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.
■ INTRODUCTIONCalcium carbonate is one of the most abundant minerals on earth, and its scaling propensity is a problem in many industrial water treatment processes. Moreover it is an important raw material in a wide range of industries. Controlling the formation of calcium carbonate is therefore of great interest, and as a result the crystallization of CaCO 3 from clear solution (spontaneous precipitation) has been investigated extensively. 1−15 In the initial phase of precipitation, or nucleation, clusters of calcium carbonate obtain a critical size. In the past decade more evidence has pointed to the existence of a two-step nucleation mechanism 16 in which the nucleation of a crystal occurs within stable mesoscopic clusters of dense liquid. These prenucleation phases were also found in calcium carbonate experiments. 17,18 Calcium carbonate nucleation, in this case the vaterite polymorph, consists of several steps. Prenucleation clusters are formed (∼1 nm) and aggregate to nanoparticles with a size distribution of around 30 nm. 18 These particles aggregate and grow out at the expense of others. The nanocrystalline domains share the same three-dimensional (3D) orientation, resulting in the development of a single crystalline structure. 18,19 Mostly, some time elapses before a measurable amount of the newly formed material is detected, the induction time, or induction period, and it marks the ability of the solution to stay in a metastable state. This induction time is heavily dependent on the measurement method and various methods have been used by others. [4][5][6][7]20,21 The application of ultrasound during crystallization and precipitation processes rec...