Mineral Procesing and Extractive Metallurgy Review
`
`ISSN: 0882-7508 (Print) 1547-7401 (Online) Journal homepage: https://www.tandfonline.com/loi/gmpr20
`
`Electrogenerated Gas Bubbles in Flotation
`
`S. VENKATACHALAM
`
`To cite this article: S. VENKATACHALAM (1992) Electrogenerated Gas Bubbles
`in Flotation, Mineral Procesing and Extractive Metallurgy Review, 8:1-4, 47-55, DOI:
`10.1080/08827509208952677
`To link to this article: https://doi.org/10.1080/08827509208952677
`
`Published online: 25 Apr 2007.
`
`Submit your article to this journal
`
`Article views: 31
`
`View related articles
`
`Citing articles: 8 View citing articles
`
`Full Terms & Conditions of access and use can be found at
`https://www.tandfonline.com/action/journalInformation?journalCode=gmpr20
`
`Tennant Company
`Exhibit 1029
`
`

`

`Mineral Processing and Extractive Metallurgy Review, 1992, Vol. 8, pp. 47-55
`Reprints available directly from the publisher
`Photocopying permitted by license only
`© 1992 Gordon and Breach Science Publishers S.A.
`Printed in the United States
`
`Electrogenerated Gas Bubbles In
`Flotation
`
`S. VENKATACHALAM
`
`Department of Metallurgical Engineering, Indian Institute of Technology, Bombay 4110 076,
`
`Electrolytic gas evolution plays a very significant part in a number of electrochemical proce sses. In the
`electrowinning of metals the evolution of gases at the electrodes is a very important phcnor renon, Gas
`evolution is common in chlorine and water electrolysis and in a number of other processes. Electro(cid:173)
`generated gas bubbles have been used in the treatment of waste water and considerable amount of
`work has been done on the electroflotation of mineral fines which are unsuited for tr eatrnent by
`conventional flotation techniques. The physical process of gas evolution viz nucleation. ~rowth and
`detachment arc discussed. The dynamic process of elcctrogcneration of gas bubbles as affected by the
`interaction amongst a large number of process variables has been dealt with in detail. Flotntion of
`mineral fines as affected by the size and charge of the bubbles is also discussed.
`
`INTRODUCTION
`
`In many electrochemical processes, electrolytic gas evolution plays a very important
`role. In the fused salt electrowinning of aluminium by the Hall process, tt e oxygen
`evolving at the carbon anode leads to the so called" Anode effect" which has been
`studied in considerable detail. Also, in the aqueous electrowinning of m etals, gas
`evolution at the electrode is important. The oxygen evolving at the nonconsumable
`anode contributes mostly to the highly energy intensive nature of the electrowinning
`process. Gas evolution is not restricted to the anodes, In the electrowinning of zinc
`hydrogen evolution at the cathodes affects the deposition of zinc. Electrolytically
`evolved gas bubbles have been shown to enhance the transport of heat md mass
`at gas evolving electrodes,
`
`THE PHYSICAL PROCESS OF GAS EVOLUTION
`
`The first step in the gas evolution is nucleation. With regard to electrogenerated
`gas bubbles, we have to discuss the influence of current or rather the current density
`at the electrodes, If the current density is very low, the dissolved gas is removed
`from the electrode in the direction of liquid bulk by molecular diffusion and con(cid:173)
`vection and no gas phase is formed, Even at high current densities this ma y happen
`
`47
`
`

`

`48
`
`S. VENKATACHALAM
`
`if the dissolved gas reacts homogeneously and quickly like chlorine. If the current
`density is sufficiently high, gas bubbles form at predetermined nucleation sites
`located at the electrode surface. Adjacent to the electrode, sufficient supersatu(cid:173)
`ration of the liquid would have occurred prior to nucleation'.
`Real surfaces contain pits, scratches and grooves of various sizes which act as
`nucleation sites. Westerheide and Westwater' have reported that nucleation on
`their micro electrodes occurred at preferred sites. Janssen and Hoogland' observed
`that bubbles nucleated on a rotating platinum wire at specific sites that depended
`on the pre treatment as well as current density.
`After nucleation, bubble growth takes place. A bubble adhering to the surface
`grows by the supply of dissolved gas from the surrounding liquid and due to the
`high internal pressure. It must be noted here only that the bubbles of a critical
`radius and larger grow while bubbles having radii less than this dimension tend to
`decay", Scriven' has presented a general analysis of a diffusion controlled growth
`of a bubble. The growth rate is given by R = 2P(Dt)l/2, where R is the radius, D
`t the time and P is a coefficient characteristic of the degree of
`the diffusivity,
`supersaturation. Westerheide and Westwater? photographed individual hydrogen
`bubbles and quantitatively compared their growth data to the diffusion square root
`of time growth dependence. Good agreement was found for a single bubble. The
`importance of mass transfer of dissolved gas to the bubble surface as the asymptotic
`mechanism by which the bubbles grow before and between coalescence has been
`established. This work has been extended to oxygen, chlorine and carbon dioxide".
`The validity of the growth law is restricted to gases where the adhering bubbles
`do not mutually interfere at moderate current densities. At current densities above
`- 1000 A/m 2 various anomalies occur'. A very important anomaly is the coales(cid:173)
`cence of bubbles on the electrode surface. While growing, two single bubbles may
`touch each other and coalesce', Bubbles of a moderate size (40 microns) establish
`themselves as central collectors and receive the smaller bubbles nucleating and
`growing around them. This has been termed as radial specific coalescence? A third
`mode of coalescence called a scavenging mechanism, that is, bubbles sliding along
`the electrode and consuming other smaller bubbles has also been notedt". Hydrogen
`bubbles produced in acid have been shown to grow by such a mechanism. Coa(cid:173)
`lescence has been shown to occur with in short times. (- 10'· seconds)", Sides and
`Tobias? have proposed a cyclic mechanism of oxygen bubble growth in basic me(cid:173)
`dium. The sequence of events, namely nucleation, growth by diffusion, coalescence
`of small bubbles, coalescence by radial motion and scavenging coalescence has
`been termed as the cyclic process of the bubble growth. The period of the cycle,
`at a current density of 100 mA/cm2, was approximately 0.1 second. However, this
`is not a general phenomena and need not occur on different electrodes evolving
`other gases. In particular, it may be noted that coalescence seems unimportant in
`hydrogen evolution from basic solution because the bubbles evolved are quite small.
`This is discussed in the latter section. Further, it may be noted that larger bubbles
`formed by the coalescence need not detach from the electrode but at least under
`certain conditions adhere to the electrode while smaller bubbles depart.
`The last stage of the process of gas evolution is the detachment of bubbles.
`Frumkin and Kabanov'" have found that the buoyant gas bubbles detach when
`
`

`

`ELECTROGENERATED GAS BUBBLES IN FLOTATION
`
`49
`
`surface adhesive forces, related to bubble contact angles, can no longe: restrain
`them. Without present knowledge, we are not in a position to predict the departure
`diameter of the bubbles. The positioning of the electrodes horizontally or vertically
`has also been shown to affect the bubble size. The mobility of the bubbles on the
`electrode surfaces is related to detachment. The relation between hysteresis 'angle,
`the contact angle and the volume of the largest bubble that sticks to i surface
`facing downward and inclined to the horizontal has been studied II.
`
`TREATMENT OF MINERAL FINES BY ELECTROFLOTATION
`
`The unsuitability of conventional flotation techniques for the treatment 0 f mineral
`fines has been attributed mainly to the large bubble size which reduces ':he prob(cid:173)
`ability of collision with fine particles, thereby decreasing collection efficiency. Be(cid:173)
`cause of small mass and low inertia, fine particles are carried along the str earn lines
`the stream lines of the bubbles and do not collide with the bubbles. To increase
`the collision probability fine bubbles can be used which may be generated in a
`number of ways. One of the methods to produce fine bubbles and carrz out the
`flotation is known as electroflotation, that is, flotation using electrogene rated gas
`bubbles.
`The gas bubbles produced in conventional flotation machines of mechanical type
`are 0.8 to 0.9 rnm, about 2 mm in pneumatic machines and about 0.1 t« 0.5 mm
`in vacuum flotation. Electroflotation permits the formation of extremely finely
`dispersed gas bubbles in the range of 10-100 microns depending upon the Co mditions
`of electrolysis", In view of the importance of bubble size in flotation, this feature
`of electroflotation offers a great advantage over conventional flotation. In addition
`to the bubble size, the bubble flux and bubble density are also important. Bubble
`flux may be defined as the number of bubbles per unit area per unit
`:ime and
`bubble density as the number of bubbles per unit volume. Continuous adjustment
`of the bubble size, bubble flux and bubble density is possible in electro Iotation.
`It is also possible to use hydrogen and oxygen gases separately or in combination.
`The electrogenerated bubbles can be used along with air which could ensure that
`the bigger particles are floated by the bigger air bubbles and the finer on es by the
`The advantage of electroflotation over conventional flotation lies not 0: ily in the
`smaller bubble size produced but the ability of hydrogen and oxygen bubbles
`generated by electrolysis to alter the flotation properties to a great extent than the
`molecular gases. This is because the nascent gases are most effective. The thin film
`of water between the mineral and the gas can be easily removed by tho: smaller
`bubbles because of the high capillary pressure inside them 13. The rninera' surfaces
`can be either activated or passivated by the electrolytic gases. This can give rise
`to an improvement or deterioration in the flotation properties of the min eral, The
`oxidation-reduction changes that can occur on the surfaces of some minerals with
`electrolytic gases is very striking. It has been shown that with certain :ninerals,
`flotation could be achieved in the absence of collector also. Chalcopyrite and pyrite
`have been shown to float with electrolytic oxygen even in the abse rce of a
`collector!'. Increased recovery and grade of tin ore (cassiterite) with electrolytic
`
`

`

`50
`
`S. VENKATACHALAM
`
`hydrogen bubbles has also been reported.P:!" In the former case, oxidation by
`electrolytic oxygen and in the latter case, reduction by electrolytic hydrogen is
`responsible for the activation of the surface":":". At this stage it must be empha(cid:173)
`sized that not all sulphide surfaces behave similar to chalcopyrite and pyrite. In
`the case of chalcocite the use of electrolytic oxygen has not been found to be
`beneficial.
`
`EFFECT OF PROCESS VARIABLES ON BUBBLE SIZE
`
`The size ofthe bubbles produced by electrolysis is dependent upon several variables.
`Figure 1 shown various parameters affecting the bubble diameter. Some of the
`important variables are discussed below.
`
`Pulp pH
`In flotation systems, the pH of the pulp plays a vital role. pH affects the charge
`on the mineral. In the case of oxide minerals H+ and OH- ions modify the electrical
`double layer and hence the zeta potential. pH may have an effect on the collector
`also. Further, OH- ions act as depressants in a number of flotation systems. The
`critical pH value beyond which flotation does not occur depends on the nature of
`mineral, the collector and its concentration. In addition to its effect on the mineral
`and the collector it has been shown that the pH of the pulp is a very important
`variable determining the bubble diameter.
`Inelectroflotation if both the gases hydrogen and oxygen are used for the flotation
`then the pH of the pulp is not expected to change. However if a diaphragm separates
`the anolyte and catholyte and either hydrogen o~ oxygen alone is used for flotation
`
`BUbble Hysteresis
`charge
`angle
`
`Bubble
`contac tangle
`
`Solution
`
`Solution
`
`Impurities
`
`Addi tives
`
`Elec trode
`
`material.v-.
`
`Electrode
`Surface Surface
`geometry roughness orientation
`
`Current
`density
`
`Electrode
`potential
`
`Polarisation Over voltage
`potential
`
`FIGURE I.
`
`Parameters Affecting Bubble Size
`
`

`

`ELECfROGENERATED GAS BUBBLES IN FLOTATION
`
`51
`
`then the pH will change. This change is drastic if the current density usee. is high.
`If hydrogen is used for flotation,
`then pH shoots up immediately since there is
`OH- ion concentration build-up. Conversely, if oxygen is used for flots.tion the
`pH reduces drastically. Hence, adjustment of the pH of the pulp is very crucial in
`electroflotation with a single gas. Figures 2 and 3 show the magnitude of th e change
`in the pH value at different current densities for evolution of hydrogen and oxygen.
`In alkaline solution hydrogen gas bubbles are small while in acidic mediuri oxygen
`gas bubbles are small. The size variation of gas bubbles tends to follow a trend
`opposite to that of excess ions, that is, in alkaline medium where the concentration
`of OH- ions is greater than that of H+ ions,
`the hydrogen bubbles ar« smaller
`compared to oxygen bubbles". The reverse is true in acid medium. It lias been
`mentioned earlier that the phenomenon of coalescence is not
`importar.t during
`hydrogen evolution in basic medium which is responsible for the production of
`
`12
`
`11
`
`9
`
`B
`
`7
`
`5
`
`:r
`
`Q.
`
`Hydrogen gas bubbles
`
`Current density
`A/m2
`
`x : 400
`.: 310
`e : 225
`&: 135
`Initial pH: 5
`
`Time,minutes __
`
`FIGURE 2. Effect of Current Density on pH for Hydrngcn Gas Bubbles
`
`

`

`52
`
`S. VENKATACHALAM
`
`1 0 , - - - - - - - - - - - - -
`
`----,
`
`9
`
`8
`
`t
`
`J:
`Q,
`
`Oxygen gas bubbles
`- - - - -
`Current density
`A/m 2
`.: 135
`e : 225
`.; 310
`x : 400
`Initial pH; 9
`
`o
`
`1~---'---~---'---~---'---_:_---'----'------'----'------'----'------'------.J
`4
`6
`Tirne , rninutes _____
`
`FIGURE 3. Effect of Current Density on pH for Oxygen Gas Bubbles
`
`small hydrogen bubbles in basic medium. It has been reported that in alkaline
`electrolytes hydrogen bubbles are about 20 microns while oxygen bubbles can be
`of 200 microns.
`
`Electrode Material
`
`The influence of cathode material on the size of the hydrogen bubbles is extremely
`pronounced in acidic media whereas the effect is less marked in alkaline media.
`In neutral media the size of the hydrogen bubbles is independent of the cathode
`
`

`

`ELECfROGENERATED GAS BUBBLES IN FLOTATION
`
`53
`
`material. The series given below shows how the size of the electrogenerated gas
`bubbles changes with change in electrode material".
`
`Hydrogen bubble size increases
`)
`
`Pb, Sn, Cu, Ag, Fe, Ni, W, Pd, Pt.
`(
`Oxygen bubble size increases
`
`Venczel" found that large bubbles formed on platinum because the bubbles grow
`uniformly and coalesced but small ones formed on copper and iron beeause the
`bubbles detached from the electrode before reaching a size for coalescence. Ibl
`and Venczel" observed that bubbbies evolved on platinum were much larger than
`those evolved on copper. The size of the electrogenerated gas bubbles has been
`found to depend on the surface geometry of the electrode also. The bubble size
`increases with increase in the thickness of the wire rnesh.P-" Experiments 'with wire
`electrodes have shown that the bubble diameter depends on the angle of ourvature
`of the electrodes'-".
`
`Current Density
`
`The influence of current density on the bubble generation is very signifu:ant. For
`the same electrode area, if we increase the current passed, current density i.rcreases.
`This increases the amount of gas evolved. Regarding the size variation 0:' bubbles
`at different current densities, there is a conflicting opinion in the literature. Janssen
`and Hoogland" found that bubble size increased with increase in currenI density.
`They attributed this to the coalescence of bubbles at higher current densities. In
`another work the same authors studied the effect of current density on gas evolving
`from platinum discs. They found that when the current density was increased
`beyond 10 Arcm? all bubbles but hydrogen evolved in base increased i.i size. It
`was found that hydrogen bubbles evolved in alkaline medium are the smallest and
`this size is not affected by current density. Oxygen bubbles evolved quickly increase
`in size beyond 30 A/m2. On the other hand, Venczel" found that bu'rble size
`decreased with current density. It may also be noted that other workerslO.12.18.20
`have reported that bubble size decreases with increase in current density. In addition
`to several variables discussed above, the effect of additives on bubble size has also
`been investigated." Venczel" added gelatin, glycerin and beta-naphthoct inolin to
`the electrolyte and in most cases bubble size decreased.
`
`CHARGE ON PARTICLES AND BUBBLES
`
`Mineral particles usually carry a charge when suspended in water arising either
`from an intrinsic charge distribution on the surface of the solid or from th e surface
`active materials adsorbed from the solution. In the same way, the bubbles in the
`flotation cell would also carry a charge either from adsorbed collector or other
`materials present in the suspension. The sign and magnitude of charge on the
`
`

`

`54
`
`S. VENKATACHALAM
`
`particles and bubbles will have an important bearing on the probability of collision
`between the particles and bubbles and hence on the flotation rate. When large
`particles collide with bubbles, the interfacial charges playa minor role on the
`kinetics of capture, provided the surface of the solid is hydrophobic. This would
`be particularly true for rough-surfaced ore particles. With very fine particles, how(cid:173)
`ever,
`it is expected that the charge on the particle and the bubble could have a
`dominant effect on the kinetics of capture. When the particle size is of the same
`order as the electrical double layer thickness, the forces of electrostatic repulsion
`could easily be much greater than the inertial forces tending to favour capture. It
`has been found that as the charge on the particles and bubbles increases,
`the
`flotation rate is decreased since the coalescence between them is inhibited by double
`layer repulsion. The maximum rate of flotation is achieved when the charge on the
`particle and bubble is zero-'. The first to show the importance of bubble and particle
`charge and its effect on the rate of flotation of small particles were Collins and
`Jameson." When the flotation system involves only one component which is to be
`removed, as far as possible, it is advantageous to keep the charges as near to zero
`as possible to promote coagulation prior to flotation as well as to help the bubbles
`and particles to coalesce. However, in mineral beneficiation circuits generally the
`requirement is to selectively remove one species from a mixture of more than one
`species. In these cases coagulation of various species is most undesirable. In such
`cases, it is necessary to ensure that the particles carry sufficient charge to prevent
`coagulation, but not too much to prevent coalescence with bubbles. Collins, Mo(cid:173)
`tarjemi and Jameson" have suggested that a charge of about 20 mv is optimum.
`
`PULSED ELECTROGENERATION OF BUBBLES
`
`The effect of interrupted (pulsed) current electrolysis on the generation of bubbles
`has been studied recently. A microcomputer controlled current source designed to
`generate the required pulses was fabricated. Studies on the pulsed electrogeneration
`of bubbles have revealed that a decrease in duty cycle at a given pH and average
`current density causes an increase in fine sized bubbles and a concomitant increase
`in bubble flux. At low duty cycle, higher current density during the 'on time' forces
`a large number of bubbles to nucleate and grow, resulting in an increased bubble
`flux. Pulsed electrolysis provides ample opportunity for the bubbles to dislodge
`since the current is interrupted after each cycle".
`
`SUMMARY
`
`In many electrochemical processes, electrolytic gas evolution playa significant role.
`The various stages in the physical process of gas evolution are nucleation, growth
`and detachment. Each of these stages is influenced by a large number of process
`variables. A very detailed analysis of the dynamic phenomenon of gas evolution
`as affected by interactions amongst the process variables is needed. Electrogener(cid:173)
`ated bubbles have been used for the flotation of mineral fines. Although it has
`
`

`

`ELECfROGENERATED GAS BUBBLES IN FLOTATION
`
`55
`
`been claimed that the electrogenerated bubbles are fine, which is suitabl ~ for fine
`particle flotation, much more work is necessary to elucidate the exact mechanism
`of fine particle flotation by electrolytic gases.
`
`References
`
`I. H. Vogt, Comprehensive Treatise of Electrochemistry, 6, 445 (1983).
`2. D. E. Westerheide and J. W. Westwater, Am. Inst, Chern. Engrs. 1. 7,357 (\961).
`3. L. J. J. Janssen and J. H. Hoogland, Electrochim. Acta 15, 1013 (1970).
`P. J. Sides, Modern Aspects of Electrochemistry; Ed. R. E. White, J. O. M. Bokris and B. E.
`4.
`Conway, 18, Plenum press, New York, 303 (1986).
`5. L. E. Scriven, Chem. Engg. Sci. 27, 1753 (1959).
`J. P. Glas and J. W. Westwater, Int. J. Heal Mass Transfer, 7, 1427 (1964).
`6.
`7. P. Sides and C. Tobias, J. Electrochem. Soc., 132, 583 (1985).
`8. R. A. Putt and C. W. Tobias, Electrochem. Soc. Meeting; Abstr. 253 (1976).
`9. G. J. Houston, Thesis, University of New South Wales. (1977).
`10. B. Kabanov and A. Frumkin, Z. Phys. Chem., 165A, 433 (1933).
`II: E. Dussan, J. Fluid Mech., 151, I (1985).
`12. V. A. Glembotsky, A. A. Mamakov and V. N. Sorokina, Electron. Obrab. Mater.B, 66 (1973).
`13. V. A. Glembotsky, A. A. Mamakov and V. N, Sorokina, Electron. Obrab. Maler, 16 46 (1973).
`14. V. A. Glembotsky, A. A. Mamakov, A. M. Ramanov and V. E. Nenno, IX Int. Min. Process
`Congress, Cagliari, 562 (1975).
`15. A. A. Mamakov, V. N. Sorokina and M. 1. Avakumov, Electron. Obrab. Maler. 28, No.4, 46
`(1969).
`16. P. Hogan, A. T, Kuhn and J. F. Turner, Trans. lnst. Min. and Me/all. 88,83 (1979).
`17. A, A. Mamakov, Modern state and perspective ofelectrolytic flotation, Ed. P. V. Nebcn " Kishinev,
`(1975).
`J. Venczel, Electrochim. Acta, 15, 1909 (1970).
`18.
`19, N. Ibl and J. Venczel, Metalloberfiache, 24, 365 (1970).
`20, D. R. Ketkar, R. Mallikarjunan and S. Venkatachalam, Int. J. Min. Process., 31, 127 (1991).
`2 I. D. R. Ketkar, R. Mallikarjunan and S. Venkatachalam,J. Electrochem. Soc. India, 37, 314 (1988).
`22. L. Janssen and J. Hoogland, Electrochim. Acta, 18, 543 (1973).
`23. G. J. Jameson, Physical aspects of fine particle flotation, Principles of Mineral Floution in the
`Wark Symposium, Ed. M. H. Jones and J. T. Woodcock, Ausl. lnst. Min. Metall., 2: 5 (1984).
`24. G. L. Collins and G. J. Jameson, Chem. Engg Sci. 32,239 (1977).
`25.. G. L. Collins, M. Motarjemi and G, J. Jameson, J. Colloid Interface Sci. 63,69 (197S).
`26. N. K. Khosla, S. Venkatachalam and P. Somasundaran, Paper communicated for publication,
`
`