Heavy Metal Recoverability Of MSWI Fly Ashes Along The Flue Gas Cooling Path

Abstract

Each year, around 75’000t of fly ash from municipal solid waste incineration (MSWI fly ash) are produced and deposited on landfills in Switzerland. MSWI fly ash consists of boiler- and electrostatic precipitator ash and contains significant concentrations of heavy metals (e.g., Zn, Cu, Cd, Pb, Sb). From the year 2026 onwards, it will be mandatory to treat Swiss MSWI fly ashes with acid leaching in order to recover the heavy metals prior to deposition. However, it has not yet been legally determined whether the boiler ash is to be treated as fly ash. It could also be treated and deposited together with the bottom ash. Currently, the data available for boiler ashes are very limited, and their metal recovery potential has not been fully explored. Therefore, detailed chemical and mineralogical characterization was performed on the different MSWI ashes that form along the flue gas path (empty pass-ash (EA), boiler ashes (BOA) and electrostatic precipitator ash (ESPA)). Using a broad combination of methods (XRF, XRD, SEM), the ashes from six Swiss MSWI plants were characterized with respect to the chemical and mineralogical composition of major- and minor phases. Important parameters to estimate the suitability of a treatment are the contents of recoverable heavy metals and the extractability of the ashes. The focus was therefore laid on matrix phases affecting leachability (e.g. alkalinity, oxidation-reduction potential), as well as on the distribution and concentration of recoverable heavy metals and their binding forms. In order to estimate the need for metal recovery before landfilling, the contents of non-mobilizable pollutants such as Sb was also recorded along the flue gas path. EA and BOA showed comparable bulk chemical and mineralogical composition and are composed of two significantly different materials: the airborne ash particles (quenched melt droplets and refractory particles) and deposits formed on heat exchanger surfaces. It is mainly the deposits that contribute to the elevated heavy metal concentration, explained by the well-developed, large (Na,K)-PbSO4 crystals and the Zn-bearing matrix sulfates. The variation in the amount and chemical composition of the deposits controls the fluctuations in the bulk composition of EA and BOA. The ESPA shows different chemical and mineralogical characteristics than EA and BOA. The ESPA is enriched in the more volatile metals Zn, Pb, Cu, Cd; which are mainly present as chlorides and sulfates. The high content of salt-bound and thus easily soluble heavy metals together with the lower alkalinity and oxidation-reduction potential indicates, that ESPA has a better leachability compared to EA and BOA. These observations suggest that individual treatment of ESPA has higher potential for heavy metal recovery. Comparing the EA and BOA, however, no significant differences could be found in the parameters affecting extractability. The obtained results provide important insights into the formation of the different ash fractions and its geochemical characteristics. The data may serve as basis for re-evaluating disposal routes of ash fractions with poor extraction properties