The Formation and Disintegration of Rafts from Different Aluminas and Fines

Raft formation is a frequently encountered problem during alumina feeding in the Hall-Héroult-process and will delay alumina from being dissolved into the melt. The mechanisms for the formation and disintegration of rafts are however not thoroughly understood yet. The current study investigates the dissolution behavior and raft structure from three different types of secondary alumina in a lab cell, with a particular attention to effect of fines, and involves both sampling of rafts and video recordings of the feeding. The mass loss rate was calculated to vary between −1.57 and −0.42 g min−1 for regular bulk alumina, and −1.15 and −0.06 g min−1 for fines. Rafts created from bulk alumina were flat with a distinct bulge or crater placed in the center of it, while rafts created from fines had a pellet-shaped structure and traces of undissolved alumina in the middle. The observed structure is due to the initial spreading of powder, confirmed by video recordings.publishedVersio

Alumina Dissolution in Cryolite Melts - Formation and Behavior of Rafts

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 furthe

CFD Modelling of Alumina Feeding

The dissolution and distribution of alumina in molten cryolite bath is a complex process, involving heat and mass transfer, phase transition and dynamics for a particle population with variable size and properties. Although single particle models can describe essential features of the process, they necessarily fail to capture features involving the interaction between particles (i.e. collisions and adhesion) and detailed coupling to the flow field, for which computational fluid dynamics (CFD) is needed. Several strategies and assumptions have been proposed in the literature, focusing on separate phenomena of relevance. Coupled models considering the full history of alumina particles have however not yet been developed. In the current work we investigate and review recent developments in coupled CFD particulate flow, aiming for general guidelines for implementation and use, with applications to both the gas-solid flow between the feeder and bath surface, and from the bath surface to dissolved alumina

Lab Scale Experiments on Alumina Raft Formation

During feeding of alumina into a Hall-Héroult cell, rafts floating on the bath surface may be formed. In this study, rafts were created in a laboratory furnace by adding 4 g secondary alumina in industrial bath. Samples were withdrawn from the bath in a time interval between 30 and 300 s. The experiments show that at 970 ∘ C, rafts will be formed within 30 s, and then slowly dissolve again with a constant rate of 0.8 g/min. Pores were found in the samples, giving extra buoyancy to the raft, thus increasing the floating time. Same experimental setup was used to investigate the effect of preheating of alumina, where it was found that coherent rafts will form up to at least 500 ∘C .acceptedVersio

The micro- and macrostructure of alumina rafts

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Lab Scale Experiments on Alumina Raft Formation

During feeding of alumina into a Hall-Héroult cell, rafts floating on the bath surface may be formed. In this study, rafts were created in a laboratory furnace by adding 4 g secondary alumina in industrial bath. Samples were withdrawn from the bath in a time interval between 30 and 300 s. The experiments show that at 970 ∘ C, rafts will be formed within 30 s, and then slowly dissolve again with a constant rate of 0.8 g/min. Pores were found in the samples, giving extra buoyancy to the raft, thus increasing the floating time. Same experimental setup was used to investigate the effect of preheating of alumina, where it was found that coherent rafts will form up to at least 500 ∘C

Alumina feeding and raft formation: Raft collection and process parameters

Alumina, alongside with electricity and carbon, is the raw material used for production of aluminium in the Hall-Héroult process. An efficient dissolution process is important to acquire stable conditions for the cell, resulting in lower energy consumption. Under certain conditions, the alumina will not dissolve upon addition but remains afloat on the bath surface as a so called raft. The conditions under which the rafts form are still not fully understood, although it is likely that their behaviour is influenced by operational conditions which in turn depend upon bath and alumina properties. In order to obtain more knowledge on the conditions for raft formation, an industrial measurement campaign was performed at Alcoa Mosjøen in which raft behaviour was recorded alongside collection of bath and alumina samples as well as the rafts themselves. The current paper describes the procedure utilized for data collection together with an analysis of bath and alumina properties, aiming to correlate these with raft flotation times. Raft floating times were found to vary between 5 and 140 s during normal operating conditions