WO1998020188A2

WO1998020188A2 - Apparatus and method for treating the cell lining of an aluminum electrowinning cell

Info

Publication number: WO1998020188A2
Application number: PCT/US1997/019026
Authority: WO
Inventors: Vittorio De Nora; Georges Berclaz; Jainagesh A. Sekhar
Original assignee: Moltech Invent SA
Current assignee: Moltech Invent SA
Priority date: 1996-10-18
Filing date: 1997-10-17
Publication date: 1998-05-14
Anticipated expiration: 1999-04-18
Also published as: WO1998020188A3

Abstract

An apparatus for treating a cathode lining (10) of an aluminum electrowinning cell when the cell is out of use, comprising: treatment means (44, 45) for treating the cell lining (10); support means (20, 30, 40) at least temporarily mounted on the cell and supporting the treatment means (44, 45) for movement thereof over the cathode lining (10); and drive means (31, 41) for moving said treatment means (44, 45) over the cathode lining (11) to carry out a treatment thereon. The support means (20, 30, 40) can further comprise length-wise support means (20, 30) for supporting the treatment means (44, 45) moveably with respect to a length direction of the cell; transverse support means (30, 40) for supporting the treatment means (44, 45) moveably with respect to a transverse direction of the cell; and means for adjusting the height of the treatment means (44, 45) over the cathode lining (10).

Description

APPARATUS AND METHOD FOR TREATING THE CELL LINING OF AN ALUMINUM ELECTRO INNING CELL

Field of the Invention

This invention relates to aluminum electro inning cells and is particularly concerned with treating a cathode lining of an aluminum production cell when the cell is out of use, i . e . after the removal of the liquid cell contents of a used cell, or before adding the cell contents in the case of a new or a renovated cell being put into service.

Background Act

Aluminum is produced conventionally by the Hall- Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolyte at temperatures up to around 950°C. A Hall-Heroult reduction cell, typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate forming the cell bottom floor. The cathode substrate is usually an anthracite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture of anthracite, coke, and coal tar.

In Hall-Heroult cells, a molten aluminum pool acts as the cathode. The carbon lining or cathode material has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminum as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.

In preparing these cells for service, various operations are carried out on the cell lining. For instance, a layer of resistor coke is applied for the purpose of protecting the cell bottom from Joule-effect heating. This layer of coke must be spread evenly over the cell bottom.

As briefly described below, various other treatments and/or protective coatings have been proposed for the cell linings, to improve the resistance of the carbon to the cell's molten, liquid contents.

For example, US Patent 5,578,174 (Sekhar) proposes conditioning cell components by impregnating and/or coating them with selected colloids.

US Patent 5,378,327 (Sekhar) proposes treating a carbon cathode with a solution, suspension or melt of a lithium compound, to improve its resistance to sodium.

US Patent 5,651,874 (Sekhar/de Nora) describes applying a slurry of refractory boride particles such as TiB₂ to a carbon cell component to produce a hard aluminum- wettable coating.

US Patent 5,486,278 (Manganiello et al) describes impregnating a carbon anode or cell sidewall with a boron based solution.

US Patent 5,343,130 (Sekhar) describes the application of aluminum phosphates in particular to cell sidewalls.

Some of these treatments can be applied to carbon blocks before they are assembled to form the cell lining. In all cases, however, it would be desirable to be able to apply the treatment to the cell bottom in si tu while the cell is not in action. Present methods, however, usually involve manual labor on the cell bottom. Specifically, coatings and treatment agents are generally applied using manual labor. For example, deposition of a layer of coke (as described above) is generally applied by a device suspended from the cell superstructure and then is manually leveled.

Summary of the Invention

A primary object of the invention is to provide an apparatus and method for treating a cathode cell lining of an aluminum electrowinning cell when the cell is not in use, without a need for the intervention of manual labor on the cell lining. The cathode cell lining to be treated is contained in a shell and includes a carbon cathode bottom and cell sidewalls forming a trough. When the cell is in use, this trough receives liquid cell contents.

According to the invention, an apparatus for treating a cathode lining of an aluminum electrowinning cell when the cell is out of use comprises: treatment means for treating the cell lining, support means at least temporarily mounted on the cell and supporting the treatment means, and drive means for moving said treatment means over the cathode lining to carry out a treatment thereon.

Further aspects of the present invention are discussed below. Considering the large surface areas of cathode lining to be covered by the treatmen't means, the support means of the cathode lining treatment apparatus may advantageously comprise length-wise support means for supporting the treatment means moveably with respect to a length direction of the cell and transverse support means for supporting the treatment means moveably with respect to a transverse direction of the cell. Furthermore, in order to adjust the height of the treatment means over the cathode lining, so as to enable treatments at different heights in respect to the support means or treatments on different types of cells, the support means should preferably include means for adjusting said height of the treatment means over the cathode lining. When the cathode lining is horizontal, a telescopic head which telescopes and which is manually adjustable is sufficient to set the treatment means at the right distance over the cathode lining, whereas when the cell bottom is inclined, an automatic system for adjusting said distance is preferable.

One way to mount the apparatus on a cell comprises at least one rail, for instance a pair of rails, mounted on a length direction of the cell and a treatment means support apparatus supporting the treatment means. The treatment means support apparatus, moveable along the rail(s), allows the treatment means to move in the length direction of the cell .

In addition, the support means can comprise means mounting the rails on the cell sidewall, a gantry spanning the cell bottom and mounted on the pair of rails, a carriage supporting the treatment means, with the carriage being moveable along the gantry, thereby allowing the treatment means to move across the cell, and drive means for driving the gantry along the rails and the treatment means across the gantry. However, a configuration where the gantry moves along one rail mounted on a cell sidewall while being supported by the opposite sidewall by means of rollers is also feasible.

On account of the high magnetic fields present in cells rooms, even adjacent to an out-of-service cell, preferably the gantry and the rail(s) are made of -non- ferromagnetic materials, for example of aluminum profiles. In cells where the cell sidewalls each include a top part of ferromagnetic materials such as a deck plate, the means mounting the rails on the cell sidewalls can comprise magnets magnetically attached to these top parts of ferromagnetic materials, assuring a firm and reliable securing of the rails in a simple manner. As an alternative to mounting on the cells deck plates, it is possible to mount the rails on aluminum girders or beams placed on the cell sides. Suspension from the cell superstructure is also possible, but not preferred.

In one embodiment, at least one rail is an upstanding profile having an upper edge provided along its length with a rack. Such upstanding rails are important to avoid any sagging which could lead to uneven treatment of the cell bottom. The drive means for driving the gantry along such upstanding rails may comprise at least one pinion which is drivably engaged with the rack on the rail, a motor carried by the gantry, and a transmission connecting the motor to this pinion. As an alternative, the gantry can be driven along a rail by having a driving belt or equivalent mounted along the rail by means of a wheel located at one end of said rail and a driving wheel driven by a motor at the other end, said gantry being attached to the driving belt. In this case, the motor for driving the gantry along the rail is immobile, i.e. not carried by the gantry.

This transmission may comprise a rotatable shaft extending across the gantry, in which case the aforesaid pinions can be adjustably mounted on end parts of the shaft to allow for adjustment. By this arrangement, the same apparatus can be fitted on cells of different widths, using one motor carried by the gantry for driving the gantry along the rails. However a configuration where only one rail is provided with a rack and pinion (s) may be preferred when combined with the roller support on the other end of the cell (as described above) in which case the motor driving the pinion may be closely located to the rail whereby no long rotatable shaft extending across the cell is required.

The gantry may comprise at least one girder carrying at least one rack extending across the cell bottom. In this case, the drive means for driving the carriage across the gantry may comprise another motor carried by the carriage, this motor driving at least one pinion which drivably engages with the rack on the girder comprising the gantry. Advantageously, the gantry has two spaced-apart parallel girders each carrying on its inside face a rack on which the carriage is supported, with the treatment means suspended between the girders.

The treatment means can, for example, be adapted to apply a material to the cell lining. For example, the treatment means can include a spray nozzle whose height is adjustable relative to the carriage and hence relative to the cell bottom. The treatment means can be connected to a supply of a material composed, for example, of a slurry of particulate, refractory, hard material in a colloidal carrier as disclosed and claimed in US Patent 5,651,874. This supply is usually external to the cell.

For example, a supply of material external to the cell may be connected to the treatment means by a flexible tubing which extends from the external supply to one end of the gantry, then along the gantry and is bent back on itself and then connected to a spray nozzle on other treatment means. From approximately mid-way along the gantry until it reaches the carriage, this flexible tubular guide is advantageously enclosed in a folded-over articulated flexible tubular guide which is arranged to roll along the gantry as the carriage moves across the gantry, so as to maintain the enclosed part of the flexible tubing with even curvature for all positions of the carriage. By this means, unwanted twisting and deformation of the tubing is avoided, thereby ensuring an even supply of the treatment material for all positions of the carriage.

The carriage may further carry at least one heater to assist drying of applied treatment material, especially when the treatment means applies liquid treatment material to the cell lining. This heater is, for instance, a blow- drier for blowing hot air onto the cell bottom.

The treatment means carried by the carriage can include at least one of the following: (a) means such as a spray nozzle for applying a slurry to the cell lining; (b) means such as a spray nozzle for applying a treatment liquid to the cell lining; (c) drying means such a blow heater; (d) means for sandblasting the cell lining, e.g. to prepare the lining for further treatments; (e) means such as a vacuum nozzle for evacuating debris from the cell lining; (f) means for depositing powdery solid material such as coke onto the carbon bottom; (g) means for raking powdery solid material such as coke deposited on the carbon bottom; (h) means for rolling powdery solid material such as coke deposited on the carbon bottom in order to level and/or to compact the deposited material; (i) means for supplying aluminum foil;

(j) means for spraying aluminum, for instance aluminum paint; (k) means for measuring the profile of the cell bottom, for the purpose of analysis; and (1) means for removing samples from the cell bottom, e.g. specimens cut/drilled from a used cell bottom for the purpose of analysis .

The apparatus can comprise a control panel, for instance, carried by the support means, such as by a gantry and adjacent to one end thereof so that the control panel moves along with the gantry. This control panel may comprise control members associated with means for setting the extent of movement and the speed of the treatment means over the cathode lining, in particular of the carriage across the gantry and of the gantry along the rail(s), means for setting treatment parameters such as the rate of delivery of material by the treatment means, and means for setting a sequence and number of movements of the gantry for any given cell bottom to be treated. Optionally, when the apparatus is so designed as to suit a sloped cathode lining configuration the control members may be associated with means setting the height of the treatment means.

This control panel is preferably associated with a memory device for storing the aforesaid settings, enabling at least some of the settings established for one cell to be applied to another cell of the same type whose lining is to be subjected to the same treatment. In this manner, the operation of the apparatus can be automated, and tedious resetting operations avoided when the apparatus is used to treat a series of like cells in the same way.

The invention also concerns a method of treating a cathode lining of an aluminum electrowinning cell when the cell is out of use. The cathode lining is contained in a shell and includes a flat carbon bottom and cell sidewalls forming a trough which receives liquid cell contents when the cell is in use. The method of treating the cathode lining includes a step of installing the support means on the cell. The method further includes a step of fitting the treatment means on the support means . The method further includes the step of adjusting the extent of movement of the treatment means over the cathode lining. For instance a programmable memory device may be used to control the extent of these movements. The method further includes the step of treating the cell lining by driving the treatment means over the cathode lining providing a selected treatment or treatments .

As described above, the support means may comprise a pair of rails installed on the cell sidewalls, a gantry and a carriage fitted on rails.

In one aspect of the method of the present invention, the apparatus is utilized on a new or a renovated aluminum electrowinning cell prior to cell start-up. In this case, the treatment means can be used to apply a material onto the cell lining. For example, at least one layer of material such as a slurry of refractory hard material is applied on the carbon cathode bottom by reciprocating the carriage across the gantry and moving the gantry along the rail(s). In another example, at least one layer of treatment material is applied on the carbon cathode bottom by reciprocating the gantry along the rail(s) and moving the carriage across the gantry.

For some applications, in particular for the application of a slurry of refractory, hard material, these two examples are combined, i.e. at least two alternate layers of material are applied on the carbon cathode bottom, one by reciprocating the carriage across the gantry and moving the gantry along the rail(s), and the other by reciprocating the gantry along the rail(s) and moving the carriage across the gantry.

Many materials can be applied to the cell lining in liquid form and drying of the applied material can be assisted by heating. The applied material can be composed of a slurry of particulate, refractory, hard material in a liquid carrier, in particular a colloidal carrier. A spray nozzle or other adequate treatment means can be used to apply a treatment liquid to impregnate at least part of the cell lining. Suitable treatment means can also be used to evacuate debris from the cell lining, or to deposit powdery solid material onto the carbon cathode bottom. Also, when powdery solid material is deposited on the carbon cathode bottom, for example even in conventional manner by unloading the material onto the cell bottom manually or from the cell superstructure, the deposited materials such as coke, can be raked or otherwise leveled by the treatment means. Powdery solid material deposited on the carbon cathode bottom can also be compacted by the treatment means, e.g. by rolling.

The treatment means can be used to apply aluminum foil onto the carbon cathode bottom or onto a layer deposited thereon. For example, the aluminum foil can be unrolled from one or more rolls carried by the gantry. Alternatively, the treatment means sprays aluminum onto the carbon cathode bottom or onto a layer deposited thereon. The sprayed aluminum can be in the form of an aluminum paint.

The method of the invention can comprise memorizing a series of settings for a given cell lining to be treated, for example, including several of the following settings:

(a) the extent of movement of the carriage and the speed of the carriage across the gantry;

(b) the extent of movement of the gantry and the speed of the gantry along the rails;

(c) treatment parameters, such as the rate of delivery of materials by the treatment means and stoppage times; and

(d) a selected sequence and number of movements of the carriage and gantry. Some or all of the memorized settings can then be used when the apparatus is fitted on another cell of the same type whose lining is subjected to the same treatment.

The apparatus can be installed on an aluminum electrowinning cell after shut-down of the cell and after removal of the liquid cell contents leaving a used cell lining. In this case, the treatment means can be used to treat the used cell lining, as discussed herein. The invention however, in particular, covers use of the apparatus for applying a coating of refractory hard material to the cell lining of an aluminum electrowinning cell prior to cell start-up, particularly when the refractory hard material is applied from a slurry of particulate, refractory, hard material in a colloidal carrier or for other purposes.

For instance, the apparatus can be used for the application of a temporary protection for the start-up of a cell, such as applying a foil of aluminum having a thickness of less than 0.1 mm, a metallization, a boron containing- solution, a polymer, a solution containing phosphates of aluminum, a colloid or combinations thereof optionally further covered with a thick sheet of aluminum having a thickness of 1 to 5 mm as described in International application PCT/US96/16643 (de Nora et al) . The apparatus can also level a layer of resistor coke atop the cell bottom, which coke is used to preheat the cell by the Joule effect, namely by passing current via anodes through the resistor coke to the cathodes.

Lastly, the apparatus can be used in the process of renovating a used cell lining of an aluminum electrowinning cell after shut-down of the cell and after removal of the liquid cell contents. The renovation steps can include measurement of the profile of the used cell bottom for the purpose of analysis, the removal of samples from selected locations of the cell bottom for the purpose of analysis, the removal of debris after breaking up the used cell lining, and so forth. Demolishing the used cell bottom, e.g. using pneumatic drills and reassembling new carbon blocks, can be carried out in the usual manners.

Brief Description of Drawings

In the accompanying drawings:

Figure 1 is a schematic transverse cross-section through an aluminum production cell when the cell is out of use, showing the cell lining, the surrounding deck plate and a preferred embodiment of an apparatus fitted over the cell lining according to the invention; and

Figure 2 is a view of a side part of the cell of Figure 1 to a larger scale.

Detailed Description

Figure 1 shows a cathode lining of 10 of an aluminum electrowinning cell when the cell is out of use. The cathode lining 10 is contained in a shell 11 which is housed in a recess in the cell-room floor 16. Lining 10 includes a carbon cathode bottom 12 and cell sidewalls 13 forming a trough 14 which, when in use, receives liquid cell contents, usually a molten cryolite electrolyte containing dissolved alumina and molten aluminum. Between the bottom 12 and sidewalls 13 is an inclined section or wedge 17 of ramming paste.

A pair of rails 20 are mounted along the cell sidewalls 13 and a gantry 30 spanning the cell bottom is mounted on rails 20 for movement along the cell sidewalls 13. A carriage 40 is movable along the gantry 30. The carriage 40 carries treatment means 44, 45 for treating the cell lining 10. An arrangement is provided for driving the gantry 30 along the rails 20, and for driving the carriage 40 across the gantry 30 to move the treatment means 44, 45 over parts of the cell lining 11 to be treated.

The gantry 30 and the rails 20 preferably are made of non-ferromagnetic materials such as aluminum profiles. Along the cell sides are deckplates 15, which are made of ferromagnetic materials and connected to the cell-room floor 16. The rails 20 are mounted along the cell sidewalls 13 by means of permanent magnets 21 magnetically attached to the deck plates 15 at an even spacing.

Each rail 20 is an upstanding aluminum profile having an upper edge provided along its length with a rack 22. The drive means for driving the gantry 30 along the rails 20 comprises pinions 23 drivably engaging with the racks 22 and rails 20, a motor 31 carried centrally by gantry 30 and a transmission 24 connecting motor 31 to pinions 23. This transmission 24 is principally composed of a rotatable shaft 24 extending across gantry 30. The pinions 23 are adjustably mounted on end parts of the shaft 24 to allow for adjustment, whereby the apparatus can be fitted on cells of different widths.

The gantry 30 comprises two spaced-apart parallel aluminum girders 33 each carrying on its inside at least one rack 32 on which the carriage 40 is supported, with the treatment means 44, 45 being suspended between the girders 33. The carriage 40 carries a motor 41. The carriage 40 is driven across the gantry 30 the motor 41. More specifically, this motor 41 drives pinions 43 which drivably engage with the racks 32 of gantry 30. The motors 31, 41 are conveniently servo or stepping motors which provide a high degree of precision under control of a control device. It is U

alternatively possible to use cage motors where high precision is not needed.

The exemplary treatment means of this embodiment includes a spray nozzle 44 whose height is adjustable relative to the carriage 40 for applying a material to the cell lining 10. In the example of Figs. 1 and 2, the spray nozzle 44 has a telescopic head. This spray nozzle 44 is connected to a supply of a treatment material. For example, the treatment material may be a slurry of particulate, refractory, hard material in a colloidal carrier via a flexible tubing 35 which connects a supply of said material external to the cell to the spray nozzle 44. This flexible tubing 35 extends from said supply to one end of gantry 30, then along the gantry 30 and is bent back on itself to be connected to the spray nozzle 44. This flexible tubing 35, from approximately mid-way along gantry 30 until it reaches carriage 40, is enclosed in a folded-over articulated flexible tubular guide 36 which is arranged to roll along gantry 30 as the carriage 40 moves across gantry 30 so as to maintain the enclosed part of the flexible tubing 35 with even curvature for all positions of carriage 40.

The spray nozzle 44 usually applies treatment material in liquid form to the cell lining 10, in which case the carriage 40 further carries at least one heater 45 to assist drying of applied treatment material. The heater 45 is, for example, a blow-drier for blowing hot air onto the cathode lining 10, at a temperature suited to the treatment.

As mentioned previously, different treatment means can be provided for carrying out different treatments. Several treatment means can be permanently fixed to the carriage 40 and/or the gantry 30, or can be removably secured so they are fitted only when a treatment is .to be carried ou . The apparatus of Figures 1 and 2 further comprises a control panel 34 carried by gantry 30 adjacent one end thereof. This control panel 34 comprises a computer control adapted for operation in the cell room with its prevailing high magnetic fields. The control panel includes means for setting the extent of movement (stroke) and the speed of carriage 40 across gantry 30. The control panel further includes means for setting the extent of movement and the speed of gantry 30 along rails 20. The control panel 34 further includes means for setting treatment parameters such as the rate of delivery of material by the spray nozzle 44 or other treatment means. The control panel further includes means for setting a sequence and number of movements of carriage 40 and gantry 30 for any given cell bottom 10 to be treated.

By means of this control panel 34, the user can set the traverse rate and stroke (along gantry 30) , the forward rate and stroke (along rails 20) , the spray idle, and the clearance of spray nozzle 44. With these settings, a program incorporated in control panel 34 can evaluate the desired movement rates of the spray nozzle 44 to take account of the wedges 17. The control panel 34 is associated with a memory device, e.g. including an insertable card, for storing the settings. This enables at least some of the settings established for one cell to be applied to another cell of the same type whose lining 10 is to be subjected to the same treatment .

A method for using the above-described apparatus to treat a cell will now be described. To treat a cell lining 10, the pair of rails 20 are installed along the cell sidewalls 13 by means of magnets 21. Then, the gantry 30 and carriage 40 are fitted on the rails 20 for movement of the gantry 30 on the rails 20 along the sidewalls 13. The apparatus is adjusted to the width of the cell by sliding of the pinions 23. The extent of movement of gantry 30 along rails 20 and of carriage 40 across gantry 30 is then adjusted so that the treatment means 44, 45 is able to move over all of cell lining 10. Treating a cell lining 10 by said treatment means 44, 45 is achieved by driving gantry 30 along rails 20 and driving carriage 40 along gantry 30. It will be appreciated that the apparatus can easily be installed on and removed from an out-of-service cell. Indeed, the apparatus should be removed from the cell before the cell is put into service.

It will be understood that modifications may be made in the present invention without departing from the scope of the following claims.

Claims

1. An apparatus for treating a cathode lining (10) of an aluminum electrowinning cell when the cell is out of use, said cathode lining being contained in a shell (11) and including a cathode bottom (12) and cell sidewalls (13) forming a trough (14) which in use contains molten cell contents, said apparatus comprising: treatment means (44, 45) for treating the cell lining (10) ; support means (20, 30, 40) at least temporarily mounted on the cell and supporting the treatment means (44, 45) for movement thereof over the cathode lining (10); and drive means (31, 41) for moving said treatment means (44, 45) over the cathode lining

(11) to carry out a treatment thereon.

2. The apparatus of claim 1, wherein the support means (20, 30, 40) comprises:

(a) length-wise support means (20, 30) for supporting the treatment means (44, 45) moveably with respect to a length direction of the cell; and

(b) transverse support means (30, 40) for supporting the treatment means (44, 45) moveably with respect to a transverse direction of the cell.

3. The apparatus of claim 1 including means for adjusting the height of the treatment means (44, 45) over the cathode lining (10) .

4. The apparatus of claim 3, wherein the means for adjusting the height has a telescopic head (44, 45) which telescopes to adjust the height of the treatment means (44, 45) over the cell lining (10) .

5. The apparatus of claim 2, wherein the support means (20, 30, 40) comprises:

(a) at least one rail (20) mounted along a length direction of the cell; and

(b) a treatment means support apparatus (30, 40) supporting the treatment means (44, 45) , with the treatment means support apparatus (30, 40) being moveable along the rail (20) , thereby allowing the treatment means (44, 45) to move in the length direction of the cell.

6. The apparatus of claim 5, wherein the support means (20, 30, 40) comprises:

(a) a pair of rails (20) ;

(b) means (21) mounting the rails (20) on the cell sidewalls (13) ;

(c) a gantry (30) spanning the cell bottom and mounted on said rail(s) (20); and

(d) a carriage (40) supporting the treatment means (44, 45), with the carriage (40) being moveable along the gantry (30), thereby allowing the treatment means (44, 45) to move across the cell.

7. The apparatus of claim 6 including, drive means for driving the gantry (30) along the rails (20) , and for driving the treatment means (40) across the gantry (30) .

8. The apparatus of claim 6, wherein the gantry (30) and the rails (20) are made of non-ferromagnetic materials.

9. The apparatus of claim 8, wherein the gantry (30) and the rails (20) are made of aluminum profiles.

10. The apparatus of claim 6, wherein each of at least two opposite cell sidewalls (13) comprises a top part (15) of ferromagnetic material and the means mounting the rails on the two facing cell sidewalls (13) comprise magnets (21) magnetically attached to said top parts (15) .

11. The apparatus of claim 7, wherein:

at least one rail (20) is an upstanding profile having an upper edge provided along its length with a rack (22); and

said drive means for driving the gantry (30) along the rails (20) comprises:

(a) at least one pinion (23) drivably engaging with the rack (22) on said at least one rail (20) ,

(b) a motor (31) carried by the gantry (30), and

(c) a transmission (24) connecting the motor (31) to said pinion (23) .

12. The apparatus of claim 11, wherein the transmission comprises at least one rotatable shaft (24) extending to the rack (22) , said pinions (23) engaging with the rack (22) being adjustably mounted on the shaft (24) to allow for adjustment whereby the apparatus can be fitted on cells of different widths.

13. The apparatus of claim 7, wherein:

(a) the gantry (30) comprises at least one girder (33) carrying at least one rack (32) extending across the cell bottom (12); and

(b) said drive means for driving the carriage (40) across the gantry (30) comprises a motor (41) carried by the carriage (40) , said motor (41) driving at least one pinion (43) which drivably engages with said rack (32) of the gantry (30) .

14. The apparatus of claim 13, wherein the gantry (30) comprises two spaced-apart parallel girders (33) each carrying on it inside at least one rack (32) on which the carriage (40) is supported with said treatment means (44, 45) being suspended between the girders (33) .

15. The apparatus of claim 1, wherein the treatment means (44) are adapted to apply a material to the cell lining (10) .

16. The apparatus of claim 15, wherein the treatment means comprise a spray nozzle (44) whose height is adjustable relative to the carriage (40) .

17. The apparatus of claim 16, wherein the treatment means (44) is connected to a supply of a material composed of a slurry of particulate refractory hard material in a colloidal carrier.

18. The apparatus of claim 6, further comprising:

a flexible tubing (35) which connects a supply of material external to the cell to said treatment means (44), said flexible tubing (35) extending from said supply to one end of the gantry (30), then along the gantry (30) and then being bent back on itself to be connected to the treatment means (44) ;

a folded-over articulated flexible tubular guide (36) enclosing said flexible tubing (35) from- approximately mid-way along the gantry (30) until it reaches the carriage (40) said guide (36) being arranged to roll along the gantry (30) as the carriage (40) moves across the gantry (30) so as to maintain the enclosed part of the flexible tubing (35) with even curvature for all positions of the carriage (40) .

19. The apparatus of claim 6, wherein: (a) the treatment means (44) applies material in liquid form to the cell lining (10) ; and

(b) the apparatus further comprises at least one heater (45) carried by the carriage (40) to assist drying of applied treatment material.

20. The apparatus of claim 19, wherein the heater (45) is a blow-drier for blowing hot air onto the cathode lining (10) .

21. The apparatus of claim 1, wherein the treatment means includes at least one of the following:

(a) means (44) for applying a slurry to the cell lining;

(b) means (44) for applying a treatment liquid to the cell lining;

(c) drying means (45) ;

(d) means for sandblasting the cell lining;

(e) means for evacuating debris from the cell lining;

(f) means for depositing powdery solid material onto the carbon bottom;

(g) means for raking powdery solid material deposited on the carbon bottom;

(h) means for rolling powdery solid material deposited on the carbon bottom;

(i) means for supplying aluminum foil;

(j) means for spraying aluminum;

(k) means for measuring the profile of the cell bottom; and (1) means for removing samples from the cell bottom.

22. The apparatus of claim 1, further comprising a control panel (34) carried by the support means, said control panel (34) comprising:

(a) means for setting the extent of movement and the speed of the treatment means (44) over the cathode lining;

(b) means for setting treatment parameters such as the rate of delivery of material by the treatment means

(44); and

(c) means for setting a sequence and a number of movements of the treatment means (44) for any given cell bottom (10) to be treated.

23. The apparatus of claim 22, wherein the control panel (34) is associated with a memory device for storing said settings, enabling settings established for one cell to be applied to another cell of the same type whose lining is to be subjected to the same treatment.

24. A method of treating a cathode lining (10) of an aluminum electrowinning cell when the cell is out of use, said cathode lining being (10) contained in a shell (11) and including a flat carbon bottom (12) and cell sidewalls (13) forming a trough (14) which in use receives liquid cell contents, said method using the apparatus of claim 1 and comprising:

(a) installing the support means (20, 30, 40) on the cell and the treatment means (44, 45) on the support means;

(b) adjusting the extent of movement of the treatment means (44, 45) over the cathode lining (10) ; and (c) treating the cell lining (10) by said treatment means (44, 45) by driving the treatment means over the cathode lining.

25. The method of claim 24, wherein the installation of the support means comprises installing a pair of rails (20) on the cell sidewalls (13) , and fitting a gantry (30) and carriage (40) on the rails (20) on the sidewalls (13) .

26. The method of claim 24 wherein the apparatus is installed on a new or a renovated aluminum electrowinning cell prior to cell start-up.

27. The method of claim 24, wherein the treatment means (44, 45) is used to apply a material onto the cell lining (10) .

28. The method of claim 25, wherein at least one layer of material is applied on the carbon cathode bottom (12) by reciprocating the carriage (40) across the gantry (30) and moving the gantry (30) along the rails (20) .

29. The method of claim 25, wherein at least one layer of material is applied on the carbon cathode bottom (12) by reciprocating the gantry (30) along the rails (20) and moving the carriage (40) across the gantry (30) .

30. The method of claim 25, wherein at least two alternative layers of material are applied on the carbon cathode bottom (12), one by reciprocating the carriage (40) across the gantry (30) and moving the gantry (30) along the rails (20) , and the other by reciprocating the gantry (30) along the rails (20) and moving the carriage (40) across the gantry (30) .

31. The method of claim 27, wherein said material is applied to the cell lining (10) in liquid form and drying of the applied material is assisted by heating.

32. The method of claim 27, wherein the applied material is composed of a slurry of particulate refractory hard material in a colloidal carrier.

33. The method of claim 27, wherein the treatment means (44) applies a treatment liquid to impregnate at least part of the cathode lining (10) .

34. The method of claim 24, wherein the treatment means evacuates debris from the cell lining (10) .

35. The method of claim 27, wherein the treatment means deposits powdery solid material onto the carbon cathode bottom (12) .

36. The method of claim 27, wherein powdery solid material is deposited on the carbon cathode bottom (12) , and the deposited powdery solid material is raked by the treatment means.

37. The method of claim 27, wherein powdery solid material is deposited on the carbon cathode bottom (12) , and the deposited powdery solid material is compacted by the treatment means.

38. The method of claim 27, wherein aluminum foil is applied onto the carbon cathode bottom (12) or onto a layer deposited thereon.

39. The method of claim 27, wherein the treatment means sprays aluminum onto the carbon cathode bottom (12) or onto a layer deposited thereon.

40. The method of claim 25, comprising the steps of:

memorizing a series of settings for a given cell lining to be treated, selected from:

(a) a setting of the extent of movement and the^" speed of the carriage (40) across the gantry (30) ; (b) a setting of the extent of movement and the speed of the gantry (30) along the rails (20) ;

(c) a setting of treatment parameters, such as the rate of delivery of material by the treatment means (44) and stoppage times; and

(d) a setting of a sequence and number of movements of the carriage (40) and gantry (30); and

using the memorized settings when the apparatus is fitted on another cell of the same type whose lining (10) is subjected to the same treatment.

41. The method of claim 24, wherein the apparatus is installed on an aluminum electrowinning cell after shutdown of the cell and after removal of the liquid cell contents, leaving a cell lining (10) , and the treatment means is used to treat the used cell lining (10) .

42. Use of the apparatus of claim 1 for applying a coating of refractory hard material to the cell lining (10) of an aluminum electrowinning cell prior to cell start-up.

43. Use according to claim 42, wherein the refractory hard material is applied from a slurry of particulate refractory hard material in a colloidal carrier.

44. Use according to claim 41, wherein the apparatus is used to level at least one layer of resistor coke applied onto the cathode cell lining which is possibly coated with refractory material, and/or to apply at least one layer of protective start-up material and/or a sheet of aluminum having a thickness of 1 to 5 mm onto the cathode lining or onto the coating of refractory material for cell start-up.

45. Use of the apparatus of claim 1 for preparing the carbon cathode bottom for cell start-up by removing any debris therefrom, and for leveling a layer of resistor coke applied onto the carbon bottom.

46. Use of the apparatus of claim 1 for renovating a used cell lining of an aluminum electrowinning cell after shut-down of the cell and after removal of the liquid cell contents .