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veal large differences in alumina concentration.
The velocity field helps to homogenise
the alumina concentration.
The above results depend strongly on the
alumina diffusion coefficient that was considered
to be 0.5 m 2 /s from the Reynolds
stress tensor. When the diffusion coefficient
is reduced, the velocity field becomes more
important. Fig. 5 shows how reducing the
diffusion coefficient impacts on the lowest
local alumina concentration in the bath. The
velocity field becomes more important, but
the global alumina concentration distribution
remains mainly defined by the diffusion
coefficient. Bubbles and Lorentz force field
act to produce much the same effect as an increase
in the alumina diffusion coefficient.
They play a key role due to the much faster diffusion
(the feeding in a cell is not continuous).
Although the maximum difference of alumina
concentration in the bath is clearly dependent
on the diffusion
coefficient,
the distribution itself
is not affected
in its shape.
Obviously, a
higher alumina diffusion
coefficient
leads to a lower
difference of concentration
in the
bath. Conversely,
greater distances
from the feeders
lead to higher differences
in concentration.
Moreover,
the situation
must be analysed
as function of time
and of the mass of
alumina fed at each
dump. The software allows us to determine
the highest difference of alumina concentration
in the bath for any type of cell design and
feeding strategy. It also considers the current
load in the cell, since Faraday’s law is satisfied
at the anode and cathode.
Fig. 5: Lowest alumina concentration function of the alumina diffusion coefficient
Example: mobile gas treatment system and
furnace covers with gas exhaust
Conclusions
A model for the velocity field in presence of
MHD and bubbles has been developed. The
velocity field is used to determine the evolution
of the alumina concentration using a nonstationary
convection-diffusion model. This
equation takes into account the feeding and
the Faraday law at the anodes and cathode.
The application to an existing cell with two
point feeders demonstrates the following:
• The local alumina concentration can vary
by up to 2-5% (depends on the bath composition)
• The pattern of
the alumina distribution
is not significantly
affected
by the velocity
field, but is mainly
Revamping solutions determined by the
diffusion process
and tailor made • The velocity
reduces the time
aluminium melting
needed to reach
and holding furnaces the stationary
state for the alumina
concentration
by a factor
of two when compared
to diffusion
only, and it therefore plays an important role
in the cell
• It would be of great interest to perform
measurements to validate the macroscopic
alumina diffusion coefficient
References
[1] O. Kobbeltvedt, S. Rolseth and J. Thonstad:
The dissolution behaviour of alumina in cryolite
bath on a laboratory scale and in point fed
industrial cells. Department of Electrochemistry,
Norwegian Institute of Technology, Trondheim,
Norway
[2] R. G. Haverkamp. PhD Thesis, University
of Auckland (1992).
[3] O. Kobbeltvedt, S. Rolseth and J. Thonstad:
On the Mechanisms of Alumina Dissolution
with relevance to Point Feeding Aluminium
Cell, Light Metals, TMS, 1996, pp.421-427
[4] R. von Kaenel, J. Antille, M. V.Romerio and
O. Besson, Impact of magnetohydrodynamic
and bubble driving forces on the alumina
concentration in the bath of a Hall-Héroult
cell, to be published in Light Metals, TMS,
2013.
Acknowledgement
The authors would like to thank Prof. Olivier
Besson from University of Neuchâtel and Prof.
Michel Romerio from The Swiss Institute of
Technology who developed the theory and
software.
Authors
René von Kaenel received his diploma of physicist
from The Swiss Federal Institute of Technology
Lausanne (EPFL) with a specialisation in plasma
physics before working for ICL in London and
specialising in computer science. In 1981 he joined
Alusuisse and became the head of the modelling
activities for smelting technology. In 2000, he received
the title of Electrolysis director in the new
Alcan organisation and further supervised Alcan’s
modelling activities. Since 1981 he has participated
in many smelter modernisation projects all over the
world, leading to large productivity increases. He
has published many articles on electrolysis cells,
casting processes and inert anode technology. In
2004 he created Kan-nak Ltd., a specialised company
for the optimisation of processes, in particular
of the Hall-Héroult process.
Dr. Jacques Antille obtained a degree in Physics at
the University of Lausanne in 1978 and his PhD at
the European Centre of Nuclear Research (CERN)
in 1984. Soon after he joined Alusuisse Technology
and Management Ltd and worked on modelling
projects of the Hall-Héroult process and casting
processes. In 2004 he joined Kan-nak S.A. where he
leads the magnetohydrodynamic studies to optimise
the electrolysis process as well as all measurement
techniques.
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