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New technology for comprehensive utilization of aluminum-chromium residue from chromium salts production

By Gloria Diaz,2014-07-08 21:26
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New technology for comprehensive utilization of aluminum-chromium residue from chromium salts productionof,for,New

    New technology for comprehensive utilization of aluminum-chromium residue

    from chromium salts production Science

    Press

    Availableonlineatwww.sciencedirect.com

    ?CIENCE

    Trans.NonfeITOUSMet.Soc.Chinal8(2008)463468

    Transacfionsof

    NonferrousMetals

    SocietyofChina

    WWW.CSU.edu.cn/ysxb/

    Newtechnologyforcomprehensiveutilizationof

    aluminumchromiumresiduefromchromiumsaltsproduction

    LIXiaobin(李小斌),QITiangui(齐天贵),JIANGXinmin(~新民),

    ZHOUQiusheng(~秋生),LIUGuihua(~Q桂华),PENGZhihung(彭志宏)',

    HANDeng.1un(韩登仑),ZHANGZhongyuan(张忠元),YANGKunshan(杨昆山)

    1.SchoolofMetallurgicalScienceandEngineering,CentralSouthUniversity,Changsha410083,China;

    2.GansuJinshiChemicalEngineeringCorporation,Ltd.,Zhangye734000,China Received20April2007;accepted24August2007

    Abstract:Colloidalaluminum.chromiumresidue(ACR)wasmass-producedinchromateproductionprocess.andthelargeenergy

    consumptionandhighrecoverycostexistedintraditionalmethodsofutilizingsuchACR.Toovercomethoseproblems,anew

    comprehensivemethodwasproposedtodealwiththeACR,andwasprovenvalidinindustry.I

nthenewprocess,thechromatewas

    separatedfirstlyfromthecolloidalACRbyripeningandwashingwithadditives.bywhichmo

    rethan95%hexavalentchromiumwas

    recovered.Thechromium-freealuminumresidue(CFAR),afterproperlydispersed,wasdig

    estedatl2Ol30?andmorethan90%

    aluminacanberecovered.Andthenthepregnantaluminatesolutionobtainedfromdigestion

    wasseededtoprecipitatealuminum

    hydroxide.Thisnewmethodcansuccessfullyrecoverbothaluminaandsodiumchromate,an

    dthusrealizethecomprehensive

    udlizationofACRfromchromateindustry.

    Keywords:utilization;aluminum

    chromateresidue(ACR);aluminumhydroxide;sodiumchromatehydrate 1Introduction

    Theproductionofchromatewithchromiteasraw

    materialisoneofthemainpartsofinorganicchemical industry.Butthewasteresiduesproducedinchromate industryresultinpollutionseriously,whichrestrainsthe furtherdevelopmentofchromate『卜21.Oneofthemain

    solidwastesinchromateplantisaluminumchromate

    residue(ACR),theamountofwhichdependsonthe

    componentsofthechromiteandtheconditionofthe

    oxidativesinteringprocessinchromateproduction.In thetypicallimefreechromiumextractionprocessfor

    sodiumchromateproduction,600-800kgACR

    (Na2Cr207?2H2022%28%,AI(OH)38%l5%,andH2O

    50%-65%,isgeneratedwhen1tsodiumchromateis

    produced.TheACRiscomposedofaluminum

    trihydrateandamountofsodiumchromatewithvirulent Cr(?,tIlatcausesseverepollutiontotheenvironment.It isnecessaryandurgenttoeliminatechromepollutionin

theproductionofchromate[251.

    Untilnow.severalprocessesforchromaterecovery fromACRshavebeenproposed,suchasroasting

    washingmethod[1,67],precipitationwithphosphateco

    precipitation[1,6-7],washingwithpressurefilter[1,7], redispersedflocculationprocess[1,78].Althoughthe

    roastingwashingmethodoperatedat700-l000?can

    obtainhighchromaterecoveryrateandhighchromate concentrationwashingsolution.thismethodwaslimited foritscostlyequipmentinvestment,intensivelabor,poor operationalcircumstanceandhighsodiumchromate recoverycost[6].111ephosphatecoprecipitationmethod

    canbeoperatedatanimprovedoperationenvironment. buttheunstableefficiency,expensiverecoverycostand beingapttoleadingsecondarypollutionmadeitdifficult torealizeindustrialproduction[1,7].Inthewashingwith pressurefiltermethodandthere.dispersedflocculation process,duetothepoorosmosisintheACRparticles, only20%-40%ofchromaterecoveryratecanbe

    achieved[7].Therefore,thosemethodsmentionedabove canhardlybeutilizedinpracticalway.

    Inaddition.thepreviousprocessesfordealingwith Correspondingauthor:LIXiao?bin;Tel:+86731?8830454;E

    mail:X.B.Li@mail.CSU.eAu.cn

    464LIXiao-bin,etaUTrans.NonferrousMet.Soc.China18(2008) theACRfocusedontherecoveryofchromatetoreduce theenvironmentpollution,butunfortunatelythe extractionofaluminumwasignored.To

    comprehensivelyutilizebothchromateandaluminumin

ACR,anewmethodwasproposedinthiswork.

    2Experimental

    2.1Materials

    TheACRusedintheexperimentswasobtained

    fromGansuJinshiChemicalEngineeringCorporation andoriginallycontained50%-65%water.Thechemical compositionofdryACRislistedinTable1.

    Table1ChemicalcompositionofdryACR(massfraction.% SpentliquorwaspreparedbydissolvingAI(OH)3 (industrialpure)inNaOH(industrialpure).Na2CO3and Na2SO4addedinthespentliquorwereinanalytical grade.Table2givesthechemicalanalysisresultsofthe spentliquorthatwasdilutedtopresetconcentration beforeexperiments.

    Table2Chemicalanalysisresultsofspentliquor(g/L 2.2Equipments

    Autoclavewitheightvolumereactionbombsof15O mL,withglycerineasheatingmedium,andtemperature controllingaccuracy4-l?.wasmadeinCSU(Central

    SouthUniversity,China).Settlingcontainerwith300mL glasscylinder,withwaterasheatinginsulationmedium, andtemperaturecontrollingaccuracy4-0.5?.was

    assembled.Theseedprecipitationtankerhadavolumeof 1.5L.withwaterasheatinginsulationmedium,andthe temperaturewascontrolledintherangeofO.5?

    automatically.

    2.3Methodology

    ACR,waterandadditiveswereweighedaccurately andputintothereactorbombs.whichwerethenputinto autoclave.There.crystallizationreactionwascarriedout

at70-80?for1-2hwithadditives,thenthebombs

    weretakenoutandcooledbywater.Theslurrywas separatedbyvacuumfilterandthefiltercakewas washedwithwater.Theseparationofaluminumand chromatewasrealizedandtheCFARwasthusobtained. Thedigestionwascarriedoutinautoclave.The CFAR.reduceragentsandspentliquorwerechargedinto thereactorbombs.digestedat120-130?.Theslurry

    obtainedwasthenseparatedbyvacuumfiltrationandthe filtercakewaswashed,driedandweighted.The concentrationofcaustic.aluminaandchromateinfiltrate wereanalysedbyvolumeanalyticalmethod.inorderto calculatethealuminumrecoveryrate.

    Thesedimentationtestwasoperatedinglass cylinderat96?inatransparentglasscontainer.The heightandvolumeofthecylinderwas300Inrnand300 mLrespectively.Putthedigestionslurryandpolymer flocculantintoglasscylindersandmixthemwithblender Thentheheightoftheslurryindifferentsettlingtime wasrecorded,thenthesedimentationcurvewasplotted andthesettlingvelocitywascalculated.

    Theseedprecipitationwasstudiedinthe

    precipitationtankers.Thepregnantaluminatesolution. chromateliquorandaluminumhydroxideseedwere chargedinthetankel~withstirrerandautomatic temperatureregulator.

    3Resultsanddiscussion

    3.1Separationofchromateandaluminum

    Thealuminumexistsintheformofamorphous

    boehmiteintheACRwhilethechromateissodium

    chromate2-hydrate.Fig.1showstheXRDpatternofthe ACR.

    Fig.1XRDpauernofACR

    TheACRisinagelstructurewherethealumina colloidforillsaspatialreticulation,inwhichthesodium chromatefullyfills.ThecolloidparticleshaveahJigh affmtothechromatesolution,soitisdicultto

    separatechromatefromthiscolloidalreticularstructure n,91.T0separatethechromatefromtheACR,the reticularstructuremustbedestroyed.Thenewmethod proposedinthispaperistoripentobreakthegel

    LIXiao.bin,etal/Trans.NonferrousMet.Soc.China18(2008)465 structIlIleinhydrosystemwithadditions.therebymaking thechromateescapefromtheACR.Andafterwashing lessthan0.5%virulentCr(?1remainsintheACR,i.e.

    morethan95%hexavalentchromiumwasrecovered. Fig.2shOWStheXRDpatternoftheCFARobtained bythenewchromateseparationprocess.Comparedwim Fig.1.thecrystallizationdegreeoftheCFARissuperior totheuntreatedACR.whichillustratesthatpartofthe colloidalaluminumhydroxideiSconvertedtohigher crystallinitymainlyduetothedestroyedcolloidal StrUctIlre.

    Fig.2XRDpatternofCFAR

    3.2DigestionofCFAR

    ThewetCFARobtainedfromthechromatesepara

    tionprocesscontains50%-65%water.Thechemical Table3Di~estionresultsofCFAR

    analysisshowsthatthedryCFARconsistsof54.44%

    AI2o3,2.19%Cr203and2.96%Si02,whichissuitable lawmaterialforaluminumhydroxideproductionby Bayerprocess.ThedigesticabilityinBayerprocessof thegroundCFARdriedat90-95?wastested.andthe

    resultsarelistedinle3.

    le3showthatthealuminarecoveryrateiS

    enhanceddramaticallywithincreasingdigestion temperatureandmolarratioofcausticsodatoalumina (ak)inspentliquor.Withthedigestiontemperatureabove l15?andtheinspentliquorbeing1.51.60,more than90%aluminarecoverycouldbeobtainedwithin30 min.Therefore.itiSprovedthattheCFARCanbe digestedwithrelativelylowertemperatureandcaustic concentrationinashortretentiontime.

    ItiSfoundthatthedispersionofCFARinspent liqouralsostronglyinfluencesthedigestioncapability (1istedinTable41,whichiSrelatedtothemicrostmcture ofCFAR.Asdiscussedabove.althoughthecrystallinity oftheCFARiSimprovedduringthechromateseparation process.mostofthealuminainresiduestillremainsin theaqueouscolloidalstructI.Thosecolloidaluminum issubjecttoaggregatingbecauseofthestrongattractive molecularforce.Thoseaggregatescannotbedispersed bylOWshearstress.Inaddition.thepoorosmosisof thoseaggregatesminimizesthereactionareaswhenthe wetCFARiSpoorlydispersed.Thusthealumina recoverywillbedominatedbythedispersiondegree whentheresidueiSnotwelldispersed.Theindustry practiceindicatesthatthealuminarecoverycanreach

    466LIXiao-bin,etal/Trans.NonferrousMet.Soc.China18(2008) morethan90%withproperdispersionandratioofliquor tosolid.

    3.3Sedimentationofdigestionslurry

    Theliquor-solidseparationisaveryimportant procedureinthealuminumhydroxideproducingprocess. Becausetheslurryhasaveryhighliquor/solidratio, theoreticallythepressurefiltershouldbeanideal separationmethodforthelowsolidcontentslurry[1O]. Butthepracticeinbothlaboratoryandindustryindicated thatthepressurefilterwasnotsuitablefortheslurry producedfromCFARdigestion,probablyduetothe highlydispersedsolidparticlesintheslurrywhichblock thepolesoftheclothfilter,andthusobstructliquidto passthrough.

    Sedimentation,asanotherimportantliquor-solid separationmethod,hadbeenwidelyadoptedinalumina productionindustry[10].Theelectricaldoublelayerson solidparticlesurfaceandtheflocculantarevery importantforthesedimentationability.Inthisstudy,a flocculantwasselectedandsomesurfacepotential adjustedadditiveswereaddedtotheslurrytoincrease thesettlingvelocity.Theexperimentresultsofthe sedimentationareplottedinFig.3,showingthatthe settlingvelocityinthefirst10minreaches1.06rn/h. 3.4Influenceofchromateonaluminahydroxide productionsystem

    ThehexavalentchromiumintheCFARwillpartly Fig.3Sedimentationcurveofdigestionslurry(Digestion

condition:temperature120?,causticconcentration125g/L,

    time30min,moleratioinspent1.6)

    dissolveandaccumulateduringtheBayerrecycling process,thusinfluencingaluminumhydroxide production.

    ZOUetal[11]andWANGetal[12]studiedthe

    equilibriumofNaOH-Na2CrO4-NaAIO2——H20system

    anddiscoveredthatthealuminasolubilityinaluminate solutioncouldbeaffectedbychromate,butthealkali concentrationintheirstudieswasmuchhigherthanthat inpopularBayerdigestionprocess.Thustheinfluenceof thechromateonthedigestionprocesswasstudiedinthis work,andtheresultislistedinTable5.

    Table5showsthatthealuminadigestionefficiency haslittlefluctuationevenastheconcentrationofsodium chromatedissolvedintherecyclereachesabout8g/L(as Cr203),whichimpliesthatafewsodiumchromateinthe recyclingliquordoesnotaffectthedigestionprocess undertheexperimentalconditions.

    Impuritiesinpregnantliquorhaveprofound

    influencesontheprecipitationrateandtheproductpurity ZOUandZHANGconsideredthataspotofchromatein pregnantliquorhaslittleeffectontheprecipitationrate andtheproductquality,whiletheprofoundinfluences wouldemergewithhighchromateconcentrationin pregnantliquor[13].Theinfluencesofdifferentchromate concentrationonprecipitationwerestudiedandthe precipitationcurvesareplottedinFig.4.

    Fig.4showsthattheprecipitationrateforaUofthe Fig.4Precipitationcurvesofpregnantliquorswithdifferent

chromateconcentration(Seedprecipitationconditions:A1203

    concentration130g/L,causticmolarratio1.6,starting

    temperature65?,endtemperature50?,seedratio2.0, chromateconcentrationmeasuredasCr203) Table5Influenceofsodiumchromateondigestionefficiencyofalumina

    Temperature/

    ?

    Time/

    mln

    CausticChromate

    concentration/(g?L.)concentration/

    (g?L.)

    Molarratioinspentliquor,DigestionmolarratioAlumina

    ktzkrecovery/%

    120

    l20

    20

    20

    l41.5

    l41.5

    8

    0

    1.64

    1.64

    I.67

    1.68

    92-3I

    91.76

    /a=lc0gIa3aJd

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