DE1092215B - Cathode and cell for the production of aluminum from aluminum oxide by fused-salt electrolysis - Google Patents

Description

GERMAN

The invention relates to a cathode and an electrolytic reduction cell for the production of Aluminum.

These cells are used for the electrolysis of an aluminum compound, generally of alumina, used in a suitable, mainly is dissolved consisting of cryolite flux and generally has a melting point above 900 ° C. Since the cells therefore have to work at a temperature in the vicinity of 1000 ° C their construction always posed considerable problems. The flux ζ. B. is metals (with the exception precious metals) and normal refractory materials are very reactive towards them. These Difficulty arises in constructing a permanent container for the molten flux. An even greater difficulty is finding suitable materials for making the electrodes to find.

Carbon (graphite) is the only material that has so far proven to be suitable for the purposes mentioned has, both for the lining of the container that holds the flux, as well as for the manufacture of the electrodes. However, there are a number of disadvantages to the use of this material with itself, not the least of which is that the electrodes, in practical operation, are horizontal must be arranged so that a single cell takes up a considerable amount of floor space and the cost of making such large cells is significant. The need for this horizontal arrangement relies on molten aluminum having a carbon cathode not wetted, which is why the carbon lining of the cell bottom is designed as a cathode on which the aluminum produced by the electrolysis precipitates as a sump of molten metal. If this arrangement is not used, there will be very poor power efficiency in the cell.

In addition, the gradual penetration of the molten flux into the carbon of the cathode causes gradual decomposition and shortening of its life. Furthermore, deposits form on the surface of the cathode, which increase the voltage drop in the cell and reduce its efficiency. It is difficult to establish good electrical contact between the cathode and the supply lines for the electrical current, and there are also considerable losses due to the electrical resistance of the carbon itself. The need to use a horizontally arranged carbon ^{cathode 1} with a resulting large area of the cell cathode and cell for extraction

of aluminum from aluminum oxide

by fusible electrolysis

Applicant:

The British Aluminum Company Limited, London

Representative: Dr. K.-R. Eikenberg, patent attorney,
Hanover, Am Klagesmarkt 10/11

Claimed priority:
Great Britain May 4, 1951 and April 15, 1952

Charles Eric Ransley, Amersham, Buckingham

(Great Britain),
has been named as the inventor

also has the consequence that high heat losses occur, which at considerable expense electrical energy must be balanced, in addition to that for the: electrolytic dismantling of the Aluminum oxide required energy is used.

In order to overcome these disadvantages, a new cathode material has been sought. An ideal Cathode must have the following properties:

1. It must have good electrical conductivity;

2. It must not react with the molten aluminum or the molten flux or be soluble therein, at least not to a substantial extent;

3. it must be able to be wetted by molten aluminum;

4. It must be able to be produced economically in the required form.

It has now been found that these requirements are met very largely that the cathode consists mainly of at least one of the substances titanium carbide and zirconium carbide. With the expression "consisting mainly of ..." is intended here and in the claims to mean a material that consists of at least 90% of the substances mentioned and otherwise the usual tech-

009 630/353

flisglien may have impurities, the content of the cathode material in free carbon is below 0.5% and preferably below 0.1 ° / »·

It is true that it was a long time ago for electrical switch contacts that carbides were used alongside other metals contain, the use of titanium carbide alone or alongside other carbides has been proposed. However, this proposal could not apply to the field of aluminum production, which is fundamentally different with regard to the stress on the electrode materials impact. In addition, a review showed that with those specified in the proposal Compositions did not produce a cathode useful for an aluminum electrolytic cell can be.

A cathode according to the invention can be used with advantage in a cell of the previously customary type the otherwise observed cathode voltage drop is reduced to a considerable extent will. It has been found, however, that their greatest value lies in their being good in a vertical or vertical position inclined position can be used. There can be considerable additional benefits from manufacturing a cell with one or more inclined cathodes can be obtained.

Titanium and zirconium are relatively common elements and their carbides are relatively common can be easily manufactured. The carbides have a relatively high electrical conductivity, are resistant to the molten flux and have a very low solubility at 1000 ° C in molten aluminum. They can be in a suitable form with good mechanical properties getting produced. Furthermore, it is possible in an effective manner the surface of a titanium carbide or to wet the zirconium carbide electrode with molten aluminum in such a way that the structure for the first time a cell with vertical or inclined electrodes becomes possible. After all, these carbides can be used without great difficulty in good mechanical and electrical connection with a metallic conductor get connected. The cathode referred to here is a plate electrode, rod electrode or of any other suitable form. You can z. B. from suitable pieces of titanium carbide, zirconium carbide or a mixture of both, supported by a support material such as carbon, being held.

Although both carbides are useful for the purposes under consideration, titanium carbide becomes zirconium carbide preferred; not only because of its lower manufacturing price, but also because of its larger one Resistance to oxidation compared to zirconium carbide. In the case of zirconium carbide, Precautions should be taken to avoid exposure to air, oxygen, or oxidizing conditions at high temperature, e.g. B. the operating temperature of the cell to switch off. For this A cathode made entirely or largely of zirconium carbide must be ground by rapid immersion be protected in molten metal or flux or by a coating of carbon, before their temperature rises significantly. Also, for sintering zirconium carbide, higher Temperatures than those required for titanium carbide in order to achieve the necessary mechanical strength. the end for this reason the following description relates mainly to the use of titanium carbide. The various operations and manufacturing processes that are specified in detail, but can also be used in the case of zirconium carbide, with the exception of cases where the Necessity to protect the zirconium carbide against oxidation, appropriate precautionary measures necessary power.

It is preferred that the carbide in the cathode be substantially pure; H. essentially free from other elements or compounds that may undesirably contaminate the aluminum produced or would lead to rapid decomposition of the cathode during operation. Nonetheless the cathodes according to the invention may initially contain small amounts of other compounds, which dissolve quickly when the cathode is put into operation for the first time, without this decomposition of the cathode takes place. These other compounds can e.g. B. small percentage amounts of titanium or zirconium oxides.

For example, a cathode according to the invention is made by pressing powdered titanium carbide with which a small proportion of a binder, e.g. B. IVo of paraffin wax dissolved in benzene, the powder having an average particle diameter of about 1 to 2 microns. The compaction can be carried out at pressures of approximately 80 to 800 kg / cm ² between dies, either at ordinary or elevated temperature. The powdery mixture can be pressed into the desired shape. If the titanium carbide powder has a low content of free carbon, it can be compacted directly and then pre-sintered in a vacuum to a temperature of 1100 ° C (approx. 1600 ° C for zirconium carbide). It can then be machined by sawing, filing or planing so that a cathode of suitable shape is obtained, although the electrode is normally already given its final shape by densification. The cathode is then sintered in vacuo at a temperature of 1600 ° C (about 2200 ° C for zirconium carbide), but the temperature can be varied to suit the density desired for the final product to produce a solid sintered part with a porosity that may be of the order of 20 percent by volume.

In general, the commercially available titanium carbide powder contains about 90% TiC, 1 to 2% free Carbon and the remainder essentially titanium oxide.

When such a powder is treated in the manner indicated, the cathode obtained is not always Suitable for the intended purposes because the free carbon content does not necessarily depend on one safe level is decreased during sintering. It has been found that a sintered titanium carbide compact, containing greater than about 0.5% free carbon when wetted by molten Aluminum likely breaks or disintegrates as a result of the formation of aluminum carbide. These Difficulty can be overcome by adding the titanium carbide powder used to produce the compacts a certain amount of calcined alumina powder is added. Preferably the aluminum oxide becomes the commercially available TiC powder added and the mixture then relatively long in a ball mill in the dry state ground until an average particle diameter of about 1 to 2 microns is present. The amount of added alumina depends to a certain extent on the content of free

Carbon in the commercially available TiC powder, but is generally about 2 to 3 percent by weight of the TiC used.

The powdery mixture of TiC and Al ₂ O ₃ is then mixed with a suitable. Binders, e.g. B. paraffin wax, which is dissolved in benzene, is added and the solvent is driven off under pressure before the mass is compacted. The compacts obtained are: annealed in a furnace, through which a stream of a neutral gas, e.g. B. hydrogen, the temperature is preferably higher than 1600 ° C, for example about 2200 ° C (about 2700 ° C for zirconium carbide). The final product contains essentially no free carbon and the remainder of the _{aluminum contained in it as Al 2} O ₃ or Al ₄ C ₃ is very little. For example, a titanium carbide powder which originally contained between 1 and 2% free carbon, after mixing with 2.5% calcined aluminum oxide and the treatment indicated above, produced a cathode molding compound which contained only 0.2% free carbon. When the Al ₂ O ₃ content was increased to 3%, a product with a content of 0.05% free carbon was obtained.

Although the content of free carbon in the finished cathode should be below 0.1%, slightly higher contents of. free carbon, provided that it does not reach 0.5%. Cathodes thus made from substantially carbon-free TiC are well wetted by molten aluminum without showing any tendency to break or crack. The density of the product before wetting is approximately 4.5 g / cm ^3, corresponding to a porosity of approximately 10 percent by volume.

As mentioned, the wettability is by molten Aluminum is an important property of cathodes. Since they are somewhat porous, the aluminum penetrates into the cathodes and impregnates them to an extent that depends on their porosity. That Coating or impregnating a Ti C compact with aluminum improves its electrical conductivity and the resistance to oxidation at high temperatures (but has little effect on the resistance to oxidation of zirconium carbide). Since the electrical conductivity of Ti C is sufficient in itself the improvement in this direction is useful, but not so important. on the other hand the increase in oxidation resistance achieved in the case of Ti C is an advantage.

When a cathode according to the invention is placed in an electrolytic cell and the current for electrolysis is applied, the aluminum deposited on the cathode automatically wets it. As a result, it is not essential to wet the cathode with aluminum prior to its introduction into the cell. However, pre-wetting improves the corrosion resistance of the cathode under the conditions outside the cell and ensures other advantages. For example, successful pre-wetting is an indication that the cathode material will perform satisfactorily in the cell if it can withstand pre-wetting. In addition, any excess of titanium that is contained in titanium carbide as TiO or TiO ₂ is washed out to a large extent, whereby a corresponding contamination of the aluminum in the cell is avoided. Finally, pre-wetting the cathode with aluminum makes it easier to connect the cathode to the outer busbars that power the cell. This connection preferably consists of an aluminum rod which is cast onto the cathode at one end.

A feature of the invention is therefore that a cathode, which in the main made of titanium carbide or zirconium carbide after being shaped with molten aluminum is pre-wetted.

The wetting outside the cell is preferably contacted by heating the sintered cathode obtained with aluminum or by immersing it in an aluminum bath at a temperature from about 1200 ° C upwards in vacuum. The aluminum wets all exposed surfaces of the Carbide and then stick as a thin film.

An electrolytic reduction cell for the production of aluminum according to the invention has an or several of the cathodes described, which are arranged so that at least one of their operating surfaces and. corresponding to the associated operating area of

ao anode is at such an angle to the horizontal extends that the liquid metal deposited on the cathode can flow away under the influence of gravity.

The metal flowing down from the cathodes collects preferably in a sump that is connected to the the lower part of the cathode communicates, and from which it is taken from time to time in the usual manner will.

As a result of the inclined or vertical arrangement of the electrodes, the bottom surface is covered by the cell is taken, compared to the previously required floor space, which can also be much smaller Electrodes are arranged so that they are in a relatively limited space of molten Working flux, which on the other hand can be surrounded by solidified flux, which by a simple Wall made of sheet steel or other suitable material held together in its desired external shape will. This makes the construction of the cell considerably cheaper. Another great benefit of the Cell construction with inclined cathodes according to the invention is that the elimination of harmful or unpleasant fumes generated during operation of the cell, as a result the much smaller area is considerably simplified. The cell can also be built to be an expensive one Loss of heat is minimized. Another benefit that comes from the inclined arrangement of the cathode results in the fact that a to and fro of the molten Aluminum is not so easily done; the distance between the electrodes can therefore be compared to the previous cells are significantly reduced, reducing the loss of electrical energy in the Flux is reduced accordingly. It is also important that due to the relatively high electrical conductivity of the cathode compared to that of a carbon cathode the voltage drop, which usually occurs when very high operating currents pass through the cathode material, is greatly reduced. A sludge build-up at the bottom of the cell, causing an undesirable voltage drop at the cathode in the horizontal cells can cause the operation of the new cell type do not bother.

It was z. B. an electrolytic reduction cell - using the usual flux - with a titanium carbide cathode and a carbon anode, both substantially vertical were arranged. It was found that the

Cathode loss was less than 0.2 volts compared to the usual 0.5 cathode loss to 0.7 volts, which occurred in the usual cells of comparable size. It also became a considerable higher efficiency of current utilization found than in a conventional cell with carbon cathode was obtained.

Further details and advantages of the invention will become apparent with reference to the drawings, the exemplary embodiments of an electrolytic cell for aluminum production represent, explained in more detail.

Fig. 1 shows a vertical cross-section along the line I-I in Fig. 2 through a cell as it is represents during operation;

Fig. 2 shows a partial section along the line II-II in Fig. 1 showing one end of the cell with no flux and molten aluminum;

FIG. 3 shows a section through a construction of a reduction cell that is modified compared to FIG along the line III-III in Fig. 4;

Fig. 4 shows a section along the line IV-IV in Fig. 3, the contents of the cell, i.e. H. the melted and solidified flux and molten aluminum sump are omitted;

FIG. 5 shows a partial section along the line V-V in FIG.

The example shown in Figs. 1 and 2 shows a cell of rectangular shape, both in plan view and in cross section. It has an outer wall 1 made of refractory insulating material, such as magnesite, and an inner wall or a carbon lining 2. Vertical partitions 3, also made of carbon, extend inward from the longitudinal side walls of the carbon lining, each partition being in contact with its outer and bottom surfaces with the corresponding areas of the carbon lining 2, but ends just before the longitudinal center line of the cell. The inner surface of each septum 3 is inclined upwardly and outwardly from its lower end relatively steeply to the horizontal, and the upper surface of the septum is located at a height slightly below the level of the molten flux 4 which, in use, fills the cell ( see. Fig. 1). The partitions 3 are arranged separately along the length of the cell and in opposing pairs, each parting wall of a pair serving to support a plate of a cathode pair which co-operates with an anode 6 which lies therebetween. Each cathode 5 consists of a rectangular plate made of sintered carbide which has been produced in the manner described above. This cathode plate rests with the central part of its outer surface on the inner surface of the corresponding diaphragm 3 and its lower edge is arranged so that it contacts the corresponding side wall of a channel 7 formed along the inner surface of the base plate of the carbon lining 2. The width of the channel extends from one partition 3 to the opposite one. As a result of this arrangement, at intervals along the cell, the pairs of oppositely arranged cathode plates 5 in a V shape, the lower edges of which are separated from one another by the width of the channel 7. The upper edges of the cathode plates, as shown in Fig. 1, protrude slightly above the upper surfaces of the partitions 3 and terminate just below the surface of the filling of molten flux 4. The anode 6 is made of carbon and has a rectangular cross-section in each horizontal cross-section Shape. At its lower end of operation, however, it has the shape of a wedge, so that the inclined rectangular surfaces 6a formed thereby extend substantially parallel to the inner surfaces of the corresponding cathode plates 5. The anode 6 is supported by an aluminum rod 8 (Fig. 1), which also

("« 111 Il Il Tj <<

serves to connect the anode to the positive Poi of the power source. The top of the anode protrudes above the surface of the molten flux 4 and through the overlying crust

ίο 4a of the solidified flux through. Since the Anode consumed during operation of the cell, it is continuously lowered in a known manner. The inclined surfaces of the anode are mounted so that the desired small distance between the electrodes is always preserved.

In the angle between one surface of each partition 3 and the adjacent longitudinal surface of the carbon lining 2 of the cell, a shallow depression 9 is formed on the base of the lining, within which lies the inner end of a rod 10 made of sintered, compressed titanium carbide of the same type as Cathode plates consists. This rod represents a power line and goes horizontally through the vertical W ^T and the carbon lining 2 through to the outside, where it is electrically connected to a contact rod 11, the inner end of which is embedded in the insulating outer wall 1 and the one with the negative pole of the power source connected for electrolysis.

However, such a power supply consisting of titanium carbide or zirconium carbide is not Subject of the invention.

When the cell is started up, the electrolysis can be initiated by one of the various known measures. For example, the anodes 6 can be lowered until they are in contact with the cathode plates 5 and the bottom of the carbon lining 2. If current is sent through the electrodes and the lining, the arrangement can thereby be heated to an appropriate temperature, for example to 700.degree. The anodes can then be lifted from the cathode plates while molten aluminum is poured into the cell to form a bottom sump 12, followed by molten cryolite containing dissolved alumina. ^R is W hen the cell up to the desired level filled, the full electrolytic current is supplied to the electrodes and the cell is brought up to full production.

During operation of the cell, the greater part of the flux mixture is in a molten state. However, a crust Ία of solidified flux forms, which bridges the gap between the carbon lining 2 of the cell and the anodes 6 and also extends down the walls of the lining. The aluminum is deposited in liquid form on the entire surfaces of the cathode plates 5 and flows on these surfaces into the sump, which extends over the bottom of the carbon lining 2 and fills the channel 7. The sump represents the electrical connection between the rods 10 and the cathode plates 5, with essentially the entire electrolysis current being conducted through the aluminum and only a small part or nothing at all going through the bottom of the carbon lining. The molten aluminum is tapped from this sump from time to time.

In the arrangement according to FIGS. 3 to 5, the type is the cell with respect to the insulating outer wall 1,

the carbon lining 2, the partitions 3, the cathode plates! 5, the anodes 6 and the channels 7 substantially the same as that in Figs.

However, the rods 10 for the power supply, which are shown in FIGS. 1 and 2, are omitted and the connections between the cathode plates 5 and the negative pole of the power source have been formed from extensions 5 a , which extend from the upper edges of the cathode plates , with which they can be formed from one part., Extend upwards, so that they protrude through the solidified crust 4 a of the flux. Each of these extensions 5 α is connected to an aluminum rod 11 in that, for example, the adjacent end of this rod is cast onto it or welded to it. The shallow depressions 9, which are provided in the design according to FIGS. 1 and 2, are not required. The operation of this cell is essentially the same as that of the cell shown in FIGS.

3 to 5 also show a hood 13 with its lower edge on the upper edge of the insulating outer wall 1 sits and its interior through a line 14 with a (not shown) Gas evacuation system is connected. In the hood there are openings for the rods to pass through, which serve as power supplies to the corresponding cathode plates. A similar hood can be provided in the arrangement as shown in FIGS.

In both of the cells shown, the aluminum becomes electrolytic on the inclined surfaces of the cathode plates 5 knocked down and runs into the sump 12. If the plates 5 before installation in the If the cell has not been wetted with aluminum, the first aluminum deposited is used for wetting their surfaces. The subsequently precipitated aluminum then runs into the swamp. In some cases it is preferable to insert cathode plates wetted with molten aluminum in the cell.

The cell can be operated in such a way that a thicker crust 4 α of solidified flux arises as shown in the drawings. In this case all of the flux in one box can be: made of steel or other suitable material.

Claims (5)

Patent claims: 1. Cathode for a cell for the production of aluminum from aluminum oxide by fused-salt electrolysis, characterized in that it consists mainly of at least one of the substances Titanium carbide and zirconium carbide. 2. Cathode according to claim 1, characterized in that the cathode material has a content of free carbon below 0.5%, preferably below 0.1%. 3. Cathode according to one of the preceding claims, characterized by pre-wetting with molten aluminum after its shaping. 4. Cathode according to claim 1 to 3, characterized in that that the cathode material is sintered and the free carbon content by intimate mixing of the selected cathode material with a small amount of aluminum oxide before Sintering is reduced. 5. Cell for the production of aluminum from aluminum oxide by fused-salt electrolysis with one or more cathodes according to one of the preceding claims, characterized in that that at least one of the operating surfaces of the cathode (5) and, accordingly, the associated operating surface (6 a) the anode (6) are arranged inclined at such an angle to the horizontal, that the liquid metal deposited on the cathode flow away under the influence of gravity can. Considered publications:
German Patent No. 622 522;
R. Kieffer and W. Hotop, "Powder Metallurgy and Sintered Materials", 1948, p. 285. 1 sheet of drawings © 009 630/353 10.60