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PAGEPAGE47.BLEACHINGBleachingSequencesPreparationofBleachChemicalsChlorination&ExtractoinOxygenBleachingHypochloriteBleachingBleachingEquipmentRecyclesofFiltratesPulpBrightening8.PREPARATIONOFSTORKFORPAPERMAKINGRepulping(Dispersion)RefiningMeteringandBlendingofFurnishes9.NON-FIBROUSADDITIVESTOPAPERMAKINGSTORKRetentiononthePaperMachineWetEndChemistryApplicationsofNon-FibrousAdditivesAlkalinePapermaking10.PAPERMANUFACTURE-WETENDOPERATIONSIntroductiontothePaperMachineApproachSystemFlowspreaderandHeadboxSheet-FormingProcessWirePart(Fourdrinier)Twin-WireFormingWhiteWaterSystemBrokeSystemPressingVacuumSystem11.PAPERMANUFACTURE-DRYENDOPERATIONSPaperDryingCalenderingProfileControlReelingPaperMachineDrivesWindingRollFinishing1.翻譯文章，每周一篇，平時(shí)成績(jī)2.參考書,外文雜志（tappi,appita,ppi,ppcetc.）圖書館3.學(xué)習(xí)方法，預(yù)習(xí)-聽(tīng)講-翻譯（主要是專業(yè)詞匯問(wèn)題）4.成績(jī)，考試+平時(shí)翻譯+提問(wèn)更正：1.p122Oioxin＝Dioxin二噁英problem2.content7bleaching7.3chlorinationh=r3.9.2wetendchemistry4.p130倒數(shù)5linecountercurrent5.p132倒數(shù)5linemodem=modern6.p153line10ids＝aids7.p153倒數(shù)3linehe＝the8.p157floes＝flocs9.p159倒數(shù)2arived＝derived10.p164lineonean＝art11.p164倒數(shù)7corrmonent＝componentThewhitenessofpulpismeasuredbyitsabilitytoreflectmonochromaticlightincomparisontoaknownstandard(usuallymagnesiumoxide).TheinstrumentmostcommonlyusedistheZeissElrephoreflectancemeterwhichprovidesadiffuselightsource.Fullybleachedsulfitepulpscantestashighas94,andunbleachedkraftpulpaslowas15Elrephounits.Unbleachedpulpsexhibitawiderangeofbrightnessvalue(Table7-1).Thesulfiteprocessproducesrelativelybrightchemicalpulps,upto65,wherethoseproducedbythekraft,sodaandsemichemicalprocessescanbequitedark.Mechanicalpulpbrightnessismainlyafunctionofthespeciesandconditionofthewoodpulped.Generally,onlythosespeciesprovidingbrightnessvaluesover55areutilizedformechnicalpulping.Celluloseandhemicelluloseareinherentlywhiteanddonotcontributetopulpcolor.Itisgenerallyagreedthat“chromophoricgroups”ontheligninareprincipallyresonsibleforcoloroxidativemechanismsarebelievedtoconvertpartofthelignin’sphenolicgroupstoquinone-likesubstancesthatareknowntoabsorblight.Heavymetalions(e.g.,ironandcopper)arealsoknowntoformcoloredcomplexeswiththephenolicgroups.Extractivematerialscancontributetothecolorofmechanicalpulpsmadefromresinouswoods.LigninPreservingorLigninRemoval?Twoapproachesareusedinthechemicalbleachingofpulps.Oneapproachistoutilizechemicalsthatselectivelydestroysomeofthechromophoricgroups,butdonotmateriallyattacklignin.Theotherapproachistoalmosttotallremoveresiduallignin.Theselectiveapproach(oftenreferredtoa“brighting”todistinguishitfromtruebleaching);usedforhigh-yieldpulpswithsignificantlignincontent.Thismethodologyislimitedinmanycasetoachievingbrightnessvalueinthe70’Toproducehigh-quality,stablepaperpulps(aswellasdissolvinggrades)bleachingmethodsthedelignifythepulpmustbeused.Theearlystages(bleachingareusuallyconsideredasacontinuation).Thedelignificationprocessstartedincooking.Thelaterstagesemployoxidizingagentstoscavengearedestroytheresidualcolor.Theentirebleachingprocessmustbecarriedoutinsuchawaythanstrengthcharacteristicsandotherpapermakingpropertiesarepreserved.7.1BLEACHINGSEQUENCESModernbleachingisachievedthroughcontinuoussequenceofprocessstagesutilizingdifferentchemicalsandconditionsineachstageusuallywithwashingbetweenstages(e.g.Figure7-1).Thecommonlyappliedorproposedchemicaltreatmentsandtheirshorthanddesignationareasfollows:Chloribnation(C)-ReactionwithelementalchlorineinacidiemediumAlkalineExtraction(E)-DissolutionofreactionproductswithNaOHChlorineDioxide(D)-ReactionwithClO2inacidiemediumOxygen(O)-ReactionwithmolecularoxygenathighpressureinalkalinemediumHypochlorite(H)-ReactionwithhypochloriteinalkalinemediumPeroxide(P)-ReactionwithperoxideinalkalinemediumOzone(Z)-ReactionwithozoneinacidiemediumThepracticeofdesignatingbleachingstagesandsequencesusingthissymbolicshorthandhasevolvedinformallyovermanyyears.However,thecomplexityofmodernbleachingpracticescoupledwithvariablesymbolismhascausedmisunderstandingsregardingbleachingpractices.Adherencetostandardizedguidelinearenownecessarytofacilitateclarityintechnicalcommunication.ThefollowingrecommendedconventionshavebeenextractedfromacomprehensiveprotocolsubmittedbytheTAPPIPulpBlechCommittee(1):1.Usingtheabovesymbols,aconventionalfive-stagebleachsequenceconsistingofchlorination,alkalineextraction,chlorinedioxide,alkalineextraction,andchlorinedioxidewouldbedesignatedasCEDED2.Whentwoormorebleachingagentsareaddedasamixtureorsimultaneously,thesymbolofthepredominantchemicalshouldbeshownfirst,andthesymbolsdepictingalloftheaddedchemicalsshouldbeenclosedinparenthesesandbeseparatedbyaplussign.Forexample,whenalesserpercentageofchlorinedioxideisaddedwithchlorineinthechlorinationstage,thesequencewouldbedesignated(C+D)EDED.Ifchlorinedioxideisthedominantchemicalspeciesinthechlorinationstage,thedesignationshouldbe(D+C)EDED.3.Whentwoormorechemicalsareaddedsequentiallywithmixinginbetweenpointsofaddition,thesymbolsdepictingtheaddedchemicalsshouldbeshowninorderofadditionandshouldbeshowninparentheses.Forexample,whenchlorinedioxideisaddedbeforechlorineinthechlorinationstage,thedesignationshouldbe,(DC)EDED.4.Iftheratioofaddedchemicalsistobeshown,thepercentagenumbershouldimmediatelyfollowthesymbolofthedesignatedchemicalandshouldbeexpressedintermsofoxidizingequivalence.Forexample,theterm(D70C30)indicatessequentialadditionof70%ClO2and30%Cl2,allexpressedasactivechlorine.Forsoftwoodkraftpulps,anumberofbleachsequencesutilizingbetweenfourandsixstagesarecommonlyusedtoachicve“full-bleach”brightnesslevelsof89-91.NumerousCEHDEDandCEDEDfull-bleachsystemsareinoperationdatingfromthe1960’sand1970Alevelof65canbeeasilyreachedwithaCEHorOHsequence.IntermediatelevelsofbrightnesscanbeachievedwithCED,DED,OCED,CEHH,CEHD,orCEHP.Inmostcases,eachbleachingstageisfollowedbyawashingstagetoremovereactionproducts,andsubsequentbleachstagesservethesamepurpose,whilecreatingadditionalreactionproducts.Forthisreason,theindustryhasoftenemployedsequenceswithalternatingacid/alkalinestages.Sulfitepulpsandhardwoodkraftpulpsare“easierbleaching”thansoftwoodkraftpulps.Bothhavelowerlignincontent,andInthecaseofsulfitepulpstheligninresidualsarepartiallysulfonatedandmorereailysolubilized.Consequently,asomewhatsimplerprocesscanbeusedtoachievedacomparablebrightnesslevel.Theinitialstagesofasequenceareusedprimarilytodelignifythepulp.Thispartofthebleachingsequencecanbeconsideredacontinuationofthedeligninficationprocesswhichstartswithcooking.Typically,significantbrighteningoccursonlyaftertheremovalofresiduallignin.Short-SequenceBleachingRecentadvancementsinequipmentdesignandprocesscontrol,togetherwiththeuseofoxygeninthealkalineextractionstage,haveresultedintheloweringofchemicalrequirementsinsubsequentstages.Thishasledtothedevelopmentofshort-sequencebleaching,whichprodeces88brightnesssoftwoodkraftpulpinonly3or4stages.Themostwidelyadoptedshortsequenceisprobaly(D+C)(EO)D.7.2PREPARATIONOFBLEACHCHEMICALSAllconventionalbleachingchemicalscanbeproducedfromair,waterandbrine(NaClsolution)utilizingvaryingamountsofenergy.Sincethenewmaterialsarecheap,itfollowsthatmanufacturingcostsaremainlyafunctionofhowmuchedergyisexpanded.OxygenTwosystemsareutilizedforoxygengeneration:cryogenicairseparationandadsorption-basedseparation.Theformermethodisuniversallyusedbychemicalprocessorswhosupplyliquidoxygeninover-the-roadtanks,andissometimesemployedforonsitegenerationinhigh-usagemills(i.e.,over80ton/day),especiallyinsituationswherecoproductnitrogencanalsobeutilized.Theadsorption-basedoxygenplantyieldsagasproductandissuitableonlyforonsitegeneration.Cryogenicplantscompressairandremovewater.CO2andotherimpuritiesinapretreatmentsystem.Thedryairisthencooledandliquifiedatcryogenictemperatyresusingturboexoandersand/orrefrigerationsystems.Finally,theliquifiedairisdistilledtoseparateitintooxygen,nitrogenandothercoproduces.Bycontrast,adsorptionsystemseparateairatambienttemperaturesusingmolecularsieveadsorbents.Theadsobentpicksupnitrogenandotherimpuritiesatthesystemfeedpresure,whiletheproduceoxygenpassesstraightthroughthebed.Afterthebedissaturatedwithnitrogen,itisregeneratedbyloweringthepressyretoreleasetheadsorbedcomponents.Adsorptionprocessesarecyclical,andmultiplebedsareusedtomaintaincontinuousoxygenproduction.ChlorineandCausticProductionChlorineandsodiumhydroxidearegenerallyproducedbyelectrolysisofbrineaccordingtothefollowingnetreactions:2NaCl+ElectricalEnergy2Na+Cl22Na+2H2O2NaOH+H2Threetypesofelectrolyticcellsareusedcommercially:diaphragmcells,mercurycells,andmembranecells.TheoperatingprincipleofeachisillustratedinFigure7-2Inthemercurycellprocess(Figure7-3),metallicsodiumformanamalgamwithmercurywhichisthenremovedfromthecellbeforethesodimreactswithwater.Therefore,themercurycellsystemproducesessentiallypureNaOH.Withthediaphragmcell,thecausticmustbeconcentratedbyevaporationtocrystallizeandseparateNaClwhichispresentinthewithdrawnceilliquor.Themembranecellisthemostrecentdevelopmentandisnowusedinmostnewelectrolysisplantsbecauseofgreateroperatingflexibilityandlowerprocessingcost.Itutilizesacationexchangemembranethatisimpervioustochlorideionflow,andthusproducesanessentiallypureNaOHproduct.Inallcases,chlorinegasformsattheanodeandisventedfromthecell.Thegasiscooled,scrubbedwithsulfuricacidtoremovemoisture,andusuallyliquifiedundercompressionandrefrigeration.Chlorineandcausticaregenerallysuppliedbythechemicalindustry.Thechlorineisshippedasliquid,usuallyintankcars.Causticismostcommonlyshippedas50%solution.Wherejustifiedbyhighconsumption,somelargeroulpmillshaveutilizedon-sitegeneration.Here,thechlorinecanbehandledasagasandthecausticatlowerconcentration,thuseliminatingsomeoftheunitoperationsnecessaryforamarketchemicalplant.Therehasbeenadramaticshiftawayfromchlorinebleachinginrecentyearsduetoenvionmentalconcerns.Aseverechlor-alkaliimbalancenowexistsintheindustry.Alternative(i.e.,non-electrolytic)methodsforproducingcausticarebeingdeveloped,forexamplebycausticizingsodiumcarbonate.HandlingofChlorineandCausticAtypicalchloringhandlingsystemisshowninFigure7-4.Bothliquidandgaseouschlorinearesafelyhandledincarbonsteeltankageandpipingaslongastheentiresystemismoisture-free.Tankcarsarepressurizedwithdryair.Liquidchlorineflowsunderthispressureheadtosteam-jacketedevaporatorsforconversiontothegasohase,andthenthechlorinegasiseithermixedwithpulpstockorusedforhypochloritemakeup.Barometriclegsarecommonlyutilizedbeforeeachpointoduseasaprecautionagainst“suckbacks”ofwaterintothedrychlorinesystem.Causticisusuallysyoredas50%solutionanddilutedto5%inacontinuousdilutionsystem.Meeasurementofeithersolutiondensityorconductivitycanbeutilizedasthebasisforanautomaticdilutionsystem.PreparationofHypochloriteHypochloritesolutionispreparedatthemillsitebyreactingchlorinewith5%-causticsilutionormilk-of-lime:2NaOH+Cl2NaOCl+NaCl+H2O2Ca(OH)2+2Cl2Ca(OCl)2+CaCl2+2H2OThesereactionsarereversible.Anexcessofalkaliisrequiredtodrivethereactiontotherightandalsopreventsubsequentdecompositionofthehypochloriteintochlorate.However,decompositionmayoccurathighertemperatureeveninthepresenceofalkali.Becausetheabovereactionareexothermic,coolingmayberequiredtomaintainthesolutionbelowthecriticallevelof50度（especiallyfollowinglimeslaking）.Mosthypochloritemakeupsystemsoperatecontinuouslyandautomaticallyutilizinganoxidation/reductionpotentialmeasuremnet,whichdependsontherelativeamountsofhypochloriteandresidualchlorinepresent.SodiumChlorateProductionSodiumchlorate(NaClO3)istheprincipalfeedstockforon-sitegenerationofchlorinedioxide(ClO2),andisthereforeanimportantbleachingchemical.Itisalsoproducedbyelectrolysisofbrine.Inthisprocess,theanodicandcathodicproductsarenotseparated(asisdoneforcausticandchlorinecells),butareallowedtoreacttoformsodiumhypochlorite(NaOCl).Excessalkaliisnotmaintainedtostabilizehypochlorite(asisdoneformillhypochloritemakeup),andformationofchlorateproceeds:3NaOClNaClO3+2NaClThesolutionisremovedandevaporated,theNaClcrystallizesouttoleaveonlyNaClO3insolution.ThesolutionisthenfirtherevaporatedtocrystallizeouttheNaClO3.Thematerialisshippedoutincrystalformandisdissolvedatthemillsiteforstorage.Thenetreactionfortheoverallchlorateproductionprocesscanbeexpressed:NaCl+3H2O+ElectricalEnergyNaClO3+3H2ChlorineDioxideGenerationChlorinedioxide(ClO2)isagasatroomtemperature/pressure,andchemicalisunstableandpotentiallyexplosiveinthepureform.Inairmixtures,itiseasilydetonatedbyexposuretoheat,light,mercury,andvariousorganicsubstances.Richmixturescanproducesebereexplosions:lowerconcentrations(below20%)produceonlymilddetonationsdescribedas“puffs”.Chlorinedioxideissomewhatmoresolubleinwaterthanchlorine(10to11gramsperliterat4度).Sinceitcannotbeshippedeitherinpureformorasaconcentratedsolution,ClO2isalwaysmanufacturedatthemillsite.Itisgeneratedasagasfromchemicalreductionofsodiumchloriteinahighlyacidicsolution,thegasisthenabsorbedincoldwatertoproduceClO2solutionataconcentrationofabout7g/liter.Numerousmethodsofon-siteClO2generationhavebeendeveloped.Originally,themethodsdifferedprimarilywithrepecttothereducingagentthatwasusedtoreactwiththechlorite.Morerecentdevelopmentshavebeenmoreconcernedwithreducingthequantityofby-productchemicals.FourseminalprocessesusedinNorthAmericaforgeneratingchlorinedioxidearelistedinTable7-2.Theseprocessesarethebasesfornumerousmodificationsandimprovementsthathavebeenintroducedovertheyears.Intheiroriginalformat,theSolvayandMathiesonsystems(whichdatafromthe1950’s)areinherentlydifficulttocontrol,andproviderelativelypoorconversionefficiency.Bothoftheseprocesses,aswellastheR2process(whichdatesfromthe1960Ina1970’smodificationoftheR2andR3processes,hydrochloriteacid(HCl)issubstitutedforalloftheNaClandpanoftheH2SO4thusreducingtheamountofby-productNa2SO4.Two1980ExamplesofGenerationSystemsTheMathiesonprocessisillustratedinFigure7-5.Thereactants(sodiumchlorate,sulfurdioxideandsulfuricacid)arefedtotheprimarygeneratoralongwithspargedairwhichactsasamixeranddiluentforthegeneratedClO2gas.Thegeneratoroperateswithacidnornalityof9-10andchloriateconcentrationof20-30g/liter.Thetemperatureiscontrolledinthe35to40度rangewithjacketcooling.Thecontentsoverflowintoasecondaryreactorandthentoastripper.Theprincipalnetreactionis2NaClO3+2NaCl+2H2SO42ClO2+Cl2+2Na2SO4+H2OThesulfuricacidactsonlyasacatalystandgoesthroughthereactionunchanged.TheR3processisshownasaplantlayoutinFigure7-6.Thenetreactionforthisprocessis:2NaClO3+SO22ClO2+Na2SO4Thereactionmixtureofsodiumchlorate,sodiumchlorideandsulfuricacidiscontrolledathighertemperaturebycontinuouscirculationthroughaheatexchanger.Thegeneratorisalsomaintainedunderavacuum,thusproducingwatervaporasacarrierfortheClO2andCl2gases.Becausethegeneratoracidsolutionisconcerntratedbyevaporation,thesodiumsulfate(saltcake)by-productcrystallizesoutofsolution:thecrystalslurryiscontinuouslypumpedtoafilterwherethesaltcakeisremovedandwashedwhiletheacidisreturnedtothereactor.HydrigenPeroxideSeveralprocessesareusedcommerciallytopriducehydrogenperoxide(H2O2),andliterallydozensofothersarebeingresearchedbychemicalcompaniesinNorthAmerica,EuoopeandJapan.Manyoftheseprocessareproprietaryandcloselyguarded.Themostwidelyusedmethodisbasedontheautooxidationofquinones.Inthisprocess,anthra-quinone(whichisdissolvedinamulticomponentorganicsolution)iscatalyticallyhydrogenatedtohydroquinone;asubsequentaerationstepproducesH2O2(whichiswater-extractedandconcentrated)andregeneratesthequinone.Anumberoffirmsarepursuinganelectrolyticroute.Forexample,thenewH-DTechprocess,whichutilizesadiaphragmcell,appearsidealforonsitegeneration.DilutecausticisfedtotheanodesidewhereO2isgenerated.ThegeneratedO2istakenoffthetopoftheanodesideand,alongwithsupplemnetalelementaloxygen,isfedtothehigh-sulface-aeracathodemadeofacarbonchipcomposite,whereitformsH2O2andcaustic.TheconcentrationofH2O2producedis30-80g/Landtheperoxide/causticratiocanbeadjustedtothatneededinthebleachingprocess.DuPonthasunveiledasystemfordirectlyreactinghydriogenandoxygenintohydrogenperoxide,whichisscheduledforcommercialimplementationinthe1990’7.3CHLORINATIONANDEXTRACTIONChlorinationandalkalineextractionhavetraditionallybeenemployedinthefirsttwostagesofapulpbleachingsequenceasameanstoeffectivelydelignifythepulp.Thechlorinereactsselectivelywiththenon-carbohydrateconstituents,renderingthemwater-solubleorsolubleinalkalinemedia.Mostofthechlorinationreactionpriductsareactuallyremovedinthesubsequentalkalineextractionstage.Theindustryiscurrentlymovingawayfromelementalchlorineasapulpbleachingagentbecauseofconcernsabouttoxicchlorinatedorganiccompoundsintheenvironment.However,mostmillswillcontinuetouseamodifiedchlorination/extractionformanyyearstocome.ChlorinationChlorinereactswithligninprimarilybymeansofsubstitutionandoxidation.Insubstitution,chlorinereplacesahydrogenontheorganicmoleculewiththesimultaneousformationofahydrochloricacidmolecule:RH+Cl2RCl+HClWhereRrepresentstheorganicmolecule.Oxidationcanbepicturedasformingelementaloxygenwhichreactswiththepulp:

H2O+Cl2O+2HClTheamountofHClwhichisformedduringchlorinationisindicativeofthereactionbalance.Generally,morethan50%oftheappliedchlorinereactsbysubstitution,whichisthepreferredreactionmode.Oxidation-typereactionscanhavearelativelymoredegradingeffectonthecellulose.Traditionally,thehigh-densitybrownstockenteringthebleachplantforchlorinationwasdilutedwithambientfreshwatertorelativelylowcinsistency(2.5-3.5%)tofacilitatemixinganddissolutionofthegaseouschlorineintothepulpstockandtodissipatetheappreciableheatofreaction.Sinceallotherbleachstagesareoperatedatmediumconsistency(10-13%)orhigher,thechlorinationstagewasconspicuousasamajorconsumerofwater.Chlorinationeffluentisnowcommonlyrecycledforstockdilutioninthesamestage,andthispracticehascausedasharpriseinchlorinationtemperature.Millsusingonlyfreshwaterusuallycarryoutchlorinationat20-30度,whilethosemillsutilizingsignificantrecyclemayoperateashighas60度.Manymillsarealsoconveningtoamedium-consistencychlorinationstagebyutilizingthelatestmixingtechnology.ThevariableaffectingchlorinationarelistedinTable7-3.TimeandtemperaturearedirectlyinterrelatedasillustratedinFigure7-7.Unfortunately,mostexistingchlorinationtowersareoffixedretentiontime,providingbetween45and90minutesundernormaloperatingconditions.Thechemicalapplicationmustbesufficienttoprovideadequatedrivingforceandensurethedesireddegreeofchlorination;theamountappliedisusuallyabout75-80%ofthefullchlorinedemandasindicatedbythekappanumbertest.Typically,chlorineapplicationsareintherangeof6-8%onoven-drypulpforsoftwoodkraftand3-4%forsulfitepulpandhardwoodkraftpulps.Measurementeutherofstockcolororoxidation/reductionpotentialataselectedpointdownstream(aftermixingandinitialreaction)canbeusedasthebasisforrapidfeedbackcontrolofchemicalapplicatin.Unfortunately,noautomaticcontrolmethodiscapableofrespondingtolarge,rapidchangeinlignincontent.Manuallappanumbertestingoftheincomingstockisstilladvisable,buttheneedcanbeminimizedbystockblendingpriortothebleachplant.Table7-3VariableaffectingchlorinationInthetraditionalchlorinationstage,itwascustomarytomaintainasmallresidualchlorineconcentrationintheretentiontowerdischargetoensurethatthereactionhadnotstoppedprematurely.However,withhigh-temperaturechlorination,thechemicalmustbetotallyconsumedinashortperiod;otherwiseseveredegradationofthecelluloseresults,pHisextremelyimportantinchlorine-watersystemsasthedeterminantofthechlorinespeciesmix.Inacidmedia,thefollowingequilibriumexists:1.Cl2+H2OHOCl+H+ClWhenabaseispresent,adifferentequilibriumexists:2.HOCl+OHOCl+H2OTherelaativeproportionofmolecularchlorine,hypochlorousacid(HOCl)andhypochloriteion(Ocl)insolutiondependsonpHasillustratedinFigure7-8.Eachchlorinespeciesisdistinctivewithrespecttoitsactiononligninandcellulose.Hypochlorousacidcanbequitedestructivetocellulose,andtherefore,bleachingwithinthepHrangeof2-9isusuallyavoided.Duringtraditionalchlorinationwithfreshwaterdilution,enoughhydrochloricacidisformedtokeepthepHbelow2.0wherethedominantspeciesismolecularchlorine.RecycleofchlorinationeffluentservestofurtherreducethepHofthechlorinationstageandtherebyalmosteliminatethedeleteriouseffectofthehypochlorousacid.Agreaterchlorideionconcentrationalsoservestoshifttheequilibrium(ofreaction#1above)towardahigherrelativeconcentrationofmolecularchlorine.OverthepasttwodecadesithasbecomeestablishedpracticeinbleacheriesutilizingchlorinedioxidetoapplyasmallportionofClO2inthechlorinationstage,displacingperhaps5to10%ofthechlorineonachemicalequivalentbasis.TheClO2servestominimizedegradationtothecellulose,especiallywhererecycleofchlorinationeffluentisnotpracticedandpHisnear2.0.Otheradvantagesclaimedarehigherfinalbrightness,betterbrightnessstablity,andreducedeffluentcolor.Numerousinvestigationswerecarriedouttodeterminethemostefficientlevelandnethodofapplication,withsomeconflictingclaims.MostmillsapplytheClO2priortotheCl2additiontoensurequickreactionandnocarryoverofresidualchemicalintothewashersystem,thusavoidingcorrosionproblems.TheImportanceofMixingAmodern,versatilechlorinationsystemisshownschematicllyinFigure7-9.Anexampleofanupflow,pump-throughtowercommonlyusedforchlorinationisdepictedinFigure7-10.Thotoughmixingofthechlorinegaswiththedilutionwaterandwiththestockisessentialtoensureuniformityofchemicaladditionisalsohelpfultowardthatobjective.Becauseofthelowsolubilityofmolecularchlorineinwater(especiallyathighertemperature),themixermustrapidlyanduniformlycombinethegas,liquidandpulp.Asubstantialinputofmechanicalenergyisrequired.Itisestimatedthat80%ormoreofthechlorineisattachedtothepulpwithintheinitial40seconds.Sincepulpcanconsumemorechlorinethanrequiredtosolubilizelignin,poormixingwillcauseaportionofthepulptobeover-chlorinatedwhileanotherportionisunder-chlorinated.Manydifferentmethodsofmixingareemployedinmodernchlorinationsystems.Aseriesofsmallagitatedtanksisadvantageousfromthestandpointofsequentialchemicaladditionandlocationofsensors.Aseriesofin-linestaticmixerswithtanksHIbetweenprovidesthesameadvantages.ExamplesofchlorinemixersareshowninFigure7-11to7-13.TheDioxinProblemIn1986,theproductionprocessforbleachedchemicalpulpwasidentifiedasacontributorofpolychlorinateddioxinanddibenzofuranstotheenvironment.Sincethesecompounds(hereafterreferredtosimplyasdioxinsandfurans)arerecognizedaspowerfultoxinsandcarcinogens,theirdetectioninbleachplanteffluentspromptedaflurryofinvestigativeactivityinEuropeandNorthAmericatoidentifypointsourcesandcorrectivemeasures.Thechlorination/extractionsequencewassubsequentlyfoundtobethemajorsourceofthesecompounds.Theindustrymovedrapidlytoimplementmeasurestoreduceoreliminatetheformationofdioxinsandfurans.Forexample,mostmillsnolongerutilizeoil-baseddefoamersortoleratewoodchipscontainingpreservativesbecausethesechemicalproductsareknowntocontainprecursorsofdioxinandfruans.Unfortunately,thewoodfurnishitselfalsoappeartocontainprecursors,andbasicprocesschangesarenecessarytofurtherreducedischargesto“non-detect”levels.Themosteffectiveactionsappeartobeeither:1.high-level(over50%)substitutionofchlorinewithchlorinedioxideinthechlotinationstage.2.step-wise,incrementalchlorinationutilizingalow“chlorinemultiple”(i.e.,ratioofappliedchlorinetokappanumber),thusavoidingconcentrationsofchlorinethatleadtotheformationofhighlysubstitutedcompounds,whilemovingaportionofthedeligninficationloaddownstream.ChlorinatedOragnicCompoundsInadditiontodioxinandfurans,ahostofchorinatedor