Recovery of Cu, Pb, Cd and Zn from synthetic mixture by selective electrodeposition in chloride solution

Abstract

Secondary fly ash, resulting from thermal treatment processes, leads to a highly concentrated chloride solution with Cu, Pb, Cd and Zn as main heavy metals when dissolved in water. The selective electrodeposition of these heavy metals has been studied in this work. The goal was to recover, under potentiostatic conditions, each heavy metal with high purity, yield and reaction rates. By changing the parameters pH and overpotential, an optimum of the three requirements was looked for. In general, Cu, Pb and Cd could be separated with purities of 99 mol% or higher. Underpotential deposition was supposed to be the main reason for the impurities in case of Cu and Pb deposition. H + reduction as side reaction could be kept small for Cu, Pb and Cd even at lower pH by carefully selecting the overpotential. The quality of the deposits obtained depended strongly on the overpotential, but hardly on the pH. The deposits of Cu, Pb and Cd were easily removable from the cathode due to a dendritic growth mechanism. Zn deposits showed compact growth and adhered to the electrode surface. In addition, the structure of the deposits, revealed by scanning electron microscope (SEM), was compared with the current transients during electrodeposition. An enhancement factor r was introduced in order to compare the different deposition rates.