  The formation of solid calcium carbonate (CaCO3) from aqueous solutions or
slurries containing calcium and carbon dioxide (CO2) is a complex process of
considerable importance in the ecological, geochemical and biological areas.
Moreover, the demand for powdered CaCO3 has increased considerably recently in
various fields of industry. The aim of this study was therefore to synthesize
fine particles of calcite with controlled morphology by hydrothermal
carbonation of calcium hydroxide at high CO2 pressure (initial PCO2=55 bar) and
at moderate and high temperature (30 and 90 degrees C). The morphology of
precipitated particles was identified by transmission electron microscopy
(TEM/EDS) and scanning electron microscopy (SEM/EDS). In addition, an X-ray
diffraction analysis was performed to investigate the carbonation efficiency
and purity of the solid product. Carbonation of dispersed calcium hydroxide in
the presence of supercritical (PT=90 bar, T=90 degrees C) or gaseous (PT=55
bar, T=30 degrees C) CO2 led to the precipitation of sub-micrometric isolated
particles (<1$\mu$m) and micrometric agglomerates (<5$\mu$m) of calcite. For
this study, the carbonation efficiency (Ca(OH)2-CaCO3 conversion) was not
significantly affected by PT conditions after 24 h of reaction. In contrast,
the initial rate of calcium carbonate precipitation increased from 4.3 mol/h in
the "90bar-90 degrees C" system to 15.9 mol/h in the "55bar-30 degrees C"
system. The use of high CO2 pressure may therefore be desirable for increasing
the production rate of CaCO3, carbonation efficiency and purity, to
approximately 48 kg/m3h, 95% and 96.3%, respectively in this study. The
dissipated heat for this exothermic reaction was estimated by calorimetry to be
-32 kJ/mol in the "90bar-90 degrees C" system and -42 kJ/mol in the "55bar-30
degrees C" system.
