E-waste: chemical processing without heat may offer efficient method of recovering metals from end-of-life products

Over 53 million tonnes (Mt) of e-waste was generated in 2019, predicted to grow to 74 Mt in 2030. Meanwhile, new electronics include about 8% of all gold produced each year – about 250 tonnes. Current methods of recovering gold and other metals from e-waste tend to use large amounts of energy, but researchers have proposed a new chemical-processing system that can be carried out at ambient temperature and pressure.

Precious metals are some of the most valuable materials in e-waste; recycling them is important in the context of resource efficiency and avoiding pollution. Recovering resources from end-of-life products has been called ‘urban mining’, and will be important to achieve the aims of the EU’s circular economy. For example, gold may be recovered from circuit boards in computers and mobile phones, and from cables and jack connectors.

The researchers developed a method using a hydrometallurgical process, using chemical reactions in liquids. This is preferable to pyrometallurgical methods (using high temperatures), say the researchers, which produce harmful gases and dust, cost more, and require a large amount of energy. For example, some standard processes require materials to be heated to 1 200°C for 12 hours to separate components¹.

The process involves leaching elements from the waste material, followed by their selective recovery from a solution via chemical reduction. The researchers demonstrated the method on jack connectors, random access memory (RAM) modules, mobile-phone printed circuit boards (PCBs) and central processing units (CPUs). They broke all of these (apart from the jacks) into small 2- or 3-centimetre pieces using scissors or hammers. They also showed how a similar process can be used on powder residue from aircraft turbine blades and pulverised catalytic converters, which contain platinum and palladium group metals.

The researchers subjected all wastes to chemical attack with aqua regia (acid mixture), which dissolves gold and other metals, at room temperature. They mixed the items with acetic acid and hydrogen peroxide, together with different levels of hydrochloric acid and, to investigate optimal ratios for leaching. They also experimented with other variables such as stirring, and analysed the resulting solutions with spectrometry to determine amounts of different metals within them.

The researchers used ascorbic acid (on e-waste), copper and iron powder (on catalytic converters and turbine residue) to remove metals from the leached solution, agitating the solution at room temperature to obtain precipitates (solid product). These were digested with aqua regia and once more analysed to ascertain purity level.

Results showed that 99% of gold from e-waste could be leached into the first solution, and, in separate tests on mobile phone PCBs and ceramic Intel CPUs, the researchers retrieved 95% and 80% of the leached gold. The final products were relatively pure, but they noted that some residual plastic remained in gold recovered from RAM. Similarly, high levels of platinum (89%) and palladium (100%) were recovered from spent catalytic converters, and 99% of palladium from turbine residues².

This efficient process provides a way of recycling precious metals and preventing pollution from e-waste and other waste streams – such as from automotive and aircraft industries – say the researchers, though further optimisation could improve the degree of purity of recovered metals. If the technology could be scaled up, it may offer a more energy-efficient method of metal recovery than currently practised by commercial firms.

Footnotes:

1. ‘PGM refining’. Johnson Matthey. Available from: https://matthey.com/en/products-and-markets/pgms-and-circularity/pgm-recycling-and-refining [Accessed 3 August 2022]

2. The purity of all these products can be further increased to over 99% by a thermal refining process.

Source:

Birloaga, I. and Vegliò, F. (2022) An innovative hybrid hydrometallurgical approach for precious metals recovery from secondary resources. Journal of Environmental Management, 307: 114567.

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“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by Science Communication Unit, The University of the West of England, Bristol.

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The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.