EP1233083A1 - Carbon bottom of electrolysis cell used in the production of aluminum - Google Patents

Abstract

Carbon bottom (18) for a cell for producing aluminum by electrolysis of molten alumina is made up of separate cathode blocks (16). Transverse conducting bars (22) under these fed current to them. The resistance of the blocks increases vertically as one moves from the center of the cell towards its walls (20).

Description

The invention relates to a coal bottom of an electrolysis cell for extraction of aluminum by melt flow electrolysis of aluminum oxide, the Carbon floor can be divided into individual cathode elements and the carbon floor or the cathode elements are electrically conductive with the side of the electrolytic cell led out current bar are connected.

For the production of aluminum by melt flow electrolysis of aluminum oxide, this is dissolved in a fluoride melt, which consists largely 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 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 takes place in a temperature range of approximately 940 to 970 ° C.

In an electrolytic cell with a carbon base according to the prior art is that of the anodes through the electrolyte melt, the liquid aluminum and electrical coal flowing to the cathodic bar Current deflected from its ideal vertical direction and receives one in Horizontal direction of the current bar against the cell wall Component. The resulting magnetic field leads to a strong flow in the Metal, connected with a curvature of the metal surface that is against the cell edge is pulled down heavily. To date, people have tried the magnetic field through targeted conductor rail routing outside the electrolysis bath to compensate and thus optimize the current flow.

The invention has for its object a coal floor of the aforementioned Way to create the flow of electricity through the cell without elaborate routing of conductor rails approximates the ideal current profile can be.

To achieve the object according to the invention, the coal floor of the middle of the cell to the edge of the cell in the direction of the current bar in vertical Has increasing electrical resistance direction.

The different electrolytic resistance of the coal floor in the z direction depending on the distance to the center of the cell causes the current due to the fact that the cathode bottom rests during operation of the electrolytic cell liquid aluminum can be approximated to the ideal vertical direction, whereby the horizontal current component decreases greatly and ideally even disappears. The coal floor according to the invention therefore enables one Increase in the electrical current flow in the cell, which is an increase in the Production equals.

A practically ideal current flow results when the electrical resistance from the center of the cell to the edge of the function ρ B = ρ 0 + ρ 1 xy 2 is approximate, where ρ ₀ and ρ _{1 are} constants and y is the distance from the center of the cell.

The carbon floor or the individual cathode elements can be in layers divided and the layers strung together in the direction of the current bar be, the individual layers from the center of the cell to the edge of the cell have increasing electrical resistance in the vertical direction. at an alternative embodiment of the coal floor according to the invention or the individual cathode elements point from the center of the cell to the Cell wall continuously in the direction of the current bar in the vertical direction and steadily increasing electrical resistance.

That for the movement of the liquid phases in an electrolysis cell for production The force field responsible for aluminum results from the vectorial Product of the current density and the magnetic field. With the use of an inventive Horizontal electricity flow is reduced, thereby reducing the force field also decreases. This results in much weaker movements in the liquid phases, i.e. the speeds in the liquid Aluminum and in the electrolyte bath decrease sharply, which affects the performance and the service life the electrolysis cell positively influenced.

• Fig. 1 einen Querschnitt durch eine Elektrolysezelle;
• Fig. 2 eine Schrägsicht auf die teilweise geschnittene Kathodenwanne der Elektrolysezelle von Fig. 1;
• Fig. 3 ein Detail von Fig. 2 in vergrösserter Darstellung;
• Fig. 4 einen Vertikalschnitt durch eine halbe Elektrolysezelle für eine analytische Modellrechnung;
• Fig. 5 die bildliche Darstellung des Stromflusses durch die Elektrolysezelle von Fig. 4 mit einem Kohleboden nach dem Stand der Technik nach einer Modellrechnung;
• Fig. 6 die Abhängigkeit des elektrischen Widerstandes eines erfindungsgemässen Kohlebodens in z-Richtung in Abhängigkeit von der Distanz zur Zellenmitte in y-Richtung;
• Fig. 7 die bildliche Darstellung des Stromflusses durch eine Elektrolysezelle gemäss Fig. 4 mit einem Kohleboden mit variablem elektrischen Widerstand in z-Richtung von der Zellenmitte in y-Richtung gemäss Fig. 6;
• Fig. 8 ein aus mehreren Schichten mit unterschiedlichem elektrischen Widerstand in z-Richtung gemäss Fig. 6 aufgebauter Kohleboden.

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

• 1 shows a cross section through an electrolytic cell.
• FIG. 2 shows an oblique view of the partially sectioned cathode tub of the electrolytic cell from FIG. 1;
• 3 shows a detail of FIG. 2 on an enlarged scale;
• 4 shows a vertical section through half an electrolysis cell for an analytical model calculation;
• 5 shows the pictorial representation of the current flow through the electrolytic cell from FIG. 4 with a carbon base according to the prior art according to a model calculation;
• 6 shows the dependence of the electrical resistance of a coal floor according to the invention in the z direction as a function of the distance to the center of the cell in the y direction;
• 7 shows the pictorial representation of the current flow through an electrolysis cell according to FIG. 4 with a carbon bottom with variable electrical resistance in the z direction from the center of the cell in the y direction according to FIG. 6;
• FIG. 8 is a coal floor made up of several layers with different electrical resistance in the z direction according to FIG. 6.

An electrolytic cell 10 shown in FIG. 1 for the production of aluminum by melt flow electrolysis of alumina exhibits one with a refractory and heat-insulating masonry 14 lined steel tub 12. On the masonry 14 at the bottom of the steel tub 12 are cathode elements 16 arranged in the form of coal blocks. The at the side joints The resulting gaps of the individual cathode elements are used for production good thermal and electrical conductivity of the connection point adjacent cathode elements 16 in a known manner with a Sealed contact mass, so that all sides thermally and electrically well conductive cathode or carbon base 18 is formed. The side walls 20 exist from individual blocks of a coal mass and lie on the masonry 14 of the steel tub 12.

As shown in FIG. 2, the electrolytic cell 10 essentially has one rectangular plan with a horizontal longitudinal direction x, one rectangular to this horizontal horizontal direction y and one on the by the Directions x, y defined plane vertical vertical direction z. The Coal base 18 is supported by current conducting or cathode bars running in the y direction 22 contacted. In the present example, the bars 22 are in the middle of the coal floor 18 separated. The cathode bars 22 can, however, also from Side wall to side wall of the steel tub 12 be continuous. Likewise can the coal floor 18 can be designed in different ways. The cathode elements 16 can, for example, with regard to their shape and extent in the x and y directions vary, or the coal floor 18 can also consist of a single one Block exist.

As shown in FIG. 3, the individual cathode elements 16 have a groove 24 on. The cathode bar 22 is in the grooves 24 via a cast iron jacket 26 poured from strung cathode elements 16. The connection between cathode bars 22 and cathode elements 16 can of course can also be carried out in other ways, for example by gluing with an electrically and thermally conductive contact mass.

As shown in FIG. 1, the electrolysis cell 10 is on during operation the coal floor 18 a layer 28 of liquid aluminum. About this Aluminum layer is a cryolite melt 30 with dissolved therein Alumina. Carbon anodes 32 dip into the cryolite melt 30 from above on. The carbon anodes 32 are anchored in these anode rods 34 on a not shown, extending in the longitudinal direction x of the electrolytic cell 10 Traverse attached. During the electrolysis, it forms on the cryolite melt 30 a crust 36 of solidified cryolite. The crust 36 is from Broken up from time to time to supply alumina to the cryolite melt. The liquid that collects as a layer 28 on the coal floor 18 Aluminum is also used from time to time, for example via a siphon removed from the electrolytic cell 10 by means of vacuum.

In the half cross section through the electrolytic cell 10 shown schematically in FIG. 4 in the y direction, the cathode bar 22 has a height h _K , the carbon floor 18 arranged above it has a height h _B and the layer 28 of liquid aluminum lying on the carbon floor 18. U ₀ means the electrical potential at the aluminum / carbon floor contact surface, U (y) the electrical potential at the carbon floor / cathode bar interface which is dependent on the electrical conductivity of the carbon floor in the z direction from the cell center to the cell edge in the direction y of the cathode bars 18. L designates half the width of the coal floor 18 in the y direction.

The cathode elements 16 or the carbon base 18 made therefrom is characterized by its electrical conductivity. The manufacturers of the cathode elements define the conductivity in the extrusion direction accordingly the y-direction and in the directions x and z perpendicular to it. at In a conventional coal floor 18, the conductivity values are shown in all directions for all cathode elements 16 the same. This condition leads to a in the y-direction against the side wall 20 of the electrolytic cell 10 increased current flow. 5, the current distribution is in a two-dimensional Model calculation assuming a constant conductivity on all sides of the coal floor 18 represented by arrows 38. Are clearly recognizable here the horizontal power components.

ρ_B

elektrischer Widerstand des Kohlebodens 18 bzw. der Kathodenelemente 16 in z-Richtung

ρ₀, ρ₁

Konstanten

h_B

Höhe des Kohlebodens 18 bzw. der Kathodenelemente 16

h_K

Höhe des Kathodenbarrens 22

L

Distanz des Kohlebodens 18 bzw. der Kathodenelemente 16 von der Zellenmitte zum Zellenrand

σ_K

elektrische Leitfähigkeit der Stromleit- oder Kathodenbarren 22

ρ₀ und ρ₁ sind Funktionen in Abhängigkeit der in Klammern gesetzten Parameter. Da die elektrischen Eigenschaften der Stromleit- oder Kathodenbarren 22 gegeben sind, sind somit die Konstanten ρ₀, ρ₁ eindeutig bestimmt.A model calculation leads to the following assuming that the electric current in the layer 28 made of liquid aluminum and in the carbon floor 18 or in the cathode elements 16 runs vertically in the z direction and in the current conducting or cathode bars 22 horizontally in the y direction Result: ρ B = ρ 0 + ρ 1 xy 2 The following applies ρ 0 = f (h B , L, h K , σ K ) ρ 1 = f (h B , H k , σ K ) ρ 1 = 1 2 x σ K xh K xh B The parameters used mean

ρ _B

electrical resistance of the carbon base 18 or the cathode elements 16 in the z direction

ρ ₀ , ρ ₁

constants

h _B

Height of the carbon base 18 or the cathode elements 16

h _K

Height of the cathode bar 22

L

Distance of the carbon base 18 or the cathode elements 16 from the cell center to the cell edge

σ _K

electrical conductivity of the current conducting or cathode bars 22

ρ ₀ and ρ ₁ are functions depending on the parameters in parentheses. Since the electrical properties of the current conducting or cathode bars 22 are given, the constants ρ ₀ , ρ ₁ are thus clearly determined.

The model calculation shows that in order to achieve a vertical current flow through the layer 28 of liquid aluminum and through the carbon base 18 or the cathode elements 16 in the vertical z direction, the electrical resistance, which is equivalent to the inverse value of the electrical conductivity, has a quadratic course depending on the distance from the center of the electrolytic cell 10 in the direction y against the side wall 20 must show an essentially square profile. The constants ρ ₀ and ρ ₁ which are included in the formula for ρ _B depend on the geometric parameters of the carbon floor 18 or the cathode elements 16 and the cathode bar 22 and on the electrical conductivity of the cathode bar 22.

6 shows the values for the electrical resistance ρ _B in the z direction determined by the model calculation for the carbon floor 18 as a function of the distance from the center of the cell M to the side edge in the y direction. Assuming this quadratic dependency of the electrical resistance from the center of the cell M to the cell edge, the model calculation gives the current distribution, illustrated in FIG. 7 by arrows 38, through a cathode base 18 with variable conductivity in the y direction. It can be clearly seen that the current runs much more vertically than in the case of a conventional cathode base shown in FIG. 5. In fact, the horizontal current flow is about ten times weaker than when using carbon floors according to the prior art with constant conductivity in the y direction.

FIG. 8 shows a possible embodiment of a carbon base 18 with the electrical resistance increasing in the z direction from the cell center M towards the cell edge in the direction y of the cathode bars 22 in accordance with the values shown in FIG. 6. In the example shown, the coal floor 18 is made up of individual layers S ₁ to S ₁₀ which run in the longitudinal direction x and abut one another laterally in the y direction. Each of these layers or blocks has an electrical resistance ρ _B in the z direction, corresponding to its distance from the cell center M, which has been calculated for this position and is shown in FIG. The electrical resistance ρ _{B of} the individual layers or blocks S ₁ to S ₁₀ is thus constant for each layer or for each block, but varies from layer to layer or from block to block. The individual layers or blocks can, for example, be glued to one another in a known manner. Of course, the individual layers S ₁ to S ₁₀ in the longitudinal direction x of the carbon base 18 can also be divided into cathode elements 16 of shorter dimensions. Although ten layers S ₁ to S ₁₀ , each with different electrical resistance ρ _B , are shown in FIG. 8 from the cell center M to the cell edge, a model calculation has shown that the desired current distribution according to FIG. 7 already has five layers or blocks with different electrical Resistance ρ _{B is} sufficient to achieve practically the same positive results as with a division into ten layers. The use of just five layers allows the individual blocks to be manufactured more easily and also leads to less expensive production.

In an alternative embodiment of the coal floor 18, the individual Cathode elements 16 also one in the extrusion or y direction of the cell center M to the cell edge continuously increasing electrical resistance exhibit. The coal floor 18 can thus be inexpensive also in one piece with a continuous from the cell center M to the cell edge and steadily increasing electrical resistance.

There is to achieve a coal floor with variable electrical resistance different manufacturing possibilities. These range from the selection specifically composite coal mass of carbon or graphite up to different admixture of aggregates.

Claims (4)

Carbon bottom of an electrolysis cell (10) for the production of aluminum by melt flow electrolysis of aluminum oxide, wherein the carbon bottom (18) can be divided into individual cathode elements (16) and the carbon bottom (18) or the cathode elements (16) are electrically conductive with the side of the electrolysis cell (10) lead-out bars (22) are connected,
characterized in that
the coal bottom (18) from the cell center (M) to the cell edge has an increasing electrical resistance (ρ _B ) in the direction (y) of the current conducting bars (22) in the vertical direction (z). Coal floor according to claim 1, characterized in that the electrical resistance (ρ _B ) from the cell center (M) to the cell edge of the function ρ B = ρ 0 + ρ 1 xy 2 is approximate, where ρ ₀ and ρ _{1 are} constants and y is the distance from the center of the cell (M). Coal floor according to claim 1 or 2, characterized in that the coal floor (18) or the individual cathode elements (16) is divided into layers (S ₁ to S ₁₀ ) and the layers (S ₁ to S ₁₀ ) in the (y) direction Current conducting bars (22) are strung together, the individual layers (S ₁ to S ₁₀ ) from the cell center (M) to the cell edge having an increasing electrical resistance (ρ _B ) in the vertical direction (z). Coal floor according to claim 1 or 2, characterized in that the carbon floor (18) or the individual cathode elements (16) from the cell center (M) to the cell edge one in the direction (y) of the current conducting bars (22) in the vertical direction (z) continuously and have steadily increasing electrical resistance (ρ _B ).

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