Urban mining: NMR helps find the golden key

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  • Published: Jan 15, 2019
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Urban mining: NMR helps find the golden key

Gold extraction

An international team has used nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS) in a study that has led to the proposal of a selective, fast, and convenient thiol-assisted gold leaching process for the urban mining of this precious metals from the electronic equipment waste stream. Credit: Angew Chem/Repo et al)

An international team has used nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS) in a study that has led to the proposal of a selective, fast, and convenient thiol-assisted gold leaching process for the urban mining of precious metals from the electronic equipment waste stream.

The concept of 'urban mining', which I have discussed in various forums during the last thirty years is finally becoming a tenable reality. It involves the recycling of precious metals from electronic gadgets. This is particularly important in the context of limited supplies of raw materials whether due to geological or political conditions.

An international team of scientists has now investigated novel methods for dissolving gold including the use of organic thiol-containing compounds to help develop more efficient commercially viable approaches to releasing elemental gold from electronic gadgets at end of life. Timo Repo of the University of Helsinki, Finland, and colleagues Minna Risnen, Eeva Heliçvaara , Feda’a Al-Qaisi, Aleksi Eronen, Henri Liljeqvist, Martin Nieger, Marianna Kemell, Karina Moslova, Jani Hmlinen, and Kalle Lagerblom discuss details in the journal Angewandte Chemie. They have used nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS) in their study to propose a selective, fast, and convenient thiol-assisted gold leaching process.

Reduced reagent load

The traditional way of recycling gold “waste” is melting: dental gold and jewellery can be recycled at close to 100 percent retrieval. Recycling of precious metals in smart phones, computers, and other electronic gadgets is much harder, and the recovery rate is still low. Despite their abundance in electronic devices, their relative content is still too low to allow for really economical urban mining.

The more modern approach to extracting gold from the waste stream has been to use hydrometallurgical cyanide leaching. Of course, this generates a large amount of hazardous waste itself and is also relatively unselective and so not a clean nor a green approach. There have been some developments that have used complexation processes to chelate gold ions with organic ligand to generate soluble complexes with sulfur-containing reagents that can then be separated and the gold condensed out. However, these experimental processes are appropriate for laboratory-scale ventures rather than large-scale industrial methods. Moreover, they still often rely on toxic materials and also generate hazardous waste.

Repo and colleagues have looked at the details of how selective gold extraction might be carried out using organic solutions. They propose an efficient gold recovery method from electronic waste with pyridinethiols and hydrogen peroxide as the extraction reagents. The process uses dimethyl formamide (DMF) as the organic solvent, and, can use elemental sulfur to reduce the reagent load if required.

Elective electrons

The pyridinethiol molecules can be described simply as a pyridine ring with a thiol, SH, group. Not only does this reagent bind well to elemental gold it readily forms soluble complexes with the metal. Moreover, the resulting complex has a favourable linear structure that arises because two pyridinethiol molecules sit either side of the gold atom in the complex. Upon oxidation, this complex transforms into a stable cationic gold-containing product in organic solution. This complex formation with two ligands is something of a speciality of gold, the energetics favouring dissolution and oxidation. As such, the researchers were able to achieve almost quantitative dissolution of gold from powder, film, or electronic boards in just twenty minutes.

The team points out that while gold requires a one-electron oxidation, other precious commonly used precious metals, platinum and palladium, require a two-electron oxidation and are therefore inaccessible to the team's pyridinethiol method. By contrast, both copper and silver form complexes with pyridinethiols, although not as effectively as gold. To avoid concomitantly extracting silver and copper components from a circuit with the desired gold, the team can use ammonia and sulfate-containing solutions, established methods, to leech out those two metals and then access the "gold finger" region of the printed circuit board with their pyridinethiol technique.

The team has looked at the precise mechanism by which their thiol-assisted gold dissolution process works and found a surprisingly large variety of sulfur-containing side products. Some of them seemed to be crucial for the process to go to oxidation. For example, S8, a common form of elemental sulphur, is something of an asset, by adding this allotrope of sulfur, the team were able to lower the quantity of ligand needed significantly. They suggest that their extraction method could mark a new basis for more efficient urban mining.

Related Links

Angew Chem Int Edn Engl 2019, online: "Pyridinethiol-Assisted Dissolution of Elemental Gold in Organic Solutions"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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