| dc.description.abstract | The results indicate that the evaporation of heavy metals in fly ash depends on the volatility of heavy metals and/or their compounds, the presence of chlorides, low melting components, and silica oxides. A water washing process at a liquid to solid ratio of 1:100 removed approximately more than 90% chloride ions, 40% Na and K, and about 20% Ca and S from the fly ash.
The washing process removed most of the soluble chlorides necessary for heavy metals to form volatile metallic chlorides, thus decreasing the evaporation of volatile Cd and Pb at 600-800℃. However, this process also removed the low melting point components which work to immobilize heavy metals during heating, thereby resulting in increasing evaporation of Cd and Pb againe at 800-1000℃. With respect to the medium to low volatile heavy metals such as Zn, Cu and Cr, no significant effects were noted on their evaporization caused by the washing process.
The addition of Silica in washed ash reduced the volatilization of heavy metals during sintering process by forming inert mineral phases. On the other hand, the presence of silica in raw fly ash samples enhanced the ability of chlorides (NaCl, KCl, CaCl2) to release chlorine by forming silicates. The available Cl then attacked the heavy metal oxides to form metallic chlorides and increased heavy metal volatilization, especially for Cu and Zn.
The results also indicates that the leaching of heavy metals were related to the fractions bound to carbonates and exchangeable cations. A washing step can significantly decrease the heavy metals located in the above fractions, thus decreasing the Cd, Pb, and Zn concentration in the TCLP leachate of the sintered ash. Heating the washed fly ash at 600-800℃ increased the Cr fraction bound to the readily exchangeable cations as the Cr was converted into soluble K2CrO4, whereas heating the washed ash at 800-1000℃resulted in the formation of Ca3Cr2(SiO4)3 and (Fe,Mg)(CrFe)2O4, thereby decreasing the Cr concentrations in the TCLP leachate.
The SiO2-containing additives were also found to have enhanced the formation of insoluble silicates and thus the Cd, Zn, and Cr leaching concentration. The additives were also found to have improved the decreased compressive strength and soundness of the washed-ash monoliths caused by the washing process. | en_US |