Local process investigations on composite electrodes: on the way to understanding design criteria for spray coated anodes in Zn electrowinning

This reaction is connected to high overpotentials, which are in Zn electrowinning about 800 mV, or 20–25% of the total cell voltage, and with that an equal amount of the total electrical energy1. Many attempts have been made to reduce the overpotential by introducing better catalysts. The main anode systems are nowadays all lead based1, which offer the best cost:value ratio for most of the industrial processes that employ oxygen evolution. The most famous approach for improvements, the dimensionally stable anode (DSA2,3), which is a titanium metal electrode covered with mixed metal oxides (MMO, which are mostly precious metals) is usually applied only in a few cases where metals are electrowon from chloride solutions. They can be also used in Cu electrowinning after the solution has been cleaned by solvent extraction. In zinc electrowinning, however, the DSA suitable for oxygen evolution has shown to fail after a few days because the manganese content of the solution leads to MnO2 growth in between the electrode base material and the active coating, which is then flaked off4. Most of the scientific approaches in that area are using the same precious mixed metal approach, where metal salts are repeatedly applied to the surface of a titanium electrode and decomposed to metal oxides. The process is very complex, it influences the performance of the anode substantially and, also because of the time consuming process in both research and manufacturing, it is very expensive5. Due to the high risk (high investment costs) and due to the impurities present, DSA anodes have not really found their way into modern tank houses, although they have been very successful in the chlor-alkali industry5. Local process investigations on composite electrodes: on the way to understanding design criteria for spray coated anodes in Zn electrowinning