desgasificado al vacío

4 June 2010

PROCESOS SIDERPROCESOS SIDERÚÚRGICOS IIIRGICOS III

Tema 4. Desgasificado al vacío

Fernando Contreras B.

4 June 2010Desgasificado al vacío Instructor: Fernando Contreras 2

Procesos Siderúrgicos III – Desgasificado al vacío

TEMARIO

1.1 Introducci1.1 Introduccióónn

1.2 1.2 VacuumVacuum technologiestechnologies

1.3 1.3 ReactionsReactions underunder vacuumvacuum

1.4 1.4 GenerationGeneration ofof vacuumvacuum

INTRODUCTION

Procesos Siderúrgicos III – Desgasificado al vacíoINTRODUCCIÓN

-- VD VD consistsconsists in in thethe generationgeneration ofof veryvery lowlow pressurepressure atat thethe interfaceinterface betweenbetween liquidliquid steelsteel & & thethe atmosphereatmosphere..

-- As a As a consequenceconsequence, , thethe equilibriumequilibrium forfor gas gas solubilitysolubility changeschanges allowingallowing thethe removalremoval ofof thethe volatilevolatile elementselements dissolveddissolved in in thethe steelsteel. .

--InitiallyInitially, VD , VD waswas usedused toto decreasedecrease H H dissolveddissolved in in

thethe steelsteel. . ThenThen VD VD becamebecame

mandatorymandatory toto produce produce

ULC ULC steelssteels & & IF IF steelssteels. .

LowLow contentcontent ofof C & N. C & N.

VACUUM TECHNOLOGIES

Procesos Siderúrgicos III – Desgasificado al vacíoINTRODUCCIÓN – General classification

Procesos Siderúrgicos III – Desgasificado al vacíoINTRODUCCIÓN – Stream degassers

LLD:Ladle to ladle

-- DegasificationDegasification ofof thethe steelsteel occursoccurs onon thethe streamstream duringduring thethe transferencetransference fromfrom oneone unitunit toto anotheranother..

LMD:Ladle to mould

TD: Furnace to ladle

Procesos Siderúrgicos III – Desgasificado al vacíoVACUUM TECHNOLOGIES – Ladle degassers

-- OccursOccurs withwith thethe liquidliquid steelsteel insideinside thethe ladleladle. . ItIt has has twotwo mainmain designsdesigns,,

-- WithWith externalexternal recirculationrecirculation: :

PartPart ofof thethe liquidliquid steelsteel isis suckedsucked out out ofof thethe ladleladle andand forcedforced intointo a a secondsecond vesselvessel wherewhere thethe vacuumvacuum isis appliedapplied andand thenthen returnedreturned toto thethe original original ladleladle. . ThreeThree typestypes ofof recirculatingrecirculating degassersdegassers havehave beenbeen developeddeveloped::

--DH: Dortmund DH: Dortmund HoerderHoerder

--RH: RH: RuhrstahlRuhrstahl HeraeusHeraeus

--RHRH--OB (KTB) Kawasaki OB (KTB) Kawasaki TopTop BlownBlown

-- WithoutWithout externalexternal recirculationrecirculation

--TheThe full full ladleladle ((steelsteel & & slagslag) ) isis permanentlypermanently underunder vacuumvacuum. . ThreeThree technologiestechnologies havehave beenbeen developeddeveloped,,

--VTD: VTD: VacuumVacuum TankTank DegasserDegasser, , oror TDTD

--VOD: VOD: VacuumVacuum OxygenOxygen DecarburizationDecarburization

--VAD: VAD: VacuumVacuum Arc Arc DegasserDegasser

Procesos Siderúrgicos III – Desgasificado al vacíoVACUUM TECHNOLOGIES – DH Dortmund Hoerder

Procesos Siderúrgicos III – Desgasificado al vacíoVACUUM TECHNOLOGIES – Ruhrstahl Heraeus

Procesos Siderúrgicos III – Desgasificado al vacíoVACUUM TECHNOLOGIES – Ruhrstahl Heraeus -Kawasaki Top Blown (RH-KTB)

Procesos Siderúrgicos III – Desgasificado al vacíoVACUUM TECHNOLOGIES –Vacuum degasser (VTD)

Procesos Siderúrgicos III – Desgasificado al vacíoVACUUM TECHNOLOGIES –Vacuum Oxygen Decarburization (VOD)

Procesos Siderúrgicos III – Desgasificado al vacíoVACUUM TECHNOLOGIES –Vacuum Arc Degassing (VAD)

Procesos Siderúrgicos III – Desgasificado al vacíoVACUUM TECHNOLOGIES –Summary

-- TheThe vacuumvacuum technologiestechnologies mostmost usedused are RH & TD are RH & TD withwith thethe correspondingcorresponding versionversion forfor oxygenoxygen blowingblowing. . ComparisonsComparisons are are shownshown nextnext::

REACTIONS UNDER VACUUM

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM –Decarburization & deoxidation

-- DeoxidationDeoxidation isis performedperformed throughthrough CC--O O reactionsreactions..

-- TheThe equilibriumequilibrium ofof thethe systemsystem isis studiedstudied withwith thethe followingfollowing reactionsreactions,,

-- DecreasingDecreasing thethe pressurepressure ofof thethe systemsystem, , thethe equilibriumequilibrium ofof thethe reactionreaction movesmoves toto thethe rightright sideside toto generategenerate CO(gCO(g).).

-- DueDue toto thethe decreasedecrease ofof CO CO partialpartial pressurepressure, , reactionreaction betweenbetween C & O C & O dissolveddissolved in in thethe bathbath becomesbecomes more intense.more intense.

--OxygenOxygen can be can be availableavailable in in thethe bathbath as as lessless stablestable oxide (oxide (FeOFeO, , MnOMnO) ) oror as as dissolveddissolved O O whichwhich come come fromfrom thethe oxidationoxidation stagestage in EAF.in EAF.

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Decarburation in Tank Degasser

-- WithoutWithout ArAr stirringstirring,,

--TheThe reactionreaction isis favoredfavored onlyonly nearnear bathbath surfacesurface ((thethe reducedreduced pressurepressure isis onlyonly effectiveeffective onon thethe surfacesurface).).

--TheThe reactionreaction raterate isis limitedlimited by C & O by C & O diffusiondiffusion in in thethe liquidliquid phasephase..

-- WithWith ArAr stirringstirring,,

--TheThe reactionreaction raterate increasesincreases remarkablyremarkably..

--C & O are C & O are broughtbrought toto thethe reactionreaction surfacesurface more more rapidlyrapidly..

--TheThe argonargon bubblesbubbles increaseincrease thethe availableavailable reactionreaction surfacesurface..

--TheThe steelsteel isis permanentlypermanently recirculatingrecirculating fromfrom thethe bottombottom toto thethe surfacesurface..

-- WithWith ArAr stirringstirring,,

--WhenWhen thethe reactionreaction occursoccurs underunder vacuumvacuum withwith strongstrong ArAr stirringstirring, , thethe diffusiondiffusion ofof [C] & [O] [C] & [O] toto thethe place place ofof reactionreaction toto formform {CO} {CO} isis enhancedenhanced..

--CO CO finallyfinally isis incorporatedincorporated intointo thethe ArAr bubblebubble & & draggeddragged out out ofof thethe systemsystem..

--ArAr & CO gas are & CO gas are draggeddragged out out throughthrough thethe vacuumvacuum systemsystem..

--DecarburizationDecarburization can be can be studiedstudied usingusing thethe [C][C]--[O] [O] equilibriumequilibrium graphgraph forfor differentdifferent partialpartial pressurepressure ofof {CO}.{CO}.

-- EquationEquation relates relates thethe operativeoperative parametersparameters::

PcoPco= = partialpartial pressurepressure ofof COCO

[O]= [O]= ppmppm dissolveddissolved in in thethe steelsteel ((eqeq))

[C]= [C]= %C%C in in eqeq..

T= T= temperaturetemperature

-- WhenWhen decarburationdecarburation startsstarts in in pointpoint a (a (atat 1 1 atmatm) ) andand thethe TD TD operatesoperates atat 0.2 0.2 atmatm, , thethe final final %[C]%[C]--ppmppm O O willwill correspondcorrespond toto pointpoint b.b.

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Decarburation in Tank Degasser (kinetics)

--TheThe kineticskinetics ofof decarburizationdecarburization followsfollows thethe relationshiprelationship::

--WhereWhere kckc isis a a constantconstant & & dependdepend onon, a) , a) specificspecific flowflow raterate ofof stirringstirring

b) b) degassingdegassing time (t)time (t)

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Decarburization Chemistry

--WeWe can can rearrengerearrenge thethe massmass actionaction lawlaw equationequation toto,,

[O]=[O]=PcoPco/[/[C]KcoC]Kco

--IfIf thethe pressurepressure aboveabove thethe steelsteel isis reducedreduced toto 0.1 0.1 atmatm, , PcoPco willwill alsoalso dropdrop toto 0.1 0.1 atmatm andand thethe equilibriumequilibrium [C].[O] [C].[O] productproduct willwill decreasedecrease accordinglyaccordingly..

--C & O C & O willwill be removed in be removed in thethe ratio 12:16 as ratio 12:16 as determineddetermined by by theirtheir respectiverespective atomicatomic massesmasses..

--TheThe injectioninjection ofof additionaladditional oxygenoxygen intointo thethe steelsteel can be can be usedused toto produce produce veryvery lowlow C C steelssteels (e g., <0.001%). (e g., <0.001%).

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Degassification (Hydrogen Equilibrium)

--TheThe reactionsreactions forfor dissolutiondissolution are:are:

--TheThe sourcessources ofof HH22 are:are:

--WaterWater whichwhich come come fromfrom humidityhumidity..

--AirAir oror anyany humidhumid material in material in contactcontact withwith thethe steelsteel

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Degassification (Hydrogen equlibrium)

--TheThe basicbasic eqeq reactionreaction isis [H]=1/2 {H[H]=1/2 {H2}g. 2}g. TheThe eqeq constantconstant isis::

--TheThe H H concentrationconcentration atat EqEq isis proportionalproportional toto thethe pressurepressure squaresquare rootroot..

--TheThe dissolutiondissolution ofof H (H (alsoalso N) in N) in purepure ironiron obeysobeys SievertSievert’’ss LawLaw: :

gas gas atomicatomic concentrationconcentration = = EqEq constantconstant x x partialpartial pressurepressure ofof gasgas

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Degassification (Hydrogen kinetics)

--ConsideringConsidering thatthat in in modernmodern VD VD thethe operatingoperating pressurepressure isis aroundaround 10 10 mbarmbar andand appliyingappliying thethe followingfollowing formula.formula.

[H] [H] ff =final H =final H contentcontent

[H] i=[H] i=initiallinitiall H H contentcontent

tt =VD =VD processprocess timetime

KKHH = H = H removalremoval constantconstant, , minmin--11

--ToTo accelerateaccelerate thethe reactionreaction kineticskinetics, , itit isis necessarynecessary toto stirrstirr thethe bathbath withwith ArAr. H . H removalremoval constantconstant dependsdepends onon thethe workingworking pressurepressure andand stirringstirring flowflow..

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Degassification (Hydrogen kinetics)

--IfIf no no ArAr stirringstirring isis promotedpromoted, , followingfollowing occursoccurs,,

--TheThe reactionreaction willwill onlyonly taketake place place atat bathbath surfacesurface..

--TheThe reactionreaction raterate isis limitedlimited by H by H diffusiondiffusion in in thethe liquidliquid phasephase..

2[H] = {H2}2[H] = {H2}

--IsIs ArAr stirringstirring isis promotedpromoted, , followingfollowing occursoccurs,,

--H H isis broughtbrought toto thethe reactionreaction surfacesurface more more rapidlyrapidly..

-- TheThe ArAr bubblesbubbles increaseincrease thethe availableavailable reactionreaction surfacesurface..

--TheThe ArAr bubblesbubbles dragdrag thethe H out H out ofof thethe steelsteel

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Degassification (Hydrogen removal)

--[H] [H] dissolveddissolved in in thethe steelsteel reactsreacts toto formform HH22 gas gas moleculemolecule whichwhich isis absorbed by absorbed by thethe ArAr bubblebubble andand draggeddragged out out ofof thethe liquidliquid steelsteel..

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Degassification (Nitrogen equlibrium)

--TheThe basicbasic eqeq reactionreaction isis [N]=1/2 {N[N]=1/2 {N2}g. 2}g.

--N N concentrationconcentration atat EqEq isis proportionalproportional toto thethe squaresquare rootroot ofof itsits partialpartial pressurepressure ((SievertSievert’’ss LawLaw). ). TheThe EqEq constantconstant isis defineddefined as as followsfollows,,

--LikeLike H, H, NitrogenNitrogen removalremoval alsoalso occursoccurs duedue toto thethe decreasedecrease ofof itsits partialpartial pressurepressure in in thethe gaseousgaseous phasephase butbut differsdiffers in in somesome particular particular aspectsaspects::--

-- VelocityVelocity isis limitedlimited by by thethe pressurepressure ofof surfacesurface active active elementselements likelike [O] & [S][O] & [S]

-- SurfaceSurface active active elementselements blockblock thethe sitessites forfor N2 N2 passagepassage toto thethe interfaceinterface..

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Degassification (Nitrogen kinetics)

--A A simplifiedsimplified formula formula forfor N N removalremoval velocityvelocity isis::

--KKNN dependsdepends onon thethe operativeoperative pressurepressure andand onon thethe stirringstirring intensityintensity..

-- N N removalremoval isis more more difficultdifficult thanthan HH’’ss sincesince N N diffusivitydiffusivity in in liquidliquid steelsteel isis lowerlower. . HenceHence, , ArAr stirringstirring has has toto be more intense.be more intense.

--N N removalremoval isis retardedretarded alsoalso by by elementselements suchsuch as Al & Ti as Al & Ti whichwhich havehave a a highhigh affinityaffinity forfor NitridesNitrides..

[[N]fN]f= final N = final N contentcontent

[[N]iN]i= = initialinitial N N contentcontent

T= VD T= VD processprocess timetime

KKNN = N = N removalremoval constantconstant consideringconsidering O & S O & S equalequal toto zerozero ((minmin--11))

--IfIf no no ArAr stirringstirring isis promotedpromoted, , followingfollowing occursoccurs,,

--ReactionReaction occursoccurs atat bathbath surfacesurface whichwhich isis reducedreduced by by presencepresence ofof S & O.S & O.

--ReactionReaction raterate isis limitedlimited by N by N diffusiondiffusion whichwhich isis slowerslower as N as N diffusivitydiffusivity isis lowerlower ((largerlarger atomatom).).

2[N] = {N2}2[N] = {N2}

--IsIs ArAr stirringstirring isis promotedpromoted, , followingfollowing occursoccurs,,

--TheThe gaseousgaseous phasephase isis generatedgenerated by by ArAr bubblebubble..

-- IncreasesIncreases thethe areaarea ofof thethe interphaseinterphase gas gas liquidliquid..

--AvailableAvailable surfacesurface forfor N N removalremoval..

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Defulfurization under vacuum

-- ItIt isis controlledcontrolled by by thethe reactionreaction::

-- DuringDuring vacuumvacuum treatmenttreatment thethe reactionreaction isis stronglystrongly favoredfavored becausebecause operatedoperated atat::

--HighHigh temperaturetemperature

--StrongStrong ArAr bubblingbubbling

--HighHigh degreedegree ofof deoxidationdeoxidation

--IncreaseIncrease ofof specificspecific slagslag//steelsteel areaarea

--ForFor desulfurizationdesulfurization underunder vacuumvacuum nearnear equilibriumequilibrium conditionsconditions can be can be reachedreached..

Procesos Siderúrgicos III – Desgasificado al vacíoREACTIONS UNDER VACUUM – Defulfurization under vacuum

-- A A equationequation has has beenbeen developeddeveloped in in termsterms ofof amountamount ofof slagslag requiredrequired toto achieveachieve a a specificspecific S S concentrationconcentration as as followsfollows::

-- EqEq considersconsiders equilibriumequilibrium conditionsconditions andand termsterms mean mean nextnext,,

WmWm = = weightweight ofof steelsteel

LsLs= = sulfursulfur distributiondistribution rationration givengiven by (by (%S%S)/[)/[%S%S]. ].

%So%So & & %S%S aimaim = = InitialInitial & & aimaim S S concentrationconcentration..

-- TheThe LsLs valuevalue dependsdepends onon thethe S S capacitycapacity ofof thethe slagslag andand isis relatedrelated toto::

--UnderUnder goodgood conditionsconditions, , valuevalue ofof LsLs=500 are =500 are assembleassemble in in practicepractice..

Example

GENERATION OF VACUUM

Procesos Siderúrgicos III – Desgasificado al vacíoGENERATION OF VACUUM – Vacuum treatment

In general In general thethe specificationspecification ofof a a VacuumVacuum SystemSystem considersconsiders,,

--VolumeVolume ofof thethe vesselvessel toto be be evacuatedevacuated..

--FlowFlow ofof ArAr toto be be injectedinjected in in thethe vesselvessel..

--LastLast operativeoperative pressurepressure toto be be attainedattained..

--PumpingPumping capacitycapacity atat thethe operativeoperative pressurepressure..

--Time Time toto reachreach thethe operativeoperative pressurepressure..

ThreeThree typestypes ofof equipmentequipment are are generallygenerally usedused,,

--LiquidLiquid ringring pumpspumps..

--MechanicalMechanical pumpspumps..

--SteamSteam ejectorsejectors..

Procesos Siderúrgicos III – Desgasificado al vacíoGENERATION OF VACUUM – Liquid ring punps

PROS,PROS,

--HighHigh suctionsuction capacitycapacity..

--NotNot affectedaffected by by liquidliquid oror dustdust entrainedentrained in in thethe processprocess gas gas streamstream

--HighHigh reliabilityreliability duedue toto simple simple designdesign

--LowLow maintenancemaintenance requiredrequired

CONS,CONS,

--CannotCannot achieveachieve a a pressurepressure belowbelow 150 150 mbarmbar ((whenwhen waterwater isis usedused forfor thethe productionproduction ofof thethe liquidliquid ringring))

Procesos Siderúrgicos III – Desgasificado al vacíoGENERATION OF VACUUM – Mechanical Pump (Roost type)

PROS,PROS,

--SteamSteam isis notnot requiredrequired thereforetherefore somesome energyenergy can be can be savedsaved..

CONS,CONS,

--MustMust treattreat relarivelyrelarively cold cold andand absolutelyabsolutely dustdust--free free fluidsfluids onlyonly. . ThereforeTherefore a a heatheat exchangerexchanger andand bagbag filterfilter mustmust procede procede thethe pumpspumps. .

--CostCost exceedsexceeds thatthat ofof thethe steamsteam jet jet pumppump systemsystem..

Procesos Siderúrgicos III – Desgasificado al vacíoGENERATION OF VACUUM – Steam Ejectors (Jet pumps)

PROS,PROS,

--HighHigh suctionsuction capacitycapacity..

--NotNot affectedaffected by by liquidliquid oror dustdust entrainedentrained in in thethe processprocess gas gas steamsteam..

--HighHigh reliabilityreliability duedue toto thethe absenceabsence ofof movingmoving partsparts..

--PracticallyPractically no no maintenancemaintenance requiredrequired..

CONS,CONS,

--RequiresRequires a a steamsteam generatorgenerator..

desgasificado al vacío

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