ISIJ International, Vol. 35 (1995), No. 3, pp. 242-249

Influence of

Wustite in

GangueSpecies onGas-conveyed

HydrogenSystems

Reduction Rate of Liquid

Shoji HAYASHIand Yoshiaki IGUCHl

Departmentof Materials Science and Engineering, NagoyaInstitute of Technology. Gokiso-cho, Showa-ku,Nagoya,Aichi -ken,466 Japan.

(Received on August 25. 7994, accepted in final form on Novemberl8. 1994)

AIaboratory scale fine particles-gas conveyedsystem wasutilized to measurethe hydrogen reduction ratesof liquid wustite containing ganguesuch as 14.5 moio/•CaO, 18.2 mol"/•Si02. or 5.1 5molo/•AI203 at 1773 K.

The N2-H2mixture having various flow rates and compositions wasflowed downwardthrough a cylindrical

reactor maintained isothermally andabatch of spherical wustite particles (meandia., 58 ,4m) wasconcurrentlyfed into the reactor at a small constant rate and reduced in a hot zone.

The reduction process was found to proceed in such a mannerthat a single metallic iron sphere wasformed in the center of a wustite droplet. Rate analysis was madeaccording to one dimensional massbalance equations for particles and gas in an isothermal steady moving bed. Under relatively small reducingpotentials, it wasconcluded that the major fraction of overall reaction resistance is attributable to chemicalreaction. Theobtained chemical reaction parameters were found to be two orders of magnitudes larger thanthose in previous COreduction. CaOraised the parameter. Si02 Iowered it. and Al203 did not affect it,

relative to gangue-free one. Under higher reducing potentials, the reduction process was estimated toinclude an appreciable diffusion resistance in the liquid partic]e.

KEYWORDS:particles-gas conveyed system; Iiquid wustite particles; hydrogen reduction; rate analysis;chemical reaction rate constant; ganguespecies; rate controlling process.

1. Introduction

The gaseous reduction of liquid iron oxides is one ofthe most important reactions in ironmaking processes,being directly concerned with the reaction behaviors of

ore and slag in the lower part of a blast furnace or thesrnelting reduction processes.

Several basic researches on gaseousreduction of liquid

wustite held in a small crucible have been reported assummarized in a few review papers.1'2) The chemicalreaction rates with carbon monoxidewere not so highto measureunder ordinary gas flow rates. However, the

rates with hydrogen were too fast to determine underlaboratory gas conditions, because the reaction ratetended to be limited by masstransfer through gas film

on the melt surface.3)

Therefore, we previously intended to utilize a dilute

fine particlesgas conveyedsystem to measurethe chemi-cal reaction rate of pure liquid wustite with hydrogen.This system has the following advantages4'5): (1) Highmassand heat transfer rates through gas film into par-ticles can be expected,4) hence it is applicable to moni-tor a rapid reaction rate appearing in hydrogen reduc-tion; (2) There is no contamination from external parts,

becauseno crucible is used; (3) Asimple reaction modelfor rate analysis could be easily constructed due tospherical particles, if someassumptions can be satisfied.

C 1995 ISIJ 242

Theobjective of this study is to utilize the similar dilute

fine phrticles-gas conveyed system in order to elucidateinfluence of ganguespecies such as CaO,Si02 or Al203on hydrogen reduction of liquid wustite and the ratecontrolling process.

2. Experimentals

Pure wustite originated from a chemical reagenthematite was prepared according to the authors' pre-vious procedure.6) This wustite was mixed with each

ganguespecies in the proportion where a homogeneousliquid phase exists at reaction temperature7~9) andcompacted into a tablet. These tablets were melted byarc heating under argon to densify andhomogenizethem,then crushed and screened between definite sizes. Theirobtained fines werepremelted in a nitrogen gas-conveyedsystem to sphere them. Thesespheres were rescreened toobtain diameters between53 - 63 /tm similarly to previouswork.6) The initial compositions such as ganguecontentand nonstoichiometry y as wustite Fel _yOin each massof spherical particles were obtained from chemicalanalysis of Fe3+ and Fe2+ as seen in Table 1, where their

melting temperatures are evaluated from some refer-

ences. 7~ I o)

The vertical type cylindrical Mullite reactor (inner

diameter DT: 2.4 cm, Iength: 80cm) is the sameemployed

Table

ISIJ International. Vol. 35 (1 995). No. 3

1. Conditions of liquid wustite particles before reduction and at initial metallization.

Additive

gangue

Before reduction At initial metallization

Ganguecontent(molo/o)

y as Fel _yOin liquid

Melting temp.(K)

Ganguecontent(molo/o)

aFo*O*Calculated

fl (~)

Nil

CaOSi02Al203

O14.5

18.2

5.15

O.08720.l05

0.07130.0785

l 64410)

- 15837)

- 15438)

- 16339)

O13.6

17.5

4.99

l .OO

0.85

0.88

0.93

0.03900.05740.03860.0273

, 1~~'::- 0.8

o:~0= 0.6

~a,~*cu 0,4

=o~0.2cu*LL o

Fig.

The activity of FetO corresponding to the mol fraction of FetO in liquid wustite, NF*,o = 14'15)NF,o+NF*,03'

30N2-H2 OHe-H2~~

oloH2

2OD

10A e

-1773 K ~~

Pure wustite

D-A-

-1--C}---

0.30.1 0.2Residencetime (S)

Relation between fractional reductionresidence time for pure liquid wustite.6)

0.4

and

0.5

particle

~~5

10*O

15

O~,g*

LL

o

1

0.8

0.6

0.4

0.2

o

509GH2eo9GH2209~H2IO9GH2

O C] ~ eSi02-wustite

-L~Z~~]- - -:- o- - - - - -

o :

0,8

0.6

0.4

0.2

O

Fig

1

0.8

0.6

0,4

0.2

O

O

co~=1'o~~=O~gLL

3o~Ha2O9GH2Io~H2 59eH2

O [] A e~~~~~~~]-~~~~~!~

-

-2~--~

~~~~h a~~~~

- -t- -e - - - -

O 0.1 0.2 0.8 0.4 0.5

Residenc:.e time (s)

Flg. 2. Relation between fractional reduction and particle

residence time for liquid wustite containing CaO.

in previous work,6) which is maintained at a constanttemperature of 1773K by heating a double spiral typeSiC element. The isothermal hot zone having a 13cmlength was found within ~~ 15K for typical downwardgas flow conditions.

N2-H2 mixtures prepared through• gas purification

systems were flowed downwardthrough the reactor. Abatch of spherical wustite particles (mean dia.; 58,tm)

was fed into the upper part of the reactor at a small

constant rate by meansof a screw feeder. The feed ratio

of particles to hydrogen F* was selected to be between0.035 - 0.4 1g/Nl, wherean extremely dilute particles-gas

conveyed system was found to be realized as indicated

by our earlier paper.6) During falling of these particles,

they are melted, reduced, solidified, and finally collected

in a cold trap. The residence time of particles in anisothermal zone T and the overall reduction rate can becontrolled by shifting both flow rate Qand hydrogencontent of gas mixtures in the range of Q=0.6-3.0

=O~;

=~C:,

=O~C:,

*LL

0.1 O.2 0,4 0.50.3

Residenoetime (S)

Relation between fractional reduction and particle

residence time for liquid wustite containing Si02'

309GH220%H2IO9GH25%H2O [] A e

~ZZ~]-

~-

243

O 0.1 0.2 0.3 0,4 0,5Residencetime (s)

Fig. 4. Relation between fractional reduction and particle

residence time for liquid wustite containing Al203'

Nl/min and 5- 50 volo/o H2, respectively.

The total iron content in particles after reduction(masso/o Fe)A is determined by the titration of K2Cr207solution (JIS M8212-1983). The fractional reduction at

bed bottom fl can be calculated from the results ofchemical analysis by Eq. (1), no matter whether including

ganguesor not.

,fl = IOOx {I - (masso/o Fe)B/(masso/o Fe)A}/(masso/o O)B.(1)

3. Results and Discussion

3.1. Results of Gas-conveyedRe4uction

Figures 1~t showthe variation of fractional reductionfl with residence time of particles T at 1773K for fourkinds of specimens. Figure I wasalready obtained in ourprevious study,6) The values of T can be calculated by

C 1995 ISIJ

ISIJ lnternational. Vol. 35 (1995), No. 3

Fig. 5. Reflective electron images of cross sections of particles after reduction at 1773K, a): CaO,N2-200/0H2,Q=1.5Nl/min, fl=0.360, b): Si02' N2-30"/oH2' Q=1.0Nl/min, fl=0.385, c): Al203' N2-200/*H2'

Q= I.5 Nl/min, fl =0.286.

Eq. (2) using Stokes's terminal velocity of a sphereUs (cm/sec),11) assuming that particles are falling at aconstant velocity given at the upper side of hot zone in

a reactor. Becausea maximumvariation of T value wasestimated to be only by 4"/o if considering density anddiameter of particles changing during reduction. Theinfluence of gangueon the value of particle density ppis ignored in this work. The value d* is corrected usingthe values pp of pure wustite (y=0.080) of 4.212) and5.613) g/cm3 at reaction and room temperatures, respec-tively.

Us=9(pp-pg)d"2/18~g, Up=U0+Us,T=L/Up

.

(2)

For all of reduction tests, someof the incubationperiods until initial metallization were observed in theearly stage. The reduction level at initial metallization

can be calculated as shownin Table I using the chemicalcompositions of liquid wustite containing gangues in

equilibrium with iron measuredby Ban-ya et al.14,15)

Since these compositions are roughly insensitive totemperature, their results obtained at 1673K are ap-plicable to the present work. The incubation periods

were found to becomelonger with less H2contents. Theyalso tended to becomeslightly shorter by CaOand longerby Si02 than those without gangues. As a whole, thevalues fl rose with increasing 1, values. Theseincrementalrates of fl with T values were enhancedwith moreoloH2and finally they seemedto be saturated. It was foundthat the reduction rate is enhancedby CaO,Iowered bySi02, and not varied by A1203.

3.2. Reduction Rate Analysis

Similarly to the authors' previous research,6) the ex-perimental results obtained in this particlesgas con-veyed system were analyzed by a simple mathematicalmodell6) which is available to a moving bed system,becauseboth reaction systemsmaybe basically the sameexcept for extremely large difference in the voidage ofthe beds.

The chemical reaction for reduction of liquid wustitecontaining ganguewith hydrogen and the standard free

energy changeof the reaction AG' (J/mol) can be expressedby Eqs. (3) and (4), respectively. The activity of liquidwustite is relative to pure liquid wustite saturated with

C 1995 ISIJ 244

HH20

N2_HGasfl

Fig. 6.

1.

2.

3.

Im

Liquid wustite

Metallic iron (s)

rfacial chemicalreaction

Elementary steps

GasphasemasstransferInterfaclal chemical reactionLlquid phasemasstransfer

Amain reaction model for a single particle.

solid iron.

FeO(in liquid wustite) +H2(g)=Fe(s) +H20(g).(3)

AG'=-30397+1928T (T 1665 1809K14))

.

(4)

From the observation of cross sections of wustiteparticles after reduction, it was found that in the earlier

periods of reaction someof dense metallic iron particles

wereentrapped inside a wustite droplet and then the ironparticles tended to grow into a completely single spherewith higher values of fl as shownin Figs. 5(a) to 5(c),

similarly to pure liquid wustite,6) though metallic ironsexist as solid state at this temperature.

Based on these observations, a main reaction modelfor a single wustite particle can be constructed as drawnin Fig. 6, where both the processes of mass transferthrough gas film and interfacial chemical reaction aretaken into account as the first analytical procedure.

The reduction rate of particles per unit bed volume in

term of H2, R* (mol H2/cm3bed • sec), can be formulatedas a reversible first-order reaction by Eq. (5), consideringshrinkage of a particle during reduction and activity ofFetO in liquid wustite containing gangueaF.,o' FetO is

usually defined to have mole fraction NF.*0=NF*o+14)NF*,o.'

R*3( I- e)(1 - af )2/3(c

- c'). .... ...

(5)ro{1/kf +K/k.(1 +aF.,oK)} """"'

ISIJ International, Vol.

Where k, (cm/sec) is an apparent forward chemicalreaction rate constant. "Apparent" means that the

parameter k, may include other influence besides

chemical reaction in somecases. Assumingboth piston

flows of particles and gas, onedimensional massbalanceequations for particles and gas in an isothermal steady

concurrent movingbedcan be constructed. In this work,

an isothermal hot zone indicated in Chap. 2 is taken as

a bed length L. Thevalues Upand 8 along the bed wereassumedto be invariant at the values given at the bedtop.

Mass balances for hydrogen and wustite Fel_yOaround an infinitesimal height dz at position z from the

bed top can be formulated by Eqs. (6) and (7),6,16) re-

spectively.

dc

dz

df MR*dz S

Applying several dimensionless parameters, n=z/L,oc = Uor0/3(1 - 8)Lkf

' X= (c - c')/(co ~ c'), ~=kf K/k.(1 +aF*,oK), and ip'=MUo(co ~c')/S, Eqs. (6) and (7) canbe converted into dimensionless Eqs. (8) and (9), re-

spectively.

dx_ - X(1 -af)2/3

..........(8)

dn oe(1 +6) """

df_

ip 'X(1 - af )2/3

.. . .... ...(9)

dn oe(1 +~) """'

Combining the condition of X=X, f=f at n=n (in

bed) and two boundary conditions, X=1, f= Oat n=0(at bed top) and X=Xl, f=fl at n= I (at bed bottom)with Eqs. (8) and (9), Eq. (10) can be derived as a coupleof integral forms from Oto f of f and from Oto nofn.

ff df_

fn=

n.....

(lO)

o '~ ~ ~dn

(ip f)(1 af)2/3o

oc(1 +6) oc(1 +5)

By setting (1 -af)1/3 =~and aip' - I =b3, the integral

term on the left side of Eq. (10) can be analytically solved

as a form of Eq. (1 l).

3

df ~ d~3(c'-f)(1 -af)2/3 ~

1~3+b

35 (1995), No. 3

rate constant k, can be converted into another expressionk. (g/cm2 , sec' atm) by Eq. (12).

k.=Mok./RT......... ..........

(12)

3.3. Results of Rate Analysis

Asshownin Figs. I - 4, incubation periods until initial

appearance of metallic iron were observed in the early

stage of reduction. Assumingthat the activity aF.*o in

liquid wustite containing gangueat the initial metalliza-

tiQn (Table l) remains invariant during reduction, rateanalysis wasactually done for the period using Eqs, (10)

and (1 1) together with the fractional reduction at bedbottom based on initial metallization fl' and the condi-

tion of x= I at fi' =0. The small amount of steamproduced before initial metallization can be neglected,

probably due to I - 2orders of magnitudes less reduction

rates of solid wustite than liquid.3'6) Thephysical prop-erties for gas were estimated using conventional expres-sions.18) The molar densities of wustite and iron in

particles p~=0.0609 and pF. =O. 127 (mol/cm3)12,19) andthe equilibrium constant of Eq. (3) as K=0.773714) wereemployed, respectively.

Figures 7- 10 show the relation between k, and Tobtained at 1773K for four kinds of specimens. Exceptfor the conditions of less o/oH2, each constant value ofk, wa~ obtained depending only upon gangue andhydrogen content, roughly not upon T value.

3.5

~ 3E~!(:l

. 2,5'O

E 2O'\_O' 1.5~D

x 1~~

0.5

Oo

Pure wustite

1773K 30N2-H2 OHe-H2e

.kH220 1o 5D A O

l In(~+b)21 1-b+b21

=- 2b2 (1 +b)21 ~2-b~ +b2 larctan

2~~b 2-b- - arctan .bb2 Ib

(11)

Whenthe condition of n= l, the fractional reduction

at bedbottom fl' andthe several experimental conditionsof T, Q, o/oH2, S, and aF.,o are substituted into Eqs. (10)

and (1 l), they lead to a rate constant k. (cm/sec). Themasstransfer coefficient through gas film arounda spherekf wascalculated by RanzMarshall's equationl 7) usingslip velocity betweengas and particles U. (cm/sec). This

245

Ae

~'~~~A'~'~'~'~-'-'-'-'-'~-X-'-'-'::::-.-.-.

e.-::::::

-.-.-.-.---- ~ ~

Fig. 7.

3,5

E 3~,g~. 2.5

~E2o~~~:' 1.5

x 1~

0.5

O

0,1 0.2 O.3 O,4 0.5

Residence time (S)

Relation between apparent chemical reaction rate

parameter and particle residence time for pure liquid

wustite.6)

eo9GH220'GH2IO'~H2 59~H2 CaO-wustiteO D1773K

A eA e

o_._._._._A o _._.___A._._._._._

~'~

~._._._.~!_

_ ~_ l.A

~'~'~' ~~f'~~c~..-.-. -~ l:r'-a-.-.a-'- --'- ty'- -.-.- .

CD"~~5~ ~'

Fig. 8.

o 0,1 0.2 0.8 0.50.4Residencetime (s)

Relation between apparent chemical reaction rate

parameter andparticle residence time for liquid wustite

containing CaO.

C 1995 ISIJ

ISIJ International, Vol. 35 (1 995), No. 3

E~t'

coj

~E

o~b_

~,

i

3,5

32,5

2

1,5

10,5

O

5O9GHe8Q~H22O'GH2I09~Hz

O C] A e Si02-wustite

i773 K

-,cl'--A-'--e"''-'-'-'-'-'-Q--

=-~C~';-'~x5~:~-"t-'~s~~~~~l~E'-~f_"~:::cj~

Fig. 9.

O 0,1 0.2 0,3 0,4 0.5

Residencetime (S)

Relation between apparent chemical reaction rate

parameter andparticle residence time for liquid wustitecontaining Si02'

8co

~~

:~

15

Eo

o~,

c:o'S::

~aC

20

15

10

5

o

1773KQ: O,eo NIlmin

A

o

e Nll, He-H2G)

3.5

Additives

D Ni] 8)

o CaOA Si02

Al203

o

Fig. 12.

1O ro GO 5040

Hydrogencontent (vol,%)

Hydrogencontent andganguedependencesof overall

reduction rate per unit surface area of particles.

~,U

co.

EO~l

~.

32.5

21,5

0.5

o

Table 2.

a09GH22O,GH2109GH25~,H2

O D A O' Al208•wustite

1773K eA

A ~ O~~~::: i': i~:~!._.~"~!T':r:::_"~:.~iiLt__:~__

i:1-'-

-d;~ D A ~]O'-'-'-~'-'-'--'~:.-.-.O~~' :]:1:~1

O

Forward chemical reaction rate parameter, k:-

Additivek: x 102 (g/cm2 ' sec ' atm)

Nil CaO Si02 Al203

This work(T= 1773K)

Nagasakaet a/,3)

(T= 1673K)

Fig. lO.

2.5

l .586)

1.6

2, 19 1.08 l .62

E-!G,.

CO.

~_*O_C.~

~.

~

0.1 0,2 0,8 0.4 0.5

Residence time (S)

Relation between apparent chemical reaction rate

parameter and particle residence time for liquid

wustite containing Al203.

2.0

1.5

1.o

0.5

0.0

1773K

O

~

AdditivesNil 6)

CaOSi02Al203

e Nll, He-H26)

Fig. 11.

o 1O 20 80 40 50Hydrogencontent (vol.%)

Hydrogen content and gangue dependencesof ap-parent chemical reaction rate constants.

Theseaverage values of k, are plotted against hydrogencontent in Fig. Il for four kinds of specimens. As awhole, k, values decrease with more o/oH2, whilst CaOraises them, Si02 Iowers them, and Al203 has no effect.

Figure 12 showsthe relation between the reduction rate

per unit surface area of particles at bed bottom based

on initial metallization vl (mol H2/cm2sec) and oloH2 atthe condition of Q=0.80 Nl/min, where the value of 1: is

nearly 0.37 sec. Thevalues of vi can be calculated by Eq.(13).

vi=roRt'

..........(13)3(1-e)~i2 """

C 1995 ISIJ 246

Below 10 "/. H2, the reduction rate vi increased linearly

with o/oH2, and the value of k, reached a nearly constantlevel depending on the kind of gangue. Therefore, underthis condition of less hydrogen contents it can beconcluded that the chemical reaction should be the ratecontrolling step.

Beyond lO~/o H2, the reduction rates vj deviatedappreciably from the linear relation and the values of k.

decreased considerably.

To investigate these results, further tests werepreviously done at 1773K to react pure wustite with aHe-30010H2mixture. These results are included in Fig.

1.Since this gas mixture has nearly two times larger

values of kf and hp than the N2-H2mixture,6) it is ofinterest how both mass and heat transfer processesthrough gas film affect the overall reduction rate.

The results obtained for the He300/0H2mixtureclearly indicated that except for slightly shorter in-

cubation periods and certainly higher fl as seen in Fig.l, the obtained value of k, was in accord with thatin the N2-30010H2mixture, as included in Fig. 11.

Therefore, it is supposed that the decrement of k, withhigh o/.H2 is attributable to an increment of diffusionresistance within the liquid phase as drawn in Fig. 6,

exclusive of both mass and heat transfer processesthrough gas film. Such supposition has been already

madein our previous paper.6)

Table 2 shows the forward chemical reaction rate

constants k~ (g/cm2 . sec ' atm). Theywereobtained at the,condition of 5o/. H2Wherethe chemical reaction shouldlimit the overall rates. The value k.' for pure wustite6)

was analogous to that reported by Ban-ya et al.3) whoreduced pure wustite melted in an iron crucible at 1673Kwith hydrogen diluted by inert gas. In this work it wasfound that CaOmakesthe value of k~ 1.4 times, Si02

ISIJ International, Vol. 35 (1 995). No. 3

'~E~!'C,

'o,

'~E

o'\o'~'

~

0.1

0,01

0,001

0,0001

1E-05

D ~ o-A-

~Lf':*~*+~l:~r

~j•~IL,A

o

This workH2reduotlon

T*1773KAddith/es

D Nil 6)

O CaOA Si02

Al203

o:

.4..;,'

Addruvee:: Nll

o CaOA SI02~ A]20e

~~5f*~~~~

e~/{,:--_______

2;' .

EI'Rehalby22)

A sl02

COreductlonT*1673KNRgasaka20)

AAA A: *~~~J

r7~:~X:;l~i:Tl18otope Exoh.

Fig. 13.

1.O 0,9 0,8 0,6 0.50,7

NFetO

Comparisonfor influence of gangueson the chemicalreaction rate constants in COand H2 reduction ofliquid wustite.1)

~] Gasphase ~] Chemical Reaction ~~ Liquid phase

~~~00

a,

O:,a

,,,

'O,p

,gEOOC9

LL

120

1oo

80

60

4o

20

O

Wustite CaO-wustite Si02-wustite Al203-wustite

~

~~

~~~~~;/'~'

~;;~_~;

;~~~;~~

.

~~;,~~'

~;.~:~

~;~;~.

~~i ~//~;

~~

-~;~;.

~~~!•1/~~

"""~//~;

~~

'/.~;

~;;~

~~

.~;~;

~.~;~~

'1/~;~';:~,

~~;

~~~

~i

,(

~:~

~~

~~

~~~

~;~;

~;~~;

~~~:~~:~.

~;,;~.

~~;~~~;

~

~;

~~;

Fig.

5 Io 20 3o 5 Io 2o 3o lo 20 30 50 5 Io 20 30Hydrogencontent (vol.'/.)

14. Fractional resistances estimated for the condition of

Q=0.80 N//min at 1773K.

makesit 0.68 times, and A1203has little effect for it tobe 1.03 times larger than gangue-free one. The influenceof gangue on COreduction rate of liquid wustite hasbeenmeasuredat 1673Kby Nagasakaet al.20) using aniron crucible. Their results indicated that the addition ofCaO,Si02 or A1203with the samecontents as this workmakesthe value of k: 2.0, 0.60 or 1.01 times higher thangangue-free one, respectively. Such influences coincidewith ours, as compared in Fig. 13, in spite of largerdifferences of two orders of magnitudes between boththe values of k: with H2and COreduction.

Figure 14 showsthe fractional resistance against total

process resistance obtained for the condition of Q=0.80Nl/min at 1773K (T=0.370-0.373sec) for fourkinds of specimens. The resistance for masstransfer in

liquid phasecan be evaluated by deviation from the linearrelation in Fig. 12. It can be seen that below 100/0H2major part of resistances is occupied by the chemicalreaction process with negligible gas phase resistance of1.4-7.8 percents, while beyond 10010H2the diffusion

process in liquid phase can not be ignored and the

diffusional resistance increases with higher "/oH2'

3.4. Reduction MechanismOurprevious study6) indicated that in the earlier stage

of reduction someof fine iron particles are entrapped in

a single liquid particle and beyond the subsequent stageof reduction these fines coalesce and grow to form asingle iron particle. Such entrapping in the liquid andthe growth as a single iron particle are probably becausethe interfacial energy betweeniron and liquid is less thansurface energy of iron under the given condition and therotation energy of a particle becomesminimumin thesituation where an iron particle of larger density existsin the center. These discussion is also appeared in thesimilar study donerecently by Nozawa~t al.23) Asshownin Fig. 5, the infiuence of gangue on these reactionbehaviors of a particle was analogous to gangue-freeones,6) though fine solidified structures were observddaccording to those expected from each ganguewustitephase diagram.8 ~ I o)

As indicated in Fig. 14, under higher reducingpotentials it was concluded that the overall reductionrate should be considerably affected by mass transfer

process in liquid wustite. This transfer process (Fig. 6)is usually considered to take place along with elec-

trochemical reactions such as Eqs. (14) and (15), ana-logous to those in solid wustite. It is also clear that thesummation of both Eqs. (14) and (15) results in theoverall chemical reaction Eq. (3).

FeO(in liquid wustite) +V" +2h ' +H2(g)

=Fe2+ (in liquid wustite) +H20(g)...............

(14)

Fe2+(in liquid wustite) =Fe(s)+V" +2h'........

(15)

WhereV" and h' are a vacancy and a positive holein liquid wustite, respectively. The following reaction

progress is considered based on photographs of Fig. 5and previous ones6): first, reaction (14) occurs on thesurface of liquid to produce Fe2+ ions there; theseFe2+ rons mterdiffuse agamstV" h' and 02- species;

whenthe content of Fe2+ near the surface exceeds theFe(s)/FeO(1) equilibrium and reaches the critical super-saturation, several iron nuclei are generated there ac-cording to reaction (15); in the earlier stage after initial

metallization (fl ~0-0.2), the growth of these nucleiwith diffusion of Fe2+ toward themandprogress of reac-tion (15) and their transfer inward a liquid occur withtheir subsequent coalescence; and during major periods(fl ~~ 0.2 - ), Fe2+ ions produced by reaction (14) diffusesteadily toward a central iron sphere under their

supersaturated state in a liquid, as seen in Fig. 5or Fig.6. This situation meansless oxygen potentials near theliquid surface than the FelFeOinterface in the figure. If

assuming a I o/o content difference of iron cation Fe2+in liquid wustite betweenboth surfaces of a central ironsphere and a liquid wustite particle and using Fick's first

law of diffusion and the interdiffusivity of species in liquid

wustite of D=3.2x l0~4cm2/sec,24) a meandiffusiondistance of iron cations in liquid wustite can be estimatedto be 15,tm at the condition of Q=O.8 Nl/min, 30 "/* H2,and 1773 K. This value is less than a particle diameter

247 C 1995 ISIJ

ISIJ International, Vol.

d~, corresponding roughly to the structures as shownin Fig. 5. Therefore, this I o/o supersaturation degree ofFe2+ ions seemsto be reasonable.

Asmentioned in Sec. 3.3, rate analysis wasperformedassuming that the activity ap.,o in liquid wustite con-taining gangue at initial metallization (Table l) re-

mains invariant during reduction. This assumption wassatisfied enoughin the cases of CaOand Al203 becausethe values of k, obtained for these both gangues keptnearly constant during reduction, dependent only uponoloH2 and gangue. However, in the case of Si02 for

50 o/. H2, it was found that values of k, decrease in thelater stage of reduction (Fig. 9, fl >0.5), probably dueto enrichment of Si02 in liquid and lowering aF.,o'Therefore, the value k, obtained in the earlier stage ofreduction (Fig. 9, fl 0.5) was plotted in Fig. 11. Forexample, it can be evaluated that Si02 enrichment in

liquid reaches as high as 25.4masso/o Si02 for fl =0.5

from initial 14.6 mass"/o Si02, being less than the satura-tion content of 43 masso/. Si02 in liquid at 1773K.

In present reactor, the isothermal hot zone having

a 13cm length was found within ~15K for typical

downwardgas flow conditions and the rate analysis wasapplied to this hot zone. However, the nonisothermal

zone beyond the melting point of particles up to this

isothermal zoneactually exists in the reactor wheresomeof reaction between liquid and gas mayproceed. Thelength of this melting zoneother than an isothermal zonewill dependon gangues as supposed from the meltingpoints in Table l. These problems will be followed asnext studies.

4. Conclusion

Utilizing the dilute particles-gas conveyed reaction

system which has a characteristics to provide high massandheat transfers through gas film to particles, hydrogenreduction tests of liquid wustite containing ganguespecies

were carried out with N2-H2gas mixtures at 1773K.The following findings were derived.

(1) The reduction process was found to proceed in

suchamannerthat major of metallic iron particles existed

as a single sphere in the central part of a wustite droplet,in spite of containing gangues.

(2) Rate analysis using one dimensional massbal-

ance equations under an isothermal steady moving bedprovided the apparent chemical reaction rate parametersk.. Under relatively small reducing potentials, it wasconcluded that the chemical reaction should be the ratelimitin*' process. Theobtained forward chemical reaction

parameters k~ werefound to be two orders of magnitudeslarger than those in previous COreduction. In this H2reduction, CaOraised the parameter, Si02 Iowered it,

and Al203 did not affect it, relative to free of gangue.(3) Under higher reducing potentials, Iess values of

k, were obtained, no matter whether including ganguesor not, supposing an appreciable increment of diffusionresistance within the liquid phase.

Nomenclature

a : parameter, = (p~-pF.)/p~ (-)

C 1995 ISIJ 248

35 (1995), No. 3

aF*,o : activity of FetO in liquid wustite equili-brated with solid iron (-)

b: parameter, aip'-1=b3c, co, cl, c': hydrogen contents in bed, at bed top, at

bed bottom, and at equilibrium betweenFe/liquid wustite containing gangue, re-spectively (mol/cm3)

d~ : diameter of wustite particle (cm)

D: interdiffusivity ofiron andoxygenin liquidwustite (cm2/sec)

DT: diameter of reactor (cm)ffl : fractional reduction in bed, at bed bottom

(-)fl' : fl based on initial metallization (-)

FetO: wustite in liquid having mole fractionNF.,o =NF.o+NF.,o,

F. : feed ratio ofparticles to hydrogen (g/Nl)

g: gravitational acceleration, =980 (cm/sec2)

AG': standard free energy change of reaction(Eq. (3)) (J/mol)

h' : positive hole in liquid wustitehp : heat transfer coefficient between gas and

particle (J/cm2 . sec ' K)k. : apparent forward chemical reaction rate

constant (cm/sec)kf : masstransfer coefficient between gas and

particle (cm/sec)k~ : forward chemical reaction rate constant

(g/cm2 . sec ' atm)k* : apparent forward chemical reaction rate

constant (g/cm2 . sec ' atm)K: equilibrium constant of reaction (Eq. (3))

(-)L : bed height (cm)

(masso/o Fe)B, (masso/o Fe)A : total iron masso/o of par-ticles before, after reduction

Mo: gramatomic weight of oxygen (g/g-atom)

M:(masSo/oO) :B

NFeo' NFe203

NFe*o

Q:

ro :rp

.

R:R* .

Rt' -

S:t:

T:Uo:Up:

molecular weight of wustite (g/mol)reducible oxygen masso/o of particles be-fore reductionmole fraction of FeO, Fe203 in liquid

(-)mole fraction of wustite in liquid defined

as NF.,o =NF,o+NF.,o, (-)volumetric flow rate of gas mixture(Nl/min)particle radius at bed top (cm)particle radius considering shrinkage at

stage of f, =(1 -af)113ro (cm)gas constant, =82.054 (cm3atm/K 'mol)reaction rate of particles per unit bedvolume in term of H2 (Eq. (5))

(mol H2/cm3bed ' sec)

R* value at bed bottom based on initial

metallization (mol H2/cm3bed • sec)

feed rate of particles (g/cm2 . sec)

reduction time (sec)

temperature (K)superficial gas velocity (cm/sec)particle velocity relative to tube wall(cm/sec)

ISIJ International, Vol. 35 (1 995), No. 3

U. : slip velocity between gas and particles

(cm/sec)

Us: Stokes's terminal velocity of a falling

sphere (Eq. (2)) (cm/sec)vj: reduction rate at bed bottom per unit

surface area of particles based on initial

metallization, Eq. (1 3) (mol H2/cm2sec)

V" : vacancy in liquid wustite

y: nonstoichiometry of wustite Fel_yO (-)z : axial distance from top level of bed (cm)

Greek letters

oe : dimensionless parameter,

= Uor0/3(1 - e)Lkf (~)8: dimensionless parameter,

=kf K/k.(1 +aF.,oK) (-)8: voidage ofbed (-)

n: dimensionless distance from top level ofbed, =z/L

'lg : viscosity of gas (g/crn ' sec)

~, ~i : parameters =(1 -af)1/3, =(1 -afl')1/3 (_)P~, PF. : molar density of wustite, iron (mol/cm3)

Pg: gas density (g/cm3)

pp : Particle density (g/cm3)

T: residence time of particles (sec)

ip': dimensionless parameter,

=MUo(co~c')/S (- )X: dimensionless hydrogen content in bed

= (c- c')1(co ~c') (-)Xo,Xl: Xat top, bottom ofbed (-)

x{ : X1 based on initial metallization (-)

Acknowledgements

The authors gratefully acknowledgethe experimentalsupports provided by undergraduate students, NagoyaInst. of Tech., Messrs. K. Mizuno. M. Abe and K.Murayama,

l)

2)

3)

4)

5)

6)

7)

8)

9)

lO)

1l)

l2)

l3)

l4)

15)

l6)

l7)

l8)

19)

20)

21)

22)

23)

24)

REFERENCEST, Nagasakaand S. Ban-ya: Tetsu-to-Hagan~, 78 (1992), 1753.T. Fuwa: Bull. Jpn. Inst. Met., 26 (1987), 365.S. Ban-ya, Y. Iguchi and T. Nagasaka: Tetsu-to-Hagan~, 70(1984), 1689.

N. J. Themelis and W. H. Gauvin: AIChEJ., 8(1962), 437.F. Tsukihashi, K. Kato, K. Ohtsuka and T. Soma: Tetsu-to-

Hagan~,68 (1982), 750.S. Hayashi and Y. Iguchi: ISIJ Int., 34 (1994), 555.E. Schtirmann and G. Kraume:Arch. Eisenhattenwes., 47 (1976),

435.

A. Muanand E. F. Osborn: PhaseEquilibria amongOxides in

Steelmaking, Addison-Wesley Pub, Co,, Mass., (1965), 61.

A. Muanand E. F. Osborn: PhaseEquilibria amongOxides in

Steelmaking, Addison-Wesley Pub. Co., Mass., (1965), 76.

A. Muanand E. F. Osborn: PhaseEquilibria amongOxides in

Steelmaking, Addison-Wesley Pub. Co., Mass., (1965), 28.

J. Szekely and N. J. Themelis: Rate Phenomenain ProcessMetallurgy, Wiley-Interscience, (1971), 607.

K. Mori and K. Suzuki: Tetsu-to-Hagan~, 54 (1968), 1123.

M. OnodaandA. Sasaki: Bull. Res. Inst. Miner. Dressin. Metall.

24 (1968), 104.

S. Ban-ya and T. Watanabe: Tetsu-to-Hagan~, 63 (1977), 1809.S. Ban-ya, A. ChibaandA. Hikosaka: Tetsu-to-Hagan~, 66 (1 980),

1484.J. Yagi, A, Moriyamaand l. Muchi: J. Jpn. Inst. Met., 32 (1968),

209.

R. B. Bird, W. E. Stewart and E. N. Lightfoot: TransportPhenomena.John Wiley &Sons, NewYork, (1960), 409.S. Omiand T, Usui: Tetsu-to-Hagan~, 59 (1973), 1888.

L. D. Lucas: Compt. Rend., 250 (1960), 1850,

T. Nagasaka, Y. Iguchi and S. Ban-ya: Tetsu-to-Hagan~, 75(1989), 74,

Y. Sasaki, S. Hara, D. R. Gaskell and G. R. Belton: Metall.Trans. B, 15B (1984), 563.S. K. EI-Rahaiby, Y. Sasaki, D. R. Gaskell and G. R. Belton:Metal/. Trans. B, 17B (1986), 307.

K, Nozawa,M. Shimizu andS. Inaba: Tetsu-to-Haganb, 79 (1 993),

443.

K. Mori and K. Suzuki: Tetsu-to-Hagan~, 54 (1968), I199.

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