Support assembly for an anode for a reduction cell for producing primary aluminum

Abstract

A support assembly (12) for supporting a prebaked anode (14) for a Hall-Hêroult cell (10) for producing primary aluminum comprises a plurality of connection plates (24), wherein each connection plate (24) is arranged at the lower end of a respective stub (22) and has a diameter (D4) that is greater than the diameter (D3) of the stub (22 ).

Description

Field of the invention

[0001] The present invention is in the field of components for producing primary aluminum through the use of Hall-Hêroult cells; in particular, the present invention relates to an innovative support assembly for an anode for a Hall-Hêroult cell, which allows the useful life of the anode operating cycle to be extended and the weight of the final anode waste to be reduced.Prior art

[0002] Primary aluminum is produced in Hall-Hêroult reduction cells. The alumina dispensed into the cells dissolves in a cryolite and aluminum fluoride (AlF 3 ) electrolytic bath at a temperature of about 950°C and is reduced to aluminum by passing direct current from the anodes to the carbon cathodes during the electrolysis process.

[0003] A support assembly for an anode, usually of the prebaked type, consists of a hook formed by an anode beam, a transition joint, a yoke, and a plurality of stubs.

[0004] For connection to the support assembly, the anode is provided at the top with a plurality of laterally fluted holes, one for each stub. A predefined amount of molten cast iron is poured into the residual space between the stub and the respective hole. Once the cast iron has solidified the stubs remain embedded in the holes of the anode, and the solidified cast iron ring becomes the mechanical and electrical junction between the anode beam and the anode.

[0005] A cell usually has several support assemblies installed, each carrying a respective anode, and the electric current flowing through each anode is substantially equal to the total line current divided by the number of support assemblies in the cell.

[0006] The normal operating period of a prebaked anode according to the prior art is substantially between 25 and 30 days and ends when the anode has reached a minimum height below which there is a risk of the cast iron ring contaminating the bath. In such a condition, it is necessary for a new support assembly, equipped with a new anode, to replace the support assembly carrying the worn anode.

[0007] The weight of the carbon in the consumed anode, which constitutes the anode butt end discard, is usually between 20 and 30% of the weight of the new anode and is recovered in a line for recovering the carbon of the anode butt ends.

[0008] To prevent cast iron rings from contaminating the molten bath and aluminum, a layer of carbon approximately 50 mm below the rings is preserved, which is part of the thickness of the anode waste. The overall thickness of the anode waste, also considering the depth of the holes, is between 150 and 200 mm.

[0009] The duration of an anode depends directly upon the total thickness of the waste, which depends directly upon the depth of the stub holes present in the upper part of the anode. According to the solutions of the prior art, the average depth H1 of the holes is approximately 120 mm. Considering the safety thickness of about 50 mm, the total thickness of the Hbutt waste is about 170 mm, which corresponds to about 26% of the height of 650 mm of a new anode.

[0010] Furthermore, in the solutions of the prior art, the density distribution of the electric currents is not uniform insofar as the currents converge towards the stubs, but only through a limited area of the cast iron rings (in particular, towards those areas of the rings having a higher carbon contact pressure). The contact resistance between the stubs and the anode is non-uniform, and the voltage drop between the stubs and the anode generally ranges from about 80 mV to 130 mV. The Joule effect inside the anode, due to the high density of local currents, generates heat and contributes to the energy loss of the cell.

[0011] Some patent documents have proposed some approaches to improve the energy efficiency of the anodes, the elimination of the use of cast iron and an improved use of the carbon. For example: The International Application WO-A1-2016 / 130014 by Norsk Hydro proposes to replace the cast iron with mechanical support elements which require anode processing and an intermediate layer of metal particles for the electrical connection between the carbon and the hook. Nevertheless, the mechanical particles must be recycled, the support assembly is different from those that are typically used in the industry and makes retrofitting on pre-existing support assemblies impossible; The International Application WO-A1-2012 / 100340 from the University of Laval proposes a seamless connection. Instead of holes for stubs, longitudinal grooves are provided and the steel part incorporated into the cast iron is a bar with a rectangular cross section; the document US 7,901,560 by Norsk Hydro proposes creating slits at the bottom of the anode to reduce energy consumption; the Storvik International Application WO-A1-2016 / 108696 and the Servico International Application WO-A1-2002 / 42525 propose yokes provided with copper parts inside steel parts and do not have a stub welded at the steel arm of a steel yoke.

[0012] Finally, further exemplary embodiments are shown in CA2838113A1 and GB2569382A, which however only indicate cast iron rings around the stubs.Object of the invention

[0013] The object of the present invention is to meet the needs of the industry whilst overcoming the drawbacks mentioned with reference to the prior art, mainly in order to increase the service life of the anode, reducing the weight of the anode butt end discard.

[0014] This object is achieved by means of a support assembly according to claim 1. The claims dependent thereon identify additional advantageous embodiments of the invention.Brief description of the figures

[0015] The features and advantages of the support assembly according to the present invention will be apparent from the description below, given by way of non-limiting example in accordance with the figures in the accompanying drawings, wherein: Figure 1 schematically depicts a Hall-Hêroult cell for producing primary aluminum and a pair of support assemblies for the respective anodes; Figures 2 to 6 refer to the prior art, and in particular: a) Figure 2 shows a support assembly assembled and connected to the anode; b) Figure 3 shows the support assembly of Figure 2, in separate parts; c) Figure 4a and 4b show an example of a cast iron ring of the support assembly, after solidification; d) Figure 5 shows a support assembly with a worn-out anode; e) Figure 6 shows a fluted hole and relative stub. Figures 7 to 17 refer to a support assembly according to the invention, and in particular: a) Figure 7 shows a support assembly assembled and connected to the anode; b) Figure 8 depicts an arm of the support assembly in Figure 7, assembled and connected to the relative anode; c) Figure 9 depicts an assembled arm of the support assembly with a corresponding fluted hole; d)Figures 10a and 10b show a ring of the support assembly, after solidification; e) Figures 11a, 11b and 11c show further embodiments of an arm of the support assembly; f) Figures 12 to 17 show further shapes of an arm of the support assembly. Description of some embodiments of the invention

[0016] Figure 1 schematically depicts a Hall-Hêroult cell 10 for producing primary aluminum and a pair of support assemblies 12 in the working position, immersed in the bath. Each support assembly 12 comprises an anode beam 16, usually made of aluminum, connected to an anode 14 made of carbon, usually of the prebaked type.

[0017] In particular, for the connection between the anode beam 16 and the anode 14, the support assembly 12 comprises a transition joint 18, welded at the lower end of the anode beam 16, and a yoke 20, usually made of steel, welded at the transition joint 18, usually bi-metal (steel-aluminum). Furthermore, the yoke 20 comprises a plurality of separate arms 20a and the support assembly comprises a plurality of stubs 22, normally made of steel, each stub 22 being welded at the respective arm 20a of the yoke 20.

[0018] The stub 22, which is usually cylindrical, has a characteristic width, i.e. a dimension that characterizes the maximum overall dimensions of the stub, for example the maximum outer diameter D3.

[0019] According to Figure 7, 8 and 9, the support assembly 12 further comprises a plurality of connection plates 24, each connection plate 24, usually cylindrical and preferably made of steel, being connected at the lower end of the respective stub 22. According to the invention, the connection plate 24, delimited at the top by an upper face 24a and at the bottom by a lower face 24b, has a maximum outer diameter D4, wherein the diameter D4 of the connection plate 24 is greater than the diameter D3 of the stub (D4 > D3).

[0020] For example, the connection plate 24 is welded at the stub 22; in a variant embodiment, the stub and the connection plate constitute a single body.

[0021] Finally, the support assembly 12 comprises a plurality of rings 26, usually made of cast and solidified iron, for connecting the support assembly 12 to the anode; according to further embodiments, the ring is made of carbon paste or consists of a carbon-based glue. Each ring 26 of cast iron joins the connection plate 24 of the respective stub 22 and the anode 14, by means of being engaged with a respective laterally fluted hole 14a of the anode 14.

[0022] To make the connection between the connection plate 24 and the anode 14, a plurality of fluted holes 14a is implemented in the upper part of the anode 14; each fluted hole 14a is provided on the lateral surface thereof with a plurality of flutes 14b, wherein each flute 14b extends predominantly axially and is oblique.

[0023] The connecting plate 24 is positioned in the respective fluted hole 14a and a predefined amount of cast iron is poured into the fluted hole. The cast iron, in solidifying, forms the ring 26. The ring 26 is firmly connected to the plate 24 by virtue of the thermal contraction that the cast iron undergoes following solidification and is firmly connected to the anode 14 by virtue of the flutes 14b; in particular, the cast iron that has solidified within the flutes 14b constitutes a plurality of flaps 26b of the ring 26.

[0024] In Figure 8, the fluted hole 14a has a diameter D2 (flutes excluded), a characteristic depth H2 and a lateral surface extension S2. Hsafety is the safety thickness of the consumed anode, Hbutt is the thickness of the anode waste, and H3 is the protrusion height of the connection plate 24 from the upper surface of the anode 14.

[0025] With respect to the prior art, depicted for example in Figure 6, it is evident that, due to the use of the connecting plate 24, the diameter D2 of the fluted hole 14a is greater than the diameter D1 of the fluted hole of the solution of the prior art; the depth H2 of the fluted hole 14a is less than the depth H1 of the fluted hole of the solution of the prior art. In particular, H4 = H1-H2 is the decrease in height between the solution of the invention and the solution of the prior art and is equivalent to the decrease in height of the anode butt end discard.

[0026] Figures 10a and 10b show an embodiment of the ring 26, consisting of a tubular main body 26a, of thickness "t", and a plurality of flaps 26b protruding radially outwards from the main body 26a, having a predominantly axial and oblique extension in relation to the central axis of the main body.

[0027] Figure 11a depicts an embodiment of the invention, wherein the connection plate 24 is a steel disk welded at the lower end of the stub 22, with a partial overlap of the height of the connection plate in relation to the height of said stub.

[0028] Figure 11b depicts a further embodiment of the invention, wherein the connection plate 24 is a steel disk welded at the lower end of the stub 22, with a total overlap of the height of the connection plate in relation to the height of said stub.

[0029] Figure 11c depicts yet another embodiment of the invention, wherein the connection plate 24 is a steel disk welded at the lower end of the stub 22, completely below the stub.

[0030] According to one embodiment of the invention, moreover, the flutes 14b of the fluted holes 14a are between 11 and 30 in number, i.e., in a greater number than those provided for in the solutions of the prior art.

[0031] In the embodiments shown in Figures 11a, 11b, and 11c, the position of the ring with respect to the stub may be modified, thus accordingly modifying the contact area. It is thus possible to control the flow of heat from the anode to the arm of the respective stub.

[0032] The heat generated by the currents inside the anode by the Joule effect is less in the solution according to the invention than in the known solutions insofar as the larger area of the surface of the fluted hole S2 reduces the current densities and contact resistance in the interface area between the connection plate (steel), the ring (cast iron) and the anode (carbon).

[0033] The contact resistance also depends upon the pressure between the lateral surface 14a of the fluted hole 14 and the carbon anode, as well as upon the profile of the lateral surface 14a which may be, for example, flat or knurled.

[0034] Figure 12 depicts a further embodiment of the invention, wherein the fluted hole 14a has a conical side surface, having an extension S2, preferably obtained by means of tool machining, and the connecting plate 24 has a conical side surface, having an extension S2.

[0035] Figure 13 shows a further embodiment of the invention, wherein the side surface of the connection plate 24 is knurled, in order to increase the electrical contact area and the mechanical support of the anode. Preferably, moreover, the fluted hole 14a has a conical lateral surface and the connecting plate 24 has a conical lateral surface.

[0036] Figure 14 is a further embodiment of the invention, wherein the support assembly 12 cooperates with a cover 28, arranged to cover the transition region between the stub 22 and the connection plate 24 and suitable to cover an end portion of the stub 22 and an upper portion of the connection plate 24. Preferably, the cover 28 rests upon an upper face 14c of the anode 14 and, by virtue of the inner shape, upon the connecting plate 24.

[0037] Preferably, the cover 28 has such an overall height H5 that, when the anode 14 is completely immersed in the cell bath, the stub is protected and may not be lapped by the liquid electrolytic bath.

[0038] For example, the cover 28 has an overall flanged ring shape and comprises two couplable half-shells 28a, 28b, held together, for example, by carbon glue or a metallic wire or carbon fiber.

[0039] Figure 15 shows a further embodiment of the invention, wherein the cover 28 consists of a tube segment, partly resting upon the upper surface 14c of the anode 14, open at the top and having a height H5.

[0040] The tube thus delimits an inner space 30, preferably filled with a plurality of protective fragments 32, e.g., consisting of unbaked carbon paste in order to prevent the liquid bath from entering and to better thermally isolate the upper part of the anode.

[0041] Figure 16 depicts a further embodiment of the invention, wherein the cover 28 has an overall flanged ring shape and consists of a lower part 28a, consisting of two half-shells 281, coupled to and resting upon the upper surface 14c of the anode 14 and upon the connecting plate 24, and an upper part 28b, consisting of two half-shells 283, 284 coupled to and resting upon the upper part 28a.

[0042] Preferably, the half-shells 281, 282, 283, 284 are joined together using carbon glue or steel tips or needles protruding from the connecting plate 24.

[0043] During the last 3 or 4 days of the anode life cycle, in order to maintain the same distance between the anode and cathode, the anode must move deeper into the liquid bath, which could submerge the upper portion of the stubs by the additional segment H4=H1-H2. The liquid bath would attack and dissolve the steel, unless the cover 28, for example made of carbon paste, is used to protect the vertical part of the stub and the upper part of the plate.

[0044] The cover may be made in one piece, in two halves or in four or more parts and is made of non-pre-baked carbon paste, or ramming paste, or carbon glue, mixed with a binding agent. For example, an agglomerate is used composed of calcined coke granules mixed with pitch granules, joined together using an organic polymeric binding agent deriving from the processing of plant products such as lignite or sucrose molasses or other organic derivatives or else using phenolic or epoxy resins. The purpose of such binders is to act from room temperature up to a temperature of about 450°C, a temperature from which the pitch begins to carbonize and act as a binder up to temperatures of about 1000°C and above.

[0045] Preferably, non-prebaked carbon paste is used, the same paste that is used for producing the anode. In this case, the cover is implemented in arranging the preformed shells around the stub and above the plate, which shells are made in one piece or in several parts.

[0046] Figure 17 shows a further embodiment of the invention, wherein the connection plate 24 is partially hollow or empty; for example, the connection plate has a cavity 34 which opens onto the lower face 24b, facing the bath of the cell. Advantageously, this feature makes it possible to lighten the connection plate and to control the flow of heat.

[0047] According to this embodiment, preferably, the support assembly 12 cooperates with a cover consisting of a tube segment 36, partly resting upon the upper surface 14c of the anode 14, which is open at the top. The tube 36 delimits the inner space 30 which is preferably filled with fragments 32.

[0048] Preferably, moreover, the connection plate 24 has one or more channels 38 connecting the upper face 24a and the cavity 34, which are suitable for allowing a portion of the fragments 32 to pass from the inner space 30 to the cavity 34. Advantageously, this allows the cavity 34 to be filled with fragments 32, limiting the entry of the molten bath into the cavity 34.

[0049] According to the invention, the ratio between the diameter D4 of the connection plate and the diameter D3 of the stub is between 1 and 3.5 (1<D4 / D3≤3.5), preferably between 1.5 and 3 (1.5≤D4 / D3≤3).

[0050] According to one aspect of the invention, the ratio D4 / H2 between the diameter D4 of the connection plate and the depth H2 of the connection plate is between 1.92 and 10 (1.92 ≤ D4 / H2 ≤ 10). For example, the ratio D4 / H2 = 220 mm / 115 mm = 1.92 or D4 / H2 = 400 mm / 40 mm = 10.

[0051] According to the solutions of the prior art, however, the ratio D3 / H1 between the diameter D3 of the stub and the depth H1 of the stub is between 1.12 (180 mm / 160 mm) and 1.83 (220 mm / 120 mm).

[0052] Advantageously, to ensure the same or better mechanical retention between the support assembly and the anode, the lateral contact area of the plate π*D4*H2 is equal to or greater than the lateral area of the stub according to prior art π*D3*H1.

[0053] According to a further aspect of the invention, the ratio D2 / H2 between the diameter of the fluted hole D2 and the depth of the fluted hole H2 is between 2.2 and 11. For example, the ratio D2 / H2 = 390 mm / 80 mm = 4.875.

[0054] According to further variant embodiments of the invention, the fluted hole has a square or rectangular section or any other polygon. Such cross-section shapes would however only be obtainable by machining the prebaked anodes. Therefore, by economy of production, a circular cross-section is a preferred embodiment.

[0055] Innovatively, according to the invention, the carbon usage efficiency of the anode and the duration of said anode are increased.

[0056] In particular, the depth H2 of the fluted holes according to the invention, which is smaller than the depth H1 of the holes according to the prior art, and the diameter D2 of the fluted holes according to the invention, which is greater than the diameter D1 of the holes according to the prior art, leads to a lower thickness of the anode butt end discard. This corresponds to a greater useful height of the anode, i.e., a "gain in height of the anode" H4 = H1-H2, and therefore an increase in the operating cycle time of the anode.

[0057] Furthermore, the fluted hole according to the invention has a contact area A2+S2 with the carbon that is greater than the contact area A1+S1 of the hole according to the prior art. Consequently, the volume V2 of the fluted hole according to the invention is equal to A2*H2 and is greater than the volume V1 = A1*H1 of the hole of the prior art. Advantageously, the carbon cost for the anode according to the invention is less than the cost according to the prior art for reducing the weight by a factor (V2-V1) * 1.6 (where 1.6 is the apparent density of carbon)

[0058] Furthermore, the fluted holes according to the invention are obtained by means of the same process as in the prior art, i.e. by vibration-press-forming the carbon paste. Therefore, advantageously, the same production line as the solutions of the prior art may be used, after modifying the lid of the vibration-press-forming station in order to adapt it to the larger diameter and smaller depth of the mandrel making the hole.

[0059] Advantageously, according to the invention, the mechanical support for the anode is improved or unaltered with respect to the solutions of the prior art. In particular, this advantage is achieved by means of the connection plate and the associated fluted holes, having a lateral surface area S2 greater than or at least equal to the area of the average lateral surface S1 of the prior art.

[0060] Furthermore, according to the invention, in order to compensate for the reduction in the depth of the holes from H1 to H2, the number of flutes of the fluted holes is greater than the number of grooves of the fluted holes according to the solutions of the prior art. In practice, the overall length of the flutes according to the invention is greater than or equal to the overall length of the grooves with respect to the prior art. In particular, the solutions of the prior art provide for a number of flutes of between 6 and 10, preferably 8; according to the invention, 11 to 30 grooves are envisaged.

[0061] Advantageously, moreover, the anodes and anode assemblies of the prior art may be subjected to retro-fitting and modified according to the present invention. It is sufficient to weld the connection plate at the stub and modify the corresponding fluted hole.

[0062] Furthermore, a further vibration-press-forming unit produces the cover, within the green anode production line or within the anode assembly line, depending on logistics and economical aspects. For example, a mixture or pellets of preheated carbon paste fills the mold and the action of a press forms and compacts the cover so that it may also be handled without baking.Embodiment example

[0063] An exemplary embodiment of the support assembly according to the invention is defined in Table 1. Table 1 Prior art Invention example Apparent density of the prebaked anodeskg / dm 3< 1.6 kg / dm 3< 1.6Weight of the anode without the holeskg1093.326kg1093.326External dimensions of the anode W x H x L mm6556501,6056556501,605Diameter / Depth Ratio D 3 / H 1 Ratio 1.83 D 4 / H 2 Ratio 4.50 Average thickness of the carbon safety barrier below the cast ironHSAFETYmm50HSAFETYmm50Thickness of the cast iron ringtmm15tmm15Depth of the grooved hole including the height of the stub or plateH 1 mm120H 2 mm80Diameter of the grooved holeD 1 mm250D 2 mm390Diameter of stub or plateD3mm220D 4 mm360Lower surface of stub and of the plateS 3 mm 2< 38, 013S 4 mm 2< 101, 788Average waste thicknessH 1 +H SAFETY mm170H 2 +H SAFETY mm130Waste weightkg285.95kg218.67Waste weight reduction Kg 0 kg 67.28 Waste weight reduction in % % 0% % 23.53% Anode useful height gainAHGmm0AHGmm40Anode useful height gain referred to 650 mm of anode height in %AHG%0%AHG%6.15%Average anode life cycle durationDays28Days30Additional anode life cycle timeDays0Days2Additional anode life cycle time in %Additional days%0%Additional days%6.67%Volume of a grooved holeV 1 dm 3< 5.89V 2 dm 3< 9.56Number of grooved holes in the anode44Total volume of the grooved holes4V 1 dm 3< 23.564V 2 dm 3< 38.23Total weight of carbon removed from the grooved holeskg37.70kg61.16Reduced weight for the anode due to the greater volume of the holes kg 0 kg 23.46 Reduction in weight of the anode in %%0%%2.15%Total reduction of waste recycling cost in % % 0% % -31.74% Kg of aluminum produced with the reference anode and 0.410 kgC / kgAl0.410kg1,969.22kg2, 133.32Increase in aluminum produced for each reference anode in %%0%%8.33%Increase in aluminum produced for each reference anode in kgkg0kg164. 10Weight of a protective flanged-ring with a thickness of about 30 mmkg0kg5.73Average voltage drop saving at the ends of the anodemV0mV70

[0064] In Table 1, the dimensions have been defined as follows: anode dimensions according to the prior art: a) Size of the anode: W 655 mm x H 650 mm x W 1,605 mm b) Hole diameter: D1 = 15+220+15 = 250 mm c) Depth of the hole: H1 = 120 mm d) Base area of the hole: A1 = 49,087 mm2 e) Volume of the hole: V1 = 5.890 dm3 size of the anode according to the invention: a) Size of the anode: W 655 mm x H 650 mm x W 1,605 mm b) Hole diameter: D2 = 15+360+15 = 390 mm c) Depth of the hole: H2 = 80 mm d) Base area of the hole: A2 = 119,459 mm2 e) Volume of the hole: V2 = 9.557 dm3

[0065] In this example, the reduction in thickness of the anode waste with respect to the solutions of the prior art is 40 mm; this increases the operating cycle duration of the anode from two to four days, corresponding to an increase in operating cycle time from 6% to 15%. The carbon weight utilization yield of the anode increases by about 10%.

[0066] The initial weight of the anode according to the present invention is about 3% lower than the solutions of the prior art, thanks to the lower weight given by the difference between the volume of the fluted holes according to the invention and those of the prior art (V2 - V1).

[0067] The cost of recycling the waste is reduced by about 35% compared to the cost of the solutions of the prior art, by virtue of the combined reduction of the thickness of the anode waste and the reduction in the anode weight according to the invention.

[0068] The volume V2 of the connecting plate makes it possible to reduce the weight of the anode by: (9.557 - 5.890) dm3 * 1.6 kg / dm3 = 5.867 kg for each hole. 5,867 kg * 4 holes = 23.46 kg or about 3% weight savings compared to an anode of the prior art.

[0069] The yield based upon the anode weight according to the invention is 6% to 15% higher than the same size of an anode of the prior art. This corresponds to an average increase in the life cycle of the anode from two to three days, which is equivalent to an increase in cycle time from 6% to 10% compared to the cycle time of the anode of the prior art. This corresponds to an average reduction of 35% of the waste recycling cost.

[0070] Additional positive effects of the anode according to the invention, resulting from the size of the holes and stubs are the greater contact area, better current distribution and a reduction in voltage drop across the anode ranging about from 50 mV to 80 mV.

Examples

embodiment example

Embodiment example

[0063]An exemplary embodiment of the support assembly according to the invention is defined in Table 1.

Table 1 Prior art Invention example

Apparent density of the prebaked anodeskg / dm 31.6 kg / dm 31.6

Weight of the anode without the holeskg1093.326kg1093.326

External dimensions of the anode W x H x L mm6556501,6056556501,605

Diameter / Depth Ratio D 3 / H 1 Ratio 1.83 D 4 / H 2 Ratio 4.50

Average thickness of the carbon safety barrier below the cast ironHSAFETYmm50HSAFETYmm50

Thickness of the cast iron ringtmm15tmm15

Depth of the grooved hole including the height of the stub or plateH 1 mm120H 2 mm80

Diameter of the grooved holeD 1 mm250D 2 mm390

Diameter of stub or plateD3mm220D 4 mm360

Lower surface of stub and of the plateS 3 mm 238, 013S 4 mm 2101, 788

Average waste thicknessH 1 +H SAFETY mm170H 2 +H SAFETY mm130

Waste weightkg285.95kg218.67

Waste weight reduction Kg 0 kg 67.28

Waste weight reduction in % % 0% % 23.53%

Anode useful height gainAHGmm0AHGmm40

Anode useful heigh...

Field of the invention

[0001] The present invention is in the field of components for producing primary aluminum through the use of Hall-Hêroult cells; in particular, the present invention relates to an innovative support assembly for an anode for a Hall-Hêroult cell, which allows the useful life of the anode operating cycle to be extended and the weight of the final anode waste to be reduced.Prior art

[0002] Primary aluminum is produced in Hall-Hêroult reduction cells. The alumina dispensed into the cells dissolves in a cryolite and aluminum fluoride (AlF 3 ) electrolytic bath at a temperature of about 950°C and is reduced to aluminum by passing direct current from the anodes to the carbon cathodes during the electrolysis process.

[0003] A support assembly for an anode, usually of the prebaked type, consists of a hook formed by an anode beam, a transition joint, a yoke, and a plurality of stubs.

[0004] For connection to the support assembly, the anode is provided at the top with a plurality of laterally fluted holes, one for each stub. A predefined amount of molten cast iron is poured into the residual space between the stub and the respective hole. Once the cast iron has solidified the stubs remain embedded in the holes of the anode, and the solidified cast iron ring becomes the mechanical and electrical junction between the anode beam and the anode.

[0005] A cell usually has several support assemblies installed, each carrying a respective anode, and the electric current flowing through each anode is substantially equal to the total line current divided by the number of support assemblies in the cell.

[0006] The normal operating period of a prebaked anode according to the prior art is substantially between 25 and 30 days and ends when the anode has reached a minimum height below which there is a risk of the cast iron ring contaminating the bath. In such a condition, it is necessary for a new support assembly, equipped with a new anode, to replace the support assembly carrying the worn anode.

[0007] The weight of the carbon in the consumed anode, which constitutes the anode butt end discard, is usually between 20 and 30% of the weight of the new anode and is recovered in a line for recovering the carbon of the anode butt ends.

[0008] To prevent cast iron rings from contaminating the molten bath and aluminum, a layer of carbon approximately 50 mm below the rings is preserved, which is part of the thickness of the anode waste. The overall thickness of the anode waste, also considering the depth of the holes, is between 150 and 200 mm.

[0009] The duration of an anode depends directly upon the total thickness of the waste, which depends directly upon the depth of the stub holes present in the upper part of the anode. According to the solutions of the prior art, the average depth H1 of the holes is approximately 120 mm. Considering the safety thickness of about 50 mm, the total thickness of the Hbutt waste is about 170 mm, which corresponds to about 26% of the height of 650 mm of a new anode.

[0010] Furthermore, in the solutions of the prior art, the density distribution of the electric currents is not uniform insofar as the currents converge towards the stubs, but only through a limited area of the cast iron rings (in particular, towards those areas of the rings having a higher carbon contact pressure). The contact resistance between the stubs and the anode is non-uniform, and the voltage drop between the stubs and the anode generally ranges from about 80 mV to 130 mV. The Joule effect inside the anode, due to the high density of local currents, generates heat and contributes to the energy loss of the cell.

[0011] Some patent documents have proposed some approaches to improve the energy efficiency of the anodes, the elimination of the use of cast iron and an improved use of the carbon. For example: The International Application WO-A1-2016 / 130014 by Norsk Hydro proposes to replace the cast iron with mechanical support elements which require anode processing and an intermediate layer of metal particles for the electrical connection between the carbon and the hook. Nevertheless, the mechanical particles must be recycled, the support assembly is different from those that are typically used in the industry and makes retrofitting on pre-existing support assemblies impossible; The International Application WO-A1-2012 / 100340 from the University of Laval proposes a seamless connection. Instead of holes for stubs, longitudinal grooves are provided and the steel part incorporated into the cast iron is a bar with a rectangular cross section; the document US 7,901,560 by Norsk Hydro proposes creating slits at the bottom of the anode to reduce energy consumption; the Storvik International Application WO-A1-2016 / 108696 and the Servico International Application WO-A1-2002 / 42525 propose yokes provided with copper parts inside steel parts and do not have a stub welded at the steel arm of a steel yoke.

[0012] Finally, further exemplary embodiments are shown in CA2838113A1 and GB2569382A, which however only indicate cast iron rings around the stubs.Object of the invention

[0013] The object of the present invention is to meet the needs of the industry whilst overcoming the drawbacks mentioned with reference to the prior art, mainly in order to increase the service life of the anode, reducing the weight of the anode butt end discard.

[0014] This object is achieved by means of a support assembly according to claim 1. The claims dependent thereon identify additional advantageous embodiments of the invention.Brief description of the figures

[0015] The features and advantages of the support assembly according to the present invention will be apparent from the description below, given by way of non-limiting example in accordance with the figures in the accompanying drawings, wherein: Figure 1 schematically depicts a Hall-Hêroult cell for producing primary aluminum and a pair of support assemblies for the respective anodes; Figures 2 to 6 refer to the prior art, and in particular: a) Figure 2 shows a support assembly assembled and connected to the anode; b) Figure 3 shows the support assembly of Figure 2, in separate parts; c) Figure 4a and 4b show an example of a cast iron ring of the support assembly, after solidification; d) Figure 5 shows a support assembly with a worn-out anode; e) Figure 6 shows a fluted hole and relative stub. Figures 7 to 17 refer to a support assembly according to the invention, and in particular: a) Figure 7 shows a support assembly assembled and connected to the anode; b) Figure 8 depicts an arm of the support assembly in Figure 7, assembled and connected to the relative anode; c) Figure 9 depicts an assembled arm of the support assembly with a corresponding fluted hole; d)Figures 10a and 10b show a ring of the support assembly, after solidification; e) Figures 11a, 11b and 11c show further embodiments of an arm of the support assembly; f) Figures 12 to 17 show further shapes of an arm of the support assembly. Description of some embodiments of the invention

[0016] Figure 1 schematically depicts a Hall-Hêroult cell 10 for producing primary aluminum and a pair of support assemblies 12 in the working position, immersed in the bath. Each support assembly 12 comprises an anode beam 16, usually made of aluminum, connected to an anode 14 made of carbon, usually of the prebaked type.

[0017] In particular, for the connection between the anode beam 16 and the anode 14, the support assembly 12 comprises a transition joint 18, welded at the lower end of the anode beam 16, and a yoke 20, usually made of steel, welded at the transition joint 18, usually bi-metal (steel-aluminum). Furthermore, the yoke 20 comprises a plurality of separate arms 20a and the support assembly comprises a plurality of stubs 22, normally made of steel, each stub 22 being welded at the respective arm 20a of the yoke 20.

[0018] The stub 22, which is usually cylindrical, has a characteristic width, i.e. a dimension that characterizes the maximum overall dimensions of the stub, for example the maximum outer diameter D3.

[0019] According to Figure 7, 8 and 9, the support assembly 12 further comprises a plurality of connection plates 24, each connection plate 24, usually cylindrical and preferably made of steel, being connected at the lower end of the respective stub 22. According to the invention, the connection plate 24, delimited at the top by an upper face 24a and at the bottom by a lower face 24b, has a maximum outer diameter D4, wherein the diameter D4 of the connection plate 24 is greater than the diameter D3 of the stub (D4 > D3).

[0020] For example, the connection plate 24 is welded at the stub 22; in a variant embodiment, the stub and the connection plate constitute a single body.

[0021] Finally, the support assembly 12 comprises a plurality of rings 26, usually made of cast and solidified iron, for connecting the support assembly 12 to the anode; according to further embodiments, the ring is made of carbon paste or consists of a carbon-based glue. Each ring 26 of cast iron joins the connection plate 24 of the respective stub 22 and the anode 14, by means of being engaged with a respective laterally fluted hole 14a of the anode 14.

[0022] To make the connection between the connection plate 24 and the anode 14, a plurality of fluted holes 14a is implemented in the upper part of the anode 14; each fluted hole 14a is provided on the lateral surface thereof with a plurality of flutes 14b, wherein each flute 14b extends predominantly axially and is oblique.

[0023] The connecting plate 24 is positioned in the respective fluted hole 14a and a predefined amount of cast iron is poured into the fluted hole. The cast iron, in solidifying, forms the ring 26. The ring 26 is firmly connected to the plate 24 by virtue of the thermal contraction that the cast iron undergoes following solidification and is firmly connected to the anode 14 by virtue of the flutes 14b; in particular, the cast iron that has solidified within the flutes 14b constitutes a plurality of flaps 26b of the ring 26.

[0024] In Figure 8, the fluted hole 14a has a diameter D2 (flutes excluded), a characteristic depth H2 and a lateral surface extension S2. Hsafety is the safety thickness of the consumed anode, Hbutt is the thickness of the anode waste, and H3 is the protrusion height of the connection plate 24 from the upper surface of the anode 14.

[0025] With respect to the prior art, depicted for example in Figure 6, it is evident that, due to the use of the connecting plate 24, the diameter D2 of the fluted hole 14a is greater than the diameter D1 of the fluted hole of the solution of the prior art; the depth H2 of the fluted hole 14a is less than the depth H1 of the fluted hole of the solution of the prior art. In particular, H4 = H1-H2 is the decrease in height between the solution of the invention and the solution of the prior art and is equivalent to the decrease in height of the anode butt end discard.

[0026] Figures 10a and 10b show an embodiment of the ring 26, consisting of a tubular main body 26a, of thickness "t", and a plurality of flaps 26b protruding radially outwards from the main body 26a, having a predominantly axial and oblique extension in relation to the central axis of the main body.

[0027] Figure 11a depicts an embodiment of the invention, wherein the connection plate 24 is a steel disk welded at the lower end of the stub 22, with a partial overlap of the height of the connection plate in relation to the height of said stub.

[0028] Figure 11b depicts a further embodiment of the invention, wherein the connection plate 24 is a steel disk welded at the lower end of the stub 22, with a total overlap of the height of the connection plate in relation to the height of said stub.

[0029] Figure 11c depicts yet another embodiment of the invention, wherein the connection plate 24 is a steel disk welded at the lower end of the stub 22, completely below the stub.

[0030] According to one embodiment of the invention, moreover, the flutes 14b of the fluted holes 14a are between 11 and 30 in number, i.e., in a greater number than those provided for in the solutions of the prior art.

[0031] In the embodiments shown in Figures 11a, 11b, and 11c, the position of the ring with respect to the stub may be modified, thus accordingly modifying the contact area. It is thus possible to control the flow of heat from the anode to the arm of the respective stub.

[0032] The heat generated by the currents inside the anode by the Joule effect is less in the solution according to the invention than in the known solutions insofar as the larger area of the surface of the fluted hole S2 reduces the current densities and contact resistance in the interface area between the connection plate (steel), the ring (cast iron) and the anode (carbon).

[0033] The contact resistance also depends upon the pressure between the lateral surface 14a of the fluted hole 14 and the carbon anode, as well as upon the profile of the lateral surface 14a which may be, for example, flat or knurled.

[0034] Figure 12 depicts a further embodiment of the invention, wherein the fluted hole 14a has a conical side surface, having an extension S2, preferably obtained by means of tool machining, and the connecting plate 24 has a conical side surface, having an extension S2.

[0035] Figure 13 shows a further embodiment of the invention, wherein the side surface of the connection plate 24 is knurled, in order to increase the electrical contact area and the mechanical support of the anode. Preferably, moreover, the fluted hole 14a has a conical lateral surface and the connecting plate 24 has a conical lateral surface.

[0036] Figure 14 is a further embodiment of the invention, wherein the support assembly 12 cooperates with a cover 28, arranged to cover the transition region between the stub 22 and the connection plate 24 and suitable to cover an end portion of the stub 22 and an upper portion of the connection plate 24. Preferably, the cover 28 rests upon an upper face 14c of the anode 14 and, by virtue of the inner shape, upon the connecting plate 24.

[0037] Preferably, the cover 28 has such an overall height H5 that, when the anode 14 is completely immersed in the cell bath, the stub is protected and may not be lapped by the liquid electrolytic bath.

[0038] For example, the cover 28 has an overall flanged ring shape and comprises two couplable half-shells 28a, 28b, held together, for example, by carbon glue or a metallic wire or carbon fiber.

[0039] Figure 15 shows a further embodiment of the invention, wherein the cover 28 consists of a tube segment, partly resting upon the upper surface 14c of the anode 14, open at the top and having a height H5.

[0040] The tube thus delimits an inner space 30, preferably filled with a plurality of protective fragments 32, e.g., consisting of unbaked carbon paste in order to prevent the liquid bath from entering and to better thermally isolate the upper part of the anode.

[0041] Figure 16 depicts a further embodiment of the invention, wherein the cover 28 has an overall flanged ring shape and consists of a lower part 28a, consisting of two half-shells 281, coupled to and resting upon the upper surface 14c of the anode 14 and upon the connecting plate 24, and an upper part 28b, consisting of two half-shells 283, 284 coupled to and resting upon the upper part 28a.

[0042] Preferably, the half-shells 281, 282, 283, 284 are joined together using carbon glue or steel tips or needles protruding from the connecting plate 24.

[0043] During the last 3 or 4 days of the anode life cycle, in order to maintain the same distance between the anode and cathode, the anode must move deeper into the liquid bath, which could submerge the upper portion of the stubs by the additional segment H4=H1-H2. The liquid bath would attack and dissolve the steel, unless the cover 28, for example made of carbon paste, is used to protect the vertical part of the stub and the upper part of the plate.

[0044] The cover may be made in one piece, in two halves or in four or more parts and is made of non-pre-baked carbon paste, or ramming paste, or carbon glue, mixed with a binding agent. For example, an agglomerate is used composed of calcined coke granules mixed with pitch granules, joined together using an organic polymeric binding agent deriving from the processing of plant products such as lignite or sucrose molasses or other organic derivatives or else using phenolic or epoxy resins. The purpose of such binders is to act from room temperature up to a temperature of about 450°C, a temperature from which the pitch begins to carbonize and act as a binder up to temperatures of about 1000°C and above.

[0045] Preferably, non-prebaked carbon paste is used, the same paste that is used for producing the anode. In this case, the cover is implemented in arranging the preformed shells around the stub and above the plate, which shells are made in one piece or in several parts.

[0046] Figure 17 shows a further embodiment of the invention, wherein the connection plate 24 is partially hollow or empty; for example, the connection plate has a cavity 34 which opens onto the lower face 24b, facing the bath of the cell. Advantageously, this feature makes it possible to lighten the connection plate and to control the flow of heat.

[0047] According to this embodiment, preferably, the support assembly 12 cooperates with a cover consisting of a tube segment 36, partly resting upon the upper surface 14c of the anode 14, which is open at the top. The tube 36 delimits the inner space 30 which is preferably filled with fragments 32.

[0048] Preferably, moreover, the connection plate 24 has one or more channels 38 connecting the upper face 24a and the cavity 34, which are suitable for allowing a portion of the fragments 32 to pass from the inner space 30 to the cavity 34. Advantageously, this allows the cavity 34 to be filled with fragments 32, limiting the entry of the molten bath into the cavity 34.

[0049] According to the invention, the ratio between the diameter D4 of the connection plate and the diameter D3 of the stub is between 1 and 3.5 (1<D4 / D3≤3.5), preferably between 1.5 and 3 (1.5≤D4 / D3≤3).

[0050] According to one aspect of the invention, the ratio D4 / H2 between the diameter D4 of the connection plate and the depth H2 of the connection plate is between 1.92 and 10 (1.92 ≤ D4 / H2 ≤ 10). For example, the ratio D4 / H2 = 220 mm / 115 mm = 1.92 or D4 / H2 = 400 mm / 40 mm = 10.

[0051] According to the solutions of the prior art, however, the ratio D3 / H1 between the diameter D3 of the stub and the depth H1 of the stub is between 1.12 (180 mm / 160 mm) and 1.83 (220 mm / 120 mm).

[0052] Advantageously, to ensure the same or better mechanical retention between the support assembly and the anode, the lateral contact area of the plate π*D4*H2 is equal to or greater than the lateral area of the stub according to prior art π*D3*H1.

[0053] According to a further aspect of the invention, the ratio D2 / H2 between the diameter of the fluted hole D2 and the depth of the fluted hole H2 is between 2.2 and 11. For example, the ratio D2 / H2 = 390 mm / 80 mm = 4.875.

[0054] According to further variant embodiments of the invention, the fluted hole has a square or rectangular section or any other polygon. Such cross-section shapes would however only be obtainable by machining the prebaked anodes. Therefore, by economy of production, a circular cross-section is a preferred embodiment.

[0055] Innovatively, according to the invention, the carbon usage efficiency of the anode and the duration of said anode are increased.

[0056] In particular, the depth H2 of the fluted holes according to the invention, which is smaller than the depth H1 of the holes according to the prior art, and the diameter D2 of the fluted holes according to the invention, which is greater than the diameter D1 of the holes according to the prior art, leads to a lower thickness of the anode butt end discard. This corresponds to a greater useful height of the anode, i.e., a "gain in height of the anode" H4 = H1-H2, and therefore an increase in the operating cycle time of the anode.

[0057] Furthermore, the fluted hole according to the invention has a contact area A2+S2 with the carbon that is greater than the contact area A1+S1 of the hole according to the prior art. Consequently, the volume V2 of the fluted hole according to the invention is equal to A2*H2 and is greater than the volume V1 = A1*H1 of the hole of the prior art. Advantageously, the carbon cost for the anode according to the invention is less than the cost according to the prior art for reducing the weight by a factor (V2-V1) * 1.6 (where 1.6 is the apparent density of carbon)

[0058] Furthermore, the fluted holes according to the invention are obtained by means of the same process as in the prior art, i.e. by vibration-press-forming the carbon paste. Therefore, advantageously, the same production line as the solutions of the prior art may be used, after modifying the lid of the vibration-press-forming station in order to adapt it to the larger diameter and smaller depth of the mandrel making the hole.

[0059] Advantageously, according to the invention, the mechanical support for the anode is improved or unaltered with respect to the solutions of the prior art. In particular, this advantage is achieved by means of the connection plate and the associated fluted holes, having a lateral surface area S2 greater than or at least equal to the area of the average lateral surface S1 of the prior art.

[0060] Furthermore, according to the invention, in order to compensate for the reduction in the depth of the holes from H1 to H2, the number of flutes of the fluted holes is greater than the number of grooves of the fluted holes according to the solutions of the prior art. In practice, the overall length of the flutes according to the invention is greater than or equal to the overall length of the grooves with respect to the prior art. In particular, the solutions of the prior art provide for a number of flutes of between 6 and 10, preferably 8; according to the invention, 11 to 30 grooves are envisaged.

[0061] Advantageously, moreover, the anodes and anode assemblies of the prior art may be subjected to retro-fitting and modified according to the present invention. It is sufficient to weld the connection plate at the stub and modify the corresponding fluted hole.

[0062] Furthermore, a further vibration-press-forming unit produces the cover, within the green anode production line or within the anode assembly line, depending on logistics and economical aspects. For example, a mixture or pellets of preheated carbon paste fills the mold and the action of a press forms and compacts the cover so that it may also be handled without baking.Embodiment example

[0063] An exemplary embodiment of the support assembly according to the invention is defined in Table 1. Table 1 Prior art Invention example Apparent density of the prebaked anodeskg / dm 3< 1.6 kg / dm 3< 1.6Weight of the anode without the holeskg1093.326kg1093.326External dimensions of the anode W x H x L mm6556501,6056556501,605Diameter / Depth Ratio D 3 / H 1 Ratio 1.83 D 4 / H 2 Ratio 4.50 Average thickness of the carbon safety barrier below the cast ironHSAFETYmm50HSAFETYmm50Thickness of the cast iron ringtmm15tmm15Depth of the grooved hole including the height of the stub or plateH 1 mm120H 2 mm80Diameter of the grooved holeD 1 mm250D 2 mm390Diameter of stub or plateD3mm220D 4 mm360Lower surface of stub and of the plateS 3 mm 2< 38, 013S 4 mm 2< 101, 788Average waste thicknessH 1 +H SAFETY mm170H 2 +H SAFETY mm130Waste weightkg285.95kg218.67Waste weight reduction Kg 0 kg 67.28 Waste weight reduction in % % 0% % 23.53% Anode useful height gainAHGmm0AHGmm40Anode useful height gain referred to 650 mm of anode height in %AHG%0%AHG%6.15%Average anode life cycle durationDays28Days30Additional anode life cycle timeDays0Days2Additional anode life cycle time in %Additional days%0%Additional days%6.67%Volume of a grooved holeV 1 dm 3< 5.89V 2 dm 3< 9.56Number of grooved holes in the anode44Total volume of the grooved holes4V 1 dm 3< 23.564V 2 dm 3< 38.23Total weight of carbon removed from the grooved holeskg37.70kg61.16Reduced weight for the anode due to the greater volume of the holes kg 0 kg 23.46 Reduction in weight of the anode in %%0%%2.15%Total reduction of waste recycling cost in % % 0% % -31.74% Kg of aluminum produced with the reference anode and 0.410 kgC / kgAl0.410kg1,969.22kg2, 133.32Increase in aluminum produced for each reference anode in %%0%%8.33%Increase in aluminum produced for each reference anode in kgkg0kg164. 10Weight of a protective flanged-ring with a thickness of about 30 mmkg0kg5.73Average voltage drop saving at the ends of the anodemV0mV70

[0064] In Table 1, the dimensions have been defined as follows: anode dimensions according to the prior art: a) Size of the anode: W 655 mm x H 650 mm x W 1,605 mm b) Hole diameter: D1 = 15+220+15 = 250 mm c) Depth of the hole: H1 = 120 mm d) Base area of the hole: A1 = 49,087 mm2 e) Volume of the hole: V1 = 5.890 dm3 size of the anode according to the invention: a) Size of the anode: W 655 mm x H 650 mm x W 1,605 mm b) Hole diameter: D2 = 15+360+15 = 390 mm c) Depth of the hole: H2 = 80 mm d) Base area of the hole: A2 = 119,459 mm2 e) Volume of the hole: V2 = 9.557 dm3

[0065] In this example, the reduction in thickness of the anode waste with respect to the solutions of the prior art is 40 mm; this increases the operating cycle duration of the anode from two to four days, corresponding to an increase in operating cycle time from 6% to 15%. The carbon weight utilization yield of the anode increases by about 10%.

[0066] The initial weight of the anode according to the present invention is about 3% lower than the solutions of the prior art, thanks to the lower weight given by the difference between the volume of the fluted holes according to the invention and those of the prior art (V2 - V1).

[0067] The cost of recycling the waste is reduced by about 35% compared to the cost of the solutions of the prior art, by virtue of the combined reduction of the thickness of the anode waste and the reduction in the anode weight according to the invention.

[0068] The volume V2 of the connecting plate makes it possible to reduce the weight of the anode by: (9.557 - 5.890) dm3 * 1.6 kg / dm3 = 5.867 kg for each hole. 5,867 kg * 4 holes = 23.46 kg or about 3% weight savings compared to an anode of the prior art.

[0069] The yield based upon the anode weight according to the invention is 6% to 15% higher than the same size of an anode of the prior art. This corresponds to an average increase in the life cycle of the anode from two to three days, which is equivalent to an increase in cycle time from 6% to 10% compared to the cycle time of the anode of the prior art. This corresponds to an average reduction of 35% of the waste recycling cost.

[0070] Additional positive effects of the anode according to the invention, resulting from the size of the holes and stubs are the greater contact area, better current distribution and a reduction in voltage drop across the anode ranging about from 50 mV to 80 mV.

Examples

embodiment example

Embodiment example

[0063]An exemplary embodiment of the support assembly according to the invention is defined in Table 1.

Table 1 Prior art Invention example

Apparent density of the prebaked anodeskg / dm 31.6 kg / dm 31.6

Weight of the anode without the holeskg1093.326kg1093.326

External dimensions of the anode W x H x L mm6556501,6056556501,605

Diameter / Depth Ratio D 3 / H 1 Ratio 1.83 D 4 / H 2 Ratio 4.50

Average thickness of the carbon safety barrier below the cast ironHSAFETYmm50HSAFETYmm50

Thickness of the cast iron ringtmm15tmm15

Depth of the grooved hole including the height of the stub or plateH 1 mm120H 2 mm80

Diameter of the grooved holeD 1 mm250D 2 mm390

Diameter of stub or plateD3mm220D 4 mm360

Lower surface of stub and of the plateS 3 mm 238, 013S 4 mm 2101, 788

Average waste thicknessH 1 +H SAFETY mm170H 2 +H SAFETY mm130

Waste weightkg285.95kg218.67

Waste weight reduction Kg 0 kg 67.28

Waste weight reduction in % % 0% % 23.53%

Anode useful height gainAHGmm0AHGmm40

Anode useful heigh...

Claims

1. A support assembly (12) for supporting a prebaked anode (14) for a Hall-Héroult cell (10) for producing primary aluminum, comprising an anode beam (16), a yoke (20) made of steel supported by the anode beam (16), having a plurality of separate arms (20a), and a plurality of stubs (22), wherein each stub (22), welded at a lower end of a respective arm (20a) of the yoke (20), has a characteristic diameter (D3), said support assembly being characterized in that it further comprises a plurality of connection plates (24) made of steel, wherein each connection plate (24) is arranged at the lower end of a respective stub (22) and has a diameter (D4), and wherein the ratio between the diameter of the connection plate (D4) and the diameter of the stub (D3) is of between 1 and 3.5 (1<D4 / D3≤3,5), preferably between 1.5 and 3 (1,5≤D4 / D3≤3).

2. A support assembly according to claim 1, wherein the connection plate (24) is welded to the stub (22).

3. A support assembly according to claim 1, wherein the connection plate (24) and the respective stub (22) form a single body.

4. A support assembly according to any one of the preceding claims, wherein the connection plate (24) has a cylindrical outer lateral surface.

5. A support assembly according to any one of the claims from 1 to 3, wherein the connection plate (24) has a frustoconical outer lateral surface.

6. A support assembly according to any one of the preceding claims, wherein the connection plate (24) has a knurled outer lateral surface.

7. A support assembly according to any one of the preceding claims, comprising a plurality of cast iron rings (26), wherein each ring (26) comprises a tubular main body (26a), internally fastened to the connection plate (24), and a plurality of flaps (26b) protruding radially outwards from the main body (26a), having axial and oblique extension, for the connection with the anode (14).

8. A support assembly according to claim 7, wherein the number of flaps (26b) is from 11 to 30.

9. A support assembly according to any one of the preceding claims, wherein the connection plate (24) is a disc welded at the lower end of the stub (22), with the height of the connection plate partially overlapping the height of the stub.

10. A support assembly according to any one of the claims from 1 to 8, wherein the connection plate (24) is a disc welded at the lower end of the stub (22), with the height of the connection plate totally overlapping the height of the stub.

11. A support assembly according to any one of the claims from 1 to 8, wherein the connection plate (24) is a disc welded at the lower end of the stub (22), completely below the stub.

12. A support assembly according to any one of the preceding claims, comprising a transition joint (18), welded on one side at the lower end of the anode beam (16) and at the other side at the yoke (20).

13. An anode assembly, comprising: - a support assembly (12) according to any one of the claims from 1 to 12; and - an anode (14), fastened to the support assembly (12).

14. An anode assembly according to claim 13, wherein the ratio (D4 / H2) between the diameter (D4) of the connection plate (24) and the characteristic depth (H2) of the respective grooved hole (14a) is of between 1.92 and 10 (1.92≤D4 / H2≤10).

15. A cover assembly, comprising: - a support assembly (12) according to any one of the claims from 1 to 12; and - a cover (28) applied to the support assembly so as to cover the transition area between the stub (22) and the connection plate (24).

16. An anode and cover assembly, comprising: - a support assembly (12) according to any one of the claims from 1 to 12; - an anode (14) fastened to the support assembly (12); and - a cover (28), applied to the support assembly so as to cover the transition area between the stub (22) and the connection plate (24).

17. A reduction cell (10) for producing primary aluminum, comprising an anode assembly according to claims 13 or 14.

18. A reduction cell (10) for producing primary aluminum, comprising an anode and cover assembly according to claim 16.

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