Characteristics of Anodic Dissolution for Al-Cu Alloys in EmImCl-AlCl3 Ionic Liquid
Authors: Nunomura Junji , Matsushima Hisayoshi , Kyo Yoshihiko , Kojima Yoichi , Ueda Mikito 
Published: November 2022

Recycled Al obtained by re-melting process used the primary Al can be produced with 3-5% of the energy required for the electrolysis. However, the Al recycled by this method contains various elements as impurities, resulting in lower quality. After this repeated recycling, the Al eventually becomes scrap that is difficult to recycle and is landfilled. As a purification process, we propose an electrorefining method using scrap Al as an anode.
Copper (Cu) is one of the major additive elements in aluminum. The Cu should be removed as far as possible in the recycling of Al scrap, since this alloy corrode from Al-Cu compounds precipitated at the grain boundaries. As Cu is a more noble element than Al, when Cu dissolves from the anode into the electrolyte due to an increase in the anode potential, the Cu ions are rapidly reduced at the cathode and electrodeposited as metallic Cu. In order to remove copper from Al scrap by electrorefining, the anodic dissolution of copper in scrap Al must be avoided. In this study, the anodic dissolution behavior of Al-Cu alloys on the metallurgical structure was investigated using Al-Cu alloys with controlled copper concentration and size of intermetallic compounds.
Al-1.0, 3.0 and 5.0 Cu casting alloy plates and cold-rolled plates were used as anode specimens. The preparation of the ionic liquid and all electrochemical measurements were carried out in Ar filled glove box. The ionic liquid electrolyte was prepared by mixing 1-ethyl-3-methyl imidazolium chloride (EmImCl) and AlCl3 in a molar ratio of 1 : 2. Electrochemical measurements were carried out in a three-electrode type cell using various Al-Cu alloys as working electrodes, Pt plates as counter electrode, and , Al wires as reference electrode. The electrolyte was heated at 323 K by hot plate stirrer, and each electrode was connected to a potentiostat for anodic polarization measurement and constant potential electrolysis. Anodic polarization curve was measured from the immersion potential to 1.5 V vs. Al/Al(III) with scanning rate of 1 mV s-1. Constant potential electrolysis was performed at potentials from 0.3 to 1.2 V, and amount of electric charge densities of 0.3, 3.0, and 30 C cm-2 were applied. The specimens after the constant potential electrolysis were rinsed by distilled water and dried, then observations of appearance, XRD measurement, and FE-SEM observation were carried out.
In the anodic polarization measurements of various Al-Cu alloys, peaks of anodic current density were observed at around 0.3 V and 0.8 V in casting alloys and Al-5.0Cu alloy with Al2Cu on the surface. Constant potential electrolysis was then carried out at potentials of 0.3, 0.8 and 1.2 V with electric charge density of 30 C cm-2 in order to investigate the dissolution behavior of Al-Cu alloys depending on the electrolysis potential. Al-5.0Cu casting alloy and cold-rolled plate after constant potential electrolysis were analyzed by SEM-EDS. Al in the matrix phase was preferentially dissolved then Al2Cu was appeared on the surface at 0.3 V. Dissolution of Al2Cu and Al in the matrix phase was occurred at higher than 0.8 V.
In summary, the potential dependence on the anodic dissolution behavior of the metallurgical structure of Al-Cu alloys in EmImCl-AlCl3 ionic liquid has been clarified. By controlling the anodic dissolution potential, the dissolution of Cu in Al2Cu into the electrolyte can be suppressed, which is expected to further improve the purity of the electrorefined aluminum.

Acknowledgement

Part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO). We would like to express our gratitude to all parties concerned.