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Novel low temperature synthesis of sodium silicate and ordered mesoporous silica from incineration bottom ash

Alam, Qadeer, Hendrix, Yuri, Thijs, Luuk, Lazaro, Alberto, Schollbach, Katrin, Brouwers, H.J.H.
Journal of cleaner production 2019 v.211 pp. 874-883
alkali treatment, bottom ash, byproducts, gels, hydrolysis, municipal solid waste, porous media, recycling, silica, sodium silicate, surface area, temperature, waste incineration, zeolites
A novel low temperature synthesis route to convert environmentally harmful silica-rich waste incineration bottom ashes into ordered mesoporous silica is reported. Bottom ash is a major by-product of municipal solid waste incineration with limited recycling options due to harmful contaminants. In this study, a low temperature alkaline dissolution process was employed to synthesize sodium silicate instead of a conventional high temperature fusion process. Moreover, the dissolution process was systematically investigated to attain fundamental insight into the hydrolysis of silica from bottom ash, which is currently lacking in the existing literature. The mineralogical composition of the ash residues before and after desilication experiments was quantified via Rietveld analysis to understand the formation of by-products, such as geopolymeric gels and zeolites. These by-products hinder the dissolution of the silica because of the following two factors: Firstly, their formation consumes part of the soluble silicate and, secondly, the precipitation of the by-products around the etching particles of bottom ash act as a passivating layer which hinders the diffusion of soluble silica away from the particle. The optimized reaction temperature and reaction time for the silica extraction was observed to be 75 °C for 48 h. A sequential extraction under these conditions can successfully attain an extraction efficiency of 70% of the silica. Subsequently, the sodium silicate derived from the bottom ash was used to synthesize mesoporous silica with a high specific surface area and purity of 870 m2/g and 99 wt %, respectively.