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xiiContributor contact detailsChapter 5Professor A. K. LahiriDepartment of MetallurgyIndian Institute of ScienceBangalore 560012IndiaCarnegie Mellon UniversityPittsburghPA 15213USAE-mail: metakl@metalrg.iisc.ernet.inChapter 11Dr G. Engberg*MIK Research AB (MIKRAB)TeknikdalenForskargatan 3SE-781 27 BorlangeSwedenChapter 6Professor Emeritus K. MukaiDepartment of Materials Science andEngineeringKyushu Institute of TechnologySensui-ChoTobata-kuKitakyushu 804 8550JapanE-mail: hiro mukai@nifty.comChapter 8Professor O. N. MohantyResearch and Development ServicesThe Tata Iron and Steel Company Ltd11T Kharagpur-721302IndiaE-mail: inChapter 9Professor Du SichenDepartment of Materials Science andEngineeringRoyal Institute of TechnologySE-100 44 StockholmSwedenE-mail: du@mse.kth.seChapter 10Professor A. W. CrambDepartment of Materials Science andEngineeringE-mail: cramb@andrew.cmu.eduE-mail: goran.engberg@mikrab.seDr L. KarlssonDalarna UniversityChapter 12Dr F. Lemoisson* and Professor L.FroyenPhysical Metallurgy and MaterialsEngineering SectionKatholieke Universiteit LeuvenKasteelpark Arenberg 44BE 3001 HeverleeBelgiumE-mail: royen@mtm.kuleuven.ac.beChapter 13Professor T. EmiTakasu 5-1B1905Urayasu-shiChiba 279-0023JapanE-mail: t-emi@yb3.so-net.ne.jp

PrefaceMetallurgy refers to the science and technology of metals. The subject area canbe considered as a combination of chemistry, physics and mechanics withspecial reference to metals. In later years, metallurgy has expanded intomaterials science and engineering encompassing metallic, ceramic andpolymeric materials.Metallurgy is an ancient subject linked to the history of mankind. Thedevelopment of civilisations from stone age, bronze age and iron age can bethought of as the ages of naturally available ceramic materials, followed by thediscovery of copper that can be produced relatively easily and iron that needshigher temperatures to produce. These follow the pattern of the Ellinghamdiagram known to all metallurgists. Faraday introduced the concept ofelectrolysis which revolutionised metal production. Today, we are able toproduce highly reactive metals by electrolysis.The prime objective to produce metals and alloys is to have materials withoptimised properties. These properties are related to structure and thus, physicalas well as mechanical properties form essential parts of metallurgy. Properties ofmetals and alloys enable the choice of materials in production engineering.The book, Fundamentals of Metallurgy is a compilation of various aspects ofmetallurgy in different chapters, written by the most eminent scientists in theworld today. These participants, despite their other commitments, have devoteda great deal of time and energy for their contributions to make this book asuccess. Their dedication to the subject is admirable. I thank them sincerely fortheir efforts.I also thank Woodhead Publishing Company for this initiative which bringsthe subject of metallurgy into limelight.Seshadri SeetharamanStockholm

Part IUnderstanding the effects of processingon the properties of metals

1Descriptions of high-temperaturemetallurgical processesH Y S O H N , University of Utah andS S R I D H A R , Carnegie Mellon University, USA1.1IntroductionMetallurgical reactions take place either at high temperatures or in aqueoussolutions. Reactions take place more rapidly at a higher temperature, and thuslarge-scale metal production is mostly done through high-temperature processes.Most metallurgical reactions occurring at high temperatures involve aninteraction between a gas phase and condensed phases, which may be moltenliquids or solids. In some cases, interactions between immiscible molten phasesare important.High-temperature metallurgical reactions involving molten phases are oftencarried out under the conditions of near equilibria among all the phases; othersuch reactions proceed under the control of interphase mass transfer withequilibria at interphase boundaries. Reactions involving gas solid contact alsooften take place under the rate control of mass transfer with chemicalequilibrium at the interface, but the chemical kinetics of the heterogeneousreactions are more often important in this case than those involving moltenphases. Even in this case, mass transfer becomes increasingly dominant astemperature increases. The solid phases undergo undesirable structural changes,such as fusion, sintering, and excessive reduction of internal porosity andsurface area, as temperature becomes too high. Thus, gas solid interactions arecarried out in practice at the highest possible temperatures before theseundesirable changes in the solid structure become damaging. In the case of hightemperature oxidation, the structure of the product oxide determines the masstransport of gases and ions.The treatment of metals in their molten state, e.g. refining and alloying,involves reactions between the melt and a gas phase or a molten slag. Interfacialreaction kinetics, mass transport in the molten or gaseous phase becomesimportant. The production of metals and alloys almost always involvessolidification, the rate of which is often controlled by the rate of heat transferthrough the mold.

4Fundamentals of metallurgy1.2Reactions involving gases and solidsSince metals occur in nature mostly as compounds (minerals), the first step inthe utilization of the naturally occurring sources is their chemical separation intoelemental forms. More often than not, the first reaction in this chemicalseparation step involves an interaction between the solid-phase minerals and areactant gas.1.2.1 Reduction of metal oxides by carbon monoxide, carbon,or hydrogenMetal oxides are most often reduced by carbon or hydrogen. The reason whyreduction by carbon is treated in this section on gas solid reaction is because theactual reduction is largely effected by carbon monoxide gas generated by thereaction of carbon dioxide with carbon, when carbon is used to reduce metaloxides in solid state. These reactions can in general be expressed as follows:MxO (s) CO (g) xM (s) CO2 (g)yC (s) yCO2 (g) 2yCO (g)Overall, MxO (s) yC (s)(1.1)(1.2) xM (1 ÿ y)CO2 (2y ÿ 1)CO(1.3)The amount y, which is determined by the pCO2 pCO ratio in the product gasmixture, depends on the kinetics and thermodynamics of the two gas solidreactions (1.1) and (1.2). In many systems of practical importance, reaction (1.1)is much faster than reaction (1.2), and thus the pCO2 pCO ratio approaches theequilibrium value for reaction (1.1). The overall rate of reaction (1.3) is thencontrolled by the rate of reaction (1.2) taking place under this pCO2 pCO ratio(Padilla and Sohn, 1979).The reduction of iron oxide is the most important reaction in metalproduction, because iron is the most widely used metal and it occurs in naturepredominantly as hematite (Fe2O3). The production of iron occupies more than90% of the tonnage of all metals produced. The most important reactor for ironoxide is the blast furnace, in the shaft region of which hematite undergoessequential reduction reactions by carbon monoxide (Table 1.1).Table 1.1Reaction3Fe2O3 CO 2Fe3O4 CO2Fe3O4 CO 3FeO CO2FeO CO Fe CO2Equil. CO/CO2ratio at 900 ëCHeat generation(900 ëC)00.252.310.3 kcalÿ8.8 kcal4.0 kcal(1.4)(1.5)(1.6)

Descriptions of high-temperature metallurgical processes5In the blast furnace, the solid charge flows downward, and the tuyere gas witha high CO/CO2 ratio flows upward. Thus, the tuyere gas first comes into contactwith wustite (FeO), the reduction of which requires a high CO/CO2 ratio, as seenabove. The resulting gas reduces magnetite (Fe3O4) and hematite (Fe2O3) on itsway to the exit at the top of the furnace.The equilibrium percentage of CO in a mixture with CO2 is shown in Fig. 1.1as a function of temperature. Again, it is seen that the equilibrium concentrationof CO for the reduction of hematite to magnetite is essentially zero; i.e., CO iscompletely utilized for the reduction. The equilibrium content of CO for thereduction of Fe3O4 to FeO and that of FeO to Fe depend on temperature. It isalso noted that wustite is a non-stoichiometric compound FexO with an averagevalue of x equal to 0.95 in the temperature range (approximately 600 ëC 1400 ëC) of its stability. The actual value of x and thus oxygen content dependon temperature and CO/CO2 ratio, as illustrated by the curves drawn within thewustite region in Fig. 1.1. Furthermore, the CO/CO2 ratio is limited by theBoudouard reaction given by eqn (1.2) and shown as a sigmoidal curve in Fig.1.1 (for 1 atm total pressure without any inert gas). Thus, the reduction reactionsindicated by the dashed lines to the left of this curve are thermodynamically notfeasible. (In practice, however, reduction by CO to the left of the Boudouardcurve is possible because the carbon deposition reaction (the decomposition ofCO) to produce solid carbon is slow.)1.1 Equilibrium gas compositions for the reduction of iron oxides by carbonmonoxide. (Adapted from Evans and Koo, 1979.)

6Fundamentals of metallurgyThe reduction of iron oxide by hydrogen is important in the production ofdirect reduced iron. This method of iron production is gaining increasingsignificance as an alternative route to the blast furnace technology with themany difficult issues facing the latter, the most important being the problemsrelated to environmental pollution and the shear size of the blast furnace. Directreduction technology for iron encompasses the processes that convert ironoxides into metallic iron in solid state without going through a molten phase. Inthis technology, iron-bearing materials are reduced by reacting with reducingsubstances, mainly natural gas or a coal, at high temperatures but below themelting point of iron. The product, direct reduced iron (DRI), is a porous solid,also known as sponge iron. It consists primarily of metallic iron with someunreduced iron oxides, carbon and gangue. Carbon is present in the range of 1 4%. The gangue, which is the undesirable material present in the ore, is notremoved during reduction as no melting and refining take place during thereduction process. The main usage of DRI products is in the electric arc furnace(EAF). However, due to its superior characteristics, DRI products have foundtheir way into other processes such as blast furnaces, basic oxygen furnaces andfoundries. Globally, DRI comprises about 13% of the charge to the EAF (Kopfleet al., 2001). Nowadays, the percentage of crude steel produced by BOF isapproximately 63%, while that of EAF is about 33%, and the balance 4% ismade up of the open hearth (OH) steel (International Iron and Steel Institute,2004). However, the contribution of EAF to the world crude steel output isexpected to increase to reach 40% in 2010 (Gupta, 1999) and 50% in 2020(Bates and Muir, 2000).Direct reduction technology has grown considerably during the last decade.The main reasons that make this technology of interest to iron and steel makersare as follows:1.2.3.4.5.6.Shortage, unpredictability and high price of scrap.The movement of EAF producers into high quality products (flat products).High capital cost of a coke plant for the blast furnace operation.Desire of developing countries to develop small steel industries andcapabilities.Availability of ores that are not suitable for blast furnace operation.Necessity for increasing iron production within a shorter time frame.There have been major developments in direct reduction processes to cope withthe increasing demand of DRI. DRI production has increased rapidly from 0.80million tons per year in 1970 to 18 million tons in 1990, 44 million tons in 2000,and 49 million tons in 2003 (MIDREX, 2004). The worldwide DRI production isexpected to increase by 3 Mt/y for the period 2000 2010 (Kopfle et al., 2001).Reduction of iron bearing materials can be achieved with either a solid orgaseous reductant. Hydrogen and carbon monoxide are the main reducing gasesused in the direct reduction' (DR) technology. These gases are largely

Descriptions of high-temperature metallurgical processes7generated by the reforming of natural gas or the gasification of coke/coal. Inreforming, natural gas is reacted with carbon dioxide and/or steam. The productof reforming is mainly H2 and CO, whereas CO is the main product from coalgasification. Reduction reactions by reducing gases take place at temperatures inthe range of 850 ëC 1100 ëC, whereas those by solid carbon occur at relativelyhigher temperatures of 1300 ëC 1500 ëC. Carburization reactions, on the otherhand, take place at relatively lower temperatures below 750 ëC. For reformingreactions, the reformed gas temperature may reach 950 ëC for the stoichiometricreformer, and 780 ëC in the case of a steam reformer.Various reactions important in direct reduction processes are listed below.Reduction reactionsFe2O3 (s) 3CO (g) 2Fe (s) 3CO2 (g)Fe2O3 (s) 3H2 (g) 2Fe (s) 3H2O (g)FeO (s) CH4 (g) Fe (s) 2H2 (g) CO (g)3Fe2O3 (s) 5H2 (g) 2CH4 (g) 2Fe3C (s) 9H2O (g)C (s) CO2 (g) 2CO (g)(1.7)(1.8)(1.9)(1.10)(1.11)Reforming reactionsCH4 (g) 0.5 O2 (g) CO (g) 2H2 (g)CH4 (g) H2O (g) CO (g) 3H2 (g)CH4 (g) CO2 (g) 2CO 2H2 (g)2CH4 (g) H2O (g) 0.5 O2 (g) 2CO (g) 5H2 (g)CH4 (g) C (s) 2H2 (g)H2O (g) C (s) CO (g) H2 (g)H2O (g) CO (g) H2 (g) CO2 rizing reactions3Fe (s) CO (g) H2 (g) Fe3C (s) H2O (g)3Fe (s) CH4 (g) Fe3C (s) 2H2 (g)3Fe (s) 2CO (g) Fe3C (s) CO2 (g)(1.19)(1.20)(1.21)Direct reduction (DR) processes have been in existence for several decades. Theevolution of direct reduction technology to its present status has included morethan 100 different DR process concepts, many of which have only been operatedexperimentally. Most were found to be economically or technically unfavorableand abandoned. However, several were successful and subsequently improved todevelop into full-scale commercial operations. In some instances, the bestfeatures from different processes were combined to develop improved processesto eventually supplant the older ones.Direct reduction processes may be classified, according to the type of thereducing agent used, to gas-based and coal-based processes. In 2000, DRIproduced from the gas-based processes accounted for 93%, while the coal-based

8Fundamentals of metallurgyprocesses produced 7%. Gas-based processes have shaft furnaces for reducing.These furnaces can be either a moving bed or a fluidized bed. The two mostdominant gas-based processes are MIDREX and HYL III, which combined toproduce approximately 91% of the world's DRI production. Fluid-bedprocesses, by contrast, have recently received attention, because of its abilityto process fine iron ores. These processes are based either on natural gas or coal.A list of the processes together with their relevant characteristics is given inTable 1.2 (MIDREX, 2001).The gaseous (or carbothermic) reduction of nickel oxide, obtained by deadroasting of nickel sulfide matte or concentrate, is an important intermediate stepfor nickel production. In the Mond process, crude nickel is obtained this waybefore undergoing refining by carbonylation. Crude nickel has sometimes beencast into anodes and electrolytically refined. Nickel laterite ores are reduced bycarbon monoxide before an ammoniacal leach. Nickel oxide reduction reactionsare simple one-step reactions, as follows:NiO (s) H2 (g) or CO (g) Ni (s) H2O (g) or CO2 (g)(1.22)Both reactions have negative Gibbs free energy values. For hydrogen reductionit is ÿ7.2 and ÿ10.3 kcal/mol, respectively, at 600K and 1000K. For reductionby CO, the free energy values are ÿ11.1 kcal/mol at both 600K and 1000K. Thethermodynamic data used here as well as below were obtained from Pankratz etal. (1984). The corresponding equilibrium ratio pH2 pH2 O is 2.4 10ÿ3 and 5.6 10ÿ3, respectively, at 600K and 1000K, and the equilibrium ratio pCO2 pCO is9.5 10ÿ5 and 3.7 10ÿ3, respectively, at the same temperatures. Therefore,the reactant gases are essentially completely consumed at equilibrium in bothcases. These reduction reactions are mildly exothermic, the standard enthalpy ofreaction ( Hrë) being ÿ2 to ÿ3 kcal/mol for hydrogen reduction and ÿ11.2 toÿ11.4 kcal/mol for reduction by carbon monoxide.Zinc occurs in nature predominantly as sphalerite (ZnS). The ZnS concentrateis typically roasted to zinc oxide (ZnO), before the latter is reduced to producezinc metal by the following reactions (Hong et al., 2003):ZnO (s) CO (g) Zn (g) CO2 (g)C (s) CO2 (g) 2CO (g)(1.23)(1.24)ZnO (s) C (s) ! Zn (g) CO (g)(1.25)The overall reaction is essentially irreversible ( Gë ÿ12.2 kcal/mol at1400K) and highly endothermic ( Hë 84.2 kcal/mol at 1400K) and thegaseous product contains a very small amount of CO2 at temperatures above1200K (Hong et al., 2003). It is also noted that this reaction is carried out abovethe boiling point of zinc (1180K), and thus zinc is produced as a vapor mixedwith CO and the small amount of CO2 from the reaction. Zinc is recovered bycondensation. Zinc vapor is readily oxidized by CO2 or H2O (produced when

Table 1.2 Characteristics of some DR processesProcessBuilderMIDREXHYLFINMETCIRCOREDIRON rgiREDSMELTSMS DemagIRON DYNAMICS elletLump/pelletFinesFinesFinesDRI/HBIDRI/HBIH

9.1 Introduction 369 9.2 Overview of process design 369 9.3 Thermodynamics and mass balance 375 9.4 Kinetics – mass transfer and heat transfer 385 9.5 Optimization of interfacial reactions 387 9.6 Micro-modelling 393 9.7 Conclusions 396 9.8 References 396 10 Solidificationand steelcastin