Increase Coarse Flotation

7/26/2019 Increase Coarse Flotation

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1

Strategies for increasing coarse particle flotation in

conventional flotation cells

Erico Tabosaa,*

, Kym Rungea,b,*

, Kristy-Ann Duffya

aMetso Process Technology & Innovation, Pullenvale, Queensland, AustraliabJulius Kruttschnitt Mineral Research Centre, University of Queensland, Indooroopilly, Queensland, Australia

Abstract

If the particle size that could be effectively recovered in a flotation cell could be increased, the product

size from grinding could be significantly coarsened, resulting in a more eco-efficient flowsheet. A number

of strategies that could potentially increase coarse particle flotation recovery are investigated in this

work by performing tests using a pilot scale 3m3Metso RCS flotation cell operated using a copper ore.

Turbulence was manipulated by changing the impeller speed, impeller size, feed pulp density and cell

aspect ratio. Tests were performed at different froth depth to enable the effect of the froth and the impact

of the turbulence on the froth phase to be evaluated independently of effects in the pulp. This work showsthat coarse particle recovery is extremely sensitive to froth phase effects with recovery optimal at shallow

froth depth and when turbulence at the pulp-froth interface is minimised.

Keywords: coarse particle flotation, froth recovery, turbulence

1. INTRODUCTION

Classically, flotation recovery is optimal for intermediate sized particles (10 to 150 m particles) with poor

recovery of the fine and coarse particles (Jowett, 1980; Trahar, 1981; King, 1982). The coarse particle size at

which recovery decreases varies significantly and is a function of the degree of liberation of the ore (Dunne,2012), degree of oxidation of the particle surfaces, valuable mineral hydrophobicity and particle density

(Schulze, 1984).

Coarse particle recovery in flotation is low for a number of compounding reasons. Firstly, they exhibit low rates

of recovery in the pulp phase this is believed to be due to (1) poor suspension of the coarse particles in the

flotation cell which reduces their effective cell residence time and therefore their probability of particle capture

and (2) poor bubble/particle stability which results in coarse particles tending to detach from bubbles in the

highly turbulent environment of the stirred flotation tank (King, 1982; Schubert and Bischofberger, 1979). To

make matters worse, coarse particles have been measured to exhibit low froth recovery in comparison to finer

particles (Ata and Jameson, 2013; Rahman et al, 2012). This is thought to be due to coarse particles, which

detach from the bubbles due to bubble bursting within the froth phase, more readily draining from the froth back

into the pulp than finer particles.

Improving coarse particle recovery achieved during flotation would have many benefits. Not only would it

result in an increase in overall recovery and therefore revenue at a specific particle size but it could also enable

flotation at a coarser feed size reducing the amount of energy used in upstream comminution processes. Forexample, if flotation could be performed at 0.3mm rather than 0.1mm, the potential energy savings in

comminution would be around 30 to 50%.

Coarse particle flotation potentially can be improved by either modifications to the design or operation of

conventional cells (Dunne, 2012), or by using alternative technologies, of which the most promising is the

fluidised bed type flotation cell (Kohmuench et al, 2010, 2013; Jameson, 2010).

*Corresponding authors

e-mail:[email protected](ManagerFlotation Process Technology) Tel.: +61 418 650 043, Fax: +61 3878 [email protected](Process EngineerFlotation Technology) Tel.: +61 459 834 576
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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Strategies for increasing coarse particle flotation in conventional flotation cellsFlotation 13 - 18-21 November 2013

Conventional impeller driven flotation cells are currently the main technology employed in the mining industry

to perform separation by flotation. Modification in this technology through a change in operation or via a

retrofitted device to increase coarse particle recovery has the potential to be rapidly adopted by industry.

This paper will present the results from a series of experiments i n Metsos 3m3RCS flotation cell performed to

test strategies that have the potential to increase coarse particle recovery in a conventional flotation

machine. Strategies tested include:

- Optimisation of froth depth (and therefore froth residence time);- Optimisation of turbulence through manipulation of impeller speed, impeller size, feed pulp density

and cell aspect ratio.

2. EXPERIMENTAL

Test work presented in this paper was performed using Metsos 3 m3RCS test rig. This rig consists of a 3 m

3

flotation tank fitted with a RCS mechanism located on an upper platform with the products from the flotation

unit discharging via gravity to a pump sump located on the bottom platform (details of which are outlined in

Runge et al, 2012 and inFigure 1).

Figure 1. Metsos 3m3transportable flotation rig operating at Rio Tintos Northparkes operation.

The rig was designed primarily for performing metallurgical testwork. It therefore incorporates easily accessible

full stream sampling points and is fully instrumented to facilitate stable operation. It has also been designed to

enable control and manipulation of the following flotation cell operating variables feed flowrate, air rate,

impeller speed and froth depth. The rig comes with three interchangeable froth launder configurations and two

sizes of RCS impeller (3 m3and 5 m

3standard RCS impellers).

For this study the rig was operated in recycle mode where the concentrate and tailing stream are mixed in the

product sump and then recycled back as new feed. This mode enables operation of the rig with unchanging feed

properties. Feed flowrate in the rig is measured by an Endress Hauser nucleonic gauge and controlled by the

pumping rate to maintain a constant feed flow. A blower is fitted to the rig to supply the required air to the

flotation impeller via a draft tube. Air mass flowrate is measured using a calorimetric air flow transmitter and

controlled using a butterfly valve located in the air line. Froth depth is measured using an ultrasonic sensor that

determines the vertical position of a ball float. Rexroth dart valves in the cell tailing box are moved up and down

to control the tail flow to achieve the desired froth depth set point.

Flotation cell

Sump tank

Feed sampling

box

Concentrate and

tailings sampling

boxes


3/15


Testing Program

Testwork presented in this paper was performed at Rio Tintos Northparkes copper operation using flotation

feed. The test program involved running the test rig at its standard operating condition (RCS3 impeller operated

at 6.2m/s at 1.7cm/s superficial gas rate) as well as at (1) reduced impeller speed, (2) with an oversized RCS5

impeller, (3) reduced cell aspect ratio and at (4) reduced pulp density. At each condition, the cell was operated at

three froth depths. Table 1 summarises the set up of the rig on each of the six days of testing that will be

presented in this paper.

On each day of testing, the cell was filled with a sample from the plant and measurements were then performed

at an as received flotation feed at two or three froth depth conditions. In the case of the evaluation of the effect

of pulp density (Test 2), a measured amount of water was then added to the rig, frother was added to result in

the same frother concentration as in the plant and measurements were then performed at a lower than plant feed

density at three froth depth conditions.

Table 1. Tests used to investigate strategies for improving coarse particle flotation.

Test No. Condition Date Pulp densityCell Aspect ratio

(height:diameter)

Impeller

size

Impeller Tip

Speed (m/s)

Superficial gas

velocity

Jg (cm/s)

Froth

depth

(cm)

1

1

23

17/10/2010 Normal 0.85 RCS3 3.5 1.7

7.4

10.813.2

2

1

23*

20/10/2010

Normal

0.85 RCS3 6.2 1.7

7.6

9.311.0

4

5

6

Low

7.6

9.3

10.6

3

1

2

3

23/10/2010 Normal 0.85 RCS5 6.2 1.7

7.7

9.2

11

4

1

2

3

01/11/2010 Normal 0.65 RCS5 6.2 1.7

8.5

11.4

9.9

512

3

09/11/2010 Normal 0.65 RCS3 3.5 1.78.09.4

10.9

6

1

23 11/11/2010 Normal 0.65 RCS3

3.5 1.7

10.4

8.511.4

4

56.2 1.7

8.5

10.4

*: Test 2 Condition 3 - standard operating condition

The feed was accessed from the flotation circuit feed in the Northparkes flotation circuit Module 1 and typically

assayed 0.4 per cent copper. Plant feed percent solids was usually 30 per cent and the flotation feed P80 was

around 75 to 100 m. There were some differences between the copper assay and percent solids of the plantsource stream and the measured feed characteristics during testing in the rig. This was a consequence of the

recycle mode of operation where a proportion of the copper and solids at steady state resides in the froth phase,

lowering the copper and solids content of the rigs feed stream.

Repeat testing (where the same test conditions were sampled repeatedly over a whole day of recycling

operation) resulted in similar copper recoveries but increasing copper concentrate grade. This increase in

concentrate grade was found to be a consequence of the water flow to concentrate (and thus entrained recovery)

dropping with time of operation of the rig. This paper will therefore predominantly analyse the copper recovery

change with a change in operational condition.


4/15


Measurements Performed

At each froth depth condition, the cell was operated for 30 minutes in recycle before representative samples of

feed, concentrate and tailing streams were collected. Feed flowrate, as measured by the Endress Hauser

nucleonic gauge, was recorded and the flowrate of the concentrate was determined by diverting it into a drum

for a period of time and measuring the volume. Samples collected were weighed wet and dry to determine

percent solids and then analysed for copper content. Samples were also sized (to 38 m using wet and dry

screening methods) and size fractions analysed for copper content.

All experimental data was balanced using Solver in Excel to produce a consistent set of information to enable

calculation of solid, water and copper recoveries.

Additional measurements were also performed to assess how the different operational condition impacts on the

key parameters which affect flotation. Turbulence was characterized using a newly developed piezoelectric

sensor, details of which are outlined in Tabosa, Runge and Holtham (2012). This sensor deflects backwards and

forwards in the pulp as a consequence of velocity fluctuation in the turbulent flow. This produces a voltage

versus frequency signal, the average of which will be used to compare the degree of turbulence in the cell at the

different tests.

Analysis Methodology

A flotation cell consists of two distinct zones a mixed pulp zone in which bubbles collide with particles in a

liquid phase and a froth zone consisting largely of particle laden air bubbles rising and moving towards the

concentrate lip. A proportion of what is recovered in the pulp phase does not report to the final concentrate as it

is returned (or lost) from the froth phase.

Thus the overall recovery of a mineral in a flotation cell is a function of its recovery in both the pulp (Rc) and

froth (Rf) phases (Dobby, 1984), as shown in Equation 1.

(1)

The drivers of recovery are very different in these two phases. In the pulp phase, recovery is largely driven by

the parameters which affect bubble particle collision, attachment and stability (e.g. bubble size, air rate,

residence time, turbulence and pulp viscosity) (Schulze, 1977; King, 1982). Whereas in the froth phase,

recovery is more a function of the time particles spend in the froth (a function of froth volume and froth

mobility), the bubble coalescence rate and the degree of liquid drainage from the froth phase (Mathe et al,

1998).

To ultimately be able to design a flotation regime which optimises coarse particle flotation recovery, one needs

to understand how the key variables of flotation impact on this recovery in both the pulp and froth phases. To

enable this type of investigation, it is important that one is able to measure the pulp and froth zone recovery

independently.

To enable this type of differentiation in this study, a changing froth depth methodology was used (Feteris et al,

1987; Vera et al, 1998). This involved performing testwork on a particular day at different froth depths. At each

condition, the overall recovery (Ro) in combination with the cell air holdup (

), cell volume (V) and feed

flowrate (Qfeed) was used to calculate the overall rate constant (Equations 2 and 3).

(2)

(3)


5/15


It is assumed that a linear relationship exists between froth depth and the overall rate constant, k, as illustrated in

Figure 2.The line is extrapolated to zero froth depth (since there should be no losses due to froth zone effects

under these conditions), giving the zero froth depth or pulp or collection zone rate constant referred to as kpulpor

kc. Equation 4 can then be used to calculate the recovery in the collection zone (Rc) based on the estimated

collection zone rate constant.

(4)

Figure 2. Using the flotation rate versus froth depth relationship to estimate the collection zone rate constant.

It can be shown that the froth recovery is equal to the flotation rate constant divided by the pulp flotation rate

constant, i.e.:

(5)

Therefore by performing tests at different froth depths and measuring recovery, the flotation rate constant can be

determined at each froth depth, the collection zone rate constant and recovery can be determined byextrapolation of the relationship depicted inFigure 2 and froth recovery can be calculated for every froth depth

as the ratio of the measured rate constant to the collection zone rate constant.

The higher the value at which the flotation rate versus froth depth line intercepts the y-axis, the higher the

recovery rate in the pulp phase. The steeper the flotation rate versus froth depth relationship, the greater the

losses in the froth phase (froth recovery) at a particular froth depth.

RESULTS

Standard Flotation Condition

The RCS 3m3flotation cell in an industrial application would usually be fitted with the standard 3m

3 impeller

and have a height to diameter ratio of 0.85. It would be operated at a standard impeller tip speed of 6.4 m/sec

and at a superficial gas rate in the order of 1.0 and a froth depth of 10 to 30 cm.

For purposes of comparison in this paper, Test 2 Condition 3 Froth Depth = 11cm will be considered to reflect

standard flotation operating conditions. In this experiment the cell is fitted with the standard 3m3impeller and

has an aspect ratio of 0.85. Impeller tip speed is 6.2m/sec (close to the standard of 6.4), pulp density was as

received from the plant and froth depth is in the usual range. The superficial gas rate was 1.7 cm/sec which is

higher than normal but not outside the range usually observed in operating plants. Feed rate was 80m3/h.

At this test condition, the flotation cell residence time was 1.7 minutes and the copper flotation recovery

achieved was 64.6% at a concentrate grade of 14.6%.

The recovery versus size relationship at this condition is shown inFigure 3.Recovery is optimal for particles

smaller than 53 micron and decreases as particle size increases. A large proportion of the copper in the feed to

Overallflotationrate-

k

Froth depth

Collection zone rate constant (k c)

k = kc, Rf= 100%

Deep froth depth

k = 0, Rf = 0


6/15


the cell is predominantly less than 38 micron with very little material above 150 micron. It should therefore be

noted that errors in the +150 micron data are considered reasonably high. Copper recovery versus size under

these standard conditions is similar in shape to that observed in the first cell in the plant.

Figure 3. Copper recovery versus size and copper distribution in the feed under standard operating conditions in the Northparkes test work

(Test 2 Condition 3, Froth depth = 11 cm, Residence Time = 1.7 min)

Using the changing froth depth method outlined above, the copper recovery in the pulp phase and the froth

recovery as a function of size was estimated at the standard condition in the test cell. Results are shown

graphically inFigure 4.

Figure 4. (a) Copper recovery estimated at zero froth depth compared to actual recovery versus size and (b) Copper froth recovery versussize estimated at the standard operating condition (Test 2 Condition 1, Froth depth = 11 cm, Residence Time = 1.7 min).

Recovery in the pulp phase (Figure 4a) does exhibit a decrease as particle size increases probably due to the

fact that in this ore these particles are not full liberated. However, the decrease is much less severe than

observed in the overall data. Froth recovery, on the other hand, is very strongly correlated with particle size

(Figure 4b) with finer particles (


7/15


recovery. In this section the sized data from the different tests will be analysed to determine the extent to which

coarse particle recovery can be increased by operating a flotation cell at a shallower froth depth.

Figure 5 shows the recovery and concentrate grade achieved when using the standard flotation conditions as a

function of froth depth. Note that the recoveries in this section (and in comparisons in later sections) have been

standardised to a residence time of 1.8 minutes to allow direct comparison. This was performed by calculating

the overall rate constant using the measured recovery and cell residence time (Equation 2 and 3) and thencalculating a recovery at 1.8 minutes using Equation 6. Most tests were performed at close to this residence time

so it does not result in a large change from the experimentally measured recovery.

(6)

Figure 5. (a) Copper recovery and (b) Copper concentrate grade achieved in the Metso RCS3 cell operated using the standard conditions(Test 2 Residence Time = 1.8 min).

Overall recovery in the cell increases only marginally as the froth depth becomes shallower. This is because the

froth recovery in the first cell of a process is reasonably high. In this case froth recovery is estimated to be 60%

at the deepest froth depth and increases to 70% at the shallowest froth depth. In contrast, copper concentrategrade decreases significantly at shallower froth depth and this is because the water recovery (and therefore the

entrainment recovery) increases significantly.

Figure 6ashows the recovery achieved at each froth depth as a function of size. It also includes an estimate of

the recovery that would be achieved if the froth depth was zero.

Figure 6. (a) Copper overall recovery and (b) Copper froth recovery as a function of size at different froth depths for the standard cell

operating conditions (Test 2 Residence Time = 1.8 min)

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12

Overallcop

perrecovery,%

Froth depth, cm

0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12

CopperConcentrateGrade(%)

Froth depth, cm

0

10

20

30

40

50

60

70

80

90

100

+150106755338-38

Overallcopperrecovery,%

Particle size, um

Test 2 - Standard Condition

FD=0 cm

FD=7.6 cm

FD=9.3 cm

FD=11 cm

0

10

20

30

40

50

60

70

80

90

100

+150106755338-38

Copperfrothrecov

ery,%

Particle size, um

Test 2 - Standard Condition

FD=7.6 cm

FD=9.3 cm

FD=11 cm


8/15


The finer particle classes exhibit similar recovery at all three froth depths. Coarse particles, on the other hand,

show a significant improvement in recovery as the froth depth decreases. For example, for a froth depth change

of 11 to 7.5 cm, the recovery of the +106 micron fraction doubles and is estimated to triple if the cell was

operated with no froth. This is a consequence of an improvement in froth recovery (Figure 6b).

Improvement in coarse copper recovery was observed when running at shallower froth depths at all test

conditions (Figure 7).

Figure 7. Copper recovery as a function of size at different froth depths for different test conditions (Residence Time = 1.8 min).

These results are in agreement with the literature, where it has been shown that froth recovery of coarse particles

can be significantly increased at shallow froth and be similar to that of the fines Figure 8 (Rahmanet al., 2012).

Figure 8. Froth recovery as a function of particle size for three different froth depths (Jg=1cm/s, 70% 60G silica in feed, feed rate =

280g/min, collector dose = 100g/t, frother dose = 20ppm). (After: Rahman et al., 2012).

It is therefore concluded that operating a cell at shallower froth depth selectively improves the recovery of

coarse particles by improving coarse particle froth recovery. This type of operation, however, results in much

higher water recoveries and entrained recoveries and therefore lower concentrate grade.

Shallow or no froth operation therefore has the potential to be used to increase coarse particle recovery in a

flotation cell. It would require a change in the way cleaning circuits are designed so that they could cope with

the higher feed flows, reject the higher amounts of entrained recovery as well as regrinding to achieve final

concentrate grade targets.

0

10

20

30

40

50

60

70

80

90

100

+150106755338-38

Overallcopperrecovery,

%

Particle size, um

Test 1

FD=0 cm

FD=7.4 cm

FD=10.8 cm

FD=13.2 cm

0

10

20

30

40

50

60

70

80

90

100

+150106755338-38


%

Particle size, um

Test 3

FD=0 cm

FD=7.7 cm

FD=11 cm0

10

20

30

40

50

60

70

80

90

100

+150106755338-38


%

Particle size, um

Test 4 FD=0 cm

FD=8.5 cm

FD=9.9 cm

FD=11.4 cm

0

10

20

30

40

50

60

70

80

90

100

+150106755338-38


%

Particle size, um

Test 5

FD=0 cm

FD=8.0 cm

FD=9.4 cm

FD=10.9 cm0

10

20

30

40

50

60

70

80

90

100

+150106755338-38


%

Particle size, um

Test 6 - Low impeller speed

FD=0 cm

FD=8.5 cm

FD=10.4 cm

FD=11.4 cm0

10

20

30

40

50

60

70

80

90

100

+150106755338-38


%

Particle size, um

Test 6 - High impeller speed

FD=0 cm

FD=8.5 cm

FD=10.4 cm


9/15


Turbulence in the Pulp phase

Anecdotally it is believed that coarse particle flotation recovery is negatively affected by turbulence

(Schulze, 1977). Under highly turbulent conditions, the probability of coarse particle detachment from bubbles

is increased in the pulp phase. In the testing program performed, a number of different methods of changing

turbulence were employed:

Tests were performed at an impeller speed half the standard speed which was expected to reduce

turbulence;

Tests were performed with an impeller with a larger diameter which was expected to increase

turbulence;

Tests were performed at a lower cell aspect ratio. A flotation cell consists of a highly turbulent region

at the bottom of the cell and a quiescent less turbulent region in the top half of the cell. By reducing

the cell aspect ratio, the proportion of the cell which is highly turbulent is increased.

At the various test cell conditions, there was a measurable significant change in both the power imparted to the

impeller per m3of slurry (Figure 9)and the localised distribution of this turbulence in the machine (Figure 10).

Figure 9. Specific power drawn by the impeller measured.

Figure 10 shows the magnitude of the turbulence as a function of height in the RCS flotation machine when

operating at low versus high impeller speed, with a 3m3impeller versus a 5m

3impeller and with a low versus

high cell aspect ratio. In all cases, there is a noticeable transition from a quiescent low turbulent zone at the top

of the cell to a highly turbulent zone in the bottom third of the cell. It is interesting to note that when operating

at higher impeller speeds, with the 5m3 impeller or at low aspect ratio, the high turbulent region increases in

height and gets closer to the pulp froth interface.

0

1

2

3

4

5

6

7

8

RCS3, IS = 3.5m/s RCS5, IS = 3.5m/s RCS3, IS = 6.2m/s RCS5, IS = 6.2m/s

SpecificPower,kW.m

-3

High cell aspect ratio Low cell aspect ratio


10/15


(a) (b) (c)

Figure 10. Magnitude of vibration signal from piezoelectric vibration sensor as a function of height in the flotation tank and impeller speedfor (a) Comparative tests performed at different impeller speed, (b) Comparative tests performed with the 3m3and 5m3 impeller mechanism

and (c) Comparative Tests performed at high and low cell aspect ratio.

Data from these tests were sized to determine how turbulence specifically affects coarse particles (in comparison

to the fines).Figure 11 shows the overall recovery achieved as a function of size at what is being considered in

this paper as the standard froth depth of 11cm.

Figure 11. Copper recovery versus size achieved in tests in which the turbulence imparted to the flotation cell has been changed using low(LIS) and high (HIS) impeller speeds; impeller size (RCS 3 or RCS 5); or low (LAR) and high (HAR) cell aspect ratio (residence

time = 1.8 minutes).

The change in turbulence is having a significant impact on the copper recovery achievable in the cell. Higher

recoveries are generally correlated with test conditions which result in low turbulence (low impeller speed,

smaller impeller, high aspect ratio). Very low recoveries are observed across all size fractions for Test 4 which

exhibits the opposite set of conditions (i.e. high impeller speed, large impeller, low aspect ratio).

Coarse particles, in particular, exhibit a negative correlation with the specific power imparted to the slurry

(Figure 12). Coarse particle recovery more than doubles between the lowest power condition and the highest

power condition.

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0 5.0

Relativecellheight(h/H)

Vibration signal, mV

Test 1 (3.5 m/s)

Test 2 (6.2m/s)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0



Test 2 (RCS3)

Test 3 (RCS5)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0 5.0



Test 1 (High cell aspect ratio)

Test 5 (Low cell aspect ratio)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0 5.0



Test 6 (3.5m/s)

Test 6 (6.2m/s)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0



Test 6 (RCS3)

Test 4 (RCS5)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0 5.0



Test 3 (High cell aspect ratio)

Test 4 (Low cell aspect ratio)

0

20

40

60

80

100

+150106755338-38

Copperrecovery,%

Particle size, um

Froth depth ~11 cm

Test 1 (HAR, RCS3, LIS)

Test 2 (Standard)

Test 3 (HAR, RCS5, HIS)

Test 4 (LAR, RCS5, HIS)

Test 5 (LAR, RCS3, LIS)




11/15


Figure 12. Copper recovery in the coarse size fract ions as a function of specific power imparted to the slurry (residence time = 1.8 minutes).

In contrast to the general rule of thumb, turbulence is not resulting in an improvement in the fines particle

recovery either, with the highest recoveries being for the lowest turbulent conditions.

The changing froth depth method was used to estimate the pulp zone recovery (froth depth = zero) and froth

recovery at each of the test conditions to determine the extent to which these losses occur in the pulp and frothphases. It should be noted that the results, especially in terms of froth recovery have a high degree of error. The

results are an interpolation of three size fraction recoveries and error in sized information is higher than for the

overall data and error can cause large deviations in the interpolated results. Because of these problems, this

information will only be viewed with the objective of determining general trends.

Figure 13 shows the copper recovery estimated if froth depth were zero in the different tests. Most tests exhibit

very similar size versus recovery curves.

Figure 13. Copper recovery versus size estimated at zero froth depth in tests in which the turbulence imparted to the flotation cell has been

changed using impeller speed, impeller size or cell aspect ratio (residence time = 1.8 minutes).

The trend in the overall results must therefore be largely a consequence of froth phase effects. Figure 14 shows

the copper overall recovery as a function of copper froth recovery and particle size estimated at 11 cm froth

depth. Large variations from size to size class are not expected and are a consequence of the high error

associated with the analysis methodology.

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8

Overallcopperrecovery,%

Overall energy dissipation rate, kW.m-3

+106um

+75um

0

20

40

60

80

100

+150106755338-38

Copperrecovery,%

Particle size, um

Froth depth = 0 cm

Test 1(HAR, RCS3, LIS)

Test 2 (Standard)

Test 3 (HAR, RCS5, HIS)






12/15


Figure 14. Copper overall recovery as a function of copper froth recovery and particle size estimated at 11 cm froth depth in tests in which

the turbulence imparted to the flotation cell has been changed using impeller speed, impeller size or cell aspect ratio.

The general trend, however, is for the froth recovery to decrease as the turbulence imparted to the pulp

increases. Test 1 presented the highest froth recoveries and is correlated with the lowest turbulent conditions

(0.82 kw/m3). Test 4, however, exhibits very low froth recoveries and is correlated with conditions which result

in the highest turbulent conditions (7.12 kw/m3). Turbulences detrimental effect on froth recovery occurs across

all size fractionsnot just specifically the coarse particle.

In summary, it is concluded that running the 3m3 test cell under highly turbulent conditions results in poorer

coarse particle recovery predominantly due to turbulence adversely affecting the froth phase and resulting in

poorer froth recovery. Turbulence does not affect coarse particle recovery in the pulp phase to a significant

degree. It should be noted that the top size in this work is not overly coarse and it cannot be discounted that

turbulence could have a significant detrimental effect in the pulp phase for coarser feeds.

These results, in combination with the changing froth depth results, highlight that coarse particles are selectively

lost in the froth phase and therefore froth recoveries need to be maximised if one is to achieve high coarserparticle flotation recoveries.

Effect of pulp density

Runge et al (2012) studied the effect of pulp density using the Metso 3m3test cell at Northparkes. In this study

pulp density was found to affect recovery in both the pulp and froth phases.

In the pulp phase, recovery increased as the pulp density decreased. At lower pulp density, the bubble size

increased and there was an observable increase in the size of the high turbulent zone presumably due to the

change in pulp density affecting pulp viscosity. The net result of these changes is more effective bubble particle

collectionthe mechanism of which is yet to be determined. Froth recovery at a particular froth depth, on the

other hand, was adversely affected by a decrease in pulp density. Presumably this is because there are lessparticles in the froth and it becomes less stable.

Potentially high pulp density is therefore a variable which could be used to increase coarse particle flotation

recovery. High density could dampen turbulence and increase recovery of coarse in the pulp phase whilst also

increasing stability in the froth phase and therefore increase coarse particle froth recovery.

Test 2 in Table 1 involved operation of the test rig at different float feed densities. In this test, the cell was

operated at standard operating conditions at three different froth depths and sampled and then water was added

to dilute the feed and samples again collected at three different froth depths.

Figure 15 shows the copper recovery achieved as a function of size at (a) the standard froth depth of 11 cm and

(b) shallower froth depth of 7.6 cm. There is very little difference in the results across all size fractions which

may indicate that under the standard operating conditions there would be no advantage in terms of coarserparticle recovery to operating at a different pulp density.

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Figure 15. Overall copper recovery as a function of particle size at (a) Standard froth depth of 11cm and (b) 7.6 cm froth depth (Test 2).

Figure 16 shows the pulp and froth recoveries estimated at high and low pulp density as a function of size in

Test 2. The high error in these results means that no conclusions can be confidently drawn from this data. Low

density tends to improve pulp recoveries and results in deterioration in froth recovery of all particle sizes.

Potentially the counteracting effects of these two zones result in very little change in the overall particle sizerecovery.

Figure 16. (a) Estimated copper recovery versus size when operating at zero froth depth and (b) estimated froth recovery as a function of

size at 11 cm froth depth for the two density feeds in Test 2

In summary, results do not show any significant difference in coarse particle recovery at different pulp densities.

Whether this would still be the case for much coarser particles (which potentially would be adversely affected

by the observed increase in turbulence at low density) or when froth stability is much poorer (e.g. coarse only

float where there are no fines to provide stabilisation or scavenger application), is unknown.

CONCLUDING REMARKS

Samples from a test program were sized to enable an assessment of the effect of different operational and design

variables on the coarse particle recovery achieved in a flotation cell. A flotation cell consists of two distinctly

different zones a pulp zone where bubbles and particles collide and attach and a froth zone where bubbles

burst, returning a proportion of the attached particles as well as entrained material back to the pulp phase.

Overall recovery is a combination of pulp and froth zone recoveries. The overall coarse particle recovery was

found to be about half that of the finer particles in all of the RCS testwork.

In the pulp zone, coarse particle recovery was marginally lower than for the fines and this is probably due to

these particles being less liberated. A change in turbulence in the flotation cell (either through a change in

impeller speed, impeller size, cell aspect ratio or pulp viscosity) did not have a significant effect on this coarseparticle pulp phase recovery. This was unexpected as the literature suggests that coarse particles should become

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unstable in a highly turbulent environment and recovery should decrease. This may be because the coarsest

particles in the feed used were -212 +150 m, which may not be coarse enough to suffer from this effect.

In the froth zone, however, coarse particle recovery was significantly lower than that of the finer particles.

Presumably coarse particles either detach more easily than the fines in the froth or they detach at an equal rate

and the coarse particles drain more readily through the froth. Froth depth was found to have a large impact on

the froth recovery of coarse particles and much less of an effect on the froth recovery of fines. Tests performed

at shallower froth depth had significantly higher coarse particle recovery and it is estimated that coarse particle

recovery would be about double that achieved under standard froth depth conditions if the cell was run with no

froth. Use of a low aspect ratio or a larger sized impeller or a higher impeller speed resulted in significantly

more losses in the froth phase and lower coarser particle froth recoveries.

It is therefore concluded from this work that the best means of increasing coarse particle recovery rates is to

operate a flotation cell under conditions which maximise froth recovery shallow or no froth depth and

conditions where turbulence at the interface is minimised (smaller impellers, high cell aspect ratio).

ACKNOWLEDGEMENTS

The authors would like to acknowledge the assistance of Metso Process Technology and Innovation

personnel involved in the testing campaign and during sample preparation and data analysis: Jaclyn

McMaster, Robert Crosbie, Kevin Cummins, Rae de Rusett, Diana Li and Shaun Baker. Rio Tintos

Northparkes operation is also gratefully acknowledged for assisting during the research work and

assay analysis.

REFERENCES

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