Date of Award

Spring 1995

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Electrical and Computer Engineering

First Advisor

Josse, Fabien

Second Advisor

Fournelle, Raymond

Third Advisor

Shana, Zack A.

Abstract

Scale formation on equipment has always represented a major problem for industries as well as households in general. Traditionally, the prevention of scale has been achieved through the use of chemicals. These chemicals, in addition to the waste they generate, often complicate the problem by also promoting the corrosion of pipes and other surfaces with which they come into contact. Various alternative methods of scale prevention have been proposed. Many of these methods involve magnetic fields and/or electric fields. This study involved the investigation of the effect of an applied electrostatic field to water on the formation of scale. Milwaukee water and two different concentrations of a prepared solution, namely a 110 ppm and 250 ppm as CaC03, were used in this study. The study utilized an accelerated scaling test (AST). The AST is a three electrode electrochemical test in which scale is deposited on a stainless steel working electrode. By recording the electrochemical current with time, one may obtain information about scale formation time and perhaps scale deposition rate. Scanning electron microscopy (SEM) was also used to characterize the morphology of the scale obtained during the AST. The results show that, in general, two samples of water, one exposed to the electrostatic field (treated), and one not exposed (untreated), have different scaling times. In general, treated samples have longer scaling times - thus partially confirming the claims that the field retards scale. From the micrographs, the treated samples appeared to be mostly in the form of calcite - the other main polymorph of calcium carbonate (CaC03) also as expected. The untreated samples give rise mostly to aragonite. However, there were samples which had combinations of both. In general, it was found that a sample (from a prepared solution) of a high concentration showed a shorter scaling time while a sample from a low concentration solution had a longer scaling time. Moreover, AST for samples at high temperatures showed shorter scaling times. The study also investigated the effect of Fe2 + and Mg2 + impurity ions on scale formation. It was found that the presence of iron impurities (Fe2 + and/or Fe3+ in concentrations above 2 ppm delayed the growth of the deposited scale. Examination of scale using SEM revealed that the deposit obtained in the presence of Fe2 + was mainly aragonite and distorted forms of it. The SEM results also showed that the scale obtained from those samples with longer scaling times, mostly the treated samples, was made of larger crystals. This reflects the longer growth periods. Finally, X-ray diffraction showed that the precipitate from treated samples from highly saturated solutions (250 ppm as CaC03) was mostly calcite and that from untreated samples was mainly aragonite. While the interaction mechanism by which the applied electric field retards scale is not well understood, it could, however be said that the field probably promotes the formation of calcite-like nuclei. Again, the exact processes leading to this assumption are not clearly understood. It may be postulated that the field changes the hydration of various ions in the water. The result could lead to charge imbalance which accelerates the substitution of co32- and other cations for HC03- and Ca2+ respectively (or vice versa) into the lattice of CaC03. This may then lead to a coordination of the cation by oxygen which preferrentially [sic] promotes the growth of calcite-like nuclei rather than aragonite.

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