WO2019123131A1

WO2019123131A1 - Anode yoke, anode hanger and anode assembly for a hall-héroult cell

Info

Publication number: WO2019123131A1
Application number: PCT/IB2018/059986
Authority: WO
Inventors: Lalit MISHRA; Kasim BASHA; Pragneshkumar CHAUHAN
Original assignee: Dubai Aluminium Pjsc
Priority date: 2017-12-18
Filing date: 2018-12-13
Publication date: 2019-06-27
Also published as: GB2569382A; GB201721141D0

Abstract

An anode yoke (1; 101) for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Heroult process, said yoke comprising: a body (2; 102) intended to be connected to said anode rod (16;1 16), and a plurality of arms (3A-3D, 4A-4D; 103A, 103B, 104A, 104B) which extends each from said body, each arm being intended to be connected to a carbon anode (11, 11'; 111), wherein each arm comprises a reduced cross section area (39, 49), said anode yoke being characterized in that said reduced cross section area comprises at least one hole (38, 38', 48; 138, 148) which leads on at least one peripheral face (34, 35, 44, 45) of said arm.

Description

ANODE YOKE, ANODE HANGER AND ANODE ASSEMBLY FOR A HALL-HEROULT

CELL

Technical field of the invention

The invention relates to the technical field of electrolysis in molten salts for making aluminium using the Hall-Heroult process. More precisely, the invention relates to improved hangers for prebaked anode blocks that can be used in said process, and in particular to improved anode yokes that connect the prebaked carbonaceous anode to the anode hanger. The invention also relates to an anode assembly comprising such anode hangers, and to a process to manufacture said anode assemblies. Prior art

Anodes (also called“anode blocks”) in the Hall-Heroult process are prebaked cuboids made from a carbonaceous material. The anode blocks are fixedly connected to so-called anode hangers. They serve two different purposes, namely to keep the carbon anodes at a predetermined distance from the cathode, and to carry the electrical current from an anode busbar (also called“anode beam”) down to the carbon anodes. Anode hangers are fixed to the overhanging anode beam in a detachable manner using clamps. They comprise an upper part called“anode rod” or“anode stem”, which is connected to the anode beam, and a lower part, called“anode yoke”. The anode yoke has a number of arms each of which terminates in a cylindrical stub that is embedded in pre-formed stubholes of the carbon anode blocks and fixed with cast iron acting as temperature- resistant, electrically conductive glue; this process is called“anode rodding” or“anode casting”.

Anode rods are usually made from aluminium, while anode yokes are usually made from low carbon structural steel. The connection between the anode rod and the anode yoke is achieved by welding: the anode rod is welded to the aluminum portion of an aluminium - steel transition joint, and anode yoke is welded to the steel portion of said transition joint. Such transition joints are commercially available products which are typically manufactured by co-rolling or explosion welding of an aluminum plate and a steel plate.

The entity“anode rod plus anode yoke” is sometimes called“anode hanger”, and the entity“anode hanger plus anode block” is called“anode assembly”.

Anode yokes (also called « anode spiders » when all stubs are not in one line) are manufactured by a number of companies throughout the world. Traditionally they are made from casting of steel in a foundry workshop. This material ensures a long lifetime, in excess of two decades, but its electrical resistivity is rather high, typically of the order of 25 - 30 mWah. Ultra Low Resistivity Steel is a widely used alternative, with a typical resistivity of the order of 10 pQcm. The electrical resistance of the anode assembly is very important factor because any additional electrical resistance in an electrolytic cell generates Ohmic losses that increase the energy consumption of the process and have a direct and immediate impact on the production cost. These losses have been estimated in terms of voltage drop to be of the order of 0.30 V for anode assemblies (see B. Woodrow, “Anode stub replacement”, Aluminium 5/2007, p. 71 -73), which can be broken down as follows: clamp drop about 0.02 V, rod drop about 0.08 V (which can be broken down as follows : aluminium rod: 0.05 V; transition joint: 0.01 V; steel yoke: 0.02 V), stub - carbon drop about 0.12 V, carbon drop about 0.08 V. These values indirectly depend on the current density of the cell: the anode block drop is proportional to the height of the anode block, and bigger anodes need bigger anode yokes. As cells operating at high current density might have higher anodes to avoid having too short anode changing cycles, the anode block drop in modern high efficiency cells is rather at least 0.13 V than 0.08 V. As an example, in EGA’s DX+ Ultra™ cells operating at 460 kA, the total voltage drop from the anode beam to the bottom of a half-spent (mid-life) anode is slightly above 0.40 V.

It would obviously be desirable to decrease the electrical resistance of the anode assembly. This can be achieved by using materials having a lower electrical resistivity. However, for the anode rod the use of thicker or shorter conductive cross sections is not a practical means to decrease their electrical resistance because the anode assemblies must fit into existing pots.

Anode rods and anode yokes with sections or inserts of low resistivity metals have been described in the patent literature. EP 0 248 452 (Norsk Hydro) describes the use of copper instead of aluminium for the anode rod. Anode rods made from copper have actually been used in some smelting plants, but since copper is expensive it would be desirable to find a less expensive solution. WO 2002/042325 (Servico) discloses the use of high-conductivity materials such as copper or aluminum as in insert in steel yokes; this would reduce the quantity of copper to be used. A similar idea is applied in CN 1626701 . WO 2016/108696 (Storvik) describes other embodiment of anode yokes having an inner portion of copper and an outer portion of steel. Other designs of anode yokes using copper inserts are described in US 6,977,031 (SRA Technologies).

Another issue related to anode assemblies is heat loss. Anodes assemblies protrude out of the pot by their metallic anode rod: this becomes the dominant pathway for heat loss. Part of the heat loss through anodes is due to thermal conduction through anode yokes, followed by radiative and convective heat losses. Heat loss and ohmic losses are not independent from each other. In metals the thermal conductivity and the electrical conductivity are both proportional to the temperature, this is the Wiedemann-Franz law. As a consequence, heat loss will be increased by using inserts of metals with a high electrical conductivity. The embodiments disclosed in the patent documents cited above will exhibit increased heat losses. While heat losses through anodes have been taken into account in model calculations that have been published as prior art, there has been no specific design of anode hangers aimed at minimizing heat losses.

Heat loss can be desirable or not, depending on the situation. When designing a new electrolysis cell, it is desirable to decrease heat losses in order to increase overall energy efficiency. When modifying an operating cell type to increase its amperage, a certain degree of heat loss is desirable to avoid overheating, knowing that any increase in amperage will increase the heat generated by the cell, and in particular its potshell, which had not initially been designed for such a high amperage.

The applicant has set himself the target to reduce heat losses of Hall-Heroult cells, and in particular to reduce heat losses through anodes without increasing the electrical resistance of the anode yokes. This problem has been solved in the present invention.

Objects of the invention

The present invention relates, among other objects, to anode yokes for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Heroult process, said yoke being intended to mechanically and electrically link at least one carbon anode block and an anode rod, said yoke comprising a body intended to be connected to said anode rod, and a plurality of arms which extends each from said body, each arm being intended to be connected to said anode formed by at least one anode block.

In general, the heat loss of a hot solid surface can be through three mechanisms: radiative, convective and conductive. Conductive heat loss requires a material connection between a hot and a cold body, convective heat loss requires a cold transporting fluid, radiative heat loss does not require anything else. The inventors have recognized that heat conduction through the anode yoke makes a significant contribution to the total heat loss related to the anode yoke. In an anode assembly electrical conduction follows roughly the same path as heat conduction. As a consequence, any means for decreasing heat conductivity will also decrease electrical conductivity. While the former may be desirable, the latter is clearly undesirable.

The problem is solved by several means that are preferably used in combination. According to a first aspect of the invention, each arm of the anode yoke comprises a reduced cross section area, and said reduced cross section area comprises at least one hole which leads on at least one peripheral face of the said arm.

Conventionally, the anode yoke has a reduced cross sectional area of the upper part of the stub. More precisely, the stub is divided into two coaxial portions: a first, lower portion intended to be connected to the anode (and that actually plunges into the anode block), having substantially the shape of a circular cylinder or of a slightly tronconical cylinder (i.e. a cylinder the diameter of which may be somewhat smaller at its bottom than at its top), and a second, upper portion having a smaller cross sectional area than said first portion. Said first portion is called the stub. Prior art (US 4,612,105 assigned to Aluminium Pechiney) has proposed to decrease heat losses by decreasing the diameter of said second (upper) portion in order to reduce conductive heat loss of the anode assembly.

The inventors have found that it is unwise to decrease the diameter of the first portion, because this portion has a major influence on the mechanical strength and the electrical resistance of the anode hanger. It may even be desirable to increase its diameter. According to the present invention, the hole does not significantly decrease the electrical conductivity of the yoke, but decreases heat conduction. This latter effect is especially significant if the yoke is covered by granular material of low thermal conductivity, such as crushed bath, in order to decrease radiative and convective heat loss. Preferably said granular material extends into the hole itself.

According to a second aspect of the invention the number of arms is at least four per anode. Indeed, the reduction in cross section of the second portion results in reduced conductive heat loss, reduced electrical conductivity and reduced mechanical strength of each arm. The inventors have found that increasing the number of arms restores the mechanical strength and (almost totally) the total electrical conductivity and yet keeps the total heat loss at a lower level.

In particular the number of arms is at least for per anode block for the so-called“inline stub” configuration (where the stubs are arranged on a single straight line), and at least six arms per anode block for the so-called“spider yoke” configuration (where the stubs are arranged on two parallel straight lines).

The stub hole depth of the anode block should not be changed with respect to prior art, although this is not an essential feature.

Such anode yokes form the first object of the invention. More precisely, the first object is an anode yoke for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Heroult process, said yoke being intended to mechanically and electrically link at least one carbon anode and an anode rod, said yoke comprising:

- a body intended to be connected to said anode rod, and

- a plurality of arms which extends each from said body, each arm being intended to be connected to said carbon anode comprising at least one anode block, wherein each arm comprises a reduced cross section area,

said anode yoke being characterized in that said reduced cross section area comprises at least one hole, and preferably one single hole, which leads on at least one peripheral face of said arm.

Said hole is advantageously a through hole.

In specific embodiments of the invention said hole leads on opposite peripheral faces of said arm, in particular on opposite side faces of said arm, and/or each arm comprises a lower portion (so-called“stub”), intended to be inserted in an orifice (so-called“stub hole”) provided in said anode block, as well as a link part which extends between said body and said stub, said hole being formed in said link part, and/or said hole is formed adjacent said stub.

In other specific embodiments which can be combined with each of the previous ones, said link part comprises a base, which is adjacent said body, as well as a branch, which extends from said base away from said body, said branch being sloped downwards with respect to said base, said hole being formed in said branch, and/or the closest dimension between one wall of said hole and said stub is inferior to 40 mm, in particular inferior to 30 mm, and/or the closest dimension between one wall of said hole and said stub is greater than 10 mm, in particular greater than 15 mm.

The ratio between the surface of said hole and the surface of said reduced cross section area can be between 0.35 and 0.5. The main front dimension of said hole, in particular its diameter, can be between 50 mm and 80 mm.

The anode yoke according to the invention can comprise at least one row of arms, each row being intended to cooperate with one respective anode block, the arms of each row being preferably arranged symmetrically with respect to a transverse axis of said yoke.

Each row can comprise two first arms; adjacent said transverse axis, which forms a first angle with respect to said transverse axis, as well as two second arms, adjacent said transverse axis, which form a second angle with respect to said transverse axis, said second angle being superior to said first angle.

Said stub can have a substantially cylindrical shape, or a frustroconical shape with a diameter that is smaller at its lower portion than in its upper portion.

According to a third aspect of the invention the diameter of said second portion is increased with respect to prior art: this fully restores the electrical conductivity.

The inventors have recognized that one of the limiting factors of the electrical conductivity of the anode toward the anode beam is the connection between the anode stub and the anode. Indeed, during manufacturing of the anode assembly from the anode hanger and the anode, the first, lower portion of the yoke is embedded in a hole provided in the upper surface of the anode block, using cast iron; this process is known to a person skilled in the art and will not be described here. In this process, cast iron acts as electrically conductive heat resistant glue.

The connection between the anode stub and the anode block implies two different connections, namely the connection between the anode stub and the cast iron on the one hand, and between the cast iron and the carbon anode block on the other hand. The stub - anode connection may be subject to shrinking during the casting process (in particular upon cooling of the cast iron poured into the hole provided in upper anode surface), and to cracking. Cracking may occur during cooling after pouring the cast iron, and also when the anode hanger is loaded, since these two connections will support the mass of the carbon anode block and any additional vertical force that may be exerted on the anode block during its use in the pot. Such additional vertical force acting upon the anode block - stub connection will occur in particular when an anode that is in its normal operating position in the pot is lifted: the solidified bath in the pot forms a solid crust englobing the anode, and this crust needs to be broken by the lifting force.

The inventors have recognized that the electrical conductivity of the anode block - stub connection can be improved by increasing the diameter of the stub. The inventors have further recognized that the conductive heat loss through the anode yoke can be decreased by decreasing the cross sectional area of the arms of the anode yoke. This decrease in cross sectional area will lead to a slight decrease in electrical conductivity of the arms. The inventors have found that it is possible to restore the electrical conductivity of the anode yoke by the combination of an increase in stub - anode diameter and, optionally, an increase of the number of arms in the yoke. Moreover, there are two further, optional features that can be applied to the anode assembly according to the invention.

The first feature is the shape of the anode yoke. According to the invention arms comprise a substantially horizontal portion and a substantially vertical portion, the latter terminating in the stub. The inventors have found that the thermal expansion of the yoke arms contributes to improve the contact (both mechanical and electrical) between the cast iron around the stub hole and the anode block. Furthermore, this contributes to a reduction of mechanical stresses in the anode block, thereby decreasing the risk of cracking of the anode carbon blocks. Said horizontal and vertical sections can be joined by an inclined section.

All other parameters related to this process, including anode design parameters and stub hole depth, can be the same as in prior art.

A second means, which is optional but highly preferred, is related to manufacturing process of the anode assembly, and more precisely to the preparation of the stub hole. Said stub hole is provided in the anode green body, and is provided with flutes in the lateral hole surface. Said flutes are convex grooves impressed into the cylinder-shaped surface of the lateral hole surface. Said flutes can have an elongated shape in the direction of the cylinder axis. Said flutes are usually inclined with respect to the vertical direction, which gives rise to a self-locking effect upon lifting of the anode rod. According to prior art, these flutes extend over substantially all of the height of the stub hole.

The inventors have found that preferably these flutes are present only in the upper half of the surface. The number of flutes can be comprised between two and ten; preferably between three and nine, and still more preferably between four and eight. They can be equally spaced around the circumference of the cylinder (or tronconical body), or can be concentrated in certain areas. According to a preferred embodiment of the invention these flutes are not present along the longitudinal axis of the anode block: this contributes to avoid cracking of the carbon anode block.

The yoke design according to the invention leads to an increase in stub temperature under normal operating conditions, this difference being about 70 °C to 100 °C. This is due to the reduction in heat loss and to the increase in current density. This increase in stub temperature results in higher thermal expansion of the stubs. According to the invention the diameter of the stub hole is increased to 15-20 % of the stub diameter. This ensures a reduction in thermal stresses and maintains a good mechanical and electrical contact between the stub and the cast iron on the one hand, and the cast iron and the carbonaceous anode material on the other hand. This improved mechanical contact is ensured 360° around the stub hole, thereby improving electrical contact. In particular, the reduction of mechanical stresses will help to minimize cracking of the anode blocks. The good electrical contact ensured a low electrical resistivity.

Another object of the invention is an anode hanger comprising an anode rod and an anode yoke according to the invention, said yoke and said anode rod being attached using attachment means, in particular permanent attachment means, in particular welding.

Another object of the invention is an anode assembly comprising an anode hanger according to the invention and at least one anode block connected to said anode yoke. In advantageous embodiments, said anode hanger has four or six arms, and holds one single anode block, or said anode hanger has eight or ten or twelve arms, and holds two anode blocks.

According to an advantageous variant of the invention the electrical contact between the cast iron and the carbon anode block can be further improved by providing a set of flutes impressed into the surface of the upper portion of the lateral stub hole surface. Said set of flutes advantageously comprises between two and ten flutes, and preferably between three and nine. Said flutes preferably have an elongated shape in the direction of the cylinder axis of said stub. In an embodiment said flutes are present only in the upper half of said lateral stub hole surface portion, and preferably extend over substantially the whole length of said upper half.

Another object of the invention is the use of an anode assembly according to the invention in an electrolysis cell for making aluminum by the Hall-Heroult process. In order to limit radiative heat loss, said anodes and arms are advantageously covered by heat resistant layer of granular material, preferably alumina or crushed bath.

A final object of the invention is a process for making aluminium in an electrolysis cell by the Hall-Heroult process, wherein said electrolysis cell is using an anode assembly according to the invention.

Figures

Figures 1 to 8 represent various embodiments of the present invention.

Figure 1 shows a perspective view of an anode assembly according to a first embodiment of the invention. Figure 2 shows a perspective view, at a greater scale, illustrating an anode yoke according to the invention, said yoke being part of the anode assembly of figure 1.

Figure 3 is a top view of the anode yoke of figure 2.

Figure 4 and 5 are cross sections along lines IV-IV and V-V of figure 3. Figure 6 is a cross section along line VI-VI showing in particular a hole provided in said anode yoke.

Figure 6a is a cross section analogous to figure 6, showing a variant of said hole.

Figure 7 shows a perspective view, analogous to figure 1 , of an anode assembly according to a second embodiment of the invention. Figure 8 shows three perspective views of anode stubs together with their cast iron mantle according to an embodiment of the invention.

Figure 9 represents an anode assembly according to another embodiment of the invention: figure 9(a) is a perspective view, figure 9(b) a top view.

The following reference numerals are used throughout the text:

Detailed description

An aluminium smelter comprises a plurality of electrolytic cells arranged the one behind the other (and side by side), typically along two parallel lines. These cells are electrically connected in series by means of conductors, so that electrolysis current passes from one cell to the next. The number of cells in a series is typically comprised between 50 and over 100, but this number is not substantial for the present invention. The cells are arranged transversally in reference of main direction of the line they constitute. In other words, the main dimension, or length, of each cell is substantially orthogonal to the main direction of a respective line, i.e. the circulation direction of current.

The Hall-Heroult process as such, the way to operate the latter, as well as the cell arrangement are known to a person skilled in the art and will not be described here in more detail. In the present description, the terms“upper” and“lower” refer to mechanical elements in use, with respect to a horizontal ground surface. Moreover, unless otherwise specifically mentioned,“conductive” means“electrically conductive”.

A typical arrangement of a Hall-Heroult electrolysis cell mainly comprises a superstructure holding a plurality of anode assemblies, as well as a potshell. The latter forms an inner volume, for the reception of a liquid electrolyte. Moreover, each anode assembly comprises at least one carbon anode, said anode plunging in said electrolyte during operation of the cell.

The general structure and operation of a Hall-Heroult electrolysis cell is known per se and will not be explained here. It is sufficient to explain that the current is fed into the superstructure, and then flows to the plurality of anodes in contact with the liquid electrolyte where the electrolytic reaction takes place. Then the current crosses the liquid metal pad resulting from the process and eventually will be collected at the cathode block.

The present invention is particularly directed to the structure of one of said above anode assemblies and, more particularly, to the structure of a yoke which is part of said anode assembly. On the other hand, the means for mounting said anode assembly on said superstructure are not part of the invention, so that they will not be described. Moreover, said anode assemblies are moveable with respect to said superstructure, by way of not shown moving means. Said moving means, which are not part of the invention, make it possible to move anode assemblies altogether and/or individually the ones with respect to the others.

Figure 1 illustrates an anode assembly according to a first embodiment of the invention, referenced 10 as a whole. Said assembly 10 first comprises at least one prebaked anode and, in the present embodiment, two adjacent anode blocks 11 and 11’. Said assembly 10 also comprises one anode hanger 15, which comprises itself an anode rod 16 and an anode yoke 1. In the present description, the anode rod is supposed to extend vertically.

Both anode blocks 11 and 11’, as well as anode rod 16, are known as such and not part of the invention. In a classic way, each anode block 11 and 11’ is formed by a massive block made of carbonaceous material. Upper face of said anodes each comprises blind orifices 12 and 12’, for the reception of attachment pads provided on said yoke, which will be described hereunder. On the other hand, anode rod 16 is typically made of aluminium. Top of said rod is provided with classic not shown means, for attachment on the anode beam of the superstructure of the cell. Bottom of said rod is attached to yoke 1 , due to means which are described hereunder.

Anode yoke 1 , which is typically made of cast steel, mainly comprises a body 2, as well as a plurality of arms. Body 2 has a global rectangular shape, viewed from top. It is formed by a plate, which extends substantially horizontally. By way of example, thickness 72 (see figure 2) of said body 2 is between 80 and 150 mm. This body defines a central seat 21 for attachment of anode rod 16. In the present embodiment, said attachment is carried out due to permanent fixation means, in particular by welding. Typically, a so called Triclad™ transition joint 22 is used.

In the present example, eight arms 3A to 3D, as well as 4A to 4D are provided. Four first arms 3A, 4A, 4B and 3B, which define a first row R1, cooperate with first anode block 11 , whereas four other arms 3C, 4C, 4D and 3C, which define a second row R2, cooperate with second anode block 11’. Let us note (figure 3) X1 the main longitudinal axis of yoke

I , which corresponds to that of body 2, as well as Y1 the main transversal axis of yoke 1 , which corresponds to that of body 2.

From top, yoke 1 is symmetrical with respect to both axes X1 and Y1. More precisely, arms 3A and 3C are symmetrical of respective arms 3B and 3D with respect to axis Y1 , arms 4A and 4C are symmetrical of respective arms 4B and 4D with respect to axis Y1 , whereas each of arms 3A, 4A, 4B and 3B is symmetrical of a respective arm 3C, 4C, 4D and 3D with respect to axis X1 . We will now describe the structure of arms 3A and 4A, bearing in mind that the structure of arms 3B, 3C, 3D is identical to that of arm 3A, and in that the structure of arms 4B, 4C, 4D is identical to that of arm 4A. The constitutive parts of arms 3A to 3D, as well as 4A to 4D, will be given the same reference numbers, with a respective final letter A to D.

With reference to figure 3. let us note L3A the main axis of arm 3A. Viewed from top, said arm 3A is slanted, with an angle a3 formed by Y1 and L3A which is between 15° and 60°. arm 3A first comprises a link part 30A (see figure 4), which is formed by a proximal region or base 31 A, adjacent the body 2, as well as an intermediate region or branch 32A, which extends from said base away from said body 2. Said arm also comprises a distal attachment stub 33A, which has a cylindrical cross section. In a way known as such, each stub is intended to be received in one respective of blind orifices 12 provided in one anode

I I , for mutual attachment of said stub and said anode. In particular, stub is sealed by means of an appropriate material, which is filled in the interstice between facing walls of said stub and said orifice.

Both base 31 A and branch 32A have a polygonal cross section, in particular a rectangular cross section. Viewed from top (see figure 3), base 31 A and branch 32A are parallel, and extend both along main axis L3A. On the other hand, viewed from front (see figure 4), base 31 A is substantially horizontal, whereas branch 32A is downward sloping. Angle b 32 between said branch and horizontal direction is between 5 and 30 °. Typically, length L31 of base 31 is between 50 and 100 mm, whereas length L32 of branch 32 is between 300 and 400 mm (both viewed from front, refer to figure 4).

Branch 32A is surrounded by peripheral faces, which comprise opposite side faces 34 and 35 (see figures 3 and 6), as well as opposite front 36 and rear 37 faces (see figure 4). By convention, side faces of one branch face a respective neighboring branch, whereas front and rear faces are respectively adjacent and opposite the body of the yoke. According to an essential feature of the invention, one hole 38 is provided in branch 32. In the illustrated example, this hole is a through hole which leads on opposite side faces 34 and 35, its main axis X38 being perpendicular to main axis L3A (see figure 6). In the illustrated example, said hole 38 is circular shaped, with a diameter D38 typically between 50 and 80 mm.

This hole defines a reduced cross section area, referenced 39 on figure 4. Said area 39 is defined, viewed from front, by front and rear faces, as well as by horizontal lines which extend respectively through top and bottom of hole 38. Let us note S38 the surface of the hole and S39 the surface of area 39 (excluding said hole). According to an advantageous feature of the invention, the ratio S38/S39 is between 0.35 and 0.65.

It should be noted that area 39 has a reduced cross section, which means that its solid surface is locally reduced, due to the presence of hole 38. This is advantageous for decreasing heat losses. However, said area 39 is surrounded by straight peripheral faces 34 to 37. Moreover, none of these faces has a surface discontinuity between said area 39 and neighboring portions. This is also advantageous, for what concerns global mechanical properties and simplicity of manufacturing process.

As seen on figure 4. hole 38 is adjacent to stub 33, which means that it is far closer to said stub, than to both body 2 and base 31. Let us note d38 (see figure 4) the closest distance between the facing walls of hole 38 and stub 33. According to an advantageous feature of the invention, said distance d38 is between 15 and 30 mm. Distance d38 should not be too small, as this would complicate the replacement of the stub for repair (and might also lead to insufficient mechanical strength of the whole arm); distance d38 should be sufficiently small such as to hinder heat conduction. Let us also note 138 (see figure 4) the closest distance between the wall of hole 38 and side face 3X

With reference to said figure 3, let us now note L4A the main axis of arm 4A. Viewed from top, said arm 4A is slanted. Angle a4 between Y1 and L4A is between 80° and 100°, a4 being far inferior to above angle a3. In other words, main axes of arms 3A and 4A are not mutually parallel. Arm 4A first comprises a link part 40A (see figure 5), which is formed by a proximal region or base 41 A, adjacent the body 2, as well as an intermediate region or branch 42A, which extends from said base away from said body. Said arm also comprises a distal attachment stub 43A, analogous to above described stub 33A.

Both base 41 A and branch 42A have a polygonal cross section, in particular a rectangular cross section. Viewed from top (see figure 3), base 41 A and branch 42A are parallel, and extend both along main axis L4A. On the other hand, viewed from front (see figure 5), base 41 A is substantially horizontal, whereas branch 42A is downward sloping. Angle b 42 between said branch and horizontal direction is between 15° and 30°, i.e. superior to above angle b 32. Typically, length L41 of base 41 is between 50 and 80 mm, i.e. inferior to above length L31 of base 31 , whereas length L42 of branch 42 is between 250 and 300 mm, i.e. inferior to above length L32 of branch 32.

Branch 42A is surrounded by peripheral faces, which comprise opposite side faces 44 and 45 (see figure 3), as well as opposite front 46 and rear 47 faces (see figure 5). One hole 48 is provided in branch 42 and leads on opposite side faces 44 and 45. In the illustrated example, said hole 48 is circular shaped, with a diameter D48 substantially identical to that D38.

This hole defines a reduced cross section area, referenced 49 on figure 5. Said area 49 is defined, viewed from front, by front and rear faces, as well as by horizontal lines which extend respectively through top and bottom of hole 48. Let us note S48 the surface of the hole and S49 the surface of area 49 (excluding said hole). According to an advantageous feature of the invention, the ratio S48/S49 is between 0.35 and 0.65, i.e. similar to above ratio S38/S39.

As for hole 38, hole 48 is adjacent to stub 43, with a distance d48 (see figure 5) analogous to distance d38. Moreover, distance 148 (see figure 5) between inner wall of hole 48 and side face 47 is similar to above distance 138.

As can be seen from figures 4 and 5, the hole 38,48 is a through hole. It remains preferably open; in service it may eventually be filled with crushed bath or alumina which are materials with low thermal conductivity. The present invention is not limited to the first embodiment, described in reference with figures 1 to 6.

Figure 6a shows a first variant, wherein hole 38’ does not lead on both opposite side faces of branch 32, but on only one of these side faces, such as 35. This embodiment is not preferred, because such a hole is less efficient than a though hole.

According to a not shown variant, hole 38 or 48 might have a shape which is different from a circle, for example a polygon, such as a rectangle. Moreover, more than one hole might be provided in said area with reduced cross section.

In the embodiment of figures 1 to 6, the stubs are arranged on two parallel straight lines (so-called “spider yoke configuration”), and one yoke 1 cooperates with two anodes. However, said yoke might cooperate either with one single anode, or with more than two anodes. In this respect, figure 7 shows an alternate embodiment wherein one yoke 101 cooperates with one single anode block 111. In this embodiment all stubs 133A,133B,133C,133D are arranged on one single straight line (so-called“inline stub” configuration) On figure 7, the mechanical elements which are analogous to those of figures 1 to 6 are given the same references, added by 100.

In the embodiments of figures 1 to 7, four arms of one yoke cooperate with one respective anode. According to not shown variants, a different number of arms might be provided for cooperation with said anode. For instance, five or six arms might be provided, which are typically arranged symmetrically with respect to transverse axis of said yoke.

Figure 8 shows three perspective views of a variant which is compatible with any of the other embodiments and variants of the invention. More precisely, figure 8 shows a stub 33 embedded in a mantle of cast iron 81 , the surface of which reproduces the inner surface of the stub hole into which the liquid cast iron has been poured; the anode is not shown on the figure. Said inner surface of the stub hole is provided with flutes, which can be seen on figure 8 as the outer surface of the cast iron mantle; said flutes are given here the reference number 80.

As mentioned above, the stub 33 has a substantially cylindrical shape, or has a frustroconical shape with a diameter that is smaller at its lower portion than in its upper portion. In this variant, the upper portion of the stub hole comprises a set flutes 80, 80’, 80”, impressed into the hole surface, said flutes 80 preferably having an elongated shape in the direction of the cylinder axis of said stub. Advantageously (and as shown on figure 8), said flutes 80 are present only in the upper half of the upper portion, and preferably extend over substantially the whole length of said upper portion. Said set of flutes comprises between two and ten flutes, and preferably between three and nine. The flutes are equally spaced around the circumference of said upper hole portion or are concentrated in certain areas. Preferably they are not present along the longitudinal axis of the anode block; this reduces cracking of the carbon material of the anode in the vicinity of the stub hole.

These flutes improve the electrical contact between the carbon anode and the anode yoke. This is because they improve the mechanical contact between the anode material and the stub and prevent slipping; this improvement in mechanical contact results in an improved electrical contact. Furthermore, the increase in the surface of the stub hole into which the flutes are imprinted leads to an increase in the contact surface between the carbon material of the anode and the cast iron, which leads to an improved electrical contact.

Figure 9 shows two views of another embodiment of the present invention in which one yoke 201 cooperates with two anodes. On this figure 9, the mechanical elements which are analogous to those of figures 1 to 6 are given the same references, added by 200. This anode yoke 201 comprises two rows R1 ,R2 of arms 203A-203F, 203G-203L, the arms of each row R1 , R2 being arranged symmetrically with respect to a longitudinal axis X1 of said yoke, thereby creating two groups G1 ,G2 of arms 233A,233B,233C, 233J,233K,233L; 233D,233E,233F,233G,233H,233I being arranged symmetrically with respect to said axis X1. The anode assembly 210 shown on figure 9 comprises two prebaked anode blocks 211 ,211’. The same yoke can also be used with one single anode block.

Example Several different anode hangers with a yoke comprising four stubs in a row for the inline stub configuration, and six stubs for the spider stub configuration, were manufactured. They were designed to replace prior art anode hangers in the same pot, comprising four anode stubs in the inline stub configuration. Each arm was provided with a hole, the diameter of which was of the order of 70 mm. This hole was provided during the casting process of the yoke. Several yoke designs were tested, especially for the arms.

It was found that by using the yoke design according to the invention the total anode assembly heat loss could be reduced by 20 % to 40 % (determined for anode yokes covered by a given, constant mass of granular material such as alumina or crushed bath). It was found that the presence of the hole leads to a slight increase in voltage drop by about 15 mV to 30 mV. This increase could be compensated in part by an increase in stub diameter from originally 185 mm to a value of 200 mm to 220 mm. The design of the arms was as short as possible to limit the conductive path and thereby conductive losses for electricity and heat, and as low as possible to facilitate the coverage of the yoke by a given, constant mass of granular material.

Good results were obtained with a yoke having arms with an inclined section (joining the horizontal and the vertical section) with an average cross section of 105 mm x 1 12 mm (cross sectional area 1 1 760 mm²) and a hole of 70 mm diameter (cross sectional area 3 848 mm²), leading to a reduction in cross sectional yoke area of 32.7 %, to a reduction of heat loss of 15 % to 20 %, and an increase in voltage drop for the full anode assembly of 5 % to 8%.

Claims

1 . An anode yoke (1 ; 101 ) for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Heroult process, said yoke being intended to mechanically and electrically link at least one carbon anode block (1 1 , 1 1’; 1 1 1 ) and an anode rod (16; 1 16), said yoke comprising:

- a body (2; 102) intended to be connected to said anode rod, and

- a plurality of arms (3A-3D, 4A-4D; 103A, 103B, 104A, 104B) which extends each from said body, each arm being intended to be connected to said anode, wherein each arm comprises a reduced cross section area (39, 49),

said anode yoke being characterized in that said reduced cross section area comprises at least one hole (38, 38’, 48; 138, 148), and preferably one single hole (38, 48), which leads on at least one peripheral face (34, 35, 44, 45) of said arm.

2. An anode yoke according to claim 1 , characterized in that said hole (38, 48; 138, 148) leads on opposite peripheral faces (34, 35, 44, 45) of said arm (3A-3D, 4A-4D), in particular on opposite side faces (34, 35, 44, 45) of said arm.

3. An anode yoke according to any of claims 1 to 2, characterized in that each arm comprises a lower portion (33, 43) (so-called“stub”), intended to be inserted in an orifice (12, 12’) (so-called“stub hole”) provided in said anode, as well as a link part (30, 40) which extends between said body (2) and said stub (33, 43), said hole (38, 48) being formed in said link part.

4. An anode yoke according to claim 3, characterized in that said hole (38, 48) is formed adjacent said stub (33, 43).

5. An anode yoke according to any of claims 3 or 4, characterized in that said link part (30, 40) comprises a base (31 , 41 ), which is adjacent said body (2), as well as a branch (32, 42), which extends from said base away from said body, said branch being sloped downwards with respect to said base, said hole (38, 48) being formed in said branch.

6. An anode yoke according to any of claims 3 to 5, characterized in that the closest dimension (d38, d48) between one wall of said hole (38, 48) and said stub (33, 43) is inferior to 40 mm, in particular inferior to 30 mm.

7. An anode yoke according to any of claims 1 to 6, characterized in that the closest dimension ( d38 , d48) between one wall of said hole (38, 48) and said stub (33, 43) is greater than arm 10 mm, in particular greater than 15 mm.

8. An anode yoke according to any of claims 1 to 7, characterized in that the ratio ( S38/S39 ) between the surface ( S38 ) of said hole and the surface ( S39 ) of said reduced cross section area is between 0.35 and 0.65.

9. An anode yoke according to any of claims 1 to 8, characterized in that the main front dimension of said hole, in particular its diameter ( D38 , D48 ), is between 50 mm and 80 mm.

10. An anode yoke according to any of claims 1 to 9, characterized in that it comprises at least one row (R1 , R2) of arms (3A-3D, 4A-4D; 103A, 103B, 104A, 104B), each row being intended to cooperate with one respective anode, the arms of each row (R1 , R2) being preferably arranged symmetrically with respect to a transverse axis (Y1 ) of said yoke.

1 1. An anode yoke according to claim 10, characterized in that each row (R1 , R2) comprises two first arms (3A-3B, 3C-3D), adjacent said transverse axis (Y1 ), which form a first angle (a3) with respect to said transverse axis (Y1 ), as well as two second arms (4A-4B, 4C-4D), adjacent said transverse axis (Y1 ), which form a second angle (b3) with respect to said transverse axis (Y1 ), said second angle being superior to said first angle.

12. An anode yoke according to any of claims 1 to 9, characterized in that it comprises two rows (R1 ,R2) of arms (203A-203F, 203G-203L), the arms of each row (R1 , R2) being arranged symmetrically with respect to a longitudinal axis (X1 ) of said yoke, thereby creating two groups (G1 ,G2) of arms (233A,233B,233C,233J,233K,233L; 233D,233E,233F,233G,233H,233I) being arranged symmetrically with respect to said axis X1 .

13. An anode yoke according to any of claims 3 to 12, wherein said stub (33,34) has a substantially cylindrical shape, or a frustroconical shape with a diameter that is smaller at its lower portion than in its upper portion.

14. Anode hanger (15; 1 15) comprising an anode rod (16; 1 16) and an anode yoke (1 ;

101 ) according to any of claims 1 to 13, said yoke and said anode rod being attached using attachment means (22), in particular permanent attachment means, in particular welding.

15. An anode assembly (10; 1 10) comprising an anode hanger (15; 1 15) according to claim 14, and at least one anode block (1 1 , 1 1’; 1 1 1 ) connected to said anode yoke.

16. An anode assembly according to claim 15, wherein said anode comprises stub holes, said stubs being fixed into said stub holes using cast iron, and wherein the upper portion of the lateral stub hole surface comprises a set flutes impressed into the surface of said upper portion, said flutes preferably having an elongated shape in the direction of the cylinder axis of said stub.

17. An anode assembly according to claim 15, characterized in that said flutes are present only in the upper half of said lateral stub hole surface portion, and preferably extend over substantially the whole length of said upper half.

18. An anode assembly according to any of claims 16 or 17, characterized in that said set of flutes comprises between two and ten flutes, and preferably between three and nine.

19. An anode assembly according to any of claims 16 to 18, characterized in that

- said flutes are equally spaced around the circumference of said lateral stub hole or are concentrated in certain areas,

- said flutes are not present along the longitudinal axis of the anode block.

20. An anode assembly according to any of claims 15 to 19, wherein said anode hanger has four, five or six arms, and holds one single anode block.

21. An anode assembly according to any of claims 15 to 19, wherein said anode hanger has six, eight ten or twelve arms, and holds two anode blocks.

22. Use of an anode assembly according to claims 15 to 21 in an electrolysis cell for making aluminum by the Hall-Heroult process.

23. Use according to claim 22, wherein said anodes and arms are covered by heat resistant layer of granular material, preferably alumina or crushed bath.

24. Process for making aluminium in an electrolysis cell by the Hall-Heroult process, wherein said electrolysis cell is using an anode assembly according to any of claims 15 to 21.