The development and testing of alternative anodes based on cobalt and lead for the electrowinning of base metals

Maryam Jozegholami Barmi

A review of literature related to the effect of introducing cobalt into an electrolysis process based on a sulphuric acid electrolyte and a lead-based anode reveals the benefits of cobalt in minimising oxygen evolution, energy consumption, cathode contamination and corrosion rate of the anode. A small amount of cobalt ions introduced into the electrolyte significantly reduces the oxygen evolution potential and the corrosion rate of the lead anode. However, harmful effects of adding cobalt ions to the electrolyte have prevented the use of cobalt in some processes and as a result its use in this way is limited. The scope of this project is to develop and study new composite anodes containing cobalt in their surface layer to determine whether they have lower oxygen evolution potential and as a consequence lower energy consumption when used in base metal electrowinning systems. In order to produce anodes with the most beneficial specifications, different composite layers containing lead and cobalt were electrodeposited on the surface of PbCaSn, Ni and Ti substrates. In the first part of the study, metal-matrix composite coatings of lead-cobalt (Pb- Co) and lead-cobalt oxide (Pb-Co3O4) were electrodeposited onto the surface of a conventional PbCaSn anode in an effort to develop an improved anode for use in base metal electrowinning. The composite-coated anodes were examined in terms of electrochemical and physical stability over several days of polarisation under typical copper electrowinning conditions. Results from scanning electron microscopy have shown that fresh Pb-Co and Pb-Co3O4 composite-coated anodes ii have a rougher surface than conventional (uncoated) PbCaSn anodes but the difference in surface area becomes insignificant after several days of polarisation under typical copper electrowinning conditions. The Tafel slope on the Pb-Co coated anode was 92 mV dec-1 and on the Pb-Co3O4 coated anode it was 90 mV dec-1, which are both significantly less than the 122 mV dec-1 measured on the conventional PbCaSn anodes. The composite-coated anodes exhibited consistently lower oxygen evolution potentials than the conventional anodes and the potential remained relatively stable throughout the polarisation period. The reduction in the operating anode potential is believed to be due to the presence of cobalt in the surface layer while the decrease in the Tafel slope shows that this reduction is most likely related to a change in the mechanism of oxygen evolution. At present there is no uniformly agreed theoretical hypothesis that explains this observed behavior. Corrosion rates estimated from 16 hr tests showed that the compositecoated 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 into the electrolyte. In the second part of this study, PbO2-CoOx and PbO2-Co3O4 composites were coated onto titanium and nickel substrates to form anodes, and tested under typical copper electrowinning conditions. The aim of depositing a well-adhered composite coating onto the surface of the dimensionally stable substrate materials was pursued using three different coatings. The performance of the produced anodes was examined in terms of oxygen evolution potential and service life. Both of the anodes with PbO2-CoOx or PbO2-Co3O4 composite deposited on titanium resulted in reduction of the oxygen evolution potential of 300 to 400 mV compared with a conventional PbCaSn anode. The anodes prepared by applying the same coatings onto a nickel substrate showed poor stability in the acidic electrolyte media used in this experiment to represent typical copper electrowinning conditions and as a result, measurements of oxygen evolution potential were not obtained. Titanium-based anodes were also produced by thermal deposition of a SnO2-Sb2O3 interlayer on the surface of a titanium substrate followed by electrodeposition of a Co-based composite coating. These resulted in lower oxygen evolution potential than conventional PbCaSn. A Tafel slope of 88 mV dec-1 was recorded for the anode with a coating of PbO2-CoOx and 47 mV dec-1 for the anode with a coating of PbO2-Co3O4, which both compare favorably to the 122 mV dec-1 which was observed for a conventional PbCaSn anode. Anodisation tests were carried out for a period of 16 hours both in the presence and in the absence of the organic additive thiourea in the electrolyte. Corrosion rates were estimated from these tests and the results also showed that the addition of thiourea appears to increase the corrosion rate of metal oxide matrix composite-coated anodes although it reduces the corrosion rate of a PbCaSn and Pb-Co3O4 composite coated anode. Nevertheless the results showed that for one of the composite-coated anodes, the titanium based anode with a SnO2-Sb2O3 interlayer and PbO2-CoOx coating, the corrosion rate was lower than a conventional PbCaSn anode. The corrosion rate of the titanium based anode with same interlayer and PbO2-Co3O4 coating was variable but could be much iv greater than for a conventional anode. Thiourea showed no obvious effect on the anode oxygen evolution potential. In addition, lead balance method was used to calculate the corrosion rate of all the composite-coated anodes and to compare this with the corrosion rate of a conventional PbCaSn anode. The results confirm that incorporation of cobalt into the surface of lead has the effect of minimising the anode oxygen evolution potential and corrosion rate during 7-day tests. Of the different composite-coated anodes tested, the Ti-SnO2-Sb2O3-PbO2-Co3O4 electrode showed the best performance and it was therefore selected to undergo a long term test at a larger scale. The results show that this composite-coated anode performs well under the typical condition of copper electrowinning for 60 days. Cross section studies of this composite-coated anode after operating for 60 days clearly showed the two distinct layers deposited on the surface of the Ti substrate (the thermally deposited interlayer and the top layer of lead-cobalt oxide). The SEM image of the composite-coated anode shows that the composite layer deposited on top of the interlayer was porous and that acid penetrated through the composite layer. This might be a reason for the gradual increase of the anode oxygen evolution potential with time. The SnO2-Sb2O3 interlayer had a great impact on blocking the acid from reaching to the substrate. This is supported by the fact that the composite-coated anode with this interlayer never underwent passivation during 60 days of anodisation.

The development and testing of alternative anodes based on cobalt and lead for the electrowinning of base metals

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