Impregnated graphitic cathode for electrolysis of aluminum. RU patent 2245396.

FIELD: metallurgy; production of graphitic cathodes.

SUBSTANCE: the invention presents an impregnated graphitic cathode for production of aluminum by electrolysis and is pertinent to the field of metallurgy, in particular, to production of the graphitic cathodes used in production of aluminum by electrolysis. The invention offers an impregnated graphitic cathode for electrolysis of aluminum and a method of its production. The cathode contains in its pores an impregnating product heat-treated. At that in the capacity of the impregnating product the cathode contains a carboniferous product heat treated under the temperature of no less than 1600°С to provide resistance to erosion at the expense of protection by the formed graphitized binding substance. The method includes production of the graphitic cathode, its impregnation by dipping into the impregnating product in vacuum and a thermal treatment. At that the graphitic cathode is produced from coke, with graphite or without it, and also from a pitch, and before impregnation it is exposed to calcination at the temperature exceeding 2400 °С. The impregnation is realized by a carboniferous product at the temperature of its viscous state and the thermal treatment of the impregnated cathode is conducted at the temperature of less than 1600 °С, but sufficient for hardening and-or sintering of the impregnating product and formation of the non-graphitized coal layer for protection of graphitizing binding substance against erosion. The technical result is an increase of service life of the graphitic cathode.

EFFECT: the invention ensures an increase of service life of the graphitic cathode.

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The object of the present invention is a graphite cathode for the electrolysis of aluminium.

In the electrolysis process used in installations for the production of aluminum electrolytic bath contains a cathode consisting of several placed side-by-side cathode blocks, metal tank, lined with refractories. This node forms the crucible, which - after seals slurry of the refractory lining is a place of transformation electrolytic solution in the aluminium under the action of electric current. This reaction occurs at a temperature, which in the General case exceeds 950°C. to withstand thermal and chemical conditions prevailing during operation of the bath, and to satisfy the need for conduction of electric current, the cathode block is made from carbon-containing materials. These materials range from poluproffesionalnye to graphitized. They are formed by extrusion or vibration compaction after mixing the following raw materials:

or a mixture of pitch, calcined anthracite and/or graphite in the case poluproffesionalnye and graphitized materials, after mixing these materials burn at approximately 1200°and graphitized cathode does not contain anthracite, a cathode made from these materials, usually called "coal cathode";

or a mixture of pitch and coke, graphite or without graphite, in the case of graphite, in this case, the material is fired at approximately 800°and then graphicsyou at temperatures above 2400°With such a cathode is called "graphite cathode".

It is known the use of carbon cathodes, which, however, have moderate electrical and thermal properties, which are not suitable for continuous operation under the operating conditions of the electrolysis modern baths, especially at high current. The need to reduce energy consumption and the possibility of increasing the current, especially in modern collections baths, has led to attempts to use graphite cathodes.

Standard processing at temperatures above 2400°in the case of graphite cathodes allows you to increase specific power and thermal conductivity, thus creating conditions for optimized operation of the electrolytic bath. The power consumption is reduced because of the drop in the electrical resistance of the cathode. Another advantage of the fall of the electric resistance is to increase the intensity of the current supplied to the bath, which gives the possibility to increase the production of aluminum. Then a high value of thermal conductivity of the cathode allows to eliminate the excess heat generated by the increased current. In addition, bath with graphite cathodes showed less electrically unstable, that is, having a smaller fluctuation of the electric potential than baths with carbon cathodes.

However, it was found that the bath is equipped with graphite cathodes, have a shorter life than the bath, are equipped with carbon cathodes. Baths with graphite cathodes fail when excessive enrichment of aluminum with iron, which is the result of corrosion of the cathode rod caused by aluminum. This metal rod reaches as a result of erosion of a graphite block. Although the observed erosion of carbon electrodes, it is much smaller and does not affect the service life of the baths, which fail for reasons not related to the erosion of the cathode.

In contrast, the graphite electrode wear occurs quickly enough, is the main cause of failure of baths for aluminum electrolysis after the premature expiration of the service life compared to life-time, registered in the case of baths, equipped with a graphite cathode. So, registered following the wear rate of various materials:

The cathode of the Rate of wear (mm/year) poluproffesionalnye coal 10-20 graphitized coal 20-40 40-80 graphite

Figure 1 depicts the cathode unit 3 with the cathode tocaptain rods 2, the source profile are indicated by position 4. Profile 5 after erosion, indicated by the dotted lines, shows that this erosion is concentrated at the ends of the cathode block. Consequently, the rate of erosion of a graphite cathode block is its "weak", so it is attractive from an economic point of view - in the context of production - can be disavowed if you cannot increase the service life.

Although their manufacture begins with raw materials, coal cathodes and graphite cathodes in the form of finished products consist of solid grains of graphite and differ significantly on the parameters of the heat treatment given to the binder. Peck graphite products processed during firing the product at a temperature close to 1200°S. Binder graphite cathode is heated during graphitization temperatures of over 2400°and therefore it is transformed into graphite.

The porosity of coal and graphite cathode is the result of sintering of the binder. However, during operation of the bath products of electrolysis, mainly fluorides of sodium and aluminum, take these pores. Therefore, these products come in contact with the carbon electrode or graphite, released from the binder.

In the document "Chemical Abstract", t, No. 22, speaks of impregnated cathodes to block pores and prevent the penetration of the reaction products. This impregnation is performed with the use of the products other than pitch and tar, which, as the experience of the inventor, not effective, because they are not moisturize carbon electrode.

The document JP 02283677 relates to electrodes for electric discharge machining. These electrodes are soaked and annealed before subjected to standard heat treatment when compiled between 2,600 and 3,000°C.

The document EP 0562591 relates to a method for impregnation of coal and graphite at room temperature using Pekov treated with resins to obtain outputs suitable products with impregnation in excess of 40%, after carbonization impregnating substances. This document is not part of the aluminum electrolysis, nor to the problem of erosion of graphite cathodes.

The document JP 54027313 refers to the electrode impregnated resins for the production of chlorine.

The objective of the invention is to develop a graphite cathode with a longer lifespan. With this purpose, this cathode contains inside then its structure carbon-containing product, annealed at a temperature of less than 1600°and to increase the resistance to erosion by protecting graphitized binder.

Carbon-containing product administered by implementing it in a graphite cathode obtained in a known manner.

Carbon-containing product, annealed at a temperature of less than 1600°ensures that graphitized filler in the pores of the cathode is protected and increases the resistance of the cathode to erosion. This product is deposited on graphitized binder substance, oblicovka, but without closing the pores, which are necessary for the flow of products coming from the electrolytic bath. Due to its location between the products from the bath and graphitized binder impregnating the product protects graphitized binder from deterioration due to reaction with the components of the bath, which migrate into the pores of the cathode. Due to heat treatment at a low temperature in comparison with graphite impregnation product is more resistant to the components of the bath.

Carbon-based product that protects graphitized substance selected from coal tar pitches and petroleum pitches.

According to one variant embodiment of the invention is a method of obtaining such a cathode is that the injected carbon-containing product in liquid form into the pores, protecting graphitized binder. For example, if carbon-containing impregnating the product is coal tar pitch, it is heated to a temperature of about 200°With, in order to obtain a satisfactory viscosity.

One method of manufacturing a cathode in accordance with the invention is that first receive the cathode in a known manner from coke - graphite or without graphite - pitch, subjected to heat treatment at a temperature in excess of 2400°With, put this cathode in the autoclave after the optional pre-heating it to a temperature corresponding to the temperature at which a sealing product has the desired viscosity, create a vacuum in the autoclave, introducing a sealing product in liquid form into the autoclave until then, while the cathode will not be completely submerged, remove the vacuum in the autoclave by discharge of compressed gas to ensure, depending on the duration of the treatment, partial or complete filling of the pores in the cathode insulating product, return the autoclave to atmospheric pressure, remove the cathode from the autoclave and finally, after a possible cooling, conduct heat treatment at a temperature of less than 1600°but sufficient for curing and/or sintering impregnating product and education thus negrification coal layer, which protects graphitized binder from erosion.

The purpose of the heat treatment carried out after impregnation, is to stabilize a sealing product. This may be necessary in special collections electrolysis baths or during preheating of the electrolysis bath and during operation of the latter.

It can be noted that the impregnation can be carried out on the entire cathode or only part of it. When you want only partial impregnation, it is necessary to make the surface of the block is impermeable or only partially immerse the block in the impregnating liquid.

To strengthen the impact of the treatment can, if desired, to spend several consecutive cycles of impregnation and calcination.

In any case, the invention can be better understood using the following description, given with reference to the accompanying drawings, representing - as a non-restrictive example graphite cathode and a device for impregnation of the cathode, and

figure 1 is a conventional image of the cathode,

figure 2 - picture of the device for impregnation of the cathode carbon-based product.

1 was described above, to show the profile of the erosion of a graphite cathode after working for some time.

Figure 2 depicts a device for impregnation containing autoclave 6, intended for making it a graphite cathode 3. This autoclave 3 may be connected via a line 8 with the tank 7 for storing carbon-containing impregnating product, as well as through the line 9 with a source of vacuum through line 10 from a source of compressed gas.

After the graphite block, intended for the formation of the cathode obtained in the usual way with the operation graphitization at a temperature in excess of 2400°With this cathode unit 3 is placed in the autoclave 6. Carbon-containing product 12 is stored in the tank 7 and is subjected to an optional heat to bring liquid to achieve a viscosity that ensures that it will easily penetrate the pores of the cathode. Graphite block 3 and the autoclave is heated to the same temperature.

The vacuum in the autoclave 6 creating, opening the line 9.

Maintaining a vacuum in the autoclave, introducing carbon-containing product 12 in the autoclave 6 up until the graphite block 3 will not be completely submerged. Since then the line 8 is closed, the vacuum is eliminated by the discharge of compressed gas through the line 10. Under the influence of the generated thus hydrostatic pressure impregnating the product penetrates into the pores in the product. Processing time calculated to provide full or partial filling of the pores in the product.

Finally, return the pressure to atmospheric pressure, remove the graphite block 3 from the autoclave and cooled, if necessary. You can then expose the graphite block heat treatment at a temperature of less than 1600°With, moreover, this heat treatment depends on the nature of the carbon-containing product 12.

Below is an example of processing of a graphite cathode.

Fully graphite cathode having a size of 650×450×3300 mm, impregnated with impregnating pitch. Impregnating pitch is coal tar pitch having a temperature of Mettler 95°and the number is not soluble in toluene matter less than 6%. Peck pre-heated to a temperature of 200°C. Immediately after reaching this temperature, the autoclave vacuum to obtain a residual vacuum of less than 10 mm Hg (760 mm Hg=101,300 PA). Then enter the hot baked in the autoclave by means of suction. At the cathode, immersed in pitch, close the inlet valve and serves gaseous nitrogen into the autoclave under a pressure of 10 bar (1 bar=10 5 PA). After pressurizing the autoclave for one hour open it, and the product cooled.

Comparison of the mass of the cathode before and after treatment shows the estimated increase in mass by 19%. Theoretical calculation based on the porosity of the product and the density of the impregnating pitch makes it possible to conclude that this absorption of all the pores of the cathode filled with impregnating substance. The product is then calcined in a reducing atmosphere at a temperature close to 1000°C. firing Operation leads to the fact that the pores open again, and in them there is a part of a sealing product. Below is a comparison of the characteristics of the impregnated cathode with characteristics not impregnated cathode.

Graphite cathode not impregnated Impregnated Change (%) bulk density 1,593 1,744 +9,5 bending Strength (MPa) 10,6 17,3 +63,5

After firing the increase in weight is 9.5%and the increase of Flexural strength is very great, and it proves that micro-cracks sealed impregnating pitch, and thus proves a good spreading of the impregnating pitch on graphitized bake.

From the foregoing it is obvious that the invention can significantly improve the existing technique by providing a graphite cathode conventional structure, electrical properties and properties of thermal conductivity which is completely preserved, and the wear is significantly limited compared with the conventional cathode.

Needless to say, that the invention is not limited to one particular variant of implementation of the cathode, to any ways of implementing the method described above as examples, on the contrary, the invention covers all variants. Thus, in particular, to expose the graphite cathode several consecutive treatments, you can use several different carbon-containing product or to conduct processing on only one surface of the block, for example - on the surface, corresponding to the ends of the cathode, while staying within the scope of the claims of the invention. Creating a vacuum, the injection pressure or full immersion is not required if you want to treat by dipping or localized heat treatment to a predetermined region of the cathode.

1. Impregnated graphite cathode for electrolysis of aluminum, containing in the pores of a sealing product and heat-treated, characterized in that a sealing product it contains carbon-containing product and heat-treated at a temperature of less than 1600°to provide resistance to erosion by protection of the resulting graphitized binder.

2. Impregnated graphite cathode according to claim 1, wherein the carbon-containing product is introduced into the pores by impregnation in a vacuum.

3. Impregnated graphite cathode according to claim 1 or 2, characterized in that the carbon-containing product, protecting the resulting graphitized binder, selected from coal tar pitches and petroleum pitches.

4. A method of manufacturing impregnated graphite cathode for electrolysis of aluminum, including the production of a graphite cathode, its impregnation by immersion in a sealing product in vacuum and heat treatment, characterized in that the graphite cathode is obtained from coke, graphite or without graphite, and of the pitch, before impregnation it is subjected to firing at a temperature in excess of 2400°With, carry out impregnation of the carbon-containing product at the temperature of its viscous state, and the heat treatment of the impregnated cathode is carried out at a temperature of less than 1600°but sufficient for curing and/or sintering impregnating product and education negrification coal layer to protect the standard binders from erosion.