Hall-Heroult type, and a method for making same

The present invention relates to an anode and a method of making same. In particular the invention relates to pre-baked anodes for electrolytic production of aluminium in electrolysis cells of Hall-Heroult type.

Commonly, pre-baked anodes are fixed to studs forming part of an anode hanger. The carbon based anode block has pre-formed bores which allow the studs to be entered into them. The fixation between the stud and the anode is performed by pouring melted cast- iron in the annular space between each individual stud and the corresponding bore in the anode. The anode hanger further comprises an anode yoke with the studs, the anode yoke further being suspended by an anode rod which is attached to an anode beam in its upper part. Often there is a bimetallic connection between the anode rod and the yoke. The use of melted cast-iron has some implications with regards to investments, for instance ovens that can melt the cast-iron, and corresponding distribution and pouring system. For the anode, the studs are in some technologies preheated.

A pre-baked anode is normally worn out after approx. 30 days in the cell due to the consumption of the carbon material it consists of. It then must be replaced. The worn out anode (butts) is transported to a facility where the studs of the anode hanger is cleaned by removing rest material of the anode together with cast iron residues. Commonly, this is performed by the use of mechanical frapping tools. The operations for assembly of a new anode and removing the butts of a worn out anode are time consuming and expensive.

US 4,574,019 relates to a process for attaching two anode blocks to an anode suspension means comprising two spades or studs by the use of an adhesive mass. The mass should be mechanical strong and have good electrical conducting properties at least at temperatures between 900 °C and 1000°C. The adhesive mass can be a mixture of solids, a binder and a hardener. The solids can be represented by metal powder such as iron, copper or aluminium. The particle size of the metal should be 5 mm at most. This type of contact mass is further described in EP 0 027 534. In accordance with the applicant's own EP 2242976 A1 there are applied electric conductive particles only between the electrical current lead and the calcinated carbonaceous material in an anode as a fill-in material. The use of electric conductive particles only without a hardening matrix, facilitates the re-use of the conductive particles. It has also been demonstrated that the electrical resistance have decreased for anodes rodded in accordance with this solution.

By the present invention, the anode voltage drop has been reduced further, due to novel designs of the yoke. First of all, in one embodiment, the yoke comprises at least two delta shaped suspension plates where their upper, central parts are attached at the lower part of an anode rod. The lower part of the plates, the rodding plates, are attached in recesses formed in the upper part of a carbonaceous anode block, and can in one embodiment be rodded according to the same principles as that shown in EP 2242976 A1. That will say, by a combination where the electric conductive rodding material consists of conductive particles only and that the current lead and the body of calcinated carbonaceous material are connected by various mechanical fixation means.

The plates of the yoke may be of a clad or composite material, preferably similar to that of a sandwich structure, that further can reduce the voltage drop and also sustain a different thermal performance. Important advantages are that this may result in less heat losses and lower voltage drop. It may be possible to design the yoke in a manner that makes it 'neutral' with regard to the heat balance in the cell when replacing a commonly used yoke. In one other embodiment, the yoke comprises a horizontally oriented suspension plate between the anode rod and the rodding plates. Similarly, this suspension plate can at least partly be of a clad or composite material, preferably similar to that of a sandwich structure, that further can reduce the voltage drop and also sustain a different thermal performance. It also allows for easier coverage with anode covering material (ACM), due to less protruding elements. Important advantages related to the above mentioned designs are that this may result in less heat losses and lower voltage drop. It may be possible to design the yoke in a manner that makes it 'neutral' with regard to the heat balance in the cell when replacing a commonly used yoke.

Still further, the yoke according to the invention will have good mechanical properties and sustain excellent stability.

These and further advantages will be achieved by the invention as defined in the accompanying claims.

The present invention will in the following be further described by figures and examples where; Fig. 1 discloses in perspective, a first embodiment of an anode hanger with yoke and anode rod,

Fig. 2 discloses the anode hanger of Fig. 1 from one side, Fig.3 discloses the anode hanger of Fig. 1 , seen in a frontal view,

Fig. 4 discloses, an anode hanger as that of Fig. 3 assembled into a carbon anode block,

Fig. 5 discloses an anode hanger assembled into a carbon anode block as that of Fig. 4, seen from one side in a cross-sectional view

Fig. 6 discloses a top view of the anode hanger assembled into an a carbon anode as shown in Fig. 5, seen from above, Fig. 7 corresponds in part to an enlarged view of the encircled area as denoted by A in Fig.

6,

Fig. 8 discloses in part an enlarged view of the encircled area as denoted by B in Fig 4, Fig. 9 discloses a completed anode assembly, seen in perspective, Fig. 10 discloses an alternative design of a mechanical fixation means, seen in a cross- section view from one end,

Fig. 11 is the same alternative design as that of Fig. 10, seen from above,

Fig. 12 is the same alternative design as that of Fig. 10 and 11 , seen in a cross-section view from one side,

Fig. 13 discloses a comparative voltage drop test of one anode according to that of the present invention and a state of the art anode,

Fig 14 discloses in a second embodiment an anode hanger with a yoke comprising a horizontal suspension plate and vertical rodding plates, seen in perspective, Fig. 15 discloses an anode block processed and prepared for rodding with the yoke,

Fig. 16 discloses in perspective an anode hanger having a yoke comprising one horizontal suspension plate rodded to an anode block, Fig 17 discloses a voltage drop test of one anode according to that of the present invention with horizontal suspension plate.

As shown in Fig. 1 , there is disclosed in perspective, an anode hanger 1 with yoke 2 and anode rod 3. The anode rod is commonly made out of aluminium or an alloy thereof, and has preferably a rectangular cross-section.

The yoke may in principle consist of four parts, two delta shaped suspension plates 4, 4' and two rodding plates 5, 5' that are interconnected with the suspension plates respectively. The suspension plates 4, 4' and the rodding plates 5, 5' can be made out of appropriate steel plate qualities, and they can be interconnected by welding or by any other appropriate means.

In one embodiment however, the rodding plate 5 and the suspension plate 4 can be made out of one piece of a metallic plate, for instance a steel plate, by appropriate processing such as cutting and possibly bending (not shown).

It should be understood that the use of the terms rodding plate and suspension plate in this document may also relate to parts of such a complete plate, either produced out of one plate as mentioned above, or out of two plates, namely suspension plate 4 integrated with rodding plate 5. The complete yoke plates produced by either ways are denoted 54, 54'.

Protrusions 8, 8" are arranged at the rodding plate 5 for interaction with recesses formed in the anode top. There are also recesses formed allowing the rodding plates to be inserted into the anode top. This will be explained later. Similar protrusions are arranged at rodding plate 5' (not shown).

It is further disclosed that the upper part of the suspension plates 4, 4' are attached to the lower part of the anode rod 3. To ensure low voltage drop between the anode rod 3 and the yoke 2, connecting members 6, 6' of good electrical conductivity can be arranged in electrical contact with the anode rod 3 and each of the suspension plates 4, 4'.

In Fig. 2, the anode hanger 1 is shown from one side. The anode rod 3 is connected to a suspension plate 4 via several connecting members 6, 6', 7, 7', 7", 9. Connecting members 7, 7', 7" can be a steel plate part of a bimetal or tri-metal as it faces the suspension plates 4, 4' of steel. The connecting members 6, 6' are preferably of aluminium to increase the contact area between the anode hanger 3 of aluminium and the bi-metal or tri-metal connection. Connecting member 9 serves as the aluminium part of the bi-metal or tri-metal. In case a tri-metal, a joining plate represented by a thin sheet of titanium is in addition arranged between the connecting member 9 and 7 (and similarly at joint 3-7" and 6'-7')

The connecting members have preferably good electrical conductivity and also a certain mechanical strength.

The bi-metallic member will in principle have two contact areas of materials that are compatible with the materials in the anode rod 3 and that of the suspension plates 4, 4' respectively. A tri-metallic connecting member will in addition comprise at third metal that enables the connection to operate appropriately at high temperatures.

For instance, the tri-metallic connecting member could consist of a steel-titanium-aluminium material based part.

Further, the Figure discloses in more details the protrusions 8, 8" in the rodding plate 5, arranged for interaction with corresponding recesses in the anode top. The protrusions can basically have a rectangular or a round cross-sectional shape. In Fig. 3 the anode hanger 1 of Fig. 1 is shown in a frontal view, where two suspension plates 4, 4' with respective rodding plates 5, 5' are attached to the anode rod 3. A connecting member 6' is shown in the contact region between these parts. In the lower part of the rodding plates 5, 5' there is shown protrusions 8, 8'. These protrusions have in this example a rectangular shape, and may penetrate the rodding plates 5, 5' in a manner where they protrude both at the outside and the inside of the rodding plates.

In Fig. 4 there is disclosed an anode hanger 1 as that of Fig. 3 assembled into a carbon anode block 20, where there is first of all processed recesses 13, 13' (slots or grooves) oriented in the length direction of the anode block, that can receive the rodding plates 5, 5' at a certain insert depth. Further, the protrusions 8, 8' arranged at the rodding plates 5, 5' are mating corresponding recesses 10, 10' in the anode block 20. Details regarding the encircled area at B will be further explained under Fig. 8. The anode block may have slots 21 , 22 open downwards for anode gas drainage, and have cantilevered external surfaces 23 at its corner regions.

Fig. 5 discloses in a cross-sectional view an anode hanger 1 assembled into a carbon anode block 20 as that of Fig. 4, seen from one side, where the anode rod 3 is attached to the suspension plate 4 with rodding plate 5 and protrusions 8, 8" engaged in corresponding undercut recesses 10, 10" formed in the anode block 20. The rodding plate 5 is entered into the recess 13. The anode block has cantilevered surfaces at 23, 23'. Details regarding the protrusion 8" and the recess 10" are shown in Figs. 6 and 7. Fig. 6 discloses the anode hanger assembled into an a carbon anode as shown in Fig. 5, seen from above, where an anode 20 is provided with recesses 10, 10", 10', 10"'. The yoke parts have protrusions 8, 8", 8', 8"' that have been entered into the recesses. The recesses can be made by applying a circular milling tool, for instance formed as a little 'saw-blade' with both axial and radial cutters of polycrystalline diamond (PCD), and further arranged at a sufficient long axle that allows the tool to process a bore by a downward movement in a first step and an undercut recess in a subsequent processing step where the tool is moved along the length axis of the anode block (20).

In Fig. 7, which shows enlarged details of the encircled area as denoted by A in Fig. 6 , the rodding plate 5 with the protrusion 8" is first engaged into a top open part 11 " of the recess 10" of the anode block 20, and then moved horizontally and further engaged into a top closed or undercut part of the recess 10" of the anode block 20. This enable the anode block 20 to be suspended by the yoke via the undercut recesses 10, 10', 10", 10"' and the protrusions 8, 8' , 8", 8"'. That implies that the weight of the anode block 20 is carried by these mechanical fixations means. Fig. 8 corresponds in part to an enlarged view of the encircled area as denoted by B in Fig. 4 and shows more details of the recesses 13 and 10. It comprises a first top open part or bore 11 similar to that shown in Fig. 7 that has been processed at an appropriate location of the recess 13. Further, the recess 10 is undercut and has two flanges 12, 12'. As the rodding plate 5 has been entered into the recess 13, and the protrusion 8 arranged at the rodding plate 5 has been entered downwards into the bore 11 and has been further moved inside the undercut part (top closed part) of the recess as shown in the Figure, it will rest in that position and being able to carry vertical loads represented by the anode block 20 via flanges 12, 12' formed in the anode block. At the same time the protrusions 8, 8', 8"' will have entered their corresponding recesses 10, 10', 10"' respectively, see Fig. 5 and 6.

Fig. 9 discloses an anode assembly with anode rod 3, suspension plates 4, 4', rodding plates 5, 5', recesses 13, 13', bore 11 , 11" (in part) and anode block 20.

Following this first rodding step, conductive electric particles are filled in to the voids between the rodding plates and the recesses in the anode block, and a collar mass can be applied to the metallic parts at the anode top as protection against corrosive attacks (not shown).

The protrusion 8, 8", 8', 8"' in the above examples may consist of cylindrical rods that are entered into a bore through the rodding plates. These rods can be fixed by a press-fit arrangement, and arranged for easy removal and exchange.

In an alternative, the shape of the protrusions can be flattened, i.e. having a more extended, planar surface acting against the recess in the anode. This will distribute the

loads of the anode block over a larger area. In one embodiment the protrusions may consist of studs that are welded to the outside and/or the inside of the respective rodding plate.

Fig. 10 discloses an alternative design of a mechanical fixation means, seen in a cross- section view from one end, where an anode block 20 has slots 21 , 22 in its bottom part, and where recesses 100, 100' are made in its top part. In the recesses there have been inserted two rodding plates, 5, 5' respectively, and bolts 14, 15 have been applied to secure the yoke plates to the anode block. Bolts 1 , 15, pass through bores in the rodding plates 5, 5' and are secured into preformed recesses or bores in the anode block. The bolts may be threaded and corresponding threads may be arranged in the rodding plate and/or the bores of the anode block 20. In addition, the suspension plates 4, 4' have in their upper parts an additional plate material 16, 17 of better electronic conductivity, such as copper or an appropriate copper alloy, that will reduce the voltage drop in the assembly significantly. The additional plate material can be either a clad material or a material electrically integrated with the yoke plates by other means. The additional plate material may also serve to optimise the thermal loss from the yoke.

Further, details of the connection between the suspension plates 4, 4' and the anode rod 3 are shown. The Figure shows two reinforcement parts 27, 28 that preferably can be welded to the suspension plates 4, 4'. The reinforcement parts are preferably of a metallic material, such as steel and will enhance the sideways stability of the yoke. It will also reduce the voltage drop between the anode rod and the yoke.

Further, there is shown a tri-metallic plate 18, 19, 26 arranged between the suspension plates 4, 4' and the anode rod 3, similar to that explained under Fig. 2.

Fig. 11 is the same alternative design as that of Fig. 10, seen from above, where the rodding plates 5, 5' are fixed to the anode block 20 by means of bolts 14, 14', 14" and 15, 15', 15".

Fig. 12 is the same alternative design as that of Fig. 10 and 11 , seen in a cross-section view from one side, where the anode block 20 has a slot 21 in its bottom part and a rodding plate 5 is inserted from the top side. The rodding plate is fixed by means of bolts 14, 14' and 14". A clad plate material is shown at 16 at the suspension plate 4, and the anode rod is shown at 3. A tri-metallic connection is shown at 18, 19, 26.

The dimensions for the suspension- and rodding-plates are dependent on what yoke that is being replaced. For instance, compared to a four stubs 0180 mm yoke, the contact area towards the carbon is doubled. A 20 mm thick rodding plate will reduce the heat conducting steel cross section by 50 %. This means that voltage drop is sacrificed by reducing the heat loss, but the voltage drop is still lower than the stub yoke because of the 100 % increase in contact area between the yoke and the carbon and the bigger bimetal cross section.

An increase in plate cross sections will increase the heat loss and reduce the voltage drop, which both could be beneficial for amperage increase of a cell.

A decrease in dimensions will reduce heat loss and increase voltage drop and is could be beneficial for reducing interpolar space.

The suspension plates can be of a steel material and of a thickness of 35 mm in case there is no clad or composite material in this part. The rodding plate can be of the same material in a thickness of 20 mm.

In one embodiment the thickness of the clad Cu material can be 8mm while the steel suspension plate is 20 mm. The rodding plate can be 20 mm thick.

Yokes with rodding plates of smaller thickness than that of the suspension plates have shown to have positive effects with regard to reducing the thermal loss via the anode yoke.

In a non-disclosed embodiment there may be applied more than two yoke plates with corresponding slots in the anode top. For instance, a central yoke plate, arranged between the two shown in the Figs, may be applied.

Fig. 13 discloses a comparative voltage drop test of one anode according to the embodiment described above having two suspension/rodding plates, named Twin-put (lower curve) and a state of the art reference anode with a yoke having stubs and where bores are made in the anode top and that the rodding is made by cast iron (upper curve). In the Figure, the curves have been made more suitable for printing by some drawing assisted by hand. The voltage drop measurement is done at corresponding locations at the anode rod and at the anode top.

The curves shows the voltage drop (vertical axis) versus time after start-up in an electrolysis cell. It will be seen from the upper curve which is the reference anode that the voltage drop can be averaged to approximately 200 mV as the current throughput has stabilized.

From the lower curve, which is an anode according to the invention, the voltage drop has stabilized to a level of approximately 120 mV.

One other comparative test between 8 anodes according to the invention as described above (Twin-put) and 8 standard anode has been performed, where the average electric resistance in the duration period (from new to consumed) has been measured and averaged. See Table 1.

Invention \ 35

1 ,23 1 ,47 1 ,4 1 ,5 1 ,76 1 ,56 1,51

(Twin-put) ' 1,47 ± 0,16

Reference 2,12 2,34 2,32 2,03 2,12 2,33 2,06 2,23 2,19 ± 0,13

Table 1. Electric resistance of twin-put anode compared to reference anode

One other embodiment of the invention is shown in Fig. 14, where an anode hanger 101 with a horizontally oriented suspension plate 104 and five rodding plates 105, 105', 105", 105"', 105"" is seen in perspective. The rodding plate 105"" has two protrusions, 108" and 108"'. Similarly, the rodding plate 105 has also two protrusions 108, 108' like this (not shown). These four protrusions will match corresponding recesses in an anode block, and being able to suspend the block.

As in Fig1 , there is disclosed connecting members 106, 106', 106" between the anode rod and the suspension plate. The connecting members can be of bimetallic and/or trimetallic types. Further, there is disclosed a spacer 144 between the anode rod 103 and the suspension plate 104. The spacer can be advantageous in that it spaces the connecting members away from the heat and corrosive gases evolved in the electrolysis process. The spacer 144 can be out of a metallic material such as steel and be welded to the suspension plate. However, in one embodiment the suspension plate can be connected to the anode rod via the connecting members but without any spacer 144 (not shown). In one other embodiment (not shown) the spacer 144 can be made out of several plates of an electrical conducting material and further being arranged in such a manner that the plates defines a gas tight void that can be filled with thermal insulation. A similar construction can be applied to the suspension plates 4, 4' in the first example. One advantage with this solution could be that the necessity for covering the anode top with anode covering material can be reduced and possibly eliminated.

Fig. 15 discloses in perspective an anode block 120 processed and prepared for rodding with the anode hanger 101. In the anode block 120 there is processed five recesses 113, 113', 113", 113'", 113"", being able to receive the corresponding rodding plates mentioned above. Further, similar to that shown in Fig. 7, there is shown/indicated bores 111 , 111', 111 ", 111 '" and undercut recesses 110, 110', 110", 110"' for receiving corresponding protrusions 108, 108', 108", 108"' at the outer rodding plates 105, 105"", and to carry the weight of the anode block 120.

Fig. 16 discloses in perspective an anode having an anode hanger comprising one anode rod 103, connecting members 106, 106', 106", a spacer 144, a suspension plate 104 rodded with an anode block 120.

It should be understood that the anode block can be secured to the current lead by means of a spike, bolt or the like interconnecting the rodding plates (105, 105"") with the anode block (120).

During the rodding steps, conductive electric particles are filled in to the voids between the rodding plates and the recesses in the anode block, and a collar mass can possibly be applied to the metallic parts at the anode top as protection against corrosive attacks (not shown).

The protrusion 108, 108", 108', 108"' in the above examples may consist of cylindrical rods that are entered into a bore through the rodding plates. These rods can be fixed by a press- fit arrangement, and arranged for easy removal and exchange.

In an alternative, the shape of the protrusions can be flattened, i.e. having a more extended, planar surface acting against the recess in the anode. This will distribute the loads of the anode block over a larger area. In one embodiment the protrusions may consist of studs that are welded to the outside and/or the inside of the respective rodding plate. Fig 17 discloses a voltage drop test of one anode according to that of the present invention (lower curves) with horizontal suspension plate versus a typical standard anode (upper curve). It can be seen that the voltage drop can be reduced with approximately 12 pohm with regard to a typical standard anode (lower curves).

Further, the clad or composite plate material applied in at the suspension plate(s) may be protected by a shield or any other appropriate means to avoid deterioration. A shield made of a steel plate that cover the clad or composite material may be appropriate. It should be understood that the main part of the recesses in the anode top for the rodding plates may be formed while the anode is in its green condition, i.e. before it is calcinated. The main part of the recesses can also be made by a anode slotting apparatus, for instance as shown in the Applicant's own publication EP 1781441 A1. Following this, the recess can then be calibrated by a rotating processing tool, either having a shape complementary to the final shape of the recess or having a dimension mince than that of the final shape. The tool can be arranged to fit a CNC machine or the similar, thus the machining (processing) will be performed in an automated manner. In the machining process, the top open recesses are made, and subsequently the same tool or any other appropriate tool is moved along the recess inside the anode top to make the top closed recesses with load carrying flanges 12, 12'.

It should be understood that the electrical conducting solids or particles can be of any appropriate metal such as steel, iron, copper, aluminium etc., or alloys of same. Further, the shape of the solids can be spherical, oval or elliptic, flaked, or have any appropriate shape. The size and particle distribution may vary. The maximum size will in general be restricted by the space to be filled. A non-homogenous distribution of particle sizes may be convenient to obtain a compact filling as possible, with little space between the particles. Apart from having good electrical conducting properties, the applied material should have good mechanical properties (crushing properties) and be able to sustain high temperatures. Magnetic properties may be advantageous for recycling reasons in the butts handling station. Further, the size of said solids can be from 0,1 millimetres and close to the minimum opening (the void) between the rodding plates and the walls of the recesses in the anode block. Commonly, the size may be up to 10 millimetres. The electrical conducting particles can be stored in a container or in an appropriate storage at a higher level than the recess in the anode to be filled. A tube fastened to this container with a valve and an appropriate opening towards the slot could regulate the correct amount of particles into the slot. Thus the transport and distribution can be performed by gravity feed. Under filling, vibration can be applied to obtain a more compact filling. The anode block and/or the yoke plates bar can be vibrated. In an alternative a vibrating stick can be applied in the recess filled with conductive particles, to generate vibration directly of the filled-in material.

Further, commonly used collar paste for anodes or other protecting substance may be used to make an encapsulating and protective layer at the top of the electrical conducting particles. Worn out anodes can commonly be handled in a rodding station where the butts is removed from the yoke (after removal of any anode covering material). Preferably the butts is cracked in a manner where it falls off mainly in a few large pieces divided along the direction of its fixation. Otherwise it can be crushed or knocked off the hanger. The butts is conventionally put on a conveyer.

In cases where there have been applied magnetically conductive particles as fill-in material, the anode yoke is preferably magnetized by appropriate means, whereby the particles will be attracted to the yoke during the butts removal. After the butts has been removed, the yoke is preferably moved to another part of the rodding station for recovery of the fill-in material. Then the yoke can be de-magnetized to let the particles come off. Any remaining particles on the previously embedded part of yoke can be removed by simple mechanical methods such as scraping, as the particles come off easily.

As a final cleaning step, the previously embedded part of the yoke can be blasted (as by sand blasting) preferably by means of a particles of the same type as or that is compatible with the actual fill-in material such as iron, steel or any other electrical conductive material. Apart from this blasting can be carried out by sand, alumina, or any other appropriate material. It should be understood that non-magnetic fill-in materials can be recovered and re-used as well, even if the separation and recover technology would be somewhat different than that described above. A sort of sieve may be applied to separate butts from fill-in material. The recovered fill-in material may come off as clusters of particles. These clusters may need to be crushed down to more or less single particles to be re-used as fill-in material determined by the opening of the actual recess to be filled. The crushing can be done by adapting conventional apparatus.