CH8451- Mechanical Operations

UNIT II PARTICLE SIZE REDUCTION

Lenin Prabhu SAssistant Professor

Dept. of Chemical Engineering

CH8451- Mechanical Operations

Even Sem. 2018 -2019

Definition

Objectives

Advantages & Disadvantages

Mechanism

Factor Affecting

Energy requirement

Methods

Size Reduction

Physical Decrement

Dimension or Decrease

of an object

So, Size Reduction means decreasing size of an object

Definition (1/2)

It refers to a mechanical process in which particles of

solids are cut or broken into small sizes by help of certain

external forces and without altering the state of aggregation

of solids.

Other terms: -

Comminution

Grinding

Milling

Pulverizing

Crushing

Definition (2/2)

Increase the surface area

Produce solid particles of desired shape, size

Separate unwanted particles effectively

Separate the valuable amongst the two constituents

Dispose solid wastes easily

Improve dissolution rate, solubility and binding strength,

dispersion properties

Mix solid particles more intimately in solid – solid

operations.

Improve the handling (storage and transportation)

characteristics.

Objectives

⧪ Content uniformity

⧪ Uniform flow

⧪ Effective drying

⧪ Increases surface area or viscosity

⧪ Uniform mixing and drying

⧪ Improve rate of absorption . Smaller the particles greater

is absorption.

⧪ Improve dissolution rate.

Advantages

⦿Drug degradation

⦿Contamination

Disadvantages

1) Impact:

Size reduction occurs by a single rigid force.

The material is stationary and is hit by an object moving at high

speed or Moving particles strikes a stationary surface.

Gravity impact: impact occurs when the moving particle strikes a

stationary surface.

E.g.: Coal dropped onto a hard steel surface.

Dynamic impact: impact occurs when the material is more or

less stationary and is hit by an object moving at high speed.

E.g.: Materials dropping in front of a moving hammer.

Mechanism (1/5)

2) Compression:

• Material is crushed by application of a pressure. The particle is

broken by two rigid forces.

• The size reduction is done between two surfaces with the work

being done by one or both surfaces.

• It suitable for reducing extremely hard and abrasive rock.

• It is used for coarse reduction of hard solids to give relatively few

fines.

E.g.: Jaw crushers, Crushing rolls

Mechanism (2/5)

3) Attrition :

• Size reduction by rubbing or scrubbing of materials each other or

against two hard surfaces.

• Here, the material is subjected to pressure as in compression, but

the surfaces are moving relative to each other.

• It may be combined with shear and impact actions.

• It is preferred for crushing the less abrasive materials such as

pure limestone and coal

E.g.: Disc attrition mill, Hammer mills

Mechanism (3/5)

4) Shear :

• It consists of a trimming or cleaving action rather than the

rubbing action associated with attrition.

• It produced by a fluid or by particle–particle interaction.

• It is usually combined with other size-reduction actions like

impact and compression.

E.g.: single-roll crushers

5) Cutting:

• It gives a particle of definite size and shape, with few or no fines

by means of a sharp blade or blades.

• The material is cut into pieces.

E.g.: Knife, cutters, scissors Rotary knife cutter

Mechanism (5/5)

Mechanism (5/5)

Method Common exampleApprox. particle size

(μm)

Approx.

increase

in

fineness

of

the

product

Cutting

Scissors

Shears

Guillotine

Cutter mill

100 – 80,000

Compression

Pestle – Mortar

Nutcrackers

Roller mill

50 – 10,000

Impact Hammer mill

Disintegrator50 – 8,000

Attrition

Pestle – Mortar

File

Roller mill

Colloidal mill

1 – 50

Impact & Attrition • Ball mill

Fluid-energy mill1 – 2000

a) Hardness:

Harder the material difficult is the size reduction.

Mohs scale is commonly used to measure the hardness of particles

‐ Moh’s Scale = 1 is for graphite

‐ Moh’s Scale < 3 is for soft material

‐ Moh’s Scale > 7 is for hard material

‐ Moh’s Scale = 10 is for diamond

b) Structure:

Normal granular materials such as coal, ores and rocks can be

effectively crushed employing the normal forces of compression,

impact, and so on. With fibrous materials a tearing action is required.

Factor Affecting (1/5)

c) Toughness:

It is the resistance of a material to impact. It is the reverse of

friability or brittleness. It can be reduced by lowering the temp.

d) Stickiness:

It cause difficulty in size reduction. Material may stick to the

surface or mesh may get choked.

Solution: Dryness Addition of inert materials such as Kaolin to

sulphur

e) Friability:

The friability of the material is its tendency to fracture during

normal handling.


f) Moisture content:

Materials do not flow well if the moisture content is higher (more

than 3 to 4% by weight). It influences properties such as Hardness,

Toughness & Stickiness. Less than 5 % moisture – Dry grinding. More

than 50 % -Wet grinding

g) Explosive Nature:

Explosive materials must be ground under wet conditions or in

the pressure of an inert environment.

h) Soapiness:

Soapiness is the measure of the coefficient of friction, μ, of the

surface of the material. If μ is low, the crushing will be more difficult.


i) Crystallinity:

Every solid material has a specific crystalline pattern, breakage

occurs when sufficient pressure is applied on the solid

j) Abrasiveness:

If material is abrasive the final powder may be contaminated

with the metal worn out from the machine.

k) Temperature Sensitivity:

Heat creates problems with materials which tend to

melt/stick/decompose. 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 or

size reduction of meat.


l) Slipperiness:

material acts as lubricant.

m) Size of the starting material

n) Quantity of material to be reduced

o) Size, Shape, Flow & Bulk density of the product

p) Ratio of feed size to product size


Size reduction may be achieved by two methods:

1) Precipitation

2) Mechanical process

Precipitation method: Substance is dissolve in appropriate solvent.

Mechanical process: Mechanical force is introduce by using different

equipments like ball mill, colloid mill etc.

Methods of Size reduction

In producing elastic deformation of the particles before

fracture occurs.

In producing inelastic deformation which results in size

reduction.

In causing elastic distortion of the equipment.

In friction between particles, and between particles and

the machine.

In noise, heat and vibration in the plant, and

In friction losses in the plant itself.

Energy utilization

Size reduction equipments Classifications

◂ Coarse crushers Large feed size to (50 - 5) mm product size

˃ Jaw crushers (Blake and Dodge), Gyratory crusher, Cone

crusher, crushing rolls (smooth and toothed rolls).

◂ Intermediate

crushers

(50 - 5) mm to (5 – 0.1) mm product size

˂ Roller mill, Cage mill, Granulator, Hammer mill, impactor,

vertical shaft impactor..

◂ Fine crushers/

Grinders

(5 – 2) to 200 mesh product size

˃ Ball mill, Pebble mill, Rod mill, Tube mill, Attrition

mill/Pulveriser.

◂ Ultrafine

Grinders

6 mm to (1– 50) μm product size

˂ Fluid energy or jet mill, hammer mill with internal

classification, colloid mill, fine impact mill.

◂ Cutting

machines

Definite size between 2 – 10 mm length

˃ Knife, cutters, scissors, dicers, slitters.

Size reduction equipments Classifications

A. Crushers (coarse and

fine)

B. Grinders (intermediate

and fine)

C. Ultrafine grinders

1. Jaw crushers

2. Gyratory crushers

3. Crushing rolls

1. Hammer mill, Impactors

2. Rolling compression mill

‐ Bowl mills

‐ Roller mills

3. Attrition mills

4. Tumbling mills

∶ Rod mills

∶ Ball mils, pebble mills

∶ Tube mills

1. Fluid energy mills

2. Agitated mills

3. Hammer mills with

internal classification

Crushers Grinders Ultrafine grinders Cutters

CompressionImpact and attrition, sometimes

combined with compressionAttrition

Characteristics

of cutter

Jaw Crushers Types

Jaw Crushers

Gyratory Crushers

Roll Crushers

Single-Roll Crushers

Double-Roll Crushers

Granulators

Coalpactors

Hammer Mills

Tumbling mills/Revolving mills

Characteristics Ball mill Rod mill Tube mill Pebble mill

Principles of

comminutionImpact

Rolling,

compression and

attrition

Impact Impact

Grinding media BallsSteel rods (metal

rods)

Small balls and

pebblesPebble

Material of

construction of

grinding media

Steel High carbon steel SteelCeramic pebbles made of flint

or porcelain

Diameter of

grinding media

12 – 125

mm50 mm - -

Feed sizeUpto 50

mmUpto 25 mm Upto 25 mm Upto 25 mm

Product size Fine Ultrafine Fine Fine

Length/diameter 1 to 1.5 : 1 1.5 to 3 : 1 3 to 4 : 1 1 to 2 : 1

Applications

Coal,

pigment,

feldspar for

pottery.

Particularly for

sticky materials

and not suitable

for tough

materials.

Same as ball

mill, but the

residence time

is more.

In paint, and pigment industries,

in cosmetic industries where

iron contamination is

objectionable. Also for high

specific gravity feed.

Ball Mills

Rod Mills

Ball Mill Vs Rod Mill

Fine Impact Mills/Pin-type mill

Fluid Energy Mill

CH8451- Mechanical Operations

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