Search

The understanding of the thermal stability of magnesium carbonates and the relative metastability of hydrous carbonates including hydromagnesite, artinite, nesquehonite, barringtonite and lansfordite is extremely important to the sequestration process for the removal of atmospheric CO2. The conventional thermal analysis of synthetic nesquehonite proves that dehydration takes place in two steps at 157, 179°C and decarbonation at 416 and 487°C. Controlled rate thermal analysis shows the first dehydration step is isothermal and the second quasi-isothermal at 108 and 145°C. In the CRTA experiment carbon dioxide is evolved at 376°C. CRTA technology offers better resolution and a more detailed interpretation of the decomposition processes of magnesium carbonates such as nesquehonite via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of non-isothermal nature reveal partial collapse of the nesquehonite structure. [ABSTRACT FROM AUTHOR]

Abstract: The mechanism for the decomposition of hydrotalcite remains unsolved. Controlled rate thermal analysis enables this decomposition pathway to be explored. Hydrotalcites containing carbonate, vanadate and molybdate were prepared by coprecipitation. The resulting materials were characterised by XRD, simultaneous TG–DTG–DTA and controlled rate thermal analysis (CRTA) to determine the stability and thermal decomposition pathway of the synthesised hydrotalcites. For the carbonate intercalated hydrotalcite dehydration takes place in three steps two of which are quasi-isothermal and one non-isothermal. Dehydroxylation and decarbonation occur separately over the 235–330 and 330–370 °C temperature range. A second non-isothermal decarbonation step is observed in the 371–541 °C range. In comparison the mixed carbonate–vanadate and carbonate–molybdate hydrotalcites show two dehydration steps and the dehydroxylation and decarbonation occur simultaneously. The observation of three dehydration steps is used to support the model of water molecules in three structurally distinct environments in the hydrotalcite interlayer. CRTA technology provides a mechanism for the decomposition of hydrotalcites. [Copyright &y& Elsevier]

Abstract: Controlled rate thermal analysis (CRTA) of a series of synthetic aurichalcite (Zn,Cu2+)5(CO3)2(OH)6 with the ratio of Cu/Zn varying from 0.1 to 0.5 proves that the dehydroxylation and carbonate loss occur as non-isothermal and isothermal decompositions. The temperature of the thermal decomposition increases as the Cu/Zn ratio increases. Thermal decomposition of aurichalcite provides a method for preparing mixed oxide catalysts at the molecular level as opposed to the particle level. CRTA technology enables separation of the processes of dehydration, dehydroxylation and decarbonation. X-ray diffraction of the products of the thermal decomposition proved to be a mixture of the oxides ZnO and Cu2O. [Copyright &y& Elsevier]