DE2107654A1 - Method for supplying aluminum oxide in the aluminum electrolysis in the fluid melt flow - Google Patents

Description

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Case 564 1 Munich 23, Cl.r «ns» tr. «

Phone. 34SO07

Swiss Aluminum AG, ..

Chippis, Switzerland J /. ^: .. ^ "

Method of feeding aluminum oxide in aluminum electrolysis in the fluoride melt flow

Priority: February 17, 1970, Switzerland, No. 229V70

When aluminum is obtained by electrolysis of aluminum oxide (Al ₂ O- ,, alumina) in molten fluoride electrolyte, the melt is located in a tank lined with carbon, which has a thermal insulation layer between the carbon lining and a steel jacket. Iron conductors (cathode bars) are embedded in the bed of the lining. The electrolytically deposited aluminum collects on the floor. Above this is the molten fluoride electrolyte in which aluminum oxide is dissolved. Anodes made of amorphous carbon are immersed in the melt from above. The melt itself is covered by a crust of solidified electrolyte and "by a layer of alumina. The alumina that is on the cell ready to be introduced into the bath warms up on the crust.

The amount of the alumina blanket on the bath, and optionally the anodes affected in this part of the cell at the same time, the heat losses can be regulated relatively well with the fleas of the alumina layer.

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The generation of heat depends heavily on the distance between the anode and the cathode (in the present case, the cathode is the liquid aluminum lying on the bottom of the cell). This distance, also called the interpolar distance, is in practice K to 6 cm. To avoid excessive deflection of the

Heat generation in the cell is desirable to do this with a practical
/ to operate a constant interpolar distance, i.e. with such an anode / cathode distance that varies only within narrow limits.

The AlgO-j concentration in the liquid cryolite is usually between 1 and 10 #. In the course of the electrolysis, the Alumina is consumed by the electrochemical process and must be replaced periodically. This is done by hitting the Electrolyte crust on the river and introduction of the overlying one Alumina in the melt.

If the AI ₂ O - ^ - concentration in the river falls below a critical value (2.5 to Γ %), the anode effect occurs; the cell voltage suddenly rises from 3.7 - 4.5 volts to values between 20 and 50 volts. The anode effect is also eliminated by breaking in the crust and adding new clay to the bath. In addition, a movement of the flow must usually be brought about by stirring, scraping, introducing gas and similar methods.

Already with decreasing Al ₂ O ^ concentration in the electrolyte increases; the cell voltage, but only slowly; from the moment of highest alumina concentration this increase is 0.1 to 0.3 volts. If you add new clay, the voltage drops

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the specified amount again. If the addition is not made in time, there is an anode effect.

The increase in voltage is explained by poorer wetting of the anodes by the molten electrolyte as the A12O-Z concentration decreases. In any case, the increase in voltage must be at the flux-anode interface, since the electrical conductivity of the electrolyte _{does not decrease with decreasing Al 2} O ^ concentration, but increases.

The increased cell voltage causes an increased energy supply to the cell. This is undesirable because the electrolyte temperature should be ^{between 9 ^ 0 and 965 0 C as constant as possible.} Too low a temperature would mean a partial excretion of the liquid cryolite, since the working temperature in the aluminum electrolysis is only slightly above the liquidus temperature. On the other hand, too high an electrolyte temperature increases metal losses through reoxidation; a part of the electrolytically deposited aluminum is in the river in solution, is in the vicinity of the anode gases, which consist of CO and COG, is reoxidized and the like. These reoxidation losses, which in practice are between 8 and 20 % , increase sharply when the temperature of the electrolyte is more than 97O ⁰ C.

For these reasons there is an interest in keeping the river temperature as constant as possible. In order to counteract the increased heat generation with decreasing Al ₂ O ^ concentration, the interpolar distance is usually reduced. This is mostly done by manual control. However, modern systems are equipped with automatic cell voltage regulators,

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which compare the cell voltage with a target value and, if differences occur, the distance between anode and cathode change so that the desired voltage is restored.

The measure is correct in itself. As soon as the cell voltage increases due to poor wetting of the anodes by the flow, the interpolar distance is reduced and the voltage is thus restored to its original value. Through this constant electrical power is supplied to the cell and the flow temperature is stabilized.

However, this type of furnace management has disadvantages. The cell voltage increases one recognizes that the AI2O3 concentrate! on has dropped is. If the control intervenes manually or automatically and the interpolar distance is reduced, the this measure increases the metal losses. These depend on both the electrolyte temperature and the interpolar distance and on the AIgO - * concentration. The less aluminum oxide Is dissolved in the flow, the more metal goes into solution. The smaller the anode - cathode distance, the more Metal is reoxidized by the anode gases.

The higher the metal losses per unit of time, the higher the specific electrical energy consumption (kWh / kg Al). This is usually between 12 and 16 kilowatt hours per kilogram of aluminum and depends heavily on the furnace guide and the cell construction.

The disadvantages outlined are covered by the present invention

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finding fixed. This refers to the introduction of aluminum oxide into the molten fluoride electrolyte in the electrolytic extraction of aluminum, which is characterized by the fact that the supply of aluminum oxide

practically
at / constant interpolar distance occurs when an upper limit value determined by the cell voltage is exceeded. The limit value is set in advance depending on the cell construction and operating conditions, e.g. that of the cell voltage at 4.5 + 0.1 to 0.15 volts. In other words, the limit value of the cell voltage is set between 4.6 and 4.65 volts. The addition of alumina takes place as soon as this limit value is exceeded, namely by hammering in the crust on the bath and stirring in the aluminum oxide. So it is not, as is often the case, waiting for the programmed service times for breaking the crust, but rather the addition of the clay as a function of exceeding a

practically fixed maximum value of the cell voltage at / constant

Interpolar distance made.

The increase in voltage can be monitored by various instruments will. For example, contact voltage transformers can be used, which generate a signal when an adjustable value is exceeded give. Process computers can also be used.

After the addition of alumina, the cell voltage is measured again to determine whether the increase that occurred before the addition of alumina the cell voltage has been reversed by the freshly dissolved aluminum oxide, i.e. whether the cell voltage has been restored

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is below the set limit . If there is no indication, it is always possible to introduce a larger amount of aluminum oxide into the electrolyte.

If, instead of a limit value for the cell voltage itself, one chooses a limit value for the rate of change of the cell voltage AiT (change in the cell voltage in the unit of time), the measures to be taken are analogous.

There are cell regulators that do not simply regulate the cell voltage, but rather determine the internal resistance of the cell by dividing the cell voltage by the V / ert of the cell current « An upper limit value of this internal resistance can also be used to determine the point in time at which the aluminum oxide is added use.

Finally, an upper limit value for the change over time can also be used the internal resistance of the inch can be selected.

The amount of clay added to the thorn bath is not accurate controllable, because when the crust hits the electrolyte, there are not precisely defined amounts of aluminum oxide in the bath reach; the actual amount depends more on the contingencies of the operation.

Prerequisite for the application of the method according to the invention ict in any case to keep the Interpolardistan constantζ during the operation of the cell.

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Incidentally, the results of the monitoring of the limit values can be found use, for example, with process computers the next Alumina addition to be programmed. In other words, the intensity and the Determine the next time the electrolyte crust will be pounded in and the addition of alumina in good time. The intensity of the impact can be determined by the number of blows of the crucible breaker chisel and / or by the amount of alumina supply prior to operation influenced v / ground. The point in time can be chosen so that an optimal driving program is created for the crust breaker.

Thanks to the addition of alumina controlled according to the method according to the invention, the production of the cells can be increased by 1 to 3% , since the metal losses due to reoxidation are reduced. In addition, the specific electrical energy consumption can be reduced by 0.5 kWh / kg Al and more.

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Claims (6)

■ Claims 1. Method of feeding alumina in aluminum electrolysis in the fluoride melt flow, characterized in that the supply takes place at a practically constant interpolar distance occurs when a cell voltage-related upper limit is exceeded. 2. The method according to claim 1, characterized in that the supply when an upper limit value is exceeded "the cell voltage he follows. 3 · The method according to claim 1, characterized in that dje Supply takes place when an upper limit value of the change in the speed of the cell voltage is exceeded. 4. The method according to claim 1, characterized in that the supply when an upper limit value of the internal resistance is exceeded the cell takes place. 5. The method according to claim 1, characterized in that the supply when an upper limit value of the change is exceeded of the internal resistance of the cell in the unit of time. 6. The method according to claim 1, characterized in that shortly after the addition of aluminum oxide, the next addition of aluminum oxide is programmed will. 109836/0987