Water Treatment'sGordian Knot'

To avoid silica-scale problems in cooling towers, plant personnel turn to unconventional methods

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The silicomolybdate test measures soluble (reactive) silica. It does not measure colloidal silica. It is worth noting that reactive silica does not refer to only monomeric silica (silicate ion). It also includes other oligomeric species such as dimers, trimers, tetramers, etc.

For all practical purposes, the silicomolybdate test offers a good indication of the soluble silica in the system. However, occasional measurements of total silica ," i.e., soluble and colloidal ," are recommended to ensure no silica is lost to precipitation and deposition. These measurements require sample treatment or elaborate instruments and techniques (e.g., inductively coupled plasma atomic emission or atomic absorption spectroscopy) and are not practiced routinely in the field. Samples usually are sent to an independent laboratory for study.

Conclusions

Silica is an undesirable scale for several reasons. It severely impedes heat transfer. It is tenacious and costly (and potentially hazardous) to remove. It is extremely prone to co-precipitation with other scales, particularly iron (hydr)oxides. It is often the limiting factor for limiting high cycles of concentration.

The amorphous character of silica deposits precludes the use of conventional crystal modification technologies. Molecules such as phosphonates that are effective mineral-scale threshold inhibitors provide virtually no benefit for silica-scale inhibition. They provide only an indirect benefit by maintaining a cooling tower free of other deposits that can act as precipitation nuclei for silica or catalyze silica precipitation in the bulk.

Just like the Gordian Knot ," the difficult, intractable and often insolvable problem ," resolution of the silica problem might require nonconventional means. Several factors, unique to the individual system, must be taken into consideration. These factors include water chemistry (presence of other scaling ions), nature of the silica (colloidal and/or reactive) in the make-up water, target cycles of concentration (need for water conservation), feasibility of mechanical silica removal (filtration and softening), capital costs (for chemicals and/or equipment) and many others. Careful selection of a general scale treatment program, combined with a silica-scale inhibitor and/or dispersant, is a good starting point for attaining a silica deposit-free cooling water system.

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Demadis is an assistant professor of chemistry at the University of Crete in Greece. He has extensive experience in chemical water treatment research and development and also in technical and field support. Contact him +3 2810 393651, or via e-mail at kostasdemadis@yahoo.com.

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