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e s e a r c h
the mass has particle diameter less
than d 0.5 ).
The results showed that there was
a marked difference in dissolution
time of the various aluminas; in fact
more than one order of magnitude.
The best dissolution behaviour was
observed for the ‘normal’ metal grade
alumina, as shown in Figure 9. In this
case relatively short dissolution times
were observed, similar to those previously
observed for commercial aluminas
in cryolite. The longest dissolution
times were observed for crushed
MG alumina, calcined at 1,600°C,
where the dissolution time was of
the order of 5 to 20 minutes in the
alumina concentration range of 1 to 2
wt-% (see. Fig. 9). Even crushed MG
alumina pre-dried to 300°C, showed
dissolution times 4 to 10 times higher
than the ‘normal’ grade alumina in
this concentration range.
In view of the observations made
in these experiments it appears that
the fineness of the alumina is a determining
factor for the dissolution
process. A high content of the alpha
phase seem to have an additional
detrimental effect, i. e. it increases the
time of dissolution. It has previously
been found that high alpha alumina
dissolves somewhat more slowly than
the gamma phase alumina in cryolitic
melts at 1,030°C [2] and it is possible
that this difference is enhanced in lowmelting
baths, where the solubility of
alumina is lower. However, no simple
relationship could be found between
Type, description Term d 0.5 /µm
Metal grade, commercial alumina MG 82.95
A-152, fine, 1 µm, highly calcined alumina, from Alcoa A152 1.86
Crushed MG * alumina, calcined at 1600°C CMG 1600 4.51
Crushed MG alumina, pre-dried at 300°C
* MG – Metal Grade
CMG 300 5.45
Table 3: Materials tested and cut sizes (see text)
these parameters and the time of dissolution.
For example, the slowest dissolving
alumina in these experiments
was the CMG 1600 material. Table 3
shows that 50% of the mass of this material
has particles with diameter 4.5
µm or less (d 0.5 =4.507 µm), compared
to 1.86 µm for the A152 material.
These materials have both been calcined
at 1,600°C and have thus been
converted to 100% α-alumina, but the
finer A152 (d 0.5 =1.86) dissolves more
rapidly than the coarser CMG 1600
(d 0.5 =4.5 µm).
conclusion
It can be concluded from this investigation
that if the only selection
criterion is the rate of dissolution in
low-melting baths, the normal industrial
grade alumina is the best choice.
The main concern will probably be
the ability to operate the bath at near
saturation concentration with respect
to alumina. Hence, the problem of
maintaining slurry and avoiding forming
sludge, i. e. alumina deposits, must
be given high priority, and operating
with fine-grained alumina might be
Fig. 9: Results from three parallel runs with ‘normal’ MG alumina in low-melting bath.
Time observed to obtain transparent melt after addition of batches of 0.5 wt-% alumina.
Time plotted as a function of the concentration of alumina dissolved in the bath, determined
from bath samples taken before the addition
the only option. In that case finely
ground low-calcined alumina appears
to be the best choice.
acknowledgement
Permission to publish given by Golden
Northwest Alumunum Holding Company,
is gratefully acknowledged.
references
[1] E.J. Frazer and J. Thonstad, “Alumina
solubility and diffusion coefficient in lowtemperature
fluoride electrolytes”, to be
published.
[2] J. Thonstad, F. Nordmo, J. B. Paulsen,
“Dissolution of Alumina in Molten Cryolite”.
Met Trans. 1972, pp. 403-408.
[3] J. Thonstad, A. Solheim, S. Rolseth,
O. Skaar, “The Dissolution of Alumina in
Cryolite Melts”, Light Metals 1988, pp.
655-661.
[4] T. Beck and R. J. Brooks. “Non-Consumable
Anode and Lining for Aluminum
Electrolytic Reduction Cell”, United States
Patent No. 5,284,562, 1994.
[5] S. Rolseth, R. Hovland, O. Kobbeltvedt,
”Alumina Agglomeration and Dissolution
in Cryolitic melts”, Light Metals 1994,
pp.351-357.
[6] O. Kobbeltvedt, S. Rolseth and
J. Thonstad, “On the Mechanism of Alumina
Dissolution with Relevance to Point
Feeding Aluminium Cells”, Light Metals
1996, pp. 421-427.
[7] R. G. Haverkamp, B. J. Welch and
J. B. Metson, ”An Electrochemical Method
for Measuring the Dissolution Rate of
Alumina in Molten Cryolite”, Bulletin
of Electrochemistry, Vol.8, pp.334-340,
1992.
authors
S. Rolseth, H. Gudbrandsen, K.S. Osen and
J. Kvello are from SINTEF Materials and
Chemistry, Trondheim, Norway.
J. Thonstad is from Department of Materials
Technology and Electrochemistry,
Norwegian University of Science and
Technology, Trondheim, Norway.
56 ALUMINIUM · 9/2009