Abstract
The invention relates to an electrolytic cell (1) for the production of aluminium (2) including collector bars structure modifications (13,14,15,16) under the cathode (4), namely a copper collector bar held in a U-shaped profile or directly embedded into the cathode. This leads to an optimized current distribution in the liquid aluminium metal (2) and/or inside the carbon cathode allowing for operating the cell at lower voltage. The lower voltage results from either a lower anode to cathode distance (ACD), and/or to lower voltage drop inside the carbon cathode from liquid metal to the end of the collector bar.
Claims
1. A cathode current collector assembly assembled in a carbon cathode of a Hall-Heroult cell for the production of aluminium, the cathode current collector assembly comprising at least one collector bar of a highly electrically conductive metal that is located under the carbon cathode, the highly-electrically conductive metal having an electrical conductivity greater than that of steel, characterized in that the or each highly electrically conductive metal collector bar comprises a central part located under a central part of the carbon cathode, said central part of the highly electrically conductive metal collector bar having at least an upper outer surface in direct electrical contact with the carbon cathode or in contact with the carbon cathode through an electrically conductive interface formed by an electrically conductive glue and/or an electrically conductive flexible foil or sheet applied over the surface of the highly electrically conductive metal collector bar; the or each highly electrically conductive metal collector bar comprises one or two outer parts located adjacent to and on one side or on both sides of said central part and a terminal end part or parts extending outwardly from said outer part(s), and said terminal end part(s) of the or each highly electrically conductive metal collector bar is/are electrically connected in series each to a steel conductor bar of greater cross-sectional area than the highly electrically conductive metal collector bar, said steel conductor bar(s) extending outwardly for connection to an external current supply.
2. The cathode current collector assembly according to claim 1, wherein the highly electrically conductive metal is selected from copper, aluminium, silver and alloys thereof, preferably copper or a copper alloy.
3. The cathode current collector assembly according to claim 1 wherein the surface of the highly electrically conductive metal that interfaces with the carbon cathode is roughened or provided with recesses such as grooves or projections such as fins to enhance contact area with the carbon cathode.
4. The cathode current collector assembly according to claim 1 comprising a conductive interface between the highly electrically conductive metal and the carbon cathode, said conductive interface being selected from a metal cloth, mesh or foam of copper, a copper alloy, nickel or a nickel alloy, or a graphite foil or fabric, or a conductive layer of glue, or a combination thereof.
5. The cathode current collector assembly according to claim 4, wherein the conductive interface comprises a carbon-based electrically conductive glue obtainable by mixing a solid carbon-containing component with a liquid component of a 2-component hardenable glue.
6. The cathode current collector assembly according to claim 1 wherein the sides and optionally the bottom of the highly electrically conductive metal collector bar directly or indirectly contact ramming paste or refractory bricks in contact with the carbon cathode.
7. The cathode current collector assembly according to claim 1, wherein the highly electrically conductive metal collector bar comprises at least one slot arranged to compensate for thermal expansion of the bar in the cathode by allowing inward expansion of the highly electrically conductive metal into the space provided by the slot(s) or wherein two or more of the highly electrically conductive metal collector bar are spaced apart from one another to allow compensation for thermal expansion.
8. The cathode current collector assembly according to claim 1, wherein the terminal end parts of the highly electrically conductive metal collector bar are electrically connected in series to the steel conductor bar that forms a transition joint, and wherein the highly electrically conductive metal collector bar and the steel conductor bar overlap one another partially and are secured together by welding, by electrically conductive glue and/or by means for applying a mechanical pressure such as a clamp or a joint secured by thermal expansion, or by a threaded connection.
9. The cathode current collector assembly according to claim 1, wherein the carbon cathode electrically contacts the open upper outer surface of the highly electrically conductive metal as a result of the weight of the carbon cathode on the highly electrically conductive metal, and thermal expansion of the highly electrically conductive metal.
10. The cathode current collector assembly according to claim 1, wherein said outer part(s) of the highly electrically conductive metal collector bar extend under or through an electrically conductive part of the cell bottom, said outer parts of the highly electrically conductive metal collector bar being electrically insulated from the electrically conductive part of the cell bottom.
11. The cathode current collector assembly according to claim 10, wherein said outer part(s) of the highly electrically conductive metal collector bar is/are insulated from the electrically conductive part of the cell bottom by being encased in an insulator, in particular by being encased in one or more sheets of insulating material such as alumina wrapped around said outer part(s) or encased in an layer of electrically insulating glue or cement.
12. The cathode current collector assembly according to claim 1 wherein said central part of the highly electrically conductive metal collector bar is held in a U-shaped profile made of a material that retains its strength at the temperatures in a Hall-Heroult cell cathode, the U-shaped profile having a bottom under said bar and on which the bar rests, optionally at least one upstanding fin, and side sections that extend on the sides and are spaced apart from or contact the sides of the highly conductive metal collector bar and said highly electrically conductive metal collector bar having at least upper part and optionally also side parts left free by the U-shaped profile to enable the highly electrically conductive metal to contact the carbon cathode directly or via a conductive interface.
13. The cathode current collector assembly according to claim 12, wherein the U-shaped profile is made of a metal such as steel, or of concrete or a ceramic.
14. the cathode current collector assembly according to claim 1 wherein the bar of the highly electrically conductive metal collector bar at least in the central part of the cathode is contained in a through-hole in the carbon cathode whereby the highly electrically conductive metal collector bar is supported on the underlying part of the carbon cathode and is surrounded by and preferably in direct electrical contact with the surface of the through hole in the carbon cathode.
15. (canceled)
16. The cathode current collector assembly according to claim 7 wherein the bar of the highly electrically conductive metal collector bar at least in the central part of the cathode is contained in a through-hole in the carbon cathode whereby the highly electrically conductive metal collector bar is supported on the underlying part of the carbon cathode and is surrounded by and preferably in direct electrical contact with the surface of the through hole in the carbon cathode.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] The invention will be further described by way of example with reference to the accompanying drawings, in which:
[0044] FIG. 1 is a schematic cross-section through a Hall-Héroult cell equipped with a collector bar according to the invention.
[0045] FIG. 2 is a cross section through a first embodiment of collector bar showing a U-shaped profile.
[0046] FIG. 3 is a cross section through a second embodiment of collector bar showing another U-shaped profile.
[0047] FIG. 4 is a graph of current density across a cathode equipped with a current collector according to the invention with a U-shaped profile, and a reference cathode.
[0048] FIG. 5A is a cross section through a cathode showing the highly conductive material of the collector bar glued to a carbon cathode.
[0049] FIG. 5B is a cross section through a cathode showing the highly conductive material of the collector bar in direct electrical contact with the carbon cathode.
[0050] FIG. 6 is a cross section through another embodiment of a cathode current collector assembly according to the invention.
[0051] FIG. 7 illustrates how the highly conductive material of the cathode current collector bar is connected to a steel bar (transition joint) for leading the current outside the cell.
[0052] FIG. 8 shows an alternative connection of the highly conductive metal of the cathode current collector bar to a steel bar leading current outside the cell.
[0053] FIG. 9 shows another alternative connection of the highly conductive material of the cathode current collector bar to a steel bar leading current outside the cell.
[0054] FIG. 10A shows the highly conductive material of the current collector bar machined to create a groove allowing for thermal expansion.
[0055] FIG. 10B shows the highly conductive material of the current collector bar machined to create a groove allowing for thermal expansion and in direct contact to a carbon cathode.
[0056] FIG. 10C shows the highly conductive material of the current collector bar, in direct contact to the carbon cathode, machined to create a slot allowing for thermal expansion and contained in a U shaped steel beam.
[0057] FIG. 11 shows the highly conductive material 15 shaped to increase the surface area between the cathode and the highly conductive material glued to the cathode block.
[0058] FIG. 12 shows the highly conductive material layer of the current collector bar, in direct contact to the carbon cathode with the upper side face and to a central folded fin of the U-shaped steel beam with the lower side face.
[0059] FIG. 13A shows the highly conductive material split into two separate conductive parts by a central vertical fin of a U-shaped steel beam, each conductive part being in direct contact to the carbon cathode from the upper sides and lateral faces.
[0060] FIG. 13B shows the highly conductive material split into two separate conductive parts by a central vertical fin of a U-shaped steel beam and electrically insulated from the carbon cathode.
[0061] FIG. 13C shows the highly conductive material split into two separate conductive parts by each of two separate vertical fins of a U-shaped steel beam and in direct contact with the carbon cathode.
[0062] FIG. 14 shows the highly conductive material on a support, and in direct contact with a carbon cathode from the upper and lateral sides.
[0063] FIG. 15 shows a slotted copper tube inserted in a hole in a graphite carbon block.
[0064] FIG. 16 shows a solid copper rod inserted in a hole in a graphite carbon block.
[0065] FIG. 17 shows two copper rods inserted in holes in a graphite carbon block, one rod having a gap for thermal expansion.
[0066] FIG. 18 is a perspective view of a copper bar bent into U-shape with two legs that are embedded in a graphite cathode block, the short section of the U-shaped copper bar being press fitted in a steel transition joint.
DETAILED DESCRIPTION
[0067] FIG. 1 schematically shows a Hall-Heroult aluminium-production cell 1 comprising a carbon cathode cell bottom 4, a pool 2 of liquid cathodic aluminium on the carbon cathode cell bottom 4, a fluoride—i.e. cryolite-based molten electrolyte 3, containing dissolved alumina on top of the aluminium pool 2, and a plurality of anodes 5 suspended in the electrolyte 3. Also shown is the cell cover 6, cathode current collector bars 7 according to the invention that lead into the carbon cell bottom 4 from outside the cell container 8 and anode suspension rods 9. As can be seen, the collector bar 7 is divided in zones. Zone 10 is insulated electrically and zone 11 is composed of layers as shown in FIG. 2, FIG. 3, FIG. 5 or FIG. 6. Molten electrolyte 3 is contained in a crust 12 of frozen electrolyte. Steel bars 18 connected in electrical series to the ends of the collector bars 7 protrude outside the cell 1 for connection to external current supplies.
[0068] Zone 10 of the collector bar is for example electrically insulated by being wrapped in a sheet of alumina or by being encased in electrically insulating glue or cement.
[0069] FIG. 2 shows a U-shaped profile 14 made of any type of temperature-resistant conductive or insulating material for example steel and the high electrically conductive material 15 such as copper inside the U-shaped profile 14, forming together the collector bar. As shown, the collector bar is optionally surrounded by a coke bed (i.e. of ramming paste) 13 to decrease the electrical resistance towards the carbon cathode. The free top surface 16 of the high conductive material can be made rough to minimize the electrical contact resistance. In one variation, the sides of the U-shaped profile do not extend to the top of the highly electrically conductive material and in another variation the sides of the U-shaped profile are wider than and spaced apart from the highly electrically conductive material.
[0070] FIG. 3 shows a U-shaped profile 14 made of any type of temperature-resistant conductive or insulating material for example steel and high conductive material 15 such as copper, forming together the collector bar in the case of using the “embedded” collector bar inside the carbon cathode 4. In this embodiment, contrary to FIG. 2 where the top of the copper/metal 15 is flush with the open top of the U-shaped profile 14, here the copper/metal 15 is separated from the two lateral sides of the U-shaped profile thereby increasing the direct electrical contact surface to the carbon cathode 4 on three sides. The lower side of the copper/metal 15 rests on the flat bottom of the U-shaped profile 14 as mechanical support.
[0071] FIG. 4 shows a typical impact of using the copper/metal bar on the current density at the surface of the cathode seen from the cathode center (point “0.0”) to the edge of the cathode (point “1.8”). These results will be discussed later.
[0072] FIG. 5A shows the cathode 4 enclosing the high electrically conductive material 15 and glue 16 around the highly conductive material, this glue being electrically conductive.
[0073] FIG. 5B shows the cathode 4 enclosing a bar 15 of high electrically conductive material of rectangular section in direct contact with the carbon cathode 4.
[0074] FIG. 6 shows the cathode 4, the high electrically conductive material 15 and glue 16 around the highly conductive material, and refractory bricks 17. The highly conductive material 15 is glued to the carbon cathode 4 but only on the lower part of the cathode, the sides and lower part of the cathode being replaced by refractory bricks 17 such as Schamotte or any type of electrically insulating or even electrically conductive material such as ramming paste.
[0075] FIG. 7 shows the cathode 4, the high electrically conductive material 15 and the glue 16 around the highly conductive material and on the contacting surfaces with a transition joint formed by a steel bar 18 leading current outside the cell. The end of the collector bar can be press fitted in a machined section in the steel bar 18, in a hole, or can be glued with the same glue. Another type of connection can be the use of a steel transition joint split in two longitudinal parts that are clamped over the collector bar by a bolted connection or weld.
[0076] FIG. 8 shows the cathode 4 from the bottom, with two edge-to-edge bars of high electrically conductive material 15 separated by an expansion gap 19 and bolted to a steel bar 18 leading the current outside the cell. By using this bolted connection use is made of the two highly conductive metal elements 15 that can be spaced apart also inside the cathode to provide a thermal expansion gap inside the cathode.
[0077] FIG. 9 shows an alternative connection where a steel bar 18 is made of two separate elements connected together by a bolted system 19. As shown, the end of the highly electrically conductive material 15 is also secured in the end of the split steel bars 18 by the same bolted system 19.
[0078] FIG. 10A shows the highly conductive material 15 of the current collector bar machined to create a central groove 17 extending over the main part of the height of the bar of highly conductive material, allowing for thermal expansion. In this example, the highly conductive material 15 is coated with electrically-conductive glue 16 which glues it to the cathode 4.
[0079] FIG. 10B shows the highly conductive material 15 of the current collector bar machined to create a central groove 17 extending over the main part of the height of the bar of highly conductive material, allowing for thermal expansion. In this example, the highly conductive material 15 is in direct contact to the carbon cathode 4. Instead of a machined groove, two or more bars of highly conductive material can be spaced from one another in spaced facing relationship.
[0080] FIG. 10C shows the highly conductive material 15 of the current collector bar machined to create a central groove 17 extending over the main part of the height of the bar of highly conductive material allowing for thermal expansion. In this example the highly conductive material 15 is in direct contact to the carbon cathode and is supported from underneath by a U-shaped steel beam 14 wider than the highly electrically conductive material.
[0081] FIG. 11 shows highly conductive material 15 whose upper surface is shaped by a series of ribs or other projections to increase the surface area between the cathode 4 and the highly conductive material 15 which is glued by a layer of electrically conductive glue 16 to the cathode block 4.
[0082] FIG. 12 shows the highly conductive material layer 15 of the current collector bar, in direct contact to the carbon cathode 4 by its upper side face and fitting over and contacting a central folded fin 14a of a U shaped steel beam 14 by its lower side face. There can be more than one vertical folded fin 14a as part of the U beam section 14.
[0083] FIG. 13A shows highly conductive material 15 split into two separate conductive parts by a central vertical fin 14a of a wide U-shaped steel beam 14, each conductive part being in direct contact to the carbon cathode 4 from its upper sides and lateral faces.
[0084] FIG. 13B shows the highly conductive material 15 split into two separate conductive parts by a central vertical fin 14a of a wide U-shaped steel beam 14, each conductive part being electrically insulated, over some segments of its length where insulation is required, namely in zone 10 (FIG. 1), from the carbon cathode 4 by a layer 20 of electrically insulating material deposited between the upper sides and the lateral faces of the conductive material and the carbon cathode 4.
[0085] FIG. 13C shows highly conductive material 15 split into two separate conductive parts by each of two separate vertical fins 14a of a U-shaped steel beam 14, each conductive part being in direct contact to the carbon cathode 4 from its upper sides and lateral faces. There can be more than two vertical fins 14a.
[0086] FIG. 14 shows a bar of highly conductive material 15 in direct contact with the carbon cathode 4 by its upper and lateral sides. The lower side of the highly conductive material 15 is supported by a “flat” steel beam 14b or by ramming paste or glue which is coextensive with and supports the highly conductive material 15. As described previously, the highly conductive material can be split by a groove or there can be more than one part of highly conductive material spaced apart from one another. The support beam 14b can be made of several layers, e.g. a steel layer over ramming paste.
[0087] FIG. 15 shows a slotted copper tube 15A inserted in a cylindrical hole in a graphite carbon block 4. The copper tube 15A is slotted along its length to provide a sufficient gap to accomodate for thermal expansion of the copper tube 15A as the cell reaches its operating temperature. The outer surface of the slotted tube 15A is preferably in direct electrical contact with the graphite of block 4.
[0088] FIG. 16 shows a solid copper rod 15B inserted in a hole in a graphite carbon block 4. In this case, expansion allowance can be achieved by precision fitting. In other words, the diameter of the cylindrical hole in the block 4 and the diameter of the rod 15B before insertion are so calculated that the rod fits comfortably in the hole and, as the temperature of the cell rises, the rod 15B expands to fit tightly in the hole.
[0089] FIG. 17 shows two copper rods inserted in holes in a graphite carbon block 4, one rod 15B being a plain cylindrical rod as in FIG. 16 and the other rod 15B′ having a diametral gap for thermal expansion.
[0090] FIGS. 15, 16 and 17 show copper bars of circular cross-section, but it is noteworthy to mention that the concept can be applied to any geometry of the hole and inserted bar/tube. The illustrated circular hole containing the copper conductor has the advantage of being sealed from underneath by the underlying carbon of the block. There is therefore no need for a supporting U-shaped beam for underneath support.
[0091] FIG. 18 is a perspective view of a particular embodiment for connecting the outer part of a highly conductive (copper) bar to a transition joint. As shown, a copper bar 15 is bent into U-shape with two legs that are embedded in grooves in the underside of a graphite cathode block 4 from which the two legs protrude. The short section 15C at the protruding end of the U-shaped copper bar 15 is press fitted in a transverse groove located towards the end of a steel transition joint 18. An end part of this transition joint 18 fits in between the two legs of the copper bar 15 and the transition joint 18 is deeper than the thickness of the legs of the copper bar 15. Overall, the cross-sectional area of the transition joint 18 is greater than the combined cross-sectional area of the two legs of the copper bar 15. A tight fit of the copper bar 15 with the transition joint 18 can be provided by thermal expansion of the copper in the transverse groove of the transition joint 18.
[0092] Further Description of the High Conductivity Collector Bars
[0093] The use of high conductivity collector bars can decrease the voltage drop from the liquid metal 2 and the end part of the collector bars. The copper or other high conductive material 15 with or without a U-shaped profile 14 or support beam 14b also helps to decrease the anode to cathode distance (ACD) allowing a decrease of the specific energy consumption, and an increase in the height of the cathode leading to increased cell lifetime.
[0094] The lengths L1, L2 and L3 (FIG. 1) are optimized in function of the busbar system and of the cell geometry in order to optimize the cell stability. Indeed, the redistribution of the current through the collector bars allows for a much better magneto-hydrodynamic cell state that will allow decreasing the ACD while increasing the current and hence minimizing the energy consumption. This is reflected by a homogeneous vertical current density in a horizontal section in the middle of the liquid metal pool.
[0095] A typical example of current density is shown in FIG. 4 for a standard cell and for a cell according to the invention in FIG. 3 or FIG. 5A. The vertical current density (Jz) depends on the location in the liquid metal, ie. Jz=Jz(x,y,z) in a (x,y,z) coordinate system. When moving from the edge of the external part of the shadow of one anode (x=−X.sub.L) to the edge of the shadow of the neighboring anode (x=X.sub.L) in an horizontal plane inside the liquid metal, the absolute value of the vertical component of the current density (|Jz(x)|) varies typically as shown in FIG. 4. When optimizing the collector bars by using a high conductivity metal 15, such as copper in direct electrical contact with the graphite cathode, contained in a U-shaped profile 14 or directly fitted into a cathode slot, |Jz(x)| is reduced by a minimum of 50% as shown in FIG. 4 (right hand part). The section of the collector bar is such that the heat extraction is minimum from the side of the carbon cathode to the end of the collector bar. In fact it is dimensioned in such a way as to obtain a temperature drop of around 200° C. outside, and a voltage drop as low as possible.
