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Fig. 6: Dissolution curve, industrial electrolyte and industrial grade
alumina. Small points: alumina concentration measured by the
alumina probe. Squares: bath samples analysed for alumina concentration
(Leco). Line: bath temperature
tored by the alumina probe in addition
to control analysis of bath samples
taken at regular intervals during
the runs. Figures 6 and 7 show the
dissolution behaviour in ‘standard’
bath and in low-melting bath respectively.
The results in Figure 6 show that
the sandy, industrial grade primary
alumina was completely dissolved
after approximately 30 minutes. This
is about 2.5 times longer than in the
comparable experiment in Figure
3, where only 2 wt-% alumina was
added.
When comparing the results shown
in Figures 6 and 7 there is a striking
difference in the initial dissolution.
The fine-grained alumina showed a
delayed response, and as expected it
did not dissolve completely, but levelled
off after 50 minutes at about 3
wt-%, which is close to saturation [1].
The undissolved alumina remained
partly as a sludge at the bottom of the
crucible and partly in suspension in
the bath.
The conclusion of these experiments
is the same as for the previous
tests, i. e. the dissolution rate in the
low-melting electrolyte was considerably
slower than the dissolution of
regular alumina in normal Hall-Heroult
bath. This is not surprising in
view of the following facts,
• Less driving force (lower
concentration gradient)
• Expected lower mass transfer
coefficient (due to lower
temperature)
• Greater tendency to clumping.
The tendency to clumping is probably
related to the ‘fineness’ of the pow-
ALUMINIUM · 9/2009
der. This also makes it more difficult
to handle (low fluidity, dusting, etc). If
it is desired to maintain a permanent
suspension of alumina in the electrolyte,
it would be preferable if it could
be achieved with a coarser-grained
alumina.
Visual observations
of alumina dissolution in
well-stirred, low melting baths
The background for these experiments
was the unexpected slow dissolution
observed for fine-grained alumina in
low-melting baths. One hypothesis
was that the alumina calcination temperature
could be an important factor,
since the fine-grained material had
been calcined at 1,600°C. The objective
of these experiments was to test
this hypothesis to see if the special
quality alumina was suited as a feed
material for
a l u m i n i u m
electrolysis.
The experiments
were
carried out in
an open furnace,
where
the low-melting
bath was
kept in a platinumcrucible.
The bath
composition
was as before
45 mol%
AlF 3 and 55
mol% NaF,
i. e. CR = 1.22.
The experi-
r e s e a r c h
Fig. 7: Dissolution curve for the first 60 minutes after addition of
industrial grade alumina to low-melting electrolyte. Small points:
alumina concentration measured by the alumina probe. Squares:
bath samples analysed for alumina concentration (Leco). Line: bath
temperature
mental set-up is sketched in Figure 8.
The melt was agitated by a propellershaped
platinum stirrer placed in the
centre of the crucible, operating at 254
rpm. A Pt/Pt10Rh (Type S) thermocouple
was also immersed in the melt,
as shown in Figure 8. The amount of
bath was 140 g, and the temperature
was 740 ± 3°C prior to each addition.
The experiments were carried out as
visual observation of the time needed
for a batch of 0.5 wt-% alumina to dissolve.
The melt became opaque immediately
after the addition. The time
counted from the moment of addition
till the bottom of the crucible became
visible again, was taken as a measure
of the time of dissolution.
Table 3 lists the various types of
alumina that were tested in these experiments.
An interesting parameter
gleaned from these curves is the ‘cut
size’ (d0.5 ), meaning that 50% of ➝
Fig. 8. Experimental set-up for visual study of alumina dissolution in
low-melting bath
55