Monitoring the effect of mineral precursor, fluid phase CO ₂-H₂O composition, and stirring on CaCO₃ crystallization in a supercritical-ultrasound carbonation process

This study focuses on the evaluation of the different factors that affect the particle size distribution of precipitated calcium carbonate formed in a wet supercritical CO₂ carbonation process and on the conversion rate from two different Ca²⁺ precursors (Ca(OH)₂ or CaO). The operating factors investigated include the composition of the fluid phase (CO₂/H₂O) in contact with the solid precursor, the calcium cation source, and the stirring mode (no agitation, vertical mechanical, and ultrasound). The calcium carbonate particles were fabricated in batch mode in a stainless steel reactor filled with the solid precursor, water, and scCO ₂ at 130 bar and 40 °C. The particle size was estimated using scanning electron microscopy, while the precipitated solid phase composition was determined by a quantitative characterization method based on X-ray diffraction. The conversion of CaO or Ca(OH)₂ to CaCO₃ varied from 50 to >90 wt % depending on the reactor fill level and the existence of a rich-water phase in equilibrium with the scCO₂-rich fluid. It was found that micrometric particles were precipitated in systems containing a large quantity of water in the fluid phase, while nanometric calcite was formed when using a reduced water percentage. The use of ultrasound stirring accelerated the kinetics of the carbonation process, thus increasing the calcium carbonate yield. The highest conversion rates were obtained using CaO as the solid precursor.