Overview of Flotation as a Wastewater Treatment Technique

Overview of otation as a wastewater treatment technique

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

a Departamento de Engenharia de Minas-PPGEM, Laboratoorio de Tecnologia Mineral e Ambiental, Universidade Federal do Rio Grande do Sul,Av. Osvaldo Aranha 99/512, 90035-190, Porto Alegre, RS, Brazil

b 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 euents 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 otation is showing a great potential due to the high throughput of modern equipment, low sludge generation and

the high eciency 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 otation 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 emulsied 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, oshore platforms, processing plants, tailing ponds, etc.(Smith, 1996; Villas Bo^oas 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 uids are discharged,

the oil/water separation becomes dicult especiallywhen the oil is emulsied, 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, otation collectors and modiers(activators or depressing agents, pH regulators);

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

Minerals Engineering 15 (2002) 139155www.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 eu-ents containing metals ions is precipitationaggrega-tion (coagulation/occulation)-settling as hydroxidesor insoluble salts. However, this method, from atechnical point of view, presents certain limitations,namely: the formation of metal hydroxide is ineective in di-

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

ing on the metal, via the reaction MOHmn OH MOHmnm;

the pH of minimum solubility of hydroxides is dier-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;

ltration may be dicult as a result of the precipitatesneness, and;

due to kinetic and scale problems, the treatment bycoagulation and settling of euent ow-rates ofabout 24 m3 s1 is very dicult and costly. Thisconstitutes a great challenge for the modern miningindustry.

1.3. Flotation processes

The use of otation has shown to have a greatpotential owing to the high throughput and eciencyof 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 otation 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 otation inenvironmental applications and is aimed to: show the potential of otation as a wastewater treat-

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

vices; serve as a bridge providing information on ota-

tion activities being conducted in various engineer-ing elds as well as in the mining andmetallurgical industry. It is believed that a cross ex-change of otation experience in mineral otationand in water and euent 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 dierent minerals from each other(Kitchener, 1985). Regarding applications of otation inwastewater and domestic sewage treatment, civil andchemical engineers have used dissolved air otation(DAF) for a number of years (Hooper, 1945). Mainapplications have been in the removal of the solids, ions,macromolecules and bers, and other materials fromwater (Matis, 1995; Mavros and Matis, 1992; Lemlich,1972; Clarke and Wilson, 1983; Zabel, 1992).

More, otation is also practiced in other elds(Kitchener, 1985; Roe, 1983; Cundeva and Stalov,1997; Kim et al., 1999; Schuugerl, 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 clarication of fruit juices.

The main dierences between conventional ota-tion of ores and otation 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 (3001500 lm) are optimal for otationof minerals (nes and coarse particles). Yet, conven-tional otation devices do not generate a sucientnumber of bubbles smaller than 600 lm. Main usesof micro-bubbles (

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

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

In mineral otation, the overall process is economi-cally attractive. In environmental application, usuallyotation means an extra cost.Other dierences are summarized in Table 1 com-

paring, among others, bubbles characteristics in dier-ent otation 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 oating 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 otation? Many advantages have been reported

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

ow 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 ota-tion devices (Da Rosa et al., 1999; Rubio, 1998a,b,2001);

thicker otation concentrates (612% w/w).Table 2 shows a partial list of current commercially

available otation devices for wastewater treatment anddrinking water treatment units.

Voronin and Dibrov (1999) have recently published aclassication of otation processes in wastewater de-contamination. They grouped dierent otation 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 otation techniques, devices and pro-

cesses

Here some recognized techniques are summarized toshow their main features.

2.1. Electro-otation (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 asemulsied oil from water, ions, pigments, ink andbers 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

Dierences between otation in mineral processing and in wastewater treatment

Parameter Froth otation of minerals Water and wastewater treatment

Feed solids content (weight/weight basis) (%) 2540

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

An electrolytic coagulation/otation (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 otation of the ocs. Bulk water passes throughthe reactor and is treated by the coupled coagulation/occulation 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 otation (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 7001500 lm diameter. Thismethod, well known inmineral processing, is utilized alsoin the petrochemical industry, for oilwater separation(oily sewage) (Zheng and Zhao, 1993; Bennett, 1988).

2.3. Dissolved air (pressure) otation (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 orices, and clouds of bubbles,30100 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: clarication of renery wastewater, wastewater recla-

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

ment plants; sludge thickening and separation of biological ocs;

Table 2

Examples of some commercially available otation devices for wastewater treatment

Supplier company Type of cell characteristics Application details

Sionex DAF Wastewater treatment to remove suspended

organic solids, dissolved oils, algae, 57 lmoocysts, volatile organic compounds, humic acid,

clarication

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 otation columns for reducing

organic reagent and kerosene from rich

electrolytes prior to electrowinning

WesTech Dissolved Air and Nitrogen (DNF) otation

systems

Wastewater treatment

OR-Tec HF IAF uses a baed, aeration system that

produces very ne bubbles

Flotation of fat, grease, suspended solids from

food, municipal and industrial waste streams

Hydroxyl Industrial Systems Positive Flotation Mechanism (PFM); dissolved

air otation processes Electrostatically

charged micro-bubbles

Dissolved air otation processes for solids, air

and grease

Aeromax Systems ZEPHYRe IAF using very ne bubbles For fat, grease, oatable solids

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

industrial processing wastes

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

euents, paper mill

BakerHughes Process ISF hydraulically operated gas otation, deg-

assing, and optional skim storage components

For oil/water separations. System in a

completely enclosed otation process

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

vacuum BAF, electrootation 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 oshore operation the treated water

discharges directly into the sea

Hydrocal CAF For treatment of laundry, food processing waters

Aquaot FF otation of aerated ocs Vehicle washing euents, removal of oil, solids,

surfactants

142 J. Rubio et al. / Minerals Engineering 15 (2002) 139155

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

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

(dissolved toxic organic chemicals); removal of algae, 57 lm Giardia oocysts, 45 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 otation method for the treatment of in-dustrial euents. It is believed that applications willrapidly expand in the waste treatment in the metallur-gical and mining eld (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 otation techniques and processes

3.1. Nozzle otation (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 otation vessel (similar to thedispersed-air conventional machines), to develop a two-phase mixture of air and water (Fig. 2). Bubbles are ofthe size 400800 lm in diameter (Bennett, 1988;Gopalratnam et al., 1988). Advantages claimed for thenozzle units, over induced air otation (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 otation

Column otation 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 otation (modied from Kiuru, 2001)

Year Development

1924 First generation: Pedersen cells. The separation tank is shallow and very low throughput, 2 m h1. The capture ofparticles by bubbles occurs in an inclined zone aside of the froth (oated product) separation tank

1960 Second generation (conventional): cells less shallow with higher loading capacity, 57 m h1

1970 DAF deeper with lters for the treated water. Higher throughput 1015 m h1

1990 Third generation: Turbulent DAF deep units, high capacity cell > 40 m h1. The capture zone is now deep andhorizontal

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 otation unit.

J. Rubio et al. / Minerals Engineering 15 (2002) 139155 143

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 concentrateproduct.

New developments in column technology includeexternal gas spargers operating with and without addi-tion of surfactant or frothers, columns with internalbaes 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 otation in the eld 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 otation (CF)

The separator and contactor can be an hydrocycloneor a simple cylinder. Thus, a centrifugal eld is devel-oped. Aeration occurs by either injecting air (or bysuction), through ow constrictions, such as staticmixers or nozzles According to Jordan and Susko(1992), medium size bubbles having 1001000 lm di-ameters are generated.

The air-sparged hydrocyclone (ASH), can be classi-ed as a centrifugal otation 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 owof the aqueous phase. Environmental applications ofASH otation have been recently reported (Beeby andNicol, 1993).

An advanced ASH type of otation has been reportedin applications to remove oil, grease, BOD, etc. BAF orbubble accelerated otation (Fig. 4) system uses thecontactorseparation 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, Electroo-tation BAF.

3.4. Jet otation

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, higheciency 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 have100600 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 otation column.

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

ber otation device.


3.5. Cavitation air otation (CAF)

Cavitation air otation utilizes an aerator (rotatingdisc), which draws ambient air down a shaft and injectsmicro-bubbles directly into the wastewater (Fig. 5).However, there is no knowledge of any fundamentalwork with this otation 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 otation in miningand metallurgy are the recovery of solvent extractionliquors losses by DAF, column and jet otation(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 elds.

A number of papers have recently been published il-lustrating techniques employed and otation 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 otation techniques(Zabel, 1992; Lazaridis et al., 1992; Rubio, 1998a,b;Matis, 1995). Principal removal methods are: precipitate otation (Silva et al., 1993; Stalidis et al.,

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

gas aphrons otation or colloidal gas aphrons(CGA);

foam otation (Clarke and Wilson, 1983); ion ota-tion (Nicol et al., 1992; Walkowiak, 1992; Schuugerl,2000);

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

ionic otation (Scorzelli et al., 1999).

4.2. Precipitate otation

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 otation 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(sulde, carbonate, etc.). In the case of anion removal,precipitation should proceed through addition of ametal cation.

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

Sebba, who established ionic otation 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 otation

This method is similar to ion otation 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 airliquid 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/otation or fractionation atlaboratory and pilot scale and some industrial applica-tions are believed to exist.

4.5. Adsorbing colloid otation

This method involves the removal of the metal ion byadsorption on a precipitate (coagula) acting as a carrier.The loaded carrier is then oated, 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 FeOH3 as themolybdenum carrier and sodium oleate as collector hasbeen reported. This method has been successful in sep-arating the molybdenum ions from CuMo concentrateltrates and meeting Chilean emission standards. Theinteresting feature is that this plant uses a rougherstage to remove rst the suspended solids and calciumions (as calcium oleate) and then the Mo ions in acleaner stage at pH about 5. Sodium oleate is alsoadded to enhance hydrophobicity and process kinetics.

4.6. Ion otation

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 otation 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 otationhas 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 eect of ionic strength (NaCland Na2SO4), frothers and surface tension was evalu-ated. Main nding are the high removal obtained for ametal collector ratio of 1:2 (98% with 0.1% v/v isopro-panol frother) and the negative eect of the highstrength (>103 M).

5. Up coming techniques and advances

5.1. Aggregation-DAF

Precipitation, coagulation and occulation have beenutilized in stages rst to destabilize highly soluble ions toform colloidal particles or precipitates. Then, coagula-tion is used to enhance particle size and nally, with thepolymer to form stable, big and hydrophobic ocs. 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 Buoc (Buck-man), the occulant. Almost complete removal (>98%)of the metal ions from solution was reported usingDAF.

Process eciency depended on the system solutionand interfacial chemistry, aggregation phenomena andDAF operating parameters. Main stages are the fol-lowing:1. ions +precipitant colloidal precipitate (310 lm),2. colloidal precipitate + occulant ocs (13 mm),3. ocs +micro (5150 lm) and mid-sized bubbles (200

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

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

The basis of the adsorptive (or sorbing) particulate (orcarrier) otation is the uptake of cation, anion or organicby readily oatable particles. This resembles oxide o-tation activation by metal ions, sulde depression byanions or adsorption of collectors or frothers. Essen-tially, APF is a variant of the adsorbing colloid otationprocess, 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 oat 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 otation) 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 (Feeris,2001). The sorbing used was a coal washing tailingmaterial from a coal industry from south of Brazil andthe otation 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 otation to remove ions

A modied Microcel column (Yoon et al., 1992) withfeed entering by the cell top (to improved solid/liquidseparation) was studied to oat loaded (with metal ions)FeOH3 precipitates as a function of pH (Souza andRubio, unpublished results). The column employs watertreated recycling procedure to generate bubbles. Thus,by pumping the ow uid 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, ow rates, etc.

Recently, Filippov et al. (2000) studied the interac-tions between supercial feed and gas velocities andrecycling pulp ow rate on bubble size distribution andits eect on Mo-precipitate otation. They conclude thatthe precipitate otation eectiveness in columns is re-lated to oc stability under turbulence created by theswarming of rising bubbles.

5.4. Dissolved air otation

DAF of iron hydroxide precipitates at workingpressures lower than 3 atm, using modied otation

units to improve the collection of fragile coagula, wasstudied at the Laboratoorio de Tecnologia Mineral eAmbiental (Feeris and Rubio, 1999). Conventional DAFotation 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 otation of hydrophilic particles. More,bubble entrapment into ocs 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 oat layer is formed. Results show high clari-cation euents 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 Henrys law) reducing and limitingprocess capacity.

Table 4

Main reported studies of APF

Adsorbing material Contaminants Author(s)

Coal jigging tailings Ni, Cu, Zn Feeris (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) Feeris et al. (1999)

Coal jigging tailings Oil Santander and Rubio (1998)

Barite Emulsied oil Santander and Rubio (1998)

Clay (hydrotalcite) Chromate, Cr6 ions Lazaridis et al. (2001)


5.5. Separation of oils and organic compounds by otation

The otation of organic bearing waters such as oilspills on water, oily sewage or oil-in-water emulsions hasbeen used in various elds 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 eect of oilconcentration, type and concentration of destabilizingagents for o/w emulsions and the type of otationtechnique to be employed (Bennett, 1988).

In the miningmetallurgical industry, residual oilywastewaters commonly discharged are waters contain-ing otation 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, emulsied or insolution in water in concentrations up to 1000 ppm. Inparticular, the presence of emulsied oil in water drop-lets around 50 lm in size causes problems in phaseseparation by conventional techniques (oil/water gravityseparation, DAF).

The otation separation of very ne oil droplets(230 lm) is even more complicated and usually re-quires ne bubbles, quiescent hydrodynamic conditionsin the cell separation zone or emulsion breakers prior tootation (Gopalratnam et al., 1988). This is due tocollection and adhesion factors, which makes the pro-cess very slow, especially when, treating high ow-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 401000 lm in size and turbulent hydrody-namic conditions. The process has low retention times,

normally 600 m d1).

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

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


5.7. Centrifugal otation cell

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

in the very near future placed on oshore platforms inBrazil. Main characteristics are the very low residencetime (high throughput), high separation eciency andlow water split. However, the otation eciency (Fig. 9)depends mainly on the degree of occulation and on thevortex nder clearance.

5.8. The FF-occulation-otation process

A new turbulent on-line occulation system assistedwith air bubbles has been developed at LTM yieldingaerated ocs (ocs with entrained and entrappedbubbles). These ocs, which rapidly oat, are

Fig. 7. Modied jet otation pilot unit (Santander and Rubio, 1997,

1998).

Fig. 8. The LTM-centrifugal otation device.

Fig. 9. Eect of occulant concentration on oil centrifugal otation

performance 33:3 l min1. Feed oil concentration 152 mg l1.


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

5.9. The multibubble otation column

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

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

otation and since that time a number of investigatorshave conrmed not only the otation of bacteria, but ofalgae and other micro-organisms (Smith, 1989;Schuugerl, 2000). Alga removal by otation 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-uents can also cause possible release of their associatedtoxins to surface and ground waters.

The jet otation process for alga removal reported byYan and Jameson (2001) appears to be an interestingapplication of otation for the treatment of algaebearing municipal waters. Alga cells such as Microcystissp. that occur commonly in wastewater maturationponds are usually very small in size (37 lm) and toinduce ecient alga cellair bubble contact, aggregatesof greater than 10 lm in size are required. Cationicpolymer occulants are found to be eective, whilenonionic or anionic polymers are not. Dierent types ofalgae appear to share common surface characteristics.The same occulant was found to be eective inocculating very dierent 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-occulation-otation device.


6.2. Proteins

Various other non-fatty organic materials, such assoluble proteins derived from soybean processing, canbe removed from water by DAF otation after precip-itation and occulation (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 otation is the aggregation of the macromol-ecules with inorganic salts and/or polymers and otationwith 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 dierent types. Most of the commonly usedplastics, such as polyvinyl chloride, polycarbonates,polyacetal, and polypropylene ether are naturally hy-drophobic and are readily oated without addition of aotation collector. Thus, process selectivity is a diculttask. However, plastics vary in their hydrophobicitiesand their critical surface tensions have been exploredusing surface-active reagents. Thus, their oatabilitiescan 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 otation machines and techniques employed inthis area, showing and discussing a variety of ap-proaches used to optimize the characteristics of suchotation 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. Theeects of the basic parameters of the process have beeninvestigated and compared with soil washing, and theadvantages of otation demonstrated (Ososkov andKebbekus, 1997).

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

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

Fig. 11. The multibubble otation 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 otation. Soil organic matter or hydrophobicimpurities in soil matrix adsorb some of hydrophobicpollutants. However, otation 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 airotation and column otation has been studied andevaluated (Misra et al., 1995; Misra et al., 1996). In suchseparations it is desired to produce a very clean material(non-oat) 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 otation depends on multiple intercon-nected factors, many considerations should be takeninto account when selecting a otation device and its

capacity and the techniques to be employed. Some ofthese factors are the following: The wastewater ow-rate (m3 h1, m3 s1 or

m3 day1) and the equipment throughput. Table 5shows examples of some reported values for otationhydraulic loading Theses values are related to thebubble size distribution generated in the dierent o-tation devices (see Figs. 12 and 13).

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

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

Table 5

Averaged hydraulic loading values reported for some otation devices

operating in mineral processing () and wastewater treatmentEquipment Hydraulic loading (m h1)

DAF 740

IAF (induced air) 36430

Column cell 50360

Jameson (jet) cell 70350

ASH (Miller cyclone) 500720

FF-occulation otation 1402160 (oil removal)

BAC 1.5500

Fig. 12. Flotation techniques/devices operating with micro-bubbles. EFElectrootation; GAGas aphrons; CAFCavitation air otation;DAFDissolved air otation.

Fig. 13. Flotation techniques/devices operating with medium sized (200800 lm) and macrobubbles (IAF > 800 lm).


sorbed on a carrier. Flocs and particulate carriers andnot coagula withstand shear and may be separated inotation devices operating with high turbulence (cen-trifugal, jet). DAF is more amenable for separation ofcoagula or precipitates. Nevertheless, DAF of aer-ated ocs 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 ux, lifting power of bubbles, eectof temperature, density, viscosity, surface tension(frothability), interfacial properties of aggregates(charge, hydrophobicity).

Flow-sheet design. Whether a rougher-cleanerscheme is needed: destiny of the oated productand the process water (possible reuse?), ltrationcharacteristics, drying, economics of the process.Figs. 12 and 13 show approximate bubble size ranges,

which have been reported in various otation 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, ota-tion devices should lead to new and better applicationsfor remediation of mineral industry contaminated wa-ters and solids. A cross fertilization of otation experi-ence in mineral otation 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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Overview of Flotation as a Wastewater Treatment Technique

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