Exploiting outer-sphere interactions to enhance metal recovery by solvent extraction

Waste electrical and electronic equipment (WEEE) such as mobile phones contain a plethora of metals of which gold is by far the most valuable. Here we describe a simple primary amide that achieves the selective separation of gold from a mixture of metals typically found in a mobile phone by extraction into toluene from an aqueous HCl solution; unlike current processes, reverse phase transfer is achieved simply using water. Phase transfer occurs by dynamic assembly of protonated and neutral amides with AuCl4 − anions through hydrogen bonding in the organic phase, as shown by EXAFS, mass spectrometry measurements and computational calculations, and supported by distribution coefficient analysis. We anticipate that the fundamental chemical understanding gained here is integral to the development of metal recovery processes, in particular through the use of dynamic assembly processes to build complexity from simplicity.

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“…1 (Figure 1), is crucial for selectivity, not only between metalates but also over chloride which is present in large excess. [10] H…”

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A Simple Primary Amide for the Selective Recovery of Gold from Secondary Resources

et al. 2016

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“…[6] Therecovery of metals using solvent extraction can offer significant environmental advantages over energy-and capital-intensive pyrometallurgical routes,e specially if as ingle metal is targeted. [10] We have now exploited this concept using the simple primary amide L (Figure 1, right), prepared from commercially available ClC(O)CH 2 CH(Me)CH 2 tBu and NH 3 ,for the extraction of H[AuCl 4 ]. [8] Currently,2 5% of global Cu recovery is carried out by hydrometallurgy using phenolic oxime reagents, [8] the development of which relied on an understanding of the coordination and supramolecular chemistry of copper.…”

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A Simple Primary Amide for the Selective Recovery of Gold from Secondary Resources

et al. 2016

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Waste electrical and electronic equipment (WEEE) such as mobile phones contains aplethora of metals of which gold is by far the most valuable.Herein asimple primary amide is described that achieves the selective separation of gold from am ixture of metals typically found in mobile phones by extraction into toluene from an aqueous HCl solution;u nlike current processes,r everse phase transfer is achieved simply using water.P hase transfer occurs by dynamic assembly of protonated and neutral amides with [AuCl 4 ] À ions through hydrogen bonding in the organic phase,asshown by EXAFS, mass spectrometry measurements,a nd computational calculations,and supported by distribution coefficient analysis.The fundamental chemical understanding gained herein should be integral to the development of metal-recovery processes,i n particular through the use of dynamic assembly processes to build complexity from simplicity.Gold is avaluable metal resource,not only as jewellery and for investment, but increasingly in modern electronics, medicine,a nd chemical catalysis. [1] Thel ow abundance of gold in its ores,c ombined with economic and societal issues associated with mining and separation operations,means that e-waste streams are attractive sources of this metal. [2] The gold content of WEEE is estimated to be 80 times that found in primary mining deposits,r epresenting 3% of the world mine supply in 2007, and its recovery would decrease the environmental and societal footprint of mining along with potential savings of 17 000 t/t in carbon dioxide emissions. [3] Furthermore,current processes for the recovery of gold from WEEE are hazardous,asthey often use toxic chemicals such as cyanide.A ss uch, governments worldwide are developing waste-recycling strategies to exploit e-waste through "urban mining", [4] with the market forecasted to have ac ompound annual growth rate of 20.6 %between 2015 and 2020 from its USD 1.66 10 9 value in 2014. [5] These strategies combine metal-leaching,-separation, and -precipitation technologies which increasingly require the use of "green" materials, reagents,a nd processes for societal and economic acceptance. [6] Therecovery of metals using solvent extraction can offer significant environmental advantages over energy-and capital-intensive pyrometallurgical routes,e specially if as ingle metal is targeted. [7] Selectivity of extraction is achieved by designing areagent that favors the phase transfer of one metal from am ixed-metal aqueous leach solution to an organic phase. [8] Currently,2 5% of global Cu recovery is carried out by hydrometallurgy using phenolic oxime reagents, [8] the development of which relied on an understanding of the coordination and supramolecular chemistry of copper.In contrast, the chemistry that underpins the recovery of gold by solvent extraction is poorly understood. Commercial reagents such as MIBK (isobutyl methyl ketone), DBC (diethylene glycol butyl ether), and 2-EH (2-ethylhexanol) recover gold as its metalate [AuCl 4 ] À from aqueous HCl by solvent extraction [...

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“…[35][36][37] It is well known that rare-earth ions are strongly hydrated in aqueous solutions (with eight or nine coordinated water molecules) and no chloride ion is present in the inner coordination sphere, even at very high chloride concentrations. 38,39 Due to the lack of coordinating chloride anions, the extraction of rare earths by Cyanex 923 from aqueous chloride solutions is inefficient.…”

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Separation of rare-earth ions from ethylene glycol (+LiCl) solutions by non-aqueous solvent extraction with Cyanex 923

et al. 2017

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The separation of a mixture of rare earths by non-aqueous solvent extraction with two immiscible organic phases has been studied. The more polar organic phase was ethylene glycol with dissolved lithium chloride and the less polar organic phase was the extractant diluted in n-dodecane. Cyanex 923 was found to be the most performant extractant amongst the investigated acidic, basic and solvating extractants: Cyanex 272, Cyphos IL 101, Aliquat 336, bis(2-ethylhexyl)amine, trioctylphosphine oxide (TOPO) and Cyanex 923. The replacement of the aqueous chloride feed solutions by non-aqueous ethylene glycol feed solutions had a profound effect on the distribution ratios and separation factors. The separation factors for extraction of pairs of rare earths from aqueous chloride solutions by Cyanex 923 are too low to be of practical use.On the contrary, a mixture of rare earths can be separated conveniently in four different groups by extraction with Cyanex 923 from ethylene glycol (+LiCl) solutions. The influence of several parameters such as the chloride concentration, the type of chloride salt, the addition of other polar solvents to the ethylene glycol phase, the addition of second extractant to the less polar organic phase, and the addition of complexing agents to the ethylene glycol phase has been studied. The extraction mechanism for extraction of ytterbium(III) was studied by slope analysis experiments. The ytterbium(III) species in the ethylene glycol phase and the extracted species in the n-dodecane phase were determined by EXAFS.Furthermore, a conceptual flow sheet for the fractionation of rare earths from an ethylene glycol (+LiCl) feed solution into different groups by extraction with Cyanex 923 has been proposed. The new extraction system is useful for extraction of scandium and for separation of scandium from the other REEs.

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Exploiting outer-sphere interactions to enhance metal recovery by solvent extraction

Cited by 62 publications

References 60 publications

A Simple Primary Amide for the Selective Recovery of Gold from Secondary Resources

A Simple Primary Amide for the Selective Recovery of Gold from Secondary Resources

A Simple Primary Amide for the Selective Recovery of Gold from Secondary Resources

Separation of rare-earth ions from ethylene glycol (+LiCl) solutions by non-aqueous solvent extraction with Cyanex 923

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