Developed at Rice University in Houston, Texas, flash Joule heating was initially developed to produce graphene from carbon sources including waste food and plastic but has since been adapted to recover rhodium, palladium, gold and silver for reuse.
A report in Nature Communications by the Rice lab of chemist James Tour shows that highly toxic heavy metals including chromium, arsenic, cadmium, mercury and lead are removed from the flashed materials, leaving a by-product with minimal metal content.
Instantly heating the waste to 3,400 Kelvin (5,660 degrees Fahrenheit) with a jolt of electricity vaporises the precious metals, and the gases are vented away for separation, storage or disposal. Tour said that with more than 40 million tons of e-waste produced globally every year, there is plenty of potential for “urban mining.”
“Here, the largest growing source of waste becomes a treasure,” 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.”
He noted an increasingly rapid turnover of personal devices like cell phones has driven the worldwide rise of electronic waste, with only about a fifth of landfill waste currently being recycled.
“We found a way to get the precious metals back and turn e-waste into a sustainable resource,” he said. “The toxic metals can be removed to spare the environment.”
Guided by lead author and Rice postdoctoral research associate Bing Deng, the researchers powdered circuit boards they used to test the process and added halides and carbon black to improve the recovery yield.
Once flashed, the process relies on evaporative separation of the metal vapours, which are transported from the flash chamber under vacuum to a cold trap where they condense into their constituent metals.
“The reclaimed metal mixtures in the trap can be further purified to individual metals by well-established refining methods,” Deng said.
The researchers reported that one flash Joule reaction reduced the concentration of lead in the remaining char to below 0.05 parts per million, the level deemed safe for agricultural soils. Levels of arsenic, mercury and chromium were all further reduced by increasing the number of flashes.
“Since each flash takes less than a second, this is easy to do,” Tour said.
The team said their process consumes about 939kWh per ton of material processed, 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.