1. CHAPTER 9 PRACTICAL EFFECTS OF PARTICLE SIZE

2. Contents Introduction Recovery of fine particles
Flotation
Entrainment
Combined effects
Improving flotation of fines
Recovery of intermediate sizes
Recovery of coarse particles
Selectivity & particle size
Discussion

3. Introduction
Particle size effects have been known for a long time but their importance is still under-rated in plant operation
The size-by-size analysis of plant operation and laboratory tests are essential for an understanding of the overal flotation performance

4. Recovery-Size Plots

5. Recovery-Size Plots
The plot may be divided into 3 regions Fine < 5 micron Difficult to float and separate Intermediate, in between Most floatable region Coarse > about 70 micron Flotation drops off depending on conditions The actual sizes separating the regions depend on the system and the conditions

6. Flotation of Fine Particles
Flotation of fines is slow
Theoretical studies have developed relationships between the flotation rate of fine particles and their size.
k  dn
Where k is the flotation rate, d is the particle diameter and the power n is between 1.5 and 2
These calculations are based on dilute systems but the result of experiments in batch flotation cells support a value of n closer to 1.

7. Batch flotation of fine particles (excess collector)

8. Recovery by entrainment
Entrainment is a very important way of transporting fine particles to the concentrate
Non-selective receovery of particles in water recovered to float product.
The more water that is recovered – the more fine particles there are that ‘float’
How is water recovered?
Carried by bubbles (water layer on and wake behind the bubbles)
‘Crowding’ effect of bubble swarms pussing water into froth

9. Water Carried by Bubble
Water film on the surface
Water in wake
Bubble rising in water

10. Preferential Recovery of Fines
Fine particles:
same concentration
inside and outside of wake
Coarse particles:
Settle out &
Conc. inside wake < outside

11. Plot below is for < 5 micron particles
Plot below is for < 5 micron particles. Slope of the curve changes with different sizes

12. Average Particle Size (µm) Recovery of quartz relative to water (%)
Results for quartz are shown in table The entrainment mechanism becomes less important as size increases
Average Particle Size (µm)
Recovery of quartz relative to water (%) 40 4 29 18 20 35 14 45 9 59
3.5 72

13. Controlling Entrainment
Use of cleaner stage(s)
Froth washing
Pulp dilution
Gas rate
Bank retention time
Froth depth …

14. Cleaner Stage

15. Froth Washing

16. Froth Washing

17. Controlling Entrainment
Use of cleaner stage(s)
Froth washing
Pulp dilution
Gas rate
Bank retention time
Froth depth …
Related to total Qg

18. Retention Time

19. Combined Flotation & Entrainment
Slow true flotation rate of fine particles together with sometimes strong entrainment of both floatable and non-floatable minerals
Common to most systems
Accounts for much of the behaviour of fine particles

20. Sulphide Systems Effect of entrainment of quartz on chalcopyrite grade in batch flotation

21. Ways to improve flotation of fines
Increase flotation time
Increase particle size by aggregation - this includes selective flocculation, carrier flotation and oil agglomeration. Some of these have specific applications, but not a lot of progress has been made.
Specific strong collectors. Once again there is not much evidence of success from this approach
Improved equipment to assist bubble paticle collision
Dissolved air flotation and other methods to produce very small bubbles aim at this
The Microfloat system generates very small bubbles and introduces them to the flotation system
Some interest is being shown in these new equipment types.

22. Recovery of intermediate particles
The recovery here is genuine flotation
The range of particle size over which good recovery is achieved varies, as shown below, depending on the mineral system and flotation conditions
This is the region where flotation is very effective.

23. Typical size recovery curves for a number of mineral & collector combinations

24. Recovery of coarse particles
Recovery of coarse particles by flotation only
No contribution from entrainment
Sometimes they can float very well with recoveries to match or exceed intermediate range

25. Recovery of coarse particles

26. Recovery of coarse particles
These results demonstrate the major characteristic of coarse particle flotation – the sensitivity of their response to collector addition and chemical environment
The first signs of collector or acivator deficiency, an excess of depressant, or unfavourable pH conditions show first in a poor flotation response of the coarse particles.
It is thought that the degree of hydrophobicity required to promote a high level of floatability increases with the size of the particle.