Water Treatment'sGordian Knot'

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

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A number of products are available commercially for silica-scale control in RO, geothermal and evaporative cooling water applications. Discussion of these products is not the intent of this article. However, much information about commercial silica-scale treatment can be found on the Web through any of the popular search engines. In addition, several proprietary technologies can be found in patent literature.

to control silica scale generally follows two approaches: inhibition and dispersion. Inhibition is defined as the prevention of silica oligomerization or polymerization. As a result, the silica remains soluble and, therefore, reactive.


Mechanism of silica-scale inhibition

Amorphous silica formation is governed by several equilibria. Silica deposition results from silicic acid self-condensation. This reaction is first-order and is catalyzed by OH- in the pH range of 5 to 10. Reports have shown that the reaction yielding a silicic acid dimer is kinetically slow in contrast to the reactions giving a trimer, tetramer, pentamer, etc., which are very fast. All these equilibria are very sensitive to pH and tend to be accelerated by metal ions that form hydroxides, e.g., Fe2+, Mg2+ or Al3+.

Polymerization of silicic acid is believed to occur through a mechanism involving a deprotonated silicon monoxide (Si-O-) and the Si center of silicate, Si(OH)4. Inhibition of this step should be critical in the inhibition of silica-scale formation. Some reports indicate that orthosilicates hydrolyze more rapidly than other silicate species such as disilicates, chain silicates, cross-linked oligomers and polymers, suggesting that bridging oxygens are much more resistant to attack than nonbridging oxygens. Above a pH of 2, this mechanism involves polymerization with condensation, catalyzed by OH-.

Silica-scale formation involves condensation between Si-OH groups formed at the material surface and Si-OH of the dissolved silicate present in water. Condensation between the Si-OH units formed at a glass surface and dissolved Si-OH can be the dominant mechanism (Hayakawa, et al.).

Silica polymerization is governed largely by pH. Unfortunately, silica is a foulant not easily cured through pH adjustments. For example, CaCO3 scale virtually can be eliminated if a cooling tower system is operated at a lower pH. With water containing a high concentration of silica, operation at a higher pH generates the problem of magnesium silicate scale. Lowering the pH (by feeding acid) does not eliminate the problem; it just shifts it from magnesium silicate to silica.

A low operational pH also increases the corrosion rates of metallic surfaces, ultimately leading to material failure. Silica solubility is very high at a pH greater than10, but this pH regime is not an operational option for cooling tower systems.

Dissolved silica precipitates out of solution principally in three ways: through surface deposition, through bulk precipitation or in living organisms.

Surface deposition.

Bulk precipitation.

This occurs as a deposit on a solid surface where the [Si(OH)4-x]x- condenses with any solid surface possessing -OH groups. If the surface contains M-OH moieties (M = metal), this reaction is enhanced further. Such pronounced silica deposition phenomena in the water treatment industry are evident on metallic surfaces that have suffered severe corrosion, with a surface covered with metal oxides/ hydroxides. Once the receptive surface is covered with silica scale, additional silica is deposited on an already-formed silica film.

In living organisms.

The precise mechanism of silica formation is not well understood. Any interference with the condensation reaction could lead to silica-scale growth inhibition. A relevant example is silica inhibition by orthoborate, which reacts with silicate ions to form borosilicates. These products are more soluble in water than are silica/metal silicates.

This occurs as colloidal silica particles grow through the condensation reaction. The particles collide with each other and agglomerate, forming larger particles.

Silica-scale inhibition monitoring

Any chemical treatment program req-uires proof of its performance. For example, programs to control CaCO3 are monitored by measuring Ca hardness in the recirculating water and by comparing the measured and theoretical amounts. Inhibitor levels also are monitored closely to ensure the presence of sufficient inhibitor and dispersant polymer.

This form of silica is called biogenic and appears in certain microorganisms such as diatoms that have the ability to remove and deposit silica from highly undersaturated solutions into precisely controlled structures of intricate design. It should be mentioned that sessile microorganisms in a biofilm-fouled heat exchanger can entrap colloidal silica. The high affinity of soluble silica toward exocellular biopolymers such as polysaccharides has been recognized (Gill, 1998).

For silica, an easy field test is based on the silicomolybdate method. This method is based on a yellow complex that forms between molybdate and silicate under certain conditions and can be measured spectrophotometrically.

Phosphate can interfere with the measurement by forming a similar complex with molybdate. Its interference can be eliminated by the action of oxalic or citric acid.

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