Kinetics study of heavy metal removal from the lead smelting slag by carbothermic reduction: Effect of phase transformation

As an environmentally hazardous waste, the lead smelting slag was considered as a potential secondary resource of valuable metals (Zn, Pb, and Sn) if recycled. In this study the removal mechanism and kinetics of Zn, Pb, and Sn from the lead slag by carbothermic reduction at 1423–1473 K were investigated. Microstructure evolution of the pellets and phase transformation of heavy metals was analyzed by XRD and SEM/EDS. The kinetics behaviors of heavy metal volatilization were associated to the phase transformation. The results showed that Zn, Pb and Sn removal were controlled by the three-dimensional internal diffusion. Interestingly, the volatilization of Zn presented a consistent kinetics behavior, because the volatile phases of Zn were always metallic Zn in the whole reaction process. However, the volatilization of Sn and Pb conformed to a two-staged kinetics model. It was because that the nonvolatile polymetallic sulfides (Fe-Sn-Pb-S) and Fe-Pb-Sn alloy generated with metallic iron at a higher metallization degree, which hindered the internal diffusion of Sn and Pb. On the basic of this, a staged kinetics model of heavy metal removal was established, and the apparent rate constants and activation energies were determined.