Chemistry Everyday for Everyone
The Iron Blast Furnace A Study in Chemical Thermodynamics Richard S. Treptow and Luckner Jean Department of Chemistry, Chicago State University, Chicago, IL 60628 Three thousand years ago, ancient metal workers first accomplished the goal of smelting iron ore into metallic iron. The furnaces they used burned charcoal or other fuels to produce the required high temperatures and reducing atmosphere. Blasts of air were injected into the fuel to promote its combustion. Improvements in the process over many centuries eventually led to the mass production of iron and to the industrial revolution. The blast furnace is often presented in general chemistry courses to illustrate the role of chemistry in the rise of civilization. Textbook presentations of the blast furnace commonly cite a set of reactions that occur in the smelting process. The reactions involve combustion of the fuel and its conversion into carbon monoxide, reduction of the iron ore, and formation of slag. Several of these reactions are reversible and can occur in the backward direction under conditions existing at some point in the furnace. Hence, it is not immediately obvious how the blast furnace accomplishes its purpose of making iron.
Figure 1. Reaction zones and temperatures of a modern blast furnace. Layers of coke alternate with layers of iron ore and limestone; coke present in the two liquids in the hearth is not shown.
This paper discusses the furnace from the perspective of chemical thermodynamics. It examines the enthalpy, entropy, and free energy change for each reaction of importance. These properties are interpreted on the molecular level and are then used to deduce the conditions necessary for each reaction to occur in its intended direction. Our discussion will rely upon the fact that thermodynamics is the indisputable judge of reaction spontaneity. Chemical kinetics will be invoked as needed to more fully explain the operation of the furnace. Blast Furnace Overview Figure 1 illustrates a typical modern blast furnace. It is a steel reactor standing approximately the height of a 10-story building. It has a refractory brick lining to enable it to withstand the intense heat generated within. A hopper at the top discharges raw materials into the furnace by use of a special mechanism that prevents gases and dust from escaping into the atmosphere. The charge consists of alternating loads of coke and a mixture of iron ore and limestone. These solids form a column that descends through the furnace with a total residence time of about eight hours.1 Nozzles at the bottom inject preheated air, often enriched with oxygen, into the furnace. The gases rapidly ascend through the column and are expelled through a pair of stacks at the top in less than 20 seconds. Blast furnaces are operated continuously. They can be run for several years before a shutdown is required to replace the refractory brick. Metallurgists classify the iron blast furnace as a countercurrent heat and mass exchanger. Figure 1 identifies its various reaction zones. Near the bottom is the active coke zone where the coke and air react to produce red-hot coals. Carbon is in excess at this point and throughout the furnace. Hence, the principal product of the combustion is carbon monoxide. The conversion of iron ore into iron takes place in the reduction zone. The metallic iron produced enters the fusion zone where temperatures are sufficiently high to melt it. The molten material percolates through the active coke and stagnant coke zones and eventually collects in the bottom of the hearth, where it is periodically tapped off. In the form of pig iron it is further processed into steel. The limestone in the charge decomposes into calcium oxide and carbon dioxide as it passes through the reduction zone. The calcium oxide combines with silicate impurities present in the iron ore to produce molten slag in the fusion zone. The slag drips through the coke and collects as the less dense liquid in the hearth. Detailed descriptions of the blast furnace operations are available in the metallurgical literature (1–8). Coke serves as both the fuel and the reducing agent of the furnace. To be kinetically effective in the latter role it must be converted into carbon monoxide. Figure 2 shows the effect of CO on iron ore. It is a cross-sectional view of an Fe2O3 particle after exposure to the gas at high temperature. The fragment is midway through a
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Chemistry Everyday for Everyone sequence of reactions that begins at the outer surface and progresses inward. The reaction sequence is Fe 2O3 → Fe3O4 → FeO → Fe. The outermost surface has already become Fe. The “petals” appearing on the particle result from the volume contraction that accompanies the formation Figure 2. Partially reduced Fe2 O3 of FeO. This contraction particle, based on photomicromakes the particle porous graphs (7 ). and enables CO to penetrate it. A mechanism has been proposed for the reduction reactions in which iron atoms migrate inward to the center of the particle leaving oxygen atoms behind to react with CO at the outer surface (2, 7). This mechanism is supported by the fact that two of the iron oxides are nonstoichiometric at high temperature.2 Model Blast Furnace Let us construct a model blast furnace to which the principles of chemical thermodynamics can be applied. For simplicity, the model will consider only the major components of an actual furnace and it will represent these often impure components with pure compounds. The steel industry employs various iron ores as the raw material for blast furnaces. The ore used by our model furnace will be hematite, a mixture of Fe2O3 with sand and rock. The final product of an actual furnace is pig iron, an alloy containing about 4% carbon and lesser amounts of other elements. The final product of the model furnace is pure iron. To understand the model blast furnace we will examine ∆H°, ∆S°, and ∆G° for each reaction over a wide temperature range. These properties will be calculated by conventional methods using ∆Hf°, S°, and ∆Gf° data from the most recently published tables of JANAF (11) and Barin (12). These references follow the current IUPAC recommendation that standard pressure is 1 bar. Readers more accustomed to a standard pressure of 1 atm should note that 1 atm = 1.01325 bar. The difference between the pressure conventions is not significant for our purposes. The sign of ∆G° indicates if a reaction is thermodynamically spontaneous at a particular temperature. The familiar equation ∆G = ∆H – T∆S reminds us that spontaneity is determined primarily by ∆H at low temperature, but that ∆S becomes important at high temperature. Strictly speaking, ∆G° is the test of spontaneity only for a reaction conducted at standard conditions.3 When nonstandard conditions must be considered, we will examine the equilibrium constant for the reaction. It can be calculated from the equation∆G° = {RT ln K.
Figure 3. Thermodynamic properties of the reaction FeO + CO → Fe + CO 2.
Figure 4. ∆G° vs. 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 Fe2O 3 → Fe3O 4 is for the reaction 3 Fe2O 3 + CO → 2 Fe3O 4 + CO2. Table 1. Thermodynamic Proper ties for Coke Combustion and Gasification Reactionsa Temperature (K)
∆H ° (kJ)
298
{393.5
2.9
{394.4
1000
{394.6
1.3
{395.9
2000
{396.8
{0.2
{396.3
C(s) + CO2(g) → 2CO(g)
C(s) + O2(g) → CO2(g)
44
∆G ° (kJ)
C(s) + O2(g) → CO2(g)
Combustion of Coke When the blast of preheated air enters the furnace it immediately encounters the bed of red-hot coke. The combustion reaction is Our model furnace uses carbon in the form of graphite to represent coke. Table 1 lists the thermodynamic properties for the reaction at three temperatures. The properties are remarkably constant with respect to temperature.
∆S ° (J/K)
298
172.5
175.8
1000
170.7
175.3
{4.7
2000
159.0
167.4
{175.7
a
Based on data of JANAF (11 ).
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Chemistry Everyday for Everyone The negative value of ∆H° can be attributed to the strong bonds in carbon dioxide. The reaction is highly exothermic and is the primary source of heat for the furnace. The value of ∆S° is near zero at all temperatures. This should be expected, since the number of moles of gas does not change in the reaction. In general, gases are the major contributors to the entropy of a system. Since T∆S° is comparatively small, ∆G° is nearly equal to ∆H° at all temperatures. Its negative value means the reaction is always spontaneous at standard conditions. We can expect the combustion to be rapid at the high temperatures which it generates.
Production of Carbon Monoxide The carbon dioxide produced at the nozzles rises into the furnace and encounters more coke. We must consider the coke gasification reaction C(s) + CO2(g) → 2 CO(g) Table 1 lists the thermodynamic properties for this reaction at three temperatures. The reaction is endothermic and it lowers the furnace temperature. The positive sign of ∆S° is predictable from the fact that the number of moles of gas increases. ∆G° becomes negative only at high temperatures where T∆S° predominates. The reaction is spontaneous above 970 K. Below this temperature the reverse process, the sooting reaction, is favored. The reversibility of the reaction causes the ratio of CO to CO2 in a system at equilibrium to be highly temperature dependent.
Reduction of Iron Ore The carbon monoxide produced in the active coke zone rises through the furnace and comes into contact with the iron ore. The reduction sequence illustrated in Figure 2 involves three steps: 3 Fe2O3(s) + CO(g) → 2 Fe3O4(s) + CO2(g) Fe3O4(s) + CO(g) → 3 FeO(s) + CO 2(g) FeO(s) + CO(g) → Fe(s) + CO2(g) The final step of the sequence imposes particular demands on blast furnace conditions. Figure 3 plots its thermodynamic properties calculated from JANAF data (11). The ∆H° and ∆S° plots are discontinuous at points where a reactant or product undergoes a phase change. For example, the major breaks at 1650 K result because FeO melts at this temperature. The reaction than becomes FeO(l) + CO(g) → Fe(s) + CO2(g) ∆H° and ∆S° become abruptly more negative because FeO has greater enthalpy and entropy as a liquid than as a solid. The discontinuities at 1809 K result because Fe melts at this temperature. The minor breaks at 1184 K are the result of a crystal structure change in Fe. The ∆G° plot displays only very slight changes in slope at the phase-change temperatures. The absence of discontinuities results from the fact that both phases have the same free energy at the temperature where they exist in equilibrium. The ∆G° plot reveals that the final step in the reduction sequence is nonspontaneous under standard conditions at all temperatures encountered in a blast furnace. Figure 4 shows the temperature dependence of ∆G° for each of the three steps in the reduction of iron ore by carbon monoxide calculated from data of JANAF (11) and Barin (12). The first and second steps (Fe2O3 → Fe3O4 and Fe3O4 → FeO) are spontaneous at standard condi-
Figure 5. PCO / PCO2 vs. temperature for equilibria in between iron oxides in an atmosphere of CO and CO2. For example, the curve labeled Fe2O 3 Fe3O 4 is for the equilibrium 3 Fe2O 3 + CO 2 Fe3O 4 + CO2 .
tions regardless of temperature. As just discussed, the final step (FeO → Fe) is disfavored at all blast furnace temperatures. To understand how the furnace manages to carry out all three reactions we must next look beyond standard conditions.
Predominance Diagram The conditions required for reduction of iron ore by carbon monoxide can be deduced by considering the chemical equilibrium associated with each step of the reaction sequence. For example, the equilibrium for the final step is FeO(s or l) + CO(g)
Fe(s or l) + CO2(g)
Imagine a system in which all four substances are present at equilibrium. A useful property of the system is PCO /PCO2 , the ratio of the partial pressures of the gases. It is the reciprocal of the equilibrium constant. Hence, we can write ∆G° = RT ln (PCO /PCO2 ) and we can calculate PCO /PCO2 as a function of temperature from the ∆G° data on hand. The results are shown in Figure 5 as the curve labeled FeO Fe. At any temperature the curve gives the PCO /PCO2 ratio for a system in which FeO and Fe equilibrate with a mixture of the two gases. The system will not be in equilibrium if the gas composition corresponds to a point off the curve. Such a system will attempt to reach equilibrium. For example, if the gas composition corresponds to a point above the curve, the reaction will proceed in the forward direction. It will continue until all FeO is consumed or until the gas composition drops to the curve. Figure 5 also displays PCO /PCO2 curves for the first and second steps in the reduction sequence. The equilibrium curves have been combined with melting point data to create Figure 6. This predominance diagram identifies the most stable form of iron existing in an atmosphere of mixed CO and CO2. Temperature and the composition of the gas phase are varied. If the atmosphere has a high P CO /PCO 2 ratio, it is a strong reducing agent and iron exists as Fe(s) or Fe(l). If the ratio is low, the atmosphere acts as an oxidizing agent and one or another of the iron oxides is most stable.
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Chemistry Everyday for Everyone The PCO /PCO2 ratio of the blast furnace atmosphere is set by the coke gasification reaction. Superimposed on the predominance diagram is a curve labeled CO2 CO, which applies to the equilibrium C(s) + CO2(g)
2 CO(g)
The curve gives PCO /PCO2 for any system in which coke is in equilibrium with a mixture of the two gases at 1 bar total pressure.4 The rapidly rising curve shows that the equilibrium shifts in the forward direction as temperature increases. The direction of the shift can be predicted from LeChâtelier’s principle and the fact that the forward reaction is endothermic. The rise in the curve means that the blast furnace atmosphere becomes a stronger reducing agent with temperature. The predominance diagram explains how the blast furnace accomplishes its purpose of converting iron ore into iron. As an Fe2O3 particle descends it encounters progressively higher temperatures and an atmosphere whose P CO /PCO 2 value increases rapidly. Above 1000 K, the gas mixture has sufficient reducing ability to completely convert the particle into Fe. The CO2 CO curve of Figure 6 reveals the fate of the furnace gases during their rapid ascent through the furnace. As the temperature decreases, the equilibrium shifts in the backward direction. The result is the conversion of carbon monoxide into soot and carbon dioxide. The PCO /PCO2 ratio of the gas and its reducing ability decrease. The equilibrium shift also has a beneficial effect. By decreasing the amount of carbon monoxide expelled from the stack, it improves the efficiency of the furnace. CO can be regarded as coke which has not yet given its full measure as a fuel or as a reducing agent. Thus, its release from the furnace is wasteful. In actual practice, the sooting reaction is slow in comparison to the residence time of the gases in the furnace and equilibrium is not achieved in the upper part of the furnace. The gas mixture expelled from a typical blast furnace has a PCO /PCO2 ratio of approximately two and a temperature of about 500 K. To improve the overall energy efficiency of the furnace the exhaust gas is commonly mixed with air and burned for its fuel value.
Formation of Slag A final blast furnace process is the removal of sand and other impurities found in the iron ore. The limestone present in the charge converts these impurities into a molten mass that readily separates from the pig iron. The first step in slag formation is calcination of the limestone: CaCO 3(s) → CaO(s) + CO2(g) Table 2 lists the thermodynamic properties of this reaction at three temperatures. ∆H° and ∆S° are both positive, as should be expected for a reaction in which a gas is generated from a solid. ∆G° is positive at low temperature and becomes negative only when T∆S° predominates. The reaction is spontaneous above 1100 K. The limestone decomposes when it reaches this temperature in its descent through the furnace. The calcium oxide produced above combines with the impurities, represented as SiO2, to form the slag by the reaction CaO(s) + SiO2(s or l) → CaSiO3(s or l) Table 2 gives the thermodynamic properties for this reaction. The values of ∆H° and ∆S° at 2000 K differ from those at the lower temperatures because SiO 2 and 46
Figure 6. Predominance diagram identifying the most stable form of iron in an atmosphere of CO and CO2 as temperature and the composition 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.
Table 2. Thermodynamic Proper ties for Slag Formation Reactionsa Temperature (K)
∆H ° (kJ)
∆S ° (J/K)
∆G ° (kJ)
CaCO3(s) → CaO(s) + CO2(g) 298
178.3
158.9
1000
169.8
145.4
130.9 24.4
2000
144.8
129.3
{113.8
CaO(s) + SiO2(s or l) → CaSiO3(s or l) 298
{82.5
7.8
{84.8
1000
{85.2
4.1
{89.3
2000
{36.5
30.7
{97.9
a
Based on data of Barin ( 12 ).
CaSiO3 are both molten at this high temperature. The slag formation reaction is thermodynamically favored at all temperatures and is rapid at the high temperatures of the fusion zone. Notes 1. Illustrations in some textbooks give the impression that the solids in a blast furnace are in a state of free fall. This is not the case. 2. Fe2O3 (hematite) is a proper compound of fixed composition. Fe3O4 (magnetite) displays constant composition at low temperature, but near the melting point its stoichiometry is variable and can be deficient in iron. FeO (wustite) is thermodynamically stable only at high temperature, where it is always deficient in iron. It can be classified as a solid solution and is often written Fe0.95O. Complete details can be found in the iron–oxygen phase diagram (9, 10 ). 3. A recent article in this Journal discusses how a reaction takes place “at standard conditions” ( 13 ). 4. The coke gasification reaction differs from reactions previously discussed in that its equilibrium constant expression is K = (PCO )2/ PCO2. Since the PCO / PCO2 ratio is not constant, it can be determined only if another condition is imposed on the sys-
Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone tem. Hence, we require that PCO + PCO2 = 1 bar. A computer spreadsheet proves invaluable for the task of calculating PCO / PCO2 over a temperature range. A quadratic equation must be solved.
Literature Cited 1. Bashforth, G. R. The Manufacture of Iron and Steel, Vol. 1; Chapman & Hall: London, 1948. 2. Huebler, J. In Iron Ore Reduction; Rogers, R. R., Ed.; Pergamon: Oxford, 1962; pp 24–56. 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. The Iron Blast Furnace: Theory and Practice; Pergamon: Oxford, 1979. 5. Rosenqvist, T. Principles of Extractive Metallurgy; McGraw-Hill: New York, 1983.
6. Walker, R. D. Modern Ironmaking Methods; Institute of Metals: London, 1986. 7. Rostoker, W.; Bronson, B. Pre-Industrial Iron: Its Technology and Ethnology; Archeomaterials Monograph: Philadelphia, 1990. 8. Moore, J. J. Chemical Metallurgy , 2nd ed.; ButterworthHeinemann: Oxford, 1994; pp 243–309. 9. Elliott, J. F.; Gleiser, M.; Ramakrishna, V. Thermochemistry for Steelmaking, Vol. II; Addison-Wesley: Reading, MA, 1963; p 406. 10. Muan, A.; Osborn, E. F. Phase Equilibria among Oxides in Steelmaking; Addison-Wesley: Reading, MA, 1965; p 28. 11. Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, 3rd ed.; National Bureau of Standards: Washington, DC, 1985. 12. Barin, I. Thermochemical Data of Pure Substances; VCH: Weinheim, Germany, 1989. 13. Treptow, R. S. J. Chem. Educ. 1996, 73, 51.
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