EP2064370A2

EP2064370A2 - Aluminium collection in electrowinning cells

Info

Publication number: EP2064370A2
Application number: EP07789757A
Authority: EP
Inventors: Thinh T. Nguyen; Frank Schnyder; René VON KAENEL; Vittorio De Nora
Original assignee: Moltech Invent SA
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 2006-06-22
Filing date: 2007-06-20
Publication date: 2009-06-03
Also published as: WO2007148297A3; WO2007148297A2; AU2007262349A1; CA2653458A1; NO20090331L; US20100294671A1

Abstract

A cell for the electrowinning of aluminium comprises an electrolysis chamber (20) in which alumina is electrolysed to produce aluminium (30) and a collection reservoir (40,40' ) in which product aluminium is collected. The electrolysis chamber and the collection reservoir are in liquid communication so that aluminium produced in the electrolysis chamber can flow from the electrolysis chamber into the collection reservoir. The electrolysis chamber contains one or more metal-based anodes (15). Each anode has an active anodic surface (16) spaced above a facing cathodic surface (31) on which aluminium is produced. The cathodic surface is formed on a structural body (12) by a layer made of molten aluminium into which product aluminium is incorporated during operation. The anodic surface and the cathodic surface have a substantially constant operative position. The cell has means ( 60, 60', 61, 61', 62) for regulating the layer of molten aluminium so the layer forms a shallow or deep continuous cathodic pool (35) that extends continuously under the entire facing active anodic surface of at least one anode. The layer regulating means are arranged to maintain during operation the cathodic surface of the cathodic pool at a substantially constant position by periodic or continuous removal of molten aluminium from the aluminium pool to the collection reservoir at a rate corresponding substantially to the rate of production of the product aluminium that is incorporated into the aluminium pool.

Description

ALUMINIUM COLLECTION IN ELECTROWINNING CELLS

Field of the Invention

This invention relates to a cell for the electrowinning of aluminium having a dimensionally stable anode facing a cathodic aluminium surface spaced by an anode-cathode gap that is constant during operation.

Background Art

The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950⁰C is more than one hundred years old and still uses carbon anodes and cathodes that are covered by a cathodic pool of molten aluminium on surface of which aluminium is produced during cell operation.

It had been proposed to replace the carbon material of the cathodes of aluminium production cells with ceramic material, in particular metal borides.

US Patent 4,396,481 (Pavlek/Lagler) discloses an aluminium electrowinning cell with a cathodic bottom made of granular refractory material, such as TiB₂, which is covered with a layer of molten aluminium. The cell bottom includes a trough for the dissolution of alumina, the trough having sidewalls projecting above the molten aluminium layer so that molten aluminium does not flow into the trough.

US Patent 4,231,853 (Rahn) discloses spaced apart cathodic boride tiles secured on the carbon floor of an aluminium electrowinning cell. In one embodiment, the cathodic tiles have inclined drained cathode surfaces on which aluminium is produced and drains to a recessed aluminium collection reservoir in the carbon floor. The collection reservoir has walls that protrude slightly above the carbon floor so as to maintain a protective pad of molten aluminium on the carbon floor. The thickness of this pad is controlled so that the pad remains below the drained cathode surfaces.

US Patent 4,544,457 (Sane/Wheeler/Kuivila) discloses an openly porous sheath made of aluminium wettable material, in particular TiB₂, filled with molten aluminium. The sheath covers a cathode block located on the bottom of an aluminium electrowinning cell, aluminium being produced on the sheath and drained therefrom onto the cell bottom where it is collected. With this drained structure, no aluminium pool is formed on the cathode in which wave motion might cause a short circuit between anode and cathode.

More recently, it has 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 and 5,651,874 (both assigned to MOLTECH Invent S. A.). Further developments of aluminium-wettable materials for use in aluminium electrowinning cells are disclosed in WO01/42168, WO01/42531, WO02/070783, WO02/096830, WO02/096831 WO2004/092449, WO2005/068390 (all assigned to MOLTECH Invent S. A.).

The use of aluminium-wettable cathode surfaces - on which aluminium is drained instead of being accumulated into a shallow or deep pool on the cathode bottom - avoids fluctuations during operation of the cathodic active surface formed by the surface of the accumulated produced aluminium. This is desirable in particular when the cell operates with dimensionally stable anodes. In this case a constant anode- cathode gap can be maintained without having to adjust the position of the anode. To prevent variations of the position of the active cathodic surface, the aluminium produced cathodically can be drained from the cathode and evacuated to an aluminium collection reservoir.

US Patent 6,682,643 (assigned to MOLTECH Invent S. A.) discloses an aluminium production cell in which the drained cathode bottom is divided into four drained cathode _ O _

sections by a longitudinally extending central aluminium evacuation groove and a central aluminium collection reservoir extending centrally across the cell. WO02/070785, WO02/097168 and WO02/097169 (all assigned to MOLTECH Invent S. A.) disclose further aluminium electrowinning cells in which product aluminium drained is from a horizontal cathode and collected in a recessed central reservoir.

It has also been proposed to provide drained-cathode aluminium production cells having an electrolysis chamber communicating with an aluminium collection chamber.

US2004/0011660 (Bradford/Barnett/Mezner) discloses an aluminium electrowinning cell having an electrolysis chamber with a vertical cathode on which aluminium is produced and drained. The product aluminium is evacuated through a hole in the bottom of the electrolysis chamber into an aluminium collection chamber that is located underneath the electrolysis chamber.

US Patent 6,419,812 (Beck/Brown) discloses an aluminium electrowinning cell having an electrolysis chamber with vertical cathodes on which aluminium is produced and drained. Product aluminium is conveyed along the cell by a collection tube extending under the vertical cathodes to a bottom part of a reservoir chamber next to the electrolysis chamber. Aluminium is forced up into the reservoir chamber by the pressure of the electrolyte height in the electrolysis chamber.

A drawback of known drained-cathode cells operating with dimensionally-stable anodes lies in the difficulty to achieve complete and permanent coverage of the drained cathode with product aluminium. Full coverage is needed to protect the cathode against molten electrolyte and to avoid non uniform wear of the cathode and non uniform distribution of the electrolysis current at the surface of the drained cathode and sub-optimal use of the anode and cathode surfaces to produce aluminium. Objects of the Invention

It is therefore a preferred object to solve the above problem.

A preferred object of the invention is to provide an aluminium electrowinning cell having a cathodic surface formed of molten aluminium which has a substantially constant position and on which aluminium is produced.

Another preferred object of the invention is to produce aluminium on a cathode which is covered with a shallow or deep pool of product aluminium having a substantially non fluctuating height (or depth) .

A further preferred object of the invention is to provide an aluminium electrowinning cell having dimensionally stable anodes with a substantially fixed position during use in the cell.

Summary of the Invention

The invention provides a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises an electrolysis chamber in which alumina is electrolysed to produce aluminium and a collection reservoir in which product aluminium is collected. The electrolysis chamber and the collection reservoir are in liquid communication so that aluminium produced in the electrolysis chamber can flow from the electrolysis chamber into the collection reservoir. The electrolysis chamber contains one or more metal-based anodes. Each anode has an active anodic surface spaced by an anode-cathode gap from a facing cathodic surface on which aluminium is produced. The cathodic surface is formed on a structural body by a layer made of molten aluminium into which product aluminium is incorporated during operation. The anodic surface and the cathodic surface have a substantially constant operative position so that the anode- cathode gap is substantially constant. In accordance with the invention, the cell comprises means for regulating the layer of molten aluminium so the layer forms a shallow or deep continuous cathodic pool that extends continuously under the entire facing active anodic surface of at least one anode and that is contained within a cathodic cavity on the structural cathodic body. The layer regulating means are arranged to maintain during operation the cathodic surface of the cathodic pool at a substantially constant position by periodic or continuous removal of molten aluminium from the aluminium pool to the collection reservoir at a rate corresponding substantially to the rate of production of the product aluminium that is incorporated into the aluminium pool.

The cathodic surface of the aluminium pool can be slightly curved due to surface tension effects. As a result of this slight curvature, the pool's height (or depth) may be not strictly uniform over its surface; however the height

(or depth) of the pool at various locations does not significantly fluctuate during operation. In such a case, if the surface of cathodic pool extends under several anodes, the position of individual anodes can be adjusted to such a slightly curved profile of the cathodic surface. In other words, the anodes located above a central part of the cathodic surface can be positioned slightly higher than the anodes that are situated at the periphery of the cathodic surface so as to adjust the anodes to the position of the facing portion of the underlying cathodic surface.

In one embodiment, the layer regulating means comprise a wall which delimits the cathodic cavity. The wall has an upper edge located at the level of the cathodic surface of the aluminium pool and is arranged to allow the flow over the edge of product aluminium to the collection reservoir that extends below the cathodic surface. The collection reservoir can have a generally U-shaped cross- section or a bottom part that extends underneath the cathodic surface. r

D

This embodiment can conveniently be retrofitted in existing cells without major modifications of the cell's structure .

In another embodiment, the cell has a reservoir chamber that contains the collection reservoir and that is adjacent to the electrolysis chamber and communicates therewith through an aluminium passage for the removal of molten aluminium from the aluminium pool to the collection reservoir. The layer regulating means comprise means for adjusting the amount of aluminium in the collection reservoir so that the position of the cathodic surface of the aluminium pool does not significantly fluctuate during operation. The layer regulating means may include means for measuring directly or indirectly the position of the cathodic surface, for example means for measuring the electrical characteristics of a current passing between the cathodic surface and at least one metal-based anode and/or one or more proximity sensors above the electrolyte for measuring the height of the electrolyte and/or aluminium pool. Furthermore, the layer regulating means can have a means to adjust the pressure above the aluminium stored in the collection reservoir.

This embodiment permits access to the product aluminium, e.g. for tapping, without having to access the electrolysis chamber which reduces de risk of unwanted interferences with the electrolysis process.

Typically, the molten aluminium in the collection reservoir is covered by a layer or molten electrolyte. This layer protects the product aluminium against oxidation and/or freezing.

Usually, the depth of the cathodic pool does not lie below 3 or 5 mm; in particular it can have a depth in the range of 1 to 15 cm such as 2 to 5 cm.

The cathodic cavity containing the cathodic pool and/or other cell components exposed to molten aluminium, and possibly also to the electrolyte, can have an aluminium- wettable surface, in particular made of an aluminium- wettable refractory material such as a titanium diboride or other boride based layer. The aluminium-wettable material advantageously includes one or more wetting agents, such as oxides of iron, copper and/or nickel. Examples of such materials are disclosed in US patents 5,364,513, 5,651,874, 6,436,250, and in PCT publications WO01/42168, WO01/42531, WO02/070783, WO02/096830 and WO02/096831, WO2004/092449, WO2005/068390 (all assigned to MOLTECH Invent S. A.).

Components which are only exposed to molten aluminium, e.g. the collection reservoir or non conductive parts of the cathodic cavity, can be covered with an electrically conductive material, e.g. aluminium-wettable material as mentioned above, and/or non conductive material. They can have a surface made of a non conductive ceramic material, such as aluminium oxide, silicon carbide, silicon nitride and/or boron nitride.

The active anodic surface can be the surface of an anode body that has a plurality of through passages for the flow of circulating electrolyte through the anode body from below to above the anode body and/or from above to below the anode body. The anode body may comprise a series of elongated members spaced apart by inter-member gaps which form said through passages, or the anode body may comprise a solid body, in particular a plate, which has through holes that form said through passages. Suitable anodes are disclosed in WO00/40781, WO00/40782, WO03/006716,

WO03/023091, WO03/023091 and WO2005/118916 (all assigned to MOLTECH Invent S. A.).

Suitable materials for metal-based, in particular oxygen-evolving, anodes include at least one metal selected from nickel, iron, cobalt and copper. For instance the anode has a metal oxide surface, in particular a surface containing at least one of iron oxide, nickel oxide and cobalt oxide. Suitable anode materials are disclosed in — o —

WO99/36591 and WO99/36592, WO99/36593 and WO99/36594, WO00/06800, WO00/06801, WO00/06802 and WO00/06803, WO00/06804, WO00/06805, WO00/40783 and WO01/42534, WO01/42536, and WO01/43208, WO02/070786, WO02/083990, WO02/083991, WO03/078695, WO03/087435, WO2004/018731, WO2004/024994, WO2004/044268 , WO2004/050956, WO2005/090641 and WO2005/090643 (all assigned to MOLTECH Invent S. A.). 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 4,614,569, 4,680,094, 4,683,037 and 4,966,674 (all assigned to MOLTECH Invent S. A. ) .

The invention further relates to a method of operating the above described cell to produce aluminium.

Usually, the cell's electrolyte is maintained at a temperature in the range of 700° to 1000⁰C, in particular 800 to 970⁰C such as 880 to 940°C.

The electrolyte can be a fluoride-containing electrolyte, for example as disclosed in WO00/06802, WO01/42535, WO02/097167, WO03/083176, WO2004/035871 ,

WO2004/074549 and WO2005/090642 (all assigned to MOLTECH

Invent S. A. ) .

The product aluminium is preferably maintained in a molten state in the collection reservoir, in particular above 700⁰C. When the reservoir is located in a chamber that is separate from the electrolysis chamber, the aluminium can be stored at the same or similar temperature as the electrolyte or at a lower temperature; in the other cases, the aluminium will usually be stored at substantially the same temperature as the molten electrolyte.

Brief Description of the Drawings

The invention will be further described by way of example with reference to the accompanying schematic drawings, in which: _ g _

- Figure 1 illustrates an aluminium electrowinning cell according to the invention having an electrolysis chamber separated from an aluminium collection reservoir chamber; and

- Figures 2 and 3 show further aluminium electrowinning cells according to the invention including an electrolysis chamber that has an aluminium collection reservoir .

Detailed Description

The aluminium electrowinning cell shown in Figure 1 has an electrolysis chamber 10 in which alumina dissolved in a molten electrolyte 20 is electrolysed to produce aluminium 30. The cell includes a collection reservoir in a reservoir chamber 40 in which product aluminium 30 is collected. The electrolysis chamber 10 is adjacent to the collection reservoir 40 and in liquid communication therewith via passage 45 so that aluminium 30 produced in the electrolysis chamber 10 can flow from the electrolysis chamber 10 into the collection reservoir 40. Passage 45 extends from the electrolysis chamber 10 to the collection reservoir 40 under a wall 50 that separates the chamber 10 and reservoir 40. Aluminium 30 collected in reservoir chamber 40 is covered with a protective layer of molten electrolyte 21.

The electrolysis chamber 10 contains dimensionally stable anodes 15. Each anode 15 has an active anodic surface

16 spaced by an anode-cathode gap 25 from a facing cathodic surface 31 on which aluminium 30 is produced. The cathodic surface 31 is formed on a cathode bottom 12 by a layer 35 made of molten aluminium 30 into which product aluminium is incorporated during operation. The anodic surface 16 and the cathodic surface 31 have a substantially constant operative position so that the anode-cathode gap 25 is substantially constant during operation.

Electrolyte 20 is maintained at a temperature in the range of 900 to 950⁰C so that a ledge 22 can form to protect cell sidewalls 13 and a crust 23 can be made to form so as to cover the electrolyte surface 24.

In accordance with the invention the cell comprises means 60,61,62 for regulating the layer of molten aluminium 30 so the layer 35 forms a shallow or deep continuous cathodic pool that extends continuously under the entire facing active anodic surfaces 16 and that is contained within a cathodic cavity on the cathode bottom 12.

The layer regulating means 60,61,62 are arranged to maintain during operation the cathodic pool 35 at a substantially constant (or non fluctuating) height by periodic or continuous removal of molten aluminium 30 from pool 35 to the collection reservoir 40 at a rate corresponding substantially to the rate of production of product aluminium 30 that is incorporated into the aluminium pool 35.

As shown in Fig. 1, the layer regulating means 60,61,62 include one or more vacuum pumps and pressure release valves 61 for adjusting the pressure in the reservoir chamber 40 above collected aluminium 30 and electrolyte layer 21, to thereby adjust the height and amount of collected aluminium 30 in chamber 40 so that the cathodic surface 31 of cathodic aluminium pool 35 remains at constant position over the cathodic bottom 12 during the production of aluminium 30 and its incorporation into cathodic layer 35. Pumps and valves 61 are controlled by a control device 60 which is itself connected to sensors 62 for measuring the electrical characteristics of cathodic bottom 12 and anodes 15 from which the height of the cathodic surface 31 of cathodic pool 35 is deducted, preferably taking into account any evolution of the bath composition during electrolysis for example by predictive models known in the art. Alternatively, this height can be measured using appropriate proximity sensors.

During cell operation, aluminium 30 is produced by the electrolysis of alumina dissolved in electrolyte 20 and the product aluminium 30 incorporated into cathodic pool 35. The distance between the anodic surfaces 16 and cathodic surface 31 is continuously monitored via sensors 62, pumps and valves 61 being operated via control device 60 so that aluminium 30 is evacuated from cathodic pool 35 to the reservoir chamber 40 via passage 45 at the rate at which aluminium 30 is produced in the anode-cathode gap 25 and incorporated into the cathodic pool 25.

The cells shown in Figures 2 and 3, in which the same numeric references designate the same elements, have recessed collection reservoirs 40' adjacent to and extending below the cathodic surface 31 of cathodic pool 35. Reservoirs 40' have a generally U-shaped cross-section. In Figure 2, reservoirs 40' are located along a peripheral part of the electrolysis chamber 10. In Figure 3, a reservoir 40' is located in a central part of the cell.

As shown in Figures 2 and 3, the cells' regulating means comprise a wall 60' that extends from reservoir 40' above cathodic bottom 12 adjacent cathodic pool 35. Wall 60' and cathodic bottom 12 form a cathodic cavity for containing the cathodic pool 35.

Wall 60' has an upper edge 61' at about the level of cathodic surface 31 and is arranged to allow the flow over edge 61' of product aluminium 30 to the collection reservoir 40 '.

During cell operation, aluminium 30 is produced by the electrolysis of alumina dissolved in electrolyte 20 and the product aluminium 30 is incorporated into cathodic pool 35. As aluminium 30 is produced and incorporated into cathodic pool 35, aluminium 30 flows over edges 61' from cathodic pool 35 into reservoirs 40 at the same rate as the rate of production of aluminium 30 and its incorporation into pool 35. Thus, the cathodic surface 31 formed by cathodic pool 35 is constantly maintained at the same level which is set by the level of edge 61' . In other words, the cathodic surface's position does not significantly fluctuate during operation, even though the cathodic surface 31 of pool 35 can be slightly curved due to surface tension effects, as shown in Figs. 2 and 3. As a result of this slight curvature, the pool's height (or depth) is not strictly uniform over its surface 31; however the height (or depth) of the pool 35 at various locations does not significantly fluctuate during operation.

When the surface 31 of cathodic pool 35 extends under several anodes 15, the position of individual anodes

15 can be adjusted to the slightly curved profile of cathodic surface 31. In other words, anodes 15 located above a central part of cathodic surface 31 can be positioned slightly higher than anodes 15 that are situated at the periphery of cathodic surface 31 to adjust to the height of the facing portion of the underlying cathodic surface 31.

Claims

1. A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising an electrolysis chamber in which alumina is electrolysed to produce aluminium and a collection reservoir in which product aluminium is collected, the electrolysis chamber and the collection reservoir being in liquid communication so that aluminium produced in the electrolysis chamber can flow from the electrolysis chamber into the collection reservoir, the electrolysis chamber containing one or more metal-based anodes, each anode having an active anodic surface spaced by an anode-cathode gap from a facing cathodic surface on which aluminium is produced, said cathodic surface being formed on a structural body by a layer made of molten aluminium into which product aluminium is incorporated during operation, said anodic surface and said cathodic surface having a substantially constant operative position so that the anode-cathode gap is substantially constant,

characterised in that the cell comprises means for regulating said layer of molten aluminium so the layer forms a shallow or deep continuous cathodic pool that extends continuously under the entire facing active anodic surface of at least one anode and that is contained within a cathodic cavity on the structural cathodic body, the layer regulating means being arranged to maintain during operation the cathodic surface of said cathodic pool at a substantially constant position by periodic or continuous removal of molten aluminium from the aluminium pool to the collection reservoir at a rate corresponding substantially to the rate of production of the product aluminium that is incorporated into the aluminium pool.

2. The cell of claim 1, wherein the layer regulating means comprise a wall which delimits the cathodic cavity, the wall having an upper edge located at the level of said cathodic surface of the aluminium pool and arranged to allow the flow over the edge of product aluminium to the collection reservoir that extends below the cathodic surface.

3. The cell of claim 2, wherein said collection reservoir has a generally U-shaped cross-section.

4. The cell of claim 2, wherein said collection reservoir has a bottom part that extends underneath the cathodic surface .

5. The cell of claim 1, comprising a reservoir chamber that contains said collection reservoir and that is adjacent to the electrolysis chamber and communicates therewith through an aluminium passage for the removal of molten aluminium from the aluminium pool to the collection reservoir, said layer regulating means comprising means for adjusting the amount of aluminium in the collection reservoir so that the position of the cathodic surface of the aluminium pool remains substantially constant during operation .

6. The cell of claims 5, wherein the layer regulating means comprise means for measuring directly or indirectly the position of the cathodic surface.

7. The cell of claim 6, wherein the measuring means comprise means for measuring the electrical characteristics of a current passing between the cathodic surface and at least one metal-based anode.

8. The cell of claim 6 or 7, wherein the measuring means comprise one or more proximity sensors above the electrolyte for measuring the height of the electrolyte and/or aluminium pool .

9. The cell of any one of claims 5 to 8, wherein the layer regulating means comprise a means to adjust the pressure above aluminium in the collection reservoir.

10. The cell of any one of claims 5 to 9, wherein the molten aluminium in the collection reservoir is covered by a layer or molten electrolyte.

11. The cell of any preceding claim, wherein the cathodic pool has a depth in the range of 1 to 15 cm.

12. The cell of claim 11, wherein the cathodic pool has a depth in the range of 2 to 5 cm.

13. A method producing aluminium in a cell that comprises:

- an electrolysis chamber containing one or more metal-based anodes in a molten electrolyte having alumina dissolved therein, each anode comprising an active anodic surface spaced by an anode-cathode gap from a facing cathodic surface that is formed on a structural body by a molten aluminium layer, said anodic surface and said cathodic surface having a substantially constant operative position so that the anode-cathode gap is substantially constant; and

- a collection reservoir that is in liquid communication with the electrolysis chamber,

said method comprising electrolysing dissolved alumina in the anode-cathode gap to evolve gas anodically and produce aluminium cathodically, product aluminium being incorporated into said aluminium layer and flowing from the electrolysis chamber into the collection reservoir,

said method being characterised in that molten aluminium layer is regulated to form a shallow or deep continuous cathodic pool that extends continuously under the entire facing active anodic surface of at least one anode and that is contained within a cathodic cavity on the structural cathodic body, the cathodic surface of the cathodic pool being maintained at a substantially constant position by periodic or continuous removal of molten aluminium from the aluminium pool to the collection reservoir at a rate corresponding substantially to the rate of production of the product aluminium that is incorporated into the aluminium pool .

14. The method of claim 13, comprising maintaining the electrolyte at a temperature in the range of 700° to 1000⁰C, 800 to 970⁰C, or 880 to 940°C.

15. The method of claim 13 or 14, comprising maintaining the aluminium in the collection reservoir in a molten state in particular above 700⁰C.