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In a water purification system, the treatment objective may include reducing silica concentration to permit increased cycles of concentration without scale. Although conventional coagulation and filtration is effective to remove most (if not all) colloidal silica, this usually is a small fraction of the total silica in natural water supplies. Conventional precipitation technologies are reasonably cost-efficient to achieve partial removal of silica. Drawbacks to precipitation are numerous. Precipitation of silica is a messy, time-consuming process and can be difficult to accomplish without upsets.
Where complete removal of silica is required, various combinations of reverse osmosis (RO) and/or ion exchange processes are used. The current state-of-the-art technology includes both multiple membrane and multiple ion exchange steps, and can produce reactive silica concentrations in the neighborhood of 0.1 ppb.
Chemical Precipitation
Silica almost never precipitates as SiO^sub 2^. Silica scale almost always contains a divalent (or trivalent) cation. Although calcium silicate is quite insoluble, this compound does not form rapidly except at very high temperatures. Although aluminum salts can be used to precipitate silica, the consequences of leaving a substantial aluminum residual in the product water makes this process undesirable. The conventional method of precipitating silica has always been co-precipitation with magnesium.
Since silica becomes part of the magnesium precipitant, some means of adding already precipitated magnesium (magnesium oxide) or of precipitating magnesium in situ is used. In situ precipitation works much better than already precipitated magnesium, probably due to surface area of the precipitant and...