5
4
B.V. L’vov, V.L. Ugolkov / Thermochimica Acta 410 (2004) 47–55
Table 9
Experimental values of the E parameter for carbonate decomposition in the isobaric (this work) and equimolar [8,16] modes
E (kJ mol 1)
−
Ei/Ee
Reaction
T (K)
Isobaric
Equimolar
Isobaric
Equimolar
CaCO3 → CaO (g)↓ + CO2
SrCO3 → SrO (g)↓ + CO2
BaCO3 → BaO (g)↓ + CO2
1020
1090
1249
820 [8]
908 [16]
1077 [16]
495 ± 6
569 ± 9
605 ± 1
254 ± 6 [8]
1.95
1.99
2.00
285.5 ± 1.3 [16]
302.1 ± 1.5 [16]
Average
1.98 ± 0.03
Decomposition of SrCO3 in CO2 was investigated by
Zemtsova et al. [19]. We used some of the reported data (the
rate constant of the Arrhenius equation, k, temperature and
partial pressure of CO2) for the calculation of the E param-
eter by the third-law method (Table 8). The absolute rate
of decomposition, J, necessary for the calculation of the E
parameter was estimated by the formula [20]:
different decomposition temperatures in cases of the iso-
baric and equimolar modes. The enthalpy for all reactions
decreases with temperature (see Tables 2–4). As a result,
the theoretical ratio E /E for CaCO3 at 1020 and 820 K for
the isobaric and equimolar modes, respectively, should be
equal to 1.98 instead of 2.00. The experimental value (1.95)
is in a better agreement with this magnitude.
i
e
We consider the agreement of experimental results with
theoretical predictions as a very strong proof of validity of
the primary dissociative evaporation mechanism for carbon-
ate decomposition and the physical approach to the inter-
pretation of kinetics of solid decomposition on the whole.
The failure of all the previous investigations into the effect
of CO2 on kinetics of carbonate decomposition may be at-
tributed mainly to shortages of the Arrhenius plots method,
especially in combination with the non-isothermal measure-
ment technique and, in case of calcite, to the strong catalytic
effect of H2O vapor on the decomposition rate [20].
J = kr0ρ
(19)
where r0 and ρ are the mean radius and density of parti-
cles of SrCO3, respectively. Taking into account that r0 =
−
6
1
3
.2 × 10 m (by estimation of the authors [19]) and r =
3
−
700 kg m , we received J and Peq values. Using Eq. (11)
and the entropy changes from Table 3, we calculated the val-
ues of the E parameter. The average value of the literature
−
1
and our data (569 ± 9 kJ mol ) is in excellent agreement
−
1
with the theoretical value (562 ± 5 kJ mol ) at 1100 K.
The only available literature data for BaCO3 decomposi-
tion in CO2 is the E parameter measured under isothermal
conditions by the Arrhenius plots method [21]. The found
−
1
value (643 kJ mol [21]), in contrast to the literature data
for calcite (Table 1), is in satisfactory agreement with our
result (605 kJ mol ).
References
−
1
[
[
[
[
[
[
[
[
[
1] J. Zawadzki, S. Bretsznajder, Ueber das Temperaturinkrement der
Reaktionsgeschwindigkeit bei Reaktionen vom Typus Afest = Bfest +
Cgas, Z. Elektrochem. 41 (1935) 215–223.
2] H. Tagawa, F. Sudo, Kinetics of thermal decomposition of limestone
in various pressures of carbon dioxide, J. Chem. Soc. Ind. Chem.
Sect. 61 (1958) 946–948 (in Japanese).
3] H. Mauras, Etude cinetique isobare de la dissociation des systemes
solides, Dissociation du carbonate de calcium, Bull. Soc. Chim.,
France (1960) 260–267.
4] P.K. Gallagher, D.W. Johnson, Kinetics of the thermal decomposition
of CaCO3 in CO2 and some observations on the kinetic compensation
effect, Thermochim. Acta 14 (1976) 255–261.
5
. Conclusions
When the experimental results obtained in this work are
compared with the theoretical predictions (Section 2.5),
it becomes apparent that they are in excellent agreement.
Firstly, the values of the E parameters for decomposition
of CaCO3 (Table 7) and SrCO3 (Table 8) in the presence
of CO2 are invariant with respect to the partial pressure of
CO2. Secondly, the decomposition rate, J, is in inverse pro-
portion to P . This conclusion is supported by the direct
measurements for CaCO3 reported in [7,9,10]. (A linear
decrease of J with P
5] K.M. Caldwell, P.K. Gallagher, D.W. Johnson, Effect of thermal
transport mechanisms on the thermal decomposition of CaCO , Ther-
3
ꢀ
mochim. Acta 18 (1977) 15–19.
CO2
6] M. Maciejewski, J. Baldyga, The influence of the pressure of the
gaseous product on the reversible thermal decomposition of solids,
Thermochim. Acta 92 (1985) 105–108.
7] J.M. Criado, M. Gonzalez, J. Malek, A. Ortega, The effect of the
CO2 pressure on the thermal decomposition kinetics of calcium
carbonate, Thermochim. Acta 254 (1995) 121–127.
ꢀ
observed by Darroudi and Searcy
CO2
[11] was connected with the effect of severe self-cooling
of samples in high vacuum. This effect was quantitatively
analyzed in [13].) Thirdly, the values of the E parameter for
decomposition CaCO3, SrCO3 and BaCO3 in the presence
and in the absence of CO2 (Table 9) are subjected to the
8] B.V. L’vov, L.K. Polzik, V.L. Ugolkov, Decomposition kinetics of
calcite: a new approach to the old problem, Thermochim. Acta
390 (1–2) (2002) 5–19.
i
e
theoretically predicted relation E = 2E . The averaged
9] E.P. Hyatt, I.B. Cutler, M.E. Wadsworth, Calcium carbonate decom-
position in carbon dioxide atmosphere, J. Am. Ceram. Soc. 41 (1958)
70–74.
i
e
value E /E is equal to 1.98 ± 0.03 instead of 2.00. The
observed underestimation for CaCO3 partly results from