Production of High-Purity Aluminum by Combining Lewis Acidic AlCl3−[C2mim]Cl Ionic Liquid and Aluminum Scrap Anode
Authors: Fu Yichen , Tsuda Tetsuya , Mikame Tomo , Hosoya Kei , Ueda Mikito , Ui Koichi , Miyake Masao , Nunomura Junji , Kojima Yoichi 
Published: December 2024

Aluminum (Al) is one of essential non-ferrous metals and is used in various places. Industrially, Al is produced using Hall-Héroult process, which requires massive amount of electricity (12,500~15,000 kWh t-1) and releases large quantities of greenhouse gas, being a high environmental impact process. In contrast, recycling Al requires only 3% of the energy needed to produce new metal. But, Al with many impurities is difficult to recycle and is eventually discarded.1 This is a waste of Al resources, so if Al with a purity equivalent to new metal can be obtained from such low-grade Al, i.e., Al scrap, using a process with a low environmental impact, it will likely attract attention as a new Al production process. To make this possible, we have focused on Al electrodeposition in Lewis acidic aluminum chloride (AlCl3) based ionic liquids (ILs) (AlCl3 molar fraction: 50 mol% < ). Due to the following electrochemical reaction in the ILs, high current efficiency can be expected:2

4[Al2Cl7]- + 3e- ⇌ Al + 7[AlCl4]- (1)
In this presentation, in order to find the appropriate electrolysis conditions, e.g., bath temperature, Lewis acidity of the ILs, and applied current densities, for electrochemical high purity Al recovery from different Al scrap anodes, electrode behavior of the Al scrap anodes was examined in Lewis acidic AlCl3−1-ethyl-3-methylimidazolium chloride (AlCl3−[C2mim]Cl) ILs.
The preparation and purification processes for AlCl3 and [C2mim]Cl were identical with those described in previous articles.2 All electrochemical experiments were conducted using a three-electrode cell. The working and counter electrodes were Cu plate cathodes defined the area (2 cm2) and Al scrap plate anodes defined the area (2 or 4 cm2), respectively. Al scraps employed in this investigation were expanded materials (A3003, A5182, and A6016) and a casting material (ADC12). For comparison, a pure Al plate was also used. The reference electrode was an Al wire immersed in the same Lewis acidic IL as the electrolyte, but separated from the electrolyte by a glass frit. The distance between the working and counter electrodes was 2 cm. The amount of IL electrolyte used was 50 mL. Electrolysis experiments were conducted at -5 mA cm-2 or more. The cathode was replaced every 25 hours, and constant current electrolysis experiments were performed for a total of 100 hours under different temperature conditions. All the experiments were carried out in an Ar gas-filled glove box with O2 and H2O < 1 ppm. The electrodes after electrolysis experiment were rinsed with dry tetrahydrofuran (THF) and ultrapure water and characterized by SEM, EDX, EPMA, and XRD.
Al electrodeposition proceeds through the reduction reaction of [Al2Cl7]− as shown in eq. (1). Therefore, if 66.7-33.3 mol% AlCl3−[C2mim]Cl, which has the highest concentration of [Al2Cl7]− in the IL system, is used, we expected that it is possible to recover Al at higher current density. However, when the anode surface area was 2 cm2, although the Al anode was a pure Al plate, the potential of the Al anode sharply increased during electrolysis within 600 sec. (Fig. 1). The 66.7-33.3 mol% AlCl3−[C2mim]Cl was not suitable for long-term electrolysis at room temperature. This phenomenon would be caused by AlCl3 precipitation on the anode surface due to a rapid increase in Lewis acidity at the IL electrolyte | anode interface.3,4 But, when a similar study using 60.0-40.0 mol% AlCl3−[C2mim]Cl, which has lower Lewis acidity, was conducted, the situation was greatly improved, and the maximum current density that can be electrolyzed for 25 hours at room temperature and without stirring was -5 mA cm-2. At 323 K, the current density reached -7.5 mA cm-
2. Furthermore, under the same condition but different surface area of the anode (4 cm2), Al electrolysis process could be achieved at a current density of -10 mA cm-2. Based on the above results, in subsequent experiments, we used a 60.0-40.0 mol% AlCl3−[C2mim]Cl as the electrolyte and set the cathode and anode areas to 2 cm2 and 4 cm2, respectively.
We conducted experiments on Al recovery under various conditions. Operations at 373 K resulted in improvements in the purity and electric power consumption rate of the recovered Al. The details will be introduced at the presentation.

References

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T. Tsuda, G. R. Stafford, and C. L. Hussey, J. Electrochem. Soc., 164, H5007 (2017), and references therein.

C. Wang and C. L. Hussey, J. Electrochem. Soc., 162, H151 (2015).

C. Wang, C. L. Hussey, et al., J. Electrochem. Soc., 163, H1186 (2016).

Figure 1