*5.5. Mixing*

The mixing rate of the reactant solutions is an important factor of the precipitation process [69]. The stirring intensity can affect the activation energy of nucleation of calcite and vaterite in the aqueous system [52]. Local non-homogeneities of the supersaturation can affect the creation of conditions conducive to the formation of one of the polymorphic forms. Homogeneous, high-shear, and constant agitation of reactant solutions enables precipitation of pure vaterite particles [49]. Also, ultrasonic agitation can be used for this purpose [13]. Ultrasounds provide a large amount of energy into the reaction systems, which produces mechanical and thermal effects and facilitates mass transfer. When CaCO3 precipitation was carried out using ultrasounds, the vaterite concentration was higher compared to the process using the same reagent mixed by a magnetic stirrer. In the carbonation method using gaseous CO2, mixing is generated by a gas flow. Therefore, an increase in the gas flow rate resulted in an increase in the vaterite precipitation [19,59,78,79].
