Researchers Discover A New Process Of Extracting Zinc From Fly Ash

Solid waste incineration in Europe produces millions of tons of fly ash yearly, most of which ends up in landfills. But this ash often contains significant quantities of precious metals such as zinc. Researchers at Chalmers University of Technology, Sweden, have discovered a brand new technique to obtain these precious metals, which can lead to significant reductions in pollutants for the environmental.

During waste incineration, the flue gases that get released are washed and the small particles that are present are separated, which leads to the formation of fly ash. Fly ash can contain many toxic substances, such as dioxins, and so it's typically classified as hazardous waste and gets sent to landfills. However, it also contains valuable metals such as zinc, which are unfortunately lost as a result.

At Chalmers University of Technology, they are testing a pilot scale and detailed over many years of research, which involves using an acid wash for waste treatment to separate the flue gases, to detach the zinc from the fly ash. Zinc can then be extracted, washed and processed into raw material. After many experiments, the researchers were able to significantly reduce the levels of toxicity.

Karin Karlfeldt Fedje, Associate Professor at Chalmers University of Technology, and researcher at the recycling and waste management company Renova AB has stated "In our pilot study, we found that 70% of the zinc present in fly ash can be recycled. 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,"

"After extraction, we incinerate the residual ash again to break down the dioxins. Ninety percent of this is then turned into bottom ash, which can be used as a construction material, for example," Karin Karlfeldt Fedje further explained.

There are many methods of waste incineration that vary across the world, but the necessity to manage vast amounts of ash after the practice is universal. In Sweden, the incineration of household waste in incinerators is common and produces around 250,000 tons of fly ash each year, which could potentially be treated in this way. The rest of Europe accounts for about ten times as much ash.

Although it is difficult to estimate how many tons of zinc are currently being lost to landfills in Sweden and other countries, this new system developed by the Chalmers researchers can be of great interest to all waste management stakeholders. It has great possibilities for relatively easy recovery of these precious metals, and could have a significant impact on the profitability of waste incineration and its role in the global economy.

"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," says Sven Andersson, Adjunct Professor at Chalmers and R&D Manager at flue gas cleaning supplier Babcock & Wilcox Vølund AB.

Karin Karlfeldt Fedje, who splits her time between Chalmers and Renova, has spent many years developing the methodology in cooperation with several external partners. One of them is Sven Andersson, together they succeeded in designing a complete procedure. As a result of their research, Renova AB and B & W Vølund have built an ash washing plant in Gothenburg, Sweden, with zinc recycling - an investment that could save the municipal waste management company hundreds of thousands of dollars each year.