Author(s)

Thomas Pfeiffer^1,2, Dirk Enke³, Robert Roth⁴, Hans Roggendorf^4*

¹Institute of Physics, Martin-Luther-University Halle-Wittenberg, Halle, Germany.
²OPTERRA Zement GmbH, Karsdorf, Germany.
³Institute of Chemical Technology, University of Leipzig, Leipzig, Germany.
⁴Institute of Physics, Martin-Luther-University Halle-Wittenberg, Halle, Germany.

Common opal was dissolved in NaOH lyes in rotating autoclaves. The starting material was characterized by X-ray diffraction and adsorption spectroscopy, thermal and chemical analysis, electron and atomic force microscopy. The opal proved to be an Opal-CT with a microstructure consisting of microcrystalline tridymite, traces of low-quartz, and amorphous parts built up by random packings of size distributed amorphous silica colloids. The dissolution conditions have been inspired by the technological process of hydrothermal water glass synthesis by dissolving silica. Temperature and time of the dissolution process as well as initial molar SiO₂:Na₂O (R_m) ratios of the starting materials were varied systematically. The particle size of the samples was varied, too, but due to the nanoscopic microstructure without greater impact on dissolution kinetics. The process products were analyzed chemically. Additionally, some of them were characterized by X-ray diffraction, viscosimetry and dynamic light scattering. Already after short dissolution times, water glasses with quite high silica concentrations of up to 27 wt.% and SiO₂:Na₂O ratios of up to 3.7 were obtained. At longer dissolution times low-quartz and analcime precipitated and the SiO₂ contents were reduced to about 22 wt.% and R_m to about 2.7. The silica contents in equilibrium with low-quartz were almost independent on temperature.

Water Glass, Opal, Silica, Hydrothermal Dissolution

Pfeiffer, T. , Enke, D. , Roth, R. and Roggendorf, H. (2017) Hydrothermal Dissolution of Opal in Sodium Hydroxide Lyes for the Synthesis of Water Glass. Advances in Chemical Engineering and Science, 7, 76-90. doi: 10.4236/aces.2017.71007.