Flash heating recovers precious metals from e-waste in seconds

E-waste is the world’s fastest-growing domestic waste stream fueled mainly by higher consumption rates of electric and electronic equipment, short life cycles, and few options for repair. Much of it is handled unsafely, causing pollution, human health hazards, and the loss of valuable finite resources.

It’s estimated that all those discarded electronic devices – such as mobile phones, laptops, printers – contain precious metals like copper, silver, and even gold, along with a wide range of valuable rare earth elements. These metals, which can be extracted, recycled, and reused, are also being thrown away with e-waste.

Now, engineers at Rice University have developed a process to extract valuable metals and rare earth minerals from the electronic waste within one second. The process should be beneficial to the environment, consume up to 500 times less energy than existing laboratory methods, and produce a by-product that’s clean enough for agricultural land.

Metals settle at the bottom of a vial after being separated from other components in a crushed circuit board through flash Joule heating.
Metals settle at the bottom of a vial after being separated from other components in a crushed circuit board through flash Joule heating. Credit: Jeff Fitlow/Rice University

In their previous work, the researcher demonstrated the flash Joule heating method to produce graphene from carbon sources like waste food and plastic that has been adapted to recover rhodium, palladium, gold, and silver for reuse.

In the new study, the team focused on the electronic waste problem. They started by grinding old discarded circuit boards into powder and then heating it to 3,400 Kelvin (5,660 degrees Fahrenheit) in milliseconds by the ultrafast electrical, thermal process. Such a high temperature vaporizes the precious metal, and the gases are then vented away for separation, storage, or disposal.

According to the researchers, the process can recover more than 80% of silver, palladium, and rhodium and over 60% of gold in a sample. The heavy metals in electronic waste, some of which are highly toxic, including chromium, arsenic, cadmium, mercury, and lead, are also removed, leaving a final waste with minimal metal content, acceptable even for agriculture soil levels.

Importantly, the researchers reported that their “scalable process” consumes about 939 kilowatt-hours per ton of material processed, which is 80 times less energy than commercial smelting furnaces and 500 times less than laboratory tube furnaces. It also eliminates the lengthy purification required by smelting and leaching processes.

“Here, the largest growing source of waste becomes a treasure,” the Rice lab of chemist James Tour said. “This will curtail the need to go all over the world to mine from ores in remote and dangerous places, stripping the Earth’s surface and using gobs of water resources. The treasure is in our dumpsters.”

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