GB941900A - Improvements in or relating to calcium carbonate - Google Patents

Abstract

The aragonite crystal form of calcium carbonate is produced by adding a continuous stream of sodium carbonate solution to an aqueous, continually agitated slaked lime slurry, which may contain caustic soda, until a concentration of 120-250 g./l. of caustic soda is formed, the temperature of the mixture being maintained between 150 DEG F. (66 DEG C.) and its atmospheric boiling point. The sodium carbonate may be added from a plurality of sources to ensure rapid mixing with the slaked lime, which is present preferably as a slight stoichiometric excess, i.e. up to 5%. Example 3 refers to lime being slaked in an agitated baffled area of a tank to which recycled caustic soda is added. The resulting slurry flows under the baffle to the main part of the tank and then through an overflow pipe to the agitated baffled area of a similar tank where sodium carbonate solution is continuously fed in. The mixture flows as before into the main part of the tank where further sodium carbonate is added until a concentration of 160 g./l. caustic soda and 151 g./l. calcium carbonate is formed, the temperature being maintained at 200-210 DEG F. The mixture then passes to a third agitated tank whence it is continuously withdrawn via an underflow pipe to a thickener where the aragonite calcium carbonate is separated from the caustic soda solution a portion of which is recycled to the slaker.