Hydrometallurgical Zinc Recovery from Municipal Solid Waste Incineration Boiler Fly Ash and Mechanical Performance of Ceramic Materials Developed from the Leaching Residue

Municipal solid waste incineration generates fly ash containing potentially hazardous elements but also metals of economic interest, creating an opportunity for combined metal extraction and residue valorisation. This study investigates fly ash from the boiler (FAB) as a secondary source of zinc and evaluates the subsequent reuse of the leaching residue in fired ceramic materials. FAB was physically, chemically and mineralogically characterised and subjected to sulphuric-acid leaching using 0.125, 0.25, 0.5 and 1 M H2SO4 solutions at 25 °C, a solid/liquid ratio of 1:20 and continuous agitation. Leaching performance was assessed through pH, electrical conductivity and zinc concentration in the liquid phase. The FAB contained 1.85 wt.% Zn, while quantitative X-ray diffraction identified 2.3 wt.% zincite (ZnO). Zinc extraction efficiencies close to 90% were achieved with 0.5 and 1 M H2SO4, with most extraction occurring within 48–72 h. The resulting solid residue was incorporated into fired-clay ceramics at 0–50 wt.%. Increasing residue content increased water absorption and open porosity and reduced compressive strength, establishing approximately 30 wt.% as the maximum technically recommended incorporation. The ceramic containing 30 wt.% leaching residue also showed low release of Cr, Pb, As, Cd and Ba under TCLP conditions, with all measured concentrations remaining well below the maximum concentrations established by the US EPA. The proposed process integrates zinc extraction and ceramic valorisation, promoting the use of the leaching residue as a secondary raw material within a circular-economy approach.