MINERAL COMPOSITION AND PHASE TRANSFORMATIONS
IN THE REFRACTORY LINING OF THE BOTTOM
OF ALUMINUM ELECTROLYSIS CELLS. PART 2.
V. V. Sharapova,
I. I. Lishchuk,
D. Yu. Boguslavskii,
and V. V. Chesnyak
Translated from Novye Ogneupory, No. 5, pp. 24 – 26, May, 2005.
Original article submitted January 21, 2005.
Specimens of post-service ShA-5-grade chamotte refractory are analyzed for total elemental aluminum and
silicon. Products of the electrolysis of cryolitic melt Na (gas), AlF, Al
O, and, possibly, Na
F are shown to dif
fuse through pores of the carbon refractory bottom lining blocks of an aluminum electrolysis cell. The lower
compounds and Na(gas) as they reach the refractory layer enter into a reaction with the refractory material.
The Gibbs free energy of hypothetic reactions in the Al – Si–O–F–Nasystem involving a gas phase has
been calculated. It is inferred that the performance characteristics of the ShA-5 refractory fail to meet standard
In a previous communication , the changes in mineral
and phase composition of ShA-5-grade chamotte refractory
specimens sampled from the bottom lining of a aluminum
electrolysis cell in service for 45.6 months have been re-
ported. In this paper, we focused attention on a thermody-
namic analysis of the processes involved in the wear of the
refractory lining of the electrolysis cell bottom.
The results of a post-service analysis of the ShA-5 re
fractory have indicated an increase in total aluminum in
some test specimens (24-1m and 33-1m) and a decrease in
total silicon, especially in specimens 6-3m, 34-3m, and
40-1m. The increase in total aluminum defies explanation as
being due to the simple penetration of metallic aluminum
into the refractory layer. The mode of occurrence and disper
sion of the penetrated metal suggest that the infiltration of
aluminum is effected via gas phase; that is, the electrolysis
products AlF and Al
penetrate through pores and joints in
the carbon bottom lining blocks to undergo disproportiona
tion during condensation from the gas phase by the reactions
3AlF (gas) = 2Al + AlF,
O (gas) = 4Al + g-Al
The low content of total silicon, tortuous crack paths,
pores with globular metal particles, and communicating
pores ¾ all these features indicate that silicon evaporates in
the form of volatile SiO species, silicon fluorides, and
. The high amount of total silicon (21.24 – 31.62%)
in the vermiculite heat-insulating layer after service (in com-
parison with the initial value of 19.61%; see Table 1 in )
lends support to this suggestion. Simultaneously, the volatile
O and AlO are removed by evaporation from the alumi
num refractory, in agreement with the chemical analysis re
sults (see Table 1 in ). Similar effects involving lower oxi
des of silicon and aluminum were reported in [2 – 5].
The occurrence of ferrosilicon and aluminum in the
post-service material suggests that a reducing medium might
have been found in the refractory layer. The most likely re
ducing agents for ShA-5 components are sodium (capable of
diffusing through the carbon-graphite material of the bot
tom), aluminum fluoride AlF [2 – 4, 6] and, possibly, sodium
F . According to [2 – 4, 8], these com
pounds are the main components of the gas phase in the liq
uid aluminum – cryolite melt system.
One will infer therefore that the volatile products of the
electrolysis of cryolitic melt — Na (gas), AlF, and, possibly,
F — are capable of diffusing through the pores of the car
bon blocks. On reaching the chamotte layer, they enter into a
reaction with chamotte components. The thermodynamic
characterization DG of reduction processes in the chamotte
Refractories and Industrial Ceramics Vol. 46, No. 3, 2005
1083-4877/05/4603-0175 © 2005 Springer Science+Business Media, Inc.
Ukrainian State Research Institute of Special Steels
(UkrNIISpetsstal’), Zaporozhe, Ukraine; Zaporozhye Aluminum
Production Kombinat, Zaporozhe, Ukraine.