Speciation of Zinc in Municipal Solid Waste Incineration Fly Ash after Heat Treatment:  An X-ray Absorption Spectroscopy Study

Abstract

Fly ash is commonly deposited in special landfills as it contains toxic concentrations of heavy metals, such as Zn, Pb, Cd, and Cu. This study was inspired by our efforts to detoxify fly ash from municipal solid waste incineration by thermal treatment to produce secondary raw materials suited for reprocessing. The potential of the thermal treatment was studied by monitoring the evaporation rate of zinc from a certified fly ash (BCR176) during heating between 300 and 950 degrees C under different carrier gas compositions. Samples were quenched at different temperatures for subsequent investigation with X-ray absorption spectroscopy (XAS). The XAS spectra were analyzed using principal component analysis (PCA), target transformation (TT), and linear combination fitting (LCF) to analyze the major Zn compounds in the fly ash as a function of the temperature. The original fly ash comprised about 60% zinc oxides mainly in the form of hydrozincite (Zn5(OH)6(CO3)2) and 40% inerts like willemite (Zn2SiO4) and gahnite (ZnAl2O4) in a weight ratio of about 3:1. At intermediate temperatures (550-750 degrees C) the speciation underlines the competition between indigenous S and Cl with solid zinc oxides to form either volatile ZnCl2 or solid ZnS. ZnS then transformed into volatile species at about 200 degrees C higher temperatures. The inhibiting influence of S was found absent when oxygen was introduced to the inert carrier gas stream or chloride-donating alkali salt was added to the fly ash.

High temperature behavior of electrostatic precipitator ash from municipal solid waste combustors

“…Struis et al [25] have detailed the quantitative Zn speciation at different heat treatment stages, under Ar or Ar+O 2 carrier gas.…”

Online elemental analysis of process gases with ICP-OES: A case study on waste wood combustion

“…Strong correlations in elemental concentrations were found among sodium, potassium and zinc against chlorine for feedstock W2. Others have reported similar findings (Poole et al, 2007;Khan et al, 2009;Jakob et al, 1996;Belevi and Langmeier, 2000;Struis et al, 2004). Under the given conditions of combustion below 1000°C and chlorine contents of at least 0.3% in the feedstock, the dominant species of alkali emissions is expected to be potassium chloride (Khan et al, 2009;Monkhouse, 2002).…”

“…In case sulfur is present in high enough amounts to compete with chlorine for reactions with zinc, volatilization of zinc can be inhibited (Struis et al, 2004). The fact that the concentrations of zinc, sodium and potassium versus the concentrations of chlorine were more tightly correlated in W2 as compared to W1, provides further evidence that in W1 the combustion efficiency was uneven and possibly more restricted to the surface layers of the feedstock particles.…”

“…Under the given conditions of combustion below 1000°C and chlorine contents of at least 0.3% in the feedstock, the dominant species of alkali emissions is expected to be potassium chloride (Khan et al, 2009;Monkhouse, 2002). Alkalis have been ascribed a carrier function for chlorine (Monkhouse, 2002), and chlorine in turn has a well-documented carrier function for the volatilization of metals (Khan et al, 2009;Struis et al, 2004;Belevi and Langmeier, 2000;Jakob et al, 1996). Two mechanisms involving chlorine can lead to a concurrent mobilization of alkalis and metals.…”

Thermal treatment of waste from the meat industry in high scale rotary kiln

“…Mn content depends on the incineration temperature. Higher process temperatures increase the content of trace elements, except for zinc, which is discharged with exhaust gases (Boman et al 2004;Struis et al 2004). Input materials containing a higher mass ratio of poultry litter (due to the very short residence time of the material in the kiln, and the resulting lack of feather afterburning) produce ash with a higher micronutrient content (Staroń et al 2016).…”