Minerals Engineering 15 (2002) 139–
www.elsevie r.com/locate/mine
155
Overview of flotation as a wastewater treatment technique J. Rubio a
a,*
a
, M.L. Souza , R.W. Smith
b
Departamento de Engenharia de Minas-PPGEM, Laboratorio de Tecnologia Tecnologia Mineral e Ambiental, Universidade Universidade Federal do Rio Grande Grande do Sul, Av. Osvaldo Osvaldo Aranha 99/512, 90035-190, 90035-190, Porto Porto Alegre, RS, Brazil b Metallurgical and Materials Engineering, Mackay Mackay School of Mines, University of Nevada-Reno, Nevada-Reno, USA
Received 7 October October 2001; accepted 12 December 2001
Abstract
The treatment treatm ent of aqueous the some minerals is one most serious seriou s environmental issues face d by and or oily of thepowders, metallurgy pollutants pollutan ts effluents are isresidual resi dual reagents, chemicals, chemic als, metal ions, oils, faced organic mayand be industries. Main valuable (Au, Ag). The use Pt, of flotation showing a great potential due to the high throughput of modern equipment, low the high efficiency the separation schemes already available. sludge generation concluded that this process and of It is will be soon water incorporat incorporated ed as a technology minerals industryin to treat these wastewaters and, when possible,Examples in the to recycle process and materials. environmental applications is fully discussed. In this paper, the use of flotation of flotation of promising emerging techniques and devices heavy metal are reported some recent advances the treatment and in of 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. Intr oduction oduction ound 1.1. Backgr ound
Process waters exiting from mining, petroleum and metallurgical operations are contaminated widespread throughout the world and can become These substances substa nces by various ch pollutants. include metal ions, oils, organic and emicals, powders, others, sometimes rendering the water useless for recycling as process water, often dangerous for the environment, and sometimes causing losses of valuable of water ces off materials (Galvin et found al., 1994). Sour contamination maysing be at mines, mills, platforms, platfor ms, processing proces plants, plant s, ponds po nds, , shore etc. etc. tailing ^as and Barreto, 1996; (Smith, 1996; Villas Bo Warhurst and Bridge, 1996). Sometimes, due to process their chemical and/or volum voecolume, e,nomical these the se pro cess water wa terss where cann ccomplexity annot ot they be treate tre ated d eco nom ically ly even eve n in cases materials. Further, when organic contain valuable fluids are discharged,
*
Corresponding author. Tel.: +55-51-3316-3540; fax: +55-51-33163530. E-mail addresses: jrubio@v ortex.ufrgs .br; http://www.lapes.ufrgs. br/Laboratorios/ltm/ltm.html (J. Rubio),
[email protected] (R.W. Smith).
the the oil/w il/wa ater ter the sepa separa rati tion on beco become mess and difficult when worse especially emulsified, oil is when the mean the droplet size is or if small stabilized (Beeby and emically emulsions are Nicol, chemically ch 1993). Smithand (1996) showed in from of detailmineral characteristics solid wastes processing liquid plants. Vari Va rious ou s techniques and technologies avai availab lable le of typical were were disand the quality and quantity ants were listed. pollutcussed of typical Thus, Thus , current curre nt and future futu re technologie techn ologiess will eventually have to deal with areas such as: process water treatment and recycling • (reuse); removal and/or recovery of ions: heavy and/or • precious metals, anions, residual organic chemicals, complexes or ch elates; cyanide and arsenic emission control, recovery or • destruction; • oil spills separation (including recovery of solvent extraction liquor s); • acid mine waters containing considerable amounts of harmful base metals such as nickel, copper, zinc, lead ate; in addition to ferrous iron and sulf ate; control and removal of residual chemical • reagents such as frothers, flotation collectors and modifiers (activators or depressing agents, pH regulators); separation of various wasted • plastics; radioactive control in aqueous effluents and • soils.
0892-6875/02/$ PII: S -0see 8 9front 2- 6 matter 8 7 5 ( 0 1 2002 1 6 - 3 Science Ltd. All rights reserved. ) 0 0 2Elsevier
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J. Rubio et al. / Minerals Engineering 15 (2002) 139–155
1.2. Conventional treatment processes
1.4. Flotation process in wastewater treatment
conventional process ions The for istreating liquid ents containing metals precipitat ion– effluegation (coagulation/flocculation)-settling as aggr salts. However, hydroxides or insoluble this view, presents method, from a point of technical certain limitations, namely: • the formation of metal hydroxide is ineffective in di- lute metal bearing effluent s; • the hydroxo precipitate tends to re-dissolve, dep end þ ð Þ ing on the metal, via the reaction MðOHÞ OH ¼
Flotation mineral (ore) had asitssuch beginning inused ing and has beenapplications cesspro for a using long in solid/ solid time separation seto lectively separate different stable froths 1985). minerals (Kitchener, from each of other Regarding applications flotation wastewater inand chemical and civil domestic sewage treatment, engineers dissolved have used air flotation (DAF) years (Hooper, 1945). for a number of Main in the removal the solids, applications have been of ions,
n
OHÞ
;
ðn þmÞ
the pH of minimum solubility of hydroxides is different for the various metals present. For example, the minimum solubility for cupric hydroxide occurs at a pH value around 9.5 while for cadmium hydroxide it occurs at pH around 11; • precipitation of metals becomes incomplete when complexing or chelating agents are present; • volumes of sludge formed are too large and with a high water content ; • filtration may be difficult as a result of the precipitates fineness, and; • due to kinetic and scale problems, the treatment by coagulation and settling of effluent flow-rates of about 2 – 4 m3 s 1 is very difficult and costly. Thi s constitutes a great challenge for the modern mining industry. •
1.3. Flotation processes
shown The potential use of owing flotation hashigh to have a throughput the great to and equipment efficien of modern now available cy Mati (Zabel, 1992; b; s, 1995; et1999; al., Parekh 1996; Dibrov,of Rubio, 1998a, andRubio Voronin 1999). Other advantages flotation are and Miller, the selectivesilver recover valuable ions such as gold, y of are palladium, pollutants), the new hich also (w available separation schemes nowprosludge generation cess. and the low in this paper summarizes general environmental applications and features to:flotation in This is aimed of wastewater show the potential of flotation as a • treat- ment technique and present some ad vances; present • devices; novel separation concepts and flotation • serve as aactivities ‘‘bridge’’being providing information on conducted flotation in various engineer ing fieldsas well as in the mining and exmetallurgical industry. change that a cross flotation It is believed of experience water in flotation and in and mineral effluent treatment should lead new to and improved procedures for industry waste treatment.
macromolecules and fibers, and other materials from water (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; Schu €gerl, 2000), such as: • analytical chemistry; • protein separation; • treatment of spent photography liquors; • odor removal; • plastics separation and recycl ing; • 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 juice s. The main differences between ‘‘conventional’’ flotation of ores and flotation applied to water treatment are the following: • The meth od of producing the gas bubbles in order to generate micro, medium or macro-bubbles. It is now widely accepted that medium size and large bubbl e diameters (300–1500 lm) are optimal for flotation of minerals (fines and coarse particles). Yet, conventional flotation devices do not generate a sufficient number of bubbles smaller than 600 lm. Main uses of micro-bubbles (<100 lm) is in applications of flotation to solid/liquid or liquid/liquid separation. Thus, the distinguishing feature between conventional mineral flotation and flotation in waste treatment is that, where extremely small (or even colloidal) particles have to be floated, microbubbles are required. • Because the species floating are usually aggregated colloids rather than dispersed ones, high shear rates
J. Rubio et al. / Minerals Engineering 15 (2002) 139–155
•
must be avoided to obviate destruction of the fragile aggregates. This is important in the clarification of effluents and introduces distinct problems not previously encountered in mineral flotation. The solids content present in the pulp system, whether diluted or not. A limiting feature of bubbles is the lifting power or carrying capacity. Microbubbles do not float dense and big particles, especially at high solids content (4–5%, w/w).
141
solid/solid/liquid Theal type processing of separation: and in solid/liquid, miner iquid1/liq liquid/liquid water solid/l 2or uid in treatment . In mineral flotation • is necessary produce itsurface e froth at thetofree stabl the to flotation cell.a of cations wastewater treatment an stable appliIn foam is not required. • In mineral flotation, the overall process is eco nomically attractive. In environmental application, •
usually flotation means an extra cost. Other differences are summarized in Table 1 comparing, among others, bubbles characteristics in different flotation devices. Flotation technology can be incorporated in mining and industrial wastewater-treatment schemes in the following ways: • as a unit process (ancillary or main process) to remove contaminants which are not separated by
other means. Depending on performance (water quality), process water can be adequately treated and recycl ed; as a treatment unit on floating solids in • thickeners (concentrates or tailings); • as an auxiliary process to bio-oxidation lagoons or sludge thickening in water reuse; as a process for removing various organics, • residuals chemicals, including petroleum, from wat er; • as a solid/liquid separation process in acid mine drainage neutralization with lime; as a primary treatment unit ahead of secondary • treatment units, such as bio-oxidation lagoons for redu cing the cost of aerobic digestion; • as a unit process for sludge thickening. Why flotation? Many advantages have been rep orted illustrating the technical and economical potential of this process: • high selectivity to recover valuables (Au, Pt, Pd, etc); • high efficiency to remove contaminants: high overflow rates, low detention periods (meaning smaller
sizes, costs); less thicker space scums needs, and savings tank in uction constr sludge than skimming and; set- tling or in gravity • low operating costs with the use of upcoming flotation devices (Da Rosa et al., 1999; Rubio, 1998a,b, 2001); • thicker flotation concentrates (6–12% w/w). Table 2shows a partial list of current commer cially available flotation devices for wastewater treatment and drinking water treatment units. Voronin and Dibrov (1999) have recently published a classification of flotation processes in wastewater decontamination. They grouped different flotation techniques based on physicochemical and technological points and divided them in adsorptive or adhesive. A number of applications are reported without mention neither the type of equipment employed nor the bubble size distribution.
2. Conventional flotation techniques, devices and processes
recognized techniques are summarized show some their main features. to Here 2.1. Electro-flotation (EF)
micro-bubbles generation The basis forofthe diluted is the electrolysis aqueous, conducting solutions with the production both gas bubbles of at date, Applications, to at an industrial electrodes. ee scale, have n the area light b in of of removal colloidal systems such as emulsified oil from from water, pigments, water ions, ink and fibers (Zabel, 1992; Zouboulis et al., 1992a,b). Advantages claimed are the clarity of the treated wastewater and disadvantages are the low throughput,
Table 1 Differences between flotation in mineral processing and in wastewater treatment
2
106 – 9
10 3 (jet/columns )
Parameter
Froth flotation of minerals
Water and wastewater treatment
Feed solids content (weight/weight basis) (%)
25–40
<
Particle size to float (lm)
10–150
Bubble size distribution (lm)
600–2000
1 Bubbles rising velocity (m h )
250–800 (approximate values)
Number of bubbles (cm
3
)
9
103 – 2
Bubbles surface area (cm2 cm 3 )
100–30
Air hold up (%)
15–25
a
Aerated flocs.
102
4 (DAF) 10–30 (jet/columns ) 1–50 (not flocculated) and 1–5 mm flocs (with polymers )a 30–100 (DAF) 100–600 (jet/columns ) 0.7–30 (DAF) 30–1000 (jet/columns ) 6 108 – 2 106 (DAF) 4000–600 (DAF) 600–100 (jet/columns ) 8–14 (DAF) 20–40 (jet/columns )
Table 2
Examples of some commercially available flotation devices for wastewater treatment Supplier company
Type of cell characteristics
Application details
Sionex
DAF
Canadian Process Technologies using
Vertical oil separation cell VOSCell –
to oils, suspended Wastewater treatment remove organic solids, dissolved algae, 5–7 lm oocysts, volatile organic compounds, humic acid, clarification Developed to remove oil and grease from produced water using natural gas as a separating medium
Canadian Process Technologies WesTech OR-Tec Hydroxyl Industrial Systems dissolved
R
natural gas as a separating medium. IAF column Dissolved Air and Nitrogen (DNF) flotation systems HF IAF – uses a baffled, aeration system that produces very fine bubbles Positive Flotation Mechanism (PFM);
Aeromax Systems Therm odyne Corporation
air flotation processes – ‘‘Electrostatically’’ charged micro-bubbles ZEPHYRe IAF – using very fine bubbles Ultra-Float ADAF – plug flow DAF device
PURAC Engineering
High capacity DAF-filter system
Baker–Hughes Process
–assing, hydraulically operated gasstorage flotation, ISF degand optional skim components BAF BAF, induced-air – air-sparged BAF BAF, vacuum BAF, electroflotation Flotation piles (underwater oil/water
ZPM
Engineering Specialties separator) Hydrocal Aquaflot
– combines secondary treatment of produced water with disposal in one vessel CAF FF – flotation of aerated flocs
the emission of H2 bubbles, electrode costs andced. maintenance and the voluminous sludge produ electrolytic coagulation/flotation An (ECF) em has been also reported using reversibleions polarity syst are aluminum electrodes. Herein, aluminum relea sed from the anodes, inducing at coagulation, and bubbles are generated the aluminum hydrogen cathodes, en- abling of the Bulk the flotation reactor and is flocs. treated the through passes bywater coupled flocculation process. Laboratory coagulation/ scale tests have that the reactor performs shown ECF aluminum sulfate better than when onal a model conventi coagulation treating colored (DOC) water, carbon with 20% more dissolved organic removed by electro-coagulation the same doses for Al (Andre et al., 2000). 2.2. Dispersed (induced) air flotation (IAF)
Bubbles are mechanically formed by aand combination high-speed mechanical agitator of ation air injec system. the technology makes use an The of force developed. The gas, introduced at the centrifugal top, the gliquid become fully intermingled and, passin afterand
Organic recovery flotationand columns for from reducing organic prior reagent kerosene rich electrolytes to electrowinnin g Wastewater treatment of fat, grease, suspended solids Flotation from food, municipal industrial wastefor Dissolved air and streams flotation processes solids, air and grease
For fat, grease, floatable solids It is a plug flow DAF device. For food or industrial processing wastes Drinking water, sludge thickener, ice-cream effluents, paper mill For oil/water separations. System in completely enclosed flotation a process For of petroleum, metal, treatment heavy laundry, food contaminated processing, screen printing, animal feed waters For offshore operation the treated water discharges directly into the sea
For treatment of laundry, food processing waters Vehicle washing effluents, removal of oil, solids, surfactants
outside thefrom impeller, form lm through a disperser a multitude of method, bubbles sizing 700–1500 well diameter. in al This known miner isoil–water processing, utilizedseparation also in (oily the sewage) petrochemical industry, (Zheng and Zhao, for 1993; Bennett, 1988). 2.3. Dissolved air (pressure) flotation (DA F)
Bubbles are formed with reduction by aair in pressure water pre-saturated higher of at pressures atmospheric. The supersaturated water forced than is clouds special orifices, and needle-valves trough or of bubbles, 30–100 in diameter, are produced just down-stream thelmconstriction of et al., 1992). (Bratby and Marai s, 1977; Lazaridis ing methodand recognized as acentury separatthen DAF was the of since early 20th particles including: has foundinmany applications • clarification of refinery wastewater, wastewater recla- mation, separation solids and other in drinking water • of treat- ment plants; sludge thickening and separation of biological • flocs;
Fig. 1. The conventional DAF unit, with water recycle to the saturator.
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, algae 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 industrial effluents. It is believed that applications will rapidly expand in the waste treatment in the metallurgical and mining field (Rubio and Tessele, 1997; Tessele et al., 1998; Rubio et al., 1996; Rubio, 1998a,b; Santander et al., 1999; Da Rosa et al., 1999). DAF development has been very rapid in the last decade and many of its earlier limitations are being solved. Table 3 reviews recent important developments in DAF. •
Fig. 2. Continuous nozzle flotation unit.
in turn is discharged into a flotation vessel (similar to dispersed-air conventional machines), develop the to2). a twophase mixture of air and water (Fig. Bubbles are the size 400–800 lm in diameter (Bennett, of etover al., induced 1988). Advantages 1988; nozzle theGopalratnam for air flotationclaimed (IAF) systems, are theunits, following: lower initial costs and energy use because a • single pump provides the mixing and air supply; lower maintenance and longer equipment life • because the unit has no high-speed moving parts to wear out. Applications reported have been exclusively in the petrochemical industry for the separation of o/w emulsions and treatment of oily metal-laden wastewater (Gopalratnam et al., 1988). 3.2. Column flotation
interest Column flotation is stillwith a subject of great growing in a steadily mineral processing number research studies of and industrial applications (Finch,
3. Emerging flotation techniques and processes 3.1. Nozzle flotation (NF)
(an This process uses a gastoaspiration uctorwhich draw air nozzle into recycled ed or an exhauster) water, Table 3 Main developments in dissolved air flotation (modified from Kiuru, 2001) Year 1924
Development 1
generation: Pedersen cells.inThe separation tank and very low product) throughput, 2 m h tank . The ‘‘capture’’ First is shallow by bubbles occurs of particles of the froth an inclined zone aside (floated separation
1960 1970 1990 and 1995
1
Second generatio n (‘‘conve ntional’’): c ells less shallow with higher loading c apacity, 5–7 m h 1 DAF deeper with filters for the treated water. Higher throughput 10–15 m h 1 Third generation: ‘‘Turbulent’’ DAF deep units, high capacity cell > 40 m h . The ‘‘capture’’ zone is now deep horizontal 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. 3. The Microcel flotation column.
Rubinstein, Finch Dobby, 1990). 1995; Infeed the columns used about the mineraland processing area, in 1994; enters one-third the way down from the slurry against aInrising top bubbles swarm of andddescends by athesparger. generate wastewater treatment, column the middle feed enters by top in of the ‘‘concentrate’’ product. developments New external in column technology gas spargers operating with and include without frothers, addition of or surfactant columns baffles and coalescers with internal for oil recovery (Gu and Chiang, the surface-active 1999). In of the presence can be obtained as in and the reagent s micro-bubbles Microcel umn plications col- Ap(Yoon etof al., 1992; flotation Yoon column Luttrell, 1994). in the field of remova waters l inintheproduction oil 1994) (Gebhardt et al., and heavy recovery of metals precipitates (Filippov et al., 2000) have been reported (Fig. 3).
Fig. BAF, bubble Theflotation cham-4.ber’’ device.accelerated flotation or BC, ‘‘bubble
of flotation has BOD, been An edadvanced ASH type report to remove oil, grease, inorapplications 4) etc. BAF (Fig. bubble accelerated flotation concept system uses the contactor–separation with detention times very low in the contactor (Colic et al., Depending bubble generation the 2001). on thenamed authors devices Induced system Air BAF, Vacuum report oflo- tation as BAF , Electr BAF.
3.3. Centrifugal flotation (CF)
3.4. Jet flotation
Thecyclone separator contactor can Thus, be an hydro oris develaandsimple a cylinder. oped. centrifugal field Aeration by occurs either injecting suction), air (or by through flow such as and static mixers or medium nozzles constrictions, (1992), According tohaving Jordan Susko size bubbles 100–1000 lm di- ameters are generated. canet al., be ),(Ye air-sparged hydrocyclone (ASH Thefied classias a centrifugal flotation unitwhereby consists aeration system 1988). It of an air is porous tube and is through sparged a jacketed numerous small bubbles thephase. highsheared into bywall velocity swirl the aqueous flow of Environmental applications ASH flotation been recently reported (Beeby of Nicol and , 1993). have
This cell appears to have and a great for liquid/liquid solid/ liquid separations for potential separations as well as mineral processing (Jameson in and Manlapig, 1991). Its main advantage is its high throughput, high et al.,efficiency and moderate equipment cost (Clayton Harbort 1991; More, consumption with no moving et al., 1994). parts, the jet cell has low The power maintenance costs. consists low cell of and an /contact aeration zone (the downcomer), a bubbledisengagement zone (the tank particle aggrega or te cleaning proper pulptank area) and a or froth (medium forming zone). The bubbles zone (the proper size) formed in this have cell may lm in diameter (Jameson and Manlapig, 100–600 1991; Clayton et al.,recently 1991). Problems with process have to been solved andand its recovery use has accuracy been extended wastewater treatment and solvent extraction liquors (Wyslouzil, 1994) of municipal waters (Yan and Jameson, 2001).
Fig. 5. CAF unit.
3.5. Cavitation air flotation (CAF )
utilizes Cavitation air flotation an aerator disc), which draws ambient air down a shaft (rotating and injects ‘‘micro-bubbles’’ directly the wastewater into 5). However, there is no of (Fig. any knowledge fundame work with this flotation technique. ntal CAF is utilized in the food industry, especially the milk in tanneries to (biological remove suspended industry, paint and solids, fats, BOD oxygen demand ).oxygen CODgreases, demand) and oils, (chemical 4. Applications and advances
foam flotation (Clarke and Wilson, 1983); ion flotaerl, (Nicol et al., 1992; Walkowiak, 1992; Schu €gtion 2000); • adsorbing particulate (colloids or aggregate) flotation (Zabel, 1992; Matis, 1995; Rubio and Tessele, 1997; Zouboulis et al., 1992a,b, 1993, 1997, 2001; McInty re et al., 1982). • ionic flotation (Scorzelli et al., 1999). •
industrial applications Main and of flotation in solvent metallurgy are the recovery mining of and extra liquors losses ction by DAF, column jet (Jameson cell), nkovic’ the , separation flotation of 2001) ions and molybdenum (Mari 1991). manganese DAF ions by (Krofta, Yet, it is other, reported there be believed that may not in examples, similar encou other those ntered to industrial fields. A number of papers have recently been published illustrating techniques employed and flotation devices. These can be summarized as following:
4.2. Precipitate flotation
4.1. Removal of ions
1959, Sebba, who established ionic in microposed the use proof or colloidal gas flotation aphrons They f o ams or simply micro-gas dispersions. are dispersions liquids introduces formed withathe use of gases in which of ator venturi genera gas to a circulating solution in a of rfactant region high su pressur e low (Sebba, 1962; Ciriello et al., velocity and 1982). This produces small bubbles, which range in from 10 toDespite 50very size lm and provide a large amount of area. the studies potential, surfa ce no industrial known and are mainly related lications are app scale (Kommlapati et al., to laboratory and pilot 1996; Save and Pangarkar, 1994).
water, oneproblems removal of Theimportant ionsinfrom of the environmental issues most today, technically possible through various flotation i s (Zabel, 1992; Lazaridis et al., techniques 1992; Principal removal b; Mati Rubio, 1998a, s, 1995). methods are: precipitate flotation (Silva et al., 1993; Stalidis et • al., 1989a,b; Lemlich, 1972; Pinfold, 1972; Mummallah and Wilson, 1981); gas • aphrons flotation or colloidal gas aphrons (CGA );
is the formation of a This prec tate its the based ionic on species, using a suitable ipi- process of subsequent removal attachment reagent, and by ‘‘concentrate’’ to air bubbles form a flotation (Huang andsolution Lemlich, 1972). Depending Liu, to 1999; on the metal concentration, the precipitation droxide formation may proceed via metal hyor as (sulfide, carbonate, a saltInwith a suitable anion remova etc.). l, precipitation of anion the case should cation. proceed through addition of a metal 4.3. Gas aphrons flotation or colloidal gas aphrons (CGA)
4.4. Foam separation or foam
flotation
method similar toorion This flotation but uses proper frother an excess of a issurfactant asubstances to stable foam. Here the removed a produce molecular, colloidal, crystalline, may be ionic or or cellular nature,to but, cases, they must in attach in all selectively the interfaces air– liquid (of (Clarke or of bubbles) and Wilson,as 1983). foams Some authors denote the accurately separation fractionation since this term descri besfoam the removal the surface active carrier compounds of in foam column. Hundreds solution in a of parpers been reported ve foam/flotation on or ha and pilot fractionation at laboratory industrial applications are believed exist.and some toscale
adsorbs the surface the rising tant at providing air bubbles, interface an of for ion thereby pairing tively collect the complex. to selecgold (1999), studied Scorzelli et al. dodecylof and Cd ions sulfatethe asremoval collector the using sodium ionic strength Na SO ), frothers effect of (NaCl and 2 4 finding and surface tension was evaluMain ated. are the high removal obtained for a metal of 1:2(98% with 0.1% v/v isopropanolcollector ratio frother) strength (>10 3 the negative effect the and of high M). 5. Up coming techniques and advances
4.5. Adsorbing colloid flotation
5.1. Aggregation-DAF
of the This method on involves the removal metalasion by adsorption a precipitate (coagula) acting a The loaded carrier is then floated, usually carrier. assisted with suitable ‘‘collector’’ surfactant. a The main carriers used with have been ferric or aluminum oleate help of sodium hydroxides collected or lauryl sulphate (Stalidisthe 1989a, et al., b). remove molybdenum recent process A DAF to ions in Chile employs this principle with the FeðOHÞ3 carrier This and sodium as as the molybdenum method oleate has been collector beenarating reported. has successful the molybdenum ions from in sepCu–Mo te The filtrates and meeting Chilean concentra emission standards. interesting feature is that uses this plant a ‘‘rougher’’ stage to remove first the suspended solids oleate) the and and then Mo calcium ions in aions (as calcium ‘‘cleaner’’ about 5. Sodium is at pH hydrophobicity addedstage andoleate process also to enhance kinetics.
Precipitation, coagulation and flocculation have utilized in stages first to destabilize highly soluble been precipitates. Then, ions to form colloidal particles or la-with tion the is used to enhance particle size and coagu finally, polymer to formhas stable, big and technique been reported hydrophobic flocs. This remove and Se processing ions to Hg, As from cyanidation streams cuits (Tessele et of gold cir- NaDTC, sodium Here al., DAF. 1998) usingwas dithiocarbamate, employed as LaCl3 precipi tant, or (BuckFeClthe were the coagulants and Bufloc 3 (> man), flocculant. Almost complete removal solution was reported 98%) of the metal ions from using DAF. Process and efficiency depended system on the interfacial chemistry, aggregation solution phenomena operating parameters. and DAF Main stages are the fol- lowing: 1. ions + precipitant ¼ colloidal precipitate (3–10 lm), 2. colloidal precipitate + flocculant ¼ flocs ( 1–3 mm), 3. flocs + micro (5–150 lm) and mid-sized bubbles (200– 600 lm) ¼ flotation by DAF and/or columns (nonturbulent regimes) .
4.6. Ion flotation
involves thespecies) removal This of ions igendmethod inactive (coll orasurface by species transport surfactant to froth as to a of counter-ion opposite charge. the surfactants perform the Here and collector, facilitating the dual role of frother onto the surfacefor colligen adsorption of the d species of an airflotation bubble. Inof some cases, a ion ligand-activator the the metal followed by a suitable surfactant has been necessary 1992; Nicol et al., 1992; Galvin et al., (Walkowiak, 1994). Despite studies performed atyears laboratory pilot scale,many only during the last few have and method industrial scale applications of this in ported (Zouboulis et al., 1992a,b; et al., been reNicol 1992). recovery scheme based onliquor, A novelhasgold ion been developed. Heapwith leach flotation containing gold cyanide is reacted a suitable surfactant and et al., 1994). Thesparged surfac- using compressed air (Galvin
5.2. Adsorbing (or sorbing) particulate flotation-APF or simply carrier flotation-CF
The basis flotation of the adsorptive (or sorbing) carrier) is thefloatable uptake of particles. cation,particulate anion or (or organic by This readily activation resembles oxide flotation by metal ions, adsorption sulfide depression anions byEssenorAPF of isemploying a variant collectors or frothers. tially, the adsorbing colloid of flotation process, particles as carrier-sorbing (absor b- ing and/or adsorbing) material for theofmetal the ion. sorbing The keycarrier to good process the selection is a having a high surface area and a high reactivity with the pollutant to be removed and it should float readily. mineral particle, The The riccarrier can be coal a or a resin, activated a by-product. polyme use organisms as sorbing materials of micro(biosorption or bio-
Table 4 Main reported studies of APF
Adsorbing material
Contaminants
Coal jigging tailings Zeolites Zeolites Pyrite Red mud Dolomite Fly ash Exchange resin Hydroxyapatite Activated coal Coal jigging tailings Barite Clay (hydrotalcite)
Ni, Cu, Zn Ni, Cu, Zn Hg, As, Se Cu, As Cu Pb
Autho r(s)
Ni
Cu Cd Dye (Rodamine B) Oil
Emulsified oil
þ6
Chromate, Cr
ions
sorptive has(Zouboulis been proposed may be alternative et al.,and 2001). an- otherflotation) from diluted of Cu, Zn and Ni The removal solutions by, APF was studied at laboratory and pilot scale (F eris 2001). The sorbing used was a coal washing tailing material from a coal industry from south of Brazil and the flotation process applied was DAF. Best results (> 95% removal) showed that the residual ions concentration is below the standards limits dictated by the local legislation. Table 4 summarizes main reported studies in this subject. 5.3. Column flotation to remove ions
A modified Microcel column et improved al., 1992) feed entering the with by was cell (Yoon topto(to separation) studied float loaded solid/liquid (with metal ions) FeðOHÞ3 precipitates as a function (Souza Rubio, unpublished results). The of pH and wat recyclingthe procedure column employs er treated to pumping flow fluid generate bubbles. Thus, by valve, through venturi drawn needle a or air is into the pipe and bubbles are pro- duced. size of bubbles be modulated addition with The of the a surfactantcan . Results showed best separation was that medium d oleate when optimizing pH, addition of obtaine sodium (as ‘‘collector’’) and operating ers conditioning, flow rates, parameters, among othetc. Recently, Filippov et al. (2000) studied the inter actions between superficial feed and gas velocities and recycling pulp flow rate on bubble size distribution and its effect on Mo-precip itate flotation. They conclude that the precipitate flotation effectiveness in columns is related to floc stability under turbulence created by the swarming of rising bubbles. 5.4. Dissolved air flotation
DAF of iron lower at hydroxidethanprecipitates pressures 3 atm, using working modified flotation
F eris (2001) Rubio and Tessele (1997) Tessele et al. (1998) Zouboulis et al. (1992a,b, 1993) Zouboulis et al. (1993) Zouboulis et al. (1993) Zouboulis et al. (1993) Duyvesteyn and Doyle (1995) Zouboulis et al. (1997) F eris et al. (1999) Santander and Rubio (1998) Santander and Rubio (1998) Lazaridis et al. (2001)
the Laborato collection of coagula, units to improve was studied de Tecnologia at the rio fragile eris and Rubio, 1999). e Mineral Ambiental (F DAF flotation was studied as a function Conventional of saturation pressur the absence and presence e in of surfactants surfactants, rator. satuin the Without pressure required the minimum satur ation for DAF to be 3 atm. But, by lowering occur was surface found to saturator, the air/water tension in ethe DAF pressur was possible at a saturation of 2atm. occur to in This behavior was found both batch operation and pilot reDAF and almost covery complete precipitates of the tests the attained. are terms Results in of was explained has be transferred minimum ‘‘energy’’ which to to bubbles cavity the phase liquid to form by a phenomenon. Since the satur stage accounts ation for about 50% of the energy costs and ng in operati considering the surfactant, lowtotal cost involved this option the appears to have a great potential. A very important feature only reported for DAF, concerns with the mechanisms of bubble/particle (aggregates) interactions other than the common ad hesion through hydrophobic forces (Fig. 6). Thus, apart from particles/bubbles collisions and adhesion, in DAF, part of the dissolved air in water, which does not convert into bubbles in the nozzle, remains in solution and ‘‘nucleate’’ at the particle surface (Solari and Gochin, 1992). This mechanism is independent on surface hydrophobicity and allows flotation of hydrophilic particles. More, bubble entrapment into flocs or coagula and aggregate entrainment are by the rising bubbles mechanisms, which make separation easier. This explains the fact that in DAF, no collector or froth is required but a thick and stable float layer is formed. Results show high clarification effluents are obtained in DAF. However, a major disadvantage is that rapid air bubble levitation speed is not attainable and hydraulic loadings are low (this is
dictated by the limiting process capacity.
Henry’s
law)
reducing
and
Fig. 6. Bubble-part icle DAF: (a) mechanisms(d)inbubbles particle–bu bble collision and adhesion; (b) bubble format ion at particle surface; (c) microentrainment by aggregates. bubble entrapment in aggregates;
5.5. Separation of oils and organic compounds by flotation
organic bearing watersemulsions such as The flotation of spills on used water,in oily sewagefields or oil-in-water oil number been has for a of various commonlyMost usedof inthe theresearch mining decades but isgynot industries. and/or metallur studies the effect separahave on the of oil from water addressed of tion oil concentration, type and agents o/w of for concentration destabilizing technique emulsions the and type of flotation to be employed (Bennett, 1988). the mining–metallurgical industry,are residual In wastewaters commonly discharged waters oily ncontai ing flotation chemicals and solvent with extraction reagent surface waters contaminated free s, wasted oil and process waters containing oil spills (Pushkarev et al., 1983). Oil in water may be dispersed, emulsified or in solution in water in concentrations up to 1000 ppm. In particular, the presence of emulsified oil in water droplets around 50 lm in size causes problems in pha se separation by conventional techniques (oil/water gravity separation, DAF). The flotation separation of very fine oil droplets (2–30 lm) is even more complicated and usually requires fine bubbles, quiescent hydrodynamic conditions in the cell separation zone or emulsion breakers prior to flotation (Gopalratnam et al., 1988). This is due to collection and adhesion factors, which makes the process very slow, especially when, treating high flowrates. IAF and DAF, have been used extensively in the removal of stable oily emulsions (Bennett, 1988; Strick-
land, 1980; Belhateche, 1995). IAF utilizes bubbles between 40–1000 lm in size and turbulent hydrodynamic conditions. The process has low retention times,
employs <5 normally min. Conversely, DAF bubbles (30–100 lm), and quiescent regimes. microbecause retention times are higher However, process is inefficient when treating(20–60 high min), effluent volume this s and high flow-rates. The Jameson cell, column flotation with CGA (prereagentized gas being bubbles) and conventional now utilized in solvent columns are extraction plants (Readett andareClayton, the devices used the Here flotation in1993). discharge aqueous streams from the solvent extraction–electrowinning recover (SX–EW) plant to ment the the organic phase. liquor lost by entrainaqueous reduce Thus, flotation caninto reduce potential environlosses organic and mental problems. 5.6. Modified jet flotation cell
A laboratory modified jet flotation cell has been in account better our (Fig. 7), to for for a studied oil coalescence and the the decrease droplet in observed amount oftype short in abandoning the conventional ). Thus, (Jameson the slurry the cellcircuit downcomer, enters a cylinder obligating the coalesced bubbles aggregates or oilflocculated the froth layer. units to leave the separation tank by Results show that cell this cell ac-oilcurate is more than yielding the conventional high re- moval levels. values and treated water with low oil Thus, 1with highly emulsified feeds having constant up to 603 lgreater of mg the removal was almost at oil, or than 80% regardless of theof initial oilcell content. It is belie that this has at a great ved potential for oil or type high organic flotation solvent removal 1 throughput values (>600 m d ).
Fig. 9. Effect of flocculant concentration on oil centrifugal 1 flotation1 performance ð33:3 l min Þ. Feed oil concentration ¼ 152mg l .
the very Main near future placed onare offshore platforms in characteristics the separation very Brazil. in low residence time (high throughput), high efficiency low water split. However, the flotation and efficiency the ce. degree of finder on flocculation(Fig. and 9) on depends the vortexmainly clearan Fig. 7. Modified jet flotation pilot unit (Santander and Rubio, 1997, 1998).
5.8. The FF-flocculation-flotation process
turbulent on-line flocculation system A new has been developed with assisted air bubbles at LTM yielding entrained aerated flocs (flocs with and bubbles). These flocs, which rapidly e‘‘float’’, ntrappedare
5.7. Centrifugal flotation cell
(coalesced) Thesions of flocculated oil separation emulflotation machine (Fig. 8) in a centrifugal has been recently performed on a pilot scale in the rio de e Ambiental Laborato Tecnologia Mineral rsidade Unive (LTM), Federal do Rio Grande do Sul, Brazil. The device will be
Fig. 8. The LTM-centrifugal flotation
device.
Fig. 10. FF-flocculation-flotation device.
molecular formed only in the ofand high weight polymers bubbles under and presence high flocculator shearing the 10). in (Fig. The air excess air flotation tank by leaves the (a centrifuge) the after very short residence the second top and flocs float aerated flocs are large s). times (within The units (some in diameter) having an density (Rubio, extremely lowmillimeters 2001).
5.9. The
‘‘multibubble’’
flotation column
Recently, F eris et al. (2001) reported data on the of colloidal ferricgenerated hydroxideinby removal in a column with bubbles anflotation static (medium-sized bubbles) and micro-bubbles mixer generated as in DAF. These authors named this column column’’. device flotation a ‘‘multibubble microbubble Using this column they modified reported better were results as compared to DAF alone. Gains reported a better air-to-solids ratio bubble surface (higher flux), improved process kinetics and improved process 11 shows some details Fig. of throughput. this flotation device. 6. Miscellaneous separations 6.1. Micro-organisms
that It has for many years, bacteriabeen can demonstrated, by froth be readily concentrated or foam
flotation andhave since that not time number of invest igators confirmed only athe flotation of bacteria, but of algae and micro-organisms other (Smith, 1989;is €gerl, Schu 2000). Alga removal by flotation other treatment a to becoming good alternative such environments, methods ingrow tropical causing problems in the algae great at a countries. rateIn reser Furthermore, proliferation of voirs. water all values algae in maturation ponds often results in solids license limits exceeding suspended EPA for elevated and pH values. Also, disof (especially blue-green algae) laden ef-charge fluents canalgae also ated toxins associ cause possible release of to their surface and ground waters. The jet flotation process for alga removal reported Jameson (2001) appears toforbe the by Yan and an interesting application of municipal flotation bearing waters. Alga treatment algae of cells such as matur cystis sp. Microation commonly that in ponds areoccur usually very small wastewater size (3–7 lm) and to induce efficient alga cell–air in bubble aggrega greater than tes of polymer 10 lm are in flocculants size arecontact, required. Cationic nonionic anionic be effective, found to while or polymers are not.surface Differentcharact types eristics. of algae The appear to share common same was flocculant found to in be effective flocculating very different types and forms of Cell alga Anabaena ). cells (e.g., cystis , Jameson Micro wasremovi shown to be of technology algae ng phosphorus simultaneously and capable enabling the continued use of maturation ponds and upgrades of existing to costly provides an treatment alternativeplants wastewater .
Fig. 11. The ‘‘multibubble ’’ flotation
column.
6.2. Proteins
include sodium lignin sulfonate, tannic acid, and Aero- sol OT (Shibata et al., 1996).
Various other non-fatty organic materials, such as soluble proteins derived from soybean processing, be removed from water can by DAF flotation after itation and flocculation (Schneider et al., 1995). precipprotein removed Soluble by this process from aqueous streams from soybean plants can was te potentially be used as supplemental animal feed. The the basis protein separation flotation for by is aggregation the macrom ecules with inorganic olof with microsalts and/or polymers and flotation bubbles. Problems ariseagents when or proteins cdispersing on- tain associated de-foaming short molecules that modify the surface properties of enhancing protein their hydrophilic character aggregates and reducing adhesion. bubble-particle
6.4. Deinking
6.3. Plastics
6.5. Soil washing
plasticswaste has Modern industrial and home created an environmental needuse to of recycle the number plastics different types. of a of Most of plastics, commonly used such as polyvinyl chloride, bonates, polycar and polyacetal, polypropylene ether without are naturally hy- drophobic and are readily floated addition of a flotation collector. Thus, process selectivity task. is a difficult vary in their hydropho However, plastics bicities surface tensions have been and critical theirusing lored exp surface-active reagents. Thus, their can be modulated by use lities suitable floatabi of depressants, which
being studied Flotation is non-volatile for removalcompounds of toxic relatively hydrophobic and heavy such as oil, PAH, or PCB from contaminated parameters the The of the basicand of with soils. investigated process haveeffects been compared washing, advantages the soil and of flotation demonstrated (Ososkov and Kebbekus, 1997). Some limited reportsof intoxic the literature point organics out that significant fraction hydrophobic a be removed from contaminated may soil by flotation. ever, investigations Howno systematic on of these substances from soil by flotation removal have been reported. Hydrophobic non-volatile organic compounds poorly adsorbed are by soil particles, which are primarily
has been used, years, Flotation for a number of of the in for paper recycling. Most paper deinking es are based on removal using surfactants and studi ink e bearing salts. (1999) Hardi calcium Finch and the main flotation machines and reviewed have techniques employed area, showing in this and discussing proaches used a variety of apto optimize the characteristics of such flotation systems.
hydrophilic. These contaminants are mainly trapped space. Trapped compounds can by be the- ported in soil pore trans soil/water slurry the surface to of es during flotation. organic matter bubbl Soil or hydroph in soil matrix flotation adsorb some impurities obicobic pollutants. However, may of hydroph remove only part of the adsorbed pollutants. 6.6. Removal of radioactive nuclides from soils
radioactive nuclides from Flotation of inate d soils and coral sand bycolumn both contam air flotation and conventional-induced studied flotation evaluated (Misrait et has been and al., 1995;toMisra et al., 1996). In such separations is desired (non-float) produce a very clean material and concentrate that contains most the a of but is still recover, a low-level radionuclides, - rial. TheThus, goalthe low-grade buttoaradioactive mate is high co trate. bulk ncenof of material dispose in a waste repository is much reduced. 7. Final remarks
the depends Since flotation on multiple con- nected factors, many considerations should inter be account when selecting taken into a flotation device and its
Some and the capacity ng:be employed. aretechniques the followito of these factors 1 3 3 1 • The wastewater flow-rate (m h , m s 1 or 3 ) the equipment throughput. m day and Table shows examples some reported values 5 of for hydraulic loading Theses values are flotation related the bubble size distribution generated to in the different flo- tation devices (see Figs. 12and 13). pollutants, whether free, nature • The ed, of inorganic-organic volatile, or mixtures. complex Their conceneffluents and standard tration in in emissions. bebestremoved. of aggregates The natutal re studies • rimendefinetothe way of to Expe willwhether remove the lutants, the form polin precipi coagula, flocs, sublate (metal-collector tates, complexes), or ad Table 5 Averaged hydraulic loading values reported for some flotation devices operating in mineral processing ( ) and wastewater treatment 1
Equipment
Hydraulic loading (m h )
DAF IAF (induced air) Column cell Jameson (jet) cell ASH (Miller cyclone) FF-flocculation flotation BAC
7–40 36–430 50–360 70–350 500–720 140–2160 (oil removal) 1.5–500
Fig. 12. Flotation operating with ‘‘micro-bubbles’’. EF ¼ Electroflotation; GA ¼ Gas aphrons; CAF ¼ Cavitation air techniques/devices flotation; DAF ¼ Dissolved air flotation.
Fig. 13. Flotation techniques/devices operating with ‘‘medium sized’’ (200–800 lm) and ‘‘macrobubbles’’ (IAF
>
800 lm).
sorbed on coagula a carrier.withstand Flocs andshear particulate carriers not and with may be and separated in flotation devices operating high turbulence (cenis more trifugal, jet). DAF amenable for separation of ated coagula flocsor is precipitates. also a good Nevertheless, DAF of aerand fast alternative. The need for collectors, optimal pH, redox • tions, residence time, air-to-solids ratio, condiair bubble surface flux, power hold up, lifting of e bubbles, of temperature, density, viscosity, ffect (frothability), surface tensi on interfacial properties aggregates (charge, hydrophobicity). of • Flow-sheet erdesign. Whether a ‘‘rough cleaner’ ’product scheme is of the needed: destiny floated the process water (possible and characteristics, drying, reuse?), filtration economics the process 12and 13 show of . Figs. approximate bubble size ranges, which in various flotation deviceshave and been technireported ques. 8. Conclusions
Flotation is ever increasingly used in waste treatment , especially in the the mining and ry. metallurgical indust Furthermore, introduction new, superior, of flotato tion devices should lead remediation new andindustry bettercontaminated applications waforters of mineral and solids. A experi mineral of flotation ence in cross fertilization flotation treat wastewater ment and in should lead new to and improved procedures in thecal mineral metallurgical industry, the chemi and and petroleum industries domestic wastewater and treatment .
Acknowledgements
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