Electrodeposition of lead-cobalt composite coatings electrocatalytic for oxygen evolution and the properties of composite coated anodes for copper electrowinning

摘要

Metal-matrix composite coatings of lead-cobalt (Pb-Co) and lead-cobalt oxide (Pb-Co_3O_4) were electro-deposited onto the surface of a conventional PbCaSn anode in an effort to develop an improved anode for use in base metal electrowinning. The developed coated anodes were examined in terms of their electrochemical and physical stability over several days of polarisation for the evolution of oxygen under typical copper electrowinning conditions. Results from scanning electron microscopy have shown that fresh Pb-Co and Pb-Co_3O_4 composite coated anodes have rougher surface than conventional (uncoated) PbCaSn anodes but the apparent differences in surface area become insignificant after several days of polarisation under typical copper electrowinning conditions. The Tafel slope on the Pb-Co anode was 92 mV dec"1 and on the Pb-Co_3O_4 it was 90 mV dec~(-1), which is significantly less than the 122 mV dec"1 measured on the conventional PbCaSn anodes. The composite anodes exhibited consistently lower oxygen evolution potentials than the conventional type and the potential remained relatively stable throughout the polarisation period. The reduction in the operating anode potential can be attributed to the presence of cobalt in the surface layer while the decrease in the Tafel slope shows that this reduction can be related to a change in the mechanism of the oxygen evolution reaction. Corrosion rates estimated from 16 h tests showed that the composite coated anodes are more stable than the conventional type during short periods of operation. It was also observed that for the Pb-Co coated anode, both the rate of corrosion and the overpotential for the oxygen evolution reaction can be further reduced by the addition of organic additives such as thiourea.