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Reconsidering water scaling tendency assessment



Evaluation methods of the CaCO3 scaling risk are still far from being well understood and in many cases lack accuracy. Most of the saturation indices and models used so far for predicting scaling are based on the thermodynamic data of calcite, which is considered to be the most stable form of the different calcium carbonate varieties. However, surpassing the solubility product of anhydrous calcium carbonate forms (calcite, aragonite, and vaterite) is not sufficient to induce spontaneous precipitation. The solubility product of the CaCO3·H2O constitutes a lower limit for spontaneous germination. Based on the thermodynamic data of the monohydrate form, a novel scaling index is established and referred to as the Monohydrated form of the Langelier Scaling Index (MLSI). The ability of the MLSI to correctly predict the scaling phenomenon was verified and compared to that of the well-known Langelier Scaling Index (LSI) for several experimental results gathered from the available literature in the scaling field. Unlike the LSI, the MLSI was able to predict the instantaneous CaCO3 precipitation in scaling water. For water in the temperature range between 25 and 60°C, a classification of scaling/equilibrium state according to MLSI values is presented. © 2006 American Institute of Chemical Engineers AIChE J, 2006