Reactive Crystallization of CaCO₃ Using CO₂/NH₃ Bubbles

—Effects of Bubble Size and Molar Ratio of CO₂/NH₃ on Polymorphism—

Abstract

In this study, we used the minute gas-liquid interfaces around CO₂/NH₃ micro-bubbles as new reaction fields where the crystal nucleation progresses and developed a crystallization technique to control the crystal structure (polymorph) of calcium carbonate (CaCO₃). At the minute gas-liquid interfaces, numerous local supersaturation regions are generated due to minimizing bubble size and mixing of NH3 gas with CO₂ gas; hence, the polymorph change of CaCO₃ is expected to occur. CaCO₃ was crystallized at 298K by a semi-batch type reaction in which CO₂/NH₃/N₂ bubbles were continuously supplied to an aqueous Ca(NO₃)₂ solution using a self-supporting bubble generator. The solution pH during crystallization was maintained constant at 9.7 by adding HNO₃ and NH₄OH solution. The average bubble size (d_bbl) was varied within the range of 40 – 1000μm by controlling the N₂ flow rate, and the molar ratio of CO₂/NH₃ (α_CO2/NH3) was set at a specified value of 0.20 – 1.00 at a constant CO₂ flow rate. By varying d_bbl and α_CO2/NH3, the effects of minimizing bubble size and mixing of NH₃ with CO₂ on the polymorphism of CaCO₃ were examined. Consequently, during reactive crystallization of CaCO₃ using CO₂/NH₃ micro-bubbles, the crystallized polymorph was changed from vaterite to aragonite when d_bbl was decreased, and the production of aragonite with minimizing bubble size was accelerated markedly with a decrease in α_CO2/NH3.