Mechanochemistry makes gold salts that are soluble in organic solvents | Research

A new one-pot mechanized method for treating elemental gold can provide organic dissolved gold salts in less than an hour. By combining potassium peroxymonosulfate, a common pool cleaner, tetraalkylammonium halide salts, and metallic gold in a solvent-free ball mill reaction, this technique avoids the toxic and harsh reagents traditionally used. used to oxidize gold.

As a precious metal, gold is resistant to most chemical processes. This quality makes it valuable for a wide range of commercial and technological applications but also requires the use of chemically aggressive reagents to process it. Current methods of refining gold using reagents include aqua regia (a mixture of concentrated hydrochloric acid or bromhydric acid with nitric acid), chlorine gas, and mercury, causing concern for the environment and workers. ‘The way gold is handled is really messy’, comments Tomislav Friščić, at the University of Birmingham, UK. ‘It’s horrible, dirty, terrible and dangerous.’

Driven by the difficulty of procuring commercially available gold salt, Friščić’s team sought a safer alternative to traditional methods of synthesizing gold salt. They added bulk gold, tetraalkylammonium halide salt and potassium peroxymonosulfate to the ball mill. After 30–60 min of grinding, a tetrahaloaurate dissolved in the body (iii) as a result the salt can be separated from the sulfate byproduct with high yield after a single extraction with ethyl acetate.

By minimizing the solvents used in the synthesis and the number of extractions and post-treatment steps that must be performed, the environmental impact of the metal gold processing into a usable salt significantly reduced. Ethyl acetate used for extraction can be reused many times.

Traditional gold production technologies do not allow the use of these reagents. ‘In industry, the temperature at which they perform these processing steps is too high. Jean-Louis Do, a doctoral student in Friščić’s group, who previously worked in the laboratory processing the rare palladium gold ore by soaking boiling sulfuric acid, explains: “You just burn the salt. , but by using mechatronics you can use these unique reagents. .

The team continued to study the gold mechanization process by milling the freshly prepared tetrahaloaureate (iii) salt with anhydrous sodium acetylacetonate for 30 min, reducing airborne compounds and moisture-stable dihaloaureate (I) Compounds. Thomas Auvray, a senior researcher in Friščić’s lab, explains: “For us, that was really exciting because within two hours we had turned the gold powder into the salt that people wanted to use. “.

“The highlight for me was that it oxidized what was supposed to be non-oxidizing,” says Nhi Ly Lu, a rare earth metal chemist from the University of Newcastle, UK. ‘The oxidation step is really the bottleneck of all the chemical processes that coordinate gold… If you have a +3 gold complex, you can do whatever you want.’

Lu would like to see this method applied to oxidize gold more selectively than other metals such as silver commonly found in complexes in natural ores. Friščić and his team agreed. One of their next areas of research will be ‘selectively oxidizing some metals over others, by adjusting the reaction conditions,’ Do said. Friščić adds, ‘if you want to be very enthusiastic, imagine you could have a mechanical process that you could tweak to selectively activate five, six or seven precious metals.’

Friščić hopes that this safer and greener gold treatment can be widely used in the industry without the need for major reforms to existing infrastructure: ‘You make exactly the same thing. yes, but now you create it in a much safer way. You don’t have to dismantle the entire industry; you can just replace a nasty, horrible step with a basic pool cleaner-based step.’