Coupled multi-scale, multi-physics simulation framework for aluminium electrolysis

Aluminium metal production through electrolytic reduction of alumina in a cryolite bath is a complex, multi-physics, multi-scale process, containing magneto-hydrodynamics (MHD), bubble flow, thermal convection, melting and solidification phenomena based on a set of chemical reactions. Through interactions of the different forces applied to the liquid bath combined with the different time and length scales, self-organised fluctuations occurs- In addition the MHD behaviour causes a complex metal pad profile and a series of surface waves due to the meta-stable condition of the metal / cryolite interface. The large aspect ratio of an industrial cell, with a foot print of 20 by 4 m and at the same time having dimensions approaching just 30 mm of height for the reaction zone, prevents an integrated approach for mathematical modelling of this large degree of freedom system. As a consequence, four different modelling approaches have been established and interlinked. Three models are used to predict details of specific physics: one to predict the electro-magnetic forces and hence the metal pad profile, a second that resolves details of the local bubble dynamics around a single anode and a third for the full cell bath flow. Results from these models are coupled to allow integration of the different phenomena into a full cell alumina distribution model. Even if several days of CPU time is needed, this modelling framework gives new insight into the process and identifies significant control parameters.