Detoxification and recycling of fly ash from waste melting systems

Molten fly ash generated by direct melting systems used in treating municipal solid waste (MSW) is designated as a specially-controlled waste in Japan due to the harmful heavy metals it contains. Proper detoxification before disposal in landfills is legally required, as in the case of fly ash produced by conventional MSW incinerators. As part of research and development of an incinerator fly ash treatment process using the calcination method, to date, the authors have clarified the removal mechanism of heavy metals such as lead (Pb) and cadmium (Cd), which are the main harmful heavy metals found in fly ash, and optimized the removal conditions based on thermodynamic and metallurgical approaches [1-3]. Molten fly ash is more difficult to treat than incinerator fly ash. For research and development of a treatment process, a new pilot plant with a design capacity of 45 kg/h was constructed in March 2004. Using this facility, it was demonstrated that harmful heavy metals such as Pb in molten fly ash can be safely removed to trace levels which satisfy the Japanese environmental standard for soil, and the pellets obtained in the process can be recycled, for example, as artificial lightweight aggregate. This paper focuses on the technology for removing Pb from molten fly ash, and in particular, discusses the volatilization removal mechanism and removal conditions. Proper proportioning of the fly ash and submaterials to obtain stronger artificial lightweight aggregate is also described.