Sustainable Solutions for Incineration Byproducts: The CLEAN Project's Innovative Approach to Municipal Solid Waste Incineration Residue Management

The fly ashes resulting from municipal solid waste incineration (MSWI FA) are classified as hazardous waste due to their high metal and soluble salt content, posing environmental risks upon reuse.
The CLEAN project, funded by the Ministry of the Environment and Protection of Land and Sea, introduced steam washing (SW) to reduce chloride and metal concentrations in FAs. This process transforms them into non-hazardous waste, suitable for stabilization, with ongoing investigations exploring the recovery of metals (Cr, Ni, Cu) aligning with the raw-secondary materials market.
Detailed elemental characterizations of solid matrices (pre- and post-SW) and liquid matrices (residual washing water, wastewater) employed X-ray fluorescence (XRF), X-ray diffraction (XRD), and inductively coupled plasma mass spectrometry (ICP-MS).
Results from a waste-to-energy plant in northern Italy indicate metals like Cr (300 mg/kg), Ni (65 mg/kg), Cu (1500 mg/kg), and Sb (1800 mg/kg) concentrating in the solid residue. After a 40% weight loss of FAs post-washing, there's an 80-90% increase in these elements per kg of ash. Residual washing water has high metal concentrations: 6000 mg/L Zn, 400 mg/L Pb, and 250 mg/L Cd, corresponding to approximately 15%, 10%, and 70% extraction from FAs (considering SW uses a 1.5-2.0 liquid-solid ratio).
Sequential extraction methods, variable pH release tests, and geochemical simulations trace metal species behavior to different speciation in FA. Many elements associate with mineralogical phases unstable at the treatment's pH (around 6), favoring mobilization (e.g., chlorides, hydroxides, carbonates, and surface adsorption).
While these chemical species pose challenges in FA, they can be valuable resources if extracted and recovered. SW yields two matrices suitable for metal recovery: a treated solid and a liquid one. For the solid matrix, increased heavy metal concentration per kg of FA favors effective extraction via targeted washing with diluted acid (e.g., HCl 2-3M), limiting reagent consumption. Analyses for the elemental characterization of wastewater are underway after absorption and recovery of metals by minerals like zeolites.