Valorization of MSWI Bottom Ash as a Function of Particle Size Distribution, Using Steam Washing

Enrico Destefanis,Giorgia Confalonieri,Francesco Turci,Davide Bernasconi,Renato Boero,Ingrid Corazzari,Erica Bicchi,Costanza Bonadiman,Giuseppe Mandrone,Alessandro Pavese,Caterina Caviglia,Quentin Wehrung,Linda Pastero

doi:10.3390/su12229461

Abstract

Treatments to reduce the leaching of contaminants (chloride, sulfate, heavy metals) into the environment from bottom ash (BA) are investigated, as a function of the ash’s particle size (s). The aim is to make BA suitable for reuse as secondary raw material, in accordance with the legal requirements. Such treatments must be economically feasible and, possibly, have to use by-products of the plant (in this case, steam in excess from the turbine). For the sake of completeness and comparison, carbonation is performed on those BA particle size classes that are not positively responsive to steam washing. BA is partitioned into four different particle size classes (s ≥ 4.75, 4.75 > s ≥ 2, 2 > s ≥ 1 and s < 1 mm, corresponding to 36, 24, 13 and 27 wt%, respectively). In the case of s ≥ 2 mm (60 wt%), steam washing is effective in reducing to under the legal limits the leaching of chlorides, sulfate and heavy metals (Zn, Cu, Cd, Pb). It has been observed that steam washing causes both removal and dissolution of thin dust adherent to the BA’s surface. BA with 2 > s ≥ 1 (~13 wt% of total BA) requires a combination of steam washing and carbonation to achieve a leaching below the legal limits. The finest BA fraction, s < 1 mm (~27 wt% of total BA), is treated by carbonation, which reduces heavy metals leaching by 85%, but it fails to sufficiently curb chlorides and sulfates.

Highlights

Bottom ash (BA) is the main solid residue from municipal solid waste incineration and represents about 20–25 wt% of the whole burnt waste [1]
The results related to the leaching tests on bottom ash (BA) (s ≥ 4.75 mm) as a function of the steam washing treatment time are shown in Figure 1 for the main contaminants
The study demonstrates that steam washing allows a successful treatment of BA

Summary

Introduction

Bottom ash (BA) is the main solid residue from municipal solid waste incineration and represents about 20–25 wt% of the whole burnt waste [1]. The incineration process produces a solid residue bearing a variety of contaminant species (i.e., chlorides and toxic metals), which lead BA to be often classified as dangerous waste [7,8]. For this reason, in several European countries (for instance: Denmark, the Netherlands, France) BA is subject to treatments to reduce its toxic species leaching into the environment and reuse it as secondary raw material to replace aggregates [9]. A variety of treatments have been designed, even as a function of particle size, to reduce the BA’s leaching of contaminants, such as chlorides, sulfates and heavy metals. Carbonation treatments in a regime of CO2-saturation and P ~2 bar, are employed for those BA particle size classes for which steam washing is not effective, for reason of completeness and comparison

Methods

Results

Conclusion