18 EhpHSamples要点.docx
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18EhpHSamples要点
18.Ep–pH-Samples
EpHmodule:
InputFileforEpHModule
Eh-pH-Diagram4.0'=Diagramtype'Heading
2'NumberofElements
Cu'NameofaElement
1.0000,1.0000'MolalityandpressureofthisElement
S
1.0000,1.0000
2'Numberoftemperatures
25.000,75.000'ValuesofTemperaturesin°C
N'Showstabilityareasofions(Y/N)
-56.6781,-54.7567'∆GvaluesofH2Oforalltemperatures
1.0000,1.0000'DielectricConstant
0'Ionstrenght
-2,2,0,14'Limitsofthediagram
Cu,0.0000,0.0000'Nameofaspeciesand∆Gvaluesforalltemperatures
CuH3,67.7313,68.0470
CuO,-30.6627,-29.5533
Cu2O,-35.3446,-34.4294
CuO*CuSO4,-189.3533,-183.9282
Cu(OH)2,-85.8077,-82.4437
CuS,-12.7800,-12.7952
Cu2S,-20.6662,-20.8844
CuSO4,-158.2417,-153.8574
Cu2SO4,-156.4294,-152.5214
CuSO4*3H2O,-334.5808,-323.1528
CuSO4*5H2O,-449.2635,-433.1954
H2SO4,-164.8848,-159.9397
H2SO4*3H2O,-345.1000,-334.0907
H2SO4*4H2O,-402.9123,-389.9613
S,0.0000,0.0000
S(M),0.0190,0.0068
SO3(B),-89.4025,-86.2283
SO3(G),-89.7786,-86.7094
Cu(+2a),15.6300,15.7317
Cu(+a),11.9450,11.0901
Cu(OH)2(a),-59.5596,-53.6263
CuSO4(a),-162.3104,-155.5767
Cu2SO3(a),-92.2243,-87.8581
H2S(a),-6.5160,-6.1670
HS(-a),2.9113,4.2276
H2SO3(a),-128.5529,-125.6145
H2SO4(a),-177.9474,-171.0575
HSO3(-a),-126.1208,-122.2317
HSO3(-2a),-121.3430,-116.7342
HSO4(-a),-180.6580,-175.4083
HSO4(-2a),-175.2501,-169.3171
HSO5(-a),-152.3532,-147.0925
S(-2a),20.5471,22.6438
S2(-2a),19.0406,21.0895
S3(-2a),17.6446,19.6504
SO2(a),-71.8354,-71.1981
SO3(a),-125.6315,-121.1608
SO3(-2a),-116.2971,-110.0146
SO4(-2a),-177.9474,-171.1338
S2O3(-2a),-123.9775,-118.6055
S2O4(-2a),-143.5482,-137.2786
S2O5(-2a),-188.0263,-180.6896
S2O6(-2a),-231.6077,-223.3179
S2O8(-2a),-266.4866,-257.3452
S3O6(-2a),-244.8178,-236.2948
EpHCase1:
MetalCorrosioninFe-H2O-system
Eh-pH-diagramsmaybeusedtoestimatecorrosionbehaviorofdifferentmetalsinaqueoussolutions.Themostcommoncorrosionphenomenonisrustformationontheironsurfaces.Thecorrosionratesandtypesdependonthechemicalconditionsintheaqueoussolution.TheEh-pH-diagramofanFe-H2O-systemmayeasilybecreatedasdescribedinChapter17.ThechemicalsystemspecificationisshowninFig.1andthecalculateddiagraminFig.2.
Thestabilityareasmaybedividedintothreegroups13:
1.Corrosionarea:
Formationofionsmeansthatmetaldissolvesintoanaqueoussolution.Forexample,Fe(+3a),Fe(+2a),FeO2(-a)andHFeO2(-a)-ionsinanFe-H2O-system.
2.Passivearea:
Formationofoxidesorsomeothercondensedcompoundsmaycreatetightfilm(impermeable)onthemetalsurfacewhichpassivatesthesurface,goodexamplesareAl2O3onaluminiumorTiO2ontitaniumsurfaces.Iftheoxidelayerisnottightenough(porous)topreventoxygendiffusionintothemetalsurface,corrosionmaycontinue.Thisisthecasewiththemostoftheironoxidesbuttheymayalsocausepassivationinfavourableconditions.
3.Immunityarea:
Allmetalsarestableiftheelectrochemicalpotentialislowenough.Mostnoblemetalsarestableevenatzeropotential,butatleast–0.6voltsareneededatthecathodeforirontoprecipitate,seeFig.2.
ThestabilityareasofwaterareshownbydottedbluelinesinEh-pH-diagrams,seeFig.2,thecolorscannotbeseeninthisB&Wcopy.Usuallyitisdifficulttoexceedtheselimitsduetotheformationofoxygenattheupperlimitandhydrogenatthelowerlimit.Insomesolutionstheselimitsmaybeexceededduethenecessaryoverpotentialofhydrogenandoxygenformation.OnthebasisofFig.2itseemsthathydrogenformationoccursoncathodebeforethemetallicironcomesstable.
TheEh-pH-diagramsmaybeusedinseveralways,forexample,
-tofindpH,potentialandtemperatureregionswhichpreventcorrosion.
-tofindoutwhichcompoundsarethecorrosionreactionproducts.
-tofindimmunematerialswhichcanbeusedasprotectivecoating.
-tofindoutametalwhichmaycorrodeinsteadoftheconstructivematerial.Forexample,thezinclayeronasteelsurface.
Fig.1.SpecificationofFe-H2O-systemforEpH-diagramat25°C.
Fig.2.Eh-pHDiagramofFe-H2O-systemat25°C.MolalityofFeis10-6M.
EpHCase2:
CorrosionInhibitorsinFe-Cr-H2O-system
Someelementsorcompoundsmaypreventcorrosionevenatverylowcontentinthechemicalsystem.Thesesubstancesarecalledcorrosioninhibitorsandtheycanbedividedintoanodicandcathodicinhibitors.Theanodicinhibitorsprimarilypreventtheanodicreactionandpassivatemetalsinthisway,thelatteronessuppressthecorrosionratebypreventingthecathodicreactionorbyreducingthecathodicarea13.
Chromateanddichromateionsarewellknownanodiccorrosioninhibitors.Smallamountsofchromateswillcreateatightcomplexoxidefilmonthesteelsurfacewhichpreventscorrosion.Theoxidefilmismainlyformedofmagnetite(Fe3O4),hematite(Fe2O3)andchromicoxide(Cr2O3).
TheinhibitorbehaviorofchromatesmaybeillustratedwithEh-pH-diagrams.TheFe-Cr-H2O-systemspecificationsareshowninFig.3.ThecalculationresultsforFe-H2OandFe-Cr-H2O-systemsareshowninFigs4and5.Asshowninthediagrams,alargeareainthecorrosionregionofironFe(+2a),Fig.4,iscoveredbytheCr2O3andCr2FeO4stabilityareasandthusprotectedfromcorrosion,Fig.5.
ItiseasytocreateEh-pH-diagramswiththeEpHmodule.However,youshouldrememberthatthistypediagramgreatlysimplifiestherealsituation.Theydonottakeintoaccount,forexample,thekineticaspectsornon-idealityofrealsolutions.Smallerrorsinthebasicthermochemicaldatamayalsohaveavisibleeffectonthelocationofthestabilityareas.Inanycase,thesediagramsgivevaluablequalitativeinformationofthechemicalreactionsinaqueoussystemsinbriefandillustrativeform.
Fig.3.SpecificationofFe-H2O-systemforEpH-diagramat100and300°C.
Fig.4.Fe-H2O-systemat100°C.Molality:
Fe10-2M,pressure1bar.
Fig.5.Fe-Cr-H2O-systemat100°C.Molalities:
FeandCr10-2M,pressure1bar.
EpHCase3:
SelectionofLeachingConditions
Thefirststepinahydrometallurgicalprocessisusuallyleachingordissolutionoftherawmaterialsinaqueoussolution.Theaimistoselectthemostsuitableleachingconditionssothatthevaluablemetalsdissolveandtherestremaininthesolidresidue.TheleachingconditionsmayeasilybeestimatedwithEh-pH-diagrams.Infavorableleachingconditionsthevaluablemetalsmustprevailinsolutionasaqueousspeciesandtheothersinsolidstate.
Roastedzinccalcineisthemostcommonrawmaterialforthehydrometallurgicalzincprocess.Itcontainsmainlyzincoxide.AnexampleofEh-pH-diagramsapplicationinzincoxideleachingisshowninFig.7,seeFig.6forchemicalsystemspecifications.ItcanbeseenfromthediagramthatacidorcausticconditionsareneededtodissolvetheZnOintosolution19.
InacidconditionsthepHofthesolutionmustbeloweredbelowavalueof5.5.InpracticalprocessesthepHmustbeevenlowerbecausetherelativeamountofzincinthesolutionincreasesifthepHisadjustedfartherfromtheequilibriumlinebetweentheZnOandZn(+2a)areas.ThedissolutionoftheZnOconsumeshydrogenionsascanbeseenfromreaction
(1).Thereforeacidmustcontinuouslybeaddedtothesolutioninordertomaintainfavorableleachingconditions.
ZnO+2H(+a)=Zn(+2a)+H2O[1]
IncausticconditionszincmaybeobtainedinsolutionbytheformationoftheanioncomplexZnO2(-2a).Theleachingreactionmaybedescribedbyequation
(2).
ZnO+H2O=ZnO2(-2a)+2H(+a)[2]
Theleachingconditionschange,forexample,ifsulfurisincludedinthechemicalsystem.TheeffectofsulfurcanbeseeninFig.10.MuchsmallerpHvaluesareneedtodissolveZnSwhichhaswidestabilityarea.Thiswillleadtotheformationofhydrogensulfidegasandionsaccordingtoreaction(3).
ZnS+2H(+a)=Zn(+2a)+H2S(g)[3]
Inoxidizingconditionsanumberofdifferentaqueousspeciesmayresultfromtheleachingreactionssuchas(4),(5)and(6),seeFig.6.Inthesereactionsitisimportanttonotethattheconsumptionofreagentsaswellasgenerationofreactionproductscontinuouslychangethesolutionconditions.Theseconditionsmustberegulatedbyfeedingmoreacidand/orremovingreactionproductsinordertothemaintainoptimumconditions.
ZnS=Zn(+2a)+S+2e-[4]
ZnS+4H2O=Zn(+2a)+HSO4(-a)+7H(+a)+8e-[5]
ZnS+4H2O=Zn(+2a)+SO4(-2a)+8H(+a)+8e-[6]
TheHSCdatabasecontainsalotofspecieswhichmayhavealongformationtime.Normallyitiswisetoselectonlysuchspecieswhichareidentifiedinrealsolutionsforchemicalsystemspecifications.AsystemspecificationwithonlycommonspeciesincludedisshowninFig.8andanotheronewithallthespeciesinFig.9.TheselectedspeciesmayhaveavisibleeffectonthediagramsascanbeseenbycomparingFigs.10and12aswellasFigs.11and13.Insomecases,diagramswithallthespeciesselectedintothecalculationsystemmaygivealsovaluableinformation,Figs.12and13.
Fig.6.Zn-H2O-systemspecifications.
Fig.7.Zn-H2O-systemat25°C.DiagramisbasedonspecificationsinFig.6.
Fig.8.Zn-S-H2O-systemspecifications,onlyidentifiedspeciesincluded
Fig.9.Zn-S-H2O-systemspecifications,allspeciesincluded.
Fig.10.Zn-S-H2O-systemat25°CbasedonspecificationsinFig.8.
Fig.11.S-Zn-H2O-systemat25°CbasedonspecificationsinFig.8.
Fig.12.Zn-S-H2O-systemat25°Cbasedonspecificationsi