CA2448311A1 - Aluminium electrowinning cells having a drained cathode bottom and an aluminium collection reservoir - Google Patents

Abstract

A cell for the electrowinning of aluminium comprises a plurality of anodes, in particular oxygen-evolving anodes (10), facing a series of pairs of cathode blocks (25), in particular graphite blocks, with aluminium-wettable drained cathode surfaces (20) placed across the cell and a longitudinal aluminium collection recess (35) which is located between the cathode blocks (25) and which extends along the cell and is at a lower level than the drained cathod e surfaces (20) so that during use aluminium produced on the drained cathode surfaces (20) drains into the aluminium collection recess (35). The aluminiu m collection recess (35) is defined by a separate reservoir body. in particula r an anthracite body, which is placed between the cathode blocks (25) and spac es them apart across the cell. The reservoir body (30) extends along the cell a nd has preferably a U-shaped cross-section across the cell.

Description

- 1 _ ALUMINIUM ELECTROWINNTNG CELLS HAVING A DRAINED CATHODE
BOTTOM AND AN ALUMINIUM COLLECTION RESERVOIR
Field of the Invention This invention relates to a cell for the electrowinning of aluminium from alumina dissolved in a-fluoride-containing molten electrolyte having an aluminium-wettable drained cathode surface and an aluminium reservoir, and a method to produce aluminium in such an aluminium electrowinning cell.
Background Art The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950°C is more than one hundred years old and still uses carbon anodes and cathodes.
Only recently has it become possible to coat carbon cathodes with a slurry which adheres to the carbon and becomes aluminium-wettable, as disclosed in US Patent 5,316,718 (Sekhar/de Nora) and US Patent 5,651,874 (de Nora/Sekar).
US Patent 5,683,559 (de Nora) proposed a new drained cathode design for aluminium production cells, where grooves or recesses were incorporated in the surface of blocks forming the cathode surface in order to channel the drained product aluminium. A specific embodiment provides an enhanced anode and drained cathode geometry where aluminium is produced between V-shaped anodes and cathodes and collected in recessed grooves.
WO98/53120 (Berclaz/de Nora) discloses an aluminium production cell provided with a cathode mass supported on a cathode shell or plate, the cathode mass having a horizontal drained cathode surface and a central channel extending along the cell for draining molten aluminium.
WO00/63463 (de Nora) discloses an aluminium production cell in which the drained cathode bottom is divided into four drained cathode sections by a longitudinally extending central aluminium evacuation groove and a central aluminium collection reservoir extending centrally across the cell on a spacer body located between and parallel to cathode blocks placed across the cell.

Objects of the Invention An object of the invention is to provide an aluminium electrowinning cell bottom and an aluminium electrowinning cell having an aluminium-wettable drained cathode which is made of conventional cell blocks which can be easily retrofitted in existing cells.
A further object of the invention is to provide an aluminium electrowinning cell having an aluminium collection reservoir from which molten aluminium can be tapped without the risk of aluminium freeing in the reservoir, and which can be easily retrofitted in existing cells.
A major object of the invention is to provide a modular assembly of an aluminium collection reservoir and cathode blocks for an aluminium production cell.
Yet another object of the invention is to provide a method to produce aluminium in an aluminium electrowinning cell provided with such a cell bottom.
Summary of the Invention The invention provides a cell for the electrowinning of aluminium from alumina. This cell comprises a plurality of anodes, in particular oxygen-evolving anodes, facing a series of pairs of cathode blocks with aluminium-wettable drained cathode surfaces placed across the cell and a longitudinal aluminium collection recess which is located between the cathode blocks and which extends along the cell and is at a lower level than the drained cathode surfaces so that during use aluminium produced on the drained cathode surfaces drains into the aluminium collection recess.
According to the invention, the aluminium collection recess is defined between the cathode blocks by a separate reservoir body which is placed between the blocks of each pair of cathode blocks spacing them apart across the cell and which extends along the cell.
The cathode blocks can be made of graphite and/or have an aluminium-wettable upper part. For example, the cathode blocks are coated with an aluminium-wettable layer.
Alternatively, the cathode blocks are made of aluminium wettable material.
Suitable aluminium-wettable materials and carbon materials for cathode blocks are disclosed in US Patent 5,651,874 (de Nora/Sekhar), and PCT applications W098/17842 (Sekhar/Duruz/liu), W001/42168 (de Nora/Duruz) and W001/42536 (Nguyen/Duruz/de Nora).
The cathode blocks may be covered with porous ceramic-based plates filled with molten aluminium whose surfaces serve as aluminium-wettable drained cathode surfaces on which aluminium is produced and from which aluminium drains into the collection reservoir.
The ceramic-based plates are preferably made of materials which are resistant and inert to molten aluminium.
The inert and resistant ceramic material may comprise at least one oxide selected from oxides of aluminium, zirconium, tantalum, titanium, silicon, niobium, magnesium and calcium and mixtures thereof, as a simple oxide and/or in a mixed oxide, for example an aluminate of zinc (~nA104) or titanium (TiA105). Other suitable inert and resistant ceramic materials can be selected amongst nitrides, carbides and borides and oxycompounds thereof, such as aluminium nitride, AlON, SiAlON, boron nitride', silicon nitride, silicon carbide, aluminium borides, alkali earth metal zirconates and aluminiumates, and their mixtures.
Preferably, the ceramic-based plates contain an aluminium-wetting agent. Suitable wetting agents include metal oxides which are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from the metal oxide and/or partly oxidised metal, such as manganese, iron, cobalt, nickel, copper, zinc, molybdenum, lanthanum or other rare earth metals or combinations thereof.
Suitable materials for producing the openly porous ceramic-based plates are described in US Patent 4,600,481 (Sane/ln~h.eeler/Gagescu/Debely/AdorianlDerivaz) and PCT/IB02/00668 (de Nora).
The aluminium-filled ceramic-based plates may extend from the cathode blocks over part of the aluminium collection recess, thereby the surface area of the aluminium-wettable drained cathode surface is increased.
The reservoir body may be made of anthracite or other carbonaceous material.

The reservoir body can be made of a plurality of sections assembled end-to-end longitudinally along the cell and/or side-by-side across the cell.
In one embodiment, the reservoir body has a generally U-shaped cross-section across the cell. The aluminium collection recess can have a triangular, rectangular or curved cross-section or a variation thereof which is suitable for the collection of aluminium. For example, the aluminium collection recess is generally U-shaped with rounded lower corners and/or an outwardly curved upper part in cross-section In another embodiment, the reservoir body has a rectangular cross-section with a planar upper surface which is at a level below the drained cathode surfaces and which constitutes the bottom surface of the aluminium collection recess . Upper parts of end faces of the cathode blocks may form the lateral surfaces of the aluminium collection recess.
The bottom part of the aluminium collection recess may be horizontal or sloping longitudinally along the cell down towards one end of the cell or the centre of the cell.
Preferably, the anodes are oxygen-evolving anodes, such as metal-based anodes, in particular metal-based anodes having an oxide-based outer part. Such anodes can be made of an iron alloy comprising nickel and/or cobalt whose surface may be oxidised.
Suitable metal-based anodes are disclosed in W000/06802, WO00/06803 (both in the name of Duruz/de Nora/
Crottaz), WO00/06804 (Crottaz/Duruz), W001/42535 (Duruz/
de Nora), W001/42534 (de Nora/Duruz) and W001/42536 (Duruz/
Nguyen/de Nora). Further oxygen-evolving anode materials are disclosed in W099/36593, W099/36594, WO00/06801, W000/06805, WO00/40783 (all in the name of de Nora/Duruz), WO00/06800 (Duruz/de Nora), W099/36591 and W099/36592 (both in the name of de Nora) .
Oxygen-evolving anodes may have an electrochemically active part coated with a slowly soluble protective layer, or a protective layer made of one or more cerium compounds, in particular cerium oxyfluoride, as disclosed in US Patents 4,614,569 (Duruz/Derivaz/Debely/ Adorian), 4,680,094 (Duruz), 4,683,037 (Duruz) and 4,966,674 (Bannochie/

Sheriff), which can be continuously replenished by in-situ electrodeposition thereon of the cerium compound(s).
The invention also relates to a method of producing . aluminium in a cell as described above. This method comprises passing an electrolysis current between the anodes and facing drained cathode surfaces in an electrolyte containing dissolved alumina to evolve gas, such as oxygen, at the anodes and produce aluminium on the drained cathode surfaces. The produced aluminium draining from the drained cathode surfaces into the aluminium collection recess defined by the separate reservoir body.
As mentioned above, the anodes may be oxygen-evolving and can be coated with a protective layer, such as a layer of one or more cerium compounds, in particular cerium oxyfluoride, in which case an amount of cerium species is preferably maintained in the electrolyte to maintain the protective cerium-based layer.
A further aspect of the invention relates to a bottom of a cell for the electrowinning of aluminium from alumina. This cell bottom comprises a series of pairs of cathode blocks with aluminium-wettable drained cathode surfaces placed thereacross and a longitudinal aluminium collection recess which is located between the cathode blocks and which extends along the cell bottom and is at a lower level than the drained cathode surfaces so that during use aluminium produced on the drained cathode surfaces drains into the aluminium collection recess.
According to the invention, the aluminium collection recess is defined between the cathode blocks by a separate reservoir body which. is placed between the cathode blocks spacing them apart across the cell bottom and which extends along the cell bottom.
The cell bottom may comprise any of the above cell bottom-related features or any combination thereof.
Another aspect of the invention relates to a method of producing aluminium in a cell comprising a cell bottom as described above. This method comprises passing an electrolysis current between anodes and the drained cathode surfaces of the cell bottom in an electrolyte containing dissolved alumina to evolve gas at the anodes and produce aluminium on the drained cathode surfaces. The produced aluminium drains from the drained cathode surfaces into the aluminium collection recess defined by the separate reservoir.
Whereas it is preferred to use non-carbon anodes to evolve oxygen during use as mentioned above, it is also possible to use carbon anodes on which carbon dioxide is produced during use.
Brief Description of the Drawings The invention will be further described by way of example with reference to the accompanying schematic drawing, in which Figure 1 illustrates a drained-cathode cell having an aluminium collection reservoir in accordance with the invention.
Detailed Description The cell shown in Figure 1 comprises a plurality of pairs of oxygen-evolving anodes 10 dipping in a molten electrolyte 5 and facing a series of pairs of cathode blocks with aluminium-wettable drained cathode surfaces 20 spaced apart across the cell and a separate longitudinal 20 reservoir body 30 which is located between the spaced apart blocks 25. The reservoir body 30 has an upper surface which defines a central aluminium collection recess 35. This recess 35 extends along and is at a lower level than the drained cathode surfaces 20 so that during use aluminium 25 produced on the drained cathode surfaces 20 drains into the aluminium collection recess 35.
The cathode blocks 25 are made of graphite and have a reduced height, e.g. 30 cm, and are coated with an aluminium-wettable layer 22 which protects the graphite from erosion and wear. Suitable aluminium-wettable layers are disclosed in the above mentioned US Patent 5,651,874, W098/17842, W001/42168 and W001/42531.
As shown in Figure 1, the reservoir body 30 is generally U-shaped and the aluminium collection recess 35 has rounded lower corners and an outwardly curved upper part.
The reservoir body 30 is made of two generally L-shaped sections 31 assembled across the cell. The reservoir sections 31 are made of anthracite-based material. The aluminium-wettable layer 22 extends in the recess 35 to protect the reservoir body 30 during use against wear and sodium intercalation.
As shown in Figure 1, the reservoir body 30 extends below the cathode blocks 25 into the refractory and insulating material 26 of the cell bottom permitting a maximisation of the capacity of the aluminium collection recess 35.
Furthermore, the reservoir body 30 has a solid base 32 which extends from above to below the bottom face of the cathode blocks 25 and provides sufficient mechanical resistance to keep the blocks 25 properly spaced. apart across the cell when exposed to thermal expansion during start-up of the cell and normal operation. As shown in dotted lines in the upper part of the reservoir body 30, longitudinally spaced apart spacer bars 33 placed across the reservoir body 30 may provide additional mechanical strength to the reservoir body 30. Such spacer bars 33 can be made of carbon material coated with an aluminium-wettable protective layer.
The cathode blocks 25 are covered with porous ceramic-based plates 21 which are filled with molten aluminium and which form the aluminium-wettable drained active cathode surfaces 20 on the cathode blocks 25. As shown in Figure 1, the aluminium-filled ceramic-based plates 21 extend from the cathode blocks 25 over part of the aluminium collection recess 35. Thus, during use projecting parts of the aluminium-wettable drained active cathode surfaces 20 are located above the aluminium collection recess 35.
The openly porous plates 21 are spaced apart over the aluminium collection recess 35 to leave an access for the tapping of molten aluminium 60 through a conventional tapping tube. The spacing between the openly porous plates 21 over the aluminium collection recess 35 can be much smaller along the remaining parts of the recess 35, thereby maximising the surface area of the active cathode surfaces 20.
The cell shown in Figure 1 comprises a series of corner pieces 42 made of the openly porous material of the plates 21 and filled with aluminium and placed at the periphery of the cell bottom against sidewalls 40. The sidewalk 40 and the surface of the electrolyte 5 are covered with a ledge _ g _ and a small crust of frozen electrolyte 6. The cell is fitted with an insulating cover 45 above the electrolyte crust 6. Further details of suitable covers are disclosed in WO99/02763 (de Nora/Sekhar), W001/31086 (de Nora/Duruz) and PCT/IB02/00669 (de Nora/Berclaz).
The cell is also provided with exhaust pipes (not shown) that extend through the cover 45 for the removal of gases produced during electrolysis.
The cell~comprises alumina feeders 15 with feeding tubes 16 that extend through the insulating cover 45 between the anodes 10. The alumina feeders 15 are associated with a crust breaker (not shown) for breaking the crust 6 underlying the feeding tube 16 prior to feeding.
In a variation, the insulating material of the sidewalk 40 and cover 45 may be sufficient to prevent formation of any ledge and crust of frozen electrolyte. In such a case, the sidewalls 40 are preferably completely shielded from the molten electrolyte 5 by a lining of the aforesaid openly porous material filled with aluminium.
The anodes 10 are preferably made of electrolyte resistant inert metal-based material. Suitable metal-based anode materials include iron alloys comprising nickel and/or cobalt which may be heat-treated in an oxidising atmosphere.
Suitable anode designs which provide optimal cell operation are disclosed in WO00/40781 and WO00/40782 (both in the name of de Nora).
The lifetime of the anode may be increased by a protective coating made of cerium compounds, in particular cerium oxyfluoride. Such coatings and cell operation therewith are disclosed in the above mentioned US Patents 4,614,569, 4,680,094, 4,683,037 and 4,966,674.
To reduce the dissolution of the anodes 10 in the electrolyte, the cell may be operated with an electrolyte 5 at reduced temperature, typically from about 850° to 940°C, preferably from 880° to 930°C. Operating with an electrolyte at reduced temperature reduces the solubility of oxides, in particular of alumina. For this reason, it is advantageous to enhance alumina dissolution in the electrolyte 5.
Enhanced alumina dissolution may be achieved by utilising an alumina feed device which sprays and _ g _ distributes alumina particles over a large area of the surface of the molten electrolyte 5. Suitable alumina feed devices are disclosed in greater detail in W000/63464 (de Nora/Berclaz). Furthermore, the cell may comprise means (not shown) to promote circulation of the electrolyte 5 from and to the anode-cathode gap to enhance alumina dissolution in the electrolyte 5 and to maintain in permanence a high concentration of dissolved alumina close to the active surfaces of anodes 10, for example as disclosed in W000/40781 (de Nora).
During operation of the cells shown in Figure 1, alumina dissolved in the electrolyte 5 is electrolysed to produce oxygen on the anodes 10 and aluminium 60 on the drained cathode surfaces 20. The product aluminium 60 drains from the cathode surfaces 20 into the reservoir 30 from where it can be tapped.

Claims (20)

1. A cell for the electrowinning of aluminium from alumina comprising a plurality of anodes, in particular oxygen-evolving anodes, facing a series of pairs of cathode blocks with aluminium-wettable drained cathode surface s placed across the cell and an aluminium collection recess which is located between cathode blocks at a lower level than the drained cathode surfaces so that during use aluminium produced on the drained cathode surfaces drains into the aluminium collection recess, characterised in that the aluminium collection recess is a longitudinal recess that extends along the cell and that is defined between the cathode blocks by a separate reservoir body which is placed between the cathode blocks spacing them apart across the cell and which extends along the cell.

2. The cell of any preceding claim, wherein the cathode blocks are made of graphite.

3. The cell of any preceding claim, wherein the cathode blocks have an aluminium-wettable upper part.

4. The cell of claim 3, wherein the cathode blocks are coated with an aluminium-wettable layer.

5: The cell of claim 1, 2 or 3, wherein the cathode blocks are made of aluminium-wettable material.

6. The cell of any preceding claim, which comprises porous ceramic-based plates placed on the cathode blocks and filled with molten aluminium.

7. The cell of claim 6, which comprises aluminium-filled ceramic-based plates that extend from the cathode blocks over part of the aluminium collection reservoir.

8. The cell of any preceding claim, wherein the reservoir body is made of anthracite.

9. The cell of any preceding claim, wherein the reservoir body is made of a plurality of sections assembled end-to-end longitudinally along the cell.

10. The cell of any preceding claim, wherein the reservoir body is made of at least two sections assembled side-by-side across the cell.

11. The cell of any preceding claim, wherein the reservoir body has a generally U-shaped cross-section across the cell.

12. The cell of claim 11, wherein the aluminium collection recess has rounded lower corners.

13. The cell of claim 11 or 12, wherein the aluminium collection recess has an outwardly curved upper part.

14. The cell of any preceding claim, wherein the anodes are oxygen-evolving anodes, such as metal-based anodes.

15. The cell of claim 14, wherein the anodes are.metal-based and have an electrochemically active outer oxide-based part.

16. The cell of claim 14 or 15, wherein the oxygen-evolving anodes are coated with a protective layer, such as a layer made of one or more cerium compounds, in particular cerium oxyfluoride.

17. A method of producing aluminium in a cell as defined in any preceding claim, said method comprising passing an electrolysis current between, the anodes and facing drained cathode surfaces. in an electrolyte containing dissolved alumina. to evolve gas, in particular oxygen, at the anodes and produce aluminium on the drained cathode surfaces, the produced aluminium draining from the drained cathode surfaces into the aluminium collection recess defined by the separate reservoir body.

18. The method of claim 17, wherein the anodes are coated with an oxygen-evolving protective layer, such as a layer made of one or more cerium compounds, in particular cerium oxyfluoride, that can be maintained by maintaining an amount of cerium species in the electrolyte.

19. A bottom of a cell for the electrowinning of aluminium from alumina comprising a series of pairs of cathode blocks with aluminium-wettable drained cathode surfaces placed thereacross and a aluminium collection recess which is located between cathode blocks at a lower level than the drained cathode surfaces so that during use aluminium produced on the drained cathode surfaces drains into the aluminium collection recess, characterised in that the aluminium collection recess is a longitudinal recess that extends along the cell bottom and that is defined between the cathode blocks by a separated reservoir body which is placed between the cathode blocks spacing them apart across the cell bottom and which extends along the cell bottom.

20. A method of producing aluminium in a cell comprising a cell bottom as defined in claim 19, said method comprising passing an electrolysis current between anodes and the drained cathode surfaces of the cell bottom in an electrolyte containing dissolved alumina to evolve gas at the anodes and produce aluminium on the drained cathode surfaces, the produced aluminium draining from the drained cathode surfaces into the aluminium collection recess defined by the separate reservoir.