Separation of platinum(IV) and palladium(II) from concentrated hydrochloric acid solutions by mixtures of amines with neutral extractants

Highlights

• A mixture of Aliquat 336 and TBP was effective for the separation of Pt(IV) and Pd(II).

• Pt(IV) was selectively extracted by this binary mixture at high HCl concentrations.

• Highest separation between Pt(IV) and Pd(II) was obtained at 6 M HCl.

• The McCabe–Thiele diagram showed that most of Pt(IV) can be extracted through three counter-current stages.

• The purity of the stripping solution of Pt(VI) was higher than 99%.

Abstract

Separation of platinum and palladium from concentrated HCl solutions by extraction with mixture of amines (Aliquat 336/Alamine 336/TOA) and neutral extractants (TBP/TOP/MIBK) have been investigated. The mixture of Aliquat 336 and TBP led to higher Pt(IV) separation factor at 6 M HCl solution. McCabe–Thiele diagram for the extraction of Pt by this mixture was constructed. Low concentration of thiourea in the HCl solution resulted in selective stripping of Pd from the loaded organic. Pt solution with 99.9% purity was obtained by extraction and stripping from the solution containing Pd. A process was proposed to separate the two metals.

Introduction

Platinum group metals (PGMs) are used as metallic catalysts in the automobile for minimizing air pollution. Among PGMs, the demand for Pt and Pd is increasing and thus the extraction and separation of these metals from secondary resources is important [1]. The separation and purification of PGMs is extremely difficult due to their similar structure and chemical behavior [2], [3]. Hydrometallurgical method, such as precipitation [4], ion exchange [5], [6], and solvent extraction [1], [3], [7], [8], [9], [10], [11], [12] are commonly employed for the separation of these metals. Generally, the solvent extraction is one of the most versatile methods in terms of separation efficiency and environmental impact [7], [11].

In treating the secondary resources by hydrometallurgical method, Pt and Pd together with other PGMs are dissolved by using concentrated hydrochloric acid concentration in the presence of an oxidizing agent. Lots of works have been reported on the extraction and separation of Pt and Pd from the leach liquors containing PGMs. Three kinds of extractant including acidic and neutral extractants and amines are employed for the separation of Pt and Pd. The process details, advantages and disadvantages of each extractant system are listed in Table 1. Literature survey indicates that acidic extractants offer high separation efficiency of Pt and Pd, when the acid concentration in leach liquor is low and concentrated HCl solution can strip the metals from the loaded organic. In the case of neutral extractants, selective extraction of Pd over Pt is possible from weak HCl solution [7] but the co-extraction of Pt is high in the concentrated HCl solution [9]. Use of amines lead to complete extraction both of Pt and Pd and these metals can be separated by selective stripping. The disadvantage of employing amines is the possibility of forming a third phase during the extraction and the complexity of the stripping process owing to the necessity to adjust the composition of the stripping solution.

Compared to neutral extractants, amines offer higher extraction efficiency of Pt and Pd [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. However in terms of separation factor, the separation efficiency of Pt and Pd by amines is low and the formation of a third phase should be overcome. Unlike single extractant, mixtures of extractants provide some opportunity to circumvent the above-mentioned disadvantages of amines. However, little information has been reported on the separation of Pt and Pd from HCl solution by using mixtures of amines and other extractants. For this purpose, solvent extraction experiments have been done from a synthetic chloride solution containing only Pt and Pd. The extraction and separation behavior of Pt and Pd was investigated as a function of the nature and concentration of extractant in the mixture, acid concentration, metal ion concentration and stripping condition. Extraction isotherm for the extraction of Pt was constructed. The effect of thiourea concentration in the HCl solution on the stripping of Pt and Pd was also investigated. A process flowsheet was developed to obtain pure Pt and Pd from the solution by employing extraction and stripping.