Extraction Properties of Palladium(II) in HCl Solution with Sulfide-Containing Monoamide Compounds

Liquid- and Solid-based Separations Employing Ionic Liquids for the Recovery of Platinum Group Metals Typically Encountered in Catalytic Converters: A Review

The wide application range and ascending demand for platinum group metals combined with the progressive depletion of their natural resources renders their efficient recycling a very important and pressing matter. Primarily environmental considerations associated with state-of-the-art recovery processes have shifted the focus of the scientific community toward the investigation of alternative recycling approaches. Within this context, ionic liquids have gained considerable attention in the last two decades chiefly sparked by properties such as tunabilty, low-volatility, and relatively easy recyclability. In this review an understanding of the state-of-the-art processes, including their drawbacks and limitations, is provided. The core of the discussion is focused on platinum group metal recovery with ionic liquid-based systems. A brief insight in some environmental considerations related to ionic liquids is also provided while some discussion on research gaps, common misconceptions related to ionic liquids and outlook on unresolved issues could not be absent from this review.

Amide-functionalised phosphonium-based ionic liquids as ligands for rhodium(iii) extraction

Seven new amide-functionalised phosphonium-based ionic liquids (APILs) with chloride anions are synthesised and applied to extraction of rhodium(iii) from HCl solution. The effects of structural modification of the APILs on the extraction performance are examined by liquid-liquid extraction using toluene as a diluent and the results compared with those for trihexyltetradecylphosphonium chloride ([P66614][Cl]), a typical commercial extractant. The performance of the APILs as rhodium(iii) extractants is influenced by three main factors: (1) the length of the alkyl chains attached to the P atom; (2) the length of the linker between the amide and phosphonium moiety; and (3) the type of amide group. A novel ligand, [3°C2P444][Cl], had outstanding performance in the effective recovery of rhodium(iii). Extraction of rhodium(iii) from a 1.0 mol dm-3 HCl solution with 0.5 mol dm-3 [3°C2P444][Cl] proceeded quantitatively (>98%) and the extraction efficiency was higher than that of the commercial extractant [P66614][Cl]. The mechanism of rhodium(iii) extraction by [3°C2P444][Cl] was investigated by slope analysis, UV-vis, and FT-IR spectroscopy. These results indicate that [RhCl4(H2O)2]- in aqueous solution is extracted by [3°C2P444][Cl] through an anion-exchange mechanism and slowly converted into a dimer, [Rh2Cl9]3-, in the organic phase.

Extraction of Pd(ii) and Pt(ii) from aqueous hydrochloric acid with 1,3-diaminocalix[4]arene: switching of the extraction selectivity by using different extraction modes

1,3-Diaminocalix[4]arene shows extraction ability toward Pd(ii) and Pt(ii), the selectivity of which can be switched by changing the concentrations of H⁺ and Cl⁻ in the aqueous phase.

Rapid and selective recovery of palladium from platinum group metals and base metals using a thioamide-modified calix[4]arene extractant in environmentally friendly hydrocarbon fluids

A novel macrocyclic calix[4]arene extractant having a long alkyl chain thioamide, 25,26,27,28-tetrakis(N-n-octylthiocarbamoyl)methoxy-5,11,17,23-tetra-tert-butylcalix[4]arene (1), was synthesized from 25,26,27,28-tetrakis(N-n-octylcarbamoyl)methoxy-5,11,17,23-tetra-tert-butylcalix[4]arene (2) using Lawesson's reagent. Extractant 1 was characterized using ¹H NMR, ¹³C NMR, FT-IR spectroscopy, and elemental analysis. The Pd(II) extraction abilities of 1 and 2 were studied in high-boiling-point and environmentally friendly hydrocarbon diluents. Pd(II) extraction experiments were conducted using single-metal Pd(II) solutions, simulated mixed palladium group metal (PGM) solutions, and acid-leached automotive catalyst residue solutions. Different experimental conditions, including the shaking time, HCl/HNO₃ concentration, Pd(II) concentration, extractant concentration, and the organic/aqueous phase ratio, were studied systematically. Extractant 1 showed very selective (> 99.9%) Pd(II) extraction from the mixed PGM/base metal solutions and the acid-leached automotive catalyst residue solution. Conversely, the Pd(II) extraction ability of extractant 2 was found to be negligible. Extractant 1 showed very fast extraction kinetics and a high extraction capacity as compared to those of the commercial extractant di-n-octyl sulfide. Effective stripping of Pd(II) from 1 was performed using HCI, HNO₃, NH₃, and HCl-thiourea solutions. Furthermore, 1 was successfully recycled over five extraction/stripping cycles. The Pd(II) extraction mechanism of 1 was studied using FT-IR spectroscopy. Extractant 1 exhibited very selective Pd(II) extraction and high acid stability, demonstrating its industrial applicability for the extraction of Pd(II) from leached automotive catalyst liquors containing PGMs and base metals.

Wastes generated by automotive industry – Spent automotive catalysts

Rhodium, ruthenium, palladium, and platinum are classified as platinum group metals (PGM). A demand for PGM has increased in recent years. Their natural sources are limited, therefore it is important, and both from economical and environmental point of view, to develop effective process to recover PGM from waste/secondary sources, such as spent automotive catalysts. Pyrometallurgical methods have always been used for separation of PGM from various materials. However, recently, an increasing interest in hydrometallurgical techniques for the removal of precious metals from secondary sources has been noted. Among them, liquid-liquid extraction by contacting two liquid phases: aqueous solution of metal ions and organic solution of extractant is considered an efficient technique to separate valuable metal ions from solutions after leaching from spent catalysts.

Comparison of the Extractabilities of Tetrachloro- and Tetrabromopalladate(II) Ions with a Thiodiglycolamide Compound

Using N,N,N',N'-tetra-2-ethylhexyl-thiodiglycolamide (TEHTDGA) in n-dodecane as the extractant, we compared the percentages of Pd(II) extracted from HCl and HBr solutions, and analyzed the structures of the Pd(II)-extractant complexes. For comparison, similar experiments were performed with di-n-hexyl sulfide (DHS), a well-known sulfide-type extractant. TEHTDGA extracted Pd(II) from both HCl and HBr solutions much faster than DHS. The Pd(II)/(TEHTDGA or DHS) stoichiometry in the organic phase was 1:2. For TEHTDGA, the extractability of Pd(II) from HBr solution was inferior to that from HCl solution, whereas the opposite was true for DHS. However, FT-IR spectroscopy and EXAFS measurements indicated that the inner-sphere structure of Pd(II) in the TEHTDGA complex was almost the same as that in the DHS system: in both cases, two of the halide ions in the tetrachloro- or tetrabromopalladate(II) ion were replaced by the sulfur atoms of two extractant molecules.

Synthesis of pincer-type extractants for selective extraction of palladium from PGMs: An improved liquid-liquid extraction approach to current refining processes

SCS pincer ligands 1–4 were synthesised, and their ability to extract Pd(II) from HCl and HNO₃ media was studied. The Pd(II) extraction properties of 1–4 were compared with those of commercial extractants (DOS and LIX^®84-I) in kerosene. 1 and 2 showed superior Pd(II) extractability (E% = 99.9) relative to DOS and LIX^®84-I from 0.1–8.0 M HCl and to DOS from 0.1–8.0 M HNO₃ and mixed HCl + HNO₃ media. The Pd(II) extraction rate, acid durability, the most suitable organic/aqueous (O/A) phase ratio, and Pd(II) loading capacity of extractants 1, 2, and DOS were evaluated. 1 and 2 exhibited a greater Pd(II) extraction rate and Pd(II) loading capacity than DOS. 1 was very stable in acid media (HCl and HCl + HNO₃), whereas 2 and DOS deteriorated in HCl + HNO₃. Selective extraction of Pd(II) by 1 and 2 was achieved from a mixed solution containing Pd, Pt, Rh, rare metals, and base metal ions that simulated the leach liquors of automotive catalysts. The back extraction of Pd(II) and reusability of extractants 1 and 2 were studied. The Pd(II) extraction mechanism of 1–4 was investigated using FT-IR, UV-visible, and NMR spectroscopy.

Isopentyl-Sulfide-Impregnated Nano-MnO₂ for the Selective Sorption of Pd(II) from the Leaching Liquor of Ores

Conventional separation methods are not suitable for recovering palladium present in low concentrations in ore leaching solutions. In this study, a novel isopentyl sulfide (S₂₀₁)-impregnated α-MnO₂ nanorod adsorbent (BISIN) was prepared, characterized, and applied for the selective adsorption and separation of palladium from the leaching liquor of ores. Batch studies were carried out, and the main adsorption parameters were systematically investigated, in addition to the relevant thermodynamic parameters, isotherms, and kinetic models. The thermodynamic parameters reflected the endothermic and spontaneous nature of the adsorption. Moreover, the experimental results indicated that the Langmuir isotherm model fits the palladium adsorption data well and the adsorption was well described by the pseudo-second-order kinetic model. The main adsorption mechanisms of palladium were elucidated at the molecular level by X-ray crystal structure analysis. Thiourea was found to be an excellent desorption agent, and the palladium-thiourea complex was also confirmed by X-ray crystal structure analysis. The results indicated that almost all of the Pd(II) (>99.0%) is adsorbed on BISIN, whereas less than 2% of the adsorbed Pt(IV), Fe³⁺, Cu²⁺, Ni²⁺, and Co²⁺ is observed under the optimum conditions. The proposed method can be used for the efficient adsorption and separation of palladium from the leaching liquor of ores.

A perspective on diverse adsorbent materials to recover precious palladium and the way forward

A perspective on diverse adsorbent materials for the recovery of palladium.

A 2-mercaptobenzothiazole-functionalized ionic liquid for selective extraction of Pd(ii) from a hydrochloric acid medium

A novel 2-mercaptobenzothiazole-functionalized ionic liquid (1,10-bis(3-methylimidazolium-1-yl)decane bis(triflic)imide two 2-mercaptobenzothiazolate, {[C₁₀(mim)₂]²⁺ 2 [MBT]⁻}) with excellent selectivity for Pd(ii) extraction was fabricated.

Rapid and selective extraction of Pd(ii) ions using the SCS type pincer ligand 1,3-bis(dimethylthiocarbamoyloxy)benzene, and its Pd(ii) extraction mechanism

Extractant 1 extracts Pd(ii) ions via pincer type S–C–S tridentate ligands, with the formation of fused six-membered chelate rings. Rapid and selective extraction by 1 of >99% Pd(ii) ions from an automotive catalyst leach liquor was observed.