2.

Size Reduction
 Introduction to Size Reduction:
• Size refers to the physical dimensions of an object.
• Reduction involves decrement or the process of decreasing the
size.
• Size reduction refers to the process of converting the object
from one physical dimension of higher order to another
dimension of small order.
• It is the operation performed to reduce the size of larger particles
into smaller ones of the desired size and shape using external
forces.

Size reduction process is also termed as comminution

or diminution or pulverization
OBJECTIVE OF SIZE REDUCTION

• To increase the surface area, because in most chemical reactions and

some unit operations (drying, adsorption, leaching, etc.) involving solid
particles, the reaction/transfer rate is directly proportional to the area of
contact between the solid and the second phase,
• To produce solid particles of desired shape, size or size ranges, and
specific surface
• To separate unwanted particles effectively
• To dispose solid wastes easily
• To mix solid particles more intimately
• To improve the handling (storage and transportation) characteristics
 Applications of Size Reduction
• Size reduction operation is carried out in coal washeries, ore processing
industries, chemical industry, Paint industry, pharmaceutical industry,
cement industry, and food processing industry.

• In the food industry - sugar, spices, grains, etc.

• In the cement industry - lime, aluminum, sand, and cement clinker.
• In the paint industry - pigments.
• In the fertilizer industry - raw phosphate rock must be reduced in size.
Size Reduction: External Forces
There are four basic ways to reduce the size of a material

1. Impact
2. Compression
3. Attrition
4. Cutting

Most of the size-reduction equipment's employ a combination of all these size

reduction methods
 Compression
• Here, the particle is broken by two forces and the size
reduction is done between two surfaces, with the
work being done by one or both surfaces.

• Compression is chosen
(i) if the material is hard and tough,
(ii) if the material is abrasive,
(iii) if the material is not sticky,
(iv) where the finished product is to be relatively
coarser in size, and
(v) when the material will break cubically.
Coarse reduction of hard solids to fines,
Coarse reduction (NUTCRACKER) reduction to a size of up to 3 mm
Impact
• Impact occurs when moving particles strikes against
a stationary phase. In the same way, particles
moving at high speed collide each other and
produce smaller particles.
• Here, the particle is subjected to a single violent
force.

Feed: Brittle, hard, Abrasive, High moisture

Medium reduction
Product: fines, intermediate, some coarse
Types of Impacts: gravity impact and dynamic impact
Example: Hammer mill
 Attrition
• It involves breaking down of the solid
material by rubbing action scrubbing
between two surfaces. It is generally
necessary for fine grinding.
• Attrition consumes more power
• It is preferred for crushing the less abrasive
materials such as pure limestone and coal
• Useful for the material is friable or not too
abrasive
• Feed: Soft; Non-Abrasive
• Product: fines
Example: Fluid energy mill.
 Cutting
• The material is cut by means of sharp
blade. It is useful for the commination
of fibrous or waxy solids.

Feed: Ductile, Fibrous

Product: definite shape / definite size

Example: Rotary knife Cutter

Properties of solids
• For a particular size-reduction operation, the choice of machine to be
used mainly depends on

(i) the size and the quantity of material to be handled, and

(ii) the nature of the product required.

• But the more important aspects about the feed material apart from its
size and quantity are its properties such as hardness, toughness,
stickiness, moisture content, friability, explosive nature, soapiness,
crystallinity, and temperature sensitivity
Hardness: The hardness of the material is its resistance to scratching/
deformation and it affects the power consumption and the wear on the
grinding machine. It refers to the surface property of a material and is
measured on the " Moh's scale" ranging from 1 to 10. Materials with a
hardness below 3 are soft, between 3 and 7 are intermediate, and above 7
are considered hard. A very hard but brittle material may not pose
significant problems in size reduction.

Moh’s hardness number

Toughness: Toughness is the resistance of a material to impact. Toughness
indicates a material's ability to resist fracture or deformation. It is the reverse of
friability or brittleness. A soft but tough material can cause more challenges in
size reduction compared to a hard but brittle substance.
Stickiness: Some materials may adhere to the grinding surface or screen
meshes during size reduction, causing difficulties. Gummy substances, in
particular, can be problematic in the size reduction process, especially if they
generate heat.
Friability: The friability of the material is its tendency to fracture during
normal handling. Generally, crystalline materials will break along well-defined
planes.
Moisture Content: Materials do not flow well if the moisture content is higher
(more than 3 to 4%by weight). They tend to cake and clog the machine which
reduces the crushing effectiveness. Too dry a condition can result in excessive
dust.
Softening Temperature: The heat generated during size reduction can result
in loss of heat-sensitive components from solids. Softening or melting may
also be important-leading to clogging. In some cases, cryogenic crushing may
be necessary using liquid nitrogen or, dry ice, e.g., in milling of spices (e.g.,
stearic acid or drug-containing oils), to soften.

Explosive Nature: Explosive materials must be ground under wet conditions

or in the presence of an inert environment.
 Factors affecting size Reduction process
Apart from the properties of solids, certain factors affecting the size-
reduction process in terms of capacity and the performance
• Presence of moisture and sticky materials in equipment's feed,
• Presence of fines in the feed,
• Segregation of feed particles in the crushing chamber,
• Lack of feed control,
• Wrong motor size
• Insufficient crusher discharge area,
• Insufficient capacity of the crushers discharge conveyor,
• Materials being extremely hard to crush,
• Surface energy of solids,
• Power consumption, and
• Selection of an appropriate crushing chamber.
Energy and Power Consumption in Size Reduction

• Material feed is distorted and

strained
• Energy is stored as mechanical
energy of stress
• Additional force beyond
ultimate strength
• Fracture occur
• New surfaces generated
 Crushing Efficiency
• Crushing efficiency (ηc) is define by amount of surface energy created by
crushing out of the total energy absorbed by the solid.

𝐸𝑠 𝐴𝑠𝑠𝑝 − 𝐴𝑠𝑠𝑓
η𝑐 =
𝑊𝐴

Es= Surface energy per unit area, J/m2

Assp and Assf = Areas per unit mass of product and feed, respectively
WA = Total energy absorbed by a unit mass of solid, J/kg
 Mechanical Efficiency
Mechanical efficiency (ηm) is defined by extent of energy absorbed by the
solid from the energy input to the machine.

𝑊𝐴
η𝑚 =
𝑊
W= Total energy/work input, J/kg
WA = Total energy absorbed by a unit mass of solid, J/kg
So, total energy input to the machine can be derived as

𝐸𝑠 𝐴𝑠𝑠𝑝 − 𝐴𝑠𝑠𝑓
𝑊=
η 𝑚 η𝐶
Power Required for Continuous Operation
• If 𝑚ሶ is the flow rate of solids to the machine then the power required, P,
by the machine is the product of the total energy input and the flow rate
as:
𝑃 = 𝑊 × 𝑚ሶ

𝐸𝑠 𝐴𝑠𝑠𝑝 − 𝐴𝑠𝑠𝑓 𝑚ሶ
𝑃=
η𝑚 η𝐶
• It can be understood that for higher flow rate and materials with high
surface energy, the power consumption would be higher.
The expressions for the specific surfaces of feed and product materials

where, Φ , Φ = Sphericity of the feed and the product materials,

f p

Dvsf, Dvsp ρ= Sauter mean diameter for the feed and the
product, m and
ρpf, ρpp = Density of the feed and the product materials, kg/m3
For the homogeneous materials,

This relation tells us that the power requirement for crushing will be
more for particles having higher surface energy and also for the higher
flow rate
 Laws of Comminution
It is almost impossible to find out the accurate amount of energy
requirement in order to effect size reduction of a given material. It is due
to these reasons:
➢There is a wide variation in the size and shape of particles both in the
feed and product.
➢Parts of input energy is wasted as heat and sound, which can not be
determined exactly.
There are three empirical laws have been proposed to relate the size
reduction with the energy input to the machine.

1. Rittinger’s law (1867)

2. Kick’s law (1885)
3. Bond’s law (1952)
 Rittinger’s law
• According to this law, the work required for size reduction is proportional to
the new surface area created.

P
W E

P
W E
For particles of constant sphericity and density, the work required will
be
1 1
𝑊𝑅 = 𝐾𝑅 −
𝐷ഥ𝑣𝑠𝑝 𝐷 ഥ𝑣𝑠𝑓
Where, 𝐷ഥ𝑣𝑠𝑝 , 𝐷
ഥ𝑣𝑠𝑓 =Sauter mean diameter for the feed and the product,
6𝐾𝐸𝑠
and 𝐾𝑅 = , is known as Rittinger’s constant.
𝜑𝜌𝑝
The inverse of Rittinger’s constant is known as Rittinger’s number.
Rittinger s law is applicable mainly to that part of the process, where new surface is being
created and holds most accurately for fine grinding where the increase in surface per unit
mass of material is predominant.
This law is applicable for feed size of less than 0.05 mm.
 Kick’s law
• This law states that the work required for crushing a given mass of
material is constant for a given reduction ratio irrespective of the initial
size. It is represented as:
𝐷ഥ𝑣𝑠𝑓
𝑊𝐾 = 𝐾𝐾 𝑙𝑛
𝐷ഥ𝑣𝑠𝑝
Where, 𝐷 ഥ𝑣𝑠𝑝 , 𝐷
ഥ𝑣𝑠𝑓 =Sauter mean diameter for the feed and the product,
and 𝐾𝐾 is known as Kick’s constant.
• The reduction ratio is the ratio of initial particle size to final particle size.

More accurate for coarse crushing where the surface area produced per unit
mass is considerably less. This law is applicable for feed size of greater than
50 mm.
Bond’s law
• This law states that the work required to form particles of size Dpp
from a very large particle size is proportional to the square root of
the surface to volume ratio Sp/Vp of the product. It is represented as:

P
 1 1
𝑊𝐵 = 𝐾𝐵 −
𝐷𝑝𝑝 𝐷𝑝𝑓

ഥ𝑝𝑓 , 𝐷
Where, 𝐷 ഥ𝑝𝑝 = particle size of the feed and the product, respectively
6
and 𝐾𝐵 = 𝐾 is known as Bond’s constant.
𝜑
This law is applicable for feed size between 0.05 and 50 mm.
The Bond s constant (Kb), is dependent on the type of machine used and, on
the material, to be crushed. And it is found more accurately using Work
Index Wi
 Work Index Wi
• It is defined as the gross energy requirement in kilowatt-hours per
short ton of feed to reduce a very large particle to a size at which
80 percentage of the product will pass through a 100-µm or 0.1 -
mm screen.
1 𝐾𝑏 = 𝑊𝑖 √Dpp
𝑊𝑖 = 𝐾𝑏
√𝐷𝑝𝑝
Now, if P is in kW, m is in tons per hour, and
(i) D is in μm then K = 10 𝑊𝑖 , and
pp b

(ii) D is in mm then K = 0.1 𝑊𝑖 = 0.3162 𝑊𝑖

pp b
Thus, if 80% of feed particles pass through a Dpf millimeter screen and 80%
of product particles pass through a pp millimeter screen, then

W P

The Bond work index provides a measure of how much energy is required to
grind a sample of materials.

For dry grinding the materials, these values are multiplied by 4/3.
Hard Very Hard
Differential form of laws
 Classification of Size Reduction Equipments

Mode of Size of feed and

Operation Method by which a force is product
applied

Coarse Crusher
Shear action of the
Impact surrounding (Large feed size to
medium 50-5 mm product
Batch Operated size)

Impact between Intermediate

Compression Non-mechanical Crusher
particles between two solid introduction of
Continuous surfaces energy (50-5 mm feed size
Operated to 5-0.1 mm
product size)
Rubbing the
materials between Thermal Shock
Impact at one two surfaces Fine
surface Crusher/grinders
Explosive (5-2 mm feed size
shattering to ≅ 200-mesh)

Crushing Ultrafine grinders

Electrohydraulic
crushing (6 mm feed size to
50-1-µm)

Cryogenic
crushing Cutting machines
Grinding
(definite size
between 2-10 mm
Ultrasonic length)
grinding
 Coarse crushers (Large feed size to 50-5 mm product size)
• Slow-speed machine for coarse reduction of large quantities of
solids
• break large pieces of solid material into small lumps
• Primary crusher - accepts anything from mine & breaks into 150
- 250 mm
• Secondary crusher - reduces lumps into 6 mm main types:
1) Jaw crushers
2) Gyratory crushers
3) Smooth-roll crushers
4) Toothed-roll crushers Gyratory crusher
Intermediate Crusher (50-5 mm feed size to 5-0.1 mm product size)
• For intermediate duty (from crushers to grinders for further
reduction)
• Reduce crushed feed to powder
• Product from intermediate grinder might be 5-0.1 mm
1) Hammer mills & impactor
2) Roller mill
3) Cage mills
4) Granulator
Fine Crusher/grinders (5-2 mm feed size to ≅ 200- mesh)
• Fine grinding which reduces the intermediate product to a finer size
• Product from fine grinder would pass a 200-mesh screen (74μm
screen) commercial grinders:
• Reduce the solid size by impact, attrition
• Ball mill
• Pebble mill
• Rod mill
• Tube mill
• Attrition mill/Pulveriser
 Ultrafine grinders: (6 mm feed size to 50-1-µm)
• Reduce solids to fine particles
• Accepts feed particles not larger than 6 mm.
• The product size is typically 1 to 50 μm.
• Reduce the solid size by attrition
Examples
1. Classifying hammer mills,
2. Fluid Energy Mills
3. Agitated Mills
4. Colloid Mills.
Cutting machines: (definite size between 2-10 mm length)

• Give particles of definite size and shape 2-10 mm in length

• Reduce the size by cutting, dicing and slitting.

Examples
• Cutters, Knife, Scissors – produce cube, thin squares or
diamonds.
 Size Reduction Equipment Classification

A. Crushers B. Grinder (intermediate and fine) C. Ultrafine grinder

(coarse and fine)
1. Jaw crushers 1. Tumbling mills: rod mill, ball mill, 1. Fluid energy mill
2. Gyratory pebble mills, tube mills 2. Agitated mills
crushers 2. Attrition mill 3. Hammer mills with
3. Crushing rolls 3. Hammer mill internal classification
Compression Impact and Attrition, sometimes Attrition
combined with compression

McCabe, W.L., Smith, J.C., and Harriott, P., “Unit Operations of Chemical Engineering”, 6 th ed., McGraw Hill, 2001.
 Methods of Feeding
• Two methods of feeding material to a crusher are possible,
Free crushing: Feeding the material at a comparatively low
rate so that the product can readily escape.
• Residence time in the machine is sort.
• And then production of appreciable quantities of
undersize material is avoided.
Choke Feeding
• Machine is kept full of material and discharge of product
is impeded so that the material remains in the crusher for
a longer period right.
• Higher degree of crushing
• The energy consumption would be very high because of
accumulated product inside the machine.
• Used only for small amounts of material when it is
desired to complete the whole of size reduction in one
operation
Coarse crushers (Large feed size to 50-5 mm product size)
Jaw crushers • Two jaw - V opening
• One – Fixed jaw – anvil jaw; Other – moving
jaw – swinging jaw
• Angle b/w jaws 20o – 30o
• Jaws are flat or slightly bulged
• crushed at upper portion & then dropped and
re-crushed at narrow end.
• Jaw open and close 250-400 times per
minute
• Eccentric drive a pitman which is connected
to toggles among which connected to moving
jaw
• Capacity: 1200 ton/hr
Distance between the two jaw plates at the feed opening is
known as gape
 The jaws themselves are usually constructed from cast steel and are fitted with replaceable
liners, made from manganese steel, or "Ni-hard", a Ni-Cr alloyed cast iron
 PITMAN

• Single toggle jaw crusher

• Double toggle jaw crusher (two shafts)

escape.
Less uniformity in product size
Single Toggle Jaw Crusher Double Toggle Jaw Crusher
Single toggle vs double toggle
Dodge crusher
• It has a large opening at the top enables it to take large feed.
• Material is crushed at upper portion, then dropped and recrushed
at narrow end.
• The constant opening at the discharge end gives the crusher an
• annoying tendency to clog.
• This type of crusher is effective for hard and abrasive materials
as the crushing force is concentrated on a single point.
• More uniformity in the product size.
Capacity of Jaw Crusher
• The theoretical capacity of a jaw crusher is
Industrial Applications
Crushers are used

1.Heavy duty mining

2.Cement industry
3.Fertilizers industry
4.Plaster of Paris Plant
 Advantages of Jaw Crusher
Simple construction makes for easy maintenance
• Stable performance
• High crushing ratio
• Low operational cost
• High capacity
• Adjustable discharge port
• It costs less energy than other crushers and it causes less
environmental problems by making less noise and less
dust.
Demerits: Jaw Crushers
• Very expensive
• The crushing action is intermittent due to which
heavy foundations required.
• It is impossible to stop jaw crusher in emergency
due to heavy flywheel.
• Restarting with choked machine is impossible.
• Flat shape particles can pass through jaws uncrushed.
Product size is in between 20-50 mm
Reduction ratio is 4:1 to 6:1
 Product size is in between 20-50 mm
Suitable for all hard materials, abrasive materials, b
not for soft and porous materials
• The smooth mantle design is an ideal choice for high abrasive applications
Features of Gyratory:
• High capacity per investment dollar.
• Handles thin, slab-like material better than a jaw crusher.
• Feeding is simpler.
• Foundation costs are lower than for jaw crusher
• Easier installation than JC since more compact design and crushing stresses
more even.
Features of Jaw Crusher:
1. Large receiving opening per investment dollar.
2. Shape of opening favors large block shaped feed better than a gyratory.
3. Handles dirty or sticky feed better, without buildup,
4. Maintenance is easier.
5. Can be easily split into sections, which makes it easier to carry it
underground.
ROLL CRUSHER
• The history of roll crushers is more than 200 years old but in recent years,
lost their popularity over jaw and gyratory crushers due to their poor wear
characteristics with hard rocks.

• Depending on the number of rolls employed, roll crushers are of two types
1. Single-roll crushers
2. Double-roll crushers.

• The single-roll crusher is one of the oldest and the simplest crushers which
are mainly used for primary crushing, whereas double-roll crushers are used
for secondary crushing.
 Single-roll crushers
• Single-roll crushers employ three different methods of size reduction -impact, shear, and compression
• Single-roll crushers have a roll assembly consisting of a roll shaft and a fabricated roll shell with integral fixed
teeth
breaker plate
Feed Material Entry:
1. Feed material enters the crusher through the feed hopper.
2. The material comes into contact with the teeth of the revolving roll.
3. Initial breakage occurs due to impact with the teeth.
Crushing Chamber : Roll
1.The rotation of the roll moves the material into the crushing chamber.
2.The chamber is formed between the breaker plate and the roll itself.
3.Material compression and shearing occur in this chamber.
Material Compression and Shearing:
1. The turning roll compresses the material against the stationary breaker plate.
2. The teeth on the roll shear the material, further breaking it down.
Sized Material Discharge:
1. Properly sized materials fall directly out through the discharge end.
2. The discharge end is fully open, without screen bars. Product discharge
3. No re-crushing of sized materials occurs, reducing power demand and fines.
Product sizes ranging from 75 to
Adjustable Product Size: 300 mm depend on machine size
1. The clearance between the breaker plate and the roll determines product size.
2. Product size can be adjusted from outside the machine using a shim arrangement.
Applications: Single-roll crushers
• Petroleum Coke
• Coal
• Limestone
• Phosphate Rock
 Advantages of Single Roll Crusher:
• Simplicity: Single roll crushers are relatively simple in design and
operation, making them easy to maintain and operate.
• Compact Design: They have a compact design, which makes
them suitable for installations with limited space.
• Uniform Product Size: Single roll crushers provide consistent
and controlled product size due to their one-roll design.
• Economical: They are cost-effective and require less maintenance
compared to some other crushing equipment.
• Versatility: Single roll crushers can handle a wide range of
materials and sizes, making them versatile for various
applications.
 Disadvantages of Single Roll Crusher
• Limited Feed Size: Single roll crushers are not suitable for
large feed sizes and may struggle with oversized materials.
• High Wear and Tear: The single roll's surface is subject to high
wear and tear, which can result in higher maintenance and
replacement costs.
• Limited Reduction Ratio: limited reduction ratios compared to
other types of crushers.
• Limited Control: The adjustable clearance between the roll and
the breaker plate may not provide as precise control over the
final product size as other crushers.
The material fed to the machine is protected by
spring loading
 Crushing Roll

Double roll

coal, limestone, clay limestone, gypsum, and iron ore

Single Roll Crusher provides up to a 6:1 ratio of reduction and double roll crusher 4:1.
Features and benefits of toothed roller crusher:
• Good Performance with Moisture Control:
• Suitable for materials with moisture content up to 15%.
• Even Discharging Size:
• Ensures consistent product size without blocking.
• Environmental Protection:
• Full-sealed design prevents dust pollution.
• Meets environmental standards.
• Sample Collection
• Convenient sample collection for analysis.
• Efficient Maintenance:
• Removable shell for easy maintenance and inspection.
• Attractive Appearance:
• Total electro-static coating for a polished look.
• Easy to clean and maintain appearance.
• Work Safety:
• All moving parts have safeguards for work safety.
Applications: Double Tooth Roller
• Crushing of Oil Seeds
• Coal
• Phosphate Rocks
• Abrasive Materials
• Lime
• Limestone
• Petroleum Coke
• Explosive Materials In Gunpowder Industries.
 Roll Crusher Capacity
• The capacity of roll crushers can be calculated using the theoretical formula:
Q (theoretical) = 188.5 N D W s d
N: Speed of rolls (rpm); D: Roll diameter (m); W: Roll width (m); s: Density of feed
material (kg/m³); d: Distance between the rolls (m).

In practice, several factors influence the actual capacity of roll crushers:

Voids between Particles: Considering the spaces between particles.
Loss of Speed: Accounting for speed loss when gripping the feed.
Feed Characteristics: The nature of the material being processed.
Machine Design: Roll size, configuration, and operating conditions.

Actual Capacity ≈ 0.25 * Theoretical Capacity

https://www.sciencedirect.com/topics/engineering/roll-crusher
 Advantages of Roll Crushers

• High tonnages up to 12000 MTPH.

• Compact in size and therefore reduced installation cost.
• Cannot mobile: lower capital cost than any other crusher.
• Large reduction ratio in relation to tonnages.
 Disadvantage of Roll Crushers
• Low reduction ratio.
• Required a high-performance electrical system as there are peak
power loads
• Can not use economically for low tonnages.
• Will not handle trash metal.
Selection of crushing rolls
While selecting the rolls for a certain duty, it is necessary to know

(i) size of feed,

(ii) size of the product, and
(iii) Amount of material to be handled.
Angle of Nip
The angle of nip is the angle formed by the tangents to the roll faces at a point
of contact with a particle to be crushed.

Angle of Bite :
Intermediate Crusher
(50-5 mm feed size to 5-0.1 mm product size)
Fine Crusher/grinders
(5-2 mm feed size to ≅ 𝟐𝟎𝟎-mesh
 Hammers - rectangular metal bars with plain, enlarged, or
sharpened edges
CASING

Hammer Mill Reduction Ratio

ROTOR

HAMMER

SCREEN
• Can grind tough fibrous solids like bark or leather, steel
turnings, hard rock, sticky clay
• Large current of air produced makes environment dusty
Intermediate hammer mills: Product size – 25 mm to 20 mesh
Fine hammer mills: It can reduce 0.1 to 15 ton/hr of feed to < 200
mesh
https://www.911metallurgist.com/blog/hammer-mill-operating-principle
 ADVANTAGES OF HAMMER MILL
• It produces specified top size without the need for a closed- circuit crushing
system.
• It produces relatively numerous size distributions with a minimum of fines
due to self-classification.
• It has a high reduction ratio and high capacity whether used for primary,
secondary or tertiary grinding.
• Relatively reasonable energy requirements.
• Brittle materials are best fractured by impact from blunt hammers.
• It is capable of grinding many different types of materials
• The machine is easy to install and operate and its operation is continuous.
• It occupies small space.
• It is easy to maintain and clean.
• It is inexpensive.
• Its ease of manufacture allows easier local construction.
DISADVANTAGES OF HAMMER MILL
• Not recommended for the fine grinding of very hard and abrasive
material due to excessive wear.
• Not suitable for low-melting sticky or plastic-like material due to
heat generation in the mill head as a result of mill fouling.
• The mill may be choked if the feed rate is not controlled, leading
to damage.
• There is a possibility of clogging of the screen.
Fine crusher (Grinders)
 The end product has the fineness in the range of 50 μm to 5 mm.
Impact mills work on the principle by impact of fast revolving hammers with the particle and by collision of particles
over the specially designed stationary grinding tracks on the walls of the grinding chamber
Applications : Attrition
• Chemical industries (fertiliser, pesticide, paints and pigments)
• Pharmaceutical industries (antibiotics, herb teas, roots, rose hips) herbs
and spice industries (savoury, rosemary, onions, turmeric)
• Food and confectionery industries (oat and potato flakes, casein, sugar,
starch, food colourings);
• Animal feed industries (soya meal, freeze-dried meat, corn cobs);
wood and chipboard industries; mineral powder industries; plastics
industries (PVC, PTFE, PE); etc.
• Fine impact used with the system of cryogenic grinding with liquid
nitrogen, and it also comes with a special design of a 10-bar (over
pressure) grinding system for grinding dust explosive materials,
acrylic resin, cornstarch, sugar, insecticides, vitamins, etc.
Ball Mill Types: Wet vs. Dry
•Wet Ball Mill
• Simple structure, minimal accessories.
• Flared discharge port with built-in spiral for easy ore discharge.
• Better performance, higher grinding efficiency.
• Lower moisture content requirement for ores.
• Less auxiliary equipment.
• Lower investment (about 5%-10% less).
•Dry Ball Mill
• More complex structure.
• Straight discharge port with air induction, dust exhaust, and collector.
• Comparative lower efficiency.
• Potential need for higher moisture content.
• More auxiliary accessories.
• Higher investment.
 Continuous Operation: Ball Mill
• Material is fed from the left side through a 60° cone.
• Product is discharged from the 30° cone on the right side.
• Balls within the mill rise and then drop onto the feed.
• Solids are ground and reduced in size through impact.
• As the shell rotates, large balls tend to move towards the feed end, while small
balls gather near the product end.
• Initial breaking of materials is achieved by the impact of large balls dropping
from a higher distance.
• Smaller particles are broken down by the impact of smaller balls from a shorter
distance.
• Adjusting the feed rate leads to obtaining coarse products.
• Altering the rotation speed affects the fineness achievable for a given capacity.
 Ball Mill Advantages:
▪ Low Installation Cost
▪ Low Power Requirement
▪ Suited for All Hardness Levels
▪ Batch & Continuous Modes
▪ Open & Closed-Circuit Grinding
▪ Safe Grinding of Explosive Materials
▪ Cost-effective Grinding Medium
l - Weight of Balls - Speed of Mill Rotation - Level of Material in the Mill Ball Mill Advantages:
DISADVANTAGES
- Low Installation Cost - LowOF BALL
Power MILLS
Requirement - Suited for All Hardness Levels - Batch & Continuous Modes - Open
•& Closed
Contamination
Circuit Grinding -of
Safeproduct
Grinding ofmay occur
Explosive as -aCost-effective
Materials result ofGrinding
wear Medium
and tear which occurs
principally from the balls and partially from the casing.
• High machine noise level especially if the hollow cylinder is made of metal,
but much less if rubber is used.
• Relatively long milling time.
• It is difficult to clean the machine after use.
 Factors effecting the size of the product
▪ Feed rate
▪ Properties of feed material
▪ Weight of the balls
▪ Diameter of balls
▪ Slope of mill.
▪ Speed of rotation mill
▪ Level of material in the mill.
 ACTION IN TUMBLING MILLS
The speed of rotation is a crucial factor for ball mills

• At low speeds of rotation, the balls are lifted

and simply roll back over feed materials.
• Size reduction is caused by attrition and little crushing
action takes place.
• Under this condition, the mill is said to be cascading
• At slightly higher speeds, the balls are carried up further inside
the mill and fall back due to gravity on to the feed particles at
the bottom. Grinding takes place by impact and the mill is said centrifugin
to be cataracting
• When the speed of rotation is continuously increased, a point is reached where the balls
inside the mill are carried along with the inner walls due to higher centrifugal force. At this
stage, the balls no longer fall, and this phenomenon is known as centrifuging.
https://www.911metallurgist.com/blog/ball-mill-critical-speed
• The minimum speed at which centrifuging occurs is referred to as the
critical speed of the mill.
• Consider at any point of time the ball is at a point A inside the mill, as
shown in Fig.
• Radius of the ball mill: R; AC=m
• Radius of the ball: r
• Angle ө==∠ACB
• Peripheral speed: v, m/s
• Mass of the ball: m, kg
Force of gravity, mg
𝑚v2
Centrifugal force=
𝑅−𝑟
• The component of gravity force opposing the centrifugal force is mg cos ө. As
long as the centrifugal force is more than the component of gravity force, the
ball will not loose its contact with the inside wall of the mill.
• Whenever and wherever these two opposing forces become equal, the ball will
loose contact and will fall down onto the particles. At this condition,𝐴 = 𝜋𝑟 2
𝑚v2
𝑚𝑔𝑐𝑜𝑠𝜃 =
𝑅−𝑟
v2
𝑐𝑜𝑠𝜃 =
𝑅−𝑟 𝑔
But, the peripheral speed is related to the speed of rotation, N by
v = 2𝜋𝑁(𝑅 − 𝑟)
4𝜋 2 N2 𝑅 − 𝑟
𝑐𝑜𝑠𝜃 =
𝑔
At critical speed , N = N and ө= 0o. Thus, cos ө = cos 0o = 1
C

4𝜋2𝑁𝑐2 𝑅−𝑟
cos ө = cos 0o = 1 =
𝑔

1 𝑔
𝑁𝐶 =
2𝜋 𝑅−𝑟

It is clearly evident from Eq. that for increased size of ball,

the critical speed increases for a given size of the mill.
https://www.911metallurgist.com/blog/ball-mill-critical-speed
 ULTRA FINE GRINDER
• Many commercial powders must contain particles averaging 1 to 20
µm in size, with substantially all particles passing a standard 325-
mesh screen that has openings 44 µm wide. Mills that reduce solids
to such fine particles are called ultrafine grinders.

• Some of the popular ultra-fine grinding machines are: Fine Impact

Mills, Spiral Jet Mills, Fluidized-Bed Opposed Jet Mills, Fluid-
Energy Mills, Classifying Hammer Mills, etc

• Ultrafine wet grinding is done in agitated mills.

 Fluid Energy Mill (Jet Mill / Micronizers / Ultrafine Grinders)
• Principle: Impact and Attrition (between materials and materials)
• Milling occurs due to high-velocity collisions between suspended particles.
The fluidized-bed opposed jet mills are better and more versatile solutions for ultrafine size
reduction of materials up to a Mohs hardness of 10.
Construction:
Due to the high-speed classifier which is
• Elliptical or loop-shaped pipe structure. rotating on the top, the classifying air
• Height: Approximately two meters. flows through the classifying wheel in
centripetal direction; the fines are sucked/
• Diameter: 20 to 200 millimeters. extracted and conveyed to the fines discharge.
• Feed inlet positioned in the airflow path. The coarse material is rejected back
• Upper side of the pipe has an outlet. to the milling area for further milling.

• Constructed with stainless steel or tough ceramics.

• Nozzles placed at the bottom of the pipe or opposed to the initial flow path of powder.
• Compressed air used at 600 kilopascals to 1.0 megapascals.
• Inert gases used to minimize compound oxidation.
• Classifier (cyclone separator or bag filter) attached to an outlet allows air to escape fine particle
https://www.youtube.com/watch?v=kq8YuyphPvk
Advantages:
• It has no moving parts
• No heat generation (so it is highly required for thermolabile substances
like sulphonamide, vitamins, and antibiotics)
• No chance of contamination
• The expansion of gases under pressure produces cooling effects.
Disadvantage:
• Not suitable for soft, Tucky, and fibrous materials
• It is expensive equipment
• Not easy to clean
Fluid energy mills are
available in different
sizes, which can grind
upto 2000 kg per hour

Typical application :
Grinding of highly pure substances such as fluorescent powders, silica gels, silicic
acids for chromatographic applications, etc.; highly abrasive materials such as tungsten
carbide, silicon carbide, boron carbide, corundum, etc.; temperature sensitive substances
like plant protectives, wax, resins, etc.; minerals like talcum,
mica, graphite, quartz; cryogenic grinding with liquid nitrogen; and toner grinding
 Cutting machine
• The feed stocks are too resilient to be broken by compression, impact or
attrition
• Applications in many manufacturing processes but are especially well
adapted to size reduction problems in making rubber and plastics.
Cutting may be accomplished by single point or multi point tools.
• Particle size ranges : (2mm -10 mm)

TYPES OF CUTTING MACHINE

1) Knife
2) Cutters
3) slitters
 Rotary Knife Cutter
• It contain a horizontal rotor turning at 200
to 900 rpm in a cylindrical chamber.
• On this rotor there are 2 to 12 flying knives
with edges tempered steel, passing with
close clearance over 1 to 7 stationary bed
knives.
• Feed particles entering from above may be
cut several times before they are small
enough to pass through a bottom screen
with 5 to 8 mm openings.
Cutting machines
 Closed Circuit Grinding
• In closed circuit grinding, the material to be ground is fed
into the grinding mill along with a certain amount of
grinding media (often steel balls).
• The grinding mill operates in a closed-loop system,
where the ground material is separated from the coarse
particles and returned for further grinding.
• The classification of particles into fine and coarse
fractions is achieved using classifiers, such as screens or
cyclones.
• The fine particles that pass through the classifier are sent
to subsequent processing steps, while the coarse particles
are returned to the mill for further reduction.
• Closed circuit grinding is often used to achieve finer
particle sizes and higher overall process efficiency
compared to open circuit grinding.
 Closed-Circuit Grinding:
Advantages:
• Enhanced Control: Closed-circuit grinding systems allow for better control over the
particle size distribution by fine-tuning the grinding parameters and the circulating load.
• Higher Efficiency: The recirculation of fine particles for further grinding in closed-circuit
systems can lead to higher overall grinding efficiency.
• Improved Product Quality: Closed-circuit systems often result in a more uniform and
narrower particle size distribution, which can improve the quality of the final product.
• Lower Energy Consumption: Closed-circuit systems can be more energy-efficient
compared to open grinding due to optimized grinding conditions and particle
classification.
Disadvantages:
• Complexity: Closed-circuit grinding systems involve additional equipment, such as
classifiers and separators, leading to a more complex setup and potentially higher
maintenance requirements.
• Higher Capital Costs: The installation and maintenance of additional equipment can lead
to higher initial costs for closed-circuit systems.
• Limited Flexibility: Closed-circuit systems might be optimized for specific feed materials
and product sizes, limiting their flexibility to handle a wide range of materials.
 Open Circuit Grinding:
• In open circuit grinding, the material enters the grinding mill
and is ground without passing through any sort of classification
device.
• Once the material is ground to the desired particle size, it exits
the mill and is collected for further processing.
• Open circuit grinding is simpler in terms of operation and
control, as there is no need for a classification step.
• However, it might result in a wider particle size distribution
and potentially coarser final product compared to closed circuit
grinding.
 Open Grinding:
Advantages:
1.Simplicity: Open grinding systems are relatively simpler in terms of design and operation. They
typically involve a single grinding stage without complex equipment or circuit configurations.
2.Flexibility: Open grinding systems can handle a wide range of feed materials and product sizes
without significant adjustments.
3.Lower Capital Costs: The equipment required for open grinding is generally less expensive to
install and maintain compared to closed-circuit systems.
4.Minimal Classification: Open grinding systems don't require elaborate classification equipment,
reducing the complexity of the process.
Disadvantages:
1.Limited Control: Open grinding lacks the level of control over the particle size distribution and
product quality that closed-circuit systems offer.
2.Reduced Efficiency: Open grinding might not achieve the same level of particle size reduction
efficiency as closed-circuit systems due to a lack of fine particle recirculation and re-grinding.
3.Less Uniform Product: The product particle size distribution can be less uniform in open
grinding, leading to potential issues in downstream processing or end use.
4.Energy Consumption: Open grinding systems may require more energy per ton of material
processed due to the absence of efficient particle classification.
Size Reduction Equipment's
• Richardson, J. F., & Harker, J. H. (2003). Coulson and Richardson’s chemical engineering
series: Volume 2. Butterworth-Heinemann
• McCabe, W.L., Smith, J.C., and Harriott, P., “Unit Operations of Chemical Engineering”,
6 th ed., McGraw Hill, 2001
• Brown, G.G. et al. (2005). Unit operations. CBS Publisher.