EvaluationofMinnesotaTaconiteWetScrubbersatMinntac.KeewatinTaconite.HibbingTaconite.andUnitedTaconitebyJohnEngesser,P.E.MinnesotaDepartmentofNaturalResources,DivisionofLands&MineralsThispaperisareviewofwatertreatmentmethodsthatcanbeusedtoincreasesulfurdioxidescrubbingatMinntac,KeewatinTaconite,HibbingTaconite,andUnitedTaconite.Thepaperwilllookattheimpactsthatmoreefficientsulfurdioxidescrubbingwillhaveonprocesswaterchemistry,tailingwaterchemistry,andironoxiderecovery.TaconiteWaterChemistryMinnesotataconitefacilitieshavebeenoperatingfor40to50years.Duringthistimetheconcentrationofdissolvedsolidsinrecycledprocesswaterandtailingwaterhasincreased.Thisincreaseintheionicstrengthoftaconite-processwaterhasoccurredbecauseofthefollowingfourchemicalreactions:1)sulfidemineraloxidationandthesubsequentformationofsulfuricacid,2)theremovalofsolubleacidgases(sulfurdioxide,hydrogenchloride,hydrogenfluoride)fromindurationexhaustbywetscrubbers,3)neutralizationofacidgasesbycarbonateminerals,and4)dissolutionofsolublesalts.Sulfideminerals,presentintaconite,oxidizeandhydrolyzetoformsulfuricacidwhenexposedtoairandwaterduringgrindingandinduration.Sulfuricacidisneutralizedbycalciumcarbonateandmagnesiumcarbonate,whicharebothpresentintaconite.Thesereactionscauseanincreaseintheionicconcentrationsofcalcium,magnesium,andsulfateinprocesswaterandtailingwater.Thefollowingreactionequationsrepresentwhatoccursduringgrinding,indurationandtailingdisposal.4FeS2+1502+14H20 74Fe(0H)3+16II'+8S04-(grindingandtailingdisposal)4FeS2+110272Fe203+8S02t(Induration)2S02+2H20+0274II'+2S04-(ScrubberReaction)II'+CaC037Ca+2+HC03-(Neutralization)II'+MgC037Mg+2+HC03·(Neutralization)Astheconcentrationofsulfateincreases,theconcentrationofbothmagnesiumandcalciumincrease.Thefollowinggraphillustratesthecorrelationbetweenhardness(calciumandmagnesium)andsulfateintaconiteprocesswater.9-':-8"7""II)6"05E.§.4"-32"1en0SulfateConcentrationCorrleatedtoCalciumandMangsiumConcentrationsinTaconiteProcessWaterR'=0.98291"..00'"-12..:l.,................".-34567Ca+Mg(mmolesfliter)•DataPoints89101Increasedsulfateconcentrationresultsinincreasedprocesswaterhardness,whichcancauseprocessingproblemsinflotationandballing.Highcalciumconcentrationsinprocesswaterresultsinprecipitationofcalciumcarbonateinpipes.Calciumsulfateprecipitationcanalsooccurincertainareasoftheplantswhenthecalciumsulfatesolubilitylimitisreached.Theultimateresultoftheincreasedwaterhardnessisanincreaseintaconiteproductioncosts.Thereareveryfewsolublesaltspresentintaconite.However,withtheadventoffluxedpellets,solublechlorides(calcium,magnesium,sodiumandpotassium)areintroducedintothewaterduringfluxstone(calciteand/ordolomite)grinding.Chlorideisvolatizedduringpelletindurationandissubsequentlyremovedbythewetscrubbersashydrochloricacid.Thefollowingreactionsrepresenttheformationofhydrochloricacidduringindurationandtheneutralizationreactionsthatoccurduringscrubbingandafterscrubbing.CaCh+H20-7CaO+2HClj(Induration)HCl+CaC03-7HC03-+Ca+2+cr(Neutralization)Smallamountsofnaturallyoccurringinsolublefluorideminerals(mostlyfluorapitite)arepresentinMinnesotataconite.Approximately90to95percentofthefluorideisrejectedtothetailingsduringmagnetiteconcentration.Theremainingfluoride(0.001%to0.002%oftheconcentrate)enterstheindurationprocess.Fluoridemineralsarealsoaddedtotheconcentratewiththeadditionofbentoniteclay,limestoneanddolomitepriortogreenballformation.Approximatelyfortypercentofthefluorideenteringtheindurationprocessisvolatilizedduringinduration.Thequantityofhydrogenfluoridethatformsduringindurationincreasesastheconcentrationofsodiumintheprocesswaterincreasesandtheconcentrationofcalciumintheprocesswaterdecreases.Wetscrubbersremovethehydrogenfluoridethatformsduringpelletinduration.Approximately0.01poundsofhydrogenfluoridedissolvesintheprocesswaterforeachtonoftaconiteprocessed.Mostofthefluoridethatdissolvesintheprocesswaterprecipitatesascalciumfluoride.Fluorideconcentrationinprocesswaterislimitedbycalciumconcentrationbecausecalciumfluorideisfairlyinsolubleinwaterandreadilyprecipitates.Thepresenceofsodiuminprocesswateraidsinthedissolutionoffluoridesaits,whichresultsinincreasedconcentrationsoffluorideinprocesswater.Theconcentrationoffluorideintaconiteprocesswatervariesfrom2mg/literto25mg/literdependingontheconcentrationofcalciumandsodium.Thefollowingtwographsdemonstratehowtheconcentrationoffluorideinprocesswaterisaffectedbytheconcentrationsofcalciumandsodiuminprocesswater.2520ECo15"-,fc010u..."5"iL0CalciumandFluorideinProcessWaterSamplesfromMinnesotaTaconitePlants.....'"•••.....o20•DataPointsi....406080CalciumCone.,ppm-TheoreticalCaF2SolubilityT1002E25a.c.:20UI:15ooell10'tl5:::IiL0oSodiumandFluorideinProcessWaterSamplesfromMinnesotaTaconitePlants•....V"'I--....•20406080100120140SodiumCone.,ppm•DataPoints160The.followingreactionsrepresentthefluoridereactionsthatoccurduringindurationandneutralizationafterscrubbing.NaF+H20HFj+NaOH(Induration)2HF+2CaC03CaF2t+Ca+2+2HC03-(Neutralization)Theadditionofsodiumcarbonateand/orsodiumhydroxidetotaconiteprocesswaterresultsintheprecipitationofcalciumcarbonate,whichdecreasestheconcentrationofcalciuminthewater.Thefollowingreactionsrepresentwhatoccurswhensodiumcarbonateandsodiumhydroxideareaddedtotaconiteprocesswater.Na2C03+Ca+22Na++CaC03tNaOH+HC03-+Ca+2Na++H20+CaC03!WaterChemistryConclusionsI)Increasedconcentrationsofsulfateintaconiteprocesswaterresultsinincreasedconcentrationsofhardness(calciumandmagnesium).Thisincreasedhardnessresultsinincreasesinpelletproductioncosts.2)Sulfateandcalciumwillprecipitatefromsolutiononcethesolubilitylimitofcalciumsulfateisreached.Thiswilloccurwhentheproductsofthemolarconcentrationofcalciumandsulfatereachthesolubilitylimit.Sodiumsulfateisextremelysoluble.3)Increasedscrubberefficiencycanresultinincreasedsulfateconcentrationintaconiteprocesswater.4)Theconcentrationofsodiumintaconiteprocesswateraffectstheamountofhydrogenfluoridethatformsduringpelletinduration.Increasedconcentrationsofsodiumresultsinincreasedformationofhydrogenfluorideduringpelletinduration.5)Theconcentrationofcalciumintaconiteprocesswateraffectstheamountofhydrogenfluoridethatformsduringpelletinduration.Increasedconcentrationsofcalciumresultsindecreasedformationofhydrogenfluorideduringpelletinduration.6)Theconcentrationofcalciuminscrubberwateraffectstheamountoffluoridethatprecipitatesfromsolutionafterhydrogenfluoridescrubbing.Increasedconcentrationsof3calciumresultsinincreasedprecipitationofcalciumfluorideanddecreasedconcentrationoffluorideinthetaconiteprocesswater.7)Precipitationofcalciumfluorideandcalciumsulfatecanlimittheamountoffluorideandsulfatethatremaininsolutionintaconiteprocesswater.8)Theadditionofsodiumcarbonateandsodiumhydroxidetotaconiteprocesswaterdecreasestheconcentrationofcalciumintaconiteprocesswater.PastHistoryofTaconiteProcessWaterTherearesomeobservationsthatcanbemadebylookingatpastpracticesatMinnesotataconitefacilities.1)Twotaconitefacilities,NorthshoreMiningandLTVMiningusedsodaash(sodiumcarbonate)watertreatmentto"soften"(precipitatecalciumcarbonate)processwater.ThetailingbasinsatthesetwofacilitiescontainwaterthathasthehighestfluorideconcentrationsofalltheMinnesotataconitefacilities.Precipitationofcalciumcarbonatedecreasedthecalciumconcentrationofthewater,whichdecreasedcalciumfluorideprecipitation;thusresultinginhighfluorideconcentrationinthetailingwater.2)ThetailingfacilityatMinntaccontainsthehighestconcentrationofsulfatebecause-+Minntacinstalledfouroncethroughwetscrubbers,whichremovesulfurdioxide.Thesulfurdioxideisoxidizedtosulfate.Thetailingbasinhasnopermitteddirectdischargeotherthantheexistingseepsandsothesulfateconcentrationinthetailingwatercontinuestoincreaseduetotherecyclingofwaterfromthetailingbasinbacktothescrubbers.Minntachasalowerfluorideconcentrationintailingwaterthandomosttaconitefacilitiesbecauseofhighcalciumconcentration,whichlimitsthefluorideconcentrationtothesolubilityofcalciumfluoride.3)Itappearsthatthelimere-circulatingscrubberatKeewatinTaconitecanlimittheamountofsulfateandfluoridethatenterthetailingwater.ThisisaccomplishedbycontinuallyadjustingthescrubberwatertotheproperpHandallowingittocirculateuntilgypsum(calciumsulfate)andcalciumfluorideprecipitationoccurs.4)Thereisnoprecipitationreactionthatlimitstheconcentrationofchlorideintaconiteprocesswater.SulfurDioxideScrubbingatTaconitePlantsTheefficiencyofsulfurdioxidescrubbingintaconiteindurationscrubbersdependsontheconcentrationofsulfurinthefuelandpre-fIredpellet,thescrubberwaterchemistry,thescrubberwaterflowrate,thecompositionoftheashfromthefuel,thecompositionofdustformedduringpelletinduration,andresidencetime(gas-watercontacttimeandreactionrates).1)Sulfurdioxidescrubbingefficiencyisaffectedbytheconcentrationofsulfurinthefuelandthepre-fIredpellet.Duringpelletinduration,thesulfurthatispresentinthefuelandpre-fIredpelletoxidizestosulfurdioxide.Anincreaseintheamountofsulfurenteringthefurnaceresultsinincreasedconcentrationofsulfurdioxideinthewastegas.Sulfurdioxidebeginstoreactwiththealkalinityinthescrubberwaterasthesulfurdioxidedissolvesinthewater.ThealkalinityandthepHofthescrubberwaterbegintodecreaseasthesulfurdioxidereacts.AsthepHofthescrubber waterdecreases,therateofsulfur4dioxidescrubbingdecreases.OncethepHofthescrubberwaterdecreasesto4.5,thereactionofsulfurdioxidewithbicarbonateisessentiallycomplete.Sulfite(SOnandbisulfite(HSOl")aremuchmorestableinwaterthanissulfurousacid(H2S0l)'SulfurousacidbeginstoformataboutpH4.SulfurdioxidescrubbingthatoccursbelowpH4.5isduetothesolubilityofsulfurdioxideinwater,whichiscontrolledbythepartialpressure(concentration)ofsulfurdioxideinthewastegas.":a•0.00250.00200.0015Co=0.0010!lcoU0.00050.0000H2S03'\/,I/\LJ"o2ConcentrationofSulfiteSpecies41\/V/\V\6pH8S03=1012142)SulfurdioxidescrubbingefficiencyisaffectedbythealkalinityandpHofthescrubberwater.ThepHoftaconiteprocesswaterisusuallyveryclosetopH8,whichisthepHatwhichthemaximumconcentrntionofbicarbonatealkalinityoccurs.Asbicarbonatereactswithsulfurdioxide,thescrubberwaterpHdecreasesandthebicarbonatealkalinitydecreases.Waterthatcontainshighbicarbonatealkalinityconcentrntioniscapableofscrubbingmoresulfurdioxidethanwaterthatcontainslowbicarbonatealkalinityconcentrntion.180160140120Ef!100o80!lc60ou4020o24CarbonateSpeciesConcentrationatVariouspH,/.//'HC03-"I\ IJ6\\\"-8pH'\,I//10/'II\\\'-..12COl=1453)Thescrubbermake-upwaterflowrateaffectstbeamountofbicarbonatetbatenterstbescrubber,whichaffectstbeamountofsulfurdioxideremovedfromtbewastegas.Higherscrubbermake-upwaterflowratesresultinmorebicarbonatealkalinityavailabletoreactwitbsulfurdioxideinthewastegas.Thetablebelowindicatesthatsulfurdioxidescrubberefficiencyisdependentontheamountofbicarbonatealkalinity(HC03')thatentersthescrubber.KeewatinTaconitedataisnotincludedinthetablebelowbecausemostofthealkalinityintbescrubberwateratKeetaccomesfromlimeadditionrathertbanthemake-upwater.Calc.WaterCalc.AirTaconiteScrubberMake-upMake-upHC03-ReactedRC03-RemainingHC03S02ScrubberS02ScrubberPlantMake-up,gpmIb/minIbmole/minIbmole/minIbmolelminEfficiencyEfficiencyMinntacL6&728004.100.06720.05960.007760%MinntacL4&528004.100.06720.02880.038453%VtacL28701.570.02570.02570.000026%Hibtac39007.670.12580.01870.10704)Sulfurdioxidescrubbingefficiencyisaffectedbytheneutralizingcapacityofashfromtbefuel.Coalashcontainslittleornoacidneutralizingcapacity;however,woodashcontainslargeamountsofCaOandMgO,whichwillneutralizeacidcomponentsintbewastegas.Theashcontentofwoodisabout2%byweight.FollowingisatabletbatliststbeashanalysesfornortbernMinnesotabirchandaspen.UsingtheMgOandCaOanalyses,theneutralizingcapacityoftbewoodashisabout0.44molespershorttonofwoodburned.Wood%FIl103%Si02%Na2O%K2O%CaO%MI(O%S03%LOIBirch1.300.858.7015.145.811.63.113.5Aspen0.595.800.1712.252.75.443.118.55)Sulfurdioxidescrubbingefficiencyisaffectedbytbeadditionoflimestonetotbeprefiredpellets.Taconiteplantsaddfrom1%to10%limestonetopre-firedpelletsdependingontbetypeofpellettbatisbeingproduced.Limestoneadditionratesof1%canbeusedforstandard(acid)pelletproduction;whereas,limestoneadditionratesof8%to10%areusedforfluxedpelletproduction.Thetaconitepelletizingprocessproducesfrom2to8poundsofdustpergrosstonofpelletsproduced.Wetscrubbersremovethisdustfromtbewastegas.Theacidneutralizingcapacityofthis"scrubberdust"duringtbeproductionofa 1 %limestonepelletisabout0.2to0.8millimolesforeachtonofstandardpelletsproduced.Theacidneutralizingcapacityof"scrubberdust"duringtbeproductionof10%limestonepelletsisabout2to8millimolesforeachtonofpelletsproduced.Thefollowingtableindicatessomecalculatedsulfurdioxidescrubberefficiencyrates.Twoefficiencyratesarelistedintbetable.Thefirstscrubberefficiencyratewascalculatedusingtbeincreaseofsulfateinscrubberwater.Thesecondscrubberefficiencyratewascalculatedusingtbeconcentrationofsulfurdioxideintbewastegasenteringtbescrubber(calculatedfrommassbalance)andtheconcentrationofsulfurdioxideexiting671%27%75%TaconitePlantKeetacthestack.ThesulfurdioxideremovedbythedustwascalculatedbasedonacalculatedremovalrateforbicarbonatealkalinitybythescrubberwaterthatoccurredfrompH8(PHoftaconiteprocesswater)tothemeasuredpHofthescrubberexitwater(listedinthetable).Thedifferencebetweenthetotalsulfurdioxideremoval(efficiency)andthesulfurdioxidethatreactedwiththebicarbonatealkalinityinthewateristheamountofsulfurdioxidethatreactedwiththedust.NoticethatforMinntacLines4&5theamountofsulfurdioxideremovedbythe"scrubberdust"wasmorethananyotherscrubberlistedinthetable.Thisisbecausewoodwasusedasafuelandwoodashhasaveryfinegrainsizeandisthereforeveryreactive.Thisdatademonstratesthatsulfurdioxidescrubberefficiencyisdependentonneutralizationofscrubberwaterbythedustandfuelash.Calc.WaterCalc.AirScrubberS02WastegasPelletS02NatGasCoalWoodPelletIbS02S02ScrubberS02ScrubberWaterScrubbingacfm,11OFProdItIhrppmscthIblbourIblhrTvoeminuteEfficiencyEfficiencypHbydust61378969362.976,200201750Acid5.9450%6.4NAMinntacL6&740795141588.083,900165000Flux5.5360%71%5.320%MinntaclA&540488341551.6136950011450Flux3.2153%6.2UtacL2Hibtac583428452139.700Acid12.5426%27%3.646887138421.9962600 0Acid1.5875%7.0..Note:Wastegasflowratesarecalculatedusmg,theoxygencontentofthewastegas,fueltype,fuelcompOSItIon,fuelrate,pre-frrepelletmoisture,andpre-firedpelletcombinedwaterandcarbondioxidecontentwiththeassumptionthatallcombustionproductsarewaterandcarbondioxide.6)Liquidgasreactionsaredependentonthegasconcentrationthatisincontactwiththeliquid.Asthegasconcentrationdecreases,theliquidgasreactionratedecreases.Inthecaseofsulfurdioxidescrubbingthegasissulfurdioxideandtheliquidiswater.Thefollowingtableindicatestheconcentrationsofsulfurdioxideinthetaconitewastegasleavingthescrubber(gasafterscrubbing).NoticethehighpHintheHibtacscrubberwaterandtheamountofalkalinityremaininginthewaterthatisstillavailabletoreactwithsulfurdioxide.However,thesulfurdioxideconcentrationinthewastegasisverylow(5.5ppm)and,therefore,thereactionisveryslow.Thegraphbelowindicatesthatthereisasulfurdioxideconcentrationlimitthatisattainableinawetventuriscrubberduetosuchthingsasresidencetimeandscrubberwaterflowrate.Scrubber ScrubberTaconiteWaterExitS02RemainingHC03PlantpHppmlbmole/minKeetac6.431.1NAMinntacL6&75.330.00.0077MinntaclA&56.224.30.0384UtacL23.6102.00.0000Hibtac7.05.50.1070735%15%39%SulfurDioxideinScrubberExitWasteGasvsScrubberWaterpH120100E.....N800III"=60".!=I.'"4020...............•-03.03.54.04.55.05.56.06.57.0ScrubberWaterpH•Minntac,Hibtac&UTac-Poly.(Minntac,Hibtac&UTac)LimeScrubberReactionseRe-circulatingLimeScrubber)ScrubberReactionswithhydratedlime(calciumhydroxide)Ca(0H)2+ H20 7Ca+2+20K+H20(limedissolution)W+OK7 H20(acidneutralization)Ca+2+ S03-7CaS03!(calciumsulfiteprecipitation)2CaS03+ O272CaS04(calciumsulfiteoxidation)2Ca+2+2S03-+02+4H2072(CaS04.2H20)!(gypsumprecipitation)Ca+2+20K+2HF7CaF2!+2H20(calciumfluorideprecipitation)Ca+2+20K+C027CaC03! + H20(highpHcalciumcarbonateprecipitation)7.5Theadditionoflimetoscrubbersincreasesacidgasscrubbingefficiency.AsthepHofthescrubberwaterincreases,theamountofacidgasremovedfromthewastegasincreases.However,thepHofthescrubberwatercannotbeincreasedtothepointwherebicarbonateand/orcarbondioxideareconvertedtocarbonatebecausecalciumcarbonatewillprecipitatefromsolutionandscalingwithinthescrubberwilloccur.AspHincreases,thehydroxide(fromlimeaddition)thatispresentinthescrubberwaterremovescarbondioxidefromthewastegasandconvertsittocarbonate,whichthenprecipitatesascalciumcarbonate.Calciumcarbonateprecipitationwillplugscrubbers,whichinturndecreasesscrubberefficiency.ThefollowingtwographsdepicttheconcentrationofcarbonateatvariouspH.Thesegraphsarebasedonwaterthathasaconcentrationof120mgCOz/liter.DependingonpHtheCO2canbecarbonicacid(H2C03),bicarbonate(HC03),and/orcarbonate(COn.8LogofConcentrationofCarbonateSpecies..,.sIg>co1lbcgu'0g>·5.5..J"2180160140S120Ec·100o80g60ou4020o42468pH1012CarbonateSpeciesConcentrationatVariouspH'\.\/.//'HC03.'\.IJ\\\6"'8pH,)/./10/'III\\\"'1214C03-14TheabovetwographsindicatethatasthepHofthewaterinare-circulatinglimescrubberincreases,theconcentrationofthecarbonateionincreases.Thecarbonateconcentrationmustbemaintainedatalowenoughconcentrationthatwillnotresultinlargeamountsofcalciumcarbonateprecipitation.Thesolubilityproductforcalciumcarbonateis4.9xlO·9.Arecirculatingscrubberthatoperatesat100Fandmaintainsthesulfateconcentrationinthewaterat1400mgSOJliterwillhavewaterthathasacalciumconcentrationof650mgCalliter.Thismeanstomaintainacalciumcarbonateprecipitationrateoflessthan1mg/liter,theconcentrationofcarbonatehastobelessthan10-5moles/liter.Examiningtheloggraphabove,thepHofthere-circulatingscrubberwatershouldbemaintainedatapHlessthan7.5to.ensurethecarbonateconcentrationislessthan10-5.9m0.00250.00200.0015Co:;0.0010ooo0.00050.0000H2S03\I\I(I/,o2ConcentrationofSulfiteSpeciesS03=r\/V/\V\'-468101214pHThesolubilityofcalciumsulfiteislessthanthesolubilityofcalciumsulfate.Theabovegraphindicatesthattheconcentrationofbisulfite(HS03")isgreaterthantheconcentrationofsulfitewhenthepHofthescrubberwaterislessthan7.Ifthesulfiteoxidationratetosulfateisnotrapidenough,thencalciumsulfitecanprecipitatefromsolution,especiallywhenthepHofthescrubberwaterisabove7.HighconcentrationsofsulfurdioxideinthewastegascanresultinhighenoughconcentrationsofsulfiteinthescrubberwaterthatprecipitationofcalciumsulfitecanevenoccuraslowaspH6.Calciumsulfiteprecipitationinthescrubbercancausescalingandpluggingproblems.Forthisreasonitisdifficulttooperatere-circulating-limescrubbersatapHthatisabove7withoutdecreasingscrubberefficiencybyformingprecipitates.ApplicationtoMinnesotaTaconiteIndurationScrubbersKeewatinTaconiteScrubber-KeewatinTaconitehasare-circulating-lime,venturi-rodscrubber.Inordertomaintainmaximumsulfurdioxidescrubberefficiency,thescrubbershouldbeoperatedbetweenpH7andpH7.5.IfitisoperatedatapHgreaterthan7.5,calciumcarbonateandcalciumsulfitescalingwilloccurinthescrubber,whichwillresultindecreasedscrubberefficiencyandcouldevenresultinshuttingdowntheprocesstocleanthescaleformationfromthescrubber.ItwouldappearthatKeewatinTaconiteiscurrentlyoperatingatnearoptimumsulfurdioxidescrubbingconditions.MinntacLine3Scrubber-Minntacinstalledare-circulating-limeventuri-rodscrubberontheLine3pelletindurationfurnaceduring2005-2006.MinntaciscurrentlynotoperatingLine3;however,whenLine3wasoperating,scalinginthescrubberwasaproblem.Thescrubbersystemwillhavetooptimizedtoensuregoodsulfurdioxidescrubbingwithoutscaling.Whenthisoptimizationisaccomplished,Line3shouldhavesulfurdioxidescrubberefficiencysimilartotheKeewatinTaconitere-circulatinglimescrubber.10MinntacLines4,5,6and7Scrubbers-Minntachas"oncethrough"venturi-rodscrubbersonlines4,5,6,and7pelletindurationfurnaces.Itiscurrentlyestimatedthatapproximately70%ofthesulfateintheMinntacbasinwaterisduetosulfurdioxidescrubbing.Thewetscrubbersarecurrentlyremovingmorethan 50%ofthewaste-gas sulfurdioxidewhenMinntacisproducingfluxedpelletsand/orusingwoodasafuel.Minntacproducesfluxedpelletsonlines6and7usingcoalandnaturalgasasfuel.Thiscombination(fluxedpellets,coalandnaturalgas)resultsinasulfurdioxidescrubbingefficiencythatisgreaterthan50%.Minntacproducesstandard(acid)pelletsandfluxedpelletsonlines4and5usingwoodandnaturalgasasfuel,whichresultsinasulfurdioxidescrubbingefficiencygreaterthan50%.Thesescrubberefficiencieswillnotbeimprovedbyaddingchemicalstoincreasescrubberwateralkalinityorbyinstallingare-circulatinglimescrubbersimilartotheKeewatinTaconiteScrubber.HibbingTaconiteCompany-HibbingTaconiteCompany(Hibtac)hasthreefurnacelineswith"oncethrough"venturirodscrubbers.Hibtacdoesnotusecoalasapelletindurationfuel.Thismeansthattheonlysulfurenteringthewastegasisfromthetaconitegreenballs(pre-firedpellets).Thecalculatedconcentrationofsulfurdioxideinthewastegasenteringthescrubberisabout25ppm.Theconcentrationofthewastegasleavingthescrubberisabout6ppm.ThescrubberwaterleavingthescrubberstillhasapHof7andcontains200ppmbicarbonatealkalinity.Thismeansthatthesulfurdioxidescrubbingreactionisgasconcentrationlimited,whichalsomeansthatsulfurdioxidescrubbingefficiencywillnotbeimprovedbyaddingchemicalstoincreasescrubberwateralkalinity.ThedataalsoshowthattheinstallationofarecirculatinglimescrubbersimilartotheKeewatinTaconiteScrubberwillnotimprovescrubbing.UnitedTaconite-Line2atUnitedTaconite(Utac)hastwore-circulatingventurirodscrubbers.Thescrubberwaterfromeachscrubberissenttotwosmallthickeners.Theunderflowfromeachthickenerispumpedbacktotheprocessatarateof435gpmoratotalof870gpmforthetwoscrubbersystems.Theoverflowfromthescrubberthickenersissentbacktothescrubberswithfreshmake-upwater.Themake-upwaterisequaltotheblowdownplusevaporativelosses.Thebicarbonatealkalinityenteringthescrubberwaterisequaltotheamountofbicarbonatealkalinitypresentinthemake-upwater.Thislimitstheamountofsulfurdioxidethatisremovedfromthewastegastotheamountofsulfurdioxidethatwillreactwiththebicarbonatealkalinityinthemake-upwaterplustheamountthatwillreactwiththedustplusasmallamountofsolublesulfurdioxide.Thisoperatingpracticelimitstheamountofsulfatethatenterstheprocesswaterandthetailingwater.Theconcentrationofsulfateinthescrubberwatercurrentlyincreasesfromabout250mg/literto800mg/liter.Thismeansthat4.0poundsofsulfateperminuteenterstheprocesswater(tailingwater)withascrubberblowdownrateof870gpm.Theacidneutralizationfortheblowdownwatercomesfromcalciumandmagnesiumcarbonatespresentintheore,whichlimitsthesolubilityoffluoridethroughcalciumfluorideprecipitation.Increasingthealkalinityofthewaterthatisre-circulatedfromthescrubberthickenertothescrubberwouldincreasesulfurdioxidescrubbingefficiency.FollowingaresomescenariosthatcouldbeconsideredtoincreasethesulfurdioxidescrubbingefficiencyforLine2atUnitedTaconite:1)Increasingthealkalinityofthescrubberwaterbyaddingsodiumhydroxideorsodiumcarbonatetothecurrentsystemwouldincreasesulfurdioxideremoval.Additionofeitherofthesechemicalswouldincreasesulfurdioxideremoval,butitalsowouldresult11inincreasedsulfateandfluorideconcentrationsinthetailingwater.Doublingsulfurdioxidescrubbingefficiencybyusingeithersodiumhydroxideorsodiumcarbonate willresultinanadditionof8.0poundsofsulfateperminutetothetailingwater(doublethecurrentrate).ThecurrentprocesswateratUnitedTaconiteis225to250mg/liter.Doublingthesulfateinputwouldresultinsulfateconcentrationsof450to500mg/literintheprocesswater.Additionofsodiumsaltswouldalsoresultinfluorideconcentrationsintheprocesswater(tailingwater)thatcouldreach20mg/liter.Useofthesesaltswillresultinincreasedconcentrationsofbothfluorideandsulfateintailingwater,whichwouldcreateenvironmentalproblemswiththetailingwater.2)Limeadditiontothescrubberwaterwillincreasesulfurdioxidescrubbingefficiencyandwillalsolimittheamountofsulfateandfluorideinthetailingwaterbyprecipitationofcalciumsulfateandcalciumfluoride.However,limeadditioncouldnotbeusedwiththecurrentscrubberdesign.Addinglimetothecurrentsystemwouldresultinscalingproblems,whichwouldresultindecreasedsulfurdioxideandparticulatescrubbingefficiency.Thecurrentscrubbersandwaterhandlingsystemwouldhavetoberedesignedtoallowforlimeadditiontothescrubberwater.3)Theinstallationofanewre-circulatinglimescrubbersimilartotheKeewatinTaconitescrubbersystemwouldresultinincreasedsulfurdioxidescrubbingefficiency.Thistypeofscrubberwouldnearlydoublethecurrentsulfurdioxidescrubbingefficiency.Theresultwouldbetheremovalofandadditional2.8poundsofsulfurdioxideperminute(700tonsperyearat95%operatingtime).Ascrubberblowdownrateof350gallonsperminutewouldberequiredinordertomaintainthecurrentrateatwhichsulfateentersthetailingwater(4.0poundsperminute).Oneofthevariablesthatmustbeconsideredwhendesigningare-circulatingscrubberisthesolidconcentrationinthewaterthatiscirculatedbacktothescrubber.Ifthesolidsconcentrationistohigh,theefficiencyofthescrubbercanbeaffectedandtheamountofblowdownhastobeincreasedtokeepthesolidsattheproperconcentration.Ablowdownrateof350gpmshouldworkifadustcollector,suchasamulticlonesystem,isinstalledpriortothescrubber;however,ifdustcollectioncannotbeinstalledpriortothescrubber,theblowdownratewouldprobablyhavetobeabout700gpm.Ascrubberblowdownrateof700gpmwouldnotresultintheprecipitationofcalciumsulfateand,therefore,theamountofsulfateenteringthetailingwaterwouldbeabout8poundsperminute.Thiswouldresultindoublingtheamountofsulfatethatiscurrentlyenteringthewater(4lb/minto8lb/miu).Theinstallationofare-circulatinglimescrubberwithoutdustcollectionpriortothescrubberwouldresultinasystemthatwouldrequireablowdownofabout700gallonsperminute.Thiswouldresultindoublingthecurrentconcentrationofsulfateandcalciumintheprocesswater.Thecalciumconcentrationintheprocesswaterwillincreasebecauselimewillneutralizetheacidgasespresentinthescrubberwater.Thesulfateconcentrationandthecalciumconcentrationoftheprocesswaterwouldincreasetoabout500mg/literand160mg/literrespectively.Thisincreaseinhardnesswouldresultinoperatingproblemsthatwouldbedifficulttopredict.Itwillbedifficulttodeterminethecostsassociatedwiththeresultingincreaseinhardnessandsulfateconcentrations.Someoftheproblemsthatwouldoccurare:12a)Increasedscalinginplantprocesspipesduetoincreasedconcentrationsofcalcium.Theprocesswaterwillessentiallybesaturatedincalciumcarbonate.Calciumcarbonateprecipitationwilloccurinpipes.Itwillalsooccuronmineralsurfacesduringgrindingwhenwatertemperaturesincrease.b)Calciumcarbonateprecipitationonmineralsurfacescanhaveanegativeimpactonmagnetiterecovery.ThereSUltingincreaseinthelossofmagnetite oretothetailingswillincreasethecostofproduction.c)CurrentlyUnitedTaconitedoesnothaveflotation,butcalciumcarbonateprecipitationonmineralsurfaceshasanegativeimpactontheflotationprocess,whichresultsinadditionalflotationreagentconsumption.d)Increasedcalciumconcentrationinprocesswaterresultsinadeteriorationofprefiredpelletqualitythatiscorrected byincreasingpre-firedpelletmoistureandincreasing binderuse.UnitedTaconiteusesorganicbinders.Pastresearchwithbentoniteclayindicatesthatanincreaseof\0mg/literincalciumand/ormagnesium(hardness)intheprocesswaterresultsina3%increaseinbentoniteclayconsumptioninordertomaintainpre-firedpelletquality.ThecurrentCa+MgconcentrationinUnitedTaconiteprocesswateris80mg/liter.Anincreaseto160mg/liter(doublethecurrentconcentration)wouldresultinatleasta24%increaseinbinderuse.Organicbindersareusuallyaffectedmorebywaterchemistrythanisbentoniteclay,whichmeanstheuseoforganicbinderwouldincreasebyatleast24%and/orthecurrentorganicbindermightnolongerwork.e)Thescrubberthickenerunderflowsolidscurrentlyaresentbacktotheprocess.Thisresultsinapproximately80to100poundsoftaconiteconcentrateperminute,whichisequivalenttoapproximately25,000longtonsofpelletsperyear.Theresultwouldbethatifmuiticlonedustcollectorscannotbeinstalledpriortothewetscrubber.Aproductionlossofapproximately25,000longtonsofpelletsperyearwouldoccur.Dustcollectorspriortowetscrubberswouldbenecessarytomaintainthecurrentprocesswaterquality.Thefollowingitemswouldhavetobeconsideredwhenevaluatinganinstallationofare-circulatinglimescrubbersystemwithdustcollectionpriortothewetscrubbersatUnitedTaconite:a)Installationofmulticlonestocollectabout20,000longtonsofconcentrateperyearwillbeneeded.Asystemtorecyclethemulticlonedustbackintothepelletprocesswillalsoneedtobeevaluated.Itdoesnotappearthatthereisenoughroominthepelletplanttoinstallmulticlonespriortothecurrentwetscrubbers.Anevaluationwillhavetobemadetodetermineiftheinstallationofmulticlonesispossible.b)Are-circulatinglimescrubberretrofitinstallationwouldprobablyrequireanewbuildingbecauseallofthenecessaryequipmentwouldnotfitintotheconfinesofthecurrentpelletplant.Thenewbuildingwouldhavetocontainalimemixingfacility,ascrubberrecirculationtank,alimeadditionsystemwithpHcontrols,ascrubberwatersolidsthickenerfortheblowdownwatertreatment,andafilteringsystemtofilterthesolids.Analternativetoafilteringsystemwouldbealinedsettlingpond.13c)Thefilteringsystemwouldhavetobesizedfor20shorttonsperdayifmulticlonesareinstalled.d)Thecurrentventurirodscrubbingsystemwouldhavetobeevaluatedtodetermineifanewroddesignisneededtohandletheincreasedsolidloadthatwouldoccurwiththere-circulatinglimescrubber.e)Anevaluationofincreasedoperatingcostsassociatedwithare-circulatinglimescrubberwouldalsobeneeded.Theseshouldinclude1.Orelossesassociatedwiththedustlossthatwilloccurduetothelossofreclaimedoredust.Thiswouldbeabout5,000longtonsperyearifprescrubberdustcollectors(multiclones)areinstalled.11.Approximately2500shorttonsoflimeperyearwillbeneededtoneutralizethescrubberwatertopH7.iii.Therewillbeapproximately8500shorttonsofsolidsperyearthatwillhavetobefilteredandhauledtoalinedlandfillordepositedinalinedsettlingpond.Thiswillinclude5600shorttonsofdust,2500shorttonsofgypsumand400shorttonsofcalciumfluoride.Thiswouldrequire2trucksperdaytohaulthematerialtoalandfill.References:I.Engesser,J ."Effectofwaterchemistry,watertreatmentandBlaineonmagnetitefilteringandmagnetiteagglomerationwithbentoniteclay."Minerals&MetallurgicalProcessing.Vol20,No.3,Augnst2003.2.Engesser,J."Reductionoffluoridedissolutionduringindurationoff-gasscrubbing."Minerals&MetallurgicalProcessing.Vol22,No.I,February2005.3.Engesser,J."EffectofPlantProcessWaterChemistryonSilicaFlotationandBalling",UniversityofMinnesotaColeraineMineralsResearchLaboratory,IronareCooperativeResearchProgram-MinnesotaDepartmentofNaturalResources.1994.4.Engesser,J."TheRemovalofSulfateandFluoridefromProcessWater-TheEffectofWaterChemistryonFilteringandFlotation"UniversityofMinnesotaColeraineMineralsResearchLaboratory,IronOreCooperativeResearchProgram-MinnesotaDepartmentofNaruralResources,1998.5.Engesser,J.''TaconiteProcessWaterandTailingWaterQuality-CationicExchangeEffectonBallingandFlotation",UniversityofMinnesotaColeraineMineralsResearchLaboratory,IronOreCooperativeResearchProgram-MinnesotaDepartmentofNaruralResources,1999.6.Engesser,J."BentoniteQuality-TheEffectofWaterChemistryandAotifreezeAgentsonBentoniteBindingCharacteristics"UniversityofMinnesotaColeraineMineralsResearchLaboratory,MinnesotaTechnologiesInc,2000.7.Engesser,J."TheReductionofFluorideDissolutionduringIndurationOff-gasScrubbing"UniversityofMinnesotaColeraineMineralsResearchLaboratory,IronOreCooperativeResearchProgram-MinnesotaDepartmentofNaruralResources,2001.8.Goetzman,H"Bleifuss,R.,Engesser,J."AnEvaluationofOrganicBindersasSubstitutesforBentonitePelletizing",Proceedingsofthe61"AnnualMeetingoftheMinnesotaSectionAJMEand49thAnnualMiningSvmposium,January1988.148..Stone,R.L."RelationshipBetweenZetaPotentialofBentoniteandtheStrengthofUnfiredPellets",AmericaInstituteofMmingMetallurgicalandPetroleumEngineersSocietyofMiningEngineersofAlMETransactionsVolume238,1967.9.Engesser,J.MinntacAgglomeratorSulfurBalance,1994.10.Engesser,J.EvelethTaconiteAgglomeratorSulfurBalance,1998.11.Engesser,J.,Niles,H.,"MercuryEmissionsfromTaconitePelletProduction",UniversityofMinnesota,DuluthNaturalResourcesResearchInstitute,1997.12.Berndt,M.,Engesser,J.,"MercuryTransportinTaconiteProcessingFacilities:(1)ReleaseandCaptureDuringInduration"IronOreCooperativeResearchProgramMinnesotaDepartmentofNaturalResources,2005.13.Benner,B.,"MercuryRemovalfromIndurationOff-GasWetScrubbers",EnvironmentalCooperativeResearchProgramMinnesotaDepartmentorNaturalResources.2001.15