EP2619353A1

EP2619353A1 - Cathode for electrolysis cells

Info

Publication number: EP2619353A1
Application number: EP11761552.6A
Authority: EP
Inventors: Christian Bruch; Frank Hiltmann; Johann Daimer; Manfred Banek
Original assignee: SGL Carbon SE
Current assignee: SGL Carbon SE
Priority date: 2010-09-20
Filing date: 2011-09-20
Publication date: 2013-07-31
Also published as: WO2012038422A1; CA2811355A1; JP2013537938A; RU2013118029A; DE102010041082A1; CN103154326A; JP5635196B2

Abstract

The invention relates to a cathode (1) for an electrolysis cell for obtaining aluminium from its oxide, comprising a lower side (1g). According to the invention, the cathode (1) is provided with a number of rods (1f) for a power supply, said rods contacting the lower side (1g) of the cathode (1) from below during operation in a power-supplying manner.

Description

Cathode for electrolysis cells

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-Heroult process" used. This is an electrolysis process in which aluminum oxide (Al 2 O ₃ ) is dissolved in molten cryolite (Na ₃ [AIF ₆ ]) and the mixture thus produced serves as a liquid electrolyte in an electrolysis cell. The basic structure of such an electrolysis cell for carrying out the Hall-Heroult process is shown schematically in FIGS. 1 a to 1 c, FIG. 1 a showing a cross section through a conventional cell, while FIG. 1 b is a side view of the cell from the outside. Fig. 1 c shows a perspective view of an electrolytic cell.

The reference numeral 1 denotes a cathode, which may be constructed, for example, from graphite, anthracite or a mixture thereof. Alternatively, graphitized coke-based cathodes can also be used. The cathode 1 is generally embedded in a skirt 2 of steel and / or refractory or the like. The cathode 1 can be constructed in one piece as well as from individual cathode blocks.

Across the length of the cell, a number of power supply bars 3 are introduced into the cathode 1, wherein only a single power supply bar 3 can be seen in the cross-sectional view of FIG. 1a. In Fig. 1 c 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. Opposite the cathode 1 there are several cuboidal anodes 4, wherein two anodes 4 are shown schematically in FIG. 1a. Fig. 1 c shows a more detailed arrangement of the anodes in an electrolytic cell. In the By carrying out the process, 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 by electric current flow, the aluminum ions moving to the molten aluminum - seen electrochemically as the actual cathode - to electrons there take. Because of the higher density, aluminum 5 accumulates in the liquid phase below the melt 6 of alumina and cryolite. The oxygen ions are reduced at the anode to oxygen, which reacts with the carbon of the anodes.

By the reference numerals 7 and 8 are schematically the negative and positive poles of a voltage source for the provision of the

Electrolysis process required voltage whose value is between about 3.5 and 5 V, characterized.

As can be seen in the side view of Figure 1 b, the enclosure 2 and thus the entire electrolysis cell has an elongated shape, with numerous power supply bars 3 are guided vertically through the side walls of the enclosure 2. 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, as shown here in FIG. 1 a, is disclosed, for example, in EP 1845174.

The high binding energy between aluminum and oxygen as well as heat and resistance losses require a high energy requirement in the production of aluminum via fused-salt electrolysis. The associated energy costs represent a large part of the process costs. Reducing these costs is one of the essential tasks to be accomplished in the field of fused-salt electrolysis.

One factor for low energy efficiency are the already mentioned power supply bars, which u. a. due to the high at the

Melting flux electrolysis temperatures occurring conventionally Steel are made. However, steel has a higher electrical resistivity than other electrical conductors that can be used in other low temperature engineering applications, such as aluminum and copper. Furthermore, the conventionally lateral horizontal supply of the electric current via the power supply bars causes an inhomogeneous current distribution in the cathode.

In addition, in the prior art, the different coefficients of thermal expansion of the cathode and power supply bars can lead to problems during startup and operation of the electrolytic cell, especially if the different coefficients of thermal expansion during contact, the so-called "pouring" with liquid cast iron, are not taken into account by suitable measures.

It is an object of the invention to provide a cathode for an electrolytic cell for the production of aluminum, which overcomes the above-described problems of the prior art, which in particular can increase the energy efficiency compared to conventional electrolysis cells and at the same time a more homogeneous cathodic current density distribution can be achieved can.

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 with a bottom is provided, according to embodiments of the invention, with a plurality of pins for a power supply, the pins contacting the underside of the cathode during operation in a current-supplying manner.

For the purposes of the invention, the term "cathode" is understood to be quite general, which may be, for example but not exclusively, a so-called "cathode" Act cathode base, which is composed of a plurality of cathode blocks, so that the core aspects of the invention - namely the above-described current-supplying contacting the bottom of the cathode with a plurality of pins from below - are realized by this cathode bottom as a whole. The term cathode, however, is also intended to refer to the substructures forming such a cathode bottom in the sense of cathode blocks. All the features which may contribute to the invention in connection with a "cathode" do so in the same way in connection with a "cathode block", without this having to be explicitly explained in the following.

Unlike the prior art, a power supply to the cathode from below is realized by the pins. By means of the number of pins from the bottom, the power supply to the cathode can be made much more homogeneous than was previously possible by a few power supply bars from the side into the cathode.

For example, in the case of a cathode bottom which is flat, such a homogeneous current distribution can be given a particularly homogeneous characteristic in that the pins are designed in accordance with the desired current flow (for example in cross-section) and arranged. For example, the pins may be arranged regularly and / or at equal distances to the adjacent pins. However, pens can also be arranged specifically according to patterns or distribution schemes, so that an inhomogeneous current distribution in the cathode is counteracted. This can be advantageous, for example, in edge regions of the cathode.

In addition, such an arrangement with a number of pins in carrying out the fused-salt electrolysis process can also achieve a reduced wave formation in the liquid aluminum which accumulates on the cathode. The reason for this is that this wave formation, as is known, by horizontal currents in the liquid aluminum, caused by an inhomogeneous cathodic current distribution, and their alternating is generated with the magnetic field. By achieved by means of cross-section, number and distribution of the pins electrical and magnetic design, this undesirable interaction can be reduced or minimized.

According to a preferred embodiment, at least one of the pins, in particular all the pins, contacts the underside at an angle of less than 30 ° to the vertical of the underside, in particular substantially perpendicularly. Such an arrangement is technically particularly simple and therefore inexpensive executable and is particularly uncomplicated in terms of load distribution and mechanical design of the pins.

In an advantageous embodiment of the cathode, the pins open into the cathode. This means that one of the cathode or its underside facing the end of the respective pin in a mounted state is partially within the cathode. As a result, a particularly good electrical contact of the pen is achieved to the cathode.

Alternatively, however, other Kontaktierungsvarianten are conceivable, which are also included in the invention. This may for example comprise an arrangement in which the cathode rests on the pins, wherein the pins do not open into the cathode. It can be improved by carbonaceous or metallic compounds such as carbonized pitch, electrically conductive adhesive or carbon masses, the electrical contact between the pin and the bottom of the cathode.

According to a particularly preferred embodiment of the invention, the pins are inserted by a screw in the bottom of the cathode. This allows, for example, a pre-assembly of the pins in the cathode before the cathode is installed in an electrolytic cell. Furthermore, a particularly secure hold and, if necessary, an uncomplicated removal for maintenance or renewal is ensured.

Particularly advantageous is the screw through a thread on a End of the pin, which faces the bottom of the cathode, and formed a mating thread in the bottom of the cathode. Since no additional screwing devices are required for this variant, but the pin itself is a screwing device, a particularly uncomplicated and material-saving screwing device is provided.

With regard to their geometry, such threaded pins can 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.

In thermal and electrical terms, it is particularly advantageous to produce threaded pins of the same material as the cathode. Thus, mechanical stresses and contact resistance are minimized.

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 results in a better energy efficiency in carrying out the melt electrolysis.

The longer the pins, the further away the busbars which supply them with power are removed from the cathode and thus from the region of high process temperatures. Also decreases with increasing distance, the strength of the caused by the current flow in the interior of the solid busbars electromagnetic fields, which in turn is favorable for the avoidance of the already mentioned wave formation in the aluminum melt. In this regard, it has been found to be particularly suitable when the pins have a length between about 100 mm and 500 mm.

One way to influence the current distribution in the cathode, represents the choice of the cross section of the pins. On the one hand, the more homogeneous the power distribution, the more pins are used and the thinner these pins. However, a lower limit will be limited by the costs and Zes defined a variety of pins. In practice, pins of a diameter between about 30 mm and 200 mm have proven to be particularly suitable in this regard. It should be noted that the individual choice is to be adapted to the structural features of the respective present cathode shape, so that it may optionally be advantageous to use pins with other dimensions.

As already mentioned, an advantage of a cathode according to the embodiments of the invention is to be able to influence the current conduction in the cathode in the sense of greater homogeneity. One means of doing this is to put the pins as close as possible. For example, it has proven to be beneficial if there are between about 4 and about 100 pins per square meter of bottom wall of the cathode.

In order to connect the pins to a voltage source, in one embodiment, a plurality of pins each at its end applied by the cathode with a common bus bar via, for example, a

Connected screw or clamp connection. Thus, as already mentioned, the bus bars are spaced from the cathode by a distance corresponding to the length of the unthreaded pins. It must be ensured that due to the thermal expansion of the material of the busbar (metal), the screw connection must be designed so that act on the pins, not too large mechanical stresses. Such a metallic screw connection between a pin and a busbar can be easily realized by those skilled in the field of connection technology, which is why the details should not be discussed in more detail here.

In a cathode according to an embodiment of the invention, since the bus bars are spaced from the cathode itself, it is not necessary for the bus bars to be made of a thermally stable material. Thus, the busbars would not be made of iron, steel or the like. Rather, the common bus bar may be made of a material containing more than 50% copper or aluminum contains. Both copper and aluminum have a lower electrical resistivity than iron or steel, so that the

Energy efficiency in fused-salt electrolysis can be increased. Of course, the busbars may also be made of pure copper or pure aluminum, or of only slightly alloyed copper or aluminum. For example, if a copper rod is used as a busbar, the specific electrical resistance to the steel used to date can be reduced to about a quarter.

According to a further embodiment, the underside of the cathode is in the form of downwardly tapered trapezoidal bodies. In this way, the current introduced from below is introduced homogeneously and uniformly into 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.

In this way, the distance between the hot cathode basin or cathode and busbar can be further increased, so that in operation at the location of the busbar substantially lower temperatures prevail than in the area of the cathode basin.

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:

Fig. 1 a, an electrolytic cell for the extraction of aluminum

Aluminum oxide according to the prior art in cross section;

Fig. 1b the electrolytic cell of Fig. 1 a in a longitudinal view of

Outside; FIG. 1 c shows an electrolysis cell for the extraction of aluminum from aluminum oxide according to the prior art in a perspective view, partially cut away; FIG.

Fig. 2a is a perspective view of a cathode according to an embodiment of the invention;

2b shows a representation of the cathode of Figure 2a from a rotated by 90 ° perspective ..; and

Fig. 3 is a sectional view of a screw connection of an electrolytic cell according to the invention.

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

With reference to FIGS. 2a and 2b, an electrolysis cell with an embodiment of a cathode according to the invention is shown from respectively different perspectives. The cathode shown is suitable for use in the recovery of aluminum from alumina according to the previously described Hall-Heroult process. On the cathode 1 side walls 1 a are arranged. The side walls 1 a may be integrally formed with the cathode 1 or connected to it and extend in the case shown along the longitudinal side of the cathode first The side walls 1 a are composed in this example of individual side wall blocks 1 a1. Likewise, in this example, the cathode 1 is composed of individual cathode blocks 1 b1.

By the side walls 1 a and the cathode 1, a basin 1 c is limited, in which in the use of the cathode in a process for melt flow electrolysis of the liquid electrolyte and the aluminum melt produced are recorded. As can be seen in the figures, the underside 1 g of the cathode 1 is trapezoidal in the form of downwardly tapered trapezoidal bodies 1 d formed, which extend parallel to each other along the width b of the cathode. The trapezoidal bodies are flat here below. In a bottom portion of the trapezoidal body 1 d pins 1 e are fixed in this. In this embodiment, the pins are designed as threaded pins similar threaded nipples for connecting graphite electrodes for electrical steel production, as can be seen schematically in Fig. 3.

In an alternative embodiment, however, other connecting means may be selected, for example a clamping connection.

The setscrews 1 e here have a circular cross-section, which is advantageous from the point of view of the homogeneity of the current density. As can be seen in FIG. 2 b, the pins 1 e have, by way of example, a slight zigzag shape with regard to their distribution. It should be noted that although a shape of the cathode 1 with trapezoidal bodies 1 d is shown on its lower side 1 g, this configuration is not mandatory. Rather, the bottom 1 g of the cathode 1 may be configured flat, as well as the top of the cathode first In such an embodiment, the pins 1 e may be arranged in any form on the underside.

For example, as shown in Fig. 2a, the pins may be arranged in a line. Of course, a plurality of such linearly arranged pins can also be arranged parallel to one another.

In each case all pins 1 e, which are arranged along a trapezoidal body 1 d, are connected at their lower sides, ie at the sides facing away from the cathode 1 sides with a common busbar 3, for example, again by screwing. In Fig. 3 is a possible screw of a pin 1 e with the busbar 3 is shown. There are in this Case two screws from the side of the busbar 3 in the bottom of a pin 1 e introduced.

Since the busbar 3, as seen in the figures, from the basin 1 c of the cathode 1 and thus in operation from the high temperature range is spaced, the temperature against which the busbar 3 must be resistant, much lower than those temperatures which the busbars in the embodiments of the prior art according to Figures 1 a to 1 c prevail. In other words, the busbars 3 are outsourced here from the high-temperature zone and can therefore be made of a less temperature-resistant but, for example, more electrically conductive material than steel.

In this embodiment, the busbars 3 are formed of an aluminum alloy. Alternatively, known copper alloys can be used. The trapezoidal bodies 1 d shown also act in the sense of increasing the distance between the basin 1 c of the cathode 1 and the busbars 3.

An optionally too strong dissipation of heat of not only electrically, but also very good thermal conductivity metals can preferably be met by suitable design, in particular geometric design.

As can be seen in the figures, in each case a plurality of busbars 3 (in each case three busbars 3) are connected to a common busbar 10 which leads to a voltage source (not shown). Collective busbar 10 and busbars 3 may be made of the same materials, for example. In an alternative, a one-piece embodiment is possible.

Depending on how soft the bottom of the cathode 1 is designed, various arrangements for the pins 1 e are possible. Is the bottom made flat, the pins 1 e may be attached, for example in the form of a regular grid on the bottom. This is particularly favorable in view of the already mentioned homogeneity of the current flow.

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 specified, for example, in DE 10261745, the relevant content of which is hereby incorporated by reference. The pins 1 e can in particular be made of the same materials as the cathode 1. 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

1 b1 cathode block

1 a side wall

1 a1 sidewall block

1 c basin

1 d trapezoidal body

1 e pen

g bottom of the cathode

2 facing

3 busbar

4 anodes

5 aluminum

6 mixture (alumina, kyrolite)

7 negative pole voltage source

8 positive pole voltage source

10 busbar

Claims

claims

A cathode (1) for an electrolytic cell for recovering aluminum from its oxide, comprising a lower surface (1 g), characterized in that the cathode (1) is provided with a number of pins (1f) for a current supply, the From below, contact the pins (1 g) of the cathode (1) during operation to supply current.

2. Cathode (1) according to claim 1, characterized in that at least one of the pins the bottom (1 g) contacted at an angle of less than 30 ° to the vertical of the bottom (1 g), in particular at least substantially perpendicular to the bottom ( 1 g).

3. Cathode (1) according to claim 1 or 2, characterized in that the pins (1f) open into the cathode (1).

4. Cathode (1) according to one or more of the preceding claims, characterized in that the pins (1f) are inserted by a screw in the bottom (1 g) of the cathode (1).

5. Cathode (1) according to claim 4, characterized in that the threaded connection by a thread at one end of the pin (1f), which faces the bottom (1 g), and a mating thread in the bottom (1 g) of the cathode (1) is formed.

6. Cathode (1) according to one or more of the preceding claims, characterized in that the pins are made of graphite.

7. Cathode (1) according to one or more of the preceding claims, characterized in that the pins have a length between about 100 mm and 500 mm.

8. Cathode (1) according to one or more of the preceding claims, characterized in that the pins (1f) have a diameter between about 30 mm and 200 mm.

9. Cathode (1) according to one or more of the preceding claims, characterized in that per square meter of cathode (1) between about 4 and about 100 pins (1f) are present.

10. Cathode (1) according to one or more of the preceding claims, characterized in that in each case a plurality of pins (1f) are connected at their from the cathode (1) remote from the end with a busbar (3).

11. Cathode (1) according to claim 8, characterized in that the busbar (3) is made of a material containing more than 50% copper and / or aluminum.

12. Cathode (1) according to one or more of the preceding claims, characterized in that the underside (1 g) of the cathode (1) trapezoidal in the form of downwardly tapered trapezoidal bodies (1 d) is formed.