UNLOCKING OPTIMAL FLOTATION: is the AIR RECOVERY the key? Jan Cilliers Royal 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

Liquid Motion and Content

Solids Motion 1.Attached Solids Particles attached to bubbles move with the froth Most 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 INTERNALCHANNEL CHANNEL 1 CHANNEL 2 Internal LaunderTwo 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

ENTRAINMENT FACTOR Ratio of gangue recovery to water recovery

Froth Recovery

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 length Air 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 loaded Stable, but move slowly Bubbles under-loaded Unstable, 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

Air Recovery and flotation performance Air rate that gives highest air recovery also gives highest mineral recovery

Froth appearance Air rate 8m 3 min -1 Air recovery 70% Air rate 12m 3 min -1 Air recovery 40%

Air Recovery Metallurgical Recovery Bubbles heavily loaded Stable, but move slowly Bubbles under-loaded Unstable, burst quickly   Optimum balance between froth stability and motion High recovery and grade Air Velocity into Flotation Cell REDUCE AIR Increase grade Increase recovery INCREASE AIR Reduce grade Increase 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 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., 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

Cu Rougher Performance: Grade-Recovery and Air Recovery 75.6% Cumulative recoveries: 76.3%

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 Profile Total air addition (m 3 min -1 ) Standard30.3 Stepped28 Sawtooth29.5

Air rate profiling: Performance

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

Air recovery optimisation: Air rate profiles Profile Total air addition (m 3 min -1 ) Standard30.3 Stepped28 Sawtooth29.5 Peak Air Recovery 28

Air recovery optimisation: Air recovery

Air recovery optimisation: Performance

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 Cilliers Royal School of Mines Imperial College London