Liquid–liquid extraction of palladium(II) from chloride media by N,N′-dimethyl-N,N′-dicyclohexylthiodiglycolamide

Highlights

• DMDCHTDGA in toluene recovers Pd(II) from HCl media with a favorable kinetics.

• DMDCHTDGA reveals robustness upon sequential SX cycles.

• A DMDCHTDGA:Pd(II) molar ratio of 2.5 is reached at saturation.

• Within the metals tested, only Fe(III) is co-extracted with Pd(II) from 4 or 6 M HCl.

• DMDCHTDGA is selective for Pd(II); Fe(III) is removed by water prior to Pd(II) stripping.

Abstract

Previous research showed that N,N′-dimethyl-N,N′-dicyclohexylthiodiglycolamide (DMDCHTDGA) in 1,2-dichloroethane is able to co-extract platinum(IV) and palladium(II) from concentrated hydrochloric acid solutions. Following a detailed study about Pt(IV), the present work focuses on the thorough investigation of Pd(II) extraction by DMDCHTDGA in toluene. The removal of Pd(II) is rather efficient from 0.5 M to 5.5 M HCl, and progressively decreases until 7.5 M HCl. Pd(II) stripping is better achieved by an acidic thiourea solution, but ammonia aqueous phases can alternatively be used for some cases. Pd(II) extraction kinetics is relatively favored (5–15 min). The robustness of DMDCHTDGA to extract Pd(II) has been confirmed through five successive reutilization experiments. The maximum loading capacity for Pd(II) reaches a 2.5 DMDCHTDGA:Pd(II) molar ratio. Pd(II) can selectively be recovered by DMDCHTDGA from 4.0 M or 6.0 M HCl complex mixtures containing equivalent concentrations of Pt(IV) and Rh(III), and fivefold Fe(III) and Al(III) concentrations, although with a slight Pt(IV) contamination.

Introduction

The platinum-group metals (PGMs) occupy a relevant place in the forecast market balance for critical raw materials, as they are considered to have small deficit supplies in 2015 and 2020 in a recent European Union report [1]. The collapse of PGMs natural resources may be considered a threat to the preservation of the everyday life in the western world, since PGMs have been gradually required for use in several technological applications. The irreplaceable catalytic properties of PGMs are currently explored in the automobile catalysts, as a means to reduce the harmful emissions of the motor engines, and also in the electronic and chemical industry [2]. Accordingly, the PGMs recycling from anthropogenic sources becomes essential, aiming to fulfill the worldwide requirements of these critical metals [3].

A few processes, all of them relying on chloride-based leaching media, have been developed for the direct hydrometallurgical leaching of PGMs from spent catalysts [4], [5], the sequential steps often applied to further isolate and concentrate PGMs being ion exchange [6], [7] and liquid–liquid extraction (solvent extraction, SX) [8]. The general compositions of these leaches may be more complex than those coming from the primary raw materials, and even the most frequent contaminants are different for many cases. This situation has been justifying the crescent commitment of the researcher’s community to further adapt already well known SX processes [9], [10], [11] or to investigate the viability of new molecules as PGMs extractants [11], [12].

Amide derivatives are one of the main families of organic compounds that has been extensively considered for PGMs extraction, particularly for Pd(II), Pt(IV) and Rh(III) [13], [14], [15], [16]. Focusing on Pd(II) recovery, sulfide containing monoamides [17] and a dithiodiglycolamide [18] have recently been proposed, as well as tertiary thioamides [19] and particular thiodiglycolamide derivatives [20], [21], [22], [23]. A cautious assessment of the Pd(II) extraction capabilities shown by these latter compounds points out to remarkable findings in terms of efficiency and selectivity for Pd(II) recovery, therefore justifying additional research.

The SX of Pt(IV) from hydrochloric acid media by N,N′-dimethyl-N,N′-dicyclohexylthiodiglycolamide (DMDCHTDGA) – Fig. 1 – in 1,2-dichloroethane (1,2-DCE) has already been cautiously investigated [23]. Furthermore, the quantitative co-extraction of Pt(IV) and Pd(II) from 8 M HCl was also described, the separation of the two metal ions from the loaded organic phase being accomplished by selective stripping [23].

It is well known that the use of chlorinated diluents is not conceivable in practical applications. However, for preliminary fundamental investigation, it is better to opt for a diluent not presenting difficulties to solubilize the extractant. As previously reported [23], DMDCHTDGA is not soluble enough in aliphatic commercial diluents of the kerosene type, but it is soluble in toluene, xylene and in other aromatic diluents. In addition to the ease of Pd(II) extraction and stripping and favorable extraction kinetics, the reutilization patterns and selectivity performance shown by DMDCHTDGA in toluene are rather promising if Pd(II) recovery from complex aqueous matrices is aimed. These encouraging results justify future attempts to investigate if more environmentally-friendly diluents can alternatively be used.