Used for a variety of applications from jewellery and electronics to space exploration, the malleable, conductive and corrosion-resistant precious metal is traditionally produced with a process involving cyanide, a notorious toxin that has been banned from industrial use in several countries.
The ore mined from the ground is crushed to powder and funnelled through a series of tanks in a process known as leaching, before the toxin is introduced to separate the gold from the ore into the leached solution.
Researchers at the Finnish university have had promising results with a method that substitutes cyanide for chloride, one of the two elements of table salt, in both the leaching and recovery phase.
Ivan Korolev, a doctoral candidate taking part in the research project, revealed that the substitution improved the recovery rate from about 64 to 84 per cent in experiments carried out by the research team.
“We started with about nine-per cent recovery, but then it grew to 25 per cent and soon we were hitting 70 per cent,” he told. “Sometimes we even achieved close to 95 per cent.”
Called electrodeposition-redox replacement (EDRR), the process combines the best features of two common methods for extracting leached gold: electrolysis, which uses electric currents to reduce gold in the leaching solution, and cementation, which adds particles of other metals to the solution to react with gold.
The method was developed by two faculty members of the school of chemical engineering at Aalto University, Mari Lundström and Kirsi Yliniemi.
“With EDRR, we apply short pulses of electricity to create thin layers of metal – in our case, copper – on the electrode and cause a reaction that encourages gold to replace the copper layer by layer,” elaborated Korolev. “The method has low energy consumption and doesn’t require the addition of any other elements.”
The research was carried out in collaboration with Metso Outotec as part of Socrates, a sustainability project initiated by the EU. Its results have been published in Chemical Engineering.
“Collaborating with Metso Outotec allowed us to develop the method in a way that’s much closer to real-world implementation,” noted Korolev.
“Extraction methods of the past have always left some valuable metals behind. Now, as demand for metals grows all the time, even these small amounts are important. I think we can still increase the yield,” he added. “Perhaps we can’t reach 100 per cent, but I believe we can hit the 90 per cent mark or more,” he added.