Kinetic study of Hubnerite (MnWO4) chlorination

Article abstract of DOI:10.1016/j.tca.2012.02.015

The kinetics of Argentinean Hubnerite (MnWO₄) chlorination using gaseous chlorine as chlorination agent was studied between 750 and 950 °C. The relative mass change during the chlorination reaction was continuously monitored using a high resolution thermogravimetric system. The starting temperature for the reaction of the manganese tungstate with chlorine was determined at about 650°C. The influence of gaseous flow rate, sample mass, chlorine partial pressure, and temperature on the reaction rate was analyzed. The dependence of the reaction rate with sample mass clearly indicates that the reaction is not occurring under chemical control, so the reaction proceeds under mixed control for sample masses greater than 0.5 mg. In those conditions, an apparent activation energy of 198 ± 9 kJ mol^-1 was obtained with an isoconversional method. Concerning the influence of chlorine partial pressure, it was determined that pressures greater than 35 kPa do not modify the kinetic regime. For the experiment at 850°C, it was found that the chlorination rate was proportional to a potential function of the partial pressure of chlorine whose exponent is around 0.85. Finally, a global rate equation that includes these parameters was developed.

Full text of DOI:10.1016/j.tca.2012.02.015

Thermochimica Acta 536 (2012) 30–40
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Thermochimica Acta
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Kinetic study of Hubnerite (MnWO4) chlorination
G.G. Fougaa,b,c,∗, R.M. Taddeoc, M.V. Boscoa,b, A.E. Bohéa,b,c
a Centro Atómico Bariloche Comisión Nacional de Energía Atómica, Avenida Bustillo, 9500, 8400 San Carlos de Bariloche, Argentina
b Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
c Centro Regional Universitario Bariloche – Universidad Nacional del Comahue, 8400 San Carlos de Bariloche, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
The kinetics of Argentinean Hubnerite (MnWO4) chlorination using gaseous chlorine as chlorination
agent was studied between 750 and 950 ◦C. The relative mass change during the chlorination reaction
was continuously monitored using a high resolution thermogravimetric system. The starting temperature
for the reaction of the manganese tungstate with chlorine was determined at about 650 ◦C. The influence
of gaseous flow rate, sample mass, chlorine partial pressure, and temperature on the reaction rate was
analyzed. The dependence of the reaction rate with sample mass clearly indicates that the reaction is
not occurring under chemical control, so the reaction proceeds under mixed control for sample masses
greater than 0.5 mg. In those conditions, an apparent activation energy of 198 9 kJ mol−1 was obtained
with an isoconversional method. Concerning the influence of chlorine partial pressure, it was determined
that pressures greater than 35 kPa do not modify the kinetic regime. For the experiment at 850 ◦C, it
was found that the chlorination rate was proportional to a potential function of the partial pressure of
chlorine whose exponent is around 0.85. Finally, a global rate equation that includes these parameters
was developed.
Received 23 February 2011
Received in revised form 8 February 2012
Accepted 11 February 2012
Available online 23 February 2012
Keywords:
Hubnerite
MnWO4
Chlorination
Kinetics
Thermogravimetry
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
natural Rutile (TiO2), or from the so-called synthetic rutile obtained
from Ilmenite (FeTiO3) or from the TiO2-rich slag produced by
The increasing use of metals in modern industry leads to a grad-
ual depletion of the primary sources of these metals. Foregoing, it
led to considerable research to develop suitable methods for using
low grade minerals, polymetallic minerals, industrial wastes, elec-
tronic scrap, etc. [1,2].
metallurgical treatment of Ilmenite. Zirconium is produced by
fluidized-bed carbochlorination of milled Zircon sand (ZrSiO4). This
ore-decomposition process is used for the co extraction of hafnium
because this metal is always found in zirconium ore. The chloride
process is also applied to the extractive metallurgy of beryllium. At
The efficient use of these resources requires the development
of such processes to enhance the extraction of metals. Chlorination
and carbochlorination are processes applied in extractive metal-
lurgy to recovering valuable metals in the form of chlorides.
Chlorination, in the case of most of the refractory metal oxides, is
one of the important intermediate steps to produce the respective
metals. The raw materials for extract refractory metals are gener-
ally refractory minerals, whose decomposition requires the use of
aggressive chemicals and high temperatures. It is well known that
chlorine possesses a high reactivity towards many compounds at
relatively low temperature. This property drove the metallurgists
to use chlorine for the extraction of valuable elements from their
bearing materials [3,4].
temperatures above 630 ◦C the components of Beryl (Be3Al2Si6O18
)
can be extracted in a stream of chlorine using carbon as reducing
agent. These halogenides are reduced to metals with other met-
als or by melt electrolysis. The Kroll process is the most widely
used in pyrometallurgical industrial process to produce Ti, Zr and
Hf in which the metal chlorides are reduced by liquid magnesium at
800–850 ◦C [5].Otherwise in the literature there are reported many
exploratory studies mentioning a possible future use of the chlori-
nation process [6–9]. The use of chlorine in metal extraction process
through pyro and hydrometallurgic methods has attracted consid-
erable attention in recent decades and the use of chlorine chemistry
may increase in the future. This is due to a number of factors, which
include the high chlorination rates resulting from the elevated
reactivity of Cl2 and other chlorinating agents; the compara-
tively moderate temperature involved in the chlorination process;
the low cost, variety, and availability of chlorinating agents like
gaseous chlorine [7,9], carbon tetrachloride [8], hydrogen chloride
[10,11], alkaline chlorides [12] and some metal chlorides [13]. Other
aspects that make chlorination and carbochlorination process
potentially interesting are: the favourable physical and chemical
Concerning to industrial process, currently, a few metals are pro-
duced by chlorination processes. Titanium is produced by reduction
of titanium tetrachloride, which is obtained by carbochlorination of
∗
Corresponding author. Tel.: +54 02944 445100; fax: +54 02944 445293.
E-mail address: fouga@cab.cnea.gov.ar (G.G. Fouga).
0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2012.02.015

G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
31
characteristics of certain metals; the properties of many chlorides
(high solubility, wide variety of oxidation states, and ease of sepa-
ration by liquid–liquid extraction or distillation); the selectivity of
the treatment and separation processes; and the development of
certain corrosion-resistant materials used for the manufacture of
reactors.
thermocouple encapsulated in silica glass, which was placed 2 mm
below the crucible. Flows of Ar(g) and Cl2(g) gases were controlled
by means of flowmeters. Non-isothermal and isothermal runs were
performed. In both cases, samples were placed in a cylindrical sil-
ica glass crucible (diameter = 0.72 cm, high = 0.42 cm). The samples
were well spread in the crucible forming a thin layer. Before reach-
ing the selected reaction temperature for each experiment, the
samples were heated to 100 ◦C in Ar(g) flow in order to remove
water. In non-isothermal runs, the Cl2(g) was introduced and the
With this in perspective, a thorough understanding of the kinet-
ics of the chlorination reactions implied in a given recovery process
is necessary for reactor design and to set the operation variables.
Scheelite (CaWO4(s)) and Wolframite (FexMn1−xWO4(s)) are
main tungsten bearing minerals. In Argentina, there are associ-
ations formed by concentrations of these minerals in different
proportions [14]. The Wolframite series consists of a complete solid
solution FexMn1−xWO4 present in nature with 0 ≤ x ≤ 1, which is
the principal source of tungsten. In the literature, there are few
research works about chlorination of mixtures of these minerals.
Menéndez et al. [15] analyzed the effect of Wolframite composition
on its reactivity with chlorine, in the presence of sulphur diox-
ide. They have been working mainly with Wolframites, including
a wide range of the ratio of divalent metals (Fe/Mn + Fe–WO4(s)),
and Scheelite (CaWO4(s)). In a previous paper, Menéndez et al. [16]
submitted a study about the main kinetic aspects of the isother-
mal and non-isothermal chlorination of a Scheelite Wolframite
concentrate with chlorine and sulphur dioxide. On that occasion,
they proposed a rate equation based on a diffusion mechanism with
spherical symmetry, which was determined with both methods.
They concluded that the control of the total kinetics corresponded
to a diffusion process through the reaction products deposited on
the unreacted concentrate. In other investigation, Menéndez et al.
[17] determined the apparent activation energy of an isomorphous
Wolframite which consisted in series of Ferberite (FeWO4(s)) and
Hubnerite (MnWO4(s)) mixtures.
samples were heated in the resulting mixture Cl2(g)–Ar(g) (PCl
=
2
35 kPa) to 950 ◦C. A linear heating rate of 5.3 ◦C min−1 was used. Rel-
ative mass changes monitored were acquired every 3 s using a data
acquisition system with the experimental setup described above.
From these measurements, it was determined the experimental
mass change as ꢀM = m0 − mt, where m0 and mt are the initial mass
and the mass at time t, respectively.
According to the following equation:
MnWO4(s) + Cl2(g) → MnClx(g) + WOxCly(g)
(1)
If the manganese chloride and tungsten oxychloride are gaseous
species the conversion degree (˛) can be defined by ꢀM/m0.
Usually, the rate of a heterogeneous solid–gas reaction can be
written, assuming separability of variables [18], as:
d˛
Rate =
= F(˛) · K(T) · F(PCl
)
(2)
2
dt
In Eq. (2) F(˛) is a function that describes the geometric evo-
lution of the reacting solid, K(T) refers to an Arrhenius equation,
and F(PCl2 ) expresses the dependence of the reaction rate on
the chlorine partial pressure. This mathematical procedure allows
to exclude mass sample effects and represents an appropriate
approach for gas–solid reaction analysis.
The present study is a kinetic investigation on a pure man-
ganese Wolframite (Hubnerite MnWO4(s)) arising from a reservoir
of Cordoba, Argentina. This study involves the evaluation of vari-
ous factors influencing on chlorination reaction kinetics, aimed to
determine the mechanism of the tungsten and manganese extrac-
tion reaction in the form of halogenated compounds like chlorides
or oxychlorides.
2.2.2. Fixed bed reactor for isothermal experiments
A fixed bed reactor was employed in order to determine the
chlorination reaction stoichiometry and to analyse the reaction
products. A schematic diagram of the reactor device is shown in
Fig. 1. This reactor is a silica glass tube where the sample was intro-
duced using a silica glass crucible. The temperature of the hot zone
(zone A) was maintained by a furnace. Zones B and C correspond
to the cold part of the reactor where volatile species were con-
densed. The chlorine gas was introduced when the system reaches
the working temperature. At the end of each run, the reactant gas
was cut off and argon stream was introduced to purge the reactor.
2. Experimental
2.1. Materials
The gases used were Ar(g) 99.99% purity (AGA, Argentina) and
Cl2(g) Ar 99.8% purity (INDUPA, Argentina). The solid starting mate-
rial was an Argentinean pure manganese tungstate (Hubnerite:
MnWO4(s)) from a reservoir located in Distrito Minero Los Mogotes,
Córdoba – Argentina.
3. Thermodynamic considerations
The first step of this study was the calculation of standard free
energy changes of the probable reaction considered in the present
work. The thermodynamic calculations for the Hubnerite chlorina-
tion reactions were performed using HSC chemistry for windows
software [19].
2.2. Experimental equipment and procedure
In the literature, there are reported several manganese chlo-
rides: MnCl(g), MnCl2(s,l,g), MnCl3(s,l,g), MnCl4(l,g) and Mn2Cl4(g)
[19,20]. In a previous research, the authors reported the formation
of MnCl2(s,l,g) as stable phase for the direct chlorination reaction
of MnO(s), Mn3O4(s) and Mn2O3(s) [21]. Therefore, the MnCl2(s,l,g)
was considered as the stable phase formed, which leads to man-
ganese content elimination through volatilization at temperatures
above 650 ◦C. Moreover, the compounds reported for tungsten are:
WCl(g), WCl2(s,l), WCl3(s,l,g), WCl4(s,l,g), WCl5(s,l,g), WCl6(s,l,g),
W2Cl10(g), WOCl(g), WOCl2(s,l), WOCl3(s,l,g), WOCl4(s,l,g),
WO2Cl(s,g), WO2Cl2(s,l,g), and physicochemical properties are
shown in Table 1 [19,20].
2.2.1. Thermogravimetric system for non-isothermal and
isothermal experiments
Mass changes occurring during Hubnerite chlorination reaction
were measured using a high resolution thermogravimetric analyser
(TGA). It consists of an electrobalance (model 2000, Cahn Instru-
ments Inc.), a vertical tube furnace, a gas line, and a data acquisition
system. This experimental setup has a sensitivity of 5 ␮g under a
gas flow rate between 2 and 8 L h−1 in the range of room tempera-
ture to 1000 ◦C.
Each sample was placed in a silica glass crucible, which hangs
from one of the arms of the electrobalance through a silica glass
wire. A silica glass hang down tube carried the gases to the sample.
The temperature of the sample was measured using a Pt–Pt (10%Rh)
The MnWO4(s)–Cl2(g) reaction can involve the formation of sev-
eral reaction products, according to Eqs. (3)–(15). These equations

32
G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
Fig. 1. Schematic diagram of fixed bed reactor.
show the possible Hubnerite chlorination reaction stoichiometry,
which foresee the formation of MnCl2(s,l,g) and tungsten chlorides
or oxychlorides.
+ 4/5O2(g)
(5)
(6)
1/3MnWO4(s) + Cl2(g) → 1/3MnCl2(s, l, g) + 1/3WCl4(s, l)
+ 2/3O2(g)
2/3MnWO4(s) +Cl2(g) → 2/3MnCl2(s, l, g) + 2/3WCl(g)
+ 4/3O2(g)
(3)
2/7MnWO4(s) + Cl2(g) → 2/7MnCl2(s, l, g) + 2/7WCl5(s, l, g)
1/2MnWO4(s) + Cl2(g) → 1/2MnCl2(s, l, g) + 1/2WCl2(s, l, g)
+ 4/7O2(g)
(7)
+ O2(g)
(4)
1/4MnWO4(s) + Cl2(g) → 1/4MnCl2(s, l, g) + 1/4WCl6(s, l, g)
2/5MnWO4(s) + Cl2(g) → 2/5MnCl2(s, l, g) + 2/5WCl3(s, g)
+ 1/2O2(g)
(8)
Table 1
Physicochemical properties of tungsten and manganese compounds.
2/7MnWO4(s) + Cl2(g) → 2/7MnCl2(s, l, g) + 1/7W2Cl10(g)
+ 4/7O2(g)
Melting point (◦C)
Boiling point (◦C)
(9)
W compounds
Oxides
WO(s)
WO2(s)
WO3(s)
Not reported
1723.85
1472
Not reported
1500 (dec)
1837
2/3MnWO4(s) + Cl2(g) → 2/3MnCl2(s, l, g) + 2/3WOCl(g)
+ O2(g)
(10)
Chlorides
WCl(g)
WCl2(s,l,g)
WCl3(s,g)
WCl4(s,l)
Not reported
588.85
550 (dec)
498
242
275
Not reported
926.85
806a
1/2MnWO4(s) + Cl2(g) → 1/2MnCl2(s, l, g) + 1/2WOCl2(s, l)
Not reported
286
346.75
WCl5(s,l,g)
WCl6(s,l,g)
W2Cl10(g)
Oxychlorides
WOCl(g)
WOCl2(s,l)
WOCl3(s,l,g)
WOCl4(s,l,g)
WO2Cl(s,g)
WO2Cl2(s,g)
Mn compounds
Chlorides
MnCl(g)
+ 3/4O2(g)
(11)
Not reported
Not reported
Not reported
Not reported
Not reported
211
Not reported
264.85
Not reported
Not reported
Not reported
227.55
2/5MnWO4(s) + Cl2(g) → 2/5MnCl2(s, l, g) + 2/5WOCl3(s, l, g)
+ 3/5O2(g)
(12)
Not reported
472a
1/3MnWO4(s) + Cl2(g) → 1/3MnCl2(s, l, g) + 1/3WOCl4(s, l, g)
Not reported
650
587
Not reported
Not reported
Not reported
1190
627
Not reported
Not reported
+ 1/2O2(g)
(13)
(14)
MnCl2(s,l,g)
MnCl3(s,l,g)
MnCl4(l,g)
Mn2Cl4(g)
Oxides
2/3MnWO4(s) + Cl2(g) → 2/3MnCl2(s, l, g) + 2/3WO2Cl(s)
+ 2/3O2(g)
MnO(s,g)
1842
535 (dec)
1347
3127
MnO2(s,g)
Mn2O3(s)
Mn3O4(s)
Oxychlorides
MnClO3(g)
Not reported
Not reported
Not reported
1562
1/2MnWO4(s) +Cl2(g) → 1/2MnCl2(s, l, g) + 1/2WO2Cl2(s, l, g)
+ 1/2O2(g) (15)
Not reported
Not reported
a
Calculated with Software HSC 6.12 chemistry.

G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
33
Fig. 2. Ellingham diagram for the Hubnerite chlorination reaction: (a) showing the
formation of different tungsten chlorides; (b) showing the formation of different
tungsten oxychlorides.
Fig. 3. (a) Phase stability diagram for Mn Cl O system at 650 ◦C, and (b) phase
stability diagram for W Cl O system at 650 ◦C.
Fig. 3 shows the phase stability diagram for: (a) Mn Cl O; and
(b) W Cl O systems at 650 ◦C. At the beginning of the chlorina-
tion, the values of partial pressure of Cl2(g) was 0.35 atm and the
oxygen was below 10−4 atm. The O2(g) is present in the argon as
an impurity.
As can be seen MnCl2(l) can be formed under the experimen-
tal conditions. At this temperature, the MnCl2(l) vapour pressure
is 2.77 × 10−4 atm. so, it is possible to observe mass loss by
volatilization. Furthermore, as can be seen in Fig. 3b, for these
experimental conditions (pCl2 = 0.35 atm and pO2 below 10−4 atm),
for the W Cl O systems the possible stable phases are WO2Cl,
WO2Cl2, WOCl2 and WOCl3.
Fig. 2(a) and (b) shows the Ellingham diagram for Eqs. (3)–(15).
They summarize the evolution of standard free energy changes per
mol of chlorine, ꢀGr◦, as a function of temperature. The positive
values of ꢀGr◦, for all the reactions considered before, are indi-
cating that they are thermodynamically unfavourable. Moreover,
the equations that include the formation◦of different tungsten oxy-
chlorides are those with the lowest ꢀGr values. As can be seen in
Table 2, the values of the equilibrium constants show that, above
750 ◦C, the reactions given by Eqs. (12)–(15) would occur with
the formation of gaseous tungsten oxychlorides species and liquid
manganese chloride. The low equilibrium constant values are indi-
cating that the reactions will not attain the complete conversion.
Nevertheless, in our experimental setup where the chlorinations
were carried out in a continuous chlorine flow, the removal of
volatile products would assure to reach the complete conversion.
4. Results and discussion
4.1. Hubnerite characterization
Table 2
Structures of the starting sample, as well as the condensed
phases and chlorination reaction residues, were well characterized
by Scanning Electron Microscopy (SEM 515, Philips XL30 Electron-
ics Instruments) and X-ray diffraction (XRD, Philips PW1310/01),
with Ni filtered Cu K␣ radiation. Multielement analysis was car-
ried out by Energy Dispersive X-Ray Fluorescence Spectroscopy
(ED-XRF, Shimadzu, EDX 800 HS).
Before analyses, the sample was pulverized using a micronized
mill grinding. Analysis of powder samples was performed to
find the particle size and size distribution by laser diffraction
Equilibrium constant for Eqs. (10)–(15), as a function of temperature.
Eq.
K
750 ◦
C
850 ◦
C
950 ◦
C
(10)
(11)
(12)
(13)
(14)
(15)
1.403 × 10−23
3.349 × 10−39
4.246 × 10−4
4.745 × 10−5
2.484 × 10−4
0.03427
1.115 × 10−20
5.415 × 10−36
0.00155
2.949 × 10−18
2.516 × 10−33
0.00456
1.306 × 10−4
6.454 × 10−4
0.09259
3.047 × 10−4
0.00144
0.2125

34
G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
Fig. 4. Histogram of particle size distribution.
(Mastersizer Micro). A histogram of particle size distribution is
shown in Fig. 4. Through the volumetric percent analysis, it was
determined that about 50% corresponds to particles which granu-
lometry is below 38 ␮m (#400 ASTM). The remaining percentages
of the sample has the following size distribution: 20% between
38 and 75 ␮m (#400–#200 ASTM), 25% between 75 and 150 ␮m
(#200–#100 ASTM) and the 5% of the sample corresponds to par-
ticles which granulometry is greater than 150 ␮m (#100 ASTM).
Fig. 5 shows the diffractogram of the starting sample with the
reference pattern. By comparing the positions and intensities of
the diffraction peaks, it can be concluded that the starting sample
is composed by a simple phase of MnWO4(s) which crystallizes
in the monoclinic system (space group P2/c). Fig. 6 shows SEM
images of this sample. Fig. 6(a) shows the average particle sizes
mostly found in the fraction of sample investigated in this work.
Fig. 6. SEM images and ED-XRF analysis of initial sample.
No porosity can be observed in the sample surfaces, Fig. 6(b). Mul-
tielement analysis carried out by ED-XRF, shows that the sample is
composed by W Mn with a good concordance with the MnWO4(s)
stoichiometry.
4.2. Non-isothermal chlorination reaction
The starting temperature for the Hubnerite chlorination was
determined by non-isothermal thermogravimetric measurements.
The experimental conditions were:
a linear heating rate of
5.3 ◦C min−1; Cl2(g)–Ar(g) flow = 4 L h−1, m0 = 10 mg and PCl
=
2
35 kPa.
In Fig. 7 the ratio between the mass change and initial sample
mass as a function of temperature is shown. The reaction starts at
approximately 650 ◦C, where the sample commences to lose mass,
denoting the formation of gaseous products.
In order to confirm that the gaseous chlorine break the bounds
in the Hubnerite structure, the reaction was interrupted at 850 ◦C
and the solid residue was analysed by X-ray diffraction. The
diffractogram shows that only unreacted Hubnerite remains in the
crucible. Meanwhile, the manganese and tungsten have formed the
respective volatile chloride and oxychloride which were dragged by
the gas stream. This continuous mass loss is due to the removal of
two possible compounds, manganese as MnCl2(g) [21] which has a
vapour pressure of 2.77 × 10−4 atm. at 650 ◦C and tungsten as any
oxychloride with high enough vapour pressure [19].
Fig. 5. XRD analysis of starting sample.

G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
35
Fig. 7. Non-isothermal chlorination reaction.
4.3. Isothermal chlorination reactions
4.3.1. Chlorination in the fixed bed reactor
Fig. 8. X-ray diffractogram of chlorination product.
In order to identify the MnWO4 chlorination reaction prod-
ucts, experiments were conducted in fixed the bed reactor shown
in Fig. 1. The experimental conditions were: T = 950 ◦C; PCl2 (g) =
1 atm; Cl2(g) flow = 2 L h−1; m0 = 500 mg. Through the analysis of
the chlorination reaction products from the fixed bed reactor,
it was confirmed that the manganese content was removed as
MnCl2(g) (Rhombohedral SG: R-3 m), which then was hydrated to
form MnCl2·2H2O(s) (Monoclinic SG: C2/m). Moreover, the tung-
sten was removed by the formation and subsequent evaporation
of WO2Cl2(g) (Orthorhombic SG: Immm), according to the reaction
given by Eq. (15). This stoichiometry was observed in all tempera-
ture range studied. Manganese chlorides and tungsten oxychloride
were collected in the so called cold zone (zones B and C in the
scheme shown in Fig. 1).
The chlorination products were characterized by SEM and X-ray
diffraction. XRD pattern obtained is showing in Fig. 8 where a good
agreement is achieved with the corresponding reference patterns
for the tungsten IV oxychloride [22] also shown in the figure.
Fig. 9 shows SEM images of condensed tungsten oxychloride.
The microscopic morphology of WO2Cl2(s) consists of intergrowth
crystals of different sizes ranging between 10 and 50 ␮m, mainly
in the form of rectangular plates. Fig. 10 shows SEM images of
condensed MnCl2(s). It can be seen that the manganese chloride
morphology is so different, revealing high porosity agglomerates
constituted by interconnected particles; these characteristic corre-
sponds to the fact that the metallic chloride is very hygroscopic.
study of this phenomena is performed by increasing the gas flow
and determining its influence on the reaction rate [23]. For a specific
flow value, there are no more changes when increasing the gaseous
flow; thus, it can be assumed that the system is not starved of
gaseous reactant. In order to determine this condition, the influence
of Cl2(g)–Ar(g) flow was analysed.
Fig. 11(a) and (b) illustrates the effects of chlorine flow rate
on the chlorination of MnWO4 at 950 ◦C and 850 ◦C, respectively.
Experiments were performed at four different flow rates: 2, 4, 6,
and 8 L h−1, which is the maximum flow rate that the experimental
system can reach, while the partial pressure of chlorine was kept
constant at 35 kPa. The sample mass used was 2 mg. In Fig. 11(a),
the behaviour observed at 950 ◦C is shown. It can be appreciated
that, when the chlorine flow rate was increased, the rate of mass
loss observed to be faster. For instance, a conversion of 0.5 was
achieved in about 186, 172, 134 and 114 s for flow rates of 2, 4,
6 and 8 L h−1, respectively. Then, it can be concluded that chlori-
nation rate increases with increasing the flow rate, and thus, the
rate-controlling regime at 950 ◦C depends on the mass transfer in
the gaseous phase outside from the boundary layer.
Fig. 11(b) shows the effect of chlorine flow rate at 850 ◦C. A
conversion of 0.5 was achieved in approximately 907 s for 2 L h−1
,
indicating a significant decrease in the chlorination rate with tem-
perature reduction. Therefore, the intrinsic reactivity of the solid
is strongly affected by the temperature. When the flow rate was
increased from 4 to 8 L h−1, it was seen that the reaction rate did
not depend on the Cl2(g)–Ar(g) flow, because it was nearly con-
stant and the obtained values were located within the experimental
uncertainty.
As discussed by Hills [24], when variations in the flow rate of the
reacting gas do not produce significant changes in the reaction rate,
it may be concluded that reacting gas starvation is absent. Hence,
the experimental results shown in Fig. 11(b) enable to conclude
that the rate of chlorine supply above 4 L h−1 is high enough for
avoiding starvation effects in Hubnerite (MnWO4(s)) chlorination
at 850 ◦C and lower temperatures.
4.3.2. Thermogravimetric experiments
4.3.2.1. Analysis of mass transfer in the gaseous phase. In order to
determine the intrinsic kinetic parameters of a heterogeneous reac-
tion, the effects of mass transfer should be firstly disregarded. The
mass transfer phenomena in the gaseous phase that could affect
the kinetic regimen are two: “exhaustion of the reagent gas” and
“diffusion through the boundary layer surrounding the solid sam-
ple”.
4.3.2.1.1. Reagent gas starvation. In order to assess whether the
system is on exhaustion of the reagent gas condition (starvation),
the influence of the Cl2(g)–Ar(g) flow over the reaction rate was
analyzed. Gas starvation occurs when the flow is low. Then, the
overall rate of reaction is linearly dependent of the gas flow. The
The next step in the present analysis is to determine if the reac-
tion rate is in the order of the gaseous reactive diffusion process

36
G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
Fig. 9. SEM images of condensed WO2Cl2(s).
through the boundary layer. The procedure used to define if the
reaction rate is controlled by the diffusion through the boundary
layer is extensively explained in the following section.
account all these factors is the well known Ranz–Marshall correla-
tion (RzM) of the molar gaseous reactive flux [25]:
4.3.2.1.2. Diffusion through the boundary layer surrounding the
solid sample. Another process that can influence the chlorination
rate is the mass transfer through the boundary layer surrounding
the solid sample, even using a gas flow high enough for avoiding
the starvation of the reagent gas. As reported by several authors
[23,25], mass transfer into the gas boundary layer surrounding solid
samples may play an important role in controlling the reaction rate.
For a reaction such as that expressed by Eq. (1), the Cl2(g) molar
transfer rate from the gas stream to the solid surface (N) is given by
the following equation [23]:
DCl
· (2.0 + 0.6Re1/2Sc1/3) · (PCBl − PCSl ) · A
Ar
2
2
2
N =
(17)
R · T · l
where N refers to the molar flow of chlorine; D is the diffusion
coefficient for chlorine diffusing through argon; Re = U · l/v and
Sc = v/D are the Reynolds and Schmidt numbers, respectively (U
is the linear fluid velocity and v is the kinematic viscosity); PCBl
2
and PCSl are the Cl2(g) partial pressures in the gas stream and
2
solid surface, respectively; A is the sample external surface (cru-
cible area = 0.43 cm2); and l is the sample characteristic dimension
(crucible diameter = 0.74 cm).
hd
Table 3 shows the values of N for PCl = 35 kPa, Cl2(g)–Ar(g)
· (PCBl − PCSl
)
2
(16)
2
N = A ·
flow = 4 L h−1, and several temperatures, as calculated by Eq. (17).
2
R · T
The values of DCl
(binary diffusion coefficient) and v (kinematic
Ar
2
viscosity), were obtained from the Chapman/Enskog theory [23,26].
For these gasses in the temperature range between 750 ◦C
and 950 ◦C, the binary diffusion coefficient can be expressed by:
where A is the external area of the reactive solid; hd is the mass
transfer coefficient (boundary layer resistance); R is the universal
gas constant; T is the temperature in K; and PCBl and PCSl are the
DCl
= −0.35 + 1.8 × 10−3 T (cm2 s−1).
Ar
2
2
Cl2(g) concentrations in the gas stream and in 2the solid surface,
The values obtained by using Eq. (17) are only approximate,
respectively, expressed as partial pressures.
because this equation is valid for a pellet in a freely flowing gas
and not for a sample contained within a crucible. Hills [24] and
Hakvoort [27] concluded that the values given by Eq. (17) are more
than one order of magnitude higher than the mass transfer rates
corresponding to powders contained within crucibles.
The value of the mass transfer coefficient (hd) depends on many
factors, such as the relative rate between the particle and fluid, par-
ticle size and fluid properties. These factors have been correlated
for various types of solid–fluid contact. An equation that takes into

G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
37
Table 3
Theoretical and experimental Cl2(g) molar transfer rates.
Temperature (◦C)
Cl2(g) molar transfer rate (mol s−1
)
NT/NE
NT theoreticala
NE experimentalb
750
775
800
825
850
1.46 × 10−06
1.49 × 10−06
1.51 × 10−06
1.54 × 10−06
1.56 × 10−06
9.47 × 10−10
1.41 × 10−09
2.41 × 10−09
3.83 × 10−09
7.26 × 10−09
1542
1057
627
402
215
Conditions used for the calculation: l = 0.74 cm, A = 0.43 cm2, PCl = 35 kPa, and
2
Cl2(g)–Ar(g) flow = 4 L h−1
.
a
Calculated from Eq. (17).
b
Obtained from the lineal range of the thermogravimetric curves.
Using this approximation, the comparison between the the-
oretical rate values given by Eq. (17) with those obtained from
experimental measurements permits to determine whether the
reaction is under mass transfer control through the boundary layer
[28,29]. When the experimental rate is in the order of the calculated
value it can be concluded that the reaction is under mass transfer
control. On the opposite, when the experimental rate is two or more
orders of magnitude smaller than the calculated value, the reaction
may be in mixed control or in pure chemical control. In the case of
reaction under diffusion control, the interfacial reaction rate will be
so fast, that the reaction rate will be controlled by diffusion through
the boundary layer and then, the values of N will tend to the theo-
retical value. For these reason, the experimental values cannot be
greater than the calculated values.
As it can be seen in Table 3, the experimental rates at 850 ◦C and
lower temperatures are three or more orders of magnitude smaller
than the RzM calculated rates. These results are indicating that, at
temperatures below 850 ◦C, the convective mass transfer through
the boundary layer is not the rate-controlling step.
4.3.2.2. Analysis of mass transfer through the sample pores. In order
to analyze the influence of sample mass on the Hubnerite chlo-
rination reaction rate, chlorination reactions were performed
with different sample masses. The experimental conditions were:
Fig. 10. SEM Images of condensed MnCl2(s).
T = 850 ◦C; PCl = 35 kPa and Cl2(g)–Ar(g) flow = 4 L h−1. The corre-
2
sponding TG curves with the reaction degree as a function of time,
are depicted in Fig. 12. It can be seen that the reaction rate mea-
sured as d˛/dt, becomes faster as the sample mass decreases from
40 to 0.5 mg. The times to achieve 50% of the reaction (t˛ = 0.5) are:
4220, 3524, 2165, 1230, 771, 431 and 245 s for 40, 20, 10, 5, 2.5,
1 and 0.5 mg respectively. Nevertheless, this behaviour is differ-
ent to those reactions under chemical control where the reaction
Fig. 11. Effect of chlorine flow rate on the chlorination of MnWO4(s) at two tem-
peratures.
Fig. 12. Influence of sample mass on the Hubnerite chlorination reaction rate.

38
G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
Fig. 13. TG curves for the chlorination of 1 mg of Hubnerite at different tempera-
tures. Right inset plot shows linear fit obtained from Eq. (22).
Fig. 14. Effect of chlorine partial pressure on the Hubnerite chlorination rate.
rate measured as d˛/dt has a constant value, meaning that the time
required to reach ˛ = 0.5 is the same for all masses studied [28–30].
Therefore, the dependence of the reaction rate with the sample
mass found in the experiments is clearly indicating that, at 850 ◦C,
the reaction is not occurring under chemical control. At 750 ◦C the
same dependence with sample mass was observed. Possible pro-
cess controls compatible with the behaviour observed are: mass
transfer through the boundary layer surrounding the solid sample
(mass transfer in the gas phase) and pore diffusion coupled with
chemical reaction (mixed control). As it was previously demon-
strated that the mass transfer in the gas phase is not the reaction
control process, it can be concluded that the MnWO4(s) chlorina-
tion reactions, at 850 ◦C, proceed under mixed control. In order to
analyze the effect of temperature and chlorine partial pressure on
the Hubnerite chlorination rates, samples less than 2.5 mg were
used for further experiments, because they showed good repro-
ducibility.
The first term on the right-hand side of Eq. (22), the constant,
is a function of the reaction degree and chlorine partial pressure.
Therefore, if the chlorine partial pressure remains constant, and
the time to attain a certain reaction degree is determined as a func-
tion of temperature, Eq. (22) allows to obtain the activation energy
from the slope of the plot ln ti vs. T−1. The values of time necessary
to calculate ln t were obtained from the curves of ˛ vs. t at differ-
ent temperatures (Fig. 13). The graphical representation of these
results are shown in the inset plot in Fig. 13 for ˛ = 0.3, 0.5, 0.7 and
0.9. An apparent activation energy of 198 9 kJ mol−1 was deter-
mined from the slope of the linear regression. The constant value of
Ea, obtained as a function of the reaction degree within the temper-
ature range, suggests that the rate controlling step does not change
during the total reaction.
4.3.2.4. Analysis of the chlorine partial pressure effect. In order to
evaluate the effect of chlorine partial pressure on the Hubnerite
chlorination rate, experiments were performed at 850 ◦C and dif-
ferent Cl2(g) partial pressures. The experimental conditions were:
Cl2(g)–Ar(g) flow = 4 L h−1; and m0 = 2 mg.
4.3.2.3. Analysis of the temperature effects. Fig. 13 shows ˛–t curves
for Hubnerite (MnWO4(s)) chlorination between 750 and 850 ◦C.
The experimental conditions were: PCl = 35 kPa; Cl2(g)–Ar(g)
2
flow = 4 L h−1, and initial mass = 1 mg. It can be seen a strong depen-
dence of the chlorination rate with temperature: the reaction is
faster as temperature increases.
Fig. 14 shows ˛–t curves for Hubnerite chlorination in the PCl
2
range between 10 and 70 kPa. As it can be seen, the reaction
becomes faster as chlorine partial pressure increases from 10 to
35 kPa. The times to achieve 50% of the reaction (t˛ = 0.5) were: 3200,
2572, and 1940 s for 10, 20 and 28 kPa respectively. For pressures
higher than 35 kPa, the chlorination rate is unaffected. The time
to achieve 50% of the reaction (t˛ = 0.5) for 35, 50, and 70 kPa was
1255 s.
As expressed earlier, the rate of a heterogeneous solid–gas reac-
tion can be expressed by Eq. (4). The activation energy (Ea) included
in the Arrhenius type equation for K(T) can be calculated from
this expression, even if functions F(PCl2 ) and F(˛) are unknown, by
applying a “model-free” method [31,32]. The analysis of Ea at dif-
ferent conversion degrees allows to determine if there is a change
rate controlling mechanism during the reaction.
To calculate the reaction order of the reaction with respect to
PCl2 , the procedure applied was analogous to that used to deter-
mine Ea. Eq. (2) was integrated, F(PCl2 ) was assumed of the form
By rearranging Eq. (2) and integrating:
ꢀ
ꢀ
˛
t
i
i
d˛
of B · PCxl (where B is a constant and x is the reaction order with
= f (˛i) =
K(T) · F(P) · dt
0
(18)
(19)
2
F(˛)
0
respect to the chlorine partial pressure, for reactions where the
overall rate is controlled by chemical kinetic). For a constant tem-
perature and applying natural logarithms, the following expression
was obtained:
f (˛i) = K(T) · F(P) · ti
Assuming that K(T) follows an Arrhenius form, rearranging and
applying natural logarithms:
− ln ti = H(˛i) + x · ln PCl
(23)
f (˛i) = k0 · e−Ea/R·T · F(P) · ti
(20)
(21)
2
ꢁ
ꢂ
f (˛i)
F(P) · k0
Ea
where ti is the time at which a conversion degree ˛ is obtained at a
temperature T; and H(˛) is a function that depends on ˛ (since T is
kept constant). From the slope of the straight lines of the −ln ti vs.
ln ti = ln
+
RT
for a given ˛i:
PCl
plot for different conversion degrees. At 850 ◦C, it was found
(g)
2
ꢃ
ꢄ
Ea
R
1
T
that the chlorination rate was proportional to a potential function
of the partial pressure of chlorine whose exponent x is around 0.85.
ln ti = cte +
·
(22)

G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
39
Table 4
Apparent rate constant k and fittings correlation coefficient, obtained from fitting
experimental data with the sphere contractile kinetic model (1 − (1 − ˛)1/3 = k · t).
Temperature (◦C)
k (s−1
)
Correlation coefficient
850
825
800
775
750
5.09 × 10−4
2.58 × 10−4
1.49 × 10−4
8.35 × 10−5
5.14 × 10−5
0.99834
0.99932
0.99893
0.99975
0.99940
4.4. A proposed kinetic equation for Hubnerite chlorination
When there is not a solid residue formation as in the case
of chlorination reactions producing high vapour pressure chlo-
rides, the reactive particle decreases in size during the course of
the reaction until its total disappearance. From the experimental
thermogravimetric curves, the ˛t values were obtained. The exper-
imental conversion vs. time curves were fitted with the following
conversion function that describes a reaction proceeding according
to a topochemically contracting geometry [23,33,34]:
Fig. 15. Experimental conversion vs. time curves for temperatures between 750 ◦
C
and 850 ◦C together with calculated conversion vs. time curves according to Eq. (25).
In a future paper, the authors will present the application of a
mixed control kinetic model in order to obtain the kinetic intrinsic
parameters of the Hubnerite chlorination reaction.
f (˛) = 1 − (1 − ˛)1/n = k · t
(24)
5. Conclusions
where ˛ is the fractional conversion, and the value of n depends on
the solid geometry, being 1, 2, or 3 for slabs, cylinders, and spheres,
respectively.
By non-isothermal thermogravimetry, it was determined that
the Hubnerite chlorination reaction begins slowly at 650 ◦C and
increases significantly above 850 ◦C.
The formation of MnCl2 and WO2Cl2 like reaction products was
determined.
Mass transfer effects in the gaseous phase, like exhaustion of the
reagent gas and diffusion through the boundary layer surround-
ing the solid sample, were analysed. It was established that the
Hubnerite chlorination reaction is not affected neither by starva-
tion (exhaustion of the reagent gas) nor by diffusion through the
boundary layer (convective mass transfer) at temperatures lower
than 850 ◦C.
Mass transfer through the sample pores was also analysed.
It was determined that the Hubnerite chlorination reaction is
strongly affected by this phenomenon at temperatures below
850 ◦C.
Analyzing the correlation coefficient of the fittings of these mod-
els, it was found that the contractile sphere model achieved the
best fit. The values of the apparent rate constant k, obtained for
the different temperatures, are shown in Table 4 together with the
correlation coefficient of the fittings. As it can be seen, the con-
tractile sphere model gives a good description of the Hubnerite
chlorination reactions for temperature between 750 and 850 ◦C,
PCl = 35 kPa; Cl2(g)–Ar(g) flow = 4 L h−1, and initial mass = 1 mg.
2
The activation energy for the chlorination reaction can be
obtained from the slope of the plot ln k vs. T−1, and, as expected
by the good correlation coefficient obtained in the fitting, the cor-
responding value of 196 5 kJ mol−1 is in a good agreement with
the previous value obtained in Section 4.3.2.3.
From Eq. (24) the following relationship between conversion
and time was established:
From the values of the rate constant calculated at different
temperatures applying the contractile sphere model, an activation
energy of 196 5 kJ mol−1 was obtained.
ꢅ
ꢆ
−Ea(aparent)
f (˛) = 1 − (1 − ˛)1/3 = k0 · B · exp
· PCxl · t
(25)
2
RT
For the experiment at 850 ◦C, it was found that the chlorination
rate is proportional to a potential function of the partial pressure
of chlorine whose exponent is around 0.85, up to a chlorine partial
pressure of 35 kPa. Above 35 kPa no effect of the chlorine partial
pressure on the chlorination rate was detected.
where 1 − (1 − ˛)2/3 is the f(˛) function that describes the geometric
evolution of the reacting solid, Ea is the apparent activation energy,
PCl is the chlorine partial pressure, and x is the exponent of poten-
2
tial function of the chlorine partial pressure dependence. The k0 · B
product was determined by the intersection of the curve of the
linear fit from the plot: −ln ti vs. ln(PCl (g)) with the “y” axis.
The corresponding conversion degr2ee vs. time curves according
to Eq. (25) are plotted in Fig. 15 together with the experimental val-
ues. It shows that a good agreement exists between experimental
and calculated conversions for all temperatures.
In the preceding sections, the effects that can influence the chlo-
rination reaction kinetics were analysed. A global apparent rate
equation that includes all these parameters has been proposed for
the authors. Based on these results, it is concluded that Hubnerite
chlorination kinetics is under mixed control in a temperature range
of 750–850 ◦C.
It was established that at temperatures below 850 ◦C the
Hubnerite chlorination reaction is under mixed control. For tem-
peratures above 850 ◦C the chlorination reaction is under mass
transfers control.
The experimental thermogravimetric curves (˛ vs. t) were used
to apply the contractile geometry kinetic models and it was found
that the contractile sphere model achieved the best fit.
Finally, a global kinetic expression for Hubnerite chlorination
reaction under mixed control was obtained.
Acknowledgements
By differentiating Eq. (25) with respect to time and rearranging,
the reaction rate expression also can be presented as follows:
The authors would like to thank the Agencia Nacional de Pro-
moción Científica y Tecnológica (ANPCyT), Consejo Nacional de
Investigaciones Científicas y Técnicas (CONICET), and Universidad
Nacional del Comahue (UNComa) for the financial support of this
work.
ꢅ
ꢆ
Ea(aparent)
d˛
dt
= (1 − ˛)2/3 · k0 · B · exp
−
· PCxl
(26)
2
R · T

40
G.G. Fouga et al. / Thermochimica Acta 536 (2012) 30–40
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