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

Description

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.

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.

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).

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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 .

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 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.

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 structural _t ur or its coating has the optimal solution searched for in consideration of capillary and surface forces here.

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.

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.

• - 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.

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.

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.

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.

• - 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.

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.

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.

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.

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.

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.

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 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.

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.

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.

Therefore, the 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.

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.

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