Feeding of Alumina in Molten Cryolite Bath

Abstract

Alumina is the principal raw-material used for the production of aluminum in the Hall- Héroult process and is fed to the electrolytic bath in batches regularly. Maintaining a stable concentration in the bath is important in order to achieve an efficient process, hence requiring alumina to disperse, dissolve and distribute fast. However, bath might freeze around the particles, creating rafts that will prevent alumina from dissolve. A better understanding in how rafts form and disintegrate is necessary in order to improve the feeding process. Studies have been conducted in both lab and industrial scale, and with numerical modeling.

In an industrial cell, the concentration of Hydrogen Fluorides (HF) was measured continuously over 43 consecutive days, in order to establish dependencies between HF- emission and alumina feeding, as well as operational conditions. The measurement revealed that HF-concentration is higher when the feeding frequency is high, and it increases rapidly after each feeding, followed by its slower decline. The longer decline might be caused by the formation of rafts, where HF can get trapped inside the structure.

Feeding of alumina was simulated in a model at room temperature, where bath and alumina were respectively replaced by water and organic particles. The effect of particle size distribution, temperature difference between particles and liquid, and gas induced convection were investigated. All of the mention parameters had a significant effect on the raft floating time, where particle size had the highest impact. Halving the average particle size resulted in an almost fivefold increase on floating time.

A method for creating and extraction of rafts in a lab cell has been developed, and further adapted for recordings from above. The effect of alumina temperature, chemical changes due to gas treatment, water content, Lithium Fluoride in the bath and fines have on raft formation was studied.

When 4 g secondary alumina is added to the melt, a raft is formed, with mass loss rates between 0.8 and 1.6 g min−1. They were found to have a porous structure in the middle and flakes of frozen bath around them, with an average porosity of 8.2%. Rafts formed from primary alumina had a lower porosity, 0.8 % on average, hence indicating that the pores in rafts are formed due to release of components added to powder during the dry scrubbing process.

Increased alumina temperature will decrease the amount of bath freezing around the dose, and rafts were seen to be formed up to 500-600 ◦C.

Rafts created from fines were smaller in size, but their mass loss was lower compared with bulk, between 0.1 and 1 g min−1. Video recordings confirmed that the lower mass is due to a poorer spreading of powder, hence resulting in lesser bath freezing.

In parallel, Computational Fluid Dynamics (CFD) in OpenFOAM was used to develop a continuous multiphase framework based on the Volume of Fluid (VOF) method for simulating the alumina feeding process. An immiscible incompressible multiphase model accounting for transfer of mass, momentum and energy was developed, and two models were implemented and verified.

A framework for modeling solidification was developed, modeling the solidified phase as a fluid with (artificially) high viscosity. The model was found to exhibit desired effects when the raft was considered to be a rigid object, with stronger damping effect for thicker layer of freeze. When alumina is assumed to be a Newtonian phase, the effect is not visible, as the spreading of the dose will inhibit a sufficient large layer of freeze to be formed.

Modeling of the alumina particles was realized by the µ(I)-rheology, which was implemented as a viscosity model. A parametric study was conducted, where an alumina dose collapsed on a flat surface. The study identified the rheology parameters µ2 and I0 to be of high importance. They are currently not been measured for alumina, and further experiments should quantify their values.

Applying µ(I)-rheology in a case where alumina is added into bath of cryolite showed that the dose spreads out and creates a flat structure with nonuniform thickness. The model also allows for pieces to disperse into the melt of detach from the main dose, which is in accordance with what has been observed in lab experiments.

The models were also successfully coupled together, where the raft shape as described above was formed and frozen bath aided in holding the structure together.