Supercritical fluid extraction of heavy metals from fly ash

Currently expensive measures for disposal of fly ash residues from coal-fired power plants (CFP) or from municipal solid waste incinerators (MSWI) are required to prevent the pollution of groundwater with water leachable metals. For the reuse in road pavement as a substitute for sand- or cement-stabilized subbases, only a minimal leachability of metals is allowed by national legislation. Supercritical fluid extraction (SFE) is a promising method to lower the leachable metal content of fly ash to such an extent that the legal demands are obeyed. This paper presents results of SFE with SC-CO2 from fly ash of both CFP and MSWI for Zn, Pb, Mn, Cd, Cu, V, Sb, Ni, Mo, Cr, and Co. Extraction from 2 kg of fly ash was carried out with a 12 L rotating extraction vessel, a constant solvent flow, and various complexing extractants. Depending on the metal, extraction efficiencies from 0 to 98% were obtained. The positive influence of time and initial moisture content was demonstrated for SFE with Cyanex 302. SFE with equimolar mixtures of TBP-D(2)EHPA resulted in synergistic effects for Pb and Cu, while extraction of Zn was insignificantly low with TBP, D(2)EHPA, or a mixture of TBP-D(2)EHPA.