Influence of sintering conditions on the reduction behaviour

Influence of sintering conditions on the reduction behaviourof pure hematite compacts

K. Higuchi *, R.H. Heerema

Department of Applied Earth Sciences, Delft University of Technology, Mijinbouwstraat 120, 2628RX Delft, The Netherlands

Received 13 July 2002; accepted 23 August 2002

Abstract

This paper is centered upon possible effects of the peak temperature attained during sintering upon subsequent reduction of

hematite. To this end, pure hematite compacts were prepared and sintered at various temperatures in the 11001400 C range, andanalysed microscopically before and after reduction. The microstructure undergoes marked changes during high temperature sin-

tering (13701400 C). This change was found to be associated with magnetite formation and subsequent re-oxidation duringcooling. Fine cracks (discontinuities) developing along grain boundaries and the coarse pores and irregular shape of re-oxidised

hematite, associated with the earlier formation of magnetite, influenced reduction behaviour particularly at 550 C, resulting inexpansion and severe loss of strength. In isothermal reduction tests of pure hematite tablets conducted between 400 and 1200 C,four types of reduction behaviour were observed. Expansion and strength after reduction were both affected by such changes of the

reduction mechanism. Reduction behaviour was influenced by both the reduction temperature and nature of the microstructure

prior to reduction.

2003 Published by Elsevier Science Ltd.

Keywords: Reduction; Agglomeration

1. Introduction

In the upper part of the shaft of the blast furnace,

iron-bearing materials such as sinter, pellets and lump

ores exhibit size degradation, which leads to a decrease

in permeability of the blast furnace and affects the sta-bility of operation. After recognition of the importance

of degradation, quality test methods such as ISO 4696

and reduction degradation index (RDI) were established

and used extensively for quality control in industrial

plants. Nakajima et al. (1987) showed that sinter suf-

fered degradation particularly in the temperature range

of 600800 C in the shaft. Furthermore Takada et al.(1986) showed that degradation of sinter was influencedby the reduction conditions as well as by the sinter

quality itself. Degradation increased with the residence

time at 500700 C in the shaft, even though the samequality of sinter was used.

Two main factors affect the reduction behaviour of

iron-bearing materials in terms of reduction rate and

size degradation, i.e., the nature of the materials and the

reduction conditions. Macroscopically, the size and

shape of the individual agglomerate particles play a role

(Kang et al., 1998a,b), and microscopically, the pore

structure, grain size and mineralogy. The microstructure

of the agglomerates is influenced by the starting mate-rials, shaping methods and sintering conditions, espe-

cially at high temperature. With regard to the reduction

conditions, much work has been published, using vari-

ous materials for testing. Natural or synthesised single

crystals of hematite were used in some investigations to

clarify the reduction mechanisms (Edstroom, 1953;Swann and Tighe, 1977; Hayes and Grieveson, 1981; Et-

Tabirou et al., 1988). In other work, finely groundnatural ore (Hayes and Grieveson, 1981), plant pellets

( UUnal and Bradshaw, 1983), or synthesised hematitecompacts were used. Even in the case of synthesised

hematite compacts, the starting materials and sample

preparation methods were different from investigation

to investigation. As a result of these differences, the

grain diameter of hematite grains and the pore structure

within the samples must differ, and hence strongly affectthe reduction characteristics. Nevertheless, information

on the influence of the microstructure of hematite

*Corresponding author.

E-mail address: [email protected] (K. Higuchi).

0892-6875/03/$ - see front matter 2003 Published by Elsevier Science Ltd.doi:10.1016/S0892-6875(02)00180-2

Minerals Engineering 16 (2003) 463477This article is also available online at:

www.elsevier.com/locate/mineng
mail to: [email protected]

compacts or agglomerates on their reduction behaviour

is limited (El-Geassy and Nasr, 1990).

In the industrial sintering process of iron ores, the

temperature profile consists of three stages, i.e., heating,

holding for a few minutes at the maximum temperature

and cooling down. The raw materials are first heated by

combustion of the coke breeze mixed in. The downwardairflow and the presence of a water condensation zone

result in a high heating rate of the raw materials, in the

range of 6001000 C/min. The maximum temperatureof the sinter bed ranges from 1200 to 1350 C. It de-pends on the content of coke breeze as a source of heat,

and on limestone and water as heat absorbers. Bed

height also has an influence on the maximum tempera-

ture by way of accumulation of heat. Afterwards, theraw materials are cooled by an air stream, the cooling

rate depending on the air flow rate. In the case of con-

stant suction sintering, it is influenced by the perme-

ability of the sintering bed as well. Hida et al. (1984) and

Hsieh and Whiteman (1989a,b) demonstrated the large

influence of the prevailing atmosphere on the formation

of mineral phases. They concluded that the industrial

sintering process takes place in a slightly reducing at-mosphere in which the partial pressure of oxygen (PO2) is101102 Pa. Under these moderately low PO2 condi-tions, the equilibrium temperature between hematite

and magnetite ranges between 900 and 1100 C (Turk-dogan, 1980). Therefore, hematite, a major constituent

of iron ore sinter, is easily decomposed to magnetite

during heating, and most of this magnetite is readily re-

oxidised to hematite during subsequent cooling.

Some areas of sintering, however, need further elu-

cidation. One of these areas concerns the fundamen-

tal sintering behaviour of iron oxide at high temperature

during induration and its influence upon the subse-

quent reduction process in the blast furnace. The

present studys objective is to explore the influence ofsintering conditions on reduction behaviour of ironoxide in terms of its degradation characteristics under

conditions relating to the upper shaft of the blast

furnace.

2. Materials and methods

Reagent Fe2O3 (purity; >99%, grain diameter;

hematite samples exposed to industrial conditions. The

reduction gas was a mixture of CO, CO2 and N2. Gas

composition was set at CO 22:5%, CO2 27:5% andN2 50% so that it would simulate the actual top gascomposition prevailing in the blast furnace. As a result,

Fe3O4 is stable below 630 C and FeO is stable abovethis temperature (Darken and Gurry, 1945). Total gasflow was maintained at 2.0 l/min. The superficial gas

velocity at standard state was 30 cm/s and sufficiently

high to prevent reduction gas starvation in the gas phase

(Yamaoka et al., 1982). The sample was first heated to

the desired temperature in N2. After stabilisation of

both the temperature and its corresponding thermal

expansion, the reduction gas was led into the chamber.

Reduction time was set mainly for a period of 30 min.Following reduction, the sample was quenched in water

to fix the existing microstructure and limit back-oxida-

tion. Reduction temperature was usually fixed at 550 C,but some tests were carried out in the temperature range

of 4001200 C.Before and after reduction, the dimensions and

the weight of a sample were measured to determine the

volumetric expansion and the degree of reduction. Thelatter was estimated using O/Fe as the mole ratio after

reduction, in terms of the mole ratio derived from the

weight change during reduction by the following equa-

tion:

O=Fe in moles OB ORFe

32 55:85

TFe 1

2 71:85 FeO DW16

55:85TFe

1

where OB is the oxygen combined with iron oxide (mol);

OR, the oxygen removed during reduction (mol); Fe, the

iron combined with iron oxide (mol), TFe; Total Fe

content before reduction (mass%); FeO, FeO content

before reduction (mass%); DW , the weight loss duringreduction (mass%).

The strength of tablets before and after the reduction

tests was measured by employing a small-scale shatter

strength test. A weight of 300 g was dropped from a

height of 30 cm above the samples twice. The results of

preliminary tests showed that the percentage of particle

size above 1 mm reflected the changes in strength of the

tablets well. Therefore the strength of samples wassubsequently defined as the percentage of particle size

over 1 mm after the drops (WA), against the total weightbefore the test (WB); [Strength of sinter 1 mm;% WA (gram)/WB (gram) 100]. Measurement error of thestrength test was within 2.4%. Some tablets were anal-

ysed by optical microscopy, X-ray diffractometry (Phi-

lips PW3710) and electron microprobe observations

(JEOL 8800).

3. Results

3.1. Influence of sintering conditions

Fig. 2 shows the changes in properties of pure he-

matite tablets as a function of the sintering temperature.

The average diameter (equivalent circular diameter) ofhematite grains and the total porosity were measured

using image analysis and a screw micrometer, respec-

tively. The magnetite content in sintered tablets was

calculated from weight loss during sintering. Looking at

the results, the changes occurring above 1370 C arestriking. All tablets sintered from 1375 C onwardcontained magnetite phase, even after cooling in air.

However, the magnetite content in tablets sintered at1375 C and 1380 C was quite small (

As for grain size and shape, hematite grain size was

extremely fine at less than 5 lm below 1100 C due tothe low diffusion rate at those temperatures. Above

1200 C, spherical hematite grains were apparent. Theirsize increased as the sintering temperature was raised,

from approximately 10 lm at 1200 C to 28 lm at

1380 C. At 1385 C, many small pores were observedinside the hematite grains and the shape of hematite

grains had become irregular. Above 1390 C, hematitegrains were large (70110 lm) and the shape of thegrains was even more irregular. Three types of pores and

two types of cracks were observed:

Fig. 3. Changes in microstructure of hematite tablets by sintering temperature: (A) crossed nicols, 250; (B) parallel nicols, 500.

466 K. Higuchi, R.H. Heerema / Minerals Engineering 16 (2003) 463477

(1) Fine pores of less than 2 lm were observed withincrystallite grains in all samples. However, these fine

pores tended to be located along grain boundaries above

1200 C.(2) Large pores (about 20 lm) were observed within

grains at 13901400 C.(3) Fine pores of less than 1 lm diameter, which were

located near the interface between hematite and mag-

netite, were observed at 13851400 C.(4) Fine cracks along grain boundaries were observed

near the surface at 13001350 C. However, theirnumber was small. No magnetite formation was ob-

served at those temperatures. Therefore these fine cracks

were most likely due to thermal shock during cooling.

Fine cracks along grain boundaries increased in numberas the sintering temperature was raised up to 1380 C.Detachment of grains during polishing was enhanced by

those fine cracks along the grain boundaries.

(5) Fine intra-particle cracks were observed at 1385

1400 C. Near the cracks magnetite phase was formed.Magnetite was formed not just at the surface, but also

inside the tablet in the surface zone. Hematite then

formed once again around the magnetite phases uponcooling.

(2), (3) and (5) were observed only at high tempera-

tures, accompanied by the formation of magnetite.

Therefore, it became clear that development of these

microstructures was associated with the formation of

magnetite and subsequent re-oxidation to hematite.

Fig. 4 shows the relationship between reduction be-

haviour (vertical expansion, strength and degree of re-duction) and the sintering temperature of pure hematite

tablets. All reduction experiments were carried out at

550 C for 30 min. Expansion showed a maximum intablets sintered at 1385 C. Strength after reductiondropped in correspondence with the expansion. The

degree of reduction was small up to a sintering tem-

perature of 1350 C. However, above 1350 C, the de-gree of reduction increased with higher sinteringtemperatures and reached almost complete transition to

magnetite in tablets that had been sintered at tempera-

ture levels of 1385 C and above.To clarify the reduction mechanism of pure hematite

tablets, long duration reduction tests were performed

until all the hematite had transformed to magnetite. The

reduction conditions were based on the following stan-

dard condition (gas composition: CO 22:5%, CO2 27:5%, N2 50%, reduction temperature: 550 C). 30min after stabilisation of the expansion curve during

reduction, the sample was quenched in water to fix the

mineralogy at the reduction temperature.

The results of the influence of the original sintering

temperature following complete transition from hema-

tite to magnetite are shown in Fig. 4 as well, the corre-

sponding curves indicated by triangles. Tablets sinteredbelow 1375 C required much longer reduction time to

reach complete transition from hematite to magnetite

than tablets sintered above 1380 C. However, ulti-mately the tablets sintered at 1375 C showed a largeexpansion. Therefore, the reason behind the limited

expansion below 1375 C in the 30 min tests must be therelatively low reduction rate due to a compact structure.

3.2. Influence of the reduction temperature

Isothermal reduction tests of pure hematite tablets

sintered at different temperatures were performed at 400

1200 C at a constant gas composition. Tablets, whichhad previously shown clearly different indurated char-acteristics and different reduction behaviour at 550 C(Fig. 4), were selected (sintered at 1375 and 1385 C,respectively, in air for 4 h). Fig. 5 shows the influence of

the reduction temperature. All reduction experiments

were performed under the following standard conditions,

except the reduction temperature (reduction time: 30

min, gas composition: CO 22:5%, CO2 27:5%, N2 50%). Wustite composition (O/Fe) in equilibrium withthe prevailing COCO2N2 gas was calculated from lit-

erature data (von Bogdandy and Engell, 1971). To clarify

Fig. 4. Influence of sintering temperature on low temperature reduc-

tion behaviour of pure hematite tablets.

K. Higuchi, R.H. Heerema / Minerals Engineering 16 (2003) 463477 467

the reduction mechanism of pure hematite tablets, re-

duction tests without time limit, to complete the transi-

tion to magnetite or wustite were also carried out. The

results are also shown in Fig. 5, as paler curves.There were two temperature ranges to explain re-

duction behaviour, i.e., low temperature (630 C) at which wustite was stable under the pre-vailing gas conditions. This indicates that changes in the

reduction mechanism observed at 630 C are related tothe change in stable phase.

Basically, expansion increased with the reductiontemperature at both sintering temperatures. However,

expansion exhibited a clear peak at a reduction tem-

perature of 550600 C in the tablets sintered at 1385 C.Strength and O/Fe changed in a manner corresponding

to the change in expansion. Influence of the sintering

temperature on reduction behaviour was clearly evident

at low temperature reduction, but became minor at high

temperature reduction. The O/Fe after 30 min followingstabilisation of the expansion was almost the same as the

O/Fe value representing complete transition. However,

in reduction experiments at 630700 C, which is near

the transition temperature between magnetite and wus-

tite, the O/Fe was higher than that of wustite. Even after

a longer period of reduction following stabilisation of

the expansion, the O/Fe did not drop further. Therefore,

the higher O/Fe observed at 630700 C must have beendue to re-oxidation of previously formed, porous wustite

to magnetite during quenching. During reduction oftablets sintered at 1375 C, expansion after completetransition increased with the reduction temperature, and

this expansion was much higher than that after 30 min

reduction. This confirms that the low expansion and

high strength observed at low temperatures after only

30 min of reduction was due to the low reduction rate

(high O/Fe) attained in that case. On the other hand,

during reduction of tablets sintered at 1385 C, the ex-pansion after complete transition was similar to that

after 30 min reduction and showed a maximum at a

reduction temperature of 600 C. Tablets fully reducedto magnetite at 400450 C were extremely weak re-gardless of the sintering temperature. Therefore, the

relatively high strength observed after 30 min reduction

at 400450 C had to be due to the low reductionrate (high O/Fe). During reduction of tablets sinteredat 1375 C, the strength increased with the reductiontemperature above 500 C. In addition, the reductiontime required to attain stabilisation of expansion was

quite long at a reduction temperature of 500 C (416min). This suggests that the reduction mechanisms be-

low and above 500 C are different.The strength of both (30 min and fully reduced)

tablets was relatively high even after complete transitionto wustite, indicating a strengthening mechanism taking

place, e.g. by repair through sintering. This strengthen-

ing mechanism is most likely due to a higher mobility of

iron ions in wustite as a result of higher vacancy con-

centration (von Bogdandy and Engell, 1971).

4. Discussion

4.1. Influence of sintering temperature on the microstruc-

ture

As shown in Figs. 2 and 3, the sintering temperature

strongly influenced the microstructure, in terms of the

pore structure, grain size and mineralogy of the tablets.

The factors determining the microstructure are as fol-

lows:

(1) Physical shrinkage and grain evolution of tablets

during sintering.(2) Chemical reaction with air or prevailing gas compo-

sition at high temperature.

As for shrinkage and grain evolution, the driving force

is minimisation of surface energy. Before sintering, the

Fig. 5. Influence of reduction temperature on the reduction behaviour

of pure hematite tablets.


green tablets had a porosity of 28% and the crystallite

grain size of hematite was less than 5 lm. Most of theporosity is represented by space between hematite par-

ticles (open pores), and the remainder is closed porosity

inside those particles. Open pores gradually diffuse to the

surface of tablets and closed pores tend to diffuse to the

interface between grains during sintering to minimisethe surface energy. This results in shrinkage of the

tablet and a decrease in porosity. Despite these trends,

porosity is not completely driven out of the tablets and

remains apparent after sintering. Growth of grains also

takes place at the contact points by diffusion in the solid

state. Larger grains grow by incorporating contacting

smaller grains to decrease their total surface area.

The chemical reaction during sintering takes placebetween iron oxides and oxygen and is represented by

decomposition of hematite and re-oxidation of formed

magnetite Eq. (2).

3Fe2O3 s 2Fe3O4 s 1=2O2 g 2

If this reaction takes place during sintering, the pore

structure of the tablets must be strongly affected be-

cause the lattice structure of both minerals is quite dif-

ferent, and may result in stress development within

hematite grains. It has been reported in the literature

that c-Fe2O3 (maghemite), a metastable phase as op-posed to a-Fe2O3 (Shultz and McCarthy, 1988), is ini-tially formed during oxidation of magnetite (Edstroom,1953; David and Welch, 1956; Ball et al., 1973;

Basantsev, 1976).

To clarify the evolution process in the microstructure

of tablets, water-quenched samples were analysed.

Green tablets were sintered at various temperatures,

from 1350 to 1400 C, in air for 4 h and subsequentlyquenched in water to freeze the mineralogy existing at

high temperature. To determine both the mineralogy

and possible strain development in hematite grains, X-

ray diffraction analysis was also carried out. The quen-

ched samples were crushed to less than 200 mesh and

mixed with 33 mass% reagent Si powder. Relative in-

tensities of the hematite(1 0 4) and magnetite(2 2 0) peaks

against the standard Si peak were quantified from re-sults of the X-ray diffraction analysis.

Fig. 6 shows the polished sections and SEM images of

a fracture surface of water-quenched tablets sintered at

1350, 1375, 1380 and 1400 C, respectively. The hema-tite to magnetite proportions in the tablets are indicated

in Fig. 6.

(a) At a sintering temperature of 1350 C, no mag-netite formation took place. The microstructure wasalmost the same as for the normal sample cooled in air,

which is shown in Fig. 3. Magnetite was formed from

1375 C onwards, as can been seen in the water-quen-ched samples in Fig. 6. After normal cooling in air, there

was no magnetite phase left at 1375 C, as shown in

Fig. 3. The difference in magnetite content before and

after cooling indicates that re-oxidation of the formed

magnetite near the surface took place during cooling in

air. In the quenched sample, re-oxidised hematite was

also observed at the surface, which must have been

formed during the water-quenching itself. The amount

of magnetite increased with the original sintering tem-perature. At 1400 C, hematite was completely decom-posed to magnetite prior to cooling. At this temperature,

11 mass% of magnetite was still found to be present after

normal cooling in air, as shown in Fig. 2. Most of the

hematite found in the tablets following normal cooling

in air must therefore have been re-oxidised. The grain

shape was changed from a regular shape to an elongate,

continuous shape at 1400 C. It is clear that a significantstructure change must have already taken place during

the decomposition of hematite, prior to cooling, at this

temperature. Hematite was subsequently formed around

these continuous grains and large pores, implying that

this happened during water-quenching. Re-oxidation of

magnetite must have taken place briefly, but at a high

rate. With regard to the lower sintering temperatures, all

of the formed magnetite was re-oxidised below 1380 Cduring air cooling, while above 1385 C, some of theformed magnetite remained even after cooling.

(b) From the observation of SEM images of the

fracture surface, significant differences were not ob-

served at 13501380 C. However, at 1400 C the imageof the fracture surface changed. At sintering tempera-

tures of 13501380 C, fracture mostly took place alongthe grain boundaries of hematite grains. No large porewas observed and the structure of the surface was

compact. Therefore the many pores seen in polished

cross-sectional images as shown in Fig. 3 must have

been caused during the polishing process due to pluck-

ing-out effects. Many fine, rounded pores (about 2 lmdiameter) were observed along the hematite grain

boundaries. Some fracture surfaces also showed intra-

particle fracture. Such fracture surfaces contained only asmall amount of fine pores. At a sintering temperature

of 1400 C, a fracture pattern could not be deducedconclusively from the observations because the fracture

surface was complex. The originally compact structure

of roundish grains disappeared and a continuous

structure (of large magnetite grains) formed, enclosing

pores of the order of 10 lm diameter.(c) X-ray diffraction analysis showed that the miner-

als present were hematite (a-Fe2O3) and magnetite, andthe presence of maghemite (c-Fe2O3) was not detected inany of the water-quenched samples. The d-spacing for

hematite in the tablets sintered between 1350 and

1385 C was virtually constant. In contrast, the d-spacingfor hematite in the tablet sintered at 1400 C showed aclear shift, which is due to the partial re-oxidation of

magnetite during quenching. Considering that hematitewas not a major constituent mineral at 1400 C, it can be


deduced that there could not have been a significant

strain within hematite grains in the case of tablets sin-

tered below 1380 C.

The conclusion is that neither the formation of

maghemite, nor strain within hematite grains can be the

reason behind the observed differences in reduction

Fig. 6. Microstructures and SEM images of the fracture surface of tablets quenched in water after sintering in air for 4 h.


behaviour. Therefore, the unusually large expansion

during reduction following sintering temperatures of

13801385 C must be associated with the formation ofmagnetite in the sintering stage.

From microstructural analysis, the mechanisms

occurring during sintering, i.e., the evolution of micro-

structures of pure hematite, are proposed and sum-marised in Fig. 7. Below 1200 C, the grain growth rateis low, resulting a small grain size of less than 5 lm.Fine pores from space among initial particles, and

original fine closed pores within particles are distributed

homogeneously. At 12001375 C, further grain growthproceeds and the crystallite grain size reaches approxi-

mately 30 lm. Many fine pores (

DG 249; 450 140:67 T JKubaschewski and Alcock; 1979

DG 237; 990 137:26 T J Turkdogan; 1980

DG RT ln a2Fe3O4 P

1=2O2

a3Fe2O3

!

ffi 12 RT ln PO2

3

Calculation results using thermodynamic data, for TH>Min air were 1403 C (Elliot and Gleiser, 1960), 1440 C(Barin and Knacke, 1973), 1422 C (Kubaschewski andAlcock, 1979) and 1383 C (Turkdogan, 1980). Thedisparity between these results was rather large at 57 C.Muan (1955) and Chaigneau (1994) directly measured

TH>M in air, and found this to be at 1390 and 1380 C,respectively. Their results show that the measured

transition temperature tends to be lower than calculated

from thermodynamic data. However, in the present

study a small amount of magnetite was observed in the

surface region even at a sintering temperature as low as

1375 C. Possible sources of deviation from the litera-ture data might be (1) inhomogeneous temperature

distribution within the muffle furnace, (2) PO2 beingslightly lower than 0:21 105 Pa due to oxygen con-sumption within the muffle furnace, (3) the influence of

impurities within the chemical reagent grade materials.

The chemical reagent grade hematite used in present

study contained a maximum impurity level of 1%.

Through sensitive TGA analysis of 99% and 99.999%

pure hematite in air, TH>M in air was found to be at 1386and 1389 C, respectively. Although factor (2) wouldappear the most likely source of the difference, the dis-

crepancy was not further pursued since under commer-

cial sintering conditions a low PO2 will always beprevalent and hence lower-temperature formation of

magnetite is to be expected.

4.2. Influence of reduction conditions on the microstruc-

ture

Fig. 8 shows the microstructures of tablets after re-

duction at different temperatures. At all reduction tem-

peratures, reduction progressed in a topochemical

fashion. At reduction temperatures of 450 and 550 C,magnetite was formed at the surface of the tablets. At

450 C, reduction progressed mainly along the grainboundaries due to a slow chemical reaction rate. It re-sulted in unreduced hematite remaining inside the to-

pochemically reducing grains. Grains did not impinge

on each other and the bulk structure was quite loose. In

contrast, at 550 C a clear reaction front appeared. Nearthe reaction front, some incompletely reduced hematite

grains surrounded by formed magnetite were visible.

The magnetite layer appeared dense, with grains grow-

ing together and with poorly defined grain boundaries as

indicated in Fig. 8. On the other hand, the magnetite

layer near the reaction front still showed a loose struc-

ture and there were spaces between grains. At 800 C, awustite layer developed on the outside of the magnetite

zone and the reaction front became more clear-cut to-

pochemical. The layer thickness of both magnetite and

wustite increased with the reduction temperature. Nounreduced hematite grains were found remaining in the

magnetite zone. In the tablets sintered at 1375 C, aporous form of wustite developed, with the pores more

or less perpendicularly facing the reduction front. A

small amount of re-oxidised hematite at the surface of

the tablet must have developed upon removal from the

furnace and during water quenching. The wustite

formed following a sintering temperature of 1385 Cshowed a different structure after reduction, in com-

parison with the one sintered at 1375 C. This was mostlikely due to the irregular shape of the initial hematite

before reduction as shown in Fig. 3. The magnetite zone

in the interior (near the reaction front) had a loose

structure, with spaces between the grains. At 1000 C,reduction was almost complete and most of the con-

stituent phase was wustite. This wustite contained lesspores, while the space between grains increased. At

1200 C, the wustite was sintered and showed a con-tinuous phase containing porosity.

Considering low temperature reduction of hematite,

two possible mechanisms can be postulated as follows:

(a) If the formed magnetite layer is dense, CO gas reacts

at the magnetite surface to form iron ions (1); theseions diffuse through the magnetite layer (2); hema-

tite and iron ions react to form magnetite at the in-

terface between hematite and magnetite (3A).

(b) If the formed magnetite layer is porous, CO gas can

readily diffuse through the pores and directly react

with hematite at the interface (3B).

4CO Fe3O4 ! 3Fe2 2e 4CO2 (step 1)Diffusion of Fe2 through the magnetite layer(step 2)

4Fe2O3 Fe2 2e ! 3Fe3O4 (step 3A)CO 3Fe2O3 ! 2Fe3O4 CO2 (step 3B)

Many past investigations reported that the mor-

phology of magnetite formed during transition from

hematite was influenced by the reduction conditions.

Bradshaw and Matyas (1976) investigated the reductionof plant pellets using a COCO2 gas mixture, and found

that porous magnetite formed below 800 C andlamellar magnetite above this temperature. Swann and

Tighe (1977) distinguished three types of magnetite re-

duced from hematite, i.e., porous, plate or transitional

types, respectively. They reported that the reduction

temperature determined the morphologies of magnetite.

On the other hand, other authors showed that the re-duction atmosphere such as partial pressure of oxygen


(Hayes and Grieveson, 1981) or CO ( UUnal and Brad-shaw, 1983) had an influence upon the morphologies of

magnetite as well. Et-Tabirou et al. (1988) observed

fracturing by open cracks during low temperature re-

duction of a hematite single crystal. Cracking was also

reported in newly formed magnetite after reduction

(Brill-Edwards et al., 1965; Bradshaw and Matyas,1976). However, these cracks apparently did not act as a

gas diffusion route, according to microstructural obser-

vation. Apparently, the reduction rate is controlled by

chemical reaction at the interface between hematite and

magnetite, and reduction proceeds topochemically as

opposed to following cracks. In the present study, po-

rous magnetite was formed and the reduction proceeded

in topochemical fashion during low temperature reduc-

tion. The observations were in line with literature databelow 550 C. Therefore, it can be concluded thatreduction at low temperature proceeds according to

Fig. 8. Microstructure of tablets after reduction (H; Hematite, M; Magnetite, W; Wustite, Re-H /Re-M; Re-oxidised hematite/magnetite during

quenching).


reduction mechanism (b). However, in the present study

it was also clearly shown that the microstructures of

tablets reduced at 450 and 550 C were different fromeach other. The transition temperature of the micro-

structure corresponded to the observations of Et-Tabi-

rou et al. (1988). Furthermore, it was found that the

morphology of the formed magnetite was influenced bythe sintering temperature, when a reduction temperature

of 550 C was employed. As shown in Fig. 8, the formedmagnetite corresponding to a sintering temperature of

1385 C was cracked, while magnetite formed followingsintering at 1375 C was not. This difference might bedue to the reduction rate, as a result of a different initial

pore structure as mentioned before. In the present study,

porous magnetite was formed even at a reduction tem-perature of 800 C. This result was different from pastobservations (Bradshaw and Matyas, 1976; Swann and

Tighe, 1977; UUnal and Bradshaw, 1983). The disparity ismost likely due to a different gas composition. In the

present study the stable phase under the prevailing

conditions at 800 C was wustite, and hence the reduc-tion mechanism was different from those reported in the

literature.With regard to higher temperature (>630 C), the

reduction of hematite to wustite can be expressed as

follows:

CO Fe1yO ! 1 y Fe2 2e CO2 (step 1)Diffusion of Fe2 through the wustite layer (step 2)

Fe3O4 1 4yFe2 2e ! 4Fe1yO (step 3A)1 4yCO 1 yFe3O4 !3Fe1yO 1 4yCO2 (step 3B)Diffusion of Fe2 through the magnetite layer (step 4)

4Fe2O3 Fe2 2e ! 3Fe3O4 (step 5A)CO 3Fe2O3 ! 2Fe3O4 CO2 (step 5B)

where y is mole fraction of vacancies in wustite.y 0:082 at the equilibrium between wustite and gas,y 0:104 at the equilibrium between wustite and mag-netite at 800 C. If the product magnetite and wustitelayers would be sufficiently dense, no gas diffusion could

take place, and reduction would proceed according to

route 1 ! 2 ! 3A ! 4 ! 5A. The diffusion rate of Feions in magnetite (4) is quite slow compared with that in

wustite (von Bogdandy and Engell, 1971), and therefore

the former controls the reduction rate at high tempera-

ture. In this reduction mechanism, the layer thickness ofmagnetite during reduction is small and 102 times that

of wustite (von Bogdandy and Engell, 1971). If only the

wustite product layer is porous, CO gas can readily

diffuse to the reaction surface on the magnetite layer.

Reduction then proceeds according to 3B ! 4 ! 5A. Inthis reduction mechanism, the magnetite layer is rela-

tively thin because the controlling step of the reduction

is diffusion of Fe ions through the magnetite layer. Ifboth the magnetite and wustite product layers are po-

rous, reduction gas can react directly at the hematite and

magnetite surfaces. Reduction then follows 3B 5B. Inthe case where the chemical reaction rate of 3B (mag-

netite to wustite) is lower than that of 5B (hematite to

magnetite), the hematite layer disappears during reduc-

tion and from then on, only reduction of magnetite to

wustite takes place. In the case where the chemical re-action rate of 3B (magnetite to wustite) is larger than

that of 5B (hematite to magnetite), the hematite layer

remains present until end of reduction. In the present

study, the layer thickness of magnetite at 800 C waslarge (Fig. 8). Therefore, reduction proceeds through

direct contact of gas with the hematite/magnetite sur-

face. It was also evident from the results that the tran-

sition to wustite was still not complete even afterstabilisation of the expansion, and hence the reduction

rate of hematite to magnetite was sufficiently high to

maintain a thick magnetite layer. Only during the final

stages of reduction did completion of the transition

from magnetite to wustite take place.

Fig. 9 shows the relationship between reduction be-

haviour and the degree of reduction (O/Fe) of tablets

sintered at several temperatures. Standard reductionconditions, as defined earlier, were used. Therefore, the

stable phase is magnetite during reduction at 450 and

550 C (O=Fe 1:33), and wustite during reduction at800 C (O=Fe 1:09). It is evident that the propertiesmeasured change with the reduction temperature. In

particular, the fact that strength after reduction de-

creases with the degree of reduction at 800 C tends tosupport the mechanism proposed for high temperature

Fig. 9. Relationship between reduction behaviour and degree of re-

duction.


reduction. If only magnetite would have formed during

the first stage of reduction, the strength after reduction

would have been decreased by the same level as during

reduction at 550 C. Looking more specifically at550 C, two reduction mechanisms can be identified.Evidently, the reduction behaviour changed depending

on the original sintering temperature at this reductiontemperature level. Since one of reasons might be the

difference in reduction rate, a number of tests were

performed to bear out this influence. To change the re-

duction rate, the partial pressure of CO (PCO) was variedby changing the proportion of N2 in the gas mix at a

constant CO2/(COCO2) ratio ( 55%). Reductiontemperature was 550 C. Pure hematite tablets sinteredbetween 1350 and 1385 C were tested. Fig. 10 shows therelationship between reduction behaviour and PCO. Ex-pansion increased slightly and the strength decreased

with the reduction rate (PCO). A similar influence of re-duction rate on expansion has been reported elsewhere

(Edstroom, 1953). Hence, the spread in reduction prop-erties at 550 C depending on the sintering temperaturewas most likely due to the reduction rate. If the tablets

contain many pores or cracks prior to reduction, thereduction rate is high, resulting in high expansion and

decrease of strength (e.g., in tablets sintered at 1380 and

1385 C). If the tablets are more dense, the reductionrate is quite low, resulting in low expansion and high

strength even after complete transition to magnetite

(tablets sintered at 1375 C).Fig. 11 shows a summary of the proposed reduction

mechanisms. The reduction mechanism changes with the

reduction temperature. Furthermore, during reduction

to magnetite at 500630 C, the reduction mechanismwas also affected by the sintering temperature.

Mechanism A: During low-temperature reduction

to magnetite (400500 C), reduction gas diffuses intoand inside tablets via pores and cracks along the grain

boundaries. However, reduction into the interior of

individual grains is limited by the low chemical reac-

tion rate at the interface between hematite and mag-

netite. Porous magnetite surrounding hematite cores

in the individual grains causes a loose structure ofthe tablets, which results in low strength. Stress de-

veloping in the surrounding magnetite, however, is

distributed homogeneously, which results in low ex-

pansion.

Mechanism B: During moderately low-temperature

reduction to magnetite (500630 C), two types of re-duction mechanism are identified. Chemical reaction

rate is increased by the increase in reduction tempera-ture, resulting in formation of a well-defined topo-

chemical layer of magnetite from the outside inward.

The straightened reduction front causes anisotropic

stress, resulting in increased expansion. However, in the

case of reduction of dense tablets, reduction proceeds

only by chemical reaction between hematite and mag-

netite. This reduction is rather slow. Therefore, the

formed magnetite is annealed and the stress is eased,which results in a moderate level of expansion and

relatively high strength (Mechanism B1). In the caseFig. 10. Influence of the reduction rate (represented by PCO on thereduction behaviour of pure hematite tablets).

Fig. 11. Schematic classification of reduction behaviour mechanisms

(magnification, 250 for A, B1 and B2, 120 for C, arrows indicatethe direction of reduction).


of reduction of more porous tablets, pores and cracks

along the grain boundaries promote reduction. There-

fore, the reduction rate is high, which results in

high expansion levels and low strength (Mechanism

B2).

Mechanism C: During high temperature reduction to

wustite (6301200 C), the high chemical reaction rateresults in concentric topochemical layers of magnetite

and wustite. Reduction rate is controlled by the chemi-

cal reaction rate. Because both the formed magnetite

and wustite layers are porous, reduction gases readily

diffuse into these regions. The thus formed, more or less

straight reaction front causes anisotropic stress, which

results in high expansion.

4.3. Relevance for plant operation

Comparing the materials used in the present study

with plant sinter, two major differences should be noted.

Generally, basic plant sinter contains 5060% bonding

phase such as SFCA (silico-ferrite of calcium and alu-

minium) (Wang et al., 1998) and silicate slag, and about40% porosity. Nevertheless, trials have been carried out

to increase the iron content in sinter in order to decrease

the energy consumption, which in turn, results in an

increase in hematite phase (Yamaguchi et al., 1999). In

addition, the microstructure of plant sinter is heteroge-

neous and large, dense unreacted grains of iron ore re-

main in the structure. If such relict hematite undergoes

high temperature sintering under relatively low PO2conditions, formation of magnetite and subsequent re-

oxidation can take place, which might result in high size

degradation during subsequent reduction. To prevent

size degradation, avoiding high sintering temperature

levels on the strand would be effective in avoiding the

formation of magnetite which would then re-oxidise

during cooling, etc. With regard to reduction conditions,

these are not isothermal in plant blast furnace opera-tion. The reduction temperature of materials increases

gradually following their charging into the furnace. At

the same time the reduction gas composition gradually

shifts to higher partial pressure of CO. Nevertheless, the

results of isothermal reduction at low temperatures are

important, because once material experiences size de-

gradation at low temperature, it is difficult to recover the

strength at higher temperatures because of the low solidstate re-sintering rate (Chaigneau et al., 1997). In the

present study, four types of reduction behaviour were

observed. In particular, reduction under conditions at

which magnetite is stable showed severe size degrada-

tion. Hence, if the reduction time under the conditions

where magnetite is stable is kept short, the strength

should be maintained during further reduction at higher

reduction temperature and less degradation would bethe result.

5. Conclusions

The main conclusions based on the present work have

been summarised in schematic overviews in Fig. 7,

concerning the effect of peak induration temperature,

and in Fig. 11, comparing the effects of low- to high

temperature reduction in its interaction with the mi-crostructure derived from induration. More general

conclusions are the following:

1. The nature of the microstructure of material coming

out of the induration stage is the determining factor

for reduction geometry and rate, which in turn affect

internal stress development, expansion of the mate-

rial and its dynamic strength.2. Peak induration conditions from 1380 to 1385 C on-

ward in air, and starting at lower temperature levels

under lower oxygen conditions, generate transition

from hematite to magnetite from the surface inward,

from occasional, individual crystallite grains at the

low end of the temperature range to massive transfor-

mation at 1400 C.3. Transition of hematite grains to magnetite during in-

duration, followed by re-oxidation to hematite upon

cooling, results in increasingly irregular grain shape

and increasing size coupled with buildup of stresses.

Consequently, fine cracking and disengagement of

grains takes place particularly in the surface region,

in addition to coarse pore formation. The result is a

relatively weak, open structure amenable to rapid re-

duction and breakdown.4. Sensitivity to weakening of the iron-bearing material

during reduction varies with the conditions em-

ployed. A high reduction rate yields lower strength

after reduction from hematite to magnetite, and from

magnetite to wustite. At low reduction temperature

(630 C)weakening is also limited due to re-strengthening ofthe wustite structure, most likely through sintering.

At moderately low temperatures (500630 C) thereduction rate is high, without noticeable repair,

and significant weakening results, particularly if an

open structure is involved.

5. Very fine closed porosity (

References

Ball, D.F., Dartnell, J., Davison, J., Grieve, A., Wild, R., Voice, E.W.,

1973. Agglomeration of Iron Ores. American Elsevier Publishing

Company Inc., New York.

Barin, I., Knacke, O., 1973. Thermochemical Properties of Inorganic

Substances. Springer-Verlag, Berlin.

Basantsev, G.P., 1976. Temperature conditions for maghematite

(Gamma hematite) formation. Russian Metall. 3, 3032.

Bradshaw, A.V., Matyas, A.G., 1976. Structural changes and kinetics

in the gaseous reduction of hematite. Metall. Trans. B 7B, 81

87.

Brill-Edwards, H., Daniell, B.L., Samuel, R.L., 1965. Structural

changes accompanying the reduction of polycrystalline haematite.

J. Iron Steel Inst. 203, 361368.

Chaigneau, R., 1994. Complex calcium ferrites in the blast furnace

process. Ph.D. Thesis, Delft University of Technology, The

Netherlands.

Chaigneau, R., Sportel, H., Trouw, J., Vos, R., Droog, J., 1997. Blast

furnace burden quality: laboratory simulation. Ironmaking Steel-

making 24, 461467.

David, I., Welch, A.J.E., 1956. Trans. Faraday Soc. 52, 1642

1650.

Darken, L.S., Gurry, R.W., 1945. The system ironoxygen, the wustite

field and related equilibria. J. Am. Chem. Soc. 67, 13981412.

Edstroom, J.O., 1953. The mechanism of reduction of iron oxides.J. Iron Steel Inst. 175, 289304.

El-Geassy, A.A., Nasr, M.I., 1990. Influence of original structure on

the kinetics and mechanisms of carbon monoxide reduction of

hematite compacts. ISIJ Int. 30, 417425.

Elliot, J.F., Gleiser, M., 1960. Thermochemistry for Steelmaking,

vol. I. Pergamon Press, London.

Et-Tabirou, M., Dupree, B., Gleitzer, C., 1988. Hematite single crystal

reduction into magnetite with COCO2. Metall. Trans. B 19B, 311

317.

Hayes, P.C., Grieveson, P., 1981. Microstructual changes on the

reduction of hematite to magnetite. Metall. Trans. B 12B, 579587.

Hida, Y., Ito, K., Okazaki, J., Sasaki, M., 1984. Influence of partial

pressure of oxygen in the atmosphere on the formation of acicular

calcium ferrite. Trans. ISIJ 24, B-331.

Hsieh, L.H., Whiteman, J.A., 1989a. Sintering conditions for simulat-

ing the formation of mineral phases in industrial iron ore sinter.

ISIJ Int. 29, 2432.

Hsieh, L.H., Whiteman, J.A., 1989b. Effect of oxygen potential on

mineral formation in lime-fluxed iron ore sinter. ISIJ Int. 29, 625

634.

Husslage, W.M., Bakker, T., Kock, M.E., Heerema, R.H., 1999.

Influence of reduction conditions on the expansion and micro-

texture of sintered hematite compacts during the transition to

magnetite. Miner. Metall. Process. 16, 2333.

Kang, H.W., Chung, W.S., Murayama, T., 1998a. Effect of iron ore

size on kinetics of gaseous reduction. ISIJ Int. 38, 109115.

Kang, H.W., Chung, W.S., Murayama, T., Ono, Y., 1998b. Effect of

iron ore shape on gaseous reduction rate. ISIJ Int. 38, 11941200.

Kubaschewski, O., Alcock, C.B., 1979. Metallurgical Thermochem-

istry, fifth ed. Pergamon Press, Oxford.

Muan, A., 1955. Phase equilibria in the system FeOFe2O3SiO2.

Trans. Am. Inst. Mining Met. Engrs. 203, 965976.

Nakajima, R., Sumigama, T., Wakimoto, K., Nagano, S., Kawata, H.,

Sakurai, M., 1987. Reduction degradation behavior of sinter in the

blast furnace shaft. Testu-to-Haganee 73, 19641971.

Shultz, D.L., McCarthy, G.J., 1988. X-ray powder data for an

industrial maghemite. Powder Diffr. 2, 104105.

Swann, P.R., Tighe, N.J., 1977. High voltage microscopy of the

reduction of hematite to magnetite. Metall. Trans. B 8B, 479487.

Takada, T., Soma, H., Irita, T., Kamisaka, E., Kimura, H., Isoyama,

M., Suzawa, A., 1986. Investigation of sinter degradation in blast

furnace using the in situ reaction vertical probe. Testu-to-Haganee

72, 203209.

Turkdogan, E.T., 1980. Physical Chemistry of High Temperature

Technology. Academic Press, New York.UUnal, A., Bradshaw, A.V., 1983. Rate processes and structural changes

in gaseous reduction of hematite particles to magnetite. Metall.

Trans. B 14B, 743752.

von Bogdandy, L., Engell, H.-J., 1971. The Reduction of Iron Ores.

Springer-Verlag, Berlin.

Wang, Z., Ishii, K., Sasaki, Y., Tsutsumi, T., Higuchi, K., Hosotani,

Y., 1998. Constitutional evaluation of high Al2O3 sinter by EPMA

sweeping area analysis. Testu-to-Haganee 84, 689696.

Yamaguchi, K., Higuchi, K., Hosotani, Y., Tanaka, T., Sato, T.,

Koizumi, F., 1999. Effective blending measurers of flux in burden

for improvement of permeability in lower part of blast furnace.

Testu-to-Haganee 85, 501508.

Yamaoka, Y., Hotta, H., Kajikawa, S., 1982. Testing method of high-

temperature properties of blast furnace burdens. Trans. ISIJ 22,

164171.

Influence of sintering conditions on the reduction behaviour of pure hematite compactsIntroductionMaterials and methodsResultsInfluence of sintering conditionsInfluence of the reduction temperatureDiscussionInfluence of sintering temperature on the microstructureInfluence of reduction conditions on the microstructureRelevance for plant operationConclusionsAcknowledgementsReferences

Influence of sintering conditions on the reduction behaviour

Documents

Transcript of Influence of sintering conditions on the reduction behaviour