Crystal-chemistry of phases in glass-ceramics prepared by reheating of municipal bottom ashes at different temperatures and duration times

Municipal solid waste (MSW) is commonly treated at high temperatures (T); the most abundant product is the so-called granular bottom ash (BA), a complex inorganic residue that shares several features with silicate rocks but also release harmful components. To overcome these aspects, new glass-ceramics were prepared by reheating BA at variable high-T (from 1000 to 1300 °C) and dwell times (t) (2/16 h); these 15 run-products (sample at 1100 °C-16 h was lost) were analysed by XRPD, SEM, EPMA, and leaching potential. XRPD reveals that BA and the 15 obtained materials contain a glass phase, which increases as T and t approach their maximum values; at the lowest T and t, several crystalline phases are observed that progressively disappear as the T and t increase. The complementary SEM and EPMA characterisations corroborate the XRPD results. The measured glass fraction by image analysis increases with T and t increase. As T moves from relatively low to high (also as t increases), the produced glass-ceramics change from crystal- to glass-dominated products; silicate crystalline phases inherited from the starting BA progressively dissolve, while spinel-group minerals persist or newly (chromite) crystallise, acting as key hosts for Fe, Cr, and Zn. Leaching tests performed on representative glass-rich products from 1200 °C demonstrate effective immobilisation of environmentally relevant elements, with concentrations below regulatory limits. The proposed approach allows for concentrating metallic elements in spinel structures at T ≥ 1200 °C; they are easily separable after comminution and magnetic sorting from the most abundant silicate glassy matrix.