Electrical and Electronics products’ waste known as e-waste, is one of the fastest-growing solid waste streams. In 2019, more than 53 million Tonnes of e-waste were produced, and by 2030, it is expected to reach 74 million Tonnes. About 8% of the annual gold production, or around 250 Tonnes, is now found in new gadgets. Recycling and reuse of precious metals like gold can improve resource efficiency.

Existing methods of recovering gold and other metals from e-waste required large amounts of energy, but over the years researchers have developed a method using a hydrometallurgical process, using chemical reactions in liquids. According to the researchers, this is better than pyrometallurgical processes (using high temperatures), which are more expensive, emit toxic fumes and dust, and demand a lot of energy. To separate the components, for instance, certain standard processes call for materials to be heated to 1200 °C for 12 hours. Researchers demonstrated the method on jack connectors, RAM modules, mobile-phone PCBs, and CPUs. They broke all these except jacks into small 2- or 3-centimetre pieces using scissors or hammers. They even showed how a similar procedure can be used on powder residue from aircraft turbine blades and pulverized catalytic converters, which have platinum and palladium group metals.

Aqua regia, an acid mixture that dissolves gold and other metals, was used by the researchers to react with all the broken wastes chemically, at room temperature. To determine the best ratios for leaching, they combined the materials with acetic acid, hydrogen peroxide, and several concentrations of hydrochloric acid. They also experimented with other factors, such as stirring, and used spectrometry to analyze the resulting solutions to figure out how much of each metal was present.

The researchers stirred the leached solution at room temperature to produce precipitates, then employed ascorbic acid (on e-waste), copper, and iron powder (on catalytic converters and turbine residue) to extract metals from the solution (solid product). These were digested with aqua regia and then examined one more time to determine the degree of purity.

In separate tests on mobile phone PCBs and ceramic Intel CPUs, the researchers recovered 85% and 80% of the gold that was in the first solution. The final products were mostly pure, but there was some residual plastic in the gold recovered from RAM. High levels of both platinum and palladium were recovered from spent catalytic converters.

This efficient process provides a way of recycling precious metals and preventing pollution from e-waste and other waste streams and through further optimization could improve the purity of recovered metals. If the technology could be scaled up, it may offer a more energy-efficient method of metal recovery than existing practice by commercial firms.

Reference: https://environment.ec.europa.eu/news/e-waste-chemical-processing-without-heat-may-offer-efficient-method-recovering-metals-end-life-2022-10-12_en

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