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J.M. Criado, M. Gonzalez / Thermochimica Acta 318 (1998) 265±269
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constant pressures are obtained depending on whether
the equilibrium is approached from above or below.
This behaviour can be understood by considering that
when the forward and reverse reactions (1) are pro-
ceeding, a progressive decrease of their rate take
place; thus equilibrium is not reached and the reaction
merely becomes very slow. Moreover, Lvov [6] has
shown, in a very recent paper, that the thermal decom-
position of alkali-earth carbonates can be improved by
water vapour. This statement is in agreement with
results obtained in a previous work [7] that shows that
the recarbonation of CaO under dry carbon dioxide
atmosphere starts at temperatures >6008C, in spite of
the fact that the calcium oxide conversion into calcium
carbonate under CO2 atmosphere is thermodynami-
cally favoured from room temperature. However, it
has been pointed out that the recarbonation tempera-
ture is dramatically decreased if humid carbon dioxide
is used, which suggests [7] that, in this case, the
reaction takes place through Ca(OH)2 as an intermedi-
ate product that is rapidly converted into calcium
carbonate.
experiment was carried out. It has been previously
shown that this method would lead to unreliable
equilibrium pressure because `pseudo-equilibrium'
would be reached for kinetic reasons.
2. Experimental
2.1. Materials
CaCO3 and PbCO3 D'Hemio a.r have been used.
CaO was proceeding from the thermal decomposition
of the corresponding carbonate.
2.2. Apparatus
A Cahn electrobalance, model RG, was employed.
The balance was connected to a conventional vacuum
system and to a gas storing system. The temperature
was measured with a thermocouple located inside the
electrobalance very close to the sample. The system
can be operated under isothermal conditions or under
heating rates that can be selected in the range from 0.5
to 25 K/min. The system can be operated at pressures
ranging from vacuum to 400 torr. This upper limit is
chosen because the noise becomes very high at higher
pressures.
The scope of the present paper is to develop a TG
method for determining the equilibrium partial pres-
sure of reactions such as [1] at different temperatures
in such a way that `pseudo-equilibria' would be
avoided. This method would take advantage of the
fact that the recarbonation of metal oxides under dry
CO2 atmosphere does not seem to take place at low
temperatures.
3. Results and discussion
The thermal decomposition of CaCO3 has been
taken as a test reaction because accurate thermody-
namic data of this reaction are available [5]. On the
other hand, this method will be used for the determi-
nation of the thermodynamic constants of the reaction:
When the temperature of a sample of CaO is
progressively raised under a de®ned pressure of
CO2, a continuous increase in the sample weight
would be expected until the equilibrium for reaction
[2] is reached. Above the equilibrium temperature,
CaCO3 will not be thermodynamically stable, and a
weight loss should be recorded in the TG diagram.
Fig. 1 includes a TG diagram obtained for a sample
of CaO at a heating rate of 10 K/min and a CO2
pressure of 30 torr. As can be observed in this ®gure,
under the experimental conditions used, the sections
of the thermogravimetric curve corresponding to the
thermal formation and the decomposition of CaCO3
do not overlap, as might be expected if a real equili-
brium situation would be reached. The sections are
connected by a broad zone of `pseudo-equilibria'
temperatures from 6808 to 7408C in which no detect-
Pb3O2CO3 , 3PbO CO2
(2)
To our knowledge, the thermodynamic data reported
by Grisafe and White [8] are the only ones available in
literature for this reaction. However, it is noteworthy
that these authors have determined the equilibrium
constants by following the decrease of the CO2 pres-
sure during the conversion of lead monoxide into
oxycarbonate from a starting pressure selected to be
higher than the one expected for the equilibrium. The
value recorded for the CO2 pressure when the steady
state is reached was taken by Grisafe and White as the
equilibrium pressure at the temperature at which the