Metals settle at the bottom of a vial after being separated from other components in a crushed circuit board through flash Joule heating. Photo: Jeff Fitlow/Rice University.In what should be a win-win-win for the environment, a process developed by researchers at Rice University to extract valuable metals from electronic waste would also use up to 500 times less energy than current lab methods and produce a by-product clean enough for use on agricultural land.
The Rice researchers adapted the flash Joule heating method, which they introduced last year to produce graphene from carbon sources like waste food and plastic, to recover rhodium, palladium, gold and silver for reuse. In a paper on this work in Nature Communications, the researchers, led by chemist James Tour, report that highly toxic heavy metals including chromium, arsenic, cadmium, mercury and lead can also be removed from the flashed materials, leaving a by-product with minimal metal content.
Instantly heating the electronic waste to 3400K (5660°F) with a jolt of electricity vaporizes 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 such '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 that 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. “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.”
Tour and his team found that flashing e-waste requires some preparation. Guided by lead author and Rice postdoctoral research associate Bing Deng, the researchers first powdered the circuit boards they used to test the process, and then added halides like Teflon or table salt and a dash of carbon black to improve the recovery yield.
Once flashed, the process relies on 'evaporative separation' of the metal vapors. These vapors are transported from the flash chamber under vacuum to another vessel, 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 scalable Rice process consumes about 939 kilowatt-hours 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. It also eliminates the lengthy purification required by smelting and leaching processes.
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.