morphology and reduction kinetics of fluxed

8/3/2019 Morphology and Reduction Kinetics of Fluxed

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Morphology and reduction kinetics of fluxediron ore pellets

S. Prakash, M. C. Goswami, A. K. S. Mahapatra, K. C. Ghosh, S. K. Das,

A. N. Sinha, and K. K. Mishra

slag than in the case of the externally fluxed iron ore pellet,impurity reversal to the metal is expected to be minimisedThe present paper describes studies of theduring smelting.1 The slag would essentially float off thereduction and sintering behaviour, followed byassimilated impurities.examination of the structural changes taking place,

Iron ore pellets are commonly used as an alternativeduring direct reduction of fluxed composite (orestarting material in conventional as well as comparativelyplus coal) pellets. The kinetics and morphologicalnew iron- and steelmaking processes. Attempts have recentlyaspects of the direct reduction of fluxed compositebeen made to employ internal reductants and fluxes topellets are compared with those for the gaseousaccelerate the reduction kinetics without affecting the physico-reduction of fluxed ore pellets without carbonchemical properties of the pellets.13 The reduction strength,addition. It is shown that the chemical reaction ishowever, depends largely on the precipitation behaviour ofthe rate limiting factor for fluxed composite pellets,iron on wustite (FeO

x) during reduction.4 Earlier authors4,5and the mass transport of reactant gas through the

have discussed some aspects of the reduction and sinteringreaction product iron layer governs the overallbehaviour of fluxed pellets. Prakash5 demonstrated significant

reduction process in the case of fluxed ore pellets. variation in the pore size distribution and changes in ironMicroscopic analysis of the fluxed compositemorphology during reduction.pellets has indicated no tendency to form a

The gaseous reduction of iron ore lumps and pelletsreduction inhibition iron shell towards the end ofhas been extensively studied, discussed, and reported in thethe reduction, as found in the case of gaseousliterature. Based on several investigations611 into the gaseousreduction of fluxed ore pellets under identicalreduction of iron oxide, the rate determining mechanismreduction process conditions. The effects ofhas been described either as the interparticle gaseous diffusion

basicity, Fe/C, temperature, and other variables onthrough the reduced layer or as the chemical reaction at

morphological and microstructural changes in thethe interface between the reduced and unreduced layers.

directly reduced fluxed composite pellets have beenThe extent of reduction and the structural changes depend

experimentally investigated in the present work.on several operating variables.8,1115 The most critical of

I&S/1422these is gaseous diffusion through the reduced layer, andthis depends upon the porosity of the pellets. Abraham and

The authors are at the National Metallurgical Laboratory,Ghosh16 predicted a significant mass transfer resistance inJamshedpur, India. Manuscript received 4 January 1999; acceptedthe oxide.5 May 1999.

Rao17 investigated the reduction kinetics of a mixture 2000 IoM Communications Ltd. of hematite and carbon powders in the temperature range

8501087C. The reduction process can be described in termsof the availability of carbon monoxide within the mixture.Although some researchers have attempted to characterise

INTRODUCTIONthe reaction mechanism and the rate controlling steps

Production of liquid iron outside the blast furnace has mathematically, there is little experimental evidence basedattracted considerable attention during the past two decades. on microscopic studies to suggest how temperature, carbonThe smelting reduction processes are intended to provide an particle size, hematite/carbon ratio in the mixture, andattractive alternative route to the conventional steelmaking addition of catalysts, binders, and fluxes affect the reductionprocess. These processes, however, require very select grades kinetics.of raw materials. Both iron ore and coal must be fed in the Investigation1821 into the interaction of CaO with Al

2O

3,

form of lumps of particular size. Fines of ore, Indian blue SiO2

, and FeOx

have shown that different concentrationsdust, or coal fines produced during mining or crushing of these compounds cause a change in iron morphologycannot be used in these processes without agglomeration. and result in different reduction conditions. According toAlthough such fines can be directly used in fluidised bed the phase diagram of Philips and Muan,18 calcium diferritereactors, sticking problems during the reduction process (CaO.2Fe

2O

3=CaFe

4O

7) is stable in the range 11551226C

cause numerous breakdowns. An alternative process is the in air at 1 atm. Edstrom19 regarded this compound as thedirect reduction of iron ore pellets, using solid or gaseous most important binding phase for the base sinter process.reductants. However, during the smelting of these pellets, A study20 of the effect of Al

2O

3on the formation of calcium

external reductant and fluxes must be added to reduce the deferrite has shown that Al2

O3

decreases the melting temper-residual oxygen content of the iron oxide in the molten slag, ature, which enhances the formation of calcium diferrite.and to give the necessary assimilation of impurities such as As is the case with CaO, it is not possible to reduce SiO

2sulphur, phosphorus, and vanadium, thereby generating slag in the temperature range 6001000C. In contrast to CaO,of suitable composition for the furnace lining, respectively. SiO

2can only form the chemical compound fayalite (Fe

2SiO

4),

The concept of the directly reduced fluxed composite pellet which cannot normally be reduced by CO. Thus, SiO2

canimplies that the fluxes and carbonaceous materials added be referred7,18 to as a sluggish compound, which hindersto the iron ore pellet feed would generate a slag of suitable the reduction of iron oxide. The apparent deviation cancomposition and adequate reduction potential in a bath be attributed to the local oxygen potential at which thesmelting process. As the impurities bound in the fluxed FeSi

2O

4was formed, and hence which differed from that

of the ambient atmosphere.composite pellet FCP can be more easily assimilated in the

ISSN 03019233194 Ironmaking and Steelmaking 2000 Vol. 27 No. 3


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Prakash et al. Reduction of fluxed iron ore pellets 195

The reduction reaction in ore plus coal pellets at hightemperature is rather complex. It involves22 devolatilisationof coal and reduction via CO and hydrogen in the volatiles,reduction via hydrogen and carbon generated by crackedhydrocarbons in the volatiles, and true direct reductionand reduction via gaseous intermediates.

The pellets should be tested to determine properties suchas porosity, green strength (impact test), isothermal reductioncharacteristics, reduction strength, compressive strength afterreduction (CSAR), and sticking characteristics. Anotherimportant factor to consider during reduction in the shaft

is clustering, which can result in uneven gas distributionduring discharge. The standard test methods23,24 are ratherinaccurate, oversimplifications of the process steps andtreatments carried out under simulated conditions. Thereis a danger, therefore, of optimising the behaviour of anore product under test conditions rather than its propertiesunder actual process conditions.

To minimise smelting costs, it is essential that the directlyreduced iron is highly metallised.25 An easily reducibleburden is necessary to achieve this without sacrificing highproductivity in the direct reduction furnace. The limitingfactor is usually the shape of the reduction curve towardsthe end of the reduction process. The time required to attain 1 Schematic diagram of experimental setup for reductionthe limiting value of reduction can, however, be minimised of pelletsby the use of suitable fluxes and additives.

The purpose of the present study was to evaluate on alaboratory scale the sintering and reduction characteristics

Apparatus and procedureof fluxed composite (ore plus coal) pellets for use in directThe reduction experiments were carried out in a resistancereduction and smelting reduction processes. Directly reducedheated vertical tube furnace (Fig. 1). The directly reducediron pellets were produced by the simultaneous reductionfluxed composite pellets were made using various pro-and sintering of fluxed composite pellets which comprisedportions of Indian blue dust, reductants, lime, and ana mixture of ore plus coal fines, fluxes, and a suitableorganic binder. The particle size of all the materials wasbinder. It was found that, for fluxed composite pellets, the200 mesh (0074 mm). The CO generated in situ servedchemical reaction was the rate controlling step, describedto maintain a reducing atmosphere inside the furnace. Theby a ln(1a) type equation, where a is the degree ofpellet samples were heated to predetermined reductionreduction. The kinetics of fluxed ore pellets made withouttemperatures of 1073, 1173, 1273, and 1323 K for a speci ficcarbon addition, however, indicated diffusion controlledreduction time. The reduced samples were then cooled inrate phenomena. In the present work the main processnitrogen, weighed, and kept in a vacuum desiccator beforeparameters to affect the kinetics and the iron precipitationchemical analysis and microscope studies.characteristics during pellet reduction, i.e. temperature,

Experiments were carried out to study the effects ofFe/C, reduction gas potential, and reduction time, have beentemperature, non-ferrous oxide content, Fe/C ratio, addi-experimentally investigated. The reduction mechanisms andtives, and basicity on the reduction and strength charac-morphological changes for both fluxed ore pellets andteristics of the pellets. Some additional isothermal reductionfluxed composite pellets are described.experiments were carried out to study the iron precipitationbehaviour during reduction. Identical pellet samples were

EXPERIMENTAL heated to the temperatures of 1073, 1173, and 1273 K forRaw materials and chemical compositions a fixed duration of 45 min. The reacted pellets were removed,

quenched by nitrogen gas, fractured, and then analysedBoth fluxed iron ore and fluxed composite ore plus coaltypes of pellets were made and tested in the laboratory using SEM.

The reduced and sintered pellets were tested for compressiveusing equipment and methods developed at the NationalMetallurgical Laboratory, Jamshedpur, India. Blue dust strength, porosity, and extent of reducibility. All the tests

were performed on pellets which were within the diameterwas used in the preparation of these pellets. Chemicalcompositions of the blue dust, coal, and flux are given in range of 10125 mm. To compare the reduction mechanisms

of fluxed iron ore (blue dust without internal reductantTable 1. Organic binder was used during balling to minimise

the quantity of acid gangue. pellets) and fluxed composite (with internal reductant) pellets,isothermal gaseous reduction of fluxed ore pellets underPellets were produced in a rotary conical drum. Their

strength characteristics, showing the ability to withstand similar experimental conditions was carried out. In this testa CON

2mixture (30 : 70) was passed through the bed;handling during transportation, were evaluated by impact

drop test.4 reduction temperatures of 1073, 1273, and 1323 K were,

Table 1 Chemical compositions* of raw materials, %

Raw material FeT

Fe2

O3

SiO2

Al2

O3

CaO MgO S P LOI FC VM Ash

Blue dust 683 976 134 040 013 002 055 Lime Traces 054 060 80 080 858 Non-coking coal 49 385 125

ash 24 428 292 230 18 Traces 0005

* FeT

total Fe; LOI loss on ignition; FC fixed carbon; VM volatile matter.

Proximate analysis.

Ironmaking and Steelmaking 2000 Vol. 27 No. 3


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196 Prakash et al. Reduction of fluxed iron ore pellets

(a)

(b)

(a)

(b)

a B2=082; b B2=133 a from Fig. 2a; b from Fig. 2b

2 Kinetic data for reduction of fluxed composite pellets3 Kinetic analysis using modified kinetic data for fluxed

composite pellet reduction

however, the same as in the former case. Tests carried out Figure 5 shows analysis of the reduction kinetics for theaccording to the Bureau of Indian Standards for evaluating isothermal gaseous reduction of fluxed ore pellets (basicityiron ore and pellets are discussed in detail elsewhere.24 of 133). Curves for the fraction of the pellets reacted, i.e.

the fraction of oxygen removed, against time at the temper-atures of 1073, 1173, 1273, and 1323 K are shown in Fig. 5 a.

RESULTS AND DISCUSSION It can be seen that the reduction rate decreases graduallyReduction kinetics with time and with an increase in fraction reacted. With

the progress of reaction, therefore, some changes seeminglyTypical results for the reduction kinetics of fluxed irontend to retard the rate of reduction. The CrankGinstlingore plus coal pellets are shown in Fig. 2 for basicities of

082 and 133. The curves do not pass through the origin

because the reaction mass underwent a short heating periodto attain the preset temperatures, i.e. time t=0 is ill defined.However, all curves have the same intercept, indicatingconsistency in results. The degree of reduction a, i.e. degreeof oxygen removal, and reduction time t values for thekinetic curves must be extrapolated accordingly. It wasassumed, however, that no reaction had taken place duringthe rather short heating period and a modified time, i.e. thetime when the pellet mass attained the specific temperature,was used for kinetic analysis. The resulting isothermalkinetic data using the modified times for the reductionexperiments on the pellets are shown in Fig. 3.

A good linear relationship was obtained between themodified time and G(a), where G (a)=ln(1a). Thisinterpretation serves only as indirect proof that a solution

loss reaction occurs between the CO2 and carbon particleswithin the pellets, and that it governs the overall reductionprocess. The kinetic data fit approximately a first order rateequation (Fig. 3). The rate constants determined from Fig. 3fit an Arrhenius type equation as shown in Fig. 4, and yieldactivation energy E values in the ranges 4950 kJ mol1and 4752 kJ mol1, respectively, for basicities of 082 and133. As the CO

2carbon reaction is an activation controlled

chemical reaction, the direct reduction component is signifi-cant in the reduction of composite pellets. The validity ofthe model has been tested using the differential approach,6resulting in E values nearly the same as those obtained

(a)

(b)

using the integral approach. The results indicate that a a B2=082; b B

2=133

higher basicity enhances the reduction kinetics, although 4 Evaluation of activation energy Eof fluxed compositethe reaction mechanism remains the same within the range pellet reduction using differential and integral

approachesof basicities (082133) investigated in the present work.



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(a)

(b)

a B2=082; b B

2=133

6 Dependence of reduction time on reductiontemperatureof fluxed composite pellets

The test results show that the characteristics of thefluxed composite pellets depend both on their total contentof non-ferrous oxides (CaO, SiO

2, Al

2O

3, etc.) and on the

composition of these oxides. Hence, it may not be possibleto obtain certain properties of the pellets simply byprescribing their basicities B

2=%CaO/%SiO

2.

In the present case, blue dust, coal, and basic additiveswere used; the quantity of non-ferrous oxides increases inproportion to the CaO content. For all mixes the following

(a)

(b)

(c)

5 a Isothermal kinetic data for reduction of fluxed relationship holds approximately: % non-ferrous oxidespellets, B2=133; b kinetic analysis of reduction of (non-FeO

x)=constant+%CaO.

fluxed pellets using data from Fig. 5a; c evaluation Figure 7 shows the dependence of non-FeOx

contents ofof activation energy using differential and integral the composite pellets on reduction time at various levelsapproaches for rate controlling steps, from Fig. 5b

of degree of reduction. Interestingly, a higher non-FeOx

content in the pellets enhances the rate of reduction. ItBrounshtein5,6 equation indicates that product layer diffusion can be inferred from this that the lime addition enhancesis the overall rate controlling step (Fig. 5b). This may be the rate of reduction. Figure 8 shows that while the greenascribed to the formation of a CaOFeOSiO

2phase. The strength is fairly independent of the coal quantity ( 030%),

fluxed pellets without an internal reductant tend to form a an increase in both lime and organic binder additivesreduction restraint product layer, which retards the rate of significantly increases the impact strength of the pellet. Inreaction. The transport of reaction gas through the product Fig. 9, porosity and CSAR are plotted against temperature.layer is a diffusion controlled reaction. The mass transfer The extra lime addition (higher basicity) surprisingly giveslimitations modify the rate, yet this does not influencethe activation energy significantly. Thus, a high activation

energy (4246 kJ mol1

) exists despite partial control of masstransfer (Fig. 5c). To achieve a higher degree of reactionrate, it is necessary to minimise the inhibiting effect of thisresistance by appropriate process control, or a measuresuch as increasing the area of the reacting surface. It is alsoimplied by the above that if fluxed composite pellets of oreplus coal can be used, the process rate can be enhanced byat least an order of magnitude.

Factors influencing pellet characteristics

Figure 6 shows typical results for the dependence ofreduction time on the reduction temperature, indicatingthat the time needed to achieve a given degree of reductiondecreases with an increase in reduction temperature. At 7 Dependence of reduction kinetics on non-ferroushigher temperatures, however, an increase in basicity seems oxides (non-FeO

x) in fluxed composite pellets: B

2=133,

T=1323 Kto cause a slight decrease in the temperature dependency.



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8 Effects of additives on green strength of fluxed

composite pellets

only a marginal decrease in porosity within the range investi-

gated. A higher lime addition and increase in reductiontemperature apparently decrease the porosity, and theysignificantly increase the CSAR. As is well known, the degreeof agglomeration increases with an increase in temperature,leading to lower porosity and generally higher strength, asindeed appears to be the case from Fig. 9. Furthermore, itis apparent that the porosity decreases more rapidly withan increase in temperature at higher lime additions. Thetemperature dependence of the CSAR is fairly dependenton an increase in the lime addition. At higher temperaturesthe lime additions seem to cause a reasonable increase inCSAR dependency on the temperature.

Microscopic analysis

In gaseous reduction of fluxed ore pellets, one reactant gasis transferred from the bulk gas stream and diffuses betweenthe grains through a solid reaction product layer. In thepresent work, the reduced fluxed ore pellets were examinedusing a Nikon optical microscope. Figure 10a depicts theformation of a dense iron shell around the surface. It isinteresting to note, however, that the reaction appears toproceed from the outside towards the centre for each grain,

a

b

(c)

10 Optical micrographs depicting a tendency of ironshell formation during reduction of fluxed pellets,

100; bdense iron shell surrounding grains of fluxedpellets, 50; c schematic diagram of grain model

forming a dense iron shell around the grain (Fig. 10b).Further growth of the iron shell occurs along a directionperpendicular to the iron/wustite interface and producestypical morphological changes, which are shown schematicallyin Fig. 10c. The shelled wustite is converted to iron by theneighbouring iron nuclei via solid diffusion. The reductionretardation noted above may be a result of slow diffusion inthe solid phase. The difference in the degree of reductionbetween two successive time intervals on the reduction timecurve (Fig. 5a) indicates that the rate of reduction decreaseswith the progress of reduction. The cause of such a retardation9 Dependence of compressive strength after reduction

(CSAR) and porosity on temperature may be the binding of iron oxide in the acid slag phase or



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b

a highly reduced; b densely sintered

11 Optical micrographs revealing microstructure offluxed composite pellets 200

the formation of dense iron shells around the remaining

a

b

c

wustite particles. If reduction stops around or before achieve-a whisker, 1173 K; b porous, 1273 K; c dense, 1323 Kment of a degree of reduction in pellets with a high iron

12 Microstructures of fluxed reduced iron pellets showingcontent, the main reason most probably is the formationmorphological changes at given temperatures

of iron shells, which hinders the ingress of reduction gas tothe inside core. The formation of the dense iron shell is themajor cause of retardation of the reduction rate observed magnification is kept about four times higher than that of

Figs. 12b and c so that the whisker morphology is clearlyat the final stage of reduction. These results correlate withthe shape of the reduction curve (Fig. 5a). This retardation visible. In addition, increased mass transfer rates are possible

in composite pellets owing to hydrocarbon liberation atmay also hinder the sintering process.Figure 11 shows optical micrographs of reduced FCPs, low temperatures between 200 and 600C ( Ref. 22) and the

influence of H2/H

2O via the gaswater shift reaction whenrevealing microstructures of highly reduced and densely

sintered iron particles ( bright contrast) and agglomerates using coal with a high volatiles content (Table 1 ). Thesereactions can affect the microstructure of the oxide grainof fine carbonaceous particles. It is interesting to note that,

despite the incomplete reaction of carbon and lack of full by the introduction of defects and have the generation ofnew paths for the reaction gases along grain boundaries.densification, the CSAR typically approaches 115 kg/pellet

(Fig. 9) . This can probably be ascribed to the formation of However, the interrelationships of these various precipitationbehaviours is rather complex.calcium ferrites, primarily diferrites, which during reduction

form a calciferous wustite that is readily reduced despite Typical SEM images of fractured reduced pellets, obtainedwith a Jeol 840 scanning electron microscope, are shownits low swelling.

Microscope studies of reduced flxed composite pellets have in Figs. 13 and 14, depicting the effect of residence timeand temperature, respectively, on changes in microstructurerevealed some interesting results of sintering, discussed below

in association with the reduction results. Figure 12 shows of the pellets. It can be seen that, with an increase in timeand temperature, the ferrite phase tends to grow on afibrous (Fig. 12a), porous (Fig. 12b), and dense (Fig. 12c)

iron growth of the fluxed composite pellets during reduction substrate of wustite, progressively producing bridges, whichconsolidate and produce aggregate particles. The reactionat temperatures 1073, 1173, and 1273 K, respectively, i.e. in

steps of 100 K. These results indicate three kinds of iron may initially proceed by the diffusion of interstitial carboninto the matrix of ore iron oxide, becomes propagatedprecipitation, which are seemingly governed by the temper-

ature and the corresponding reduction potential during by interfacial reaction gas expulsion and reaction withiron oxide, and is then followed by shrinkage during ironthe reduction process. The visual differences could also be

attributed to thermal expansion in the pellets, and variations precipitation at the hypostoichiometric iron oxide FeOx

layer. This may eventually cause pores to expand owing toin packing density during pelletisation. In Fig. 12a, the



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a

b

c

a 20 min; b 45 min; c 60 min

a

b

c

a 1173 K; b 1273 K; c 1323 K13 Microstructural changes in fluxed composite reducediron pellets with increase in residence time: Fe/C=77, 14 Microstructures of fractured fluxed compositeB2=20, T=1323 K reduced iron pellets showing effect of temperature:

Fe/C=77, B2=20, T=60 min

internal gas pressure. These pores are further increased as Figure 15 shows the effect of lime addition on the reductiona result of the reduction of wustite to metallic iron. characteristics of the pellets. The reaction rate was found

Figure 14 depicts the fluid phase formation. The fluid to increase with higher lime addition. The SEM images inphase presumably comprises an amalgamation offluid oxides Fig. 15 indicate significant changes in the morphology of(slag: CaOFeOSiO

2) and molten metal mass (reduced iron precipitation owing to the presence of CaO, and this

wustite), which can drip off the wustite surface. It does not must account for the change in reducibility. It is possibleclose the pores, however, as the slag generally does not wet that an increasing CaO percentage decreases the solubilitythe carbon. It spreads quickly across the carbon particles of FeO in the slag phase and inhibits fayalite formation.owing to an apparent lowering of interfacial tension when It is also fairly likely that additional CaO may changethe reactions take place. Subsequent transport of carbon into the swelling behaviour and porosity thereby enhancingthe melt may occur via a dissolutionprecipitation reaction reducibility, as CaO favours the nucleation of porous iron.mechanism, in which the carbon precipitates out of theslag phase and wustite/carbon interface. The dissolution of

CONCLUSIONScarbon into the metallic fraction of the moment mass canThe reduction kinetics of fluxed composite pellets obeysdecrease the melt temperature, thereby resolidifying thefirst order type models, and shows a significant dependencecarbon deficient iron phase and preventing dripping of theon temperature. The overall rate of iron oxide reductionfluid oxides. Further transport of carbon can also occur viaof fluxed composite pellets is seemingly controlled by thesolid state diffusion through intermediate iron oxide in the

reduced ore layer, and the reduction continues. gasification of carbon by CO2

and, being highly endothermic,



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described in the present work. It has been shown by SEMthat lime additions influence the iron morphology andenhance the reduction and strength properties of the pellets.Binder is not seemingly a significant parameter beyond acertain value. However, this parameter is very importantin attaining the green strength of the pellets. This in turnaffects the compressive strength after reduction (CSAR) ofthe pellets.

The range of carbon and CaO contents for which aniron shell is likely to form depends on variables such asthe other gangue contents and the reduction temperature.

Therefore the optimum values for carbon and CaO contentscannot be generalised. The reduction and simultaneoussintering of the fluxed composite reduced pellets is not alinear continuum. It rather seems to be a disconnected anddiscursive montage of overlapping phenomena.

ACKNOWLEDGEMENTS

The investigators wish to thank Professor P. RamachandraRao, Director National Metallurgical Laboratory (NML),Jamshedpur for permission to carry out this work andfor the use of NML facilities during the course of theinvestigation.

REFERENCES

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b

c

16. . . and . : Ironmaking Steelmaking, 1979,a B2=082; b B

2=151; c B

2=20

6, 14.15 Microstructural changes in fractured pellets with17. . . : Metall. T rans. B, 1971, 2B, 1439.increase in basicity: Fe/C=50, T=90 min, T=1323 K18. . and . : J. Am. Ceram. Soc., 1958, 41, 445.19. . . : Jernkontorets Ann., 1956, 140, 101.

requires a high temperature to proceed at an appreciable20.

. , . , . ,and

. : Proc. 1st Int.rate. The most critical phenomenon in the reduction of fluxed Cong. on Science and technology of ironmaking (ICSTI-94),

pellets is gaseous diffusion through the reduced layer. Mass Sendai, June 1994, ISIJ, 86.21. . , . , . , . , and . : Proc. 1st Int.transfer limitations modify the rate, but seemingly do not

Cong. on Science and technology of ironmaking (ICSTI-94),influence the activation energy significantly. Thus, a highSendai, June 1994, ISIJ, 92.level of activation energy exists despite partial control by

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morphology and reduction kinetics of fluxed

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