Note: Descriptions are shown in the official language in which they were submitted.
Quenchinq anode effect by anode_rockin~
This invention relates to a method of quenching anode effect
during the production of aluminum by electrolytic smelting.
The invention is particularly suitable for the automatic
operation of pots and potlines.
Aluminum is normally produced by the Hall-Heroult method which
involves the use of an electrolytic cell operating in the tempera-
ture range 950 - 980C, decomposing alumina dlssolved in a
bath of molten cryolite. Normally, over 100 such cells, or pots,
are connected in series.
The individual electrolytic cell consists of a flat vessel with
low sides, built of steel plates. Inside this steel shell there
is a refractory layer which surrounds a carbon lining. The
carbon lining contains the molten bath, which consists of cryo-
lite with various additives. The carbon walls of the vessel are
usually covered with frozen bath which continues some way out
along the bottom. Most of the carbon bottom is free of frozen
bath and serves as the cathode. The entire bottom is covered
with molten aluminum, extracted from the alumina, and this
aluminium has a higher specific gravity than the molten bath.
The anode, which is made of carbon, is partly immersed in the
bath which, as mentioned above, consists mainly of molten cryo-
lite and dissolved aluminaO The distance between the bottom of
the anode and the molten metal on the cathode is called the ACD
(anode-cathode distance) or the interpolar distance, and is in
the region of 2-7 cm.
The bath can also contain other substances to influence the
solubility of the alumina and the freezing point of the bath.
This subject is discussed in a number of publications, but, as
will be understood, these additives are of no interest in connec-
tion with the present invention. The bath around the anode is
covered with a crust of frozen bath. On top of this crust the
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alumina is laid so that it is pre-heated before being pushed
down into the bath.
During normal operations, the electrolytic reduction process
takes place with a voltage drop from anode to cathode of
between 4 and 6 V, dependent upon the cell design chosen. At
the beginning of the reduction process, the concentration of
alumina in the bath can lie around G-8~. The electrolytic
process decomposes the alumina. The metal thus extracted sinks
down on to the cathode, whilst the oxygen is liberated on the
underside of the anode. The anode is thus oxidized and is con-
sumed, whilst at the same time the alumina content in the bath
is reduced. When the alumina concentration in the bath falls
below a certain critical value - about 2~ - depending upon the
temperature of the bath and the current density of the anode
etc., the steady electrolysis process is replaced by an anode
e~fect. This reveals itself in the form of a sudden increase in
the resistance of the electrolytic cell which, in the course of
seconds, can increase tenfold. This increase in resistance is
normally attributed to a layer of gas under the anode, see for
example Norwegian patent No. 123318.
To indicate the presence of an anode effect, an electric
incandescent lamp is connected across the cell. It lights up on
anode effect because of the higher voltage. The normal practice
is to endeavour to quench or terminate an anode effect as soon
as possible, because it brings with it a number of disadvantages.
The sharp increase in resistance can result in a loss of energy
in other pots in the series, thus disturbing operations. The
extra energy resulting from this higher resistance causes the
pot undergoing anode effect to heat excessively causing the
electrolyte to evaporate with consequent large direct loss of
bath vapours, mainly fluorides. This emission of fluorides to
the atmosphere requires costly scrubbing.
If an anode effect is allowed to remain active too long, the
operatlon of the pot is substantially disturbed and this requires
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extra efforts on the part of personnel to bring the pot back
into normal operation, whilst at the same time output falls.
To quench anode effect, the alumina concentration in the bath
under the anode must be raised. The earlier method of doing
this was that the operator manually broke ~he crust around the
anode, whereby the alumina which had been laid on top of the
crust for pre-heating, fell down into the melt. The operator
used a heavy iron implement to mix the alumina into the bath and
then to rake under the anode with vigorous movements. Another
method of quenching anode effect was to knock a hole in the crust
and insert a wooden pole down into the bath under the anode. The
gas liberated from the pole resulted in a powerful stirring of
the bath, thus quenching the anode effec~. The pole has sub-
sequently in a number of instances been replaced by an air lance,
i.e. a tube which is used to blow compressed air down into the
bath and thus to bxing about a particularly vigorous stirring
under the anode.
These methods of quenching anode effect require heavy work in a
hot and gas~filled atmosphere. A number o~ attempts have there-
fore been made to mechanize and automate these operations.
British patent no. 853056 describes a method whereby an audio-
frequency vibrator i~ used for quenching anode effect. How
this procedure quenches anode effect is not explained. `
In US patent no. 2560854, a procedure is described for terminat-
ing anode effect by slowly swinging the anode from a region in
the electrolyte with low concentration of alumina to a region
with a higher alumina concentration. It is clear that this
method will mix alumina and bath and thus help to quench the
anode effect, but it requires complicated and expensive equip-
ment.
Norwegian patent no. 123318 discloses a method of quenching anode
effect which is built on the principle that the entire anode is
lowered ~o between 30 and 6 % of the normal interpolar distance,
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and then raised again. This lowering is started when the voltage
drop across the cell exceeds 150 % of the normal working voltage,
and the anode ls raised again under the control of a timing
device incorporating a cam mechanism. Whilst the anode is in its
lower position, an apparatus for feeding alumina to the bath is
activated, so that an alumina concentration of 2-6 ~ in the bath
is achieved. This procedure will doubtless reduce the amount of
manual work involved in quenching anode effect, but it has
nevertheless certain disadvantages. When the anode is lowered,
the level of the ba-th rises because the anode must, of necessity,
displace fluid. The pot shells have therefore to be somewhat
higher than usual. As the bath rises, it will come into contact
with the crust along the sides of the pot. This will, in time,
result in a mixing of bath and alumina which solidifies forming
a crust. This crust will have better heat conductivity than
pure alumina and will thus iead to heat loss along the sides.
In those cases where bath overflows in connection with this
so-called anode pumping, the heat content in the bath which flows
out over the crust will be dissipated to the surroundings, and
the spilt bath causes extra work.
In modern smelters, computers are used to control many of the
pot operations. An important parameter in control strategy is
the reference resistance of a pot. This is around 20 microohm
and is defined as the pot voltage minus the decomposition voltage
divided by the potline amperage.
In addition to the disadvantages inherent in NP 123318, there
is also the increase in the reference vol~age, which means a
higher consumption of energy in production.
The inventors have now found a method of quenching anode effect
3n whereby the above-mentioned difficultles are avoided. The
principle of the method is that the anode is lowered at the one
end whilst at the same time the other end is raised, when anode
effect has occurred and is to be quenched. In other words,
instead of lowering the entire anode, only a part of it is
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lo~ered, whilst the opposite end is raised. The anode is thus
rocked about an axis, which axis may lie either in the longi-
tudinal direction of the anode or in the transverse direction.
This method is called anode rocking and has obvious advantages:
1) The surface of the bath remains at the same level. The bath
will merely flow from the one end to the other. This can be
advantageous for mixing of low and high alumina concentration
in the bath. 2) The wearing face of the anode will be inclined,
so that the gas and the gas film can more readily escape.
3) It is not necessary to make the cathode shells higher in order
to contain the bath. 4) The energy consumption is kept down
because the thermal insulation efficiency of the crust is not
impaired by its being mixed with molten bath and alumina.
5) The loss of energy through molten bath flooding over the crust
is avoided. 6) There is no spilling of bath on to the floor.
Thus, the present invention provides a method of quenching anode
effect when producing aluminium by electrolytic reduction charac-
terized by anode rocking whereby the anode, when anode effect
has started, is placed in an inclined position so that its ~'
wearing surface forms an angle of up to 20 degrees with the `
horizontal and the anode is ~lternately rocked about either its
longitudinal or transverse axis and then allowed to rest in its
inclined position until the anode effect is quenched, whereupon
the anode is returned to its normal horizontal position.
It is obvious that the performance of anode rocking imposes
certain requirements on the mechanical equipment, but the type
of equipment in general use today in modern pots can usually be
readily adapted, only minor additional installations being
required.
The anode is usually suspended in jacks, which are operated by
lifting motors at each end of the anode. These motors have to
be operated independently of one another, only through impulses
from a common control device.
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The working program for anode rocklng can be easily fitted into
existing programs.
The work rhythm for a modern potroom prepared for the quenching
of anode effect by anode rocking will comprise the usual
operations in the usual sequence: alumina feed on to the crust,
crust breaking outside anode effect and crust breaking durlng
anode effect, followed by anode rocking.
Even if these operations are performed in all potlines, the
routines will not be identical in the different plants.
On the basis of trials, the inventors have arrived at a prefer-
red work method, but all methods are dependent on the apparatus
and the operational characteristics of the method regarding time
periods for anode rocking.
On the occurrence of anode effect, i.e. on exceeding a preset
cell voltage (reference voltage), anode rocking is initiated
by a control device, and lasts from 0-80 seconds whilst the
anode is in an inclined positionO This is followed by a resting
period of about 60 seconds to see whether the anode effect will
reoccur. If it does not do so, the control device completes the
order cycle for anode effect quenching by straightening up the
anode and placing it in its normal position. Should, on the
other hand, a new anode effect occur, the anode will be lowered
about 10 mm whilst still ln the inclined position, but will be
returned
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to normal position after a short time, for example lo seconds,
whereupon the order cycle will be terminated by straightening
up the anode.
The times and durations stated here refer to given pot equipment.
It will be obvious that the invention can equally we].l be per-
formed using other parameter values, with this reducing the
scope of the invention.
During anode rocking, it has been shown that the lowest paxt of the
anode carries a considerably higher current than normal, loo~ or
more but, surprisingly enough, this has not proved to have any
harmful effect. Weither has it been found that anode rocking
has had any adverse mechanical consequences or operational com
plications.
It is pointed out that the desired effect, the quenching of anode `
effect, is probably due to two aspects of anode rocking:
1) The bath is set in motion in a manner which will lead to the
mixing in of bath with a higher alumina content under the anode.
2) wnen the anode is placed in an inclined position, i~ is easier
for gas bubbles and film und~r the anode to escape.
Movement of the bath will also be favourable to promote the
escape of gas film and bubbles. To achieve the desired effect,
the wearing face of the anode must adopt an angle of up to
20 from the horizontal.
