RU2371523C1 - Composite material for moistened cathode of aluminium electrolytic cell - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: composite material for moistened cathode of aluminium electrolytic cell relates to field of non-ferrous metallurgy and, particularly to production techniques of aluminium by method of electrolysis of cryolite-aluminous melts. Composite material consists of moistened by liquid aluminium high-melting compound - titanium boride and binding agent, where in the capacity of binding agent it is used high-aluminous cement, herewith correlation of components of titanium boride: cement is selected 9:1.

EFFECT: it is provided electroconductivity of material, bringing to increasing of manufacturability, reduction of energy and labour costs, improvement of engineering-and-economical performance of manufacturing process of moistened material and product on its basis without reduction of level of functional and performance attributes.

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Description

The invention relates to the field of non-ferrous metallurgy, in particular to technology for the production of aluminum by electrolysis of cryolite-alumina melts.

In the current modern technology of electrolysis in horizontal electrolyzers, the cathode is a carbon-graphite hearth not wetted by molten aluminum, which is associated with a number of known disadvantages (increased wear and degradation of the hearth, refractory and heat-insulating lining, increased energy consumption, etc.). An aluminum-wetted cathode has significant advantages that eliminate these drawbacks, which is why it is a desirable element in the current horizontal technology and a necessary condition for the implementation of promising designs of electrolyzers, such as bathtubs with a drained cathode or with a vertical arrangement of electrodes [Sorlie M., Oue N.A. Cathodes in aluminum electrolysis. 2 ^nd edition. Aluminum-Verlag, 1994. 408 p.].

The wettable cathode of an aluminum electrolyzer, both horizontal and vertical, can be implemented in various ways, in particular by applying a wettable coating to a carbon-graphite or other conductive base in the form of a composite layer of a certain thickness containing a powder, usually titanium diboride, or by lining the conductive base with tiles from a composite based on the same diboride. Titanium diboride is the main functional component, highly conductive, well wetted by aluminum and, at the same time, weakly interacting with aluminum and electrolyte. Another main component of a wettable composite material is a binder - an organic substance (polymer resins, pecks) or inorganic (colloidal solutions of oxides, true solutions of complex salts, etc.) of nature.

In the manufacture of a wettable cathode material during heat treatment, the organic binder, undergoing pyrolysis, forms carbon, and the inorganic binder forms the corresponding oxide. In this case, the components of the material are bonded into a monolithic solid composite. The wettability of aluminum is achieved due to the high volume content in the finished composite of titanium diboride.

Known technical solutions for the implementation of a wettable coating of the bottom of aluminum electrolytic cells, variants of which are described in numerous patents (see, for example, Sekhar D.A., de Nora B. Suspension, carbon-containing cell component, method for applying refractory boride, method for protecting carbon-containing component, mass a carbon-containing component, a component of an electrochemical cell, a method for increasing oxidation stability, a cell for aluminum production and use of the cell. / RF Patent No. 2135643. 08.28.1999). In the proposed composite coating material, the binder is the so-called "colloidal alumina" - a stabilized colloidal suspension of an alumina nanopowder with particle sizes of 10-50 nm. During firing, solid-phase reactions occur between the diboride and the binder in the coating material with the formation of interfacial aluminates (for example, Al ₂ TiO ₅ ), which ensures the coherence and strength of the coating material. Such a binder is valuable in that:

- practically does not interact with molten aluminum and does not dissolve in it, as opposed to a carbon bond, which determines its great chemical wear resistance and provides the possibility of long-term operation of the cathode material;

- due to the formation of corundum from the binder also provides high mechanical wear resistance of the cathode;

- does not introduce additional extraneous impurities in the cathode aluminum.

A common drawback of cathode coatings is their small thickness, usually from 1 to 20 mm, which limits their service life due to the dissolution of the main functional component - titanium diboride - in aluminum melt, as well as mechanical damage, delamination, etc.

The closest analogue of the invention - the prototype - on the set of essential features is the technical solution described in the patent Sekhar JA, Duruz JJ, de Nora V. Production of bodies of refractory borides. / US Pat. N, 5,753,163. 05/19/1998. A wettable cathode material was implemented on an inorganic binder - “colloidal alumina”, from which it is proposed to produce bulk products, for example, tiles with a thickness of at least 3 mm, glued to a carbon-graphite cathode and giving it the wetting property of aluminum. The material was produced by injection molding or pressing under pressure of a powder slip, consisting of> 90% titanium diboride and a colloidal alumina solution (<10% in terms of dry Al ₂ O ₃ ) with further heat treatment-firing of green workpieces at temperatures up to 1600 ° C in an atmosphere of inert gas - argon to prevent the oxidation of diboride. Indicate that with a higher alumina content, the material becomes non-conductive.

Significant disadvantages of the proposed solution are: low manufacturability of manufacturing bulk products and high energy intensity of the final operation of the technology - firing, which is carried out at high temperature, as well as the use of expensive "colloidal alumina" as a binder.

The objective of the invention is the creation of an aluminum-wetted cathode material, which can be used both for the manufacture of bulk lining products (bricks, tiles, etc.) for the bottom of a horizontal electrolyzer, and a monolithic cathode in the form of plates, bars of the necessary design for a vertical electrolysis apparatus.

Thus, the technical result obtained by using the proposed invention consists in improving manufacturability, reducing energy and labor costs, improving technical and economic indicators of the production process of wettable material and products based on it without reducing the level of functional and operational properties.

The technical result is achieved by the fact that in the composite material for a wettable cathode of an aluminum electrolyzer, consisting of a refractory compound wettable by liquid aluminum - titanium diboride and a binder, it is new that high-alumina cement is used as a binder, and the ratio of the components of titanium diboride: cement is 9: 1, to ensure the electrical conductivity of the material.

These signs allow us to conclude that the claimed technical solution meets the criterion of "novelty." When studying other well-known technical solutions in this technical field, the features that distinguish the claimed invention from the prototype are not identified, and therefore they provide the claimed technical solution with the criterion of "inventive step".

The essence of the proposal is that industrial high-alumina cement is used as a binder in the cathode composite material based on titanium diboride, and the ratio of components of titanium diboride: cement is chosen so as to ensure the electrical conductivity of the material and the entire cathode, i.e. 9: 1. The conductivity of the material must be sufficient to start the electrolyzer. With a lower ratio, the material has too low electrical conductivity, and with a larger ratio, its mechanical strength is reduced.

The claimed composite material for a wettable cathode differs from the prototype in that high-alumina cement is used as a binder, produced on an industrial scale and, therefore, affordable and cheap.

To test the proposed technical solution, the manufacturing technology of the composite material of the TiB ₂ / Al ₂ O _{3 system} , its testing and testing of functional properties, a number of options for the material composition of the material were tested under laboratory conditions. A titanium diboride powder of the -44 micron fraction was used and Alit-97 industrial high-alumina cement manufactured by Aliter-Axi LLC. The cement was further screened from a coarse filler on a 500 micron sieve. The manufacturing technology of composite samples followed the recommendations of the manufacturer for the use of cement: mixing the initial powders for 5-10 minutes, mixing with a minimum amount of water and mixing, filling the mold with simultaneous vibration, curing the mixture, drying with a gradual increase in temperature to 150 ° C, further temperature increase and annealing at 650 ° С for 1 h. Annealing was carried out in a closed container under a carbon backfill to avoid possible oxidation of titanium diboride at elevated temperatures.

Table
Compositions and properties of synthesized samples of materials No pp Composition, wt.% Resistance, Ohm Relation density,% Strength, MPa one 50 TiB ₂ -50 Alit-97 > 10 ⁶ 64 ... 68 60 ... 70 2 65 TiB ₂ -35 Alit-97 > 10 ⁶ 63 ... 65 50 ... 60 3 80 TiB ₂ -20 Alit-97 ~ 10 ... 100 62 ... 65 30 ... 40 four 90 TiB ₂ -10-97 Alite <1 60 ... 64 15 ... 20

The strength of the cathode material after the final heat treatment, as well as the resistance to thermal stresses, were quite high, they were controlled qualitatively by shock and hard thermal cycling (rapid heating to 800 ° C — cooling in air at room temperature), and they were also subjected to compression measurements. No visible damage from thermal shock was observed. The compressive strength decreases with a decrease in the cement content, but remains at a level acceptable for the target technical application (see table). The electrical resistance of samples with a size of about 15 × 15 × 60 mm was evaluated semi-quantitatively using an electronic ohmmeter. Acceptable values were considered at a level of not more than 10 ohms. As in the prototype, a high level of electrical conductivity of the composite was observed in compositions with titanium diboride content of about 90%.

Samples made from the proposed composite material were tested as vertical cathodes in a laboratory electrochemical cell under the following conditions: cryolite ratio KO = 1.8, the electrolyte is saturated with alumina, the electrolysis temperature is 920 ° C, the working current density at the cathode is 0.92 A / cm ² (total cell current - 31 A), test duration - 5 hours. Graphite rods, periodically replaced during electrolysis, were used as anodes. At the initial stage of electrolysis (up to 20-30 min), a sharp decrease in the operating voltage of the cell was observed, associated with a decrease in ohmic losses in the cathode as its surface is wetted and an aluminum layer is formed. Visual inspection after the test showed that during the electrolysis the cathodes were wetted and covered with an aluminum film, while the external dimensions and shape of the cathode did not change after the tests.

In the inventive composite material for a wettable cathode of an aluminum electrolyzer, high-alumina cement is used as a binder, produced on an industrial scale and, therefore, affordable and cheap.

The combined advantages of the technology of manufacturing products from aluminum wettable composite material using cement provide the technological and economic efficiency of the proposed solution. In addition, the manufacturing technology of wettable material on cement is universal and allows you to apply the material in the form of a coating, to produce bulk products in the form of tiles, bricks, etc. for lining carbon-graphite materials of an aluminum electrolyzer, cast the blocks “in place”, directly on the bottom of the electrolyzer, and also obtain cathodes for promising vertical designs of electrolyzers of arbitrary sizes and shapes by casting or pressing.

Claims (1)

A composite material for a wettable cathode of an aluminum electrolyzer, consisting of titanium diboride wetted by liquid aluminum and a binder, characterized in that it contains high alumina cement as a binder with a ratio of titanium diboride: cement components of 9: 1.