substance
such as steric acid or drug containing oil.
Ex: That may be affected to avoid this problem pulling of mill should be done.
05). Moisture content – It contains influence a number of properties that an
affect size reduction.
Ex: Hardness, toughness & stickiness.

6. In general, the materiel should be dry or wet & not nearly dame. Usually less than
5% of moisture is suitable for dry substance & more than 50% for wet substance.

7. HAMMER MILL
Principle: The hammer mill obtained on the principle of impact b/w rapidly
moving hammer mounded on a rotated & the powder materiel.
Construction: The hammer mill can be horizontal or vertical soft type; it is usually
made up of stainless steel with impact surface made up of extremely resisting
materiel. The hammer may have several shapes. The blade of hammer can be flat
edges or shark edge. The unit is enclosed with a chamber containing removable
screen through which the materiel passes.
Working:
The hammers are allowed to be an continues motion (8000-15000 revolution per
mint.) the feed materiel is placed into hopper which flows vertically down & then
horizontal, while hammers are in continues motion. The rotating hammer’s beat
the materiel to yield smaller particle then these particles passes through the
screen & collection in the receiver.
Uses=
 It is used for grinding of powder’s which depend on speed of hammer.
 It is used to mill dry materiel, wet filter presses cakes, ointment etc.

8. Advantages-
 It is easily to setup & use.
 It occupies small space.
 It is versatile.
Disadvantages-
❖ The screen may get clogged.
❖ Hammer cannot be used to mill sticky, fibrous & hard materie.

9. BALL MILL
Principle-
Th ball mill work on the principle of impact between the rapidly moving ball & the
powder materiel, both enclose in a hollow cylinder. At low speed the ball role
over each other attrition will be predominant mode of action. Thus, in boll mill
impact or attrition or both are responsible for size reduction.
Construction:
It consists of a hollow cylinder which is mounted on a metallic frame in such a way
that it can be rotated on its longitudinal access length of cylinder is greater than
its diameter. The cylinder is made up of mettle & usually lime with frame.
The cylinder contain boll the size of boll depends upon size feed. The boll is made
up of steel or iron.
Working- the drug to e ground is put into the cylinder. A fixed number of bolls are
introduced & the cylinder is closed.
The mill Is allowed to rotate on its longitudinal access. The speed of rotation Is
very important at low speed the boll role over each other & attrition will be
predominate mode of action.

10. At correct speed, the centrifugal course occurs as a result the ball are picked up
by the mill boll & carried nearly to the top where they contact with the boll & foul
to bottom. In this way impact occurs & size reduction is affective.
At very high speed the boll is through out to the boll by the centrifugal force,
hence grinding will not occurs.
Uses=
 It is used of grinding of particle size having diameter 100-5 mm
 Stainless steel was are preferred for production of ophthalmic & parenteral
products.
Advantages-
 It can produce very fine powder.
 It can be used for batch operation.
 It is midtable for both wet & dry grinding process.
 Toxic substance also be granted into small a particle.
Disadvantages:
❖ It is very noisy mill/machine.
❖ It very slow process.
❖ Where occurs from the wall as well as casing which may result in
contamination of the product

11. FLUID ENERGY MILL
Principle- fluid energy mill obtains in the principle of impact & attrition. In this
equipment the feed is suspended within a high velocity collagen between the
suspended particle.
Construction-
It consists of elliptical pipe has a Hight of about 2 meter & diameter of 20-200
mm, the ill surface is made up of stainless steel, grinding nozzles may be plays
tangential or opposed to initial flow path of powder. Compressed air of 600 kilo
pascal -1 mega pascal is used.
Working:
Powder is introduced through the inlet. The air entering to the grinding nozzles
transport the powder to elliptical pipe. The suspended particle colloid with each
other & break. The resultant small particle is carried to outlet & removed y
cyclone separator whereas the courses particle undergoes recirculation in
chamber, where they colloid again with new incoming feed.
The powder remains in the mill until its size reduced sufficiently.

12. Uses=
 This mill is used to reduce the particle size of most of the drugs such as
antibiotics & vitamins.
Advantages-
 Heat level substance can be mild.
Ex. Vitamins & antibiotics.
 It is rapid & affective method for reducing powder to 30 mm or less.
Disadvantages -
❖ This mill is not suitable for milling of soft & fibrous materiel.
❖ This equipment is expensive.

13. EDGE RUNNER MILL
Principle= The size reduction is done by crushing (compression), due to the heavy
weight of stones. Shearing course is also involved during the movement of stones.
Construction =
It consists of two heavy rollers (several tones). The roller moves on the bed which
is made up of stone. Each roller has central shaft & revolve its axis.
Working=
The materiel to be ground is placed on the bed with the help of stopper. It is kept
in the path of the stone wheel.
The stones revolve on their axis. The outer part of the wheel here to travel
greater distance then inner so that size reduction is achieved by shearing as well
as crushing. At the last powder is collected & pass through the sims.
Uses=
 It used of grinding tough materiel to fine particle.
Advantages-

14.  This mill does not require attrition during operation.
Disadvantages:
❖ This mill occupies more space
❖ It is not use for sticky materiel.
❖ The contamination of product with roller is possible.

15. END RUNNER MILL
Principle- The materiel is getting crushed due to weight of heavy pestle &
shearing force which get applied during the movement of these stones.
Construction & Working:
It can be considered as a mechanical mortar & pestle, where the mortar is shallow
& the bottom of the pestle is flat rather than round. It consists of bed of stone or
mild steel with an eccentrically placed, vertical cylindrical dumb bell-shaped roll
supported by a horizontal shaft such when the shaft is rotated, friction between
contacting surfaces of the bed & stone results in rotation of the roll & grinds the
materiel placed on the bed. A scraper forces the materiel to the grinding surface.
It can be used for size reduction of crystalline or brittle materiel.
Uses=
 It is suitable of fine grinding.

16. Disadvantages -
❖ End runner mill is not suitable for drugs, which are in unbroken or slightly
broken condition.