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RESEARCH
Cathode wear in Hall-Héroult cells
K. Tschöpe, E. Skybakmoen, A. Solheim, SINTEF Materials and Chemistry; T. Grande, NTNU
Laboratory tests for cathode wear identify
some variables as important, and eliminate
others. These results are compared
with models and with industrial potline
experience.
Introduction
The research team Electrolysis in SINTEF
Materials and Chemistry offers a high level of
competence in the field of light metals production,
molten salt chemistry, and particularly,
the process of aluminium electrolysis. The
activities cover fundamental as well as applied
research in close collaboration with the
industry and NTNU (Norwegian University of
Science and Technology). This paper reviews
the recent activities in the Durable Materials
in Primary Aluminium Production (DuraMat)
project on cathode wear, a phenomenon that
is of great interest for all primary aluminium
producers.
The Hall-Héroult process has for more than
125 years been the only commercial method
for primary aluminium production. To date,
this process has survived the attempts to replace
it by alternative methods such as carbothermal
reduction, electrochemical reduction
of anhydrous aluminium chloride, and electrolysis
based on inert electrodes.
Tremendous scientific and technological
efforts have been made to improve the efficiency
of the process; e. g. the potline amperage
has been increased from 50 kA in 1940
to currently 400-500 kA [1]. Modern cells
may operate at specific energy consumptions
as low as 12.5 kWh/kg Al, and the current efficiency
is typically in the range of 92-96%.
Careful choice of new lining materials, and in
particular, the increase of the graphite content
in cathode blocks, is one of the factors that
made this possible. Anthracitic carbon has
been gradually replaced by the now state-ofthe-art
graphitised cathode blocks. While this
has allowed additional energy savings and
increased productivity through the increased
electrical conductivity, these benefits need to
be weighed against higher material costs and
lower wear resistance.
eventually leads to direct
contact between
the metal and the
collector bar. Consequently,
cathode wear
is usually the limiting
factor for the service
life of aluminium reduction
cells. The cross
sectional view of cathode
blocks often reveals
a W-shaped wear
pattern, and even a
WW-pattern has been
observed, as shown in
Fig. 1 [3]. Typical wear
rates are in the range
of 2-6 cm/year [4].
The cell service life is
a crucial economic
parameter, which
makes it important to
understand the wear
mechanism(s).
It is generally agreed
that formation, dissolution
and transport
of aluminium carbide are important factors
that influence the cathode wear. Aluminium
carbide can be formed chemically as well as
electrochemically, according to reaction (1)
Fig. 1: Visualisation of the wear profile. Cathode surface of a shutdown
cell after 2,088 days in operation (a); plotted image using the laser scanning
method, the blue colour corresponds to less wear and the red colour
indicates the highest wear (b); longitudinal wear profile of all 19 cathodes
showing the WW-shaped wear pattern (c) [3].
and (2), respectively. However, the underlying
mechanism(s) suggested are mainly based
on theoretical considerations and are still a
matter of discussion [5-10].
Characteristics of cathode wear
Wear is generally defined as net removal of
material from a surface [2]. Carbon blocks
wear excessively along their periphery, which
a) b)
Fig. 2: Schematic drawing of the experimental set-up with a quartz glass tube and position of the sample
(a) as well as the sample appearance and its exposed cross section after embedding and polishing (b)
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