WO2006129267A2

WO2006129267A2 - Electrolytic cell with improved feed device

Info

Publication number: WO2006129267A2
Application number: PCT/IB2006/051708
Authority: WO
Inventors: Georges Berclaz; Vittorio De Nora
Original assignee: Moltech Invent S.A.
Priority date: 2005-06-02
Filing date: 2006-05-30
Publication date: 2006-12-07
Also published as: WO2006129267A3

Abstract

A cell for the electrowinning of a metal from a compound of the metal dissolved in a molten electrolyte (1), comprises a cell trough containing the molten electrolyte (1) in which the metal compound is dissolved and a cell cover (5) located above the molten electrolyte (1). The cell further comprises an anode (10) that has an anode body (30) and an elongated anode stem (20) having a bottom end (21) connected to the anode body (30). The anode stem (20) extends through a stem passage (6) in the cell cover (5) and suspends the anode body (30) in the molten electrolyte (1). The anode body has a bottom surface (31) which is immersed in the molten electrolyte (1) and which is active for the electrolysis of the metal compound. A delivery duct (22) for supplying particles of the metal compound (3) to the electrolyte (1) extends along the anode stem (20) through the stem passage (6) . The delivery duct (22) is arranged to deliver the metal compound particles (3) to the electrolyte (1) at an exit point (26) of the duct (22) located between the cell cover (5) and the anode body (30). The cell cover (5) may comprise: one or more preformed components, usually one or more insulating plates, placed above the electrolyte; and/or an electrolyte crust formed in-situ above the electrolyte by freezing its surface.

Description

ELECTROLYTIC CELL WITH IMPROVED FEED DEVICE

Field of the Invention

The present invention relates to a cell for the electrowinning of a metal from a compound thereof dissolved in a molten electrolyte. The cell is fitted with a device

5 for feeding particles of the metal compound to the molten electrolyte .

Background of the Invention

The feed device of the invention can be used in various molten salt electrolysis cells in particular for 10 aluminium electrowinning.

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 and has not undergone any great change Ib or improvement, in particular in the way in which alumina is fed to the molten electrolyte for its subsequent dissolution and electrolysis.

Conventional cells are usually operated with a crust of frozen electrolyte above the molten electrolyte. This

20 crust needs to be periodically broken to form an opening for feeding alumina into the molten electrolyte situated underneath. Various systems have been provided to locally break the frozen electrolyte crust and feed alumina into the molten electrolyte, for instance as described in US Patents

2b 3 '664, 946 (Schaper/Springer/Kyburz ) , 4,049,b29 (Golla) , 4,437,964 (Gerphagnon/Wolter ) , b, 04b, 168 (Dalen/Kvalavag/ Nagell), b,108,bb7 (Nordquist) , b, 294, 318 (Grant/Kristoff ) , b, 324, 408 and b, 423, 968 (both in the name of Kissane) .

One drawback of feeding alumina to the molten

30 electrolyte by initially breaking the electrolyte crust resides in the introduction of a mass of frozen electrolyte into the molten electrolyte which generates a thermal shock therein. Moreover, after the crust is broken cold alumina is added to the molten electrolyte which freezes the bath,

3b forming dense alumina and/or electrolyte aggregates increasing the chance of sludging. With the trend towards automated systems, the frequency of feeding alumina has been increased. Feeding may take place every 20 to 90 min . , sometimes even shorter, for instance every 1 to 5 min. as described in US Patent 5 3,673,075 (Kibby) , while smaller amounts of alumina are introduced with each feed. The advantages are in particular maintaining a more constant concentration of dissolved alumina in the electrolyte and reducing the temperature variation in the electrolyte. A typical automated break and ^' 0 feed system comprises a pneumatically-operated crust breaker beam and an ore bin capable of discharging a fixed amount of alumina (K. Grjotheim & B. J. Welsh, "Aluminium: Smelter Technology ", 1988, 2^nd Edition, Aluminium Verlag GmbH, D- 4000 Dϋsseldorf 1, pp. 231-232) .

15 US Patent 5,476,574 (Welsh/Stretch/Purdie) discloses a feeder arranged to continuously feed alumina to an aluminium electrowinning cell. The feeder is associated with a point breaker which is operated to maintain a hole in a frozen electrolyte crust on the surface of the molten

?0 electrolyte.

In order to enhance dispersion, dissolution and control of the amount of fine particulate alumina fed to the electrolytic bath, various alumina feed devices have been developed involving fluidisation of alumina powder by using 25 compressed gas such as compressed air, for instance as disclosed in US Patents 3,901,787 (Niizeki/Watanabe/ Yamamoto/Takeuchi/Kubota) , 4,498,818 (Gudmundur/Eggertsson) and 4,525,105 (Jaggi) .

Despite substantial efforts to enhance the feeding

30 of alumina as described above, feeding is still locally limited to one or more feeding points over the electrolytic bath between suspended carbon anode blocks using vertical point feeders. Furthermore, with the above-described processes it is still necessary to form periodically or to

35 maintain continuously as many holes in the frozen electrolyte crust above the molten bath as there are feeding points .

A solution to the latter problem has been presented in DE 37 21 311 (Schlepers). This document discloses an 40 aluminium electrowinning cell having a Sδderberg or a preformed carbon anode in an electrolyte covered by a crust. The cell has an alumina feed system which includes an alumina feed duct extending into the cell through the anode stem and exiting under the anode. Alumina is fed to the gap spacing the anode and the cathode by blowing alumina particles along the stem, through the anode under the anode surface into the electrolyte. Thus, there is no need to 5 break the electrolyte crust each time alumina is fed to the electrolyte. However, the system requires a powerful blower to provide enough pressure to push alumina under the anode. Moreover, alumina particles pushed directly down into the anode-cathode gap are more likely to settle at the bottom of ^' n the cell before being able to dissolve, which leads to sludging the cell.

Several devices have been disclosed for supplying alumina to the molten electrolyte of a crustless aluminium electrowinning cell operating with non-carbon anodes and 15 with a thermic insulating cover.

WO03/006717 (Berclaz/Duruz ) discloses a device for feeding alumina to a thermally insulated aluminium electrowinning cell in which metered quantities of alumina are dropped from a dosing system onto a divider that divides 20 the metered quantities into batches and that directs these batches into a plurality of feeding tubes which guide the batches to different areas of the cell's molten electrolyte.

Dispersive spraying of alumina has been proposed for crustless aluminium production cells. WO00/63464 (de

2b Nora/Berclaz) discloses an aluminium electrowinning cell with a thermally insulated crustless molten electrolyte and inter-alia an alumina feeding tube extending horizontally above the molten electrolyte. The feeding tube has a series of openings along its length for spraying sideways alumina

30 fed along the tube.

WO2003/014420 (Nguyen/de Nora) discloses an aluminium electrowinning cell which has an alumina feeder with a feeding pipe which extends through the cell sidewalls and which has a horizontal end portion that extends over the 3b molten electrolyte. Alumina is fed over the cell's molten electrolyte from the pipe ' s end portion along the end portion's axial direction.

Despite these improvements of the different alumina supply systems there is still a need to improve the supply

40 of a particulate feed, e.g. alumina, to the molten electrolyte of a cell for the electrowinning of a metal, such as aluminium, in particular to a cell having an insulating cover.

Summary of the Invention

The invention relates a cell for the electrowinning 5 of a metal from a compound of the metal dissolved in a molten electrolyte. The cell comprises the following features :

- a cell trough containing the molten electrolyte in which the metal compound is dissolved;

10 - a cell cover located above the molten electrolyte;

- an anode that has an anode body and an elongated anode suspension stem having a bottom end connected to the anode body, the stem extending through a stem passage in the cell cover to suspend the anode body in the molten

Ib electrolyte, the anode body having a bottom surface which is immersed in the molten electrolyte and which is active for the electrolysis of the metal compound; and

- a delivery duct for supplying particles of the metal compound to the electrolyte via an exit point of the duct,

20 the delivery duct extending along the anode suspension stem through the stem passage, the exit point being arranged to deliver the metal compound particles to the electrolyte at a location between the cell cover and the anode body.

?5 The cell cover may comprise: one or more preformed components, usually one or more insulating plates, placed above the electrolyte; and/or an electrolyte crust formed in—situ above the electrolyte by freezing its surface. When the cell cover comprises a frozen electrolyte crust, such

30 crust is preferably formed at cell start-up, the level of electrolyte in the cell being then lowered, e.g. by removing electrolyte and/or by not compensating product aluminium upon tapping, to form a gap between the electrolyte surface and the crust that forms the cover or part thereof.

35 Such a frozen electrolyte crust can be supported by the anode stems. For instance, the stems are provide with flanges or other protruding members at the level of the crust so as anchor the crust to the stems. The cell cover above the molten electrolyte is usually located and spaced above the surface of the molten electrolyte so that the exit point of the delivery duct is located between the cell cover and the molten electrolyte. b Suitable preformed cell cover plates are for instance disclosed in WO99/02763 (de Nora/Sekhar) , WO02/070784 and US2003/0102228 (both de Nora/Berclaz ) . Such an arrangement of cover components thermally insulates the surface of the molten electrolyte and substantially prevents 10 formation of an electrolyte crust on the molten electrolyte. However, for the purpose of the invention, the surface of the electrolyte does not need to be entirely crust free, but at least the feeding area should be free from any frozen electrolyte crust for optimal operation.

Ib It is also possible to combine an in-situ formed frozen electrolyte crust spaced above and extending over substantially the entire electrolyte surface with one or more preformed cover components located above the electrolyte crust. In this latter case, the preformed cover

20 components may be less thermally insulating. Alternatively, the cover over the electrolyte can be made of an in-situ formed crust without preformed cover components.

Usually, the delivery duct is arranged to deliver the metal compound particles above the molten electrolyte to

2b its surface. However, the exit point of the delivery duct could be located below the surface of the molten electrolyte .

Preferably, the delivery duct is arranged to deliver the metal compound to a part of the electrolyte which during

30 use circulates so as to enhance dissolution of the metal compound by the stirring effect of the circulating molten electrolyte .

The delivery duct (or several ducts in combination) can be arranged to deliver and disperse metal compound over

3b an expanse which includes at least part of the perpendicular projection onto the molten electrolyte surface of the anode body. The size of the expanse may be at least the active anode surface or a fraction of this surface, in particular from a quarter to a half of the full surface area.

40 Typically, the size of the expanse is at least 0.1 m², usually 0. b or 1 or 2 m² to 6 or 10 m² or more. Conveniently, the size of this expanse corresponds approximately to the perpendicular projection on the surface of the molten electrolyte of the active anode surface. For example, the expanse covers entirely or at least partly the perpendicular projection onto the molten electrolyte surface of an anode body. The metal compound feeding area may correspond to the feeding area on the surface of the molten electrolyte of an anode body.

In one embodiment, the feeding area corresponds to a projection onto the surface of the electrolyte of the active anode surfaces, this projection possibly being smaller or greater than the corresponding area(s) of the active anode surfaces. This feeding area is usually, but not necessarily, situated directly above the active anode surfaces. The feeding area typically occupies an expanse of the molten electrolyte surface which can be about the same size as the surface area of the corresponding active anode surfaces. However, when anodes co-operate with special electrolyte circulation means, for instance as disclosed in WO00/40782 (de Nora), the size of the feeding area may be smaller than the actual size of the active anode surfaces. In practice, powder metal compound may even be supplied over substantially the entire surface of the molten electrolyte. This is particularly advantageous in configurations where at least part of the electrolyte flows through an open anode structure to the inter-electrode gap.

In one embodiment, the anode body 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 delivery duct being arranged to deliver the metal compound particles to the circulating electrolyte above 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, WO00/40781, WO03/006716, WO03/023091 and WO03/023091 (all in the name of de Nora) and WO2005/118916 (de Nora/Nguyen) . At least part of the electrolyte rich in dissolved metal compound may flow down around the open anode body into the inter-electrode gap to be electrolysed and then electrolyte depleted in metal compound can rise to the feeding area through the open anode structures.

Whether or not electrolyte rich in dissolved metal compound flows around the anodes, dissolution of the metal b compound is improved by using the inventive feeding device in combination with the foraminate anode body. The improvement is not bound to a specific electrolyte circulation path. Either electrolyte flows from the feeding area down through the anode body, or electrolyte flows

10 through the anode body up to the feeding area, or both flow patterns are combined.

The delivery duct may extend through the stem passage of the cover along a central part of the stem. For instance, the delivery duct and the anode suspension stem

Ib are generally concentric. In another embodiment the part of the delivery duct that extends through the stem passage of the cover is located along a peripheral part of the anode stem. For example, the delivery duct is delimited from above to below the stem passage by a groove extending along the

20 stem and by a plate that covers the groove or, in another example, the delivery duct is delimited from above to below the stem passage by a tubular member surrounding the anode stem, the delivery duct extending between the anode stem and the tubular member.

2b The exit point of the delivery duct is usually arranged to deliver the metal compound at a downward angle to the longitudinal direction of the elongated stem, in particular a downward angle in the range of Ib to 60 deg. Typically, the delivery duct comprises at its exit point an

30 inclined surface, such as a surface of a ramp or an inclined tubular end portion, for guiding the metal compound particles. However, when the metal compound supply system operates with a blower, the metal compound particles may also be delivered at an upward angle from the delivery duct

3b or through a disperser (e.g. nozzle).

The anode stem can be associated with a plurality of delivery ducts, typically two, three four or more delivery ducts that can be evenly distributed at the periphery of the stem.

40 The bottom end of the elongated anode stem can have a plurality of cross members which are connected to different points of the anode body (e.g. two, three, four or more points), in particular around a central part of the anode body, the exit point of each delivery duct being located so as to deliver the particles of said metal compound between the cross members. b Alternatively, the bottom end of the elongated stem can be connected to a single point of the anode body, usually a centrally located point of the anode body's upper face .

A cell according to the invention can comprise a 10 plurality of the above described anodes, each associated with one or more delivery ducts extending along the stems.

The cell may be a cell for the electrowinning of aluminium from an aluminium compound such as alumina dissolved in a molten electrolyte, in particular a fluoride- ^'5 based electrolyte.

To overcome a prior art prejudice when electrolysis is carried out at high temperature which is for instance the case for aluminium electrowinning, it is preferred to supply preheated particulate to the molten electrolyte to minimise

?0 electrolyte freezing caused by contact with "cold" solid particulate and by the possibly endothermic dissolution reaction of the particulate in the molten electrolyte which for instance happens with the dissolution of alumina. Ideally, the fed particulate supplies at least part of the

?5 energy needed for its dissolution. Heat may be provided to the particulate during the feeding process by contact with hot air, by using a heater or possibly with a burner providing a flame which may also be used to drive the particulate along the feeding tube. The particulate may be

30 preheated before feeding, for instance by heating a reservoir in which it is stored and from which it is delivered through the feeding tube to the molten electrolyte. More generally, the particulate may be heated before and/or during delivery.

35 Therefore, the feeding means is preferably associated with a heater arranged to heat the particulate before it is delivered from the end opening over the molten electrolyte .

Another aspect of the invention relates to a method

40 for electrowinning a metal from a compound of the metal dissolved in a molten electrolyte of the above described cell. For example, aluminium can be electrowon from an aluminium compound such as alumina dissolved in a fluoride- containing molten electrolyte. The method comprises:

- electrolysing the dissolved metal compound between the 5 active surface of the anode body and a facing cathode to produce said metal; feeding particles of the metal compound into the delivery duct through the passage in the cell cover along the anode stem from where the particles are delivered at the ^' n exit point of the delivery duct between the cover and the anode body to the electrolyte for its dissolution; and

- circulating electrolyte containing the dissolved and/or dissolving metal compound to the active anode surface for its electrolysis.

15 A further aspect of the invention relates to an elongated anode suspension stem having an upright operative position and a bottom end for suspending an anode body during use. The stem comprises a duct for delivering a particulate material. The delivery duct extends along the

?0 stem and has an exit point arranged for delivering the particulate material above the bottom end when the stem is in its operative position, so that during use the supplied particulate material exits the delivery duct above an anode body suspended by the stem.

25 The anode suspension stem can include any of the above described feature or combination of features relating to the anode stem.

Yet another aspect of the invention relates to an anode that comprises the above anode stem and an anode body 30 which is connected to the stem's bottom end.

The concept of this invention may be adapted to any aluminium electrowinning cell and is particularly suitable for cells operating with metal-based anodes at reduced temperatures, typically below 94O⁰C, such as in the range of

3b 730° to 91O⁰C or 850° to 88O⁰C, for instance cells as disclosed in the abovementioned WO00/40781, WO00/40782, WO03/006716 and WO2005/118916 operating with metal-based oxygen-evolving grid-like anodes provided with vertical through openings for the circulation of electrolyte and the

40 escape of anodically produced oxygen.

The aluminium electrowinning cell can be operated with a fluoride-containing electrolyte, for example as disclosed in WO00/06802 (Duruz/de Nora/Crottaz ) , WO01/42535 (Duruz/de Nora), WO02/097167 (Nguyen/de Nora), WO03/083176 (de Nora/Duruz), WO2004/035871, WO2004/074549 (both de Nora/Nguyen/Duruz) and WO2005/090642 (Nguyen/de Nora) . b Suitable materials for oxygen-evolving anodes include alloys of iron, nickel, cobalt and/or copper which may be heat-treated in an oxidising atmosphere as disclosed in WO00/06802, WO00/06803 (both in the name of Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz ) , 0 WO01/42534 (Duruz/de Nora) WO01/42535 (de Nora/Duruz) WO01/42536 (Duruz/Nguyen/de Nora), WO02/083991 WO03/078695 (both Nguyen/de Nora), WO2005/090641 (de Nora/ Nguyen) and WO2005/090643 (Nguyen/de Nora) . Further oxygen-evolving anode materials are disclosed in WO03/087435 (Nguyen/de 5 Nora), WO2004/018731 (Nguyen/de Nora), WO2004/024994 (Nguyen/de Nora), WO99/36593, WO99/36594, WO00/06801, WO00/06805, WO00/40783 (all in the name of de Nora/Duruz), WO00/06800 (Duruz/de Nora), WO99/36591, WO99/36592 (both in the name of de Nora) and WO03/087435 (Nguyen/de Nora) . 0 The anode can comprise an applied cerium oxyfluoride-based outermost coating, for example as disclosed in WO02/070786 (Nguyen/de Nora) and WO02/083990 (de Nora/Nguyen) . Such a coating may be applied before or during use and maintained during use by the presence of 5 cerium species in the electrolyte.

Advantageously, the aluminium electrowinning cell comprises an aluminium-wettable cathode, in particular a carbon cathode covered with an aluminium-wettable coating to increase the lifetime of the cathode. The cathode may be a 0 drained cathode whereby the anode-cathode gap and the voltage drop though the electrolyte can be reduced.

Suitable cell troughs and cell bottoms for aluminium production are for example disclosed in WO00/63463 (de Nora), WO01/31086 (de Nora/Duruz), WO01/31087 (Duruz/de b Nora), WO01/42168 (de Nora/Duruz), WO01/42531 (Nguyen/Duruz/de Nora), WO02/096831 (Nguyen/de Nora), EP 1 146 146 (de Nora), WO02/070783, WO02/070785, WO02/097169, WO03/023091, WO02/097168 (all de Nora) and WO03/083176 (de Nora/Nguyen) . Brief Description of the Drawings

The invention will be further described by way of example with reference to the accompanying schematic drawings, in which: - Figure 1 illustrates an aluminium electrowinning cell according to the invention having an anode with delivery ducts for feeding alumina to the electrolyte shown partly in section along line B-B of Fig. 2;

- Figure 2 is a plan view of the anode of Fig. 1; - Figure 3 schematically illustrates an aluminium electrowinning cell according to the invention having another anode with delivery ducts for feeding alumina to the electrolyte;

- Figure 4 is a cross-section of the anode's stem along line C-C of Figure 3; and

Figure 5 schematically shows an aluminium electrowinning cell according to the invention having yet another anode with delivery ducts for feeding alumina to the electrolyte . Detailed Description

The aluminium electrowinning cell partly shown in Figure 1 comprises a molten electrolyte 1 covered with a preformed insulating cover 5 and an anode 10 having a vertically elongated anode stem 20 with a bottom end 21 connected to an anode body 30 that is suspended in electrolyte 1. Anode body 30 has a bottom face 31 that is active for the oxidation of oxygen and faces a cathode (not shown) .

Anode stem 20 extends through a stem passage 6 of cover 5. An insulating ring 7 located around stem 20 at the upper end of passage 6 inhibits heat loss through stem passage 6 along anode stem 20.

Anode stem 20 is associated with four delivery ducts 22 for supplying alumina 3 (and/or electrolyte constituents, such as aluminium fluoride, sodium fluoride and additives such as calcium and potassium fluorides) through cover 5 to electrolyte 1. Each delivery duct 22 has a tubular inlet 23 and is delimited by a longitudinal recessed groove 24 in anode stem 20 and a plate-like member 25 that covers the groove 24. Each delivery duct 22 has an outlet 26 that is formed by an uncovered lower extremity of groove 24 and that is located above anode body 30 and above the surface of electrolyte 1. Outlet 26 has an inclined ramp 27 for 5 directing a particulate flow exiting delivery duct 22.

As shown in Fig. 2, anode body 30 is a grid-like body made of parallel anode members 32. Gaps 33 space apart anode members 32 and form passages for a circulation 2 of electrolyte 1 from and/or to the active anode surface 31. 10 The flow 2 of electrolyte 1 and the geometry of the anode body 30 is explained in greater detail in the abovementioned WO00/40781, WO00/40782, WO03/006716 and WO2005/118916.

The bottom end of stem 20 is divided into four cross members 21 in an X-configuration, each member is connected

Ib in the middle of a quadrant (delimited by dotted lines A and B) of anode body 30. Delivery ducts 22 are located between members 21 so that the alumina feed exiting from outlet 26 of delivery ducts 22 is directed to electrolyte 1 between the four members 21 as indicated by arrows 3 in Figures 1

20 and 2.

Figures 3 and 4 schematically illustrate another aluminium electrowinning cell according to the invention in which the same numeric references designate the same elements, Figure 4 being an enlarged cross-sectional view 2b (size ratio 2:1) of stem 20 shown in Figure 3 along broken line C-C.

Delivery duct 22 shown in Figures 3 and 4 extends down along the middle of anode stem 20 and exits between the cell cover b and the molten electrolyte 1 on opposite sides 30 of stem 20 along two inclined ramps 27 machined into stem 20. Anode stem 20 has a bottom end which is connected to a single location of anode body 30 in a central part thereof.

Figure b illustrates a further aluminium electrowinning cell according to the invention in which the 35 same numeric references designate the same elements.

Delivery duct 22 shown in Figure 5 extends down between the anode stem 20 and a surrounding tubular member 25 that is substantially concentric with stem 20. Below tubular member 25, anode stem 20 has a downwardly inclined 40 ledge 27 for directing a downcoming alumina supply from delivery duct 22 over the surface of electrolyte 1. During operation of the cells shown in Figures 1 to 5, alumina particles are fed from outside the cell, usually from an alumina reservoir (not shown) along delivery ducts 22 located at the periphery of anode stem 20 (Figures 1, 2 and 5) or inside stem 20 (Figures 3 and 4) through cover 5 to the exit point 26 and over ramp 27 from where the particles are delivered over electrolyte 1 as indicated in Figures 1 and 2 by arrows 3.

Bayer-process alumina or other suitable grades of alumina, may be utilised. For instance, partly dehydrated alumina particles, modified alumina, and alumina particles of different shapes and sizes may be used. To enhance dispersion of the alumina powder above the molten electrolyte surface, and to facilitate its dissolution into the molten electrolyte, the alumina powder is preferably composed of particles in the range of 20 to 200 micron, preferably from 30 to 50 micron.

Alumina 3 supplied to electrolyte 1 dissolves therein and is circulated (by electrolyte flow 2 that is promoted by anodically evolved oxygen and the geometry of anode body 30) between the active anode surface 31 and a facing cathode where it is electrolysed to produce aluminium cathodically and oxygen anodically.

The spreading of alumina 3 delivered at exit points 26 and ramp 27 over electrolyte 1 can be enhanced by supplying alumina 3 under pressure. For this purpose, the alumina feed system can advantageously be associated with a blower (not shown) . See the abovementioned WO00/40781,

WO00/40782, WO03/006716 and WO2005/118916 for further details on the electrolyte circulation with such anodes.

As mentioned above, the supplied alumina (and/or other electrolyte constituents) may also be preheated to inhibit freezing of the electrolyte when fresh alumina is fed and enhance dissolution of the fed alumina. In a variation the cover may include or consist of an electrolyte crust as discussed above.

Claims

1. A cell for the electrowinning of a metal from a compound of the metal dissolved in a molten electrolyte, comprising:

- a cell cover located above the molten electrolyte;

- an anode that has an anode body and an elongated anode suspension stem having a bottom end connected to the anode body, the anode stem extending through a stem passage in the cell cover to suspend the anode body in the molten electrolyte, the anode body having a bottom surface which is immersed in the molten electrolyte and which is active for the electrolysis of the metal compound;

- a delivery duct for supplying particles of the metal compound to the electrolyte via an exit point of the duct, the delivery duct extending along the anode suspension stem through the stem passage, the exit point being arranged to deliver the metal compound particles to the electrolyte at a location between the cell cover and the anode body.

2. The cell of claim 1, wherein the delivery duct is arranged to deliver the metal compound particles above the molten electrolyte.

3. The cell of claim 1 or 2, wherein the delivery duct is arranged to deliver the metal compound to a part of the electrolyte which during use circulates so as to enhance dissolution of the metal compound.

4. The cell of any preceding claim, wherein the anode body 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 delivery duct being arranged to deliver the metal compound particles to the circulating electrolyte above the anode body.

5. The cell of claim 4, wherein the anode body comprises a series of elongated members spaced apart by inter-member gaps which form said through passages.

6. The cell of claim 4, wherein the anode body comprises a solid body, in particular a plate, which has through holes that form said through passages.

7. The cell of any preceding claim, wherein the delivery duct extends through the stem passage of the cover along a central part of the anode stem.

8. The cell of any one of claims 1 to 6, wherein the delivery duct extends through the stem passage of the cover along a peripheral part of the anode stem.

9. The cell of claim 8, wherein the delivery duct is delimited from above to below the stem passage by a groove extending along the anode stem and by a plate that covers the groove.

10. The cell of claim 8, wherein the delivery duct is delimited from above to below the stem passage by a tubular member surrounding the anode stem, the duct extending between the anode stem and the tubular member.

11. The cell of any preceding claim, wherein the exit point of the delivery duct is arranged to deliver the metal compound at a downward angle to the longitudinal direction of the anode elongated stem, in particular a downward angle in the range of 15 to 60 deg.

12. The cell of claim 11, wherein the delivery duct comprises at its exit point an inclined surface, such as a surface of a ramp or inclined tubular end portion, for guiding the metal compound particles.

13. The cell of any preceding claim, wherein the anode stem is associated with a plurality of delivery ducts.

14. The cell of claim 13, wherein the bottom end of the elongated anode stem has a plurality of cross members which are connected to different points of the anode body, in particular around a central part of the anode body, the exit point of each delivery duct being located so as to deliver the particles of said metal compound between the cross members .

15. The cell of any preceding claim, which comprises a plurality of anodes, each associated with one or more delivery ducts.

16. The cell of any preceding claim, wherein the cell cover comprises one or more preformed components, in particular plate-like components.

17. The cell of any preceding claim, wherein the cell cover comprises an in—situ formed crust of frozen electrolyte.

18. The cell of any preceding claim, wherein said metal compound is an aluminium compound, in particular alumina.

19. A method for electrowinning a metal from a compound of the metal dissolved in a molten electrolyte of a cell as defined in any preceding claim, the method comprising: electrolysing the dissolved metal compound between the active surface of said anode body and a facing cathode to produce said metal; feeding particles of the metal compound into the delivery duct through the passage in said cell cover along said anode stem from where the particles are delivered at said exit point of the duct between said cover and said anode body to the electrolyte for its dissolution; and circulating electrolyte containing the dissolved and/or dissolving metal compound to the active anode surface for its electrolysis.

20. The method of claim 19, wherein aluminium is electrown from a dissolved aluminium compound, such as alumina.

21. An elongated anode suspension stem having an upright operative position and a bottom end for suspending an anode body during use, the anode stem comprising a duct for delivering a particulate material, the delivery duct extending along the anode stem and having an exit point arranged for delivering the particulate material above said bottom end when the anode stem is in its operative position, so that during use the supplied particulate material exits the delivery duct above an anode body suspended by the anode stem.

22. The anode suspension stem of claim 21, which is a stem for suspending an anode body in a cell as defined in any one of claims 1 to 15.

23. An anode comprising an anode stem as defined in claim 21 or 22 and an anode body which is connected to the anode stem's bottom end.