RU2518032C1 - Compound for material of cathode wetted coating in aluminium electrolyser - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: in powdered compound for material of cathode wetted coating in aluminium electrolyser that contains a functional component of titanium diboride, a binding agent and an inert filler the binding agent includes two types of binding agents simultaneously - inorganic one and namely a saturated acid solution of aluminium trichloride and organic one and namely a polymer with high coke value.

EFFECT: improving moulding characteristics of the powdered compound, increasing mechanical strength and electroconductivity of the material synthesised out of it, improving quality and serviceability of the final cathode wetted coating thus improving its service life and cost-performance ration of the electrolyser.

Description

The invention relates to the field of non-ferrous metallurgy, in particular to the electrolytic production of aluminum from cryolite-alumina melts.

At present, a large number of technical solutions have been proposed for the implementation of an aluminum-wettable coating material for the carbon-graphite cathode of the electrolyzer, including titanium diboride powder (TiB ₂ ) as a main functional component, which is a solid, electrically conductive, high-temperature compound wetted by aluminum, but chemically weakly interacting with molten aluminum and fluoride electrolyte.

Due to the high melting point of TiB ₂ , the insufficient resistance to thermal stresses of ceramics, and the high cost of the initial powders, it is proposed to be used in the application under consideration as part of non-sintered composites, the strength of which is ensured by high-temperature binders formed during firing of inorganic or organic substances present in the initial composition -relations [Sorlie M., Oue N.A. Cathodes in aluminum electrolysis. 2 ^nd edition. Aluminum-Verlag, 1994. - 408 p.]. Additions of fillers inert to aluminum to powders of corundum or carbon in various forms reduce the total content of diboride and significantly reduce the cost of the finished material wetted by aluminum. Therefore, the initial composition, from which a wettable cathode material with adequate application properties is formed and as a result of firing, should contain TiB ₂ powder, filler powder and a binder having the property of a low-temperature binder for bonding powder components at low temperatures and the properties of a high-temperature binder after firing.

Numerous technical solutions are known that are proposed for the implementation of wettable cathode materials from compositions with binders on an organic basis [US No. 3,400,061. Publ. 09/03/1968; GB No. 1138522. Publ. 01/01/1969; US No. 4,466,996. Publ. 08/21/1984; US No. 4,526,911. Publ. 07/02/1985; RU No. 2418888, C25C 3/08, publ. 05/20/2011; et al.] - these are polymeric compounds with a high coke number — pitches or artificial resins (for example, thermosetting phenol-formaldehyde resin), which after thermal degradation leave up to 50-60% of the carbon residue in the form of a strong coke skeleton.

The main advantages of the material on an organic (carbon) binder:

- good electrical conductivity of the finished calcined material, provided not only by the diboride component, but also by carbon;

- the coefficient of thermal expansion of the diboride-carbon material differs little from the carbon-graphite material of the hearth, which provides a strong bond between them and reduces the possibility of delamination, warping and cracking of the coating or lining layer;

- the presence of carbon in the composite partially protects the diboride phase from oxidation, which is important during the start-up of the cell.

A disadvantage of the carbon-bonded cathode material is its increased wear rate due to the dissolution of the carbon component in the form of Al ₄ C ₃ carbide, which is soluble in aluminum, which forms liquid aluminum-carbon at the interface at the electrolysis temperature.

Technical solutions are also known for compositions for a wettable coating of the bottom of aluminum electrolyzers on an inorganic binder, the so-called "colloidal alumina" - a stabilized colloidal suspension of aluminum oxide nanopowder with particle sizes of 10-50 nm [US No. 5,534,119. Publ. 07/09/1996; RU No. 2135643, C25C 3/06, C25C 3/08, C23C 20/08, C04B 35/58, publ. 08/27/1999; US No. 2001/0046605. Publ. 11/29/2001; et al.] or corundum refractory cement [RU No. 2412284, C25C 3/08, C04B 35/58, publ. 02/20/2011]. When firing such compositions, solid-phase reactions occur between the diboride and the binder, with the formation of complex interfacial aluminum titanates, which ensures the coherence and strength of the material.

Alumina binder has the following advantages:

- practically does not interact with molten aluminum and does not dissolve in it, in contrast to the carbon bond, which determines its chemical wear resistance and provides the possibility of long-term operation of the wetted 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.

The closest technical solution, selected as a prototype, is a composition based on TiB ₂ / acidic, saturated solution of aluminum chloride [RU No. 2412283, C25C 3/08, C23C 20/08, C04B 35/58, publ. 02/20/2011], which serves to obtain the material of the protective, wettable coating TiB ₂ / Al ₂ O ₃ synthesized during the firing of the electrolyzer. The composition is technologically advanced and can be applied on a carbon-graphite base in the form of paint, mastic or used for the manufacture of bulk material. The resulting coating material has chemical and abrasive wear resistance, good wettability with liquid aluminum.

However, despite the high content of titanium diboride, the finished material synthesized from the composition has an insufficient level of strength and electrical conductivity. A significant content of titanium diboride determines the high cost, which makes it economically impractical to use, especially for the production of bulk products.

The technical result obtained by using the invention consists in improving the molding characteristics of the powder composition, increasing the mechanical strength and electrical conductivity of the material synthesized from it, due to the simultaneous introduction of inorganic and organic binders in the composition, as well as improving the quality and service properties of the final wetted material cathodic coating, which helps to increase its service life and increase the technical and economic indicators of the cell.

The technical result is achieved by the fact that in the composition for a wettable coating material of the cathode of an aluminum electrolyzer containing titanium diboride powder, an organic or inorganic binder, inert fillers, it is new that both types of binders are contained simultaneously - inorganic and organic, and the thermosetting phenol formaldehyde is the organic binder resin.

For a wettable cathode material intended to be used both as bulk products and as a thin layer, it is important that, along with the wettability of aluminum, it possesses a sufficient level of strength and electrical conductivity. In this regard, the objective of the invention is to develop such a composition for a material of a protective, wettable coating of the cathode of the electrolyzer, which would provide the necessary quality of the coating at an affordable cost. For this purpose, it is proposed to simultaneously introduce both known types of ligaments — organic and inorganic — into the powder composition for the wettable coating material of the bottom of the aluminum electrolyzer, which, on the one hand, improves the strength of the finished material, and on the other hand, provides good electrical conductivity and better adhesion to the bottom due to the presence of carbon. In addition, the presence in the material of both types of binder having a different chemical nature and, therefore, a different mechanism of destruction in contact with liquid aluminum, contributes to a longer service life of the wetted material.

The known composition for a material of a protective, wettable coating of the bottom of an aluminum electrolyzer, consisting of TiB ₂ / inorganic binder / additives, is modified in such a way that two types of bundles are taken as a binder - inorganic (saturated acidic solution of aluminum trichloride), which yields highly dispersed aluminum oxide during thermolysis , similar to colloidal alumina, and organic - a polymer with a large coke number (phenol-formaldehyde resin, which gives a strong coke residue during thermolysis). An inert, non-conductive corundum powder may be used as an inert additive.

From the prototype, the claimed composition for the material of the wettable coating of the cathode of an aluminum electrolyzer is different in that both types of binders are used simultaneously — inorganic and organic. As an organic binder, rezol phenol-formaldehyde resin is used.

These differences allow us to conclude that the proposed technical solution meets the criterion of "novelty." Signs that distinguish the claimed method from the prototype are not identified in other technical solutions in the study of this and related fields of chemistry and, therefore, provide the claimed solution with the criterion of "inventive step".

The essence of the proposal is that two types of binders are introduced into the composition in the form of thermally unstable chemical compounds of inorganic (saturated acid solution of aluminum trichloride) and organic (rezol phenol-formaldehyde resin) nature. During thermal degradation, during heating, they leave highly dispersed, reactive residues that easily form chemical bonds due to the synthesis of new interfacial compounds, which ensures the binding of the composition powders into a solid, monolithic body. Further, the residual carbon from the organic binder, uniformly distributed in the composition, forms a continuous, conductive network, significantly increasing the electrical conductivity of the material and making it possible to lower the content of expensive titanium diboride, replacing it with a non-conductive, but chemically and mechanically wear-resistant and cheap filler powder - corundum. And in addition: two physicochemical binding mechanisms with different chemical nature increase the resistance of the material against degradation processes during cathode operation and increase the service life.

The specified solution of aluminum chloride, proposed in the prototype, it is advisable to use for the following reasons: during thermal degradation, it leaves a nanodispersed residue uniformly distributed over the developed surface of the powder component of the composition and already at a content of about 5-6% effectively binds the powders into artificial stone. This solution is simple and technologically advanced to obtain, the starting components are affordable and cheap, it is stable during long-term storage.

Thermosetting phenol-formaldehyde resin is also known to be most effective in this application because of the high coke number, strong coke residue crosslinked, the lowest cost and availability among other synthetic polymers with the required properties, which is important for the implementation of large-tonnage production in industry. The advantage of this binder over cheaper pitch is its greater efficiency and manufacturability: the resin is completely soluble in volatile alcohols and ketones, which makes it easy to enter it into a powder mixture and obtain a homogeneous composition, and the solvent can be quickly removed during the drying process. If wettable cathode products, for example in the form of tiles, are made separately, the solvent can be easily regenerated. In addition, the resin has a lower curing temperature, which is also a technological advantage.

An experimental verification of the proposed technical solution was carried out with different composition and temperature of the heat treatment of the samples. The titanium diboride powder of the fraction — 44 μm was used; the inert additive was the corundum powder of the fraction — 63 μm. An inorganic binder is a saturated acidic solution of aluminum trichloride (indicated below by ATX), synthesized on the basis of recommendations [Yu. V. Karyakin Pure chemicals / Yu.V. Karyakin, Angelov I.I. - M .: Chemistry, 1974. - 408 pp.] From industrial hydroxide Al (OH) ₃ (this is a transparent, sticky yellowish solution with a density of about 1.33 g / cm ³ , the dry residue after calcination is about 13.5%), as an organic binder - rezol phenol-formaldehyde resin (FFS) grade CT1138 (TU 6-07-487-95).

When the mass content of the main functional component of titanium diboride in the finished, calcined material is less than 30%, its wettability sharply worsens with aluminum and therefore the experiments were mainly carried out on compositions without inert fillers or with a TiB ₂ content close to the minimum possible. The number of binders of the composition: 10% - ATX solution and 6% - dry FFS, was selected based on the achievement of effective binding and obtaining the necessary consistency and humidity of the composition for semi-dry pressing of samples.

The components are thoroughly mixed, the samples for measurements are molded in a steel mold at a pressure of 100 MPa. Firing is carried out in a closed container under a layer of carbon backfill to avoid oxidation of the diboride and carbon components at a temperature of 1323 K with preliminary drying at 473 K. The compressive strength is measured on an IP-100 measuring press, the specific resistance of samples at room temperature is evaluated on rectangular bars using multimeter, after metallization of the ends of the bar with silver paste PP-17S (TU 6365-006-59839838-2004).

The effect of the composition composition at two main heat treatment temperatures on the properties of pressed samples is shown in table 1.

Table 1 The effect of composition on material properties No pp Composition, wt. % Drying and firing temperatures, K Compressive strength, MPa Beats resistance, Ohm · m × 10 ³ one* 35TiB ₂ -55Al ₂ O ₃ -10ATX 473 8.6 ~ 10 ⁵ one 35TiB ₂ -55Al ₂ O ₃ -10ATX 1323 76.6 1.44 2 * 35TiB ₂ -49Al ₂ O ₃ -10ATX-6FFS 473 64.6 ~ 10 ⁵ 2 35TiB ₂ -49Al ₂ O ₃ -10ATX-6FFS 1323 129.4 1.19 3 * 84TiB ₂ -10ATX-6FFS 473 86.7 twenty 3 84TiB ₂ -10ATX-6FFS 1323 136.6 0.13 four* 94TiB ₂ -6FFS 473 41 fifteen four 94TiB ₂ -6FFS 1323 240 0,07

As can be seen from the above data, the simultaneous presence of inorganic and organic ligaments in the initial composition leads to significant hardening of the compact material both at low and high heat treatment temperatures. At the same time, an acceptable low resistivity of the material is ensured in cheap samples with a filler, which is favorable for reducing electrical losses. At a qualitative level, a higher manufacturability of the composition was also noted: molded samples have greater strength and less tendency to delamination during molding.

The technologically proposed composition can be used in the form of layers of paint, mastic applied to a carbon graphite hearth and fired when starting the electrolyzer, pre-made by semi-dry pressing and fired tiles for lining the cathode elements, or a pressed layer of the required thickness on the hearth blocks during the manufacturing of the latter. The difference of the composition in possible applications lies in its rheological properties, which are decisively determined by the solvent content.

The advantages of the proposed technical solution are that it becomes possible to replace a significant part of the diboride with an inert, non-conductive filler (corundum), which increases the functional properties of the finished composite material and reduces cost indicators, as well as improving the molding characteristics of the powder composition, increasing mechanical strength and electrical conductivity the material synthesized from it, due to the simultaneous introduction of an inorganic (saturated acidic solution of trichloride and aluminum) and organic (thermosetting phenol-formaldehyde resin) in the binder composition, as well as improving the quality, utility properties of the final material wettable cathodic coating, which contributes to an increase in its service life and improvement of technical and economic parameters of the cell.

Claims (1)

Composition for coating a wettable cathode of an aluminum electrolyzer, containing titanium diboride powder, a binder and inert fillers, characterized in that it contains a binder in the form of inorganic and organic bonds, moreover, a thermosetting phenol-formaldehyde resin is used as an organic bond, and a saturated acidic one is used as an inorganic bond aluminum trichloride solution.