PHYS MET FERR ALLOY
PHYS MET FERR ALLOY MAT E 443
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This 15 page Class Notes was uploaded by Henriette Ernser on Saturday September 26, 2015. The Class Notes belongs to MAT E 443 at Iowa State University taught by Ralph Napolitano in Fall. Since its upload, it has received 31 views. For similar materials see /class/214475/mat-e-443-iowa-state-university in Materials Engineering at Iowa State University.
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Date Created: 09/26/15
Mm 443 e asmmshs ecmmz n a quotmum ChopTer 2 The recovery of iron from ore sham s m emu s h M 01 wtuh w WWW 01 w hm quoty s s New was quotem 5 mm mm sums mm samy meme 5mm may s emsquot m s M us My afsmitmg h we m M s em a mad m s m hm Wm mums mmwmqwem Introduction Aldwugh ebuhdem eerhbsuhd sh geheeeuy ex s s as an axxde he is mush mare s eb1e hen me e1hs sh under yphel amasphent sshdmshs The dxstavery af meQal smemhg h u w when me e1hs e 4 he he cambushan gases were alsa Canhxned Due s he buxld up af Cambushan gases is p m h me e1 smemhg was bsm Where he essenhal requuemehs were mm s be 1 hee n a reducmg atmasphere and m a mehlbeenhg she th h s realmahan he prathte af me s 1 memhg advanted mpxdly as a me eshhslsgy Aldwugh madem me ads are mush mare saphxshtated he Challenges af redutmg me e1s mm hex axxde s e es remsm essenhally he Mehlhs me ene1s generally favar hex axxde s e es m an e emmem temperatures and ash pressuresFar m h s xs emdehwy mspethan af he examemxc eat 2imFeeoz aZFeMOquot meat 1 n n Mat E 443 r Cumse Nutes 7 Chapter 2 RE Nupulittmu Table 21 Enthalpy and entropy of formation for common oxides of iron Rauction m n AH j AS I Fe 02 gt R90 1 1 264429 64 7 2Fe02 gt F6203 2 3 806665 2439 3Fe 202 gt F93 O4 3 4 1 092 861 298 6 The heat of reaction AH is relatively independent of temperature Of course the enthalpy alone is not sufficient for assessment of relative stability Under conditions of constant temperature and pressure we employ the Gibbs free energy The overall standard Gibbs free energy change for the reaction is given y AG AH 4A3 2 AGquotltO 3 AG vgto Plotting the free energy change as a function of temperature as shown 6 schematically in Fig21 we note that the yaxis intercept T0K gives AH and the slope of the line gives AS Such AH o TAS 0 a plot is known as an Ellingham Diagram We note also that the xaxis intercept indicates a condition where AG 0 and therefore indicates an quot AH0 equilibrium temperature Writing 2 in terms of the equilibrium constant K Flg39z39 139 SChemOhc pIOf Of AG ovs39 femperafure for an oxida on reacfion AG iRTInK 3 we see that the equilibrium temperature indicated by the xaxis intercept is for the condition K1 Considering now the case where mn1 as an example ie the rst reaction in Table 21 the oxidation reaction writben per mole of 02 is given as 2Fe 02 gt 2FeO 4 200 100 73909 0 and the equ111br1um constant 1s g1ven by 400 1000 2000 3000 4000 5000 a 1 200 K 7 5 Po2 300 400 where p02 is the equilibrium partial 500 pressure of ox en To examine more closely the effect of both temperature and 39600 atmosphere on reaction equilibria we plot the free energy change as a function of Fig22 AG vs femperafure for fhe temperature ie Eq 2 Note again that oxida on reac on 2FeO22FeO Mat E 443 e Cumse Nutes 7 Chapter 2 RE Nupulittmu the intercept at T0K gives the standard enthalpy change of the reaction and the negative of the slope gives the standard entropy change The point at which the line crosses the AG 0 axis gives the equilibrium temperature corresponding to K1 which indicates an equilibrium partial pressure of oxygen equal to unity In this case from 2 we compute the corresponding equilibrium temperature AH 2644 Teq 4068K 6 43 0065 Accordingly the plot in Fig22 shows that metallic iron will spontaneously oxidize at temperatures less than 4068K in one atmosphere of oxygen due to the negative value of AGquot Needless to say it would not be wise to choose this oxygen rich environment if one is interested in reducing iron ores to metallic iron We can easily reveal the effect of pm by computing the change in AG per mole associated with a change in pressure from one atmosphere to another pressure AG Tpoz AG 7 p0 1 00 p02 00 1 7 1 1 AGTp02 AH ms RT In In 7 3 O AGT p02 2 AH 7 TAS RTIn p02 9 Because we consider many oxidation 200 reactions simultaneously rather than 150 105 102 add the last term in 9 to each of the oxidation reactions and combining the 3 100 101 temperature dependence into a term of g 50 the form Rlnp02AS it makes more pOZ 10El sense to plot by the quantity RTlnp02 EA 0 on the Ellingham diagram 50 1 04 independently This practice was first 400 considered by Richardson and is 072 shown in Fig23 The constant quot150 073 pressure lines can be considered to 200 represent the effective AG 0 line for 0 1000 2000 3000 4000 the indicated partial pressure of TM oxygen Therefore the intersection of an oxidation reaction line with a given Fig2 3 The variofion Wifh femperafure of Richardson line indicates the Gibbs free energy change realized by equilibrium temperature for that faking one mole of O2 from I aim fo fhe particular value of p02 The effects of DdiCOled PIGSSUre dim both temperature and oxygen partial pressure are now easily recognized We have seen that decreasing the oxygen in our system is one way of promoting reducing conditions Of course an oxygenpoor environment could be achieved through vacuum methods but such practices would be very inef cient As our predecessors discovered reducing conditions can be effectively promoted by employing a gaseous species that is itself oxidized preferentially by the oxygen present For example consider a system of iron oxide FeO and Mat E 443 r Cumse Nutes 7 Chapter 2 RE Nupulittmu carbon monoxide gas in the presence of a partial pressure of oxygen ie FeO in a gaseous mixture of CO and 02 In this case the two relevant oxidation reactions are 2Fe o2 gt 2FeO AGquot 5233 0130T k 10 2co 02 a 2002 AG 5648 0174T k 11 These two reactions are plotted in Fig24 where their intersection indicates an equilibrium temperature of 8182 K The overall reaction written per onehalf mole 02 is given by FeO co 9 Fe 002 AG 130 0022T k 12 where the CO is oxidized and the FeO is 0 reduced to metallic iron The equilibrium temperature ie AG 0 is then 100 7 AH 180 qu 8182K 13 2200 7 AS 0022 O E 3 Interestingly 02 does not appear in the 79A39300 7 overall reaction However other gaseous species are present with the equilibrium 400 constant written as K pcoz pc0and with the free ener chan e iven b 2Fe 0 FeO gy g g y 39500 7 2co 0 2002 p a CO FeoCOFeCOZ AG 7RTnKRTnp 14 600 502 0 400 800 1200 1600 2000 T K Since AG 0 at T8182K we know that Fig24 Ellingham diagram showing Eqs1012 K E 1 pco In the same manner that the partial pressure of oxygen was handled earlier we can see the effect of the atmosphere by plotting lines of constant pcopcoz as shown on the Ellingham Richardson diagram in Fig25 Note here the point 11C where the 2COOz2COZ reaction intersects the free energy axis Analogous to the lines of constant P02 discussed previously lines originating at point C are for constant values of pcopcoz as indicated by the scale on the right side of the diagram Considering once again our equilibrium temperature of 8182K for the reaction in 12 we see that higher temperatures indicate lower values of pcopcoz at equilibrium Thus an atmosphere of pcopcoz1 is reducing with respect to FeO Similarly at lower temperatures pcopcoz1 is an oxidizing atmosphere with respect to Fe This illustrates the relationship between the requirements for reducing atmosphere and high temperature for the reduction of metallic iron Mat E 443 Course Notes GmpterZ RE Napolitano from the oxide state The required temperature for smelting decreases with an increase in the reducing power of the gas as indicated by the pcopcoz ratio Fundamental Principles The utilization of metallic iron as an engineering material requires a process for the reduction of iron from iron ore In principle this is done by exposing the various oxidebased ores to a reducing environment and by providing a means for separation of the reduced metal from the residual oxides The essential reaction for the reduction of Fe from ore is FeO RO e RO1 15 x y u v v LIV V a 0 7 III396 Io 5 Ioquot HZIHZO s 1 s 5 4 3 W602 Io Io I0 6 I0 I0 0393 lo0 0 2Iol loz 10 Ioa l04 Io m6 I0 Io a 25 102 III mega l Vlo H 3 Io m IO C H wIz no I0 539 l05 los Io a E lov 05 IO 200 no I05 II31 Io39s 5 Io9 loa ozo 2 7250 CHANGES OF STATE ELEMENT 0me lom oa W22 quot MELTING POINT M M 2 BOILING POINT BI H suaLIMATIoM POINT s s l0 mm ID Z 1 I I I I I I I 1 loll was 250 500 750 I000 Izsa I500 I750 20 o TEMPERATURE 390 loquot v ABSOLUTE ZERO 02 390 A44 moi lols 0I5 Ion AAA Fm I I Haw Io 6 0395 I0 Iol losn Io m mi Io p02 am lovso 040 was was Tu W32 Fio25 Free enerov of formation of oxides of the standard where the ore is reduced fully to Fe and the reducing agent R is oxidized by one state If we write this with respect to one mole of iron production we have lerxoy Lro a Fe 1 Roy 1 16 X UVX VX ML E 443 a CamacNotes acmth R E Napolrmno The selectLon of carbon as a redLLang agent Ls essentLal to the blast furnace process for reasons L L L L L L L L L L L L L of carbon rnonoXLde The redLLctLon of the panchal ores hernatLte and magnetlte accordmg to L L L gpepowgco apegcop mamaer 17 1 4 4 F9304 lECO gtFe L Eco AHrVS 90 k 18 Both of these reactLons are exotherrnLc suggestmg spontaneoLLs redLLctLon of the ores The two L L L L LL L 20239 49 k L 9quot 0 wt RT 19 L L L L L y L L L L L lel be shown Ln the next sectLon the net reactLon Ls endotherrnLc Neglectmg the ternperatLLre dependence of lH Lt Ls easlly shown from 2 and 3 t at 20 where Lt Ls meedLately emdent that anreaslng the reactlon temperature of an endothermlc process lel force the reactLons further toward co pletL To tth oLnt no rnentLon has been rnade of reactLon kmetlcs Because the redLLctLon of Lron lnvolves solLd ore reactants sttLLsLon of the redLLang gas Ls an Lrnportant factor ReactLon t th t l l H the ste and shape of the solLds Ln the reactor In adstLon we have assLLrned an eanronrnent of pLLre carbon onoXLde but have proLLLded no rneans for Lts LntrodLLctLon These LssLLes are consLdered Ln the followmg sectLons The Modern Blast Furnace Process The blast furnace process was developed to recover rnetalch Lron from rarLoLLs oXLde res Lron reqLLLres the heatmg of large Fig26 BGST furnaces aft5 Sfee Gary Works Mat E 443 r Cumse Nutes 7 Chapter 2 RE Nupulittmu amounts of material to relatively high temperatures a major contributor to the cost is the energy for heating ie the heating fuel In addition the reduction reactions shown above indicate that a significant amount of reducing gas is required This is another major expense A key to the success of the blast furnace as an industrial process is the use of carbon in the form of coke to satisfy both of these requirements at once Rather than injecting the reducing gas into an externally heated charge of ore coke is physically mixed with the ore and blasted with hot air The coke is readily oxidized in accord with the following equation C 02 gt co AH 1105 k 21 This is the primary combustion reaction in the blast furnace where both heat 1105 kImol and reducing gas C0 are evolved Thus by combusting the fuel inside the reactor very efficient heating is achieved and the necessary reactant gas is produced and automatically mixed with the ore An additional effect of using coke as a fuel and reacting it with hot air is that carbon will react with any moisture in the blast air evolving H2 which is another useful reducing gas CH20 gtCOH2 AH1314 k 22 This reaction absorbs heat as indicated by the positive value of AH The oxidation of carbon through this reaction however produces two moles of reducing gas for each mole of carbon reacted The H2 gas acts in a manner similar to C0 compare to eqs 3 and 4 in the reduction of Fe gFeZO3 H2 aFegH20 AH4895 k 23 1 4 4 EFe3O4 H20 a Fe H20 AH 5104 k 24 To best utilize the action of the reducing gases the reactor is built in a counter ow stack configuration and the primary combustion is performed by introducing the blast air into the lower portion of the stack The evolved gases then rise through the stack where they preheat the charge and reduce much of the ore according to eqs17182324 These are termed secondary reduction reactions The primary reduction reactions occur in the lower furnace where the blast air is introduced FeOCO aFeC02 AH 1611k 25 FeOH2 aFeH20 AH2510k 26 These are sometimes referred to as direct reduction where eqs17182324 are referred to as indirect reduction Note here that the biproducts of these direct reduction processes are C02 and H20 Each of these further reacts with carbon from the coke to produce more reducing gas The reaction of H20 is shown in eq 8 and the reaction of C02 with carbon is given by C co2 co AH 17259 k 27 Two clear reaction sequences are initiated by the primary combustion and emerge here for primary reduction in the lower furnace where the blast air is injected These are i the reduction by C0 as described by eqs 25 and 27 and ii the reduction by H2 as described by eqs 22 and 26 These are summarized below where it is shown that the sums of these two reaction sequences are identical Mat E 443 r Cumse Nutes 7 Chapter 2 RE Nupulittmu Table 22 Direcf Reduc on Reac ons in Lower Furnace Reduction 171 CO AH Reduction 171 H2 AH FeOCO gtFe002 161 k FeOH2 gtFeH20 251k C 002 a CC 1726 k C H20 gt CC H2 1314k FeOC FeCO 1565 k FeOC FeCO 1565 k Note that the direct reduction reactions absorb a great deal of heat These are necessary reactions however because they generate the CO gas required for the indirect reduction reactions It has been found that the most ef cient process is one where approximately one third of the iron is reduced through eqs 25 and 26 with the remaining iron being reduced through indirect reduction This requires that the furnace be designed for effective use of the gases produced in the direct reduction stage The basic design of the blast furnace is discussed in the next section Blast Furnace Design and Operation A blast furnace iron production plant includes four fundamental equipment groups i the raw materials handling equipment which is used for measuring and mixing the correct proportions of iron ore coke and limestone and hoisting them to the top of the blast furnace ii the blast furnace proper where the ore is reacted to produce iron iii the cast house where hot metal tapped from the furnace is poured into a transfer ladle and iv the air handing facility which produces and distributes the hot blast air A general overview of the blast furnace plant is shown in Fig27 but in the subsequent discussion we are concerned only with the blast furnace reactor The blast furnace reactor consists of three general components These are i the reactor vessel where the iron ore is reacted with the coke air and ux to produce iron slags and reaction gases ii the blast furnace top where the ore coke and ux are charged into the vessel and where the reaction gases are collected and iii the inlet and outlet system where blast air is injected and where slag and iron are tapped from the vessel The reactor vessel itself can be further divided into three parts The upper portion of the reactor is called the stack This is where the furnace charge is preheated by counter owing reaction gases from below and where partial reduction occurs according to eqs 17182324 This portion of the vessel is conical in shape and is tapered so that it is more narrow on the top than at the bottom The lower portion of the reactor is known as the bash This section is also conical in shape but is inverted so that vessel diameter is largest at the boundary between the stack and the bosh The bosh is where melting starts and where primary coke combustion takes place Iron is reduced here in accord with the reactions in Table 22 At the bottom of the reactor is a thick refractory basin called the hearth where the molten metal and slag collect The air inlet and tapping components are also located at the lower portion of the reactor Blast air is injected through ports in the sidewalls called tuyeres located above the hearth in the lower portion of the bosh Below the tuyeres is the slag notch where the slag is tapped from the furnace into a slag ZVIat E 443 Course Notes GmpterZ RE Napolitano ladle At the base of the hearth is the iron notch where iron is tapped from the bottom of the molten pool An overview of reactions which occur in the blast furnace reactor are quIunarized in Fig28 Ol39l39 bridge P lu IM39UIIH39I B Orclmusfercar Q Int hluxllinulnfurlmtc I On smmge urd I Sunk lint R quot h rr D Slutkhmlse ark R uluvlinrluthickener Dl Ore and limestone bins JH Bush IL I39m WI 3 him hi 110 T cm H tum prvcipimmr 11 gummy s 1mnlllukxlmImclmrnvr C luurth T I lol lukm t39UlIIICL l ion from slow F 3 Bustle pipe U Slow C l L l Imn notch Ul hlslnn39nvr H S pmulhellhuist luglzu IL l mnlumimlchamber I plu idge L Iml hon L hoclmruhxun yur j lilusl furnace l lmn lrnugh quot lixlmIN gas line lnshwk l liluonlurvalve 41 lugskimmer V Tnkl hlns l linv Irnm lwhm39vr 1A2 Gusupluke IAS Imm uuner X Surplm Lauliln39 143 R ving hopper M Y SlmL II39HH m39v mlw lilnes lune 14 Dulrxlmtur N a Z jilxluuuu mm 0 I Du culcl39cr Fig 27 Overview of o blast furnace plonf Furnace Charge ore coke limestone Exhaus r 639 Gas Dus r Removal Ho r Blas r Air Fig 28 A schema c summan of fhe blasf furnace process Mat E 443 r Cuurse Nutes r Giapter Z RE Napulitanu Raw Materials for the Production of Iron The charge of the blast furnace is principally composed of iron ore to be reduced to metallic iron coke which serves as a fuel and as a source of reducing gas and limestone which is a slagpromoting ux Historically the availabilty of these essential ingredients have in large part contributed to the feasibility of iron production in various locations around the globe Iron Ores quotThe said are is placed in a strung lye then after it is taken nut and put on a well burning re it develops the culur ufthefmnusities which issuefrum it One bluws on it very gently with a small belluws and its evilness is discerned in the different culurs uf copper that appear there It has been known from the earliest days of iron smelting that the properties of the ore are critical to the quality and utility of the iron or steel produced With iron comprising more than 4 of the earth s crust there is a virtually limitless supply of ironbearing minerals distributed widely around the planet Despite this abundance there is a much smaller quantity of materials with suf cient quality to merit commercial mining These valuable minerals are termed ores and generally include oxides carbonates sulphides and silicates Some common ores of iron are listed in Table 23 Table 23 Common ores used for the production of iron Oxide Ores Stoicneometn Iron Content wt M dgnetite F6304 72 36 Hem dtite Fe203 69 94 llmenite FeTiOa 36 8 Limonite FeOOH HFeO2 62 85 The principal factors that contribute to the metallurgical quality of an iron ore are location iron content water content and impurity content Because of the tremendous costs involved with transporting raw materials the smelting of iron is generally carried out in proximity to naturally occurring are deposits A listing of major ore deposits along with an assessment of the quality factors is given in Table 24 Principal domestic US ore deposits are shown in Fig 28 Iron ore in the asmined condition is generally not in a suitable physical form to be used as a blast furnace feed These ores must be processed for the requisite chemical andor physical characteristics Such processing is known as beneficiation The methods of ore bene ciation include crushing screening blending grinding 39 U quot 39 g and 00 39 0 Examples of re ned ores are shown in Fig29 Mm 443 rCourstNottsrompth RE Nayokmm Table 24 Principal from Ore reseNes and pOTen 39GOres mw at Audude Addmmul mrlluiumdmxmwu mm Dquot Mummr Bureau Mmmmol PaLml mm w emu lrmcomm meTvmr lamCmKum WMWW Caumh mm A mm w u x nMUV a um mm as mm 215 omnm Sm mmm 4aquot 7 me use 7 2m 5410 lass mu m Jew 7 11m 77 1 31 mm Vulwur39umym u mm M 7 Mammy mm 17 x w r Luxemme m 15m 70 LSJJ mm m we 7 7 and m m 1115 mm 2151 m1 am Tm m m 3051 umaxmmquot 1 m mum r65 7 um zuw uig nz ya man Cud mh lu a no mm an M Human m Tu Afnm gm rm 2 4 m mm m m a w lt v mm 7 Wm mm um am 573 mum M7 me m w mu m m m1 3 7 n 4w 4 s 7 7 aw um 4 Lang 7 7 may u mm ms m mu 1 57 an m sue m 11 w w m mu m a 15 m y hulrdm 1507 Muse w Mankxlvmn m was 7 xwzmma s a 5 MatEm 4mm row as Nam Fig28 Principal ms ore depos s MeTalurgfca Coke The sexemhh of EDkE as a blast furnace fuel Ts essentxal to the eehhhmm pmaumhh of mm Constituents m Dal are eamhh aha hydngn but smallEr amsuhts of BxygEn mtrugen and sulfur are assh present Dunng the fumallun pmeess vanuus margamts may also become ka2 must orgaan matenass many of the constxtuents of Dal are hughly ymatue These temperatures The eyhmtmh of these gases leaves behmd a EarbDnaEEDus resxdue mth a pumus m ta AkR V n wxyxeld k h a h the pmpemes of the EDkE may vary greatly For thxs reason coals are gEnEmHy blended to pmvxde Consistent pErf rmanEE aha chemxstry Early EDkE pmaumhh mvulved heatth the EDkE m a ehhtmuea atm sphem mth an admitted in the ehmbustmh of Vulatxles The combustmn assh pmvxded heat to further a tu t ZVIat E 443 Course Notes GmpterZ RE Napolitano Fig29 Four common forms of iron ore blast furnace chdrge hydrocarbon fuels however the practice of internal combustion of volatiles gave way to the by product coking process where all of the evolved gases are recovered Some of these may be returned to the furnace for heating but the remainder may be utilized for other industrial or commercial applications Approximately 35by weight of the initial coal charge is recovered as gases and processed The typical yield from one ton of US coking coal is shown in Table 25 Table 25 Typicdl Coking Products Coking product Bldst furndce Collte Collte Breeze Coke oven Gds Tdr Ammonium Sulfdte Ammonid Liquor Light Oil Yield per ton of cool 1200 1400 lb 100 200 lb 9500 1 1500 ft3 8 12 gal 20 28 lb 15 35 gal 25 4 gal Mat E 443 r Cumse Nutes 7 Chapter 2 RE Nupulittmu Fluxes and Slags As discussed in the preceding sections the smelting of iron involves two fundamental processes The first is the reduction of iron from its ore state to a metallic state The second is the physical separation of the metallic iron from the resulting mechanical mixture Indeed the utilization of uxes and slags is essential to the successful performance of these processes Fluxes and slags are cooperative in nature and must be considered in concert The primary function of a ux is to render the various refractory constituents Within the ore more easily fusible so that they may be removed from the mixture promoting the smelting process As the reduction of iron proceeds various other elements which are simultaneously reduced may react With the iron Accordingly the secondary function of a ux is to provide a material With which these elements will react preferentially leaving the iron unaffected The action of a ux will generate a fused product of refractory ore constituents and oxidized metallic impurities called a slag Dissolved in such a slag these undesirables may be easily separated and removed from the melt Fluxes are generally characterized as acid basic or neutral Blastfurnace processes utilize basic uxes typically limestone CaCOs or dolomite CaMgCOs or a mixture of the two The desired mixture is determined by the amount of sulfur that must be removed With limestone forming the more effective slag for sulfur removal Pig iron may be re ned into steel using acid or basic processes Silica SiOz in the form of sand or gravel may be used as an acid ux In many acid processes however the silica refractories lining the vessel provide adequate uxing and no additions are required Alumina is another common material found in the refractory components of the blast furnace and steelmaking furnaces For this reason it is often present in the slag and may function as an acid ux or basic ux
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