Sulfidation treatment of molten incineration fly ashes with Na2S for zinc, lead and copper resource recovery

The present study focuses on the conversion of heavy metals involved in molten incineration fly ashes to metal sulfides which could be thereafter separated by flotation. The sulfidation treatment was carried out for five molten incineration fly ashes (Fly ash-A to Fly ash-E) by contacting each fly ash with Na2S solution for a period of 10 min to 6 h. The initial molar ratio of S2− to Me2+ was adjusted to 1.20. The conversion of heavy metals to metal sulfides was evaluated by measuring the S2− residual concentrations using an ion selective electrode. The formation of metal sulfides was studied by XRD and SEM–EDS analyses.In the case of Fly ash-A to Fly ash-D, more than 79% of heavy metals of zinc, lead and copper was converted to metal sulfides within the contacting period of 0.5 h owing to a fast conversion of metal chlorides to metal sulfides. By contrast, the conversion of about 35% was achieved for Fly ash-E within the same contacting period, which was attributed to a high content of metal oxides. Further, the S2− to Me2+ molar ratio was reduced to 1.00 to minimize Na2S consumption and the conversions obtained within the contacting period of 0.5 h varied from 76% for Fly ash-D to 91% for Fly ash-C. Finally, soluble salts such as NaCl and KCl were removed during the sulfidation treatment, which brought about a significant enrichment in metals content by a factor varying from 1.5 for Fly ash-D to 4.9 for Fly ash-A.