Metals Removal from Municipal Waste Incinerator Fly Ashes and Reuse of Treated Leachates

Incinerator fly ash from municipal solid waste is considered as a hazardous waste and can release toxic metals such as Pb and Cd into the environment. This research verifies the performance of a sequential, closed circuit treatment process involving chemical leaching (alkaline washings followed by an acid washings) and precipitation (neutralization at pH=5 and 7). This method also includes the recirculation of treated leachates during the acid washing steps. In total, ten recirculation loops were executed in laboratory pilot scale. The three alkaline leaching steps effectively solubilized the leachable Pb. Two acid leaching steps were required to solubilize Cd, Al, and Zn. Toxic metals, Cd and Pb, were removed from the fly ash at 72 and 30%, respectively. The toxicity characteristic leaching procedure (TCLP), the synthetic precipitation leaching procedure (SPLP), and a simple test using neutral water were conducted on the treated ash in order to validate the process. The results obtained were below the norm for the TCLP and near 0.01 mg/L of Cd and Pb in solution for the other two tests. The removal efficiency during the precipitation step were 21% Cd, 99% Pb, 100% Al, and 63% Zn. The metallic residue produced at pH=7 contained 23% Zn which is potentially recyclable in the metallurgical industry. The recirculation of treated leachates reduced water consumption for the decontamination process by 60%.