Solid products and rate-limiting step in the thermal half decomposition of natural dolomite in a CO2 (g) atmosphere

Natural dolomite powders obtained from caves which give unusual high resistance building materials, have been decomposed in a Knudsen cell at high CO2 pressures in the temperature range of 913–973 K. XRD traces for the final solid products, after the first half thermal decomposition, have shown, that beside the XRD patterns for the calcite and MgO, the existence of a new structure with major peaks at 2θ equal to 38.5 and 65°. This finding has been ascribed to a solid solution of MgO in calcite. The kinetic analysis of the TG curves yield a total apparent enthalpy (ΔH∗) for the decomposition equal to 440±10 kJ mol−1 for a range of fraction decomposed (α) varying between 0.2 and 0.7. This value is much closer to the theoretical expected at 950 K value ΔH=486 kJ mol−1 for the dolomite decomposition in CO2 environment, where CaO, MgO and oxides of solid solution can be the solid reaction products. The rate determining step is the transport of CO2 across the reacting interface through an high activated thermal process due to solid state diffusion of CO32− in the bulk and/or the grain boundaries phases of CaCO3 and/or of the solid solution. The microstructure evolution of the solid products follows a shear-transformation mechanism. At temperatures below 943 K, porous product particles are characterized by a monomodal narrow pore size distribution around 0.05 μm. At higher temperatures, a critical level of tensions inside the particles is reached and a bimodal pore size distribution around 1 and 0.05 μm is formed.