Synthesis of Spherical Calcite Nanoparticles by Reactive Crystallization of CaCO₃ from Ca(OH)₂ Suspension Using CO₂ Fine Bubbles and Antisolvent

In this study, CO₂ fine bubble injection and addition of ethanol （EtOH） as an antisolvent for carbonate were applied to the reactive crystallization of CaCO₃ from a Ca(OH)₂ suspension to produce spherical calcite nanoparticles. In the regions surrounding the gas-liquid interfaces of the CO₂ fine bubbles, the local increase in the Ca²⁺ and CO₃^2– concentrations is due to the electric charge on the fine bubble surface and acceleration of CO₂ mass transfer as a result of a reduction in the bubble diameter. Simultaneously, EtOH accumulation resulting from the electric charge on the fine bubble surface leads to not only the decrease in the solubility of CaCO₃ but also the increase in the solubility of CO₂ with increasing EtOH concentration at the minute gas-liquid interfaces. Thus, a local high supersaturation of CaCO₃ is attained in the region around the fine bubble surfaces. Furthermore, the particle shape can be altered by changing the growth conditions in the presence of EtOH in the bulk solution. Hence, spherical calcite nanoparticles are expected to be produced. At a reaction temperature of 278 K and a CO₂ flow rate of 26.8 mmol/(L･min), CO₂ bubbles with an average diameter (d_bbl) of 40–2000 μm were continuously supplied into the Ca(OH)₂ suspension of EtOH-water with a different weight percentage of EtOH (W_EtOH) to crystallize CaCO₃ particles and Ca(OH)₂ was converted to CaCO₃. During reactive crystallization of CaCO₃ from Ca(OH)₂ suspension free of EtOH, minimizing the CO₂ bubble diameter led to an enhancement of production and micronization of cubic calcite particles, and the average size of approximately 0.9 μm was obtained at d_bbl of 40 μm. When the W_EtOH increased to 70 wt.% at a d_bbl of 40 μm, spherical calcite nanoparticles with an average size of 48 nm were produced within a reaction time of 30 min. However, at W_EtOH below 50 wt.%, cubic calcite particles were obtained. Therefore, high-yield production of spherical nano-sized particles with the most stable calcite form was achieved by CO₂ fine bubble injection combined with EtOH addition during the reactive crystallization of CaCO₃ from a Ca(OH)₂ suspension.