Volatile Separation and Recovery of Iridium from Oxygen Evolution Electrodes Using Calcium Oxide

Iridium, a platinum–group metal, is used in the form of a mixture of iridium and tantalum oxides in the catalytic layer of oxygen–generating electrodes owing to the unique catalytic properties and chemical stability of Ir. The recovery of Ir from end–of–life products is important because of its low production, uneven geographical distribution of Ir sources, and high supply risks. However, recovery of Ir requires the dissolution of Ir in an aqueous solution, a procedure which involves the use of a strong acid and is, therefore, dangerous and environmentally hazardous. Moreover, if metals other than Ir dissolve in the aqueous solution during the recovery of Ir, harmful effluents and gases would be generated and the separation of Ir from other metals would be difficult. In this study, we developed a method that involves the extraction of only Ir from the catalyst layer of an oxygen–generating electrode and simultaneous recovery of Ir as a Ca–Ir composite oxide, where the composite oxide is soluble in hydrochloric acid. Only iridium oxide was volatilized from the catalyst layer of the oxygen–generating electrode and brought into contact with CaO via the gas phase. The composite oxide obtained was dissolved in hydrochloric acid and analyzed; the analysis revealed that Ir was highly soluble in hydrochloric acid and that the composite oxide did not contain Ta. © 2023 The Japan Institute of Metals and Materials.