THE SEEDED GROWTH OF CALCIUM-SULFATE DIHYDRATE CRYSTALS IN NACL SOLUTIONS UP TO 6-M AND 90-DEGREES-C

The seeded crystal growth rate of calcium sulfate dihydrate was measur ed as a function of supersaturation in NaCl electrolyte solutions from 0 to 6 m at temperatures of 25, 50, 70, and 90-degrees-C. The growth followed a second-order parabolic rate law with activation energies gr eater than 53 kJ/mol which suggested the surface reaction as the rate- limiting step. It was observed that the rate constant and the activati on energy are solution composition dependent. The rate constant increa ses with NaCl concentration up to 3 molal and then begins to fall slig htly. The activation energy dropped from 61 kJ/mol in the pure Ca-SO4- H2O system to 53 kJ/mol in 3.0 m NaCl solutions. The electrolyte effec t was similar to the crystal solubility behavior in aqueous electrolyt e solutions resulting from the variation of the mean activity coeffici ent of the crystal with ionic strength. The rate constant is proportio nal to the crystal solubility and inversely proportional to the edge w ork or the interfacial tension as expected in the classic BCF model. ( C) 1994 Academic Press. Inc.