Separation of Ir(IV) and Rh(III) from strong hydrochloric acid solutions by solvent extraction with amines

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Highlights

  • Ir(IV) was selectively extracted over Rh(III) by amines from HCl solution.

  • Higher separation factor between Ir and Rh was obtained at low HCl concentration.

  • Solution aging depressed the extraction of Rh(III).

  • The mixture of HCl and thiourea stripped selectively Ir from the loaded Aliquat336.

Abstract

The separation of Ir(IV) and Rh(III) from chloride solution by extraction with amines (TOA, TEHA, Alamine 308, and Aliquat 336) was investigated. Ir(IV) was selectively extracted over Rh(III) by these extractants. HCl concentration affected little the extraction of metals. The highest separation factor of 82.5 between the two metals was obtained from 1 M HCl solution by extraction with Aliquat 336. Solution aging until 2 weeks depressed the extraction of Rh(III). The mixture of HCl and thiourea stripped Ir(IV) selectively from the loaded amines and thus promoted the separation.

Graphical abstract

Effect of HCl concentration on stripping of Ir(IV) and Rh(III) from Aliquat 336 by the mixture 1 M thiourea and HCl.

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Introduction

Iridium (Ir) and rhodium (Rh) are platinum group metals (PGMs) used as a constituent in diverse advanced materials. Since the demand for Ir and Rh is increasing, the recovery of these metals from spent resources has attracted much attention. In most of the hydrometallurgical processes, the PGMs in the spent resources are leached by employing strong hydrochloric acid solution in presence of an oxidizing agent. Either solvent extraction or ion exchange is commonly employed to separate the PGMs from the resulting HCl solutions [1], [2], [3], [4], [5]. Many works have been done on the separation systems of PGMs such as, Pt-Pd [1], [6], [7], [8], [9], Pt-Pd-Rh [10], [11], and Pt-Rh [12], [13], [14], [15]. Compared to these systems, few spent resources contain Ir and Rh. That is the reason why there are a few papers on the separation of Ir and Rh. In the recovery of PGMs from ores or other resources, Ru and Os are separated firstly by oxidative distillation. In the separation of the resulting four PGMs, Pt and Pd are selectively extracted over Ir and Rh. Then it is necessary to separate Ir and Rh from the raffinate after the extraction of Pt and Pd.

The separation of Ir(IV) and Rh(III) is difficult because of their similar chemical properties and formation of complicated complexes. In hydrochloric acid solution, stable chlorocomlexes of Ir(IV), such as IrCl5 and IrCl62− are formed [16], [17], while that of Rh(III) was not stable [18]. The aquation/conversion of extractable RhCl63− complexes to RhCl5(H2O)2− is influenced by the chloride ion concentration and solution aging [19]. The separation of Ir(IV) and Rh(III) by solvent extraction can be classified into two methods according to the addition of SnCl2. Firstly, amines or neutral extractants are employed in the absence of SnCl2 to selectively extract Ir(IV) over Rh(III) [20], [21]. Since Rh(III) can be extracted by these extractants, the control of extractant concentration is important to obtain high separation efficiency of Ir(IV). Since SnCl2 is a reducing agent, the presence of SnCl2 in the chloride solution results in oxidation-reduction reaction of Ir(IV) and Rh(III) in the solution. Lee et al., [21] have reported that the Ir(IV) can be selectively stripped from the loaded tri-octyl/dodecyl amine (Alamine 336) or tributyl phosphate (TBP)containing Rh(III) by using stannous chloride solution. IrCl62− and RhCl63− are reduced to IrCl63− and Rh(I), respectively in the presence of SnCl2. Moreover, the Rh(I) can form complexes with SnCl3 such as [Rh(SnCl3)5]4−, which can be selectively extracted over Ir(III) by TBP or Alamine 336 [17], [22]. The main disadvantage of the second method is that the co-extraction percentage of Sn with Rh is high and thus another separation step should be employed to separate Rh and Sn.

Comparing the two methods, the first method seems to be more economical than the second one. In order for the first method to be applicable commercially, it is important to obtain an optimum extraction condition to minimize the co-extraction of Rh(III) and a stripping condition to separate Ir(IV) and Rh(III) from the loaded organic phases. For this purpose, the extraction and separation behavior of Ir(IV) and Rh(III) in the absence of SnCl2 using various amines was investigated as a function of hydrochloric acid and extractant concentration and solution aging. Moreover, an optimum stripping condition to separate Ir(IV) and Rh(III) was obtained.

Section snippets

Experimental

A synthetic solution was prepared by dissolving appropriate amounts of H2IrCl6 (99.5%) and RhCl3.xH2O (99.9%) purchased from Alfa-Aesar. The value of x in the rhodium reagent was determined by measuring the concentration of a solution containing a certain amount of the reagent. The composition of the synthetic solution was Ir(IV)-200 mg/L and Rh(III)-120 mg/L. The acidity of the solution (1–8 M) was controlled by adding HCl (Daejung Co.) solution. Amines such as tri-n-octylamine (TOA, Samchun Pure

Effect of HCl concentration

Alamine 308, TOA, TEHA, and Aliquat 336 were employed to investigate the extraction and separation behavior of Ir(IV) and Rh(III). The effect of hydrochloric acid concentration on the extraction of Ir(IV) and Rh(III) from synthetic solutions containing Rh(III)-120 mg/L and Ir(IV)-200 mg/L was studied in the HCl concentration range of 1.0–8.0 M. In these experiments, the concentration of each extractant was fixed at 0.01 M and the ratio of organic to aqueous phase was unity. The obtained results are

Conclusions

The extraction and separation of Ir (IV) and Rh(III) from synthetic chloride solutions was investigated by employing amine extractants (TOA, TEHA, Alamine 308, and Aliquat 336) in the HCl concentration range from 1 to 8 M. In our experimental ranges, the extraction percentage of Ir(IV) was much higher than that of Rh(III). There was not much difference in the extraction percentage of both metals with HCl concentration. The separation factor between Ir(IV) and Rh(III) decreased with the increase

Acknowledgments

This work was supported by a grant operated by KEITI of the Ministry of Environment of Korea. The authors would like to thank for the financial support. We gratefully thank the Gwangju branch of the Korea Basic Science (KBSI) for ICP data.

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