CONGRESO CONAMET/SAM 2004
A THERMOGRAVIMETRIC STUDY OF COPPER CHLORINATION
G. De Micco(1,2), A.E. Bohé(3,4)and D.M. Pasquevich(1,3)
(1)
Comisión Nacional de Energía Atómica (C.N.E.A.), Bustillo 9500, (8400) San Carlos de Bariloche, Argentina,
[email protected]
(2)
Universidad Nacional de Cuyo, Instituto Balseiro, Bustillo 9500, (8400) San Carlos de Bariloche, Argentina.
(3)
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina.
(4)
Universidad Nacional del Comahue, Centro Regional Universitario Bariloche, (8400) San Carlos de Bariloche,
Argentina.
ABSTRACT
Metals and alloys chlorination is being studied as part of the HALOX process which belongs to CNEA. The aim
of this project is to develop a possible way of conditioning for the spent nuclear fuels from research reactors.
The chlorination of metallic copper under isothermal conditions was studied by thermogravimetry in the range of
temperatures between 100 and 800°C. The effects of gas flow rate was analyzed. The starting temperature for the
reaction of copper with chlorine was determined at about 150°C by non-isothermal measurements. The products
of reaction at the different temperatures were analyzed by X-ray diffraction and their physicochemical behavior
was studied in chlorine and argon atmospheres.
Keywords: copper chlorination, thermogravimetric study
1. INTRODUCTION
2. THEORY AND CALCULATIONS
Metal chlorination processes have their technological
application in treating metal ores and recycling
valuable materials [1, 2]. They are also important
when studying metal surface corrosion rates, and to
obtain anhydrous chlorides [3]. With this propose,
copper chlorination has been studied by other authors,
the chlorination products obtained were CuCl and
CuCl2 , however the kinetic and mechanism of the
reaction is not well established yet for many reaction
conditions. The surface reaction at low chlorine
exposure was deeply studied [4, 5]
The Ellingham diagram Fig. 1 corresponding to
copper species which take part in the chlorination is
shown below. The ΔG° values were calculated by mol
of chlorine, this means that it is the minority reactive,
as for example, in the first stages of the reaction when
there is excess copper to react with the incoming
chlorine flow, or when starvation or diffusion are the
controlling processes.
The objective of this work is to analyze the kinetics of
the copper chlorination reactions at different
temperatures by TG measurements, to identify the
reaction products and the effect of chlorine flow. The
volatilization of copper chlorides has been also
analyzed for temperatures over 400°C in chlorine and
argon atmospheres.
Thermodynamic analysis show that CuCl is the most
likely product to be obtained; whether in liquid or
solid state. However, formation of CuCl2 from
metallic copper can not be excluded since it also has a
negative value of ΔG°. The reaction of CuCl with Cl2
to give CuCl2 is straight forward for temperatures
below 400°C, at higher temperatures the
decomposition of CuCl2 is predictable.
CONGRESO CONAMET/SAM 2004
4. RESULTS AND DISCUSION
200
CuCl = CuCl(g)
2CuCl+Cl2(g)=2CuCl2
150
4.1 Non-Isothermal TG
100
ΔG (KJ)
por mol de Cl2
50
2Cu+Cl2(g)=2CuCl(g)
0
-50
Cu+Cl2(g)=CuCl2
-100
-150
-200
2Cu+Cl2(g)=2CuCl(l)
-250
2Cu+Cl2(g)=2CuCl
-300
0
200
400
600
800
For general characterization of the copper chlorinating
process, a non isothermal TG measurement was
carried out. The mass change vs temperature curve is
shown in Fig. 2 the heating rate was 5°C/s. As it can
be seen there, the reaction starts at about 150°C with a
mass gain followed by a mass loss starting at 500°C.
The mass gain is due to the formation of condensed
copper chlorides, whereas the mass loss is due to
CuCl2 decomposition followed by CuCl volatilization
as will be discussed below.
1000
Temperatura (°C)
120
Figure 1: Ellingham Diagram
As it can be seen in the diagram, for temperatures over
about 750°C the formation of gaseous CuCl will
compete with the formation of condensed products.
Finally volatilization of solid CuCl although having a
positive value of ΔG° for all the range of temperatures
has to be taken into account in our flowing system.
The partial pressure of CuCl(g) is about 10-6 atm for
temperatures over 750°C [6] which suggests that the
mass change will be detectable, and the flux
conditions may enhance the volatilization process.
The melting and boiling points for this species are
presented on Table I
Table I. Physical Properties [7]
Compound Melting Point (°C)
Boiling Point(°C)
CuCl
422
1366
CuCl2
498
Forms CuCl at 993
3. EXPERIMENTAL PROCEDURE
The TG measurements were performed on a
thermogravimetric analyzer based on a Cahn 2000
electrobalance adapted to work with gaseous chlorine.
The system is described elsewhere [8]. The samples
consisted in sheets of commercial copper (99.9% of
purity) of about 20mg. Isothermal and non-isothermal
measurements were made with a partial pressure of
chlorine of 33.3kPa in a Cl2-Ar flow which varied
between 2.1 and 7.9lt h-1.
Since copper chlorides are hygroscopic, samples were
isolated and handled inside a glove box before X-ray
diffraction data collection.
% Mass Change
(According to initial mass of copper)
CuCl2
100
CuCl + ½ Cl2=> CuCl2
CuCl2(l) => CuCl + ½ Cl2
80
60
Cu + ½ Cl2=> CuCl
40
CuCl(l) => CuCl(g)
20
Cu
0
0
100
200
300
400
500
600
Temperature °C
Figure 2: Non-Isothermal TG curve
4.2 Effect of flow rate
The TG curves for chlorination reaction at 400°C and
Cl2-Ar fluxes between 2.1 and 7.9 l h-1 were obtained
for constant partial pressure of chlorine of 33.3 kPa.
The reaction rate was found to increases when
increasing the gaseous flow, this indicates that the
metal is highly reactive for temperatures above 400°C
and the process that is rate determining is starvation.
For temperatures below 200°C, the rate is maximum
at the beginning of the reaction; however it is at least
one order of magnitude slower than for 400°C what is
indicating that gaseous starvation is no more the
controlling process.
4.3 Isothermal TG
The isothermal chlorination curves were obtained for
temperatures between 100 to 825°C.
In general the chlorination reactions start with a mass
gain which corresponds to the formation of condensed
copper chlorides, after that, for temperatures over
500°C, the mass continuously decrease until complete
volatilization whereas for the lower temperatures the
mass tends to a constant value. For temperatures
below 500°C all the reaction proceeds without mass
loss, the amount of mass gained increases with
temperature up to about 450°C. However, the mass
CONGRESO CONAMET/SAM 2004
gains in the isothermal reactions are always lower than
the mass gain in the non-isothermal case.
80
% Mass Change
(according to initial mass of copper)
70
500°C
400°C
60
300°C
50
In Fig. 4 we show typical TG curves one
corresponding to volatilization in chlorine and the
other in argon obtained for 500°C. The argon
volatilization TG curves clearly show two sections of
different slope during the mass loss. If we assume that
the first corresponds to CuCl volatilization plus CuCl2
decomposition, and the second only to CuCl
volatilization the amounts of CuCl2 and CuCl can be
calculated.
40
700°C
30
600°C
20
200°C
10
100°C
0
0
2500
5000
7500
10000
Time (s)
Figure 3: Isothermal TG curves
4.3.1 High Temperatures Analysis
The isothermal TG curves are shown in Fig. 3. The
mass balances corresponding to the different reactions
are shown in table II
Table II: Mass Changes
Reaction
Cu + 12 Cl 2 → CuCl
Cu + Cl 2 → CuCl 2
% Mass Gain (According to
initial mass of copper)
55.8
111.6
Form the TG curves obtained in chlorine, the rate of
volatilization was estimated. With that value, we
calculated the mass loss due to volatilization during
simultaneous volatilization and decomposition
processes i.e. the first part of the curve corresponding
to the sharper slope; this quantity was then discounted
of the total mass loss in the same part or the curve to
obtain the amount of chlorine that belongs to CuCl2.
The amount of CuCl in the maximum of the curve is
equal to the total mass loss due to volatilization, (i.e.
the mass loss from the change of slope point till the
end of the curve plus the mass loss due to
volatilization during simultaneous decomposition and
volatilization calculated before) minus the amount of
CuCl that comes from CuCl2 decomposition already
obtained. Finally, the amount of CuCl(g) formed
during the mass gain arises from the difference
between the initial mass of copper and the total mass
of copper contained in the copper chlorides in the
maximum of the curve just calculated.
The procedure described is presented in equations (1)
to ( 4):
(1)
G = A+ B −C
The TG curves obtained for temperatures in the range
between 275° and 625°C show that the mass gain is
higher than that needed for complete reaction to form
CuCl, the difference corresponds to CuCl2 formation.
To evidence the amount of CuCl2 formed during the
reaction at different temperatures; experiments were
carried out in which the chlorine flow has been
interrupted after the mass gain. The objective of doing
this was to distinguish the rates of decomposition and
volatilization by enhancing the decomposition
reaction. In chlorine atmosphere, although both
processes occur, the change in the slope of the TG
curve is almost imperceptible for the majority of the
temperatures. The rate of mass loss can be linear fitted
and the corresponding slope gives the CuCl
volatilization rate, as is this process that predominates
throughout the time interval.
When the flux of chlorine is switched to argon the
sample continues gaining mass and after some
seconds starts to lose mass. By properly adjusting the
time of chlorine cut off it is possible to obtain similar
percentages of mass gain to those in chlorine
atmosphere with a difference no higher than +/-5%.
G: mass gain from the beginning of the reaction to the
maximum of the TG curve.
A: mass of chlorine that forms CuCl2 in the maximum
of the TG curve.
B: mass of chlorine that forms CuCl in the maximum
of the TG curve.
C: mass of copper that leaves the sample as CuCl(g),
during the mass gain.
A = 2 (S1 − VCi t1 )
B=
(S 2 + VCl t1 ) PCl
C = M0 −
(2 )
2
PCuCl
−A
(S 2 + VCl t1 ) PCu
PCuCl
M0: mass of the copper sample
Px: molecular or atomic weight of specie x
(3)
(4 )
CONGRESO CONAMET/SAM 2004
S1: mass change form the maximum of the TG curve
to the change of slope point
S2: mass change form the change of slope point to the
en of the TG curve
t1: interval of time from the maximum of the TG curve
to the change of slope point
VCl:
volatilization
atmosphere.
rate
of
CuCl
in
chlorine
To determined the point where the slope changes for
each temperature, the last part of the curves
corresponding to volatilization in argon was fitted and
an error band of 0.1% was considered; the
corresponding slope change point was then
determined by the first point belonging to the first part
of curve (the mass loss due to decomposition plus
volatilization) that reaches the error band. The curves
are shown schematically in Fig. 4 and the results for
temperatures between 400 to 750°C are shown in
Table III as percentage of initial mass of copper that
forms CuCl, CuCl2 and CuCl(g) respectively.
beginning there is few CuCl2 and the time interval is
bigger.
During the mass gain the volatilization conditions are
similar to those of argon atmosphere volatilization,
because chlorine in the reaction surface is being
exhausted by fast reaction with metallic copper, (as
was discussed on section 4.2.). On the other hand,
during decomposition, the volatilization conditions are
similar to volatilization in chlorine atmosphere,
because there is chlorine in the reaction surface arising
from the CuCl2 decomposition.
As can be seen in Table III we obtained that the
amount of CuCl2 is maximum near 450°C and over
that temperature decreases with increasing
temperature.
Table III: Percentages of initial mass of copper that
form CuCl, CuCl2 and CuCl(g) at different
temperatures
Temperature (°C)
Comp.
400 450 500 550 600 650 700 750
CuCl2 31.0 54.1 50.1 27.8 20.4 22.4 13.4 8.7
CuCl2 => CuCl + 1/2Cl2
CuCl => CuCl(g)
80
Cl2 Atmosphere
CuCl2 => CuCl + 1/2Cl2
CuCl => CuCl(g)
70
S1
CuCl => CuCl(g) ( VCl )
60
Change of slope
t1
50
CuCl(g) 1.0 2.4 3.4 1.2 2.0 7.8 10.4 13.7
CuCl => CuCl(g)
S2
500°C
0
500
1000
Ar Atmosphere
1500
2000
2500
3000
3500
Time (s)
Figure 4: TG Curves for argon and chlorine
volatilization.
For temperatures below 500°C the samples in chlorine
do not lose mass. This is in accordance with CuCl2
melting point (498°C) suggesting that solid CuCl2
retards CuCl volatilization. In this conditions the mass
loss due to volatilization is entirely compensated with
the mass gain arising from the slow formation of
CuCl2 and as a result the mass remains almost
constant. However some volatilization do occur for
temperatures over 350°C in a flowing system; this was
confirmed by us in other experiments where
condensed copper chlorides were found in the wall of
the reactor, away from the chlorination sample, which
could only be due to gaseous phase transport.
For this reason, the volatilization during
decomposition was neglected for temperatures below
450°C because there is an important amount of solid
CuCl2 over the CuCl and the interval of time involved
is small. However, it can not be neglected during the
mass gain of the same reaction because at the
The formation of CuCl2 at 600°C, although being
thermodynamically unlikely, was confirmed by XRD
of a sample obtained interrupting the chlorination
reaction during the mass gain by quenching with
liquid air. The profile is shown in Fig. 5. The amount
of CuCl is minimum at 450°C then increases with
temperature, and remains almost constant above
550°C. Finally CuCl(g) increases with temperature,
specially over 600°C.
∅
100
T:600°C
Reaction interrupted
during mass gain
80
Relative Intensity
% Mass Change
(According to initial mass of copper)
CuCl 39.8 27.4 28.8 41.2 44.5 40.3 43.3 43.8
60
* CuCl
∅ CuCl2
40
*∅
20
∅
∅
*
*
0
0
20
40
60
80
100
2θ
Figure 5: XRD profile of the chlorination products at
600°C.
CONGRESO CONAMET/SAM 2004
4.3.1.1 Volatilization Process
4.3.2 Low Temperature Analysis
The rate of volatilization of the copper chlorides in
chlorine atmosphere was determined for the range
between 500 and 825°C. The TG curves show a
practically constant slope during the whole
volatilization process. At the beginning of the mass
loss there is a liquid containing CuCl and CuCl2 and
decomposition of CuCl2 occur together with
volatilization of CuCl, however the velocities of this
to process are so similar that the change in the slope is
almost imperceptible for the majority of the
temperatures. The change in the slope is well
evidenced for the non-isothermal measurement (Fig 2)
in those conditions the amount of CuCl2 formed
reaches a maximum value indicating that almost all
copper has formed CuCl2 in this way the process of
decomposition predominates in first place followed by
volatilization leading to the slope change.
An activation energy of 109+/-3kJ/mol for the
volatilization process in chlorine was calculated. The
experimental data of lnv vs. 1/T are shown in Fig 6a.
-2
For the lower temperatures up to about 275°C the
reaction rate reaches a near cero value at intermediate
stages and although there is still metallic copper to
react, the reaction practically stops. The percentages
of mass gain are below the value that corresponds to
complete formation of CuCl (table II). The products of
low temperature reactions consist in a brownish
powder layer which can be easily removed and below
this, there is another white layer in contact with
unreacted metallic copper. The products for a
chlorination at 200°C were analyzed by XRD and the
results are shown on Fig. 7. We obtained that the
brownish powder consist of CuCl and CuCl2, but only
CuCl was detected in the surface of reaction which
was partially covered with the white layer. This is
indicating that in the copper surface only CuCl is
being formed and CuCl2 is being produced by further
reaction of CuCl with chlorine. The results suggests
that as was observed in the case of CuCl volatilization
that seems to be prevented by the presence of solid
CuCl2; in the case of the lower temperature
chlorination reactions CuCl2 may be responsible for
the passivity observed at low degrees of reaction.
Volatilization in Argon Atmosphere
-3
-4
200°C
-5
-6
-7
-9
Ea: 97.5 +/- 3 kJ mol
reacted powder
80
b
& Cu
* CuCl
∅ CuCl2.2H2O
60
-10
0.0009
-2
0.0010
0.0011
0.0012
0.0013
0.0014
0.0015
Volatilization in Chlorine Atmosphere
-3
-4
-5
-6
-7
∗
-1
-1
a
Ea: 109 +/- 3 kJ mol
-8
0.0009
0.0010
0.0011
0.0012
0.0013
-1
1/T K
Figure 6: Activation Energies
We also study the volatilization of CuCl in argon
atmosphere. The rate for each temperature was
calculated from the slope of the curves obtained
switching the flux of chlorine by argon during the
mass gain. For temperatures between 450 to 650°C the
first portion of the mass loss corresponds to CuCl2
decomposition plus volatilization (Fig. 4) for this
reason, the rate was calculated with the last part of the
curves. The value of the activation energy obtained
was 97 +/- 3 kJ/mol, and the lnv vs. 1/T plot is shown
in Fig. 6b.
The enthalpy of vaporization of cuprous chloride for
temperatures between 450 to 800°C is 219 kJ/mol for
the monomer (CuCl) and 50 kJ/mol for the trimmer
(Cu3Cl3) [6]. This values which are completely
different to the activation energy that we obtained,
mean that the volatilization process we measured is
not being controlled by diffusion in the gaseous phase.
40
Relative Intensity
lnv
-8
∗
100
∅
∅ ∅
∅
∅ ∅ ∅ ∅ ∅∅
∅
20
0
100 0
20
80
9
60
6
40
3
20
0
∗
40
∅
60
80
&
100
reacted sheet
∗
& ∗
20
40
20
40
&
& ∗
60
80
60
80
0
0
100
2θ
Figure 7: XRD profiles of the residue of a 200°C
chlorination.
5. CONCLUSIONS
Copper chlorination reaction has been measured in the
range between 100 and 800°C, the starting
temperature for the reaction was determined at 150°C.
The rate controlling process for the formation of
chlorides was found to be starvation under our
experimental conditions at least over 400°C.
Form the analysis of the TG curves obtained above
400°C in chlorine and argon atmospheres, the relative
CONGRESO CONAMET/SAM 2004
quantities of the different chlorination products i. e.
condensed and gaseous CuCl and condensed CuCl2
were estimated. We found that the results are in
accordance with thermodynamics predictions,
although the formation of CuCl2 at high temperatures
(over 500°C) is not obvious.
The volatilization process was analyzed for the higher
temperatures in chlorine and argon atmospheres, the
activation energies obtained in both cases was around
100kJ/mol, being this value half of the vaporization
enthalpy for CuCl in the same range of temperatures.
For the lower we obtained that the metal reacts to
form CuCl and the CuCl2 was only found above this
CuCl, indicating that it is being form by further
chlorination of CuCl and not directly from metallic
copper.
6. REFERENCES
[1] N. Kanari, I. Gaballah, E. Allain, Thermochemica
Acta, 373, 2001, pp. 75-93
[2] A. E. Bohé, J. J. Andrade Gamboa, D. M.
Pasquevich, Materials Science and Technology, 13,
1997, pp.865-871
[3] J. P. Remeika and B. Battlogg, Mat. Res. Bull., 15,
1980, pp.1179-1182.
[4] W. Sesselmann and T. J. Chuang, Surface Science,
176, 1-2, 1986, pp. 32-66.
[5] W. Sesselmann and T. J. Chuang, Surface Science,
176, 1-2, 1986, pp. 67-90.
[6] A. Roine. Outokumpu HSC chemistry for
Windows, 93001-ORGT version 2.0 Outokumpu
Research Oy Information Service, (1994).HSC
[7] Robert H. Perry, “Perry Manual del Ingeniero
Químico”, Ed. Mc Graw-Hill, sixths edition, 1992.
[8] D. M. Pasquevich, A. M. Caneiro, Thermochimica
Acta, 156 (2) 1989, pp. 275-283