RU2299278C2 - Wettable coating on aluminum cell hearth applying method - Google Patents

Abstract

FIELD: non-ferrous metallurgy, namely production of aluminum by electrolysis of cryolite-alumina melts.

SUBSTANCE: method is realized during period of starting electrolysis bath directly from industrial cryolite-alumina electrolyte containing additives being sources of boron, titanium and(or) zirconium at condition when simultaneous deposition of metallic aluminum is not realized. Characteristics are set according to type and concentration of target or modifying additives added to electrolyte, electric current density values, cryolite relation, temperature and duration of deposition process. With use of measuring and monitoring instruments it is registered, mainly (more than 90%) deposition of diborides in coating together with presence of small quantities of associated chemical compounds. Liquid aluminum rapidly and totally wets surfaces of coated samples of carbon-graphite cathode blocks.

EFFECT: possibility for providing dense coating layer highly bonded with surface of cell hearth.

4 cl, 1 tbl

Description

The invention relates to the field of non-ferrous metallurgy, in particular to the production of aluminum by electrolysis of cryolite-alumina melts in electrolyzers, where the bottom (bottom) serves as a cathode, which is made from coal-graphite blocks. The surface of carbon-graphite blocks is not wetted by the liquid aluminum metal that is released, which creates known technological problems (magneto-hydrodynamic instability of the metal layer, increased wear of blocks, etc.), and also reduces the service life of industrial units due to the impregnation and penetration of fluoride salts through the cathode blocks to oxide lining, contributes to the degradation of blocks due to "sodium expansion". One of the effective methods for eliminating these drawbacks is to create an aluminum-wettable coating on the bottom of the electrolytic cell from materials resistant to molten aluminum and electrolyte. Borders of refractory metals, in particular titanium and zirconium - electrically conductive materials with high hardness, oxidation resistance and a sufficient level of adhesion to liquid aluminum are considered the leaders among the candidate materials for resistance in liquid aluminum, taking into account a reasonable cost.

A large number of methods and compositions have been patented for creating wettable hearth coatings by applying a layer of paint or mastic based on titanium boride powder with a carbon or oxide binder [1, 2]. The layer (or layers) deposited on the carbon-graphite cathode block in advance or during the installation of the electrolyzer is dried and fired to obtain a durable, wear-resistant, aluminum-wetted composite coating.

The technological possibilities of using such materials for wettable coatings are limited by the high cost and complexity of the synthesis of metal borides. In addition, the thermal expansion coefficients differ from the corresponding values for carbon graphite materials, which leads to cracking and peeling of the coating obtained on the surface of the carbon blocks before the firing procedure of the electrolysis bath. One cannot ignore the great risk of oxidative destruction of the coating during firing and starting up the bath.

Methods of forming a wettable boride coating on conventional [3] and “dry” inclined cathodes [4-6] by introducing small amounts of titanium and boron-containing components into the electrolyte are proposed. But if such methods are applied, the metal boride layer grows at a rate of the order of tenths of a millimeter per year. Obviously, such growth rates are clearly insufficient and cannot be considered as a direct coating method.

Another method is also known - electrochemical deposition of a boride coating during electrolysis of a molten electrolyte containing in the form of small additives the main coating components - titanium and boron. In this way, a wettable coating was obtained under laboratory conditions on graphite [7] by electrolysis of a melt based on sodium fluoride with the addition of 2-5 wt.% TiO ₂ and the same amount of Na ₂ B ₄ O ₇ . When the current density in the range of 0.05-0.3 A / cm ² recorded the formation of a dense and durable layer with a thickness of 0.02-3 mm, which was well wetted by liquid aluminum.

In a similar way, it is proposed to synthesize boride coatings from borate melts [8, 9] and alkali metal fluoride melts into which titanium and boron-containing components are introduced in the form of complex fluoride salts (fluoroborates and hexafluorotitanates) [10].

These methods are divided into two groups: (1) precipitation from oxide (only oxygen-containing) melts, (2) precipitation from fluoride, chloride, mixed fluoride-chloride melts. The introduction of oxygen-containing compounds in the composition of fluoride melts leads to poor quality of the resulting coating or makes the precipitation of borides impossible. In addition, the deposition techniques of such coatings are effective when using metal substrates; carbides are mainly formed on carbon-graphite materials.

A known method of obtaining a uniform, durable coating of titanium boride on metal and carbon graphite electrodes at current densities of 0.2-0.6 A / cm ² and introducing into the molten cryolite electrolyte 4 wt.% Al ₄ B ₂ O ₉ and 2 wt.% CaTiO ₃ [11]. But the proposed composition of the electrolyte (cryolite ratio - 3, 1, the content of calcium fluoride 1%, alumina 4%) makes it impossible to implement the proposed method in industrial electrolyzers due to the predominant deposition of titanium oxides.

Known methods that allow in principle the electrochemical deposition of a wettable boride coating have a very significant drawback: application to the cathode blocks can be carried out, as a rule, only outside the aluminum electrolysis cell, before its installation and firing. In addition to the need to create additional technological units for such application, after installing the bath, it is necessary to additionally cover the interblock joints, as well as protect the coating from oxidation during subsequent firing. With this scheme, the very meaning of electrochemical deposition technology is lost. The method becomes economically attractive and technologically feasible provided that the coating is applied “in situ” - directly during the start-up of the electrolyzer after its firing.

The closest set of essential features to the claimed is the method of applying a wettable coating, proposed in the patent [12], which is selected as the closest analogue. A method is proposed for creating an electrochemically deposited, wettable boride coating directly on a mounted hearth of an industrial electrolyzer. The deposition of the coating is carried out from a molten electrolyte based on sodium borates and tetraborates with the addition of titanium dioxide or alkali metal titanates at a temperature of 900-1000 ° C and a current density of 0.005-0.1 A / cm ² . Carbon materials or soluble titanium anodes are used as anodes. After the deposition process is completed, the electrolyte is removed from the bath (for example, washed off with water) and a regular bath is started.

Allowing the electrolytic coating directly on the mounted bottom of the electrolyzer, the known method nevertheless has significant drawbacks: (1) an additional operation of heating the bath is necessary, (2) high consumption of expensive borates and tetraborates used as an electrolyte, (3) as a result of the subsequent regular firing and starting the electrolyzer may peel off the coating due to the difference in thermal expansion coefficients, as well as its oxidation. These disadvantages determine, in essence, the inappropriateness of the industrial implementation of the method.

The objective of the proposed technical solution: obtaining electrodeposited boride wettable coating of improved quality and performance at low unit cost. This can be achieved by the technical result obtained by using the invention: the coating is applied “in situ” - directly during the start-up of the electrolyzer after its firing - using cheap additives compared to metal borides in industrial cryolite-alumina electrolyte, while ensuring the density and cohesion of the layer coatings with the surface of the hearth, independence from differences in the coefficients of thermal expansion of the coating layer and the material of the hearth, oxidation of the main component of the coating is excluded.

The known method of applying a wettable coating to the bottom of an aluminum electrolyzer is modified so that the application process is carried out during the start-up of the bath after firing the electrolyzer directly from an industrial cryolite-alumina electrolyte containing additives-sources of boron, refractory metals, in the absence of deposition of aluminum metal, which provides a change in parameters method, such as type and concentration of electrolyte additives, current density, cryolite ratio, temperature and deposition time. In this case, oxygen and / or fluorine-containing derivatives of boron are used as a source of boron, namely boron oxide, tetraborates of alkali and alkaline-earth metals, tetrafluoroborates of alkali and alkaline-earth metals in an amount of 0.5-2 wt.% Based on oxide boron, and as a source of refractory metals - additives of oxygen- and / or fluorine-containing derivatives of these metals, namely oxides, oxygen- or fluorine-containing compounds with cations of alkaline or alkaline-earth metal in an amount of 0.5-4 wt.% based on metal oxide precipitation ix carried out at a bath ratio equal to 2.3-2.9, a current density of 0.1-0.9 A / cm ^2, a temperature of 950-970 ° C during deposition wettable coating further introduced into the electrolyte boron, metal- components to provide a coating of the desired thickness, and during the initial application or regeneration of an existing coating in the post-launch period, coating is deposited after aluminum is removed from the bath.

From the closest analogue of the claimed method differs in that:

(1) - coating is carried out directly in the starting period from an industrial cryolite-alumina electrolyte;

(2) - deposition is carried out at a cryolite ratio of 2.3-2.9, a current density of 0.1-0.9 A / cm ² , a temperature of 950-970 ° C;

(3) - oxygen and / or fluorine-containing derivatives of boron are used as a source of boron, namely boron oxide tetraborates of alkali and alkaline-earth metals, tetrafluoroborates of alkali and alkaline-earth metals in an amount of 0.5-2 wt.% Based on boron oxide, and as a source of refractory metals - additives of oxygen- and / or fluorine-containing derivatives of these metals, namely oxides, oxygen- or fluorine-containing compounds with cations of alkali or alkaline earth metal in an amount of 0.5-4 wt.% based on oxide metal;

(4) - when a wettable coating is deposited, additional components containing boron and refractory metals are added to the electrolyte;

(5) - for the initial application or regeneration of an existing coating in the post-launch period, the coating is deposited after removal of aluminum from the bath.

Thus, the claimed method meets the criterion of "novelty."

If these synthesis conditions are not observed, obtaining a high-quality coating of the required phase and chemical composition becomes impossible: along with borides of refractory metals, foreign products (for example, titanium oxides) co-precipitate in significant quantities, the microstructure of the coating worsens, its connectivity with the base.

Comparison of the claimed solutions not only with the prototype, but also with other technical solutions in the art did not allow them to identify signs that distinguish the claimed solutions from the prototype, which makes it possible to conclude that the criterion of "inventive step".

To verify and comparatively test the effectiveness of the method, a number of boride coatings were tested over a range of parameters with the most stringent conditions for deposition, including those that can be implemented by industrial electrolysis, as well as using available reagents manufactured by the industry. For example, in most of the tested examples, the synthesis was carried out at a high alumina content of about 8%. With a decrease in the alumina content in the electrolyte to values of 2–3%, typical for electrolytes of industrial electrolyzers, the conditions and possibilities for the deposition of borides improve, and the content of impurity coprecipitating phases decreases. Examples of tested electrochemical wettable diboride coatings are summarized in the table below.

For preparative electrochemical deposition of a boride coating, an AutoLab PGSTAT 30 potentiostat with an AutoLab BSTR20A power amplifier was used as a measuring tool. The substrates were electrodes made of an “amorphous” hearth block with a graphite content of about 30%. Precipitation was carried out in a 1 liter crucible with 1 kg of electrolyte at a temperature of 950-970 ° C. A cryolite-alumina melt with a certain amount of dissolved alumina at a controlled cryolite ratio, in which additives containing boron and refractory metals in a given amount were introduced, was used as the main electrolyte composition. As refractory metals, titanium and zirconium were used.

The qualitative and quantitative composition of the coating was monitored by x-ray phase analysis at room temperature in a high-resolution Guinier IMAGE FOIL Guinier Camera 670 (using the JCPDS PDF-2 database).

Table No pp Application Conditions Control results one KO - 2.9 Composition - 95% TiB ₂ , 5% Alumina Content - 8% Ti ₂ O ₃ The content of additives is 1.07% TiO ₂ , Current output - about 0.93% B ₂ O ₃ 35% Current density: 0.45 A / cm ² Wetted by aluminum Duration - 30 minutes. 2 KO - 2.7 Composition - 92% TiB ₂ , 8% Alumina Content - 8% Ti ₂ O ₃ Additives - 0.53% TiO ₂ and 0.47% B ₂ O ₃ Current efficiency - 31% The current density is 0.45 A / cm ² Wetted by aluminum Duration - 30 minutes. 3 KO - 2.5 Composition - 87% TiB ₂ , 13% Alumina Content - 8% Ti ₂ O ₃ Additives - 0.53% TiO ₂ and 0.47% B ₂ O ₃ Current efficiency - 29% The current density is 0.25 A / cm ² Wetted by aluminum Duration - 50 minutes.

No pp Application Conditions Control results four KO - 2.3 Composition - 81% TiB ₂ , 19% Alumina Content - 8% Ti ₂ O ₃ Additives - 0.53% TiO ₂ and 0.47% B ₂ O ₃ Current efficiency - 25% The current density of 0.42 A / cm ² Wetted by aluminum Duration - 60 minutes. 5 KO - 2.7 Composition - 63% TiB ₂ , 37% Alumina Content - 8% Ti ₂ O ₃ Additives - 2.55% Na ₂ В ₄ O ₇ * 10Н ₂ O, 3.41% Current efficiency - 15% Al ₂ O ₃ , 1.07% TiO ₂ , 0.75% CaO Wetted by aluminum The current density is 0.15 A / cm ² Duration - 60 minutes. 6 KO - 2.9 Composition - 100% ZrB ₂ Alumina Content - 8% Current efficiency - 47% Additives - 1.65% ZrO ₂ and 0.93% B ₂ O ₃ Wetted by aluminum The current density is 0.12 A / cm ² Duration - 60 minutes. 7 KO - 2.9 Composition - 100% ZrB ₂ Alumina content - 4% Current efficiency - 53% Additives - 1.65% ZrO ₂ and 0.93% B ₂ O ₃ Wetted by aluminum The current density is 0.33 A / cm ² Duration - 60 minutes. 8 KO - 2.9 Composition - 98% TiB ₂ , 2% Alumina Content - 8% Ti ₂ O ₃ Additives - 2.78% Na ₂ TiF ₆ and 2.93% NaBF ₄ Current efficiency - 41% The current density is 0.4 A / cm ² Wetted by aluminum Duration - 60 minutes. 9 KO - 2.9 Composition - 100% ZrB ₂ , Alumina content - 4% Current efficiency - 59% Additives: 3.79% K ₂ ZrF ₆ and 2.93% NaBF ₄ Wetted by aluminum The current density is 0.25 A / cm ² Duration - 60 minutes.

In laboratory tests, when the deposition was carried out under conditions close to optimal, the predominant deposition of metal borides in the coating was unambiguously recorded along with the presence of accompanying chemical compounds in small quantities. Experiments on the wettability of the coating showed that liquid aluminum quickly and completely wetted the surface of the coated carbon-graphite samples already when they were immersed in the melt, while coatings applied in the form of composite paints or mastics usually required a wetting current application or longer contact time liquid aluminum.

From the test results shown in the table, it follows that in the range of the proposed synthesis parameters and type of reagents, the target technical result is obtained.

The proposed method of applying a wettable coating is preferably as follows.

(1) Filling the electrolyzer after firing with circulating electrolyte in the required standard amount.

(2) Introduction to the electrolyte of said additives in the declared concentration.

(3) Electrochemical deposition of the coating using standard anodes at standard temperature and current density of 0.1-0.9 A / cm ² .

(4) The introduction of additional quantities of additives as they are consumed during the deposition of the boride coating in order to maintain the concentration of reagents in the optimal range and obtain a coating of the desired thickness.

(5) The termination of the introduction of additives and the removal of the electrolyzer to the normal mode of operation after obtaining the coating of the desired thickness: as the additives are consumed, the limiting current of the release of metal boride decreases sharply, the emission of aluminum begins at the cathode.

The proposed method is an effective technical solution to create a cheap, wettable coating of the hearth and ultimately to increase the technological parameters of the process, increase the service life of aluminum electrolytic cells.

Information sources

1. Sekhar D.A., de Nora V. Suspension, carbon-containing cell component, method for applying refractory boride, method for protecting carbon-containing component, mass of carbon-containing component, electrochemical cell component, method for increasing oxidation resistance, cell for aluminum production and cell use / / Patent of Russia No. 2135643. 08/27/1999.

2. Boxall L.G., Buchta W.M., Cook A.V., Nagle D.C., Townsend D.W. Aluminum cell cathode coating method // U.S. Patent N 4,466,996. Aug. 21, 1984.

3. Gorlanov E.S., Barantsev A.G. A method of obtaining and maintaining a boride-containing refractory metal protective coating of carbon blocks // Patent of Russia №2221086. 10/01/2004.

4. Towsend D.W. Supersaturation plating of aluminum wettable cathode coatings during aluminum smelting in drained cathode cells, US patent 5028301, 2.07.1991.

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7. Shapoval V.I., Zarutsky I.V., Malyshev V.V., Uskova N.N., Current problems of electrochemistry of titanium and boron, synthesis of titanium diboride and its intermetallic compounds in ionic melts // Successes in Chemistry. 68 (1999) 1015.

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Claims (4)

1. The method of applying a wettable coating of the bottom of an aluminum electrolyzer, including electrolytic deposition of a coating of refractory metal borides from molten electrolyte on a mounted bottom of an aluminum electrolyzer, characterized in that the deposition is carried out directly in the starting period of electrolysis after firing the electrolyzer from an industrial cryolite-alumina electrolyte containing additives - sources of boron in an amount of 0.5-2 wt.% calculated on boron oxide, and sources of refractory metals in quantities e 0.5-4 wt.% calculated on metal oxide, with a cryolite ratio of 2.3-2.9, at 950-970 ° C and a current density of 0.1-0.9 A / cm ² . 2. The method according to claim 1, characterized in that oxygen and / or fluorine-containing derivatives of boron are used as a source of boron: boron oxide, tetraborates of alkali and alkaline earth metals, tetrafluoroborates of alkali and alkaline earth metals, and as a source of refractory metals - additives of oxygen- and / or fluorine-containing derivatives of these metals: oxides, oxygen- or fluorine-containing compounds with cations of an alkali or alkaline-earth metal. 3. The method according to claim 1, characterized in that when the coating is deposited, additional sources of boron and sources of refractory metal are added to the electrolyte to make up for their consumption. 4. The method according to claim 1, characterized in that for coating or regeneration of the coating in the post-launch period of electrolysis, the deposition of the coating is carried out after removal of aluminum from the cell.