This paper describes a new concept for treatment of air pollution-control (APC) residues from waste incineration and characterises the wastewater and stabilised residues generated by the process. The process involves mixing of APC-residues with a ferrous sulphate solution and subsequent oxidation of the suspension (Ferrox-process 1996). The process results in a significant reduction in the leaching of salts and heavy metals from the residue, by washing out most of the salts and by binding the heavy metals in the iron oxides formed. In the laboratory, a semidry gas-cleaning residue and a fly ash were treated by the process. The generated wastewater contained low concentrations of heavy metals (e.g. Pb: 27-39 microg l(-1) and Cd: 2.6-4.6 microg l(-1)), but high concentrations of salts (e.g. Cl, Na, K, and Ca). The treatment process redUced the leaching of Pb from the residues by more than two orders of magnitude at fixed pH as determined by pH-static leaching tests. Likewise, the leaching of Cd, Zn and Cu was significantly reduced. The effect on elements that form oxyanions (e.g. Cr) is marginal and in the current process there is no reduction in the release of Hg.
Stabilization of air pollution control residues by coprecipitation with ferrous iron and subsequent thermal treatment
(at 600 and 900 °C) has been examined as a means to
reduce heavy metal leaching and to improve product stability.
Changes in mineralogy and metal binding were analyzed
using various analytical and environmental techniques.
Ferrihydrite was formed initially but transformed upon thermal
treatment to more stable and crystalline iron oxides
(maghemite and hematite). For some metals leaching studies
showed more substantial binding after thermal treatment,
while other metals either volatilized or destabilized with
respect to leaching. Pb, in particular, exhibited increased
reactivity following the formation of an ordered iron
oxide structure at 900 °C. The thermal treatment had a
positive effect on Cr release, which was reduced significantly
at 900 °C in the presence of organic matter. Thermal
treatment of the stabilized residues produced structures
with an inherently better iron oxide stability. However, the
concentration of metals in the leachate generally increased
as a consequence of the decreased solubility of metals in
the more stable iron oxide structure.
Amorphous iron (hydr)oxides are used to remove heavy metals from wastewater and in the treatment of air pollution control residues generated in waste incineration. In this study, iron oxides containing heavy metals (e.g., Pb, Hg, Cr, and Cd) were treated at 50, 600, and 900 °C to simulate their transformations caused by heat treatment prior to disposal or aging at a proper disposal site. The transformations were investigated by XRD, SEM, XANES, EXAFS, surface area measurements, pH static leaching tests, and extractions with oxalate and weak hydrochloric acid. It was found that at 600 and 900 °C the iron oxides were transformed to hematite, which had a greater thermodynamic stability but less surface area than the initial products. Heat treatment also caused some volatilization of heavy metals (most notably, Hg). Leaching with water at pH 9 (L/S 10, 24 h) and weak acid extraction showed that heat treatment caused a part of the metals bound in the oxides to be released, thus increasing metals leachability by 1-2 orders of magnitude depending on the metal. Pb and Cd were released in particularly significant concentrations, suggesting less incorporation into the iron oxides after heat-induced transformation. For Pb, this transformation of the chemical state of the bound metal was clearly supported by the X-ray absorption fine structure (XAFS) studies. A fraction of the bound Cr remained stable even after treatment at the highest temperature used in the study. It was concluded that the heat treatment of iron oxides may be advantageous to improve the thermodynamic stability of the product but that thermal treatment at both 600 and 900 °C significantly reduced the binding capacity for heavy metals.
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