WO2007105124A2

WO2007105124A2 - Aluminium electrowinning cell with reduced heat loss

Info

Publication number: WO2007105124A2
Application number: PCT/IB2007/050610
Authority: WO
Inventors: Vittorio De Nora; René VON KAENEL; Thinh T. Nguyen
Original assignee: Moltech Invent S.A.
Priority date: 2006-03-10
Filing date: 2007-02-26
Publication date: 2007-09-20
Also published as: WO2007105124A3

Abstract

A drained-cathode cell (10) for the electrowinning of aluminium (25) from alumina dissolved in a fluoride- containing molten electrolyte (30) has: a cell bottom (11) and cell sidewalls (12', 15, 16) forming a cavity for containing the electrolyte (30) up to an operative electrolyte level (31); and aluminium-wettable drained cathode surfaces (13) on which during use aluminium is produced and from which aluminium is drained. The sidewalls have one or more sections (12') extending from the cell bottom to about the electrolyte level or thereabove. The sidewall sections are thermally insulated so as to inhibit formation of a ledge of frozen electrolyte thereon and inhibit heat losses therethrough. An upper part (13') of at least one sidewall section has an aluminium-wettable drained cathode surface on which during use aluminium is produced and drained along and over the entire sidewall section exposed to molten electrolyte from about the operative electrolyte level down to the cell bottom. The product aluminium draining along and over the sidewall section inhibits exposure to molten electrolyte of this sidewall section between the electrolyte level and the cell bottom.

Description

ALUMINIUM ELECTROWINNING CELL WITH REDUCED HEAT LOSS

Field of the Invention

The invention relates to an aluminium electrowinning cell with minimal thermic losses.

Background of the Invention The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 95O⁰C is more than one hundred years old.

This process, conceived almost simultaneously by Hall and Heroult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.

The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by prebaked anthracite- graphite or all graphite carbon blocks at the cell floor bottom which acts as cathode. The side walls are also covered with prebaked anthracite-graphite carbon plates.

To increase the efficiency of aluminium production numerous drained-cathode cell designs have been developed, in particular including sloping drained cathode surface, as for instance disclosed in US Patents 3,400,061 (Lewis/Altos/Hildebrandt) , 4,602,990 (Boxall/ Gamson/Green/Stephen) , European Patent Application No. 0 393 816 (Stedman). Advantageous drained cathode cells designs and materials therefor have been disclosed in US Patents 5,362,366, 5,368,702, 5,683,559, 6,436,273, 6,682,643, 6,783,656, and in WO99/02764, WO01/42168, WO01/42531, WO02/070783, WO02/070785, WO02/096830, WO02/096831, WO02/097168, WO02/097169, WO03/023091, WO03/023092, WO2004/092449 and WO2005/068390 (all assigned to MOLTECH Invent S.A.). These cell designs permit reduction of the inter-electrode gap and consequently reduction of the voltage drop between the anodes and cathodes. However, drained cathode cells have not as yet found wide acceptance in industrial aluminium production .

It has been proposed to decrease energy losses during aluminium production by increasing the thermal insulation of the sidewalls of aluminium production cells. In conventional cells, the major heat losses occur at the sidewalls, the current collector bars and the cathode bottom, which account for about 35%, 8% and 7% of the total heat losses respectively, and considerable attention is paid to providing a correct balance of these losses. Further losses of 33% occur via the carbon anodes, 10% via the crust and 7% via the deck on the cell sides .

A manner to limit thermal losses involves the use of a thermal insulation covering the electrolyte, as for instance disclosed in US Patents 5,368,702, 6,402,928,

6,656,340, and publications WO02/070784 and WO03/102274

(all assigned to MOLTECH Invent S. A.) as well as in US

Patent 5,415,742 (La Camera/Tomaswick/Ray/Ziegler ) , and Publications WO02/06565 (D 'Astolfo/Hornack) and US

2001/0035344 (D 'Astolfo/Lazzaro) and US 2001/0037946

(D'Astolfo/Moor) .

Furthermore, it has been proposed to increase the thermal insulation of the sidewalls to avoid thermal losses therethrough. However, suppression of the thermal gradient through the sidewalls prevents bath from freezing on the sidewalls and consequently leads to exposure of the sidewalls to highly aggressive molten electrolyte and molten aluminium.

Several proposals have been made in order to increase the sidewall resistance for ledgeless cell operation. US Patent 2,915,442 (Lewis) discloses inter- alia use of silicon carbide or silicon nitride as sidewall material. US Patent 3,256,173 (Schmitt/Wittner ) describes a sidewall lining made of a honeycomb matrix of coke and pitch in which particulate silicon carbide is embedded. US Patent 5,876,584 (Cortellini) discloses sidewall lining material of silicon carbide, silicon nitride or boron carbide having a density of at least 95% and no apparent porosity.

Even though certain materials provide a good resistance against attack of the molten electrolyte, the sidewalls of known ledgeless cells are exposed to erosion at the interface between the molten electrolyte and the molten aluminium which accumulates on the bottom of the cell. Despite formation of an inert film of aluminium oxide around the molten aluminium metal, cryolite operates as a catalyst which dissolves the protective aluminium oxide film at the aluminium/cryolite interface, allowing the molten aluminium metal to wet the sidewalls along the molten aluminium level. As opposed to aluminium oxide, the oxide-free aluminium metal is reactive at the cell operating temperature and combines with constituents of the sidewalls, which leads to rapid erosion of the sidewalls about the molten aluminium level. A solution to this problem has been proposed in US 6,258,246 (assigned to MOLTECH Invent S. A.) by providing an aluminium electrowinning cell with a cathode bottom arranged to drain aluminium away from the sidewalls so as to prevent reaction of the crustless sidewalls with aluminium.

While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations and reduction of thermal losses, none suggest the invention.

Summary of the Invention

An object of the invention is to provide an aluminium electrowinning cell operating with reduced thermic losses. Another object of the invention is to provide an aluminium electrowinning cell in which electrolyte is inhibited from freezing on the sidewalls that are efficiently protected against electrolyte attack. One main aspect of the invention concerns a drained-cathode cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises: a cell bottom and cell sidewalls forming a cavity for containing the electrolyte up to an operative electrolyte level; and aluminium- wettable drained cathode surfaces on which during use aluminium is produced and from which aluminium is drained. The sidewalls comprise one or more sections extending from the cell bottom to about the electrolyte level or thereabove. The or each sidewall section is thermally insulated so as to inhibit formation of a ledge of frozen electrolyte thereon and inhibit heat losses therethrough . According to the invention, an upper part of at least one sidewall section has an aluminium-wettable drained cathode surface on which during use aluminium is produced and drained along and over the entire sidewall section exposed to molten electrolyte from about the operative electrolyte level down to the cell bottom so that the product aluminium draining along and over the sidewall section inhibits exposure to molten electrolyte of this sidewall section between the electrolyte level and the cell bottom. The cathodic sidewall section can extend slightly above the electrolyte level to accommodate for electrolyte fluctuations. Alternatively, the cathodic sidewall section may end slightly below the electrolyte level. The upper end of the cathode sidewall section may be spaced by a distance of up to 5, 10, 15 or 20 cm blow or above the electrolyte level. When the cathodic sidewall section ends slightly below the electrolyte level, the upper part of the sidewall is preferably protected for example by the peripheral part of an electrolyte crust, as described below.

At least one sidewall section may have an aluminium-wettable drained cathode section that extends from its upper part down toward the cell bottom. Thus aluminium can be produced substantially over the entire sidewall section. Typically, the cell has a pair of facing sidewalls made of one or more of these aluminium-wettable drained sidewall sections. For instance, the cell has a generally rectangular bottom with a length that is greater than its width and longitudinal facing sidewalls extending along the cell's length, the longitudinal sidewalls being made of one or more of said sidewall sections .

Each sidewall can be made of one or more of these aluminium-wettable drained sidewall sections. For instance, the cell has a periphery delimited by the sidewalls, the entire periphery of the cell being made of these aluminium-wettable drained sidewall sections.

These aluminium-wettable drained sidewall sections can be removable so that, if necessary, they can be replaced during cell operation.

The cell can be operated with or without a crust of frozen electrolyte over the molten electrolyte. In one embodiment, at least one cathode surface of a sidewall section has an upper end that is contacted by a crust of frozen electrolyte which inhibits molten electrolyte form contacting a portion of the sidewall that is located above the sidewall ' s cathodic surface and that is not covered with molten aluminium. The crust may contact the cathodic sidewalls about the electrolyte level, i.e. at a location that is slightly above or below the level of the molten electrolyte, typically at a distance of a few centimetre above or below this level, such as 5, 10 or 15 cm thereabove or therebelow. When the crust is spaced above the electrolyte level, it usually means that the crust has been formed by letting the surface of the electrolyte freeze before lowering the electrolyte level, for instance after tapping product aluminium without addition of electrolyte. Usually, the cell has, in addition to the sidewall section with the cathode surface, one or more further cathodes. For instance the cell may have further cathode bodies resting or suspended on the cell bottom and/or a cathodic cell bottom, in particular an aluminium-wettable drained bottom or a bottom covered with a shallow pool of aluminium. Such further cathode (s) can have an inclined cathode surface. Examples of cell bottoms and cathodes are for instance disclosed in US Patents 5,368,702, 5,651,874, 5,683,559, 6,436,273, 6,682,643, 6,783,656, and in WO01/42168, WO01/42531, WO02/070783, WO02/070785, WO02/096830, WO02/096831, WO02/097168, WO02/097169, WO03/023091, WO03/023092, WO2004/092449 and WO2005/068390 (all assigned to MOLTECH Invent S. A. ) .

The cell may have a metal-based anodes, such as an anode comprising 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 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. ) . Typically, the metal-based anode comprises an active body having a grid-like or plate-like foraminate structure that is parallel to the facing cathode. Examples of such anode bodies are disclosed in WO00/40781, WO00/40782 and WO03/006716 (all assigned to MOLTECH Invent S.A.).

The invention also relates to a method of producing aluminium in a cell as described above. This method comprises electrolysing the dissolved alumina to produce aluminium on the aluminium-wettable drained cathode surface of the upper part of at least one sidewall section. The cathodically produced aluminium drains along and over the entire sidewall section exposed to molten electrolyte from about the operative electrolyte level down to the cell bottom so that the product aluminium draining along and over the sidewall section inhibits exposure to molten electrolyte of the sidewall section between the electrolyte level and the cell bottom.

Brief Description of the Drawings The invention will be further described with reference to the accompanying schematic drawings, in which :

Fig. 1 is a schematic cross-sectional view of a comparative aluminium electrowinning cell; Fig. 2 is a schematic cross-sectional view of an aluminium electrowinning cell according to the invention; and

Fig. 3 is a schematic cross-sectional view of another aluminium electrowinning cell according to the invention.

Detailed Description

Fig. 1 discloses a comparative cell 10 for the electrowinning of aluminium 25 by the electrolysis of alumina dissolved in a fluoride-containing molten electrolyte 30. The cell has a trough formed by bottom 11 and sidewalls 15 containing the molten electrolyte 30.

Electrolyte 30 can in particular contain a mixture of aluminium fluoride and sodium fluoride possibly including one or more additives such as potassium, calcium, magnesium and lithium fluorides. The temperature of electrolyte 30 is normally in a range from above the melting point of aluminium to 1000⁰C, usually above 700 or 75O⁰C and below 985 or 97O⁰C. Typically, the temperature is in a range from 860 to 96O⁰C such as 900° to 95O⁰C. Suitable electrolytes are disclosed in US patents 5,725,744, 6,372,099, 6,521,116, WO01/42535 WO02/097167, WO2004/035871 and WO2004/074549 (all assigned to MOLTECH Invent S. A.)

A pair of generally prismatic cathode blocks 12 with sloping aluminium-wettable surfaces 13 on which aluminium is produced and drained, are located on bottom ll in a pool of product aluminium 25 collected at the bottom of blocks 12.

Cathode blocks 12 can be made of any suitable aluminium-wettable material that is resistant to the cell operating conditions, in particular to molten aluminium and electrolyte at high temperature. The cathode blocks can be made of a carbon body covered with an aluminium- wettable refractory material such as a titanium diboride or other boride based layer, or of a ceramic body made of aluminium-wettable material. 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 and WO2005/068390 (all assigned to MOLTECH Invent S.A.).

The same aluminium-wettable materials can advantageously be used to make or coat bottom 11 and/or sidewalls 15. The sidewalls can also be made of or coated with silicon carbide, silicon nitride and/or other conventional materials.

Cathodes 12 are located between metal-based anodes 20 that have an inclined generally coplanar arrangement of elongated spaced apart anode members

(rods) 21 on which oxygen is evolved. Each arrangement of anode members 21 is generally parallel to a facing cathode surface 13. Anodes 20 are suspended by stems 22 in electrolyte 30. Such anodes 20 are the most advanced and efficient oxygen-evolving anodes and incorporate means to improve the circulation of electrolyte 30 in order to ensure optimal distribution of dissolved alumina in the gaps spacing cathodes 12 and anodes 20. In contrast to carbon anodes, such oxygen-evolving anodes 20 are provided with an active body of reduced thickness, i.e. the arrangement of members 21. These anodes 20 can be arranged to permit circulation of electrolyte 30 through gaps in the active surface formed by members 21 from and into the anode-cathode gap. Suitable anode designs can be found in co-pending applications WO99/02764, WO00/40781, WO00/40782, WO03/006716 and WO2005/118916 (all assigned to MOLTECH Invent S.A.), which show electrochemically active anode structures fully immersed in a molten electrolyte, and suspended from an electrically conductive stem which is partly immersed in the molten electrolyte, the stem feeding to the active structure current from a current source via a busbar in the cell superstructure. However, other anode configurations may also be used, such as configurations disclosed in US Patents 5,362,366 and 6,797,148 and in PCT publication WO93/25731 (all assigned to MOLTECH Invent S.A.).

Suitable materials which could be used as electrochemically active anode materials are disclosed in

US Patents 6,077,415, 6,103,090, 6,113,758, 6,248,227,

6,361,681, 6,365,018, 6,379,526, 6,521,115, 6,562,224,

6,878,247 and PCT publications WO00/40783, WO01/42534,

WO02/070786, WO02/083990, WO02/083991, WO03/014420, WO03/078695, WO03/087435, WO2004/018731 , WO2004/024994,

WO2004/044268, WO2004/050956 , WO2005/090641 ,

WO2005/090642 and WO2005/090643 (all assigned to MOLTECH

Invent S. A.) . Stem 22 can be made of the same materials or, advantageously, of the stem material disclosed in WO2004/035870 (assigned to MOLTECH Invent S. A.)

Cell 10 has a crust 35 of frozen electrolyte covering the molten electrolyte and a ledge 36 of frozen electrolyte formed on sidewalls 15 to protect them against molten electrolyte 30. Crust 35 and ledge 36 are maintained by a thermal gradient through sidewalls 15, which leads to a heat loss during use.

Thus, the formation and maintenance of ledge 36 to protect sidewalls 15 leads to a significant energy consumption via heat dissipation. As mentioned above, typically 35% of the heat loss is caused by dissipation through the sidewalls used for the formation and maintenance of ledge 36 of frozen electrolyte.

Figs 2 and 3, in which the same numeric references designate the same elements, disclose cells in accordance with the invention, in which significant heat loss through the sidewalls is avoided.

The cells of Figs 2 and 3 include sidewalls 12', 15, 16 having sections 12' extending from cell bottom 11 to about the electrolyte level 31. As shown in Fig. 3, sections 12' may end slightly below electrolyte level 31, e.g. typically up to 5 or 15 cm therebelow, whereas in Fig. 2, sections 12' extend up to level 31. In a variation, such sections may extend above the electrolyte level, typically by the same distance i.e. up to 5 or 15 cm thereabove.

Each sidewall 12', 15, 16 includes a thermic insulating layer 16 which is sufficient to inhibit formation of a ledge of frozen electrolyte in the cell along section 12' and thus inhibits heat losses therethrough .

An upper end 13' of at least one sidewall section 12' has an aluminium-wettable drained cathode surface on which during use aluminium 25 is produced and drained along and over the entire sidewall section 12' from about electrolyte level 31 down to cell bottom 11 so that the product aluminium draining along and over the sidewall section 12' inhibits exposure to molten electrolyte 30 of sidewall 12', 15, 16 between electrolyte level 31 and cell bottom 11. As shown in Figs 2 and 3, cathodic sidewall sections 12' cooperate with facing parallel oxygen- evolving anodes 20. Cathodic sidewall sections 12' can be made of the same material as cathodes 12.

Upper ends 13' of sidewall section 12' contact a peripheral part of crust 35 of frozen electrolyte. This crust 35 protects the upper part of sidewall 15 which is not covered by cathodic sidewall section 12', in particular against electrolyte projections and gases above electrolyte level 31 and, as shown in Fig. 2, against molten electrolyte 30 when the cathodic sidewall section 12' ends slightly below electrolyte level 31.

The insulating layer 16 shown in Fig. 2 extends along the entire height of sidewall 15. Such an insulating layer 16 provides a maximum barrier against - ] 1 -

heat dissipation. However, such an extensive insulation 16 reduces also the possibility of a crust formation extending along sidewall 15 significantly below electrolyte level 31. Because of this, the electrolyte level 31 can be adjusted to maintain at least a thin layer of frozen electrolyte along sidewall 15 above cathodic sidewall section 12' and avoid that this section 12' ends to far below electrolyte level 31. A borderline configuration meeting these conditions is shown in Fig. 2, in which an uppermost end 13' of sidewall section 12' reaches the lowermost end of crust 35 so as to protect the entire wall 15 against molten electrolyte 30 or other products of cell 10.

In Fig. 3, insulating layer 16 extends along sidewall 15 only to about the electrolyte level 31. Such a layer 16 provides a sufficient barrier against heat loss but nevertheless allows for greater heat dissipation above the electrolyte level. This leads to the formation of a more substantial electrolyte crust 35 on upper end 13' of cathodic sidewall section 12' and thus to a greater protection of wall 15 and safer operation.

On the left-hand side of Fig. 2, cell 10 is shown at start-up before formation of crust 35 by freezing molten electrolyte 30. In this transitional state, the upper end of sidewall 15 is not protected against molten electrolyte 30 and other products of cell 10 such as electrolyte vapours. However, a protective crust 35 as shown on the right-hand side of Fig. 2 can be formed quickly, typically between a couple of hours and half a day, so that the sidewall 15 is not noticeably attacked during this short period of time while it is exposed to an aggressive environment. Further protection of the sidewall 15 during start up can be provided by impregnating or coating the upper end of sidewall 15 with a protective material, as for example disclosed in US Patents 5,486,278, 5,534,130, 5,753,382, 6,194,096 and 6,228,424 (all assigned to MOLTECH Invent S.A.).

During operation of cells 10 shown in Figs 2 and

3, alumina continuously or periodically fed to electrolyte 30 is dissolved therein and then electrolysed between anodes 20 and facing cathodes 12 and cathodic sidewall sections 12', respectively, to produce oxygen anodically and aluminium cathodically . Product aluminium is drained along cathodes 12 and cathodic sidewall sections 12' and collected at their bottom on cell bottom 11 to form a pool of molten aluminium 25.

According to the invention, the product aluminium 25 is drained along and over the entire sidewall 12', 15, 16 exposed to molten electrolyte 30 from about electrolyte level 31 down to the cell bottom 11 so that the product aluminium 25 draining along and over the sidewall 12', 15, 16 inhibits exposure thereof to molten electrolyte 30 between electrolyte level 31 and the cell bottom 11.

As shown in Figs 1 to 3, electrolyte level 31 is spaced by a small gap below crust 35. This gap is useful for the evacuation of gas produced during electrolysis and can be formed by removal of a small amount of electrolyte 30 after formation of crust 35, by evaporation of electrolyte 30 or by tapping product aluminium 25 without full compensation with additional electrolyte .

In a variation the cells shown in Figs 2 and 3 can be operated with top parts of the cathodic sidewalls extending above the electrolyte level, in particular in a crustless cell fitted with an insulating cover such as a cover disclosed in US Patents 5,368,702, 6,402,928, 6,656,340, and publications WO02/070784 and WO03/102274 (all assigned to MOLTECH Invent S.A.). While the invention has been described in conjunction with specific embodiments thereof, it is evident that many modifications and variations will be apparent to those skilled in the art in the light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations which fall within the scope of the appended claims .

Claims

1. A drained-cathode cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising: a cell bottom and cell sidewalls forming a cavity for containing the electrolyte up to an operative electrolyte level; and aluminium- wettable drained cathode surfaces on which during use aluminium is produced and from which aluminium is drained, the sidewalls comprising one or more sections extending from the cell bottom to about the electrolyte level or thereabove, the or each sidewall section being thermally insulated so as to inhibit formation of a ledge of frozen electrolyte thereon and inhibit heat losses therethrough,

characterised in that an upper part of at least one sidewall section has an aluminium-wettable drained cathode surface on which during use aluminium is produced and drained along and over the entire sidewall section exposed to molten electrolyte from about the operative electrolyte level down to the cell bottom so that the product aluminium draining along and over the sidewall section inhibits exposure to molten electrolyte of said sidewall section between the electrolyte level and the cell bottom.

2. The cell of claim 1, wherein at least one sidewall section has an aluminium-wettable drained cathode section that extends from its upper part down toward the cell bottom.

3. The cell of claim 1 or 2, which has at least one pair of facing sidewalls made of one or more of said aluminium-wettable drained sidewall sections.

4. The cell of claim 3, which has a generally rectangular bottom with a length that is greater than its width and longitudinal facing sidewalls extending along said length, the longitudinal sidewalls being made of one or more of said sidewall sections. - i c -

5. The cell of any preceding claim, wherein each sidewall is made of one or more of said aluminium- wettable drained sidewall sections.

6. The cell of any preceding claim, which has a 5 periphery delimited by the sidewalls, the entire periphery of the cell being made of said aluminium- wettable drained sidewall sections.

7 The cell of any preceding claim, wherein at least one cathode surface of a sidewall section has an upper _^O end that is contacted by a crust of frozen electrolyte which inhibits molten electrolyte form contacting a portion of the sidewall that is located above said cathode surface and that is not covered with molten aluminium.

÷b 8. The cell of claim 7, wherein said crust contacts the sidewalls about the electrolyte level.

9. The cell of any preceding claim, which has, in addition to the sidewall section with the cathode surface, one or more further cathodes.

20 10. The cell of claim 9, wherein said one or more further cathodes comprise a cathodic cell bottom, in particular an aluminium-wettable drained bottom or a bottom covered with a shallow pool of aluminium.

11. The cell of claim 9 or 10, wherein said one or more 25 further cathodes comprise at least one inclined cathode surface .

12. The cell of any preceding claim, comprising metal- based anodes.

13. The cell of claim 12, wherein at least one metal- 30 based anode comprises at least one metal selected from nickel, iron, cobalt and copper.

14. The cell of claim 12 or 13, wherein at least one metal-based anode comprises a metal oxide surface, in particular a surface containing at least one of iron

3b oxide, nickel oxide and cobalt oxide. - 1 b -

15. A method of producing aluminium in a cell as defined in any preceding claim, the method comprising electrolysing the dissolved alumina to produce aluminium on said aluminium-wettable drained cathode surface of the upper part of at least one sidewall section, cathodically produced aluminium draining along and over the entire sidewall section exposed to molten electrolyte from about the operative electrolyte level down to the cell bottom, the product aluminium draining along and over the sidewall section to inhibit exposure to molten electrolyte of said sidewall section between the electrolyte level and the cell bottom.

16. The method of claim 15, comprising forming a crust of frozen electrolyte.

17. The method of claim 16, wherein the crust contacts an upper end of at least one cathode surface of a sidewall section so as to inhibit molten electrolyte form contacting a portion of the sidewalls that are located above said cathode surface and that are not covered with molten aluminium.

18. The method of claim 17, wherein said crust contacts the sidewalls about the electrolyte level.

19. The method of any one of claims 16 to 18, wherein the crust is spaced above the electrolyte level.