dc.description.abstract | This study investigated the feasibility of melting fly ash for a recycling purpose, by using chemical energy released by the reaction of waste-derived thermite. Typical thermite tested in this study comprised of strong-exothermic aluminum and iron(Ⅲ) oxide, simulating aluminum dross from aluminum foundries and iron oxides from the fly ash and byproducts of steelworks in industrial practice. The self-propagating characteristics of the targeted thermite treating municipal solid wastes incinerator(MSWI) fly ash was evaluated by varying fly ash content in the starting mixture (thermite mixed with fly ash) from 5% to 35% by weight. The distribution of major elements (Al, Fe, Ca, and Si), and the partitioning of heavy metals during thermite type melting process were determined. The recovered alloy and slag were analyzed for their composition and engineering properties. The results indicate that the self-propagating temperature required a maximum fly ash content less than 30%, corresponding to a melting temperature higher than 2017K in this study. The maximum reaction temperature reached was found to be 3055K for thermite without addition of fly ash, depending on the fraction of heat loss from the thermite reactor. It was also noted that the recovery of slag increased with increasing MSWI fly ash content in the starting mixture whereas greater than 91% alloy, mainly iron, was recovered. The major elements in thermite and MSWI fly ash, including Al, Fe, Ca, Si, were evaluated. It was found expectedly that most of the iron was recovered in alloy, and aluminum in slag and secondary fly ash (generated from melting process, SFA). The distribution of Al to SFA decreased with increasing ash addition, showing the violent character of the thermite reaction was weakened. Calcium and silicon existed in MSWI fly ash as calcium oxides and silicon oxide, and were mostly recovered in slag.
In the thermite reactions with half of the Al replaced by Mg, and with 100% excess of stoichiometric Al, it was found that the volatility of Pb was decreased; whereas the volatility of Cd and Zn was enhanced. Moreover, the recovered slag showed stable vitrified structure with extremely low TCLP leaching concentration of heavy metals which complies with current regulatory thresholds. The slag has a specific gravity less than 2, with apparent porosity ranging from 2-25%, and water adsorption from 1.6-16%. Most of the slag samples has a linear expansion less than 15%. The ability of anti-acid(base) corrosion for all slag samples is greater than 80%.
This study demonstrated that a thermite reaction of aluminum and iron oxide treating MSWI fly ash was demonstrated to be a feasible approach to recover metallic resources and slag for construction materials. | en_US |