Water Treatment'sGordian Knot'

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

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The solubility of amorphous silica is important to the operation of water-dominated production processes. In areas such as Texas, New Mexico, Arizona, parts of California, southern Europe, the Pacific Rim and Latin America, the water used for industrial applications contains high silica concentrations (50 parts per million [ppm] to 100 ppm, expressed as silicon dioxide [SiO2]). These concentrations result from quartz (crystalline SiO2) dissolution from rock formations into the groundwater.

The potential for silica-scale deposition poses serious problems in water with a high dissolved silica content. In fact, silica-scale problems could be termed the Gordian Knot of water treatment ," such problems are extremely difficult to resolve. Personnel responsible for power plants, evaporative cooling systems, semiconductor manufacturing and geothermal systems must monitor water silica levels very closely.

Silica precipitation/deposition frequently is encountered in evaporative cooling systems, where salt concentrations increase through partial evaporation of the cooling water. Silica solubility in water generally is 150 ppm to 180 ppm, depending on water chemistry and temperature. This imposes severe limits on water users, leading either to operation at very low cycles of concentration and consuming enormous amounts of water, or to use of chemical water treatment techniques that prevent silica-scale formation and deposition.

Silica and/or silicate deposits are particularly difficult to remove once they form. Harsh chemical cleaning (based on hydrofluoric acid) or laborious mechanical removal usually is required. In addition, the potential for silica scale with high calcium and magnesium levels limits the use of polysilicate as a steel and aluminum inhibitor, especially in low-hardness cooling waters.

Silica speciation and deposition

Silica-scale formation is a highly complex process. It is usually favored at a pH of less than 8.5, whereas magnesium silicate scale forms at a pH of more than 8.5. Data suggest silica solubility is largely independent of pH in the range of 6 to 8. Silica exhibits normal solubility characteristics, which increase proportionally to temperature. In contrast, magnesium silicate exhibits inverse solubility.

Silica formation is actually a polymerization event. When silicate ions polymerize, they form a plethora of structural motifs, including rings of various sizes, cross-linked polymeric chains of different molecular weights, oligomeric structures, etc. The resulting silica scale is a complex and amorphous product (colloidal silica) ," a complicated mixture of the above components.

Operation in a high-pH regime is not necessarily a solution for combating silica scale. Water system operators must take into account the presence of magnesium (Mg2+) and other scaling ions such as calcium (Ca2+). A pH adjustment to greater than 8.5 might result in massive precipitation of magnesium silicate if high levels of Mg2+ are present or in calcium carbonate (CaCO3)or calcium phosphate if high levels of these ions are overlooked.

Silica precipitation also can be aggravated by the presence of metal ions such as iron (Fe2+/3+) or aluminum (Al3+) and their hydroxides. Corroded steel surfaces (e.g., on pipes or heat exchangers) are very prone to silica fouling. Iron oxides/hydroxides act as deposition matrices for silica (either soluble or colloidal) deposits.

General Treatment Guidelines for Silica-Scale Control

Silica-scale control

Current practices for combating silica-scale growth in industrial waters include operation at low cycles of concentration, prevention of other-scale formation, pretreatment and inhibitor or dispersant use.

Operation at low cycles of concentration

Keep in mind that Mg2+ levels also should be taken into account at a pH level greater than 7.5. In this case, the product (ppm Mg as CaCO3) 3 (ppm SiO2 as SiO2) should be below 20,000 ppm. The figure gives additional rules-of-thumb for silica-bearing process waters.

is a common practice, but one that consumes large amounts of water. In a cooling tower operating at a pH of less than 7.5, silica generally should be maintained below 200 ppm (as SiO2). For a pH greater than 7.5, silica should be maintained below 100 ppm (as SiO2).

Prevention of other-scale formation

Pretreatment

indirectly interferes with the propensity of silica scale to co-precipitate with other scales. The method is based on prevention of other scaling species such as CaCO3 or calcium phosphate and indirectly benefits the whole cooling tower operation. CaCO3 precipitates provide a crystalline matrix in which silica can be entrapped and grow. In environments in which CaCO3 or any other mineral precipitate is prevented completely, higher silica levels generally are tolerated in the process water than in those environments in which other scales are controlled ineffectively (Gill, 1998).

In addition, silica can be removed through reverse osmosis (RO) and ion exchange techniques, as well as desilicizers. RO membranes are not immune to silica scale, which forms as a gelatinous mass on the membrane surface. It then can dehydrate, forming a cement-like deposit.

The use of inhibitors or dispersants

involves reactive or colloidal silica removal in precipitation softeners through an interaction between silica and a metal hydroxide. Both iron hydroxide, Fe(OH)3, and aluminum hydroxide, Al(OH)3, have shown silica-removal capabilities, although magnesium hydroxide, Mg(OH)2, is considered more effective.

Dispersion, on the other hand, is the prevention of particle agglomeration to form larger-size particles and the prevention of the adhesion of these particles onto surfaces.

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