Particle Size Reduction – Pharma Notes


Size Reduction

Size reduction is a process of reducing large solid unit
masses into small unit masses, coarse particle or fine particles.

Size reduction process is also referred to as commination
and grinding.

When the particle size of solid reduced by mechanical means
it is known as milling.

Size reduction operation divided into two category –

Grinding & Cutting – If material is solid

Emulsification & Atomization – if material
is liquid


Dissolution and therapeutic efficacy


Flow ability



Mechanism of size

Impact – this
involve hammer or bar at high speed (hammer mill). Particle Size 50-8,000 µm.

particle crushed b/w rollers by the application of force (roller mill). Particle
Size 50-10,000 µm.

Cutting –
the material cut by a sharp blade (cutter mill), Particle Size 100-80,000 µm.   

arising from particles scraping against one another or rubbing action (colloidal
mill, roller mill). Particle Size 1-50 µm

Impact and
Attrition –
ball mill, fluid energy mill, Particle Size 1-2,000 µm.   

Advantage of size

Content uniformity

Uniform flow

Effective extraction of drug

Effective drying

Improve physical stability

Improve dissolution rate

Improve rate of absorption

Increase surface area and viscosity

Facilitates bioavailability, uniform mixing and

Disadvantage of size

Drug degradation

Poor mixing


Factors affecting
size reduction

– Hardness, toughness, abrasiveness, stickiness, material structure,
bulk density, physiological effect Particle shape, Polymorphism, Temperature,
and Feeding rate.

Hardness – It is
easier to break soft material than hard materials. Ex: For iodine hammer mill
is used.

Fibrous – These are
tough in nature. A soft, tough material has more difficulty than a hard,
brittle substance. Ex: Raowulfia, Ginger. Here cutters can be used.

Elastic / Sticky –
Become soft during milling. Ex: synthetic gums, waxes, resins. Low melting
substances should be chilled before milling. These are milled using hammer,
colloid or fluid energy mill.

Melting point –
Waxy substances, fats and oils are softened during size reduction due to heat
generated. This is avoided by cooling the mill and the substance.

Hygroscopic –
Certain substances absorb moisture content rapidly. This wet mass hampers the
milling process. Ex: Potassium carbonate. Closed system such as porcelain ball
mill is used.

Moisture content – presence
of more than 5% moisture influences hardness, toughness, stickiness of substance,
in general material having moisture content below 5% are suitable for dry
grinding and above 50% for wet grinding.

Purity required
the size reduction of such hard substance leads to the abrasive wear of milling
parts, causing contamination.

Laws governing size

• Griffith Equation –
The amount of force to be
applied depends on the crack length.

• Kicks Law – Work
required to reduce the size of a given quantity of material is constant for the
same reduction ratio regardless of the original size.

• Rittinger ’s Law – Worked
use for particulate size reduction is directly proportional to the new surface

• Bond’s Law – Worked
used to reduce particle size is proportional to the square root of the diameter
of the particle produced.

Classification of

Hammer mill

Ball mill

Fluid energy mill or ultra-fine grinder or

EDGE Runner Mill

END Runner Mill

Quadro co mill

Multi Mill

Roller Mill

Rotary Mill


Hammer mill


It operates on the principle of impact between
rapidly moving hammers mounted on rotor and the stationary powder material.


Consists of a metal casing, enclosing a central
shaft, to which 4 or more swinging hammers are attached.

Lower part of casing consists of a screen,
through which material can pass and collected in a suitable receiver.

Construction &

A hammer mill is essentially a steel drum
containing a vertical or horizontal rotating shaft or drum on which hammers are

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 fast rotation of hammers and is collected under the screen

Impact mill using high speed rotor up to 10000
rpm with a peripheral speed up to 7600 meter per minute

Dry materials, wet filter press cake, wet
granules, ointments and slurries

Size reduction up to 20 to 40µ

Circular hole design for fibers and herringbone
design for crystalline materials and continuous operation jump- gap screen for
abrasive and clogging materials.

Micro pulverizer for nonabrasive materials

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