Extraction of silicon dioxide from waste colored glasses by alkali fusion using potassium hydroxide

Abstract

A process of extraction of SiO₂ from waste colored glasses by alkali fusion using KOH was investigated. In the present study, the waste colored bottles of green, blue, brown or black were selected as sample waste glasses. These colored bottles were qualitatively confirmed to contain the elements of Na, Mg, Al, Si, K, Ca, Ti, Mn, Fe and Cu from energy dispersive X-ray spectroscopic analysis. The condition for alkali fusion of KOH and the waste glasses was optimized, i.e., for each bottle, the composition was KOH:crushed glass bottle = 90:10 wt%, melting temperature was 360°C and melting time was 2 h. For each waste colored bottle, the potassium silicate solution was prepared by using the potassium silicate obtained by alkali fusion. After mixing HCl with the obtained potassium silicate solution, Si(OH)₄ was then precipitated by boiling the solution which became to be a very strong acid. After drying the Si(OH)₄ separated from the solution, the SiO₂ powder with purity of 99.9% was obtained. The yield of 97.75 ± 1.05% for the SiO₂ powder extracted from the bottles was confirmed from quantitative analysis, indicating that the bottles selected in the present study contain about 60 wt%SiO₂. From the results of the yield and purity, the established process was found to have a potential applicability as a recycling process of waste glasses. Particularly, for alkali fusion of KOH, the melting temperature of 360°C was lower than those for alkali fusion of NaOH (500°C) and carbonate fusion of Na₂CO₃ (900°C). This suggested that the energy consumption during alkali fusion of KOH and the glasses in an electric furnace was lower than those for the fusions of NaOH and Na₂CO₃. In addition, ideal recycling and reusing processes of waste glasses and the final wastes produced after extracting SiO₂ from the waste glasses were also discussed. For the KCl solution which is obtained after extracting SiO₂, the KCl can be resolved into K and Cl by electrolysis (: 2KCl → 2K⁺ − 2Cl⁻ and 2K + 2H₂O → 2KOH + H₂ ↑) and the HCl can be also prepared by the chemical reaction of the H₂ and Cl₂ (: H₂ + Cl₂ → 2HCl). These reactions suggested that the KOH and HCl can be reused as the alkali flux and acid solution, respectively. From these results and discussions, the established recycling process of waste glasses was expected that cost reduction would result from selection of KOH as an alkali flux and reuse of the final wastes.