Improving the mechanical characteristics and restraining heavy metal evaporation from sintered municipal solid waste incinerator fly ash by wet milling

Abstract

The milling process has a verified stabilizing effect on the leaching of heavy metals into the environment from municipal solid waste incinerator (MSWI) fly ash. The aim of this current study is to further improve and confirm the effectiveness of the process by exploring its effects on the evaporation of heavy metals and on the mechanical characteristics of the sintered MSWI fly ash. The chemical composition of the MSWI fly ash is first altered by the addition of water treatment plant sludge (WTS) and cullet, and then processed to produce sintered specimens suitable for reuse as an aggregate. In the experiments, fly ash, WTS and cullet (40%: 30%: 30%, respectively) were mixed and milled for 1 h. Samples were sintered for 60 min at temperatures of 850, 900, 950 and 1000 °C. Test results confirm that milling increased the compressive strength of the sintered specimens. The compressive strength of unmilled specimens sintered at 900 °C was only 90 kg/cm 2, but that of milled specimens was 298 kg/cm 2 when sintered at only 850 °C. There was also an improvement in the soundness ranging from 11.04% to 0.02% and a reduction in the evaporation rates of Pb, Cd, Cu, Cr and Zn from 54–64%, 43–49%, 38–43%, 30–40% and 14–35% (900–1000 °C) to 19–21%, 19–21%, 14–19%, 12–19% and 14–17% (850–1000 °C), respectively. The improvement in compressive strength was attained by the combination in the liquid sintering stage of powdered ash with the amorphous material. The amorphousness of the material also helped to seal the surface of the fly ash, thereby reducing the evaporation of heavy metals during the heating process.