Thermal investigations of direct iron ore reduction with coal

Article abstract of DOI:10.1016/S0040-6031(03)00398-8

In this paper, fundamental mechanisms for iron ore reduction in coal-ore mixtures have been investigated using several advanced experimental techniques. Firstly, the thermal properties of coal-ore mixtures were studied and apparent specific heat of coal-ore mixtures against temperature was obtained at a heating rate of 10°C/min. Several exothermic and endothermic peaks were observed which were related to the decomposition reactions and reduction. The flue gases from the mixture were analysed using a mass spectrometer. Secondly, the X-ray diffraction (XRD) and the iron phase analytical techniques were applied to identify the iron phase changes with the temperature. It has been found that coal devolatilisation and iron oxides reduction occur simultaneously during the heating of the mixture. H₂ and CO gases produced from coal devolatilisation and char gasification were responsible for the reduction of iron oxides at these temperatures. Iron oxides undergo step-wise reduction over the whole process. The results in this work provide a fundamental understanding for the direct reduced ironmaking processes.

Full text of DOI:10.1016/S0040-6031(03)00398-8

Thermochimica Acta 410 (2004) 133–140
Thermal investigations of direct iron ore reduction with coal
Gui-su Liu , Vladimir Strezov , John A. Lucas , Louis J. Wibberleyd
a
b,∗
c
a
Niksa Energy Associates, 1745 Terrace Drive, Belmont, CA 94002, USA
b
Newbolds Applied Research, The University of Newcastle, Cnr Frith and Gavey Sts, Mayfield, NSW 2304, Australia
Discipline of Chemical Engineering, Faculty of Engineering and Built Environment, The University of Newcastle,
c
University Drive, Callaghan 2308, NSW, Australia
BHP Billiton Newcastle Technology Centre, Off Vale Street, Shortland 2307, NSW, Australia
d
Received 8 April 2003; received in revised form 16 June 2003; accepted 25 July 2003
Abstract
In this paper, fundamental mechanisms for iron ore reduction in coal–ore mixtures have been investigated using several advanced experi-
mental techniques. Firstly, the thermal properties of coal–ore mixtures were studied and apparent specific heat of coal–ore mixtures against
◦
temperature was obtained at a heating rate of 10 C/min. Several exothermic and endothermic peaks were observed which were related to the
decomposition reactions and reduction. The flue gases from the mixture were analysed using a mass spectrometer. Secondly, the X-ray diffrac-
tion (XRD) and the iron phase analytical techniques were applied to identify the iron phase changes with the temperature. It has been found that
coal devolatilisation and iron oxides reduction occur simultaneously during the heating of the mixture. H and CO gases produced from coal
2
devolatilisation and char gasification were responsible for the reduction of iron oxides at these temperatures. Iron oxides undergo step-wise
reduction over the whole process. The results in this work provide a fundamental understanding for the direct reduced ironmaking processes.
©
2003 Elsevier B.V. All rights reserved.
Keywords: Coal; Iron ore reduction; Direct reduced iron; Thermal property
1
. Introduction
misation of the DRI process, however, requires knowledge
of the thermal properties of the coal–ore mixtures and mech-
anism of the reduction reactions, which have still not been
well understood. It is therefore necessary to have an insight
into fundamental mechanisms for these complex reactions.
The aim of this work is to investigate coal–ore reactions of
their mixtures during heating, using several advanced mea-
suring techniques. Thermal property, gas analysis, and iron
phase changes during heating of the mixtures were obtained,
through which fundamental insights into coal and iron ore
behaviour and their interactions are provided.
Substantial developments in direct reduction ironmaking
(DRI) technologies have been recently conducted providing
sustainable mean, for metallurgical operations. The largest
advantage of the DRI technologies relies on the fact that
DRI does not require cokemaking and sintering. Both coke-
making and sintering, being at the front end of the conven-
tional blast furnace ironmaking technology, are considered
as costly for the new process construction and are consis-
tently causing environmental concerns. The DRI process, on
the other hand, consists of carbothermic reduction of iron
oxide directly with the volatiles liberated during coal de-
volatilisation as well as the carbon monoxide regenerated
from coal char. This process provides an advanced utilisa-
tion opportunity for the high volatile coals, which were oth-
erwise unusable in the steel industry.
2. Experimental
Coal C1 and iron ore O1 both from Australian origin were
used in this work. The coal has 33.5% volatile matter and
6.5% ash in air-dry base, and has 85.1% C, 5.5% H, 2.1%
N, 0.7% S and 6.6% O in daf base. The major components
of the iron ore are 62.1% Fe, 4.33% SiO2 and 1.98% Al2O3.
There has been extensive work performed on iron ore re-
duction of coal–ore mixtures and its kinetics [1–20]. Opti-
◦
∗
Both samples were dried under vacuum at 80 C for 2 h
Corresponding author. Tel.: +61-2-4968-6771;
before proceeding with experiments. The mean particle size
of coal and iron ore was −80 and −50 ␮m, respectively.
fax: +61-2-4968-6777.
E-mail address: vladimir.strezov@newcastle.edu.au (V. Strezov).
0
040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0040-6031(03)00398-8

1
34
G.-s. Liu et al. / Thermochimica Acta 410 (2004) 133–140
Fig. 1. The cross-sectional diagram of the instrument used for thermal property measurements.
A mixture of coal and iron ore particles at a mass ratio of
1
000
2
0:80 was used throughout the study.
Several measuring techniques were used for investigating
9
8
00
00
sample surface
the reactions between coal and ore in the study. The thermal
property measurements were performed using the computer
aided thermal analysis technique [21,22], with the instru-
ment shown in Fig. 1. This method provides an opportunity
for dynamic thermal analysis by heating the sample with
controlled heat flux while continuously monitoring boundary
temperature conditions within the sample. The heat transfer
is, then, calculated using inverse numerical modelling tech-
nique. There was a substantial effort in the past [21–27] to
apply inverse methods in a variety of different thermal stud-
ies. It was shown that, with controlled heating conditions it
is possible to obtain accurate thermal characterisation of the
materials.
700
600
500
400
graphite
sample centre
3
2
1
00
00
00
0
0
1000
2000
3000
4000
5000
6000
Time (s)
Fig. 2. Time–temperature history during heating of coal–ore mixture at a
mass ratio of 20:80 and heating rate of 10 C/min.
◦
2
In this work, the sample with 30 mm in length and
where Q is the heat flux (W/m ), F1
the radiation
−2
1
0.6 mm in diameter was packed in a silica glass tube to
shape factor, σ the Stefan–Boltzmann constant (5.67 ×
3
−8
g s
10 W/m K ), and T and T are the temperatures of the
2 4
the density of 1860 kg/m , and heated by radiation from
a surrounding graphite cylinder. The heating rate of the
furnace was typically maintained at 10 C/min, controlled
graphite and the surface of the sample (K), respectively.
The one-dimensional heat conduction in the sample is
given by the following expression:
◦
through a type K thermocouple embedded in the graphite.
The sample was maintained under inert atmosphere with
an argon flow of 5 ml/min through the glass sample tube.
Sample temperatures were continuously measured by ther-
mocouples positioned at the surface and in the centre of the
sample. The measured data were acquired at a frequency of
ꢀ
ꢁ
∂
T
∂
∂T
∂r
ρCp
= k
r
(2)
∂
t
∂r
3
where ρ is the density (kg/m ), Cp the specific heat (J/kg K),
k the thermal conductivity (W/m K), T the temperature (K),
t the time (s) and r the radius (m).
1
Hz, and the typical temperatures for one experimental run
for coal–ore mixture in a ratio of 20:80 are shown in Fig. 2.
The heat flux at the surface of the sample was calculated by
assuming heat transfer from the graphite heating element to
the sample was performed predominantly by radiation and
was estimated using Eq. (1). For this purpose and to ensure
uniform emissivity of the glass, the outside surface of the
sample glass tube was coated with a thin layer of carbon
soot, prior to positioning the sample tube centrically to the
graphite heating cylinder.
The sample was numerically divided into a number of
nodes (n) across the radius. For each node an estimate was
made based on the heat balance principle (i.e. heat accu-
mulated in the node equals to the difference between the
incoming and outgoing thermal energies). The boundary
conditions of the system were the temperatures measured
at the centre and surface of the sample, and the heat flux
calculated according to Eq. (1). A computational matrix
was then generated using Eq. (3) to estimate the volumetric
specific heat based on the initial mass of the heated sam-
ple. For closer familiarising with the equation evaluation
procedure the reader is referred to [21–24].
4
4
s
Q = F1−2σ(T − T )
(1)
g
2
πn ꢀxQ(t)
ρCp =
ꢂ
(3)
n−1
i=1
t
i
t−1
)
i
2
t
0
t−1
2
t
n
2
(
ꢀx π/4 ꢀt)(T − T ) + (ꢀx π/ꢀt)(n − (1/4))(T − T nt −1) +
(2π ꢀx i/ꢀt)(T − T
0

G.-s. Liu et al. / Thermochimica Acta 410 (2004) 133–140
135
3
6.E+06
5.E+06
where ρCp is the volumetric specific heat (J/m K), n the
Mixture of C1+O1
t
number of nodes, T the temperature expressed in K of the
i
node i for the time t (s) and Q(t) the heat flux expressed in
W/m for time t.
4
3
.E+06
.E+06
2
The volumetric specific heat in this case is apparent, which
means that it includes heats developed due to decomposi-
tion, transformation and reaction. If, for instance, endother-
mic heat is developed in the sample, it results in increase
of the apparent specific heat, hence the heat is consumed
by the sample. On the contrary, if an exothermic reaction
is developed during heating, the apparent specific heat will
show decrease of its values. The performance of the above-
mentioned measurement method was tested previously [22]
on a range of different materials and accuracy of approxi-
mately ± 2% was found in the measurement data. Maximum
2.E+06
Single O1
1
0
.E+06
.E+00
Single C1
0
100 200 300 400 500 600 700 800 900 1000
Temperature (ºC)
Fig. 3. Specific heats of single coal C1, single ore O1, and their mixture
at 20:80 mass ratio.
◦
◦
ued to evolve at 1000 C, with a maximum rate at 785 C.
The specific heat of O1 iron ore was also measured, which
exhibited several sharp peaks. The first peak at 120 C
◦
temperature was limited to 1000 C, which was the maxi-
◦
mum limit of the furnace used. Results are normally plotted
against average of the two sample temperatures.
represents the endothermic water vaporisation, and the sec-
◦
Thermal studies of direct iron ore reduction were, in this
work, incorporated with analysis of the gaseous products us-
ing a mass spectrometer Prima 600 connected to the gas out-
let of the apparatus described above. Argon gas was flown
at a rate of 100 ml/min across the sample for the purpose of
these measurements. The water vapour and larger molecu-
lar weight volatiles generated during heating of the samples
were condensed at the outlet of the furnace prior to the gas
analyser. The volume percentages of gases as a function of
sample temperature were then obtained. The coal–ore sam-
ples, which were heated up to certain temperatures in the
above furnace, were further collected and analysed using an
X-ray diffraction (XRD), and an iron phase analysing tech-
nique. The XRD uses the “fingerprint” of a crystalline mate-
rial to allow identification of unknown phases in a mixture.
Rapid identification of unknown phases can be possible us-
ing search/match software available at the unit. The samples
were also sent to chemical laboratory for carbon and iron
phase analysis.
ond at 340 C was caused by dehydroxylation of goethite
(FeO(OH), hydrated iron oxide) with formation of hematite
◦
(Fe O ). The third endothermic peak at around 600 C was
2
3
due to breakdown of kaolinite (Si Al O (OH) ) [29]. The
4
4
10
8
fourth peak, caused by magnetic transformation of Fe O
2
3
◦
[30], appeared at 685 C, whereas the exothermic trough at
◦
843 C was most likely caused by the partial reduction of
hematite from the carbon present in the ore.
The specific heat of the mixture at a mass ratio of 20:80
◦
exhibits similarities with the pure iron ore, prior to 600 C.
Dehydroxylation and decomposition of kaolinite are the
dominant reactions for the mixture, with peaks being similar
to those of single iron ore. Primary coal devolatilisation has
◦
initiated below 600 C as shown for single C1 coal, however
the rate is reduced at the presence of iron oxides [3,4]. Above
◦
600 C, the curve was substantially different from that of
either the single iron ore or coal. An exothermic reaction
◦
occurred at around 690 C, followed by two strong endother-
mic reactions. These reactions are most likely due to the
iron ore step-wise reduction, i.e. the reduction of hematite
(
Fe2O3), magnetite (Fe3O4) and wustite (FeO), respectively.
Fig. 4 shows the specific heats of iron ore reduction
3
. Results
of mixtures of C1–O1 and coke–O1 at a mass ratio of
3
.1. Apparent specific heat
The apparent specific heats for single coal C1 and iron
8
7
6
5
4
3
2
1
0
.E+06
.E+06
.E+06
.E+06
.E+06
.E+06
.E+06
.E+06
.E+00
◦
Coke+O1
ore O1 were measured at a heating rate of 10 C/min under
Ar atmosphere, as shown in Fig. 3. Coking coal C1 ex-
hibits a rapid and significant exothermic reaction occurring
◦
between 420 and 460 C, which is thought to be caused
by both physical and chemical related changes in the coal
plastic region, i.e. tar formation, tar vaporisation and reso-
lidification. Following the tar formation, the secondary
devolatilisation and hydrogen release occurred over a tem-
C1+O1
◦
perature range of 500–1000 C. Previous work [28] has
0
100 200 300 400 500 600 700 800 900 1000
◦
Temperature (ºC)
shown that the evolution of CO for C1 initiated at 450 C
and completed at about 950 C, with a maximum evolution
rate at 720 C, whereas H2 started from 495 C and contin-
◦
Fig. 4. Specific heats of mixtures of C1–O1 and coke–O1 at a 20:80 mass
ratio.
◦
◦

1
36
G.-s. Liu et al. / Thermochimica Acta 410 (2004) 133–140
2
0:80. Similar to that of C1–O1 mixture, the specific heat
Unlike that of C1–O1 mixture, at higher temperatures no
significant peaks were observed for coke–O1 mixture until
the sample reached 920 C. At temperatures above 920 C,
similar to the C1–O1, a rapid increase of specific heat was
observed for coke–O1 mixture, due to char gasification with
CO2.
of coke–O1 mixture only exhibits two endothermic peaks
at 340 and 595 C, respectively. These reactions are of the
same origin as the dehydroxylation and decomposition reac-
tions of the pure iron ore in the same temperature range. The
intensity of the peak at 595 C in coke–O1 is smaller than
◦
◦
◦
◦
C1–O1 mixture. The reason is that the peak for the latter is a
sum of the endothermic reactions of iron ore decomposition
and the secondary devolatilisation of coal.
The above observation suggests that CO and H2 volatiles
generated from coal decomposition are the predominant re-
ductants for iron oxides at lower temperatures, whereas the
iron ore reduction by char gasification dominates above
◦
9
20 C.
6
5
4
3
2
1
0
.E+06
.E+06
.E+06
.E+06
.E+06
.E+06
.E+00
5.E-02
CH4 of C1
4
3
2
1
.E-02
.E-02
.E-02
.E-02
3
.2. Product gas analysis
The gas analysis was performed by connecting a mass
specific heat
spectrometer to the outlet of the heating furnace. Fig. 5
shows the weight percentages of product gases CH4, H2,
CO and CO2 of the coal–ore mixture as a function of tem-
perature, in comparison with the specific heat. The CH4 was
predominantly produced from coal devolatilisation, and re-
CH4 of C1+O1
0.E+00
1000
0
0
0
0
200
400
600
800
Temperature (ºC)
◦
leased between 450 and 800 C, with a maximum value at
6
5
4
3
2
1
0
.E+06
.E+06
.E+06
.E+06
.E+06
.E+06
.E+00
8.E-03
◦
◦
6
00 C. The H2 started to release at 500 C, and the con-
7
6
5
.E-03
.E-03
.E-03
H2 of C1
centration decreased with increasing the temperature from
◦
◦
5
30 C, and disappeared from 600 to 700 C, which indi-
cated a great consumption of H2 gas due to the significant
4.E-03
specific heat
3
2
1
.E-03
.E-03
.E-03
7
6
5
4
3
.E+06
.E+06
.E+06
.E+06
.E+06
5.E-02
H2 of C1+O1
0.E+00
4
3
.E-02
.E-02
200
400
600
8001
000
Temperature (ºC)
6
5
4
3
2
1
0
.E+06
.E+06
.E+06
.E+06
.E+06
.E+06
.E+00
0.3
0.25
0.2
0.15
0.1
0.05
0
specific heat
2.E-02
.E-02
CO of C1+O1
2.E+06
1.E+06
0.E+00
1
CO of coke+O1
specific heat
0.E+00
1000
0
200
400
600
800
Temperature (ºC)
CO of C1
7
6
5
4
3
.E+06
.E+06
.E+06
.E+06
.E+06
1.4E-01
1.2E-01
1.0E-01
8.0E-02
6.0E-02
4.0E-02
2.0E-02
0.0E+00
200
400
600
800
1000
Temperature (ºC)
6
5
4
3
2
1
0
.E+06
.E+06
.E+06
.E+06
.E+06
.E+06
.E+00
0.25
0.2
CO2 of C1+O1
0
0
0
0
.15
specific heat
specific heat
.1
2.E+06
1
0
.E+06
.E+00
CO of coke+O1
.05
2
CO2 of C1
800
0
200
400
600
800
1000
200
400
600
1000
Temperature (ºC)
Temperature (ºC)
Fig. 5. Mass spectrometric analysis of gas evolution of C1 + O1 mixture
Fig. 6. Mass spectrometric analysis of gas evolution of coke + O1 mixture
during heating.
during heating.

G.-s. Liu et al. / Thermochimica Acta 410 (2004) 133–140
137
reduction of the iron oxide. The H2 increased again from
duced from iron ore reduction by CO predominantly from
volatile, and increased dramatically at temperatures above
780 C, indicating the carbon gasification has become sig-
nificant within this temperature range. Rapid increasing of
both CO and CO2 indicates that significant carbon consump-
tion occurred due to gasification.
◦ ◦
7
00 C, reaching a peak at 780 C, and decreased again.
◦
Compared to the single coal C1, the H2 concentration of
C1–O1 mixture is much lower due to the consumption of H2
as a reductant gas for iron oxides, with the peak temperature
similar to each other, as shown in Fig. 5. The CO started to
◦
release through by the coal devolatilisation at 450 C, and
Fig. 6 shows the mass spectrometric analysis of coke and
O1 mixture at the same mass ratio. The CO and CO2 con-
centrations were extremely low and remained constant be-
◦
decreased with the temperature again from 530 to 700 C,
which was due to the consumption by reduction. The CO
◦
◦
◦
concentration increased rapidly from 700 C, followed by
low 700 C. They started to increase at 700 C, and became
◦
◦
a decrease at about 950 C. The CO2 concentration exhib-
ited an inverse trend of CO below 780 C which was pro-
rapid from 800 C. It is more than likely that direct reduc-
◦
tion of iron oxide by solid carbon occurred between 700 and
Fig. 7. X-ray diffraction analysis of mixture of C1 + O1 heated at different temperatures.

1
38
G.-s. Liu et al. / Thermochimica Acta 410 (2004) 133–140
◦
8
00 C with the CO2 as a primary product gas. Significant
3.4. Iron phases analysis
◦
carbon gasification occurred at temperatures above 800 C,
and the reduction of iron oxide occurred through reducing
gaseous phase, for example CO and H2 produced from car-
bon gasification with CO2 and H2O, respectively.
Fig. 8 shows the quantitative iron phase and carbon con-
tent as a function of temperature. The analysis showed that
the Fe3 started to reduce at 450 C the reduction was al-
+
◦
2+
◦
most completed at 870 C. The Fe percentage increased
from 450 C, reaching maximum at 870 C, and decreased
◦
◦
3
.3. XRD analysis
◦
with further increasing of the temperature up to 1100 C.
◦
Coal and ore samples were prepared in a furnace at vari-
ous temperatures followed by cooling with a rate of approx-
The metallic iron rapidly increased from 870 C and reached
◦
up to 98.7% at 1200 C, which is consistent with the XRD
◦
imately −100 C/min, and were further analysed by XRD.
results. Also included in Fig. 8 is the carbon content of the
sample as a function of temperature. The carbon was reduced
from 17 to 14% as temperature increased from 25 to 800 C.
The rate of carbon release was slow, and was identical to
the rate of coal devolatilisation. The carbon reduced rapidly
at temperatures above 800 C, which is demonstrated to be
due to carbon gasification according to the analysis of coke
and O1 mixture. The concentration of carbon in the reduced
iron was found to be approximately 3.5%.
◦
Two of the samples heated at temperatures above 1000 C
◦
were prepared separately in a muffle furnace. Fig. 7 shows
the XRD analysis of coal–ore mixture made at different tem-
peratures. At 25 C (i.e. original coal and iron ore mixture),
the sample was dominated by Fe2O3 and carbon, with mi-
nor phases of goethite (FeO(OH)) and kaolinite. At 450 C,
◦
◦
◦
only the disappearance of goethite was evident, due to the
dehydroxylation reaction at 340 C as indicated by the spe-
◦
cific heat. The first step reduction of Fe2O3 forming Fe3O4
◦
was initiated below 530 C, and completed at a temperature
3.5. Scanning electron microscope (SEM) analysis
◦
between 620 and 670 C. The rate for the first step reduc-
tion within 530–580 C was much higher than that within
80–670 C. It was evident that the kaolinite disappeared
between 580 and 620 C, due to the thermal decomposi-
tion as found by the specific heat study presented above.
The second step reduction of Fe3O4 to form FeO initi-
ated between 670 and 740 C, and completed below 870 C.
Rapid reduction rate was indicated between a temperature
range of 800–870 C. The third step reduction of FeO to
form metallic Fe initiated from 870 C. Small amount of
metallic Fe was formed at 870 C. At 1000 C, the metal-
lic Fe dominated the sample, with a minor phase of FeO.
The 1100 and 1200 C samples were primarily metallic Fe
◦
Fig. 9 shows the cross-sectional SEM images of iron ore
particles in the coal–ore mixture made at different temper-
atures. The chemical analysis can be carried out through an
energy dispersion system (EDS) with the SEM. In the sam-
◦
5
◦
◦
ple prepared at 560 C, it was evident that ore particles were
◦
◦
heterogeneous, with mineral grand (deep gray area) included
in the ore particles. The ore particle exhibited a dense phase,
with some macropores randomly scattering throughout the
◦
◦
◦
particle. In the 670 C sample, cracks were observed within
◦
◦
the particle, indicating the thermal decomposition occurred
below this temperature. From the specific heat and XRD
analysis, it is obvious that the reduction of Fe O has also
◦
2
3
phase.
occurred below this temperature. However, it is impossible
1
00
20
16
12
8
8
0
0
0
0
6
4
2
Fe (3+) in total Fe (%)
Fe (2+) in total Fe (%)
Metallic Fe in total Fe (%)
C (total) %
4
0
0
0
100 200 300 400 500 600 700 800 900 1000 1100 1200
Temperature (ºC)
Fig. 8. Iron phases change and carbon content as a function of temperature.

G.-s. Liu et al. / Thermochimica Acta 410 (2004) 133–140
139
Fig. 9. Scanning electron microscopic (SEM) images of iron ore particles in coal–ore samples at different temperatures. Below each image is the
temperature at which the sample was prepared, followed by the length of scale bar.
to detect the phases through the brightness of the area. The
ore particle in sample heated to 840 C exhibited a devel-
Carbon gasification with CO2 and H2O occur at temper-
atures above 800 C to produce CO and H2 for iron oxides
◦
◦
oped porous system due to the reduction reaction. Some
bright area can be seen in the sample prepared at 950 C,
which was confirmed to be metallic iron through the EDS
reduction, which are represented by the following reactions:
◦
C + CO2 → 2CO
C + H2O → H2 + CO
(5)
(6)
analysis. The bright spots were seen severely in the samples
◦
heated to 950 and 1000 C.
The CO2 and H2O source for gasification reactions is gen-
erated from the reduction of iron oxide by CO and H2, re-
spectively, which is detailed below.
Reduction of iron oxides in a direct reduction system has
been known to occur by gaseous phases, for example CO
and H2, rather than the solid carbon [20]. It has been shown
from the present study that the iron ore (Fe2O3) undergoes
stepwise reductions by CO and H2, which are represented
by the following reactions:
4
. Discussion
Several measuring techniques have been applied to char-
◦
acterise the coal–ore mixtures under 10 C/min heating rates,
providing consistent understanding of fundamental reactions
during the reduction process. It can be concluded that coal
and iron ore reactions are complex and are coupled ex-
tensively in the coal–ore mixture system. Furthermore, the
changes in the reactions are heavily dependant on the heat
transfer parameters including size, shape, sample density
and heating rate. From here it is noteworthy mentioning that
the results presented in this work depict reaction mechanism
for the current heating conditions and parameters.
3
Fe2O3 + CO ↔ 2Fe3O4 + CO2
(7)
(8)
3Fe O + H ↔ 2Fe O + H O
2
3
2
3
4
2
Fe3O4 + CO ↔ 3FeO + CO2
Fe O + H ↔ 3FeO + H O
FeO + CO ↔ Fe + CO2
FeO + H2 ↔ Fe + H2O
(9)
3
4
2
2
(10)
(11)
(12)
When heated, coal particles undergo severe devolatilisa-
tion producing solid coke, tar and light gases, as illustrated
by the following reaction:
◦
The Fe2O3 reduction initiated below 580 C, and has com-
◦
coal → char(coke) + tar + gases (e.g. H , CO)
(4)
pleted by 670 C. The Fe3O4 was reduced between 670 and
8
2
◦
70 C to form FeO. The FeO was reduced thereafter by
Coal goes through the primary devolatilisation between 420
and 460 C, releasing tar, followed by secondary reactions
CO generated predominantly from carbon gasification. The
reduction of iron oxides was almost completed by 1200 C,
◦
◦
at temperatures above with the release of light gases. CH4 is
released primarily between 450 and 700 C, whereas H2 and
CO are released at higher temperatures becoming a reductant
source for ore reduction.
resulting in formation of metallic iron. It has been found that
the volatile in the coal plays an important role in iron ore
reduction, providing reductant adequate for complete reduc-
tion of Fe2O3 and Fe3O4.
◦

1
40
G.-s. Liu et al. / Thermochimica Acta 410 (2004) 133–140
[6] S.K. Dutta, A. Ghosh, Metall. Mater. Trans. B 25B (1994) 15–26.
5
. Conclusions
[7] R. Haque, H.S. Ray, A. Mukherjee, ISIJ Int. 31 (1991) 1279–1285.
[
8] R. Haque, H.S. Ray, A. Mukherjee, Metall. Trans. B 24B (1993)
From the current work the following conclusions have
been driven out:
511–520.
[9] B.-H. Huang, W.-K. Lu, ISIJ Int. 33 (1993) 1055–1061.
[
10] S. Mookherjee, H.S. Ray, A. Mukherjee, Thermochim. Acta 95
1985) 247–256.
◦
•
At 10 C/min heating rate and under Ar atmosphere,
(
9
8.7% of Fe2O3 in the initial iron ore was reduced to
metallic Fe at 1200 C in the 20:80 mass ratio mixture of
coal and iron ore.
Fe2O3 was reduced to Fe3O4 between 500 and 670 C by
CO and H2 as products of coal devolatilisation.
Fe3O4 was reduced to FeO between 740 and 870 C by
H2 and CO as products of coal devolatilisation and CO
as a product of char gasification. Rate of reduction was
lower between 740 and 800 C and rapid between 800 and
[11] S. Mookherjee, H.S. Ray, A. Mukherjee, Thermochim. Acta 95
◦
(1985) 235–246.
[12] S. Mookherjee, H.S. Ray, A. Mukherjee, Ironmaking Steelmaking
◦
13 (1986) 229–235.
•
•
[
[
[
13] Y.K. Rao, Metall. Trans. 2 (1971) 1439–1447.
14] C.E. Seaton, J.S. Foster, J. Velasco, Trans. ISIJ 23 (1983) 490–496.
15] T. Sharma, Ironmaking Steelmaking 20 (1993) 362–365.
◦
[16] S. Sun, W.-K. Lu, in: Proceedings of the 1996 Steelmaking Confer-
ence, Pittsburgh, 24–27 March 1996, p. 1062.
[17] S. Sun, W.-K. Lu, ISIJ Int. 39 (1999) 123–129.
◦
[18] Q. Wang, Z. Yang, J. Tian, W. Li, J. Sun, Ironmaking Steelmaking
4 (1997) 457–460.
◦
8
70 C.
2
◦
•
FeO was reduced to Fe between 870 and 1200 C mainly
by CO from char gasification. The rate of reduction was
rapid between 950 and 1100 C.
[19] Q. Wang, Z. Yang, J. Tian, W. Li, J. Sun, Ironmaking Steelmaking
25 (1998) 443–447.
◦
[20] R.J. Fruehan, Metall. Trans. B 8B (1977) 279–286.
[
[
[
21] V. Strezov, J.A. Lucas, L. Strezov, Metall. Trans. B 31B (2000)
125–1131.
22] V. Strezov, J.A. Lucas, L. Strezov, Therm. Anal. Calorim. 72 (2003)
07–918.
1
Acknowledgements
9
23] J.V. Beck, B. Blackwell, C.R. St Clair, Inverse Heat Conduction—III
The authors are grateful for the financial support provided
by the Australian Research Council. The support from BHP
Research Newcastle, Australia is also acknowledged.
Posed Problems, Wiley, New York, 1985.
[24] C.J. Geankoplis, Transport Process and Unit Operations, Allyn and
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11–14 September 2000, Barcelona.
[
[
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