FROTHER CHARACTERIZATION IN A LABORATORY

FLOTATION CELL

C.O. Gomez, P. Castillo, M. Rebolledo and J. Alvarez

Universidad of Concepcin

Departamento de Ingeniera Metalrgica

Edmundo Larenas 285

Concepcin, Chile

(cesar.gomez@mcgill.ca)

Frothers play two major roles in flotation: preservation of the formation bubble

size and stabilization of the froth. Many tests have been proposed to characterize these

two functions but, in general, the focus has been on only one of these roles and

measurements are done at conditions quite different from those existing in industrial

cells. A technique to characterize the two roles simultaneously at flotation

hydrodynamic conditions, based on the use of a laboratory flotation column, was

developed and successfully used to screen and select a replacement candidate for a

particular concentrator. Although operations require to test and compare frothers

regularly, the problems associated with the installation and use of a laboratory flotation

column for on-site testing are a serious limitation for the implementation of the

technique. The objective of this work is the development of a frother characterization

test using a laboratory mechanical cell, a common piece of equipment in operations. It is

believed that measurement of the CCC and determination of the water overflow rate, in

these units, will provide data to establish differences between frothers and characterize

the strength of their roles. The work reported at this stage is the result of tests completed

to demonstrate this concept.

INTRODUCTION

Flotation is the most important mineral separation technology used to concentrate

metal sulphides for metals production such as copper. It is based on the collection and

separation of hydrophobic particles on the surface of bubbles. The separaton requires

several consecutive processes to occur: formation of bubble-particle aggregates by

collision of bubbles and particles, rising and concentration of these aggregates in a top

layer, and finally their removal into a concentrate stream. Flotation machines operates

with two zones for these processes to proceed: a pulp or collection zone in which the

aggregates are formed and rise, and a froth or cleaning zone where the aggregates

concentrate and drain water as they rise to overflow the cell and report to the concentrate.

The efficiency of the separation, as expected, has been closely linked to the

characteristics of the bubble and particle size distributions because of their obvious

relevance in the formation of bubble-particle aggregates. In the last few years, however,

studies have shown that frothers play several roles in flotation that profoundly affect both

bubble size and hydrodynamics in both zones of a flotation machine. Frothers not only

reduce bubble size, but also increase collection zone gas holdup by slowing down bubble

rise velocity, and water overflow rate by affecting the froth structure and its ability to

drain water. It has been demonstrated that measurement of bubble size [1], gas holdup

[2] and water overflow rate [3] could be used to classify frothers. The two most relevant

roles for industrial operations are preserving bubble formation size, which is connected

with bubble size control, and defining water carrying rate into the froth and water

drainage in the froth layer, which is associated with froth stabilisation. The magnitude of

these effects depends on the frother in use and its concentration, and knowing their

values are necessary not only for frother selection, but also to decide operating conditions

leading to stable operation and acceptable metallurgical performance. In a given

concentrator, with cell size and type long established, and mineral and particle

characteristics varying within a narrow margin, frother selection and distribution offer an

alternative for operators to make adjustments after feed changes or to improve

performance.

Frother characterisation efforts have been, in general, focused on determining

either the effect of frothers on bubble size and velocity in water and pulp, which is

important for the processes ocurring in the collection zone, or on bubble coalescence and

water drainage in the froth layer, of interest to understand the phenomena happening in

the froth zone. In most cases, work has been accomplished using batch techniques,

designed to measure one variable, and run at conditions quite different from those

existing in industrial cells. The results obtained, although useful for classifying (ranking)

frothers, cannot be used to understand the result of frother replacement or of

concentration changes in plant operations. These approaches fail to recognize and

account for the strong interactions existing between the two zones; for example, it was

demonstrated that water overflow rate correlated with gas holdup in the collection zone

[3]. The need for a method for capturing both roles at hydrodynamic conditions closer to

those existing in flotation machines was been recognized [1,6].

A frother characterisation method was proposed to test frothers through

simultaneous measurements in the collection and froth zone of a laboratory flotation

column [8,9]. The approach offered data collected at true flotation conditions and, at the

same time, including the effect of interactions between zones. Air was dispersed using a

porous SS sparger and was well instrumented to be automatically operated with a

constant froth layer. The procedure included the measurement of bubble size, gas holdup

in the collection zone and water overflow rate for a selected froth height, when the

column was run continuously with a frother solution. Frothers were characterized and

classified using three parameters: the critical coalescence concentration (CCC)

determined from the bubble size decrease as the frother concentration was increased, the

minimum gas holdup to have overflow for a selected froth depth, and the slope of the

increase of overflow rate with gas holdup. Although the approach was tested without

solids, it was successfully used to aid a sponsor to replace the frother in use by one of

three options offered by chemical suppliers [7].

Operations require testing and comparing frothers regularly, but the problems

associated with the installation and operation of a laboratory flotation column for on-site

testing are a severe limitation for the implementation of the technique, in spite of the

advantages of running tests with the local process water and solids. The final objective

of the work described in this communication is the development of a frother

characterization technique based on measurements in a laboratory mechanical cell, a

piece of equipment normally available in operations. Although a laboratory mechanical

cell is basically a batch unit, it is believed that measurement of the CCC and of water

overflow rate, will provide data to establish differences between frothers and to

characterize their strength in the preservation of bubble size and the stabilization of the

froth. The work reported at this stage is the result of tests completed to demonstrate this

concept.

METHODOLOGY

Laboratory Mechanical Cell

The unit selected for this work (Figure 1) was an automated mechanical flotation

machine (Edemet), which allowed continuous monitoring and control of the relevant

operating variables, particularly impeller speed and air flow rate. Impeller speed was

electronically controlled using a variable frequency driver while an internal compressor

provided a steady flow of clean (filtered) and oil-free air. A side loop in the gas supply

line allowed injection of other gases or addition of an instrument such as an on-line flow

meter/controller. The cell electronics provided the capacity to drive up to six peristaltic

pumps to schedule continuous or staged reagent additions and to control water or pulp

level in the cell, and to collect signals and display values from pH and ORP (oxidation-

reduction potential) electrodes. Rotating paddles, which could be programmed to run

continuously at a selected speed or to turn around once after a given time, were also

available. The largest cells provided (5 L) and their SS associated impellers were used in

the testing (Figure 2).

Figure 1 - Full view (a) and control panel (b) of mechanical cell used in this work

Figure 2 - Impellers and associated cells available

Keyboard

Screen

Rotameters

Gas line side loop

Rotating paddles

5-L cell (a)

(b)

Impeller A Impeller D

A control front panel (Figure 1b) integrated the utilization of the installed

components and facilitated the operation of the unit. A combined 4-line screen and a 16-

key digital keyboard allowed to program and monitor flotation and conditioning times, to

set impeller speed, and to display ongoing values of operating variables (impeller speed,

pH, ORP). Air flow was manually controlled and monitored using one of two rotameters

installed in the front panel (ranges 1-10 L/min and 2-20 L/min).

Bubble size measurement

Bubble size measurement was accomplished by collecting and processing bubble

images [4, 5]. Briefly, the technique involves a device for bubble sampling and imaging,

which was mounted on top of the cell (Figure 3), and automated image processing using

image analysis software (Image J). Bubble images were collected using a digital camera

(Nikon model D5100) fitted with a macro lens (Nikon 60-mm), a combination that

produced high magnification images (4928x3264 pixels), and resolutions between 170

and 180 pixels/mm. The imaging chamber provided backward illumination, and spread

bubbles into a single plane to make possible precise bubble focusing and eliminate

overlapping bubbles, which facilitated automatic image processing. An image collection

frequency of one every second was used for avoiding to include some bubbles twice.

Results are generally reported as an average bubble size; the Sauter mean diameter D32

(volume to interfacial area ratio) was used in this work.

Figure 3 - Installation of bubble size measuring device

Test Procedure

The development of a standard test to characterize frother roles using a laboratory

mechanical cell requires finding operating conditions that detect differences in the ability

of frothers to preserve the original bubble size obtained during generation, and to reduce

water drainage when bubbles are concentrated and rise in close contact through a froth

layer. Characterization of these two roles was considered through the measurement of:

Critical coalescence concentration (CCC), which requires the measurement of bubble size as a function of frother concentration (values selected 2, 5, 10, 15, 30,

60 y 100 ppm); and

Rate or total water that overflows the cell for one or more frother concentrations.

The procedure to follow for running tests and collect measurements with

acceptable reproducibility must consider that frothers, in many cases, have limited

solubility and are slow to solubilize. Frothers may be volatile and solutions may

decompose at room temperature and if exposed to daylight. Large volumes (10 to 12 L)

of frother solutions in the ppm range are necessary to fill both the cell and the bubble size

measuring device; the same solution must be used as a fraction of that filling the bubble

imaging device is released into the cell during bubble size measurement.

The procedure devised to run the tests included three aspects: preparation of the

frother solution, measurement bubble size, and determination of water overflow rate or

volume:

Preparation of frother solutions. The solution to be used in the bubble size and water

overflow measurements was prepared by dilution from a 100-ppm stock solution. The

stock solution (30 L necessary for the tests at the seven concentrations) required 3 g and

30 kg of the frother and water, respectively. The mixture was maintained overnight with

a gentle agitation to ensure complete dissolution of the frother, and then stored in a dark

cabinet. An effort was made to run the bubble size and water overflow measurements the

same day the stock solution was prepared, to avoid concentration changes by

volatilization and decomposition, which would go unaware.

Bubble size measurement. The measurement is initiated by filling the cell (with 4.5 L)

and the device for collecting bubble images with the selected frother solution. The cell

was started with the impeller type, impeller speed, gas flow rate and amount of water

selected for the test. After 1 minute of conditioning, the stopper closing the bubble

collection tube was removed for bubbles to be sampled and directed to the exposing area.

After 3 minutes to allow a steady flow of rising bubbles to reach the imaging point, 200

to 300 images were collected one every second (Figure 4). The images were processed

using the program Image J, which calculated area and pixel value statistics for every

object (group of neighboring pixels darker than a threshold, selected by the program) in

the image; area was used to calculate bubble size as the diameter of the circle of the same

area, and circularity used as criterion to accept objects as bubbles (larger than 0.8).

Figure 4 - Typical bubble images: left (2-ppm solution) and right (60-ppm solution)

Determination of water overflow rate. The cell was filled completely with the selected

frother solution (5.46 L) while agitated with an impeller speed of 1500 rpm; this value

was higher than that used to measure the CCC to increase the volume of water

overflowing the cell. Air was then delivered at a rate of 15 L/min. Overflowing water

was collected at several time intervals (30, 60, 90, 120, 180, 240 y 300 s) and weighed.

RESULTS AND DISCUSSION

Operating conditions for testing

A full gas dispersion characterization of the cell was not available. A quick

screen of the different impellers and cells, as well as of operating conditions, resulted in

the selections indicated in Table 1. Impellers D generated smaller bubbles than impellers

A and the largest cell volume facilitated the installation of the bubble size measuring

device.

Table 1 Variable ranges and values selected for the tests

Variable Range Selected value

Impeller Type A or D Type D

Cell size (L) 1,5 - 2.7 - 5.0 5.0

Impeller speed (rpm) 600 - 1600

600 (Conditioning)

1000 (CCC)

1500 (Water overflow)

Gas flow rate (L/min) 0 - 20 15

Samples of about ten frothers currently used in Chilean operations were obtained

from several sponsors and suppliers. These frothers are not identified by name or

chemical formula as confidentiality issues couldn't be resolved for all cases in time for

this publication.

Frother characterization

Measurement of frother CCCs. The CCC is determined from the decrease in bubble size

obtained as the frother concentration is increased. The results obtained for five of the

frothers tested in this work are shown in Figure 5, which made possible, even from visual

inspection, to detect variations in the ability of the different frothers to preserve the

formation bubble size.

Figure 5 - CCC measurement results for various frothers

CCC was calculated by fitting separate exponential functions to the results for

high concentrations (30, 60 and 100 ppm), to characterize the effect of surface tension on

bubble size, and for low concentrations (2, 5 and 10 ppm) with surface tension effects

subtracted, to characterize preservation of the formation bubble size by controlling

coalescence at the hydrodynamic conditions existing in the cell. Bubble size as a

function of frother concentration was determined by addition of these two contributions

(lines in plots of Figure 5), and the CCC was calculated as the concentration at which

0

1

2

3

4

0 25 50 75 100

FROTHER CONCENTRATION, ppm

Frother 5

BU

BB

LE

DIA

ME

TE

R D

32, m

m

0

1

2

3

4

0 25 50 75 100

FROTHER CONCENTRATION, ppm

Frother 3

BU

BB

LE

DIA

ME

TE

R D

32, m

m

CCC = 7.1 ppm

0

1

2

3

4

0 25 50 75 100

FROTHER CONCENTRATION, ppm

Frother 7

Frother 8

BU

BB

LE

DIA

ME

TE

R D

32, m

m

CCCs = 12.1 and 12.0 ppm

0

1

2

3

4

0 25 50 75 100

FROTHER CONCENTRATION, ppm

Frother 4

BU

BB

LE

DIA

ME

TE

R D

32, m

m

CCC = 14.5 ppm

coalescence increased bubble size by 0.1 mm over the value predicted considering only

surface tension effects. The value 0.1 mm was selected because it corresponds to the

error associated with bubble size measurement for a 95% C.I. (average and standard

deviation of 5 measurements, 0.738 and 0.035 mm, respectively).

The results showed noticeable CCC differences, for example 7.1 and 14.5 ppm

for Frothers 3 and 4, respectively. The case of Frother 5 was unique as it showed limited

ability to control coalescence. Frothers 7 and 8 were the same product but provided by

different suppliers; the similar results obtained were an indication of the reproducibility

of the technique.

Determination of water overflow rate. Froth stabilization is associated with the amount

of water carried by bubbles into the froth layer, which has been demonstrated to be

affected by the frother in use. The alternative considered to characterize this function

was by measuring water overflow as a function of time. Results obtained for three of the

frothers in the form of curves of cumulative fraction of overflow water (relative to the

volume at the start of the test) and of water overflow rate, as a function of time, are

included in Figures 6a and 6b, respectively.

Figure 6 - Water overflow volume (a) and rate (b) as a function of time

These results showed that most of the water overflowed in the first 30 s, which

was expected for a batch flotation test started with the cell completely filled with water

and no level control. The results also indicated that differences between frothers could

be established considering only the final volume of water (after 5 minutes no more water

overflowed the cell). A comparison of the final water volumes obtained at the highest

frother concentrations used (60 and 100 ppm), for the same five frothers selected to

illustrate the CCC results, showed that the sequence from smallest to largest water

volume (Frothers 5, 3, 8, 7 and 4) was the same at both concentrations (Figure 7),

although in the case of Frothers 4 and 5, significant differences (increase and decrease,

respectively) were observed.

0

5

10

15

20

25

30

0 100 200 300

O/F

WA

TE

R R

AT

E,

g/s

TIME, s

Frother 6Frother 7Frother 8

(b)

0

5

10

15

20

25

30

0 100 200 300

O/F

WA

TE

R ,

%

TIME, s

Frother 6

Frother 7

Frother 8

(a)

Figure 7 - Final water overflow volumes collected at two concentrations

Plots of the variation of the final volume as a function of frother concentration

(Figure 8) showed, in general, three behaviours: (i) a volume steadily increasing with

concentration (Frother 4); (ii) a volume reaching a maximum before the largest

concentration (Frothers 3, 7 and 8); and (iii), a volume increasing at two rates (Frother 5).

It is important to point out that the frothers used in this work were commercial frothers,

which in many cases are blends of two or three major components; The different

behaviours observed may be a consequence of changes in the proportion of the mixture

components or in the relative amount of one of the components in the cell during the test.

Figure 8 - Final water overflow volume as a function of frother concentration

0

10

20

30

40

50

0 25 50 75 100 125

OV

ER

FL

OW

WA

TE

R, %

FROTHER CONCENTRATION, ppm

Frother 3Frother 4Frother 5Frother 7Frother 8

0

10

20

30

40

50

3 4 5 7 8 3 4 5 7 8

OV

ER

FL

OW

WA

TE

R, %

FROTHER

60 ppm

100 ppm

The integration of the CCC and water overflow volume results showed that

strength for controlling bubble size didn't guarantee strength for stabilizing the froth, and

vice versa. For the five frothers used to illustrate the results of this work, the sequence

for the ability to control bubble size (Frothers 3, 7, 8, 4 and 5) was not the same as that

indicating strength for froth stabilization (Frothers 4, 7, 8, 3 and 5). Frother 3 was

stronger than Frothers 7 and 8 for controlling bubble size but all three showed similar

strength as froth stabilizers. Frother 4 was weak for controlling bubble size but very

strong for stabilizing the froth, while Frother 5 was weak for both functions.

CONCLUSIONS AND FUTURE WORK

The results obtained in this work demonstrated that the concept devised to

develop a frother characterization test, using a laboratory mechanical cell, was successful

in the detection of differences in the ability of frothers to both controlling bubble size and

stabilizing the froth. The approach selected to characterize these two functions was

measurement of the CCC and determination of the water overflow volume, respectively.

The results were collected in air-water tests using samples of commercial frothers

provided by sponsors. Both measurements, CCC and water overflow volume, showed

clear differences between frothers, and sorting their strength was straightforward. The

results also demonstrated that a strong or weak frother for bubble size control didn't

necessarily mean a strong or weak frother for froth stabilization.

Future work will focus on the development of a standard test to classify frothers.

Tests will use different types of frothers, this time available as reagents, to avoid blends

and contaminants. A search for operating conditions maximizing differences between

frothers will be carried out. Although the cell in use is well instrumented and offers

control of impeller speed and reagent addition, which can be used to automatically stage

varying operating conditions, an effort will be made to propose a test to be run in a

standard cell.

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