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By Mary-Lou Considine 17 March 2021 4 min read

Biomining is an innovative way of mining valuable metals from printed circuit boards like these, which usually end up in landfill. Image by CSIRO

Thanks to digitisation and our insatiable demand for new mobile phones, computers, flat-screen TVs and other electronic goods, electronic waste – or e-waste – has become one of the world’s fastest growing waste streams.

The United Nations estimated that, in 2019, a record 54 million metric tonnes of e-waste was generated globally, of which only 17 per cent was recycled. The UN also found that e-waste volumes are rising three times faster than the world’s population.

E-waste contains widely used metals like copper and iron, but is also rich in precious metals like gold, silver and platinum. Yet currently, with much of the world’s e-waste ending up in landfill –contaminating soil, groundwater and waterways – only a fraction of these metals are recovered.

In Australia, e-waste is growing three times faster than any other municipal waste stream, with most going to landfill – apart from 40 per cent of computers and TVs, items specifically covered under the 2011 National Television and Computer Recycling Scheme (NTCRS).

Finding the best ‘brews’ and flows

CSIRO’s Dr Anna Kaksonen says that, apart from its volume, e-waste poses a major recycling challenge due to its complex nature – a hard-to-separate tangle of plastics, glass, ceramics, as well as an array of metals that includes mercury and lead.

She sees great environmental and economic benefit in applying biomining, a relatively new area of mining research, to e-waste recycling. Broadly, biomining refers to the use of microbes like bacteria and fungi to extract and recover metals through processes such as bioleaching and bioprecipitation.

Electron microscope image of biomining microbes (green) and mineral precipitates (yellow) (artificially coloured). © CSIRO

“Biomining is already used at a large scale for extracting base metals from low-grade sulfide ores and biooxidising refractory sulfidic gold concentrates before cyanidation,” says Dr Kaksonen. “About 10–15 per cent of the world’s copper and about 5 per cent of the world’s gold is mined this way.”

In a recently published research paper, a team of Australian scientists, including Dr Kaksonen, investigated the potential of using Acidithiobacillus ferrooxidans – a naturally occurring, hardy bacterial species able to live in acid mining waste – to extract base metals from Australia’s e-waste.

The scientists were particularly interested in the prospect of bioleaching metals from printed circuit boards (PCBs). PCBs contain a mix of plastics, ceramics and metals like gold, silver, copper, zinc, aluminium and nickel.

Thanks to a recent grant from the Western Australian Government’s New Industries Fund, Dr Kaksonen and a CSIRO team are now embarking on lab-scale research to identify an effective sequence of biological unit processes to enable bacterial leaching of both base and precious metals from ground PCBs.

“The research will help us identify which of the metals present in e-waste are the most feasible to mine,” adds Kaksonen.

The two key processes in biomining are dissolving the metals into solution, then recovering the target metals out of solution. “Microbes can be used for both processes, but the current project will focus on the first one,” says Dr Kaksonen.

“At the moment, the first step is usually done using conventional hydrometallurgy, which uses strong chemical acids, or cyanide in the case of gold, but this results in a toxic waste-stream.”

Pyrometallurgy – using the high temperatures in smelters to recover metals from PCBs – is another approach to metals recovery from PCBs that is widely used overseas.

However, smelters require significant capital investment and continuous, high-volume feedstock. This makes them economically viable in countries like Japan or in Europe, but not Australia. Pyrometallurgy is also energy-intensive and produces toxic emissions such as dioxin.

Sourcing the e-waste ‘ore’

CSIRO’s commercial partner in the research is Total Green Recycling, established in 2008 in Perth by brothers James and Michael Coghill.

James Coghill says while Total Green Recycling always encourages ‘second-life’ reuse of electronic equipment – his company offers data-scrubbing services to customers – it has also developed a production line of manual and automated processes for sorting and separating e-waste into component materials for decontamination, onselling to manufacturers, or further processing.

TGR was also awarded a New Industries Fund grant to create a mobile e-waste recovery centre in a shipping container, which could be moved between different municipalities and events – further lowering the recycling barrier, says Mr Coghill, for people and businesses wanting to dispose of unwanted technology conveniently, securely and cleanly.

TGR is supplying the CSIRO researchers with shredded, size-reduced PCBs, which will be used to bioleach metals with bacterial catalysts.

“So the innovation will be in identifying the most effective microbes, solutions and flow-sheet of unit processes that give the best result,” says Dr Kaksonen.

Scalability and the prospect of urban mining

The CSIRO research will run for a year, and Dr Kaksonen hopes a successful outcome will lead to a subsequent project to scale-up and assess the economic feasibility of the process, in terms of cost versus the value of metals recovered.

“The e-waste flows in Australia are comparatively small by volume,” she says. “So the question is, will biomining PCBs be economical in Australia, with its smaller-scale waste recycling sector?

“The concentration of metals in e-waste can be higher than in ore bodies. For example, there is up to 100 times more gold in a tonne of mobile phones than in a tonne of gold ore. That’s why some people refer to biomining e-waste as ‘urban mining’.”

Mr Coghill thinks a local e-waste biomining would give Australia “more capability and create more jobs”.

“We would be more resilient as a country, should supply chains be disrupted as they were during COVID.”

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