Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ANODE SUPPORT APPARATUS
Field of the invention
This invention relates to an apparatus and method for supporting anodes in an
electrolysis cell producing for example aluminium, and in particular an
apparatus and
5'method for adjusting the position of these anodes in a cell.
Background of the invention
The electrolysis of alumina to produce aluminium by the Hall-Heroult process
is well
known and involves an electrochemical reaction. This process includes an
electrolytic
cell comprising an electrolytic tank having a plurality of cathodes and
anodes. The
aluminium oxide is supplied to a cryolite based bath in which the aluminium
oxide is
dissolved. The electrolytic process is most effective at bath temperatures
between
940 C and 970 C. The anodes comprise carbonaceous anode blocks and aluminium
stems which provide the mechanical and electrical link to the anode beam from
which
the anodes are suspended. The anodes are partially immersed in the electrolyte
to
provide an anode-cathode separation distance through which an electrical
current
passes. During the electrolytic process, aluminium is produced at the cathode
and
forms a molten aluminium layer on top of the cathode with the cryolite bath
floating on
the top of the aluminium layer. For efficient operation of the electrolytic
cell, the anode-
cathode gap should be set and maintained, either at a predetermined optimum
distance
or within an optimum range. If the anode-cathode gap is too large, this causes
a
significant voltage drop between the electrodes resulting in an unwanted
increased
power generation in the electrolyte. If the gap is too small the electrolytic
process
becomes unstable and inefficient.
With conventional carbon anodes, the anode blocks are continuously consumed
during
the electrochemical reaction producing mainly CO2 gases. As a consequence of
the
continuous consumption of the anodes the anode-cathode gap increases over
time. In
order to maintain the gap distance, the cell voltage is continuously monitored
and the
position of the anodes periodically reset to maintain the optimum anode-
cathode gap.
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Due to the continuous consumption, the anodes have a limited lifetime of about
four
weeks after which they have to be replaced by new anodes. It would be
appreciated by
those skilled in the art that changing more than one of all the anodes in an
electrolytic
cell in a short period of time will lead to severe interference with the
chemical and
thermal processes. Therefore, it is common practice to change only one anode
per day
in a given cell so that each anode of that cell has a different age between
zero and
approximately 28 days.
In a conventional electrolytic cell design, the movable anode support, called
an anode
beam, is supported by the superstructure. The anodes are clamped directly to
this
anode beam which provides mechanical support and supplies electrical current
to the
anodes. Conventionally a single clamp per anode is used for both, mechanical
fixation
and electrical contact of the anode stem with the anode beam. The distance
between
the anodes and the aluminium layer on top of the cathode is then adjusted by
raising
and lowering the whole anode beam. Hence during the operational life of the
anodes,
there is a need to raise and lower the anode beam to maintain the anode-
cathode gap
within optimum ranges.
Due to the consumption of the anodes, the downward movement of the anode beam
prevails and ranges at about 15 to 20 mm per day. As a result of the downward
movement the anode beam will reach is lowest possible position on the
superstructure
after two to three weeks and must then be raised with the help of an auxiliary
anode
raising beam which is temporarily positioned on top of the cell
superstructure. This
anode raising beam is equipped with devices which hold the anodes in place
while the
anode clamps are manually opened by operators to allow the anode beam to move
back to its upper position. When the anode beam has reached its upper
position, all the
anode clamps have to be closed again by the operators.
These clamping devices also exert a high lateral pressure on the anode stems
to
maintain electrical contact between the anode stems and anode beam while the
anode
beam is travelling upward sliding along the anode stems. The process of
raising the
anodes poses a significant safety risk as the pressure exerted on the anode
stems can
drop resulting in a loss of the electrical contact between the anode stems and
anode
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beam. In such a situation, dangerous electrical arcs develop until the
substation of the
aluminium smelting plant trips due to the open electrical circuit. In such an
instance, the
operators executing the anode beam raising are at risk of burn injuries.
Accordingly, it is an object of the present invention to provide an apparatus
for raising
the anode beam automatically without human interaction which addresses one or
more
of the problems with the existing arrangements.
Summary of the invention
In one aspect of the invention, there is provided
an apparatus for supporting anodes above a cathode in an electrolysis cell
including a superstructure, an anode beam to which a plurality of individual
anodes are attached each anode having a respective anodestem for attachment
to the anode beam by a main clamp, the anode beam being adjustably mounted
to the superstructure,
an auxiliary clamp for each anode stem, and at least one electrical beam
supported by the superstructure, the electrical beam having connectors
providing
electrical connection between the electrical beam and the anode stems.
In a preferred form of this aspect of the invention, the auxiliary clamps are
fixed in
position relative to the superstructure.
Unlike all commonly known cell technologies, the anode beam of this invention
is only a
mechanical fixation for the anode stems to provide vertical movements of the
anodes for
the control of the anode-cathode distance. The anode beam no longer carries
out the
task of conducting electricity to the anodes. This task is provided by the at
least one
electrical beam preferably fixed at the centre top of the cell superstructure
and a
plurality of flexibles. These flexibles preferably are made of a multitude of
aluminium
foils which are welded to the electrical beam and attached by way of bolted or
clamped
connections to the top of the anode stems.
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In this way, the task of mechanical fixation and adjustment of the height of
the anodes
can be maintained separate to the electrical connection to the anode stems.
Hence, an
uninterrupted electrical contact is provided to the electrical beam while a
safer fixation of
the anode stems is provided to the movable anode beam.
In a second aspect, the invention provides an apparatus for adjusting the
distance
between the anodes and cathode of electrolysis cells including an anode beam
to which
a plurality of individual anodes are attached by respective anode stems, the
anode stem
of each anode being maintained in position relative to the anode beam by a
main clamp,
an auxiliary clamp for each anode stem, and a means to control the operation
of the
main and auxiliary clamps to allow engagement and disengagement of the clamps.
The apparatus may further be provided with a superstructure. The auxiliary
clamp is
preferably secured to the superstructure to support the auxiliary clamp during
anode
beam raising.
In a preferred form of the first and second aspects of the invention, the
engaging and
disengaging of the main and auxiliary clamps are controlled by a process
control
computer. The disconnecting and reconnecting of the electrical flexibles
necessary for
the operation of anode change are done by specialized tools attached to the
manipulator cranes. The electrical flex connection will only be opened to
execute the
anode change at the end of the useful life of the anode block.
In this invention, the operation of raising the anode beam is no longer done
with an
auxiliary raising beam holding the anodes in place as described above.
Instead, the two
sets of clamps are used for this operation. In order to raise the anode beam,
the
process control computer will close the auxiliary clamps to maintain the
present
positions of the anodes while disengaging the main clamps. When the main
clamps of
all anodes of the cell are in an open position, the anode beam can be raised
freely.
There is no need to maintain an electrical contact between anode beam and
anode
stems by applying pressure on the stems since the electrical conduction
remains
uninterrupted through the continuous flexible-stem connection.
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Accordingly to a third aspect of the invention, there is provided a method of
raising an
anode beam in an electrolysis cell described above including the steps of
engaging the auxiliary clamps to maintain the position of the anodes relative
to
the superstructure,
5 disengaging the main clamps,
moving the anode beam,
reengaging the main clamps, and
disengaging the auxiliary clamps.
With this invention, it is possible to perform automated anode beam raising on
a much
more frequent basis (e.g. everyone to two days) than with a conventional anode
beam
arrangement where the labour intensive beam raising occurs every two to three
weeks.
For this reason, the overall anode beam travel distance can be reduced
significantly.
Description of the drawings
Figure 1 is a sectional view of an electrolysis cell and conventional anode
support of the
prior art, and Figure 2 is a sectional view of an electrolysis cell and anode
support
according to an embodiment of the invention.
Detailed description of the preferred embodiments
The electrolysis cell of the prior art includes a steel outer shell 12 having
a bottom
refractory lining 14 and sidewall refractory lining 16. The bottom refractory
lining
supports a cathode 18 having steel collector bars 20 for conducting
electricity to
busbars (not shown) on the outside of the steel shell 12. An anode assembly
comprising
an anode block 24 connected to an anode stem 26 via a steel yoke 22 and stubs
28 is
shown. The anode stem 26 is both mechanically and electrically connected to an
anode
beam 30 by a clamping arrangement 32 fixed to the anode beam. Mechanical drive
trains (not shown) are provided to raise and lower the anode beams in order to
vary the
anode-cathode gap (in reality the gap between anodes immersed in the
electrolyte and
the liquid aluminium layer on top of the cathode) to a desired distance.
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The design of the anode raising and lowering equipment is based on the
assumption
that all anodes will be consumed at a constant rate and so generally, all of
the anodes
in the cell can be raised or lowered concurrently to maintain the gap within
the optimum
range. Inside the cell superstructure, a number of alumina hoppers (34) is
provided to
supply alumina to point feeders (not shown) which are generally positioned
between the
pairs of anodes.
Such anode beam systems rely on a clamp connection providing sufficient
mechanical
pressure to maintain the anode stems in a fixed position relative to the anode
beam with
this same connection providing the electrical connection between the anode
beam and
the anode stems. Therefore such a system comprises both the electrical
connection
function and the mechanical fixation function. Furthermore, the removal and
addition of
the anodes is provided by a crane which is positioned above the electrolytic
cell.
Because of the design of the cell superstructure, the crane must necessarily
be much
higher in order to clear the superstructure.
While the details of a conventional electrolysis cell are those shown in
Figure 1 the
embodiment of this invention is shown in Figures 2. For ease of understanding,
structures in the embodiment of the invention which are similar to a
conventional
electrolysis cell have been given identical numbering. The anode support
structure in
accordance with the invention includes an electrical beam 50 which is
preferably fixed in
position and connected to anode stems 26 by electrical connectors referred to
as
flexibles 52. These flexibles 52 maintain the electrical connection to the
anode stems
while giving the stems the ability to move relative to the electrical beam 50.
The flexibles
52 are generally formed from a plurality of aluminium sheets shaped into a
laminar
block of aluminium with the laminar structure extending in the direction of
current flow.
One end of the flexibles is welded to the electrical beam 50 while the other
end is
connected to the top of the anode stems by way of a bolted or clamped
connection 60.
The anode supporting structure comprises a main clamp 54 attached to the
vertically
movable anode beam 30 and an auxiliary clamp 56 fixed to the cell
superstructure base
plate 58. The anode beam is moved by a mechanical drive train (not shown)
which is
similar to that of a conventional anode support structure. There is one main
clamp 54
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and one auxiliary clamp 56 for each anode stem 26. The main clamp 54 of all
anodes of
a cell are continuously engaged except for during the operation of raising the
anode
beam. A control means such as pneumatic cylinders or electrical motors is
provided to
operate the engagement and disengagement actions of the main clamps 54 and
auxiliary clamps 56.
An alumina hopper 62, smaller than the prior art hopper, is also provided in
the
superstructure between the anode beams 30.
In order to raise the anode beam the auxiliary clamps 56 of all anodes in a
given cell are
closed by the process control computer. Subsequently the main clamps 54 are
opened
so that the mechanical drive train can lift the anode beam 30 freely with the
anodes
being held in their position by the auxiliary clamps 56 and the electrical
connection,
provided by the flexibles 52 is uninterrupted. Once the anode beam has reached
its
upper position the main clamps 54 are closed followed by the opening or
disengaging of
the auxiliary clamps 56. The whole operation of disengaging and reengaging the
clamps
as well as raising the anode beam is fully automated and requires no operator
intervention.
