Recent studies on the hydrothermal treatment of MSWI fly ash have improved the understanding of stabilization and extraction of heavy metals [17–22]. Bayuseno and Schmahl [17] indicated that a hydrothermal process together with the addition of alkaline could change MSWI fly ash into stable minerals, and facilitate the immobilization of heavy metal ions. The products could be only destroyed with difficulty under strongly acidic conditions. Wang et al. [18–19] found that the addition of Na2 CO3 removed more