Investigation of three-liquid-phase extraction systems for t(3)

K.Xieetal./SeparationandPuri cationTechnology76 (2010) 191–197193

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pH value

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2

pH value

Fig.1.PartitioningofTi(IV),Fe(III)andMg(II)intheTESscontaining25wt.%TRPO,15wt.%PEG2000and20wt.%(NH4)2SO4atdifferentpHvalues.

thefactsthatMg(II)iscommonlyusedtosalt-outPEG[20–22]andTRPOhasweakextractabilityfordivalentcations[23],itisreason-ablethatMg(II)preferthesalt-richphaseintheTESs.TheextremelylowextractabilityofTi(IV)andFe(III)inthePEG-richphasewerealsoobservedforthePEG-inorganicsaltaqueoustwo-phasesys-temsandcanbecorrelatedtotheions’largenegativeGibbsfreeenergiesofhydration[24].Sincethemiddlephaseseemeduselessinextractingmetalions,thefunctionoftheTESwascomparabletothatofaTRPO–waterliquid–liquidextractionsystem.

AscanbeseenfromFig.1(a),morethanhalfamountofTi(IV)andFe(III)weredistributedintotheTRPO-richtopphase.Ti(IV)extractionisbetterthanFe(III)asTi(IV)hasastrongercomplex-ationwithTRPO.ExtractionpercentageofTi(IV)bythetopphasewas71.85%atpH0.5andslightlydeceasedto64.8%atpH2.0,whilethatofFe(III)increasedfrom55%atpH0.5tonearly65%atpH2.0.Thebehavioragreeswiththereportedliterature[9]anditmaybeunderstoodthataqueousaciditycanaffectthephasemiscibil-ity,thechemicalpropertiesoftheextractantandparticularlythespeciationofmetalionsintheaqueousphase.Thesulfatedegreeoftitaniumwasenhancedbyincreasingthesulfuricconcentra-tion,thusformingmoreneutralspecies,whicharepreferentiallytransferredtotheTRPO-richtopphase.Fig.1(b)showsthedistri-butionratiosofTi(IV)andFe(III)betweenthetopphaseandthebottomphase,andtheseparationfactorˇTi(IV)/Fe(III),t/m.Themax-imumˇTi(IV)/Fe(III),t/misonly4.Hence,Ti(IV)andFe(III)couldnotbeseparatedfromeachotherdespitetheycouldbeseparatedfromMg(II).

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N235N

235+TRPO

D2EHPAEHPA+TRP

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2PO

D

TROrganic solvent

Fig.2.PartitioningofTi(IV),Fe(III)andMg(II)intheTESscontaining15wt.%PEG

2000,20wt.%(NH4)2SO4and25wt.%organicextractantatpH0.5.TheextractantsareN235,D2EHPA,TRPO,amixtureofN235andTRPO(1:1,wt.%)andamixtureofD2EHPAandTRPO(1:1,wt.%).

3.2.PEG2000–(NH4)2SO4basedTESswithdifferentorganicextractants

Fourorganicextractants,whichwereN235,N1923,D2EHPAandTBP,wereusedassubstituteforTRPOintheformationofTESs.N1923ledtofullygelwhenitwasinvolvedinamixturecontain-ing15wt.%PEG2000and20wt.%(NH4)2SO4atpH0.5.IncaseofN235,D2EHPAandTBP,stableTESswereobtained.However,approximatelyone fthoftotalTBPenteredintothemiddlephaseoftheTEScontaining25wt.%TBP,15wt.%PEG2000,and20wt.%(NH4)2SO4.TheintersolubilityofTBPandPEG2000wouldmaketherecyclingofTBPandPEG2000aheavyproblem.Therefore,N235andD2EHPAwerepreferredasextractantsastheyprovidedbetterphaseseparationbehaviors.TheextractionpercentagesofTi(IV),Fe(III)andMg(II)inTESsusingN235,D2EHPA,N235–TRPO(1:1,wt.%),andD2EHPA–TRPO(1:1,wt.%)werecalculatedandcom-paredwiththatofTRPOatpH0.5.

TheresultsweregiveninFig.2.AscanbeseenfromFig.2,almostcompleteTi(IV)and70%ofFe(III)wereextractedintotheD2EHPAorganicphase,leavingMg(II)inthebottomaqueousphase.TheextractabilityofTi(IV)andFe(III)bydifferentextrac-tantsdecreasedfollowingtheorder:D2EHPA>TRPO>N235.Thiscouldbeconnectedwiththenatureofextractedmetalspeciesandtheextractionmechanism.TakingTi(IV)forexample,inthesulfatesolutions,Ti(IV)ispresentasmonomericdivalentcations,assumedtobeTiO2+[25],andformstwoweaktitanylsulfatocom-plexesTiOSO4andTiO(SO4)22 ,withTiOSO4predominating[26].Theequilibriumshavebeenwrittenas:TiO2++SO42 TiOSO4

(6)TiOSO4+SO42 TiO(SO4)22

(7)

ExtractionoftitaniumwithD2EHPAisprobablygovernedbyacationexchangemechanismwithD2EHPA(representedasthedimerH2A2)accordingtothereaction:TiO2+(a)+2H2A2(t) TiOH2A4(t)+2H+(a)

(8)

where(a)and(t)denotespeciesintheaqueousphasesandthetopphase,respectively.

Becausethestabilityconstantsassociatedwiththetwotitanylsulfatocomplexesaremuchsmallerthanthatofthetitanium–D2EHPAcomplex,theequilibria(6)and(7)shiftedtothe