WO2010068992A1

WO2010068992A1 - An aluminium reduction cell and method for operating same

Info

Publication number: WO2010068992A1
Application number: PCT/AU2009/001643
Authority: WO
Inventors: Duncan Hedditch; Jeffrey Keniry; Christopher Peter Jones; Craig Tischler
Original assignee: ALUMINIUM SMELTER DEVELOPMENTS Pty Ltd
Current assignee: ALUMINIUM SMELTER DEVELOPMENTS Pty Ltd
Priority date: 2008-12-18
Filing date: 2009-12-17
Publication date: 2010-06-24
Anticipated expiration: 2011-06-18

Abstract

A method of operating an aluminium reduction cell including a rodless anode block is described. The method includes supplying aluminium or an aluminium alloy in a solid or molten state to adjacent an interface between a current receiving face of the anode block and a fixed conductor. During operation of the cell, the softened or molten aluminium or aluminium alloy is drawn between the current receiving face of the rodless anode block and the fixed conductor to act as an electrical resistance reducing contact material between the two.

Description

AN ALUMINIUM REDUCTION CELL AND METHOD FOR OPERATING SAME

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 Soderberg 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 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; supplying aluminium or an aluminium alloy in a solid or molten state to adjacent an interface between the current receiving face of the anode block and the contact face of the fixed conductor; and allowing supplied solid aluminium or aluminium alloy to soften or melt; wherein the softened or molten aluminium or aluminium alloy is drawn between the current receiving face of the rodless anode block and the contact face of the fixed conductor as the anode is displaced in the cell towards the cathode to act as an electrical resistance reducing contact material between said faces during operation of the cell.

According to a second aspect of the invention, there is provided an aluminium reduction cell apparatus 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; a contact material receiving zone adjacent an interface between the current receiving face of the rodless anode block and the contact face of the fixed conductor from which softened or molten contact material is drawn in to said interface as the anode is displaced in the cell towards the cathode to reduce electrical resistance between said faces; and a dispenser which supplies aluminium or aluminium alloy in a solid or molten state to said contact material receiving zone.

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 is expected that a 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 700 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 aluminium or aluminium alloy 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. 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. The individual displacement of each anode block is advantageous because the anode blocks may be consumed at different rates during operation of the cell. 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 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 support means for the or each anode block 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 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.

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.

Electrical current is delivered to the anode block across an interface between the contact plate of the fixed conductor and the anode block. A contact material receiving zone is formed adjacent/above the interface between the two faces.

Preferably, current is supplied to the or each anode block in such a way as to provide an even consumption of the anode. Accordingly, the or each anode block can have two current receiving faces each in electrical contact with a respective contact face of a respective fixed conductor. Each current receiving face will have a contact material receiving zone as described above. 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 face 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. In 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.

Contact material is supplied to the contact material receiving zone by a dispenser. The contact material is a material separate to the fixed conductor and separate to the anode block, but which contacts both at the interface. The contact material is electrically conductive, thermally expandable and capable of reducing resistance across the anode-to- conductor connection.

The electrical resistance reducing contact material is aluminium or an aluminium alloy. References to aluminium alone hereinafter 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 at the operating temperatures and pressures is advantageous to the working of the invention as will be appreciated from the following description.

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

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 contact material is preferably supplied as a solid to the contact material receiving zone. However, the contact material could be supplied in molten form. The contact material could be heated until molten in a vessel and then supplied to the aluminium contact material receiving zone as a liquid. The supplied molten aluminium would likely at least partially solidify in the current receiving zone thereby behaving like the solid described below to some extent. However, the supply of aluminium in a molten state may not be desirable given the handling issues.

Thus, advantageously, the contact material is supplied as a solid which melts or softens in the current receiving zone immediately adjacent the interface between the anode face and the contact plate of the fixed conductor due at least to the temperature of the cell environment. The solid is conveniently supplied in particulate form. Any shape of particles or combinations of shapes could be supplied. Random shapes could be used and cubes or other shapes having edges could be used. However, shapes that are able to roll or rotate about at least one axis are preferred, e.g. sphere, tubes, cylinders, since these can work their way into the current receiving zone as described in detail below. Balls are most preferred because they are able to rotate about any axis (as bearings) so are more likely to be trapped adjacent the interface between the contact plate and the current receiving face of the anode and be drawn into the interface as the anode is displaced.

In a preferred embodiment, the solid aluminium or aluminium alloy contact material comprises particles having an average size in the range of from about 3 mm to about 10 mm, preferably about 5 mm to about 8 mm. However, particles of different sizes could be supplied and any combinations of sizes could be supplied. The solid contact material particles should not be so large that they are not able to locate within the gap between the anode block and the contact plate and/or do not adequately soften once in use in the cell as described in more detail below. Furthermore, the solid particles of contact material should not be so small that they readily oxidise, are difficult to handle and/or are potentially explosive. 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 based on finely divided aluminium, are likely to oxidise readily because of the high surface to volume ratio.

As mentioned above, the contact material is delivered to the contact material receiving zone adjacent the interface between the faces of the conductor plate and anode block by a dispenser. In a preferred embodiment, the dispenser is a chute or chutes disposed so as to deliver the contact material. Other means of supplying the contact material include, for example, a tube or tubes that direct the contact material to the aluminium contact material receiving zone. The contact material is dispensed upon demand during use of the cell.

The aluminium is supplied or dispensed from a source external to the cell. The external source could be a container with an inlet and an outlet for releasing the contact material upon demand. The outlet could be connected to the abovementioned chute for delivery to the contact material receiving zone. New material could be continually or intermittently fed into the inlet of the container to ensure there is sufficient contact material for use.

In order to supply the molten or solid contact material to the interface, the contact material receiving zone can be tapered towards it. As the contact material is supplied by the dispenser, the material may fall (under the force of gravity) into the wedge formed by the taper adjacent the interface. Preferably, the contact material receiving zone is formed partly by the current receiving face of the anode block. In one embodiment, the wedge is formed between a support member for the contact plate and the current receiving face of the anode block, with the support member tapering towards the contact plate. The high operating temperatures of the cell in combination with the heat generated by the electrical current passing through the interface will cause at least some of the solid aluminium contact material in the receiving zone to soften and possibly melt.

As the anode is displaced towards the cathode, the material of the anode block may be crushed locally by the solid contact material. As the anode is displaced, the bulk of the contact material will remain in the receiving zone; however, at least some of the contact material will soften or melt. The propensity for the contact material to soften or melt will depend upon the yield point of the aluminium (or alloy) as well as the local temperatures and pressures. The temperature may be greater immediately adjacent the interface between the face of the anode block and the contact plate of the fixed conductor, meaning that some of the contact material may be soft or molten while the remainder is solid.

Any contact material that has softened or melted will be drawn by the moving current receiving face of the anode block into the interface and will be smeared and/or squeezed between the contact face of the fixed conductor and the current receiving face of the anode block. Effectively, the contact material will be drawn between the current receiving face of the rodless anode block and the contact face of the fixed conductor as the anode is displaced in the cell towards the cathode to act as an electrical contact resistance reducing material between said faces.

Eventually, the contact material will be drawn or wiped from the bottom of the contact plate where it is no longer in contact with the anode block. This contact material will be lost into the aluminium reduction cell. This loss of contact material is the reason that a continual supply is needed in the contact material receiving zone.

Aluminium (and its alloys) do not typically wet the rough surface of the anode block, so it is not expected that a continuous thin film will be formed in the interface between the anode block and contact plate. However, if such a film does form over limited areas, it may be most advantageous in reducing electrical contact resistance. The areas that do not have contact material disposed therebetween may allow for direct bearing of the contact face of the fixed conductor to the anode block.

In some embodiments, the contact plate of the fixed conductor can comprise substantially vertical or inclined from vertical channels or grooves therein into which the contact material will soften and/or melt and be smeared. The advantage of this arrangement is that the land between the grooves should provide areas of the contact plate that directly bear onto the anode for support, although it should be understood that some aluminium may smear over the land.

The grooves can have any shape and width in the contact plate. The grooves can act to trap solid contact material and cause it to slide against the anode block surface as the block is moved. In some embodiments, the grooves are able to receive the solid contact material into them. In other embodiments, the grooves are smaller than the solid contact material, so the balls (for example) may get trapped and bite into the anode block and/or the contact plate. Preferably, the grooves taper with the narrow part of the taper towards the bottom of the contact plate to reduce any tendency for molten contact material to run-off and be lost into the cell. Optionally, the grooves could terminate before the bottom of the contact plate so that aluminium cannot easily escape through run-off.

Since the contact material is a resource, it is desirable from a commercial perspective to keep its use to a minimum. Accordingly, enough contact material is supplied to provide the desired reduction in contact resistance and it is preferably only replenished by the dispenser as it is lost/consumed in the reduction cell. The amount of contact material supplied can be determined by trial and error, with a fixed amount supplied at predetermined intervals e.g. daily, weekly or monthly or when a new anode block is put in position. Altemativel y, the contact material can be supplied in response to resistance measurements taken at the anode block/contact plate interface. Once the voltage drop increases beyond a predetermined acceptable value, more material could be supplied. 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 amount of contact material and the frequency that it needs to be replenished to achieve the required current dispersion in the anode.

When disposed between the contact plate and the anode block, the contact material may fill and/or possibly deform some surface irregularities between the two. The thermal expansion of the contact material covered by the contact plate of the fixed conductor may also increase the microscopic surface area 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 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 temperature gradient in the anode could be controlled, so that the optimum temperature for softening and/or melting the contact material is achieved at the place on the anode 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 e.g. a blanket or another anode.

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; and

FIGURE 3 is a cross-sectional view of a preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Figure 3 is a cross-sectional schematic of part of an anode block 10 and a fixed conductor 12 having a contact plate 14. Contact plate 14 is the portion of fixed conductor 12 in electrical contact with a current receiving face 11 of anode block 10. A contact material receiving zone 16 is above an interface 18 between the contact plate 12 and the current receiving face 11 of anode block 10.

A support member 19 of the fixed conductor 12 tapers outwardly and downwardly towards the contact plate 14, to form a wedge-shaped contact material receiving zone 16 between the support member 19 and the current receiving face 11 of anode block 10. As contact material 20 is supplied from a dispenser chute 22, it is delivered to the contact material receiving zone 16 adjacent and above the interface 18 between the contact plate 14 and the anode block 10. Figure 3 is not drawn to scale and, at any one time, contact plate 14 may be in contact with more contact material than shown.

In Figure 3, the contact material is shown as solid, particulate aluminium contact material in the form of balls 20 having a diameter of about 6 mm (not to scale). In the wedge- shaped receiving zone 16, the balls roll downwards towards interface 18. The current receiving face 11 of anode block 10 may be crushed locally by some of the solid balls in the wedge. As the temperature of the balls immediately adjacent the interface 18 increases, some of them may soften and possibly melt. The softening/melting may be assisted by the pressure exerted on the balls trapped in the wedge. The softened/partially melted balls 20 are shown schematically in Figure 3 as partially flattened spheres 20a. The softened or molten contact material is drawn into the interface 18 between the anode block 10 and contact plate 12 by the downwards displacement of the anode block thereby increasing contact and reducing electrical contact resistance between anode block 10 and contact plate 12 of conductor 14.

Eventually, aluminium contact material will be lost from the bottom 24 of contact plate 12 into the reduction cell (not shown). To compensate for this loss and to provide continual resistance reduction, more balls 20 can be supplied from the dispenser via chute 22. By way of example only, there may be between 30 and 60 displacement events of the anode block each day to compensate for anode block consumption, each displacement event being in the range of from about 0.5 mm to 2 mm.

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. 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; supplying aluminium or an aluminium alloy in a solid or molten state to adjacent an interface between the current receiving face of the anode block and the contact face of the fixed conductor; and allowing supplied solid aluminium or aluminium alloy to soften or melt; wherein the softened or molten aluminium or aluminium alloy is drawn between the current receiving face of the rodless anode block and the contact face of the fixed conductor as the anode is displaced in the cell towards the cathode to act as an electrical resistance reducing contact material between said faces during operation of the cell.

2. A method of operating an aluminium reduction cell according to claim 1, wherein the supplied aluminium or aluminium alloy comprises small solid particles capable of rotating about at least one axis.

3. A method of operating an aluminium reduction cell according to claim 2, wherein the small solid particles are spheres.

4. A method of operating an aluminium reduction cell according to claim 2 or 3, wherein the small solid particles have an average size in the range of from about 3 mm to about 10 mm, more preferably about 5 mm to about 8 mm.

5. A method of operating an aluminium reduction cell according to any one of the preceding claims, wherein the aluminium or aluminium alloy is supplied from an external source via a dispenser.

6. A method of operating an aluminium reduction cell according to any one of the preceding claims, 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.

7. A method of operating an aluminium reduction cell according to any one of the preceding claims, 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.

8. A method of operating an aluminium reduction cell substantially as hereinbefore described with reference to the drawings.

9. An aluminium reduction cell apparatus 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; a contact material receiving zone adjacent an interface between the current receiving face of the rodless anode block and the contact face of the fixed conductor from which softened or molten contact material is drawn in to said interface as the anode is displaced in the cell towards the cathode to reduce electrical resistance between said faces; and a dispenser which supplies aluminium or aluminium alloy in a solid or molten state to said contact material receiving zone.

10. An aluminium reduction cell according to claim 9, wherein the contact material receiving zone is tapered towards the interface between the current receiving face of the rodless anode block and the contact face of the fixed conductor.

11. An aluminium reduction cell according to claim 10, wherein the taper is formed by a support member for the contact face of the fixed conductor.

12. An aluminium reduction cell according to any one of claims 9 to 11, wherein the aluminium or aluminium alloy supplied by the dispenser comprises small solid particles capable of rotating about at least one axis.

13. An aluminium reduction cell according to claim 12, wherein the small solid particles are spheres.

14. An aluminium reduction cell according to claim 12 or 13, wherein the small solid particles have an average size in the range of from about 3 mm to about 10 mm, more preferably about 5 mm to about 8 mm.

15. An aluminium reduction cell according to any one of claims 9 to 14, wherein the dispenser includes a chute or tube.

16. An aluminium reduction cell according to any one of claims 9 to 15, wherein the anode comprises a plurality of said rodless anode blocks.

17. An aluminium reduction cell substantially as hereinbefore described with reference to the drawings.

18. Aluminium when prepared by the method according to any one of claims 1 to 8 or using the aluminium reduction cell according to any one of claims 9 to 17.