EP0091914B1 - Cathode of a cell for the electrolysis of a melt, for the preparation of aluminium - Google Patents

Cathode of a cell for the electrolysis of a melt, for the preparation of aluminium Download PDF

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EP0091914B1
EP0091914B1 EP82902974A EP82902974A EP0091914B1 EP 0091914 B1 EP0091914 B1 EP 0091914B1 EP 82902974 A EP82902974 A EP 82902974A EP 82902974 A EP82902974 A EP 82902974A EP 0091914 B1 EP0091914 B1 EP 0091914B1
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solid cathode
cathode
solid
aluminum
open
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German (de)
French (fr)
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EP0091914A1 (en
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Tibor Kugler
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Alcan Holdings Switzerland AG
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Schweizerische Aluminium AG
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    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25CPROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/08Cell construction, e.g. bottoms, walls, cathodes

Definitions

  • the invention relates to a wettable solid-state cathode which can be used in a melt-flow electrolysis cell for the production of aluminum and comprises an aluminide of at least one transition metal of groups IV A, V A and VI A of the periodic system of the elements.
  • the electrolysis generally takes place in a temperature range of about 940 to 970 ° C.
  • the electrolyte becomes poor in aluminum oxide.
  • At a lower concentration of about 1 to 2% by weight of aluminum oxide in the electrolyte there is an anode effect, which results in a voltage increase of, for example, 4 to 4.5 V to 30 V and above.
  • the aluminum oxide concentration must be increased by adding new aluminum oxide (alumina).
  • cathodes made of titanium diboride, titanium carbide, pyrolytic graphite, boron carbide and other substances are proposed, mixtures which can be sintered together, for example, also being used.
  • the usual interpolar distance of approximately 5 cm can be reduced as far as the other parameters allow, for example circulation of the electrolyte in the interpolar gap and maintenance of the electrolysis temperature.
  • The. reduced interpolar distance causes a significant reduction in energy consumption and avoids the formation of non-uniformities in the thickness of the aluminum layer.
  • DE-OS 28 38 965 shows solid-state cathodes made of individually interchangeable elements, each with at least one power supply.
  • the interchangeable elements are made of two different materials from two mechanically rigidly interconnected parts that are resistant to thermal shock - an upper part protruding from the molten electrolyte into the separated aluminum and a lower part arranged exclusively in the liquid aluminum .
  • the upper part at least in the area of the surface, remains unchanged from aluminum-wettable material, while the lower part or its coating consists of an insulator material that is resistant to the liquid aluminum.
  • DE-OS 3045349 relates to an exchangeable wettable solid-state cathode, which consists of an aluminide from at least one metal from the group, formed from titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, without a binding phase made of metallic aluminum, consists.
  • the non-aluminum components of the aluminide thus belong to group IV A, V A and / or VI A of the periodic system of the elements.
  • the chemical and thermal resistance of the aluminides allows them to be used in both molten electrolytes and molten aluminum, although they are of limited solubility in the latter. However, this solubility drops sharply as the temperature drops.
  • the solubility of a metallic non-aluminum component of the aluminide in the liquid aluminum is approximately 1%.
  • the cathode elements are therefore alloyed until the deposited liquid aluminum is saturated with one or more of the transition metals in the aluminide.
  • the elements of the aluminides alloyed during the electrolysis process are recovered from the deposited metal by cooling it to approximately 700 ° C.
  • the crystallizing aluminide can be removed from the liquid metal by known means and used again for the production of cathode elements. This creates a material cycle with relatively low losses.
  • the inventor has set himself the task of creating solid-state cathodes based on aluminides with a service life that corresponds to one or more anode service lives, with the purchase price of the cathode and the handling costs being substantially reduced.
  • the solid cathode consists essentially of a support body and an open-pore structure, at least in the region of the working surface, impregnated with aluminum saturated with transition metal / s, which structure can be fed continuously from aluminide supplies.
  • the working surface is the surface which, when the cathode is inserted in the electrolysis cell, points in the direction of the anode and through which direct electrical current flows.
  • the Alumi nium ions reduced to elemental aluminum.
  • the working surfaces of the cathodes are therefore appropriately slightly inclined so that the deposited aluminum, which forms a film on the wettable cathode, can flow off.
  • the work surfaces of the corresponding anodes which, for. B. can consist of flammable carbon or non-flammable oxide ceramics are inclined accordingly. This tendency also has an advantageous effect here: the oxygen or C0 2 formed can escape better from the molten electrolyte.
  • the open-pore structure is anchored on the support body or part of it. If this support body consists of an electrically non-conductive material, the open-pore structure impregnated with aluminum saturated with transition metal / s must reach at least liquid metal when the solid-state cathode is inserted, so that the electrical current can flow through this impregnation alloy and, if appropriate, through the structure.
  • the carrier body therefore preferably consists at least partially of a material which is electrically conductive at 900 to 1000 ° C. and is resistant to the melt flow. In this case, the current can flow mainly through the support body. Apart from the electrical conductivity, it is essential that the material of the support body is cheap and easy to form. For these reasons, carbon is particularly well suited for the supporting body.
  • the solid-state cathodes are therefore preferably designed as individually replaceable elements which stand on the cell bottom. Damaged elements can be quickly replaced.
  • the risk of damage can be significantly reduced if the solid-state cathodes are designed as elements which float in the melt flow and have lateral spaces.
  • the molten electrolyte has a density of 2.1 g / cm 3
  • the liquid aluminum has a density of 2.3 g / cm 3 .
  • the density of a floating cathode must be between these two values.
  • the density of the cathode material used is too low, appropriate pieces of iron can be used, but these must be evenly distributed and completely encased by the cathode material.
  • the weight of the iron pieces to be used is calculated so that the apparent density of the entire solid cathode is between 2.1 and 2.3 g / cm 3 .
  • the density of the cathode material used is too high, correspondingly closed cavities are formed in the cathode material.
  • Solid cathodes with the correct density float like rafts in the liquid aluminum they are preferably held at the desired distance from each other and from the cell shelf by appropriately trained spacers.
  • the open-pore structure on the one hand must be sufficiently permeable to the aluminum saturated with transition metal / s, but on the other hand it must not allow it to flow out without resistance.
  • These requirements can be formed by sintered fine-grained granules or preferably by a fiber structure in the form of a felt or a woven fabric.
  • the fibers are a few micrometers thick and are preferably made of carbon.
  • the open-pore structure impregnated with transition metal / s saturated aluminum is continuously fed from cavities arranged in the supporting body, into which the open-pore structure projects, or from another location the open-pore structure on which solid aluminide can be held.
  • the aluminum produced during the electrolysis process flows along the diagonally arranged open-pore structure, mixes with the impregnating aluminum saturated with transition metal (s) and would gradually reduce the transition metal content in it so that the open-pore structure would be attacked and gradually dissolved. This is prevented, however, by the open-pore structure being able to be fed continuously from aluminide stocks.
  • the transition metal extracted from the saturated aluminum becomes constantly replaced by new ones, so that the open-pore structure remains permanently impregnated with aluminum saturated with transition metal / s.
  • the open-pore structure in particular a 1-5 mm thick felt made of carbon fibers, is coated with a thin, well-adhering layer of titanium carbide or titanium diboride.
  • the layers that are preferably less than 0.4 ⁇ m thick are produced, for example, by CVD (Chemical Vapor Deposition). If the aluminum impregnating the felt is permanently saturated with titanium, the wettable coating is not dissolved, which can increase the life of the felt.
  • a felt consisting of coated carbon fibers has the further advantage that if the coating is faulty, the entire work surface will not become unusable, but only individual fibers will be dissolved prematurely.
  • the main advantage of the invention is thus that simple ceramic means can be used to replace expensive ceramic moldings by supporting bodies made of a cheap, easily moldable material with an open-pore surface structure impregnated with aluminum saturated with transition metal / s.
  • the solid-state cathodes according to the invention are also particularly suitable for converting existing aluminum melt flow electrolysis cells.
  • solid cathodes 10 and anode blocks 12 arranged in pairs form the electrode units of the electrolytic cell.
  • the solid-state cathode 10 consists of a shaped support body 14 made of carbon and a felt 16 made of carbon fibers coated with titanium carbide and fastened on the working surface facing the anode body 12. Rags of this approximately 4 mm thick felt 16 extend into a cavity 18 in the carrier body 14, which is filled with a titanium aluminide 19 which is dough at electrolysis temperature and which consists, for example, of 80% by weight aluminum and 20% by weight titanium.
  • the feet 20 of the support body 14 stand in correspondingly shaped recesses in the carbon base 22 of the electrolytic cell.
  • the density of the solid-state cathode 10 must therefore be greater than that of the liquid aluminum 24.
  • the felt 16 is impregnated with aluminum saturated with titanium, which system forms the cathode.
  • the deposited aluminum mixes with the titanium-saturated aluminum in the felt and flows to the center of the electrode element, depending on the inclination of the working surface of the solid-state cathode.
  • the felt 16 acts like a wick in the oil, liquid alloy is drawn from the cavity 18 with the pasty titanium aluminide, thus replacing the current losses. Without this replacement of the spent titanium, the deposited aluminum would dissolve the titanium carbide coating on the carbon fibers and render the cathode surface non-wettable.
  • a solid-state cathode 10 and an anode block 12 form a pair of electrodes.
  • the support body 14 consists of an insulating material, for example of highly sintered aluminum oxide, aluminum oxide-containing ceramics, silicon carbide or silicon nitride-bonded silicon carbide.
  • the felt 16 In order to ensure that the electrical direct current flows away, the felt 16 always extends along as far as possible all side surfaces of the support body 14 into the liquid aluminum 24.
  • the cavity 18 is trough-shaped, with a relatively large opening, and is filled with solid titanium aluminide granules which consist, for example, of 55% by weight of aluminum and 45% by weight of titanium.
  • the felt 16 does not reach down into the cavity 18;
  • the aluminum impregnating the felt 16 with titanium is saturated by the convection of the molten electrolyte 26.
  • the deposited aluminum flows through an opening 28 in the support body 14.
  • the apparent density of the entire solid cathode, at working temperature, must be between the density of the molten electrolyte and the molten aluminum. In the case of supporting bodies 14 made of carbon, this is achieved by inserting iron pieces 30 into closed cavities, for example in the form of a ring.
  • solid cathodes 10 attached to a cathodic suspension system 36 and anode bodies 12 attached to an anodic suspension system 38 are alternatively arranged.
  • the felt 16 is fed by means of a sleeve 34 which is placed over the support rods of the support body 14 and which is made of a solid aluminum minid exist.
  • the cathodes and anodes can be shifted to the right in the direction of the arrow.
  • a mechanism known per se ensures that the same interpolar distances exist between the anode and cathode after each shift.
  • anodes 12 or cathodes 14 arranged on the left have to be displaced more than those arranged on the right.
  • Burned-off anodes, along with the cathode, are removed on the right. A sufficiently large space has now been created on the left side so that the cathode can be reinserted together with a new anode.

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  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
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  • Organic Chemistry (AREA)
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Description

Die Erfindung bezieht sich auf eine benetzbare, in einer Schmelzflußelektrolysezelle zur Herstellung von Aluminium einsetzbare Festkörperkathode mit einem Aluminid von wenigstens einem Übergangsmetall der Gruppen IV A, V A und VI A des periodischen Systems der Elemente.The invention relates to a wettable solid-state cathode which can be used in a melt-flow electrolysis cell for the production of aluminum and comprises an aluminide of at least one transition metal of groups IV A, V A and VI A of the periodic system of the elements.

Für die Gewinnung von Aluminium durch Elektrolyse von Aluminiumoxid wird dieses in einer Fluoridschmelze gelöst, die zum größten Teil aus Kryolith besteht. Das kathodisch abgeschiedene Aluminium sammelt sich unter der Fluoridschmelze auf dem Kohleboden der Zelle, wobei die Oberfläche des flüssigen Aluminiums die Kathode bildet. Am Anodenbalken befestigte, bei konventionellen Verfahren aus amorphem Kohlenstoff bestehende Anoden tauchen von oben in die Schmelze ein. An den Kohleanoden entsteht durch die elektrolytische Zersetzung des Aluminiumoxids Sauerstoff, der sich mit dem Kohlenstoff der Anoden zu C02 und CO verbindet.For the production of aluminum by electrolysis of aluminum oxide, this is dissolved in a fluoride melt, which consists largely of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the carbon bottom of the cell, the surface of the liquid aluminum forming the cathode. Anodes attached to the anode bar and made of amorphous carbon in conventional processes are immersed in the melt from above. At the carbon anodes, the electrolytic decomposition of the aluminum oxide produces oxygen, which combines with the carbon of the anodes to form CO 2 and CO.

Die Elektrolyse findet im allgemeinen in einem Temperaturbereich von etwa 940 bis 970° C statt. Im Laufe der Elektrolyse verarmt der Elektrolyt an Aluminiumoxid. Bei einer unteren Konzentration von etwa 1 bis 2 Gew.-% Aluminiumoxid im Elektrolyten kommt es zum Anodeneffekt, der sich in einer Spannungserhöhung von beispielsweise 4 bis 4,5 V auf 30 V und darüber auswirkt. Spätestens dann muß die Aluminiumoxidkonzentration durch Zugabe von neuem Aluminiumoxid (Tonerde) angehoben werden.The electrolysis generally takes place in a temperature range of about 940 to 970 ° C. In the course of electrolysis, the electrolyte becomes poor in aluminum oxide. At a lower concentration of about 1 to 2% by weight of aluminum oxide in the electrolyte, there is an anode effect, which results in a voltage increase of, for example, 4 to 4.5 V to 30 V and above. Then at the latest the aluminum oxide concentration must be increased by adding new aluminum oxide (alumina).

Es ist bekannt, bei der Schmelzflußelektrolyse zur Herstellung von Aluminium benetzbare Festkörperkathoden einzusetzen. Dabei werden Kathoden aus Titandiborid, Titankarbid, pyrolytischem Graphit, Borkarbid und weiteren Subtanzen vorgeschlagen, wobei auch Gemische, die beispielsweise zusammengesintert sein können, eingesetzt werden.It is known to use wettable solid-state cathodes in the melt flow electrolysis for the production of aluminum. Here, cathodes made of titanium diboride, titanium carbide, pyrolytic graphite, boron carbide and other substances are proposed, mixtures which can be sintered together, for example, also being used.

Bei benetzbaren Kathoden kann die übliche Interpolardistanz von ca. 5 cm so weit herabgesetzt werden, als es die übrigen Parameter, beispielsweise Zirkulation des Elektrolyten im Interpolarspalt und Aufrechterhalten der Elektrolysetemperatur, erlauben. Die. reduzierte Interpolardistanz bewirkt einen in bedeutendem Maße herabgesetzten Energieverbrauch und vermeidet die Ausbildung von Ungleichmäßigkeiten in bezug auf die Dicke der Aluminiumschicht.In the case of wettable cathodes, the usual interpolar distance of approximately 5 cm can be reduced as far as the other parameters allow, for example circulation of the electrolyte in the interpolar gap and maintenance of the electrolysis temperature. The. reduced interpolar distance causes a significant reduction in energy consumption and avoids the formation of non-uniformities in the thickness of the aluminum layer.

Im Gegensatz zu im Kohleboden der Zelle fest verankerten benetzbaren Kathoden zeigt die DE-OS 28 38 965 Festkörperkathoden aus einzeln auswechselbaren Elementen mit je mindestens einer Stromzuführung. In einer Weiterentwicklung nach der DE-OS 3024 172 werden die auswechselbaren Elemente aus zwei mechanisch starr miteinander verbundenen, gegen Wärmeschocks widerstandsfähigen Teilen - einem vom schmelzflüssigen Elektrolyten in das abgeschiedene Aluminium hineinragenden oberen und einem ausschließlich im flüssigen Aluminium angeordneten unteren Teil - aus verschiedenen Materialien hergestellt. Der obere Teil besteht, mindestens im Bereich der Oberfläche, unverändert aus mit Aluminium benetzbarem Material, während der untere Teil bzw. dessen Beschichtung aus einem gegen das flüssige Aluminium beständigen Isolatormaterial besteht.In contrast to wettable cathodes firmly anchored in the carbon bottom of the cell, DE-OS 28 38 965 shows solid-state cathodes made of individually interchangeable elements, each with at least one power supply. In a further development according to DE-OS 3024 172, the interchangeable elements are made of two different materials from two mechanically rigidly interconnected parts that are resistant to thermal shock - an upper part protruding from the molten electrolyte into the separated aluminum and a lower part arranged exclusively in the liquid aluminum . The upper part, at least in the area of the surface, remains unchanged from aluminum-wettable material, while the lower part or its coating consists of an insulator material that is resistant to the liquid aluminum.

Die DE-OS 3045349 hat eine auswechselbare benetzbare Festkörperkathode zum Gegenstand, welche aus einem Aluminid von mindestens einem Metall der Gruppe, gebildet aus Titan, Zirkon, Hafnium, Vanadium, Niob, Tantal, Chrom, Molybdän und Wolfram, ohne Bindephase aus metallischem Aluminium, besteht. Die Nicht-Aluminiumkomponenten des Aluminids gehören also zur Gruppe IV A, V A und/oder VI A des periodischen Systems der Elemente.DE-OS 3045349 relates to an exchangeable wettable solid-state cathode, which consists of an aluminide from at least one metal from the group, formed from titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, without a binding phase made of metallic aluminum, consists. The non-aluminum components of the aluminide thus belong to group IV A, V A and / or VI A of the periodic system of the elements.

Die chemische und thermische Widerstandsfähigkeit der Aluminide erlaubt, daß sie sowohl im schmelzflüssigen Elektrolyten als auch in geschmolzenem Aluminium eingesetzt werden können, obwohl sie in letzterem begrenzt löslich sind. Diese Löslichkeit fällt jedoch bei sinkender Temperatur steil ab.The chemical and thermal resistance of the aluminides allows them to be used in both molten electrolytes and molten aluminum, although they are of limited solubility in the latter. However, this solubility drops sharply as the temperature drops.

Bei Arbeitstemperatur der Aluminiumelektrolysezelle, welche im Bereich von 900 bis 1000°C liegt, beträgt die Löslichkeit einer metallischen Nicht-Aluminiumkomponente des Aluminids im flüssigen Aluminium ungefähr 1%. Die Kathodenelemente werden also ablegiert, bis das abgeschiedene flüssige Aluminium mit einer oder mehreren der im Aluminid befindlichen Übergangsmetalle gesättigt ist.At the working temperature of the aluminum electrolysis cell, which is in the range from 900 to 1000 ° C., the solubility of a metallic non-aluminum component of the aluminide in the liquid aluminum is approximately 1%. The cathode elements are therefore alloyed until the deposited liquid aluminum is saturated with one or more of the transition metals in the aluminide.

Die Elemente der während des Elektrolyseprozesses ablegierten Aluminide werden aus dem abgeschiedenen Metall zurückgewonnen, indem dieses auf ungefähr 700°C abgekühlt wird. Das auskristallisierende Aluminid kann mit bekannten Mitteln aus dem flüssigen Metall entfernt und wieder zur Herstellung von Kathodenelementen eingesetzt werden. Damit entsteht ein Materialkreislauf mit verhältnismäßig geringen Verlusten.The elements of the aluminides alloyed during the electrolysis process are recovered from the deposited metal by cooling it to approximately 700 ° C. The crystallizing aluminide can be removed from the liquid metal by known means and used again for the production of cathode elements. This creates a material cycle with relatively low losses.

Der Erfinder hat sich die Aufgabe gestellt, auf der Basis von Aluminiden arbeitende Festkörperkathoden mit einer Lebensdauer, die einer oder mehreren Anodenstandzeiten entspricht, zu schaffen, wobei der Anschaffungspreis der Kathode und die Handhabungskosten wesentlich reduziert werden sollen.The inventor has set himself the task of creating solid-state cathodes based on aluminides with a service life that corresponds to one or more anode service lives, with the purchase price of the cathode and the handling costs being substantially reduced.

Die Aufgabe wird erfindungsgemäß dadurch gelöst, daß die Festkörperkathode im wesentlichen aus einem Tragkörper und einer mindestens im Bereich der Arbeitsoberfläche befindlichen offenporigen mit an Übergangsmetall/en gesättigtem Aluminium imprägnierten Struktur, welche aus Aluminidvorräten kontinuierlich speisbar ist, besteht.The object is achieved in that the solid cathode consists essentially of a support body and an open-pore structure, at least in the region of the working surface, impregnated with aluminum saturated with transition metal / s, which structure can be fed continuously from aluminide supplies.

Als Arbeitsfläche wird diejenige Fläche bezeichnet, welche bei in die ELektrolysezelle eingesetzter Kathode in Richtung zur Anode weist und vom elektrischen Gleichstrom durchflossen wird. Auf dieser Arbeitsfläche werden die Aluminiumionen zu elementarem Aluminium reduziert. Die Arbeitsflächen der Kathoden sind deshalb zweckmäßig leicht geneigt, damit das abgeschiedene Aluminium, welches auf der benetzbaren Kathode einen Film bildet, abfließen kann.The working surface is the surface which, when the cathode is inserted in the electrolysis cell, points in the direction of the anode and through which direct electrical current flows. The Alumi nium ions reduced to elemental aluminum. The working surfaces of the cathodes are therefore appropriately slightly inclined so that the deposited aluminum, which forms a film on the wettable cathode, can flow off.

Die Arbeitsflächen der korrespondierenden Anoden, welche z. B. aus brennbarem Kohlenstoff oder unbrennbarer Oxidkeramik bestehen können, sind entsprechend geneigt. Auch hier wirkt sich diese Neigung vorteilhaft aus: der entstehende Sauerstoff bzw. das C02 kann besser aus dem schmelzflüssigen Elektrolyten entweichen.The work surfaces of the corresponding anodes, which, for. B. can consist of flammable carbon or non-flammable oxide ceramics are inclined accordingly. This tendency also has an advantageous effect here: the oxygen or C0 2 formed can escape better from the molten electrolyte.

Die offenporige Struktur ist auf dem Tragkörper verankert oder Bestandteil davon. Falls dieser Tragkörper aus einem elektrisch nicht leitenden Material besteht, muß die offenporige, mit an Übergangsmetall/en gesättigtem Aluminium imprägnierte Struktur bei eingesetzter Festkörperkathode bis mindestens zum flüssigen Metall reichen, damit der elektrische Strom durch diese Imprägnierlegierung und gegebenenfalls durch die Struktur fließen kann. Der Tragkörper besteht deshalb vorzugsweise mindestens teilweise aus einem bei 900 bis 1000°C elektrisch gut leitenden und gegen den Schmelzfluß beständigen Material. In diesem Fall kann der Strom hauptsächlich durch den Tragkörper fließen. Abgesehen von der elektrischen Leitfähigkeit ist wesentlich, daß das Material des Tragkörpers billig und gut formbar ist. Aus diesen Gründen ist Kohlenstoff für den Tragkörper besonders gut geeignet.The open-pore structure is anchored on the support body or part of it. If this support body consists of an electrically non-conductive material, the open-pore structure impregnated with aluminum saturated with transition metal / s must reach at least liquid metal when the solid-state cathode is inserted, so that the electrical current can flow through this impregnation alloy and, if appropriate, through the structure. The carrier body therefore preferably consists at least partially of a material which is electrically conductive at 900 to 1000 ° C. and is resistant to the melt flow. In this case, the current can flow mainly through the support body. Apart from the electrical conductivity, it is essential that the material of the support body is cheap and easy to form. For these reasons, carbon is particularly well suited for the supporting body.

Bei Manipulationen an der Traverse bzw. dem Anodenbalken und insbesondere beim Auswechseln der Anode ist die Kathode stets der Gefahr mechanischer Beschädigung ausgesetzt. Vorzugsweise sind deshalb die Festkörperkathoden als einzeln auswechselbare Elemente ausgebildet, welche auf dem Zellenboden stehen. Beschädigte Elemente können so rasch ausgewechselt werden.If the crossbeam or the anode bar is tampered with and in particular when the anode is replaced, the cathode is always exposed to the risk of mechanical damage. The solid-state cathodes are therefore preferably designed as individually replaceable elements which stand on the cell bottom. Damaged elements can be quickly replaced.

Die Beschädigungsgefahr kann wesentlich herabgesetzt werden, wenn die Festkörperkathoden als im Schmelzfluß schwimmende Elemente mit seitlichen Zwischenräumen ausgebildet sind. Bei einer Temperatur von 900 bis 1000° C hat der schmelzflüssige Elektrolyt eine Dichte von 2,1 g/cm3, das flüssige Aluminium eine solche von 2,3 g/cm3. Die Dichte einer schwimmenden Kathode muß zwischen diesen beiden Werten liegen.The risk of damage can be significantly reduced if the solid-state cathodes are designed as elements which float in the melt flow and have lateral spaces. At a temperature of 900 to 1000 ° C, the molten electrolyte has a density of 2.1 g / cm 3 , the liquid aluminum has a density of 2.3 g / cm 3 . The density of a floating cathode must be between these two values.

Wenn die Dichte des eingesetzten Kathodenmaterials zu gering ist, können entsprechende Eisenstücke eingesetzt werden, die jedoch gleichmäßig verteilt und vom Kathodenmaterial vollständig umhüllt sein müssen. Das Gewicht der einzusetzenden Eisenstücke wird so berechnet, daß die scheinbare Dichte der gesamten Festkörperkathode zwischen 2,1 und 2,3 g/cm3 liegt.If the density of the cathode material used is too low, appropriate pieces of iron can be used, but these must be evenly distributed and completely encased by the cathode material. The weight of the iron pieces to be used is calculated so that the apparent density of the entire solid cathode is between 2.1 and 2.3 g / cm 3 .

Wenn die Dichte des eingesetzten Kathodenmaterials dagegen zu groß ist, werden im Kathodenmaterial entsprechend geschlossene Hohlräume ausgebildet.If, on the other hand, the density of the cathode material used is too high, correspondingly closed cavities are formed in the cathode material.

Festkörperkathoden mit der richtigen Dichte schwimmen wie Floße im flüssigen Aluminium, sie werden vorzugsweise von entsprechend ausgebildeten Distanzhaltern im gewünschten Abstand voneinander und vom Zellenbord gehalten.Solid cathodes with the correct density float like rafts in the liquid aluminum, they are preferably held at the desired distance from each other and from the cell shelf by appropriately trained spacers.

Wird bei schwimmenden Kathoden durch eine Fehlmanipulation die Anode gegen die Festkörperkathode gedrückt, so kann diese ausweichen und erleidet keinerlei Beschädigung.If the anode is pressed against the solid-state cathode due to incorrect manipulation in the case of floating cathodes, this can deflect and suffer no damage.

Die offenporige Struktur muß für das an Übergangsmetall/en gesättigte Aluminium einerseits genügend durchlässig sein, darf es aber andererseits nicht widerstandslos ausfließen lassen.The open-pore structure on the one hand must be sufficiently permeable to the aluminum saturated with transition metal / s, but on the other hand it must not allow it to flow out without resistance.

Je nach dem Material der offenporigen Struk- tur bzw. deren Beschichtung muß hier unter Berücksichtigung von Kapillar- und Oberflächenkräften die optimale Lösung gesucht werden.Depending on the material of the open-pore structural t ur or its coating has the optimal solution searched for in consideration of capillary and surface forces here.

Diese Anforderungen können durch zusammengesinterte feinkörnige Granalien oder vorzugsweise durch eine Faserstruktur in Form eines Filzes oder eines Gewebes ausgebildet. Die Fasern sind einige Mikrometer dick und bestehen vorzugsweise aus Kohlenstoff.These requirements can be formed by sintered fine-grained granules or preferably by a fiber structure in the form of a felt or a woven fabric. The fibers are a few micrometers thick and are preferably made of carbon.

Die kontinuierliche Speisung der offenporigen, mit an Übergangsmetall/en gesättigtem Aluminium imprägnierten Struktur erfolgt, je nach geometrischer Form der Festkörperkathode und der chemischen Zusammensetzung des verwendeten Aluminids, aus im Tragkörper angeordneten Hohlräumen, in welche die offenporige Struktur hineinragt, oder von einer anderen Stelle auf der offenporigen Struktur an welcher festes Aluminid gehaltert werden kann.Depending on the geometric shape of the solid cathode and the chemical composition of the aluminide used, the open-pore structure impregnated with transition metal / s saturated aluminum is continuously fed from cavities arranged in the supporting body, into which the open-pore structure projects, or from another location the open-pore structure on which solid aluminide can be held.

Aus wirtschaftlichen Gründen und wegen der wissenschaftlich guten Erforschung werden vorzugsweise Titanaluminide eingesetzt. Je nach dem prozentualen Titangehalt haben diese Aluminide bei der Elektrolysetemperatur im Bereich von 900 bis 1000°C verschiedene Aggregatzustände:

  • - Aluminide mit weniger als 37,2 Gew.-% Titan sind bei Elektrolysetemperatur zähflüssig bis teigig. Diese können also nicht als feste Formkörper, sondern nur als Schüttkathode in Hohlräumen des Tragkörpers eingesetzt werden.
  • - Aluminide mit einem Titangehalt oberhalb 37,2 (bis 63) Gew.-% Titan dagegen können auch als feste Formkörper mit der offenporigen Struktur in Verbindung gebracht werden.
For economic reasons and because of the scientifically good research, titanium aluminides are preferably used. Depending on the percentage of titanium, these aluminides have different physical states at the electrolysis temperature in the range of 900 to 1000 ° C:
  • - Aluminides with less than 37.2 wt .-% titanium are viscous to pasty at electrolysis temperature. These can therefore not be used as solid shaped bodies, but only as a bulk cathode in cavities of the supporting body.
  • - In contrast, aluminides with a titanium content above 37.2 (up to 63)% by weight of titanium can also be associated with the open-pore structure as solid moldings.

Das während des Elektrolyseprozesses erzeugte Aluminium fließt entlang der schräg angeordneten offenporigen Struktur, mischt sich dabei mit dem imprägnierenden, an Übergangsmetall/en gesättigten Aluminium und würde in diesem allmählich den Übergangsmetallgehalt derart weit herabsetzen, daß die offenporige Struktur angegriffen und allmählich aufgelöst würde. Dies wird jedoch verhindert, indem die offenporige Struktur kontinuierlich aus Aluminidvorräten speisbar ist. Das dem gesättigten Aluminium entzogene Übergangsmetall wird laufend durch neues ersetzt, so daß die offenporige Struktur dauernd mit an Übergangsmetall/ en gesättigtem Aluminium imprägniert bleibt.The aluminum produced during the electrolysis process flows along the diagonally arranged open-pore structure, mixes with the impregnating aluminum saturated with transition metal (s) and would gradually reduce the transition metal content in it so that the open-pore structure would be attacked and gradually dissolved. This is prevented, however, by the open-pore structure being able to be fed continuously from aluminide stocks. The transition metal extracted from the saturated aluminum becomes constantly replaced by new ones, so that the open-pore structure remains permanently impregnated with aluminum saturated with transition metal / s.

Beim bevorzugt eingesetzten Titanaluminid wird die offenporige Struktur, insbesondere ein 1-5 mm dicker Filz aus Kohlenstoffasern, mit einer dünnen, gut haftenden Schicht aus Titankarbid oder Titandiborid beschichtet. Die bevorzugt weniger als 0,4 um dicken Schichten werden beispielsweise durch CVD (Chemical Vapor Deposition) hergestellt. Wenn das den Filz imprägnierende Aluminium dauernd mit Titan gesättigt ist, wird die benetzbare Beschichtung nicht aufgelöst, wodurch die Lebensdauer des Filzes vervielfacht werden kann.In the case of the preferably used titanium aluminide, the open-pore structure, in particular a 1-5 mm thick felt made of carbon fibers, is coated with a thin, well-adhering layer of titanium carbide or titanium diboride. The layers that are preferably less than 0.4 μm thick are produced, for example, by CVD (Chemical Vapor Deposition). If the aluminum impregnating the felt is permanently saturated with titanium, the wettable coating is not dissolved, which can increase the life of the felt.

Ein aus beschichteten Kohlefasern bestehender Filz weist weiter den Vorteil auf, daß bei fehlerhafter Beschichtung nicht die ganze Arbeitsoberfläche unbrauchbar wird, sondern nur einzelne Fasern vorzeitig aufgelöst werden.A felt consisting of coated carbon fibers has the further advantage that if the coating is faulty, the entire work surface will not become unusable, but only individual fibers will be dissolved prematurely.

Der wesentliche Vorteil der Erfindung besteht also darin, daß mit einfachen Mitteln teure keramische Formkörper durch Tragkörper aus einem billigen, gut formbaren Material mit einer offenporigen, mit an Übergangsmetall/en gesättigtem Aluminium imprägnierten Oberflächenstruktur ersetzt werden können.The main advantage of the invention is thus that simple ceramic means can be used to replace expensive ceramic moldings by supporting bodies made of a cheap, easily moldable material with an open-pore surface structure impregnated with aluminum saturated with transition metal / s.

Die erfindungsgemäßen Festkörperkathoden sind insbesondere auch für das Umrüsten von bestehenden Aluminiumschmelzflußelektrolysezellen geeignet.The solid-state cathodes according to the invention are also particularly suitable for converting existing aluminum melt flow electrolysis cells.

Die Erfindung wird anhand der in der Zeichnung dargestellten Ausführungsbeispiele näher erläutert. Die schematischen teilweisen Vertikalschnitte aus Elektrolysezellen zeigen in

  • - Fig. 1 eine Festkörperkathode mit leitendem Tragkörper und entsprechend ausgebildeter Anode,
  • - Fig. 2 eine Festkörperkathode mit einem Tragkörper aus elektrisch isolierendem Material und entsprechend ausgebildeter Anode,
  • - Fig. 3 im schmelzflüssigen Aluminium schwimmende Festkörperkathoden aus elektrisch leitfähigem Material und entsprechend ausgebildeter Anode, und
  • - Fig. 4 alternativ angeordnete Festkörperkathoden aus elektrisch leitfähigem Material und entsprechend ausgebildete Anoden.
The invention is explained in more detail using the exemplary embodiments shown in the drawing. The schematic partial vertical sections from electrolysis cells show in
  • 1 shows a solid-state cathode with a conductive support body and a correspondingly designed anode,
  • 2 shows a solid-state cathode with a supporting body made of electrically insulating material and a correspondingly designed anode,
  • - Fig. 3 in the molten aluminum floating solid cathodes made of electrically conductive material and appropriately trained anode, and
  • - Fig. 4 alternatively arranged solid cathodes made of electrically conductive material and appropriately designed anodes.

Nach der in Fig. 1 dargestellten Ausführungsform bilden paarweise angeordnete Festkörperkathoden 10 und Anodenblöcke 12 die Elektrodeneinheiten der Elektrolysezelle. Die Festkörperkathode 10 besteht aus einem geformten Tragkörper 14 aus Kohlenstoff und einem auf der dem Anodenkörper 12 zugewandten Arbeitsfläche befestigten Filz 16 aus mit Titankarbid beschichteten Kohlenstoffasern. Lappen dieses etwa 4 mm dicken Filzes 16 reichen in einen Hohlraum 18 im Tragkörper 14, welcher mit einem bei Elektrolysetemperatur teigigen Titanaluminid 19, das aus beispielsweise 80 Gew.-% Aluminium und 20 Gew.-% Titan besteht, gefüllt ist.According to the embodiment shown in FIG. 1, solid cathodes 10 and anode blocks 12 arranged in pairs form the electrode units of the electrolytic cell. The solid-state cathode 10 consists of a shaped support body 14 made of carbon and a felt 16 made of carbon fibers coated with titanium carbide and fastened on the working surface facing the anode body 12. Rags of this approximately 4 mm thick felt 16 extend into a cavity 18 in the carrier body 14, which is filled with a titanium aluminide 19 which is dough at electrolysis temperature and which consists, for example, of 80% by weight aluminum and 20% by weight titanium.

Die Füße 20 des Tragkörpers 14 stehen in entsprechend geformten Ausnehmungen des Kohlenstoffbodens 22 der Elektrolysezelle. Die Dichte der Festkörperkathode 10 muß also größer als diejenige des flüssigen Aluminiums 24 sein.The feet 20 of the support body 14 stand in correspondingly shaped recesses in the carbon base 22 of the electrolytic cell. The density of the solid-state cathode 10 must therefore be greater than that of the liquid aluminum 24.

Während des Elektrolyseprozesses wird auf dem mit an Titan gesättigten Aluminium imprägnierten Filz 16, welches System die Kathode bildet, Aluminium abgeschieden. Das abgeschiedene Aluminium mischt sich mit dem an Titan gesättigten Aluminium im Filz und fließt, entsprechend der Neigung der Arbeitsfläche der Festkörperkathode, zur Mitte des Elektrodenelementes. Der Filz 16 wirkt wie ein Docht im Öl, aus dem Hohlraum 18 mit dem teigigen Titanaluminid wird flüssige Legierung nachgezogen und so die laufenden Verluste ersetzt. Ohne diesen Ersatz des verbrauchten Titans würde das abgeschiedene Aluminium die Titankarbidbeschichtung auf den Kohlenstoffasern auflösen und die Kathodenoberfläche unbenetzbar machen.During the electrolysis process, aluminum is deposited on the felt 16 impregnated with aluminum saturated with titanium, which system forms the cathode. The deposited aluminum mixes with the titanium-saturated aluminum in the felt and flows to the center of the electrode element, depending on the inclination of the working surface of the solid-state cathode. The felt 16 acts like a wick in the oil, liquid alloy is drawn from the cavity 18 with the pasty titanium aluminide, thus replacing the current losses. Without this replacement of the spent titanium, the deposited aluminum would dissolve the titanium carbide coating on the carbon fibers and render the cathode surface non-wettable.

Durch die verhältnismäßig kleine Öffnung des Hohlraumes 18 kann nur wenig des zirkulierenden schmelzflüssigen Elektrolyten 26 eintreten, die Speisung mittels Konvektion ist also klein.Due to the relatively small opening of the cavity 18, only a little of the circulating molten electrolyte 26 can enter, so the supply by means of convection is small.

In Fig. 2 bilden eine Festkörperkathode 10 und ein Anodenblock 12 ein Elektrodenpaar. Der Tragkörper 14 besteht aus einem isolierenden Material, beispielsweise aus hochgesintertem Aluminiumoxid, aluminiumoxidhaltigen Keramiken, Siliziumkarbid oder siliziumnitridgebundenem Siliziumkarbid. Damit der Abfluß des elektrischen Gleichstromes gewährleistet ist, reicht der Filz 16 entlang möglichst aller Seitenflächen des Tragkörpers 14 stets bis in das flüssige Aluminium 24 hinein. Der Hohlraum 18 ist trogförmig, mit verhältnismäßig großer Öffnung, ausgebildet und mit festen Titanaluminidgranalien gefüllt, welche beispielsweise aus 55 Gew.-% Aluminium und 45 Gew.-% Titan bestehen.2, a solid-state cathode 10 and an anode block 12 form a pair of electrodes. The support body 14 consists of an insulating material, for example of highly sintered aluminum oxide, aluminum oxide-containing ceramics, silicon carbide or silicon nitride-bonded silicon carbide. In order to ensure that the electrical direct current flows away, the felt 16 always extends along as far as possible all side surfaces of the support body 14 into the liquid aluminum 24. The cavity 18 is trough-shaped, with a relatively large opening, and is filled with solid titanium aluminide granules which consist, for example, of 55% by weight of aluminum and 45% by weight of titanium.

Der Filz 16 reicht dagegen nicht in den Hohlraum 18 hinunter; die Sättigung des den Filz 16 imprägnierenden Aluminiums mit Titan erfolgt durch die Konvektion des schmelzflüssigen Elektrolyten 26.In contrast, the felt 16 does not reach down into the cavity 18; The aluminum impregnating the felt 16 with titanium is saturated by the convection of the molten electrolyte 26.

Das abgeschiedene Aluminium fließt durch eine Öffnung 28 im Tragkörper 14 ab.The deposited aluminum flows through an opening 28 in the support body 14.

Die in Fig.3 dargestellten schwimmenden Festkörperkathoden 10 füllen, an die Anodenkörper 12 angepaßt, die gesamte Elektrolysewanne, indem ihre umlaufend ausgebildeten Distanzhalter 32 satt aneinander liegen. Die scheinbare Dichte der gesamten Festkörperkathode muß, bei Arbeitstemperatur, zwischen der Dichte des schmelzflüssigen Elektrolyten und des geschmolzenen Aluminiums liegen. Dies wird bei Tragkörpern 14 aus Kohlenstoff durch die Einlage von Eisenstücken 30 in geschlossene Hohlräume, beispielsweise in Form eines Ringes, erreicht.The floating solid-state cathodes 10 shown in FIG. 3, adapted to the anode bodies 12, fill the entire electrolysis bath by their circumferentially designed spacers 32 lying snugly against one another. The apparent density of the entire solid cathode, at working temperature, must be between the density of the molten electrolyte and the molten aluminum. In the case of supporting bodies 14 made of carbon, this is achieved by inserting iron pieces 30 into closed cavities, for example in the form of a ring.

In Fig. 4 sind an einem kathodischen Aufhängesystem 36 befestigte Festkörperkathoden 10 und an einem anodischen Aufhängesystem 38 befestigte Anodenkörper 12 alternativ angeordnet. Die Speisung des Filzes 16 erfolgt durch über die Tragstangen der Tragkörper 14 gestülpte Manschette 34, welche aus einem festen Aluminid bestehen.4, solid cathodes 10 attached to a cathodic suspension system 36 and anode bodies 12 attached to an anodic suspension system 38 are alternatively arranged. The felt 16 is fed by means of a sleeve 34 which is placed over the support rods of the support body 14 and which is made of a solid aluminum minid exist.

Falls die Anodenkörper 12 aus Kohlenstoff bestehen, also abbrennen, können Kathoden und Anoden in Pfeilrichtung nach rechts verschoben werden. Ein an sich bekannter Mechanismus sorgt dafür, daß nach jedem Verschieben überall die gleichen Interpolardistanzen zwischen Anode und Kathode bestehen.If the anode bodies 12 consist of carbon, ie burn off, the cathodes and anodes can be shifted to the right in the direction of the arrow. A mechanism known per se ensures that the same interpolar distances exist between the anode and cathode after each shift.

Deshalb müssen die links angeordneten Anoden 12 bzw. Kathoden 14 mehr verschoben werden als die rechts angeordneten.Therefore, the anodes 12 or cathodes 14 arranged on the left have to be displaced more than those arranged on the right.

Abgebrannte Anoden werden, zusammen mit der Kathode, rechts entnommen. Auf der linken Seite ist nun ein genügend großer Zwischenraum entstanden, so daß die Kathode, zusammen mit einer neuen Anode, wieder eingesetzt werden kann.Burned-off anodes, along with the cathode, are removed on the right. A sufficiently large space has now been created on the left side so that the cathode can be reinserted together with a new anode.

Claims (10)

1. A wettable solid cathode which may be used in a fusion electrolysis cell for the production of aluminium and has an aluminide of at least one transition metal from groups IV A, V A and VI A of the periodec table of elements, characterized in that the solid cathode (10) consists essentially of a carrier body (14) and an open-pored structure (16) which lies at least in the region of the working face and is impregnated with aluminium saturated with transition metal or metals and which may be fed continuously from supplies (19,34) of aluminide.
2. A solid cathode as in claim 1, characterized in that the solid cathode (10) is made as an interchangeable member.
3. A solid cathode as in claim 1 or 2, characterized in that its carrier body (14) consists at least partially of material which at 900 to 1000°C is electrically highly conductive and resistant to the melt, preferably carbon.
4. A solid cathode as in at least one of the claims 1 to 3, characterized in that the carrier body (14) has at least one cavity (18) for receiving the aluminide, into which the openpored structure (16) preferably projects.
5. A solid cathode as in at least one of the claims 1 to 4, characterized in that the open- pored structure (16) consists of fine-grain granules sintered together.
6. A solid cathode as in at least one of the claims 1 to 4, characterized in that the open- pored structure (16) consists of fibres, preferably of a felt or weave.
7. A solid cathode as in claim 6, characterized in that the open-pored structure (16) consists of a felt of carbon fibres, preferably 1-5 mm thick.
8. A solid cathode as in at least one of the claims 1 to 7, characterized in that in the case of the use of titanium aluminide the open-pored structure (16) is coated with titanium carbide or titanium diboride, preferably 0.4 ¡.tm thick.
9. A solid cathode as in at least one of the claims 1 to 8, characterized in that the solid cathode (10) at 900-1000° C has an apparent density which lies between those of the electrolyte and of the liquid aluminium, preferably between 2.1 and 2.3 g/cm3, and is provided with spacers (32).
10. A solid cathode as in claim 9, characterized in that for achieving the correct apparent density uniformly distributed pieces (30) of iron sheated by cathode material are inserted.
EP82902974A 1981-10-23 1982-10-14 Cathode of a cell for the electrolysis of a melt, for the preparation of aluminium Expired EP0091914B1 (en)

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AT82902974T ATE15079T1 (en) 1981-10-23 1982-10-14 CATHODE FOR A MOLTEN FLOW ELECTROLYSIS CELL FOR THE PRODUCTION OF ALUMINUM.

Applications Claiming Priority (2)

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CH6788/81 1981-10-23
CH6788/81A CH648870A5 (en) 1981-10-23 1981-10-23 CATHOD FOR A MELTFLOW ELECTROLYSIS CELL FOR PRODUCING ALUMINUM.

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EP0091914A1 EP0091914A1 (en) 1983-10-26
EP0091914B1 true EP0091914B1 (en) 1985-08-21

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IT (1) IT1152748B (en)
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US4462886A (en) 1984-07-31
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DE3142686C1 (en) 1983-02-03
NO832198L (en) 1983-06-17
EP0091914A1 (en) 1983-10-26
IT1152748B (en) 1987-01-07
WO1983001465A1 (en) 1983-04-28
CA1209526A (en) 1986-08-12

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