The mechanisms of crystallization and transformation of calcium carbonates

Abstract

The mechanisms of the formation and transformation of calcium carbonates and the inhibition by the phosphorus containing compounds were studied. Supersaturated solutions form the unstable polymorph, amorphous calcium carbonate, at first, followed by the transformation to the metastable polymorph which finally transforms to stable form, calcite. Transformation proceeds by a mechanism of recrystallization. The growth and transformation is strongly inhibited by a phosphorus containing compounds. Details of the adsorption mechanism were reported. The zeta potentials of calcium carbonates were determined at various solution conditions. INTRO D U CTI 0 NIn order to obtain reliable data of the solubility of solid in solution, it is quite important to study the mechanism of crystal growth by taking into consideration the solution equilibria of lattice ions. That is, the formation and transformation of crystals are often significantly retarded by a quite low amount of coexisting substances (inhibitors) in solution. If the transformation of an unstable form to the stable one is prohibited by inhibitors, the solution is saturated with unstable (more soluble) crystals even if the amount of these crystals is very small. The solubility experiments under these conditions lead to erroneous results.Consequently, it is quite important to understand the details of the mechanism of crystallization in order to obtain reliable solubility data. The present paper deals with the mechanism of crystal growth, transformation and the effect of phosphorus containing compounds, which are known as one of the strongest inhibitors, with respect to the most typical and well understood ionic crystal, calcium carbonate. The adsorption of these compounds on the crystal surface and the surface potential of the crystals were reported.Calcium carbonate is one of the most common and widely dispersed minerals, occurring as limestone, chalk, and also biominerals. Because of harmless properties and inexpensiveness, it has been used for a variety of purposes such as neutralizing agent, filler, flux and cement. Furthermore, it is formed as undesirable precipitation such as scales on walls of heat exchangers and pipes. Thus, the problem of the crystal growth of calcium carbonate is situated not only at an important position in inoraganic chemistry, geochemistry and biomineralization, but also in the fields of chemical industries and water treatments. Practically crystalline CaCO3 has three polymorphs (ref. 1): Calcite (solubility product: log Ksp = -8.48 at 25 "C) is the thermodynamically most stable form and most calcium carbonate occurs in the form of calcite in nature. Less stable form, aragonite (log K,, = -8.34), is mainly found in the biosynthetic CaC03 such as shells and corals. The most unstable polymorph, vaterite (log K,, = -7.91), rarely occurs in natural fields, but plays an important role in the calcium carbonate formation from solution. A huge number of studies have been carried out on the formation and transformations of CaCO3 polymorphs. Coexis...

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