Consolidation of Inorganic Precipitated Silica Gel

Abstract

Abstract:Colloidal gels are possible intermediates in the generation of highly porous particle systems. In the production process the gels are fragmented after their formation. These gel fragments compact to particles whose application-technological properties are determined by their size and porosity. In the case of precipitated silica gels, this consolidation process depends on temperature and pH, among other parameters. It is shown that these dependencies can be characterized by oedometer measurements. Originally, the oedometer test (one-dimensional compression test) stemmed from soil mechanics. It has proven to be an interesting novel examination method for gels. Quantitative data of the time-dependent shrinkage of gel samples can be obtained. The consolidation of the gels shows a characteristic dependence on the above parameters.

Influence of pH, Temperature and Sample Size on Natural and Enforced Syneresis of Precipitated Silica

“…This work succeeds some of our previous work [3][4][5][6][7][8]. We refer to the literature [1,2,9-11] for a profound understanding of the polymerization and syneresis process of silica.…”

“…According to Figure 1, monomeric silicic acid (Si(OH) 4 ) is formed due to mixing of sodium silicate and sulfuric acid. The very low solubility of monomeric silicic acid (cS i(OH) 4 « 192 mg/L, corresponding to r cS i(OH) 4 « 2 mmol/L for pH < 7 and ϑ = 25˝C [2]) in combination with concentrations of c Si(OH) 4 « 10 5 mg/L at typical process conditions lead to a high supersaturation of S " c SipOHq 4 cS ipOHq 4 « 500.…”

“…The very low solubility of monomeric silicic acid (cS i(OH) 4 « 192 mg/L, corresponding to r cS i(OH) 4 « 2 mmol/L for pH < 7 and ϑ = 25˝C [2]) in combination with concentrations of c Si(OH) 4 « 10 5 mg/L at typical process conditions lead to a high supersaturation of S " c SipOHq 4 cS ipOHq 4 « 500. This supersaturation is depleted by polymerization of silanol groups (Si-OH)…”

“…These two mechanisms, namely the formation of new siloxane bonds and the reduction of the pressure difference through drainage of the pore liquid and subsequent contraction of the solid skeleton, lead to natural syneresis. Contrarily, we define "enforced" syneresis as the superposition of this "naturally" present pressure difference with an external one [4,5].…”

“…Analysis of the influences on syneresis of the principal process parameters, such as temperature, pH and ionic strength, is very time-consuming. It has been shown, that syneresis can be accelerated by applying an external, compressive force to the gel sample [4,5], resulting in an additional pressure difference. This additional pressure difference superimposes the "naturally" present pressure difference.…”