Effects of Basicity Index on Incinerator Fly Ash Melting Process and Stabilization

The generation of hazardous industrial waste in Taiwan has rapidly increased, reaching 1.5 million tons produced annually in 2021. Most of this waste was burned in incinerators, with about 15% (225,000 tons) of it converted into fly ash. Incinerator fly ash primarily consists of heavy metals, dioxins, chlorides, and silica. Historically, fly ash disposal has only relied on cement solidification, contributing to insufficient landfill capacity and soil-pollution concerns. To address these issues, the melting process has been a feasible solution, wherein the heavy metals can be encapsulated within a vitrified structure to prevent them from leaching out. However, the melting point of fly ash is too high, so this study aimed to explore the optimal basicity index for fly ash to conduct the melting process. Basicity indices are estimated by the ratio of CaO/SiO₂, and the melting point of the fly ash can be decreased during the melting process with the right basicity index. In this study, the characteristics of incinerator fly ashes from industrial waste and laboratory waste were initially investigated. With their basicity indices adjusted with two sources of silica, the fly ashes were tested at 1100~1400 °C to observe whether they melted. The vitrified slags were subsequently subjected to TCLP, XRF, and ICP tests to verify their stability. In summary, we discovered that fly ash could be melted through the melting process with the basicity index adjusted to under 1.28, with the silica source as either glass or silica sand powder. After melting, the heavy metals were confirmed to be stabilized in the vitrified slags. Consequently, the melting process could be an alternative solution for fly ash disposal that is sustainable and eco-friendly.