GB2566674A - Electrolytic cell for aluminium production, with individual anode drives - Google Patents

Abstract

This superstructure comprises a frame (102) of substantially rectangular shape, an anode beam (104) fixed with respect to said frame, said superstructure comprising a plurality of anode holders (4A-N, 6A-N) each comprising holding means (130) for holding a respective suspended anode (1A-N, 2A-N) of said cell. The superstructure also comprises moving means (10,12,14,15,16,17,21A-N,31A-N,22A-N,32A-N) adapted to move a single anode holder independently from the others, along a vertical direction. The moving means comprises drive means (1) and transmission means (12,14,15,16,17,21A-N, 31A-N, 32A-N). Thus each holder (4A-N, 6A-N) is slidably mounted in a respective pocket or guide (60A-N, 70A-N) provided in said anode beam (104). The transmission means may compose a common transmission means (12,14,15,16,17), which works with the drive means (10). A number of individual transmission means (Figure 3, 21A-N, 31A-N, 22A-N, 32A-N) operate with the common transmission means and the drive means (10) so that motion is imparted to a single anode holder separate from the other anode holders.

Description

Electrolytic cell for aluminum production, with individual anode drives

Technical field of the invention

The invention relates to an improvement of an electrolysis cell (also called “pot”) for producing aluminium by fused salt electrolysis using the Hall-Heroult-process. More precisely it relates to a cell with individual anode drives.

Prior Art

The anode-cathode distance (ACD), or more precisely, the vertical spacing between the anode and the subjacent electrolyte-metal interface (knowing that the liquid metal is denser than the electrolyte and forms a pad on the top of the cathode) is a critical parameter for the control of an electrolytic Hall-Heroult cell. As the anodes burn off during the electrolysis process, the ACD is not constant and needs to be adjusted regularly. In conventional cells there are two means to adjust the ACD: collectively by a vertical movement of the anodic frame to which all anode rods are attached, or individually, by using a crane. Individual adjustment is necessary in particular when anodes that are burnt off or defective are replaced.

Apparatus and methods for automatically regulating the ACD are described in US 3,627,666 (Pechiney) and US 3,900,373 (Olin Corp.).

Modern process control systems of electrolysis cells are designed to allow a vertical displacement of the anodic frame with an increment of about 1 mm, using a mechanical jack system driven by an electrical motor that is fed with individual electrical pulses of a preselected duration. The accuracy of the adjustment of the ACD is not only determined by the control of the duration of the timed pulse that is sent to the electric motor but also by perturbative effects such as the varying resistance to anode movement imparted by the frozen crust: sticking of the anodes to the frozen crust or sticking of crust to the anodes may affect the response of the anode height change to a timed pulse sent to the motor.

Individual anode adjustment can be done only by a crane, with an increment of the order of 0.5 cm. However, the use of a crane is a cumbersome procedure as it requires the presence of an operator next to the pot, for supervision and to lift and put back the hood. In daily practice, the adjustment of the individual ACD by a crane is infrequent.

It would be desirable to be able to adjust the ECD for each anode or pairs of anodes more easily. Indeed, in an electrolytic cell it is desirable that each anode carries about the same current, for several reasons: it leads to higher energy efficiency, it favors uniform anode consumption, it contributes to overall pot stability, it avoids overloading certain anodes which can then show a burn-off in the contact area of the stubs (in this case the anode can fall down into the pot and burn).

In 1987 J.P.R Huni (“A-275 - Individual Anode Control’’, TMS Light Metals 1987, p. 199-202) described Alcan’s A-275 cell in which “every anode (..,) has its own mechanical drive and flexible current lead to the fixed anode busbar. This construction lends itself to individual anode current monitoring which is used to determine the cell anode current distribution as well as the current stability of each anode. This data is then used by a control logic to identify the anode(s) which need to be moved up or down to equilize the current distribution and/or cure individual anode instability before the appearance of the instability on the cell voltage. Individual anode drives are also used to simplify the setting at anode change.”

A detailed description of the mechanical design of the individual anode drives of Alcan’s A275 cell has been given in US 4,414,070 (Alcan International Ltd). Unlike the cell described in US 3,575,827, this cell has two rows of anodes. The anode drive system uses a plurality of screwjacks driven by a common drive system, together with individually engageable slipping frictional clutches associated with gear means located on the output shafts. The use of a friction clutch is an essential means to protect the motor in case of jamming of the system.

Individual screwjacks or pneumatic jacks had been known earlier, but they have not proven to be sufficiently reliable. In particular, raising (or even descending) the anodes need to overcome the resistance of the crust formed by solidified electrolyte that sticks to the anodes. This implies a risk of jamming the motor of the screw jack, and also a risk of uncontrolled movement upon breaking of the crust in the case of screwjacks or pneumatic jacks.

US 3,575,827 (Johnson) describes an unusual cell design in which the anodes are arranged side-by-side in one single row. Each anode is equipped with two lateral screw jacks (one on each end). Each jack seems to have its own motor which would allow an individual vertical movement of each anode; however, this is a very expensive design, and cannot be used as such in conventional cells with two rows of anodes.

WO 2016/128822 (Rio Tinto International Ltd) describes a novel design of the superstructure for a cell with two rows of anodes. Two opposite anodes share a common anode bus bar. The superstructure is designed so as to allow a common vertical movement of all the anodes, or an individual vertical movement of an individual anode bus bar, leading to the common vertical movement of two opposite anodes. This novel design does not address significant issues such as the collection of exhaust gases, and does not fully address the problem of the control of the ACD for individual anodes.

US 3,245,898 (Swiss Aluminium Ltd.) describes a rack and pinion device for positioning anodes: the anodes are connected to a rack trough a plate, and the rack is drive, by its connection to a pinion carried on a motor shaft.

Other patents have tried to overcome the use of electrical motors: US 4,039,419 (Aluminum Company of America) describes a system for positioning anodes by using a piston-cylinder means, whereas US 4,210,513 (Aluminum Company of America) describes another system for positioning anodes individually in which mechanical shafts are driven by pneumatic means. This requires a huge supply of compressed air in the plant which is expensive.

Object of the invention

A first object of the invention is a superstructure for an electrolytic cell, suitable for the HallHeroult electrolysis process, said superstructure comprising a frame of substantially rectangular shape, an anode beam fixed with respect to said frame, a plurality of anode holders each comprising holding means for holding a respective suspended anode of said cell, said superstructure comprising also moving means adapted to move one single anode holder independently from the others, along a substantially vertical direction, said moving means comprising drive means, as well as transmission means, for transmitting movement imparted by drive means to each holder, said superstructure being characterized in that each holder is slidably mounted into a respective pocket or guide, provided in said superstructure.

Said transmission means may comprise common transmission means adapted to engage with drive means as well as a plurality of individual transmission means adapted to selectively engage with common transmission means, each individual transmission means being adapted to impart motion from drive means to one single anode holder independently from the others.

The superstructure may comprise a plurality of clutches, each being interposed between common transmission means and a respective individual transmission means.

Each clutch may be adapted to have an engaged position and a disengaged position, said superstructure comprising spring means adapted to maintain said clutch in its engaged position, said superstructure also comprising pneumatic means adapted to pneumatically disengage said clutch.

Said pneumatic means may comprise a common pneumatic distribution unit and individual pneumatic distribution lines, each connecting pneumatic distribution unit to a respective clutch.

According to an advantageous embodiment, each clutch is a spring-applied gear-tooth clutch.

According to an advantageous embodiment, each individual transmission means comprise a driving screw adapted to cooperate with a driving nut adapted to move said holder.

Said superstructure may comprise a plurality of auxiliary insulating members adapted to electrically insulate said driving screw with respect to said holder.

According to an advantageous embodiment, each auxiliary insulating member is attached with one driving nut.

Said auxiliary insulating member may comprise a tubular body provided with said inner threaded wall, as well as means for the attachment of said tubular body with respect to said holder.

According to an advantageous embodiment, said tubular body extends partly in a not threaded groove provided in a front wall of said holder, said groove extending along a portion of circle, viewed from above.

Said anode beam may be provided with a not threaded notch, facing said groove of the holder, to ensure a free passage of tubular body.

According to an advantageous embodiment, said not threaded notch and said groove substantially form a circle, the diameter thereof is slightly superior to that of tubular body.

Each holder may be a massive block substantially shaped as a parallelepiped, made of aluminium.

Walls of said guide may comprise so called anti-jamming walls intended to face said holder, said anti-jamming walls being made of a material different from aluminium, in particular of copper.

Said anti-jamming walls may comprise profiles made of said material different from aluminium.

Said guide may lead only to outer front wall of anode beam, opposite the other row of anodes.

Side walls of said holder may comprise a shoulder adapted to cooperate with a corresponding shoulder provided in facing side walls of said guide.

Said holder may have fixing means, in particular removable fixing means, for fixing a first end of an electric connector, second end of said connector being attached, in particular permanently attached, to anode beam.

Said connector may be formed by a plurality of flexible members of a conductive material.

Said holder may comprise clamping means for selective attachment of an anode rod with respect to said holder.

According to a first alternative of the invention, said pocket or guide is provided in said anode beam.

According to another alternative of the invention, said pocket or guide is provided in said frame of said superstructure.

According to a first embodiment of the invention, the whole holder is slidably mounted into said pocket or guide, so that said holder does not protrude outside said guide, viewed from top.

According to another embodiment of the invention, only part of said holder is slidably mounted into said pocket or guide. In particular, said holder may comprise a main body located outside said guide, viewed from top, as well as at least one projection extending from said body, said projection being slidably mounted into said pocket or guide.

Another object of the invention is an electrolytic cell, in particular an electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a superstructure as above described, as well as a plurality of anodes and anode rods, each holder being adapted to selectively attach a respective anode rod with respect to said holder.

Each anode rod may be associated with an inverter, so that a first row of anodes may move vertically in a first direction, whereas the other row of anodes may move vertically in the opposite direction.

Still another object of the invention is an aluminium electrolysis plant comprising at least one line of electrolytic cells of substantially rectangular shape, said cells being arranged side by side, and said plant further comprising means for electrically connecting said cells in series and for connecting the cathodic busbar of a cell to the anode beam of a downstream cell, characterized in that more than 80% of the electrolytic cells in at least one of said line, and preferably each electrolytic cell of said line, is an electrolytic cell as above described.

Still another object of the invention is a method for making aluminium by the Hall-Heroult electrolysis process using electrolytic cells of substantially rectangular shape, characterized in that said method is carried out in an aluminium electrolysis plant as above described.

The present invention is for use in electrolytic aluminium reduction cells implementing the Hall-Heroult process. In these cells a plurality of suspended anode assemblies are supported to a superstructure, the height of each anode being controlled, and being capable to be vertically raised and lowered, by a respective individual jack. The system comprises a small number (and preferable one single) of common geared motors connected through multiple drive shafts, miter gearboxes, clutches and jacks. The common geared motors are capable of driving anodes in different operation modes such as individually, in batches (groups), and all anodes together. The system is equipped with jacks and respective clutches connected to a common drive shaft from where the motion is transmitted when required. Anode positions can be controlled through electronic positional devices (encoders) integrated within each jacking system. The electrical connectivity between the anode beam and the anode rod is ensured by an anode rod sliding mechanism with stationary anode beam.

Each anode is attached, via its anode rod, to a respective holder which is slidably received in a respective guide of the anode beam. This makes it possible to guide the vertical motion of the holder, when imparted by the motor and the transmission chain. At least part of the walls of the fixed anode beam, facing the walls of moving holder, are made of an electrically conductive material different from aluminium, typically copper. This makes it possible to avoid any jamming phenomenon between said holder and said anode beam.

Figures

Figures 1 to 14 represent various embodiments of the present invention.

Figures 1 and 2 are respectively a top view and a perspective view of an electrolysis cell according to the invention, wherein the superstructure and the anodes are schematically shown, whereas means for moving these anodes are more clearly illustrated.

Figure 3 is a perspective view, showing at a greater scale some mechanical elements of the cell of figure 1, said elements being located at the centre of said cell with respect of longitudinal dimension thereof.

Figure 4 is a top view, showing the central mechanical elements of the cell illustrated on figure 3.

Figure 5 is a cross-section along a horizontal plane, showing the central mechanical elements of the cell illustrated on figure 4.

Figure 6 is a top view analogous to figure 4, rotated by 90 degrees, showing at a greater scale detail VI of figure 4.

Figure 7 is a cross-section analogous to figure 5, showing at a greater scale detail VII of said figure 5.

Figure 8 is a perspective view showing a holder, which belongs to the electrolysis cell according to the invention.

Figure 9 is a top view showing a guide provided in an anode beam part of the electrolysis cell according to the invention, said guide allowing the sliding of the holder illustrated on figure 8.

Figure 10 is a cross-section along line V-V of figure 5, showing further details of said guide as well as an insulating member which cooperates with said guide.

Figure 11 is a cross-section analogous to figure 7, showing at still a greater scale detail XI of said figure 7.

Figure 12 is a top view, analogous to figure 4, showing another embodiment of the invention wherein said guide is provided in the frame of the superstructure.

Figure 13 is an end view, showing the embodiment of figure 12.

Figure 14 is a top view, analogous to figure 4, showing another embodiment of the invention wherein only part of said holder is mounted in said guide.

100	Superstructure	102	Fixed frame of 100
106	Top of 102	104	Anode beam
108	Posts of 102	1,2	Rows of Anodes
1A-1N	Anodes of row 1	2A-2N	Anodes of row 2
TA-TN	Anode rods of row 1	2Ά-2ΊΜ	Anode rods of row 2
L	Longitudinal axis	T	Transversal axis
10	Main motor	12	Main gear box
14,15	Common gear boxes	16,17	Main shafts
1A-1N	Anodes of row 1	2A-2N	Anodes of row 2
20A-20N	Auxiliary gear boxes of row 1	30A-30N	Auxiliary gear boxes of row 2
21A-21N	Auxiliary drive shafts of row 1	31A-31N	Auxiliary drive shafts of row 2
22A-22N	Auxiliary drive screws of row 1	32A-32N	Auxiliary drive screws of row 2
23A-23N	Clutches of row 1	33A-33N	Clutches of row 2
24A-24N	Pneumatic lines of row 1	34A-34N	Pneumatic lines of row 2
108	Pneumatic distribution system	40	Top of 4
4A-4N	Holders of row 1	6A-6N	Holders of row 2
41,42	Inner/outer front wall of 4	43,44	Side walls of 4

45,46	Top/bottom wall of 4	47	Shoulder
W41/W42	Width of 41/42	H4	Height of 4
48	Groove of 41	T4	Thickness of 4
50	Cover	51	Body of 50
52	Threaded hole of 51/ Driving nut	53	Protrusions of 50
54	Foots of 53	55	Holes of 54
56	Bolts in 55	122	Intermediate plate
120	Electric connector	121	Flexible members of 120
80	Insulating member	81	Cylindrical body of 80
82	Bottom of 80	90	Clearance
84/85	Inner/outer faces of 83	86	Flaps of 80
60A-60N	Guides of row 1	63	Profile
70A-70N	Guides of row 2	61,62	Profiles
61’-63’	Flaps of 61-63	W61.W62	Width of Profiles 61,62
110,111	Top/bottom wall of 104	112,113	Outer front/thin wall of 104
D63	Distance of 63	114	Notch
130	Clamp	131	Fixed part of 130
132	Rods of 131	133	Jaw of 130
201’	Anode rod	204	Holder
260	Guide	300	Superstructure
302	Frame of 300	304	Anode beam
320	Connector
401’	Anode rod	404	Holder
404’	Body of 404	405,405’	Projections of 404
460	Guide	460’,460”	Slots of 460
504	Anode beam

Detailed description

An aluminium smelter comprises a plurality of electrolytic cells arranged the one behind the other (and side by side), typically along two parallel lines. These cells are electrically connected in series by means of conductors, so that electrolysis current passes from one cell to the next. The number of cells in a series is typically comprised between 50 and over 100, but this figure is not substantial for the present invention. The cells are arranged transversally in reference of main direction of the line they constitute. In other words the main dimension, or length, of each cell is substantially orthogonal to the main direction of a respective line, i.e. the circulation direction of current.

The Hall-Heroult process as such, the way to operate the latter, as well as the cell arrangement are known to a person skilled in the art and will not be described here in more detail. In the present description, the terms “upper” and “lower” refer to mechanical elements in use, with respect to a horizontal ground surface. Moreover, unless otherwise specifically mentioned, “conductive” means “electrically conductive”.

Figures 1 and 2 describe a typical arrangement of a Hall-Heroult electrolysis cell, which substantially comprises a not shown potshell, a superstructure and a plurality of anodes. This superstructure, referenced 100 as a whole, is schematically represented on said figures 1 and 2. It mainly comprises a fixed frame 102 and a metallic anode frame 104, hereafter called “anode beam”, which extends at the outer periphery of the fixed frame 102. Said frame 102 is essentially formed by an elongated roof 106, the respective ends thereof resting on supporting posts 108. Let us note L the main longitudinal axis of said cell, as well as T its main transversal axis.

Anodes, which are known as such and are schematically represented on figure 1, are arranged in two rows 1 and 2, each comprising 14 anodes 1A, 1B, ..., 1M, 1N, and 2A, 2B, ..., 2M, 2N in the present embodiment. Said anodes, which are known as such, are schematically illustrated on figures. They are associated to classic anode rods TA-TN and 2’A-2’N, so as to allow their attachment to said superstructure.

The general structure of a Hall-Heroult electrolysis pot is known per se and will not be explained here. It is sufficient to explain that the current is fed into the anode beam, flows from the anode beam to the plurality of anodes in contact with the liquid electrolyte where the electrolytic reaction takes place. Then the current crosses the liquid metal pad resulting from the process and eventually will be collected at the cathode block. Only the passage of current from anode beam to the plurality of anodes will be described hereafter more in detail.

The present invention is more particularly directed to the means for individually moving each anode with respect to the fixed frame. The term “individually” means that each anode may be moved along a substantially vertical direction, independently from the other anodes. These means are hereafter called moving means. In the present embodiment, they generally comprise drive means, individual moving members each cooperating with a respective anode, as well as transmission means, adapted for transmitting the movement imparted by drive means to the different moving members.

As mentioned above, figures 1 and 2 illustrate the whole elongated superstructure. These figures also represent, in a schematic manner, the two rows of anodes as well as the moving means cooperating with said anodes. Figures 3 to 5 show, at a greater scale, moving means which cooperate with central anodes 1G, 1H, 2G and 2H, with respect to longitudinal dimension L. Figures 6 and 7 show more in detail, at still a greater scale, mechanical elements associated to single anode 1G.

Drive means of the invention first comprise a motor, designated as a whole with reference 10, which is of a type known as such. Preferably, an electric rotary motor is used, such as a triphasic motor. This motor 10 is associated with a gearbox 12 known as such, also called primary gearbox. This motor and this gearbox are attached to the roof 106 of the fixed frame 102, part of the superstructure 100, by any appropriate means, in particular removable means. This motor and this gearbox may be received in a not shown protecting coffer, which is drilled with an opening for permitting access to an operator. This motor and this gearbox are provided substantially at a midpoint of the rows of anodes, i.e. in the vicinity of the middle of these rows, according longitudinal dimension.

Primary gearbox 12 engages with two identical miter gear boxes 14 and 15, via respective intermediate shafts 14’ and 15’, which extends along an axis which is parallel to axis T. Gear boxes 14 and 15 are also called main or common gear boxes, since each of these gear boxes is common to a respective row 1 or 2 of anodes. Each common gear box engages with a respective so called common or main shaft 16 or 17, adapted to rotate about its longitudinal axis, which is parallel to main axis L of the cell.

Each main shaft cooperates with a plurality of so called individual or auxiliary miter gear boxes, the number of which corresponds to the number of anodes of the cell. Auxiliary miter gear boxes of row 1 are referenced 20A, 20B, ..., 20M and 20N, whereas auxiliary miter gear boxes of row 2 are referenced 30A, 30B, ..., 30M and 30N. As more clearly shown on figure 4, each auxiliary gear box cooperates with a so called individual or auxiliary drive shaft, adapted to move one single respective anode. Auxiliary drive shafts of row 1 are referenced 21 A, 21B, ..., 21M and 21N, whereas auxiliary drive shafts of row 2 are referenced 31 A, 31B, ..., 31M and 31N. Finally, each auxiliary drive shaft is adapted to impart the rotation of a respective driving screw, about its main axis which is substantially vertical. Driving screws of row 1 are referenced 22A, 22B, ..., 22M and 22N, whereas driving screws of row 2 are referenced 32A, 32B, ..., 32M and 32N.

A clutch is interposed between each auxiliary gear box and each auxiliary drive shaft, so as to selectively engage or disengage said shaft. Clutches of row 1 are referenced 23A, 23B,

..., 23M and 23N, whereas clutches of row 2 are referenced 33A, 33B, ..., 33M and 33N. Figure 6 shows more in detail the structure of one 23G of these clutches, bearing in mind that all clutches have the same structure.

The output pin 20’ of auxiliary gear box 20G engages with the input 23’ of said clutch 23G, whereas the output 23” of said clutch engages with said auxiliary drive shaft. Another key feature is the engaging I disengaging mechanism of the clutch. The inventors have found that friction clutches, which are recommended in US 4,414,070, are unreliable upon ageing: they cannot provide a sufficiently precise control of the amplitude of vertical movement over a sufficiently long time of operation. For this reason the inventors do not use friction clutches; this solution is discarded in US 4,414,070 as presenting a risk of jamming the motor. Indeed, in a friction clutch the friction mechanism can protect the motor if the driving mechanism is stuck. According to the invention, the clutches are spring-applied gear-tooth clutches that are normally engaged and can be pneumatically disengaged. To this end, the superstructure is provided with a pneumatic distribution system 108, which is connected to the plurality of clutches via respective pneumatic distribution lines 24A, 24B, ..., 24M and 24N, as well as 34A, 34B, ..., 34M and 34N.

Each driving screw cooperates with a respective driving nut, provided in a respective holding member, or holder, adapted to hold a respective anode. Holders of row 1 are referenced 4A, 4B, ..., 4M and 4N, whereas holders of row 2 are referenced 6A, 6B, ..., 6M and 6N. Figure 8 shows more in detail the structure of one 4 of these holders, bearing in mind that all holders have the same structure.

Said holder 4 is a massive block made of aluminium, shaped substantially as a parallelepiped. With reference in particular to figure 8, it first comprises two opposite front walls, i.e. a so called inner front wall 41, adjacent the other row of anodes, as well as a so called outer front wall 42. As more clearly shown on figure 10, said inner front wall 41 forms a shoulder, which defines a top 40 of said holder. Said top 40 has a greater thickness than main part of holder 4, in other words it protrudes in direction of the other row of anodes. Holder 4 also comprises opposite side walls 43 and 44, as well as a top wall 45 and a bottom wall 46. Each side wall is provided with a shoulder 47 (see figure 8), so that the width of inner front wall 41 is a bit superior to that of outer wall 42.

By way of example:

- the width W41 (see figure 8) of inner front wall 41 of said holder, i.e. the maximum distance between opposite side walls, is substantially equal to W61 (see figure 9).

- the width W42 (see figure 8) of outer front wall 42 of said holder, i.e. the minimum distance between opposite side walls, is substantially equal to W62 (see figure 9).

- the height H4 (see figure 8) of said holder, i.e. the distance between top and bottom walls, is equal to the stroke length required with some allowances.

- the thickness T4 (see figures 8 and 10) of main part of said holder, i.e. the distance between opposite front walls, is substantially equal to W63 (see figure 9)

Inner front wall 41 of said holder 4 is provided with a groove 48, shaped substantially as a half circle. This groove is not threaded, so as to receive an auxiliary insulating member 80, the structure thereof will be described hereafter more in detail, said member 80 being adapted to cooperate with above described driving screw 22. Moreover a thin wall 113 of said anode beam is provided with a not threaded notch 114, shaped as a half circle, the section of which substantially corresponds to that of above described groove 48 provided in said holder 4. Therefore, both groove 48 and notch 114 substantially define a circle.

Top 40 of said holder is provided with a not threaded through hole 49, to allow the passage of said member 80. Said top 40 is provided with a cover 50, attached by any appropriate means, in particular removable means as shown on figure 10. Said cover 50 comprises first a central body 51, extending over top wall 45 of holder and projecting with respect to inner front wall 41 thereof. Said body is provided with a threaded through hole 52, the peripheral walls thereof form a driving nut adapted to cooperate with the above mentioned driving screw 22.

Said cover also comprises two lateral protrusions 53, projecting with respect to side walls 43, 44 of holder. Each protrusion is in particular provided with a respective foot 54, 55, each intended for fixing an electric connector 120. As shown in particular on figure 3, said connector 120 is formed by several flexible members 121 which extend substantially in a parallel way, the one above the other. Each flexible member, which is known as such, is for example an aluminium sheet or strip. It is elongated viewed from side and, by way of example, is substantially rectangular in cross-section. Each flexible member has two opposite axial ends, the first one being attached to a respective foot 54.

Relative attachment of flexible member and foot is preferably of the removable type, by way of example with bolts 56 extending in holes 55 provided in said foots. On the other hand relative attachment of connector 120 and anode beam 104 is preferably of the permanent type, typically by welding. In the illustrated example, second axial end of connector 120 is welded to an intermediate plate 122 (see figure 3), the latter being also welded to top wall 110 of anode beam 104. Above welds are aluminium/aluminium welding seam of known type. As a not shown alternative, intermediate plates are optional. In this case, connector 120 is longer than shown on figure 3 and its second axial end is directly welded on anode beam itself.

Turning back to figure 10, auxiliary member 80 will now be described. It is made of an insulating material, preferably a material which can be used in high temperatures under humid conditions. In particular, said material can be a composite material (fiberglass-polymer or carbon fiber - polymer) such as the one marketed under the trade reference NEMA G7®. Said member 80 comprises a main cylindrical body 81 having a thin wall, said body 81 being provided with a closed bottom 82. It is to be noted that, for sake of clarity, the different mechanical elements of figure 11 are not illustrated at the real scale. Auxiliary member 80 is firmly attached to holder nut 52, by any appropriate means. Peripheral walls of the hole 52, shown on figure 10, form a driving nut adapted to engage with a respective driving screw 22 as shown on figure 10. Moreover, as shown on figure 11, outer face 85 of cylindrical body 81 defines a so called functional clearance 90 with facing walls of holder 4 and anode beam 104.

Each holder 4A-4N and 6A-6N is slidably received in a respective pocket or guide 60A-60N and 70A-70N, provided in anode beam 104. As shown in particular on figures 5 and 6, each holder is substantially fully received into the inner volume of said guide. In other words, said holder 4 does not substantially protrude with respect to front wall of anode beam, viewed from top. Figure 9 shows more in detail the structure of one 60G of these guides, bearing in mind that all guides have the same structure. Said guide leads both to top wall 110 and bottom wall 111 of anode beam 104 (see also figure 3), as well as to outer front wall 112 thereof, opposite to the other row of anodes. Opposite side walls of said guide are formed by profiles made of a material different from aluminium, in particular copper. Said profiles are fixed to the aluminium body of anode beam by any appropriate means, in particular removable fixing means.

First profiles 61, opposite to wall 112, define a first width W61 of said guide 60, whereas second profiles 62, adjacent to wall 112, define a second width W62 of said guide 60, which is inferior to first width W61. Said widths W61 and W62 are slightly superior respectively to above defined widths W41 and W42, so that holder might be moved within the guide. The respective differences (W61-W41) and (W62-W42) are typically between 2 mm and 6 mm, and preferably between 3 mm and 5 mm, at room temperature.

Inner front wall of said guide 60G is formed by above mentioned so called thin wall 113 of the anode beam itself, as well as by two further copper profiles 63, provided at both ends of said thin wall 113. Each profile advantageously projects forward, with respect, to said wall, so that holder does not directly contact anode beam, while moving.

The use of above profiles 61 to 63, not made of aluminium, has specific advantages. This makes it possible to avoid any jamming phenomenon between said holder and said anode beam. It is to be noted that such jamming would be likely to occur, in case of direct aluminium-aluminium contact. As shown on figure 9, upper ends of profiles 61 to 63 form flaps 61’ to 63’, so as to allow the attachment of said profiles on anode beam 110. Attachment is performed by any appropriate means, preferably by removable means such as bolting.

Each holder 4A-4N and 6A-6N receives a clamp 130, of a known type, for attachment of a respective anode 1A-1N and 2A-2N to this holder. As more clearly shown on figure 7, each clamp 130 is first formed by a fixed part 131, which comprises several rods 132 which extend through horizontal passages 49, transversally provided in said holder (see figure 8). Said clamp is also formed by a mobile part 133, or jaw, adapted to cooperate with anode rod 1’G. Mobile part is adapted to have two functional positions, i.e. a first or free position wherein it allows free vertical motion of anode rod, as well as a maintaining position wherein it firmly maintains said anode rod with respect to holder.

Figures 12 and 13 show a first variant of the invention. On these figures, the mechanical elements analogous to those of figures 1 to 11 are given the same references, added by 200. The embodiment of figures 12 and 13 differs from that of figure 1 to 11, in particular in that guides 260 are not provided in anode beam 304, but are provided in the frame 302 of the superstructure 300. Guides 260 lead each to lateral wall 302’ of this frame, which is opposite the other row of anodes. Moreover, each holder 204 is slidably mounted into said guides, in a way similar to that described in reference to above figures 1 to 11. Holders 204 and anode beam 304 are linked by connectors 320, similar to those 120 of first embodiment.

Figure 14 shows a second variant of the invention. On this figure, the mechanical elements analogous to those of figures 1 to 11 are given the same references, added by 400. The embodiment of figure 14 differs from that of figure 1 to 11, in particular in that only part of holder 404 is mounted into a respective guide 460. On figure 14, this guide 460 is provided in anode beam 504, bearing in mind that it may be provided in the frame of the superstructure. Holder 404 comprises a massive body 404’, as well as two projections 405 and 405’ which extend horizontally from said body. Moreover anode beam 504 is provided with two vertical slots 460’ and 460”, which form said guide 460. Each projection 405, 405’ is slidably mounted in a respective slot 460, 460’.

All the details, described in reference with figures 1 to 11, may be combined with the specific embodiments of figures 12 and 13, as well as of figure 14. These details concern in particular the structure of the guides or pockets, the nature of moving means, the presence of an insulation member such as 80, as well as the presence of anti-jamming walls such as 61 to 63.

The processing of the above electrolysis cell will now be described, with reference to the embodiment of figures 1 to 11. It shall be noted that this processing also applies for the embodiments of figures 12 to 14. For various purposes, it is necessary to raise and lower the anodes 1A-1N and 2A-2N collectively and/or individually during the operation of the cell.

On the one hand, to maintain the anode-cathode distance within the relatively narrow limits required for satisfactory cell efficiency, the anodes must be moved up or down together in correspondence with changes in the level of the interface between the molten metal pool and the electrolyte. Such changes in level may be caused by progressive accumulation of produced metal in the pool, or by extraction of metal from the pool. On the other hand, when an individual anode block has been substantially consumed by reaction with oxygen, the remnant of the block must be raised out of the cell and replaced. The present invention provides means for effecting either individual or collective vertical movement of the anodes, in particular for above purposes.

In normal operation, during a very substantial fraction of the operational time, which typically amounts to 90 to 95%, all the clutches will be engaged, which replaces moving all the anodes by means of anode beam motion. In other words, the normal state of the clutches is “engaged”. However, the spring force has been selected to a given maximum value (preferably 350 Nm) in order to protect the motor and the gear box if the driving mechanisms gets stuck (a normal force is 60 Nm): above 350 Nm the soring-loaded clutch will disengage.

With all the clutches engaged, the two rows of anodes can be raised or lowered in unison by operating the motor to drive the whole transmission chain in the appropriate direction. Each individual driving screw 22 with its threaded face 49 engages with a respective driving nut 52. When screw 22 rotates, it drives said driving nut 52 in vertical translation, but the anode beam does not move vertically since screw does not engage with the not threaded notch 114. On the other hand, each auxiliary member 80 does vertically move, so does each respective holder 4, since holder 4 and member 80 are firmly attached. The displacement of said holder brings about a corresponding vertical movement of anode rod and anode, supported by said holder. The amplitude of the jack, formed by driving screw and driving nut, is for example typically about 400 mm. All the anodes are then displaced together with the same velocity and same extent of displacement.

The use of auxiliary insulating member 80 has specific advantages. First, it ensures a reliable driving of holder 4, anode rod and anode. Moreover, it enables an electrical insulation between holder 4 and driving screw. Indeed it is not desirable that the anode potential be transmitted to the shafts, gear boxes, clutches and motors; these should be at a neutral potential. This problem is not addressed in the cited prior art documents, and it is not apparent how it is dealt with in these documents.

When the desired extent of anode movement has been achieved, the motor is stopped and the anodes halt. If, during such movement, the anodes (or any of them) have jammed, or if there has been any potentially excessive torque development in the system for any other reason, the clutch or clutches involved simply slip, at their preset or predetermined torque limit, thereby preventing development of destructive torque such as could burn out the motor or damage other system components.

If it is desired to move only a single anode (for example, to raise a largely consumed anode entirely out of the bath for replacement) or to move only some of the anodes, the clutches associated with the other anodes are first disengaged, and the motor is then operated in the appropriate direction for moving the selected anode or anodes in the desired vertical direction. Again, motor operation continues only until the anode reaches its selected new level.

In addition, in a variant, the moving assembly according to the invention is selectively operable to raise the anodes of one row 1 or 2, while simultaneously lowering the anodes of the other row 2 or 1. These motions are carried out in such a way that the anodes all move exactly equal amounts from the equilibrium position at equal speeds in equal times, because all of the anodes are mechanically linked together and driven by the one bidirectional motor. In this variant inverters are added to the gear boxes of each anode rod, so as to invert the movement. This allows “pumping”, i.e. to agitate the bath as may be desired to quench anode effects. The bath level does not significantly change under these conditions so long as the number of anodes raised equals the number of anodes lowered and given that the areas of the two sets of anodes in plan view are the same. Said inverters are advantageously controlled by pneumatic means.

According to the invention, providing pockets for sliding reception of holders has specific advantages. First, it does not substantially increase the global footprint of the whole superstructure. In this respect, as shown for example on figure 4, said holders are fully integrated into the whole volume of the anode beam, viewed from top. Moreover, these pockets make it possible to guide said holders and, as a consequence, the anodes attached thereto during their vertical motion. This ensures a safe and efficient operation of the cell.

The present invention is not limited to the illustrated embodiments.

In the present embodiments, drive means comprise one single motor for the whole cell. As a not shown variant, two motors may be provided, i.e. one per row of anodes. As a further not shown variant, each jack may be provided with an individual motor.

In the present embodiment, each anode rod holds one single anode. As a not shown variant, two anodes or more, such as four anodes, may be used per anode rod.

In the present embodiment, each anode holder holds one single anode rod. As a not shown variant, two anode rods or more, such as four anode rods, may be used per anode holder.

In the present embodiment, one single anode holder is associated to one single jack. As a not shown variant, two anode holders or more, such as four anode holders, may be associated to one jack.

In the present embodiment, the anode movement is either up or down. As a not shown variant, the variation may be simultaneous up and down of different anodes. This requires a second pneumatic line with inverted gearbox and pneumatic valves.

Claims (30)

1. A superstructure (100; 300; 500) for an electrolytic cell, suitable for the Hall-Heroult electrolysis process, said superstructure comprising

- a frame (102; 302; 502) of substantially rectangular shape,

- an anode beam (104; 304; 504) fixed with respect to said frame,

- a plurality of anode holders (4A-4N, 6A-6N; 204; 404) each comprising holding means (130) for holding at least one respective suspended anode (1A-1N, 2A-2N) of said cell,

- moving means (10, 12, 14, 15, 16, 17, 21A-21N, 31A-31N, 22A-22N, 32A-32N) adapted to move one single anode holder independently from the others, along a substantially vertical direction,

- said moving means comprising drive means (10), as well as transmission means (12, 14, 15, 16, 17, 21A-21N, 31A-31N, 22A-22N, 32A-32N), for transmitting movement imparted by drive means to each holder, said superstructure being characterized in that at least part of each holder (4A-4N, 6A-6N; 204; 404) is slidably mounted into a respective pocket or guide (60A-60N, 70A-70N; 260; 460), provided in said superstructure (100; 300; 500).

2. A superstructure according to claim 1, characterized in that transmission means comprise common transmission means (12, 14, 15, 16, 17), adapted to engage with drive means (10), as well as a plurality of individual transmission means (21A-21N, 31A-31N, 22A-22N, 32A32N) adapted to selectively engage with common transmission means, each individual transmission means being adapted to impart motion from drive means to one single anode holder independently from the others.

3. A superstructure according to claim 1 or 2, characterized in that it comprises a plurality of clutches (23A-23N, 33A-33N), each being interposed between common transmission means and a respective individual transmission means.

4. A superstructure according to claim 3, characterized in that each clutch (23A-23N, 33A33N) is adapted to have an engaged position and a disengaged position, said superstructure comprising spring means adapted to maintain said clutch in its engaged position, said superstructure also comprising pneumatic means (24A-24N, 34A-34N, 108) adapted to pneumatically disengage said clutch.

5. A superstructure according to claim 4, characterized in that pneumatic means comprise a common pneumatic distribution unit (108), and individual pneumatic distribution lines (24A24N, 34A-34N), each connecting pneumatic distribution unit (108) to a respective clutch.

6. A superstructure according to any of claims 3 to 5, characterized in that each clutch is a spring-applied gear-tooth clutch.

7. A superstructure according to any of claims 1 to 6, characterized in that each individual transmission means comprise a driving screw (22A-22N, 32A-32N), adapted to cooperate with a driving nut (52) adapted to move said holder (4).

8. A superstructure according to claim 7, characterized in that it comprises a plurality of auxiliary insulating members (80), adapted to electrically insulate said driving screw (22) with respect to said holder (4).

9. A superstructure according to claim 8, characterized in that each auxiliary insulating member (80) is attached with one driving nut (52).

10. A superstructure according to claim 9, characterized in that said auxiliary insulating member (80) comprises a tubular body (81), as well as means for the attachment of said tubular body to said driving nut (52).

11. A superstructure according to claim 10, characterized in that said tubular body (81) extends partly in a not threaded groove (48) provided in a front wall (41) of said holder (4), said groove extending along a portion of circle, viewed from above.

12. A superstructure according to any of claims 1 to 11, characterized in that each holder (4A-4N, 6A-6N) is a massive block substantially shaped as a parallelepiped, made of aluminium.

13. A superstructure according to claim 12, characterized in that walls of said guide (60A60N, 70A-70N) comprise so called anti-jamming walls (61, 62, 63), intended to face said holder (4), said anti-jamming walls being made of a material different from aluminium, in particular of copper.

14. A superstructure according to claim 13, characterized in that said anti-jamming walls (61, 62, 63) comprise profiles made of said material different from aluminium.

15. A superstructure according to any of claims 1 to 14, characterized in that said pocket or guide (60A-60N, 70A-70N) is provided in said anode beam (104).

16. A superstructure according to claims 11 and 15, characterized in that anode beam is provided with a not threaded notch (114), facing said groove (48) of the holder, to ensure a free passage of tubular body (81).

17. A superstructure according to claim 16, characterized in that said not threaded notch (114) and said groove (48) substantially form a circle, the diameter thereof is slightly superior to that of tubular body.

18. A superstructure according to any of claims 15 to 17, characterized in that said guide leads only to outer front wall (112) of anode beam (104), opposite the other row of anodes.

19. A superstructure according to any of claims 15 to 18, characterized in that side walls of said holder comprise a shoulder (47), adapted to cooperate with a corresponding shoulder provided in facing side walls of said guide.

20. A superstructure according to any of claims 15 to 19, characterized in that said holder has fixing means (54), in particular removable fixing means, for fixing a first end of an electric connector (120), second end of said connector being attached, in particular permanently attached, to anode beam.

21. A superstructure according to claim 20, characterized in that said connector (120) is formed by a plurality of flexible members (121) of a conductive material.

22. A superstructure according to any of claims 1 to 14, characterized in that said pocket or guide (260) is provided in said frame (302) of said superstructure.

23. A superstructure according to any of claims 1 to 22, characterized in that the whole holder (4A-4N, 6A-6N; 204) is slidably mounted into said pocket or guide (60A-60N, 70A70N; 260), so that said holder does not protrude outside said guide, viewed from top.

24. A superstructure according to any of claims 1 to 22, characterized in that only part of said holder (404) is slidably mounted into said pocket or guide (460).

25. A superstructure according claim 24, characterized in that said holder (404) comprises a main body (404’) located outside said guide (460), viewed from top, as well as at least one projection (405, 405’) extending from said body, said projection being slidably mounted into said pocket or guide (460).

26. A superstructure according to any of claims 1 to 25, characterized in that said holder comprises clamping means (130), for selective attachment of an anode rod with respect to said holder.

27. An electrolytic cell, in particular an electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a superstructure (100) according to any of previous claims, as well as a plurality of anodes (1A-1N, 2A-2N) and anode rods (TA-TN, 2’A-2’N), each holder (4A-4N, 6A-6N) being adapted to selectively attach a respective anode rod with respect to said holder.

28. An electrolytic cell according to preceding claim, characterized in that each anode rod is associated with an inverter, so that a first row of anodes may move vertically in a first direction, whereas the other row of anodes may move vertically in the opposite direction.

29. An aluminium electrolysis plant comprising at least one line of electrolytic cells of substantially rectangular shape, said cells being arranged side by side, and said plant further comprising means for electrically connecting said cells in series and for connecting the cathodic busbar of a cell to the anode beam of a downstream cell, characterized in that more than 80% of the electrolytic cells in at least one of said line, and preferably each electrolytic cell of said line, is an electrolytic cell according to claim 27 or 28.

30. A method for making aluminium by the Hall-Heroult electrolysis process using electrolytic cells of substantially rectangular shape, characterized in that said method is carried out in an aluminium electrolysis plant according to claim 29.