Mutual Separation of Palladium (II) and Platinum (IV) from Hydrochloric Acid Solutions Using m-Phenylene Diamine-Containing Agents

Abstract

Platinum group metals (PGMs), especially Pd and Pt, are important industrial elements owing to their extensive use in electrical devices, dental materials, catalysts, and jewelry [1,2]. Given the scarcity of the PGMs primary resources, the recycling of these metals from secondary sources or post-consumer scrap is essential [3]. Solvent extraction is regarded as a typical and practical method to recover PGM ions [4-8]. For example, Pd(II) and Pt(IV) can be extracted from hydrochloric acid (HCl) solutions with di-n-octyl sulfide [9] and tri-n-butyl phosphate [10] extractant agents, respectively. However, the use of volatile organic solvents as extractant diluents negatively affects the recovery process owing to their toxicity and environmental load. Mutual separation of PGMs, which is required in the recovery process, is complicated because of the similar physical and chemical properties of the metal components. PGM ions are known to be recovered via coordination or ion-pair mechanisms [11]. Pd and Pt are typically recovered following a coordination mechanism with the trend Pd(II) >> Pt(IV) [12]. On the other hand, Pd and Pt chloro-complex anions (i.e., [PdCl4] 2and [PtCl6] ) were extracted to a similar extent via an ion-pair mechanism [11,13]. Therefore, mutual separation of Pd and Pt is generally performed by combining coordination and ion-pair recovery strategies, respectively. However, the utilization of different mechanisms within the recovery process requires several recovery agents specially designed for the corresponding recovery mechanism. In addition, the recovery of PGMs via a coordination mechanism is typically accompanied with base metals such as Cu, Fe, Al, and Zn [12]. In this study, we report on the selective precipitation and mutual separation of Pd and Pt using new precipitating agents (i.e., m-phenylene diamine-containing compounds). These precipitating agents were prepared by condensation of aniline or 4-phenoxyaniline with 3,5-bis(trifluoroacetamido)benzoyl chloride followed by deprotection with hydrazine. The precipitation behaviors of Pd and Pt with the as-prepared precipitating agents were examined by using Pdand Pt-containing HCl solutions with or without base metals. The precipitating agents prepared in this study enabled mutual separation of Pd and Pt from a HCl solution containing base metals via an ion-pair mechanism.