Silica almost never precipitates as “SiO2”. 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" (as Mg(OH) 2) is used. In situ precipitation works much better than already precipitated magnesium, probably due to surface area of the precipitant and proximity to a silica molecule.