Comparing Borate Fusion and Microwave Digestion in the Challenging Dissolution and Quantification of Inorganic Elements from Bottom Ash

Municipal solid waste incineration bottom ash like many other solid wastes is a heterogeneous material, making complete mineralization and accurate quantification challenging, especially for samples containing silica, aluminum oxides, and related blends with other elements. This study aimed to compare the effectiveness of two mineralization processes: microwave-assisted acid digestion using a combination of hydrofluoric acid (HF), nitric acid (HNO₃), and hydrogen peroxide (H₂O₂) under different digestion times (3, 6, and 9 h) and borate fusion using lithium metaborate (LiBO₂) as a flux (1:4 ash-to-flux ratio) at high temperatures (900-1050 °C). Elemental quantification of Si, Ca, Fe, Al, Na, and K was performed using ICP-AES. Among the tested digestion times, the 6 h microwave digestion showed better mineralization and was therefore compared to the borate fusion. Silica recovery reached 189 ± 6 g/kg of ash when using borate fusion, compared to 156.5 ± 16 g/kg of ash with microwave digestion. A similar trend was observed for Ca, Fe, and Al. This effectiveness is attributed to the high temperatures (900-1050 °C) and strongly alkaline conditions, which enabled the effective decomposition of complex silicates and oxides. These findings show borate fusion as a reliable and efficient technique for the mineralization of complex materials such as bottom ash for further reliable quantitative analyses. This approach could favor the recovery of metal species from various waste streams and enable the circular management of metal resources.