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

This study investigates the effects of water-washing (extraction) process o n the removal of the major elements and on heavy metals, their speciation i n the washing process, and their thermal stability in the washed ash. A liq uid-to-solid ratio (L/S) ranging from 2 to 100 was used in the washing proc ess, followed by the sintering of the washed-ash pellets at 600-900 degrees 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 t han 30% of the Cr, were found to be leachable at L/S = 2. The washing proce ss resulted in a concentrated heavy metal content in the ash afterwards. Si ntering the washed ash showed a reduction in total concentration (RTC) of h eavy metal decreased as the L/S increased, suggesting that the heavy metals evaporated less and/or were better incorporated into the sintered ash matr ix. The formation of relatively low-melting point calcium-containing alumin osilicates, as identified by X-ray diffraction (XRD) techniques, was though t to be contributable to heavy metal stability in the washed ash, by acting as binder for chemical-reactions-accompanying sintering. The removal of mo st of the chlorides from the ash was also considered as a possibility to mi nimize volatile metal oxide formation, thus increasing the thermal stabilit y of the heavy metals. However, the washing process then had less effect on the heavy metal leachability reduction. Results from this work should be u seful in increasing the recycling safety of fly ash by increasing its therm al stability and decreasing heavy metal leachability. (C) 2001 Elsevier Sci ence B.V. All rights reserved.