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Rice University develops a process to extract valuable metals from electronic waste that reportedly uses up to 500 times less energy than current lab methods and produces a byproduct clean enough for agricultural land. The flash Joule heating method introduced last year to produce graphene from carbon sources like waste food and plastic has been adapted to recover rhodium, palladium, gold and silver from discarded electronics for reuse.
A new report in the journal “Nature Communications” by Rice University chemistry professor James Tour shows how highly toxic heavy metals such as chromium, arsenic, cadmium, mercury and lead are removed from the flashed materials, leaving a byproduct with minimal metal content.
“We found a way to get the precious metals back and turn e-waste into a sustainable resource,” says Tour, who is also a professor of computer science and of materials science and nanoengineering. “The toxic metals can be removed to spare the environment.”
Tour says 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 says. “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 says an increasingly rapid turnover of personal devices like cell phones has driven the worldwide rise of electronic waste, with only about 20% of landfill waste currently being recycled.
“Precious metal recovery from electronic waste, termed urban mining, is important for a circular economy,” the Tour report explains.
“Present methods for urban mining, mainly smelting and leaching, suffer from lengthy purification processes and negative environmental impacts. Here,” the report states, “a solvent-free and sustainable process by flash Joule heating is disclosed to recover precious metals and remove hazardous heavy metals in electronic waste within one second.”
Instantly heating the waste to 3,400 Kelvin (5,660 degrees Fahrenheit) with a jolt of electricity vaporizes the precious metals, and the gases are vented away for separation, storage or disposal.
Once flashed, the process relies on “evaporative separation” of the metal vapors. The vapors are transported from the flash chamber under vacuum to another vessel, a cold trap, where they condense into their constituent metals.
The scalable Rice process reportedly consumes about 939 kilowatt-hours of electricity per ton of material processed, 80 times less energy than commercial smelting furnaces and 500 times less than laboratory tube furnaces, according to the researchers. Flash Joule heating also eliminates the lengthy purification required by smelting and leaching processes.
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