Effects of a water-washing process on heavy metal speciation and leaching efficiency in MSWI fly ash during acid extraction

Fly ash from municipal waste incinerators form a major environmental problem as they are polluted with heavy metals. But the metal value can be partly removed and recovered by extracting with acid solution. Experiments have been performed to investigate the effect of water washing-acid extraction process (WWAE process) for treatment of MSWI fly ash. The chemical composition of fly ash has been analyzed. The effects of water-washing process on the removal of the major elements and on heavy metals, their speciation in the washing process, the effects of acid extraction have been elucidated and heavy metals stability in the extracted washed ash residuals have been evaluated. The results indicate that more than 60% of the Cl, and more than 47% of the major elements of Na, K, and Ca, as well as more than 18% of the Pb, were found to be leached at L/S=20. The washing process resulted in more concentrated heavy metal content in the washed fly ash. A comparison of the results of acid extraction of raw ash and washed fly ash demonstrated that increment of extracted amount of Zn improved evidently after washing, while Pb improved indistinctively. Leaching with more concentrated acid yielded greater heavy metal content from washed fly ash. The removal of most of the exchangeable and carbonate fractions from the ash, which were sensitive to the change of ion intensity and pH, was thought to be contributing to the rapid extraction of Pb and Zn as the L/S increased within 1M. The results of sequential extraction procedure reveal that the zinc and lead fractions in the extracted washed ash shifted from being mostly bound to the Fe-Mn oxides and carbonate, to being mostly bound to residual form, which is stable in the environment. Results from this work showed WWAE process can increase the environmental safety of fly ash by increasing its stability and decreasing heavy metal leachability.