Bottom and fly ashes: constitution

Daniel Vollprecht (Montanuniversität Leoben)

Incineration by grate or fluidized bed furnaces represents the state-of-the-art treatment of non-recyclable fractions of municipal solid waste (MSW) as it reduces its volume and mass, destructs organic contaminants and prevents the release of methane from landfills allowing the safe and sustainable landfilling of the resulting ashes. Furthermore, MSW incineration unlocks the resource potential of metals intergrown with non-metallic particles in mixed MSW. However, the mineral fraction of MSWI ashes represents a longstanding challenge in waste management.

This lecture deals with the formation of MSWI ashes by evaporation, calcination, solid state reactions and partial melting focusing on the fate of heavy metals and their distribution between bottom and fly ashes. Then a bridge is built via the mechanical processing of MSWI bottom ashes for the recycling of ferrous and non-ferrous metals and incineration remainders such as glass, ceramics and building materials to incineration products composed of salts, ash phases and melt products.

Based on a comprehensive description of the chemical and mineralogical composition of MSWI bottom and fly ashes with special emphasis on heavy metals, it concludes with a chapter on the weathering of MSWI bottom ashes by oxidation, hydration and carbonation which is relevant in case of a recycling of the mineral fraction of bottom ashes for civil engineering purposes.

Finally, the lecture concludes that either a part or the entire MSWI bottom and fly ashes must represent the final sink for contaminants in a circular economy, but another part may be used as secondary raw material depending on primary raw material availability and quality.