Sedimentationandwaterelectrolysiseffectsinelectrof

SedimentationandWaterElectrolysisEffectsinElectrofiltrationofKaolinSuspensionO.LarueandE.VorobievChemicalEngineeringDept.,UniversityofCompie`gne,Compie`gne,BP20529-60205,FranceDOI10.1002/aic.10227PublishedonlineinWileyInterScience().Filtrationcombinedwithanelectricfield(electrofiltration)enhancesthefiltrationkineticsofaqueoussuspensions.Althoughelectrophoresisandelectroosmosislargelycontributetoelectrofiltrationbehavior,sideeffectssuchaswaterelectrolysis,heatproduction,andgravitationalsedimentationofparticlesmaybeinfluential.Amathemat-icalmodelwasdevelopedtoincludegravitationalsedimentationofparticlesandelec-trolyticgasinelectrofiltrationtheory.Themodelsuccessfullyrepresentedelectrofiltrationdataobtainedfromkaolinsuspensionswithalaboratoryhorizontalfilter.Electrolyticgascontributedtoanincreaseof13%inelectrofiltrationrate.Settlingparticlesincreasedcakeresistancetodownwardelectrofiltrationflowandcouldsubstantiallyoffsetelectro-kineticeffectsathighsettlingvelocities.Becauseofelectrolysisionicproducts,adecreaseofpHoccurredinthefilterduringelectrofiltration,causingthecoagulationofkaolinparticlesandanincreaseinhydrauliccakepermeability.Subsequentfiltrationcouldthentakeplacerapidlywithoutanyelectricfield.Theoverallprocessingtimewasdecreasedbyhalfwhenanelectricfieldisappliedonlyfor37%oftheprocessingtime.Thiscoagulationphenomenonseemstobeapromisingmeanstoacceleratethefiltrationkineticswithouttheneedforfull-timeelectrofiltration.©2004AmericanInstituteofChemicalEngineersAIChEJ,50:3120–3133,2004IntroductionInherentpropertiesoffineparticlesdispersedinaqueoussuspensionsareattheoriginofextendedcakeformationtimes.Acombinationofmechanicalandelectricalfiltration(thatis,electrofiltration)canbeeffectiveinenhancingfiltrateremoval.InaDCelectricfield,asignificantaccelerationoffiltrationratewasobservedforvariousmineralsuspensionssuchasgelati-nousbentonite(Moulik,1971),kaolin(VorobievandJany,1999),quartzsand(WeberandStahl,2002),calciumcarbonate(Yukawaetal.,1976),andforbiologicalmaterials(Iritanietal.,1992).Previousauthorsshowedthatthefiltrationratebecomesmorepronouncedwithincreasingfieldstrengthandtheelectrokineticpotentialofparticles.Particlesinaqueoussolutionsacquireasurfacechargecreatinganelectrokineticpotential,causedbyiondissociation,ionabsorption,oriondissolution.Anelectricfieldappliedbetweentwoelectrodescanthenmovesuspendedparticlesrelativetotheliquid(elec-trophoresis)ormovetheliquidrelativetoafixedlayerofchargedparticles(electroosmosis).Inelectrofiltration,bothphenomenatakeplace.Inanelectricfieldofproperpolarity,electrophoresisreducestherateofcakeformationandin-creasestherateofpermeationthroughthefiltercakebyelec-troosmosis.ADCelectricfieldcanalsoincreasetheefficiencyofotheroperationsperformedonindustrialfilters.Numerouspublica-tionsshowthatcombiningaDCelectricfieldwithmechanicalcompressionoffiltercakesisveryeffective(pressurizedelec-troosmoticdewateringordeliquoring;Bartonetal.,1999;Iwataetal.,1991;Orsatetal.,1999).Recently,theapplicationofDCelectricfieldsduringthewashingoperation(electro-washing)wasstudied.Theveryinterestingeffectofselectiveremovalofchargedspeciesduringelectrowashingwasdem-onstrated(Tarletonetal.,2003).ElectricallyassistedfiltrationCorresponden[e-mail]©2004AmericanInstituteofChemicalEngineers3120AIChEJournalDecember2004Vol.50,No.12andcompressionoffiltercakesarenowadvancedatthepilotscaleandhaverevealedpromisingresults.Electricaldevicesareeasilyadaptabletothetechnologyofindustrialfilterssuchasplatefilterpress(KondoandHiraoka,1990)andbeltfilterpress(Raatsetal.,2002).Difficultiesstillsubsistintheindustrializationoftheelec-trofiltrationprocess.Acontinuouselectricfieldisrathercriti-cizedforitshigh-energyrequirements,andcurrentflowsideeffectssuchasheatproductionandreactionsattheelectrodes.Eventhoughthesesideeffectsmightsignificantlyimprovetheelectrofiltrationprocess,onlyfewauthorsareconcernedwiththem.Theelectrolyticgases(O2andH2)generatedduringwaterelectrolysisattheelectrodescanaccelerateelectrofiltra-tionandslightlyimprovetheresidualmoistureofthefinalfiltercake(WeberandStahl,2002).Waterelectrolysisalsogener-atestheionicspeciesH3O
andOHthatmaydestabilizethecolloidalsuspension,leadingtotransientcoagulationofparti-cles.Itisworthmentioningthatjustasinelectrowashing,displacementofionicspeciesintheelectricfieldisimportantintheelectrofiltrationprocess.BecauseoftheproductionofH3O
attheanodeandOHatthecathodeandtheelectro-phoreticmigrationoftheseionicproducts,strongpHgradientsbetweentheelectrodesmaybeexpectedinelectrofiltration.ChangesinpHduringelectrofiltrationmayalsoaffecttheelectrokineticeffectsandthustheelectrofiltrationrate.Usually,theelectrodesusedinelectrofiltrationaremadefrominsolublematerials(suchaspreciousmetals,conductiveceramics).How-ever,incasesolubleanodesareused(suchasaluminumoriron),anodeelectrolysisgeneratesionsthatcanbeveryeffec-tiveinthecoagulationofthesuspendedparticles(Larueetal.,2003).ThecoagulationeffectinducedbypHorbyotherelectrolyticproductsmayincreasethefiltercakep