Increase Productivity

Reduce Operating Costs

How Rock Breaks ?
Improve Long Term Sustainability
Rock Breakage During Blasting

Purpose & Topics

Purpose: to get a basic understanding of rock breakage in key

mining and milling processes

Topics:
• Rock breakage in blasting process
• Ore breakage in crushing
• Ore breakage in grinding

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Rock Breakage During Blasting

Rock Blasting Process

Blast results
Explosive • fragmentation
energy • muckpile profile
• Dilution
Rockmass
• Damage
• Vibrations
Blast geometry & • Flyrock
Initiation pattern

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Rock Breakage During Blasting

Detonation Phase
• Detonation: an explosion in a reactive
material which is characterised by a
shock wave moving at greater speed
than sonic velocity and accompanied
by a chemical reaction whose energy
release supports the propagation of
the shock.
• Upon detonation explosive chemicals
are converted into large quantity of
hot, high pressure gases.
• Detonation Pressure is the pressure
in the CJ plane, behind the
detonation front and can be
estimated as
• 𝑃𝑑 = 0.25 ∗ 𝜌𝑜 ∗ 𝐷2
• Pd in Pa; o in kg/m3 , D:
detonation velocity in m/s
• Explosion pressure is the pressure in
behind the CJ plane and is usually
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 Rock Breakage During Blasting

Shock Wave Phase

Open fracture • Explosion gases expand the

blasthole until it reaches an
Tensile wave equilibrium with the stress
Compressive wave
• The explosive energy
Spalling expanded until this point is
defined as shock energy
• Energy during this phase is
used to develop the fracture
Radial cracks network
Expanded blasthole
Original blasthole
Crushed zone

Compressive stress > Dyn Compressive strength – leads to crushing

Tangential stress > Dyn tensile strength – leads to radial cracking

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Rock Breakage During Blasting

Gas Expansion Phase

p
i
Hydrostatic
p zone
i

p
i

Free face

pi
Explosion gases penetrate into fracture network at
approximately 0.1- 0.4 times the sonic velocity
State of stress
on an element in
hydrostatic zone

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Rock Breakage During Blasting

Burden Movement Phase

Stemming
ejection

Original blasthole
Crushed zone

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Rock Breakage During Blasting

Single Hole Break-Out

Crater from a
single hole

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 Rock Breakage During Blasting

Phases of the bench blasting

12m bench at a quarry with BxS = 4x5 m

Phase Speed, m/s Duration

VOD 4000-6000 2-3 ms
Shock wave 4000-6000 2-3 ms
Cracking ≈1000 5-10 ms
Gas flow 100-500 20-80 ms
Bench starts 20-80 ms
Burden moves 10-20 Some seconds

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 Rock Breakage During Blasting

Explosive – Rock Interaction

Ideal blasting energies: useful energy(shock&heave)+ lost energy

Energy partitioning theory by Lownds (1991)

Cut-off
Pressure, 100MPa

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Rock Breakage During Blasting

Explosive – Rock Interaction

PCJ Kinetic Heave Energy - used in fragmentation

Effective Shock Energy

Equilibrium Effective Heave Energy – used in displacement

point

Escape
point
Pc

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Rock Breakage During Blasting

Explosive – Rock Interaction

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Rock Breakage During Blasting

Explosive – Rock Interaction

Emulsion, 165mm, hard rock ANFO, 165mm, hard rock

VOD>Vp Vs<VOD<Vp VOD<Vs

Stress wave shape and mode determines energy

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Rock Breakage During Blasting

Rock Response to Energy

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Rock Breakage During Blasting

Rock Response to Energy

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Rock Breakage During Blasting

Fragmentation
Fragmentation has two Liberation
mechanisms
• failure of the intact rock
• liberation of in-situ matrix

Breakage
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 Optimum Fragmentation?

• Top size 100

• Dig times Top size
• Fill factors
• Crusher blockages
•

Percent passing
Leaching
• Mid - range 50
• Dig times
• Grinding
• Fines Mid - range
• Dig times
• Fill factors Fines
• Grinding
0
• Leaching 1 10 100 1000

Size mm
Optimum size depends on the requirements of the downstream processes

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Optimum Fragmentation

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 Impact of Fragmentation – Load and Haul

12
Benchmark Blast
10
Finer fragmentation
8
Frequency

6 Pay load = 63.1 mt

FragIndex = 6.9
4

0 30%
2500 3000 3500 4000 4500 5000 5500 6000

Loading Productivity (tonne/hr)

Pay load = 84.5 mt
FragIndex = 4.6

(Michaud and Blanchet, 1995)

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 Impact of Fragmentation – Load and Haul

• A case study from a gold mine in Asia

• Waste fragmentation optimisation
• Dig rate improved by 25-30%.
• 4.0M USD cost savings for the LOM

Dig rate before Dig rate after % Increase

Mine to Mill, Mine to Mill,
t/h t/h
EX 1900 845 1100 30.2
EX 1200 638 798 25.1

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 Fragmentation Impact on Crushing

• Big top size can jam the crusher

• Reduced top size increases throughput
• Reduced top size reduces crusher power
• Reduced top size allows choke feeding and smaller gap
setting
• Finer mill feed
100

90

80

70
Percent Passing

60

50

40
PC product - 2006
data
30
Sagfeed - 2006
survey
20
29-5-08 survey

10
"Dec-08 curvey"

0
0.1 1 10 100 1000
Size

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Impact on sag mill feed size
(Kanchibotla and Valery 2010; Esen et al, 2007)
Fragmentation impact on SAG mill

Copper operation – 36ft SAG mill

Dance et al., 2006

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Optimum Feed size for SAG millsll

100

90
Top Size
80

70
% Cumulative Passing

Critical Size
60

50

40

30

20

10
Fines
0
1 10 100 1000
Size (mm)

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Optimum Feed size for AG millsl

100

90
Top Size
80

70
% Cumulative Passing

60 Critical Size
50

40

30

20
Fines
10

0
1 10 100 1000
Size (mm)

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Optimum Feed size for Multi Stage Crushing & Grindingll

100

90

80

70
% Cumulative Passing

60

50

40

30
finest possible
20

10

0
1 10 100 1000
Size (mm)

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 Strength of Blast Fragments (Pre Conditioning)

Open joint
Micro fractures
and liberation of Tensile wave
valuable mineral

Spalling

Radial fracture

Compressive
wave
Crushed zone

Surface area of micro-cracks may be 10 - 100 times larger

than the surface area of fragments (Revnivstev 1988)

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 Strength of Blast Fragments (Pre Conditioning)

• Katsabanis (2008, 2009) showed that

rock damage increased with an
increase of powder factor.
• Adamson and Parra (2017) showed a
similar result (about 30% reduction in
ore strength).

(Katsabanis, 2009)

(Akesson et al, 2004)

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Rock Breakage During Blasting

Explosive Selection

High VOD Medium VOD • Rock properties

High density High density
• Blast objectives
Strength

High VOD Low VOD Low VOD High VOD

Low density Low density High density High density

Movement
Fractures

Low VOD High VOD

Low density Low density

Fragmentation

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Rock Breakage During Blasting

Powder Factor

• Explosive consumption per unit

volume or weight of rock
Problems with PF
o Simple to estimate
o Used for design purpose • Desired blast results?
o To control costs
• Rock mass properties?
• Explosive energy and partition?
• Explosive efficiency?
1.2 g/cc 0.8 g/cc
Explosive Explosive • Energy distribution?
VOD 5500 m/s VOD 4000 m/s

• Timing and pattern?

What is the difference ?

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Rock Breakage During Blasting

Electronic Blasting System

• It can add value to improve fragmentation

and reduce vibration
• Shock wave interaction due to ultra-short
delay (1-3ms) use
• About 46% improvement in X50 at
Chuquicamata Mine, Chile
• Ore/waste segregation blasting (Chiappetta, 2010)
time

overlap of
tensile tails of overlap of
blast waves leading
compressive
wave parts

P-wave fronts
blast- S blast-
hole hole
(Vanbrabant and Escobar, 2008)
(Rossmanith, 1997)
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Rock Breakage During Blasting

Summary
• Rock breakage in blasting is a function of rock mass
characteristics, blast energy input and its distribution
• Rock fragmentation in blasting is due to liberation of existing
fractures and creation of new fractures in the intact rock mass
• Optimum fragmentation depends on the downstream processes
• Explosive selection should be based on the intended blast
outcomes, rock mass characteristics and hydrology of the bench
• Powder factor alone is not the best criterion to design blast.
Blast design should take into account:
• Rock mass characteristics
• Intended blast outcomes
• Energy distribution
• Delay timing

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• Confinement conditions and energy release
 Breakage Theory During Crushing
and Grinding

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Rock Breakage During Crushing & Grinding

Purpose & Topics

Purpose: to get a basic understanding of breakage interactions

during comminution and how these impact the outcomes of the
Mine-to-Mill® processes

Topics:
• Comminution Theory
• Breakage Mechanisms in comminution machines

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Crushing & Grinding

Crushing and Grinding

Throughout the years

Early machines mimicked Newer machines moved
there have been many
breakage by hammers and the bed of materials being
types of machines used to
rollers broken, often by rotation
break rocks

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Crushing & Grinding

Size Reduction Technology

Different technologies more efficient at different sizes

CRUSHING GRINDING

• Jaw Crusher • Tumbling mill

(SAG, ball)
• Gyratory Crusher
• Stirred Mill (Tower
• Impact crusher mill)
(VSI and HSI)

FINER

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Rock Breakage During Crushing & Grinding

Breakage in Comminution Machines

Why do we need breakage?

LIBERATION

• To separate the components of the ore and concentrate the

valuables.
• Different degrees of liberation for different processes: HMS,
flotation, leaching.
• Size as an indirect measure of liberation.

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Rock Breakage During Crushing & Grinding

Breaking Rocks

There are different breakage

mechanism
• Single Vs Multiple particle
breakage

• High Energy vs Low Energy

particle breakage

• Compressed bed breakage

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Rock Breakage During Crushing & Grinding

Breaking SMALL Particles requires MORE ENERGY

Larger particles have more and bigger flaws. When particles

become smaller, flaws become fewer and smaller. When particles
become very fine (single grains) their resistance to breakage
increases sharply.

In comminution theory, at least, size matters!!

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 Rock Breakage During Crushing & Grinding

General Form of the Energy-Size Relationship

106

105 𝒅𝒙
𝒅𝑬 = −𝑲 × 𝒏
Energy Consumption (kWh/tonne)

104 Hukki (1961)

𝒙
(integrated)
Rittinger (1867)
103 (slope= -1.0) A decrease in grain size
1 1  A decrease in liberation P80
102 𝐸=𝐾 −
𝑥2 𝑥1  A massive increase in
energy consumed
101 Bond (1952)
(slope= -0.5)
100 Kick (1885)
1 1 (slope=0)
𝐸=𝐾 −
𝑥2 𝑥1 𝑥1
10-1 𝐸 = 𝐾 × 𝑙𝑛
𝑥2
10-4 10-2 100 102 104 106
Size mm

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Rock Breakage During Crushing & Grinding

Blending and Rock Breakage

• Paradox: hard parts help to break the rock

• Stress the rock (squeeze or hit)
• Softer deforms further than harder
• Stresses multiply at the boundaries
• Soft components break to save harder ones
• Important for SAG mill ore blends

Harder rocks survive ➔ Dominate Throughput

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 Rock Breakage During Crushing & Grinding

What are we trying to understand?

Amount of energy required T10 (%)

to break a particle 38.1

+ 19.8

The resulting size 10.4

distribution of product 4.4

particles A*b =23 (hard ore) JKRBT

0.2 0.5 1.0 2.0

Breakage Energy (kWh/t)

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 Crushing and Grinding Equipment
and Mechanisms

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Crushing & Grinding

Crushing and Grinding Equipment

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Crushing & Grinding

Jaw Crushers
Features
• cheap, heavy duty
• able to treat a large top size of
material but not able to produce a
particularly fine product
• generally only used in primary
crushing
• especially useful for treating a
scalped oversize stream

Key parameters
• Single or double toggle – affects
movement of jaw
• Open and closed side settings
• Throw
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Gyratory/Cone Crushers
Crushing & Grinding

Gyratory (primary)/ Cone(secondary, tertiary) crushers are reliable, heavy duty crushers
which are power efficient, but often more expensive then the other types.

Gyratory Crushers Cone Crushers

• Long tapered chamber • secondary and tertiary crushing
• slowly munches large rocks • different gap geometry to primary
crushers, flatter mantle
• sub-size falls straight through
• Reduction ratios are lower. 4:1
• Idle a lot of the time compared to 8:1

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Rock Breakage During Crushing & Grinding

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Crushing & Grinding

Gyratory Crusher

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Rock Breakage During Crushing & Grinding

Energy Required for Breakage in Mills

Energy from Height - I

Vacuum

?m 1 kWh/t

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Rock Breakage During Crushing & Grinding

Energy Required for Breakage in Mills

Energy from Height - II

Vacuum

360 m 1 kWh/t

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Rock Breakage During Crushing & Grinding

Energy Required for Breakage in Mills

Energy from Height - III

100 mm dia. rock

70 mm dia ball

?m 1 kWh/t
10 mm dia rock

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Rock Breakage During Crushing & Grinding

Energy Required for Breakage in Mills

Energy from Height - IV

100 mm dia. rock

70 mm dia ball

0.36 m 1 kWh/t
10 mm dia rock

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Crushing & Grinding

Large Trunnion Mounted SAG Mills

Up to 42 ft Diameter, 28 MW Gearless Drives

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Crushing & Grinding

Modern Ball Mills

OYU TOLGOI CITIC PACIFIC – SINO IRON

24 ft Diameter, 11.4 MW 26 ft Diameter, 15.6 MW

As large as 28x44 ft, 22MW

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Rock Breakage During Crushing & Grinding

Breakage Mechanisms AG/SAG Mills

Shoulder
Region

Ball

Drop
Height Rock

Impact Zone Attrition

Toe Region Zone

Power and breakage both directly linked to charge motion

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Crushing & Grinding

Charge Motion

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Crushing & Grinding

Charge Motion

Cascading and Cateracting Varying Mill Speed

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Rock Breakage During Crushing & Grinding

Breakage Mechanisms

High Energy Low Energy

Impact Abrasion Attrition

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Rock Breakage During Crushing & Grinding

Breakage with Rods and Balls

• In a ball mill particles are “nipped” between steel balls

• In a rod mill they are broken between the rods

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Rock Breakage During Crushing & Grinding

Breakage Theory Summary

• Input energy related to resultant product size.

o A specific material characteristic.

• Complicated sets of "indices" or “adjustment factors"

are applied in the relationship.

• More breakage energy per unit mass is required as

particles become finer.
o Only the Bond relationship reflects this.

• Process Engineers are interested not only in the energy

required to break a particle, but also the resulting size
distribution of the product particles.
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Rock Breakage During Crushing & Grinding

Breakage Theory Summary

• The underlying assumption of all single point size

measures is that the shape of the size distribution
remains relatively constant regardless of breakage
history.
• This is usually true for rod and ball mills but often in
serious error for crushers, AG mills and SAG mills.

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SAG Mill Monitoring and Control

Variable ore
= variable operation
= variable grind
= variable recovery
= lower recovery

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Targets

Automatic control system stabilises mill load

Product size varies
“What are the control conditions to target for
optimal mill operation?”
Test 1 Window SAG Feedrate

Throughput tph? SAG Load SAG Pow er

200 1900

grind size? 190 1850

180 1800

Feed rate (tph) and Mill load (t)

170 1750

160 1700

Power (kW)
150 1650

140 1600

130 1550

120 1500

110 1450

100 1400
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12:00 12:30 13:00 13:30 14:00 14:30 15:00
Time (hh:mm)
 Impact of Mill Instability Ball Size

Speed
Grinding
rate
Ball and Rock
Filling

MILL
FEED

Feed type Mill contents

Hardness Feed PSD

Density Grate Size
Size dist
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PRODUCT
Lost Opportunity

Unstable mill operation leads to:

• Cost of stabilisation control
• Sacrificing grind control
• Losing recovery

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Mill Response Observations

3 000
20% balls
35% balls
35% balls
2 800

2 600 20% balls

2 400
power, kW

2 200

2 000

1 800

1 600

1 400
35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115

Load, t

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Merensky SAG mill
- Paul Green

50
tph Power 2400
Grind
45

Power, kW, throughput, tph*10

%-75um 2200

40 2000
% -75um

1800
35
1600
30
1400
25
1200

20 1000
20 25 30 35 40 45 50

% filling
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Mill Power, kW power throughput
Kopanang
Low aspect closed circuit, high filling

100 Power 2200

tph
2000
95
% -75um, throughput, tph

1800
90
1600

Power, kW
85 1400
Grind 1200
80
1000
75 800

600
70
throughput %-75um Grind 400
65
throughput power, kW power 200

60 0
20.0 25.0 30.0 35.0 40.0 45.0

% filling

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Observations

• Throughput & power peaks do not coincide

• dramatic affect of mill speed

• Peak values change with speed

• Grind coarsens with speed

• Throughput increases with speed

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Opportunity

Varying mill speed for control

• grind is responding dramatically
• unknown by operator or control system

Complex response
• opportunity to manipulate mill performance
• operating window - throughput & grind
• family of operating conditions
• throughput–grind trade-off
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Mine-to-Mill™ Philosophy

Breakage Leverage in the Mine-to-Mill Value Chain

How can we
Leverage these

Energy
mechanisms?

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Rock Breakage in Crushing & Grinding

The Production Value Chain

Blasting

Crushing
How can we
High Energy Breakage Leverage these
mechanisms?
Low Energy Breakage

Optimized Process Operation

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QUESTIONS?