Rapid Dissolution of Noble Metals in Organic Solvents

Abstract

The dissolution of elemental noble metals (NMs) such as gold, platinum, palladium, and copper is necessary for their recycling but carries a high environmental burden due to the use of strong acids and toxic reagents. Herein, a new approach was developed for the rapid dissolution of elemental NMs in organic solvents using mixtures of triphenylphosphine dichloride or oxalyl chloride and hydrogen peroxide, forming metal chloride salts directly. Almost quantitative dissolution of metallic Au, Pd, and Cu was observed within minutes at room temperature. For Pt, dissolution was achieved, albeit more slowly, using the chlorinating oxidant alone but was inhibited on addition of hydrogen peroxide. After leaching, transfer of Pt IV and Pd II chloride salts from the organic phase into a 6 m HCl aqueous phase facilitated their separation by precipitation of Pt IV using a simple diamide ligand. In contrast, the retention of Au III chloridometalate in the organic phase allowed its selective separation from Ni and Cu from a leachate solution obtained from electronic CPUs. This new approach has potential application in the hydrometallurgical leaching and purification of NMs from ores, spent catalysts, and electronic and nanowastes.

Sustainable and Selective Modern Methods of Noble Metal Recycling

“…Organic media can be applied for selective and sustainable dissolution. From moderately toxic halogens in alcohols [17] or noxious organic aqua regia [105] to more sustainable systems using recyclable PPh 3 Cl 2 or oxalyl chloride in the presence of H 2 O 2 can be employed to dissolve Au and other noble metals in acetonitrile [106] . Less corrosive and volatile system based on halogens trapped as X 3 − in ionic liquids [107, 108] (Figure 5a) or halogens in deep eutectic solvents like ethaline [109] are also known.…”

“…Size, charge and shape driven differentiation of ions as well as highly specific supramolecular interactions offer great selectivity as well as lesser environmental impact when greener compounds and systems are used. Selective transfer of [AuCl 4 ] − is possible by aqueous‐to‐ionic liquid, [118] aqueous‐to‐surfactant phase by cloud point extraction [119] or by aqueous‐to‐organic solvent extraction with simple amides [120, 121] (Figure 5g) or phosphines [106, 118] . Further to this with green and renewable diethyl carbonate, a more sustainable alternative to frequently used methyl isobutyl ketone and dibutyl carbitol that is capable of selectively transport of Au III over Cu II from important secondary source—copper anode slime—solution.…”

“…From moderately toxic halogens in alcohols [17] or noxious organic aqua regia [105] to more sustainable systems using recyclable PPh 3 Cl 2 or oxalyl chloride in the presence of H 2 O 2 can be employed to dissolve Au and other noble metals in acetonitrile. [106] Less corrosive and volatile system based on halogens trapped as X 3…”

“…There also exists excellent green alternatives such as glucose and modified chitosan resins for silver recycling. [106,114] The silver in its + 1-oxidation state can coordinate to various sites within these carbohydrate derivatives and will be reduced and precipitate as a silver nanoparticles. Such green lixiviants are highly promising for further extractions of silver and other noble metals.…”

“…Selective transfer of [AuCl 4 ] À is possible by aqueous-to-ionic liquid, aqueous-to-surfactant phase by cloud point extraction [119] or by aqueous-to-organic solvent extraction with simple amides [120,121] (Figure 5g) or phosphines. [106,118] Further to this with green and renewable diethyl carbonate, a more sustainable alternative to frequently used methyl isobutyl ketone and dibutyl carbitol that is capable of selectively transport of Au III over Cu II from important secondary source-copper anode slime-solution. Aqueousto-aqueous media transfer through polymer inclusion membrane is also possible with amic acid based extractants.…”