Hydrometallurgical treatment of municipal solid waste fly ash for simultaneous detoxification and metal recovery

Fly ash from municipal solid waste (MSW) incinerators is frequently classified as a characteristic hazardous waste, due to low levels of Pb and Cd. In some cases the fly ash also contains a significant amount of Zn, up to 15%. The objective of this investigation was to lay the foundation for a potential hydrometallurgical process for simultaneous detoxification and recovery of metal values, particularly Zn, Pb, and Cd. This study has provided a more complete characterization of the By ash particularly with regard to those parameters which are important to a hydrometallurgical process, such as the forms in which Pb and Zn occur and leaching characteristics in several different lixiviants including various concentrations of hydrochloric, acetic, and sulfuric acid, Results show that Pb is present in at least two forms, PbSO4 and PbCl2. Similarly, three forms of Zn are indicated: (I) a water soluble form, probably chloride or sulfate or both, (2) an acid soluble form, likely Zn oxide, and (3) a much less soluble form. These sources represent roughly 26%, 44%, and 30%, respectively, of the total Zn content. Extraction of Pb; Cd, and Zn can be accomplished quickly and effectively in a 1 M HCl + 1 M NaCl solution, at a solid to liquid ratio of 1:10, resulting in extractions of approximately 85, 83, and 65%, respectively. A Zn concentration of about 15 g/L after cementation can be achieved, which is suitable for electrowinning. In addition the leaching residue is not hazardous according to the TCLP test. Pb and Cd in the leaching solution can be recovered by cementation with Zn dust. Suggested areas for future investigation are presented.