What is gold recycling

Gold recycling

The recovery of precious metals from electronic waste ("urban mining") is becoming more and more important, but there are still hardly any environmentally friendly processes that are really economically viable. Now, in an international cooperation, Finnish scientists have taken a closer look at how certain sulfur-containing compounds can dissolve gold. Like her in the magazine applied Chemistry such thiol-assisted gold leaching could be carried out selectively and rapidly.

Gold "scrap" is traditionally recycled by remelting it: In dental gold and jewelry, the recycling percentage is even close to 100%. It is much more difficult to recover the precious metals built into them from cell phones, computers and other small electronic devices. The recycling rate is still in the lower double-digit range. Their relative share in electronic scrap is simply too low for urban mining to actually pay off economically.

The raw material gold is normally extracted hydrometallurgically by cyanide leaching. This method produces mountains of hazardous waste and is not particularly selective. For some time now, people have therefore started to dissolve gold as a complex in organic solutions: Sulfur compounds are well suited for this. However, the process must be scalable and avoid toxic waste. Timo Repo at the University of Helsinki and colleagues have now investigated selective gold extraction in organic solutions in more detail. According to their results, it should be possible to dissolve gold from electronic scrap with pyridine thiol compounds and hydrogen peroxide, the organic solvent dimethylformamide and - optionally, to reduce the amount of reagents - elemental sulfur.

Pyridine thiol is the nitrogen-containing aromatic compound pyridine with an additional thiol group, SH. Pyridinthiol not only complexes elementary gold atoms, but also forms an elongated, linear structure, which is very favorable for gold complexes, with one pyridinthiol molecule on each side of the gold atom. If this complex is oxidized, it turns into a stable gold salt in organic solution. The fact that exactly two molecules surround the gold atom as ligands is typical of this noble metal and energetically favors its dissolution and oxidation. The scientists observed that gold powder, foils or gold residues in circuit boards completely dissolved within 20 minutes.

How selective is the dissolution and how does gold differ from other precious metals? During oxidation, platinum and palladium lose two electrons, gold only one. Therefore, they are not reactive enough with the method. This is different with copper and silver: Both metals form complexes with pyridinthiols, albeit not as easily. Therefore, using established methods, the scientists first extracted copper and silver with ammonia and sulphate salt solutions, before they selectively extracted the gold from a circuit board with the pyridine thiols.

On examining the mechanism of this gold dissolution, the authors found that a surprising number of by-products were formed. Some of them even seemed to be very important for the course of the reaction, elemental sulfur for example. This fact could even be exploited: the scientists simply added sulfur to the reaction from the outside, thereby reducing the amount of thiol reagent to be used. The process could make urban mining more efficient.