Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/20—Automatic control or regulation of cells

Definitions

• This invention relates to a method of quenching anode effect during the production of aluminium by electrolytic smelting.
• the invention is particularly suitable for the automatic operation of pots and potlines.
• Aluminium is normally produced by the Hall-Heroult method which involves the use of an electrolytic cell operating in the temperature range 950° - 980° C, decomposing alumina dissolved 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 cryolite 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 aluminium, 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 cryolite and dissolved alumina.
• 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 connection with the present invention.
• the bath around the anode is covered with a crust of frozen bath. On top of this crust the alumina is laid so that it is pre-heated before being pushed down into the bath.
• the electrolytic reduction process takes place with a voltage drop from anode to cathode of between 4 and 6 V, depending upon the cell design chosen.
• concentration of the alumina in the bath can lie around 6-8 %.
• 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 consumed, whilst at the same time the alumina content is the bath is reduced.
• the anodes of the reduction cell are kept in an inclined position at least until the lowered under surface is flattened because of the different electrical load of the different parts of the anode, whereby this flattening,depending upon the angle of inclination of the anode will normally occur after 1 to 24 hours.
• the anode is tilted in a different way, in order to provide an inclined under surface again.
• the alumina concentration in the bath under the anode must be raised.
• the previous method of doing this was that the operator manually broke the 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 effect.
• the pole has subsequently 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 und thus to bring about a particularly vigorous stirring under the anode.
• British Patent No. 853056 describes a method whereby an audiofrequency vibrator is used for quenching anode effect. How this procedure quenches anode effect is not explained.
• Norwegian Patent No. 123318 discloses a method of quenching anode effect which is built on the principle that the entire anode is lowered to between 30 and 6 % of the normal interpolar distance 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 is 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.
• the inventors have now found a method of quenching anode effect, whereby the above mentioned difficulties are avoided.
• the principle of the method is that the anode is temporarily lowered at the one end whilst at the same time the other end is raised, when anode effect has ocurred and is to be quenched.
• the anode is thus for a short period rocked about an axis, which axis may lie either in the longitudinal 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.
• 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.
• the method of quenching anode effect according to the invention as characterizing step contains the placing of the anode into an inclined position for a short period when anode effect has started, so the wearing surface forms an angle of 0.1 - 20° with the horizontal and placing it then back into its normal position.
• the wearing surface of the anode can be held stationary in the inclined position during the lasting of the anode effect, that is usually for about 1-120 sec, and for an additional preset period of about 1-120 sec.
• the anode also can be moved up and down in the inclined position for a predetermined period of a few seconds to about 3 minutes.
• the anode also can be moved so that the angle formed by its wearing surface with the horizontal varies for a predetermined period of a few seconds to about 3 minutes.
• the above indicated possibilities of anode rocking can be parts of a working programme.
• such a working programme can comprise the following steps: Rocking of the anode, when anode effect has started for a period of about 1 - 80 sec, whilst the anode is in an inclined position, followed by a resting period of about 60 sec, whereafter, in case that anode effect does not reoccur, the anode is placed back into its normallposition.
• the inclined anode is lowered for about 10 mm for a short time,for'example 10 sec, and is then placed back into its normal position.
• the order to start or stop and also the duration and scope of anode rocking are preferably given by a control device which measures operational characteristics and organizes the other operating manoeuvres for the pot line, whereby the measured operational characteristic preferably is the voltage across the cell. When this voltage exceeds a certain preset value, anode rocking is initiated.
• 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.
• the working programme for anode rocking can be easily fitted into existing programmes.
• anode rocking is initiated by a control device, and preferably lasts from 1-80 sec, whilst the anode is in an inclined position. This is followed by a resting period of preferably about 60 sec to see whether the anode effect will reoccur. If it does not so, the control device completes the order cycle for anode effect quenching by straightening up the anode and placing it into its normal position.
• a preset cell voltage reference voltage
• the anode will be lowered about 10 mm, whilst stillin the inclined position, but will be returned into normal position after a short time, for example 10 sec, whereupon the order cycle will be terminated by straightening up the anode.
• Temporary movement of the bath will also be favourable to promote the escape of gas film and bubbles.
• the wearing face of the anode must adopt an angle of between 0.1 - 20° from the horizontal.during such a-movement.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)

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Citations (2)

• 1979
  • 1979-04-12 EP EP79101144A patent/EP0017653A1/fr not_active Withdrawn

Patent Citations (2)

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