THE PRECIPITATION OF CALCIUM-CARBONATE IN AQUEOUS-SOLUTIONS

The stability of calcium carbonate supersaturated solutions was studied between 25-80-degrees-C and pH 8.00-9.00. The stability diagrams showed narrow supersaturation ranges, for which very reproducible induction times could be obtained. Investigation of the precipitation process by a plethostatic (i.e., in which a constant number of moles is maintained) experimental approach, enabled us to identify vaterite as the initially forming crystalline phase, throughout the pH and temperature range investigated. The precipitation took place via a polynuclear mechanism. Ethane-1-hydroxyethylideno-1,2-diphosphonic acid (EHDP) caused not only retardation in the rates of spontaneous precipitation but resulted also in stabilization of the initially forming vaterite. An apparent activation energy of 46 kJ mol-1 was obtained for the precipitation reaction of vaterite. Seeded growth experiments on calcite seed crystals over the temperature range 10-37-degrees-C, gave second order kinetics suggesting a spiral growth mechanism. In all cases, calcite was found to be the only phase forming. The presence of additives such as orthophosphate and oxalate reversed the surface charge of the calcite particles while metal ions such as Cd2+ or Zn2+ resulted in a marked reduction of the rates of precipitation by adsorption on the active growth sites on the surface of the calcite crystals.