Characterization of MSWI fly ashes along the flue gas cooling path and implications on heavy metal recovery through acid leaching.

M Wolffers,S Schlumberger,S.V Churakov,U Eggenberger

doi:10.1016/j.wasman.2021.08.022

M Wolffers, S Schlumberger + Show 2 more

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https://doi.org/10.1016/j.wasman.2021.08.022

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Journal: Waste management (New York, N.Y.)	Publication Date: Oct 1, 2021
Citations: 13	License type: cc-by

Affiliation: University of Bern

Abstract

This study reports on detailed chemical and mineralogical characterization of the different municipal solid waste incineration ashes forming along the flue gas path of plants with separate dust removal and neutralization. In pursuit of optimizing heavy metal recovery through acid leaching, the metal extractability from empty pass ashes (EA), boiler ashes (BOA) and the electrostatic precipitator ashes (ESPA) was evaluated and compared. The focus was laid on matrix phases affecting leachability (e.g. alkalinity, oxidation-reduction potential), as well as on distribution and concentration of recoverable heavy metals and their binding forms. The data showed, that EA and BOA are geochemically similar and are essentially composed of two different materials: the heavy metal poor airborne ash particles and the Zn- and Pb-rich sulfate deposits that condensate on heat exchanger surfaces. Variation in relative amount and chemical composition of the deposits is responsible for fluctuations in bulk composition of EA and BOA. The ESPA shows different chemical and mineralogical characteristics than EA and BOA. The ESPA is enriched in the volatile heavy metals Zn, Pb, Cu, Cd and Sn, which are mainly incorporated in chlorides and sulfates. The high content of salt-bound and thus easily soluble heavy metals together with the lower alkalinity and lower oxidation-reduction potential indicates, that ESPA has a better leachability compared to EA and BOA. The EA and BOA, on the other hand, do not show any significant differences in leachability. The data may contribute to a basis for re-evaluating disposal routes of ash fractions with poor extraction properties.

Highlights

Heavy metal recovery from residues of municipal solid waste incin eration (MSWI) is a sustainable technology for returning valuable metals back into the raw materials cycle
Volatile heavy metals of economic interest (e.g. Zn, Cu, Sn) and potentially environmentally hazardous heavy metals (e.g. Pb, Cd, Sb) are highly enriched in electrostatic precipitator ashes (ESPA) compared to empty pass ashes (EA) and boiler ashes (BOA)
A drop in Ca and an increase in Na, K concentration is observable from the middle of the boiler on and there is a strong Pb peak for BOA3, with concentrations almost 5x higher than in ESPA

Summary

Introduction

Heavy metal recovery from residues of municipal solid waste incin eration (MSWI) is a sustainable technology for returning valuable metals back into the raw materials cycle. Dry- and semi dry flue gas cleaning systems use additives as neutralizing chemicals and produce solid flue gas cleaning products. In other countries, these flue gas cleaning residues are often collected together with FA and the hazardous mixture is termed air pollution control residues (APCr) (Quina et al, 2008; Quina et al, 2018). The wet flue gas cleaning system consists of a multi-stage scrubber and the APCr arise in liquid form as scrub water. Sulfur oxides (SO2, SO3) and residual heavy metals are separated with the addition of NaOH, resulting in neutral or alkaline scrub water. Pure gypsum can be precipitated from the sulfate bearing alkaline scrub water in order to avoid landfilling of a potential secondary raw material

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