Kinetics and Thermodynamics of Decomposition of Dolomite to a Metastable Solid Product

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Abstract
The decomposition of dolomite, CaMg(CO3)2, in vacuum was studied by the torsion‐effusion and torsion‐Langmuir methods. Complete decomposition yields poorly crystalline MgO and CaO of high total surface area, but the decomposition product when the reaction is 50% complete does not give coherent X‐ray scattering. Extrapolation of the effusion pressures to pressures for zero orifice area yields thermodynamic data that disagree with well‐established data for dolomite and its thermodynamically stable products. The data are consistent with the interpretation that dolomite and CO2 are at equilibrium with a glasslike solid which has a free energy of formation of (17,600 – 8.8T)cal from O.SCaO + 0.5Mg0. From the torsion‐Langmui data, the apparent activation enthalpy and entropy of decomposition to this product are calculated to be 46.5 kcal and 22.3 cal/degree per mol of CO2. The ratio of the free surface decomposition pressure to the metastable equilibrium pressure is 1 × 10−4 at 850°K. Earlier anomalously low pressures measured for decomposition of Mg(OH)2 are also consistent with the interpretation that the solid product was glasslike.