Solubilization of Rhodium in Hydrochloric Acid Using an Alkali Metal Salt Method
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I.
INTRODUCTION
PLATINUM group metals (PGMs) play important roles in a wide range of fields and are extensively employed by the automobile industry in catalysts for exhaust gas purification systems. Accordingly, the demand for PGMs is expected to increase over the next few decades due to, among other factors, the rapid expansion of motor vehicle use in developing countries.[1] However, the amount of PGMs currently produced is very small and PGM-producing areas are very limited. For example, palladium (Pd) is obtained from mines in the largest amount (207 tons), followed by platinum (Pt, 180 tons) and rhodium (Rh, 23 tons).[2] Furthermore, these amounts are not increasing, and this situation will lead to a significant increase in supply risk in the near future. Thus, the recovery of PGMs from spent catalysts and other sources is becoming increasingly important. For the recovery of PGMs from scrap materials, dissolution into acid is essential. This is because separation and purification processes used for PGMs (e.g., solvent extraction, precipitation separation, and
RYO KASUYA and KATSUHIRO NOMURA are with the Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98, Anagahora, Shimo-shidami, Moriyama-ku, Nagoya, Aichi 463-8560, Japan. Contact e-mail: [email protected] HIROKAZU NARITA is with the Environmental Management Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan. Manuscript submitted May 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
ion-exchange resin methods) rely on the formation of PGM complex ions in solution. However, since PGMs are precious metals, they can not be ionized (oxidized) by protonic acids like hydrochloric acid (HCl). Therefore, acids such as aqua regia that contain strong oxidizing agents are used to dissolve PGMs. Aqua regia is a 3:1 mixture of HCl and nitric acid (HNO3), wherein HNO3 acts as an oxidizing agent for HCl so as to generate chlorine gas (Cl2) and nitrosyl chloride (NOCl), as shown in Eq. [1].[3,4] 3HCl þ HNO3 ! Cl2 þ NOCl þ 2H2 O
½1
However, while these oxidizing agents are capable of oxidizing PGMs, they present serious problems in terms of very high toxicity and corrosivity. For example, the corrosion of chromium steel, from which components in gas-treatment plants are typically fabricated, occurs much more rapidly in Cl2 gas than in HCl gas.[5] Furthermore, the generation of nitrate-nitrogen is unavoidable in dissolution processes using aqua regia. Since excessive intake of nitrate-nitrogen is harmful to human health, it is strictly regulated in the Guidelines for Drinking-water Quality as set by the World Health Organization (WHO).[6] In Japan, the upper limit for tap water is 10 mg-N/L,[7] while the limit applied to factory effluent is higher at 100 mg-N/L.[8] In addition, provisional effluent standards are applied in several fields. For example, the provisional effluent standard value for the precious
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