Froth Flotation Innovation and Research

Dr Pablo Brito-Parada

Department of Earth Science and Engineering

Royal School of Mines

Imperial College London

2

Agricola, De Re Metallica, 1556

If we remove metals from the service of man, all methods of preserving the course of life are done away with.

If there were no metals, men would pass a horrible and wretched existence in the midst of wild beasts; they would return to the forest.

… Will anyone be so foolish or obstinate as not to allow that metals are necessary?

• World leading research in flotation froth physics, based in the Department of Earth Science and Engineering, Imperial College London

• Development of fundamentally based models and measurement techniques to characterise the structure and behaviour of 2 and 3-phase foams

• Understanding the physical processes in froth flotation has lead to improvements in operating conditions successfully implemented at industrial scale

The Froth and Foam Research Group (FFRG)

Conceptually simple…

…but chemically and physically complex

A fundamental understanding of the phenomena is the key

Froth flotation

Froth flotation

Los Colorados,

Escondida

Flotation Froths

• Even when it works really badly, it still works pretty well!

• It’s relatively cheap

• Low incentive for big, risky changes

Nonetheless....

• We use huge amounts of energy to grind up the rocks

• We lose quite a lot of the valuable mineral

• We collect quite a lot of the gangue minerals

So what’s the problem with flotation?

• Much and very good work has gone into understanding and manipulating better the surface chemistry

• Equipment manufacturers have reduced cost and increased energy efficiency (through size, mostly)

• Our approach is to focus on the physics of the froth, where the physical separation happens

• Break down the process to its smallest components, build models and do experiments

• Identify new approaches to improvement

How do you improve flotation?

• Flotation froth model, leading to....

• Experimental methods for the froth

• Back to the model, then leading to industrial implementation

• Some other fun experiments

• Some conclusions

Outline of this talk

• Froth motion

• Liquid flow in the froth

• Solids motion

A Froth Model - components

A model that predicts the flowing patterns of the froth

Laplace’s equation gives velocity

Froth Motion

Boundary conditions:

1. Shape of the tank2. Air entering the froth that

overflows and doesn’t burstAir recovery (%)

Where is the liquid? Froth structure and the physics of froths

Films between bubbles

Plateau borders

Liquid Motion and Content in the Froth

Mesh adaptivity Accurate prediction of liquid content

Focussing resolution where it is needed

Liquid drainage

•A narrow column - two vertical, parallel plates, 5mm spacing

• Flowing foam with a single layer of bubbles

• Accurate, automated image analysis possible

Experiments for foam flow and coalescence

Foam wall – operation, coalescence and bursting

Foam wall – operation, coalescence and bursting

Population balance model predicts accurately the bubble size distribution with height

Bubble size and bursting modelsvalidated using experimental data

Now the motion of solids…

1. Attached (valuable) solids Particles attached to bubbles move with froth Most particles detach due to bubble coalescence (>95%)

2. Unattached solidsValuable and gangueMove in the Plateau bordersFollow the liquid and settleOverflow into concentrate

• Difficult experiments…

• Combine simulations and experiments

What do the attached particles do to the bubbles in the froth?

• Filmed at 4000 fps, shown 133 times slower than reality

• Galena particle in top meniscus.

• The film ruptures on or near the particle.

• Reflection and refraction from container and liquid obfuscate the image.

Galena bridging a film

3mm

Experiment and simulation

The experimental system is modelled based on minimum surface energy and compared with video data.

Rendered and ray-traced

Surface Evolver modelHigh speed video still

Film failure is away from particle

•Particle interaction with liquid vapour interface is complex.

• Model cross-section shows the film is forced together away from the particle – this is a new observation.

Film appears to touch at particle surface The film does not actually meet at the

particle surface

Side view Cut away side view

Back to the motion of solids…

1. Attached (valuable) solids Particles attached to bubbles move with froth Most particles detach due to bubble coalescence (>95%)

2. Unattached solidsValuable and gangueMove in the Plateau bordersFollow the liquid and settleOverflow into concentrate

Mineral and gangue particlesExample of motion in Plateau borders

Valuable Mineral

Gangue Minerals

Mineral grade through the froth

Potential for froth launder designInwards vs Outwards froth flow

INTERNAL

CHANNELCHANNEL 1 CHANNEL 2

Internal Launder Two Launders

The behaviour of the different phases at every point in a 3D froth

3D Simulation of Froths

Are the valuable and gangue particle models accurate?

How can we tell?

Positron Emission Particle Tracking(PEPT)

It allows the visualization of the path line of the

particle, measured with a PET camera

PEPT measures the trajectory of a tracer particle

• Particle labelled with a radionuclide that decays via positron emission

• Range in size from a 100 μm to several mm

x

y

z

Cape Town PEPT facilities

Positron Emission Particle Tracking

Model validation

Tracking particles in flotation using PEPT

First data from the original Birmingham system

Particle tracks from PEPT Cape Town

Hydrophobic Hydrophilic

PEPT Raw Data – 500mm hydrophobic tracer

Averaged tracer speed (mm/s)

Hydrophobic Hydrophilic

How does this help industrial operation?

How does air recovery help us?

Measuring air recoveryAir rate effect and flotation performance

Air recovery

A model that predicts the flowing patterns of the froth

Laplace’s equation gives velocity

Froth Motion

Boundary conditions:

1. Shape of the tank2. Air entering the froth that

overflows and doesn’t burstAir recovery (%)

Froth

concentrate

Air

overflowing

the weir as

froth

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 recovery in industry

Measuring froth stability: Air recovery

39

Air In

Air leaving through bursting

Air flowing over lip

Overflowing froth height

Air Recovery =Volumetric flowrate air overflowing

Air flowrate into cell

Volumetric flowrate air overflowing= overflowing velocity

x overflowing froth height x lip length

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

Why is there a Peak Air Recovery (PAR)?

Air rate that gives highest air recovery also gives highest mineral recovery- big economic value

Air Recovery and Flotation Performance

Experiments where bubbles and particles get together

Particle attaching to a bubble

Galena attachment (-212 +150μm)

Loaded bubble coalescence

Galena +45-106μm 500ppm SIBX

Droplets falling onto a bubble film

• Flotation is the largest tonnage separation operation in the world

• It works pretty well, to make it work better is hard, but important

• Improvement has huge financial and sustainability benefits

• The physics of the froth has largely been neglected, that is our focus

• Complex models and novel experiments

Summary and Conclusions I

Air Recovery was identified and explored this way; now an industrial control variable

Novel experimental systems allow us to do new things:• Measure & model coalescence and bursting• Track particles in real systems• Do high-speed observations

Fundamental research is necessary and does improve industrial operations

Summary and Conclusions II

This research was performed in the Rio Tinto Centre for Advanced Minerals Recovery at

Imperial College London

Acknowledgement

Froth Flotation Innovation and Research

Dr Pablo Brito-Parada

Department of Earth Science and Engineering

Royal School of Mines

Imperial College London