On Stabilizing Aluminium Electrolysis Cells with Oscillating Currents
Ibrahim Mohammad, PhD Defense, Advised by Professor Douglas H. Kelley
Wednesday, January 11, 2023
Humankind produced 67.1 million metric tons of primary aluminium (Al) metal in 2021, nearly all via the Hall-Héroult electrolytic process in which electrical current liberates molten Al from alumina dissolved in an electrolyte inside an electrolysis cell. That year, Al production required 843 TWh of electrical energy, 3.4% of the world total. About 40% of the electrical energy does not produce any aluminium and is instead lost in the form of heat in the poor electrically conducting electrolyte where the alumina is dissolved. Thinning the electrolyte layer could decrease loss but has been limited by the Metal Pad Instability (MPI), which causes Al cells to slosh out of control if the electrolyte is not sufficiently thick. The MPI is a magnetohydrodynamic process that depends on a coupled resonance between hydrodynamic gravity wave modes, driven by the cell’s electrolytic current, and appears as a circulating traveling wave on the Al-electrolyte interface that grows exponentially.
Parametric instabilities can often be decoupled by introducing a new frequency that frustrates the resonance, so we hypothesized that adding an oscillation to the current would prevent the MPI. To test this hypothesis, I start by extending a mechanical analogue of the MPI to include an oscillating current component and show that the extended model is stabilized by the oscillating current. I also explore the stability of the extended mechanical model at different oscillation amplitudes and frequencies. Then, I show in high fidelity numerical simulations of a TRIMET 180 kA electrolysis cell that adding an oscillating current component prevented the MPI and allowed for stable operation at 11.6% lower electrolyte thickness than with steady current only. I analyze the Al-electrolyte interface evolution, through the fast Fourier transform and projecting the interface displacement onto the hydrodynamic gravity wave modes, to show that the oscillating current excites standing gravity wave modes that frustrate the MPI. Finally, I examine the impact of the oscillation current amplitude on stability and investigate the potential effects exciting the standing waves might have on the Al cell’s current efficiency. This novel method of stabilizing Al electrolysis cells could allow Al production at lower cost, with less energy, and a smaller carbon footprint.