ABSTRACT
A different DTA/DSC endotherm shape would be expected for calcination reactions such as CaC03(s) = CaO(s) + C02c9) (Figure 3.13). Given that the partial pressure of C02 in dry air is
0.033% [12], the thermodynamic9 decomposition temperature of CaC03 should be 526.4°C. This temperature, marked on the figure, does not correspond to any visible deviation from the baseline. Apparently, only a minute amount of CaC03 must decompose to locally meet the partial pressure requirements of C02 for this and somewhat higher temperatures, and such decomposition has an undetectable thermal effect. However, the equilibrium partial pressure of carbon dioxide increases exponentially with temperature, requiring a much more rapid decomposition; hence, the DTA trace begins to deviate from the baseline with increasing temperature. At 896.4°C, the temperature at which the equilibrium partial pressure of C02 reaches
1 atm, the reaction rate would be expected to significantly increase, using arguments similar to those used for the boiling of water. Other factors, such as the diffusivity of C02 gas out of the inwardly growing (into the CaC03 grain) porous oxide product layer and the rate of heat flow into the ( endothermic) reaction zone [2], also may act to slow the decomposition process. Thermodynamically, decomposition reactions such as these are reversible; that is, given a C02 atmosphere and adequate time, CaC03 should form from CaO upon cooling.
