An anode hanger

Abstract

An anode hanger (10) for electrolytic production of aluminium, comprising an anode yoke (11) comprising a yoke head (13), at least two yoke arms (12) extending from the yoke head (13), at least two yoke legs (14) provided on the at least two yoke arms (12), a yoke core (16) of aluminium extending from the yoke head (13) through the yoke arms (12) to the yoke legs (14), a yoke mantle (17) of steel, a stud (15) of steel fixed to each respective yoke leg (14), the anode hanger (10) comprises a copper element (18) connecting each stud (15) to the yoke core (16) of each respective yoke leg (14), for reducing voltage drop of the anode hanger (10) from the yoke core (16) to the studs (15).

Description

AN ANODE HANGERField of the invention

[0001] The invention relates to an anode hanger, and more specifically to an anode hanger for electrolytic production of aluminium, comprising a copper element.Background of the invention

[0002] Electrolysis is the chemical process which takes place at the electrodes when electric current is passed through an electrolyte in contact with electrodes. In the process, compounds which are dissociated into ions in the electrolyte is reduced at the cathode and oxidized at the anode, by means of the electric current. An electrolysis processes is the electrolysis of alumina solved in a molten salt electrolyte, for example an electrolyte of molten cryolite. In industrial production of aluminium, cells of the Hall-Heroult type are connected electrically in series, and the solution of alumina in molten cryolite is brought to a temperature up to 980 °C by the heating effect of the current traversing through the cell. The cells are arranged in rows in a potline, and the current travels through the potline via the busbar system, from the cathode current collector bars in the cell bottom to the anode rods in the next cell, through the electrolyte to the cathode collector bars, and further to the next cell in the row.

[0003] In the electrolysis of alumina for production of aluminium, energy losses due to heat loss and reduced electrical current efficiency is a very significant part of the total cost, and a better electrical current efficiency would lead to significant savings. The terms voltage drop, conductivity, resistance and current efficiency are used interchangeably in the following as it is found natural and are used in general by skilled persons. The relationship between the terms, for example by the Ohm's law and Faraday's law for electrolysis are assumed well known to a person skilled in the art to which the present invention is related.

[0004] In the Hall-Heroult type cells for production of aluminium the anodes are usually consumable carbon blocks connected to current conducting anode hangers via an anode yoke (also known as cross bar) with anode studs (also commonly referred to as stubs, studs, and bolts), to which the anode carbon block is fixed. The anode hanger is further attached to an anode frame, through which the current is applied, and which comprises means for inter alia replacing anodes and adjusting the height of the anode in the bath. The electrical current is passed from said current conducting devices through the carbon of the anode and into the electrolyte where electrolysis takes place, and further into the cathode in the cell bottom, as described above.

[0005] A voltage drop appears all over the electrolysis cell, including the anode hanger. The electrical current through a typical electrolysis cell is typically in a range from 100 kA to 300 kA, and even higher in modern cells. Hence, only a small reduction of the voltage drop will significantly reduce electrical losses. Approximately 50 % of the energy put in to the electrolysis cells is lost as heat, and around 50 % of the total heat loss is from the top of the electrolysis cell. Of the 50 % lost from the anode top approximately 25 % is lost from the anode hangers. The anode hangers are an important heat sink for the cell, and the lower the heat production in the anode hangers is, the more heat can be transported out of the area where the aluminium is produced. The portion of the anode hangers closest to the area where the aluminium is produced and majority of the heat is generated, is the most important portion of the anode hanger with respect to heat loss i.e. the studs and anode yoke.

[0006] At present, materials such as iron- and steel alloys (herein generally referred to as steel) are used in the current conducting devices, and some designs include a yoke core of copper or aluminium to minimize the voltage drop, as well as to influence the path of the current through and out of the cathode cell bottom. The portion of the anode hanger or studs for incorporation into the anode carbon is typically manufactured from steel. The upper, upwardly extending portion of the anode hanger, i.e. the anode rod (also referred to as stem), is normally manufactured from aluminium and connected to the anode yoke via a bimetal transition. It has proven difficult to form a stable and lasting contact between aluminium and steel, due to both chemical reactions between the two metals and the fact that steel has a smaller expansion coefficient than aluminium. This problem is particularly applicable to the contact between the studs and the anode yoke, and is one of the main factors behind reduced life expectancy for an anode hanger comprising a yoke core of aluminium.

[0007] In the aluminium production industry, the need for good heat and electrical conductors that are resistant towards corrosive environments dictates use of a metal of good conductivity, such as copper and aluminium. Iron or steel is used in areas which are exposed for corrosion and high temperatures, such materials also have very good mechanical properties, however the electrical conductivity is poor compared to copper and aluminium. Copper is a much more expensive material than aluminium, and to keep costs down, the use of copper in anode hangers today is less common, and avoided due to the cost.

[0008] There is therefore a need for an improved anode hanger to reduce or eliminate the above mentioned disadvantages of known techniques. It is an objective of the invention to achieve this and to provide further advantages over the state of the art.Summary of the invention

[0009] It is an object of the invention to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the prior art and solve at least the above mentioned problem.

[0010] According to a first aspect, there is provided an anode hanger for electrolytic production of aluminium, comprising an anode yoke comprising a yoke head, at least two yoke arms extending from the yoke head, at least two yoke legs provided on the at least two yoke arms, a yoke core of aluminium extending from the yoke head through the yoke arms to the yoke legs, a yoke mantle of steel, a stud of steel fixed to each respective yoke leg, the anode hanger comprises a copper element connecting each stud to the yoke core of each respective yoke leg, for reducing voltage drop of the anode hanger from the yoke core to the studs.

[0011] According to an embodiment, the copper element is spaced from the yoke mantle such that the copper element is embedded in the stud and the yoke core.

[0012] According to an embodiment, the stud comprises a recess, and the copper element is provided in the recess and protruding from the recess into the yoke core.

[0013] According to an embodiment, the copper element is connected by means of an intermetallic connection to the stud and a heat shrink connection to the yoke core.

[0014] According to an embodiment, the stud is cylindrical, and the copper element is cylindrical, and a longitudinal axis of the copper element is coincident with a longitudinal axis of the stud.

[0015] According to an embodiment, the stud is welded to the yoke mantle.

[0016] According to an embodiment, the at least two yoke legs are arranged symmetrical on each side of, and spaced from the yoke head.

[0017] According to an embodiment, the anode yoke comprises two yoke arms and four yoke legs, the yoke legs are arranged in-line and symmetrical on each side of, and spaced from the yoke head.Brief description of the figures

[0018] The aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying figures. The figures are provided to illustrate the general structures of the invention. Like reference numerals refer to like elements throughout.

[0019] Figure 1 shows a perspective view of an embodiment of an anode hanger.

[0020] Figure 2 shows a cross section in a front view of the anode hanger, through a symmetrical plane of the anode hanger.

[0021] Figure 3 shows a cross section in a side view of the anode hanger, through a center line of a stud and a copper element.

[0022] Figure 4 shows an isolated perspective view of a stud and a copper element.Detailed description of the invention

[0023] The invention will now be described with reference to the accompanying figures, in which preferred example embodiments of the invention are shown. The invention may, however, be embodied in other forms and should not be construed as limited to the herein disclosed embodiments. The disclosed embodiments are provided to fully convey the scope of the invention to the skilled person.

[0024] It is to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claims, the articles "a", "an" and "the" are intended to mean that there are one or more of the elements or steps unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

[0025] Referring initially to figures 1 and 2, an embodiment of an anode hanger 10 is shown. The anode hanger 10 comprises an anode yoke 11. The anode yoke 11 comprises yoke arms 12 extending from a yoke head 13. The yoke arms 12 terminates in one or more yoke legs 14. To each yoke leg 14 a stud 15 is fixed. In the embodiment shown in figures 1 and 2, the anode yoke comprises two yoke arms 12. Each yoke arm 12 comprises two yoke legs 14. The four yoke legs 14 are arranged in-line and symmetrical on each side of and spaced from the yoke head 13. In other embodiments, the anode yoke 11 may comprise two, three, five or more yoke legs 14 (with a corresponding number of studs 15), arranged in-line or rotationally symmetrical about the yoke head 13. A yoke leg 14 may also be provided centered below the yoke head 13, resulting in an uneven number of yoke legs 14. In embodiments with an uneven number of yoke legs 14, a yoke arm 12 may also connect a centered yoke leg 14 to the yoke head 13, resulting in an uneven number of yoke arms 12.

[0026] The anode hanger 10 may additionally comprise an anode rod or stem (not shown), connected to the yoke head 13. The anode yoke 11 is thus connected and fixed to an anoderod and an anode frame, for conducting electric current to the anode hanger 10 for its use in the electrolytic process in metal production. An anode rod is not illustrated in the shown embodiment, but may preferably be made of aluminium. The anode yoke 11 is connected to the anode rod at the yoke head 13, and the anode rod may comprise a cross-section corresponding to the yoke head 13.

[0027] The anode yoke 11 comprises a yoke core 16 encompassed by a yoke mantle 17. The yoke core 16 is thus an inner portion of the anode yoke 11, and the yoke mantle 17 is an outer portion of the anode yoke 11. The yoke core 16 is exposed (i.e. not covered by the yoke mantle 17) at the yoke head 13 and at the yoke legs 14. When the anode hanger 10 is fully assembled, the anode rod seals the exposed yoke core 16 at the yoke head 13, and the studs 15 seal the exposed yoke core 16 at the yoke legs 14.

[0028] In figures 1 and 2, the exposed upper area of the yoke head 13 is shown. In the cross section view of figure 2 it is clearly illustrated that the anode yoke 11 comprises a yoke core 16 and a yoke mantle 17. The yoke core 16 is preferably made of aluminium, and the yoke mantle 17 is preferably made of steel. The yoke core 16 extends from the yoke head 13, through the yoke arms 12 and the yoke legs 14, to the studs 15 arranged at the yoke legs 14. The shown embodiment comprises four yoke legs 14 and four respective studs 15. Although the yoke head 13 shown in figure 1 has an upper rectangular configuration, and the yoke legs 14 and studs 15 comprise a generally cylindrical shape with a substantially circular horizontal cross section, other shapes and cross section geometries are contemplated. E.g. one or more of the aforementioned elements, as well as the yoke arms 12 may have a uniform or non-uniform cross section over a respective length, such as e.g. a conical, square, or an elliptic cross-section.

[0029] Aluminium may include pure aluminium metal or an aluminium alloy. Pure aluminium metal may comprise essentially pure aluminium metal of at least 99 %. The aluminium may also include aluminium alloys of the 6000 series; 6XXX aluminium alloys. The steel may comprise steels that are normally used in anode hangers, anode yokes and studs, and includes steel alloys.

[0030] The anode hanger 10 comprises copper elements 18. The copper elements 18 are shown in figure 2, and are further described with reference to figures 3 and 4. Each copper element 18 connects a stud 15 to a respective yoke leg 14. More particularly, the copper element 18 connects the stud 15 to the yoke core 16 of a respective yoke leg 14. The shown embodiment thus comprises four copper elements 18, one copper element 18 provided at each yoke leg 14. The presence of a copper element 18 between the stud 15 and the yoke leg 14 increases the conductivity between the yoke core 16 and the stud 15, thereby minimizing the voltage drop. Copper is a highly conductive material, and by introducing the copper elements 18 the electrical loss over the anode hanger 10 is reduced. The copperelements 18 extends into the yoke core 16, but are preferably spaced from the yoke mantle 17. As such, the copper elements 18 are in direct contact only with the studs 15 to which they are connected and the yoke core 16.

[0031] Because the inclusion of the copper elements 18 in the anode hanger 10 does not affect the outer shape of the yoke mantle 17 or the outer shape of the anode yoke 11, the anode hanger 10 can easily be implemented in existing smelters that already has the same basic design of the anode yokes. There is thus no need for changes in the production facility or rodding area. The volume of the copper element 18 is small compared to the volume of the yoke core 16. As such, the cost of the copper element 18 is correspondingly small, e.g. compared to a yoke core made from copper. However, the inclusion of the copper element 18 increases the conductivity of the anode hanger 10 greatly compared to an anode hanger with a yoke core consisting of aluminium.

[0032] The electrical resistivity for aluminium does not increase as much with the temperature as steel do, therefore the anode hanger 10 in which the anode yoke 11 has a core of aluminium will have a more stable heat loss and thermal conductivity compared with an all steel anode yoke with increased temperature. At normal operation of an aluminium electrolysis cell the temperature on the anode yoke 11 is in the range 350-450 °C, which is well below the melting point of aluminium. Because the anode hanger 10 is provided with the copper elements 18, electrical current efficiency over the anode hanger 10 is increased.

[0033] Referring now to figures 3 and 4, an embodiment of a stud 15 and copper element 18 is shown. The stud 15 may preferably be made of massive steel, typically of the same steel type as the yoke mantle 17. The stud 15 is preferably cylindrical or conical having a general circular cross section. However, the stud 15 may have other shapes, such as an elongated body having e.g. elliptical, rectangular, conical, square, hexagonal or other polygonal shaped horizontal cross-sections. The stud 15 preferably has a flat, or substantially flat, lower end, i.e. the portion of the stud 15 configured for fixation to an anode carbon. A stud 15 may typically have a diameter of 160 mm and a height of 265 mm.

[0034] The stud 15 preferably comprises a recess 19. The copper element 18 is provided in the recess 19, and protrudes from the stud 15. The copper element 18 may be a bar or similar cylindrical element. The copper element 18 may typically have a diameter of 50 mm and a height of 165 mm. The recess 19 preferably comprises a corresponding shape to that of a portion of the copper element 18. If the copper element 18 is cylindrical, the depth of the recess 19 may correspond to the diameter of the copper element 18. A longitudinal axis of the copper element 18 may preferably be coincident with a longitudinal axis of the stud

[0035] When provided in the anode hanger 10, the portion of the copper element 18 protruding from the recess 19 of the stud 15 is encompassed by the yoke core 16, and therefore allows for maximum contact between the copper element 18 and the aluminium in the yoke core 16, for increased electrical conductivity. A ratio where approximately 1 / 3 of the volume of the copper element 18 is provided in the recess 19 and where the remaining 2 / 3 of the volume of the copper element 18 is provided in the yoke core 16 has proven to be especially effective.

[0036] The stud 15 is made from a steel material, and the copper element 18 and stud 15 may advantageously form an intermetallic connection such as a continuous metal-to-metal diffusion bond. An intermetallic connection is optimized for minimizing voltage drop from the copper element 18 to the stud 15. The intermetallic connection may be provided by means of heating the stud 15 and / or the copper element 18 as is known in the art of generating intermetallic connections. The heating may comprise induction heating. The copper element 18 may thus be provided in the recess 19 of the stud 15 prior to assembly with the anode yoke. An inductor may be positioned in close proximity to at least a portion of the stud 15, and electric energy may be supplied to the inductor. The stud 15 is then induction heated to a temperature sufficiently high and for a time sufficiently long to molt at least an outer portion of the copper element 18. The stud 15 and copper element 18 are subsequently cooled, allowing the intermetallic connection to form, and the copper element 18 is solidified.

[0037] Even though copper and aluminium have approximately similar conductivity at the operational temperatures of the anode hanger 10, providing the copper element 18 between the yoke core 16 and the stud 15 has surprisingly proven to greatly increase the electrical conductivity of the anode hanger 10. This effect is due both to the intermetallic connection between the copper element 18 and the stud 15, and the copper element 18 being embedded in the yoke core 16. Electrolytic production of aluminium by using an anode hanger 10 comprising a copper element 18 as described herein thus reduces the amount of electrical power needed.

[0038] Further, the longevity of the anode hanger 10 is increased with the copper element 18. As previously argued, it is difficult to achieve optimal contact between aluminium and steel, and the contact between these two metals also degrade over time. With copper it is easier to achieve optimal contact between copper and steel, and the bonding between copper and steel has a better longevity. Typically, an anode hanger 10 as described herein, comprising a copper element 18, may last 3-4 times longer than an anode hanger with a yoke core of aluminium but without the copper element. Copper has a higher expansion coefficient than steel and aluminium, and therefore the contact between both the copper element 18 and the stud 15, and between the copper element 18 and the yoke core 16 may actually increase during use, as heat is generated.

[0039] An upper portion of the stud 15 may comprise a conical portion 20 such as a chamfer or a tapered shape. The upper portion of the stud 15 is configured for fixing to the yoke leg 14, and the conical portion 20 may provide a suitable gap formed between the yoke leg 14 and the stud 15. The yoke leg 14 and the stud 15 may preferably be welded together, and the conical portion 20 may provide easy and efficient welding of the stud 15 to the yoke leg 14. More particularly, the stud 15 may be welded to the yoke mantle 17 of the yoke leg 14. A strong welding seam 21 may thus be provided around the periphery of the stud 15 and corresponding yoke leg 14. The welding seam 21 may preferably be a flat, single bevel butt weld. Because the copper element 18 is spaced from the yoke mantle 17, the portion of the copper element 18 not in contact with the stud 15 is thus embedded in aluminium, and the copper element 18 is not affected by the welding seam 21. Any changes in the microstructure of the stud 15, yoke leg 14 and / or the welding seam 21 does not influence the connection between the copper element 18 and the stud 15.

[0040] During production of the anode hanger 10, the yoke mantle 17 may be cast as a separate, hollow part. The copper elements 18 are fixed to the studs 15, and the studs 15 comprising the copper elements 18 are welded to the yoke mantle 17. The yoke mantle 17 may subsequently be filled with molten aluminium. The yoke mantle 17 may thus also be referred to as a yoke mold or a steel mold. The molten aluminium form the yoke core 16. The molten aluminium fills the yoke mantle 17 and embeds the portions of the copper elements 18 protruding from the studs 15. As the aluminium solidifies, the yoke core 16 forms a heat shrink connection to the copper elements 18. The solidifying may beneficially be directionally, by controlling the cooling of the yoke core 16 from the yoke legs 14 to the yoke head 13.

[0041] While the invention has been described with reference to the embodiments mentioned above, it is to be understood that modifications and variations can be made without departing from the scope of the present invention, and such modifications and variations shall remain within the field and scope of the invention, as defined by the appended claims.

Claims

Claims1. An anode hanger (10) for electrolytic production of aluminium, comprising; an anode yoke (11) comprising; a yoke head (13); at least two yoke arms (12) extending from the yoke head (13); at least two yoke legs (14) provided on the at least two yoke arms (12); a yoke core (16) of aluminium extending from the yoke head (13) through the yoke arms (12) to the yoke legs (14); a yoke mantle (17) of steel; a stud (15) of steel fixed to each respective yoke leg (14); the anode hanger (10) comprises a copper element (18) connecting each stud (15) to the yoke core (16) of each respective yoke leg (14), for reducing voltage drop of the anode hanger (10) from the yoke core (16) to the studs (15).

2. The anode hanger (10) according to claim 1, wherein the copper element (18) is spaced from the yoke mantle (17) such that the copper element (18) is embedded in the stud (15) and the yoke core (16).

3. The anode hanger (10) according to claim 1 or 2, wherein the stud (15) comprises a recess (19), and the copper element (18) is provided in the recess (19) and protruding from the recess (19) into the yoke core (16).

4. The anode hanger (10) according to any one of the previous claims, wherein the copper element (18) is connected by means of an intermetallic connection to the stud (15) and a heat shrink connection to the yoke core (16).

5. The anode hanger (10) according to any one of the previous claims, wherein the stud (15) is cylindrical, and the copper element (18) is cylindrical, and a longitudinal axis of the copper element (18) is coincident with a longitudinal axis of the stud (15).

6. The anode hanger (10) according to any one of the previous claims, wherein the stud (15) is welded to the yoke mantle (17).

7. The anode hanger (10) according to any one of the previous claims, wherein the at least two yoke legs (14) are arranged symmetrical on each side of, and spaced from the yoke head (13).

8. The anode hanger (10) according to any one of the previous claims, wherein the anode yoke (11) comprises two yoke arms (12) and four yoke legs (14), the yoke legs (14) are arranged in-line and symmetrical on each side of, and spaced from the yoke head (13).

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