Synthesis of Single Phase Aragonite Precipitated Calcium Carbonate in Ca(OH)_2-Na_2CO_3-NaOH Reaction System

摘要

The formation behavior of precipitated calcium carbonate polymorphs was investigated in three different supersaturation levels. Because the most easily adjustable and influential variable determining supersaturation is the ion concentration of the major reactants - Ca~(2+) and CO_3~(2-) - the supersaturation can be adjusted by changing the ion concentration of these two ions. At high supersaturation, free energy is necessary for a decrease in nucleation, promoting the formation of a sphere-shaped vaterite, while aragonite and calcite were seen to co-exist at medium supersaturation. At low supersaturation, aragonite was mainly formed by mixing with some calcite. Hence, we considered that lower supersaturation was necessary to obtain a single phase aragonite. Furthermore, we found that the solubility of Ca(OH)_2 was decreased with the addition of NaOH by a common ion effect. Thus, it is possible to perform an experiment at a lower Ca~(2+) concentration. The aragonite was synthesized by adding the Na_2CO_3 solution to the Ca(OH)_2 slurry containing several concentrations of NaOH solution at 75℃ and under the addition rate of Na_2CO_3 at 3 ml/min. The formation yield of calcite decreased when the NaOH concentration was increased. In conclusion, in the case of the reaction of the 2.5 M NaOH solution over 210 minutes, single-phase aragonite with an aspect ratio of 20 was obtained.