Particle size is one of the most important factors affecting the process
ability of powder including theirmixing, flow and compaction properties.
Particle size reduction is significant to enhance surface area and
flowproperty. Larger sized particles face issues with solubility as they
require more time for dissolving. In certainspecial cases particles are not
soluble in any media. In those cases, particles are to be dissolved in a
suspensionof particle size same as that as that of the dissolving
particles. The importance of particle size in absorption isthat with the
decrease in particle size, there is an increase in surface area of particle
which leads to increasedabsorption itself.
The process of reducing larger solid unit masses using various techniques
like jaw crusher, gyratorycrusher, roll crusher, hammer mill and ball mill
into smaller or finer particles is called as size reduction. The principle
of attrition and impact is mainly used here.
The process of size reduction, also known as comminution or diminution or
pulverization is chieflyachieved by two methods – precipitation and
mechanical. Precipitation involves the use of dissolution inappropriate
solvent and mechanical process involves subjugation to mechanical forces
using grinding machines.The former is used mainly for the production of
bulk drugs or inorganic raw materials and chemicals likemagnesium carbonate
or calcium carbonate.
Size reduction is influenced by various factors. These include various
physical properties like moisturecontent, hardness, stickiness, toughness,
slipperiness, abrasiveness etc. Other material properties like coarse
andbulk density of the product, material structure, flow, shape and size
also influence size reduction.
The main purpose of size reduction is to increase the surface area of the
particles. Other advantages ofsize reduction include enhanced and uniform
mixing of powders due to the narrow size range of the particles,rapid rate
of absorption, reduced sedimentation rate, improved physical appearance and
increased stability inthe case of emulsions.
Mechanisms of Size Reduction
The mechanisms have demonstrated that stresses of varied nature are
required to achieve sizereduction.The common mode ofsize reduction are
explained as follows [Table 1].
General Parts of Size Reduction Equipment
A size reduction equipment has three main components, namely Hopper,
Milling chamber and
Discharge chute (Figure 9).
Figure 9: General Parts of Size Reduction Equipment (Three basic
Advantages of size reduction
Improved mixing and minimized segregation
Improved chemical reactivity
Improved surface area
Rapid dissolution and increased absorption of drugs in the case of drugs
Slower rate of settling and creaming in the case of suspensions and
Less gritty cosmetic products.
Types of Size Reduction
Size reduction is a unit operation process and the operations include
grinding, compression and impactforces. The types of forces involved are
compression, impact and attrition forces.
During size reduction the material subjected is under due stress and the
resulting internal strains leads todeformation or fracture of the material.
The factor that affects the degree of deformation includes the hardnessof
material, structure of materials, the amount of heat generated in the
material and its tendency to crack. Also,harder materials require higher
energy to fracture. The nature of materials determines the type of impact
itrequired for size reduction. For example, softer materials require
shearing forces and a combination of shearingforces and impact is necessary
for fibrous materials.
Some of the common mechanical size reduction methods are listed as follows:
Equipment for cutting:
Extensively used in food industry for cutting of vegetable or meat
products. Abowl chopper is a common example.
Equipment for milling:
These include both grinders and crushers. Grinders make use of impact
andshear forces for size reduction. Examples include hammer mills, roller
mills and hammer mills. Crushers can becommonly seen in usage in sugar cane
industry where, sugar cane is subjected to compression forces.
Equipment for homogenization:
In homogenization, two immiscible liquids are combined together intoan
emulsion thereby dispersing the particles. During this the liquid is forced
through a narrow nozzle resultingin shear, impact, turbulence and
compressive forces acting on the particles in the liquid.
Size reduction laws:
1. Kicks law
states that the energy required to reduce the size of particles is directly
proportional to the ratio ofthe initial size to the final size of the
2. Rittinger’s law
states that the energy required for size reduction is proportional to the
change in surface areaof the pieces.
3. Bond’s law
states that the work required to form particles of size Dp from very large
feed is proportional tothe square root of the surface to volume ratio of
Equipment for size reduction:
1. Jaw crushers
The jaw crusher is V shaped consisting of two jaws – one movable and the
other fixed making an angleof 10-20 ° between them. The materials for size
reduction are introduced between the two jaws at the top andcompressive
force is exerted by the movable jaw. The size reduction happens from top to
bottom with largerlumps in the upper part of the jaw and successively
broken and plunged in to the lower narrower portion below.
Further, the cracked pieces are reduced at the next stroke. Normally, the
jaw crusher is operated at 300 timesper minute.
2. Gyratory crusher
The solid materials are fed between circular jaws. The solid materials are
introduced into a V–shapednotch between the metal casing and the head. Size
reduction happens in this V-section where the materials arecaught up and
the comminuted particles pass out from the bottom. The speed of operation
varies from 100 to450 gyrations per minute. The gyratory crusher has added
advantages like lower energy consumption andreduced maintenance compared to
3. Roll crushers
Roller crusher consist of two equal sized and smoothened rolls, one
rotating in clockwise and the otherin anti-clockwise direction with the
same speed. The two rollers are sufficiently heavy, rotating on the
sameparallel horizontal axes. The rolls can also be jagged depending on the
intended need. The coefficient of frictionbetween the material to be size
reduced and the surface of roller is the primary factor influencing the
extent ofsize reduction. Larger particles can also be accommodated in a
roll crusher as the size reduction is influenced byimpact, shear and
compression – all acting together.
4. Hammer mill
Hammer mill consist of a rapidly moving hammer connected to a high-speed
rotor in a cylindricalcasing. Hammers upto 4 numbers are mounted on a
horizontal shaft. It works on the principle of impactbetween the hammer and
the particles to be size reduced. In hammer mill, the materials are
introduced throughthe top of metal casing and are directly exposed to the
hammer in operation thereby leading to size reduction.Later, the particles
pass through a screen at the bottom and are collected at the receiver. The
critical factorsaffecting size reduction includes – feed rate, size of the
screen and rotor speed. A hammer mill is typicallyoperated between
5. Ball mill
The ball mill consists of hollow cylindrical rotating shell made of steel
lined with porcelain or highcarbon steel plate. Upto 50 % of its volume,
the shell is filled with balls made of steel or pebbles. The balls areof
constant weight and varying size which depend on the amount of feed. The
size reduction happens due to thegrinding of the balls against the material
to be comminuted. The particle size and shape of the material to
becomminuted depends on the size of the ball, speed of rotation of the
shell and feed rate. When the shell isrotated, centrifugal forces inside
the shell carry the ball along the mill wall and get dropped whenreaching a
height due to gravitational force. This ensures the grinding of the
material. Some of the added advantages ofball mill includethe fine grinding
of a large spectrum of materials and grinding of toxic substances due to
theclosed environment in a ball mill.
Advances in Size Reduction Technologies
In micronization, high pressure gas or air are exposed to particles thereby
causing particle collision.Micronizing is the process of reducing the
particles to less than 20 µm. This process is particularly effective inthe
pharmaceutical industry leading to enhanced bioavailability, solubility and
optimized formulation with rightmix of the excipients.
Low friction shearing is the basic concept behind Gran-U-Lizer technology.
Designed by the ModernProcess Equipment Corporation, USA, the method
involves shearing of the particles between two rolls. Thistechnology can
reduce particle size between 4760 to 50 µm. It is widely used in various
food millingapplications like – coffee, pepper, salt, sugar, rice etc.
Gran-U-Lizer technology is also utilized in theminimization of various
Jet-O-Mizer particle size reducer
Designed and manufactured by the Fluid Energy Processing and Equipment
Company, USA. The Jet-O-Mizer is versatile machinery as along with size
reduction it also blends the ingredients, free entrappedsolvents and coats
the products with waxes/oils thereby saving time and energy. The
Jet-O-Mizer is capable ofproducing particle size in the range of 0.5 to 45
Jet-Stream Homogenization (Micro fluidization)
Developed by Microfluidics International, USA, this method is based on the
jet-stream principle. Theparticle size reduction is brought about by the
particle collision and cavitation due to the introduction of jetstream at
high pressure through the interaction chamber. Along with particle size
reduction, they avertagglomeration of the particles resulting in the
production of products with longer shelf life.