FIELD OF THE INVENTION

This invention relates to an aluminium production cell of the type comprising a carbon cathode fitted with an elongated cathode current collector bar of highly electrically conductive metal such as copper provided with an outer protective layer of a temperature-resistant metal such as steel that is in contact with the carbon cathode.

BACKGROUND OF THE INVENTION

Aluminium is produced by electrolysis of alumina dissolved in cryolite based electrolytes at a temperature up to 980°C. A typical Hall-Heroult cell for aluminium production cell is composed of a steel shell, an insulating lining of refractory materials and a carbon cathode that holds the liquid metal. The cathode is usually composed of cathode blocks with embedded collector bars to extract the current flowing through the cell.

WO 01/63014 describes a collector bar construction for use in a Hall-Heroult reduction cell to produce aluminium. Each collector bar includes a core of relatively high electrical conductivity material (copper or a copper alloy) and an outer housing of a more chemically resistant material than the core material, typically of steel. This steel layer/coating/casing was applied directly on the copper core. Each collector bar includes a section that is cast or glued in a channel of the cathode block. When the cell is put into service, the end surface that fits against the cathode and its side surfaces is typically rodded with cast iron. In this usual configuration, the outer layer of steel is in contact with the carbon cathode through the layer of cast iron.

WO 01/63014 was a development of previous proposals in US 3’551’319 and US 5’976’333 for a current collector with a copper core coated with a ferrous (steel) coating, both of which proposals also required rodding with cast iron between the ferrous/steel outer coating and the carbon cathode.

Some proposals have been made to dispense with the need for rodding with cast iron or ramming paste. For example, WO 2018/019910 describes a collector bar of copper with a thin protective steel layer bar, that includes an inclined part such that the cell does not require rodding with cast iron or ramming paste. Other proposals dispense with such an outer protective layer of steel on the cathode current collector. For example, US Patent 11 ’ 136’682 presents a solution with a copper connector bar that is in direct contact with the carbon cathode, or in contact with the carbon cathode through an electrically conductive interface formed by an electrically conductive glue and/or an electrically conductive flexible foil or sheet applied over the surface of the copper connector bar.

SUMMARY OF THE INVENTION

The invention relates to an aluminium production cell with a carbon cathode equipped with an optimized current collector bar designed to reduce the energy consumption and increase the cell lifetime.

The invention also provides a solution that allows inexpensive implementations of copper or copper alloy current collector bars inside carbon cathode blocks.

The invention in particular concerns an aluminium production cell of the type comprising a carbon cathode and an elongated cathode current collector bar of highly electrically conductive metal provided with an outer protective layer of a temperature- resistant metal that is in contact with the carbon cathode.

According to the main aspect of the invention, the elongated cathode current collector bar of highly electrically conductive metal is coated with a carbon-based layer comprising ramming paste; ramming paste containing electrically-conductive particles; carbon particles; carbon pieces, solid carbon layers; carbon paste and/or carbon glue containing electrically conductive particles^ and the outer protective layer of temperature-resistant metal is disposed over and/or around the carbon-based layer.

The inventive solution has the advantage that it can be directly implemented in the cell as it avoids rodding with cast iron or ramming paste.

Furthermore, the collector bars can be made significantly smaller in size than steel conductor bars of conventional cells, leading to a longer cell life.

Lastly, the current collector bars can be designed to reach a very low electrical resistance. In other words, to provide a very low cathode voltage drop (CVD) that opens up options for operating the cells with a low energy consumption. The inventive cell can be implemented with the following preferred features.

The carbon-based layer may for example consist of ramming paste, in particular highly electrically conductive ramming paste with an electrical resistivity less than 40 pQ-m at operating conditions, in particular ramming paste containing electrically conductive particles such as steel or copper particles; or it may consist of ramming paste together with carbon particles or carbon pieces; or the carbon-based layer may consist of carbon glue containing electrically conductive particles for example steel chips or copper chips.

Ramming paste is well known in the aluminium production industry and is traditionally used as a carbon interface between the carbon cathodes and the side lining for creating an expansion joint that seals the cell and prevents liquid metal infiltrations. Ramming paste is also used as conductive interface for rodding the cathode current feeder to the carbon cathodes when the current feeder is installed.

Ramming pastes can have different compositions. Typical ramming pastes are prepared from calcined anthracite, graphite powder and metallurgical coke with coal tar as binding agent.

One usual composition is for example Nningxia Carbonvalley Cold Lining Paste Type- K which comprises a high content (80-90% by weight) of graphitize anthracite and a lower content (10-20% by weight) of coal tar pitch.

Various other types of commercially available ramming paste are for instance the CleO2 clean ramming pastes from Carbone Savoie and SmartRam RO20 Ramming Paste from GrafTech International Holdings Inc.

The patent literature also describes various types of ramming paste. For example, CN102850072A describes the production of cold ramming paste by mixing and ramming the following raw materials, by gross weight: 45-70% of calcined anthracite, 8-20% of artificial graphite, 13-15% of coal tar and 3-5% of anthracite or wash oil. US5676807A describes an aluminium production cell with ramming paste that consists essentially of 50-98% weight % of carbonaceous materials, l-60weight% of fillers and 1.30 weight % of binder.

Depending on their composition, ramming pastes can have different electrical properties. For the present applications, use is preferably made of a highly electrically conductive ramming paste, i.e. with an electrical resistivity lower than 40 pQ-m (CleO2 quotes a resistivity of 55 pQ-m at 20°C after baking and 37 pQ-m at 1000°C). The resistivity of any ramming paste can be decreased by using steel or copper chips.

The carbon-based layer is typically from 1 mm to 3 cm thick.

A carbon piece could be a thin slice of a cathode with typical electrical resistivity ranging from 10 pQ-m to 30 pQ-m, preferably as low as possible. The crushing strength of such material is not an issue as it is always above 20 MPa but cannot be lower than 10 MPa. Ramming paste can be used with carbon foils whose electrical properties are less critical as the thickness is very low (1 mm to 2 mm thick). When carbon glue with electrically conductive particles is used, one needs to check that the compressive stress generated by the copper bar expansion when going from room temperature to the operating temperature above 900°C will not exceed the crushing strength of the cathode.

The highly electrically conductive metal of the current collector bar is preferably copper or a copper alloy.

The shape and dimensions of the current collector bar of high electrically conductive metal in particular copper or a copper alloy do not need to be precise. Any suitable process can be used to produce the collector bars. However, the electrical properties of the high electrically conductive metal at the operating temperature must be significantly different to those of the steel or other temperature-resistant metal. In particular, the electrical conductivity of high electrically conductive metal at the operating temperature above 900°C should be at least 5 times greater than that of the temperature-resistant metal, for example of standard steel bars.

The temperature-resistant metal of the outer protective layer is preferably made of steel, for example carbon steel or alloy steel, and such outer protective layer of steel is typically from 0.1 to 10 mm thick.

Instead of steel, the temperature-resistant metal of the outer protective layer can alternatively be made of nickel, or any other metallic sheet. In principle, the temperature resistant material should be stable at temperatures up to 1000°C.

In preferred embodiments, the layer of protective temperature-resistant metal is preferably a layer of steel from 1.0 mm to 3.0 mm thick and the carbon-based layer is preferably a layer consisting of ramming paste, or comprising predominantly ramming paste, with a thickness of 1 millimeter to 3 centimeters.

Preferably, the protective layer of steel, or any optional other metallic conductive layer, is no more than 2 mm thick and is applied to the surface of a groove or slot in the cathode groove before applying a carbon-based layer of one millimeter to a few centimeters thick, this carbon-based layer separating the steel protective layer from a copper or copper alloy bar inside the cathode slot. Advantageously, the carbon-based layer is made from a high conductive ramming paste. Alternatively, but less preferred, the protective carbon-based layer is made of a carbon glue whose electrical conductivity is improved by adding steel or copper chips and/or steel or copper particles.

In one configuration, the outer protective layer is made of two L shaped elements of steel assembled over and around the carbon-based layer.

In another configuration, the cathode collector bar comprises a cylindrical core of copper or copper alloy and the protective layer of temperature-resistant metal is a tube in which the cylindrical core of copper or copper alloy is inserted with an intermediate carbon-based layer of carbon paste or carbon glue containing electrically conductive particles. In this way, the copper or copper alloy applies a homogeneous pressure.

BRIEF DESCRIPTION OF FIGURES

The invention will be further described by way of example with reference to the accompanying schematic drawings.

Figure 1 is a schematic cross-section through a Hall-Heroult aluminium-production cell equipped with a collector bar according to the invention.

Figure 2 is a schematic vertical cross section through two cell cathodes.

Figure 3 schematically shows another aluminium-production cell according to the invention.

Figure 4 is a schematic vertical cross section through two further cell cathodes. Figure 5 is a schematic view of a carbon cathode with cathode collector bars viewed from underneath.

Figure 6 schematically shows a cathode with two L -shaped steel plates around a cathode collector bar.

Figure 7 is a schematic side view of a cathode with grooves that receive copper current collector bars embedded in ramming paste.

Figure 8 is a schematic cross-section of the cathode of Figure 7 with its embedded current collector bars.

Figure 9 is a picture of steel chips incorporated in one example of a cathode collector bar.

Figure 10 is a schematic vertical section through another cathode collector bar.

Figure 11 is a picture of copper chips incorporated in a further example of a cathode collector bar.

Figure 12 is a schematic vertical section through this further example of a cathode collector bar.

DETAILED DESCRIPTION

Figure 1 is a schematic cross-section through a Hall-Heroult cell equipped with a copper current collector bar 1 according to the invention on the left side and with a standard steel collector bar 2 on the right side. Figure 1 shows two anodes 3, liquid metal 4, and liquid bath 5 which flows around the liquid metal 4 to create a solidified ledge 6. Below the liquid metal 4 is a carbon cathode 7 fitted underneath with metallic current collector bars 1, 2. The liquid bath 5 may also diffuse inside the carbon cathode 7 to reach the metallic collector bars 1, 2. The inventive cathode collector bar 1 is significantly smaller than the conventional steel current collector bar 2 and is connected to an external steel connector bar end 8 providing a conventional connection to existing external busbars (not shown).

Figure 2 is a schematic vertical section through two examples of a cell cathode. On the left is a cathode 9 incorporating the inventive collector bar design. On the right is a carbon cathode 10 with a conventional a steel current collector bar 12 surrounded by cast iron 11, i.e. by the usual rodding process. The left cathode 9 is fitted with a copper current collector bar 13 according to the invention that is coated with a layer 14 of high conductive ramming paste which is in turn coated with a thin protective steel layer 15 in contact with the carbon cathode 9.

Figure 3 schematically shows another Hall-Heroult aluminium-production cell comprising a carbon cathode 7 in the cell bottom, apool 4 of liquid cathodic aluminium on the carbon cathode 7 in the cell bottom, a cryolite-based molten electrolyte 5 containing dissolved alumina on top of the aluminium pool 4, and a plurality of anodes 3 suspended in the electrolyte 5, the cell being enclosed in a cell container 16. Also shown on the left are two examples of copper cathode current collector bars 1 and 20 according to the invention.

The copper cathode current collector bar 1 has an area 19 that is electrically insulated from the carbon cathode 7 to prevent too much current flowing at the sides of the cathode 7. The length of the insulating area 19 can be set so that the maximum vertical current density at the surface of the carbon cathode 7 can be significantly reduced, whereby electro-erosion is also reduced leading to a longer cell life.

An enlarged externally-extending steel bar 20 connects the copper bar 1 to an external busbar 18 outside the cell. The part of the steel bar 20 inside the cell can have a length such that the steel bar 20 partially penetrates into the carbon cathode 7 (not shown), or can be located outside the carbon cathode 7 as shown at 21 on the left side of Figure 3. The total length of the enlarged steel current collector bar 20 (on the left) can be shorter or longer than the length of the narrower copper current collector bar 1 (on the right) such that the current flowing outside the cell container 16 on the left (or on the right) is not necessarily the same. This can be useful for compensating external busbars asymmetry (most often not desired) and/or optimizing magneto-hydrodynamic effects.

If the design of busbars 18 is such that the external busbars electrical resistance is not symmetrical, it is detrimental for cell performance, and this can be compensated by changing the electrical resistance inside the cell at the cathode level. For this, the copper current collector bar 1 can penetrate inside the steel bar 20. The length of penetration depends on the amount of heat the cell is designed to dissipate. This length can be different to provide a different electrical resistance. To achieve similar effects, the electrically conductive bar 1 can be electrically insulated from the cathode 7 in area 19 over a distance of 0 to 50 cm depending on the desired global electrical resistance of each side of the cell. The choice of solution depends on the desired velocity field inside the liquid metal. When the bar 1 is insulated over some distance, the current at the surface of the cathode 7 is not the same and the Lorentz forces acting on the liquid metal are modified.

Between the copper current collector bars 1 and the carbon cathode 7 is a carbon-based interface possibly fitted with an external steel plate as further detailed in Figure 4.

Figure 4 shows two carbon cathodes 9 each in the form of a block fitted with a copper current collector bar 13 coated with a layer 14 of high conductive ramming paste externally coated with a steel casing 15 that is fitted in and in contact with the surface of a groove in the bottom of the cathode 9. The ramming paste of layer 14 may be impregnated with steel or copper chips or particles as shown at 22 to make it much more electrically conductive. The ramming paste of layer 14 on top and/or on one side of the copper bar 13 can be replaced by solid carbon material such as a thin carbon block 23 shown in the right-hand part of Figure 4.

Figure 5 shows a view of a carbon cathode 9 seen from underneath. A copper bar 13 is inserted inside a larger steel bar 8 (for connection to external busbar) over a distance which minimizes the cathode’s electrical resistance and optimizes heat loss through the steel bar 8. In this way, the reduction of voltage drop achieved in the cathode 9 can contribute to a saving in the cell voltage, hence saving energy in the aluminium production process.

Inside the cathode 9, the copper cathode collector bar 13 protrudes inwardly from the steel bar 8 and is protected by a layer 22 of high conductive ramming paste which itself is covered by and protected by a thin steel layer 15. The thin steel layer 15 prevents possible undesirable chemical reactions from affecting the copper of the cathode collector bar 13.

On the right side of Figure 5, an externally projecting larger steel bar 8 carries the current coming from several copper cathode collector bars 24, for example two bars 24 as illustrated. The number of copper cathode collector bars 24 may be chosen as function of the desired cathode resistance target (energy minimization) and the cathode surface current density target (cell life maximization). Obviously, increasing the copper bar section and/or number of copper bars 24 decreases the cathode resistance. The length of each copper cathode collector bar 24 can be different, and the electrically insulated distance along the copper cathode current collector bars 24 can also be varied to optimize cell life, cell magneto -hydrodynamic stability and lower electrical resistance.

Figure 6 shows a cathode 9 with two L-shaped steel plates 25, 26 providing a protection layer over a copper current collector bar 13 that is surrounded by a carbonbased layer 22 that will expand and push the two steel plates 25,26 towards the carbon cathode 9. As shown, one branch of each of the L-shaped steel plates 25,26 extends against opposite sides of the collector bar 13 with its carbon-based layer, whereas another branch of each of the L-shaped steel plates 25,26 overlaps with one another over the top of the collector bar 13 with its carbon-based layer. The two L-shaped steel plates 25,26 create a U-shaped steel protection that is directly applied towards the faces of a groove in the cathode 9.

Figure 7 shows from below a cathode 9 with two grooves 27 extending from end-to- end through the cathode 9. In the top-shown groove 27 two copper bars 13 are surrounded with high conductive ramming paste 14. The bottom- shown groove 27 is shown empty but will also be filled with two copper bars 13, like the top-shown groove 27.

At the center of the cathode 9 (Figure 7 shows only * of a full cathode, corresponding to the left or right part of Figure 5), the copper bars 13 are stopped over some distance as quasi no current is flowing at that level. This leaves a gap in the groove 27 over some distance, typically 10 cm to 50 cm. The gaps in the grooves 27 are preferably filled with ramming paste 14.

The surfaces of the grooves 27 are covered with two L shaped steel plates 15 (Figure 8) of thickness 2 mm and extending over the full length of the cathode 9. The plates 15 extend from one end of the cathode 9 to the other end. An externally-extending steel bar is insulated over some distance 28 inside the carbon cathode 9. The position of the shell is shown 29 and the steel bar is connected outside to a busbar 18 using aluminium flexes 30. Figure 8 shows two grooves in the bottom of the cathode 9 filled with high conductive ramming paste 14 with the surface of the grooves covered with U-shaped steel plates 15 or two L shaped steel plates that enclose layers of high conductive ramming paste 14 each surrounding two copper current collector bars 13.

EXAMPLES

The invention will be further illustrated by the following Examples of specific embodiments of the elongated cathode current collector bar.

Example 1

This example concerns a cathode and its copper collector bar protected by ramming paste with the addition of 25% steel chips. Figure 9 is a picture of the steel chips. Figure 10 shows a vertical section through the cathode 9. The thickness of the ramming paste is 2 cm. The ramming paste is Nningxia Carbonvalley Cold Lining Paste Type- K with a resistivity of 60 pQ m at room temperature. The steel chips are 5 mm in diameter and between 2 cm to 10 cm in length. When compressed in the ramming paste, the steel chips form electrical shortcuts in all directions and many chips touch the copper bar and a steel L-shaped protection (as in Figure 6). The equivalent resistivity is estimated to be lower than 5 pQ-m.

Figure 10 is a vertical section through the cathode 9 having in its bottom face a groove of dimensions 110 mm in height and 73 mm in width with rounded comers of 20 mm radius. The groove contains a copper bar 13 that is 7 cm in height and 3 cm in width. The copper bar 13 is surrounded by a 20 mm thick layer 22 consisting of high conductive ramming paste containing steel chips. The layer 22 is enclosed in two L- shaped steel plates 25,26 are 105 mm in height and 45 mm in width with a thickness of 1.5 mm. The radius “R20” of the steel plates 25,26 is 20 mm, and the radius of the groove in the carbon block / cathode 9 is also 20 mm. The two L -shaped steel plates 25, 26 are also bent with a radius of 20 mm at the corners. One of the plates 25,26 is slightly bent at the top such that when the plates are superposed inside the groove, the comers touch the carbon cathode 9. The height of the L-shaped steel plates 25,26 (i.e. 105 mm) ensures that the steel does not protmde into the carbon block of cathode 9. The groove in the cathode 9 is fully sealed only with ramming paste 22 for the two centimeters under the copper bar 13. Example 2

This example, shown in Figure 12, concerns a cathode 9 and its copper collector bar 13 protected by high conductive ramming paste with the addition of 25% copper chips. The copper chips are shown in Figure 11. These copper chips are arranged so that they do not come in contact with the steel plate 25,26 to avoid any diffusion of steel into the copper, welding both elements together and preventing an easy separation at the end of the cell life. The ramming paste is Nningxia Carbonvalley Cold Lining Paste Type-K with a resistivity of 60 iiO ni at room temperature. The copper chips are 2 mm in diameter and between 2 cm to 20 cm in length. When compressed in the ramming paste, they form electrical shortcuts in all directions and many chips are touching the copper bar and the steel L-shaped protection. The equivalent resistivity is estimated lower than 1 pQ-m.

Figure 12 is a vertical section through the cathode 9 with a groove in the bottom face of cathode 9. This groove is 110 mm in height and 73 mm in width with rounded comers of 20 mm radius. The copper bar 13 is 7 cm in height and 3 cm in width. The copper bar is surrounded by a 20 mm layer 27 consisting of a mixture of high conductive ramming paste and copper chips. The layer 27 is enclosed in L-shaped steel plates 25, 26 that are 105 mm in height and 45 mm in width with a thickness of 1.5 mm. The radius of the steel plates 25,26 is 20 mm and the radius of the groove in the carbon block/cathode 9 is also 20 mm. The two L-shaped steel plates 25, 26 are also bent with a radius of 20 mm at the comers. One of the plates 25, 26 is slightly bent at the top such that when the plates 25,26 are superposed inside the groove, their corners touch the carbon cathode 9. The height 105 mm of the L-shaped steel plates 25, 26 ensures that the steel does not protmde the carbon block 9. The groove in cathode 9 is fully sealed with only ramming paste 23 extending over two centimeters under the copper bar 13.