Nov 5 2018
A set of 17 elements at the bottom of the periodic table is closely entwined with modern life. Most of these metals, referred to as rare earths (REs), are highly magnetic and are used in green power, computing, and other technologies.
Schematic view of the chelator-assisted wet-milling in a ball-mill, and comparison of the Y-yields (%) from EoL-FL in rotary and planetary ball-mills. (Image credit: Kanazawa University)
Conversely, protecting the supply of these REs is a major political and scientific challenge because of the growing prices, the complexity of mining, and legal issues.
A number of REs, for example, europium (Eu) and yttrium (Y), are utilized as phosphors in fluorescent lamps (FLs). While these fluorescent lamps are progressively replacing conventional incandescent lighting, their lifespan is rather limited. Therefore, end-of-life FLs are possibly a huge source of REs—an example of “technospheric mining”—but still polluting and harsh processes are required to extract these metals from the spent phosphors. Now, a research team, headed by Kanazawa University in Japan, has come up with a cleaner technique.
As specified in Waste Management, the Kanazawa researchers did not use acid extractants to dissolve the REs trapped in the spent lamps, and instead turned to chelator chemistry. Chelators are organic compounds that contain nitrogen or oxygen elements and bond to metals via electron donation. This enables them to mildly leach out REs from the solid mass of a spent phosphor, but without any necessity for strong acids.
An ideal type of chelator compound is known as amino-polycarboxylates. These are already used to remove toxic metals from solid waste. We found they were also very efficient at extracting REs from spent phosphors—especially yttrium and lanthanum, which are used in the more chemically reactive red phosphors. The best performance was by the chelator EDTA, probably because it forms the strongest complexes with the metals.
Ryuta Murase, Study Co-Author.
In order to boost the extraction rate, the researchers added another ingredient, called mechano-chemical energy, to their process. “Planetary ball-milling,” in which a solid is ground into fine particles between layers of hard and tiny balls in a rotating chamber, was found to increase the yield of REs when carried out during chelator treatment. This is because, after they are milled, the greater surface area of the pulverized phosphors offered easier access to the leachable metals inside.
We worked hard to optimize the process in every detail, including temperature, pH, milling speed, ball size, and other factors. Our efforts paid off, and the most economically important RE metals were leached out from spent lamps with recoveries from 53% to 84%. Recycling REs will be vital for sustainable technology, and we hope to show that it can be done cleanly and efficiently.
Hiroshi Hasegawa, Study Corresponding Author.