Magneticseparationofferrihydritefrom

Magneticseparationofferrihydritefromwastewaterbymagneticseedingandhigh-gradientmagneticseparationNurayKarapinar*TechnologyDepartment,GeneralDirectorateofMineralResearchandExploration,06520Ankara,TurkeyReceived22May2002;receivedinrevisedform5March2003;accepted6March2003AbstractFerrihydrite,amemberofironoxidesfamily,hasbeenusedasanadsorbentfortheremovalofheavymetalionsfromindustrialwastewater.Thesuccessoftheoperationdependsmainlyontheefficientremovalofferrihydritefromtheaqueousphase.Hence,theemphasisofthisstudywasgivenontheseparationofferrihydritebyhigh-gradientmagneticseparator(HGMS)toovercomesolid/liquidseparationdifficultiesofferrihydrite.ThispaperclarifiestheseedingofferrihydritewithmagnetitemineralanditseffectsontheoperationparametersinHGMS.Magneticseedingtechniquewasusedtomakemagneticseparationavailablefortheremovalofferrihydrite,andthemagnetitemineralwaschosenasaseedingmaterial.Themethodclearlyinvolvestheattachmentofferrihydritetoamagneticseedmaterialandsubsequentmagneticseparationofferrihydrite–magnetitecoagulates.Inseedingprocess,finelydividedmagnetiteparticleswereentrappedinferrihydriteprecipitateevolvingbyincreasingpHofsolution.InrelationwiththehydrolysispropertiesofFe(III)ion,thereisapHrangewheretheseedingperformanceisoptimal.Iron/magnetiteratiohadamarkedeffectontheseparationofseededferrihydriteprecipitatebyHGMS.ThisratiodeterminestheoperationparameterssuchasmagneticfieldstrengthandflowrateonwhichthecostandperformanceofHGMSdepend.Testresultsshowedthatwhenusedwithmagneticseedingtechnique,HGMShasthepotentialtoovercomeseparationdifficultiesassociatedwithadsorption-basedtreatmenttechniqueswithitsadvantagesofhigh-performance,high-capacityandlow-spacerequirements.D2003ElsevierScienceB.V.Allrightsreserved.Keywords:ironoxides;ferrihydrite;magneticseeding;solid/liquidseparation;magneticseparation0301-7516/03/$-seefrontmatterD2003ElsevierScienceB.V.Allrightsreserved.doi:10.1016/S0301-7516(03)00029-2*Tel.:[phone]/2116;fax:[phone].E-mailaddress:[e-mail](N.Karapinar).(2003)45–541.IntroductionBecauseofthestringenttreatmentanddisposalregulations,theuseofsorptionprocessesfortheremovalofheavymetalsfromwastewaterhasbeenseenasapromisingmethod.Inorganicsorbentsarebecominginterestingrecentlyinthisregardasbeingtheirlowercostwhencomparedtocommercialsorbentssuchasactivatedcarbonandion-exchangeresins.Theapplicationofseveralindustrialby-productsandmineralparticleshasrecentlybeenproposedasalternativesorbentsfortheremovaloftoxicions.Becauseofthewell-knownimportanceofironoxidesinattenuatingheavymetaltransport,severalinvestigatorshavesuggestedironoxidesasappropriateadsorbent.Amongtheironoxides,ferrihydritehasbeenknownasaquiteefficientsorbentformetalionremovalfromindustrialwastewater(Schultzetal.,1987;EdwardsandBenjamin,1989),andmanylaboratory-basedadsorptionstudieshavebeenperformedtoinvestigatethisphenomenon(e.g.,DavisandLeckie,1978;BenjaminandBloom,1981;DzombakandMorel,1986).However,solid/liquidseparationofferrihydritehasbeenacceptedasalimitationfactorforthewastewatertreatmentapplicationsbecauseofthedifficultyindewateringofferrihy-dritesuspensions(Andersonetal.,1991;Benjaminetal.,1996).Magneticseparationisamethodfortheseparationofparticlesonthebasisoftheirmagneticpropertiesandhasbeenusedinthemineralprocessingandtherecycleindustriesformanyyearsforconcentrationandpurificationrequirements.Efficientuseofmagneticseparationmethodsinmineralprocessinghasencourageditsuseinwastewatertreatment.Inthe1970s,theintroductionofhigh-gradientmagneticseparators(HGMS)developednotonlyfortherecoveryofusefulmaterialsbutalsorapidtreatmentof,wastewater,hasmadethemagneticseparationmethodapplicabletothehandlingofweeklysubmicronparticles(Svoboda,1987).Asamatteroffact,ithasbeenwellknownthatmagneticseparationhasbeenusedfortheremovalofmagneticsolidparticlesinwastewater.Recentprogressinthisfieldistheremovalofnonmagneticwaterpollutantssuchasvirus,algaeanddissolvedpollutantsbymagneticseparationthatreliesontheusageofmagneticseedingtechniquetoenhancethemagneticpropertiesofpollutantstoberemoved(e.g.,Andersonetal.,1983;Anandetal.,1985;Treshimaetal.,1986;VanValsenetal.,1991;DeReuver,1994;GilletandDiot,1999;FranzrebandHo¨ll,2000).InspiteofthefactthatthesubjectwasfirststudiedbyDeLatour,1973,andtillnowanumberofreportshavebeenpublishedonthesubject(e.g.,Moffatetal.,1994;Prengeretal.,1994;Karapinar,2003),magneticseparationmethodisanotwell-knownprocessinwastewatertreatmentfacilities.Inthisregard,magneticseparationmethodmayovercomeseparationdifficultiesassociatedwithFeoxidesadsorbentbyconsiderablyacceleratingsolid/liquidseparationprocess,andhenceincreasingthethroughput.However,todothis,ironionscanbeprecipitatedaferritesludgewithmagneticproperties(Tamauraetal.,1991;Barradoetal.,1998),orprecipitatedslurriesareseededwithveryfinemagnetitepowder(Treshimaetal.,1986;Andersonetal.,1991).Whentheferrihydriteismademagnetic,fluidcanbetreateddirectlyinHGMSwithoutaneedforsludgesedimentation.Thisstudy