WO2010068991A1

WO2010068991A1 - A rodless anode block for an aluminium reduction cell

Info

Publication number: WO2010068991A1
Application number: PCT/AU2009/001642
Authority: WO
Inventors: Duncan Hedditch; Jeffrey Keniry; Christopher Peter Jones; Craig Tischler
Original assignee: Aluminium Smelter Developments Pty Ltd
Priority date: 2008-12-18
Filing date: 2009-12-17
Publication date: 2010-06-24

Abstract

A rodless anode block is described having a current receiving face for engagement with a fixed conductor. The current receiving face includes an array of vertically spaced recesses, at least a portion of which contain an electrical resistance reducing contact material comprising aluminium or aluminium alloy. The contact material is disposed in the recesses so as to define with the current receiving face a substantially smooth surface over which the fixed conductor can slide when in engagement therewith. Also described is a method of operating an aluminium reduction cell comprising the rodless anode block of the invention displaceably supported in the cell.

Description

A RODLESS ANODE BLOCK FOR AN ALUMINIUM REDUCTION CELL

FIELD OF THE INVENTION

The present invention relates to the operation of an aluminium reduction cell comprising at least one rodless anode. In particular, the invention relates to reducing electrical resistance between a rodless anode and a fixed cell conductor during operation of an aluminium reduction cell.

BACKGROUND The state-of-the-art approach to aluminium reduction has relied for some time on a metal rod to support the pre-baked carbon anode in the cell. To support the anode, the rod has stubs that mate with holes in the anode. The connection between the rod and the anode is formed by pouring molten cast iron into the gap, which solidifies as a thimble in each hole to hold the two together (Figure l(a)). The electrical circuit of the cell includes the rod, the stubs, the cast iron thimbles, and the anode. While this was undoubtedly an improvement over the previous Sόderberg technology, the pre-baked, rodded anode has a drawback, namely that not all of the anode carbon can be usefully consumed by the reduction process. A protective layer of carbon must be left between the bottom surface of the anode and the thimbles to prevent the cast iron dissolving in the electrolyte and contaminating the aluminium produced by the cell. At some point in the process, the consumed anode (or butt) must be replaced with a new rodded anode. A modern smelter therefore contains a material reclamation loop which involves anode butts, bath material removed with the butts, rods and cast iron thimbles. The capital and operational costs associated with this material reclamation loop are significant.

Consequently, there are economic incentives to eliminate these essentially wasteful procedures by developing rodless anode technology that allows complete consumption of anode carbon within the cell.

Figure l(b) is a schematic of a rodless anode. The anode is supported by contact pads compressed against its outside faces. The contact pads can comprise a part of the cell conductor. As the bottom of the anode is consumed, the anode can be pushed downwards through the pads, while the top surface is free for the placement of a replacement anode which, if necessary, can be held in position with a suitable binder. In this way, consumption of the anode is continuous and the recycling of butt material is not necessary.

A key technical challenge for rodless anode technology is to efficiently pass electrical current into the carbon anode from the fixed components of the cell's super-structure. The electrical resistance across the contact surface is dependent upon the contact pressure. Low electrical resistance generally requires large applied stresses. In rodded technology, large applied stresses, of about 5 MegaPascals (MPa), are obtained across the electrical contact between the carbon anode and the cast iron thimble because of the differential rates of thermal expansion between cast iron/steel and carbon as the temperature of the assembly is elevated to its operating temperature. Typically, the operating voltage drop across the cast iron thimble is around 120 mV. The aim in rodless anode technology is to at least match this performance. However, it is not practical to design a rodless anode system based on these high contact pressures. When scaled to the size of industrial anodes, the required compression forces amount to several tens of tonnes.

Accordingly, methods which reduce electrical contact resistance across a rodless carbon anode to conductor interface are desirable. These methods can be advantageously employed during operation of the aluminium reduction cell.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided, in an aluminium reduction cell, a rodless anode block, said rodless anode block having a current receiving face for engagement with a contact face of a fixed conductor provided in the aluminium reduction cell, said current receiving face of the rodless anode block including an array of vertically spaced recesses, at least a portion of said array of vertically spaced recesses containing an electrical resistance reducing contact material comprising aluminium or aluminium alloy, the contact material being disposed in said recesses so as to define with the current receiving face a substantially smooth surface over which the contact face of the fixed conductor can slide when in engagement therewith.

According to a second aspect of the invention, there is provided an aluminium reduction cell including the rodless anode block according to the first aspect of the invention. In this aspect there is provided: a cathode; an anode comprising a rodless anode block displaceably supported in the cell; a fixed conductor having a contact face in electrical contact with a current receiving face of the rodless anode block, the rodless anode block being displaceable towards the cathode relative to the fixed conductor; said current receiving face of the rodless anode block including an array of vertically spaced recesses, wherein a respective one or some of said recesses are opposed to the contact face of the fixed conductor according to the displacement of the rodless anode block relative to the fixed conductor; and an aluminium or an aluminium alloy contact material provided in the one or some of said recesses opposed to the contact face of the fixed conductor, said contact material acting to reduce electrical contact resistance between the rodless anode block and the contact face of the fixed conductor during operation of the cell.

According to a third aspect of the invention there is provided a method of operating an aluminium reduction cell including a cathode and an anode comprising a rodless anode block according to the first aspect of the invention displaceably supported in the cell, the method comprising the steps of: applying an electrical current to the rodless anode block via a fixed conductor having a contact face in electrical contact with a current receiving face of the anode block; forming aluminium at the cathode and thereby consuming the rodless anode block; displacing the rodless anode block towards the cathode relative to the fixed conductor as the rodless anode block is consumed; wherein said current receiving face of the rodless anode block includes an array of vertically spaced recesses, a respective one or some of said recesses being opposed to the contact face of the fixed conductor according to the displacement of the anode block relative to the fixed conductor; and wherein an aluminium or an aluminium alloy contact material is provided in the one or some of said recesses opposed to the contact face of the fixed conductor, said contact material acting to reduce electrical contact resistance between the rodless anode and the contact face of the fixed conductor during operation of the cell.

By "electrical resistance reducing" it is meant that the contact material reduces the voltage drop across the anode-to-conductor interface compared to the same arrangement in the absence of the contact material. In the absence of the contact material, the contact stresses required to achieve low electrical resistance, that is a voltage drop of preferably no more than is seen in rodded technology, would be of the order of 5 MPa. Using the electrical resistance reducing contact material in accordance with the present invention, it has been found, in a test apparatus, that low electrical resistance across the fixed conductor-to-anode contact can be achieved using only about 500 kilopascals (kPa) of contact pressure. Figure 2 shows that electrical contact resistance, and hence the electrical losses in operation are reduced as the applied force between the anode/conductor increases. A trend change is observed at around 500 to 700 kPa, beyond which the applied stress must increase significantly for an ever decreasing reduction in contact resistance. Accordingly, in preferred embodiments, the contact pressure between the fixed conductor and the rodless anode block is in the range of from about 300 to 700 kPa.

One way of applying a contact pressure of the order of 300 to 500 kPa across the conductor-to-anode connection is disclosed in US 5,071,534 to Norsk Hydro, which is hereby incorporated in its entirety.

Most advantageously, the resistance reducing contact material reduces the voltage drop across the anode-to-conductor connection to the minimum that could possibly be obtained at the connection. Although preferably the voltage drop is less than or similar to that of the stub and thimble connection of the rodded anode (i.e. a drop of no more than about 120 mV), drops of at most about 200 mV, more preferably at most about 150 mV, could be tolerated depending upon the commercial operation. In a most preferred embodiment, the voltage drop would be about 100 mV or less.

The anode can be a carbon anode of the type typically used in an aluminium reduction cell. Generally, the aluminium reduction cell will comprise a plurality of rodless anode blocks which together form the anode. Advantageously, the or each of the anode blocks is in accordance with the first aspect of the invention. For convenience, the invention is sometimes described with reference to only one rodless anode block, but the context should make clear where the description relates to more than one anode block. The or each anode block is typically prismoidal, preferably cuboidal, comprising a top face or surface, a bottom face and opposed pairs of side faces between the top and bottom faces. Preferably, the anode block has a flat top surface without stub holes or a crown. Preferably, each face of the anode block is rectangular rather than tapered (the latter is common in conventional anodes), although tapered side faces can be accommodated. The anode used in the cell of the present invention is rodless. By "rodless" it is meant that the or each anode block does not have a rod (as used in a conventional rodded anode cell) inserted into its body to support the anode block in the cell and which can provide current to the anode block.

In a preferred embodiment, a plurality of anode blocks are arranged in the cell side by side to one another. Such an arrangement is referred to below as a "cassette". Each of the anode blocks in a cassette can be individually displaceably supported in the cell by a support means that is described in more detail below, but that can be any means of suspending the anode block in the electrolyte bath of the cell during use. Preferably, each anode block is displaceably engaged on opposite sides by the support means, so each anode block is effectively gripped between opposed members of the support means. In one embodiment, the support means comprises a yoke having a pair of support members or girders each having a metal contact plate at a distal end. Figure l(b) shows the two metal support members and metal contact plates (the yoke joining the pair is not shown). Each metal contact plate acts as a contact pad that is pressed against the respective side face of the anode block to support it in the cell. The contact plate can be in one or plural parts as desired.

The support means can also be the means by which electrical current is fed to the anode block, although there need only be one fixed electrical connection for each anode block.

The steel support member and metal contact plate or other support means can therefore be conductive and can together form the fixed conductor with the contact plate being the contact face that engages with the current receiving face of the anode block. Alternatively, the fixed conductor is independent of the support means, although it may also be supported by the support means.

Preferably, the or each anode block has two current receiving faces each in electrical contact with a contact face of a respective fixed conductor. Each such current receiving face of the anode block will be in accordance with the invention, that is have an array of vertically spaced recesses in at least said one or some of which an electrical resistance reducing contact material comprising aluminium or aluminium alloy is provided.

The contact plate of the fixed conductor can be formed from any suitable conductive metal, but preferably a corrosion resistant metal such as stainless steel is used. In a preferred embodiment, the contact plate is coated or formed from a material that is stable in the presence of aluminium, i.e. a material that does not react with aluminium at the temperatures used in the reduction cell. The material can be a refractory compound that is electrically conductive, for example, titanium diboride.

By "fixed" conductor, it is meant that the conductor is not displaceable towards or away from the cathode relative to the support structure under normal operating conditions of the cell. The support structure may move with the fixed conductor to maintain normal operating conditions of the cell.

Preferably, current is supplied to the or each anode block in such a way as to provide an even consumption of the anode. To achieve this, preferably, the contact plate of the fixed conductor extends along substantially the entire length (from one of the opposed side faces to the other) of the current receiving face of the anode block, that is preferably at least about 75 % of the length of the block. As noted above, preferably, the anode block has two opposed current receiving faces along each of which the contact plate of a respective fixed conductor extends for substantially the entire length. In one embodiment in which the anode block is elongate, the or each current receiving face is a longitudinal face. I n embodiments in which the contact plate comprises plural parts, preferably the plural parts together extend along substantially the entire length of the current receiving face of the anode block.

The support means associated with each anode block in a cassette can allow each anode block to be individually displaceable. In other words, each anode block in the cassette can be displaceable relative to the other anode blocks, as well as to the respective fixed conductor. The individual displacement of each anode block is advantageous because the anode blocks may be consumed at different rates during operation of the cell. The cassette of individually displaceable anode blocks can be supported by a support superstructure, which allows for movement of the entire cassette relative to the cathode. Such movement may be required as aluminium metal pools on the cathode, thereby decreasing the distance between the bottom faces of the anode blocks and the cathode. The superstructure, including the cassette, can be jacked up as the aluminium pools at the cathode and jacked back down once the aluminium has been drained from the cell to substantially maintain a desired distance between the anode and the cathode. The extent and frequency of the jacking movements will depend upon how much aluminium has pooled at the cathode and how much and how frequently it is drained. The pooling can be continuously monitored, for example using external computers, as would be appreciated by the skilled addressee.

The array of vertically spaced recesses on the or each current receiving face of the rodless anode block may be formed during moulding of the 'green' anode block (that is, the shaped anode block material prior to baking, optionally containing one or more binders) or by machining before or after baking of the anode block. The vertical array may comprise recesses having any suitable shape or any suitable combination of shapes randomly distributed over the surface. Preferably, the recesses are vertically spaced in rows. In one embodiment, each recess extends across the full width of the current receiving face of the anode block, or at least the width covered by the contact plate of the fixed conductor. Alternatively or in addition, the array of recesses may comprise a vertical array of a series of recesses extending across the width of the current receiving face, each recess in the series being of shorter length than said width. This embodiment may be advantageous when the contact plate comprises plural parts, since the recesses can be positioned only in the portions of the anode face that will be in contact with each part. References to recess or recesses should be understood to include a series of recesses, unless the context requires otherwise.

At any one time during operation of the aluminium reduction cell, a respective recess (or some recesses) on the or each current receiving face of the rodless anode block is aligned with the contact plate of the fixed conductor. For that recess (or each of those recesses), the contact plate of the fixed conductor effectively covers or overlies the recess or at least a portion of the recess. At least the recess(es) opposed to the contact plate of the fixed conductor are provided with an electrical resistance reducing contact material.

In some embodiments, the rows of recesses or series of recesses extend across the anode face horizontally. However, the rows could be vertically spaced at an inclined angle across the anode face. Alternatively, the recesses could from a grid-like pattern across the anode face. By connecting the array between levels in a grid, the arrangement will benefit from differential expansion both vertically and horizontally. If the contact material is cast into the anode block, a grid-like pattern could improve performance, because the contact material will solidify in an expanded condition and contract as it cools. The tension in the grid will apply contact pressure between the anode block and the contact material.

When recesses are inclined at an angle to horizontal, only a portion of a recess may, at any one time, be opposed to the contact plate of the fixed conductor. In these cases, it is believed that the contact material that is not opposed to the contact plate of the fixed conductor will remain substantially solid such that the contact material between the conductor and anode body can remain in the portion of the recess without escaping from an end of the recess.

The vertical spacing of the recesses in the array is such that there are portions of the current receiving face of the anode block that do not have recesses therein. These areas free from recesses can be referred to as "land". The total surface area of the current receiving face comprising land can be greater than the surface area having recesses. The portions of land are not adapted to receive contact material, but some contact material may seep and be smeared onto these portions. It is believed that the land areas are advantageous since they allow for direct bearing of the contact plate of the fixed conductor (or other support means if present) on the bare anode surface. This direct bearing may assist in supporting the anode if the contact material liquefies and, in that form, has reduced ability to support a shear stress.

Since the contact material is a resource, it is desirable from a commercial perspective to keep its use to a minimum. Accordingly, the recesses are advantageously shallow since the resistance reducing contact material provides only a "contact" between the anode block and the contact plate of the fixed conductor. However, the required depth of the recesses will depend upon the shape of the anode and the required current distribution at the bottom face of the anode. In other words, if the anode block is large and the current delivered by the fixed conductor cannot adequately penetrate the anode body such that there is an uneven current distribution at the bottom face of the anode, it may be advantageous to have the contact material extending a greater distance into the anode body than would be the case for a smaller block. For most commercial operations, the desired current density at the bottom face of the block will be in the range of from about 0.5 to about 1.5 A/cm². Preferably, the current density at the bottom face of the anode block is about 0.85 A/cm². The skilled person will be able to determine the required opening and depth of recesses required to achieve this current dispersion in the anode.

It is preferable for the contact material to have relatively greater contact with the anode material than with the material of the contact plate of the fixed conductor by a ratio of at least 2 : 1. It is believed that by maximising contact of the contact material with the anode surfaces and by presenting a smaller amount of contact material to the contact plate of the fixed conductor, the contact material can be brought closer in temperature to the anode block i.e. slightly hotter. By way of example only, where the anode block has the dimensions of about 1500 mm x 600 mm x 500 mm, recesses in the form of elongate grooves are preferred. The grooves are preferably about 8 to 10 mm in depth and about 3 mm to 5 mm in height.

The recesses formed in the current receiving face of the anode block are incapable of receiving a stub or rod such as is used to support the anode in a conventional rodded process. To insert a rod into an anode a substantial hole must be formed in the green anode prior to baking. The hole typically has a diameter of 200 mm and a depth of 150 mm and sometimes has spiral fluted sides. The hole for the stub must be larger than the stub itself to allow for the stub to be embedded using, for example, liquefied cast iron that solidifies around the stub as a thimble.

As the rodless anode is consumed, the distance between the anode and the cathode will increase. To overcome this, as described above, the anode is displaceable. As the anode is displaced towards the cathode, the contact material in the recess(es) previously between the anode block and the contact plate of the fixed conductor may no longer be opposed to the contact plate of the fixed conductor. Accordingly, the recesses should be vertically spaced to ensure that following a displacement, at least one recess or a part thereof is opposed to the contact plate of the fixed conductor. Advantageously, there is more than one recess disposed between the contact plate of the fixed conductor and the anode, to compensate for any failure in electrical conductivity of contact material in a recess and/or to prevent or at least reduce any bottle-necking of electrical current.

The spacing of the array of recesses along the height of the anode may also depend upon the height of the contact plate, since a contact plate having a greater height may cover more recesses than a contact plate of lesser height. The array of recesses are preferably vertically spaced approximately equi-distant from one another. The recesses are advantageously vertically spaced along at least substantially the entire height of the anode block to allow them to be used, in turn, as the anode block is consumed. Each next recess or series of recesses disposed between the anode block and the contact plate of the fixed conductor following displacement of the anode block provides fresh or further contact material to reduce electrical resistance across the contact.

During operation of the cell at elevated temperatures, the electrical resistance reducing contact material within a recess opposed to the contact plate thermally expands. However, the engagement of the contact face with the current-receiving face prevents or restricts the contact material from expanding out of that recess. The resulting thermal expansion under pressure is believed to cause the contact material to fill and/or possibly deform some surface irregularities within the recess and thereby increase the microscopic surface area of contact between the material of the anode block in the recess and the contact material. The thermal expansion of the contact material covered by the contact plate of the fixed conductor also increases the microscopic surface ar ea of contact between the contact material and the contact plate, thereby reducing contact resistance across that interface also. The thermal expansion of the contact material may provide at least a 1 % increase in volume and is thought to provide greater surface contact at the microscopic level across the anode-to-conductor connection than would be achieved by using practical means of mechanical compression alone.

The contact material is a material separate to the fixed conductor and separate to the anode block, but which contacts both. The contact material is electrically conductive, thermally expandable and capable of reducing resistance across the anode-to-conductor connection. Preferably, the contact material has electrical conductivity and a thermal expansion coefficient greater than the anode block containing it.

The electrical resistance reducing contact material is advantageously aluminium or an aluminium alloy. References to aluminium alone should be understood to include aluminium alloys. Preferably, an unalloyed aluminium is used for the lower yield point of the material. The deformation of aluminium is advantageous to the working of the invention as will be appreciated from the above description.

If an aluminium alloy is used however, preferably, the alloy is selected from 1000 series (of the International Alloy Designation System). Alloys in the 1000 series have a minimum 99% aluminium content by weight. However, alloys from other compositions can be used if convenient.

Aluminium is suitable because it has low inherent electrical resistance and the electrical contact is able to withstand the operating environment of the reduction cell. The environment of the cell includes a combination of elevated temperature (the electrolyte in the cell can operate up to about 1000 °C), radiant heat, and corrosive chemicals such as cryolite, hydrogen fluoride and sulphur dioxide.

Since the contact material is ultimately transmitted into the cell, it should be selected to be compatible with the electrolysis process and not contaminate the product. An aluminium contact material is particularly suitable because any aluminium metal which is transferred to the cell adds to the metal produced by the cell. Furthermore, any oxidised aluminium (alumina) that is formed during the displacement of the anode towards the bath redissolves in the cryolite to be reduced to aluminium by the cell processes.

The recesses allow the aluminium contact material to be mechanically retained within the anode face as the anode is moved relative to the fixed conductor. If the contact material is not mechanically retained, i.e. if the aluminium contact material were applied as a surface coating, for example, by spraying, the aluminium contact material is more likely to peel as the anode block is moved relative to the contact plate, particularly given the contact pressures involved and the weak bonds normally formed between anode materials, such as carbon, and aluminium.

It is undesirable to lose contact material by oxidation before it has performed its function. Contact material applied in a powder or cement form, for example, aluminium powders and/or cements applied to an anode surface, based on finely divided aluminium, are likely to oxidise readily because of the high surface to volume ratio. Accordingly, the contact material used is contained in the recesses in the anode face and the portion exposed is relatively small. Preferably, oxygen and other corrosive gases are kept away from the contact material to prevent reactions that lead to non-conductive compounds.

The aluminium contact material could be applied in molten form and poured into the recesses of the current receiving face of the anode block. Alternatively, the contact material can be applied to the recesses as a solid in the form of strips or rods or other shape which correspond to the shape of the recesses. For example, where the recesses are grooves in the anode face with a rectangular cross-section, the aluminium contact material could be provided as bars with a corresponding rectangular cross-section. However, grooves with other cross-sectional shapes are possible e.g. of triangular or part-circular cross-section, provided the solid contact material can be press fitted into the grooves. For a groove of part-circular cross-section, dowels of solid aluminium could be pressed into the grooves. Where the recesses are in the form of a series of depressions, for example, circular depressions or other short recesses extending along part of the width of the anode block face, correspondingly shaped elements of solid contact material could be applied to the recesses, or the contact material may be cast therein.

When in place, the contact material contained in the recesses of the current receiving face of the anode block defines a substantially smooth current receiving face over which the contact plate of the fixed conductor can slide when in engagement therewith. In some embodiments, the amount of contact material applied to the recesses is slightly more than is required to fill the recessed volume. For example, when the contact material is applied as a solid, it can be applied so as to be slightly proud of the surface, so that when the contact material softens there is excess material to enhance contact, i.e. more contact material than is required to fill the recesses could be applied. In some embodiments, about 5 % or about 10 % more than required by volume could be applied. This may equate to about 0.5 mm to about 3 mm of contact material protruding from the recess. This ensures there is excess contact material in a recess to take up any manufacturing tolerances or irregular shapes of the recessed surface. However, the volume of contact material applied to the recesses should permit the contact face of the fixed conductor to engage with the land portion of the anode face once the contact material has softened or melted. Too much excess contact material can be disadvantageous if it is able to run out of the recess and pool above the contact plate of the fixed conductor. Preferably, therefore, the contact material protrudes from the recess by no more than 0.5 mm.

Alternatively, or in addition, the contact material is supplied in another form. In one embodiment, the metal contact plate of the fixed conductor is sacrificial and is formed of aluminium or an alloy thereof. As the contact plate softens due to the temperature rise caused by the electrical current, the material forming the plate will be forced into corresponding recesses formed in the anode surface. While it is possible to provide the contact material in this way, a disadvantage of this arrangement may be that the contact plates have reduced longevity in the reduction cell or have a part which must be replaced.

Further alternatively or in addition, the contact material could be progressively introduced to the recess(es) between the anode block and the contact plate during use in the cell e.g. by an external feeding means. For example, solid contact material in the form of a wire or strip could be continuously fed though an aperture in the contact plate to the recesses. Alternatively, contact material could be introduced into a gap above or to one side of the contact plate so as to fill accessible recesses, or into channels in the contact plate or anode to thereby progressively fill the recesses. These embodiments have the advantage of ensuring that the contact material is delivered to the interface between the anode-conductor at a controlled optimum rate and pressure.

Preferably, the temperature of the contact material increases to near the desired operating temperature just before it is disposed between the anode block and the contact plate of the fixed conductor. When the contact material is opposed to the contact plate, the additional heating as a result of the electrical resistance is preferably just enough to cause the contact material to reach the optimum temperature for thermal expansion. As described above, it is thought that the resistance reducing contact material thermally expands in the recesses in the anode face, but the material does not necessarily have to melt in order to reduce the contact resistance. At about 200 °C, aluminium softens and expands as the yield stress begins to decrease. The resulting applied stresses on the soft material can push it into more intimate contact with the anode material. As heating continues, the aluminium melts (at about 660 °C). By thermal expansion and softening, the aluminium can take up any gaps between the anode block and the contact plate of the fixed conductor. However, the further expansion on melting may not be required to achieve the acceptable resistance reduction (or voltage drop). Before all of the contact material melts, it is possible that there is some localised melting of the material due to some portions of the contact passing slightly higher current and being hotter than others.

If desired, the temperature gradient in the anode block could be controlled, so that the optimum temperature for softening the contact material is achieved at the place on the anode block where the contact material is located. The temperature of the anode could be controlled, for example, by adding insulating material to the top surface of the anode block e.g. a blanket or another anode piece.

Many benefits flow from the use of rodless anode blocks and the elimination of the carbon butt recycling loop. For example, the capacity of anode baking furnaces can be reduced by about 20 % because all carbon placed in the cell is consumed and no recycling of butt material is required. Significant environmental emissions and occupational exposure issues are avoided or at least substantially reduced by the elimination of the anode butt replacement in cells. Furthermore, rodding operations and all the associated plant equipment and infrastructure are not required. The thermal stability of the cell is also improved, because there are no anode changing operations where cold anodes are introduced to the cell.

BRIEF DESCRIPTION OF THE FIGURES Preferred embodiments of the invention will now be described with reference to the following drawings, which are intended to be exemplary only, and in which: FIGURE l(a) is a schematic of a conventional rodded anode;

FIGURE l(b) is a schematic of a rodless anode arrangement;

FIGURE 2 is a graph showing electrical resistance as a function of applied stress for anode carbon samples in contact with a metal plate at room temperature;

FIGURES 3(a) to 3(d) are schematics of examples of recess formations in an anode face;

FIGURE 4 is a schematic showing the resistance reducing material disposed between the anode and the conductor;

FIGURE 5 is a graph showing the influence of temperature on the electrical resistance of two aluminium-carbon contacts in a sample stack;

FIGURE 6 is a graph showing measured contact voltage drop, adjusted for current density and losses in the anode block from an arrangement in accordance with an embodiment of the present invention; and

FIGURE 7 is a graph showing the measured contact voltage drop, adjusted for current density and losses in the anode block for a connection comprising contact material in accordance with an embodiment of the present invention and a connection in the absence of contact material.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Figures 3(a) to 3(d) show examples of recesses 12 in a portion of anode block 10. It will be appreciated that the Figures show only a portion of the anode surface, are not to scale, and the anode block may comprise more recesses than shown. Furthermore, the recesses shown are in the form of grooves (Figures 3 (a) and 3(c)) or circular depressions (Figures 3(b) and 3(d)). It should be appreciated that the recesses can have any combination of shapes in the anode surface.

In Figures 3 (a) to 3(d), the recesses or series of recesses are shown extending across the full width of the anode face in vertically spaced rows. For example, Figure 3 (a) shows the recesses as grooves extending across the width of the anode face. Figure 3(b) shows a series of recesses aligned across the width of the anode face. In some embodiments, the recesses can extend horizontally (Figures 3(a) and 3(b)). However, the recesses could extend at an inclined angle across the anode face (Figures 3(c) and 3(d)).

The dashed-line box 14 shown on Figures 3(a) and 3(c) is an illustration of the location of the contact plate of the fixed conductor placed on the anode block. It should be understood that as anode block 10 moves relative to dashed-line box 14 in the direction of the arrow shown, different recesses are opposed to the contact face of the fixed conductor. A similar dashed-line box on Figure 3(c) shows that in some cases, portions of recesses rather than the entire recess may be opposed to the contact plate of the fixed conductor.

Figure 4 is a cross-sectional schematic of the top portion of anode block 10 and contact plate 16. Figure 4 is effectively a side view of the schematic shown in Figure 3(a). Contact plate 16 is the portion of fixed conductor 18 in electrical contact with the current receiving face of anode block 10. The current flowing into anode block 10 is depicted by arrows, although it should be understood that the arrows are for illustrative purposes only. In Figure 4 the contact plate is shown in contact with three separate recesses filled with aluminium. However, Figure 4 is not drawn to scale and, at any one time, contact plate 16 may be in contact with more or less aluminium.

In Figure 4, the aluminium strips shown schematically have a square cross-section. The aluminium is inserted into the grooves so as to be slightly proud of the carbon surface. As the contact plate is forced or pressed towards the anode face and the temperature of the cell increases, the aluminium will soften and may eventually melt. The contact material thermally expands in the recesses opposed to the contact plate, thereby increasing the contact and reducing the electrical contact resistance between anode block 10 and contact plate 16 of conductor 18.

As anode block 10 moves relative to dashed-line box 14 in the direction of the arrow shown, different recesses will be opposed to the contact plate of the fixed conductor. By way of example only, there may be between 30 and 60 displacement events each day to compensate for anode block consumption, each displacement event being in the range of from about 0.5 mm to 2 mm.

To gain a better appreciate of the invention, embodiments of the invention will now be described by the following non-limiting examples.

EXAMPLES

Example 1 - The effectiveness of aluminium as a contact material

Tests were conducted to establish the electrical contact resistance across an aluminium- carbon contact as a function of pressure and temperature in bench-scale tests.

The test specimen consisted of a cylinder of anode carbon with a cross-sectional area of 18 cm². Intermediate aluminium layers were inserted. The stack was fitted with thermocouples to record the temperature and with voltage taps to calculate the contact resistance. The stack was enclosed in a heated box and compressed by a lever. A micro- ohm meter recorded the total stack resistance.

The contact pressure was set to 407 kPa, and the test current was set to 50 A (this gives a current density of 2.8 A/cm²). Figure 5 shows the results from the two aluminium layers recorded in a single test run.

The contact resistance is strongly influenced by the electrical resistivity of the metal. The electrical resistivity of pure aluminium is 2.733 x 10^"8 Ωm at 25 ⁰C and rises linearly to

10.005 x 10^"8 Ωm at 900 K (627 ⁰C), which is just below the melting point of aluminium (660 °C). As the melting point of aluminium approaches, the metal softens and the actual contact area between the metal and the carbon begins to increase. The additional contact area provides less constriction of the electrical current and the contact resistance falls at higher temperatures.

As the temperature increases, the effects of the rising electrical resistivity and the loss of strength combine to produce the hump in the curve of Figure 5. It can be seen from Figure 5 that the hump in the curve (the Hump Zone) is divided into two regions where low resistance is achieved. The regions are:

• less than about 300 °C (the Cold Zone)

• between 500 °C and the melting point of aluminium 660 °C (the Soft Zone).

The aluminium in the Cold Zone and the Soft Zone have different characteristics that affect how the contact material operates in practice. Electrical heating in the Cold Zone warms the contact and can push the temperature of the contact onto the upward slope. This results in additional electrical resistance and more heating. The Cold Zone is unstable in this sense, as there is a tendency to move out of the Cold Zone towards higher temperatures.

Conversely, the Soft Zone is stable because the gradient of the curve is generally downwards. As the temperature increases, the amount of heating decreases and the rise in temperature slows. If the temperature of the contact begins to cool, the amount of heating increases. For these reasons it is thought practical to operate commercially in the Soft Zone.

Example 2 - Preparation of the experimental rig

Carbon anodes having dimensions 740 mm x 620 mm x 520 mm, and a weight of approximately 400 kg were prepared by a stone mason. A series of narrow slots (10 mm deep x 3 mm wide) were cut in the current receiving faces of the block that are engaged by the contact plates. Thin strips of extruded aluminium bar were placed in these slots. The aluminium is tightly fitted to the carbon to take advantage of the difference in the coefficients of thermal expansion between carbon and aluminium. This ensures that as the contact heats up, the materials are pressed into good contact so that the electrical resistance is reduced.

Based on the size of the anode block, a 4000 A current source was provided. The current density at the base of the anode was 0.87 A/cm².

A test stand was constructed to support the anode. The stand conducted current to two contact patches on either side of the anode. The test stand had four jacking screws that allowed the anode to be displaced vertically downwards through the contacts. The support structure could be adjusted to vary the clamping pressure acting on the contacts.

The anode sat in a bed of activated carbon, which acted as a carbon pile resistor. By adjusting the pressure on the carbon pile resistor, the amount of heat dissipated within the apparatus could be controlled and maintained. About 20 kW of electrical energy was dissipated in the equipment mostly across the carbon pile resistor. Gas burners were also installed under the carbon pile resistor to provide additional heating. Heating at the bottom of the anode, from both sources, was used to simulate the heat generated by the aluminium reduction process, so that the heat transfer through the equipment was similar to what might be experienced in a full scale reduction cell.

To prepare for the tests, an anode was mounted in the equipment and the contact pressure set to about 400 kPa. The carbon pile resistor was installed and adjusted until the overall voltage drop across the equipment was between 6 VDC and 7.5 VDC. The jacking screws were adjusted so that any backlash was taken up.

A Datataker was installed to record data at 10 second intervals throughout the test period. It recorded the test current, temperatures of the contact plates, temperatures within the carbon anode, and voltage drops to determine the contact resistance and current flow through each of the contacts. The test commenced with the lighting of the gas burners, and the initiation of the data collection equipment. When the lower parts of the block reached 200 °C, a generator set was started and the current source adjusted to produce 4000 A.

A range of operating temperatures was achieved by waiting as the anode warmed owing to the passage of the current. During testing, the applied stress i.e. the force of the contact plate to the current receiving face of the anode was measured at about 407 kPa.

As the equipment changed in temperature, the overall resistance changed and the current source required manual regulation to maintain the current at 4000 A ± 1 %. The test current was maintained for the duration of the experiment. After 10 to 12 hours the contacts had reached 600 °C and the test was terminated.

In order to demonstrate the effect of lowering the anode relative to the contact plate, small jacking movements of approximately 1 mm were made approximately every half an hour. This emulates the frequent anode position movements that might occur in a typical rodless anode aluminium reduction cell to simulate the consumption of carbon by the reduction process.

Experiments were conducted on a number of anodes with aluminium attached to the anode surface. A control experiment was also conducted where a plain carbon anode without aluminium was used for the test.

Results from a first experiment

Figure 6 shows the voltage drop across the anode-to-conductor interface as the temperature is increased. The effect of regular and small jacking displacements can be seen by steep reductions in voltage drop followed by gradual increases until the next displacement. The decrease in voltage drop following each displacement is attributed to the fresh or further contact material that is provided between the conductor and the anode body. For comparison, the voltage drop expected from a conventional rodded anode (i.e. stub- and-thimble joint) is marked on the graph at about 120 mV. The graph of Figure 6 illustrates the advantageous voltage drop achieved using an embodiment of the method of the present invention. The graph also shows that the influence of the jacking movement becomes smaller as the temperature increases and the aluminium is softer.

Results from a second experiment

Figure 7 shows the difference in the voltage drops in the presence and absence of contact material. In the absence of contact material, there is a higher voltage drop over the range of temperatures measured.

The low resistance contact with aluminium in the face of the carbon anode performed as predicted by the small scale testing of Example 1. The same broad shape is apparent with the Cold Zone being separated from the Soft Zone by a temperature range where the voltage drop is relatively high. The voltage drop recorded in the Soft Zone is favourably comparable to the benchmark performance of conventional rodded anode (typically around 12O mV).

The effect of jacking events is also evident in the trace of the graph of Figure 7. These are the sharp reductions in the measured voltage across the contact followed by a gradual recovery. The effect is most significant in the Hump Zone where the aluminium is not quite at the temperature where it is fully soft. Displacement of the contact is able to smear the aluminium into closer contact with the carbon, but some relaxation occurs and the voltage drop recovers. In the Soft Zone, (and the Cold Zone as well) the influence of the jacking movement is not as easily detected in the trace.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

1. In an aluminium reduction cell, a rodless anode block, said rodless anode block having a current receiving face for engagement with a contact face of a fixed conductor provided in the aluminium reduction cell, said current receiving face of the rodless anode block including an array of vertically spaced recesses, at least a portion of said array of vertically spaced recesses containing an electrical resistance reducing contact material comprising aluminium or aluminium alloy, the contact material being disposed in said recesses so as to define with the current receiving face a substantially smooth surface over which the contact face of the fixed conductor can slide when in engagement therewith.

2. The rodless anode block according to claim 1, wherein the array of vertically spaced recesses comprises vertically spaced rows of recesses.

3. The rodless anode block according to claim 2, wherein each row comprises an elongate groove.

4. The rodless anode block according to claim 2 or 3, wherein each row comprises a series of recesses.

5. The rodless anode block according to any one of claims 2 to 4, wherein each row extends horizontally across at least a portion of the current receiving face of the anode.

6. The rodless anode block according to any one of the preceding claims, wherein the contact material was introduced as a solid into at least some of the recesses.

7. The rodless anode block according to any one of the preceding claims, wherein the contact material was cast into at least some of the recesses.

8. The rodless anode block according to any one of the preceding claims, wherein the contact material protrudes from at least some of the recesses by no more than about 3 mm, preferably no more than about 1.5 mm, more preferably no more than about 0.5 mm.

9. A rodless anode block substantially as hereinbefore described with reference to the drawings and/or examples.

10. An aluminium reduction cell including: a cathode; an anode comprising a rodless anode block displaceably supported in the cell; a fixed conductor having a contact face in electrical contact with a current receiving face of the rodless anode block, the rodless anode block being displaceable towards the cathode relative to the fixed conductor; said current receiving face of the rodless anode block including an array of vertically spaced recesses, wherein a respective one or some of said recesses are opposed to the contact face of the fixed conductor according to the displacement of the rodless anode block relative to the fixed conductor; and an aluminium or aluminium alloy contact material provided in the one or some of said recesses opposed to the contact face of the fixed conductor, said contact material acting to reduce electrical contact resistance between the rodless anode block and the contact face of the fixed conductor during operation of the cell.

1 1. The aluminium reduction cell according to claim 10, wherein a voltage drop between the contact face of the fixed conductor and the rodless anode block is no more than about 200 mV, preferably no more than about 180 mV, more preferably no more than about 150 mV, even more preferably no more than about 130 mV, most preferably no more than about 100 mV.

12. The aluminium reduction cell according to claim 10 or 11, wherein the array of vertically spaced recesses comprises vertically spaced rows of recesses.

13. The aluminium reduction cell according to claim 12, wherein each row comprises an elongate groove.

14. The aluminium reduction cell according to claim 12 or 13, wherein each row comprises a series of recesses.

15. The aluminium reduction cell according to any one of claims 12 to 14, wherein each row extends horizontally across at least a portion of the current receiving face of the anode.

16. The aluminium reduction cell according to any one of claims 10 to 15, wherein the current distribution at a bottom face of the anode is in the range of from about 0.5 to about 1.5 A/cm².

17. The aluminium reduction cell according to any one of claims 10 to 16, wherein the contact material has relatively greater contact with the anode material of the rodless anode block than with the material of the contact plate by a ratio of at least 2 : 1.

18. The aluminium reduction cell according to any one of claims 10 to 17, wherein the contact material was introduced as a solid into at least some of the recesses.

19. The aluminium reduction cell according to any one of claims 10 to 18, wherein the contact material was cast into at least some of the recesses.

20. The aluminium reduction cell according to claim 10, wherein the rodless anode block is in accordance with any one of claims 1 to 9.

21. The aluminium reduction cell according to any one of claims 10 to 20, wherein the anode comprises a plurality of said rodless anode blocks.

22. A method of operating an aluminium reduction cell including a cathode and an anode comprising a rodless anode block displaceably supported in the cell, the method comprising the steps of: applying an electrical current to the rodless anode block via a fixed conductor having a contact face in electrical contact with a current receiving face of the anode block; forming aluminium at the cathode and thereby consuming the rodless anode block; displacing the rodless anode block towards the cathode relative to the fixed conductor as the rodless anode block is consumed; wherein said current receiving face of the rodless anode block includes an array of vertically spaced recesses, a respective one or some of said recesses being opposed to the contact face of the fixed conductor according to the displacement of the anode block relative to the fixed conductor; and wherein an aluminium or aluminium alloy contact material is provided in the one or some of said recesses opposed to the contact face of the fixed conductor, said contact material acting to reduce electrical contact resistance between the rodless anode and the contact face of the fixed conductor during operation of the cell.

23. The method according to claim 22, wherein a voltage drop between the contact face of the fixed conductor and the rodless anode block is no more than about 200 mV, preferably no more than about 180 mV, more preferably no more than about 150 mV, even more preferably no more than about 130 mV, most preferably no more than about 100 mV.

24. The method according to claim 22 or 23, wherein the array of vertically spaced recesses comprises vertically spaced rows of recesses.

25. The method according to claim 24, wherein each row comprise an elongate groove.

26. The method according to claim 24 or 25, wherein each row comprises a series of recesses.

27. The method according to any one of claims 24 to 26, wherein each row extends horizontally across at least a portion of the current receiving face of the anode.

28. The method according to any one of claims 22 to 27, wherein the current distribution at the bottom face of the anode is in the range of from about 0.5 to about 1.5 A/cm².

29. The method according to any one of claims 22 to 28, wherein the contact material has relatively greater contact with the anode material of the rodless anode than with the material of the contact plate by a ratio of at least 2 : 1.

30. The method according to any one of claims 22 to 29, wherein the contact material was introduced as a solid into at least some of the recesses.

31. The method according to any one of claims 22 to 30, wherein the contact material was cast into at least some of the recesses.

32. The method according to claim 22, wherein the rodless anode block is in accordance with any one of claims 1 to 9.

33. The aluminium reduction cell according to any one of claims 22 to 32, wherein the anode comprises a plurality of said rodless anode blocks.

34. Aluminium when prepared using the aluminium reduction cell according to any one of claims 10 to 21 or by the method according to any one of claims 22 to 33.