WO2011148347A1 - Hall-heroult cell cathode design - Google Patents

Hall-heroult cell cathode design Download PDF

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Publication number
WO2011148347A1
WO2011148347A1 PCT/IB2011/052325 IB2011052325W WO2011148347A1 WO 2011148347 A1 WO2011148347 A1 WO 2011148347A1 IB 2011052325 W IB2011052325 W IB 2011052325W WO 2011148347 A1 WO2011148347 A1 WO 2011148347A1
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WIPO (PCT)
Prior art keywords
carbon cathode
cell bottom
cathode cell
aluminium
carbon
Prior art date
Application number
PCT/IB2011/052325
Other languages
French (fr)
Inventor
René VON KAENEL
Jacques Antille
Original Assignee
Kan-Nak S.A.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Kan-Nak S.A. filed Critical Kan-Nak S.A.
Publication of WO2011148347A1 publication Critical patent/WO2011148347A1/en

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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes

Definitions

  • the invention relates to the production of aluminium using the Hall-Heroult process; in particular to the optimization of the cathode for the decrease of energy consumption and maximization of the current efficiency.
  • Aluminium is produced by the Hall-Heroult process, by electrolysis of alumina dissolved in cryolite based electrolytes at temperature going up to 1000°C.
  • a typical cell is composed of a steel shell, an insulating lining of refractory materials and a carbon cathode holding the liquid metal.
  • the cathode is itself composed of a number of cathode blocks in which collector bars are embedded at their bottom to extract the current flowing through the cell.
  • WO2008/062318 proposes the use of a high conductive material in complement to the existing steel collector bar and gives reference to WO 02/42525, WO 01/63014, WO 01/27353, WO 2004/031452 and WO 2005/098093 that disclose solutions using copper inserts inside the collector steel bars.
  • US patent 4,795,540 splits the cathode in sections as well as the collector bars.
  • WO2001/27353 and WO2001/063014 use high conductive materials inside the collector bars.
  • US2006/0151333 covers the use of different electrical conductivities in the collector bars.
  • WO 2007/1 18510 proposes to increase the section of the collector bar when moving towards the center of the cell for changing the current distribution at the surface of the cathode.
  • US 6,231 ,745 presents the use of a copper insert inside the steel collector bar.
  • EP 2 139 446 describes cathode block arrangements to modify the surface geometry of the cathode in order to stabilize the waves at the surface of the metal pad and hence to minimize the ACD (anode to cathode distance).
  • the resistivity of molten cryolite is very high, typically 220 ⁇ "1 ⁇ 1 and the ACD cannot be decreased too much due to the formation of magneto-hydrodynamic instabilities leading to waves at the metal-bath (metal - cryolite electrolyte) interface.
  • the existence of waves leads to a loss of the current efficiency of the process and does not allow decreasing the energy consumption under a critical value.
  • the current density is such that the voltage drop in the ACD is minimum 0.3 V/cm.
  • the ACD is 3 to 5 cm, the voltage drop in the ACD is typically 1.0 V to 1.5 V.
  • the magnetic field inside the liquid metal is the result of the currents flowing in the external busbars and the internal currents.
  • the internal local current density inside the liquid metal is mostly defined by the cathode geometry and its local electrical conductivity.
  • the magnetic field and current density produce the Lorentz force field which itself generates the metal surface contour, the metal velocity field and defines the basic environment for the magneto-hydrodynamic cell stability.
  • the cell stability can be expressed as the ability of lowering the ACD without generating unstable waves at the surface of the metal pad.
  • the level of stability depends on the current density and induction magnetic fields but also of the shape of the liquid metal pool.
  • the shape of the pool depends on the surface of the cathode and the ledge shape.
  • the Prior Art solutions respond only marginally to the needed magneto-hydrodynamic status and liquid metal pool shape to satisfy good cell stability (low ACD).
  • the invention provides a carbon cathode bottom of a Hall-Heroult cell for the production of aluminium, on which in use there is a pool of molten cathodic aluminium above which there is a molten fluoride-based electrolyte in which during operation anodes are suspended, the top of the carbon cathode cell bottom having on opposite sides generally flat ledges.
  • the top of the carbon cathode cell bottom has a recessed central area for receiving in use a deeper central part of the cathodic aluminium pool.
  • the recessed central area of the carbon cathode cell bottom has a volume A for receiving cathodic aluminium which leads to an improved current distribution in the liquid metal and a damping factor for the waves resulting in a better magneto-hydrodynamic cell stability allowing for a lower ACD and lower cathode voltage drop.
  • this volume A is equal to or greater than 20% of the volume of the top part of the carbon cathode cell bottom calculated as an envelope that encloses the recessed central area and that lies between the level of said ledges and the deepest part of the recessed central area.
  • the volume A of the recessed central area of the carbon cell bottom is about 25% to 40% and usually at most 50% of the volume of the top part of the carbon cathode cell bottom calculated as before.
  • the maximum depth d of the aluminium pool in the recessed central area 5 is in a range of typically 5 cm to 20 cm, and is usually larger than the fluctuating depth D of the aluminium pool over the ledge 31 , which varies between 3 cm to 10 cm after tapping.
  • the depth D is kept as small as possible for minimizing the total amount of liquid metal in the cell.
  • the recessed central area of the carbon cathode cell bottom has inclined surfaces that are inclined slightly to the horizontal and that lead down to a deepest central part of the recessed central area, and these inclined surfaces of the recessed central area are extended laterally by steeper inclined surfaces that join to the top surface of said ledges.
  • the carbon cathode cell bottom usually comprises a plurality of current-carrying bars that lead into the carbon cathode cell bottom from outside the cell, these current-carrying bars extending across the carbon cathode cell bottom between said ledges.
  • the inventive carbon cathode cell bottom advantageously comprises a plurality of inserts in the carbon cathode cell bottom, each insert comprising at least one highly conductive metal or at least one material of low electrical conductivity, contained in an enclosure within the carbon cathode cell bottom.
  • At least one of these inserts can comprise a wall or shell of metallic material that remains solid at cell operating temperature, such as steel, containing a highly conductive metallic material that is liquid at cell operating temperature, such as ferro-aluminium alloys.
  • the inventive carbon cathode cell bottom comprises inserts that are located above the level of current-carrying bars that lead into the carbon cathode cell bottom from outside the cell, such inserts typically being parallel to or perpendicular to the current-carrying bars.
  • the carbon cathode cell bottom comprises inserts that are located between and in alignment with opposite sections of current-carrying bars that lead into the carbon cathode cell bottom from outside opposite sides of the cell.
  • the opposite ledges of the carbon cell bottom extend under and beyond the position of the shadow of where anodes are suspended in use, and wherein side parts of the recessed central area in the carbon cell bottom also extend under parts of the position of the shadow of where anodes are suspended in use.
  • Another aspect of the invention is a Hall-Heroult cell for the production of aluminium from alumina dissolved in a molten fluoride-containing electrolyte, comprising a carbon cathode cell bottom as explained above, a pool of molten cathodic aluminium on the carbon cathode cell bottom, a fluoride-based electrolyte containing dissolved alumina on the aluminium pool, and a plurality of anodes suspendable in the electrolyte.
  • the invention also pertains to a method of producing aluminium in such a cell, comprising passing electrolysis current through the electrolyte from the anodes to produce aluminium that is collected in the cathodic aluminium pool.
  • a further aspect of the invention pertains to the inserts as discussed above, i .e. independently of the design of the carbon cathode cell bottom.
  • a carbon cathode bottom of a Hall-Heroult cell for the production of aluminium on which in use there is a pool of molten cathodic aluminium above which there is a molten fluoride-based electrolyte in which during operation anodes are suspended, wherein the carbon cathode cell bottom comprises a plurality of inserts therein, each insert comprising at least one highly conductive metal or at least one material of low electrical conductivity, contained in an enclosure within the carbon cathode cell bottom.
  • Figure 1 is a schematic cross-section through a Hall-Heroult cell with a cathode cell bottom according to the first aspect of the invention
  • Figure 2 is a similar cross-sectional view but where the cathodic cell bottom comprises inserts
  • Figure 3 is a schematic view from the side of adjacent cathode cell bottom blocks showing a plate for containing the molten metal at the cell ends;
  • FIGS 4 and 5 show alternative arrangements of inserts
  • Figure 6 shows a carbon block with a cap for fitting an insert
  • Figure 7 shows another arrangement of inserts, as in Figure 6.
  • Figure 8 shows a cross-section of a cell with a corresponding diagram illustrating the modification of the current density of the inventive cell compared to a standard cell.
  • Figure 1 schematically shows a Hall-Heroult aluminium-production cell 1 comprising a carbon cathode cell bottom 4 according to the invention, a pool 2 of liquid cathodic aluminium on the carbon cathode cell bottom 4, a fluoride- i.e. cryolite- based molten electrolyte 21 containing dissolved alumina on top of the aluminium pool 2, and a plurality of anodes 22 suspended in the electrolyte 21. Also shown is the cell container 23, current collector bars 24 that lead into the carbon cell bottom 4 from outside the cell container 23 , cell covers 25 and anode suspension rods 26. As can be seen, the aluminium pool 2 and the molten electrolyte 21 are contained in a crust 27 of frozen electrolyte.
  • the carbon cathode cell bottom 4 has on its top opposite sides flat ledges 31 between which the top of the carbon cathode cell bottom 4 has a recessed central area 5 that receives a deeper central part of the cathodic aluminium pool 2.
  • the recessed central area 5 of the carbon cathode cell bottom 4 has a volume A receiving cathodic aluminium, which volume A is equal to or greater than 20% of the volume of the top part B of the carbon cathode cell bottom 4 calculated as an envelope that encloses the recessed central area 5 and that lies between the level of 31 and the deepest part of the recessed central area 5.
  • the recessed central area 5 of the carbon cathode cell bottom 4 has inclined surfaces 32 that are inclined slightly to the horizontal and that lead down to a deepest central part of the recessed central area 5 and these inclined surfaces 32 of the recessed central area 5 are extended laterally by steeper inclined surfaces 33 that join to the top surface of ledges 31.
  • Other shapes of the recessed central area are possible, but generally its width is from 40-60% the width of the carbon cathode cell bottom 4, and its maximum depth is about 1 ⁇ 4 to 1/10 its width.
  • the opposite ledges 31 extend under and beyond the position of the shadow of where anodes 22 are suspended in use.
  • Side parts of the recessed central area 5 also extend under parts of the position of the shadow of where anodes 22 are suspended in use, as shown in Fig. 1.
  • the first aspect of the invention is thus a cell bottom of an electrolytic cell for the production of aluminium, that includes the following features:
  • Increasing the depth of the metal pool provides a deeper pool for at least 1/3 of the area in between the ledges 31 leading to an improved current distribution in the liquid m eta l a n d a damping factor for the waves resulting in a better magneto- hydrodynamic cell stability allowing for a lower ACD and lower cathode voltage drop;
  • the volume A of metal in the recessed part of the pool ie. volume of metal that would be "inside the cathode” if the cathode would be a parallelepiped, must be larger or equal to 20% of the volume of the parallelepiped carbon cathode (A+B);
  • the liquid metal 2 can be confined by using a terminal carbon slab (E) (Fig 3).
  • the second aspect of the invention shown in Figs. 2, 4, 5 and 7 is the use of inserts 3, 6, 7 to improve the current distribution in the cathode, or to further improve the current distribution using a cathode cell bottom according to the first aspect of the invention.
  • the carbon cathode cell bottom advantageously comprises a plurality of inserts 3, 6, 7 in the carbon cathode cell bottom 4, each insert comprising at least one highly conductive metal or at least one material of low electrical conductivity, contained in an enclosure within the carbon cathode cell bottom.
  • Figure 2 shows a cell bottom according to the invention as in Fig. 1 where the cell bottom 4 is fitted with inserts 3 that are located above, and transversally to, the cathode current collector bars 24 that extend all the way across the cell.
  • inserts 3 that are located above, and transversally to, the cathode current collector bars 24 that extend all the way across the cell.
  • one insert 3 is located on each side of the cell, in the area just below the ledges 32 and adjacent to the edges of the central recessed part 5.
  • Figure 4 also shows a cell with transverse inserts 6 and 7 located above the level of the current collector bars.
  • two inserts 6,7 are located on each side of the cell, one 6 located adjacent to an edge of the central recessed part 5 under the part of the ledge 31 free from the frozen crust 27, and an outer insert 7 is located close to where the crust 27 contacts ledge 31.
  • insert 6 is conductive and insert 7 is non-conductive.
  • Figure 5 shows a solution for which the collector bars 24A and 24B do not cross the cathode celll bottom 4.
  • a central insert 33 is placed in between the short collector bars 24A, 24B under the pool 2, i.e. in extension of the collector bars 24A, 24B.
  • the ends of the insert 33 are sealed with glued carbon elements 34.
  • the insert 33 alters the conductivity of the cathode body but does not participate in current collection and extraction by the collector bars 24A, 24B.
  • Inserts inside the carbon cathode will strongly affect the local resistivity; such inserts can be introduced in the cathode during the production of the cathode blocks or after by machining the cathode blocks;
  • Inserts can be placed in such a way that the current density at the surface of the carbon cathode becomes as low and as constant as possible when moving from the cell center towards the sides;
  • Inserts may be placed from the top, the sides or the bottom of the blocks; Inserts can be made of one highly electrical conductive material or a plurality of conductive materials leading to a higher electrical conductivity than the cathode blocks itself, i.e. inserts 6 of Figure 4;
  • Inserts can be made of low conductive materials or insulating materials allowing for displacing the current in the cathode for achieving a low and constant current density at the surface of the cathode blocks, i.e. inserts 7 of Figure 4;
  • Inserts can be made of a wall or shell of metallic material that will remain solid at its operating temperature (750°C to 1000°C) such as steel filled with a highly conductive material that may be liquid at its operating temperature such as ferro-aluminium alloys; in this case, the wall will have an opening at the top allowing for gas to escape;
  • inserts in the cathode can decrease the voltage drop inside the cathode between the liquid metal 2 and the end of the collector bars 24.
  • the inserts are constituted by holes or cavities in the cathode blocks 4 filled for example with a high conductive material such as aluminium and sealed. In case of the use of aluminium, the insert will reach a temperature that is above the melting point.
  • the mass of filling material (aluminium or preferably a ferro- aluminium alloy) is calculated in such a way that the cavity would be fully filled if the metal reaches 1000 °C, which is never the case.
  • the holes are sealed with carbon in order to avoid any material (aluminium) leak.
  • the sealing (Fig. 6) can be realized with a glued carbon plate 8 that is slit into a groove 9 to avoid the leak of the insert material in the cavity 10.
  • the filling material eg. ferro-aluminium alloy
  • the filling material can also be confined in a steel container or shell that is itself introduced in the cathode holes/cavities.
  • the diameter in calculated in such a way that the steel will be under pressure at the operating temperature towards the carbon cathode but not to exceed the crushing strength of the cathode block.
  • the hole/cavity in the carbon block can be filled with aluminium and/or an alloy made of aluminium and iron and/or copper and/or any good conductive material with an electrical conductivity that is at least higher than the electrical conductivity of the cathode block.
  • the mass of conductive material is such that its volume fills the hole/cavity once the operating conditions are reached.
  • the liquid aluminium will be in contact with the steel.
  • an Al-Fe alloy is used. Any high conductive material such as copper can replace the aluminium if it is economically viable.
  • the holes/cavities in the cathode for receiving the inserts are very important. They should be placed in such a way that the current density in the liquid metal pad in the cell stabilizes the cell from a magneto-hydrodynamic point of view. As the magneto- hydrodynamic behavior of the cell is strongly modified it will allow the ACD to be decreased before observing MHD instabilities. It will also reduce the maximum current density at the surface of the carbon cathode meaning that electro-erosion will be minimized. Lower electro- erosion will contribute to a longer cell lifetime.
  • a pressure release When a container is used for the inserts, a pressure release must be implemented by using holes at the top of the container. The gas will be released though the holes and will spread though the porous carbon cathode.
  • Figure 7 shows a solution where the inserts 10/11 are above and parallel to the collector bars 24.
  • the inserts 10/1 1 are placed directly above the collector bars 24 or in between the collector bars 24 and are made of the same materials as previously mentioned.
  • the hole/cavities are sealed at the end by carbon materials 1 1 or an insulating material that is compatible with the thermal expansion of carbon such that the hole/cavity remains tight to liquid.
  • the length L1 and diameter D1 of the insert are optimized in function of the busbar system in order to optimize the cell stability. Indeed, the redistribution of the current through the inserts allows for a much better magneto-hydrodynamic cell state that will allow to decrease the ACD while increasing the current and hence minimizing the energy consumption, ie. the voltage.
  • a typical example of current density is shown for a standard cell and for a cell according to the invention in Figure 8.
  • Jz Jz(x,y,z) in a (x,y,z) coordinate system as shown on Figure 8.
  • the absolute value of the vertical component of the current density varies typically as shown in Figure 8 (Standard).
  • is reduced by 25% as minimum (Invention).

Abstract

The invention relates to an electrolytic cell (1) for the production of aluminium (2) including cathode surface shape modifications (5) and/or additional insert(s) (3,6,7) inside the carbon cathode (4) leading to an optimized current distribution in the liquid metal 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.

Description

HALL-HEROULT CELL CATHODE DESIGN
Field of Invention
The invention relates to the production of aluminium using the Hall-Heroult process; in particular to the optimization of the cathode for the decrease of energy consumption and maximization of the current efficiency.
Background of the Invention
Aluminium is produced by the Hall-Heroult process, by electrolysis of alumina dissolved in cryolite based electrolytes at temperature going up to 1000°C. A typical cell is composed of a steel shell, an insulating lining of refractory materials and a carbon cathode holding the liquid metal. The cathode is itself composed of a number of cathode blocks in which collector bars are embedded at their bottom to extract the current flowing through the cell.
A number of patent publications have proposed different approaches for minimizing the voltage drop between the liquid metal to the end of the collector bars. WO2008/062318 proposes the use of a high conductive material in complement to the existing steel collector bar and gives reference to WO 02/42525, WO 01/63014, WO 01/27353, WO 2004/031452 and WO 2005/098093 that disclose solutions using copper inserts inside the collector steel bars. US patent 4,795,540 splits the cathode in sections as well as the collector bars. WO2001/27353 and WO2001/063014 use high conductive materials inside the collector bars. US2006/0151333 covers the use of different electrical conductivities in the collector bars. WO 2007/1 18510 proposes to increase the section of the collector bar when moving towards the center of the cell for changing the current distribution at the surface of the cathode. US 6,231 ,745 presents the use of a copper insert inside the steel collector bar. EP 2 139 446 describes cathode block arrangements to modify the surface geometry of the cathode in order to stabilize the waves at the surface of the metal pad and hence to minimize the ACD (anode to cathode distance).
The resistivity of molten cryolite is very high, typically 220 Ω"1πϊ1 and the ACD cannot be decreased too much due to the formation of magneto-hydrodynamic instabilities leading to waves at the metal-bath (metal - cryolite electrolyte) interface. The existence of waves leads to a loss of the current efficiency of the process and does not allow decreasing the energy consumption under a critical value. On average in the aluminium industry, the current density is such that the voltage drop in the ACD is minimum 0.3 V/cm. As the ACD is 3 to 5 cm, the voltage drop in the ACD is typically 1.0 V to 1.5 V. The magnetic field inside the liquid metal is the result of the currents flowing in the external busbars and the internal currents. The internal local current density inside the liquid metal is mostly defined by the cathode geometry and its local electrical conductivity. The magnetic field and current density produce the Lorentz force field which itself generates the metal surface contour, the metal velocity field and defines the basic environment for the magneto-hydrodynamic cell stability. The cell stability can be expressed as the ability of lowering the ACD without generating unstable waves at the surface of the metal pad. The level of stability depends on the current density and induction magnetic fields but also of the shape of the liquid metal pool. The shape of the pool depends on the surface of the cathode and the ledge shape. The Prior Art solutions respond only marginally to the needed magneto-hydrodynamic status and liquid metal pool shape to satisfy good cell stability (low ACD).
Summary of the Invention
A thorough study of cell magneto-hydrodynamic instabilities using a specialized software tool led to the conclusion that there is still a large possibility of improving the interaction of the magnetic field with the local current density in the liquid metal by modifying the local current density. The invention is based on the observation that changing the cathode geometry and conductivity will lead to new current distribution inside the liquid metal.
Therefore, in a first aspect the invention provides a carbon cathode bottom of a Hall-Heroult cell for the production of aluminium, on which in use there is a pool of molten cathodic aluminium above which there is a molten fluoride-based electrolyte in which during operation anodes are suspended, the top of the carbon cathode cell bottom having on opposite sides generally flat ledges. According to this aspect of the invention, between said ledges, the top of the carbon cathode cell bottom has a recessed central area for receiving in use a deeper central part of the cathodic aluminium pool.
The recessed central area of the carbon cathode cell bottom has a volume A for receiving cathodic aluminium which leads to an improved current distribution in the liquid metal and a damping factor for the waves resulting in a better magneto-hydrodynamic cell stability allowing for a lower ACD and lower cathode voltage drop. Notably, this volume A is equal to or greater than 20% of the volume of the top part of the carbon cathode cell bottom calculated as an envelope that encloses the recessed central area and that lies between the level of said ledges and the deepest part of the recessed central area.
Typically, the volume A of the recessed central area of the carbon cell bottom is about 25% to 40% and usually at most 50% of the volume of the top part of the carbon cathode cell bottom calculated as before. Generally speaking, the maximum depth d of the aluminium pool in the recessed central area 5 is in a range of typically 5 cm to 20 cm, and is usually larger than the fluctuating depth D of the aluminium pool over the ledge 31 , which varies between 3 cm to 10 cm after tapping. Advantageously, the depth D is kept as small as possible for minimizing the total amount of liquid metal in the cell.
In preferred designs, the recessed central area of the carbon cathode cell bottom has inclined surfaces that are inclined slightly to the horizontal and that lead down to a deepest central part of the recessed central area, and these inclined surfaces of the recessed central area are extended laterally by steeper inclined surfaces that join to the top surface of said ledges.
The carbon cathode cell bottom usually comprises a plurality of current-carrying bars that lead into the carbon cathode cell bottom from outside the cell, these current-carrying bars extending across the carbon cathode cell bottom between said ledges.
The inventive carbon cathode cell bottom advantageously comprises a plurality of inserts in the carbon cathode cell bottom, each insert comprising at least one highly conductive metal or at least one material of low electrical conductivity, contained in an enclosure within the carbon cathode cell bottom.
At least one of these inserts can comprise a wall or shell of metallic material that remains solid at cell operating temperature, such as steel, containing a highly conductive metallic material that is liquid at cell operating temperature, such as ferro-aluminium alloys.
In some designs, the inventive carbon cathode cell bottom comprises inserts that are located above the level of current-carrying bars that lead into the carbon cathode cell bottom from outside the cell, such inserts typically being parallel to or perpendicular to the current-carrying bars.
In another design, the carbon cathode cell bottom comprises inserts that are located between and in alignment with opposite sections of current-carrying bars that lead into the carbon cathode cell bottom from outside opposite sides of the cell.
Usually, the opposite ledges of the carbon cell bottom extend under and beyond the position of the shadow of where anodes are suspended in use, and wherein side parts of the recessed central area in the carbon cell bottom also extend under parts of the position of the shadow of where anodes are suspended in use.
Another aspect of the invention is a Hall-Heroult cell for the production of aluminium from alumina dissolved in a molten fluoride-containing electrolyte, comprising a carbon cathode cell bottom as explained above, a pool of molten cathodic aluminium on the carbon cathode cell bottom, a fluoride-based electrolyte containing dissolved alumina on the aluminium pool, and a plurality of anodes suspendable in the electrolyte.
The invention also pertains to a method of producing aluminium in such a cell, comprising passing electrolysis current through the electrolyte from the anodes to produce aluminium that is collected in the cathodic aluminium pool.
A further aspect of the invention pertains to the inserts as discussed above, i .e. independently of the design of the carbon cathode cell bottom. According to this aspect there is provided a carbon cathode bottom of a Hall-Heroult cell for the production of aluminium, on which in use there is a pool of molten cathodic aluminium above which there is a molten fluoride-based electrolyte in which during operation anodes are suspended, wherein the carbon cathode cell bottom comprises a plurality of inserts therein, each insert comprising at least one highly conductive metal or at least one material of low electrical conductivity, contained in an enclosure within the carbon cathode cell bottom. These inserts can have the features mentioned above or in the following description.
Brief Description of the Drawings
The invention will be further described by way of example with reference to the accompanying drawings, in which:
Figure 1 is a schematic cross-section through a Hall-Heroult cell with a cathode cell bottom according to the first aspect of the invention;
Figure 2 is a similar cross-sectional view but where the cathodic cell bottom comprises inserts;
Figure 3 is a schematic view from the side of adjacent cathode cell bottom blocks showing a plate for containing the molten metal at the cell ends;
Figures 4 and 5 show alternative arrangements of inserts;
Figure 6 shows a carbon block with a cap for fitting an insert;
Figure 7 shows another arrangement of inserts, as in Figure 6.
Figure 8 shows a cross-section of a cell with a corresponding diagram illustrating the modification of the current density of the inventive cell compared to a standard cell.
Detailed Description
Figure 1 schematically shows a Hall-Heroult aluminium-production cell 1 comprising a carbon cathode cell bottom 4 according to the invention, a pool 2 of liquid cathodic aluminium on the carbon cathode cell bottom 4, a fluoride- i.e. cryolite- based molten electrolyte 21 containing dissolved alumina on top of the aluminium pool 2, and a plurality of anodes 22 suspended in the electrolyte 21. Also shown is the cell container 23, current collector bars 24 that lead into the carbon cell bottom 4 from outside the cell container 23 , cell covers 25 and anode suspension rods 26. As can be seen, the aluminium pool 2 and the molten electrolyte 21 are contained in a crust 27 of frozen electrolyte.
The carbon cathode cell bottom 4 according to the invention has on its top opposite sides flat ledges 31 between which the top of the carbon cathode cell bottom 4 has a recessed central area 5 that receives a deeper central part of the cathodic aluminium pool 2.
The recessed central area 5 of the carbon cathode cell bottom 4 has a volume A receiving cathodic aluminium, which volume A is equal to or greater than 20% of the volume of the top part B of the carbon cathode cell bottom 4 calculated as an envelope that encloses the recessed central area 5 and that lies between the level of 31 and the deepest part of the recessed central area 5.
As shown, the recessed central area 5 of the carbon cathode cell bottom 4 has inclined surfaces 32 that are inclined slightly to the horizontal and that lead down to a deepest central part of the recessed central area 5 and these inclined surfaces 32 of the recessed central area 5 are extended laterally by steeper inclined surfaces 33 that join to the top surface of ledges 31. Other shapes of the recessed central area are possible, but generally its width is from 40-60% the width of the carbon cathode cell bottom 4, and its maximum depth is about ¼ to 1/10 its width.
As shown, the opposite ledges 31 extend under and beyond the position of the shadow of where anodes 22 are suspended in use. Side parts of the recessed central area 5 also extend under parts of the position of the shadow of where anodes 22 are suspended in use, as shown in Fig. 1.
The first aspect of the invention is thus a cell bottom of an electrolytic cell for the production of aluminium, that includes the following features:
Increasing the depth of the metal pool provides a deeper pool for at least 1/3 of the area in between the ledges 31 leading to an improved current distribution in the liquid m eta l a n d a damping factor for the waves resulting in a better magneto- hydrodynamic cell stability allowing for a lower ACD and lower cathode voltage drop;
The volume A of metal in the recessed part of the pool, ie. volume of metal that would be "inside the cathode" if the cathode would be a parallelepiped, must be larger or equal to 20% of the volume of the parallelepiped carbon cathode (A+B); At each end of the cathode, the liquid metal 2 can be confined by using a terminal carbon slab (E) (Fig 3).
The second aspect of the invention, shown in Figs. 2, 4, 5 and 7 is the use of inserts 3, 6, 7 to improve the current distribution in the cathode, or to further improve the current distribution using a cathode cell bottom according to the first aspect of the invention. As shown in Figures 2, 4, 5 and 7 the carbon cathode cell bottom advantageously comprises a plurality of inserts 3, 6, 7 in the carbon cathode cell bottom 4, each insert comprising at least one highly conductive metal or at least one material of low electrical conductivity, contained in an enclosure within the carbon cathode cell bottom.
Figure 2 shows a cell bottom according to the invention as in Fig. 1 where the cell bottom 4 is fitted with inserts 3 that are located above, and transversally to, the cathode current collector bars 24 that extend all the way across the cell. In this example, one insert 3 is located on each side of the cell, in the area just below the ledges 32 and adjacent to the edges of the central recessed part 5.
Figure 4 also shows a cell with transverse inserts 6 and 7 located above the level of the current collector bars. Here two inserts 6,7 are located on each side of the cell, one 6 located adjacent to an edge of the central recessed part 5 under the part of the ledge 31 free from the frozen crust 27, and an outer insert 7 is located close to where the crust 27 contacts ledge 31. As explained below, insert 6 is conductive and insert 7 is non-conductive.
Figure 5 shows a solution for which the collector bars 24A and 24B do not cross the cathode celll bottom 4. Here a central insert 33 is placed in between the short collector bars 24A, 24B under the pool 2, i.e. in extension of the collector bars 24A, 24B. The ends of the insert 33 are sealed with glued carbon elements 34. Thus, the insert 33 alters the conductivity of the cathode body but does not participate in current collection and extraction by the collector bars 24A, 24B.
The use of inserts implies the following features:
Inserts inside the carbon cathode will strongly affect the local resistivity; such inserts can be introduced in the cathode during the production of the cathode blocks or after by machining the cathode blocks;
Inserts can be placed in such a way that the current density at the surface of the carbon cathode becomes as low and as constant as possible when moving from the cell center towards the sides;
Inserts may be placed from the top, the sides or the bottom of the blocks; Inserts can be made of one highly electrical conductive material or a plurality of conductive materials leading to a higher electrical conductivity than the cathode blocks itself, i.e. inserts 6 of Figure 4;
Inserts can be made of low conductive materials or insulating materials allowing for displacing the current in the cathode for achieving a low and constant current density at the surface of the cathode blocks, i.e. inserts 7 of Figure 4;
Inserts can be made of a wall or shell of metallic material that will remain solid at its operating temperature (750°C to 1000°C) such as steel filled with a highly conductive material that may be liquid at its operating temperature such as ferro-aluminium alloys; in this case, the wall will have an opening at the top allowing for gas to escape;
Changing the depth of the metal pool combined with inserts minimizes the cathode voltage drop inside the carbon cathode together with decreasing the ACD;
Further description of the inserts
Use of inserts in the cathode can decrease the voltage drop inside the cathode between the liquid metal 2 and the end of the collector bars 24. The inserts are constituted by holes or cavities in the cathode blocks 4 filled for example with a high conductive material such as aluminium and sealed. In case of the use of aluminium, the insert will reach a temperature that is above the melting point. The mass of filling material (aluminium or preferably a ferro- aluminium alloy) is calculated in such a way that the cavity would be fully filled if the metal reaches 1000 °C, which is never the case.
The holes are sealed with carbon in order to avoid any material (aluminium) leak. The sealing (Fig. 6) can be realized with a glued carbon plate 8 that is slit into a groove 9 to avoid the leak of the insert material in the cavity 10.
The filling material (eg. ferro-aluminium alloy) can also be confined in a steel container or shell that is itself introduced in the cathode holes/cavities. The diameter in calculated in such a way that the steel will be under pressure at the operating temperature towards the carbon cathode but not to exceed the crushing strength of the cathode block.
The hole/cavity in the carbon block can be filled with aluminium and/or an alloy made of aluminium and iron and/or copper and/or any good conductive material with an electrical conductivity that is at least higher than the electrical conductivity of the cathode block. The mass of conductive material is such that its volume fills the hole/cavity once the operating conditions are reached. In case of use of aluminium in a steel casing, the liquid aluminium will be in contact with the steel. In order to minimize the diffusion of Fe in the Al, an Al-Fe alloy is used. Any high conductive material such as copper can replace the aluminium if it is economically viable.
Very important is the disposition of the holes/cavities in the cathode for receiving the inserts. They should be placed in such a way that the current density in the liquid metal pad in the cell stabilizes the cell from a magneto-hydrodynamic point of view. As the magneto- hydrodynamic behavior of the cell is strongly modified it will allow the ACD to be decreased before observing MHD instabilities. It will also reduce the maximum current density at the surface of the carbon cathode meaning that electro-erosion will be minimized. Lower electro- erosion will contribute to a longer cell lifetime.
When a container is used for the inserts, a pressure release must be implemented by using holes at the top of the container. The gas will be released though the holes and will spread though the porous carbon cathode.
Figure 7 shows a solution where the inserts 10/11 are above and parallel to the collector bars 24. The inserts 10/1 1 are placed directly above the collector bars 24 or in between the collector bars 24 and are made of the same materials as previously mentioned. The hole/cavities are sealed at the end by carbon materials 1 1 or an insulating material that is compatible with the thermal expansion of carbon such that the hole/cavity remains tight to liquid.
The length L1 and diameter D1 of the insert are optimized in function of the busbar system in order to optimize the cell stability. Indeed, the redistribution of the current through the inserts allows for a much better magneto-hydrodynamic cell state that will allow to decrease the ACD while increasing the current and hence minimizing the energy consumption, ie. the voltage.
This is reflected by a homogeneous vertical current density in a horizontal section in the middle of the liquid metal pool. The number of inserts and the distribution in the cathode blocks is not important as long as the current density becomes homogeneous.
A typical example of current density is shown for a standard cell and for a cell according to the invention in Figure 8. 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 as shown on Figure 8. When moving from the edge of the external part of the shadow of one anode (x= -XL) to the edge of the shadow of the neighboring anode 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 Figure 8 (Standard). When optimizing the cathode by using inserts and/or modified cathode shape, |Jz(x)| is reduced by 25% as minimum (Invention).

Claims

1. A carbon cathode bottom of a Hall-Heroult cell for the production of aluminium, on which in use there is a pool of molten cathodic aluminium, the top of the carbon cathode cell bottom having on opposite sides generally flat ledges, characterized in that between said ledges, the top of the carbon cathode cell bottom has a recessed central area for receiving in use a deeper central part of the cathodic aluminium pool, wherein the recessed central area of the carbon cathode cell bottom has a volume A for receiving cathodic aluminium, which volume A is equal to or greater than 20% of the volume of the top part of the carbon cathode cell bottom calculated as an envelope that encloses the recessed central area and that lies between the level of said ledges and the deepest part of the recessed central area.
2. The carbon cathode cell bottom of claim 1 wherein the volume A of the recessed central area of the carbon cell bottom is at most 50% of the volume of the top part of the carbon cathode cell bottom calculated as before.
3. The carbon cathode cell bottom of claim 1 or 2 wherein the recessed central area of the carbon cathode cell bottom has inclined surfaces that are inclined slightly to the horizontal and that lead down to a deepest central part of the recessed central area.
4. The carbon cathode cell bottom of claim 3 wherein said inclined surfaces of the recessed central area are extended laterally by steeper inclined surfaces that join to the top surface of said ledges.
5. The carbon cathode cell bottom of any preceding claim comprising a plurality of current-carrying bars that lead into the carbon cathode cell bottom from outside the cell, said current-carrying bars extending across the carbon cathode cell bottom between said ledges.
6. The carbon cathode cell bottom of any preceding claim further comprising a plurality of inserts in the carbon cathode cell bottom, each insert comprising at least one highly conductive metal or at least one material of low electrical conductivity, contained in an enclosure within the carbon cathode cell bottom.
7. The carbon cathode cell bottom of claim 6 wherein at least one insert comprises a wall or shell of metallic material that remains solid at cell operating temperature, such as steel, containing a highly conductive metallic material that is liquid at cell operating temperature, such as ferro-aluminium alloys.
8. The carbon cathode cell bottom of claim 6 or 7 wherein at least one insert comprises at least one material of low electrical conductivity.
9. The carbon cathode cell bottom of claim 6, 7 or 8 comprising inserts which are located above the level of current-carrying bars that lead into the carbon cathode cell bottom from outside the cell.
10. The carbon cathode cell bottom of claim 9 wherein the inserts are parallel to or perpendicular to the current-carrying bars.
1 1. The carbon cathode cell bottom of any one of claims 6 to 10 comprising inserts that are located between and in alignment with opposite sections of current-carrying bars that lead into the carbon cathode cell bottom from outside opposite sides of the cell.
12. The carbon cathode cell bottom of any preceding claim wherein the opposite ledges extend under and beyond the position of the shadow of where anodes are suspended in use, and wherein side parts of the recessed central area also extend under parts of the position of the shadow of where anodes are suspended in use.
13. A Hall-Heroult cell for the production of aluminium from alumina dissolved in a molten fluoride-containing electrolyte, comprising a carbon cathode cell bottom as claimed in any preceding claim, a pool of molten cathodic aluminium on the carbon cathode cell bottom, a fluoride-based electrolyte containing dissolved alumina on the aluminium pool, and a plurality of anodes suspendable in the electrolyte.
14. A method of producing aluminium in a cell according to claim 13, comprising passing electrolysis current through the electrolyte from the anodes to produce aluminium that is collected in the cathodic aluminium pool.
15. A carbon cathode bottom of a Hall-Heroult cell for the production of aluminium, on which in use there is a pool of molten cathodic aluminium, characterized in that the carbon cathode cell bottom comprises a plurality of inserts therein, each insert comprising at least one highly conductive metal or at least one material of low electrical conductivity, contained in an enclosure within the carbon cathode cell bottom.
PCT/IB2011/052325 2010-05-28 2011-05-27 Hall-heroult cell cathode design WO2011148347A1 (en)

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