Tuneable separation of gold by selective precipitation using a simple and recyclable diamide

Abstract

The efficient separation of metals from ores and secondary sources such as electronic waste is necessary to realising circularity in metal supply. Precipitation processes are increasingly popular and are reliant on designing and understanding chemical recognition to achieve selectivity. Here we show that a simple tertiary diamide precipitates gold selectively from aqueous acidic solutions, including from aqua regia solutions of electronic waste. The X-ray crystal structure of the precipitate displays an infinite chain of diamide cations interleaved with tetrachloridoaurate. Gold is released from the precipitate on contact with water, enabling ligand recycling. The diamide is highly selective, with its addition to 29 metals in 2 M HCl resulting in 70% gold uptake and minimal removal of other metals. At 6 M HCl, complete collection of gold, iron, tin, and platinum occurs, demonstrating that adaptable selective metal precipitation is controlled by just one variable. This discovery could be exploited in metal refining and recycling processes due to its tuneable selectivity under different leaching conditions, the avoidance of organic solvents inherent to biphasic extraction, and the straightforward recycling of the precipitant.

Fig. 1. Schematic of the precipitation process and its selectivity.

a Chemical structure of L and schematic of the precipitation process. b Percentage metal(s) removed by precipitation from a 0.01 M mixed-metal solution in 2 M (orange and green bars) or 6 M (blue and yellow bars) HCl following the addition of either 0.2 mmol L (10-fold excess L relative to metal, orange and blue bars) or 0.02 mmol L (equimolar, green and yellow bars). c Selective metal precipitation and stripping sequence. Blue bars: Percentage metal removed by precipitation from a 0.01 M mixed-metal solution in 6 M HCl. Green bars: percentage of metal stripped from the precipitate by a 2 M HCl wash. Yellow bars: percentage of metal stripped from the precipitate after a subsequent wash with deionised water. Experiments carried out in triplicate.

Fig. 2. Structural characterisation of precipitates by X-ray crystallography.

For clarity, all hydrogens except those involved in hydrogen bonding are omitted (displacement ellipsoids are drawn at 50% probability). Atom colours: red = O; cyan = N; grey = C; green = Cl; yellow = Au, pale grey = Pt, dark blue = Co; white = H. a X-ray crystal structure of [HL][AuCl₄] showing the intermolecular proton-chelate structure and the arrangement of AuCl₄⁻ within the rhombohedral clefts derived from the phenyl and methyl substituents of the infinite chain of protonated diamides, C(H)—Cl(Au) 3.43–3.97 Å; N1-C3-C3′-N1′ 54.9(3)°. b X-ray crystal structure of [HL]₂[PtCl₆](H₂O) showing the arrangement of PtCl₆²⁻ and a molecule of water within the intermolecular proton-chelated structure. For clarity, only one of the two [HL] cations is shown. c X-ray crystal structure of [HL][H₃O(H₂O)₂][CoCl₄] showing the intermolecular proton-chelate structure and the layered arrangement of CoCl₄²⁻ and H₃O⁺ water cluster between the infinite ribbon chain of protonated diamides.

Fig. 3. Non-covalent interaction (NCI) analysis.

Non-covalent bonding interactions 3D isosurface plots (s = 1.0 au, −0.00025 (dark blue scale) < ρ < 0 (green scale) au) (top), and 2D ρ vs s plots (bottom) for the crystal structures of [HL][AuCl₄], [HL]₂[PtCl₆].H₂O, and [HL][H₃O(H₂O)₂][CoCl₄]. Atom colours: cyan = C, red = O, dark blue = N; yellow = Cl; white = H; brown = Au; grey = Pt or Co.