Cathode collector bar of aluminum production electrolytic cell

Abstract

An aluminum production cell equipped with an optimized current collector bar designed to reduce energy consumption and increase cell life. The electrolytic cell comprises a carbon cathode (7, 9) having an elongated cathode current collector bar (1, 13, 24) of a highly conductive metal, in particular copper, provided with an outer protective layer (15) of a high temperature resistant metal, in particular steel, in contact with the carbon cathode (7, 9). Optimization is achieved by a cathodic current collector rod (1, 13, 24) coated with a highly conductive metal with a carbon-based layer (14, 22) comprising a tamping paste alone or together with carbon particles, carbon blocks, a solid carbon layer, a carbon paste containing conductive particles and / or a carbon glue, an outer protective layer (15) of a high temperature resistant metal being arranged above and / or around the carbon-based layer (4, 22).

Description

Technical Field

[0001] The present invention relates to an aluminium production electrolytic cell of the type comprising a carbon cathode fitted with an elongated cathode current collector bar of a highly conductive metal, such as copper, provided with an outer protective layer of a temperature-resistant metal, such as steel, in contact with the carbon cathode. Background Art

[0002] Aluminium is produced by electrolysis of aluminium oxide dissolved in a cryolite-based electrolyte at temperatures up to 980°C. A typical Hall-Héroult cell for aluminium production consists of a steel shell, a refractory insulating lining and a carbon cathode, which contains the liquid metal. The cathode usually consists of a cathode block with an embedded collector bar to extract the current flowing through the cell.

[0003] WO01 / 63014 describes a collector bar structure for use in Hall-Héroult reduction electrolytic cells for the production of aluminium. Each collector bar comprises a core of a relatively high conductivity material (copper or copper alloy) and an outer shell of a material that is more chemically resistant than the core material, typically steel. This steel layer / coating / casing is applied directly to the copper core. Each collector bar comprises a portion cast or glued in the channel of the cathode block. When the electrolytic cell is put into use, the end face adjacent to the cathode and its sides are usually reinforced with cast iron rods. In this conventional configuration, the outer layer of steel is in contact with the carbon cathode through the cast iron layer.

[0004] WO 01 / 63014 is an improvement on previous proposals in US 3'551'319 and US 5'976'333 for current collectors having a copper core coated with an iron (steel) coating, both of which also required cast iron rodding between the iron / steel outer coating and the carbon cathode.

[0005] Some solutions have been proposed to avoid the need for cast iron rodding or ramming paste. For example, WO 2018 / 019910 describes a copper collector bar with a thin protective steel layer bar, which includes an inclined portion so that the electrolytic cell does not need to be rodded with cast iron or ramming paste.

[0006] Other solutions dispense with this steel outer protective layer on the cathode current collector. For example, US Pat. No. 11,136,682 proposes a technical solution in which a copper connecting rod is in direct contact with the carbon cathode or is in contact with the carbon cathode through a conductive interface formed by a conductive glue and / or a conductive flexible foil or sheet applied on the surface of the copper connecting rod. Summary of the invention

[0007] The present invention relates to an aluminium production electrolytic cell with carbon cathodes equipped with optimised current collector bars designed to reduce energy consumption and increase cell life.

[0008] The invention also provides a technical solution which allows the inexpensive implementation of copper or copper alloy current collector bars inside the carbon cathode block.

[0009] The invention relates in particular to an aluminium production electrolytic cell of the type comprising a carbon cathode and an elongated cathode current collector bar of a highly conductive metal provided with an outer protective layer of a refractory metal in contact with the carbon cathode.

[0010] According to the main aspects of the present invention, an elongated cathode current collector rod of a highly conductive metal is coated with a carbon-based layer, which includes a ramming paste; a ramming paste containing conductive particles; carbon particles; carbon blocks; a solid carbon layer; a carbon paste and / or carbon glue containing conductive particles, and an outer protective layer of a high temperature resistant metal is arranged above and / or around the carbon-based layer.

[0011] The advantage of the technical solution of the present invention is that it can be realized directly in the electrolytic cell, because it avoids the use of cast iron rods or ramming the paste.

[0012] Furthermore, the collector bars can be manufactured in dimensions significantly smaller than conventional pot cell steel conductor bars, resulting in a longer pot cell life.

[0013] Finally, the current collector bars can be designed to achieve very low resistance. In other words, to provide very low cathode voltage drop (CVD), providing the option of operating the electrolyser with low energy consumption.

[0014] The electrolytic cell of the present invention can be implemented with the following preferred features.

[0015] The carbon-based layer may, for example, consist of a ramming paste, in particular a highly conductive ramming paste having a resistivity of less than 40 μΩ∙m under operating conditions, in particular a ramming paste containing conductive particles, such as steel or copper particles; or it may consist of a ramming paste together with carbon particles or carbon blocks; or the carbon-based layer may consist of a carbon paste containing conductive particles, such as steel chips or copper chips.

[0016] Ramming paste is well known in the aluminum production industry and is traditionally used as a carbon interface between the carbon cathode and the side liner to form an expansion joint that seals the electrolytic cell and prevents liquid metal penetration. Ramming paste also serves as a conductive interface to rod the cathode current feeder to the carbon cathode when installing the current feeder.

[0017] Ramming paste can have different compositions. A typical ramming paste is prepared from calcined anthracite, graphite powder and metallurgical coke with coal tar as binder.

[0018] One common composition is, for example, Ningxia Carbonvalley ColdLining Paste Type-K, which contains a high content (80-90% by weight) of graphitized anthracite and a low content (10-20% by weight) of coal tar pitch.

[0019] Various other types of commercially available tamping pastes are, for example, CleO2 clean tamping paste from Carbone Savoie and SmartRam RO20 tamping paste from GrafTech International Holdings Inc.

[0020] Patent literature also describes various types of ramming pastes. For example, CN102850072A describes the production of cold ramming paste by mixing and ramming the following raw materials (by total weight): 45-70% calcined anthracite, 8-20% artificial graphite, 13-15% coal tar and 3-5% anthracite or washed oil. US5676807A describes an aluminum production electrolytic cell with a ramming paste, which is mainly composed of 50-98 wt% carbonaceous material, 1-60 wt% filler and 1.30 wt% binder.

[0021] Depending on its composition, the ramming paste can have different electrical properties. For the present application, it is preferred to use a highly conductive ramming paste, i.e. one with a resistivity below 40 μΩ∙m (CleO2 quotes a resistivity of 55 μΩ∙m at 20°C and 37 μΩ∙m at 1000°C after baking). The resistivity of any ramming paste can be reduced by using steel or copper shavings.

[0022] The carbon-based layer is typically 1 mm to 3 cm thick.

[0023] The carbon block can be a thin sheet of the cathode with a typical resistivity in the range of 10 μΩ∙m to 30 μΩ∙m, preferably as low as possible. The crush strength of this material is not an issue as it is usually above 20 MPa, but cannot be below 10 MPa. Ramming paste can be used with carbon foil, the electrical properties of which are less critical because it is very thin (1 mm to 2 mm thick). When using carbon paste with conductive particles, it is necessary to check that the compressive stresses generated by the expansion of the copper rods do not exceed the crush strength of the cathode when going from room temperature to operating temperatures above 900°C.

[0024] The highly conductive metal of the current collector bar is preferably copper or a copper alloy.

[0025] The shape and dimensions of the current collector bars of highly conductive metals, in particular copper or copper alloys, do not need to be precise. Any suitable process can be used to produce the current collector bars. However, the electrical properties of the highly conductive metals at the operating temperature must be significantly different from those of steel or other high temperature resistant metals. In particular, at operating temperatures above 900°C, the electrical conductivity of the highly conductive metals should be at least 5 times higher than that of high temperature resistant metals (e.g. standard steel bars).

[0026] The high temperature resistant metal of the outer protective layer is preferably made of steel, such as carbon steel or alloy steel, and the outer protective layer of such steel is usually 0.1 mm to 10 mm thick.

[0027] Instead of steel, the refractory metal of the outer protective layer can be made of nickel or any other metal sheet. In principle, the refractory material should remain stable at temperatures up to 1000°C.

[0028] In a preferred embodiment, the protective refractory metal layer is preferably a 1.0 mm to 3.0 mm thick steel layer, and the carbon-based layer is preferably a layer consisting of ramming paste, or a layer mainly comprising ramming paste, with a thickness of 1 mm to 3 cm.

[0029] Preferably, the protective layer of steel or any optional other metal conductive layer is not more than 2 mm thick and is applied to the surface of the groove or slot in the cathode groove before applying a carbon base layer of one millimeter to several centimeters thick, which separates the steel protective layer from the copper or copper alloy rod in the cathode slot. Advantageously, the carbon base layer is made of a highly conductive ramming paste. Alternatively, but less preferably, the protective carbon base layer is made of carbon glue, the conductivity of which is increased by adding steel or copper chips and / or steel particles or copper particles.

[0030] In one configuration, the outer protective layer is made of two L-shaped steel elements assembled over and around the carbon base layer.

[0031] In another configuration, the cathode collector bar comprises a cylindrical core of copper or copper alloy, and the protective layer of high temperature resistant metal is a tube with an intermediate carbon-based layer of carbon paste or carbon glue containing conductive particles, wherein the cylindrical core of copper or copper alloy is inserted into the tube. In this way, the copper or copper alloy applies uniform pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention will be further described by way of example with reference to the accompanying schematic drawings.

[0033] Figure 1 is a schematic cross section through a Hall-Héroult aluminium production electrolysis cell equipped with collector bars according to the invention.

[0034] Figure 2is a schematic vertical cross section through two electrolytic cell cathodes.

[0035] Figure 3 Another aluminium production electrolysis cell according to the invention is schematically shown.

[0036] Figure 4 is a schematic vertical cross-section through two further electrolytic cell cathodes.

[0037] Figure 5 is a schematic diagram of a carbon cathode with a cathode collector rod viewed from the bottom.

[0038] Figure 6 A cathode with two L-shaped steel plates surrounding the cathode collector bar is schematically shown.

[0039] Figure 7 is a schematic side view of a cathode having grooves that receive copper current collector bars embedded in the tamping paste.

[0040] Figure 8 yes Figure 7 Schematic cross-section of a cathode and its embedded current collector bar.

[0041] Fig. 9 is a picture of steel shavings contained in one example of a cathode collector bar.

[0042] Fig.10 is a schematic vertical section through another cathode collector bar.

[0043] Fig.11 is a picture of copper filings contained in another example of a cathode collector bar.

[0044] Fig.12 is a schematic vertical cross-section through this other example of a cathode collector bar. DETAILED DESCRIPTION

[0045] Figure 1 is a schematic cross section through a Hall-Héroult electrolysis cell equipped with a copper current collector bar 1 according to the invention on the left and with a standard steel current collector bar 2 on the right. Figure 1 Two anodes 3, liquid metal 4, and a liquid bath 5 are shown, the liquid bath 5 flowing around the liquid metal 4 to form a solidified ledge 6. Below the liquid metal 4 is a carbon cathode 7, the bottom of which is fitted with metal collector bars 1, 2. The liquid bath 5 can also diffuse within the carbon cathode 7 to reach the metal collector bars 1, 2. The cathode collector bar 1 of the present invention is significantly smaller than a conventional steel current collector bar 2, and is connected to an external steel connector bar end 8, which provides a conventional connection to an existing external busbar (not shown).

[0046] Figure 2 are schematic vertical sections through two examples of electrolytic cell cathodes. On the left is a cathode 9 incorporating the collector bar design of the 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 rod joining process). The cathode 9 on the left is equipped with a copper current collector bar 13 according to the invention, which is coated with a highly conductive ramming paste layer 14, which in turn is coated with a thin protective steel layer 15 in contact with the carbon cathode 9.

[0047] Figure 3 Another Hall-Héroult aluminum production electrolytic cell is schematically shown, comprising a carbon cathode 7 in the cell bottom, a liquid cathode aluminum pool 4 on the carbon cathode 7 in the cell bottom, a cryolite-based molten electrolyte 5 containing dissolved aluminum oxide above the aluminum pool 4, and a plurality of anodes 3 suspended in the electrolyte 5, the cell being enclosed in a cell container 16. Two examples of copper cathode current collector bars 1 and 20 according to the invention are also shown on the left.

[0048] The copper cathode current collector bar 1 has a region 19 electrically insulated from the carbon cathode 7 to prevent excessive current from flowing on the sides of the cathode 7. The length of the insulating region 19 can be set so that the maximum vertical current density at the surface of the carbon cathode 7 can be significantly reduced, thereby also reducing electrical corrosion, resulting in a longer electrolytic cell life.

[0049] The enlarged externally extending steel rod 20 connects the copper rod 1 to the external busbar 18 outside the electrolytic cell. The portion of the steel rod 20 inside the electrolytic cell can have a length such that the steel rod 20 partially penetrates the carbon cathode 7 (not shown), or can be located outside the carbon cathode 7, such as Figure 3 As shown at 21 on the left side. The overall length of the enlarged steel current collector bar 20 (on the left side) can be shorter or longer than the length of the narrower copper current collector bar 1 (on the right side), so that the current flowing outside the electrolytic cell container 16 on the left side (or on the right side) is not necessarily the same. This can be used to compensate for external busbar asymmetry (usually the most undesirable) and / or optimize magnetohydrodynamic effects.

[0050] If the design of the busbar 18 is such that the external busbar resistance is asymmetric, this is detrimental to the performance of the electrolytic cell, which can be compensated by changing the resistance inside the electrolytic cell at the cathode level. To this end, the copper current collector bar 1 can be penetrated into the steel bar 20. The length of the penetration depends on the heat dissipation designed for the electrolytic cell. The length can be different to provide different resistances. To achieve a similar effect, the conductive bar 1 can be electrically insulated from the cathode 7 in the region 19 at a distance of 0 to 50 cm, depending on the desired overall resistance on each side of the electrolytic cell. The choice of solution depends on the velocity field required inside the liquid metal. When the bar 1 is insulated for a distance, the current at the surface of the cathode 7 is not the same, and the Lorentz force acting on the liquid metal changes.

[0051] Between the copper current collector bar 1 and the carbon cathode 7 is a carbon-based interface which may be provided with an external steel plate, such as Figure 4 Further details are shown.

[0052] Figure 4 Two carbon cathodes 9 are shown, each in block form, equipped with a copper current collector bar 13 coated with a highly conductive ramming paste layer 14 and coated externally with a steel sleeve 15 which fits in a recess in the bottom of the cathode 9 and contacts its surface. The ramming paste of layer 14 may contain steel or copper shavings or steel or copper particles, as shown at 22, to make it more conductive. The ramming paste of layer 14 above and / or on one side of the copper bar 13 may be replaced by a solid carbon material, such as Figure 4 A thin carbon block 23 is shown in the right part.

[0053] Figure 5 A view of a carbon cathode 9 from the bottom is shown. The copper rod 13 is inserted inside a larger steel rod 8 (for connection to an external busbar) at a distance that minimizes the resistance of the cathode and optimizes heat losses through the steel rod 8. In this way, the reduction in voltage drop achieved in the cathode 9 helps to save pot cell voltage and thus energy in the aluminum production process.

[0054] Inside the cathode 9, the copper cathode collector bar 13 projects inwardly from the steel bar 8 and is protected by a highly conductive ramming paste layer 22, which is itself covered and protected by a thin steel layer 15. The thin steel layer 15 prevents possible undesired chemical reactions affecting the copper of the cathode collector bar 13.

[0055] exist Figure 5On the right side of the copper cathode collector bar 24, the externally protruding larger steel bar 8 carries the current from several copper cathode collector bars 24, such as the two bars 24 shown. The number of copper cathode collector bars 24 can be selected as a function of the desired cathode resistance target (energy minimization) and cathode surface current density target (maximization of electrolyzer life). Obviously, increasing the copper bar cross-section and / or the number of copper bars 24 will reduce the cathode resistance. The length of each copper cathode collector bar 24 can be different, and the electrical insulation distance along the copper cathode current collector bar 24 can also be varied to optimize the electrolyzer life, electrolyzer magnetohydrodynamic stability and lower resistance.

[0056] Figure 6 A cathode 9 is shown with two L-shaped steel plates 25, 26 providing a protective layer on a copper current collector bar 13 which is surrounded by a carbon based layer 22 which will expand and push the two steel plates 25, 26 towards the carbon cathode 9. As shown, one branch in each L-shaped steel plate 25, 26 extends against the opposite side of the carbon based layer of the collector bar 13, while the other branch in each L-shaped steel plate 25, 26 overlaps each other over the top of the carbon based layer of the collector bar 13. The two L-shaped steel plates 25, 26 form a U-shaped steel protection which is applied directly to the surface of the groove in the cathode 9.

[0057] Figure 7 The cathode 9 is shown from below and has two grooves 27 extending from one end to the other through the cathode 9. In the upper groove 27 shown, two copper rods 13 are surrounded by a highly conductive tamping paste 14. The bottom groove 27 shown is shown empty but will also be filled with two copper rods 13 similar to the upper groove 27 shown.

[0058] At the center of cathode 9 ( Figure 7 Only 1 / 2 of the complete cathode is shown, corresponding to Figure 5 The copper rod 13 is stopped at a certain distance, because almost no current flows at this level. This leaves a gap in the groove 27 for a certain distance, typically 10 cm to 50 cm. The gap in the groove 27 is preferably filled with tamping paste 14.

[0059] The surface of the groove 27 is covered with two L-shaped steel plates 15 with a thickness of 2 mm ( Figure 8 ) and extends over the entire length of the cathode 9. Plates 15 extend from one end of the cathode 9 to the other. Externally extending steel bars are insulated a certain distance 28 inside the carbon cathode 9. The housing is located as shown at 29 and the bars are externally connected to the busbars 18 using aluminum flexes 30.

[0060] Figure 8Two grooves filled with highly conductive tamping paste 14 are shown in the bottom of the cathode 9, the groove surface being covered with a U-shaped steel plate 15 or two L-shaped steel plates surrounding a layer of highly conductive tamping paste 14, each of which surrounds two copper current collector bars 13.

[0061] Example

[0062] The invention will be further illustrated by the following examples of specific embodiments of the elongated cathode current collector bar.

[0063] Example 1

[0064] This embodiment relates to a cathode and a copper collector bar thereof, which are protected by a ramming paste with 25% steel chips added. Fig. 9 It is a picture of steel shavings. Fig.10 A vertical section through cathode 9 is shown. The thickness of the ramming paste is 2 cm. The ramming paste is Ningxia Tan Gu K-type cold ramming paste with a resistivity of 60 μΩ∙m at room temperature. The steel chips are 5 mm in diameter and range in length from 2 cm to 10 cm. When the steel chips are compressed in the ramming paste, they form an electrical short circuit in all directions, with multiple chips contacting the copper rod and the steel L-shaped protection (such as Figure 6 The equivalent resistivity is estimated to be less than 5 μΩ∙m.

[0065] Fig.10 is a vertical section through the cathode 9, which has a groove with a height of 110 mm and a width of 73 mm on its bottom surface, with a fillet radius of 20 mm. The groove contains a copper rod 13, which has a height of 7 cm and a width of 3 cm. The copper rod 13 is surrounded by a 20 mm thick layer 22, which consists of a highly conductive ramming paste containing steel chips. The layer 22 is surrounded by two L-shaped steel plates 25, 26 with a height of 105 mm, a width of 45 mm and 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 bent at the corners with a radius of 20 mm. One of the plates 25, 26 is slightly bent at the top so that when the plates overlap in the groove, the corner contacts the carbon cathode 9. 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 in the cathode 9 is completely sealed with tamping paste 22 only to two centimeters below the copper rod 13.

[0066] Example 2

[0067] like Fig.12 As shown, this embodiment involves the cathode 9 and its copper collector bar 13, which is protected by a highly conductive ramming paste with 25% copper chips added. Fig.11As shown. These copper chips are arranged so that they are not in contact with the steel plates 25, 26 to avoid diffusion of steel into copper, welding the two elements together and preventing easy separation at the end of the life of the electrolytic cell. The ramming paste is Ningxia Tan Gu K-type cold ramming paste with a resistivity of 60 μΩ∙m at room temperature. The copper chips have a diameter of 2 mm and a length ranging from 2 cm to 20 cm. When the copper chips are compressed in the ramming paste, they form an electrical short circuit in all directions, with multiple chips contacting the copper rod and the steel L-shaped protection. The equivalent resistivity is estimated to be less than 1 μΩ∙m.

[0068] Fig.12 is a vertical section through the cathode 9, which has a groove on its bottom surface. The groove has a height of 110 mm, a width of 73 mm and a fillet radius of 20 mm. The copper rod 13 has a height of 7 cm and a width of 3 cm. The copper rod is surrounded by a 20 mm layer 27, which consists of a mixture of highly conductive ramming paste and copper chips. The layer 27 is surrounded by L-shaped steel plates 25, 26 with a height of 105 mm, a width of 45 mm and 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 at the corners with a radius of 20 mm. One of the plates 25, 26 is slightly bent at the top so that when the plates 25, 26 overlap in the groove, their corner contacts the carbon cathode 9. The height of the L-shaped steel plates 25, 26 is 105 mm, ensuring that the steel does not protrude from the carbon block 9. The groove in the cathode 9 is completely sealed with tamping paste 23 only to two centimeters below the copper rod 13.

Claims

1. An aluminum production electrolytic cell comprising a carbon cathode (7, 9) and an elongated cathode current collector bar (1, 13, 24) of a highly conductive metal, the elongated cathode current collector bar (1, 13, 24) being provided with an outer protective layer (15) of a high temperature resistant metal in contact with the carbon cathode (7, 9), It is characterized in that The elongated cathode current collector rod (1, 13, 24) of the highly conductive metal is coated with a carbon-based layer (14, 22), the carbon-based layer (14, 22) comprising a ramming paste; a ramming paste containing conductive particles; carbon particles; carbon blocks; a solid carbon layer; a carbon paste and / or carbon glue containing conductive particles, and the outer protective layer (15) of the high temperature resistant metal is arranged above and / or around the carbon-based layer.

2. The aluminum production electrolytic cell according to claim 1, wherein: The carbon-based layer (14, 22) is composed of a rammed paste or a rammed paste containing conductive particles.

3. The aluminum production electrolytic cell according to claim 1, wherein: The carbon-based layer (14, 22) includes a ramming paste and carbon particles, carbon blocks and / or carbon foils.

4. The aluminum production electrolytic cell according to claim 1, wherein: The carbon-based layer comprises a ramming paste containing conductive particles, such as steel or copper.

5. The aluminum production electrolytic cell according to claim 1, wherein: The carbon-based layer (14, 22) includes carbon paste containing conductive particles such as steel or copper.

6. Aluminium production electrolysis cell according to any one of the preceding claims, wherein: The carbon-based layer (14, 22) is 1 mm to 3 cm thick.

7. Aluminium production electrolysis cell according to any one of the preceding claims, wherein: The carbon-based layer (14, 22) has a resistivity lower than 40 μΩ∙m.

8. Aluminium production electrolysis cell according to any one of the preceding claims, wherein: The high-conductivity metal of the current collecting rod (1, 13, 24) is copper or a copper alloy.

9. Aluminium production electrolysis cell according to any one of the preceding claims, wherein: The high temperature resistant metal of the outer protective layer (15) is steel.

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

11. Aluminum production electrolytic cell according to claim 9 or 10, wherein: The outer protective layer (15) of steel is 0.1 mm to 10 mm thick.

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

13. The aluminum production electrolytic cell according to any one of claims 1 to 11, wherein: The cathode collector rod (1, 13, 24) comprises a cylindrical core of copper or copper alloy, and the protective layer (15) of high temperature resistant metal is a tube having an intermediate carbon-based layer (14, 22) of carbon paste or carbon glue containing conductive particles, wherein the cylindrical core of copper or copper alloy is inserted into the tube.

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