Application of Lead Dioxide Anodes for Oxygen Evolving Actions

Application of Lead Dioxide Anodes for Oxygen Evolving Actions

Lead dioxide anodes have been widely used as oxygen evolving counter electrodes in a sulfuric acid electrolyte free of halide ions. Its main application is the electrowinning of copper, zinc, nickel, and chromium.

The technology is designed to permit easy removal of the metal after deposition on the cathode. The metals are refined in open tank cells with parallel, alternate lines of cathode and anode plates; for copper, the tanks are similar in size to a swimming pool and each anode plate is a fraction of a square meter.

The anode current density is generally in the range of 10–100 mA cm2. The anode material is a lead alloy with 6–15% Sb and 0–1% Ag. The antimony hardens the alloy and improves its mechanical properties while the silver reduces the overpotential for oxygen evolution and hence the energy consumption for the process.

In the process operating conditions, the anodes have an easily seen corrosion coating but with careful handling have a lifetime of several years. These lead anodes have received competition from DSA coatings (Ti/IrO2 based dimensionally stable anodes), but the low cost and acceptable performance of the lead dioxide lead to their retention in many plants.
 
Another process that used a PbAg alloy anode in a sulfuric acid anolyte for the hydrodimerization of acrylonitrile to adiponitrile. This process employed divided, parallel plate cells in a bipolar stack. The catholyte was a complex mixture of organics and a proton permeable membrane was used to separate it from the anolyte and hence maintain a clean sulfuric acid anolyte. In these conditions, the lead dioxide anodes could maintain a current density of 450 mA cm2 .