Ultrasonic Crystallization of Calcium Carbonate in Presence of Seawater Ions

Ultrasonic technique was applied to remove the scaling compound CaCO3 as a pretreatment method in desalination process. Supersaturated solution of calcium carbonate was treated with an ultrasonic probe to precipitate CaCO3, followed by filtration to remove the precipitates. The fundamentals of nucleation, growth, crystal form and morphology of CaCO3 in the presence of the major seawater ions and ultrasound were investigated. Naked eye observation on induction time showed that ultrasound accelerates the nucleation by 60-85% though the major seawater ions delay the occurrence of nucleation up to 3 times longer. Continued ultrasound treatment after nucleation accelerates the growth rate by 2-5% only. Metastable vaterite with morphology of spherical aggregates constitutes ~ 90% of the obtained precipitates in most solution systems except F− and Na+, which favors calcite formation. Ultrasound treatment makes the aggregated crystals more fragile and breakable. As a pretreatment method, ultrasound technique not only accelerates the precipitation rate of CaCO3, but also has an effect in inhibiting the crystals growing into compact aggregates.