Abstract

The main objective of the Hall-Héroult process, which is applied to produce primary aluminium, is the electrolytic reduction of alumina (Al2O3) dissolved in an electrolyte. The electrolyte mainly consists of cryolite (Na3AlF6) and is present in a molten condition at around 960°C. Alumina, as the raw material of the process, is permanently fed to the bath. After the injection, alumina particles have to reach the temperature of the surrounding bath, before they start to dissolve. The dissolution rate is controlled by thermal and chemical conditions as well as the transport and mixing of particles and electrolyte. The present model describes the feeding and dissolution of alumina particles to the bath. It is implemented in the OpenFOAM® framework using the Lagrangian approach for the coupling of continuous (electrolyte and liquid aluminium) and dispersed phase (alumina). The model solves the 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 because of gas bubbles arising under the anodes, is considered by the model. The velocity field is adopted from previously executed MHD simulations (described in part II). With the help of the developed model, 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 has to be ensured, that the full amount of fed alumina is dissolved. The sinking of undissolved alumina to the bottom of the cell has to be prevented as it has a negative impact on cell efficiency. The results show a dependency of dissolution rate and total dissolution time on the bath temperature. Even more important is 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, chemical conditions of the bath, velocity distribution, and turbulent mixing are studied in detail.