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Kanazawa University-led researchers recycle rare-earth elements from spent fluorescent lamps by chemical/mechanical treatment

A team led by Kanazawa University in Japan has developed a cleaner method for the recycling of several rare earths (REs) such as yttrium (Y) and europium (Eu) used as phosphors in fluorescent lamps (FLs).

End-of-life FLs are a potentially huge source of REs, but harsh and polluting processes are needed currently to extract these metals from the spent phosphors. As reported in Waste Management, instead of using acid extractants to dissolve the REs trapped in the spent lamps, the Kanazawa team turned to chelator chemistry.

Rare-earths (REs) are key components for the transition to a greener energy profile and low carbon society. The elements turn out to be of limited availability in the market, due to the supply-demand issues, exponential price rises, or geopolitics, which has led to a focus on the exploration of secondary sources for RE reclamation. End-of-life (EoL) nickel-metal hydride batteries, permanent magnets, and fluorescent lamps (FL) have been the primary sources for recyclable REs, while the recovery of REs in EoL FL (Ce, Eu, La, Tb, or Y) includes comparatively fewer processing steps than the other potential sources. In the current work, we proposed a simple, energy-efficient protocol for EoL FL processing, using chelators in combination with ball milling.

—Hasegawa et al.

Chelators—organic compounds containing elements such as N or O—bond to metals through electron donation. This allows them to gently leach out REs from the solid mass of a spent phosphor, without the need 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.

—co-author Ryuta Murase

To bolster the extraction rate, the team added a second ingredient to their process: mechano-chemical energy. Planetary ball-milling—i.e., grinding a solid into fine particles between layers of small, hard balls in a rotating chamber—was found to raise the yield of REs when performed during chelator treatment. This is because once milled the greater surface area of the pulverized phosphors provided easier access to the leachable metals within.

184950_web

Schematic view of the chelator-assisted wet-milling in a ball-mill, and comparison of the Y-yields (percent) from EoL-FL in rotary and planetary ball-mills. Credit: Kanazawa University.

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.

—corresponding author Hiroshi Hasegawa

Resources

  • Hiroshi Hasegawa, Zinnat A. Begum, Ryuta Murase, Kento Ishii, Hikaru Sawai, Asami S. Mashio, Teruya Maki, Ismail M.M. Rahman (2018) “Chelator-induced recovery of rare earths from end-of-life fluorescent lamps with the aid of mechano-chemical energy,” Waste Management, Volume 80, Pages 17-25 doi: 10.1016/j.wasman.2018.08.049.

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