Abstract
The production of silica is performed by mixing an inorganic, silicate-based precursor and an acid. Monomeric silicic acid forms and polymerizes to amorphous silica particles. Both further polymerization and agglomeration of the particles lead to a gel network. Since polymerization continues after gelation, the gel network consolidates. This rather slow process is known as “natural syneresis” and strongly influences the product properties (e.g., agglomerate size, porosity or internal surface). “Enforced syneresis” is the superposition of natural syneresis with a mechanical, external force. Enforced syneresis may be used either for analytical or preparative purposes. Hereby, two open key aspects are of particular interest. On the one hand, the question arises whether natural and enforced syneresis are analogous processes with respect to their dependence on the process parameters: pH, temperature and sample size. On the other hand, a method is desirable that allows for correlating natural and enforced syneresis behavior. We can show that the pH-, temperature- and sample size-dependency of natural and enforced syneresis are indeed analogous. It is possible to predict natural syneresis using a correlative model. We found that our model predicts maximum volume shrinkages between 19% and 30% in comparison to measured values of 20% for natural syneresis.
Highlights
Precipitated silica (SiO2) is an amorphous solid and is applied in many fields of industrial applications
We found that our model predicts maximum volume shrinkages between 19% and 30% in comparison to measured values of 20% for natural syneresis
“Natural syneresis” of silica can be measured with a pycnometer device adapted to determine the influence of the process parameters
Summary
Precipitated silica (SiO2) is an amorphous solid and is applied in many fields of industrial applications. The liquid contained in the pores is expulsed due to a pressure difference that is induced between the gel and its environment These processes are known as syneresis and are highly important for the final product properties [1,2]. Natural syneresis is measured with an adapted pycnometer device, whereas a specially designed pressure cell is proposed for the observation of enforced syneresis. These devices have already proven to be suitable [5]. A brief review of the state-of-the-art in theory of precipitation of silica is given Following this part, an experimental set-up is proposed that permits the measurement of natural and enforced syneresis. Thereafter, the correlative model for the prediction of natural syneresis is derived
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