EP0091914A1

EP0091914A1 - Cathode of a cell for the electrolysis of a melt, for the preparation of aluminium.

Info

Publication number: EP0091914A1
Application number: EP82902974A
Authority: EP
Inventors: Tibor Kugler
Original assignee: Alusuisse Holdings AG; Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1981-10-23
Filing date: 1982-10-14
Publication date: 1983-10-26
Also published as: DE3142686C1; IT1152748B; WO1983001465A1; IT8223834A0; CA1209526A; EP0091914B1; CH648870A5; NO832198L; US4462886A

Abstract

La cathode solide (10) est constituée essentiellement d'un corps porteur (14) et d'une structure poreuse (16) à pores ouverts, se trouvant au moins dans la zone de travail. La structure poreuse est imprégnée resp. imbibée d'aluminium saturé par au moins un métal de transition du groupe IV A, V A et VI A du système périodique des éléments. Cette structure (16) peut être alimentée de manière continue à partir de réserves de composés d'aluminium (19, 34). Un feutre d'une épaisseur de quelques millimètres, composé de fibres de carbone enduites, s'est révélé comme particulièrement adapté en tant que structure poreuse à pores ouverts. Les cathodes solides (10) sont peu coûteuses et peuvent être fabriquées en matériau malléable.The solid cathode (10) essentially consists of a carrier body (14) and a porous structure (16) with open pores, located at least in the working area. The porous structure is impregnated resp. soaked in aluminum saturated with at least one transition metal from group IV A, V A and VI A of the periodic system of elements. This structure (16) can be supplied continuously from reserves of aluminum compounds (19, 34). A felt with a thickness of a few millimeters, composed of coated carbon fibers, proved to be particularly suitable as a porous structure with open pores. Solid cathodes (10) are inexpensive and can be made of malleable material.

Description

Cathode for a melt flow electrolysis cell for the production of aluminum

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 has an aluminide of at least one transition metal from groups IVA, VA and VIA of the periodic system of the elements.

For the production of aluminum by electrolysis of aluminum oxide, this is dissolved in a fluoride melt, which largely consists 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 consisting 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 CO2 and CO.

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, the anode effect occurs, which results in an increase in voltage from, 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).

It is known to use wettable solid-state cathodes in the melt flow electrolysis for the production of aluminum. Cathodes made of titanium diboride, titanium carbide, pyro-graphite, boron carbide and other substances are

OMP beaten, mixtures which, for example, can be sintered together, are also used.

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 results in a significantly reduced energy consumption and avoids the formation of non-uniformities in relation to the thickness of the aluminum layer.

In contrast to wettable cathodes which are firmly anchored in the carbon bottom of the cell, DE-OS 28 38 965 shows solid body cathodes made of individually interchangeable elements, each with at least one power supply. In a further development according to DE-OS 30 24 172, the interchangeable elements are arranged from two mechanically rigidly connected parts that are resistant to thermal shock - an upper part protruding from the molten electrolyte into the separated aluminum and an exclusion cloth in the liquid aluminum lower part - made of different materials. 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 30 45 349 relates to an exchangeable wettable solid-state cathode, which consists of an aluminide of at least one metal from the group formed from titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, without binding phase made of metallic aluminum. The non-aluminum components of the aluminide thus belong to group III A, IV A and / or VI A of the periodic system of the elements.

OMP v The chemical and thermal resistance of the aluminides allows them to be used both in the _. molten electrolytes as well as in molten aluminum can be used, although they are of limited solubility in the latter. However, this solubility drops sharply as the temperature drops.

At the working temperature of the aluminum electrolysis cell, which is in the range of 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 liquid aluminum deposited is saturated with one or more of the transition metals in the aluminide. _t elements of-. The aluminides which have been removed 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 working on the basis of aluminides with a service life which corresponds to one or more anode service lives, with the purchase price of the cathode and the handling costs being to be substantially reduced.

According to the invention, the object is achieved in that the solid-state cathode essentially consists of a support body and an open-pore one located at least in the region of the working surface. structure impregnated with transition metal / s saturated aluminum, which can be fed continuously from aluminide supplies.

OWPI The working surface is the surface which, when the cathode is inserted in the electrolytic cell, points in the direction of the anode and the direct electrical current _. will flow through. The aluminum ions are reduced to elemental aluminum on this work surface. The working surfaces of the cathodes are therefore expediently slightly inclined so that the deposited aluminum, which forms a film on the wettable cathode, can measure off.

The working surfaces of the corresponding anodes, which e.g. can consist of combustible carbon or incombustible oxide ceramics are inclined accordingly. This tendency also has an advantageous effect here: the oxygen or CO2 which is 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 to the liquid metal when the solid cathode is inserted, so that the electrical current through this impregnation alloy and possibly through the structure can The support body therefore preferably consists at least partially of a material which is highly electrically conductive at 900 to 1000 ° C. and is resistant to the melt flow. In this case, the current can mainly flow 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 cathode is always in the case of manipulations on the traverse or the anode bar and in particular when changing the anodes

O PI

^* ° 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.

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 ^. The density of a floating cathode must be between these two values.

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 pieces of iron 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 .

If, on the other hand, 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 fins in the liquid aluminum; they are preferably held at the desired distance from one another and from the cell shelf by appropriately designed spacers.

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.

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. Depending on the material of the open-pore structure or its coating, the optimal solution must be sought, taking capillary and surface forces into account.

These requirements can be met by sintered fine-grained granules or preferably by a fiber structure. This fiber structure is expediently designed in the form of a felt or a fabric. The fibers are a few micrometers thick and preferably consist of carbon.

Depending on the geometrical shape of the solid cathode and the chemical composition of the aluminide used, the open-pore structure, which is impregnated with transition metal / saturated aluminum, is supplied continuously from cavities arranged in the supporting body, into which the open-pore structure protrudes , or from another location on the open-pore structure at which solid aluminide can be held.

For economic reasons and because of the scientifically good research, titanium alu inides 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 rather only as bulk cathodes in cavities of the supporting body.

- On the other hand, aluminides with a titanium content above 37.2 (up to 63)% by weight of titanium can also be associated as solid moldings with the open-pore structure. The aluminum produced during the electrolysis process flows along the diagonally arranged open-pore structure and mixes with the impregnating aluminum saturated with transition metal / s and in this would gradually reduce the ^• transition metal content to such an extent 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 is continuously replaced by new one, so that the open-pore structure remains impregnated with aluminum saturated with transition metal / s.

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, which 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, as a result of which the life of the felt can be multiplied.

A felt consisting of coated carbon fibers has the further advantage that if the coating is defective, the entire work surface does not become unusable, but only individual fibers are 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.

__ OMPI The solid-state cathodes according to the invention are also particularly suitable for converting existing aluminum melt flow electrolysis cells.

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 support body made of electrically insulating material and anode designed accordingly

- Fig. 3 is floating in the molten aluminum cathode Fest¬ body of electrically conductive Mate¬ rial and- ^{'correspondingly} shaped anode, and

4 alternatively arranged solid-state cathodes made of electrically conductive material and correspondingly designed anodes.

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 cathode 10 consists of a shaped support body 14 made of carbon and a felt 16 attached to the work surface facing the anode body 12 made of carbon fibers coated with titanium carbide. 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 dough at electrolysis temperature, which consists, for example, of 80% by weight aluminum and 20% by weight titanium.

The feet 20 of the support body 14 are in a correspondingly OMPI formed 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.

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 aluminum saturated with titanium in the felt and flows according to the inclination of the working surface of the solid-state cathode to the center of the electrode element. The felt 16 acts like a wick in the oil, liquid alloy is drawn from the cavity 18 with the pasty titanium aluminide and thus the current losses are replaced. Without this replacement of the used titanium, the deposited aluminum would dissolve the titanium carbide coating on the carbon fibers and render the cathode surface non-wettable.

Due to the relatively small opening of the cavity 18, only a small amount of the circulating molten electrolyte 26 can occur, so the supply by means of convection is small.

2, a solid-state cathode 10 and an anode block 12 form a pair of electrodes. The supporting body 14 consists of an insulating material, for example of hochgesinter¬ tem alumina, alumina-containing ceramics, silicon carbide _or silicon carbide siliziumnitridgebundenem. 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 relatively large opening is formed and filled with solid Titanaluminidgranalien, which for example consist of 55 wt .-% aluminum and 45 wt .-% titanium. 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.

The deposited aluminum flows through an opening 28 in the support body 14.

The floating solid-state cathodes 10 shown in FIG. 3, adapted to the anode bodies 12, fill the entire electrolysis tank by their spacers 32, which are formed all around, lie snugly against one another. The apparent density of the entire solid cathode, at working temperature, must lie between the density of the molten electrolyte and the molten aluminum. In the case of support bodies 14 made of carbon, this is achieved by inserting iron pieces 30 into closed cavities, for example in the form of a ring.

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 sleeves 34 which are placed over the support rods of the support bodies 14 and which consist of a solid aluminide.

If the anode bodies 12 consist of carbon, ie burn them 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.

Therefore, the anodes 12 and cathodes 14 arranged on the left have to be displaced more than those arranged on the right.

OMPI Burned-off anodes are removed together with the cathode on the right. A sufficiently large space has now been created on the left-hand side so that the cathode can be reinserted together with a new anode.

Claims

-12 claims

1. wettable solid-state cathode that can be used in a melt-flow electrolysis cell for the production of aluminum with an aluminide of at least one transition metal from groups IV A, VA and VI A of the periodic system of the elements, characterized in that the solid-state cathode (10) consists essentially of a supporting body ( 14) and an open-pored structure (16) located at least in the area of the work surface and impregnated with aluminum saturated with transition metal / s, which structure can be fed continuously from aluminide stocks (19, 34).

2. Solid-state cathode according to claim 1, characterized gekennzeich¬ net that the solid-state cathode (10) is designed as an exchangeable element.

3. Solid-state cathode according to claim 1 or 2, characterized gekenn¬ characterized in that the support body (14) at least partially from 900 to 1000 ° C electrically well conductive and resistant to the melt flow material, preferably carbon.

4. Solid cathode according to at least one of claims 1 to 3, characterized in that the support body (14) at least one cavity (18) for receiving the

Has aluminide, in which the open-pore structure (16) preferably protrudes.

5. Solid cathode according to at least one of claims 1 to 4, characterized in that the open-pore structure (16) consists of sintered fine-grained Graalen.

OMPI

6. Solid cathode according to at least one of claims 1 to 4, characterized in that the open-pore structure (16) consists of fibers, preferably a felt or fabric.

7. Solid cathode according to claim 6, characterized in that the open-pore structure (16) consists of a preferably 1-5 mm thick felt made of carbon fibers.

8. Solid cathode according to at least one of claims 1 to 7, characterized in that when titanium aluminide is used, the open-pore structure ( _. 16) is coated with titanium carbide or titanium diboride, preferably 0.4 μm thick.

9. Solid cathode according to at least one of claims 1 to 8, characterized in ^' that the solid cathode (10) at 900-1000 ° C has an apparent density between. Those of the electrolyte and the liquid aluminum, preferably between 2, 1 to 2.3 g / cm ³ , and is provided with spacers (32).

10. Solid-state cathode according to claim 9, characterized in that regularly distributed iron pieces (30) sheathed by the cathode material are used to achieve the correct apparent density.

OMPI