The current recycling process for aluminum is predominantly cascade recycling with grade down of quality. After repeated cascade recycling, aluminum accumulates many alloying elements and is disposed of in landfills. We have proposed the application of electrorefining to aluminum recycling, which allows the removal of alloying elements of aluminum. Al-Si alloys are one of the widely used in Al alloys, but the alloys with different Si concentrations and microstructures have been produced. To investigate anodic dissolution characteristics depending on the alloying elements and alloy structure, electrorefining experiments were conducted using casting and cold-rolling Al-Si alloys.

AlCl₃-NaCl-KCl molten salt was prepared at 423 K in the atmosphere. The anode of casting Al-Si alloys were ADC12 alloy (82.8 wt.% Al -10.6 % Si -2.8 % Cu -1.3% Fe -0.9% Zn) and AC4C alloy (90.2 wt.% Al -7.38 % Si -0.6 % Mn -0.5% Fe), cold-rolling Al-Si alloy was Al-11%Si alloy (89.2 wt.% Al - 10.7 % Si). Cu plate is used as cathode. Inter-electrode distance between anode and cathode is 2 cm. Electrorefining experiments were carried out with current densities of 10 mA cm^-2 and electrolysis time of 50 hour.

After the electrorefining, surface color of the Al-Si alloys changed from metallic to dark and Si concentration of the surface increased compare with before electrorefining by XRF analysis. From cross-sectional SEM image of Al-Si alloys, mesh structure formed on the surface of ADC12 alloy and AC4C alloy, and did not form on Al-11%Si alloy after rinsed by distilled water. From the EDS analysis and XRD measurement, these surface layers were mainly crystalline Si. These layers are considered to form by aluminum dissolution from ADC12 and AC4C casting alloys. From the EPMA analysis, Cu and Fe are also detected on surface layers. And these surface layers were also found to be layered, with the bulk Si microstructure remaining intact. On the other hand, cold-rolling alloy has a finely and uniformly distributed Si structure due to cold-rolling in the manufacturing process, and the surface layer generated by electrorefining was thought to have been washed away with distilled water because of its low adhesiveness. However, the XRD analysis of the recovered washed-out material showed crystalline Si, suggesting that the same dissolution behavior occurred in the cold-rolled alloy as in the casting alloys.

In conclusion, more than 99% Al was recovered from casting and cold-rolling Al-Si alloys at the cathode in electrorefining. At the anode, Al of matrix preferentially dissolves, and Si, Cu and Fe elements remained on the surface, forming surface layers. The surface layer, which has a porous structure, did not offer significant resistance to dissolution of Al from the matrix, and electrorefining proceeded steadily.

Acknowledgement

Part of the experiments reported are supported by the New Energy and Industrial Technology Development Organization (NEDO).