Scaling calculations are usually only based on the silicate concentration in the feed water, because suitable test methods for the interaction of silica with other ions are missing. As the contents of the other ions, i.e. above all calcium and magnesium are not considered, this method is incorrect and can lead to substantial problems.

In this paper first the results of laboratory experiments investigating the influence of different cations on the behaviour of supersaturated solutions of silica in different test waters are discussed. Subsequently, the results of the new membrane-based test method considering the influence of Ca2+ and Mg2+, the pH-value and the addition of antiscalants are discussed.

Different methods of analysis made it possible to distinguish three groups of silicates: ‘monomeric’, ‘polymeric’ and ‘filterable’. Ca2+ and Mg2+-ions have a strong influence on the formation of filterable silicate and on the kinetics of the formation of the different silicate species.

The ‘polymeric silica’ is mainly responsible for the membrane scaling, and the kinetic of its formation is strongly influenced by the cations and the pH-Value.

The antiscalant Osmotech 1309 makes it possible to operate the plant at significantly higher recovery rates, resp. at higher SiO2-concentrations.

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