IngentaConnect Thermal evolution of Mg-Al-CO3 hydrotalcites

Authors: T.S. Stanimirova; N. Piperov; N. Petrova; G. Kirov

Source: Clay Minerals, Volume 39, Number 2, 1 June 2004 , pp. 177-191(15)

Abstract:

The thermal decomposition of hydrotalcite (HT), with chemical composition Mg_1-xAl_x(OH)₂(CO₃)_x/2.(1-3x/2)H₂O, (0.20 < x

0.33), is a complex sequence of dehydration, dehydroxylation and decarbonization and leads to the formation of a series of metaphases: HT-D (dehydrated HT), HT-B (partially dehydroxylated HT) and MO (mixed oxides with periclase-like structure). The evolution of water and CO₂ in natural and synthetic hydrotalcites (a Mg/Al ratio between 2:1and 3.7:1 ), heated to 800ºC, was investigated by differential thermal analysis, thermogravimetry and evolved gas analysis. At least six endothermic and two exothermic effects were established by computer-aided resolving of the curves. The formation of each HT metaphase was related to the release of a discrete number of water molecules depending on the Al content in the samples and each appeared as a corresponding endothermic peak in the DTA curves. The exothermic processes associated with the crystallization of HT-B and MO metaphases were specified by decomposition of DTA curves. The evolution of CO₂ during the thermal decomposition of the carbonate groups was found to be different for the samples studied. The preservation of CO₃ even at high temperatures was established for synthetic samples with a high Al content. The release of volatile H₂ O and CO₂ (which comprise ~40% of the sample mass) provokes fine cracking both along and across the layers.