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Chemistry Everyd ay for Everyone

JChemEd.chem.wisc.edu Vol. 75 No. 1 January 1998 Journal of Chemical Education 43

Three thousand years ago, ancient metal workersfirst accomplished th e goal of smelting iron ore int o me-tallic iron. The furnaces they used burned charcoal orother fuels to produce the required high tempera tur esand reducing atmosphere. Blasts of air were injected int oth e fuel to promote its combust ion. Impr ovement s in th eprocess over many centuries eventually led to the massproduction of iron an d to the indu str ial revolut ion. Theblast furnace is often presented in general chemistrycourses to illustrate the role of chemistry in the rise ofcivilization.

Textbook presentations of the blast furnace com-monly cite a set of reactions that occur in the smeltingprocess. The rea ctions involve combu stion of the fuel a ndits conversion in to carbon m onoxide, reduction of the ironore, and form at ion of slag. Several of these rea ctions a rereversible and can occur in the backward direction un-der conditions existing at some point in the furnace.Hence, it is not imm ediately obvious h ow the blast fur-na ce accomplishes it s pur pose of makin g iron.

This paper discusses the furnace from the perspec-tive of chemical t herm odyna mics. It examines t he ent ha lpy,entropy, and free energy change for each reaction ofimportance. These properties are interpreted on the mo-lecular level and a re t hen u sed to deduce the condit ionsnecessary for each reaction to occur in its intended di-rect ion. Our discussion wil l re ly upon the fact thatthermodynamics is the indisputable judge of reactionsponta neity. Chem ical kinetics will be invoked as neededto more fully explain th e operation of th e furna ce.

Blast Furnace Overview

Figure 1 illustrat es a typical modern blast furnace.It is a steel reactor st anding approximat ely the h eightof a 10-story building. It ha s a r efra ctory brick lining toenable it to withst and th e inten se heat generated within.A hopper at the top discharges raw materials into thefurnace by use of a special mechanism that preventsgases and du st from escaping into the a tmosphere. Thecharge consists of alternating loads of coke and a mix-tu re of iron ore a nd limest one. These solids form a col-umn that descends through the furnace with a t otal resi-dence time of about eight h ours.1 Nozzles at t he bottominject preheat ed air, often enriched with oxygen, int o thefurnace. The gases rapidly ascend through the columnand a re expelled thr ough a pair of stacks at the t op inless than 20 seconds. Blast furnaces are operated con-

tinuously. They can be run for several years before ashu tdown is required t o replace the r efractory brick.

Metallurgists classify the iron blast furnace as acount ercurr ent hea t an d mass exchan ger. Figure 1 iden-tifies its various reaction zones. Near the bottom is theactive coke zone wher e the coke an d air r eact to producered-hot coals . Carbon is in excess a t th is point andthroughout the furnace. Hence, the principal product ofth e combu stion is car bon m onoxide.

The conversion of iron ore into iron takes place inthe reduction zone. The meta llic iron pr oduced enters t he fusion zone where tempera tur es are sufficiently high t omelt it . The molten mat erial percolates t hrough the ac-tiv e coke and stagnant coke zones and even tu ally collectsin the bottom of the hearth, where it is periodically

ta pped off. In th e form ofpig iron it is fur th er processedinto steel. The limestone in th e cha rge decomposes intocalcium oxide and carbon dioxide as it passes throughthe reduction zone. The calcium oxide combines withsilicate impurities present in the iron ore to producemolten slag in the fusion zone. The slag drips throughthe coke and collects as the less dense liquid in thehear th . Detailed descriptions of th e blast furn ace opera-tions ar e available in th e metallurgical literatu re (18 ).

Coke serves as both t he fuel and th e reducing agentof the furnace. To be kinetically effective in the latterrole it must be converted int o car bon m onoxide. Figure 2shows t he effect of CO on iron ore. It is a cross-sectionalview of an F e2O3 part icle after exposure to the gas at high

t e m p e r a t u r e . T h e f r a g m e n t i s m i d w a y t h r o u g h a

The Iron Blast Furnace

A Study in Chemical Thermodynamics

Richard S. Treptow and Luck ner Jean

Department of Chemistry, Chicago State University, Chicago, IL 60628

Figure 1. Reaction zones and temperatures of a modern blast fur-

nace. Layers of coke alternate with layers of iron ore and lime-

stone; coke present in the two liquids in the hearth is not shown.

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4 4 Journal of Chemical Education Vol. 75 No. 1 January 1998 JChemEd.chem.wisc.edu

sequence of rea ctions t ha tbegins at t he outer sur facea n d p r o g r e s s e s i n w a r d .The reaction sequence isFe 2O3 Fe3O4 FeO Fe. The outermost surfacehas a l ready become Fe .The petals appea ring onthe par ticle result from t he

vo lume con t rac t ion tha taccompan ies the format ionof FeO. This contractionmakes the pa rticle porousand enab les CO to pen-etra te it. A mechanism ha s

been pr oposed for t he r eduction reactions in wh ich ironatoms m igrat e inward to the center of the par ticle leav-ing oxygen atoms behind to react with CO a t t he outersur face (2, 7). This mechanism is supported by the factth at two of the iron oxides are n onstoichiometric at hightemperature .2

Model Blast Furnace

Let us constr uct a m odel blast furnace to which t heprinciples of chemical thermodynamics can be applied.For simplicity, the model will consider only the majorcomponents of an actual furnace and it will representthese often impure components with pure compounds.The steel industr y employs various iron ores as the rawmaterial for blast furnaces. The ore used by our modelfurnace will be hematite, a mixture of Fe2O3 with sandand rock. The final product of an actual furnace is pigiron, an alloy containing about 4% carbon and lesseramounts of other elements. The final product of themodel fur na ce is pure iron.

To understand the model blast furnace we will ex-amine H, S ,and G for ea ch reaction over a wide

tempera tur e ra nge. These properties will be calculatedby conventional methods using Hf, S , and Gf datafrom the most recently published tables of JANAF (11)and Bar in (12). These references follow the currentIUPAC recommen dat ion tha t sta ndar d pressure is 1 bar.Readers more accustomed to a standard pressure of 1atm should note that 1 atm = 1.01325 bar. The differ-ence between th e pressu re conventions is n ot significan tfor our pur poses.

The sign ofGindicates if a reaction is ther mody-nam ically sponta neous at a par ticular t emperatur e. Thefamiliar equation G =H TS reminds us that spon-tan eity is determined prima rily by H at low tempera-ture , but that S becomes important at high tempera-tur e. Strictly speaking, Gis the t est of sponta neity onlyfor a reaction conducted at standard conditions.3 Whennonst an dar d conditions mu st be considered, we will ex-amine t he equilibrium consta nt for the r eaction. It canbe calculated from th e equat ionG = R T ln K.

Combustion of Coke

When the blast of preheated air enters t he furnaceit immediately encounters the bed of red-hot coke. Thecombus tion rea ction is

C(s) + O2(g) CO2(g)

Our model furnace uses carbon in the form of graphiteto repr esent coke. Table 1 lists t he t herm odynam ic proper-ties for the reaction at t hree temper at ures. The propert iesare remarkably constant with respect to temperature.

Figure 2. Partially reduced Fe2O3particle, based on photomicro-

graphs (7).

aBased on data of JANAF (11).

noitsubmoCekoCrofseitreporPcimanydomrehT.1elbaT

snoitcaeRnoitacifisaGdna a

erutarepmeT)K(

H

)Jk(S

)K/J(G

)Jk(

O+)s(C2

)g( OC2

)g(

892 5.393 9.2 4.493

0001 6.493 3.1 9.593

0002 8.693 2.0 3.693

OC+)s(C2

)g( )g(OC2

892 5.271 8.571 1.021

0001 7.071 3.571 7.4

0002 0.951 4.761 7.571

Figure 4. Gvs. temperature for the three steps in the reduction

of iron ore by carbon monoxide. Each curve applies to a different

reaction. For example, the curve labeled Fe2O3Fe3O4 is for the

reaction 3Fe2O3 +CO2Fe3O4 +CO2.

Figure 3. Thermodynamic propertiesof the reaction FeO+CO

Fe+CO2.

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The nega tive value ofH can be at tribut ed to the str ongbonds in carbon dioxide. The r eaction is highly exoth er-mic and is the primary source of heat for the furnace.The value ofS is near zero at a ll temperat ures. Thisshould be expected, since the nu mber of moles of gas doesnot change in the reaction. In general, gases are thema jor cont ribut ors to the ent ropy of a system. Since TS is compara tively small, Gis nearly equal to Hat a l ltemperat ures. Its negative value mean s the r eaction is

always spontaneous at standard conditions. We can ex-pect th e combustion to be rapid at th e high tem perat ureswhich it generates.

Production of Carbon Monoxide

The carbon dioxide produced at th e nozzles rises int oth e fur na ce an d encoun ter s more coke. We mus t considerthe coke gasification reaction

C(s) + CO2(g) 2 CO(g)

Table 1 lists the thermodynamic properties for this reac-tion at thr ee temperat ures. The reaction is endothermicand it lowers th e fur nace temper at ure. The positive signofS is predictable from the fact that the number ofmoles of gas increases. G becomes negative only at

high temperatures where TS predominat es. The reactionis spont aneous above 970 K. Below this t empera tu re th ereverse process, the sooting reaction, is favored. Therevers ibilit y of the r eaction causes t he r at io of CO to CO2in a system at equil ibr ium to be highly temperaturedependent.

Reduction of Iron Ore

The carbon monoxide produced in the active cokezone rises through the furnace and comes into contact

with the iron ore. The reduction sequence illustrated inFigure 2 involves thr ee steps:

3 Fe2O3(s) + CO(g) 2 Fe3O4(s) + CO2(g)

Fe3

O4

(s) + CO(g) 3 FeO (s) + CO2

(g)

FeO (s) + CO(g) Fe(s) + CO2(g)

The final step of the sequence imposes particulardemands on blast furnace conditions. Figure 3 plots itsth erm odyna mic properties calculat ed from J ANAF dat a(11). The Hand S plots are discontinuous at pointswhere a r eactan t or product u ndergoes a pha se chan ge.For example, the major breaks at 1650 K result becauseFeO melts at th is temperat ure. The reaction th an becomes

FeO(l) + CO(g) Fe(s) + CO2(g)

H and S become abruptly more negative becauseFeO has greater ent halpy and entropy as a liquid tha nas a solid. The discontinuities at 1809 K result because

Fe melts at th is temperatu re. The minor breaks at 1184K are t he result of a crystal str ucture change in Fe. TheG plot displays only very slight changes in slope atthe phase-change temperatures. The absence of discon-tinuities results from t he fact t hat both ph ases have thesame free energy at the temperature where they existin equilibrium. The G plot r eveals th at the final stepin the r eduction sequence is nonsponta neous under st an-dard conditions at all temperatures encountered in ablast furna ce.

Figure 4 shows the tempera tur e dependence ofGfor each of the three steps in the reduction of iron oreby car bon monoxide calculat ed from dat a of J ANAF (11)and Barin (12). The first a nd second st eps (Fe2O3 Fe3O4and Fe3O4 FeO) are spontaneous at standard condi-

tions regardless of temperature. As just discussed, thefinal step (FeO Fe) is disfavored at all blast furnacetemperat ures. To understand h ow the furna ce man agesto carry out all three reactions we must next look be-yond standard conditions.

Predominance Diagram

The conditions requir ed for r eduction of iron ore bycarbon monoxide can be deduced by considering thechemical equilibrium associated with each step of thereaction sequence. For exam ple, the equilibrium for t hefina l step is

FeO(s or l) + CO(g) Fe(s or l) + CO2(g)

Imag ine a sys tem in which a l l fou r subs tances a represent at equilibrium. A useful property of the systemis PCO/PCO2 , the r atio of the par tial pressur es of the gases.It is the reciprocal of the equilibrium constant. Hence,we can write G = R T ln (PCO/PCO2 ) and we can calcu-la te PCO/PCO2 as a function of temperatur e from t he Gdata on ha nd. The results are shown in Figure 5 as thecurve labeled FeO Fe. At any tempera tur e the curvegives th e PCO/PCO2 rat io for a system in which FeO andFe equilibrat e with a m ixtur e of the two gases. The sys-tem will not be in equilibrium if th e gas composit ion cor-responds t o a point off th e cur ve. Such a syst em will at-tempt to reach equilibrium. For example, if th e gas com-

position corr esponds to a point a bove the curve, th e re-action will proceed in th e forwa rd direction. It will con-tinue until all FeO is consumed or until the gas compo-sition dr ops to the curve. Figure 5 a lso displays PCO/PCO2curves for the first and second steps in the reductionsequence.

The equilibrium curves have been combined withmelting point da ta to creat e Figure 6. Thispredominancediagram identifies the m ost sta ble form of iron existingin an atmosphere of mixed CO and CO2. Temperatu reand the composition of the gas phase are varied. If theatm osphere has a h igh P CO/PCO 2 rat io, it is a str ong re-ducing agent a nd iron exists a s Fe(s) or F e(l). If the ra tiois low, the atmosphere acts as an oxidizing agent andone or a nother of th e iron oxides is most st able.

Figure 5. PCO/PCO2 vs. temperature for equilibria in between iron

oxides in an atmosphere of CO and CO2. For example, the curve

labeled Fe2O3 Fe3O4 is for the equilibrium 3 Fe2O3 + CO

2 Fe3O4 + CO2.

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Figure 6. Predominance diagram identifying the most stable form of

iron in an atmosphere of CO and CO2 as temperature and the com-

position of the gaseous phase are varied. The dashed line applies to

the equilibrium C(s) + CO2(g)

2 CO(g) at 1 bar total pressure.

Th e PCO/PCO2 rat io of the blast furn ace atmosphereis set by the coke gasification reaction. Superimposedon the predominance diagram is a curve labeled CO2

CO, which a pplies to the equilibrium

C(s) + CO2(g) 2 CO(g)

The cur ve gives PCO/PCO2 for any system in which cokeis in equilibrium with a m ixture of the t wo gases at 1 bartotal pressure.4 The rapidly rising curve shows that the

equilibrium shifts in t he forwar d direction as t empera-tu re increases. The direction of the shift can be pr edictedfrom LeChtelier s principle and th e fact th at t he for-ward react ion is endothermic. The r ise in the curvemeans that the blas t furnace a tmosphere becomes astronger reducing agent with t emperatu re.

The predominance diagram explains how the blastfurna ce accomplishes its pur pose of converting ir on oreinto iron. As an Fe2O3 particle descends it encountersprogressively higher temperatures and an atmospherewhose P CO/PCO 2 value increases rapidly. Above 1000 K,the gas mixture has sufficient reducing ability to com-pletely convert th e par ticle int o Fe.

The CO2 CO curve of Figur e 6 reveals t he fat e of

the furna ce gases during their ra pid ascent th rough thefurnace. As t he t emperatu re decreases, the equilibriumshifts in t he backward direction. The result is the conver-sion of carbon monoxide into soot and carbon dioxide.Th e PCO/PCO 2 ratio of the gas and its reducing abilitydecrease. The equilibrium shift also has a beneficialeffect. By decreasing the amount of carbon monoxideexpelled from t he s ta ck, it impr oves t he efficiency of thefurnace. CO can be regarded as coke which has not yetgiven its full measure as a fuel or as a reducing agent.Thus, its r elease from t he furnace is wasteful. In a ctua l

practice, the sooting reaction is slow in comparison tothe residence time of the gases in t he furna ce and equi-librium is not achieved in th e upper pa rt of the furn ace.The gas mixture expelled from a typical blast furnace

has a PCO/PCO 2 ratio of approximately two and a tem-pera tu re of about 500 K. To impr ove th e overa ll energyefficiency of the furnace the exhaust gas is commonlymixed with air a nd bur ned for its fuel value.

Formation of Slag

A final blast furna ce process is th e removal of san dand other impurities found in th e iron ore. The limestonepresent in the charge converts these impurities into amolten mass that readily separates from the pig iron.The first st ep in slag form ation is calcination of th e lime-stone:

CaCO3(s) CaO(s) + CO2(g)

Table 2 lists the t herm odyna mic propert ies of th is reac-

t ion a t three temperatures . H and S are both posi-tive, as should be expected for a rea ction in wh ich a gasis generat ed from a solid. Gis positive at low tempera -ture and becomes negative only when TS predomi-nates. The reaction is spontaneous above 1100 K. Thelimestone decomposes when it r eaches t his tempera tur ein its descent t hrough th e furn ace.

The calcium oxide produced above combines with th eimpurities, represented as SiO 2, to form th e slag by thereaction

CaO (s) + SiO2(s or l) CaSiO3(s or l)

Table 2 gives the thermodynamic properties for this re-action. The values ofHand S at 2000 K differ fromthose a t the lower tempera tu res because S iO

2a n d

CaSiO3 are both molten at this high temperature. Theslag form at ion r eaction is th ermodynamically favored ata l l temperatur es and is rapid a t the high temperaturesof th e fusion zone.

Notes

1. Illustrations in some textbooks give the impression thatthe solids in a blast furnace are in a state of free fall. This is notthe case.

2. Fe2O3 (hematite) is a proper compound of fixed composi-tion. Fe3O4 (magnetite) displays constant composition at low tem-perature, but near the melting point its stoichiometry is variable and

can be deficient in iron. FeO (wustite) is thermodynamically stableonly at high temperature, where it is always deficient in iron. It canbe classified as a solid solution and is often written Fe0.95O. Com-plete details can be found in the ironoxygen phase diagram (9, 10).

3. A recent article in this Journaldiscusses how a reaction

takes place at standard conditions (13).4. The coke gasification reaction differs from reactions pre-

viously discussed in that its equilibrium constant expression isK= (PCO)

2/PCO2. Since the PCO/PCO2 ratio is not constant, it canbe determined only if another condition is imposed on the sys-

noitamroFgalSrofseitreporPcimanydomrehT.2elbaTsnoitcaeR a

erutarepmeT)K(

H

)Jk(S

)K/J(G

)Jk(

OCaC3

)s( OC+)s(OaC2

)g(

892 3.871 9.851 9.031

0001 8.961 4.541 4.42

0002 8.441 3.921 8.311

OiS+)s(OaC2

)lros( OiSaC3

)lros(

892 5.28 8.7 8.48

0001

2.58 1.4

3.98

0002 5.63 7.03 9.79

aBased on data of Barin (12).

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tem. Hence, we require that PCO + PCO2 = 1 bar. A computerspreadsheet proves invaluable for the task of calculating PCO/PCO2over a temperature range. A quadratic equation must be solved.

Literature Cited

1 . B a s h for t h , G . R . The Manufacture of Iron and Steel, Vol. 1;

Chapm an & Ha ll: London, 1948.2 . H u e bl er , J . I n Iron Ore Reduction ; Rogers, R. R., Ed.; Per gamon:

Oxford, 1962; pp 2456.

3. Aspects of Modern Ferrous Metallurgy; Kirkaldy, J. S.; Ward, R.G., Eds.; University of Toronto: Toronto, 1964.

4. Peacey, J. G.; Davenport, W. G.Th e Iron Blast Furn ace: Th eory and

Practice; Pergam on: Oxford, 1979.5 . Rosenqvis t , T. Principles of Extractive Metallurgy; McGraw-Hill:

New York, 19 83.

6 . Walker, R. D. Modern Ironmaking Methods; Institute of Metals:

London, 1986.7. Rostoker, W.; Bronson, B. Pre-Ind ustrial Iron: Its Technology and

Ethnology; Archeomateria ls Monograph : Philadelphia, 1990.

8. M oor e , J . J . C hem ica l M e ta l lu rgy , 2 n d e d . ; B u t t e r w o r t h -

Heinem an n: Oxford, 1994; pp 243309.

9. Ell iott , J . F. ; Gleiser, M.; Rama krishna, V. Thermochemistry for

Steelmaking, Vol. II; Addison-Wesley: Readin g, MA, 1963; p 406 .

10. Muan, A.; Osborn, E. F. Phase Equilibria among Oxides in Steel-

making; Addison-Wesley: Read ing, MA, 1965; p 28.

11. Chas e, M. W., Jr.; Davies, C. A.; Downey, J. R., J r.; Frurip, D. J .;

McDonald, R. A.; Syverud, A. N.JAN AF Th ermochemical Tables,3rd ed.; National Bureau of Standards: Washington, DC, 1985.

12. B a r in , I . Therm ochem ica l D a ta o f P ure Subs tances ; VCH:Weinheim , Germa ny, 1989.

13. Treptow, R. S .J. Chem. Educ. 1996, 73, 51.

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