Polymorph control of calcium carbonate by reactive crystallization using microbubble technique

Abstract

In this study, we used the minute gas–liquid interfaces around CO 2/NH 3 microbubbles as new reaction fields where the crystal nucleation progresses and developed a crystallization technique to control the polymorphism of calcium carbonate (CaCO 3). In the regions around the gas–liquid interfaces of CO 2/NH 3 microbubbles, Ca 2+ and CO 3 2− ion concentrations can be adjusted because of the characteristic of the electric charge on the bubble surface and the decrease in CO 2 concentration based on unit bubble caused by minimization of bubble size, and because of the pH difference between local pH at the gas–liquid interface and overall pH in the bulk liquid caused by mixing of NH 3 with CO 2; hence, the polymorph change of CaCO 3 is expected to occur. CaCO 3 was crystallized at 298 K by a semi-batch type reaction in which CO 2/NH 3/N 2 bubbles were continuously supplied to an aqueous Ca(NO 3) 2 solution using a self-supporting bubble generator. The solution pH during crystallization was maintained at a constant level of 6.9–12.0 by adding HNO 3 and NH 4OH solution. The average bubble size ( d bbl) was varied in the range of 40–1000 μm by controlling the N 2 flow rate, and the molar ratio of CO 2/NH 3 ( α C O 2 /N H 3 ) was set at a specified value of 0.20–1.00 at a constant CO 2 flow rate. The following results were obtained by varying solution pH, d bbl, and α C O 2 /N H 3 : at a constant d bbl of 40 μm and α C O 2 /N H 3 of 0.20, vaterite and calcite were major products at a solution pH lower than 9.0 and at a solution pH greater than 11.0, respectively, while aragonite was crystallized predominantly in the solution pH range of 9.7–10.5; at a constant solution pH of 9.7 the crystallization of aragonite was accelerated remarkably with a decrease in α C O 2 /N H 3 and d bbl.