Characterization of municipal solid waste incineration fly ash before and after electrodialytic treatment

Abstract

A MSWI fly ash was characterized with respect to leaching properties (pH-static leaching, availability test), mineralogy (X-ray powder diffraction (XRPD)), and morphology and element distribution (scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX)) before and after electrodialytic treatment to study possible changes in the ash characteristics due to the treatment. It was shown that although a significant amount of the initially present heavy metals had been removed from the ash during the electrodialytic treatment, the leachability of several of the residual metals had actually increased, presumably due to mineral dissolution and chelating of metals by residual citrate in the ash. Ammonium citrate had been added to the ash before and during electrodialytic treatment to increase the heavy metal desorption. The morphology and mineralogy of the ash was also altered as a result of the treatment. XRPD examinations showed that a depletion of Cl, Na and K in the treated ash was due to wash-out of the highly soluble minerals sylvite (KCl) and halite (NaCl). Indications of formation of secondary minerals (Ca-sulphates, ettringite) and precipitation of Ca-citrate in the treated ash were also seen. SEM investigations confirmed a changed morphology in the treated ash; probably due to the formation of secondary minerals.