Cathodic Behavior of Impurities in the Process of Aluminum Extraction by Soluble Anode Electrolysis

Recycling and re-using resources that could otherwise be damped as wastes have been promoted across numerous industries. Recycling promotes circular economy and has significant benefits for the environment, economy and community. With the increasing production of large amounts of aluminum globally, the generation of scrap aluminum alloys has also increased. To this end, developing effective strategies for recycling aluminum-containing, solid wastes has been promoted as a viable option for achieving efficient utilization of aluminum resources and environmental protection.

Most aluminum-containing solid wastes contains two main components: Al and Si. Carbon-based electrothermal reduction scheme is widely used for large-scale preparation of coarse Al-Si alloys. Although this technique has proved useful for recycling scrap aluminum at the industrial level, its application is limited by the presence of impurities on the cathode, which has remarkable effects on the purity and quality of the resulting products. Nevertheless, there have been limited attempts to characterize impurities at the cathode during the extraction of electrolytic aluminum. Additionally, the reasons behind the increase in the cell voltage and the corresponding reduction in the aluminum product purity and current efficiency are poorly understood.

Herein, a team of researchers from the Northeastern University School of Metallurgy: Mr. Yuyao Huang, Professor Yaowu Wang, Mr. Jinzhong Yang, Professor Yuezhong Di and Professor Jianping Peng investigated the characteristics and behaviors of impurities on the cathode during electrolytic aluminum extraction using coarse Al–Si alloy as the soluble anode. The mechanisms responsible for the increase in the cell voltage and decrease in the current efficiency and aluminum purity over time were examined. Their work is currently published in the peer-reviewed Journal of the Electrochemical Society.

The research team showed that in the presence of molten salt electrolyte, both silicon and titanium elements were relatively stable, with solubilities of about 7.5 ×10^-2 mg g^-1. The precipitation of the elements added as impurities was in the order Fe > Al > Si > Mn > Ti >Mg > Ca. An increase in the impurity concentration of these elements in the molten salt electrolyte increased the cell voltage, with the greatest effect observed in Ca and Mg elements. The Ca and Mg continuously entered the anode into the molten salt electrolyte via chemical dissolution. However, the impurities that failed to dissolve under normal operating conditions entered the electrolyte and precipitated into the cathode, resulting in a cell voltage increase, reduction in aluminum purity and current efficiency.

In summary, the authors reported the cathode behavior of impurities during the aluminum extraction process and the underlying mechanism. They had few important recommendations to improve the aluminum extraction process and limit the effects of Mg and Ca. Strict control over the electrochemical dissolution of Si and Fe in the soluble anode could help improve the purity of the aluminum product. It was necessary to adjust the Ca and Mg in the anode to a low level during the production of the soluble anode to enhance the stability of the cell voltage after prolonged extraction. Finally, replacing the electrolyte before the voltage exceeds the normal range was important. In a statement to Advances in Engineering, Professor Yaowu Wang, the corresponding author explained the findings would improve the recycling of aluminum-containing solid wastes.