Use of chlorination for chromite upgrading

Article abstract of DOI:10.1016/S0040-6031(00)00423-8

The chlorination of a chromite concentrate was studied between 600 and 1000°C. The reaction products were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and chemical analyses. Chlorination of a chromite concentrate at about 700°C allowed the extraction of about 50% of the iron, thus doubling the Cr/Fe ratio in the treated concentrate. Aluminum chloride was tested as a chlorinating agent in the presence of a reducing atmosphere. The effect of temperature on the kinetics of chromite chlorination was studied between 900 and 1040°C using thermogravimetric analysis (TGA). Temperature effects changed significantly with reaction extent. The initial stage of the chlorination was characterized by an apparent activation energy of about 112 kJ/mol, while a value of about of 269 kJ/mol was found for reaction extents greater than 0.4. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0040-6031(00)00423-8

Thermochimica Acta 351 (2000) 109±117
Use of chlorination for chromite upgrading
N. Kanaria, I. Gaballaha,*,1, E. Allainb
aMineral Processing and Environmental Engineering Team, LEM, CNRS, UMR 7569, ENSG, INPL, BP 40, 54501 Vandúuvre, France
bSchool of Mines and Metallurgy, Center for Pyrometallurgy, 210 Fulton Hall, University of Missouri-Rolla, 65401 Rolla, MO, USA
Received 26 July 1999; accepted 26 January 2000
Abstract
The chlorination of a chromite concentrate was studied between 600 and 10008C. The reaction products were analyzed by
scanning electron microscopy (SEM), X-ray diffraction (XRD), and chemical analyses. Chlorination of a chromite concentrate
at about 7008C allowed the extraction of about 50% of the iron, thus doubling the Cr/Fe ratio in the treated concentrate.
Aluminum chloride was tested as a chlorinating agent in the presence of a reducing atmosphere.
The effect of temperature on the kinetics of chromite chlorination was studied between 900 and 10408C using
thermogravimetric analysis (TGA). Temperature effects changed signi®cantly with reaction extent. The initial stage of the
chlorination was characterized by an apparent activation energy of about 112 kJ/mol, while a value of about of 269 kJ/mol was
found for reaction extents greater than 0.4. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Chlorination; Chromite upgrading; Aluminum chloride
1. Introduction
certain cases, chlorine technology can replace other
processes and lead to a better environmental plan. For
example, the production of titanium oxide from the
carbochlorination of titanium-bearing materials is
more advantageous from an environmental point of
view than the sulphate process [1].
Recently, Gaballah et al. [2] used chlorination for
the recovery of valuable metals (Pb, Zn, and Cu)
contained in Jarosite. The resulting residues contained
essentially iron oxides that are environmentally accep-
table. Allain et al. [3,4] investigated the chlorination
and the carbochlorination of Ta and Nb in tin slags at
temperatures up to 10008C to recover these refractory
metals. The chlorination technique was also used
successfully for the recovery of Co, Ni, Mo, and V
from spent hydrore®ning catalysts [5].
The high reactivity of chlorine towards various
compounds at moderate temperatures makes chlorine
technology a competitive alternative for the extraction
of several valuable metals. Further, the periodic imbal-
ance between the world consumption of NaOH and
Cl2 generates troubles for the producers, as storage of
chlorine gas is costly and dangerous. Besides, in
* Corresponding author. Tel.: ‡33-383-5963;
fax: ‡33-8356-9385.
1
Â
Laboratorie Environnement et Mineralurgie (LEM), rue du
Doyen M. Roubault, BP 40, 54501 Vandoeuvre Cedex, France;
Centre National de la Recherche Scienti®que (CNRS), rue Michel-
Â
Â
Ange, 75794 Paris Cedex, France; Ecole Nationale Superieure de
Â
Geolgie (ENSG), rue du Doyen M. Roubault, BP 40, 54501
Different applications using chromite concentrates
and ores require a high Cr/Fe ratio [6]. The increase in
this ratio by physical means seems to be limited
Vandoeuvre Cedex, France; Institut National Polytechnique de
Ã
Lorraine (INPL), 2 rue de la foret de Haye, 54501 Vandoeuvre
Cedex, France.
0040-6031/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 0 ) 0 0 4 2 3 - 8

110
N. Kanari et al. / Thermochimica Acta 351 (2000) 109±117
because most of the iron is included in the spinel
of ®ve major elements contained in the chromite
concentrate. This concentrate consists of about 48%
Cr2O3 and was of metallurgical grade. It was char-
acterized by a Cr/Fe ratio close to 3.2. The distribution
of these oxides in the chromite and the gangue was
revealed by microprobe analysis. Results of this ana-
lysis showed (Table 1) that the chromite was com-
posed of Cr, Fe, Mg, and Al oxides. Calculations using
data obtained from the microprobe analysis indicated
the following formula for the chromite mineral:
structure of chromite (MN2O4: MˆFe2‡
, Mg;
NˆFe3‡, Cr, Al). For this reason, chlorination of
chromite concentrates has been tested at various par-
tial pressures of oxygen (Cl2‡O2, Cl2, and Cl2‡CO).
It was suggested that this technique would be appro-
priate for improving poor chromite concentrates and/
or ores [7±10].
The literature concerning the thermodynamic and
kinetic aspects of the chlorination of simple oxides
contained in the chromite (FeO, Fe2O3, Cr2O3, MgO,
Al2O3, and SiO2) using different chlorinating agents is
vast. Results of this bibliographic survey have been
summarized in [7±10]. This literature review includes
few papers concerning the chlorination of chromium-
bearing materials at low temperatures. However, no
publications have been devoted to the chlorination of
chromite ores and concentrates in the absence of a
reducing atmosphere.
2‡
(Fe , Mg0.70)(Cr1.56, Al0.37, Fe30:‡07)O4. This formula
0:30
can also be expressed as simple spinels having the
following weight composition: 30.9% FeCr2O4,
51.0% MgCr2O4, 13.7% MgAl2O4, and 4.4%
Fe3O4. These different spinels have the same crystal-
line structure and lattice parameters, making their
identi®cation by XRD almost impossible. A similar
chromite ore composition has been reported by
[11,12].
This paper focuses on the chlorination of chromite
concentrates to selectively remove iron as FeCl3. In
addition, the use of aluminum chloride as a chlorinat-
ing agent has been reported. The effect of temperature
on the reaction of chromite mineral with Cl2 has also
been studied by using the thermogravimetric analysis
(TGA) technique.
The gangue is essentially composed of magnesium
and silicon oxides along with iron and aluminum
oxides (Table 1). Results of microprobe and XRD
analyses indicate that olivine and serpentine were the
main phases present in the gangue. The combination
of the results of both chemical and microprobe ana-
lyses showed that the chromite represented about
80 wt.% of the concentrate.
The chlorination tests were performed using several
grams of chromite concentrates. The horizontal
experimental setup shown schematically in Fig. 1
was used to perform chlorination tests of chromite
concentrate. This setup is composed of a gas metering
unit followed by a gas puri®cation unit and an elec-
trical horizontal furnace. When using aluminum chlor-
ide as a chlorinating agent, an additional furnace
heated at 1308C was used for the volatilization of
AlCl3 (Fig. 1). The gaseous reaction products are
recovered through condensors and the outlet gases
are treated before their release into the atmosphere.
The reaction products were examined by XRD, SEM,
and conventional chemical analyses.
The sample used to investigate the temperature
effect on chlorination was the chromite mineral con-
tained in the concentrate mentioned earlier. It was
separated from the gangue by a series of successive
treatments using heavy media. The chlorination tests
were carried out on 40 mg of chromite sample and
using the experimental setup described in [8].
2. Materials and experimental procedure
The sample of chromite concentrate was obtained
from an Albanian mine. Scanning electron micro-
scopy (SEM), X-ray diffraction (XRD), microprobe
and chemical analyses were used in the physical
characterization of this concentrate. Table 1 gives
the results of the chemical and microprobe analysis
Table 1
Composition of chromite concentrate (wt.%)
Chemical analyses
Oxide
Microprobe
Chromite
Average
Gangue
Cr2O3
FeO
47.7
13.4
17.8
8.8
59.4
13.9
14.2
9.5
0.7
3.7
MgO
Al2O3
SiO2
42.0
1.4
7.1
0.1
39.7

N. Kanari et al. / Thermochimica Acta 351 (2000) 109±117
111
Fig. 1. Horizontal experimental setup.
3. Results
at temperatures close to 10008C. The chlorination
residues were analyzed to determine their iron con-
tent. The curve of iron extraction as a function of
temperature is given in Fig. 2(b). For a reaction time of
2 h, about 20 and 90% of the iron contained in the
chromite was extracted at 600 and 10008C, respec-
tively. However, the weight loss obtained at 10008C
indicates that other oxides, besides iron, had partially
reacted with the chlorine. A calculation based on the
chemical composition of the chromite suggests that
the majority of iron chromite (FeCr2O4) had reacted at
10008C.
3.1. Chlorination of chromite concentrate using the
horizontal experimental setup
3.1.1. Chlorination with chlorine
The chromite concentrate was treated using a gas
mixture of chlorine and nitrogen having a Cl2/N2 ratio
equal to 1. The investigated temperature range was
600±10008C and the reaction time was 2 h. Fig. 2(a)
shows the percentage weight loss (% WL) of the
sample as a function of temperature. The interaction
of the chromite concentrate with chlorine starts at
about 6008C. About 25% of the sample is chlorinated
Table 2 lists the results of SEM analysis of the
several residues and condensates produced by the
Fig. 2. Evolution of % WL of sample (a) and % iron extraction (b) as a function of temperature during the chlorination of chromite
concentrate for 2 h.

112
N. Kanari et al. / Thermochimica Acta 351 (2000) 109±117
Table 2
chlorine and a gas mixture containing Cl2‡N2 (Cl2/
N2ˆ1) were used, while the reaction time was varied
from 0.5 to 8 h. Chromium and iron contents of the
residues were determined by chemical analysis. Fig.
3(a) plots the evolution of the % WL of the chromite
concentrate versus reaction time. It is obvious from
Fig. 3(a) that about 20% of the sample was chlorinated
during the ®rst half-hour of the treatment. A longer
reaction time of 8 h led to a weight loss between 40
and 50%, depending on the composition of the chlor-
inating gas mixture.
Extraction of iron and chromium as a function of the
reaction time is illustrated in Fig. 3(b). About 80% of
the iron is removed within 30 min of chlorination with
both gas mixtures. Extraction of chromium was about
19 and 13% for chlorination by Cl2 and Cl2‡N2,
respectively. For a reaction time of 30 min, the Cr/
Fe weight ratios of the chlorination residues were
equal to about 18 and 14 using Cl2 and Cl2‡N2,
respectively. Beyond 2 h of treatment, the extraction
of chromium increased gradually with time and about
50% of the chromium was extracted after a reaction
time of 8 h.
XRD analyses of the products obtained at different temperatures
during the chlorination of chromite concentratea
T (8C)
Residues
Condensates
C1
C2
Raw sample Cr, Mg, Si, Fe, Al
600
Cr, Si, Mg, Fe, Cl, Al Cl, Fe
NC
800
Cr, Si, Mg, Al, Fe
Cl, Fe, Cr Cl, Fe, Cr
Cl, Mg Cl, Mg, Fe, Cr
1000
Cr, Si, Mg, Al, eFe
a C1 and C2: condensate nos. 1 and 2 recovered at different
temperatures of the condenser, (TC > TC ); NC: no condensate; e:
traces.
1
2
chlorination of chromite concentrate in the experi-
mental conditions mentioned above. Chromium
remained a major element even in the residue obtained
at 10008C. However, the presence of chromium in the
condensates at temperatures higher than 8008C sug-
gests that the chromium compounds had partially
reacted with chlorine. A similar observation can be
made for the magnesium compounds at 10008C.
Although iron was detected in all the residues
obtained, its content decreased as the chlorination
temperature increased. XRD results indicate that the
spinel (Fe, Mg)(Cr, Al)2O4 was the predominant phase
in all the chlorination residues. The condensates were
only slightly crystallized. Finally, the chromium chlor-
ide was identi®ed in the condensate obtained at
10008C.
One may note that shorter chlorination times will
only lead to partial removal of iron and lower extrac-
tion extents of chromium. Thus, the chlorination of
chromite ores or low grade concentrates using a gas
mixture of chlorine and nitrogen at about 10008C
generates a residue having a high Cr/Fe ratio and
meeting the standard requirements for the metallurgi-
cal production of ferro-chromium.
Another series of chlorination experiments of chro-
mite concentrate was carried out at 10008C. Pure
Fig. 3. Evolution of % WL of sample (a) and % Fe and % Cr extraction (b) as a function of time during the chlorination of chromite
concentrate using Cl2 and Cl2‡N2 at 10008C.

N. Kanari et al. / Thermochimica Acta 351 (2000) 109±117
113
(2)) seem to be the most favorable at temperatures
higher than 4008C:
3
2
32 FeO ‡ AlCl3 ! FeCl2 ‡ 21 Al2O3
(1)
(2)
(3)
12 Fe2O3 ‡ AlCl3 ! FeCl3 ‡ 12 Al2O3
12 Cr2O3 ‡ AlCl3 ! CrCl3 ‡ 12 Al2O3
38 FeCr2O4 ‡AlCl3 ! FeCl2 ‡34 CrCl3 ‡21 Al2O3
(4)
3
8
38 MgCr2O4 ‡ AlCl3 ! MgCl2 ‡ 43 CrCl3
3
8
‡ 12 Al2O3
(5)
3
32 MgAl2O4 ‡ AlCl3 ! MgCl2 ‡ 2Al2O3 (6)
2
3
34 O2 ‡ AlCl3 ! Cl2 ‡ 12 Al2O3
(7)
2
Fig. 4. Evolution of the free-energy changes (DG0) as a function of
temperature for the chlorination with AlCl3 of several compounds
contained in the chromite [13,14].
Results of the chlorination of chromite concentrate
with Cl2‡AlCl3 at 10008C are given in Fig. 5. The
evolution of % WL of the sample versus time during
the chlorination with Cl2‡AlCl3 is traced in Fig. 5(a).
Comparison of the results of Figs. 3(a) and 5(a)
indicates that the presence of aluminum chloride
did not increase the % WL of the sample. Identical
®gures were obtained for the extraction of iron and
chromium during the chlorination with Cl2‡AlCl3
(Fig. 5(b)) and chlorine alone (Fig. 3(b)).
3.1.2. Chlorination in the presence of aluminum
chloride
From the thermodynamic point of view, aluminum
chloride appears to be a good chlorinating agent. The
evolution of the free-energy changes (DG0) as a
function of temperature for the reactions of AlCl3
with the main oxides of chromite (Eqs. (1)±(6)) is
traced in Fig. 4. All envisaged reactions are thermo-
dynamically feasible up to at least 9008C. However,
the chlorination reactions of iron oxides (Eqs. (1) and
While the chlorination reactions of chromite
compounds with AlCl3 are thermodynamically
more favorable than those of its chlorination by
chlorine [8], experimental results do not agree with
Fig. 5. Evolution of % WL of sample (a) and % Fe and % Cr extraction (b) as a function of time during the chlorination of chromite
concentrate using Cl2‡AlCl3 at 10008C.

114
N. Kanari et al. / Thermochimica Acta 351 (2000) 109±117
this result. This could be attributed to the applied
experimental conditions that do not enhance the
role of aluminum chloride signi®cantly during the
chlorination of chromite concentrate due to the
following reasons:
3.2. TGA of the chlorination of chromite
3.2.1. Non-isothermal tests
The non-isothermal chlorination test of chromite
mineral was investigated from room temperature to
10008C using TGA. Fig. 6 gives the % WL of the
sample versus temperature between 500 and 10008C.
The chromite started to react with chlorine at tem-
peratures higher than 6008C and a ®nal weight loss of
about 43% was obtained at 10008C. About 35% of the
sample was chlorinated and volatilized at a tempera-
ture close to 9508C. The derivative of the % WL traced
in Fig. 6 showed clearly a maximum in the reaction
rate at about 7258C. At higher temperatures, the
reaction rate slowed down and increased again at
temperatures higher than 9758C.
1. The partial vapor pressure of aluminum chloride
in Cl2‡AlCl3 gas mixture was lower than that of
chlorine.
2. The diffusion of AlCl3 towards non-reacted core is
expected to be lower than that of Cl2.
3. The oxygen partial pressure in the reactor allows
partial oxidation of gaseous AlCl3, generating
Al2O3 that is thermodynamically favorable (Eq.
(7) and Fig. 4).
4. The Al2O3 generated can act as a diffusion barrier
for further interaction of the chlorinating gas
mixture with the chromite.
3.2.2. Isothermal tests
To determine the effect of temperature, isothermal
tests of chromite chlorination were performed
between 900 and 10408C. Results are given in Fig. 7
as % WL versus time. At 10008C, about 10 min were
needed to chlorinate 35% of the sample and more
than 120 min were necessary to obtain a % WL of
about 70. It is clear from Fig. 7(b) that the slope
of % WL versus time changes after the chlorination
of 35% of chromite.
Different kinetic equations [15] can be used to
describe mathematically the evolution of the reaction
extent as a function of time between Xꢀ0.35 and
Xꢁ0.40. Eq. (8) seems to be the most appropriate
To avoid some of these disadvantages, another
series of experiments were conducted between 800
and 10008C, using CO as a reducing agent. A gaseous
mixture of AlCl3‡CO‡N2 (CO/N2ˆ1) was used for
the chlorination of the chromite concentrate. The
majority of iron was chlorinated and extracted from
this concentrate at 8008C. XRD analysis revealed the
presence of FeCl2 in the condensates. This observation
indicates that the chlorination of iron chromite
(FeCr2O4) occurred according to Eq. (4). About
40% of the chromium was extracted using such
treatment.
Fig. 6. Evolution of % WL of the sample and derivative of WL vs. temperature during non-isothermal chlorination of chromite.

N. Kanari et al. / Thermochimica Acta 351 (2000) 109±117
115
Fig. 7. TGA results of chromite chlorination using chlorine between 900 and 10408C for (a) 35% WL and (b) for 70% WL.
to describe the evolution of reaction extent as a
function of time in the investigated temperature range.
Results are represented in Fig. 8(a) and (b). The
average correlation coef®cient for data ®tting was
about 0.999.
1. Chromite contains several elements having differ-
ent reactivities towards chlorine.
2. Chlorination of iron and to some extent of
chromium (ꢃ35% WL) probably generates porous
chromite grains.
Eq. (8) describes pore diffusion control of a reaction
of porous or non-porous solids with a porous product
layer applied to spheres:
3. Formation of a layer of magnesium chloride can
act as a barrier for further reaction.
4. Iron chloride enhances the vapor pressure of other
produced chlorides due to the formation of
gaseous binary chlorides [16] etc.
2=3
3ꢂ1 X† ‡ 2ꢂ1 X† ˆ kt
1
(8)
However, the effect of temperature on the chlorina-
tion and the volatilization of products was estimated
by constructing an Arrhenius diagram. Fig. 9 gives an
example of the Arrhenius plots for % WL lower than
35% and higher than 40%. An Arrhenius diagram was
where k is the rate constant and t is the chlorination
time.
Nevertheless, the kinetics of the chromite chlorina-
tion is complex due to the simultaneity of several
phenomena as follows:
Fig. 8. Mathematical ®tting of the chlorination data using Eq. (8) for the reaction extents: (a) Xꢀ0.35 and (b) Xꢁ0.40.

116
N. Kanari et al. / Thermochimica Acta 351 (2000) 109±117
Fig. 9. Arrhenius diagram of chromite chlorination between 900 and 10408C.
calculated using the value of the chlorination rate
constant, k, from the linear relationship of data repre-
sented in Fig. 8(a) and (b). The value of the apparent
activation energies changes with the reaction extent.
An apparent activation energy, Ea, of about 112 kJ/
mol, was calculated for the chlorination of chromite
up to 35% WL. This value probably corresponds to the
chlorination of FeCr2O4. A higher value of Ea of about
269 kJ/mol characterizes the chlorination process at
higher reaction extents and probably describes the
chlorination of MgCr2O4.
However, special caution to control the partial pres-
sure of oxygen in the chlorination reactor is needed.
Between 900 and 10408C, the temperature coef®-
cient of the reaction of chromite mineral with Cl2
changes with the extent of reaction. The average value
of the apparent activation energy was about 112 kJ/
mol for reaction extents up to 0.35. The chlorination
process for a reaction extent higher than 0.4 proceeded
with a value of Ea of about 269 kJ/mol.
These results con®rm that upgrading of the chro-
mite concentrate, at a temperature lower than 9008C,
will operate mainly through the selective extraction of
iron allowing the increase in Cr/Fe without signi®cant
loss of chromium.
Acknowledgements
This work was performed in the frame of the
contract N8 BRE2-CT92-0173 thanks to the ®nancial
support of the European Union (DG-XII). The authors
thank Dr. H.L. Schmidt for discussion, suggestion and
help.
4. Conclusions
They also would like to thank Dr. J.C. Mugica, Dr.
R. Solozabal (INASMET, San Sebastian, Spain) and
Dr. M. Coelho (INETI, Lisbon, Portugal) for technical
The chlorination of chromite at 8008C could be
employed to upgrade chromium concentrates. At this
temperature and for a reaction time of 2 h, more than
60% of the iron was removed during the treatment of a
chromite concentrate. The chlorination at 10008C
using diluted chlorine for short times (<30 min) leads
to a selective removal of iron and low extraction
extents of chromium. The Cr/Fe ratio could be
increased easily to the required level.
Â
discussions. Authors are indebted to Dr. A. Bonazebi,
Dr. M. Djona, Dr. S. Ivanaj, Dr. N. Menad, Dr. N.
Mirghaffari for discussions and help on different
subjects and to Mrs. C. Richard for technical and
administrative support.
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Aluminum chloride can be used as a chlorinating
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