KINETICS OF CaCO3 PRECIPITATION IN THE PRESENCE OF SHMP

摘要

The present work examines the kinetics of CaCO3 precipitation in a continuous stirred tank vessel in the presence of the inhibiting additive sodium hexametaphosphate (SHMP). The degree of CaCO3 precipitation and the degree of SHMP adsorption on the crystallizing particles were both measured in a series of experiments performed at the same initial supersaturation level. The parameters varied were the initial SHMP concentration and the residence time in the precipitation vessel. Variation of the SHMP dosage level gave the anticipated trend of a decrease in the degree of CaCO3 precipitation with an increase in inhibitor concentration. However, unexpectedly, at all SHMP dosage levels, the degree of CaCO3 precipitation was found to diminish at increasing residence times. The rate of SHMP adsorption on the crystallizing CaCO3 particles was found to vary inversely with the square root of residence time. This result suggests that adsorption of the SHMP on crystallizing CaCO3 particles is a slow, diffusion-controlled process. Precipitation is increasingly inhibited as the level of adsorbed SHMP increases at the augmented residence time. A diffusion-controlled adsorption model enabled successful correlation of all kinetic data.