METHOD OF SELECTIVE PRECIPITATION OF METALS USING AMIDE COMPOUNDS

Abstract

A method of separating a metal from a solution comprises adding to the solution a compound having a structure represented by Formula (I): wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group: and Z is a C2-C6 hydrocarbyl group or an aryl group.

Claims

1. A method of separating a metal from a solution, the method comprising adding to the solution a compound having a structure represented by Formula (I): ##STR00017## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group.

2. A method according to claim 1, wherein R.sub.1 and/or R.sub.2 is phenyl.

3. A method according to claim 1, wherein R.sub.3 and/or R.sub.4 is an unsubstituted C1-C8 alkyl group.

4. A method according to claim 1, wherein Z is (CH.sub.2).sub.2 or (C.sub.6H.sub.4).

5. A method according to claim 1, wherein the method comprises separating the metal from the solution by precipitation.

6. A method according to claim 1, wherein the compound has a structure represented by Formula (II): ##STR00018## wherein Z is a C2-C6 hydrocarbyl group or an aryl group.

7. A method according to claim 1, wherein the compound has a structure represented by Formula (III): ##STR00019##

8. A method according to claim 1, wherein the solution comprises one or more precious metals selected from the list consisting of gold, platinum, palladium, ruthenium, rhodium, iridium and osmium, optionally wherein the solution further comprises tin and/or gallium.

9. A method according to claim 1, comprising precipitating gold, platinum, tin and/or gallium from the solution.

10. A method according to any preceding claim 1, wherein the solution is an aqueous acid solution of HCl at a concentration of about 0.1-8 M.

11. A method according to claim 1, comprising selectively precipitating gold from the solution, wherein the compound is added at a molar ratio of about 1:1 to about 1.1:1 relative to gold in the solution.

12. A method according to claim 11, wherein the method comprises adjusting the concentration of the acid in the solution, to about 0.1-4 M.

13. A method according to claim 1, comprising co-precipitating gold and platinum from the solution, wherein the concentration of the acid in the solution is at least about 6 M, and wherein the compound at added at a molar ratio in excess of 1:1 relative to gold.

14. A method of selectively separating gold from a solution containing gold and one or more other metals, the method comprising adding to the solution a compound having a structure represented by Formula (I): ##STR00020## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group, wherein the compound is added in a molar ratio of about 1:1 to about 1.1:1 relative to gold.

15. A method of selectively separating gold from a solution containing gold and one or more other metals, wherein the solution is an aqueous solution of a strong acid at a concentration of about 0.1-4 M, wherein the method comprises adding to the solution a compound having a structure represented by Formula (I): ##STR00021## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group.

16. A method of sequentially separating gold, and one or more other metals, from a solution containing gold and one or more other metals, the method comprising: (i) adding to the solution a compound having a structure represented by Formula (I): ##STR00022## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group, wherein the compound is added in a molar ratio of about 1:1 to about 1.1:1 relative to gold, so as for form a first precipitate comprising gold; (ii) separating the first precipitate from the solution; and (iii) adding a further amount of the compound to the solution so as to form a second precipitate comprising one or more other metals.

17. A method of sequentially separating gold, and one or more other metals, from a solution containing gold and one or more other metals, wherein the solution is an aqueous solution of a strong acid at a concentration of about 0.1-4 M, the method comprising: (i) adding to the solution a compound having a structure represented by Formula (I): ##STR00023## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group, so as for form a first precipitate comprising gold; (ii) separating the first precipitate from the solution; and (iii) adjusting the concentration of the acid in the solution, to about 4-8 M, so as for form a second precipitate comprising the one or more other metals.

18. A method of sequentially separating, from an acidic solution containing gold and one or more other metals, the one or more other metals, then gold, the method comprising: (i) adding to the solution a compound having a structure represented by Formula (I): ##STR00024## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group, so as to form a co-precipitate comprising gold and one or more other metals; (ii) washing the co-precipitate in an aqueous acidic solution so as to strip one or more other metals from the co-precipitate and yield a third precipitate; and (iii) washing the third precipitate in deionised water so as to strip gold from the third precipitate.

19. A method of separating gold and platinum from a solution containing gold, platinum and one or more other metals, the method comprising adding to the solution a compound having a structure represented by Formula (I): ##STR00025## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group, wherein the solution is an aqueous solution of a strong acid at a concentration of at least 6 M.

20. A method of separating platinum from a solution containing platinum and one or more other precious metals, the solution being substantially free of gold, the method comprising adding to the solution a compound having a structure represented by Formula (I): ##STR00026## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group, wherein the solution is an aqueous solution of a strong acid at a concentration of at least 6 M.

21. A method of separating tin from a solution containing tin and one or more other metals, the solution being substantially free of gold, platinum and iron, the method comprising adding to the solution a compound having a structure represented by Formula (I): ##STR00027## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group, wherein the solution is an aqueous solution of a strong acid at a concentration of at least 6 M.

22. A method of separating gallium from a solution containing gallium and one or more other metals, the solution being substantially free of gold, platinum, iron, and tin, the method comprising adding to the solution a compound having a structure represented by Formula (I): ##STR00028## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each independently a substituted or unsubstituted aryl group or a substituted or unsubstituted C1-C8 hydrocarbyl group; and Z is a C2-C6 hydrocarbyl group or an aryl group, wherein the solution is an aqueous solution of a strong acid at a concentration of at least 6 M.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0197] Embodiments of the present disclosure will now be given by way of example only, and with reference to the accompanying drawings, which are:

[0198] FIG. 1 Schematic representation of a selective precipitation process according to an embodiment, using compound L;

[0199] FIG. 2 Graph showing percentage metal(s) removed by precipitation from a 0.01 M mixed-metal solution in 2 M or 6 M HCl following the addition of either 0.2 mmol of compound L (10-fold excess L relative to metal) or 0.02 mmol of L (equimolar);

[0200] FIG. 3 Graph illustrating a selective metal precipitation and stripping sequence.

[0201] FIG. 4 Graph illustrating the selectivity for gold in the presence of 28 other elements from ICP-MS standard solutions;

[0202] FIG. 5 X-ray crystal structure of [HL][AuCl.sub.4] showing the intermolecular proton-chelate structure and the arrangement of the AuCl.sub.4.sup. anions within the rhombohedral clefts derived from the phenyl and methyl substituents of the infinite chain of protonated diamides;

[0203] FIG. 6 X-ray crystal structure of [HL][H.sub.3O(H.sub.2O).sub.2][CoCl.sub.4] showing the intermolecular proton-chelate structure and the layered arrangement of the CoCl.sub.4.sup.2 anions and H.sub.3O.sup.+ water cluster between the infinite ribbon chain of protonated diamides;

[0204] FIG. 7 Percentage of gold precipitated from 2, 4 or 6 M HCl solutions of 0.01 M HAuCl.sub.4 over time (conditions: 0.02 mmol L stirred at 500 rpm with 2 mL HAuCl.sub.4 in 2, 4 or 6 M HCl at 20 C.);

[0205] FIG. 8 Percentage of gold precipitated from 0-2 M HCl solutions of 0.01 M HAuCl.sub.4 (conditions: 0.02 mmol L stirred at 500 rpm with 2 mL HAuCl.sub.4 in 0-2 M HCl solutions for 1 h at 20 C.);

[0206] FIG. 9 Percentage of gold precipitated after 5 minutes from 2 M HCl solutions of 0.005 M HAuCl.sub.4 at varying temperatures (conditions: 0.02 mmol L stirred at 500 rpm with 2 mL HAuCl.sub.4 in 2 M HCl solutions for 5 minutes at 20, 40, and 80 C.);

[0207] FIG. 10 Graph showing percentage metal(s) removed by precipitation from a 0.01 M mixed-metal solution of precious metals in 6 M HCl following the addition of either 0.2 mmol of compound L (10-fold excess L relative to metal) or 0.02 mmol of L (equimolar);

[0208] FIGS. 11-13 Percentage of metal precipitated from solutions of 0.02 M metal salt at different HCl concentrations, for three different compound variants;

[0209] FIGS. 14-18 Percentage of metal precipitated from solutions of 0.02 M metal salt at different HCl concentrations, for three different compound variants, for each metal;

[0210] FIGS. 19-20 show the effect of the length of the linker group between the two amide groups, in an embodiment of the compound;

[0211] FIGS. 21-22 show the effect of the use of an aryl linker group between the two amide groups, in an embodiment of the compound;

[0212] FIGS. 23-24 show the effect of changing the substituent group on the nitrogen atoms of the two amide groups, in an embodiment of the compound;

[0213] FIGS. 25(a)-(c) show alternative embodiments of the compound.

DETAILED DESCRIPTION

Methods and Compounds

[0214] All solvents and reagents were used as received from Sigma-Aldrich, Fisher Scientific UK, Alfa Aesar, Acros Organics or VWR International. Deionised water was obtained from a MilliQ purification system.

[0215] The exemplary compound used herein (compound L) was prepared according to the method described in Kaufmann, L. et al. Substituent effects on axle binding in amide pseudorotaxanes: comparison of NMR titration and ITC data with DFT calculations. Org. Biomol. Chem., 2012, 10, 5954-5964.

[0216] Compound L was the compound of Formula (III):

##STR00015##

Precipitation Procedure for 0.01 M Mixed Metal Solutions

[0217] Hydrochloric acid solutions (2 M and 6 M) were prepared by dilution of concentrated hydrochloric acid with deionised water. Mixed-metal solutions (0.01 M) were typically prepared by dilution of 0.1 M stock solutions of each individual metal salt solution in 2 or 6 M HCl.

[0218] Solid compound L (0.2 mmol or 0.02 mmol) was added to a vial with a magnetic stir bar and the metal-containing aqueous solution (2 mL) added. The mixture was stirred for 1 hour at room temperature (20 C.) at 500 rpm after which the stir bar was removed and the vial centrifuged. The supernatant was decanted and samples prepared for ICP-OES analysis to measure the uptake of metal by L. Samples were diluted by 100 in 2% nitric acid prior to ICP-OES analysis. This procedure was repeated in triplicate.

Selective Precipitation of Gold from 28 Other Elements Procedure

[0219] The following ICP multi-element standard solutions were used: Transition metal mix 3 for ICP supplied by Sigma Aldrich comprising 100 mg L.sup.1 Au, Ir, Os, Pd, Pt, Rh, Ru in 10% hydrochloric acid and ICP multi-element standard solution IV comprising 1000 mg L.sup.1 Ag, Al, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, K, Li, Mg, Mn, Na, Ni, Pb, Sr, Tl, Zn in dilute nitric acid.

[0220] Each solution (1 mL) was diluted to 10 mL using either 2 M HCl or 6 M HCl, resulting in solutions of 10 ppm Au, Ir, Os, Pd, Pt, Rh, Ru and 100 mg L.sup.Al, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, K, Li, Mg, Mn, Na, Ni, Pb, Sr, Tl, Zn. The solutions were filtered prior to use in precipitation experiments due to the precipitation of silver chloride, which was subsequently excluded from ICP-OES analysis. The precipitation method used for the 0.01 M mixed-metal solutions was followed.

Selective Precipitation of Gold from Waste Printed Circuit Boards

[0221] End-of-life printed circuit boards were supplied by Edinburgh School of Chemistry workshop. Gold-tipped sections of the circuit boards (22.85 g) were cut off and soaked in 100 mL aqua regia for 24 hours. This solution was then diluted with deionised water to 250 mL and the metal content analysed by ICP-OES.

[0222] An aliquot of the e-waste solution (2 mL) was stirred with L (0.0059 g, 0.02 mmol, excess with respect to the gold concentration) for 1 hour at room temperature after which the stir bar was removed and the vial centrifuged. The supernatant was decanted and samples prepared for ICP-OES analysis to measure the uptake of metal. Samples were diluted by 1000 and 20 in 2% nitric acid prior to ICP-OES analysis. This procedure was repeated in triplicate.

Crystallisation Procedures

[0223] [HL][AuCl.sub.4]: Light yellow prisms were grown at RT from a 0.01 M solution of HAuCl.sub.4 in 2 M HCl layered on a 0.1 M solution of L in chloroform. [HL][H.sub.3O(H.sub.2O).sub.2][CoCl.sub.4]: Translucent dark blue plates were grown at RT from a mixture of 0.01 M CoCl.sub.2 and L in 10 M HCl.

Timed Gold Precipitation Experiments

[0224] Solutions of HAuCl.sub.4 (0.01 M) were prepared in 2, 4 or 6 M HCl.

[0225] Solid L (0.02 mmol) was added to a vial with a magnetic stir bar and the relevant aqueous metal solution (2 mL) was added. The mixture was stirred for between 1 minute* and 55 minutes after which the stir bar is removed and the vial centrifuged for 5 minutes. The supernatant was decanted and samples prepared for ICP-OES analysis to measure the uptake of metal. Samples were diluted by 100 in 2% nitric acid prior to ICP-OES analysis.

[0226] *One-minute experiments were not centrifuged and instead stirred for 30 seconds before removing the stir bar and allowing any solids to settle for an additional 30 s. A clear 0.1 mL aliquot was then sampled immediately and prepared for ICP-OES analysis.

Quantitative NMR Solubility Experiments

[0227] .sup.1H NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer. 2 M and 6 M HCl solutions were diluted from concentrated HCl in D.sub.2O. A 0.1 M solution of L in 2 M or 6 M HCl was prepared by adding L (0.0178 g) to an NMR tube along with the relevant HCl/D.sub.2O solution (0.55 mL) and 1 M tert-butanol in D.sub.2O (0.05 mL) as an internal standard. Any undissolved solids were allowed to settle to the bottom of the NMR tube before acquiring .sup.1H NMR spectra.

[0228] .sup.1H NMR spectra were acquired for 2 M HCl solutions between 300-350 K in 10 K increments and for 6 M HCl solutions between 300-330 K in 10 K increments; attempts to acquire additional spectra beyond 330 K for these latter samples were unsuccessful due to excessive line broadening of the spectra and difficulties with sample locking.

Selective Stripping Experiments with H-Tube Apparatus

[0229] Solid L (0.2 mmol) was added to one side of the H-tube with a stir bar. The metal-containing aqueous solution (2 mL) was then added to the solids and the mixture stirred for 1 hour at room temperature at 500 rpm, after which it was passed through the glass frit of the H-tube with the aid of compressed air or N.sub.2 gas. The filtrate was collected for ICP-OES analysis to determine metal uptake. The solids were subsequently washed with 2 M HCl (32 mL) for 30 mins, with each 2 mL solution being passed through the glass frit of the H-tube. The solids were then washed with ultrapure deionised water (52 mL) in the same manner. The use of a H-tube allows for all solids to be retained in the same vessel to minimise any loss of metal due to material transfer. This procedure was repeated in duplicate.

ICP-OES Analysis

[0230] ICP-OES analysis was carried out on a Perkin Elmer Optima 5300DC Inductively Coupled Plasma Optical Emission Spectrometer. Samples in 2% nitric acid were taken up by a peristaltic pump at a rate of 1.3 mL min.sup.1 into a Gem Tip cross-flow nebulizer and a glass cyclonic spray chamber. Argon plasma conditions were 1500 W RF forward power and argon gas flows of 12, 1.0, and 0.6 L min.sup.1 for plasma, auxiliary, and nebulizer flow, respectively. ICP-OES calibration standards were obtained from VWR International, Merck Millipore, or Sigma-Aldrich. Selected emission wavelengths are detailed in the supplementary information. Data are rounded to 3 significant figures after incorporating the appropriate dilution factors (typically 100 unless otherwise stated).

X-Ray Crystallography

[0231] X-ray crystallographic data were collected at 100 K or 120 K on an Oxford Diffraction Excalibur diffractometer using graphite monochromated MoK.sub. radiation equipped with an Eos CCD detector (=0.71073 ), or at 100 K or 120 K on a Supernova, Dual, Cu at Zero Atlas diffractometer using CuK.sub. radiation (=1.5418 ), or at 100 K on a Bruker APEX-II CCD diffractometer using graphite monochromated MoK.sub. radiation (=0.71073 ). Structures were solved using ShelXT direct methods or intrinsic phasing and refined using a full-matrix least-square refinement on |F|.sup.2 using ShelXL. All programs were used within the Olex suites. All non-hydrogen atoms were refined with anisotropic displacement parameters. H-atom parameters were constrained to parent atoms and refined using a riding model except H1 and H2, which were located in the difference Fourier maps and refined with isotropic displacement parameters. All X-ray crystal structures were analysed and illustrated using Mercury 4.1.0.

Data Availability

[0232] X-ray data are available free of charge from the Cambridge Crystallographic Data Centre (https://www.ccdc.cam.ac.uk/data_request/cif) under reference numbers CCDC-2084239 ([HL][AuCl.sub.4]) andCCDC-2084241 [HL][H.sub.3O(H.sub.2O).sub.2][CoCl.sub.4].

Results and Discussions

[0233] Referring to FIG. 1 there is shown a schematic representation of a selective precipitation process according to an embodiment, using compound L for Formula (III), according to a first embodiment.

[0234] As can be seen in FIG. 1, compound L is added to a mixed-metal solution 10, causing precipitation of a gold-containing precipitate 12. The gold-containing precipitate 12 is filtered from the solution 10. The gold-containing precipitate 12 is then washed with deionised water to retrieve gold from the precipitate 12. Filtering the resulting mixture yields an aqueous solution of gold 14, and a recycled compound L. An advantage of this approach is the ability to reuse compound L, for example to repeat the process.

[0235] Table 1 below describes precipitation experiments with Au dissolved in various aqueous matrices. Conditions: 2 mL Au solution contacted with 0.059 g L for 1 hour, room temperature. Solution filtered and diluted 100 in 2% HNO.sub.3 prior to ICP-OES analysis. *HAuCl.sub.4 used. **Au.sup.0 added to sulfuric acid solution with a few drops of 30% hydrogen peroxide added to aid dissolution of Au. All solutions were diluted 100 prior to ICP-OES analysis.

TABLE-US-00001 TABLE 1 Initial Au Au concentration % concentration after contact with L Pre- Aqueous matrix (mg L.sup.1) (mg L.sup.1) cipitated 100% Aqua regia * 1920 6.00 99.7 20% Aqua regia * 1920 4.00 99.8 2 mol L.sup.1 H.sub.2SO.sub.4 and 1940 12.2 99.4 2M NaCl ** 2 mol L.sup.1 H.sub.2SO.sub.4 and 1930 9.00 99.5 2M NaBr **

[0236] Table 2 below describes precipitation of HAuCl.sub.4 by L from 2 M HCl followed by its release from L as HAuCl.sub.4 using deionised water. All solutions were diluted 100 prior to ICP-OES analysis.

TABLE-US-00002 TABLE 2 Au % concen- % Stripping tration Precipita- (cumu- Sample (mg L.sup.1) tion lative) 0.01M HAuCl.sub.4 in 2M HCl (feed 1940 solution) 2M HCl solution after contact with 8.00 99.6% L Deionised water after contact with 103 5.3 [HL][AuCl.sub.4] solids (2 mL) 1.sup.st wash Deionised water after contact with 605 36.6 [HL][AuCl.sub.4] solids (2 mL) 2.sup.nd wash Deionised water after contact with 826 79.3 [HL][AuCl.sub.4] solids (2 mL) 3.sup.rd wash Deionised water after contact with 147 86.9 [HL][AuCl.sub.4] solids (2 mL) 4.sup.th wash Deionised water after contact with 139 94.1 [HL][AuCI.sub.4] solids (2 mL) 5.sup.th wash

[0237] FIG. 2 is a graph showing percentage metal(s) removed by precipitation from a 0.01 M mixed-metal solution in 2 M or 6 M HCl following the addition of either 0.2 mmol of compound L (10-fold excess L relative to metal) or 0.02 mmol of L (equimolar).

[0238] The uptake of gold by L from mixtures of metals in HCl is highly selective. The addition of 0.2 mmol of solid L to a mixed-metal solution comprising 0.01 M each of Au, Al, Cu, Ni, Fe, Zn, Pt, Pd, and Sn in 2 M HCl results in near quantitative removal of Au with minimal co-precipitation of other metals (<5%, FIG. 2, (a) orange bars). It is notable that using stoichiometric L results in gold uptake only (i.e., 0.02 mmol, FIG. 2, (b) green bars) which contrasts with SX conditions where an excess of extractant is required, thus highlighting the enhanced atom economy of this precipitation method. At 6 M HCl using excess L, complete uptake of Fe, Sn, and Pt is also seen, alongside Au, from the above mixture of metals (FIG. 2, (c) blue bars), and is likely due to an increased propensity to form the chloridometalates FeCl.sub.4.sup., SnCl.sub.6.sup.2, and PtCl.sub.6.sup.2 at higher HCl concentrations. Using stoichiometric L, however, a return to selective gold uptake is seen (FIG. 2, (d) yellow bars), which shows that a process could be designed to sequentially precipitate Au then, depending on the feed stream, Fe, Sn, or Pt. This is significant as leach solutions from gold ores (typically pyrite or arsenopyrite) are rich in iron, while those derived from e-waste have high concentrations of tin (Rao, M. D., Singh, K. K., Morrison, C. A. & Love, J. B. Challenges and opportunities in the recovery of gold from electronic waste. RSC Adv. 10, 4300-4309 (2020). Furthermore, the selectivity shown between Pt(IV) and Pd(II) at 6 M HCl is notable as this separation is integral to precious metal refining processes currently based on SX (Narita, H., Kasuya, R., Suzuki, T., Motokawa, R. & Tanaka, M. in Encyclopedia of Inorganic and Bioinorganic Chemistry, 2021, 1-28). The selectivity seen under stoichiometric conditions also suggests that the preference for gold precipitation is not wholly dependent on the ease of formation of HAuCl.sub.4 compared with other chloridometalates, but that the chemical structures of the precipitates also define the sequence of separation (see structural analysis later).

[0239] FIG. 3 is a graph illustrating a selective metal precipitation and stripping sequence.

[0240] (a) orange bars: Percentage metal removed by precipitation from a 0.01 M mixed-metal solution in 6 M HCl.

[0241] (b) green bars: percentage of metal stripped from the precipitate by a 2 M HCl wash.

[0242] (c) blue bars: percentage of metal stripped from the precipitate after a subsequent wash with DI water.

[0243] As can be seen from FIG. 3, the selective uptake of Au at 2 M HCl compared with the requirement for 6 M HCl to load Fe, Sn and Pt permits a selective stripping process to be undertaken. As such, loading L with Au, Fe, Pt and Sn at 6 M HCl (FIG. 3, (a) orange bars), followed by a wash with 2 M HCl results in dissolution of Fe, Sn and Pt only, with Au retained on the solids (FIG. 3, (b) green bars). Washing the isolated solids with DI water releases the Au into solution and recycles L (FIG. 3, (c) blue bars).

[0244] Referring now to FIG. 4, the selectivity of compound L for Au uptake was evaluated further by adding an excess to mixed-metal ICP-OES standard solutions (diluted in 2 M or 6 M HCl), comprising 29 metals at 100 or 10 ppm concentrations. Analysis of the concentrations of metals that remain in solution reveals that even in this competitive environment, L is highly selective for gold, with 70% uptake after 24 hours; thallium (at 10%) is the only other element that shows appreciable uptake at this acid concentration (FIG. 4). Raising the concentration of HCl to 6 M increases the uptake of Au to >99% but decreases selectivity, with Tl (95%), Ga (>99%), and Fe (70%) also precipitated; however, these metals could in principle be removed from the precipitate by a 2 M HCl wash (see above with reference to FIG. 3). Interestingly, no Pt uptake is seen and is due to it being present as Pt(II) (i.e., PtCl.sub.4.sup.2) and not Pt(IV) (i.e., PtCl.sub.6.sup.2), showing that the structure and charge of the chloridometalate is important to the precipitation process.

[0245] In FIG. 4, it is believed that the negative adsorption efficiencies of some of the metals above are considered to be due to contamination of the samples from elements commonly present in water and on the experimental tools.

[0246] With reference to Table 3 below, gold was selectively separated from end-of-life printed circuit boards dissolved in aqua regia (diluted to 20%), with 98% Au precipitation after 1 hour and no co-precipitation of any of the other elements present.

TABLE-US-00003 TABLE 3 Au Cu Ni Pb Sn Sample (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) Feed solution 180 15400 105 818 1650 Feed solution after 3.45 15400 108 829 1660 contact with L % metal precipitated 98.1 0 0 0 0

[0247] Thus, Table 3 illustrates the selective precipitation of HAuCl.sub.4 from a 20% aqua regia mixed-metal solution derived directly from waste printed circuit boards.

[0248] Referring to FIG. 10, there is shown a graph depicting percentage metal(s) removed by precipitation from a 0.01 M mixed-metal solution of precious metals in 6 M HCl following the addition of either 0.2 mmol of compound L (10-fold excess L relative to metal) or 0.02 mmol of L (equimolar). This graph highlights that, at equimolar amounts of compound L, selective precipitation of gold over other precious metals occurs. In addition, this graph shows that, in an excess amount of L, selective co-precipitation of gold and platinum is achieved. Therefore, in a mixture of previous metals containing gold and platinum, selective co-precipitation of gold and platinum can be achieved. In addition, if the mixture of precious metals is substantially free of gold, this graph demonstrates that it is possible to selectively separate platinum from the mixture of precious metals by using compound L in a relatively high concentration (at least 6 M) of acid (in this example, HCl).

[0249] FIG. 5 shows an X-ray crystal structure of [HL][AuCl.sub.4] showing the intermolecular proton-chelate structure and the arrangement of the AuCl.sub.4.sup. anions within the rhombohedral clefts derived from the phenyl and methyl substituents of the infinite chain of protonated diamides, with interactions between HL.sup.+ and AuCl.sub.4.sup. of C(H)Cl(Au) 3.43-3.97 ; N1-C3-C3-N1 54.9(3).

[0250] Layering a solution of 0.01 M HAuCl.sub.4 in 2 M HCl on a 0.1 M chloroform solution of L results in controlled crystallisation. The X-ray crystal structure (FIG. 5) shows a chemical formula of [HL][AuCl.sub.4] in which the unique proton H1 is bound between adjacent amide O-atoms O1 and O1a (O1O1a=2.420(3) ), forming an intermolecular proton chelate between amide units that assemble into an infinite supramolecular chain motif. While the linking of the diamides in [HL][AuCl.sub.4] is similar to that seen for HAuCl.sub.4 complexes of the diamidodurene RC(O)N(R)CH.sub.2(C.sub.6Me.sub.4)CH.sub.2N(R)C(O)R (Shaffer, C. C. & Smith, B. D. Macrocyclic and acyclic supramolecular elements for co-precipitation of square-planar gold (iii) tetrahalide complexes. Org. Chem. Frontiers, 2021, 8, 1294-1301, (2021), the positioning of the AuCl.sub.4.sup. anions is different. In the latter example, the -rich aryl group interacts strongly through face-to-face -bonding with the planar AuCl.sub.4.sup. anion, whereas for [HL][AuCl.sub.4] the phenyl and methyl substituents within the ribbon-like structure of the protonated diamides provide rhombohedral clefts that host the AuCl.sub.4.sup. guest. This demonstrates the uniqueness of metal separation using the present methodology.

[0251] FIG. 6 shows an X-ray crystal structure of [HL][H.sub.3O(H.sub.2O).sub.2][CoCl.sub.4] showing the intermolecular proton-chelate structure and the layered arrangement of the CoCl.sub.4.sup.2 anions and H.sub.3O.sup.+ water cluster between the infinite ribbon chain of protonated diamides.

[0252] The discovery that full uptake of gold from solution occurs using a stoichiometric amount of compound L suggests that a dissolution-precipitation, not a surface-deposition mechanism, is occurring. This is supported by analysis of the rate of gold uptake at various concentrations of HCl (See FIGS. 7 and 8), which is found to be related to the extent of dissolution of L. Addition of HAuCl.sub.4 to a solution of L in 12 M HCl results in the rapid and wholesale precipitation of [HL][AuCl.sub.4]. Dissolution of L in 2 M HCl, as determined by quantitative .sup.1H NMR spectroscopy, is minimal at 0.02 M, while heating this solution to 350 K increases the concentration of dissolved L to 0.1 M. Increasing the concentration of HCl to 6 M results in a 16-fold increase to 0.32 M at 300 K. The increase in dissolved L mirrors the increase in quantity of [HL][AuCl.sub.4] precipitated from 2 M HCl over 5 minutes, which doubles on raising the temperature from 20 to 40 C. and from 40 to 80 C. (see FIG. 9).

[0253] FIGS. 11-18 relates to the investigation of the effect of modifying the end group (R.sub.1, R.sub.2 in Formula (I)) on the precipitating behaviour of the compound according to an embodiment.

[0254] In the experiments relating to FIGS. 11-18, the compound tested was a compound of Formula (IV):

##STR00016##

[0255] in which the R substituent was either H (FIG. 11), OMe (FIG. 12) or Cl (FIG. 13).

[0256] In each case, the respective graph shows the percentage of metal (gold, iron, tin, platinum or gallium) precipitated from solutions of 0.02 M metal salt at different HCl concentrations, following the addition of 0.2 mmol/L of the compound of Formula (IV) (i.e. 10-fold excess compound relative to metal).

[0257] It can be seen that, for all three compound variants, gold always precipitates a low concentration of HCl, namely from about 0.1M HCl. This is consistent with the results of FIG. 8 for compound L.

[0258] In addition, the selectivity of the compound, for all three variants, is shown as other metals begin to precipitate at around 3-6 M HCl.

[0259] For completeness, it will be noted that, in FIG. 11, the plot for tin was overlapped by the gallium plot. Also, in FIG. 13, the plot for iron was overlapped by the tin plot.

[0260] FIGS. 14-18 show similar data as the results of FIGS. 11-13, but presented for each metal (gold, iron, platinum, gallium and tin) respectively. Again, it can be seen that, for all three compound variants, gold (FIG. 14) always precipitates a low concentration of HCl, namely from about 0.1M HCl. In addition, the selectivity of the compound, for all three variants, is shown as other metals (FIGS. 15-18) begin to precipitate at around 3-6 M HCl.

[0261] FIG. 19 is a graph showing the effect of the length of the linker group between the amide groups, on the precipitation behaviour of a solution of iron chloride. Conditions were: 0.2 mmol of compound contacted with 2 mL 0.01 M FeCl.sub.3 in 6-12 M HCl for 24 hours at RT, 500 rpm. The plots were obtained for four variants of the linkage represented in FIG. 20.

[0262] It will be noted that the tested compound relates to a secondary diamide. In contrast to the observations made for compound L above, compound L11 of FIG. 20(a) was surprisingly ineffective at precipitating iron after contacting L11 with 6 M HCl solutions for 24 hours, but precipitation was observed from about 9 M. Secondary diamides were also found to lack sufficient solubility to be effective in the present application in the selective precipitation of precious metals. The insolubility of the secondary diamides in acid is believed to be a result of strong intermolecular hydrogen bonding between NH and CO groups of adjacent amides, which is not present in tertiary amides.

[0263] As the length of the alkyl spacer is varied from 2 carbons to 6 carbons FIGS. 20(b)-20(d), Fe(III) precipitation was seen to occurs at slightly lower HCl concentrations, although still not as readily as compound L above.

[0264] Whilst this experiment was carried out on variants of a secondary diamide compound, the results show that varying the length of the size of the linker group between C2 and C6 does not significantly alter the precipitating behaviour of the compound, and this observation could reasonably be expected to also apply for a tertiary diamide.

[0265] FIG. 21 is a graph showing the effect of the use of an aryl linker group between the two amide groups, on the precipitation behaviour of a solution of iron chloride. Conditions were: 0.2 mmol of compound contacted with 2 mL 0.01 M FeCl.sub.3 in 6-12 M HCl for 24 hours at RT, 500 rpm. The plots were obtained for two variants of the phenyl linkage, as represented in FIG. 22.

[0266] The two phenyl linker derivatives at the meta (FIG. 22a) and para (FIG. 22b) positions were found to precipitate Fe(III) from 7 M HCl onwards, showing that an aromatic linker between the amide groups may be envisaged as an alternative to a C2-C6 alkyl linker.

[0267] FIG. 23 is a graph showing the effect of changing the substituent group on the nitrogen atoms of the two amide groups, on the precipitation behaviour of a solution of iron chloride. Conditions were: 0.2 mmol of compound contacted with 2 mL 0.01 M FeCl.sub.3 in 6-12 M HCl for 24 hours at RT, 500 rpm. The plots were obtained for two variants of the substituents, as represented in FIG. 24.

[0268] It can be seen that precipitation behaviour was very effective for each of methyl, ethyl, and t-butyl substituents.

[0269] FIGS. 25(a)-(c) show alternative embodiments of the compounds that were tested. Conditions were: 0.2 mmol of compound contacted with a 2 M or 6 M HCl multi element solution for 24 hours, RT.

[0270] The solution comprised: [0271] 10 mg L.sup.1 Au, Ir, Os, Pd, Pt, Rh, Ru; and [0272] 100 mg L.sup.1 Al, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, K, Li, Mg, Mn, Na, Ni, Pb, Sr, Tl, Zn/

[0273] For the 2 M HCl solution, it was observed that the compound of FIG. 25(a) (cyclohexyl end substituent), led to about 92% precipitation of Au, and about 94% precipitation of Tl. The compound of FIG. 25(b) (t-butyl end substituent), led to about 16% precipitation of Au, and about 96% precipitation of Tl. The compound of FIG. 25(c) (methoxy phenyl end substituent), led to about 50% precipitation of Au, and about 81% precipitation of Tl.

[0274] For the 6 M HCl solution, it was observed that the compound of FIG. 25(a) (cyclohexyl end substituent), led to about 99% precipitation of Au, Ga and Tl, and about 71% precipitation of Fe. The compound of FIG. 25 b) (t-butyl end substituent), led to about 82% precipitation of Au, 90% precipitation of Tl, 87% precipitation of Ga, and about 52% precipitation of Fe. The compound of FIG. 25(c) (methoxy phenyl end substituent), led to about 99% precipitation of Au, Tl, Ga and Fe.

[0275] Thus, the present data demonstrate the applicability of the present compounds and methodology in highly selective separation of metals by precipitation. The present method is tuneable by varying the concentration of acid, e.g., HCl, such that different metals can be selectively precipitated depending on the metal feed stream. Advantageously, the present method allows recycling of the compounds and does not rely on the use of organic solvents and may provide a simple solution towards environmentally benign metal separation and/or recycling.

[0276] It will be appreciated that the described embodiments are not meant to limit the scope of the present invention, and the present invention may be implemented using variations of the described examples.