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e s e a r c h
On the dissolution of alumina in a
low-melting electrolyte for aluminium production
S. Rolseth, J. Thonstad, H. Gudbrandsen, K.S. Osen, and J. Kvello, Trondheim
In studies of inert anodes for
aluminium production, so-called
low-melting electrolytes have been
tested, operating at temperatures
as low as 750°C. Dissolution of
alumina may then become critical,
because of lower solubility
and lower rate of dissolution. The
rate of alumina dissolution was
tested in a particular electrolyte
operating at 750°C, using a very
fine-grained alumina as well as industrial
grade alumina. The rate of
dissolution was markedly slower
in the low-melting electrolyte, and
the fine-grained material dissolved
more slowly than commercial
grade alumina, because it showed
greater tendency to agglomeration
and because it was calcined
at high temperature. However,
crushed samples of commercial
alumina also dissolved more slowly
than the normal grade.
Introduction
In the conventional Hall-Héroult process
for aluminium electrolysis, the
cryolite-based (Na 3 AlF 6 ) electrolyte
normally contains 10-13 wt% excess
AlF 3 , 3-6 wt% CaF 2 and 2-4 wt% Al 2 O 3 ,
operating at about 960°C. In modern
cells the alumina feeding is performed
by so-called point feeders, whereby
alumina is fed frequently in small
batches. This ensures rapid and usually
trouble-free supply of alumina to the
molten electrolyte (often called bath).
When trying to replace the carbon
anode by oxygen-evolving, so-called
inert anodes, it is desirable to lower
the electrolyte temperature in order
to reduce the corrosion rate of the
anode material. This is achieved by
increasing the content of excess AlF 3 ,
in some cases as far as ~37 wt% AlF 3
(55 mol% NaF, 45 mol% AlF 3 , molar
ratio NaF/AlF 3 , CR=1.22), allowing
an operating temperature of about
750°C. At the same time the solubility
of alumina decreases from about
10 wt% to about 3 wt% [1]. Thereby
the rate of dissolution of alumina may
become critical.
When a batch of alumina is being
fed to aluminium cells, rapid and
complete dissolution is desirable in
order to control the concentration
of alumina in the bath and to avoid
so-called anode effects and to avoid
accumulation of undissolved alumina
(sludge). Dissolution tests performed
in cryolite melts at around 1,000°C
have shown that when the alumina
gets effectively dispersed in the bath,
the dissolution is very rapid, i. e. it is
completed in less than 10 s [2], but
the dissolution process is normally
slowed down because the alumina
has a tendency to form clumps/aggregates.
When cold alumina is added as
a batch to the molten bath, it is difficult
to achieve complete dispersion
of the alumina particles. When hitting
the bath, the alumina spreads out,
and bath freezes on to the alumina.
This results in the formation of flakeshaped
agglomerates. The formation
of such agglomerates strongly reduces
the contact area between alumina and
bath compared to what would be the
case if all the alumina grains were effectively
dispersed in the bath.
A large contact area between alumina
and bath is obviously important
in order to ensure rapid heating and
dissolution of the alumina, since the
dissolution process, which is strongly
endothermic, has been found to be
mass transfer controlled [3]. The importance
of this becomes even more
evident if the alumina is added to a
low-melting bath rich in aluminium
fluoride, where the alumina solubility
is lower [1].
One remedy would be to establish
a large contact area between alumina
and bath. Beck and Brooks [4] have
patented a process where very finegrained
alumina is used in low-melting
baths, keeping the particles in
suspension in a bath agitated by gasinduced
convection.
The purpose of the present work was
to study the dissolution rate of finegrained
alumina when added batchwise
to a low-melting bath with the
composition given above. The results
are compared with the well-documented
behaviour of regular alumina
in conventional baths at around
960°C.
experimental
As mentioned above the process of
alumina dissolution in cryolite-based
melts involves the formation and
break-up of agglomerates. In a laboratory
cell for studies of alumina dissolution,
the convection pattern in the
melt should be as close as possible to
that existing in industrial cells. That
is difficult to achieve on a laboratory
scale. In industrial cells the anode gas
escaping up along the anode side sets
up strong convection, making the bath
move upwards close to the anode side
and down in the middle of the channel
between two anodes. At the same time
the bubbles, when reaching the bath
surface, create an undulating surface
with bath splashing over newly
formed alumina agglomerates.
effect of convection in the bath
The objective with the laboratory
set-up was to combine the effects of
convection and an undulating bath
surface. Convection in the bath was
ensured by mechanical stirring of the
melt with an impeller. Bubbling of argon
gas through the bath was intended
to simulate the surface effect created
by escaping gas bubbles. A description
of the gas stirrer arrangement is
given elsewhere [5].
The inner diameter of the crucible
was 20 cm. The amount of bath was
6500 g, and the composition of the
industrial type electrolyte that was
tested initially was 10 wt% AlF 3 , 5
wt% CaF 2 , and the initial Al 2 O 3 concentration
was 2 wt%, the balance be-
52 ALUMINIUM · 9/2009