ISIJ International, Vol, 31 (1991), No, l, pp. 17-23

Factors Governing the Strength of Agglomerated Granules after Sintering

Eiki KASAl, Shengli WUand YasuoOMORl

Research Institute of Mineral Dressing and Metallurgy, TohokuUniversity, Katahira, Aoba-ku, Sendai, Miyagi-ken, 980 Japan.

(Received on M~y2, 1990; accepted in the final form on July 20, 1990)

Segregations of materials exist moreor less in iron ore sintering beds and they are essential for the sintering process.Therefore, control of the segregations can be a key technology to improve the performance of the process. Granuledesign for raw materials seemsto be an effective technology to realize it. However, there is still insufticient information

available for establishing the technology. This study focused to estimate factors governing the strength of sintered

granules and following results were obtained: Fluidity of the fine mixtures in high temperatures can be qualitatively esti-

matedconsidering liquidus temperature indicated in an appropriate phasediagram which corresponds to the main compo-nents ofthe mixtures. Fair linear relations wereobtained betweenthefluidity ofthe adhering layers of each type of modelgranules and the strength of the sintered granules. Chemical composition of the adhering layers and fraction of large

(+0.01 mm)pores and loss on ignition of the core particles are the estimated dominant factors governing the strength of

sintered granules.

KEYWORDS:sintering of iron ores; raw materials; granule design; fluidity cf melt; Iiquidus temperature; strength;

agglomeration.

1. Introduction

Thenormal size rangre ofraw materials used in thesintering process of iron ores is less than 8mm. Sucha wide size distribution causes somesegregations in

sintering beds, e.g., chemical composition and heat

source. A certain extent of the segregations are pre-ferable for an appropriate sintering performance be-

cause a partial and inhomogeneousfusion of rawmaterials is essential for the agglomeration of thegranules of sinter mixtures. However, extreme seg-regation will promote excess inhomogeneity of gasflow through the sintering beds and temperature pro-files in the beds, and therefore will result in a decreasein yie]d and hence productivity of sinter products.

From this point of view, the quantitative control ofthe segregations of raw materials is a key factor so thatthe sintering process can be optimized depending onthe variation of the materials, without serious chantgesof operational conditions.

Several new processesl-6) have recently been pro-posed, that provide for the control of the segregations

by designing mixtures for raw materials. The HPS,~lybrid ~elletized ~inter, process5) has been commer-cially applied by a Japanese steel mill, which does

not require sibcrniflcant flow of melts during sintering

of the materials and is different from conventionalsintering processes. The characteristic of the HPSprocess is the simultaneous suppression of segregations ;.fluxing materials by using large quantities of fine iron

ore (pellet feeds) and burnt lime) and heat source by

meansof the secondary granulation of flne (- I mm)coke to the surface of completely pre-tgranulatedpellets. Taking account of current raw materialsfor sintering, however, complete granulation is dif-

ficult for the materials having wide size distribution.

Further, there may be a restriction in the HPSprocess, that it is necessary to use high grade iron

ores because the use of an small amount of fluxingmaterials is essential to suppress the fusion of materials

and the flow of the formed melts during sintering.

The other processes positively utilize the segregations

to control the formation of melts and its flow in sin-

tering beds. These processes can be sorted into twotypes : One is that materials are segregated within

a granulel-8,6) and another is that two separatedgranulation lines are used so as to realize a segrega-tion whencharged in sintering machines.4)

The technology which aims the control of the seg-regation during mixing and g)ranulation of raw mate-rials is often called " granule design ". In order toestablish this technology) systematic and quantitativeinvestigations are necessary, e.g., the relations amongthe physical and chemical properties ofraw materials,reactivities of ores and fluxing materials, properties ofmelts formed as results of the reactions and, of course,properties of sinter products. As a first step for suchinvestigations, the authors tried to evaluate the rela-

tive fluidity of fine mixtures in hit)cth temperatures andfound that the fluidity of the adhering layers relates

clearly to the strength of the sintered granules.7) This

paper describes the results of further experimentsusint)cr various types of model granules consisting ofdifferent kinds of core particles and adhering layers.

It is discussed about the main factors governing thestrength on the basis of the experimental results.

2. Materials Used

Chemical compositions of materials used in the

C1991 ISI J 17

ISIJ International, Vol. 31 (1991), No, l

Material

Ore POre MOre HOre RAlumina sphere ASintered alumina SAPorous ceramics sphere PCLimestone

Table l. Chemical

T. Fe

68.76

69.30

62.40

57.OO

4.lO

O.55

FeO

27.OO

O.32

0.16O. 15

composition

Si02

l .79

0,13

4.63

5.05

6.92

13.38

54.05

2.20

of materials

CaO

used.

O.ll

O.05

O.09

0.31

6.87

53.77

Al203

o.

o.

2.

92.

84.

27.

o.

39

223917

6558

9730

MgO

O.04

O.05

O.02

0.lO

l .60

l.16

O.46

lg, Ioss

O.44

3.34

l0.27

O.60

41.50

experiments are listed in Table l. Mand H arehematite ores from Brazil and Australia, respectively.

Ris limonite ore and P is magnetically separated ore.Dense alumina spheres A, sintered alumina SAandporous ceramic spheres PCwere used as psudo oresfor the comparisons with the iron ores.

Taking account of a normal size distributionl2) re-ported for the adhering layers of granules in the rawmixtures, the particle size of materials used for thetablets and the adhering layers of the model granules

prepared in the present experiments was set less than0.25 mm. Size distribution of such fine materials is

listed in Table 2.

Table 3showsapparent density and porosity, mea-sured using a mercury intrusion porosimetry, of thematerials used as core particles of the modelgranules.

Their particle size was in the range between 1.68 and2.00 mm. The measurementswere made for thematerials after calcination at I OOO'Cfor 10 min ex-cept R' because the high temperature characteristics

of the ores are important for this study. R' which

was ore R calcined at 1350'C for 10 min was also

used in order to examine the effect of the calcining

temperature. Ore Mis denser than the other ores.

The ore R' becamedenser by 5.7 o/o in porosity dueto the calcination at 1350'C comparing to that cal-

cined at I OOO'C(ore R). This suggests that thedensification of the ore occurs moreat the higher tem-perature.8)

Fig. I showspore size distribution of the materials.

Comparingores RandHwhich have similar porosity,

~has more large pores greater than 0.01 mm. Morethan 90 o/o of the pores of SA are the large poresgreater than 0.01 mm. Ore R' has less pores, smaller

than 0.01 mm,than ore R, whilst there is no signifi-

cant difference on the pores greater than 0.01 mm.

3. SamplePreparation

3. 1. Tablet

Ores M. Hand Rand limestone were crushed tothe size less than 0.25 mm. Ore P was used as-re-ceived because all the particles were less than 0.25

mm. Theseores and limestone were well mixed andweighed out by 0.98 g each. Then, the mixtures

were pressed in the steel cylinder of 9.3 mminside

diameter with the compressive pressure of 290 MPa.The resultant height of the tablets was approximate-ly 5mm. CaOconcentratlon of the tablets werechangedfrom 5 to 45 masso/o'

Table 2.

Screen size

Size distribution

the tablets andmodel granules.

(mm)

Ore M(mass o/o)

Ore R (mass o/o)

ore H (mass o/o)

Ore P (mass o/o)

Limestone (mass o/o)

o)~(

Q,

o~oooe'

E::

o>

Table 3.

-o.

of

the

044 -o.

77.354

. 247

. 970

. 273

. 5

fine materials usedadhering layers of

074 -o.

13.3

23. 8

25.825

. Ol0.2

l05

3,24.34,92,54.1

-o.

forthe

149 -O4.7

l0.911.3

2.08.1

.250

l,5

6.8lO, l0.34,l

Apparent density and porosity of materials

used as core particles of model granules.

Material

Ore MOre HOre ROre R/ *

Alumina sphere ASintered alumina SAPorous ceramic sphere PC

* Calcined ore R at

Apparent density(kglm3)

l 350'C for

4290

3220

3060

2860

3330

2860

l 520

10 min

Porosity(o/o)

8.31

20. 7l

21 .62

l4.90

0.91

13.42

21.54

R- POrosity : 21.6010

R~__ ,1 14.go/o

III l

- I 11 II

PC 21'5 olo

la

8

6

4

2oo

8

6

4

2o

IO

8

6

4

2c

H 20.7'/- SA 13.4 '/.

M 8. 31olo A 0.9 1'lo

-20 -1,4 -OS -0,2 04 lO l~ 22 -2~ -1,4 -0,8 -O.Z 0.4 1O i6 22lO l~ 22 -2~ -1,4 -o 8 -oZ 0.4 lO i6 22

log D (lJm)Fig. l. Pore size distribution ofmaterials used as core par-

ticles of modelgranules,

3.2. Model Granule

The core particles were the size between I,68 and2.00 mm. Fine mixture of iron ores and limestone

18

rslJ International, Vol. 31 (1991), No, l

whoseparticle size were less then 0.25 mmwasgranu-lated as adhering layers on the core particles. Thegrranules having size between 2.32 and 2.84 mmweresubjectecl to sintering experiments. The fine ore used

was normally ore M, but ores Hand R were also

used for a few types of model granules. CaOcon-centration of the adhering layers was varied as 4levels, 10, 15, 20 and 30 masso/o' Mass ratio of the

cores to the adhering fines was in the range between0.7 and 0.9.

TheCaOconcentration of the tablets and gra,nules,

expressed in this paper, is the converted value onthe basis after calcination. The tablets and modelgranules were submitted to the sintering experimentsin order to examine the relative fluidity of flne mix-tures in high temperatures and strength of the sintered

granules, respectively. The relative fluidity waseval-

uated measuring the projection area, A1" of the speci-

menin the vertical direction to the disk surface. Thestrength of the sintered model granules was evaluated

by a "shatter " method. The detai]s of the ex-perimental procedure were reported in a prevlouspaper.7)

4. Results andDiscussion

4.1. Factors Influencing ihe Relative Fluidity ofFine Mix-tures in High Temperatures

Limestone is a major fluxing material of the sin-

tering process of iron ores. Therefore, the effects of

CaOconcentration was first examined. The results

of the Ap measurementsfor the tablets madefrom the

mixtures of fine ore Mand limestone are shown in

Flg. 2. CaOconcentration of the tablet waschant:)cred

as 5 Ievels between 5 and 30 masso/o' A2' decreased

once with increasing holding temperature due to

shrinkage and then increased because of the flow ofthe materials due to melt formation. The tempera-ture, at which A1' started to increase, Iowered and the

slope of A1' rose with increasing CaOconcentration.

Theexperiments that the holding temperature wasfixed to 1220'C were conducted so as to see further

the effect of CaOconcentration (see Fig. 3). Thefine ores used were P, M, Hand R. CaOconcen-tration of the tablets waschangedfrom 5to 45 masso/o'

The curve of A1' for each tablet has a peak at about30 mass~/0 CaO. A for ta-blets P is lar~ger than the

!'

other tablets in the range between 20 and 40 masso/o

CaO. Especially, the value at the peak is more thantwo times of A,, for the other tablets. As fbr thetablets except P, it is found that whenCaOconcen-tration at the peak is hit)crher, the value of the peak is

smaller.

The results were examined referring to phase dia-

t)crrams and chemical composition of the tablets. Mand Hare hematite ores and Rcan be also regardedto be a hematite ores in the present study since ccnTl-

bined water decomposesat lower temperature thanthe experlmental temperatures. Fig. 4showsthb= par-tial phase diagram of CaO-Fe203-Si02 system.9)

The three dotted lines dravVn in the figure indicate

the possible chemical composition of' the tablets pre-pared using ores M, H and R and the limestone.

The differences among them are ong'inatecl fromthose of Si02 content in the ores.

As for tablet M, the liquidus temperature lowerswith an increase in CaOconcentration up to about21 masso/o' In other words, when the tablets are

350

i~E~E~ 300

ci~ 250

~ 200

c.. 150

oLCL 100

50

P eh/1

--- o---H ---

J~442

R CI

~_ _ _ ~.__-~qf

-~•-'

Hold fngtemperature :

1220 'C

o~t---

170

EE 150

130

."~110

=.

~ 90

~70

50

CaOmasso/o

5-c]-10 ----o ----15 e20 - --- A ----

30- o -

~~~:I~-_eA~__lo'~~~.

~A!'

9'

~0--C;

A.l

o.~5

.~i

llO

p'

11oo

p

L~,,fl4,1

c "\ ,~

,dlh. ~.

*~)*~

Fig. 3.

Fig. 2.

O iO 40 5020 30

CaO {mass~)

Changein the projection arca with CaOconcentra-tion of tablets prcparecl using mixtures ol' rlne iron

ores and limestone.

20 -r:oo

Hema-\i~G tite

.\~;~'e,

t~:~~~~

CS(;eF~ 30~S~)~)

f'5~~;h~~~

C2F_____

20

1150 1200 1250 1300 1350 1400

Holding temperature CTh)('C)

Relationship betwcen the projection area and the

holding temperature for the tablets prepared using

the mixtures of fine iron ore Mand limestone.

ko

Magne- V1tite ~)

~;i~

10 ~2~~li~

_~i_ ol

-- -H

M~O

60

Fig. 4.

70 CF 9Cl80 CF2Fe203{mclsso/o)

Possible chemical composition of thc tablets M, Hand Rshownon the pha,se diagram of CaO-Fe20sSi02 system.

19

ISIJ International, Vol. 31 (1991), No. l

held at a certain temperature, e.g., 1200'C, the ratio

of solid to liquid decreases with increasing CaOcon-centration. Since the decrease in the ratio of' solid

phase causes an increase in fluidity, it can be assumedthat the fluidity of the mixture is larger when its

liquidus temperature is lower. Contrary, the liquidus

tempera,ture of the tablet Mrises whenCaOconcen-tration is more than 21 masso/o' The liquidus tem-peratures are about 1220'C and more than 1300'CwhenCaOconcentrations are 20 and 30 masso/o' re-spectively. Therefore, the fluidity of the tablet hav-ing 20 masso/o CaOis likely to be greater than that

having 30 masso/o CaOat the sametemperature.However, Fig. 2shows the opposite result to this.

The detail of the reason for this is still not clear,

although a constituent segregation in the tablets maybe one of the reasons, which is explained as follows:

The particle size range of ores and limestone used is

less than 0.25 mm. This meansthat there is such asegregation whose magnitude is never greater than0.25 mm. It can be supposed that the segregationdid not disappear, at least, in the earlier stage of thesintering. This appears to be a reason that CaOconcentration at the peak of the relative fluidity shifts

from that corresponding to the minimumliquidus

temperature due to mutual interaction of liquids (or

semi-molten materials) having different chemical com-positions and hence different fluidity. Such a shift

maybe toward the higher CaOconcentration.

CaOconcentrations corresponding to the lowestliquidus temperatures for the tablets are 21 masso/ofor M, 24 masso/o for Hand 25 masso/o for R (see

Fig. 4). Thus, CaOconcentration of the lowestliquidus temperature increases with increasing Si02content in the ores. This phenomenonagain coin-

cides with the trend of differences of CaOconcen-tration at the peaks of the relative fluidity for the ores(see Fig. 3). However, the peak of the relative flu-

idity is located at higher CaOconcentrations thanthat corresponding to the lowest liquidus temperatureby 5or 6masso/o for every kind of ore.

Fig. 5 shows the partial phase diagram of CaO-FeO-Fe203system.ro) The dotted line drawn in thefigure indicates the possible chemical composition for

the mixtures of ore Pand the limestone. It can beassumedfrom the figure that the fluidity of tablet Pincreases with increasing CaOconcentration until 22masso/o and then decreases. The magnetite ore Pwill be oxidized during the sintering experimentssince they were conducted in air. The change ofcomposition due to the oxidation can be approxi-mated by a parallel shift of the dotted line to theFe203 direction. Even in this case, there does notseem to be qualitatively a significant difference be-

cause the region showing a low liquidus temperaturelies in parallel to the FeO-Fe203axis. TheCaOcon-centration corresponding to the minimumliquidus

temperature on the dotted line is approximately 22masso/o' This value is greater, by about 8masso/o'

than that at the peak of the relative fluidity for the

ore P (see Fig. 3). It is interesting that such a phe-

nomenonis similar to those observed for ores M, H

FeO (mass'/.)50 3040 20 lO

t/~ P

050

1l1

40

~OoL I~Oo

L

1

Q30(:tO

l:~:Q

20Qli•LQ

'J

IO

\t MAGNETITE

5o 70 O60 80 10090Fe203 (mass'/.)

Fig. 5. Possible chemical composition of the tablet Pshownon the phase diagram of CaOFeO-Fe203system.

and R.

4.2. The Influence of Chemical Composition of the Adher-ing Layers and ll'ind of Core Particles on the Strength

of the Sintered Model Granules

The strength of sintered model granules was mea-sured by a " shatter " method.7) The granules weredropped twice onto a steel plate 10 mmthick fromthe height of 2m. Then, the masspercentage of theparticles having size larger than 4.76 mmwas thendetermined as an index of strength and is namedcoalescence index, CI, in this paper.

Fig. 6 shows the results for various types of the

model granules prepared using the core particles

shown in Table 2 and fine mixtures of ore Mandlimestone. The relationship between CI and hold-

ing (or sintering) temperature was shown in thefigure arranged by CaOconcentration of the adher-ing layers. The strength of each granules increasedwith rising holding temperature and increasing CaOconcentration in the adhering layer. There is a trendthat the granules consisting of denser core particles

such as A and Mhave higher strength while porouscore particles such as SA, H and R show lowerstrength. Contrastively, the strength of granules R'is comparable to those of A and Min spite of its

relatively high porosity.

Suchan order of the strength for the used types ofthe model granules was almost stable independent of

CaOconcentration in the adhering layers in the pres-

ent conditions. However, the behavior of PCwaspeculiar. Its strength was lowest when CaOcon-centration of the adhering layers wasnot greater than15 masso/o' while a relative increase in the strength

was observed for 20 masso/o CaO. PCitself starts to

soften at 1220'C and contains muchmore Si02 thanthe other used core particles. Such characteristics

appear to be a reason for the peculiar behavior of

PC. The results obtained at the holding temperatureof 1220'C were arranged in Fig. 7so as to clarify theeffect of CaOconcentration in the a.dhering layers onthe strength ofsintered granules. CI rapidly increased

with increasing CaOconcentration up to 20 masso/o

for every type of granules. However, CI for granules

A and M, which had denser core particles, did not

20

ISIJ International, Vol 31 (1991), No. l

lOo

~~~,90 core partide ~~~~l80 N-o- ~~l;~1"II70 H~

ASA

20(a] 10mQsso/oCaO (b] 15mass%CQOlO

OiOO

go

Bo.

1O50

I50

40

30

20ie) 20masso/oCaO (d) 30masso/oCaOio

oi i50 i300iaoo il50 i200 i250i250i200

lOO

i i50

Fig. 6.

i200 iaoo i 150 i300i200 i250i250

Holding tompcraturo ITh] ('c]

Changein the coalescence index with the holding temperature for

the several types of modelgranules.

change significantly between 20 and 30 masso/o CaO.

4.3. The Relaiion bet~)een the Relative Fluidity of the

Adhering Layers of Model Granules and TheirStrength after Sintering

It wasobtained that the relative fluidity, A2" of theflne mixture increased with increasing CaOconcen-tration (see Fig. 3) and the strength of sintered

granules also increased (see Fig. 7). Therefore, it canbe assumedthat there is a relation between the rela-

tive fluidity of the adhering layers and the strength ofthe sintered granules. Fig. 8 shows the relationship

betweenA2' for the flne mixtures used as the adheringlayers of the model granules and CI for the sinteredgTranules. These are mainly the results for the gran-ules whoseadhering layers consisted of the fine ore

Mand limestone. However, in the case that the

cores were A, the ore Mwasoccasionally replaced byflne ore Hor R. There was no difference in the ob-tained values of A1' due to the replacement of the fine

ore. Fair linear correlations were found betweenAf'and CI and the correlation coefficient was more than0.8 for every type of model granules. Such correla-tions were valid through CI of about 90 mass~/o andCI did not vary significantly above this value. Theslopes of the lines were almost the sameand the value

was in the range between 0.6 and 0.67, when the

cores were the iron ores. Whenthe cores were thepseudo ores, however, the average of the slopes was0.78. A reason for this difference maybe the differ-

ence of chemical composition of core particles, butcould not be clarified in the present study. It wasassumedin the subsequent analysis that the slope,i.e., the effect of the relative fluidity, A1" of the ad-hering layers on the strength of the sintered granules,CI, is samefor every type of the granules.

4.4. Dom,inant Factors Governing the Strength of the ModelGranules after Siniering

Therelative fluidity of the adhering materials is one

~t~u,

I~

CJ

c:

u,

o

iOO

90

BO

70

GO

50

40

30

20

iO

O

Temperclture1220'c

/!///r'

!1

/l ,

///

Dl// Jaf/

I' ll

Q //

//////

/

O,/

/l,/

l!D

Core particie

A -----.-----

M-o-SA -----1-----

R-pc ---.-A.----

Fig. 7,

O iO 20 30 40

CaO [massX]

Change in the coalescence index for

model granules with CaOconcentrationof' the adhering layers of the granules,

iOO

go

80

70~~u' En'o uuE

- 50

ux dO

E:

V iOO

fo go.

BO

70

GO

50

40

M:H:R:

CI (olo) = 38.4 +O.60APCI (o/o) = 23 ,2 + 0.67APCI fo/o) = 14.4 + a.65AP

(r= 0.81 )(r= 0.87)

(r= 0~1)

DD

oo

oo

O(O

~

D

o

o o,1

oD

DD

Core pqrticle

M-o-H-R-D-

A:CI(o/o)=19.1 t O.86APSA: CI (olo)= I.54 ~ 0.89APPC:CI (o/e)=-5.71' 0.74Ap

(r= 0.86)

(r= 0.9 2)

(r= 0.94)

,,

'eeee'e e~///I

' e I:!1':/11e 1

11

111lll II'

Corepartcle

A-e-SA-1-PC-(-

Fig. 8.

50 50 70 BO 90 iOO i iO i20

Projection arEa [Apl imm2]

Relationship between the projection area of the ad-hering layers of granules and the coalescence indexof the sintered granules.

of the factors influencing the strength of the sinteredgranules (see Fig. 8). Thedifference ofCI amongthe

types of granules at arbitrary A1' suggests that there

are other lactors governingr the strength. Since thegranules consisting of denser core particles tended tohave higher CI, porosity of the core is likely a factor.

Fig. 9shows the aspects of the model granules A, Mand R after sintering at 1240'C, whose adheringlayers contained 15 masso/o CaO. It can be seen that

21

ISIJ International, Vol. 31 (1991). No. l

sufrircient liquid lbrmed and i)onded the t)o'ranules for

A and M, while, on the bo'ranules ll), unevennesswasremaincd on the surlace of the granules and their

bondings were still insufrilcient. Fi~)o'. 10 is a micro-scopic view ol' a cross section oi' each sintered granules.

The boLmdary betwecn the core particles and the

Fig.

A

l Omm,~J'

9. Appcarancc of granulcs, whose cores are aluminasphcres A, hematite ore Mand limonitc ore Randaclhcring laycrs contained 15 masso/(r CaO,after sin-

tering at 12400C.

f~"~:}J ' ' -~~*~,~.

+ ,~ ;,\*

v '•.

Fig.

O.5,mfh

10. Microscopic photographs of cross-sectional viewsof the sintered granules shownin Fig. 9.

formed liquid wasclear for the granules AandM, andthe liquid acts as the bonding matrix amongthe coreparticles. As for granule R, however, the boundarywas not clear and the very porous texture formed atthe center parts of granules where a core particleexisted. Further, the shape of the granules remainedunchangedand the contact (bonding) area betweengranules was quite small. This seemedto result in

the relatively lower strength obtained for the granulesR. Although the details of this phenomenawere notexamined in the present study, it can be assumedas

a reason that an effective amount of the liquid de-

creases due to the penetration into the pores of the

core and the fluidity of the liquid is relatively loweredby the fusion of the core into the liquid because the

contact area between cores and liquids is larger for

the ore R.Ores Hand Rhave a similar porosity (see Table

2), but the difference of CI between the grranules Hand Rwas more than 10 o/o' There is a differenceof the pore size distributions betweenthe two ores andore R has more pores being larger than 0.01 mm(see Fig. I ). The sintered granule R' has higherstrength similar to A and M(see Figr, 6(b)) despiteporosity and pore size distribution of the ore R' is

muchdifferent from those of Aand M. Ore R' hadbeen calcined) therefore, no decomposition of com-bined water occurred during sintering of the granuleR'.

In this way, several factors which likely determinethe strength of granules after sintering can be pointedout. A Iinear regression analysis was carried out for

the strength using various parameters including suchfactors. The following equation was obtained as anappropriate result;

CI = 30.5 -~0.726Ar'~ I . 17Pl ~I .231...

.(1)g .*...

where, Pl (volo/o) : the porosity corresponding to the

pores being larger than 0.01 mmlg (masso/o) : Ioss on ignition of core particle.

A correlation coefficient of 0.90 was obtained com-paring the CI calculated by Eq. (1) with that ob-tained by the experiments (see Fig. I I). Thegranule

PCcould not be arranged in the similar way. It

maybe because property of PCas core particles was

iOO

90:~(.'

ul BOul

OE 70

O_ 50

1:,Q'* 50::,

u,oa, 40

~:30

2020 30 40 50 BO 70 BO 90 iOO

Calculated [CI] (masso/Q)

Fig. 11. Comparisonbetweencalculated andmeasuredcoa-lescence indices for the several types of modelgranules sintered under various conditions.

22

ISIJ International, Vol. 31 (1991), No. l

significantly changed during' sintering due to theself-softening.

The regression coefficient of lg is considerably large

in Eq. (1) in terms of' the effective value for CI. Ig

mayalmost originate in decomposition of combinedwater of core particles. If the formed steam due tothe decomposition remained as bubbles in the liquid

formed around core particles, it would increase in the(structural) viscosity of the liquid and decrease in the

strength of the sintered granules. The decomposi-tion of combined water normally finishes prior to

500'C and liquid ibrmation occurs above 1200'C.Hence, if the inside temperature of a granules is

homogeneous,the decrease in the strength due to the

above mechanismwill not happen. It can happen,however, if a certain temperature distribution wouldbe bcriven by an adiabatic condition and endothermicreactions in the granule and so forth. In order to

confirm this, it is necessary to analyze further thethermal condition in the granule considering heat-

and masstransfer and reactions. In addition, recrys-tallization of limonite ores has been observedll) at

over 1200'C and such a phenomenonmayalso affect

the strength. These will be a further subject of this

study.

5. Conclusions

The sintering experiments were conducted usingthe tablets and model granules in order to obtainfundamental information for the granule design ol'

raw mixtures of the iron ore sintering process, ibcusing

on the strength of sintered granules. The results ob-tained were summarizedas fol]ows :

(1) The relative fluidity of the flne mixtures athigh temperatures can be qualitatively estimated con-sidering liquidus temperature shownin an appropriatephase diagram which correspond to the compositionof' main components. However, CaOconcentration

at the peak of the measured relative fluidity wasseveral mass par cents higher than that estimated

from the phase diagram ibr every used iron ores.(2) A fair linear relation was obtained between

the relative fluidity oi' the adhering layers oi' modelgranules and the strength of the sintered modelgran-ules for each type of granules. Thedifferences amongthe slopes of the relations for the used granule types

were not large, therefore, the slope was likely inde-

pendent on the kind of core particles.

(3) The dominant factors governing the strengthof the model granules after sintering were pointed outby the results of a linear regression analysis ibr themeasuredstrength of sintered granules. Fraction ofthe pores being larger than 0.01 mmand loss onignitions are the estimated factors on the core parti-cles.

1)

2)

3)

4)

5)

6)

7)

8)

9)

lO)

ll)

12)

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