Resource Recovery from Municipal Solid Waste Incineration Fly Ash via Sulfuric Acid Digestion: Extraction of Zn and Cu with Calcium Sulfate Production

The sustainable management of municipal solid waste incineration (MSWI) fly ash (FA) and air pollution control (APC) residues remains challenging due to their hazardous nature and underutilized valuable metal content. This study evaluates sulfuric acid digestion for selective metal extraction method that minimizes calcium dissolution, supporting ash valorization. The process parameters (temperature, solid-to-liquid ratio, H2SO4 concentration, digestion time) were investigated for five different ashes (A–E) resulting from different MSWI technologies (rotary kiln, grate furnace and fluidized bed furnace). At 40 °C, a liquid-to-solid ratio of 5 and 98% H2SO4, > 99% copper extraction was achieved for ashes B and C, while other samples showed 45–75% extraction. For ash E, highest copper recovery was obtained at 70–80% acid concentrations. Zn extraction exceeded 90% in all ashes except for Ash D, where the extraction was 42.5%. Calcium dissolution remained negligible (< 1%) across all samples, with XRD confirming anhydrite (CaSO4) as the main phase in residues, which could be further processed into gypsum. The process also prevented silica gel formation and yielded amorphous aluminum sulfate, offering potential for conversion to valuable products like gibbsite (Al(OH)3) or boehmite (AlOOH). These findings demonstrate the feasibility of a tailored acid digestion approach for recovering metals and valorizing FA/APC residues.