Abstract
The properties and the conditions of formation of a metastable hydrated form of calcium carbonate (CaCO 36H 2O) are described. This compound often crystallises when Essex river waters are softened by the lime/soda process. Formation of the hydrated salt is favoured by low water temperatures and by high orthophosphate levels. Calcium carbonate hexahydrate forms pure well-defined crystals which are considerably more soluble in water than calcite, the usual product of lime/soda softening. The efficiency of the softening process is very much reduced when the hydrated compound crystallises, and increased quantities of chemicals are required both for softening and for subsequent stabilisation of the softened water. Considerable success has been achieved on the practical scale in reducing the incidence of formation of the hexahydrate by precipitating phosphate before softening using ferric salts and by addition of pre-formed calcite nuclei in the form of commercial Whiting; this has led to substantial economies in chemical consumption.
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