A Controlled Composition Study of Calcium Carbonate Crystal Growth: The Influence of Scale Inhibitors

The precipitation of calcium carbonate is of considerable importance in a wide variety of scale forming situations since carbonate concentrations in the environment are usually quite appreciable. As a first-formed solid phase, calcium carbonate may induce the growth of other scale minerals on its surface. Since the solubility of calcium carbonate polymorphs decreases with increasing temperature, the problem is also aggravated by the relatively high ambient temperatures in many processes. This paper describes a new highly reproducible, seeded crystal growth method in which the activity of ion species is automatically maintained constant during the crystal growth reaction so that the kinetics can be studied over a wide range of supersaturation. The rate of calcite growth, over a 30X range of ionic strength, is proportional to the square of the relative supersaturation ([{Ca2+}{CO32−}]1/2−Kso1/2)2 expressed in terms of the activities of the ionic species. The rate is directly proportional to the seed concentration over a 3X range of the latter. The constant composition method has also been used to investigate the mineralization rate of a rotating disc of calcite (∼ 1 cm2 area) having very well defined hydrodynamic parameters. The rate of reaction (per m seed surface) is in striking agreement with that obtained using seed crystals (typically 500 cm2) despite the large difference in available growth sites. The evidence points to a surface controlled crystallization mechanism. Traces of orthophosphate and typical phosphonate scale inhibitors markedly inhibit the rate of calcium carbonate scale formation. Even with the sustained driving force provided by the constant composition method, a concentration of 9.1×10−8 M hydroxyethylidene-1, 1-diphosphonic acid completely inhibits the crystallization of calcite for more than 100 hours. The effectiveness of this scale inhibitor is clearly due to adsorption at active growth sites on the developing crystal surfaces.