MANAGEMENT OF MSW INCINERATION BOTTOM ASHES: DISCHARGE SYSTEMS AND LAYOUTS OF TREATMENT PLANTS

Abstract

Recovery of ferrous and non-ferrous metals from MSWI bottom ash has become a common practice in recent years. In fact, the concentration of such metals can be greater than that in natural ores, making their recovery economically advantageous. In addition, significant environmental benefits are associated to their recycling, and their separation is crucial in view of the recovery of the inert fraction of the bottom ash. The paper reports an overview of the different options for the bottom ash extraction system from the furnace and for its treatment for metal recovery. Usually, bottom ash is extracted through ”wet” discharge systems, which means that it is quenched in a water bath. Recent studies and some real scale applications have shown that, compared to the wet technology, the “dry” discharge allows to obtain a slag with higher quality and with less agglomeration between the particles, allowing a better separation of the non-ferrous metals and the possibility to recover also the fine fraction, smaller than 5 mm. This is particularly interesting since the metals content of this fraction is not negligible and nowadays it is not recovered, with a consequent loss of resources and economic advantages. In addition, the recovered metals result of good quality, cleaner and less oxidised than those recovered from wet bottom ash. For these reasons, the dry discharge of bottom ash and the consequent treatment of the fine fraction is likely to gain more and more importance in the context of the recycling of materials from incineration residues.