Microstructure, kinetic, structure, thermodynamic analysis for calcite decomposition: free-surface and powder bed experiments

This paper describes studies of incongruent vaporization reactions that were carried out in Berkeley and Genoa from 1970 to about 1987, but that have not previously been described in terms of the overall research objectives and the extent to which those objectives were achieved. When the research was initiated, it was already known that for metals and some simple salts the rates of vaporization in vacuum are predictable as functions of temperature if the thermodynamic properties of the solid and vapors were known. That simple behavior was not shown by the few congruent decomposition reactions that had been quantitatively studied. Our review focuses on calcite decomposition because it illustrates the daunting array of variables—heat transfer, CO2 pressure, crystal structure and crystal orientation, particle size, particle shape, particle-pore distribution, surface energy of the CaO reaction product, and interface strain—all of which could influence the reaction kinetics. In the body of the paper we review the theoretical bases of our kinetic evaluations and experimental studies for single crystals and powder beds. As a major conclusion we identify an easily achievable range of experimental conditions within which investigators in different laboratories have obtained concordant measurements of calcite decomposition rates. This is an important result because a prominent international committee concluded a decade ago that goal had not yet been achieved. Another important conclusion is that the rate limiting step of calcite decomposition probably occurs in a CO2-depleted surface layer or layers of calcite. At high relative pressures of CO2 the process is kinetically reversible, but at low relative pressures, it is highly irreversible. We think the latter rate is determined by a strain-induced collapse of CO2-deficient calcite at its interface with crystalline CaO rods with simultaneous expulsion of CO2 gas. Such a mechanism seems not to have been suggested before, but it probably accounts for some of the exothermic events observed during heating of solids that decompose incongruently. The role of particle size on such a mechanism has been investigated and discussed.