The CLEAN project: a sustainable methodology to reduce the environmental impact of MSWI fly ash and to recover strategic metals

Municipal solid waste fly ash (MSWI FA) is classified as hazardous waste due to its high metal and soluble salt content, giving rise to environmental risks upon its reuse. Multi-step washing proved highly effective, notably in chloride removal, yet often requires substantial water volumes (Ferraro et al., 2019). Therefore, water washing poses challenges including high water consumption, substantial wastewater generation, heavy metal enrichment, and potential secondary environmental contamination. The CLEAN project, funded by the Ministry of the Environment and Protection of Land and Sea, introduced the steam washing (SW) to reduce chloride and metal concentrations in FAs, exploiting the steam available in waste-to-energy plant, as a sustainable solution to decrease the volume of these residues and make them more suitable for stabilization, since the reduction of alkali chlorides (including KCl, NaCl) can improve fly ash mechanical and geotechnical properties as mechanical strength. While SW wastewater is rich in trace elements, investigations were conducted to recover heavy metals (Cr, Ni, Cu) for the raw-secondary materials market. Detailed elemental characterizations of solid matrices (pre- and post-SW) and liquid matrices (leachates, wastewater) were carried out, by X-ray fluorescence (XRF), X-ray diffraction (XRD), and inductively coupled plasma mass spectrometry (ICP-MS). The results of leachates analyses of samples treated by SW show that steam washing was efficient in removing especially water-soluble chlorides including sodium chloride, potassium chloride (up to 80%) and sulfates (by 30%); while for heavy metals, like Cd, Zn, Pb, the removal was up to 80%, even at short time of treatment (15-30 minutes). It was noted a decrease of concentration all for trace elements, excluding Sb, (only 70% and higher than the legal limits), which still requires particular attention and study. Results about solid residues indicate metals like Cr (300 mg/kg), Ni (65 mg/kg), Cu (1500 mg/kg), and Sb (1800 mg/kg). After a 40% weight loss of FAs post-washing, there is a 80-90% surge 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). Analyses for the elemental characterization of wastewater are underway after absorption and recovery of metals by minerals like zeolites. Three synthetic zeolites and a natural zeolite has already tested showing that Zn, Cd and Pb are successfully removed (reduction by about three orders of magnitude), using zeolites with low Si/Al (Si/Al~ 1.3; 1-2); whereas Cd is better adsorbed by more alkaline zeolite.