11

ISIJ Internationdl, Vol. 29 (1989). No. l, pp. 24-321

Sintering

Industrial

Conditions for Simulating

lron Ore Sinter

the Formation of Mineral Phases in

Li-Heng HSIEH1)and J.A. WHITEMAN2)

1)Researchand DevelopmentDepartment, China Steel Corporation, Kaohsiung, 8i 233, Taiwan, RO.C.University of Sheffield, Sheftield. S1 3JD, UK

(Received on March22. 1988, accepted in the fmal form on September9. 1988)

2) TheDepartmentof Metallurgy,

,

1

In industrial iron ore sintering, the raw material is heated in a reducing atmosphereandcooled in an oxidizing atmosphereIn order to study the effects of gas atmosphere in industrial sintering, small tablet specimenscontaining powderedcommer-cial iron ore, Iimestone, quartz and kaolin were heated in controlled gas atmospheres to examine the effects of gaseousatmosphere, heating temperature and cooling condition on the formation of minerals in sinter. The results obtained aresummarizedas follows

In the heating stage ot laboratory sintering, with a decrease of partial pressure of oxygen, the magnetite content increasesand hematite content decreases The calcium ferrite content is found also to decrease at the low sintering temperature(1 210'C) However, at a higher sintering temperature (1 255'C), a mediumoxygen potential (5 x 10-3 atm) produces the

most calcium ferrite. In the air cooling stage, magnetite mayreact with the silicate melt and oxygen to generate a large

amountof columnar calcium ferrite.

Atypical microstructure of the bondcomposedof columnar calcium ferrite, granular magnetite grain and glassy silicate in

a normal industrial sinter can be simulated reasonably by heating a specimen to 1255'C for 4min in the gaseousmixture

CO= I "/•, C02=24 "/* and N2 = 75 o/o and then cooling it slowly in air

KEYWORDS:agglomeration; sintering; iron ore; simulation; atmosphere; mineral phases.

,,

1.Introdn cti on

In industrial iron ore sintering, the raw material is

heated in a low oxygen partial pressure initially andthen cooled under a high partial pressure of oxygen.Generally, the whole sintering reaction occurs in anon-homogeneousand non-equilibrium state. Be-

cause the gas atmosphere in the reaction is not exactlyknown, manyresearchersl-8) have simplifled simula-tion expcriments by heating tablet specimens in air

instead of a controlled gas atmosphere. Y. Hicla et

al.9) studied the effects of gas atmosphere on theminerals of sinter during the heating stage. How-ever, from the viewpoint of thermodynamics if thereal situation in industrial sintering is to be studied,the gas atmosphereencountered in industrial sinteringshould be simulated in the experiment. The aim ofthis study is to examine the effects of gas atmosphere,heating temperature and cooling conditions on min-eral formation in sinter. By comparing the micro-strLlctures of experimental and industrial sinter it

should be possible to select the most suitable sinteringconditions for laboratory simulation.

ore (from Brazil), Iimestone, silica sand and kaolin.

Each was screened and crushed to obtain a particlesize smaller than 0.25 mmbefore use. Table I showsthe chemical compositions of the raw materials. Theraw material mixture which gave an overall chemicalcomposition ol' sinter corresponding to Ca0=13.5wto/a' Si02=6.5 wto/o' and Al203=3'O wto/o was usedto make the tablet specimens for sintering experi-

ments.Cylindrical tablets 6mmdiameter and approxi-

mately 6mmin height were produced by pressing

0.4g of the mixture of raw materials mixed with

8wto/o water into a cylindrical mould for I min. Thepressure of 0.45 kgjmm2wasproduced by a vertically

loading piston. Before the tablet wasused for sinter-

ing, it wasdried at I lO'C for 3h. To investigate thereaction, whenlarge ore particles were present, tablets

were also prepared with a large (about 2mmdiam-

Table l. Chemical compositions of raw materials.

Raw Chemical composition (o/o)

material TFe Si02 Al203 CaOMgOIg. Ioss

d'

,

~

~

2. Experimental Procedure

2.1, Rac() Material and Tablet SamplePreparation

The raw materials for this study were MBRrron

MBRironore

LimestoneSilica sandKaolin

67. 7

0.3

0.4

O.72

3.61

99. 9

46. 3

O.80

l .07

38. 6

52. o 0.3

0.9

41.1

13.0 d

24 C1989 ISIJ

Thermocoup[efor contro[ling

temperature

Fig. I .S]ntermg apparatus

Vacuumpump Thermocoup[e

ermocoup[e Alumina reaction tube

contro[lingFurnace

mperature Sarnpling bar

/ \LJJ

/ /Combustion boat

water Water cooling

coo[ ingDrying ur

Air N2 /'-- Silicone oil bubbler

(Coo[ ing gas)

Gasouttet

N2

aratus.COCO, (F

unit

CO, (ReactionAir

gas)

eter) particle of iron ore in the centre of tablet.

13502. 2. Sintering

The sintering apparatus is shown in Fig. 1, andconsists essentially of a t.ube furnace. A gas mixtureof controlled composition may'De passed through the

reaction tube (25.4 mmdiameter). The furnace waskept at a fixed controlling temperature before thespecimen in the middle of a small flreclay combustionboat 72 mmlong by 16 mmwide by 10 mmdeep waspushed into the hot zone of the furnace. After theboat had stayed in the hot zone for the desired time,it waswithdrawn for cooling by one of two methods.Either rapid cooling was used in which the boat wasdirectly removedto the cool end of the reaction tube,

or slow cooling wasemployed in which the boat wasmoved first to a location in the reaction tube at

l 140'C for 2min before removal to the cool end ofthe tube.

In the reaction tube a Pt-Pt• 130/0Rh thermocoupleat 10 mmawayfrom the front of the combustion boat

was used for controlling the sintering temperature.The actual sintering temperature was taken by aver-aging temperatures measuredon the top surface ofthe specimen and I mmbelow the top surface in thespecimen during sintering. Fig. 2showsan exampleof the temperature/time profiles measuredby the ther-

mocouples at various positions of the specimen. It

can be seen that the temperature above the specimenin the gas is muchhigher than that in the specimen.Typical temperature/time proflles for sintering experi-

ments are shownin Fig. 3.

In industrial sintering, generally it takes around2min to heat the raw material to maximumtempera-ture at around 1300'C. In this sintering apparatus,whenthe temperature of reaction tube wascontrolled

at 1393'C, the specimen could be heated to 1255'Cin 4min and to 1223'C in 2min (as shown in Fig.

3). However, the apparatus is not capable of work-ing at significantly higher temperatures (aroundl 500'C required in the reaction tube). The ob-servations madeat there somewhatlower tempera-tures will be relevant to the industrial sintering cycle.

Increasec.1 holding time will compensatefor the lower

uaJ

='

ro

(v~LE

~!

Fig. 2.

Fig.

1300

1250

1200

11oo

1100

1050

1000

Controlling temperature(betore the specimen chaTged)

/

/,' ./~~f •1'~~

f ./~

f~l

I~'

~~~~ Above the specimen

j --- an the top surfaceof specimen

1mmbelow the topsurface In the specimen

O 2 3 4 51Time (minufes )

Temperature/time profiles at various positions dur-ing the sintering.

1400

1200

u1000

Q,

::'

ro 800cvOLE~(u cH ~OO

400

200

Heating stage cooling sta9e

~ l~S[ow ccoling

_ /

ItRapid 1

coo[ing {

Max sinteringtemperatufle 1255~C

Max slntering

temperature 1210 'C

O 2 4 6 8 Io

Time (minutes)

3. Typical temperature/time profiles of sintering experiments.

5

maximumtemperature to someextent.DLlring the experiments, the atmosphere was con-

trolled by passing the various gaseous mixtures into

the reaction tube. Normally industrial iron ore sin-

ter does not contain wtistite, and therefore, the sinter

produced in the experiments should not containwustite. From the following chemical reaction andthe standard free energy equation'

Fe304+CO= C02+3FeOAG= 7120-9. 15T

where, AG: standard free energy (cal/mol)

T: absolute temperature (K)around the sintering temperature (1 300'C), if theratio ol' C(.)/(C0+ C02) is lower than 0.09, the forma-tion of wustite can be prevented. Therefore the gasmixtures chosen for the experiments in the heating

stagc are as shownin Table 2.

The gases used in sintering experiments were CO,C02, N2, and air. Before the mixed gas flowed into

the tube, it passed through a drying unit (500 mmlong by 30mmdiameter) filled with desiccant CaS04'The flow rate was typically 400 mljmin and the rateof flow of each gas was measuredby a flow meter.The working range of flow meter for each gas is N2:lOO to 1200 ml/min, C02: 20 to 250 ml/min, CO:

Table 2. Gas compositions for sintering experiments.

CO N CO Po,C02ONo ("//"2)

C0+C02 (atm)' (o/*)(9/~2)

("/o)

l2345

8lOOO

OOO.5

521

9224

ooo

o75

99. 5

9579

o,08

o,04

3x l0-8*

l > l0-7*

5> 10-3

5x l0-2

O.21

* at 1300'C (calculated value, see Table 3)

Table 3.

3 to 90 ml/min, Air: 3 to 90 ml/min and 40 to 700mljmin, respectively. In order to obtain a partic-ular controlled atmosphere, before the specimen waspushed into the tube, the tube was evacuated andthen the gaseousmixture waspassed through the tubefor at least 8min ii' the gas mixture was the sameasthe preceding test, otherwise for at least 30min. Thecooling gas (N2 or air) was directly led into the tube20 s belbre the end of the heating stage to compensatefor the time taken for the gas to reach the specimen.Theexhaust gas outlet wasconnected to a silicone oil

bubbler.

2.3. )Vficrostructure Analysis

After sintering, the specimens were mounted in

epoxy resin and then vacuumimpregnated. General-ly, t-he specimens were polished to expose a planesection corresponding to I mmbelow and parallel tothe original top surface. Whena large ore particle

waspresent, the specimens were polished to expose aplane section perpendicular to the top sur'lace on adiameter of the specimen so that a cross section of the

ore particle could be seen. These sections were pol-ished by using silicon carbide paper to I OOOgrit em-ploying ethanol as a lubricant, and finally to l!4

micron finish by using diamond paste. During thewhole process, the specimens were not exposed to

water.After polishing, the specimens were examined un-

der an optical microscope. The volume proportionsof phases in sinter were estimated by using the pointcounting method. Around I OOOpoints were countedfor the whole polished surface of a specimen.

3. Results

3.1. Effect of GaseousAtmosphereduring Heating Stage

The tablet specimenswere heated to different tem-

Experimental conditions.

Heating condition Cooling condition

Specimen Maximum Max. gas GaseouscompositionTimetemperature temperature (min)('C) CO CO('C)

2 N2

("/~)

02

P~, of gasmixture

(atm)Speed Gas

SlS2S3S4S5S6S7S8S9SIOSllS12S13S14S15S16

l 255

l 255

l 255

l 255

l 255

l 210

1210

l 255

l 255

1255

l 255

l 210

l 255

l 255

1255

1230

l 345

1345

1345

l 345

1345

l 291

l 291

l 345

l 345

l 345

1345

1291

l 345

l 345

1345

1315

4444444444444434

8lOOOlO81OOl

O

l

9224

ooo

24

o9224

oo

24

24

o2424

o75

99. 5

9579

75

79

o75

99. 5

7975

75

99. 5

7575

OO0.5

21

O21

OO0.5

21

OO0.5

OO

9x l0-8

4x l0-7

5x 10-3

5x l0-2

O.21

9x 10-8

0.21

9x l0-8

4x 10-7

5x l0-3

0.21

9x 10-8

4x l0-7

5x l0-3

4> l0-7

2x l0-7

RapidRapidRapidRapidRapidRapidRapidSlowSlowSlowSlowSlowSlowSlowSlowSlow

N2N2

As heating

As heating

As heating

N2As heating

AirAirAirAirAirAirAirAirAir

* If the gas mixture contains COand C02'energy equation :

C0+11202= C02AG= -67 500+20.75T

Po, was calculated by using the following chemical

where, AG: standard free energy (cal/mol)

T: maximumgas temperature (K).

reaction and the standard free

~

1

1'

~

1

1

~'

,

dld

,I

l

,

,

26

peratures in various gas mixtures and then cooled

rapidly for this study; the experimental conditions arerecorded in Table 3 (specimens Sl to S7).

Fig. 4 shows the effect of 02 partial pressure onthe microstructure obtained after heating the speci-

mens to 1255 'C followed by rapid cooling. At alow partial pressure of oxygen9x 10-8 atm (calculated

from gas composition and maximumgas tempera-ture) the predominant phase is magnetite (66 o/o) with13 o/o calcium ferrite and 21 o/o glassy silicate, as

shownin Fig. 4(A). Fig. 4(B) showsa microstructureobtained at an intermediate oxygen potential (5 x10-3 atm). Less magnetite and more calcium ferrite

appear. The sinter contains 48 o/o magnetite, 31 ~lo

calcium ferrite and 21 o/o glassy silicate. The micro-

structure obtained at the highest oxygen potential

(0.21 atm) is shownin Fig. 4(C) and this sinter con-tains 54 o/o hematite, 6 o/o magnetite, 5o/o calciunl

ferrite, and 350/0 glassy silicate. There are thus clear

trends of a decrease in magnetite content as the oxy-

gen potential increases. The increases of hematite

and glassy silicate contents with oxygen potential arealso found above a certain oxygen potential. Thesetrends are shownin Fig. 5together with data for other

specimens corresponding to intermediate oxygen po-tentials. The most interesting observation is that the

calcium ferrite content goes through a maximumat

ii~~~

~~~~~~

Fig. 4.

C02=92o/o

(Po, =9X l0-8 atm)Magnetite: 66 ?•~

Calcium ferrlte:

(B) SpecimenS302=0.5 ~;,

N2=99.5 ",/o

(Po, =5X l0-3 atm)Magnetite: 48 o/o

Calcium ferrite :31 qlo

Glassy silicate:

21 O/b

N2=79o/o

(P()2 =0.21 atm)Hematite : 54 Ol

Magnetite: 6 o/o

Calcium ferrite:

~; o!J l{)

Glassy silic.ate:

35 o/o

F: Calcium fe'rriteH: HematiteS: C~.lassy silic.ateM: Magnetite

Microstructures of specim.ens heated for 4min to

1255'C in various gaseous atmospheres and thencooled rapidly.

60

~e 50

c(1,

40couQ, 30

?ji

E 20oI

Ca,

cOua,

Q,

cO1rO

Z

10

70

60

50

40

30

20

10

80

70

_ 60

~;(D~: 50ou(1'

- AO~~2

E 30::,

C;75

V 20

10

~~ 30'l'

~v(L,

207~u_

u,10

>v,u)Jg(J)

I 1255 'C Rapid cooting

A 1210 'C Rapid coo[ing

e 1255 'C S[ow cooLhg in air

ee'//,eI I

I

l

A

.

^\.\.

e e e

IeA

e-\e-\e

J'

A 1

I

I I-/I~IA

e .:~,JL

Fig.

C8 / co] C1 '!. co] [0.5'/,02] C5'/, 02] C21'l,02 lGas compositiom

(P02 )

5. Effects of sintering conditions (heating gas atmo-spherc, temperaturc and cooling process) on volumeproportion of each phase in sinter. Gascomposi-tions are detailed in Table 1.

27

!ti

Fig.

(B) SpecimenS7~;ii~; 02=21 ("

lO'

~#.~~~' ---i~;~~~;;=;='~

(poE;;;0.21 atm)Hematite: 17o/o

Calcium ferrite:

79 ollO

Glassy silicate :4 tl,~]

6. Microstructures of specimens heated for 4m.in to

1210OCin various atmospheres and then cooledrapidly.

an intermediate level of oxygen potential.

At a lower maximumheating temperature of

l 210'C the microstructure is rather different in thatcalcium ferrite proportion on heating at a lower oxy-gen potential (9x 10-8 atm) (cooled rapidly) is rela-tively low but very great after heating in air, asshownin Fig. 6. A comparison between the micro-structures produced after heating to 1255 andl 210'C in air (cooled rapidly) is shownin Figs. 4(C)and 6(B). It can be seen that a large amount ofcalcium ferrite (74 o/o) formed at 1210'C is replacedby hematite and glassy silicate at the higher tempera-ture of 1255'C. However, the specimens heated inthe low oxygen potential atmosphere (gas composi-tion C0=1o/o' C02=24o/o' and N2=75o/o) showsthat a small amountof calcium ferrite (lO o/o) formedat 1210'C is replaccd by magnetite and glassy silicate

at the higher temperature 1255'C (from Fig. 5).

Thesephenomenashowthat during the heating stage,the calcium fcrrite transforms to hematite and silicate

melt at the high oxygen potential bllt transforms tomagnetite, silicate melt and oxygen at the low oxygenpotential.

In all specimens, except those heated in air, themicrostructure at the surface of the specirnen wasdif-

ferent from that of the interior, and the surface con-tains moremagnetite and less calcium ferrite.

3.2. Effect of Oxidization during Cooling

Whenthe specimens were cooled slowly in air

rather than cooled rapidly to simulate the coolingsituation in industrial iron ore sintering (as shownin

Table 3, spechnens S8 to S12), different results wereobtained. Figs. 7(A), 7(B) and Fig. 8show that thespecimensheated in the low and intermediate oxygenpotentral to 125 ) and 1210'C followed by a slow cool-ing in air contain high levels of ca]cium ferrite (53-59 o/o)' This is matched by a generally low levels

of magnetite (24-27v/~) and glassy silicate (10-

(A) SpecimenS8

C0=8o//o'

C02=92o/J

(P02=9X l0-8 atm)Hematite: 4 olo

Magnetite : 27 olo

Calcium fcrrite :57 o/o

Glassy silicate:

12 o//o

(B) SpecimenSIO02=0.5 o/~,

l\T _nn ~; oli~2-t':'..J Io

(P02=5X l0-3 atm)Hematite : 10 o/o

Magnetite : 27 o/o

Calcium ferrite :53 o/)

Glassy silicate :OllO Io

(C) SpecimenS1102=21 Ol/o'

N2=79o/o

(P(): =0.2 1atm)Hematite: 53 olo

Magnetite : 2o/o

Calcium ferrite :12 ~,/o

Glassy silicate :33 o/o

Fig. 7.

Fig. 8.

H: Hematite F: Calcium ferrite

M: Magnetite S: Glassy silicate

Microstructures of specinlens heated for 4min to

1255'C in various gaseous atmospheres and thencooled slowly in air.

lviicrostructurc of spccimen (S12) heated fbr 4minto 1210'C in the gas mixture C0=1/o' C02=Ol

24 o/o' and N2=75,~ (Po,=9X l0-8 atm) and thencooled slowly in air. This specimen contains 7o/o

hematite, 24 o/~ magnetite, 59 ~~ calcium ferrite

and 10 o/ glassy silicate.,*o

12 o/o)' The reoxidized hematite content is in the

range 4 to 10 ~/o (comparison of Figs. 4(A) & 4(B)and Figs. 7(A) & 7(B)); this is generally at the sur-face of the specimen, as shown in Fig. 9. Thcsetrends are also shownclearly in Fig. 5.

In comparison with the mineral compositions ofrapidly cooled specimens, the specimenscooled slowlyin air contain muchmore calcium ferrite and reoxi-dized hematite, and less magnetite and glassy silicate.

This shows that during the air cooling stage, a large

lb

,

,

~

dl

~

~,

J

~

28

Fig. 9. Surface microstructure of the specimen (s9) heatedfor 4min to 1255'C in the gaseous mixture C0=l "/*, C02=24"/~, and N2=75"/o and then cooled

slowly in air.

amountof calcium ferrite is generated from the reac-tion of magnetite with silicate melt and oxygen andsomereoxidi7.ed hematitc is formed. This is a reversetransformation oi' calcium ferrite at the low oxygenpotential in heating stage.

Froma comparison of Figs. 4(C) and 7(C), it canbe seen that for specimens heated in air, the mineralcompositions after rapid cooling and slow cooling aresimilar. Both contain almost the samelarge amountofhematite (54 and 53 r/o' respectively). Theslightly

higher calcium ferrite content (7 o/o) in the slowcooling specimen approximately corresponds to the-

amount of the reaction of magnetite and silicate in

cooling stage. Therefore, during the cooling stage inair, the hematite phase formed in the heating stage(at a high oxygen potential) is quite stable; it doesnot tend to react with silicate melt to generate cal-

cium fcrrite.

3.3. Effect of Sintering Conditions on the Reaction of lron

Ore

Those tablets with a 2mmdiameter iron ore par-ticle at the centre were heated under various condi-tions and then cooled slowly in air in order to studythe boundary reaction of the iron ore particle, asshownin Table 3specimens S13 to S16.

It can be seen in Figs. lO(A) and lO(B) that a mag-netite layer reduced from iron ore particle is produced

on the unreacted hematite. The thickness of thatlayer increases with the reduction potential of the

gaseous atmosphcre, and some reoxidized hematite

grows in the magnetite layer. Acomparison betweenFigs. 10(A) and 10(C) reveals that the thickness ofthe magnetite layer decreases with the heating time.

Whenthe heating temperature decreases, from Figs.

lO(A) and lO(D), it can also be seen that the thick-

ness of the magnetite layer decreases.

From the above, the gaseous atmosphere, heating

temperature and heating time are all factors affecting

the thickness of the magnetite layer on the unreactedhematite particles which maybe used to assess thedegree of reductron mmdustnal smter

3.4. Comparison between the Experimental Sinter and the

Industrial Sinter

The industrial sinter used for comparison wasmadein a 400mmx 400mmsinter pot. The productionconditions are given in Table 4. In order to assess

Fig. 10. Microstruc.tures of iron

specimens heatcd underthen cooled slowly in air.

Table

(A) SpecimenS13Heated for 4min

to 1255'CC.as composition:

CO= I o,~,

C02=24 o/o'

N2=75o/o

(P(:)2 =4> I0-7

atm)

(B) SpecimenS14Heated for 4min

to 12~]_)5'C

Gasc.omposition:

n n F:, o'v2=v." /(]'

N2=99.5 (yo

(P02=5> l0-3

atm)

(C) SpecimenS15Heated for 3min

to 1255'CGascomposition :

Tlle sameas (A)

to 1230'CGascomposition:

Thesameas (A)

ore boundaries in the

various conditions and

4. Informatlon of pot smter

Ratio ofiron ores

Chemicalcomposition

of sinter

Quality ofsinter

(JIS Standard)

Hammersley:Mt. NewmanCVRD:MBR:ISCOR:

20 ol~

17 o/o

26 o/*

29 o/o

8010

T.Fe :Si02 :

CaO:Al203

MgOFeO:

57.5 o/o

5. 13 o/o

9.62 o/o

1.83 o/o

l.80/0

6.0 o/o

TJ(+10mmo/o) : 58

RDI(-3 mmo/o) : 44

RI (o/o) : 70

Coke rate (kg/t55sinter) :

the degree of reduction of this industrial sinter, thethickness of the magnetite layer on the unreacted iron

ore which camefrom the samesource as that used in

experiments was observed and compared to that ofthe experimental sinters. However,generally the real

thickness of that layer appears on the largest section

of iron ore particle. Therefore, only the magnetite

29

Fig. Il. Mic.rostructure of iron ore boundary in the in-

dustrial sinter.Fig. 14. Microstructure of high magnetite content area in

the industrial sinter.

Fig. 12, Microstructure oi' commonbond in the industrial

sinter,

Fig. 13. Microstructure of specimen (S9) heated for 4minto 1255'C in the gaseousmixturc C0=I ~•(), C02=24 o/o' and N2=75o/o (P02=4> l0-7atm) andthen cooled slowly in air.

layers on the large sections (>3 mm)of iron ore par-ticles were counted.

The thickness of the magnetite layer varies in theindustrial sinter, in some cases the thickness is dif-

ferent even around a single unreacted iron ore par-ticle. This mayin part be a sectioning effect but it

could be that the degree of reduction is different fromone place to another in industrial sinter. Fig, Il

shows a typical magnetite layer which is similar inthickness to the magnetite layer of thc experimentalsinter heated to 1255'C in the gaseous mixture of

CO I yo' C02 24 o/o' and N2=75o/o (Po,=4XI0-7atm), as shownin Fig. 10(A).

Amixture of columnar calcium ferrite with granu-lar magnetite grain and a little glassy silicate is the

most commonbond in this industrial sinter, as shownin Fig. 12. This microstructure is quite similar tothat of the specimenheated to 1255'C in the gaseousmixture of C0=1~/o' C02=24o/o' and N2=75o/o

then cooled slowly in air (as shownin Fig. 13). How-

Fig. 15. Microstrllcture of' specimen (S2) heatcd for 4minto 1255'C in the gaseous mixture C0=1O/o'

C02=24?/o' and N2=75~'i, (Po.=4XI0-7atm)and then coolcd rapidly in nitrogen.

ever, there are also regions of the microstructure whichcontain a large amountof magnetite with a little cal-

cium ferrite and glassy silicate as shown in Fig. 14,

and these occupy about 5 ~/o of area fraction. Thismicrostructure is qultc similar to that of a specimenheated to 1255'C in the gaseous mixture of C0=l o/o' C02=24ol:), and N2=75o/o then cooled rapidly(as shown in Fig. 15). These observations supportthe suggestion that the mineral formation in mast ofthe industrial sinter is affected by the air cooling proc-ess. The typical structure of the bond in this in-

dustrial sinter can be simulated by heating the speci-

mento 1255'C in the gaseous mixture mentionedabove and then cooling it slowly in air.

4. Discussion

(1) During the heating stag_e of laboratory sin-

tering, the specimen is initially in a flowing gaseousmixture which controls the atmosphere of the speci-

men. Whenthe specimen is heated, a considerable

amount of carbon dioxide and oxygen can be pro-duced by the calcination of limestone and the trans-formation of hematite into magnetite respectively inthe specimen. Inside the specimen, these gases will

continuously mix or react with the flowing gas.Therefore, if the specimen is heated in the gaseousatmosphere of low oxygen potential, at the beginningof heating the oxygen potential of the centre of thetablet will be higher than that of the controlled atmo-sphere and then gradually approach it. The oxygenpotential at the surface of the specirnen maybe close

to that of the controlled atmosphere. Onthe otherhand, even whenthe specimen is small (6 mmdiam-

J

J'

1

~

J

30 j

eter by 6mmheight), the heat distribution through-

out the whole specimen is not homogeneous. Natu-rally, the temperature of the surface is slig_ htly hig_ her

than that ofthe centre during heating and the cooling

rate at the surface is also higher. For these reasons,with the specimen heated in the gaseous atmosphereof low oxygen potential and then coo]ed rapidly, the

surface of the specimen contains moremagnetite andless calcium ferrite than the centre. Y. Hida et al.9)

also found that the surface contained moremagnetite.

In the case of the specimen cooled slowly in air,

normally the oxygen potential is higher at the surface

and decreases towards the centre. Therefore, reoxi-

dized hematite tends to form on the surface of the

specimen. However, if during the heating stage, the

oxygenpotential of the centre does not decrease great-ly, reoxidized hematite mayform in the centre of the

tablet during cooling. For example, the reoxidized

hematite may form near the unreacted ore in the

centre.Becauseof the non-homogeneousmicrostructure in

the specimen, it is important that comparisons aremadebetween the sameflxed section of each of the

specimens.(2) The rapidly cooled specimens used to study

the microstructure produccd in the heating stage of

sintering only approximate the real conditions in sin-

tering, because during the rapid cooling stage, apartglassy silicate phase solidified from the silicate melt,it is still possible to form somesmall crystals of iron

oxide minerals. In this investigation, however, the

rapidly cooled specimenscontain only a small amountof the small crystal minerals. With the specimensheated in the low oxygen potential atmo_spheres andthen cooled rapidly, the outline appears slig_ htly soft,

which corresponds to the specimenscontaining mainlysolid phases (around 80 o/) iron oxide minerals, asshown in Figs. 4(A) and 4(B)) in the heating stage.

Therefore, during the rapid cooling stage, there is notmuchiron oxide mineral formation. The rapid cool-

ing process used to study the microstructure in heating

stage is reasonable.(3) For specimens heated to 1210'C and then

cooled rapidly, with a decrease of oxygen potential,

the amountof magnetite increases but the amountof

calcium ferrite decreases. This agrees with the result

of Y. Hida et al.9) However, whenthe heating tem-perature increases to 1255'C, at high oxygen poten-tial (air) the calcium ferrite transforms to hematiteand silicate melt, and greatly decreases in amount.Therefore, the specimen heated in the mediumoxy-gen potential (5 x l0-3 atm) contains the most calciumferrite. The observation that with an increase ofheating temperature, the calcium ferrite transforms to

hematite and silicate melt in the high oxygen poten-tial, but transforms to magnetite silicate melt andoxygen in the low oxygen potential agrees with that

proposed by M. Sasaki et al.10)

During the cooling stage, the calcium ferrite haspieviously been supposed to form from the solidifica-

tion of the melt.3,4,ro) However, in this study it hasbeen clearly shown that a large amount of calciumferrite also maybe generated by the reactio_ n of mag-

netite with silicate melt and oxygen.According to the experimental results, the reducing

atmosphereduring the heating stage, the temperaturedistribution and the cooling conditions are all impor-

tant [~ctors affecting the minerals of' sinter. There-fore, whendoing the experiments to simulate indus-

trial sintering, those factors should be taken into con-sideration.

(4) From the results of the rapidly cooled speci-

mens, it can be deduced that in the heating stage of

sintering, calcium ferrite is generated initially andthen decomposed; the decomposition temperature is

lower than the maximumheating temperature of

l 255'C (from Fig. 4). Since generally the reaction

does not reach equilibrium, once calcium ferrite candecomposethe proportion decomposedwill increase

with holding time. The specimen contains less cal-

cium ferrite and more magnetite and silicate melt in

the low oxygen potential atmosphere, but less calciumferrite and more hematite and silicate melt in the

high oxygen potential atmosphere. On the other

hand, the amo_unt o_f decomposedcalcium ferrite also

increases with the maximumheating temperature(from Fig. 5). Although the maximumtemperatureused in this work is lower than that used in commer-cial sintering, it is likely that the samesequence of

reactions occurs.(5) In industrial iron ore sintering, because raw

materials of various chemic.al compositions and non-uniform particle size distributions are mixed, from amicroscopic point of view, the reactions of sintering

are non-homogeneous. Generally, the material nearcoke particles is more reduced in the heating stage.

In the cooling stage, the material beside the openpores throngh which air passes is cooled in a high

oxygen potential atmosphereand the material distant

from the open pores maybe cooled in an atmosphereof low oxygen potential. Therefore, it is probablethat a small part of material situated near coke anddistant from the pores is cooled in an atmosphere of

low oxygen potential and preserves a microstructuresimilar to that formed in the heating stage.

Since both the reduction degree during heating

and the effect of air cooling vary even in a single sam-l of sinter the different resultant microstructurespe ,

can not be simulated by o_ne experiment. Therefore

an experiment which can closely simulate the typical

microstructure of industrial sinter is probably the mostinstructive.

In this investigation, the industrial sinter is from anormal sintering. Its typical microstructure com-posed of columnar calcium ferrite, granular magnetitegrain and glas~'y si]icate can be simulated by heatingthe specimen for 4min to 1255'C in the gas mixtureof C0=1o/o' C02=24o/.,, and N2=75o/o and thencooling it slowly in air. Thercfore l,'t seerns reason-able- to use this as a standard procedure to simulate

the normal industrial sintering. However, with the

various raw material mixtures and operating condi-

tions of different industrial sintering, the degree of

reduction and the temperature profile maybe verydifferent. For this reason, the heating atmosphereand the temperature profile for simulating various

31

industrial sintering should be different.

5. Conclusion

In order to examine the effects of gaseous atmo-sphere, heating temperature and cooling condition onthe formation of minerals in sinter, small tablet speci-

menscontaining powderedcommercial iron ore, Iime-stone, quartz, and kaolin wcre heated in the con-trolled gas atmospheres. The results obtained aresummarizedas follows:

(1) In the heating stage of laboratory sintering,

with a decrease of partial pressure of o_xygen, themagnetite content increases and hematite content de-creases. The calcium ferrite content is found also todecrease- at the low sintering temperature (1 210'C).However, at a higher sintering temperature (1 255'C),the calcium ferrite transforms to hematite and silicate

melt greatly at the high oxygen potential; ancl there-fore, the mediurn oxygen potential (5 x l0-3 atm) pro-duces the most calcium ferrite.

(2) The air cooling stage has a significant effect

on the formation of minerals. In this stage, mag-netite mayreact with the silicate melt and oxygen toproduce a large amount of calcium ferrite and also

mayoxidize to reoxidi7.ed hematite. However, thehematite phase (fo_ rmed in the heating stage) does nottend to react with silicate melt to generate calciumferrite.

(3) The thickness of magnetite layer on the un-reacted iron ore particle in sinter maybe used t.o

assess the degree of reduction during thc sintering.(4) A typical microstructure of the bond com-

posed of columnar calcium ferrite, granular magnetite

grain and glassy silicate in a normal industrial sinter

can be simulated reasonably by heating a specimen tol 255'C for 4mmmthe gaseo_us mixture C0=1o/o'

C02=24o/o' and N2=75o/o and then cooling it slowlyin air.

Acknowledgements

The authors would like to express their apprecia-tion to China Steel Corporation, Taiwan, and theDepartment of Metallurgy of Shefficlcl University,U.K., for supporting this work. Manythanks arealso due to Dr. A. W. Bryant of the Department ofMetallurgy, Sheflield University, for his assistance.

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