P3Ek644
`TROFLOCCULATION
`State-of =the-Art Electrof locculation
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`J.P.F. Koren and U. Syversen
`0stfold Research Foundation, PO Box 276, N-1601 Fredrikstad, Norway
`
`The electroflocculation principle has been known since the beglnnlng of the century, but until
`recently it has not been used in industrial applications. The ‘Purifier’ described in the paper is an
`electroflocculation unit which can separate oil, organic substances and heavy metals from water. It
`is most competitive with concentrations of less than 5000 parts per million oil or organlc
`substances in water. The paper describes the major working prlnclples of the ‘Purlfler’.
`
`consumption of 0.48 kWh/m3 of water, but they used a residence
`time of 10-20 min.l41
`
`0 substances is a problem in many different industries.
`
`il-polluted water and water polluted with other organic
`
`Demands from the authorities and general public for a cleaner
`environment will increase, and the authorities might reduce the
`legal effluent concentrations to 20 mg of organic substances per
`litre of effluent. When the effluent is characterised as hazardous
`waste, it is illegal to drain it into the sewer. It might only consist of
`0.6% oil, while the rest is pure water. These huge volumes of low-
`concentration oily waste are expensive to treat. If the oil could be
`separated from the water it would be much less costly to treat the
`oily waste, because of the lower volume. The oily sludge can
`probably be incinerated, and the water reused in the industrial
`process with a reduction in the cost of new process water.
`The electroflocculation unit is capable of separating many kinds
`of organic substances and heavy metals in addition to oil. The
`degree of separation is in most cases above 99%, and the power
`consumption is about 1 kWh/m3 of wastewater. Current units can
`treat about 1 m3/h of wastewater in a continuous process. The best
`results are when the wastewater contains 5000 ppm organic
`substances or less,
`
`Electrof lotation
`In flotation processes, air or gas is bubbled through a liquid
`containing particles which float or are emulsified in the water. The
`process consists of four basic steps: (1) gas bubble generation, (2)
`contact between gas bubble and oil drop, (3) gas bubble
`adsorption on the surface of the particle, and (4) the gas bubbles
`and oil drops rising to the surface.16] At the surface a layer of foam
`will be created. This foam consists of gas bubbles and the flotated
`particles, and can be removed by skimming. The rate of flotation
`depends on several parameters, such as the surface tension
`between the water, particles and gas bubbles; the gas bubble
`diameter; the size of the particles; the water’s residence time in the
`electrolytic cell and the flotation tank; the particle and gas bubble
`zeta potentials; and the temperature, pH and particle size
`distribution.
`There are many different flotation methods. The conventional
`process is to use a compressor to blow air through nozzles in the
`bottom of the flotation tank. The problem is the distribution of the
`air bubbles, and to make small enough bubbles. Small gas bubbles
`are more efficient than larger gas bubbles, since they have a larger
`surface area per unit volume of gas. Smaller gas bubbles also have
`the advantage that they have lower buoyancy, and so will have a
`longer residence time in the electrolyte. This increases the
`possibility for collisions between bubbles and oil particles.
`Another method is dissolved-air flotation, which gives a better
`bubble distribution in the water. The disadvantages is that it is not
`a continuous process, and it is difficult to control the bubble flux.
`During the process air is injected into the water under pressure;
`when the pressure is released, the water is supersaturated with air,
`which is released as air bubbles.I6] This is the same process that
`happens when a bottle of carbonated water or beer is opened.
`A method which follows the same principle, but which uses a
`very low pressure, is vacuum flotation. The water is saturated with
`air at atmospheric pressure, and when a vacuum is applied, air
`bubbles will be released.I61 This process has the same advantages
`and disadvantages as dissolved-air flotation.
`Electroflotation is a continuous method. The bubbles are
`generated by electrolysis of water; the water flows between two
`electrodes, and is reduced to hydrogen at the cathode and oxidised
`to oxygen at the anode. One advantage is that the gas bubbles
`generated are essentially at the same, very small size. However, the
`power consumption can be high if the process is not well designed
`and optimised. Another advantage is that it is easy to adjust the
`gas bubble flux, by varying the current across the electrodes. The
`distribution of the gas bubbles is also good, because the bubbles
`are produced over the whole area of the electrode.
`
`History
`Electrolytic processes to separate oil in wastewater were described
`in the patent literature as early as 1903. The process was used to
`treat condensed water from steam engines, before it entered the
`steam boiler as feedwater. The unit used iron sheets as the anode
`material; the iron was oxidised during the process, and had to be
`potential of 160 V and with a fairly high current.[ 8
`replaced after a while. The electrolysis cell o erated with a
`This process was further developed by Weintraub, Gealer
`Golovoy and Dzieciuch into a continuous process to clean oily
`wastewater from metal-cutting, forming, rolling and finishing
`operations. An electrolytic cell which can treat 3.8 l/min of
`wastewater was designed and patented in 1980. The wastewater
`which was fed into the unit contained from 300 to 7000 mg oil/litre
`of water. The processed water contained less than 50 mg/l for 90%
`of the time, and less than 26 mg/l for 83% of the time. The unit can
`be improved to reach an effluent oil concentration of 10 mg/l. The
`power consumption was calculated to be 1.6 kWh/m3.[’]
`One of the first experiments with electroflotation was in 191 1,
`treating domestic sewage in the United States. This method has not
`become generally used because the electrodes tended to scum after
`a while and, because of this, the efficiency decreased with time.[’]
`Flotation processes are used extensively in the mining industry.
`In 1904, it was proposed for the first time to use electrolysis to
`make gas bubbles to ‘float’ minerals. The process was used in some
`mines in Broken Hill, Australia. However, the operation was not
`successful, because the power consumption was too high and
`because the technology was not well enough developed.[31
`In 1946, Rivkin et uZ.[~] obtained a patent for a method for
`Electropreclpitation
`electroflotation of ore. They designed several laboratory-scale
`electroflotation cells, which gave an improved flotation rate
`Electroprecipitation is a flocculation process where the flocculat-
`compared to other flotation techniques. This method is not
`ing agent is ions of metal which are precipitated from the anode.
`reported to have been taken into commercial use by the
`The metal ions will settle in the electrolyte, but on the way down
`indust~.[~1 Other experiments with electrolytic cells have been
`they collide with particles in the electrolyte, and adsorb onto the
`reported, but it appears that electmflotation of minerals is still at
`surface of these particles. The best anode material is iron or
`the laboratorylexperimental stage. This is because only a limited
`aluminium, because they give trivalent ions; most other cheap and
`amount of reseerch has been done on the mechanisms and the
`easy accessible metals give bivalent ions. Trivalent ions have a
`design of the ~ell.1~1
`higher ability to adsorb onto particles in the water than bivalent
`Kaliniichuk et! al. have described an electroflocculation unit
`ions, because they have a higher charge density.
`which can be used to separate an oil-in-water em~1sion.I~~ They
`The mechanism which breaks down emulsions in the water
`achieved a degree of separation of more than 99% w3th a poiver
`phase is irot fdib Gndel-tGGd. ’#eintraub et al. suggested thlt the
`0015-1882/95/U8$7.00 0 1995 Elsevier Science Ltd
`Flitration & Separation
`February 1995
`153
`
`~
`
`__
`
`__
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`Tennant Company
`
`
`Exhibit 1130
`
`

`

`TROFLOCCULATION
`
`breakdown of emulsions is brought about with the assistance of
`hydroxyl radicals which are generated during ferrous-ion oxida-
`tion. The reaction sequence is:
`Fe2+ +02 + H+ = Fe3+ + HOz
`(4
`Fe2++H02+H+ = Fe3+ + HzOz
`( b )
`Fe2+ + Hz02 -+ Fe3+ + HO + OH-
`(4
`-+ Fe3+ + OH-
`Fez+ + HO
`(4
`The ferrous ion/hydrogen peroxide solution taking part in
`reactions (c) and (d) is recognised as Fenton's reagent, and is a
`powerful oxidising system. The emulsion is destabilised by both
`oxidative destruction of the chemical emulsifier and by neutralisa-
`tion of the emulsion/droplet charge.[']
`No mechanism has been suggested with aluminium as the
`anode. Aluminium ions are very unstable, and it is suggested that
`aluminium ions react with hydroxyl ions and make a network as
`soon as they are released from the anode. The network of
`aluminium hydroxide will adsorb onto colloidal particles. Colloi-
`dal particles are defined as particles which in the dispersed state
`have an extension in at least one dimension of between 1 nm and 1
`pm. The upper limit is not distinct, since particles which are larger
`than 1 pm are also treated as colloidal particles if they have
`properties like colloidal particles.[71
`The network of aluminium ions is built up from a chain with
`three hydroxyl ions per aluminium ion. The chains can be several
`hundred &ngstrvms long, but only a few particle diameters thick.
`The chains are created by a condensation polymer reaction
`mechanism. The process depends on pH and temperature to create
`the correct crystals. At 25OC the pH in the water must be between 4
`and 10, and at 100°C between 3 and 7, in order to create large
`crystals. Outside these pH ranges the aluminium ions will react to
`make less complex compounds with hydrogen and oxygen.[']
`
`Electrof locculation
`Electroflocculation is a combination of the processes of electro-
`flotation and electroprecipitation. Our electroflocculation unit
`consists of an electrolytic cell with an aluminium anode and a
`stainless-steel cathode. The anode must be more easily oxidisable
`than the cathode to give the correct effect. Balmer and F o ~ l d s [ ~ ]
`tried many different electrode materials, such as iron, steel, copper,
`brass, zinc, alloys of aluminium, bronze and phosphor bronze. All of
`these materials produced enough flocs, and gave a high degree of
`separation. They concluded that the cheapest and most easily
`accessible electrode materials should be used.
`An electrolytic cell can be designed in many different ways.['']
`Here we will discuss a specific unit, which has been designed and
`patented by Jan Sundell; this unit is called the 'Purifier'. The
`distance between the electrodes is 3 mm; this distance is an
`important design variable when it comes to optimising the
`operating costs of the unit. The operating costs are dependent on
`the power consumption, which can be expressed as
`P = U x I
`(1)
`where P is the power consumption 0 , U is the voltage (V) and I
`is the current (A). Using Ohm's Law (U = R x I, where R is the
`resistance in ohms), it is also possible to rewrite Eqn. 1 as
`
`(2)
`
`(3)
`
`P = R x 1 2
`U2
`p = -
`R
`The relationship between current and power consumption is shown
`in Eqn. 2, and a change in current will change the power
`consumption in the second power. The amount of gas which
`evolves at the electrodes is dependent on the current flowing across
`the electrodes.
`To reduce the power consumption without changing the current
`and the degree of separation, one can reduce the resistance in the
`electrolyte. Reducing the distance between the electrodes or
`increasing the conductivity of the electrolyte will reduce the power
`consumption without changing the degree of separation, because
`the current is not changed. Ohm's Law states that the power
`consumption will decrease as the distance between the electrodes
`is reduced and as the conductivity of the electrolyte increases.
`The electrolyte's conductivity will affect the power consumption
`in the same way as the distance between the electrodes. According
`te Ohm's La;;; a high conductivitj~ in tine eiectroiyte and a small
`154
`
`distance between the electrodes gives a low power cons~mption.['~
`In some types of wastewater the conductivity is too low, and it is
`necessary to add some salts to increase the number of dissolved
`ions in the electrolyte. The simplest method is to add table salt
`(NaCI), but it has also been reported that 0.01N CaCl2 has been
`used to increase the conductivity of the electrolyte.[']
`There is an optimum arrangement with a certain power
`consumption and a certain degree of separatien. When the current
`in the electrolytic cell is increased, the gas bubble flux increases;
`this increases the separation effect. However, when the concentra-
`tion of gas bubbles is increased, the possibility that two gas bubbles
`collide also increases. This reduces the separation effect since
`larger gas bubbles are less effective than smaller gas bubbles,
`because they have a smaller surface area/volume ratio. In addition,
`gas bubbles have a lower conductivity than the electrolyte; this
`increases the power consumption.
`When the gas bubble concentration increases, the result is that
`the degree of separation increases as the current increases up to a
`certain level. The concentration of the gas bubbles gives a large
`contribution to the electrolyte resistance, and eventually too many
`of the gas bubbles will coalesce. The degree of separation will then
`slowly decrease as the current across the electrodes increases.[' '1
`At a certain point, increasing the power consumption will no longer
`affect the degree of separation.
`
`The electrode reactiolfs
`The anode and cathode Factions are as follows:
`2Hz0 + 2e-= H2(g) + 20H-
`(cathode) E = -0.83 V
`2H20 = Oz(g) + 4Hf + 4e-
`(anode) E = +0.40 V
`+ 3e-
`(anode) E = -1.66 V
`AI(,) = ~ l ~ +
`2Al(,) +6H20 = 3Hz(g) + 60H- + 2A13+ (Total) E = +OS3 V
`Hydrogen gas will evolve on the cathode, and oxygen gas will evolve
`on the anode; oxygen gas will only evolve at high current densities.
`It is an advantage that hydroxyl ions are developed at the cathode,
`because they maintain the pH in the electrolyte. To create the
`correct aluminium complexes, the pH must be close to 7.
`There are many mechanisms which are at work in the
`electrolytic cell. These include an electrophoresis mechanism,
`which makes the negatively charged oil particles attracted to the
`anode. This results in a faster flocculation than would be the case
`with conventional flocculation or flotation methods.[']
`
`Gas bubble formation
`The gas bubbles which are created by electrolysis have an
`important function in the separation process. Reay and Ratcliff
`found that the rate of flotation of polystyrene
`articles is
`dependent on the gas bubble- and particle diameter.$'] The rate
`of flotation at a constant gas rate varies as dkrtiele and d A l e . This
`means that the rate of flotation is at its highest when the particles
`are as large as possible and when the gas bubbles are as small as
`possible. Collins and Jameson found that the rate of flotation of
`polystyrene latex particles with a diameter of 4 - 20 pm vaned as
`d'.5
`.["] They also found that the exponent showed little
`&$%ence
`on the charge on the particle, but that the actual
`rate of flotation was strongly influenced by the particle charge,
`which was varied in the 30 - 60 mV range.
`There is a specific ratio between the sizes of the particles and
`between gas bubbles and particles.[' P A direct collision makes the
`the bubbles where there is a higher robability of direct collisions
`gas bubble adsorb onto the particle, and many direct collisions
`during a short time give a high rate of flotation.
`If a collision between a particle and a gas bubble is to be
`successful, the energy barrier represented by the water film
`between the actual particle and the actual gas bubble must be
`overcome. The collision velocity and the particle mass determine
`the energy which is available during the collision; larger particles
`hit the gas bubbles with a greater force than smaller gas bubbles.
`The relative velocities of a particle and a gas bubble increase with
`the difference in their dimensions. If the gas bubble hits a relatively
`small particle, the force from the collision will make the particle
`rebound from the gas bubble. The highest probability for a
`successful collision is when the gas bubble collides perpendicular
`to the particle surface. There are distribution curves available for
`air bubbles and mineral particle diameter to find the optimum
`flotability. This is commo!
`in the mining industry.['31
`Collins and Jamesoni'"' reported that the gas bubble collection
`February 1995 FUtratlon 4% Separatlon
`
`

`

`TROFLOCCULATION
`
`when one gas bubble rolls across thecathode surface, and coalesces
`with every gas bubble which comes into its path. This gas bubble
`will often grow larger than necessary before it detaches. Sides (Ref.
`16, p. 312) reported that when the electrode was tilted a few
`degrees from the horizontal, the bubbles coalesced and made a
`large front of gas bubbles, which ‘scavenged‘ other bubbles in its
`path. This is unfavourable for the rate of flotation, because gas
`bubbles with smaller diameters are more effective than gas bubbles
`with larger diameters.
`The third step is detachment. Its occurence is dependent on the
`bubble contact angle and the size of the gas bubble. Pulsating
`electrolysis gives the gas bubbles a shock, which makes the gas
`bubbles detach earlier than they would without the pulses. A
`surface-active substance in the electrolyte will reduce the surface
`tension between the electrolyte, the surface of the cathode and the
`gas bubbles, and make the gas bubbles detach sooner. Venczel (Ref.
`16, p. 316) added gelatine, glycerine and beta-naphthochinolin to
`the electrolyte; in most cases the bubble diameter decreased. The
`reason suggested is that the surface-active substance results in
`increased wettability of the electrode.
`The ‘Purifier’ reported here has been tested with soap in the
`electrolyte. This test gave a higher degree of separation with oil-in-
`water emulsions. The disadvantage is that a surface-active
`substance works as an emulsifier and stabilises the oil emulsion,
`but it is probably possibleto find surface-active substances which
`increase the wettability of the cathode without working as an
`emulsifier for pollutants h the water.
`
`-
`
`-
`
`-
`
`The ‘Purifier’, an electroflocculation unit designed by
`Jan Sundell
`Before the wastewater enters the electrolytic cell, it is coarsely
`filtered in a hydrocyclone. This removes the larger particles, and
`improves the total degree of separation. It was mentioned earlier
`that there is a problem with the anode, which will tend to scum.[’]
`This problem is solved in the Purifier. Jan Sundell has designed and
`patented a system which keeps the anode clean all of the time, and
`makes the anode wear evenly.
`In the electrolytic cell aluminium ions precipitate from the
`anode and flocculate the unwanted particles like oil, most kinds of
`organic compounds and heavy metals. At the same time the
`hydrogen gas bubbles which form at the cathode cause the
`flocculated particles to float. The residence time of the wastewater
`in the electrolytic cell is less than 1 s, but this depends on the flow
`velocity of the water. The water flows into a sedimentation tank,
`where the residence time is short, but depends on the temperature,
`the concentration of impurities etc.
`This process forms both a sediment and a layer floating on the
`surface, which are removed at regular intervals. The sediment is
`drained regularly, and the surface layer is skimmed off. The sludge
`is dewatered in a filter, and the removed water is recycled into the
`electrolytic cell. The sedimentation tank is tapped for clean water
`at regular intervals.
`If the wastewater conductivity is too low, it is necessary to add
`
`I
`
`c
`
`efficiency for bubbles less than 100 pm in diameter can be
`described by
`
`(4)
`
`where N = 1.90 when ppTtlde/pflutd = 1.0, and N = 2.05 when
`ppa7.tl~e/p,luld = 2.5. This assumes that the stream around the
`sphere follows Stokes’ Law for a flow around a rigid sphere; that
`electrical interactions between bubbles and particles do not have
`any effect on the particle trajectory or on E,, and that bubble
`motions are not affected by the presence of the particles.Ii2I
`
`The size of the gas bubbles
`The following conditions affect the gas bubble departure
`diameter :[14]
`0 Gas bubble contact angle.
`0 Cathode surface morphology.
`0 Current density.
`0 Polarisation potential.
`0 Gas bubble charge.
`Parameters which are fixed or which are impractical to adjust in
`order to obtain fine-sized gas bubbles are pH, temperature, reagent
`concentration and the electrode material.
`Khosla et a1.1i41 and Glembotskiy et uZ.IlS1 have reported that
`the gas bubble size decreases with increasing current density.
`Glembotskiy et aL1lsl have also found that the hydrogen gas bubble
`size increases with increasing temperature, and that the smallest
`gas bubbles were formed at pH 7. Khosla et uZ.[’~]
`have also
`successfully used pulsed electrolysis to generate very small gas
`bubbles; with a pulse cycle time varied in the range of 30 ms and
`the current density varied from 2 to 0.5 A/m2, they observed an
`increasinf number of small gas bubbles ( < 10- 15 pm).
`Others[ii3 have reported gas bubble diameters of 20 pm during
`electroflotation with a constant current density.
`
`Gas bubble formation on the cathode
`There are three basic steps in the development of gas bubbles:
`nucleation, growth and detachment. Nucleation occur in energy-
`favourable places like pits and scratches. These places have a
`higher voltage than the rest of the cathode surface (see Reference
`16, p. 305). Khosla et uZ.[’~]
`reported that with pulsed electrolysis
`the energy-favourable places are less important as nucleation sites,
`and that nucleation is more uniform over the cathode surface.
`Growth is driven by expansion cause? by a high internal
`pressure and transport of dissolved gas through the gas/liquid
`interface because of supersaturation of gas in the electrolyte.[i41
`Another mechanism which make gas bubbles grow is coalescence.
`This occurs when two gas bubbles touch, and coalesce into a single
`gas bubble. Sides (Reference 16, p. 309) described a mechanism
`called radial specific coalescence, and observed that smaller gas
`bubbles moved radially towards a larger gas bubble and coalesced.
`Another mechanism which make two gas bubbles coalesce is
`
`M
`
`Electrolytic
`
`*I [:I
`
`Sludge
`
`Water and
`Salt
`
`Pure water
`
`Osmosis
`
`Flotation
`
`Figure 1. Flowsheet for
`the Purlfler.
`
`Recycled
`Water
`
`HydroQclone -
`
`155
`
`

`

`TROFLOCCULATION
`
`sodium chloride to increase the conductivity. If the recipient
`cannot accept salt water, it will be necessary to remove the salt.
`This is done with the use of a reverse osmosis unit. The unit
`produces two streams, one with pure water and the other with a
`mixture of salt and water. The latter stream is recycled back to the
`electrolytic cell for reuse of the salt.
`The Purifier has achieved prominent results with the separation
`of different kinds of pollution from water. Especially good results
`have been achieved in the separation of oily emulsions, and it has
`also achieved a high degree of separation with several kinds of
`organic substances and heavy metals. Table 1 gives values which
`were achieved during testing of the Purifier (the analysis was
`performed by the KM Laboratory in Karlstad).
`The US company Environomics has also developed a process
`which uses the electroflocculation principle, which has achieved
`good results for separation of polyaromatic hydrocarbons (PAHs) ,
`oil, organic compounds and heavy metals fiom Wastewater.
`
`Acknowledgment
`This work has been financed by the Letten F. Saugstads Fund.
`
`Degree of
`separation
`
`Example
`
`Figure 2. The Purifier unit, including electrolytic cell,
`sedimentation tank and process control system.
`
`Table 1. Results from test runs with the Purlfier.
`
`After the
`Before the
`Purifier, mgll Purifier, mg/l
`
`1. Oil in water
`2. Aliphats in cooling water
`Aromats in cooling water
`3. Lead, PbZ+
`Copper, cu2+
`Zinc, Zn2+
`
`350
`260
`62
`5.3
`110
`160
`
`0.3
`2.4
`0.6
`< 0.05
`1.2
`0.22
`
`Car wash unit
`Ship
`
`Printing office
`
`99.9%
`99.1%
`99.0%
`> 99%
`98.9%
`99.9%
`
`References
`1 Weintraub, M.H., Gealer, RL, Golovoy, A and Dzieciuch, M.A: ‘Development of
`electrolytic treatment of oily waste water’, Environmental hgress, February
`1983, 2(l), p. 32.
`2 Fukui, Y. and Yuu, S.: ‘CollecGon of submicron particles in electro-flotation’,
`Chem. Eng. Sci., 1980,36, pp. 1097- 1105.
`3 Mallikaduan, R. and Venktachalam, S.: ‘Elektroflotation - A review’.
`International Symposium on Electrochemistry in Mineral & Metal Processing
`(165th meeting of the Electrochemical Society), Cincinnati, Ohio, USA, May 1984,
`pp. 233 - 256.
`4 Kaliniichuk, E.M., Vasilenko, LI., Shchepanyuk, V.Yu., Sukhoverkhova, N.A and
`Makarov, LA: ‘Treating refinery waste waters to remove emulsified oils by
`electrocoagulation and electroflotation’, Int. Chem. Eng., July 1976,16(3), p. 434.
`5 Hosny, AY.: ‘Electroflotation technique for removing petroleum oil waste‘,
`Bull. Electrochem., Janualy 1991, 7 , p. 38.
`6 Chambers, D.B. and Cottrell, W.RT.: ‘Flotation: two fresh ways to treat
`effluents’, Chem. Eng., August 1976, 83, p. 95.
`7 Merk, P.C.: ‘Overtlate og kolloidk-
`jemi, 3rd edition’ (Institutt for Indus-
`triell Kjemi, Norges Tekniske
`Hegskole, Norway, 1991.
`8 Diggle, J.W. and Viih, AK: ‘Oxides
`and oxide films’, in: Alwitt, RS.: ‘The
`aluminium-water system, vol. 4’
`(Marcel Dekker, New York, 1976),
`Chap. 3, pp. 169 - 254.
`9 Balmer, LM. and Foulds, AW.:
`‘Electroflocculation/electroflotation
`for the removal of oil from oil-in-water
`emulsions’, Filhvction & Separation,
`November/December 1986, 23(6), p.
`366.
`10 Hogan, P. and Kuhn, AT.: ‘Die
`Electroflotation bei der Abwasserbe-
`handlung’, O b m h x - S u & c e 9 1977,
`18(10), p. 255.
`11 Hosny, AY.: ‘Separation of oil from
`oil/water emulsions using an electro-
`flotation cell with insoluble electro-
`des’, Filtration & Separation,
`September/October 1992, 29(5), pp.
`419 -423.
`12 Collins, G.L and Jameson, GJ.:
`‘Experiments on the flotation of fine
`particles - The influence of particle
`size and charge’, cha. Eng. Sci.,
`1976, 31, pp. 985-991; Reay, D. and
`Ratcliff, G.A: Canadian J. C h a .
`Eng., 1975, 63, p. 481.
`13 Klassen, V.I. and Mokrousov, V A :
`‘An introduction to the theory of
`flotation’ (Butterworths, London,
`1963).
`14 Khosla, N.K, Venkatachalam, S.
`and Somasundaran, P.: ‘Pulsed elec-
`trogeneration of bubbles for electro-
`flotation’, J. Applied Electrocha.,
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`15 Glembotskiy, V.A, Mamakov, AA
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