Mechanochemical interaction of alkali metal metasilicates with carbon dioxide: 1. Absorption of CO2 and phase formation

Processes proceeding during the mechanochemical activation of alkali metal metasilicates M₂SiO₃ (where M is Li, Na, K) are studied in the air and in an atmosphere of carbon dioxide. At the initial stage of activation in a centrifugal planetary mill in an atmosphere of carbon dioxide, the main portion of supplied mechanical energy is expended for grinding and the mechanosorption of CO₂ occurs in the regime of cleavage, i.e., on the freshly formed surfaces of particles. As the time of activation increases, the specific surface area becomes constant, which, however, does not substantially affect the rate of interaction between carbon dioxide and silicates. The absorption of CO₂ occurs in the regime of friction on the active sites of already formed surfaces and is accompanied by the tribodiffusion of gas molecules into structurally disordered layers of particles. With identical amounts of supplied energy, the CO₂/M₂SiO₃ molar ratio in the samples activated in the medium of carbon dioxide increases in the Li < Na < K series. The main product of mechanically induced interactions between Li₂SiO₃ and CO₂ is the X-ray amorphous carbonate-silicate phase. In the case of sodium and potassium metasilicates, the reaction of mechanochemical substitution occurs to form corresponding carbonates, hydrocarbonates, and amorphous silica. It is shown that the character of mechanochemical interaction between M₂SiO₃ and CO₂ depends on the change in the Gibbs energy of the transformation of silicate into corresponding carbonate, as well as on the melting temperature and the hygroscopicity of silicate.