Separation of heavy metals from hazardous lead slag by carbothermic reduction and thermal volatilization: Effect of phase transformation on Sn, Pb, Zn removal

In this study, the carbothermic reduction and volatilization were applied to recover valuable metals from lead slag. The removal and volatilization mechanisms, especially the migration and distribution behavior of heavy metals, were investigated. The results showed that the removal ratio of Sn was lower than those of Pb and Zn. Increasing the temperature was beneficial to the removal of Zn, but unfavorable for the removal of Sn and Pb. SEM/EDS analysis of the reduced pellets and the flue dust showed that ZnFe2O4 and ZnS in the raw slag were reduced to Zn for volatilization and then reoxidized and formed ZnO(s) in the flue. At lower temperatures, SnO2 and FeOx·PbO·SiO2 in the raw slag was reduced to the volatile SnO and PbO. However, SnO and PbO were further reduced to nonvolatile metallic Sn and Pb at higher temperatures. Fe-Sn and Fe-Pb alloys easily formed when metallic Sn and Pb coexisted with metallic Fe, leading to lower removal ratios. For the metallization of Fe, the iron grains grew up gradually and easily formed joined crystals at higher temperatures, which inhibited the volatilization of Sn and Pb.