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Kinetic studies have been made of the thermal decomposition of precipitated calcium carbonate, powdered calcite, and regular fragments of calcite crystals. The powdered materials were examined in the form of pellets, which were prepared by compacting the powder to about 70% of its theoretical density. The work was done at one atmosphere of pressure in a flow of air containing various amounts of carbon dioxide. It was observed that the decomposition of the pellets, which were prepared in a variety of shapes, was characterized by the same advancing interface mechanism as that observed for single specimens of crystal fragments. When the rates of decomposition were normalized for the change in of interfacial area accompanying decomposition, it was possible to correlate the observed rates of decomposition for a variety of pellet shapes, and to relate these rates, as a function of particle size and pellet roughness, to the rates of decomposition of large fragments of calcite crystals. The activation energy for the decomposition reaction was found to be 40.6 kcal./mole. At a constant temperature, the decrease in reaction rate with increasing carbon dioxide pressure was found to be proportional to the difference between the equilibrium dissociation pressure and the back pressure of carbon dioxide. A reaction mechanism based on diffusion through a constant thickness of active calcium oxide is suggested.

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A simplified method has been developed for the determination of the activation energy of a heterogeneous reaction having linear kinetics. The thermal decomposition of calcium carbonate has been used as an example and it has been shown that when the logarithm of the weight loss per unit area of powder compacts is plotted against the reciprocal of absolute temperature, for a run done with a linear heating rate, a linear relationship is obtained. When the logarithms of the heating rate and the absolute temperature are included, results for a variety of heating rates can be completely correlated. Heating rates of less than 5 degrees per minute are recommended. An activation energy of 48.4 ± 2 kilocalories per mole has been estimated for the decomposition reaction and 7 ± 3 kilocalories per mole for the formation reaction.