Cathode current collector bars for aluminum production cells

Abstract

The aluminum production cell includes an optimized current collector bar designed to reduce energy consumption and extend cell life. The cell includes a carbon cathode (7, 9) having an elongated cathode current collector bar (1, 13, 24) of a highly conductive metal, particularly copper, provided with an outer protective layer (15) of a refractory metal, particularly steel, in contact with the carbon cathode (7, 9). The optimization is achieved by coating the highly conductive metal cathode current collector bar (1, 13, 24) with a carbon-based layer (14, 22) including ramming paste alone or with carbon particles, carbon flakes, a solid carbon layer, or a carbon paste and / or carbon glue containing conductive particles, and the outer protective layer (15) of refractory metal is provided on and / or around the carbon-based layer (4, 22).

Description

[Technical Field]

[0001] The present invention relates to an aluminum production cell of the type that includes a carbon cathode to which is attached an elongated cathode current collector bar of a highly conductive metal such as copper that is provided with an outer protective layer of a refractory metal such as steel in contact with the carbon cathode. [Background technology]

[0002] Aluminum is produced by electrolyzing alumina dissolved in a cryolite-based electrolyte at temperatures up to 980 °C. A typical Hall-Hell cell for aluminum production consists of a steel shell, an insulating lining of refractory material, and a carbon cathode that holds the liquid metal. The cathode usually consists of a cathode block with embedded current collector bars to extract the current flowing through the cell.

[0003] WO 01 / 63014 describes a current collector bar structure for use in a Hall-Heroult reduction cell for producing aluminum. Each current collector bar includes a core of a relatively highly conductive material (e.g., copper or a copper alloy) and an outer housing, typically steel, of a material that is more chemically resistant than the core material. This steel layer / coating / casing is applied directly onto the copper core. Each current collector bar includes a section that is cast or bonded into a channel in a cathode block. When the cell is in use, the cathode and its mating end faces are typically loaded with cast iron. In this typical configuration, the outer steel layer is in contact with the carbon cathode through a layer of cast iron.

[0004] WO 01 / 63014 is a development of earlier proposals in U.S. Pat. Nos. 3,551,319 and 5,976,333 for a current collector having a copper core coated with an iron (steel) coating, both of which also required a cast iron loading between the iron / steel outer coating and the carbon cathode.

[0005] Several proposals have been made to eliminate the need for loading with cast iron or ramming paste, for example WO 2018 / 019910 describes a copper current collector bar with a thin protective steel layer bar that includes a sloped portion so that the cell does not require loading with cast iron or ramming paste.

[0006] Other proposals omit this outer protective steel layer on the cathode current collector. For example, U.S. Patent No. 11,136,682 presents a solution using a copper connector bar that is in direct contact with the carbon cathode or that is in contact with the carbon cathode through a conductive interface formed by a conductive adhesive and / or a conductive flexible foil or sheet applied to the surface of the copper connector bar. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] WO 01 / 63014 [Patent Document 2] U.S. Patent No. 3,551,319 [Patent Document 3] U.S. Patent No. 5,976,333 [Patent Document 4] International Publication No. 2018 / 019910 [Patent Document 5] U.S. Patent No. 1,113,682 Summary of the Invention

[0008] The present invention relates to an aluminum production cell having a carbon cathode with optimized current collector bars designed to reduce energy consumption and extend cell life.

[0009] The present invention also provides a solution that allows for the inexpensive implementation of copper or copper alloy current collector bars within a carbon cathode block.

[0010] The present invention particularly relates to an aluminum production cell of the type which includes a carbon cathode and an elongated cathode current collector bar of highly conductive metal provided with an outer protective layer of refractory metal in contact with the carbon cathode.

[0011] According to a main aspect of the present invention, an elongated cathode current collector bar of a highly conductive metal is coated with a carbon-based layer comprising ramming paste, ramming paste containing conductive particles, carbon particles, carbon flakes, a solid carbon layer, carbon paste containing conductive particles and / or carbon glue, and an outer protective layer of a refractory metal is provided on and / or around the carbon-based layer.

[0012] The solution of the invention has the advantage that it can be mounted directly in the cell, since it avoids loading with cast iron or ramming paste.

[0013] Additionally, the current collector bars can be significantly smaller in size than the steel conductor bars of conventional cells, resulting in longer cell life.

[0014] Finally, the current collector bars can be designed to reach very low electrical resistance, which in turn provides a very low cathode voltage drop (CVD), opening up the option to operate the cell with low energy consumption.

[0015] The cell of the present invention can be implemented with the following preferred features.

[0016] The carbon-based layer may consist of, for example, a ramming paste, in particular a highly conductive ramming paste having an electrical resistivity of less than 40 μΩ·m under operating conditions, in particular a ramming paste with conductive particles such as steel or copper particles, or may consist of a ramming paste with carbon particles or carbon flakes, or the carbon-based layer may consist of a carbon glue containing conductive particles, for example steel or copper flakes.

[0017] Ramming paste is well known in the aluminum manufacturing industry and is conventionally used as the carbon interface between the carbon cathode and the side lining to create an expansion joint that seals the cell and prevents liquid metal ingress. Ramming paste is also used as a conductive interface for loading the cathode current feed to the carbon cathode when the current feed is installed.

[0018] Ramming pastes can have different compositions. A typical ramming paste is prepared from calcined anthracite, graphite powder, and metallurgical coke with coal tar as binder.

[0019] One common composition, for example, is Nningxia Carbonvalley's low-temperature lining paste Type-K, which contains a high content (80-90 wt%) of graphitized anthracite and a low content (10-20 wt%) of coal tar.

[0020] Various other types of commercially available ramming pastes are, for example, CleO2 clean ramming paste from Carbone Savoie and RO20 ramming paste from GrafTech International Holdings Inc.

[0021] The patent literature also describes various types of ramming pastes. For example, Chinese Patent Publication No. 102850072 describes the production of low-temperature ramming paste by mixing and ramming the following raw materials by weight: 45-70% calcined anthracite, 8-20% artificial graphite, 13-15% coal tar, and 3-5% anthracite or washing oil. U.S. Patent No. 5,676,807 describes an aluminum production cell with a ramming paste consisting essentially of 50-98% by weight of carbonaceous material, 1-60% by weight of filler, and 1.30% by weight of binder.

[0022] Depending on their composition, ramming pastes can have different electrical properties. For this application, preferably a highly conductive ramming paste is used, i.e. one with an electrical resistivity of less than 40 μΩ·m (ClO2 exhibits a resistivity of 55 μΩ·m at 20°C and 37 μΩ·m at 1000°C after firing). The resistivity of any ramming paste can be reduced by using steel or copper tips.

[0023] The carbon-based layer is typically 1 mm to 3 cm thick.

[0024] The carbon pieces can be thin slices of cathode with a typical electrical resistivity ranging from 10 μΩ·m to 30 μΩ·m, preferably as low as possible. The crushing strength of such materials is not a problem, as they are always above 20 MPa but never below 10 MPa. Ramming paste can be used with carbon foil, whose thickness is so thin (1 to 2 mm) that its electrical properties are not very important. When carbon glue with conductive particles is used, it is necessary to ensure that the compressive stress caused by the expansion of the copper bar when going from room temperature to operating temperatures above 900°C does not exceed the crushing strength of the cathode.

[0025] The highly conductive metal of the current collector bar is preferably copper or a copper alloy.

[0026] The shape and dimensions of current collector bars made of highly conductive metals, especially copper or copper alloys, do not need to be precise. Any suitable process can be used to manufacture the collector bars. However, the electrical properties of the highly conductive metal at operating temperatures must be significantly different from those of steel or other refractory metals. In particular, the electrical conductivity of the highly conductive metal at operating temperatures above 900°C must be at least five times greater than that of a refractory metal, such as a standard steel bar.

[0027] The heat-resistant metal of the outer protective layer is preferably made of steel, for example carbon steel or alloy steel, and such steel outer protective layers are typically from 0.1 to 10 mm thick.

[0028] Instead of steel, the heat-resistant metal of the outer protective layer can alternatively be made of nickel, or any other metal sheet. As a rule, the heat-resistant material must be stable at temperatures up to 1000°C.

[0029] In a preferred embodiment, the protective refractory metal layer is preferably a layer of steel having a thickness of 1.0 mm to 3.0 mm, and the carbon-based layer is preferably a layer consisting of or comprising primarily ramming paste having a thickness of 1 millimeter to 3 centimeters.

[0030] Preferably, the protective steel layer, or any optional other metallic conductive layer, is no more than 2 mm thick and is applied to the surface of the cathode groove or slot before applying a carbon-based layer, 1 mm to several centimeters thick, which separates the protective steel layer from the copper or copper alloy bar in the cathode slot. Advantageously, the carbon-based layer is made from a highly conductive ramming paste. Alternatively, but less preferably, the protective carbon-based layer is made from a carbon glue whose conductivity has been improved by adding steel or copper chips and / or steel or copper particles.

[0031] In one configuration, the outer protective layer is made of two L-shaped elements of steel assembled over and around a carbon-based layer.

[0032] In another configuration, the cathode current collector bar comprises a cylindrical core of copper or copper alloy, and the protective layer of refractory metal is a tube into which the cylindrical core of copper or copper alloy is inserted with an intermediate carbonaceous layer of carbon paste or glue containing conductive particles, thus applying uniform pressure to the copper or copper alloy. [Brief explanation of the drawings]

[0033] The invention will now be further explained, by way of example only, with reference to the accompanying schematic drawings, in which: [Figure 1] 1 is a schematic cross-sectional view of a Hall-Heroult aluminum production cell with a current collector bar according to the present invention. [Figure 2] FIG. 1 is a schematic vertical cross-sectional view of two cell cathodes. [Figure 3] FIG. 2 is a schematic diagram of another aluminum production cell according to the present invention. [Figure 4] FIG. 10 is a schematic vertical cross-sectional view of two further cell cathodes. [Figure 5] FIG. 1 is a schematic diagram of a carbon cathode with a cathode current collector bar viewed from below. [Figure 6] FIG. 1 is a schematic diagram of a cathode having two L-shaped steel plates around a cathode current collector bar. [Figure 7] FIG. 1 is a schematic side view of a cathode having grooves for receiving copper current collector bars embedded in ramming paste. [Figure 8] FIG. 8 is a schematic cross-sectional view of the cathode of FIG. 7 with its embedded current collector bar. [Figure 9] 1 is a photograph of a steel tip incorporated into an example cathode current collector bar. [Figure 10] FIG. 2 is a schematic vertical cross-sectional view of another cathode current collector bar. [Figure 11] 10 is a photograph of a copper chip incorporated into a further example of a cathode current collector bar. [Figure 12] FIG. 10 is a schematic vertical cross-sectional view of this further example of a cathode current collector bar. DETAILED DESCRIPTION OF THE INVENTION

[0034] FIG. 1 is a schematic cross-sectional view of a Hall-Hellou cell with a copper current collector bar 1 according to the present invention on the left and a standard steel current collector bar 2 on the right. FIG. 1 shows two anodes 3, a liquid metal 4, and a liquid bath 5 that flows around the liquid metal 4 to create a solidified ledge 6. Below the liquid metal 4 is a carbon cathode 7 with metallic current collector bars 1, 2 attached underneath. The liquid bath 5 may also diffuse into the interior of the carbon cathode 7 to reach the metallic current collector bars 1, 2. The cathode current collector bar 1 of the present invention is significantly smaller than the conventional current collector bar 2 and is connected to an external steel connector bar end 8 to provide a conventional connection to an existing external bus bar (not shown).

[0035] Figure 2 shows schematic vertical cross-sections of two example cell cathodes. On the left is a cathode 9 incorporating the current collector bar design of the present invention. On the right is a carbon cathode 10 having a conventional steel current collector bar 12 surrounded by cast iron 11, i.e., by the usual loading process. Attached to the left cathode 9 is a copper current collector bar 13 according to the present invention, coated with a layer 14 of highly conductive ramming paste, which is coated with a thin protective steel layer 15 in contact with the carbon cathode 9.

[0036] 3 shows a schematic representation of another Hall-Heroult aluminum production cell including a carbon cathode 7 at the bottom of the cell, a pool 4 of liquid cathode aluminum above the carbon cathode 7 at the bottom of the cell, a cryolite-based molten electrolyte 5 containing dissolved alumina above the aluminum pool 4, and a plurality of anodes 3 suspended in the electrolyte 5, the cell 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 present invention.

[0037] The copper cathode current collector bar 1 has a region 19 that is electrically insulated from the carbon cathode 7 to prevent excessive current flow on the sides of the cathode 7. The length of the insulating region 19 can be set to significantly reduce the maximum vertical current density on the surface of the carbon cathode 7, thereby reducing galvanic corrosion and extending cell life.

[0038] An enlarged, externally extending steel bar 20 connects the copper bar 1 to an external bus bar 18 outside the cell. The portion of the steel bar 20 within the cell can have a length such that the steel bar 20 penetrates partially 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 FIG. 3. The overall length of the enlarged steel current collector bar 20 (left side) can be shorter or longer than the length of the thinner copper current collector bar 1 (right side) so that the currents flowing outside the cell vessel 16 on the left (or right) side are not necessarily the same. This can be useful to compensate for asymmetry in the external bus bar (which is undesirable in most cases) and / or to optimize magnetohydrodynamic effects.

[0039] If the busbar 18 is designed such that the external busbar electrical resistance is not symmetrical, this is detrimental to cell performance and can be compensated for by modifying the electrical resistance inside the cell at the cathode level. For this purpose, the copper current collector bar 1 can penetrate the interior of the steel bar 20. The length of the penetration depends on the amount of heat the cell is designed to dissipate. This length can be varied to provide different electrical resistances. To achieve a similar effect, the conductive bar 1 can be electrically insulated from the cathode 7 within the region 19 over a distance of 0 to 50 cm, depending on the desired overall electrical resistance on each side of the cell. The choice of solution depends on the desired velocity field inside the liquid metal. If the bar 1 is insulated over a certain distance, the currents on the surface of the cathode 7 will not be the same, and the Lorentz force acting on the liquid metal will be modified.

[0040] Between the copper current collector bar 1 and the carbon cathode 7 is a carbon-based interface, optionally with an external steel plate attached, as further detailed in FIG.

[0041] Figure 4 shows two carbon cathodes 9 in the form of blocks, each fitted with a copper current collector bar 13 coated with a layer 14 of highly conductive ramming paste that is coated externally with a steel casing 15 that fits in contact with the surface of the groove at 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 conductive. The ramming paste of layer 14 on and / or over the copper bar 13 may be replaced with a solid carbon material such as a thin carbon block 23 shown on the right-hand side of Figure 4.

[0042] Figure 5 shows a view of the carbon cathode 9 seen from below. The copper bar 13 is inserted inside the larger steel bar 8 (for connection to an external busbar) over a distance that minimizes the electrical resistance of the cathode and optimizes heat loss through the steel bar 8. The reduction in voltage drop achieved at the cathode 9 in this way can contribute to cell voltage savings and therefore energy savings in the aluminium production process.

[0043] Inside the cathode 9, the copper cathode current collector bar 13 projects inward from the steel bar 8 and is protected by a layer 22 of highly conductive ramming paste which is itself covered and protected by a thin steel layer 15. The thin steel layer 15 prevents any possible undesirable chemical reactions from affecting the copper of the cathode current collector bar 13.

[0044] On the right side of Figure 5, a larger, protruding steel bar 8 carries current coming from copper cathode current collector bars 24, e.g., two bars 24 shown. The number of copper cathode current collector bars 24 can be selected as a function of the desired cathode resistance target (minimizing energy) and cathode surface current density target (maximizing cell life). Obviously, increasing the number of copper bar sections and / or copper bars 24 reduces cathode resistance. The length of each copper cathode current collector bar 24 can vary, and the electrical insulation distance along the copper cathode current collector bar 24 can also be varied to optimize cell life, magneto-rheological stability of the cell, and low electrical resistance.

[0045] 6 shows a cathode 9 having two L-shaped steel plates 25, 26 that provide a protective layer over a copper current collector bar 13 surrounded by a carbonaceous layer 22 that expands and presses the two steel plates 25, 26 toward the carbon cathode 9. As shown, one branch of each of the L-shaped steel plates 25, 26 extends to opposite sides of the current collector bar 13 with its carbonaceous layer, while the other branch of each of the L-shaped steel plates 25, 26 overlaps each other above the current collector bar 13 with its carbonaceous layer. The two L-shaped steel plates 25, 26 form a U-shaped steel protector that is applied directly against the face of the groove in the cathode 9.

[0046] 7 shows a cathode 9 from below with two grooves 27 extending end-to-end through the cathode 9. In the groove 27 shown at the top, two copper bars 13 are surrounded by highly conductive ramming paste 14. The groove 27 shown at the bottom is shown empty, but like the groove 27 shown at the top, it will also be filled with two copper bars 13.

[0047] At the centre of the cathode 9 (FIG. 7 shows only half of the whole cathode, corresponding to the left or right part of FIG. 5), the copper bar 13 is stopped over a certain distance, since apparently no current flows at this level. This leaves a gap in the groove 27 over a certain distance, typically 10 cm to 50 cm. The gap in the groove 27 is preferably filled with ramming paste 14.

[0048] The surface of the groove 27 is covered with two L-shaped steel plates 15 (Figure 8) of 2 mm thickness which extend the whole length of the cathode 9. The plates 15 extend from one end of the cathode 9 to the other. The externally extending steel bars are insulated for some distance 28 inside the carbon cathode 9. The position of the shell is indicated at 29 and the steel bars are connected externally to the busbars 18 using aluminium flex 30.

[0049] FIG. 8 shows two grooves at the bottom of the cathode 9 filled with highly conductive ramming paste 14, the surfaces of which are covered with a U-shaped steel plate 15 or two L-shaped steel plates each enclosing a layer of highly conductive ramming paste 14 surrounding two copper current collector bars 13. Example

[0050] The present invention is further illustrated by the following examples of specific embodiments of elongated cathode current collector bars.

[0051] Example 1 This example concerns a cathode and its copper current collector bar protected by a ramming paste containing 25% steel chips. Figure 9 is a photograph of the steel chips. Figure 10 shows a vertical cross-section of cathode 9. The ramming paste is 2 cm thick. The ramming paste is Nningxia Carbonvalley's low-temperature lining paste Type-K, which has a resistivity of 60 μΩ·m at room temperature. The steel chips are 5 mm in diameter and between 2 cm and 10 cm long. When compressed in the ramming paste, the steel chips form electrical shortcuts in all directions, with many of the chips contacting the steel bar and the steel L-shaped protector (as shown in Figure 6). The equivalent resistivity is estimated to be less than 5 μΩ·m.

[0052] Figure 10 shows a vertical cross-section of cathode 9, which has a groove at its base measuring 110 mm high and 73 mm wide with rounded corners of a 20 mm radius. The groove contains a copper bar 13 measuring 7 cm high and 3 cm wide. The copper bar 13 is surrounded by a 20 mm thick layer 22 of highly conductive ramming paste containing steel chips. Layer 22 is enclosed in two L-shaped steel plates 25, 26, each 105 mm high, 45 mm wide, and 1.5 mm thick. The steel plates 25, 26 have a 20 mm radius ("R20"), 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 at the corners with a 20 mm radius. One of the plates 25, 26 is slightly bent at the top so that the corner will contact the carbon cathode 9 when the plates are overlapped inside the groove. The height of the L-shaped steel plates 25, 26 (i.e. 105 mm) ensures that the steel does not protrude into the carbon block of the cathode 9. The groove of the cathode 9 is completely sealed with ramming paste 22 only for 2 cm below the copper bar 13.

[0053] Example 2 This example, shown in Figure 12, involves a cathode 9 and its copper current collector bar 13 protected by a highly conductive ramming paste containing 25% copper chips. The copper chips are shown in Figure 11. These copper chips are positioned so that they do not come into contact with the steel plates 25 and 26 to avoid any diffusion of steel into the copper, welding both elements together and preventing them from easily separating at the end of the cell's life. The ramming paste is Nningxia Carbon Valley's low-temperature lining paste Type-K, which has a resistivity of 60 μΩ·m at room temperature. The copper chips are 2 mm in diameter and between 2 cm and 20 cm in length. When compressed in the ramming paste, they form electrical shortcuts in all directions, with many chips contacting the steel bar and the steel L-shaped protection. The equivalent resistivity is estimated to be below 1 μΩ·m.

[0054] Figure 12 shows a vertical cross-section of the cathode 9, which has a groove on its bottom surface. The groove is 110 mm high and 73 mm wide, with rounded corners with a 20 mm radius. The copper bar 13 is 7 cm high and 3 cm wide. The copper bar is surrounded by a 20 mm layer 27 of a highly conductive ramming paste and copper chip mixture. Layer 27 is enclosed in L-shaped steel plates 25, 26, each 105 mm high, 45 mm wide, and 1.5 mm thick. The steel plates 25, 26 have a 20 mm radius, and the groove in the carbon block / cathode 9 also has a 20 mm radius. The two L-shaped steel plates 25, 26 are also bent at the corners with a 20 mm radius. One of the plates 25, 26 is slightly bent at the top so that its corner contacts the carbon cathode 9 when the plates 25, 26 are overlapped inside the groove. The 105 mm height of the L-shaped steel plates 25, 26 ensures that no steel protrudes into the carbon block 9. The groove of the cathode 9 is completely sealed only by ramming paste 23 which extends for 2 cm below the copper bar 13.

Claims

1. 1. An aluminum production cell comprising a carbon cathode (7, 9) and an elongated cathode current collector bar (1, 13, 24) of highly conductive metal provided with an outer protective layer (15) of refractory metal in contact with the carbon cathode (7, 9), 1. An aluminium production cell comprising: an elongated cathode current collector bar (1, 13, 24) of highly conductive metal coated with a carbon-based layer (14, 22) comprising ramming paste, ramming paste with conductive particles, carbon particles, carbon flakes, a solid carbon layer, a carbon paste with conductive particles and / or carbon glue; and an outer protective layer (15) of refractory metal provided on and / or around the carbon-based layer.

2. 2. The aluminum production cell of claim 1, wherein the carbon-based layer (14, 22) comprises a ramming paste or a ramming paste containing conductive particles.

3. The aluminum production cell of claim 1 , wherein the carbon-based layer (14, 22) comprises a ramming paste in combination with carbon particles, carbon flakes, and / or carbon foil.

4. 10. The aluminum production cell of claim 1, wherein the carbon-based layer comprises a ramming paste containing conductive particles, for example of steel or copper.

5. 2. The aluminum production cell of claim 1, wherein the carbon-based layer (14, 22) comprises a carbon glue containing conductive particles, for example of steel or copper.

6. 6. An aluminium production cell according to any one of claims 1 to 5, wherein the carbon-based layer (14, 22) is between 1 mm and 3 cm thick.

7. 7. The aluminum production cell of claim 1, wherein the carbon-based layer (14, 22) has an electrical resistivity of less than 40 μΩ·m.

8. 8. An aluminium production cell according to any one of claims 1 to 7, wherein the highly conductive metal of the current collector bars (1, 13, 24) is copper or a copper alloy.

9. 9. An aluminium production cell according to any one of claims 1 to 8, wherein the refractory metal of the outer protective layer (15) is steel.

10. 10. The aluminum production cell of claim 9, wherein the outer protective layer (15) is made of carbon steel or alloy steel.

11. 11. An aluminium production cell according to claim 9 or 10, wherein the outer protective layer (15) of steel is between 0.1 mm and 10 mm thick.

12. 12. An aluminium production cell according to claim 9, 10 or 11, wherein the outer protective layer (15) is made of two L-shaped elements of steel assembled on and around the carbon-based layers (14, 22).

13. 12. An aluminum production cell according to any one of claims 1 to 11, wherein the cathode current collector bar (1, 13, 24) comprises a cylindrical core of copper or copper alloy, and the protective layer (15) of refractory metal is a tube into which the cylindrical core of copper or copper alloy is inserted with an intermediate carbon-based layer (14, 22) of carbon paste or carbon glue containing conductive particles.

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