UNLOCKING OPTIMAL FLOTATION: is the AIR RECOVERY the key?

1. UNLOCKING OPTIMAL FLOTATION:is the AIR RECOVERY the key? Jan Cilliers Royal School of Mines Imperial College London

2. 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

3. 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) What is the Air Recovery? Air leaving froth by bursting at top surface Air overflowing the weir as froth Froth concentrate Air into the cell

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

5. 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

6. A Flowing Froth Model - components • Froth motion • Liquid flow in the froth • Solids motion

7. Froth Structure:The Physics of the Froth Films between bubbles Plateau borders

8. Froth motion from pulp to concentrate Laplace equation gives velocity Boundary conditions: • Shape of tank and launders • Air entering the froth that overflows: AIR RECOVERY (%)

9. Froth Flow in Radial Equipment Designs

10. Liquid Flow in the Froth Three balanced forces act on the liquid in Plateau borders: Gravity, capillary and viscous dissipation

11. Liquid Motion and Content

12. Solids Motion • 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

13. Valuable Mineral Gangue Minerals Mineral and Waste ParticlesExample of motion in Plateau borders

14. Mineral grade in froth

15. Froth Launder Design:Effect of forcing froth to flow inwards or outwards INTERNAL CHANNEL CHANNEL 1 CHANNEL 2 Internal Launder Two Launders

16. Tracking particles in flotation using PEPT Model validation

17. Tracking particles in flotation using PEPT Model validation

18. Simplified Equations for Flotation Modelling Water flowrate to concentrate Entrainment factor Froth recovery (α<0.5)

19. Water flowrate to concentrate

20. ENTRAINMENT FACTOR Ratio of gangue recovery to water recovery

21. Froth Recovery

22. 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

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

24. 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 recovery.. a reminder Air leaving froth by bursting at top surface Air overflowing the weir as froth Froth concentrate Air into the cell

25. Measuring the air recovery Air leaving through bursting Air Recovery = Volumetric flowrate air overflowing Air flowrate into cell Volumetric flowrate air overflowing = overflowing velocity x overflowing froth height x lip length Overflowing froth height Overflowing velocity Air flowing over lip Air In

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

27. Why is there a Peak in Air Recovery (PAR)? Optimum balance between froth stability and motion Air Recovery Bubbles heavily loaded Stable, but move slowly Bubbles under-loaded Unstable, burst quickly Air Velocity into Flotation Cell

28. Predicting air recovery – theory

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

30. Froth appearance • Air rate 8m3 min-1 • Air recovery 70% • Air rate 12m3 min-1 • Air recovery 40%

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

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

33. Air rate profiling The air rate profile in a flotation bank affects the performance • Two strategies: • Determine the best air rate profile • Vary distribution of a set total air addition • Determine the optimal total air addition • Vary the total air addition with a set air profile

34. Air rate profiling approaches • 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)

35. Air Profiling Strategies • 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 • Determine the optimal total air addition

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

37. Introduction: Previous work • Determine the best air rate profile • 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

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

39. Study performed in two stages • Air rate profiling tests • Air recovery optimisation (PAR) tests First direct comparison of the two approaches

40. Stage 1: Air rate profiles • Air rate profiling tests • Three profiles tested, the ‘Standard’ and two others, all adding same total air • Air recovery optimisation

41. Air rate profiling: Air rate profiles

42. Air rate profiling: Performance

43. 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

44. Stage 2: Peak Air Recovery test • Air rate profiling test • Air recovery optimisation • Preliminary tests to find PAR air rates • Test conducted at PAR air rates • Total air added same as ‘Standard’ profile

45. Air recovery optimisation: Preliminary tests

46. Air recovery optimisation: Air rate profiles

47. Air recovery optimisation: Air recovery

48. Air recovery optimisation: Performance

49. Air recovery optimisation: Performance of first cell • Effect of air rate: • Recovery maximum at PAR air rate • Upgrade ratio decreases with increasing air rate

50. 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