O. J. Murphy et al. (eds.), Electrochemistry in Transition
`© Plenum Press, New York 1992
`
`Tennant Company
`Exhibit 1028
`
`

`

`22
`
`KLAUS MULLER
`
`little undesired convection (though probably more than has been thought for a long time,
`according to recent Soviet work‘).
`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 ofthe 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:
`® a stream of the fluid to be treated (e.g., effluent, fruit juice, or a mineral slurry, part
`or all of which may have been pretreated),
`© streams of fluid containing chemicals aiding flotation (the point of entry actually is
`upstream from the tank),
`@ dc powerto the electrodes.
`Out of the EF tank come:
`
`® a purified fluid stream, usually from the bottom,
`® concentrated residue (sludge), usually from the top,
`® the electrolysis gases (some dissolved in the off-fluid, some contained in the sludge,
`and somedirectly 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 1m. 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 EF units
`are relatively quiet in their operation.
`Moretechnical 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 importanceofthe 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
`>
`
`vided by Dr. E. Baer.°®)
`
`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 whereit is
`removed;
`it
`then undergoes
`degassing (venting) and further
`dehydration. Purified effluent
`leaves the EF unit. (Kindly pro-
`
`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 offinely divided matter (liquid and/orsolid; e.g.,
`oil emulsions). In such matter, the charge may be a highly important factorin 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 treatmentof 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 muchso will be a question of solution
`pH andelectrode polarity). Unfortunately, bubbles have been muchless 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 concerningthefine details
`of the kinetics and mechanism of individual EF steps. Panoy and Kravchenko® 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 aldiscussed 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®®; they also reported that several companies in Japan have succeeded in
`treating effluents 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.
`Amongrelevant 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.
`Elsewhere,”particle interactions have been discussed in the contextof 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-j.m bubbles were producedat 0.2-mm-diameter
`wire,t 130-ym bubbles at 1.5-mm wire, and there is a sharp size distribution maximum),
`temperature and electrode material have influence on bubble size, and an optimum current
`density of 20 to 30 mA/cm? wasreported." Typical bubble sizes in EF are between 20 and
`70 zm. Bubble rise velocities decrease with increasing electrolyte concentration and with
`
`+In a recent example,” 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 foundfor relatively large bubbles”). Clean bubblesrise faster
`(there is an analogy to the fall of mercury drops: the clean interfaces are mobile, which is
`responsible for the higher speed), while bubblesrigidified with surfactant rise more slowly."
`Bubbie charge was foundto be positive at pH < 2 and negative at pH > 3; thatis, the bubbles
`have anisoelectric point or point of zero charge at a pH of 2 to 3,which was confirmed
`quantitatively by bubble electrophoresis."* Double-layer structure appears to be governed
`by negative adsorptionofions (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, andfinally a period where growth is sustained faradaically.©
`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 componentof 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 avoided.“ The benefits of using
`small bubbles seem to be large: flotation rates rise with the inverse third power of bubble
`radius."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 bubbles.”This mechanism is helped by the fact that the microbubbles actually are
`stabilized by the sheathsof 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 affordedbythis 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 troublesomeeffluents back to usable water.
`
`2.3. Early Hopes and Failures
`
`To look back is not the purpose of this contribution. May it suffice, therefore, to say
`that the history and applications of EF have been reviewed, for example, by Kuhn,°that
`many companies and workers have been involvedin the past, and that spectacular separation
`and cleanup operations have been described (from hog farm effluents to diamondfines), yet
`it must be suspected that manyinitially 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 powert and advancedelectrode designs, a hog farm’s effluent volume and
`
`+ However, it has been reported that a poultry waste digester could produce biogas to the extent of
`about 10 W h/day per caged layer.“\?”
`
`

`

`ELECTROFLOTATION
`
`25
`
`its contaminant load probably are too high, dissolved-air flotation (or an altogether
`different method) may have provided a solution at a lowerpriceor, lastly, environ-
`mental concerns may not yet have motivated a cleanupatall.
`
`2.4, Breakthrough to a Viable Technology
`
`The following is a list of requirements which must be metfor fluid treatment by EF.
`
`© The fluid must be sufficiently 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.
`®@ 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 (hydroxideflocs) as well as surface modification of the minuteparticles
`by special chemicals. Also, this requirement implies limitations with respect to the
`specific gravity and numberdensity (concentration) of the suspendedparticles.
`@ 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 EFlifting act. Also, any chemicals added
`should not contribute to cleanup problems, and they should preferably be recycled
`(such as iron and aluminum forfloc).
`
`What has persistently caused trouble in long-term operation was incrustation of the
`electrodes, particularly the cathodes. The phenomenonis 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 developmentof 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 (noless so than
`the success of the stable metal-oxide anodes in chloralkali electrolysis).
`The structure of these electrodes has been disclosed as being Ti/TiO,_,, Pt”; they are
`available from Heraeus Hanau, Germany, and their different 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,??”» EF was not
`a central topic. Literature reviews precedingthe “electrode breakthrough” (see above) should,
`of course, be digested “with a grain of salt.” One speculates that investment requirements
`for the process have become somewhathigher 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.2*" Romanov hasrestated a great many of the salient points
`
`

`

`26
`
`KLAUS MULLER
`
`=====—====== A
`
`
`
`===
`Bi==Ss=ES=
`==
`
`MIITNANTNNRNT
`
`FIGURE 2. Heraeusactivated titanium electrodefor 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, efficient use of natural
`resources.°'>?) These are the reasons why EF has the potential of being among the key
`technologies with electrochemical background in the 21st century.
`(33)
`and this predates the
`Booksare 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 dewatering°” (which appears to have application to EF sludge), all the other
`electrochemical water treatments,°» and, of course, all membrane technologies which would
`be applicable to the EF effluent 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 readeris invited to look through the section
`on applications that follows. For plants, one reference is to Dr. Baer Verfahrenstechnik in
`Frankfurt,°® who kindly provided photos and processing schemes of operating plants (see
`Figs. 3 and 4). The Baer technology andits applications to wastewaters from railroads, army
`vehicles, and steel and photochemical plants has been described in a recent review from
`Italy.°” In the United Kingdom, Simon-Hartley had been one ofseveral 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. ELECTROFLOTATION APPLICATIONS
`
`3.1. Oil-Water Emulsions
`
`Spent emulsions and effluents in the form of oil-in-water emulsions are among the
`effluents hardest to treat. They come from metal working (cutting oil; in the United Kingdom,
`
`

`

`ELECTROFLOTATION
`
`27
`
`10° 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 spillage.© 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.““°-Here the effluents 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 townclarification 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/m?and 6 to 9-V.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 m?/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 metertotal (including the 20% makeup
`water), as compared to SFr 0.90 per cubic meter for town water, which meansthat 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 plantis one ofthe reference setups. Other plants operate for the German Federal
`Railways, the German Army, Alitalia in Rome Fiumicino (see the schemein Fig. 4), Opel
`in Riisselsheim near Frankfurt, Daimler-Benz, Fiat, Michelin Torino, and the 3M Ferrania
`plant in the troubled Val Bormidain northern Italy, and also in Czechoslovakia and Hungary.
`Theability to deal with phenol, cyanide, and heavy metals, the possibility of reducing chromate
`andtreating nitrite, and the fact that the purification effect is very high yet energy requirements
`are modest(said to be comparable to those in compressed-air flotation, surprisingly)°® and
`
`a ee'
`7
`
`:
`
`ee
`
`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.°°)
`
`

`

`28
`
`KLAUS MULLER
`
`fo ]0
`
`pao
`7
`
`MECHANISCHE VORKLARUNG
`
`Lt
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`
`@®@
`eT Te
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`CHEM. FALLUNG
`
`=
`x
`
` @
`
`
`=a M4
`
`
`“TE
`
`ELEKTRO- FLOTATION
`
`P
`
`SCHLAMM=SILO
`
`SCHLAMM ~
`ENTWASSERUNG
`
`}@®
`
`SANDFILTER
`
`cH
`AKTIVKOHLE ~ FILTER
`
`@,
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`
`NEUTRALISATION
`
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`Wu
`
`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 carbonfiltration, and neutralization. (Kindly provided by Dr. E.
`Baer.©®)
`
`chemical requirements minimal imply that the installations can be adapted to a numberof
`tough effluent situations. EF plants with a capacity of up to at least 300 m°/h have been
`built; the process is designed for tough situations and oughtto be situated at points close to
`effluent 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 treatment) or by reduction
`to Cr(III) with metallic iron followed by EF. Zinc also responds well, though at different
`optimum pH values: >9-10 versus <9 for Cr(VI). Nickel and copper are eliminated most
`readily at pH7.“® Multimetal
`recovery by EF after
`ferrocyanide precipitation was
`described.@”
`Cadmium and cyanide from a finishing process were removed from the effluent by EF
`according to a laboratory demonstration, 2-6 g of Cd were removed per kilowatt-hour in
`a system to which seawater was addedas a bottom layer, in the space holding the horizontal
`electrodes; magnesium oxide acted as the floc. Mercury could be removed byelectroflotation
`when precipitated as the sulfide.“
`Electrochemical machining (ECM) producesparticularly 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©”), sludge removalwill extend electrolyte life, and purification
`schemes have been described long ago.©! 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.©”
`
`3.3. Dairy Industry
`
`Dairy effluents are high-strength waste; the BODS values are typically ten times those
`of domestic sewage.©*) The quantities of effluent produced can be very large, and important
`quantities of fat, proteins, and lactose are lost.” Effluent cleanup could becomeattractive
`because of the recovered values.” 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 CaO-
`protein interface structures formingat the bubblesin 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 efficiency of EF relative to
`more conventional separation techniques (Swift, Chicago®’*®), Grease recovery for nonfood
`applications was found to be feasible. A recent example of successful laboratory evaluation
`comes from Bratislava,©” where 99.8% of the initial fat was removed and the fat-protein
`concentrate obtained was sufficiently pure to use in feeds. Manyother food industries present
`effluent problems, and EF wastried for sugar plants,” in the manufacture of starch from
`corn,” in the isolation of nutrient yeast,” and in the purification of grape°*and recently
`apple juice.In the treatment of palm oil mill effluents,’ 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 effluent is concerned. However,
`energy requirements are high. A value of 300 A h/m® wasreported 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 Vv,voltages between 3
`and 530 V (!) have been reported, and hence excess energy consumption between 0.5 and
`80 kW h/m?. Apparently the problems have not been resolved, though the savings potential
`of EF relative to conventional processes has been calculated to be enormous®’-©’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
`conference.)
`
`3.6. Cellulose and Paper Industry
`
`Early applications used the EF processwith integrated electrocoagulation (soluble anodes
`in the EF tank producingflocforflotation, e.g., Mg anodes for sulfate cellulose effluents).”
`For cardboard manufacturing effluents, insoluble anodes were used and disinfectant anodi-
`cally produced.”A 1980 Soviet patent describes EF of lignin from wood-processing
`wastewaters (using Ti anodes with an active MnO, layer).‘””
`
`

`

`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 efficient method. Over 99% of the zinc along with manyof the other pollutants could
`be extracted from the effluent; a pilot plant was operated.”>”©
`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 proposed,‘’”since 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 wastewaters.”
`
`3.8. Chemical Industry
`
`Electrocoagulation and EF was proposed for the treatment of effluents from the produc-
`tion of acetylene by oxidative methane pyrolysis.®” Here the efficiency of purification was
`found to be very good, at the price of about 1 kWh ofelectric power and 10 g of aluminum
`per cubic meterof the effluent. Work concerning the recovery of process water in a heavy-oil
`extraction facility has been mentioned earlier™ 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” for the treatment of paint effluents,
`considering the large volumeto 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. Someresults 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 U.S. Coast Guard.®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 kWh wasfor EFitself, 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 seawater.“” A Japanese patent describes a three-reactor scheme
`[electrolysis to produce Al(OH);, flocculation, electroflotation with graphite electrodes] for
`the treatment of bilge wastewater.®>
`
`3.11. Urban Effluents
`
`Kuhndescribed the treatment of urban effluents 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. Morerecently, Electricité de France in collaboration with the Société Lyonnaise
`des Eaux haslooked into possibilities for simultaneous EF anddisinfection (anodic chlorine)
`of urban sewage.°°®”) Ferric chloride and an anionic polymer had to be added, and 200 A/m?
`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 betrue 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.°'°?) 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 30kWh/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 environmentalthreat.
`Optimistic papers have come from the Soviet Union for a long time, for example, about
`EF of chromite.” A single American document found EF tobe 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).°*” Many more instances have been reported, such as EF of polymetallic tin ores°”
`or pyrite.°” 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
`improvementoverordinary flotation wasfoundin the case of manganese ore processing.“1°'”)
`Anglesite fines EF was studied quite recently.“°°)
`Operating experience had been reported early in the treatment of mining wastewaters,
`for example, for molybdenum and uranium recovery") in the United States or nickel
`recovery°>) in the Soviet Union. In the latter case, the effluent contained 14 g Ni/m? 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/m*), and EF performed with 0.2 kW h/m*, 70-80 A/m72, a flow rate
`of 2m?/(m?-h), and an effluent with <0.1 g Ni/m’.
`
`3.13. Silver Recovery
`
`A technological schemefor a 99% recovery ofsilver from the effluents of a photochemical
`plant was described by Polish workers.‘'Silver levels were reduced from 70-80 down to
`less than 0.5g/m>. Ferrous sulfate and sodium hydroxide were added together with a
`flocculating agent. A 1.3 m° vessel was reported to handle 9 m*/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
`reagents.” A currentefficiency of 85~98% was reportedforits collection in the froth layer.
`It can be further used for magnesium production,for on-site effluent treatment,“and even
`for oil-spill cleanups.
`
`3.15. Radioactive and Toxic Metal Effluents
`
`interesting methods for the
`Shvedov and Yakushev,"'''” after reviewing several
`removal of radioactive material from the process or waste stream, have examined the
`possibilities for the removal of °’Sr and '*’Csfrom wastes by electrocoagulation and electroflo-
`tation. A hydroxide collector (soluble titanium electrodes were reported to be most effective)
`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 of traces of toxic metal from effluents
`by flotation with adsorbing colloids,“'?) which will only be successful when the very small
`bubbles of EF are used.
`
`3.16. Biotechnology
`
`In biotechnology, microbiological processes are used to make or destroy products. In
`both cases, liquids and solids (the latter including the microorganisms) must subsequently
`be separated.“!!) Dissolved-air flotation and EF are