Patent Application
20160208399
The present subject matter relates to electrode assemblies for use in the electrolytic reduction of refined materials, including, but not limited to, use in aluminum reduction cells. The subject matter more specifically relates to the reduction of electrical resistance and improvement of the current distribution through the anode and cathode assemblies to reduce power consumption, to improve reduction cell performance and to improve operational life of the anodes and cathodes.
The Hall-Heroult process is a well known process for aluminum synthesis by electrolytic reduction of alumina. The process uses either pre-baked or in-situ baked carbon anodes paired in a reduction cell with opposing carbon cathodes that are separated from the anodes by a molten electrolyte (‘bath’) that contains dissolved alumina, and a molten metal pad of conductive aluminum overlying the top of the cathode. During the process an electrical current is passed between the anode and cathode to reduce the dissolved alumina into molten aluminum and to evolve carbon monoxide/or dioxide from the bottom of the anode. The conductive molten aluminum sinks to the aluminum layer or ‘metal pad’ on top of the carbon cathode, thereby becoming part of the electrical circuit. Alumina is added to the bath from an overhead hopper, while the molten aluminum is withdrawn from the reduction cell by intermittent siphoning to a mobile crucible.
The pre-baked carbon anode is attached to, and supported by, the lower end of a vertical conductor rod, and the upper end of the conductor rod is clamped to an electric buss beam. The vertical conductor rod is made of aluminum or copper and is typically joined to the carbon anode through a lower, horizontal steel yoke and from one to eight (or more) round bar steel stubs attached to the yoke.
The carbon anodes are formed with prepared recesses (stub holes) in which the corresponding stubs are fitted. The stubs and anode are then joined by filling the space between them with cast iron or with carbon paste adhesive. When using cast iron for the bond the solidified iron annulus between the stub and carbon anode is referred to as a ‘thimble’. The carbon anodes are mostly consumed in the reduction process by combining with the evolved oxygen from the reduced alumina. The anode assemblies have an operational duty life of typically 20-30 days in the reduction cell. A typical anode assembly of this type is disclosed in U.S. Pat. No. 3,398,081 to Bonfils et al.
Pre-baked cathode assemblies include a pre-baked carbon (carbonaceous or graphitic) cathode connected by cast iron or carbon paste to one or more steel current collector bars that are positioned in slots in the underside of the cathode. The collector bars are longer than the cathode and protrude through the reduction cell sidewall for connection to an electric buss. The cathodes have an operational duty life of typically 5-10 years, during which period the carbon corrodes, preferentially in electrical paths of lowest resistance and highest current density. The cathode blocks undergo sodium absorption from the bath, aluminum infiltration and vertical heaving from thermal stresses due to uneven temperature profile throughout the carbon, which conditions gradually increase the electrical resistance of the cathode assembly. When the resistance becomes too high or the drawn aluminum contains too much iron, that indicates dissolving collector bars, the reduction cell is removed from service to be relined with new material.
The electrical energy required to reduce alumina to aluminum and to heat the incoming alumina to the reduction temperature is a minor fraction of the total electrical energy typically consumed between the anode buss to cathode buss connections. The balance of energy consumed is from heat generated due to electrical resistance through the various components and connection interfaces of the reduction cell circuit, which heat is removed to the environment. The factors influencing electrical resistance at the interfaces include the electrical resistance of the adjoining materials, the surface area and cleanliness of the interface, and the contact pressure between the adjoining materials. Although the cast iron connections used in the anode and cathode assemblies are electrically conductive, they still exhibit significant electrical resistance which generates ohmic heating that does not directly contribute to the electrolytic reduction process. The high electrical resistance is at least partly due to the shrinkage which the iron undergoes during solidification. This problem can be exacerbated by differential thermal expansion of the various materials of the electrode assemblies when the cell is heated to operating temperature.
Past efforts to improve the efficiency of electrolytic processes for metal production have either focused on improving the electrical current distribution through the anodes and cathodes, and/or reducing resistance between various components of the electrode assemblies. For example, a number of patents and published patent applications disclose thimbles having extensions to increase surface contact area with the anode, improving current distribution and reducing resistance. Examples of such constructions are disclosed in U.S. Pat. Nos. 4,552,638, 4,557,817 and 4,824,543. Others have attempted to change vertical current distribution through the thimble by adding undercuts or lateral extensions to the thimble (eg. U.S. Pat. No. 4,621,674). Others have attempted to modify the contact pressure between the carbon electrodes and other components of the electrode assemblies, for example by providing an expanded graphite lining in the cathode collector bar slots (eg. U.S. Pat. No. 7,776,190 to Hiltmann et al.), or providing tapered, straight, or threaded connectors which are assembled with a low pressure fit with stress relief slots in the connection pin, or assembled with a loose fit, which pin expands and tightens only with increasing temperature (eg. U.S. Pat. Nos. 3,179,736, 3,390,071, 3,489,984 and 3,499,831). Others have used alternate materials and methods to provide separate mechanical and electrically conductive connections between the stub and anode (e.g. International Publication No. WO 2009/099335).
Despite the past efforts to improve the efficiency of electrolytic processes, there remains a need for anode and cathode assemblies which will help to achieve further reductions in the amount of energy consumed per kilogram of produced metal, reduced carbon emissions and carbon losses, and longer electrode life.
In an embodiment, there is provided an electrode assembly for use in a reduction cell for the production of metal, the electrode assembly comprising: (a) an electrically conductive carbon electrode block having a first surface and a second surface, wherein the first surface faces an interior of the reduction cell when the electrode assembly is in use; (b) an electrically conductive metal member having a first end and a second end, wherein the first end of the metal member is connected to the carbon electrode block in an electrically conductive manner, and the second end of the metal member is adapted for connection to a buss bar in an electrically conductive manner; (c) a solid, conductive metal insert at least partly received in the carbon electrode block, wherein the insert extends into the carbon electrode block from the second surface thereof; and wherein the metal insert is received in the carbon electrode block with an interference fit, such that the insert exerts a lateral force on the carbon electrode block.
According to one aspect, the electrode is a pre-baked carbon anode, wherein the first surface of the carbon electrode block is a bottom surface thereof, and wherein the electrically conductive metal member comprises a vertical conductor rod. The electrically conductive metal member may further comprise a vertical stub at its first end, the carbon electrode block having a top surface opposite the bottom surface, a recess formed in the top surface, with an end of the vertical stub being received in the recess. The second surface in which the insert extends may comprise an inner surface of the recess, the inner surface being selected from a bottom surface and a side surface of the recess. For example, the insert may extend into the bottom surface of the recess and extend vertically downwardly therefrom, and/or the insert may extend into the side surface of the recess and extend radially outwardly therefrom, with a portion of each said insert optionally protruding from the bottom surface or the side surface of the recess, the protruding portion optionally including an enlarged head. The insert may be inclined downwardly and outwardly from said second surface. A plurality of said inserts may be provided in the bottom surface and/or the side surface of the recess, with each of said inserts being at least partly received in the carbon electrode block.
According to another aspect, the recess may be provided with a conductive metal lining through which an electrically conductive connection is formed between the stub and the carbon electrode block, and wherein the insert or the plurality of inserts is in direct, electrically conductive contact with the conductive metal lining of the recess. At least a portion of the conductive metal lining may comprise a cast portion which is formed in situ between the stub and the carbon electrode block. For example, a portion of the conductive metal lining may comprise a solid preform which is combined with the cast portion during formation of the metal lining, and wherein the insert or the plurality of inserts is in direct, electrically conductive contact with the preform prior to formation of the cast portion. The preform may comprise a bottom plate which is in contact with a bottom surface of the recess.
According to yet another aspect, the electrode assembly may further comprise a plurality of said vertical stubs at its first end, the stubs being spaced apart from one another, with the carbon electrode block having a plurality of said recesses formed in its top surface, with an end of each of the vertical stubs being received in a corresponding one of the recesses; and wherein each of the vertical stubs is secured to the vertical conductor rod through an electrically conductive metal yoke. The electrode assembly may further comprise a plurality of electrically conductive bypass members, each of which bypasses the yoke and one of the vertical stubs, wherein each of the bypass members has a first end connected to the vertical conductor rod through an electrically conductive connection, and a second end connected to the carbon electrode block through an electrically conductive connection. The carbon electrode block is provided with a plurality of said inserts in the top surface thereof, and wherein the second end of each of the bypass members is connected to at least one of the inserts. The second end of each bypass member may be secured to the top surface of the electrode by at least one of the inserts, and wherein the second end of each bypass member includes an expandable or flexible portion.
According to yet another aspect, the carbon electrode block has a top surface opposite the bottom surface, and is provided with a plurality of said inserts in the top surface thereof, wherein the electrode assembly further comprises a collar-shaped connector having a side wall to receive the first end of the electrically conductive metal member and to provide an electrically conductive connection between the electrically conductive metal member and the carbon electrode block, and wherein the collar-shaped connector further comprises at least one attachment portion which is connected to the side wall and extends outwardly therefrom, each of the attachment portions being connected to at least one of the inserts to provide an electrically conductive connection between the attachment portion and the at least one of said inserts. The plurality of said inserts may be distributed across the top surface of the carbon electrode block, with the attachment portion(s) connected to each of the inserts. The electrically conductive metal member may further comprise a vertical stub at its first end, wherein each of the vertical stubs is secured to the vertical conductor rod through an electrically conductive metal yoke; the electrode assembly further comprising a plurality of said collar-shaped connectors, and the sidewall of each collar-shaped connector receiving an end of one of the vertical stubs; and each of the attachment portions is conductively connected to all of the vertical stubs through the collar-shaped connectors.
According to yet another aspect, carbon electrode block has a top surface opposite the bottom surface, and is provided with a plurality of said inserts in the top surface thereof, wherein the electrode assembly further comprises a yoke assembly through which an electrically conductive connection is provided between the electrically conductive metal member and the carbon electrode block; wherein the yoke assembly comprises a plurality of curved metal struts, each having an upper end and an opposed lower end, the upper end being secured by an electrically conductive connection to the lower end of the electrically conductive metal member, the lower end being secured to the carbon electrode block with an electrically conductive connection by at least one of said inserts. The lower ends of the struts may extend outwardly away from one another and from the electrically conductive metal member, and the yoke assembly may include a pair of said struts, oppositely disposed relative to one another.
According to yet another aspect, the electrode is a pre-baked carbon cathode, wherein the first surface of the carbon electrode block is a top surface thereof and the carbon electrode block has a bottom surface opposite the top surface, and wherein the electrically conductive metal member comprises a current collector bar having an end received in a slot in the bottom surface, the current collector bar and the bottom surface being substantially parallel. A cast iron layer may be provided in the slot, between the current collector bar and the carbon electrode block. The second surface in which the insert extends may comprise an inner surface of the slot, the inner surface of the slot being selected from a top surface and a side surface of the slot. The insert may have a flat head which is received between the cast iron layer and the carbon electrode block, and a conductive metal liner and/or a conductive metal washer may be received between the flat head of the insert and the carbon electrode block.
According to yet another aspect, the current collector bar is provided with one or more collector bar anchors, each said anchor having a first end attached to the collector bar and a second end embedded in the cast iron layer. For example, the first end of each said anchor comprises a threaded shank which is received in a threaded bore in the collector bar.
According to yet another aspect, the second surface in which the insert is received comprises the bottom surface of the carbon electrode block, the current collector bar having a flat bottom surface which is substantially co-planar with the bottom surface of the carbon electrode block. An electrically conductive metal connector may be attached to the bottom surface of the carbon electrode block and to the flat bottom surface of the current collector bar, to provide an electrically conductive connection between the current collector bar and the carbon electrode block. The electrically conductive metal connector may be attached to the top surface of the carbon electrode block by said insert, and is in electrically conductive contact with said insert. The electrically conductive metal connector is in the form of one or more layers of flat metal strap. The strap may have an expandable portion to permit deformation of the electrically conductive metal connector in response to differential thermal expansion of the current collector bar and the carbon electrode block, along an axis defined by the current collector bar. Alternatively, the electrically conductive metal connector may comprise a flexible cable connector, which may be provided with lug ends.
According to yet another aspect, the electrode assembly may comprise a plurality of said inserts provided in the bottom surface of the carbon electrode block, with each of said inserts being at least partly received in the carbon electrode block, and the electrode assembly may further comprise a plurality of said electrically conductive metal connectors, each providing an electrically conductive connection between the current collector bar and at least one of the inserts. The inserts may be spaced apart along a length of the carbon electrode block, and/or the inserts may be of different dimension to control the resistance of the top surface of the carbon block relative to the external portion of the collector bar.
According to yet another aspect, a thickness of the insert and the width of the bore are sized relative to one another such that an interface contact pressure between the insert and the adjoining carbon electrode block is at least about 1 kPa. For example, the interface contact pressure may be less than about 10 MPa, and/or the interface contact pressure may be between about 1 MPa and about 10 MPa. A maximum interface contact pressure may be less than about one half of the breaking pressure required to break the surrounding carbon electrode block.
According to yet another aspect, the second end of the metal member, which is adapted for connection to the buss bar, includes a connecting surface which is adapted to mate with the buss bar, and wherein the connecting surface is electroplated or clad with corrosion resistant conductive material.
According to yet another aspect, the insert is received in a bore either pre-drilled or formed in the carbon electrode block.
According to yet another aspect, the electrode is pre-baked and the insert is inserted into the carbon electrode block either before or after the electrode is pre-baked.
According to yet another aspect, the current collector bar is supported within the slot of the carbon electrode block by at least one hanger assembly comprising a tongue portion slidably received in a slot portion. The slot portion may be secured to the carbon electrode block by one or more of said conductive metal inserts, and the tongue portion is secured to the current collector bar.
According to yet another aspect, there is provided a magnetic mounted cooling fin for removable attachment to a steel potshell of a reduction cell for the production of metal. The magnetic mounted cooling fin comprises: (a) a bottom plate having a bottom surface and a top surface, wherein the bottom surface is adapted to be received against the potshell; (b) one or more fins extending from the top surface of the bottom plate; (c) one or more magnets having a curie point of at least about five hundred degrees Celsius, said one or more magnets being secured to said bottom plate.
According to yet another aspect, the magnets of the magnetic mounted cooling fin may comprise rare earth magnets and/or non-ferrous magnets. For example, the magnets may comprise Samarium-Cobalt or Aluminum-Nickel-Cobalt (Alnico) alloyed magnets.
According to yet another aspect, the magnetic mounted cooling fin may further comprise a thermal break attached to the bottom surface of the magnet, situated between the magnet and the potshell, wherein the thermal break comprises a thin layer of a non flammable material with low thermal conductivity.
According to yet another aspect, there is provided a magnetic mounted blanket for removable attachment to a steel potshell of a reduction cell for the production of metal. The magnetic mounted blanket comprises: (a) one or more layers of a flexible, high temperature resistant material having a melting point of at least about 600 degrees Celsius; and (b) a plurality of magnets are attached to said material to hold the blanket to the potshell.
According to yet another aspect, the magnets of the magnetic mounted blanket have a curie point and holding power such that they lose adequate holding power at a predetermined temperature which corresponds to an unacceptably high temperature of said steel potshell. The magnets may be comprised of a ferrous, or nonferrous or rare earth alloy.
According to yet another aspect, the material of the magnetic mounted blanket may comprise flexible glass or ceramic fibre cloth.
In order that the claimed subject matter may be more fully understood, reference will be made to the accompanying drawings, in which:
Certain embodiments are now described below with reference to the Hall-Heroult process for aluminum synthesis by electrolytic reduction of alumina. However, it will be appreciated that the embodiments described herein may be modified for use in other electrolytic reduction or electrolysis processes for the production of metals or chemicals that may also use metal electrodes joined to carbon anodes or cathodes, including but not limited to the electrolytic reduction of Lithium, Sodium and Magnesium.
Although pre-baked anode technology has developed over time to larger anodes with larger or more stubs, and to larger reduction cells with higher applied amperage, the subject matter disclosed herein may be applied to all forms and configurations of pre-baked carbon anodes and pre-baked carbon cathodes. Furthermore, the subject matter disclosed herein for the thimble anchors or preform may be applied either before or after the anode is baked, the carbon material of the anode being referred to herein as “green” prior to baking.
Each electrode assembly 12, 14 comprises an electrically conductive carbon electrode block having a first surface and a second surface, wherein the first surface faces the interior 16 of the reduction cell 10 when in use. In this regard, each anode assembly 12 includes a carbon anode electrode block 18 (also referred to herein as the “anode block 18”) and each cathode assembly includes a carbon cathode electrode block 20 (also referred to herein as the “cathode block 20”), which are described in more detail below. The first surface of each anode block 18 is the lower surface 22, and the first surface of each cathode block 20 is the upper surface 30. As shown in
The interior 16 of reduction cell 10 contains a molten electrolyte bath 26 which contains dissolved alumina, and a molten metal pad 28 of conductive aluminum overlying the top surface 30 of the cathode block 20. As alumina is reduced to aluminum, it sinks into the molten pad 28 and becomes part of the electrical circuit. Alumina is added to the bath 26 from an overhead hopper 32, while the molten aluminum is withdrawn from the metal pad 28 by intermittent siphoning to a mobile crucible (not shown). The interior 16 of reduction cell 10 is enclosed by sidewall refractories 34 and bottom refractory 36. The cell 10 further comprises an outer metal shell 11 (referred to herein as the “potshell”) which encloses the refractories 34, 36.
Each electrode assembly 12, 14 further comprises an electrically conductive metal member having a first end and a second end, wherein the first end of the metal member is connected to the carbon electrode block 18 or 20 in an electrically conductive manner, and the second end of the metal member is adapted for connection to a buss bar in an electrically conductive manner.
In this regard, the electrically conductive metal member 37 of each anode assembly 12 comprises a vertical conductor rod 38 which is typically made of aluminum or copper, the first (lower) end 40 of conductor rod 38 being connected to the top surface 42 of the anode block 18, and the second (upper) end 44 of which is connected to an anode buss bar 46, for example by clamping or the like. It will be appreciated that the electrically conductive metal member 37 carries electric current from the anode buss bar 46 to the anode block 18, and also suspends the anode block 18 in the bath 26.
The electrically conductive metal member 37 of each anode assembly 12 further comprises one or more vertical stubs 48 at its first end 40. In the reduction cell shown in
The electrically conductive metal member 37 of each anode assembly 12 further comprises an electrically conductive metal yoke 50 through which the stubs 48 are connected to the lower end 40 of the vertical conductor rod 38. The yoke 50 comprises a horizontal member such as a thick conductive plate of steel or other conductive metal. Alternatively, the yoke 50 may be integrally formed with the stubs 48.
The electrically conductive metal member of each cathode assembly 14 comprises a current collector bar 52. A section of the collector bar 52 is located within the cell 10 and is connected to the cathode block 20. One or both ends of the collector bar 52 are located external of the cell 10 and connected to a cathode buss 24 located outside the cell 10. Therefore, as shown in
As shown most clearly in the isolated views of
The cathode block 20 and the current collector bar 52 may be connected by a layer of cast iron 57 or carbon paste provided between the interior surfaces of the slot 56 and the current collector bar 52. In the illustrated embodiment, the layer 57 of cast iron or carbon paste is provided over the top surface 55 and the two side surfaces 59 of collector bar 52 which are received within slot 56, with the bottom surface being free of layer 57. In another embodiment, the layer 57 of cast iron or carbon paste may also be provided over the bottom surface 53 of current collector bar 52, so as to join both sides of layer 57 across the bottom surface 53 of the collector bar 52.
During operation, the cathode blocks 20 undergo sodium absorption from the bath 26, aluminum infiltration from the metal pad 28, and thermal stresses between the top and bottom surfaces 30, 54 due to uneven temperature profile throughout the carbon, which conditions gradually cause an upward bowing of the cathode block 20 which increases the electrical resistance of the cathode assembly 14. When the resistance becomes too high, or the cathode block 20 corrodes to the extent of allowing aluminum contact with the collector bars 52, the reduction cell 10 is removed from service to be relined with new material.
The stubs 48 and recesses 58 have diameters such that an annular gap is provided between the vertical side surface of each stub 48 and the cylindrical side surface 62 of each recess 58.
Upon rapidly freezing the thimble 64 in situ from molten iron, the iron undergoes solidification shrinkage. As shown in
Similarly, with reference to
The electrical current distribution through the anodes and cathodes follow the paths of least resistance, passing from the power connection points at the anode buss 46 to the external ends of the collector bars 52. This electrical current behavior results in uneven electrical resistance and current density across the first surfaces of the blocks 22, 30 to the bars 38 and 52 respectively, which in turn may cause uneven anode consumption and uneven cathode wear and corrosion. The uneven electrical resistance across the top surface 30 of the cathode block 20 may also contribute to horizontal electrical flow through the aluminum metal pad 28 due to the aluminum's very low electrical resistance, which horizontal electrical flow generates electromagnetic flow currents and turbulence (waves) in the metal pad 28. This turbulence may force a larger than normal anode to cathode distance (ACD) to be maintained to avoid short circuits between the bottom surface of anode block 18 and the top surface 30 of cathode block 20, which additional ACD incurs extra electrical resistance through the bath 26, consuming more electrical energy than otherwise required for the reduction process.
The present subject matter addresses the above problems by providing solid conductive metal inserts 76 received in the carbon electrode blocks 18 and/or 20 of the electrode assemblies 12, 14, with an interference fit, and with or without accompanying bores 68 (also referred to herein as “boreholes”) in the carbon electrode blocks 18 and/or 20. One end of each insert 76 is embedded into the carbon of the electrode block 18 or 20, with a controllable lateral interface contact pressure between the insert 76 and the carbon, which pressure provides a low resistance electrical path into the carbon, thereby reducing power consumption. The application of the interference fit inserts 76 varies between the anode 12 and cathode 14, but follows the same general principle.
As defined herein, an interference fit is one which produces a high ‘lateral’ interface pressure between the insert 76 and the carbon material of the electrode block 18 or 20, i.e. the pressure being directed radially outwardly against the carbon material. The high lateral interface pressure reduces the electrical resistance across the ‘lateral’ interface between the insert 76 and the carbon material. Alternatively, or in addition to the high lateral interface pressure, the interference fit may provide an axial (i.e. along an axis defined by insert 76) compression or interference fit with high contact pressure between the outer surface of the insert 76 and the carbon material. To produce an interference fit the inserts 76 must be inserted into the solid carbon material, as opposed to being formed by solidifying a molten metal poured or injected into a recess or cavity, which would be subject to shrinkage upon solidification and which, in any event, would not achieve significant interface pressure to provide an interference fit.
The controlled, high contact pressure between the insert 76 and the carbon electrode block 18 or 20 may cause minor carbon material failure local to the interface (by crushing or scaling), which limited carbon failure is acceptable provided that such carbon failure does not extend to cracks through the carbon electrode block 18 or 20 which may introduce high electrical resistance areas across the crack, or which cracks may propagate during operation causing the anode or cathode electrode block 18 or 20 to physically break apart. Use of inserts 76 providing an interference fit therefore requires consideration of the mechanical stresses induced by thermal expansion of the insert within the carbon anode or cathode electrode block 18 or 20 when placed in operation, and the strength of the anode or cathode electrode block 18 or 20 relative to the location of the insert 76 determined by the lateral thickness of carbon surrounding the insert 76. The interface pressure may change during the operational life of the anode 12 or cathode 14 due to material creep, depending on the temperature and thermal behaviour of the adjoining materials. The breaking strength of the anode 12 or cathode 14 may also change due to its temperature and condition over its operational life. The interface pressure and resulting electrical resistance of the connection can be controlled by adjusting the external dimensions of the insert 76 and/or the dimensions of bores 68 (e.g. drilled pilot holes, etc.) in the carbon material, in order to control the amount of interference fit, contact area and resulting contact pressure.
The interference fit between the insert 76 and the carbon material of the electrode block 18 or 20 is such that an interface contact pressure between the insert 76 and the carbon material of the adjoining electrode block 18 or 20 is at least about 0.1 kPa. For example, the interface contact pressure is typically up to about 10 MPa, and/or between about 1 MPa and about 10 MPa. Interface contact pressures in excess of 10 MPa, although possible, do not significantly further reduce the electrical resistance across the interface and increase the risk of cracking of the carbon substrate.
To avoid cracking the carbon electrode block 18 or 20, the pressure applied by the interference fit may be less than about one half of the stress required to break the carbon electrode block 18 or 20. The breaking pressure depends on the strength of the carbon material of the carbon electrode block 18 or 20 and the minimum width and thickness of carbon material surrounding the insert 76. It will be appreciated that the breaking pressure may vary in different regions of the electrode block 18 or 20, being higher in the middle of the block than near its edges.
The total applied pressure caused between the insert and the surrounding carbon material of the electrode block 18 or 20 depends on the contact area between them. Limiting the surface area of the insert 76 within the carbon material, with or without a bore 68, can enable a high interface pressure with a limited total applied pressure. Selecting a maximum applied pressure of less than approximately one half of the minimum yield strength of the surrounding carbon should provide an adequate margin of error, such that the anode or cathode electrode block 18 or 20 does not break from extra pressure that may result during anode and cathode operation, possibly due to greater thermal expansion of the inserts 76 versus the carbon of the carbon electrode block 18 or 20, and the potential weakening of the carbon electrode block 18 or 20 from bath absorption and air burn during its operational cycle. In the alternative, the interface pressure for each insert 76, with or without a bore 68, may be theoretically determined using Lames equation for interference fit. The selection of dimensions, locations and quantity of inserts 76 are to take into consideration an acceptable current density through the inserts 76 and desired current distribution through the anode or cathode electrode block 18 or 20. The maximum interface pressure may also be tested experimentally for each insert 76 location prior to implementation in the reduction cell 10.
The inserts 76 will typically be inserted into the electrode block 18 or 20 after it has been formed and baked to a hardened state, producing an interference fit as described above. However, it will be appreciated that the inserts 76 may be inserted into the electrode block 18 or 20 after it has been formed, and while it is in a green state, i.e. prior to baking. Where an insert 76 is installed into the carbon of the electrode block 18 or 20 prior to baking, the carbon is relatively soft and the interface contact pressure between the insert 76 and the surrounding carbon will initially be low. The electrode block 18 or 20 will then be hardened by baking, with the insert 76 installed therein, and a good electrical bond between the insert 76 and the surrounding carbon of the electrode block 18 or 20 will result.
The current density and resistance through the inserts 76 and between the inserts 76 and the surrounding carbon material of electrode block 18 or 20 will generate ohmic heating that will heat the inserts 76, which temperature should be maintained below the point at which the yield strength of the heated insert 76 falls below two times the desired interface contact pressure between the insert 76 and the surrounding carbon material, taking into account any additional load that any insert 76 may carry, for example due to the mass of the suspended carbon block 18 in the case of anode 12.
When the inserts 76 are used within the anode assembly 12, specifically between the anode block 18 and the cast iron thimble 64, the insert 76 is sometimes referred to herein as a ‘thimble anchor’. The thimble anchor 76 enhances the mechanical and electrical connections between the thimble 64 and the anode block 18. A portion of the thimble anchor 76 is embedded in the anode block 18 with an interference fit, as defined above, between the thimble anchor 76 and the surrounding carbon material of the anode block 18. The total interface contact area between the insert and the anode block 18 depends on the diameter, embedded length and quantity of the inserts 76 used. When using large diameter inserts 76, then a predrilled borehole 68 may be provided for the insert 76 to be embedded into, in order to limit the maximum lateral interface contact pressure to avoid fracture of the carbon of the anode block 18. The inserts 76 may be impact driven if nail style, or screwed into the carbon if screw or bolt style, either with or without a borehole 68 to provide the desired interface contact pressure. The inserts 76 may also be in the form of expansion anchors that are loose fit inserted into predrilled boreholes 68 and then tightened to impose the desired lateral interface contact pressure between the insert 76 and the inner surface 74 of the borehole 68
When used as thimble anchors, the inserts 76 perform two functions, firstly that of providing an electrical conductor between the cast iron thimble 64, which is cast in-situ from molten iron around the non-embedded end of the thimble anchor 76, and the anode block 18. Secondly, because one end of the thimble anchor 76 protrudes from a surface of the anode block 18 into the stub hole 58, the thimble anchor 76 modifies the freeze profile of the cast iron thimble 64 by providing a heat sink on the inner wall of stub hole 58 which promotes the solidification or freezing of the iron across the full width of the space between the stub 48 outer surface and the inner wall of stub hole 58 at the locations of the inserts 76, while there is molten metal above the insert 76 to fill in for the solidification shrinkage. This modified freeze profile will reduce or eliminate the usual shrinkage gap between the thimble 64 and the inner wall of stub hole 58 at those locations, thereby enabling a tighter contact and lower electrical resistance between the stub 48, thimble 64 and wall of stub hole 58 when the assembly 12 heats up in the reduction cell 10.
In the embodiments shown in
Depending on the diameter and type of thimble anchor 76, it may be necessary to provide the inner surface of the stub hole 58 of
As shown, the thimble anchors 76 embedded partially in the bottom surface 60 of stub hole 58 may extend vertically downwardly from the bottom surface 60, although the thimble anchors 76 in bottom surface 60 may instead be sloped relative to the vertical. The partially embedded thimble anchors 76 in the side surface 62 may extend horizontally and radially outwardly from the side surface 62. Alternatively, the thimble anchors 76 in the side surface 62 may be sloped relative to the horizontal, extending downwardly and outwardly from the side surface 62, and one such sloped anchor 76 is shown in
In this embodiment, where the inserts 76 comprise thimble anchors, they will be formed of a material such as cast iron or carbon steel which can be recycled with the metal comprising the thimble 64.
The thimble anchors 76 provide the electrical connection between the thimble 64 and the anode block 18, while the cast iron thimble 64 will bond the anode block 18 to the stub 48. The thimble anchors 76 and stub 48 together create a good electrical connection through those components of the electrode assembly 12 even when it is cold. Further, the provision of at least one thimble anchor 76 in the bottom surface 60 of stub hole 58 with a cast iron bond will enable current distribution through the bottom surface 60 of the stub hole 58 through the stub 48. This typically does not occur when the stubs 48 sit directly on the bottom surface 60 of the stub hole 58 during iron pouring, which prevents a cast iron connection forming between the bottom of the stub 48 and the bottom surface 60 of stub hole 58.
Care must be taken if the thickness of the cast iron under the stub 48 is too great, thereby preventing the thimble stripping press from breaking and stripping the thimble 64 from the stub 48. To prevent this from happening, the bottom surface of the thimble 64 can be provided with a weaker breaking area. For example, as shown in
In another aspect, also shown in
The preform 82 described above will typically be fastened to the bottom surface 60 of stub hole 58 after baking of the anode block 18. However, according to an alternate embodiment, the preform 82 may be inserted into the stub hole 58 while the anode block 18 is in a relatively soft, green state, in which case the preform 82 may be partially embedded in the bottom surface 60 of stub hole 58. In such an embodiment, the preform 82 may be formed into the anode block 18 during the anode forming process, i.e. at the time that the stub hole 58 is formed. Alternatively, the preform 82 may be inserted into the stub hole 58 after it is formed. Where the preform 82 is inserted into the green anode block 18, the inserts 76 may optionally be integrally formed with the preform 82.
Although
Insert 76c shown in
Although the inserts 76 described and shown herein generally have cylindrical shanks, it will be appreciated that this is not necessary. Rather, the inserts may be of any convenient cross-section, including square-shaped, rectangular-shaped, star-shaped, fluted, etc.
In another embodiment now described with reference to
The embodiment shown in
The anode block 18 is provided with a plurality of conductive metal inserts 76 in its top surface 42, forming an interference fit as defined above with the carbon material of the anode block 18. As discussed above, the inserts 76 are provided with or without bores 68, depending on the diameters of the inserts 76 and the strength of the substrate material.
The inserts 76 are at least partly received in the top surface 42. It can be seen from the drawings that the inserts 76 are located in top surface 42 such that the second ends 92 of the bypass members 88 are connected to the anode block 18 by at least one of the inserts 76. In the illustrated embodiment, each of the second ends is connected to the anode block 18 by two of the inserts 76. High pressure contact between the second ends 92 and the inserts 76 may be provided by using inserts 76 provided with compression washers 77 to maintain interface pressure, or using other conductive connections, including but not limited to brazing, welding or use of a nut 79 welded to the second end 92, or a locking thread 81 in the second end 92, ensuring an electrical connection with the insert 76. These latter two options are illustrated in
It will be appreciated that the attachment of the bypass members 88 to inserts 76 also results in the second ends 92 of the bypass members 88 being secured to the top surface 42 of the anode block 18 with an electrically conductive connection.
The first ends 90 of the bypass members 88 are fastened to the vertical conductor rod 38 by means of fusion welding, soldering, brazing, interference fit fastener, screw, bolt, rivet, clamp or other mechanical or fusion connection which forms an electrical conduction path from the vertical conductor rod component 38 to the bypass member 88. In the embodiment of
Although
According to another embodiment, the inserts 76 are used in combination with conductive connectors similar to the bypass members 88 described above, to replace and eliminate the conventional iron connection without, however, changing the main basis of the shapes of the electrodes, to allow a user to transition from traditional assemblies to the low resistance assemblies disclosed herein. Small changes to the shape of the carbon electrode block 18, 20 may be included in this embodiment to enable the use of the inserts 76, with or without bores 68.
The connector 94 also has at least one attachment portion 98 which is connected to the sidewall 96 in an electrically conductive manner, and may be integrally formed therewith. Each attachment portion 98 extends outwardly from the sidewall 96 of connector 94 and is secured and conductively connected to the top surface 42 of the anode block 18 by one or more conductive inserts 76, with or without a bore 68. High pressure contact between the connector 98 and the insert 76 may be provided by the same means described above with reference to FIGS. 6 and 7 and 14a to 14e, for example by using inserts 76 provided with compression washers 77, to maintain interface pressure, or using other conductive connections, including but not limited to brazing, welding or use of a backing plate, ensuring an electrical connection.
In the embodiment of
As discussed above, the inserts 76 are used to connect components such as the collar-shaped connector 94 to the top surface 42 of the anode block 18, and also to conduct electricity into the block 18 through the inserts 76 themselves. The use of inserts 76 allows for modification of electrical resistance and current distribution through the anode assembly 12 by adjusting the material of inserts 76, as well as the length, diameter, contact surface area, quantity, location and interference fit or contact pressure between the insert 76 and the anode block 18. These adjustments may enable a more consistent electrical resistance from the vertical conductor rod 38 to any point on the bottom surface 22 of the block 18, which promotes consistent current density, lower overall electrical resistance, more consistent anode consumption resulting in a flatter bottom surface 22 of the block 18 during its operational life. The flatter anode bottom surface 22 may enable a higher portion of the block 18 to be consumed before it is necessary to remove the anode 12 from operation, thus reducing recycle volumes and reducing the ongoing replacement costs of anode replacement.
The use of inserts 76, with or without bores 68, may also be applied in a cathode assembly 14, between the cathode block 20 and the current collector bar 52. The collector bar 52 is typically bonded to the cathode block 20 by a layer of cast iron 57 between the collector bar 52 and the corresponding recess 56 (also referred to in this embodiment as “slot 56”), in the bottom surface 54 of carbon cathode collector block 20. The current collector bar 52 is bonded to the carbon cathode collector block 20 with molten cast iron 27 while it is upside down (See
As will be appreciated, there may be multiple current collector bars 52 connected to each cathode block 20. Each block 20 is installed across the width of the reduction cell 10 and multiple blocks 20 are installed beside each other to line the bottom floor of the reduction cell 10. As shown in
In accordance with an embodiments shown in
Although the strap connectors 112 are shown in
Alternately, as shown in
It will be appreciated that the use of strap connector 112 allows the layer 57 of cast iron or carbon paste to be bypassed or eliminated, as the strap connector provides a direct, electrically conductive connection between the current collector bar 52 and the bottom surface 54 of the cathode block 20. As shown in
As with the anode assemblies discussed above, the use of inserts 76 allows modification of the electrical resistance and current distribution of the cathode assembly 14 from the end of the collector bar 52, where it exits the reduction cell 10, to the top surface 30 of the cathode block 20, by adjusting the insert material, length, diameter, quantity, position and interference fit or contact pressure relative to the carbon material of the cathode block 20. For example, the current distribution and resistance profile of the cathode assembly 14 may be made more even across the top surface 30 by using longer or more inserts 76 towards the centre of block 20, in relation to its ends as illustrated in
Due to differential thermal expansion of the current collector bar 52 it is also desirable to provide strap connectors 112 with means for permitting differential thermal expansion. As shown in
It is appreciated that the strap connectors 112 may not be limited to singular pieces of conductor but may be comprised of multiple layers of thin straps which flex more easily than solid pieces while providing a similar electrical resistance. For example, the strap connectors 112 in
According to another embodiment, illustrated in
The yoke assembly 126 comprises a pair of curved metal struts 128, which may be identical to one another. Each of the struts has an upper end 130 and an opposed lower end 132. The upper end 130 of each strut 128 is bonded through mechanically and electrically conducting attachment to opposing side surfaces of the lower end of the vertical conductor rod 38, which is shown as having a rectangular cross section with four vertical side surfaces. The mating surface of the struts 128 and the rod 38 may be electroplated, or bonded with suitable surface material to enable subsequent bonding of the strut 128 to the rod 38 by welding, brazing or other electrical connection. Following attachment of the struts 128 to the rod 38, one or more mechanical through fasteners 136, such as but not limited to bolts with washers and nuts, are inserted through the struts 128 and rod 38, and are adequately tightened to remove cyclical physical stress on the electrical joint due to loading and unloading the weight of suspended anode block 18 from the rod 38.
The two struts 128 are arranged in opposed, face-to-face relation with each other. In this embodiment they are joined together by one curved connector or brace 134, which contributes to the mechanical strength of the struts 128 while enabling some flexure of the struts 128 under thermal expansion stress. The struts 128 may be made of a single alloy of metal or clad or cored with different conductive metal.
The lower ends 132 of the struts 128 are curved outwardly away from the vertical conductor rod 38 and may be formed with multiple sections, separated by slits 138. The lower ends 132 mate with the top surface 42 of the anode block 18, and for this purpose the lower ends 132 are provided with one or more holes to enable the installation of one or more inserts 76 through the lower ends 132 and into the top surface 42 of the anode block 18, with or without bores 68. The inserts 76 therefore carry the weight of the anode block 18 from the struts 128 and provide an electrical connection from the vertical conductor rod 38 to the anode block 18. To provide added resistance against withdrawal of the inserts 76, at least some of the bores 68 may be angled from the vertical, toward the vertical conductor rod 38, so that the inserts 76 received in these bores 68 will be “toed in” toward one another. Other inserts 76 may be angled from the vertical in other directions to provide improved current distribution within the anode block 18. If, as shown in
It is appreciated that the lower ends 132 of the struts 128 may be connected to the vertical conductor rod 38 by other means than mechanical fasteners 136, provided that electrical conduction is maintained between the two parts during operation.
As compared to the traditional rod assembly, the configuration of the low resistance yoke assembly 126 eliminates one fusion weld on the bimetallic transition joint, increases the electrical contact surface area of the strut 128 to rod 38 connection through connection on both sides of the rod 38, it removes the physical stresses in the bimetallic connection from the weight of the suspended load by use of the through bolt connections, it eliminates the yoke to stub welded connection, it eliminates the condition of stub toe-in that the traditional yoke & stub assembly suffers due to repeated thermal stress and material creep at high temperatures, it enables the use of inserts 76 with toed in orientation to carry high loads through the cross section of the inserts 76. These benefits provide a low electrical resistance configuration with long life.
According to convention, the upper end 44 of the aluminum or copper vertical conductor rod 38 is temporarily attached to anode buss bar 46 with a buss clamp (not shown). The electrical resistance through the mating surface(s) of the rod 38 and buss 46 is dependent on the cleanliness, surface area and clamp pressure between the mating surfaces. Over repeated use the surfaces of the rod 38 and buss 46 may become oxidized or pitted from arcing which introduces surface roughness and a surface oxide layer of relatively high electrical resistance. The electrical resistance of the rod 38 to buss 46 connection may be reduced by coating (for example by cladding or electroplating) the mating surface 140 of the buss 46 (
One or both ends of each steel current collector bar 52 are connected by a bolted connection to the cathode buss 24 flex connectors (not shown). Also, a portion of each collector bar 52 is in electrical contact with the cathode block 20 through the cast iron layer 57, as explained above with reference to
The current collector bars 52 are typically comprised of an electrically conductive metal such as steel, which has a melting point which is substantially higher than the maximum operating temperature of the reduction cell. However, typical steel collector bars have higher electrical resistance than the electrical resistance of the aluminum metal pad, and therefore the current entering from the bath into the metal pad will preferentially conduct itself horizontally through the metal pad toward the sidewall of the cell before conducting downward through the cathode assembly 14 to the external busbar connection.
To reduce electrical resistance within the cathode assembly and horizontal electrical currents in the metal pad, an embodiment of a cathode assembly 14 shown in
The collector bar 52 of
The collector bars 52 remove heat from the cell to the environment through thermal conduction through the collector bar 52 and by convective, radiation and conductive cooling of the exposed portion of the collector bar 52 situated outside of the cell. This heat loss must be taken into account when balancing the heat loss of the cell. In an embodiment illustrated in
The following embodiments described with reference to
The conductive inserts 76 embedded in the surfaces of slot 56 do not significantly protrude into the cast iron layer 57. Rather, the heads of inserts 76 in
An alternate arrangement for permitting axial expansion movement of the cast iron layer 57 relative to the cathode block 20 is now described with reference to
As shown in
Instead of using a liner, the current distribution may be enhanced by providing more inserts 76 or, as shown in
In the case of the anode 12, the inserts 76 embedded in the anode block 18 have heads which protrude into the stub hole 58 and are embedded in the thimble 64, to modify the shape of the freezing iron to reduce shrinkage gap between the cast iron and the stub hole wall. This reduces the electrical resistance between those components. As shown in
The quantity, depth, dimensions and locations of the collector bar anchors 152 may be modified to adjust the electrical resistance between the top surface 30 of the cathode block 20 and the collector bar 52 in order to make the electrical resistance as consistent as possible. In order to enhance the heat sink effect, the collector bar anchors 152 may be provided with enlarged heads, as shown in
The left hand side of current collector bar 52 is provided with fixed collector bar anchors 156 which are secured to the outer surface of current collector bar by fusion bonding, such as by welding, brazing or soldering. The fixed anchors 156 may be in the form of cylindrical studs or rods, and the protruding ends may be tapered as shown. The fixed anchors 156 will act as heat sinks, and have a similar effect on the freeze profile of the cast iron layer 57, as indicated by the freeze isotherms 29 on the left hand side of
As shown in
In addition, the cast iron layer 57 along each side surface 59 of collector bar 52 varies in height along the length of the collector bar 52, being higher in the middle portion of collector bar 52 and slot 56, and lower proximate to the ends of collector bar 52 and slot 56. Accordingly, each cast iron layer 57 has a top edge 180 which varies in distance from the top surface 55 of collector bar 52 along the length of slot 56, where any spaces caused by the profiling of the top edge 180 may be filled with insulating refractory material 178. In the illustrated embodiment, the top edge 180 of cast iron layer 57 is shaped in an approximate arc profile, however, the shape can be varied from that which is shown, such as straight or polygonal profiles. Also, in this embodiment, the top edge 180 of cast iron layer 57 is substantially level with the top surface 55 of collector bar 52 at the midpoint of cathode block 20.
With the profiled top edge 180 of cast iron layers 57, the resistance of the cathode carbon between the top surface 30 of the cathode block 20 and the contacting surface of the iron layers 57 presents a changing electrical resistance along the length of the cathode block 20 that is offset by the changing electrical resistance along the length of the collector bar 52, thereby presenting a near uniform electrical resistance from anywhere along the length of the top surface 30 of the cathode block 20 to the external end of the collector bar 52. The current distribution across the width of the top surface 30 of the cathode block 20 may be made more uniform by the altering the dimensions of the slot 56 and the iron connection surface between the cathode block 20 and collector bar 52. The shape profiles of the top edges 180 of the iron layers 57, and the distance from the top edge 180 of the iron layers 57 to the top surface 55 of the collector bar 52, may also be varied in adjacent cathode assemblies 14 so as to balance the current distribution across the width of the reduction cell 10. Cast iron may be substituted equally with carbon paste used to connect the cathode block and the collector bar.
The insulating refractory material 178 between the cathode block 20 and the collector bar 52 serves to reduce the rate of heat transfer from the cathode block 20 to the collector bar 52. This material 178 may be castable, using a temporary form, or may be preformed and positioned into the cathode slot 56 prior to positioning of the collector bar 52 and casting of the iron layer 57.
As shown in
In the embodiment of
With the conductive hanger assembly 160, relative thermal expansion of the collector bar 52 and the cathode block 20 results in axial longitudinal movement of the tongue portion 162 within slot portion 164, thereby providing support for the collector bar 52 while avoiding the creation of thermal stresses.
If eliminating the layer 57 of cast iron or paste the gap remaining between the collector bar 52 and the inner surface of the cathode slot 56 may be filled with thermal insulation to reduce heat transfer between the cathode block 20 and the collector bar 52.
The thermal balance of the cell must be maintained throughout the life of the cell, so as to maintain the bath at about 25-50 degrees Celsius above its freezing temperature, while enabling the bath to freeze against the sidewall for corrosion protection of the sidewall refractory, and to prevent excessive freezing of the bath on the anode and cathode surfaces. However, the thermal conductivity of the cell changes over time due to corrosion wear of the sidewall and the cathode blocks 20, and the energy (watts) input to the cell may change with the cell's electrical resistance. According to the embodiments shown in
In accordance with the embodiment of
Each cooling fin 186 may comprise an extruded aluminum shape with a bottom plate 188 that includes a bottom surface 190 and a top surface 192, where the bottom surface 190 contacts the potshell 11. Extending from the top surface 192 are one or more fins 194. The cooling fins 186 may be comprised of aluminum, and may include a surface treatment of anodization, such as a coloured anodization, which increases the emissivity of the cooling fin material in order to increase its ability to radiate heat to the environment compared to a non-anodized aluminum cooling fin.
The cooling fins 186 may be magnetically held against the potshell 11 by one or more rare earth and/or non-ferrous magnets 196 having a curie point of at least about five hundred degrees Celsius. For example, the magnets 196 may comprise Samarium-Cobalt or Aluminum-Nickel-Cobalt (Alnico) alloyed magnets. Such magnets 196 maintain their magnetic force at high temperatures, and hold the cooling fin 186 against the side of the steel potshell 11 when it has heated beyond normal design parameters due to the wear of the internal refractory lining.
The magnets 196 are retained in the bottom plate 188. For example, the bottom surface 190 of the bottom plate 188 may be provided with one or more cavities 198 for retaining magnets 196, and the magnets 196 may be retained therein by retaining screws 200 or other mechanical means, and/or by bonding. The bottom plate 188 may have a groove 202 along an edge of the bottom surface 190 of plate 188 to enable prying the cooling fin 186 off of the potshell 11 for removal.
The cooling fins 186 may further comprise a thin thermal break 204 between the magnets 196 and the steel potshell 11, through which the heat transfer into the magnets 196 will be reduced but the magnetic force will be maintained. The thermal break 204 may be attached to the external surfaces 197 of magnets 196, and may comprise a non flammable material with low thermal conductivity. The thermal break 204 improves the performance of the magnets 196 by keeping them at a cooler temperature than without the thermal break 204. For clarity, the thermal break 204 is shown as being removed from the external surface of the lower magnet 196 in
In accordance with the embodiment of
According to the present embodiment, the temporary thermal insulation is applied to the exterior of the potshell 11 in the form of one or more magnetically mounted blankets 206, one of which is shown in
A plurality of magnets 208 are attached to or captured within the construct of the blanket 206, in adequate location and quantity to hold the blanket 206 against the potshell 11 and thereby reduce the convective and radiation cooling of the potshell 11 to the environment. The magnets 208 may be ferrous, nonferrous or may be comprised of a rare earth alloy. The magnets 208 may be selected by strength and curie point to lose holding power in the event that the temperature of the potshell 11, or a portion thereof, reaches an unacceptably high level. Also, the magnets 208 are of low enough magnetic force to enable manual removal of the blankets 206 from the potshell 11.
Although specific embodiments have been described herein, the claims are not limited to these embodiments. Rather, the disclosure includes all embodiments which may fall within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2838113 | Dec 2013 | CA | national |
This application claims priority to and the benefit of Canadian Patent Application No. 2 838 113 filed Dec. 16, 2013 and U.S. Provisional Patent Application No. 62/081,187 filed Nov. 18, 2014, the contents of each are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2014/051178 | 12/8/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62081187 | Nov 2014 | US |
