03 - Overview of Flotation as a Wastewater Treatment Technique

Overview of flotation as a wastewater treatment technique

J. Rubio a,*, M.L. Souza a, R.W. Smith b

a Departamento de Engenharia de Minas-PPGEM, Laborat�oorio de Tecnologia Mineral e Ambiental, Universidade Federal do Rio Grande do Sul,

Av. Osvaldo Aranha 99/512, 90035-190, Porto Alegre, RS, Brazilb Metallurgical and Materials Engineering, Mackay School of Mines, University of Nevada-Reno, USA

Received 7 October 2001; accepted 12 December 2001

Abstract

The treatment of aqueous or oily effluents is one of the most serious environmental issues faced by the minerals and metallurgy

industries. Main pollutants are residual reagents, powders, chemicals, metal ions, oils, organic and some may be valuable (Au, Pt,

Ag). The use of flotation is showing a great potential due to the high throughput of modern equipment, low sludge generation and

the high efficiency of the separation schemes already available. It is concluded that this process will be soon incorporated as a

technology in the minerals industry to treat these wastewaters and, when possible, to recycle process water and materials. In this

paper, the use of flotation in environmental applications is fully discussed. Examples of promising emerging techniques and devices

are reported and some recent advances in the treatment of heavy metal containing waters and emulsified oil wastes are dis-

cussed. � 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Flotation machines; Pollution; Flocculation; Flotation bubbles; Environmental; Wasteprocessing

1. Introduction

1.1. Background

Process waters exiting from mining, petroleum andmetallurgical operations are widespread throughout theworld and can become contaminated by various pollu-tants. These substances include powders, chemicals,metal ions, oils, organic and others, sometimes render-ing the water useless for recycling as process water, oftendangerous for the environment, and sometimes causinglosses of valuable materials (Galvin et al., 1994). Sourcesof water contamination may be found at mines, mills, offshore platforms, processing plants, tailing ponds, etc.(Smith, 1996; Villas Booas and Barreto, 1996; Warhurstand Bridge, 1996).

Sometimes, due to their chemical complexity and/orvolume, these process waters cannot be treated eco-nomically even in cases where they contain valuablematerials. Further, when organic fluids are discharged,

the oil/water separation becomes difficult especiallywhen the oil is emulsified, and worse when the meandroplet size is small or if the emulsions are chemicallystabilized (Beeby and Nicol, 1993).

Smith (1996) showed in detail characteristics of liquidand solid wastes from mineral processing plants. Vari-ous techniques and technologies available were dis-cussed and the quality and quantity of typical pollutantswere listed.

Thus, current and future technologies will eventuallyhave to deal with areas such as:• process water treatment and recycling (reuse);• removal and/or recovery of ions: heavy and/or pre-

cious metals, anions, residual organic chemicals,complexes or chelates;

• cyanide and arsenic emission control, recovery or de-struction;

• oil spills separation (including recovery of solvent ex-traction liquors);

• acid mine waters containing considerable amounts ofharmful base metals such as nickel, copper, zinc, leadin addition to ferrous iron and sulfate;

• control and removal of residual chemical reagentssuch as frothers, flotation collectors and modifiers(activators or depressing agents, pH regulators);

• separation of various wasted plastics;• radioactive control in aqueous effluents and soils.

Minerals Engineering 15 (2002) 139–155www.elsevier.com/locate/mine

*Corresponding author. Tel.: +55-51-3316-3540; fax: +55-51-3316-

3530.

E-mail addresses: [email protected]; http://www.lapes.ufrgs.

br/Laboratorios/ltm/ltm.html (J. Rubio), [email protected] (R.W.

Smith).

0892-6875/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0892-6875 (01 )00216-3

1.2. Conventional treatment processes

The conventional process for treating liquid efflu-ents containing metals ions is precipitation–aggrega-tion (coagulation/flocculation)-settling as hydroxidesor insoluble salts. However, this method, from atechnical point of view, presents certain limitations,namely:• the formation of metal hydroxide is ineffective in di-

lute metal bearing effluents;• the hydroxo precipitate tends to re-dissolve, depend-

ing on the metal, via the reaction MðOHÞþmn OHð�Þ ¼

MðOHÞ�mðnþmÞ;

• the pH of minimum solubility of hydroxides is differ-ent for the various metals present. For example, theminimum solubility for cupric hydroxide occurs at apH value around 9.5 while for cadmium hydroxideit occurs at pH around 11;

• precipitation of metals becomes incomplete whencomplexing or chelating agents are present;

• volumes of sludge formed are too large and with ahigh water content;

• filtration may be difficult as a result of the precipitatesfineness, and;

• due to kinetic and scale problems, the treatment bycoagulation and settling of effluent flow-rates ofabout 2–4 m3 s�1 is very difficult and costly. Thisconstitutes a great challenge for the modern miningindustry.

1.3. Flotation processes

The use of flotation has shown to have a greatpotential owing to the high throughput and efficiencyof modern equipment now available (Zabel, 1992;Matis, 1995; Rubio et al., 1996; Rubio, 1998a,b;Voronin and Dibrov, 1999; Parekh and Miller, 1999).Other advantages of flotation are the selective recoveryof valuable ions such as gold, palladium, silver (whichare also pollutants), the new separation schemes nowavailable and the low sludge generation in this pro-cess.

This paper summarizes general features of flotation inenvironmental applications and is aimed to:• show the potential of flotation as a wastewater treat-

ment technique and present some advances;• present novel separation concepts and flotation de-

vices;• serve as a ‘‘bridge’’ providing information on flota-

tion activities being conducted in various engineer-ing fields as well as in the mining andmetallurgical industry. It is believed that a cross ex-change of flotation experience in mineral flotationand in water and effluent treatment should lead tonew and improved procedures for industry wastetreatment.

1.4. Flotation process in wastewater treatment

Flotation had its beginning in mineral (ore) process-ing and as such has been used for a long time in solid/solid separation applications using stable froths to se-lectively separate different minerals from each other(Kitchener, 1985). Regarding applications of flotation inwastewater and domestic sewage treatment, civil andchemical engineers have used dissolved air flotation(DAF) for a number of years (Hooper, 1945). Mainapplications have been in the removal of the solids, ions,macromolecules and fibers, and other materials fromwater (Matis, 1995; Mavros and Matis, 1992; Lemlich,1972; Clarke and Wilson, 1983; Zabel, 1992).

More, flotation is also practiced in other fields(Kitchener, 1985; Roe, 1983; Cundeva and Stafilov,1997; Kim et al., 1999; Sch€uugerl, 2000), such as:• analytical chemistry;• protein separation;• treatment of spent photography liquors;• odor removal;• plastics separation and recycling;• harvesting or removal of algae;• deinking of printed paper;• separation or harvesting of micro-organisms;• removal of sulfur dyes, seed hulls, serum, resins and

rubber, impurities in cane sugar; and• clarification of fruit juices.

The main differences between ‘‘conventional’’ flota-tion of ores and flotation applied to water treatment arethe following:• The method of producing the gas bubbles in order to

generate micro, medium or macro-bubbles. It is nowwidely accepted that medium size and large bubblediameters (300–1500 lm) are optimal for flotationof minerals (fines and coarse particles). Yet, conven-tional flotation devices do not generate a sufficientnumber of bubbles smaller than 600 lm. Main usesof micro-bubbles (<100 lm) is in applications of flo-tation to solid/liquid or liquid/liquid separation.Thus, the distinguishing feature between conven-tional mineral flotation and flotation in waste treat-ment is that, where extremely small (or evencolloidal) particles have to be floated, micro-bubblesare required.

• Because the species floating are usually aggregatedcolloids rather than dispersed ones, high shear ratesmust be avoided to obviate destruction of the fragileaggregates. This is important in the clarification of ef-fluents and introduces distinct problems not previ-ously encountered in mineral flotation.

• The solids content present in the pulp system,whether diluted or not. A limiting feature of bubblesis the lifting power or carrying capacity. Micro-bub-bles do not float dense and big particles, especiallyat high solids content (4–5%, w/w).

140 J. Rubio et al. / Minerals Engineering 15 (2002) 139–155

• The type of separation: solid/solid/liquid in mineralprocessing and solid/liquid, solid/liquid1/liquid2 orliquid/liquid in water treatment.

• In mineral flotation it is necessary to produce a stablefroth at the free surface of the flotation cell. In appli-cations to wastewater treatment an stable foam is notrequired.

• In mineral flotation, the overall process is economi-cally attractive. In environmental application, usuallyflotation means an extra cost.Other differences are summarized in Table 1 com-

paring, among others, bubbles characteristics in differ-ent flotation devices.

Flotation technology can be incorporated in miningand industrial wastewater-treatment schemes in thefollowing ways:• as a unit process (ancillary or main process) to re-

move contaminants which are not separated by othermeans. Depending on performance (water quality),process water can be adequately treated and recycled;

• as a treatment unit on floating solids in thickeners(concentrates or tailings);

• as an auxiliary process to bio-oxidation lagoons orsludge thickening in water reuse;

• as a process for removing various organics, residualschemicals, including petroleum, from water;

• as a solid/liquid separation process in acid minedrainage neutralization with lime;

• as a primary treatment unit ahead of secondary treat-ment units, such as bio-oxidation lagoons for reduc-ing the cost of aerobic digestion;

• as a unit process for sludge thickening.Why flotation? Many advantages have been reported

illustrating the technical and economical potential ofthis process:• high selectivity to recover valuables (Au, Pt, Pd, etc);• high efficiency to remove contaminants: high over-

flow rates, low detention periods (meaning smaller

tank sizes, less space needs, savings in constructioncosts); thicker scums and sludge than in gravity set-tling or skimming and;

• low operating costs with the use of upcoming flota-tion devices (Da Rosa et al., 1999; Rubio, 1998a,b,2001);

• thicker flotation concentrates (6–12% w/w).Table 2 shows a partial list of current commercially

available flotation devices for wastewater treatment anddrinking water treatment units.

Voronin and Dibrov (1999) have recently published aclassification of flotation processes in wastewater de-contamination. They grouped different flotation tech-niques based on physicochemical and technologicalpoints and divided them in adsorptive or adhesive. Anumber of applications are reported without mentionneither the type of equipment employed nor the bubblesize distribution.

2. Conventional flotation techniques, devices and pro-

cesses

Here some recognized techniques are summarized toshow their main features.

2.1. Electro-flotation (EF)

The basis for the micro-bubbles generation is theelectrolysis of diluted aqueous, conducting solutionswith the production of gas bubbles at both electrodes.Applications, to date, at an industrial scale, have beenin the area of removal of light colloidal systems such asemulsified oil from water, ions, pigments, ink andfibers from water (Zabel, 1992; Zouboulis et al.,1992a,b).

Advantages claimed are the clarity of the treatedwastewater and disadvantages are the low throughput,

Table 1

Differences between flotation in mineral processing and in wastewater treatment

Parameter Froth flotation of minerals Water and wastewater treatment

Feed solids content (weight/weight basis) (%) 25–40 <4 (DAF)

10–30 (jet/columns)

Particle size to float (lm) 10–150 1–50 (not flocculated) and

1–5 mm flocs (with polymers)a

Bubble size distribution (lm) 600–2000 30–100 (DAF)


Bubbles rising velocity (m h�1) 250–800 (approximate values) 0.7–30 (DAF)


Number of bubbles (cm�3) 9� 103–2� 102 6� 108–2� 106 (DAF)

2� 106–9� 103 (jet/columns)

Bubbles surface area (cm2 cm�3) 100–30 4000–600 (DAF)


Air hold up (%) 15–25 8–14 (DAF)


aAerated flocs.

J. Rubio et al. / Minerals Engineering 15 (2002) 139–155 141

the emission of H2 bubbles, electrode costs and main-tenance and the voluminous sludge produced.

An electrolytic coagulation/flotation (ECF) systemhas been also reported using reversible polarity alumi-num electrodes. Herein, aluminum ions are releasedfrom the anodes, inducing coagulation, and hydrogenbubbles are generated at the aluminum cathodes, en-abling flotation of the flocs. Bulk water passes throughthe reactor and is treated by the coupled coagulation/flocculation process. Laboratory scale tests have shownthat the ECF reactor performs better than conventionalaluminum sulfate coagulation when treating a modelcolored water, with 20% more dissolved organic carbon(DOC) removed by electro-coagulation for the same Aldoses (Andre et al., 2000).

2.2. Dispersed (induced) air flotation (IAF)

Bubbles are mechanically formed by a combination ofa high-speed mechanical agitator and an air injectionsystem. The technology makes use of the centrifugalforce developed. The gas, introduced at the top, and theliquid become fully intermingled and, after passing

through a disperser outside the impeller, form a multi-tude of bubbles sizing from 700–1500 lm diameter. Thismethod, well known inmineral processing, is utilized alsoin the petrochemical industry, for oil–water separation(oily sewage) (Zheng and Zhao, 1993; Bennett, 1988).

2.3. Dissolved air (pressure) flotation (DAF)

Bubbles are formed by a reduction in pressure ofwater pre-saturated with air at pressures higher thanatmospheric. The supersaturated water is forced troughneedle-valves or special orifices, and clouds of bubbles,30–100 lm in diameter, are produced just down-streamof the constriction (Bratby and Marais, 1977; Lazaridiset al., 1992).

DAF was recognized as a method of separatingparticles in the early 20th century and since then hasfound many applications including:• clarification of refinery wastewater, wastewater recla-

mation,• separation of solids and other in drinking water treat-

ment plants;• sludge thickening and separation of biological flocs;

Table 2

Examples of some commercially available flotation devices for wastewater treatment

Supplier company Type of cell characteristics Application details

Sionex DAF Wastewater treatment to remove suspended

organic solids, dissolved oils, algae, 5–7 lmoocysts, volatile organic compounds, humic acid,

clarification

Canadian Process Technologies Vertical oil separation cell VOSCellR – using

natural gas as a separating medium.

Developed to remove oil and grease from

produced water using natural gas as a separating

medium

Canadian Process Technologies IAF column Organic recovery flotation columns for reducing

organic reagent and kerosene from rich

electrolytes prior to electrowinning

WesTech Dissolved Air and Nitrogen (DNF) flotation

systems

Wastewater treatment

OR-Tec HF IAF – uses a baffled, aeration system that

produces very fine bubbles

Flotation of fat, grease, suspended solids from

food, municipal and industrial waste streams

Hydroxyl Industrial Systems Positive Flotation Mechanism (PFM); dissolved

air flotation processes – ‘‘Electrostatically’’

charged micro-bubbles

Dissolved air flotation processes for solids, air

and grease

Aeromax Systems ZEPHYRe IAF – using very fine bubbles For fat, grease, floatable solids

Thermodyne Corporation Ultra-Float ADAF – plug flow DAF device It is a plug flow DAF device. For food or

industrial processing wastes

PURAC Engineering High capacity DAF-filter system Drinking water, sludge thickener, ice-cream

effluents, paper mill

Baker–Hughes Process ISF – hydraulically operated gas flotation, deg-

assing, and optional skim storage components

For oil/water separations. System in a

completely enclosed flotation process

ZPM BAF – air-sparged BAF, induced-air BAF,

vacuum BAF, electroflotation BAF

For treatment of petroleum, heavy metal,

laundry, food processing, screen printing, animal

feed contaminated waters

Engineering Specialties Flotation piles (underwater oil/water separator)

– combines secondary treatment of produced

water with disposal in one vessel

For offshore operation the treated water

discharges directly into the sea

Hydrocal CAF For treatment of laundry, food processing waters

Aquaflot FF – flotation of aerated flocs Vehicle washing effluents, removal of oil, solids,

surfactants


• removal/separation of ions;• treatment of ultra-fine minerals (Gochin and Solari,

1983);• removal of organic solids, dissolved oils and VOCs

(dissolved toxic organic chemicals);• removal of algae, 5–7 lm Giardia oocysts, 4–5 lm

cryptosporidium oocysts, humic water treatment, al-gae from heavily algae laden waters, etc.The DAF process (see Fig. 1) is by far the most

widely used flotation method for the treatment of in-dustrial effluents. It is believed that applications willrapidly expand in the waste treatment in the metallur-gical and mining field (Rubio and Tessele, 1997; Tesseleet al., 1998; Rubio et al., 1996; Rubio, 1998a,b; Sant-ander et al., 1999; Da Rosa et al., 1999). DAF devel-opment has been very rapid in the last decade and manyof its earlier limitations are being solved. Table 3 reviewsrecent important developments in DAF.

3. Emerging flotation techniques and processes

3.1. Nozzle flotation (NF)

This process uses a gas aspiration nozzle (an eductoror an exhauster) to draw air into recycled water, which

in turn is discharged into a flotation vessel (similar to thedispersed-air conventional machines), to develop a two-phase mixture of air and water (Fig. 2). Bubbles are ofthe size 400–800 lm in diameter (Bennett, 1988;Gopalratnam et al., 1988). Advantages claimed for thenozzle units, over induced air flotation (IAF) systems,are the following:• lower initial costs and energy use because a single

pump provides the mixing and air supply;• lower maintenance and longer equipment life be-

cause the unit has no high-speed moving parts towear out.Applications reported have been exclusively in the

petrochemical industry for the separation of o/w emul-sions and treatment of oily metal-laden wastewater(Gopalratnam et al., 1988).

3.2. Column flotation

Column flotation is still a subject of great interest inmineral processing with a steadily growing number ofresearch studies and industrial applications (Finch,

Fig. 1. The conventional DAF unit, with water recycle to the

saturator.

Table 3

Main developments in dissolved air flotation (modified from Kiuru, 2001)

Year Development

1924 First generation: Pedersen cells. The separation tank is shallow and very low throughput, 2 m h�1. The ‘‘capture’’ of

particles by bubbles occurs in an inclined zone aside of the froth (floated product) separation tank

1960 Second generation (‘‘conventional’’): cells less shallow with higher loading capacity, 5–7 m h�1

1970 DAF deeper with filters for the treated water. Higher throughput 10–15 m h�1

1990 Third generation: ‘‘Turbulent’’ DAF deep units, high capacity cell > 40 m h�1. The ‘‘capture’’ zone is now deep and

horizontal

1995 Fourth generation: co-current type of cell with the capture occurring in the same tank (Cocco-DAF). They resemble

more the high capacity cells used in mineral processing, but with micro-bubbles (Eades and Brignall, 1995)

Fig. 2. Continuous nozzle flotation unit.


1995; Rubinstein, 1994; Finch and Dobby, 1990). In thecolumns used in the mineral processing area, feed slurryenters about one-third the way down from the top anddescends against a rising swarm of bubbles generatedby a sparger. In wastewater treatment, feed enters bythe column top in the middle of the ‘‘concentrate’’product.

New developments in column technology includeexternal gas spargers operating with and without addi-tion of surfactant or frothers, columns with internalbaffles and coalescers for oil recovery (Gu and Chiang,1999). In the presence of the surface-active reagentsmicro-bubbles can be obtained as in the Microcel col-umn (Yoon et al., 1992; Yoon and Luttrell, 1994). Ap-plications of column flotation in the field of oil removalin production waters (Gebhardt et al., 1994) and in therecovery of heavy metals precipitates (Filippov et al.,2000) have been reported (Fig. 3).

3.3. Centrifugal flotation (CF)

The separator and contactor can be an hydrocycloneor a simple cylinder. Thus, a centrifugal field is devel-oped. Aeration occurs by either injecting air (or bysuction), through flow constrictions, such as staticmixers or nozzles According to Jordan and Susko(1992), medium size bubbles having 100–1000 lm di-ameters are generated.

The air-sparged hydrocyclone (ASH), can be classi-fied as a centrifugal flotation unit (Ye et al., 1988). Itconsists of an aeration system whereby air is spargedthrough a jacketed porous tube wall and is sheared intonumerous small bubbles by the high-velocity swirl flowof the aqueous phase. Environmental applications ofASH flotation have been recently reported (Beeby andNicol, 1993).

An advanced ASH type of flotation has been reportedin applications to remove oil, grease, BOD, etc. BAF orbubble accelerated flotation (Fig. 4) system uses thecontactor–separation concept with very low detentiontimes in the contactor (Colic et al., 2001). Depending onthe bubble generation system the authors report devicesnamed as Induced Air BAF, Vacuum BAF, Electroflo-tation BAF.

3.4. Jet flotation

This cell appears to have a great potential for solid/liquid separations and for liquid/liquid separations aswell as in mineral processing (Jameson and Manlapig,1991). Its main advantage is its high throughput, highefficiency and moderate equipment cost (Clayton et al.,1991; Harbort et al., 1994). More, with no moving parts,the jet cell has low power consumption and low main-tenance costs. The cell consists of an aeration/contactzone (the downcomer), a bubble-particle or aggregatedisengagement zone (the tank proper pulp area) and acleaning or froth forming zone (the tank proper zone).The bubbles (medium size) formed in this cell may have100–600 lm in diameter (Jameson and Manlapig, 1991;Clayton et al., 1991). Problems with process accuracyhave been recently solved and its use has been extendedto wastewater treatment and recovery of solvent ex-traction liquors (Wyslouzil, 1994) and municipal waters(Yan and Jameson, 2001).

Fig. 3. The Microcel flotation column.

Fig. 4. The BAF, bubble accelerated flotation or BC, ‘‘bubble cham-

ber’’ flotation device.


3.5. Cavitation air flotation (CAF)

Cavitation air flotation utilizes an aerator (rotatingdisc), which draws ambient air down a shaft and injects‘‘micro-bubbles’’ directly into the wastewater (Fig. 5).However, there is no knowledge of any fundamentalwork with this flotation technique. CAF is utilized in thefood industry, especially in the milk industry, paint andtanneries to remove suspended solids, fats, oils, greases,BOD (biological oxygen demand) and COD (chemicaloxygen demand).

4. Applications and advances

Main industrial applications of flotation in miningand metallurgy are the recovery of solvent extractionliquors losses by DAF, column and jet flotation(Jameson cell), the separation of molybdenum ions(Marinkovic’, 2001) and manganese ions by DAF(Krofta, 1991). Yet, it is believed that there may beother, not reported examples, similar to those encoun-tered in other industrial fields.

A number of papers have recently been published il-lustrating techniques employed and flotation devices.These can be summarized as following:

4.1. Removal of ions

The removal of ions from water, one of the mostimportant issues in environmental problems today, istechnically possible through various flotation techniques(Zabel, 1992; Lazaridis et al., 1992; Rubio, 1998a,b;Matis, 1995). Principal removal methods are:• precipitate flotation (Silva et al., 1993; Stalidis et al.,

1989a,b; Lemlich, 1972; Pinfold, 1972; Mummallahand Wilson, 1981);

• gas aphrons flotation or colloidal gas aphrons(CGA);

• foam flotation (Clarke and Wilson, 1983); ion flota-tion (Nicol et al., 1992; Walkowiak, 1992; Sch€uugerl,2000);

• adsorbing particulate (colloids or aggregate) flotation(Zabel, 1992; Matis, 1995; Rubio and Tessele, 1997;Zouboulis et al., 1992a,b, 1993, 1997, 2001; McIntyreet al., 1982).

• ionic flotation (Scorzelli et al., 1999).

4.2. Precipitate flotation

This process is based on the formation of a precipi-tate of the ionic species, using a suitable reagent, and itssubsequent removal by attachment to air bubbles toform a flotation ‘‘concentrate’’ (Huang and Liu, 1999;Lemlich, 1972). Depending on the metal solution con-centration, the precipitation may proceed via metal hy-droxide formation or as a salt with a suitable anion(sulfide, carbonate, etc.). In the case of anion removal,precipitation should proceed through addition of ametal cation.

4.3. Gas aphrons flotation or colloidal gas aphrons(CGA)

Sebba, who established ionic flotation in 1959, pro-posed the use of colloidal gas aphrons or micro-foamsor simply micro-gas dispersions. They are dispersions ofgases in liquids formed with the use of a venturi gener-ator which introduces a gas to a circulating surfactantsolution in a region of high velocity and low pressure(Sebba, 1962; Ciriello et al., 1982).

This produces very small bubbles, which range in sizefrom 10 to 50 lm and provide a large amount of surfacearea. Despite the potential, no industrial applicationsare known and studies are mainly related to laboratoryand pilot scale (Kommlapati et al., 1996; Save andPangarkar, 1994).

Fig. 5. CAF unit.


4.4. Foam separation or foam flotation

This method is similar to ion flotation but uses anexcess of a surfactant or a proper frother to produce astable foam. Here the substances removed may be ionicor molecular, colloidal, crystalline, or cellular in nature,but, in all cases, they must selectively attach to the air–liquid interfaces (of foams or of bubbles) (Clarke andWilson, 1983). Some authors denote the separation asfoam fractionation since this term accurately describesthe removal of the surface active carrier compounds insolution in a foam column. Hundreds of parpers havebeen reported on foam/flotation or fractionation atlaboratory and pilot scale and some industrial applica-tions are believed to exist.

4.5. Adsorbing colloid flotation

This method involves the removal of the metal ion byadsorption on a precipitate (coagula) acting as a carrier.The loaded carrier is then floated, usually assisted with asuitable ‘‘collector’’ surfactant. The main carriers usedhave been ferric or aluminum hydroxides collected withthe help of sodium oleate or lauryl sulphate (Stalidiset al., 1989a,b).

A recent DAF process to remove molybdenum ionsin Chile employs this principle with the FeðOHÞ3 as themolybdenum carrier and sodium oleate as collector hasbeen reported. This method has been successful in sep-arating the molybdenum ions from Cu–Mo concentratefiltrates and meeting Chilean emission standards. Theinteresting feature is that this plant uses a ‘‘rougher’’stage to remove first the suspended solids and calciumions (as calcium oleate) and then the Mo ions in a‘‘cleaner’’ stage at pH about 5. Sodium oleate is alsoadded to enhance hydrophobicity and process kinetics.

4.6. Ion flotation

This method involves the removal of ions (colligendor surface inactive species) by transport to froth as acounter-ion to a surfactant species of opposite charge.Here the surfactants perform the dual role of frotherand collector, facilitating the adsorption of the colligendspecies onto the surface of an air bubble. In some cases,a ligand-activator for the flotation of the metal ionfollowed by a suitable surfactant has been necessary(Walkowiak, 1992; Nicol et al., 1992; Galvin et al.,1994). Despite many studies performed at laboratoryand pilot scale, only during the last few years have ap-plications of this method in industrial scale been re-ported (Zouboulis et al., 1992a,b; Nicol et al., 1992).

A novel gold recovery scheme based on ion flotationhas been developed. Heap leach liquor, containing goldcyanide is reacted with a suitable surfactant and spargedusing compressed air (Galvin et al., 1994). The surfac-

tant adsorbs at the surface of the rising air bubbles,thereby providing an interface for ion pairing to selec-tively collect the gold complex. Scorzelli et al. (1999),studied the removal of Cd ions using sodium dodecyl-sulfate as collector and the effect of ionic strength (NaCland Na2SO4), frothers and surface tension was evalu-ated. Main finding are the high removal obtained for ametal collector ratio of 1:2 (98% with 0.1% v/v isopro-panol frother) and the negative effect of the highstrength (>10�3 M).

5. Up coming techniques and advances

5.1. Aggregation-DAF

Precipitation, coagulation and flocculation have beenutilized in stages first to destabilize highly soluble ions toform colloidal particles or precipitates. Then, coagula-tion is used to enhance particle size and finally, with thepolymer to form stable, big and hydrophobic flocs. Thistechnique has been reported to remove Hg, As and Seions from processing streams of gold cyanidation cir-cuits (Tessele et al., 1998) using DAF. Here NaDTC,sodium dithiocarbamate, was employed as precipitant,LaCl3 or FeCl3 were the coagulants and Bufloc (Buck-man), the flocculant. Almost complete removal (>98%)of the metal ions from solution was reported usingDAF.

Process efficiency depended on the system solutionand interfacial chemistry, aggregation phenomena andDAF operating parameters. Main stages are the fol-lowing:1. ions +precipitant¼ colloidal precipitate (3–10 lm),2. colloidal precipitate + flocculant¼ flocs (�1–3 mm),3. flocs +micro (5–150 lm) and mid-sized bubbles (200–

600 lm)¼ flotation by DAF and/or columns (non-turbulent regimes).

5.2. Adsorbing (or sorbing) particulate flotation-APF orsimply carrier flotation-CF

The basis of the adsorptive (or sorbing) particulate (orcarrier) flotation is the uptake of cation, anion or organicby readily floatable particles. This resembles oxide flo-tation activation by metal ions, sulfide depression byanions or adsorption of collectors or frothers. Essen-tially, APF is a variant of the adsorbing colloid flotationprocess, employing particles as carrier-sorbing (absorb-ing and/or adsorbing) material for the metal ion. The keyto the process is the selection of a good sorbing carrierhaving a high surface area and a high reactivity with thepollutant to be removed and it should float readily.

The carrier can be a mineral particle, a polymericresin, activated coal or a by-product. The use of micro-organisms as sorbing materials (biosorption or bio-


sorptive flotation) has been proposed and may be an-other alternative (Zouboulis et al., 2001).

The removal of Cu, Zn and Ni from diluted solutionsby APF was studied at laboratory and pilot scale (F�eeris,2001). The sorbing used was a coal washing tailingmaterial from a coal industry from south of Brazil andthe flotation process applied was DAF. Best results(>95% removal) showed that the residual ions concen-tration is below the standards limits dictated by the locallegislation. Table 4 summarizes main reported studies inthis subject.

5.3. Column flotation to remove ions

A modified Microcel column (Yoon et al., 1992) withfeed entering by the cell top (to improved solid/liquidseparation) was studied to float loaded (with metal ions)FeðOHÞ3 precipitates as a function of pH (Souza andRubio, unpublished results). The column employs watertreated recycling procedure to generate bubbles. Thus,by pumping the flow fluid through a venturi or needlevalve, air is drawn into the pipe and bubbles are pro-duced. The size of the bubbles can be modulated withaddition of a surfactant.

Results showed that best separation was obtainedwhen optimizing medium pH, addition of sodium oleate(as ‘‘collector’’) and operating parameters, among oth-ers conditioning, flow rates, etc.

Recently, Filippov et al. (2000) studied the interac-tions between superficial feed and gas velocities andrecycling pulp flow rate on bubble size distribution andits effect on Mo-precipitate flotation. They conclude thatthe precipitate flotation effectiveness in columns is re-lated to floc stability under turbulence created by theswarming of rising bubbles.

5.4. Dissolved air flotation

DAF of iron hydroxide precipitates at workingpressures lower than 3 atm, using modified flotation

units to improve the collection of fragile coagula, wasstudied at the Laborat�oorio de Tecnologia Mineral eAmbiental (F�eeris and Rubio, 1999). Conventional DAFflotation was studied as a function of saturation pressurein the absence and presence of surfactants in the satu-rator. Without surfactants, the minimum saturationpressure required for DAF to occur was found to be 3atm. But, by lowering the air/water surface tension inthe saturator, DAF was possible at a saturation pressureof 2 atm.

This behavior was found to occur in both batch andpilot DAF operation tests and almost complete re-covery of the precipitates was attained. Results areexplained in terms of the minimum ‘‘energy’’ whichhas to be transferred to the liquid phase to formbubbles by a cavity phenomenon. Since the saturationstage accounts for about 50% of the total operatingenergy costs and considering the low cost involved inthe surfactant, this option appears to have a greatpotential.

A very important feature only reported for DAF,concerns with the mechanisms of bubble/particle (ag-gregates) interactions other than the common adhesionthrough hydrophobic forces (Fig. 6). Thus, apart fromparticles/bubbles collisions and adhesion, in DAF, partof the dissolved air in water, which does not convert intobubbles in the nozzle, remains in solution and ‘‘nucle-ate’’ at the particle surface (Solari and Gochin, 1992).This mechanism is independent on surface hydrophob-icity and allows flotation of hydrophilic particles. More,bubble entrapment into flocs or coagula and aggregateentrainment by the rising bubbles are mechanisms,which make separation easier. This explains the fact thatin DAF, no collector or froth is required but a thick andstable float layer is formed. Results show high clarifi-cation effluents are obtained in DAF. However, a majordisadvantage is that rapid air bubble levitation speed isnot attainable and hydraulic loadings are low (this isdictated by the Henry’s law) reducing and limitingprocess capacity.

Table 4

Main reported studies of APF

Adsorbing material Contaminants Author(s)

Coal jigging tailings Ni, Cu, Zn F�eeris (2001)

Zeolites Ni, Cu, Zn Rubio and Tessele (1997)

Zeolites Hg, As, Se Tessele et al. (1998)

Pyrite Cu, As Zouboulis et al. (1992a,b, 1993)

Red mud Cu Zouboulis et al. (1993)

Dolomite Pb Zouboulis et al. (1993)

Fly ash Ni Zouboulis et al. (1993)

Exchange resin Cu Duyvesteyn and Doyle (1995)

Hydroxyapatite Cd Zouboulis et al. (1997)

Activated coal Dye (Rodamine B) F�eeris et al. (1999)

Coal jigging tailings Oil Santander and Rubio (1998)

Barite Emulsified oil Santander and Rubio (1998)

Clay (hydrotalcite) Chromate, Crþ6 ions Lazaridis et al. (2001)


5.5. Separation of oils and organic compounds by flotation

The flotation of organic bearing waters such as oilspills on water, oily sewage or oil-in-water emulsions hasbeen used in various fields for a number of decades butis not commonly used in the mining and/or metallurgyindustries. Most of the research studies on the separa-tion of oil from water have addressed the effect of oilconcentration, type and concentration of destabilizingagents for o/w emulsions and the type of flotationtechnique to be employed (Bennett, 1988).

In the mining–metallurgical industry, residual oilywastewaters commonly discharged are waters contain-ing flotation chemicals and solvent extraction reagents,surface waters contaminated with free wasted oil andprocess waters containing oil spills (Pushkarev et al.,1983). Oil in water may be dispersed, emulsified or insolution in water in concentrations up to 1000 ppm. Inparticular, the presence of emulsified oil in water drop-lets around 50 lm in size causes problems in phaseseparation by conventional techniques (oil/water gravityseparation, DAF).

The flotation separation of very fine oil droplets(2–30 lm) is even more complicated and usually re-quires fine bubbles, quiescent hydrodynamic conditionsin the cell separation zone or emulsion breakers prior toflotation (Gopalratnam et al., 1988). This is due tocollection and adhesion factors, which makes the pro-cess very slow, especially when, treating high flow-rates.IAF and DAF, have been used extensively in the re-moval of stable oily emulsions (Bennett, 1988; Strick-land, 1980; Belhateche, 1995). IAF utilizes bubblesbetween 40–1000 lm in size and turbulent hydrody-namic conditions. The process has low retention times,

normally <5 min. Conversely, DAF employs micro-bubbles (30–100 lm), and quiescent regimes. However,because retention times are higher (20–60 min), thisprocess is inefficient when treating high volume effluentsand high flow-rates.

The Jameson cell, column flotation with CGA (pre-reagentized gas bubbles) and conventional columnsare now being utilized in solvent extraction plants(Readett and Clayton, 1993). Here the flotationdevices are used in the discharge aqueous streamsfrom the solvent extraction–electrowinning (SX–EW)plant to recover the organic liquor lost by entrain-ment into the aqueous phase. Thus, flotation canreduce organic losses and reduce potential environ-mental problems.

5.6. Modified jet flotation cell

A modified jet flotation cell has been studied in ourlaboratory (Fig. 7), to account for a better oil dropletcoalescence and for the decrease in the amount of shortcircuit observed in the conventional (Jameson type cell).Thus, the slurry abandoning the downcomer, enters acylinder obligating the coalesced or flocculated oil-bubbles aggregates units to leave the separation tank bythe froth layer. Results show that this cell is more ac-curate than the conventional cell yielding high oil re-moval values and treated water with low oil levels. Thus,with highly emulsified feeds having up to 603 mg l�1 ofoil, the removal was almost constant at or greater than80% regardless of the initial oil content. It is believedthat this type of flotation cell has a great potential for oilor organic solvent removal at high throughput values(>600 m d�1).

Fig. 6. Bubble-particle mechanisms in DAF: (a) particle–bubble collision and adhesion; (b) bubble formation at particle surface; (c) micro-bubble

entrapment in aggregates; (d) bubbles entrainment by aggregates.


5.7. Centrifugal flotation cell

The separation of flocculated (coalesced) oil emul-sions in a centrifugal flotation machine (Fig. 8) has beenrecently performed on a pilot scale in the Laborat�oorio deTecnologia Mineral e Ambiental (LTM), UniversidadeFederal do Rio Grande do Sul, Brazil. The device will be

in the very near future placed on offshore platforms inBrazil. Main characteristics are the very low residencetime (high throughput), high separation efficiency andlow water split. However, the flotation efficiency (Fig. 9)depends mainly on the degree of flocculation and on thevortex finder clearance.

5.8. The FF-flocculation-flotation process

A new turbulent on-line flocculation system assistedwith air bubbles has been developed at LTM yieldingaerated flocs (flocs with entrained and entrappedbubbles). These flocs, which rapidly ‘‘float’’, are

Fig. 7. Modified jet flotation pilot unit (Santander and Rubio, 1997,

1998).

Fig. 8. The LTM-centrifugal flotation device.

Fig. 9. Effect of flocculant concentration on oil centrifugal flotation

performance ð33:3 l min�1Þ. Feed oil concentration¼ 152 mg l�1.


formed only in the presence of high molecular weightpolymers and bubbles and under high shearing in theflocculator (Fig. 10). The air excess air leaves theflotation tank (a centrifuge) by the top and the flocsfloat after very short residence times (within seconds).The aerated flocs are large units (some millimeters indiameter) having an extremely low density (Rubio,2001).

5.9. The ‘‘multibubble’’ flotation column

Recently, F�eeris et al. (2001) reported data on theremoval of colloidal ferric hydroxide by flotation in acolumn with bubbles generated in an static mixer(medium-sized bubbles) and micro-bubbles generated asin DAF. These authors named this column flotationdevice a ‘‘multibubble column’’. Using this modifiedmicrobubble column they reported better results ascompared to DAF alone. Gains reported were a betterair-to-solids ratio (higher bubble surface flux),improved process kinetics and improved processthroughput. Fig. 11 shows some details of this flotationdevice.

6. Miscellaneous separations

6.1. Micro-organisms

It has been demonstrated, for many years, that bac-teria can be readily concentrated by froth or foam

flotation and since that time a number of investigatorshave confirmed not only the flotation of bacteria, but ofalgae and other micro-organisms (Smith, 1989;Sch€uugerl, 2000). Alga removal by flotation is becoming agood alternative to other treatment methods in tropicalcountries. In such environments, the algae grow at agreat rate causing problems in all water reservoirs.Furthermore, proliferation of algae in maturation pondsoften results in values exceeding EPA license limits forsuspended solids and elevated pH values. Also, dis-charge of algae (especially blue-green algae) laden ef-fluents can also cause possible release of their associatedtoxins to surface and ground waters.

The jet flotation process for alga removal reported byYan and Jameson (2001) appears to be an interestingapplication of flotation for the treatment of algaebearing municipal waters. Alga cells such as Microcystissp. that occur commonly in wastewater maturationponds are usually very small in size (3–7 lm) and toinduce efficient alga cell–air bubble contact, aggregatesof greater than 10 lm in size are required. Cationicpolymer flocculants are found to be effective, whilenonionic or anionic polymers are not. Different types ofalgae appear to share common surface characteristics.The same flocculant was found to be effective inflocculating very different types and forms of alga cells(e.g., Microcystis, Anabaena). Jameson Cell technologywas shown to be capable of simultaneously removingalgae and phosphorus enabling the continued use ofmaturation ponds and provides an alternative to costlyupgrades of existing wastewater treatment plants.

Fig. 10. FF-flocculation-flotation device.


6.2. Proteins

Various other non-fatty organic materials, such assoluble proteins derived from soybean processing, canbe removed from water by DAF flotation after precip-itation and flocculation (Schneider et al., 1995). Solubleprotein removed by this process from aqueous wastestreams from soybean plants can potentially be used assupplemental animal feed. The basis for protein sepa-ration by flotation is the aggregation of the macromol-ecules with inorganic salts and/or polymers and flotationwith micro-bubbles. Problems arise when proteins con-tain associated de-foaming agents or short dispersingmolecules that modify the surface properties of proteinaggregates enhancing their hydrophilic character andreducing bubble-particle adhesion.

6.3. Plastics

Modern industrial and home use of plastics has cre-ated an environmental need to recycle waste plastics of anumber of different types. Most of the commonly usedplastics, such as polyvinyl chloride, polycarbonates,polyacetal, and polypropylene ether are naturally hy-drophobic and are readily floated without addition of aflotation collector. Thus, process selectivity is a difficulttask. However, plastics vary in their hydrophobicitiesand their critical surface tensions have been exploredusing surface-active reagents. Thus, their floatabilitiescan be modulated by use of suitable depressants, which

include sodium lignin sulfonate, tannic acid, and Aero-sol OT (Shibata et al., 1996).

6.4. Deinking

Flotation has been used, for a number of years, inpaper deinking for paper recycling. Most of the studiesare based on ink removal using surfactants and calciumbearing salts. Finch and Hardie (1999) have reviewedthe main flotation machines and techniques employed inthis area, showing and discussing a variety of ap-proaches used to optimize the characteristics of suchflotation systems.

6.5. Soil washing

Flotation is being studied for removal of toxic andrelatively non-volatile hydrophobic compounds such asheavy oil, PAH, or PCB from contaminated soils. Theeffects of the basic parameters of the process have beeninvestigated and compared with soil washing, and theadvantages of flotation demonstrated (Ososkov andKebbekus, 1997).

Some limited reports in the literature point out that asignificant fraction of toxic hydrophobic organics maybe removed from contaminated soil by flotation. How-ever, no systematic investigations on removal of thesesubstances from soil by flotation have been reported.

Hydrophobic non-volatile organic compounds arepoorly adsorbed by soil particles, which are primarily

Fig. 11. The ‘‘multibubble’’ flotation column.


hydrophilic. These contaminants are mainly trapped inthe soil pore space. Trapped compounds can be trans-ported to the surface of soil/water slurry by bubblesduring flotation. Soil organic matter or hydrophobicimpurities in soil matrix adsorb some of hydrophobicpollutants. However, flotation may remove only part ofthe adsorbed pollutants.

6.6. Removal of radioactive nuclides from soils

Flotation of radioactive nuclides from contaminatedsoils and coral sand by both conventional-induced airflotation and column flotation has been studied andevaluated (Misra et al., 1995; Misra et al., 1996). In suchseparations it is desired to produce a very clean material(non-float) and a concentrate that contains most of theradionuclides, but is still a low-level radioactive mate-rial. The goal is high recover, but a low-grade concen-trate. Thus, the bulk of material to dispose of in a wasterepository is much reduced.

7. Final remarks

Since the flotation depends on multiple intercon-nected factors, many considerations should be takeninto account when selecting a flotation device and its

capacity and the techniques to be employed. Some ofthese factors are the following:• The wastewater flow-rate (m3 h�1, m3 s�1 or

m3 day�1) and the equipment throughput. Table 5shows examples of some reported values for flotationhydraulic loading Theses values are related to thebubble size distribution generated in the different flo-tation devices (see Figs. 12 and 13).

• The nature of pollutants, whether free, complexed,volatile, inorganic-organic or mixtures. Their concen-tration in effluents and in standard emissions.

• The nature of aggregates to be removed. Experimen-tal studies will define the best way to remove the pol-lutants, whether in the form of coagula, precipitates,flocs, sublate (metal-collector complexes), or ad-

Table 5

Averaged hydraulic loading values reported for some flotation devices

operating in mineral processing (�) and wastewater treatment

Equipment Hydraulic loading (m h�1)

DAF 7–40

IAF (induced air) 36–430�

Column cell 50–360�

Jameson (jet) cell 70–350�

ASH (Miller cyclone) 500–720�

FF-flocculation flotation 140–2160 (oil removal)

BAC 1.5–500

Fig. 12. Flotation techniques/devices operating with ‘‘micro-bubbles’’. EF¼Electroflotation; GA¼Gas aphrons; CAF¼Cavitation air flotation;

DAF¼Dissolved air flotation.

Fig. 13. Flotation techniques/devices operating with ‘‘medium sized’’ (200–800 lm) and ‘‘macrobubbles’’ (IAF > 800 lm).


sorbed on a carrier. Flocs and particulate carriers andnot coagula withstand shear and may be separated inflotation devices operating with high turbulence (cen-trifugal, jet). DAF is more amenable for separation ofcoagula or precipitates. Nevertheless, DAF of aer-ated flocs is also a good and fast alternative.

• The need for collectors, optimal pH, redox condi-tions, residence time, air-to-solids ratio, air hold up,bubble surface flux, lifting power of bubbles, effectof temperature, density, viscosity, surface tension(frothability), interfacial properties of aggregates(charge, hydrophobicity).

• Flow-sheet design. Whether a ‘‘rougher-cleaner’’scheme is needed: destiny of the floated productand the process water (possible reuse?), filtrationcharacteristics, drying, economics of the process.Figs. 12 and 13 show approximate bubble size ranges,

which have been reported in various flotation devicesand techniques.

8. Conclusions

Flotation is ever increasingly used in waste treatment,especially in the mining and metallurgical industry.Furthermore, the introduction of new, superior, flota-tion devices should lead to new and better applicationsfor remediation of mineral industry contaminated wa-ters and solids. A cross fertilization of flotation experi-ence in mineral flotation and in wastewater treatmentshould lead to new and improved procedures inthe mineral and metallurgical industry, the chemicaland petroleum industries and domestic wastewatertreatment.

Acknowledgements

Authors thank all the students and colleagues re-sponsible for the friendly atmosphere at the LTM-Uni-versidade Federal do Rio Grande do Sul and to allinstitutions supporting research in Brazil.

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