Municipal waste incinerator fly ash: Supercritical fluid extraction of metals

The amount of residues such as fly ash from municipal waste incinerators and coal-fired power plants is growing. Fly ash is usually contaminated with toxic heavy metals that leach out on contact with water and pollute the groundwater. Therefore, isolated and expensive disposal of the ash is required. Reuse of ash as a filler for cement or pavements only allows minimum leachability of metals and maximum leaching values of various metals from reused fly ash are prescribed by national legislation. Supercritical-fluid extraction (SFE) offers a method to reduce the metal content so that leachability is reduced and the demands of legislation are observed. This paper presents results of metal extraction from municipal waste incinerator ash using supercritical CO₂. Initial experiments with a 12dm³ rotating extraction vessel and constant solvent flow showed extraction efficiencies of between 10% and 52%. The influences of complexing-agent concentration and process time are studied on divalent metals such as Zn²⁺, Pb²⁺, Cu²⁺, Sb²⁺, Ni²⁺, and Cd²⁺.