The gold standard: How Notre Dame could revolutionise extraction

Notre Dame University researchers have created a molecule that could deliver environmental and efficiency benefits for gold extraction. Australian Mining writes.

For all of mining’s technological leaps in recent years some things remain firmly in the past. Nowhere is this more apparent than the use of potassium cyanide in gold leaching, a process that has remained largely unchanged since its inception in the mining industry over a century ago.

Sodium cyanide is useful for the extraction of gold from gold-bearing ore, but it is extremely poisonous and can cause significant environmental damage. Researchers at the microscopy-focused Notre Dame Integrated Imaging Facility, led by Professor Bradley Smith of the University of Notre Dame in Indiana, may have found another way.

“From a purely technical standpoint, it is truly remarkable that the 125-year old process of cyanide leaching is still in operation today even in the world’s most technically-advanced countries,” Smith tells Australian Mining.

“There is likely no other major industrial process that has endured for so long without disruption.”

Smith and his team released an academic article for the Journal of the American Chemical Society (JACS) entitled, ‘Macrocyclic receptors for precious gold, platinum or palladium coordination complexes’ that details a new process for precious metals extraction.

Their research led to the creation of a type of molecule that in the words of the team’s paper can, “selectively encapsulate anionic, square-planar chloride and bromide coordination complexes of gold(III), platinum(II), and palladium(II).”

In more general terms, the team created a molecule (referred to as a molecular complexation agent) that is capable of attaching itself to molecules in gold-containing ore for the process of separating and purifying gold chloride and gold bromide from a leach solution.

By-products of this reaction are then converted into relatively harmless chloroauric acid, allowing for simple reduction via industrial solvents. The process is not just limited to gold, it can be applied to other precious metals such as palladium and platinum, which share a similar chemical structure with gold.

Smith, a chemistry and biochemistry professor, led the study, accompanied by doctoral student Wenqi Liu and Notre Dame Molecular Structure Facility director Allen Oliver, using funds from the National Science Foundation.

The research was originally driven by curiosity rather than any financial or industrial incentive; Liu found a way to synthesise arene tetralactam macroycycles, complexation agents designed as supramolecular hosts for the capture of dye molecules.

These molecules hold the same shape as gold chloride molecules, a serendipitous and coincidental discovery that allowed the project to evolve into a distinct study of gold-selective precipitation.

The team’s process eliminates the need for water, which has the potential to reduce tailings waste and encourage recycling of solvents. Once perfected, Smith’s team hopes to hold financial potential in mining situations where cyanide is not a feasible choice, or as a method of extracting gold from waste streams.

“Many alternative lixiviants have been explored over the years as potential replacements for cyanide, especially oxidising solutions that convert the ore into gold chloride or gold bromide,” Smith explains.

“While these alternative methods are environmentally more benign, there are various technical challenges due in large part to the lower stability of gold chloride and gold bromide, which complicates the purification steps.”

The research is expected to host potential applications for several industries, such as recycling, water purification, process chemistry and pharmaceuticals, but it is the mining industry that is front and centre when it comes to precious metals.

“We are hoping to form collaborations with mining experts who can help us to first identify the quantitative performance benchmarks that will produce a broad positive impact, and then enable us to achieve those benchmarks,” Smith concludes.

This article originally appeared in the August issue of Australian Mining.

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