Enhancing the nobility of metal processing

March 2018
MECHANOCHEMISTRY

Tomislav Friščić, assistant professor in the Department of Chemistry, McGill University

Tomislav Friščić, assistant professor in the Department of Chemistry, McGill University. Photo credit: McGill University

The images from villages in Africa or Asia resemble a 21st-century Dickensian nightmare: open air cottage industries, ostensibly dedicated to the virtuous purpose of recycling, rely on harsh combinations of acids that often flow freely into local soil and waterways. This could be where your old cell phone or television ultimately winds up, since the electronic hearts of these products feature mixtures of highly valued metals that are well worth recovering but chemically challenging to separate — creating toxic hotbeds in exchange for jobs.

Such images have prompted many manufacturers to adopt recycling strategies with less devastating health and environmental consequences, but fundamental chemistry has continued to compromise the efficiency of these efforts. Now a team of researchers at McGill University may have written a new ending for this story, one based on principles of green chemistry that eliminate the use of solvents and intermediate steps. Instead, these scientists have substituted techniques based on nothing more offensive than the use of brute force to initiate reactions that minimize the impact of these recycling operations.

The extent of that impact has been defined by the characteristics of noble metals such as gold, iridium, palladium, or platinum, which are found in so much of our high-tech hardware. These elements are highly valued for their resistance to corrosion and oxidation but they are just as resistant to extraction from chemical complexes like ores or alloys.

“The problem is that they’re so difficult to activate,” explains McGill chemistry professor Tomislav Friščić. “This is not a case where you need a little bit of cyanide for a little bit of metal. To dissolve this stuff and turn it into a soluble form, you need a lot of very aggressive reagents, say elementary chlorine or chlorine in the presence of peroxides.”

He still recalls his initial astonishment when he learned that one of the cheapest and easiest agents for the job — commonly found at those polluted electronic recycling sites — is aqua regia, the harsh mixture of nitric and hydrochloric acid with alchemical roots dating back to the 15th century.

“If you had told me to use aqua regia to do a metal extraction, I would say maybe you should read some more books,” he says.

Friščić was therefore quite receptive when graduate student Jean-Louis Do proposed a project to remove unpleasant actors like aqua regia from noble metal processing. Do pointed to a recently developed one-pot synthesis strategy known as “telescoping,” which places successive chemical reactions within a single reactor vessel where the product of each step becomes the basis for the next.

“You’re moving the compound from one step in the reaction to another without having to isolate it,” observes Friščić. “People interested in green chemistry want that because you avoid all these intermediate steps — separation, extraction, filtration.”

Telescoping was first demonstrated in 2014 by a team led by Stuart James at Queen’s University Belfast, UK, which used a series of twin-screw extruders similar to those used to combine ingredients for products in the food industry. By arranging these devices in a line, the output of each extruder became the input of the next, along with other reagents that were added along the way. The resulting “telescope” became the sole vessel for multi-component organic syntheses that were successfully completed using little or no solvent, the impetus for reactions instead being provided by the mechanochemical shear forces and compression of the turning screws.

Friščić’s team at McGill applied this same method to the challenge of noble metal extraction. Their results, published earlier this year in Angewandte Chemie, confirmed the viability of using a comparable mechanical system to achieve direct, room-temperature conversion of palladium and gold metals into water-soluble salts or metal-organic catalysts, all without the use of any harsh solvents.

“I was impressed by the fact that we can make chlorine, bromine, and iodine derivatives of palladium and gold,” says Friščić. “We could take a mixture of palladium and platinum or palladium, platinum, and gold, and in the first milling run we can leave behind pure platinum. We can actually separate these elements.”

Above all, as the initial proponents of telescoping have insisted, the equipment that they employed at a table-top scale already has industrial-scale equivalents hard at work in factories around the world. That prospect sets the stage for a similar bulk extraction of noble metals, one that could create jobs without the need for toxic consequences.

“I’m convinced we can solve some big problems using this technology, in terms of selective separation of element mixtures,” insists Friščić, adding that above all he finds the solution to be an elegant one.

“This is how I think chemistry should be,” he concludes. “We should take the simplest possible starting material, in this case the element itself. You extrude from this metal new compounds, step by step. It’s a bit of a poetic thing for me.”