Zinc Speciation in Fine and Coarse Fly Ash Particles Collected In-Flight at a Waste Incinerator

TL;DR

This study examines the chemical forms of zinc in fine and coarse fly ash particles from a waste incinerator to guide their safe reuse.

Contribution

The study reveals distinct zinc speciation in fine and coarse fly ash particles, offering insights for targeted recycling strategies.

Findings

Fine particles contain zinc mainly as potassium zinc chlorides, while coarse particles have zinc in stable forms like silicates.

About 50% of zinc is found in the fine fraction despite coarse particles making up most of the fly ash mass.

Abstract

Safe and optimized utilization of waste-to-energy (WtE)
fly ash
(FA) requires a detailed understanding of the physicochemical properties
of its metal constituents. This study provides a comprehensive analysis
of the chemical form of Zn in fine (<1 μm) and coarse (>1
μm) FA particles, hypothesized to originate from different formation
mechanisms. Size-selective aerosol sampling was performed during standard
operation in the flue gas channel at a WtE facility. Additionally,
FA samples from the air pollution control filters at the facility
and boiler deposits were analyzed. Speciation was determined primarily
using synchrotron-based X-ray absorption spectroscopy, complemented
by XRD, SEM-EDS, and total elemental analysis. Significant differences
in terms of elemental composition, crystalline phases, and Zn chemical
forms were observed between fine- and coarse FA particles. Fine particles
were dominated by Cl, K, and Na with Zn almost exclusively present
as potassium zinc chlorides. Coarse particles were heterogeneous,
with Zn occurring in stable forms such as aluminate, ferrite, and
silicates (e.g., gehlenite). The major elemental constituents were
Ca, Si, and Al. Although coarse particles constitute the major mass
of the FA, about 50% of the Zn was found in the fine fraction. These
findings support strategies for efficient secondary use and recycling
of FA, such as targeted Zn extraction from fine particles and potential
utilization of the Ca-rich coarse particles in construction, reducing
the reliance on virgin materials.

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