Modelling of the Aluminium Electrolysis Process: Feeding and Dissolution of Alumina Particles

Abstract

The present numerical study describes the feeding and dissolution behaviour of alumina particles in the electrolytic bath of industrial Hall-Héroult cells. The dissolution rate is controlled by thermal and chemical conditions as well as the transport and mixing of particles in the electrolyte. The model presented is implemented in the OpenFOAM® framework using the Lagrangian approach for the coupling of continuous (electrolyte and liquid aluminium) and dispersed phase (alumina particles). The model solves the particle motion, mass transfer from dispersed to continuous phase, the energy equation, and the transport of dissolved concentration. Moreover, the influence of thermal conditions and turbulent mixing due to gas bubbles arising under the anodes is considered by the model. The velocity field is adopted from a previously executed magnetohydrodynamic (MHD) simulation. With the help of the model developed, various studies are carried out. The focus corresponding to the increased flexibility of electric power consumption is on the thermal behaviour during cold operating conditions of the electrolysis cell. It is important to ensure that all the alumina feed is dissolved. The sinking of undissolved alumina to the bottom of the cell must be prevented as it impairs cell efficiency. The results show that the dissolution rate and total dissolution time depend on the bath temperature. A greater effect is observed for the bath superheat defined by the difference of bath temperature and liquidus temperature of the bath. Furthermore, the influence of process parameters like grain size and feeding masses are studied in detail.