Cathode current collector/connector for a Hall-Heroult cell

Abstract

An electrolytic cell for the production of aluminium including collector bars under the cathode, namely a copper collector bar whose external terminal end is connected by a conductor element providing electrical connection of the collector bar to an external bus. This conductor element comprises a flexible connector strip of the same or a different highly conductive metal as the conductor bar, such as copper.

Claims

1. A cathode current collector and connector assembly assembled in a carbon cathode of a Hall-Heroult cell for the production of aluminium, said assembly comprising: at least one collector bar of a copper or copper alloy that is located under the carbon cathode is in direct electrical contact with the carbon cathode, wherein the or each collector bar comprises one or two terminal end part or parts extending outwardly up to inside or outside a cell outer cover to a connector, whereat said terminal end part(s) of the or each collector bar are electrically connected in series each to a conductor element providing connection to an external bus, wherein said conductor element providing electrical connection of the collector bar to an external bus comprises a flexible connector strip which is made of copper or copper alloy.

2. The cathode current collector and connector assembly according to claim 1, wherein the flexible connector strip is a flexible strip having at its ends connecting pieces of solid copper with rings or hooks for connection directly or indirectly to the terminal part of the collector bar and to an external bus.

3. The cathode current collector and connector assembly according to claim 1, wherein said terminal part(s) of the collector bars comprise in the vicinity of said connector a zone of reduced cross-sectional area wherein the cross-sectional area of said zone of the terminal part is less than the cross-sectional area of the remainder of said terminal part(s).

4. The cathode current collector and connector assembly according to claim 3, wherein the zone of reduced cross-sectional area comprises at least one opening, or recess or part of reduced thickness in the terminal end part of the collector bar.

5. The cathode current collector and connector assembly according to claim 1, wherein said connector comprises a conductor block of the same copper or copper alloy as the collector bar, a different electrically conductive metal or a different copper alloy from the collector bar, and wherein the conductor block is attached to the terminal end part of the collector bar(s) such that it protrudes from above and below and/or laterally from either side of said terminal end part.

6. The cathode current collector and connector assembly according to claim 5, wherein the collector bar comprises two spaced-apart arms joined at an external end by a cross-piece, wherein the conductor block is externally connected to the cross-piece, and wherein the two spaced-apart arms each comprise adjacent to the connection with the cross-piece, a said zone wherein the cross-sectional area of each arm is less than the cross-sectional area of the remainder of said arms.

7. The cathode current collector and connector assembly according to claim 5, wherein the conductor block is connected to the flexible connector strip which is made of a plurality of strips or braids or embossed sections of highly conductive metal and wherein the conductor block is made of aluminium, copper or alloys thereof.

8. The cathode current collector and connector assembly according to claim 5, comprising a bimetallic plate between facing surfaces of the conductor block and the collector bar.

9. The cathode current collector and connector assembly according to claim 1, wherein said terminal part(s) of the collector bars comprise an outer protective casing of metal extending up to the vicinity of said connector.

10. The cathode current collector and connector assembly according to claim 9, wherein the space between the collector bar and the protective casing is optionally filled with a compressible material of low electrical conductivity and low thermal conductivity.

11. The cathode current collector and connector assembly according to claim 1, wherein at least one cathode is composed of carbon and/or graphite in a proportion of at least 50% by weight of carbon.

12. The cathode current collector and connector assembly according to claim 1, wherein an upper part of the cathode contains at least one refractory hard metal compound like TiB2 and a lower part of the cathode is made of carbon.

13. A Hall-Heroult cell for the production of aluminium fitted with a cathode current collector and connector assembly according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be further described by way of example with reference to the accompanying drawings, in which:

(2) FIG. 1A is a schematic cross-section through a Hall-Héroult cell equipped with a prior art collector and connector bar arrangement.

(3) FIG. 1B is a schematic view of a Hall-Héroult cell equipped with a collector and connector bar arrangement according to the invention.

(4) FIG. 2 is a schematic perspective view showing the connection of a copper collector bar to an external busbar.

(5) FIG. 3 is a diagrammatic view showing one possibility of providing a zone of reduced cross-section in a collector bar.

(6) FIG. 4 illustrates the temperature decrease at the end of the collector bar.

(7) FIG. 5 shows bent bars with reduced section and connecting area.

(8) FIG. 6 shows examples of holes of different shapes to decrease the cross-section of the collector bar.

(9) FIG. 7 illustrates another way of reducing the cross-section of a collector bar.

(10) FIG. 8 shows two examples of flexible copper strips.

(11) FIGS. 9A and 9B illustrate a test set up for comparing the effects of a collector bar without a zone of reduced cross-section to one with a zone of reduced cross-section.

DETAILED DESCRIPTION

(12) FIG. 1 schematically shows a Hall-Heroult aluminium-production cell 1 according to WO 2016/079605 comprising a carbon cathode cell bottom 4, a pool 2 of liquid cathodic aluminium on the carbon cathode cell bottom 4, a fluoride—i.e. cryolite-based molten electrolyte 3, containing dissolved alumina on top of the aluminium pool 2, and a plurality of anodes 5 suspended in the electrolyte 3. Also shown is the cell cover 6, cathode current collector bars 7 according to the invention that lead into the carbon cell bottom 4 from outside the cell container 8 and anode suspension rods 9. As can be seen, the collector bar 7 is divided in zones. Zone 10 is insulated electrically and zone 11 is composed of layers. Molten electrolyte 3 is contained in a crust 12 of frozen electrolyte.

(13) An essential consideration of WO 2016/079605 was that steel bars 18 of enlarged cross-sectional area were connected in electrical series to the ends of the collector bars 7 and protrude outside the cell 1 for connection to external current supplies. Zone 10 of the collector bar is for example electrically insulated by being wrapped in a sheet of alumina or by being encased in electrically insulating glue or cement.

(14) FIG. 1B schematically shows a Hall-Héroult cell equipped with a collector and connector bar arrangement according to the invention. Here, a copper collector bar 7 is connected directly to the main busbar 40 via an intermediate aluminium block 20 and a flexible copper connector 30.

(15) FIG. 2 is an enlarged scale perspective view showing an example of the connection of the copper collector bar 7 to the external busbar 40. As shown, in this example, the collector bar 7 comprises two parallel spaced-apart arms joined at an external end by a cross-piece. The aluminium conductor block 20, which is wider than and much higher than the spaced-apart arms 7, is externally connected to the cross-piece. The two spaced-apart arms each comprise adjacent to the connection with the cross-piece, a zone 15 wherein the cross-sectional area of each arm is less than the cross-sectional area of the remainder of said arms, in this example by having circular holes in the opposite arms, adjacent to the connection area.

(16) The aluminium conductor block 20 is massive compared to the collector bars 7 and is attached to the cross-piece of the collector bar(s) such that it protrudes from above and below the terminal section of the collector bars 7 and laterally from either side. As shown, the protruding bottom part of the conductor block 20, opposite the collector bars 7, is connected by a flexible copper connector 30 connected at its other end to the busbar 40, this flexible connector 30 sagging in the middle.

(17) The conductor block 20 when made of aluminium can for example typically measure 220×120×50 mm but this block 20 can be dispensed with when using a copper flex.

(18) FIG. 3 is a diagrammatic view showing one possibility of providing a zone 16 of reduced cross-section in a collector bar, namely by reducing the thickness along and adjacent to the cross-piece.

(19) FIG. 4 illustrates the temperature decrease at the end of the collector bar. The temperature is typically close to 950° C. inside the carbon cathode and decreases when leaving the cathode to reach about 200° at the copper-bar/flex interface.

(20) FIG. 5 shows bent bars 7 with reduced section in the connecting area 17. The bent areas are used to bolt the end of the copper bar to the copper flex(es) and/or to the solid interface 20.

(21) FIG. 6 shows examples of holes of different shapes to decrease the cross-section of the collector bar 7 in a zone 15. FIG. 6a shows a circular or optionally oval opening. FIG. 6b shows an opening of narrow rectangular shape rounded at its edges. 6c shows a square opening with rounded edges and 6d a lozenge shape with rounded edges. FIG. 6e shows an array of five round openings grouped together.

(22) FIG. 7 illustrates another way of reducing the cross-section of a collector bar 7 by compression between two rollers 22 to form a zone 15 of reduced cross-section shaped by the rollers.

(23) FIG. 8 shows two examples of flexible copper strips 30 for joining the block 20 to the external bus 40. Each flexible strip 30 is made up of an indented or ribbed or braided copper strip 32 having at either end a solid copper connector 34 for connection to the block 20 or the bus 40. The connectors 34 have a central circular opening for making the connection, so one end of the copper bar 7 can be bolted to one end of the flexible strip 30 or to the underside of a block 20, and the other end of the flexible strip 30 can be clamped to the main busbar 40.

(24) In order to realize a very low contact voltage over time, a special electrically-conductive metallic foam such as ECOCONTACT™ can be used at the copper-aluminium contact (30/20) and at the copper-copper contact (30/40).

(25) These copper flexible strips 30 can advantageously be used to replace current aluminium flexes. The advantages of copper flexes when compared to the aluminium flexes are numerous: Fast implementation Highly flexible easing the procedure Lower voltage drop Easy to find the right section No mechanical stress on the copper bar.

(26) The reduction of external voltage can be significant:

(27) FIGS. 9A and 9B illustrate a test set up for comparing the effects of a collector bar without a zone of reduced cross-section to one with a zone of reduced cross-section.

(28) As shown in FIG. 9A, a collector bar 7 without a zone of reduced cross-section is connected to an aluminium block 20 which is in turn connected to an external bus 40 by a flexible copper connector 30. FIG. 9B shows a comparable set-up except that the collector bar 7 has a zone 15 of reduced cross-section, formed namely by opposite pairs of grooves in the opposite sides of the two arms making up the collector bar 7. The two set-ups were subjected to identical test conditions and the temperature of the bars measured. The temperature at the end of the collector bars, i.e. at the location of the terminal cross-piece was respectively 241° C. for the collector bar without a zone of reduced cross-section, and 218° C. for the collector bar with a zone of reduced cross-section.