US3492208A - Electrolytic cells and methods of operating same - Google Patents

Electrolytic cells and methods of operating same Download PDF

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Publication number
US3492208A
US3492208A US518090A US3492208DA US3492208A US 3492208 A US3492208 A US 3492208A US 518090 A US518090 A US 518090A US 3492208D A US3492208D A US 3492208DA US 3492208 A US3492208 A US 3492208A
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cell
molten
aluminum
carbon
metal
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George Cresswell Seager
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British Aluminum Co Ltd
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British Aluminum Co Ltd
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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

  • a system for producing aluminum includes a cathodic structure in which molten aluminum is collected in a pool to a depth approaching two inches.
  • the cathodic structure has a substantially horizontal molten aluminum contracting surface which has a cathodic expansion of less than about 3%, which is wettable by molten aluminum and which comprises a mixture of refractory hard substance with at least 5% calcined carbon, the surface having a tapping depression therein.
  • This invention relates to improvements in electrolytic cells and method of operating same and is concerned with electrolytic cells for the production of aluminum.
  • the aluminum reduction cell in common commercial use today is generally of the classic Hall/Heroult design with carbon anodes and a substantially fiat carbon lined bottom which functions as part of the cathodic system.
  • An electrolyte is used in the production of aluminum by electrolytic reduction of alumina which consists primarily of molten cryolite with dissolved alumina and which may contain other materials such as fluorspar. Molten aluminum resulting from the reduction of alumina collects as a molten metal pool over the carbon lined bottom and acts as a liquid metal cathode.
  • the carbon anodes extend into the cell from above and make contact with the electrolyte.
  • Current collector bars usually of steel are embedded in the carbon lined bottom and complete the connection to the cathodic system.
  • Such commercial cells are generally operated by maintaining a minimum depth of liquid aluminum on the floor of the cell. This minimum depth is usually at least 2" and is required due to the fact that at lesser depths the aluminum would break up into globules, as it does not wet the carbon floor. If a continuous pool of molten aluminum is not obtained, it is not possible to establish a uniform interpolar distance, i.e., the distance between the anode and the surface of the molten aluminum. If the metal forms globules these move about, the inter-polar distance varies and eflicient operation at the optimum inter-polar distance cannot be established.
  • the surface areas presented by such globules to the electrolyte is considerably greater than the substantially continuous surface of the pool of molten metal and as the aluminum backreacts with the electrolyte and tends to go back into solution the result is that more metal goes back into solution than would otherwise be the case and the process becomes uneconomic.
  • the depth of metal required to overcome these problems leads to another problem resulting from the electromagnetic effects which are usually associated with the reduction cell and which produce circulation of the liquid metal and cause the thickness or depth to vary and hence restrict the possible reduction of the inter-polar spacing. This also produces a loss in efliciency since power is lost to the electrolyte interposed between the anode and cathode. Any restriction on the reduction of the anode to cathode spacing also restricts the achieve- 3,492,208 Patented Jan. 27, 1970 ment of maximum power efliciency and limits the ability to improve electrolytic cell operation.
  • refractory hard substances as materials for contacting the liquid metal pool and leading current from the cell has been previously proposed in US. 3,028,324.
  • Such substances are broadly defined as being wettable by molten aluminum under electrolytic cell operating conditions, having a low solubility in molten aluminum and molten cryolite, having an electrical conductivity at least as good as carbon, having a resistance to attack by molten aluminum and other cell constituents at least as good as carbon and being substantially dimensionally stable in a cathodic structure in an electrolytic cell.
  • Refractory hard substances include refractory hard metals and mixtures of refractory hard metals as well as mixtures containing refractory hard metals and aluminum compounds such as borides, nitrides and carbides, compounds of rare earth metals, chromium and combinations of the above.
  • the expression refractory hard metals refers to the carbides, borides, silicides and nitrides of the transition metals in the fourth to sixth groups (Hubbards Periodic Chart of the atoms).
  • the preferred refractory hard substances include the borides and carbides of titanium and zirconium and mixtures thereof.
  • refractory hard metals frequently contain up to /z% carbon as impurities and this is generally considered to be pure refractory hard metal.
  • the amount of refractory hard metal in a cathodic structure may, however, be reduced virtually without sacrifice of the function thereof.
  • a cathodic structure having a surface intended to contact molten cell constituents, which surface is formed from a mixture of refractory hard substance and at least 5% carbon and is wettable by molten aluminum and has a cathodic expansion of less than about 3% It is preferred, however, that the mixture should contain at least 20% by weight of the refractory hard substance.
  • the carbon component of the mixture preferably comprises a carbonaceous material which has been heat treated at a temperature about 1500 C. to improve its stability under cathodic conditions in the cell.
  • the refractory hard substance is disposed as a molten constituent contacting layer on the floor of the cell to provide a cathodic structure which is drained, i.e., the molten metal is prevented from forming a pool on the cell floor and is drained into a collecting well.
  • the cathodic structure is provided with an inclination to the horizontal so that the metal produced by electrolysis at the face of the cathodic structure is drained off into a substantially centrally disposed well having an appreciable capacity so that the cell need only be tapped at periodic intervals of time.
  • An electrolytic cell using a drained and wetted cathode represents a considerable improvement over conventional cells.
  • By draining the I cathodic surface so that only a thin film of molten aluminum remains in contact with the cathode and functions as part of the electrical circuit it is possible to employ very short anode to cathode spacing and at the same time maintain high current efficiency. Power losses can be reduced by decreasing the electrical resistance in the cell. Electrical resistance can be decreased without sacrificing current efiiciency by decreasing the interpolar distance and thereby decreasing voltage losses due to electrolytic resistance, or enabling increased current densities to be employed to increase the output from a given cell by increasing the current through it.
  • Aluminum which is produced in the drained and wetted cathode cell by the electrolysis of alumina is drained off the cathode surface so that only a thin substantially uniform film of molten metal remains thereon, since the surface is wettable by molten aluminum, i.e., molten aluminum adheres as a liquid to the surface.
  • Molten aluminum drained from the cathode surface collects in a pool in the well which is located so that the molten metal pool is not an essential part of the electrical system, i e., the molten metal pool is not essential for conducting cathodic current from the cell and the well may be periodically tapped dry of aluminum.
  • the well must be capable of holding a substantial volume of molten metal so that it need only be tapped at acceptably long intervals of time.
  • the level of molten aluminum in the well must be necessarily lower than that of the sloping cathode surface.
  • the concentration of a considerable volume of aluminum in the well results in an imbalance of the temperature conditions existing Within the cell and can act as a heat sink. If the collecting well is located outside the active area of the anodes, i.e., remote from the shadow of the anodes, it will be at a temperature lower than the temperature prevailing in the active area and this can lead to precipitation and collection of solidified bath constituents in the well in addition to unbalancing the cell temperature gradients.
  • an electrolytic reduction cell for the production of aluminum having a substantially level floor surface composed of refractory hard substance.
  • a small depression is provided to facilitate tapping of the molten metal but having a volume which is insignificant in relation to the volume of molten metal which accumulates on the floor of the cell during cell operation.
  • a method of operating an electrolytic reduction cell for the production of aluminum which comprises the steps of leading current from the cell through a substantially level floor surface exposed to the interior of the cell and composed of refractory hard substance and maintaining the depth of molten aluminum which accumulates on the floor surface at a value of not greater than 2 inches.
  • the fact that the floor of the cell is composed of a refractory hard substance which is wettable by molten aluminum prevents the formation of globules of aluminum at the small depths envisaged and the step of maintaining this depth at a value of below 2 inches avoids the disadvantages associated with the electromagnetic elfects and enables small inter-polar distances of less than 1.75 inches and preferably from A to 1 /2 inches to be maintained with consequent increase in the power efiiciency of the cell without decreasing the current efficiency appreciably.
  • the molten metal is collected as a pool over the floor of the cell so that there is no localized heat sink to disturb the temperature gradients.
  • the build-up in depth of the molten metal on the floor of the cell during cell operation compensates for the consumption of the anode material during operation so that the inter-polar spacing is substantially maintained at a reasonably constant value.
  • the anodes are readjusted to the desired spacing between their active face and the surface of molten metal.
  • a small depression is desirably provided to facilitate entry of a tapping nozzle, its capacity is insignificant in relation to the volume of metal which is allowed to collect in a pool on the floor of the cell before tapping.
  • this small depression is full and the metal pool on the cell floor presents a continuous surface to the electrolyte.
  • This depression may therefore be partly or wholly within the active area of the anodes, i.e., below the shadow of the anode or anodes, which may be either of the Soderberg or pre-baked type.
  • FIG. 1 is a plan view of an electrolytic cell according to the invention
  • FIG. 2 is a section taken on the line 11-11 of FIG. 1, and
  • FIG. 3 is a section taken on the line 111111 of FIG. 1.
  • the electrolytic cell of this example comprises a bottom 1 of refractory material such as thermal insulating brick over which is disposed a layer 2 of firebric-k. On top of the firebnck are supported a number of steel cathode bus-bars 3 over and between which is a layer 4 of rammed and fired carbon of a depth of about 12 inches.
  • the side walls 5 of the cell are formed from alumina and lined with carbon bricks 6.
  • the carbon layer 4 is identified as the bottom layer in Table 1 below and has the composition there described. It is formed with a central depression 7 of about 9 inches in diameter and from 1 to 2 inches deep.
  • a graded layer 8 of refractory hard substance is disposed over the rammed carbon layer 4 and is effectively formed in three layers, the top layer of which is the richest in refractory hard metal.
  • the lowest or third layer of the layer 8 may have 30 by weight of refractory hard metal and the balance carbon and binder, the intermediate or second layer 60% by weight of reefractory hard metal and the balance carbon and binder, and the top layer by weight of refractory hard metal and the balance carbon and binder.
  • the refractory hard metal component may consist essentially of 70% by weight of titanium diboride and 30% by weight of titanium carbide although higher percentages of TiB may be preferred and commercially pure TiB can be used.
  • the carbon constituent of the layer 8 is desirably a carbonaceous materialsuch as anthracite or petroleum coke heat treated at a temperature above 1500" C. to improve its stability under cell operating conditions.
  • the layer 8 has a cathodic expansion of less than about 3% as defined in the co-pending application Ser. No. 325,228.
  • composition of the layers 8 and 4 is set out below in Table I.
  • the depression in the layer 4 is reproduced at 9 in the layer 8 and is provided for the purpose of accommodating a tapping nozzle.
  • Four carbon anodes 10 extend into the cell to afford an operative area of about 1000 square inches and have feet 11 embedded therein connected to hangers 12 supported on anode bus-bars indicated by the lines 13.
  • the cell contains the usual molten flux constituents with dissolved alumina and an electric current passes from the anodes through the layers 8 and 4 to the bus-bars 3.
  • Molten aluminum is deposited on the floor of the cell which is wetted thereby.
  • the molten aluminum forms a pool the level of which is indicated at 14 and the depth of which is maintained at a depth of less than 2 inches.
  • the cell is tapped by inserting a tapping spout into the depression and the metal is sucked out, care being taken that no molten flux constituents are removed.
  • the anodes are lowered to the desired interelectrode spacing between their operative faces and either the wetted surface of the layer 8 or the surface of any remaining molten metal as the case may be, this spacing being less than 1.75 inches and desirably less than 1 /2 inches. As the molten metal pool builds up the operative faces of the anodes burn away so that the inter-electrode spacing is approximately maintained below the Value specified.
  • Table II illustrates the comparable operations of a conventional cell, a wetted drained cathode cell as described int-he co-pending application Ser. No. 325,228 and a fiat wetted cathode cell in accordance with the present invention.
  • the inter-polar distances quoted in the table represent an average value over the anode surface which becomes curved during use and the minimum distance may be A to /2 inch less than those given in the table.
  • inter-polar distances of less than 1.75 inches can be achieved without difficulty and cells according to the present invention are desirably operated with an inter-polar distance in the range of A to 1 /2 inches.
  • the pool of molten aluminum presents a continuous surface to the anodes so that there is no significant inequality in the erosion or burning away of the latter so that either pre-baked or Soderberg anodes may be used.
  • the depression 9 which is merely provided to facilitate tapping, has a volume which is insignificant in comparison with that of the aluminum which is collected on the floor of the cell and has no effect on cell operation and does not interrupt the continuity of the surface of the molten aluminum pool.
  • a not insignificant advantage of the present invention is that existing conventional cells may be readily modified by incorporating a layer 8 over the existing carbon floor without any great capital cost and without any requirement for structural alterations.
  • the method of operating an electrolytic cell for the production of aluminum which comprises a shell defining a receptacle, electrolyte containing dissolved aluminum compound within the receptacle, an anodic system including at least one anode, and a cathodic system comprising a cathodic structure disposed within the receptacle contacting a molten aluminum pool exposed to molten constituents during cell operation, the cathodic structure comprising a continuous horizontal molten aluminum pool contacting surface, having a shallow tapping depression therein, the molten aluminum pool contacting surface comprising composite cathode material said material being a mixture of refractory hard substance and at least about 5% calcined carbon, and having a cathodic expansion of less than about 3% and being wettable by molten aluminum comprising:
  • said composite cathode structure comprises a bottom layer of about 30% by weight refractory hard metal and the bal- 7 ance carbon and binder, an intermediate layer of about 60% by weight of refractory hard metal and the balance carbon and binder, and a top layer of about 90% by weight refractory hard metal and the balance carbon and binder.
  • the refractory hard substance component of the composite cathode material comprises 70% by weight of titanium diboride and 30% by weight of titanium carbide.
  • the composite cathode material comprises a mixture of at least 20% refractory hard substance and at least about 5% carbon.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electrolytic Production Of Metals (AREA)
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US518090A 1965-01-06 1966-01-03 Electrolytic cells and methods of operating same Expired - Lifetime US3492208A (en)

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GB578/65A GB1138522A (en) 1965-01-06 1965-01-06 Improvements in or relating to electrolytic cells for the production of aluminium and methods of operating same

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US (1) US3492208A (US06229276-20010508-P00022.png)
AT (1) AT274400B (US06229276-20010508-P00022.png)
CH (1) CH456169A (US06229276-20010508-P00022.png)
DE (1) DE1533439A1 (US06229276-20010508-P00022.png)
ES (1) ES321480A1 (US06229276-20010508-P00022.png)
FR (1) FR1466660A (US06229276-20010508-P00022.png)
GB (1) GB1138522A (US06229276-20010508-P00022.png)
NL (2) NL6600097A (US06229276-20010508-P00022.png)
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Cited By (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4308115A (en) * 1980-08-15 1981-12-29 Aluminum Company Of America Method of producing aluminum using graphite cathode coated with refractory hard metal
WO1984000565A1 (en) * 1982-07-22 1984-02-16 Martin Marietta Corp Aluminum cathode coating cure cycle
US4466995A (en) * 1982-07-22 1984-08-21 Martin Marietta Corporation Control of ledge formation in aluminum cell operation
US4466996A (en) * 1982-07-22 1984-08-21 Martin Marietta Corporation Aluminum cell cathode coating method
US4481052A (en) * 1983-01-28 1984-11-06 Martin Marietta Corporation Method of making refractory hard metal containing tiles for aluminum cell cathodes
US4526911A (en) * 1982-07-22 1985-07-02 Martin Marietta Aluminum Inc. Aluminum cell cathode coating composition
US4544457A (en) * 1982-05-10 1985-10-01 Eltech Systems Corporation Dimensionally stable drained aluminum electrowinning cathode method and apparatus
US4582553A (en) * 1984-02-03 1986-04-15 Commonwealth Aluminum Corporation Process for manufacture of refractory hard metal containing plates for aluminum cell cathodes
US4747924A (en) * 1984-10-03 1988-05-31 Sumitomo Light Metal Industries, Ltd. Apparatus for producing neodymium-iron alloy
US6258246B1 (en) * 1998-05-19 2001-07-10 Moltech Invent S.A. Aluminium electrowinning cell with sidewalls resistant to molten electrolyte
DE102010038669A1 (de) * 2010-07-29 2012-02-02 Sgl Carbon Se Kathodenblock für eine Aluminium-Elektrolysezelle und ein Verfahren zu seiner Herstellung
CN102691075A (zh) * 2012-06-27 2012-09-26 云南铝业股份有限公司 一种曲面阴极铝电解槽通电前挂阳极方法
WO2013068412A3 (de) * 2011-11-09 2013-10-24 Sgl Carbon Se Kathodenblock mit gewölbter und/oder gerundeter oberfläche
CN110475908A (zh) * 2017-03-31 2019-11-19 美铝美国公司 电解生产铝的系统和方法

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CH576005A5 (US06229276-20010508-P00022.png) * 1972-03-21 1976-05-31 Alusuisse

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3256173A (en) * 1960-10-28 1966-06-14 Alusuisse Electrolytic furnace with lined cathode pots for the production of aluminum
US3330756A (en) * 1951-05-04 1967-07-11 British Aluminum Company Ltd Current conducting elements
US3383294A (en) * 1965-01-15 1968-05-14 Wood Lyle Russell Process for production of misch metal and apparatus therefor
US3400061A (en) * 1963-11-21 1968-09-03 Kaiser Aluminium Chem Corp Electrolytic cell for production of aluminum and method of making the same

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3330756A (en) * 1951-05-04 1967-07-11 British Aluminum Company Ltd Current conducting elements
US3256173A (en) * 1960-10-28 1966-06-14 Alusuisse Electrolytic furnace with lined cathode pots for the production of aluminum
US3400061A (en) * 1963-11-21 1968-09-03 Kaiser Aluminium Chem Corp Electrolytic cell for production of aluminum and method of making the same
US3383294A (en) * 1965-01-15 1968-05-14 Wood Lyle Russell Process for production of misch metal and apparatus therefor

Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4308115A (en) * 1980-08-15 1981-12-29 Aluminum Company Of America Method of producing aluminum using graphite cathode coated with refractory hard metal
US4544457A (en) * 1982-05-10 1985-10-01 Eltech Systems Corporation Dimensionally stable drained aluminum electrowinning cathode method and apparatus
WO1984000565A1 (en) * 1982-07-22 1984-02-16 Martin Marietta Corp Aluminum cathode coating cure cycle
US4466995A (en) * 1982-07-22 1984-08-21 Martin Marietta Corporation Control of ledge formation in aluminum cell operation
US4466996A (en) * 1982-07-22 1984-08-21 Martin Marietta Corporation Aluminum cell cathode coating method
US4526911A (en) * 1982-07-22 1985-07-02 Martin Marietta Aluminum Inc. Aluminum cell cathode coating composition
US4481052A (en) * 1983-01-28 1984-11-06 Martin Marietta Corporation Method of making refractory hard metal containing tiles for aluminum cell cathodes
US4582553A (en) * 1984-02-03 1986-04-15 Commonwealth Aluminum Corporation Process for manufacture of refractory hard metal containing plates for aluminum cell cathodes
US4747924A (en) * 1984-10-03 1988-05-31 Sumitomo Light Metal Industries, Ltd. Apparatus for producing neodymium-iron alloy
US6258246B1 (en) * 1998-05-19 2001-07-10 Moltech Invent S.A. Aluminium electrowinning cell with sidewalls resistant to molten electrolyte
DE102010038669A1 (de) * 2010-07-29 2012-02-02 Sgl Carbon Se Kathodenblock für eine Aluminium-Elektrolysezelle und ein Verfahren zu seiner Herstellung
WO2013068412A3 (de) * 2011-11-09 2013-10-24 Sgl Carbon Se Kathodenblock mit gewölbter und/oder gerundeter oberfläche
CN103958740A (zh) * 2011-11-09 2014-07-30 西格里碳素欧洲公司 具有半球形和/或圆形表面的阴极块
CN102691075A (zh) * 2012-06-27 2012-09-26 云南铝业股份有限公司 一种曲面阴极铝电解槽通电前挂阳极方法
CN110475908A (zh) * 2017-03-31 2019-11-19 美铝美国公司 电解生产铝的系统和方法
CN110475908B (zh) * 2017-03-31 2022-10-14 美铝美国公司 电解生产铝的系统和方法

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NL129768C (US06229276-20010508-P00022.png)
GB1138522A (en) 1969-01-01
FR1466660A (fr) 1967-01-20
AT274400B (de) 1969-09-10
DE1533439A1 (de) 1970-01-02
ES321480A1 (es) 1967-04-01
CH456169A (fr) 1968-05-15
NL6600097A (US06229276-20010508-P00022.png) 1966-07-07
NO118766B (US06229276-20010508-P00022.png) 1970-02-09

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