Precious metals from electronic waste in seconds – sciencedaily
In what should be a win-win for the environment, a process developed at Rice University to extract precious metals from electronic waste would also use up to 500 times less energy than current laboratory methods and produce a sufficiently clean by-product for agricultural land.
The Joule flash heating method introduced last year to produce graphene from carbon sources such as food waste and plastic has been adapted to recover rhodium, palladium, gold and silver for their reuse.
A report in Nature Communication by chemist James Tour’s Rice lab also shows that highly toxic heavy metals including chromium, arsenic, cadmium, mercury and lead are removed from flashed materials, leaving a byproduct with minimal content of metal.
Instant heating of waste to 3,400 Kelvin (5,660 degrees Fahrenheit) with an electric discharge vaporizes the precious metals and the gases are vented for separation, storage or disposal. Tour said that with more than 40 million tons of electronic waste produced worldwide each year, there is great potential for “urban mining”.
“Here, the biggest growing source of waste becomes a treasure,” Tour said. “It will reduce the need to go all over the world to mine for minerals in remote and dangerous places, by scouring the Earth’s surface and using gobs of water resources. The treasure is in our dumpsters.”
He noted that an increasingly rapid turnover of personal devices like cellphones has led to the global increase in electronic waste, with only about 20% of landfill waste currently being recycled.
“We have found a way to recover the precious metals and turn electronic waste into a sustainable resource,” he said. “Toxic metals can be removed to save the environment.”
The lab found flashing electronic waste requires some preparation. Guided by senior author and postdoctoral research associate Rice Bing Deng, the researchers sprayed circuit boards they used to test the process and added halides, like Teflon or table salt, and a pinch of black. of carbon to improve the recovery efficiency.
Once flashed, the process relies on an “evaporative separation” of the metal vapors. The vapors are transported from the vacuum flash chamber to another container, a cold trap, where they condense into their constituent metals. “The mixtures of metals recovered from the trap can be further purified into individual metals by well-established refining methods,” said Deng.
The researchers reported that a Joule flash reaction reduced the concentration of lead in the remaining coal to less than 0.05 parts per million, the level deemed safe for agricultural soils. The levels of arsenic, mercury and chromium were all further reduced by increasing the number of lightning bolts.
“As each flash takes less than a second, it’s easy to do,” Tour said.
The scalable Rice process consumes about 939 kilowatt hours per tonne of material processed, which is 80 times less energy than commercial smelters and 500 times less than laboratory tube furnaces, according to the researchers. It also eliminates the time-consuming purification required by the melting and leaching processes.
The co-authors of the article are former student Rice Duy Xuan Luong, graduate students Zhe Wang and Emily McHugh, and researcher Carter Kittrell. Tour holds the TT and WF Chao Chair in Chemistry as well as Professor of Computer Science, Materials Science and Nanotechnology. The Air Force Office of Scientific Research (FA9550-19-1-0296) and the Department of Energy (DE-FE0031794) supported the research.
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