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Synthesis of Ir–CaO Complex Oxides and Their Solubilities in Hydrochloric Acid

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Abstract

Iridium (Ir) is widely used in electrochemical, electrical, and chemical applications. Because of its low production volume, Ir recycling is crucial. Dissolution is necessary for recycling, and the conventional dissolution method involves using a highly hazardous strong acid. In this study, we investigated a method to dissolve Ir in hydrochloric acid (HCl) by employing the principle that acid solubility improves when a platinum group metal becomes a complex oxide. The conditions for the formation of Ir and CaO complex oxides were investigated via X-ray diffraction (XRD) analysis of Ir and CaCO3 powders that were mixed and heated. The solubility of Ir was also investigated by dissolving the sample with HCl after heating and analyzing it via inductively coupled plasma-optical emission spectroscopy (ICP-OES). The formation of Ca2IrO4 and Ca4IrO6 was confirmed, and both compounds exhibited high solubility in HCl. The solubilities of Ca2IrO4 and Ca4IrO6 in hydrochloric acid were also compared. The decomposition temperatures of the experimental complex oxides were verified thermodynamically. The dissolution mechanism of Ir via complex oxides is also discussed.

Graphical Abstract

In this study, in order to develop a new recycling method for Ir, the conditions for the formation of a complex oxide of Ir and CaO and its solubility in hydrochloric acid (HCl) were investigated. In the experiment, Ir and CaCO3 powders were mixed and heated, and analyzed by X-ray diffraction (XRD) to investigate the conditions for the formation of complex oxides. In addition, after heating, the samples were dissolved in hydrochloric acid and analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) to investigate the solubility of Ir. Figure 1 shows the results of dissolving a sample heated for one hour at a given temperature with 20 mL of concentrated HCl. The dissolution conditions were 3 h at 353 K. The acid solubility of Ir was shown to be enhanced by heating at temperatures above 973 K. In particular, the dissolution rate was higher when heated at 1473 K and 1573 K. The XRD profiles for the heating temperatures of 873K-1073K, in which changes in solubility appeared, are shown in Fig. 2. At 873 K, the metal remains metallic Ir after heating. At 973 K, the formation of Ca2IrO4, a complex oxide of Ir and CaO, was observed, but metallic Ir was also observed to remain. At 1073 K, metallic Ir was no longer identified, indicating that most of the metal was Ca2IrO4. As described above, the acid solubility of Ir is improved by forming a complex oxide with CaO, and it can be dissolved in oxidant-free HCl. In this paper, we also provide a thermodynamic verification of the experimental decomposition temperatures for a complex oxide of Ir and CaO. The dissolution mechanism of Ir by complex oxidation was also discussed.

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Acknowledgements

We would like to thank Editage (www.editage.com) for its English language editing services.

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Authors and Affiliations

  1. Chiba Institute of Technology, Narashino, Chiba, Japan

    Ryoji Sanekata, Yuto Kobayashi & Takashi Nagai

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  1. Ryoji Sanekata
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  2. Yuto Kobayashi
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  3. Takashi Nagai
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Correspondence to Takashi Nagai.

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Sanekata, R., Kobayashi, Y. & Nagai, T. Synthesis of Ir–CaO Complex Oxides and Their Solubilities in Hydrochloric Acid. J. Sustain. Metall. 9, 240–248 (2023). https://doi.org/10.1007/s40831-022-00643-4

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