The inhibition of calcium carbonate precipitation in water softening

The cold-lime water softening reaction was simulated by the precipitation of calcium carbonate following the addition of lime water, to prepared solutions of calcium bicarbonate. A laboratory batch crystalliser was used. The overall rate of calcium ion removal was followed using an ion selective electrode and samples were removed for Coulter Counting to enable the variation in crystal size with time to be studied. The effect on the crystallisation rate due to some organic materials likely to be present in raw water or added during processing was studied. These experiments were done with and without added calcium carbonate to simulate conditions in a retained sludge type of reactor. The calcium ion measurements enabled the induction period before precipitation, the half-life of the precipitation and the precipitation rate to be determined. High molecular weight polyacrylamides of anionic, cationic and nonionic type, had no significant effects whereas an extract of water humic substance and low molecular weight polyacrylate had a marked effect on the induction period, half life and precipitation rate. A starch was intermediate in its effects. The optimum does and the adsorption isotherms of the flocculants on calcium carbonate were also determined. The crystal size distribution over the range 2-37 um was determined by Coulter Counter for several runs. Although high product recovery was obtained attempts to use the population balance method for a detailed study of the nucleation kinetics were less successful. From the results obtained-possible reasons for the observed differences in inhibition behaviour were discussed, with particular reference to the solution diffusivity of the species concerned.