Sorption of atmospheric carbon dioxide and structural changes of Ca and Mg silicate minerals during grinding: II. Enstatite, åkermanite and wollastonite

Abstract

Processes which occur during dry grinding of the magnesium and calcium silicate minerals enstatite MgSiO 3, åkermanite Ca 2MgSi 2O 7 and wollastonite CaSiO 3, have been studied with the use of diffuse reflectance Fourier transform infrared spectroscopy (DRIFT), powder X-ray diffraction, 29Si MAS-NMR spectroscopy, CO 2-content analysis, and BET surface area measurements. It has been found that the extensive sorption of atmospheric CO 2 (carbonization) by the ground mineral that was reported earlier for diopside CaMgSi 2O 6 also occurs for the minerals included in this study. The shape and position of the double peak of the CO 3 2−-group in the 1430–1515 cm −1 region in the FT-IR spectra of ground minerals are similar to that of ground diopside. CO 2-content analysis and BET surface area measurements show that carbonate groups are present not only at the surface but also in the bulk of the ground minerals. The degree of carbonization of silicate depends on its calcium content. The higher the fraction of Ca in octahedral positions in the silicate structures, the higher the degree of carbonization. For synthetic wollastonite CaSiO 3, which is a pure calcium silicate, the degree of carbonization is 102.6 mg CaCO 3 per gram of sample after 36 h of grinding. The structure of silicate appears to be less important for the degree of carbonization during grinding.