Effects of a water-extraction process on heavy metal behavior in municipal solid waste incinerator fly ash

Abstract

This study investigates the effects of water-washing (extraction) process on the removal of the major elements and on heavy metals, their speciation in the washing process, and their thermal stability in the washed ash. A liquid-to-solid ratio (L/S) ranging from 2 to 100 was used in the washing process, followed by the sintering of the washed-ash pellets at 600–900 °C in a tubular furnace. The results indicate that more than 65% of the Cl, and more than 50% of the major elements of Na, K, and Ca, as well as more than 30% of the Cr, were found to be leachable at L/S=2. The washing process resulted in a concentrated heavy metal content in the ash afterwards. Sintering the washed ash showed a reduction in total concentration (RTC) of heavy metal decreased as the L/S increased, suggesting that the heavy metals evaporated less and/or were better incorporated into the sintered ash matrix. The formation of relatively low-melting point calcium-containing aluminosilicates, as identified by X-ray diffraction (XRD) techniques, was thought to be contributable to heavy metal stability in the washed ash, by acting as binder for chemical-reactions-accompanying sintering. The removal of most of the chlorides from the ash was also considered as a possibility to minimize volatile metal oxide formation, thus increasing the thermal stability of the heavy metals. However, the washing process then had less effect on the heavy metal leachability reduction. Results from this work should be useful in increasing the recycling safety of fly ash by increasing its thermal stability and decreasing heavy metal leachability.