Modern life is intimately intertwined with a set of 17 elements at the foot of the periodic table. Known as rare earths (REs), many of these metals are highly magnetic and find use in computing, green energy and other technologies. However, due to rising prices, legal issues and the difficulty of mining, safeguarding its supply is a major scientific and political challenge.
Several REs, such as yttrium (Y) and europium (Eu), are used as phosphorus in fluorescent lamps (FLs). These bulbs increasingly replace traditional incandescent lighting, but have a limited shelf life. End-of-life FLs are thus a potentially large source of REs – an example of "technosphere mining" – but severe and polluting processes are required to extract these metals from spent phosphors. Now a team led by Kanazawa University in Japan has developed a cleaner method.
As reported in Waste Management, instead of using acid extractors to dissolve ERs trapped in spent lamps, the Kanazawa team turned to chelating chemistry. Chelators – organic compounds containing elements such as N or O – bind to metals through the electron donation. This allows them to gently release the ERs from the solid mass of a spent phosphor without the need for strong acids.
"An ideal type of chelating compound is known as amino-polycarboxylates," explains co-author Ryuta Murase. "We have found that they were also very efficient in extracting REs from spent phosphorus – especially yttrium and lanthanum, which are used in chemically reactive red phosphors. by the EDTA chelator, probably because it forms the strongest complexes with metals. "
To strengthen the extraction rate, the team added a second ingredient to their process: mechano-chemical energy. "Planetary ball milling" – grinding a fine particulate solid between layers of small and hard balls in a rotating chamber – was found to increase ER yields when performed during chelating treatment. This is because, once milled, the larger surface area of the phosphorous powder provided easier access to the leachable metals inside it.
"We work hard to optimize the process in every detail, including temperature, pH, milling speed, ball size and other factors," says corresponding author Hiroshi Hasegawa. "Our efforts have been rewarded, and the most important RE metals have saved from spent lamps with recoveries of 53% to 84%. Recycling REs will be vital to sustainable technology, and we hope to show that this can be done cleanly and efficiently. "
Materials provided by Kanazawa University. Note: Content can be edited for style and size.