Basicity of Iron Ore Pellete

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Transactions of the Japan Institute of Metals, Vol. 22, No.5 (1981), pp. 309 to 314

Crushing Strength of Metallised Iron

Pellets after Hydrogen Reduction

under Rising Temperature Conditions

By Shigeji Taniguchi*, Munekazu Ohmi* and Toshio Nakajima**

The room temperature crushing strength was tested for the metallised iron pellets obtained by

hydrogen reduction of six kinds of self-fluxing pellets and two kinds of acid pellets under rising

temperatures at a rate of 0.19 K/s up to 1273 K.

The degree of swelling during the reduction was small for the all pellets compared with that

under isothermal reduction, but the strength was unexpectedly low. This was found to be due to

the formation of large cracks in some pellets. The formation of microcrack in the remaining slag

phase also appeared to have further decreased the strength.

The strength tended to increase as the basicity of the pellets increases up to about unity and

above this it decreased remarkably. This was attributed to the strength of the slag phase at low

temperatures where the reduction step from hematite to magnetite was almost completed with the

development of considerable stresses.

The effect of the increase in the slag content was cancelled out by the appearance of relatively

large macropores in such pellets.

The acid pellets also resulted in low crushing strengths because they had little slag phase and

hence the number of the remaining bonding between the iron particles in the pellets became small.

(Received September 19. 1980)

I. Introduction

Several factors influencing the crushingstrength of metallised iron pellets have been

discussed in some detail in the previousstudies(1)-(3). These studies have revealed that

the degree of swelling during the reduction

largely influences the strength, because the

number of the bonding between the iron par-

ticles composing the pellet decreases as the

degree of swelling increases. Partial disintegra-

tion of the iron particlesv" and the formation

of relatively large cracks'P were also found to

have some influence.However, the metallised iron pellets were

obtained by the isothermal reduction at various

temperatures with a constant gas composition.

If the actual apparatus by which the metallised

iron pellet is produced on a commercial scale,

such as a shaft furnace, is considered, the

* Department of Metallurgical Engineering,

Faculty of Engineering, Osaka University, Suita,

Osaka 565, Japan.

** Department of Metallurgical Engineering,Faculty of Engineering, Osaka University. Now,

Murata Machinery, Ltd., Kyoto 601, Japan.

information on the reduction behaviour under

rising temperature conditions and varying gas

composition may be more useful from a

practical viewpoint.As a first fundamental access to such a

process, the present study was made on six

kinds of self-fluxing pellets and two kinds of

acid pellets for examining the effect of the

basicity and the slag content on the strength

of the final reduction product.

n. Experimental

Six kinds of self-fluxing pellets, Pellets I to

6, and two kinds of acid pellets, Pellets 7 and 8,were used in the present study. Microstructures

of Pellets 1 to 4 were shown in the previous

studyv" which dealt with the influence of the

basicity of the original pellet on the crushing

strength of the pellets after isothermal reduc-

tion. The slag contents of Pellets 1, 3 and 4

were approximately 10% while that of Pellet

2 was 7.3 % . Pellets 1 to 4 have a varying

basicity ranging from 0.64 to 2.54 and increas-

ing in this order, and Pellets 5 and 6 have larger

slag contents with basicity around 0.9. Table 1

summarises chemical compositions and a few



310 Shigeji Taniguchi, Munekazu Ohmi and Toshio Nakajima

Table 1 Chemical compositions (mass % ) and other properties of Pellets 1 to 8.

Firing CrushingNo. T.Fe FeO CaO Si02 Slag CaO/Si02 temperature Porosity strength

(K) (%) (kN)

62.96 0.92 2.50 3.90 10.3 0.64 1553 19.4 3.102 64.86 0.50 1.93 1.91 7.3 1.01 1553 21.5 4.31

3 62.82 0.56 4.08 2.61 10.2 1.56 1553 29.0 2.52

4 62.57 0.43 4.52 1.78 10.6 2.54 1553 28.9 2.73

5 60.81 2.27 4.78 5.02 13.9 0.95 1593 23.3 5.89

6 58.66 1.84 5.19 6.00 16.3 0.87 1553 22.5 5.42

7 64.90 2.51 0.41 3.65 22.7 2.67

8 65.91 0.14 0.78 2.83 30.6 2.57

other properties of Pellets 1 to 8.

The individual pellets ranged from 3.0 to

3.5 g in mass and around 12mm in diameter.

An X-ray diffractometer analysis revealed that

the iron oxides in the all pellets were almost

hematite, though a metallographic examination

confirmed the presence of traces of magnetite

in the self-fluxingpellets.

Photograph 1 shows microstructures of

Pellets 5 to 8 before reduction. Pellets 5 and 6

have a similar structure with iron ore particles

surrounded by the slag phase. Relatively large

macropores are noticeable in these pellets.

On the other hand, the acid pellets have verylittle slag phase and a few Si02 particles, and

are mainly composed of hematite bond.

Iron oxides and slag phases were identifiedby etching treatments(4)-(6). Although it was

impossible to identify all slag phases present

by the etching treatments only, calcium silicate

and calcium ferrite were at least identified.

The slag phase in Pellet 1 is mainly calcium

silicate and those in Pellets 2, 5 and 6 are

calcium silicate and calcium ferrite in smaller

quantities. The amount of calcium ferrite

increased as the basicity further increased.

The apparatus used for the kinetic study and

swelling measurement were the same as those

described previously'Pv" except for a tempera-

ture programme controller employed for con-

trolling the heating rate.

The reduction was carried out with a 50%

H2-50%N2 gas mixture flowing at a rate of

66.7 cm3(STP)/s under rising temperature con-

ditions up to 1273 K at a rate of 0.19 K/s.

This heating rate was found to give rise to ahighest crushing strength for Pellet 1 after the

reduction in a preliminary test which involved a

few heating rates.

The crushing strength was tested at room

temperature in a similar way to that in the

previous study+". The conventional metal-

lographic examination was carried out for

partially and totally reduced specimens with

Photo. 1 Microstructures of Pellets 5 to 8 before reduction. White: hematite, gray: magnetite

or slag, and dark: pore (mounting plastic).



Crushing Strength of Metallised Iron Pellets after H Reduction under Rising Temperature Conditions 311

an optical and a scanning electron microscopes.

Further details of the experimental procedureswere given elsewhere(1)(2).

ID. Results

Figure 1 shows reduction curves of the all

pellets and the specimen temperature during

the reduction. The number in the figure cor-

responds to the pellet number. The acid pellets,

7 and 8, were reduced faster than the self-

fluxing pellets. Pellets 5 and 6 containing larger

slag contents were reduced fairly slowly com-

pared with the other pellets.

Swelling behaviour of the all pellets during

the reduction are summarised in Fig. 2(a)

c

o

: e 60

"¥_ 40

o

400

1000 ~

'". .

800 :;

~s

600 E. .

0-

9.0 10.8

Time (ks)

Fig. 1 Reduction curves of Pellets 1 to 8 and the

specimen temperature.

20 40Degree of

60 80("I. )reduction

Degree of reducti on (.,,)

Fig. 2 Swelling curves of Pellets 1 to 8.

and (b). The pellets can be classified into three

groups according to their swelling behaviour.

The first group consists of Pellets 2 to 4 the

degree of swelling of which passes a maximum

with the final apparent volume slightly larger

than the original one. The second group

includes Pellets 1, 5 and 6 showing that the

degree of swelling simply increases with the

progress of the reduction, though Pellet 1 is

somewhat exceptional during the final reduction

stage. The acid pellets form the third group

which is characterised by the swelling curve

showing a marked decrease after a maximum.

This resulted in the final pellet volume to be

almost equal to or less than the original one.

The crushing strength of the metallised ironpellets is plotted against their original basicity

in Fig. 3. The acid pellets resulted in low

strength, while in the case of self-fluxingpellets

the strength tended to increase as the basicity

increases up to about unity, beyond which

the strength is lowered again .

A visual inspection revealed that the pellets

of low strengths such as Pellets 1, 3, 4, 7 and 8

were associated with relatively large cracks as

shown in Photo. 2.

Microstructures of the metallised iron pelletswere also examined and are shown in Photo. 3.

Each pellet shows its characteristic feature.

100

1.5

6 2

z.>t:

.J:: 1.0/-01

c

k / '< II. . . 5iii

f~ I01c

~ 0.5::J. . .o

1 8

0

Basi ct t y (-)

Fig. 3 Variation of the crushing strength of the

metallised iron pellet with its original basicity.




For example, spongy iron particles are sur-

rounded by the remaining slag phase in Pellet 6,

Photo. 3(f), or relatively large iron particles

are fractured in Pellet 7, Photo. 3(g).

However, it is difficult to understand the

physical nature of the bonding phase in these

pellets from these photographs only. Moreover,

these microstructures give little indication

which is strongly related to the variation of

the crushing strength with the original pellets.

Therefore, an examination with a scanning

electron microscope was carried out further.

Some characteristic features are shown in

Photo. 4. Generally, the metallised iron pellet

from the self-fluxing pellet is composed of

spongy iron particles and the remaining slagphase between them as shown in Photo. 4(a).

The bonding between iron particles or the iron

particle and the slag phase are maintained well.

However, in Pellets 3 and 4 the remaining slag

is associated with sharp cracks as shown in

Photo. 4(b). This kind of failure was found

in the pellets reduced only by 20% . In the caseof the pellet of a large slag content, the slag

Photo. 2 Features of the metaIlised iron pellets. phase maintains its original network structure

Photo. 3 Optical micrographs of the metaIlised iron pellets. (a) to (h) correspond to Pellets

1 to 8 respectively.



Crushing Strength of MetalJised Iron Pellets after H Reduction under Rising Temperature Conditions 313

Photo.4 Scanning electron micrographs showing characteristic features; (a) Pellet 2, 98.9%

reduction, (b) Pellet 4,98.1 %, (c) Pellet 5, 97.0%, and (d) Pellet 8, 100%.

which firmly grips the iron particles as shown

in Photo. 4(c). Contrary to this, the acid pelletshave very little slag phases and hence the poor

contact, as shown in Photo. 4(d), may have

resulted.

IV. Discussion

Previous studies(1)~(3) showed that the crush-

ing strength of a few kinds of metallised iron

pellets obtained by isothermal reduction de-

pends largely upon the degree of swelling during

the reduction, because the number of thebonding between the iron particles composing

the pellet decreases as the degree of swelling

increases. Then, the present results were at

first examined from the same viewpoint and an

effort was made to explain the crushing strength

in terms of the degree of swelling.

It was, however, very difficult to recognise

any clear correlation between them, since the

degree of swelling varied little with the kind of

the pellet and was small compared with that

during isothermal reduction. The degree of

swelling larger than 20 % was often observed in

the previous studies. The type of swelling

behaviour seems to have no major influence,

too.

Therefore, a consideration was extended to

the physical nature of the bonding phase in

the pellet, next. The crack formation is at-

tributable to the stresses developed during

the reduction step from hematite to mag-

netite(7). The currently proposed mechanism

for this process was briefly discussed in the

previous studyv", The strength of the bonding

phase in the pellet at temperatures where this

reduction step takes place is consideredresponsible for the crack formation, since if the

bonding phase is strong few crack may form

and vice versa.

In the case of the acid pellet, the bonding

phase is mainly hematite and when they are

converted into magnetite a large part of the

bonding is broken because of their crystal-

lographic disregistry-'". The pellet shape is

maintained by the remaining bonding during

the subsequent reduction. During the final

stage of reduction the sintering of the ironproduced takes place resulting in the decrease

in the apparent pellet volume; however, the

bonding between the iron particles cannot be

improved as shown in Photo. 4(d) and thus

the strength cannot be restored either. This

view is consistent with a conclusion of the

previous study'P,

The difference in the strength between Pellets

7 and 8 can be attributed to the fact that Pellet

8 has a higher initial porosity than Pellet 7.

The higher porosity may have resulted in a

higher rate of reduction, Fig. 1, and a higher

degree of final swelling.

The higher initial porosity and the higher

degree of final swelling imply a smaller number

of bonding.

In the case of the self-fluxing pellet, the slag

phase acts as the major bonding phase. It is

important to note that a hematite briquette

containing calcium silicate was reported'?'

to be stronger than that containing calcium

ferrite below about 1070 K. This may imply

that calcium silicate is stronger than calcium




ferrite in the temperature range. In this tem-

perature range the reduction from hematite

to magnetite was almost completed in the

present study as the metallographic study of

the partially reduced pellets indicated.

The amount of calcium silicate in the slag

phase increases as the basicity increases up to

about unity. This resulted in the strongly

bound iron particles as shown in Photo. 4(a).

When the amount of the slag phase is large, it

remains forming a strong network structure

and grips the spongy iron particles firmly as

shown in Photo. 4(c).

The further increase in the basicity resulted

in the calcium ferrite in the slag phase which

resisted less strongly to the stresses formingmany microcracks in it, as shown in Photo.

4(b). These considerations based on the

strength of the slag phase can be supported by

the previous study'P'.

The crushing strength after the reduction of

the pellet containing a larger amount of slag,

e.g. Pellet 6, would be expected to be some-

what higher than that of the pellet containing

a smaller amount of slag, e.g. Pellet 2, with

similar basicity. However, the increase in the

slag content resulted in larger macropores inthe pellet as shown in Photo. lea) or (b). These

larger macropores can provide sites for stress

concentration. This is a possible reason for the

strength even though the outer surface of the

pellet is very sound as shown in Photo. 2. The

effect of the increase in the slag content was

thus cancelled out resulting in similar strengths.

V. Summary

The acid pellet resulted in low strength,

because of the crack formation due to the

absence of the bonding phase which can resist

the stresses developed during the hematite to

magnetite reduction step. The basicity of the

self-fluxing pellet largely influence the strength

because the crack formation depends upon the

strength of the slag phase. The highest strength

was obtained when the basicity was about

unity. The effect of the increase in the slag

content was cancelled out by the appearance

of large macropores in such pellets.

Acknowledgements

The authors are grateful to Kobe Steel, Ltd.

and Nakayama SteelWorks, Ltd. for the supply

of the pellets.

REFERENCES

(1) S. Taniguchi and M. Ohmi: Trans. nM, 19 (1978),

581.

(2) S. Taniguchi, M. Ohmi and H. Fukuhara: Trans.

ISIJ, 18 (1978), 633.

(3) S. Taniguchi and M. Ohmi: Trans. nM, 21 (1980),

433.(4) The 54th Committee Jap. Soc. Prom. Sci.: Trans.

ISIJ, 7 (1967), 126.

(5) M. Asada, Y. Omori and K. Sanbongi: Tetsu-to-

Hagane, 54 (1968), 14 (in Japanese).

(6) K. Kunii, R. Nishida, H. Koizumi and M.

Nakagawa: Tetsu-to-Hagane, 54 (1968), 266

(in Japanese).

(7) R. L. Bleifuss: Proc. ICSTIS, I, Suppl. Trans.

ISIJ, 11 (1971), 52.

(8) R. Baro, H. Moineau and J. J. Heizmann:

Advances in X-ray Analysis, 11 (1968), 473.

(9) H. Brill-Edwards, H. E. N. Stone and B. L.

Daniell: J. Iron Steel Inst., 207 (1969),1565.

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Basicity of Iron Ore Pellete