Conditions Simulating the Formation Mineral Phases
Transcript of Conditions Simulating the Formation Mineral Phases
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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