Effect of ultrasonic treatment on early growth during CaCO ₃ precipitation

The present study focuses on the effect of ultrasound (42 kHz, 7.1 W cm ^-2) on the free drift precipitation of CaCO ₃ from a clear, supersaturated solution. To delineate the way ultrasound exerts its effect, we applied different treatment periods (time windows). Applying ultrasound during the first 10 min of the experiment did not result in any significant effect which rules out an influence on primary nucleation. The application of ultrasound starting later in the experiment enhanced precipitation of CaCO ₃. The dominant mechanism responsible for enhanced precipitation is deaggregation during the early growth phase (nuclei to crystals conversion regime). This effect is attributed to shear induced by micromixing and/or shear/stress induced by (supersonic) shockwaves, as a result of cavitation. With ultrasound applied, online pH and scattering measurements displayed a reduction in induction time and an increase in the volumetric precipitation rate. Scanning electron microscopy analysis shows that ultrasound increases the total number of particles that has, in addition, a more uniform size distribution compared with the untreated experiment. Consequently the available surface area for growth is higher resulting in a higher volumetric precipitation rate. With and without ultrasound the formed polymorph was predominantly vaterite with small amounts of calcite.