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.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.