Towards a coupled multi-scale, multi-physics simulation framework for aluminium electrolysis

Abstract

Aluminium metal production through electrolytic reduction of alumina in a cryolite bath is a complex, multi-physics, multi-scale process, including 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 occur. Moreover, 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 footprint 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 where all relevant physics are included in a single mathematical model of this large degree of freedom system. In order to overcome these challenges, different modelling approaches have been established in ANSYS FLUENT; 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. The current paper outlines each of the approaches and presents how the coupling between them can be realized in a complete framework, aiming to provide new insight into the process. 2 Highlights• Scales of the process in length and time are identified for aluminium electrolysis.• Four modelling approaches describing essential features are presented.• Coupling between modelling approaches is discussed and demonstrated.

CFD Modelling of Alumina Feeding

“…As pointed out in the pioneering work of Kaszas et al [4] -raft formation and its effects have not been thoroughly investigated yet, for instance related to the partitioning between particles which disperse and particles which agglomerate. Advanced models based on computational fluid dynamics (CFD) can potentially accelerate the understanding of these phenomena, as demonstrated for anodic gas evolution [7,8] and for alumina precipitation in the Bayer process [9,10] over the last decade…”

“…Particular interest to alumina has been given to mixing and distribution of alumina under the influence of the different forces present, e.g. bubble induced motion and MHD [7,14,15,16]. Typically, alumina is not treated as a separate phase but as a specie present in the bath -i.e.…”

“…Both are based on an Eulerian frame-work with special sub-routines for identification and evolution of the free interface. The various free surface methods have been studied and used extensively due to their many industrial applications such as bubble flow and metal heaving [7], and solidification [25], the latter of which being of particular interest for the current application.…”

A heat and mass transfer problem for the dissolution of an alumina particle in a cryolite bath

“…For example, Hofer [11] uses a finite element formulation for a model for the Hall-Héroult cell in which the fluid velocity and pressure, the concentration of dissolved alumina, and the position and sizes of the alumina particles are tracked, in order to determine the influence of the flow on the dispersion of both the alumina particles and the dissolved alumina. The finite element framework has also been used by other authors to investigate various aspects of the Hall-Héroult cell [12][13][14]. In order to be computationally tractable, all these simulations rely upon sub-grid models which in turn depend upon closure laws formulated on a rigorous mathematical basis.…”

Explanation of Bubble Nucleation Mechanisms: A Gradient Theory Approach

“…13,14 In the general analysis of electrochemical systems, the surface tension between the gas bubble and the surrounding liquid is typically considered to be constant. 11,15 The presence of dissolved gas in the liquid has been proposed to change this surface tension by Lubetkin, 16 and observed in molecular dynamics simulation. 17 The change in surface tension is critical in electrochemical systems where supersaturation drives the nucleation.…”