Kinetics of crystal growth of calcium sulfate dihydrate. The influence of polymer composition, molecular weight, and solution pH

A seeded crystal growth technique has been used to study the influence of solution pH and temperature ranging from 25 to 50 °C on the performance of polymers of varying functional groups as calcium sulfate dihydrate (CaSO4•2H2O, gypsum) crystal growth inhibitors. Results indicate that at constant temperature and at a constant solution pH, crystallization in the presence of polyacrylic acids is preceded by an initial slow growth reaction, hereafter called induction period, following which crystal growth of gypsum proceeds with a rate close to that in pure supersaturated solution. Results suggest that at a constant pH, polyacrylic acid concentration, molecular weight, and temperature greatly affect the duration of induction period. Kinetic data collected as a function of solution pH in the range 2.5 to 9.0 suggest that solution pH has a marked effect on the induction period. The observed dependence of induction period on solution pH may be explained in terms of the degree of ionization of polyacrylic acid. Among the polymers of varying functional groups studied, i.e. carboxyl, sulfonate, acrylamide, dimethyldiallylammonium, etc., only those polymers having carboxyl group showed marked inhibitory activity for the growth of gypsum crystals.