The rare earths are misnamed: these seventeen chemical species are still more abundant on Earth than gold, and the most common, cerium (Ce), is even more abundant than copper. But they are still very difficult to exploit. Extracting them from deposits and then separating them from other species and from each other is very resource- and energy-intensive. However, the properties of these materials make them very useful for the most advanced technologies, especially in electronics, in electric vehicle motors or in wind turbines. Their supply is therefore a major issue. Bing Deng, James Tour and their colleagues from Rice University, Houston, USA, have developed a technique to facilitate the extraction of certain wastes with much greater efficiency than existing methods.
Researchers focused on three main types of waste containing rare earths: fly ash, which is produced when carbon is burned and released from coal-fired power plants; bauxite tailings, or “red mud,” a toxic by-product of a common aluminum manufacturing technique; and finally e-waste. According to a previous study, $15 billion worth of rare earths was wasted in 2015. And of 750 million tons of fly ash produced worldwide each year, 500 tons of rare earths are hidden, as James Tour” We have mountains! »
But they remain prisoners of this waste and are heavily mixed together because of their similar chemical properties. Current recovery techniques often involve a first step called “leaching”, a hot washing of solid waste with a very acidic solution. This first step loosens the rare earths and separates them from the solid waste residue. The solution obtained is then refined through a series of successive extractions to separate the rare earths from the impurities, then between them. These processes release highly polluting, toxic and radioactive substances into nature. The researchers therefore tried to facilitate this first dissolution step with a less aggressive starting solvent.
In the experiment conducted by Bing Deng, James Tour and their colleagues, the rare earth waste is first placed in a quartz tube pressed between two electrodes. The assembly is electrified quickly and with very high intensity. Thanks to the Joule effect (a dissipation of heat that a material loses when it conducts electricity), the material is heated to around 3,000°C in one second! At such a temperature, the rare earths and the species that contain them then dissolve much more easily. The solution extraction yield is doubled with a 150 times less concentrated hydrochloric acid solution.
The authors of this process had already used this “instantaneous” Joule effect to produce graphene from any solid carbon source or to extract precious metals from e-waste. Here, this method of rare earth recovery, by bringing a material to a very high temperature in a very short time, would work with the same efficiency regardless of the type of waste, fly ash, bauxite residue or e-waste. All at a cost of 600 kilowatt-hours per ton of material processed for about $12, the researchers said.
Ultimately, this type of process would allow industries that require rare earths to no longer depend on mining, thereby reducing the costs, environmental impact, and geopolitical tensions associated with exploiting these chemical species.