EP0787833A1 - Conductor arrangement for electrolytic cells - Google Patents

Abstract

Arrangement of current rails from the cathode bar ends of an electrolysis cell, especially for the production of aluminium, to the anode beam ends of the follow-on cell, wherein the cathode bar ends on both sides of the cell (10n) are each connected to a part current rail (A,B) and the part current rails are led under the follow-on cell (10n+1) to a current collector rail (C) below the cell lying in the longitudinal direction (X) towards the downstream end of the anode beam.

Description

The invention relates to a rail arrangement for guiding the direct electrical current from the cathode bar ends of a longitudinal electrolysis cell, in particular for the production of aluminum, via conductor rails to the traverse ends of the subsequent cell.

For the production of aluminum by electrolysis of aluminum oxide, this is dissolved in a fluoride melt, which largely consists of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the carbon bottom of the cell, the surface of the liquid aluminum forming the cathode. Anodes, which are attached to anode bars or traverses and which consist of amorphous carbon in conventional processes, are immersed in the melt. The electrolytic decomposition of the aluminum oxide produces oxygen at the carbon anodes, which combines with the carbon of the anodes to form CO ₂ and CO. The electrolysis generally takes place in a temperature range from about 940 to 970 ° C. In the course of electrolysis, the electrolyte becomes poor in aluminum oxide. At a lower concentration of 1 to 2% by weight of aluminum oxide in the electrolyte, there is an anode effect, which results in an increase in the voltage from, for example, 4 to 5 V to 30 V and above. Then, at the latest, the crust formed from solidified electrolyte material must be hammered in and the aluminum oxide concentration increased by adding new aluminum oxide.

In normal operation, the electrolysis cell is usually operated periodically, even if there is no anode effect by breaking in the crust and adding alumina.

The cathode bars are embedded in the carbon bottom of the electrolysis cell, the ends of which penetrate the electrolysis tank on both long sides. These iron bars collect the Electrolysis current, which flows via the busbars arranged outside the cell, the risers, the anode bars or traverses and the anode rods to the carbon anodes of the subsequent cell. The ohmic resistance from the cathode bars to the anodes of the subsequent cell causes energy losses in the order of up to 1 kWh / kg of aluminum produced. There have therefore been repeated attempts to optimize the arrangement of the busbars with respect to the ohmic resistance. However, the vertical components of magnetic induction formed must also be taken into account, which - together with the horizontal current density components - generate a force field in the liquid metal obtained through the reduction process.

In an aluminum smelter with longitudinal electrolysis cells, the current is conducted from cell to cell as follows: The direct electrical current emerges from cathode bars arranged in the carbon bottom of the cell. The ends of the cathode bars are connected to the busbars via flexible bands, which run parallel to the row of electrolytic cells. From these busbars running along the long sides of the cells, the current is led via other flexible belts and via risers to the two ends of the traverse of the subsequent cell. Depending on the type of furnace, the current distribution between the nearer and the far end of the traverse, based on the general current direction of the cell row, varies from 100/0% to 50/50%. The vertical anode rods, which carry the carbon anodes and feed with electrical current, are attached to the crossbar by means of locks.

• Die erste Strömungskomponente, welche im Prinzip eine Zirkulationsbewegung entlang der inneren Zellenwände ist, hat besonders schädliche Auswirkungen in bezug auf die Stabilität der Elektrolysezelle. Diese erste Komponente entsteht durch den Einfluss der benachbarten Elektrolysezellenreihe, welche den elektrischen Strom zum Gleichrichter zurückführt. Der Drehsinn der Rotation hängt davon ab, ob die benachbarten Zellenreihe links oder rechts, bezogen auf die allgemeine Richtung des Gleichstromes, von der Zelle liegt.
• Die zweite Strömungskomponente besteht darin, dass in jeder Zellenhälfte (in bezug auf die Längsrichtung) je eine Zirkularströmung entsteht, wobei die Strömungsrichtungen gegenläufig sind. Diese Rotationsart hängt von der Stromverteilung zwischen den Steigleitungen ab.
• Die dritte Strömungskomponente schliesslich besteht aus vier in den Zellenquadranten ausgebildeten Rotationen, wobei die diagonal gegenüberliegenden Rotationsrichtungen gleich sind. Diese Rotationen entstehen durch die ungleiche Stromverteilung in den Stromschienen und der Traverse von einem Zellenende zum anderen.

From a magnetic point of view, the current supply with direct electrical current is not particularly favorable. Superposition of three flow components creates movements in the liquid metal:

• The first flow component, which is in principle a circulation movement along the inner cell walls, has particularly harmful effects with regard to the stability of the electrolytic cell. This first component is created by the influence of the neighboring row of electrolytic cells, which returns the electrical current to the rectifier. The direction of rotation of the rotation depends on whether the neighboring row of cells is on the left or right of the cell in relation to the general direction of the direct current.
• The second flow component is that a circular flow occurs in each cell half (with respect to the longitudinal direction), the flow directions being opposite. This type of rotation depends on the current distribution between the risers.
• Finally, the third flow component consists of four rotations formed in the cell quadrants, the diagonally opposite directions of rotation being the same. These rotations result from the uneven distribution of current in the busbars and the crossbar from one end of the cell to the other.

The superimposition of these three flow components causes the speed of the metal flows within the cell to differ greatly. Where all three flow components run in the same direction, there is a high metal speed.

In view of these circumstances, the inventor has set himself the task of providing a rail arrangement of the type mentioned at the outset with which the greatest possible compensation of the electromagnetic force fields generated by the various current flows can be achieved.

To achieve the object according to the invention, a part of the cathode bar ends on each long side of the cell is combined to form a partial current bar, the partial current bars being brought together under the subsequent cell to form a common current bar and the common current bar under the cell in the longitudinal direction thereof to the downstream end of the traverse is led.

The rail arrangement according to the invention for longitudinal electrolysis cells is suitable for arrangements with currents up to 170 KA.

In a preferred rail arrangement, the partial current rails are arranged under each cell in the longitudinal center thereof and perpendicular to the longitudinal axis thereof, and the busbar runs in the longitudinal axis of the cell.

The partial busbars expediently run under each cell between support supports of the cathode trough, the busbar crossing the support supports. The arrangement of partial busbars and busbars is preferably arranged approximately halfway up the height of the support beams.

With the busbar configuration according to the invention, both the stationary state of the cell is improved by reducing the level differences in the liquid metal surface and the stability of the cell in the non-stationary state is reduced by the interference influences during cell operation.

- Fig. 1

einen Querschnitt durch eine Elektrolysezelle;

- Fig. 2

das Prinzip der magnetischen Kompensation.

Further advantages, features and details of the invention emerge from the following description of a preferred exemplary embodiment and with reference to the drawing; this shows schematically in

- Fig. 1

a cross section through an electrolytic cell;

- Fig. 2

the principle of magnetic compensation.

According to FIG. 1, an electrolysis cell 10 has a steel trough 12 which is lined with thermal insulation 14 and receives a carbon base 16. Cathode bars 18 are embedded in the carbon base 16, the ends of which extend through the steel trough 12 on both longitudinal sides. The cathode bars 18 are connected to busbars 22 via flexible current strips 20. The steel trough 12 is arranged at a distance h from the floor 26 and rests on steel supports 24.

The principle of magnetic compensation by means of the special conductor rail guidance results from the consideration of FIG. 2, which has the arrangement according to the invention for a series of electrolytic cells 10 with a nominal current strength of 140 KA. The general direction of the direct electrical current is designated I. The numbers in parentheses in FIG. 2 refer to the number of cathode bars, which are each brought together to form individual busbars. The current distribution within a cell depends on the current strength for the same cell type. Since there is no linear relationship between current intensity and current distribution, the current distribution, i.e. the exact number of cathode bar units combined to form individual busbars for a specific current density is calculated using magnetohydrodynamic models.

In the present example, the electrolytic cell 10 _{n is} equipped with 20 cathode bar ends on each longitudinal side of the cell, of which 26 cathode bar units feed the upstream end of the anode bar or the traverse 28 of the subsequent cell 10 _{n + 1} and 14 units the downstream end. 3 cathode bar units from each long side of the cell 10 _n are combined to form a partial busbar A, B and are guided along the longitudinal center m of the subsequent cell 10 _{n + 1} below the cell to its longitudinal axis x. In the middle of the cell's longitudinal axis x the two partial busbars A, B unite to form a busbar C which leads along the longitudinal axis x to the downstream end of the traverse 28.

The two partial busbars A, B run between the steel beams 24. The busbar C crosses the steel beams 24 in openings 25 provided for this purpose. The arrangement consisting of the partial busbars A, B and the busbar C, which has the shape of a "T", is located at a height a above the floor 26, which corresponds to approximately half the height h of the steel beams 24.

The magnetic influence of the partial busbars A, B and the busbar C is increased by the proximity of the electrolysis metal and by the ferromagnetic environment present as a result of the steel trough 12 and the steel support 24. The small distance between the busbars A, B and the busbar C to the electrolysis metal allows the current to be reduced by dividing the busbars into a "T". Magnetohydrodynamic calculations in the present case lead to the results summarized in the table below. Rail guide Current (KA) Stationary analysis Stability analysis Vmax (cm / s) Vmetal (cm / s) Δh (mm) Growth factor (1 / S) .10 ^-2 without "T" 140 28 7.8 37 1.5 with "T" 140 20th 6.6 28 .44 Vmax = maximum speed in the liquid metal
Vmetal = mean quadratic velocity in the liquid metal
Δh = level difference of the liquid metal surface

The calculated values clearly show the superiority of the conductor rail arrangement according to the invention in the form of a “T” compared to a conventional rail arrangement. The most important information comes from the stability analysis. The maximum of the growth factor associated with the excitation states is for the magnetically optimized rail guide in the form of a "T" by a factor of 3 less than the rail guide without "T". This results in a significant improvement in the stability of the electrolytic cell.

Claims (4)

Rail arrangement for guiding the direct electrical current from the cathode bar ends of a longitudinal electrolysis cell, in particular for the production of aluminum, via conductor rails to the traverse ends of the subsequent cell,
characterized,
that a part of the cathode bar ends on each long side of the cell (10 _n ) is combined to form a partial busbar (A, B), the partial busbars under the subsequent cell (10n _{+ 1} ) being brought together to form a busbar (C) and the busbar below the cell is guided in the longitudinal direction (x) to the downstream end of the crossmember (28). Rail arrangement according to Claim 1, characterized in that the partial current rails (A, B) are arranged under each cell (10) in the longitudinal center (m) and perpendicular to their longitudinal axis (x) and the busbar (C) in the longitudinal axis (x) the cell runs. Rail arrangement according to Claim 1 or 2, characterized in that the partial current rails (A, B) run under each cell (10) between support carriers (24) of the cathode trough (12) and the busbar (C) crosses the support carriers (24). Rail arrangement according to claim 3, characterized in that the partial busbars (A, B) and the busbar (C) are arranged under each cell (10) approximately at half the height (a) to the height (h) of the support beams (24).

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