Oscillating currents stabilize aluminum cells for efficient, low carbon production*

In an aluminum smelter, a large electrical current (∼105A) is passed vertically through an electrolyte layer, where aluminum oxide is dissolved, and a layer of molten Al that lies below the electrolyte. The current can amplify resonant motions on the Al-electrolyte interface, producing a circulating traveling wave that can grow out of control . Thick electrolyte layers prevent this metal pad instability (MPI) but sacrifice efficiency because the electrolyte is a poorconductor. We simulated a TRIMET 180 kA smelter using MHD-Valdis, a high-fidelity software package common in the Al industry. When the layer thickness (anode-cathode distance, ACD) was 4.0 cm and the current was 180 kA, the MPI occurred, as expected. Including an oscillating current component with half-amplitude 19.8 kA and frequency 0.045 Hz prevented the MPI and replaced it with stable standing waves. Their dynamics caused electromagnetic forces that opposed the MPI. Oscillations at other frequencies that drove standing waves could likewise prevent the MPI. We also found that an oscillating current component can halt the MPI in progress. Additional simulations showed that stable operation without the oscillating current required a 4.3 cm ACD, but with oscillation, required only a 3.8 cm ACD — reducing heat power by 12% and overall powerby 4%. Similar reductions worldwide might save 34 TWh/year and reduce greenhouse gas emissions by 13 Mton/year.