Solubilization of Ir in Hydrochloric Acid via Occlusion in CaMnO3

Abstract

In the recovery of platinum group metals (PGMs) via dissolution, aqua regia (AR) has been used, owing to its strong oxidation power. During conventional dissolution, PGMs are oxidized from metals (M) to metal ions (Mⁿ⁺). The generation of nitrate-nitrogen (N in HNO₃) when using AR is unavoidable, and strict regulatory guidelines have been established recently, owing to the associated health risks. To address this issue, we developed a new dissolution process for PGMs using only hydrochloric acid (HCl), involving occlusion into perovskite-type oxides ((La_0.7Sr_0.2Ba_0.1)ScO_3−δ, CaMnO₃ (CM), etc.). In this study, we elucidated the mechanism by which CM occludes Ir, which is one of the PGMs and poorly soluble even in AR, and the dissolution mechanism of Ir-occluded CM perovskite-type oxides Ca(Mn,Ir)O₃ (CMIs) in HCl. The high reactivity of CM toward Ir in air was attributed to Ir oxidation during firing in air; the presence of Ca and O2 site (Schottky) defects in CMIs, which acceralated the solid-state reaction of MnO and IrO₂; and the formation of Mn^3+/4+–Ir^4+/5+ redox pairs in CMIs. The higher solubility of CMIs in HCl originates from the reduction of Mn (from Mn^3+/4+ to Mn²⁺) and Ir (from Ir^4+/5+ to Ir³⁺) ions and the oxidation of HCl (from 2Cl^– to Cl₂).