Gold bar from e-waste. Image: Flinders University.
An interdisciplinary team of experts in green chemistry, engineering and physics at Flinders University in Adelaide, Australia, says it has developed a safer and more sustainable approach to extract and recover gold from ore and electronic waste.
Published in the leading journal Nature Sustainability, the gold-extraction technique promises to reduce levels of toxic waste from mining and shows that high purity gold can be recovered from recycling valuable components in printed circuit boards in discarded computers.
The project team, led by Professor of Chemistry Justin Chalker, applied this integrated method for high-yield gold extraction from many sources – even recovering trace gold found in scientific waste streams.
The progress toward safer and more sustainable gold recovery was demonstrated for electronic waste, mixed-metal waste, and ore concentrates.
“The study featured many innovations including a new and recyclable leaching reagent derived from a compound used to disinfect water,” said Chalker, who leads the Chalker Lab at Flinders University’s College of Science and Engineering.
“The team also developed an entirely new way to make the polymer sorbent, or the material that binds the gold after extraction into water, using light to initiate the key reaction.”
Extensive investigation into the mechanisms, scope and limitations of the methods are reported in the new study, and the team now plans to work with mining and e-waste recycling operations to trial the method on a larger scale.
“The aim is to provide effective gold recovery methods that support the many uses of gold, while lessening the impact on the environment and human health,” Chalker added.
The new process uses a low-cost and benign compound to extract the gold. This reagent (trichloroisocyanuric acid) is widely used in water sanitation and disinfection. When activated by salt water, the reagent can dissolve gold.
Next, the gold can be selectively bound to a novel sulfur-rich polymer developed by the Flinders team. The selectivity of the polymer allows gold recovery even in highly complex mixtures.
The gold can then be recovered by triggering the polymer to “un-make” itself and convert back to monomer. This allows the gold to be recovered and the polymer to be recycled and re-used.
Global demand for gold is driven by its high economic and monetary value but is also a vital element in electronics, medicine, aerospace technologies and other products and industries. However, mining the previous metal can involve the use of highly toxic substances such as cyanide and mercury for gold extraction – and other negative environmental impacts on water, air and land including CO2 emissions and deforestation.
The aim of the Flinders-led project was to provide alternative methods that are safer than mercury or cyanide in gold extraction and recovery.
The team also collaborated with experts in the US and Peru to validate the method on ore, in an effort to support small-scale mines that otherwise rely on toxic mercury to amalgamate gold.
Gold mining typically uses highly toxic cyanide to extract gold from ore, with risks to the wildlife and the broader environment if it is not contained properly. Artisanal and small-scale gold mines still use mercury to amalgamate gold. Unfortunately, the use of mercury in gold mining is one of the largest sources of mercury pollution on Earth.
Chalker said interdisciplinary research collaborations with industry and environmental groups will help to address highly complex problems that support the economy and the environment.
“We are especially grateful to our engineering, mining, and philanthropic partners for supporting translation of laboratory discoveries to larger scale demonstrations of the gold recovery techniques,” he said.
Lead authors of Flinders’ major new study – postdoctoral research associates Max Mann, Thomas Nicholls, Harshal Patel and Lynn Lisboa – extensively tested the new technique on piles of electronic waste, with the aim of finding more sustainable, circular economy solutions to make better use of ever-more-scarce resources in the world. Many components of electronic waste, such as computer processing units and RAM cards, contain valuable metals such as gold and copper.
“This paper shows that interdisciplinary collaborations are needed to address the world’s big problems managing the growing stockpiles of e-waste,” Mann said.