Nucleation of calcium carbonate at different initial pH conditions

The precipitation of calcium carbonate from mixing calcium chloride and potassium bicarbonate/carbonate solutions at 25 degrees C has been studied doing unseeded batch pH-drift experiments. The record of pH has been used to determine induction times. The pH and pCa measurements were used for calculations of species concentration and supersaturation. SEM indicated that vaterite was the predominant phase formed. The dependence of the induction time on supersaturation was analyzed with different models. The results indicate that a good fit is obtained from the plot of In t(i) versus 1/1n(2) beta but the small size of the critical nucleus indicates that an atomistic model is more realistic. The nucleation parameters have been determined using an empirical expression of the growth rate of vaterite taken from the literature, together with the experimentally measured induction times.