Gomez Finch (2007)

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Gas dispersion measurements in flotation cells

C.O. Gomez , J.A. Finch

McGill University, Canada

Received 7 June 2006; received in revised form 30 January 2007; accepted 28 March 2007

Available online 12 April 2007

Abstract

Progress in science and engineering relies on accurate and precise measurements. Flotation is no exception. It is arguable that

progress in both fundamental understanding (modeling) and plant practice has been limited by the lack of instruments and sensors

capable of functioning in operating flotation machines. The McGill Mineral Processing group has developed instruments and

procedures to measure three variables in industrial cells: superficial gas velocity, gas-holdup and bubble size distribution. This

communication describes advances on equipment and operating procedures, and introduces a protocol for sensor installation and

documentation that makes possible the correction of measurements to bubble-collection-point conditions.

2007 Elsevier B.V. All rights reserved.

Keywords: Flotation; Gas dispersion measurements; Gas velocity; Gas holdup; Bubble size

1. Gas dispersion sensors

Gas dispersion is the collective term for superficial

gas (air) velocity (or gas rate, volumetric air flow rate per

unit cross-sectional area of cell, Jg), gas-holdup

(volumetric fraction of gas in a gasslurry mix, g),

and bubble size distribution (Db). The sensors and

devices we have used to measure these variables in

flotation machines have been described in the literature

(Gomez and Finch, 2002). They have been used tocharacterize gas dispersion in cells ranging from micro-

units (e.g. 50 mL Hallimond tube variations) to tank cells

up to 160 m3 in volume.

The emphasis here will be on developments since the

first review (Gomez and Finch, 2002) including

integration of the measurements, and use of the units

to characterize the response of cells and circuits. Data

from some 20 sites on 5 continents have been collected

to date. The devices, initially designed for stationary

installation in flotation columns, were modified for

sequential measurements in circuits of mechanical

machines. The requirements were for compact, robust

units that could be multiplexed and were adaptable for

on-line measurement. The designs have been refined to

a point that continuous (gas velocity and gas holdup) or

relatively high frequency (bubble size) measurements

are possible within the plant environment on individualand multiple flotation cells.

1.1. Gas velocity sensor (Fig. 1)

The gas velocity sensor is an automated version of

the inverted graduated cylinder used by plant engineers

in the past, i.e., it is based on the collection of bubbles

from the pulp zone into a vertical tube partially

immersed in the cell (Gomez and Finch, 2002; Gomez

et al., 2003a). The sensor is constructed around a piece

Int. J. Miner. Process. 84 (2007) 51 58

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Corresponding author.

E-mail address:[email protected](C.O. Gomez).

0301-7516/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.minpro.2007.03.009
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of PVC plastic tube of 10 cm diameter which is large

enough to minimize bubble-sampling biases common in

small tubes. The collected bubbles rise until they reach

the liquid surface in the tube and burst. If the tube is

closed at the top, the gas accumulates, pressure

increases, and the slurry (and any froth layer) is pusheddown the tube. The signals from pressure sensors are

permanently recorded in a data collection computer. The

rate of descent is related to the superficial gas velocity

and can be monitored by the slope of the pressure

change (Gomez et al., 2003a). The measurement begins

when valve A (Fig. 1a) is closed to accumulate gas

within the tube and pressure increases, and ends when

the tube is full of gas. Simultaneous measurements in

many cells are possible by opening and closing

(manually or automatically using solenoid valves) the

valves on the tubes installed in each cell.Calculation of the gas velocity from the rate of

pressure change requires the bulk density of the fluid

(aerated pulp) which provides the backpressure to the

gas accumulating in the tube. By including a second

tube (Fig. 1a) with its bottom end closer to the pulp/froth

interface and with valve B closed (i.e., second tube is

always full of air) then, knowing the immersion length

difference between the two tubes, the bulk density can

be calculated from the pressure difference measured

after the first tube fills with gas (this second tube is

referred to as the bubblertube to distinguish from the

original sensor tube).A continuous version of the sensor based on the use of

an orifice has been developed (Fig. 1b) (Torrealba-Vargas

et al., 2003). In this case the sensor includes two valves

and is operated with valve A closed and valve B open to

direct air to the orifice until a steady state pressure is

reached (rate of air entering the tube from the cell equals

that leaving the tube through the orifice). From a prior

calibration of the flow through the orifice the volumetricflow of air (and hence Jg after dividing by tube cross-

sectional area) is known. A major advantage is that

measurement of bulk density is not required, but practical

limitations are that different orifices are required to cover

a range inJgand sometimes froth builds in the tube and

reaches the orifice which biases the measurement of the

steady state pressure. Efforts to solve this latter problem

are in progress.

1.2. Gas-holdup sensor (Fig. 2)

Gas holdup represents the volumetric proportion of

gas in the pulp. The sensor is based on Maxwell's model

that relates the concentration of a non-conducting

dispersed phase to the conductivities of the continuous

phase and the dispersion (Tavera et al., 1996). This

means two measurements, made in two tubes (flow

cells), one that measures the conductivity of aerated

slurry and the second, by inducing a siphon action to

eliminate the bubbles, measures the conductivity of the

air-free slurry (Fig. 2). The ratio of the two signals is used

to solve Maxwell's model to estimate the fraction of non-

conducting phase (i.e., gas holdup) in the dispersion.The conductivity cells (noteFig. 2) have 3 concentric

conductivity rings the two outer at the same polarity to

Fig. 1. Schematic of the two versions of the gas velocity sensor.

52 C.O. Gomez, J.A. Finch / Int. J. Miner. Process. 84 (2007) 5158

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constrain the field inside the cell. Some prior knowledge

of the pulp liquid-phase conductivity, which in plants

with highly saline pulps may reach 100 mS/cm, is

needed to setup the conductivity meter. Fluctuations in

pulp conductivity during measurements are not a

problem since the technique is one of relative difference

between the open and siphon cells, and not absolute

values (Gomez et al., 2003b). The method gives a

continuous signal.

The key for effective operation lies in excludingbubbles from the siphon cell. The cell design (a conical

bottom) is a consequence of this requirement. The

conical opening hinders entry of rising bubbles, and the

cell contents quickly develop a higher density than the

dispersion outside the cell, generating a flow out

through the bottom opening, replenished by pulp from

the top, which completes the exclusion of bubbles from

the siphon cell. To ensure that no bubbles are entrained

into the cell from the top, the pulp velocity entering the

top of the cell must be lower than the terminal velocity

of the smallest size bubble present in the system. Thiscondition is met by selecting the bottom-opening size.

1.3. Bubble size measurement (Fig. 3)

Bubble size (Db) measurement is a full characteriza-

tion of the entire size distribution made from images

collected using McGill's unique bubble-viewing cham-

ber (Hernandez-Aguilar et al., 2002). The viewing

chamber (and camera and light source) is mounted on an

aluminum frame to facilitate installation and operation

(Fig. 3). In operation, bubbles are collected via a

sampling tube and directed into a viewing area where

they are exposed under pre-set lighting conditions to be

imaged (Fig. 4). Operation follows a sequence of steps

to create a continuous flow of rising bubbles through the

chamber. The unit is initially filled with water through

the top opening (normally plant process water) to

provide the liquid column in which the bubbles rise to

provide a clear image (particles in the slurry do not enter

the unit) for up to several minutes (till particles

dislodged from bursting bubbles cloud the viewing

chamber). As a consequence of its elevated locationabove the flotation cell pulp level, the pressure inside the

viewing chamber remains below atmospheric during

operation.

Although the accuracy of the technique is hard to

establish (there is no gold standard against which to

compare) the approach is now widely used, and

continues to evolve as improvements dictated by field

experiences are implemented. The characteristic that

distinguishes the technique is the quality of the images

Fig. 3. Schematics of bubble viewer depicting main components.

Fig. 2. Schematics and picture of the gas-holdup sensor showing details of the open and siphon conductivity cells.

53C.O. Gomez, J.A. Finch / Int. J. Miner. Process. 84 (2007) 5158

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attained by photographing bubbles against backlighting

(i.e., the bubble shadow is imaged) as they slide up a 15-

degree sloped window. This window spreads the bubbles

into a near monolayer, providing both an unambiguous

plane of focus and reducing bubble overlap and ghost

images of bubbles in the background.

The measurement involves three steps: image collec-

tion, image processing, and data analysis; progress in

each area has been substantial. In image collection, the

latest cylindrical bubble viewer design and the support

structure automates integration with the video-camera

and light source while facilitating transportation, instal-lation and cleaning of the unit between measurements.

Safety issues related to window integrity have been

addressed by incorporating a pressure relief valve and

using tempered glass. Software has been developed to

drive the camera for collecting, naming, and storing

images at a user-defined rate and directly in the format

necessary for image processing (lengthy image transfer

and digitization are no longer required). Parameters to

consider during image collection are magnification

(normally 25 pixels/mm in plant work), and image

number and frequency (normally five hundred at a rate oftwo every second). Experience has demonstrated that the

most effective way of operating the camera is using a

fixed shutter speed (1/1000 or 1/2000 of a second,

depending on the intensity of the light) in auto-iris mode.

In image processing, software developed at McGill

drives Northern Eclipse, a commercial image analysis

program, to automatically handle all images collected in

one measurement. For every image (for the camera

currently in use, a set of positioned pixels in a matrix of

720 535 elements each representing light intensity

through a grayscale value from black 0 to white 255),

properties such as area, perimeter, and minor and major

axes are determined for every object discriminated. The

processing requires the selection of threshold (maximum

grayscale intensity value of a pixel to discriminate objects

from background), and up to six bubble selection criteria

(e.g., minimum major and minor axes size, aspect ratio

range, and shape factor) to establish what objects are

bubbles. In industrial measurements, as the collection of

images advances, solid particles released when the

bubbles burst at the surface of the liquid in the viewing

chamber progressively reduce the background light in-

tensity, and the threshold needs continual adjustment. The

software includes an option to process with a thresholdautomatically calculated for every image utilizing an

algorithm based on the picture intensity histogram.

Processing starts by creating a binary image when pixel

intensities are compared to the threshold (pixels with

values below or equal to the threshold are given a value of

1, while those with higher values are given a value of 0).

Groups of adjacent1pixels are considered objects. Two-

dimensional properties of individual objects are derived

from the pixel arrangement and converted to sizes using

the magnification. Objects complying with all selection

criteria are accepted as bubbles.Use of a magnification of 25 pixels/mm means

minimum bubble sizes of around 0.2 mm (smallest

circular object is considered to be 5 pixels in diameter).

As the number of bubbles counted is normally between

five and ten thousand, the information generated in a

single test is massive (an eighteen-column Excel file

with as many rows as bubbles counted). To facilitate

examination of the files, software was developed to

calculate, analyze and display size distributions from the

files stored in a Windows subdirectory. The program

retrieves the data and, based on the equivalent diameter

(diameter of a circle with the same area as that exposed

Fig. 4. Bubble viewer in operation.


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in the image), sorts the bubbles into frequency

distributions with bin sizes (arithmetic or geometric

intervals) defined by the user. Frequency and cumula-

tive size distributions are displayed in plots with both

linear and logarithmic size scales, and average diameters

such as D10 or D32 are calculated as well. A usefuloption for comparison purposes is the simultaneous

display of up to five size distributions, in frequency or

cumulative form.

2. Sensor installation

The literature includes several reports of measure-

ments performed during the development stage of the

sensors. As the sensors became more robust and reliable

for use in industrial cells, simultaneous measurements to

characterize gas dispersion demonstrated that an instal-lation strategy was necessary. Because the volume of a

mass of gas depends on its pressure and temperature, gas

dispersion measurements will be affected by the location

and geometry of the sensors. At the same time, as the

conditions existing at the measurement points are not the

same for the three sensors (and are different from those

prevailing within the flotation cell at the bubble-collection point), corrections are necessary. In bubble

size determination, for example, diameters are measured

from images of bubbles in the bubble viewer which

operates under vacuum; bubbles are larger than their

sizes in the cell. A similar situation occurs with the

measurement of gas velocity where a volume of gas is

accumulated under variable pressure at conditions

different from those existing at the location where

bubbles enter the tube. Corrections to common condi-

tions are straightforward for bubble size and gas velocity

by applying Boyle's law, given that air behaves as anideal gas under flotation conditions.

Fig. 5. Location of the sensors and relevant lengths and distances for calculation and correction of gas dispersion parameters.


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Installation of the gas dispersion sensors during cell

characterization exercises requires careful documenta-

tion to be effective; integration of the gas dispersion

results with metallurgical data requires information

collected at the same time under strictly controlled

conditions. An adequate response to the non-homoge-neity of gas dispersion in industrial cells is to locate all

sensors as close together as possible.

Respecting the inevitable restrictions imposed by

local situations, it is recommended that sensors are

operated through the same sampling port, to collect

bubbles from the same radial position at the same depth

(Fig. 5) to facilitate correction of measurements to cell

conditions. In general, similar results are obtained at the

same radial distance, but relative distance to baffles or to

the feed entrance, or different impeller rotation direction

in two cells may lead to different values. In the case ofdepth, installation must be carefully selected consider-

ing that there will be only one position where the three

gas dispersion parameters will be known (reference

plane inFig. 5at the middle ring of the gas-holdup open

cell). Although gas velocity and bubble size can be

easily corrected to different depths our current inability

to do the same for gas holdup makes this reference depth

unique. No method has been found to correct a gas

holdup resulting from the flow of a swarm of different

diameter bubbles.

To try to have all sensors sampling the same bubble

population, the gas velocity sensor and the bubbleviewer are installed with the bottom end of their

sampling tubes immersed to the same depth as that of

the open cell of the gas-holdup sensor (Fig. 5). The

second tube (bubbler) used to measure the bulk density

(required for the dispersion above the bottom end of the

gas velocity sensor tube) is installed at about 0.25 m

below the interface (or about 0.5 m below the cell lip if

froth depth is unknown), with a recommended minimum

immersed length difference (HBD) of 0.5 m.

Calculations and corrections of the measurements

require some of the distances indicated inFig. 5: 1) totallength of theJgsensor tube (HL); 2) length of the sensor

tube above the cell overflow (HO); 3) immersed length

difference between the Jg sensor and bubbler tubes

(HBD); 4) distance between the middle ring and the

bottom end of the open cell in the gas-holdup sensor

(HE); 5) total length of the bubble viewer collection tube

(H1); 6) vertical distance between camera focal point

and bottom of bubble viewer (H2); 7) vertical distance

between camera focal point and water level in the

bubble viewer (H3); and 8), froth depth. There is a small

variation (a few cm) inH3

during measurement as more

bubbles accumulate at the top of the bubble viewer; it is

our practice to use an average of the distance before and

after images are collected.

3. Calculation and correction of gas dispersion

measurements

3.1. Gas velocity

Operation of the double-tube sensor provides the data

for calculating gas velocity. The data, pressure vs. time

in both tubes, is processed to derive the slope dp / dt

(normally an average of five values) and then to derive

the average of pressures p1and p2when both tubes are

full of air. The bulk density is first calculated from

averages of the pressures p1 and p2, and the immersed

length difference between the sensor and bubbler tubes

(HBD):

b p1p2

HBD1

The gas velocity calculation is based on the following

equation relating this variable to the slope of the pressure

vs. time curve when the gas is accumulating in the tube.

This equation was obtained applying a mass balance to

the tube volume filled with gas (a cylinder with variable

volume and pressure), and using the ideal gas equation to

relate mass to pressure and volume, and the weight of a

column of fluid to relate pressure to volume:

Jg;L Patm bHL

b Patm b HLHO

dp

dt cm=s 2

Distances (defined in Fig. 5) are measured in cm,

while pressurespiare manometric and Piare absolutes,

both in cm H2O. The calculated gas velocity Jg.Lcorresponds to that entering the sensor tube at a depth

(HL HO). To correct the measured gas velocity to a

different location (different hydrostatic pressure) Boy-

le's law is used. To calculate, for example, the gas

velocity at the reference plane (Jg,ref

) the following

equation is used:

Jg;ref Jg;LPL

PrefJg;L

Patmp1Patmp1 bHE

3

In plant measurements, gas velocities at the reference

plane are typically about 3% larger than the values

obtained from Eq. (2).

3.2. Gas holdup

Gas holdup is calculated utilizing Maxwell's equa-

tion (Eq. (4)) with averages of the conductivity kover


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time (typically 5 min) for both the open and siphon cells,

which measure the conductivity of the dispersion

(bubbles plus pulp) kd, and pulp only kp, respectively:

eg 1 kd

kp

10:5 kdkp4

3.3. Bubble size

In the case of bubble size, images are collected at the

point where the focal plane of the camera meets the

sloped window (a vertical distance H3 from the gas

liquid interface in the bubble viewer); at this point the

absolute pressure is below atmospheric. To correct

bubble sizes to the cell reference plane, the pressure in

the bubble viewerPBV needs to be measured, or in itsabsence, estimated from equalizing two hydrostatic

pressures at the bubble-collection point, namely that

from the air-water column inside the bubble viewer and

collection tube (water density assumed), and that from

the pulp and froth column in the cell:

PBV H2O H1H2H3 Patmp1 5

The pressure p1 is known (measured to estimate the

bulk density) and with distances H1, H2, and H3 the

pressure in the bubble viewer can be calculated:

PBV Patmp1 H2O H1H2H3 6

Once PBVis known, measured bubble sizes Db,imagecan be corrected to reference conditions (Db,ref) again

using Boyle's law:

Db;ref Db;image

ffiffiffiffiffiffiffiffiffiffiffiffiPimage

Pref

3

r Db;image

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiPBV H2OH3

Patmp1 bH3

3

s

7

In plant measurements, bubble sizes at the reference

plane are about 8% smaller than the values obtaineddirectly from image analysis.

Although the volume of gases is also affected by

temperature, no corrections for this variable are nor-

mally applied because gas temperature differences are

usually not large. If large temperature differences are

detected between the gas in the pulp and that accu-

mulated during the measurement of Jg or Db, then a

temperature correction in Eqs. (2), (3) and (7) has to be

considered. It is recommended that pulp tempera-

tures be measured in case results from cells in differ-

ent banks, circuits and plants need to be compared or

related.

4. Concluding statements

Sensors for measuring superficial gas velocity, gas-

holdup and bubble size distribution in industrial flotation

machines have been refined to a point that continuous

(in the case of the gas velocity and gas holdup) or rela-tively high frequency measurements are possible within

the plant environment on individual and multiple flotation

cells. Hardware and software improvements have simpli-

fied measurement and reduced the time required for data

processing. The techniques will continue to evolve as

modifications dictated by use are put into effect.

As the sensors have become more robust and reliable,

simultaneous measurements were practiced to better

characterize cell response to changes in operating

conditions. In response to the non-homogeneity of gas

dispersion in industrial cells an installation protocol,which requires careful documentation to be effective,

was necessary to integrate measurements in one cell and

to compare with those collected in different units.

Gas dispersion measurements are affected by sensor

location and geometry. Because the volume of a mass of

gas depends on its pressure and temperature, corrections

are necessary. Conditions existing at the measurement

points are not the same for the three sensors, and are

different again from those prevailing within the cell at

the bubble-collection point. Corrections are straightfor-

ward for bubble size and gas velocity by applying

Boyle's law, considering that air behaves as an ideal gasunder flotation conditions. However, no method is

available to correct gas holdup to conditions different

from those existing at the measuring point. This makes

the location of the gas-holdup sensor the reference point.

Standardization of sensor installation and correction

procedures has taken the technique to a stage where it

can be effectively transferred to operations. Units have

been supplied to several groups along with training.

Work will continue towards our long-term goal,

establishing a relationship between gas dispersion mea-

surements, notably bubble surface area flux, and metal-lurgical performance.

Acknowledgements

The effort and dedication of current and former

graduate students, Dr. Alejandro Uribe-Salas, Dr.

Francisco Tavera, Franklin Cortes-Lopez, Ralph

Dahlke, Marta Bailey, Dr. Jose Hernandez-Aguilar, Dr.

Jorge Torrealba-Vargas, Helin Girgin, Jason Doucet,

Claudio Acua and Davin Knuutila in the development

and testing of these sensors is gratefully recognized.

Funding for sensor development is from two Natural


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Sciences and Engineering Research Council of Canada

grants; one sponsored by Noranda, Inco, Falconbridge,

Teck Cominco, COREM and SGS Lakefield Research,

and the other by Amira International under the P9N

project, and is greatly appreciated.

References

Gomez, C.O., Finch, J.A., 2002. Gas dispersion measurements in

flotation machines. CIM Bulletin 95 (1066), 7338.

Gomez, C.O., Torrealba-Vargas, J.A., Dahlke, R., Finch, J.A., 2003a.

Measurement of gas velocity in industrial flotation cells.

In: Lorenzen, L., Bradshaw, D.J. (Eds.), Proceedings of the XXII

International Mineral Processing Congress, pp. 17031713.

Gomez, C.O., Cortes-Lopez, F., Finch, J.A., 2003b. Industrial testing

of a gas holdup sensor for flotation systems. Minerals Engineering

3, 17031713.

Hernandez-Aguilar, J.R., Gomez, C.O., Finch, J.A., 2002. A technique

for the direct measurement of bubble size distribution in industrial

flotation cells. Proceedings 34th Annual Meeting of the Canadian

Mineral Processors. CIM, pp. 389402.Tavera, F.J., Gomez, C.O., Finch, J.A., 1996. Novel gas holdup probe

and application in flotation columns. Trans. Instn Min. Metall

(Sec. C: Mineral Process. Extr. Metall.), 105, pp. C99C104.

Torrealba-Vargas, J.A., Gomez, C.O., Finch, J.A., 2003. Continuous

air rate measurement in flotation cells: a step towards gas

distribution management. In: Gomez, C.O., Barahona, C. (Eds.),

Proceedings Copper 2003. Mineral Processing, vol. 3, pp. 91102.


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