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Dr. C. EswaraiahScientist

Mineral Processing Department

Institute of Minerals & Materials Technology, Bhubaneswar

COMMINUTION (Crushing and Grinding)

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1. Introduction to Mechanical Operations

2. Introduction to Comminution

3. Particle characteristics

4. Different Mechanisms of Size Reduction

5. Comminution-Governing Laws

6. Crushers Introduction

LECTURE-1

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MixingFiltration

Settling

and

Sedimentation

Size

Reduction

MECHANICAL

OPERATIONS

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COMMINUTION

Comminution is derived from the Latin comminuere = to makesmall.

One of the oldest technologiest used in human history.

The Size reduction is applied to all the ways in which solids are cut

or broken into smaller pieces.

Modern industrial civilisation cannot exist without exploiting wide

range of comminution technologies

Core processing phase in mining and mineral production.

Why comminution is an interesting research area to apply

optimization and simulation?

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INTRODUCTION

Particle breakage is an essential component of mineralprocessing and is important in other industry sectors.

The aim is to efficiently reduce the particle size with maximum

throughput at minimum operating cost.

Example: Tumbling mills that are used in most of the

comminution operations are very inefficient as much of the

energy is wasted in impacts which do not break particles, or

simply consumed in the generation of unwanted product sizes.

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INTRODUCTION

The difficulty is mainly associated with the complex physicalphenomenon involved in the process characterized by highly dynamic

and non-linear behavior.

For efficient mill operation and energy utilization a good knowledge of

the charge dynamics in tumbling mill is essential.

However, dynamics of the charge inside any particulate material

processing equipment is complex and difficult to characterize in terms of

operating and design variables.

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Reasons for optimization & simulation

For example: It has been estimated that 1.5 % of electrical energy in

USA is consumed in comminution processes.

realistic improvements in energy efficiency in comminution could

result 20 billion kWh energy savings in USA. That is about 15 % of

Autralias entire annual consumption of electrical energy.

in mineral processing plants 30-50% of power draw is used by

comminution (and for hard ores up to 70 %).

The benefits in optimization can be:

reduced unit operating costs (/ton treated).

increased throughput and production value.

improved downstream process performance, when the feed size

specifications met better.

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How important is grinding ?

grinding operations in the mineral processing industry consume about

50% of the total processing cost.

in the US alone it was estimated that size reduction operations

consume 5% of the total electric power.

at the same time these processes have energy efficiencies of less

than 5% due to wasted collisions between grinding media that do not

catch mineral ore in between, or result in generating ultra-fine

particles.

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How important is grinding ?

on a survey of the energy consumed in a number ofCanadian copper concentrators it was shown that the

average energy consumption in kWh/ton was 2.2 for

crushing, 11.6 for grinding and 2.6 for flotation (Joe,

1979).

it can be shown, using Bonds equation, that 19% extraenergy must be consumed in grinding one screen size

finer on a 2 screen series.

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PRINCIPLES OF COMMINUTION

stresses may be generated by tensile or

compressive loading

the presence of cracks or flaws in the

matrix which acts as sites for stress

concentration

increase in stress at crack tip is

proportional to square root of the length

of crack perpendicular to the stress

direction.

Strain of crystal lattice resulting

from tensile or compressive stress

Stress concentration at a crack tip

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METHODS OF SIZE REDUCTION

Solids may be broken in eight or nine different ways but only four of them

are commonly used in size reduction machines.They are

(1) compression, (Jaw crusher)

In general, compression is used for coarse reduction of hard solids,

to give relatively few fines

(2) impact, (Ball mill)Impact gives coarse, medium or fine products

(3) attrition or rubbing (pulverizer)

The attrition yields very fine products from soft, nonabrasive

materials.

(4) cutting. ( pair of shears)

Cutting gives the definite particle size and some times definite shape

with few or no fines.

Cont.

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Shatter

Induced by rapid application of

compressive stress.

occurs in industrial autogenous,

rod and ball mills.

PATTERN OF FRACTURE WHEN A

SINGLE PARTICLE BREAKS

Cont..

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Cleavage

occurs at preferred surfaces.

produce several relatively

large fragments together with

much finer particles

Cont..

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Attrition and Chipping:

occurs when the particle is largeand the stresses are not largeenough.

occurs in autogenous

generates a significant numberof particles that are muchsmaller than the parent size(birth process). parent particle is

not destroyed (no death process)

in contrast shatter and cleavagegive rise to both birth and deathprocesses

Cont..

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OBJECTIVES OF COMMINUTION

production of a final product, e.g. construction material, coal

production of a product for the following benefication steps

-Classification

-sorting [liberation of the resources, value mineral]

-Leaching

-others (thermal, biological, etc. processing steps)

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Evolution of Comminution Technologies

Chronology:

Manual comminution

Hammer mechanism (ca. 1512)

Edge runner (ca. 1800)

Ball mill (ca. 1890)

Vertical roller mill (ca. 1930)

High pressure grinding rolls (ca. 1985)

Horomill

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Manual comminution (16th century)

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Hammer mechanism (16/17th century)

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Edge runner (18/19th century)

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Why size Reduction?

stringent market specification

easy transport and handling of solid particles

increases the reactivity of solids especially in catalytic

reactions

it permits separation of unwanted ingredients by

mechanical separations

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in mineral processing, it is required to break the lump minerals to

separate valuable minerals from intimately associated with gangue

minerals.

chemical reaction, the chemical reactivity of fine particles is

greater than that of the coarse particles.

colouring of pigment is considerably affected by the size of the particle.

for more intimate mixing of the solids can be achieved if the

particle size is small.

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Comminution mechanisms in different grinding systems

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CHARACTERISTICS OF SOLID PARTICLES

Particle shape

Particle size

Specific surface of mixture

Average particle size

Crushing mechanisms

LECTURES-2 & 3

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PARTICLE SHAPE

this is defined as the surface-volume ratio for a sphere of

diameter Dp divided by the surface-volume ratio for the particlewhose nominal size is Dp

the shape of an individual particle is conveniently expressed in terms

of the sphericity (s).

sphericity is independent of the particle size, for non-spherical

particle sphericity is defined by the relation

---------- (1)

where Dp = equivalent diameter or nominal diameter of particle

sp = surface area of one particle

vp = volume of one particle

pp

p

s sD

v6

p

p

p

s

vs

D6

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PARTICLE SIZE

Particle size is expressed in terms of diameter if it is a sphericalparticle. Particles that are not equi--dimensional it is characterised by

second longest major dimensions.

Example: for needle like particles, particle size Dp would refer tothickness rather than length of the particle

Equivalent diameter: Dpe

diameter of sphere having the same ratio of surface to volume

on the actual particle.

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MIXED PARTICLE SIZES

in a sample of uniform particles of diameter (Dp

), the total volume of

particles is m/p where m and p arethe total mass of the sampleand the density of the particles, respectively. Since the volume of

one particle is vp,

the no. of particles in the sample N is

ppvmN

the total surface area of the particles is , from equation (1) & (2)

---------- (2)

---------- (3)

Equation (2) & (3) can be applied to each fraction through which

surface areas can be estimated.

pps

pDmNsA

6

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SPECIFIC SURFACE OF MIXTURE

specific surface of mixture is defined as the area occupied by the

mixture of particle per unit volume.

. (4)

where xi= mass fraction in a given increment

Dpavg= arithmetic average of smallest and largest particle

diameter in increment.

n

i pi

i

pspnps

n

ppsppsw D

x

D

x

D

x

D

x

A12

2

1

1 6666

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AVERAGE PARTICLE SIZE

The average particle size for the mixture of particles is defined as givenbelow.

where = Volumesurface mean diameter

= Sphericity of the particle

Aw = Specific surface area of particles in m2/m3

p = Density of particles

pw

sA

D

s

6

n

i pi

i

s

Dx

D

1

1

ssD

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Ari thm et ic mean d iameter

T

n

i

pii

n

i

i

n

i

pii

NN

DN

N

DND

1

1

1

Mass mean diameter

n

i

piiw DxD1

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Volume mean d iameter

Number of partic les in m ixture

n

i pi

i

V

Dx

D

1

1

3

pp aDv

n

i Vp

n

i pi

i

p

wDaD

x

aN

1

3

1

3

111

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CRUSHING MECHANISMS

There are four mechanism by which size reduction

occurs.

Compression

Impact

Shear

Attrition

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COMPRESSION

as the name implies, crushing by compression is done between two

surfaces, with the work being done by one or both surfaces.

jaw crushers uses this mechanism of compression hence suitable forreducing extremely hard and abrasive rock.

however, some jaw crushers employ attrition as well as compression and

are not as suitable for abrasive rock due to the rubbing action

Cont.

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IMPACT

in crushing terminology, impact refers to the sharp, instantaneous

collision of one moving object against another.

both objects may be moving, such as a baseball bat connecting with

a fast ball, or one object may be motionless, such as a rock being struck

by hammer blows.

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Impact is most useful in following

circumstances

when a cubical particle is needed

when finished product must be well graded

and must meet intermediate sizing

specifications, as well as top and bottom

specifications

when ores must be broken along naturalcleavage lines in order to free and separate

undesirable inclusions (such as mica in

feldspar)

Continued

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shear consists of a trimming or cleaving action rather than the

rubbing action associated with attrition.

Example, :single-roll crushers employ shear

.Continued

SHEAR

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Shear crushing is used

when material is somewhat friable and

has a relatively low silica content

for primary crushing with a reduction

ratio of 6 to 1

when a relatively coarse product is

desired, usually larger than 11/2" (38mm)

top size

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attrition is a term applied to the reduction of materials by scrubbing it

between two hard surfaces.

Example :Hammer mills operate with close clearances between the hammers

and the screen bars and size is reduced by attrition combined with shear andimpact reduction.

Continued

ATTRITION

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Attrition crushing is most useful in the

following circumstances:

when material is friable or not too abrasive

when a closed-circuit system

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CHARACTERISTICS OF AN IDEAL

CRUSHER

have a large capacity

require a small power input per unit of product

yield a product of the single size or the size distribution required

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Energy Consumed in Crushing

1. Deforming the particle to its elastic limit

2. Compacting particles after fracture

3. Overcoming friction between particles

4. Elastically deforming milling surfaces

5. Deformation of fractured particles

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Crushing Efficiency

Crushing efficiency is defined as the ratio of the surface energy created by

crushing to the energy absorbed by the solid.

where

is the efficiency

es is the energy per unit area, feet times pounds force per square foot

Awb is the surface area per unit mass of product

Awa is the surface area per unit mass of feed

Wn is the power input to the crusher

c

wawbsn

AAeW

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Factors Influencing Choice of Size Reduction

Equipment

Feed and Product Size

Feed size Product size

Coarse Crushers 1500 - 40mm 50 - 5mm

Intermediate Crushers 50 - 5mm 5 - 0.1mm

Fine Crushers. grinders 5 - 2mm

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Nature of Material

1. Hardness - very hard materials are better in low speed or low

contact machines2. Structure - fibrous materials need tearing or cutting action

3. Moisture content - materials with 5 - 50% moisture do not flow

easily and can be difficult to process

4. Friability

5. Stickiness - sticky materials need easily cleaned machines

6. Soapiness - if coefficient of friction is low, crushing may be difficult7. Explosives - need inert atmosphere

8. Hazardous to health - need good confinement

9. Closeness of distribution

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Principal Types of Size Reduction Equipment

are as Follows

1. Jaw Crusher (coarse and fine)2. Gyratory Crusher

3. Crushing Rolls

B. Grinders (Intermediate and fine)

1. Hammer Mills2. Rolling Compression Mills

3 .Attrition Mills

4 Tumbling Mills

C. Ultrafine Grinders

1. Hammer Mills with Internal Classification

2. Fluid Energy Mills

3 .Agitated Mills

D. Cutting Machines

1.Knife Cutters; Dicers; Slitters

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STAGES OF SIZE REDUCTION

Explosive shattering from infinite to 1 m

Primary crushing from1 m to 100 mm

Secondary crushing from

100 mm to 10 mm

Coarse grinding from10mm to 1mm

Fine grinding from1mm to 100 microns

Very fine grinding from100micron to 10 micron

Super fine grinding below 10 microns

BASIC CRUSHING PLANT FLOW SHEET

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BASIC CRUSHING PLANT FLOW SHEET

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Choke feeding

discharge is restricted by inserting a screen in the outlet, so material

stays choked in the action zone until reduced to a small enough size

long residence time results in undersize particles and additional energy

consumption useful to prevent oversize, and a large reduction ratio can

be achieved.

Closed circuit grinding

residence time kept short, but classifier system at the outlet separates

oversize material and recycles it

more energy efficient, with narrower range of final particle size

additional cost of classifier system

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COMMINUTIONGOVERNING LAWS

RITTINGERS LAW

KICKS LAW

GENARALIZED LAW

BONDS LAW

RITTINGERS LAW

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The work required in crushing is proportional to the new surface created.

Crushing efficiency (c) is constant and for a given machine and feed material is

independent of the sizes of feed and product. If the sphericities aand bare equal

and the mechanical efficiency is constant, various constants in efficiency equation

can be combined into a simple constant Krand Rittingerslaw is

RITTINGER S LAW

Rittingers law is applicable for coarse crushing

where P = power

= feed rate

and are the volume surface mean diameters of feed and product respectively.

This applies where energy input per unit mass is not too high. Kr

can be

determined from the experiments conducted on machine of the type to be used

and material to be crushed.

sasb

r

DDK

mP

11

m

saD

sbD

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Kicks proposed a law based on the stress analysis of plastic deformation

within the elastic limits, which states that the work for crushing a given mass of

material is constant for the same reduction ratio, that is, the ratio of an initial

particle size to the final particle size.

according to Kicks law, the energy required for size reduction is given by

KICKS LAW

Kicks law is accurate in crushing range above 10mm size

sb

sak

D

DK

m

Pln

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A generalized relation for both cases is the differential equation

when

n = 1 leads to Kicks law

n = 2 Rittingers law

sn

s

D

DKd

m

Pd

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Work required to produce particle of size Dpfrom very coarse feed isproportional to the square root of the surface to volume ratio of the product

p

p

v

s

BONDS LAW

=

pD

6

p

p

v

s

p

b

D

K

m

P

Kb= constant, depends on the type of machine and on the material

This is more realistic in estimating the power requirement of commercial

crushers and grinders.

Bonds law applicable size range: 10-1000 m (Ball Mills and Rod Mills)

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WORK INDEX (Wi)

Work index, Wi is defined as the gross energy requirement in

kilowatthours per ton of feed needed to reduce a large feed to such a size

that 80 percent of the product passes a 100-m screen.

This definition leads to a relation between Kb and Wi. If Dpis in millimeters,

P in kilowatts, and in tons per hour

If 80 percent of the feed passes a mesh size of Dpa millimeters and 80

percent of the product passes a mesh size of Dpb millimeters, it leads to

the equation

The work index includes the friction in the crusher, and the power

given by above equation is gross power.

papb DDm

P 113162.0

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Typical work indexes for dry crushing#or wet grinding

Material Specific gravity Work index, Wi

Bauxite

Cement clinker

Clay

Coal

CokeIron ore (Hematite)

Limestone

Quartz

Trap rock

2.20

3.15

2.51

1.4

1.313.53

2.66

2.65

2.87

8.78

13.45

6.30

13.00

15.1312.84

12.74

13.57

19.32

# for dry grinding, multifly by 4/3

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Grinding tough materials at cryogenic temperatures means that they are

brittle enough to be ground to very fine particle sizes.

Cryogenic Grinding

Cryo-grinding systems, inject controlled amounts of liquid nitrogen toregulate the heat of the grinding mill and the allow higher material

throughputs at any time of year.

Inert Grinding

Organic materials have an increased risk of explosion when veryfine particles combine. The use of nitrogen to inert the atmosphere

during the grinding process minimises safety hazards associated with

explosions.

CRYOGENIC-GRINDING PROCESS

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Raw material passing along a conveyor is cooled using controlled amounts of liquid

nitrogen which allows for finer grinding and increased throughputs.

CRYOGENIC GRINDING PROCESS

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readily available, easy handling, low technical input, easy control, good heat transition, and inert atmosphere.

Economic aspect of Cryogenic Grinding

The utilisation of liquid nitrogen (LN2) has undoubtedly numerous

advantages in applications, such as:

Applications

New Materials preparationSpace ResearchExtracting Metals & Plastics from Electronic waste

target Industries like

Mineral based IndustriesChemicals, Food & PharmaceuticalsRubber,Plastics,Manufacturing etc.,.