Iron Resources and Direct Iron Production

Direct reduction (DR) is defined as the production ofmetallic iron by the reduction of iron ore (iron oxide)below the melting temperature of any materials in-volved. The product of a DR process is called directreduced iron (DRI). Because of its low level ofimpurities compared to scrap, DRI is used mainly as ahigh quality feedstock in steelmaking, primarily in theelectric arc furnace (EAF).

The most common method of converting iron ore tometallic iron utilizes a blast furnace (BF). This is acountercurrent reactor in which iron ore is reduced bygas in the shaft, and melted (smelted) in the hearth toproduce pig iron. Approximately 93% of the worldsiron is produced in this way. However, conditions inthe steel industry have changed dramatically over thepast 25 years, such that for new capacity, alternativeprocesses may be preferred. Some new smeltingreduction processes for pig iron have been developed,existing DR processes have been improved, and newDR processes developed. Compared to the BF, DRprocesses have lower capital requirements, producefewer emissions, and can use a wider choice of fuels.

Commercial production of DRI began in the 1950s,but did not achieve significant growth until the 1970s.World DRI production was 31Mt in 1995, andreached 43Mt in 2000. This is about 7.5% of total ironproduction (US Geological Society [USGS] 2000).The reason for the rapid increase in DRI productionwas to meet the demand for a high purity supplementfor ferrous scrap for EAF steelmaking. Steel producedby the EAF accounted for nearly 40% of the worldproduction of 830Mt in 2000.

1. Raw Materials

The principal raw materials for DRI production areiron oxide concentrates, iron oxide pellets, and naturalgas. A few DR processes that use rotary kilns or rotaryhearth furnaces use solid hydrocarbon reductants,such as lignite and coal. Recently, the offgas from theCOREX direct smelting furnace has been adapted foruse in DRI production. Continued interest is expectedin DR processes based on natural gas, but insome locations, coal-based DR processes may offeradvantages.

1.1 Iron Oxide

Available sources of iron oxide include high-gradelump ore, beneficiated iron ore fines, iron ore pellets,and agglomerates from dusts produced by the BF,basic oxygen furnace, and the EAF. Most DRI isproduced in shaft furnaces, which require a uniform-sized coarse feed. Due to the high gas velocities andabrasive conditions in shaft furnaces, fine particles arenot suitable as charge materials. They tend to be

carried out with the gas stream, from which they mustbe collected and recirculated. Fluidized bed DRprocesses are exceptions. Shaft furnaces use pellets(produced in the same way as pellets for the BF), orlump ore. Raw material for pellets is produced bycrushing and grinding low-grade iron orestypicallyof the taconite class and finer than 325 mesh(0.044mm)and magnetically separating the ironoxide (magnetite, Fe

$O

%) from the siliceous gangue.

The fine particles are reconstituted into moist pelletsabout 1cm in diameter, and then indurated by heatingto temperatures approaching 1300C. This is sufficientto bring about complete oxidation to recrystallizedhematite (Fe

#O

$).

There are some key differences in the pellet chem-istry for DRI versus BF use. In DRI production, theprimary chemical change is the removal of oxygen andthe addition of some carbon; the other constituentsremain with the DRI. In smelting, the formation of aslag allows substantial removal of the ore contamin-ants. For this reason, the iron content of DRI pelletsshould be as high as possible and preferably " 67%.Pellet reducibility, strength, and swelling specificationsare similar to those of BF pellets. Coal-based processeshave the potential disadvantage of contributing coalash oxides to the product.

1.2 Natural Gas

Natural gas-based DR processes account for about92% of worldwide production of DRI. Natural gasconsists primarily of methane (CH

%), together with

small amounts of other hydrocarbons, nitrogen, andcarbon dioxide. Natural gas cannot be used directly inthe reduction of iron ore because it decomposes toform soot at a temperature below that which ironoxide can be reduced. Natural gas is used in three mainways: first, as a feedstock for producing the reducinggas, second as a fuel for supplying the necessary heat inthe furnace and gas reformer, and third as a coolantand carburizing agent for freshly-prepared DRI. Amajor constraint on the specification for natural gas isits sulfur content; if above 10ppm, it can deactivatesome types of reformer catalyst. Techniques areavailable to remove sulfur if necessary.

1.3 Coal

A wide variety of coals are suitable for producingreducing agents for DR processes, but coal itself is notused to reduce iron oxide. When a coal}ore mixture isheated, the coal devolatilizes to a hydrocarbon-richgas and char. The gas is burned above the bed toprovide some of the necessary heat, and a supplemen-tal fuel, such as natural gas, oil, or pulverized coal isused to provide the rest. The char becomes an effectivereducing agent only above about 950C. Some of thedesired specifications for coal are low ash and sulfurcontent, high char reactivity, and high ash fusion

1

Iron Resources and Direct Iron Production

temperature. Coal can also be used as a feedstock forthe production of reducing gas for gas-based proces-ses. Coal is oxidized in a slagging gasifier, and the hotoffgas used to reduce pellets in a shaft furnace.

2. Principles, Operations, and Products

The reduction of iron ore is accomplished by a series ofgas-solid reactions similar to those taking place in thestack of the BF. Even carbothermic reduction of ironore does not take place by direct contact betweencarbon and iron oxide, but rather involves an in-termediate step whereby carbon monoxide is generat-ed by carbon reacting with carbon dioxide. Thereducing gas for shaft furnace and fluidized bed DRprocesses is produced by reforming natural gas withsteam and}or carbon dioxide. Reducing gas may alsobe produced by partial oxidation of natural gas andother hydrocarbons with oxygen. Thermodynamiccalculations involving gas-solid reactions were madewith the F*A*C*T Equilib program (CRCT).

2.1 Reduction of Iron Oxide

In the simplest instance, iron production consists ofremoving oxygen from iron oxide with a suitablereducing agent, such as carbon monoxide (CO) orhydrogen:

Fe#O

$3CO! 2Fe3CO

#;

DH at 25Cflfi25kJ (1)

Fe#O

$3H

#! 2Fe3H

#O;

DH at 25Cfl99kJ (2)

In the case of coal used as a reductant, the CO#formed

in Eqn. (1) reacts with the char to form CO accordingto:

CCO#! 2CO; DH at 25Cfl172kJ (3)

All of the DR processes depend, in part, on Eqns.(1) and (2), which occur at reasonable rates only aboveabout 700C. Equation (3) occurs at a reasonable rateonly above about 9501000C, depending on charreactivity. The manner in which hematite (Fe

#O

$) is

converted to magnetite (Fe$O

%), wustite (Fe

xO), and

iron is very complex. If the starting oxide material isvery dense, the reduction usually proceeds topo-chemically in accord with the shrinking core model.That is, the outside of each reacting particle or pellet iscovered with a layer of metallic iron, beneath whichare well-defined layers of wustite and magnetite, andan unreacted core of hematite. If the oxide is veryporous, the reducing gases penetrate easily and re-duction to metallic iron proceeds throughout. Withina single particle, there may be many interconnectedpores along which the reduction steps proceed. Theterm sponge iron is used to describe DRI produced

from porous iron oxide. Under certain conditions,iron ore pellets may expand dramatically duringreduction (catastrophic swelling). This is attributed tothe formation of filamentary iron growing fromwustite at a limited number of sites.

Wustite has a defect structure containing both ferricand ferrous iron. In equilibrium with iron, the O}Feratio is nearly constant at 1.05 over the temperaturerange 5651200C. In equilibrium with magnetite, theO}Fe ratio varies between 1.05 at 565C and 1.17 at1200C. Wustite is unstable with respect to theformation of iron and magnetite below 565C; un-reduced wustite in DRI therefore tends to decomposeto iron and magnetite as it cools.

Each reduction step is limited by the usual mass-action principles, so that CO and H

#cannot be fully

utilized in iron oxide reduction alone. Figure 1 showsthe percentage CO in a COCO

#mixture, and the

percentage H#

in a H#H

#O mixture for the co-

existence of wustite (Fe!.*&

O) and metallic iron, andmagnetite and wustite (Fe

xO, where x varies from

about 0.85 to 0.95). There is nearly complete con-version of H

#to H

#O, and CO to CO

#for the reduction

of Fe#O

$to Fe

$O

%; hence this phase boundary is not

shown in Fig. 1. In the conversion of wustite to iron,H

#becomes more effective with increasing tempera-

ture, while the opposite is true for CO. This is becauseH

#reduction is endothermic, while CO reduction is

exothermic. At 820C, H#

and CO have equal capa-bility as reducing gases. Most gas-based DR processesuse mixtures of CO and H

#; and the gas composition

satisfies the equilibrium for the water-gas reaction:

CO#H

#!COH

#O; DH at 25Cfl41kJ (4)

Solid carbon is not an effective reducing agent forwustite below about 1000C for two reasons. First, theDG for reduction to iron and CO is positive up toabout 700C; second, the kinetics of Eqn. (3) are slowbelow about 1000C.

Fig. 1 shows that the product gas from the reductionof wustite to iron will have about twice as much(COH

#) as (CO

#H

#O). The high proportion of H

#and CO in the product gas from reduction of wustiteto iron is not as serious a limitation in the overallconversion as it might initially seem. This is becausethe gas can be used to reduce hematite to wustite, or beused as a fuel to provide the necessary heat. Inaddition, the exit gas can be processed to remove mostof the CO

#and H

#O, and be recycled back into the

furnace.

2.2 Production of Reforming Gas

The data presented in Fig. 1 show that near 900C, thegas in equilibrium with iron and wustite has about65% (COH

#). This composition is a weak function

of temperature and the H}C ratio in the gas. To beeffective, a reducing gas for DRI must thereforecontain over 65% (COH

#). The higher the per-

2

Iron Resources and Direct Iron Production

Figure 1Fe-O-C (solid) and Fe-O-H (dashed) equilibrium curves(intersection at about 820C). The heavy dashed linemarked a

cfl 1 is for (pCOpCO

#)fl 1 bar. Points W and

W refer to similarly labeled points on Fig. 5, and are the(%CO%H

#) values at 900C for a gas with H}Cfl 6.

Point W is for iron-wustite (Fe!.*&

O) equilibrium, andpoint W is for wustite (Fe

!.))O) magnetite equilibrium.

centage (COH#), the more wustite that can be

reduced by a given amount of gas. Therefore, a majorobjective of a gas-based DRI plant is to produce largequantities of highly reducing gas. This is generallydone by reacting a hydrocarbon feedstock (such asnatural gas) with oxygen-containing gases so that thedesired ratio is obtained. This is called catalyticreforming, and is carried out in different ways.

(a) Steam reforming. Paraffinic-bonded hydrocarbonfeedstock reacts with steam to form a reducing gasaccording to:

CnH

#n+#nH

#O! nCO(2n1)H

#(5)

Reforming is carried out over a catalyst aboveatmospheric pressure and at temperatures between750C and 1050C. Modern plants operating withimproved catalysts use only a slight excess of steam toproduce a gas with over 90% (COH

#). This is

referred to as one-step reforming. Up to 2% unreactedCH

%can be present in the product. Equation (5) is

strongly endothermic: heat is supplied by radiantburners to heat the reformer tubes, and recuperationof heat from the flue gas is used to improve the thermalefficiency. In one-step reforming of natural gas, thenatural gas proportions such that approximately 40%is used as fuel and 60% is being reformed. Reduceroffgas may be used as a fuel in place of natural gas.Reforming may also be carried out using dewateredoffgas from the reduction furnace in place of part, ornearly all, of the steam. In the case of CH

%, the

reforming reaction becomes:

CH%xH

#O(1fix)CO

#! (2fix)CO(x2)H

#(6)

(b) Partial oxidation. Paraffinic hydrocarbons can beoxidized by supplying just enough oxygen to formCO and H

#.For methane (CH

%):

CH%"

#O

#!CO2H

#(7)

Compared to reformer reactions, partial oxidationproduces less gas per unit of hydrocarbon; however ituses less fuel because Eqn. (7) is slightly exothermic.Partial oxidation also produces a reducer gas with alower H}C ratio. It may also be used on liquidhydrocarbon feedstock, or even coal. Compared tosteam reforming, higher pressures and temperaturesare typically required to produce a gas with high(COH

#). Historically, partial oxidation has not been

widely used in DR processes, but new advances in thetechnology may change this.

(c) In situ reforming. Most gas-based processes takeadvantage of the natural catalytic activity of freshDRI to carry out some reforming in the reductionreactor. Oxygen and makeup natural gas are injectedinto the reducing gas just before entry to the furnace.The role of oxygen is to produce some CO

#, H

#O,

and heat. Reforming occurs on freshly reduced, hotDRI according to a combination of Eqns. (5)(7). Inaddition, some carburization of the DRI may occur.In a recent development, all reforming is carried outin situ; no reformer is needed. Offgas from the reduc-tion furnace is tempered by removing most of thewater and CO

#. It is then reheated to about 40C

above the typical reducer gas entry temperature. Oxy-gen and natural gas are added in the amounts indi-cated by Eqn. (7). In situ reforming is particularlyattractive for hydrocarbon feedstock with sulfurlevels above those permissible for catalytic reform-ing. The constant renewal of DRI in the furnacemeans that fresh uncontaminated catalyst is alwayspresent for the reforming reactions.

2.3 Product Characteristics

DRI can be produced in powder, pellet, lump, orbriquette form. The powder, pellets, and lumps retainthe shape of the iron oxide material fed into the DRprocess. The removal of oxygen leaves voids, givingthe DRI a spongy appearance when viewed through amicroscope. Thus, DRI in these forms has a lowerapparent density, greater porosity, and more surfacearea than iron ore. In the hot briquetted form, it isknown as hot briquetted iron (HBI). Fluidized bedDRI is usually processed to HBI due to the difficulty ofusing fines in steelmaking, and the reactivity of fines tooxygen, even when cool. Typical physical properties ofDRI forms are shown in Table 1.

HBI is produced by molding hot (ca. 700C) DRIinto pillow-shaped briquettes using a pocketed rollpress. The lower surface area and higher density of

3

Iron Resources and Direct Iron Production

Table 1Physical characteristics of forms of DRI.

Parameter Pellets}lump HBI

Density (tm$):bulk 1.61.9 2.42.8apparent 3.5 5.05.5

Porosity (%) 50 15Nominal size (mm) 635 3050110

HBI makes it 100 times more resistant to reoxidation.The high density, strength, and minimum waterabsorption make it ideally suited for merchant appli-cations where shipping, handling, and storage charac-teristics are important.

DRI containing several percent C may be producedwhere the benefit of added carbon in steelmakingoutweighs the added cost. The carbon can be as ironcarbide (cementite, Fe

$C), or graphitic carbon. The

carbon contained in shaft-furnace DRI is typicallyover 90% Fe

$C. Fluidized bed processes for producing

iron carbide from ore fines have been developed, butnone are presently operating.

DRI retains the chemical purity of the iron ore fromwhich it is produced. It therefore tends to be very lowin residual elements such as copper, chromium, tin,nickel, and molybdenum. Typical ranges of DRIchemistry are 9094% total iron, 8389% metalliciron, 6.59% iron oxide, 0.82.5% carbon, 2.86%gangue, 0.0050.09% phosphorus, and 0.0010.03%sulfur.

DRI normally has at least 90% of the oxygenremoved, with the unreduced oxide present as wustite.Processes producing solid with ! 90% reduction areclassified as prereduction processes. Prereduced iron isnot acceptable for steelmaking, but can be used as afeed for ironmaking (e.g., feed for a BF).

Although it is theoretically possible to convert allthe iron oxide to metal, it is not economically feasible.Reduction slows significantly in the last stages, and tocomplete the reduction would require low productionrates. In practice, the DRI is discharged with a smallamount of iron oxide remaining. In addition, duringcooling prior to discharge, iron in the DRI reacts withthe CO and CH

%in the cooling gas to form cementite

according to:

2CO3Fe!Fe$CCO

#;

DH at 25Cflfi147kJ (8)CH

%3Fe!Fe

$C2H

#;

DH at 25Cfl100kJ (9)

The carbon content can be adjusted within limits byoperating changes in the DR process, and is typicallybetween 1% and 2.5% Calthough it can be higherby adjusting the composition and amount of coolinggas. During melting in an EAF, the iron oxide andcarbon in the DRI react to form metallic iron and CO.The CO evolution enhances the steelmaking reactions,

and the oxygen used to oxidize the extra carbonimproves the energy balance. Most steelmakers preferslightly more carbon than is required to balance theremaining iron oxide.

The gangue content of DRI is typically comprisedof oxides such as SiO

#, Al

#O

$, CaO, MgO, TiO

#, K

#O,

Na#O, MnO, etc., and for gas-based processes, is

dictated by the chemistry of the iron ore used. Thephosphorus is normally in the form of P

#O

&. Sulfur

content depends on the sulfur level in the ore andreductant, and the amount of sulfur released orabsorbed by the DRI during reduction. The ganguecontent of DRI produced by coal-based processes maybe considerably higher, owing to the retention of someof the coal ash by the DRI.

When handling, shipping, and storing DRI, careshould be taken to avoid oxidation. This requires thematerial be kept cool and dry. If not, oxidation of DRIcan take place by two mechanisms: corrosion andreoxidation. Corrosion occurs when the metallic ironin DRI is wetted with fresh or salt water, and reactswith oxygen from the air to form rust, Fe(OH)

$. The

corrosion reactions continue as long as water ispresent. Corrosion is very exothermic, but as long aswater is present, the temperature does not reach muchabove 100C. Reoxidation occurs when the warmmetallic iron in corroding DRI reacts with oxygen inthe air to form either Fe

$O

%or Fe

#O

$. The reaction

continues as long as the DRI remains hot, and oxygenis available. Owing to the exothermic nature ofreoxidation, and the insulating nature of bulk DRI,the DRI temperature increases and accelerates thereoxidation rate. In comparison, HBI is almost twiceas dense as DRI and thus does not absorb much water.It is much more resistant to corrosion and reoxidation.Several methods of passivating DRI have been de-veloped, but none are as effective as hot briquetting.

3. Direct Reduction Processes

A wide variety of apparatus has been developed for theproduction of DRI: retort furnaces, reverberatoryfurnaces, shaft furnaces, rotary kilns, grate kilns,rotary hearth furnaces, fluidized bed furnaces, electricfurnaces, plasma arc furnaces, and combinations ofthese. The only types of apparatus that operatecommercially are the shaft furnace, the fluidized bedfurnace, the rotary kiln, and the rotary hearth furnace.Four principal processes produce over 95% of thetotal DRI. To facilitate discussion of the differentoperating processes, broad categories are shown inTable 2. The principal processes are compared inTable 3.

3.1 Shaft Furnace Processes

The dominant DR technology involves the shaftfurnace, using reformed natural gas as the reducing

4

Iron Resources and Direct Iron Production

Table 2Classification of DRI processes.

Gas reductionprocesses

Solid reductionprocesses

Shaft processes Rotary kiln processesMIDREX Krupp-CODIRHYLa SL}RNPurofer DRC

ACCAR}OSILFluidized bed processesFior}FINMET Shaft and hearth processesIron Carbide Kinglor-MetorCircored FASTMET

INMETCO

a When using in situ reforming, all reducing gas is generated internally.

agent. The MIDREX and HYL processes togetheraccount for about 90% of all DRI production. Figure2 shows the main features of the shaft furnace used bythese processes for DRI production. Early versions ofthe HYL process used a series of fixed-bed shaftfurnaces, but since 1980, HYL has used a moving bedshaft furnace. Ore pellets are charged through gasseals or rotary valves, and preheated in the upper zoneof the furnace. Spent reducing gas exits the top of thefurnace and flows to scrubbers. The DRI is cooled andcarburized in a lower section of the furnace, thendischarged through seals or valves. The cooling gas ismainly natural gas, with some reformed gas. The DRIdischarge temperature is about 700C if charged hotto a melting furnace or made into HBI, and about60C for cold DRI.

Current versions of the MIDREX and HYL proces-ses differ in the way the reducing gas is prepared. In theMIDREX process, the top gas is cleaned in a scrubberwhich also acts as a dehumidifier to remove much ofthe H

#O. After cleaning, part of the gas is used as a

fuel. The rest of the gas is compressed to about 2 barsgauge, natural gas is added, and the mixture is fed tothe reformer. Steam is not added to the reformer;instead, the CO

#and H

#O in the recycled top gas

reform the natural gas according to Eqn. (6). Thereformed gas exits the reformer at 900C to 970C.Before entering the shaft furnace, the gas can betrimmed by adjustments to its temperature and}orcomposition. Figure 3 shows a flowsheet for the gasprocessing section of a standard MIDREX plant.

In the HYL reformer, the top gas is cleaned andscrubbed, and then most of the CO

#is removed. This

processed top gas is not sent to the reformer, butinstead is blended with reformer gas produced bysteam reforming of natural gas. Gas leaving thereformer is cooled to remove water, and mixed withthe cleaned top gas. The blended stream is heated to" 900C, and may be trimmed by adjustments totemperature and composition before entering the shaftfurnace. As an alternative, HYL may use in situreforming, in which case steam reforming is elim-

Figure 2Shaft furnace typical of that used by MIDREX and HYLfor gas-based production of DRI. Hot reducing gases fromreformer may be trimmed by additions of oxygen and}ornatural gas. Reducing gas enters shaft furnace around theperiphery at the bottom of the reduction zone through abustle pipe that distributes the gases evenly. MIDREXfurnaces have burden feeders below the reduction zone toassure uniform burden descent and to break up anyclusters that may have formed during reduction. The topgas scrubber removes dust and most of the water vaporfrom the gas.

inated, and natural gas and oxygen are added to theheated top gas and reformed in contact with DRI inthe shaft furnace. Figure 4 shows a flowsheet for thegas processing section of a standard HYL plant.

(a) Shaft furnace reactions. The relationship betweenthe transfer of oxygen from solid to gas in a counter-current shaft furnace can be shown by a Rist dia-gram. Here the degree of oxidation of the solid isplotted versus the degree of oxidation of the gas. Agas of some initial degree of oxidation X

Renters the

reduction furnace and removes oxygen from an in-coming solid to produce DRI of some degree of oxi-dation Y

R. Because the moles of gas and moles of

iron remain constant, there is a linear relationship be-tween the change in degree of oxidation of the solidand gas. This line is called the operating line for theprocess.

The degree of oxidation of the gas X can beexpressed as a ratio of moles of oxidized gas per mole

5

Iron Resources and Direct Iron Production

Table 3Comparison of the principal direct reduction processes. Data based on 2000 production figures of 43 Mt.

Parameter MIDREX HYL SL}RN Fior}FINMET

World production (10 t) 29 (68%) 9.5 (22%) 1.2 (2.8%) 1.0 (2.3%)Reduction vessel Shaft Shaft Rotary kiln Fluidized bedReductant source Natural gas Natural gas Coal Natural gasIron oxide form Pellet}lump Pellet}lump Pellet}lump Sized finesDRI:

form Pellet}lump or HBI Pellet}lump or HBI Pellet}lump HBImetallization (%) 9295 9295 9293 9293carbon content (%) 13.5 0.55.5 0.20.5 11.5

Reduction:temperature (C) 7601000 8501030 10001100 690780pressure (kPa) 30 500 0 1120time (h) 46 46 810 67

Reformer type Catalytic H#OCO

#Catalytic steam None Catalytic steam

Reducing gas H}C ratio 34 819 910Reducing gas(H

#CO)}(H

#OCO

#)

1112 1119 1214

Mean residence time (h)a 57 68 810 1.52Range unit capacity installed(10 tyr")

0.331.5 0.251.1 0.040.18 0.4

Consumption per ton of DRI:iron oxide (t) 1.42 1.45 1.47 1.6c

natural gas (GJ) (LHV) 9.4 1011 1527coal (t) 0.8b

electricity (kWh) 95115 090 6080 250water (m$) 1.21.5 1.8 23 2.5

a Based on ore entry to DRI discharge. b Dolomite is also added at the rate of 60kgt" of DRI. c Increased consumption caused by need to rejectore below certain size.

Table 4Heat effects for major parts of the shaft furnace production of DRI for the conditions shown as solid line on Fig. 5. Heatloss is 150kJ.

Process Heat effect

Cool 110 moles of reducing gas from 930 to 25C (23.7% CO, 71.3% H#, 1.2% CO

#, 3.8% H

#O) fi3060kJ

Reduce 15.23 moles (2.43kg) of Fe#O

$to DRI at 25C (92.7% Fe, 7.3% Fe

!.*&O) 750kJ

Heat 110 moles of product gas to 330C (8.0% CO, 47.2% H#, 10.1% CO

#, 34.7% H

#O) 1110kJ

Heat 30.56 moles (1.73kg) of DRI to 910C 1050kJ

of gas. The degree of oxidation of the iron Y isexpressed as a ratio of the atoms of oxygen per atom ofiron:

Xfl (CO#H

#O)}(CO

#H

#OCOH

#) (10)

Yfl (1.5Fe$+Fe#+)}FeflO}Fe (11)

Figure 5 shows a Rist diagram (Rist and Bonnivard1963) for the reduction of hematite in a countercurrentshaft furnace at 900C using a mixed reducing gas ofH}Cfl 6. The dashed line represents conditions forcomplete reduction of hematite to iron (Y

Rfl 0) with

an incoming gas consisting only of CO and H#(X

Rfl

0). The critical operating parameter is given by thevalue of X at point W, the value of which depends ontemperature and the H}C ratio in the gas as given bythe equilibrium constants of equations used in con-structing Fig. 1. The operating line must pass through,

or to the left of, point W. For a very reducible ore andlong residence times, the operating line will passthrough point W. For a gas having a H}C ratio of 6and ideal conditions as described above, three moles ofreducing gas are required to produce one mole of iron.The extension of the operating line to the top of thediagram gives the degree of oxidation of the gasleaving the top of the shaft. The distance between theoperating line and the phase boundary lines on a Ristdiagram indicates the difference in oxygen potentialbetween the gas and solid. Clearly, the gas above pointW is highly reducing to the solid charge all the way tothe top.

The shaft furnace operates at greatest efficiencywhen the operating line passes closest to point W.However, at the high gas flows used to achieve thedesired throughput rate, the gas never reaches equi-librium with the solid. The operating line of a practical

6

Iron Resources and Direct Iron Production

Figure 3Major unit operations for the gas processing section of atypical MIDREX plant. The top gas is divided intostreams at splitter SP. Natural gas is added to the streamsent to the reformer. The other part of the stream is usedas fuel for the reformer burner B. The flue gas from thereformer heater (shown as a dotted line) is used to preheatthe burner air and the gas to the reformer.

furnace therefore always passes to the left of point W.In reality, the incoming gas is slightly oxidized and theproduct is not completely metallized. The operatingline for this situation is shown in Fig. 5 as a solid linewith an incoming gas X

Rfl 0.05 and DRI leaving at

YRfl 0.06. The operating line was drawn at 110% of

the gas that would have been required had theoperating line passed through point W. This conditionrequires 3.83 moles of reducing gas to produce DRIcontaining 1 mole of metallic iron.

As important as the equilibrium constraints arethose posed by the heat balance. The incoming gasmust bring in sufficient thermal energy to heat the oreto its reduction temperature (commonly " 900C),and to provide heat for the chemical reactions. Thecalculated heat effects for the main parts of a typicalshaft furnace reduction process are shown in Table 4.The incoming gas at 930C is 95% (COH

#) (X

Rfl

0.05), and is 110% of the amount of reducing gasrequired by an operating line passing through point Wat 900C. The DRI product is 92.7% metallic iron (Y

R

fl 0.06) and exits the reduction zone at 910C. A heatloss of 150kJ (10kJ per mole of Fe

#O

$in) is assumed.

Figure 4Major unit operations of the gas processing section of atypical HYL plant. Top gas is split into two streams (SP),one for use as part of the burner fuel (with natural gas,NG) to heat the reducer gas, and the other as feed to thereformer after CO

#removal (in SC). Additional reducing

gas is produced in a standard steam reformer, thendewatered in quencher Q, and added to the cleaned topgas. Reformer steam is produced in a series of boilers andheat exchangers, with heat from the reformer flue gas(shown as dotted lines) and hot water (shown as dashedline). An alternate HYL reducing gas process uses in situreforming to replace the steam reformer circuit. Naturalgas is added before HX1, and oxygen is added to the hotreducing gas before it enters the shaft furnace.

3.2 Fluidized Bed Processes

A fluidized bed is a well-stirred reactor, characterizedby excellent gas-solid contact, and uniformity oftemperature and composition across the bed. Fluid-ized bed processes accounts for about 2% of DRIproduced. Gas utilization in a single bed process isunfavorable, so all commercial processes use two ormore beds in series; the gases and solids flow counter-currently. Fluidized bed processes are typically usednext to sources of iron ore, thus saving the cost ofpelletizing. The product DRI is almost always con-verted to HBI.

The CIRCORED process uses two fluidized bedsfor reduction. The ore is preheated in two fluidizedbeds, and then prereduced in a circulating fluidizedbed reactor, with high gas velocities and a retentiontime of 2030 minutes. The prereduced ore then passesinto a conventional fluidized bed reactor for finalreduction. After a retention time of 24 hours, it isflash-heated to 680C prior to roll briquetting. Theoperating pressure is 4 bars gauge, and the temperatureis kept below 630C to prevent sticking. Natural gas isused as a feedstock to a steam reformer with a CO

#

7

Iron Resources and Direct Iron Production

Figure 5Rist diagram for a continuously operating shaft furnacewith reduction taking place at 900C with a reducing gasof H}Cfl 6. The Y-axis is the O}Fe in the solid (in thiscase, hematite feed), and the X-axis is the degree ofoxidation of the reducing gas. The dashed line is theoperating line for idealized complete metallization ofhematite with a reducing gas consisting only of CO andH

#, and reaching equilibrium with wustite and iron at

point W (see corresponding points W and W on Fig. 1).The solid line represents an operating line for a practicalprocess, producing DRI with Y

Rfl 0.06 and using an

initial reducing gas of XR

fl 0.05. Gas flow is 110% of thatrequired for an operating line passing through point W.Point W represents conditions of solid wustite of Fe

!.))O,

and point M represents solid magnetite in equilibrium withwustite. Point M represents magnetite in equilibrium withhematite, and point H represents the incoming hematite.The intersection of the operating line with the top axisrepresents the degree of oxidation of the gas exiting thefurnace (after Rist and Bonnivard 1963).

removal system, such that the reducing gas is mainlyhydrogen.

The Fior}FINMET process consists of a fluidizedbed dryer, four reducing reactors, and a roll briquetter.The reducing gas is produced by steam reforming ofnatural gas, in a manner similar to that shown in Fig.4. Reactor temperatures range from 550C in theprereduction reactor to 780800C in the final re-duction reactor. The throughput is enhanced byoperating at pressures between 11 and 13 bars gauge;this greatly decreases the linear gas flow as comparedto lower pressure operation. Ore fines between 13mmand 150lm are preferred.

Figure 6Sketch of flowsheet for rotary kiln production of DRI,typical of that used in the SL}RN and CODIR processes.The flux is a mixture of raw dolomite and limestone.

3.3 Kiln Processes

The rotary kiln is a revolving horizontal cylindercomprising a shell with an internal refractory lining.The furnace is inclined at an angle of 34 degrees fromthe horizontal, so the burden travels through thefurnace by rotation and gravity. Coal, flux, and ore(lump or pellets) enter the high end of the kiln, andpass into a heating zone where the coal is devolatilized,the flux is calcined, and the ore is preheated to thereduction temperature. A retention time of severalhours is required. The ore is reduced in the reductionzone according to Eqn. (1), while a portion of theadditional CO is generated by Eqn. (3). Part of theprocess heat is supplied by combustion of coal volatilesand CO leaving the bed with air introduced into thefree space above the bed, and partly by combustion ofcoal or other fuel at the discharge end burner with adeficiency of air. The temperature profile in the kiln iscontrolled by radial air ports in the preheating zone,and axial air ports in the reduction zone. The productis cooled in a rotary cooler, and screened andmagnetically separated to recover the DRI and char.DRI fines are briquetted, and the char is recycled tothe kiln to increase fuel efficiency. The offgases fromthe kiln are cleaned and burned in an afterburner.

Kiln DR processes (mainly SL}RN and CODIR)account for about 8% of worldwide DRI production.The processes are similar except for the method ofcooling. A flowsheet of a typical kiln process is shownin Fig. 6. The latest installations incorporate a wasteheat boiler to recover energy from the kiln offgas.

8

Iron Resources and Direct Iron Production

4. Further Developments

Recent gains in productivity and decreased cokeconsumption in BFs mean that the BF will continue todominate iron production for the foreseeable future.However, DR processes offer a high purity scrapsubstitute for EAF steelmaking where natural gasprices are low, and scrap prices justify. The increasedfraction of steel produced by the EAF indicates thatDRI production will likely increase faster than totalsteel production. Recent improvements in shaft-fur-nace DRI processes involve the use of flux-coatedpellets, which allow higher temperature, greaterproductivity, and increased oxygen injection (whichdecreases specific energy consumption). Current coal-based DRI processes have not established a generallycompetitive position and, unless significant techno-logical improvements are made, their commercialadoption will occur only in special and limited cir-cumstances. One emerging coal-based technology isthe rotary hearth furnace, in which pellets of coal andiron ore are heated above 1300C by natural gasburners for a retention time of less than 13 minutes.The hot pellets may be melted in an EAF to producepig iron, or charged hot to an EAF steelmakingfurnace.

Another challenge to the BF comes from a variety ofsmelting reduction processes, such as the COREX andHIsmelt processes. Their advantage comes from theability to use coal as a reductant instead of the morecostly and environmentally problematic coke requiredby the BF. Smelting reduction processes separatereduction and smelting into two reactors. Coal, oxygen

Copyright 2001 Elsevier Science Ltd.All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmittedin any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording orotherwise, without permission in writing from the publishers.

Encyclopedia of Materials : Science and TechnologyISBN: 0-08-0431526

pp. 43024310

(or preheated oxygen enriched air), prereduced ore,and flux are charged to the smelting furnace, where hotmetal, slag, and an offgas of high (COH

#) content is

produced. This offgas is used as the reducing gas in aseparate shaft or fluidized bed prereduction furnace.Smelting reduction may play a future role wherecoking coals are in short supply, or cokemaking isrestricted because of stringent environmental regula-tions. It may also play a role in rounding out thecapacity of a plant where hot metal needs are insuf-ficient to justify the addition of a new BF. A BF is noteconomical at less than 2000000Mt}yr, while smelt-ing reduction may be economical at a quarter of this.

See also: Metal Extraction: Phase Stability Diagrams

Bibliography

Centre for Research in Computational Thermochemistry(CRCT) http:}}www.crct.polymtl.ca}fact}fact.htm

Feinman J, MacRae D (eds.) 1999 Direct Reduced IronTechnology and Economics of Production and Use. The Ironand Steel Society, Warrendale, PA

HYLSAMEX http:}}www.hylsamex.com.mx}HYLMIDREX http:}}www.midrex.comRist A, Bonnivard G 1963 Reduction of an iron oxide bed with

a gas. Re. Metall. 60, 2327. BISI Trans. No. 3679US Geological Survey 2000 Minerals Yearbook for Iron and

Steel, Iron and Steel Scrap, and Iron Ore. US GeologicalSurvey, Reston, VA

Wakelin D H (ed.) 1999 The Making, Shaping and Treating ofSteel. Ironmaking Volume. 11th edn. AISE Foundation,Pittsburgh, PA

A. E. Morris

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