A method called ‘flash Joule heating,’ which was originally developed to produce graphene from carbon sources like food waste, has been adapted by researchers at Rice University to recover rhodium, palladium, gold and silver from electronic waste.
In a report published in the journal Nature Communications, the scientists explain that the process uses up to 500 times less energy than current lab methods and since it removes highly toxic heavy metals including chromium, arsenic, cadmium, mercury and lead from the flashed materials, it also produces a byproduct clean enough for agricultural land.
Flash Joule heating consists of heating waste to 3,400 Kelvin (5,660 degrees Fahrenheit) with a jolt of electricity that vaporizes the precious metals contained in the feed. The gases are vented away for separation, storage or disposal.
The researchers first powdered circuit boards and 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 vapours. The vapours are transported from the flash chamber under vacuum to another vessel, a cold trap, where they condense into their constituent metals.
According to lead author Bing Deng, the reclaimed metal mixtures in the trap can be further purified to individual metals by well-established refining methods.
Deng and his team 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, something that is easy to do because each flash takes less than one second.
The group also said that the scalable 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. It also eliminates the lengthy purification required by smelting and leaching processes.
In their view, these are important findings that may promote the idea of urban mining, particularly taking into account that the increasingly rapid turnover of personal devices like cell phones has driven the worldwide rise of electronic waste, which now reaches 40 million tons per year with only about 20% of landfill waste currently being recycled.
“Here, the largest growing source of waste becomes a treasure,” James Tour, senior author of the study, said in a media statement. “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.”