LOW-PROFILE ALUMINUM CELL POTSHELL AND METHOD FOR INCREASING THE PRODUCTION CAPACITY OF AN ALUMINUM CELL POTLINE

Abstract

An aluminum reduction cell having a shell structure with a pair of longitudinally extending sidewalls, a pair of transversely extending endwalls, a bottom wall, and an open top having an upper edge. The aluminum reduction cell also has a transverse support structure with transverse bottom beams located under the shell structure and extending transversely between the sidewalls, each of the transverse bottom beams having a pair of opposed ends. The aluminium reduction cell also has compliant binding elements fixed to the transverse support structure, each extending vertically along an outer surface of one of the sidewalls for applying an inwardly directed force said sidewall. The compliant binding elements are in the form of cantilever springs. Each spring has a metal member with a lower end which is secured to the transverse support structure, and a compliant, upper free end which is movable inwardly and outwardly in response to expansion and contraction of the shell structure.

Claims

1-33. (canceled)

34. An aluminum reduction cell, comprising: (a) a shell structure comprising a pair of longitudinally extending sidewalls, a pair of transversely extending endwalls, a bottom wall, and an open top having an upper edge; (b) a transverse support structure comprising a plurality of transverse bottom beams located under the shell structure and extending transversely between the sidewalls, each of the transverse bottom beams having a pair of opposed ends; and (c) a plurality of compliant binding elements fixed to the transverse support structure, each extending vertically along an outer surface of one of the sidewalls, for applying an inwardly directed force said sidewall; wherein the compliant binding elements are in the form of cantilever springs, each comprising a metal member having a lower end which is secured to the transverse support structure, and a compliant, upper free end which is movable inwardly and outwardly in response to expansion and contraction of the shell structure.

35. The aluminum reduction cell according to claim 34, wherein the ends of the transverse bottom beams do not substantially extend beyond the sidewalls of the shell structure.

36. The aluminum reduction cell according to claim 35, wherein the lower end of each of the compliant binding elements is rigidly secured to one of the ends of one of the transverse bottom beams.

37. The aluminum reduction cell according to claim 34, wherein each of the compliant binding elements extends vertically along an outer surface of one of the sidewalls.

38. The aluminum reduction cell according to claim 37, wherein each of the compliant binding elements is in contact with the outer surface of the sidewall along at least a portion of its length.

39. The aluminum reduction cell according to claim 34, wherein the upper end is located at or below the upper edge of the shell structure.

40. The aluminum reduction cell according to claim 39, wherein at least some of the compliant binding elements are attached, rigidly or flexibly, over parts of their length, to the sidewall.

41. The aluminum reduction cell according to claim 39, wherein each of the compliant binding elements is of sufficient length such that a main point of load transfer to the sidewalls is approximately at the tops of cathode blocks lining the bottom wall of the aluminum reduction cell.

42. The aluminum reduction cell according to claim 34, wherein each of the compliant binding elements comprises a metal plate.

43. The aluminum reduction cell according to claim 42, wherein the metal plate has a thickness, width and composition such that the upper end is compliant, and such that the compliant binding element maintains an inwardly directed compressive force on the shell structure during outward dilation and inward contraction of the shell structure.

44. The aluminum reduction cell according to claim 43, wherein the thickness and/or width of each of the compliant binding elements is varied along its length, with the upper end of the compliant binding element being reduced in width and/or thickness relative to the lower end, such that the upper end is more compliant than the lower end.

45. The aluminum reduction cell according to claim 34, wherein each of the compliant binding elements is designed such that, during normal operation of the aluminum reduction cell, they are at a first applied load; and such that, in response to an expected reduction in process temperature, the compliant binding elements are at a second load which is greater than a minimum binding load; wherein the minimum binding load is a load at which forces opposing contraction of a lining of the aluminum reduction cell are overcome, thereby preventing formation of gaps in the lining during contraction in response to a thermal cycle comprising a deviation of about +/−100-150° C. from a normal operating temperature of the aluminum reduction cell.

46. The aluminum reduction cell according to claim 34, wherein the compliant binding elements comprise a mild or low-alloy steel.

47. The aluminum reduction cell according to claim 34, wherein the compliant binding elements have a depth of no more than about 200 mm.

48. The aluminum reduction cell according to claim 47, wherein the compliant binding elements have a depth from about 50 mm to about 200 mm.

49. The aluminum reduction cell according to claim 34, wherein the compliant binding elements are provided with adjustment means, and wherein the adjustment means are located between the upper ends of the compliant binding elements and the shell structure.

50. A method for improving the productivity of an aluminum reduction cell potline housed in an enclosure having a length and a width; wherein the potline comprises a plurality of existing aluminum reduction cells, each of said existing cells including an existing potshell and an existing support structure and having a first footprint defined by an area of the existing potshell and the existing support structure, wherein the existing potshell and the existing support structure each have a length extending across the width of the enclosure, and the length of the existing support structure is greater than the length of the existing potshell; the method comprising: (a) removing one or more of said existing aluminum reduction cells from the potline; and (b) inserting one or more new aluminum reduction cells with a potshell according to claim 34 into the potline, wherein each of the new cells comprises a new potshell and a new base structure and is inserted into a space vacated by one of the existing cells; wherein each of the new cells has a second footprint which is substantially the same as the first footprint, and wherein the new potshell has a length which is substantially the same as a length of the new support structure, such that the area of the new potshell is greater than an area of the existing potshell.

51. The method according to claim 50, whereupon increasing the width of the cells results in an increase in the operating current of the cells, so that the current density of the cathode remains substantially the same as before the capacity increase.

52. An aluminum reduction potline, comprising aluminum reduction cells connected in series, and further comprising: (a) support plinths; (b) bus-bars and risers; (c) superstructures, carrying anodes; (d) off-gas ducts; (e) a feed distribution system; and (f) other known ancillaries; where the aluminum reduction cells are furnished with aluminum reduction cells according to claim 34.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order that the claimed subject matter may be more fully understood, references will be made to the accompanying drawings, in which:

[0030] FIG. 1: A pair of conventional potshells in their bays, showing supports and bus bars.

[0031] FIG. 2: One of the conventional potshells of FIG. 1, shown without the busbars.

[0032] FIG. 3: Transverse cross-section of the conventional potshell of FIG. 2, showing lining, and transverse structure.

[0033] FIG. 4: Potshell according to an embodiment of the invention.

[0034] FIG. 5: Enlarged, partial cross-section of potshell of FIG. 4, showing lining and transverse structure.

[0035] FIG. 6: Transverse cross-section of potshell of FIG. 4.

[0036] FIG. 7: Transverse cross-section of transverse bottom beams and compliant binding elements of the potshell of FIG. 4, including a first type of adjustment means.

[0037] FIG. 8: Enlarged view of one of the compliant binding elements and adjustment means of FIG. 7.

[0038] FIG. 9: Transverse cross-section of transverse bottom beams and compliant binding elements of the potshell of FIG. 4, including a second type of adjustment means.

[0039] FIG. 10: Enlarged view of one of the compliant binding elements and adjustment means of FIG. 9.

[0040] FIG. 11: Graph of Installed Cost of Capacity vs. Potshell Weight comparing prior art to present invention.

[0041] FIG. 12: Schematic representation showing load-displacement behavior of a potshell.

[0042] FIG. 13: Graph showing the relationship between elastic deflection and member depth, for a mild steel member 1 m in length.

DETAILED DESCRIPTION OF EMBODIMENTS

[0043] In the following description, specific details are set out to provide examples of the claimed subject matter. However, the embodiments described below are not intended to define or limit the claimed subject matter. It will be apparent to those skilled in the art that many variations of the specific embodiments may be possible within the scope of the claimed subject matter.

[0044] FIGS. 4 and 5 illustrate an aluminum reduction cell potshell 10 (sometimes referred to herein as “reduction cell 10” or “potshell 10”) according to an embodiment, with some of the components thereof eliminated for clarity, and located in a single reduction cell bay. It will be understood by the reader that the potshell 10 may be furnished with a support structure, superstructure, collector bars, and bus bars in order to produce aluminum by the Hall-Heroult process. These elements, being common to reduction cells, are omitted from the following description unless needed for clarity of the content specific to the embodiment.

[0045] The reduction cell potshell 10 comprises a shell structure 12 (also referred to herein as a “shoebox 12”) comprising a pair of longitudinally extending sidewalls 14, a pair of transversely extending endwalls 16, a bottom wall 18, and an open top having an upper edge 22 about its perimeter. As shown, the shell structure 12 is substantially rectangular in shape, with the sidewalls 14 being longer than the endwalls 16.

[0046] The sidewalls 14 and endwalls 16 of potshell 10 are protected from the bath by refractory wall blocks 34 lining their inner surfaces. The bottom wall 18 is lined with a carbon-based bottom composed of graphitic or graphitized cathode blocks 26 (of a type not prone to excessive long-term chemical growth) furnished with current collector bars 28, the ends of which extend through the sidewalls 14.

[0047] When a plurality of reduction cells 10 are combined to form a potline (not shown), the reduction cells 10 are lined up beside each other, each in their respective reduction cell bay, with the sidewalls 14 of adjacent reduction cells 10 in parallel, opposed relation to one another. The potline is housed within an enclosure (not shown) having a length and a width, with the sidewalls 14 of the reduction cells 10 extending across the width of the enclosure and the endwalls 16 of the reduction cells 10 extending along the length of the enclosure. The enclosure is typically a building with a width sufficient to accommodate a single potline.

[0048] Each reduction cell bay further comprises one or more longitudinal busbars (not shown in FIG. 4) extending along each of the sidewalls 14, and one or more transverse busbars extending along each of the endwalls 16. The longitudinal busbars 36 (FIG. 6) are conductively connected to the ends of the current collector bars 28 of the cathode blocks 26. The longitudinal busbars are spaced from the sidewalls 14 and the transverse busbars are spaced from the endwalls 16, forming a defined envelope in which the potshell 10 resides. The arrangement of the bus bars in the embodiment shown in FIG. 4 will have the same appearance and structure as the bus bars shown in prior art FIG. 1.

[0049] The shell structure 12 and its contents are supported on a base structure 40 which includes a plurality of stiff, horizontally extending, transverse bottom beams 46 extending substantially parallel to endwalls 16, and may also comprise a plurality of stiff, horizontally extending, longitudinal bottom beams 44 extending parallel to sidewalls 14. The bottom beams 44, 46 (also referred to herein as “support members”) are located below the bottom wall 18 of the shell structure 12 and may form a criss-crossing network of horizontal support beams to support the weight of the reduction cell 10 and its contents.

[0050] The transverse bottom beams 46 together define a transverse support structure. As can be seen from the drawings, the transverse bottom beams 46 are located almost entirely underneath the shell structure 12, and the ends of the transverse bottom beams 46 do not substantially extend beyond the sidewalls 14 of the shell structure 12. Thus, the transverse bottom beams 46 do not add significantly to the footprint of the reduction cell 10.

[0051] The endwalls 16 are furnished with an endwall reinforcement, known as an endwall structure, to supply the reaction forces necessary in the longitudinal direction. The endwall structure is of any suitable conventional design, and is not described herein in detail.

[0052] In addition to the transverse bottom beams 46, the transverse support structure comprises a plurality of compliant binding elements, described below, which are connected to the transverse bottom beams 46.

[0053] The transverse support structure comprising the plurality of stiff horizontal transverse bottom beams 46 is located below the bottom wall 18 of the shoebox 12. The transverse bottom beams 46 are designed to withstand the vertical loads; namely the weight of the shoebox 12 and its contents and maintenance loads that are applied to the structure. The transverse bottom beams 46 also reinforce the shoebox 12 against buckling, and the bending moment applied by the compliant binding elements in response to the expansion of the lining, which includes the refractory wall blocks 34 and the cathode blocks 26.

[0054] The potshell 10 further comprises a plurality of compliant binding elements 60 (also referred to herein as “vertical binding elements 60”), each extending vertically along the outer surface of one of the sidewalls 14 of the shell structure 12, i.e. in the space between one of the sidewalls 14 and an adjacent longitudinal busbar. Thus, it can be seen that the vertical binding elements 60 are located substantially within the outer perimeter of the reduction cell 10, and do not contribute significantly to the footprint of the reduction cell 10.

[0055] Each of the vertical binding elements 60 has a lower end which is secured to the transverse support structure, and more specifically is rigidly secured to one of the transverse bottom beams 46. For example, as shown in FIGS. 4 and 5, each of the vertical binding elements 60 is rigidly secured to an end of one of the transverse bottom beams 46.

[0056] Each of the vertical binding elements 60 has an opposite upper end or free end, which is located at or below the upper edge 22 of the shell structure 12. Thus, the vertical binding elements 60 do not add to the height of the potshell 10. For example, the upper ends of the vertical binding elements 60 may be located below the upper edge 22 of the shell structure 12, and may be located at substantially the same level as the upper surfaces of cathode blocks 26.

[0057] Each of the vertical binding elements 60 may comprise a vertical cantilever spring or cantilever plate comprising a metal member, which may comprise a metal plate, attached at its lower end to one of the transverse bottom beams 46. The cantilever springs are of sufficient length so that the main point of load transfer to the shoebox 12 is at approximately the elevation of the top of the cathode blocks 26, as mentioned above.

[0058] The thickness, width and composition of the metal members are selected such that the free upper end of each vertical binding element 60 is compliant, such that it is outwardly movable in response to thermal and/or chemical outward dilation of the shell structure 12, and inwardly movable in response to a thermal contraction of the shell structure 12, while maintaining an inwardly directed compressive force on the shell structure 12. For example, the thickness and/or width of the vertical binding elements 60 may be varied along the length of the vertical binding element 60. As shown in the drawings, for example, the upper ends of the vertical binding elements 60 may be reduced in width and/or thickness as compared to the lower ends, such that the upper ends are more compliant than the lower ends.

[0059] The compliant binding elements 60 may be designed so that during normal operation they are at a first load, termed the operating load, so that in response to an expected reduction in process temperature (thermal cycle), the associated shrinkage of the lining does not cause a reduction in the applied load below a second load, termed the minimum binding load.

[0060] The minimum binding load may be defined as the load at which the calculated frictional and other forces opposing the contraction of the lining are overcome, thereby preventing the formation of gaps in the lining during contraction in response to the thermal cycle.

[0061] The thermal cycle may be defined as a departure from the normal operating temperature, consistent with the limits of normal current aluminum cell operating practice, typically in the range +/−100-150° C. of the normal operating temperature.

[0062] The advantage of the present embodiment is that increased compliance of the structure, provided by vertical binding elements 60 in the form of cantilever springs, reduces the load that must be developed during normal operation to maintain the minimum binding load during a thermal cycle. This relies on the fact that the less stiff a structure is, the less the reaction load changes when it is deflected. This is illustrated in FIG. 12, which shows the load-displacement characteristics for a stiff structure, and a compliant one. Although both structures maintain the minimum binding load during a thermal cycle, the stiff structure needs a substantially higher operating load to do so.

[0063] The cantilever spring of the compliant binding element 60 may be designed using sizes and materials of construction (typically mild or low-alloy steels) so that it deforms principally within the plastic range of the materials of construction above the design operating load. The materials of construction are selected so as have sufficient ductility to accommodate the expected thermal and chemical growth of the lining, as calculated based on the expansion properties of the lining materials or estimated from operating experience. Stronger materials can be selected for the compliant binding elements 60 to reduce their size and increase the elastic range, if desired.

[0064] The sizes of the vertical binding element 60 may be selected to be no more than about 200 mm in depth (thickness), to maximize the advantages obtained from the invention. This can be seen, for example, by comparing the cross-section of FIG. 6 with the prior art cross-section of FIG. 3, in which the vertical binding elements comprise rigid beams having a depth of about 300 mm to 500 mm. This permits the use of longer cathode blocks 26 in the shell structure 12 of FIG. 6, as compared to that of FIG. 3.

[0065] To further illustrate the benefits of the vertical binding elements 60 according to the present embodiment, FIG. 13 shows the relationship between elastic deflection and member depth, for a mild steel member 1 m in length. For example, selecting a cantilever spring in the range of about 200-50 mm can increase the elastic deflection range of the compliant binding element by 150-600%, relative to conventional potshell stiffeners. In an embodiment, each of the compliant binding elements 60 extends between about 75 mm-150 mm in the transverse direction from the inside of the shell structure 12 over substantially the entire height of the compliant binding element 60.

[0066] The inventors have found minimum depth of the vertical binding elements 60 is limited by the requirement to achieve the operating load during heat-up of the lining. If the vertical binding elements 60 are excessively compliant, the initial lining expansion may be insufficient to reach the operating load. If this happens the reduction cell 10 will be at increased risk of metal infiltration during the early part of the campaign, before any chemical expansion has taken place. To overcome this limitation, the compliant binding elements 60 can be furnished with adjustment means that can be introduced between the free upper ends of the vertical binding elements 60 and the shell structure 12.

[0067] A first type of adjustment means is shown in FIGS. 4-8. As shown, the upper end of the compliant binding element 60 is shaped such that a slot 88 is provided between the sidewall 14 of shell structure 12 and an upper portion of the compliant binding element 60, including the upper end thereof. The slot 88 may include a sloped surface 92 which is outwardly sloped toward the upper end of the compliant binding element 60, thereby increasing the depth of the slot 88 at the upper end of the compliant binding element 60. At least partly received in the slot 88 is a wedge 90 that is fitted against the sloped surface 92, inbetween the upper end of the compliant binding element 60 and the outer surface of sidewall 14. The wedge 90 may be driven downwardly from above to increase the outward deflection of the upper end of the compliant binding element 60. The driving of the wedge 90 can be achieved by various means, for example by using a hammer, a portable hydraulic jack reacting against a suitable bracket, or any other suitable means. As shown in the close-up of FIG. 8, for example, a bracket 94 may be secured to the sidewall 14 above the upper end of the compliant binding element 60 and the wedge 90. The bracket 94 has a threaded aperture 96 which receives a screw 98, having a lower end which engages the upper (wide) end of the wedge 90. Threading the screw 98 into the aperture 96 will drive the wedge 90 downwardly into the slot 88, thereby increasing deflection of the upper end of the compliant binding element 60. Turning the screw 98 in the opposite direction will permit the wedge 90 to move upwardly in slot 88 to decrease deflection of the upper end of the compliant binding element 60.

[0068] As will be appreciated, the wedges 90 can be withdrawn over the campaign in response to the growth of the lining. This can facilitate expansion of the reduction cell 10 without encroaching on other constraints.

[0069] A second type of adjustment means is shown in FIGS. 9 and 10. As shown, the upper end of the compliant binding element 60 is reduced in depth so as to form a slot 100 between the upper end of the compliant binding element 60 and the outer surface of the sidewall 14. The slot 100 may have a rectangular shape as shown in FIGS. 9 and 10, and is sized and shaped to receive a pressure block 102. As can be seen from the enlarged view of FIG. 10, the upper end of the compliant binding element 60 has a threaded aperture 106 into which a screw 108 is threaded, an end of the screw 108 engaging the pressure block, the screw 108 being substantially perpendicular to sidewall 14. The pressure block 102 may have a recess 104 which aligns with the threaded aperture 106 and which receives the end of the screw 108, and which prevents the screw 108 from being dislodged during movements of the potshell 10 and lining. As will be appreciated, threading the screw 108 into the threaded aperture 106 will apply load to the pressure block 102, increasing the outward deflection of the upper end of the compliant binding element 60. Conversely, turning the screw 108 in the opposite direction will reduce the load on the pressure block 102, and decrease the outward deflection of the upper end of the compliant binding element 102.

[0070] The purpose of the adjustment means described above is to force additional deflection of the compliant binding element 60 after the lining has been heated to operating temperature, and after the carbon paste has been substantially baked, but before molten electrolyte or metal is introduced. The additional deflection provided by the adjustment means is sufficient to deflect the upper end of the compliant binding elements 60 by an amount, that when added to the expansion of the lining, will produce a reaction force in the compliant binding elements 60 equal to the desired operating load.

[0071] Therefore, providing the compliant binding elements 60 with the adjustment means described above allows the depth of the compliant binding elements 60 to be further reduced without reducing the performance of the aluminum reduction cell 10.

[0072] As discussed above, the profile (width and thickness dimensions) of the cantilever springs (i.e. compliant binding elements 60) can be varied along their length to achieve a greater or lesser compliance of the structure. Also, the compliant binding elements 60 can be attached, flexibly or rigidly, over parts of their length to the sidewall 14, while maintaining the freedom of movement of their upper ends, as may be suitable for a particular embodiment.

[0073] It should be clear to those skilled in the art that the compliant binding elements 60 as described herein can be used in combination with other spring elements, such as coil springs, disk springs, wave springs, leaf springs, or torsion bars to achieve greater compliance than is possible with the cantilever spring arrangement of the compliant binding elements 60 alone.

[0074] As will be appreciated, the embodiments described herein permit an increase of the capacity of an existing potline that is limited by current density on the surfaces of the anodes and cathodes. This benefit is illustrated by way of the following example:

[0075] A potline has 300 aluminum cells in two pot rooms, limited by current density, operating at 280 kA. The existing cells are of a conventional design having external and internal dimensions, and other characteristics according to Table 1.

TABLE-US-00001 TABLE 1 With Low-Profile Original Potshells Number of Cells 300 300 Pot-to-Pot Spacing (m) 6.5 6.5 Cell External Width (m) 4 4 Cell External Length (m) 11 11 Cathode Length 2.8 3.1 Stiffener Depth - Each Side (m) 0.30 — Compliant Binding Depth - Each Side (m) — 0.15 Endwall Structure Depth - Each Side (m) 0.5 0.5 Electrode Area (m{circumflex over ( )}2) 28 31 Operating Current (kA) 280 310 Current Density (A/cm{circumflex over ( )}2) 1.00 1.00 Capacity Increase — 11%

[0076] As can be seen from the above table, the production capacity of the potline is increased by 11% by replacing the existing aluminum cells with low-profile cells having identical external dimensions and larger internal area. The increase in internal area is used to house larger anodes and cathodes. The current of the potline, and hence the production capacity, are increased without exceeding the current density limit.

[0077] It will be clear to those skilled in the art that in order to accommodate the larger anodes and cathodes, the superstructures will need to be modified.

[0078] It will also be clear to those skilled in the art that the increased production of aluminum may be associated with additional heat generation within the cell. The greater requirement for heat rejection can be met by mounting conductive cooling fins to the potshell exterior at the bath elevation, or increasing the convective heat transfer by other means, for example, forced air cooling.

[0079] It will also be clear, that the rectifiers, anode plant, rod shop, off-gas system, crane, pot tending machines, cast-house and other ancillaries may need to be modified, if they do not have sufficient extra capacity, to take full advantage of the improvements provided by the present invention.

[0080] It will also be clear to those skilled in the art that the present invention can be applied to the construction of new potlines, with the object of reducing the capital intensity of installed capacity.

[0081] Prior art FIG. 1 illustrates a pair of prior art aluminum reduction cells 10′ arranged side-by-side in a potline. The prior art reduction cells 10′ include a number of elements which are similar or identical to the reduction cells 10 described above. Like reference numerals are used to identify these like elements of prior art reductions cells 10′, and the above descriptions of these elements apply to the prior art figures unless indicated otherwise in the following description.

[0082] Also shown in FIG. 1 are longitudinal bus bars 36 extending along sidewalls 14 and spaced therefrom, and transverse bus bars 38 extending along the endwalls 16 and spaced therefrom. Although not shown in the drawings showing reduction cells 10, it will be appreciated that similar or identical bus bars 36, 38 will be included in the reduction cells 10 according to the invention. Also shown in FIG. 1 is the base structure of the prior art reduction cells 10′.

[0083] Prior art FIG. 2 illustrates one of the prior art aluminum reduction cells 10 with the bus bars removed, to more clearly show the rigid, vertical binding elements 58 provided along the sidewalls.

[0084] Prior art FIG. 3 is a transverse cross section through one of the aluminum reductions cells 10′, again showing the rigid, vertical binding elements 58, having a depth of 300-500 mm.

[0085] FIG. 12 shows the load-displacement characteristics for a stiff structure as shown in prior art FIGS. 1-3, and a compliant one in accordance with the present invention.

[0086] The above-described implementations of the present application are intended to be examples only. Alterations, modifications and variations may be effected to the particular implementations by those skilled in the art without departing from the scope of the application, which is defined by the claims appended hereto.