Treatment of fly ashes for inertization and adsorbent purposes

The increasing generation of municipal solid waste incinerator (MSWI) fly ash, classified as hazardous waste due to its high content of heavy metals and soluble salts, poses a significant environmental challenge. Traditional disposal through landfilling is becoming increasingly unsustainable, not only because of the declining availability of landfill capacity, but also owing to the risk of pollutant leaching and long-term environmental contamination. In the context of the circular economy, the transformation of MSWI fly ash from a waste product into a valuable secondary raw material represents a strategic priority. However, the highly heterogeneous composition of fly ash complicates its handling and disposal, often requiring tailored management pathways depending on its physicochemical characteristics. This study investigated ball milling as an innovative dual-purpose treatment for MSWI fly ash, targeting both inertization (detoxification) and activation (enhancement of surface area and adsorption properties). Mechanochemical treatments are nowadays promising, achieving metastable materials with no solvent involved and modest energy consumption. Ball milling was tested under four different conditions, with two different ash feedstock type and two milling ball dimensions combined. The effectiveness of inertization was evaluated by measuring the quantity of chlorine and lead in leachates, as they are the only two components exceeding the toxic threshold in untreated fly ash. Chlorine and lead content are assessed using potentiometric titration and atomic absorption spectroscopy, respectively. Activation performance was evaluated through SO₂ and H₂S adsorption tests, complemented by acid-base curves and surface area analysis. Results demonstrated that ball milling effectively reduced 80 % of chlorines and 55 % of lead leaching content in untreated fly ash, with higher yields achieved with bigger balls. However, when applied to pre-inertized ash, the process facilitated the release of chlorine with both ball sizes. Whereas for lead, occurred only with larger balls and decreased with smaller ones (29 %). Only with a reduction in ball size and a pre-treatment, it is achieved an ash that does not exceed both toxic pollutant thresholds. Ball milling improved surface area and enhanced adsorption capacity for SO₂ and H₂S. This was proven with good maximum adsorption capacity for both pollutants (42,02 mg SO2/g FA and 19,61 mg H2S/g FA). The acid-base curve provided valuable insights into ash categorization in a type C material, and coupled with BET analysis, some mechanisms governing surface interactions with target gases were discovered. Ball milling represents an economical and environmentally sustainable treatment option with promising performance outcomes. Conversely, optimal results require integration with complementary treatment processes to maximize both inertization efficiency and activation potential for MSWI fly ash valorisation.