DE102010041081B4 - Cathode for electrolysis cells - Google Patents

Abstract

A cathode (1) for an electrolytic cell for recovering aluminum from its oxide in an electrolytic bath, comprising an upper part (1a) facing the electrolytic bath and a lower part (1b) provided with terminals (1b1) for the power supply, characterized in that the upper part (1a) and the lower part (1b) are detachably connected together at least in sections via an intermediate layer (1c).

Description

The invention relates to a cathode for an electrolytic cell for the production of aluminum by fused-salt electrolysis.

For the industrial extraction of aluminum from its oxide is currently the so-called "Hall-Héroult process" used. This is an electrolysis process, in which aluminum oxide (Al ₂ O ₃ ) is dissolved in molten cryolite (Na ₃ [AlF ₆ ]) and the mixture thus produced serves as a liquid electrolyte in an electrolytic cell The basic structure of such an electrolytic cell for carrying out the Hall-Heroult's method is schematic in the 1a to 1c shown, where 1a shows a cross section through a conventional cell while 1b is a side view of the cell from the outside. 1c shows a perspective view of an electrolytic cell.

With the reference number 1 is a cathode referred to, which may be constructed for example of graphite, anthracite or a mixture thereof. Alternatively, graphitized coke-based cathodes can also be used. The cathode 1 is generally in a mount 2 embedded steel and / or refractory material or the like. The cathode 1 can be constructed both in one piece and from individual cathode blocks.

Across the length of the cell is into the cathode 1 a number of power supply bars 3 introduced, wherein in the cross-sectional view of 1a only a single power supply bar 3 it can be seen in 1c It can be seen that each cathode block, for example, two power supply bars can be provided. The power supply bars serve to supply the cell with the electricity needed for the electrolysis process. The cathode 1 opposite there are several typically cuboid anodes 4 , where in 1a two anodes 4 are shown schematically. 1c shows a more detailed arrangement of the anodes in an electrolytic cell. In carrying out the process is by applying a voltage between the cathode 1 and the anodes 4 the aluminum oxide dissolved in cryolite is split into aluminum and oxygen ions, the aluminum ions moving toward the molten aluminum - seen electrochemically as the actual cathode - in order to absorb electrons there. Because of the higher density aluminum accumulates 5 in the liquid phase below the molten mixture 6 made of aluminum oxide and cryolite. The oxygen ions are reduced at the anode to oxygen, which reacts with the carbon of the anodes.

With the reference numerals 7 and 8th are schematically the negative and positive poles of a voltage source for the supply of the voltage required in the electrolysis process, the value between, for example, about 3.5 and 5 V, characterized.

As in the side view of 1b can be seen, the enclosure points 2 and thus the entire electrolysis cell has an elongated shape, with numerous power supply bars 3 perpendicular through the side walls of the enclosure 2 are guided. Typically, the longitudinal extent of currently deployed cells is between about 8 and 15 meters, while the width dimension is about 3 to 4 meters. A cathode like this one in here 1a is shown, for example, in the EP 1845174 disclosed.

In conventional cathode blocks, substantially all components are made of only one material. However, this does not do justice to the fact that various requirements are placed on the various parts of a cathode for a fused-salt electrolysis process. Thus, in the region of the electrolytic bath or in that part of the cathode which comes into contact with the molten aluminum in the process described, material loss takes place by wear of the cathode material, in particular by chemical and mechanical processes in the electrolysis process, such as by flow movements. It is therefore necessary to renew the cathode from time to time, d. H. in this case, replace the entire lining of the electrolytic cell. In general, such a change takes place approximately every 1500 to 3000 days. Moreover, compromises have to be made with regard to the individual components with regard to their optimal design, as the requirements for the individual components are in part incompatible with one another. In addition, it is necessary, due to the frequent replacement of the entire material, such. As cathode blocks, ramming mass, side enclosure and insulation material to dispense with high quality materials, so as not to increase the aluminum production costs excessively.

It is therefore an object of the invention to provide a cathode for an electrolysis cell for the production of aluminum, which can overcome the aforementioned disadvantages of the prior art, with the particular material cost savings can be achieved while the cathode can be optimized in terms of their function.

This object is achieved by a cathode with the features of claim 1. Preferred embodiments are specified in the subclaims.

A cathode for an electrolytic cell for recovering aluminum from its oxide in an electrolytic bath according to the embodiments of the invention comprises: a) an upper portion facing the electrolytic bath, and b) a lower portion provided with terminals for the power supply. According to the invention, the upper part and the lower part are detachably connected to each other at least in sections via an intermediate layer. The upper part represents a bottom pan, which is in use in direct contact with the electrolytic bath.

The term "cathode" in the context of the present invention refers to the upper part in conjunction with the lower part. For the purposes of the invention, the term cathode is generally understood.

It may be z. B. - but not exclusively - to act as a so-called cathode bottom, which is composed of a plurality of cathode blocks, so that the core aspects of the invention - namely the structure described above from an upper part in conjunction with a lower part - of this cathode bottom as a whole will be realized. The term cathode, however, is also intended to refer to the substructures forming such a cathode bottom in the sense of cathode blocks. All features that can contribute to the invention in conjunction with a "cathode" do so in the same way in conjunction with a cathode block " this would have to be explained explicitly below.

Due to the two-part design of the cathode, it is possible to optimize the different functional areas in the production. Thus, the upper part serves to accommodate in the process the liquid electrolyte as well as the final product, namely the molten aluminum.

The upper region, which may also be referred to as the "consumption fraction" of the cathode, should be as resistant as possible to wear, for example as a result of mechanical, thermal and / or chemical stress, with respect to its construction. In any case, due to the fact that the upper area occasionally needs to be replaced due to the consumption of cathode material in the electrolysis reaction, the cost of the upper area material should be kept low. In contrast, the lower part of the cathode is to be designed for optimum current supply and current distribution. Due to this division into two parts, as is the feature of the present invention, now the two parts (upper part and lower part) can be manufactured separately from each other and then joined together via the intermediate layer. Thus, each part can be optimized in terms of its function, without this affecting the function of the other part negative. Thus, for example, the lower part of higher quality, expensive, but also less resistant to wear material are formed because it is not affected by the wear or wear-related replacement of the upper part. Overall, this results in a considerable savings in material costs, since not in every case a whole cathode is affected by the exchange, or all cathode blocks have to be replaced.

Another advantage of the invention is that the lower part can be protected by the intermediate layer from chemical influences from the electrolytic bath. Thus, the intermediate layer not only allows a first separate in an upper part and a lower part structure, but also helps that the advantage that the lower part is made of high quality material, not by penetrating to the lower part corrosive liquids or gases, such as z. B liquid aluminum or electrolyte components, is nullified.

The intermediate layer, which connects the upper part to the lower part, may for example be made of graphite foil, in particular a graphite foil. A graphite foil is particularly well suited to prevent penetration of liquid and / or gaseous bath constituents, such as liquid aluminum or electrolyte components, into the lower part or at least largely prevent it, wherein the actual function of the entire cathode is not significantly changed. The graphite foil as intermediate layer has similar electrical properties as the components of the cathode, in particular as the lower part. Graphite foil, which is produced by at least partial densification of expanded graphite, is particularly well suited to act as a separating layer against chemical influences from the electrolytic bath because of their anisotropy in the film surface and thus very low permeability perpendicular to the film. Graphite foil also acts to balance differences in surface structure between the upper part and the lower part, as well as thermal expansion and contraction movements, especially of the upper part. Graphite foil has a low electrical contact resistance to other carbon materials and a very good electrical conductivity Although the electrical resistivity perpendicular to the graphite foil is higher than in foil surface, a very low absolute electrical resistance can be achieved due to the very small thickness of graphite foil.

In the case of the construction of the cathode of individual cathode blocks, the intermediate layer is preferably not according to the size of the Cathode blocks provided, but advantageously covers a larger area than the respective lower part of the cathode blocks. The intermediate layer may advantageously have an area which corresponds to the size of the total cathode.

The intermediate layer can be formed with a very small thickness. For example, the layer may only be a single graphite foil. As a suitable film thickness, for example, the range between 1 mm and 5 mm has been found. This thickness is sufficient to perform the described functions and on the other hand thin enough that the properties of the film do not significantly affect the functionality of the entire cathode.

It may also be advantageous to use a plurality of layered graphite foils or to use graphite foils with larger thicknesses. The intermediate layer may be adjusted as desired or necessary with regard to its specific electrical conductivity and / or its electrical contact resistance. For this purpose, a coating of the intermediate layer can be provided, which reduces a contact resistance. It is also possible to specifically increase the specific electrical conductivity of the graphite foil in the thickness direction by known measures.

A suitable current flow within the cathode is used according to the prior art to keep the loss of material on the cathode surface in the interior of the cathode basin as evenly as possible. Since an optimization of the current flow in embodiments of the invention can be made specifically on the lower part, it is possible to make the upper part with respect to its design and thus its manufacture correspondingly simple.

In a cathode according to the invention, the upper part may be integrally formed with a side wall of the electrolysis cell. This means that the bottom wall and side walls are molded in one piece. As a result, problems of sealing and jointing between the bottom wall and side walls are avoided.

Since the lower part of the cathode does not come into contact with the liquid electrolyte or the aluminum melt when used in a molten electrolysis process, the resistance to mechanical or chemical wear in this part is not a criterion. Thus, this part is subject to little or even no maintenance susceptibility and does not need to be replaced at regular intervals, as is the case in the upper part. For this reason, higher quality materials can be used for the lower part. Such a material is, for example, highly conductive graphite, since a significant disadvantage of graphite, namely its low mechanical wear resistance, does not come into play for this application.

The lower part can be produced according to a preferred embodiment, for example using needle coke as starting material. As is known, needle coke is the highest quality petroleum or pitch coke, with the name derived from its needle-like structure. Needle coke is characterized, inter alia, by its lower thermal expansion coefficient and its low electrical resistivity after graphitization, in the longitudinal direction of the needle-shaped structure. This is particularly advantageous in the lower part of the cathode, where the streams flow at high density. By suitable design, the alignment of the acicular coke particles can be achieved in a vertical position. The reduction in electrical resistivity causes a lower voltage drop across the cathode and helps to achieve better energy efficiency in fused-salt electrolysis. As energy costs account for a large part of the total cost of the process, this can bring significant savings.

The upper part of the cathode can be made of any known materials suitable for use as a cathode. In particular, calcined anthracite, coke or graphite are to be mentioned as starting materials in this context. The starting material is ground and sorted by particle size. A defined mixture of the fractions of the grain is mixed with pitch and then formed from the upper part. Following this, one or more production steps take place at elevated temperature, distinguishing between a graphitized, a graphitic and an amorphous cathode material based on the heat treatment temperature and starting materials.

Advantageously, the cathode may have a vertical power supply. This is to be understood as meaning a vertical introduction of current into the lower part of the cathode from below. This advantageously makes it possible to avoid an uneven current distribution in the cathode as in a conventional horizontal power supply.

According to one embodiment of the cathode according to the invention, the lower part may be provided with vertical pins as power supply lines. These pins can be designed as threaded pins, the lower part threaded holes as connections for receiving the threaded pins In the tapped holes, pins provided with an external thread can be screwed vertically or approximately perpendicularly into the lower part of the cathode. In this way, in the context of fused-salt electrolysis, the current can be introduced approximately perpendicularly into the cathode. The power supply can be kept very homogeneous by the number and diameter of the pins of the geometry of the cathode is adjusted.

The geometry of the pins may advantageously correspond to the geometry of threaded nipples for graphite electrodes for electrical steel production. With regard to current distribution, mechanical strength and screwability, this geometry has proven to be particularly good. The relatively large cross-section of the pins causes a high electrical current flow, the length of a sufficiently large distance of the cathode and thus the electrolysis cell from the power supply bar, so that a strong cooling is possible.

According to a preferred embodiment, the pins are made of graphite. As a result, a high thermal stability of the pins and a low electrical resistance can be achieved, which leads to a reduction in the specific energy costs in carrying out the fused-salt electrolysis.

In terms of a homogeneous power supply, it has also proven to be favorable when the lower part of the cathode is in the form of a downwardly tapered trapezoidal body. In this way, the vertically or approximately perpendicularly introduced stream is homogeneously and evenly distributed in the upper part of the cathode. Preferably, in the case of the construction of the cathode of individual cathode blocks at least some of the cathode blocks of the cathode have such a downwardly tapered trapezoidal body, which advantageously extend parallel to each other. The trapezoidal bodies may extend, for example, in the longitudinal direction of the cathode or perpendicular thereto.

It should be noted that in the context of the invention, the term "approximately perpendicular" shall be used to encompass all directions which include an angle of less than about 20 ° to the vertical. It is intended to encompass "vertical" but in the broadest sense all vertical feeders that are not horizontally oriented in a conventional manner

The invention will now be explained in more detail with reference to the accompanying drawings with reference to a non-limiting embodiment. In the drawing show:

1a schematically an electrolytic cell for the extraction of aluminum from alumina according to the prior art in cross section;

1b the electrolytic cell of 1a in a longitudinal view from the outside;

1c an electrolytic cell for the extraction of aluminum from alumina according to the prior art in a perspective view, partially cut;

2a a perspective view of a cathode unit according to an embodiment of the invention; and

2 B a representation of the cathode unit of 2a from a 90 ° rotated perspective.

In the figures, like reference numerals are used to designate like or corresponding elements throughout the several views.

With reference to the 2a and 2 B is an electrolytic cell with an embodiment of a cathode according to the invention 1 shown from different perspectives. The cathode shown 1 is suitable for use in the recovery of aluminum from aluminum oxide by the Hall-Héroult process. The electrolytic cell is here with two side walls 1a1 provided, which together with a bottom wall 1a2 pick up the electrolysis bath. In the case shown, the side walls extend 1a1 along the long side of the cathode 1 , The side wall 1a1 is from single sidewall blocks 1a3 built up. The bottom wall 1a2 represents an upper or first part 1a the cathode 1 dar. The cathode 1 is in this embodiment of individual cathode blocks 11 built up.

A lower part 1b the cathode 1 includes in the embodiment shown a number of terminals 1b1 which are in a lower region of trapezoidal bodies 1b2 are formed, which are tapered down V-shaped. The connections 1b1 For example, in the form of internal threads (not visible in the figures) may be formed, in each case a pin 9 with corresponding external thread for the power supply to the cathode 1 take. Several of the pens 9 are at theirs the connections 1b1 opposite sides with power supply bars 3 connected, which to busbars 10 lead to the cathode 1 to connect to the corresponding pole of a voltage source.

The upper part 1a and the lower part 1b stand over an intermediate layer 1c in conjunction, which may be, for example, a graphite foil. This allows the top of the cathode to be removed without damaging the bottom. At the same time ensures the graphite foil that no liquid aluminum or electrolyte penetrates to the lower part and acts as a separation layer in this case has the graphite foil despite poorer specific electrical conductivity perpendicular to the film plane compared to the conductivity within the film plane because of its small thickness of for example a few millimeters a very low absolute electrical resistance and causes a very good electrical contact between the upper part and the lower part, so that the functionality of the cathode is not disturbed. In addition, the intermediate layer resembles an expansion of the two parts 1a . 1b for example due to thermal fluctuations.

Because the upper part 1a and the lower part 1b are formed separately from each other, the two parts can be made of different materials and have different properties with respect to thermal expansion and electrical resistance. So each part can be optimized especially with regard to its function. In particular, the upper part 1a be designed so that it can withstand wear, for example due to mechanical abrasion and uneven electrochemical decomposition as well as possible.

In contrast, the lower part should 1b be designed with a view to the most homogeneous power management and energy efficiency. For this he can be optimized with regard to the materials used, since the relatively quickly wearing upper part 1a which needs to be replaced more frequently, separated from the lower part 1b will be produced. So you can also expensive materials such as needle coke can be chosen to the durable lower part 1b to optimize the desired homogeneous power distribution.

As suitable materials for the power supply bars 3 In particular, copper and aluminum have proven their low electrical resistivities. Because the power supply bars through the pins 9 from the cathode 1 are spaced, they are strongly cooled and it is therefore not necessary to form them from high temperature resistant steel. Due to the lower electrical resistivity of said metals for the power supply bars 3 less energy is converted into waste heat and the energy efficiency of fused-salt electrolysis can be significantly increased. Also the shown rejuvenations 1d The trapezoidal body acts as an increase in distance between the upper part 1a the cathode 1 and the current-carrying power supply bar 3 and thus a cooling of the power supply bars 3 supportive.

As far as the materials for cathode are concerned, all materials known and suitable to the person skilled in the art for the electrolysis of aluminum from its oxide can be used. Suitable materials are for example in the DE 10261745 the content of which is hereby incorporated by reference. The pencils 9 can be made of the same materials as the cathode 1 be prepared. Graphite has proved to be particularly favorable in this context due to its temperature resistance and because of its low electrical resistivity.

LIST OF REFERENCE NUMBERS

1

cathode

1a

upper part

1a1

Side wall

1a2

bottom wall

1a3

Sidewall block

1b

lower part

1b1

connection

1b2

trapezoidal body

1c

interlayer

2

mount

3

Power supply bar, busbar

4

anode

5

aluminum

6

Electrolysis bath mixture (aluminum oxide, cryolite)

7

negative pole voltage source

8th

positive pole voltage source

9

pen

10

Collecting bar

11

cathode block

Claims (10)

Cathode ( 1 for an electrolytic cell for recovering aluminum from its oxide in an electrolytic bath, comprising an upper part facing the electrolytic bath (US Pat. 1a ) and a lower part ( 1b ), which with connections ( 1b1 ) is provided for the power supply, characterized in that the upper part ( 1a ) and the lower part ( 1b ) at least in sections via an intermediate layer ( 1c ) are releasably connected together. Cathode ( 1 ) according to claim 1, characterized in that the intermediate layer ( 1c ) is made of graphite. Cathode ( 1 ) according to claim 1 or 2, characterized in that the intermediate layer ( 1c ) is a graphite foil. Cathode ( 1 ) according to one or more of the preceding claims, characterized in that the lower part ( 1b ) is prepared using needle coke as a starting material. Cathode ( 1 ) according to one or more of the preceding claims, characterized in that the lower part ( 1b ) has a vertical power supply. Cathode ( 1 ) according to one or more of the preceding claims, characterized in that the lower part ( 1b ) with threaded holes as connections ( 1b1 ) is provided for receiving grub screws. Cathode ( 1 ) according to one or more of the preceding claims, characterized in that the upper part ( 1a ) is made with anthracite, coke or graphite. Cathode ( 1 ) according to one or more of the preceding claims, characterized in that the lower part ( 1b ) in the form of a downwardly tapered trapezoidal body ( 1b2 ) is trained. Cathode ( 1 ) according to one of the preceding claims, characterized in that the cathode ( 1 ) a plurality of cathode blocks ( 11 ), in particular from a plurality of cathode blocks ( 11 ), wherein the cathode blocks ( 11 ) are constructed in particular geometrically and / or structurally the same or the same acting and / or in particular are arranged laterally adjacent to each other. Electrolytic cell for recovering aluminum from its oxide, characterized in that it comprises a cathode ( 1 ) according to one or more of claims 1 to 9.

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