Decomposition and Microwave Dielectric Characterization of MSWI Fly Ash and the Role of Carbon-Based Additives in Microwave-Assisted Volatile Metal Recovery

This study evaluates the thermal behavior, dielectric properties, and metal volatilization potential of municipal solid waste incineration (MSWI) fly ash (FA) and air pollution control (APC) residues under microwave heating. Thermogravimetric analysis (TGA), combined with mass spectrometry (MS), and dielectric property measurements were employed to examine decomposition mechanisms and microwave absorption properties. TGA–MS revealed multi-step decomposition patterns, with early mass loss attributed to hydrated salts, followed by carbonate and sulfate breakdown. Zinc was released at both intermediate and high temperatures, depending on the sample composition. Dielectric measurements showed limited microwave responsiveness of raw ashes at low temperatures, with significant increase at high temperatures, particularly in sulfide- and Fe-rich samples. Microwave treatment with carbon-based additives improved heating rates and metal volatilization. Graphite provided the fastest heating due to high conductivity, while biochar achieved superior volatilization of Pb, Cd, Sb, and Zn owing to its reactive and porous structure. Blast furnace sludge (BFS) was less effective compared to other additives, although higher content partially enhanced heating and metal release. Despite high volatilization ratios of the most volatile metals, residues in some cases showed elevated Mg and Zn concentrations due to overall mass loss and retention in refractory phases.