無機高分子聚合鐵鹽類混凝劑制備方法評述

無機高分子聚合鐵鹽類混凝劑制備方法評述

摘要：綜述了無機高分子聚合鐵鹽類混凝劑的幾種生產制備方法。對用硫酸亞鐵經(jīng)催化氧化法和直接氧化法制備聚合硫酸鐵，用各種含鐵工業(yè)廢渣制備復合聚鐵，以及采用生物和電化學方法制備聚鐵類混凝劑等方法的優(yōu)缺點等作了較全面評述。還對固體聚合鐵的生產及液體聚合鐵混凝劑的穩(wěn)定性進行了討論，對目前聚合鐵生產中存在的問題及發(fā)展方向提出了建議。

關鍵字：聚合硫酸鐵

硫酸亞鐵

混凝劑

硫鐵礦燒渣ProgressonManufacturingMethodofInorganicPolymerCoagulantPolyferricsaltcategoryAbstract:Thispapergivesareviewofthemanufacturingmethodofinorganichighpolymercoagulantpolymerizedferricsulfate.ThepresentsituationofthestudiesandtheproductionofPFSisdiscussed.ItpointsoutthemeritandtheshortcomingofthemanufacturingmethodsofPFSwithferroussulfate,pyritecinderandotherslagscontainingiron.SomesuggestionsaboutpresentproblemsanddevelopmentdirectionintheproductionofPFShavebeenputforward.Keywords

Polyferricsulfate,ferroussulfate,coagulant,pyritecinderIntroduction

Polymerizedferricsulfate(PFS)isanewinorganichighpolymercoagulant.ItsmolecularformulaisFe2(OH)n(SO4)3-n/2]m,(n>2,m=f(n)).Itcanbeusedinwastewatertreatmentandthepurificationofdrinkingwater[1].Comparedwithgeneralinorganiccoagulants,forexample,ferroussulfate,ferricchloride,aluminumchloride,aluminumsulfate,alumflocandpolymerizedaluminumchloride(PAC),PFSisthebestcoagulant.Itscoagulatingabilityisstrong,sinkingspeedisquick,andproducingcostislow.Inaddition,theleavingofaluminumismoreinthewatertreatedwiththecoagulantscontainingaluminum,anditisharmfultothehealthofhumanbody[2].Thisrestrictsthedevelopmentofaluminumcoagulants,andparticularlyindrinkingwatertreatment.Nevertheless,theleavingofferricislittleinthewatertreatedwithPFS,anditisnotharmfultohumanbody.Atthesametime,PFSisalsosuperiortoorganiccoagulantssuchaspolyacrylamide(PAM)insomeindustrialwastewatertreatment[3].WhenPFSsubstitutedforPAMinwatertreatment,theharmofmonomerAM,whichwasleftinwaterafterPAMwasdegraded,couldbeavoided.SotheapplicationprospectofPFSisverygoodandthemarketcompetitionpowerisstrongerinindustrialwastewatertreatment,industrailusingwater,andlifedrinkingwater.

In1972,JapanputforwardPFSasoneofcoagulants,andappliedforthefirstpatent[4]ofPFSmadefromferroussulfate.Thenindustrialscopeproductionandapplicationinwatertreatmentwereformed.ChinastartedthestudiesofPFSin80'searly.ThemanufactureofPFSmadefromferroussulfatewasdevelopedsuccessfullybyTianjinResearchAcademiesofChemicalIndustryunderMinistryofChemicalIndustryofChinain1982.TheproductionofPFSwasindustrialized[5]in1984.Hereafter,popularizingapplicationstepbystepinWuhanironandsteelcompany,etc.Now,PFShasbeenuniversalconcernedandappliedinChinaasanewinorganichighpolymercoagulant[6].Atpresent,themajormanufacturingmethodofPFSistheoxidationmethodofferroussulfate.Ferroussulfateandsulfuricacidareimportantrawmaterialsinthismethod.PFScanbemadebytheoxidationofferroussulfateandthehydrolysisandthepolymerizationofferricsulfate.Anothermethodisthatsomeironoredregs(orwasteresidue)andsulfuricacidareusedastherawmaterialsofPFS.Ironoredregsaredissolvedinsulfuricacid,thenhydrolysis,polymerization.1ManufactureofPFSwithferroussulfate

Ingeneral,ferroussulfate,whichisaby-productproducedintheproductionprocessoftitaniumwhite(titaniumdioxide),andsulfuricacidareusedastherawmaterialsofPFS.Besides,acidwashingwasteliquids,scrap-iron,andsulfuricacidcanalsobeused.Inthemanufacturing

processofPFS,thereactionsofoxidation,hydrolysis,andpolymerizationareallexistentialatthesametime.Becauseofthecauseofdynamics,theoxidizingreactionofferroussulfateisslowerunderaciditycondition.Sotheratecontrollingstepofthereactionsystemistheoxidizingreaction.Thisreactionmaybedirectedoxidationorcatalyzedoxidationaccordingtodifferentoxidizingprocess.1.1Methodofdirectedoxidation

Themethodofdirectedoxidationisthatferroussulfatewasoxidizeddirectlyintoferricsulfatebyoxidizer.Theoxidizerusedherewashydrogenperoxide[7],manganesedioxide,potassiumchlorate[8],sodiumpersulfate,sodiumhypochloriteornitricacidandsoon.Theoxidizingreactioncanbecompletedwithintwohoursundertheconditionofroomtemperaturewhenhydrogenperoxideisusedasoxidizer:

2FeSO4+H2O2+H2SO4=Fe2(SO4)3+2H2O（1）Then,hydrolysisandpolymerization:

Fe2(SO4)3+nH2O=Fe2(OH)n(SO4)3-n/2+n/2H2SO4

（2）

m[Fe2(OH)n(SO4)3-n/2]=[Fe2(OH)n(SO4)3-n/2]m（3）Thetimeoftheoxidizingreactionmaybeshortenedfurtherundertheconditionof50℃.

Themethodofdirectedoxidationhassomemerits,suchassimpleprocess,easyoperation,andshorterreactiontime,buttheconsumptionofoxidizersishighandthepriceisexpensive.SoPFSmadebythemethoddoesnothavemarketcompetitionpower.

Inthemethodofdirectedoxidation,literatures[9]putforwardanotherwaywhichwaseasytooperateandpopularize.Thewaywasbasedonsomefeaturesoftheinstabilityofferroushydroxideandbeingeasytobeoxidizeintoferrichydroxide.Firstly,ferroussulfatewaschangedintoferroushydroxidebyaddingalkaliinthesolutionofferroussulfate:

FeSO4＋2NH3·H2O＝Fe(OH)2＋(NH4)2SO4

(4)Secondly,ferroushydroxideforminginthereactionabovewasoxidizedintoferrichydroxideafterbeingplacedinair:

4Fe(OH)2＋O2＋2H2O＝4Fe(OH)3

(5)Then,oxidizersolutionwasobtainedthoughthereactionbetweenhydrogenperoxideandferroussulfateaccordingtotheequation(1).Finally,PFSwasmadebymixingtheferrichydroxidesuspensionandtheoxidizersolutionwithin2hoursunderthestirredcondition.

2Fe(OH)3＋H2SO4＋Fe2(SO4)3?[Fe2(OH)n(SO4)3-n/2]m＋H2O(6)

Thesereactionsmentionedabovecantakeplaceinroomtemperature.TheprocessingequipmentofthemethodaresimpleanditiseasytooperateandthecostofPFSislowtoo.

Literature[9]reportedthatthestabilityofPFSmadeinthiswaywaspoorer.Infact,theinstabilityofPFSmadeinothermethodsmentionedinfollowingisexistenttoo.ThecriticalfactorsaffectingthestabilityarepH,Fe3+/SO42-oraddingstabilizerinPFS.1.2Methodofcatalyzingoxidation

Themethodofcatalyzingoxidationisthatferroussulfateisoxidizedintoferricsulfatebyoxidizerundertheeffectofcatalyst.ItisamajormethodofmanufacturingPFSwithferroussulfate.Manystudies[10-15]havebeenmadeinthisaspect.Undertheconditionsofhavingoxidativecatalyst

andsuitabletemperature,PFScanbemadethroughtheoxidationofferroussulfate,hydrolysis,andpolymerization.Thecatalystsusedinthereactionofcatalyzingoxidationweresodiumnitrite,manganesedioxideorchloride.Atpresent,themostproductioncompaniesofPFSusethismethod.

ThemanufactureofPFSwithferroussulfateandsulfuricacidwasreportedfirstlybyJapanpatent[4].Whensodiumnitriteandairwereusedascatalystandoxidizerrespectively,themechanismofcatalyzingoxidationcanbeshowedasfollows:

2FeSO4+H2SO4+2NaNO2＝2Fe(OH)SO4+Na2SO4+2NO

(7)

FeSO4+NO＝Fe(NO)SO4

(8)

2Fe(NO)SO4+1/2O2+H2SO4＝Fe2(SO4)3+2NO+H2O(9)

2NO+O2＝2NO2(10)

2NO+1/2O2＝N2O3

(11)

2FeSO4+N2O3+H2O＝2Fe(OH)SO4+2NO(12)or

2FeSO4+N2O3+H2SO4＝Fe2(SO4)3+2NO+H2O(13)

2FeSO4+NO2+H2O＝2Fe(OH)SO4+NO

(14)or

2FeSO4+NO2+H2SO4＝Fe2(SO4)3+NO+H2O(15)

Ferricsaltsproducedinthesereactionswerehydrolyzedandpolymerized:

2mFe(OH)SO4+(1-n/2)mH2SO4＝[Fe2(OH)n(SO4)3-n/2]m+(2-n)mH2O

(16)or

Fe2(SO4)3+m×nH2O＝[Fe2(OH)n(SO4)3-n/2]m+nm/2H2SO4(17)

ItcanbeseenfromthereactionsabovethatthemechanismoftheoxidizingreactionofferroussulfatehadbeenchangedowingtotheparticipationofNOwhichmadetheoxidizingreactioneasy.Thecatalyzingoxidationtimewasshortenedto4~5hoursfromoriginal17hours.Evenso,thetimewasstilllongerandtheconsumptionquantityofcatalystwasalsomore.Meanwhile,nitrogenoxidesweredischargedintoairinthemanufacturingprocessofPFSandthisresultedintheenvironmentalpollution.

Forthesakeofshorteningthereactiontime,somemeasuressuchasheatingandhighpressureandstrongstirring,wereoftenadoptedinthemanufacturingprocess.Amongthem,themethodoftwostepsoxidationcouldshortenthetime.Firstly,ferroussulfatewasoxidizedintoferricsulfateundertheheatingcondition.Secondly,bothferroussulfateandhydrogenperoxidewereaddedinthesolutionofferricsulfatemadeaboveaccordingtoproportion.Then,PFSwasobtainedafteronehourreactionunderthestirredcondition.

Inordertoavoidpollution,Shortenfurtherreactiontime,andreduceenergyconsumption,themanufacturingtechnologyandthemanufacturingmethodofPFShadbeeninvestigatedthoroughly.Someprogresseshadbeenmade[16],forexample,thecatalysisoxidizingreactiontakenplaceinahermeticcontainerandtheproportionofSO42-/Fe3+wascontrolledinthereactionsystem.Inaddition,takingtheplaceofair,oxygenwasusedasoxidizer.Insuchsituation,PFSwasobtainingundertheconditionofstirringstronglywithin1.5~2hoursand55~90℃and29.4-147′104Pa.Neverthelesstheseconditionsofhightemperatureandhighpressureandstrongstirringwillincreasethedifficultyoftechnologyandtheinvestmentofequipments.Furtherimprovementwasthatthespeedofaddingsulfuricacidwascontrolledandsodiumnitritecatalystwasaddedinbatchesintheoxidizingprocessofferroussulfate.Undertheconditionsofnormalpressureandabove40℃,theoxidizingreactioncouldachievedwithin2-3hourswhenoxygenwasusedasoxidizer[13].Thisreportthoughtthatthemethodofaddingsulfuricacid,thatisthePHofreactionsystemwascontrolledincertainscope,wasthekeyofshorteningthereactiontime.BecausetheoxidizingreactionofferroussulfatewasverydifficultintheconditionoflowerPH,butitcouldtakeplaceeasilywhenthePHwashigherthan2.

Inaddition,literature[17]developedanewmanufacturingtechnologyofPFSwithanatomizationmethod,thatis,undertheconditionsofnormalpressureandheating,theatomizationdripofferroussulfatewascontactedwithnitrogendioxideincirculationstate.Theaimofthisprocesswastoraisethecontactareabetweennitrogendioxideandferroussulfatesolution,quickenreactionvelocity,andshortenproductioncycle.TheprocesspioneeredanewwayfortheproductionofPFS.Ofcourse,thiswillalsoincreasethedifficultyoftechnologyandtheinvestmentofequipment.

Moreover,itwasreportedthatthebestkindofcatalysthadbeenfound[18]alreadyuptonow.Thiscatalystwasnotonlyhigheffectivebutnonpoisonous.Whenitwasusedintheoxidizingreactionofferroussulfate,thereactioncouldbecompletedintwohours.WehopeitcanbeusedintheindustrialproductionofPFSearly.

InthemanufacturingprocessofPFSabove-mentioned,thereactionofferrousandnitrogenoxidesbelongstoareactionbetweenliquidandgas.Inordertoincreasetheircontactareaandquickenthereactionspeed,somemeasures,suchasheating,high-pressure,strongstirring,andspraying,wereoftenused.So,thetechnologyofthemethodiscomplicatedandtheinvestmentcostofequipmentisalsohigher.

Besides,inotherreport,potassiumchlorateorhydrogenperoxidewereusedasasubstituteforairoroxygeninthesystemofsodiumnitrite.Althoughthiscanquickenthespeedofcatalysisoxidizingreaction,itincreasestheproductcost,too.1.3Effectofcatalystpromoteronthereactionofcatalyzedoxidation

BecausetheoxidizingreactionisaratecontrollingstepinthemanufacturingprocessofPFS,shorteningthereactiontimehasalreadybecomeafocalpointconcerned.Sopeoplehadmadethoroughstudies[3,6,18]attheaspectsofoxidizer,catalyst,andcatalystpromoter.Literature[3]hadmadesomedetailedstudiesonthekindandtheadditionofcatalysts,theselectionofcatalystpromoters,thetemperatureandtheacidityofreactionsystem.Thisliteraturethoughtthatsodiumiodideasacatalystpromotercouldquickenthereactionspeed.Atthesametime,italsothoughtthatcontrollingthePHofthereactionsystemwasakeytoquickenthespeedofoxidizingreaction.Itdiscussedhowsodiumiodidequickensthespeedoftheoxidizingreaction,too.

Onthebasisofinspectingtheeffectofcatalystsandoxidizersonthespeedofcatalysisoxidizingreaction,literature[6]reportedthatthetracecatalystpromoterofHG-1,HG-2,orHG-3couldnotablyraisethespeedinthereactionsystemofoxygenandsodiumnitrite.Thismadenotonlytheconsumptionofcatalystdecreasesoneathirdbutthereactiontimesshortentwothirds.Inaword,sodiumnitritewasgenerallyusedasthecatalystintheprocessofcatalysisoxidizingreaction.Itshoudbepointedoutthatsodiumnitriteisacarcinogen,andsomenitricoxideswereletintheprocess.Thisresultsinenvironmentalpollution.Inordertosimplifytechnology,cutdownthecostofproductanddecreasepollution,thedevelopmentdirectionofPFSmadewiththecatalysisoxidizingmethodshouldbestudyingandimprovingfurtherthemanufacturingtechnology,anddevelopingnewcatalystsandcatalystpromoterswhicharehigheffectiveandnonpoisonous.2ManufactureofPFSwithslagorwasteresidue

ItisagoodmethodofturningwasteintousefulproducttomanufacturePFSfromslagorwasteresiduecontainingoxidecompoundofiron.Itcannotonlysolvethethree-wasteproblem,andalsocutdownthecostofPFS,simplifymanufacturingtechnology,havebettereconomicbenefit.2.1ManufactureofPFSwithpyritecinder

PyritecindercanbeasarawmaterialofPFS.ThisisoneofmajormethodspreparingPFSwithslagorwasteresidue.Avastamountofpyritecinderisproducedintheproducingcourseofsulfuricacid.Itsmaincomponentsareferricoxideandferriferrousoxide.Attheaspectofthecomprehensiveutilizationofpyritecinder,thegreaterpartofpyridecinderwasusedasanassistantintheproductionofcement.

Ingeneralsituation,totalironcontentisabout65~65%inpyritecinderandferriccontentis96.5%intotaliron.Afterpyritecinderisdissolvedinsulfuricacid,thefiltratingcandirectlybeusedinthemanufactureofPFS.Thishasdevelopedanotherbetterwayforthecomprehensiveutilizationofpyritecinder.ThemanufacturingmethodofPFSusingpyritecinderiseconomical,anditscostisalsolower.Atpresent,manyprogresseshadbeenmadeinthisaspect[19-22].

ThemanufacturingprocessofPFSusingpyritecindermaybedividedintotwosteps.Thefirststepisthatpyritecinderisdissolvedinsulfuricacid:

Fe3O4+4H2SO4=Fe2(SO4)3+FeSO4+4H2O

(18)

Fe2O3+3H2SO4=Fe2(SO4)3+3H2O(19)Thesecondisoxidation,hydrolysisandpolymerization:

FeSO4＋2H2SO4＋O2＝2Fe2(SO4)3＋2H2O(20)

Fe2(SO4)3+nH2O＝Fe2(OH)n(SO4)3-n/2+(n-2)H2SO4

(21)

mFe2(OH)n(SO4)3-n/2＝[Fe2(OH)n(SO4)3-n/2]m(22)

Inthesecondstep,ferroussulfatemaybeoxidizedwithdifferentmethods.Oneisdirectedoxidation[19].Anotheriscatalysisoxidation[41].Moreover,whenthecontentofferroussulfateisverylowinthefiltrating,theoxidationmaybeleftoutandboththehydrolysisandpolymerizationcantakeplacedirectly[21,22].Inthemanufacturingprocess,itisimportanttoimprovethedissolvingrateofpyritecinderfurtherlyinsulfuricacid.Author'sstudiesindicated[23,24]thatthedissolvingrateisconcernedwithsulfuricacidconcentration,temperature,andtheratiobetweensolidandliquid.Becausethedissolvingcourseofpyritecinderisanexothermicreaction,thedissolvingratemayachieve95%undertheeffectofthereactionheat.Thecontentofferrousonlyoccupies3%ofthetotalironcontentinthefiltrating.PFScanbeobtainedthroughthehydrolyzingandthepolymerizingofferricsulfateundercertaincondition.Oxidizerisnotneededinthemanufacturingprocess.Thishasnotonlyeliminatedthepollutionofnitricoxides,andalsosavedspendingandcutdownfurtherthecostofproduct.

WhenPFSmadebyourselveswasusedinthetreatmentofpapermakingwastewater(CODCr=1000mg/L),theremovalrateofCODCrwasmorethan90%astheadditionofPFSwas150mg/L(thatis,[Fe3+]=150mg/L).Inaddition,theremovalrateofCODCrwasmorethan60%inthetreatmentofsaccharinsodiumslatwastewater(CODCr=15000mg/l)astheadditionofthePFSwas800mg/L.

Inordertoraisethedissolvingratefurtherly,pyritecindermaybepreprocessed.First,

ferricoxideinpyritecinderwasreducedasferrousoxideundertheeffectsofbothhightemperatureandreducer.Pyritecinderwaschangedintoreductiondregs.Second,thereductiondregswasdissolvedinsulfuricacid,obtainingmid-productferroussulfate.Final,PFSwasmadethroughoxidationandhydrolysisandpolymerizationasmentionedin1.2.

Itwassaidthatthismethodcanraisetheutilizationratioofpyritecinder,neverthelesstherewasnotonlyoxidizingreactionbutalsoreducingreactioninthemanufacturingprocessofPFS.Soitwillresultinlongerproducingperiodandhigherconsumption,anditspopularizationandapplicationwillbeverydifficult,too.

2.2ManufactureofPFSwithotherslagscontainingiron

Siderite,magnetiteore,ironmud,naturalironsandandopen-hearthfurnacedust[25-28]arealsotherawmaterialsofPFSbesidespyritecinder.Thecontentsofferriferrousoxideandferricoxidearehighinthem,too.

Literature[26]hadobtainedbetterresultinthemanufactureofPFSusingironmud.Ironmudisthewasteproducedintheproducingprocessofdyemediums.Itsmaincomponentareferriferrousoxideandalittleferricoxide,ferrousoxide,andironbesides.InChina,thetreatmentmethodofironmudisgenerallytostoreuportobury.Thiscauseseasilyenvironmentalpollutionagain,andtheusefulresourceinironmudisalsowasted.ItisagoodwayofwastecomprehensiveutilizationforironmudtobeusedastherawmaterialofPFS.

NaturalironsandisakindofrichmineralresourcesoftheDabieshanMountainsarea.Totalironcontentis59.6%init,andferriccontentis41.1%.ItisagoodrawmaterialofpreparingPFS.Literature[27]reported,usingnaturalironsandandsulfuricacidastherawmaterials,highconcentrationPFS(220g/L)couldbemadethroughpickling,oxidizing,andpolymerizing.Then,theliquidPFSwasconcentratedintosolidbyspray-dryingprocess.

Open-hearthfurnacedustisanotherwasteusedalsotomanufacturePFS.Itisthesmokeanddirtintheopen-hearthfurnacesteelmakingwastegas.Thecontentofferricoxideismorethan90%init.Literature[28]reportedthatthedustcouldbeusedastherawmaterialofPFS:Firstly,thedustwasdissolvedinalkalisolutionforremovingsomeimpurityinit.Thecontentofferricoxidewasmorethan99.5%inthedustpurified.ThenPFScouldbemadethroughaciddissolving,hydrolysisandpolymerization.WhenthePFSwasusedtotreatwashingwastewaterofblastfurnacegas,thetreatedefficiencywasverygood.Moreover,becausethereisnototherharmfulioninthePFS,itisverysuitableforthePFStotreatdrinkingwater.

Inotherreports,wasteresiduecontainingferricoxidewasusedtomanufacturingPFS,too.Undercertaintemperatureandpressure,PFScouldbeobtainedbydissolving,hydrolysis,polymerizationwithoutoxidizer.

Onthewhole,themanufactureofPFSbyslagsorwasteresidueisaneconomicalmethod.Thismethodissimplerthanthecatalysisoxidationmethodmentionedabove.Certainly,therearestillsomeproblemsthatshouldbesolvedfurtherlyinthemanufacturingprocessofPFS.Forexample,thedissolvingrateofslagsshouldberaisedagain,soastoraisetheutilizationrateofrawmaterial.Inaddition,thereisexcessiveacidgenerallyinthesolutionofferricsulfateobtainedbydissolvingslagsinsulfuricacid.Thiswillincreasetheconsumptionquantityofotherassistantsinthecourseofhydrolysisandpolymerization.Iftherearesomeharmfulheavymetalsinslagsandwasteresidue,therewillbesomeheavymetalionsinPFS.Thiswillrestrictitsapplicationindrinkingwatertreatment.3ManufactureofPFSwithothermethods3.1Methodofbiologicoxidation

Usingpyrite,coal,sulfur,andhydrogensulfideasrawmaterials,PFScanbemadewiththemethodofbiologiccatalyzingoxidationundertheinfluenceofiron-sulphurbacteria,suchasthiobacillusferrooxidansandthiobacillusthiooxidans.Literature[29]investigatedsomefactorswhichaffectingthemanufacturingofPFSwithbiologicoxidation.ItreportedthatthesefactorswerepH,theadditionofpyrite,temperature,stirring,andreactiontime.UndertheconditionsofpH2.0~2.3,2.8gpyrite/L,30℃,and300r.p.m.,amid-productwasobtainedin10hoursreaction.ThenPXTcoagulant(thatisPFS)wasmadethroughacidification,concentration,andaging.WhenthePXTwasusedtotreatthehighSSashwaterofpowerplant,itcoulddecreasenotonlySSbutpHandF-.

Intheprocessofbiologicoxidation,boththiobacillusferrooxidansandthiobacillusthiooxidansplayanimportantrole.Theyarethepeculiarmicro-organismswhichcanobtaincarbonresourcesfromCO2inairandliveininorganicenvironment.Theycanoxidizeironandreducesulphurintoferricandsulfuricacid,respectively.

ThepreparationofPFSwithbiologicoxidationisaverygoodway.ThewaydoesnotneedanyotherrawmaterialsbesidespyriteandalittledilutesulfuricacidusedtoadjustthepHofreactionsystem.Itischaracterizedbysimpleproces,savingenergy.Nevertheless,manystudiesshouldbedoneifthemethodofbiologicoxidationisusedinindustrialproductionofPFS.Atpresent,thismethodisstillintheinitialstageofstudyanddevelopment.3.2MethodofelectrolyticoxidationItshouldbepointedoutthattheelectrolyticoxidationofferroussulfateisalsoapreparingmethodsofPFS.First,ferroussulfatewasoxidizedintoferricsulfatewithD.C.electricityinelectrolyticbath.Then,PFSwasobtainedthroughthehydrolysisandthepolymerizationofsulfateferric.However,theelectrolyticoxidationexpendsenormouselectricenergy.Itcannotbespreadedandappliedinindustry.4ManufactureofsolidPFS

Ingeneral,theproductofPFSisliquidstate.Thisproducthasthemeritofbeingeasytouseinwastewatertreatmentprocess,butitsshortcomingisthatthecostoftransportationishigherthansolidstatebecauseofitsvalidcomponentlowerthansolid.SosomestudiesaboutthemanufactureofsolidPFShadbeenmade.

SolidproducthadbeendevelopedthoughconcentratingliquidPFS[9,12,15,27],suchasheatingevaporization,stoving,andvacuumevaporationanddrying.Wecallthesemethodsthetwo-stepprocessasitincludesboththepreparationofliquidPFSandtheconcentration.KreplkaJ[30,31]reportedthatsolidPFScouldbemanufacturedbyoxidizingthesolutionofferroussulfatewithairundertheconditionsof120~180℃and0.3~2.5MPa.Inaddition,PrasakTP[32]developedasimplemethodofmanufacturingsolidPFS:firstly,preparingsolidbasicferricsulfatebyoxidatingferroussulfatewithairinarevolvingblastfurnaceundertheconditionsof200℃and3hours,andthenobtainingsolidPFSbymakingthesolidbasicferricsulfatereactswithsulfuricacid.Theshortcomingofthemethodisthattheoxidationofferroussulfateisnotcompleteinthefirststep.Usingsolidrawmaterials,LiKechang[33]alsosucceededinmanufacturingsolidPFSthroughsolidphasereaction.

Recently,WangDuoren[34]reportedthatsolidPFScouldalsobemadebyone-stepprocess.First,mixturingthesolutionofpotassiumchlorateandthesuspensionofferroussulfate.Then,obtainingsolidgranulatedPFSundertheconditionsof100℃andpH2-3after1hourreaction.

AlthoughhavingovercometheshortcomingofthehightransportationcostofliquidPFS,solidPFSconsumesenormousheatenergyinmanufactureprocess,andithavetobedissolvedagainbeforebeingusedinflocculationprocess.SothesuitablemethodofmanufacturingPFSshouldbetomakehighconcentrationliquidPFS.

5StabilityofPFS

IfliquidPFSislong-termstored,Fe(OH)SO4sedimentwillbeproducedinPFSsolution,thatis,liquidPFSisunstable.ThestabilityofPFSisaveryuniversalandveryimportantproblem,anditmustbeconsideredinthemanufactureandtheapplicationofPFS,butthisproblemisreportedseldominmostliteraturesaboutPFS.

LiuChangrangetal.[9]hadstudiedthestabilityofPFSandpointedoutthatliquidPFS,whichwasmadefromferroussulfatewithcatalysisoxidationmethod,wasunstable.OnestabilizerwasaddedinliquidPFSinordertostrengthenthestabilityofPFS.Inaddition,ShuWangenetal.[35]andWangShicaietal.[36]hadstudiedthestabilityofpolymerizedferricchlorideandpolymerizedaluminumsulfate,respectively.TheirexperimentalresultsshownthatstablepolymerizedferricchloridecouldbemadebyaddingstabilizerS-3insolution,andtartaricacidwasagoodstabilizerofpolymerizedaluminumsulfate.

Author'sstudiesindicatethatliquidPFSmadewithothermethodsalsohastheinstability.ThestabilityofPFSisrelatedtothePHofsolutionandtheconcentrationofSO42-insolution.So,besidesaddingsomestabilizersinPFS,bothadjustingthepHandcontrollingtheconcentrationofSO42-areaeffectivemethod,too.

Finally,itshouldbepointedoutthatchangingliquidPFSintosolidthroughevaporationanddryingisagoodwayofsolvingtheinstabilityofliquidPFS.6Conclusion

Comparingwithothercoagulants,PFSisthebestkindofinorganicwatertreatmentagentsanddoesn’tproducetwicepollution.Ithasawideapplicationprospect.Thecatalysisoxidizingmethodofferroussulfatehadbeenpopularizedandappliedinindustrialproduction.ItisamajormanufacturingmethodofPFSatpresent.However,therearestillsomeproblemswhichshouldbesolvedinmanufacturingprocess.