How Dimensionally Stable Anodes(DSA) are Applied for the Chlor-alkali Industry?

How Dimensionally Stable Anodes(DSA) are Applied for the Chlor-alkali Industry?

During the first half of the 20th century, Chlor-alkali cells were equipped with graphite anodes. Despite having been a well established technology for the anodic evolution of chlorine, the disadvantages related to the consumption of graphite were evident. The oxidation of graphite, not only caused the contamination of the chlorine but also reduced the service life of the anodes to a maximum of 24 months. Besides, the consumption of graphite increases the gap between electrodes leading to an increase in energy consumption over the electrode lifespan.

In 1965, Beer invent dimensionally stable anodes(DSA) for the Chlor-alkali industry. Beer refers that the electrode consists of a metallic substrate (titanium, tantalum, zirconium, niobium) and a coating made of at least two oxides. One of the oxides contains one metal from the platinum group (platinum, iridium, rhodium, palladium, ruthenium, or osmium) and the second is an oxide of one of the following metals: manganese, lead, chromium, cobalt, iron, titanium, tantalum, zirconium and silicon. Beer claimed titanium as the preferred metal for the substrate since titanium forms a spontaneous protective titanium oxide layer that prevents the substrate from electrolyte attack and does not exhibit significant resistance to the current flow.

These features made titanium the most suitable material to be used as a substrate for anodes in Chlor-alkali cells. Regarding the choice of the active metal, the selection of ruthenium, in the oxide form, was first based on its price compared with platinum or iridium. Attempts to improve the stability of the coating led to the addition of titanium oxide as a stabilizer of the metal oxide. Therefore, the final composition for the first commercial anodes was a titanium substrate coated with a binary ruthenium-titanium oxide film. Nowadays the composition of DSA for chlorine evolution has remained almost unchanged. Only the addition of iridium oxide to the original composition was a relevant milestone. The addition of iridium oxide was shown to prevent the loss of ruthenium, leading to the extension of the service life of the anodes from 2-3 years to 8 years.