Waste Wood Fly Ash Treatment in Switzerland: Effects of Co-Processing with Fly Ash from Municipal Solid Waste on Cr(VI) Reduction and Heavy Metal Recovery

Mirjam Wolffers,Gisela Weibel,Urs Eggenberger

doi:10.3390/pr9010146

Abstract

In Switzerland, waste wood fly ash (WWFA) must be treated before deposition on landfills due to its high pollutant load (Cr(VI) and heavy metals). Acid fly ash leaching, the process used for heavy metal recovery from municipal solid waste incineration fly ash (MSWIFA), represents a possible treatment for heavy metal depletion and Cr(VI) reduction in WWFA. The co-processing of WWFA with MSWIFA during acid fly ash leaching was investigated in laboratory- and industrial-scale experiments with different setups. Of interest were the effects on heavy metal recovery efficiency, the successful outcome of Cr(VI) reduction and consumption of neutralizing chemicals (HCl, H2O2). Detailed chemical and mineralogical characterization of two WWFA types and MSWIFA showed that MSWIFA has higher concentrations in potentially harmful elements than WWFA. However, both WWFA types showed high concentrations in Pb and Cr(VI), and therefore need treatment prior to deposition. Depending on the waste wood proportion and quality, WWFA showed chemical and mineralogical differences that affect leaching behavior. In all experimental setups, successful Cr(VI) reduction was achieved. However, WWFA showed higher consumption of HCl and H2O2, the latter resulting in a particularly negative effect on the recovery of Pb and Cu. Thus, co-processing of smaller WWFA portions could be expedient in order to diminish the negative effects of Pb and Cu recovery.

Highlights

Accepted: 11 January 2021The demand for renewable heat and energy production using the CO2 neutral energy source wood has been growing enormously in Switzerland over the last decades—leading to strongly increasing amounts of wood ashes
Notable is the higher concentration in S and heavy metals in WWFA100 compared to WWFA40, the latter showing higher concentrations in matrix elements (Ca, Si, K, Mg) in contrast
The concentrations of the main- and subsidiary constituents in the three municipal solid waste incineration fly ash (MSWIFA) samples vary within 10–20%, but Cu shows variations by more than

Summary

Introduction

The demand for renewable heat and energy production using the CO2 neutral energy source wood has been growing enormously in Switzerland over the last decades—leading to strongly increasing amounts of wood ashes. In Switzerland, an annual load of 60,000 t wood ash arises from automatic firings through the energetic use of natural wood (e.g., forest) and from the thermal utilization of waste wood (e.g., coated, painted wood) [1]. A quarter thereof represents wood ash from waste wood enriched in heavy metals and Cr(VI). The coarse-grained grate ash arises directly from the grate and is equivalent to bottom ash residing from municipal solid waste incineration (MSWI). This is the biggest fraction with roughly

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