Chemical stabilization of MSW incinerator fly ashes

In this work, the relationship between heavy metal content of fly ash and that of the solid wastes incinerated was correlated and compared. It is found that the former is a function of the latter. Hence, it is important to prevent heavy metal-rich wastes from being incinerated in order to reduce the content of toxic metals in the fly ash. The leachability of fly ash from incineration was usually beyond the scope of toxicity standard and must be properly treated before discharge. Secondly, chemical stabilization for the heavy metals in fly ash was explored. Among the chemicals used, it was found that sodium hydroxide was not suitable for the adequate extraction of the heavy metals from the fly ash. Ethylenediaminetetraacetic acid disodium salt (EDTA) was also tested and seems to be effective for the leaching of toxic metals from the fly ash. On the other hand, sodium sulfide and thiourea are one of excellent chemicals for the effective treatment of fly ashes, since they convert soluble and leachable toxic metals into non-leachable and insoluble forms such as lead and zinc sulfide or their similar forms of thiourea. These chemical species are supposed to be stable in nature. A comparison between chemical stabilization noted above and cement or asphalt solidification methods is made. Chemical stabilization processes, especially using sodium sulfide as the chemical agent, are strongly recommended for the practical uses, in terms of the volume expansion and environmental safety of the stabilized products and cost balances, in comparison with the traditional cement or asphalt solidification methods.