Amorphous Silica: An Idiosyncratic, Water-Born Precipitate/Deposit That Requires Exotic Mitigation Approaches

摘要

Colloidal/amorphous silica (SiO2·xH_2O) is one of the most undesirable deposits in the watertreatment industry. It is not as widespread as other mineral scale deposits (eg. CaCO_3), but itsformation and deposition onto critical equipment surfaces can be catastrophic. Its control is mainlybased on two approaches: (a) removal of soluble and colloidal silica before entering the system,and (b) use of chemical inhibitors. Nevertheless, silica scale inhibition is not a simple task. Thereason lies with the chemical nature of colloidal silica. Silica is a random, three-dimensionalpolymer that forms by propagation of Si-O bonds, which form by polycondensation of monosilicicacid. The amorphous nature of silica precludes use of the traditional scale inhibition approaches(usually phosphonate additives and polyacrylate-based polymers) that are principally followed formineral scales. Hence, silica can be tagged as an idiosyncratic scale. Not unexpectedly, itrequires specialized inhibition strategies.This presentation focuses on the discovery, design and application of organic additives (preferablygreen and environmentally friendly) that have some effect on silicic acid polycondensation. Thispaper reports the inhibition efficiency of a variety of polymeric additives and some combinations inretarding silicic acid polymerization is supersaturated aqueous solutions. There are three classesof polymeric additives: (1) neutral, (2) cationic of varying positive charge density, (3) zwitterionic,and (4) combinations/blends of anionic and cationic polyelectrolytes. All aforementioned additivesand/or blends exhibited inhibition features from good to excellent. Inhibition efficiency was found todepend on a multitude of factors, such as cationic charge density, inhibitor dosage, time, etc.These results are discussed in detail.