Recovery of lead and zinc from fly ash generated from municipal incineration plants by means of acid and/or alkaline leaching

Fly ash from municipal waste incineration (MWI) is regulated as a hazardous waste because it contains considerable amounts of heavy metals and hazardous organic materials such as dioxin. Primary fly ash contains 0.8 wt.% Zn and 0.12 wt.% Pb, 1.37 wt.% Fe while secondary fly ash contains 40.18 wt.% Zn, 10.7 wt.% Pb and 2.12 wt.% Fe in addition to large amount of sodium chloride and potassium chloride. Therefore, it should be treated for detoxification or for the recovery of these metals as secondary resources. The recovery of these heavy metals by acid leaching using different acids such as sulfuric, hydrochloric and acetic acid was carried out. It was found that sulfuric acid leaching is effective where most of Zn was dissolved and also hydrochloric or acetic acid leaching was effective where most of Pb and Zn were dissolved. However, the acids dissolve also some impurities together with Pb and Zn from the fly ash. Therefore, alkaline leaching using NaOH as leachate was carried out. It was found that alkaline leaching suffers from low solubility of Zn. In order to improve Zn solubility, washing the solid residue using 2 or 5 wt.% HCl was carried out. It was found that almost all Pb (more than 97 wt.%) and about 68 wt.% Zn were leached from secondary fly ash by this method. This investigation has shown that hydrometallurgical treatment of fly ash by means of alkaline leaching followed by dilute acid washing is a potentially feasible method. A recommended flow sheet for alkaline leaching is proposed.