WO2004031452A1

WO2004031452A1 - Collector bar providing discontinuous electrical connection to cathode block

Info

Publication number: WO2004031452A1
Application number: PCT/CA2003/001484
Authority: WO
Inventors: Robert Leblanc; Pascal Thibeault
Original assignee: Alcan International Limited
Priority date: 2002-10-02
Filing date: 2003-09-26
Publication date: 2004-04-15
Also published as: AU2003271461A1

Abstract

A collector bar (16, 17) and cathode block (15) are used in an electrolyte reduction cell for the production of aluminum. A ferrous collector bar is mounted in a slot (21) in the cathode block, being embedded in the slot by an electrically conductive embedding material, such as cast iron. Discontinuous embedding portions (25, 26) are used with insulated gaps (27, 28) between them. The conductive embedding portions are concentrated in a central region of the slot and the insulated gaps are in the outer extremities, such that a greater portion of electrical current is forced through the conductive portions toward the center of the cathode block during operation of the cell. This results in a more uniform distribution of current along the cathode block.

Description

COLLECTOR BAR PROVIDING DISCONTINUOUS ELECTRICAL CONNECTION TO CATHODE BLOCK

Technical Field

This invention relates to electrolytic cells for the production of aluminum from alumina and, more particularly, to a cathode collector bar having a discontinuous electrical connection to the cathode block.

Background Art

Aluminum is produced by electrolytic reduction of alumina in cryolite electrolyte. This is done in a Hall- Heroult reduction cell which is typically operated at low voltages and very high electrical currents. The high electrical current enters the reduction cell through an anode structure and then passes through the cryolite bath, through a molten aluminum metal pad and then enters a carbon cathode block. The electrical current is carried out of the cell by cathode collector bars which connect to external electric bus bars.

The flow of electrical current through the aluminum pad and the carbon cathode block flows along paths of least resistance. The electrical resistance in a conventional cathode collector bar is proportional to the length of the current path from the point the electric current enters the cathode collector bar to the nearest external bus. The lower resistance of the current path starting at points on the cathode collector bar closer to the external bus cause the flow of current through the molten aluminum pad and carbon cathode blocks to be skewed in that direction. Particularly with graphite-containing cathode blocks, the dominant cause of failure is highly localized erosion of the cathode surface that exposes the collector bar to the aluminum metal. It has been established that there is a link between rapid wear rate, the location of the area of maximum wear, and the non-uniformity of the cathode current distribution. There is, therefore, a continuing need to develop and provide a more even cathode current distribution so that localized cathode wear rates will be decreased and thus the life of the cell increased.

It is known that electrical conductivity of steel is very poor relative to the aluminum metal pad such that the outer third of the collector bar, nearest the side of the electrolytic cell, carries the majority of the load, thereby creating a very uneven cathode current distribution within each cathode block.

One method that has been used to achieve a more uniform cathode current distribution has been to increase the electrical conductivity of the collector bar. This has resulted in reduced localized current density and consequent wear rates. One method that has been found to achieve this increased electrical conductivity has been by the use of copper inserts in the collector bar, such as described in Pate, U.S. Patent 5,976,333 issued November 2, 1999 and in Homley et al. U.S. Patent 6,231,745 issued May 15, 2001.

While the use of the copper core with greater conductivity is clearly helpful in achieving a more uniform cathode current distribution, it serves only to reduce the severity of the non-uniform current distribution and still fails to provide a balanced cathode current distribution along the width of the cathode block. It is therefore the object of the present invention to provide still further improvements to the cathode current distribution. Disclosure of the Invention

The present invention relates to the combination of a carbonaceous cathode block and a ferrous cathode collector bar adapted for use in an electrolytic reduction cell for the production of aluminum. It comprises a carbonaceous cathode block having an exterior surface into which is formed an elongated slot extending the width of the cathode block. This slot has an interior bottom face and side faces and a current collector bar is seated in the slot in electrically conductive contact with the carbon block by way of discontinuous embedding portions of electrically conductive bonding material formed between the side faces of the elongated slot and side faces of the collector bar. The embedding portions of conductive bonding material comprise a plurality of discrete electrically conductive embedding portions separated by non-conductive portions therebetween. The electrically conductive embedding portions are in greater concentration in a central region of the cathode block than in outer extremities of the cathode block so that a greater portion of electrical current is directed toward the center of the cathode block during operation of the reduction cell. This effectively forces the current away from its usual path of least resistance toward the edges of the block. The result is a more even current distribution along the cathode block.

Thus, the fundamental feature of this invention is the discovery that the cathode current distribution can best be controlled by controlling the areas of electrical contact between the collector bars and the cathode block. By providing most of the electrical contact area toward the center of the cathode block and providing only a small area of electrical contact toward the edges of the cathode block, the greater proportion of the current is forced into the center of the cathode block through the contact areas rather than allowing it to follow its usual path of least resistance.

The electrically conductive bonding material is preferably cast iron, although other materials may be used such as carbon glue or a carbonaceous paste. While the greatest benefit from these discontinuous embedding portions of cast iron are achieved with a collector bar with a copper insert, it can also be used to advantage with an ordinary steel collector bar.

A single collector bar may be inserted in a slot extending substantially across a cathode block. Alternatively, two half-width cathode collector bars may be used, each extending to about the center line of the cathode block where they may be separated by a gap in the middle of the block. The gap may be filled by a carbon block or ceramic fiber or other suitable filling materials .

When a single collector bar is used extending across the cathode block, it is possible to have a major central electrically conductive embedding portion with at least one smaller embedding portion on each side of and separated from the central embedding portion. With a pair of half-width collector bars, a major electrically conductive embedding portion is provided adjacent the inner end of each half-width bar with at least one smaller embedding portion spaced outwardly from the major inner portion. The embedding portions are preferably concentrated along the inner or central 70% of the collector bar or bars.

A sufficient lateral space is provided between the side faces of the elongated slot and the side faces of the collector bar for easy addition of the cast iron filler and to guarantee a good electrical contact between the adjacent side faces. The non-conductive gaps between the electrically conductive embedding portions are preferably filled by an electrically insulating material, such as a ceramic fiber blanket material. The concentration of electrically conductive embedding portions along the inner 70% of the collector bar is typically achieved by a single cast iron embedding portion. It is, however, also possible to divide this single electrically conductive embedding portion into several smaller portions with small gaps therebetween, provided these electrically conductive embedding portions serve the objective of forcing more of the current paths toward the center of the cell so that the desired even current distribution is achieved. In a particularly preferred embodiment of the invention, inner major cast iron embedding portions extends outwardly from the central region of the cathode block about 20 to 50% of the distance from the center to the edges of the cathode block. This is preferably accompanied by one or more pairs of minor cast iron embedding portions located between the above major embedding portions and the edges of the block. These minor portions have a length about 5 to 20% of the distance between the edge and the center of the cathode block.

The use of the discontinuous embedding portions according to this invention is effective in controlling the cathode current distribution. It does, however, tend to cause additional voltage drop in the cell, resulting in increased operating costs. The copper insert helps to overcome this voltage drop and it is also helped by increasing the overall cross-section of the collector bar. The cross-sectional area of copper insert is preferably about 5% to 50% of the cross-sectional area of the collector bar. This copper insert in combination with an increased cross-sectional area of the collector bar is able to substantially compensate for the additional voltage drop caused by the discontinuous embedding portions. (Preferably the collector bars have a cross- sectional area of at least 6000 square mm and also preferably have an orthogonal shape with a width of at least 60 mm and a height of at least 100 mm.

Brief Description of the Drawings In the drawings which illustrate the present invention:

Fig. 1 is a schematic illustration in cross section of a portion of an aluminum reduction cell;

Fig. 2 is a perspective view of a cathode collector bar seated in a groove of a carbon block embedded in accordance with the invention;

Fig. 3 is a plan view of the cathode block of Fig. 2; Fig. 4 is a side elevation of the cathode block of Fig. 2; and Fig. 5 is a plot of current density as a function of distance from the center of the cathode block for different collector bars and embedding arrangements.

Best Modes For Carrying Out The Invention

Referring now to Fig. 1, an aluminum reduction cell 10 includes anodes 11 extending into an electrolytic bath 12 over a molten aluminum pad 13. Beneath these is located a carbon cathode block 15 having a slot in the bottom face thereof into which extends a pair of conductor bars 16 and 17 divided at the center of the cell by a gap 18. Each of the current collector bars 16 and 17 contains a copper core 20. The outer ends of the current collector bars 16 and 17 extend through the side walls of the cell

10 where they connect to external bus bars. The electrical current enters the cell through the anodes 11 and then passes through the electrolytic bath 12 and molten aluminum pad 13. The electrical current then enters the carbon cathode block 15 and is carried out of the cell by the current collector bars 16, 17.

The collector bars are typically made of a ferrous material such as mild steel. They are preferably rectangular or square in cross section. When a copper insert is used, it may be square, round or any convenient cross section. The cross sectional area of the copper insert is preferably about 5% to 50% of the cross sectional area of the collector bar, and the copper insert is preferably enclosed within the collector bar.

Figs. 2, 3 and 4 show a preferred embedding pattern according to the invention. A slot is provided in the bottom of cathode block 15 (shown in the inverted position in Fig. 2) . A pair of half-width collector bars 16 and 17 are placed in slot 21 with a space between the sides of the collector bars and the side walls of the slot and a central gap 18 between the bars 16, 17. A generous space for the collector bars is provided in the slot so that sufficient embedding cast iron can be added to provide a good electrical contact in the embedding portions. The collector bars were then embedded with cast iron in the pattern shown. Thus, a pair of major embedding portions 25 extend outwardly from the inner ends of the collector bars 16 and 17. A further pair of minor embedding portions 26 are located between the major embedding portions 25 and the outer ends of the cathode block. Gaps 27 is provided between the major and minor embedding portions 25 and 26 and further gaps 28 are provided between the minor embedding portions 26 and the edges of the cathode block. These gaps 27 and 28 are filled by an electrically insulating material, preferably a ceramic fiber blanket material.

While a pair of half-width collector bars have been described above, the generally same procedure may be used with a single collector bar extending across the cathode block. It is also possible to use half-width cathode blocks or a plurality of adjacent blocks rather than the long cathode block shown. Thus, throughout the present description and claims any reference to "cathode block" includes two or more adjacent blocks that together are equivalent to the single block shown. Furthermore, any reference to a central region of the cathode block includes a central region of the two or more adjacent blocks and therefore a central region of the cell.

Example 1

A series of tests were conducted to illustrate the effectiveness of the present invention.

For the tests, a collector bar arrangement was used as shown in Figs. 2, 3 and 4. The cathode block 15 had a width of 2742 mm and each collector bar 16, 17 had cross sectional dimensions of 112 mm x 150 mm with a length of 1676 mm. When a copper insert was used in the collector bars, the insert had cross sectional dimensions of 45 mm x 45 mm with a length of 1335 mm. The major electrically conductive embedding portions 25 each had a length of 608 mm, while the minor electrically conductive embedding portions 26 each had a length of 122 mm. The gaps 27 each had a length of 304 mm and the gaps 28 each had a length of 168.9 mm. Using the embedding arrangements of this invention as described above, two collector bars were installed. One collector bar was a standard mild steel collector bar as described above while the other was a mild steel collector bar containing a copper insert as described above. The two bars were embedded in the pattern described above. Two additional collector bars were installed using standard embedding technology. The results obtained are shown in Fig. 5. In this graph, curve A is the embedding arrangement according to the invention using a collector bar with a copper core operating at a voltage of 292.2 mV. Curve B is a standard mild steel collector bar embedded in accordance with the present invention operating at a voltage of 420.3 mV. The curve C represents a graphitized cathode block with a standard collector bar and a standard embedding pattern. Curve D represents the current density distribution along a 30% graphite cathode block with a standard collector bar and standard embedding pattern.

It can be seen that the embedding arrangement in accordance with this invention provides a greatly improved uniformity in current density using collector bars both with and without a copper core.

Claims

Claims:

1. The combination of a carbonaceous cathode block and a current collector bar adapted for use in an electrolytic reduction cell for the production of aluminum, comprising a carbonaceous cathode block having an exterior surface into which is formed an elongated slot extending the width of the cathode block, said slot having an interior bottom face and side faces, and a current collector bar comprising a steel bar with a copper core seated in said slot in electrically conductive contact with the carbon block by way of discontinuous embedding portions of electrically conductive bonding material formed between the side faces of the elongated slot and side faces of the collector bar, said embedding portions of conductive bonding material comprising a plurality of discrete electrically conductive embedding portions separated by non-conductive portions therebetween, with the electrically conductive embedding portions in greater concentration in a central region of the slot than in outer extremities thereof, and thereby being adapted to direct a greater portion of electrical current toward the center of the carbonaceous cathode block during operation of the reduction cell resulting in a more even current distribution along the cathode block.

2. The combination according to claim 1, wherein the cross-sectional area of the copper core is in the range of 5 to 50% of the cross-sectional area of steel current collector bar.

3. The combination according to claim 2, wherein the cross-sectional area of the steel current collector bar is of a size such that the size of the steel current collector bar in combination with the copper insert substantially compensate for voltage drops created by the discontinuous embedding portions.

4. The combination according to claim 2, wherein the steel current collector bar has a cross-sectional area of at least 6000 square mm.

5. The combination according to claim 4, wherein the steel current collector bar has an orthogonal shape with a width of at least 60 mm and a height of at least 100 mm.

6. The combination according to claim 1, wherein the electrically conductive bonding material is cast iron.

7. The combination according to claim 1, wherein the current collector bar comprises a pair of axially aligned collector bars with a gap therebetween, with each bar having a length approximately half the width of the cathode block.

8. The combination according to claim 1, wherein the current collector bar comprises a single bar extending the width of the cathode block, said bar being embedded by a major central embedding portion and at least one minor embedding portion spaced outwardly from said central embedding portion.

9. The combination according to claim 6, wherein each half-width collector bar is embedded by at least two separate electrically conductive embedding portions.

10. The combination according to claim 6, wherein the electrically conductive embedding portions are concentrated along the inner 70% portion slot extending across the cathode block.

11. The combination according to claim 6, wherein the non-conductive portions comprise embedding portions of electrically insulating material.

12. The combination according to claim 11, wherein the electrically insulating material comprises a ceramic fiber material .

13. The combination according to claim 9, wherein the electrically conductive embedded portions comprise a pair of major cast iron embedding portions extending in opposite directions outwardly from a central region of the cathode block about 20 to 50% of the distance from the center to the edges of the cathode block and a pair of minor cast iron embedding portions located between said major embedding portions and the edges of the cathode block, with insulating portions between the major and minor electrically conductive embedded portions and between the minor electrically conductive embedded portions and the edges of the cathode block.

14. The combination according to claim 13, wherein each said minor cast iron embedded portion extends along the slot about 5 to 20% of the distance between the edge and center of the cathode block.

15. The combination according to claim 14, wherein a central gap is provided between the pair of half-width collector bars.