Rhombohedral-scalenohedral calcite transition produced by adjusting the solution electrical conductivity in the system Ca(OH)2-CO2-H2O

This work is aimed to investigate the effects of the adjustment of the electrical conductivity (K-25) during the semicontinuous carbonation of Ca(OH)(2) suspension (slaked lime) on the morphology of the precipitated calcite (CaCO3) particles. The experiments were carried out at 30, 45. and 60 degreesC. A gradual morphological change from rhombohedral to scalenohedral shapes was produced with an increase Of K-25, from 1 to 7 mS/cm at each temperature. The explanation of this morphological change is given in terms of the increase of both the supersaturation and the ratio between concentrations of charged species containing calcium and carbonate ([Ca]ch/[CO3]ch) in the aqueous phase as the K-25 set-point increases, prior to the precipitation process. In addition to the rise of the supersaturation this change most probably takes place because the increase of the [Ca]ch/[CO3]ch ratio affects the growth rate of the rhombohedral {104} and scalenohedral {21-1} faces in a different manner: (i) favoring the equality between the surface coverage of Ca and CO3 on the stoichiometric {104} face, thus enhancing the formation of CaCO30 growth units and then its growth rate and (ii) inhibiting the growth of the {21-1} face by adsorption of the excess calcium species. (C) 2003 Elsevier Science (USA). All rights reserved.