O. J. Murphy et al. (eds.), Electrochemistry in Transition
`© Plenum Press, New York 1992
`
`Tennant Company
`Exhibit 1128
`
`

`

`22
`
`KLAUS MULLER
`
`little undesired convection (though probably more than has been thought for a long time,
`according to recent Soviet workm).
`Flotation processes separate suspended matter from fluids. In EF, this occurs without a
`size classification. The gas bubbles become attached to the suspended particles, and these
`are lifted to the top of the fluid, where they are collected as a sludge; hence, the suspended
`matter and the fluid can be used or disposed of separately. For a separation of species
`originally present as true solutes, precipitation or coprecipitation must precede flotation. For
`the separation of colloidal matter, coagulation/ flocculation or adsorption must precede
`flotation.
`
`EF equipment can be sketched as shown in Fig. 1. Into the EF tank go:
`
`0 a stream of the fluid to be treated (e.g., eflluent, fruit juice, or a mineral slurry, part
`or all of which may have been pretreated),
`0 streams of fluid containing chemicals aiding flotation (the point of entry actually is
`upstream from the tank),
`0 dc power to the electrodes.
`Out of the EF tank come:
`
`0 a purified fluid stream, usually from the bottom,
`0 concentrated residue (sludge), usually from the top,
`0 the electrolysis gases (some dissolved in the off-fluid, some contained in the sludge,
`and some directly vented).
`
`Floor-space requirements for the EF tank depend on throughput; they will be between
`square meters and tens of square meters. Tank height is on the order of l m. Throughputs
`of operating plants are cubic meters to hundreds of cubic meters per day. This is much below
`the scale of (nonelectro) flotation equipment found in the minerals industry. The BF units
`are relatively quiet in their operation.
`More technical background will be provided in the last part of the present section and
`in the sections on applications that follow.
`
`2.2. The Double—Layer Connection
`
`Legion are the examples where fundamental inquiries into the structure and properties
`of the electrical double layer at interfaces have been justified by the importance of the results,
`in terms of practical applications or their understanding. Colloid science constitutes the area
`where the loop between theory and practice has been closed most successfully. A similar
`claim cannot be made in the case of EF. So where is the double-layer connection?
`
`Chemicals/Air
`
`Venting
`-—>
`
`
`
`FIGURE 1. Schematic of elec-
`troflotation unit. Effluent enters
`from the left, after appropriate
`addition of chemicals. Flota-
`tion and separation occur in
`the central unit holding elec-
`trodes (+ and -—). Sludge col-
`lects at the top from where it is
`removed;
`it
`then undergoes
`degassing (venting) and further
`dehydration. Purified effluent
`leaves the EF unit. (Kindly pro—
`vided by Dr. E. Baer.(36))
`
`Purified effluent
`
`

`

`ELECTROFLOTATION
`
`23
`
`It is somewhat indirect. It will not suffice to say that all interfaces are charged, though
`this is a correct opening statement.
`
`2.2.1. The Suspended Matter
`
`The substrate in EF is a suspension of finely divided matter (liquid and/ or solid; e.g.,
`oil emulsions). In such matter, the charge may be a highly important factor in stability (before
`the treatment, at the stage of the “problem”) and destabilization (during pretreatment;
`emulsions and similar, stable colloidal systems must be broken prior to separation by EF).
`In a technical report on the treatment of oil-sand production recycle water from the University
`of Alberta?) which reads like a dissertation on the subject, the double layer (around the
`particles) has been discussed as the starting point of an apparently successful practical
`evaluation of EF.
`
`2.2.2. The Bubbles
`
`The “motor” in any flotation process (not merely in EF) is the gas bubbles. They, too,
`are charged, and their charge, too, will be more important, the finer they are. The claim is
`made that, when the bubbles are generated electrolytically, the parent electrode influences
`their charge; also, electrolytic bubbles are finer (how much so will be a question of solution
`pH and electrode polarity). Unfortunately, bubbles have been much less popular than mercury
`drops in double—layer studies. Moreover, though it may be attractive to think of the elegant
`attraction of positively charged bubbles to negatively charged particles, flotation actually has
`an optimum at zero zeta potentials of the bubbles and is most efficient when the particles
`are at their point of coagulation/ flocculation.
`
`2.2.3. Electroflotation and the Hamaker Constant
`
`Not many papers can be consulted which provide information concerning the fine details
`of the kinetics and mechanism of individual EF steps. Panov and Kravchenkom discussed
`the inevitable supersaturation which must exist in the solution (because of the Kelvin equation)
`when very small bubbles are present in equilibrium. Kul’skii et al.” discussed the effect of
`the degree of contaminant dispersion on coagulant and energy requirements. Fukui and Yuu
`studied flotation kinetics with model dispersions to see the effect of bubble diameter and
`bubble charge‘s’é); they also reported that several companies in Japan have succeeded in
`treating eflluents by EF. Fukui and Yuu’s treatment considers the effect of Hamaker constants
`and zeta potentials as the important factors in particle collection by charged bubbles.
`Among relevant papers in colloid chemistry, Watanabe’s") is a fairly recent review of
`the oil/ water interface; adsorption and the interaction between model drops were considered.
`Elsewherefs) particle interactions have been discussed in the context of flotation in terms of
`surface potential and surface charge of the particles.
`Bubble properties have been studied for a long time. Small bubble sizes are found for
`the cathodic gas in alkaline solutions, but for the anodic gas in acidic solutions”); bubble
`size also is a function of electrode diameter (40-um bubbles were produced at 0.2-mm-diameter
`wire,’r 130-,um bubbles at 1.5-mm wire, and there is a sharp size distribution maximum),(1°)
`temperature and electrode material have influence on bubble size, and an optimum current
`density of 20 to 30 mA/cmz was reportedm) Typical bubble sizes in EF are between 20 and
`70 um. Bubble rise velocities decrease with increasing electrolyte concentration and with
`
`T In a recent examplefm’ optimum EF treatment of meat processing wastewater was achieved with a
`wire diameter of 0.2—0.5 mm, mesh grid size of 2.5—5.0 mm, and an inclination of the electrodes of
`30—45° to the horizontal.
`
`

`

`24
`
`KLAUS MULLER
`
`surfactant addition (this was found for relatively large bubbles‘m). Clean bubbles rise faster
`(there is an analogy to the fall of mercury drops: the clean interfaces are mobile, which is
`responsible for the higher speed), while bubbles rigidified with surfactant rise more slowly.(13)
`Bubble charge was found to be positive at pH < 2 and negative at pH > 3; that is, the bubbles
`have an isoelectric point or point of zero charge at a pH of 2 to 3,0) which was confirmed
`quantitatively by bubble electrophoresisf”) Double-layer structure appears to be governed
`by negative adsorption of ions (e.g., H+ or OH‘) at the water/ gas interface. In bubble growth,
`there is an induction time (supersaturation must be reached), a period where growth is
`sustained by diffusion, and finally a period where growth is sustained faradaicallyf")
`In the area of flotation kinetic studies, some more papers are available which take the
`same direction as Fukui and Yuu’s. It was noticed that a finite time is required for bubble
`and particle to become permanently joined“) Charge on the particles and bubbles may be
`detrimental, and the molecular component of the forces may be preponderant; maximum
`floatability was found to occur at the isoelectric point“) There is a hydrodynamic factor,
`since liquid streaming around the bubbles is important in the stage preceding attachment;
`the Reynolds number in surfactant-free systems (but this would appear to be a rather
`exceptional case!) should be between 1 and 40. Microbubbles are likely to get attached to
`hydrophobic sites of the floc, and high shear should be avoidedf“) The benefits of using
`small bubbles seem to be large: flotation rates rise with the inverse third power of bubble
`radius.(17’18) In model systems, it was found that minute particles first will become attached
`to, or deposit on, the surface of small bubbles, and such aggregates then are floated by larger
`gas bubblesflg) This mechanism is helped by the fact that the microbubbles actually are
`stabilized by the sheaths of colloidal particles which become attached”)
`
`2.2.4. From the Double Layer to Troubled Waters
`
`It must be doubted, despite the theoretical work cited in Section 2.2.3, that double-layer
`theory has as yet been a quantitative help in putting EF processes to work. Its relevance and
`value as a guide is evident, and anybody with a background in double—layer structure and/ or
`colloid science should enjoy the introduction to EF afforded by this background. It is gratifying
`to see that a process in which double-layer properties (but not merely the electrostatic ones)
`play a basic role can convert some of the most troublesome effluents back to usable water.
`
`2.3. Early Hopes and Failures
`
`To look back is not the purpose of this contribution. May it suflice, therefore, to say
`that the history and applications of EF have been reviewed, for example, by Kuhnfzo) that
`many companies and workers have been involved in the past, and that spectacular separation
`and cleanup operations have been described (from hog farm effluents to diamond fines), yet
`it must be suspected that many initially successful operations did not continue forever. The
`reasons are:
`
`(i) technical complications: many of the early engineers in the field put the steps of
`flocculant generation, emulsion breaking, and flocculation right into the EF tank’s
`electrode region, which may have brought an untractable situation in the long run;
`(ii) technical difficulties: viz., formation of undesirable deposits on the electrodes and
`undesirable corrosion of the electrodes; and
`(iii) competition and price: a chicken farm might not be able to support the bill for
`electric power? and advanced electrode designs, a hog farm’s efliuent volume and
`
`T However, it has been reported that a poultry waste digester could produce biogas to the extent of
`about 10 Wh/day per caged layerfuo)
`
`

`

`ELECTROFLOTATION
`
`25
`
`its contaminant load probably are too high, dissolved-air flotation (or an altogether
`different method) may have provided a solution at a lower price or, lastly, environ—
`mental concerns may not yet have motivated a cleanup at all.
`
`2.4. Breakthrough to a Viable Technology
`
`The following is a list of requirements which must be met for fluid treatment by EF.
`
`0 The fluid must be sutficiently conductive for economy of the electrolysis process
`producing the gas; if it is not, electrolyte must be added (e.g., industrial waste brines
`or seawater), and it has been suggested that a special electrolyte loop be created only
`around the electrodes.
`
`0 The suspended matter in the fluid must be floatable; this may require prior steps such
`as emulsion breaking/ coagulation, aggregation/ flocculation, or the attachment to
`carrier particles (hydroxide flocs) as well as surface modification of the minute particles
`by special chemicals. Also, this requirement implies limitations with respect to the
`specific gravity and number density (concentration) of the suspended particles.
`0 There may be optimization requirements such as using one or both electrolysis gases,
`simultaneously producing disinfectant (anodically from chloride ions), or using
`cathodic pH variation and anodic dissolution to produce hydroxide particles, but all
`this must occur without interference with the EF lifting act. Also, any chemicals added
`should not contribute to cleanup problems, and they should preferably be recycled
`(such as iron and aluminum for floc).
`
`What has persistently caused trouble in long-term operation was incrustation of the
`electrodes, particularly the cathodes. The phenomenon is not perfectly understood; precipita-
`tion by pH variation, electrophoretic deposition, or cathodic (and anodic) electrodeposition
`may be involved. Mechanical cleaning of the electrodes not only is cumbersome, disruptive,
`and expensive, but also actually very difficult. A good solution to this problem, polarity
`change of the electrodes during operation, which in itself will not upset the EF process, was
`unsuccessful because of excessive corrosion until the quite recent development of stable,
`nonconsumable electrodes which will operate without corrosion and passivation as anode
`and as cathode, in alternation. This must be regarded as a true breakthrough (no less so than
`the success of the stable metal-oxide anodes in chloralkali electrolysis).
`The structure of these electrodes has been disclosed as being Ti/Ti02_x, Ptm); they are
`available from Heraeus Hanau, Germany, and their diflerent applications (in addition to
`electroflotation) have been described. Important points are their high surface area and highly
`open design (see Fig. 2). Among other companies supplying platinized titanium electrodes,
`Engelhard can be mentioned.
`
`2.5. Where to Look
`
`While practical applications recently have multiplied (see below), other strong tech-
`nologies for separations and water treatment are available or under development, and in
`some recent symposia about water treatments and separation technologies,(22’23) EF was not
`a central topic. Literature reviews preceding the “electrode breakthrough” (see above) should,
`of course, be digested “with a grain of salt.” One speculates that investment requirements
`for the process have become somewhat higher than figures provided occasionally in the more
`distant past, on account of superior electrode design, though operating costs should be
`relatively lower now. With this in mind, the interested reader can go back and consult earlier
`reviews and book chapters.(24'3°) Romanov has restated a great many of the salient points
`
`

`

`26
`
`KLAUS MULLER
`
`-_--:u
`a=—u.=q.a.
`a”n.
`u.
`q.
`u.
`c.
`n.r..-c_
`r.-
`
`
`
`c.r.a.5-.- lflfi'flMff‘l'F'fi’F'fl'J'
`
`FIGURE 2. Heraeus activated titanium electrode for electroflotation. (Kindly provided by Dr. B. Busse,
`Heraeus Elektroden GmbH, D—6463 Freigericht/Hanau.)
`
`about EF (speed, selectivity, process parameters) in his 1985 reviews, but the new aspects
`rightly stressed by him are the environment and the rational, eflicient use of natural
`resources.(31’32) These are the reasons why EF has the potential of being among the key
`technologies with electrochemical background in the let century.
`(33)
`and this predates the
`Books are not available on the subject, except one in Russian,
`“breakthrough.” There have been few recent reviews on EF as such, but one should watch
`for information about processes which will favorably complement EF, such as, for instance,
`electroacoustic dewateringm) (which appears to have application to EF sludge), all the other
`electrochemical water treatmentsf”) and, of course, all membrane technologies which would
`be applicable to the EF efliuent water.
`Electroflotation is one of the easiest subjects to search in data banks by computer.
`However, as is often the case for technical matter, not all that is published works, and not
`all that works is published. With this in mind, the reader is invited to look through the section
`on applications that follows. For plants, one reference is to Dr. Baer Verfahrenstechnik in
`Frankfurt,(36) who kindly provided photos and processing schemes of operating plants (see
`Figs. 3 and 4). The Baer technology and its applications to wastewaters from railroads, army
`vehicles, and steel and photochemical plants has been described in a recent review from
`Italy.(37) In the United Kingdom, Simon-Hartley had been one of several successful suppliers
`of EF equipment in the past. A manufacturer might specialize in EF or offer it as an option
`in a line of flotation equipment.
`
`3. ELECTROFLOTA TION APPLICA TIONS
`
`3.1. Oil—Water Emulsions
`
`Spent emulsions and effluents in the form of oil-in-water emulsions are among the
`efliuents hardest to treat. They come from metal working (cutting oil; in the United Kingdom,
`
`

`

`ELECTROFLOTATION
`
`27
`
`108 gal/ yr in 1974)“) and engine cleaning operations (road vehicles, aircraft, ships), and
`also from rolling mills, petroleum refineries, chemical processing, general manufacturing,
`tankers, and spillagef”) The grease, detergents (emulsifiers), and often the metal content
`make them unfit for discharge and cause problems in ordinary waste treatment installations.
`A successful treatment could yield recycle water and recycle oil.
`Details for the Swissair plant
`in Zurich Kloten were made available soon after
`commissioning.(4°‘44) Here the eflluents from workshops, galvanic shops, and aircraft mainten-
`ance (especially engine cleaning) are treated in the plant shown in Fig. 3. The system has an
`80% yield of technically pure water for recycling and offers a high degree of environmental
`protection; the load left to the Kloten town clarification plant is very low. The overall process
`is a combination of EF and reverse osmosis. The inorganics are heavy metals, cyanide,
`alkalies, acids, and salts; the organics are detergents, various oils, and solvents. Emulsification
`is strong. The original installation used cathodes of ferritic stainless steel and anodes of
`platinized titanium. Operation was at 15 to 25 A/m2 and 6 to 9V. Sixty to 80 ppm of aluminum
`(as alum) must be added as the primary flocculant, and a further 1.5 ppm of an anionic
`polymer. The total investment was 17 million SFr for a capacity of 40 m3/ h in the EF plant.
`Full operation started in June 1977. The plant is operating; the original cathodes have been
`replaced by the newly developed electrodes mentioned above. The 1977 operating costs were
`SFr 0.43 per cubic meter for EF and SFr 1.24 per cubic meter total (including the 20% makeup
`water), as compared to SFr 0.90 per cubic meter for town water, which means that decontami-
`nation and environmental protection costs were SFr 0.34 per cubic meter of the effluent.
`Baer plants have been commissioned for similar situations in a number of countries.
`The Swissair plant is one of the reference setups. Other plants operate for the German Federal
`Railways, the German Army, Alitalia in Rome Fiumicino (see the scheme in Fig. 4), Opel
`in Riisselsheim near Frankfurt, Daimler-Benz, Fiat, Michelin Torino, and the 3M Ferrania
`plant in the troubled Val Bormida in northern Italy, and also in Czechoslovakia and Hungary.
`The ability to deal with phenol, cyanide, and heavy metals, the possibility of reducing chromate
`and treating nitrite, and the fact that the purification eflect is very high yet energy requirements
`are modest (said to be comparable to those in compressed-air flotation, surprisingly)(36) and
`u
`.
`I
`
`p Q a E‘WWWE — ”7
`
`FIGURE 3. View of two EF units at the Swissair Kloten (Zurich) installations. The overall scheme
`includes reverse osmosis. (Kindly provided by Dr. E. Baer.(36))
`
`

`

`28
`
`KLAUS MULLER
`
`uw'
`
`®
`
`®
`-
`
`'
`
`MECNANISCHE VDRKLARUNG
`
`
`J,
`L
`J,
`0,},
`<9”
`®,.- ® IN I
`
`©
`‘
`
`CHEM. FALLUNG
`
`_
`
`«i
`. w —— miA
`
`3'3
`
`
`
`SCHLAMM-SILU
`
`ENYWASSERUNG
`504mm —
`
`®
`
`SANDFILTER
`
`Y ‘
`
`8
`
`.
`
`ELEKYRD~FLDYATIGN
`
`I I
`
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`AKTIVKONLE—FKLYER
`
`n77
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`7
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`-l
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`0 53- h 'T
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`.L
`
`@ l
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`
`FIGURE 4. Full process scheme of the EF units at the Alitalia Fiumicino (Rome) installations. The
`consecutive operations include mechanical preclarification, chemical precipitation, electroflotation,
`sludge dehydration, sand and active carbon filtration, and neutralization. (Kindly provided by Dr. E.
`Bach‘s”)
`
`chemical requirements minimal imply that the installations can be adapted to a number of
`tough effluent situations. EF plants with a capacity of up to at least 300 m3/ h have been
`built; the process is designed for tough situations and ought to be situated at points close to
`eflluent generation, before any dilution has occurred.
`
`3.2. Metal-Plating Shops, Electrochemical Machining
`
`The effluents from metal-plating shops have a high content of metals which are too
`valuable and too toxic for direct discharge. The particularly undesirable Cr(VI) was found
`to be readily removed by an electrocoagulation—electroflotation treatmentm) or by reduction
`to Cr(III) with metallic iron followed by EF. Zinc also responds well, though at difierent
`optimum pH values: >9—10 versus <9 for Cr(VI). Nickel and copper are eliminated most
`readily at pH 7 .(46) Multimetal
`recovery by EF after
`ferrocyanide precipitation was
`describedf‘m
`Cadmium and cyanide from a finishing process were removed from the efliuent by EF
`according to a laboratory demonstration”); 2—6 g of Cd were removed per kilowatt—hour in
`a system to which seawater was added as a bottom layer, in the space holding the horizontal
`electrodes; magnesium oxide acted as the floc. Mercury could be removed by electroflotation
`when precipitated as the sulfide.(49)
`Electrochemical machining (ECM) produces particularly high concentrations of suspen—
`ded and ionic metal dissolution products. Apart from metal recovery (which may be of
`interest even in the case of iron(5°)), sludge removal will extend electrolyte life, and purification
`schemes have been described long ago.‘5 1) It was found that oxygen should not be used for
`
`

`

`ELECTROFLOTATION
`
`29
`
`EF in ECM, since Fe(II) (green electrolytes) is oxidized by Fe(III) hydroxide (red elec-
`trolytes); a diaphragm between the electrodes was said to help.(52)
`
`3.3. Dairy Industry
`
`Dairy eflluents are high-strength waste; the BODS values are typically ten times those
`of domestic sewagef”) The quantities of effluent produced can be very large, and important
`quantities of fat, proteins, and lactose are lost.(54’ Effluent cleanup could become attractive
`because of the recovered valuesfss) To this end, the effluent is adjusted with HCl to pH 4,
`which is the average isoelectric point for milk proteins (casein, albumin, globulin); a fat—
`protein complex will precipitate and can be floated by EF.
`No more than pilot operations have been described. It was pointed out that the Ca0—
`protein interface structures forming at the bubbles in these applications should be temperature-
`dependent, and hence temperature control of dairy effluent EF could be the way to optimize
`the process“)
`
`3.4. Food Industries
`
`Meat factories have provided early, drastic examples of the efliciency of EF relative to
`more conventional separation techniques (Swift, Chicagoan”). Grease recovery for nonfood
`applications was found to be feasible. A recent example of successful laboratory evaluation
`comes from Bratislavafsg) where 99.8% of the initial fat was removed and the fat—protein
`concentrate obtained was sufficiently pure to use in feeds. Many other food industries present
`eflluent problems, and EF was tried for sugar plants,(6°) in the manufacture of starch from
`corn,“” in the isolation of nutrient yeast,‘62) and in the purification of grapem) and recently
`apple juice.‘63’ In the treatment of palm oil mill effluentsf“) EF was combined with anodic
`oxidative destruction of soluble constituents.
`
`3.5. Livestock Farming
`
`From many countries, EF applications to farm effluents have been reported. The technical
`feasibility appears to have been demonstrated insofar as the efliuent is concerned. However,
`energy requirements are high. A value of 300 A h/m3 was reported for a hog farm,(“) where
`the effluent carried 6 kg of suspended solids per cubic meter. Probably because of electrode
`passivation (fouling, scale formation), instead of the expected 1.9 V,(“) voltages between 3
`and 530V (1) have been reported, and hence excess energy consumption between 0.5 and
`80 kW h/m3. Apparently the problems have not been resolved, though the savings potential
`of EF relative to conventional processes has been calculated to be enormous‘67‘69)——provided
`higher reliability could be achieved.
`'
`A fairly positive report on the EF treatment of egg-washing wastewaters and subsequent
`land disposal of the solid waste, including a description of the facility, was given at a 1984
`conferencefm)
`
`3.6. Cellulose and Paper Industry
`
`Early applications used the EF process with integrated electrocoagulation (soluble anodes
`in the EF tank producing floc for flotation, e.g., Mg anodes for sulfate cellulose effluents).‘7”
`For cardboard manufacturing efl’luents, insoluble anodes were used and disinfectant anodi—
`cally produced.(72’73) A 1980 Soviet patent describes EF of lignin from wood-processing
`wastewaters (using Ti anodes with an active MnOz layer).(74)
`
`

`

`30
`
`KLAUS MULLER
`
`3.7. Fiber, Textile, and Leather Industry
`
`In rayon production, where a high pollutional load of carbon disulfide, hydrogen sulfide,
`sulfur, coagulated viscose, hemicellulose, surfactants, etc. arises, EF was examined and found
`to be an eflicient method. Over 99% of the zinc along with many of the other pollutants could
`be extracted from the efliuent; a pilot plant was operated.(75’76)
`For the decontamination of textile production effluents containing organic dyes and
`surfactants, a scheme using dimensionally stable anodes (titanium anodes with an active
`surface layer of isomorphic metal oxides) was proposedfm sinée active chlorine is required.
`Owing to the oxidation step, energy requirements here are very high.
`For the recovery of concentrated mercerizing liquor, formation of a peroxide adduct
`and its separation by EF were reported“) Apparently successful tests of EF were reported
`for tannery wastewatersf”)
`
`3.8. Chemical Industry
`
`Electrocoagulation and EF was proposed for the treatment of effluents from the produc—
`tion of acetylene by oxidative methane pyrolysisfso) Here the efficiency of purification was
`found to be very good, at the price of about 1 kW h of electric power and 10 g of aluminum
`per cubic meter of the effluent. Work concerning the recovery of process water in a heavy-oil
`extraction facility has been mentioned earlierm; speed and price of recycle water production
`were found to be acceptable.
`
`3.9. Paint and Print Shops
`
`A relatively pessimistic outlook has been presented‘sl) for the treatment of paint effluents,
`considering the large volume to be treated and the requirements for electrolyte addition. Yet
`settling efficiencies demonstrated for EF were two orders of magnitude better than in
`nonelectrolytic treatment. There should be room for reevaluation. Some results were reported
`from this trade in Refs. 27 and 82.
`
`3.10. Shipboard Applications
`
`A detailed feasibility study of the use of electroflotation for bilge water purification was
`performed for the US. Coast Guardf“) The only preliminaries required are the addition of
`NaOH to pH 10 and of 10 to 15 ppm of anionic polyelectrolyte. Simulated bilge and ballast
`water were purified to <10-ppm residual oil when influent streams contained as much as
`3000 to 4000 ppm of emulsified oil. Pilot plant data were used to estimate system costs, which
`at the time appeared very modest. Energy consumption was estimated to be 5.6 kW h/ 1000
`gallons, of which only 1.2 kW h was for EF itself, with the rest for pumps, stirrers, and control
`equipment. Cathode scale formation was a problem then. For seawater duty, Pt—Ir mesh
`spotwelded to a niobium substrate was proposed as the anodes. A more recent report deals
`with diesel fuel emulsions in seawaterfg“) A Japanese patent describes a three—reactor scheme
`[electrolysis to produce Al(OH)3, flocculation, electroflotation with graphite electrodes] for
`the treatment of bilge wastewaterf“)
`
`3. 1 7. Urban Effluents
`
`Kuhn‘24) described the treatment of urban efliuents by EF under conditions where
`seawater is available. Then flocculant is produced from the magnesium ions at the cathode
`
`

`

`ELECTROFLOTATION
`
`31
`
`owing to alkalinity induced by hydrogen evolution. At the time, a large plant was operating
`on Guernsey. More recently, Electricité de France in collaboration with the Société Lyonnaise
`des Eaux has looked into possibilities for simultaneous EF and disinfection (anodic chlorine)
`of urban sewage.(86’87) Ferric chloride and an anionic polymer had to be added, and 200 A/ m2
`were employed. Coliform counts decreased by six powers of ten, yet the activated sludge
`was found to remain viable, and hence available for recycling, an important consideration
`since EF would not be a stand-alone operation here. In this study, it was concluded that
`electrocoagulation [coagulation with the aid of anodically dissolved iron and anodically
`generated alkalinity (added lime)] is too expensive, while disinfection was regarded as
`economically competitive even in large plants. Scale formation on and corrosion of the
`electrodes were cited as problems—this would no longer be true in view of recent develop-
`ments.
`
`In the Soviet Union, EF was proposed as the preferred alternative to settling tanks in
`the case of arctic settlements“) For ordinary conditions, EF was suggested as a feasible
`technology for the thickening of excess sludge produced in regular effluent purification
`schemes“); for technical details, see Ref. 90.
`
`3.12. Mining
`
`Hogan et al.(9"92’ have carefully reviewed and analyzed the situation of EF in mining
`applications. Against a theoretical minimum energy requirement of 1.6, true requirements
`are expected not to fall below 30 kW h/ton of ore floated, and, apart from that, settling
`tendencies are too high in most instances for trouble-free operation, except in the case of
`extreme fines posing an environmental threat.
`Optimistic papers have come from the Soviet Union for a long time, for example, about
`EF of chromite?” A single American document found BF to be a viable possibility for the
`beneficiation of strategically important, low-grade U.S. chrome ores (oxygen bubbles and
`the acidic environment developing in the anolyte compartment are favorable for chromite
`flotation).(94) Many more instances have been reported, such as EF of polymetallic tin ores“)
`or pyrite.(96'99) An early paper worth reading is that about diamond EF,”°°) where details
`were given and EF was reported to compare favorably to the conventional process.
`Mining slurries, which are a problem of considerable magnitude resulting from the
`working of increasingly poorer ore deposits, are somewhat more likely candidate substrates
`for a technical EF operation. Many examples have been analyzed; an instance of large
`improvement over ordinary flotation was found in the case of manganese ore processing.(1°1'1°2)
`Anglesite fines EF was studied quite recently.(1°3)
`Operating experience had been reported early in the treatment of mining wastewaters,
`for example, for molybdenum and uranium recoveryuo“) in the United States or nickel
`recoveryuos) in the Soviet Union. In the latter case, the effluent contained 14 g Ni/m3 and
`also Cu, Co, Fe, Zn, and Pb. Lime (0.2 kg/m’) was used for precipitation, then poly(vinyl
`alcohol) was added (1 g/m3), and EF performed with 0.2 kW h/m3, 70—80 A/mz, a flow rate
`of 2 m3/(m2-h), and an effluent with <0.1 g Ni/m3.
`
`3. 13. Silver Recovery
`
`A technological scheme for a 99% recovery of silver from the effluents of a photochemical
`plant was described by Polish workersfm) Silver levels were reduced from 70-80 down to
`less than 0.5 g/ma. Ferrous sulfate and sodium hydroxide were added together with a
`flocculating agent. A 1.3 m3 vessel was reported to handle 9 m3/ h and yield a concentrate
`containing 10—20% silver.
`
`

`

`32
`
`KLAUS MULLER
`
`3.14. Magnesium from Seawater
`
`Electroflotation was described as an efficient method to collect magnesium hydroxide
`from seawater. It precipitates because of cathodic alkalinity, without addition of any
`reagentsfm) A current efficiency of 85-98% was reported for its collection in the froth layer.
`It can be further used for magnesium production, for on-site effluent treatmentfmg) and even
`for oil-spill cleanups.
`
`3.15. Radioactive and Toxic Metal Effluents
`
`interesting methods for the
`Shvedov and Yakushev,“09'“” after reviewing several
`removal of radioactive material from the process or waste stream, have examined the
`possibilities for the removal of ”Sr and 137Cs from wastes by electrocoagulation and electroflo-
`tation. A hydroxide collector (soluble titanium electrodes were reported to be most eflective)
`or ferrocyanide collector and the correct pH value (close to six) should be used. Further
`improvements were seen when a precipitate of nickel ferrocyanide was present (the nickel
`ions come from the anodic dissolution of stainless steel) and soap is added.
`Arguments have been presented for the removal