Prediction of the limit of the metastable zone in the "CaCO3CO2H2O" system

The surpassing of the solubility product of the anhydrous forms of calcium carbonate—calcite, aragonite, and vaterite—is not sufficient to induce spontaneous precipitation. The existence of a metastable zone, in the nucleation of the calcium carbonate, is still an experimental phenomenon. A thermodynamic demarcation of the metastable zone in the “CaCO3CO2H2O” system, where only a secondary nucleation can occur, has been delimited for the first time. Through experimental exploration of a large supersaturation field, results obtained by the bubbling method are treated with the use of thermodynamic data of different varieties of CaCO3. At temperatures ranging between 25 and 60°C, a primary nucleation (spontaneous precipitation) occurs when the ionic activity product of the calco–carbonic solution surpasses the solubility product of CaCO3·H2O. No spontaneous nucleation occurs when the ionic activity product stabilizes between the solubility products of calcite and monohydrated calcium carbonate, which means that the solution remains in a metastable state. The metastability can be broken by seeding with calcium carbonate crystals (aragonite in this case) and then the germination is a secondary one. A model for the prediction of the limit of the metastable zone, presented in this report, is in agreement with experimental results. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1908–1916, 2004