MORPHOLOGY AND GROWTH-RATE OF CALCIUM-CARBONATE CRYSTALS IN A GAS-LIQUID-SOLID REACTIVE CRYSTALLIZER

The polymorphism of calcium carbonate crystal was studied by introduci ng CO2 gas through a gas sparger or a double-tube gas injection nozzle into a pH-stat crystallizer containing calcium chloride solution. Sev eral operating variables were investigated and the pH of solution and calcium ion concentration were the important factors that affect the f ormation of polymorphs of calcium carbonate. At ambient temperature ei ther calcite or vaterite was the major product, depending on the pH an d calcium ion concentration. Then the same crystallization system was employed to study the growth kinetics of calcite crystals, using a dou ble-tube gas injection nozzle instead of a gas sparger, which cannot b e used to suppress nucleation, to introduce the CO2 gas into the cryst allizer. The growth rates of calcite seeds increased with increasing s upersaturation ana crystal size. As a conclusion, the polymorphs and g rowth rate of calcium carbonate are controllable in a gas-liquid-solid reactive crystallizer, using a pH-statoperation mode. (C) 1997 Elsevi er Science Ltd.