Evaluation of Simple Amides in the Selective Recovery of Gold from Secondary Sources by Solvent Extraction

Abstract

The recycling of metals from end-of-life secondary sources such as electronic waste remains a significant environmental and technological challenge currently detrimental to the development of circular economies. The complex nature of electronic waste, containing a myriad of different elemental metals, means that sophisticated yet simple separation methods need to be developed in order to recycle these valuable and often critical metal resources. In this work simple 2 primary, secondary, and tertiary amides are appraised as reagents that selectively transport gold from aqueous to organic phases in a solvent extraction experiment. While the strength of extraction of gold from single metal solutions is ordered 3 o >2 o >1 o , the 3 o and 2 o amides are ineffective at gold transport from mixed-metal solutions of concentrations representative of smartphones due to the formation of a third, dense phase. Increasing the polarity of the organic phase can negate third phase formation but at the expense of selectivity. The identities of the species that reside in the organic and third phases have been studied by a combination of slope analysis, mass spectrometry, NMR spectroscopy, and computational methods. These techniques show that protonation of the amide L occurs at the oxygen atom, resulting in the protonated dimer HL2 + which acts as a receptor for AuCl4 − to form dynamic supramolecular aggregates in the organic phase. The characterization of a tin complex in the third phase by X-ray crystallography supports these conclusions and furthermore, suggests the preference for the chelation of the proton by two amide molecules instead of the transport of hydronium into the organic phase and its subsequent use as structural template.

Tantalum Recycling by Solvent Extraction: Chloride Is Better than Fluoride

“…(Figure 4), which results in a L:Ta ratio of approximately 2 (slope of 1.71 ± 0.13), suggesting the formation of an ion pair such as [HL 2 ][TaCl 6 ] in the organic phase. This type of behaviour is similar to that seen previously for Au extraction using L in which experimental and computational analysis showed that two amides chelate a proton through the oxygen atoms to form a charge-diffuse cation that can interact with diffusely charged metalate anions through classical (N-H•••X) and non-classical (C-H•••X) hydrogen bonds [23,24]. Direct structural characterisation of the extracted species has so far proven inconclusive, although 1 H NMR studies provide an indication as to how the receptor [HL2] + is interacting with the metalate ( Figure 5).…”

“…The simple primary amide, L, shows excellent performance for tantalum recovery by solvent extraction under high chloride conditions. The extraction mechanism is identified as similar to that seen for the recovery of gold by L, in which charge-diffuse protonated receptors are formed that preferentially interact with the charge diffuse monoanionic metalate [AuCl 4 ] − [23,24]. 1 H and 13 C NMR data are consistent with a strong outer-sphere interaction between [HL 2 ] + and [TaCl 6 ] − .…”

“…Deionised water was produced using a Milli-Q purification system. The syntheses of 3,5,5-trimethylhexanamide, L and [H(C 11 H 23 NO) 2 ] 2 [SnCl 6 ] were carried out according to the literature [23,24]. ICP-OES analysis was carried out on a Perkin Elmer Optima 8300DC Inductively Coupled Plasma Optical Emission Spectrometer.…”

“…These changes in the 1 H NMR spectra for Ta-loaded L are similar to those seen for the solid third phase formed between [SnCl 6 ] 2− and a tertiary amide version of L (Figures S2 and S3). In this latter case, the solid-state structure showed that the two amide ligands chelate the single proton through the amido oxygen atoms, with the cation subsequently interacting with the outer-sphere of the [SnCl 6 ] 2− octahedron through non-classical C-H hydrogen bonds [24,26,27]. The similarity in the 1 H NMR spectra for Ta/L, and also a relatively large downfield shift of the carbonyl carbon from 174 ppm to 180 ppm in the 13 C NMR spectrum ( Figure S4) upon Ta loading strongly suggest that a similar structure is formed in the organic phase, with no direct interaction of the amido-oxygen donor with the Ta centre; i.e., an amide complex such as TaCl 5 (L) is not formed.…”

Permeation of AuCl₄^- across a Liquid Membrane Impregnated with A324H⁺Cl^- Ionic Liquid

“…Among these residues, the materials generated from the electronic and communication industries are of importance [1,2], because they contained, among other valuable metals, gold. Thus, there is an interest, from worldwide researchers, to investigate methods for the recovery of this precious metal from these residues or, in general, gold-bearing wastes [3][4][5][6][7][8].…”