JOt,;R~AL OF APPLIED ELECTROCHE~I(STRY 21 (19911986-990

P. SOMASVNDARANHenr.l" Krumb School of ,\lines" Columbia Universit.v. Nt"'" York. .VY 10017. U.S.A

itnl try 199t~ fe"'istd. . ,fr-r('h 1991

Fine bubbles of the size required for many processes such as electro flotation can be generated byelectrolysis. A large number of factors such as electrode material, electrode surface/morphologicalproperties. pH and current density affect the gas bubble size distribution. This work is aimed at studieson the effect of interrupted current (pulsed) electrolysis on the generation of gas bubbles. Amicrocomputer-controlled current source designed to generate the required pulses is described alongwith typical results obtained with this system. It was observed that a decrease in duty cycle at a givenpH and average current density causes an increase in fine sized bubbles and concomitant increase inbubble flux. A mechanism based on local potential gradients is proposed to explain this phenomenon.

l. Introduction parameters to obtain efficient generation of the desiredfine sized bubbles. Thus. there is need for a techniqueto generate bubbles of desired bubble size distributionlargely independent of the solution conditions. Pulsedelectrolysis can be carried out by using a waveformgenerator along with a potentiostat capable of fastslew rates. PC programmable power supplies usuallyhave low slew rates and may not prove suitable as apulsed current source for a capacitive load such as anelectrolytic cell in the frequency range discussed in thiswork. In this work an IBM/PC controlled pulsedcurrent source having a high voltage sle\\; rate hasbeen described. The pulsed source can be controlledby appropriate software which can be tailored to anindividual application.

2. Literature review

Much of the experimental work addressing the gen-eration of bubbles has been concerned with nucleateboiling. gas generation during electrolytic processingof materials as well as mass transfer/hydrodynamicaspects [I. 2]. A special consideration however forelectro.flotation of mineral fines is the need to gen-erate fine ( < 50 JlTn) dispersed gas bubbles. Fine par-ticle flotation which is limited due to the diminishingprobability of bubble-particle collisions. can- beimproved by increasing the number of fine sizedbubbles [3]. Thus. the present work is addressedtowards the generation of small sized bubbles. Animportant outcome of smaller bubble size is an increasein bubble _ffu.~ (the number of bubbles traversing unitcross-section per unit time) as well as the number ofbubbles per unit volume for a given volume of gasevolved. These two factors have been shown to have astrong influence in fine particle flotation [3].

A primary concern in processes such as electro.flotation is the generation of fine bubbles of knownsize at a known bubble flux [3-7]. Various investi-gators [3. 4-16] have studied electrolytic generation ofgas bubbles and the physical parameters governing thebubble size. The occurrence of increased hydrogenbubble size in the acidic pH region and that of oxygenin the alkaline region is well known. Bubble contactangle [9-13]. electrode condition (for example. surfaceroughness) [3. 16. 17]. hydrogen overvoltage [3]. cur-rent density [3. 9-11. 17]. polarization potential [3] andbubble charge [15. 18] have been suggested by manyworkers to be among the factors governing the bubbledeparture diameter. For a given set of system par-ameters such as pH. temperature. reagent concen-tration and electrode material. the bubble propertiesare fixed and it becomes impractical to adjust these

The physical process of gas evolution can be dividedinto three stages: nucleation, growth and detachment.Bubbles nucleate at electrode surfaces from solutionshighly supersaturated with product gas and grow bydiffusion of dissolved gas to the bubble surface or bycoalescence at the electrode with other bubbles [19].They detach from the electrode when buoyancy orliquid shearing forces pulling the bubbles away over-come the forces binding them.

Surface inhomogeneities such as cracks are gener-ally considered high energy nucleation sites due to theavailability of atomic ledges as high energy anchoragepoints. This phenomenon has been a subject of detailedinvestigation in nucleate boiling [13]. crystal1izationand solidification [20. 21]. For bubble generation invacuum or pressure release flotation (analogous toprecipitation) the above phenomenon is important.During the eleCtrolysis it is general1y agreed that thepreferred nucleation sites are at surface inhomogen-

986 OO~I.89IX/91 503.00 + .\2 :t \99\ Chapman &. Hall

N. K. KHOSl.-\.. S. VENKA T ACHALA.\I{

Dt'partment of .\.leta//urgica/ Engint'ering. Indian Institute of Techno/ag.\". Po','ai. Bomba.\' 4000i6. India

Pt:LSED ELECTROGE~ERATIO~ OF BL"BBLES FOR ELECTROFLOTATION 987

cities such as fissures. cracks and SCT'.1tches [13. 17]as well as local inhomogeneities resulting: in donor-acceptor [3] and low overpotential sites [3. 17). Thedependence of thl: voltage gradient at the tip of an«dle el.:ctrode upon its curvature is a ~.ell knownphenom~non. Occurrence of such sharp points on anel.:ctrode and the consequent presence of high local

, ,.;.. ,.. " , "it,." O' I . , ., T L~,"- .~ ",. 0"'" '-..,~ ,,'0"" .., ',..,;" Vu, f1L

importance of the role of voltage gradients towardsnucleation is clear from the observation that on wireand mesh electrodes bubble size d~pends largely onelectrode curvature (and thus potential gradient)almost independent of the current density [3. 13].However. bubble growth rates are not strongly depen-dent on the diameter of wire electrodes [17]-

Various mechanisms have been proposed to deter-mine the bubble departure diameter. Coehn [8] pro-posed that electrostatic attraction between the bubbleand the electrode is instrumental in determining thedeparture diameter. Stronger attraction requires alarger buoyant force for detachment necessitatinglarger bubble diameters. Bubble charge studies [18]conducted on platinum anodes at 10-40 kA m -1 tend

to support this mechanism. However. electrostaticforces are predominant at current densities (10' A m -~).larger than those encountered in electroftotation[9-11, 14. 16]. Using high speed photography. Glasand Westwater [17] observed that contact angle isnot as significant a factor as surface roughness orelectrode material in determining the growth rate ofbubbles.

A relation between the hydrogen overvoltage andthe bubble size has been observed [4]-

Increasing bubble size

Pb Sn Cu Ag Fe \\0' PdNi

Increasing overvoltage

microcomputer. Thus. the requisite wavefoJm is gen-erated by the computer in the background while thecomputer is simultaneously available for compu-tational and other control worK. The DA output is fedto the power amplifier (PA). The high current supply(PS) is capable of providing a peak current of 5 A. Theuse of a 12-bit DA operating under direct memoryaccess of an 18M/PC operating at 8 MHz offers aresolution of 0.024°;, for current (maximum current of4A) and minimum pulse cycle time of 3.2ms with0.5'10 resolution.

Software was developed for the generation of pulses(Fig. 2) of variable duty cycle at a selected averagecurrent. The average current (i.. - id/lOO) was com-puted from the area under the curve (shaded) andthe duty cycle (d) as the percentage of cycle time (l~ Iduring which the current ftows through thc cell. Thus.for a square wave. the duty cycle is 500,/0. The softwarewas designed to generate a high peak current nucleationpulse at the start of each cycle. The contribution of thenucleation pulse to the total energy was negligible« 0.2';.).

Ela:trolysis was carried out in a quartz cell mounted

It was also observed that under clean conditions con-tact angle is dependent on the overvoltage [4].

Thus bubble size and flux in aqueous electrolysisseems to depend upon (a) surface morphologicalfactors including donor acceptor and low overpoten-tial sites and (b) the inherent overvoltage property ofthe electrode material.

t

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u

3. Experimental details

The pulsed d.c. source was designed as a circuit boardto be interfaced to a microcomputer giving the systemftexibility in its configuration as a constant current!voltage source (Fig. I). User interface is more versatiledue to software control of the equipment. The circuitis mapped on the 1/0 bus of the microcomputer usingthe switch Sol and address decoder (DEC). The micro-computer sends data to the digital to analog converter(DA) through buffers (BUF). The timer (TIM) isprogrammed to generate timing pulses to update DAunder the direct memory a~ (DMA) mode of the

~-~ - -t- - t I, - cy---,

Time ($) -

Fig. 2. Pulse wa\"CShape

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'00:z

on a stand backlit with Cl bright light source. A shut-tered video cam~ra (1/1000 shutter Spced) fitted withmicroscope objective SOX to JOOX) was used to recordbubble dimensions on a video recorder. A videographicprinter was used to print the picture rrames ror subse-quent bubble size measurement. The quartz cell wasdesigned to circulate the electrolyte (I M Na:SQj).The electrolyte (pH 10.0) was circulated by means orperistaltic pump and a large external reservoir or theelectrolyte was maintained to ensure minima' pH..:.ullb':. i::i,:t:'.:~l),c circu:.\ti;:JJI ..:~v i...~iiitated removalor the generated gases.

The platinum plate cathode (I cm~) was carerullypolished and thoroughly degreased. Care was taken toexclude surrace active contaminants. The externalglass reservoir for the electrolyte was always keptcovered to prevent air-borne contamination. Thebubbles were obser'w.ed between 2 to 2.5 mm verticallyabove the electrode.

Experiments were carried out ror the electrogener-ation or hydrogen bubbles with a pulsed currenthaving a step waveform (Fig. 2). The solutions wereelectro lysed with an average current density rangingfrom 0.5 to 2.0 Am-: and duty cycle ranging from 5to 100%. A cycle time of 30 ms was used and a record-ing period of 5 min was used at each experimentalcondition. Bubble size distribution was computed byexamining at least 150-200 bubbles in sharp focus.

8 - 16 - 24 32 ~

Log bubble diam. (~m)

Fig. 4. Bubble size distribution at a~erage currenl density of1 Am-~ at pH 10.0 and duty cycles 0(0/0): tOO (8'. SO ()(). 2.S (OJ.to (.) and S (6).

puted from the video prints by taking the averageof bubbles found in sharp focus. It can be seen fromFigs 3-6 that at a given average current density, adecrease in duty cycle results in an increase of bubblesof smaller diameter. This is expected to significantlyincrease the bubble flux.

4. ResultsS. Discussion

Initial experimental work on electrolysis with rec-tangular wavefonn having a leading high energynucleation pulse. showed no significant differencefrom the electrolysis carried out wjthout the leadingnucleation pulse. Thus all subsequent work was con-ducted wjthout a leading nucleation pulse.

The results obtained for bubble size distribution asa function of current density and duty cycle arepresented in Figs 3-6. Bubble diameters were com-

]O, -

The chief effect of pulsing observed during electrolysisis the occurrence of an increased number of small sized« 1001Spm) bubbles (Figs 3-5). Thus at low dutycycle, higher current during the 'on' time forces alarger number of bubbles to nucleate and grow result-ing in an increased bubble flu~. The requirement ofhigh current density for nucleation and growth hasbeen observed by Brandon [18) to be due to diffusion

25..

25

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.20

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..

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8 210 S6Log bubble diam. (11m)

Fig- 5. Bubble size distribution 3t awraac cu~nt density or0.5 A m-: at pH 11>.0 and duty cycles or (-I.): so (x). 25 (e) and10(0).

F~

989Pl;LSED ElECTROGE~ERATION OF Bl;BBlES FOR ElECTROFLOTATtON..\l

higher instantaneous currents under pulsed conditionslargely offsets the effect or local potential gradientsand also results in a high degree or local gas super.saturation. thus resulting in homogeneous nucleation.At short times i.e. I < 10 ms. bubbles grow by actingas sinks for the previously generated dissolved gas inthe supersaturated solution adjacent to the electrodell~}. ~uch growth IS hydruuJllu",":al:) conlrUllCd by

liquid inertia.

S.2. Gro~.lh and derachmenl

Growth is driven by the high internal excess pressureinside the bubble which is related to the surface ofliquid-vapour interface. From 10 ms onwards. dif-fusion of dissolved gas to the bubble contact perimeterbecomes rate controlling [22}. Under pulsed conditionsof low duty cycle. growth is either entirely inertialor inertial and diffusion controlled. Under conditionsof low current density (high duty cycle). while mostnuclei may grow in a similar manner during the inertialphase. only nuclei at the lowest overpotential sitesgrow during the diffusion control phase due to con-centration polarization. Frequent interruptions incurrent during pulsed electrolysis also promote bubbledetachment. Bubble size increase by coalescence at theelectrode [t. 19}. as well as by coalescence in bulk.results in further increase in bubble diameter.

~40nsily oft. ~5 to).

averagen from1sity. a)ubblesfic3ntly

6. Conclusions

:trolysisl;llt siz~d:>" dutyforces ;l"" resutt-:m~nt ofI\\"th hasdiffusion

1. Pulsed electrolysis can be used to regulate bubblesize independent of the average current density and

other such parameters.2. During water electrolysis. increase in current den-sity results in finer bubbles which may find applicationin processes such as electroftotation.

Acknowledgements

The authors acknowledge the assistance of Dr D. R.Ketkar, Department of Metallurgical Engineering,Indian Institute of Technology. Bombay. in the experi-

mental work. This work was supported by NationalScience Foundation. (Grant No. INT-87{16926.)

References

.Ii:

(1)

(3)

(4)

of dissolved gas away from the electrode. Thus aninduction time is observed for bubble nucleation. Theinduction time is the time required to attain thenecessary supersaturation of the electrolyte withrespect to the dissolved gas at the electrode surface.Thus bubbles. once nucleated. require a relatively highcurrent density for sustained growth. Studies withpl:ltinum electrodes have shown that bubble detach-ment is greatly facilitated by the interruption of cur-rent as \I."ell as by a reduction in the potential [18).Pulsed electrolysis provides ample opportunity for thebubbles to dislodge since the current is interruptedafter each cycle.

To study the effect of the presence of surface activereagents during pulsed electrolysis. experiments wereperformed in the presence of I x 10-5 M sodiumdodecylsulphonate. the concentration chosen beingtypical for froth flotation systems. Results plotted inFig. 6 show that the effect of surfactant at high dutycycle (500,/0). as well as low duty cycle (5%). is notsignificant. An important effect of surfactant. namelythe modification of the bubble-electrode contact anglehas been shown to have very little effect in the case ofhydrogen bubbles [17].

The following interpretation of the mechanisms issuggested for the nucleation and growth of bubblesduring pulsed electrolysis.~j

---(6 Nucleation$, [5)

[6)

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t density of). 25 (8) and

990 N. K. KHOSLA. ETAl.

171 (15)

(16)

(17]

(18)

(19)(20)

(81£91

(IOJ

(III

till(I))

(14)[21)

(22)

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