Influence of additives on the precipitation of gypsum in sodium chloride solutions

The kinetic of precipitation of calcium sulfate dihydrate (gypsum) in sodium chloride solutions was investigated at various supersaturation from 1.05–2.49, pH = 8.50, 25°C and ionic strength 0.5 M. The rate of precipitation showed a quadratic dependence upon the relative supersaturation which suggests a surface-controlled growth mechanism. Influence of a number of amino acids on the rate of reaction has been studied at several additive concentrations. The addition of amino acids even at relatively low concentration (10−6 mol 1−1) markedly retard the rates of precipitation. Moreover, the effect was enhanced as the relative supersaturations decreased. Additionally the synergistic effect of one amino acid and other inhibitors was studied. The retardation effect of these additives hasbeen attributed to the blocking of active sites by adsorption of these additives at the crystal surfaces. Inhibition of crystallization of gypsum by amino acids can be interpreted in terms of a Langmuir adsorption isotherm. This research will help to understand the usefulness of these compounds for industrial applications such as the control of scale.