Mechanical activation of alkaline residues in view of the application of accelerated carbonation

Mineral carbonation is a permanent and safe way for the sequestration of anthropogenic CO2. Carbonation reactions of metal oxides- and hydroxides-bearing alkaline solid waste occur spontaneously at atmospheric temperature and pressure, but proceed slowly to obtain an industrial viable process. In order to enhance mineral carbonation it is necessary a careful selection of the process routes and the process conditions and to use pre-treatments that increase the material reactivity. Mechanical activation is a pre-treatment method that increases mineral dissolution rates due to particle size reduction, specific surface increase and structural disorder production. Many experiments were conducted by increasing the grinding time and the carbonation time of the selected materials (steelmaking slag, incinerator ashes and track ballast) to evaluate how increasing grinding time influences the carbonation rate. The two key factors for the selection of the materials for the laboratory tests were the presence of alkali metals and alkaline earth metals and the time of mechanical activation. A jaw crusher and a continuous ring mill were used to reduce the samples size and / or to obtain homogeneous samples, while a disc mill and a vario-planetary mill were used for mechanical activation. The chemical composition of the samples was determined with X-ray fluorescence, a simultaneous DSC / TGA analyzer was utilized to define the carbonate content, the CO2 uptake and the calcium conversion efficiency and X-ray diffraction was used to determine the mineralogical composition of the materials and the changes in the crystal structure after the experiments. X-ray diffraction and thermogravimetric analyses of mechanically activated alkaline residues show respectively that with increasing the grinding time the crystallinity of the materials decreases and the carbonate content, the CO2 uptake and the calcium conversion efficiency generally increase.