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Reclaiming precious metals hidden in plain sight

Incineration of solid waste produces millions of tonnes of waste fly ash in Europe each year. Most of it ends up in landfill, despite the significant precious metal content. A unique method developed by Chalmers University of Technology in Sweden can now help extract zinc and other metals.  

‘In our pilot study, we found that 70% of the zinc present in fly ash can be recycled,’ says Karin Karlfeldt Fedje, associate professor at Chalmers University and researcher at the recycling and waste management company Renova AB. ‘The zinc is not extracted as a pure metal, which would be a much more intensive process, but instead as a zinc-rich product, which can be sold to the metal industry and processed further in currently existing industry production lines.’

How does it work?

The method involves using an acid wash which releases zinc and other metal ions from the ash. The zinc is recovered from the leachate as zinc hydroxide using chemical precipitation, which can then be further refined using metal industry processes to generate high purity zinc metal. The leached fly ash can be re-incinerated in order to destroy toxic dioxins.

During the pilot study, 75–150 kilograms an hour of fly ash from a Swedish Waste-to-Energy plant was mixed with scrubber liquids from the same flue gas treatment system in a continuously stirred vessel. The resulting slurry was dewatered in a vacuum belt filter. Hydroxide precipitation of the resulting leachate, followed by filtration of the formed crystals in a membrane filter press, produced a filter cake with up to 80% weight zinc hydroxide.

Positive effects

In further refinement to the method, the researchers have been able to significantly reduce the level of toxicity. ‘After extraction, we incinerate the residual ash again to break down the dioxins. About 90% of this is then turned into bottom ash, which can be used as a construction material, for example,’ says Karlfeldt Fedje.

In Sweden, incineration of household waste in waste-to-energy plants is common, and results in around 250 000 tonnes of fly ash every year. The rest of Europe accounts for around ten times that amount. The researchers admit it is hard to estimate how many tonnes of zinc are currently lost through landfill.

‘And yet, the technology for extracting zinc from fly ash could have several positive effects, such as reducing the need for mining virgin zinc raw material, lower levels of toxicity in the ash, and greatly reduced landfill contributions. It can be a vital contribution to society’s efforts towards a more circular economy,’ comments Sven Andersson, adjunct professor at Chalmers.

He teamed up with Karlfeldt Fedje to create the new methodology, in collaboration with several industry players. Together, they were able to design a full-scale process.

As a next step, they are building an ash washing facility with zinc recycling in the city of Gothenburg. This is hailed as an investment that could save the municipality hundreds of thousands of euro every year.

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