Optimized Metal Recovery from Fly Ash from Municipal Solid Waste Incineration

Abstract

Switzerland plays a pioneering role in sustainable waste management with a long tradition of waste incineration and the prohibition to landfill unburnt municipal solid waste since 2000. In recent years, the focus has been laid on further reduction of pollutants from incineration residues because the revised Swiss Waste Ordinance prescribes the recovery of metals from fly ash starting in 2021. Fly ash collected in the heat recovery section and the electrostatic precipitator contains high concentration of aluminosilicates, oxides, soluble salts, heavy metals and toxic organic compounds. Metals are either carried along with the flue gas as particles, forming enriched mineral aggregates or vaporized and condensed as complex chlorides or sulphates in fly ash. An efficient treatment of fly ash promises considerable ecological and economic benefits due to an improved quality for disposal and the recovery of the metals contained. At present acidic fly ash leaching (FLUWA) is the state‐of‐the‐art process, where up to 80% of Zn and minor amounts of Pb and Cu are recovered. This thesis contributes considerably to a better understanding of fly ash composition and leaching behaviour as a basis for improved metal separation. Detailed analyses of fly ash describing the chemical associations of metals were the basis for nearly 200 leaching experiments of various fly ashes. The achieved data set contains valuable information regarding the binding environment of metals in fly ash and the leaching behaviour covering a wide range of pH‐values, redox conditions, liquid to solid ratios, temperatures and leaching times. It could be shown that acidic fly ash leaching under oxidative conditions as well as a secondary leaching step using concentrated sodium chloride solution leads to an almost complete mobilization of Pb, Cu and Cd. Based on these findings at laboratory scale, an optimization of the acidic fly ash leaching on industrial‐scale was tested. It could be shown, that the trends of the chemical processes and metal recovery pointing in the right direction. Difficulties have been experienced in keeping and monitoring stable process conditions at the given system technology. The results of this thesis may also serve as a decision support for the upcoming implementation aid by the Federal Office for the Environment (FOEN), where the criteria for fly ash treatment and metal recovery efficiency have to be formulated.