Enhancement in hydrophobicity of low rank coal by surfactants — A critical overview

Review

Enhancementinhydrophobicityoflowrankcoalbysurfactants—Acriticaloverview

ShobhanaDey⁎

CouncilofScientificandIndustrialResearch-NationalMetallurgicalLaboratory,Jamshedpur,India

abstractarticleinfo

Articlehistory:Received30December2010Receivedinrevisedform14September2011Accepted15October2011Availableonline9December2011

Keywords:LowrankHydrophobicityOxidationWettabilitySurfacechargeTheflotationoffine(−0.5mm)lowrankoroxidizedcoalisdifficulttoachievewiththecommoncoalflota-tioncollectorslikekerosene,fueloilordieseloil(oilycollector).Thepresenceofsmallamountsofoxygenisenoughtocauseoxidation.Theoxidationofcoalsstartswiththephysicaladsorptionofoxygenonthesurfacetoformanoxycomplexfollowedbychemicaladsorptionofoxygentoformpolarphenolic–OH,carbonyls,phenolsandperoxidetypeoxygenatedmoietiesbytheruptureofcyclicrings.Theadditionofpromoter,sur-factantoroxygenatedfunctionalgroupstothecollectormoleculemarkedlyenhancestheflotationoflowerrankandoxidizedcoalsduetothehydrogenbondingwiththepolarpartofthecoalsurfaceandthereagent.Theperformanceofthesereagentsiscomparedwiththatofoilycollectors,namelykerosene,dodecane,non-ylbenzeneandpolarpartofthesurfactanthavinganoxygenatom.Themodeofadditionofnon-ionicsurfac-tantwithoilycollectoralsohasamajorroleintheflotationresponse.Theadditionofnon-ionicsurfactantaftertheoilycollectorhasshownapositiveeffectonyieldandgrade.©2011ElsevierB.V.Allrightsreserved.

Contents

1.Introduction..............................................................1512.Literaturereview............................................................1522.1.Formsofoxygenoncoalsurface.................................................1522.2.Generationofsurfacechargeatcoal/waterinterface........................................1522.3.Wettabilityofoxidizedcoal...................................................1532.4.Wettabilityofoxidizedcoalinpresenceofsurfactants.......................................1532.5.Effectofcoalparticle–promoterinteractionsonflotation......................................1542.6.Flotationofoxidizedcoals....................................................1542.6.1.Mechanismofadsorption................................................1552.6.2.Collectorspreading...................................................1552.6.3.Interactionoftheoxygenatednon-ionicpartofcollectorsoncoalsurface...........................1562.6.4.Structuraleffectofreagentsonflotationresponse.....................................1563.Summary...............................................................157References.................................................................157

1.Introduction

Coalflotationmakesuseofthenaturalhydrophobicityofthecar-

bonaceousmatterincoal.Toenhancethehydrophobicityofthecoal

particles,oilycollectors,suchasdieseloilandkeroseneareusually

added.Forhigher-rankcoals,thereagentconsumptioninflotation

islowbecauseofthenaturalhydrophobicityofthecoal.However,theoxidizedcoalsorlowrankcoalsaredifficulttofloatwithcom-

monlyusedfueloilorkeroseneandlargeamountsofcollectorsare

requiredtoachievesatisfactoryyields[1].Thepoorfloatabilityis

duetothepresenceofgreateramountsofoxygencontentandabun-

danceofhydrophilicsurfacefunctionalgroups[2,3,4,5].Itwas

reportedbyQuastandReadett[6]thatsub-bituminouscoalshave

anaverageoxygencontentof18%,withcarboxylicgroupsconstitut-

ingaboutonethirdofthisamount.Therefore,theamountofadhesion

ofoildropletsontolowrankcoalsisverysmall,andtheuseofoil

alonedoesnotimprovefl

otation.FuelProcessingTechnology94(2012)151–158

⁎Fax:+916572271709.E-mailaddress:sd@nmlindia.org

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0378-3820/$–seefrontmatter©2011ElsevierB.V.Allrightsreserved.doi:

10.1016/j.fuproc.2011.10.021ContentslistsavailableatSciVerseScienceDirect

FuelProcessingTechnology

journalhomepage:www.elsevier.com/locate/fuprocTherearemanycoalprocessingplantsthatarefacingproblemsinfloatingoxidizedcoal.Thesourcecanbefromoutcropcoal,strip

mines—wheretheoverburdenwasremovedseveralyearsbefore

miningstartedorevenfromimproperstockpilingoveraperiodof

time.Oxidationaltersbothphysicalandchemicalpropertiesofthe

coalsurfaceandreducesthefloatability.Thesurfaceoxidationofcoal

isreflectedbydecreaseinpHofthepulp.Tosolvethisproblemreagents

containingpromoterarerecommended,alongwithlargeamountof

fuels[7].

Infinecoal,oxidationbyweatheringorcoalskeptatminesiteor

duringstorageandtransportationresultsintheformationofoxygenat-

edfunctionalgroups.Carboxyl,phenolicandcarbonylarethemost

commonlyfoundfunctionalitiesonthecoalsurfaceandtheirconcen-

trationcanbedetermined[8].Thisreducesthehydrophobicityofthe

coalsurfacebyincreasingthenumberofsitesthatformhydrogen

bondswithwatermolecules.Itisgenerallyacceptedthatoxidationof

coal(whetheritbeexposedartificiallyortotheatmosphere)proceeds

inthreestages.StageIisthesuperficialoxidationcharacterizedbythe

formationofcoal–oxygencomplexeswithacidicproperties.InStage

II,theorganiccomponentsofcoalformalkalinesolublehydroxycar-

boxylicacidscalledhumicacids.InStageIII,thehumicacidsdegrade

tosimplewater-solubleacids[9].Adsorptionofoxygenonthecoalsur-

faceisexothermicand,besidesthemoietiesformedonthecoalsurface,

thereactionproductslikeCO,CO2andH2Omaybereleasedfromthe

structure[10].Themostsusceptiblelinkagestooxidationwerefound

tobetheα-CH2groupstopolyaromatics[11,12,13,14].Mitchelletal.

[15]revealedaninterestingpointonoxidationthatblue-lightirradia-

tionisalsoastrongagenttooxidizethevitrinitesurfaces.Sarikaya

etal.[16]observedthatuponoxidationtheflotationyieldreduces

from95%to24%forabituminouscoalusingalcoholtypefrotheronly.

Theoxidationofthesurfacemakesthecoalmoredifficulttofloatwith

oilycollectors[17–22].Anoilycollectorcannotspreadonthesurface

ofthecoalparticles.Thepresentpaperhighlightsthestudyofoxidation

ofthecoalsurface,adsorptionbehaviorofsurfactantsonoxidizedor

lowrankcoal,itseffectonfloatabilityandimprovementoftheirhydro-

phobicitybyaddingreagentsindifferentsequences.

2.Literaturereview

Coalflotationisacomplexphenomenoninvolvingseveralphases

(particles,oildropletsandairbubbles).Thesephasesthensimulta-

neouslyinteractwitheachotherandwithotherspeciesviz,molecules

ofsurfactants/promotingreagentsanddissolvedionsinwater.The

structureofsurfactants/collectormolecules,thetypeofelectrical

chargepossessingthecollector,itsdispersionabilityandthetypeof

bonding(physisorptionorchemisorption)thatformswiththecoalsur-

faceareallimportantfactorsforimpartinghydrophobicityonthecoal

surface.Molecularorientationsofthereagentmoleculesalsoplayan

importantroleontheadsorptionprocess[23–25].Theflotationstudies

performedwithlowrankcoalsindicatedthatthemixturesofoilscould

beusedtoimproveflotationrecoveryandselectivity[26].Asitis

known,oilemulsionsarethermodynamicallyunstableduetoincreased

interfacialarea.Therefore,tostabilizetheemulsions,mechanicalenergy

orasurfactantisneeded.Mechanicalemulsificationisusuallystable

onlyforashortperiodoftime,whileemulsionsproducedwithsurfac-

tantsarestableoveralongperiodoftime[27].Itwasshownthatthe

emulsificationofthekeroseneproducedbythehighintensitystirring

shapeddropletsofabout20μminsize,whileemulsificationwiththe

additionofsurfactants(anionicandcationic)reducedthedropletsize

toabout1.5–2.0μm[28,29].Thisstabilizationoftheoildropletsusing

surfactantsisduetothedecreaseinsurfacefreeenergywithadsorption

attheoil/waterinterfaceofthesurfactants.

Surfactants,whenaddedtogetherwithoil,arelikelytoimprovecoalflotationbyseveralmechanisms.Whenaddedasemulsifiers,theyaid

dispersionoftheoilintofinedroplets[27].Theincreasednumbersof

oildropletsassistflotationkineticsbyincreasingtheprobabilityofcoalparticle–oildropletcollisions[30].Surfactantsalsolowertheener-

gyrequiredtospreadthecollectoroil[31]acrossthecoalsurface

throughadsorptionatthecoal/waterinterfaceandcoal/oilinterface.

Atlowconcentrations,suchsurfactantsmakecoalhydrophobic,where-

as,athighconcentration,theymakeithydrophilic[32].Ifthesurfactant

promotesthespreadingandadhesionofoil,thehydrophobicityofcoal

willincrease,leadingtoahighprobabilityofadhesionoftheairbubble

tothecoalparticle[31,33].Incontrast,ifthewettingofthecoalby

waterisincreasedbythesurfactant,whichusuallyhappensathighcon-

centrations,therecoveryofthecoalwilldecrease.

Chanderetal.[34]carriedoutflotationoflowrankcoalof−150μm

sizewithdodecaneanditwasshownthatsubstantialamountofreagent

wasrequiredtoachievethesatisfactoryyield.Theyalsoperformedcoalflotationwithnon-ionicandionicsurfactants.Someofthewatersoluble

surfactantsmakecoalmorehydrophilicorwettablewhenpresentin

sufficientquantities.Incontrast,thesamesurfactantsincreasethe

hydrophobicitywhenpresentinsmallamount.Therefore,theflotation

characteristicsofcoalslargelydependonthesesurfaceproperties.

2.1.Formsofoxygenoncoalsurface

QuastandReadett[6]reportedthewettabilityofsub-bituminous

coalsurfacebythepresenceofoxygencontentof18%andthecarboxylic

groupsconstitutingaboutonethirdofthisamount.Considerablecon-

troversyexistsregardingtheformsofoxygenincoalandthemeansof

analyzingforthem.ThevaluesinTable1forcarboxyl(–COOH)andhy-

droxyl(–OH,bothphenolandalcohol)arecitedfromPennStateCoal

DataBankandavarietyofothersources[3,35–38].Itwasfoundthat

theoxygenfunctionalitiesalsoincludeether(C–O–C),ketone(C=O)

andmethoxy(–OCH3)groupinadditionto–OHand–COOH.Itwas

observedbyKorobetskiietal.[39]thatsmallamountsofresidualoxygen

aresufficienttobringaboutoxidation.Naturaloxidationmainlyaffects

theexternalsurfacesofcoal,hence,forbetterflotationresultsthesize

reductionmustberetardedaslongaspossible[40].Weatheringofthe

coalparticlesdevelopedcrackswhoseextentwasafunctionofcoal

rank.Lowrankcoalparticlesdevelopedextensivecrackswhereashigh

rankcoalshardlyseemtobeaffectedphysically[41,42].

2.2.Generationofsurfacechargeatcoal/waterinterface

Sincecoalisaveryheterogeneoussubstance,themechanismofsur-

facechargegenerationisverycomplex.CampbellandSun[43]have

proposedasimplifiedmodelbasedontheearlierworkofGlembotskii

[44].Thismodelassumesthatwhenafreshlycrushedcoalsurfaceisex-

posedtotheatmosphere,theexposedcarbonatomsinthelatticereact

withatmosphericoxygenandformoxidizedsurfaceswithcharging

characteristicssimilartooxideminerals.Inthiscase,hydroxylandhy-

droniumionswillbethepotentialdeterminingions.

Kelebeketal.[45]investigatedthegenerationofchargingmechanism

forlowrankligniticcoalswouldbedifferentduetothelargeamountsof

oxygen-containingfunctionalgroups.Thechargingmechanismforlignite

wasexplainedbasedonthepresenceofoxygen-containingweaklyacidic

groups(e.g.,phenolicOHandCOO).Dependingonthemoisturecontent,

thesurfacespossessthesegroupsindehydrated(oxideform)orin

hydratedforms.Thesurfacechargecanbedeterminedmainlybythe

Table1Approximatecontentofvariousoxygenfunctionalgroupsincoal(basedonthedataofBlometal.[35]).

%CTotalOC–O–Cðer–OHC=O–COOH–OCH39511----856232--75166711-65a28107281

aAEuropeanbrowncoal.152S.Dey/FuelProcessingTechnology94(2012)151–158