The mechanism of the thermal decomposition of calcium carbonate

Abstract

The decomposition of single sintered spheres of calcium carbonate has been investigated using a thermo-balance, modified to allow the simultaneous measurement of the temperature and weight of a decomposing sample. It has been shown that the decomposition reaction takes place on a definite boundary between the undecomposed carbonate, and the layer of porous lime formed outside it. This boundary moves towards the centre of the compact but remains spherical in shape. The rate of the reaction is controlled by the transfer of heat to this reaction boundary, and by the transfer of CO 2 away from it. This reaction mechanism is at variance with the mechanism proposed by previous workers who have suggested that the reaction is controlled by a chemical step at the reaction interface. Their conclusions have been based on the experimental observation of certain characteristics of the reaction that have been assumed to be symptomatic of chemically controlled reactions. This paper shows that transport controlled reactions can also display these characteristics, and so they do not provide any evidence as to the mechanism controlling the reaction. The modified thermo-balance has been used to measure the diffusion coefficient and thermal conductivity of the porous lime layer, and the mass and heat transfer coefficients to the surface of the compact. The values so obtained agree with values obtained from the literature. Three reaction models have been developed from which it is possible to predict the progress of the reaction for a single sphere in a constant environment. Predictions made by the two more complicated models are shown to agree very well with the experimental results.