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.

Subject
Water, Precipitation (solids), Correlation, Diffusion, Additives, Adsorption, Scale inhibitors, Solubility, Dosages, Supersaturation, Experimental data, Chemical inhibition, Corrosion inhibitors
Keywords:
CaCO3 precipitation, residence time, sodium hexametaphosphate, inhibition, diffusion-controlled adsorption
© 2007 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2007
Association for Materials Protection and Performance (AMPP)
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