Reactive Crystallization of CaCO3 Using CO2/NH3 Bubbles-Effects of Bubble Size and Molar Ratio of CO2/NH3 on Polymorphism-:—Effects of Bubble Size and Molar Ratio of CO2/NH3 on Polymorphism—

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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 (dbbl) was varied within the range of 40 – 1000μm by controlling the N2 flow rate, and the molar ratio of CO2/NH3 (αCO2/NH3) was set at a specified value of 0.20 – 1.00 at a constant CO2 flow rate. By varying dbbl and αCO2/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 dbbl was decreased, and the production of aragonite with minimizing bubble size was accelerated markedly with a decrease in αCO2/NH3.
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