NO20180882A1

NO20180882A1 - Anode hanger, and method of production thereof

Info

Publication number: NO20180882A1
Application number: NO20180882A
Authority: NO
Inventors: Nils Sande
Original assignee: Storvik As
Priority date: 2018-06-22
Filing date: 2018-06-22
Publication date: 2019-12-23
Also published as: NO345111B1; WO2019245386A1

Description

Anode hanger, and method of production thereof

Technical field

The present invention relates to an anode hanger for an anode in an electrolysis cell for the production of aluminium by electrolysis of alumina solved in a melted electrolyte. More specifically the present invention relate to an anode yoke, and a method for the production of such anode yoke.

Background art

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 important electrolysis processes is 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.

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, and 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.

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.

A voltage drop appears all over the electrolysis cell, of which the most significant voltage drop appears over the electrolyte. However, voltage drop also appears over the current conducting devices comprising the anode hangers and the anode studs, and the current collectors of the cathode. 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 this 50 % lost from the anode top approximately 25 % is lost from the anode hangers, i.e. from studs, yoke and stem. 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 and into the anode hangers. The part of the anode hangers closest to the area where the aluminium is produced and majority of the heat is generated, is the most important part of the anode hanger with respect to heat loss i.e. the studs and yoke.

The heat balance of the electrolysis pot is automatically regulated by keeping the electrical resistance over the pot at a certain fixed pre-set level. A reduced electrical resistance in the anode cathode inter polar distance (ACD) or other parts of the cell, will automatically lead to an increase in the ACD. But it is a major difference if the reduced electrical resistance is close to the area where the aluminium is produced (in the ACD) or in an area farther away from the ACD. If the reduction in the electrical resistance is in the part of the cell not influencing the heat balance of the cell, the set resistance of the cell need to be reduced to avoid increasing the heat input into the ACD area of the cell, i.e. it gives a saving in the specific energy consumption of the cell, but is not a potential for increasing the amperage. If the reduction in electrical resistance in the cell is in an area influencing the heat balance of the cell, an increase in the ACD gives a potential for increasing the amperage by keeping the net heat input into the cell constant before and after the increase in amperage. How much the amperage can be increased and still end up with the same ACD as before and after the amperage increase, is normally calculated by using a special designed heat balance models.

Reduced electrical resistance in the cell can be achieved in three different ways, i.e. by shortening the distance the current is transported in the ACD, reducing the current density in the bath where the aluminium production is taking place, or reducing the electrical resistance in the current conducting devices.

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 outer or inner parts of copper and/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. Electrical cathode collector bus bars are typically manufactured from massive steel, at least in the part which is to be incorporated into the cathode. Ends extending from the electrolysis cell could be of another material with a better electrical conductivity, such as for example copper. Also the part of the anode hanger or anode studs for incorporation into the anode carbon is typically manufactured from steel. The upper, upwardly extending part of the anode hanger, i.e. the anode rod (also referred to as stem in the present context), is normally manufactured from aluminium, connected to the lower steel containing part of the anode hanger, the anode yoke, via a bimetal transition. Several anode hanger designs contain a plurality of welds, typically manual welds performed in difficult welding positions, with may result in poor quality with low conductivity and strength.

In patent publication NO 162083 an anode hanger for holding a carbon containing anode in cells for production of aluminium has been described. The anode hanger disclosed in NO 162083 for use in Hall-Heroult-process cells for production of aluminium, includes an upper part of a metal such as aluminium, copper or steel, which is joined to an anode beam, and a lower current conducting steel part which is fastened to the upper part. The lower steel part comprises a yoke with downward extending studs whereto the carbon containing anode is secured. The upper part is fastened to the lower steel part by means of a cast-joining of aluminium or copper.

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 (Cu) and aluminium (Al). Iron/steel are used in areas which are exposed for corrosion and high temperatures, this material(s) also have very good mechanical properties, however the electrical conductivity is relatively poor compared with Cu and Al.

It is an object of the invention to provide an improved anode hanger which reduces or eliminates disadvantages of known anode hangers. Thus, it is an objective of the present invention to achieve this and to provide further advantages over the state of the art, which advantages will become evident in the following description.

Summary of invention

In a first aspect the present invention relates to an anode hanger for fixedly holding anodes in cells for the electrolytic production of aluminium according to the Hall-Heroult process, comprising

an anode yoke having a yoke head, at least two yoke arms, at least two yoke shoulders and at least two yoke legs, to each respective yoke legs a stud is fixed, the anode yoke comprises

an inner portion of aluminium metal or aluminium alloy having a first surface, and an outer portion of steel material covering and bonded to a substantial part of said first surface of the inner portion,

an exposed upper area of the inner portion of aluminium at the yoke head, wherein an interface between said inner portion and said outer portion provides a substantially continuous metal-to-metal diffusion bond between said inner portion and said outer portion.

In an embodiment the said outer portion is covering at least 85%, or 90%, or 95% of an outer surface of said inner portion.

In an embodiment the at least 95%, or 98%, or 99.5%, or 99.95% of said interface between said inner portion and said outer portion is providing a metal-to-metal diffusion bond between said inner portion and said outer portion.

In an embodiment the metal-to-metal bond is substantially a metallic bond at atomic level.

In an embodiment the inner portion of aluminium is made of pure aluminium of at least 99 % Al, or an aluminium alloy chosen from 6XXX aluminium alloys.

In an embodiment the two yoke legs are arranged symmetrical on each side of, and spaced from the head of the yoke. The said two yoke legs are arranged via respective shoulders and arms to the head of the yoke.

In an embodiment the yoke comprises three yoke legs, two of which yoke legs are arranged symmetrical on each side of, and spaced from the head of the yoke via respective arms and shoulders, and the third yoke leg is immediately below the yoke head.

In an embodiment the yoke comprises two arms, with respective shoulders, and four yoke legs, the four legs are arranged in-line and symmetrical on each side of, and spaced from the head of the yoke. Two of the yoke legs are arranged to respective shoulders, while the other two yoke legs are arranged immediately below the arms.

In an embodiment the yoke comprises four arms with respective shoulders, each shoulder having a yoke leg, the four arms with respective shoulders and legs are arranged and spaced from the yoke head at respective 0, 90, 180 and 270 degree positions of a circle about the head with the yoke head at the circle centre.

In an embodiment the yoke comprises four arms, the arms being arranged and spaced from the yoke head at respective 0, 90, 180 and 270 degree positions of a circle about the head with the yoke head at the circle centre, where the arms at 90 and 270 degree positions, or at 0 and 180 degree positions, are extended and split into two branches each having a shoulder and carrying a respective one of four studs, while the two nonsplit arms each carrying a stud, providing the total of six studs, three of the six studs are arranged in-line with each other on each of two parallel lines spaced from and at respective sides of the head.

In an embodiment the anode hanger further comprises a rod of aluminium material welded to the exposed upper area of the inner portion of aluminium at the yoke head.

In a second aspect the present invention relates to a method of producing the anode hanger according to the first aspect or any of the embodiments of the first aspect, the method comprising

providing a cast mould having a suitable pattern for casting an outer portion of steel material;

providing a mould core, having a shape corresponding to a suitable inner portion of aluminium metal or alloy;

casting the outer steel portion by pouring molten steel into the casting mould comprising the mould core;

after solidification of the steel outer part, removing the core, thereby obtaining the outer portion of steel material having a an internal hollow space and an opening in the upper yoke head part and in the at least two yoke legs lower ends; arranging a feederhead at the opening in the yoke head part of the said outer portion of steel material;

filling the said internal hollow space and feederhead with molten aluminium metal or alloy;

heat treating the aluminium filled outer portion of steel material comprising the feederhead at a temperature above the melting temperature of the aluminium metal or alloy; and

directionally solidifying the liquid aluminium metal or aluminium alloy by controlling the cooling from the lower end of the cast part, towards the feederhead.

In an embodiment of the method the holes in the at least two legs are plugged by a mould insert, or by fixing a stud to each yoke leg.

In an embodiment of the method the temperature for heat treatment is up to 1000 °C.

In an embodiment of the method the period for heat treatment is up to 15 hours.

In an embodiment of the method the period for heat treatment is up to 8 hours.

In an embodiment of the method the controlled cooling is performed by gradually immersing the heat treated cast product in a cooling bath comprising a cooling liquid..

In an embodiment of the method the controlled cooling is performed in a period of up to 45 minutes.

In an embodiment of the method the controlled cooling is performed in a period of up to 15 minutes.

In an embodiment, the method further comprises welding the steel studs to each yoke leg.

Brief description of drawings

Figure 1 : Illustrates an anode hanger according to the present invention; front view of a four stud embodiment.

Figure 2: Illustrates an anode hanger according to the present invention; top view of a four stud embodiment.

Figure 3 : Illustrates an anode hanger according to the present invention; crosssection through line A-A view of a four stud embodiment.

Figure 4: Illustrates details in part I (Figure 3) of an anode hanger according to the present invention; a front view cross-section of the connection between the anode yoke and the anode rod.

Figure 5: Illustrates an anode hanger according to the present invention; side view of a four stud embodiment.

Figure 6: Illustrates details in part II (Figure 5) of an anode hanger according to the present invention; a side view cross-section of the connection between the anode yoke and the anode rod.

Figures 7A,B: Illustrates studs according to the present invention; Fig. 7A illustrates a side view of a stud, Fig. 7B illustrates a cross-section through line A-A in Fig. 7A.

Figure 8: Illustrates a two yoke legs, two studs embodiment.

Figure 9: Illustrates a three yoke legs, three studs embodiment.

Figure 10: Illustrates a four yoke legs, four studs embodiment.

Figure 11 : Illustrates a six yoke legs, six studs embodiment.

Detailed description

In the following the anode hanger of the invention will be described and explained by way of example and with reference to the accompanying drawings in which the same reference numbers refer to the same or technically equivalent elements, and wherein the terms “inner portion of an aluminium material” and “aluminium core” are used interchangeably, and the terms “outer portion of a steel material” and “steel lining” are used interchangeably. In the present invention the aluminium material includes pure aluminium metal or essentially pure aluminium metal of at least 99 % Al. The aluminium material also includes aluminium alloys of the 6000 series; 6XXX aluminium alloys. The steel material should be such steels that are normally used in anode hangers.

Reference is first made to the drawings of figures 1, 2, and 3, illustrating in a front view, in a top view, and in a front cross-section view, respectively, an anode hanger according to the present invention. The line A- A in figure 2 is to indicate the projection of the section plane for the cross section view of figure 3. This anode hanger embodiment comprises an anode rod (also denoted stem herein) 110, an anode yoke, comprising yoke “shoulders” 113, yoke “arms” 112, yoke “head” 111, and four yoke “legs” 117, to each of which four legs 117 a stud 114 is fixed. In the four stud embodiment shown in figures 1-3 the four legs are arranged in-line and symmetrical on each side of and spaced from the head 111 of the yoke. In figure 2 it is also shown an exposed upper area 121 of the head part of the aluminium core of the yoke head 111, at which the anode yoke is connected electrically to a source of electric current for its use in the electrolytic process in metal production. In the cross section view of figure 3 it is clearly illustrated that the anode yoke has an outer portion of a steel material and an inner portion of aluminium metal. Further it is illustrated feature details of this embodiment in which the aluminium forming a core of the anode yoke extends from the exposed upper area 121 in the yoke head 111, to a part of the yoke at which the yoke arms 112 and shoulders 113 transits into the legs 117, at which the studs 114 are arranged. Preferably, the inner portion of aluminium material does not extend completely to the lowermost end of the steel lining yoke leg 117, but ends before so that the outer steel portion forms a short sleeve at the lower end of the yoke leg 117. The studs 114 constitutes a substantially all steel material part fixed to the anode yoke leg.

Although the yoke head 111 shown in figure 2 has a rectangular shape, while the legs 117 and stud 114 parts are of cylindrical shape with a substantially circular horizontal cross section, other shapes and cross sections geometries are contemplated. E.g. one or more of the aforementioned parts, as well as the shoulders and arms of the anode yoke 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.

The studs 114, as illustrated in drawings in figures 7A and 7B, are substantially massive steel, typically of the same steel type as the anode yoke steel lining. The studs 114 are preferably cylindrical or conical having a general circular cross section, however the studs may have other shapes, such as an elongated body having a e.g. elliptical, rectangular, conical, square, hexagonal or other polygonal shaped horizontal crosssection. The studs 114 should have a flat, or substantially flat, lower end, i.e. the part of the stud being fixed to the anode carbon. The upper part of the studs 114, which are fixed to the anode yoke legs 117, should advantageously have a conical or tapered shape 119, with a central head 120. The shape and dimensions (diameter (d), height (h)) of the central head 120 corresponds to the dimensions of the steel sleeve in the lower part of the yoke legs 117. The central head 120 is advantageously cylindrical, however it should be understood that other shapes are possible. It should also be noted that the upper part of the stud may have a shape different from conical, however the upper end, which is fixed to the yoke leg, should have a shape such that a suitable gap is formed between the yoke leg and the stud where the two parts are welded together. Due to manufacturing process a conical shape is preferred. A conical (tapered) shape is also favourable for providing a strong welding seam 116. The shape of the central stud head 120 has a diameter and cross-section corresponding to the lower end of the aluminium core in the yoke leg 117. Thus, the stud receiving hole (i.e. the steel sleeve) in the anode yoke legs 117 ensures an intimate contact between the aluminium core and the steel stud head 120, while the conical peripheral shape of the studs forms a gap between the stud 114 and the yoke leg 117, for securely welding the stud 114 to the yoke leg 117.

The angle, a, defined by the gap between the conical peripheral of the stud 114 and the horizontal part of the yoke leg 117 (see figure 3), should be between about 20-60 °. An angle within this range will provide a suitable gap between the steel stud and yoke leg for securely welding the stud to the yoke by a steel-steel weld 116. The cylindrical central head is essentially not affected by the welding, thus any changes in the steel microstructure and/or the weld seam 116 will not influence the steel-aluminium interface area where the stud steel contact the aluminium core in the anode yoke.

In a second embodiment the anode hanger has two studs, see fig. 8 illustrating a crosssection viewed from the front. The overall shape and the main parts of the yoke of this two stud embodiment of an anode hanger according to the present invention are substantially the same as those of the first embodiment explained above. Preferably, in the two stud embodiment, the studs 114 are fixed to the yoke legs 117 symmetrical on each side of and spaced from the head 111 of the yoke. The studs and fixation of the studs to the two yoke legs 117 are also substantially the same as explained in the first embodiment. As explained above for the first embodiment of the four stud embodiment, other shapes and cross sections geometries are contemplated also for the second two stud embodiment of the anode hanger.

In a third embodiment the anode hanger has three studs, see fig. 9 illustrating a crosssection viewed from the front. The overall shape and the main parts of the yoke of this three stud embodiment of an anode hanger according to the present invention are substantially the same as those of the first, and second embodiments explained above. Preferably, in the three stud embodiment, two studs 114 are fixed to the yoke legs 117 symmetrical on each side of and spaced from the head 111 of the yoke, and the third yoke leg 117, to which the third stud 114 is fixed, is immediately below the yoke head 111. The studs and fixation of the studs to the two yoke legs are also substantially the same as explained in the first embodiment. As explained above for the first embodiment, other shapes and cross sections geometries are contemplated also for the second three stud embodiment of the anode hanger.

In a fourth embodiment of the invention the anode hanger has four arms 112 and shoulders 113, each shoulder having a yoke leg 117 to which a stud 114 is fixed, see fig.

10 illustrating a top-view. The overall shape and the main parts of the yoke of this four shoulder, four stud embodiment of an anode hanger according to the present invention are substantially the same as those of the first embodiment explained above. Taking the second embodiment, two stud embodiment, as reference in which the two studs 114 via respective legs 117, shoulders 113 and arms 112 are attached to the head 111 at respective 0 and 180 degree positions of a circle about the head 111 with the head 111 at circle center, a substantial difference lies in that there are provided two additional studs 114 and respective arms 112 arranged in 90 and 270 degree positions of the circle, and the two additional studs 114 fixed to yoke legs 117, via said respective shoulder and arms to the head 111, correspondingly. The studs and fixation of the studs to the four yoke legs are also substantially the same as explained in the first embodiment. As explained above for the first embodiment, other shapes and cross sections geometries are contemplated also for this four stud embodiment of the anode hanger.

In a fifth embodiment of the invention the anode hanger has six studs, see fig. 11 illustrating a top-view. The main parts of the yoke of this six stud embodiment of an anode hanger according to the present invention correspond to similar main parts of the first, second, third, and fourth embodiments explained above, however, taking the fourth embodiment of a four stud anode hanger as reference in which the four studs 114 via respective legs 117, shoulders 113 and arms 112 are attached to the head 111 at respective 0, 90, 180, and 270 degree positions of a circle about the head 111 with the head 111 at circle center, a substantial difference lies in that the arms 112 at 90 and 270 degree positions (or 0 and 180 degree positions) are extended and split into two branches each carrying a respective one of four studs, bringing the total of studs 114 to six, with three of the six studs arranged in-line with each other on each of two parallel lines spaced from and at respective sides of the head 11 l.The studs and fixation of the studs to the six yoke legs are also substantially the same as explained in the first embodiment. As explained above for the first embodiment, other shapes and cross sections geometries are contemplated also for the fifth six stud embodiment of the anode hanger.

The anode yokes according to the described embodiments are fixed to an anode rod 110 for conducting electric current to the anode for its use in the electrolytic process in metal production. The anode rod 110 is preferably made of aluminium or an aluminium alloy, and should have a cross-section corresponding to the exposed upper area 121 of the head part of the aluminium core of the yoke head 111. The end of the anode rod 110, which is to be fixed to the anode yoke, advantageously has a conical (tapered) shape in its peripheral and a central substantially flat part, see figures 4 and 6, which shows enlarged details of sections I and Π in figures 3 and 5 respectively. Due to the conical/tapered peripheral shape of the rod end, there is a gap between the rod end and the exposed upper area 121 of the head part of the aluminium core of the yoke head 111. An angle, β, is defined by the inclination angle of the conical rod end to the horizontal yoke head 111/upper area 121 of the head part if the aluminium core. Suitably, the angle, β, is between 20-60 e.g. 25-50 which will ensure a proper gap between the rod and the upper end of the aluminium core 121 in the yoke head 111 for securely fixing the rod to the anode yoke by aluminium-aluminium welding 115. It should be noted that the end part of the anode rod, which is to be welded to the yoke head, may have another peripheral shape than conical, however the end should have a design such that a gap or groove is formed, for providing a strong weld connection 115 between the anode rod and the anode yoke head. The anode hanger according to the present invention avoids a bimetal plate/weld for connecting the yoke with the rod, thus the drawbacks related to a bimetal connection are avoided. Bimetallic plates used in the conventional yoke design, which are needed to be able to connect the steel yoke and the aluminium rod, increase the voltage drop, are costly to repair and are mechanically weak points. Normally, a security pin is provided in conventional anode hangers with bimetallic plates, in case the bimetallic weld is damaged, i.e. to avoid the anode to drop in case of failure of the connection.

In a method for the manufacture of the anode hanger according to the present invention, the anode yoke is formed by pouring molten steel material at a high temperature (typically above 1400 °C for casting steel) into a properly shaped casting mould, inside which casting mould has been placed, before pouring of the molten steel, a premade core body (e.g. sand core) corresponding to a suitable form of an aluminium core. The casting of the outer steel portion may be performed according to conventionally steel casting techniques, e.g. by using sand moulds. After the steel material has solidified the core body is removed in its entirety and the surface of the steel material, including the hollow space, is cleaned by e.g. sandblasting or another method known in the art.

Before casting of the aluminium core, a feederhead is arranged at the yoke head opening of the steel lining, and the holes in the yoke legs are plugged. The feederhead should have a volume at least large enough to compensate any shrinkage during solidification of the aluminium core material. Thereafter, the hollow space in the steel material and the feederhead arranged at the anode yoke head are filled with molten aluminium. The steel-aluminium anode yoke, with the feederhead, is thereafter treated in a furnace at a temperature where the aluminium is kept in molten state. The heat treatment temperature is from the melting temperature of the aluminium material up to about 1000 °C, preferably the heat treatment temperature should be up to about 720 °C. The period for the heat treatment should be up to about 15 hours, e.g. up to about 8 hours. The period for the heat treatment should be at least 30 minutes, e.g. 1 hour, or 2 hours. During the heat treatment, where the aluminium core material is kept in molten state in the steel lining, interdiffusion in the solid steel-molten aluminium interface takes place, thereby forming diffusion bonding between the two metal materials.

After the heat treatment the steel-aluminium anode yoke is subjected to a controlled cooling, such that the aluminium metal core solidifies directionally from the yoke leg ends, towards the shoulders and arms (the cross bar), and further towards the yoke head and feederhead. By the controlled directional solidification of the aluminium material forming the core, any contraction of the aluminium material during solidification is compensated by the reservoir of molten aluminium in the feederhead, thus contraction cavities are avoided in the aluminium core. The controlled directional cooling may be performed by gradually immersing the heat treated steel-aluminium yoke in a cooling bath, comprising a cooling liquid. The cooling liquid may be oil-water, or other suitable liquid. The cooling of the cast product is controlled such that the total cooling time for the directional solidification is up to 45 minutes. In some instances the controlled cooling time may be up to about 15 minutes. The controlled cooling time should be at least 2 minutes. It should be understood that large cast products will generally be subjected to longer cooling times compared to smaller cast products. The feederhead is cut away after the cooling, leaving the yoke head with the aluminium core head 121 exposed.

In an alternative method for the manufacture of the anode yoke, instead of plugging the holes in the steel lining yoke leg, the studs are welded to the steel lining yoke legs before casting the aluminium material core. The casting process, heat treatment and controlled cooling is otherwise carried out as described above.

By the said heat treatment diffusion bond occurs in the solid-liquid interface between the steel and aluminium. By the prolonged heat treatment formation of a solid metal-tometal diffusion bond at atomic level is achieved at substantially all interfaces between the aluminium metal core and the surrounding steel material. By the term “substantially” in this context, it is meant that at least 95 % of the said interface between the inner portion of an aluminium material and the outer portion of steel is providing a solid metal-to-metal diffusion bond at atomic level. Advantageously, at least 98 %, e.g.

99.5 % or 99.95 % of the interface between the inner portion of an aluminium material and the outer portion of steel is providing a solid metal-to-metal diffusion bond at atomic level.

The rod 110 (stem) for providing electrical connection to the anode yoke is welded by an aluminium-aluminium weld as explained above. With the aluminium casted so as to be exposed in the upper part of the anode yoke, surrounded by the steel lining, it is not necessary to do any manual welding to attach aluminium plates at the upper part of the yoke to reduce the electrical resistance compared to conventional steel yokes. The yoke according to the present invention can be attached to the rod by a simple aluminiumaluminium weld between the aluminium core in the yoke and the aluminium rod. By avoiding a bimetal plate the risk of fracture is significantly reduced. A conventional bimetal weld normally tolerates only up to about 300 °C. The need of a security pin between the rod and the anode yoke is also eliminated by the aluminium-aluminium weld.

The anode hanger and anode yoke according to the present invention can easily be implemented in existing smelters that already has the same basic design of the yokes, which means that less needs for changes, e.g. various machines, in the rodding area are needed.

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

The aluminium is implemented in the yoke, i.e. not in the studs. However, since the aluminium is placed so close to the studs the heat loss from the studs are effected, and will make it possible to increase the amperage in the anode hanger/yoke, which again leads to an increased production. Calculations and industrial tests shows a 50-60 mV reduction in the voltage drop in the new anode hanger/yoke design compared to a conventional steel yoke. A 1 mV reduction equals a potential of energy saving of about 1.34 MWh pr. year for a large plant.

Further, with aluminium casted into the yoke it is possible to have different dimensions of the aluminium “arms”, “shoulders” and/or “legs” on different parts of the yoke, which may be advantageous with an asymmetric connection between the carbon anode and the yoke.

It is estimated that the electrical resistance of the anode hanger according to the invention could be up to 40-150 mV lower than the electrical resistance of other comparable anode hangers. Consequently, the electrical power dissipated in the anode hanger is reduced accordingly, having the effects of extending the lifetime of the anode hanger, reducing cost and resources used for maintaining and replacing anode hangers in a facility for electrolytic cell metal production, and a favourable overall reduction in energy consumption in the process that makes use of the anode hanger.

A further effect of the technology of the present invention is increased pot efficiency and a considerably better refractory lining lifetime. This effect is due to the fact that the current can be equally distributed through the cell through the design of the aluminium core in the anode yoke, which will provide an even longer lifetime to the refractory lining

Other advantages achieved by the anode hanger (yoke) according to the present invention is possible less “cowboy effects” to the anode yoke due to better thermal transport of the heat. Reduced cowboy effect extends the lifetime of the anode hanger and less need for repairing/maintenance.

The design of the anode yoke is also optimised such that the arms extends to the shoulders, which shoulders are designed such as the alumina powder feeded into the cell easily slides off the hanger and does not build up. Further, due to the better thermal transport, the temperature of the anode hanger (yoke) is lower, thereby reducing the corrosion of the hanger.

Due to the lower weight of the anode hanger transport costs are reduced, which is also an environmental advantage.

Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the claims.

Claims

P a t e n t c l a i m s

1.

An anode hanger for fixedly holding anodes in cells for the electrolytic production of aluminium according to the Hall-Heroult process, comprising

an anode yoke having a yoke head (111), at least two yoke arms (112), at least two yoke shoulders (113) and at least two yoke legs (117), to each respective yoke legs (117) a stud (114) is fixed, the anode yoke comprises

an exposed upper area (121) of the inner portion of aluminium at the yoke head (111), wherein an interface between said inner portion and said outer portion provides a substantially continuous metal-to-metal diffusion bond between said inner portion and said outer portion.

2.

The anode hanger of claim 1, wherein said outer portion is covering at least 85%, or 90%, or 95% of an outer surface of said inner portion.

3.

The anode hanger of any one of claims 1 and 2, wherein at least 95%, or 98%, or 99.5%, or 99.95% of said interface between said inner portion and said outer portion is providing a metal-to-metal diffusion bond between said inner portion and said outer portion.

4.

The anode hanger of claim 3, wherein the metal-to-metal diffusion bond is substantially a metallic bond at atomic level.

5.

The anode hanger of any one of the previous claims, where the inner portion of aluminium is made of pure aluminium of at least 99 % Al.

6.

The anode hanger of any one of the previous claims 1-4, where the inner portion of aluminium is made of an aluminium alloy chosen from 6XXX aluminium alloys.

7.

The anode hanger of any one of the previous claims, where the two yoke legs (117) are arranged symmetrical on each side of, and spaced from the head (111) of the yoke.

8.

The anode hanger of any one of the previous claims 1-6, where the yoke comprises three yoke legs (117), two of which yoke legs are arranged symmetrical on each side of, and spaced from the head (111) of the yoke via respective arms (112) and shoulders (113), and the third yoke leg (117) is immediately below the yoke head (111).

9.

The anode hanger of any one of the previous claims 1-6, where the yoke comprises two arms (112), with respective shoulders (113), and four yoke legs (117), the four legs are arranged in-line and symmetrical on each side of, and spaced from the head (111) of the yoke.

10.

The anode hanger of any one of the previous claims 1-6, where the yoke comprises four arms (112) with respective shoulders (113), each shoulder having a yoke leg (117), the four arms (112) with respective shoulders (113) and legs (117) are arranged and spaced from the yoke head (111) at respective 0, 90, 180 and 270 degree positions of a circle about the head (111) with the yoke head (111) at the circle centre.

11.

The anode hanger of any one of the previous claims 1-6, where the yoke comprises four arms (112), the arms (112) being arranged and spaced from the yoke head (111) at respective 0, 90, 180 and 270 degree positions of a circle about the head (111) with the yoke head (111) at the circle centre, where the arms (112) at 90 and 270 degree positions, or at 0 and 180 degree positions, are extended and split into two branches each carrying a respective one of four studs, while the two non-split arms (112) each carrying a stud (114), providing the total of six studs (114), three of the six studs are arranged in-line with each other on each of two parallel lines spaced from and at respective sides of the head (111).

12.

The anode hanger of any one of the previous claims, further comprising a rod (110) of aluminium material welded to the exposed upper area (121) of the inner portion of aluminium at the yoke head (111).

13.

Method of producing an anode hanger according to any of the claims 1-12, the method comprising

providing a mould core, having a shape corresponding to a suitable inner portion of aluminium metal or aluminium alloy;

filling the said internal hollow space and feederhead with molten aluminium metal or aluminium alloy;

- heat treating the aluminium filled outer portion of steel material comprising the feederhead at a temperature above the melting temperature of the aluminium metal or alloy;

14.

Method according to claim 13, wherein the holes in the at least two legs are plugged by mould inserts, or by fixing a stud to each yoke leg.

15.

Method according to any of claims 13 and 14, wherein the temperature for heat treatment is up to 1000 °C.

16.Method according to any of claims 13-15, wherein the period for heat treatment is up to 15 hours.

Method according to claim 16, wherein the period for heat treatment is up to 8 hours.

18.

Method according to any of claims 13-17, wherein the controlled cooling is performed by gradually immersing the heat treated cast product in a cooling bath comprising a cooling liquid..

19.

Method according to claim 18, wherein the controlled cooling is performed in a period of up to 45 minutes.

20.

Method according to any of claims 13-19, further comprising welding steel studs to each yoke leg.