Hydrothermal Dissolution of Opal in Sodium Hydroxide Lyes for the Synthesis of Water Glass

Abstract

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 SiO 2 :Na 2 O (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 SiO 2 :Na 2 O ratios of up to 3.7 were obtained. At longer dissolution times low-quartz and analcime precipitated and the SiO 2 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.

Hydrothermal Dissolution of Low‐Quartz in Sodium Hydroxide Lyes: Kinetics and Equilibrium

“…Rowe reported a SiO 2 concentration of 27.5 wt % to be in equilibrium with low‐quartz at 200 °C. These high SiO 2 concentrations occurred transiently in opal dissolution tests and are believed to be due to oversaturation during seed formation. Rowe applied crystalline sodium silicates as starting materials but no low‐quartz.…”

“…Final silica contents c ∞,NW obtained for Dörentrup W6 (▴), Sikron SF800 (□), and Mikrosil SP6 (+) quartz flours by the proposed calculation and silica contents of opal dissolution (○; dissolution time 20 h, ) as function of dissolution temperature.…”

“…The solubility of silica in general was already treated in . In the present report, the literature concerning quartz solubility and dissolution kinetics is further evaluated.…”

“…In a previous paper (Pfeiffer et al ) opal was used as starting material for hydrothermal dissolution of silica materials in NaOH lyes. There liquid water glasses with SiO 2 contents up to 27 wt % and molar SiO 2 :Na 2 O ratios (= R m ) of 3.4 were obtained at intermediate dissolution times.…”

Generation of High Quality Biogenic Silica by Combustion of Rice Husk and Rice Straw Combined with Pre- and Post-Treatment Strategies—A Review

“…1400 • C) [50,56]. Water glass can also be produced by hydrothermal treatment of sand with lyes, which is also an energy consuming procedure [57]. The traditional process is not only expensive, but also it is hazardous to the environment because during the production of 1 ton of silica, approximately 0.23 ton carbon dioxide, 0.74 ton sodium sulfate and 20 tons of waste water are produced, and it violates the principle of sustainable development [50].…”