Carbothermic reduction kinetics of nanocrystallite Fe2O3/NiO composites for the production of Fe/Ni alloy

Article abstract of DOI:10.1016/j.jallcom.2008.02.026

Nanocrystalline Fe-Ni alloy has been fabricated by reduction of Fe₂O₃/NiO composite materials with solid carbon. Nanocrystalline compacts of pure and 26% NiO doped iron oxide were reduced at 800-1100 °C by solid carbon. The reduction behavior was investigated using advanced quadrupole mass spectrometer. XRD, SEM and TEM analysis were used to characterize the reduced samples. The aim of work is to clarify the reduction behavior of Fe₂O₃/NiO composite materials with solid carbon which is an important step for production of nanocrystalline Fe-Ni alloys. The influence of reduction conditions on the structural characteristics of the products was extensively studied to get clear comprehension of reduction process. It was found that the reduction rate increases with temperature. The experimental measurements were used to elucidate the carbothermic reduction mechanism of iron oxide doped with NiO. The reactions proceeded in a stepwise manner and Boudouard reaction is the major controlling step particularly at low temperatures whereas more contribution to solid state reaction control seems to have an effect at high temperatures.

Full text of DOI:10.1016/j.jallcom.2008.02.026

Journal of Alloys and Compounds 463 (2008) 585–590
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Carbothermic reduction kinetics of nanocrystallite Fe2O3/NiO composites
for the production of Fe/Ni alloy
K.S. Abdel-Halima,∗, M.H. Khedrb, M.I. Nasra, M.Sh. Abdel-wahabb
a Central Metallurgical Research & Development Institute (CMRDI), P.O. Box 87, Helwan, Cairo, Egypt
b Chemistry Department, Faculty of Science, Beni-Suief University, Beni-Suief, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanocrystalline Fe–Ni alloy has been fabricated by reduction of Fe2O3/NiO composite materials with solid
carbon. Nanocrystalline compacts of pure and 26% NiO doped iron oxide were reduced at 800–1100 ◦C
by solid carbon. The reduction behavior was investigated using advanced quadrupole mass spectrometer.
XRD, SEM and TEM analysis were used to characterize the reduced samples. The aim of work is to clarify
the reduction behavior of Fe2O3/NiO composite materials with solid carbon which is an important step
for production of nanocrystalline Fe–Ni alloys. The influence of reduction conditions on the structural
characteristics of the products was extensively studied to get clear comprehension of reduction process.
It was found that the reduction rate increases with temperature. The experimental measurements were
used to elucidate the carbothermic reduction mechanism of iron oxide doped with NiO. The reactions
proceeded in a stepwise manner and Boudouard reaction is the major controlling step particularly at low
temperatures whereas more contribution to solid state reaction control seems to have an effect at high
temperatures.
Received 18 December 2007
Received in revised form 8 February 2008
Accepted 11 February 2008
Available online 26 March 2008
Keywords:
Nanocrystallite
Fe2O3
NiO
Reduction
Solid carbon
Kinetics and mechanism
Fe–Ni alloy
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
and NiFe2O4 were first reduced and then followed by the reduction
of iron oxides. The produced metallic Ni and Fe can form a series
The reduction processes of iron oxides doped metal oxides
include many complicated reactions and take place in a step-
wise manner via formation of a series of intermediate oxides and
metal ferrites. The reduction behavior of iron oxides has been
extensively studied using H2, CO, H2/CO gas mixtures and even
with solid carbon under isothermal or non-isothermal conditions
[1–10]. Nasr et al. [5] studied the reducibility of different compacts
made of high grade iron ore fines containing 5–20 wt.% petroleum
coke at 950–1100 ◦C in an inert atmosphere. They found that the
reduction process proceeds in a stepwise manner. The overall
reduction process is controlled by Boudouard reaction and the
apparent activation energy was calculated and given in the range
of 29–55 kcal/mol. Sarkisyan [7] studied the mechanism and kinet-
ics of the reduction of Fe2O3–NiO system with carbon and with
H2 under isothermal conditions. It was observed that the mechan-
ical mixture of NiO and Fe2O3 mixed with carbon was reduced
separately without significant interaction. The product was a con-
tinuous series of solid solutions of different compositions. Cores et
al. [9] investigated the reduction mechanism of NiO–Fe3O4 with
coal and H2. The reduction proceeded in two stages in which NiO
of solid solutions. The mechanism of solid state reduction in the
Fe2O3–NiO–Cr2O3–C system was studied using thermal analyses
[10]. It was reported that the presence of transitional compound
such as FeCr2O4 and Fe2O3 promotes the complete reduction of
Cr2O3 at temperatures lower than that required for reducing pure
Cr2O3. The whole reduction process may be classified as five dis-
tinguishable reduction stages and the reduced metals in the final
product exist primarily in the form of solid solutions, such as
Cr–Fe–Ni which mainly contain Fe, Ni and Fe, Cr and small amounts
of carbides.
On the other hand, a great variety of mechanical, physical and
chemical methods are being applied for the production of ferroal-
loys. One of the most important methods is the reduction of ferrites
or metal oxides mixtures. Szekely and Hasoglu [11] investigating
the reduction of Ni–Fe2O3 mixture in H2 and found that maximum
selectivity of reduction was obtained at lower temperatures where
the chemical reaction is the rate controlling step. The isothermal
reduction behavior of iron oxides doped with different ratios of
NiO was investigated at 900–1100 ◦C [12]. It was found that the
rate of reduction at both initial and final stages increased with the
increase of dopant content at all reduction temperatures. This was
attributed to the formation of nickel ferrite (NiFe2O4) phase and the
consequent increase in the total porosity of compacts, ferronickel
alloy was produced during reduction at all temperatures studied.
∗
Corresponding author. Tel.: +20 25010642; fax: +20 25010639.
E-mail address: khaledsaad@cmrdi.sci.eg (K.S. Abdel-Halim).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.02.026

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K.S. Abdel-Halim et al. / Journal of Alloys and Compounds 463 (2008) 585–590
However, Fe–Ni alloys are important materials for their low thermal
expansion [13,14] and their remarked magnetic properties [15,16].
The isothermal reduction behavior of nanocrystallite NiFe2O4 pow-
der was investigated in pure hydrogen at 800–1100 ◦C [17]. It was
concluded that nanocrystalline Fe–Ni alloy (Fe0.64Ni0.36) can be
obtained from NiFe2O4 powders. The reduction rate increased with
increasing reduction temperature in both the initial and final reduc-
tion stages. Grain growth and coalescence of the formed Fe0.64Ni0.36
grains took place by increasing the reduction temperature.
The present study is designated to prepare nanosized Fe–Ni
alloys from pure metal oxides prepared by classical chemical meth-
ods. Reduction by solid carbon was investigated in binary system
oxide (Fe2O3–NiO) which chosen as the first step study and the
obtained results will be used for further investigation of the ternary
system Fe2O3–NiO–Cr2O3.
The microstructures of the reduced compacts together with the
kinetics data obtained from reduction process were used to eluci-
date the solid state reduction mechanism of nanosized Fe2O3/NiO
composite materials.
2. Experimental work
Nanosized Fe2O3 powder with average crystal size 40 nm was prepared using self
flash combustion technique through drying of pure ferric acetate basic compound
[(CH3COO)2Fe·OH] at 150 ◦C for 24 h and then heated on a hot plate at 600 ◦C for
2 h. Also, nanocrystallite NiO powder with average 100 nm was prepared using the
same technique by drying nickel acetate at 150 ◦C for 24 h and then decomposed on
a hot plate at 600 ◦C for 2 h.
Both powders were mixed in stoichiometric ratio with ultra pure solid carbon
in a ball mill for 2 h. The produced powder is moistened with 10% water, then equal
weights of 1.2 g were pressed in a cylindrical mould of 1 cm in inner diameter at
150 kg/cm2.
The reduction behavior of the prepared compacts was investigated using two
different techniques, thermo-gravimetric technique to measure the total weight loss
of carbon and oxygen as a function of time and effluent gas analysis method for the
outlet gases. In the thermo-gravimetric experiments, the compacts were reduced
at 800–1100 ◦C. The reduction assembly and gas flow system used in this study
was previously mentioned [5]. At a given temperature, the platinum basket was
hanged on the balance and settled in the middle zone of the tube furnace in a flow
of pure argon gas at a constant rate of 0.5 l/min. The weight loss was recorded using
an automatic sensitive electronic balance. After constant weight was achieved, the
platinum cell was pulled up at the upper part of the reaction tube and kept away
from the hot zone. When the temperature reached less than 200 ◦C, the sample was
quenched in pure acetone.
The reduced compacts were characterized with X-ray diffraction analysis (JSX-
60P JEOL diffract meter with a copper target), optical microscope (MEJI CK3900),
scanning electron microscope (JEOL-JSM-5410) and TEM (JEOL-JSM1230).
Since thermo-gravimetric technique offers only the general behavior of reduc-
tion processes via measuring the total weight loss of carbon and oxygen together,
so, another series of reduction experiments were carried out using advanced
quadrupole mass spectrometer to study the actual reduction mechanism of metal
oxides with solid carbon through measuring and following up the concentration of
CO and CO2 in the outlet gas. In these experiments, the compact was placed in a small
platinum boat and held in a tubular horizontal flow reactor made of silica (length
120 cm, i.d. 3.8 cm). Pure argon gas was passed over the compact at a constant rate
of 0.5 l/min. The reactor was inserted in a tube furnace heated to the required reduc-
tion temperature (800–1100 ◦C). Monitoring of the individual gas concentration in
the outlet gases mixture during the reduction is performed using advanced gas ana-
lyzer (QMS sample analysis HPR 20, Hidden Analytical, Warrington, UK). The degree
of reduction and total carbon solution loss were calculated by the oxygen and carbon
balance based on the CO and CO2 concentrations obtained from gas analysis.
Fig. 1. The weight loss behavior of samples reduced at 800–1100 ◦C. (a) Fe2O3/C and
(b) Fe2O3/NiO/C.
oxygen is plotted against time of reduction. It can be observed that
weight loss is greatly affected by reduction temperature. At rela-
tively low temperature (800 ◦C), both pure iron oxide and doped
compacts do not attain complete weight loss, 91% and 93% weight
loss were obtained, respectively. This can be attributed to the for-
mation of dense metallic layer and the difficulty of solid state
reduction of lower iron oxides i.e. wustite (FeO) at low reduction
temperature. The presence of wustite phase was confirmed by XRD
analysis as shown in Table 1 and Fig. 2 where FeO together with
metallic iron phases were detected. Although this hardly reducible
phase of wustite formed during isothermal reduction of compacts
at all temperatures but it can be completely reduced by solid car-
bon only at higher reduction temperatures (>900 ◦C) [5]. Increasing
the reduction temperature to 900 ◦C causes increasing in the per-
centage of total weight loss. Fe2O3/C compact shows 95% weight
loss while Fe2O3 doped with NiO compact shows complete weight
loss percentage (100%). Thus, it can be reported that the presence
of NiO enhanced the reduction rate of iron oxides [12]. At relatively
high reduction temperatures (1000–1100 ◦C), the weight loss is the
highest and the compacts are completely reduced.
3. Results and discussion
3.1. Reducibility of metal oxides with solid carbon
Table 1
Phase identification of compacts completely reduced with solid carbon
The solid state reduction behavior of two different kinds of
nanosized materials namely pure Fe2O3/C and Fe2O3/NiO/C com-
pacts was investigated. Firstly, the reducibility of both compacts
was tested in thermo-gravimetric apparatus. The general trend
of reduction process for both compacts isothermally reduced at
800–1100 ◦C is shown in Fig. 1. The total weight loss of carbon and
Temperature (◦C)
800
Fe2O3/C
Fe2O3/NiO/C
Fe, traces of
wustite (FeO)
Fe
Iron nickel (Fe0.64 Ni0.36)–Kamacite
(Fe–Ni) (38.7 nm)
1100
Iron nickel (Fe0.64 Ni0.36)–Kamacite
(Fe–Ni) (34.6 nm)

K.S. Abdel-Halim et al. / Journal of Alloys and Compounds 463 (2008) 585–590
587
Fig. 2. XRD patterns of Fe2O3/C compact reduced at 800 ◦C.
Generally, the reduction of metal oxides with solid carbon takes
place in a stepwise manner and the reduction kinetics cannot be
understood well from the percentage of weight loss only. Accord-
ingly, the reduction behavior of the pure and doped compacts
reduced at 800–1100 ◦C was followed using gas analyzer system.
The extents of reduction were calculated as a function of time and
the reduction curves were plotted as given in Fig. 3. For each reduc-
tion curve, the rate of reduction was highest at early stages and
gradually decreased with time till the end of the experiment. It can
be seen that the extent of reduction increased as the reduction tem-
perature increased both at the initial and final reaction stages. The
microstructure of the reduced samples was examined by SEM as
shown in Fig. 4. At 800 ◦C (Fig. 4a) the structure was characterized
by a dense matrix of iron metal, whereas increasing the reduction
temperature to 1100 ◦C (Fig. 4b) led to formation of tubular grains
with high porosity.
Fig. 3. Typical reduction curves of samples reduced at 800–1100 ◦C. (a) Fe2O3/C and
(b) Fe2O3/NiO/C.
3.2. Formation of Fe–Ni alloys
detected in different composition at all the investigated reduction
temperatures. At relatively low reduction temperatures (800 ◦C),
iron–nickel alloy was formed as a solid solution of Fe and Ni
phases together with Fe0.64Ni0.36 phase as the main formed phase.
With increasing the reduction temperature to 1000–1100 ◦C, Fe–Ni
alloy was formed in Fe0.64Ni0.36 phase only as deduced from XRD
patterns. The average crystallite size of the produced alloy was
calculated from XRD data using a software TOPAZ 2 through the
diffractions peaks from Scherer’s formula [18] as shown below:
The presence of NiO in the iron oxide compacts plays a con-
siderable role in the rate of reduction and greatly affects the
morphological changes accompanying reduction process. Fig. 5
shows the effect of NiO content on the reduction behavior of iron
oxide compacts reduced with solid carbon at 1100 ◦C. It can be
noticed that the presence of NiO increases the rate of reduction
where NiO is reduced first then the metallic phase of Ni acts as a
catalyst and promotes the reduction rate of iron oxides [12].
Table 1 shows the phase identification of partially and com-
pletely reduced Fe2O3/NiO/C samples at lower and higher reduction
temperatures. It can be seen that the phase of iron–nickel alloy was
0.9ꢀ
D =
ˇ cos ꢁ
Fig. 4. SEM photomicrographs of Fe2O3/C compact reduced at: (a) 800 ◦C and (b) 1100 ◦C.

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K.S. Abdel-Halim et al. / Journal of Alloys and Compounds 463 (2008) 585–590
Fig. 5. Effect of NiO content on the reduction behavior of iron oxide compacts
reduced with solid carbon at 1100 ◦C.
Fig. 7. TEM photomicrograph of Fe2O3/NiO/C compact completely reduced at
1100 ◦C.
cant change in the microstructure due to the swelling phenomena
accompanying the reduction of Fe2O3/NiO/C compacts. Agglomer-
ated grains in a highly porous structure can be observed for samples
examined by TEM as shown in Fig. 7 where coalescence of particles
takes place.
where D is the crystal size, ꢀ is the X-ray wavelength, ˇ is the
broadening of the diffraction peak and ꢁ is the diffraction angle.
Measuring the crystal size of the formed Fe–Ni alloys showed
that nanocrystallite size in the range of about 34.6 nm have been
formed at 1100 ◦C (Table 1). It is known that such kind of Fe–Ni alloy
(Fe0.64Ni0.36) has a face centered cube (FCC) crystalline structure. It
is also characterized by a soft magnetic nature with ferromagnetic
state where the most stable state is the FexNi1−x ferromagnetic state
at x = 0.7 and 0.6 [13,14]. Fig. 6 shows SEM analysis of the inter-
nal structure of Fe2O3/C and doped compacts completely reduced
at 1100 ◦C. Homogenous particles of metallic iron phases were
observed in a dense structure (Fig. 6a) for non-doped samples. The
presence of NiO during reduction of iron oxide causes a signifi-
Fig. 6. SEM photomicrographs of compacts reduced at 1100 ◦C. (a) Fe2O3/C and (b)
Fe2O3/NiO/C.
Fig. 8. Variation of gas composition (CO and CO2) with time for samples reduced at
900 ◦C. (a) Fe2O3/C and (b) Fe2O3/NiO/C.

K.S. Abdel-Halim et al. / Journal of Alloys and Compounds 463 (2008) 585–590
589
3.3. Reduction kinetics and mechanisms
as in the most of reduction processes with solid carbon [5]. The oxi-
dation of carbon is considerably slower than the reduction of metal
oxides.
It is agreed that the reduction reactions of metal oxides with
solid carbon might be occurred according the following equations:
However, its well known from literatures [4,5,11,12] that the
reduction reaction of iron oxides preceded in a stepwise manner
(Fe2O3 → Fe3O4 → FeO → Fe). The ratio of CO2/(CO + CO2) in the exit
gas was measured experimentally and correlated with the corre-
sponding reduction extent, respectively as shown in Fig. 9a and
b for pure and doped compacts reduced at 800–1100 ◦C respec-
tively. The obtained experimental data showed similar trend to
that of the theoretical equilibrium gas composition (straight lines).
The product gases obtained at the initial stages of reduction (<25%
reduction extents) had the composition similar to the equilibrium
transformation Fe3O4–FeO. At higher reduction extents, the gas
composition showed a drop and was kept relatively constant till
the end of reduction (FeO–Fe equilibrium conversion) [6].
The results obtained can be used for analyzing kinetic data and
elucidate the mechanism of reduction of metal oxides by solid car-
bon. Eq. (I) is a rate equation and can be used to elucidate the order
of reaction [5];
2Fe2O3 + 3C = 4Fe + 3CO2
2NiO + C = 2Ni + CO2
Fe2O3 + NiO = NiFe2O4
NiFe2O4 + 2C = Ni + 2Fe + 2CO2
CO2 + C = 2CO
(1)
(2)
(3)
(4)
(5)
(6)
(7)
Fe2O3 + 2CO = 2Fe + 2CO2
NiO + CO = Ni + CO2
The overall rate of reduction depends mainly on the relative
rates of sub-reactions. It is reasonable to expect that the overall
rate is controlled by the oxidation of carbon (Boudouard reaction,
reaction (5)) [19–21]. The oxidation of carbon is considerably slower
than the reduction of iron oxides below 1100 ◦C. At higher temper-
atures, the reaction rate is expected to be controlled by reactions
(1), (2) and (5) proceeding in series [20].
ln(A − R) = −kt + ln A.
(I)
where R is the percent of reduction, k is the reaction rate constant
and A is a constant depending on reduction temperature and reac-
tion mode. The value of A can be obtained by plotting the relation
between R and Cu which represent the percent of carbon utiliza-
tion (carbon solution loss) as shown in Fig. 10. Straight lines were
obtained which means that the weight of oxygen removed depends
on the weight of carbon consumed (R = ACu) and the reaction is first
The variation of gas composition CO and CO2 in the product gas
can be used to clarify the mechanism of the reaction as shown in
Fig. 8a and b for pure and doped compacts, respectively reduced at
900 ◦C. Both metal oxides and carbon are in contact. The CO gener-
ated diffuses and reacts with oxides. The CO2 diffuses back into the
carbon to generate more CO, according to Boudouard reaction (Eq.
(5)). It can be observed that the rate of formation of CO2 is greater
than that of CO particularly at the beginning of the reaction and
then slows down. It is reasonable to expect that Boudouard reac-
tion is the major controlling step particularly at low temperatures
Fig. 9. Variation of gas composition with reduction extent for compacts reduced at
Fig. 10. Variation of carbon utilization and extent of reduction for compacts reduced
800–1100 ◦C. (a) Fe2O3/C and (b) Fe2O3/NiO/C.
at: (a) 900 ◦C and (b) 1000 ◦C.

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4. Conclusion
Both reduction temperature and NiO content enhanced the
rate of reduction of iron oxides. At lower reduction temperatures
(800–900 ◦C), pure compacts do not attain complete reduction with
solid carbon which can be attributed to the formation of dense
metallic iron and hardly reducible phases which can be reduced
only at higher temperatures. Iron–nickel alloys were detected in
different compositions at all the investigated reduction tempera-
tures, mostly formed in nanosized Fe0.64Ni0.36 phase particularly
at higher reduction temperatures.
The presence of NiO in the iron oxide compacts greatly affects
the morphological structure of reduced samples and was always
associated with swelling phenomena of iron oxides particles.
The variation of gas composition CO and CO2 in the prod-
uct gas was used to clarify the kinetics and mechanism of the
reduction with solid carbon. The reduction process takes place by
gaseous reduction mechanism and Boudouard reaction is the major
rate controlling step particularly at low temperatures. The calcu-
lated values of activation energy revealed that solid state reaction
seems to have an effect in the reduction process. The controlling
mechanism is prone to changes with temperature and percent of
reduction.
Fig. 11. Arrhenius plots for: (a) Fe2O3/C and (b) Fe2O3/NiO/C.
order. Plotting ln(A − R) against (t) gives linear relationships which
prove the validity of Eq. (I) and revealed that the reduction pro-
cess takes place by gaseous reduction mechanism and Boudouard
reaction is major rate controlling step. However, the controlling
mechanism is prone to changes with temperature and percent con-
version.
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