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A solid-state electrolysis process for upcycling aluminium scrap

Abstract

The recycling of aluminium scrap today utilizing a remelting technique downgrades the quality of the aluminium, and the final sink of this downgraded recycled aluminium is aluminium casting alloys1,2,3,4,5,6,7,8,9. The predicted increase in demand for high-grade aluminium as consumers choose battery-powered electric vehicles over internal combustion engine vehicles is expected to be accompanied by a drop in the demand for low-grade recycled aluminium, which is mostly used in the production of internal combustion engines2,7,10,11. To meet the demand for high-grade aluminium in the future, a new aluminium recycling method capable of upgrading scrap to a level similar to that of primary aluminium is required2,3,4,7,11. Here we propose a solid-state electrolysis (SSE) process using molten salts for upcycling aluminium scrap. The SSE produces aluminium with a purity comparable to that of primary aluminium from aluminium casting alloys. Moreover, the energy consumption of the industrial SSE is estimated to be less than half that of the primary aluminium production process. By effectively recycling aluminium scrap, it could be possible to consistently meet demand for high-grade aluminium. True sustainability in the aluminium cycle is foreseeable with the use of this efficient, low-energy-consuming process.

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Fig. 1: Global aluminium cycle in 2020 and 2040.
Fig. 2: The schematic and the electrochemical principle of the proposed SSE process.
Fig. 3: Results for the electrolysis of the AC2A casting alloy in molten MgCl2–NaCl–KCl–5mol%AlF3.
Fig. 4: Comparison of the SSE process with other industrial aluminium processes.

Data availability

All data are available in the main text or the online supplementary materials.

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Acknowledgements

We thank Y. Sasaki (Tohoku University) and E. Webeck (TEQED) for their input to the discussions. We thank K. Kobayashi and K. Watanabe for their experimental assistance. We also thank W. Takayanagi (LAIMAN) for illustration of graphic images. Financial support was provided by the Grant-in-Aid for Scientific Research, JSPS KAKENHI grant nos. 20H02492, 20K15069, 21H04610 and 21K17918, and the New Energy and Industrial Technology Development Organization, NEDO grant no. P21003. The cooperation of Hoei Metal Co. Ltd. is also gratefully acknowledged.

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Authors and Affiliations

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Contributions

H.Z. and T.N. conceived the idea of the study. X.L., T.H. and O.T. undertook the experiments. Z.Z. and K.M. undertook the scenario analysis. X.L., Z.Z., K.M., H.Z. and T.N. composed the manuscript and discussed the content of the manuscript.

Corresponding authors

Correspondence to Hongmin Zhu or Tetsuya Nagasaka.

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The authors declare no competing interests.

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Nature thanks Daniel Copper, Yaowu Wang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Results for the electrolysis of the AC2A casting alloy in the molten LiCl-KCl-5mol%AlF3.

(a) The anode and cathode potentials and (b) the cell voltage during the recycling of aluminium casting alloy (AC2A) using SSE (electrolyte: LiCl-KCl-5 mol%AlF3; electrolysis temperature: 500 °C). EPMA results of (c) the initial aluminium casting alloy (AC2A) and (d) the anode slime, showing the elemental distribution.

Extended Data Fig. 2 Results for the electrolysis of the AD12 die-casting alloy in the molten LiCl-KCl-5mol%AlF3.

(a) The anode and cathode potentials and (b) the cell voltage while recycling the aluminium die-casting alloy (AD12) using SSE (electrolyte: LiCl-KCl-5 mol%AlF3; electrolysis temperature: 500 °C). (c) The composition by ICP analysis and (d) XRD results of the initial aluminium die-casting alloy (AD12), anode slime and the cathode deposition. EPMA results of (e) the initial aluminium die-casting alloy (AD12) and (f) the anode slime, showing the elemental distribution.

Extended Data Fig. 3 The calibrated potential of the used Ag/AgCl reference electrode.

(a) The controlled current, (b) the electrode potential, and (c) the expansion of the black square area in (b).

Extended Data Fig. 4 The system boundary of the global aluminium cycle.

The fabrication, use, scrap processing and metallurgical processes are shown in blue and the products are shown in yellow. Process losses are shown in grey. Internal combustion engine vehicles are abbreviated as ICEVs. Hybrid electric vehicles are abbreviated as HEVs. Battery electric vehicles are abbreviated as BEVs. End-of-life is abbreviated as EoL.

Extended Data Table 1 Description of symbols used in Fig. 1
Extended Data Table 2 The conductivity and cost of typical ionic liquids and the molten salts used in our study
Extended Data Table 3 The estimated energy consumption for the practical SSE process
Extended Data Table 4 A comparison of the primary aluminium process, remelting process, three-layer electrolysis and SSE

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Supplementary Tables 1–13.

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Lu, X., Zhang, Z., Hiraki, T. et al. A solid-state electrolysis process for upcycling aluminium scrap. Nature 606, 511–515 (2022). https://doi.org/10.1038/s41586-022-04748-4

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