International Journal of Mineral Processing 99 (2011) 78–83

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International Journal of Mineral Processing

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On the gas dispersion measurements in the collection zone of flotation columns

E. Matiolo, F. Testa, J. Yianatos 1, J. Rubio ⁎Laboratório de Tecnologia Mineral e Ambiental (LTM), Departamento de Engenharia de Minas-PPGEM, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500/75,91501-970, Porto Alegre, RS, Brazil

⁎ Corresponding author at: Mining Engineering Depdo Rio Grande do Sul, Brazil. Tel.: +55 51 33089479; fa

E-mail addresses: juan.yianatos@usm.cl (J. Yianatos)URL: http://www.ufrgs.br/ltm (J. Rubio).

1 Chemical Engineering Department, Santa María UniChile.

0301-7516/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.minpro.2011.03.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 September 2008Received in revised form 21 February 2011Accepted 11 March 2011Available online 29 March 2011

Keywords:Gas hold-upColumn flotationBubble sizeBubble superficial area flux

This work shows the results of gas dispersion parameters in a fully controlled laboratory column flotation cell,namely gas hold-up (εg), superficial gas velocity (Jg) and bubble size distribution, measured directly by imageanalyses using the LTM-BSizer. Gas hold-up and bubble size (and their distribution) were found to be stronglydependent on Dowfroth 250 concentration and superficial gas velocity. A fairly linear relationship betweenexperimental εg and bubble superficial area flux (Sb) was established, and results are compared to thosecalculated using drift flux analysis. Data obtained are discussed in terms of solution, hydrodynamics andinterfacial phenomena. Possible implications on the role of the frother in the energy dissipation in bubblegeneration and on interfacial tensions are explored.

artment, Universidade Federalx: +55 51 33089477., jrubio@ufrgs.br (J. Rubio).

versity, P.O. 110-V, Valparaíso,

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The properties of bubbles (gas) dispersion or hydrodynamicconditions clearly play an important role in froth flotation andapplied flotation to effluent treatment. Recent developments(Schwarz and Alexander, 2006; Dahlke et al., 2005; Hernández et al.2003; Grau and Heiskanen, 2003; Chen et al., 2001; Finch et al., 2000;Deglon et al., 2000; Gorain et al., 1997, 1998, 1999) have permittedreliable measurements of some gas dispersion parameters, namelygas hold-up (εg), bubble size (db), gas rate (Qg) or superficial rate (Jg)and bubble surface area flux (Sb):

Jg =Q gA

ð1Þ

where A is the cell cross-sectional area, and the bubble surface areaflux (Sb) defined by:

Sb =6⋅Jgdb

ð2Þ

Many attempts to relate these parameters to flotation performancehave beenmade by a number of authors (Hernández et al., 2003; Grauet al., 2005; Grau and Heiskanen, 2003; Kracht et al., 2005; Deglon

et al., 1999; Gorain et al., 1998). Jameson et al. (1977) derived that thefirst-order flotation rate constant (k) is given by:

k =0:25⋅Ec⋅Jg

dbð3Þ

where Ec is the collection efficiency, the term ending up in terms ofsurface area flux, yielding:

k = 0:25⋅Ec⋅Sb ð4Þ

Some data reported suggests that bubble surface area flux (Sb) andgas hold-up are related by the following relation (Finch et al., 2000):

Sb = 5:5⋅εg ð5Þ

This relation would have advantages, because the gas hold-up iseasier to measure and would solve the problem of poor bubble sizemeasurements (Tavera et al., 2001). This appears to be the case forflotation columns andmechanical cells, both laboratory and plant scale,over the approximate range Sbb130 s−1 and εgb25%. Yet, Heiskanen(2000) claims that, in practice, the Sb better matches with the flotationrates of the fine fractions and suggests thatmore experimental work onthe k–Sb relationship with different mineralogical species, is needed.

With regard to the Sb values, Deglon et al. (2000); Power andFranzidis (2000) and Gorain et al. (1997) found that for normal (non-flooding) operating conditions, bubble surface area flux lied typicallywithin the range of 30–70 s−1.

Recently, some of these hydrodynamic plant data have beencombined, whereby typical mean bubble diameter ranged betweendb=1–1.5 mm and Jg=1–2 cm/s. Herein, theoretical and practical

79E. Matiolo et al. / International Journal of Mineral Processing 99 (2011) 78–83

considerations of their border limits have been reported (Yianatosand Henríquez, 2007).

In summarizing, bubble surface area flux has been related toflotation performance and claimed to be a key “machine variable”.However, most of the available data have been taken, calculated ormeasured in conventional cell and only a few have been done incolumn flotation cells.

The effect of frothers has been revised lately, in terms of airdispersion into fine bubbles, froth stabilization, interactions at thewater/gas interface and with collector molecules adsorbed into solidparticles, water carrying effect and their main effect on bubble sizeand uprising velocity (Finch et al., 2006, Melo and Laskowski, 2006,Grau et al., 2005, Nguyen et al., 2003).

This article is a contribution to the general discussion on thepossible implications of frothers in bubble generation (energy andsize) -interfacial tensions and system hydrodynamics.

2. Experimental

2.1. Column flotation

A laboratory flotation column, 2.54 cm diameter and 2.20 m totalheight, made of Plexiglas was used as the experimental apparatus(Fig. 1) for gas dispersion measurements in a two-phase system (air/water). Bubble generator used was a porous stainless cylinder with anominal porous size of 5 μm. Fig. 1 shows the set-up for thehydrodynamic measurements.

2.2. Gas hold-up measurement

For the experiments in a two-phase system (water/air), therequired amount of Dowfroth 250 was added to 30 L of tap water in

Fig. 1. Experimental set-up of the

natural pH (around 6–7). This solution was continuously agitatedusing a stirrer and introduced after 10 min in the column, with aperistaltic pump. The interface level was controlled by a peristalticpump located at the column bottom discharge and the air rate(injected directly into the bubble generator) was measured by a massflowmeter and regulated with a pinch valve (Fig. 1). Gas hold-up (εg)in the two-phase system (air–water) was measured by pressuredifference over a section of length, L of 83 cm in the collection zonejust below the froth. Pressure was sensed by water-filled manometersand the fractional gas hold-up was determined by:

εg =ΔHL

ð6Þ

where ΔH is the difference in the manometer readings.

2.3. Bubble size measurement procedure

Bubble size measurements were made using the LTM-BSizer(Rodrigues and Rubio, 2003) whereby the image analysis systemincluded the bubble capture cell, a microscope and a CDD camera(Fig. 2).

This technique (LTM-BSizer) employs a sampler to draw bubbles,rising in a column, into a special viewing chamber and exposes themto a digital camera, after they have decelerated and stopped. Thuscommon problems related to the movement of bubbles, namely focus,illumination, photographic speed and bubbles overlapping are allovercome. Results obtained are in good correlation with those valuesreported with the traditional image analysis method and show thatusing this technique, accurate size distributions can be produced,conveniently and efficiently (Rodrigues and Rubio, 2003, 2007).

laboratory flotation column.

Fig. 2. The LTM-BSizer for the determinations of the bubble size distribution (Rodrigues and Rubio, 2003).

80 E. Matiolo et al. / International Journal of Mineral Processing 99 (2011) 78–83

3. Results and discussion

Fig. 3 shows the size distribution of the generated bubbles andFig. 4 summarizes the mean bubble diameter (Sauter) at differentDowfroth concentrations (10 to 40 mg L−1) for a superficial gasvelocity fixed at 0.49 cm s−1. Herein, values obtained from drift-fluxcalculations (Yianatos et al., 1988), are enclosed for comparisonreasons.

Results show that when the frother concentration increases, thebubbles become smaller with the Sauter mean diameters decreasingfrom about 1000 μm to about 470 μm. This minimum (reaching aplateau) for the bubble size (and its size distribution) also depends onsparger and flow mixer (if venturi, needle valve, and static mixer)designs. Rodrigues and Rubio, 2003 found, for a similar system andfrother (used in this work), bubbles can be as small as 200 μm with aventuri, for the same frother concentration range. The higher fluidvelocity (and the higher energy to be dissipated ahead) inside theventuri, compared to a porous (perforated) cylinder, explains thedifference in bubble size and in the hold-up values.

Explanations for this behavior have been given by a number ofauthors and the bubbles size decrease appears to be associated to theenergy required for bubbles generation. This may be related to adecrease of the liquid–air interfacial tension, when the frotherconcentration increases, but there is no agreement on this issue.

A frother may produce a dramatic effect on bubbles sizedistribution (and narrowed) but may not substantially decrease theair/water interfacial tension. In the absence of frother, big (not-measurable) bubbles are formed while for concentrations higher than5 mg/L, the average bubbles size decreased substantially. Cho andLaskowski (2002), studied the effect of frothers on the size of bubbles,in single and multi-hole spargers and a flotation cell. These authorsfound that the size of bubbles strongly depends on frotherconcentration only when multi-hole spargers are utilized or whenenergy (arising from the liquid turbulence) is transferred, producingcavity phenomenon, and small bubbles are formed. At low frotherconcentrations, lower than a “critical coalescence concentration”(CCC), the bubble size is much larger, indicating a sort of coalescence

as the main mechanism determining the size. This coalescencephenomenonmight be prevented at frother concentrations exceedingthis CCC value, which is empirically determined as the froth dosage atthe break point of the slope (Melo and Laskowski, 2006, Laskowski,2003, Sweet et al., 1997, Grau et al., 2005).

Conversely, Finch et al. (2008) (to appear) believe that there is noagreed mechanism on how frothers act to reduce bubble size. Theseauthors claim that there would be a breakup mechanism, phenomenaassociated with bubble shape, with its hydrodynamics in pulps(velocity and surface flows). The resulting force would be associatedwith surface tension gradients and it would impulse this breakupphenomenon.

The fact is, when bubbles are formed in pure water, where littlepressure transfer exists, large bubbles are produced. At operatingpressures lower than 3 atm and in the absence of frothers, there is nosufficient energy to overcome attrition and nucleation to form smallbubbles. However, a decrease in solution/air interfacial tension provideslow energy nucleation sites and minute bubbles are generated.

Results obtained in previous works (Takahashi et al., 1979, Férisand Rubio, 1999, Féris et al., 2000) show that the minimum “energy”,ΔF, to be transferred to the liquid phase to form bubbles by a cavityphenomenon (arising from the liquid turbulence) is given by thefollowing equation (Takahashi et al., 1979):

ΔF =5:3⋅Π⋅γ3

Po−Pað Þ3 ð7Þ

where:

γ air–water surface tension (Nm−1)Pa flotation cell atmospheric pressure (atm or Pascal units);Po flow entrance pressure (atm or Pascal units) or pressure in

the sparger

Thus, the energy required to generate bubbles will be smaller withlower air–liquid interfacial tensionsandwithhigherpressuredifferences

Fig. 3. Bubble size distribution at different Dowfroth 250 concentrations and at constant Jg, 0.49 cm s−1.

81E. Matiolo et al. / International Journal of Mineral Processing 99 (2011) 78–83

across the sparger (inlet e outlet). Thus, when lowering the air–liquidinterfacial tension (Fig. 5) in a few units (7–10mN/m−1 units), theliquid–solid attrition will be smaller at the sparger outlet (or inside ifwater is used as carrier for the air). This results in a faster fluid flowvelocity and in a rapid bubble formation, after nucleation and cavitation.This would explain the initial very sharp inclination of the curve (Figs. 4and 5) and the onward plateau appear to be independent on thisdecrease in surface tension.

Furthermore, many authors find that there is a poor correlationbetween bubble size and surface tension but not many works dealwith errors committed in their experimental work. Fig. 5 for instance,poses the problem of the data accuracy and reliability, whichhighlights this ignored fact. It seems to be clear that surface tensionmeasurements depend on equipment (method) used, temperatureand on water quality. This situation is clearly shown in Fig. 5, wherecurves are different even if the frother sample is the same. Thus,

400

500

600

700

800

900

1000

1100

0 5 10 15 20 25 30 35 40 45

[DF 250], mg.L-1

Bub

ble

diam

eter

, µm

LTM-BSizer

Drift flux

Fig. 4. Bubble size (Mean Sauter Db) as a function of DF 250 concentration. Comparisonbetween bubbles sizes measured using the LTM-BSizer (Jg, constant at 0.49 cm/s andJw=0) and calculated by a drift flux method (Yianatos et al. 1988).

0

5

10

15

20

25

30

0.2 0.4 0.6 0.8 1.0 1.2

Superficial gas rate, cm·s-1

Gas

Hol

dup,

%

[DF250], mg/L 510203040

Fig. 6. Gas hold-up values as a function of superficial gas rate (Jg), at different DF 250frother concentration and JW=0.66 cm/s.

10

15

20

25[DF250], mg/L

510203040

as H

oldu

p, %

82 E. Matiolo et al. / International Journal of Mineral Processing 99 (2011) 78–83

despite using Nima or Kruss tensiometers, which employ the sameinteraction of a platinum ring (DuNouy) with the surface being tested,using very pure and ions free water, the experimental values werefairly different (assuming that not many differences would beencountered between 24 and 27 °C). Unfortunately, Grau et al. data(2005) do not include temperature, neither informed about waterquality (measurements appear to be done using a platinum ring).

0

5

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Superficial gas rate, cm·s-1

G

Fig. 7. Gas hold-up values as a function of superficial gas rate (Jg), at different DF 250frother concentration and JW=0 cm/s.

3.1. Gas dispersion parameters

Figs. 6 and 7 show the experimental gas hold-up values as afunction of superficial gas velocity (Jg) for Dowfroth 250 concentra-tions 5 to 40 mg L−1, and Jw of 0.66 and 0 cm/s, respectively. Thesefigures show a typical increase in gas holdup with Jg. Also, for aconstant Jg, the higher frother dosage the higher the gas hold-up. Yet,increasing the frother dosage, the hold-up values increase probablydue to bubble size decreasing and again, after a certain concentration,this trend tends to level off (20–40 mg/L Dowfroth 250, in this case).According to Fig. 4, the bubbles decrease dramatically in size, but onlyat very low dosages. According to some authors, the CCC is reached athigher frother dosages, thus ceasing the increase of gas hold-upbecause the bubbles size reduction stops. Also, comparison of Figs. 6and 7 shows that increasing the superficial liquid rate Jw from 0 to0.66 cm/s, the gas hold-up increases for the same frother dosage andsuperficial gas rate Jg.

Fig. 8 shows the relationship between the bubble surface area flux(Sb) and the gas hold-up (εg), with JW=0 cm/s, comparingexperimental data (LTM-BSizer) and values calculated by the drift-flux relationship (Yianatos et al., 1988). Experimental data shows a

56

60

64

68

72

76

0 10 20 30 40 50 60

Surf

ace

tens

ion,

mN

.m-1

Nima (this work, 24 ºC)Krüss (this work, 27 ºC)Rodrigues and Rubio, 2003, Krüss 25ºCGrau et al., 2005

[DF 250], mg.L-1

Fig. 5. Surface tension of aqueous solutions as a function of frother (DF 250)concentration. Values determined in different periods, conditions and authors.

good correlation and results confirm that for the range of interest, therelationship between bubble surface area flux and gas hold-up isreasonably linear (as predicted by drift-flux), with a better fit at low Jg(closer to a countercurrent flow). This finding reinforces the fact thatthe gas hold-up can be considered as a good estimate of bubblesurface area flux to characterize the flotation process (i.e. to correlatethe kinetic rate constant in the collection zone).

Fig. 9 shows the effect of the superficial liquid flowrate Jw on theSb–εg relationship, calculated by drift-flux. According to thisprediction an increase in Jw will increase the gas hold-up, keepingthe gas rate and bubble size constant. Otherwise, for a constant gashold-up the bubble surface area flux will decrease by increasing Jw, ifthe bubble size increases.

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25

Gas Holdup, %

Bub

ble

surf

ace

area

flu

x (S

b), s

-1

Jg, cm/s0.330.490.660.820.49 LTM BSizer

Fig. 8. Relationship between the bubble surface area flux (Sb) and the gas hold-up (εg),with JW=0 cm/s.

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Gas Holdup, %

Bub

ble

surf

ace

area

flu

x (S

b), s

-1

Jw = 0 cm/s

Jw = 0.66 cm/s

Fig. 9. Relationship between the bubble surface area flux (Sb) and the gas hold-up (εg),with JW=0 and Jw=0.66 cm/s.

83E. Matiolo et al. / International Journal of Mineral Processing 99 (2011) 78–83

4. Conclusions

Gas dispersion parameters were measured in a well characterizedlaboratory column flotation cell. Gas hold-up and bubble size (andtheir distribution) were found to be strongly dependent on Dowfroth250 concentration and on superficial gas velocity. A good relationshipbetween experimental gas hold-up and bubble surface area flux (Sb)was found for experimental values and with those calculated usingdrift flux analysis for Jw=0. Data obtained are discussed in terms ofsolution, hydrodynamics and interfacial phenomena. The role of thefrother in the energy dissipation in flow constrictors and in bubblegeneration and interfacial tensions was explored.

Acknowledgments

The authors gratefully acknowledge CAPES and CNPq for thescholarships awarded to E. Matiolo and F. Testa. Special thanks to Prof.R. T. Rodrigues, Camila Centeno and Alexandre Englert, from UFRGS, forthehelp in thebubble size and surface tensionmeasurements. J. Yianatosand J. Rubio wish to thank Conycit-Fondecyt, grant no. 7070233, forallowing the visit of J. Rubio to Chile and the research on flotation atU. Santa Maria, Chile.

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