MoyoGomez Finch (2007)

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CANADIAN METALLURGICAL QUARTERLY, VOL 46, N O 3

CHARACTERIZING FROTHERS USING WATERCARRYING RATE

P. MOYO, C.O. GOMEZ and J.A. FINCH

Department of Mining, Metals and Materials Engineering, McGill University,Montreal, Quebec, Canada H3A 2B2

[email protected]

( Received in revised form March 2007 )

Abstract The recovery of fine hydrophilic particles in flotation is related to the recovery of water.Water is carried by bubbles into and through the froth. The amount transported (entrained) depends onbubble size, gas rate and the subject of this paper, frother type. To isolate the effect of frother type fromthat of bubble size, gas holdup is used as the correlating variable. Measurements were made using abubble column (no solids) operated continuously with the overflow recycled. Using an automaticallycontrolled 7 cm foam depth, the overflow rate of water is used as a measure of water carrying rate.It is shown that the common frothers can be grouped into four families based on the water carrying rate-gas holdup relationship.

Rsum La rcupration de la gangue fine hydrophile lors de la flottation est relie la rcupration deleau. Leau est transporte par les bulles dans et travers la mousse. La quantit transporte (entrane)dpend de la taille de bulle, du dbit de gaz et, le sujet de cet article, du type de mousse. Pour isoler leffetdu type de mousse de celui de la taille de bulle, on utilise la rtention de gaz comme variable de corrlation.On a effectu des mesures en utilisant une colonne bulles (sans solides) opre en continu avec recyclagedu trop-plein. En utilisant une profondeur de mousse automatiquement contrle de 7 cm, le taux detrop-plein deau est utilis comme mesure du taux de transport de leau. On montre que lon peut grouper les moussants communs en quatre familles bases sur la relation taux de transport de leau-rtention de gaz.

INTRODUCTION

Frothers aid the production of fine bubbles which enhancesflotation rate and promotes froth formation. The generallyaccepted mechanism is that frothers reduce coalescence [1]although possible effects on bubble break-up are entertained[2, 3]. While the primary task is to collect particles, bubblesalso transport water. The transport of water governs therecovery of hydrophilic particles by entrainment which playsa large role in controlling grade [4]. Understanding water recovery is perhaps second only to particle recovery inpredicting and controlling flotation performance.

The interest here is to measure water transport as a

means to characterize frothers. There is a generalunderstanding that the type of frother does influence water transport. Frothers such as alcohols are considered to givedry froths while polygylcols give wet froths [5].Measurements of water recovery show a dependence onlocation of frother type [6-9] as do measurements of liquidflux in foams [10]. On a more fundamental level, frother type has been shown to control film thickness on bubblesblown in air [11].

To determine the role of frother type (or chemistry) inwater transport, it must be isolated from other controllingfactors primarily aeration rate, bubble size, froth depth andthe presence of hydrophilic and hydrophobic solids. Thepresent work uses an air-water system in a bubble columnwith froth (foam) depth and aeration rate controlled. Isolatinga frother chemistry effect from its effect through control of bubble size is not so straightforward. One approach is to useconcentrations above the critical coalescence concentration(CCC) [7] on the assumption that different frother types thenyield the same concentration-independent bubble size. It isnot clear how robust this assumption is. In principle, theproblem could be managed by measuring bubble size and

using bubble surface area flux as the correlating parameter [13]. Although bubble size measurement techniques continueto evolve, they remain intensive exercises where the selectionof appropriate bubble size metric for the present task is notimmediately clear. A possible alternative to bubble size is touse gas holdup. The advantages include easy measurement inair-water systems, an apparent relationship with bubblesurface area flux [14] and it has been used in a related manner to diagnose the fatty acid system [15].

Canadian Metallurgical Quarterly, Vol 46, No 3 pp 215-220, 2007 Canadian Institute of Mining, Metallurgy and Petroleum

Published by Canadian Institute of Mining, Metallurgy and PetroleumPrinted in Canada. All rights reserved


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EXPERIMENTAL

Apparatus

A column 10 cm internal diameter and 105 cm high wasemployed (Figure 1). It was equipped with eleven 2.5 cmwide stainless steel ring electrodes flush mounted to make

ten conductivity cells which were used to control foamdepth and to estimate gas holdup. (The term foam is usedrather than froth to distinguish it is a two-phase system,i.e., no solids are present.) The cell constant was determinedfor each cell to convert conductance to conductivity. Thecells were connected to a computer and conductivity wasrecorded automatically. Operation was continuous with thefeed (frother solution) controlled by a pump (Cole Palmer model 7520-25) with overflow water recycled to the feedtank (not shown).

The conductivity readings were used to detect thefoam/solution interface and the feed rate was manipulated tocontrol its position (i.e., control foam depth). The electrodeswere separated by acrylic sections 7.5 cm wide below the

foam and 5 cm wide across the foam zone to provide tighter depth control. A series of foam depths were employed; for thecurrent purpose, a depth of 7 cm was used.

A calibrated mass flow meter was used to control the air flow rate and a vertical porous sparger was used to generatebubbles. A temperature sensor (Thermopar type K) was usedto monitor temperature to correct the conductivity values tothe standard temperature of 25 C.

Measurements

Gas Holdup : Gas holdup is estimated from conductivity usingMaxwells model [16] for a non-conducting dispersed phase(bubbles in this context) in a conducting continuous medium:

(1)

where k d is the dispersion conductivity (aerated solution) andk w the continuous medium conductivity (un-aerated solution).

Water Carrying Rate (J wo): At steady state, the overflow wasmeasured by collecting and weighing over known periods of time. The value was checked against a pump calibration (atsteady state feed and overflow rates are equal). The mass wasconverted to volume (density assumed 1 g/cm 3) and divided bythe column cross-section area to give the water carrying rate.

Procedure

Table I summarizes the frothers examined and testconditions. The frothers were used as supplied. (Theinclusion of n-pentanol, not a frother used industrially, waspartly because short chain alcohols can be present incommercial supplies of other flotation reagents notablyxanthate collectors.) Salt (potassium chloride) was added to

e

k

k k

k

g

d

w

d

w

=

-

+

1

1 0 5.

216


Fig. 1. The bubble column set-up showing the electrodes, overflow and feedpump

Table I Summary of frothers, suppliers and test conditions

Frother Supplier Concentration range Jg, cm/sppm mol/L (x10 -3)

n-pentanol Sigma Aldrich 100-150 1.1 1.7 1.25-2.5n-hexanol Sigma Aldrich 30-80 0.29-0.78 1.0-2.5

n-heptanol Fischer Scientific 30-80 0.25-0.69 1.0-1.75n-octanol Sigma Aldrich 30-80 0.23-0.61 0.75-1.75

MIBC Sigma Aldrich 30-80 0.29-0.78 1.0-2.5Ethoxylated C 6 alcohol Flottec 20-50 0.14-0.34 0.75-2.0

Dowfroth 200 Flottec 50-100 0.25-0.5 1.0-2.0Dowfroth 250 Flottec 50-80 0.2-0.32 0.75-1.75

F-150 Flottec 30-50 0.075-0.125 0.75-1.50

Wateroverflow

collectionpoint

Electroderings

Feed pump

Feed entrypoint

Air

Poroussparger


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CHARACTERIZING FROTHERS USING WATER CARRYING RATE 217


raise the conductivity of the solution to about 0.40 mS/cm toincrease measurement sensitivity. For each condition, theconductivity of the unaerated solution ( k w) was obtainedprior to and at the end of the test. To start, the height of solution in the column was adjusted to leave about 25 cm atthe top for foam to build. The level was then adjusted to thetarget foam depth (7 cm) and controlled automatically bymanipulation of the feed rate. At steady state, the dispersionconductivity values ( k d ) were logged and water overflowwas collected for 3 to 5 minutes depending on the flow rateand weighed and water carrying rate ( J wo) was determined.The gas holdup value reported is the mean of all the cellsbelow the foam zone. The J wo - e g trend was thendetermined for the range of frother types.

RESULTS

Establishing Gas Holdup As Correlating Variable

The water carrying rate, J wo, as a function of gas rate, J g, for

n-hexanol at three concentrations is shown in Figure 2a.Linear trends are evident which depend on frother concen-tration. Figure 2b shows the corresponding gas holdup, e g,vs J g data and Figure 2c shows the results of combining andplotting J wo as a function of e g. There is now a singleeffectively linear trend. Figure 3 likewise shows that J wo vs

J g for two spargers of different porosity becomes a uniquefunction of e g. From this point, all tests used the 5 m mporosity sparger. Both plots show an intercept between an e gof 15 to 20% which means foam could not reach therequired 7 cm at gas holdups below this point. The trends inFigures 2c and 3b establish gas holdup as the correlatingvariable.

Frother Characterization

A comparison of straight chain alcohols (n-alcohols) is shownin Figure 4. An increase in the water carrying rate with carbonchain length (C 5-C 8) is observed. Combining all data onalcohols, they classify into three groups (Figure 5). The resultfor the two C 6 alcohols, MIBC and n-hexanol were similar,but the ethoxylated C 6 alcohol was different, being similar ton-octanol.

Figure 6 shows a comparison of polyglycol frothers. Thetwo Dowfroths show an increase in J wo with molecular weight. The F150 gave the highest J wo of the frothers tested.Figure 7 combines all of the results showing the frothersclassify into four major groups (or families).

DISCUSSION

The measurement task was to isolate an effect of frother chemistry on the water carrying rate independent of the rolefrother plays through control of bubble size. A direct routewas considered, using bubble surface area flux, but excluded

Fig. 2c. Water carrying rate as a function of gas holdup (same system asFigure 2a)

Fig. 2a. Water carrying rate as a function of gas rate for three concentrationsof n-hexanol

Fig. 2b. Gas holdup as a function of gas rate (same system as Figure 2a)

J g , cm/s

J w o , c

m / s

30ppm

50ppm

80ppm

0.3

0.25

0.2

0.15

0.1

0.05

0

0.5 1 1.5 2 2.5 3

J g , cm/s

e g ,

%

30ppm50ppm80ppm

30

25

20

15

10

0 0.5 1 1.5 2 2.5 3

eg , %

J w o , c

m / s

30ppm

50ppm

80ppm

0.5

0.4

0.3

0.2

0.1

0

10 15 20 25 30 35


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P. MOYO, C.O. GOMEZ and J.A. FINCH218


Fig. 3b. Water carrying rate as a function of gas holdup (same system as inFigure 3a)

Fig. 3a. Water carrying rate as a function of gas rate for two sparger porosi-ties (50 ppm n-octanol)

Fig. 5. Water carrying rate as a function of gas holdup for all alcoholstested

Fig. 4. Water carrying rate as a function of gas holdup for n-alcoholstested

Fig. 7. Water carrying rate as a function of gas holdup for all frotherstested

Fig. 6. Water carrying rate as a function of gas holdup for polyglycolstested

J g , cm/s

J w o , c

m / s

5 mm

20 mm

0.5

0.4

0.3

0.2

0.1

0

0 0.5 1 1.5 2 2.5

eg , %

J w o , c

m / s

Pentanol

Hept, Hex, MIBC

Octanol, Ethoxy

0.5

0.4

0.3

0.2

0.1

010 15 20 25 30 35

eg , %

J w o ,

c m / s

5 mm20 mm

0.5

0.4

0.3

0.2

0.1

0

10 15 20 25 30 35

eg , %

J w o ,

c m / s

DF200DF250

F150

0.5

0.4

0.3

0.2

0.1

0

10 15 20 25 30 35

eg , %

J w o ,

c m / s

Pentanol

Hexanol

Heptanol

Octanol

0.5

0.4

0.3

0.2

0.1

010 15 20 25 30 35

eg , %

J w o , c

m / s

PentanolHept, Hex, MIBC, DF200Octanol, Ethoxy, DF250F150

0.5

0.4

0.3

0.2

0.1

0

10 15 20 25 30 35


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CHARACTERIZING FROTHERS USING WATER CARRYING RATE 219


in favour of gas holdup on the basis of simplicity. The resultwas a unique dependence of J wo on e g for a given frother which established gas holdup as a useful correlating variable.

The J wo - e g relationship was treated as linear (above aminimum gas holdup required to establish the target foamdepth) and showed essentially the same slope for each frother type. (As a note in passing, at greater depths linearity wasapproximately preserved but slopes were no longer similar among frother types.) The frothers tested were grouped intofour families. The classification fits the qualitativeassessment that alcohols transport less water (give drierfroths) than polyglycols. The two categories overlap withMIBC showing similar rates as DF200. Among the alcohols,the two C 6 alcohols, MIBC and n-hexanol, gave a similar response and the ethoxylated C 6 alcohol (i.e., now a C 8) gavea similar result to n-octanol. This suggests that as far as thewater carrying rate is concerned, the number of carbons isimportant but not whether the chain is branched. Thisobservation has led directly to the development of newfrothers based on alkoxylation [17].

The concentration range used here (Table I) is higher than relevant to plant practice where 10 ppm may beconsidered average [18]. High concentration is needed in

the absence of the stabilizing effect of hydrophobic particles.Most frothers are not foaming agents in the strict senserequiring solids to complete that task. The presence of solidscan be expected to influence the overflow water rate. Meloand Laskowski [7], for example, note that MIBC switchedfrom giving the lowest water recovery in two-phaseexperiments to the highest when floating coal. This isprobably related to the impact solid particles exert on water flow in the froth.

It is unlikely that a standard solid can be agreed on for frother characterization making the two-phase system apractical starting point. Selecting frother equivalents on thebasis of two-phase testing has proved viable [17]. Thepossibility of using talc as a model hydrophobic solid is being

explored [19].The measurement of J wo is a combination of water transport into and through the froth. There is currently consid-erable focus on modelling water transport in the froth [20-22]which will need to include a parameter for frother chemistry.There may be a role of frother type on transport into the froth.Bascur and Herbst [23] modelled this by introducing a filmthickness on the bubble which could be related to frother properties. Table II shows that direct measurements of film

thickness (on a bubble blown in air) do appear to correlatewith water carrying rate [3].

CONCLUSIONS

Characterizing frothers using the water carrying rate, J wo, asa function of gas holdup e g, was explored. The main findingswere:1. For the conditions used, the J

wo - e

grelationship for

each frother was linear with an intercept on the axisindicating the minimum e g capable of supporting therequired foam depth (7 cm).

2. For the frothers tested, the J wo - e g relationshipidentified four classes (families), the order being, fromlowest to highest J wo (at a given e g)pentanol < heptanol, hexanol, MIBC, DF200 < octanol,ethoxylated C 6 alcohol, DF250 < F150

3. For alcohols, the water carrying rate appeared to dependon chain length but not on branching.

ACKNOWLEDGEMENTS

Funding was through the Chair in Mineral Processing atMcGill University, under the Collaborative Research andDevelopment Program of NSERC (Natural Sciences andEngineering Research Council of Canada) with industrialsponsorship from (using the original names, 2001) INCO,Falconbridge, Noranda, Teck Cominco, COREM and SGSLakefield Research. Extensive discussions with Jan Nesset,Stphanie Glinas and especially Frank Cappuccitti (Flottec)are gratefully acknowledged.

REFERENCES

1. SME Handbook of Mineral Processing, 1985, N.L. Weiss, ed.,American Institute of Mining, Metallurgical and Petroleum Engineers,Inc., Kingsport Press, Kingsport, TN, pp 5-85 5-87.

2. R.A. Grau, J.S. Laskowski and K. Heiskanen, Effects of Frothers onBubble Size, Minerals Engineering , article in press.

3. J.A. Finch, S. Glinas and P. Moyo, Frother-Related Research atMcGill University, Minerals Engineering , 2006, vol. 19,pp. 726-733.

4. J. Lynch, N.W. Johnson, E.V. Manlapig and C.G. Thorne, Mineral andCoal Flotation Circuits, vol 3, Developments in Mineral Processing ,Elsevier Scientific Publishing Company, pp 21-43.

5. Cytec Mining Chemicals Handbook , 2002, 75-78.

Table II Water carrying rate and thickness of film on bubble blown in air 1

Frother Water carrying rate, J wo (cm/s) Water film thickness (nm)(at e g = 25%, Figure 7)

n-pentanol 0.07 < 160MIBC 0.18 < 160

DF 250 0.25 600

F 150 0.35 11001 The film comprises two parts; the one recorded here is the inner bound layer [3] .


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6. Z. Ekmekci, D.J. Bradshaw, S.AAllison, P.J. Harris, Effects of Frother Type and Froth Height on the Flotation Behaviour of Chromite in UG2Ore, Minerals Engineering , 2003, vol. 16, pp. 941-949.

7. F. Melo and J.S. Laskowski, Fundamental Properties of FlotationFrothers and Their Effect on Flotation, Minerals Engineering , 2006,vol. 19, pp. 766-773.

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Engineering , 2000, vol. 13, pp. 365-372.15. R. Espinosa-Gomez, J.A. Finch and W. Bernett, Coalescence and

Froth Collapse in the Fatty Acid System, Colloids & Surfaces , 1988,vol. 32, pp. 197-209.

16. J.C. Maxwell, A Treatise of Electricity and Magnetism , 1892, 3 rd ed.,Vol. 1, Part II, Chapter 9: 435-449.

17. F. Capuccitti, J.A. Finch, Development of New Frothers throughHydrodynamic Characterization of Frother Chemistry, Proceedings

39 th Annual Meeting of the Canadian Mineral Processors of CIM , 2007,compiled by J. Folinsbee, pp. 399-412.

18. S. Glinas, J.A. Finch and F. Cappuccitti, Frother Analysis: Procedureand Plant Experience, Proceedings 37 th Annual Meeting of theCanadian Mineral Processors of CIM , 2005, compiled by J. Starkey,pp. 569-576.

19. J. Quinn, Exploring the Role of Salts on Gas Dispersion and FrothProperties, 2006, Masters Thesis, McGill University, Montreal.

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International Mineral Processing Congress , 3-8 September 2006,Istanbul, Turkey, pp. 756-761.

21. S.J. Neethling, J.J. Cilliers, Modeling Flotation Froths, Inter. J. Min. Proc ., 2003, vol. 72, pp. 267-287.

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