A golden opportunity to recover precious metal from waste

An interdisciplinary team of experts in green chemistry, engineering and physics at Flinders University claims to have found a safer and more sustainable approach to extract and recover gold from ore and electronic waste. Their gold-extraction technique, described in the journal Nature Sustainability, could reduce levels of toxic waste from mining and shows that high-purity gold can be recovered from recycling components in discarded computers.

Global demand for gold is driven by its high economic and monetary value, but it is also a vital element in electronics, medicine, aerospace technologies, and other products and industries. However, mining the precious metal can involve the use of highly toxic substances such as cyanide and mercury for gold extraction — with 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 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 ‘unmake’ itself and convert back to monomer. This allows the gold to be recovered and the polymer to be recycled and reused, further increasing the green credentials of this method.

“The study featured many innovations, including a new and recyclable leaching reagent derived from a compound used to disinfect water,” said Matthew Flinders Professor Justin Chalker, who led the project team.

“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 [UV] light to initiate the key reaction.”

The project team applied their method for high-yield gold extraction from many sources — including electronic waste and mixed-metal waste — with the team even recovering trace gold found in scientific waste streams. They 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.

“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 noted.

Extensive investigation into the mechanisms, scope and limitations of the methods are reported in the new study. The team now plans to work with mining and e-waste recycling operations to trial their technique on a larger scale.

Image shows gold recovered from electronic waste.