Composition of fly ash from a grate firing municipal solid waste incineration plant with a focus on the composition and distribution of zinc-bearing minerals

Municipal solid waste incineration (MSWI) generates fly ash as a minor but chemically and mineralogically complex byproduct, considered both an environmental risk and a potential secondary resource. This study investigates fly ash collected from eight outlets at the ARGO MSWI plant in Roskilde, Denmark using SEM-automated quantitative mineralogy, and X-ray diffraction analyses to assess mineralogy and grain morphology of the fly ash with focus on the nature and distribution of zinc-bearing minerals. Fly ash particles exhibit rounded morphologies with inclusions. They show a consistent sequence of minerals in rims around the grains—silicates, oxides, carbonates, sulfates, and chlorides—interpreted to reflect the crystallization order during cooling. Agglomeration is driven by alkaline phases such as arcanite and halite, which bind smaller particles into loosely structured clusters. Zincite formation is favored in alkaline intermediate temperature zones, co-occurring with arcanite. Zinc concentrations increase from 0.5 wt% in the combustion chamber to 5 wt% in the bag filter. The main Zn-mineral changes from Zn-oxides in the combustion chamber to Zn-silicates in the bag filter. The abundance of hydrated Zn-silicates in the bag filter suggests condensation of zinc here, followed by mineral reactions with silicates under moist conditions facilitated by halite. These findings provide a mineralogical framework for understanding fly ash formation and support future targeted zinc recovery strategies in MSWI systems.