| dc.description.abstract | Alumina is the raw-material used for aluminum production in the Hall-Héroult process. Though the process is used for several years, some phenomena is not fully investigated yet. One of those is the formation of floating agglomerates, known as rafts, when alumina is added to the cryolitic bath in the cell. An efficient feeding of alumina will reduce emissions and the energy consumption in the process.
Experiments are carried out in a lab furnace, where the goal is to create a reproducible method that investigate the formation and dissolution of rafts as a function of time, as well as changes in structure. Alumina is added into molten cryolite, and samples are pulled out at time intervals between 30 and 300 seconds. The experiments shows that at 970 degrees Celsius, rafts will quickly form, and then slowly dissolve again with a constant rate of 0.8 g/min. Lowering the temperature resulted in very different results, illustrating the effect heat transfer has on the dissolution process. The effect of this should be investigated further. Pores where found in the samples, and the origin of those should be investigated, as pores will give extra buoyancy to the raft, thus increasing the flotation time.
In parallel, Computational Fluid Dynamics is used to create a model with the same dimensions as the experimental one. Simulations are done in OpenFOAM with a multiphase solver at isothermal conditions. These simulations show that air can get trapped as the alumina fall into cryolite. This can have an important effect on the structure of the raft as well as its floating ability, and should be investigated further | |
