Thermal and Carbothermic Decomposition of Na2CO3 and
Li2CO3
JONG-WAN KIM and HAE-GEON LEE
In order to elucidate the decomposition mechanism of Na2CO3 and Li2CO3 in mold-powder systems
employed in the continuous casting of steel, decompositions of Na2CO3 and Li2CO3 were investigated
using thermogravimetric (TG) and differential scanning calorimetric (DSC) methods at temperatures
up to 1200 ⬚C, under a flow of argon gas. For the case of pure Na2CO3, the thermal decomposition
started from its melting point and continued as the temperature was increased, but at a very slow
rate. For Li2CO3, however, the decomposition occurred at much faster rates than that for Na2CO3.
When carbon black was added to the carbonate particles, the decomposition rates of both Na2CO3
and Li2CO3 were significantly enhanced. From mass-balanced calulations and X-ray diffraction (XRD)
analyses of the reaction products, it is concluded that decompositions of Na2CO3 and Li2CO3 with
carbon black take place according to the respective reactions of Na2CO3 (l) ⫹ 2C (s) ⫽ 2Na (g) ⫹
3CO (g) and Li2CO3 (l) ⫹ C (s) ⫽ Li2O (s) ⫹ 2CO (g). It was found that liquid droplets of Na2CO3
were initially isolated due to carbon particles surrounding them, but, as the carbon particles were
consumed, the liquid droplets were gradually agglomerated. This effected a reduction of the total
surface area of the carbonate, resulting in a dependence of the decomposition rate on the amount of
carbon black. For the case of Li2CO3, on the other hand, hardly any agglomeration occurred up to
the completion of decomposition, and, hence, the rate was almost independent of the amount of
carbon black mixed. The apparent activation energies for the decomposition of Na2CO3 and Li2CO3
with carbon black were found to be similar and were estimated to be 180 to 223 kJ mole⫺1.
I. INTRODUCTION
IN the continuous casting of steel, mold powder plays
an important role in the lubrication and control of heat
transfer between the cast strand and the mold, the insulation
of the surface of the molten steel pool in the mold, the
protection of liquid steel against reoxidation, and the absorption of inclusions. Tight control of the melting behavior
and physico-chemical properties of the mold powder is,
therefore, essential for the production of quality cast products. Recently, it has become common practice to add 1 to
20 wt pct Na2O to control the melting behavior and viscosity
of the mold powder.[1] Li2O is also added (particularly in
high-speed casting[2]), because of its ability to reduce both
the viscosity and solidification temperature. In general, these
oxides are added in the form of Na2CO3 and Li2CO3, which
are subsequently decomposed during the melting of the mold
powder. Therefore, it is important to clarify the decomposition and melting mechanisms of these carbonates in moldpowder mixtures. It was reported that an increase in the
melting rate of mold powder by the addition of carbonates
is due to the increase of the thermal conductivity of the
mold powder by gases generated during the decomposition
of the carbonates.[3] In the present study, the kinetics of
the thermal decomposition of Na2CO3 and Li2CO3 and the
carbothermic decomposition of these carbonates with carbon
black have been investigated by employing thermogravimetric (TG) and differential manning calorimetric (DSC)
techniques.
II. EXPERIMENTAL
The chemical compositions of the materials used in the
present study are given in Table I. The mean particle sizes
of Na2CO3, Li2CO3, and the carbon black were 200, 20,
and 0.11 ␮m in diameter, respectively. A thermal analyzer
equipped with both TG and DSC functions was employed.
The analyzer enabled simultaneous analysis of the TG and
DSC measurements with a detection accuracy of 1 ␮g.
Experiments on the decomposition of pure Na2CO3, Li2CO3,
and carbonates with carbon black were conducted in an inert
atmosphere under a flow of argon gas. The gaseous flow
rate was maintained at 5 ⫻ 10⫺5 m3 min⫺1. The sample mass
was in the range of 15 to 25 mg, depending on experimental
conditions, and Al2O3 powder was used as a reference material. For the decomposition of pure carbonates, both the
samples and references were contained in a platinum crucible, whereas the carbonates mixed with carbon black were
contained in an alumina crucible. Both crucibles had a 5
mm i.d. and 6 mm height. The samples were heated to the
desired temperature at a heating rate of 10 K min⫺1.
III. RESULTS
JONG-WAN KIM, Researcher, Technical Research Laboratories,
POSCO, Pohang, Gyungbuk, 790-785 Korea, is Research Assistant, Department of Materials Science and Engineering, Pohang University of Science
and Technology. HAE-GEON LEE, Professor, is with the Department of
Materials Science and Engineering, Pohang University of Science and
Technology, Pohang 790-784, Korea.
Manuscript submitted January 5, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS B
A. Decomposition of Pure Na2CO3 and Li2CO3
The results of the thermal decomposition of both pure
Na2CO3 and Li2CO3 are shown in Figure 1. The two sharp
endothermic peaks shown in the figure correspond to the
melting temperatures of Na2CO3 and Li2CO3, i.e., 850 ⬚C
VOLUME 32B, FEBRUARY 2001—17
Fig. 1—TG-DSC curves for the pure Na2CO3 and Li2CO3 (heating rate:
10 K min⫺1).
Table I Chemical Composition of the Materials Used in
This Study
Na2CO3
Li2CO3
Carbon black
Na2O
Li2O
Total C
56
—
—
—
38.8
—
11.1
16
99.9
(Mass Percent)
CO2
40.7
58.6
—
Fig. 2—Decomposition path of Li2CO3 superimposed on the Li2CO3-Li2O
phase diagram.
with increasing temperature. It is also seen that the decomposition terminates at 1076 ⬚C, and the total mass loss is 56.3
pct. No further mass loss was observed, even though the
temperature was increased to 1100 ⬚C. If Li2CO3 decomposes
according to the reaction given in Eq. [3], the theoretical
mass loss would be 59.5 pct.
Li2CO3 (l) ⫽ Li2O (s) ⫹ CO2 (g)
and 720 ⬚C, respectively.[4] Once the carbonates have been
melted, the mass of the sample begins to decrease. It is also
seen in the figure that the very weak endothermic heat flow
after melting reflects the slow rate of the thermal decomposition of both carbonates. It is apparent that the Li2CO3 mass
loss is more appreciable when compared with that of
Na2CO3. Up to 1200 ⬚C, the weight loss of Na2CO3 is 26
pct (3.9 mg), and the reaction is not completed. The decomposition of Na2CO3 has been reported to occur in two consecutive steps:[5,6]
Na2CO3 (l) ⫽ Na2O (s) ⫹ CO2 (g)
Na2O (l) ⫽ 2Na (g) ⫹ –21 O2 (g)
[1]
[2]
Motzfeldt reported that the Na2O content in Na2CO3 (l)
was on the order of 1 mass pct, and the overall rate of
decomposition was extremely slow.[5] In the present study,
X-ray diffraction (XRD) analyses showed that sample
remaining after the experiment was all Na2CO3, and Na2O
was not detected. The carbon content in the sample before
and after the decomposition experiment was the same, confirming that the sample remaining after the experiment was
all Na2CO3. Considering the previous results, it can, thus,
be concluded that the thermal decomposition of Na2CO3
occurs in two steps, as given by Eqs. [1] and [2], and that
reaction [1] is extremely slow and, thus, controls the overall
decomposition reaction.
On the other hand, the decomposition rate of Li2CO3 is,
as seen in Figure 1, faster than that of Na2CO3 and increases
18—VOLUME 32B, FEBRUARY 2001
[3]
The remaining materials after the decomposition experiment
are Li2O and LiOH ⭈ H2O, as identified by XRD analysis.
The hydrate is thought to have formed after the experiment
from a reaction with moisture in the air. Therefore, it can
be concluded that Li2CO3 decomposes into Li2O and CO2
(Eq. [3]) and that no further decomposition of Li2O into Li
(g) and O2 (g) occurs. As the thermal decomposition proceeds, the system will be composed of Li2CO3 and Li2O, in
which the relative proportion of Li2O will continue to
increase. Figure 2 shows the path of compositional change
of the system during the thermal decomposition of Li2CO3,
plotted on the Li2CO3-Li2O phase diagram (open circles).[7]
It is seen that, at the early stage of decomposition, the mixture
of Li2CO3-Li2O forms a homogeneous liquid phase. Once
the amount of Li2O reaches 26 mol pct, the liquid solution
becomes saturated with Li2O at the corresponding temperature. On further processing, the proportion of solid Li2O
continues to increase until all the Li2CO3 has been completely decomposed. In Figure 2, as all the vertical distances
between neighboring points are set to be the same, meaning
that the temperature interval (i.e., the time interval) between
the neighboring points is the same due to the constant rate
of the temperature increase (10 K min⫺1), then the horizontal
distance between neighboring points represents the relative
amount of Li2CO3 decomposed between the temperature
(i.e., time) interval. In other words, the horizontal distance
between the neighboring points represents the average rate of
decomposition of Li2CO3 at the corresponding temperature
range. It is clearly seen that the horizontal distance between
the neighboring points increases with temperature, which
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 3—TG-DSC curves for carbothermic decomposition of Na2CO3 and
Li2CO3 (CB ⫽ carbon black, and heating rate: 10 K min⫺1).
Fig. 4—Weight loss of Na2CO3-carbon black mixture for the different mol
ratios. (Heating rate: 10 K min⫺1) (The arrows indicate the points where
carbothermic decomposition is complete.)
implies that the decomposition rate increases with temperature. One might argue that solid Li2O formed at the later
stage of decomposition would hinder the decomposition by
blocking the surface. However, this is not seen from the
figure, probably due to the overwhelming effect of temperature on the rate enhancement.
B. Decomposition of Na2CO3 and Li2CO3 Mixed with
Carbon Black
Due to its fine size, carbon black is most effective in
delaying the melting of mold powder and, hence, is commonly used in producing mold-powder mixtures. Figure 3
shows the results of TG-DSC analysis for carbothermic
decomposition of both Na2CO3 and Li2CO3 with carbon
black. The mixing ratio of carbonate to carbon black was
80:20 mass pct for both carbonates, with the initial total
mass being 24 mg. It is seen that the system (carbonate ⫹
carbon black) begins to lose its mass at around the melting
point of the corresponding carbonate, and that the rate of
mass loss increases with temperature. Eventually, the mass
loss comes to an end for each carbonate. For Na2CO3, it is
seen that its behavior of mass loss in coexistence with carbon
black is notably different from that of Na2CO3 without carbon black. While the mass loss due to thermal decomposition
of pure Na2CO3 is small (26 pct, or 3.9 mg), as seen in
Figure 1, the addition of carbon results in an almost complete
mass loss of the system (92 pct, or 22 mg), as seen in Figure
3. On the other hand, for the mixture of Li2CO3 and carbon
black, the system experiences a relatively small mass loss
of only 60 pct (14.4 mg). The heat-flow curves given in
Figure 3 indicate that the carbothermic decompositions of
these carbonates are endothermic. The endothermic peak
shown in each heat-flow curve represents the melting point
of the corresponding carbonate. It is noted that these peaks
are not as sharp as those shown in Figure 1, which appears
to be due to concurrent occurrence of melting and carbothermic decomposition.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 5—Weight loss of Li2CO3-carbon black mixture for the different mol
ratios. (Heating rate: 10 K min⫺1) (The arrows indicate the points where
carbothermic decomposition is complete.)
In order to clarify the effect of carbon black on the decomposition of the carbonates, an investigation was carried out with a
number of different mixing ratios of carbonate to carbon black,
and the results are summarized in Figures 4 and 5. It is seen
that the decomposition of Na2CO3 is greatly affected by the
mixing ratio (Figure 4). The higher the proportion of carbon
black is, the higher the rate of decomposition. The addition
of carbon black to Li2CO3 is also effective in increasing the
decomposition rate (Figure 5). However, when the mixing ratio
of carbon black to Li2CO3 is higher than 0.5:1, the rate is seen
to be hardly affected by the mixing ratio. Another interesting
VOLUME 32B, FEBRUARY 2001—19
Fig. 6—SEM micrographs of mixture of carbonates with carbon black: (a) Na2CO3 ⫹ carbon black (1:1 mol) and (b) Li2CO3 ⫹ carbon black (1:1 mol).
feature that is seen in Figures 4 and 5 is that the behavior of
mass loss may be classified into two different types: one type
showing no change in mass after a rapid mass loss, and the
other showing a gradual change in mass even after a rapid mass
loss. The arrows shown in Figures 4 and 5 indicate the points
at which the continual rapid mass loss terminates. In order to
understand how the carbon black affects the rate of decomposition of the carbonates, a micrographic analysis was carried out,
as seen in Figures 6 and 7. It is seen in Figure 6 that Na2CO3
particles are about 10 times larger in size (diameter) than Li2CO3
particles, and that carbon black particles are present by being
attached on the carbonate surface. Figure 7 shows the change
in morphology of particles at different mixing ratios of carbonate
to carbon black after a 30 pct decomposition of the carbonates.
It is clearly seen that coalescence takes place in the case of
Na2CO3, and the extent of coalescence greatly depends on the
mixing ratio of carbonate to carbon black. For the 1:1.5 molar
ratio of Na2CO3 to carbon black, an extensive coalescence has
occurred after a 30 pct decomposition (Figure 7(a)), whereas
no appreciable coalescence is observed for a 1:3 mixing ratio
(Figure 7(c)). On the other hand, Li2CO3 particles hardly
undergo coalescence, irrespective of the mixing ratios employed
in the present study (Figures 7(d) through (f)). It should be
noted that both Na2CO3 and Li2CO3 are entirely liquid phases
at the temperature at which the accumulated decomposition has
reached 30 pct (Figures 3 through 5). Therefore, it is clear that
carbon plays an important role in preventing coalescence from
taking place. Since carbonate decomposition is considered to
occur at the particle surface and particle coalescence reduces
the effective surface area, the difference between Na2CO3 and
Li2CO3 in the dependency of their decomposition rate on the
carbon-black mixing ratio can be due to the difference in coalescence behavior between the two carbonates.
IV. DISCUSSION
A. Reaction Mechanisms of the Carbothermic
Decomposition of Carbonates
The carbothermic decomposition of the carbonates may
be represented by the following two consecutive reactions:
20—VOLUME 32B, FEBRUARY 2001
M2CO3 ⫹ C ⫽ M2O ⫹ 2CO (g)
[4]
M2O ⫹ C ⫽ 2M ⫹ CO (g)
[5]
where M is either Na or Li.
If a carbonate undergoes decomposition by the previous
two-step consecutive reactions, each mole of the carbonate
would require 2 moles of carbon for complete decomposition. On the other hand, if a carbonate decomposes only to
an oxide form, according to Eq. [4], then the second reaction,
Eq. [5], would, not occur, and, hence, decomposition of each
mole of the carbonate needs only 1 mole of carbon for the
completion of decomposition. Figures 8 and 9 show mass
losses according to the carbothermic decomposition of
Na2CO3 and Li2CO3, respectively, as a function of the molar
ratio of carbon black to carbonates. The initial total mass
(carbonate ⫹ carbon black) was fixed at 15 mg for all
the cases. The observed values given in Figures 8 and 9
correspond to the accumulated weight losses at the points
indicated by arrows in Figures 4 and 5, respectively. Two
continuous lines superimposed in the figures represent the
theoretical mass losses that are expected to occur when
decomposition terminates after the reaction, according to
Eq. [4], and after the two-step reactions, according to Eqs.
[4] and [5], respectively. It should be noted that, as the total
sample weight is constant (15 mg), the net amount of a
carbonate decreases with increasing the carbon-to-carbonate
ratio. Therefore, the weight loss reaches the maximum (theoretically complete loss of the sample mass) when the carbonto-carbonate ratio satisfies the exact stoichiometric requirement, i.e., 1:1 for Eq. [4] and 2:1 for Eqs. [4] and [5]. If either
carbon or carbonate exists surplus against a stoichiometric
requirement, the total weight loss decreases accordingly, due
to the excess amount remaining. It is clearly seen from
Figures 8 and 9 that the observed results for Na2CO3 are in
line with the theoretical prediction for the two-step decomposition represented by Eqs. [4] and [5], whereas the results
for Li2CO3 agree well with the prediction by Eq. [4]. Therefore, it can be concluded that the carbothermic decomposition of Na2CO3 and Li2CO3 can be represented by the overall
reactions of Eqs. [6] and [7], respectively:
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 7—SEM micrographs of shape of mixture during decomposition reaction. Na2CO3 ⫹ carbon black mol ratio: (a) 1:1.5; (b) 1:2; (c) 1:3, Li2CO3 ⫹
carbon black tool ratio; (d ) 1:0.5; (e) 1:1; and ( f ) 1:2.
Na2CO3 (l) ⫹ 2C (s) ⫽ 2Na (g) ⫹ 3CO (g)
[6]
Li2CO3 (l) ⫹ C (s) ⫽ Li2O (s) ⫹ 2CO (g)
[7]
It might be argued that the following reactions could also
occur:
M2CO3 (l) ⫽ M2O (s) ⫹ CO2 (g)
CO2 (g) ⫹ C ⫽ 2CO (g)
[8]
[9]
However, the reaction according to Eq. [8] is much too slow
METALLURGICAL AND MATERIALS TRANSACTIONS B
when compared with the reactions of Eqs. [6] or [7], and,
hence, the proportion of carbonate decomposition, according
to Eqs. [8] and [9], in existence of carbon should be insignificant (Figures 4 and 5).
As for carbothermic decomposition of Na2CO3, the present work is in agreement with literature.[5,6,8] For Li2CO3,
no previous report has been found in the literature on carbothermic decomposition with carbon black. However, XRD
analysis of what was left after carbothermic decomposition
has confirmed the existence of Li2O and LiOH⭈H2O, the
VOLUME 32B, FEBRUARY 2001—21
Fig. 8—Comparison of observed weight loss with calculated weight loss
based on the two different reaction equations.
Fig. 10—Comparison of standard free energy for the decomposition reactions of carbonates with carbon.
further decomposition of Li2O → Li is thermodynamically
not likely to proceed due to positive free-energy changes.
Equilibrium partial pressures of CO for decomposition of
Na2O and Li2O by carbon at 1100 K are calculated to be
10⫺2.6 and 10⫺11.5atm,[4] respectively. The previous thermodynamic analysis strongly supports the experimental observations of the present study, i.e., Na2CO3 decomposes
according to Eq. [6], whereas Li2CO3 decomposes according
to Eq. [7].
B. Effect of the Relative Amount of Carbon Black
Fig. 9—Comparison of observed weight loss with calculated weight loss
based on the two different reaction equations.
latter being considered to have formed after quenching by
reaction with moisture in the atmosphere. In order to understand the difference in decomposition behavior between the
two carbonates, the standard free-energy changes of various
reactions are compared at different temperatures in Figure
10. It is clear that the sequential decomposition of Na2CO3
→ Na2O → Na is quite feasible to occur when coexisting
with carbon. On the other hand, the decomposition of Li2CO3
is expected to be different, that is, while the decomposition
of Li2CO3 → Li2O in existence of carbon is feasible to occur,
22—VOLUME 32B, FEBRUARY 2001
It was seen earlier in Figure 7 that there is a distinct
difference between Na2CO3 and Li2CO3 in the behavior of
coalescence during carbothermic decomposition. Na2CO3
particles tend to agglomerate to form a large lump, particularly when the amount of carbon black is not in excess of
what is required for complete decomposition of Na2CO3.
Since coalescence reduces the effective interfacial area for
reaction between the carbonate and carbon black, the rate
of the decomposition reaction will decrease accordingly.
Figure 11 shows the change in decomposition rates of
Na2CO3 with temperature for different carbonate-to-carbon
black ratios, relative to that for the carbonate-to-carbon black
ratio of 1:2. It is seen that the relative rate sharply decreases
with increasing temperature. This appears to imply that the
coalescence takes place at the early stage of decomposition,
where the carbonate begins to melt. Based on the previous
analysis together with the micrographical observations given
in Figure 7, it may be concluded that the effect of the relative
amount of carbon black on the carbothermic decomposition
of Na2CO3 is closely related to the coalescence among
Na2CO3 particles.
For Li2CO3, on the other hand, the carbothermic decomposition rate appears to be hardly affected by the relative
amount of carbon black (Figure 5). Also, there is no significant coalescence occurring during carbothermic decomposition, irrespective of the relative carbon black content
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 11—Comparison of decomposition rate ratio between the different
mol ratios of the mixture of Na2CO3 and carbon black.
(Figures 7(d) through (f)). The apparent differences between
Na2CO3 and Li2CO3 are as follows.
(1) Li2CO3, particles are much smaller than Na2CO3 particles (20 ␮m, versus 200 ␮m, in average diameter).
(2) The product of the carbothermic decomposition of
Li2CO3 is Li2O (melting point ⫽ 1453 ⬚C), whereas
that of Na2CO3 is Na (g), which vaporizes out from
the system.
The product of Li2O will either dissolve into the Li2CO3
melt or stay on the particle surface. If the carbothermic
decomposition reaction is faster than the dissolution rate of
Li2O into the melt, or the melt has been saturated with Li2O,
then Li2O will accumulate on the surface. If either or both
of the aforementioned is the case, coalescence of Li2CO3
particles will be hindered by the solid Li2O residing at the
particle’s surface. Therefore, the fact that Li2CO3 particles
do not agglomerate during carbothermic decomposition can
be attributed to Li2O (s) at the surface, mainly, and to its
small size, in part. On the other hand, in the case of Na2CO3,
the particle surface always remains in a liquid state because
the reaction products are only gas phases (Na, CO) and, as
mentioned before, the solubility of Na2O is less than 1 pct.[5]
Therefore, the coalescence between carbonate particles is
much easier, resulting in the high dependency of the reaction
rate on the relative amount of carbon black.
C. Kinetics of the Carbothermic Decomposition
The Freeman–Carroll method[9] is frequently employed
in analysis of TG-DSC experimental data. In the method,
the rate expression for the disappearance of a reactant from
the mixture is assumed:
dX
⫺ ⫽ kX n
dt
[10]
where X is the amount of the reactant at time t, k is the
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 12—Kinetics of thermal decomposition of Na2CO3 with carbon black.
specific rate constant, and n is the empirical order of irreversible reaction with respect to the reactant.
It is assumed that the specific rate constant can be
expressed by a simple Arrhenius equation,
k ⫽ Ae⫺E/RT
[11]
where A is the frequency factor, E is the apparent activation
energy, R is the gas constant, and T is the absolute
temperature.
From Eqs. [10] and [11], Freeman–Carroll derived the
following equation:
⫺
冢冣
E
1
⌬
2.3R
T
⌬ log Wr
⌬ log
⫽ ⫺n ⫹
Wr ⫽ wc ⫺ w
冢 dt 冣
dw
⌬ log Wr
[12]
[13]
where wc is the weight loss at completion of the reaction,
and w is the total weight loss up to time t.
Applying Eq. [l2], values of the order of reaction (n) and
activation energy (E ) for the reaction of interest can be
obtained. The plots of Eq. [12] for the decomposition of
Na2CO3 with carbon black, for the different molar ratios,
are shown in Figure 12. The activation energies for the molar
ratios of 1:2 and 1:3 show similar values of 197 and 183
kJ mol⫺1, respectively. For the molar ratios smaller than 1:2,
however, the activation energy drastically decreases, viz.,
118 kJ mol⫺1 for the molar ratio of 1:1.5 and 70 kJ/mol⫺1
for the molar ratio of 1:1. Such lower activation energies
might be due to the lower reaction rate because of the reduction of the reaction surface area, as discussed previously.
Therefore, it is thought that the calculated activation energies
for molar ratios lower than the critical molar ratio of 1:2
are not meaningful, because the effective reaction surface
area changes significantly during the decomposition. Figure
13 shows the plots of Eq. [12] for the decomposition of
Li2CO3 with carbon black, for the different molar ratios. All
VOLUME 32B, FEBRUARY 2001—23
mixtures show similar activation energies of 195, 223, 193,
and 191 kJ mol⫺1 for the molar ratios of 1:0.5, 1:1, 1:2,
and 1:3, respectively. Such similar activation energies are
ascribed to no significant change in the reaction surface area
during the decomposition. The activation energies also give
similar values to those of Na2CO3 with carbon black, for
molar ratios of 1:2 and 1:3. Thus, it can be concluded that
the activation energy for the thermal decomposition of the
mixture of the carbonates with carbon black lies in the range
from 180 to 223 kJ mol⫺1. The activation energies found in
the present study are similar to the reported values: 246 kJ
mol⫺1 for the spontaneous thermal decomposition of
Na2CO3[6] and 189 kJ mol⫺1 for the reaction of Na2CO3 with
bastnasite concentrate.[10] The term n in Eq. [12], which
represents the order of reaction as seen in Eq. [10], can be
found from the intercept of the regression lines in Figures
12 and 13 with the ordinate. In the present study, the value
of n is found to fall mostly in the range of 0.3 ⫾ 0.2.
Theoretically, this value should represent the order of reaction for carbothermic decomposition of the carbonates. As
can be seen in Figures 12 and 13, however, the resolution
of the experimental data becomes worse as the value of
the abscissa approaches zero. Therefore, it appears that the
application of the Freeman–Carroll method to the present
study does not give reliable information on the order of
reaction.
V. CONCLUSIONS
Decomposition mechanisms of Na2CO3 and Li2CO3 particles were investigated using the TG and DSC methods at
temperatures up to 1200 ⬚C. The following observations and
conclusions were obtained.
1. Thermal decomposition of pure Na2CO3 took place from
its melting point and continued to occur as the temperature was increased, but at a very slow rate.
2. For Li2CO3, however, the decomposition occurred at
much faster rates than that for Na2CO3.
3. When carbon black was mixed with the carbonates, the
decomposition rates of both Na2CO3 and Li2CO3 were
significantly enhanced.
4. From mass-balance calculations and XRD analyses of
the reaction products, it is concluded that decompositions
of Na2CO3 and Li2CO3 with carbon black occur by the
respective reactions of
Na2CO3 (l) ⫹ 2C (s) ⫽ 2Na (g) ⫹ 3CO (g) and
Li2CO3 (l) ⫹ C (s) ⫽ Li2O (s) ⫹ 2CO (g).
5. It was identified that liquid droplets of Na2CO3 were
initially isolated due to the carbon particles surrounding
them, but, as the carbon particles were consumed, the
liquid droplets were gradually agglomerated. This
24—VOLUME 32B, FEBRUARY 2001
Fig. 13—Kinetics of thermal decomposition of Li2CO3 with carbon black.
effected a reduction of the total surface area of the carbonate, resulting in a dependence of the decomposition rate
on the amount of carbon black.
6. For the case of Li2CO3, on the other hand, agglomeration
hardly occurred up to completion of decomposition, and,
hence, the rate was almost independent of the amount of
carbon black mixed. This is attributed to the combined
effects of the existence of carbon black and solid-product
Li2O at the surface of each liquid Li2CO3 droplet.
7. The apparent activation energies for the decomposition
of Na2CO3 and Li2CO3 with carbon black were found to
be similar and were estimated to be 180 to 223 kJ mol⫺1.
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METALLURGICAL AND MATERIALS TRANSACTIONS B