Petrogenesis of municipal solid waste combustion bottom ash

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A petrographic study was conducted on a suite of bottom ash particles from 3 different modern municipal solid waste combustors. The object of the study was to evaluate the mineralogical characteristics and formation process of the bottom ash by using standard geological techniques of light microscopy, electron microscopy, and X-ray microanalysis. This information was subsequently used to model the bottom ash petrogenesis based upon an examination of the mineralogy, melt structure, and composition of the ash. Bottom ash can be divided into two major groups: 1) refractory waste products and 2) melt products. The refractory waste products consist largely of rock and mineral fragments, various waste metals, and unmelted glass shards. The melt products consist of two distinct glasses: 1) isotropic glass, and 2) opaque glass. Complex silicate minerals are precipitated from and are abundant in the isotropic glass whereas both metal oxide and silicate minerals are precipitated from the opaque glass. The isotropic and opaque glasses formed simultaneously in different locations on the combustor grate. The contrast in melting (liquidus) temperatures shown by these glasses suggests that the isotropic melts were produced at localized hot spots (1500°C to 1650°C) and the opaque melts formed at cold spots (1150°C to 1400°C) on the grate. This could be the result of heterogeneous distribution of combustible municipal solid waste on the grate or from localized hot spots where air is introduced through the grates. In some instances the two glasses then had the opportunity to variably mix with each other. Fe-oxides represent waste metal fragments that were assimilated by melting and later recrystallized. Bottom ash is produced via a co-mingled two melt system that forms melilite-bearing, alkaline, volcanic-like rocks. The great similarity of the bottom ash residues between these 3 different MSW combustors suggests that, despite variable combustor designs and heterogeneous waste feed, high temperature combustion of MSW produces bottom ash of fairly uniform composition and structure that formed via the petrogenetic process described above. Alterations to the combustion process or implementations of secondary treatment technologies may render the bottom ash residue into a more environmentally stable material better suited for aggregate or long term secure disposal in landfills.