NZ529852A - Aluminium electrowinning cells having a drained cathode bottom and an aluminium collection reservoir - Google Patents

Abstract

A cell for the electrowinning of aluminium has a cell bottom that comprises: a series of carbon cathode blocks (25) having an aluminium-wettable upper surface (22); aluminium-wettable openly porous plates (21) which are placed on the upper surfaces of the carbon cathode blocks and which are filled with molten aluminium to form an aluminium-wetted drained active cathode surface (20) above the upper surfaces of the carbon cathode blocks ; and a recess (35) formed in a reservoir body (30) and located at a level below the upper surfaces of the carbon cathode blocks and which during use collects molten aluminium (60) drained from the aluminium-wettable drained active cathode surface. The openly porous plates on the upper surfaces of the carbon cathode blocks extend over at least part of the recess such that part of the aluminium-wetted drained active cathode surface is located over the aluminium collection recess.

Description

529852 WO 02/097169 PCT/IB02/01884 ALUMINIUM ELECTROWINNING 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 a cell bottom 5 with 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 10 the electrolysis of alumina, dissolved in molten cryolite containing salts, 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 15 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 20 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 25 is produced between V-shaped anodes and cathodes and collected in recessed grooves.

W098/53120 (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 3 0 drained cathode surface and a central channel extending along the cell for draining molten aluminium.

WOOO/63463 (de Nora) discloses an aluminium production cell in which the drained cathode bottom is divided into four drained cathode sections by a 35 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.

Previously, it had been proposed to replace the carbon material of the cathodes of aluminium production 5 cells with ceramic material. For example, US Patent 4,560,448 (Sane/Wheeler/Kuivila) discloses a porous component made of aluminium repellent material covered with an aluminium-wettable metal boride coating which during use is maintained by saturating the molten aluminium 10 infiltrating the porous component with coating constituents. US Patent 4,650,552 (de Nora/Gauger/Fresnel/Adorian/Duruz) discloses an aluminium production cell component produced from a powder mixture of alumina and aluminium. US Patent 4,600,481 (Sane/Wheeler/Gagescu/Debely/Adorian/Derivaz) discloses a component of an aluminium production cell which is made of an openly porous matrix, e.g. an alumina matrix, , filled with molten aluminium. The openly porous matrix may comprise an aluminium-wettable coating made of a boride or nickel. The infiltration of the matrix with aluminium is 20 carried out at a temperature of 1000° to 1500°C.

Objects of the Invention It is an object of the invention to provide a cell bottom and an aluminium electrowinning cell, in particular with oxygen-evolving anodes, having an aluminium-wettable 25 drained cathode bottom and an aluminium collection reservoir from which molten aluminium can be tapped.

Another object of the invention is to provide an aluminium electrowinning cell having a cell bottom with an aluminium collection recess and an aluminium-wettable 30 cathode surface of great surface area.

A major object of the invention is to provide an aluminium electrowinning cell having a cell bottom with a drained cathode and an aluminium collection recess, the aluminium production yield relative to surface area of the 35 cell bottom being maximised.

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 freezing in the reservoir, and which 40 can be easily retrofitted in existing cells.

Summary of the Invention The invention provides a cell for the electrowinning of aluminium from alumina, in particular alumina dissolved in a fluoride-containing molten electrolyte. The cell has a 5 cell bottom that comprises: a cathode, in particular a series of carbon cathode blocks, having an aluminium-wettable upper surface; aluminium-wettable openly porous plates which are placed on the upper surface of the cathode and which during use, and preferably also prior to use, are 10 filled with molten aluminium to form an aluminium-wetted drained active cathode surface above the upper surface of the cathode; and a recess which extends at a level below the upper surface of the cathode and which during use collects molten aluminium drained from the aluminium-wettable drained 15, active cathode surface.

In accordance with the invention, openly porous plates on the upper surface of the cathode extend over at least part of the recess such that part of the aluminium-wetted drained active cathode surface is located over at 20 least part of the recess.

During use, the openly porous aluminium-wettable plates are filled with aluminium and a bottom part of the plates is usually wetted by molten aluminium which also wets the aluminium-wettable surface of the cathode. The aluminium 25 in the openly porous plates ensures optimal electrical conductivity from the cathode to the entire aluminium-wettable active cathode surface on the openly porous plates, even where the plates extend over the aluminium collection recess.

The openly porous plates extending over at least part of the aluminium collection recess preferably cover a substantial part of the recess to maximise the surface area of the aluminium-wettable active cathode surface. For instance the plates may leave only a small vertical opening 35 above the recess sufficient to let product aluminium drain from the aluminium-wettable active cathode surface through the opening into the aluminium collection recess. For instance the opening is a gap, typically located between one or more pairs of plates placed over and across the recess, 40 preferably having a width of a couple or a few centimetres. Usually, an access to the aluminium collection recess is left for the tapping of molten aluminium. Such an access may _ 4 _ be provided by placing openly porous plates which do not extend over the aluminium collection recess at the tapping location or by providing a hole of sufficient size, e.g. from about 20 to 40 cm diameter, in or between openly porous 5 plates covering the aluminium collection recess at the tapping location.

The aluminium-wettable openly porous plates are preferably made of ceramic-based materials which are resistant and inert to molten aluminium. The inert and 10 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 (ZnAl04) or titanium 15 (TiAl05) . 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 20 and aluminiumates, and their mixtures.

Preferably, the aluminium-wettable openly porous 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, 25 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 30 plates are described in US Patent 4,600,481 (Sane/Wheeler/Gagescu/Debely/Adorian/Derivaz) and PCT/IB02/00668 (de Nora).

The cathode can be made of carbon cathode blocks arranged in pairs across the cell bottom, spaced apart or 35 end-to-end. Alternatively, the cathode can be made of carbon cathode blocks, each cathode block extending across substantially the entire cell bottom.

The cathode can be made of graphite or another carbonaceous material.

The cathode may be coated with a layer of aluminium-wettable material which forms the aluminium-wettable upper cathode surface.

Suitable aluminium-wettable layers and carbonaceous 5 materials for the cathode are disclosed in US Patent 5,651,874 (de Nora/Sekhar), and PCT applications W098/17842 (Sekhar/Duruz/liu), WOOl/42168 (de Nora/Duruz) and WO01/42536 (Nguyen/Duruz/de Nora).

In one embodiment, at least part of the aluminium 10 collection recess is formed in a reservoir body that is juxtaposed to the cathode. This body may comprise anthracite or another carbonaceous material. For example, the reservoir body is located between the blocks of at least one pair of carbon cathode blocks and spaces them apart across the cell 15 bottom. The aluminium collection recess may be generally U-shaped.

The cell may comprise a series of oxygen-evolving anodes facing the cathode. Suitable oxygen-evolving metal-based anodes are disclosed in W000/06802, W000/06803 (both 20 in the name of Duruz/de Nora/Crottaz) , WOOO/06804 (Crottaz/ Duruz), WOOl/42535 (Duruz/de Nora) , W001/42534 (de Nora/ Duruz) and WOOl/42536 (Duruz/Nguyen/de Nora). Further oxygen-evolving anode materials are disclosed in W099/36593, W099/36594, WOOO/O68OI, W000/06805, W000/40783 (all in the 25 name of de Nora/Duruz) , WOOO/O68OO (Duruz/de Nora) , W099/36591 and W099/36592 (both in the name of de Nora).

Oxygen-evolving anodes may be coated with a protective layer made of one or more cerium compounds, in particular cerium oxyfluoride, as disclosed in US Patents 30 4,614,569 (Duruz/Derivaz/Debely/Adorian), 4,680,094 (Duruz), 4,683,037 (Duruz) and 4,966,674 (Bannochie/Sheriff).

Another aspect of the invention relates to a bottom of a cell for the electrowinning of aluminium from alumina. This cell bottom comprises a cathode, in particular a series 35 of carbon cathode blocks, having an aluminium-wettable upper surface, aluminium-wettable openly porous plates which are placed on upper surface of the cathode, and a recess which extends at a level below the upper surface of the cathode. Openly porous plates covering the upper surface of the 40 cathode extend over at least part of the aluminium collection recess.

A further aspect of the invention relates to a cell for the electrowinning of aluminium comprising a bottom as described above. In such a cell, the aluminium-wettable openly porous plates are filled with molten aluminium to 5 form an aluminium-wetted drained active cathode surface above the upper surface of the cathode.

Yet a further aspect of the invention relates to a method of producing aluminium in a cell as described above. This method comprises passing an electrolysis current 10 between anodes and the aluminium-wetted drained active cathode surface in an electrolyte comprising dissolved alumina to evolve gas at the anodes and produce aluminium on the drained active cathode surface. The produced aluminium drains into the aluminium collection recess from the drained 15 active cathode surface over the openly porous plates which extend over at least part of the recess.

As mentioned above the cell may evolve oxygen on the anodes. The anodes may be coated with a protective layer, such as a layer made of one or more cerium compounds, in 20 particular cerium oxyfluoride, in which case the protective layer may be maintained by maintaining an amount of cerium species in the electrolyte, as disclosed in the above mentioned US Patents 4,614,569, 4,680,094, 4,683,037 and 4,966,674. Furthermore, the cell may be operated with an 25 electrolyte at reduced temperature, e.g. 880° to 930° or even 940°C, to reduce dissolution of the metal-based anodes.

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 30 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 one embodiment of a 35 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 oxygen evolving anodes 10 dipping in a molten electrolyte 5 40 and facing a cell bottom. The cell bottom comprises: a series of pairs of spaced apart carbon cathode blocks 25 WO 02/097169 PCT/IB02/01884 placed across the cell and having an aluminium-wettable upper surface 22 formed by an aluminium-wettable layer. The upper surfaces 22 are covered with aluminium-wettable openly porous plates 21 which are filled with molten aluminium to 5 form an aluminium-wetted drained - active cathode surface 20 above the upper surfaces 22 of the carbon cathode blocks 25.

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 which forms the upper surface 22 10 and 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 WOOl/42531. The aluminium-wettable openly porous plates 21 covering the coated cathode blocks 25 can be made of the 15 material disclosed in PCT/IB02/00668 (de Nora).

The cell bottom further comprises a centrally-located recess 35 which extends at a level below the upper surfaces 22 of the carbon cathode blocks 25 and which during use collects molten aluminium 60 drained from the aluminium-20 wettable drained active cathode surface 20.

The aluminium collection recess 35 is formed in a reservoir body 30 which is placed between the blocks 25 of each pair of cathode blocks and spaces them apart across the cell. As shown in Figure 1, the recess 35 formed in the 25 reservoir body 30 is generally U-shaped with 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 30 aluminium-wettable layer forming the upper surfaces 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 35 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 40 cathode blocks 25 and provides sufficient mechanical resistance to keep the blocks 25 properly spaced apart WO 02/097169 PCT/EB02/01884 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 5 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.

In accordance with the invention, the openly porous 10 plates 21 placed on the upper surfaces 22 of the carbon cathode blocks 25 and located in the central region of the cell bottom extend over part of the aluminium collection recess 35 so that during use the protruding part of the aluminium-wetted drained active cathode surface 20 is 15 located over the 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 through a conventional tapping tube. The spacing between the openly porous plates 20 21 over the aluminium collection recess can be much smaller along the remaining parts of the recess 35, thereby maximising the surface area of the active cathode surface 20.

The cell shown in Figure 1 comprises a series of corner 25 pieces 41 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 sidewalls 40 and the surface of the electrolyte 5 are covered with a ledge and a small crust of frozen electrolyte 6. The cell is 30 fitted with an insulating cover 45 above the electrolyte crust 6. Further details of suitable covers are disclosed in W099/02763 (de Nora/Sekhar), W001/31086 (de Nora/Duruz) and PCT/IB02/00669 (de Nora/Berclaz).

The cell is also provided with exhaust pipes (not 35 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 40 crust breaker (not shown) for breaking the crust 6 underlying the feeding tube 16 prior to feeding.

WO 02/097169 PCT/IB02/01884 In a variation, the insulating material of the sidewalls 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 5 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 inqlude iron alloys comprising nickel and/or 10 cobalt which may be heat-treated in an oxidising atmosphere.

Suitable anode designs which provide optimal cell operation are disclosed in WOOO/4D781 and W000/40782 (both in the name of de Nora) .

The lifetime of the anode may be increased by a 15 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 20 electrolyte, the cell may be operated with an electrolyte 5 at reduced temperature, typically from about 850° to 940°C, preferably from 880° to 93 0°C. Operating with an electrolyte at reduced temperature reduces the solubility of oxides, in particular of alumina. For this reason, it is advantageous 25 to enhance alumina dissolution in the electrolyte 5.

Enhanced alumina dissolution may be achieved by utilising an alumina feed device which sprays and distributes alumina particles over a large area of the surface of the molten electrolyte 5. Suitable alumina feed 30 devices are disclosed in greater detail in WOOO/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 35 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 40 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 over the openly porous plates 21 that extend over part of the reservoir 30 into the reservoir 30 from where it can be tapped.

Hence, aluminium is produced on the drained active cathode surface 20 which covers not only the cathode blocks 25 but also part of the reservoir 3 0, thereby maximising the useful aluminium production area (i.e.' the drained cathode surface 22) of the cell. ritt *3 07 04 200 CLMSPAMD - li - 02730626 IB02018^

Claims (17)

1. A cell for the electrowinning of aluminium from alumina having a cell bottom that comprises: a cathode, in particular a series of carbon cathode blocks, having an aluminium-wettable upper surface; aluminium-wettable openly porous plates which are placed on the upper surface of the cathode and which are filled with molten aluminium to form an aluminium-wetted drained active cathode surface above the upper surface of the cathode; and - a recess at a level below the upper surface of the cathode and which during use collects molten aluminium drained from the aluminium-wettable drained active cathode surface, wherein openly porous plates on the upper surface of the cathode extend over part of the recess such that part of the aluminium-wetted drained active cathode surface is located over the recess.

2. The cell of claim 1, wherein the cathode is made of carbon cathode blocks arranged in pairs across the cell bottom.

3. The cell of preceding claim 1 or 2, wherein the cathode is made of graphite.

4. The cell of any preceding claim, wherein the cathode is coated with a layer of aluminium-wettable material forming the aluminium-wettable upper cathode surface.

5. The cell of any claim, wherein at least part of the aluminium collection recess is formed in a reservoir body that is juxtaposed to the cathode.

6. The cell of claim 5, wherein the reservoir body comprises anthracite.

7. The cell of claim 5 or 6 when depending on claim 2, wherein the reservoir body is located between the blocks of at least one pair of carbon cathode blocks and spaces them apart across the cell bottom.

8. The cell of any preceding claim, wherein the aluminium collection recess is generally U-shaped. AMENDED SHEET Pnrted 07 04 2003 CLMSPAM . ■ • " 12 - * ,

9. The cell of any preceding claim, which comprises a series of oxygen-evolving anodes facing the aluminium-wettable openly porous plates.

10. The cell of claim 9, 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.

1,1, A method of producing aluminium in a cell as defined in any preceding claim, comprising passing an electrolysis current between anodes and the aluminium-wetted drained active cathode surface in an electrolyte conprising dissolved alumina to evolve gas at the anodes and produce 0 aluminium on the drained active cathode surface, the produced aluminium draining into the aluminium collection recess from the drained active cathode surface over the openly porous plates which extend over part of the recess.

12. The method of claim 11, comprising evolving oxygen on the anodes.

13. The method of claim 12, wherein the anodes have a protective coating, such as a coating made of one or more cerium compounds, in particular cerium oxyfluoride, said method comprising maintaining an amount of cerium species in the electrolyte to maintain the protective layer.

14. The method of any one of claims 11 to 13, wherein the electrolyte is at a temperature in the range from 850° to 940°C.

15. A bottom of a cell for the electrowinning of aluminium from alumina, comprising a cathode, in particular a series of carbon cathode blocks, having an aluminium-wettable upper surface, aluminium-wettable openly porous plates which are placed on the upper surface of the cathode, and an aluminium collection recess which extends at a level below the upper surface of the cathode, wherein openly porous plates covering the upper surface of the cathode extend over part of the aluminium.collection recess.

16. A cell for the electrowinning of aluminium comprising a bottom as defined in claim 15, wherein the aluminium-wettable openly porous plates are filled with molten aluminium to form an aluminium-wetted drained active cathode surface above the upper surface of the cathode. a AMENDED SHEET ("jfnrm CLMSPAMD- 02730626-IB02018$ - 13 -

17. A method of producing aluminium in a cell as defined in claim 16, comprising passing an electrolysis current between anodes and the aluminium-wetted drained active cathode surface in an electrolyte comprising dissolved alumina to evolve gas at the anodes and produce aluminium on the drained active cathode surface, the produced aluminium draining into the aluminium collection recess from the drained active cathode surface over the openly porous plates which extend over part of the recess. AMENDED SHEET 29 03 2003 14 19. 20. # 21. A cell according to claim 1 substantially as herein described or exemplified. A method according to claim 11 substantially as herein described or exemplified. A bottom of a cell according to claim 15 substantially as herein described or exemplified. A Cell according to claim 16 substantially as herein described or exemplified. A method according to claim 17 substantially as herein described or exemplified. END OF CLAIMS