WO2010118465A1

WO2010118465A1 - Support for rodless anode

Info

Publication number: WO2010118465A1
Application number: PCT/AU2010/000414
Authority: WO
Inventors: Christopher Peter Jones
Original assignee: Aluminium Smelter Developments Pty Ltd
Priority date: 2009-04-16
Filing date: 2010-04-15
Publication date: 2010-10-21
Also published as: CA2779727A1

Abstract

A continuous pre-baked or rodless anode assembly 1 for an aluminium smelting cell, comprising one or more pre-baked anodes (2, 3, 4) arranged in a cassette to have two exposed end faces, electrical contact means (5, 6, 7) at the exposed end faces of the anode(s), and between the anodes in the case of multiple anodes in the cassette, the cassette including clamping force applying means including elongate clamping means (9) and tensioning means (10), said elongate clamping means transferring tensioning forces applied by said tensioning means as a clamping force to the electrical contact means (5, 6, 7) at the exposed end faces of the anodes, to hold the one or more anodes in the cassette, and to keep the electrical contact means (5, 6, 7) in electrical contact with the one or more anodes.

Description

SUPPORT FOR RODLESS ANODE

FIELD OF THE INVENTION

The present invention relates to the operation of an aluminium reduction cell comprising rodless, or continuous, pre-baked anodes. In particular, the invention relates to the means of applying the force required to support the anode(s), and to achieve electrical contact.

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 gaps, which solidifies as a thimble in each hole to hold the two together. 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 Sδderberg technology, the pre-baked, rodded anode has drawbacks, including 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.

Once secured in the anode, the rod is clamped to the anode beam. The anode beam is part of the superstructure of the aluminium reduction cell and is part of the electrical circuit. When mounted like this, the base of the carbon anode is suspended in the molten bath of electrolyte. The passage of electrical current through the cell consumes carbon and produces aluminium at the cathode. The aluminium produced is added to the liquid metal cathode in the bottom of the pot and the surface of the cathode rises with time. As it is important to maintain the distance between the carbon anode and the liquid aluminium cathode, adjustments are made to the height of the anode beam by large jacks. In general the rate of addition of aluminium is slightly faster than the rate of consumption of carbon and there is a small net upwards movement of the anode beam.

A normal part of the operating routine is to remove the molten aluminium from the cell by tapping at a frequency between 24 to 48 hours. A crucible is brought to the cell which is fitted with a siphon. The siphon is inserted into the cell and sucks out some of the molten aluminium which reduces the level of aluminium pooled on the cathode. As the level of the cathode falls, the jacks on the superstructure automatically adjust to maintain the distance between the anode and the cathode. The result is that the carbon anode moves downwards.

Considering the combined effect of the daily rise and the fall at tapping, the net movement of the anode is typically downwards, for example, by about 20 mm/day. This is the rate at which the height of the carbon is reduced.

Eventually, if the bottom of the carbon were allowed to be continually consumed, the thimbles in the top of the anode would eventually protrude through the bottom surface of the anode. This would expose the thimbles to corrosion by the molten bath, and contaminate the aluminium being produced with iron. Consequently, a rodded carbon anode might be typically limited to a service life of, say, 24 days. After this time there is still typically about 50 mm of carbon between the thimble and the bottom of the anode. At this point, the anode and the associated rod are removed from service and replaced by a new anode and rod. The carbon on the removed anode (the butt) is stripped from the rod. The rod is cleaned up and set in a new anode block, and the carbon butt is crushed and recycled within the anode production process. The anode change and butt recycle processes expose production operators to elevated temperatures, dust, fumes and noise. Generally, the present requirement to recycle and replace rodded anodes is a wasteful, costly, environmentally undesirable, hazardous, and time consuming process. Consequently, there are several strong incentives to eliminate the processes required when using rodded anodes by developing rodless anode technology that allows complete consumption of anode carbon within the cell.

In achieving this, problems arise, including providing a suitable mechanism for applying a support force to the anode, which allows the position of the anode with respect to the cell to be altered as the anode is consumed, without loosening the mechanism. In the case of multiple anodes, the mechanism needs to apply the required clamping force to the anodes to ensure proper electrical contact between the anodes. The key issues for a suitable clamping system are as follows:

• it should not significantly impinge upon the footprint of the anodes within the cell.

• it should provide a reliable clamping force adequate to maintain the electrical connection and support the anodes • it should maintain the clamping force without premature creep of the stressed components due to the elevated temperatures.

• it should be robust and durable in the face of very aggressive working conditions.

• it should be simple and economical to manufacture and maintain.

The invention therefore provides a continuous pre-baked or rodless anode assembly for an aluminium smelting cell, comprising one or more pre-baked anodes arranged in a cassette, electrical contact means in physical and electrical contact with the anode(s), and between the anodes in the case of multiple anodes in the cassette, the cassette including clamping force applying means including elongate clamping means and tensioning means, said elongate clamping means transferring tensioning forces applied by said tensioning means as a clamping force to the electrical contact means to hold the one or more anodes in the cassette, and to keep the electrical contact means in electrical contact with the one or more anodes.

In one form the anode(s) has opposed faces which are exposed for contact by the electrical contact means. In one form, the electrical contact means includes one or more contact plates engaging the opposed faces of the anode

In one form, the elongate clamping means includes the electrical contact means at the opposed fa ces and elongate clamping means extending between the electrical contact means at the exposed end faces.

The use of elongate thin members to hold the anode assembly has the advantage of being a mechanically simple, tension-only device which occupies minimal space in the smelting cell, and which is otherwise able to withstand the environment of the smelting cell. The force applying means can be located above the fume cover of the cell, which in a preferred arrangement is located at a level just above the deck plate of the cell; this thereby reduces the exposure of the force applying means to the aggressive cell environment.

In one form, the elongate thin members may be strap-like members fonned from a metal resistant to the environment of the electrolytic cell and having the necessary tensile strength to apply tension to the anodes in the array. The use of strap-like members is not essential as the strap-like members may be replaced by cables or any similar long thin member capable of applying tension forces to the anodes in the cassette.

The tensioner means may include a tension applying screw means engaging one or more disc springs or another suitable means of maintaining tension such as springs or compressible material suitable for the high temperature operating environment; this ensures that appropriate tension is applied to the strap connected to the tensioner means.

The elongate clamping means comprises spaced elongate members connected to each other by means which transfers tensioning forces applied by the tensioner means to the clamping means around the exposed corners of the anode(s) in the cassette.

The means for transferring tension forces between adjacent elongate thin members may comprise flexible chain means which may be contained within one or more protective sleeves to protect the chain from the environment of the electrolytic cell. The contact means may comprise conductive plate means attached to a frame which extend up and over the top of each anode.

In order that the invention may be more readily understood, an embodiment will now be described with reference to the accompanying drawings in which:

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a simplified sectional plan view through a typical cassette embodying the invention showing the contact means at the ends of and between the anodes and the force applying means in schematic form;

Figure 2 is a sectional plan view similar to Figure 1 showing one embodiment of the invention;

Figure 3 is a detailed view of the tensioner means used in the embodiment of Figure 2, and

Figure 4 is a detailed view of the connecting means between adjacent force applying strap means used in the embodiment of Figure 2.

DESCRIPTION OF EMBODIMENTS

Referring now to Figure 1, it will be noted that each cassette 1 supports anodes 2, 3 and 4, together with end contact plates 5 and 6 and intermediate contact plates 7 which engage opposed faces of the anodes. The array of anodes and contact plates is held together by a force applying means or clamping strap arrangement 9 which includes a tensioner means

10, an embodiment of which will be described further below. The strap arrangement 9 and tensioner 10 apply the required tension forces to the end contact plates 5 and 6 to ensure that the anodes 2, 3 and 4 are in electrical contact with the end contact plates 5 and 6 and the intermediate contact plates 7. The force applied is sufficient for this purpose but still allows the anodes to be pushed through the force applying means as the anodes are consumed and new anodes are added to the cassette as described further below.

While Figures 1 and 2 illustrate a three anode cassette 1 embodying the invention, it will be appreciated that the cassette may support more, or less, than three anodes, including one anode, although ideally, the number of anodes in each cassette should not exceed about six.

Since the embodiment of the invention relates to the force applying means for holding the anode or anodes in the cassette in electrical and physical contact, the manner in which the cassette is supported in an electrolytic cell does not form part of the invention and will not be described in particular detail since any suitable arrangement within the capability of a person skilled in the art may be used while still reaping the benefits of the present invention.

As illustrated in more detail in Figure 2, the strap arrangement 9 includes a front strap 11, a back strap 12 and end straps or plates, which in the present embodiment, are the end contact plates 5 and 6. If needed, further plates may be added to the contact plates, for example, if the contact plates are defined by narrow strips (not shown) engaging narrow portions of the anodes.

The tensioner means 10, which is illustrated in further detail in Figure 3, includes a force applying bolt and nut arrangement 15 mounted in a frame comprising side plates 16 and cross plate 17 through which the bolt arrangement 15 passes, the arrangement being secured by bolts, as illustrated, to the end plate 6. The bolt arrangement 15 is secured by any suitable means to the front strap 11 so that the necessary tension forces may be applied to this strap. To ensure that the appropriate tension forces are applied, a stack of disk springs is positioned between the cross plate 17 and the tension applying nut 18. It will be appreciated that the use of disc springs is not essential as they may be replaced by a suitable coil spring, or some other suitable resilient means.

To transfer the required tension forces between the front strap 11 and the end strap or plate 6, a flexible chain member 19 is secured to the respective strap/plates 11 and 5, and enclosed within a protective sleeve 20, as illustrated in greater detail in Figure 4. A similar chain connector 21 is secured to the other end of the end strap/plate 5 and the back strap 12, the back strap 12 being secured to the plate 6 in any suitable manner, whereby the tension means 10 operates to tension the end plates 5 and 6 and the intermediate plates 7 and the anodes 2, 3 and 4 in the cassette 1, to ensure that electrical contact between the respective anodes and the contact plates is maintained.

While the above arrangement is presently preferred for strap tensioners, the use of a cable in place of the straps may obviate the need for flexible connecting chains.

The connection of the tensioner means 10 to the front strap 11 provides access to the tensioner so that the tension forces applied may be adjusted as required. While in the present illustrations the force applying means 9 is located in the lower regions of the anode cassette, it will be appreciated that this mechanism may if desired be located above the fume cover of the electrolytic cell to which the cassette 1 is fitted to isolate the mechanism from the cell environment.

As the anodes 2, 3 and 4 are consumed during the reduction process, the anodes 2, 3 and 4 are periodically incrementally pushed down through the contact plates or pads of the cell by any suitable jacking arrangement (not illustrated). This displacement of the anode block disturbs the electrical contact at the conduction pad as the anode surface slips past the pad. The low resistance connection between the contact pad and the anode surface must be maintained during this disturbance. In one embodiment the low resistance connection is achieved by the softening or melting of aluminium as it enters the contact region as a result of the passage of electrical current through the contact, and by heat transfer to the contact area from the operating cell.

As mentioned above, the necessary electrical connections to the superstructure of the cell and the means of conducting potline current to the anodes does not form part of the invention and may be achieved in any suitable manner, such as by flexible bus connectors for example. As defined above, the anodes are supported by contact plates or pads. The anode is supported by the force applying means with the 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 is held in position with a suitable glue. In this way, consumption of the anode is continuous and the recycling of butt material is not necessary.

As described the anodes are supported in the cell by the force applying means generating a clamping force in the order of about 50 kN to about 100 kN per cassette. This clamping force is required both to adequately support the block without risk of slippage and to help provide an acceptably low voltage drop across the sliding joint interface. 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 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.

The application of the clamping forces in the manner defined above meets the following requirements:

• it does not compromise the space available for productive anode area in the cell; ideally not reducing the percentage of area of the cell covered with anodes with respect to conventional pre-bake technology. • it is able to maintain the clamping force without premature creep of the stressed components due to the elevated temperatures. • it is robust in the face of very aggressive working conditions of corrosive fume and elevated temperatures.

• it is able to clamp a number of anodes but no greater than 20% of the total number anodes in the cell; (in the event of needing to change a cassette for maintenance, a change-out at any one time of greater than 20% provides an unacceptable disruption to the cell operation

• it is simple and economical to manufacture and maintain.

As the anode carbon is consumed in the reduction process (the normal operation of the reduction cell) it is necessary to replace this material. This is done by applying conductive adhesive paste to the intervening surfaces and gluing a new block on top of the old one.

This addition of a new anode block is done on a periodic basis. As the old anode is consumed so the new one takes its place and the process is repeated. Any suitable method of installing new anodes to the cassette 1 may be adopted, and since any such installation method does not form part of the present invention, it will not be further described in this specification.

In this way the operation of the cell is not thermally or physically disturbed by the addition of replacement anode carbon. Once the cell is first started, the anode temperature at the working surface remains essentially constant for the life of the cell's operation, eliminating bath freezing on the bottom surfaces of introduced anodes, and cell temperature excursions due to opening the crust, factors which often cause operational problems in pre-bake cells fitted with conventional rodded anodes. Additionally, fume evolution is not increased during the anode replacement process, unlike with a conventional pre-bake cell, where the hot surface of the electrolyte is exposed to the atmosphere for several minutes on each anode change, resulting in very significant fume evolution to the atmosphere.

The rodless anode arrangement described above differs from current industrial practice in the following respects:

• The anode block does not have a cast-in rod assembly; • No spent anodes (butts) are generated. This means that all of the following manufacturing processes are eliminated. o Anode butt removal and transportation o Anode butt cooling and fume capture o Anode butt cleaning o Anode butt stripping o Anode butt crushing and sizing o Anode butt dust handling and disposal o Anode cover (bath) recycling o Rod repair and cleaning o Cast-iron recycling preparation and handling o Anode rodding (casting)

• No cast iron is used in the process;

• In one embodiment the anode has aluminium at the electrical connection interface for current conduction. The aluminium at the electrical connection interface operates near its melting point at the point of current entry to the anode;

• Anodes are arranged in cassettes;

• The electrical conduction path does not change length/resistance significantly during normal operation; • Replacement anode blocks are glued onto the top of the working anode;

• The anode working surface (bottom face) temperature is held constant during addition of replacement anodes;

• The anode-cathode distance is held constant during the replacement anode addition;

• The cell crust is not opened during anode replacement. The following aspects have also been incorporated in developing the technology:

• If there is an operational issue with one anode, the whole cassette may be removed, the repair made and the cassette reinserted in the cell;

• The electrical resistance of the anode-busbar connection is similar to that in an equivalent pre-bake cell.

The benefits of the rodless anode technology include: Reduced Capital Costs

• Elimination of the rodding room;

• Reduction in the size of the anode baking furnace; • Elimination of gas treatment on the baking furnace to remove fluorides

• Reduction in the size of the green anode mill;

• Elimination of the rod repair facility;

• Elimination of some pot tending machines;

• Elimination of butt cooling and processing facilities; • Elimination of bath removal facilities;

• Reduction of the size of the bath processing plant;

• Elimination of the cost of rods and yokes; and

• Reduction in the size of pot gas treatment centres.

Reduced Operating Costs:

• Reduced labour in pot room, rodding room, green mill etc;

• Elimination of energy use by redundant facilities (see list above);

• Reduction of energy use by elimination of anode butt recycle

• Reduction of operating supplies; • Reduction of waste disposal costs;

• Elimination of rod repair costs;

• Reduced carbon consumption resulting from sodium contamination;

• Cell energy savings (from elimination of butt waste heat);

• Reduced fluoride attack on the baking furnace flue and refractory; • Reduced loss of fluoride to roof emissions;

• Reduced gross carbon conversion cost.

Other Benefits:

• Reduced iron contamination of aluminium; • Complete elimination of dust generation from rodding room operations, butt cooling and anode cleaning; • Elimination of emissions from anode setting;

• Significantly reduced dust generation from much reduced bath re-processing;

• Significantly reduced dust generation as anode-cover handling is virtually eliminated;

• Complete elimination of fluoride emissions from carbon plants; • Elimination of cross-cell contamination due to anode cover, as anode cover is not transferred between cells;

• Simpler cell chemistry management owing to removal of feedback loops in the process;

• Simpler plant layouts and logistical efficiencies; • Substantial health and safety benefits for pot room operator by reducing exposure to pot room dust and fume and eliminating anode setting over open cells.

• Elimination of thermal shock during anode replacement.

The invention has been described by way of non-limiting example only and many modifications and variations may be made thereto without departing from the spirit and scope of the invention.

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

CLAIMS:

1. A continuous pre-baked or rodless anode assembly for an aluminium smelting cell, comprising one or more pre-baked anodes arranged in a cassette, electrical contact means in physical and electrical contact with the anode(s), and between the anodes in the case of multiple anodes in the cassette, the cassette including clamping force applying means including elongate clamping means and tensioning means, said elongate clamping means transferring tensioning forces applied by said tensioning means as a clamping force to the electrical contact means to hold the one or more anodes in the cassette, and to keep the electrical contact means in electrical contact with the one or more anodes.

2. The anode assembly of claim 1, wherein the anode has posed faces which are exposed for contact by the electrical contact means.

3. The anode assembly of claim 2, wherein the electrical contact means includes one or more contact plates engaging the opposed faces of the anode(s).

4. The anode assembly of any one of claims 1 to 3, wherein the elongate clamping means includes the electrical contact means at the opposed faces and elongate clamping means extending between the electrical contact means at the opposed faces.

5. The anode assembly of claims 1 to 4, wherein the elongate clamping means extends around the one or more anodes and at least one of the contact means in the cassette.

6. The anode assembly of any one of claims 1 to 5, wherein the clamping force required to maintain electrical contact and to support the anode(s) is moulded by means of a strap around at least one of the contact means and the anode(s).

7. The anode assembly of any one of claims 1 to 6, wherein the clamping force is applied externally to a grouping of multiple anodes.

8. The anode assembly of any one of claims 1 to 7, wherein the elongate clamping means selected from strap-like members, cables, or any long relatively thin member capable of applying tension forces to the anode(s) in the cassette while resisting the environment of an electrolytic cell and without occupying additional space in the electrolytic cell.

9. The anode assembly of claim 8, wherein the elongate clamping means are strap-like members formed of metal resistant to the environment of the electrolytic cell and having the necessary tensile strength to apply tension to the anodes in the array.

10. The anode assembly of claims 8 or 9, wherein the elongate clamping means comprises spaced elongate members connected to each other by means which transfers tensioning forces applied by the tensioning means to the clamping means around the exposed comers of the anode(s) in the cassette.

11. The anode assembly of claim 10, wherein the means for transferring tension forces between adjacent elongate members includes flexible chain means.

12. The anode assembly of claim 11, wherein the flexible chain means is contained within one or more protective sleeves to protect the chain means from the environment of the electrolytic cell.

13. The anode assembly of any one of claims 1 to 12, wherein the tensioning means includes tension applying means engaging one or more resilient means formed to ensure that appropriate tension is applied and maintained during operation to the elongate clamping means connected to the tensioner means.

14. The anode assembly of claim 13, wherein the resilient means is selected from disc springs, coil springs or similar means.

15. The anode assembly of claim 14, wherein the resilient means comprises an array of disc springs engaged by a tensioning screw.

16. The anode assembly of claims 1 to 15, wherein the contact means includes conductive plate means attached to a frame which extends over the top of the or each anode.