1. Flotation Circuits

2. Contents Grinding & flotation Stage operation & Recycle
Co-current & counter-current
Circuit layout
Plant sampling & analysis
Optimisation

3. Grinding & Flotation Flotation circuits cannot be considered alone
Comminution prepares feed for later separation
With flotation separation the objective is to make the feed size distribution well suited to the flotation process
This means grinding so that
minerals are well liberated
top size is not too large to be captured by bubbles
But grind is not be too fine so fine values are lost
An overall balance between grinding and flotation is needed.

4. Grinding & Flotation Chemical effects can also be due to comminution
Grinding with steel balls can affect the surface chemistry of the minerals
Also attempts to regrind flotation streams may affect pulp chemistry
Such effects are complex & require careful study
Optimisation of the process must rely on careful test work guided by calculations aimed to match the feed size distribution to the size-recovery behaviour of the flotation process.

5. Flotation Stages
The aim in flotation – or any separation – is to achieve both grade and recovery at the same time
Single stage operation may impose unacceptable limits
Multiple stage operation is generally needed

6. Single Stage Operation
Consider the grade-recovery relationship for a bank of 10 cells treating a feed of 10% values
(k = 1 min-1) and 90% gangue (k = 0.1 min-1)
Hence the maximum grade is achieved at minimum (zero) recovery
If a higher product grade than this is needed (eg 60% values in this example) it is impossible to achieve this with a single stage operation

8. Two Stage Operation with Scavenger
Consider recycling the concentrate from the later cells to the head of the rougher

9. Two Stage Operation with Scavenger
Consider a constant overall recovery 50% τ
/ τ
0.5 1 5 10
Large sc R
Concentrate grade %
44.6
49.2
52.5
65.6
72.6
Near 100

10. Two Stage Operation with Scavenger

11. Two Stage Operation with Scavenger
The maximum grade is now not necessarily at the minimum recovery
In general the maximum grade is achieved at some intermediate and finite recovery
The improved grade is achieved at the cost of increased total residence time (ie increased plant capacity)
A larger plant is needed to treat the feed plus recycle

12. Two Stage Operation with Cleaner
Consider a similar idealized arrangement, this time recycling cleaner tails to the rougher

13. Two Stage Operation with Cleaner
Consider again constant total recovery of 50% τ
/ τ
Near 0
0.1
0.5 1 10
100
Large Cl R
44.6
Concentrate grade %
44.7
60.7
84.7
87.7
92.5
Near 100

14. Two Stage Operation with Cleaner

15. Two Stage Operation with Cleaner
Once again the whole region can be reached
Maximum grade is again at zero recovery, but since this grade is now 100% no limitation
Once again increased grade is achieved at the expense of increased total residence times, plant capacity and recycles

16. Multiple Stages
Although it seems theoretically possible to achieve any result with a two stage circuit, it is generally much more efficient, in terms of utilising plant capacity, to have a multi-stage circuit arranged in counter flow

17. Multiple Stages
It becomes much more difficult to represent results simply since there are too many degrees of freedom to permit a simple plot
Results are calculated for a particular circuit
The number of scavenger stages and cleaning stages can vary
Up to about 5 or more cleaning stages are used for difficult separations

18. Circuit Calculations
The calculation of flotation circuit performance is extremely difficult due to the complexity of the process
Attempts have been made to develop circuit simulation packages to assist the metallurgist in circuit design and optimisation
These packages are based on the kinetic model of flotation
The first stage is to survey and analyse the circuit

19. Circuit Sampling & Analysis
A thorough investigation and analysis of circuit performance must be based on taking steady state samples of the process streams then careful analysis of the samples
The aim is to get an acurate picture of the way that the circuit is performing

20. Circuit Sampling & Analysis
Circuit should be set in a steady operating mode
Then samples accumulated over an extened period, perhaps a few hours
The samples are immediately weighed wet, filtered and dried, then weighed again ( so getting pulp density estimates)
The dried samples then sized & size fractions assayed (& given mineralogical analysis)
Some reference tonnage is needed, generally the feed tonnage, and any other pulp flow measurements that can be taken are recorded
In modern plants the process control system will record many plant operating parameters, so a printout of these data sheets is essential

21. Circuit Sampling & Analysis

22. Circuit Sampling & Analysis
Here 8 process streams in total giving the following mass balances for total and component flows
Overall balance = (1)
Rougher balance = (2)
Scavenger balance = (3)
Cleaner balance = (4)
Recycle balance = 2 (5)

23. Circuit Sampling & Analysis
Not generally possible to sample all streams
Major feed and product streams are most important (1,4 & 8)
Some internal streams may be difficult or impossible to sample, eg 2 if 1, 6 & 7 are fed to the feed box of the rougher
As many samples as possible should be taken so long as they are reliable samples.

24. Circuit Sampling & Analysis
Programs exist that carryout mass balance estimates on whole circuit simultaneously
Rather than making individual balances these programs use experimental results to make best estimates of all the flows in the circuit
Estimates must be made of the reliability of the respective analyses so that due weights can be given to the analyses in the overall calculation
The result is a balanced estimate of component flows that serves as basis for the circuit analysis
A sampling survey is a major exercise, but is the only way to find out what is happening in circuit

25. Overall Circuit Performance
Total circuit performance should be presented in terms of the major components
Attempt this on a mineral basis, if necessary by estimation of minerals from assays
The size by size behaviour is required to interpret the circuit performance

26. Overall Circuit Performance

27. Overall Circuit Performance
Look at the shape of the mineral recovery curve
At what sizes is mineral lost?
Look at the shape of the feed to the plant
Does it match the recovery curve?
Repeat this for all the major components including the gangue components
The recovery of the other materials determines the concentrate grade
It is just as important to define the gangue recovery since the aim is to minimise this

28. Overall Circuit Performance
A plot of selectivity is a useful way of indicating the effect of size on separation
It can highlight the “cost” of additional mineral recovery

29. Stage Performance
Look at individual stages – rougher, scavenger, cleaner – in the same way
Which stages are limiting the overall recovery?
Are the feeds to the stages appropriate?
Are the recycles appropriate?

30. Flotation Performance
Why is the recovery limited?
Why is the grade limited?
Is there adequate flotation capacity?
Is the grind size too large?
Do the reagent conditions need adjustment?

31. Flotation Performance & Mineralogy
A full mineralogical investigation will help solve many of these problems. It will tell you:
How the losses are divided between
Liberated mineral
Locked mineral
What is diluting the concentrate
Liberated gangue components
Locked components

32. Flotation Performance - Mineralogy
If the liberation is appropriate then losses from locked particles will be not be excessive
Also if there is an excess of fine liberated mineral being lost then the grind might be too fine.
If there are significant losses of liberated material then there are two possibilities:
The chemical conditions may not be appropriate – this is particularly true at the coarse sizes (check in the lab)
The physical flotation conditions may not be optimised ie inadequate residence time, poor cell design, etc (check in the plant)

33. Mineralogical performance of galena recovery to lead concentrate

34. Losses of galena to the zinc concentrate

35. Losses of galena to the final tailings

36. Notes on lead performance
Galena recovery is about 85%
Most of this (~70%) is liberated mineral
Remainder is locked mainly with sphalerite
Losses of galena to the zinc concentrate (a double problem since it is wasted and also dilutes the zinc concentrate) are a little over 4%
About half of this is liberated fine galena
Most of the remainder is locked with sphalerite
Losses of galena to the final tail are about 9%
Mainly these losses are fine liberated galena
The other major losses are galena locked with silicates and other minerals

37. Comments No indication of loss of coarse liberated galena
If reagent conditions were poorly optimised, recovery would tend to drop at the coarser sizes
Note the grind is already quite fine
The loss of fine liberated material might perhaps be overcome with additional flotation capacity (these are the slow floating particles)
Such a change must be evaluated economically (would the additional metal recovered pay for the extra equipment?) and
Technically (additional entrainment of other fine minerals might lower concentrate grades – there is already entrainment of fine galena to the zinc concentrate).

38. Comments
Loss of locked lead might perhaps be overcome by additional grinding
Once again there is a cost involved and such a change would produce more fine galena - already the major component of the losses
In fact since the losses are roughly split between locked and liberated galena it seems that the grind is probably about right
A similar analysis should be carried out with the other components in feed, particularly sphalerite