UNLOCKING OPTIMAL FLOTATION:

is the AIR RECOVERY the key?

Jan CilliersRoyal School of Mines

Imperial College London

Outline

The Origins of Air Recovery• Modelling Flotation Froths• Useful froth equations

Air Recovery Application• Measuring air recovery• Air rate effect and flotation performance• Bank air profiling using air recovery

•Air leaves a flotation cell by bursting on the top of the froth or overflowing into the concentrate.

•The AIR RECOVERY is the fraction of the air that that overflows (and does not burst)

Air leaving froth by bursting at top surface

Air into the cell

Air overflowing the weir as froth

Froth concentrate

What is the Air Recovery?

The Origins of Air Recovery

• Modelling Flotation Froths• Useful froth equations

Froth Flotation and Froth Physics

The surface chemistry determines whether the minerals can be separated

The froth physics determines how well the separation happens

Requires a froth-phase model describing the physics

A Flowing Froth Model - components

• Froth motion

• Liquid flow in the froth

• Solids motion

Froth Structure:The Physics of the Froth

Films between bubbles

Plateau borders

Froth motion from pulp to concentrate

Laplace equation gives velocity

Boundary conditions:

1. Shape of tank and launders

2. Air entering the froth that overflows:

AIR RECOVERY (%)

Froth Flow in Radial Equipment Designs

Liquid Flow in the Froth

Three balanced forces act on the liquid in Plateau borders:

Gravity, capillary and viscous dissipation

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Liquid Motion and Content

Solids Motion1. Attached SolidsParticles attached to bubbles

move with the frothMost particles are detached due

to coalescence (>95%)

2. Unattached Solids:

Particles move in the Plateau borders

Follow the liquid, settle and disperse

Overflow into concentrate

Mineral and Waste Particles Example of motion in Plateau borders

Valuable Mineral

Gangue Minerals

Mineral grade in froth

Froth Launder Design:Effect of forcing froth to flow inwards or outwards

INTERNAL INTERNAL CHANNELCHANNEL

CHANNEL 1CHANNEL 1 CHANNEL 2CHANNEL 2

Internal Launder Two Launders

Tracking particles in flotation using PEPT

Model validation

Tracking particles in flotation using PEPT

Model validation

Simplified Equations for Flotation Modelling

Water flowrate to concentrate

Entrainment factor

Froth recovery

(α<0.5)

Water flowrate to concentrate

)1(2

2

bubble

gcolumnl d

JAkQ

ENTRAINMENT FACTOR Ratio of gangue recovery to water recovery

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Froth Recovery

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Froth Modelling Summary

• Froth physics determines the effectiveness of the flotation separation

• Complex froth zone simulators are available for operation and design

• Simplified models have been developed for liquid recovery, froth recovery and

entrainment, based on the physics

All the froth models include THE AIR RECOVERY

Air Recovery Application

• Measuring air recovery• Air rate effect and flotation performance

• Bank air profiling using air recovery

Air leaves a flotation cell by bursting on the top of the froth or overflowing into the concentrate.

The AIR RECOVERY is the fraction of the air that that overflows (and does not burst)

Air leaving froth by bursting at top surface

Air into the cell

Air overflowing the weir as froth

Froth concentrate

Air recovery.. a reminder

Measuring the air recovery

Air Recovery =

Volumetric flowrate air overflowing

Air flowrate into cell

Volumetric flowrate air overflowing

= overflowing velocity x overflowing froth height x lip lengthAir In

Air leaving through bursting

Air flowing over lip

Overflowing velocity

Overflowing froth height

Air Recovery shows a maximum (PAR) at a specific air rate

Air Recovery

Bubbles heavily loadedStable, but move slowly

Bubbles under-loadedUnstable, burst quickly

Optimum balance between froth stability and motion

Air Velocity into Flotation Cell

Why is there a Peak in Air Recovery (PAR)?

Predicting air recovery – theory

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Air Recovery and flotation performance

Air rate that gives highest air recovery also gives highest mineral recovery

Froth appearance

• Air rate 8m3 min-1

• Air recovery 70%

• Air rate 12m3 min-1

• Air recovery 40%

Air Recovery

Metallurgical Recovery

Bubbles heavily loadedStable, but move slowly

Bubbles under-loadedUnstable, burst quickly

Optimum balance between froth stability and motionHigh recovery and grade

Air Velocity into Flotation Cell

REDUCE AIRIncrease gradeIncrease recovery

INCREASE AIRReduce gradeIncrease recovery

Why does the Air Recovery affect flotation?

Air Recovery Application

• Measuring air recovery• Air rate effect and flotation performance

• Bank air profiling using air recovery

The air rate profile in a flotation bank affects the performance

• Two strategies:

1. Determine the best air rate profile– Vary distribution of a set total air addition

1. Determine the optimal total air addition– Vary the total air addition with a set air profile

Air rate profiling

Air rate profiling approaches

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1. Different air profiles with same total addition (e.g. Cooper et al., 2004)

2. Different air addition with the same profile

(Hadler et al., 2006)

Air Profiling Strategies

1. Determine the best air rate profile– Vary distribution of the total air addition

– Increasing profile typically improves performance e.g. Cooper et al., 2004; Gorain, 2005; Hernandez-Aguilar and Reddick, 2007; Smith et al., 2008

1. Determine the optimal total air addition

Determining the air rate profile

• Increasing profile typically yields better performance Higher cumulative grade for same cumulative recovery (e.g. Cooper et al., 2004)

Introduction: Previous work

1. Determine the best air rate profile

1. Determine the optimal total air addition– Best performance at air rate giving

Peak Air Recovery (PAR)

e.g. Hadler et al., 2006; Hadler and Cilliers, 2009

25%26%27%28%29%30%31%32%33%34%35%

0% 20% 40% 60% 80% 100%

Cumulative Recovery (% Cu)

Cu

mu

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Cu

)

As Found Peak Air Recovery

Cu Rougher Performance: Grade-Recovery and Air Recovery

75.6%

Cumulative recoveries:

76.3%

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Study performed in two stages

1. Air rate profiling tests

2. Air recovery optimisation (PAR) tests

First direct comparison of the two approaches

Stage 1: Air rate profiles

1. Air rate profiling tests– Three profiles tested, the ‘Standard’ and

two others, all adding same total air

2. Air recovery optimisation

Air rate profiling: Air rate profiles

0

2

4

6

8

10

Standard Stepped Sawtooth

Air

flo

wra

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m3 m

in-1

Profile

Total air addition

(m3 min-1)

Standard 30.3

Stepped 28

Sawtooth 29.5

Air rate profiling: Performance

0

40

80

120

160

20 30 40

Cumulative recovery / %

Cu

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Standard

Stepped

Sawtooth

0

20

40

60

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Re

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/ %

Stepped Sawtooth Standard

Air rate profiling: Findings

• Order of cumulative Cu recovery is same as cumulative air recovery

– Sawtooth > Stepped > Standard

Mineral recovery and air recovery qualitatively linked

Stage 2: Peak Air Recovery test

1. Air rate profiling test

2. Air recovery optimisation– Preliminary tests to find PAR air rates– Test conducted at PAR air rates– Total air added same as ‘Standard’ profile

Air recovery optimisation: Preliminary tests

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20%

40%

60%

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5 7 9 11

Air flowrate / m3 min-1

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%

Cell A

Cell B

Air recovery optimisation: Air rate profiles

0

2

4

6

8

10

Standard Stepped Sawtooth PAR

Air

flo

wra

te /

m3

min

-1 Profile

Total air addition

(m3 min-1)

Standard 30.3

Stepped 28

Sawtooth 29.5

Peak Air

Recovery

28

Air recovery optimisation: Air recovery

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20

40

60

80

A B C DCell

Cu

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Standard Stepped

Sawtooth PAR

Air recovery optimisation: Performance

0

40

80

120

160

20 30 40 50

Cumulative recovery / %

Cu

mu

lati

ve u

pg

rad

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tio

Standard

Stepped

Sawtooth

PAR

Air recovery optimisation: Performance of first cell

• Effect of air rate:– Recovery

maximum at PAR air rate

– Upgrade ratio decreases with increasing air rate

Air profiling using air recovery: Summary

• Air profiling can significantly improve flotation performance• The performance improvement is a froth effect; rate kinetics alone cannot explain it

• The air rate giving the highest air recovery (PAR) also gives the best flotation • The PAR method simultaneously determines the optimal bank air rate and distribution

• Froth physics determines the effectiveness of flotation• Froth models indicate important variables – this is the

origin of AIR RECOVERY

• Air recovery is affected by air rate; there is an air rate at which the air recovery is a maximum (PAR)

• The Peak Air Recovery (PAR) methodology simultaneously establishes the correct air addition rate and the best air rate profile for a flotation bank

• Significant improvements observed; plant control strategy

Summary and Conclusions

Acknowledgements

• Rio Tinto Centre for Advanced Mineral Recovery at Imperial College London

• Froth and Foam Research team

• Intellectual Property Rights • The peak air recovery-based froth flotation optimisation

methodology is protected by a PCT-stage patent application, covering most of the countries of the world, with additional protection in Chile and Peru

Questions?

UNLOCKING OPTIMAL FLOTATION:

is the AIR RECOVERY the key?

Jan CilliersRoyal School of Mines

Imperial College London