Treatment of semiconductor wastewater by dissolved air flotation

7 Pages·0.089 MB·English

by  Chuang T.C., Huang C.J., Liu J.C.

Treatment of Semiconductor Wastewater by Dissolved Air
Flotation
Tzu-Chieh Chuang1; C. Jane Huang2; and J. C. Liu3
Abstract: Treatment of fluoride-containing wastewater from the semiconductor industry by dissolved air flotation was investigated.
Calcium chloride was added to the wastewater to form calcium fluoride precipitate (CaF ), which was then removed from the aqueous
2
solutionbyflotation.Whenthemolarratio,@Ca21#/@F2#, wasequalto0.5,residualfluorideconcentrationwaslowerthan15mg/L.All
flotation reactions were completed in 5 min. Experimental results indicated that the sodium oleate ~SOl! dosage was the major factor in
determining the removal efficiency of CaF . Effective flotation efficiency ~.95%! of CaF was found in the pH range of 5.5–9.5.
2 2
Flotation efficiency increased slightly with increasing saturation pressure ~3 to 5 kg/cm2! and recycle ratio ~20–40%!. The presence of
sulfateandphosphateresultedinhigherresidualfluorideconcentrationandlowerflotationefficiency.Theinterferencecouldbeovercome
by increasing @Ca21#@F2# to 1.0. However, a higher dosage of SOl was needed,An ionic strength higher than 0.1 M did not affect the
removal of CaF , but rendered residual fluoride concentrations higher.
2
DOI: 10.1061/~ASCE!0733-9372~2002!128:10~974!
CEDatabasekeywords: Fluoride;Wastewater; Flotation;Wastewater treatment.
Introduction and higher aspect ratios, and increased wafer size ~12 in.! are
The semiconductor industry has recently experienced rapid expectedtocontributetonewdemandsinthecleaningsteps,and
growth at an unprecedented rate in Taiwan. The industry gener- the supply of water will become even more difficult.
ated a gross production value of 16 billion U.S. dollars in 1999, Hydrofluoricacid~HF!iswidelyusedtoremovesilicondiox-
accounting for 6% of the national gross national product. This ide from wafers and to clean the quartz were used in the manu-
expansionhascausedgreatconcernduetothelargequantitiesof facturing process. Spent concentrated HF is usually recycled and
waterrequiredforsemiconductorwafermanufacturing.Newres- reused in other industries ~Gilles and Loehr 1994!. However, a
ervoirs are being built and a major upgrade of water treatment large amount of fluoride-containing wastewater is generated in
plants is now being considered in the science-based industrial rinsing and cleansing operations. Wastewater generated from a
park in Hsinchu County. Unit processes of a semiconductor fab- semiconductor fabrication facility is generally segregated into
ricationfacilityincludephotolithography,oxidation,etching,dop- three streams: fluoride containing, acid base, and chemical me-
ing, and planarization. Since requested cleanness of a wafer is chanical polishing. Amount of fluoride-containing wastewater
high, ultrapure water is mainly used in rinsing the silicon wafers ranges from 350 to 700 m3/d with varying concentrations, from
duringfabrication.Alocalsurveyindicatesthattheusageofwater 50 to 1,000 mg/L, for a typical 8 in. semiconductor fabrication
ranges from 10 to 12 gal/in.2 for typical 8 in. wafer fabrication facility.Adding calcium salts to form CaF2 precipitate, followed
facilities. The number is generally lower than that in U.S. chip by the addition of polymeric flocculent to facilitate CaF2 sludge
manufactures ~DeGenova and Shadman 1997!. It implies that sedimentation mainly treats the wastewater in most semiconduc-
local manufacturers are facing higher pressure for water conser- tor fabrication facilities in Taiwan. Though the current
vation and wastewater reuse. Currently more than 40% of water precipitation–sedimentation process treats fluoride-containing
and more than 70% of process wastewater are recycled among wastewater effectively, there exist some problems to be solved.
Taiwanesesemiconductormanufacturers.Theintroductionofnew Solid–liquid separation may not be effective owing to the very
materialssuchaslow-kdielectrics,shrinkinglinewidth~0.15mm! fineCaF particles,andfluorideisredissolvedoncetheeffluentis
2
combined with other streams of wastewater containing waste
1Dept. of Chemical Engineering, National Taiwan Univ. of Science acids. In addition, land acquisition is difficult and costly in the
andTechnology,Taipei,43KeelungRd.,Section4,106Taiwan. faceofrapidexpansionofthesemiconductorindustryinTaiwan.
2Dept. of Chemical Engineering, National Taiwan Univ. of Science Besides,itisveryhardtofindlandfillsiteforthefinaldisposalof
andTechnology,43KeelungRd.,Section4,Taipei,106Taiwan. CaF sludge.
2
3Dept. of Chemical Engineering, National Taiwan Univ. of Science For pollution prevention and waste minimization purposes,
andTechnology, 43 Keelung Rd., Section 4,Taipei, 106Taiwan ~corre- local plant managers and engineers have practiced wastewater
spondingauthor!.E-mail:[email protected] segregation, equipment modification, and wastewater reuse. To
Note. Associate Editor: Robert G. Arnold. Discussion open until
minimize CaF sludge generation, a crystallization process has
March 1, 2003. Separate discussions must be submitted for individual 2
beendevelopedfortreatingfluoride-containingwastewaterfroma
papers.Toextendtheclosingdatebyonemonth,awrittenrequestmust
semiconductor manufacturer ~Giesen etal. 1998!. In addition,
befiledwiththeASCEManagingEditor.Themanuscriptforthispaper
process combining microbiological and chemical reaction has
was submitted for review and possible publication on August 20, 2001;
approved on January 21, 2002. This paper is part of the Journal of been proposed for mixed acid drainage containing fluoride~Goto
Environmental Engineering, Vol. 128, No. 10, October 1, 2002. andYamasaki1999!.Analternativemethod,currentlyunderprac-
©ASCE,ISSN0733-9372/2002/10-974–980/$8.001$.50perpage. tical application, is to use Al salt as a coagulant and adsorbent,
andreclaimitafterwards~ToyodaandTaira2000!.Wehavedem-
onstrated that dispersed air flotation ~DiAF! utilizing calcium
chloridecantreatfluoride-containingwastewaterfromasemicon-
ductor manufacturer satisfactorily ~Huang and Liu 1999!. As an
extensionofourpreviouswork,thisstudyaimstoinvestigatethe
application of dissolved air flotation ~DAF! for the treatment of
fluoride-containing wastewater. The basic difference between
DiAF and DAF is in the method of generating air bubbles in the
flotation column. Air is supplied directly to the bottom of the
columnthroughaspargerintheDiAF.IntheDAFsystem,some
fractionofwastewaterisrecycledandpressurized.Airisreleased
into the column as the pressurized wastewater flows into the flo-
tationcolumnunderatmosphericpressure.Inaddition,itisnoted
that both the collector and frother are needed in the DiAF. For
instance, sodium dodecyl sulfate ~SDS! plays both roles in the
DiAF of fluoride-containing wastewater ~Huang and Liu 1999!.
However,onlythecollectorisrequiredintheDAF.Thatexplains
why DAF is more popular in industrial applications. The advan-
tage of DAF over DiAF is probably due to the fact that air
bubblesaremoreconcentratedwithasmallersizeinDAF~Matis
and Mavros 1991!. The high air/solid ratio thus results in the
better solid/liquid separation efficiency.
DAFhasbeenwidelyutilizedinvariousindustries.Ithasbeen Fig.1. Bench-scaleapparatusofdissolvedairflotation
extensively used in the mineral industry for separation and puri-
fyingpurposes.Itiseffectiveinwatertreatment,especiallyinthe
removal of algae, color, and turbidity ~Edzwald etal. 1992; Materials and Methods
Edzwald1995;Matsuietal.1998!.Ithasalsobeenappliedinthe
treatmentofwastewaterssuchas,soybeanproteinprocessingand Wastewaterwassampledfromanintegratedcircuit~IC!manufac-
bakery ~Schneider etal. 1995; Liu and Lien 2001!. Compared turer. In contains 760 mg/L of fluoride, 166.8 mg/L of nitrate,
withotherwastewatertreatmentunits,DAFpossessesadvantages 0.28 mg/Lof calcium, and traces of sulfate.The suspended solid
of~MalleyJr.andEdzwald1991;LinandHuang1994;Ferguson concentrationwasverylow~;1.5mg/L!.Therefore,asuspended
etal. 1995!: solid was presumed to be solely a CaF precipitate unless other-
2
wise defined. Wastewater was purged with N gas for 30 min to
1. Flexible and effective in treating wastewater with a wide 2
prevent carbonate interference in the precipitation reaction. It
range of concentration;
should be pointed out that it might be more practical to process
2. Rapid operation and small space requirement due to rela-
full scale without striping inorganic carbon, since it will be in-
tively higher hydraulic loading;
cluded in the water during pressurization and flotation. A mea-
3. Smallamountofsludgegeneratedwithhighersolidcontent;
sured amount of CaCl ~Merck, 99%!, NaNO ~Merck, 99%!,
and 2 3
Na SO ~Merck,99%!,orKH PO ~Acros,99%!,andwastewater
4. Low-energy input and moderate cost. 2 4 2 4
were added to a 1 L volumetric flask, placed on a stirrer, and
Mineral flotation is a well-established commercial technique.
stirredfor30mintoallowtheprecipitationreactiontoproceedto
The sparingly soluble calcium minerals such as calcite, fluorite
completion before being subject to flotation. A fixed amount of
(CaF2), apatite,andscheelitearelargelyconcentratedbyflotation SOl ~J. T. Baker! stock solution ~5000 mg/L! was added as a
processes. For effective separation, selective adsorption of the collector.AftervaluesofpHwereadjustedby1NNaOH~Acros,
collectorsatmineral/waterinterfacesisessential.Oneofthemost 98%! and 1 N HNO ~Janssen, 68%!, distilled water was used to
widely used collectors is sodium oleate ~SOl! that floats fluorite fill the flask to the m3 ark. Initial suspended solid concentrations
preferentiallyfromasolutioncontainingcalciteandfluorite~Rao were measured. Wastewater was then transferred to the flotation
etal. 1991a!.Therefore, the mechanism of the adsorption of SOl column. Suspended solid concentration was measured using the
on natural and synthetic fluorite has been extensively studied, modified method in which the membrane filter with a smaller
mainly because of the important role of adsorption in flotation pore size ~0.2 mm, MFS! was used to prevent the very fine CaF
2
~Raoetal.1991a;FreeandMiller1996;Mielczarskietal.1999!. particles from flowing through the filter ~Huang and Liu 1999!.
Ithastobepointedoutthatthespeciesandioncompositioninthe All measurements were triplicated and the average value was
aqueous phase affect the fluorite separation behaviors signifi- taken. The fluoride concentration was measured when the flota-
cantly ~Hicyilmaz etal. 1997; Mielczarski etal. 1999!. On the tion reaction was completed. Ion chromatography ~Dionex, DX-
premisethatthedifferencesinsolutionchemistrybetweenfluorite 100! equipped with a column ~IonPac AS4A-SC! and an auto
ore suspension and CaF -containing wastewater can be well de- sampler~SpectraAS1000!wasusedtodeterminefluorideconcen-
2
lineated,DAFappearstobeanexcellentalternativetechniquefor tration.
Abench-scale, batch-type apparatus was used in the DAF ex-
treating the fluoride-containing wastewater.The major objectives
periments~Fig.1!.Theflotationcolumnismadeofacrylicwitha
ofthecurrentworkweretostudythefeasibilityofutilizingDAF
heightof60cmandaninnerradiusof8cm.Thefoamdischarge
in treating fluoride-containing wastewater, and to investigate im-
port is 24 cm, and the sampling port is 5 cm from the bottom of
portant parameters in the design and operation of DAF.
the column.Air is pumped by the compressor into the 3 L satu-
Fig.2. Effectofmolarratio,@Ca21#/@F2#, ontreatmentefficiency Fig.3. EffectofNaOlconcentrationontreatmentefficiency
rator equipped with a release valve that automatically vents at a
pressureof7.5kg/cm2. Thecompressorwasturnedoffwhenthe of calcium-sodium oleate micelles ~Mielczarski etal. 1999!. The
saturator reached a designated pressure.At least 30 min of satu- advantage of using high @Ca21#/@F2# in treating fluoride-
rationtimewasallowedbeforeflotationexperiments.Therecycle containing wastewater, as practiced among many local IC manu-
ratiowasadjustedafterthewastewaterwasplacedintheflotation facturers,isthattheresidualfluoridecanbemuchlowerthanthe
column. The recycled ratio is defined as the amount of pressur- effluent standard ~15 mg/L!. However, possible drawbacks, as
izedwaterflowsintothecolumndividedbytheamountofwaste- evidenced in our work, include higher cost of chemicals and
water. A sample was taken at certain time intervals during the largeramountofsludgegenerated.Carefulassessmentandcalcu-
preliminaryflotationexperiments.Flotationtimewasthenkeptat lations should first be considered. As for the current study,
5minforfollowingexperimentssincethekineticstudyshoweda @Ca21#/@F2# was chosen as 0.5 for following experiments.
negligibleincrementinflotationefficiencywithalongerflotation
time. When the reaction was completed, the fluoride concentra-
EffectofSodiumOleateConcentration
tionandthesuspendedsolidconcentrationintheremainingsolu-
tion were then measured. Fig. 3 demonstrates the effect of SOl concentration on the treat-
mentefficiency.Residualfluorideconcentrationrangedfrom10.3
to 10.9 and was not affected by SOl. This implied that most
Results and Discussion calcium had reacted with fluoride to form CaF precipitate, and
2
little remained in the solution to consume oleate. However, SOl
concentration significantly affected flotation efficiency. The re-
Effectof Ca2¿ (cid:213) F(cid:192)
(cid:134) (cid:135) (cid:134) (cid:135) moval of CaF was 6.2% in the absence of SOl. The removal
2
Fig.2demonstratestheeffectofmolarratio,@Ca21#/@F2#, onthe efficiencies increased with SOl concentration and reached 97.1%
treatment efficiency. Experiments were conducted under condi- ontheadditionof75mg/LofSOl.Asananioniccollector,SOlis
tionsthatthemixingtimeforCaF toprecipitatewas30min,SOl widely used in the mineral industry in purifying fluorite. It has
2
of 150 mg/L, pH value at 5.560.1, saturation pressure at 4 been indicated that the adsorption of oleate on fluorite surfaces
kg/cm2, and recycle ratio of 30%. When @Ca21#/@F2# was 0.5, involves the formation of monolayer at low oleate concentration
residual fluoride concentration was 10.6 mg/L, and 98.8% of duetochemisorption,followedbytheformationofmultilayersof
CaF wasremoved.Theresidualfluorideconcentrationdecreased sodium dioleate precipitate at higher oleate concentration ~Rao
2
toastablevalue~;5.0–6.0mg/L!as@Ca21#/@F2# increasedfur- etal. 1991b; Mielczarski etal. 1999!. The hydrophobicity of
ther. However, the removal efficiency of CaF deteriorated when CaF surfaces is enhanced when oleate is adsorbed ~Hicyilmaz
2 2
@Ca21#/@F2# was higher than 1.0. It has been pointed that the etal. 1997!. Driven mainly by hydrophobic interaction, hydro-
relative kinetics of the fluorite dissolution and the oleate interac- phobic particle-bubble attachment is always accompanied by the
tion with calcium on the mineral surface and in the solution are rupture of the film of vicinal water on surfaces, by direct contact
the key factors which govern the formation of adsorbed layer betweenparticleandbubbleandbytheformationofathreephase
~FreeandMiller1996!.Calciumhigherthanastoichiometriccon- contact boundary ~Lu 1991!. This is the mechanism of how SOl
centrationhasagreatinfluenceontheinterfacialreactions.Itmay plays the role of an effective collector in the flotation CaF . The
2
react with oleate to form a calcium oleate complex, or lower the concentrationofSOlwassetat75mg/Lforthefollowingexperi-
realconcentration~activity!ofoleateionsasaresultofformation ments.
Table1. EffectofPressureonDissolvedAirFlotationEfficiency
P53 kg/cm2 P54 kg/cm2 P55 kg/cm2
RemovalofSS~%! 95.7 97.0 98.0
Residualfluoride~mg/L! 10.2 10.8 10.3
Note:@Ca21#/@F2#50.5,@NaOl#575mg/L,pH55.560.1,recycleratio
530%.
EffectofPressureandRecycleRatio
The pressure effect was examined under the same experimental
conditionsastheformerrunsexceptthattheoleateconcentration
was chosen at 75 mg/L ~Table 1!. Removal of CaF was 95.7%
2
when the pressure was 3 kg/cm2. Slightly higher removal effi-
ciency ~97.0%! was found when the pressure was 4 kg/cm2. It
increased to 98.0% when the pressure was elevated to 5 kg/cm2.
It was concluded that effective treatment efficiency could be
achieved within this pressure range, and an increase in pressure
had some positive effect on the removal efficiency of CaF . The
2
quantity of an air bubble released in the flotation column is de-
terminedbypressureutilized,accordingtoHenry’slaw.Whenthe
pressure is too low, the air/solid ratio may be too low to bring
aboutgoodseparationefficiency.Moreover,thesizeofairbubble
is also affected by pressure, as having been indicated that the
higher the pressure, the finer the air bubble ~Edzwald 1995!. Fig.4. EffectofpHontreatmentefficiency
Thoughthebubblesizewasnotmeasured,airbubblesseemedto
beconcentratedandveryfineinthepressurerangeexamined,and 5.5and7.5,andincreasedto13.3mg/LunderpHof9.5~Fig.4!.
the up-flow velocity of an air bubble was not too high as well.
This is in agreement with thermodynamic prediction that over
These are possible reasons why satisfactory efficiency was
99% of fluoride precipitates as CaF when the pH is higher than
achieved in the pressure range and why the pressure change had 5.0~HuangandLiu1999!.Thecomp2etitionbetweenhydroxylion
an insignificant effect on the flotation removal of CaF2. and fluoride ion ~Free and Miller 1997; Hicyilmaz etal. 1997!
IntheoperationofDAF,thepressuredoesnotchangemuchas
probably caused the elevated residual fluoride concentration at a
anoperationalparameter.Itrangestypicallyfrom483to585kPa.
pHof9.5.TheremovalofCaF wasbetterthan97.1%inthepH
Theamountofairsuppliedisusuallyadjustedthrougharecycled 2
range. Owing to the affinity of oleate and the surfaces of CaF
ratio ~Edzwald etal. 1992!. Depending on the types of applica- 2
when pH values were higher than 4.0, the oleate can become
tion,therecycledratiovaries.Forexample,itisrelativelylowfor adsorbed without difficulty ~Hicyilmaz etal. 1997; Rao etal.
watertreatmentandisveryhighforsludgethickening.Theremay 1991b!. This is why CaF was effectively removed in the pH
exist an appropriate recycled flow rate, as the separation effi- 2
range.
ciency in DAF deteriorates when the recycled flow rate is either
too high or too low ~Plummer etal. 1995!. The effect of the
recycledratioontheDAFefficiencyoffluoride-containingwaste- EffectofIonicStrength
water was investigated. Removal of CaF was 93.4% when the
2 In the dispersed air flotation of CaF , the removal efficiency de-
recycled ratio was 20% ~Table 2!. The removal efficiency in- 2
creases with increasing ionic strength. It is due to the larger and
creased to 96.7% when the recycled ratio increased to 30%. It
fragile air bubble under high salinity, and the decreased electro-
increasedto99.2%whentherecycledflowratewas40%.Appar-
static adsorption of the anionic collector, such as SDS, on CaF
ently the removal efficiency of CaF2 increased with increasing surfaces ~Lin and Huang 1994; Huang and Liu 1999!.The effec2t
recycled ratio.
of ionic strength on the DAF of CaF was studied under same
2
experimental conditions. It was found that the residual fluoride
EffectofpH concentration was 10.7 mg/L under a background condition
~Table3!.AstheconcentrationofNaNO increasedto0.1M,the
Experiments were conducted to study the pH effect. The 3
residual fluoride concentration increased to 14.6 mg/L. As the
@Ca21#/@F2# was equal to 0.5, SOl concentration of 75 mg/L,
NaNO concentration increased to 0.3 and 0.5 M, the residual
pressure at 5 kg/cm2, and recycle rate at 30%. Results indicated 3
fluoride concentrations were 22.2 and 22.7 mg/L, respectively. It
that residual fluoride concentration was 10.5 mg/L under pH of
demonstratedthattheprecipitationreactionoffluoridewithCaCl
2
Table 2. Effect of Recycle Ratio on Dissolved Air Flotation Effi- Table 3. Effect of Ionic Strength on Dissolved Air Flotation Effi-
ciency ciency
Ratio520% Ratio530% Ratio540% I50.1M I50.3M I50.5M
RemovalofSS~%! 93.4 96.7 99.2 RemovalofSS~%! 95.7 97.0 98.0
Residualfluoride~mg/L! 11.8 10.8 10.2 Residualfluoride~mg/L! 14.6 22.2 22.7
Note: @Ca21#/@F2#50.5, @NaOl#575 mg/L, pH55.560.1, P55 Note: @Ca21#/@F2#50.5, @NaOl#575 mg/L, pH55.560.1, P55
kg/cm2. kg/cm2, recycleratio530%.
Fig.5. EffectofSO 22 ontreatmentefficiency Fig.6. EffectofPO 32 ontreatmentefficiency
4 4
was hindered under high ionic strength and could not meet the tionfrom1,62969~background!to1,646621(@SO 22#550mg/
4
effluent standard of fluoride ~15 mg/L!. A higher dosage of cal- L!, 1,657625 (@SO 22#5100 mg/L!, and 1,75761,921
4
ciummayberequired.AsfortheremovalofCaF , theefficiency (@SO 22#5500 mg/L!. The direct competition of sulfate with
2 4
ofhigherthan96.7%wasstillfoundwhentheionicstrengthwas fluoride for calcium resulted in the residual fluoride concentra-
changed from background to 0.5 M. This is quite different from tions exceeding effluent standards. Nevertheless, it seemed cal-
thepronouncedeffectofionicstrengthfoundinDiAFofCaF . It cium sulfate precipitate was removed from the aqueous phase
2
might be attributed to the fact that oleate was adsorbed onto the without difficulty.
surfaces of CaF through chemisorption, which is not much af-
2
fected by changes in ionic strength ~Hu etal. 1986; Kellar etal.
InterferenceofPhosphate
1990!.ThedecreaseofthezetapotentialofCaF andthedecrease
2
in electrostatic attraction between oleate and CaF under high The effect of phosphate on the treatment efficiency was studied.
2
ionic strength thus had little effect on the adsorption. Similar Phosphate is known to inhibit kinetics of calcium fluoride crys-
findingshavebeenreportedinutilizingSOlasacollectorforthe tallization,andcompeteswithfluorideforcalciumioninthepre-
flotation of apatite and dolomite ~Moudgil and Chanchani 1985!. cipitation reactions ~Shyu and Nancollas 1980!. Not only CaF ,
2
ThismaybeamajoradvantageofSOloverothercollectorsinthe butalsocalciumphosphate,areexpectedtopresentassuspended
flotationreaction.Itcanalsoberegardedasapotentialadvantage solid in the system, and the amount of precipitate is expected to
of DAF over DiAF processes. increase with increasing phosphate concentration. This is con-
firmed in our experimental results that the total suspended solid
concentrationsbeforetheflotationreactionindeedwere1,62969
InterferenceofSulfate ~background!, 1,604666 (@PO 32#550 mg/L!, 1,70766
4
Mixed acids are commonly used in the semiconductor industry (@PO 32#5100 mg/L!, and 1,904618 (@PO 32#5500 mg/L!.
4 4
for water etching and quartz cleansing ~Gilles and Loehr 1994!. The presence of phosphate significantly affected the treatment
Sulfate, nitrate, and phosphate ions may sometimes be found in efficiency of DAF. The removal of suspended solid (CaF and
2
fluoride-containing wastewater. It is essential to study the inter- calciumphosphate!decreasedfrom96.1to79.1%whenthephos-
ference of anions. Fig. 5 shows the effect of sulfate on dissolved phate concentration was 50 mg/L ~Fig. 6!. It then decreased to
airflotationofCaF . TheremovalofCaF wasnotaffectedwhen 65.8%, as the phosphate concentration became 100 mg/L, and to
2 2
the wastewater was spiked with 50–500 mg/L of Na SO . This 49.0% at a phosphate concentration of 500 mg/L. It is apparent
2 4
indicatesthatoleateisadsorbedpreferentiallyincompetitionwith that the presence of phosphate markedly inhibits flotation effi-
sulfate for adsorption sites on CaF surfaces ~Popping etal. ciency. The inhibitory effect may be induced by that the adsorp-
2
1992!. Nevertheless, it was also found that residual fluoride con- tion of phosphate ion at CaF surfaces that tends to block the
2
centrations were 19.8, 21.3, and 36.3 mg/L at sulfate concentra- active crystal growth sites ~Amjad 1991!. There might not be
tions of 50, 100, and 500 mg/L, respectively. Thermodynamic enoughSOltocoverthesefinerprecipitatesthathavearelatively
calculationbyGEOCHEM~SpositoandMattigod1979!indicates largersurfacearea.Itisalsopossiblethatcalciumphosphatepre-
that sulfate may form precipitation with calcium. In fact, it is cipitateconsumesSOlaswell.However,furtherworkisrequired
speculatedthatCaSO precipitatemayhaveformedinthesystem. to elucidate the phenomenon, though the interference of phos-
4
This is evidenced by the increase of suspended solid concentra- phateonthesystemisclear.Inaddition,itwasalsofoundthatthe
Table 4. Improvement of Dissolved Air Flotation Efficiency under Summary
AnionInterferences
@NaOl# @NaOl# @NaOl# ThestudyinvestigatedtheapplicationofDAFtothetreatmentof
5150mg/L 5200mg/L 5250mg/L fluoride-containing wastewater of semiconductor industry. The
main results of this work may be summarized as follows:
@SO 22#5100mg/L
4 1. SOl was an effective collector for DAF of fluoride-
RemovalofSS~%! 24.7 54.0 94.2
containing wastewater. The adsorption and precipitation of
Residualfluoride~mg/L! 5.2 4.8 4.8
oleate on the CaF surfaces increases the hydrophobicity.
2
@RPeOm4o3v2a#l5of1S0S0~m%g!/L 29.4 65.4 95.9 TThheeraeffienxiitsytsoafnaiorpbtuimbballes@Ctoa2C1a#F/@2Fs2ur#f.acCeaslcisiutmhushiegnhhearntcheadn.
Residualfluoride~mg/L! 5.2 4.8 4.8 stoichiometricamounttendstodecreasesolid–liquidsepara-
Note: @Ca21#/@F2#51.0, pH55.560.1, P55 kg/cm2, recycle ratio tion.
530%. 2. Satisfactory removal efficiency was found in the pressure
range from 3 to 5 kg/cm2, and recycled flow rate of 20–
40%.
3. FlotationefficiencydidnotchangesubstantiallyinthepHof
5.5–9.5,thoughcompetitioneffectofOH-wasobservedata
residualfluorideconcentrationsincreasedfrom10.5to19.0,29.9, pHof9.5.Residualfluorideconcentrationbecamehigheras
and73.11mg/L,atphosphateconcentrationsof50,100,and500 the ionic strength increased. Nevertheless, the flotation effi-
mg/L,respectively.Thecompetitionforcalciumbetweenfluoride ciency of CaF was not affected. It is probable due to the
2
and phosphate is apparent. In another words, the presence of a high gas–solid ratio, and to the chemisorption of oleate to
high concentration of phosphate not only decreases solid–liquid the CaF surfaces.
2
separation efficiency, but also renders the residual fluoride con- 4. Thetreatmentefficiencydecreasesinthepresenceofsulfate
centration exceedingly high. To overcome anion interference, it and phosphate. It could be explained by the competition be-
was found that by increasing the SOl concentration and molar tween sulfate and phosphate with oleate for adsorption sites
rpartoiov,ed@C~Ta2a1bl#e/@4F!2.#, the removal of CaF2 could be markedly im- @oCnaC21aF#/2@sFu2r#faccaens.iHmipgrhoevrectohnecehnintrdaetrioedntorfeaotlmeaetnetaenfdfichieignhceyr.
It is suggested that different streams of wastewater need to
be collected and treated separately.
ApplicationofDissolvedAirFlotation
Judging from previous results, the importance of the segregated Acknowledgments
collectionandtreatmentofdifferentstreamsofwastewaterneeds
to be emphasized. Experiences of Taiwanese semiconductor The writers thank the National Science Council of Taiwan for
manufacturers have shown that it is common to find contamina- financial support of this work under Contract No. NSC-89-2211-
tionoffluoride-containingwastewaterwithsulfateandphosphate, E-011-006. The kind suggestions from Dr. Marco A. P. Reali at
mainly through cross connection and inappropriate operation.As the University of Sao Paulo on the design and operation of the
evidencedbythiswork,thepresenceofsulfateandphosphatenot DAF apparatus are appreciated.
only raises the residual fluoride concentration in the effluent, but
also decreases the removal efficiency of CaF sludge. Though
2
certain measures could mitigate the interference, the contamina- References
tion still could limit the possible utilization of CaF sludge as a
2
potential resource. Another practical problem is that calcium Amjad,Z.~1991!.‘‘Inhibitionofcalciumfluoridecrystalgrowthbypoly-
phosphate precipitates tend to accumulate on the reactor wall, electrolytes.’’Langmuir,7~10!,2405–2408.
DeGenova,J.,andShadman,F.~1997!.‘‘Recovery,reuse,andrecycleof
baffles, and weir, and require routine clean up and maintenance,
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