EP0041045B1 - Cathode for molten-salt electrolysis cell - Google Patents

Abstract

A solid cathode in a fused salt electrolytic cell for the production of aluminum is made up of individually exchangeable elements (10). These cathode elements are made up of two parts which are rigidly joined together and which are resistant to thermal shock. The upper part (12) which projects from the molten electrolyte (30) into the precipitated aluminum (26), or the coating on this part (12), is made of a material which, at working temperature, is a good electrical conductor, is chemically resistant and is wet by aluminum. The lower part (14,16), which is exclusively in the liquid aluminum (26), or the coating on this part (14,16) is on the other hand made of an insulating material which can withstand molten aluminum.

Description

The invention relates to a cathode made of individually replaceable elements for a melt flow electrolysis cell, in particular for the production of aluminum.

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 anode bars 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 940-970 ° C. In the course of electrolysis, the electrolyte becomes poor in aluminum oxide. At a lower concentration of approximately 1-2% by weight aluminum oxide in the electrolyte, there is an anode effect, which results in a voltage increase of, for example, 4-4.5 V to 30 V and above. Then, at the latest, the crust formed from solidified electrolyte material must be hammered in and the aluminum oxide concentration increased by adding new aluminum oxide (alumina).

It is known to use wettable cathodes in the melt flow electrolysis for the production of aluminum. In the deposition of aluminum, cathodes made of titanium diboride, titanium carbide, pyrolytic graphite, boron carbide and other substances are proposed for electrolytic cells belonging to the prior art, mixtures of these substances which can be sintered together also being used.

Compared to conventional electrolytic cells with an interpolar distance of approx. 6-6.5 cm, cathodes that can be wetted with aluminum and are not or only slightly soluble in aluminum offer decisive advantages. The cathodically deposited aluminum already flows when a very thin layer is formed on the cathode surface facing the active anode surface. It is therefore possible to remove the deposited liquid aluminum from the gap between the anode and cathode and to feed it to a sump located outside the gap.

Thanks to the thin aluminum layer on the cathode surface, the irregularities with respect to the thickness of the aluminum layer, which are well known from conventional electrolysis, do not form - under the influence of electromagnetic and convectional forces. Therefore, the interpolar distance can be reduced without sacrificing current efficiency, i.e. H. a significantly lower energy consumption per unit of reduced metal is achieved.

An electrolysis cell is proposed in US Pat. No. 3,400,061, in which cathodes which are wettable from carbon are attached to the cell bottom. The cathode plates are slightly inclined with respect to the horizontal, towards the center of the cell. The clear width of the gap between the anode and cathode, i. H. the interpolar distance is much smaller than with conventional cells. However, this makes the circulation of the electrolyte between the anode and cathode difficult. When aluminum is deposited, the cryolite melt becomes very poor in alumina, making the cell susceptible to anode effects. Only a small part of the total floor area of the cell is available for collecting the liquid metal. In order not to let the scooping intervals become uneconomically small, the sump must therefore be deep, which in turn requires increased insulation of the cell floor.

In addition, it should be noted that the connection between the carbon base and the wettable cathode plates places difficult demands on the connection mass and increases the electrical resistance of the cell base. As with conventional electrolysis cells, the cell bottom is made of electrically conductive, ie weakly heat-insulating carbon material.

Wettable cathodes are also used according to the process of DE-OS 26 56 579. In this prior publication, the circulation of the cryolite melt is improved in that the cathode elements are anchored in the electrically conductive cell bottom and protrude in the area below the anodes from the aluminum sump collected on the entire remaining cell bottom surface. In the present case, the cathode elements consist of tubes, closed at the bottom, made of aluminum-wettable material, the tubes being completely filled with aluminum.

Above the aluminum sump, i.e. H. between the tubes, gaps between the cathode elements facilitate the circulation of the electrolyte. The height of these gaps or tubes is chosen so that there is no significant current transfer between the anode and the aluminum sump. The power supply lines to the cathode elements shown in the examples of the above-mentioned DE-OS all have the disadvantages of power supply through the carbon base. The flow of the electrolyte is a vortex flow around the cathode element and takes place without a preferred direction, so the distribution of the alumina concentration is not optimal.

A further development of the above DE-OS can be found in US Pat. No. 4,177,128. The pipes, which optionally consist of electrically conductive or electrically non-conductive material are provided with a precisely adapted cover made of electrically conductive material, which is connected to the liquid aluminum in the tube via a downward extension. According to this embodiment, however, more titanium diboride is used in electrically conductive pipes than in the above-mentioned DE-OS, while electrically insulating pipes are hardly sufficiently resistant to the molten cryolite. In addition, sludge is formed in non-hermetically sealed pipes, which dissolves poorly and can practically not be removed.

A major disadvantage of all of these previously discussed embodiments with wettable cathodes is that they are firmly anchored in the carbon bottom of the cell. For economic reasons, a material must therefore be selected for the wettable cathodes whose lifespan is at least the same or better than the operating life of the cell lining. This would result in the use of a cheaper material with a shorter operating time or a simpler manufacturing technology. that failure of only a small part of the cathode elements, for example due to operating or manufacturing errors, would result in failure of the entire electrolytic cell. The carbon floor with the cast-in cathode bars is in itself extremely sensitive to manufacturing defects.

The applicant has therefore proposed in DE-OS 28 38 965 a wettable cathode for a melt flow electrolysis furnace, in particular for the production of aluminum, which consists of individually interchangeable elements, each with at least one power supply. With this embodiment of easily replaceable cathode elements, the most serious, the above-mentioned disadvantages have been eliminated, but some inconveniences remain. The electrically conductive wettable elements consist of relatively expensive material that is difficult to machine. There are limits to the size and geometric shape of the elements.

The inventor has set himself the task of creating a cathode from individually interchangeable elements for a melt flow electrolysis cell for the production of aluminum, which can be produced more economically, in particular with regard to shaping and processing.

• - der obere Teil bzw. dessen Beschichtung aus einem bei Arbeitstemperatur elektrisch gut leitenden, chemisch beständigen und von Aluminium benetzbaren Material, und
• - der untere Teil bzw. dessen Beschichtung aus einem gegen das flüssige Aluminium beständigen Isolatormaterial besteht.

The object is inventively achieved in that the elements of two mechanically rigidly connected to each other, resistant to thermal shock parts - a projecting from the molten electrolyte into the separated aluminum upper and in the liquid aluminum exclusively ^- mi- nium arranged lower part - formed of different materials are, where

• - The upper part or its coating made of a material that is highly electrically conductive at working temperature, chemically resistant and wettable by aluminum, and
• - The lower part or its coating consists of an insulator material resistant to the liquid aluminum.

The upper parts of the elements consist of materials described in the relevant literature for wettable cathode plates which meet the requirements. Examples include titanium diboride, titanium carbide, titanium nitride, zirconium diboride, zirconium carbide, zirconium nitride and mixtures of two or more materials, which may optionally contain a small amount of mixed boron nitride.

The electrically conductive, preferably plate-shaped upper parts of the elements protrude into the liquid aluminum, but they do not touch the carbon bottom of the cell.

The lower parts of the elements or their coating, on the other hand, need not be wettable by aluminum or have electrical conductivity. They only have to be compatible with molten aluminum, have sufficient mechanical strength and high thermal shock resistance. Materials that meet these conditions sufficiently are much cheaper than those used for the upper parts or their coating, which are wettable by aluminum and. electrically conductive materials.

Furthermore, molded parts made of insulator material used for the lower part of the elements are much easier to manufacture, which - together with the lower production costs for the materials - is expressed in the fact that mass production of lower parts is 10 to 20 times cheaper than that of upper parts. Examples of such insulator materials that never come into contact with the molten electrolyte include highly sintered aluminum oxide, aluminum oxide-containing ceramics, silicon carbide or silicon nitride-bonded silicon carbide. These materials have a higher specific weight than aluminum and are erosion-resistant, which is important because of the sludge present in the circulating aluminum.

Both the lower and the upper part of a cathode element can - instead of being designed as a homogeneous solid body - a core made of a less expensive, mechanically stable material, such as. B. steel, titanium or graphite, which is coated by a known method with at least one of the corresponding materials. If graphite is used as the core material, the composite body can be produced using a sintering process.

The cathode elements preferably consist of several sub-elements. The electrically conductive sub-elements forming the upper part are expediently of the simplest possible geometric shape, for. B. 1-2 cm thick, vertically arranged plates, the distance between the plates being greater than their thickness. The easily formable and editable sub-elements made of insulating material forming the lower part form a support or a support structure for the upper sub-elements.

With combinations of sub-elements, it is possible to combine simple, electrically conductive hard metal parts without mechanical or other post-processing after sintering, i.e. with possibly large deviations in the actual mass, to form a sufficiently dimensionally stable assembly, which is stressed by lifting tools when inserting or removing them from the Allow electrolysis cell without destroying the relatively sensitive upper parts as a result of impacts, bending stresses, etc. Mechanical influences occurring during the operation of the electrolytic cell are also less dangerous.

The dimensions and thus the weight of the electrically conductive parts used, which bring the greatest economic burden by far, are significantly smaller than in all known cells with solid-state cathodes.

The horizontal surface dimensions of the cathode elements are expediently designed in such a way that an integer multiple between 1 and 7 corresponds to the horizontal surface dimensions of the anode above. However, the horizontal geometric dimensions of a cathode element and the corresponding anode of the same order of magnitude are preferred.

• - Defekte Kathodenelemente können ohne Betriebsunterbruch ersetzt werden.
• - Bei in bezug auf den Ofengang oder Wirkungsgrad unbefriedigenden Elektrolysezellen können anders gestaltete Kathodenelemente eingesetzt werden.

When inserting or replacing a cathode element, the anode above it can be removed for a short time. This is a key advantage for the following reasons:

• - Defective cathode elements can be replaced without interrupting operation.
• - If the electrolysis cells are unsatisfactory in terms of furnace operation or efficiency, differently designed cathode elements can be used.

As already described in DE-OS 2838965, the type of power supply from the power source to the cathode surface is of crucial importance for the furnace operation: the electrolyte located between the anode and the cathode element is exposed to a magnetohydrodynamic pumping action under the influence of the electrolytic current and the magnetic field.

• Figur 1 einen teilweisen Vertikalschnitt durch den aktiven Bereich einer Elektrolysezelle, in Längsrichtung der von Aluminium benetzbaren Kathodenplatten
• Figur 2 einen Vertikalschnitt an der Stelle II-II von Fig. 1, in Querrichtung der Kathodenplatten
• Figur 3 einen Horizontalschnitt durch III-III von Fig. 2
• Figur 4 einen vertikalen Längsschnitt durch eine Variante von Kathodenplatten
• Figur 5 einen vertikalen Längsschnitt durch eine weitere Variante von Kathodenplatten.

The invention is explained in more detail with reference to exemplary embodiments schematically shown in the drawing. Show it :

• 1 shows a partial vertical section through the active area of an electrolysis cell, in the longitudinal direction of the cathode plates wettable by aluminum
• Figure 2 is a vertical section at point II-II of Fig. 1, in the transverse direction of the cathode plates
• 3 shows a horizontal section through III-III of FIG. 2nd
• 4 shows a vertical longitudinal section through a variant of cathode plates
• 5 shows a vertical longitudinal section through a further variant of cathode plates.

A cathode element 10 with an upper part made of the electrically conductive aluminum-wettable plates 12 and a lower part made of aluminum-compatible shaped plates 14, 16 is shown in FIGS. 1-3. In the present example, the wettable cathode plates 12 are mechanically stably connected to insulator plates 14 of the same dimensions by means of round bolts 18. The bolts 18 are preferably made of the more easily machinable and cheaper insulator material; they do not come into contact with the molten electrolyte.

The support plates 14 made of insulator material have recesses 20 on their underside, which in turn fit in a form-fitting manner in recesses 22 of the support plates 16 likewise made of insulator material.

As a result, a mechanically stable cathode element 10 is formed with simple means, in which a group of cathode plates 12 which can be wetted by aluminum is joined to form a unit by means of a support structure made of much cheaper material. The mass of this cathode element 10 is large enough not to be displaced or carried away by the bath currents.

If a further increase in mechanical stability is desired, intermediate pieces, e.g. B. in the form of wedges, and / or cements resistant to liquid aluminum can be used. The elements can also adapt adequately to thermal expansions afterwards.

• - Das flüssige Aluminium 26 kann frei zirkulieren, dadurch wird die Ausbildung eines Bodenschlammes verhindert
• - Es werden Materialkosten eingespart
• - Das Kathodenelement 10 kann besser in die Zelle eingesetzt bzw. daraus entfernt werden.

The support plates 16 have cutouts 24 on their underside, which are provided essentially for three reasons:

• - The liquid aluminum 26 can circulate freely, thereby preventing the formation of a soil sludge
• - Material costs are saved
• - The cathode element 10 can be better inserted into or removed from the cell.

The electrically conductive cathode plates 12 have the interpolar distance d from the burning carbon anode 28. During the electrolysis process, the electrolyte is quickly used up in a narrow gap between the cathode plates and the anode. The cathode plates 12 are relatively narrow; therefore, the bath flow can rapidly renew the electrolyte depleted in aluminum oxide in the interpolar gap, even if the value is greatly reduced compared to the normal value of 6-6.5 cm for d. The deposited metal forms an uninterrupted film on the wettable cathode plates 12 and flows down to the metal sump 26.

The surface 32 of the liquid aluminum 26 must always lie in the area of the wettable cathode plates 12, in particular when scooping, this metal level must never sink into the area of the insulator plates 14, 16. This would be both a power cut and corrosive Destruction of the insulator plates mean.

The direct electrolysis current flows from the anodes 28 via the electrolytes 30 in the interpolar gap to the cathode plates 12, then passes into the liquid aluminum 26 and finally flows via the carbon bottom 34 into the iron cathode bars 36.

It can be seen from FIG. 2 that the working surface of the anode 28 copies the shape of the cathodes. For this reason, plates extending over the entire width of the anode work surfaces are preferably used according to the invention.

In principle, it would also be possible to form wettable cathodes which, for example, have tubes known from the prior art. As a result, however, corresponding recesses would form in the working surface of the anodes, which would result in the formation of gas bags which reduce the current efficiency during the electrolysis process.

Guide grooves 35 can be formed in the carbon base 34 of the electrolytic cell, which prevent the cathode elements 10 from slipping sideways.

4 shows a variant of a cathode plate 12. The formation of a window 38 allows material to be saved and improves the flow conditions in the electrolyte. On the underside, the plate 12 has a dovetail 40 which can be inserted into a corresponding recess in the carrier plate 14. The support structure made of insulator material is then designed so that the plates cannot be moved laterally.

Another variant of wettable cathode plates 12 is shown in FIG. 5. Both the formation of a window 38 and the bevelled underside are intended on the one hand to save wettable cathode material and on the other hand to optimize the flow conditions in the bath. The cathode plate 12 is fastened in a support plate 14 by means of an extension 42 directed downwards in the center.

• - sie teurer und schwieriger herstellbar sind, und
• - sich an den Uebergangsstellen zu den gut leitenden Kathodenplatten 12 Kontakterscheinungen und -erosion ausbilden würden.

The term "insulator material" used in the description also covers poorly conductive materials. In contrast, electrically good conductive materials are never used for the support structure because

• - they are more expensive and difficult to manufacture, and
• - 12 contact phenomena and erosion would form at the transition points to the highly conductive cathode plates.

A support structure 14, 16 per se is not the subject of the invention; any suitable variant used in other fields of technology can be used for this purpose.

The cathode elements 10 according to the invention can also be used to convert existing electrolysis cells by simply placing units adapted to the anode dimensions and the metal level on the carbon floor. As a result, the interpolar distance can be reduced at low additional costs, and the current yield can thereby be increased. In particular, it should be noted that the retrofitting can be carried out without decommissioning the electrolytic cell and that subsequent replacement of defective cathode elements does not pose any problems.

Claims (10)

1. Cathode of individually exchangeable elements for a fusion electrolysis cell for the production of aluminium, characterised in that the elements (10) are made of two parts of different materials, which are joined rigidly together by mechanical means, and are resistant to thermal shock - one upper part (12) projecting from the molten electrolyte (30) into the separated aluminium (26), and a lower part (14,16) arranged exclusively in the molten aluminium (26), while

- the upper part (12) or its coating consists of a material which, at the working temperature, is a good electrical conductor, is chemically resistant, and is wetted by aluminium, and

- the lower part (14, 16) or its coating consists of an insulating material which is resistant to the liquid aluminium.

2. Cathode according to claim 1, characterised in that the upper part of the element (10) consists of plates (12) which are arranged vertically and the horizontal surface of which extends preferably over the whole region of the working surface of the corresponding anode (28).

3. Cathode according to claim 2, characterised in that the distance between the plates (12) is greater than their thickness.

4. Cathode according to one of the claims 1 to 3, characterised in that the elements (10) are stabilised further by intermediate pieces and/or cement which can withstand liquid aluminium.

5. Cathode according to at least one of the claims 1-4, characterised in that a whole number multiple between 1 and 7 of the horizontal surface dimensions of an element (10) corresponds to the horizontal surface dimensions of the overlying anode (28).

6. Cathode according to at least one of the claims 1 to 5, characterised in that windows (38) are formed in the electrically conductive cathode plates (12).

7. Cathode according to at least one of the claims 1-6, characterised in that the upper (12) and/or lower parts (14, 16) have a core of steel, titanium or graphite, which is coated with at least one appropriate material.

8. Cathode according to at least one of the claims 1-7, characterised in that the upper part (12) or its coating consists of titanium diboride, titanium carbide, titanium nitride, zirconium diboride, zirconium carbide, zirconium nitride or mixtures thereof.

9. Cathode according to claim 8, characterised in that the material of the upper part (12) or of its coating contains a small amount of boron nitride mixed into it.

10. Cathode according to at least one of the claims 1-9, characterised in that the lower part (14, 16) or its coating consists of highly sintered aluminium oxide, ceramics containing aluminium oxide, silicon carbide or silicon carbide bonded with silicon nitride.

Images