Nickel Metal Hydride (NiMH) batteries are widely used in hybrid vehicles. The anode of this battery consists a mischmetal alloy of lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd) [1]. These four elements are part of rare earth elements group, and among them, praseodymium and neodymium are considered critical materials, since they are in increasing high demand and facing supply uncertainty and near zero recycling. On the basis of the historic data, the annual demand for REEs has been increasing at a rate of 8.6% annually [1,4], and it is predicted that the rate could increase to above 20% annually to accommodate the increased utilization of renewable power generation (in particular wind and solar) and electrified transportation in the society [5]. To address the sustainability hurdles associated with NiMH battery and REE supply, new strategies have been initiated to mine these elements from secondary sources. Waste electrical and electronic equipment (WEEE) including NiMH batteries contain considerable amounts of REEs, which make them an attractive source.
The number of NiMH batteries manufactured up to this date is significant, and the global annual production is also increasing, as Toyota plans to increase the sale of Prius hybrid model by 2020 [2]. Although REEs account for more than 30 wt% of a NiMH battery, the global recycling rates of REEs from end of life NiMH batteries is less than 1% [1,3]. Considering the great opportunity, it is essential to develop efficient processes for the recovery of REEs from this class of WEEE.
Current recycling practices rely on either pyrometallurgy or hydrometallurgy. The former in highly energy intensive and the latter relies on large volumes of acids and organic solvents, generating large volumes of hazardous residues. This study is focused on developing an innovative and sustainable process for the urban mining of REEs from NiMH battery. The developed process relies on supercritical fluid extraction (SCFE) utilizing carbon dioxide as the solvent, which is inert, safe, and abundant. This process is very efficient because it runs at low temperature, and does not produce hazardous waste, while recovering about 90% of REEs. Furthermore, we proposed a mechanism for the SCFE of REEs, where we considered a trivalent REE state bonded with three Tri-n-butyl phosphate (TBP) molecules and three nitrates model for the extracted rare earth TBP complex. The SCFE process is an efficient and environmentally friendly process to valorize postconsumer NiMH battery without utilizing hazardous reagents; therefore, it minimizes the negative impacts of process tailings. This novel process proves to be a promising technique that can help realize the technological potential of REE recovery from post consumer WEEE, particularly NiMH battery

References:
[1] Tunsu, C.; Petranikova, M.; Gergorić, M.; Ekberg, C.; Retegan, T. Reclaiming rare earth elements from end-of-life products: a review of the perspectives for urban mining using hydrometallurgical unit operations. Hydrometallurgy 2015, 156, 239–258.
[2] Toyota. Worldwide sales of TMC hybrids top 5 million units, URL: http://corporatenews.pressroom.toyota.com/releases/worldwide+toyota+hybrid+sales+top+6+million.htm?view_id=35924.
[3] Ekberg, C.; Petranikova, M.; Koniecko, I. H.; Retegan, T.; Tunsu, C. Hydrochemical routes to recycle NiMH batteries and fluorescent lamps. In Critical Metal Symphosium; Sendai, Japan, 2016; pp 1–6.
[4] Tang, K. Recycling the rare earth elements from waste NiMH Batteries and magnet scraps by pyrometallurgical processes. 2014, No. November 2015.
[5] Alonso, E.; Sherman, A. M.; Wallington, T. J.; Everson, M. P.; Field, F. R.; Roth, R.; Kirchain, R. E. Evaluating rare earth element availability: a case with revolutionary demand from clean technologies. Environ. Sci. Technol. 2012, 46, 3406–3414.