Achieving end-of-waste status for municipal solid waste incineration fly ash: the phoenix process from mineralogical characterization to life cycle assessment

Municipal solid waste incineration (MSWI) fly ash is considered hazardous waste in Europe because it contains heavy metals, soluble chlorides, and possibly persistent organic pollutants. Traditionally, this waste is managed by stabilizing or solidifying it and then sending it to hazardous landfills, which is expensive, uses a lot of land, and does not fit with circular economy goals. This thesis examines the Phoenix process, created by A2A Ambiente S.p.A. and will be installed at the Integrated Center of Corteolona and Genzone (PV). The Phoenix process turns fly ash into two End-of-Waste (EoW) products: synthetic sand for construction and chloride salt for road de-icing, using granulation, carbonation, and salt recovery. First, the thesis reports the comparison of Phoenix to other fly ash treatment technologies found through a structured technology search (Carbon8, Ash2Salt, FLUWA/FLUREC, HALOSEP, NOAH, NORSEP, and Renova) made by A2A Ambiente in collaboration with Chem4Tech group of the University of Brescia. The report has been critically reviewed and completed. Unlike these methods, Phoenix aims to convert all the input ash into useful products, avoiding leftover waste for landfill. The thesis then presents the results of chemical, eco-toxicological, leaching, and technical-standard tests on Phoenix sand and salt from several production batches, checking if they meet EN 13055-1/2, UNI EN 16811, DM 186/2006, DM 127/2024, and REACH/CLP/POPs standards. Phoenix sand meets the chemical, eco-toxicological, and technical requirements tested here: it is non-hazardous, meets EN 13055-1/2 lightweight-aggregate standards in all tested batches, and poses little risk in both indoor plaster and road subbase scenarios, although about a third of the batch-elution samples showed higher sulfate and chromium leaching. Mineralogical and sequential-extraction tests, done with the Universities of Padova and Turin, show that this chromium release comes from chromate trapped in ettringite and similar cement phases, and that most of the released chromium is trivalent. The study suggests using reducing agents and adsorbents to reduce this release. A separate comparison of binders (white cement versus activated slag, or “loppa”) shows that slag-based binders reduce chromium and molybdenum release compared to white cement, but result in weaker bonds between particles, as seen under a scanning electron microscope. These findings are used to judge how environmentally and technically robust the Phoenix process is compared to other methods, and to point out the main regulatory and technical issues that could affect its wider use.