MHD EFFECTS OF THE ALUMINA DISSOLUTION IN ALUMINIUM ELECTROLYSIS CELLS

The numerical modelling of high amperage aluminium electrolysis cells requires to account for a variety of their individual features: electric current and associated magnetic field distribution, ferromagnetic parts effect, the velocity field in the two overlying fluid layers, their interface deformation and wave motion, referred to as MHD instability. The normal electrolytic process is regularly disrupted due to the anode changes and the feeding of alumina (Al2O3) particulate material ensuring the continuity of the electrolytic aluminium production. The proposed modelling technique of alumina dissolution uses Lagrangian tracking for feed material particles of different sizes accounting for their inertia, drag in the turbulent flow, the electrolyte layer shape and the electromagnetic force at the location. The feed material initially forms rafts of frozen electrolyte+alumina, which gradually disperse and dissolve depending on the local flow, turbulent diffusion, and the instantaneous concentration level below saturation until reaching a quasi-steady concentration distribution. The concentration of solution is continuously depleted due to the electrolytic metal production. Modelling is applied to illustrate optimization of the commercial cell performance, while avoiding regions of low concentration responsible for increased fluoride gas release due to the low voltage anode effect.