MEANING OF SIZE REDUCTION The operation of size reduction is carried

out in coal washeries, ore processing industries,

Size – refers to the physical dimension of an chemical industry, paint industry, cement
object. industry, food processing industry and
Reduction – refers to decrement or the process of pharmaceutical industry.
decreasing the size.
FACTORS AFFECTING SIZE REDUCTION
size reduction – is a mechanical process of
breaking down solids into smaller size particle
without altering the aggregation of solids. the • Hardness - is a surface property of the
mechanical breakdown of solids does not change material, although it is frequently
the chemical composition and only alters the confused with a property that could be
state of aggregation of solids. it is also called better described as strength. Thus, it is
communition. it is necessary if the starting possible for a material to be very hard,
material is too coarse, and the final product but if it is brittle also then size reduction
needs to be a fine powder. generally, this process may present no special problems.
is done by two methods, the precipitation, and • Toughness - is sometimes more important
the mechanical method. than hardness, a soft but tough material
• The Precipitation Method is where the may present more problems in size
substance dissolves in an appropriate reduction than a hard but brittle substance
solvent and then is finely precipitated by for example it is difficult to break rubber
the addition of another solvent. than a stick of black board chalk.
• The Mechanical Method applies the • Abrasiveness - is a property of hard
substance with a mechanical force using materials (particularly those of mineral
crushing equipment and grinding origin) and may limit the type of
equipment like a ball, hammer, or a cutter machinery that can be used. During the
mill, etc. grinding of some very abrasive
substances the final powder may be
IMPORTANCE OF SIZE REDUCTION
contaminated with more than 0.1 percent
• To increase the surface in order to increase of metal worn from the grinding mill.
the rate of a physical or chemical process. • Stickiness - is a property that causes
In most reactions and unit operations considerable difficulty in size reduction
(e.g., leaching) involving solid particles, for material may adhere to the grinding
the rate increases by increasing the area surfaces, or the meshes of the screen may
of contact between solid and second become choked. If the method used for
phase since the rate is proportional to the size reduction produces heat then
area of contact between the phases substances that are gummy or resinous
involved. may be troublesome to the size reduction
• to effect the separation of two constituents process. The reverse of this property,
in cases where one is dispersed in small which can be described as Slipperiness,
isolated pockets. for want of a better term, can also give
• to meet stringent specifications regarding rise to size reduction difficulties, since
the size of commercial products. the material acts as a lubricant and lowers
the efficiency of the grinding surfaces.
• to improve dissolution rate, solubility,
binding strength and dispersion properties. • Softening temperature - During size
reduction process sometimes heat is
• Many solid materials exist/present in sizes generated which may cause some
that are too large to be used directly. Thus, substances to soften, and the temperature
such materials must be reduced in size at which this occurs can be important.
before use. Waxy substances, such as stearic acid, or
• To achieve intimate mixing. drugs containing oils or fats are examples
• To dispose solid waste easily. that may be affected. Some methods can
• To improve the handling characteristics. be used to overcome this like cooling the
APPLICATIONS OF SIZE REDUCTION mill, either by a water jacket or by
passing a stream of air through the
 equipment. Another alternative is to use occur when a forceful particle is strike
liquid nitrogen, as discussed earlier. against a stationary object. Ex. Hammer •
• Material structure - Some substances are Attrition - Arising from particles scraping
homogeneous in character, but the against one another or against a rigid surface.
majority show some special structure, for Ex. File
example, mineral substances may have • Shear or Cutting – Produced when the
lines of weakness along which the particle is compressed between the two
materials splits to form flake-like edges of two hard surfaces moving
particles, while vegetable drugs have a tangentially. Also, the material is crushed
cellular structure often leading to long by means of sharp blade. Ex. Scissors
fibrous particles.
• Moisture content - Moisture content • Compression – Particle disintegration by two
influences a number of properties that can rigid forces. Ex. Nutcraker
affect size reduction, for example,
hardness, toughness or stickiness. In SIZE REDUCTION
general, materials should be dry or wet
and not merely damp. Usually, less than 5
percent of moisture is suitable if the MACHINE/EQUIPMENT
substance is to be ground dry or more
than 50 if it is being subjected to wet
grinding
• Physiological effect - Some substances are CRUSHERS - are slow-speed machines
very potent and small amounts of dust employed for the coarse reduction of large
have an effect on the operators. To avoid quantities of solids. Jaw crushers, gyratory
this dust, enclose mills must be used, crushers and roll crushers are different types of
systems that can extract air are also crushers. They operate by compression and can
desirable, and wet grinding also, if break large lumps of hard materials. They find
possible, as it eliminates the problem application in rockary and mining industries.
entirely.
• Jaw Crusher - Jaw Crusher: In a jaw crusher
• Purity required - certain types of size feed is admitted between two jaws, set to form a
reduction apparatus cause the grinding V open at the top. One jaw, fixed, or anvil, jaw,
surfaces to wear, and such methods must is nearly vertical and does not move; the other,
be avoided if a high degree of purity of the swinging jaw, reciprocates in a horizontal
product is needed. Similarly, some plane. It makes an angle of 20 to 30° with the
machines will be unsuitable if cleaning anvil jaw. It is driven by an eccentric so that it
between batches of different materials is applies great compressive force to lumps caught
difficult. between the jaws. The jaw faces are flat or
• Ratio of feed size to product ratio - slightly bulged; they may carry shallow
Machines that produce a fine may be horizontal groves. Large lumps caught between
necessary to carry out the size reduction the upper parts of the jaws are broken, drop into
process in several stages with different the narrower space below, and are re crushed the
equipment, for ex, preliminary crushing, next time the jaws close. After sufficient
followed by coarse grinding and then fine reduction they drop out the bottom of the
grinding. machine. The jaws open and close 250 to 400
times per minute. There are 2 types of jaw
• Bulk density - The capacity of most batch crushers; Blake Jaw Crusher and Dodge Jaw
mills depends on volume, whereas Crusher.
processes usually demand solid materials o In Blake jaw crusher, the swing jaw is
by weight. Hence, all other factors being
pivoted at upper position, the greatest amount of
equal, the output of the machine is
motion is at the bottom which means less
related to the bulk density of the
choking.
substance.
o In Dodge jaw crusher, the swing jaw is
pivoted at bottom position, it has a greater
MECHANISMS OF SIZE REDUCTION tendency to choke
• Impact - In this, the substance is subjected
to hammers or bars at high speed. Impact also
 the shaft and turns slowly because of friction
with the material being crushed. The speed of
the crushing head is typically 125 to 425
gyrations per minute. Gyratory crusher also
BLAKE JAW DODGE JAW CRUSHER works on the principle on compression.
CRUSHER
Roll Crusher - Roll crushers apply shear or
movable jaw is movable jaw is pivoted at the
compression to the material that is going to be
pivoted at the top bottom
reduced in size. This action occurs between two
maximum maximum movement is at the top hardened surfaces. The speed and diameter of
movement is at the the rolls, the desired capacity, and the feed and
bottom product size distribution are all critical in the
proper sizing and operation of this type of
no tendency tendency to choke (no freedom machine.
to from choking)
choke/clog GRINDERS – Grinding means sub-dividing the
(freedom solids to a finer product than crushing. The size
from reduction machines employed for an intermediate
choking) duty are referred to as Grinders. A grinder is
suitable for high suitable for low production rates often charged with the product from a crusher
production rates which it reduces to powder.
• Hammer Mills - Hammer mill is the most
large reduction ratio large reduction ratio is possible
is not possible
widely used grinding mill and among the
oldest. Hammer mills consist of a series
low maintenance high maintenance of hammers (usually four or more)
hinged on a central shaft and enclosed
comparatively made comparatively made in smaller within a rigid metal case. It produces size
in large sizes sizes reduction by impact.

does not give gives uniform product

uniform product • Ball Mills - A ball mill also known as
pebble mill or tumbling mill is a milling
commonly widely seldom/less widely used machine that consists of a hallow
used cylinder containing balls; mounted on a
metallic frame such that it can be rotated
DIFFERENCE BETWEEN BLAKE JAW CRUSHER AND DODGE JAW
CRUSHER
along its longitudinal axis. The balls
which could be of different diameter
• Gyratory Crusher - The gyratory crusher was occupy 30 – 50 % of the mill volume and
invented by Charles Brown in 1877. The name its size depends on the feed and mill size.
gyratory comes from the fact that while the The large balls tend to break down the
spindle is revolving around the axis of the frame coarse feed materials and the smaller
generating a conical surface, at the same time it balls help to form fine product by
rotates about its own axis due to frictional drag. reducing void spaces between the balls.
A gyratory crusher may be looked upon as a jaw Ball mills grind material by impact and
crusher with circular jaws, between which
attrition.
material is being crushed at same point at all
times. A conical crushing head gyrates inside a
• Fluid Energy Mill - A fluid energy mill is
funnel-shaped casing, open at the top. The
crushing head is carried on a heavy shaft pivoted a size reduction unit in which size
at the top of the machine. An eccentric drives the reduction results from attrition between
bottom end of the shaft. At any point on the rapidly moving particles of the material
periphery of the casing, therefore, the bottom of being ground. A source of compressed
the crushing head moves toward, and then away air or gas or high pressure superheated
from, the stationary wall. Solids caught in the V- steam that enters the grinding chamber
shaped space between the head and the casing through nozzles in the periphery at high
are broken and rebroken until they pass out the speed provides energy to the particles to
bottom. The crushing head is free to rotate on
 achieve high velocities. In fluid energy
mill there is no moving parts and no
grinding media.

COMPARISON OF CRUSHERS AND

GRINDERS

CRUSHERS GRINDERS

The solid particles are The solid

reduced in size by particles are
compression. reduced in size
by impact and
attrition.

It is aimed at breaking large It is aimed at

pieces of solid material into reducing
small lumps. crushed feed
to powders.

This operation is This operation can

performed on dry feed be performed on
material. dry as well as wet
feed.

In crushing operation, the In grinding

reduction ratio often operation, the
exceeds 6 to 8. reduction ratio as
high as 100 as
possible.

Crushers are of two Grinders are of two

types, e.g., primary types, e.g., fine
crushers and secondary grinders and
crusher. ultrafine grinders.

Crushers are heavy Grinders are relatively

duty, low speed light duty, high speed.
machines.

Energy consumption Energy

per unit mass of consumption
product is low due to per unit mass
coarse particle of product is
production. high due to
fine particle
production.
I. Separation of Solids which further belts run at right angles to the feed
Separation is extremely common in chemical belt. The first pair of balls are weakly magnetized
manufacturing, In fact, much processing and served to draw off any iron ore present. The
equipment is devoted to separate one phase or second pair are strongly magnetized and attracted
oner material from the other the wolframite, which is weakly magnetic. These
Types of Separation: machines are capable of treating 10 tons of ore a
1.1 Diffusional Separation day
a technique used for the separation of 2.3 Leaching
homogeneous mixtures. This separation includes the process of extracting substances from a
the transfer of material between the phases solid by dissolving them in a liquid, either
including distillation, crystallization and naturally or through an industrial process. In the
absorption. chemical processing industry, leaching has a
1.2 Mechanical Separation variety of commercial applications, including
A technique used for the separation of separation of metal from ore using acid, and sugar
heterogeneous mixtures. These are based on the from sugar beets using hot water.
physical differences between the particles such as III. Classification of Solids
size, shape or density. It can be applied for Particle classification is an important unit
separating solids from solids, solids from liquids process in many industries and finds demanding
and also solids from gases. applications in mineral processes, food
II. Separation Techniques processing, pharmaceutical, and toner or powder
2.1 Sieves or sifters paint industries. Particle classification is defined
are devices for separating wanted elements as a method sorting an initial distribution of
from unwanted material or for characterizing the particles to achieve a desired degree of
particle size distribution of a sample, typically uniformity, according to a chosen parameter.
using a woven screen such as a mesh or net or Classifiers are designed so that the fine and coarse
metal. Sieving is a simple technique for separating particles have different velocities or motion
particles of different sizes. Coarse particles are directions. The fine and coarse products are
separated or broken up by grinding against one- collected according either to time or to the
another and screen openings. Depending upon the locations in the classifiers. A classifier is usually a
types of particles to be separated, sieves with complex system since a number of forces can be
different types of holes are used. Sieves are also involved in determining trajectories of particles.
used to separate stones from sand. Sieving plays These forces include drag forces by fluids,
an important role in food industries where sieves gravitational forces, centrifugal forces, and so on.
(often vibrating) are used to prevent the IV. Classification Techniques
contamination of the product by foreign bodies. Classifiers are traditionally grouped into
2.2 Mechanical Screening wet and dry classifiers. The difference between
often just called screening, is the practice dry and wet methods is the medium of suspension
of taking granulated ore material and separating it being used. Wet classifiers use liquids as the
into multiple grades by particle size. This practice medium of suspension while dry classifiers use
occurs in a variety of industries such as mining gases. However, several technologies can be
and mineral processing, agriculture, operated under either dry or wet conditions.
pharmaceutical, food, plastics, and recycling. A Classification techniques can be also classified
screening machine consist of a drive that induces into mechanical and non-mechanical classifiers
vibration, a screen media that causes particle regardless the working medium. Mechanical
separation, and a deck which holds the screen classifiers incorporate moving parts which
media and the drive and is the mode of transport influences the motion of particles. Non
for the vibration mechanical classifiers use fluid drag only to
2.3 Magnetic Separation separate particles.
Magnetic separation is a process in which V. Wet classifiers
magnetically susceptible material is extracted Wet classification refers to the process of
from a mixture using a magnetic force. This separating particulates in a liquid suspension into
separation technique can be useful in mining iron fractions according to particle size or density by
as it is attracted to a magnet. In the machine, the methods other than screening. Wet classifiers
raw ore is fed onto a conveyor belt which passes work as a result of difference in settling rate
underneath two pairs of electromagnets under between fine and coarse particles. The basic
principles of classification is that fine particles classification uses air as the working medium, it
have a slower settling velocity than coarse often refers to air classification. As a complement
particles of same density or light particles have a of sieving, air classification can separate smaller
slower settling velocity than heavy particles of or larger particles than commercial sieving sizes.
same size. The applications of wet classifiers were 6.1 Elutriation
found in the treatment of raw materials for Elutriation is a process of washing fines by
example effecting a simple sand-slime separation air while allowing the coarse product settling. The
resulting in two products. bulk of unclassified powders are usually
introduced to the elutriator from the middle inlet.
The air stream flows upwards and washes the fine
5.1 Sedimentation Classifier particles from the bulk. The lighter or finer
Spiral classifiers and rake classifiers are particles are lift against gravity to a fines collector
two types of sedimentation classifiers and are overhead. The denser or larger particles are too
mostly used in separating coarse particles from a heavy to be raised and fall against airflow into the
mixture. A typical spiral classifier consists of a coarse collector below. The cut size is controlled
sloping elongated round bottom tank and a sand- by varying the gas velocity.
raking spiral. The raw materials are fed to the 6.2 Fluidized bed classifiers
central section of the pool and flow to the weir Segregation may occur when a binary
while the suspended particles settle down. Four mixture is fluidized in a fluidized bed. Measures
zones can be identified in the poor, which are are taken to prevent segregation, which makes
stationary zone, moving zone, hindered settling classification by fluidized bed unusual. This is
and free settling zone. The cut size depends on a because segregation is often incomplete under
number of parameters including the height of the normal operations. In order to make classification
weir, angle of tank slope, viscosity of the mixture, processes feasible, some modifications to the
and so on. Secondary fresh water may be fluidized bed must be made to enhance the degree
supplemented to clean the coarse fraction before of particle segregation.
discharging. Spiral classifiers produce coarse 6.3 Cross flow classifiers
particles in good quality while they are generally Designed so that the direction of airflows
not used in separating fine particles. The rake is perpendicular to the gravity. The gas is injected
classifiers have almost similar configurations with to the classifier horizontally from the inlet on the
spiral classifiers. The only difference is that the left wall. The material inlet is nearby the gas
rakes move following a rectangular direction in nozzle and the powders are fed downwardly into
rake classifiers. the classifier. The particles are spread to a fan-
5.2 Hydraulic Classifier shape in the chamber. The particles are separate
A classifier in which particles are sorted since the coarse powders and the fine powders
by specific gravity in a stream of hydraulic water have different trajectories in the separation zone
that rises at a controlled rate; heavier particles due to the fluid drag forces and gravitational
gravitate down and are discharged at the bottom, forces. The coarse powders settle quicker than the
while lighter ones are carried up and out. Also fines. By inserting the several plates at certain
known as hydrosizer. distances to the gas inlet, the classified particles
5.3 Hydro clones are collected into fractions
Commonly called a Cyclone. Unlike the 6.4 Cascade air classifier
others it has no moving parts and is worked in The Zigzag classifiers consist of several
conjunction of another piece of equipment, a inclined branch pipes vertically arranged to form a
pump. Unlike the rake and spiral classifiers, the zigzag separation zone. These inclined pipes have
overflow of the cyclone is the exit point for the rectangular cross-section and are inclined
fine material instead of the other way around as it alternately to the left and to the right at the same
is in the case of the other two. Extending from the angle. An inlet is opened at the top or the middle
overflow discharge into the body, which is the of the classifier in order that the materials to be
feed entry point of the cyclone, classified are fed into it. The coarse fractions fall
VI. Dry classifiers to the bend where they need to cross through the
Dry classification is widely used in many classifying airflow coming from below. The
industrial processes. In comparison to wet separation of fines takes place at each bend and
classification, dry classification does not need several bends are required to increase the
drying and slurry treatment. When dry sharpness of separation.
6.5 Inertial air classifier
These classifiers separate fine powders
from 40 to 400 µm. Both particles and airflow
enter the classifier from the top and flow
downwardly. An outlet is set on the sidewall and
inclines by 45 degrees. The fine particles are
carried by air and discharged from this outlet. The
coarse particles proceed straight down by inertia
and collected below. A secondary air is introduced
just below the fines outlet and the airflow washes
the remaining fine particles from the falling coarse
fraction, particularly dislodging the adhering fines
from the surface of the coarse particles. A
curvilinear chamber is provided to direct the
secondary airflows to the fine’s outlet.
6.6 Vortex air classifier
Belong to centrifugal classifiers. It is the
first industrial design which separates particles at
size cut about 10 µm. An advantage of the vortex
air classifiers is that de-agglomeration occurs
during classification which leads to good
dispersion the raw materials in airflow. The
disadvantage is that the product to air ratio has
significant effects on the cut size. To solve this
problem, a vane is installed in the classifier to
create a forced vortex in which the circumferential
velocity component is determined by the rotor
speed.
6.7 Rotating wheel classifiers
Use rotating blades to create the air vortex
or the centrifugal field. The advantage of a
rotating wheel classifier is the elimination of any
external compressor to send air as in a vortex air
classifier. In addition, the volumetric flow rate of
air required for classification is much less in the
rotating wheel classifier.
6.8 Circulating air classifiers
Are widely used in cement industry. It has
generally complex interior geometry. Particles are
fed from the top onto a plate where the particles
are scattered by rotation of the plate. Circulating
airflows carry the fine particles to the outer wall
(annular chamber). The air flows back to the
classification chamber via vanes and leave the fine
particles fall into the fines collector. The coarser
particles remain in the interior chamber and falls
into the coarse collector below.
6.9 Electrostatic classifier
The conventional classifiers have in
general cut size above 1 µm. However, finer end
products are required to obtain better product
characteristics for example in toner industry.
Electrostatic classification is a process by which
aerosol particles are fractionated according to
their electric mobility.