Optimization of Operating Parameters of a Carbon Free Aluminum Electrolysis Cell with Vertical Inert Cu/Ni/Fe Anodes
Student: Daníel Þór Gunnarsson
Year: 2023
Supervisors: Guðrún Arnbjörg Sævarsdóttir, Guðmundur Gunnarsson
Abstract:
By replacing the carbon anodes in aluminum production with inert anodes, the evolution of carbon dioxide would be replaced with oxygen evolution. The main setback of inert anodes is their wear rate in the corrosive electrolyte used to dissolve alumina. The objective of this thesis was to optimize operation for a vertical inert electrode cell using Cu-Ni-Fe anodes and TiB_2 cathodes in a low temperature KF-NaF-AlF_3 electrolyte.
A total of eleven experiments were conducted in this thesis: seven in a 60 A aluminum production laboratory cell, and four in a see-through furnace. The goals for the 60 A cell experiments can be split into three categories: experimenting with higher current density, running electrolysis for a longer duration, and running electrolysis close to the electrolyte's liquidus. The see-through cell experiments were made to study the bubble formation in oxygen evolving aluminum electrolysis.
The high current density experiment was conducted at 1.30 A/cm^2 and resulted in a current efficiency of 91%. Other experiments have been conducted at 0.70-0.80 A/cm^2 with good results being around 80% current efficiency. The longest period of time an electrolysis ran was for 50 hours, limited by the alumina crucible used to contain the electrolyte. That experiment resulted in the best aluminum purity of 99.21% with the highest impurity being Si. When looking only at anode constituents, the purity of aluminum was 99.84%, pointing to a very low wear rate of the anodes. SEM-EDS scans revealed the oxide layer on the anode had grown significantly. The liquidus of the electrolyte was 28°C lower than expected in the low liquidus experiments due to dissolved alumina. More experiments are required to see if an electrolysis can run stably near the liquidus temperature. The oxygen bubbles measured in the see-through furnace ranged from 0.70-1.06 mm in diameter, growing with decreased current. Contact angles could not be measured due to technical difficulties. The anodes were observed to catastrophically corrode when submerged at equal to or less than 1.6 V.