**Kazuya Matsumoto \*, Yuto Sezaki, Sumito Yamakawa, Yuki Hata and Mitsutoshi Jikei**

Department of Materials Science, Graduate School of Engineering Science, Akita University, 1-1 Tegatagakuen-machi, Akita-shi, Akita 010-8502, Japan; sezaki.y@yahoo.ne.jp (Y.S.); smi.kun.12@gmail.com (S.Y.); wimper\_h\_1114@outlook.jp (Y.H.); mjikei@gipc.akita-u.ac.jp (M.J.)

**\*** Correspondence: kmatsu@gipc.akita-u.ac.jp; Tel.: +81-18-889-2745

Received: 16 February 2020; Accepted: 28 February 2020; Published: 29 February 2020

**Abstract:** The selective recovery of platinum-group metals (PGMs) remains a huge challenge. Although solvent extraction processes are generally used for PGM separation, the use of organic solvents is problematic because of their toxicity and environmental concerns. Here, we have developed a new PGM recovery method by precipitation from hydrochloric acid (HCl) solutions containing Pd(II), Pt(IV), and Rh(III) using aliphatic primary amines as precipitants. Pt(IV) was precipitated using the amines with alkyl chains longer than hexyl independent of HCl concentration. The precipitation of Pd(II) required longer alkyl amines than octyl, regardless of the HCl concentration. Rh(III) was recovered by precipitation at high HCl concentrations using the amines longer than hexyl. The mutual separation of Pt(IV), Rh(III), and Pd(II), in this order, was successfully achieved by changing the HCl concentrations and alkyl chain lengths of the amines. X-ray photoelectron spectroscopy and thermogravimetric analysis evidently showed that the metal-containing precipitates were ion-pair complexes composed of metal chloro-complex anions and ammonium cations.

**Keywords:** platinum-group metal; metal precipitation; ion-pair; aliphatic primary amine; hydrochloric acid

### **1. Introduction**

Platinum-group metals (PGMs), particularly Pd, Pt, and Rh, are of crucial importance because of their wide range of applications, such as automobile catalysts and electrical devices [1–5]. Despite the increasing demand for PGMs, their availability remains limited due to their scarcity in Nature and regional maldistribution. Therefore, efficient recovery and separation processes are necessary to recycle PGMs from post-consumer scrap. Typically, solvent extraction is regarded as a practical method to recover and separate PGM ions from metal-containing aqueous solutions. For example, tertiary amines [6,7], organophosphates [8–11], and organosulfides [12,13] have been reported to act as Pd(II) or Pt(IV) extractants. However, solvent extraction processes require organic solvents, which are used as diluents of extractants or extractants themselves, and the use of organic solvents is problematic because of their toxicity and environmental concern. Furthermore, the selective recovery of PGMs remains a massive challenge due to the similarity in their properties.

The recovery mechanisms of PGMs by solvent extraction are generally classified into two types, namely, ligand-metal coordination and ion-pair formation [14]. For the coordination mechanism, metal extraction generally occurs in the order of Pd(II) >> Rh(III) >> Pt(IV); and Rh(III) and Pt(IV) are regarded as kinetically inert [15]. Furthermore, the order of extractability via ion-pair formation for the PGM chloro-complexes formed in an aqueous hydrochloric acid (HCl) solution is reported to be [MCl4] <sup>2</sup><sup>−</sup> [MCl6] <sup>2</sup><sup>−</sup> > [MCl6] <sup>3</sup><sup>−</sup> > aqua species [16]; while the chloro-complex anions of PGMs

formed in HCl are reported to be [PdCl4] <sup>2</sup>−, [PtCl6] <sup>2</sup>−, and [RhCl6] <sup>3</sup>−, as well as the chloro-aqua complexes of Rh(III) ([RhCl4(H2O)2] <sup>−</sup> and [RhCl5(H2O)]2<sup>−</sup>) [14]. Therefore, the extractability of PGMs in ligand-metal coordination and ion-pair formation evidently indicates the difficulty in the selective recovery of Pt(IV) in priority to Pd(II) as well as the Rh(III) recovery in advance of Pd(II) and Pt(IV) by conventional solvent extractions.

Recently, we developed PGM recovery methods based on precipitation using aromatic primary amines as precipitants [17–20]. The selective precipitation of Pt(IV) from HCl solutions containing Pd(II) and Pt(IV) was successfully achieved using 4-hexyloxyaniline at high HCl concentrations [17]. Furthermore, the preferential and selective Rh(III) recovery from the mixture of Pd(II), Pt(IV), and Rh(III) in HCl was successfully accomplished using aromatic primary monoamines [18] or diamines [19,20] as precipitants. The successful precipitation of Rh(III) was achieved based on the formation of unique ion-pair complexes comprising [RhCl6] <sup>3</sup><sup>−</sup> and anilinium cations of the amines. Although the potential of aromatic primary amines for PGM precipitants has been revealed, studies on the capability of aliphatic primary amines for PGM precipitants, as well as PGM extractants have been limited. Therefore, investigations of the availability of aliphatic primary amines for PGM recycling are valuable in the viewpoint of recycling industry.

Herein, we present a new PGM separation method by precipitation from the mixed solution of Pd(II), Pt(IV), and Rh(III) in HCl using aliphatic primary amines as precipitants. Preferential and selective precipitation of Pt(IV) at low HCl concentrations was achieved using *n*-heptylamine and *n*-octylamine. Moreover, Rh(III) was recovered by precipitation in preference to Pd(II) at high HCl concentrations using *n*-heptylamine and *n*-octylamine. Thereafter, we successfully achieved the mutual separation of Pt(IV), Rh(III), and Pd(II) in this order by changing the HCl concentrations and the alkyl chain lengths of the amines. The mechanism of the selective precipitation of PGMs was studied by the analysis of the metal-containing precipitates.

## **2. Materials and Methods**
