Effect of monovalent salts on morphology of calcium carbonate crystallized in Couette-Taylor reactor

Monovalent ionic additives, Na⁺, K⁺ and NH ₄⁺ impact on the morphology and agglomeration of CaCO ₃ crystals. As increasing the additive concentration, the regular shaped crystals such as rhombohedron and spindle are changed to irregular one due to the inclusion of Na⁺ and K⁺ into the crystal structure. The inclusion of Na⁺ and K⁺ is detected using ICP-AES. The partition of coefficients of Na⁺ and K⁺ are estimated as 9.74×10^-4, 9.73×10^-4, respectively and the amount of inclusion in the crystals is about 2×10³ ppm. However, the inclusion of ions does not modify a crystal structure of calcite. Since NH₄⁺ is large in radius, it is not included in crystal but shifts the spindle shape of crystal to the rhombohedral one. It is interesting to find that such modification of crystal morphology begins to appear at high additive concentration (0.05 M). In addition, the crystal agglomeration is promoted because the electric repulsive charge is reduced as increasing the additive concentration.