Oxidation Potentials in Iron and Steel Making

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  • Oxidation Potentials in Iron and Steel Making

    J. W. MATOUSEK1,2

    1.Englewood, CO, USA. 2.e-mail: jwmatousek@q.com

    The state of oxidation of a pyrometallurgical process given by the partialpressure of oxygen and the temperature (the oxidation potential) is one of theimportant properties monitored and controlled in the smelting and refining ofiron and the nonferrous metals. Solid electrolyte sensors based on ZrO2 and areference electrode such as Cr/Cr2O3 to measure the oxygen pressure foundearly application in the steel industry, followed soon after in copper, nickel,lead, and zinc smelting. Similar devices are installed in automobile postcom-bustion/exhaust trains as part of emission control systems. The current dis-cussion reviews this technology as applied in the primary steps of iron andsteel making and refining.

    INTRODUCTION

    Previous papers in this series explored the con-cepts of oxidation potentials (given as the base-tenlogarithm of the equilibrium oxygen partial pres-sure in bars and the temperature) in studying andcontrolling pyrometallurgical extraction pro-cesses.13 Applications to copper, nickel, lead, andzinc smelting and metallurgical slags in generalwere examined. It is fitting to close these discus-sions with a return to where the technology be-ganwith the making of iron and steel.

    TECHNOLOGY

    Variations of Fig. 1 have been a central fixture inthe education of every extractive metallurgist sinceat least the 1951 publication of Basic Open HearthSteelmaking.4 The solid line represents theapproximate equilibrium at 1600C between steelsof the carbon content given and atmospheres ofcarbon monoxide. The broken line indicates thedirection of the trend toward the conditions ob-served in production furnaces.

    The measurement of carbon in steel has long beenan established practice. The direct measurement ofoxygen began to look practical with the 1960s pub-lications of Fisher and Ackermann,5,6 Fitterer7 andFruehan et al.,8 describing sensors based on solidelectrolytes such as stabilized zirconia (ZrO2) cou-pled with the reference electrodes Ni/NiO, Fe/FeO,Mo/MoO2, and Cr/Cr2O3; an example of the latter isgiven by Eq. 1. The difference in activities (concen-trations) between oxygen dissolved in iron and the

    reference electrode generates an electromotive force(emf) that is related to the two activities by theNernst equation (Eq. 2). Since the 1970s, solid-electrolyte oxygen sensor use has been standardpractice in a variety of smelting and refining oper-ations; Fig. 2 illustrates the range over which theyhave found application.1

    Cr; Cr2O3== ZrO2== O Fe; Fe (1)

    E 2:3RT=zF log p O2 Fe= p O2 ref

    (2)

    where E is the cell emf in millivolts (mV), R is thegas constant, T is the absolute temperature, z is thecharge transfer (four in the case of O2), F is theFaraday constant (96,500 J/V mol), and log p(O2) isthe base-ten logarithm of the oxygen pressure inbars.

    IRON AND STEELMAKING

    Classical ironmaking begins with the blast fur-nace smelting of iron ore (magnetite, Fe3O4, orhematite, Fe2O3) with coke and fluxes. The productpig iron contains approximately 4% carbon andvirtually no oxygen, as suggested by Fig. 1. The ironis made into steel with the removal of carbon (andother impurities) by oxidation in basic oxygen fur-naces (BOFs) or formerly in Bessemer converters(pig iron charge) and open hearth furnaces (pig ironand scrap charge). Modern electric arc furnaces(EAFs) start with scrap iron and steel, bypass theprimary smelting step, and proceed directly to

    JOM, Vol. 65, No. 11, 2013

    DOI: 10.1007/s11837-013-0713-1 2013 TMS

    1584 (Published online August 14, 2013)

  • melting followed by refining in ladle furnaces andcontinuous casting. These latter two operations arealso standard in BOF practice. It is in the refiningand casting that solid electrolyte oxygen sensorsplay an important role in quality control.

    Figure 3 illustrates the range of products andoxygen levels in steels treated with various deoxi-dizers.9 The upper product is a rimmed steel. Theseterms, rimmed, semikilled, and killed, describe thebehavior of molten steel as it freezes in an ingot moldand the macrostructures of cross-sectioned ingots.10

    For reference, from Eq. 3 at 1600C the maximumsolubility of oxygen in pure, liquid iron is 2290 partsper million (ppm) at log p(O2) = 8.2.4,9,10

    log O ppm 6:734 6320=T at saturation (3)

    THEORY AND PRACTICE

    Equation 4 relates the oxygen content of steel andcell voltage of a Cr/Cr2O3 sensor.

    8 From this, theNernst equation, and tables of standard thermody-namic data, Eqs. 59 were written for 1600C.Equation 8 would give the potential noted above,8.2 at 2,290 ppm [O], as 7.9. The data used todevelop Eq. 8, however, did not extend to concen-trations greater than 1000 ppm [O]. As previously,the standard electrode potential for Cr2O3(1120 mV at 1600C) in Eq. 10 was taken to bethat for Eq. 11the opposite sign of the formationreaction.1 This was designated for emphasis in theoriginal paper as E. At 1600C and an oxygenconcentration of 40 ppm, the value of E in Eq. 10 is

    Mo/MoO2 Reference, Lead and Zinc1200 oC

    Cr/Cr2O3 Reference, Iron, 1600 oCy = -92.87x - 1120

    -800

    -600

    -400

    -200

    0

    200

    400

    600

    800

    -16.0 -14.0 -12.0 -10.0 -8.0 -6.0 -4.0 -2.0 0.0

    log pO2 (bar)

    E, m

    V

    Ni/NiO Reference, Copper, 1150 oCy = -70.56x - 590

    Air Reference, Copper, 1150 oCy = -70.56x - 48

    Cr/Cr2O3 Reference, Lead and Zinc Smelting, 1200 oC

    Fig. 2. Applications of solid electrolyte oxygen sensors in smelting and refining.

    0 50 100 150 200 250 300[O], ppm

    Deo

    xidi

    zer

    Fe/Mn

    Si/Mn, semi-killed

    Si/Mn/Al, semi-killed

    Si/Mn/Ca, semi-killed

    Al, killed

    Fig. 3. Oxygen contents of deoxidized steels.

    0

    100

    200

    300

    400

    500

    600

    0.0 0.5 1.0 1.5 2.0 2.5wt% [C]

    [O], p

    pm T = 1600 oC p(CO) = 1 bar%[C] x %[O] = 0.0020

    Fig. 1. Oxygen and carbon dissolved in molten iron at 1600C.

    Oxidation Potentials in Iron and Steel Making 1585

  • zero and log p(O2) = 12.1. At oxygen concentra-tions less than this and oxidation potentials morenegative than 12.1, the Cr/Cr2O3 reference elec-trode is positive relative to the working electrode, asseen in Fig. 2.

    Equations 49 are written for T = 1600C =1873 K.

    E 217 log O ppm 348 (4)

    log O ppm 0:0046E 1:604 (5)

    E 92:87 log p O2 Fe 1120 (6)

    log p O2 Fe 0:0108E 12:06 (7)

    log p O2 Fe 2:34 log O ppm 15:81 (8)

    log O ppm 0:427 log p O2 Fe 6:76 (9)

    E E 2:3RT=zF log p O2 FeE

    E

    1873K

    1120 mV (10)

    2=3 Cr2O3 2=3 Cr O2DG

    749:0 0:169T; kJ=mol O2(11)

    Engineers new to this field should be prepared tofind in the technical literature every possible rear-rangement of terms in Eqs. 2 and 10 and everyconceivable choice of signs, within the equationsand those assigned to E and E. The polarity of the

    thermoelectric emf between the dissimilar metalcouple of reference and working electrodes mustalso be noted.

    Equation 4 is shown graphically in Fig. 4. Thedata points were extracted from trade literature forsteelmaking operations.11

    INDUSTRIAL APPLICATIONS

    Table I summarizes typical composition rangesfor the slags of nonferrous and ferrous productionfurnaces: Cu(Ni) flash, Cu(Ni) Peirce-Smith con-verter, lead blast, iron blast, Bessemer converter,basic open hearth, basic oxygen, electric arc, andrefining ladles. The slags may be classified as ferro-silicates or calcium-ferro-silicates, containing vary-ing proportions of alumina and magnesiagener-ally in the olivine family of minerals. Thethermodynamic properties of the various systemsare determined by the temperature, slag compo-nents, oxidation of ferrous to ferric iron, and ap-proach to equilibrium between the slag and metal ormatte. The ratio of ferric to ferrous iron (Fe+3/Fe+2)for a given temperature can be related to the oxi-dation potential. Equilibrium is approached in theflash, blast, open hearth, and electric smelting fur-naces, but less so in the converters and ladle refin-ing furnaces. The primary function of nonferrousslags is to absorb iron oxides in a form suitable fordiscard; that of ferrous slags is to absorb theimpurities oxidized during refining. Slag productionrelative to the quantity of metal (or matte) is higherin nonferrous smelting than in the production ofiron and steel. The times between metal and slagtaps in nonferrous smelting are measured inhoursminutes in the production of iron and steelexcept in the blast and open hearth furnaces.

    Taking the carbon content of blast furnace pig iron tobe 4%, the oxidation potential can be estimated fromFig. 1 and Eq. 8 to be around 13 for a temperature of1600C. This would correspond to a CO/CO2 ratio of1400 in the gases above the slag. But it is also inter-esting to note that blast furnace slags contain virtuallyno iron and to recall a suggestion made by Schuh-mann16 oxide slags without iron do not necessarilyeven have an oxygen activity; that is, they may nothave oxidation potentials. This statement could be re-placed by one that stresses that slags must contain oneof the transition metals in order for oxidation/reductionreactions to occur and, therefore, for there to be anoxidation potentialbut it is still an intriguing point.

    Richards13 in 1918, gave the composition of aBessemer steel blown from blast furnace pig iron as0.04% C, 0.02% Si, 0.01% Mn, and probably lessthan 0.3% oxygen (3000 ppm) with a temperatureof 1500C. The maximum soluble oxygen at 1600Cwas given above as 2290 ppm with log p(O2) = 8.2;at 1500, [O] = 1477 ppm and log p(O2) = 9.0.

    Oxidation potentials in open hearth furn

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