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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