In situ kinetic measurements of gas–solid carbonation of Ca(OH)2 by using an infrared microscope coupled to a reaction cell
Résumé
Gas–solid carbonation experiments were carried out by using an infrared microscope coupled to a reaction cell. The hydroxide ions (OH) consumption and the production of molecular water (H2O) and carbonate (CO32−) vibration bands were directly monitored as a function of time. Herein, we demonstrated that the gas–solid carbonation of calcium hydroxide (or portlandite) was exclusively activated by initial adsorbed water-molecules (water activity ≈ 0.6 in the lab room) at low temperature (30 °C) and low CO2 pressure (0.5–1.5 bar). We assume that carbonation reaction was then rapidly autocatalysed by the water production and followed by a passivation step due to the formation of a dense layer of carbonate around the reacting particles of portlandite. The fast carbonation and passivation steps were satisfactory fitted by using a kinetic pseudo-second-order model. Moreover, the infrared measurements provided complementary insights with relevance to the reaction mechanism of gas–solid carbonation of calcium hydroxide. Herein, the formation of metastable aragonite was identified and a hydrated calcium carbonate was suspected during carbonation process. On the other hand, when initial adsorbed water onto reacting particles was removed by in situ vacuum drying (P < 10−5 mbar, T = 110 °C) prior to injection of CO2 in the reaction cell (water activity ≈ 0), the carbonation of calcium hydroxide particles was no more detected by infrared spectroscopy at low temperature (30 °C). However, there was evidence for a very limited carbonation reaction at higher temperature (300 °C) and low CO2 pressure (<1 bar). Another carbonation mechanism is required to explain this observation, for example the migration of oxygen atoms from the solid towards adsorbed CO2.