Primary Production of Aluminium and Its Alloys in Molten Salts with Oxygen Evolving Anodes: Overview

Abstract

Carbon dioxide and other greenhouse gases are emitted during the production of primary aluminium through the Hall-Heroult process. Moreover, the process requires specific energy consumption of as high as 14.685 kWh/tonne of Al. There is an urgency to replace the existing process with a more environmentally and economically friendly process. One such process involves the use of inert anodes instead of traditional carbon anodes, where Al2O3 → 2Al + 1.5O2 occurs instead of 1.5C + Al2O3 → 2Al + 1.5CO2. The life span of inert anodes can be improved by using them at low operating temperatures, however, alumina solubility and dissolution rate decrease at low operating temperatures. Instead of using a traditional NaF-AlF3 molten solution, NaF-KF-AlF3 or KF-AlF3 could be used as low-temperature electrolytes. Introducing KF salt into the NaF-AlF3 mixture increases the alumina solubility and reduces the liquidus temperature of the molten salt. Another factor that affects the liquidus temperature of the mixture and alumina solubility in the electrolyte is the cryolite ratio , where M = Na or K (or both). It should be noted that the cells with AlF3-rich NaF electrolytes suffer with problems such as high cell voltage, low aluminium purity and unstable frozen side ledges. The addition of CaF2, MgF or LiF to the electrolyte mixture could improve the physical-chemical properties of the electrolyte. For instance, the addition of CaF2 to KF-AlF3 molten salts improves the electrical conductivity of the electrolyte. Specific energy consumption of the cell can be significantly reduced by using a vertical electrode cell with reduced anode-cathode distance (ACD). However, the major problem with low ACD is the contamination of reduced aluminium at cathode by anode products. This problem can be solved by using suspension electrolytes where the electrolyte contains excess about of Al2O3 in the electrolyte which obstructs the transportation of anode products into the aluminium diffusion layer on the cathode, thus maintaining high aluminium purity. In this paper, we summarise the developments obtained in designing the inert anode cells. We mainly focus on three main components: Inert anodes, wettable cathodes, and low-temperature electrolytes. The influence of additives on the properties of electrolytes will be discussed in detail. We also discuss the energy consumption, environmental impact and economic aspects of producing aluminium using vertical electrode cells.