A study on heavy metal extractability and subsequent recovery by electrolysis for a municipal incinerator fly ash

The extractability of heavy metals (Pb, Cd, Cr and Cu) from a municipal incinerator fly ash (designated MIFA S) using an acidic or basic extractant and their subsequent recovery by electrowinning using an electrolytic cell of the fluidized-bed type was investigated. The extractants studied were HCl, HNO 3, H 2SO 4, HOAc, NaOAc (pH=8.15), and NaOAc (pH=3) having 0.1 N in concentration. Of which, NaOAc (pH=3) outperformed the others and was selected for further studies. A 2 4-1 fractional factorial experimental design was adopted using liquid-to-solid (L/S) ratio, extractant concentration, extraction time, and initial pH as the experimental factors. The results show that L/S and extraction time played an important role in Pb extraction while L/S ratio and extractant concentration were important for Cr extraction. However, no controlling parameter was determined for Cd extraction, but a lower extraction efficiency was obtained if an initial pH of greater than 5 was used. For the study of electrolytic recovery, the experiments were carried out based on a 2 3-1 fractional factorial design using current density, initial pH, and operating temperature as the experimental factors. It was determined that current density was important to electrolytic recovery of Pb and Cu, but with an opposite effect. The other two experimental factors were insignificant, 96.70% of Pb and 93.69% of Cu were recovered from the leachate of MIFA S when 0.4 A-h of electrical charge was applied. The results obtained suggest that the recovery of heavy metals from MIFA S by solvent extraction and electrolytic recovery is technically feasible.