The
effect
of
chemical
composition
on
the
leaching
behaviour
of
electric
arc
furnace
(EAF)
carbon
steel
slag
during
a
standard
leaching
test
D.
Mombelli
a,*
,
C.
Mapelli
a,
S.
Barella
a,
C.
Di
Cecca
a,
G.
Le
Saout
b,
E.
Garcia-Diaz
b aDipartimentodiMeccanica,PolitecnicoDiMilano,ViaLaMasa1,Milano20156,Italy
b
CentredesMatériauxdesMinesd’Alès(C2MA),EcoledesMinesd’Alès(InstitutMinesTelecom),AvenuedeClavières6,30100Alèscedex,France
Keywords:
Electricarcfurnace(EAF)slag Leachingbehaviour Standardleachingtest Chemicalcomposition Ternarydiagrams
ABSTRACT
Theelectricarcfurnace(EAF)slagcouldbeexploitedinseveralfieldsofapplication,suchaslandfilling,
roadconstructionsandconcreteproduction.However,theiruseislimitedbythepresenceofpolluting
chemicalelements(chromium(Cr),barium(Ba),Vanadium(V),etc.)thatcanbedangeroustohumans
andtheenvironment.Thus,chemicalandstructuralstabilityisafundamentalrequirement,especially
whentheslagmaycomeincontactwithwater.Therefore,theinteractionbetweenslagandwateriskey,
inordertoclassifytheslagasasaferawmaterial.Inthiswork,theeffectofslagchemicalcompositionon
thechemicalleachingofaboutseventyEAFcarbonsteelslagsofdifferentproductionsteelgradeswas
investigated.Standardleachingtests(24hat10l/kg)indeionizedwateronslagbulkswereperformed
andtheresultswerecorrelatedwiththeslagchemicalcomposition.Thesurveyhasmadepossible
definingthesafechemicalcompositionareasonthemainternarydiagrams,abletotransferstabilityto
theslag.Theresultsobtainedindicatethewater/slagratioasthemostimportantfactorinthereleaseof
pollutingsubstances,alsoidentifyingacriticalscenarioforslagrecycling.
Introduction
Steelslagsaretodaylargelyutilizedinroads,asconstruction andpavingmaterials,sincetheirphysicalpropertiesaresimilar,or sometimesbetter,thannaturalmaterialslikegravel.
Environmentalconcernsregardingutilizationandlandfillingof steelslagsfocusedonthecontentofheavymetalsandparticularly on the leachable quantities, where chromium (Cr) has gained specialattentionduetoitstoxicityinthehexavalentstate[1,2]. Otherchemicalspecies,i.e.barium(Ba),vanadium(V), molybde-num(Mo),requiremonitoringduetotheirhightoxicityonhumans andtheenvironment[3,4].Theidentificationofleachingbehaviour ismandatoryforslag usein unboundapplications,i.e.unpaved roads,armourstoneorgabions,becauseinsuchconditions,slagis subjecttoacontinuouscycleofwettinganddrying.Severalstudies
[5–9]haveinvestigatedtheleachingbehaviourofslagaggregates, and highlight that such aggregates could potentially release dangerouschemicalelements(especially Ba,V and Cr),as well as unbound slag, when coming into contact with water. In
particular,therecyclingofsuchaslagissubordinatedwithrespect totheseverelimitsimposedbynationallegislativedecreesthat changefromcountrytocountryinEuropeanUnion.Alsothelistof substancesandchemicalstobemonitoredchangesfromcountry to country. For examples, Italian environmental legislation requiresthemonitoringof 22parameters,namelypH,COD,As, Ba,Be,Cd,Co,Cr(tot),Cu,Hg,Mo,Ni,Pb,Sb,Se,V,Zn,Cl,CN,F, NO3,SO42,whereasinGermanyonlyfewparametersmustbe controlled(pH,EC,Cr,Mo,VandFluorides).InFranceandSpain almostthesameelementsarerestricted(particularlyfocusedon heavymetals).ThelimitsonCdandSeinSpainarethemoststrictly (less than 1
m
g/l) amongst the considered European countries. Moreover,GermanlegislationdoesnotrequireanycontrolonBa leaching,whereastheBalimitsinFranceandSpainaretwicethe levelofItalianlimits.AlsotheVis considereddifferentlyinthe variousEUcountries:severelimitsaresetinItaly,Germanyand Spain,butnotinFrance[10–15].To determine the concentration of leached elements every country adopts a different test criteria that contributes to confusion about slag environmental safety (Table 1), in spite of the European Committee of Standardization (CEN) is consideringtheintroductionofcommonharmonizedstandards
[16,17].
* Correspondingauthor.
Inthisstudy,theItalianregulationwastakenasreferencesince verycriticaltestcriteriaareadopted,withhighliquid-to-solidratio (L/S)coupledwithasmallaggregatesizethatcouldenhancethe leachingofsuchachemicals.
Eveniftheconditionsunderwhichtheleachatesareproduced vary as a result of different physical parameters like pH, temperature, or flow conditions, and influence the resulting concentrationsofmetalsintheleachates[18],theslagchemical compositioncoversanimportantroleonimmobilization,orthe releaseoftheabovementionedtoxicelements.Forexample,MgO concentration in the slag seems to be favourable for Cr immobilization but detrimental for the leaching of other pollutingelements, i.e. Ba and V [19]. The solution behaviour oftheseelementshaveaparaboliclawasafunctionofCaOand MgOconcentration,whereastheirdependenceonsilica concen-trationisgenerallydescribedbyalinearlaw[20].Currently,the role of chemical composition on carbon steel slag leaching behaviourhasnot been extensivelyandsystematically investi-gated,andisoftenonlydedicatedtoCrleaching[21,22].Forthese reasons, an in-depth understandingof therole of the average chemical composition on slag leaching behaviour in standard conditions(deionizedwateratpH7,roomtemperature,L/S:10l/ kg)couldgiveimportantindicationstosteelmakersabouthowto properlymanageormodifytheirslagtoimprovetheir environ-mentalsustainabilityandrecycling.
Inthis study, differentclasses ofcarbon steelEAFslag from differentsteelproductions(reinforcedbarsteel,highalloyedsteel and qualitysteel)wereinvestigated, in ordertocorrelate their leachingbehaviourinstandardconditionswithchemicalfeatures. The main goal of the present workis to forecast the leaching behaviourofaslagbyaveragechemicalcomposition,anddefinea correctivepathtotransformslagintoasafeby-product.Chemical composition and slag-water interaction were investigated and correlated with the results of standard leaching tests (in accordancewithEN 12,457-2)performedonbulk slag4mmin size. The effects of the main chemical species (CaO, MgO, Al2O3,...) on Ba, Cr and V leaching is discussed. To better understandhow the slag interacts withwater during standard leaching tests, qualitative tests were performed on 4mm of crushedslag grainsatdifferentL/Sratios(10and 100l/kg).The analysesallowedfortheidentificationofsafechemical composi-tion areas on the main ternary diagrams that ensure full slag stability.
Experimentalprocedure
TheEAFslagswereprovidedbydifferentItalianandEuropean electricsteelworksandassociatedwithdifferentsteelproductions. Sixty-nine different samples were collected and investigated, classifying thedifferentslag intothree groupsaccordingtothe amount of iron and chromiumoxide (Table 2).The number of samplesforeachgroupisindicatedinTable2.
The slag chemical composition was determined by ED-XRF analysisthroughtheAmetec SpectroXeposspectrometer inHe atmosphereon5gofpowderedslag.
The averagechemical compositionof theslag is reportedin
Table3,andtheirpositiononternarydiagramsisshowninFig.1. A morphological and microstructural investigation was per-formedbymeansofXRDandSEManalyses.
X-raydiffraction(XRD)datawerecollectedusingaBrukerD8 Advancediffractometerina
u
–u
configurationemployingtheCu Ka
radiation(l=1.54Å)withafixeddivergenceslitsize0.5anda rotatingsamplestage.Grindedsamples(averagediameter15m
m) werescannedbetween10and80(stepsizeof0.007)withthe Vantecdetector.The morphological and microstructural characterizationwas performedby ZeissEVO50scanningelectronmicroscopy(SEM) equippedwithanOxfordIncaEDSprobe.Theslagwasmouldedin araldite-basedresin,beforebeinggroundandpolished.
Standard leaching tests were performed according to EN 12,457-2 ongranulatedslag rangedfrom4 to0.1mm. Thefine fractionbelow0.1mmwasdischarged.Theslagtestportionwas broughtincontactwithfixedvolumeofdeionizedwater(10l/kg) andstirredat10rpmbyarotatorymixerfor24h.Thetestisbased on the assumption that equilibrium or near-equilibrium is achieved between the liquid and solid phases during the test. Theconcentrationlimitstakenasareferenceareindicatedinthe Italianlegislativedecrees(D.M.03August2005N.201[10]andD. M. 05 April 2006 N. 186 [11]). Leached concentrations were measuredbyICP–OES(inducedcoupledplasma–opticalemission spectroscopy,detectionlimit30
m
g/l).To investigate slag-water interaction, a crushed particle (approximately 4mm in size) for every slag was leached as a standardleachingtestrequiredbutvaryingtheL/Sratio(10,100l/ kg).SlagswerecharacterizedbySEM,beforeandafterelutiontests, to detect morphological alterations, and tomake a qualitative comparison on the effects caused by water erosion. The same
Table1
EnvironmentallegislationandleachingteststandardsfordifferentEuropeancountries.
Country Italy Germany France Spain(BasqueCountry) Environmentallegislation DLGS152-2006 EBV TLGesteinStB04 SETRA RD34/2003
Guidedelaitierssidérurgique RD49/2009 Leachingteststandard EN12457-2 DIN19528 DIN38414-4 EN12457-4 EN12457-3
DIN19529
L/Sratio 10l/kg 2l/kg 10l/kg 10l/kg Firststep2l/kg Secondstep8l/kg Samplegranulometry <4mm <32mm <32mmor8–11mm <10mm <4mm Testduration 24h 24h 24h 24h Firststep6h
Secondstep18h
Table2
Slagclassificationandnumberofsamplesaccordingtochemicalcompositionand steelgrades.
GroupID No.ofsamples FeOx Cr2O3 Production
A 13 >25% <5% Reinforcingbarsteel B 10 >25% >5% Highalloyedsteel C 46 <25% <5% Qualitysteel
Table3
Chemicalcompositionrange(min–max)oftheinvestigatedslag(%weight). GroupID MgO Al2O3 SiO2 CaO FeOx Cr2O3 V2O5 Ba
A 2–5 10–15 15–20 15–25 30–50 2–5 0.1–0.2 0.05– 0.1 B 1–3 1–3 5–25 15–25 30–50 5–30 1–2 ND C 5–15 5–15 10–40 20–50 5–30 0.5–5 0.05–0.4 0.1–0.5
Fig.2. LeachingdataboxplotsofinvestigatedEAFcarbonsteelslag:(a)Ba,(b)V,(c)Crand(d)pH.Theconcentrationlimitsindicatedarethemoststrictprovidedbythe differentenvironmentallawsappliedinthedifferentcountries.
particlewasrepeatedlytreatedatthedifferentL/Sratioandthe sameareawasanalysedaftereachcondition.
Resultsanddiscussion
Chemical,crystallographicandmicrostructuralcharacterizations Otherdifferences,withrespecttothefirstclassification,canbe highlightedamongstthedifferentslagtypologies.Infact,slagfrom thereinforcingbarsteelproduction(groupA)ischaracterizedby highiron(averagely40%wt.)andaluminiumoxide(above10%wt.) content (Table 3). High oxygen flow is required during steel production duetothe highheterogeneityof thescrap used,in termsofsizeandchemicalcompositionandthehighcontentof pollutingelements.
TheslagsbelongingtogroupBaregenerallytypifiedbyalow contentofMgOandAl2O3(lessthan3%wt.),andbyahighfraction ofCr,VandMooxides,derivedfromboththehighalloyedscrap usedduring manufacturingand from theferro-alloys additions duringtherefining(Table3).
Theslagassociatedtoqualitysteels(groupC)ismainlyformed byCaO(sometimesoverthe50%wt.)andasmallquantityofFeO (limitedoxidationofthesteelscrapstolimittheFeOaround15– 20%wt.)(Table3).QualitysteelsrequireaverylowPandScontent, andthereforealargeamountofCaOisnecessarytomaintainhigh slagactivity.
Despiteofthesignificantdifferencesinchemicalcomposition, crystallographicandmicrostructuralanalysisoftheinvestigated
slag highlighted the occurrence of five main phases1 [24–36]: wustite, (Mg,Cr,Fe,Al)-spinel, bredigiteand/or larnite, brown-millerite,gehlenite.
Otherphases,likekirschsteinite,weredetected,generallyina samplefeaturinghighFeOcontentandamoderateMgOfraction (groupAandB),whereasmerwiniteisformedinslagwithhigh levelsofMgOandCaO(groupC).SlagsbelongingtogroupCwere alsocharacterizedbyasmallfractionoftri-calciumsilicate and freelime,sincethesesamplesarecharacterizedbythehighestCaO content that cannot be completely complexed by SiO2. In two samples(onebelongingtogroupBandonetogroupC)theXRD patternpointedoutaparticularlyintensepeakassociatedto Ca-chromite.
Theroleplayedbythesecrystallinephasesduringleachingtests isdiscussedindetailinthefollowingparagraphs.
Standardleachingtestresults
Standard leaching test results, performed according to EN 12,457-2,onslagparticlesrangedfrom4to0.1mmhighlightthat carbonsteelEAFslagmanifestonlyBa,VandCrleaching,whereas theotherspecies werealwaysundertheregulatorylimits [37]. Theseresultswereobservedalsoinpreviousexperiments[38–40]
andforthisreasononlyBa,CrandVbehaviourwerediscussed.The Ba, V and Cr mean and median values surpassed not only the maximumItalianconcentrationpermitted,butalso,insomecases,
Fig.3. ComparisonamongstBa,V,CrandpHconcentrationsinleachate:(a)Ba–VandBa–Crrelationships;(b)V–Crrelationship;(c)pH-Baand(d)pH-Vrelationships.Red linesrefertoBa,VandCrItalianleachinglimits,1mg/l,250and50mg/lrespectively.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferred tothewebversionofthisarticle.)
1
thoseimposedbytheotherEuropeancountries[12–15](Fig.2). GroupAslagonlyseemedtobeproblematicforBa,whereasVand Crvaluesarealwaysfarenoughfromregulationlimits(Fig.2a). GroupBwas mainlycharacterized by V leaching even ifsome samplesalsoinducedCrleaching(Fig.2b).GroupCslagshowedBa andCrrelease,whereasV,inmostofthesamples,compliedwith thelimit,evenifallthevanadiumcontentcharacterizingthesteel scrapsisabsorbedbytheslaginoxidizedform(Fig.2c).EvenifV doesnotseemproblematicforthislastslaggroup,thedetectionof highVconcentrationreleasedbysomesamplesdoesnotexcludeV leachingforthiskindofslag.ThisresultindicatesthatCrreleaseis always associated to the release of other elements, namely V, whereasBaleaching canoccurwithor withouttheleachingof othersubstances(Fig.3).Moreover,BaandCrleachingseemsto interestcarbonsteelslagthemost(AandCgroups),andVleaching isonlyprevalentinhighalloyedsteelslag(Bgroup).
SomeoutlierswereidentifiedforBaandCr.Inthefirstcase, threesamplesofgroupAreleasedanunexpectedhigh concentra-tionofBa.Currently,noexplanationshavebeenfound,sincethe microstructureofthosesamplesdidnotindicateparticularphases orunusualconfigurations.This behaviourcoulddependonthe highMgOcontentandwillbediscussedinthenextparagraphs. Theoutliersamplesthatleachedanunexpectedconcentrationof CrbelongtogroupBandgroupC,respectively.Thesamplesare thosefeaturedbyCa-chromiteandthisphaseisprobablythemain phaseinvolvedinCrleaching.
TheleachatepHcompliedwiththesafetyrangeproposedby theregulationinmostoftheanalysedsamples,anditsaverage valuesincreaseinalinearformfromgroupAtogroupC,andis probablyduetothedifferentcontentofCaOandFeOintheslag (Fig.2d).LeachatefromgroupCreachedthehighestpHvaluesand insomecasessurpassedthemaximumadmittedvalue(12pH). Overthat valuetheenvironmentbecomes causticand mustbe classifiedascorrosive,furtherlimitingthepossibilitytousesteel slagforcivilpurposes[41].
Ba concentrationwas highin leachates featured bylow pH numbers.BaintheslagismainlypresentintheformofBaOand, since it is an alkaline oxide, it tends todissolve first in water together with the other alkaline elements. However, if the environmentreachesalkaline pH,theBaOmigration shouldbe reduced[42].Thishypothesisisconsistentwiththeexperimental evidence(Fig.3c)andshouldexplainthelowBaconcentrations whentheleachatereachedhighpHvalues(between11and13).
Vanadiumoxide isconsideredanamphotericspecies[43,44]
andinalkalinesolutionstendstobehaveasacidoxide[45].Since the EAF slag is basic slag, the leachate produced from their dissolutiongenerallyreachapHin therange 11–13.Thiscould
explainthehighconcentrationofsuchelementsinleachatehaving apHintherange11–11.5[46].However,whenthepHreachesvery high values (above 12), the solubility of V can be drastically reduced(Fig.3d).
BaandVseemtohaveanoppositetendencywhendissolvedin water (Fig. 3a). Firstly, no samples manifested Ba and V concentrationovertheimposedlimitsintheleachateatthesame time.Infact,inallsampleswithBaleaching,Vcomplieswiththe limits, and vice versa. This behaviour could depend on, and contemporarycontrol,thefinalleachatepHandshouldberuledby slagchemicalcompositionandphasedistribution.Inthegraphin
Fig. 3, it is possible to note some slag fall in the “safe area” (confined betweenV and Baregulation limits). Thoseslags are generallycharacterizedbyacrystallinegehlenitemicrostructure thattrapsandinhibitschemicalleaching,duetopoorhydraulic properties[47–50].
These resultssuggest that thecorrelationbetweenchemical composition and leachingtest results couldhelp identifysome common aspects governing the interaction between slag and water.Forexample,thecorrelationbetweenCr2O3contentinslag andCrconcentrateintheleachatecanbeexcluded.Infact,BandC slagleachateshadalmostthesameaverageCrconcentrationeven iftheCr2O3contentofthefirstgroupisfivetimesmorethanthe latterones.Inaddition,thesameconclusioncanbereachedfor iron oxide, since different FeO/Fe2O3 content featured in the differentslaggroups.
Leachingtestoncrushedslagparticles
DifferentslagbelongingtogroupsA,BandCwereinvestigated, andsomeexamplesarereportedinFigs.4and5.Thesameparticle was repeatedly treated at thedifferent L/S ratio and the same surfacewasanalysedaftereachtestcondition.Theliquid-to-solid ratio is one of the fundamental factors ruling the interaction betweenslagandtheenvironment.Itseffectisevident:increasing thewatervolumecausedanincreaseintheamountofdissolved slag(Fig.4).Infact,instandardconditions,theinteractionbetween waterandslagwassuperficialandisonlyofinteresttoalimited portionofthesurfaceparticle(Fig.4b),butbyincreasingtheL/S ratio, the surface of the slag particle appeared to be largely consumed (Fig. 4c). Thisimplies that during thestandard test, masslossisnotappreciable,eveniftheslag-waterinteractionis enoughtochangetheleachatepHanddriveheavymetalleaching. However,notallthesurfacereactswithwater,butonlyspecific phasesthatareinvolvedinhydrationanddissolutionprocesses.In particular,SEM-EDSanalysisonthesurface(beforeandafterthe test) indicated that the Ca-rich silicates, i.e. larnite and
Fig.4.SEM-BSEmicrographiesofcrushedslagparticles(belongingtogroupC)leachedatdifferentL/Sratios:(a)as-received;(b)10l/kg;(c)100l/kg.B:brownmillerite;E: eutectic3CaOSiO2–FeO;L:larniteandW:wustite.
brownmillerite,drivetheslagdissolution(BandLmarksinFig.4). Thedissolved larnitefractionrapidlyincreaseswhereas brown-millerite dissolution is clearly evident only at high L/S ratios. Wustite(W)remainsunalteredwhereasinthecomplexeutectic phase3CaO
SiO2–FeO(E)a largeamountof CaOandSiO2 were dissolved,creatingacharacteristicstripedmorphology(Table4). Similarresultswereobtainedfortheotherslagsamples,fullyin agreement with those previously reported by other authors[51,52].
However,notalltheinvestigatedslagbehavedinthesameway. Forexample,thesurfaceoftheslagreportedinFig.5appeared completely unaltered after immersion at 100l/kg. This slag complied with leaching limits, thus enabling the possibility to discern between safe and unsafe slag. In this case, the slag microstructurewasmainlyformedbykirschsteinite (Kmarkin
Fig.5),gehlenite(G)andwustite(W)that,aspreviouslystatedby thesameauthors[46],seemstoensurecompletestabilitytothe slag. In fact, the particlestructure appeared unaltered and the phasechemicalcompositionmeasuredbyEDSprobepointedout negligiblesilicon,calciumandirondissolution(Table4).
Theseresultshighlighttheimportantroleofcrystallinephases in slag leaching, as previously stated by several authors
[6,7,33,34,51–54].Inparticular,thestabilityofwustitewas well documentedby Belhadj[55],whereas larnite,and its polymor-phism bredigite, as well as brownmillerite own hydraulic properties,sincetheyarethesameconstituents ofcementand concreteandbehaveinthesamewaywhenhydrated[36,56,57]. Whilethemicrostructureis strictlyrelatedtotheslagchemical
composition,apredictionontheleachingattitudeoftheslagcan be evaluatedbycouplingchemical information, CaO–SiO2–FeOx andCaO–SiO2–Al2O3 ternarydiagrams.Infact,ternarydiagrams providereliableindicationsaboutthecrystallinephase/phasesin theslagmatrix.ReferringtothesamplesshowedinFigs.4and5, thereisanagreementbetweentheirpositiononternarydiagrams, andXRDandSEMevidences.
Discussion Barium
TheMgOconcentrationinslagcontrolstheBaleachingwitha quadratic equation (Fig. 6a). If a slag contains enough Mg-complexing oxides (i.e. FeO), barium oxide could dissolve in calciumsilicateslattice(i.e.
b
-Ca2SiO4).Unfortunately,thisphase hashydraulicpropertiesanddissolvesduringcontactwithwater. Incontrast,inhighmagnesiaslag,MgOcouldreplacebarium(and calcium)incalciumsilicates,reducingtheBaOsolubilityinsuch phases(i.e.a
-Ca2SiO4)[58,59].Thus,whenBaisboundincalcium silicates, its release could be enhanced because of the high reactivityofthesephaseswithwater.Inbothcases(withhighor lowMgOcontent),Bacouldeasilybereleased.Inaddition,theMgO effectcouldbedividedasafunctionofitsaveragecontentinthe slag.Infact,asindicatedinFig.7a,twogroupscanbeidentified: thefirstischaracterizedbyMgOcontentinthe1–5%wt.range,the latterfrom5to14%wt.Thefirstgroupconsistsmostprevalentlyof slagingroupsAandB,whereasthelatterismostlyinCgroupslag.Table4
SEM-EDSpointanalysisofcrushedslagparticlesbeforeandafterleachingtestat100l/kg(%atomic).B:Brownmillerite;E:eutectic3CaOSiO2–FeO;L:larnite;G:gehlenite;K:
kirschsteinite;W:wustite.
Sample Phase Condition Mg Al Si Ca Ti V Cr Mn Fe Ba
Fig.4 W Beforeleaching 14.27 19.83 1.87 20.45 43.48 0.11 Afterleaching100l/kg 16.30 18.02 0.02 2.11 20.15 43.39 0.01 E Beforeleaching 2.25 22.96 66.95 0.05 0.67 2.45 4.48 0.20 Afterleaching100l/kg 4.04 5.99 14.93 28.59 0.44 0.14 2.71 15.22 27.85 0.08 B Beforeleaching 1.00 25.79 1.18 49.09 1.41 0.43 8.48 1.03 11.47 0.10 Afterleaching100l/kg 1.03 23.28 2.46 48.45 1.60 0.36 10.50 1.53 10.79 L Beforeleaching 1.49 31.22 66.01 0.38 0.23 0.50 0.17 Afterleaching100l/kg 3.49 4.66 4.37 29.26 5.02 53.20
Fig.5 W Beforeleaching 8.34 0.06 1.00 9.06 81.52 0.02 Afterleaching100l/kg 10.35 0.43 0.10 1.10 8.81 79.15 0.05 K Beforeleaching 10.30 39.79 36.67 3.30 9.94
Afterleaching100l/kg 12.50 43.69 33.72 2.32 7.70 0.07 G Beforeleaching 3.28 21.98 29.86 39.78 0.66 4.44
Afterleaching100l/kg 4.25 23.08 28.53 31.34 1.40 11.38 0.01 Fig.5. SEM-BSEmicrographiesofcrushedslagparticles(belongingtogroupA)leachedatdifferentL/Sratios:(a)as-received;(b)10l/kg;(c)100l/kg.G:Gehlenite;K: kirschsteinite;W:wustite.
Nevertheless,theMgOeffectonBareleaseisthesameforboththe groups.
ThesameconclusionscanbereachedfortheCaOcontentofthe slag.AsreportedinFig.6b,theBaleachingseemstobefavouredby highCaOconcentrations.Mostlikely,highCaOcontentleadtothe formationofhigh-hydraulicsilicatesthateasilyreactwithwater (i.e. Ca3SiO5, Ca2SiO4). The hydration process facilitates the migrationofspeciessuchasCa,MgorBaintowater.
TheeffectofSiO2onslaghydraulicbehaviourdependsonCaO content and for this reason no correlation between SiO2 and leachedBawasfound.Thus,SiO2fractionisnotausefulindexto forecastslagleachingbehaviour,andforthisreasonisnotreported. Althoughseveralauthors[32–34,46]reportedthebeneficialeffect oftheadditionofsilicaonretainingbehaviour,inthiscase,theslag analysedwasnotchemicallytreatedandthesilicacontentisonly relatedtotheprocessroute.
Vanadium
V behaviour(Fig. 6c)seemed tobecontrolledby CaO,SiO2, Al2O3andMgOcontentintheslag.ThehighertheCaOfractionthe lowerVconcentrationthereisintheleachate.Thisresultcomplies withtheV-pHrelationshipandcorrelateswithotherresearchin thesamefield.Specifically,someyettobepublishedresultshave suggested an experimental correlation between Ca and V. Increasing the Ca leaching, V concentration in the leachate solutionslightlydecreases.Valsoseemedtohavea logarithmic dependence on the aluminium and magnesium oxide weight fractions.AdirectcorrelationbetweenV2O5intheslagandleached Vhasbeenfound.
Vanadium oxides are usually associated to silicates and calcium–ferrite. Brownmillerite-type (4CaO
Al2O3Fe2O3) phases could also beformed with a low Al2O3 amount thanksto thereactionbetweencalcium–ferriteandvanadiumand/orchromium oxide.Essentially,SEM-EDSinvestigationshighlighted brownmil-lerite,constitutedwithhighVand/orCroxideinplaceofAl2O3.For example,thebrownmilleriteidentifiedinFig.4isfeaturedhighCr content(c.a.9%at.)andnon-negligibleVcontent(c.a.0.5%at.).In othersamples,mainlybelonginggroupC, brownmilleritehad a compositionrangedfrom0.5to1.0Vand5–10Cr(% at.).Since brownmilleritehashydraulicpropertiesandhydratesduringthe firstcontactperiod betweenslag andwater,in poorAl2O3slag, higherVconcentrationcanbeeluted.Inaddition,AlandMgoxides are the main constituents, together with magnetite, of spinel phases. The analysis carried out on the different slag samples investigatedemphasizedthevanadiumtendencytoalsobebound inspinelphases.IftheslagisfeaturedbylowcontentofMgOand Al2O3, non-stable spinels can occur, promoting V leaching. Furthermore, in these conditions, as demonstrated by Kuhn etal.[21,22],Crcanalsobeeasilyreleased.
HighMgOcontentintheslaginhibitVleaching,butenhanceBa leaching. Theabovementioned considerationsmayexplain the conflictingbehaviourbetweenBaandV.Effectively,groupBslag manifestedtherelease ofV but notBa. Theiraveragechemical composition(reportedinTable3)ischaracterizedbylowMgOand Al2O3content,thatseemtofavourVleaching.Onthecontrary,the slagcharacterizedbyhigherAl2O3andMgOcontent,indicateda higherBaconcentrationintheleachedsolution.Moreover,forthe slag featured bya higher CaO fraction, the V concentration in leachatesolutionsneverreachedworryingvalues,withanaverage concentrationoflessthan50
m
g/landlimiteddatadispersion.On the other hand, slag featured by a low CaO concentration manifestedVleaching,liketheothercases.HighSiO2contentin theslagseemedtoimproveitsV-retainingbehaviour(Fig.6c).SiO2 over the 15%wt. seems tobe enough tocompletely prevent V release. Thisresult confirms thegood V-retaining behaviour of acidicslag,asdemonstratedinpreviouswork[34,35].Linearproportionaldependencewasinsteadretrievedbetween V2O5,Cr2O3 and leached V (Fig.6d). The higherthe Cr2O3, the higherthespinelfractionandsizeintheslagmicrostructure.High Cr2O3 content imposes the presence of higher spinel-forming species(i.e.MgO,Al2O3)inordertoensurestablespinelformation
[21,22].Iftheslagchemicalcomposition,doesnotallow forthe
spinel-formingspeciesinvolvedtobeproperlycombined,unstable spinelsareformed,inducingCrandVleaching.Thecoolingrate and the cooling procedures also control the spinel formation
[12,18],butthistypeofcorrelationwasnotanalysedinthepresent work.
Chromium
Concerning the Crrelease behaviour, no indication or clear correlationwerefoundbetweenCrconcentrationintheeluteand slagchemicalcomposition.Thus,asdescribedbyKuhnetal.[21,22]
themainfactorrulingCrimmobilizationistheformationofspinel structure withcompositionsimilartonaturalMgCr2O4.Evenif, nowadays,theonlyreliableforecastingtooltopredictCrleaching seemstobethesp-factor(Eq.(1))proposedbyKuhnetal.[21,22], itsefficacyhasnotbeencompletelyverifiedforcarbonsteelslag. spfactor¼0:2MgOþ1:0Al2O3þnFeOn0:5
Cr2O3ð%wt:Þ ð1Þ
wherenistheopticalelectronegativityofironions,dependsonthe oxidationstateoftheEAF-slagandcanassumeavalueofbetween 1and4asafunctionoftheoxidationnumberassumedbytheFe ions.
Infact,asreportedinFig.6e,severalsamplesthathavean sp-factorover the25%wt. (value consideredenoughto preventCr leaching).releasedCr,whereassomesamplesfallingwithinthe unsafe zone (between 5 and 25%wt.), didnot manifest any Cr leaching. It seems more reliable to focus on the effect of slag basicity(intermsofCaO/SiO2ratio)onCrleaching(Fig.6f).High slag basicityis associatedwithahighCr6+/Cr3+ratio[60].Thus, increasedslagbasicity,decreasestheCrthatcanbeboundinthe spinel(intheCr3+formasCr2O3 [60–62]),increasingthemean chromiumcontentin theslag matrix.In addition,highbasicity induceshighMgOandCaOactivityinthemoltenslag,resultingin the formationof a Ca-chromitephase insteadof a more stable MgCr2O4[63].Ca-chromiteisunstableanditsroleinCrleachingis wellknown[64,65].
Theaverageslagchemical compositioncouldhelptopredict slagproperties,forboththeleachingelementsandmicrostructural features, since the relationship between these two aspects is
Fig.7.Identificationofsafe(green)andrisk(red)chemicalcompositionareason(a)CaO–SiO2–FeOand(b)CaO–SiO2–Al2O3ternarydiagramstopredictleachingbehaviourin
established.Infact,thedangerousheavymetalsinvolvedinthe leachingprocessfromEAFcarbonsteelslagarecontrolledbythe oxidespecies(i.e.CaO,MgO,...)andspecificcrystallinephases (i.e.larnite,brownmillerite,...).
Ausefulindicationaboutthechemicalcompositionofsafesolid slagisindicatedintheternarydiagramsshowninFig.7.Theslag fall in the green areas were featured by stable matrix, mainly formedbygehleniteand/orkirschsteiniteandinmostofthecases theywerecompliantwiththeleachinglimits,whereastheslagfall intheredareaswerecharacterizedbylarnitematrixandsubjected tothereleaseofpollutingsubstances.SincetheEAFslagcanbe considereda quinquenaryoxidesystem, theeffectof theother oxidesspeciesformingtheslag(i.e.Al2O3forC–F–Sdiagramand MgO for C–A–S) can be determined by other pseudo-ternary diagrams for quaternary systems [66–70]. In particular, Al2O3 contentover5%byweightcreatestheformationofgehleniteand anorthiteintheCaO–SiO2–FeOsystem,closingtheCaO
SiO2fields. ByincreasingtheAl2O3concentration,however,theolivinefield disappears [69,70]. On the contrary, MgO tends to close the gehleniteandanorthitefiledintheCaO–SiO2–Al2O3systemfrom 5%byweight.Thisaspectcouldexplainwhyslagfeaturedbyahigh concentrationof MgO release high Ba concentration and form gehlenite structure with difficulty [66–68]. For example, the outliersofgroupAsufferedthisproblem:eveniftheirpositionon the C–A–S ternary diagram is in the field of gehlenite, their microstructure did not signal gehlenite. On the contrary, the matrixwas characterizedbyhighMg-di-calciumsilicate, which induced the high Ba leaching highlighted during the standard leachingtest.Thusacorrectbalanceofchemicalcompositioncouldallowthe formationofstableandsafeslag. Inparticular,MgOshouldnot exceed5%byweightforhighoxidizedslag(groupsAandB)andthe 7%forlessoxidizedones(groupC),whereasAl2O3shouldbeinthe range7–10%wt.Nonetheless,CaOshouldbecontrolled,toavoid reachingan overly high concentrationthat could stimulate Cr leachingandcreatedustingandvolumeinstabilityproblems.
Since theslag hasto beprepared toobtain steels withthe appropriatequalityandproperties,athermo-chemical stabiliza-tion/inertization process (outside the EAF) should be applied independently on all the recyclable slag in order to avoid dangerousenvironmental contaminations. In particular, as pro-posedby Mombelliet al. [51] theadditionof suitable external oxides during the deslagging operation could improve the sustainability of the slag, moving the chemical composition in thesafeareasofternarydiagrams.
Evenifwater-slaginteractiononlyintereststheslagsurface,itis enoughtodissolveanimportantamountofdangeroussubstances and to drive the leachate pH to caustic values. These results, althoughqualitative,indicated thatliquid-on-solid ratioplaysa fundamentalroleonslagleachingbehaviour.Thecharacterization oftheslagleachingbehaviourthroughonlystandardleachingtests couldbemisleadinginordertoassesssafetyorhazardattitudeof thisby-product.Ifslagsaretobeusedasunboundstonematerial, the continuous wetting-drying cycles may lead to dangerous substanceaccumulationintotheenvironmenteveniftheslagwere declaredsafeafterastandardelutiontest.
Theseconsiderationssuggestthatthecharacterizationofslags should not only undergo the standard elution test but also a modified test in which the effect of liquid-on-solid ratio is investigated.
Conclusions
In this paper different EAF carbon steel slag associated to differentsteel production were investigated to understandthe mechanisms that control the leaching of polluting substances
duringleaching tests instandard conditions. Theslag chemical compositioninfluencestheleachingbehaviouroftheslagandthus, itcanbeusedtoforecasttheoutcomeofleachingtest.
Onthebasisoftheobtainedresults,thefollowingconclusions canbeattained:
slagcharacterizedbyhighCaOandMgOcontent(althoughthey indicatedcausticleachatepH) had betterretainingbehaviour against V release. Acidic slag (with high SiO2 content) was featuredbyabettertrappingattitudeagainstVrelease; highCaOand MgO concentrations lead tohighBa release.A
correctbalanceofthebasicoxidespeciescouldreducetheslag environmentalimpact,reducingtheoverallelementleaching; highbasicityslag,intermsofCaO/SiO2ratio,couldenhanceCr
leaching;
water-slaginteractioninterestsonly theslag surfacebut was enoughtodissolveimportantamountsofdangeroussubstances andtodrivetheleachatepHtocausticvalues;
ternarydiagramscanbeusedtopredicttheleachingbehaviour ofslagbuttheeffectofotheroxidesonternarysystemsmustbe taken into account due to their significant effects on phase equilibrium;
a correct balance of chemical composition could allow the formationofstableandsafeslag,eventuallyproviding oppor-tunechemicalcorrectionoutsidetheEAF.
In order to satisfactorily reuse EAF slag for civil purposes (aggregatesforasphalt,inertgravel,...)somepractical recom-mendations canbe highlighted: MgO shouldnot exceed 5% by weightforhighoxidizedslagandthe7%forlessoxidizedones, Al2O3shouldbeintherange7–10%wt.andCaOshouldnotexceed the30%byweighttofitthesafeareasonternarydiagrams.
Iftheslagchemicalcompositionisouttherecommendedrange, achemical compositioncorrectionbytheadditionofstabilizers (i.e. SiO2, Al2O3,...) would be applied outside the EAF. Slag characterizedbystableandnon-leachablemicrostructurecanbe safelyusedalsoinunboundapplications,namelyunpavedroads, armourstoneorgabions.
AppendixA Listofphases.
Phase Definition Formula Akermanite Calcium-magnesiumsilicate Ca2MgSi2O7
(2CaOMgO2SiO2)
Anorthite Calcium-aluminiumsilicate CaAl2Si2O8(CaOAl2O32SiO2)
Bredigite adi-calciumsilicate Ca7Mg(SiO4)4
(7CaOMgO4SiO2)
Brownmillerite Tetra-calciumaluminate ferrite
Ca2(Al,Fe)2O5
(4CaOAl2O3Fe2O3)
Gehlenite Calcium-aluminiumsilicate Ca2Al(AlSi)O7
(2CaOAl2O3SiO2)
Kirschsteinite Calciumolivine CaFeSiO4(CaOFeOSiO2)
Larnite borgdi-calciumsilicate Ca2SiO4(2CaOSiO2)
Mayenite Calciumaluminate Ca12Al14O33(12CaO7Al2O3)
MgCr-spinel Magnesiochromite(spinel) MgCr2O4/(Mg,Fe)(Al,Cr)2O4
Wustite Ironoxide (Fe,Mg,Mn)O/MgOFeO
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