Study On Selective Leaching And Electrowinning Of Lead From Zinc Oxide Dust Using Alkaline Na₂EDTA

The fume dust of zinc oxide was produced in the process of fuming the pyrometallurgical slags of lead smelting, which contains high content impurities such as lead, fluorine and chlorine. It can only be used to produce zinc chemical products or metal zinc after removing the fluorine and chlorine. But the acid leaching residues contain low lead, which are unfavorable to return to the lead smelting. Considering the high chelating ability between lead and EDTA, a new hydrometallurgical process has been proposed for the selective extraction of lead from the ZOD by using alkaline EDTA solution to leach and electrowin lead in this work.Firstly, adopting double equilibrium method, the equilibrium relationship among [Pb2+]T,[Zn2+]T,[EDTA4-]T,[NaOH]T,[EDTA4-] and pH was also discussed in Pb(II)-Zn(II)-EDTA4--OH--H2O system. The optimum alkaline concentration for selective extraction of lead was determined as0.3M.Using zinc oxide dust as raw material, technical studies on alkaline EDTA leaching of lead was carried out. The experimental results show that the leaching ratio of Pb and Zn is89.92%and0.94%respectively under the following optimized conditions:L/S ratio5:1, stirring speed650rpm, EDTA0.12M, NaOH0.5M, temperature70℃for120mins.The electrochemical behaviors of anodic and cathodic process in the Pb2+-Na+-EDTA4--OH--H2O solution were investigated by using cyclic voltammetry, linear polarization. It was indicated that anodic process is mainly the oxygen evolution reaction, but lead complex in the solution was oxidlized to PbO2, the oxidization rate increased as the increase of Pb concentration, NaOH concentration and temperature, adding1.5ml/L H3PO4could inhibit PbO2generation effectively. In the cathode, the reduction of lead occurred through Pb2+but not complex species, existence of the pre-conversion reaction, the cathodic reaction was a quasi-reversible process controlled by mass transfer with a diffusion coefficients of1.08×10-5cm2/s, the addition of H3PO4had little effect on cathodic reaction, lead electrodeposition potential shifts towards positive values as the increase of Pb concentration, NaOH concentration and temperature.In the electrowinning experiments, the optimum conditions were5g/L lead left in the electrolyte, temperature40℃, current density200A/m2, H3PO4concentration1.5ml/L. Under this condition, the average lead current efficiency and the purity of cathode lead were over94%and98%, respectively, and958kWh of direct current were consumed per ton of lead powder. Below13%of EDTA was lost after five cycles.The results show that using alkaline EDTA solution to leach and electrowin lead to deal with the zinc oxide dusts has the following advantages:shorter flowsheet, higher efficiency and better selectivity.