HYDROMETALLURGICAL TREATMENT OF MUNICIPAL SOLID-WASTE FLY-ASH FOR SIMULTANEOUS DETOXIFICATION AND METAL RECOVERY

Fly ash from municipal solid waste (MSW) incinerators is frequently cl assified as a characteristic hazardous waste, due to low levels of Pb and Cd. In some cases the fly ash also contains a significant amount o f Zn, up to 15%. The objective of this investigation was to lay the fo undation for a potential hydrometallurgical process for simultaneous d etoxification and recovery of metal values, particularly Zn, Pb, and C d. This study has provided a more complete characterization of the By ash particularly with regard to those parameters which are important t o a hydrometallurgical process, such as the forms in which Pb and Zn o ccur and leaching characteristics in several different lixiviants incl uding various concentrations of hydrochloric, acetic, and sulfuric aci d, Results show that Pb is present in at least two forms, PbSO4 and Pb Cl2. Similarly, three forms of Zn are indicated: (I) a water soluble f orm, probably chloride or sulfate or both, (2) an acid soluble form, l ikely Zn oxide, and (3) a much less soluble form. These sources repres ent roughly 26%, 44%, and 30%, respectively, of the total Zn content. Extraction of Pb; Cd, and Zn can be accomplished quickly and effective ly in a 1 M HCl + 1 M NaCl solution, at a solid to liquid ratio of 1:1 0, resulting in extractions of approximately 85, 83, and 65%, respecti vely. A Zn concentration of about 15 g/L after cementation can be achi eved, which is suitable for electrowinning. In addition the leaching r esidue is not hazardous according to the TCLP test. Pb and Cd in the l eaching solution can be recovered by cementation with Zn dust. Suggest ed areas for future investigation are presented.