The Problems of Lead Alloys(Pb-Ag Anode) in the Electrowinning of Zinc

The Problems of Lead Alloys(Pb-Ag Anode) in the Electrowinning of Zinc

The electrolytic production of zinc is normally operated at a higher (than copper) current density of about 500 A m-2. The Pb-Ca-Sn anodes are not suitable for use at this high current as it will lead to high energy consumption.

Tin cannot be used as an alloying element because it will dissolve into the electrolyte and reduce the cathode current efficiency in addition to contaminating the cathode zinc. Instead, lead alloys containing 0.45 to 1% silver (Pb-Ag anode) have been used for many years. Silver is alloyed with the lead anode to reduce the rate of corrosion and improve the conductivity of the anode. In addition, the addition of silver also lowers the oxygen evolution overpotential by approximately 120 mV compared to pure lead. A small amount of silver oxide may be formed on the surface of the anode in addition to the lead oxides. However, the poor mechanical properties are the disadvantage of Pb-Ag anodes in that they are relatively weak and bend quite easily when struck by cathode sheets as they are removed or inserted to the cells. Therefore, calcium to the extent of 0.05 to 0.08% is sometimes added to the alloys to improve the mechanical properties.

Due to the high cost of silver metal, other alloying elements have been investigated in an effort to reduce or replace silver. Thus, cobalt has been found to be a potential additive in lead alloys as it can improve the stability of a lead anode. However, Pb-Co anodes have not to date been used in any industrial application because of the difficulties in producing suitable Pb-Co alloys. The addition of cobalt ions to the electrolyte in zinc electrowinning is not advantageous, since it will adversely affect the current efficiency.

The formation of a dense adherent protective layer of PbO2/MnO2 on the surface of the anode is normally a very slow process. It can take from three to six months for the surface anode to be fully established. A number of different surface pretreatment methods for Pb-Ag and Pb-Ag-Ca alloy anodes have been developed to accelerate its formation, such as chemical oxidation in a KMnO4-H2SO4 electrolyte at 70οC, KF electrochemical pretreatment, sandblasting and shot-peening. All of the methods aim to roughen the anode surface to enhance the generation of a hard adherent corrosion layer in a relatively short time.

The permanganate chemical method, however, is sensitive to the rate of cooling after the pretreatment process, which, if too rapid, can cause cracking and curling of the anodic layer. The electrochemical preconditioning process in fluoride containing solutions provides extra resistance to corrosion due to the formation of the lead difluoride layer between the lead substrate and the lead dioxide coating. Abrasive blasting and peening are carried out with relatively coarse silica sand, glass beads, or steel at high pressures above 500 kPa. The adherent oxide layer forms soon after installation and remains intact for at least two months allowing a stable PbO2-MnO2 layer to form underneath. Sandblasting is currently the most common preconditioning method since peening with steel shot or glass beads can result in significant deformation of the surface of the lead anode and actual warping of the entire anode if not carried out carefully.